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Molecular Manganese Compounds as Single-Molecule Magnets: A Molecular Approach to Nanoscale Magnets

Permanent Link: http://ufdc.ufl.edu/UFE0010832/00001

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Title: Molecular Manganese Compounds as Single-Molecule Magnets: A Molecular Approach to Nanoscale Magnets
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Copyright Date: 2008

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Source Institution: University of Florida
Holding Location: University of Florida
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System ID: UFE0010832:00001

Permanent Link: http://ufdc.ufl.edu/UFE0010832/00001

Material Information

Title: Molecular Manganese Compounds as Single-Molecule Magnets: A Molecular Approach to Nanoscale Magnets
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0010832:00001


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MOLECULAR MANGANESE COMPOUNDS AS SINGLE-MOLECULE MAGNETS: A MOLECULAR APPROACH TO NANOSCALE MAGNETS By NICOLE E. CHAKOV A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Nicole E. Chakov

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I dedicate this document to my family, for their love and unending support of me as I pursue my career goals.

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iv ACKNOWLEDGMENTS First and foremost, I would like to thank my research advisor, Professor George Christou, for the opportunity to pursue my goal of earning a doctoral degree in chemistry in his research group. His guidance, unwav ering support and constant encouragement during my time as a member of his group have been invaluable, allowing me to draw on my own creativity to achieve my research pote ntial. I would also like to thank my other committee members, Dr. Daniel Talham, Dr. David Powell, Dr. McElwee-White and Dr. Stephen Hill, for their insightful comments and words of encouragement. I would like to take this opportunity to thank the many research scientists with whom I have worked during my doctoral studies. These collaborators include Dr. Wolfgang Wernsdorfer, who provided essentia l single crystal measurements on numerous compounds below 1.8 K using his unique micr o-SQUID apparatus. I also express my sincerest gratitude to the crystallographers who worked diligently to solve numerous challenging crystal structures. These include Dr. Khalil Abboud and his staff at UFCXC, Dr. Arnie Rheingold and Dr. Lev Zahkarov at the University of Southern California at San Diego, as well as Dr. Maren Pink and the IUMSC staff. Special appreciation is also given to Dr. Grgory Chabbousan t, Dr. Hans Gdel, Dr. Reto Basler and Andreas Sieber for their INS measurements on three Mn12 samples and to Dr. Monica Soler for her involvement in the synthetic portion of this project. Additionally, my genuine gratitude goes to Dr. Stephen Hill, Dr. Rachel Edwards and Dr. Konstantin Petukhov in the UF Physics Department for their HFEPR measurem ents, which generated such excitement in

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v the area of single-molecule magnetism. Their patience and understand ing as they taught me the fundamentals, from understanding a HF EPR experiment to interpreting the terms in a spin Hamiltonian of a SMM, were exceptio nal. I also thank Dr. Andrew Kent and Dr. Enrique del Barco in the Physics Departme nt at New York University for their magnetization measurements. The initial intere st of Dr. Kent in the study of a high symmetry Mn12 complex has resulted in a better unde rstanding of the magnetic behavior exhibited by a “model” Mn12 complex – a much debated topic for researchers in the SMM area. Lastly, I would like to thank Dr. Naresh Dalal, Andrew Harter and Dr. Randy Achey for 55Mn NMR measurements on si ngle crystals of two Mn12 clusters, a study that was the first of its kind on a SMM. The friendships that I made both in Gain esville and Bloomington were an integral part of this journey for me, providing me w ith a special support network. I would like to express my appreciation to the entire Christou group, pa st and present, for their thoughtful discussions, companionship and for helping me to grow as a chemist. To Gosia, I am especially grateful for her si ncere friendship, for always cheering me up and for all the moments we spent togeth er talking, laughing and celebrating. Finally, and most importantly, I would like to express my sincerest gratitude to my family, my mother, father, grandmother and sist er, who have always been there for me in every way. Their encouragement, pride and unwavering confidence in me throughout my pursuit of my doctoral degree have been invalu able. I am forever grateful for their love and support, for the understanding ear with which they listen and for their enduring belief in me.

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...............................................................................................................x LIST OF FIGURES.........................................................................................................xiv ABBREVIATIONS........................................................................................................xxii ABSTRACT...................................................................................................................xxii i CHAPTER 1 GENERAL INTRODUCTION................................................................................1 2 SINGLE-MOLECULE MAGNETS. A Mn12 COMPLEX WITH MIXED CARBOXYLATE-SULFONATE LIGATION: [Mn12O12(O2CMe)8(O3SPh)8(H2O)4].....................................................................17 2.1 Introduction......................................................................................................17 2.2 Results and Discussion....................................................................................19 2.2.1 Syntheses...........................................................................................19 2.2.2 Description of Structures..................................................................22 2.2.2.1 X-ray crystal structure of [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] ( 2 )...........................22 2.2.2.2 X-ray crystal structure of [Mn4O4(O2PPh2)6] ( 5 )...............28 2.2.3 Magnetochemistry of Complexes 2 and 3 ........................................34 2.2.3.1 DC studies..........................................................................34 2.2.3.2 AC studies..........................................................................37 2.2.3.3 Relaxation studies..............................................................39 2.2.3.4 Hysteresis studies below 1.8 K..........................................42 2.2.4 Magnetochemistry of Complex 5 ......................................................43 2.3 Conclusions......................................................................................................45 2.4 Experimental....................................................................................................46 2.4.1 Syntheses...........................................................................................46 2.4.2 X-ray Crystallography......................................................................48

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vii 3 NOVEL MIXED-VALENCE MnIII/MnIV CLUSTERS FROM THE USE OF BENZENESELENINATE LIGANDS: [Mn7O8(O2CMe)(O2SePh)8(H2O)] AND [Mn7O8(O2SePh)9(H2O)]..............................................................................51 3.1 Introduction......................................................................................................51 3.2 Results and Discussion....................................................................................53 3.2.1 Syntheses...........................................................................................53 3.2.2 Description of Structures..................................................................55 3.2.2.1 X-ray crystal structure of [Mn7O8(O2CMe)(O2SePh)8(H2O)] ( 6 )...............................55 3.2.2.2 X-ray crystal structure of [Mn7O8(O2SePh)9(H2O)] ( 7 ).....60 3.2.3 Magnetochemistry of Complexes 6 and 7 ........................................63 3.2.3.1 DC studies..........................................................................63 3.2.3.2 AC studies..........................................................................66 3.2.3.3 Hysteresis studies below 1.8 K..........................................68 3.2.3.4 Origin of the relaxation barrier..........................................71 3.3 Conclusions......................................................................................................74 3.4 Experimental....................................................................................................75 3.4.1 Syntheses...........................................................................................75 3.4.2 X-ray Crystallography......................................................................76 4 AN INVESTIGATION OF THE REACTIVITY OF Mn12 COMPLEXES WITH DIMETHYLARSINIC ACID: NEW Mn4 CUBANE COMPLEXES AND Mn16 SINGLE-MOLECULE MAGNETS...................................................79 4.1 Introduction......................................................................................................79 4.2 Results and Discussion....................................................................................81 4.2.1 Syntheses...........................................................................................81 4.2.2 Description of Structures..................................................................85 4.2.2.1 X-ray crystal structure of [Mn4O4(O2AsMe2)6] ( 9 )...........85 4.2.2.2 X-ray crystal structure of {[Mn4O4(O2AsMe2)6](NO3)}2 ( 10 )....................................89 4.2.2.3 X-ray crystal structure of [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] ( 11 )................92 4.2.2.4 X-ray crystal structure of [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] ( 12 )...........................98 4.2.3 Magnetochemistry of Complexes 9 and 10 ....................................103 4.2.4 Magnetochemistry of Complexes 11 and 12 ..................................109 4.2.4.1 DC studies........................................................................109 4.2.4.2 AC studies........................................................................112 4.2.4.3 Hysteresis studies below 1.8 K........................................114 4.3 Conclusions....................................................................................................116 4.4 Experimental..................................................................................................118 4.4.1 Syntheses.........................................................................................118 4.4.2 X-ray Crystallography....................................................................120

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viii 5 NEW POLYNUCLEAR Mn CLUS TERS FROM THE USE OF THE HYDROPHOBIC CARBOXYLATE LIGAND 2,2-DIMETHYLBUTYRATE.............................................................................125 5.1 Introduction....................................................................................................125 5.2 Results and Discussion..................................................................................127 5.2.1 Syntheses.........................................................................................127 5.2.2 Electrochemistry.............................................................................130 5.2.3 Description of Structures................................................................134 5.2.3.1 X-ray crystal structure of [Mn12O12(O2CPet)16(MeOH)4] ( 15 )................................134 5.2.3.2 X-ray crystal structure of [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] ( 16 )....139 5.2.3.3 X-ray crystal structure of [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] ( 17 )......................143 5.2.3.4 X-ray crystal structure of [Mn4O2(O2CPet)6(bpy)2] ( 18 )..........................................146 5.2.4 Magnetochemistry of Complexes 15 19 .......................................149 5.2.4.1 DC studies of 15 ...............................................................149 5.2.4.2 AC studies of 15 ...............................................................151 5.2.4.3 DC and AC susceptibility studies of complexes 16 19 ............................................................154 5.3 Conclusions....................................................................................................158 5.4 Experimental..................................................................................................160 5.4.1 Syntheses.........................................................................................160 5.4.2 X-ray Crystallography....................................................................163 6 SINGLE-MOLECULE MAGNETS: STRUCTURAL CHARACTERIZATION, MAGN ETIC PROPERTIES AND 19F NMR SPECTROSCOPY OF A Mn12 FAMILY SPANNING THREE OXIDATION LEVELS...............................................................................................................165 6.1 Introduction....................................................................................................165 6.2 Results and Discussion..................................................................................167 6.2.1 Syntheses and Electrochemistry.....................................................167 6.2.2 Description of Structures................................................................171 6.2.2.1 X-ray crystal structure of [Mn12O12(O2CC6F5)16(H2O)4] ( 20 )..................................171 6.2.2.2 X-ray crystal structure of (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] ( 21 )......................176 6.2.2.3 X-ray crystal structure of (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] ( 22 ).....................178 6.2.3 19F Nuclear Magnetic Resonance Spectroscopy.............................180 6.2.4 Inelastic Neutron Scattering Spectroscopy.....................................185 6.2.5 Magnetochemistry of Complexes 20 22 .......................................192 6.2.5.1 DC studies........................................................................192 6.2.5.2 AC studies........................................................................198

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ix 6.2.5.3 Relaxation studies using AC and DC data.......................204 6.2.5.4 Variable-frequency AC susceptibility studies.................209 6.2.5.5 Hysteresis studies below 1.8 K........................................213 6.3 Conclusions....................................................................................................216 6.4 Experimental..................................................................................................217 6.4.1 Syntheses.........................................................................................217 6.4.2 X-ray Crystallography....................................................................220 7 EFFECT OF SYMMETRY ON MA GNETIC BEHAVIOR: A STUDY OF THE HIGH-SYMMETRY Mn12 SINGLE-MOLECULE MAGNET [Mn12O12(O2CCH2Br)16(H2O)4]...........................................................................224 7.1 Introduction....................................................................................................224 7.2 Results and Discussion..................................................................................233 7.2.1 Synthesis.........................................................................................233 7.2.2 Electrochemistry.............................................................................233 7.2.3 X-Ray Crystal Structure of [Mn12O12(O2CCH2Br)16(H2O)4] ( 26 )..236 7.2.4 1H Nuclear Magnetic Resonance Spectroscopy..............................243 7.2.5 Single Crystal 55Mn Nuclear Magnetic Resonance Spectroscopy..247 7.2.6 Magnetochemistry of Complex 26 ..................................................257 7.2.6.1 DC studies........................................................................257 7.2.6.2 AC studies........................................................................260 7.2.6.3 Relaxation studies using AC and DC data.......................265 7.2.6.4 Hysteresis studies below 1.8 K........................................267 7.2.7 Single Crystal High-Freque ncy Electron Paramagnetic Resonance....................................................................................268 7.3 Conclusions....................................................................................................282 7.4 Experimental..................................................................................................283 7.4.1 Synthesis.........................................................................................283 7.4.2 X-ray Crystallography....................................................................284 8 GENERAL CONCLUSIONS..............................................................................286 APPENDIX A BOND DISTANCES AND ANGLES.................................................................295 B LIST OF COMPOUNDS.....................................................................................349 C PHYSICAL MEASUREMENTS........................................................................351 LIST OF REFERENCES.................................................................................................355 BIOGRAPHICAL SKETCH...........................................................................................371

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x LIST OF TABLES Table page 2-1 [Mn12O12(O2CR)16(H2O)4] derivatives, together with pKa values of the conjugate acid of the carboxylate ligand..................................................................................19 2-2 Crystallographic data for [Mn12O12(O2CMe)8(O3SPh)8(H2O)4]4CH2Cl2...............23 2-3 Bond valence sum calculations for 2 4CH2Cl2.........................................................26 2-4 Bond valence sum calculations for selected oxygen atoms in 2 4CH2Cl2...............26 2-5 Comparison of selected bond distances () and angles ( ) for [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O ( 1 ), [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] ( 2 ) and [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4] ( 4 )................................................................29 2-6 Crystallographic data for [Mn4O4(O2PPh2)6]...........................................................30 2-7 Bond valence sum calculations for 5 ........................................................................33 2-8 Bond valence sum calculations for selected oxygen atoms in 5 ..............................33 3-1 Crystallographic data for [Mn7O8(O2CMe)(O2SePh)8(H2O)]6MeCN and [Mn7O8(O2SePh)9(H2O)]2CH2Cl2...........................................................................57 3-2 Bond valence sum calculations for 6 6MeCN and 7 2CH2Cl2.................................59 3-3 Bond valence sum calculations for selected oxygen atoms in 6 6MeCN and 7 2CH2Cl2.................................................................................................................59 3-4 Comparison of selected bond distances () and angles ( ) for [Mn7O8(O2CMe)(O2SePh)8(H2O)] ( 6 ) and [Mn7O8(O2SePh)9(H2O)] ( 7 )................62 4-1 Crystallographic data for [Mn4O4(O2AsMe2)6]5H2OC5H12 and {[Mn4O4(O2AsMe2)6](NO3)}2MeCN12H2O.......................................................88 4-2 Bond valence sum calculations for 9 5H2OC5H12 and 10 MeCN12H2O............88 4-3 Bond valence sum calculations for selected oxygen atoms in 9 5H2OC5H12 and 10 MeCN12H2O...................................................................................................89

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xi 4-4 Comparison of selected bond distances () and angles ( ) for [Mn4O4(O2AsMe2)6] ( 9 ) and {[Mn4O4(O2AsMe2)6](NO3)}2 ( 10 )............................91 4-5 Comparison of selected bite distances () for MeCO2 -, PhSO3 -, Ph2PO2 -, PhSeO2 and Me2AsO2 ligands in 1 2 4 5 6 7 9 and 10 .....................................92 4-6 Crystallographic data for [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4]32MeCN and [Mn16O8Sr4(O2CPh)16(O2AsMe2)24]16MeCN..................................................94 4-7 Bond valence sum calculations for 11 32MeCN and 12 16MeCN..........................96 4-8 Bond valence sum calculations for selected oxygen atoms in 11 32MeCN and 12 16MeCN..............................................................................................................96 4-9 Comparison of selected bond distances () and angles ( ) for 11 32MeCN and 12 16MeCN............................................................................................................102 4-10 Distribution of spin states for 9 ..............................................................................104 4-11 Spin states of 9 in the |ST, SA, SB> format ordered as a function of energy...........107 4-12 Selected MnOMn angles () of 9 ......................................................................107 5-1 Anodic / cathodic peak curre nt ratios at the indicated scan rates for the -0.07 V and -0.21 V reduction waves of 13 and 15 .............................................................134 5-2 Crystallographic data for [Mn12O12(O2CPet)16(MeOH)4]2MeCN, [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)]CH2Cl2, [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2]H2OHO2CPet and [Mn4O2(O2CPet)6(bpy)2]2H2O..............................................................................136 5-3 Bond valence sum calculations for 15 2MeCN......................................................137 5-4 Bond valence sum calculations for selected oxygen atoms in 15 2MeCN............138 5-5 Bond valence sum calculations for 16 CH2Cl2....................................................141 5-6 Bond valence sum calculations for selected oxygen atoms in 16 CH2Cl2..........142 5-7 Bond valence sum calculations for 17 H2OHO2CPet............................................146 5-8 Bond valence sum calculations for selected oxygen atoms in 17 H2OHO2CPet...146 5-9 Bond valence sum calculations for 18 2H2O.........................................................148 5-10 Bond valence sum calculations for selected oxygen atoms in 18 2H2O................148 6-1 Anodic / cathodic peak cu rrent ratios at the indicated scan rates for the 0.64 V and 0.30 V reduction waves of 20 ..........................................................................171

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xii 6-2 Crystallographic data for [Mn12O12(O2CC6F5)16(H2O)4]3CH2Cl2, (NMe4)[Mn12O12(O2CC6F5)16(H2O)4]4.5CH2Cl2H2O, and (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4]6C7H8........................................................174 6-3 Bond valence sum calculations for 20 3CH2Cl2, 21 4.5CH2Cl2H2O, and 22 6C7H8.................................................................................................................175 6-4 Bond valence sum calculations for selected oxygen atoms in 20 3CH2Cl2, 21 4.5CH2Cl2H2O, and 22 6C7H8.......................................................................175 6-5 Interatomic distances () of Mn(11) in 20 .............................................................176 6-6 Selected interatomic distances () and angles ( ) for [Mn12O12(O2CC6F5)16(H2O)4] ( 20 ), (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] ( 21 ), and (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] ( 22 ).....................................................181 6-7 Solution 19F NMR spectral data for 20 22 .............................................................184 6-8 Experimental and calculated energi es and relative intensities of the INS transitions of isomers a and b in [Mn12O12(O2CC6F5)16(D2O)4] ( 23 )....................188 6-9 Zero-field splitting parameters D and 0 4B (in cm-1) for 23 25 ...............................190 6-10 M/NB vs H/ T fitting parameters for 20 22 ...........................................................194 6-11 Arrhenius parameters for wet and dried samples of 20 22 ....................................203 6-12 Fitting parameters of the m and m AC susceptibility vs frequency data for 20 22 .......................................................................................................................212 7-1 Debye-Waller thermal parameters for atoms of MeCO2 ligands in 1 ...................231 7-2 Crystallographic data for [Mn12O12(O2CCH2Br)16(H2O)4]4CH2Cl2.....................237 7-3 Bond valence sum calculations for 26 4CH2Cl2.....................................................240 7-4 Bond valence sum calculations for selected oxygen atoms in 26 4CH2Cl2...........240 7-5 Comparison of selected intera tomic distances () and angles ( ) for [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O ( 1 ) and [Mn12O12(O2CCH2Br)16(H2O)4] ( 26 )......................................................................242 7-6 Solution 1H NMR spectral data for 1 and 26 .........................................................244 7-7 Comparison of parameters obtained from 55Mn NMR spectra of an aligned powder and single crystal of 26 ..............................................................................252 8-1 Comparison of selected bite distances () for MeCO2 -, PhSO3 -, Ph2PO2 -, PhSeO2 and Me2AsO2 ligands in 1 2 4 5 6 7 9 and 10 ...................................289

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xiii A-1 Selected interatomic distances () and angles () for [Mn12O12(O2CMe)8(O3SPh)8(H2O)4]4CH2Cl2......................................................295 A-2 Selected interatomic distances () and angles () for [Mn4O4(O2PPh2)6]..............300 A-3 Selected interatomic distances () and angles () for [Mn7O8(O2CMe)(O2SePh)8(H2O)]6MeCN...........................................................303 A-4 Selected interatomic distances () and angles () for [Mn7O8(O2SePh)9(H2O)]2CH2Cl2.........................................................................307 A-5 Selected interatomic distances () and angles () for [Mn4O4(O2AsMe2)6]5H2OC5H12..........................................................................309 A-6 Selected interatomic distances () and angles () for {[Mn4O4(O2AsMe2)6](NO3)}2MeCN12H2O.....................................................312 A-7 Selected interatomic distances () and angles () for [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4]32MeCN............................................317 A-8 Selected interatomic distances () and angles () for [Mn16O8Sr4(O2CPh)16(O2AsMe2)24]16MeCN.......................................................319 A-9 Selected interatomic distances () and angles () for [Mn12O12(O2CPet)16(MeOH)4]2MeCN..................................................................323 A-10 Selected interatomic distances () and angles () for [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)]CH2Cl2..................................329 A-11 Selected interatomic distances () and angles () for [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2]H2OHO2CPet............................................331 A-12 Selected interatomic distances () and angles () for [Mn4O2(O2CPet)6(bpy)2]2H2O..............................................................................333 A-13 Selected interatomic distances () and angles () for [Mn12O12(O2CC6F5)16(H2O)4]3CH2Cl2..................................................................334 A-14 Selected interatomic distances () and angles () for (NMe4)[Mn12O12(O2CC6F5)16(H2O)4]4.5CH2Cl2H2O.......................................339 A-15 Selected interatomic distances () and angles () for (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4]6C7H8........................................................344 A-16 Selected interatomic distances () and angles () for [Mn12O12(O2CCH2Br)16(H2O)4]4CH2Cl2..............................................................347

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xiv LIST OF FIGURES Figure page 1-1 Representations of magnetic dipole arrangements in (i) paramagnetic, (ii) ferromagnetic, (iii) antiferromagnetic, and (iv) ferrimagnetic materials...................3 1-2 Typical hysteresis loop of a magnet...........................................................................4 1-3 ORTEP representation showing (a) the [Mn12O12]16+ core and (b) the [Mn12O12(O2CMe)16(H2O)4] complex with peripheral ligation.................................8 1-4 Simplified representation of th e exchange interactions in the [Mn4 IVMn8 III(3-O)12]16+ core of 1 .............................................................................9 1-5 ORTEP representation of the [Mn12O48] core of a typical Mn12 complex showing the relative disposition of the JT elongation axes....................................................10 1-6 Representative plots of the potential energy versus (a) the orientation of the ms vector ( ) along the z axis and (b) the ms sublevel for a typical Mn12 complex......11 1-7 In-phase and out-of-phase AC susceptib ility signals for a dried, microcrystalline sample of [Mn12O12(O2CR)16(H2O)4].......................................................................12 1-8 Magnetization hysteresis loops for a typical Mn12 complex....................................13 1-9 Representation of the change in energy of ms sublevels as a function of applied magnetic field...........................................................................................................15 2-1 ORTEP representation of [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] ( 2 )........................24 2-2 ORTEP representation of [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] ( 2 ) showing the relative disposition of the JT elongation axes..........................................................25 2-3 ORTEP representation of [Mn4O4(O2PPh2)6] ( 5 ).....................................................31 2-4 ORTEP representation of the packing of 5 ..............................................................31 2-5 Plot of MT vs T for dried, microcrystalline samples of (a) 2 C6H14 and (b) 3 0.2C6H14...........................................................................................................34 2-6 Plot of M/NB vs H/ T for dried, microcrystalline samples of (a) 2 C6H14 and (b) 3 0.2C6H14...........................................................................................................36

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xv 2-7 Contour plot of the error surface for the D vs g fit for (a) 2 C6H14 and (b) 3 0.2C6H14...........................................................................................................37 2-8 Plot of the in-phase and out-ofphase AC susceptibility signals vs T for dried, microcrystalline samples of (a) 2 C6H14 and (b) 3 0.2C6H14....................................38 2-9 Plot of the natural logari thm of relaxation rate vs 1/ T for (a) 2 C6H14 and (b) 3 0.2C6H14 using M versus T data....................................................................40 2-10 Relaxation time vs T studies for a single crystal of 2 4CH2Cl2. (a) Plot of magnetization vs time decay. (b) Arrh enius plot generated using AC M data and DC magnetization decay data............................................................................41 2-11 Magnetization hysteresis l oops for a single crystal of 2 4CH2Cl2...........................42 2-12 Plot of MT vs T for a dried, microcrystalline sample of 5 2H2O.............................44 2-13 Plot of the in-phase and out-ofphase AC susceptibility signals vs T for a dried, microcrystalline sample of 5 2H2O..........................................................................45 3-1 ORTEP representation of [Mn7O8(O2CMe)(O2SePh)8(H2O)] ( 6 )............................56 3-2 ORTEP representations of (a) the [Mn7O8]9+ core of 6 and (b) the relative disposition of th e elongation axes............................................................................58 3-3 ORTEP representation of the packing of 6 along the a axis of the crystal..............60 3-4 ORTEP representation of [Mn7O8(O2SePh)9(H2O)] ( 7 )...........................................61 3-5 Plot of MT vs T for dried, microcrystalline samples of 6 2H2OMeCN () and 7 2H2O ( )................................................................................................................63 3-6 Plot of M/NB vs H/ T for a dried microcrystalline sample of 6 2H2OMeCN......64 3-7 Plot of M T vs T for 6 2H2OMeCN in the 2.0-10.0 K range from AC and DC susceptibility measurements.....................................................................................66 3-8 Plot of the in-phase and out-of-phase AC susceptibility si gnals for a dried, microcrystalline sample of 6 2H2OMeCN...........................................................67 3-9 Magnetization hysteresis l oops for a single crystal of 6 6MeCN............................69 3-10 Curie-Weiss plot for 6 2H2OMeCN.....................................................................70 3-11 Relaxation time vs T studies for a single crystal of 6 6MeCN. (a) Magnetization vs time decay plots. (b) Arrhenius plot generated using the resulting relaxation time vs T data...........................................................................................................71 4-1 ORTEP representation of [Mn4O4(O2AsMe2)6] ( 9 ).................................................86

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xvi 4-2 ORTEP representation of the cation of {[Mn4O4(O2AsMe2)6](NO3)}2 ( 10 )............90 4-3 ORTEP representation of [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] ( 11 ).............93 4-4 ORTEP representation of 11 showing the crystallographic D2 symmetry...............94 4-5 ORTEP representation of the asymmetric unit of [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] ( 11 )......................................................95 4-6 Schematic representation of the ei ght chelating and/or bridging modes in 11 ........97 4-7 ORTEP representation of [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] ( 12 ).......................98 4-8 ORTEP representation of the asymmetric unit of [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] ( 12 ).................................................................99 4-9 ORTEP representation of the side-view of 12 .......................................................100 4-10 Schematic representation of the nine chelating and/or bridging modes in 12 .......101 4-11 Representation of the pairwise exchange interactions J, J and J between numbered Mn ions of 9 ..........................................................................................104 4-12 Plot of MT vs T for a dried, microcrystalline sample of 9 H2O.............................105 4-13 Ordering of the spin states of 9 ..............................................................................106 4-14 Plot of the in-phase and out-ofphase AC susceptibility signals vs T for a dried, microcrystalline sample of 9 H2O..........................................................................108 4-15 Plot of MT vs T for dried, microcrystalline samples of (a) 11 2MeCN and (b) 12 ......................................................................................................................110 4-16 Plot of the in-phase and out-ofphase AC susceptibility signals vs T for a dried, microcrystalline sample of 11 2MeCN..................................................................113 4-17 Magnetization hysteresis l oops for a single crystal of 11 32MeCN.......................114 5-1 Cyclic voltammogram and differe ntial pulse voltammogram for (a) 15 and (b) 13 in CH2Cl2.....................................................................................................132 5-2 Scan rate dependence of oxidation wave at -0.07 V of 13 (a) Cyclic voltammogram at the indicated scan rate s. (b) Plot of cathodic and anodic peak current dependence vs 1/2......................................................................................133 5-3 Scan rate dependence of oxidation wave at -0.21 V of 15 (a) Cyclic voltammogram at the indicated scan rate s. (b) Plot of cathodic and anodic peak current dependence vs 1/2......................................................................................133

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xvii 5-4 ORTEP representation of [Mn12O12(O2CPet)16(MeOH)4] ( 15 )..............................135 5-5 ORTEP representation of the [Mn12O44(MeOH)4] core of complex 15 emphasizing the relative disposi tion of the JT elongation axes.............................137 5-6 Space-filling diagram of complex 15 including all non-hydrogen atoms..............139 5-7 ORTEP representation of [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] ( 16 ).140 5-8 ORTEP representation of the [Mn6O30CH2] core of 16 emphasizing the relative disposition of the JT elongation axes.....................................................................142 5-9 ORTEP representation of [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] ( 17 )...................143 5-10 ORTEP representations of (a) the [Mn9O6(OH)(CO3)]12+ core of 17 and (b) the relative dispositions of the elongation axes............................................................144 5-11 ORTEP representation of [Mn4O2(O2CPet)6(bpy)2] ( 18 ).......................................147 5-12 Plot of MT vs T for a dried, microcrystalline sample of 15 2CH2Cl2....................149 5-13 Determination of ground state spin of 15 2CH2Cl2. (a) Plot of M/NB vs H/ T for a dried, microcrystalline sample. (b) Error surface for the D vs g fit....................150 5-14 Plot of the in-phase and out-ofphase AC susceptibility signals vs T for a dried, microcrystalline sample of 15 2CH2Cl2.................................................................152 5-15 Plot of the natural logari thm of relaxation rate vs 1/ T for a dried, microcrystalline sample of 15 2CH2Cl2 using AC M data...................................154 5-16 Plot of MT vs T for a dried, microcrystalline sample of 16 CH2Cl24H2O.........155 5-17 Determination of ground state spin of 17 HO2CPet. (a) Plot of MT vs T for a dried, microcrystalline sample. (b) Plot of M T vs T in the 2.0-30.0 K range from AC and DC susceptibility measurements......................................................157 5-18 Determination of ground state spin of 19 MeNO2. (a) Plot of MT vs T for a dried, microcrystalline sample. (b) Plot of M T vs T in the 2.0-30.0 K range from AC and DC susceptibility measurements......................................................157 5-19 Determination of ground state spin of 18 2H2O. (a) Plot of MT vs T for a dried, microcrystalline sample. (b) Plot of M T vs T in the 2.0-30.0 K range from AC and DC susceptibility measurements.....................................................................158 6-1 Cyclic voltammogram and diffe rential pulse voltammogram of 20 in CH2Cl2.....169 6-2 Scan rate dependence of reduction waves of 20 (a) Cyclic voltammogram with corresponding plot of cathodic and anodic peak current dependence vs 1/2 for (b) 0.64 V reduction wave and (c) 0.30 V reduction wave....................................170

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xviii 6-3 ORTEP representation of [Mn12O12(O2CC6F5)16(H2O)4] ( 20 )...............................172 6-4 ORTEP representation of the anion of (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] ( 21 ).............................................................177 6-5 Packing diagram for 21 emphasizing intermolecular -stacking of carboxylate aromatic rings.........................................................................................................178 6-6 ORTEP representation of the anion of (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] ( 22 )...........................................................179 6-7 19F NMR (282 MHz) spectra at ~ 23 C in CD2Cl2 of 20 22 .................................182 6-8 Neutron energy loss side INS spectrum of 23 25 ..................................................186 6-9 Neutron energy loss side INS spectrum of (a) 23 at 20 K and (b) 24 at 9.9 K......186 6-10 Energy level diagram for 23 calculated using the determined INS parameters.....189 6-11 Plot of MT versus T for dried, microcrystalline samples of (a) 20 (b) 21 and (c) 22 2.5C7H8........................................................................................................193 6-12 Plot of M/NB versus H/ T for dried, microcrystalline samples of (a) 20 (b) 21 and (c) 22 2.5C7H8.................................................................................................195 6-13 Contour plot of the error surface for the D vs g fit for (a) 20 (b) 21 and (c) 22 2.5C7H8........................................................................................................196 6-14 M T vs T plots for vacuum-dried [Mn12] complex 20 [Mn12]complex 21 and [Mn12]2complex 22 2.5C7H8.................................................................................199 6-15 M vs T plots for vacuum-dried [Mn12] complex 20 [Mn12]complex 21 and [Mn12]2complex 22 2.5C7H8.................................................................................201 6-16 m vs T plots for wet crystals of [Mn12] complex 20 [Mn12]complex 21 and [Mn12]2complex 22 ...............................................................................................202 6-17 Plots of relaxation time vs 1/ T for wet crystals of complexes 20 22 using AC M data...................................................................................................................205 6-18 Relaxation time vs T studies for a single crystal of 20 3CH2Cl2. (a) Magnetization vs time decay plots. (b) Arrhenius plot generated using AC M and DC decay data..................................................................................................206 6-19 Relaxation time vs T studies for the slower -relaxing species of 21 4.5CH2Cl2H2O. (a) Mmagnetization vs time decay plots. (b) Arrhenius plot generated using AC M and DC decay data..........................................................207

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xix 6-20 Relaxation time vs T studies for the faster-relaxing species of 21 4.5CH2Cl2H2O. (a) Magnetization vs time decay plots. (b) Arrhenius plot generated using DC decay data..............................................................................207 6-21 Relaxation time vs T studies for a single crystal of 22 6C7H8. (a) Magnetization vs time decay plots. (b) Arrhenius plot generated using AC M and DC decay data.........................................................................................................................20 8 6-22 Argand plot of m vs m for wet crystals of (a) 20 3CH2Cl2 at 4.0 K, (b) 21 4.5CH2Cl2H2O at 3.4 K and (c) 22 6C7H8 at 2.2 K.......................................210 6-23 Plot of (a) the in-phase and (b) the ou t-of-phase AC magnetic susceptibility vs frequency at 4.0 K for 20 3CH2Cl2........................................................................211 6-24 Plot of (a) the in-phase and (b) the ou t-of-phase AC magnetic susceptibility vs frequency at 3.4 K for 21 4.5CH2Cl2H2O..........................................................211 6-25 Plot of (a) the in-phase and (b) the ou t-of-phase AC magnetic susceptibility vs frequency at 2.2 K for 22 6C7H8............................................................................212 6-26 Magnetization hysteresis l oops for a single crystal of 20 3CH2Cl2.......................214 6-27 Temperature dependence of the magne tization hysteresis loops for a single crystal of 21 4.5CH2Cl2H2O...............................................................................215 6-28 Scan rate dependence of the magnetizati on hysteresis loops for a single crystal of 21 4.5CH2Cl2H2O..........................................................................................215 6-29 Magnetization hysteresis l oops for a single crystal of 22 6C7H8...........................216 7-1 Representations of the structure of 1 (a) Photograph of seven crystals and (b) packing diagram showing the orientation of molecules of 1 relative to each other...............................................................................................................225 7-2 Magnetization hysteresis l oops for a single crystal of 1 at the indicated transverse field sweep rate at 0.04 K......................................................................226 7-3 Edge dislocation along the y axis with the extra plane y z inserted at z > 0..........228 7-4 ORTEP representation of the two-fold disorder of the MeCO2H solvent molecules of crystallization in 1 .............................................................................229 7-5 ORTEP representation of the hydr ogen-bonding interaction between an MeCO2H molecule of crystallization and an MeCO2 ligand bridging MnIIIMnIII pairs in 1 .................................................................................................................230

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xx 7-6 ORTEP representation of (a) [Mn12O12(O2CMe)16(H2O)4] with MeCO2H solvent molecules and (b) [Mn12O12(O2CCH2Br)16(H2O)4] with CH2Cl2 solvent molecules................................................................................................................230 7-7 ORTEP representation of the coordi nation sphere of Mn(2) and Mn(3) of 1 ........231 7-8 Depiction of the six isomers of 1 that differ in the nu mber of hydrogen-bonded MeCO2H molecules of cr ystallization....................................................................232 7-9 Cyclic voltammogram and diffe rential pulse voltammogram for 26 in CH2Cl2....235 7-10 Scan rate dependence of the reduction waves of 26 (a) Cyclic voltammogram with corresponding plot of cathodic and anodic peak current dependence vs 1/2 for (b) 0.57 V reduction wave a nd (c) 0.21 V reduction wave...............................236 7-11 ORTEP representation of [Mn12O12(O2CCH2Br)16(H2O)4] ( 26 )............................238 7-12 Seven different geometric isomers of Mn12 SMMs...............................................239 7-13 ORTEP representation of the anis otropy axes of Mn(1) and Mn(2) in 26 .............241 7-14 ORTEP representation of the packing of 26 ..........................................................242 7-15 1H NMR (300 MHz) spectrum at ~ 23 C in CD2Cl2 of 26 ....................................243 7-16 ORTEP representation of the fluxional pr ocess between the water ligand and the axial carboxylate bridging a MnIIIMnIII pair in a typical Mn12 molecule...............244 7-17 ORTEP representation of [Mn12O12(O2CCH2Br)16(H2O)4] ( 26 ) showing the D2 d symmetry typical of a Mn12 molecule in solution..................................................245 7-18 Array of inversion recovery data of 26 in CD2Cl2 at 23 C....................................246 7-19 1H NMR (300 MHz) spectrum at ~ 23 C in CD3CN of 1 .....................................246 7-20 Comparison of zero-field 55Mn NMR spectra of (a) an aligned dried, microcrystalline sample of 26 (b) a single crystal of 26 4CH2Cl2 and (c) a single crystal of 1 ..............................................................................................................249 7-21 ORTEP representation of 26 emphasizing the three crystallographically independent Mn ions within the cluster.................................................................250 7-22 Angular dependence of 55Mn NMR in the ab plane of a single crystal of 26 4CH2Cl2.............................................................................................................254 7-23 Angular dependence of 55Mn NMR in the ac plane of a single crystal of 26 4CH2Cl2.............................................................................................................255

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xxi 7-24 Temperature dependence of the 55Mn NMR spectrum of a single crystal of 26 4CH2Cl2 at the indicated temperatures..............................................................256 7-25 Plot of MT vs T for a dried, microcrystalline sample of 26 ...................................257 7-26 Determination of the ground state spin of 26 (a) Plot of M/NB versus H/ T for a dried, microcrystalline sample. (b) Error surface for the D vs g fit.......................259 7-27 Plot of the in-phase and out-ofphase AC susceptibility signals vs T for a dried, microcrystalline sample of 26 ................................................................................261 7-28 Plot of the out-of-phase AC susceptibility signals vs T for dried, microcrystalline 26 and for wet crystals of 26 4CH2Cl2...................................................................262 7-29 Plots of (a) m and (b) m vs frequency for wet crystals of 26 4CH2Cl2...............264 7-30 Argand plot of m vs m of wet crystals of 26 4CH2Cl2 at 4.6 K..........................265 7-31 Relaxation time vs T studies on a single crystal of 26 4CH2Cl2. (a) Magnetization vs time decay plots in zero field. (b) Plot of relaxation time vs 1/ T for 26 using AC M and DC decay data..........................................................267 7-32 Magnetization hysteresis l oops for a single crystal of 26 4CH2Cl2.......................268 7-33 Plot of HFEPR peak positions deduced from easy-axis measurements.................271 7-34 Plot of the frequency of the ms sublevels of the S = 10 ground state of 26 versus applied magnetic field............................................................................................271 7-35 Contour plot of the a ngle-dependent EPR spectra.................................................272 7-36 Comparison of hard plane EPR spectra of 26 4CH2Cl2 measured under different conditions...............................................................................................................273 7-37 Angle dependence of the EPR spectrum of 26 4CH2Cl2........................................274 7-38 Fits to eq 7-12 for the frequency depe ndence of the hard plane spectra for the S = 10 state and for the S = 9 state of 26 ................................................................275 7-39 Comparison of EPR spectrum of 26 4CH2Cl2 with simulated spectrum...............277 7-40 Angle dependence of several of the most important resonances............................277 7-41 EPR spectra for a single crystal of 26 4CH2Cl2 measured at (a) different temperatures at 51.5 GHz and (b) different frequencies........................................278 7-42 Determination of relative energy of S = 9 excited state. (a) Temperature dependence of the area of the 9 and 9 resonances. (b) Schematic for the energy levels of both the S = 10 and S = 9 states in zero magnetic field...............279

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xxii ABBREVIATIONS But tertiary butyl BVS bond valence sum CF crystal field CH2Cl2 dichloromethane CV cyclic voltammogram DFT density f unctional theory DPV differential pulse voltammogram Et ethyl Fc (Fc+) ferrocene (ferrocenium) G gauss fwhm full-width at half maximum HFEPR high-frequency electron paramagnetic resonance INS inelastic neutron scattering IR infrared JT Jahn-Teller Me methyl MeCN acetonitrile MeOH methanol NMR nuclear magnetic resonance ORTEP Oak Ridge thermal ellipsoid plotting program Pet tertiary pentyl Ph phenyl PS II photosystem II QTM quantum tunneling of magnetization SCM single-chain magnet sp square pyramidal SQUID superconducting quantum interference device T tesla tbp trigonal bipyramidal THF tetrahydrofuran TIP temperature-independent paramagnetism WOC water-oxidizing complex ZFS zero-field splitting

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xxiii Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy MOLECULAR MANGANESE COMPOUNDS AS SINGLE-MOLECULE MAGNETS: A MOLECULAR APPROACH TO NANOSCALE MAGNETS By Nicole E. Chakov August 2005 Chair: George Christou Major Department: Chemistry The primary reason for the current inte rest in high nuclearity manganese carboxylate clusters is the study of singlemolecule magnets (SMMs), or molecular nanomagnets. A SMM possesses a significant energy barrier to re laxation of its magnetization vector and, consequently, exhibi ts out-of-phase AC susceptibility signals and magnetization hysteresis l oops, both properties characteristic of a superparamagnetlike particle. Hence, SMMs function as m onodisperse, nanoscale magnetic particles below their blocking temperature and have potential applications for high density information storage as well as qubits in qua ntum computers. The SMMs with the largest energy barriers to date are the [Mn12O12(O2CR)16(H2O)4] (Mn12) family. However, the isolation of a SMM that beha ves as a magnet at technological ly relevant temperatures continues to elude researcher s despite (i) the preparati on of numerous novel 3d metal carboxylate clusters possessing di fferent topologies and (ii) the expansion of existing families of SMMs with new derivatives. Such an approach is essential both for the

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xxiv correlation of structural featur es with magnetic properties and for the design of a rational synthetic method for improved SMMs. Progress to ward this end has already resulted in the isolation of a number of SMMs. With this same goal in mind, a variety of synthetic strategies have been developed to access new Mn12 derivatives as well as other polynuclear Mnx complexes. Such approaches include (i) ligand substitution reactions of Mn12 complexes to derivatize them with non-carboxylate ligands; (ii) intr oduction of a bulky carboxylate ligand to destabilize the Mn12 complex and allow access to other new polynuclear Mn clusters; and (iii) chemical reduction of a Mn12 derivative to facilita te a thorough study of a Mn12 family spanning three oxidation levels. Ye t another strategy involves single crystal measurements of Mn12 SMMs by previously unemployed techniques, specifically 55Mn nuclear magnetic resonance, to directly pr obe the magnetic structur e and individual metal ions in these clusters. Each of the complexes isolated as a result of this investigation has been characterized by spectro scopic, electrochemical and magnetochemical techniques. These studies provide useful in sight into the influence of small modifications, including variation of the peripheral li gation or oxidation level, on bot h the isolated product and the magnetic behavior of a Mn12 species.

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1 CHAPTER 1 GENERAL INTRODUCTION From the ancient times to the modern era, the discovery and subsequent utilization of magnetic materials have been central to significant technological advancements that have dramatically affected civilization and humankind.1 Beginning with the Greeks, who first recognized that the mixed-valent iron oxide, magnetite, attracts elemental iron, the magnetic properties of materials have long been pursued for technological progress. Approximately two thousand years ago, the Ch inese exploited the ma gnetic properties of magnetite to build the first magnetic devi ce – the compass. Following this early development, magnets have become essentia l to every modern society and have found uses in magnetomechanical applications, acoustic instruments, information and telecommunications devices, electrical motors and generators, magnetic shielding as well as numerous others.1,2 Modern day magnetic materials include magnetic alloys and oxides, particularly ferrites such as MgFe2O4, which can function in transformer cores, magnetic recording or info rmation storage devices.2,3 The magnetic field associated with a ma gnetic substance is the result of an electrical charge in motion, sp ecifically the spin and orbital angular momenta of electrons within atoms of a material. While all matter is composed of atoms containing one or more electrons, only a small handful of materials behave as magnets. In most substances, atoms have closed electron shells, i.e., electrons with magnetic fields aligned in opposite directions are paired with each other. Such materials with no magnetic moment are called diamagnets.4,5 Hence, the crucial element that dis tinguishes a magnetic substance, or a

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2 paramagnet, from a diamagnet is the existence of a magnetic moment that arises from at least one unpaired electron. The various types of magnetic materials ar e grouped according to their response or susceptibility, to an applied magnetic field. The el ectron pairs of a di amagnet interact with an applied field, generating a repulsive fi eld that weakly repels the diamagnet from the applied field; the sign of is negative. In contrast, a paramagnet is attracted to an applied magnetic field; the sign of is positive. The strength of the attraction is governed both by the number of unpaired electrons in the material as well as the nature of the interactions of its electron spins.1,2,6,7 Both the temperature dependence as well as the absolute magnitude of are measures by which the vari ous types of paramagnetism are distinguished.7 Simple paramagnetic behavior is obs erved in substances in which the magnetic moments of unpaired elec trons are independent of each other. In the absence of a magnetic field, individual magnetic moment s are randomly oriented. As a field is applied, the moments align parall el, albeit weakly, to the fiel d; this alignment is opposed by the randomizing effect of thermal energy (F igure 1-1). The susceptibility of these materials is inversely proportional to te mperature as defined by the Curie Law ( = C/ T ), where C is the Curie constant.4-8 There are other paramagnetic materials however that display a temperature dependence unlike th at of a simple paramagnet. In these substances, the magnetic moments of the unpair ed spins are not independent, but rather interact with each other, either in a cooperative manner when there is a parallel alignment of the magnetic moments or in a non-coopera tive way when there is an overall antiparallel alignment of magnetic moments. Th e former describes ferromagnetic behavior while the latter is associated with an antiferromagnetic or ferrimagnetic response;

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3 antiferromagnetism refers to a complete canceling of magnetic moments while ferrimagnetism corresponds to the situation in which magnetic moments align in an antiparallel fashion but a non-zero magnetization results.1,2,4,7,8 Examples of ferromagnets include iron, cobalt, nickel and several rare ea rth metals and their a lloys while magnetite, Fe3O4, is a ferrimagnet.4 ParamagneticFerromagneticAntiferromagnetic Ferrimagnetic Figure 1-1. Representations of magnetic dipole arrangements in (i) paramagnetic, (ii) ferromagnetic, (iii) antiferromagnetic, and (iv) ferrimagnetic materials. At all temperatures, ferro-, antiferroand ferrimagnets are composed of domains, or tiny regions in which all the spins are aligned parallel. The transition from independent to cooperative behavior in these materials is associated with a critical temperature, Tc. Above Tc, there is enough thermal energy to cause a random alignment of each domain with respect to its neighbor, maximizing th e entropy while minimizing the magnetization of the system. The application of a strong ma gnetic field induces the alignment of all of the domains with the field and hence, with each other, imparting a net magnetization to the material. As alignment occurs, the in teraction of spins becomes strong enough to overcome dipole interactions and entropy c onsiderations that maintain the random alignment of the domains.6,8 When a magnetic field is applied and then removed at a temperature below Tc, the magnetization induced by the fi eld does not entirely disappear, and in some cases can remain equal to the fiel d-induced magnetization. This is in contrast to the behavior observed for paramagnetic systems in which the spins immediately

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4 randomly reorient following removal of th e field. For suppression of the remnant magnetization, a coercive field in the oppos ite direction is ap plied, inducing the realignment of the spins in the opposite di rection and resulting in a hysteresis loop (Figure 1-2). For information storage, a small coercive field with a relatively rectangularshaped hysteresis loop is crucial so that th e two magnetic orientations of the spin can represent zero and one in the binary di gital system used by current technology.4,7,8 The requirement for information storage is that th e system remains at a temperature at which the material exhibits hysteresis while the removal of the stored information involves heating to a temperature above Tc.8 M H Ms Ms Figure 1-2. Typical hysteresis loop of a ma gnet, where M is magnetization, H is the applied magnetic field and Ms is the saturation valu e of the magnetization. Economic studies estimate that the inform ation storage area of the nanomagnetic market generated $3.4 billion in revenues in 2003 and is expected to reach $4.07 billion in 2004.9 Considering the economic impact of this technology as we ll as the projected continual importance of magnets for storage de vices as well as in other areas (average annual growth rate of 22.6%),9 much research is being devoted towards the discovery and development of new, improved magnetic ma terials. The overwhelming trend of this research is towards miniaturization of magnetic storage media; the first hard disk drive, “RAMAC” introduced by IBM in 1957 had a storage capacity of only 2000 bits in-2 while

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5 storage density reached approximately 10 Gbits in-2 in 2000 – an increase by a factor of five million.10 Because of the need for the storag e of greater quantities of digital information on smaller surfaces areas, th e development of ma gnetic particles of nanoscale dimensions is a necessity. Progress to wards this end involves the use of smaller materials of nanoscale dimensions that beha ve as “permanent” magnets with functional temperatures in the practical range for technological use. One approach towards this end involves the fragmentation of bulk ferromagnets or ferrimagnets such as ferro-spinels. For example, crystals of magnetite can be broken down such that each fragment is smaller in size than a single domain (20-200 nm); these nanoscale magnetic particles are known as superparamagnets. The magnetic moments within one superparamagnetic particle are ferrimagnetically aligned due to short range order. Alignment of the superparamagnets is induced by the application of a magnetic field, resulting in a remnant magnetization.12,13 The reversal of the magnetization direction of a single domain re quires an energy to overcome the crystal field anisotropy. Hence, slow magnetization relaxation is not related to domain formation as with a traditional ferromagnet, but rather involves an energy barrier that arises in part from the magnetic anisotropy associated with the shape of the particles. Giving no control in size versus properties and a non-uniform respons e to an applied field, the major drawback with this approach involves the wide distribution of par ticle sizes that result from fragmentation.11 Recently, new fragmentation techniques based on scanning tunneling microscopy and biomineralization have been devised as a method of improvement of the non-uniform particle size obstacle.12,13

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6 Yet another strategy cu rrently being explored in deta il is the development of new magnets using molecules as building blocks Such materials, called molecule-based magnets, have the potential to demonstrate ch aracteristics that are unattainable by the conventional metal/intermetallics and metal-oxide magnets used currently. These properties include low-temperat ure processability, high magne tic susceptibilities, high solubility, compatibility with polymers for composites, biocompatibility, transparency, semiconducting and insulating properties, hi gh remnant magnetizations, as well as several other desira ble characteristics.1,14 Paramagnetic organic or inorganic molecules with a large number of unpaired electrons are typically used as the building blocks for the preparation of these molecule-based magnets which rely on long-ra nge intermolecular interactions to account for their magnetic be havior. Reported in 1967, the first moleculebased magnet, [Fe(dtc)2Cl], where dtc = diethyldithiocarbamato, was found to have an S = 3/2 ground state with ferromagnetic ordering at 2.46 K.8,15 Subsequently, there was very little published activity in the area until 1987 when Miller and co-workers reported a molecular ferromagnet composed of alternat ing stacks of metallocenium donor cations (D+) and organic radical acceptor anions (A-), each with a single unpaired electron. For the complex [Fe(C5Me5)2]+, where D+ is the decamethylferrocenium cation and Ais the tetracyanoethylene anion, [TCNE]-, the ordering temperature was found to be 4.8 K.16,17 From these studies, it was determined that th e position of adjacent chains relative to one another had a significant effect on the bul k magnetic properties of the material. In order to gain more insight on the effect of chain arrangement on magnetic properties, Kahn and co-workers prepared compounds containing ferromagnetic chains of CuII-bridge-MnII moieties.18 Antiferromagnetic coupling was observed between the

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7 chains of MnIICuII(pba)(H2O)3, where pba = 1,3-propylenebis(oxamate). By changing the ligand only very slightly contrasting data were observed as the compound, MnIICuII(pbaOH)(H2O)3, where pbaOH = 2-hydroxy-1,3-propylenebis(oxamate), showed overall ferromagnetic coupling with Tc = 4.6 K.18 Such studies emphasized the importance of selected bridging gr oups that aid in the formati on of 2D or 3D lattices and facilitate communication between the magnetic centers in the molecular building blocks. Based on the magnetic data collected from these and other similar molecular-based magnetic compounds, an attractive solution ai med at the improvement of the magnetic properties became apparent – molecules containing several transition metal ions, such as Mn, Fe, V, Ni, and Co, can potentially ex hibit behavior similar to that of superparamagnets. Several polynuclear metal complexes that act as nanoscale magnetic particles have been prepared, resulting in the ra pid development of an exciting new area of high-spin metal clusters term ed single-molecule magnets (SMMs).19 For numerous reasons, SMMs represent an exciting area, promising several advantages over conventional nanoscale magnetic particles. Such advantages include (i ) the preparation of purified compounds by solution methods, resulti ng in a product with a single, sharply defined size; (ii) possible vari ation in peripheral carboxylate ligation such that small or bulky, hydrophilic or hydrophobic lig ands can be incorporated into the synthesis; (iii) solubility in several organic solvents, providing access to indu strial applications; and (iv) the possibility of reaching sub-nanos cale dimensions, resulting in the potential development of even better memory storage devices.19 The magnetic behavior of a SMM arises from the combination of a large ground state spin, S, and a large ne gative magnetic anisotropy as ga uged by the axial zero-field

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8 splitting parameter (ZFS), D.11,20 Both in the fields of inorganic and organic chemistry, there is an intense search underway for such potentially useful highspin molecules. In 1993, it was discovered that this exceptional combination of high-spin ground state and large, negative magnetic anisotropy is displayed by the complex, [Mn12O12(O2CMe)16(H2O)4] 2MeCO2H 4H2O ( 1 ), resulting in na noscale-like magnetic behavior and the subsequent classification of the molecu le as a SMM (Figure 1-3).11,21-23 Probably the most intens ely studied SMM, [Mn12O12(O2CMe)16(H2O)4] has an S = 10 ground state spin and is only one in a class of well-characterized Mn12 complexes of the general formula, [Mn12O12(O2CR)16(H2O)4], where R = Me, Et, Ph as well as numerous other groups.19 (a) (b) Figure 1-3. ORTEP representation in P ov-Ray format showing (a) the [Mn12O12]16+ core and (b) the [Mn12O12(O2CMe)16(H2O)4] complex with peripheral ligation. MnIV green; MnIII blue; O red; C gray. In these dodecanuclear complexes, a non-plan ar ring composed of eight alternating MnIII and eight triply bridging oxide ions surrounds a central [Mn4 IVO4]8+ cubane moiety. Peripheral ligation is provided by sixteen br idging carboxylate ligands and four terminal water molecules. The large ground state spin ar ises from exchange interactions between

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9 the S = 3/2 spins of the MnIV ions and the S = 2 spins of the MnIII ions. The magnitude of the various exchange interactions be tween the magnetic centers in the Mn12 molecule has recently been determined by the fitting of a Heisenberg spin Hamiltonian to reproduce high-field magnetization data collected on samples of complex 1 .24 A simplied representation of the [Mn4 IVMn8 III(3-O)12]16+ core shows the four most important exchange pathways: (i) J1 relates each MnIV bridged by two 3-O ions to a MnIII; (ii) J2 relates each MnIV bridged by one 3-O ion to a MnIII; (iii) J3 refers to coupling between MnIV ions and; (iv) J4 refers to coupling between MnIII ions (Figure 1-4). The determined values of the exchange interactions found in this study are J1 = -119 K, J2 = -118 K, J3 = 8 K and J4 = 23 K.24 On the basis of these J values, the cluster can be approximately described as four MnIII/MnIV dimers with spin S = 1/2 that are ferromagnetically coupled to the four remaining MnIII ions with spin S = 2, accounting for the S = 10 ground state spin of the molecule. 9 10 11 12 8 7 6 5 2 3 4 1 J3 J4 J4J4J4J4J4J4J4 J1J1J1J1 J2J2J2J2 Figure 1-4. Simplified representation of the exchange interactions in the [Mn4 IVMn8 III(3-O)12]16+ core of 1 MnIV ions (green) are numbered 1-4; MnIII ions (blue) are numbered 5-12; O red.

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10 Each of the eight MnIII ions on the outer periphery of the complex undergoes a Jahn-Teller (JT) distortion as expected for a high-spin d4 ion in near-octahedral geometry. The distortion takes place in th e form of an elongation of two trans bonds. The approximately parallel alignment of the elongation axes of the eight MnIII ions accounts for a high degree of molecular anisotropy; th e anisotropy of a cluster is primarily a consequence of the single-ion anisotropies of the constituent ions w ithin the cluster and of the relative orientations of the magnetic axes of these i ons with respect to each other (Figure 1-5). Figure 1-5. ORTEP representation in Pov-Ray format of the [Mn12O48] core of a typical [Mn12O12(O2CR)16(H2O)4] complex, showing the rela tive disposition of the JT elongation axes indicated as solid black bonds. MnIV green; MnIII blue; O red. Hence, the magnetic anisotropy of a Mn12 cluster is primarily of the axial type, with the x and y directions approximately equivalent to each other while different from the z direction. The magnetic mome nt of an individual Mn12 molecule preferentially lies in the z direction, or the easy-axis, of the molecule A consequence of this Ising type of zerofield splitting is that the S = 10 ground state sp in is divided into 21 (2S + 1) sublevels, each characterized by a spin projection quantum number, ms, where –S ms S. The energy of each sublevel is given as E (ms) = ms 2D, giving rise to a double well potential (Figure 1-6). Because the value of the axial ZFS parameter D for a SMM is negative

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11 (-0.50 cm-1 for 1 ), the ms = 10 sublevels lie lowe st in energy while the ms = 0 sublevel lies highest. Consequently, there is a potential energy barrier between the “spin-up” (ms = -10) and “spin-down” (ms = +10) orientations of the magnetic moment. ms= +10 +9 +8 +7 +6 +5 +4 +3 +2 +1 ms= 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 ms= -10 ms Energy 100|D| ms= 0 ms= +10 +9 +8 +7 +6 +5 +4 +3 +2 ms= -10 -9 -8 -7 -6 -5 -4 -3 -2 +1 -1Energy Orientation of msvector ( ) (a)(b) Figure 1-6. Representative plots of the potential energy versus (a) the orientation of the ms vector ( ) along the z axis and (b) the ms sublevel for a Mn12 complex with an S = 10 ground state, expe riencing zero-field splitting,2S ˆ Dz. To reverse the spin of the molecule from along the z (spin up) to the + z (spin down) axis of the molecule, a potential energy barrier, U = S2|D| 72 K for 1, must be overcome. For this reason, SMMs exhib it slow magnetization relaxation at low temperatures. Experimental evidence for this behavior is supported by the appearance of frequency-dependent signals ( M signals) in out-of-phase AC magnetic susceptibility measurements, as shown in Figure 1-7, and of hysteresis loops in magnetization versus DC field scans.11 To ensure that the slow magneti zation relaxation shown by a SMM is intrinsic to the molecule itself and not to long-range interactions, experiments have been carried out on frozen solutions and on polycrystalline samples embedded in a polymeric matrix in which single molecules are in finitely separated from their neighbors.25

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12 Temperature (K) 246810 0 10 20 30 40 50 Temperature (K) 246810 0.0 0.5 1.0 1.5 2.0 2.5 3.0 1000 Hz 250 Hz 50 Hz 1000 Hz 250 Hz 50 Hz MT (cm3K mol-1) M(cm3mol-1)(a)(b) Figure 1-7. In-phase (as MT) and out-of-phase (as M ) AC susceptibility signals for a dried, microcrystalli ne sample of [Mn12O12(O2CR)16(H2O)4] at the indicated oscillation frequencies. The splitting of the S = 10 spin into 21 sublevels to give a double well potential in zero applied magnetic field is a consequen ce of the first term in the Hamiltonian describing the giant spin of an individual Mn12 molecule (eq 1-1): 'B y x zH ˆ O ˆ S ˆ g H ) S ˆ S ˆ E( S ˆ D H ˆ4 2 2 2 (1-1) where D is the axial anisotropy constant, z is the spin projection operator along the easy-axis of the molecule, E is the rhombic anisotropy constant, x and y are the x and y projections of the total spin operator the third term represents the Zeeman interaction with an applied magnetic field H,4O ˆ includes fourth order terms in the crystal field, and represents environmental couplings such as hyperfine, dipolar and exchange interactions.26 In an applied magnetic field, th e potential energy barrier becomes asymmetric and the degeneracy of the ms sublevels is remove d. According to the Zeeman term of the spin Hamiltonian, the ms = -10 sublevel is stabilized in energy with respect to other ms states and, hence, is prefer entially populated, giving a net magnetization. As the field is cycled to zero, the ms sublevels again become degenerate

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13 in energy as described in Fi gure 1-6. However, molecules remain trapped in the ms = -10 sublevel as the magnetization is frozen by the exis tence of the large energy barrier (U = S2|D| for integer spin systems and U = (S2-)|D| for non-integer spin systems), resulting in very slow magne tization relaxation, i.e., a remnant magnetization. Reversal of the magnetization direction is accomplished by cycling the field first to zero and then to a strong magnitude in the opposite directi on, generating a hystere sis loop (Figure 1-8). Hence, the energy barrier that arises from the spin and anisotropy of a SMM imparts a memory effect; molecules in the “spin up” or “spin down” states can represent zero and one for information storage purposes. -1 -0.5 0 0.5 1 -1-0.500.51 M/Ms 0H (T) 0.004 T/s 1.3 K 1.8 K 2 K 2.2 K 2.4 K 2.6 K 2.8 K 3 K 3.6 K M/Ms -1 -0.5 0 0.5 1 H0(T) 4 mT/s 1.3 K 1.8 K 2 K 2.2 K 2.4 K 2.6 K 2.8 K 3 K 3.6 K -1 0 0.5 1 -0.5 Figure 1-8. Magnetization hystere sis loops for a typical [Mn12O12(O2CR)16(H2O)4] complex in the 1.3-3.6 K temperature range at a 4 mT/s field sweep rate. M is normalized to its saturation value, Ms. In contrast to the hysteresis loops of tr aditional ferrior fe rromagnetic materials, such as those of magnetite or chromium dioxide, respectively, the plots of magnetization versus magnetic field of many SMMs show st eps that occur at re gular intervals. The observed steps correspond to an increase in the relaxation rate of magnetization that occurs when there is an energy coincidence of ms sublevels on the opposite sides of the potential energy barrier (F igure 1-9). For these critical field values, H = nD/gB, at which

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14 steps occur, quantum tunneling of the magnetiz ation (QTM) is allowed, resulting in an increase in the relaxation rate of the molecule.20,27-30 Such predicted, but never before observed behavior was first re ported in 1996 for molecules of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O.27 Hence, the relaxation of the magnetization of a SMM occurs not just by thermal activation ove r the energy barrier, but also by quantum tunneling of the ma gnetization through the energy barrier. A transverse component contained in the Hamilto nian of the molecule must be present to promote tunneling through the energy barrier however. Such transverse components include second and fourth order tr ansverse anisotropy terms, E and 4 4B, that can be provided in three ways: (i) by low-symmetry components of the crystal field; (ii) by a magnetic field provided by magnetic nuclei; and (iii) by a magnetic field provided by neighboring molecules. This quantum phenomenon is not unique to 1, but is also exhibited by many other SMMs, including the octanuclear FeIII oxo-hydroxo cluster, [Fe8O2(OH)12(tacn)6]8+ (tacn = 1,4,7-triazacyclononane ), where ground state tunneling was first observed, i.e., tunneling between th e lowest energy ms levels.31 In contrast, qua ntum tunneling of magnetization in molecules of 1 occurs between ms sublevels higher in energy than the lowest lying ms = 10 states. Here, mol ecules in the ground state ms = -10 sublevel are thermally activated to higher lying ms sublevels through which tunneling of the magnetization occurs. Relaxation of the magnetization to the ms = +10 sublevel follows in a process termed therma lly-activated quantum tunneli ng. Although tunneling provides a route for rapid reversal of magnetization and, hence, a less attractive memory storage

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15 device, an investigation into the gap between the quantum an d classical unde rstanding of magnetism can be made using these complexes as models. H = 0 H = 0 H = 0 Figure 1-9. Representation of the change in energy of ms sublevels as the magnetic field is swept from zero to a non-zero valu e. Resonant magnetization tunneling occurs when the ms sublevels are aligned betw een the two halves of the diagram. Due to the strong need for SMMs with ev en larger S values and more negative D values, numerous synthetic strategies aimed at the improvement of these materials have been considered. However, in contrast to the relative ease with which synthetic routes are developed in other fields of chemistry, th e preparation of polynuclear metal complexes presents a considerable challenge. One of th e primary goals of this research is the development of new synthetic methods aimed at the preparation of new SMMs. Three of the strategies employed towards this goal in clude (i) ligand substitution reactions of Mn12 complexes to derivatize them with non-carboxylate ligands in a site-specific manner, in order to direct or enhance reac tivity at selected sites, and al so to distort the geometry of

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16 the [Mn12O12]16+ core, thereby changing the magnitude s of the exchange interactions between the Mn centers as well as the spin and magnetic anisot ropy of the molecule; (ii) the introduction of a bulky, strongly electron donating carboxylate ligand to reductively destabilize the Mn12 complex, thereby making it more reactive and more likely to afford new polynuclear Mn clusters which may behave as SMMs; and (iii) the chemical reduction of a neutral Mn12 derivative to facilitate a thorough study of a family of neutral, oneand two-electron reduced Mn12 species with identical peripheral ligation using inelastic neutron scattering (INS) as well as other techniques. An additional research aim involves th e study of single crysta ls of high-symmetry Mn12 singlemolecule magnets by previously un employed techniques, specifically 55Mn nuclear magnetic resonance, to directly probe the magne tic structure and indivi dual metal ions in the core of these interesting molecules. Such studies provide insight into the necessary pathways to gain access to clusters that behave as single-molecule magnets at more technologically relevant temperatures.

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17 CHAPTER 2 SINGLE-MOLECULE MAGNETS. A Mn12 COMPLEX WITH MIXED CARBOXYLATE-SULFONATE LIGATION: [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] 2.1 Introduction One of the primary reasons for the cu rrent interest in high nuclearity Mn carboxylate clusters is the st udy of single-molecule magnets (SMMs). A SMM possesses a significant energy barrier to relaxation (re orientation) of its magnetization vector, owing to a combination of a large ground state spin (S) value and a large easy-axis type of magnetoanisotropy (negativ e axial zero-field splitting pa rameter, D). The upper limit of the energy barrier is given by S2|D| or (S2-)|D| for integer a nd non-integer spins, respectively.13,19,22,23 The SMMs with the largest energy barriers to date are the [Mn12O12(O2CR)16(H2O)4] (Mn12) family, and they have thus received a great deal of attention. Amongst the many studies that we have performed on Mn12 complexes have been attempts to derivatize them with non-carboxylate ligands in a site-specific manner, in order to direct or enhance reactivity at select ed sites and thus make regioselective reactions feasible. Such selectivity woul d be important for achieving clean and controllable reactivity in a complex contai ning so many carboxylat e groups, and would make more feasible important objectives such as the binding of groups that might enhance the shape anisotropy or magnetic properties of the Mn12 complexes, and/or facilitate the binding of the latter to surfaces or to each other to give dimers (or higher aggregates) of Mn12 species. One objective has been to re place just the eight axial, or just

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18 the eight equatorial carboxyl ate groups with non-carboxylat e ones, and thus obtain a mixed-ligand derivative with th e two types of groups in specific positions on the Mn12 molecule. Unfortunately, our previous attempts have met with limited success, and we have been unable to substitute all axial or all eq uatorial carboxylate ligands with non-carboxylate groups to date. For example, four (but no more) of the eight axial carboxylate groups could be replaced with NO3 groups using nitric acid, giving [Mn12O12(O2CCH2But)12(NO3)4(H2O)4];32 additional equivalent s of nitric acid caused decomposition. This incorporation of nitrate was a useful step forward but did not fulfill our desire to functionalize a ll axial sites with non-carboxylate ligands. Si milarly, we have been able to replace eigh t of the carboxylate groups with diphenylphosphinates (Ph2PO2 -) to give [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4],33 but the steric bulk of the Ph2PO2 groups resulted in them distributing themselves e qually between axial and equatorial sites. However, we have now achieved the above goal by the introduc tion into all the axial sites of benzenesulfonate groups, PhSO3 -, with retention of the Mn12 structure and its SMM properties. This also represents the first incorporation of S-based ligands into the Mn12 SMMs. The synthesis, st ructure, and magnetic pr operties of the obtained complex are described. In addition, reactivity studies on this new Mn12 complex have led to the preparation of a tetranuclear Mn cluster, [Mn4O4(O2PPh2)6], possessing a manganese-oxo cubane core [Mn4O4]6+. The cluster contains non-carboxylate diphenylphosphinate ligands and maintain s the central cubane core of the Mn12 cluster, but it is not a SMM.

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19 2.2 Results and Discussion 2.2.1 Syntheses Our previous development and use of ligand substitution reactions on readily available complex 1 have allowed us access to [Mn12O12(O2CR)16(H2O)4] derivatives with a large variety of substituents (Table 2-1). Table 2-1. [Mn12O12(O2CR)16(H2O)4] derivatives, together with pKa values of the conjugate acid of the carboxylate ligand. [Mn12] derivative pKa 34-37 [Mn12O12(O2CEt)16(H2O)3] 4.87 [Mn12O12(O2CPh)16(H2O)4] 4.20 [Mn12O12(O2CCH2Cl)16(H2O)4] 2.86 [Mn12O12(O2CCHCl2)16(H2O)4] 1.29 [Mn12O12(O2CCH2But)16(H2O)4] 5.00 [Mn12O12(O2CPet)16(H2O)4] 5.03 [Mn12O12(O2CC6F5)16(H2O)4] 1.48 [Mn12O12(O2CCH2Br)16(H2O)4] 2.90 [Mn12O12(O2CC6H4-p-OMe)16(H2O)4] 4.47 [Mn12O12(O2CC6H4-p-F)16(H2O)4] 4.14 [Mn12O12(O2CC6H4-o-(NO2))16(H2O)4] 2.22 [Mn12O12(O2CC6H4-p-(NO2))16(H2O)4] 3.44 [Mn12O12(O2CC6H3-o,p-(NO2)2)16(H2O)4] 1.42 The ligand substitution reaction (eq 2-1) is an equilibrium that must be driven to completion by (i) using a carboxylic acid with a much lower pKa than that of acetic acid (4.76); and/or (ii) using an excess of RCO2H; and/or (iii) removing the acetic acid as its toluene azeotrope. [Mn12O12(O2CMe)16(H2O)4] +16 RCO2H [Mn12O12(O2CR)16(H2O)4] + 16 MeCO2H (2-1) The latter is particularly useful for incorporating car boxylate groups whose conjugate acid has a pKa comparable to, or even higher than th at of acetic acid. The substitution reaction has been successfully employed for lig ands such as benzoate (and substituted benzoates), and a variet y of alkanecarboxylates.13,38 Of relevance to the present work is

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20 the fact that the s ubstitution procedure has already been extended to non-carboxylate ligands in the successful incorporation of Ph2PO2 groups,33 as mentioned above. Thus, the reactions of 1 with another type of organi c acid were explored, namely benzenesulfonic acid (pKa = 2.55).The reaction of complex 1 with eight equivalents of benzenesulfonic acid (PhSO3H) in MeCN was followed by several cycles of addition of toluene and its removal under vacuum. Note that since only partial replacement of acetate groups was being sought, excess acid could not be added, and the removal of acetic acid as the toluene azeotrope thus was an essentia l step to ensure comp lete reaction. This procedure successfully led to the crystallization a nd isolation of [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2) in essentially quantita tive (96%) yield (eq 2-2). Recrystallization from CH2Cl2/hexanes gave dark brown crystals of 24CH2Cl2 suitable for X-ray crystallography. [Mn12O12(O2CMe)16(H2O)4] + 8 PhSO3H [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] + 8 MeCO2H (2-2) Similarly, the reaction of 1 with four equivalents of PhSO3H (eq 2-3) led to the formation of [Mn12O12(O2CMe)12(O3SPh)4(H2O)4] (3), whose identity was established by elemental analysis and spectroscopic comparison with 2. Reactions of PhSO3H with other Mn carboxylate complexes, including [Mn3O(O2CR)6(py)3]0,+ and [Mn4O2(O2CPh)9(H2O)]-, have also been carried out, but attempts to crystallographically characterize the products have all been unsuccessful to date. The carboxylate substitution with benzenesulfonate is reversible on treatment of 2 with eight equivalents of MeCO2H in CH2Cl2, supporting the feasibility of using 2 for reactions directed at site-specific

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21 replacement of axial PhSO3 ligands by some added reagen t, taking advantage of the good leaving properties of this group. [Mn12O12(O2CMe)16(H2O)4] + 4 PhSO3H [Mn12O12(O2CMe)12(O3SPh)4(H2O)4] + 4 MeCO2H (2-3) Reactions of complex 1 with various non-carboxy lic acids, including HNO3, Ph2PO2H and PhSO3H have resulted in the partially-substituted Mn12 complexes, [Mn12O12(O2CCH2But)12(NO3)4(H2O)4],32 [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4] (4),33 [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2) and [Mn12O12(O2CMe)12(O3SPh)4(H2O)4] (3)39 as discussed. Despite many attempts, however reactions aimed at the isolation of a Mn12 complex with ligation by only non-carboxylate li gands have been ineffective. This is likely due to a combination of effect s, including steric hindrance of Ph2PO2 ligands about the Mn12 cluster and poor solubility of the PhSO3 ligand in various solvents. Hence, the reaction of 2 with Ph2PO2H was of particular interest ; a mixed non-carboxylate ligand strategy might be effectiv e route to obtain a Mn12 cluster with ligation by at least more than eight non-carboxylate li gands, if not by sixteen such ligands. Thus, complex 2 in MeCN was treated with eight equivalents of Ph2PO2H. Acetic acid was removed from the reaction system by multiple cycles of addition and removal of toluene. After 12 hours, the resulting deep brown solution was separated by filtration from some white powder. The filtrate was evaporated to dryness and crystallization from CH2Cl2/toluene gave dark brown crystals of [Mn4O4(O2PPh2)6] (5) in ~50% yield. Accompanying the dark brown crystalline product was some white powde r which was subsequently identified by spectroscopic methods as a mixture of the excess ligands, diphenylphosphinic acid and benzenesulfonic acid. The overall transformation of 2 into 5 is summarized in eq 2-4. The

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22 average oxidation state of th e starting material is +3.33 while that of the product (5) is +3.5. Thus, the formation of complex 5 appears to involve the oxidation of 2 followed by structural rearrangement. [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] + 18 Ph2PO2H 3 [Mn4O4(O2PPh2)6] + 8 MeCO2H + 8 PhSO3H + 4 H2O + 2 H+ + 2 e(2-4) 2.2.2 Description of Structures 2.2.2.1 X-ray crystal structure of [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2) A labeled ORTEP40 plot in PovRay format of complex 2 is shown in Figure 2-1, together with a stereoview. Th e crystallographic data and stru cture refinement details are collected in Table 2-2, and selected interato mic distances and angles are listed in Table A-1. Complex 24CH2Cl2 crystallizes in the triclinic space group 1 P with the asymmetric unit consisting of one Mn12 molecule and four CH2Cl2 molecules of crystallization. The Mn12 cluster possesses an overall st ructure similar to that of 1, with a central [MnIV 4O4] cubane held within a non -planar ring of eight MnIII ions by eight 3-O2ions (Figure 2-1). The oxidation levels of the Mn ions were assigned quantitatively on the basis of bond valence sum calculations,41 indicating a mixed-valence, trapped-valence MnIII 8MnIV 4 complex (Table 2-3 and 2-4). A bond valence sum (BVS) is an empirical value based on crystallographically determined metal-ligand distances that is routinely used to determine the oxidation level of an atom in a molecule. The valence of the ith atom, Vi, can be defined in terms of the sum of the individual bond valences, sij, of the atom i with those atoms, j, in its coordination sphere (eq 2-5).42 jj b R R ij iije s V] / ) [(0 (2-5)

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23 The valences of the individual bonds, sij, can be calculated fro m the observed bond length in the crystal structure of a molecule using eq 2-5, where Rij is the observed bond length, R0 is the length expected for a bond of unit valence and b is an experimentally determined constant equal to 0.37. Values of R0 for Mn-O bonds for Mnn+ (n = 2, 3, 4, and 7) are available,43 allowing the application of this calcu lation to determine the oxidation states of the Mn centers in our clusters. The cal culations can also be extended to include inorganic oxygen atoms and are a useful m eans of assessing the protonation levels of such atoms in a complex. In addition to transition metal clusters, bond valence sum analysis has been used to determine th e oxidation states of metal centers in metalloenzymes44 and superconductors.45 Table 2-2. Crystallographic data for [Mn12O12(O2CMe)8(O3SPh)8(H2O)4]4CH2Cl2. Parameter 24CH2Cl2 formulaa C68H80S8Cl8Mn12O56 fw, g mol-1 2992.76 space group 1 P a, 15.9971(6) b, 16.0923(7) c, 22.0964(9) deg 94.742(2) deg 90.307(2) deg 104.262(2) V 3 5492.1(4) Z 2 T C -80(2) radiation, b 0.71073 calc, g cm-3 1.810 cm-1 17.69 R 1 ( wR 2), %c d 3.79 (9.90) a Including solvent molecules. b Graphite monochromator. c R 1 = || Fo| – | Fc|| / | Fo|. d wR 2 = [ [ w ( Fo 2 Fc 2)2] / [ w Fo 2 )2]]1/2 where S = [[ w ( Fo 2 – Fc 2)2] / ( n p )]1/2, w = 1/[ 2( Fo 2) + ( mp )2 + np ], p = [max( Fo 2, 0) + 2 Fc 2]/3, and m and n are constants.

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24 O37 O39 O38 O3 O6 O7 O36 O4 O29 O42 O30 O31 Mn1 Mn3 O47 O5 O14 O15 O19 O9 O10 O20 O24 O25 O28 O23 S6 Mn6 O26 O27 O35 Mn7 O34 O16 O21 O32 O46 O12 Mn12 O18 Mn4 Mn10 O51 O8 S8 O33 S7 O1 O13 O2 S2 S4 O57 O11 Mn5 Mn11 O40 S5 Mn2 S3 O56 O50 O53 O48 O52 Mn9 O49 O45 S1 O54 O43 O55 O44 O22 O17 Mn8 Figure 2-1. ORTEP representa tion in PovRay format of [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2) with thermal ellipsoids at the 50% probability level except for the C atoms, together with a stereopair. For clarity, only the ipso C atoms of the benzenesulf onate groups are shown. Mn blue; O red; S green; C gray. The eight MnIII ions separate into two groups of four MnIII ions each. In the first group, each MnIII ion is coordinated to a single MnIV ion via two oxide bridges [Mn(1), Mn(3), Mn(5), Mn(7)], while in the second group each MnIII ion is coordinated to two MnIV ions via two oxide bridges [M n(2), Mn(4), Mn(6), Mn(8)].29 Four water molecules, eight -carboxylate and ei ght -benzenesulfonate groups complete the peripheral ligation about the [Mn12O12] core of the complex. The eight MnIII centers exhibit a Jahn-Teller

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25 (JT) distortion, as expected for a high-spin d4 ion in near-octahedral geometry (Figure 2-2). As is almost always the case for MnIII, the JT distortion in 2 takes the form of an axial elongation of two trans bonds. Again as is usually th e case, the JT elongation axes avoid the Mn-O2bonds, the shortest and strongest in the molecule, and th us the JT axes are all axially disposed, roughl y perpendicular to the [Mn12O12] disk-like core. As a result, there is a near parallel alignment of the eight MnIII JT elongation axes. This is also the origin of the significant magnetic anisotropy in the z direction that greatly influences the observed magnetic properties ( vide infra ). Figure 2-2. ORTEP representa tion in PovRay format of [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2) showing the relative disposition of the Jahn-Teller elongation axes indica ted as solid black bonds. For clarity, only the ipso C atoms of the benzenesulfonate groups are shown. Mn blue; O red; S green; C gray. The MeCO2 groups occupy the eight equatorial sites of 2, while the PhSO3 ligands are located at the eight axial sites above and below the disk-like [Mn12O12] core. This selectivity in axial vs equatorial binding sites can be rationalized on the basis of the relative basicities of acetate vs benzenesulfonate. The pKa value of PhSO3H is 2.55 while that of MeCO2H is 4.76.37 The more basic, stronger donor MeCO2 ligands favor occupation of the equatorial sites where shor ter, stronger Mn-O bonds can be formed, to the benefit of the overall energy stabilization of the molecule.46 The less basic PhSO3

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26 ligands thus occupy axial positions where they bridge either MnIIIMnIII or MnIIIMnIV pairs and thus have one or both of their O atoms on the JT elongation axes. There are twenty axial coordination sites, of which sixteen lie on JT elo ngation axes, and these bonds are thus lengthened by 0.1-0.2 , and t hus weakened relative to the equatorial, nonJT elongated Mn-carboxylate bonds. The same rationalization based on relative acid pKa values also explained the selective ax ial vs equatorial disposition in mixedcarboxylate [Mn12O12(O2CR)8(O2CR)8(H2O)4] complexes.46 Table 2-3. Bond valence sum calculationsa for complex 24CH2Cl2. Atom Mn2+ Mn3+ Mn4+ Mn(1) 3.293 3.012 3.162 Mn(2) 3.167 2.897 3.041 Mn(3) 3.232 2.957 3.104 Mn(4) 3.284 3.004 3.154 Mn(5) 3.270 2.991 3.140 Mn(6) 3.191 2.918 3.064 Mn(7) 3.289 3.008 3.158 Mn(8) 3.277 2.997 3.146 Mn(9) 4.261 3.898 4.092 Mn(10) 4.198 3.840 4.032 Mn(11) 4.253 3.890 4.084 Mn(12) 4.169 3.813 4.003 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value. Table 2-4. Bond valence sum calculationsa for selected oxygen atoms in complex 24CH2Cl2. Atom Vi Assignment Atom Vi Assignment O(46) 2.189 O2O(54) 2.071 O2O(47) 2.092 O2O(55) 2.052 O2O(48) 2.082 O2O(56) 2.101 O2O(49) 2.079 O2O(57) 2.089 O2O(50) 2.123 O2O(4) 0.286 H2O O(51) 2.209 O2O(5) 0.290 H2O O(52) 2.100 O2O(26) 0.287 H2O O(53) 2.077 O2O(27) 0.285 H2O a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0.

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27 In all [Mn12O12(O2CR)16(H2O)4] complexes studied to date, the four water ligands coordinate only to the four MnIII ions in the second group described above [Mn(2), Mn(4), Mn(6), Mn(8)],20 either one water on each Mn, two each on two Mn, or similar. Indeed, complex 2 similarly has two water liga nds, O(4) and O(5), on a MnIII ion of the second group [Mn(2)], and two water lig ands, O(26) and O(27), on another MnIII ion of the same group [Mn(6)]. This trans disposition two pairs of H2O ligands has also been observed for [Mn12O12(O2CPh)16(H2O)4], [Mn12O12(O2CC6H4p -Cl)16(H2O)4] and others.20,29 The formation of complex 3 from 1 represents an abst raction of only four carboxylate groups from a [Mn12O12(O2CR)16(H2O)4] complex. This has been previously achieved exclusively at the ax ial ligands bridging the MnIIIMnIII pairs, where the four bridging carboxylates are the only ones to ha ve both their O atoms on JT elongation sites and thus are the most susceptible to substitution. Both [Mn12O12(O2CR)12(NO3)4(H2O)4] (R = CH2But and Ph)32 and, subsequently, [Mn12O12(O2CPh)12(O2P(OPh)2)4(H2O)4]47 have been crystallographically confirmed to have their non-carboxylate bridging groups at these positions. Thus, although we have not sought the crystal structure of complex 3, it is very likely that it has the four PhSO3 groups in axial positions bridging the same MnIIIMnIII pairs as the NO3 and (PhO)2PO2 derivatives. Comparison of 2 with the related clusters [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) and [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4] (4) reveals that there is no significant distortion of the [Mn12O12]16+ core upon coordination of the noncarboxylate benzenesulfonate groups (Table 2-5). The apparent lack of distortion is in contrast to that seen within the core of 4

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28 in which the MnIIIO2-MnIII bond angles differ significantly and the MnIIIMnIII bond distances are on average 0.1 longer than the corresponding values of 1. Additional differences are reflected in the linearity of the Mn4 units that span the core of the cluster, with 1 and 2 having angles of the type MnIIIMnIVMnIV equal to almost 180 while the corresponding angle in 4 is only ~171.33 The similarity of complexes 1 and 2, and the difference between them and 4 are likely reflective of th e distribution of ligands around the cluster. In 4, the acetate groups at the four axial MnIIIMnIII and four of the equatorial MnIIIMnIII carboxylate sites have been replaced by relatively bulky diphenylphosphinate groups, giving rise to a distortion of the co re of the complex from steric effects. 2.2.2.2 X-ray crystal structure of [Mn4O4(O2PPh2)6] (5) A labeled ORTEP40 plot in PovRay format of complex 5 is shown in Figure 2-3, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 2-6, and selected interato mic distances and angles are listed in Table A-2. Complex 5 crystallizes in the cubic space group P 213 with four crystallographically independent molecules in the unit cell, each slightly diffe rent with respect to the orientation of the phenyl groups of the diphenylphosphinate ligands.48 The bond distances and angles of the four molecules also vary within a small range. For the sake of brevity, references to specific atoms in the following discussion implicitly include their symmetry-related partners.

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29Table 2-5. Comparison of selected bond distances () and angles ( ) for [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1), [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2) and [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4] (4). Parametera 1 2 4 MnIV – Oc (ax) 1.8954(13) 1.863(2) – 1.876(2) 1.894(2) MnIV – Oc (eq) 1.9116(11), 1.9196(10) 1.9157(19) – 1.9393(19) 1.905(2), 1.960(2) MnIV – Or 1.8592(11), 1.8795(11) 1.854(2) – 1.874(2) 1.855(2), 1.862(2) MnIV – Oax 1.9131(13) 1.907(2) – 1.945(2) 1.915(3) MnIII b – Or 1.8770(11), 1.8983(11) 1.883(2) – 1.909(2) 1.890(2), 1.932(2) MnIII c – Or 1.8860(11), 1.8964(11) 1.880(2) – 1.910(2) 1.873(2), 1.992(3) MnIII b – Oeq 1.9371(13), 1.9393(12) 1.920(2) – 1.942(2) 1.935(3), 1.963(3) MnIII c – Oeq 1.9655(12), 1.9884(13) 1.929(2) – 1.962(2) 1.910(3), 1.998(3) MnIII b – Oax 2.111(5)*, 2.228(3), 2.2318(13) 2.163(2) – 2.261(2) 2.167(3), 2.195(3) MnIII c – Oax 2.117(11), 2.151(17)* 2.174(2) – 2.190(2) 2.066(3) MnIII c – Ow 2.1735(15) 2.216(2) – 2.222(2) 2.208(3) MnIII bMnIII c 3.321, 3.414 3.352 – 3.454 3.410, 3.547 MnIVMnIV 2.8166(4), 2.8166(4), 2.9271(4) 2.8054( 6) – 2.9478(6) 2.8492(9), 2.9461(10) MnIII bMnIV 2.7643(3) 2.7905(6) – 2.8016(6) 2.7726(8) MnIII cMnIV 3.445, 3.448 3.416 – 3.466 3.420, 3.452 Or – MnIV – Or 84.96(5) 83.27(9) – 84.13(9) 84.90(11) Or – MnIII b – Or 83.95(5) 81.70(9) – 82.74(8) 82.06(10) Or – MnIII c – Or 93.29(5) 94.83(8) – 95.52(9) 95.78(10) MnIII bMnIVMnIV 178.380(14) 175.99(2) – 178.21(2) 171.45(2) a Oc = cubane O2-, Or = ring O2-, Oax = axial carboxylate, Oeq = equatorial carboxylate, Ow = water; b MnIII b atoms: Mn(2) in 1, Mn(1, 3, 5 and 7) in 2, and Mn(3) in 4; c MnIII c atoms: Mn(3) in 1, Mn(2, 4, 6 and 8) in 2, and Mn(2) in 4. Involves the disordered acetate ligand bridging Mn(2) and Mn(3).

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30 The cluster consists of a cubane [Mn4O4]6+ core in which there is one unique [Mn(2)] and three symmetry-related Mn ions [Mn(1)] (Figure 2-3). Six diphenylphosphinate ligands that bridge Mn pairs across each of the six [Mn2O2] faces complete the coordination sphere of the complex. The unit cell contains sixteen Mn4 molecules; eight are located on the cell corners (8 ), twelve are located on cell edges (12 ), six are located on the cell faces (6 ) and seven complete molecules are situated within the cell dimensions. The Mn4 molecules in the cubic P 213 lattice are stacked in columns, with all of the molecu les approximately oriented in the same way with respect to the cell axes. There is no ev idence of any interaction between molecules in the same column or in different columns (Figure 2-4). Table 2-6. Crystallographic data for [Mn4O4(O2PPh2)6] (5). Parameter 5 formulaa C72H60P6Mn4O16 fw, g mol-1 1586.83 space group P 213 a, 29.9772(10) b, 29.9772(10) c, 29.9772(10) deg 90 deg 90 deg 90 V 3 26938.5(16) Z 16 T C -123(2) radiation, b 0.71073 calc, g cm-3 1.565 cm-1 9.46 R 1 ( wR 2), %c d 11.20 (20.58) a Including solvent molecules. b Graphite monochromator. c R 1 = || Fo| – | Fc|| / | Fo|. d wR 2 = [ [ w ( Fo 2 Fc 2)2] / [ w Fo 2 )2]]1/2 where S = [[ w ( Fo 2 – Fc 2)2] / ( n p )]1/2, w = 1/[ 2( Fo 2) + ( mp )2 + np ], p = [max( Fo 2, 0) + 2 Fc 2]/3, and m and n are constants.

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31 P2a O6a O5a Mn2 P2b O6b O6 P2 P1 O3 O4 O5b O3b P1a O4a O3a O5 O2 O1 O1b Mn1 Mn1b Mn1a P1b O7b O1a Figure 2-3. ORTEP representati on in PovRay format of [Mn4O4(O2PPh2)6] (5) with thermal ellipsoids at the 50% probability level, together with a stereopair. For clarity, only the ipso C atoms of the diphenylphosphinate groups are shown. Mn blue; O red; P orange; C gray. (a)(b) b a c b c a Figure 2-4. ORTEP representation in PovR ay format of the packing of complex 5 along (a) one of the faces and (b) a selected C3 rotation axis of the unit cell. Mn blue; O red; P orange; C gray.

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32 All of the Mn centers are six-coordinate with near-octahedral geometry. Charge considerations require a mixed-valence MnIII 2MnIV 2 system. A bond valence sum41 analysis to determine the oxidation levels of the Mn centers is not conclusive however. The results of the BVS analyses for the Mn and inorganic O atoms are summarized in Tables 2-7 and 2-8. Similarly, an exam ination of the bond distances around the Mn centers does not reveal obvious Jahn-Teller (JT) distortion ax es on any of the Mn centers as would be expected for a high-spin MnIII (d4) ion in near-octahedral geometry. The absence of such JT elongation axes is in cont rast to the related dist orted-cubane clusters with [MnIII 3MnIV(3-O)3(3-X)]6+ (X = Cl-, F-, Br-, N3 -, NCO-, or RCO2 -) cores, in which there are clear JT elongation axes of two trans bonds on each of the MnIII ions.12 On average, the JT elongated bond distance in [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) and a representative distorted-cubane complex [Mn4O3Cl4(O2CMe)3(py)3], is 2.188 and 2.146 , respectively, approximately 0.10.2 longer than similar non-JT elongated bond distances.49,50 The Mn-O bond distances in 5 are in the range of 1.841(6) – 2.051(7) ; distinguishable JT el ongation or compression axes are not obvious on this basis. Symmetry considerations require that three Mn ions are crystallographically identical with equivalent bond distances and angles, making MnIII ions indistinguishable from MnIV ions. An electronically delocalized syst em or a static disorder of trappedvalence MnIII and MnIV sites can account for this behavior. Comparison of the MnMn separations acro ss the faces of the cube with related Mn complexes having cubane cores indicates that the distances in 5 most closely resemble those in the central [Mn4(3-O)4]16+ core of members of the Mn12 family. Averaged over the four independent mol ecules of the unit cell, the mean MnMn

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33 distance is 2.928 . Simila r distances are found in Mn12 complexes while those in the distorted-cubane Mn4 complexes are slightly shorter. Unlike 5, the Mn12 complexes have no carboxylate-bridged Mn2 pairs in the cubane core. Comparison of the O2-MnO2angles in the cores of complexes 1 and 5 reveals additional similarities. The average O2-MnO2angle in 1 is 82.7 while the corresponding angle in 5 is 81.7. A evaluation of the OO distances of the carboxylate ligands in 1 with the non-carboxylate ligands in 2 and 5 shows that Ph2PO2 groups may be preferred for formation of the cubane complex. The average OO distances for th e bridging ligands acetate, benzenesulfonate, and diphenylphosphinate are 2.24, 2.42, and 2.57 , respectively. Table 2-7. Bond valence sum calculationsa for complex 5. Atom Mn2+ Mn3+ Mn4+ Mn(1) 3.751 3.431 3.602 Mn(2) 3.553 3.249 3.411 Mn(3) 3.489 3.192 3.351 Mn(4) 4.172 3.816 4.006 Mn(5) 3.891 3.559 3.737 Mn(6) 3.274 2.995 3.144 Mn(7) 3.415 3.123 3.279 Mn(8) 3.887 3.556 3.733 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value. Table 2-8. Bond valence sum calculationsa for selected oxygen atoms in complex 5. Atom Vi Assignment O(1) 2.189 O2O(2) 2.092 O2O(48) 2.082 O2O(49) 2.079 O2O(50) 2.123 O2O(51) 2.209 O2O(52) 2.100 O2O(53) 2.077 O2a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0.

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34 2.2.3 Magnetochemistry of Complexes 2 and 3 2.2.3.1 DC studies Solid-state, variable-temperature magnetic susceptibility measurements were performed on vacuum-dried, powdered samples of complexes 2C6H14 and 30.2C6H14, suspended in eicosane to prevent to rquing. DC magnetic susceptibility ( M) data were collected in the 2.00-300 K range in a 5.0 kG magnetic field (Figure 2-5). The MT versus T behavior is similar to thos e of previously studied [Mn12O12(O2CR)16(H2O)4] complexes with S = 10 ground states, exhibiting a nearly temperature-independent MT value of 21-22 cm3 K mol-1 (2) and 18-19 cm3 K mol-1 (3) in the 150-300 K range which then increases rapidly to a maximum value of 45-46 cm3 K mol-1 (2) and 44-45 cm3 K mol-1 (3) at ~20 K before decreasing rapidly at lo wer temperatures. The maximum indicates a large ground state spin (S) valu e, and the low temperature d ecrease is primarily due to Zeeman and zero-field splitting (ZFS) effects. Temperature (K) 050100150200250300 15 20 25 30 35 40 45 50 MT (cm3K mol-1) Temperature (K) 050100150200250300 15 20 25 30 35 40 45 50 MT (cm3K mol-1)(a)(b) Figure 2-5. Plot of MT vs temperature for dried, microc rystalline samples of (a) complex 2C6H14 and (b) complex 30.2C6H14 in eicosane. M is the DC molar magnetic susceptibility measured in a 5.0 kG field.

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35 A theoretical treatment of the su sceptibility data via the Kambe51 method to determine the individual pairwise Mn2 exchange interactions was not possible owing to the topological complexity and low symmetry of the Mn12 core. Instead, the ground state spin of complexes 2 and 3 was determined from magnetization (M) measurements in the 1.80-5.00 K temperature range and 2.0-7.0 T field range, where N is Avogadro’s number, B is the Bohr magneton, and H is the appl ied magnetic field. The obtained data are plotted as M/NB vs H/ T in Figure 2-6. For a system occupying only the ground state and experiencing no ZFS, the various isofield lines would be superimposed and M/NB would saturate at a value of g S. The non-superposition of the isofield lines Figure 2-6 is indicative of the presence of strong ZFS. Th e data were fit using the method described elsewhere49,52 that involves diagonalization of the spin Hamiltonian matrix, assuming only the ground state is occupied at thes e temperatures and including axial ZFS (2S ˆ Dz), Zeeman interactions, and a full powder average of the magnetization.53,54 The obtained fit of the data, shown as solid lines in Figure 2-6, gave S = 10, g = 1.96 and D ~ -0.34 cm-1 (-0.49 K) for complex 2 and S = 10, g = 1.78 and D ~ -0.37 cm-1 (-0.53 K) for complex 3. Thus, complexes 2 and 3 retain the same spin as 1, with fitting parameters typical for [Mn12O12(O2CR)16(H2O)4] clusters. The poorer quality of the fit than is typically found for [Mn12O12(O2CR)16(H2O)4] complexes likely results from population of an excited state of each cluster that is lower-lying than normal; thus, the obtained D values must be considered as only a rough approximation.46 The S = 10 value shows that, as previously seen with the mixed-carboxylate [Mn12O12(O2CR)8(O2CR)8(H2O)4] clusters, the presence of two types of ligands with distin ctly different basiciti es does not affect the large ground state S value of the Mn12 complex.46

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36 10203040 10 12 14 16 18 20 2 T 3 T 4 T 5 T 6 T 7 T M/NB 10203040 11 12 13 14 15 16 3 T 4 T 5 T 6 T 7 T M/NBH/T (kGK-1) H/T (kGK-1)(a)(b) Figure 2-6. Plot of re duced magnetization M/NB vs H/ T for dried, microcrystalline samples of (a) complex 2C6H14 and (b) complex 30.2C6H14 in eicosane at the indicated applied fields. The solid lines ar e the fit of the data; see the text for the fit parameters. To confirm that the obtained minima were the true global minima and to assess the uncertainty in the obtained g and D values, root-mean square D vs g error surfaces for the fits were generated w ith the program GRID,55 which calculates the relative difference between the experimental M/NB data and those calculated fo r various combinations of D and g This is shown as a contour plot for complexes 2 and 3 in Figure 2-7 for the D = -0.10 to -0.70 cm-1 and g = 1.7 to 2.2 ranges (2) and the D = -0.15 to -0.65 cm-1 and g = 1.5 to 2.05 ranges (3). One very soft fitting minimum for 2 is observed; the contour describes the region of minimum error encompassing D -0.24 to -0.49 cm-1 and g = 1.70 to 2.20, giving fitting parameters of D = -0.36 0.12 cm-1 and g = 1.95 0.25. Similarly, the region of minimum error for 3 is D -0.28 to -0.51 cm-1 and g = 1.70 to 1.92, giving fitting parameters of D = -0.39 0.12 cm-1 and g = 1.81 0.10.

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37 g 1.71.81.92.02.12.2 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 D (cm-1) g 1.61.71.81.92.0 -0.6 -0.5 -0.4 -0.3 -0.2 D (cm-1)(a)(b) Figure 2-7. Two-dimensional contour plot of the error surface for the D vs g fit for (a) complex 2C6H14 and (b) complex 30.2C6H14 in eicosane. The asterisk indicates the the soft minimum. 2.2.3.2 AC studies In an AC susceptibility experiment, a weak field (typically 1 5 G) oscillating at a particular frequency ( ) is applied to a sample to probe the dynamics of the magnetization (magnetic moment) relaxation. An out-of-phase AC susceptibility signal ( M ) is observed when the rate at which the magnetizat ion of a molecule relaxes is close to the operating frequency of the AC field, and there is a corresponding decrease in the in-phase (M T ) signal. At low enough temperature, wher e the thermal energy is lower than the barrier for relaxation, the magnetization of th e molecule cannot relax fast enough to keep in-phase with the oscillating field. Theref ore, the molecule will exhibit a frequencydependent M signal indicative of slow magnetiza tion relaxation. Frequency-dependent M signals are an important indicator of SMMs.

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38 M(cm3mol-1) Temperature (K) 246810 0.0 0.5 1.0 1.5 2.0 500 Hz 50 Hz 10 Hz 0 10 20 30 40 50 (b)MT(cm3K mol-1) Temperature (K) 246810 0.0 0.5 1.0 1.5 2.0 500 Hz 50 Hz 10 Hz 0 10 20 30 40 50 (a)M(cm3mol-1) MT(cm3K mol-1) Figure 2-8. Plot of the in-phase (as M T ) and out-of-phase ( M ) AC susceptibility signals vs temperature for dried, microcrystalline samples of (a) complex 2C6H14 and (b) complex 30.2C6H14 in eicosane at the indicated oscillation frequencies. AC susceptibility studies were performed on complexes 2C6H14 and 30.2C6H14 in the 1.80-10.0 K range in a 3.5 G AC field oscillating at frequencies ( ) up to 1488 Hz. The in-phase ( M) signal (as M T ) and out-of-phase ( M ) signal are plotted vs temperature in Figure 2-8. Clearly eviden t are a frequency-dependent decrease in M T at T < 10 K, concomitant with the appearan ce of a frequency-dependent out-of-phase ( M ) signal. These indicate that the magnetization of both 2 and 3 cannot relax fast enough to stay in-phase with the oscillating field, a nd this is a strong indi cation that complexes 2 and 3 are single-molecule magnets (SMMs). Note that at each AC oscillation frequency for complex 2 there is only one M peak, and complex 2 does not therefore exhibit JT isomerism. The latter is the pr esence of an abnormally orient ed JT axis, equatorial with respect to the Mn12 disk, that leads to a lower barr ier to magnetization relaxation and a

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39 M peak at consequently lower temperatures.56-59 Complex 2 is thus present as exclusively the normal, slower-relaxing JT isomer. This is in contrast to complex 3 where in addition to the frequency-dependent signa l in the 4-7 K range, there is also a frequency-dependent out-of-phase signal in the 2-3 K range that corresponds to a fasterrelaxing species. Loss of solvent molecules as the material is dried under vacuum likely causes the conversion of a small fraction of slower-relaxing molecules of complex 3 to the faster-relaxing isomer almost certainly by re-orientation of a normally-oriented JT elongation axis to an abnormal position.56b 2.2.3.3 Relaxation studies The M versus T plots were used as a source of kinetic data to calculate the effective energy barrier (Ueff) to magnetization relaxation. At a given oscillation frequency ( ), the position of the M peak maximum is the temperature at which the angular frequency ( = 2 ) of the oscillating field e quals the relaxation rate (1/ where is the relaxation time) at which the magneti zation vector of a molecule relaxes between one orientation along the easy-axis (the z axis) to the opposite one. The relaxation rates at a given temperature can thus be obtained from = 1/ at the maxima of the M peaks. The peak maxima were accurately determin ed by fitting the peaks to a Lorentzian function. The magnetization relaxation of a SMM obeys the Arrhenius relationship (eq 2-6), the characteristic behavior of a th ermally-activated Orbach process,60 where Ueff is the effective anisotropy energy barrier, k is the Boltzmann constant, and 1/ 0 is the pre-exponential term.

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40 (1/ ) = (1/ 0) exp(-Ueff/k T ) (2-6a) ln(1/ ) = ln(1/ 0) – Ueff/k T (2-6b) The frequency dependence of the M peak for complexes 2 and 3 was determined at eight oscillation frequencies up to 1488 Hz. Plots of ln(1/ ) vs 1/ T for 2 and 3 using M versus T data are separately shown in Figure 2-9, with the least-squares fits to eq 2-6 shown as solid lines. From the slope and intercept, it was determined that Ueff = 45.5 cm-1 (65.5 K) and 1/ 0 = 1.1 108 s-1 for complex 2, and Ueff = 47.2 cm-1 (67.9 K) and 1/ 0 = 1.6 108 s-1 for complex 3. 0.140.160.180.200.220.24 3 4 5 6 7 8 9 10 0.140.160.180.200.220.24 3 4 5 6 7 8 9 10 1/T (K-1) 1/T (K-1)ln(1/ ) ln(1/ )(a)(b) Figure 2-9. Plot of the natural l ogarithm of relaxation rate, ln(1/ ), vs inverse temperature for (a) complex 2C6H14 and (b) complex 30.2C6H14 using M versus T data at different frequencies. The solid line is a fit to the Arrhenius equation; see the text for the fit parameters. To supplement the AC data and thus provi de for a more accurate kinetic analysis, additional relaxation versus temp erature data were obtained for 2 at temperatures below 1.8 K, the operating minimum of our SQUID instrument, from DC magnetization decay vs time measurements. These data we re obtained on a single crystal of 24CH2Cl2 using a micro-SQUID apparatus. First, a large DC fi eld of 1.4 T was applied to the sample at

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41 about 5 K to saturate its magnetization in one direction, and the temperature was then lowered to a chosen value between 0.04 and 2. 0 K. When the temperature was stable, the field was swept from 1.4 T to zero at a rate of 0.14 T/s, and then the magnetization in zero field was measured as a function of tim e. This gave a set of relaxation time ( ) vs T data, which were combined with the AC data a nd used to construct an Arrhenius plot of vs 1/ T (Figure 2-10). 0 0.1 0.2 0.3 0.4 0.5 0.6 0.11101001000 t (s) 2.0 K 1.7 K 1.3 K 2.2 K 2.4 K 4.0 K 3.9 K 3.8 K 3.7 K 3.6 K 3.5 K 3.4 K 3.3 K 3.2 K 3.1 K 3.0 K 2.9 K 2.8 K 2.7 K 2.6 KM/Ms 10-510-310-11011031051071090.10.20.30.40.50.60.70.8 DC AC 1/T (K-1) (s)(a) (b) Figure 2-10. Relaxation time vs temperature studies for complex 24CH2Cl2. (a) Plot of magnetization vs time decay in zero fiel d. The magnetization is normalized to its saturation value, Ms. (b) Arrhenius plot using AC M data () on a microcrystalline sample and DC magnetization decay data ( ) on a single crystal. The dashed line is the fit of th e data in the thermally-activated region to eq 2-6; see the text for the fitting parameters. The data above 2.5 K (1/ T = 0.4 K-1) were fit to the Arrhenius relationship (eq 2-6), and the fit (dashed line in Figure 2-1 0b) gave an effec tive energy barrier (Ueff) of 67 K and a pre-exponential factor ( 0) of 6.610-9 s. The Ueff value falls within the normal range 42-50 cm-1 (60-72 K) observed previously for several [Mn12O12(O2CR)16(H2O)4] complexes.19,22,23 Below ~2.5 K, the vs 1/ T plot of Figure 2-10b deviates from linearity as the thermally-activated relaxation rate diminishes and the relaxation via quantum tunneling through the anisotropy barrier become s more important. Eventually, at low

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42 enough temperature, the plot would plateau and become te mperature-independent, as expected for the relaxation now being onl y via quantum tunneling as the thermallyactivated relaxation rate becomes insignificant. 2.2.3.4 Hysteresis studies below 1.8 K Since complex 2 is a SMM, it should exhibit hysteresis below its blocking temperature, TB, in a magnetization versus DC fiel d plot. Figure 2-11a shows such magnetization vs field scans for 24CH2Cl2 at different temperat ures in the 2.0-4.0 K range and a constant sweep rate of 4 mT/s. Hysteresis loops were indeed observed below 4.0 K, whose coercivities increase with decr easing temperature, as expected for a SMM. The SMM behavior of complex 2 is further emphasized in Figure 2-11b where magnetization vs field scans are shown at 2.5 K for different field sweep rates. -1 -0.5 0 0.5 1 -1-0.500.51 0.280 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/s M/Ms 0H (T) 2.5 K 2.5 K M/Ms 1 -1 0.5 -0.5 0 0.280 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/s 1 -1 -0.5 0.5 0 H0(T) -1 -0.5 0 0.5 1 -1-0.500.51 2.0 K 2.5 K 3.0 K 3.5 K 4.0 K 4 mT/s H0(T)M/Ms(a) (b) Figure 2-11. Magnetization (M) vs magnetic fiel d hysteresis loops for a single crystal of complex 24CH2Cl2 at (a) the indicated temperat ures and fixed sweep rate and (b) the indicated sweeping rates at 2.5 K. M is normalized to its saturation value, Ms. As expected for a SMM, the coercivity of the hysteresis l oops at zero field increases with increasing scan rates. The loops are not completely smooth, instead showing the step-like features characteris tic of quantum tunneli ng of the magnetization

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43 (QTM) through the anisotropy barrier. Such st eps are a common feature of the hysteresis loops of many types of SMMs.28 However, the steps in the loops for complex 2 in Figure 2-11 are much less well defined than usual for [Mn12O12(O2CR)16(H2O)4] complexes.61 Instead, they are rather broad and poorly resolved, although clearly present at field positions of zero, ~ 0.65 and ~ 1.3 T. Examination of many crystals gave comparable results, the steps occasionally being almost totally smeared out. Such broadening of QTM steps is not uncommon, particular ly in larger SMMs such as Mn18 62 and Mn30 63 where no sign of a step feature is ev ident even though the presence of QTM is confirmed from other types of data such as temperatureindependent relaxation rates and quantum hole digging. The primary origins of step broade ning are weak intermolecular interactions (exchange and dipolar), and a distribution of molecular environments arising from disordered solvent molecules of crystalliza tion, partial solvent loss, ligand disorder, crystal defects, and similar.62,63 A distribution of molecular environments results in a distribution in D values, and thus a distribution in step positions, which depend upon the D value. As a result, the observed steps are broadened, or even smeared out if the broadening is sufficient. Since intermolecular in teractions are likely to be comparable in all Mn12 complexes and the latter usually give well de fined steps, it is likely that the main causes of step broadening in 2 are the other reasons listed above, particularly the disorder in the CH2Cl2 solvents of crystallization, their partial loss on removing crystals from their mother liquor, and the slight crystal damage that results from the latter. 2.2.4 Magnetochemistry of Complex 5 DC magnetic susceptibility data were collected in the 5.00-300 K range on a powdered microcrystalline sample of 52H2O restrained in eicosane to prevent torquing. The MT per molecule smoothly decreases from 6.58 cm3 K mol-1 at 300 K to

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44 0.78 cm3 K mol-1 at 5 K (Figure 2-12). The value at 300 K is less than 9.75 cm3 K mol-1, the spin-only value expected for a 2MnIII, 2MnIV complex with noninteracting metal centers, indicating the presence of appreciable antiferromagnetic interactions between the manganese ions and suggesting a small ground state spin. 050100150200250300 0 1 2 3 4 5 6 7 MT (cm3K mol-1)Temperature (K) Figure 2-12. Plot of MT vs temperature for a dried, microcrystalline sample of complex 52H2O in eicosane. M is the DC molar magnetic su sceptibility measured in a 5.0 kG field. This was confirmed by AC magnetic sucepti bility measurements carried out on a dried, microcrystalline sample of 52H2O in a 3.5 G AC field os cillating at 1000 Hz. In Figure 2-13 is shown the in-phase AC susceptibility, plotted as M T versus T together with the out-of-phase AC susceptibility ( M ), in the 1.8-10 K temperature range to minimize the possibility of populating excited states. The downward sloping M T versus T plot confirms that depopulation of excited states with greater S value than the ground state is occurring as the temper ature decreases. Extrapolation of the plot to 0 K, where only the ground state will be populated, gives a M T value of ~ 0.5 cm-3 K mol-1. This value is approximately that expected for an S = 0 state with g = 2. Also evident from the AC studies is the lack of an out-of-phase AC susceptibility signal ( M ). Such a signal is

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45 indicative of the onset of slow magnetic relaxation and confirms that complex 5 does not function as a single-molecule magnet in accord with the small ground state spin of the molecule. Temperature (K)M(cm3mol-1) MT(cm3K mol-1) 246810 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Figure 2-13. Plot of the in-phase (as M T ) and out-of-phase ( M ) AC susceptibility signals vs temperature for a dried, microcrystalline sample of complex 52H2O in eicosane in a 3.5 G AC field oscillating at 1000 Hz. 2.3 Conclusions Complete derivatization of all the axial sites with noncarboxylate ligands has been achieved for the first time in an Mn12 complex by replacement of the axial carboxylate ligands of 1 with PhSO3 groups. Incorporation of the noncarboxylate, S-based ligands is driven by the greater acidity of PhSO3H over MeCO2H, with the less basic PhSO3 ligands occupying axial s ites with at least one O atom lying along a MnIII JT elongation axis. The resultant mixed-ligand product reta ins both the high ground state spin value and SMM properties of the parent [Mn12O12(O2CR)16(H2O)4] complexes, with the complex

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46 possessing an S = 10 ground state spin and disp laying frequency-dependent peaks in the out-of-phase AC magnetic susceptibility. Complex 2 is also a potential starting point for further chemistry, and reactivity studies on 2 with diphenylphosphinic acid have afforded a clean route for the preparation of a tetranuclear clus ter possessing a [Mn4O4]6+ cubane core. Although 5 does not retain the SMM properties of its parent compound, it provides further insight into the behavior of 2 when treated with additional non-carboxylate ligands. Complex 2 is also the first Mn12 species with sulfur-cont aining ligands and thus represents a useful expansion of this family of SMMs. In addition, with the more basic MeCO2 groups blocking the equatorial sites, regioselective chemistry at the axial positions should now be feasible with a variety of anionic groups. Such reactions are currently under investigation. 2.4 Experimental 2.4.1 Syntheses All manipulations were performed under aerobic conditions us ing chemicals as received, unless otherwise stated. [Mn12O12(O2CMe)16(H2O)4] ]2MeCO2H4H2O (1) was prepared as described elsewhere.50 [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2). A solution of complex 1 (1.00 g, 0.49 mmol) in MeCN (75 cm3) was treated with PhSO3H (0.61 g, 3.9 mmol) in MeCN (25 cm3). The solution was stirred overnight and the solvent was removed. Toluene (25 cm3) was added to the residue, and the solution wa s evaporated to dryness. The addition and removal of toluene was repeated three mo re times. The residue was dissolved in CH2Cl2 (50 cm3) and filtered. An equal volume of hexanes was added to the filtrate, and the solution was allowed to stand at room temperature for 4 da ys. Dark brown crystals of 24CH2Cl2 were collected by filtration, washed with hexanes, and dried in vacuo ; yield

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47 ~96%. A sample for crystallography was mainta ined in contact with the mother liquor to prevent the loss of in terstitial solvent. Anal. Calcd (found) for 2C6H14 (C70H86Mn12O56S8): C, 30.69 (30.72); H, 3.16 (2.99); N, 0.00 (0.13). Selected IR data (cm-1): 1506 (s), 1446 (vs), 1245 (m), 1203 (m), 1127 (s), 1037 (m), 1018 (s), 996 (m), 981 (m), 757 (w), 732 (m), 690 (s), 671 (s), 607 (vs), 564 (s). [Mn12O12(O2CMe)12(O3SPh)4(H2O)4] (3). A solution of complex 1 (0.50 g, 0.24 mmol) in MeCN (75 cm3) was treated with PhSO3H (0.15 g, 0.97 mmol) in MeCN (25 cm3). The solution was stirred overnight, a nd the solvent was then removed. Toluene (25 cm3) was added to the residue, and the solution was evaporated to dryness. The addition and removal of toluene was repe ated three more times. The residue was dissolved in CH2Cl2 (25 cm3) and filtered. An equal volume of hexanes was added to the filtrate, and the solution allowed to stand at room temperature for ~ 4 days. Dark brown crystals of 3 were collected by filtration, washed with hexanes, and dried in vacuo ; yield ~81%. Anal. Calcd (found) for 3 (C48H64Mn12O52S4): C, 25.50 (25.73); H, 2.85 (3.24); N, 0.00 (0.02). Selected IR data (cm-1): 1576 (m), 1506 (m), 1447 (vs), 1330 (m), 1192 (s), 1128 (s), 1039 (m), 1019 (s), 997 (m), 761 (w), 731 (m), 673 (vs), 646 (s), 607 (vs). [Mn4O4(O2PPh2)6] (5). To a stirred solution of complex 24CH2Cl2 (0.50 g, 0.17 mmol) in CH2Cl2 (50 cm3) was added a solution of Ph2PO2H (0.29 g, 1.3 mmol) in CH2Cl2 (25 cm3). The resulting solution was stirred for 12h. The solvent was removed in vacuo Toluene (25 cm3) was added to the residue, and the solution was again evaporated to dryness. The addition and removal of to luene was repeated two more times and the residue was redissolved in CH2Cl2 (40 cm3) and filtered through Celite. Diffusion of toluene into the CH2Cl2 solution slowly produced dark brown hexagonal crystals, and

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48 these were suitable for X-ray crystallography if maintained in contact with the mother liquor to prevent the loss of interstitial solvent. After three weeks, the crystals were isolated by filtration, washed with toluen e, and dried under vacuum; yield ~51%. The dried crystals are hygroscopi c. Anal. Calcd (found) for 52H2O (C72H64P6Mn4O18): C, 53.29 (53.05); H, 3.97 (3.71); N, 0.00 (0.01). Selected IR data (KBr, cm-1): 1481 (s), 1312 (w), 1260 (w), 1184 (m), 1132 (vs), 1106 (s), 1067 (m), 1032 (vs), 986 (vs), 974 (vs), 859 (w), 755 (vs), 732 (vs), 694 (vs), 630 (m), 562 (s), 542 (vs), 524 (s), 513 (s), 485 (s). 2.4.2 X-ray Crystallography Data were collected using a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). A suitable crystal of 24CH2Cl2 was attached to a glass fiber using silicone grease and transferred to the goniostat where it was cooled to -80 C for characterization and data collection. The structure was so lved by direct methods (SHELXTL)64 and standard Fourier techniques. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions ans re fined with the use of a riding model. Cell parameters were refined usi ng up to 8192 reflections. A full s phere of data (1381 frames) was collected using the -scan method (0.3 frame width). Th e first 50 frames were re-measured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1%). Absorptio n corrections by integration were applied based on measured indexed crystal faces. A preliminary search of reciprocal space for 24CH2Cl2 revealed a set of reflections with no symmetry and no systematic absences An initial choice of the centrosymmetric

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49 space group 1 P was confirmed by the subsequent solution and refinement of the structure. The asymmetric unit contains the Mn12 cluster and four CH2Cl2 molecules of crystallization. Two of the dichloromethane solvent molecules were disordered. The Cl atoms in one CH2Cl2 molecule were disordered over three positions and their site occupancy factors refined to 85:17:8%. The Cl atoms in the remaining disordered CH2Cl2 molecule were disordered over two (main) positions and the occupancies refined to 56:44%. A total of 1406 parameters were refine d in the final cycle of refinement using 16674 reflections with I > 2 (I) to yield R 1 and wR 2 of 3.79% and 9.90%, respectively. The final difference Fourier map was essentiall y featureless, the largest peak being 0.813 e -3 and the deepest hole being -0.784 e -3. Data were collected on a Bruker D8 plat form goniometer equipped with a SMART APEX CCD area detector and a grap hite monochromator utilizing MoK radiation ( = 0.71073 ). A suitable single crystal of 5 was mounted on a glass fiber using silicone grease and transf erred to the diffractometer where it was cooled to -123 C for characterization and data co llection. The intensity data were collected using the -scan methods with a scan step = 0.03. The first 50 frames were re-measured at the end of data collection to monitor instrument and crystal stability (maximum correction was < 1%). The data were corrected for Lorent z and polarization effects using the Bruker SAINT software and an absorption corre ction was performed using the SADABS program supplied by Bruker AXS (Tmin/Tmax = 0.804). The structure was solved by direct met hods and standard Fourier techniques, and was refined on F2 using full-matrix leastsquares method (SHELXTL).64 All non-hydrogen atoms were refined anisotropi cally. Hydrogen atoms were placed in

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50 calculated positions and refined with th e use of a riding model. A total of 1175 parameters were refined in the fi nal cycle of refinement using 21864 [Rint = 0.0935] independent reflections with I > 2 (I) to yield R 1 and wR 2 of 3.70 and 10.14%, respectively. The final difference Fourier map was reasonably clean, max/min residual electron density are +0.99/-1.029 e -3. The Flack parameter is 0.05(2), i.e., the found structure is an absolute structure of 5.

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51 CHAPTER 3 NOVEL MIXED-VALENCE MnIII/MnIV CLUSTERS FROM THE USE OF BENZENESELENINATE LIGANDS: [Mn7O8(O2CMe)(O2SePh)8(H2O)] AND [Mn7O8(O2SePh)9(H2O)] 3.1 Introduction There are many motivations for the preparation of new polynuclear Mn clusters, not least of which is the structural beauty th at such complexes often display. However, a more practical and major objective is the se arch for new examples of molecules with significant values of ground state spin, S. Indeed, Mn chemistry has proven a fertile source of such species. In cases where S is fairly large and there is also a significant magnetoanisotropy of the easy-axis (or Ising) ty pe, as reflected in a negative value of the zero-field splitting (ZFS) parameter D, then such molecules will have a significant energy barrier to relaxation of the magnetization vect or and will thus function as single-molecule magnets (SMMs). The upper limit of the energy barrier is given by S2|D| or (S2-)|D| for integer and half-integer S va lues, respectively. Experimental evidence for SMMs is provided by the observation of frequency-dependent, out-o f-phase AC susceptibility signals ( M ), and by hysteresis loops in magnetizati on vs DC field scans, both properties characteristic of a superparamagnet-like particle.21,23 Additionally, several SMMs display step-like features in the hys teresis loops, a consequence of quantum tunneling of the magnetization (QTM).27,61,65 The two preparative strategies that have typically been employed previously for the preparation of new SMMs (and new Mnx clusters in general) ar e the following: (i) ligand substitution of some or all of the peripheral lig ands in preformed compounds with

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52 retention of the core structure;13,38,46 and (ii) structural tr ansformation of a given Mnx core to a new structural type by r eaction with some suitably chos en chelate or other reagent.11 Of the SMMs known to date, the [Mn12O12(O2CR)16(H2O)4] family is the most thoroughly studied, exhibiting SMM behavi or at the highest temperatures.13,21-23,38,46 A number of Mn12 derivatives have been prepared th rough the use of lig and substitution reactions on the parent Mn12 complex (R = Me), making possible the tunability of solubility and redox proper ties of the clusters.13,38,46 Mn12 compounds are also good starting materials for the preparation of othe r high nuclearity Mn complexes. They have been used in this capacity to prepare a num ber of high nuclearity pr oducts, and some of these have also proven to be new additions to the SMM family, including the largest Mn carboxylate cluster obtained to date, [Mn84O72(O2CMe)78(OMe)24(MeOH)12(H2O)42(OH)6].66 As part of this general characteriza tion of the reactivit y properties of Mn12 complexes, we have recently been studyi ng the replacement of some or all of the carboxylate ligands with non-carboxylate ones. This has included the replacement of the carboxylate ligands in a site-specific manner, enhancing reactivity at selected sites and making site-specific reactions feasible. Publis hed progress along these lines includes the site-selective replacement of some of the carboxylate groups with nitrate, diphenylphosphinate, and benzenesulf onate anions (by reactions of [Mn12O12(O2CMe)16(H2O)4] with the corresponding c onjugate acids) to give [Mn12O12(O2CCH2But)12(NO3)4(H2O)4],32 [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4],33 and [Mn12O12(O2CMe)8(O3SPh)8(H2O)4],39 respectively. In add ition, other groups have reported the replacement of four carboxylate groups with diphenylphosphates to give

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53 [Mn12O12(O2CPh)12(O2P(OPh)2)4(H2O)4].47 In all of these cases, the [Mn12O12]16+ core is retained intact, although occasio nally slightly distorted comp ared with the all-carboxylate parent compound. The present report is an extension of these studies into a new di rection, the reaction of [Mn12O12(O2CMe)16(H2O)4] with benzeneseleninic acid (PhSeO2H). The latter is significantly different from both Ph2PO2 and PhSO3 in containing only a threecoordinate central atom; however, it is also unlike carboxylate and ni trate groups in that the Se possesses a stereoactive lone pair. In fact, we have found that the PhSeO2H causes rupture of the [Mn12O12] core and gives products of an unprecedented structural type containing a [MnIII 3MnIV 4O8]9+ core. Herein we report the sy ntheses, single crystal X-ray structures, and magnetic propert ies of two related examples of this new type of Mn7 complex. 3.2 Results and Discussion 3.2.1 Syntheses Our initial attempts to introduce PhSeO2 groups into high-nuclearity Mn aggregates involved the reaction of benzenesel eninic acid with several trinuclear and tetranuclear complexes containing the [Mn3(3-O)] and [Mn4(3-O)2] cores, respectively. These were known from previous work to yield magnetically interesting complexes upon reaction with a chelating ligand.52,62,67-69 However, despite many attempts, such reactions were ineffective, and spectroscopic charac terization of the products indicated that no reaction between the Mn complexes and PhSeO2H had occurred. This is likely due to a combination of effects, including the poor sol ubility of the ligand in the MeCN solvent and the essentially identical acid dissociation constants of PhSeO2H (pKa = 4.79) and

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54 MeCO2H (pKa = 4.76);70 a large difference in pKa values facilitates substitution of MeCO2 ligands. We thus turned our attention to reactions of the acid with [Mn12O12(O2CMe)16(H2O)4], thinking that this might allow at least some PhSeO2 groups to be incorporated around the [Mn12O12] core, as had been found with PhSO3 -. Thus, complex 3 in distilled MeCN was treated w ith eighteen equivalents of PhSeO2H, which slowly dissolved. After 12 hours, the resu lting deep brown solution was separated by filtration from some brown powder, and fr om the filtrate was obtained a dark brown crystalline product. It was immediately obvi ous from the infrared spectrum that the reaction had led to transformation of the [Mn12O12] core of 1, and the product was subsequently identified by X -ray crystallography as [Mn7O8(O2CMe)(O2SePh)8(H2O)] (6), obtained in ~35% yield. When the same reaction system was ma intained for longer reaction times, the amount of the brown precipitat e steadily increased. After 48 hours, the precipitate was collected by filtration and recrystallized from CH2Cl2/Et2O to give crystals of [Mn7O8(O2SePh)9(H2O)] (7) in 40% yield. The overal l transformations to give 6 and 7 are summarized in eq 3-1, which has been formulated for product 6; that for 7 would be very similar. [Mn12O12(O2CMe)16(H2O)4] + 8 PhSeO2H [Mn7O8(O2CMe)(O2SePh)8(H2O)] + 7 H2O + 5Mn3+ + 15 MeCO2 (3-1) Charge considerations and inspection of metric parameters indicate 6 and 7 to be mixed-valence 3MnIII, 4MnIV, with a trapped-valence situation ( vide infra ). Clearly, the overall conversion of [Mn12O12(O2CMe)16(H2O)4] (8MnIII, 4MnIV) into these products

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55 must involve a complicated mechanism involvi ng fragmentation and recombination steps. There are almost certainly other species left in the colored filt rates after the product crystals are collected, and these are likely MnIIIacetate species, but we have not explored these further. Since complexes 6 and 7 are so similar, differing only in the identity of one ligand, acetate vs benzeneseleninate, we we re fortunate that they have significantly different solubilities in MeCN or their isolation in pure form would not have been possible. Complex 6 is fairly soluble in MeCN whereas 7 is not. Thus, at shorter reaction times we were able to obtain 6 from the filtered reaction solution, whereas at longer reaction times substitution of its remaining acetate by another PhSeO2 group converts it to 7, which precipitates from the solution. 3.2.2 Description of Structures 3.2.2.1 X-ray crystal structure of [Mn7O8(O2CMe)(O2SePh)8(H2O)] (6) A labeled ORTEP40 plot in PovRay format of complex 6 is shown in Figure 3-1, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 3-1, and selected interato mic distances and angles are listed in Table A-3. The complex crystallizes in the triclinic space group 1 P with the Mn7 molecule in a general position. The structure consists of a [MnIII 3MnIV 4(3-O)5(-O)3]9+ core (Figure 3-2a), with the peripheral ligati on provided by eight bridging PhSeO2 ligands, one bridging MeCO2 ligand, and one terminal H2O molecule. Bond valence sum (BVS) calculations41 indicate a mixed-valence, trappedvalence complex; Mn(3), Mn(4), and Mn(5) are MnIII, while the remaining Mn centers Mn(1), Mn(2), Mn(6), and Mn(7) are MnIV (Table 3-2 and 3-3). Each MnIV ion is in a distorted oc tahedral environment. The two outer MnIII ions, Mn(3) and Mn(5), are five-c oordinate with square pyramidal geometry ( = 0.06 and 0.07, respectively, where is 0 and 1 for ideal square pyramidal

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56 and trigonal bipyramidal geometries, respectively71), while the remaining MnIII ion, Mn(4), has a very distor ted octahedral geometry. Se8 O8 O7 O23 O24 O22 O20 Se6 Se7 Mn7 Mn6 O19 O21 O16 O14 O6 O4 Mn4 Se1 O11 O12 O2 O1 Mn1 Mn2 O18 O17 Se4 Se5 O10 O9 O15 O13 O5 O3 O25 Mn3 Mn5 O27 O26 Se2 Se3 Figure 3-1. ORTEP representa tion in PovRay format of [Mn7O8(O2CMe)(O2SePh)8(H2O)] (6) with thermal ellipsoids at the 50% probability level except for the C atoms, together with a stereopair. For clarity, the hydrogen atoms have been omitted, and only the ipso C atoms of the phenyl groups are shown. Mn bl ue; O red; Se yellow; C gray. At first glance, the viewpoints of Figures 31 and 3-2a suggest that a useful way of describing the core of 6 is as a Mn6 ring with a seventh Mn in the middle. However, this is not a good description, because the Mn7 unit is very far from planar. This is emphasized in the side-view of Figure 3-2b. As can be seen, a much better dissection of the core is as a central [MnO2MnO2Mn]+ unit (i.e., 3MnIII) to whose bridging oxide ions on each side is attached an [MnO2Mn]4+ unit (2MnIV). The three MnIII ions in the central

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57 unit are ligated on one side by the bridging acetate and terminal water groups (O(25) – O(27)), and these MnIII ions would thus all be fi ve-coordinate ex cept that one [MnO2Mn]4+ unit swivels about its bridging oxide atoms O(4) and O(6) to bring O(7) within bonding distance of Mn(4), making the latter six-coordi nate (Mn(4)-O(7) = 2.278 ). Table 3-1. Crystallographic data for [Mn7O8(O2CMe)(O2SePh)8(H2O)]6MeCN and [Mn7O8(O2SePh)9(H2O)]2CH2Cl2. Parameter 66MeCN 72CH2Cl2 formulaa C62H63Se8N6Mn7O27 C56H51Cl4Se9Mn7O27 fw, g mol-1 2340.45 2393.02 space group 1 P P 21/ m a 10.7432(8) 10.7776(7) b 15.1584(11) 27.7484(8) c 24.3955(18) 13.4463(9) deg 99.639(2) 90 deg 91.197(2) 105.670(2) deg 105.776(2) 90 V 3 3759.9(5) 3871.8(4) Z 2 2 T C -100(2) -100(2) radiation, b 0.71073 0.71073 calc, g cm-3 2.067 2.053 cm-1 50.94 55.48 R 1 ( wR 2), %c d 5.91 (14.96) 8.73 (24.55) a Including solvent molecules. b Graphite monochromator. c R 1 = || Fo| – | Fc|| / | Fo|. d wR 2 = [ [ w ( Fo 2 Fc 2)2] / [ w Fo 2 )2]]1/2 where S = [[ w ( Fo 2 – Fc 2)2] / ( n p )]1/2, w = 1/[ 2( Fo 2) + ( mp )2 + np ], p = [max( Fo 2, 0) + 2 Fc 2]/3, and m and n are constants. Discrete examples of the [Mn(-O)2Mn]4+ core in dinuclear complexes are fairly common.72 But note that only recently, in the [Mn8O10(O2CMe)6(H2O)2(bpy)6]4+ cation,73 has the [MnIV 2(-O)(3-O)]4+ variant of these units, as found in 6, been previously observed. Note also that the [MnO2Mn]4+ units can alternatively be described as [MnO2(O2SePh)Mn]4+ units since there is also a PhSeO2 bridging them. Thus, they are also very similar to the common dinuclear complexes containing the triply bridged

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58 [Mn2(-O)2(-O2CR)]2+,3+ core.74,75 In contrast, the central [MnO2MnO2Mn]+ fragment, containing a linear Mn3 unit, has never been seen before in a discrete Mn3 complex, although it is a commonly encountered sub-fragme nt of several higher nuclearity clusters, such as certain Mn10,76 Mn11 77 and Mn18 62 species. Overall, the complete Mn7 complex possesses C1 symmetry. O2 O1 Mn1Mn2 O3O5 Mn4 Mn3 Mn5 O4 O6 O7 Mn6 Mn7 O8(a) (b)O27 O26 O25 Mn5 Mn3 O7 O8 Mn6 Mn7 O4 O6 O3 O5 Mn1 Mn2 O1 O2 Mn4 Figure 3-2. ORTEP representations in PovRay format of (a) the [Mn7O8]9+ core of complex 6 and (b) the relative disposition of the elongation axes, indicated as solid black bonds. The central MnIII ion, Mn(4), displays a Jahn-Te ller (JT) axial elongation, as expected for a high-spin MnIII (d4) ion. Normally, JT elongation axes avoid Mn-oxide bonds, almost always the strongest and shortest in the molecule but in this case the oxide O(7) cannot in any case appr oach too closely and the re sulting Mn(4)-O(7) distance (2.278(6) ) is very long for a Mn-oxide bond, even a JT elongated one. The other JT elongated bond is Mn(4)-O(27) (2.180(7) ). For the two square pyramidal MnIII ions, Mn(3) and Mn(5), their local z axis is oriented parallel to the JT axis of Mn(4). Thus, the long, apical bonds of the sp geometries (Mn(3)-O(25) = 2.289(8 ) and Mn(5)-O(27) = 2.172(8) ) are parallel to th e long bonds at Mn(4). In effect, then, there is parallel alignment of the three MnIII distortion axes, which will dominate the magnetic anisotropy

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59 (i.e., the magnitude of the ZFS parameter, D) of the complete Mn7 molecule. This will be of relevance to the magnetic discussion later. Table 3-2. Bond valence suma calculations for complexes 66MeCN and 72CH2Cl2. 66MeCN 72CH2Cl2 Atom Mn2+ Mn3+ Mn4+ Mn2+ Mn3+ Mn4+ Mn(1) 4.165 3.810 3.999 4.136 3.783 3.972 Mn(2) 4.049 3.704 3.888 3.584 3.278 3.442 Mn(3) 3.108 2.842 2.984 4.141 3.787 3.976 Mn(4) 3.185 2.914 3.059 3.137 2.869 3.012 Mn(5) 3.158 2.888 3.032 Mn(6) 4.134 3.781 3.969 Mn(7) 4.182 3.825 4.016 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value. Table 3-3. Bond valence sum calculationsa for selected oxygen atoms in complexes 66MeCN and 72CH2Cl2. 66MeCN 72CH2Cl2 Atom Vi Assignment Atom Vi Assignment O(1) 1.850 O2O(11) 1.761 O2O(2) 1.647 O2O(12) 2.049 O2O(3) 2.024 O2O(13) 1.994 O2O(4) 1.976 O2O(14) 1.876 O2O(5) 2.091 O2O(15) 1.617 O2O(6) 1.977 O2O(16) 1.911 O2O(7) 1.856 O2O2O(8) 1.726 O2O2a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0. The Se geometry is pyramidal, owing to the presence of a stereochemically active lone pair of electrons. The only previous example of the PhSeO2 group bridging in the manner seen in 6 was the polymer [Ph3SnO2SePh]n in which the PhSeO2 ligand symmetrically bridges two Sn atoms.78 There is evidence for st rong intermolecular SeO interactions in 6 involving Se and oxide O atom s, forming chains along the a axis of the crystal (Figure 3-3). The Se(1) O(8) distance (2.749(6) ) is longer than the sum of the

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60 covalent radii of Se and O (1.89 ), but much shorter than the sum of their van der Waals radii (3.40 ). This interaction likely involves O(p) to Se(d) donation into empty Se d-orbitals. Additionally, the Se(6)-O(11) contact (3.327(6) ), and perhaps also Se(7)-O(12) (3.643(6) ), likely contribut e to the intermolecu lar interaction. Se1 O8 O12 Se7 O11 Se6 Figure 3-3. ORTEP representati on in PovRay format at the 50% probability level of the packing of complex 6 along the a axis of the crystal. For clarity, the hydrogen atoms have been omitted, and only the ipso C atoms of the phenyl groups are shown. There is precedent for these types of interactions; there are several examples in the literature of intramolecular SeO nonbonded interactions, including those in selenoiminoquinones,79 selenoxocine,80 and selenazofurin.81 However, none of these previous examples involve the same kind of seleninate ligan ds as present in 6. There are also numerous weak inter-chain contacts appare nt in packing diagrams between aromatic rings and/or solvent molecules, but the di sorder in these groups discussed in the Experimental Section complicates their clear visualization. 3.2.2.2 X-ray crystal structure of [Mn7O8(O2SePh)9(H2O)] (7) A labeled ORTEP40 plot in PovRay format of complex 7 is shown in Figure 3-4, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 3-1, and selected interato mic distances and angles are listed in Table

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61 A-4. The complex crystallizes in the monoclinic space group P 21/ m with the Mn7 molecule lying on a mirror plane. For the sake of brevity, references to specific atoms in the following discussion implicitly include their symmetry-related partners. The structure of 7 is very similar to that of 6, except that the MeCO2 group in the latter has been replaced with a ninth PhSeO2 group. Se5 O11 O16 O10 O10a O9a O9 Se4a Se4 Mn3 Mn3a O8a O8 O7a O7 O13a O13 Mn4 O1 O14 O15 Se1 O3 O3a O4a O4 Se2a O5a Se2 O5 Mn2a Mn2 O6 O6a O12a O12 O2a O2 Mn1a Mn1 Se6 Se3 Se3a Figure 3-4. ORTEP representati on in PovRay format of [Mn7O8(O2SePh)9(H2O)] (7) with thermal ellipsoids at the 50% probab ility level except for the C atoms, together with a stereopair. For clarity, the hydrogen atoms have been omitted, and only the ipso C atoms of the phenyl groups are shown. Mn blue; O red; Se yellow; C gray. Bond valence sum calculations41 again indicate a 3MnIII, 4MnIV trapped-valence situation (Table 3-3). The core ha s the same structure as that for 6 in Figure 3-2a, with

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62 five-coordinate Mn(2) be ing square pyramidal ( = 0.08). The PhSeO2 for MeCO2 substitution causes almost insi gnificant perturbation of th e core. The long Mn(4)-O(16) bond (2.276(9) ) is very similar to that in 6, and the three MnIII distortion axes are again essentially parallel. As observed with complex 6, there is a strong intermolecular interaction between Se atoms and oxide O atoms, forming chains along the a axis of the crystal, with weak in ter-chain contacts. The shortest, st rongest contact is Se(1)O(11) (2.754(6) ). Complexes 6 and 7 possess a structure that is quit e different from that of any previously characterized Mn7 complex. These include [MnII 3MnIII 4(OMe)12(dbm)6] (dbmis the anion of dibenzoylmethane),82 the [MnII 4MnIII 3(OH)3(hmp)9Cl3]3+ cation (hmpis the anion of 2-(hydroxylmethyl)pyridine),76,83 and the [MnII 4MnIII 3(teaH)3(tea)3]2+ cation (teaH3 = triethanolamine).84 Table 3-4 shows a compar ison of selected Mn-O bond distances and angles for the complexes 6 and 7. This comparison confirms that the bond distances in the [Mn7O8]9+ cores are almost superimposable. Table 3-4. Comparison of selected bond distances () and angles ( ) for [Mn7O8(O2CMe)(O2SePh)8(H2O)] (6) and [Mn7O8(O2SePh)9(H2O)] (7). Parametera 6 7 MnIV – O 1.788(6) – 1.852(5) 1.800(6) – 1.845(7) MnIII – O 1.880(6) – 1.944(6) 1.873(8) – 1.963(7) MnIV – Ocarb 1.909(7) – 2.002(6) 1.923(7) – 2.010(8) MnIII – Ocarb 1.909(7) – 1.950(6) 1.903(10) – 1.935(8) MnIII – Oelong 2.172(8) – 2.180(7) 2.001(17) – 2.160(12) MnIII Ow 2.289(8) 2.001(17) MnIVMnIV 2.722(2) – 2.758(2) 2.740(3) – 2.761(3) MnIIIMnIII 2.829(2) – 2.856(2) 2.8429(19) O – MnIV – O 82.6(2) – 83.7(3) 81.5(3) – 83.1(3) MnIIIMnIIIMnIII 174.10(8) 174.45(12) a O = bridging oxide ion, Ocarb = bridging PhSeO2 or MeCO2 carboxylate, Oelong = bridging bridging PhSeO2 or MeCO2 carboxylate situated along an MnIII elongation axis, Ow = water

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63 3.2.3 Magnetochemistry of Complexes 6 and 7 3.2.3.1 DC studies Variable-temperature DC susceptibility measurements were performed in the 5.0300 K range on powdered microcrystalline samples of 62H2OMeCN and 72H2O, restrained in eicosane to prevent torquing, in a 5 kG field (Figure 3-5). For complex 6, the MT value of 12.0 cm3 K mol-1 at 300 K decreases gradually with decreasing temperature to 3.4 cm3 K mol-1 at 5.0 K. For complex 7, the MT value of 12.8 cm3 K mol-1at 300 K decreases gr adually to 4.1 cm3 K mol-1 at 5.0 K. The spin-only ( g = 2) value for a molecule compos ed of non-interacting MnIII 3MnIV 4 ions is 16.5 cm3 K mol-1. Hence, the molecules appear to have a ppreciable intramolecular antiferromagnetic interactions. Temperature (K) 050100150200250300 2 4 6 8 10 12 14 MT (cm3K mol-1) Figure 3-5. Plot of MT vs temperature for dried, micr ocrystalline samples of complex 62H2OMeCN () and 72H2O ( ) in eicosane. M is the DC molar magnetic susceptibility measured in a 5.0 kG field. Each complex contains three MnIII and four MnIV centers, with total spin (S) values therefore ranging from 0 to 12. The low symme try and size of the molecules makes very difficult a matrix diagonaliza tion approach, and completely precludes application of the Kambe equivalent operator method51 to determine the various Mn2 pairwise exchange

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64 interaction constants (J) in the molecule Efforts were instead concentrated on determining the ground state S value of the complexes. Thus, magnetization data were collected in the 0.10-70 kG and 1.8-10.0 K field and temperature ranges (Figure 3-6). H/T (kGK-1) 010203040 0 1 2 3 4 0.1 T 0.5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T M/NB Figure 3-6. Plot of re duced magnetization (M/NB) vs H/ T for a dried microcrystalline sample of 62H2OMeCN; the DC field value of each of the isofield plots is indicated. The saturation value in the highest fields is approaching the value of ~4 expected for an S = 2 spin. However, it was not possi ble to obtain a reasonabl e fit for these data. This is likely due to the intermolecula r exchange interactions mediated by the intermolecular SeO contacts that were detect ed in the crystal structures and which are not incorporated in the fitting model. In addi tion, there may be lowlying excited states that are populated even at very low temper atures. The latter is unfortunately a common problem in higher nuclearity clusters, due to a high density of spin states and/or the presence of spin frustration effects. Spin frus tration, in its general sense, is the presence of competing antiferromagnetic exchange inte ractions of comparable magnitude, which can often prevent (frustrate) the antiparallel alignment of all spins. Such spin frustration effects have been previo usly described in detail.85,86 The ground state spin value and the energies of low-lying excited states become sensitive to the precise magnitude of the

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65 competing exchange interacti ons. The triangular arrangement of coupled metal ions, as found in 6 and 7, is the textbook topology for spin frustration if the couplings are antiferromagnetic, since the spin s cannot all be antiparallel to all their neighbors. It is anticipated, based on the properties of dinuc lear systems, that all the oxide-bridged MnIVMnIV, MnIVMnIII and MnIIIMnIII pairwise exchange interactions in 6 and 7 will possibly be antiferromagnetic, and thus the pres ence of spin frustration in this core is expected. The strongest exchange interaction within the molecule is very likely a strong antiferromagnetic coupl ing within the MnIV 2 units. This MnIV 2 interaction in several dinuclear complexes with the [MnIV 2(-O)2( -O2CR)] core is in the range from -37 to -67 cm-1, depending strongly on the MnIV-O-MnIV angle,72a,75 and it is likely that the MnIV 2 interactions in 6 and 7 are of similar magnitude. This is also the magnitude expected for the various MnIIIMnIV interactions.74 The lowest temperature data in Figur e 3-5 and the saturation value of M/NB in Figure 3-6 suggest a small ground st ate spin value of S ~ 2 for 62H2OMeCN and 72H2O, but this is not the safest way to de termine the ground state spins because of potential complications from Zeeman effects from the applied DC field. A better way is to measure the AC susceptibility, which does no t involve use of a DC field. Such studies were carried out with a 3.5 G AC field oscill ating at frequencies up to 1488 Hz. In fact, the AC in-phase susceptibilities ( M T ) for the two complexes are essentially superimposable, and turn out to be very similar to the DC susceptibilities ( MT ). The AC M T and DC MT for 6 are compared in Figure 3-7.

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66 Temperature (K) 024681012 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 MT and MT (cm3K mol-1) Figure 3-7. Plot of M T vs temperature for complex 62H2OMeCN in the 2.0-10.0 K range from AC magnetic susceptibility measurements (), and including the DC MT data ( ) from Figure 3-5 for this te mperature range. The AC data were measured with a 3.5 G AC field oscillating at 997 Hz. The strongly sloping M T vs T plot is strongly indicativ e of the population of low-lying excited states, since occupati on of only the ground state would give an essentially temperature-independe nt value. Extrapolation sugges ts the plot is heading to a MT value of ~3 cm3 K mol-1, the value of an S = 2 state with g = 2, before a noticeable downturn at the lowest temperatur es due to relaxation effects ( vide infra ). We conclude that the ground state spin of 6 and 7 is S = 2, but that there are several low-lying excited states that are significantly populated even be low 10 K. Note that th e extrapolation from higher temperatures also avoids complicati ons from weak intermolecular interactions. 3.2.3.2 AC studies With such a small ground state, it seemed unlikely that complexes 6 and 7, even with significant magnetic anisotropy as a result of their three parallel MnIII elongation axes, would possess a significantly large barr ier to magnetization relaxation to be SMMs. As a result, we did not expect to see an out-of-phase AC susceptibility signal ( M ), an indicator of slow magnetizati on relaxation. However, with AC oscillation frequencies up

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67 to 997 Hz (Figure 3-8), a frequency-dependent M signal was indeed observed for complex 6, or rather the tail of a M signal whose peak lies at a temperature significantly below 1.8 K. It is very im portant to note that similar M signals were seen whether we used crystals kept wet with mother liquor (i.e., 66MeCN) or those that had been filtered and dried (i.e., 62H2OMeCN), showing that for this compound the drying and consequent change in solvation conten t has no significant effect on the magnetic properties. Since we did not believe 6 could be a SMM, we instead suspected that this signal might be due to a spin-chain behavior resulting from the intermolecular exchange interactions mediated by the short SeO cont acts observed in the crystal structure. We thus decided to investigate the magnetic properties at lower temperatures. Temperature (K) 246810 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 1000 Hz 250 Hz 50 Hz 2.5 3.0 3.5 4.0 M(cm3mol-1) MT(cm3K mol-1) Figure 3-8. In-phase and out-of-phase ( M ) AC susceptibility signals for a dried, microcrystalline sample of 62H2OMeCN at the indicated AC oscillation frequencies.

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68 3.2.3.3 Hysteresis studies below 1.8 K Single-molecule magnets (SMMs) or single-chain magnets (SCMs)87-91 below their blocking temperature, TB, will exhibit hysteresis in their magnetization vs DC field response, the classical property of a magnet. Such studies were therefore performed on a single crystal of 66MeCN using a micro-SQUID apparatus;92 crystals were kept wet with mother liquor to prevent any damage from lo ss of solvent. Measurements were performed in the 0.04 to 7.0 K temperature range, usi ng different field sweep rates from 0.008 to 0.56 T/s. The sensitivity and time resolution of a micro-SQUID magnetometer allows the study of very small single crystals in good c ontact with a thermal bath. The temperature dependence at a fixed sweep rate of 0.07 T/s is shown in Figure 39a. Hysteresis loops become evident in the scans at 4 K, but they only have a small coercivity. The latter increases, but only slightly, with decreasing temperature down to 0.5 K, and then is constant down to 0.04 K. In Figure 3-9b is shown the sweep rate dependence of the loops at a constant temperature of 0.04 K. A sma ll decrease in coercivi ty is observed with decreasing sweep rate, but again the change is only slight. This behavior is not that expected for a SMM, for which one would expect a greater dependence of the coercivity on the temperature and on the sweep rate. Instea d we believe that the hysteresis behavior is the consequence of the one-d imensional chain structure of 6 and 7 in the solid state. Thus, complex 6 (and by implication 7, given its very similar st ructure) appears to be a new example of the small but growing fa mily of single-chain magnets (SCMs).93 This implies that the intermolecular interactions mediated by the SeO contacts are ferromagnetic in nature, leading to a parallel alignment of spins of the interacting Mn7 units along the chain, a significant barrier (versus thermal ener gy) to magnetization reversal, and the resu lting hysteresis loops.

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69 -1 -0.5 0 0.5 1 -1.2-0.8-0.400.40.81.2 0.04 K 0.5 K 1 K 2 K 4 K 7 KM/Ms 0.07 T/s0H (T) -1 -0.5 0 0.5 1 -1.2-0.8-0.400.40.81.2 0.560 T/s 0.140 T/s 0.035 T/s 0.002 T/s 0.04 K0H (T)M/Ms(a) (b) Figure 3-9. Magnetization vs DC field plots for a si ngle crystal of complex 66MeCN at (a) the indicated temperatures and a fixe d field sweep rate of 0.07 T/s; and (b) four sweep rates and a fixed temperat ure of 0.04 K. The magnetization is normalized to its maximum value, Ms. However, closer inspection of the loop s in Figure 3-9 indicates that this ferromagnetic coupling between molecules w ithin the chains is not sufficient to completely account for the observed hysteresi s behavior. Following saturation of the magnetization in one direction wi th a large field, the revers al of the direction of the magnetization should occur after the fi eld has been swept past zero for a ferromagnetically-coupled chain. However, this is clearly not the case in Figure 3-9; the reversal of the magnetization direction begi ns before zero field. This feature is characteristic of the presence of antiferroma gnetic interactions, and we assign these as being inter-chain interactions mediated by th e weak contacts observed in the crystal structure. Similar behavior has been observed in some SMM systems, including [CeMn8O8(O2CMe)12(H2O)4]94 and the Fe19 SMMs.95 Since the hysteresis loops are suggest ive of antiferroma gnetic inter-chain interactions, we sought additional evidence for their presence, and this was obtained from the Curie-Weiss plot of the lowest temperature DC susceptibility data, shown in

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70 Figure 3-10. The Weiss consta nt is obtained from the x -axis intercept, and this is -2.0 K, confirming the weakest interaction in the system to be antiferromagnetic. This is assigned to the inter-chain interaction. Temperature (K) -2024681012 0.0 0.5 1.0 1.5 2.0 2.5 1/ M(cm-3mol) Figure 3-10. Curie-Weiss plot for 62H2OMeCN showing a negative (antiferromagnetic) intercept assigned to inter-chain interactions. To characterize the system further, we collected magnetization decay data to determine the barrier to magnetization relaxation. The magnetization of the sample was saturated in one direction with a large a pplied field at 5 K. The temperature was decreased to a chosen value in the 0.04-1.0 K range, the ap plied field was removed, and the magnetization of the sample was monitore d with time. The resulting data are shown in Figure 3-11a. The decay data at each temp erature were analyzed to give a set of relaxation time ( ) vs temperature data, which were used to construct the Arrhenius plot of Figure 3-11b, based on the Arrhen ius relationship of eq 3-2, where 0 is the pre-exponential factor, Ueff is the mean effective barrier to relaxation, and k is the Boltzmann constant. = 0 exp(Ueff/k T ) (3-2)

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71 The fit of the thermally-activ ated region above ~0.5 K, s hown as the dashed line in Figure 3-11b, gave Ueff = 9.87 cm-1 = 14.2 K and 1/ 0 = 1.9 x 10-9 s-1. Below this temperature, the relaxation time levels off a nd becomes temperature-independent at ~104 s. This temperature-independence of th e relaxation rate is the characteristic signature of quantum tunneling of the ma gnetization through the anisotropy barrier. 10-310-210-11001011021031040510152025 1/T (K-1) (s) 0.02 0.04 0.06 0.08 0.1 0.12 0.11101001000M/Mst (s) 0.15 K 0.10 K 0.04 K 0.2 K 0.25 K 0.3 K 0.4 K 0.5 K 0.6 K 0.7 K 0.8 K 0.9 K 1 K (a) (b) Figure 3-11. Relaxation time vs temperat ure studies for a single crystal of 66MeCN. (a) Magnetization vs time decay plots at the indicated temperatures. (b) Arrhenius plot using the resulting relaxation time ( ) vs T data. The dashed line is a fit of the thermally-activated region to the A rrhenius equation. See the text for the fit values. 3.2.3.4 Origin of the relaxation barrier The picture that emerges from the above magnetic analysis of complex 6 is that there is a significant barrier to magnetization relaxation of 14.2 K in this compound. As a result, at low enough temperatures the compound exhibits the ch aracteristic behavior of a magnet, namely magnetization hysteresis. This cannot be rationalized on the basis of single-molecule magnetism, because the spin of the molecule is only S = 2 and this is unlikely to give a significant barrier to ma gnetization relaxation. The upper limit (U) of the latter for an integer spin system is given by U = S2|D|, where D is the axial anisotropy

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72 (zero-field splitting) parameter. Even assuming a value of D of -1.0 cm-1, which is actually larger (more negative) than typical for Mnx clusters, would still only give a barrier U of 4.0 cm-1 (5.8 K). When one then takes into account that the actual (or effective) barrier (Ueff) is usually considerably less th an U, because the magnetization can tunnel through the barrier via higher-lying ms levels rather than going over the top, then it becomes impossible to convincingly rati onalize the observed effective barrier (Ueff) of 9.87 cm-1 = 14.2 K on the basis of the proper ties of the individual molecules of 6. Instead, as we mentioned earlier, we interp ret the observed behavior as a result of the one-dimensional chains formed between Mn7 molecules, giving a single-chain magnet (SCM). The latter is a relatively new phenomenon in molecular magnetism, but it is nevertheless now well es tablished with several well-documented examples.87-91 In such a system, the magnetization relaxation barrier and the resulting slow relaxation rates are caused by one-dimensional intermolecular exchange interactions between the constituent spin carriers. Such chains may be either homometallic90 or heterometallic,88,91 and can even comprise an alternating me tal/organic radical arrangement.89 According to Glauber theory fo r one-dimensional Ising chains,93 the magnetization relaxation rate follows an Arrhenius la w (eq 3-3) with a barrier given by 8JS2 (for the -2J i j convention), where S is the spin of th e repeating unit and J is the exchange parameter between these units. = 0 exp(8JS2/k T ) (3-3) If the repeating unit also possesses intrinsic anisotr opy and thus a magnetization relaxation barrier of U = S2|D|, then there will be two cont ributions to the total relaxation barrier, that from the anisot ropy of the molecular unit and that from the interaction

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73 between these units. This was clearly de scribed in a recent paper by Miyasaka et al on a series of related SCMs,88 and the corresponding Arrhenius re lationship for such a system can then be expressed by eq 3-4, where = (8J + |D|)S2 is the barrier to magnetization relaxation, or rather its upper li mit in the absence of tunneling. = 0 exp( /k T ) (3-4) The results described in the present work can be rationalized within the framework of the above model. The Mn7 cluster has a small but nevertheless significant spin of S = 2, and in the presence of some easy-axis (or Ising) type anisot ropy, as expected for axially-elongated MnIII, there will be a small barrier to magnetization relaxation. This is insufficient to provide a singl e-molecule magnet, but the interactions between the Mn7 units (Figure 3-3) provide an additional contri bution to the barrier. As a result, and even allowing for the diminution of the barrier by t unneling effects, there is still a la rge enough barrier (Ueff) of 14.2 K to yield out-of-phase AC susceptibility signals and hysteresis. The relaxation barrier for complex 6 of 14.2 K is unfortunately rather small compared with other SCMs in the lite rature, which fall in the 50 – 154 K range.87-91 In addition, there are noticeable inter-chain in teractions, so the best description of 6 is as an SCM with weak inter-chain coupling, in the same way that SMMs with weak intermolecular interactions are also known.94,95 The important thing is that in these SMMs, and also complex 6, the intermolecular or inter-chain interactions are weak enough to be considered a perturbation of the SMM or SCM behavior, respectively, rather than strong enough to give a true th ree-dimensional ordered solid. Clearly the frequency-dependent AC data and the sweep ra te and temperature-dependent coercivities in the hysteresis loops rule out 6 as a 3-dimensional magnet. Thus, regardless of its low

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74 anisotropy barrier, it is appropriate to call 6 a SCM. It would also be the first example to consist of a chain of polynuclear metal clus ters rather than m ononuclear or dinuclear repeating units, and it is also the first to e xhibit the temperature-i ndependent relaxation regime in the Arrhenius plot characteristic of quantum tunne ling. It thus represents a significant new addition to the SCM family. 3.3 Conclusions The use of benzeneseleninic acid (PhSeO2H) in attempted ligand substitution reactions with [Mn12O12(O2CMe)16(H2O)4] (1) instead causes a structural change and has afforded two new heptanuc lear Mn clusters, [Mn7O8(O2CMe)(O2SePh)8(H2O)] (6) and [Mn7O8(O2SePh)9(H2O)] (7), which possess a novel [MnIII 3MnIV 4(3-O)5(-O)3]9+ core and represent the first examples of transition metal clusters ligated by PhSeO2 groups. Magnetic studies suggest a low ground state spin value of S = 2 and the appearance in the AC susceptibility of out-of-phase signals char acteristic of slow ma gnetization relaxation. Studies down to 0.04 K reveal that these sp ecies are not new additions to the growing family of single-molecule magnets (SMMs). Instead, the slow relaxation is caused by single-chain magnetism behavior, with the rela xation barrier arising from a combination of the molecular anisotropy and the excha nge interaction betw een the individual Mn7 molecules. These complexes are thus intere sting for a number of reasons, including aesthetically pleasing structur es and their average oxidati on state of +3.6, which is unusually high for a high nuclearity Mn cluster. It will be intere sting to see in future work whether the PhSeO2 allows access to other high oxidation state species. Finally, it is important to remember for the future that the observation of the M signals in Figure 2-8 for an ostensibly molecular compound did not presage the observation of the characteristic properties of a SMM. This represents an important

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75 reminder that it is by not safe to take the appearance of M signals as sufficient proof that a SMM has been prepared. 3.4 Experimental 3.4.1 Syntheses All manipulations were performed under aerobic conditions us ing materials as received, except where otherwise noted. [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) was prepared as described elsewhere.50 [Mn7O8(O2CMe)(O2SePh)8(H2O)] (6). To a stirred solution of complex 1 (0.50 g, 0.24 mmol) in MeCN (75 cm3) was added solid PhSeO2H (0.83 g, 4.4 mmol) in portions, and the mixture was stirred for 12h. The d eep brown solution containing some brown powder was filtered through Celite. The volume of the filtrate was reduced by half by roto-evaporation, and it was then allowed to evaporate slowly in air. Crystals formed slowly over one week, and these were suitable for X-ray studies if maintained in contact with the mother liquor to pr event the loss of in terstitial solvent. After one week, the crystals were isolated by filtration, washed with small volumes of MeCN, and dried under vacuum; yield ~35%. The dried crys tals are hygroscopic, and analyze for 62H2OMeCN. Anal. Calcd (found) for C51H50.5Se8N0.5O29Mn7: C, 28.25 (28.26); H, 2.44 (2.37); N, 0.32 (0.29). Selected IR data (KBr, cm-1): 1636 (w), 1542 (m), 1474 (m), 1441 (m), 1419 (m), 1172 (w), 1094 (m), 1063 (m), 1021 (w), 997 (w), 747 (vs), 712 (vs), 686 (vs), 623 (s), 599 (s), 528 (s), 439 (m). [Mn7O8(O2SePh)9(H2O)] (7). A solution of complex 1 (0.50 g, 0.24 mmol) in MeCN (75 cm3) was treated with solid PhSeO2H (0.83 g, 4.4 mmol) in portions. The reaction mixture was stirred for 48h, during wh ich time the amount of a brown precipitate continuously increased. The latter was co llected by filtration, re-dissolved in CH2Cl2

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76 (30 cm3) and filtered through Celite. Vapor diffusion of Et2O into the CH2Cl2 solution slowly produced crystals, and these were suit able for X-ray crystallo graphy if maintained in contact with the mother liquor to prevent th e loss of interstitial so lvent. After 2 weeks, crystals were isolated by filtration, washed with Et2O, and dried under vacuum; yield 40%. The dried material is hygroscopic, analyzing for 72H2O. Anal. Calcd (found) for C54H51Se9O29Mn7: C, 28.71 (28.65); H, 2.28 (2.19); N, 0.00 (0.02). Selected IR data (KBr, cm-1): 3054 (w), 1650 (w), 1635 (w), 1576 (w), 1540 (w), 1475 (w), 1441 (m), 1418 (w), 1336 (w), 1176 (w), 1065 (m), 1021 (w), 998 (w), 744 (vs), 709 (vs), 687 (vs), 622 (s), 599 (s), 530 (s). 3.4.2 X-ray Crystallography Data were collected using a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Suitable single crystals of 66MeCN and 72CH2Cl2 were attached to glass fibers using silicone grease and transf erred to the goniostat where they were cooled to -100 C for characterization and data co llection. Each structure wa s solved by direct methods (SHELXTL)64 and standard Fourier techniques, a nd was refined using full-matrix leastsquares methods. All non-hydrogen atoms were refined anisotropi cally. Hydrogen atoms were placed in calculated positions and refined with the use of a riding model. Cell parameters were refined usi ng up to 8192 reflections. For each complex, a full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were re-measured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1%). Absorption corrections by integration were applied based on measured indexed crystal faces.

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77 A preliminary search of reciprocal space for 66MeCN revealed a set of reflections with no symmetry and no systematic absences An initial choice of the centrosymmetric space group 1 P was subsequently confirmed by the su ccessful solution of the structure. The asymmetric unit contains the Mn7 molecule and six disordered MeCN molecules. The solvent molecules could not be mode led properly, and the program SQUEEZE,96 a part of the PLATON97 package of crystallographic soft ware, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. The phenyl rings in two of the PhSeO2 ligands [C(7)-C(12) and C(43)-C(48)] were disordered. Their site occupancy factors were dependently refined to 85:15% and 70:30%, respectively. Atoms of the minor disorder position in each case were refined with isotropic thermal parameters. Both Se(4) and Se(5) were diso rdered about two (main) positions and the occupancies refined to 76:24% and 51:49% respectively; the phe nyl rings in these ligands were not involved in the disorder Additionally, the acet ate ligand bridging Mn(4)-Mn(5) was disordered about two positi ons, where the minor disorder position was bridging Mn(3) and Mn(4). This disorder wa s very minor and coul d not be modeled; a peak of 1.68 e -3 appears where the acetate central atom [C(49)] should be, but the corresponding methyl peak [C( 50)] could not be found and pr operly refined. A total of 830 parameters were refined in the final cycl e of refinement using 33358 reflections with I > 2 (I) to yield R 1 and wR 2 of 5.91% and 14.96%, respectively. The final difference Fourier map was essentially featureless, the largest peak being 1.68 e -3 and the deepest hole being -0.78 e -3. For complex 72CH2Cl2, an initial survey of a portion of reciprocal space located a set of reflections with a monoclin ic lattice. Analysis of the fu ll data set revealed that the

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78 space group was P 21/ m The asymmetric unit co ntains half of the Mn7 molecule and one disordered CH2Cl2 molecule. The solvent molecule was disordered and could not be modeled properly, so the program SQUEEZE96 was used to calculate the solvent disorder area and remove its contributi on to the overall intensity data The phenyl rings in two of the PhSeO2 ligands [C(1)-C(6) and C(51)-C(56)] were disordered about the crystallographic mirror plane that bisects the molecule. Their site occupancy factors were fixed at 50%. Additionally, th e phenyl rings in two PhSeO2 ligands not centered over the mirror plane [C(31)-C(36) and C(41)-C(46)] we re disordered over two sites. Their site occupancy factors were depende ntly refined to 53:47% and 50:50%, respectively; atoms of minor disorder positions we re refined with isotropic thermal parameters. Se(2) and its phenyl ring were disordered about two positions and the occupancies dependently refined to 67:33% and 32:68%, respectively. Finally, Se(6) and its phenyl ring [C(61)-C(66)] bridge Mn(2) and Mn(4) and, because of the mirror symmetry, Mn(4) and Mn(2a). The disorder is with a water molecule. The site occupancy factors were refined to 50%. As a precaution, the structure was also solved and refined in space group P 21, the mirror symmetry removed, but the amount of disorder was not changed compared with the structure in space group P 21/ m A total of 328 parameters were refined in the final cycle of refinement using 29004 reflections with I > 2 (I) to yield R 1 and wR 2 of 8.73% and 24.55%, respectively. The final difference Four ier map was essentiall y featureless, with the largest peak and deepest hole being 1.704 and -1.540 e -3, respectively.

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79 CHAPTER 4 AN INVESTIGATION OF TH E REACTIVITY OF Mn12 COMPLEXES WITH DIMETHYLARSINIC ACID: NEW Mn4 CUBANE COMPLEXES AND Mn16 SINGLE-MOLECULE MAGNETS 4.1 Introduction The search for new polynuclear 3d transition metal complexes has intensified over recent years largely as a result of the s uperparamagnetic-like properties exhibited by a number of such clusters.19,22,23,27 The magnetic behavior results from a combination of high ground state spin (S) and la rge, negative magnetic anisot ropy as gauged by the axial zero-field splitting parame ter (D) of a molecule.12,21-23 Together, these two parameters give rise to an energy barrier (U) for magne tization reversal, the magnitude of which is given by S2|D| and (S2-)|D| for integer and non-inte ger spin systems, respectively.19 The first complex to display such superparamagne tic-like behavior as evidenced by hysteresis loops obtained from magnetiza tion vs DC field studies was [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1).27 Subsequent studies proved the origin of the magnetic properties to be intrin sic to the molecule, i.e., molecules of 1 are discrete magnetically non-interacting units, and such co mplexes have been consequently termed single-molecule magnets (SMMs).25 Much research has been devoted to the ar ea of single-molecule magnetism, with the principal objective being th e preparation of a SMM that behaves as a magnet at technologically relevant temperatures, i.e., at least 77 K (that of liquid N2). The goal has been approached primarily by two means: (i) the preparation of novel 3d metal carboxylate clusters possessing di ffering topologies that may be have as SMMs and (ii)

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80 the addition of new derivatives to already existing families of SMMs so that structural features may be correlated with magnetic properties and ultimately, a rational synthetic method for improved SMMs may be developed. Progress toward this end has resulted in the isolation of a number of SMMs most of which are Mn clusters.13,32,33,38,39,46,47,56-59,6163,66,94,98-114 Several complexes comprising V,115 Co,116 Ni117 and Fe95,118 also behave as SMMs however. By far, single-molecule magne ts composed of Mn centers are the most abundant, and this likely results from the spin associated with the accessible oxidation states of Mn and from the magnetic anisot ropy that arises from the presence of JahnTeller elongation axes of MnIII ions. Some of the most well-studied families of singlemolecule magnets include Mn12, [Mn12]and [Mn12]2complexes.13,38,46,61,100,113 An important avenue of approach invol ves the study of the reactivity of SMMs with various ligands, chelati ng, non-carboxylate and otherwis e. With this strategy, a knowledge base of the reactivity properties of various SMMs has been developed and new polynuclear clusters that behave as hi gh-spin molecules and/or SMMs have been accessed. Reported progress along these lines in cludes the characterization of clusters obtained from the reactions of Mn12 complexes with non-carboxylate ligands.32,33,39,47,58,101 Such studies were first reported fo r nitric acid, wher e reactions with [Mn12O12(O2CCH2But)16(H2O)4] with four equivalents of HNO3 yielded [Mn12O12(O2CCH2But)12(NO3)4(H2O)4].32 Similarly, reactions of diphenylphosphinic acid (Ph2PO2H) with [Mn12O12(O2CR)16(H2O)4] (R = Me (1) Ph (8)) gave [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4]33 and [Mn12O12(O2CPh)7(O2PPh2)9(H2O)4].58 In each case, the [Mn12O12]16+ core was retained with the s ubstitution causing only a slight

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81 distortion. In further work, we and other gr oups have extended these studies to include reactions with (PhO)2PO2H,47 PhSO3H,39 MeSO3H,101 and more recently, PhSeO2H.119 As part of our continuing interest in the reactivity of non-car boxylate ligands with [Mn12O12(O2CMe)16(H2O)4], we decided to shift the fo cus of our investigation to a previously unexplored reactant, namely dimethylarsinic acid (Me2AsO2H). The latter is very similar to Ph2PO2H, which was already shown to yield new Mn12 derivatives upon reaction with either 1 or 8,33,58 and a survey of the literatu re shows that oftentimes, the Me2AsO2 analogue of a complex with ligation by Ph2PO2 can be prepared.120 However, we were still interesting in exploring this strategy as a potential r oute to new topologies in Mn chemistry and possibly to new singl e-molecule magnets. We herein report the syntheses, single crystal X-ray structures and magnetic properties of two [Mn4O4] cubane clusters obtained from reactions of Me2AsO2H with [Mn12O12(O2CMe)16(H2O)4] and of two related Mn16 complexes isolated from reactions of Me2AsO2H with [Mn12O12(O2CPh)16(H2O)4] (8). We show that these Mn16 complexes are new additions to the family of SMMs. 4.2 Results and Discussion 4.2.1 Syntheses As already discussed, various non-carboxyl ate ligands have previously been found to be useful reactants fo r the synthesis of new Mn12 derivatives. Reactions of 1 with either four or eight equivalents of Ph2PO2H, (PhO)2PO2H, PhSO3H, MeSO3H, and HNO3 have afforded a handful of new Mn12 SMMs32,33,39,47,58,101 while the use of PhSeO2H transformed the core of 1 into a new structural type in Mn chemistry that was found to behave as a single-chain magnet (SCM).119 Thus, the reaction of 1 with Me2AsO2H was of particular interest. A solution of 1 in MeCN was treated with 18 equivalents of

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82 Me2AsO2H, which immediately dissolved, changi ng the solution color from dark brown to deep red. After 12 h, several cycles of addition of toluene and its removal under vacuum were completed to facilitate the removal of MeCO2H from the reaction solution; the acid dissociation constant of Me2AsO2H (pKa = 6.27)121 is very high relative to MeCO2H (pKa= 4.76), i.e., Me2AsO2H is much less acidic than MeCO2H, and hence this was thought to be an essential step to ensure ligand substitution. Extraction of the soluble component of the residue into CH2Cl2 and diffusion of pentane into the resulting solution leads to the formation of a dark red crysta lline product. It was immediately obvious from the infrared spectrum of th e material that the [Mn12O12] core of 1 was not retained, and the product was subsequently identifi ed by X-ray crystallography as [Mn4O4(O2AsMe2)6] (9), obtained in ~75% yield. The transformation of 1 into 9 is summarized in eq 4-1. Previous findings have similarly shown that [Mn4O4(O2PPh2)6] can also be isolated from an attempted ligand substitution reaction of [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] with Ph2PO2H,39 suggesting that such reactions might be an effective route to obtain manganese-oxo complexes possessing a cubane structure. [Mn12O12(O2CMe)16(H2O)4] + 18 Me2AsO2H 3 [Mn4O4(O2AsMe2)6] + 16 MeCO2H + 4 H2O + 2 H+ + 2 e(4-1) Subsequent studies sh owed that complex 9 can be obtained from the same reaction without the repeated addition and removal of toluene, and hence, the substitution of MeCO2H need not be driven by the removal as its toluene azeotrope. In an attempt to obtain a structural analogue of 9 with ligation by more biol ogically releva nt carboxylate groups, reactions of 9 with MeCO2H, EtCO2H, Cl2CHCO2H and ButCH2CO2H were performed. However, despite many attempts, such reactions were ineffective, and

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83 spectroscopic characterization of the products indicated th at no reactions between 9 and the various carboxylic acids had occurred. Th is can likely be attributed to the strong basicity of the Me2AsO2 group coordinated to the Mn centers as reflected in the relatively high pKa value of its conjugate acid (pKa = 6.27). Similarly, as part of our continuing efforts to obtain a tetranuclear Mn cluster incorporating a Ca2+ ion, a topology that has potential relevance to the water-oxidizing comple x (WOC) found in photosystem II (PS II),122 we focused our attention on the reaction of 9 with various Ca2+ salts, including Ca(NO3)2, Ca(O2CMe)2 and Ca(ClO4)2. However, our efforts were again without success, with no reactions occurri ng. We thought that the addition of Ca2+ to the reaction in situ might be a more effective route towa rds the incorporation of at least one Ca2+ ion. Thus, a solution of 1 in a 30:1 mixture of MeCN and MeOH was treated with 18 equivalents of Me2AsO2H and 1.5 equivalents of Ca(NO3)2. MeOH was added to the reaction system to aid in the dissolution of Ca(NO3)2, which is not readily soluble in MeCN. The solution color changed from dark brown to deep red as observed with 9, and after 20 min, a small amount of brown powde r was separated from the reaction solution by filtration. To the filtrate was added 1,2-dichloroethane and after 2 weeks, dark red crystals of {[Mn4O4(O2AsMe2)6](NO3)}2 (10) were obtained in ~24% yield. The infrared spectrum of 10 is nearly superimposable with that of 9, except for a prominent signal that corresponds to a NO3 anion. Spectroscopic characteriza tion of precipitates isolated from reactions solutions left undisturbed for more than ~ 2 weeks suggest the formation of Mn oxides. The deep red color of the filtrate following colle ction of the pr oduct crystals suggests that there are almost certainly other species remainin g in the solution. We have

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84 not explored these further how ever. The transformation of 1 into 10 is summarized in eq 4-2. [Mn12O12(O2CMe)16(H2O)4] + 18 Me2AsO2H + 3 NO3 3 [Mn4O4(O2AsMe2)6](NO3) + 16 MeCO2H + 4 H2O + 2 H+ + 4 e(4-2) We extended this work to include reactions of Me2AsO2H with [Mn12O12(O2CPh)16(H2O)4] (8), thinking that the difference in solubility properties of the MeCO2 and PhCO2 ligands might lead to the isolat ion of a substantially different product; crystallization kinetics and relative solubili ty as well as numerous other factors are expected to play an important role in the determination of the identity of the isolated product. Hence, treatment of a solution of complex 8 in a 10:3 solvent mixture of MeCN and MeOH with 18 equivalents of Me2AsO2H and 1 equivalent of Ca(NO3)2 leads to the formation of a deep red solution after ~20 min. Some brown powder was removed from the reaction mixture by filt ration, and diffusion of Et2O into the resulting filtrate leads to the formation of crystalline [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] (11). Similarly, diffusion of Et2O into the filtrate obtained from the reaction of 8 with 18 equivalents of Me2AsO2H and 1 equivalent of Sr(ClO4)2 in a 10:1 solvent mixture of MeCN and MeOH yields the related complex [Mn16O8Sr4(O2CPh)16(Me2AsO2)24] (12). Although the final yield for both complexes is very low, 5% and 7%, respectively, the preparations are reproducible. The reaction system is unques tionably very compli cated, involving the conversion of [Mn12O12(O2CPh)16(H2O)4] (8) to both 11 and 12 via fragmentation and recombination steps. Th e poor solubility of 11 and 12 in MeCN is likely an important consideration for the rationaliz ation of the isolation of pur e crystalline pr oducts although other factors undoubtedly are also important for the preferrentia l crystallization of 11 and

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85 12. The transformation of 8 into 11 and 12 is summarized in eq 4-3 and eq 4-4, respectively. 4 [Mn12O12(O2CPh)16(H2O)4] + 84 MeAsO2H + 12 Ca(NO3)2 + 16 e3 [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] + 12 NO3 + 4 PhCO2 + 36 PhCO2H + 40 H2O (4-3) 4 [Mn12O12(O2CPh)16(H2O)4] + 72 MeAsO2H + 12 Sr2+ + 16 e3 [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] + 16 PhCO2H + 8 H+ + 40 H2O (4-4) MeOH is required for the isolation of the products and likely functions as a reducing agent, providing the needed electr ons to the reaction system; only precipitates were obtained from those reactions carried out in MeCN alone. Spectroscopic analysis of crystalline materials indicates that complexes 11 and 12 are also formed from reactions with EtOH instead of MeOH. The yield and qual ity of the crystals are strongly affected by the ratio of MeCN and MeOH solvents, w ith the optimum ratios herein reported. 4.2.2 Description of Structures 4.2.2.1 X-ray crystal structure of [Mn4O4(O2AsMe2)6] (9) A labeled ORTEP40 plot in PovRay format of complex 9 is shown in Figure 4-1, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 4-1, and selected interato mic distances and angles are listed in Table A-5. The complex crystallizes in the triclinic space group 1 P with the Mn4 molecule in a general position. The structure consists of a [MnIII 2MnIV 2(3-O)4]6+ core with the peripheral ligation provi ded by six bridging Me2AsO2 ligands.

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86 As3 O9 O10 O3 O2 Mn1 O5 O7 As1 O6 As5 O13 O11 As4 O12 O4 As6 O15 O8 As2 O16 Mn3 O1 Mn2 Mn4 O14 Figure 4-1. ORTEP representati on in PovRay format of [Mn4O4(O2AsMe2)6] (9) with thermal ellipsoids at the 50% probab ility level except for the C atoms, together with a stereopair. Hydrogen atoms have been omitted for clarity. Mn blue; O red; As pink; C gray. The rela tive dispositions of the JT elongation axes are indicated as solid black bonds. Assignment of the oxidation states of the metal centers was done qualitatively by charge consideration and also by comp arison of the bond distances around the Mn centers. These assignments were confirme d quantitatively by bond valence sum (BVS) calculations,41 indicating that Mn(1) and Mn(3) are MnIV while Mn(2) and Mn(4) are MnIII (Table 4-2). All of the Mn centers ar e six-coordinate, with near-octahedral geometry. A BVS calculation was also carried out for the inorganic O atoms, confirming that the the triply bridging O atoms are all deprotonated (Table 4-3). Both of the octahedral MnIII centers in 9 displays a JT ax ial elongation of two trans bonds as expected for a high-spin MnIII (d4) ion; the elongation axes ar e oriented along the

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87 O(2)-Mn(2)-O(11) and O(4)-M n(4)-O(10) bonds. The axial MnIII-O bonds lengths are in the range of 2.066(3) – 2.291(3) , and are significantly longer than equatorial MnIII-O bonds in the range of 1.897(3) – 1.974(3) . The MnIV ions, Mn(1) and Mn(3), exhibit shorter Mn-O bond lengths in the range of 1.838(3) – 1.959(3) and lack JT elongation axes, consistent with their assignment as MnIV. Normally, the JT elongation axes avoid Mn-oxide bonds, almost always the strongest and shortest in the molecule, but the cubane core of 9 requires that the JT elongation axes of Mn(2) and Mn(4) be situated in an abnormal position containing a core O2ion, O(2) and O(4), respectively. The JT elongation axes are oriented nearly parallel to each other and this alignment will dominate the magnetic anisotropy (i.e., the ma gnitude of the ZFS parameter, D) of the complete Mn4 molecule. This will be of relevance to the magnetic discussion later ( vide infra ). Overall, the complete Mn4 complex does not possess es any crystallographic symmetry element although the virtual symmetry is Td. There are a handful of othe r reported examples of Me2AsO2 groups symmetrically bridging Fe,123 Mo120,124 and W125 ion pairs in the manner seen in 9. However, to our knowledge complex 9 is the first example of the use of the Me2AsO2 ligand in Mn chemistry. Two other discrete examples of the [MnIII 2MnIV 2(3-O)4]6+ core as found in 9 have been previously observed in the valence-delocalized compound [Mn4O4(O2PPh2)6]48 (5) and the trapped-valence [Mn4O4(O2P(OPhp -Me)2)6]126 complex. Hence, complex 9 is only the second example of a mixe d-valence, trapped-valence [MnIII 2MnIV 2(3-O)4]6+ molecule. The only other complexes possessi ng manganese-oxo cubane cores include the trapped-valence [MnIIIMnIV 3(3-O)4(O2PPh2)6]127 and [MnIII 4(3-O)2(3OMe)2(HOMe)(O2P(OPhp -Me)2)6]126 molecules reported by Dismukes and others and

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88 the distorted cubane clusters of general formula [MnIII 3MnIV(3-O)3(3-X)]6+, where X = Cl-, F-, Br-, N3 -, NCO-, or RCO2 -, reported by Christou and coworkers.12,49,128 Table 4-1. Crystallographic data for [Mn4O4(O2AsMe2)6]5H2OC5H12 and {[Mn4O4(O2AsMe2)6](NO3)}2MeCN12H2O. Parameter 95H2OC5H12 10MeCN12H2O formulaa C17H58As6Mn4O21 C25H97.5As12N2.5Mn8O50 fw, g mol-1 1267.91 2572.08 space group 1 P 1 P a 11.7278(7) 14.0401(7) b 13.7179(8) 14.3205(7) c 14.0559(9) 21.1805(2) deg 99.8690(10) 103.945(2) deg 90.4520(10) 99.819(2) deg 111.7540(10) 94.437(2) V 3 2062.9(2) 4041.2(3) Z 2 2 T C -55(2) -100(2) radiation, b 0.71073 0.71073 calc, g cm-3 2.041 2.114 cm-1 60.41 61.78 R 1 ( wR 2), %c d 3.70 (10.14) 5.15 (12.40) a Including solvent molecules. b Graphite monochromator. c R 1 = || Fo| – | Fc|| / | Fo|. d wR 2 = [ [ w ( Fo 2 Fc 2)2] / [ w Fo 2 )2]]1/2 where S = [[ w ( Fo 2 – Fc 2)2] / ( n p )]1/2, w = 1/[ 2( Fo 2) + ( mp )2 + np ], p = [max( Fo 2, 0) + 2 Fc 2]/3, and m and n are constants. Table 4-2. Bond valence suma calculations for complexes 95H2OC5H12 and 10MeCN12H2O. 95H2OC5H12 10MeCN12H2O Atom Mn2+ Mn3+ Mn4+ Mn2+ Mn3+ Mn4+ Mn(1) 4.172 3.816 4.006 3.886 3.554 3.731 Mn(2) 3.476 3.179 3.338 3.665 3.353 3.520 Mn(3) 3.853 3.524 3.700 3.841 3.513 3.688 Mn(4) 3.184 2.913 3.058 4.098 3.748 3.935 Mn(5) 4.136 3.783 3.971 Mn(6) 4.124 3.772 3.960 Mn(7) 3.228 2.952 3.100 Mn(8) 4.177 3.821 4.011 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value.

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89 Table 4-3. Bond valence sum calculationsa for selected oxygen atoms in complexes 95H2OC5H12 and 10MeCN12H2O. 95H2OC5H12 10MeCN12H2O Atom Vi Assignment Atom Vi Assignment O(1) 1.927 O2O(13) 1.909 O2O(2) 1.822 O2O(14) 1.804 O2O(3) 1.898 O2O(15) 1.904 O2O(4) 1.754 O2O(16) 1.913 O2O(29) 1.888 O2O(30) 1.921 O2O(31) 1.996 O2O(32) 1.799 O2a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0. 4.2.2.2 X-ray crystal structure of {[Mn4O4(O2AsMe2)6](NO3)}2 (10) A labeled ORTEP40 plot in PovRay format of complex 10 is shown in Figure 4-2, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 4-1, and selected interato mic distances and angles are listed in Table A-6. Complex 10MeCN12H2O crystallizes in the triclinic space group 1 P with two independent Mn4 anions in general positions. The structure of 10 is very similar to that of 9, except that the oxidation levels of the Mn ions in the latter (MnIII 2MnIV 2) have been changed; 10 contains two [MnIIIMnIV 3(3-O)4]7+ molecules each with the peripheral ligation provided by six bridging Me2AsO2 ligands. Two nitrate cations serve as counterions. Bond valence sum (BVS) calculations41 indicate a mixed-valence, trappedvalence complex; Mn(2) and Mn(7) are MnIII, while the remaining Mn centers Mn(1), Mn(3), Mn(4), Mn(5), Mn(6) and Mn(8) are MnIV (Table 4-2). Similarly, a BVS calculation carried out to asse ss the protonation levels of th e inorganic O atoms confirms that the triply bridging cubane O atoms are deprotonated (Table 4-3).

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90 As1 O1 O5 O3 O2 As2 O4 O7 As4 O8 As3 O6 As6 O12 O11 O16 O10 As5 O14 Mn1 Mn4 O13 Mn3 O9 Mn2 O15 As8 O20 O19 Mn7 O30 Mn5 O17 As7 O18 O26 As11 O25 Mn6 O21 As9 O31 O27 As12 O28 O24 As10 O32 O22 O29 O23 Mn8 Figure 4-2. ORTEP representation in PovRay format of the cation of {[Mn4O4(O2AsMe2)6](NO3)}2 (10) with thermal ellipsoids at the 50% probability level except for the C atoms, together with a stereopair. Hydrogen atoms have been omitted for clarity. Mn blue; O red; As pink; C gray. The relative dispositions of the JT elongati on axes are indicated as solid black bonds. All of the Mn centers are six-coordinate with near-octahedral geometry. Each MnIII ion exhibits an axia l elongation of two trans bonds as is typical of the Jahn-Teller distortion that is exp ected of a high-spin MnIII (d4) ion. The elongation axes are oriented along the O(7)-Mn(2)-O(14) and O(20)-Mn(7)-O(32) bonds. For each MnIII ion, the average axial bond length [2.004 for Mn(2 ) and 2.165 for Mn(7)] is ~0.1-0.2

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91 longer than the average equa torial bond length [1.923 for Mn(2) and 1.936 for Mn(7)]. The JT elongation axes of Mn(2) and Mn (7) are oriented near ly perpendicular to each other and this point will be of particular importance to the magnetic properties of 10 ( vide infra ) because the orientation of the MnIII JT axes relative to one another is the main factor that determines the overall magnetic anisotropy of the molecules. In contrast, Mn(1), Mn(3), Mn(4), Mn(5), Mn(6) a nd Mn(8) exhibit shor ter average Mn-O bond lengths [1.912 ] and lack Jahn-Teller elongation axes, consistent with MnIV ions. Table 4-4 shows a comparison of average Mn-O bond distances and angles for the two [Mn4O4(O2AsMe2)6]z [ z = 0 (9) and +1 (10)] complexes. This comparison confirms that the bond distances in the [Mn4(3-O)4] cores are almost superimposable. Table 4-4. Comparison of selected bond distances () and angles ( ) for complexes [Mn4O4(O2AsMe2)6] (9) and {[Mn4O4(O2AsMe2)6](NO3)}2 (10). Parametera 9 10 MnIV – Oc (ax) 1.899(3) – 1.959(3) 1.883(5) – 1.924(5) MnIV – Oc (eq) 1.838(3) – 1.950(3) 1.865(5) – 2.004(6) MnIII – Oc (ax) 2.139(3) – 2.291(3) 2.029(6) – 2.242(5) MnIII – Oc (eq) 1.897(3) – 1.974(3) 1.907(5) – 1.985(4) MnIV – Oeq 1.931(3) – 1.958(3) 1.864(4) – 1.971(5) MnIV – Oax 1.906(3) – 1.951(3) 1.885(5) – 1.917(5) MnIII – Oeq 1.909(3) – 1.938(3) 1.892(5) – 1.925(5) MnIII – Oax 2.066(3) – 2.095(3) 1.978(5) – 2.087(5) MnIVMnIV 2.8857(8) 2.8323(14) – 2.9281(15) MnIIIMnIII 3.0546(8) –––––––––– MnIVMnIII 2.8638(8) – 3.0171(8) 2.9102(15) – 3.0079(14) Oc – MnIV – Oc 80.53(11) – 88.76(12) 80.2(2) – 881.(2) Oc – MnIII – Oc 76.77(10) – 85.46(11) 76.39(18) – 82.0(2) a Oc = cubane O2-, Oax = axial carboxylate, Oeq = equatorial carboxylate. Comparison of the OO bite distances of the Me2AsO2 ligands in complexes 9 and 10 and the Ph2PO2 ligands in 5 with the corresponding distances of the MeCO2 and PhSO3 ligands in 1 and 2, respectively, shows that dimethylarsinate and diphenylphosphinate groups may be preferre d for formation of the cubane complex

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92 (Table 3-5). The OO distances for the Me2AsO2 -, Ph2PO2 and PhSeO2 ligands bridging two Mn ions are considerably longer th an the corresponding distances for the MeCO2 and PhSO3 ligands and these long distances may be required to accommodate the geometry of the cubane topology. Table 4-5. Comparison of selected bite distances () for MeCO2 -, PhSO3 -, Ph2PO2 -, PhSeO2 and Me2AsO2 ligands in complexes 1, 2, 4, 5, 6, 7, 9 and 10. Acid Compound [Mnx] Bite Distancea pKa 36,70,121,129 MeCO2H 1 [Mn12] 2.235 – 2.249 4.76 PhSO3H 2 [Mn12] 2.400 – 2.422 2.55 Ph2PO2H 4 [Mn12] 2.561 – 2.565 2.32 5 [Mn4] 2.566 – 2.617 PhSeO2H 6 [Mn7] 2.654 – 2.799 4.79 7 [Mn7] 2.640 – 2.836 MeAsO2H 9 [Mn4] 2.791 – 2.828 6.27 10 [Mn4] 2.749 – 2.847 a Bite distance refers to the OO distance between O atoms of the ligand coordinated to Mn ions in each selected cluster. 4.2.2.3 X-ray crystal structure of [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] (11) A labeled ORTEP40 plot in PovRay format of complex 11 is shown in Figure 4-3. The crystallographic data and structure refineme nt details are collected in Table 4-6, and selected interatomic distances and angles are listed in Table A-7. The complex crystallizes in the or thorhombic space group I 222, with the Mn16 molecule lying on a 222 symmetry site and hence, possessing D2 crystallographic symmetry with three perpendicular C2 crystallographic rotation axes bisec ting the molecule (Figure 4-4). For the sake of brevity, references to specific atoms in the following discussion implicitly include their symmetry-related pa rtners. The structure of the Mn16 molecule consists of four [MnIII 4(-O)2] “butterfly” units [atoms Mn(1), Mn(2), Mn(3), Mn(4), O(7) and O(11) and their symmetry equivalents] with the peri pheral ligation to each subunit provided by six bridging Me2AsO2 ligands and one bridging PhCO2 ligand.

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93 Mn4a Mn4 Mn2a Mn3a Mn1a Mn1 Mn2 Mn3 Mn4c Mn4b Mn1c Mn1b Mn2c Mn3c Mn3b Mn2b Ca1a Ca1 O11a O7a O7 O11 O7c O11c O11b O7b Ca1b Ca1c Figure 4-3. ORTEP representa tion in PovRay format of [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] (11). For clarity, the hydrogen atoms have been omitted and only the ipso C atoms of the phenyl groups are shown. Mn blue; O red; As pink; Ca yellow; N light blue; C gray. The asymmetric unit of 11 is shown in Figure 4-5. The dihedral angle between the Mn(1)-Mn(2)-Mn(3) and Mn(2)-Mn(3)-Mn(4) planes is 72.4. The two 3-oxide atoms O(7) and O(11) lie 0.31 and 0.78 below their respective Mn3 planes. On one side of the [Mn4O2] “butterfly” subunit is another [Mn4O2] subunit that is bridged and connected to Mn ions and Me2AsO2 groups of the former by a [Ca2(O2CPh)2(O2AsMe2)(NO3)2] moiety. On the other side of the [Mn4O2] “butterfly” subunit is another [Mn4O2] subunit attached to the former by a single Me2AsO2 ligand [As(8), O(14)] th at is coordinated to Mn(4).

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94 C2C2 Figure 4-4. ORTEP representation in PovRay format of complex 11 showing the crystallographic D2 symmetry. Another C2 rotation axis (not shown) passes through the center of the molecule. For clarity, the hydrogen atoms have been omitted and only the ipso C atoms of th e phenyl groups are shown. Mn blue; O red; As pink; Ca yellow; N light blue; C gray. Table 4-6. Crystallographic data for [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4]32MeCN and [Mn16O8Sr4(O2CPh)16(O2AsMe2)24]16MeCN. Parameter 1132MeCN 1216MeCN formulaa C160H280N28Ca4As28Mn16O92 C192H272N16Sr4As24Mn16O88 fw, g mol-1 7365.52 7239.89 space group I 222 P 21/ c a 17.1626(16) 20.862(3) b 23.758(2) 36.594(5) c 31.136(3) 18.644(2) deg 90 90 deg 90 101.370(2) deg 90 90 V 3 12696(2) 13953(3) Z 8 2 T C -100(2) -100(2) radiation, b 0.71073 0.71073 calc, g cm-3 2.066 1.725 cm-1 50.80 43.47 R 1 ( wR 2), %c d 8.76 (22.86) 8.72 (22.12) a Including solvent molecules. b Graphite monochromator. c R 1 = || Fo| – | Fc|| / | Fo|. d wR 2 = [ [ w ( Fo 2 Fc 2)2] / [ w Fo 2 )2]]1/2 where S = [[ w ( Fo 2 – Fc 2)2] / ( n p )]1/2, w = 1/[ 2( Fo 2) + ( mp )2 + np ], p = [max( Fo 2, 0) + 2 Fc 2]/3, and m and n are constants.

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95 Mn1 As8 O1 O2 O3 O4 As3 O5 O9 O10 O7 O8 As3 O6 O11 O12 As4 O13 O14 O15 O16 O17 As7 As5 Mn2 Mn3 Mn4 Ca1 O81 O92 N1 O91 O82 O18 O19 As6 As1 Figure 4-5. ORTEP representation in PovR ay format of the asymmetric unit of [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] (11). For clarity, the hydrogen atoms have been omitted and only the ipso C atoms of the phenyl groups are shown. Mn blue; O red; As pink; Ca yellow; N light blue; C gray. The Mn oxidation states were determined by qualitative inspection of the relative Mn-O bond distances, as well as quantitative calculati on of bond valence sum (BVS)41 (Table 4-7), indicating a 16 MnIII trapped-valence situation. The protonation levels of the inorganic O atoms were also established by bond valence sum calculatio ns and the results are collected in Table 4-8. All of the Mn at oms are six-coordinate with near-octahedral geometry and each exhibits a Jahn-Teller (JT) elongation by ~0.1-0.2 of two trans bonds as expected for a high-spin d4 ion. The JT elongation axes of Mn(2) and Mn(3) along the O(9)-Mn(2)-O(91) and O(10)-Mn(3 )-O(92) bonds are aligned parallel with respect to each other and are easily discerna ble; the axial JT elonga ted bond distances are in the range of 2.135(16) – 2.31(2) wh ile the non-JT elongated equatorial bond distances are only in the range of 1.838(13) – 1.999(14) . The JT elongation axes of Mn(1) and Mn(4) are not clearly defined, however. For Mn(1), th e axial bond distances, Mn(1)-O(3) = 2.117(14) and Mn(1)-O(5) = 2.06(2 ) , are very similar to one pair of equatorial trans bonds, Mn(1)-O(1) = 2.016(16) a nd Mn(1)-O(6) = 2.14(2) . The

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96 remaining equatorial bonds, Mn(1)-O(2) = 1.949(8) and Mn(1)-O(7) = 1.858(7) , exhibit distances typical for MnIII-Oeq and MnIII-O2bonds, respectively. Similarly for Mn(4), all four equatorial bonds, [Mn(4)-O(9) = 2.15(2) , Mn(4)-O(15) = 1.990(17) , Mn(4)-O(10) = 2.184(18) and Mn(4)-O(16) = 2.041(17) ] are elongated with respect the axial ones [Mn(4)-O(11) = 1.915(8) a nd Mn(4)-O(14) = 1.888(9) ]. The abnormal distances around Mn(1) and Mn(4) suggest a stat ic disorder of a JT elongation axis about two bonds, O(3)-Mn(1)-O(5) and O(1)-Mn(1)-O (6) for Mn(1) and O(9)-Mn(4)-O(15) and O(10)-Mn(4)-O(16) for Mn(4). Ca(1) is eight -coordinate with bond distances in the range of 2.344(16) – 2.66(2) . Table 4-7. Bond valence suma calculations for complexes 1132MeCN and 1216MeCN. 1132MeCN 1216MeCN Atom Mn2+ Mn3+ Mn4+ Mn2+ Mn3+ Mn4+ Mn(1) 3.093 2.829 2.970 3.120 2.854 2.996 Mn(2) 3.062 2.801 2.940 3.207 2.934 3.080 Mn(3) 3.384 3.095 3.250 3.243 2.967 3.115 Mn(4) 3.078 2.815 2.956 3.178 2.907 3.051 Mn(5) 3.007 2.750 2.887 Mn(6) 3.212 2.938 3.085 Mn(7) 3.231 2.955 3.103 Mn(8) 3.037 2.777 2.916 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value. Table 4-8. Bond valence sum calculationsa for selected oxygen atoms in complexes 1132MeCN and 1216MeCN. 1132MeCN 1216MeCN Atom Vi Assignment Atom Vi Assignment O(7) 2.052 O2O(5) 2.054 O2O(11) 1.993 O2O(6) 2.052 O2O2O(33) 2.043 O2O2O(34) 2.029 O2a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0.

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97 As shown in Figure 4-6, the eight benzoa te ligands exhibit two types of binding modes: four bridge Mn ions in the common syn syn mode (I) and four are chelating Ca(1) in mode II. Additionally, there are four NO3 anions that are chelating Ca(1) and bridging to the nearest symmet ry-related Ca(1) ion in the syn anti syn bridging mode III. Of the 28 dimethylarsinate ligands eight are bridging Mn ions in the syn syn bridging mode (IV), eight are bridging three Mn ions in the syn syn anti 3 bridging mode (V), eight are bridging two Mn and one Ca ion in the syn syn anti 3 bridging mode (VI), two are bridging Mn ions in the anti anti bridging mode (VII), and the final two are bridging two Mn and two Ca ions in the 4 bridging mode (VIII). O C O Ph Ca Mn O As O Mn Me Me syn, syn, (IV) Mn O C O Mn Phsyn, syn, (I) O N O O CaCa(II)syn, anti, syn, (III)syn, syn, anti, 3(V) Mn O As O Mn Me Me Mn Mn O As O Mn Me Me Ca syn, syn, anti, 3(VI) O As O Me Me MnMn anti, anti, (VII) Ca O As O Ca Me Me MnMn 4(VIII) Figure 4-6. Schematic representation of th e eight chelating and/ or bridging modes found in complex 11. Complex 11 is only the second st ructurally characteri zed molecular species containing both Mn and Ca ions, th e first being the recently reported [Mn13Ca2O10(OH)2(OMe)2(O2CPh)18(H2O)4] complex.130

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98 4.2.2.4 X-ray crystal structure of [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] (12) A labeled ORTEP40 plot in PovRay format of complex 12 is shown in Figure 4-7. The crystallographic data and structure refineme nt details are collected in Table 4-6, and selected interatomic distances and angles are listed in Table A-8. The complex crystallizes in the monoclinic space group P 21/ c with the Mn16 molecule lying on a crystallographic C2 rotation axis. For the sake of brevit y, references to specific atoms in the following discussion implicitly include their symmetry-related partners. The structure of 12 is very similar to that of 11 with essentially two exceptions: (i) the Ca2+ ions and chelating NO3 groups in the latter have been replaced by Sr2+ ions and chelating PhCO2 groups; and (ii) four of the Me2AsO2 bridging ligands in the latter have been replaced with PhCO2 groups. Mn8 Mn1 Mn6 Mn7 Mn5 Mn4 Mn2 Mn3 Mn8a Mn1a Mn5a Mn4a Mn6a Mn7a Mn2a Mn3a Sr2 Sr1 O34 O33 O6 O5 O33a O34a O5a O6a Sr1a Sr2a Figure 4-7. ORTEP representa tion in PovRay format of [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] (12). MnIII elongation axes are shown as solid black bonds. For clarity, the hy drogen atoms have been omitted and only the ipso C atoms of the phenyl groups are shown. Mn blue; O red; As pink; Sr green; C gray.

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99 Hence, the structure of the Mn16 molecule consists of four [MnIII 4(-O)2] “butterfly” units [atoms Mn(1), Mn(2), Mn(3 ), Mn(4) and Mn(5), Mn(6), Mn(7), Mn(8) and their symmetry equivalents] with the peri pheral ligation to each subunit provided by five bridging Me2AsO2 and two bridging PhCO2 ligands. The asymmetric unit of 12 is shown in Figure 4-8. The dihedral angle betw een the Mn(1)-Mn(2)-Mn(3) and Mn(2)-Mn(3)-Mn(4) planes is 71.2. The two 3-oxide atoms O(5) and O(6) lie 0.72 and 0.28 below their respective Mn3 planes. Similarly, the dihedral angle between the Mn(5)-Mn(6)-Mn(7) and Mn(6)-Mn(7)-Mn(8) pl anes in the other crystallographically independent [Mn4O2] subunit is 72.3. The 3-oxide atoms O(33) and O(34) lie 0.29 and 0.74 below their respective Mn3 planes. Mn1 As1 O1 O2 O3 O4 As2 O5 O9 O10 O7 O8 As3 O6 O11 O12 As6 O13 O14 O15 O16 O17 O18 As7 As4 Mn2 Mn3 Mn4 Sr1 Sr2 O19 O20 O21 O22 O23 O24 O25 O26 As10 O27 O28 Mn5 O44 O43 As11 O42 O41 As13 O40 O39 As16 O31 O32 As8 O34 O33 Mn8 Mn7 O29 O30 O35 O36 As9 Mn6 O38 O37 Figure 4-8. ORTEP representation in PovR ay format of the asymmetric unit of [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] (12). For clarity, the hydrogen atoms have been omitted and only the ipso C atoms of the phenyl groups are shown. Mn blue; O red; As pink; Sr green; C gray. Again, one Me2AsO2 ligand coordinated to Mn(1) bridges one [Mn4O2] subunit to Mn(8) of a neighboring [Mn4O2] subunit. One the other side, a [Sr2(O2CPh)4(O2AsMe2)] moiety bridges and connects the former [Mn4O2] “butterfly” subunit to another through Mn ions and Me2AsO2 ligands. Bond valence sum calcu lations, again, indicate a 16 MnIII

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100 trapped-valence situation (Table 4-4). The core has the nearly same structure as 11, the main difference involves two relatively weak bonds in 12 [Mn(1)-O(11) = 2.56(1) and Mn(8)-O(32) = 2.72(2) ] that are markedly shorter in 11. The substitution of Sr2+ for Ca2+ and the replacement of the NO3 groups in 11 for the PhCO2 groups in 12 causes an almost insignificant perturbation of the core otherwise. A side-view of 12 emphasizes the planarity of the Mn16 molecule as shown in Figure 4-9. Figure 4-9. ORTEP representa tion in PovRay format of the side-view of complex 12, emphasizing the planarity of the molecule. For clarity, the hydrogen atoms have been omitted and only the ipso C atoms of the phenyl groups are shown. Mn blue; O red; As pink; Sr green; C gray. All of the Mn atoms are six-coordinate with near-octahedral geometry and show the expected Jahn-Teller (JT) distortion, which takes the fo rm of an axial elongation in every case. The average JT elongated bond di stance is 2.229 , approximately 0.30 longer than the average nonJT elongated bond distance of 1.925 (disregarding the weak Mn(1)-O(11) and Mn(8)-O(32) bond distan ces). The JT elongation axes of Mn(2) and Mn(3) and their symmetry partners are al igned approximately parallel to each other within each [Mn4O2] subunit while essentially perpendicular to those of Mn(1) and Mn(4). Similarly, the JT elongation axes of Mn (6) and Mn(7) are aligne d parallel to each other, but perpendicular to Mn(5) and Mn(8). Hence, as with 11, there is an overall nearly

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101 random alignment of the magnetic anisotropy axes of the MnIII ions that will strongly affect the magnetic properties to be discussed below. As shown in Figure 4-10, the 16 benzoate ligands exhibit four types of binding modes: four bridge Mn ions in the common syn syn mode (I), four are bridging three Mn ions in the syn anti syn bridging mode II, four are ch elating Sr(1) or Sr(2) in mode III, and the final four are chelating Sr(1) or Sr(2) and bridging Sr(2) or Sr(1), respectively, in the syn anti syn bridging mode IV. Of the 24 dimethylarsinate ligands, eight are bridging Mn ions in the syn syn bridging mode (V), four are bridging three Mn ions in the syn syn anti 3 bridging mode (VI), eight are bridging two Mn and one Sr ion in the syn syn anti 3 bridging mode (VII), two ar e bridging Mn ions in the anti anti bridging mode (VIII), and the final two are bridging two Mn and two Sr ions in the 4 bridging mode (IX). O C O Ph Sr Mn O As O Mn Me Me syn, syn, (V) Mn O C O Mn Phsyn, syn, (I) O C O Ph SrSr(III)syn, anti, syn, (IV)syn, syn, anti, 3(VI) Mn O As O Mn Me Me Mn Mn O As O Mn Me Me Sr syn, syn, anti, 3(VII) O As O Me Me MnMn anti, anti, (VIII) Sr O As O Sr Me Me MnMn 4(IX) Mn O C O Mn Mn Phsyn, anti, syn, (II) Figure 4-10. Schematic representation of the nine chelating and/or bridging modes found in complex 12. The [Mn4O2] repeating “butterfly” unit found in 11 and 12 has been found in its discrete form in complexes of the general formula [Mn4O2(O2CR)7(bpy)2](ClO4)52 and as

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102 a subunit in various othe r clusters, including [Na2Mn9O7(O2CPh)15(MeCN)2],131 [Mn22O12(O3PPh)8(O2CEt)22(H2O)8],132 [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2],112 [Mn18O14(O2CMe)18(hep)4(hepH)2(H2O)2](ClO4)2 (hepH = hydroxyethylpyridine),62 [Mn21O14(OH)2(O2CMe)16(hmp)8(pic)2(py)(H2O)](ClO4)4 (hmpH = hydroxymethylpyridine and pic = picolinate)114 and numerous others. However, the overall structure of 11 and 12 is quite different from that other previously characterized Mn16 complexes. These include [MnIV 6MnIII 10O16(OMe)6(O2CMe)16(MeOH)3(H2O)3],111d [MnIV 6MnIII 10O16(OMe)6(O2CR)16(MeOH)6] (R = CH2Ph, CH2Cl and CH2Br),111i and [MnIII 8MnII 8(O2CMe)16(teaH)12] (teaH3 = triethanolamine).106 Complex 12 is also only the second example of a mixe d metal Mn/Sr cluster; [Mn14SrO11(OMe)3(O2CPh)18(MeCN)2]133 was the first such species. Table 4-9. Comparison of selected bond distances () and angles ( ) for complexes 1132MeCN and 1216MeCN. Parametera 11 12 Mnw – Oa 1.858(7) – 1.915(8) 1.866(8) – 1.900(10) Mnb – Oa 1.838(13) – 1.999(14) 1.872(10) – 1.926(8) Mnb – Ob (ax) 2.135(16) – 2.230( 12) 2.122(12) – 2.209(9) Mnb – Oc (ax) 2.299(13) – 2.31( 2) 2.305(9) – 2.475(17) Mnb – Od (eq) 1.897(14) – 1.962( 17) 1.890(9) – 1.947(10) MnbMnw 3.097(4)-3.409(4) 3.081(3) – 3.438(3) MnbMnb 2.837(3) 2.822(3) – 2.826(3) X2+X2+ 3.870(6) 4.018(2) X2+Mnw 3.178(3) 3.370(2) – 3.381(3) OaMnbOa 80.5(6) – 85.5(6) 81.6(4) – 82.8(4) MnwOaMnb 104.6(6) – 129.6(10) 108.1(5) – 131.3(4) MnwMnbMnb 49.23 – 65.51 48.91 – 66.68 MnwMnbMnw 92.75 – 93.49 93.21 – 94.50 a Mnb atoms: Mn(2 and 3) in 11 and Mn(2, 3, 6 and 7) in 12; Mnw atoms: Mn(1 and 4) in 11 and Mn(1, 4, 5 and 8) in 12; X2+ atoms: Ca(1) in 11 and Sr(1 and 2) in 12; Oa = triply bridging O2-; Ob = axial bridging PhCO2 -; Oc = axial bridging Me2AsO2 -; Od = equatorial bridging Me2AsO2 -.

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103 In Table 4-9 is shown a comparison of selected bond distan ces and angles of complexes 11 and 12, emphasizing the structural si milarities between the two Mn16 molecules. Slight differences in the values should be treated with caution and are probably reflective of the disorder associated with several PhCO2 and Me2AsO2 ligands in both 11 and 12. 4.2.3 Magnetochemistry of Complexes 9 and 10 Variable-temperature DC susceptibility measurements were performed on dried, microcrystalline samples of 9H2O and 103ClCH2CH2Cl, restrained in eicosane to prevent torquing, in a 1.0 kG field in the range of 5.0-300 K. The isotropic (Heisenberg) spin Hamiltonian describi ng an exchange-coupled MnIII 2MnIV 2 tetranuclear complex such as 9 is given by eq 4-5, using the numb ering scheme of Figure 4-11, where S1 = S3 = 3/2 and S2 = S4 = 2, and it is assumed that pairwise magnetic exchange interactions between pairs of MnIII and MnIV ions are nearly equivalent such that J12 = J23 = J14 = J34 = J. = – 2J( 24) – 2J( 13) – 2J( 12 + 14 + 23 + 34) (4-5) The eigenvalues of the spin Hamiltonian may be determined using the Kambe vector coupling method51 with the following coupling scheme, where A = 1 + 3, B = 2 + 4, T = A + B, and are given in eq 4-6. E(ST,SA,SB) = – J [SB(SB + 1)] – J[SA(SA + 1)] – J[ST(ST + 1) – SA(SA + 1) – SB(SB + 1)] (4-6) The overall degeneracy of the system is 400, made up of 60 individual spin states with total spin (ST) values in the range of 0 to 7 (Table 4-10).

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104 Mn1Mn4Mn3Mn2J JJJJJ Figure 4-11. Representation of the pair wise exchange interactions J, J and J between numbered Mn ions of complex 9. Table 4-10. Distribution of spin states for complex 9. ST na 2ST + 1 n (2ST + 1) 0 4 1 4 1 10 3 30 2 13 5 65 3 13 7 91 4 10 9 90 5 6 11 66 6 3 13 39 7 1 15 15 a number of spin states with the indicated ST values A theoretical M versus T expression was derived for complex 9 from the use of the Van Vleck equation,134 and was modified to include a fraction ( p ) of paramagnetic impurity (assumed to be mononuclear MnII), and temperature-independent paramagnetism (TIP). The latter was kept constant at 400 10-6 cm3 K mol-1. The resulting equation was used to fit the experimental M vs T data for complex 9 which are plotted as MT vs T in Figure 4-12. The MT value decreases rapidly with decreasing temperature from of 8.4 cm3 K mol-1 at 300 K to 4.6 cm3 K mol-1 at 15 K, and then it sharply increases to 5.7 cm3 K mol-1 at 5.0 K. The spin-only ( g = 2) value for a molecule composed of noninteracting MnIII 2MnIV 2 ions is 9.75 cm3 K mol-1. Hence, antiferromagnetic coupling dominates the overa ll intramolecular exch ange interactions.

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105 Temperature (K)MT (cm3K mol-1) 050100150200250300 4 5 6 7 8 9 Figure 4-12. Plot of MT vs temperature for a dried, microcrystalline sample of complex 9H2O in eicosane. M is the DC molar magnetic susceptibility measured in a 1.0 kG field. The solid line is the fit of the data to the theoretical equation; see the text for the fit parameters. Data below 15 K were ignored and the m odified Van Vleck equation was used to fit the observed temperature dependence of the molar magnetic susceptibility as a function of the three exchange coupling parameters, J, J and J, and an isotropic g value. The obtained fit (solid line in Figure 4-12) gave J = -0.49 cm-1, J = -14 cm-1, J = -3.6 cm-1, g = 1.91 and p = 4.7 10-4. The obtained values indica te that the ground state of 9 is doubly degenerate specifically |ST, SA, SB> = |0,0,0> and |ST, SA, SB> = |1,0,1>. There are six other spin states within 15 cm-1 of the ground state (Figure 4-13 and Table 4-11). The results of the fit confirm the pr esence of antiferromagnetic exchange interactions (negative J values) between the Mn ions in complex 9 as was expected by comparison of 9 to various dinuclear [Mn2(-O)2(-O2CR)]z + ( z = 1, 2, 3) systems in which MnIIIMnIII, MnIIIMnIV and MnIVMnIV pairwise exchange inte ractions were found to be antiferromagnetic. The Mn-O-Mn angles are expected to play an important role in determining both the sign and magnitude of J, J and J (Table 4-12). The weakly, negative (antiferromagnetic) J values are consistent with the relatively acute

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106 MnIII-O-MnIII, MnIII-O-MnIV, and MnIV-O-MnIV angles of ~96, ~97 and ~97. Also consistent with expectations is that the MnIVMnIV pairwise exchange interaction is the strongest in the molecule. Unfortunately, only qualitative comp arisons between the calculated J values and those f ound in the literature are possi ble; to our knowledge there are no examples of [Mn4(3-O)4(-O2CR)6]z ( z = 0 or 1) systems for which J values have been reported. Spin frus tration in the core of 9 is also expected as all of the couplings are antiferromagnetic and the spin s cannot all be antiparallel to all of their neighbors. 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 E (cm-1) Figure 4-13. Ordering of the spin states usi ng the calculated exchange parameters, J, J and J, and the Van Vleck equation of complex 9.

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107 Table 4-11. Spin states of complex 9 in the |ST, SA, SB> format ordered as a function of their energy calculated using the exchange parameters, J, J and J, and the Van Vleck equation. |ST,SA,SB> E (cm-1) |ST,SA,SB> E (cm-1) |0,0,0> 0.00 |2,2,3> 113.82 |1,0,1> 0.00 |1,2,3> 116.46 |0,1,1> 0.98 |3,2,3> 128.34 |1,2,1> 2.94 |4,3,3> 131.28 |3,3,1> 5.00 |0,3,3> 133.92 |2,3,1> 5.88 |3,1,3> 140.90 |2,2,1> 9.32 |4,2,3> 150.12 |1,1,1> 14.62 |5,3,3> 160.32 |2,1,1> 21.88 |4,1,3> 162.68 |3,2,1> 23.84 |4,0,4> 168.96 |4,3,1> 26.78 |2,3,4> 178.58 |2,0,2> 28.16 |5,2,3> 179.16 |2,3,2> 32.28 |3,1,4> 191.72 |1,1,2> 36.40 |3,3,4> 193.10 |1,2,2> 43.86 |6,3,3> 196.62 |0,2,2> 45.62 |3,2,4> 211.94 |3,3,2> 46.80 |4,3,4> 214.88 |2,2,2> 51.12 |2,2,4> 215.46 |1,3,2> 55.82 |4,2,4> 233.72 |2,1,2> 63.68 |1,3,4> 240.18 |3,2,2> 65.64 |5,3,4> 243.92 |4,3,2> 68.58 |4,1,4> 246.28 |3,1,2> 78.20 |5,2,4> 262.76 |3,0,3> 84.48 |5,1,4> 275.32 |4,2,2> 87.42 |6,3,4> 280.22 |1,3,3> 87.72 |1,1,0> 281.60 |2,3,3> 94.98 |6,2,4> 299.06 |5,3,2> 97.62 |2,2,0> 311.62 |2,1,3> 99.98 |7,3,4> 323.78 |3,3,3> 109.50 |3,3,0> 342.62 Table 4-12. Selected MnOMn angles () relevant to magnetic exchange parameters J, J and J for complex 9. Parametera Range MnIIIOMnIII 93.21(11) – 98.14(12) MnIIIOMnIV 93.25(11) – 100.18(11) MnIVOMnIV 97.11(12) – 97.69(11) a O = triply bridging O2ion = O(1), O(2), O(3) and O(4); MnIII ions = Mn(2) and Mn(4); MnIV ions = Mn(1) and Mn(3)

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108 The energies of the spin states calcul ated using the J parameters indicate the presence of low-lying excited states of greater S value th an the ground state. This was confirmed by AC magnetic suceptibility measurements carried out on a dried, microcrystalline sample of 9H2O in a 3.5 G AC field oscillating at 1000 Hz. In Figure 4-14 is shown the in-phase AC susceptibility, plotted as M T versus T together with the out-of-phase AC susceptibility ( M ), in the 1.8-10 K temperature range to minimize the possibility of populating excited states. MT(cm3K mol-1) M(cm3mol-1)Temperature (K) 246810 -0.2 0.0 0.2 0.4 0.6 0.8 0.0 0.5 1.0 1.5 2.0 Figure 4-14. Plot of the in-phase (as M T ) and out-of-phase ( M ) AC susceptibility signals vs temperature for a dried, microcrystalline sample of complex 9H2O in eicosane in a 3.5 G AC field oscillating at 1000 Hz. The downward sloping M T versus T plot confirms that depopulation of excited states with greater S valu e than the ground state is o ccurring as the temperature decreases. Extrapolation of the plot to 0 K, where only the ground state will be populated, gives a M T value of ~ 0.8 cm-3 K mol-1. This value is intermediate between 0.0 cm3 K

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109 mol-1 and 1.0 cm3 K mol-1, that expected for an S = 0 state and an S = 1 state with g = 2, respectively. Also evident from the AC st udies is the lack of an out-of-phase AC susceptibility signal ( M ). Such a signal is indicative of the onset of slow magnetic relaxation and confirms that complex 9 does not function as a single-molecule magnet in accord with the small ground st ate spin of the molecule. Unfortunately, application of the Kambe vector coupling method51 to determine the pairwise exchange interactions in complex 10 is not feasible. Although the crystallographic symmetry of 10, like 9, is Ci, it is not reasonable to assume a simplifying approximation to analyze the exchange interactions as was done for 9. As discussed previously, there are two independent Mn4 molecules, each with significantly differing angles and interatomic distances. As an al ternative, density f unctional theory (DFT) calculations on both 9 and 10 are currently in progress to better understand the factors that control the magnitude of the J values in these types of Mn cubane complexes. 4.2.4 Magnetochemistry of Complexes 11 and 12 4.2.4.1 DC studies Variable-temperature DC susceptibility measurements were performed on dried, microcrystalline samples of 112MeCN and 12, restrained in eicosane to prevent torquing, in a 1.0 kG field in the range of 5.0-300 K. Evident from a comparison of the MT vs T plots is that the magnetic behavior of the two complexes is very similar (Figure 4-15). For 11, MT smoothly decreases with decr easing temperature from 43.9 cm3 K mol-1 at 300 K to 16.7 cm3 K mol-1 at 5.0 K. Similarly for 12, MT gradually decreases with decreasing temperature from 42.9 cm3 K mol-1 at 300 K to 15.8 cm3 K mol-1 at 5.0 K. For both Mn16 complexes, the value of MT at 300 K is less than that expected for a

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110 MnIII 16 complex with non-interacting metal centers (48 cm3 K mol-1 for g = 2), suggesting the presence of appreciable intramol ecular antiferromagnetic interactions. (a) (b)Temperature (K)MT (cm3K mol-1) 050100150200250300 15 20 25 30 35 40 45 Temperature (K)MT (cm3K mol-1) 050100150200250300 15 20 25 30 35 40 45 Figure 4-15. Plot of MT vs temperature for dried, mi crocrystalline samples of (a) complex 112MeCN and (b) complex 12 in eicosane. M is the DC molar magnetic susceptibility measured in a 1.0 kG field. Each complex contains sixteen MnIII centers, with total spin (S) values therefore ranging from 0 to 32. A matrix diagonizati on approach to evaluate the various Mn2 pairwise exchange interaction constants (J) is made challenging by the low symmetry and high nuclearity of the molecules. Such a me thod would involve the diagonalization of a matrix of dimensions 153 109 by 153 109, and this is clearly not reasonable. An equivalent operator approach base d on the Kambe vector coupling method51 is also not feasible. Hence, we concentrated on dete rmining on the ground state spin S of the complexes by the fitting of va riable-temperature and -field magnetization data collected on polycrystalline samples in the 0.1-70 kG and 1.8-10.0 K field a nd temperature ranges. Despite many attempts to fit the data using the program MAGNET,54 which assumes only the ground state of a molecule is populated, sa tisfactory fits were not obtained. This is likely due to the presence of low-lying excite d states that are populat ed even at the very low temperatures employed. Unfortunately, the latter are commonly encountered in high

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111 nuclearity clusters; weak exchange interactio ns between the constituent ions and/or spin frustration effects within the molecule give rise to a high density of spin states. Reasonable fits were not obtai ned even using only very weak field and low temperature magnetization data (0.1-0.5 T and 1.8-4 K) wh ere no interference from Zeeman effects in the applied DC field, i.e., excited states with larger S values crossing in energy with the ground state S, is expected. The topology of 11 and 12 promotes spin frustration, consis ting of eight edge-sharing [Mn3O] triangular subunits found in the four [Mn4O2] “butterfly” units that comprise the Mn16 molecule. Such spin frustration effects, whereby competing antiferromagnetic exchange interactions of comparable magnitude prevent the antiparallel alignment of spins, were previously reported for the related [MnIII 4O2(O2CMe)7(bpy)2](ClO4) “butterfly” complex.52 Very weak exchange coupling parameters (J) between the constituent MnIII ions were found and it was shown further that the ground state spin of the “butterfly” molecule was very sensitive to the precise magnitude of the competing exchange interactions.52 Hence, the triangular arrangement of coupled Mn ions, as found in 11 and 12, is the classic topology for spin frustration effects, accounting for a high density of mol ecular spin states close in energy to the ground state spin, S. Thus, a more accurate and reliable approach of determining the ground state spin of such molecules as 11 and 12 is to measure the AC magnetic susceptibility, a method which does not involve the use of a DC field.66,106,111i,112,119 Such studies have recently proven crucial in the de termination of the ground state spin of other molecules reported by our group and were therefore carried out on complexes 11 and 12.66,106,111i,112,119

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112 4.2.4.2 AC studies In an AC magnetic susceptibility experiment a weak field (typically in the 1 5 G range) oscillating at a pa rticular AC frequency ( ) is applied to a sample to investigate the dynamics of its magnetization relaxation. If the magnetization vector of the molecule can reorient at the frequency of the oscillat ing AC field, then th ere is no out-of-phase ( M ) susceptibility signal, a nd the in-phase signal ( M) is equal to the DC magnetic susceptibility. However, if the magnetization vector of the molecule cannot keep up with the oscillating field, then an out-of-phase si gnal is observed, indicating an energy barrier to magnetization relaxation that is comparable in energy to that of the thermal energy. The M signal is dependent on the frequency of th e oscillating AC field, i.e., faster the oscillation of the AC field, the hi gher the temperatures at which the M signal is observed, and is accompanied by a frequency-de pendent decrease in the in-phase signal (as M T ). Such signals are a characteristic signature of the superparamagnet-like properties of a SMM, however, should not be taken as proof that a molecule behaves as a SMM; intermolecular interac tions and phonon bottlenecks have been shown to also lead to such signals. Thus, AC magnetic sus ceptibility data were collected on dried, microcrystalline samples of 112MeCN and 12 in the temperature range 1.8-10 K in a zero DC field and a 3.5 G AC field oscillating at several frequencies in the 25-997 Hz range. The data for complex 112MeCN are plotted as M T and M in Figure 4-16 and show a frequency-dependent M signal below 3 K; the M signals at the given frequencies are merely the tails of peaks whose maxima clearly lie below the 1.8 K operating limit of our SQUID instrument.

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113 MT(cm3K mol-1) M(cm3mol-1)Temperature (K) 246810 -0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 100 Hz 50 Hz 25 Hz 8 9 10 11 12 13 14 Figure 4-16. Plot of the in-phase (as M T ) and out-of-phase ( M ) AC susceptibility signals vs temperature for a dried, microcrystalline sample of complex 112MeCN in eicosane at the i ndicated oscillation frequencies. The in-phase signal, plotted as M T appears frequency-dependent at the lowest temperatures and decreases gradually with decreasing temperature, hence confirming the population of low-lying excite d states of the molecule since occupation of only the ground state would give an e ssentially temperature-indepe ndent value. The downward sloping of the M T vs T plot indicates depopulation of excited states with S values greater than the ground state with decreasing temperature. Extrapolation to 0 K, where only the ground state will be populated, suggests the plot is heading to a M T value of ~10 cm3 K mol-1. This is the value expected for an S = 4 state with g ~ 2.00. Similar results were obtained for complex 12, and we conclude that the ground state spin of 11 and 12 is S = 4. The out-of-phase M signals exhibited by 11 and 12 appear at very low temperatures, suggesting only very small effective energy barriers to magnetization

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114 relaxation. Nevertheless, we decided to i nvestigate the magnetic behavior at lower temperatures, where the origin of the slow magnetization behavior could be resolved. 4.2.4.3 Hysteresis studies below 1.8 K Hysteresis loops are the definitive property of a magnet and can provide unquestionable proof that a molecule behave s as a single-molecule magnet. Thus, DC magnetization field studies below 1.8 K were collected on wet crystals of complex 1132MeCN in order to determine whether th e slow magnetization relaxation suggested by the tails of the M signals in the AC magnetic su sceptibility was due to singlemolecule magnetism or to some other effect, such as phonon bottleneck. DC magnetization field scans were performed in the 0.04-7.0 K temperature range using a fixed field sweep rate of 0.070 T/s as s hown in Figure 4-17a. Magnetization responses measured at different field sweep rates in th e range from 0.004 to 0.56 T/s at a constant temperature of 0.04 K are given in Figure 4-17b. -1 -0.5 0 0.5 1 -1.2-0.8-0.400.40.81.2 0.560 T/s 0.280 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s M/Ms 0H (T) 0.04 K 1.2 0.8 0 -0.8 -1.20.4 -0.4 H0(T) 0.04 K M/Ms -1 -0.5 0 0.5 1 M/Ms1 0.5 0 -0.5 -1 0.017 T/s 0.280 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.560 T/s 0.004 T/s 0.008 T/s -1 -0.5 0 0.5 1 -1.2-0.8-0.400.40.81.2 0.04 K 0.5 K 1 K 2 K 7 K M/Ms 0H (T) 0.07 T/s M/Ms -1 -0.5 0 0.5 1 M/Ms1 0.5 0 -0.5 -1 1.2 0.8 0 -0.8 -1.20.4 -0.4 H0(T) 0.07 T/s 0.5 K 1 K 2 K 0.04 K 7 K H0(T) H0(T)(a) (b) Figure 4-17. Magnetization vs DC field plots for a single crystal of complex 1132MeCN at (a) the indicated temperatures and a fixed field sweep rate of 0.07 T/s; and (b) the indicated sweep rates and a fixed temperature of 0.04 K. The magnetization is normalized to its maximum value, Ms.

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115 Hysteresis loops become evident in the scan s at 7 K, and their coercivites increase with decreasing temperature and with increa sing field sweep rate, as expected for the superparamagnetic-like behavi or of a SMM. Thus, complex 11 (and given the structural similarities, 12) are new additions to the family of SMMs. In contrast to the hysteresis loops of several other SMMs, such as Mn12 complexes, the loops for 11 do not show the steplike features indicative of quantum tunneling of magnetization (QTM) between the energy states of the molecule.27,61,104,113 The absence of such steps is a common finding for higher nuclearity SMMs62,63,66,98,105,106,111g,111h,111i,114 and is primarily a consequence of a distribution of local molecular environments owing to solvent and ligand disorder, both of which are prevalent in 11 and 12. In addition, weak interm olecular interactions and low-lying excited states likely also contribute to broade ning of the steps. Although the loops are smooth, there is a read ily discernable step at zero field that likely corresponds to tunneling through the anisotropy ba rrier through the degenerate ms sublevels of the S = 4 spin manifold. Tunneling through ms sublevels of excited state manifolds probably also contributes to the magnitude of the zero-field step. Unfortunately, due to the extremely fast ma gnetization relaxation at zero field, DC magnetization decay measurements are not feas ible; this is typically the method by which the effective energy barrier to magnetization reversal (Ueff) of a SMM is determined, especially in cases where only an the tail of a M signal (and not the complete peak) is exhibited by a molecule. It is apparent, how ever, from the hysteresis measurements that the effective relaxation barrier for complex 1132MeCN is unfortunately rather small and this is likely due to efficient tunneling path ways in the ground state and/or excited state spin manifolds. The energy barrier (U), given by S2|D| for an integer spin system such as

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116 11 and 12, would be of moderate magnitude comp ared to other SMMs as S = 4 and a nonzero, negative D value is expected given th e orientation of the anisotropy axes of the MnIII ions in a non-antiparallel manner. This parameter reflects only thermal activation over the magnetization energy barrier and di sregards relaxation by quantum tunneling, however, and is almost certainly greater than Ueff. Values of D are often obtained by monitoring the separations betw een the steps that occur at periodic field values in the hysteresis loops of a SMM or by single crystal high-frequency EPR (HFEPR) measurements. Alternatively, in instances such as this when there are no steps or only small crystals unsuitable for HFEPR measurements can be obtained, D is usually determined from fits of variable-temperatu re and -field magnetization measurements. We have not been able to obtain an estimate of D, however, and explanations for this have already been described. As such, we have been unable to determine values of both Ueff and U for either complex 11 or 12 to date, but despite the low anisotropy barriers, the frequency-dependent AC susceptibility data and the sweep rate and temperaturedependent coercivities of the hysteresis loops of 11 establish that these Mn16 molecules are new SMMs. 4.3 Conclusions In summary, attempted ligand substitution reactions of [Mn12O12(O2CR)16(H2O)4] (R = Me and Ph) with dimethylarsinic acid (Me2AsO2H) have been explored and found to cause a transformation of the [Mn12O12] core. Two general structural types were isolated from these reactions, which vary only slight ly in the conditions: (i) mixed-valence, trapped-valence tetranuclear complexes 9 and 10 which possess a manganese-oxo cubane core and the general formula [Mn4(3-O)4(O2AsMe2)6]0,+ and (ii) trapped-valence MnIII 16 complexes 11 and 12 which possess a novel structural topology comprised of four

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117 [MnIII 4(-O)2] “butterfly” units. All four complexes represent the first examples of manganese clusters ligated by Me2AsO2 groups. DC and AC magnetic susceptibility studies on 9 suggest a small ground state spin and show that the complex does not behave as a single-molecule magnet. Similar studies are currently in progress for complex 10 as are more sophisticated DFT calculations on both 9 and 10. These studies should provide considerab le insight to our understanding of their magnetic properties. Similarly, magnetic studies on 11 and 12 suggest a ground state spin value of S = 4 and tails of peaks in the out-of-phase AC susceptibility indicate the onset of slow magnetization re laxation. Hysteresis loops ob tained from magnetization vs DC field scans establish that 11 (and by analogy 12) are new members of the growing family of single-molecule magnets. Hence, our efforts to extend the explor ation of the reactivity of various non-carboxylate ligands with Mn12 complexes continues to prove a useful strategy for the preparation of new polynuclear Mn clusters, some of whic h have interesting magnetic properties. Like the relate d non-carboxylic acid, PhSeO2H (pKa = 4.79), the use of Me2AsO2H as a reactant causes a structural rearrangement of the [Mn12O12] core. On this basis, we conclude that a combination of relatively high pKa value and large OO bite distance compared to MeCO2 are important factors to be c onsidered for the isolation of new structural types from reactions of Mn12 complexes with non-carboxylate ligands. Other studies with related non-carboxylate ligands will undoubtedly help us to better understand the causes of the structural transformations.

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118 4.4 Experimental 4.4.1 Syntheses All manipulations were performed under aerobic conditions us ing materials as received, except where otherwise noted. [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) and [Mn12O12(O2CPh)16(H2O)4] (8) were prepared as described elsewhere.22,50 [Mn4O4(O2AsMe2)6] (9). A solution of complex 1 (0.50 g, 0.24 mmol) in MeCN (75 cm3) was treated with Me2AsO2H (0.60 g, 4.3 mmol) in MeCN (25 cm3). The solution color changed from dark brown to deep red as it was stirred overnight. The solvent was removed in vacuo Toluene (25 cm3) was added to the residue, and the solution was again evaporated to dryness. The addition and rem oval of toluene was repeated three more times. The residue was redissolved in CH2Cl2 (50 cm3) and filtered through Celite. Diffusion of pentane into the CH2Cl2 solution produced large red crystals, and these were suitable for X-ray crystallogr aphy if maintained in contact with the mother liquor to prevent the loss of interstitial solvent. After 6 days, the crystals were isolated by filtration, washed with pentane and dried in vacuo ; yield 75%. The dried material is hygroscopic, analyzing for 9H2O. Anal. Calcd (found) for C12H38As6Mn4O17: C, 12.83 (12.79); H, 3.41 (3.48); N, 0.00 (0.00). Selected IR data (KBr, cm-1): 3016 (m), 2965 (m), 2926 (m), 2878 (m), 1653 (w), 1568 (s), 1473 (w), 1420 (s), 1272 (m), 1094 (s), 808 (vs), 652 (m), 622 (s), 565 (m), 500 (s), 481 (s). {[Mn4O4(O2AsMe2)6](NO3)}2 (10). A solution of complex 1 (0.20 g, 0.10 mmol) in MeCN (30 cm3) was treated with solid Me2AsO2H (0.24 g, 1.7 mmol) and Ca(NO3)24H2O (0.034 g, 0.15 mmol). MeOH (1 cm3) was added and the solution was stirred for 30 min. A small amount of dark brown precipitate was removed from the deep red solution by filtration through Celite. To the filtrate was added 1,2-dichloroethane (80

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119 cm3) and the solvent was allowed to evaporate sl owly in air. Crystals formed slowly over two weeks, and these were suitable for X-ray studies if maintained in contact with the mother liquor to prevent the loss of interstitial solvent. After two weeks, the crystals were isolated by filtration, washed with large am ounts of 1,2-dichloroethane, and dried under vacuum; yield ~24%. Anal. Calcd (found) for 103ClCH2CH2Cl (C30H84As12Cl6N2O38Mn8): C, 13.69 (13.67); H, 3.22 (3. 36); N, 1.06 (0.96). Selected IR data (KBr, cm-1): 3422 (vs), 1559 (s), 1419 (m), 1384 (vs), 1273 (w), 828 (vs), 796 (vs), 652 (m), 616 (w), 590 (w), 490 (m). [Mn16Ca4O8(O2CPh)8(O2AsMe2)28](NO3)4 (11). To a stirred slurry of complex 8 (0.25 g, 0.087 mmol) in MeCN (10 cm3) was added solid Me2AsO2H (0.22 g, 1.6 mmol) and solid Ca(NO3)24H2O (0.021 g, 0.087 mmol). MeOH (3 cm3) was added to the slurry with stirring for 20 min. The deep red solution containing some brown powder was filtered through Celite. Diffusion of Et2O into the solution slowly produced crystals, and these were suitable for X-ray crystallography if maintained in contact with the mother liquor to prevent the loss of interstitial solvent. After 3 days, crystals were isolated by filtration, washed with MeCN, and dried under vacuum; yield 5%. Anal. Calcd (found) for 112MeCN (C116H214N6As28Ca4O92Mn16): C, 22.11 (22.04); H, 3.42 (3.65); N, 1.33 (1.18). Selected IR data (cm-1): 1597 (m), 1550 (m), 1394 (s), 1269 (w), 799 (vs), 721 (m), 649 (m), 588 (m), 484 (s). [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] (12). To a stirred slurry of complex 8 (0.25 g, 0.087 mmol) in MeCN (10 cm3) was added solid Me2AsO2H (0.22 g, 1.6 mmol) and solid Sr(ClO4)2H2O (0.025 g, 0.087 mmol). MeOH (1 cm3) was added to the slurry with stirring for 20 min. The deep red solution containing some brown powder was filtered

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120 through Celite. Diffusion of Et2O into the solution slowly produced crystals, and these were suitable for X-ray crysta llography if maintained in cont act with the mother liquor to prevent the loss of interstitial solvent. After 5 days, crystals were isolated by filtration, washed with MeCN, and dried in vacuo ; yield 7%. Anal. Calcd (found) for 12 (C160H224As24Sr4O88Mn16): C, 29.19 (29.42); H, 3.43 (3. 66); N, 0.00 (0.00). Selected IR data (cm-1): 3421 (m,br), 2930 (w), 1597 (s), 15 50 (s), 1394 (vs), 1268 (w), 1023 (w), 833 (vs), 800 (vs), 720 (m), 675 (m), 667 (w), 651 (m), 613 (w), 592 (w), 490 (m), 459 (w), 447 (w), 416 (w). 4.4.2 X-ray Crystallography Data were collected on a Bruker P4 (9) and Siemens SMART PLATFORM (10, 11 and 12) platform goniometer equipped with a SMART APEX CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Suitable single crystals of 95H2OC5H12, 10MeCN12H2O, 1132MeCN and 1216MeCN were attached to glass fibers using silicone greas e and transferred to th e goniostat where they were cooled to -55 C (9) and -100 C (10, 11 and 12) for characterization and data collection. Each structure was so lved by direct methods (SHELXTL)64 and standard Fourier techniques, and was refined on F2 using full-matrix least-squares methods. All non-hydrogen atoms were refined anisotropi cally. Hydrogen atoms were placed in calculated positions and refined with the use of a riding model. Cell parameters were refined using up to 8192 reflecti ons. The intensity data for 9 was collected using the Phiscan method with a scan step Phi = 0.03. For 10, 11 and 12, a full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were re-measured at the end of data collecti on to monitor instrument and crystal stability

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121 (maximum correction on I was < 1%). Data for 9 were corrected for Lorentz and polarization effects using th e Bruker SAINT software and an absorption correction was performed using the SADABS program supplied by Bruker AXS (Tmin/Tmax = 0.657). Absorption corrections by integration were applied based on measured indexed crystal faces for 10, 11 and 12. A preliminary search of reciprocal space for 95H2OC5H12 revealed a set of reflections with no symmetry and no system atic absences. An initial choice of the centrosymmetric space group 1 P was subsequently confirmed by the successful solution of the structure. The asy mmetric unit contains the Mn4 molecule, five water and one pentane molecules of crystallization. The pe ntane molecule was significantly disordered and could not be modeled properl y. Hence, the program SQUEEZE,96 a part of the PLATON97 package of crystallographic software was used to calculated the solvent disorder area and remove its contribution to the overall intensity data. Correction of the X-ray data by SQUEEZE for 9 was the same as the required value, 84 electron/cell. A total of 388 parameters were refined in the final cycle of refinement using 9507 (Rint = 0.0218) independent reflections with I > 2 (I) to yield R 1 and wR 2 of 3.70% and 10.14%, respectively. The final difference Fo urier map was reasonably clean, the largest peak being 1.796 e -3 and the deepest hole being -1.122 e -3. Complex 10MeCN12H2O crystallizes in the triclinic space group 1 P with the asymmetric unit consisting of two [Mn4]+ molecules, two NO3 anions, half of an MeCN and twelve water molecules of crystallization. One of the NO3 anions [N(3), O(36), O(37), O(38)] was disordered over two sites and was contrained to be geometrically similar to the whole nitrate anion. The site occupancy factors were dependently refined to

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122 50:50% and atoms were refined with isotropic thermal parameters. The H atoms of the water molecules of crystallizat ion could not be located. The Me2Asgroup of a Me2AsO2 ligand [As(6), C(11), C(12)] in one of the Mn4 molecules was disordered over two positions, whose site occupancy factors were dependently refined to 76:24%. A total of 875 parameters were refined in the final cycl e of refinement using 26916 reflections with I > 2 (I) to yield R 1 and wR 2 of 5.15% and 12.40%, respecti vely. The highest peak in the electron density map is 0.82 from an As at om and thus was attributed to anisotropy. The final difference Fourier map was essentiall y featureless, the largest peak being 2.117 e -3 and the deepest hole being -1.162 e -3. For complex 11, an initial survey of a portion of reciprocal space located a set of reflections with a orthorhombic lattice. Analys is of the full data set revealed that the space group was I 222. The asymmetric unit contains one-quarter of a Mn16 molecule and 8 MeCN molecules of crystallization. The latter 8 solvent molecules were disordered and could not be modeled properl y, so the program SQUEEZE,96 a part of the PLATON97 package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. The phenyl rings in the two PhCO2 ligands [C(82)-C(87) and C(92 )-C(97)] were disordered and could not be resolved to satisfaction. The As atoms and their co rresponding Me groups in two of the Me2AsO2 ligands [As(6), C(61), C(62) and As(8), C(101), C(102)] were disordered over the crystallographic C2 rotation axes; the cluster is located on a 222 symmetry site. Their site occupancy factors were fixed at 50%. Additi onally, the As and corresponding Me atoms of one Me2AsO2 ligand not situated on a crystallogra phic rotation axis [As(7), C(71) and C(72), were disordered over two main pos itions. Their site occupancy factors were

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123 dependently refined to 49:51. These disord ers contribute significantly to the poor refinement and relatively high final R values. Over ten data sets were collected on different crystals of 11, and all have significant disordering of the Me2AsO2 ligands. In a few data sets, we were unable to locate even the carbon atoms of th eir Me groups. All of these data sets suggest the same structure and they refined to different levels of acceptability. The best data set is herein reported. A total of 380 parameters were refined in the final cycle of refinement using 26952 reflections with I > 2 (I) to yield R 1 and wR 2 of 8.76% and 22.86%, respectively. The final difference Fourier map was reasonably clean, with the largest p eak and deepest hole being 1.496 e -3 and -1.451 e -3, respectively. Complex 1216MeCN crystallizes in the monoclinic space group P 21/ c with the asymmetric unit consisting of half a Mn16 molecule and 8 MeCN molecules of crystallization. The latter eight molecules were disordered and could not be modeled properly. Thus, the program SQUEEZE,96 a part of the PLATON97 package of crystallographic software, was used to calcula ted the solvent disorder area and remove its contribution to the overall intensity data The phenyl rings in two of the PhCO2 ligands [C(8)-C(13) and C(64)-C(69)] were disorder ed. Their site occupancy factors were dependently refined as rigi d bodies to 53:47 and 39:61, re spectively. All other phenyl rings were also refined as rigi d bodies, but were either not di sordered or the disorder was minor and could not be resolved. The As atoms and their corresponding Me groups [As(4)/As(5), As(11)/As(12), As(13)/As(14), and As(15)/As(16)] in four of the Me2AsO2 ligands were disordered about two (main) positions, and the occupancies dependently refined to 62:38, 50:50, 58:42 and 47:53, re spectively. A total of 967 parameters were

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124 refined in the final cycle of refineme nt using 61172 reflections with I > 2 (I) to yield R 1 and wR 2 of 8.72% and 22.12%, respectively. The final difference Fourier map was reasonably clean, with the largest p eak and deepest hole being 2.115 e -3 and -2.709 e -3, respectively.

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125 CHAPTER 5 NEW POLYNUCLEAR Mn CLUSTERS FROM THE USE OF THE HYDROPHOBIC CARBOXYLATE LIGAND 2,2-DIMETHYLBUTYRATE 5.1 Introduction One of the principal motivations for the continuing exploration of Mn carboxylate chemistry is the established potential of this area as a rich source of single-molecule magnets (SMMs),12,13,38,46,61-63,98-114 a field that began in 1993 when it was shown that [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) functions as a magnet at temperatures below its blocking temperature, TB. A SMM possesses a significant potential energy barrier (U) to relaxation of its magnetizati on vector, arising from the combination of a large ground state spin, S, and a large, ne gative magnetoanisotropy of the easy-axis (or Ising) type (negative axial zero-field splitting parameter, D). The upper limit of this energy barrier is given by S2|D| and (S2-)|D| for integer and half-integer S values, respectively.19,22,23,135 Experimental evidence for the slow, superparamagnet-like magnetic relaxation of a SMM is provided by th e observation of hysteresis, the classical macroscale property of a magnet, in magnetizati on versus DC field scans, and also by the observation of frequency-de pendent, out-of-phase AC su sceptibility signals ( M ).21,136 The calculated energy barrier, U, of complex 1 is 50 cm-1 (72 K), arising from S = 10 and D = -0.50 cm-1 (-0.72 K).19 This energy barrier allows molecules of complex 1 to function as individual magnets at temperatures below 10 K. The half-life for magnetization decay is so long that it is hardly measurable if molecules of 1 are magnetized at 1.5 K by applying a magnetic field and then removing the field.19 After

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126 more than a decade of resear ch in this area, complex 1 and carboxylate substituted derivatives still remain the molecules that function as magnets at the highest temperatures, exhibiting the most promise for use in high density memory storage devices or in quantum computing. This is in spite of the preparati on of many SMMs of a different structural type, as well as research in other areas such as molecular magnetism. There are two main routes to new SMMs. The first is the modification of a given structural type in some way that does not ch ange the core structure. For example, such studies on Mn12 complexes have included varia tion of the peripheral carboxylate ligation,13,38,46 variation of the oxidation level by cluster reduction,13,61,100 replacement of some of the Mn centers with either Fe or Cr,137,138 and replacement of some of the carboxylate ligands with non-carboxylate groups.32,33,39,47,58,101 The second is the use of harsher conditions, such as the reaction of a cluster with a chelating ligand, that will often cause a core structural and/or nuclearity change leading to new structural types.11,102-107 These methods have together afforded many new Mnx species when a Mn12 complex was employed, such as [Mn12O12(O2CCH2But)16(H2O)4],56 [Mn30O24(OH)8(O2CCH2But)32(H2O)2(MeNO2)4],63 and [Mn84O72(O2CMe)78(OMe)24(MeOH)12(H2O)42(OH)6].66 A particularly fruitful starting material fo r a variety of reactions of the second type has been [Mn12O12(O2CCH2But)16(H2O)4] (13), where ButCH2CO2H is 3,3-dimethylbutyric acid ( t -butylacetic acid).63,108,139 For example, the reaction of 13 with MeOH led to the isolation of the novel Mn21 cluster [Mn21O24(OMe)8(O2CCH2But)16(H2O)10],108 while reductive destabilization with a reducing agent such as phenol led to the new Mn8 cluster,

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127 [Mn8O2(O2CCH2But)14(HO2CCH2But)4].139 In addition, the Mn30 cluster mentioned above, [Mn30O24(OH)8(O2CCH2But)32(H2O)2(MeNO2)4], was obtained by simply recrystallizing 13 from a CH2Cl2/MeNO2 solvent mixture.63 It has been postulated that the strong basicity of the ButCH2CO2 ligand, as reflected in the relatively high pKa value of its conjugate acid (pKa = 5.00), combined with th e bulky and hydrophobic nature of the But group are the main reas ons for the interesting pr oducts obtained with this carboxylate. As part of our continuing intere st in SMMs in general and in Mn12 complexes in particular, we have extended these inves tigations of the infl uence of bulky, hydrophobic groups on the nature of the obtained products. We report our findings from the use of the related carboxylic acid, 2,2dimethylbutyric acid (PetCO2H, where Pet is the t -pentyl group, -CMe2Et) with a pKa of 5.03, which is similar to ButCH2CO2H. We herein describe the syntheses, singl e crystal X-ray structures, a nd magnetic properties of the products obtained by th e introduction of PetCO2 groups into Mn12 complexes. 5.2 Results and Discussion 5.2.1 Syntheses In order to introduce the hydrophobic PetCO2 group onto the Mn12 core, we employed the previously developed carboxyl ate substitution reaction that involves the treatment of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) with an excess of RCO2H.13,38 Thus, a solution of complex 1 in MeCN was treated with an excess of PetCO2H in CH2Cl2. The reaction is an equilibrium that was driven to completion by several cycles of removal of acetic acid as its toluene azeotrope (28:72%; b.p. 101 C at one atmosphere) under reduced pressure (eq 5-1). The product was subsequently

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128 crystallized with MeNO2 and identified by infrared spect roscopy and elemental analysis as [Mn12O12(O2CPet)16(H2O)4] (14). [Mn12O12(O2CMe)16(H2O)4] + 16 PetCO2H [Mn12O12(O2CPet)16(H2O)4] + 16 MeCO2H (5-1) The stability of complex 14 to recrystallization from CH2Cl2/MeNO2 or CH2Cl2/MeOH was investigated both in the pr esence and absence of the free carboxylic acid. We had previously observed56 that [Mn12O12(O2CCH2But)16(H2O)4] (13) can be recrystallized without change from these solvent systems in the presence of ButCH2CO2H. However, in the absence of the latter, the Mn30 and Mn21 clusters, respectively, mentioned earlier were obtained.63,108 Somewhat different results were obtained with 3: in the presence of PetCO2H, recrystallization from CH2Cl2/MeOH and CH2Cl2/MeNO2 gave [Mn12O12(O2CPet)16(MeOH)4] (15) and [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] (16), respectively. In the absence of PetCO2H, each recrystallization gave materi als that, by infrared spectroscopy and elemental analysis, have not retained the Mn12 core but that are not 15 or 16. However, we have been unable to date to identify these products. The crystallization solutions from the reaction of 14 with PetCO2H in a solvent mixture of CH2Cl2/MeNO2 were allowed to stand for several weeks in an effort to increase the yield of complex 16, a new structural type. Instead, several products crystallized, including complexes 14 and 16, and a number of other materials. The reaction system is unquestionably very comp licated, with several species no doubt in equilibrium in solution. The in itial crystalline products were separated by filtration, the filtrate was concentrated to dryness by roto-evaporation, a small amount of PetCO2H was

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129 added to the residue, and the latter was dissolved in CH2Cl2. Layering the solution with MeNO2 slowly gave small crystals of [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] (17) containing a bridging CO3 2group. This was an unexpected re sult that can be rationalized by reasoning that the CO3 2ligand in 17 likely arises from the oxidation of the O2CH2 2ligand in 16, which itself likely originates from the hydrolysis of CH2Cl2, a process for which there is preced ent in the literature.140 Alternatively, the CO3 2ligand in 17 may be from the reaction of 16 with atmospheric CO2 as has been previously observed with a series of hydroxo complexes [M(HB(3,5i Pr2pz)3)]2(OH)2 (HB(3,5i Pr2pz)3 = hydrotris(3,5-diisopropyl-1pyrazolyl)borate and M = MnII, FeII, CoII, NiII, CuII and ZnII).141 Although mechanistic studies are not f easible for this complicated reaction system, it can be noted that neither complexes 16 nor 17 crystallize from identical reactions carried out in CHCl3 instead of CH2Cl2. In addition, the infrared spectra of materials obtained from similar reactions using [Mn12O12(O2CBut)16(H2O)4] and ButCO2H suggest that structural analogues of complexes 16 or 17 are not obtained. Although these results support the sugges tion that the O2CH2 2and CO3 2ligands originate from CH2Cl2 and not atmospheric CO2 or carboxylate groups, the exact mechanism remains unclear. We also investigated the reactivity of the Pet-substituted Mn12 cluster 14 under the various conditions explored earlier for the But-substituted derivative, specifically with chelates and reducing agents. Reactions of complex 14 with 2,6-pyridinedimethanol (pdmH2), 2,2 -bipyridine (bpy) and phenol were carried out in CH2Cl2. Large crystals of [Mn4O2(O2CPet)6(bpy)2] (18) were obtained in high yi eld from the reaction of 14 with bpy. IR spectra of the solids obtaine d from reactions with either pdmH2 or phenol

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130 suggested the product to be [Mn8O2(O2CPet)14(HO2CPet)4], the Pet analog of [Mn8O2(O2CCH2But)14(HO2CCH2But)4]139 mentioned earlier. However, not enough material was isolated from these reactions fo r more definitive characterization. To further understand the reaction system, recrystal lization was explored of complexes 13 and 14 from solvent mixtures other than CH2Cl2/MeNO2, in the absence of the corresponding acid. As mentioned earlier, the Mn30 complex was obtained by dissolution of complex 13 in this mixture of solvents; in fact, this transformation to Mn30 requires the presence of MeNO2. Hence, several solvents were explored in place of the CH2Cl2, and among these was tetrahydrofuran (THF). Indeed, we obtaine d black needle-like crystals from a dark brown solution of 13 in THF/MeNO2. Preliminary X-ray analysis identified the product as [Mn9O7(O2CCH2But)16(THF)2] (19), but we have been unable to obtain suitable crystals for a high quality structur e refinement. The transformation of 13 into 19 is summarized in eq 5-2. The average oxidation st ate of the starting material is +3.33 while that of the product (19) is only +3. Thus, the formation of complex 19 appears to involve the reduction of 13 followed by structural rearrangement. [Mn12O12(O2CCH2But)16(H2O)4] + 2 THF + H+ + 4 e[Mn9O7(O2CCH2But)13(THF)2] + 3 Mn3+ + 3 ButCH2CO2 + 9 OH(5-2) 5.2.2 Electrochemistry Previous studies have established the Mn12 clusters to exhibit interesting electrochemical behavior.13,61,100 They normally display several oxidation and reduction processes of which at least one reduction wave is usually reversible by the common electrochemical standards (CV peak separations, DPV peak broadness, ianodic/icathodic peak current ratio, and linearity of peak current vs 1/2 plots, where is the scan rate). In addition, as expected the re dox processes are very sensitiv e to the electron withdrawing

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131 or donating ability of the carboxylate lig and, and we have reported the E1/2 values for a number of Mn12 derivatives. The range of E1/2 values (vs Fc/Fc+) for the first one-electron reduction ranges from 0.91 V for the R = CHCl2 complex to 0.00 V for the R = p -C6H4OMe complex. Similarly, the second reduction ranges from 0.61 V for the R = CHCl2 complex to -0.50 V for the R = Et complex. The potential of the first oxidation process varies from 1.07 V for R = p -C6H4CF3 to 0.70 V for R = p -C6H4OMe or p -C6H4Et. By reduction with either one or two equivalents of I-, both the oneand twoelectron reduced forms of some Mn12 species have been isolated and structurally characterized (I-/I2 couple occurs at 0.21 V in CH2Cl2 vs Fc/Fc+).13,61 However, despite the reversible nature of the oxidation processes of several characterized Mn12 clusters, a one-electron oxidized Mn12 cluster has not yet been isolated. This is likely due in part to the high potentials associated with this process. The introduction of a strongly electron donating ligand onto the Mn12 complex would likely make mo re feasible the preparation of a one-electron oxidized Mn12 derivative by moving the firs t oxidation process to more accessible potentials, and we decided to explore this. The cyclic voltammogram (CV) and diffe rential pulse volta mmogram (DPV) of 15 are shown in Figure 5-1. There is a quasi-re versible oxidation wave at -0.21 V and an irreversible reduction wave at -0. 87 V. The CV and DPV profiles of 13 are very similar; a quasi-reversible oxidation wave occurs at -0.07 V and an irreversible reduction appears at -0.82 V. The oxidation processes of both co mplexes meet the standard electrochemical requirements for quasi-reversible electron transfer. A study of the scan rate ( ) dependence for the oxidation waves showed a linear dependence of peak current with respect to 1/2, indicating that the pro cess is diffusion-controlled as shown in Figure 5-2

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132 for complex 13 and in Figure 5-3 for complex 15. This relationship is described by the Randles-Sevcik equation (eq 5-3) ip = (2.687 105)n3/21/2D1/2AC (5-3) where n is the number of electrons appeari ng in the half-reaction of the redox couple, is the scan rate, A is the electrode area, D is th e diffusion coefficient of the analyte and C is the concentration of the analyte. -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 10 A 5 A -0.21 V -0.87 V -0.07 V -0.82 V 2 A 10 APotential (V)Potential (V)Current Current(a) (b) Figure 5-1. Cyclic voltammogram at 100 mV s-1 (top) and differential pulse voltammogram (bottom) for (a) complex 15 and (b) complex 13 in CH2Cl2 containing 0.1 M NBun 4PF6 as supporting electrolyte. The indicated potentials are vs Fc/Fc+. In Table 5-1 are included values of the anodic peak current / cathodic peak current ratio (ia/ic) for the -0.07 and -0.21 V oxidation waves of complexes 13 and 15, respectively. The ia/ic ratio has a approximate value of ~1 over the range of scan rates, providing support of the quasi -reversible nature of the oxidation processes.

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133 Potential (V) -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 250 mV/s 300 mV/s 350 mV/s 400 mV/s 450 mV/s 500 mV/s 550 mV/s 600 mV/s 650 mV/s 10 A 1416182022242628 -30 -20 -10 0 10 20 30 Current (A)1/2r2= 0.998 r2= 0.999Current(b) (a) Figure 5-2. Scan rate dependence of oxidation wave at -0 .07 V of complex 13 in CH2Cl2 containing 0.1 M NBun 4PF6 as supporting electrolyte. (a) Cyclic voltammogram at the indicated scan ra tes. (b) Plot of cathodic (top) and anodic (bottom) peak current dependence vs 1/2. Potential (V) -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 150 mV/s 200 mV/s 250 mV/s 300 mV/s 350 mV/s 400 mV/s 450 mV/s 500 mV/s 550 mV/s 10 A 1/2 101214161820222426 -30 -20 -10 0 10 20 30 Current (A)r2= 0.998 r2= 0.993Current(b) (a) Figure 5-3. Scan rate dependence of oxidation wave at -0 .21 V of complex 15 in CH2Cl2 containing 0.1 M NBun 4PF6 as supporting electrolyte. (a) Cyclic voltammogram at the indicated scan ra tes. (b) Plot of cathodic (top) and anodic (bottom) peak current dependence vs 1/2.

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134 Table 5-1. Anodic peak current / cathodic peak current ratios at the i ndicated scan rates in mV s-1 for the -0.07 V and -0.21 V reduction waves of complex 13 and complex 15, respectively. 13 15 Scan Rate (mV s-1) ia/ic Scan Rate (mV s-1) ia/ic 100 0.68 100 1.04 125 0.78 150 0.99 150 0.78 200 1.01 175 0.86 250 1.01 200 0.82 300 1.03 225 0.92 350 1.04 250 0.93 400 1.05 275 0.97 450 1.06 300 0.95 500 1.07 325 1.02 550 1.08 350 1.02 600 1.09 375 1.05 650 1.11 400 1.06 700 1.11 425 1.08 450 1.09 475 1.11 500 1.12 525 1.13 550 1.15 575 1.14 600 1.16 625 1.17 650 1.19 5.2.3 Description of Structures 5.2.3.1 X-ray crystal structure of [Mn12O12(O2CPet)16(MeOH)4] (15) A labeled ORTEP40 plot in PovRay format of complex 15 is shown in Figure 5-4, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 5-2, and selected interato mic distances and angles are listed in Table A-9. The complex crystallizes in the triclinic space group 1 P with the asymmetric unit consisting of the Mn12 molecule and two MeCN molecules of crystallization. The structure of 15 is very similar to other previously characterized neutral Mn12 complexes,13,22 consisting of a central [MnIV 4O4] cubane that is surrounded by a

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135 non-planar ring of eight MnIII atoms that are bridged and connected to the cubane by eight 3-O2ions. The eight MnIII ions separate into two groups of four MnIII ions each. In the first group, each MnIII ion is coordinated to a single MnIV ion via two oxide bridges [Mn(5), Mn(7), Mn(9), Mn(11)], while in the second group each MnIII ion is coordinated to two MnIV ions via two oxide bridges [M n(6), Mn(8), Mn(10), Mn(12)].29 Peripheral ligation is by sixteen bridging PetCO2 ligands and four terminal MeOH groups, which are bound in a 1:1:2 fashion to Mn(8), Mn( 10) and Mn(12), respect ively (Figure 5-4). O31 O30 O171 O170 O121 O120 O15 O20 O21 O60 O61 Mn11 O7 O6 O16 O131 O70 O71 O140 O141 O9 Mn5 Mn9 O50 O51 O150 O151 O12 Mn8 O40 O41 O90 O91 O101 O10 O11 O13 O111 Mn7 O100 O110 Mn1 O2 O3 Mn4 Mn2 O4 O81 O80 O14 O5 Mn12 O160 O161 O8 Mn3 O130 O1 Mn6 Mn10 Figure 5-4. ORTEP representati on in PovRay format of [Mn12O12(O2CPet)16(MeOH)4] (15) with thermal ellipsoids at the 50% probability level except for the C atoms, together with a stereopair. Fo r clarity, the hydrogen atoms have been omitted, and only the quaternary C atoms of the ligands are shown. MnIV green; MnIII blue; O red; C gray.

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136Table 5-2. Crystallographic data for [Mn12O12(O2CPet)16(MeOH)4]2MeCN, [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)]CH2Cl2, [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2]H2OHO2CPet and [Mn4O2(O2CPet)6(bpy)2]2H2O. Parameter 152MeCN 16CH2Cl2 17H2OHO2CPet 182H2O formulaa C104H198N2Mn12O48 C81.5H151Cl1Mn6O32 C79H151Mn9O39 C56H86N4Mn4O16 fw, g mol-1 2903.96 2008.16 2219.49 1291.07 space group 1 P P 21/ c C m c 21 P 21/ c a 16.285(3) 15.980(2) 25.9274(14) 12.9936(10) b 16.556(3) 21.255(3) 19.6851(10) 18.8216(14) c 27.839(5) 30.570(4) 21.1978(12) 14.0822(10) deg 83.524(3) 90 90 90 deg 74.242(3) 101.675(2) 90 107.1740(10) deg 70.340(3) 90 90 90 V 3 6801(2) 10168(2) 10819.0(10) 3290.4(4) Z 2 4 4 4 T C -100(2) -100(2) -55(2) -55(2) radiation, b 0.71073 0.71073 0.71073 0.71073 calc, g cm-3 1.404 1.309 1.356 1.339 cm-1 11.51 8.21 10.89 8.17 R 1 ( wR 2), %c d 8.36 (19.93) 6.97 (17.69) 6.59 (18.05) 5.30 (15.42) a Including solvent molecules. b Graphite monochromator. c R 1 = || Fo| – | Fc|| / | Fo|. d wR 2 = [ [ w ( Fo 2 Fc 2)2] / [ w Fo 2 )2]]1/2 where S = [[ w ( Fo 2 – Fc 2)2] / ( n p )]1/2, w = 1/[ 2( Fo 2) + ( mp )2 + np ], p = [max( Fo 2, 0) + 2 Fc 2]/3, and m and n are constants.

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137 The oxidation levels of the Mn ions a nd protonation levels of the inorganic O atoms were assigned by analysis of the Mn-O bond distances, charge considerations, and bond valence sum (BVS) calculations41 (Table 5-3 and 5-4). As expected, each of the MnIII ions exhibits Jahn-Tel ler (JT) elongation of two trans bonds, as expected for a highspin d4 ion in near-octahedral geometry. Th e JT elongation axes of the eight MnIII ions of the outer ring are aligned approximately para llel to each other, roughly perpendicular to the [Mn12O12] disk-like core of the molecule (Figure 5-5). Figure 5-5. ORTEP representation of the [Mn12O44(MeOH)4] core of complex 15, emphasizing the relative disposition of the Jahn-Teller elongation axes indicated as solid black bonds. MnIV green; MnIII blue; O red; C gray. Table 5-3. Bond valence sum calculationsa for complex 152MeCN. Atom Mn2+ Mn3+ Mn4+ Mn(1) 4.108 3.757 3.944 Mn(2) 4.126 3.774 3.962 Mn(3) 4.075 3.727 3.913 Mn(4) 4.169 3.813 4.003 Mn(5) 3.290 3.009 3.159 Mn(6) 3.274 2.995 3.144 Mn(7) 3.272 2.993 3.142 Mn(8) 3.253 2.975 3.123 Mn(9) 3.289 3.008 3.158 Mn(10) 3.284 3.004 3.153 Mn(11) 3.306 3.024 3.174 Mn(12) 3.236 2.960 3.107 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value.

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138 Table 5-4. Bond valence sum calculationsa for selected oxygen atoms in complex 152MeCN. Atom Vi Assignment Atom Vi Assignment O(1) 2.008 O2O(9) 2.146 O2O(2) 1.968 O2O(10) 2.142 O2O(3) 1.200 O2O(11) 2.054 O2O(4) 2.018 O2O(12) 2.198 O2O(5) 2.118 O2O(13) 1.234 OHO(6) 2.189 O2O(14) 1.264 OHO(7) 2.091 O2O(15) 1.145 OHO(8) 2.162 O2O(16) 1.152 OHa The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0. In addition to sixteen bridging carboxylat e ligands, either three or four water molecules are coordinated to the four MnIII ions in the second group described above [Mn(6), Mn(8), Mn(10), Mn(12)],20 in all but two of the pr eviously characterized Mn12 complexes; the exceptions are the [Mn12O12(O2CMe)12(dpp)4] complex in which there are four five-coordinate MnIII centers and no coordinating water molecules109 and the [Mn12O12(O2CMe)16(MeOH)4] complex in which there are four coordinated MeOH ligands.110 Both complexes have been only recently reported. Instead of water ligands, complex 15 has four MeOH ligands c oordinated to the three MnIII ions of the second group [O(13) to Mn(8), O(14) to Mn(10), O(15) and O(16) to Mn(12)]. This 1:1:2 ligand distribution pattern has also been observed for [Mn12O12(O2CC6H4p -Me)16(H2O)4]3H2O.20,29,57 The Mn12 molecules in the triclinic 1 P lattice are stacked in columns along the b axis of the crystal; all of the molecules are oriented in the same manner with respect to the cell axes. The disordered Pet groups encapsulate the [Mn12O12]16+ core of 15 as shown in Figure 5-6, clearly separating individual Mn12 molecules from their neighbors. Similarly, But groups have been found to result in the same separation between molecules

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139 in other Mn aggregates with ligation by ButCH2CO2 56,63 Hence, there is no evidence of intermolecular interactions in 15. Disordered MeCN solvent molecules of crystallization are situated in the large voids separating the columns although their cl ear visualization is precluded by the use of the program SQUEEZE96 to remove their overall electron density as discussed in the Experimental Section. Figure 5-6. Space-filling diagram of complex 15 including all non-hydrogen atoms. Mn blue; O red; C gray. 5.2.3.2 X-ray crystal structure of [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] (16) A labeled ORTEP40 plot in PovRay format of complex 16 is shown in Figure 5-7, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 5-2, and selected interato mic distances and angles are listed in Table A-10. The complex crystallizes in the monoclinic space group P 21/ c with the Mn6 molecule in a general position. The structure consists of a [MnIII 6(3-O)2(4-O2CH2)]12+ core (Figure 5-7) with th e peripheral ligation provid ed by eleven bridging PetCO2 ligands, one bridging MeCO2 ligand, and two terminal PetCO2H ligands. Bond valence sum calculations41 indicate that all of th e distorted octahedral Mn centers are at the +3

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140 oxidation level (Table 5-5). Th e protonation levels of the inorganic O atoms are collected in Table 5-6. Mn6 Mn5 Mn4 Mn3 Mn2 Mn1 O130 O131 O1 O2 O40 O41 O90 O91 O80 O81 O20 O21 O10 O11 O3 O4 O110 O111 O30 O31 O120 O121 O70 O71 O100 O101 O140 O141 O51 O50 O61 O60 C131 Figure 5-7. ORTEP representa tion in PovRay format of [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] (16) at the 50% probability level except for the C atoms, together with a stereopair. For clarity, the hydrogen atoms have been omitted, and only the quaternary C atoms of the ligands are shown. MnIII blue; O red; C gray. The six manganese ions are arranged in a twisted boat conformation, which can be described as two trinuclear [Mn3(3-O)]7+ units linked at one of their edges by two -PetCO2 and one 4-O2CH2 2groups. The dihedral angl e between the Mn(2)-Mn(3)Mn(4) and Mn(1)-Mn(5)-Mn(6) pl anes is 57.0. The central 3-O2ion O(3) in one triangular unit is slightly (0.04 ) out of the Mn3 plane that it brid ges [Mn(1), Mn(5) and

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141 Mn(6)]. The other 3-O2ion O(4) is essentially coplan ar with the three Mn centers [Mn(2), Mn(3) and Mn(4)]. The remaining two Mn2 edges within each triangular unit are bridged by two -PetCO2 groups. There is an additional PetCO2 group bridging Mn(1) and Mn(5), but the corresponding pair, Mn(2) and Mn(4), in the other triangular unit is bridged by an acetate group. Peripheral ligati on at both Mn(3) and Mn(6) is completed by two monodentate PetCO2H groups. The Mn-O(3) and Mn-O(4) bond distances and Mn-O(3)-Mn and Mn-O(4)-Mn angles within each triangular unit are inequivalent, and the triangles are thus scalene. Only two other examples of a 4-O2CH2 2bridging unit as found in 16 have been previously obs erved in the compounds (NBun 4)[CH2Mo4O15H]142 and [Fe6O2(O2CH2)(O2CCH2But)12(py)2],143 a compound very similar to 16. Complexes containing somewhat similar bridging motifs include Na[Fe4(dhpta)2(-O)(-OH)(ala)2] and [Mn4(dhpta)2(-O)(-OH)(-O2CMe)2]4(H5dhpta = 2hydroxytrimethylenedinitriloacetic acid and ala = alanine), where eith er four Fe or Mn ions are bridged by a hydrogen-bonded O2-HOunit rather than a single diolate unit as in 5.144,145 The overall structure of 16 as two M3 triangular units linke d at one edge has not been seen before in Mn chemistry, although it is commonly encountered in Fe chemistry.146-149 Table 5-5. Bond valence sum calculationsa for complex 16CH2Cl2. Atom Mn2+ Mn3+ Mn4+ Mn(1) 3.214 2.940 3.087 Mn(2) 3.229 2.954 3.101 Mn(3) 3.170 2.900 3.044 Mn(4) 3.196 2.923 3.069 Mn(5) 3.240 2.964 3.112 Mn(6) 3.163 2.893 3.038 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value.

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142 Table 5-6. Bond valence sum calculationsa for selected oxygen atoms in complex 16CH2Cl2. Atom Vi Assignment O(3) 2.019 O2O(4) 2.002 O2O(130) 1.996 O2O(131) 2.014 O2O(110) 2.149 O2O(111) 1.012 OHO(140) 1.222 OHO(141) 2.176 O2a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0. Each of the octahedral MnIII centers in 16 displays a JT axial elongation (Figure 5-8). Normally, JT elongation axes avoid Mn -oxide bonds, almost always the strongest and shortest in the molecule, but in the case of Mn(3) and Mn(6) the JT elongation axis is situated in an abnormal position containing a core O2ion, O(4) and O(3), respectively. The JT elongation axes of Mn(2) and Mn(4) ar e oriented nearly pa rallel to each other along Mn-O(carboxylate) bonds. In addition, the JT elongation axes of Mn(1) and Mn(5) are along Mn-O(carboxylate) a nd Mn-O(gem-diolate) bonds, a nd are almost parallel to each other, but essentially perpendicular to those of Mn(2) and Mn(4). This will be of relevance to the magnetic discussion later ( vide infra ). Mn6 Mn5 Mn1 Mn2 Mn3 Mn4 O110 O30 O21 O120 O11 O121 O31 O20 O71 O70 O2 O4 O40 O10 O131 O41 O130 O101 O60 O100 O1 O91 O50 O3 O81 O61 O80 O141 O51 O90 Figure 5-8. ORTEP representation of the [Mn6O30CH2] core of complex 16, emphasizing the relative disposition of the Jahn-Te ller elongation axes indicated as solid black bonds.

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143 5.2.3.3 X-ray crystal structure of [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] (17) A labeled ORTEP40 plot in PovRay format of complex 17 is shown in Figure 5-9, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 5-2, and selected interato mic distances and angles are listed in Table A-11. Complex 17 has crystallographic Cs symmetry, the mirror plane containing Mn(1), Mn(2) and Mn(3). The structure consists of a [MnIII 9(3-O)6(-OH)(3-CO3)]12+ core with peripheral ligation provide d by twelve bridging PetCO2 ligands and two terminal water molecules. Mn3 Mn5 Mn4 Mn6 Mn6a C1 O6 O6a O5 O4 O19 O19a O16 O16a O15 O15a O20 O21 O14 O14a O2 O2a O8 O7 O13a O13 Mn5a O12 O12a O11a O11 O17 O17a O1 O1a O9 O9a Mn1 Mn2 O18 O18a Mn4a O3 O3a O10 O10a Figure 5-9. ORTEP representa tion in PovRay format of [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] (17) at the 30% probability level except for the C atoms, together with a ster eopair. For clarity, the hydrogen atoms have been omitted, and only the quaternary C atoms of the ligands are shown. MnIII blue; O red; C gray.

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144 The base of the [Mn9O6(OH)(CO3)] core may be viewed as containing a [Mn7O4] subunit consisting of two [Mn4O2] “butterfly” units [atoms Mn(1), Mn(2), Mn(4), Mn(4a) and Mn(2), Mn(3), Mn(5), Mn(5a)] fused at five-coordinate Mn(2) (Figure 5-10a). These two fused “butterfly” units form a basketlike subunit of the molecule. The resultant [Mn3O4] base of the basket [Mn(1), Mn(2), Mn(3), O(1), O(2)] is not pl anar but bent with Mn(1)-Mn(2)-Mn(3) = 169.1. The remain ing two Mn atoms, Mn(6) and Mn(6a), represent the “handle” of the basket and are connected to the fusedbutterfly unit by a 1, 1, 1, 3-CO3 2group through atoms O(5), O(6) and O(6a) and two 3-O2ions [O(3) and O(3a)]. One OHion [O(4)] and one carboxylate ligand bridge Mn(6) and Mn(6a) across the mirror plane of the molecule. (a)(b)Mn6 Mn6a C1 O6 O6a O5 O4 Mn1 Mn2 Mn3 O2 O2a O1 O1a Mn5 Mn5a O3 O3a Mn4 Mn4a Mn6 Mn6a C1 O6 O6a O5 O4 Mn1 Mn2 Mn3 O2 O2a O1 O1a Mn5 Mn5a O3 O3a Mn4 Mn4a O19O19a O16O16a O15O15a O18 O18a O10 O10a O17a O11a O12a O13a O17 O11 O12 O13 O14 O14a O21 O8 O7 O9 O9a O20 Figure 5-10. ORTEP representations in PovRay format of (a) the [Mn9O6(OH)(CO3)]12+ core of complex 17 and (b) the relative disposi tions of the el ongation axes, indicated as solid black bonds. MnIII blue; O red; C gray. Bond valence sum (BVS) calculations41 indicate that all the Mn ions are in the +3 oxidation state (Table 5-7) and the protona tion levels of the inorganic O atoms are collected in Table 5-8. Atom Mn(2) is fivec oordinate with exactly square pyramidal (sp) geometry ( = 0, where is 0 and 1 for ideal square pyramidal and trigonal bipyramidal

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145 geometries, respectively71) as a result of the crystallographically imposed mirror plane symmetry. Atoms Mn(4) and Mn(5) are also fi ve-coordinate with sp and distorted sp geometries ( = 0.01 and 0.22, respectively). The remaining MnIII centers all possess distorted octahedral geomet ries. Each octahedral MnIII displays a clear JT axial elongation, with axial Mn-O bonds approximate ly 0.20 longer than equatorial ones. For Mn(2), the axial Mn(2)-O (8) length [2.192(6) ] is longer than basal lengths [1.900(3) -1.906(4) ], as expected for sp geometry. Similarly for square pyramidal Mn(4) and Mn(5) [Mn(4)-O(11) = 2.081(5) an d Mn(5)-O(12) = 2.041(5) ]. In effect, there is parallel alignment of the JT distortion axes of the three MnIII ions of the [Mn3O4] base [Mn(1), Mn(2), Mn(3)], whereas there is an overall random alignment of the distortion axes of the remaini ng Mn centers (Figure 5-10b). Th is will be of relevance to the magnetic discussion later ( vide infra ). The Mn9 core of 17 is very similar to that in [Mn9O7(O2CPh)13(py)2],150 [Na2Mn9O7(O2CPh)15(MeCN)2]131 and [K2Mn9O7(O2CBut)15(HO2CBut)2].151 The core also appears as a subunit in the larger cluster [Mn22O12(O3PPh)8(O2CEt)22(H2O)8].132 The primary difference between complex 17 and these other Mn9 clusters is the triply bridging CO3 2ligand that fuses the “handle” to the [Mn7O4] “basket” subunit. Triply bridging carbonate ions are somewh at rare in discrete transiti on metal clusters, especially in Mn chemistry. Known clusters with such 1, 1, 1, 3-CO3 2ligands include a handful of molecules containing Cu,152 Z,152d,152e,153 V,154 and Mo.155 To our knowledge, complex 17 is the first example of a triply bridging CO3 2ion in molecular Mn chemistry. There is, however, a doubly bridging, 1, 1,-CO3 2ligand in MnII chemistry, although it was not crystallographically confirmed.141 A heterometallic Mn16 cluster in which a carbonate

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146 group bridges equatorial Ba2+ ions has also been reported.156 The carbonate ligand in 17 is not planar; the carbon atom C(1) is slight ly (0.18 ) out of the plane formed by the three oxygen atoms [O(5), O(6) and O(6a)]. All previous 1, 1, 1, 3-CO3 2groups are planar, and we assign the slig htly pyramidal structure in 17 to the strain the CO3 2group experiences in the bridging mode it adopts. The bond distances C(1 )-O(5) (1.274(8) ) and C(1)-O(6) and C(1)-O(6a) (1.303(5) ) are in agreement with a CO3 2ion. The C-O bond lengths in formaldehyde, free carbonate ani on, and methanol are 1.22, 1.33 and 1.43 , respectively. Table 5-7. Bond valence sum calculationsa for complex 17H2OHO2CPet. Atom Mn2+ Mn3+ Mn4+ Mn(1) 3.277 2.997 3.147 Mn(2) 3.070 2.808 2.948 Mn(3) 3.039 3.027 3.177 Mn(4) 3.178 2.906 3.051 Mn(5) 3.186 2.914 3.059 Mn(6) 3.311 3.028 3.179 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value. Table 5-8. Bond valence sum calculationsa for selected oxygen atoms in complex 17H2OHO2CPet. Atom Vi Assignment O(1) 2.137 O2O(2) 2.116 O2O(3) 2.164 O2O(4) 1.568 OHO(20) 0.270 H2O O(21) 0.259 H2O a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0. 5.2.3.4 X-ray crystal structure of [Mn4O2(O2CPet)6(bpy)2] (18) A labeled ORTEP40 plot in PovRay format of complex 18 is shown in Figure 5-11. The crystallographic data and structure refineme nt details are collected in Table 5-2, and

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147 selected interatomic distances and angles are listed in Table A-12. Complex 182H2O crystallizes in the monoclinic space group P 21/ c The asymmetric unit contains half the molecule and two H2O solvent molecules. The structure consists of a [MnIII 2MnII 2(3-O)2]6+ core with peripheral ligat ion provided by six bridging PetCO2 ligands and two terminal bipyridine molecules. BVS calculations41 indicate that Mn(1) and Mn(1a) are in the +3 oxidation state whil e Mn(2) and Mn(2a) are in the +2 oxidation level (Table 5-9) and also that tr iply bridging O(1) and O(1a) are O2(Table 5-10). Each MnII center is in a distorted octahe dral environment while the two MnIII centers are fivecoordinate with distorted s quare pyramidal geometry ( = 0.16).71 Mn2 N2 N1 N2a N1a Mn2a Mn1 Mn1a O1a O1 O6 O7 O7a O6a O4 O5 O2 O3 O4a O2a O3a O5a Figure 5-11. ORTEP representati on in PovRay format of [Mn4O2(O2CPet)6(bpy)2] (18) at the 50% probability level except for C at oms, together with a stereopair. For clarity, the hydrogen atoms have been omitted, and only the quaternary C atoms of the ligands are shown. MnIII blue; MnII orange; O red; C gray; N cyan.

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148 The complex possesses an exactly planar array of four Mn atoms bridged by two 3-oxide atoms, O(1) and O(1a), one above and one below the Mn4 plane. The distance of the 3-oxygen atoms above or below the Mn4 plane is 0.60 . The [MnIII 2MnII 2(3-O)2]6+ core can be considered as two edge-sharing [Mn3O] units. Each edge of the Mn4 rhombus is bridged by either one or two -PetCO2 groups. The edges bridged by only one t -pentylate ligand have a slight ly longer MnMn separation [3.465 ] than those bridged by two t -pentylate groups [3.274 ]. The central Mn(1) Mn(1a) separation is significantly shorter [2.775 ], consistent with the two oxide bridges. Two terminal bpy groups complete the peripheral ligation, one at each end of the molecule. The local z axes of the two five-coordinate MnIII centers, Mn(1)-O(4) = 2.089(2) and Mn(1a)-O(4a) = 2.089(2), are oriented parallel to each other. The overall symmetry of the complex is Ci. The overall structure of complex 18 is nearly identical to that observed in [Mn4O2(O2CMe)6(bpy)2].52 Table 5-9. Bond valence sum calculationsa for complex 182H2O. Atom Mn2+ Mn3+ Mn4+ Mn(1) 3.163 2.893 3.037 Mn(2) 2.060 1.919 1.959 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value. Table 5-10. Bond valence sum calculationsa for selected oxygen atoms in complex 182H2O. Atom Vi Assignment O(1) 2.039 O2a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0.

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149 5.2.4 Magnetochemistry of Complexes 15 19 5.2.4.1 DC studies of 15 Variable-temperature DC susceptibility measurements were performed in the 5.0-300 K range on a powdered mi crocrystalline sample of 152CH2Cl2, restrained in eicosane to prevent torquing, in a 5 kG field (Figure 5-12). The value of MT in the 150-300 K range, 20-22 cm3 K mol-1, rapidly increases to a maximum of 53 cm3 K mol-1 at 15 K before decreasing rapi dly at lower temperatures. The maximum value indicates a large ground state spin (S) value and the decrease of MT at low temperatures is primarily due to Zeeman and zero-field splitting (ZFS) effects. 050100150200250300 15 20 25 30 35 40 45 50 55 Temperature (K)MT (cm3K mol-1) Figure 5-12. Plot of MT vs temperature for a dried, microcrystalline sample of complex 152CH2Cl2 in eicosane. M is the DC molar magnetic susceptibility measured in a 5.0 kG field. A theoretical treatment of the susceptibil ity data using the Kambe vector coupling approach51 was not feasible owing to the topolo gical complexity and low symmetry of the Mn12 cluster. Instead, our efforts were focused on the determination of the ground state spin by variable-temperature and -field magnetization (M) measurements in the 1.8-4.00 K temperature and 0.5-7 T field ra nges. The data are plotted as reduced magnetization (M/NB) versus H/ T in Figure 5-13a, where N is Avogadro’s number, B is

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150 the Bohr magneton, and H is the applied ma gnetic field. For complexes populating only the ground state and experiencing no ZFS, the magnetization follows the Brillouin function and the isofield lines all supe rimpose and saturate at a value of g S. The data in Figure 5-13a show that the isofield lines do not superimpose for complex 15, indicating that the ground state is zero-f ield split. The data were fit using the method described elsewhere49,52 that involves diagonalization of the spin Hamiltonian matrix, assuming only the ground state is occupied at these temperatures, and including axial ZFS Zeeman interactions and a full powder average of the magnetization.54 Fitting of the data gave S = 10, g = 2.03, and D = -0.38 cm-1. These values are t ypical of neutral Mn12 complexes; complex 15 has the same spin as its parent complex 1. Thus, replacement of the water ligands with methanol groups does not signi ficantly perturb the properties of the Mn12 complex.112 gD (cm-1)(a) (b) 010203040 8 10 12 14 16 18 20 0.5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T M/NBH/T (kGK-1) 1.81.92.02.12.2 -0.6 -0.5 -0.4 -0.3 -0.2 Figure 5-13. Determination of ground state sp in. (a) Plot of reduced magnetization M/NB vs H/ T for a dried, microcryst alline sample of complex 152CH2Cl2 in eicosane; the DC field value of each of the isofield plots is indicated. (b) Twodimensional contour plot of the error surface for the D vs g fit for complex 152CH2Cl2. The asterisk indicates the soft minimum. In order to confirm that the obtained parame ters were the true global rather than a local minimum, and to assess the uncertainty in the obtained g and D values, a root-mean

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151 square D vs g error surface for the fit was ge nerated using the program GRID.55 The error surface is shown in Figure 5-13b as a contour plot for the D = -0.10 to -0.60 cm-1 and g = 1.8 to 2.25 ranges. One soft fitting minimum is observed; the contour describes the region of minimum error from D -0.30 to -0.49 cm-1 and g 1.94 to 2.14, giving fitting parameters of D = -0.39 0.10 cm-1 and g = 2.0 0.1. 5.2.4.2 AC studies of 15 To confirm, as expected, that this new Mn12 complex is a SMM, and to assess further the influence of its terminal MeOH groups instead of the usual H2O ones, AC magnetic susceptibility studies were carried out on a dried, microcrystalline sample of 152CH2Cl2 in the 1.8-10 K range in a 3.5 G AC field with oscill ation frequencies ( ) up to 1488 Hz. All other previously characterized Mn12 complexes exhibit at least one frequency-dependent out-of-phase ( M ) signal that is accompanied by a frequencydependent decrease in the in-phase ( M) signal, indicating slow magnetization relaxation. Although such signals are not sufficient proof of the SMM property, they are a strong indicator that a complex behaves as a S MM. Indeed, frequency-dependent out-of-phase ( M ) AC susceptibility signals ar e clearly exhibited by complex 152CH2Cl2 (Figure 5-14). In fact, there are two such signals, corresponding to two distinct relaxation processes, a higher-temperature (HT) peak at ~ 6 K and a lower-temperature (LT) peak at ~ 2.5 K. The signals are accompanied by two frequency-dependent decreases in the inphase M T plot, first at T ~ 7.5 K and then at T ~ 3 K, respectively, indicating that the magnetization of 15 cannot relax fast enough to stay in -phase with the oscillating field and that complex 15 is most likely a SMM. The value of M T in the temperatureindependent region of Figure 5-14 is especially useful for estimating the ground state spin without interference from ev en a small DC field. The M T value above 7 K of ~54 cm3 K

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152 mol-1 for 15 corresponds to an S = 10 system with g = 1.98, consistent with the DC magnetization results above. MT(cm3K mol-1) M(cm3mol-1)Temperature (K) Temperature (K) 246810 0 1 2 3 1000 Hz 250 Hz 50 Hz 0 10 20 30 40 50 60 Figure 5-14. Plot of the in-phase (as M T ) and out-of-phase ( M ) AC susceptibility signals vs temperature for a dried, microcrystalline sample of complex 152CH2Cl2 in eicosane at the indicat ed oscillation frequencies. The presence of two peaks in the out-of-phase AC susceptibility plot of 15 is typical of Mn12 complexes, and has been shown previ ously to be due to Jahn-Teller (JT) isomerism, in which complexes differ in the relative orientation of one or more MnIII JT elongation axes.56-59 The LT (faster-relaxing) isomer is the one with the abnormal orientation of the JT axis towards the bridging oxide ions, whereas the HT (slowerrelaxing) form is that with all JT axes avoiding bridging oxide ions. Normally, the HT form is the predominant form in wet crystals, with the LT form often becoming more

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153 pronounced in dried solids as interstitial solven t of crystallization is lost. Indeed, the HT:LT ratio in dried solid of 15 is 3:1 (Figure 5-14), whereas it is 19:1 in wet crystals. The significantly smaller amount of the LT form in the latter also ra tionalizes why it is not observed in the X-ray crystal structure of the complex, since the small fraction of the LT form and the likely static disorder of the abnormally-oriented ax is over multiple sites will dilute its effect on th e structural parameters. At the M peak maximum, the magne tization relaxation rate (1/ where is the relaxation time) is equal to the angular frequency (2 ) of the AC field, and thus M versus studies provide rate vs T kinetic data,157 and these were fit to the Arrhenius equation (eq 5-4). This is th e characteristic behavior of a thermally-activated Orbach process, where Ueff is the effective energy barrier, k is the Boltzmann constant, and 1/ 0 is the pre-exponential term. 1/ = 1/ 0 exp(-Ueff/k T ) (5-4) The frequency dependencies of the M peaks for 15 were determined at different oscillation frequencies in the 51500 Hz range. A plot of ln(1/ ) vs 1/ T using this M vs T data is shown in Figure 5-15, with the least-sq uares fit to eq 5-4 shown as solid lines for both the HT ( ) and LT () signals. The effective energy barrier to magnetization relaxation (Ueff) for the HT signal (slower-relaxing species) is 43 cm-1 (62 K) while that of the LT signal (faster-relaxing species) is much smaller, 24 cm-1 (35 K). These are very similar to the values previously found for such HT and LT signals within the [Mn12O12(O2CR)16(H2O)4] family.56 The values of the pre-exponential factor, 1/ 0, 1.29108 s-1 for the HT peak and 2.98 1010 s-1 for the LT peak, are also within the range normally found for the [Mn12O12(O2CR)16(H2O)4] family.

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154 0.100.150.200.250. 300.350.400.450.50 3 4 5 6 7 8 9 1/T (K-1)ln(1/ ) Figure 5-15. Plot of the natural l ogarithm of relaxation rate, ln(1/ ), vs 1/ T for a dried, microcrystalline sample of complex 152CH2Cl2 in eicosane using AC M data; ( ) corresponds to the HT peak (s lower-relaxing species) and () corresponds to the LT peak (f aster-relaxing species). The solid lines are fits to the Arrhenius equation; see the text for the fit parameters. Finally, we should add that we have not complemented the above results on 15 with single crystal hysteresis studies using a micro-SQUID apparatu s to investigate the effect, if any, of the bulky carboxylate groups on the hysteresis loops. While we are very interested in this general point, such studies are instead currently in progress in great detail with the ButCH2CO2 derivatives, and these result s will be reported separately. 5.2.4.3 DC and AC susceptibility studies of complexes 16 19 Variable-temperature DC susceptibility measurements were performed on dried, microcrystalline samples of 16CH2Cl24H2O, 17HO2CPet, 182H2O and 19MeNO2, restrained in eicosane to prev ent torquing, in a 1 kG field in the range of 5.0-300 K. For 16, MT smoothly decreases from 15.8 cm3 K mol-1 at 300 K to 3.9 cm3 K mol-1 at 5.0 K (Figure 5-16). The value at 300 K is less than the 18 cm3 K mol-1 spin-only value ( g = 2.0) expected for a MnIII 6 complex with non-interacting metal centers, indicating the presence of appreciable antif erromagnetic interactions between the manganese centers. For 17, the MT value of 4.0 cm3 K mol-1 at 300 K decreases gradually with decreasing

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155 temperature to 0.35 cm3 K mol-1 at 5.0 K (Figure 5-17a), while the MT value for 19 of 19.3 cm3 K mol-1at 300 K decreases gr adually to 2.4 cm3 K mol-1 at 5.0 K (Figure 5-18a). The spin-only ( g = 2) value for a molecule composed of non-interacting MnIII 9 ions is 27 cm3 K mol-1. The MT value of 7.4 cm3 K mol-1 at 300 K for complex 18 gradually decreases with decreasing temperature to 3.5 cm3 K mol-1 at 5.0 K (Figure 5-19a). The value of MT at 300 K is less than the spin-only value for a unit composed of non-interacting MnII 2MnIII 2 ions (14.75 cm3 K mol-1). Hence, the molecules all appear to have appreciable intramolecular antiferromagnetic interactions and relatively low or zero ground state spin values. 050100150200250300 2 4 6 8 10 12 14 16 18 Temperature (K)MT (cm3K mol-1) Figure 5-16. Plot of MT vs temperature for a dried, microcrystalline sample of complex 16CH2Cl24H2O. M is the DC molar magnetic su sceptibility measured in a 1 kG field. Complex 16 contains six MnIII centers with total spin values ranging from 0 to 12 while complexes 17 and 19 each contain nine MnIII centers with total spin values ranging from 0 to 18. Again, it is not possible to eas ily evaluate the various exchange parameters between the Mn centers as a re sult of the complexity and low symmetry of the clusters. In addition, magnetization data collected in the 0.1-70 kG and 1.8-10.0 K field and temperature ranges could not be satisfactorily fit to a model that assumes only the ground

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156 state is populated. This beha vior is typical of (i) comp lexes with low ground state S values, which cross with excited states with la rger S values as a result of Zeeman effects in the applied DC field, and/or (ii) lowlying excited states from weak exchange interactions and/or sp in frustration effects85,86 within the complicated topologies present in most of these complexes. The exception to the latter is tetranuclear complex 7, but even this has MnII/MnIII interactions, which ar e typically very weak, and spin frustration within the triangular Mn3 subunits. In any event, further analysis of the exchange interactions within 18 was not pursued since the DC magne tic susceptibility behavior of such “butterfly” clusters has been already thoroughly studied.52 Overall, the low temperature DC magnetic susceptibility studies and the profiles of the susceptibility vs T plots suggest small ground state spin values of S 2 for complexes 16CH2Cl24H2O, 17HO2CPet, 182H2O and 19MeNO2. Further assessment of the values was carried out using AC susceptibility studies in a 3.5 G AC field oscillating at frequencies up to 997 Hz. While the appearan ce of an out-of-phase AC susceptibility signal ( M ) is indicative of the onset of slow magnetic relaxa tion, the in-phase signal ( M) can indicate, as mentione d earlier, the ground state S value and whether low-lying excited states are populated even at very low temperatures. Specifically, a temperatureindependent M T vs T plot indicates that only the ground state spin state is populated, while a sloping M T vs T plot indicates population of low-lying excited states. As expected on the basis of the DC studies, the M T vs T plots of each of the complexes are strongly sloping with decreasing values of M T with decreasing temperature, indicating population of excited states with greater S va lues than the ground state. Comparison of the AC susceptibility data ( M T ) with the DC data ( MT ) over the same T range shows

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157 that the two are essentially superimposable for each of the clusters, as shown in Figure 5-17b for representative complex 17HO2CPet. 05101520253035 0 1 2 3 4 5 6 7 MT and MT (cm3K mol-1)Temperature (K) 050100150200250300 0 1 2 3 4 Temperature (K)MT (cm3K mol-1)(a) (b) Figure 5-17. Determination of ground state spin. (a) Plot of MT vs temperature for a dried, microcrystalline sample of complex 17HO2CPet. M is the DC molar magnetic susceptibility measured in a 1 kG field. (b) Plot of M T vs temperature for complex 17HO2CPet in the 2.0-30.0 K range from AC magnetic susceptibility measurements (), and including the DC MT data ( ) for this temperature range. The AC data were measured with a 3.5 G AC field oscillating at 997 Hz. 051015202530 0 2 4 6 8 Temperature (K) Temperature (K) 050100150200250300 0 2 4 6 8 10 12 14 16 18 20 22 (a) (b)MT and MT (cm3K mol-1) MT (cm3K mol-1) Figure 5-18. Determination of ground state spin. (a) Plot of MT vs temperature for a dried, microcrystalline sample of complex 19MeNO2. M is the DC molar magnetic susceptibility measured in a 1 kG field. (b) Plot of M T vs temperature for complex 19MeNO2 in the 2.0-30.0 K range from AC magnetic susceptibility measurements (), and including the DC MT data ( ) for this temperature range. The AC data were measured with a 3.5 G AC field oscillating at 997 Hz.

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158 05101520253035 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Temperature (K) 050100150200250300 3 4 5 6 7 8 Temperature (K)(a) (b)MT and MT (cm3K mol-1) MT (cm3K mol-1) Figure 5-19. Determination of ground state spin. (a) Plot of MT vs temperature for a dried, microcrystalline sample of complex 182H2O. M is the DC molar magnetic susceptibility measured in a 1 kG field. (b) Plot of M T vs temperature for complex 182H2O in the 2.0-30.0 K range from AC magnetic susceptibility measurements (), and including the DC MT data ( ) for this temperature range. The AC data were measured with a 3.5 G AC field oscillating at 997 Hz. Linear extrapolation suggests that the plot for 17 is heading for MT 0.6 cm3 K mol-1 at 0 K, a value consistent with at most an S = 1 ground state spin, and more likely S = 0. Similarly, the AC data for 16, 18 (Figure 5-19b) and 19 (Figure 5-18b) suggest S 2 (and probably 1 or 0), but with very low-lyi ng excited states that are populated even at 1.8 K and which make a more precise assignment of a ground state value very difficult. 5.3 Conclusions The use of the basic, hydrophobic carboxylic acid, t -pentylic acid, as a reactant in Mn12 chemistry has led to the isolation of seve ral new Mn clusters, two of which are new structural types in Mn chemistry. The reaction of [Mn12O12(O2CPet)16(H2O)4] (14) with MeOH in the presence of t -pentylic acid has led to the isolation of [Mn12O12(O2CPet)16(MeOH)4] (15), a new member of a sub-class of Mn12 clusters in which the water ligands that coordi nate to either three or four MnIII ions in the outer ring of the cluster have been replaced by another ligand, MeOH. Magnetic studies on 15

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159 suggest that the complex retains an S = 10 gr ound state spin as expected for a neutral Mn12 cluster. Additionally, peaks in the out-o f-phase AC susceptibility indicate the onset of slow magnetization re laxation and suggest that 15 functions as a SMM. In contrast, recrystallization of 14 from CH2Cl2/MeNO2 in the presence of t -pentylic acid causes a structural change a nd affords two new structural types in Mn chemistry, [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] (16) and [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] (17). Complex 16 contains only the third example of a 4-methanediolate unit, while complex 17 possesses a non-planar triply bridging CO3 2anion and is the first example of a disc rete complex with Mn ions bridged by a carbonate group. These investigations emphasize the differences in the reactivity of the nearly identical Mn12 complexes, 13 and 14; recrystallization of 13 under similar conditions results in th e retention of the [Mn12O12]16+ core. In addition, the reaction of 14 with the chelating ligand, 2,2 -bipyridine, has afforded [Mn4O2(O2CPet)6(bpy)2] (18). Structural rearrangement of the [Mn12O12]16+ core is also observed when 13 undergoes reactions with other chelating ligands. Further investigations to probe the reactivity of 13 with various solvents, including THF, have le d to the isolation of a new enneanuclear cluster, [Mn9O7(O2CCH2But)13(THF)2] (19). Magnetic studies on complexes 16-19 suggest ground state spin values of S 2 and show that the complexes do not function as single-molecule magnets. Electrochemical studies on both complexes 13 and 15 reveal that in contrast to most other characterized Mn12 clusters, these molecules exhibit irreversible reduction processes, supporting the proposal that reductive destabiliz ation of the [Mn12O12] core followed by structural rearrangement occurs to yield new clusters of differing nuclearity

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160 and topology. While the strongl y electron donating nature of the ligands coordinated to the metal centers in 13 and 15 accounts for the unwillingness of the cluster to accept another electron reversibly, it also accounts for the quasi-r eversible oxidation wave, a process that occurs at a pote ntial nearly one volt lower than previously found for other Mn12 clusters. 5.4 Experimental 5.4.1 Syntheses All manipulations were performed under aerobic conditions us ing materials as received, except where otherwise noted. [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1)50 and [Mn12O12(O2CCH2But)16(H2O)4] (13)13 were prepared as described elsewhere. [Mn12O12(O2CPet)16(H2O)4] (14). A solution of complex 1 (2.0 g, 0.97 mmol) in MeCN (75 cm3) was treated with a solution of HO2CPet (3.89 cm3, 31.0 mmol) in CH2Cl2 (25 cm3). The solution was stirred overnight, and the solvent was then removed in vacuo The residue was dissolved in toluene (20 cm3), and the solution was again evaporated to dryness. The addition and removal of toluene was repeated two more times. The residue was then dissolved in CH2Cl2 (75 cm3) and treated again with HO2CPet (3.89 cm3, 31.0 mmol) in CH2Cl2 (25 cm3). After 3 h, three more cycles of addition and removal of toluene were performed. The residue was re-dissolved in CH2Cl2 (50 cm3), the solution was filtered, and MeNO2 (50 cm3) was added. The solution was then maintained undisturbed at 4 C for 4 days. The resulting bl ack crystals were collected by filtration, washed with MeNO2, and dried in vacuo ; the yield was ~83%. Anal. Calcd (found) for 14CH2Cl2 (C97H186Cl2O48Mn12): C, 40.87 (40.78); H, 6.58 (6.53); N, 0.00 (0.00). Selected IR data (cm-1): 1584 (s), 1557 (s), 1524 (s), 1499 (m), 1477 (vs), 1459 (m), 1419 (vs), 1374 (s), 1356 (m), 1326 (m), 1284 (m), 1229 (w), 1209 (m), 1061 (w), 1006

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161 (w), 931 (w), 883 (w), 801 (w), 786 (w), 726 (m), 680 (m), 613 (s), 545 (m), 508 (w), 473 (w), 440 (w). [Mn12O12(O2CPet)16(MeOH)4] (15). A solution of complex 14 (0.40 g, 0.14 mmol) in CH2Cl2 (20 cm3) was treated with HO2CPet (2.0 cm3, 16.0 mmol) and MeOH (10 cm3). The solution was maintained undisturbed at 4 C for 6 days. The resulting black crystals were collected by filtration, washed with MeOH, and dried in vacuo ; the yield was ~82%. A sample for crystallography was maintained in contact with the mother liquor to prevent the loss of intersti tial solvent. Anal. Calcd (found) for 152CH2Cl2 (C102H196Cl4O48Mn12): C, 40.95 (40.79); H, 6.60 (6.68); N, 0.00 (0.00). Selected IR data (cm-1): 1587 (s), 1559 (s), 1524 (s), 1479 (vs), 1459 (m), 1419 (vs), 1374 (s), 1359 (m), 1324 (m), 1289 (m), 1206 (m), 1179 (m), 1069 (w), 1008 (w), 933 (w), 873 (w), 803 (w), 783 (w), 721 (m), 670 (m), 638 (m), 615 (s), 543 (m), 510 (w), 478 (w), 445 (w). [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] (16). A solution of complex 14 (0.40 g, 0.14 mmol) in CH2Cl2 (20 cm3) was treated with HO2CPet (2.0 cm3, 16.0 mmol). The solution was layered with MeNO2 (80 cm3), and black crystals slowly grew. After one week, the crystals were collected by filtration, washed with MeNO2, and dried in vacuo ; the yield was ~10%. A sample for crysta llography was maintained in contact with the mother liquor to prevent th e loss of inters titial solvent. Dried so lid appeared to be hygroscopic. Anal. Calcd (found) for 16CH2Cl24H2O (C81.5H159Cl1O36Mn6): C, 47.06 (47.09); H, 7.70 (7.73); N, 0.00 (0.11). Selected IR data (cm-1): 1710 (m), 1627 (vs), 1539 (vs), 1477 (vs), 1459 (s), 1437 (m), 1417 (s), 1359 (m), 1326 (w), 1286 (s), 1204 (s), 1066 (m), 1051 (m), 1008 (m), 971 (m), 928 (w), 886 (w), 803 (m), 781 (m), 763 (w), 678 (m), 635 (s), 603 (m), 545 (m), 475 (m), 445 (m).

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162 [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] (17). To a solution of complex 14 (0.40 g, 0.14 mmol) in CH2Cl2 (20 cm3) was added HO2CPet (2.0 cm3, 16.0 mmol). The solution was layered with MeNO2 (80 cm3), and black crystals of 16 slowly grew. After two weeks, these were removed by filtration thr ough Celite, and the filtrate was evaporated to dryness in vacuo The residue was re-dissolved in CH2Cl2 (10 cm3), HO2CPet (1.3 cm3, 10.0 mmol) was added, and the solution layered with MeNO2 (40 cm3). Black crystals slowly grew over 4 weeks, and these were su itable for X-ray studies if maintained in contact with the mother liquor to prevent the loss of interstitial so lvent. After 4 weeks, the crystals were isolated by filtration, washed with small volumes of MeNO2, and dried in vacuo ; the yield was ~5%. Anal. Calcd (found) for 17HO2CPet (C79H149O38Mn9): C, 43.10 (43.16); H, 6.82 (6.77); N, 0.00 (0.02). Selected IR data (cm-1): 1697 (w), 1557 (vs), 1537 (vs), 1474 (vs), 1459 (w), 1419 (vs), 1379 (m), 1364 (m), 1326 (w), 1289 (m), 1204 (m), 1066 (w), 1051 (w), 1006 (w), 808 (w ), 783 (w), 690 (w), 665 (w), 653 (w), 613 (m), 413 (w). [Mn4O2(O2CPet)6(bpy)2] (18). To a solution of complex 14 (0.25 g, 0.090 mmol) in CH2Cl2 (10 cm3) was added 2,2 -bipyridine (0.23 g, 1.4 mmol), and the resulting solution was stirred for 3 h and filtered thr ough Celite. The filtrate was layered with MeNO2 (25 cm3). Black crystals slowly grew, and these were suitable for X-ray crystallography if maintained in contact with the mother liquor to prevent the loss of interstitial solvent. After 3 days, the crystals were isolated by filtration, washed with MeNO2, and dried in vacuo ; the yield was 35%. Anal. Calcd (found) for 182H2O (C56H86N4O16Mn4): C, 52.10 (52.17); H, 6.71 (6.65); N, 4.34 (4.34). Selected IR data (cm-1): 1584 (vs), 1547 (s), 1476 (s), 1442 (m), 1420 (vs), 1370 (m), 1357 (m), 1290 (m),

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163 1205 (w), 1161 (w), 1054 (w), 1016 (w), 804(w) 786 (w), 764 (s), 738 (m), 696(m), 653 (m), 630 (s), 415 (m). [Mn9O7(O2CCH2But)13(THF)2] (19). To a solution of complex 13 (0.20 g, 0.072 mmol) in THF (30 cm3) was added MeNO2 (50 cm3). The solution was filtered, and the filtrate was allowed to stand undisturbed at 4 C. Black crystals formed slowly over two weeks, and these were suitable for X-ray studie s if maintained in contact with the mother liquor to prevent the loss of in terstitial solvent. Af ter 2 weeks, the crys tals were isolated by filtration, washed with MeNO2, and dried in vacuo ; the yield was ~30%. Anal. Calcd (found) for 19MeNO2 (C86.5H160.5N0.5O36Mn9): C, 45.61 (45.36); H, 7.10 (7.16); N, 0.31 (0.30). Selected IR data (cm-1): 1564 (vs), 1479 (m), 1434 (s), 1409 (vs), 1362 (m), 1306 (w), 1274 (w), 1234 (m), 1196 (w), 1149 (w ), 1046 (w), 973 (w), 903 (w), 803 (w), 733 (m), 693 (s), 665 (s), 633 (s), 605 (m), 568 (w), 515 (w), 463 (w). 5.4.2 X-ray Crystallography Diffraction intensities were collected at 173 K (152MeCN, 16CH2Cl2) and 218 K (17H2OHO2CPet, 182H2O), respectively, with a Br uker SMART 1000 and a Bruker P3 diffractometer equipped with a CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Space groups were determined on the basis of systematic absences (16, 18), intensity statistics (15), or both (17). Absorption corrections were applied based on measured indexed crystal faces (15, 16) and by SADABS (17, 18). Structures were solved by direct methods a nd standard Fourier techniques, and they were refined on F2 using full-matrix least-squares procedures. The MeCN molecules in 15 and the HO2CPet group in 17 are disordered, and they were treated by the SQUEEZE96 program, a part of the PLATON97 package of crystallographic software; the correction of the X-ray data was 361 electron/cell and the

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164 required value is 256 electron/cell (17). A common feature in each of the structures is a disorder of Me and Et groups in some bridging t -pentylate groups. The Me and Et groups are disordered in six groups in 15, seven groups in 16, and two groups in 17 and 18. Additionally, the Me gro up of the MeOH ligand coor dinated to Mn(12) in 15 was disordered over two positions [C(14) and C(15)], and the o ccupancies refined to 61:39%. The models for these disorders included refi nements over two sites with correlated site occupation factors. Non-hydrogen atoms were refined anisotropically, except those of disordered groups, which were refined w ith isotropic thermal parameters. In 17, all carbon atoms in the Pet groups were refined with isotropic thermal parameters. Hydrogen atoms were placed in calculated positions a nd refined with a riding group model. The H atoms in some highly disordered gro ups, solvent water molecules and the OHgroup in 17 were not included. Restrictions for the CC distances in ligands were used in the refinement of 17. The non-centrosymmetric crystal of 17 was treated as a racemic twin; the Flack parameter is 0.52(2). Maximum and minimum peaks on the residual densities in 15-18 are in the rang e between 1.093 e -3 and -0.643 e -3.

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165 CHAPTER 6 SINGLE-MOLECULE MAGNETS: STRU CTURAL CHARACTERIZATION, MAGNETIC PROPERTIES AND 19F NMR SPECTROSCOPY OF A Mn12 FAMILY SPANNING THREE OXIDATION LEVELS 6.1 Introduction Slow magnetization relaxation in molecu les at low temperatures was first recognized in 1993 when hysteresis loops, the fundamental property of a magnet, were exhibited by single molecules of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1).23 This finding led to the birth of a new research area, whose objective includ es the discovery of a single-molecule magnet (SMM) that exhibits magnetization hysteresis at technologically relevant temperatures.12,19,22,31,66,98,99,119,101-108 Such a breakthrough would ultimately enable the use of molecules such as [Mn12O12(O2CR)16(H2O)4] complexes in a number of applications, one of the most important of these being high density information storage at the molecular level. The basis for such “molecular memory” is the storage of one bit of digital information on a single molecule.158,159 In addition to the obvious applications of these complexes in nanotechology, there are other important fundamental properties that can be better understood through the study of SMMs. Close examination of the hysteresis loops of 1 reveals a predicted quantum phenomenon that had never before been observed – quantum tunneling of magnetization (QTM).20,27,28,61 This realization has prompted an explosion of research involving single-molecule ma gnetism in the physics area.135,136,160 Molecules of 1 are of sub-nanoscale dimensions, ~1.5 nm in diameter Hence, they represent the point at which classical and quantum worlds meet, being m acroscopic entities which display quantum

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166 effects.27,136,159,161 The presence of QTM is evidenced by step-like features that occur at regular intervals in magnetization vs DC field sweeps.27,61,65,113,162 One consequence of QTM, a process by which a molecule exists as a quantum superposition of states, is that SMMs can potentially function as qubits in quantum computers.163 In recent years there has been a rapid in crease in the number of molecules which behave as SMMs. Complexes of varying nuc learity, topology and peripheral ligation have been prepared through the use of various transition metals. These include V,115 Co,116 Ni,117 Fe,95,118 and particularly Mn. 13,32,33,38,39,46,47,56 -59,61-63,66,94,98-114 However, although several new SMMs have been isolat ed and characterized, the temperature at which the reversal of magnetization is fr ozen (i.e., the blocking temperature, TB) is still only ~3 K, that which is exhibited by complex 1. The properties of a molecule that give ri se to an energy barrier for magnetization reversal and hence allow it to behave as a SMM include a large ground state spin, S, and a large, negative magnetic anisotropy as gauge d by the zero-field splitting parameter, D.12,19,21-23 Researchers are striving to gain a thorough understanding of the structural influences of a molecule on both S and D in order to design new SMMs with increased blocking temperatures. Hence, the study of the effects of chemical and/or physical variations of a molecule is extremely useful. Mn12 complexes have been modified in a number of ways,13,32,33,38,39,46,47,5 8,61,100,101,137,138 the magnetic properties measured to gauge the effect of the changes on the behavi or. One of the most exciting changes made to this system was the preparation and isolation of the one -electron reduced Mn12 species, (PPh4)[Mn12O12(O2CEt)(H2O)4].13 This was shortly followed by the isolation of the twoelectron reduced species, (PPh4)2[Mn12O12(O2CCHCl2)16(H2O)4].61 Both complexes

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167 behave as SMMs, albeit at slightly lower temperatures than the neutral Mn12 species. A complete study, including hyste resis measurements on single cr ystals, of such a family of Mn12 complexes is lacking in the literature however. We herein describe the results of this work, the syntheses, single crystal X-ray structures and magnetic prope rties of a family of Mn12 clusters with identical peripheral ligation spanning three oxidation levels: [Mn12O12(O2CC6F5)16(H2O)4] (20), (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] (21) and (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] (22). Low temperature magnetization measuremen ts and AC susceptibility measurements on all three complexes show a progressive decrease in the magnetic anisotropy parameter, D, and hence, the effective en ergy barrier for magnetization reversal, Ueff, as the Mn12 complex is successively reduced. Para meters obtained from inelastic neutron scattering (INS) measurements on deuterat ed analogues of the complexes are in agreement with the trend.164 6.2 Results and Discussion 6.2.1 Syntheses and Electrochemistry Employing the previously developed ligand substitution procedure, which involves the treatment of 1 with an excess of a carboxylic acid, pentafluorobenzoate ligands were introduced onto the Mn12 complex.13,38 The transformation of 1 into 20 is summarized in eq 6-1. [Mn12O12(O2CMe)16(H2O)4] + 16 C6F5CO2H [Mn12O12(O2CC6F5)16(H2O)4] + 16 MeCO2H (6-1) The ligand substitution reaction is an equilibr ium that favors the product side when the pKa of the added acid is lower th an that of acetic acid; the pKa of C6F5CO2H is 1.4837 while that of MeCO2H is 4.76. A consequence of this c onsiderable difference is that the

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168 fully-substituted derivative can be obtained in high yield when only a slight excess of C6F5CO2H is used to treat complex 1. In addition, acetic acid removal from the reaction system under low pressure as its toluene azeo trope (28/72%; b.p. 101 C at atmospheric pressure) guarantees complete substitution.13,38 The redox chemistry of a number of Mn12 derivatives has been thoroughly explored.13,61,100,113 Cyclic voltammograms typically ex hibit at least one quasi-reversible reduction wave and frequently a second qua si-reversible one as well. The reduction potentials are highly dependent on the electr on donating or withdrawing nature of the carboxylate ligand (RCO2 -). The first reduction potential occurs over a wide range, from 0.00 V for R = C6H4p -OMe to 0.91 V for R = CHCl2 (vs the Fc/Fc+ couple).13,61 In previous reports, it was shown that with the use of stoichiometric amounts of Ias a reducing agent (I-/I2 couple occurs at 0.21 V in CH2Cl2 vs Fc/Fc+), both the oneand twoelectron reduced Mn12 species could be ge nerated in bulk in CH2Cl2. The isolation and structural characte rization of both (PPh4)[Mn12O12(O2CEt)16(H2O)4] and (PPh4)2[Mn12O12(O2CCHCl2)16(H2O)4] as well as others have been described.13,61,100 The cyclic voltammogram (CV) and diffe rential pulse volta mmogram (DPV) for complex 20 are shown in Figure 6-1. Two reducti on processes are displayed at 0.64 V and 0.30 V, each of which appears quasi-revers ible. This reversibility was assessed on the basis of the usual electrochemical cr iteria, including CV peak separations, anodic/cathodic peak current ratio, DPV peak broadness, and proportionality of the peak current to the square root of the scan rate ( ). In the latter, a study of the scan rate dependence for the 0.64 V and 0.30 V reduction processes showed a linear dependence of the peak current with 1/2, indicating that the reducti ons are diffusion-controlled

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169 processes (Figure 6-2). As expected, the E1/2 values are rather posit ive, in accord with the strongly electron withdraw ing substituents in C6F5CO2 -, and both the first and second reductions are consequently within the capability of the reducing agent, I-. Potential (V) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Current0.30 V 0.64 V 5 A 1 A Figure 6-1. Cyclic voltammogram at 100 mV s-1 (top) and differential pulse voltammogram (bottom) of complex 20 in CH2Cl2 containing 0.1 M NBun4PF6 as supporting electrolyte. The i ndicated potentials are vs Fc/Fc+. In Table 6-1 are included values of the anodic peak current / cathodic peak current ratios for the 0.64 and 0.30 V reduction waves of complex 20. The ia/ic ratio of the 0.64 V reduction wave has an approximate value of nearly one over the ra nge of scan rates, providing support of the quasi-r eversible nature of the one-electron reduction process. The ia/ic ratio of the second reduction wave at 0.30 V is nearly constant at a value of ~0.30 over the range of scan rates.

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170 Potential (V) -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 (a) 5 ACurrent 100 mV/s 125 mV/s 150 mV/s 175 mV/s 200 mV/s 225 mV/s 250 mV/s 10121416182022 -20 -10 0 10 20 30 10121416182022 -30 -20 -10 0 10 20 30 (c) (b)Current (A)1/2r2= 0.998 r2= 0.994Current (A)1/2r2= 0.997 r2= 0.997 Figure 6-2. Scan rate dependence of reduc tion waves at 0.64 V and 0.30 V of complex 20 in CH2Cl2 containing 0.1 M NBun4PF6 as supporting electrolyte. (a) Cyclic voltammogram at the indicated scan ra tes with corresponding plot of cathodic (top) and anodic (bottom) p eak current dependence vs 1/2 for (b) 0.64 V reduction wave and (c) 0.30 V reduction wave. Thus, [Mn12O12(O2CC6F5)16(H2O)4] (20) was treated with one and two equivalents of NMe4I in MeCN; the formation of I2 was confirmed by its extraction into a hexanes phase. Removal of MeCN in vacuo and recrystallization of the residue from CH2Cl2 with heptane or heptane/toluene (1:1) gave black crystals of the desired oneand two-electron reduced complexes, 21 and 22 in good yield ( 70%) and analytical purity. The transformation of 20 into both 21 and 22 is summarized in eq 6-2, where n = 1 or 2.

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171 [Mn12O12(O2CC6F5)16(H2O)4] + n NMe4I (NMe4)n[Mn12O12(O2CC6F5)16(H2O)4] + 2 n I2 (6-2) Table 6-1. Anodic peak current / cathodic peak current ratios at the i ndicated scan rate in mV s-1 for the 0.64 V and 0.30 V reduction waves of complex 20. 0.64 V Reduction Wave 0.30 V Reduction Wave Scan Rate (mV s-1) ia/ic Scan Rate (mV s-1) ia/ic 100 -0.66 100 -0.27 125 -0.68 125 -0.27 150 -0.71 150 -0.29 175 -0.71 175 -0.30 200 -0.72 200 -0.31 225 -0.71 225 -0.31 250 -0.74 250 -0.32 275 -0.75 275 -0.34 300 -0.76 300 -0.32 325 -0.75 325 -0.32 350 -0.75 350 -0.32 375 -0.72 375 -0.31 400 -0.74 400 -0.32 425 -0.74 425 -0.32 450 -0.74 450 -0.32 475 -0.72 475 -0.33 500 -0.74 500 -0.33 6.2.2 Description of Structures 6.2.2.1 X-ray crystal structure of [Mn12O12(O2CC6F5)16(H2O)4] (20) Labeled ORTEP40 plots in PovRay format of complexes 20, 21, and 22 are presented in Figures 6-3, 6-4 and 6-6, together with ster eoviews. The crystallographic data and structure refinement details are collected in Table 6-2, and selected interatomic distances and angles are listed in Tabl es A-13, A-14 and A-15, respectively. Complex 203CH2Cl2 crystallizes in th e triclinic space group 1 P (Figure 6-3). The structure consists of a central [MnIV 4O4] cubane core that is surrounded by a non-planar ring of eight MnIII ions that are bridged and conn ected to the cubane by eight 3-O2ions. The eight MnIII ions separate into two groups of four MnIII ions each. In the first group,

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172 each MnIII ion is coordinated to a single MnIV ion via two oxide br idges [Mn(1), Mn(4), Mn(5), Mn(8)], while in the second group each MnIII ion is coordinated to two MnIV ions via two oxide bridges [Mn(9) Mn(10), Mn(11), Mn(12)].29 Peripheral ligation is provided by sixteen bridging C6F5CO2 ligands and four terminal H2O molecules [O(33), O(36), O(41), O(44)], which are bound in a 2:2 fashion to two trans MnIII ions, Mn(9) and Mn(12), respectively (Figure 6-3). This disposition of two pairs of H2O ligands has also been observed for [Mn12O12(O2CPh)16(H2O)4], [Mn12O12(O2CC6H4p -Cl)16(H2O)4], [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] and others.20,29,39 O25 O26 O24 O38 O39 O37 O20 O34 O35 O14 O33 O36 O8 Mn5 Mn10 O7 O40 O5 O21 O43 O41 O44 O42 O30 O6 O22 Mn4 O31 O47 O46 O15 O48 O11 Mn11 O2 O45 O16 Mn1 O13 O12 Mn12 O32 O29 Mn8 O1 O17 O23 O3 O27 Mn2 O10 O4 O28 Mn7 O18 O9 Mn3 Mn6 O19 Mn9 Figure 6-3. ORTEP representati on in PovRay format of [Mn12O12(O2CC6F5)16(H2O)4] (20) with thermal ellipsoids at the 50% probability level except for the C atoms, together with a stereopair. For clarity, only the ipso C atoms of the pentafluorobenzoate groups are shown. MnIV green; MnIII blue; O red; H2O yellow; C gray.

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173 All of the Mn centers are six-coordi nate, with near-octahedral geometry. Assignment of the oxidation states of the meta l centers was done qualitatively by charge consideration and also by comparison of the bond distances around the Mn centers. These assignments were confirmed quantitatively by bond valence sum (BVS) calculations,41 indicating that Mn(2), Mn(3), Mn(6) and Mn(7) are MnIV while the remaining Mn centers are MnIII (Table 6-3). The protonation levels of the inorganic O atoms were confirmed by a similar BVS calculation and are collected in Table 6-4. The structure of 20 is very similar to other previously characterized neutral Mn12 complexes. The eight MnIII centers exhibit a Jahn-Teller (JT) distorti on, as expected for a high-spin d4 ion in near-octahedral geometry. As is almost always the case for MnIII ions, the JT distortion takes the form of an axial elongation of two trans bonds, typically lengthening bond di stances by 0.1-0.2 . The JT elongation axes avoid the Mn-O2bonds, the shortest and strongest in the molecule, and thus the JT axes are all axially disposed, roughly perpendicular to the [Mn12O12] disk-like core. As a result, there is a near parallel alignment of the eight MnIII JT elongation axes. This is also the origin of the significant magnetic anisotropy in the z direction that greatly influences the observed magnetic properties ( vide infra ).

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174Table 6-2. Crystallographic data for [Mn12O12(O2CC6F5)16(H2O)4]3CH2Cl2, (NMe4)[Mn12O12(O2CC6F5)16(H2O)4]4.5CH2Cl2H2O, and (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4]6C7H8. Parameter [Mn12] (20) [Mn12]1(21) [Mn12]2(22) formulaa C115H14Cl6Mn12F80O48 C120.5H30Cl9Mn12N1F80O48.5 C162H80Mn12N2F80O48 fw, g mol-1 4555.19 4765.74 5001.53 space group 1 P P 2/ c C 2/ c a 18.519(4) 33.6633(2) 21.7892(15) b 18.895(4) 17.3311(9) 27.0937(19) c 26.169(5) 27.2564(2) 31.623(2) deg 70.073(2) 90 90 deg 72.616(2) 94.2090(2) 91.640(2) deg 73.364(3) 90 90 V 3 8040(3) 15859.1(2) 18661(2) Z 2 4 4 T C -80(2) -80(2) -100(2) radiation, b 0.71073 0.71073 0.71073 calc, g cm-3 1.882 1.996 1.780 cm-1 11.77 12.48 9.40 R 1 ( wR 2), %c,d 8.80 (21.79) 5.34 (13.25) 5.81 (14.31) a Including solvent molecules. b Graphite monochromator. c R 1 = || Fo| – | Fc|| / | Fo|. d wR 2 = [ [ w ( Fo2 Fc2)2] / [ w Fo2 )2]]1/2 where S = [[ w ( Fo2 – Fc2)2] / ( n p )]1/2, w = 1/[ 2( Fo2) + ( m p )2 + n p ], p = [max( Fo2, 0) + 2* Fc2]/3, and m and n are constants.

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175 Table 6-3. Bond valence sum calculationsa for complexes 203CH2Cl2, 214.5CH2Cl2H2O, and 226C7H8. 203CH2Cl2 214.5CH2Cl2H2O 226C7H8 Atom Mn2+ Mn3+ Mn4+ Mn2+ Mn3+ Mn4+ Mn2+ Mn3+ Mn4+ Mn(1) 3.265 2.987 3.136 4.193 3.835 4.026 4.187 3.830 4.021 Mn(2) 4.189 3.832 4.023 4.126 3.774 3.962 4.184 3.827 4.018 Mn(3) 4.126 3.774 3.962 4.128 3.776 3.964 3.299 3.018 3.168 Mn(4) 3.304 3.022 3.173 4.121 3.770 3.957 3.284 3.004 3.153 Mn(5) 3.343 3.058 3.210 3.297 3.007 3.157 3.262 2.984 3.133 Mn(6) 4.172 3.816 4.006 3.268 2.989 3.138 2.101 1.922 2.017 Mn(7) 4.239 3.878 4.071 3.264 2.985 3.134 Mn(8) 3.248 2.971 3.119 2.121 1.940 2.036 Mn(9) 3.289 3.008 3.158 3.296 3.015 3.166 Mn(10) 3.278 2.998 3.148 3.297 3.016 3.166 Mn(11) 3.333 3.039 3.201 3.252 2.974 3.122 Mn(12) 3.259 2.981 3.130 3.265 2.987 3.135 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value. Table 6-4. Bond valence sum calculationsa for selected oxygen atoms in complexes 203CH2Cl2, 214.5CH2Cl2H2O, and 226C7H8. 203CH2Cl2 214.5CH2Cl2H2O 226C7H8 Atom Vi AssignmentAtom Vi AssignmentAtom Vi Assignment O(1) 2.135 O2O(1) O22.027 O(1) 1.994 O2O(2) 2.042 O2O(2) O21.980 O(2) 2.007 O2O(3) 1.995 O2O(3) O21.987 O(3) 2.141 O2O(4) 2.053 O2O(4) O22.030 O(4) 2.001 O2O(5) 2.156 O2O(5) O22.192 O(6) 2.139 O2O(6) 2.128 O2O(6) O22.081 O(7) 2.102 O2O(7) 2.109 O2O(7) O22.142 O(20) 0.308 H2O O(8) 2.168 O2O(8) O21.979 O(23) 0.264 H2O O(9) 2.057 O2O(9) O22.123 O(10) 2.042 O2O(10)O22.142 O(11) 2.141 O2O(11)O22.138 O(12) 2.199 O2O(12)O22.081 O(33) 0.293 H2O O(17)H2O0.323 O(36) 0.327 H2O O(46)H2O0.341 O(41) 0.293 H2O O(47)H2O0.296 O(44) 0.301 H2O O(48)H2O0.286 a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0.

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176 The JT elongation axis of Mn(11) is not clearly defined, however. The axial bonds, Mn(11)-O(45) = 2.015(8) and Mn(11)-O(48) = 2.010(9), are slightly shorter than is typical for an axially-elongated JT bond of a MnIII ion. In addition, the equatorial bonds, Mn(11)-O(47) = 2.053(10) and Mn(11)-O(2) = 1.978(8), are slight ly longer than is typical for a MnIII-Oeq and MnIII-O2bond, respectively, in a Mn12 complex (Table 6-5). These abnormal distances around Mn(11) suggest a static disorder between a normal, axially disposed JT elongati on axis and one abnormally-ori ented in the plane of the molecule. We have previously called such mol ecules (differing only in the orientation of one or more JT axes) Jahn-Teller isomers.56-59 Examples include [Mn12O12(O2CCH2But)16(H2O)4],56 [Mn12O12(O2CC6H4p -Me)16(H2O)4]57 and [Mn12O12(O2CCF3)16(H2O)4].59 Table 6-5. Interatomic distances () of Mn(11) in complex 20. Bond Distance Typea Mn(11)-O(11) 1.884(7) Or Mn(11)-O(2) 1.978(8) Or Mn(11)-O(46) 1.978(10) Oeq Mn(11)-O(47) 2.053(10) Oeq Mn(11)-O(48) 2.010(9) Oax Mn(11)-O(45) 2.015(8) Oax a Or = ring O2-, Oax = axial carboxylate, Oeq = equatorial carboxylate 6.2.2.2 X-ray crystal structure of (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] (21) Complex 214.5CH2Cl2H2O crystallizes in the monoclinic space group P 2/ c The anion consists again of a central [MnIV 4O4] cubane surrounded by a non-planar ring of eight Mn atoms bridged by eight 3-O2ions, sixteen bridging C6F5CO2 ligands and four terminal H2O molecules (Figure 6-4). Examin ation of the Mn-O bond lengths unequivocally establishes that the added electron has converted a formerly MnIII atom to a MnII ion [Mn(8)]. JT axial elongations are obs erved at the other seven ring Mn atoms,

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177 as expected for a high-spin d4 ion in near-octahedral geomet ry. All of the JT elongation axes are approximately perpendicular to the plane of the molecule. The Mn oxidation levels were determined by a BVS41 calculation (Table 6-3 and 6-4). Therefore, the [Mn12]anion is a trapped-valence MnII, Mn7 III, Mn4 IV cluster. O29 O28 O23 O24 O20 O22 O19 O30 O25 O31 O47 O48 O8 Mn7 Mn6 O7 O21 O6 O16 O15 O17 O46 O43 O42 O5 Mn5 O41 O40 O37 O36 O38 O11 Mn10 O10 O35 O34 Mn9 O32 O12 Mn12 O39 O44 Mn11 O9 O33 O27 O2 O26 Mn3 O3 O4 O45 Mn4 O14 O1 Mn1 Mn2 O18 Mn8 O13 Figure 6-4. ORTEP representation in PovRay format of the anion of (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] (21) with thermal ellipsoids at the 50% probability level except for the C atoms, together with a stereopair. For clarity, only the ipso C atoms of the pentafluorobenzoate groups are shown. MnIV green; MnIII blue; MnII purple; O red; H2O yellow; C gray. A static disorder between one carboxylate group and an adjacent water molecule is observed in the crys tal structure of 21. Hence, the structure is a mixture of two isomers, the 2:2 form (77%) and the 1:1:2 form (23%), the notation describi ng the distribution of

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178 the four H2O molecules among the Mn ions. Figur e 6-4 shows the form with 77% occupancy, where the carboxylat e is bridging Mn(6) and Mn(5 ) [O(21)-C(64)-O(18)] and the water molecule [O(17)] is coordinated to Mn(12). The minor form has the carboxylate bridging Mn(12) and Mn(5) [O(17)-C(57)-O (18)] and the water molecule [O(21)] coordinated to Mn(6). The [Mn12]molecules are stacked in columns with all of the molecules oriented in the same way with respect to the cell axes. There is a -stacking interaction of the pentafluorophenyl ri ngs bridging Mn(5) and Mn(6) between [Mn12]anions in adjacent columns along the 01 1 direction of the cell as shown in Figure 6-5 as dashed lines. The dihedral angle between the rings is 1 and the ring separation is ~3.5 . a b c Figure 6-5. Packing diagram for 21, emphasizing intermolecular -stacking of carboxylate aromatic rings (dashed lines). 6.2.2.3 X-ray crystal structure of (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] (22) Complex 226C7H8 crystallizes in the monoclinic space group C 2/ c (Figure 6-6). The [Mn12]2cluster is located on a crystallographic C2 rotation axis perpendicular to the

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179 plane of the molecule and passing through the central cuba ne unit. The anion again consists of a central [MnIV 4O4] cubane surrounded by a non-planar ring of eight Mn atoms bridged by eight 3-O2ions, sixteen bridging C6F5CO2 ligands and four terminal H2O molecules, which are bound in a 2:2 fashion to two trans Mn atoms, Mn(6) and Mn(6a), respectively. O24a O11 O10 O13 O14 O15 O17 O22a O21a O18a O23a O7 Mn3 Mn4 O6 O12 O3 O18 O21 O20 O23 O22 O24 O4 O16 Mn5 O10a O13a O14a O17a O15a O6a Mn4a O12a O16a Mn6a O7a Mn6 O11a O9a Mn3a O20a O4a O9 O8 Mn1a O2a O1a O8a Mn2a O5 O2 Mn1 Mn2 O19 O3a Mn5a O19a O1 O5a Figure 6-6. ORTEP representation in PovRay format of the anion of (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] (22) with thermal ellipsoids at the 50% probability level except for the C atoms, together with a stereopair. For clarity, only the ipso C atoms of the pentafluorobenzoate groups are shown. MnIV green; MnIII blue; MnII purple; O red; H2O yellow; C gray. Analysis of the Mn-O bond lengths again es tablishes that the central cubane Mn atoms are MnIV ions, and that the added electrons have gone one each on two formerly

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180 MnIII atoms converting them to MnII ions [Mn(6) and Mn(6a)], to which the water molecules are coordinated. The distances around Mn(6) and Mn(6a) are typical of MnII ions in near-octahedral geometry. The other six outer ring Mn atoms display bond distances and JT axial elongations typical of MnIII. All JT elongation axes are perpendicular to the plane of the molecule. The Mn oxidation levels were confirmed by a BVS41 calculation (Table 6-3 and 6-4). Thus, the [Mn12]2anion is a trapped-valence Mn2 II, Mn6 III, Mn4 IV species. Table 6-6 shows a comparison of aver age Mn-O bond distan ces for the three [Mn12O12(O2CC6F5)16(H2O)4]z [ z = 0 (20), 1(21) and 2(22)] complexes. This comparison confirms that the bond distances in the [Mn12O12] cores are almost superimposable except for thos e positions th at involve MnII ions being compared with MnIII ions. 6.2.3 19F Nuclear Magnetic Resonance Spectroscopy In order to assess the structures and st ability of the complexes in solution, a 19F NMR spectroscopic investigation of complexes 20, 21 and 22 in CD2Cl2 was carried out. Chemical shifts and T1 times for 20-22 are listed in Table 6-7. 1H NMR spectroscopy has proven in the past to be a c onvenient tool for the study of Mn12 complexes, and the use of 19F NMR spectroscopy in the present work is a useful complement, providing a rare example of this type of NMR spect roscopy on a paramagnetic system. The 19F NMR spectrum of complex 20 in CD2Cl2 in Figure 6-7 (bottom) show s eight resonances with a 4:2:2:1:2:1:2:4 in tegration ratio. As with other Mn12 derivatives, there are only three distinct types of br idging carboxylates in the NMR sp ectrum: (i) four axial ligands bridging MnIIIMnIV pairs, (ii) four ax ial ligands bridging MnIIIMnIII pairs, and (iii) eight equatorial ligands bridging MnIIIMnIII pairs.

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181Table 6-6. Selected interatomic distances () and angles ( ) for [Mn12O12(O2CC6F5)16(H2O)4] (20), (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] (21), and (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] (22). Parametera 20 21 22 MnIV – Oc (ax) 1.898(6) – 1.906(6) 1.874(3) – 1.915(3) 1.889(3) – 1.893(3) MnIV – Oc (eq) 1.895(7) – 1.927(6) 1.905(3) – 1.967(3) 1.921(4) – 1.945(4) MnIV – Or 1.850(7) – 1.894(6) 1.818(3) – 1.903(3) 1.830(4) – 1.877(4) MnIV – Oax 1.914(7) – 1.931(6) 1.924(3) – 1.951(3) 1.940(4) – 1.947(3) MnIIIb – Or 1.874(7) – 1.915(7) 1.842(3) – 1.919(3) 1.841(4) – 1.904(4) MnIIIc – Or 1.865(6) – 1.903(6) 1.860(3) – 1.915(3) 1.878(4) – 1.882(4) MnIIIb – Oeq 1.909(8) – 1.960(6) 1.919(3) – 1.981(3) 1.944(5) – 1.980(4) MnIIIc – Oeq 1.956(6) – 1.978(7) 1.947(3) – 2.031(4) 1.967(4) – 1.979(4) MnIIIb – Oax 2.161(7) – 2.244(10) 2.164(3) – 2.236(4) 2.171(4) – 2.268(4) MnIIIc – Oax 2.148(7) – 2.162(8) 2.069(4) – 2.163(3) 2.151(4) – 2.180(4) MnIIIc – Ow 2.168(7) – 2.212(7) 2.207(4) – 2.221(4) –––––––––– MnIId – Or –––––––––– 2.080(3) – 2.153(3) 2.075(4) – 2.117(4) MnIId – Oeq –––––––––– 2.135(3) – 2.164(3) 2.126(5) – 2.139(5) MnIId – Oax –––––––––– –––––––––– –––––––––– MnIId – Ow –––––––––– 2.179(4) – 2.200(4) 2.219(4) – 2.281(4) Or – MnIV – Or 83.9(3) – 84.8(3) 83.36(13) – 84.46(13) 84.07(16) – 84.26(16) Or – MnIIIb – Or 82.3(3) – 83.4(3) 82.53(13) – 83.91(13) 82.49(15) – 83.09(16) Or – MnIIIc – Or 91.7(3) – 95.1(3) 93.64(13) – 95.31(13) 93.88(15) Or – MnIId – Or –––––––––– 88.63(12) 93.37(14) a Oc = cubane O2-, Or = ring O2-, Oax = axial carboxylate, Oeq = equatorial carboxylate, Ow = water; b MnIIIb atoms: Mn(1, 4, 5 and 8) in 20, Mn(5, 7, 9 and 11) in 21, and Mn(3 and 5) in 22; c MnIIIc atoms: Mn(9, 10, 11 and 12) in 20, Mn(6, 10 and 12) in 21, and Mn(4) in 22; d MnII atoms: Mn(8) in 21, and Mn(6) in 22. Distances around Mn(11) in complex 20 are not considered because of disorder in the structure.

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182 -170 -170 -160 -160 -150 -150 -140 -140 -130 -130 -120 -120 -110 -110 -100 -100 -90 -90 -80 -80 -70 -70 -60 -60 -170 -170 -160 -160 -150 -150 -140 -140 -130 -130 -120 -120 -110 -110 -100 -100 -90 -90 -80 -80 -70 -70 -60 -60 -170 -170 -160 -160 -150 -150 -140 -140 -130 -130 -120 -120 -110 -110 -100 -100 -90 -90 -80 -80 -70 -70 -60 -60 o eq (III-III) o ax (III-IV) o eq (III-III) o ax (III-IV) o eq (III-III) o ax (III-IV) m eq (III-III) m eq (III-III) p eq (III-III) p ax (III-IV) m ax (III-IV) p ax (III-IV) p ax (III-III) m ax (III-III) p ax (III-III) p e q (III-III) p ax (III-IV) m ax (III-IV) m eq (III-III) p ax (III-III) m ax (III-III) m ax (III-IV) p eq (III-III) m ax (III-III) Figure 6-7. 19F NMR (282 MHz) spectra at ~ 23 C in CD2Cl2 of 20 (bottom), 21 (middle) and 22 (top). Solution studies of Mn12 molecules at room temperatur e have shown that there is a fluxional process that is fa st on the NMR timescale that rapidly exchanges the water ligands with one type of axial carboxylate ligand,32,165 the one that has both its O atoms located on the JT elongation axes of the MnIIIMnIII pairs. This introduces dihedral mirror planes, which make all of the equatori al carboxylate groups e quivalent. In effect, the solution state symmetry of a Mn12 molecule is D2d, giving three resonances in a 1:1:2 relative integration ratio (axial:axial:equatorial). Hence, 20 should exhibit three resonances each for the o-, m and p positions of the three types of ligands, giving a total

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183 of nine signals. Only ei ght are observed however; the o resonance of the axial ligands bridging MnIIIMnIII pairs cannot be clearly located, a nd we have concluded that it is probably too paramagnetically broadened to be observed. The resonances in the 19F NMR spectra of 20-22 are shown in Figure 6-7. The spectra are all very similar, exhibiting eight signals each with only minor shifts of the resonances. Table 6-7. Solution 19F NMR spectral data for [Mn12O12(O2CC6F5)16(H2O)4]0, -, 2complexes. 20 21 22 Peaka Assignmentb T1 c Peaka Assignmentb T1 c Peaka Assignmentb T1 c -77.8 o eq (III-III) 2.8 -86.6 o eq (III-III) 2.4 -98.2 o eq (III-III) 2.2 -114.4 o ax (III-IV) 3.4 -109.8 o ax (III-IV) 3.4 -115.3 o ax (III-IV) 3.3 -133.7 p eq (III-III) 35.5 -138.8 p eq (III-III) 30.2 -139.3 m ax (III-III) 7.5 -138.2 p ax (III-III) 22.5 -139.7 p ax (III-III) 19.3 -143.3 p ax (III-III) 16.6 -142.2 m ax (III-III) 11.2 -140.3 m ax (III-III) 9.3 -145.1 p eq (III-III) 24.8 -147.1 p ax (III-IV) 24.8 -149.6 p ax (III-IV) 22.3 -153.0 p ax (III-IV) 19.3 -152.9 m ax (III-IV) 15.0 -153.4 m ax (III-IV) 12.8 -154.0 m ax (III-IV) 11.5 -167.6 m eq (III-III) 19.0 -163.5 m eq (III-III) 15.1 -158.4 m eq (III-III) 13.1 a ppm, at ~ 23 C. b ax = axial, eq = equatorial; IIIIII and III-IV refer to ligands bridging MnIIIMnIII and MnIIIMnIV pairs, respectively; o = ortho, m = meta and p = para. c ms. Spin-lattice relaxation time ( T1) measurements were performed to aid in the assignment of the peaks. Unpaired electrons on metal centers relax at very fast rates, providing efficient pathways for nuclear relaxation; in genera l, the longitudinal relaxation time is directly related to the distance of a nucleus from a paramagnetic center.166,167 Resonances were also assigne d on the basis of relative inte gration ratios, peak broadness ( r-6 dependence, where r is the distance to the paramagnetic centers), and comparisons with Mn12 derivatives possessing other carboxylate groups.13,61 For each of the three complexes, the T1 times can be separated into three distinct groups, two resonances with very short T1 times, three with intermediate T1 times and three with long T1 times. The resonances with the shortest times were assigned to the o -F nuclei; these are the resonances that are broadened and shifted the most. The resonances with the longest T1

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184 times were assigned to the p -F nuclei while those with the intermediate T1 times were assigned to the m -F nuclei. The paramagnetic shifts are expected to have both contact (through-bond) and dipolar (through-space) contributions.166-168 Unpaired spin density on the paramagnetic metal centers is primarily in d orbitals, and -symmetry overlap with the carboxylate system will result in direct delocalization of positive spin density from the metal onto the -CO2 system. Direct delocalization of the positive spin density onto the o and p positions of the aromatic pentafluorophenyl rings then occurs. This results in direct delocalization of the positive spin density from the aromatic ring to the o and p -F nuclei, giving downfield paramagnetic shifts of thes e NMR resonances. Spin polarization effects give a negative spin density at the m -positions of the pentafl uorophenyl rings, giving an upfield paramagnetic shift of these resona nces. Indeed, the observed alternating downfield-upfield-downfield shifts of the ortho meta para resonances in 20-22 are characteristic of a dominant -spin delocalization mechanism for the contact shifts.167,169 Isotropic shifts from dipolar co ntributions likely also affect the paramagnetic shifts of the fluorine nuclei in these magnetically anisotr opic molecules, and probably are the reason for the difference in the magnitude of the shifts of the o and p resonances of a particular carboxylate group, which are us ually much more similar on the basis of only a delocalization mechanism.167,168 If the added electron(s) in 21 and 22 is (are) trapped on the NMR timescale, then unique resonances for carboxyl ates coordinated to the MnII site(s) should be observed. This is not seen. Instead, 21 and 22 display the same D2d solution symmetry as 20,

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185 indicating that the added el ectrons are detrapping at a fast rate versus the 19F NMR timescale among the outer ring of Mn ions. 6.2.4 Inelastic Neutron Scattering Spectroscopy To obtain a larger ground state spin S a nd/or a more negative anisotropy D, it is crucial to better understand th e effects of chemical and physical variations on these magnetic parameters. Consequently, we have undertaken an inelastic neutron scattering (INS) study of these three Mn12 derivatives. In such systems, INS is a powerful tool to obtain detailed magnetic information includi ng magnetic anisotropy parameters (axial, rhombic, and up to the fourth order) as it do es not rely on an applied magnetic field. Previous experiments on complex 1170 and other molecular magnets171 have shown that the anisotropy parameters can be derived with great accuracy. In Figure 6-8 are shown the low temperat ure neutron energy loss side spectra of [Mn12O12(O2CC6F5)16(D2O)4] (23), (NMe4)[Mn12O12(O2CC6F5)16(D2O)4] (24) and (NMe4)2[Mn12O12(O2CC6F5)16(D2O)4] (25) taken at 1.8 K (23 and 25) and 2.0 K (24), respectively. Two peaks were obser ved in the spectra of complexes 23 and 24. The peaks labeled Ia and Ib are centered at about 10 and 9.3 cm-1, respectively, in the spectrum of 23 while Ic and Id are observed at about 7.4 and 6.9 cm-1, respectively, in the spectrum of 24. The spectrum of 25 shows no resolved peaks in the observed energy region, but weak intensity between about 3 and 6 cm-1. The asterisk marks the en ergy transfer position of an impurity of complex 24 in 25. In Figure 6-9 are shown the spectra of complexes 23 and 24 at 20 K and 9.9 K, respectively. For 23, new transitions, labeled IIa – VIa and IIb – VIb, appear at energies between about 3 and 9 cm-1 as the temperature is increased. Three new peaks appear in 24 upon raising the temperature to 9.9 K, labeled IIc – IVc and

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186 IId – IVd. The solid lines (blue) in each spectru m in Figures 6-8 and 6-9 correspond to the sum of the gaussians underlined (das hed green lines) in the analysis. Energy Transfer (cm-1) 46810 Intensity (arb. units) IeIfIIeIIf Energy Transfer (cm-1) 46810 Intensity (arb. units) IcId Energy Transfer (cm-1) 46810 Intensity (arb. units) IaIb(a)(b) (c) Figure 6-8. Neutron energy loss si de INS spectrum of (a) complex 23 at 1.8 K, (b) complex 24 at 2.0 K and (c) complex 25 at 1.8 K obtained at an incident neutron wavelength = 6.0 . The solid lines (blu e) correspond to the sum of the gaussians underlined (dashed green lin es) in the analysis; see the text for the fit parameters. Energy Transfer (cm-1) 46810 Intensity (arb. units) Energy Transfer (cm-1) 46810 Intensity (arb. units) (a) (b)IaIbIIaIIbIIIaIIIbIVaIVbVa,bVIa,b IcIdIIcIIdIIIcIIIdIVcIVd Figure 6-9. Neutron energy loss si de INS spectrum of (a) complex 23 at 20 K and (b) complex 24 at 9.9 K obtained at an in cident neutron wavelength = 6.0 . The solid lines (blue) correspond to the sum of the gaussians underlined (dashed green lines) in the analysis; see the text for the fit parameters.

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187 The average energy of transitions Ia and Ib of complex 23 in Figure 6-8a is centered at about 9.8 cm-1. Assuming that this corresponds to the first allowed transition between the ms = 10 and ms = 9 sublevels in the S = 10 ground state, its energy is given by 19D, which means that D ~ -0.51 cm-1. Taking this value, the obser vation of two transitions at 1.8 K for complex 23 in Figure 6-8a is puzzling. Accord ing to Boltzmann statistics, just the ms = 10 state is populated at 1.8 K (99.9% ). Thus, due to the INS selection rule within a given S state, ms = 1, only one transition should be observed, as is the case for complex 1.170a The observation of two peaks in the low temperature spectra of 23 is attributed to two isomers present in the sa mple, either due to Jahn-Teller isomerism known in Mn12 compounds56-59 or to structural isomers.172,173 Thus, the peaks Ia and Ib correspond to transitions from ms = 10 to ms = 9 energy levels in isomer a and isomer b, respectively. The ratio of a to b is about 3:1 with the assumption that the INS intensities are proportional, w ithin a good accuracy, to the am ount of each isomer in the sample. The same explanation is valid for complex 24, where the observation of two peaks is also seen. Here, the two peaks labeled Ic and Id correspond to the isomers c and d (ratio c/d = 0.7). A closer examination of the data in Figure 6-9c also reveals the existence of two isomers in 25, but in this case higher en ergy states are already populated at 1.8 K. Thus, hot peaks superimpose with the cold ones and lead to the broad band observed between 3 and 7 cm-1. The two isomers identified are named e and f. The ratio e/f could not be determined from the present data. On the basis of this information, data ar e fitted with gaussians of equal width for each sample, shown as dashed lines (green) in Figures 6-8 and 6-9. The width (fwhm) of the gaussians for the elevated temperatur e data are fixed to the width of the

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188 corresponding transition I at low temperature (about 60 eV). Exceptions are transitions Va/Vb and VIa/VIb, respectively. For those it is not po ssible to locate their four individual positions, because they are superimposed. Thus, Va/Vb and VIa/VIb are treated as one transition for their position with fwhm of about 100 eV. The results of this procedure are presented in Table 6-8 for the neutral species 23, and the resulting energy splitting patterns for the two isomers a and b in 23 are shown in Figure 6-10. Table 6-8. Experimental (neutron energy loss side) and calculated energies and relative intensities of the INS transitions of isomers a and b in [Mn12O12(O2CC6F5)16(D2O)4] (23) at = 6.0 .a Isomer Transition Energy (cm-1) Normalized Intensity (arb. units) 2.0 K 20.0 K exp calc exp calc exp calc a Ia 10.03(2)10.01 1.00(3) 1.00 0.37(2) 0.44 IIa 8.48(2) 8.51 0.29(2) 0.41 IIIa 7.11(2) 7.16 0.24(2) 0.31 IVa 5.98(3) 5.94 0.19(2) 0.24 Va 4.86(2) 4.83 0.18 VIa 3.79(2) 3.82 0.14 b Ib 9.30(5) 9.28 1.00(8) 1.00 0.35(2) 0.42 IIb 7.90(1) 7.94 0.29(2) 0.41 IIIb 6.69(2) 6.72 0.24(2) 0.33 IVb 5.65(1) 5.61 0.18(2) 0.25 Vb 4.86(2) 4.59 0.20 VIb 3.79(2) 3.65 0.16 a The intensities were scaled to a value of 1.0 for transitions Ia and Ib at 2.0 K. For the calculation the following sets of parameters were used: for isomer a, D = -0.463 cm-1 and 0 4B = -2.09 10-5 cm-1 with 2 = 7.7 10-4; for isomer b: D = -0.437 cm-1 and 0 4B = -1.69 10-5 cm-1 with 2 = 5.4 10-4.

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189 Isomer a Isomer bEnergy (cm-1)IIaIIIaIaIVaVaVIaVIbVbIVbIIIbIIbIb 10 9 8 7 6 5 4 3 2 1 0 ms 20 30 40 50 10 0 Figure 6-10. Energy level diagram for complex 23 calculated using the parameters given in Table 6-8. In the following analysis, the bands V and VI are not included, due to their overlap problem mentioned above. The experiment al energy patterns in Figure 6-10 are reproduced by applying the following axial Hamiltonian for the magneto-crystalline anisotropy (eq 6-3), where 0 4B is the axial fourth order term to the zero-field splitting, and 2 2 2 z 4 z 0 41 S S 3 ) 1 S(S 6 S 25 ) 1 S(S 30 S 35 O ˆ ˆ ˆ ˆ. 0 4 0 4 2 zO B 1 S S 3 1 S D H ˆ ˆ ˆ (6-3) Small deviations from tetragonal symmetry have been neglected in our analysis as they only marginally influence the low energy leve ls. Calculations using only the D parameter cannot reproduce the experimental data. Thus, the 0 4B term is significant. This allows us to reproduce the deviations of our experimental peak positions from a pure Land pattern, observed for complexes 23 and 24 For 25 the data do not contain enough information to extract the 0 4B parameter and thus this is neglected in the analysis of 25 Fitting the data with the Hamiltonian (eq 6-3) allowed the determination of the zero-field splitting parameters for the six isomers. The results are collected in Table 6-9.

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190 Table 6-9. Zero-field splitting parameters D and 0 4B (in cm-1) for complexes 23 24 and 25 Parameter Isomer a [Mn12] b [Mn12] c [Mn12]1d [Mn12]1e [Mn12]2f [Mn12]2D -0.463 -0.437 -0.368 -0.332 -0.274 -0.241 0 4B (10-5) -2.09 -1.69 -1.59 -1.80 2 (10-3) 0.8 0.5 0.4 3.8 4 7 Using INS, not only is information on peak positions obtained, but also on their relative intensities. Sinc e the intensity of a given transi tion is a function of the underlying wave functions of the initial and final states, it is very se nsitive to the applied model parameters. Comparing experimental intens ities and calculated ones is therefore an excellent way to validate the chosen mode l. The differential magnetic neutron cross section for a transition i j is given by eq 6-4.174 ) ( exp 4 d d d2 ) ( 2 2 e 2 2Q 'QF k k c m e N EW (6-4) i j i j j j i j j i iE E Q Q S S exp) R R ( Q 2j iˆ ˆ Q, , In this equation k and k are the wavenumbers of the in coming and scattered neutrons, Q is the scattering vector, exp(-2W) is the Debye – Waller factor, is the neutron energy, and i and j are the cluster wave functions with energies Ei and Ej, respectively. gi is the g factor and F( Q ) is the magnetic form factor, iR is the space vector of the ith Mn ion in the cluster and and stand for the spatial coordinates x, y and z. e and me are the charge and mass of the electron, respectively, c is the speed of light, and = -1.91 is the gyromagnetic constant of the neutron. For a powder sample, eq 6-4 must be averaged in

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191 Q-space. For the relative intensities we are then left with eq 6-5, where S is the spin component perpendicular to the scattering vector Q and pi is the Boltzmann factor of level i.175 The results of these calculations are given in Table 6-9 for both isomers of complex 23 The overall agreement with the experimental data is very good. f i j i ip E,2 2S d d d (6-5) The S = 10 ground state of the neutral compound is empirically understood by considering the ferrimagnetic ar rangement first suggested for 1 As one MnIII ion is reduced to a MnII, one would intuitively expect a gr ound state spin S = 21/2 instead of 19/2. With one more electron reduction step we recover S = 10 again, but we would naively expect S = 11. This pattern clearly shows that the canonical picture of the Mn12 ground state should be handled with care, as the wrong ground state can easily be predicted. This is due to several factors; the ground state is extremely dependent on the competition between the various exchange coup lings in the cluster and any change, for example converting one MnIII to a MnII ion, can considerably modify the low energy levels. We know that in 1 low energy spin states lie cl ose in energy to the ground state, the first one as low as 40 cm-1,170b,170c and a minor change in the exchange couplings can easily alter the energy level ordering. The anisotropy parameter D decreases as these Mn12 complexes get successively reduced. The D values given in Table 6-9 are approximate values because the fitting model assumes axial symmetry, and these complexes are all crystallographi cally occupying non-axial sites.113 The upper limit for the energy barrier U can be evaluated for th e three compounds as the following: U = S2|D| = 45 cm-1 for z = 0, U = (S2 – )|D| = 31 cm-1 for z = 1and U = S2|D| = 26 cm-1 for z = 2-.

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192 Using INS spectroscopy, we have clearly s hown the effect of electron reduction in Mn12 derivatives. The ground state spin varies from S = 10 for the neutral and two-electron reduced compounds to S = 19/2 for the one-electron reduced species. Only the knowledge of all the exchange couplings will permit a definitive rationalization of the ground state evolution as a function of elect ron reduction. The anisotropy is clearly reduced upon addition of electrons to the neutral species, as e xpected for the loss of a JT elongation axis as a MnIII ion is converted to a MnII ion. 6.2.5 Magnetochemistry of Complexes 20 22 6.2.5.1 DC studies Variable-temperature DC susceptibility measurements were performed on powdered, microcrystalline samples of 20 21 and 22 2.5C7H8, restrained in eicosane to prevent torquing, in a 5.0 kG fiel d in the range of 5.0-300 K. The MT versus T dependences of complexes 20 and 22 2.5C7H8 are similar to those of previously studied [Mn12O12(O2CR)16(H2O)4] complexes with S = 10 ground states, exhibiting a nearly temperature-independent value of 17-19 cm3 K mol-1 ( 20 ) and 18-20 cm3 K mol-1 ( 22 2.5C7H8) in the 150-300 K range which then increa ses rapidly to a maximum of 47-49 cm3 K mol-1 ( 20 ) and 49-53 cm3 K mol-1 ( 22 2.5C7H8) at ~15 K before decreasing rapidly at lower temper atures (Figure 6-11).13,22 The data for the [Mn12]complex 21 show a similar temperature dependence. The MT value of 21 slowly increases from 19.65 cm3 K mol-1 at 300 K to a maximum of 51.35 cm3 K mol-1 at 15.0 K and then decreases rapidly at lower temperatures. The maximum value indicates a high ground state spin (S) value for each of the complexes, with the low temperature decrease primarily due to a combination of Zeeman and zero-fiel d splitting effects. The spin-only (g = 2) values for units composed of non-interacting MnIV 4MnIII 8 ( 20 ), MnIV 4MnIII 7MnII ( 21 ) and

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193 MnIV 4MnIII 6MnII 2 ( 22 ) ions are 31.5 cm3 K mol-1, 32.9 cm3 K mol-1 and 34.3 cm3 K mol-1, respectively. The MT value at 300 K of each of the complexes is less than that expected for non-interacting metal ions, indicating th e presence of appreci able intramolecular exchange interactions. Temperature (K) 050100150200250300 20 30 40 50 MT (cm3K mol-1) Temperature (K) 050100150200250300 15 20 25 30 35 40 45 50 55 MT (cm3K mol-1) Temperature (K) 050100150200250300 15 20 25 30 35 40 45 50 55 MT (cm3K mol-1)(a)(b) (c) Figure 6-11. Plot of MT versus T for dried, microcrystalline samples of (a) [Mn12] complex 20 (b) [Mn12]complex 21 and (c) [Mn12]2complex 22 2.5C7H8 in eicosane. M is the molar DC magnetic susceptibility measured at 5.0 kG. As with other Mn12 clusters, a theoretical treatment of the DC magnetic susceptibility data51 as a means of determining the magn itude of the exchange parameters between the Mn ions is not simple. Instead, we concentrated only on determining the ground state spin of the molecules using magne tization measurements in the 1-70 kG and 1.80-10.0 K ranges. The data, plotte d as reduced magnetization (M/NB) vs H/T, are

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194 shown for the three complexes in Figure 612, where N is Avogadro’s number and B is the Bohr magneton. For a system occupyi ng only the ground state and experiencing no zero-field splitting (ZFS), the various isofield lines woul d be superimposed and M/NB would saturate at a value of gS. The non-superimposition of the isofield lines clearly indicates the presence of ZFS. The data were fit using MAGNET,54 which assumes only the ground state is populated at these temperat ures and is based on the method described elsewhere involving diagonaliza tion of the spin Hamiltonian matrix, including axial ZFS (2S ˆ Dz) and Zeeman interactions, and in corporating a full powder average.53 The best fits are shown as solid lines in Figure 6-12 for complexes 20 22 and the fitting parameters are listed in Table 6-10. Attempts to fit the data with S = 17/2 ( 21 ) or S = 9 ( 22 ) gave unreasonable g values of 2.26 and 2.28, respectively; a g value significantly greater than 2 is not expected for a MnIV 4MnIII 7MnII or MnIV 4MnIII 6MnII 2 cluster, and these possibilities were therefor e discounted. Reasonable (but not as good) fits for 21 and 22 can also be obtained with other S values, however: the data for 21 can be fit with S = 21/2, g = 2.01 and D = -0.32 cm-1, and that for 22 with S = 11, g = 1.87 and D = -0.24 cm-1. Nevertheless, note that the S = 10 ( 20 ), S = 19/2 ( 21 ) and S = 10 ( 22 ) ground state values obtained from the best fits of the magnetization data are in agreement with these same values obtained from INS measurements on dried, microcrystalline deuterated analogues of 20 22 .164 Table 6-10. M/NB vs H/T fitting parameters for [Mn12]z (z = 0, 1-, 2-) complexes. Parameter [Mn12] ( 20 ) [Mn12]( 21 ) [Mn12]2( 22 ) S 10 19/2 10 g 1.87 2.04 2.05 D, cm-1 -0.40 -0.34 -0.29 D, K -0.58 -0.49 -0.42 D/g, cm-1 0.21 0.17 0.14 U, K 58 44 42

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195 (a)(b) (c) H/T (kG/K) 010203040 6 8 10 12 14 16 18 1 T 2 T 3 T 4 T 5 T 6 T 7 T M/NB H/T (kG/K) 010203040 6 8 10 12 14 16 18 20 1 T 2 T 3 T 4 T 5 T 6 T 7 T M/NB H/T (kG/K) 010203040 0 5 10 15 20 0.1 T 0.5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T M/NB Figure 6-12. Plot of M/NB versus H/T for dried, microcrystalline samples of (a) [Mn12] complex 20 (b) [Mn12]complex 21 and (c) [Mn12]2complex 22 2.5C7H8 in eicosane at the indicated applied fields. The solid lines are the fit of the data; see the table for the fit parameters. In order to confirm that the obtained para meters were the true global rather than local error minima, and to assess the uncertainty in the obtained g and D values, rootmean square D vs g error surfaces for the fits were generated using the program GRID.55 The error surfaces for 20 22 are shown in Figure 6-13 as contour plots for the D = -0.10 to -0.70 cm-1 and g = 1.7 to 2.1 ranges ( 20 ), D = -0.10 to -0.55 cm-1 and g = 1.8 to 2.25 ranges ( 21 ), D = -0.05 to -0.50 cm-1 and g = 1.9 to 2.2 ranges ( 22 ). For each, one very soft fitting minimum is observed indicating sign ificant uncertainty in the fit values; the lowest indicated contour describe s the region of minimum error for 20 from D -0.33 to

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196 -0.48 cm-1 and g 1.81 to 1.93, giving fitting parameters of D = -0.40 0.08 cm-1 and g = 1.87 0.06, for 21 from D -0.30 to -0.39 cm-1 and g 1.99 to 2.09, giving fitting parameters of D = -0.34 0.04 cm-1 and g = 2.04 0.05, and for 22 from D -0.26 to -0.32 cm-1 and g 2.01 to 2.09, giving fitting parameters of D = -0.29 0.03 cm-1 and g = 2.05 0.04. gD (cm-1)gD (cm-1)gD (cm-1)(a)(b) (c) 1.701.751.801.851.901.952.002.052.10 -0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 1.801.851.901.952.002.052.102.152.20 -0.55 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 1.901.952.002.052.102.152.20 -0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 Figure 6-13. Two-dimensional contour plot of the error surface for the D vs g fit for (a) [Mn12] complex 20 (b) [Mn12]complex 21 and (c) [Mn12]2complex 22 2.5C7H8. The asterisk indicates the soft minimum. The soft (shallow) fitting minima an d the resulting large uncertainties are undoubtedly the primary reason that the g values are slightly > 2 in some cases; this is not expected for Mn. We did not feel it appropriate to fix g at 2.0, preferring to quote in Table 6-10 the values given by the fit. The similarly significant uncertainties in the D values quoted in Table 6-10 mean that they are most useful for relative comparisons, and

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197 their absolute values should be treated with some caution. Also note that the D/g values obtained from the magnetization fits will be useful for comparisons with the more reliable D/g values obtained from hysteresis studies (vide infra). More reliable values of D have also been obtained from the INS measurements: 164 this sensitive technique detected two species present in each sample with D values of -0.463 and -0.437 cm-1 for 20 -0.368 and -0.332 cm-1 for 21 and -0.274 and -0.241 cm-1 for 22 in satisfying agreement with the single values obtained from the magnetization fits given in Table 6-10. A comparison of the fitting parameters show s that the ground state spin of each of the complexes changes only slightly as the Mn12 molecule is progressively reduced. The spin value does not change at all on tw o-electron reduction, suggesting that the complexes act as “spin buffers”, accepting elect rons with little or no change to the S value.61 This behavior is in cont rast to that observed with the axial magnetic anisotropy parameter, D; as the Mn12 molecule is progressively re duced, the magnetic anisotropy decreases, as reflected in the decreasing absolu te value of D. This trend is expected; the molecular anisotropy of a cluster is primarily a consequence of the single-ion anisotropies of the constituent metal ions and the relative or ientations of the anisotropy axes of these ions with respect to each other. The primar y source of the magnetic anisotropy in a Mn12 molecule is from Jahn-Teller distorted MnIII ions; MnII and MnIV are fairly isotropic ions. The addition of electrons to the neutral Mn12 molecule on reduction converts a formerly MnIII ion to a MnII ion. This results in a decrease in the number of MnIII ions and a concomitant decrease in the molecular anisotropy.

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198 The combination of a large ground state sp in and a negative magnetic anisotropy that has been decreased but is still reasonabl y large implies that in addition to complex 20 the reduced complexes, 21 and 22 might still function as SMMs as has been previously found for other onea nd two-electron reduced species.13,61,100 In order for these molecules to behave as SMMs, they must possess a sufficiently large energy barrier to magnetization reversal such that they di splay slow magnetization relaxation rates. The upper limit (U) of this ener gy barrier is given by U = S2|D| and U = (S2-)|D| for integer and non-integer spin s, respectively.12,19,22,23 These values of U for 20 22 are included in Table 6-10. Both AC susceptibility studies and DC magnetization studies below 1.8 K using a micro-SQUID92 apparatus were therefore performed. 6.2.5.2 AC studies On the basis of similar measurements on other [Mn12], [Mn12]and [Mn12]2systems,13,61,100 we expected that 20 22 would exhibit at least one frequency-dependent out-of-phase AC suscep tibility signal ( M ), an indicator of slow magnetization relaxation. Such signals have been observed for all Mn12 SMMs and are considered a signature of the SMM property altho ugh only hysteresis loops obtained from magnetization vs DC field scans provide defi nitive evidence of the SMM property. Key kinetic parameters can be obtained from M vs T data, including Ueff, the true or effective barrier to relaxation. Ueff is smaller than U, since quantum tunneling of the magnetization (QTM) represents a short-cut thr ough the barrier via higher lying ms levels of the ground state S spin manifold. In addition, the in-phase AC susceptibility ( M) is invaluable for the determination of the ground-state spin of a molecule.66,106,111i,112,119 In Figure 6-14 are shown the MT vs T plots for 20 22 at different AC oscillation frequencies. The MT values are temperature-indepe ndent down to the temperatures at which decreases due to

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199 slow magnetization relaxation are reac hed. The temperature-independent MT values show that only the ground st ates of the molecules are populated at these temperatures, and can be used to calculate the ground stat e S values without complications from a DC field. The MT values of ~ 48, ~ 47, and ~ 54 cm3 K mol-1 for 20 21 and 22 respectively, correspond to S = 10 / g = 1.87, S = 19/2 / g = 1.95, and S = 10 / g = 1.98, respectively, consistent with the fits of the DC magnetization data above. 0 10 20 30 40 50 0 10 20 30 40 50 Temperature (K) 246810 0 10 20 30 40 50 60 MT (cm3K mol-1) Figure 6-14. M T vs T plots for vacuum-dried [Mn12] complex 20 (top), [Mn12]complex 21 (middle), and [Mn12]2complex 22 (bottom) at 1000 (), 250 () and 50 Hz ().

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200 The sharp decreases in the MT signal at the lowest temperatures are accompanied by the appearance of frequency-dependent out-of-phase ( M ) signals, the characteristic signature of a superparamagnet-like species such as a SMM. These were studied in detail, for both dried samples and those maintained in contact with their mo ther liquor to avoid solvent loss. Previous studies have shown that environmen tal factors strongly influence the magnetic behavior of a SMM. Disordered solvent molecules of crystallization, the loss of such solvent molecules, and other environmental factors such as the sitesymmetry of a molecule (the crystal space gr oup) have accounted for: (i) a broadening or smearing out of QTM steps in hysteresis l oops such that the l oops appear smooth, without the periodic st eps expected for a SMM;62,63,66,114,111i (ii) the appearance of a second peak in the out-of-phase AC susceptibi lity that is indicative of a second relaxation process56 and (iii) differences in th e effective energy barrier (Ueff) for magnetization reversal between seemingly identical molecules.61 Hence, it is important to probe the relaxation behavior of SMMs by measurements on both vacuum-dried samples and on wet samples maintained in mother liquor to prevent solvent loss. For this reason, AC susceptibilities were measured in a 3.5 G AC field oscillating at frequencies up to 1500 Hz for dried samples 20 21 and 22 2.5C7H8 and wet crystals of 20 3CH2Cl2, 21 4.5CH2Cl2H2O and 22 6C7H8, facilitating an assessment of the effect of solvent loss on the magnetic properties. A comparison of the M vs T plots for dry and wet samples of 20 at three AC oscillation frequencies is shown in the top pa nels of Figures 6-15 and 6-16, respectively. In each plot are shown two peaks corresponding to two distinct rela xation processes. In the dry sample, the higher-temperature (HT) peak at ~ 6 K predominates with a relative

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201 integration ratio of approximately eleven times that of the lower-temperature (LT) peak at ~ 2.5 K, whereas in the wet sample, the HT signal predominates over the LT form by more than thirty-fold. M(cm3mol-1) Temperature (K) 246810 Figure 6-15. M vs T plots for vacuum-dried [Mn12] complex 20 (top), [Mn12]complex 21 (middle), and [Mn12]2complex 22 (bottom) at 1000 (), 250 () and 50 Hz ().

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202 m(cm3) Temperature (K) 246810 Figure 6-16. m vs T plots for wet crystals of [Mn12] complex 20 (top), [Mn12]complex 21 (middle), and [Mn12]2complex 22 (bottom) at 1000 (), 250 () and 50 Hz (). Because the integration ratio of HT:LT is mu ch larger for the wet crystals than for the dry sample, it seems reasonable that the LT peak arises as a result of the loss of highly volatile CH2Cl2 solvent molecules of crystallization. This solvent loss is deliberate and complete in the vacuum-dried sample, gi ving two molecular environments that each give a M signal. The presence of two M signals is commonly encountered for Mn12 complexes and has been previously a ttributed to Jahn-Teller isomerism,56-59 a phenomenon by which molecules differ in the relative orientation of one or more MnIII JT

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203 elongation axis. The LT M signal corresponds to the is omer with at least one abnormally-oriented JT elongation axis towa rds a bridging oxide ion while the HT M signal corresponds to the isomer with eight normally-oriented JT elongation axes. Consistent with most previ ous findings, the LT form in 20 is the minor species not just in the dried, microcrystalline sample, but especially in wet crystals. The X-ray crystal structure of 20 reveals that the JT elo ngation axis of Mn(11) is not clearly defined as already discussed, suggesting that a static disorder of the a bnormally-oriented JT axis of this ion might exist and that the JT elongati on axis of Mn(11) might be reorienting as interstitial solvent molecules of crystalliz ation are lost. Note that instead of molar susceptibility, the figure ordinates for a samp le maintained in mother liquor are simply the total magnetization (plotted as mT and m ) as the mass of a wet sample cannot be accurately determined. A comparison of the Ueff and pre-exponential factor 1/ 0 values in Table 6-11 for the HT peak of the wet and dr y samples shows that the relaxation process is probably the same in each sample. Table 6-11. Arrhenius parameters (Ueff and 0) for weta and driedb complexes 20 22 0, s Ueff, K Wet a,c 20 e 2.910-9 64 21 f 4.910-9 53 22 g 3.310-8 28 Dried b,c 20 h 3.310-9 66 21 h 4.010-9 54 22 i 2.710-8 29 Wet a,d 20 e 8.110-9 59 21 f j 3.010-10 49 21 f k 1.310-10 21 22 g 9.110-8 25 a crystals maintained in contact with mother liquor. b vacuum-dried crystals. c AC data only. d combined AC and DC data. e 20 3CH2Cl2 f 21 4.5CH2Cl2H2O g 22 6C7H8 h unsolvated. i 22 2.5C7H8 j slower-relaxing species. k faster-relaxing species.

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204 The M vs T plot for dry and wet samples of both 21 and 22 2.5C7H8 at three frequencies are shown in the middle and bottom panels of Figures 6-15 and 6-16, respectively. Only one peak is observed for each molecule at temperatures above 1.8 K, suggesting effectively only one molecular environment, a small range of axial and rhombic anisotropy parameters D and E, and consequently a small range of Ueff values. For 21 the crystal structure rev eals a static disorder betw een a carboxylate group and an adjacent water molecule, as described earlier, giving a mixture of isomers (77:23%) differing in the distribution of the four H2O ligands. However, only one M peak is seen, suggesting either (i) the two isom ers have very similar D and Ueff values and thus relax at essentially the same rate, or (ii) the differences in D and Ueff are so large that the fasterrelaxing form gives signals at < 1.8 K, th e operating limit of our SQUID instrument. Studies to lower temperatures are needed to address this point further (vide infra). In accord with the decreasing MnIII content, the M signals shift to lower temperatures on progressive reduction: 6-8 K [Mn12], 4-6 K range for [Mn12]-, and 2-4 K range for [Mn12]2-. This shift reflects the decreasing energy barrier to magnetization relaxation, Ueff. As expected, the change in Ueff with decreasing MnIII content is not linear. Ueff depends on a number of f actors, including S, the axial (D) and rhombic (E) ZFS parameters, fourth order spin Hamilt onian parameters, and others. Qualitative comparisons, however, are reasonable, a nd display a monotonic decrease of Ueff with MnIII content. 6.2.5.3 Relaxation studies using AC and DC data The temperature at the peak of the M signal is the point at which the rate of reorientation (relaxation) of the magnetizati on vector equals the operating frequency of the oscillating field. This is t hus a source of kinetic data, and Ueff can be determined by

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205 fitting the resulting relaxation rate (1/ ) vs T data to the Arrhen ius equation (eq 6-6), where is the relaxation time. = 0exp(Ueff/kT) (6-6) This is the characteristic behavior of a ther mally-activated Orbach process, where k is the Boltzmann constant and 1/ 0 is the pre-exponential term. The frequency dependence of the M peak for 20 22 was determined at eight oscill ation frequencies in the 5-1500 Hz range. Plots of vs 1/T for wet crystals of 20 22 are shown in Figure 6-17, with the leastsquares fit to eq 6-6 shown as a solid line. The Ueff and 0 values for both dried, microcrystalline samples and wet crystals ar e compiled in Table 6-11 and are similar to values previously obtained for [Mn12], [Mn12]and [Mn12]2systems. Within experimental uncertainty, the Ueff and 0 values for wet and dried samples are almost identical. ) 0.10.20.30.40.5 10-410-310-210-1 Ueff= 53 K Ueff= 28 K Ueff= 64 K (s)1/T (K-1) Figure 6-17. Plots of relaxation time ( ) vs 1/T for wet crystals of complexes 20 (), 21 () and 22 () using AC M data. The solid lines are the fits to the Arrhenius equation. See Table 6-11 for the fit parameters. To supplement the AC data, additional relaxation vs time measurements were obtained at temperatures below 1.8 K by DC magnetization decay vs time measurements. These data were obtained on si ngle crystals using a micro-SQUID apparatus. First, a

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206 large DC field of 1.4 T was applied to the samp le at ~ 5 K to saturate its magnetization in one direction, and the temperature was lowe red to a chosen value between 0.04 and 3.5 K. When the temperature was stable, the fi eld was swept from 1.4 to 0 T at a rate of 0.14 T/s, and then the magnetization in zero field was measured as a function of time. This gave a set of relaxation time ( ) vs T data, which were combined with the AC data and used to construct an Arrheniu s plot. The data are plotted as vs 1/T in Figures 6-18, 6-19, 6-20 and 6-21 together with the fits (dotted lines) to eq 6-6 for complex 20 the slower-relaxing species of complex 21 the faster-relaxing species of complex 21 and for complex 22 respectively; the fit parameters are given in Table 6-11. Good fits of the combined AC and DC data were obtained for 20 and 22 ; the Ueff for the HT JT isomer of 20 is 59 K, a typical value for Mn12, while Ueff for 22 is 25 K. These can be compared with 64 and 28 K, respectively, for wet crystals over the smaller T range of Figure 6-17. 0 0.25 0.5 0.75 1 0.11101001000M/Ms t (s) 3.5 K 3.4 K 3.3 K 3.2 K 3.1 K 3.0 K 2.9 K 2.8 K 2.7 K 2.6 K 2.5 K 2.4 K 2.3 K 2.2 K 2.1 K 2 K 1.3 K 1.5 K t (s) M/Ms 1 0.75 0.5 0.25 0 0.11 10 100 1000 3.5 K 3.4 K 3.3 K 3.2 K 3.1 K 3.0 K 2.9 K 2.8 K 2.7 K 2.6 K 2.5 K 2.4 K 2.3 K 2.2 K 2.1 K 2 K 1.3 K 1.5 K(a) (b) 10-310-11011031050.10.20.30.40.50.6 1/T (K-1) DC AC (s) Figure 6-18. Relaxation time vs temperat ure studies for a single crystal of 20 3CH2Cl2. (a) Plot of magnetization vs time decay in zero field. The magnetization is normalized to its saturation value, Ms. (b) Plot of relaxation time ( ) vs 1/T for using AC M and DC decay data. The dotted lin es are fits to the Arrhenius equation. See Table 6-11 for the fit parameters.

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207 (a) (b) 0 0.2 0.4 0.6 0.8 0.010.11101001000 1.8 K 1.7 K 1.6 K 1.5 K 1.4 K 1.3 K 2.4 K 2.3 K 2.2 K 2.1 K 2 K 1.9 K 2.5 K 2.6 K 10-510-310-110110310510700.20.40.60.81 DC AC 1/T (K-1) t (s) (s) M/Ms Figure 6-19. Relaxation time vs temperature studies for the slower-relaxing species of complex 21 4.5CH2Cl2H2O. (a) Plot of magnetization vs time decay in zero field. The magnetization is normalized to its saturation value, Ms. (b) Plot of relaxation time ( ) vs 1/T for using AC M and DC decay data. The dotted lines are fits to the Arrhenius equation. See Table 6-11 for the fit parameters. (a) (b) 0.7 0.8 0.9 1 0.010.11101001000 1.3 K 0.92 K 1.15 K 1.07 K 0.7 K 0.8 K 0.6 K 0.5 K 0.4 K 0.04 K 0.2 K 0.3 K 1.8 K 1.7 K 1.6 K 1.5 K 1.4 K 10-410-21001021041060246810 (s) M/Ms1/T (K-1) t (s) Figure 6-20. Relaxation time vs temperature studies for the faster-relaxing species of complex 21 4.5CH2Cl2H2O. (a) Plot of magnetization vs time decay in zero field. The magnetization is normalized to its saturation value, Ms. (b) Plot of relaxation time ( ) vs 1/T for using AC M and DC decay data. The dotted lines are fits to the Arrhenius equation. See Table 6-11 for the fit parameters.

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208 (a) (b) 10-510-310-1101103105107012345 (s)1/T (K-1) DC AC 0 0.25 0.5 0.75 1 0.11101001000 t (s) 1.8 K 1.7 K 1.6 K 1.5 K 1.4 K 1.3 K 1.07 K 1.2 K 1.12 K 0.95 K 1 K 0.88 K 0.82 K 0.75 K 0.7 -0.04 KM/Ms Figure 6-21. Relaxation time vs temperature studies for a single crystal of complex 22 6C7H8. (a) Plot of magnetization vs time decay in zero field. The magnetization is normalized to its saturation value, Ms. (b) Plot of relaxation time ( ) vs 1/T for using AC M and DC decay data. The dotted lines are fits to the Arrhenius equation. See Table 6-11 for the fit parameters. There is an interesting discrepanc y between the AC and DC data for 21 in Figure 6-19, which give Ueff = 53 K, 0 = 4.910-9 s and Ueff = 49 K, 0 = 3.010-10 s, respectively. Thus, the slope s of the plots (i.e., the Ueff values) are almost the same within experimental uncertainty, but the significant difference in 0 values (almost an order of magnitude) causes a noticeable offset. Both th e AC and DC measurements were repeated several times on different crystals with the same result. We are not sure of the exact origin of this difference, but it may be due to differences caused by the differing mounting conditions; the AC data were obtaine d on crystals, maintained in mother liquor, that were quickly transferred to the SQUID and cooled to low temperatures to avoid solvent loss, whereas the DC data were on sing le crystals in Apiezon grease. Crystals of 21 4.5CH2Cl2H2O are very sensitive to solvent loss and readily fracture even under grease, making handling very difficult. Note that hysteresis loops described below also

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209 show greater complexity for 21 4.5CH2Cl2H2O compared with those for 20 3CH2Cl2 and 22 6C7H8. 6.2.5.4 Variable-frequency AC susceptibility studies To further understand the nature of the magnetization relaxation processes in these clusters, more detailed AC experiment s were carried out on wet crystals of 20 3CH2Cl2, 21 4.5CH2Cl2H2O, and 22 6C7H8. At a fixed temperature of 4.0 K ( 20 ), 3.4 K ( 21 ) and 2.2 K ( 22 ), the in-phase (m) and out-of-phase (m ) components of the AC magnetic susceptibility were measured as the frequency ( ) of oscillating AC field was varied in the range of 0.1-1500 Hz. For a single relaxation process, the m and m behavior as a function of angular frequency ( ) is given by eqs 6-7 and 6-8, respectively, while for a distribution of single relaxati on processes, the m and m behavior is expressed by eqs 6-9 and 6-10, respectively, 2 21 ) ( ) ( ts T s (6-7) 2 21 ) ( ) ( ts T (6-8) ) 1 ( 2 1 1) ( ) 2 / sin( ) ( 2 1 )] 2 / sin( ) ( 1 )[ ( ) ( t Ts T s (6-9) ) 1 ( 2 1 1) ( ) 2 / sin( ) ( 2 1 ) 2 / cos( ) )( ( ) ( ts T s (6-10) where s is the adiabatic susceptibility, T is the isothermal susceptibility, = 2 is the angular frequency, and is the magnetization relaxation time. An additional parameter, which takes a value between 0 and 1 and gauges the width of the distribution, is included in the expressions for a distribut ion of single relaxation processes. Plots of m vs m (Cole-Cole or Argand plots) are shown for complexes 20 22 in the top, middle, and

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210 bottom panels of Figure 6-22, respectively.176 A least-squares fitting of the data to a single relaxation process is show n as a dashed line while the fitting to a distribution of single relaxation processes is shown as a so lid line; clearly, significantly improved fits are obtained for the latter. On this basis, it is concluded that in wet crystals of complexes 20 22 the magnetization relaxes via only a single process and that there is a distribution in this single relaxation process (i.e ., a distribution of relaxation barriers). m(cm3)m(cm3) (a)(b) (c) m(cm3) m(cm3)m(cm3)m(cm3) Figure 6-22. Argand pl ot of m vs m for wet crystals of (a) complex 20 3CH2Cl2 at 4.0 K, (b) complex 21 4.5CH2Cl2H2O at 3.4 K and (c) complex 22 6C7H8 at 2.2 K. The dashes lines (blue) are a leastsquares fitting of the data to a single relaxation process as described by eqs 67 and 6-8. The solid lines (red) are a least-squares fitting of the data to a di stribution of single relaxation processes as described by eqs 6-9 and 6-10; se e Table 6-12 for the fit parameters. In Figures 6-23, 6-24 and 6-25 are show n plots of m vs frequency and m vs frequency for wet crystals of complexes 20 22 As expected, least-squares fitting of the data to a distribution of single relaxation processes (solid red li ne) is better than that to

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211 only a single relaxation process (dashe d blue line). The relaxation times ( ) obtained from the two fitting schemes are very similar for each complex, and the main difference in the fitting parameters occurs in the values of the adiabatic and isothermal susceptibility. Frequency (Hz) Frequency (Hz) 0.1110100100010000 0.1110100100010000 m(cm3) m(cm3)(a) (b) Figure 6-23. Plot of (a) the in-pha se (m) and (b) the out-of-phase (m ) AC magnetic susceptibility vs frequency at 4.0 K for complex 20 3CH2Cl2. The dotted lines (blue) are a least-squares fitting of the data to a single relaxation process. The solid lines (red) are a least-squares fitting of the data to a di stribution of single relaxation processes; see Table 6-12 for the fit parameters. Frequency (Hz) Frequency (Hz)m(cm3) m(cm3)(a) (b) 0.1110100100010000 0.1110100100010000 Figure 6-24. Plot of (a) the in-pha se (m) and (b) the out-of-phase (m ) AC magnetic susceptibility vs frequency at 3.4 K for complex 21 4.5CH2Cl2H2O. The dotted lines (blue) are a least-squares f itting of the data to a single relaxation process. The solid lines (red) are a le ast-squares fitting of the data to a distribution of single re laxation processes; see Table 6-12 for the fit parameters.

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212 Frequency (Hz) 0.010.1110100100010000 Frequency (Hz) 0.010.1110100100010000 m(cm3) m(cm3)(a) (b) Figure 6-25. Plot of (a) the in-pha se (m) and (b) the out-of-phase (m ) AC magnetic susceptibility vs frequency at 2.2 K for complex 22 6C7H8. The dotted lines (blue) are a least-squares fitting of the data to a single relaxation process. The solid lines (red) are a least-squares fitting of the data to a di stribution of single relaxation processes; see Table 6-12 for the fit parameters. Table 6-12. Least-squares fitting parameters of the in-phase (m) and out-of-phase (m ) AC magnetic susceptibility vs fre quency data at 4.0 K for complex 20 3CH2Cl2, 3.4 K for complex 21 4.5CH2Cl2H2O and 2.2 K for complex 22 6C7H8 to a single relaxation process or to a distribution of a single relaxation processes. Compound Single relaxation process Distribution of single relaxation processes m m m m 20 3CH2Cl2 S (cm3) 2.1910-5 0.50 S (cm3) 1.8910-5 -7.00 T (cm3) 1.6910-4 0.50 T (cm3) 1.7410-4 -7.00 (s) 0.0515 0.0526 (s) 0.0525 0.0525 0.216 0.137 21 4.5CH2Cl2H2O S (cm3) 9.4810-6 6.50 S (cm3) 8.2610-6 -7.00 T (cm3) 4.4010-5 6.50 T (cm3) 4.5710-5 -7.00 (s) 0.0401 0.0403 (s) 0.0407 0.0401 0.249 0.170 22 6C7H8 S (cm3) 2.6410-5 0.500 S (cm3) 2.1410-5 -7.00 T (cm3) 3.8210-4 0.500 T (cm3) 3.8610-4 -7.00 (s) 0.0133 0.0133 (s) 0.0133 0.0133 0.155 0.0963 The width of the distribution is gauged by the fitting parameter ; average values of obtained from fitting of plots of m vs frequency and m vs frequency to a distribution of single relaxation processes are 0.177 ( 20 ), 0.210 ( 21 ), and 0.126 ( 22 ). A complete listing of the fitting parameters is included in Table 6-12. These results are consistent

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213 with similar studies done on other Mn12 complexes;100 the main conclusion is that the samples have a distribution of energy relaxati on barriers, i.e., a dist ribution of D values due to a distribution in local molecular envi ronments. It should be noted that at the temperature was which the measurement was carried out for complex 21 only the HT (slower-relaxing) isomer was studied. Similar measurements at much lower temperatures are needed to study the LT (f aster-relaxing) isomer of 21 6.2.5.5 Hysteresis studies below 1.8 K The AC measurements suggest that complexes 20 22 function as SMMs, and this was confirmed by hysteresis loops obtained fr om magnetization vs DC field scans. These were performed on single crystals of 20 3CH2Cl2, 21 4.5CH2Cl2H2O, and 22 6C7H8 using a micro-SQUID apparatus.92 In Figure 6-26 are shown the temperature and scanrate dependence studies. The coercivities cl early increase with decreasing temperature and increasing scan rate, as expected for the superparamagnet-like behavior of SMMs. The loops also clearly show the steps at pe riodic values of applied field due to QTM, which causes a surge in the relaxation as ms levels on opposite sides of the S = 10 double well potential energy barrier come into res onance at those field positions. The field separation, H, between the steps is proportional to D and is given by eq 6-11. B D H g (6-11) Measurement of the step positions in Figure 6-26 gave an average H of 0.475 T, and a resulting |D|/ g value of 0.22 cm-1, consistent with both the valu es obtained from fits of the magnetization data for dried 20 (0.21 cm-1) and from the INS studies (0.22 cm-1).164

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214 -1 -0.5 0 0.5 1 -1-0.500.51 M/Ms 0H (T) 0.004 T/s 1.3 K 1.8 K 2 K 2.2 K 2.4 K 2.6 K 2.8 K 3 K 3.6 K M/Ms -1 -0.5 0 0.5 1 H0(T) 4 mT/s 1.3 K 1.8 K 2 K 2.2 K 2.4 K 2.6 K 2.8 K 3 K 3.6 K -1 0 0.5 1 -0.5 -1 -0.5 0 0.5 1 -1-0.500.51 0.560 T/s 0.280 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s M/Ms 0H (T) 2.4 K M/Ms 1 0.5 0 -1 1 0.5 H0(T) 0 -0.5 -1 -0.5 2.4 K 0.560 T/s 0.280 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s(a) (b) Figure 6-26. Magnetization hysteresi s loops for a single crystal of 203CH2Cl2: (a) temperature dependence at a fixed sc an rate of 4 mT/s; (b) scan-rate dependence at a fixed temperature of 2. 4 K. M is normalized to its saturation value, Ms. The corresponding hysteresis loops on 21 are shown in Figures 6-27 and 6-28. They clearly reveal two differe nt species within the crystal. Approximately 28% of the molecules have smaller Ueff and D values than the remaining 72%. Analysis of the step positions of the faster-relaxing species (28%) gave an average H 0.18 T and |D|/ g 0.042 cm-1, whereas the slower-relaxing species (72%) gave H 0.33 T and |D|/ g 0.15 cm-1. The logical conclusion is that the two isomers of 21 detected in the crystal structure in ~ 77:23% ratio corres pond to the two species detected in the hysteresis loops of Figures 627 and 6-28 in a ~ 72:28% ratio. The much smaller D value of the minor component and the resulting small Ueff would be consistent with the AC M signal occurring at < 1.8 K and thus not obser ved in Figures 6-15 and 6-16. Of course, we cannot rule out that the same ~ 3:1 ratio of two species in the crystal structure and the hysteresis loops may just be a coincidence, and the faster-relaxing sp ecies may in reality be due to a JT isomer with an abnormally-ori ented JT axis statically disordered about multiple MnIII ions and thus not showing up in the structural parameters of the X-ray

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215 structure. Finally, examination of the loop co ercivities for each sub-species shows that both exhibit the temperature and scan-rate dependence of a SMM. -1 -0.5 0 0.5 1 -1-0.500.51 M/Ms 0H (T) 0.04 K 1.5 K 1.5 K 0.04 K 0.008 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.280 T/s 0.002 T/s 0.004 T/s 1 0.5 0 -0.5 -1 1 0.5 0 -0.5 -1 -1 -0.5 0 0.5 1 -1-0.500.51 M/Ms 0H (T) 0.002 T/s 1.3 K 1.9 K 1.15 K 1.5 K 1.4 K 1.6 K 1.8 K 1.7 K 1.05 K 0.9 K 0.8 0.04 K 2 mT/s 0.8 –0.04 K 0.9 K 1.05 K 1.15 K 1.3 K 1.4 K 1.5 K 1.6 K 1.7 K 1.8 K 1.9 K 1 0.5 0 -0.5 -1 1 0.5 0 -0.5 -1 (a) (b)M/MsH0(T)M/MsH0(T) Figure 6-27. Magnetization hysteresi s loops for a single crystal of 214.5CH2Cl2H2O: (a) temperature dependence at a fixed scan rate of 2 mT/s; (b) scan-rate dependence at a fixed temperature of 0.04 K. Loops for both the fasterand slower-relaxing species are shown. M is normalized to its saturation value, Ms. -1 -0.5 0 0.5 1 -1-0.500.51 0.560 T/s 0.280 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/s M/Ms 0H (T) 0.04 K 0.04 K 0.004 T/s 0.280 T/s 0.140 T/s 0.560 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.002 T/s 1 0.5 0 -0.5 -1 1 0.5 0 -0.5 -1 -1 -0.5 0 0.5 1 -1-0.500.51 0.280 T/s 0.140 T/s 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/s M/Ms 0H (T) 1.5 K 1.5 K 0.002 T/s 0.140 T/s 0.070 T/s 0.280 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s 1 0.5 0 -0.5 -1 1 0.5 0 -0.5 -1 (a) (b)H0(T) H0(T)M/MsM/Ms Figure 6-28. Magnetization hysteresi s loops for a single crystal of 214.5CH2Cl2H2O: (a) scan-rate dependence at a fixed te mperature of 1.5 K of the slowerrelaxing species; (b) scan-rate dependen ce at a fixed temperature of 0.04 K of the faster-relaxing species. M is normalized to its saturation value, Ms.

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216 The corresponding hysteresis loops on 22 are shown in Figure 6-29, and they are overall similar to those for 20. There is only one species pres ent, and the step pattern is well defined. Again, the coerci vities increase with decreasi ng temperature and increasing scan rate, as expected for a SMM, an d as seen previously for another [Mn12]2salt.61 Analysis of the step positions gave |D|/ g 0.15 cm-1, a value consistent with that obtained from both INS164 and magnetization vs field studies on a dried sample of this complex. -1 -0.5 0 0.5 1 -1-0.500.51 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/s M/Ms 0H (T) 1.0 K M/Ms1 0.5 0 -0.5 -1 1.0 K H0(T)1 0.5 0 -0.5 -1 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/s -1 -0.5 0 0.5 1 -1-0.500.51 M/Ms 0H (T) 0.002 T/s 1.3 K 1.15 K 1.4 K 1 K 0.9 K 0.4 0.04 K 1.5 K 0.8 K 0.7 K 0.6 K 0.5 K 1 K 0.9 K 1.5 K 1.4 K 0.8 K 2 mT/s 1.3 K 0.7 K 0.5 K 0.6 K 1.15 K 0.4 –0.04 K H0(T)1 0.5 0 -0.5 -1 M/Ms1 0.5 0 -0.5 -1 (a) (b) Figure 6-29. Magnetization hysteresi s loops for a single crystal of 226C7H8: (a) temperature dependence at a fixed sc an rate of 4 mT/s; (b) scan-rate dependence at a fixed temperature of 2. 4 K. M is normalized to its saturation value, Ms 6.3 Conclusions The goal of this investigation was to st udy the effect of reduction on the magnetic properties of a family of Mn12 SMMs with identical peripheral ligation. The syntheses, crystal structures and magnetic properties ha ve been obtained on such a family with pentafluorobenzoate ligands spa nning three oxidation levels. 19F NMR spectra have been obtained on all three complexes, establishing re tention of the structures on dissolution, with fast de-trapping (on the 19F NMR timescale) of the a dded electrons leading to effective D2d solution symmetry in every case. AC a nd DC susceptibility studies on dried,

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217 microcrystalline samples of 20-22 establish that S = 10, 19/2, and 10 for 20-22, respectively, and show a decrease in the |D| value with decreasing MnIII content, i.e., the magnetic anisotropy decreases with reduction, and the magnetization re laxation rate thus increases. Magnetization vs DC field scan s exhibit hysteresis, establishing all the compounds to be SMMs. The hysteresis loops al so exhibit the steps characteristic of QTM. AC susceptibility measurements carried out on wet crystals maintained in mother liquor to prevent the loss of interstitial solvent and on dried, microcrystalline samples gave only small differences in Ueff and 0 between wet and dry samples. However, measurements on wet crystals do avoid certain complications associated with solvent loss, such as an increase in the am ount of the faster-relaxing form of 20. This again emphasizes the importance of comparing data from different techniques using samples maintained in the same way. For example, the data from micro-SQUID hysteresis studies on wet single crystals are best compared to magnetization studies on similarly wet crystals. 6.4 Experimental 6.4.1 Syntheses All manipulations were performed under aerobic conditions us ing materials as received, except where otherwise noted. [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) was prepared as described elsewhere.50 [Mn12O12(O2CC6F5)16(H2O)4] (20). A slurry of complex 1 (2.0 g, 0.97 mmol) in CH2Cl2 (50 cm3) was treated with HO2CC6F5 (3.7 g, 0.017 mol). Th e solution was stirred overnight and the solvent was removed in vacuo Toluene (20 cm3) was added to the residue, and the solution was again evaporated to dryness. The addition and removal of

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218 toluene was repeated two more times. The remaining solid was redissolved in CH2Cl2 (50 cm3) and treated again with HO2CC6F5 (0.41 g, 1.9 mmol). After 12 h, three more cycles of addition and remova l of toluene were performed. The residue was redissolved in CH2Cl2 (50 cm3) and filtered. An equal volume of heptane was added and the solution was allowed to stand undistur bed at room temperature fo r 4 days. The resulting black crystals were collected by filtration, washed with heptane, and dried in vacuo ; yield 91%. Anal. Calcd (found) for 20 HO2CC6F5 (C119H9F85Mn12O50): C, 31.86% (31.90%); H, 0.21% (0.38%). A crystallography sa mple was grown slowly from CH2Cl2/heptane and maintained in mother liquor to avoid so lvent loss. Selected IR data (KBr, cm-1): 1699 (w), 1604 (vs), 1552 (s), 1507 (s ), 1471 (m), 1417 (vs), 1351 (s), 1302 (m), 1231 (s), 1152 (vs), 1091 (m), 1015 (m), 857 (s), 797 (w), 778 (vs), 692 (m), 667 (m), 632 (s, br), 549 (m), 520 (m), 500 (m), 441 (w), 418 (w). For the preparation of the INS sample, the H2O molecules were exchanged with D2O by treatment of 20 (6.0 g, 1.4 mmol) in MeCN (100 cm3) with D2O (0.50 cm3, 0.028 mol) under a nitrogen atmosphere. The soluti on was stirred overnigh t and the solvent was removed in vacuo The residue was redissolved in CH2Cl2 (150 cm3) and filtered. Crystallization of the residue was performed as described above. Anal. Calcd (found) for [Mn12O12(O2CC6F5)16(D2O)4] 23 (C112D8F80Mn12O48): C, 31.22% (31.30%); H, 0.00% (0.00%). (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] (21). Solid NMe4I (0.047 g, 0.23 mmol) was added to a stirred dark brown solution of complex 20 (1.0 g, 0.23 mmol) in MeCN (70 cm3). The solution was stirred overnight w ith no noticeable color change. Hexanes (25 cm3) were added to the reaction solution to facilitate the extraction of I2, the hexanes

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219 layer removed, and the MeCN layer evaporat ed to dryness. The residue was redissolved in MeCN (50 cm3), hexanes (25 cm3) were again added to extract further I2, separated, and the MeCN solution evaporated to dryness. This process was repeated six more times to complete the removal of I2. The residue was dissolved in CH2Cl2 (50 cm3) and filtered. An equal volume of heptane was added and th e solution was allowed to stand undisturbed for 4 days. The resulting black microcrystal line product was recrystallized twice more, filtered, washed with heptane and dried in vacuo ; yield 74%. Anal. Calcd (found) for 21 (C116H20F80N1Mn12O48): C, 31.85% (32.00%); H, 0.46% (0 .47%); N, 0.32% (0.33%). X-ray quality crystals were grown slowly from CH2Cl2/heptane and maintained in mother liquor to avoid solvent loss. Selected IR data (KBr, cm-1): 1649 (s), 1604 (s), 1557 (m), 1522 (s), 1492 (vs), 1417 (vs), 1387 (vs), 1294 (m ), 1116 (m), 996 (s), 948 (m), 936 (m), 823 (w), 758 (s), 746 (s), 708 (m), 653 (m), 608 (m), 583 (m), 560 (m), 525 (m), 505 (w), 455 (w), 420 (w). For the preparation of the INS sample, the H2O molecules coordinated to the complex were exchanged with D2O by treatment of 21 (6.0 g, 1.3 mmol) in MeCN (100 cm3) with D2O (0.48 cm3, 0.027 mol) under a nitrogen atmosphere. The solution was stirred overnight and th e solvent was removed in vacuo The residue was redissolved in CH2Cl2 (150 cm3) and filtered. Crystallization of the residue was performed as described above. Anal Calcd. (found) for (NMe4)[Mn12O12(O2CC6F5)16(D2O)4] 24 (C116H12D8F80N1Mn12O48): C, 31.79% (31.79%); H, 0.28% (0.21%); N, 0.32% (0.30%). (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] (22). Solid NMe4I (0.094 g, 0.47 mmol) was added to a stirred dark brown solution of complex 20 (1.0 g, 0.23 mmol) in MeCN (70 cm3). The solution was stirred overnight w ith no noticeable color change. Hexanes

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220 (25 cm3) were added to the reaction solution to facilitate the extraction of I2 as for 21, and the separated MeCN solution was evaporated to dryness. The residue was redissolved in MeCN (50 cm3), hexanes (25 cm3) were again added, and the solution was evaporated to dryness. This process was repeated six more times to complete the removal of I2. The residue was dissolved in CH2Cl2 (50 cm3) and filtered. Crystals were obtained by addition of a mixture of heptan e and toluene to the CH2Cl2 solution. The resulting black microcrystalline product was recr ystallized twice, filtered, wa shed with heptane and dried in vacuo ; yield 70%. Anal Calcd. (found) for 22 (C120H32F80N2Mn12O48): C, 32.40% (32.39%); H, 0.73% (0.68%) ; N, 0.63% (0. 63%). X-ray quality crystals were grown slowly from CH2Cl2/heptane-toluene (1:1), and maintained in mother liquor to avoid solvent loss. Selected IR data (KBr, cm-1): 1616 (vs), 1523 (vs), 1489 (vs), 1381 (vs), 1356 (s), 1291 (m), 1105 (m), 993 (s), 950 (m), 930 (m), 832 (w), 764 (s), 752 (s), 738 (s), 583 (s, br), 508 (m). For the preparation of the INS sample, the H2O molecules coordinated to the complex were exchanged with D2O by treatment of 22 (6.0 g, 1.3 mmol) in MeCN (100 cm3) with D2O (0.46 cm3, 0.026 mol) under a nitrogen atmosphere. The solution was stirred overnight and the solvent was removed in vacuo The residue was redissolved in CH2Cl2 (150 cm3) and filtered. Crystallization of the residue was performed as described above. Anal Calcd. (found) for (NMe4)2[Mn12O12(O2CC6F5)16(D2O)4] 25 (C120H24D8F80N2Mn12O48): C, 32.34% (32.47%); H, 0.54% (0.41%); N, 0.63% (0.70%). 6.4.2 X-ray Crystallography Data were collected using a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Suitable single crystals of 203CH2Cl2, 214.5CH2Cl2H2O and 226C7H8 were attached

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221 to glass fibers using silicone grease and tr ansferred to the goniostat where they were cooled to -80 C (20 and 21) and -80 C (22) for characterization and data collection. Each structure was solved by direct methods (SHELXTL)64 and standard Fourier techniques, and was refined using full-matrix leastsquares methods. All non-hydrogen atoms were refined anisotropically. Cell parameters were refined using up to 8192 reflections. For each complex, a full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were re-m easured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1%). Absorption corrections by integration were applied based on measured indexed crystal faces. A preliminary search of reciprocal space for 203CH2Cl2 revealed a set of reflections with no symmetry and no system atic absences. An initial choice of the centrosymmetric space group 1 P was subsequently confirmed by the successful solution of the structure. The asymmetric unit contains the Mn12 molecule and three disordered CH2Cl2 molecules of crystallization. The solven t molecules were significantly disordered and could not be modeled properl y. Hence, the program SQUEEZE,96 a part of the PLATON97 package of crystallographic software was used to calculate the solvent disorder area and remove its contribu tion to the overall intensity data. The pentafluorophenyl rings in two of the C6F5CO2 ligands [C(9)-C(14) and C(16)-C(21)] were disordered over two positions, whose site occupancy factors were dependently refined to 70:30% and 54:46%, respectively. Hydrogen atoms of the water ligands were not located and therefore not included in th e final cycle of refinement. A total of 2266 parameters were refined in the final cycle of refinement using 10424 reflections with I >

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222 2 (I) to yield R 1 and wR 2 of 8.80% and 21.79%, respectively. The final difference Fourier map was essentially featurele ss, the largest peak being 1.211 e -3 and the deepest hole being -0.702 e -3. For complex 214.5CH2Cl2H2O, an initial survey of a portion of reciprocal space located a set of reflections with a monoclinic lattice. Analysis of the full data set revealed that the space group was P 2/ c The asymmetric unit contains the Mn12 molecule, two onehalf NMe4 + cations, 4.5 CH2Cl2 molecules, and half of a water molecule of crystallization. Hydrogen atoms of the cations and solvent molecules were calculated in ideal positions and were refi ned with the use of a riding model. The hydrogen atoms of the four coordinated water molecules were located from a difference Fourier map and refined freely. A disorder was observed i nvolving a water molecule [O(17)] and a pentafluorobenzoate ligand [O(21)-C(64)-C(18) ]. Each was refined in two positions with their site occupancies being 77:23%. Atoms of the minor disorder position were refined with isotropic thermal parameters. A total of 2479 parameters were refined in the final cycle of refinement using 18144 reflections with I > 2 (I) to yield R 1 and wR 2 of 5.34% and 13.25%, respectively. The final difference Fourier map was essen tially featureless, with the largest peak and deepest hole being 1.704 and -1.540 e -3, respectively. A preliminary search of reciprocal space for 226C7H8 revealed a set of reflections with a monoclinic lattice. An initial choice of the space group C 2/ c was subsequently confirmed by the successful so lution of the structure. The asymmetric unit contains half of the Mn12 molecule, two toluene molecules in ge neral positions, two one-half toluene molecules, each of which is disordered over a center of inversion, and one NMe4 + cation. Hydrogen atoms were calculated in ideal pos itions and were refined with the use of a

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223 riding model. The pentafluorophe nyl rings in two of the C6F5CO2 ligands [C(16)-C(21) and C(51)-C(56)] were disordered over two sites, whose site occupancy factors were dependently refined to 58:42% and 50:50% respectively. A tota l of 1415 parameters were refined in the final cycle of refi nement using 7055 reflections with I > 2 (I) to yield R 1 and wR 2 of 5.81% and 14.31%, respectively. Th e final difference Fourier maps were reasonably clean, the maximum and minimum residual electron density being 0.933 e -3 and -0.620 e -3, respectively.

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224 CHAPTER 7 EFFECT OF SYMMETRY ON MAGNETIC BEHAVIOR: A STUDY OF THE HIGHSYMMETRY Mn12 SINGLE-MOLECULE MAGNET [Mn12O12(O2CCH2Br)16(H2O)4] 7.1 Introduction Mesoscopic physics, an area of research that focuses on the boundary between classical and quantum behavior, has become a very attractive and intensely studied field, particularly since the discovery of quantum effects in single-molecule magnets (SMMs) in 1996.19,27,92,135,136,160,177,178 The crossover between the clas sical and quantum regimes of behavior can be explored e xperimentally by observing signat ures of quantum mechanical behavior, including quantum tunneling of magnetization (QTM), quantum phase interference and quantum cohere nce, in macroscopic systems.160,162,163 Materials that exhibit such mesoscopic quantum phenomena have potential applications in numerous areas, including quantum computing and magnetic information storage,135,158,159 and as such, are actively investigated both for f undamental scientific and for technological interests.20,28,179,180 Single-molecule magnets, or molecules that function as si ngle-domain magnetic particles are ideal systems for the inves tigation of quantum effects on macroscopic magnetization, offering numerous advantages over higher dimensionality, more complex magnetic systems. These advantages are la rgely a consequence of their well-defined structure; a single crystal of a SMM is an ordered ensemble of non-interacting molecules with a defined orientation with respect to the cell axes (Figure 7-1). The magnetic

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225 response is amplified by the number of mo lecules in the crystal and macroscopic measurements give direct access to single-molecule properties. (b) (a) Figure 7-1. Representations of the structure of 1. (a) Photograph of seven crystals and (b) packing diagram showing the orientation of molecules of 1 relative to each other with respect to the z axis (long axis) of an individual crystal. In 1996 it was reported that the dodecanuc lear mixed-valence, trapped-valence manganese-oxo cluster [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1), exhibits quantum tunneling of magnetization (QTM) as evidenced by a previously unseen field dependence of the relaxation rate of the magnetization (Figure 7-2).27 With this observation of quantum tunneling of magnetization as step-like features in the hysteresis loops of 1, the interface between classical and quantum behavi or was finally bridged. Not only unique to this molecule, the quant um phenomenon was also observed in the octanuclear FeIII oxo-hydroxo cluster [Fe8O2(OH)12(tacn)6]8+ (tacn = 1, 4, 7-triazacylononane)31 and has since been seen in numerous other SMMs. Subsequently, much research was devoted to an in-depth understanding of the magnetic behavior of single-molecule magnets, specifically complex 1. This cluster was the first known member of the Mn12 SMM family and until recently was the most ideal candidate for study by numerous techniques, owing to its crystallographic te tragonal symmetry.

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226 -1 -0.5 0 0.5 1 22.533.544.55 M/Ms 0Hz (T) 0.04 K 0.007 T/s 0.14 T 0.07 T 0 T 0H0.49 T 0.84 T 0.77 T 0.70 T 0.63 T 0.56 T 0.42 T 0.35 T 0.28 T 0.21 Ttr 0.04 K 7 mT/s M/Ms1 0.5 0 -0.5 -1 Hz(T) 4 3.5 3 2.5 2 4.55 Figure 7-2. Magnetization (M) vs fiel d of a single crystal of complex 1 at the indicated field sweep rate at 0.04 K. Transv erse fields are applied along the z axis of the crystal and M is normalized to its saturation value, Ms. The symmetry of a SMM strongly influe nces its magnetic behavior, dictating selection rules as described by certain terms in the spin Hamiltonian of the molecule, an example of which is given in eq 7-1. H O B O B S g H ) S S E( S D H4 4 4 4 0 4 0 4 B 2 2 2ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ y x z (7-1) The first term represents the second order uni axial magnetic anisotropy of the molecule (D is the axial anisotropy constant and z is the spin projection operator along the easyaxis of the molecule), the second term repres ents the second order transverse anisotropy (E is the rhombic anisotropy constant and x and y are the x and y projections of the total spin operator ), the third term represents the Zeeman interaction with an applied magnetic field H, the third and fourth term s represent the fourth order uniaxial and transverse anisotropy (0 4B and 4 4B are the fourth order uniaxia l and transverse anisotropy constants), respectively, and represents environmental couplings such as hyperfine, dipolar and exchange interactions.26 Quantum tunneling of magnetization is predicted when there are transverse terms in the spin Hamiltonian that do not commute with z.

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227 Such terms are provided by: (i) allowed transverse anisotropy terms for systems in which x y z directions; (ii) transverse components of the magnetic fields due to dipolar interactions and/or hyperfine nuc lear fields; and (iii) the Z eeman interaction associated with the transverse component of the external magnetic fi eld. Mixing of degenerate ms sublevels on opposite sides of the double we ll potential describi ng the S = 10 ground state spin of the Mn12 molecule occurs in the presence of transverse interactions, causing a small tunnel splitting, and allowing relaxation of the magnetization by quantum tunneling through the two ms sublevels separated in energy by the tunnel splitting. A few of the techniques that have been used to study the magnetic behavior of the model Mn12 system, [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O, include inelastic neutron scattering (INS), high-frequency electr on paramagnetic resonance (HFEPR), lowtemperature hysteresis/magnetization measurements, and 55Mn nuclear magnetic resonance. Although these techni ques have provided substa ntial information about the magnetic properties of the cluster, many que stions still remain concerning certain abnormal features exhibited by 1 at low temperatures. These include the absence of tunneling selection rules and multiple, broad EPR absorption peaks. Until recently, it was assumed on the basis of X-ray diffraction studies that molecules of 1 possess strict axial symmetry, i.e., the second order tran sverse anisotropy constant, E, is zero. The remaining terms in the spin Hamiltonian allow quantum tunneling of magnetization only from even-numbered ms to odd-numbered ms sublevels. However, hysteresis loops coll ected on single crystals of 1 reveal that in addition to evento-odd resonances, there are al so steps that correspond to t unneling from odd-to-even ms sublevels. These resonances are forbidden by a spin Hamiltonian that assumes strict axial

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228 symmetry, and hence there is not an obvious ex planation to account for these resonances. Several reasons have been proposed to accoun t for this strange magnetic behavior of 1 however. These include crystal dislocations th at give rise to lo cal rotations of the anisotropy axes (Figure 7-3). Th ese local rotations give rise to a broad distribution of tunneling rates and account for odd tunneling re sonances. Unfortunately, the predicted distributions are considerably broader than those observed experimentally.181 Very recently, on the basis of X-ray studies, Cornia and others have proposed a more realistic model involving a discrete di sorder associated with th e acetic acid molecule of crystallization of 1. This disorder also gives rise to a locally varying rhombicity and hence a distribution of tunneli ng rates. However, the predic ted distribution in this case more closely resembles that wh ich is experimentally observed.173 xz Figure 7-3. Edge di slocation along the y axis with the extra plane y z inserted at z > 0. Until recently, there was only one Mn12 complex that met the requirements for strict axial symmetry.50 Unfortunately, the majority of mono-substituted neutral Mn12 clusters crystallize in space groups in which the crystallograp hic symmetry is two-fold or less. In fact, the only two mono-substituted published Mn12 clusters with 3-fold or greater symmetry are [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) and [Mn12O12(O2CCH2Br)16(H2O)4] (26).50,182 Each of these complexes crystallizes in a

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229 tetragonal space group, 4 I for 1 and I 41/ a for 26, with the molecules possessing S4 crystallographic symmetry. The difference of space group is of no consequence for the magnetic properties of these complexes; inst ead, the primary difference between the two clusters involves the nature of the solvent molecules of crystalliz ation. In the case of 1, the cluster crystallizes with four H2O and two MeCO2H molecules. The acetic acid molecules are disordered over sites with tw o-fold symmetry so the site occupancy is 50:50% (Figure 7-4). C9 C10 O14 O15 H15 C10A C9A O14A O15A H15A Figure 7-4. ORTEP representati on in PovRay format of the two-fold disorder of the MeCO2H solvent molecules of crystallization in complex 1. A crystallographic C2 rotation axis (not shown) pa sses through the center of the two MeCO2H molecules. Disordered acetic acid molecules have been proposed to give rise to a locally varying rhombic anisotropy that brea ks the axial symmetry of complex 1 and accounts for some of the abnormal behavior of the molecule.172 This disordered acetic acid molecule is strongly hydrogen-bonded to an ace tate ligand on the cluster that bridges two MnIII ions (Figure 7-5) [O(15)O(6) = 2.866( 6) and O(15)-H(15)O(6) = 174.6(3)]. Complex 26 also possesses crystallographic axial symmetry, but instead of having acetic acid molecules of crystallizati on, the molecule crystallizes with four relatively inert dichloromethane molecules (Figure 7-6b).

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230 C10 C9 O14 O15 C4 C3 O7 O6 H15 Figure 7-5. ORTEP representati on in PovRay format of th e hydrogen-bonding interaction between an MeCO2H molecule of crystallizatio n [O(14), O(15), C(9), C(10), H(15)] and an MeCO2 ligand [O(6), O(7), C(3), C(4)] bridging MnIIIMnIII pairs in complex 1. (a) (b) Figure 7-6. ORTEP representation in PovRay format of (a) [Mn12O12(O2CMe)16(H2O)4] with MeCO2H solvent molecules of crystallization and (b) [Mn12O12(O2CCH2Br)16(H2O)4] with CH2Cl2 solvent molecules of crystallization. Mn blue; O red; C gr ay; Br pink; Cl green; H light gray. The abnormal features in the hyste resis loops and EPR spectra of 1 constitute a key reason for an in-depth study of complex 26, to see if the same features are observed in the absence of hydrogen-bonding of a carboxylate ligand of the clus ter with the solvent. The acetate ligand that is involved in this strong hydrogen-bonding interaction is the one

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231 shown in Figure 7-5, consisting of atoms O(6) O(7), C(3), and C(4). Atom O(6) and the neighboring methyl C(4) atom exhibit unusually large displace ment parameters at room temperature compared with the remaining ace tate ligands, possibly reflecting unresolved disorder (Table 7-1). To investigate this issu e in more detail, Cornia and others collected a new set of X-ray diffraction data at 83 K.173 Preliminary structure refinement showed abnormal elongation of the displacements ellip soids of atoms O(6) and C(4) and to a lesser extent, O(7) and C(3). This MeCO2 ligand was consequently modeled over two positions, A and B, shown in green and purple colors, respectively; site occupancy factors refined to 0.46 and 0.54, respectively. When the acetate ligand occupies the A position, the hydrogen-bonding interaction with the MeCO2H solvent molecule exists (Figure 7-7). Mn3 O11 O2 O12 H12a O9 O3 O5 O12 H12b O14 C10 C9 O15 H15 O8 O7 O6 C3 C4 Figure 7-7. ORTEP representation in PovRay format with thermal ellipsoids at the 50% probability level of the coordination sphere of Mn(2) and Mn(3) of complex 1, showing the model of the MeCO2 ligand over two positions. A = green and B = purple. Table 7-1. Debye-Waller thermal parameters for (i) atoms of MeCO2 ligand involved in hydrogen-bonding interaction with MeCO2H solvent molecules of crystallization and (ii) atoms of MeCO2 ligands not involved in this interaction. Beq, 2 C O Disordered MeCO2 15.06 [C(4)] 4.35 [O(6)] Normal MeCO2 4.07 – 4.38 2.19 – 3.25

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232 The new study shows that the disorder of th e acetic acid molecule is transmitted to the Mn12 units via a strong hydrogen-bonding inte raction. Since the number of hydrogenbonded acetic acid molecules which surround each Mn12 unit can, in principle, range from zero to four, up to six isomeric forms of the cluster can be envisioned and these are shown in Figure 7-8.173 Two of them, the ones in which there are zero ( n = 0) or four ( n = 4) hydrogen-bond interactions possess S4 symmetry, while the remaining isomers have either C2 ( n = 2, trans ) or C1 ( n = 1 and n = 2, cis ) symmetry. Non-axial isomers must then be present in the lattice, wh ich provides a possible explanation for the intriguing tunneling behavior of complex 1. 6.25 % 25 % 12.5 % 25 % 25 % 6.25 % n = 4 n = 3 n = 2 ‘ trans ’ n = 2 ‘ cis ’ n = 1 n = 0 Figure 7-8. Depiction of th e six isomers of complex 1 that differ in the number of hydrogen-bonded MeCO2H molecules of crystalliza tion as discussed in the text. The red arrow is representative of such a hydrogen-bond interaction. It is therefore of importance to study th e magnetic behavior of a high symmetry Mn12 cluster that consists of only one species with strict axial symmetry, i.e., [Mn12O12(O2CCH2Br)16(H2O)4]4CH2Cl2 (264CH2Cl2). We herein describe the results of

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233 this work, the synthesis, single crys tal X-ray structure, single crystal 55Mn NMR spectrum, high-frequency electron parama gnetic resonance (HFEPR) spectra and magnetic properties of the high symmetry Mn12 complex. 7.2 Results and Discussion 7.2.1 Synthesis In a manner similar to the preparation of most Mn12 derivatives, we employed the previously developed carboxylate substitution procedure that involves the treatment of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) with an excess of RCO2H to introduce the BrCH2CO2 ligands onto the Mn12 core.13,38 Thus, a solution of complex 1 in a solvent mixture of MeCN and CH2Cl2 was treated with an excess of BrCH2CO2H. The transformation of 1 into 26 is summarized in eq 7-1. [Mn12O12(O2CMe)16(H2O)4] + 16 BrCH2CO2H [Mn12O12(O2CCH2Br)16(H2O)4] + 16 MeCO2H (7-1) The ligand substitution reaction is an equilibr ium that favors the product side when the pKa of the reactant acid is lower th an that of acetic acid; the pKaof BrCH2CO2H is 2.90 while that of MeCO2H is 4.76.34 Acetic acid was removed fr om the reaction system by multiple cycles of addition and removal of under reduced pressure as its toluene azeotrope (28:72%; b.p. 101 C at one atmosphe re). This procedure successfully led to the isolation and crystallization of [Mn12O12(O2CCH2Br)16(H2O)4] (26) in nearly quantitative yield (~ 96%). Crystallization from CH2Cl2/hexanes gave dark brown crystals of 264CH2Cl2 suitable for X-ray crystallography. 7.2.2 Electrochemistry Electrochemical studies on Mn12 complexes have revealed that they have a rich redox chemistry involving several oxida tion and reduction processes, the E1/2 potentials

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234 of which are very sensitive to the electr on withdrawing or dona ting ability of the carboxylate (RCO2 -). By ligand substitution of complex 1 (R = Me) with an appropriate electron withdrawing RCO2 group, the E1/2 reduction potentials have been shifted to relatively positive values. The shift of potentia ls can be correlated to the relative pKa values of the carboxylic acids. As the pKa of the acid decreases, the electron withdrawing ability of the RCO2 group increases; an electron withdrawing substituent causes the carboxylate to become less basic, reducing th e electron density on the metal center and thereby making the aggregate easier to reduc e and concomitantly harder to oxidize. On this basis, E1/2 potentials have been shifted to chem ically accessible values, leading to the isolation of the oneand two-electron reduced Mn12 species. 13,61,100 The cyclic voltammogram (CV) and diffe rential pulse volta mmogram (DPV) of 26 are typical of the Mn12 family of complexes, and are s hown in Figure 7-9. There are two quasi-reversible reduction waves at 0.57 and 0.21 V. Standard electrochemical requirements for quasi-reversible electron transfer, including CV peak separations, anodic/cathodic peak current ratio, DPV peak broadness, are fulfilled by each redox process. A study of the scan rate ( ) dependence of each reduction showed a linear dependence of peak curr ent with respect to 1/2, indicating that the processes are diffusion-controlled. This relationship is descri bed by the Randles-Sevcik equation (eq 7-2) ip = (2.687 105)n3/21/2D1/2AC (7-2) where n is the number of electrons appeari ng in the half-reaction of the redox couple, is the scan rate, A is the electrode area, D is th e diffusion coefficient of the analyte and C is the concentration of the an alyte. As expected, the E1/2 values of the first and second

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235 reduction process are shifted to more positive values than those of 1 (0.18 and -0.06 V, respectively) and are very similar to t hose of the monochloroacetate-substituted Mn12 derivative (R = ClCH2CO2 -), 0.60 and 0.30 V, respectivel y. This was expected on the basis of the similarity of the pKa values of ClCH2CO2H (2.85) and BrCH2CO2H (2.90). 0.0 0.2 0.4 0.6 0.8 1.0 Potential (V) Current0.21 V 0.57 V 10 A 4 A Figure 7-9. Cyclic voltammogram at 100 mV s-1 (top) and differential pulse voltammogram (bottom) for complex 26 in CH2Cl2 containing 0.1 M NBun4PF6 as supporting electrolyte. The i ndicated potentials are vs Fc/Fc+.

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236 Potential (V) 0.0 0.2 0.4 0.6 0.8 1.0 (a)Current 10 A 250 mV/s 300 mV/s 350 mV/s 400 mV/s 450 mV/s 500 mV/s (c) (b)Current (A)1/2r2= 0.999 r2= 0.999Current (A)1/2r2= 0.999 r2= 0.999 810121416182022242628 -30 -20 -10 0 10 20 30 810121416182022242628 -20 -10 0 10 20 30 40 50 Figure 7-10. Scan rate dependence of th e reduction waves at 0.57 V and 0.21 V of complex 26 in CH2Cl2 containing 0.1 M NBun4PF6 as supporting electrolyte. (a) Cyclic voltammogram at the indicat ed scan rates with corresponding plot of cathodic (top) and anodic (bottom) peak current dependence vs 1/2 for (b) 0.57 V reduction wave and (c) 0.21 V reduction wave. 7.2.3 X-Ray Crystal Structure of [Mn12O12(O2CCH2Br)16(H2O)4] (26) A labeled ORTEP40 plot in PovRay format of complex 26 is shown in Figure 7-11, together with a stereoview. The crystallographi c data and structure re finement details are collected in Table 7-2, and selected interato mic distances and angles are listed in Table A-16. Complex 264CH2Cl2 crystallizes in th e tetragonal space group I 41/ a (Figure 7-11). The Mn12 molecule is located on a crystallographic S4 improper rotation axis

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237 perpendicular to the plane of the molecule and passing through the central cubane unit. For the sake of brevity, references to specifi c atoms in the following discussion implicitly include their symmetry-relate d partners. The complex has the same structure as previously characterized [Mn12O12(O2CR)16(H2O)4] complexes, possessing a central [MnIV 4O4]8+ cubane moiety held within a non-planar ring of eight MnIII ions by eight 3-O2ions. The eight MnIII ions separate into two groups of four MnIII ions each. In the first group, each MnIII ion is coordinated to a single MnIV ion via two oxide bridges [Mn(2)], while in the second group each MnIII ion is coordinated to two MnIV ions via two oxide bridges [Mn(1)].29 Peripheral ligation is pr ovided by sixteen bridging BrCH2CO2 and four water molecules [O(1)]. Table 7-2. Crystallographic data for [Mn12O12(O2CCH2Br)16(H2O)4]4CH2Cl2. Parameter 264CH2Cl2 formulaa C36H48Cl8Mn12Br16O48 fw, g mol-1 3470.09 space group I 41/ a a 26.9948(16) b 26.9948(16) c 12.7245(11) deg 90 deg 90 deg 90 V 3 9272.6(11) Z 4 T C -100(2) radiation, b 0.71073 calc, g cm-3 2.480 cm-1 87.86 R 1 ( wR 2), %c,d 5.68 (15.43) a Including solvent molecules. b Graphite monochromator. c R 1 = || Fo| – | Fc|| / | Fo|. d wR 2 = [ [ w ( Fo2 Fc2)2] / [ w Fo2 )2]]1/2 where S = [ [ w ( Fo2 – Fc2)2] / ( n p )]1/2, w = 1/[ 2( Fo2) + ( m p )2 + n p ], p = [max( Fo2, 0) + 2* Fc2]/3, and m and n are constants.

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238 Mn3 Mn3a Mn3b Mn3c O5a O6a O11c O12c O5 O6 O11b O12b O11 O12 O6b O5b O11a O12a O6c O5c Mn2c O2c O3c O1c O1a O1 O1b Mn2b Mn2a Mn2 Mn1 Mn1b Mn1a Mn1c O3b O2b O7b O8b O9b O3a O8a O9a O7a O2a O7 O8 O2 O9 O10 O4 O4c O4b O4a O10c O10a O10b O7c O8c O9c O3 Figure 7-11. ORTEP representati on in PovRay format of [Mn12O12(O2CCH2Br)16(H2O)4] (26) with thermal ellipsoids at the 50% pr obability level except for the C and Br atoms, together with a stereopa ir. For clarity, the hydrogen atoms have been omitted. MnIV green; MnIII blue; O red; H2O yellow; Br pink; C gray. In all [Mn12O12(O2CR)16(H2O)4] complexes studied to date, the four water ligands coordinate only to the four MnIII ions in the second group desc ribed above [Mn(1)], either one water on each Mn, two each on two Mn, or similar.20 Seven different isomeric forms of monosubstituted, neutral Mn12 clusters which differ in th e arrangement of the four water ligands have been identified: (1,1,1,1), (2,2), (1,1,1), (1,1,2), (1,2,1), (2), and (0) (Figure 7-12). In complex 26, the four water ligands are coor dinated to Mn(1) in a 1:1:1:1

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239 pattern (Figure 7-12a). Th is disposition of one H2O ligand on each MnIII that is bridged to two MnIV ions by two oxide bridges has also been observed for [Mn12O12(O2CMe)16(H2O)4]50 and [Mn12O12(O2CC12H9)16(H2O)4] (C12H9 = biphenyl). (a) (b) (c)(d)(e) (f) (g) Figure 7-12. Seven different geometric isomers of Mn12 SMMs found in structurally characterized molecules: (a) (1,1,1,1); (b) (2,2); (c) (1,1,1); (d) (1,1,2); (e) (1,2,1); (f) (2); (g) (0) isomer. All of the Mn centers are six-coordinate with near-octahedral geometry. The Mn oxidation levels were qualitatively determ ined by charge consideration and also by evaluation of the bond distances around the Mn centers. These assignments were confirmed quantitatively by bond valence sum (BVS)41 calculations, indicating that Mn(1) and Mn(2) are MnIII and the remaining Mn center, Mn(3), is MnIV (Table 7-3). The protonation levels of the inorganic O atoms were also confirmed by a BVS calculation and the results are colle cted in Table 7-4. The eight MnIII centers exhibit a Jahn-Teller (JT) distortion, as expected for a high-spin d4 ion in near-octahedral

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240 geometry. As is almost always the case for MnIII ions, the JT distortion takes the form of an axial elongation of two trans bonds. The elongation typica lly lengthens bond distances by 0.1-0.2 . The JT elo ngation axes avoid the Mn-O2bonds, the shortest and strongest in the molecule, and thus are all axially disposed, roughly perpendicular to the [Mn12O12] disk-like core. As a result, there is a near parallel alignment of the eight MnIII JT elongation axes. This is the predominant f actor that determines the overall magnetic anisotropy of the molecule and hence, gr eatly influences the observed magnetic properties ( vide infra ). Table 7-3. Bond valence sum calculationsa for complex 264CH2Cl2. Atom Mn2+ Mn3+ Mn4+ Mn(1) 3.2665 2.9878 3.1368 Mn(2) 3.3000 3.0185 3.1689 Mn(3) 4.1305 3.7781 3.9664 a The underlined value is the one closest to th e actual charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value. Table 7-4. Bond valence sum calculationsa for selected oxygen atoms in complex 264CH2Cl2. Atom Vi Assignment O(3) 2.019 O2O(4) 2.002 O2O(130) 1.996 O2a The oxygen atoms is O2if Vi 2, OHif Vi 1, and H2O if Vi 0. The local JT axes of the two MnIII ions, Mn(1) and Mn(2), are canted away from the crystallographic symmetry ( z ) axis by = 34.0 and 7.9, respectively; the local MnIII symmetry axes are defined by the OO elongation axis of the MnIII coordination sphere (Figure 7-13). The direction of the local JT ax is is the direction of the principal hyperfine field of the MnIII ion, and the difference in of the two MnIII ions will be of relevance to the 55Mn spectroscopic measurements ( vide infra ).

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241 Mn1 O7 O1 O5 O2 O11 O3 O8 O6 O2 O12 Mn2 z z O9 Figure 7-13. ORTEP representation in PovRay format of the anisotropy axes (dashed lines) of Mn(1) and Mn(2) in complex 26, where is the angle of the local MnIII anisotropy axis with resp ect to the cr ystallographic z axis. MnIII blue; O red; Br pink; C gray. There is evidence of weak intermolecu lar interactions between separate Mn12 molecules. The Br(8)Br(8) distance show n as a dashed line between neighboring molecules in Figure 7-14 is 3.487 , signifi cantly longer than the sum of the covalent radii of Br (2.28 ), but much shorter than the sum of their van der Waals radii (3.70 ).183 This interaction is complicated by th e disorder over seven sites of the BrCH2CO2 ligand involved in this interaction. The Br Br distance given is between Br(8) and Br(8) on adjacent Mn12 molecules; the site occupancy fact or of Br(8) is only 19% however as will be discussed in the Experimental Section. The CH2Cl2 solvent molecules of crystallization are included in the packing diag ram in Figure 7-14 and it is clear that there are four CH2Cl2 solvent molecules per Mn12 unit. There is no evidence of hydrogenbonding and/or other types of interactions between the solvent molecules and the Mn12 molecule as have been found in 1 by Cornia and others.173 In Table 7-5 is shown a comparison of selected interatomic di stances and angles for complexes 1 and 26; the closeness of the parameters emphasizes the stru ctural similarities of the two complexes.

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242 a b c Figure 7-14. ORTEP representation in PovR ay format of the packing of complex 26. For clarity, the hydrogen atoms have been omitted. Representative pairs of interacting BrBr atoms between nei ghboring molecules are indicated by the dashed lines. Mn blue; O red; Br pink; C gray; Cl green. Table 7-5. Comparison of selected in teratomic distances () and angles ( ) for [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) and [Mn12O12(O2CCH2Br)16(H2O)4] (26). Parametera 1 26 MnIV – Oc (ax) 1.8954(13) 1.902(4) MnIV – Oc (eq) 1.9116(11), 1.9196(10) 1.922(4), 1.934(4) MnIV – Or 1.8592(11), 1.8795(11) 1.865(4), 1.878(4) MnIV – Oax 1.9131(13) 1.923(4) MnIII b – Or 1.8770(11), 1.8983(11) 1.871(4), 1.902(4) MnIII c – Or 1.8860(11), 1.8964(11) 1.898(4), 1.898(4) MnIII b – Oeq 1.9371(13), 1.9393(12) 1.932(5), 1.948(5) MnIII c – Oeq 1.9655(12), 1.9884(13) 1.959(4), 1.970(5) MnIII b – Oax 2.111(5)*, 2.228(3), 2.2318(13) 2.187(5), 2.220(5) MnIII c – Oax 2.117(11), 2.151(17)* 2.114(5) MnIII c – Ow 2.1735(15) 2.193(5) Or – MnIV – Or 84.96(5) 83.75(17) Or – MnIII b – Or 83.95(5) 82.91(17) Or – MnIII c – Or 93.29(5) 92.35(17) MnIVMnIV 2.8166(4), 2.8166(4), 2.9271(4) 2.8180(14), 2.8180(14), 2.9851(16) MnIII bMnIII c 3.321, 3.414 3.332, 3.388 MnIII bMnIVMnIV 120.688(13), 122.594(13) 119.24(5), 121.28(5) 178.380(14) 177.25(4) MnIII bMnIV 2.7643(3) 2.7938(12) MnIII cMnIV 3.445, 3.448 3.460, 3.461 a Oc = cubane O2-, Or = ring O2-, Oax = axial carboxylate, Oeq = equatorial carboxylate, Ow = water; b MnIIIb atoms: Mn(2) in 1 and Mn(2) in 26; c MnIIIc atoms: Mn(3) in 1 and Mn(1) in 26. Involves the acetate ligand disord er bridging Mn(2) and Mn(3).

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243 7.2.4 1H Nuclear Magnetic Resonance Spectroscopy In order to assess the soluti on state structure and stabilit y of the complex, an NMR spectroscopic investigation of complex 26 in CD2Cl2 was carried out. Chemical shifts and T1 times are listed in Table 7-6. 1H NMR spectroscopy has been used extensively in the past to investigate the be havior in solution of [Mn12O12(O2CR)16(H2O)4]z ( z = 0, 1-, and 2-) complexes.13,46,57,165 MnIII has a relatively fast elec tron relaxation time. As such, electron relaxation cannot f acilitate nuclear relaxation efficiently, making the MnIII ion generally suitable for NMR spectroscopic study. The 1H NMR spectrum of complex 26 in CD2Cl2 in Figure 7-15 shows three resonances with a 1:2:1 integration ratio. ax (III-III) 0 0 10 10 20 20 30 30 40 40 50 50 eq (III-III) ax (III-IV) * s a Figure 7-15. 1H NMR (300 MHz) spectrum at ~ 23 C in CD2Cl2 of 26; ax = axial, eq = equatorial, s = solvent protio-impurity; a = HO2CCH2Br impurity, = solvent impurities. As with other Mn12 derivatives, there are only thre e distinct types of bridging carboxylates in the NMR spectrum: (i ) four axial ligands bridging MnIIIMnIV pairs, (ii) four axial ligands bridging MnIIIMnIII pairs, and (iii) eight e quatorial ligands bridging MnIIIMnIII pairs. Solution studies of Mn12 molecules at room temperature have shown that there is a fluxional process that is fa st on the NMR timescale that rapidly exchanges

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244 the water ligands with one t ype of axial carboxylate ligand; 32 the one that has both its O atoms located on the JT elongation axes of the MnIIIMnIII pairs (Figure 7-16). This introduces dihedral mirror planes which make all of the equatorial carboxylate groups equivalent. In effect, the so lution state symmetry of a Mn12 molecule is D2d, giving three resonances in a 1:1:2 relative integration ratio (axial:axial:equatoria l) as shown in Figure 7-17. Table 7-6. Solution 1H NMR spectral data for complexes 1 and 26. Compound Peaka Assignmentb T1c R = CH2Br (26) 47.8 ax (III-III) 2.9 37.0 eq (III-III) 3.1 17.8 ax (III-IV) 5.1 R = Me (1) 48.7 ax (III-III) 3.6 41.7 eq (III-III) 3.4 14.0 ax (III-IV) 5.6 a ppm, at ~ 23 C. b ax = axial, eq = equatorial; II I-III and III-IV refer to the ligand bridging a MnIIIMnIII and MnIIIMnIV pair, respectively. c ms. Figure 7-16. ORTEP representation of the fluxi onal process that takes place between the water ligand and the axia l carboxylate bridging a MnIIIMnIII pair in a typical [Mn12O12(O2CR)16(H2O)4] molecule. MnIV green; MnIII blue; O red; C gray; H2O yellow.

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245 vvC2C2 Figure 7-17. ORTEP representati on in PovRay format of [Mn12O12(O2CCH2Br)16(H2O)4] (26) showing the D2d symmetry typical of a [Mn12O12(O2CR)16(H2O)4] molecule in solution. Another C2 rotation axis (not s hown) passes through the central cubane unit of the molecule. MnIV green; MnIII blue; Br pink; C gray; O red (except those O atoms in orange that are involved in the fluxional process). Longitudinal (spin-lattice) relaxation times ( T1) were determined using the inversion-recovery pulse method (180-90) to aid in the assignment of the peaks. Electrons on metal centers relax at very fa st rates, providing e fficient pathways for nuclear relaxation. In general, the longitudinal relaxation time is directly related to the distance of a nucleus fr om a paramagnetic center.166,167 The sequence consists of a 180 pulse that inverts the magnetization as far as possible out of its e quilibrium direction, followed by a variable delay ( ) and finally a 90 pulse to monitor the magnetization. In Figure 7-18 are shown several spectra of 26 obtained at selected values of showing the evolution of the magnetization relaxation as a function of time. Resonances were also assigned on the basis of relative in tegration ratios, peak broadness (r-6 dependence, where r is the distance to the paramagnetic centers), and comparisons with Mn12 derivatives

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246 possessing other carboxylate groups. The 1H NMR spectrum of complex 1 in CD3CN is shown in Figure 7-19 for comparison; similarly, T1 times for 1 are included in Table 7-6. 50 40 30 20(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) Figure 7-18. Array of i nversion recovery data of 26 in CD2Cl2 at 23 C. The number of transients measured at each value is 32. The delay times were (a) 4.16 10-5 s, (b) 8.32 10-5 s, (c) 1.16 10-4 s, (d) 3.33 10-4 s, (e) 6.66 10-5 s, (f) 1.33 10-3 s, (g) 2.66 10-3 s, (h) 5.32 10-3 s, (i) 1.06 10-2 s, (j) 2.13 102 s, and (k) 4.26 10-2 s after the 180 pulse. 0 0 10 10 20 20 30 30 40 40 50 50 ax (III-III) eq (III-III) ax (III-IV) s Figure 7-19. 1H NMR (300 MHz) spectrum at ~ 23 C in CD3CN of 1; ax = axial, eq = equatorial, s = the solvent protio-impurity; = H2O.

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247 The isotropic shifts of th e nuclei are expected to ha ve both contact (through-bond) and pseudocontact, or dipolar (through-space), contributions166-168 (eq 7-3). contact 0 dipolar 0 iso 0 (7-3) The dipolar shift arises from dipolar inter actions, or direct th rough-space interactions between the nuclear and electronic magnetic moments and the contact shift involves the interaction of electronic a nd nuclear magnetic moments via the metal-ligand bonding framework. The contact component of th e isotropic shift is dominated by two mechanisms: (i) direct delocalization whereby unpaired spin density is delocalized onto a ligand orbital of appropriate symmetry via dir ect overlap of the magnetic orbitals with the ligand orbitals and (ii) spin polarization wher eby unpaired spin density polarizes the spin density in an orthogonal, filled orbital. Direct delocalization of unpair ed spin density in the d orbitals on the paramagnetic metal center to the -CO2 system occurs by -symmetry overlap. Similarly, direct delocation of the positive spin density onto the -CH2Br nuclei occurs, giving downfield paramagne tic shifts of these NMR resonances. In addition, pseudocontact (dipolar) contri butions also likely affect the hydrogen paramagnetic shifts; derivatives such as 26 exhibit too wide of a range of shifts for nuclei located in the three different sites of the cluster on the basis of only a delocalization mechanism. 7.2.5 Single Crystal 55Mn Nuclear Magnetic Resonance Spectroscopy In addition to our examination of the solu tion state structure a nd stability of both 26 (and for comparison 1), a solid state NMR spectroscopic investigation of complexes 1 and 26 was carried out. This technique can be used to directly probe the magnetic structure and individual Mn i ons in these interesting molecules, and for this reason, we

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248 have carried out 55Mn NMR measurements on singl e crystals of complexes 1 and 26 and on an aligned dried, microcrystalline sample of 26.184 Such a study allows the assessment of the influence of the hydrogen-bonding of the MeCO2H solvent molecules of crystallization in 1 on the symmetry of the Mn12 core by comparison of the resulting NMR spectrum with that of 26, which has no significant such hydrogen-bonding perturbation of its Mn12 core. In addition, comparisons can be made between the NMR spectra obtained on an aligned powder and single crystal of 26. As will be shown, there are significant differences between the single crystal and aligned powder spectra. Specifically, the single crystals afford significantly higher spectral resolution than the aligned powder, and moreover, that significant differences in the chemical structure of 26 are induced upon vacuum-drying and powdering of the sample. Furthermore, angular dependence studies are now possible because of the use of high quality, large single crystals. Such studies were c onducted with rotations in the ab and ac planes in 1 T and 2 T fields, respectively. A comparison of 55Mn NMR signals obtained from (a) [Mn12O12(O2CCH2Br)16(H2O)4] (26) powder, (b) a si ngle crystal of [Mn12O12(O2CCH2Br)16(H2O)4]4CH2Cl2 (264CH2Cl2), and (c) a single crystal of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1), all in zero external field is presented in Figure 7-20. The observed peaks are assigned to the three types of Mn ions expected assuming S4 symmetry (Figure 7-21), following Goto and co-workers185 and Furukawa et al,186 who reported oriented powder NMR spectra for 1.

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249 220240260280300320340360380400 226228230232234 227.8 MHz 279.6 MHz 370.5 MHz Intensity (arb. units)Mn(1) Mn(2) Mn(3)Frequency (MHz)232.2 MHz 284.6 MHz 354.4 MHz 279.4 MHz 366.2 MHz 229.7 MHz 231.7 MHz 230.2 MHz (a) (b) (c) Figure 7-20. Comparison of zero-field 55Mn NMR spectra of (a) an aligned dried, microcrystalline sample of [Mn12O12(O2CCH2Br)16(H2O)4] (26), (b) a single crystal of [Mn12O12(O2CCH2Br)16(H2O)4]4CH2Cl2 (264CH2Cl2) and (c) a single crystal of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1). Assignments were made on the basis of individual resonance profiles, including central frequency and quadrupole splitting, a nd also by comparison of spectra with those already reported for powdered samples of complex 1.185 The position and width of the three peaks in the NMR spectrum can be explained by a combination of large internal hyperfine field and a small quadr upolar interaction. The internal field determines the resonance frequency of the lines and descri bes the interaction of the nuclear magnetic dipole moment with the local magnetic mome nt of the electron spin on the manganese centers. The quadrupole effect yields the obser ved line width and is related to the local

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250 electric field gradient symmetr y at the Mn sites. A nucleus with an electric quadrupole moment that is surrounded by a non-spherical el ectron distribution will interact with the electric field gradient of the asymmetric electron cloud. 55Mn is a such nucleus with an electric quadrupole moment as th e nuclear spin quantum number I equals 5/2. Hence, it is essential to the interpretation of the NMR spectrum that the electron distribution about the Mn centers within the cluster be evaluated. Mn3 Mn3a Mn3b Mn3c Mn2c Mn2b Mn2a Mn2 Mn1 Mn1b Mn1a Mn1c Figure 7-21. ORTEP representation in PovRay format of complex 26, emphasizing the three crystallographically independent Mn ions within the cluster. MnIV green; MnIII blue; O red; H2O yellow; Br pink. The MnIV site within the central cubane core is in an essentially non-distorted octahedral environment, and for this reason, the width of the peak corresponding to this Mn ion should be negligible. This is in contra st to that which is e xpected for the each of the MnIII ions which possess an elongated octahe dral coordination due to the Jahn-Teller distortion of these ions. The qua drupolar nucleus interacts with the electric field gradient of the aspherical distribution of electrons about these ions giving a large quadrupolar splitting. The direction of the electric field gradient local symmetry axis is the axis of elongation of the oxygen oc tahedron surrounding the MnIII ion; the angle between the

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251 this axis and the molecular z axis is 34.0 for Mn(1) and 7.9 for Mn(2). As this angle increases, the quadrupolar splitting is expected to decrease. Hence, we expect that the experimentally observed quadrupolar splitting of the peak corresponding to Mn(1) will be smaller than that of the peak corresponding to Mn(2), and this is one point that is helpful in the assignment of the peak s in the spectrum. Also of us e is the relationship between and the magnitude of the internal field; as increases, the magnitude of the internal field also increases. Hence, the peak for Mn(1 ) should occur in the spectrum at a higher resonance frequency than does the peak of Mn (2). A knowledge of the angle of the local principal axis of the electric field gradient with respect to the molecular z axis allows us to definitively assign these two peaks in the NMR spectrum to the corresponding MnIII ions, consistent with previous assignments.185,186 Specifically, the peak centered at 230 MHz [labeled Mn(3) in Figure 721] is assigned to the four MnIV ions in the central [Mn4O4] cubane core of the Mn12 molecules. The second signal at 284 MHz [labeled Mn(2)], with well-resolved quadrupolar splitting in the crystals, is assigned to four of the eight MnIII ions in the outer ring, and the broa d signal around 355 MHz [labeled Mn(1)] is assigned to the remaining four MnIII ions. Hyperfine fields can be calculated using 55n= 10.5 MHz/T, giving the field values coll ected in Table 7-7 for an aligned powder and single crystal of 26. Two significant conclusions ar e evident from Figure 7-20: (i) Comparison of the oriented powder (Figur e 7-20a) and single crystal (Figure 7-20b) spectra of 26 demonstrates the impressive increase in the spectral resolution gained using a crystal. A significant broadeni ng of all of the peaks in Figure 7-20a as compared to those in Figure 7-20b and, in particular, the complete loss of quadrupolar splitting on the Mn(2) peak a nd up to 16 MHz shifts in th e peak positions [e.g., for

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252 Mn(1)] provide clear evidence that aligned pow ders could lead to erroneous structural and magnetic information. Table 7-7. Comparison of parameters obtained from 55Mn NMR spectra of an aligned powder and single crystal of 26. 26 Peaka Frequency (MHz) FWHM (MHz) Hyperfine Field (T) Aligned Powder 1 227.8 6.52 21.69 2 279.6 24.74 26.60 3 370.5 16.67 35.25 Single Crystal 1 232.2 2.89 22.11 2 284.6 2.91 27.10 3 354.4 13.82 33.76 a Using the numbering scheme described in the text Indeed, the single crystal data enable measurement of the quadrupole coupling parameter e2qQ through the quadrupole splitting, Q, using the equation for the energy, where I and m are the nuclear spin and its projection quantum numbers, and is the angle between the internal magnetic field (molecular z axis) and the local symmetry axis of the electric field gradient tensor.185,187 )] 1 ( 3 [ 2 1 cos 3 ) 1 2 ( 4 H E2 2 2 0 I I m I I qQ e mn m (7-4) From the crystallographic data for 26, it is known that the local Jahn-Teller axes, and hence the principal hyperfine fi eld directions of Mn(2) and Mn(1) ions, are canted away from the c axis at angles of 7.9 and 34.0, respectively. With I = 5/2, Q = 5.15 0.05 MHz for the Mn(2) peak and Q = 3.3 0.10 MHz for the Mn(1) peak, resulting in e2qQ values of 35.33 0.35 and 41.43 1.26 MHz, re spectively. No such information can be obtained from the oriented powder data. Vacuum-drying and crushing the crystals obviously changes the environment of the Mn ions, most likely due to the pressure and temperature change placed on the crystals duri ng this process. Furt her studies of these

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253 effects are underway to determine the exact cau se of the transformation. More significant to this work is the knowledge that an a ligned powder does not re present a statistical average of the crystal, as is seen by the la rge shift of the peaks in the aligned powdered spectra, suggesting that studies on aligned powders should by treated with caution. (ii) Comparison of the si ngle crystal spectrum of 1 (Figure 7-20c) with those reported earlier using oriented powders shows qualitative di fferences. In particular, the splitting reported on the Mn(3) peak, at ~230 MHz, was earlier assi gned to a partially resolved quadrupolar interacti on. In contrast, our data dem onstrate three gaussian lines (Figure 7-20), instead of five lines expected from this interpretation. Rather, these peaks could originate from structural isomers of 1 (with very little or no quadrupolar splitting), as was inferred from X-ray analysis172 and supported by recent high-field EPR26,188 and magnetization data.189 Additional measurements are needed to identify the origin of the powdering effects. From the line positions ( obtained from fitting with three gaussians) shown in the inset of Figure 7-20c and 55 n, we deduce that the three variants of 1 have internal fields of 21.89, 21.93 and 22.07 T for the [Mn4O4] core. These should be contrasted with the average value of 21.8 T reported by Goto and co-workers185 and 22.11 T for 26. The higher resolution afforded by the single crystal of complex 26 allows the investigation of the anisotr opic behavior of the hyperfin e field of the Mn(3) site. Measurements were made with a Zeeman field of 1 T in the ac plane and 2 T in the ab plane, which allows resolution for observing the critical structures in the spectra. For example, in Figure 7-22, we observe an evolut ion of the zero-field single peak, splitting first into two peaks at low angles and then in to four distinct peaks when the field is at

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254 large angles with respect to the a or b axis. This result can only be attributed to four magnetically distinct (but crys tallographically equivalent) MnIV ions in the [Mn4O4] core. Our preliminary analysis suggests that the hyp erfine fields at each of the atoms in the core are canted away from the c axis with a small but fin ite perpendicular component. Consequently, this implies that the interac tion among the local electr onic moments in the core leads to a spin structure that is not purely ferromagnetically aligned, as suggested earlier by EPR data.190 Because 26 is even more symmetric than 1, our results imply that earlier conclusions based on the hyperfine fields being parallel to the easy-axis need to be reexamined. 234236238240242244246 Intensity (arb. units)Frequency (MHz)48 32 28 16 0 01020304050236 238 240 242 244 Frequency (MHz) qHintc-axisq a ?b(a) (b)H0q a ?b(degrees) Figure 7-22. Angular dependence of 55Mn NMR in the ab plane of a single crystal of complex 264CH2Cl2. Angular dependence of the Mn(3) peak of 26 was also conducted from the easy to hard axis, as shown in Figure 7-23. Rotation in the ac plane was prepared by cooling the crystal in zero field, so signals from both ms = 10 states are discernable, separated by

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255 twice the Zeeman frequency at the applied field of 1 T. The angular variation in the ac plane is described by the eq 7-5, where CF represents the central frequency and is the angle between the applied field H0 and the c axis of the crystal. Peak frequency = ) )(cos )(H 2 (0 55 CF / (7-5) As the crystal is rotated, the effective field, Heff, felt by the nuclei is the sum of the projection of H0 onto the hyperfine field, HN (Heff = HN H0cos). Thus, a maximum in the splitting is seen when H0 is parallel to the c-axis ( = 90). One interesting result is the splitting observed when the crystallographic c-axis is perpendicular to the external field, H0. When the crystal is in this configuration a nd an external field is applied, we observe two peaks as opposed to the expected one peak. We tentatively assign this to two different orientations of th e hyperfine fields at the MnIV sites. H0perpendicular to easy axis H0parallel to easy axis Angle (degrees)0102030405060708090100 -10 Frequency (MHz)243 240 237 234 231 228 225 222 y = 232.2 + 10.5 cos(x) y = 232.2 + 10.5 cos(x) Figure 7-23. Angular dependence of 55Mn NMR in the ac plane of a single crystal of complex 264CH2Cl2. The 55Mn spectrum gives information on the coupling of the nuclear magnetic moments of the Mn nuclei with the local magne tic moments of the electrons, and for this reason, the spectrum is only visible at very low temperatures. At te mperatures above the blocking temperature TB of a SMM, that is the temp erature at which the molecule

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256 functions as a magnet, the magnetization vect ors of the electrons are rapidly changing orientations. Hence, the manganese nuclei rela x at very fast rates because of the fast electronic relaxation and the p eaks are broadened to such an extent that the spectrum appears to be featureless. Once the temper ature at which measurements are taken is below TB, the local magnetic moments couple to only one orientation of the local magnetic moments of the electrons of the Mn center, giving a sharp signal. Hence, low temperatures are essential for this technique (Figure 7-24). Frequency (MHz) 220240260280300320340360380Intensity (arb. units) 1.7 K 1.9 K 2.1 K 2.3 K 2.5 K 2.7 K 2.9 K 3.1 K 3.3 K Figure 7-24. Temperature dependence of the 55Mn NMR spectrum of a single crystal of complex 264CH2Cl2 at the indicated temperatures. In conclusion, the ability to probe individual Mn sites within the Mn12 core represents a powerful new tool to study factors such as sy mmetry lowering in SMMs due to extrinsic perturba tion. Single crystal 55Mn NMR spectroscopy affords a significant resolution enhancement over oriented powder spectroscopy, allowing information to be obtained that would otherwise not be achieve able, including more accurate measurements of hyperfine fields and quadrupole coupling cons tants. It also shows that vacuum-drying and powdering of these SMM crystals introduc ed a significant struct ural perturbation.

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257 Angular dependence studies show that there are further studies to be done to better understand the magnetic structure of th is molecule. The higher symmetry of 26, a result of the absence of hydrogen-bonding of la ttice solvent molecules with the Mn12 molecule, produces a less convoluted spectrum than 1, suggesting that 26 is a model system to study for understanding magnetic tunneling in the Mn12 family. 7.2.6 Magnetochemistry of Complex 26 7.2.6.1 DC studies Variable-temperature DC magnetic susceptibility ( M) data were collected on a microcrystalline powdered sample of 26, restrained in eicosane to prevent torquing, in a 5.0 kG magnetic field in the 5.0-300 K range (Figure 7-25). The MT versus T dependence is similar to thos e of previously studied [Mn12O12(O2CR)16(H2O)4] complexes with S = 10 ground states, exhibitin g a nearly temperature-independent value of 19-20 cm3 K mol-1 in the 150-300 K range which then increases rapidly to a maximum of 51 cm3 K mol-1 at 15 K before decreasing ra pidly at lower temperatures.13,22 Temperature (K)MT (cm3K mol-1) 050100150200250300 15 20 25 30 35 40 45 50 55 Figure 7-25. Plot of MT vs temperature for a dried, mi crocrystalline sample of complex 26 in eicosane. M is the DC molar magnetic susceptibility measured in a 5.0 kG field.

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258 The maximum suggests a large ground state spin (S) value for the complex, with the sharp decrease at low temperatures primarily due to a combination of Zeeman and zero-field splitting effects. The spin-only (g = 2) value for a unit composed of noninteracting MnIV 4MnIII 8 ions is 31.3 cm3 K mol-1. The MT value at 300 K of 26 is less than that expected for noninteracting metal ions, indicating the presence of appreciable intramolecular exchange interactions. A matrix diagonalization of the spin Ham iltonian of the complex to evaluate the various Mn2 exchange parameters would allow the determination of all of the possible spin states and their energi es and hence, is a method by which the ground state spin of 26 could be determined. Such a matrix diagonaliza tion approach is not so simple however as it involves a matrix of 1.0 108 by 1.0 108 dimensions. Additionally, as with other Mn12 complexes, it is not feasible to a pply the Kambe equivelent operator method51 to evaluate the exchange parameters (J) betw een the Mn ions because of the size and complexity of the molecule. Thus, variable-t emperature, variable-field DC magnetization (M) data were collected in the 1.8-4.0 K range at applied DC fields (H) ranging from 1-70 kG. The data in Figure 7-26a ar e shown as reduced magnetization (M/NB) plotted versus H/T, where M is the magnetization, N is Avogadro’s number, B is the Bohr magneton, and H is the magnetic field. Fo r complexes populating only the ground state and experiencing no zero-field splitting (ZFS) the magnetization follows the Brillouin function and the isofield lines all supe rimpose and saturate at a value of gS. The nonsuperimposition of the isofield lines clea rly indicates the presence of ZFS. The M/NB versus H/T data were fit usin g the program MAGNET54 that assumes only that the ground state is populated at these temperatures and magnetic fields.53 A spin

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259 Hamiltonian including a isotropic Zeeman inte ractions and axial zero-field splitting ZFS (2S ˆ Dz) was used to least-squares-fit the data. Th e matrix was diagonalized on each cycle, and a powder average was calculated. The best f it is shown as solid li nes in Figure 7-26a, and the fitting parameters were S = 10, g = 1.87, and D = -0.38 cm-1 = -0.54 K. The values are typical members of the Mn12 family. gM/NBH/T (kGK-1)D (cm-1)(a) (b) 010203040 0 2 4 6 8 10 12 14 16 18 0.1 T 0.5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 1.71.81.92.02.12.22.3 -0.6 -0.5 -0.4 -0.4 -0.3 -0.2 -0.1 Figure 7-26. Determination of the ground state spin. (a) Plot of M/NB versus H/T for a dried, microcrystalline sample of complex 26 in eicosane at the indicated applied fields. The solid lines are the f it of the data; see the text for the fit parameters. (b) Two-dimensional contour plot of the error surface for the D vs g fit for complex 26. The asterisk indicates the minimum. Other S values were also e xplored in the fitting; attempts to fit the magnetization data either with an S = 9 or a S = 11 ground state spin gave fitting parameters of g = 2.08 and D = -0.47 cm-1 and g = 1.71 and D = -0.32 cm-1, respectively. Each fit was of similar quality to that obtained for S = 10, but the g value is too high in the case of S = 9; a g value significantly greater than that of a free electron (2.0023) is unreasonable. Similarly, the g and D values obtained for the S = 11 fit are slightly lower than that expected for a neutral Mn12 species. In order to confirm that th e obtained parameters were the true global rather than local minimum, and to assess the uncertainty in the obtained g and D

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260 values, a root-mean square D vs g error surface for the fit was generated using the program GRID.55 The error surface is shown in Figure 7-26b as a contour plot for the D = -0.07 to -0.70 cm-1 and g = 1.7 to 2.3 ranges. One soft fitting minimum is observed; the contour describes the region of minimum error from D -0.32 to -0.44 cm-1 and g 1.81 to 1.93, giving fitting parameters of D = -0.38 0.06 cm-1 and g = 1.87 0.06. 7.2.6.2 AC studies In order to probe the dynamics of the relaxation of the magnetization of the complex, AC susceptibility studies were coll ected on a dried, microcrystalline sample of 26 and on wet crystals of 264CH2Cl2 in the 1.8-10 K range in a 3.5 G AC field with eight oscillation frequencies ( ) from 5 to 1488 Hz. In Figure 7-27 are shown the resulting inphase (as MT) and out-of-phase ( M ) AC susceptibility signals for a vacuum-dried sample of 26. In an AC susceptibility experiment a weak field (typically 1-5 G) oscillating at a particular frequency ( ) is applied to a sample. The magnetization vector of the molecule oscillates with the AC field, and there is no out-of-phase AC susceptibility signal ( M ) until the temperature is lowered to a value at which the barrier to magnetization relaxation is comparable to the thermal energy. A frequency-dependent M signal is observed and there is a concomitant frequency-de pendent decrease in the in-phase ( M) signal. The value of MT in the temperature-inde pendent region provides especially useful support for conclusions drawn concerning the gr ound state spin of a cluster from DC magnetization measurem ents described above. A value of MT that is independent of temperature indicates the gr ound state is well-isolated, and hence, DC magnetization measurements within the temperat ure-independent range, can be fitted to give the true ground state spin of the molecule. The MT value of ~ 50 cm3 K mol-1 for

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261 26 corresponds to an S = 10 system with g = 1.91, consistent with the DC magnetization results above. MT(cm3K mol-1) M(cm3mol-1)Temperature (K) 246810 0 1 2 3 1000 Hz 250 Hz 50 Hz 0 10 20 30 40 50 Figure 7-27. Plot of the in-phase (as MT) and out-of-phase ( M ) AC susceptibility signals vs temperature for a dried, microcrystalline sample of complex 26 at the indicated oscillation frequencies. A comparision of the M vs T plot for dry and wet samples of 26 for a field oscillating at 1000 Hz can be made using the top and bottom porti ons of Figure 7-28, respectively. The dried, microcrystalline sample of 26 exhibits two signals in the out-ofphase AC susceptibility, a higher-temperat ure (HT) peak at ~ 6 K and a lowertemperature (LT) peak at ~ 2.5 K. Th e signals are accompanied by two frequencydependent decreases in the in-phase MT plot, first at T ~ 7 K and then at T ~ 3 K, respectively, and correspond to two distinct relaxation pr ocesses. Consistent with measurements on other Mn12 complexes, the HT peak of 26 predominates over the LT form by a factor of more than ten. The LT sp ecies arises as a result of the loss of highly

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262 volatile CH2Cl2 solvent molecules of crystallization and corresponds to a fraction of molecules in which the magnetization relaxa tion occurs by a different mechanism. 246810 Temperature (K)m(cm3) M(cm3mol-1) Figure 7-28. Plot of the out-of-phase AC suscep tibility signals vs temperature for dried, microcrystalline complex 26 (top) and for wet crystals of complex 264CH2Cl2 (bottom) at 1000 Hz. The presence of two such peaks in the M vs T plots of Mn12 molecules is very common and has been previously attributed to Jahn-Teller isomerism, a situation whereby molecules differ in the relati ve orientation of at least one MnIII JT elongation axis.56-59 The LT (faster-relaxing) isomer is the one with the abnormal orientation of the JT axis towards the bridging oxide ions, wher eas the HT (slower-relaxing) species is that with all JT axes avoiding bridging oxide ions. The M vs T plot obtained from crystals of 26 maintained in mother liquor shows only the HT M signal at ~ 7 K, the signal expected at T ~ 3 K corresponding to the LT isomer is virtually absent. Note that instead of molar susceptibility, the figur e ordinates for a sample main tained in mother liquor are

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263 simply the total magnetization (plotted as m ) as the mass of the sample cannot be accurately determined. This finding emphasizes the importance that measurements be made on wet crystals whenever possible in order to minimize the range of molecular environments. These AC susceptibility measurements at a given oscillat ion frequency as a function of temperature were supplemented w ith further AC susceptibility measurements on wet crystals of 264CH2Cl2 at a constant temperature and a variable frequency of oscillation of the 3.5 G AC field. Such m easurements have become a routine method of studying the nature of the magnetizat ion relaxation process in SMMs100,103,113 as well as spin glasses.176,191 At a fixed temperature of 4.6 K, the in-phase (as m) and out-of-phase (m ) components of the AC magnetic susceptib ility were measured as the frequency of the AC field was varied from 0.1 to 1488 Hz As with similar measurements on other Mn12 complexes,100,113 the data were best fit to a distribution of single relaxation processes rather than to a singl e relaxation process. The m and m behavior as a function of angular frequency ( ) for a single relaxation process is given by eqs 7-6 and 7-7, respectively, while for a di stribution of single relaxati on processes, the m and m behavior is expressed by eqs 7-8 and 7-9, respectively, 2 21 ) ( ) ( ts T s (7-6) 2 21 ) ( ) ( ts T (7-7) ) 1 ( 2 1 1) ( ) 2 / sin( ) ( 2 1 )] 2 / sin( ) ( 1 )[ ( ) ( t Ts T s (7-8) ) 1 ( 2 1 1) ( ) 2 / sin( ) ( 2 1 ) 2 / cos( ) )( ( ) ( ts T s (7-9)

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264 where s is the adiabatic susceptibility, T is the isothermal susceptibility, = 2 is the angular frequency, and is the magnetization relaxation time. The primary importance of this measurement involves an additional parameter a value between 0 and 1 that is included in the expressions for a distribution of single relaxation pro cesses as a gauge of the width of the distribution. In Figure 729 are shown plots of m vs frequency and m vs frequency for wet crystals of 264CH2Cl2. A least-squares fitting of the data to a single relaxation process is shown as a dashed line while the fitting to a distribution of single relaxation processes is shown as a solid line. Frequency (Hz) Frequency (Hz)m(cm3) m(cm3)(a) (b) 0.1110100100010000 0.1110100100010000 Figure 7-29. Plots of (a) m vs frequency and (b) m vs frequency at 4.6 K for wet crystals of 264CH2Cl2. The dashed lines are a leastsquares fitting of the data to a single relaxation process as descri bed by eqs 7-6 and 77; the solid lines are a least-squares fitting of the data to a distribution of single relaxation processes as described by eqs 7-8 and 7-9; see the test for the fitting parameters. Clearly, significantly improve d fits are obtained for the latter, and hence, it is concluded that the magnetization relaxes via a distribution of single relaxation processes. The relaxation times ( ) obtained from the two fitting schemes are very similar, = 0.0393 s (single relaxation process) and = 0.0392 s (distributi on of single relaxation processes) and the main difference in the fitting parameters occurs in the values of the

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265 adiabatic and isothermal su sceptibility. A Cole-Cole (o r Argand) plot of m vs m can be used to determine the number of distinct ma gnetization relaxation pro cesses. Such a plot of m vs m for wet crystals of 264CH2Cl2 is shown in Figure 7-30, and the symmetric shape of the plot is indicativ e of only one relaxation process.176 m(cm3)m(cm3) Figure 7-30. Argand plot of m vs m of wet crystals of 264CH2Cl2 at 4.6 K. The dashed line is a least-squares fitting of the data to a single relaxation process as described by eqs 7-6 and 7-7. The solid line is a least-squares fitting of the data to a distribution of single relaxation processe s as described by eqs 7-8 and 7-9. Hence, the magnetization in wet crystals of 264CH2Cl2 relaxes via a single process and there is a distribution in this single rela xation process, the widt h of which is gauged by the fitting parameter The average value of obtained from the fitting of plots of m and m vs frequency to a distribut ion of single relaxation proce sses is 0.149. These results are very similar to those obtained on other Mn12 SMMs100,113 and are consistent with a narrow range of energy relaxation barriers due to a distribution of environments of the molecules. 7.2.6.3 Relaxation studies using AC and DC data AC susceptibility studies at several osci llation frequencies can be used as a means of determining the effective energy barrier, Ueff, to magnetization relaxation of a

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266 molecule, since at the M peak maximum the magnetization relaxation rate (1/ where is the relaxation time) is equa l to the angular frequency (2 ) of the AC field. Hence, outof-phase AC measurements at different osci llating field frequencies are a valuable source of rate vs T kinetic data157 that can be fitted to the A rrhenius equation (eq 7-10), where Ueff is the effective energy barrier to relaxation, k is the Boltzmann constant, is the relaxation time and 1/ 0 is the pre-exponential factor. = 0exp(Ueff/kT) (7-10) To supplement these AC data and to provide for a more accurate analysis over a wider range of temperatures, DC magnetizati on decay data were collected and combined with the AC M vs T data. These data were obtained on a single crystal of 264CH2Cl2 using a micro-SQUID apparatus. First, a la rge DC field of 1.4 T was applied to the sample at about 5 K to saturate its magnetiz ation in one direction, and the temperature was lowered to a chosen value between 1.3 a nd 4.4 K. When the temperature was stable, the field was swept from 1.4 to 0 T at a rate of 0.14 T/s, and then the magnetization in zero field was measured as a function of tim e (Figure 7-31a). An Arrhenius plot was constructed using the combined data sets and is shown in Figure 7-31b as vs 1/T. The fit of the thermally-activated region above ~ 3.3 K gave 0 = 2.9 10-9 s and Ueff = 75 K, consistent with values obtained from similar maeasurements on other neutral Mn12 species.

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267 0 0.2 0.4 0.6 0.8 1 0.11101001000M/Ms t (s) 3.6 K 3.4 K 3.3 K 3.2 K 3.1 K 3.0 K 2.9 K 2.8 K 2.7 K 2.6 K 2.5 K 2.4 K 2.3 K 2.2 K 2 K 1.3 K 3.8 K 4.4 K 4.2 K 4.0 K 3.3 K 3.2 K 3.1 K 3.0 K 3.4 K 3.6 K 3.8 K 4.0 K 4.2 K 4.4 K 2.9 K 2.8 K 2.7 K 2.6 K 2.5 K 2.4 K 2.3 K 2.2 K 1.3 K 2 K t (s) 0.11 10 100 1000 1 0.8 0.2 0 0.4 0.6M/Ms(a) (b) DC AC 0.10.20.30.40.50.61/T (K-1)10-310-1101103105 (s) Figure 7-31. Relaxation time vs temper ature studies on a single crystal of 264CH2Cl2. (a) Magnetization vs time decay plots in zero field. M is normalized to its saturation value, Ms; (b) Plot of relaxation time ( ) vs 1/T for complex 26 using AC M and DC decay data. The solid line is a fit to the Arrhenius equation. See the text for the fitting parameters. 7.2.6.4 Hysteresis studies below 1.8 K Like all of the reported Mn12 derivatives, complex 26 is also expected to function as a SMM. Out-of-phase AC su sceptibility signals exhibited by 26 are not sufficient proof of the SMM property however, and fo r this reason, hysteresis loops from magnetization vs DC field scans on an aligned si ngle crystal of 264CH2Cl2 using a micro-SQUID apparatus were obtained. The re sulting hysteresis loops are given in Figure 7-32, showing the temperature dependence at a constant field sweep rate of 2 mT/s and the field sweep rate dependen ce at a constant temperature of 3 K. The loops are very typical of neutral Mn12 complexes, exhibi ting well-defined steps that correspond to quantum tunneling of magnetization (QTM). Th ese sharp increases in the magnetization relaxation rate occur at regular intervals of the applied field, and the field separation, H, between the steps is proportiona l to D as given in eq 7-11.

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268 Bg D H (7-11) Measurement of the step positions in Figure 7-32 gave an average H of 0.45 T and a |D|/g value of 0.21 cm-1 (assuming g = 2.0). This is consistent with value of D obtained from magnetization vs field measurements on a dried sample of complex 26. The coercivities of the hysteresis loops increase with decreasing temperature at a constant field sweep rate and increase with increasing sweep rate at a c onstant temperature; this is as expected for the superparam agnetic properties of a SMM. -1 -0.5 0 0.5 1 -1-0.500.51 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/sM/Ms0H (T) 3 K H0(T)1 0.5 0 -0.5 -1 M/Ms1 0.5 0 -0.5 -1 3 K 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/s -1 -0.5 0 0.5 1 -1-0.500.51M/Ms0H (T) 0.002 T/s 3.6 K 3.2 K 3.0 K 2.8 K 2.7 K 2.6 K 2.5 K 2.4 K 2.3 K 2.2 K 2.1 K 3.6 K 3.2 K 3.0 K 2.8 K 2.7 K 2.6 K 2.4 K 2.3 K 2.2 K 2.1 K 2.5 K M/Ms1 0.5 0 -0.5 -1 2 mT/s H0(T) 1 0.5 0 -0.5 -1 (a) (b) Figure 7-32. (a) Magnetization (M) vs ma gnetic field hysteresis loops for complex 264CH2Cl2 at the indicated temperatures and sweep rate. (b) Magnetization (M) vs magnetic field hysteresis loops for complex 264CH2Cl2 at the indicated sweeping rates at 3.0 K. M is normalized to its saturation value, Ms. 7.2.7 Single Crystal High-Frequenc y Electron Paramagnetic Resonance In order to obtain detailed information on the spin Hamiltonian and to verify the S = 10 ground state spin of complex 264CH2Cl2, a high-frequency electron paramagnetic resonance (HFEPR) spectroscopy study was performed on a single crystal. This technique is ideally suited for complexes that have appreciable zero-field splitting192 and since the microwave energies employed are re latively large (110-550 GHz), it is possible

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269 to observe direct transitions between the ze ro-field split sublevels of the high ground state spin. HFEPR has been used in this capacity to characterize the ground state of several high-spin complexes.111b,111a,177,193,194 Detailed analysis of th e EPR spectra at various frequencies and angles gives direct access to the energy differences between spin levels and because changes in the relative intensi ties of EPR peaks reflect changes in the Boltzmann population of states, the sign and precise values of the zero-field splitting parameters can be determined. This then enables a precise determination of the spin Hamiltonian parameters. In an ideal case, th e spin of the ground state can be determined by simply counting the number of peaks in the EPR spectrum, and the zero-field splitting parameter can be evaluated from the spacing between successive peaks. The giant spin Hamiltonian describi ng the S = 10 ground state spin of 26 is given by eq 7-12: 4 4 4 4 0 4 0 4 B 2O B O B S g H S D Hˆ ˆ ˆ ˆ ˆ z (7-12) where the first term is the second order uniax ial anisotropy with a negative D value, the second term is the Zeem an interaction, and 0 4Oˆ and 4 4Oˆ represent the fourth order uniaxial and transverse anisotropies. In contrast to 1, a detailed structural analysis of 26 shows the absence of any symmetry-lowering solvent disorder. On this basis, only anisotropy terms that obey the crystallogra phic symmetry are allowed in the spin Hamiltonian, i.e., the E parameter should equal zero. Our initial studies on 26 at lower frequencies confir med that the ground state spin of the molecule is S = 10, with a low-lying S = 9 excited state.195 Though we did obtain a reasonable estimate of the zero-field splitti ng (ZFS) parameters from lower frequency data (D = -0.456 cm-1 and 0 4B = 2.0 10-5 cm-1) (vide infra), this precise determination

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270 should be carried out at higher frequencie s (up to frequencies above the zero-field splitting energy of the ground stat e transition), and, in par ticular, the angle-dependent studies must be performed in order to obtain the transverse anisotropy terms (with fields applied close to the hard plane). To accura tely determine the uniaxial ZFS parameters, the magnetic field is applied along the easy-ax is. In this situation, the transverse anisotropy terms operate at very high orders of perturbation theory and, essentially, vanish. Thus, the energy of each spin state is solely determined by the unaxial anisotropy terms and the Zeeman energy msgBH. The energy difference diagram between adjacent levels can then be constructed accordingly as function of magnetic field. Experimentally, such a diagram is obtained by identifying the fields at which the EPR absorptions occur from EPR spectra taken at various frequencies. As shown in Figure 7-33 the solid squares mark the EPR absorptions observed in EPR sp ectra taken at differe nt frequencies. The Hamiltonian parameters, D, 0 4B, and the easy-axis g value, gz, are obtained by minimizing the square difference between the data and simulation, which is shown in Figure 7-34a for a field applie d along the easy-axis of the molecule. A similar simulation for a field applied perpendicula r to the easy-axis (i.e., in th e hard plane) is shown in Figure 7-34b. Though there are three paramete rs to be determined, the huge volume of data involved in the least squares analysis yields a precise determination of all three parameters. The solid lines plotted in Figur e 7-33 present the optimal fit with D = -0.468 cm-1, 0 4B = -2.5 10-5 cm-1, and gz = 1.97.

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271 0123456 200 250 300 350 400 E/h (GHz)Field (Tesla) Easy Axis Data 0123456Magnetic Field (T) 200 250 300 350 400Frequency (GHz) Figure 7-33. Plot of HFEPR peak positions deduced from easy-axis measurements at different frequencies in the range fr om 240 GHz to 360 GHz at 15 K. Solid lines represent a single fit to the data using the Hamiltonian described in the text. 0123456 0 50 100 150 200 250 300 350 -1500 -1000 -500 0 500 1000 1500 2000 2500 Frequency (GHz)0123456 0 50 100 150 200 250 300 Magnetic Field (T) Magnetic Field (T) Frequency (GHz)-1500 -1000 -500 0 500 1000 1500 (a) (b)Energy Energy Figure 7-34. Plot of the energy and frequency of the ms sublevels of the S = 10 ground state of 26 versus applied magnetic field wh ere (a) the applied magnetic field is parallel to the easy-axis and (b) th e applied magnetic field is perpendicular to the easy-axis. The solid lines are a simulation produced using the optimal fit parameters given in the text.

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272 When the DC magnetic field is in the ha rd plane, the good quantum number is no longer solely determined by the zero-field spin Hamiltonian, but also by the field direction. In this situation, the transverse anisotropy, which essentially vanishes in the axial case, becomes a zero order perturbation to the Hamiltonian, and the energies of spin levels are determined by both unaxial and tr ansverse ZFS parameters. Therefore, to determine the transverse anis otropy, EPR spectra were take n at 51.3 GHz, and the field was rotated within the hard plane.8,196 A contour plot of the EPR absorption intensity taken over a 125 angle range, is shown in Figure 7-35. There is a clear four-fold shift of the EPR absorption peaks, and the fine structure previously observed in 1 is not seen in 26, hence confirming the absence of disc rete solvent disorder in this Mn12 molecule. The four-fold shift is caused by the pres ence of the transverse anisotropy 4 4B term. The precise determination of 4 4B is obtained by simulating the four-fold sh ift with 4 4B = 3 10-5 cm-1 and gx,y = 1.945 using the previously de termined axial parameters. -20020406080 0 1 2 3 4 5 6 -20020406080 0 1 2 3 4 5 6 Angle (degree)Field (Tesla) -20020406080Angle (degrees) 6 5 4 3 2 1 0Magnetic Field (T) 8 6 4 2 Figure 7-35. Contour plot of the angle-depende nt EPR spectra with the field rotating in the hard plane. Darker shades correspond to stronger EPR absorption. Superimposed on the data is a single f it to all of the peak positions (blue lines).

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273 It should be noted that the spectra we obt ained in Figure 7-35 ar e not as sharp as our earlier study.195 This is attributed to the poor qu ality of the crystal in this study relative to the crystal previous ly measured; this can be seen visually by the poor surface quality of the crystal in this study. The crystal was thermo-cycled in situ to 300 K and recooled to 15 K and the resulting EPR spect rum shows even broader resonant peaks. The comparisons of previously obtained spect ra and the present data before and after thermo-cycling are shown in Figure 7-36 and the comparison indicates that the broadening of the EPR absorptions is lik ely caused by random solvent loss and poor handling of the crystals in the present study. 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Magnetic Field (T)123456 7Transmission (arb. units) 8 6 4 2 Figure 7-36. Spectra of complex 264CH2Cl2 taken under different conditions. The spectrum shown in black is from our ear lier studies and represents the best crystal that was cooled under optimal conditions. The remaining spectra shown in red and green were obtained from the present studies, measured after the crystal was cooled only once and then after thermo-cycling, respectively. In addition to our studies with the field applied along the easy-axis of the molecule to determine the spin Hamiltonian parameters and to characterize the ground state spin of the molecule, we also undertook a detailed investigation of the EPR spectrum with the applied field in the hard plane of the molecu le. Such an investiga tion allows a study of the spin-energy levels of th e ground state as well as excite d spin states of complex 26. In

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274 order to align the hard plane (x, y) of the sample with respect to the applied DC magnetic field, angle dependent measur ements of the EPR spectra were performed and these are shown in Figure 7-37. 23456789 hard plane 4' 6' 3 2 4 6 8 5 7 10 9 8' 7' 16.2 -3.6 14.4 12.6 10.8 9.0 7.2 5.4 3.6 1.8 -1.8 Cavity TransmissionMagnetic Field (T) Figure 7-37. Angle dependen ce of the EPR spectrum of 264CH2Cl2 in the range of 3.6 on either side of the hard plane, with an angular step of 0.36. In general, the hard plane spectra of complex 264CH2Cl2 look very similar to those of complex 1.196,197 In the high-field limit (gBH0 > |D|S), a total of 20 EPR transitions with the 2S + 1 (S = 10) multiplet are expected, as shown in Figure 7-34b by solid curves. The resonances correspond to transi tions within the Zeeman-split ms zero-field sublevels and comprise half of this total. The quantization axis is defined by the uniaxial crystal field tensor in the zero-field limit and is along the z direction. In the high-field limit however, the quantization axis points along the applied field vector; the 10 resonances then corresp ond to transitions from ms = even-to-odd transitions, e.g. ms = -10

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275 to -9. Because the resonances originate fr om pairs of levels (ms) which are (approximately) degenerate in zero field, one expects the resonance frequencies, when plotted against field, to tend to zero as the field tends to zero, as shown in Figure 7-38. The simulation has been performed by exact diagonalization of eq 7-12, and this procedure is described in detail elsewhere.196,197 In order to fit the experimental data (open circles), crystal field parameters obtained from our earlier studies of 264CH2Cl2 with the field along the easy-axis, D = -0.456 cm-1, 0 4B = -2.0 10-5 cm-1, were used. 198 These Hamiltonian parameters are very close to the accepted crystal field parameters for 1 (D = -0.454 cm-1, 0 4B = -2.0 10-5 cm-1),170a,188,197 thus emphasizing the close similarity of physical properties of these two Mn12 derivatives. 02468 0 40 80 120 9 t o 84 t o 26 t o 58 t o 71 0 t o 9Magnetic Field (T) Frequency (GHz) Magnetic Field (T) Frequency (GHz) Figure 7-38. Fits to eq 7-12 for the frequenc y dependence of the hard plane spectra for the S = 10 state (solid black curves, open circles) and for the S = 9 state (dotted red curves, solid circles); the crystal field parameters for this simulation are given in the text. Open a nd closed circles are experimental data at frequencies of 51.5, 65.4 and 76.9 GHz. In earlier investigations of 1 it was pointed out that EPR spectra obtained for a field applied perpendicular to the easy-axis of th e molecule revealed a number of anomalous transitions which were labeled ,196,197 as opposed to the resonances which nicely fit the

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276 accepted S = 10 Hamiltonian (eq 7-12). Initially, these transitions were tentatively ascribed to the ms = odd-to-even transitions (e.g., ms = -9 to -8).196 However, they should not be observable below a cutoff frequency, wh ich is about 95 GHz at high fields for the given CF parameters, as depicted in Fi gure 7-38. In full agreement with these calculations we do not observe these resonances until we slightly misalign the hard plane of the sample with respect to the applie d magnetic field (Figure 737). Indeed, at 3.6 away from the hard plane, the resonances become highly pronounced. Meanwhile, the 10, 8 and 6 resonances disappear over this same angle range and there is even an approximately 0.75 range over which neither 10 or 9 are observed and, although 4 and 2 peaks remain visible at = 3.6, it is clear that their intensities diminish substantially. This symmetry eff ect between the out-of-plane angle dependence of the and resonances was recently reported for 1, and is discussed in more detail elsewhere.26 For comparison, Figure 7-39b shows simu lations of the EPR spectra for the same angle range, generated using the program SIM.199 These simulations agree well with our observations, i.e., the peaks disappear, and the 9 appears, as the field as tilted away from the hard plane. The simulations predict accurately the angles at which the peaks disappear and 9 peaks appear. This contrasts the behavior seen in molecules of 1, where a significant overlap of the and peaks has been attributed to a distribution of tilts of the easy axes of the molecules (up to 1.7), induced by a discrete disorder associated with the two acetic acid molecules of crystallization.26 A small distribution of tilts can be inferred from the present data as seen from the overlap of the the 10 and 9 resonances (Figure 7-40), and from the absence of some of the features in the simulations

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277 which disperse strongly with a ngle. However, the width of th e distribution must be on the order of, or less than, the angle resolution em ployed in these measurements, i.e., ~ 0.2. 23456789 4' 6' 3 2 4 6 8 5 7 10 9 8' 7'Cavity TransmissionMagnetic Field (T) 23456789 14.4 12.6 10.8 9.0 7.2 5.4 3.6 1.8 10 2 4 6 8 3 5 9 7 4' 6' 8' 7'Cavity transmissionMagnetic Field (T) Figure 7-39. Comparison of EPR spectrum of 264CH2Cl2 with simulated spectrum. (a) Angle dependence of the EPR spectrum in th e range of 3.6 on either side of the hard plane, with an angular step of 0.36. (b) SIM199 simulations of the EPR spectrum for the field tilted up to 3.6 away from the hard plane. -3.6-1.80.01.83.6 6 7 8 Angle (degree) 8 9 9 10Magnetic Field (T) 9 Figure 7-40. Angle dependence of severa l of the most important resonances.

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278 It is also apparent from the data in Figure 7-37 that, for the most part, the resonances are extremely symmetric and much sharper that those of 1.26 In particular, none of the fine structures seen in the hard axis spectra of 1 are seen for the present complex; e.g., the pronounced high-field shoulders on the resonances. Consequently, we conclude that the discrete solvent disorder26,172,173,188,189 that is now well established in 1 is absent in 26. This observation is consistent with the full compliment of four solvent molecules per formula unit, and suggests that complex 26 probably represents a more suitable candidate for measurements of quant um effects in high-symmetry S = 10 SMMs. In Figure 7-41a is shown the temperature dependence of the EPR spectra of 25 at 51.5 GHz for a field applied perpendi cular to the easy-axis to within an accuracy of 0.1, as inferred from the angle dependent data in Figure 7-37. As the temperature is increased from 10 K up to 40 K, an extra resona nce is found at 7.42 T. Since this peak is located between the 10 and 8 resonances, it is labeled as 9. Similar peaks at 65.4 and 76.9 GHz were also observed, which are located, respectively, at 7.77 and 8.22 T between the 8 and 10 peaks (Figure 7-41b). (a) (b) 2345 51.5 GHz 965.4 GHz 76.9 GHzCavity Transmission 10 2 4 6 8 9 40 K 35 K 30 K 25 K 20 K 15 K 10 KCavity Transmission Magnetic Field (T)23456789 6789Magnetic Field (T) Figure 7-41. EPR spectra for a single crystal of complex 264CH2Cl2 measured at (a) different temperatures at 51.5 GHz and (b) different frequencies, specifically 51.5, 65.4 and 76.9 GHz.

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279 The positions of 9 peaks as a function of frequency are plotted as solid circles in Figure 7-40. It is tempting to attribute these resonances to the onset of the 9 transitions, which could occur if a small minor ity of molecules have their easy-axes tilted with respect to the majority of molecules, whose easy-axes are exactly perpendicular to the applied magnetic field, as has recently been found for 1.26 However, a careful angledependent study of the EPR spectra shows that the positions of 9 and 9 exhibit completely different angle dependencies. The temperature dependencies of the intensities of the 9 and 9 peaks reveal even more disc repancies. The areas under the 9 and 9 peaks were calculated by integration and plotte d as a function of temperature (Figure 7-42a). This procedure does not employ any f itting functions and is sensitive only to the noise background of the data; th e corresponding uncertainty is de picted by error bars in Figure 7-42a. Again, the nature of the 9 resonance is different from the 9 resonance and, thus, cannot be explained within the framework of the S = 10 picture. (a) (b)0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 S = 9 S = 10Energy (K)0 102103 9 at 3.6 9Peak area (a.u.) 10203040Temperature (K) 9 data = 36 K 9 data = 40 K = 44 K Figure 7-42. Determination of relative ener gy of S = 9 excited state. (a) Temperature dependence of the area of the 9 and 9 resonances. The curves through the solid circles represent the calculated 9 resonance areas assuming = 36 K, = 40 K and = 44 K. (b) Schematic for the energy levels of both the S = 10 and S = 9 states in zero magnetic field. The S = 9 state is located at an energy = 40 2 K above the bottom of the S = 10 state.

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280 Further examination of Figures 7-41 and 7-42 re veals that the 9 resonance diminishes in intensity as the temperature d ecreases to 15 K, becoming invisible at 10 K. This fact proves beyond any doubt that the 9 resonance originates from an excited state of the Mn12 molecule. We therefore conclude that, at low frequencies, the 9 resonance corresponds to a transi tion within an excited state of complex 26. For comparison, Figure 7-38 includes dashes red curves corresponding to fits to the 9 data; this fit assumes S = 9 and, due to the limited number of data points, allows only for the variation of D. For an odd total spin state, the low fiel d limiting behavior of odd-to-even ms and even-to-odd ms transitions is the reverse of that for an ev en total spin state. Consequently, one does expect the frequency of the ms = -9 to -8 transition to go to zero in the low field limit within the S = 9 manifold. The fit to the data for S = 9 yields the Hamiltonian parameter D = -0.430 cm-1 (-0.62 K), which is 5% smaller than for S = 10. The low frequency 9 resonance data lie perfectly on the S = 9 curves Therefore, the anisotropy barrier for the S = 9 state is |D|S2 50 K, which is 23% smaller than that for S = 10 (65 K). Having established that the low-frequency 9 transition corresponds to an S = 9 state, we can estimate its approximate location relative to S = 10. Using the CF parameters for both S = 10 and S = 9 states, the energy levels E10(ms) and E9(ms) for the two states, and both partition functions for a given temperature T, where is the energy difference between the bottoms of the S = 10 and S = 9 manifolds were calculated using eq 7-13. 10 10 m T ) (m E s 10s s 10) (m/e Zand 9 9 m T ) ) (m E ( s 9s s 9) (m/e Z (7-13) The area under the 9 peak, at a given temperature, is proportional to the difference in populations of the corres ponding levels (eq 7-14).

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281 ) ( ) ( ) (9 10 )} 9 ( ) 8 ( { 8 9 99 9T Z T Z e Z N N T AT E E, ,/ (7-14) Thus, by varying the only parameter we have found that the S = 9 manifold is located at = 40 2 K above the bottom of the S = 10 st ate (Figure 7-42b). This implies that the S = 9 state lies very close to the ms = 6 excited state within the S = 10 multiplet. We have also performed simila r calculations of the temp erature dependence of the 9 peak area for the values of = 36 K and = 44 K, and both dependencies were inconsistent with our experiemental data, as depicted in Fi gure 7-42a. The obtained lo cation of the S = 9 excited state at = 40 2 K is in perfect agr eement with recent calculations.200 In summary, detailed analysis of the EP R spectra obtained on a single crystal of 264CH2Cl2 show very similar ZFS parameters to the acetate-substituted derivative. However, our spectroscopic analysis does not show any evidence of second order transverse anisotropy, which confirms that the discrete solven t disorder is not present in the molecules. It is worth noting that the cr ystals are very susceptible to solvent loss, even more so than crystals of complex 1, and hence, must be treated very carefully prior to measurements. Similar frequency and temperature dependent EPR studies with the applied field in the hard plane of the molecu le reveal the existence of an S = 9 state located only 40 2 K above the S = 10, ms = 10 ground state. This result is in perfect agreement with theoretical predictions.200,201 The effects of the coexistence of an excited S = 9 state and the groun d S = 10 state in the Mn12 molecule are not known, and we hope these investigations will stimulate further studies. Our experiment s also indicate that 26 is an intrinsically cleaner system than 1, which we believe to be connected with the fact that the former possesses a full compliment of four solvent molecules per formula unit. Thus,

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282 further investigations of th e title compound may provide furt her insights into the quantum magnetization dynamics of giant spin (S = 10) SMMs. 7.3 Conclusions The goal of this investiga tion included (i) a detailed study of the structure, magnetic properties, and high-frequency elec tron paramagnetic resonance spectrum of [Mn12O12(O2CCH2Br)16(H2O)4] and (ii) the confirmation of the origin of the abnormal features present in the hysteresis l oops and EPR spectra of molecules of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O by comparison of the former to those obtained on the high-symmetry Mn12 molecule in the present work. To assess the presence or absence of in teractions between the Mn12 molecule and interstitial solvent molecules, our X-ray crystal structure of 26 included the refinement of the four CH2Cl2 solvent molecules of crystallization, and a detailed structural analysis subsequently revealed almost no symmetry-l owering contacts between Mn12 molecules. This is in contrast to molecules of 1, where hydrogen-bonding interactions between the Mn12 molecule and acetic acid molecules of crys tallization break the tetragonal symmetry of the molecule, giving rise to up to six isomers in the crystalline lattice. Similar to other typical neutral Mn12 species, DC studies es tablish that the ground state spin of 26 is S = 10 and that the molecule po ssesses a significant uniaxial magnetic anisotropy. Magnetization vs DC field scan s exhibit hysteresis, establishing that 26 is a single-molecule magnet. AC susceptibility studies carried out on wet crystals maintained in mother liquor to prevent th e loss of inters titial solvent and on a dr ied, microcrystalline sample showed that solvent loss increases th e fraction of faster-relaxing molecules in a sample of 26. This result is consistent with differences observed between the 55Mn NMR spectra obtained on a single crystal of 26 maintained in mother liquor and on a dried,

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283 microcrystalline sample of 26 and emphasizes the importance of the environment and local symmetry of a molecule on the resul ting magnetic properties of a SMM. More indepth HFEPR measurements on a single crystal of 26 reveal that in contrast to 1, there is no second order transverse magnetic anisotropy. Further analysis s hows that the spectra obtained on a single crystal of 26 are generally much sharper, with more symmetric resonance profiles, than those of 1. The higher symmetry of 26, a result of the absence of hydrogen-bonding of lattice solv ent molecules with the Mn12 molecule, suggests that 26 is a model system to better our understandi ng of the quantum magnetization dynamics in the Mn12 family. 7.4 Experimental 7.4.1 Synthesis All manipulations were performed under aerobic conditions us ing materials as received, except where otherwise noted. [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) was prepared as described elsewhere.50 [Mn12O12(O2CCH2Br)16(H2O)4] (26). To a solution of complex 1 (2.0 g, 0.97 mmol) in a mixture of MeCN (50 cm3) and CH2Cl2 (50 cm3) was added BrCH2CO2H (4.3 g, 31 mmol). The mixture was stirred for 1 hr, and the solvent was removed in vacuo. Toluene (20 cm3) was added to the residue, and the solution was again evaporated to dryness. The addition and removal of to luene was repeated two more times. The remaining solid was redissolved in CH2Cl2 (75 cm3) and treated again with BrCH2CO2H (4.3 g, 31 mmol). After 1 h, three more cycles of addition and removal of toluene were performed. The residue was redissolved in CH2Cl2 (100 cm3) and filtered. Hexanes (25 cm3) were added and the solution was allo wed to stand undisturbed at room temperature for 4 days. The resulting black cr ystals were collected by filtration, washed

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284 with hexanes, and dried in vacuo; yield 96%. A sample for crystallography was maintained in contact with the mother liquor to prevent the loss of interstitial solvent. Anal. Calcd (found) for 26 (C32H40Mn12O48Br16): C, 12.28 (12.61); H, 1.29 (1.30); N, 0.00 (0.00). Selected IR data (cm-1): 1720 (w), 1597 (vs), 1574 (vs), 1557 (s), 1534 (s), 1419 (vs), 1402 (vs), 1359 (s), 1209 (m), 1116 (w), 958 (w), 896 (w), 733 (m), 680 (s), 645 (s), 603 (s), 553 (s), 525 (m). 7.4.2 X-ray Crystallography Data were collected using a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). A suitable single crystal of 264CH2Cl2 was attached to a glass fiber using silicone grease and transferred to the goniosta t where it was cooled to -100 C for characterization and data collection. The structure was solved by direct methods (SHELXTL)64 and standard Fourier techniques and was re fined using full-matrix leas t-squares methods. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and refined with the use of a riding model. Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames we re re-measured at the end of data collection to monitor instrument and crystal stab ility (maximum correction on I was < 1%). Absorption corrections by integrati on were applied based on measured indexed crystal faces. An initial survey of reciprocal space reveal ed a set of reflections with a tetragonal lattice. Analysis of the full data set revealed that the space group was I41/a. The asymmetric unit contains one-quarter of the Mn12 molecule and one disordered CH2Cl2 molecule. The bromine atoms [Br(2) and Br(3)] in two of the BrCH2CO2 ligands were

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285 slightly disordered. Their site occupancy f actors were each dependently refined to 94:6%. A more complex disorder was observed for Br(4). Seven possi ble positions for the atom were located, and the occupancies of the seven atoms refined to a sum of 1.0. The disorder of Br(4) was coupled as a resu lt of proximity with the disordered CH2Cl2 molecule of crystallization. The CH2Cl2 molecule was disordered over three positions and was refined using a model having only one C atom and two Cl atoms, each disordered over three positions. Three possible positions for each Cl atom were located, and the occupancies of the three atoms refined to a sum of 1.34 and 0.75 for Cl(1) and Cl(2), respectively. The corresponding C atom disorder could not be resolved. A total of 321 parameters were refined in the fi nal cycle of refinement using 3860 re flections with I > 2 (I) to yield R1 and wR2 of 5.68% and 15.43%, respectively. The final difference Fourier map was essentially featurele ss, the largest peak being 1.905 e -3 and the deepest hole being -1.185 e -3.

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286 CHAPTER 8 GENERAL CONCLUSIONS In summary, the strategies and methods employed in the present work have resulted in: (i) the expansion of existing families of single-molecule magnets with new members which differ appreciably by one or more fact ors, e.g. peripheral ligation or oxidation level; (ii) the prepar ation of several novel Mnx clusters, some of which behave as either new SMMs or new SCMs; (iii) the realiza tion of the importance of single crystal measurements on crystals maintained in moth er liquor to prevent th e loss of in terstitial solvent in lieu of previously employed measur ements on dried, polycrystalline studies of SMMs. Such studies are valuable for the determ ination of the various factors, structural, synthetic and otherwise, that infl uence the magnetic properties of a Mnx cluster, and ultimately might prove useful for the rational design of an improved single-molecule magnet that behaves as a magnet at t echnologically relevant temperatures. Our investigation of the reactivity of Mn12 complexes with various non-carboxylate ligands has led us to the conclude the impor tance of the following properties of the noncarboxylate ligand: solubility steric considerations, aci d dissociation constant and geometry (i.e., OO bite di stance). A combination of thes e factors and others strongly influences the identity of the products obtained from reactions carried out under seemingly identical conditions with different non-carboxylic acids. Our initial interest in the expansion of our understanding in this area was spurred by the isolation of the diphenylphosphinate-substituted Mn12 derivative. Although the [Mn12O12] core of the molecule was retained upon subs titution of the organic groups, structural distortions were

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287 evident upon close examination of the bond dist ances and angles of the new derivative. Such ligand-induced distortions were subseque ntly shown to not significantly perturb the magnetic properties of the Mn12 molecule, but the strategy of fine-tuning the magnitude of the exchange interactions between the Mn centers by distorting the geometry still appeared to be a promising means by which the magnetic anisotropy or ground state spin of the Mn12 molecule might be improved. An additional aim of the use of non-carboxylate organic ligands includes th e site-specific replacement of carboxylate ligands on the Mn12 molecule. Such replacement of certain ligands, e.g. all axial or all equatorial, is essential to ma ke regioselective reactions pos sible in a complex with so many carboxylate ligands and make more feas ible important objectives such as the binding of groups that might enhance the shap e anisotropy or magnetic properties of the Mn12 complexes, and/or facilitate the binding of the latter to surfaces or to each other to give dimers of Mn12 species. Unfortunately, the diphe nylphosphinate groups distributed themselves equally between four axial and four equatorial sites about the Mn12 molecule, and this is likely a consequence of the steric bulk of the ligan d. This conclusion is further supported by our inability to isolate a fully-substituted diphenylphosphinate Mn12 derivative. To this end, we studied the reactivity of Mn12 complexes with a similar organic acid, benzenesulfonic acid, both to obtain a mixed-ligand derivative with two types of ligands in spec ific positions on the Mn12 molecule and to study the effects of substitution on the Mn12 structure and on the resulting ma gnetic properties. We obtained a new Mn12 derivative in which be nzenesulfonate groups o ccupy only axial positions about the Mn12 core with acetate ligands in the e quatorial sites. The distribution of ligands was rationalized on the basis of the relative basicities of acetate and

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288 benzenesulfonate, with the more basic, stronger donor aceta te ligands favoring occupation of the equatorial sites where shor ter, stronger Mn-O bonds can be formed, to the benefit of the overall energy stabiliz ation of the molecule, and the less basic benzenesulfonate ligands thus occupying axial positions with one or both of their O atoms on the more labile, JT elongation axes Hence, as with diphenylphosphinic acid, the use of benzenesulfonic acid has allowed access to a novel mixed-ligand Mn12 derivative that retains the SMM properties of its parent Mn12 complex and might ultimately prove practical for regioselective reactions. Examination of the bond distances and angles of the new benzenesulfonate-substituted Mn12 derivative shows that there is virtually no distortion of the [Mn12O12] core. This is in contrast to the diphenylphosphinate-substituted derivative, and considering the similar acid dissociation constants, 2.32 for Ph2PO2H and 2.55 for PhSO3H, can be rationalized by considering the dissimilar average OO bite distances of th e ligands in the two derivatives, 2.56 for Ph2PO2 and 2.42 for PhSO3 (Table 8-1). The latter is mu ch closer to that which is found in the acetate-substituted deri vative of 2.24 , and as a result, we conclude that a combination of steric bulk of the phenyl groups of the Ph2PO2 ligand and the large OO bite distance prevent the formation of a fully-substituted Mn12 derivative. This conclusion is supported by the isolation of a mangan ese-oxo cubane cluster, and not a fullysubstituted Mn12 derivative, from the reaction of the benzenesulfonate-substituted Mn12 derivative with diphenylphosphinic acid. We a ttribute our inability to isolate a fullysubstituted benzenesulfonate Mn12 derivative to the relatively insoluble nature of the ligand.

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289 Table 8-1. Comparison of selected bite distances () for MeCO2 -, PhSO3 -, Ph2PO2 -, PhSeO2 and Me2AsO2 ligands in complexes 1, 24CH2Cl2, 4, 5, 66MeCN, 72CH2Cl2, 95H2OC5H12 and 10MeCN12H2O. Ligand Compound, [Mnx] Bite Distancea pKa 171 MeCO2H 1, [Mn12] 2.235 – 2.249 4.76 PhSO3H 2, [Mn12] 2.400 – 2.422 2.55 Ph2PO2H 4, [Mn12] 2.561 – 2.565 2.32 5, [Mn4] 2.566 – 2.617 PhSeO2H 6, [Mn7] 2.654 – 2.799 4.79 7, [Mn7] 2.640 – 2.836 MeAsO2H 9, [Mn4] 2.791 – 2.828 6.27 10, [Mn4] 2.749 – 2.847 a Bite distance refers to the OO di stance between O atoms of the carboxylate coordinated to Mn ions in each selected cluster. Our results with diphenylphosphinic acid and benzenesulfonic acid are in contrast to those results obtained from similar a ttempted ligand substitution reactions with benzeneseleninic acid and dimethylarsinic acid, where the [Mn12O12] core was completely transformed into products w ith considerably different topological arrangements. The use of benzeneseleninic acid (PhSeO2H) has afforded two new heptanuclear Mn clusters, which possess a nove l structural type in Mn chemistry and represent the first examples of transition metal clusters ligated by benzeneseleninate groups. The pKa of benzeneseleninic acid (4.79) is very similar to that of acetic acid (4.76), but there is a significant difference in the OO bite distances; such distances are in the range of 2.64 – 2.84 for the two Mn7 clusters obtained. Hen ce, the rupture of the [Mn12O12] core followed by structural rearrangement confirms our suspicions that there is a limit to the number of organic ligands with a significantly increased OO bite distance that can be ligated to a Mn12 derivative without causing structural transformation. Magnetic studies on the two Mn7 clusters reveal that these species are not new additions to the growing family of single-molecule ma gnets. Instead, the slow relaxation is caused

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290 by single-chain magnetism behavior, with the relaxation barrie r arising from a combination of the molecular anisotropy a nd the exchange inte raction between the individual Mn7 molecules. These two clusters are the first single-chain magnets (SCMs) which are composed of polynucle ar metal clusters and are al so the first SCMs to show quantum tunneling of magnetization. As an extension of this work, we studied the reactivity of Mn12 derivatives with an even more basic ligand with an even greater OO bite distance, dimethylarsinic acid. Again as expected, attempted ligand substituti on reactions caused a transformation of the [Mn12O12] core, giving two tetranuclear complexes possessing a manganese-oxo cubane core and two Mn16 complexes possessing a novel struct ural topology comprised of four [MnIII 4(-O)2] “butterfly” units. All four complexes represent the first examples of manganese clusters ligated by dimethylarsinate groups and ma gnetic studies establish that the two Mn16 complexes behave as single-molecule magnets. A comparison of dimethylarsinic acid to the similar react ant diphenylphosphinic acid reveals two main dissimilarities: (i) a large difference in pKa (6.27 for Me2AsO2H and 2.32 for Ph2PO2H) and (ii) a considerable difference in steric bulk of the substituents, i.e., Me vs Ph. For each acid, the bite distance is relatively larg e compared with that of acetate and on this basis, we might expect that at least a partially-substituted Mn12 complex would be obtained with dimethylarsinate ligands. Howe ver, even with the replacement of the bulky phenyl groups with sterically unhindered methyl groups, we did not isolate even a partially-substituted Mn12 derivative as was found w ith diphenylphosphinic acid. The most obvious explanation for this result is the high basicity of the dimethylarsinate ligand as reflected in the relatively high pKa value. Other factors incl uding relative solubilities

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291 and crystallization kinetics almo st certainly play important ro les in determining the exact species that preferrentially cr ystallizes from the reactions however. In conclusion, these studies have proven that an e ffective strategy for the prepar ation of new polynuclear Mn aggregates involves the use of a chelating ligand with (i) a pKa significantly higher than that of acetic acid; (ii) a OO (or otherwise) bite distan ce that is appreciably greater than that found in typical Mn12 derivatives; and (iii) a comb ination of the above-mentioned properties. The other approach used in the pres ent work to obtain new polynuclear Mnx clusters that might possess a significan t enough magnetic anisotropy and high ground state spin to behave as single-molecule magnets involves the use of a bulky, hydrophobic carboxylate ligand to destabilize the Mn12 molecule and induce a structural rearrangement. This idea was stimulated by the preparation of novel high nuclearity single-molecule magnets from reactions of a t-butylacetate-substituted Mn12 derivative. It has been postulated that th e strong basicity of the ButCH2CO2 ligand, as reflected in the relatively high pKa value of its conjugate acid (pKa = 5.00), combined with the bulky and hydrophobic nature of the But group are the main reasons for the interesting products obtained with this carboxylate and as a result, we have extended these investigations of the influence of bulky, hydrophobic groups on the nature of the obtained products through the use of the relate d carboxylic acid, 2,2-dime thylbutyric acid with a pKa of 5.03, which is similar to ButCH2CO2H. Reactions of the 2,2-dimethylbutyrate-substituted Mn12 derivative similar to those which le d to the isolation of new polynuclear Mnx clusters in the case of the t-butylacetate-substituted Mn12 complex afforded several new Mn clusters, two of which are new structural types in Mn chemistry and one of which is

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292 only the second member of a new sub-class of Mn12 derivatives in which the water ligands that coordinate to either three or four MnIII ions in the outer ring of the cluster have been replaced by another ligand, MeOH. Magnetic studies reveal that the new Mn12 complex likely still functions as a single-mo lecule magnet, with an effective energy barrier to magnetization reversal that is comparable to that of the parent Mn12 complex. Unfortunately, magnetic studies on the other new clusters, reveal insufficiently small ground state spin values to allow larg e enough energy barriers to magnetization relaxation. As such, these new complexes do not function as single-molecule magnets. However, despite this outcome, the intr oduction of the hydrophobic, bulky ligand onto the Mn12 complex did allow access to new Mn aggr egates as predicted, confirming our suspicions that a Mn12 complex substituted with a strongly electron donating ligand is easily destabilized and can under go structural rearrangement. As important as development of new synt hetic methodologies for the preparation of new single-molecule magnets with novel topolo gies is a more thor ough understanding of the structural factors that influence the ground state spin and magnetic anisotropy of a known molecule. Chemical variation of a know n SMM is useful towards this end and such modifications on Mn12 complexes include variation of the peripheral carboxylate ligation, variation of the oxida tion level by cluster reduction, replacement of some of the Mn centers with either Fe or Cr, and repla cement of some of the carboxylate ligands with non-carboxylate groups. With each perturbation, the magnetic properties of the resulting clusters were measured to gauge the effects and from these studies we have realized the importance of symmetry, solvent or ligand disorder, intermolecular interactions, the loss of interstitial solvent molecules as well as numerous others. To extend further our

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293 knowledge in this area, we have investigat ed the effect of reduction on the magnetic properties of a family of Mn12 single-molecule magnets with identical peripheral ligation spanning three oxidation levels. Low temper ature magnetization measurements and AC magnetic susceptibility measurements on all three complexes show that (i) the ground state spin of each of the complexes change s only slightly and (ii) the axial magnetic anisotropy decreases as the Mn12 molecule is progressively reduced. This trend is consistent with our expectations as the successive reduction decreases the number of MnIII ions, the primary source of the magnetic anisotropy in a Mn12 molecule. To investigate the effect of so lvent loss on the magnetic properties, AC susceptibility measurements were carried out on crystals ma intained in mother liquor to prevent the loss of interstitial solvent and on dried, microcrystalline samples and only small differences in the obtained kinetic paramete rs were found between wet and dry samples. These studies did reveal however, that measurements on we t crystals are without certain complications associated with solvent loss and minimize local environmental differences between neighboring molecules in a crysta l, giving a narrower range of anisotropy parameters and also effective energy barri er to magnetization reversal. This finding emphasizes the importance that parameters obtained from measurements made on wet crystals, such as single crystal micro-S QUID hysteresis measurements, should not be quantitatively compared with those values obtained from measurements on dried, microcrystalline samples, but rather should be compared only with those obtained from similar measurements on wet crystals. Similarly, our work on the the tetragonal symmetry bromoacetate-substituted Mn12 complex has shown that that measurements on single crystals produces data of far

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294 superior quality than that obtained from similar measurements on dried, microcrystalline samples of single-molecule magnets. Eviden ce for this conclusion is supported by a comparison of the 55Mn NMR spectra obtained on a single crystal and on a powdered sample of the bromoacetate-substituted Mn12 complex, where the single crystal spectrum showed several features which were absent in the powder spectrum, including wellresolved quadrupolar splitting of two peaks. The single crystal 55Mn NMR study of this complex represents the first of its kind; there are only two other 55Mn NMR spectroscopic studies of Mn12 SMMs and these were carried out on dried, polycrystalline samples. HFEPR measurements on the cluster reveal that in contrast to the “model” high symmetry acetate-substituted Mn12 derivative, there is no measureable rhombic anisotropy term. This is consistent with our structural analysis, wh ich revealed almost no symmetry-lowering contacts between Mn12 molecules, i.e., no interaction of the interstitial solvent molecules with the Mn12 molecules as seen with the “model” system. Hence, we conclude that this Mn12 complex is a cleaner, truly axial Mn12 single-molecule magnet and is a model system to study fo r understanding magnetic tunneling in the Mn12 family. Consistent with previous research, this work has confirmed that Mn12 complexes represent excellent starting material s, affording a diversity of novel Mnx topologies. In conclusion, the strategies employed in the present work have proven effective methods by which a better understanding of the factors that affect both th e spin and magnetic anisotropy of a molecule has been attained. Su ch studies will prove us eful for the rational design of a synthetic method for an improved single-molecule magnet.

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295 APPENDIX A BOND DISTANCES AND ANGLES Table A-1. Selected interatomic distances () and angles () for [Mn12O12(O2CMe)8(O3SPh)8(H2O)4]4CH2Cl2 (24CH2Cl2). Mn(1)-O(46) 1.883(2) Mn(6)-O(26) 2.220(2) Mn(1)-O(47) 1.909(2) Mn(6)-O(27) 2.222(2) Mn(1)-O(1) 1.925(2) Mn(7)-O(52) 1.895(2) Mn(1)-O(42) 1.942(2) Mn(7)-O(53) 1.903(2) Mn(1)-O(38) 2.163(2) Mn(7)-O(29) 1.920(2) Mn(1)-O(43) 2.238(2) Mn(7)-O(28) 1.931(2) Mn(1)-Mn(9) 2.7905(6) Mn(7)-O(33) 2.171(2) Mn(2)-O(56) 1.903(2) Mn(7)-O(30) 2.254(2) Mn(2)-O(47) 1.907(2) Mn(7)-Mn(12) 2.7934(6) Mn(2)-O(3) 1.948(2) Mn(8)-O(46) 1.895(2) Mn(2)-O(2) 1.954(2) Mn(8)-O(52) 1.906(2) Mn(2)-O(5) 2.216(2) Mn(8)-O(36) 1.935(2) Mn(2)-O(4) 2.221(2) Mn(8)-O(40) 1.951(2) Mn(3)-O(57) 1.899(2) Mn(8)-O(34) 2.174(2) Mn(3)-O(56) 1.901(2) Mn(8)-O(37) 2.184(2) Mn(3)-O(7) 1.932(2) Mn(9)-O(46) 1.854(2) Mn(3)-O(6) 1.933(2) Mn(9)-O(47) 1.8643(19) Mn(3)-O(8) 2.198(3) Mn(9)-O(54) 1.865(2) Mn(3)-O(11) 2.261(2) Mn(9)-O(44) 1.923(2) Mn(3)-Mn(10) 2.8015(6) Mn(9)-O(48) 1.9258(19) Mn(4)-O(51) 1.880(2) Mn(9)-O(49) 1.9260(19) Mn(4)-O(57) 1.910(2) Mn(9)-Mn(12) 2.8069(6) Mn(4)-O(14) 1.929(2) Mn(9)-Mn(10) 2.8117(6) Mn(4)-O(15) 1.962(2) Mn(9)-Mn(11) 2.9353(6) Mn(4)-O(9) 2.176(2) Mn(10)-O(49) 1.866(2) Mn(4)-O(16) 2.190(2) Mn(10)-O(56) 1.871(2) Mn(5)-O(51) 1.883(2) Mn(10)-O(57) 1.872(2) Mn(5)-O(50) 1.905(2) Mn(10)-O(55) 1.9174(19) Mn(5)-O(20) 1.926(2) Mn(10)-O(12) 1.924(2) Mn(5)-O(19) 1.940(2) Mn(10)-O(54) 1.9393(19) Mn(5)-O(17) 2.185(3) Mn(10)-Mn(11) 2.8054(6) Mn(5)-O(21) 2.255(3) Mn(10)-Mn(12) 2.9478(6) Mn(5)-Mn(11) 2.8016(6) Mn(11)-O(51) 1.859(2) Mn(6)-O(50) 1.889(2) Mn(11)-O(50) 1.870(2) Mn(6)-O(53) 1.910(2) Mn(11)-O(55) 1.876(2) Mn(6)-O(25) 1.946(2) Mn(11)-O(22) 1.907(2) Mn(6)-O(24) 1.952(2) Mn(11)-O(49) 1.9227(19)

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296 Table A-1. Continued. Mn(11)-O(48) 1.925(2) Mn(12)-O(52) 1.874(2) Mn(11)-Mn(12) 2.8117(6) Mn(12)-O(54) 1.9157(19) Mn(12)-O(48) 1.863(2) Mn(12)-O(55) 1.935(2) Mn(12)-O(53) 1.874(2) Mn(12)-O(31) 1.945(2) O(46)-Mn(1)-O(47) 82.12(8) O(56)-Mn(3)-O(6) 95.09(9) O(46)-Mn(1)-O(1) 177.95(9) O(7)-Mn(3)-O(6) 86.43(10) O(47)-Mn(1)-O(1) 95.88(9) O(57)-Mn(3)-O(8) 96.67(9) O(46)-Mn(1)-O(42) 95.24(9) O(56)-Mn(3)-O(8) 96.19(10) O(47)-Mn(1)-O(42) 174.95(10) O(7)-Mn(3)-O(8) 87.28(11) O(1)-Mn(1)-O(42) 86.72(9) O(6)-Mn(3)-O(8) 87.17(10) O(46)-Mn(1)-O(38) 92.17(9) O(57)-Mn(3)-O(11) 88.57(9) O(47)-Mn(1)-O(38) 90.80(9) O(56)-Mn(3)-O(11) 89.08(9) O(1)-Mn(1)-O(38) 88.32(10) O(7)-Mn(3)-O(11) 87.57(10) O(42)-Mn(1)-O(38) 93.61(10) O(6)-Mn(3)-O(11) 87.81(10) O(46)-Mn(1)-O(43) 88.72(9) O(8)-Mn(3)-O(11) 173.02(10) O(47)-Mn(1)-O(43) 87.72(9) O(57)-Mn(3)-Mn(10) 41.66(6) O(1)-Mn(1)-O(43) 90.75(10) O(56)-Mn(3)-Mn(10) 41.63(6) O(42)-Mn(1)-O(43) 87.92(9) O(7)-Mn(3)-Mn(10) 135.93(7) O(38)-Mn(1)-O(43) 178.16(9) O(6)-Mn(3)-Mn(10) 134.80(7) O(46)-Mn(1)-Mn(9) 41.30(6) O(8)-Mn(3)-Mn(10) 106.26(8) O(47)-Mn(1)-Mn(9) 41.69(6) O(11)-Mn(3)-Mn(10) 80.72(6) O(1)-Mn(1)-Mn(9) 136.66(7) O(51)-Mn(4)-O(57) 94.94(9) O(42)-Mn(1)-Mn(9) 134.85(7) O(51)-Mn(4)-O(14) 172.60(10) O(38)-Mn(1)-Mn(9) 98.57(7) O(57)-Mn(4)-O(14) 92.45(10) O(43)-Mn(1)-Mn(9) 81.04(6) O(51)-Mn(4)-O(15) 90.63(10) O(56)-Mn(2)-O(47) 95.52(9) O(57)-Mn(4)-O(15) 174.09(9) O(56)-Mn(2)-O(3) 91.05(9) O(14)-Mn(4)-O(15) 81.98(10) O(47)-Mn(2)-O(3) 171.95(10) O(51)-Mn(4)-O(9) 87.32(9) O(56)-Mn(2)-O(2) 171.70(10) O(57)-Mn(4)-O(9) 93.84(9) O(47)-Mn(2)-O(2) 91.19(9) O(14)-Mn(4)-O(9) 92.04(10) O(3)-Mn(2)-O(2) 82.69(10) O(15)-Mn(4)-O(9) 84.42(10) O(56)-Mn(2)-O(5) 98.37(9) O(51)-Mn(4)-O(16) 92.36(9) O(47)-Mn(2)-O(5) 84.36(9) O(57)-Mn(4)-O(16) 92.83(9) O(3)-Mn(2)-O(5) 90.10(10) O(14)-Mn(4)-O(16) 87.42(10) O(2)-Mn(2)-O(5) 87.12(10) O(15)-Mn(4)-O(16) 88.92(10) O(56)-Mn(2)-O(4) 87.14(9) O( 9)-Mn(4)-O(16) 173.33(10) O(47)-Mn(2)-O(4) 96.15(9) O(51)-Mn(5)-O(50) 81.70(9) O(3)-Mn(2)-O(4) 88.80(10) O(51)-Mn(5)-O(20) 176.10(10) O(2)-Mn(2)-O(4) 87.29(10) O(50)-Mn(5)-O(20) 95.35(9) O(5)-Mn(2)-O(4) 174.40(9) O(51)-Mn(5)-O(19) 96.10(9) O(57)-Mn(3)-O(56) 82.11(9) O(50)-Mn(5)-O(19) 174.94(10) O(57)-Mn(3)-O(7) 96.15(9) O(20)-Mn(5)-O(19) 86.64(10) O(56)-Mn(3)-O(7) 176.27(10) O(51)-Mn(5)-O(17) 93.77(9) O(57)-Mn(3)-O(6) 175.46(10) O(50)-Mn(5)-O(17) 92.48(9)

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297 Table A-1. Continued. O(20)-Mn(5)-O(17) 88.89(10) O(29)-Mn(7)-Mn(12) 136.12(7) O(19)-Mn(5)-O(17) 92.22(10) O(28)-Mn(7)-Mn(12) 135.86(7) O(51)-Mn(5)-O(21) 86.30(9) O(33)-Mn(7)-Mn(12) 103.31(6) O(50)-Mn(5)-O(21) 90.37(9) O(30)-Mn(7)-Mn(12) 80.82(6) O(20)-Mn(5)-O(21) 91.18(10) O(46)-Mn(8)-O(52) 94.83(8) O(19)-Mn(5)-O(21) 84.93(10) O(46)-Mn(8)-O(36) 172.45(9) O(17)-Mn(5)-O(21) 177.13(9) O(52)-Mn(8)-O(36) 92.72(9) O(51)-Mn(5)-Mn(11) 41.20(6) O(46)-Mn(8)-O(40) 90.54(9) O(50)-Mn(5)-Mn(11) 41.61(6) O(52)-Mn(8)-O(40) 174.29(9) O(20)-Mn(5)-Mn(11) 135.37(7) O(36)-Mn(8)-O(40) 81.91(9) O(19)-Mn(5)-Mn(11) 135.21(7) O(46)-Mn(8)-O(34) 88.53(9) O(17)-Mn(5)-Mn(11) 101.58(7) O(52)-Mn(8)-O(34) 94.23(9) O(21)-Mn(5)-Mn(11) 80.35(6) O(36)-Mn(8)-O(34) 90.76(10) O(50)-Mn(6)-O(53) 95.42(8) O(40)-Mn(8)-O(34) 83.94(9) O(50)-Mn(6)-O(25) 171.13(10) O(46)-Mn(8)-O(37) 93.61(9) O(53)-Mn(6)-O(25) 91.72(9) O(52)-Mn(8)-O(37) 93.20(9) O(50)-Mn(6)-O(24) 90.77(9) O(36)-Mn(8)-O(37) 86.12(10) O(53)-Mn(6)-O(24) 171.61(10) O(40)-Mn(8)-O(37) 88.40(10) O(25)-Mn(6)-O(24) 82.68(10) O(34)-Mn(8)-O(37) 172.07(9) O(50)-Mn(6)-O(26) 96.48(9) O(46)-Mn(9)-O(47) 84.13(9) O(53)-Mn(6)-O(26) 86.16(9) O(46)-Mn(9)-O(54) 92.89(9) O(25)-Mn(6)-O(26) 89.24(10) O(47)-Mn(9)-O(54) 90.04(9) O(24)-Mn(6)-O(26) 87.53(10) O(46)-Mn(9)-O(44) 94.16(9) O(50)-Mn(6)-O(27) 85.54(9) O(47)-Mn(9)-O(44) 94.29(9) O(53)-Mn(6)-O(27) 101.45(9) O(54)-Mn(9)-O(44) 172.07(9) O(25)-Mn(6)-O(27) 87.89(11) O(46)-Mn(9)-O(48) 95.57(8) O(24)-Mn(6)-O(27) 84.64(10) O(47)-Mn(9)-O(48) 173.32(9) O(26)-Mn(6)-O(27) 171.94(9) O(54)-Mn(9)-O(48) 83.31(8) O(52)-Mn(7)-O(53) 82.74(8) O(44)-Mn(9)-O(48) 92.39(9) O(52)-Mn(7)-O(29) 95.83(9) O(46)-Mn(9)-O(49) 175.18(9) O(53)-Mn(7)-O(29) 176.84(10) O(47)-Mn(9)-O(49) 99.39(8) O(52)-Mn(7)-O(28) 176.12(10) O(54)-Mn(9)-O(49) 83.85(8) O(53)-Mn(7)-O(28) 95.73(9) O(44)-Mn(9)-O(49) 88.88(9) O(29)-Mn(7)-O(28) 85.52(10) O(48)-Mn(9)-O(49) 80.54(8) O(52)-Mn(7)-O(33) 96.02(9) O(46)-Mn(9)-Mn(1) 42.08(6) O(53)-Mn(7)-O(33) 93.08(9) O(47)-Mn(9)-Mn(1) 42.94(6) O(29)-Mn(7)-O(33) 89.87(10) O(54)-Mn(9)-Mn(1) 98.82(6) O(28)-Mn(7)-O(33) 87.60(9) O(44)-Mn(9)-Mn(1) 88.86(6) O(52)-Mn(7)-O(30) 90.68(8) O(48)-Mn(9)-Mn(1) 137.55(6) O(53)-Mn(7)-O(30) 86.39(9) O(49)-Mn(9)-Mn(1) 141.90(6) O(29)-Mn(7)-O(30) 90.81(9) O(46)-Mn(9)-Mn(12) 89.26(6) O(28)-Mn(7)-O(30) 85.66(9) O(47)-Mn(9)-Mn(12) 131.98(7) O(33)-Mn(7)-O(30) 173.16(8) O(54)-Mn(9)-Mn(12) 42.76(6) O(52)-Mn(7)-Mn(12) 41.88(6) O(44)-Mn(9)-Mn(12) 133.67(7) O(53)-Mn(7)-Mn(12) 41.89(6) O(48)-Mn(9)-Mn(12) 41.35(6)

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298 Table A-1. Continued. O(49)-Mn(9)-Mn(12) 85.94(6) O(54)-Mn(10)-Mn(11) 85.43(6) Mn(1)-Mn(9)-Mn(12) 121.51(2) Mn (3)-Mn(10)-Mn(11) 123.71(2) O(46)-Mn(9)-Mn(10) 135.94(7) O(49)-Mn(10)-Mn(9) 42.97(6) O(47)-Mn(9)-Mn(10) 89.82(6) O(56)-Mn(10)-Mn(9) 89.48(6) O(54)-Mn(9)-Mn(10) 43.38(6) O(57)-Mn(10)-Mn(9) 134.61(7) O(44)-Mn(9)-Mn(10) 129.85(7) O(55)-Mn(10)-Mn(9) 85.85(6) O(48)-Mn(9)-Mn(10) 85.75(6) O(12)-Mn(10)-Mn(9) 130.36(7) O(49)-Mn(9)-Mn(10) 41.34(6) O(54)-Mn(10)-Mn(9) 41.35(6) Mn(1)-Mn(9)-Mn(10) 124.39(2) Mn (3)-Mn(10)-Mn(9) 123.94(2) Mn(12)-Mn(9)-Mn(10) 63.290(16) Mn(11)-Mn(10)-Mn(9) 63.010(16) O(46)-Mn(9)-Mn(11) 135.90(6) O(49)-Mn(10)-Mn(12) 82.95(6) O(47)-Mn(9)-Mn(11) 139.48(6) O(56)-Mn(10)-Mn(12) 138.00(6) O(54)-Mn(9)-Mn(11) 83.02(6) O(57)-Mn(10)-Mn(12) 137.87(6) O(44)-Mn(9)-Mn(11) 89.38(6) O(55)-Mn(10)-Mn(12) 40.29(6) O(48)-Mn(9)-Mn(11) 40.33(6) O(12)-Mn(10)-Mn(12) 88.89(6) O(49)-Mn(9)-Mn(11) 40.26(6) O(54)-Mn(10)-Mn(12) 39.83(6) Mn(1)-Mn(9)-Mn(11) 177.17(2) Mn (3)-Mn(10)-Mn(12) 177.21(2) Mn(12)-Mn(9)-Mn(11) 58.585(15) Mn(11)-Mn(10)-Mn(12) 58.450(15) Mn(10)-Mn(9)-Mn(11) 58.392(15) Mn(9)-Mn(10)-Mn(12) 58.276(15) O(49)-Mn(10)-O(56) 90.70(9) O(51)-Mn(11)-O(50) 83.27(9) O(49)-Mn(10)-O(57) 92.16(9) O(51)-Mn(11)-O(55) 92.73(9) O(56)-Mn(10)-O(57) 83.62(9) O(50)-Mn(11)-O(55) 91.12(9) O(49)-Mn(10)-O(55) 83.92(8) O(51)-Mn(11)-O(22) 96.08(10) O(56)-Mn(10)-O(55) 174.51(9) O(50)-Mn(11)-O(22) 93.39(9) O(57)-Mn(10)-O(55) 97.62(8) O(55)-Mn(11)-O(22) 170.51(9) O(49)-Mn(10)-O(12) 171.70(9) O(51)-Mn(11)-O(49) 95.85(9) O(56)-Mn(10)-O(12) 94.36(9) O(50)-Mn(11)-O(49) 174.52(9) O(57)-Mn(10)-O(12) 94.94(9) O(55)-Mn(11)-O(49) 83.52(8) O(55)-Mn(10)-O(12) 90.87(9) O(22)-Mn(11)-O(49) 92.08(9) O(49)-Mn(10)-O(54) 83.46(8) O(51)-Mn(11)-O(48) 175.22(9) O(56)-Mn(10)-O(54) 98.27(8) O(50)-Mn(11)-O(48) 99.92(8) O(57)-Mn(10)-O(54) 175.23(9) O(55)-Mn(11)-O(48) 83.68(8) O(55)-Mn(10)-O(54) 80.09(8) O(22)-Mn(11)-O(48) 87.32(9) O(12)-Mn(10)-O(54) 89.29(9) O(49)-Mn(11)-O(48) 80.64(8) O(49)-Mn(10)-Mn(3) 99.84(6) O(51)-Mn(11)-Mn(5) 41.84(6) O(56)-Mn(10)-Mn(3) 42.45(6) O(50)-Mn(11)-Mn(5) 42.58(6) O(57)-Mn(10)-Mn(3) 42.38(6) O(55)-Mn(11)-Mn(5) 100.24(6) O(55)-Mn(10)-Mn(3) 139.64(6) O(22)-Mn(11)-Mn(5) 88.70(6) O(12)-Mn(10)-Mn(3) 88.32(7) O(49)-Mn(11)-Mn(5) 137.36(6) O(54)-Mn(10)-Mn(3) 140.22(6) O(48)-Mn(11)-Mn(5) 141.92(6) O(49)-Mn(10)-Mn(11) 43.01(6) O(51)-Mn(11)-Mn(10) 89.28(7) O(56)-Mn(10)-Mn(11) 133.11(7) O(50)-Mn(11)-Mn(10) 133.06(7) O(57)-Mn(10)-Mn(11) 90.09(7) O(55)-Mn(11)-Mn(10) 42.88(6) O(55)-Mn(10)-Mn(11) 41.73(6) O(22)-Mn(11)-Mn(10) 133.52(7) O(12)-Mn(10)-Mn(11) 132.51(7) O(49)-Mn(11)-Mn(10) 41.46(6)

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299 Table A-1. Continued. O(48)-Mn(11)-Mn(10) 85.94(6) O(54)-Mn(12)-O(31) 90.02(9) Mn(5)-Mn(11)-Mn(10) 122.52(2) O(55)-Mn(12)-O(31) 91.81(9) O(51)-Mn(11)-Mn(12) 135.64(7) O(48)-Mn(12)-Mn(7) 98.46(6) O(50)-Mn(11)-Mn(12) 91.00(6) O(53)-Mn(12)-Mn(7) 42.69(6) O(55)-Mn(11)-Mn(12) 43.27(6) O(52)-Mn(12)-Mn(7) 42.47(6) O(22)-Mn(11)-Mn(12) 128.21(7) O(54)-Mn(12)-Mn(7) 140.70(6) O(49)-Mn(11)-Mn(12) 85.86(6) O(55)-Mn(12)-Mn(7) 139.05(6) O(48)-Mn(11)-Mn(12) 41.25(6) O(31)-Mn(12)-Mn(7) 88.63(6) Mn(5)-Mn(11)-Mn(12) 125.58(2) O(48)-Mn(12)-Mn(9) 43.07(6) Mn(10)-Mn(11)-Mn(12) 63.308(16) O(53)-Mn(12)-Mn(9) 132.98(7) O(51)-Mn(11)-Mn(9) 136.18(6) O(52)-Mn(12)-Mn(9) 90.13(6) O(50)-Mn(11)-Mn(9) 140.17(6) O(54)-Mn(12)-Mn(9) 41.38(6) O(55)-Mn(11)-Mn(9) 83.04(6) O(55)-Mn(12)-Mn(9) 85.66(6) O(22)-Mn(11)-Mn(9) 88.16(6) O(31)-Mn(12)-Mn(9) 131.09(6) O(49)-Mn(11)-Mn(9) 40.34(6) Mn(7)-Mn(12)-Mn(9) 123.35(2) O(48)-Mn(11)-Mn(9) 40.35(6) O(48)-Mn(12)-Mn(11) 42.93(6) Mn(5)-Mn(11)-Mn(9) 175.99(2) O(53)-Mn(12)-Mn(11) 88.20(6) Mn(10)-Mn(11)-Mn(9) 58.599(15) O(52)-Mn(12)-Mn(11) 133.51(7) Mn(12)-Mn(11)-Mn(9) 58.424(15) O(54)-Mn(12)-Mn(11) 85.68(6) O(48)-Mn(12)-O(53) 90.32(9) O(55)-Mn(12)-Mn(11) 41.65(6) O(48)-Mn(12)-O(52) 91.22(9) O(31)-Mn(12)-Mn(11) 133.32(7) O(53)-Mn(12)-O(52) 84.10(9) Mn(7)-Mn(12)-Mn(11) 122.18(2) O(48)-Mn(12)-O(54) 83.63(8) Mn(9)-Mn(12)-Mn(11) 62.991(16) O(53)-Mn(12)-O(54) 173.46(9) O(48)-Mn(12)-Mn(10) 82.94(6) O(52)-Mn(12)-O(54) 98.46(8) O(53)-Mn(12)-Mn(10) 136.41(6) O(48)-Mn(12)-O(55) 83.74(8) O(52)-Mn(12)-Mn(10) 138.80(6) O(53)-Mn(12)-O(55) 96.68(8) O(54)-Mn(12)-Mn(10) 40.42(6) O(52)-Mn(12)-O(55) 174.91(9) O(55)-Mn(12)-Mn(10) 39.86(6) O(54)-Mn(12)-O(55) 80.24(8) O(31)-Mn(12)-Mn(10) 90.00(6) O(48)-Mn(12)-O(31) 172.78(9) Mn(7)-Mn(12)-Mn(10) 178.21(2) O(53)-Mn(12)-O(31) 95.86(9) Mn(9)-Mn(12)-Mn(10) 58.434(15) O(52)-Mn(12)-O(31) 93.11(9) Mn (11)-Mn(12)-Mn(10) 58.242(15)

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300 Table A-2. Selected interatomic di stances () and angles () for [Mn4O4(O2PPh2)6] (5). Mn(1)-O(1) 2.008(6) Mn(5)-O(13) 1.841(6) Mn(1)-O(1a) 1.860(6) Mn(5)-O(13a) 1.999(6) Mn(1)-O(2) 1.919(5) Mn(5)-O(14) 1.909(5) Mn(1)-O(3) 1.910(7) Mn(5)-O(15) 1.930(5) Mn(1)-O(4a) 1.998(8) Mn(5)-O(16) 1.964(7) Mn(1)-O(5) 1.960(6) Mn(5)-O(17) 1.928(6) Mn(1)-Mn(1a) 2.908(3) Mn(5)-Mn(5a) 2.874(2) Mn(1)-Mn(2) 2.947(3) Mn(5)-Mn(6) 2.9694(18) Mn(2)-O(1) 1.951(7) Mn(6)-O(13) 2.007(7) Mn(2)-O(6) 1.967(7) Mn(6)-O(18) 1.972(5) Mn(3)-O(7) 1.891(5) Mn(7)-O(19) 1.904(7) Mn(3)-O(7a) 2.051(7) Mn(7)-O(19b) 2.045(9) Mn(3)-O(8) 1.978(6) Mn(7)-O(20) 1.973(5) Mn(3)-O(9) 1.921(6) Mn(7)-O(21) 2.020(7) Mn(3)-O(10b) 1.977(8) Mn(7)-O(22b) 1.915(7) Mn(3)-O(11) 1.997(7) Mn(7)-O(23) 2.007(8) Mn(3)-Mn(3a) 2.986(3) Mn(7)-Mn(7a) 2.970(3) Mn(3)-Mn(4) 2.888(2) Mn(7)-Mn(8) 2.884(3) Mn(4)-O(7) 1.873(6) Mn(8)-O(19) 1.909(7) Mn(4)-O(12) 1.928(6) Mn(8)-O(24) 1.943(7) O(1a)-Mn(1)-O(1) 81.0(4) O( 3)-Mn(1)-Mn(1a) 130.8(2) O(1a)-Mn(1)-O(2) 83.2(3) O( 3)-Mn(1)-Mn(1b) 86.4(2) O(1a)-Mn(1)-O(3) 171.8(3) O( 3)-Mn(1)-Mn(2) 134.0(2) O(1a)-Mn(1)-O(4a) 87.5(3) O(4a)-Mn(1)-Mn(1a) 83.4(2) O(1a)-Mn(1)-O(5) 90.6(3) O( 4a)-Mn(1)-Mn(1b) 132.2(2) O(2)-Mn(1)-O(1) 79.4(3) O( 4a)-Mn(1)-Mn(2) 127.9(2) O(2)-Mn(1)-O(3) 90.2(3) O( 5)-Mn(1)-Mn(1a) 133.77(19) O(2)-Mn(1)-O(4a) 91.5(3) O(5)-Mn(1)-Mn(1b) 128.4(2) O(2)-Mn(1)-O(5) 167.7(3) O( 5)-Mn(1)-Mn(2) 84.17(18) O(3)-Mn(1)-O(1) 93.0(3) Mn(1a)-Mn(1)-Mn(1b) 60.0 O(3)-Mn(1)-O(4a) 97.7(3) Mn(1a)-Mn(1)-Mn(2) 60.44(3) O(3)-Mn(1)-O(5) 94.9(3) O(1)-Mn(2)-O(1a) 80.2(3) O(4a)-Mn(1)-O(1) 166.0(3) O(1)-Mn(2)-O(6) 89.9(3) O(4a)-Mn(1)-O(5) 98.8(3) O( 1)-Mn(2)-O(6a) 167.7(3) O(5)-Mn(1)-O(1) 89.1(3) O(1a)-Mn(2)-O(6) 91.0(3) O(1)-Mn(1)-Mn(1a) 82.87(18) O(6a)-Mn(2)-O(6) 97.6(3) O(1)-Mn(1)-Mn(1b) 39.36(18) O(1)-Mn(2)-Mn(1) 42.64(19) O(1)-Mn(1)-Mn(2) 41.15(18) O(1)-Mn(2)-Mn(1a) 82.7(2) O(1a)-Mn(1)-Mn(1a) 43.2(2) O(1a)-Mn(2)-Mn(1) 38.23(18) O(1a)-Mn(1)-Mn(1b) 85.4(2) O(6)-Mn(2)-Mn(1) 85.06(19) O(1a)-Mn(1)-Mn(2) 40.5(2) O(6)-Mn(2)-Mn(1a) 133.6(2) O(2)-Mn(1)-Mn(1a) 40.74(16) O(6a)-Mn(2)-Mn(1) 128.1(2) O(2)-Mn(1)-Mn(1b) 40.74(16) Mn(1)-Mn(2)-Mn(1a) 59.12(7) O(2)-Mn(1)-Mn(2) 84.2(3) Mn(1b)-O(1)-Mn(1) 97.4(3)

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301 Table A-2. Continued. Mn(1b)-O(1)-Mn(2) 101.3(3) O(7)-Mn(4)-Mn(3b) 87.7(2) Mn(2)-O(1)-Mn(1) 96.2(3) O(12)-Mn(4)-Mn(3) 85.49(19) Mn(1a)-O(2)-Mn(1) 98.5(3) O(12)-Mn(4)-Mn(3a) 136.2(2) O(7)-Mn(3)-O(7a) 79.5(4) O(12)-Mn(4)-Mn(3b) 129.2(2) O(7)-Mn(3)-O(8) 83.0(3) Mn(3a)-Mn(4)-Mn(3) 62.26(6) O(7)-Mn(3)-O(9) 169.2(3) Mn(4)-O(7)-Mn(3) 100.2(3) O(7)-Mn(3)-O(10b) 89.0(3) Mn(4)-O(7)-Mn(3a) 94.7(3) O(7)-Mn(3)-O(11) 91.9(3) Mn(3)-O(7)-Mn(3a) 98.4(3) O(8)-Mn(3)-O(7a) 79.0(3) Mn(3a)-O(8)-Mn(3) 98.0(4) O(8)-Mn(3)-O(9) 89.4(3) O(13)-Mn(5)-O(13a) 83.1(4) O(8)-Mn(3)-O(10b) 92.0(3) O(13)-Mn(5)-O(14) 84.4(2) O(8)-Mn(3)-O(11) 167.0(3) O(13)-Mn(5)-O(15) 172.2(3) O(9)-Mn(3)-O(7a) 91.5(3) O(13)-Mn(5)-O(16) 91.4(3) O(9)-Mn(3)-O(10b) 99.0(4) O(13)-Mn(5)-O(17) 90.1(2) O(9)-Mn(3)-O(11) 93.8(3) O(14)-Mn(5)-O(13a) 80.2(2) O(10b)-Mn(3)-O(7a) 166.2(3) O(14)-Mn(5)-O(15) 89.0(3) O(10b)-Mn(3)-O(11) 99.9(3) O(14)-Mn(5)-O(16) 90.2(3) O(11)-Mn(3)-O(7a) 88.3(2) O(14)-Mn(5)-O(17) 170.7(3) O(7)-Mn(3)-Mn(3a) 84.5(2) O(15)-Mn(5)-O(13a) 91.7(3) O(7)-Mn(3)-Mn(3b) 42.8(2) O(15)-Mn(5)-O(16) 92.7(3) O(7)-Mn(3)-Mn(4) 39.66(18) O(15)-Mn(5)-O(17) 95.9(3) O(7a)-Mn(3)-Mn(3a) 38.79(15) O(16)-Mn(5)-O(13a) 169.3(3) O(7a)-Mn(3)-Mn(3b) 81.96(16) O(16)-Mn(5)-O(17) 97.4(3) O(7a)-Mn(3)-Mn(4) 40.27(17) O(17)-Mn(5)-O(13a) 91.8(2) O(8)-Mn(3)-Mn(3a) 40.98(19) O(13)-Mn(5)-Mn(5a) 86.52(18) O(8)-Mn(3)-Mn(3b) 40.98(19) O(13)-Mn(5)-Mn(5b) 43.69(18) O(8)-Mn(3)-Mn(4) 82.7(3) O(13)-Mn(5)-Mn(6) 41.6(2) O(9)-Mn(3)-Mn(3a) 84.7(2) O(13a)-Mn(5)-Mn(5a) 39.50(16) O(9)-Mn(3)-Mn(3b) 130.4(2) O(13a)-Mn(5)-Mn(5b) 83.75(16) O(9)-Mn(3)-Mn(4) 131.7(3) O(13a)-Mn(5)-Mn(6) 42.26(19) O(10b)-Mn(3)-Mn(3a) 132.9(3) O(14)-Mn(5)-Mn(5a) 41.16(15) O(10b)-Mn(3)-Mn(3b) 84.4(2) O(14)-Mn(5)-Mn(5b) 41.16(15) O(10b)-Mn(3)-Mn(4) 128.7(2) O(14)-Mn(5)-Mn(6) 85.6(2) O(11)-Mn(3)-Mn(3a) 126.8(2) O(15)-Mn(5)-Mn(5a) 85.8(2) O(11)-Mn(3)-Mn(3b) 134.7(2) O(15)-Mn(5)-Mn(5b) 130.1(2) O(11)-Mn(3)-Mn(4) 85.78(19) O(15)-Mn(5)-Mn(6) 133.90(19) Mn(3a)-Mn(3)-Mn(3b) 60.0 O(16)-Mn(5)-Mn(5a) 131.3(2) Mn(3)-Mn(3b)-Mn(4) 58.87(3) O(16)-Mn(5)-Mn(5b) 85.97(19) O(7)-Mn(4)-O(7a) 84.7(3) O(16)-Mn(5)-Mn(6) 133.0(2) O(7)-Mn(4)-O(12) 91.2(3) O(17)-Mn(5)-Mn(5a) 131.20(19) O(7a)-Mn(4)-O(12) 89.2(3) O(17)-Mn(5)-Mn(5b) 133.80(17) O(7b)-Mn(4)-O(12) 172.9(3) O(17)-Mn(5)-Mn(6) 85.27(17) O(12a)-Mn(4)-O(12) 94.5(3) Mn(5a)-Mn(5)-Mn(5b) 60.0 O(7)-Mn(4)-Mn(3) 40.12(17) Mn(5a)-Mn(5)-Mn(6) 61.06(3) O(7)-Mn(4)-Mn(3a) 45.1(2) O(13)-Mn(6)-O(13a) 78.9(2)

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302 Table A-2. Continued. O(13)-Mn(6)-O(18) 90.5(2) O(19b)-Mn(7)-Mn(7a) 83.3(2) O(13)-Mn(6)-O(18a) 164.4(2) O(19b)-Mn(7)-Mn(7b) 39.5(2) O(18)-Mn(6)-O(13a) 87.9(2) O(19b)-Mn(7)-Mn(8) 41.3(2) O(18)-Mn(6)-O(18a) 100.6(2) O(20)-Mn(7)-Mn(7a) 41.18(17) O(18)-Mn(6)-Mn(5a) 125.38(19) O(20)-Mn(7)-Mn(7b) 41.18(17) O(18)-Mn(6)-Mn(5) 83.55(17) O(20)-Mn(7)-Mn(8) 83.2(3) O(18a)-Mn(6)-Mn(5) 132.52(19) O(21)-Mn(7)-Mn(7a) 82.8(2) O(13)-Mn(6)-Mn(5) 37.50(16) O(21)-Mn(7)-Mn(7b) 130.1(2) O(13)-Mn(6)-Mn(5a) 81.12(16) O(21)-Mn(7)-Mn(8) 129.2(2) O(13a)-Mn(6)-Mn(5) 42.07(16) O(22b)-Mn(7)-Mn(7a) 132.2(2) Mn(5a)-Mn(6)-Mn(5) 57.88(5) O(22b)-Mn(7)-Mn(7b) 85.7(2) Mn(5)-O(13)-Mn(5b) 96.8(3) O(22b)-Mn(7)-Mn(8) 131.5(3) Mn(5)-O(13)-Mn(6) 100.9(3) O(23)-Mn(7)-Mn(7a) 133.1(2) Mn(5b)-O(13)-Mn(6) 95.7(2) O(23)-Mn(7)-Mn(7b) 127.7(2) Mn(5)-O(14)-Mn(5b) 97.7(3) O(23)-Mn(7)-Mn(8) 85.1(2) O(19)-Mn(7)-O(19b) 81.9(4) Mn(7a)-Mn(7)-Mn(7b) 60.0 O(19)-Mn(7)-O(20) 83.7(3) Mn (7b)-Mn(7)-Mn(8) 59.01(4) O(19)-Mn(7)-O(21) 88.4(3) O(19)-Mn(8)-O(19b) 85.5(4) O(19)-Mn(7)-O(22b) 171.1(4) O(19)-Mn(8)-O(24) 92.6(3) O(19)-Mn(7)-O(23) 90.1(3) O(19)-Mn(8)-O(24b) 173.6(4) O(20)-Mn(7)-O(19b) 80.1(3) O(24)-Mn(8)-O(19b) 88.2(3) O(20)-Mn(7)-O(21) 88.9(3) O(24)-Mn(8)-O(24b) 93.5(3) O(20)-Mn(7)-O(22b) 91.0(3) O(19)-Mn(8)-Mn(7) 40.8(2) O(20)-Mn(7)-O(23) 167.5(3) O(19)-Mn(8)-Mn(7b) 88.0(3) O(21)-Mn(7)-O(19b) 166.1(3) O(19b)-Mn(8)-Mn(7) 45.0(3) O(21)-Mn(7)-O(22b) 98.7(3) O(24)-Mn(8)-Mn(7b) 128.8(2) O(21)-Mn(7)-O(23) 101.7(3) O(24)-Mn(8)-Mn(7a) 137.6(2) O(22b)-Mn(7)-O(19b) 90.2(3) O(24)-Mn(8)-Mn(7) 86.52(19) O(22b)-Mn(7)-O(23) 93.7(3) Mn(7a)-Mn(8)-Mn(7) 61.98(8) O(23)-Mn(7)-O(19b) 88.3(3) Mn(7)-O(19)-Mn(8) 98.3(3) O(19)-Mn(7)-Mn(7a) 43.0(3) Mn(7)-O(19)-Mn(7a) 97.5(3) O(19)-Mn(7)-Mn(7b) 85.6(3) Mn(8)-O(19)-Mn(7a) 93.6(4) O(19)-Mn(7)-Mn(8) 40.9(2) Mn(7)-O(20)-Mn(7a) 97.6(3)

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303 Table A-3. Selected interatomic distances () and angles () for [Mn7O8(O2CMe)(O2SePh)8(H2O)]6MeCN (66MeCN). Mn(1)-O(1) 1.788(6) Mn(4)-O(26) 2.180(7) Mn(1)-O(2) 1.839(7) Mn(4)-O(7) 2.278(6) Mn(1)-O(3) 1.914(6) Mn(4)-Mn(5) 2.829(2) Mn(1)-O(11) 1.944(6) Mn(4)-Mn(6) 3.018(2) Mn(1)-O(9) 1.961(6) Mn(4)-Mn(7) 3.036(2) Mn(1)-O(13) 1.998(6) Mn(5)-O(5) 1.880(6) Mn(1)-Mn(2) 2.722(2) Mn(5)-O(6) 1.907(6) Mn(2)-O(1) 1.813(6) Mn(5)-O(18) 1.909(7) Mn(2)-O(2) 1.844(7) Mn(5)-O(21) 1.950(6) Mn(2)-O(5) 1.905(6) Mn(5)-O(27) 2.172(8) Mn(2)-O(12) 1.948(6) Mn(5)-O(16) 2.530(7) Mn(2)-O(10) 1.992(7) Mn(5)-Mn(7) 3.126(2) Mn(2)-O(15) 2.002(6) Mn(6)-O(8) 1.831(6) Mn(3)-O(3) 1.893(6) Mn(6)-O(7) 1.848(6) Mn(3)-O(4) 1.898(6) Mn(6)-O(4) 1.899(7) Mn(3)-O(17) 1.918(6) Mn(6)-O(23) 1.909(7) Mn(3)-O(19) 1.923(6) Mn(6)-O(20) 1.963(6) Mn(3)-O(25) 2.289(8) Mn(6)-O(14) 1.994(6) Mn(3)-O(14) 2.478(7) Mn(6)-Mn(7) 2.758(2) Mn(3)-Mn(4) 2.856(2) Mn(7)-O(8) 1.819(6) Mn(3)-Mn(6) 3.111(2) Mn(7)-O(7) 1.852(5) Mn(4)-O(5) 1.895(6) Mn(7)-O(6) 1.886(6) Mn(4)-O(3) 1.907(6) Mn(7)-O(24) 1.935(7) Mn(4)-O(4) 1.944(6) Mn(7)-O(16) 1.960(6) Mn(4)-O(6) 1.949(6) Mn(7)-O(22) 1.964(6) O(1)-Mn(1)-O(2) 83.7(3) O(11)-Mn(1)-Mn(2) 91.2(2) O(1)-Mn(1)-O(3) 93.3(3) O( 9)-Mn(1)-Mn(2) 139.3(2) O(2)-Mn(1)-O(3) 90.0(3) O(13)-Mn(1)-Mn(2) 130.64(18) O(1)-Mn(1)-O(11) 92.8(3) O(1)-Mn(2)-O(2) 82.8(3) O(2)-Mn(1)-O(11) 90.0(3) O(1)-Mn(2)-O(5) 92.3(3) O(3)-Mn(1)-O(11) 173.9(3) O(2)-Mn(2)-O(5) 91.3(3) O(1)-Mn(1)-O(9) 176.4(3) O( 1)-Mn(2)-O(12) 92.7(3) O(2)-Mn(1)-O(9) 97.0(3) O( 2)-Mn(2)-O(12) 89.8(3) O(3)-Mn(1)-O(9) 90.3(3) O( 5)-Mn(2)-O(12) 174.9(3) O(11)-Mn(1)-O(9) 83.6(3) O( 1)-Mn(2)-O(10) 179.0(3) O(1)-Mn(1)-O(13) 89.4(3) O(2)-Mn(2)-O(10) 97.0(3) O(2)-Mn(1)-O(13) 173.0(3) O(5)-Mn(2)-O(10) 88.7(3) O(3)-Mn(1)-O(13) 91.5(3) O(12)-Mn(2)-O(10) 86.3(3) O(11)-Mn(1)-O(13) 89.3(3) O(1)-Mn(2)-O(15) 90.9(3) O(9)-Mn(1)-O(13) 89.8(3) O(2)-Mn(2)-O(15) 173.7(3) O(1)-Mn(1)-Mn(2) 41.24(19) O(5)-Mn(2)-O(15) 89.8(3) O(2)-Mn(1)-Mn(2) 42.4(2) O(12)-Mn(2)-O(15) 89.7(3) O(3)-Mn(1)-Mn(2) 92.92(19) O(10)-Mn(2)-O(15) 89.2(3)

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304 Table A-3. Continued. O(1)-Mn(2)-Mn(1) 40.57(19) O(3)-Mn(4)-O(7) 99.8(2) O(2)-Mn(2)-Mn(1) 42.3(2) O(4)-Mn(4)-O(7) 75.4(2) O(5)-Mn(2)-Mn(1) 93.05(19) O(6)-Mn(4)-O(7) 74.4(2) O(12)-Mn(2)-Mn(1) 91.0(2) O(26)-Mn(4)-O(7) 160.8(2) O(10)-Mn(2)-Mn(1) 139.3(2) O(5)-Mn(4)-Mn(5) 41.25(18) O(15)-Mn(2)-Mn(1) 131.42(19) O(3)-Mn(4)-Mn(5) 141.3(2) O(3)-Mn(3)-O(4) 83.5(3) O( 4)-Mn(4)-Mn(5) 136.79(19) O(3)-Mn(3)-O(17) 93.9(3) O(6)-Mn(4)-Mn(5) 42.22(18) O(4)-Mn(3)-O(17) 177.1(3) O(26)-Mn(4)-Mn(5) 83.0(2) O(3)-Mn(3)-O(19) 173.5(3) O(7)-Mn(4)-Mn(5) 92.84(14) O(4)-Mn(3)-O(19) 90.7(3) O(5)-Mn(4)-Mn(3) 141.48(19) O(17)-Mn(3)-O(19) 91.9(3) O(3)-Mn(4)-Mn(3) 41.09(19) O(3)-Mn(3)-O(25) 89.0(3) O(4)-Mn(4)-Mn(3) 41.34(18) O(4)-Mn(3)-O(25) 89.4(3) O(6)-Mn(4)-Mn(3) 136.16(19) O(17)-Mn(3)-O(25) 89.3(3) O(26)-Mn(4)-Mn(3) 91.5(2) O(19)-Mn(3)-O(25) 94.0(3) O(7)-Mn(4)-Mn(3) 91.69(14) O(3)-Mn(3)-O(14) 90.0(2) Mn(5)-Mn(4)-Mn(3) 174.10(8) O(4)-Mn(3)-O(14) 74.4(2) O(5)-Mn(4)-Mn(6) 135.9(2) O(17)-Mn(3)-O(14) 106.9(3) O(3)-Mn(4)-Mn(6) 92.4(2) O(19)-Mn(3)-O(14) 85.4(3) O(4)-Mn(4)-Mn(6) 37.72(19) O(25)-Mn(3)-O(14) 163.8(2) O(6)-Mn(4)-Mn(6) 82.01(18) O(3)-Mn(3)-Mn(4) 41.46(18) O(26)-Mn(4)-Mn(6) 130.1(2) O(4)-Mn(3)-Mn(4) 42.59(18) O(7)-Mn(4)-Mn(6) 37.67(14) O(17)-Mn(3)-Mn(4) 134.7(2) Mn(5)-Mn(4)-Mn(6) 118.37(7) O(19)-Mn(3)-Mn(4) 133.17(19) Mn(3)-Mn(4)-Mn(6) 63.88(5) O(25)-Mn(3)-Mn(4) 83.75(19) O(5)-Mn(4)-Mn(7) 90.58(19) O(14)-Mn(3)-Mn(4) 84.85(14) O(3)-Mn(4)-Mn(7) 137.2(2) O(3)-Mn(3)-Mn(6) 89.9(2) O(4)-Mn(4)-Mn(7) 83.72(19) O(4)-Mn(3)-Mn(6) 35.0(2) O(6)-Mn(4)-Mn(7) 36.95(18) O(17)-Mn(3)-Mn(6) 146.6(2) O(26)-Mn(4)-Mn(7) 126.3(2) O(19)-Mn(3)-Mn(6) 83.7(2) O(7)-Mn(4)-Mn(7) 37.49(13) O(25)-Mn(3)-Mn(6) 123.98(18) Mn(5)-Mn(4)-Mn(7) 64.31(5) O(14)-Mn(3)-Mn(6) 39.82(14) Mn(3)-Mn(4)-Mn(7) 118.05(7) Mn(4)-Mn(3)-Mn(6) 60.59(5) Mn(6)-Mn(4)-Mn(7) 54.20(5) O(5)-Mn(4)-O(3) 100.4(3) O(5)-Mn(5)-O(6) 83.8(3) O(5)-Mn(4)-O(4) 173.6(3) O( 5)-Mn(5)-O(18) 96.2(3) O(3)-Mn(4)-O(4) 81.9(3) O( 6)-Mn(5)-O(18) 176.2(4) O(5)-Mn(4)-O(6) 82.3(3) O( 5)-Mn(5)-O(21) 172.1(3) O(3)-Mn(4)-O(6) 174.0(3) O( 6)-Mn(5)-O(21) 90.2(3) O(4)-Mn(4)-O(6) 94.9(3) O(18)-Mn(5)-O(21) 89.3(3) O(5)-Mn(4)-O(26) 90.9(3) O(5)-Mn(5)-O(27) 90.1(3) O(3)-Mn(4)-O(26) 95.0(3) O(6)-Mn(5)-O(27) 89.4(3) O(4)-Mn(4)-O(26) 94.9(3) O(18)-Mn(5)-O(27) 94.5(4) O(6)-Mn(4)-O(26) 90.3(3) O(21)-Mn(5)-O(27) 95.1(3) O(5)-Mn(4)-O(7) 98.3(2) O( 5)-Mn(5)-O(16) 87.5(3)

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305 Table A-3. Continued. O(6)-Mn(5)-O(16) 72.7(2) O(8)-Mn(6)-Mn(3) 130.8(2) O(18)-Mn(5)-O(16) 103.4(4) O(7)-Mn(6)-Mn(3) 93.19(18) O(21)-Mn(5)-O(16) 85.7(3) O(4)-Mn(6)-Mn(3) 34.96(18) O(27)-Mn(5)-O(16) 162.1(3) O(23)-Mn(6)-Mn(3) 141.73(19) O(5)-Mn(5)-Mn(4) 41.67(19) O(20)-Mn(6)-Mn(3) 84.6(2) O(6)-Mn(5)-Mn(4) 43.38(18) O(14)-Mn(6)-Mn(3) 52.73(19) O(18)-Mn(5)-Mn(4) 137.3(2) Mn(7)-Mn(6)-Mn(3) 118.75(7) O(21)-Mn(5)-Mn(4) 133.3(2) Mn(4)-Mn(6)-Mn(3) 55.53(5) O(27)-Mn(5)-Mn(4) 81.6(2) O(8)-Mn(7)-O(7) 82.8(2) O(16)-Mn(5)-Mn(4) 84.83(14) O(8)-Mn(7)-O(6) 93.5(3) O(5)-Mn(5)-Mn(7) 88.1(2) O(7)-Mn(7)-O(6) 86.8(3) O(6)-Mn(5)-Mn(7) 34.28(19) O(8)-Mn(7)-O(24) 88.4(3) O(18)-Mn(5)-Mn(7) 141.9(3) O(7)-Mn(7)-O(24) 91.7(3) O(21)-Mn(5)-Mn(7) 84.0(2) O(6)-Mn(7)-O(24) 177.4(3) O(27)-Mn(5)-Mn(7) 123.5(2) O(8)-Mn(7)-O(16) 174.4(3) O(16)-Mn(5)-Mn(7) 38.77(14) O(7)-Mn(7)-O(16) 91.9(3) Mn(4)-Mn(5)-Mn(7) 61.06(5) O(6)-Mn(7)-O(16) 88.2(3) O(8)-Mn(6)-O(7) 82.6(2) O(24)-Mn(7)-O(16) 89.7(3) O(8)-Mn(6)-O(4) 95.8(3) O( 8)-Mn(7)-O(22) 96.4(3) O(7)-Mn(6)-O(4) 87.6(3) O( 7)-Mn(7)-O(22) 175.5(3) O(8)-Mn(6)-O(23) 87.5(3) O(6)-Mn(7)-O(22) 88.8(3) O(7)-Mn(6)-O(23) 92.1(3) O(24)-Mn(7)-O(22) 92.8(3) O(4)-Mn(6)-O(23) 176.6(3) O(16)-Mn(7)-O(22) 89.0(3) O(8)-Mn(6)-O(20) 95.8(3) O(8)-Mn(7)-Mn(6) 41.09(18) O(7)-Mn(6)-O(20) 175.5(3) O(7)-Mn(7)-Mn(6) 41.75(17) O(4)-Mn(6)-O(20) 88.3(3) O(6)-Mn(7)-Mn(6) 90.60(18) O(23)-Mn(6)-O(20) 92.1(3) O(24)-Mn(7)-Mn(6) 89.66(18) O(8)-Mn(6)-O(14) 172.1(3) O(16)-Mn(7)-Mn(6) 133.64(19) O(7)-Mn(6)-O(14) 90.2(2) O(22)-Mn(7)-Mn(6) 137.3(2) O(4)-Mn(6)-O(14) 87.2(3) O(8)-Mn(7)-Mn(4) 87.8(2) O(23)-Mn(6)-O(14) 89.4(3) O(7)-Mn(7)-Mn(4) 48.44(19) O(20)-Mn(6)-O(14) 91.6(2) O(6)-Mn(7)-Mn(4) 38.39(18) O(8)-Mn(6)-Mn(7) 40.76(18) O(24)-Mn(7)-Mn(4) 140.07(19) O(7)-Mn(6)-Mn(7) 41.88(16) O(16)-Mn(7)-Mn(4) 90.27(19) O(4)-Mn(6)-Mn(7) 92.70(19) O(22)-Mn(7)-Mn(4) 127.2(2) O(23)-Mn(6)-Mn(7) 89.30(19) Mn(6)-Mn(7)-Mn(4) 62.57(5) O(20)-Mn(6)-Mn(7) 136.47(19) O(8)-Mn(7)-Mn(5) 128.2(2) O(14)-Mn(6)-Mn(7) 131.97(17) O(7)-Mn(7)-Mn(5) 93.09(19) O(8)-Mn(6)-Mn(4) 88.1(2) O(6)-Mn(7)-Mn(5) 34.70(18) O(7)-Mn(6)-Mn(4) 48.88(19) O(24)-Mn(7)-Mn(5) 143.46(19) O(4)-Mn(6)-Mn(4) 38.78(18) O(16)-Mn(7)-Mn(5) 53.93(19) O(23)-Mn(6)-Mn(4) 140.9(2) O(22)-Mn(7)-Mn(5) 83.9(2) O(20)-Mn(6)-Mn(4) 126.9(2) Mn(6)-Mn(7)-Mn(5) 117.07(7) O(14)-Mn(6)-Mn(4) 89.73(19) Mn(4)-Mn(7)-Mn(5) 54.63(5) Mn(7)-Mn(6)-Mn(4) 63.24(5) Mn(1)-O(1)-Mn(2) 98.2(3)

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306 Table A-3. Continued. Mn(1)-O(2)-Mn(2) 95.3(3) Mn(7)-O(6)-Mn(5) 111.0(3) Mn(3)-O(3)-Mn(4) 97.5(3) Mn(7)-O(6)-Mn(4) 104.7(3) Mn(3)-O(3)-Mn(1) 130.7(3) Mn(5)-O(6)-Mn(4) 94.4(3) Mn(4)-O(3)-Mn(1) 121.3(3) Mn(6)-O(7)-Mn(7) 96.4(3) Mn(3)-O(4)-Mn(6) 110.0(3) Mn(6)-O(7)-Mn(4) 93.5(2) Mn(3)-O(4)-Mn(4) 96.1(3) Mn(7)-O(7)-Mn(4) 94.1(2) Mn(6)-O(4)-Mn(4) 103.5(3) Mn(7)-O(8)-Mn(6) 98.2(3) Mn(5)-O(5)-Mn(4) 97.1(3) Mn(6)-O(14)-Mn(3) 87.4(2) Mn(5)-O(5)-Mn(2) 133.7(3) Mn (7)-O(16)-Mn(5) 87.3(2) Mn(4)-O(5)-Mn(2) 122.3(3)

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307 Table A-4. Selected interatomic distances () and angles () for [Mn7O8(O2SePh)9(H2O)]2CH2Cl2 (72CH2Cl2). Mn(1)-O(14) 1.800(6) Mn(2)-Mn(3) 3.113(2) Mn(1)-O(15) 1.845(7) Mn(3)-O(11) 1.822(5) Mn(1)-O(12) 1.886(7) Mn(3)-O(16) 1.843(5) Mn(1)-O(3) 1.932(7) Mn(3)-O(13) 1.894(8) Mn(1)-O(4) 1.983(8) Mn(3)-O(10) 1.923(7) Mn(1)-O(6) 2.010(8) Mn(3)-O(9) 1.952(8) Mn(1)-Mn(1a) 2.760(3) Mn(3)-O(7) 2.005(7) Mn(2)-O(13) 1.873(8) Mn(3)-Mn(3a) 2.741(3) Mn(2)-O(12) 1.902(7) Mn(3)-Mn(4) 3.033(3) Mn(2)-O(5) 1.903(10) Mn(4)-O(12) 1.913(7) Mn(2)-O(8) 1.935(8) Mn(4)-O(13) 1.963(7) Mn(2)-O(2) 2.001(17) Mn(4)-O(1) 2.160(12) Mn(2)-Mn(4) 2.8427(19) Mn(4)-O(16) 2.276(9) O(12)-Mn(1)-O(3) 173.8(3) O( 2)-Mn(2)-Mn(4) 88.2(5) O(14)-Mn(1)-O(4) 177.2(4) O(13)-Mn(2)-Mn(3) 34.5(2) O(15)-Mn(1)-O(4) 98.8(4) O(12)-Mn(2)-Mn(3) 89.5(2) O(12)-Mn(1)-O(4) 89.9(4) O( 5)-Mn(2)-Mn(3) 147.9(4) O(3)-Mn(1)-O(4) 84.1(4) O( 8)-Mn(2)-Mn(3) 82.3(3) O(14)-Mn(1)-O(6) 90.2(3) O( 2)-Mn(2)-Mn(3) 125.5(7) O(15)-Mn(1)-O(6) 171.7(3) Mn(4)-Mn(2)-Mn(3) 61.03(6) O(12)-Mn(1)-O(6) 91.5(3) O(11)-Mn(3)-O(16) 83.1(3) O(3)-Mn(1)-O(6) 89.9(3) O(11)-Mn(3)-O(13) 94.6(4) O(4)-Mn(1)-O(6) 89.3(4) O(16)-Mn(3)-O(13) 87.3(3) O(14)-Mn(1)-Mn(1a) 40.0(2) O(11)-Mn(3)-O(10) 89.4(4) O(15)-Mn(1)-Mn(1a) 41.6(2) O(16)-Mn(3)-O(10) 92.1(3) O(12)-Mn(1)-Mn(1a) 92.6(2) O(13)-Mn(3)-O(10) 175.9(3) O(3)-Mn(1)-Mn(1a) 91.2(3) O(11)-Mn(3)-O(9) 96.3(3) O(4)-Mn(1)-Mn(1a) 140.3(3) O(16)-Mn(3)-O(9) 176.3(4) O(6)-Mn(1)-Mn(1a) 130.2(2) O(13)-Mn(3)-O(9) 89.1(3) O(13)-Mn(2)-O(12) 84.5(3) O(10)-Mn(3)-O(9) 91.6(3) O(13)-Mn(2)-O(5) 176.4(4) O(11)-Mn(3)-O(7) 173.7(3) O(12)-Mn(2)-O(5) 92.7(4) O(16)-Mn(3)-O(7) 90.9(3) O(13)-Mn(2)-O(8) 88.9(4) O(13)-Mn(3)-O(7) 86.9(3) O(12)-Mn(2)-O(8) 171.7(4) O(10)-Mn(3)-O(7) 89.0(3) O(5)-Mn(2)-O(8) 94.1(5) O(9)-Mn(3)-O(7) 89.8(3) O(13)-Mn(2)-O(2) 92.0(7) O(11)-Mn(3)-Mn(3a) 41.21(19) O(12)-Mn(2)-O(2) 95.5(5) O(16)-Mn(3)-Mn(3a) 41.94(19) O(5)-Mn(2)-O(2) 86.1(8) O( 13)-Mn(3)-Mn(3a) 92.6(2) O(8)-Mn(2)-O(2) 89.8(6) O(10)-Mn(3)-Mn(3a 89.66(19) O(13)-Mn(2)-Mn(4) 43.4(2) O(9)-Mn(3)-Mn(3a) 137.5(3) O(12)-Mn(2)-Mn(4) 42.0(2) O(7)-Mn(3)-Mn(3a) 132.72(19) O(5)-Mn(2)-Mn(4) 133.4(3) O(11)-Mn(3)-Mn(4) 87.6(3) O(8)-Mn(2)-Mn(4) 132.1(4) O(16)-Mn(3)-Mn(4) 48.4(3)

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308 Table A-4. Continued. O(13)-Mn(3)-Mn(4) 39.0(2) O(13a)-Mn(4)-Mn(2) 136.7(2) O(10)-Mn(3)-Mn(4) 140.5(2) O(1)-Mn(4)-Mn(2) 87.55(6) O(9)-Mn(3)-Mn(4) 127.9(3) O(16)-Mn(4)-Mn(2) 91.86(6) O(7)-Mn(3)-Mn(4) 89.8(2) Mn(2)-Mn(4)-Mn(2a) 174.45(12) Mn(3a)-Mn(3)-Mn(4) 63.13(3) O(12)-Mn(4)-Mn(3a) 136.1(2) O(11)-Mn(3)-Mn(2) 128.6(3) O(13)-Mn(4)-Mn(3a) 82.9(2) O(16)-Mn(3)-Mn(2) 93.0(2) Mn(2)-Mn(4)-Mn(3a) 117.56(8) O(13)-Mn(3)-Mn(2) 34.1(2) O(12)-Mn(4)-Mn(3) 91.7(2) O(10)-Mn(3)-Mn(2) 142.0(2) O(13)-Mn(4)-Mn(3) 37.3(2) O(9)-Mn(3)-Mn(2) 84.5(3) O(1)-Mn(4)-Mn(3) 127.5(3) O(7)-Mn(3)-Mn(2) 53.3(2) O(16)-Mn(4)-Mn(3) 37.29(14) Mn(3a)-Mn(3)-Mn(2) 118.15(5) Mn(2)-Mn(4)-Mn(3) 63.88(5) Mn(4)-Mn(3)-Mn(2) 55.08(5) Mn (3a)-Mn(4)-Mn(3) 53.73(7) O(12)-Mn(4)-O(12a) 99.9(4) Mn (3a)-O(11)-Mn(3) 97.6(4) O(12)-Mn(4)-O(13) 81.8(3) Mn(1)-O(12)-Mn(2) 132.2(4) O(12)-Mn(4)-O(13a) 173.5(3) Mn(1)-O(12)-Mn(4) 122.5(3) O(13)-Mn(4)-O(13a) 95.8(4) Mn(2)-O(12)-Mn(4) 96.4(3) O(12)-Mn(4)-O(1) 94.0(3) Mn(2)-O(13)-Mn(3) 111.5(4) O(13)-Mn(4)-O(1) 92.2(3) Mn(2)-O(13)-Mn(4) 95.6(3) O(12)-Mn(4)-O(16) 98.9(3) Mn(3)-O(13)-Mn(4) 103.7(3) O(13)-Mn(4)-O(16) 74.6(3) Mn(1)-O(14)-Mn(1a) 100.1(4) O(1)-Mn(4)-O(16) 160.0(4) Mn(1a)-O(15)-Mn(1) 96.8(5) O(12)-Mn(4)-Mn(2) 41.7(2) Mn(3)-O(16)-Mn(3a) 96.1(4) O(12a)-Mn(4)-Mn(2) 141.4(2) Mn(3)-O(16)-Mn(4) 94.3(3) O(13)-Mn(4)-Mn(2) 41.0(2)

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309 Table A-5. Selected interatomic distances () and angles () for [Mn4O4(O2AsMe2)6]5H2OC5H12 (95H2OC5H12). Mn(1)-O(2) 1.855(3) Mn(2)-Mn(3) 2.8638(8) Mn(1)-O(3) 1.873(3) Mn(2)-Mn(4) 3.0546(8) Mn(1)-O(1) 1.899(3) Mn(3)-O(4) 1.838(3) Mn(1)-O(9) 1.906(3) Mn(3)-O(8) 1.933(3) Mn(1)-O(5) 1.931(3) Mn(3)-O(1) 1.950(3) Mn(1)-O(7) 1.940(2) Mn(3)-O(12) 1.951(3) Mn(1)-Mn(4) 2.8646(8) Mn(3)-O(15) 1.958(3) Mn(1)-Mn(3) 2.8857(8) Mn(3)-O(3) 1.959(3) Mn(1)-Mn(2) 2.9375(8) Mn(3)-Mn(4) 3.0171(8) Mn(2)-O(4) 1.897(3) Mn(4)-O(2) 1.899(3) Mn(2)-O(13) 1.909(3) Mn(4)-O(16) 1.929(3) Mn(2)-O(6) 1.921(3) Mn(4)-O(14) 1.938(3) Mn(2)-O(1) 1.933(3) Mn(4)-O(3) 1.974(3) Mn(2)-O(11) 2.066(3) Mn(4)-O(10) 2.095(3) Mn(2)-O(2) 2.139(3) Mn(4)-O(4) 2.291(3) O(2)-Mn(1)-O(3) 83.47(11) O( 2)-Mn(1)-Mn(2) 46.55(9) O(2)-Mn(1)-O(1) 86.57(12) O( 3)-Mn(1)-Mn(2) 85.87(8) O(3)-Mn(1)-O(1) 84.09(11) O( 1)-Mn(1)-Mn(2) 40.38(8) O(2)-Mn(1)-O(9) 92.87(12) O( 9)-Mn(1)-Mn(2) 139.28(8) O(3)-Mn(1)-O(9) 93.71(12) O( 5)-Mn(1)-Mn(2) 88.28(9) O(1)-Mn(1)-O(9) 177.78(12) O( 7)-Mn(1)-Mn(2) 131.07(8) O(2)-Mn(1)-O(5) 93.62(12) Mn(4)-Mn(1)-Mn(2) 63.519(19) O(3)-Mn(1)-O(5) 173.95(12) Mn(3)-Mn(1)-Mn(2) 58.909(19) O(1)-Mn(1)-O(5) 90.45(12) O( 4)-Mn(2)-O(13) 93.12(12) O(9)-Mn(1)-O(5) 91.73(12) O(4)-Mn(2)-O(6) 173.07(12) O(2)-Mn(1)-O(7) 175.09(11) O(13)-Mn(2)-O(6) 91.67(13) O(3)-Mn(1)-O(7) 92.17(11) O(4)-Mn(2)-O(1) 81.66(11) O(1)-Mn(1)-O(7) 90.75(11) O(13)-Mn(2)-O(1) 171.85(12) O(9)-Mn(1)-O(7) 89.65(11) O(6)-Mn(2)-O(1) 93.02(12) O(5)-Mn(1)-O(7) 90.50(12) O( 4)-Mn(2)-O(11) 92.54(13) O(2)-Mn(1)-Mn(4) 40.82(8) O(13)-Mn(2)-O(11) 96.52(13) O(3)-Mn(1)-Mn(4) 43.25(8) O(6)-Mn(2)-O(11) 91.89(13) O(1)-Mn(1)-Mn(4) 89.35(8) O(1)-Mn(2)-O(11) 89.99(12) O(9)-Mn(1)-Mn(4) 88.83(8) O(4)-Mn(2)-O(2) 85.46(11) O(5)-Mn(1)-Mn(4) 134.37(9) O(13)-Mn(2)-O(2) 95.19(11) O(7)-Mn(1)-Mn(4) 135.13(8) O(6)-Mn(2)-O(2) 89.12(12) O(2)-Mn(1)-Mn(3) 87.22(8) O(1)-Mn(2)-O(2) 78.23(10) O(3)-Mn(1)-Mn(3) 42.27(8) O(11)-Mn(2)-O(2) 168.21(11) O(1)-Mn(1)-Mn(3) 42.12(8) O(4)-Mn(2)-Mn(3) 39.18(8) O(9)-Mn(1)-Mn(3) 135.73(9) O(13)-Mn(2)-Mn(3) 132.30(10) O(5)-Mn(1)-Mn(3) 132.48(9) O(6)-Mn(2)-Mn(3) 135.74(9) O(7)-Mn(1)-Mn(3) 88.04(7) O(1)-Mn(2)-Mn(3) 42.72(8) Mn(4)-Mn(1)-Mn(3) 63.291(19) O(11)-Mn(2)-Mn(3) 88.28(9)

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310 Table A-5. Continued. O(2)-Mn(2)-Mn(3) 82.83(7) O(8)-Mn(3)-Mn(4) 129.98(8) O(4)-Mn(2)-Mn(1) 85.51(8) O(1)-Mn(3)-Mn(4) 84.06(8) O(13)-Mn(2)-Mn(1) 134.19(9) O(12)-Mn(3)-Mn(4) 141.53(9) O(6)-Mn(2)-Mn(1) 87.56(9) O(15)-Mn(3)-Mn(4) 86.39(8) O(1)-Mn(2)-Mn(1) 39.53(8) O(3)-Mn(3)-Mn(4) 40.10(7) O(11)-Mn(2)-Mn(1) 129.29(9) Mn(2)-Mn(3)-Mn(4) 62.522(19) O(2)-Mn(2)-Mn(1) 39.01(7) Mn(1)-Mn(3)-Mn(4) 58.014(18) Mn(3)-Mn(2)-Mn(1) 59.643(19) O(2)-Mn(4)-O(16) 170.58(11) O(4)-Mn(2)-Mn(4) 48.48(9) O(2)-Mn(4)-O(14) 92.63(11) O(13)-Mn(2)-Mn(4) 88.56(9) O(16)-Mn(4)-O(14) 94.21(12) O(6)-Mn(2)-Mn(4) 126.73(10) O(2)-Mn(4)-O(3) 79.67(11) O(1)-Mn(2)-Mn(4) 83.30(8) O(16)-Mn(4)-O(3) 92.20(11) O(11)-Mn(2)-Mn(4) 140.98(9) O(14)-Mn(4)-O(3) 165.14(11) O(2)-Mn(2)-Mn(4) 37.98(7) O(2)-Mn(4)-O(10) 91.52(12) Mn(3)-Mn(2)-Mn(4) 61.197(19) O(16)-Mn(4)-O(10) 93.97(12) Mn(1)-Mn(2)-Mn(4) 57.079(18) O(14)-Mn(4)-O(10) 97.66(11) O(4)-Mn(3)-O(8) 176.71(12) O( 3)-Mn(4)-O(10) 95.25(11) O(4)-Mn(3)-O(1) 82.69(11) O(2)-Mn(4)-O(4) 81.26(11) O(8)-Mn(3)-O(1) 94.07(11) O(16)-Mn(4)-O(4) 92.31(11) O(4)-Mn(3)-O(12) 92.26(12) O(14)-Mn(4)-O(4) 89.68(11) O(8)-Mn(3)-O(12) 88.46(12) O(3)-Mn(4)-O(4) 76.66(10) O(1)-Mn(3)-O(12) 92.90(12) O(10)-Mn(4)-O(4) 169.95(10) O(4)-Mn(3)-O(15) 92.44(12) O(2)-Mn(4)-Mn(1) 39.69(8) O(8)-Mn(3)-O(15) 90.68(12) O(16)-Mn(4)-Mn(1) 132.67(8) O(1)-Mn(3)-O(15) 170.28(11) O(14)-Mn(4)-Mn(1) 132.15(9) O(12)-Mn(3)-O(15) 95.70(13) O(3)-Mn(4)-Mn(1) 40.55(7) O(4)-Mn(3)-O(3) 88.76(12) O(10)-Mn(4)-Mn(1) 89.18(8) O(8)-Mn(3)-O(3) 90.15(11) O( 4)-Mn(4)-Mn(1) 80.80(7) O(1)-Mn(3)-O(3) 80.53(11) O( 2)-Mn(4)-Mn(3) 82.68(8) O(12)-Mn(3)-O(3) 173.18(12) O(16)-Mn(4)-Mn(3) 88.04(8) O(15)-Mn(3)-O(3) 90.99(11) O(14)-Mn(4)-Mn(3) 127.12(9) O(4)-Mn(3)-Mn(2) 40.69(8) O(3)-Mn(4)-Mn(3) 39.72(8) O(8)-Mn(3)-Mn(2) 136.13(8) O(10)-Mn(4)-Mn(3) 134.95(8) O(1)-Mn(3)-Mn(2) 42.25(8) O(4)-Mn(4)-Mn(3) 37.46(7) O(12)-Mn(3)-Mn(2) 89.91(9) Mn(1)-Mn(4)-Mn(3) 58.695(19) O(15)-Mn(3)-Mn(2) 133.06(8) O(2)-Mn(4)-Mn(2) 43.88(9) O(3)-Mn(3)-Mn(2) 86.46(7) O(16)-Mn(4)-Mn(2) 130.50(8) O(4)-Mn(3)-Mn(1) 88.11(9) O(14)-Mn(4)-Mn(2) 84.63(8) O(8)-Mn(3)-Mn(1) 89.06(8) O(3)-Mn(4)-Mn(2) 81.02(8) O(1)-Mn(3)-Mn(1) 40.77(8) O(10)-Mn(4)-Mn(2) 135.33(8) O(12)-Mn(3)-Mn(1) 133.23(9) O(4)-Mn(4)-Mn(2) 38.31(7) O(15)-Mn(3)-Mn(1) 131.03(9) Mn(1)-Mn(4)-Mn(2) 59.403(19) O(3)-Mn(3)-Mn(1) 40.04(8) Mn(3)-Mn(4)-Mn(2) 56.281(18) Mn(2)-Mn(3)-Mn(1) 61.45(2) Mn(1)-O(1)-Mn(2) 100.09(12) O(4)-Mn(3)-Mn(4) 49.29(9) Mn(1)-O(1)-Mn(3) 97.11(12)

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311 Table A-5. Continued. Mn(2)-O(1)-Mn(3) 95.04(12) Mn(1)-O(3)-Mn(4) 96.20(11) Mn(1)-O(2)-Mn(4) 99.49(12) Mn(3)-O(3)-Mn(4) 100.18(11) Mn(1)-O(2)-Mn(2) 94.44(12) Mn(3)-O(4)-Mn(2) 100.12(13) Mn(4)-O(2)-Mn(2) 98.14(12) Mn(3)-O(4)-Mn(4) 93.25(11) Mn(1)-O(3)-Mn(3) 97.69(11) Mn(2)-O(4)-Mn(4) 93.21(11)

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312 Table A-6. Selected interatomic distances () and angles () for {[Mn4O4(O2AsMe2)6](NO3)}2MeCN12H2O (10MeCN12H2O). Mn(1)-O(5) 1.897(5) Mn(5)-O(19) 1.885(5) Mn(1)-O(3) 1.922(5) Mn(5)-O(17) 1.897(4) Mn(1)-O(15) 1.923(4) Mn(5)-O(29) 1.903(4) Mn(1)-O(1) 1.929(5) Mn(5)-O(21) 1.907(5) Mn(1)-O(13) 1.930(5) Mn(5)-O(30) 1.908(4) Mn(1)-O(14) 1.956(5) Mn(5)-O(31) 1.917(5) Mn(1)-Mn(4) 2.8791(14) Mn(5)-Mn(6) 2.8691(15) Mn(1)-Mn(2) 2.9236(15) Mn(5)-Mn(8) 2.8756(14) Mn(1)-Mn(3) 2.9281(15) Mn(5)-Mn(7) 2.9178(13) Mn(2)-O(9) 1.901(5) Mn(6)-O(32) 1.872(4) Mn(2)-O(15) 1.907(5) Mn(6)-O(23) 1.892(5) Mn(2)-O(2) 1.925(5) Mn(6)-O(18) 1.905(4) Mn(2)-O(16) 1.960(5) Mn(6)-O(26) 1.911(5) Mn(2)-O(7) 1.978(5) Mn(6)-O(29) 1.920(5) Mn(2)-O(14) 2.029(6) Mn(6)-O(31) 1.925(5) Mn(2)-Mn(4) 2.9102(15) Mn(6)-Mn(8) 2.8323(14) Mn(2)-Mn(3) 2.9400(16) Mn(6)-Mn(7) 3.0000(13) Mn(3)-O(14) 1.884(5) Mn(7)-O(24) 1.892(5) Mn(3)-O(12) 1.905(6) Mn(7)-O(27) 1.912(5) Mn(3)-O(6) 1.906(5) Mn(7)-O(30) 1.954(5) Mn(3)-O(16) 1.924(5) Mn(7)-O(29) 1.985(4) Mn(3)-O(10) 1.971(5) Mn(7)-O(20) 2.087(5) Mn(3)-O(13) 2.004(6) Mn(7)-O(32) 2.242(5) Mn(3)-Mn(4) 2.9016(15) Mn(7)-Mn(8) 3.0079(14) Mn(4)-O(13) 1.883(5) Mn(8)-O(28) 1.864(4) Mn(4)-O(4) 1.887(5) Mn(8)-O(32) 1.865(5) Mn(4)-O(11) 1.889(6) Mn(8)-O(31) 1.914(4) Mn(4)-O(16) 1.906(5) Mn(8)-O(22) 1.916(5) Mn(4)-O(8) 1.911(5) Mn(8)-O(25) 1.917(5) Mn(4)-O(15) 1.967(5) Mn(8)-O(30) 1.923(5) O(5)-Mn(1)-O(3) 92.7(2) O( 1)-Mn(1)-O(14) 91.6(2) O(5)-Mn(1)-O(15) 173.5(2) O(13)-Mn(1)-O(14) 82.0(2) O(3)-Mn(1)-O(15) 90.8(2) O(5)-Mn(1)-Mn(4) 131.80(17) O(5)-Mn(1)-O(1) 91.1(2) O( 3)-Mn(1)-Mn(4) 88.62(15) O(3)-Mn(1)-O(1) 92.0(2) O(15)-Mn(1)-Mn(4) 42.85(14) O(15)-Mn(1)-O(1) 94.2(2) O( 1)-Mn(1)-Mn(4) 137.04(15) O(5)-Mn(1)-O(13) 91.6(2) O(13)-Mn(1)-Mn(4) 40.36(14) O(3)-Mn(1)-O(13) 94.2(2) O(14)-Mn(1)-Mn(4) 84.83(15) O(15)-Mn(1)-O(13) 82.8(2) O(5)-Mn(1)-Mn(2) 136.65(16) O(1)-Mn(1)-O(13) 173.2(2) O(3)-Mn(1)-Mn(2) 130.60(16) O(5)-Mn(1)-O(14) 92.9(2) O(15)-Mn(1)-Mn(2) 40.03(14) O(3)-Mn(1)-O(14) 173.2(2) O(1)-Mn(1)-Mn(2) 88.36(15) O(15)-Mn(1)-O(14) 83.2(2) O(13)-Mn(1)-Mn(2) 85.39(16)

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313 Table A-6. Continued. O(14)-Mn(1)-Mn(2) 43.78(16) Mn(1)-Mn(2)-Mn(3) 59.92(4) Mn(4)-Mn(1)-Mn(2) 60.20(4) O(14)-Mn(3)-O(12) 173.8(2) O(5)-Mn(1)-Mn(3) 88.92(16) O(14)-Mn(3)-O(6) 91.9(2) O(3)-Mn(1)-Mn(3) 137.06(16) O(12)-Mn(3)-O(6) 91.5(2) O(15)-Mn(1)-Mn(3) 84.80(14) O(14)-Mn(3)-O(16) 83.8(2) O(1)-Mn(1)-Mn(3) 130.93(16) O(12)-Mn(3)-O(16) 92.1(2) O(13)-Mn(1)-Mn(3) 42.88(17) O(6)-Mn(3)-O(16) 171.7(2) O(14)-Mn(1)-Mn(3) 39.42(15) O(14)-Mn(3)-O(10) 92.5(2) Mn(4)-Mn(1)-Mn(3) 59.95(4) O(12)-Mn(3)-O(10) 92.4(2) Mn(2)-Mn(1)-Mn(3) 60.32(4) O(6)-Mn(3)-O(10) 94.9(2) O(9)-Mn(2)-O(15) 173.2(2) O(16)-Mn(3)-O(10) 92.5(2) O(9)-Mn(2)-O(2) 92.8(2) O(14)-Mn(3)-O(13) 81.8(2) O(15)-Mn(2)-O(2) 93.0(2) O(12)-Mn(3)-O(13) 92.9(2) O(9)-Mn(2)-O(16) 91.8(2) O(6)-Mn(3)-O(13) 92.2(2) O(15)-Mn(2)-O(16) 82.0(2) O(16)-Mn(3)-O(13) 80.2(2) O(2)-Mn(2)-O(16) 170.6(2) O(10)-Mn(3)-O(13) 171.1(2) O(9)-Mn(2)-O(7) 91.2(2) O(14)-Mn(3)-Mn(4) 85.43(15) O(15)-Mn(2)-O(7) 91.77(19) O(12)-Mn(3)-Mn(4) 88.41(16) O(2)-Mn(2)-O(7) 94.8(2) O( 6)-Mn(3)-Mn(4) 132.19(18) O(16)-Mn(2)-O(7) 93.3(2) O(16)-Mn(3)-Mn(4) 40.52(14) O(9)-Mn(2)-O(14) 94.7(2) O(10)-Mn(3)-Mn(4) 132.93(16) O(15)-Mn(2)-O(14) 81.7(2) O(13)-Mn(3)-Mn(4) 40.16(14) O(2)-Mn(2)-O(14) 92.3(2) O(14)-Mn(3)-Mn(1) 41.23(16) O(16)-Mn(2)-O(14) 79.2(2) O(12)-Mn(3)-Mn(1) 133.76(19) O(7)-Mn(2)-O(14) 170.6(2) O(6)-Mn(3)-Mn(1) 88.62(16) O(9)-Mn(2)-Mn(4) 132.01(17) O(16)-Mn(3)-Mn(1) 83.46(15) O(15)-Mn(2)-Mn(4) 42.09(14) O(10)-Mn(3)-Mn(1) 133.72(16) O(2)-Mn(2)-Mn(4) 135.13(15) O(13)-Mn(3)-Mn(1) 40.93(15) O(16)-Mn(2)-Mn(4) 40.48(14) Mn(4)-Mn(3)-Mn(1) 59.19(4) O(7)-Mn(2)-Mn(4) 87.85(14) O(14)-Mn(3)-Mn(2) 43.18(17) O(14)-Mn(2)-Mn(4) 82.77(15) O(12)-Mn(3)-Mn(2) 133.23(16) O(9)-Mn(2)-Mn(1) 136.45(15) O(6)-Mn(3)-Mn(2) 135.10(17) O(15)-Mn(2)-Mn(1) 40.44(13) O(16)-Mn(3)-Mn(2) 41.26(14) O(2)-Mn(2)-Mn(1) 88.03(15) O(10)-Mn(3)-Mn(2) 87.49(16) O(16)-Mn(2)-Mn(1) 83.02(15) O(13)-Mn(3)-Mn(2) 83.69(15) O(7)-Mn(2)-Mn(1) 132.18(15) Mn(4)-Mn(3)-Mn(2) 59.76(4) O(14)-Mn(2)-Mn(1) 41.84(15) Mn(1)-Mn(3)-Mn(2) 59.76(4) Mn(4)-Mn(2)-Mn(1) 59.14(4) O(13)-Mn(4)-O(4) 91.9(2) O(9)-Mn(2)-Mn(3) 88.83(16) O(13)-Mn(4)-O(11) 91.5(2) O(15)-Mn(2)-Mn(3) 84.73(14) O(4)-Mn(4)-O(11) 90.5(3) O(2)-Mn(2)-Mn(3) 131.56(16) O(13)-Mn(4)-O(16) 83.8(2) O(16)-Mn(2)-Mn(3) 40.36(14) O(4)-Mn(4)-O(16) 173.9(2) O(7)-Mn(2)-Mn(3) 133.56(16) O(11)-Mn(4)-O(16) 94.0(2) O(14)-Mn(2)-Mn(3) 39.47(14) O(13)-Mn(4)-O(8) 174.8(2) Mn(4)-Mn(2)-Mn(3) 59.47(4) O(4)-Mn(4)-O(8) 91.7(2)

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314 Table A-6. Continued. O(11)-Mn(4)-O(8) 92.2(2) O(29)-Mn(5)-Mn(6) 41.59(14) O(16)-Mn(4)-O(8) 92.3(2) O(21)-Mn(5)-Mn(6) 133.19(16) O(13)-Mn(4)-O(15) 82.8(2) O(30)-Mn(5)-Mn(6) 83.98(14) O(4)-Mn(4)-O(15) 93.4(2) O(31)-Mn(5)-Mn(6) 41.80(13) O(11)-Mn(4)-O(15) 173.2(2) O(19)-Mn(5)-Mn(8) 134.42(14) O(16)-Mn(4)-O(15) 81.8(2) O(17)-Mn(5)-Mn(8) 134.38(16) O(8)-Mn(4)-O(15) 93.3(2) O(29)-Mn(5)-Mn(8) 84.85(14) O(13)-Mn(4)-Mn(1) 41.57(16) O(21)-Mn(5)-Mn(8) 88.86(15) O(4)-Mn(4)-Mn(1) 88.74(15) O(30)-Mn(5)-Mn(8) 41.55(14) O(11)-Mn(4)-Mn(1) 133.00(18) O(31)-Mn(5)-Mn(8) 41.31(12) O(16)-Mn(4)-Mn(1) 85.15(14) Mn(6)-Mn(5)-Mn(8) 59.08(4) O(8)-Mn(4)-Mn(1) 134.84(15) O(19)-Mn(5)-Mn(7) 87.70(14) O(15)-Mn(4)-Mn(1) 41.67(13) O(17)-Mn(5)-Mn(7) 133.88(15) O(13)-Mn(4)-Mn(3) 43.33(18) O(29)-Mn(5)-Mn(7) 42.43(13) O(4)-Mn(4)-Mn(3) 135.15(16) O(21)-Mn(5)-Mn(7) 134.50(15) O(11)-Mn(4)-Mn(3) 88.55(17) O(30)-Mn(5)-Mn(7) 41.53(14) O(16)-Mn(4)-Mn(3) 41.00(14) O(31)-Mn(5)-Mn(7) 88.61(13) O(8)-Mn(4)-Mn(3) 133.17(15) Mn(6)-Mn(5)-Mn(7) 62.44(3) O(15)-Mn(4)-Mn(3) 84.78(13) Mn(8)-Mn(5)-Mn(7) 62.55(3) Mn(1)-Mn(4)-Mn(3) 60.86(4) O(32)-Mn(6)-O(23) 93.5(2) O(13)-Mn(4)-Mn(2) 86.59(16) O(32)-Mn(6)-O(18) 173.1(2) O(4)-Mn(4)-Mn(2) 133.71(17) O(23)-Mn(6)-O(18) 93.3(2) O(11)-Mn(4)-Mn(2) 135.77(19) O(32)-Mn(6)-O(26) 90.8(2) O(16)-Mn(4)-Mn(2) 41.86(15) O(23)-Mn(6)-O(26) 94.4(2) O(8)-Mn(4)-Mn(2) 88.25(14) O(18)-Mn(6)-O(26) 87.6(2) O(15)-Mn(4)-Mn(2) 40.52(14) O(32)-Mn(6)-O(29) 88.1(2) Mn(1)-Mn(4)-Mn(2) 60.66(4) O(23)-Mn(6)-O(29) 91.0(2) Mn(3)-Mn(4)-Mn(2) 60.78(4) O(18)-Mn(6)-O(29) 92.9(2) O(19)-Mn(5)-O(17) 91.2(2) O(26)-Mn(6)-O(29) 174.6(2) O(19)-Mn(5)-O(29) 95.8(2) O(32)-Mn(6)-O(31) 82.85(19) O(17)-Mn(5)-O(29) 92.02(19) O(23)-Mn(6)-O(31) 172.5(2) O(19)-Mn(5)-O(21) 89.4(2) O(18)-Mn(6)-O(31) 90.55(19) O(17)-Mn(5)-O(21) 91.6(2) O(26)-Mn(6)-O(31) 92.2(2) O(29)-Mn(5)-O(21) 173.6(2) O(29)-Mn(6)-O(31) 82.42(19) O(19)-Mn(5)-O(30) 93.16(19) O(32)-Mn(6)-Mn(8) 40.63(14) O(17)-Mn(5)-O(30) 173.4(2) O(23)-Mn(6)-Mn(8) 134.02(14) O(29)-Mn(5)-O(30) 82.62(19) O(18)-Mn(6)-Mn(8) 132.66(15) O(21)-Mn(5)-O(30) 93.4(2) O(26)-Mn(6)-Mn(8) 89.96(14) O(19)-Mn(5)-O(31) 175.59(19) O(29)-Mn(6)-Mn(8) 85.78(13) O(17)-Mn(5)-O(31) 93.1(2) O(31)-Mn(6)-Mn(8) 42.30(13) O(29)-Mn(5)-O(31) 83.08(19) O(32)-Mn(6)-Mn(5) 87.93(15) O(21)-Mn(5)-O(31) 91.5(2) O(23)-Mn(6)-Mn(5) 132.06(16) O(30)-Mn(5)-O(31) 82.47(18) O(18)-Mn(6)-Mn(5) 88.37(15) O(19)-Mn(5)-Mn(6) 137.35(16) O(26)-Mn(6)-Mn(5) 133.57(16) O(17)-Mn(5)-Mn(6) 89.45(15) O(29)-Mn(6)-Mn(5) 41.14(13)

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315 Table A-6. Continued. O(31)-Mn(6)-Mn(5) 41.59(14) Mn(6)-Mn(7)-Mn(8) 56.26(3) Mn(8)-Mn(6)-Mn(5) 60.57(4) O(28)-Mn(8)-O(32) 93.8(2) O(32)-Mn(6)-Mn(7) 48.26(15) O(28)-Mn(8)-O(31) 173.8(2) O(23)-Mn(6)-Mn(7) 86.61(14) O(32)-Mn(8)-O(31) 83.34(19) O(18)-Mn(6)-Mn(7) 133.41(15) O(28)-Mn(8)-O(22) 90.8(2) O(26)-Mn(6)-Mn(7) 138.95(14) O(32)-Mn(8)-O(22) 175.3(2) O(29)-Mn(6)-Mn(7) 40.59(13) O(31)-Mn(8)-O(22) 91.99(19) O(31)-Mn(6)-Mn(7) 86.08(13) O(28)-Mn(8)-O(25) 94.8(2) Mn(8)-Mn(6)-Mn(7) 62.01(3) O(32)-Mn(8)-O(25) 91.8(2) Mn(5)-Mn(6)-Mn(7) 59.57(3) O(31)-Mn(8)-O(25) 90.80(19) O(24)-Mn(7)-O(27) 94.4(2) O(22)-Mn(8)-O(25) 88.6(2) O(24)-Mn(7)-O(30) 167.65(19) O(28)-Mn(8)-O(30) 92.2(2) O(27)-Mn(7)-O(30) 93.33(19) O(32)-Mn(8)-O(30) 86.9(2) O(24)-Mn(7)-O(29) 90.95(19) O(31)-Mn(8)-O(30) 82.16(18) O(27)-Mn(7)-O(29) 165.8(2) O(22)-Mn(8)-O(30) 92.1(2) O(30)-Mn(7)-O(29) 79.37(18) O(25)-Mn(8)-O(30) 172.94(19) O(24)-Mn(7)-O(20) 97.2(2) O(28)-Mn(8)-Mn(6) 134.61(15) O(27)-Mn(7)-O(20) 96.1(2) O(32)-Mn(8)-Mn(6) 40.81(13) O(30)-Mn(7)-O(20) 91.47(19) O(31)-Mn(8)-Mn(6) 42.61(13) O(29)-Mn(7)-O(20) 96.21(19) O(22)-Mn(8)-Mn(6) 134.54(14) O(24)-Mn(7)-O(32) 94.06(19) O(25)-Mn(8)-Mn(6) 89.71(14) O(27)-Mn(7)-O(32) 89.55(19) O(30)-Mn(8)-Mn(6) 84.75(13) O(30)-Mn(7)-O(32) 76.39(18) O(28)-Mn(8)-Mn(5) 133.22(17) O(29)-Mn(7)-O(32) 76.94(17) O(32)-Mn(8)-Mn(5) 87.86(15) O(20)-Mn(7)-O(32) 166.91(18) O(31)-Mn(8)-Mn(5) 41.40(14) O(24)-Mn(7)-Mn(5) 131.11(15) O(22)-Mn(8)-Mn(5) 88.38(15) O(27)-Mn(7)-Mn(5) 133.66(15) O(25)-Mn(8)-Mn(5) 131.91(15) O(30)-Mn(7)-Mn(5) 40.33(13) O(30)-Mn(8)-Mn(5) 41.15(13) O(29)-Mn(7)-Mn(5) 40.31(13) Mn(6)-Mn(8)-Mn(5) 60.34(3) O(20)-Mn(7)-Mn(5) 87.22(13) O(28)-Mn(8)-Mn(7) 87.98(15) O(32)-Mn(7)-Mn(5) 80.35(12) O(32)-Mn(8)-Mn(7) 48.06(15) O(24)-Mn(7)-Mn(6) 87.94(14) O(31)-Mn(8)-Mn(7) 86.05(13) O(27)-Mn(7)-Mn(6) 127.97(16) O(22)-Mn(8)-Mn(7) 131.45(16) O(30)-Mn(7)-Mn(6) 79.70(13) O(25)-Mn(8)-Mn(7) 139.86(15) O(29)-Mn(7)-Mn(6) 39.01(13) O(30)-Mn(8)-Mn(7) 39.50(13) O(20)-Mn(7)-Mn(6) 135.16(14) Mn(6)-Mn(8)-Mn(7) 61.73(3) O(32)-Mn(7)-Mn(6) 38.54(11) Mn(5)-Mn(8)-Mn(7) 59.41(3) Mn(5)-Mn(7)-Mn(6) 57.98(3) Mn(4)-O(13)-Mn(1) 98.1(2) O(24)-Mn(7)-Mn(8) 132.29(16) Mn(4)-O(13)-Mn(3) 96.5(2) O(27)-Mn(7)-Mn(8) 86.71(14) Mn(1)-O(13)-Mn(3) 96.2(2) O(30)-Mn(7)-Mn(8) 38.74(14) Mn(3)-O(14)-Mn(1) 99.4(2) O(29)-Mn(7)-Mn(8) 79.98(13) Mn(3)-O(14)-Mn(2) 97.3(2) O(20)-Mn(7)-Mn(8) 130.14(14) Mn(1)-O(14)-Mn(2) 94.4(2) O(32)-Mn(7)-Mn(8) 38.23(12) Mn(2)-O(15)-Mn(1) 99.5(2) Mn(5)-Mn(7)-Mn(8) 58.04(3) Mn(2)-O(15)-Mn(4) 97.4(2)

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316 Table A-6. Continued. Mn(1)-O(15)-Mn(4) 95.5(2) Mn(5)-O(30)-Mn(7) 98.1(2) Mn(4)-O(16)-Mn(3) 98.5(2) Mn(8)-O(30)-Mn(7) 101.8(2) Mn(4)-O(16)-Mn(2) 97.7(2) Mn(8)-O(31)-Mn(5) 97.29(19) Mn(3)-O(16)-Mn(2) 98.4(2) Mn(8)-O(31)-Mn(6) 95.10(19) Mn(5)-O(29)-Mn(6) 97.3(2) Mn(5)-O(31)-Mn(6) 96.6(2) Mn(5)-O(29)-Mn(7) 97.26(19) Mn(8)-O(32)-Mn(6) 98.6(2) Mn(6)-O(29)-Mn(7) 100.4(2) Mn(8)-O(32)-Mn(7) 93.7(2) Mn(5)-O(30)-Mn(8) 97.31(19) Mn(6)-O(32)-Mn(7) 93.20(19)

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317 Table A-7. Selected interatomic distances () and angles () for [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4]32MeCN (1132MeCN). Mn(1)-O(7) 1.858(7) Mn(3)-O(13) 1.962(17) Mn(1)-O(2) 1.949(8) Mn(3)-O(92) 2.135(16) Mn(1)-O(1) 2.016(16) Mn(3)-O(10) 2.299(13) Mn(1)-O(5) 2.06(2) Mn (3)-Mn(4) 3.131(5) Mn(1)-O(3) 2.117(14) Mn(4)-O(14) 1.888(9) Mn(1)-O(6) 2.14(2) Mn(4)-O(11) 1.915(8) Mn(1)-Mn(2) 3.403(4) Mn(4)-O(15) 1.990(17) Mn(1)-Mn(3) 3.409(4) Mn(4)-O(16) 2.041(17) Mn(2)-O(4) 1.897(14) Mn(4)-O(9) 2.15(2) Mn(2)-O(12) 1.902(13) Mn(4)-O(10) 2.184(18) Mn(2)-O(7) 1.938(15) Ca(1)-O(1) 2.344(16) Mn(2)-O(11) 1.999(14) Ca(1)-O(17) 2.37(2) Mn(2)-O(91) 2.230(12) Ca(1)-O(3) 2.373(17) Mn(2)-O(9) 2.31(2) Ca(1)-O(17a) 2.39(2) Mn(2)-Mn(3) 2.837(3) Ca(1)-O(82) 2.54(2) Mn(2)-Mn(4) 3.097(4) Ca(1)-O(2) 2.542(10) Mn(3)-O(11) 1.838(13) Ca(1)-O(81) 2.603(12) Mn(3)-O(8) 1.900(18) Ca(1)-O(18) 2.66(2) Mn(3)-O(7) 1.909(16) Ca(1)-Ca(1a) 3.870(6) O(7)-Mn(1)-O(2) 167.7(4) O(4)-Mn(2)-O(9) 97.4(7) O(7)-Mn(1)-O(1) 92.4(6) O(12)-Mn(2)-O(9) 87.1(7) O(2)-Mn(1)-O(1) 77.2(6) O(7)-Mn(2)-O(9) 88.6(6) O(7)-Mn(1)-O(5) 99.7(7) O(11)-Mn(2)-O(9) 79.8(6) O(2)-Mn(1)-O(5) 87.6(7) O(91)-Mn(2)-O(9) 168.7(7) O(1)-Mn(1)-O(5) 93.0(8) O( 4)-Mn(2)-Mn(3) 137.8(5) O(7)-Mn(1)-O(3) 88.1(6) O(12)-Mn(2)-Mn(3) 135.2(4) O(2)-Mn(1)-O(3) 84.4(6) O( 7)-Mn(2)-Mn(3) 42.1(5) O(1)-Mn(1)-O(3) 85.0(4) O(11)-Mn(2)-Mn(3) 40.2(3) O(5)-Mn(1)-O(3) 172.0(7) O(91)-Mn(2)-Mn(3) 81.3(4) O(7)-Mn(1)-O(6) 96.6(7) O( 9)-Mn(2)-Mn(3) 91.6(6) O(2)-Mn(1)-O(6) 93.2(6) O( 4)-Mn(2)-Mn(4) 141.0(4) O(1)-Mn(1)-O(6) 169.4(5) O(12)-Mn(2)-Mn(4) 86.1(4) O(5)-Mn(1)-O(6) 91.0(6) O( 7)-Mn(2)-Mn(4) 88.8(4) O(3)-Mn(1)-O(6) 89.6(8) O(11)-Mn(2)-Mn(4) 36.8(3) O(4)-Mn(2)-O(12) 86.6(6) O(91)-Mn(2)-Mn(4) 124.8(4) O(4)-Mn(2)-O(7) 96.8(6) O( 9)-Mn(2)-Mn(4) 43.9(6) O(12)-Mn(2)-O(7) 174.8(5) Mn(3)-Mn(2)-Mn(4) 63.51(14) O(4)-Mn(2)-O(11) 176.2(5) O(11)-Mn(3)-O(8) 174.2(6) O(12)-Mn(2)-O(11) 95.9(5) O(11)-Mn(3)-O(7) 85.5(6) O(7)-Mn(2)-O(11) 80.5(6) O(8)-Mn(3)-O(7) 95.2(6) O(4)-Mn(2)-O(91) 93.7(6) O(11)-Mn(3)-O(13) 90.6(7) O(12)-Mn(2)-O(91) 91.8(5) O(8)-Mn(3)-O(13) 88.4(8) O(7)-Mn(2)-O(91) 91.9(4) O(7)-Mn(3)-O(13) 175.5(7)

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318 Table A-7. Continued. O(11)-Mn(2)-O(91) 89.1(4) O(14)-Mn(4)-Mn(3) 139.4(7) O(11)-Mn(3)-O(92) 93.0(5) O(11)-Mn(4)-Mn(3) 32.7(4) O(8)-Mn(3)-O(92) 92.7(7) O(15)-Mn(4)-Mn(3) 85.6(5) O(7)-Mn(3)-O(92) 91.4(5) O(16)-Mn(4)-Mn(3) 124.1(5) O(13)-Mn(3)-O(92) 91.1(7) O(9)-Mn(4)-Mn(3) 87.2(6) O(11)-Mn(3)-O(10) 78.4(5) O(10)-Mn(4)-Mn(3) 47.2(3) O(8)-Mn(3)-O(10) 95.8(7) Mn(2)-Mn(4)-Mn(3) 54.20(6) O(7)-Mn(3)-O(10) 89.7(5) Mn(1)-O(7)-Mn(3) 129.6(10) O(13)-Mn(3)-O(10) 87.2(7) Mn(1)-O(7)-Mn(2) 127.3(10) O(92)-Mn(3)-O(10) 171.3(6) Mn(3)-O(7)-Mn(2) 95.0(3) O(11)-Mn(3)-Mn(2) 44.6(4) Mn(4)-O(9)-Mn(2) 87.8(8) O(8)-Mn(3)-Mn(2) 137.4(5) Mn(4)-O(10)-Mn(3) 88.5(5) O(7)-Mn(3)-Mn(2) 42.9(4) Mn(3)-O(11)-Mn(4) 113.1(6) O(13)-Mn(3)-Mn(2) 133.9(6) Mn(3)-O(11)-Mn(2) 95.3(3) O(92)-Mn(3)-Mn(2) 82.8(5) Mn(4)-O(11)-Mn(2) 104.6(6) O(10)-Mn(3)-Mn(2) 92.2(5) O(1)-Ca(1)-O(17) 96.9(6) O(11)-Mn(3)-Mn(4) 34.2(3) O(1)-Ca(1)-O(3) 72.6(3) O(8)-Mn(3)-Mn(4) 139.9(5) O(17)-Ca(1)-O(3) 146.4(5) O(7)-Mn(3)-Mn(4) 88.3(4) O(1)-Ca(1)-O(17a) 143.6(5) O(13)-Mn(3)-Mn(4) 87.2(5) O(17)-Ca(1)-O(17a) 70.8(5) O(92)-Mn(3)-Mn(4) 127.1(4) O(3)-Ca(1)-O(17a) 98.4(6) O(10)-Mn(3)-Mn(4) 44.2(5) O(1)-Ca(1)-O(82) 138.6(6) Mn(2)-Mn(3)-Mn(4) 62.29(14) O(17)-Ca(1)-O(82) 91.3(5) O(14)-Mn(4)-O(11) 171.9(9) O(3)-Ca(1)-O(82) 118.0(6) O(14)-Mn(4)-O(15) 88.8(7) O(17a)-Ca(1)-O(82) 77.1(4) O(11)-Mn(4)-O(15) 88.4(6) O(1)-Ca(1)-O(2) 60.7(5) O(14)-Mn(4)-O(16) 96.3(8) O(17)-Ca(1)-O(2) 79.5(4) O(11)-Mn(4)-O(16) 91.5(6) O(3)-Ca(1)-O(2) 67.5(5) O(15)-Mn(4)-O(16) 94.6(5) O(17a)-Ca(1)-O(2) 83.2(4) O(14)-Mn(4)-O(9) 96.1(8) O(82)-Ca(1)-O(2) 160.1(4) O(11)-Mn(4)-O(9) 86.0(7) O(1)-Ca(1)-O(81) 90.7(5) O(15)-Mn(4)-O(9) 172.7(8) O(17)-Ca(1)-O(81) 130.6(5) O(16)-Mn(4)-O(9) 90.2(9) O(3)-Ca(1)-O(81) 82.3(5) O(14)-Mn(4)-O(10) 92.4(7) O(17a)-Ca(1)-O(81) 123.7(5) O(11)-Mn(4)-O(10) 79.9(5) O(82)-Ca(1)-O(81) 55.0(3) O(15)-Mn(4)-O(10) 87.3(7) O(2)-Ca(1)-O(81) 142.8(3) O(16)-Mn(4)-O(10) 171.1(5) O(1)-Ca(1)-O(18) 75.5(6) O(9)-Mn(4)-O(10) 87.2(4) O(17)-Ca(1)-O(18) 50.7(6) O(14)-Mn(4)-Mn(2) 144.3(6) O(3)-Ca(1)-O(18) 145.4(6) O(11)-Mn(4)-Mn(2) 38.7(4) O(17a)-Ca(1)-O(18) 115.3(6) O(15)-Mn(4)-Mn(2) 126.7(5) O(82)-Ca(1)-O(18) 78.9(7) O(16)-Mn(4)-Mn(2) 84.2(4) O(2)-Ca(1)-O(18) 107.5(7) O(9)-Mn(4)-Mn(2) 48.3(6) O(81)-Ca(1)-O(18) 85.0(6) O(10)-Mn(4)-Mn(2) 87.8(4) Ca(1)-O(17)-Ca(1a) 108.9(5)

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319 Table A-8. Selected interatomic distances () and angles () for [Mn16O8Sr4(O2CPh)16(O2AsMe2)24]16MeCN (1216MeCN). Mn(1)-O(44) 1.874(9) Mn(6)-O(36) 1.896(9) Mn(1)-O(5) 1.890(9) Mn(6)-O(33) 1.908(8) Mn(1)-O(3) 1.957(10) Mn(6)-O(41) 1.931(9) Mn(1)-O(7) 2.013(9) Mn(6)-O(38) 2.166(12) Mn(1)-O(1) 2.070(10) Mn(6)-O(32) 2.475(17) Mn(1)-O(11) 2.56(1) Mn(6)-Mn(7) 2.826(3) Mn(1)-Mn(2) 3.107(3) Mn(6)-Mn(8) 3.151(3) Mn(1)-Mn(3) 3.157(3) Mn(7)-O(34) 1.905(9) Mn(2)-O(2) 1.890(9) Mn(7)-O(33) 1.906(9) Mn(2)-O(6) 1.907(9) Mn(7)-O(28) 1.919(11) Mn(2)-O(15) 1.917(9) Mn(7)-O(39) 1.947(10) Mn(2)-O(5) 1.926(8) Mn(7)-O(37) 2.122(12) Mn(2)-O(9) 2.184(10) Mn(7)-O(30) 2.341(10) Mn(2)-O(7) 2.399(9) Mn(7)-Mn(8) 3.081(3) Mn(2)-Mn(3) 2.822(3) Mn(8)-O(43) 1.891(9) Mn(2)-Mn(4) 3.438(3) Mn(8)-O(34) 1.900(10) Mn(3)-O(5) 1.872(10) Mn(8)-O(42) 1.952(11) Mn(3)-O(6) 1.916(8) Mn(8)-O(30) 1.985(10) Mn(3)-O(13) 1.922(9) Mn(8)-O(40) 2.103(11) Mn(3)-O(4) 1.929(8) Mn(8)-O(32) 2.72(2) Mn(3)-O(10) 2.209(9) Sr(1)-O(23) 2.468(10) Mn(3)-O(11) 2.305(9) Sr(1)-O(14) 2.523(9) Mn(3)-Mn(4) 3.377(3) Sr(1)-O(19) 2.58(3) Mn(4)-O(6) 1.866(8) Sr(1)-O(16) 2.585(9) Mn(4)-O(14) 1.947(9) Sr(1)-O(22) 2.596(10) Mn(4)-O(8) 1.952(10) Sr(1)-O(20) 2.66(2) Mn(4)-O(17) 1.973(9) Sr(1)-O(21) 2.676(10) Mn(4)-O(16) 2.180(9) Sr(1)-O(17) 2.718(9) Mn(4)-O(12) 2.203(10) Sr(1)-Sr(2) 4.018(2) Mn(5)-O(33) 1.878(8) Sr(2)-O(22) 2.514(10) Mn(5)-O(29) 1.970(10) Sr(2)-O(26) 2.525(15) Mn(5)-O(35) 1.977(9) Sr(2)-O(27) 2.543(9) Mn(5)-O(18) 1.986(9) Sr(2)-O(25) 2.552(15) Mn(5)-O(27) 2.200(9) Sr(2)-O(35) 2.568(10) Mn(5)-O(31) 2.246(12) Sr(2)-O(23) 2.603(10) Mn(5)-Mn(6) 3.388(12) Sr(2)-O(18) 2.743(10) Mn(5)-Mn(7) 3.432(12) Sr(2)-O(24) 2.750(12) Mn(6)-O(34) 1.893(10) O(44)-Mn(1)-O(5) 174.6(5) O( 3)-Mn(1)-Mn(3) 82.7(3) O(44)-Mn(1)-O(3) 90.0(4) O( 7)-Mn(1)-Mn(3) 89.2(3) O(5)-Mn(1)-O(3) 89.7(4) O( 1)-Mn(1)-Mn(3) 124.2(3) O(44)-Mn(1)-O(7) 93.9(4) Mn(2)-Mn(1)-Mn(3) 53.54(6) O(5)-Mn(1)-O(7) 85.7(4) O(2)-Mn(2)-O(6) 174.8(4)

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320 Table A-8. Continued. O(3)-Mn(1)-O(7) 171.0(4) O( 5)-Mn(3)-O(10) 91.4(4) O(44)-Mn(1)-O(1) 93.7(4) O( 6)-Mn(3)-O(10) 90.3(3) O(5)-Mn(1)-O(1) 91.7(4) O(13)-Mn(3)-O(10) 93.6(4) O(3)-Mn(1)-O(1) 95.7(4) O( 4)-Mn(3)-O(10) 90.5(4) O(7)-Mn(1)-O(1) 92.1(4) O( 5)-Mn(3)-O(11) 85.9(4) O(44)-Mn(1)-Mn(2) 144.4(3) O(6)-Mn(3)-O(11) 89.2(3) O(5)-Mn(1)-Mn(2) 35.9(3) O(13)-Mn(3)-O(11) 89.1(4) O(3)-Mn(1)-Mn(2) 125.5(3) O(4)-Mn(3)-O(11) 89.6(4) O(7)-Mn(1)-Mn(2) 50.5(3) O(10)-Mn(3)-O(11) 177.3(4) O(1)-Mn(1)-Mn(2) 86.4(2) O(5)-Mn(3)-Mn(2) 42.8(3) O(44)-Mn(1)-Mn(3) 141.9(4) O(6)-Mn(3)-Mn(2) 42.3(3) O(5)-Mn(1)-Mn(3) 32.8(3) O(13)-Mn(3)-Mn(2) 138.5(3) O(2)-Mn(2)-O(15) 89.9(4) O(4)-Mn(3)-Mn(2) 130.4(3) O(6)-Mn(2)-O(15) 93.9(4) O(10)-Mn(3)-Mn(2) 80.3(2) O(2)-Mn(2)-O(5) 94.2(4) O(11)-Mn(3)-Mn(2) 97.6(3) O(6)-Mn(2)-O(5) 81.6(4) O( 5)-Mn(3)-Mn(1) 33.1(2) O(15)-Mn(2)-O(5) 171.6(4) O( 6)-Mn(3)-Mn(1) 88.2(3) O(2)-Mn(2)-O(9) 89.5(4) O(13)-Mn(3)-Mn(1) 141.9(3) O(6)-Mn(2)-O(9) 93.8(4) O( 4)-Mn(3)-Mn(1) 85.0(3) O(15)-Mn(2)-O(9) 94.9(4) O(10)-Mn(3)-Mn(1) 124.2(3) O(5)-Mn(2)-O(9) 92.4(4) O(11)-Mn(3)-Mn(1) 53.2(3) O(2)-Mn(2)-O(7) 85.5(4) Mn(2)-Mn(3)-Mn(1) 62.32(7) O(6)-Mn(2)-O(7) 90.4(3) O( 6)-Mn(4)-O(14) 90.8(4) O(15)-Mn(2)-O(7) 98.2(4) O(6)-Mn(4)-O(8) 97.0(4) O(5)-Mn(2)-O(7) 74.9(3) O( 8)-Mn(4)-O(16) 94.7(4) O(9)-Mn(2)-O(7) 165.9(4) O(17)-Mn(4)-O(16) 80.9(3) O(2)-Mn(2)-Mn(3) 134.1(3) O(6)-Mn(4)-O(12) 96.6(3) O(6)-Mn(2)-Mn(3) 42.6(2) O(14)-Mn(4)-O(12) 88.9(4) O(15)-Mn(2)-Mn(3) 135.8(3) O(8)-Mn(4)-O(12) 88.6(4) O(5)-Mn(2)-Mn(3) 41.3(3) O(17)-Mn(4)-O(12) 90.1(4) O(9)-Mn(2)-Mn(3) 83.5(3) O(16)-Mn(4)-O(12) 170.4(4) O(7)-Mn(2)-Mn(3) 90.6(2) O(14)-Mn(4)-O(8) 172.1(4) O(2)-Mn(2)-Mn(1) 84.9(3) O(6)-Mn(4)-O(17) 170.5(4) O(6)-Mn(2)-Mn(1) 89.9(3) O(14)-Mn(4)-O(17) 82.6(4) O(15)-Mn(2)-Mn(1) 138.5(3) O(8)-Mn(4)-O(17) 89.9(4) O(5)-Mn(2)-Mn(1) 35.1(3) O(6)-Mn(4)-O(16) 91.9(3) O(9)-Mn(2)-Mn(1) 126.1(3) O(14)-Mn(4)-O(16) 86.6(4) O(7)-Mn(2)-Mn(1) 40.4(2) O(33)-Mn(5)-O(29) 97.6(4) Mn(3)-Mn(2)-Mn(1) 64.14(7) O(33)-Mn(5)-O(35) 90.3(4) O(5)-Mn(3)-O(6) 82.8(4) O(29)-Mn(5)-O(35) 172.1(4) O(5)-Mn(3)-O(13) 174.9(4) O(33)-Mn(5)-O(18) 169.8(4) O(6)-Mn(3)-O(13) 97.2(4) O(29)-Mn(5)-O(18) 89.4(4) O(5)-Mn(3)-O(4) 89.5(4) O(35)-Mn(5)-O(18) 82.9(4) O(6)-Mn(3)-O(4) 172.3(4) O(33)-Mn(5)-O(27) 91.7(4) O(13)-Mn(3)-O(4) 90.4(4) O(29)-Mn(5)-O(27) 92.0(4)

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321 Table A-8. Continued. O(35)-Mn(5)-O(27) 88.7(4) O(34)-Mn(7)-O(30) 75.5(4) O(18)-Mn(5)-O(27) 80.6(4) O(33)-Mn(7)-O(30) 90.1(3) O(33)-Mn(5)-O(31) 96.5(4) O(28)-Mn(7)-O(30) 96.0(4) O(29)-Mn(5)-O(31) 90.6(5) O(39)-Mn(7)-O(30) 87.9(4) O(35)-Mn(5)-O(31) 87.5(4) O(37)-Mn(7)-O(30) 169.9(4) O(18)-Mn(5)-O(31) 90.8(4) O(34)-Mn(7)-Mn(6) 41.8(3) O(27)-Mn(5)-O(31) 171.0(4) O(33)-Mn(7)-Mn(6) 42.2(2) O(34)-Mn(6)-O(36) 176.4(5) O(28)-Mn(7)-Mn(6) 138.2(3) O(34)-Mn(6)-O(33) 82.1(4) O(39)-Mn(7)-Mn(6) 134.2(3) O(36)-Mn(6)-O(33) 97.8(4) O(37)-Mn(7)-Mn(6) 83.3(3) O(34)-Mn(6)-O(41) 91.6(4) O(30)-Mn(7)-Mn(6) 90.7(3) O(36)-Mn(6)-O(41) 88.2(4) O(34)-Mn(7)-Mn(8) 35.9(3) O(33)-Mn(6)-O(41) 172.9(4) O(33)-Mn(7)-Mn(8) 89.3(3) O(34)-Mn(6)-O(38) 93.0(4) O(28)-Mn(7)-Mn(8) 135.9(4) O(36)-Mn(6)-O(38) 90.6(4) O(39)-Mn(7)-Mn(8) 86.9(4) O(33)-Mn(6)-O(38) 90.4(4) O(37)-Mn(7)-Mn(8) 129.8(3) O(41)-Mn(6)-O(38) 93.2(4) O(34)-Mn(8)-O(30) 84.9(4) O(34)-Mn(6)-O(32) 89.6(5) O(42)-Mn(8)-O(30) 171.4(4) O(36)-Mn(6)-O(32) 86.8(5) O(43)-Mn(8)-O(40) 93.0(4) O(33)-Mn(6)-O(32) 85.2(4) O(34)-Mn(8)-O(40) 90.2(4) O(41)-Mn(6)-O(32) 91.5(4) O(42)-Mn(8)-O(40) 99.2(5) O(38)-Mn(6)-O(32) 174.6(5) O(30)-Mn(8)-O(40) 88.7(4) O(34)-Mn(6)-Mn(7) 42.1(3) O(43)-Mn(8)-Mn(7) 142.9(3) O(36)-Mn(6)-Mn(7) 138.7(3) O(34)-Mn(8)-Mn(7) 36.0(3) O(33)-Mn(6)-Mn(7) 42.2(3) O(42)-Mn(8)-Mn(7) 127.6(3) O(41)-Mn(6)-Mn(7) 132.5(3) O(30)-Mn(8)-Mn(7) 49.4(3) O(38)-Mn(6)-Mn(7) 81.8(3) O(40)-Mn(8)-Mn(7) 84.0(3) O(32)-Mn(6)-Mn(7) 97.0(3) O(43)-Mn(8)-Mn(6) 143.1(3) O(34)-Mn(6)-Mn(8) 33.9(3) O(34)-Mn(8)-Mn(6) 33.8(3) O(36)-Mn(6)-Mn(8) 142.5(3) O(42)-Mn(8)-Mn(6) 83.9(3) O(33)-Mn(6)-Mn(8) 87.2(3) O(30)-Mn(8)-Mn(6) 88.9(3) O(41)-Mn(6)-Mn(8) 85.8(3) O(40)-Mn(8)-Mn(6) 123.8(3) O(38)-Mn(6)-Mn(8) 126.6(3) Mn(7)-Mn(8)-Mn(6) 53.91(7) O(32)-Mn(6)-Mn(8) 56.4(4) Mn(3)-O(5)-Mn(1) 114.1(4) Mn(7)-Mn(6)-Mn(8) 61.78(8) Mn(3)-O(5)-Mn(2) 96.0(4) O(34)-Mn(7)-O(33) 81.8(4) Mn(1)-O(5)-Mn(2) 109.0(4) O(34)-Mn(7)-O(28) 171.3(5) Mn(4)-O(6)-Mn(2) 131.3(4) O(33)-Mn(7)-O(28) 96.4(4) Mn(4)-O(6)-Mn(3) 126.4(4) O(34)-Mn(7)-O(39) 94.2(4) Mn(2)-O(6)-Mn(3) 95.1(4) O(33)-Mn(7)-O(39) 175.9(4) Mn(1)-O(7)-Mn(2) 89.1(3) O(28)-Mn(7)-O(39) 87.4(4) Mn(8)-O(30)-Mn(7) 90.4(4) O(34)-Mn(7)-O(37) 94.7(4) Mn(5)-O(33)-Mn(7) 130.2(4) O(33)-Mn(7)-O(37) 90.8(4) Mn(5)-O(33)-Mn(6) 127.0(5) O(28)-Mn(7)-O(37) 93.9(5) Mn(7)-O(33)-Mn(6) 95.6(4) O(39)-Mn(7)-O(37) 90.5(5) Mn(6)-O(34)-Mn(8) 112.3(5)

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322 Table A-8. Continued. Mn(6)-O(34)-Mn(7) 96.2(4) O(22)-Sr(2)-O(26) 125.5(5) Mn(8)-O(34)-Mn(7) 108.1(5) O(22)-Sr(2)-O(27) 89.9(3) O(23)-Sr(1)-O(14) 142.7(3) O(26)-Sr(2)-O(27) 86.6(5) O(23)-Sr(1)-O(19) 111.4(7) O(22)-Sr(2)-O(25) 83.8(7) O(14)-Sr(1)-O(19) 85.2(7) O(26)-Sr(2)-O(25) 51.5(9) O(23)-Sr(1)-O(16) 83.0(3) O(27)-Sr(2)-O(25) 117.7(6) O(14)-Sr(1)-O(16) 67.4(3) O(22)-Sr(2)-O(35) 142.9(3) O(19)-Sr(1)-O(16) 78.2(8) O(26)-Sr(2)-O(35) 85.1(5) O(23)-Sr(1)-O(22) 73.7(3) O(27)-Sr(2)-O(35) 69.8(3) O(14)-Sr(1)-O(22) 117.2(3) O(25)-Sr(2)-O(35) 132.9(7) O(19)-Sr(1)-O(22) 137.2(8) O(22)-Sr(2)-O(23) 72.9(3) O(16)-Sr(1)-O(22) 142.8(3) O(26)-Sr(2)-O(23) 128.2(5) O(23)-Sr(1)-O(20) 91.6(8) O(27)-Sr(2)-O(23) 145.0(3) O(14)-Sr(1)-O(20) 122.8(9) O(25)-Sr(2)-O(23) 91.0(7) O(19)-Sr(1)-O(20) 50.0(11) O(35)-Sr(2)-O(23) 106.2(3) O(16)-Sr(1)-O(20) 121.6(8) O(22)-Sr(2)-O(18) 84.1(3) O(22)-Sr(1)-O(20) 88.2(10) O(26)-Sr(2)-O(18) 137.8(4) O(23)-Sr(1)-O(21) 122.2(3) O(27)-Sr(2)-O(18) 61.6(3) O(14)-Sr(1)-O(21) 79.9(3) O(25)-Sr(2)-O(18) 167.8(7) O(19)-Sr(1)-O(21) 108.1(8) O(35)-Sr(2)-O(18) 59.1(3) O(16)-Sr(1)-O(21) 146.2(3) O(23)-Sr(2)-O(18) 86.0(3) O(22)-Sr(1)-O(21) 48.7(3) O(22)-Sr(2)-O(24) 119.2(3) O(20)-Sr(1)-O(21) 82.8(6) O(26)-Sr(2)-O(24) 89.1(5) O(23)-Sr(1)-O(17) 87.3(3) O(27)-Sr(2)-O(24) 146.3(3) O(14)-Sr(1)-O(17) 59.0(3) O(25)-Sr(2)-O(24) 84.4(6) O(19)-Sr(1)-O(17) 133.0(8) O(35)-Sr(2)-O(24) 76.5(3) O(22)-Sr(1)-O(17) 88.7(3) O(23)-Sr(2)-O(24) 48.0(3) O(22)-Sr(1)-O(17) 88.7(3) O(18)-Sr(2)-O(24) 102.1(4) O(20)-Sr(1)-O(17) 177.0(10) Sr(2)-O(22)-Sr(1) 103.6(4) O(21)-Sr(1)-O(17) 95.3(3) Sr(1)-O(23)-Sr(2) 104.8(4)

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323 Table A-9. Selected interatomic distances () and angles () for [Mn12O12(O2CPet)16(MeOH)4]2MeCN (152MeCN). Mn(1)-O(10) 1.870(4) Mn(6)-O(140) 2.150(6) Mn(1)-O(11) 1.879(4) Mn(6)-O(101) 2.168(6) Mn(1)-O(1) 1.913(4) Mn(7)-O(10) 1.898(4) Mn(1)-O(110) 1.913(5) Mn(7)-O(11) 1.916(4) Mn(1)-O(2) 1.921(4) Mn(7)-O(41) 1.942(5) Mn(1)-O(3) 1.940(4) Mn(7)-O(90) 1.956(5) Mn(1)-Mn(7) 2.7873(14) Mn(7)-O(100) 2.153(6) Mn(1)-Mn(4) 2.8263(14) Mn(7)-O(111) 2.162(5) Mn(1)-Mn(2) 2.8399(14) Mn(8)-O(12) 1.870(4) Mn(1)-Mn(3) 2.9656(14) Mn(8)-O(11) 1.904(4) Mn(2)-O(12) 1.860(4) Mn(8)-O(40) 1.930(5) Mn(2)-O(5) 1.883(4) Mn(8)-O(51) 1.988(5) Mn(2)-O(4) 1.908(4) Mn(8)-O(150) 2.156(6) Mn(2)-O(2) 1.912(4) Mn(8)-O(13) 2.239(6) Mn(2)-O(81) 1.930(5) Mn(9)-O(5) 1.897(5) Mn(2)-O(1) 1.934(4) Mn(9)-O(12) 1.898(4) Mn(2)-Mn(9) 2.7914(15) Mn(9)-O(61) 1.930(5) Mn(2)-Mn(3) 2.8348(16) Mn(9)-O(50) 1.960(6) Mn(2)-Mn(4) 2.9421(14) Mn(9)-O(151) 2.160(6) Mn(3)-O(6) 1.869(4) Mn(9)-O(80) 2.191(7) Mn(3)-O(7) 1.894(4) Mn(10)-O(6) 1.877(5) Mn(3)-O(4) 1.898(5) Mn(10)-O(5) 1.886(5) Mn(3)-O(3) 1.914(4) Mn(10)-O(60) 1.949(6) Mn(3)-O(2) 1.939(4) Mn(10)-O(120) 1.970(6) Mn(3)-O(130) 1.940(5) Mn(10)-O(170) 2.138(6) Mn(3)-Mn(11) 2.7920(15) Mn(10)-O(14) 2.241(6) Mn(3)-Mn(4) 2.8265(15) Mn(11)-O(6) 1.885(4) Mn(4)-O(8) 1.869(4) Mn(11)-O(7) 1.915(5) Mn(4)-O(9) 1.873(4) Mn(11)-O(30) 1.941(5) Mn(4)-O(3) 1.901(5) Mn(11)-O(121) 1.951(5) Mn(4)-O(1) 1.903(4) Mn(11)-O(171) 2.156(6) Mn(4)-O(160) 1.918(5) Mn(11)-O(131) 2.162(6) Mn(4)-O(4) 1.939(4) Mn(12)-O(7) 1.872(5) Mn(4)-Mn(5) 2.7699(15) Mn(12)-O(8) 1.873(5) Mn(5)-O(9) 1.886(4) Mn(12)-O(31) 1.969(5) Mn(5)-O(8) 1.903(4) Mn(12)-O(20) 1.978(5) Mn(5)-O(70) 1.931(5) Mn(12)-O(15) 2.208(6) Mn(5)-O(21) 1.951(5) Mn(12)-O(16) 2.211(5) Mn(5)-O(141) 2.165(6) O(13)-C(12) 1.404(11) Mn(5)-O(161) 2.214(6) O(14)-C(13) 1.393(13) Mn(6)-O(10) 1.886(4) O(15)-C(15) 1.41(3) Mn(6)-O(9) 1.893(4) O(15)-C(14) 1.447(18) Mn(6)-O(91) 1.964(5) O(16)-C(16) 1.443(10) Mn(6)-O(71) 1.972(5)

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324 Table A-9. Continued. O(10)-Mn(1)-O(11) 84.97(19) O(12)-Mn(2)-O(5) 84.44(19) O(10)-Mn(1)-O(1) 92.67(19) O(12)-Mn(2)-O(4) 173.72(19) O(11)-Mn(1)-O(1) 88.85(19) O(5)-Mn(2)-O(4) 100.78(19) O(10)-Mn(1)-O(110) 92.8(2) O(12)-Mn(2)-O(2) 92.87(18) O(11)-Mn(1)-O(110) 92.61(19) O(5)-Mn(2)-O(2) 89.07(19) O(1)-Mn(1)-O(110) 174.5(2) O(4)-Mn(2)-O(2) 83.79(18) O(10)-Mn(1)-O(2) 173.97(18) O(12)-Mn(2)-O(81) 93.4(2) O(11)-Mn(1)-O(2) 99.82(18) O( 5)-Mn(2)-O(81) 91.4(2) O(1)-Mn(1)-O(2) 83.82(18) O( 4)-Mn(2)-O(81) 90.0(2) O(110)-Mn(1)-O(2) 90.67(19) O(2)-Mn(2)-O(81) 173.8(2) O(10)-Mn(1)-O(3) 95.38(18) O(12)-Mn(2)-O(1) 94.32(18) O(11)-Mn(1)-O(3) 172.1(2) O(5)-Mn(2)-O(1) 172.4(2) O(1)-Mn(1)-O(3) 83.22(18) O(4)-Mn(2)-O(1) 80.04(18) O(110)-Mn(1)-O(3) 95.28(19) O(2)-Mn(2)-O(1) 83.52(18) O(2)-Mn(1)-O(3) 79.37(17) O(81)-Mn(2)-O(1) 96.1(2) O(10)-Mn(1)-Mn(7) 42.67(13) O(12)-Mn(2)-Mn(9) 42.54(14) O(11)-Mn(1)-Mn(7) 43.27(13) O(5)-Mn(2)-Mn(9) 42.58(14) O(1)-Mn(1)-Mn(7) 98.17(14) O(4)-Mn(2)-Mn(9) 143.09(14) O(110)-Mn(1)-Mn(7) 86.53(14) O(2)-Mn(2)-Mn(9) 97.28(13) O(2)-Mn(1)-Mn(7) 142.64(13) O(81)-Mn(2)-Mn(9) 87.25(17) O(3)-Mn(1)-Mn(7) 137.99(13) O(1)-Mn(2)-Mn(9) 136.85(13) O(10)-Mn(1)-Mn(4) 88.42(13) O(12)-Mn(2)-Mn(3) 135.65(14) O(11)-Mn(1)-Mn(4) 130.11(14) O(5)-Mn(2)-Mn(3) 89.99(15) O(1)-Mn(1)-Mn(4) 42.09(13) O(4)-Mn(2)-Mn(3) 41.72(14) O(110)-Mn(1)-Mn(4) 137.14(14) O(2)-Mn(2)-Mn(3) 42.96(12) O(2)-Mn(1)-Mn(4) 85.69(12) O(81)-Mn(2)-Mn(3) 130.80(17) O(3)-Mn(1)-Mn(4) 42.09(13) O(1)-Mn(2)-Mn(3) 85.67(14) Mn(7)-Mn(1)-Mn(4) 120.44(5) Mn(9)-Mn(2)-Mn(3) 123.54(5) O(10)-Mn(1)-Mn(2) 135.11(15) O(12)-Mn(2)-Mn(1) 87.88(14) O(11)-Mn(1)-Mn(2) 88.93(14) O(5)-Mn(2)-Mn(1) 130.30(15) O(1)-Mn(1)-Mn(2) 42.69(13) O(4)-Mn(2)-Mn(1) 86.08(13) O(110)-Mn(1)-Mn(2) 131.96(14) O(2)-Mn(2)-Mn(1) 42.33(12) O(2)-Mn(1)-Mn(2) 42.07(13) O(81)-Mn(2)-Mn(1) 138.12(16) O(3)-Mn(1)-Mn(2) 85.23(13) O(1)-Mn(2)-Mn(1) 42.13(13) Mn(7)-Mn(1)-Mn(2) 123.64(5) Mn(9)-Mn(2)-Mn(1) 119.20(5) Mn(4)-Mn(1)-Mn(2) 62.56(3) Mn(3)-Mn(2)-Mn(1) 63.01(4) O(10)-Mn(1)-Mn(3) 134.72(13) O(12)-Mn(2)-Mn(4) 133.86(14) O(11)-Mn(1)-Mn(3) 139.45(14) O(5)-Mn(2)-Mn(4) 140.93(15) O(1)-Mn(1)-Mn(3) 82.37(14) O(4)-Mn(2)-Mn(4) 40.50(13) O(110)-Mn(1)-Mn(3) 93.08(14) O(2)-Mn(2)-Mn(4) 82.60(12) O(2)-Mn(1)-Mn(3) 40.02(12) O(81)-Mn(2)-Mn(4) 93.15(17) O(3)-Mn(1)-Mn(3) 39.36(12) O(1)-Mn(2)-Mn(4) 39.56(12) Mn(7)-Mn(1)-Mn(3) 177.28(5) Mn(9)-Mn(2)-Mn(4) 176.41(5) Mn(4)-Mn(1)-Mn(3) 58.36(4) Mn(3)-Mn(2)-Mn(4) 58.55(4) Mn(2)-Mn(1)-Mn(3) 58.41(4) Mn(1)-Mn(2)-Mn(4) 58.49(3)

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325 Table A-9. Continued. O(6)-Mn(3)-O(7) 84.80(19) Mn(2)-Mn(3)-Mn(1) 58.58(3) O(6)-Mn(3)-O(4) 90.8(2) O(8)-Mn(4)-O(9) 85.47(19) O(7)-Mn(3)-O(4) 87.3(2) O(8)-Mn(4)-O(3) 89.6(2) O(6)-Mn(3)-O(3) 172.4(2) O(9)-Mn(4)-O(3) 91.14(19) O(7)-Mn(3)-O(3) 100.65(19) O(8)-Mn(4)-O(1) 173.2(2) O(4)-Mn(3)-O(3) 84.20(19) O(9)-Mn(4)-O(1) 98.03(19) O(6)-Mn(3)-O(2) 94.16(18) O(3)-Mn(4)-O(1) 84.54(18) O(7)-Mn(3)-O(2) 170.5(2) O( 8)-Mn(4)-O(160) 93.2(2) O(4)-Mn(3)-O(2) 83.33(18) O( 9)-Mn(4)-O(160) 90.8(2) O(3)-Mn(3)-O(2) 79.59(17) O( 3)-Mn(4)-O(160) 176.7(2) O(6)-Mn(3)-O(130) 92.3(2) O(1)-Mn(4)-O(160) 92.6(2) O(7)-Mn(3)-O(130) 90.3(2) O(8)-Mn(4)-O(4) 95.95(19) O(4)-Mn(3)-O(130) 175.9(2) O(9)-Mn(4)-O(4) 174.38(19) O(3)-Mn(3)-O(130) 93.0(2) O(3)-Mn(4)-O(4) 83.44(18) O(2)-Mn(3)-O(130) 99.1(2) O(1)-Mn(4)-O(4) 80.02(18) O(6)-Mn(3)-Mn(11) 42.16(13) O(160)-Mn(4)-O(4) 94.6(2) O(7)-Mn(3)-Mn(11) 43.16(14) O(8)-Mn(4)-Mn(5) 43.22(14) O(4)-Mn(3)-Mn(11) 94.00(13) O(9)-Mn(4)-Mn(5) 42.73(13) O(4)-Mn(3)-Mn(11) 94.00(13) O(3)-Mn(4)-Mn(5) 95.54(13) O(3)-Mn(3)-Mn(11) 143.76(13) O(1)-Mn(4)-Mn(5) 140.74(14) O(2)-Mn(3)-Mn(11) 136.30(13) O(160)-Mn(4)-Mn(5) 87.68(16) O(130)-Mn(3)-Mn(11) 86.50(16) O(4)-Mn(4)-Mn(5) 139.13(13) O(6)-Mn(3)-Mn(4) 133.78(16) O(8)-Mn(4)-Mn(1) 132.34(15) O(7)-Mn(3)-Mn(4) 88.58(15) O(9)-Mn(4)-Mn(1) 89.13(13) O(4)-Mn(3)-Mn(4) 43.12(13) O(3)-Mn(4)-Mn(1) 43.16(13) O(3)-Mn(3)-Mn(4) 42.01(13) O(1)-Mn(4)-Mn(1) 42.35(13) O(2)-Mn(3)-Mn(4) 85.37(13) O(160)-Mn(4)-Mn(1) 134.22(17) O(130)-Mn(3)-Mn(4) 133.51(16) O(4)-Mn(4)-Mn(1) 85.90(12) Mn(11)-Mn(3)-Mn(4) 121.34(5) Mn(5)-Mn(4)-Mn(1) 120.59(5) O(6)-Mn(3)-Mn(2) 86.61(15) O(8)-Mn(4)-Mn(3) 86.89(14) O(7)-Mn(3)-Mn(2) 128.33(16) O(9)-Mn(4)-Mn(3) 132.89(15) O(4)-Mn(3)-Mn(2) 41.99(13) O(3)-Mn(4)-Mn(3) 42.36(13) O(3)-Mn(3)-Mn(2) 85.85(14) O(1)-Mn(4)-Mn(3) 86.46(14) O(2)-Mn(3)-Mn(2) 42.23(13) O(160)-Mn(4)-Mn(3) 136.06(18) O(130)-Mn(3)-Mn(2) 140.95(16) O(4)-Mn(4)-Mn(3) 41.99(13) Mn(11)-Mn(3)-Mn(2) 116.43(5) Mn(5)-Mn(4)-Mn(3) 119.39(5) Mn(4)-Mn(3)-Mn(2) 62.62(4) Mn(1)-Mn(4)-Mn(3) 63.29(4) O(6)-Mn(3)-Mn(1) 133.68(14) O(8)-Mn(4)-Mn(2) 135.53(14) O(7)-Mn(3)-Mn(1) 140.07(14) O(9)-Mn(4)-Mn(2) 138.21(14) O(4)-Mn(3)-Mn(1) 82.69(13) O(3)-Mn(4)-Mn(2) 83.04(13) O(3)-Mn(3)-Mn(1) 40.02(12) O(1)-Mn(4)-Mn(2) 40.31(13) O(2)-Mn(3)-Mn(1) 39.59(12) O(160)-Mn(4)-Mn(2) 93.75(16) O(130)-Mn(3)-Mn(1) 97.08(16) O(4)-Mn(4)-Mn(2) 39.72(13) Mn(11)-Mn(3)-Mn(1) 174.88(5) Mn(5)-Mn(4)-Mn(2) 178.22(6) Mn(4)-Mn(3)-Mn(1) 58.35(3) Mn(1)-Mn(4)-Mn(2) 58.95(3)

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326 Table A-9. Continued. Mn(3)-Mn(4)-Mn(2) 58.83(4) O(41)-Mn(7)-O(100) 87.0(2) O(9)-Mn(5)-O(8) 84.16(18) O(90)-Mn(7)-O(100) 88.4(2) O(9)-Mn(5)-O(70) 95.7(2) O(10)-Mn(7)-O(111) 87.53(19) O(8)-Mn(5)-O(70) 174.9(2) O(11)-Mn(7)-O(111) 85.74(19) O(9)-Mn(5)-O(21) 175.8(2) O(41)-Mn(7)-O(111) 90.1(2) O(8)-Mn(5)-O(21) 94.0(2) O(90)-Mn(7)-O(111) 87.0(2) O(70)-Mn(5)-O(21) 85.9(2) O(100)-Mn(7)-O(111) 174.6(2) O(9)-Mn(5)-O(141) 95.3(2) O(10)-Mn(7)-Mn(1) 41.89(13) O(8)-Mn(5)-O(141) 93.8(2) O(11)-Mn(7)-Mn(1) 42.22(13) O(70)-Mn(5)-O(141) 91.2(2) O(41)-Mn(7)-Mn(1) 136.06(16) O(21)-Mn(5)-O(141) 88.6(2) O(90)-Mn(7)-Mn(1) 134.35(15) O(9)-Mn(5)-O(161) 85.0(2) O(100)-Mn(7)-Mn(1) 106.61(15) O(8)-Mn(5)-O(161) 84.6(2) O(111)-Mn(7)-Mn(1) 78.59(13) O(70)-Mn(5)-O(161) 90.3(2) O(12)-Mn(8)-O(11) 93.68(19) O(21)-Mn(5)-O(161) 91.2(2) O(12)-Mn(8)-O(40) 173.8(2) O(141)-Mn(5)-O(161) 178.4(2) O(11)-Mn(8)-O(40) 92.2(2) O(9)-Mn(5)-Mn(4) 42.37(13) O(12)-Mn(8)-O(51) 91.1(2) O(8)-Mn(5)-Mn(4) 42.26(13) O(11)-Mn(8)-O(51) 171.8(2) O(70)-Mn(5)-Mn(4) 136.71(17) O(40)-Mn(8)-O(51) 82.8(2) O(21)-Mn(5)-Mn(4) 135.19(16) O(12)-Mn(8)-O(150) 95.4(2) O(141)-Mn(5)-Mn(4) 101.07(15) O(11)-Mn(8)-O(150) 99.6(2) O(161)-Mn(5)-Mn(4) 78.03(14) O(40)-Mn(8)-O(150) 85.4(2) O(10)-Mn(6)-O(9) 93.67(18) O(51)-Mn(8)-O(150) 86.6(2) O(10)-Mn(6)-O(91) 91.5(2) O(12)-Mn(8)-O(13) 92.1(2) O(9)-Mn(6)-O(91) 172.5(2) O(11)-Mn(8)-O(13) 92.8(2) O(10)-Mn(6)-O(71) 174.3(2) O(40)-Mn(8)-O(13) 85.8(2) O(9)-Mn(6)-O(71) 91.5(2) O(51)-Mn(8)-O(13) 80.4(2) O(91)-Mn(6)-O(71) 83.6(2) O(150)-Mn(8)-O(13) 165.1(2) O(10)-Mn(6)-O(140) 95.0(2) O(5)-Mn(9)-O(12) 83.05(19) O(9)-Mn(6)-O(140) 92.2(2) O(5)-Mn(9)-O(61) 95.3(2) O(91)-Mn(6)-O(140) 82.0(2) O(12)-Mn(9)-O(61) 177.4(2) O(71)-Mn(6)-O(140) 87.2(2) O(5)-Mn(9)-O(50) 172.6(3) O(10)-Mn(6)-O(101) 95.7(2) O(12)-Mn(9)-O(50) 95.8(2) O(9)-Mn(6)-O(101) 97.6(2) O(61)-Mn(9)-O(50) 85.6(2) O(91)-Mn(6)-O(101) 87.3(2) O(5)-Mn(9)-O(151) 98.8(2) O(71)-Mn(6)-O(101) 81.3(2) O(12)-Mn(9)-O(151) 93.3(2) O(140)-Mn(6)-O(101) 165.1(2) O(61)-Mn(9)-O(151) 88.9(3) O(10)-Mn(7)-O(11) 83.18(18) O(50)-Mn(9)-O(151) 88.5(2) O(10)-Mn(7)-O(41) 177.2(2) O(5)-Mn(9)-O(80) 84.9(2) O(11)-Mn(7)-O(41) 95.2(2) O(12)-Mn(9)-O(80) 86.9(2) O(10)-Mn(7)-O(90) 95.02(19) O(61)-Mn(9)-O(80) 91.0(3) O(11)-Mn(7)-O(90) 172.6(2) O(50)-Mn(9)-O(80) 87.7(3) O(41)-Mn(7)-O(90) 86.3(2) O(151)-Mn(9)-O(80) 176.3(2) O(10)-Mn(7)-O(100) 95.5(2) O(5)-Mn(9)-Mn(2) 42.20(14) O(11)-Mn(7)-O(100) 99.0(2) O(12)-Mn(9)-Mn(2) 41.51(13)

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327 Table A-9. Continued. O(61)-Mn(9)-Mn(2) 136.52(19) O(7)-Mn(12)-O(15) 93.5(2) O(50)-Mn(9)-Mn(2) 135.19(18) O(8)-Mn(12)-O(15) 94.6(2) O(151)-Mn(9)-Mn(2) 103.83(14) O(31)-Mn(12)-O(15) 86.5(2) O(80)-Mn(9)-Mn(2) 78.75(18) O(20)-Mn(12)-O(15) 84.9(2) O(6)-Mn(10)-O(5) 93.86(19) O(7)-Mn(12)-O(16) 94.2(2) O(6)-Mn(10)-O(60) 174.7(3) O(8)-Mn(12)-O(16) 93.0(2) O(5)-Mn(10)-O(60) 91.4(2) O(31)-Mn(12)-O(16) 85.2(2) O(6)-Mn(10)-O(120) 90.8(2) O(20)-Mn(12)-O(16) 86.9(2) O(5)-Mn(10)-O(120) 172.3(2) O(15)-Mn(12)-O(16) 168.9(2) O(60)-Mn(10)-O(120) 83.9(3) Mn(4)-O(1)-Mn(1) 95.6(2) O(6)-Mn(10)-O(170) 94.1(2) Mn(4)-O(1)-Mn(2) 100.1(2) O(5)-Mn(10)-O(170) 97.8(2) Mn(1)-O(1)-Mn(2) 95.18(19) O(60)-Mn(10)-O(170) 85.0(3) Mn(2)-O(2)-Mn(1) 95.61(19) O(120)-Mn(10)-O(170) 88.0(3) Mn(2)-O(2)-Mn(3) 94.81(18) O(6)-Mn(10)-O(14) 92.9(2) Mn(1)-O(2)-Mn(3) 100.39(18) O(5)-Mn(10)-O(14) 92.2(2) Mn(4)-O(3)-Mn(3) 95.63(19) O(60)-Mn(10)-O(14) 87.1(3) Mn(4)-O(3)-Mn(1) 94.8(2) O(120)-Mn(10)-O(14) 81.5(3) Mn(3)-O(3)-Mn(1) 100.62(19) O(170)-Mn(10)-O(14) 167.4(2) Mn(3)-O(4)-Mn(2) 96.29(18) O(6)-Mn(11)-O(7) 83.78(18) Mn(3)-O(4)-Mn(4) 94.89(19) O(6)-Mn(11)-O(30) 177.9(2) Mn(2)-O(4)-Mn(4) 99.8(2) O(7)-Mn(11)-O(30) 94.7(2) Mn(2)-O(5)-Mn(10) 131.8(3) O(6)-Mn(11)-O(121) 95.6(2) Mn(2)-O(5)-Mn(9) 95.2(2) O(7)-Mn(11)-O(121) 173.0(3) Mn(10)-O(5)-Mn(9) 127.8(2) O(6)-Mn(11)-O(171) 94.0(2) Mn(3)-O(6)-Mn(10) 135.5(3) O(7)-Mn(11)-O(171) 97.3(2) Mn(3)-O(6)-Mn(11) 96.1(2) O(30)-Mn(11)-O(171) 87.5(2) Mn(10)-O(6)-Mn(11) 123.1(2) O(121)-Mn(11)-O(171) 89.7(3) Mn(12)-O(7)-Mn(3) 132.8(3) O(6)-Mn(11)-O(131) 86.2(2) Mn(12)-O(7)-Mn(11) 127.7(2) O(7)-Mn(11)-O(131) 84.6(2) Mn(3)-O(7)-Mn(11) 94.3(2) O(30)-Mn(11)-O(131) 92.3(3) Mn(4)-O(8)-Mn(12) 134.4(2) O(121)-Mn(11)-O(131) 88.4(3) Mn(4)-O(8)-Mn(5) 94.5(2) O(171)-Mn(11)-O(131) 178.1(2) Mn(12)-O(8)-Mn(5) 128.8(2) O(6)-Mn(11)-Mn(3) 41.74(13) Mn(4)-O(9)-Mn(5) 94.9(2) O(7)-Mn(11)-Mn(3) 42.56(13) Mn(4)-O(9)-Mn(6) 133.4(2) O(30)-Mn(11)-Mn(3) 136.56(17) Mn(5)-O(9)-Mn(6) 123.8(2) O(121)-Mn(11)-Mn(3) 135.53(17) Mn(1)-O(10)-Mn(6) 134.5(2) O(171)-Mn(11)-Mn(3) 102.77(15) Mn(1)-O(10)-Mn(7) 95.44(19) O(131)-Mn(11)-Mn(3) 78.70(16) Mn(6)-O(10)-Mn(7) 123.1(2) O(7)-Mn(12)-O(8) 93.25(19) Mn(1)-O(11)-Mn(8) 132.8(2) O(7)-Mn(12)-O(31) 91.8(2) Mn(1)-O(11)-Mn(7) 94.51(19) O(8)-Mn(12)-O(31) 174.8(2) Mn(8)-O(11)-Mn(7) 127.3(2) O(7)-Mn(12)-O(20) 175.3(2) Mn(2)-O(12)-Mn(8) 135.2(2) O(8)-Mn(12)-O(20) 91.3(2) Mn(2)-O(12)-Mn(9) 95.9(2) O(31)-Mn(12)-O(20) 83.7(2) Mn(8)-O(12)-Mn(9) 123.6(2)

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328 Table A-9. Continued. C(12)-O(13)-Mn(8) 122.1(5) C(14)-O(15)-Mn(12) 120.7(8) C(13)-O(14)-Mn(10) 120.2(5) C(16)-O(16)-Mn(12) 123.8(6) C(15)-O(15)-Mn(12) 120.8(13)

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329 Table A-10. Selected interatomic distances () and angles () for [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)]CH2Cl2 (16CH2Cl2). Mn(1)-O(3) 1.838(3) Mn(4)-O(40) 1.963(4) Mn(1)-O(1) 1.934(3) Mn(4)-O(31) 1.966(4) Mn(1)-O(100) 1.997(3) Mn(4)-O(121) 2.216(4) Mn(1)-O(81) 2.055(4) Mn(4)-O(71) 2.231(4) Mn(1)-O(130) 2.088(3) Mn(5)-O(3) 1.850(3) Mn(1)-O(91) 2.152(4) Mn(5)-O(41) 1.931(4) Mn(2)-O(4) 1.856(3) Mn(5)-O(101) 1.956(4) Mn(2)-O(130) 1.913(3) Mn(5)-O(60) 2.017(4) Mn(2)-O(2) 1.948(4) Mn(5)-O(131) 2.128(3) Mn(2)-O(20) 1.996(4) Mn(5)-O(50) 2.176(4) Mn(2)-O(10) 2.173(4) Mn(6)-O(51) 1.903(4) Mn(2)-O(70) 2.211(4) Mn(6)-O(80) 1.923(4) Mn(3)-O(21) 1.935(4) Mn(6)-O(61) 1.955(4) Mn(3)-O(30) 1.947(4) Mn(6)-O(90) 1.981(4) Mn(3)-O(120) 1.947(4) Mn(6)-O(3) 2.065(3) Mn(3)-O(11) 1.982(4) Mn(6)-O(141) 2.323(4) Mn(3)-O(4) 2.015(3) O(130)-C(131) 1.410(6) Mn(3)-O(110) 2.277(5) O(131)-C(131) 1.398(6) Mn(4)-O(4) 1.866(3) C(131)-H(13d) 0.9900 Mn(4)-O(131) 1.903(3) C(131)-H(13e) 0.9900 O(3)-Mn(1)-O(1) 175.89(15) O( 2)-Mn(2)-O(10) 89.01(17) O(3)-Mn(1)-O(100) 93.33(14) O(20)-Mn(2)-O(10) 86.55(16) O(1)-Mn(1)-O(100) 88.39(15) O(4)-Mn(2)-O(70) 92.69(15) O(3)-Mn(1)-O(81) 91.54(15) O(130)-Mn(2)-O(70) 102.01(15) O(1)-Mn(1)-O(81) 86.21(15) O(2)-Mn(2)-O(70) 84.12(17) O(100)-Mn(1)-O(81) 170.20(15) O(20)-Mn(2)-O(70) 81.93(16) O(3)-Mn(1)-O(130) 94.06(14) O(10)-Mn(2)-O(70) 167.04(16) O(1)-Mn(1)-O(130) 89.24(14) O(21)-Mn(3)-O(30) 87.83(17) O(100)-Mn(1)-O(130) 101.42(14) O(21)-Mn(3)-O(120) 172.06(16) O(81)-Mn(1)-O(130) 86.70(14) O(30)-Mn(3)-O(120) 91.99(17) O(3)-Mn(1)-O(91) 91.66(14) O(21)-Mn(3)-O(11) 89.50(16) O(1)-Mn(1)-O(91) 84.77(14) O(30)-Mn(3)-O(11) 161.36(16) O(100)-Mn(1)-O(91) 85.01(14) O(120)-Mn(3)-O(11) 88.14(16) O(81)-Mn(1)-O(91) 86.35(14) O(21)-Mn(3)-O(4) 92.06(15) O(130)-Mn(1)-O(91) 171.11(14) O(30)-Mn(3)-O(4) 96.78(15) O(4)-Mn(2)-O(130) 92.94(14) O(120)-Mn(3)-O(4) 95.84(15) O(4)-Mn(2)-O(2) 176.41(17) O(11)-Mn(3)-O(4) 101.75(15) O(130)-Mn(2)-O(2) 89.34(15) O(21)-Mn(3)-O(110) 85.53(16) O(4)-Mn(2)-O(20) 92.86(15) O(30)-Mn(3)-O(110) 80.71(17) O(130)-Mn(2)-O(20) 172.82(15) O(120)-Mn(3)-O(110) 86.61(16) O(2)-Mn(2)-O(20) 85.07(15) O(11)-Mn(3)-O(110) 80.69(17) O(4)-Mn(2)-O(10) 93.80(16) O(4)-Mn(3)-O(110) 176.58(15) O(130)-Mn(2)-O(10) 88.84(15) O(4)-Mn(4)-O(131) 92.74(14)

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330 Table A-10. Continued. O(4)-Mn(4)-O(40) 177.90(15) O(80)-Mn(6)-O(61) 88.67(17) O(131)-Mn(4)-O(40) 89.37(15) O(51)-Mn(6)-O(90) 87.87(16) O(4)-Mn(4)-O(31) 93.38(15) O(80)-Mn(6)-O(90) 90.36(17) O(131)-Mn(4)-O(31) 172.99(16) O(61)-Mn(6)-O(90) 162.29(16) O(40)-Mn(4)-O(31) 84.52(16) O(51)-Mn(6)-O(3) 96.08(15) O(4)-Mn(4)-O(121) 90.71(14) O(80)-Mn(6)-O(3) 91.95(15) O(131)-Mn(4)-O(121) 89.13(14) O(61)-Mn(6)-O(3) 95.61(14) O(40)-Mn(4)-O(121) 89.29(15) O(90)-Mn(6)-O(3) 102.09(14) O(31)-Mn(4)-O(121) 87.38(15) O(51)-Mn(6)-O(141) 87.89(17) O(4)-Mn(4)-O(71) 90.98(14) O(80)-Mn(6)-O(141) 84.13(17) O(131)-Mn(4)-O(71) 99.62(14) O(61)-Mn(6)-O(141) 78.89(15) O(40)-Mn(4)-O(71) 88.71(15) O(90)-Mn(6)-O(141) 83.42(15) O(31)-Mn(4)-O(71) 83.70(15) O(3)-Mn(6)-O(141) 173.29(15) O(121)-Mn(4)-O(71) 171.00(14) Mn(1)-O(3)-Mn(5) 123.77(17) O(3)-Mn(5)-O(41) 177.89(15) Mn(1)-O(3)-Mn(6) 117.95(16) O(3)-Mn(5)-O(101) 93.50(15) Mn(5)-O(3)-Mn(6) 118.14(16) O(41)-Mn(5)-O(101) 85.88(16) Mn(2)-O(4)-Mn(4) 124.19(17) O(3)-Mn(5)-O(60) 93.31(15) Mn(2)-O(4)-Mn(3) 117.25(16) O(41)-Mn(5)-O(60) 87.12(16) Mn(4)-O(4)-Mn(3) 118.56(17) O(101)-Mn(5)-O(60) 171.21(16) C(131)-O(130)-Mn(2) 113.9(3) O(3)-Mn(5)-O(131) 93.12(13) C(131)-O(130)-Mn(1) 115.0(3) O(41)-Mn(5)-O(131) 88.97(14) Mn(2)-O(130)-Mn(1) 126.61(17) O(101)-Mn(5)-O(131) 99.79(14) C(131)-O(131)-Mn(4) 117.3(3) O(60)-Mn(5)-O(131) 85.38(14) C(131)-O(131)-Mn(5) 111.9(3) O(3)-Mn(5)-O(50) 92.11(14) Mn(4)-O(131)-Mn(5) 126.40(17) O(41)-Mn(5)-O(50) 85.80(15) O(131)-C(131)-O(130) 109.4(4) O(101)-Mn(5)-O(50) 88.39(15) O(131)-C(131)-H(13d) 109.8 O(60)-Mn(5)-O(50) 85.80(15) O(130)-C(131)-H(13d) 109.8 O(131)-Mn(5)-O(50) 169.99(14) O(131)-C(131)-H(13e) 109.8 O(51)-Mn(6)-O(80) 171.97(17) O(130)-C(131)-H(13e) 109.8 O(51)-Mn(6)-O(61) 90.63(17) H(13d)-C(131)-H(13e) 108.2

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331 Table A-11. Selected interatomic distances () and angles () for [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2]H2OHO2CPet (17H2OHO2CPet). Mn(1)-O(1) 1.885(3) Mn(4)-O(15) 1.943(4) Mn(1)-O(9) 1.933(4) Mn(4)-O(11) 2.081(5) Mn(1)-O(7) 2.111(7) Mn(4)-Mn(6) 3.1767(10) Mn(1)-O(5) 2.425(7) Mn(5)-O(3) 1.865(3) Mn(1)-Mn(2) 2.8126(14) Mn(5)-O(2) 1.882(4) Mn(2)-O(1) 1.900(3) Mn(5)-O(13) 1.923(4) Mn(2)-O(2) 1.906(4) Mn(5)-O(17) 1.975(4) Mn(2)-O(8) 2.192(6) Mn(5)-O(12) 2.041(5) Mn(2)-Mn(3) 2.8313(15) Mn(6)-O(4) 1.849(2) Mn(3)-O(2) 1.868(4) Mn(6)-O(3) 1.885(3) Mn(3)-O(14) 1.936(5) Mn(6)-O(19) 1.928(4) Mn(3)-O(20) 2.245(6) Mn(6)-O(16) 1.969(4) Mn(3)-O(21) 2.261(10) Mn(6)-O(18) 2.155(4) Mn(4)-O(1) 1.861(3) Mn(6)-O(6) 2.321(4) Mn(4)-O(3) 1.882(3) O(5)-C(1) 1.274(8) Mn(4)-O(10) 1.934(4) O(6)-C(1) 1.303(5) O(1)-Mn(1)-O(1a) 81.4(2) O(2)-Mn(3)-O(14a) 177.7(3) O(1)-Mn(1)-O(9) 95.16(15) O( 2)-Mn(3)-O(14) 96.0(2) O(1)-Mn(1)-O(9a) 172.72(18) O(14)-Mn(3)-O(14a) 84.6(3) O(9)-Mn(1)-O(9a) 87.5(2) O(2)-Mn(3)-O(20) 90.74(17) O(1)-Mn(1)-O(7) 94.74(19) O(14)-Mn(3)-O(20) 91.4(2) O(9)-Mn(1)-O(7) 91.9(2) O( 2)-Mn(3)-O(21) 91.0(3) O(1)-Mn(1)-O(5) 90.38(15) O(14)-Mn(3)-O(21) 86.9(3) O(9)-Mn(1)-O(5) 83.20(18) O(20)-Mn(3)-O(21) 177.7(3) O(7)-Mn(1)-O(5) 173.2(2) O( 2)-Mn(3)-Mn(2) 41.89(11) O(1)-Mn(1)-Mn(2) 42.22(10) O(14)-Mn(3)-Mn(2) 137.07(17) O(9)-Mn(1)-Mn(2) 136.14(12) O(20)-Mn(3)-Mn(2) 96.19(15) O(7)-Mn(1)-Mn(2) 83.9(2) O(21)-Mn(3)-Mn(2) 86.1(3) O(5)-Mn(1)-Mn(2) 102.85(14) O(1)-Mn(4)-O(3) 90.49(14) O(1)-Mn(2)-O(1a) 80.6(2) O(1)-Mn(4)-O(10) 93.93(16) O(1)-Mn(2)-O(2a) 175.19(18) O(3)-Mn(4)-O(10) 169.19(19) O(1)-Mn(2)-O(2) 98.88(15) O( 1)-Mn(4)-O(15) 168.4(2) O(2)-Mn(2)-O(2a) 81.2(2) O(3)-Mn(4)-O(15) 89.98(15) O(1)-Mn(2)-O(8) 92.72(18) O(10)-Mn(4)-O(15) 83.72(17) O(2)-Mn(2)-O(8) 92.08(19) O( 1)-Mn(4)-O(11) 99.1(2) O(1)-Mn(2)-Mn(1) 41.80(10) O(3)-Mn(4)-O(11) 99.88(16) O(2)-Mn(2)-Mn(1) 139.04(11) O(10)-Mn(4)-O(11) 89.16(19) O(8)-Mn(2)-Mn(1) 81.40(18) O(15)-Mn(4)-O(11) 92.2(2) O(1)-Mn(2)-Mn(3) 139.68(10) O(1)-Mn(4)-Mn(6) 98.50(10) O(2)-Mn(2)-Mn(3) 40.87(11) O(3)-Mn(4)-Mn(6) 32.52(11) O(8)-Mn(2)-Mn(3) 87.72(18) O(10)-Mn(4)-Mn(6) 136.78(15) Mn(1)-Mn(2)-Mn(3) 169.12(5) O(15)-Mn(4)-Mn(6) 75.99(12) O(2)-Mn(3)-O(2a) 83.3(2) O(11)-Mn(4)-Mn(6) 128.80(13)

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332 Table A-11. Continued. O(3)-Mn(5)-O(2) 90.94(16) O(16)-Mn(6)-O(6) 83.19(16) O(3)-Mn(5)-O(13) 172.4(2) O(18)-Mn(6)-O(6) 173.50(17) O(2)-Mn(5)-O(13) 92.3(2) O(4)-Mn(6)-Mn(4) 102.71(16) O(3)-Mn(5)-O(17) 90.89(18) O(3)-Mn(6)-Mn(4) 32.46(10) O(2)-Mn(5)-O(17) 159.4(2) O(19)-Mn(6)-Mn(4) 143.06(14) O(13)-Mn(5)-O(17) 83.7(2) O(16)-Mn(6)-Mn(4) 76.41(11) O(3)-Mn(5)-O(12) 99.57(18) O(18)-Mn(6)-Mn(4) 121.44(12) O(2)-Mn(5)-O(12) 105.5(2) O(6)-Mn(6)-Mn(4) 52.55(12) O(13)-Mn(5)-O(12) 86.1(2) Mn(4)-O(1)-Mn(1) 126.33(19) O(17)-Mn(5)-O(12) 94.5(2) Mn(4)-O(1)-Mn(2) 129.48(16) O(4)-Mn(6)-O(3) 93.14(18) Mn(1)-O(1)-Mn(2) 95.98(15) O(4)-Mn(6)-O(19) 94.4(2) Mn(3)-O(2)-Mn(5) 121.8(2) O(3)-Mn(6)-O(19) 172.31(18) Mn(3)-O(2)-Mn(2) 97.24(16) O(4)-Mn(6)-O(16) 173.7(2) Mn(5)-O(2)-Mn(2) 128.2(2) O(3)-Mn(6)-O(16) 89.27(17) Mn(5)-O(3)-Mn(4) 123.00(19) O(19)-Mn(6)-O(16) 83.08(19) Mn(5)-O(3)-Mn(6) 121.98(17) O(4)-Mn(6)-O(18) 92.7(2) Mn(4)-O(3)-Mn(6) 115.02(17) O(3)-Mn(6)-O(18) 91.57(16) Mn(6)-O(4)-Mn(6a) 126.4(3) O(19)-Mn(6)-O(18) 89.68(19) C(1)-O(5)-Mn(1) 118.6(5) O(16)-Mn(6)-O(18) 93.04(17) C(1)-O(6)-Mn(6) 133.5(3) O(4)-Mn(6)-O(6) 91.35(18) O(5)-C(1)-O(6) 118.0(3) O(3)-Mn(6)-O(6) 83.12(15) O(6)-C(1)-O(6a) 118.3(5) O(19)-Mn(6)-O(6) 95.10(18)

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333 Table A-12. Selected interatomic distances () and angles () for [Mn4O2(O2CPet)6(bpy)2]2H2O (182H2O). Mn(1)-O(1a) 1.8537(18) Mn(2)-O(1) 2.0906(18) Mn(1)-O(1) 1.8560(18) Mn(2)-O(5) 2.1201(19) Mn(1)-O(2) 1.957(2) Mn(2)-O(3) 2.184(2) Mn(1)-O(6a) 1.965(2) Mn(2)-O(7) 2.212(2) Mn(1)-O(4) 2.089(2) Mn(2)-N(1) 2.259(2) Mn(1)-Mn(1a) 2.7755(8) Mn(2)-N(2) 2.281(2) O(1a)-Mn(1)-O(1) 83.13(8) O(5)-Mn(2)-O(3) 91.87(8) O(1a)-Mn(1)-O(2) 159.64(9) O(1)-Mn(2)-O(7) 89.56(7) O(1)-Mn(1)-O(2) 92.51(9) O(5)-Mn(2)-O(7) 94.47(8) O(1a)-Mn(1)-O(6a) 94.04(9) O(3)-Mn(2)-O(7) 173.39(8) O(1)-Mn(1)-O(6a) 169.43(9) O(1)-Mn(2)-N(1) 165.17(8) O(2)-Mn(1)-O(6a) 86.62(10) O(5)-Mn(2)-N(1) 88.97(8) O(1a)-Mn(1)-O(4) 103.54(9) O(3)-Mn(2)-N(1) 94.81(8) O(1)-Mn(1)-O(4) 102.84(8) O(7)-Mn(2)-N(1) 87.18(8) O(2)-Mn(1)-O(4) 96.82(10) O(1)-Mn(2)-N(2) 93.62(8) O(6a)-Mn(1)-O(4) 87.72(10) O(5)-Mn(2)-N(2) 160.42(9) O(1a)-Mn(1)-Mn(1a) 41.60(5) O(3)-Mn(2)-N(2) 86.07(9) O(1)-Mn(1)-Mn(1a) 41.54(5) O(7)-Mn(2)-N(2) 88.57(9) O(2)-Mn(1)-Mn(1a) 130.95(8) N(1)-Mn(2)-N(2) 71.84(9) O(6a)-Mn(1)-Mn(1a) 134.78(7) Mn(1a)-O(1)-Mn(1) 96.87(8) O(4)-Mn(1)-Mn(1a) 107.76(7) Mn(1a)-O(1)-Mn(2) 122.80(10) O(1)-Mn(2)-O(5) 105.72(8) Mn(1)-O(1)-Mn(2) 111.95(9) O(1)-Mn(2)-O(3) 86.93(7)

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334 Table A-13. Selected interatomic distances () and angles () for [Mn12O12(O2CC6F5)16(H2O)4]3CH2Cl2 (203CH2Cl2). Mn(1)-O(2) 1.874(8) Mn(6)-O(3) 1.904(6) Mn(1)-O(1) 1.915(7) Mn(6)-O(10) 1.917(6) Mn(1)-O(15) 1.933(9) Mn(6)-O(27) 1.920(7) Mn(1)-O(14) 1.958(10) Mn(6)-Mn(7) 2.941(2) Mn(1)-O(13) 2.184(8) Mn(7)-O(12) 1.850(7) Mn(1)-O(16) 2.207(8) Mn(7)-O(11) 1.868(6) Mn(1)-Mn(2) 2.779(2) Mn(7)-O(4) 1.898(6) Mn(2)-O(1) 1.864(7) Mn(7)-O(10) 1.909(7) Mn(2)-O(2) 1.864(6) Mn(7)-O(28) 1.920(7) Mn(2)-O(10) 1.905(6) Mn(7)-O(9) 1.922(6) Mn(2)-O(17) 1.914(7) Mn(7)-Mn(8) 2.796(2) Mn(2)-O(4) 1.920(7) Mn(8)-O(11) 1.903(7) Mn(2)-O(3) 1.926(6) Mn(8)-O(31) 1.909(8) Mn(2)-Mn(6) 2.814(2) Mn(8)-O(12) 1.910(6) Mn(2)-Mn(7) 2.840(2) Mn(8)-O(30) 1.927(10) Mn(2)-Mn(3) 2.962(2) Mn(8)-O(29) 2.227(8) Mn(3)-O(6) 1.874(7) Mn(8)-O(32) 2.244(10) Mn(3)-O(5) 1.880(6) Mn(9)-O(8) 1.865(6) Mn(3)-O(9) 1.906(6) Mn(9)-O(1) 1.880(7) Mn(3)-O(4) 1.907(6) Mn(9)-O(35) 1.957(8) Mn(3)-O(3) 1.927(6) Mn(9)-O(34) 1.978(7) Mn(3)-O(18) 1.931(6) Mn(9)-O(36) 2.168(7) Mn(3)-Mn(4) 2.795(2) Mn(9)-O(33) 2.212(7) Mn(3)-Mn(7) 2.813(2) Mn(10)-O(5) 1.891(6) Mn(3)-Mn(6) 2.834(2) Mn(10)-O(7) 1.903(6) Mn(4)-O(5) 1.876(7) Mn(10)-O(39) 1.956(6) Mn(4)-O(6) 1.913(6) Mn(10)-O(38) 1.964(6) Mn(4)-O(21) 1.926(8) Mn(10)-O(37) 2.148(7) Mn(4)-O(20) 1.960(6) Mn(10)-O(40) 2.162(8) Mn(4)-O(22) 2.161(7) Mn(11)-O(11) 1.884(7) Mn(4)-O(19) 2.195(7) Mn(11)-O(46) 1.978(10) Mn(5)-O(7) 1.874(7) Mn(11)-O(2) 1.978(8) Mn(5)-O(8) 1.909(6) Mn(11)-O(48) 2.010(9) Mn(5)-O(25) 1.934(7) Mn(11)-O(45) 2.015(8) Mn(5)-O(24) 1.947(7) Mn(11)-O(47) 2.053(10) Mn(5)-O(26) 2.161(7) Mn(12)-O(12) 1.869(7) Mn(5)-O(23) 2.172(7) Mn(12)-O(6) 1.875(7) Mn(5)-Mn(6) 2.785(2) Mn(12)-O(42) 1.963(9) Mn(6)-O(8) 1.869(7) Mn(12)-O(43) 1.974(8) Mn(6)-O(7) 1.894(6) Mn(12)-O(44) 2.201(8) Mn(6)-O(9) 1.895(7) Mn(12)-O(41) 2.211(8) O(2)-Mn(1)-O(1) 82.7(3) O( 1)-Mn(1)-O(15) 171.5(3) O(2)-Mn(1)-O(15) 93.6(3) O(2)-Mn(1)-O(14) 177.7(3)

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335 Table A-13. Continued. O(1)-Mn(1)-O(14) 96.8(4) O(1)-Mn(2)-Mn(7) 129.4(2) O(15)-Mn(1)-O(14) 86.7(4) O(2)-Mn(2)-Mn(7) 89.2(3) O(2)-Mn(1)-O(13) 89.9(3) O(10)-Mn(2)-Mn(7) 41.9(2) O(1)-Mn(1)-O(13) 84.3(3) O(17)-Mn(2)-Mn(7) 137.5(2) O(15)-Mn(1)-O(13) 88.0(4) O(4)-Mn(2)-Mn(7) 41.64(19) O(14)-Mn(1)-O(13) 87.8(4) O(3)-Mn(2)-Mn(7) 85.4(2) O(2)-Mn(1)-O(16) 93.8(3) Mn(1)-Mn(2)-Mn(7) 120.96(7) O(1)-Mn(1)-O(16) 93.4(3) Mn(6)-Mn(2)-Mn(7) 62.68(6) O(15)-Mn(1)-O(16) 94.5(4) O(1)-Mn(2)-Mn(3) 139.1(2) O(14)-Mn(1)-O(16) 88.5(3) O(2)-Mn(2)-Mn(3) 135.5(3) O(13)-Mn(1)-O(16) 175.4(3) O(10)-Mn(2)-Mn(3) 82.15(19) O(2)-Mn(1)-Mn(2) 41.8(2) O(17)-Mn(2)-Mn(3) 92.6(2) O(1)-Mn(1)-Mn(2) 42.0(2) O(4)-Mn(2)-Mn(3) 39.13(17) O(15)-Mn(1)-Mn(2) 132.5(3) O(3)-Mn(2)-Mn(3) 39.78(19) O(14)-Mn(1)-Mn(2) 137.1(3) Mn(1)-Mn(2)-Mn(3) 177.50(8) O(13)-Mn(1)-Mn(2) 78.6(2) Mn(6)-Mn(2)-Mn(3) 58.70(5) O(16)-Mn(1)-Mn(2) 102.4(2) Mn(7)-Mn(2)-Mn(3) 57.97(5) O(1)-Mn(2)-O(2) 84.3(3) O(6)-Mn(3)-O(5) 84.1(3) O(1)-Mn(2)-O(10) 88.2(3) O(6)-Mn(3)-O(9) 88.3(3) O(2)-Mn(2)-O(10) 92.6(3) O(5)-Mn(3)-O(9) 92.3(3) O(1)-Mn(2)-O(17) 93.1(3) O(6)-Mn(3)-O(4) 101.0(3) O(2)-Mn(2)-O(17) 94.2(3) O(5)-Mn(3)-O(4) 173.5(3) O(10)-Mn(2)-O(17) 173.2(3) O(9)-Mn(3)-O(4) 83.8(3) O(1)-Mn(2)-O(4) 170.9(3) O(6)-Mn(3)-O(3) 171.1(3) O(2)-Mn(2)-O(4) 96.4(3) O(5)-Mn(3)-O(3) 95.1(3) O(10)-Mn(2)-O(4) 82.7(3) O(9)-Mn(3)-O(3) 82.8(3) O(17)-Mn(2)-O(4) 96.0(3) O(4)-Mn(3)-O(3) 79.2(3) O(1)-Mn(2)-O(3) 99.9(3) O( 6)-Mn(3)-O(18) 91.3(3) O(2)-Mn(2)-O(3) 174.5(3) O( 5)-Mn(3)-O(18) 93.2(3) O(10)-Mn(2)-O(3) 84.0(3) O( 9)-Mn(3)-O(18) 174.3(3) O(17)-Mn(2)-O(3) 89.1(3) O( 4)-Mn(3)-O(18) 90.7(3) O(4)-Mn(2)-O(3) 78.9(3) O( 3)-Mn(3)-O(18) 97.6(3) O(1)-Mn(2)-Mn(1) 43.4(2) O(6)-Mn(3)-Mn(4) 42.99(18) O(2)-Mn(2)-Mn(1) 42.1(2) O(5)-Mn(3)-Mn(4) 41.9(2) O(10)-Mn(2)-Mn(1) 98.38(19) O(9)-Mn(3)-Mn(4) 96.6(2) O(17)-Mn(2)-Mn(1) 87.1(2) O(4)-Mn(3)-Mn(4) 143.7(2) O(4)-Mn(2)-Mn(1) 138.44(19) O(3)-Mn(3)-Mn(4) 136.98(19) O(3)-Mn(2)-Mn(1) 142.7(2) O(18)-Mn(3)-Mn(4) 87.0(2) O(1)-Mn(2)-Mn(6) 87.8(2) O(6)-Mn(3)-Mn(7) 88.17(19) O(2)-Mn(2)-Mn(6) 135.0(2) O(5)-Mn(3)-Mn(7) 134.85(19) O(10)-Mn(2)-Mn(6) 42.76(18) O(9)-Mn(3)-Mn(7) 42.91(18) O(17)-Mn(2)-Mn(6) 130.6(2) O(4)-Mn(3)-Mn(7) 42.19(19) O(4)-Mn(2)-Mn(6) 85.26(19) O(3)-Mn(3)-Mn(7) 86.07(19) O(3)-Mn(2)-Mn(6) 42.42(18) O(18)-Mn(3)-Mn(7) 131.45(19) Mn(1)-Mn(2)-Mn(6) 123.22(7) Mn(4)-Mn(3)-Mn(7) 122.07(7)

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336 Table A-13. Continued. O(6)-Mn(3)-Mn(6) 129.1(2) O(8)-Mn(5)-O(26) 89.4(3) O(5)-Mn(3)-Mn(6) 88.69(19) O(25)-Mn(5)-O(26) 85.5(3) O(9)-Mn(3)-Mn(6) 41.6(2) O(24)-Mn(5)-O(26) 97.8(3) O(4)-Mn(3)-Mn(6) 84.9(2) O(7)-Mn(5)-O(23) 90.1(3) O(3)-Mn(3)-Mn(6) 41.98(18) O(8)-Mn(5)-O(23) 84.9(3) O(18)-Mn(3)-Mn(6) 139.4(2) O(25)-Mn(5)-O(23) 89.6(3) Mn(4)-Mn(3)-Mn(6) 118.95(7) O(24)-Mn(5)-O(23) 88.1(3) Mn(7)-Mn(3)-Mn(6) 62.76(5) O(26)-Mn(5)-O(23) 172.2(3) O(6)-Mn(3)-Mn(2) 139.90(19) O(7)-Mn(5)-Mn(6) 42.60(18) O(5)-Mn(3)-Mn(2) 134.8(2) O(8)-Mn(5)-Mn(6) 41.94(19) O(9)-Mn(3)-Mn(2) 82.1(2) O(25)-Mn(5)-Mn(6) 135.7(2) O(4)-Mn(3)-Mn(2) 39.5(2) O(24)-Mn(5)-Mn(6) 134.3(2) O(3)-Mn(3)-Mn(2) 39.75(18) O(26)-Mn(5)-Mn(6) 100.44(19) O(18)-Mn(3)-Mn(2) 94.6(2) O(23)-Mn(5)-Mn(6) 78.8(2) Mn(4)-Mn(3)-Mn(2) 176.52(7) O(8)-Mn(6)-O(7) 83.9(3) Mn(7)-Mn(3)-Mn(2) 58.84(5) O(8)-Mn(6)-O(9) 172.0(3) Mn(6)-Mn(3)-Mn(2) 58.04(5) O(7)-Mn(6)-O(9) 97.9(3) O(5)-Mn(4)-O(6) 83.2(3) O(8)-Mn(6)-O(3) 88.4(3) O(5)-Mn(4)-O(21) 178.3(3) O(7)-Mn(6)-O(3) 91.6(3) O(6)-Mn(4)-O(21) 95.8(3) O(9)-Mn(6)-O(3) 83.7(3) O(5)-Mn(4)-O(20) 95.8(3) O(8)-Mn(6)-O(10) 98.1(3) O(6)-Mn(4)-O(20) 174.5(3) O(7)-Mn(6)-O(10) 175.4(3) O(21)-Mn(4)-O(20) 85.1(3) O(9)-Mn(6)-O(10) 79.6(3) O(5)-Mn(4)-O(22) 92.7(3) O(3)-Mn(6)-O(10) 84.3(3) O(6)-Mn(4)-O(22) 91.8(3) O(8)-Mn(6)-O(27) 92.5(3) O(21)-Mn(4)-O(22) 88.7(3) O(7)-Mn(6)-O(27) 93.8(3) O(20)-Mn(4)-O(22) 93.6(3) O(9)-Mn(6)-O(27) 95.1(3) O(5)-Mn(4)-O(19) 86.6(3) O(3)-Mn(6)-O(27) 174.5(3) O(6)-Mn(4)-O(19) 86.0(3) O(10)-Mn(6)-O(27) 90.2(3) O(21)-Mn(4)-O(19) 92.0(3) O(8)-Mn(6)-Mn(5) 43.05(18) O(20)-Mn(4)-O(19) 88.6(3) O(7)-Mn(6)-Mn(5) 42.1(2) O(22)-Mn(4)-O(19) 177.7(3) O(9)-Mn(6)-Mn(5) 139.79(19) O(5)-Mn(4)-Mn(3) 41.98(18) O(3)-Mn(6)-Mn(5) 97.93(19) O(6)-Mn(4)-Mn(3) 41.9(2) O(10)-Mn(6)-Mn(5) 140.6(2) O(21)-Mn(4)-Mn(3) 136.7(2) O(27)-Mn(6)-Mn(5) 86.4(2) O(20)-Mn(4)-Mn(3) 136.0(2) O(8)-Mn(6)-Mn(2) 86.71(19) O(22)-Mn(4)-Mn(3) 98.98(19) O(7)-Mn(6)-Mn(2) 133.9(2) O(19)-Mn(4)-Mn(3) 79.08(19) O(9)-Mn(6)-Mn(2) 86.5(2) O(7)-Mn(5)-O(8) 83.4(3) O( 3)-Mn(6)-Mn(2) 43.02(19) O(7)-Mn(5)-O(25) 178.3(3) O(10)-Mn(6)-Mn(2) 42.43(18) O(8)-Mn(5)-O(25) 95.0(3) O(27)-Mn(6)-Mn(2) 131.6(2) O(7)-Mn(5)-O(24) 94.6(3) Mn(5)-Mn(6)-Mn(2) 121.73(7) O(8)-Mn(5)-O(24) 172.7(3) O(8)-Mn(6)-Mn(3) 130.5(2) O(25)-Mn(5)-O(24) 87.0(3) O(7)-Mn(6)-Mn(3) 90.03(19) O(7)-Mn(5)-O(26) 94.6(3) O(9)-Mn(6)-Mn(3) 41.93(19)

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337 Table A-13. Continued. O(3)-Mn(6)-Mn(3) 42.61(19) O(10)-Mn(7)-Mn(2) 41.83(18) O(10)-Mn(6)-Mn(3) 85.54(19) O(28)-Mn(7)-Mn(2) 135.6(2) O(27)-Mn(6)-Mn(3) 136.9(2) O(9)-Mn(7)-Mn(2) 85.2(2) Mn(5)-Mn(6)-Mn(3) 122.19(7) Mn(8)-Mn(7)-Mn(2) 120.81(8) Mn(2)-Mn(6)-Mn(3) 63.26(5) Mn(3)-Mn(7)-Mn(2) 63.19(6) O(8)-Mn(6)-Mn(7) 137.39(19) O(12)-Mn(7)-Mn(6) 137.4(2) O(7)-Mn(6)-Mn(7) 137.7(2) O(11)-Mn(7)-Mn(6) 136.9(2) O(9)-Mn(6)-Mn(7) 39.94(18) O(4)-Mn(7)-Mn(6) 82.1(2) O(3)-Mn(6)-Mn(7) 82.89(19) O(10)-Mn(7)-Mn(6) 39.87(18) O(10)-Mn(6)-Mn(7) 39.7(2) O(28)-Mn(7)-Mn(6) 93.5(2) O(27)-Mn(6)-Mn(7) 92.8(2) O(9)-Mn(7)-Mn(6) 39.3(2) Mn(5)-Mn(6)-Mn(7) 179.13(8) Mn(8)-Mn(7)-Mn(6) 179.00(9) Mn(2)-Mn(6)-Mn(7) 59.09(6) Mn(3)-Mn(7)-Mn(6) 58.96(5) Mn(3)-Mn(6)-Mn(7) 58.28(5) Mn(2)-Mn(7)-Mn(6) 58.23(5) O(12)-Mn(7)-O(11) 84.8(3) O(11)-Mn(8)-O(31) 96.6(3) O(12)-Mn(7)-O(4) 89.3(3) O(11)-Mn(8)-O(12) 82.3(3) O(11)-Mn(7)-O(4) 92.8(3) O(31)-Mn(8)-O(12) 174.2(3) O(12)-Mn(7)-O(10) 172.3(3) O(11)-Mn(8)-O(30) 176.0(4) O(11)-Mn(7)-O(10) 97.1(3) O(31)-Mn(8)-O(30) 84.8(4) O(4)-Mn(7)-O(10) 83.2(3) O(12)-Mn(8)-O(30) 96.0(3) O(12)-Mn(7)-O(28) 93.2(3) O(11)-Mn(8)-O(29) 85.8(3) O(11)-Mn(7)-O(28) 91.2(3) O(31)-Mn(8)-O(29) 90.7(3) O(4)-Mn(7)-O(28) 175.5(3) O(12)-Mn(8)-O(29) 83.5(3) O(10)-Mn(7)-O(28) 94.2(3) O(30)-Mn(8)-O(29) 90.5(4) O(12)-Mn(7)-O(9) 98.5(3) O(11)-Mn(8)-O(32) 92.1(3) O(11)-Mn(7)-O(9) 175.0(3) O(31)-Mn(8)-O(32) 94.1(3) O(4)-Mn(7)-O(9) 83.6(3) O(12)-Mn(8)-O(32) 91.6(3) O(10)-Mn(7)-O(9) 79.1(3) O(30)-Mn(8)-O(32) 91.5(5) O(28)-Mn(7)-O(9) 92.3(3) O(29)-Mn(8)-O(32) 174.9(3) O(12)-Mn(7)-Mn(8) 42.8(2) O(11)-Mn(8)-Mn(7) 41.67(19) O(11)-Mn(7)-Mn(8) 42.6(2) O(31)-Mn(8)-Mn(7) 136.6(3) O(4)-Mn(7)-Mn(8) 97.0(2) O(12)-Mn(8)-Mn(7) 41.2(2) O(10)-Mn(7)-Mn(8) 139.71(19) O(30)-Mn(8)-Mn(7) 136.1(3) O(28)-Mn(7)-Mn(8) 87.4(2) O(29)-Mn(8)-Mn(7) 77.6(2) O(9)-Mn(7)-Mn(8) 141.1(2) O(32)-Mn(8)-Mn(7) 97.8(2) O(12)-Mn(7)-Mn(3) 87.1(2) O(8)-Mn(9)-O(1) 91.7(3) O(11)-Mn(7)-Mn(3) 134.6(2) O(8)-Mn(9)-O(35) 174.1(3) O(4)-Mn(7)-Mn(3) 42.44(18) O(1)-Mn(9)-O(35) 93.7(3) O(10)-Mn(7)-Mn(3) 86.26(18) O(8)-Mn(9)-O(34) 92.6(3) O(28)-Mn(7)-Mn(3) 133.9(2) O(1)-Mn(9)-O(34) 175.2(3) O(9)-Mn(7)-Mn(3) 42.46(19) O(35)-Mn(9)-O(34) 82.1(3) Mn(8)-Mn(7)-Mn(3) 120.56(7) O(8)-Mn(9)-O(36) 92.7(3) O(12)-Mn(7)-Mn(2) 131.0(2) O(1)-Mn(9)-O(36) 95.2(3) O(11)-Mn(7)-Mn(2) 89.8(2) O(35)-Mn(9)-O(36) 89.3(3) O(4)-Mn(7)-Mn(2) 42.2(2) O(34)-Mn(9)-O(36) 82.4(3)

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338 Table A-13. Continued. O(8)-Mn(9)-O(33) 93.9(3) O(42)-Mn(12)-O(44) 88.8(4) O(1)-Mn(9)-O(33) 94.5(3) O(43)-Mn(12)-O(44) 85.4(3) O(35)-Mn(9)-O(33) 83.3(3) O(12)-Mn(12)-O(41) 91.3(4) O(34)-Mn(9)-O(33) 87.3(3) O(6)-Mn(12)-O(41) 93.8(3) O(36)-Mn(9)-O(33) 168.1(3) O(42)-Mn(12)-O(41) 82.3(4) O(5)-Mn(10)-O(7) 95.1(3) O(43)-Mn(12)-O(41) 87.9(4) O(5)-Mn(10)-O(39) 90.2(3) O(44)-Mn(12)-O(41) 169.5(4) O(7)-Mn(10)-O(39) 173.6(3) Mn(2)-O(1)-Mn(9) 134.6(4) O(5)-Mn(10)-O(38) 172.0(3) Mn(2)-O(1)-Mn(1) 94.7(3) O(7)-Mn(10)-O(38) 91.9(3) Mn(9)-O(1)-Mn(1) 127.7(4) O(39)-Mn(10)-O(38) 83.0(3) Mn(2)-O(2)-Mn(1) 96.0(4) O(5)-Mn(10)-O(37) 92.6(3) Mn(2)-O(2)-Mn(11) 132.5(4) O(7)-Mn(10)-O(37) 94.2(3) Mn(1)-O(2)-Mn(11) 122.4(3) O(39)-Mn(10)-O(37) 81.9(3) Mn(6)-O(3)-Mn(2) 94.6(3) O(38)-Mn(10)-O(37) 90.6(3) Mn(6)-O(3)-Mn(3) 95.4(3) O(5)-Mn(10)-O(40) 93.4(3) Mn(2)-O(3)-Mn(3) 100.5(3) O(7)-Mn(10)-O(40) 90.6(3) Mn(7)-O(4)-Mn(3) 95.4(3) O(39)-Mn(10)-O(40) 92.7(3) Mn(7)-O(4)-Mn(2) 96.1(3) O(38)-Mn(10)-O(40) 82.8(3) Mn(3)-O(4)-Mn(2) 101.4(3) O(37)-Mn(10)-O(40) 172.0(3) Mn(4)-O(5)-Mn(3) 96.2(3) O(11)-Mn(11)-O(46) 174.9(4) Mn(4)-O(5)-Mn(10) 123.9(3) O(11)-Mn(11)-O(2) 93.6(3) Mn(3)-O(5)-Mn(10) 133.4(3) O(46)-Mn(11)-O(2) 90.6(4) Mn(3)-O(6)-Mn(12) 133.0(3) O(11)-Mn(11)-O(48) 92.9(3) Mn(3)-O(6)-Mn(4) 95.1(3) O(46)-Mn(11)-O(48) 83.9(4) Mn(12)-O(6)-Mn(4) 128.6(4) O(2)-Mn(11)-O(48) 94.2(3) Mn(5)-O(7)-Mn(6) 95.3(3) O(11)-Mn(11)-O(45) 93.4(3) Mn(5)-O(7)-Mn(10) 123.6(3) O(46)-Mn(11)-O(45) 89.2(4) Mn(6)-O(7)-Mn(10) 131.7(3) O(2)-Mn(11)-O(45) 94.3(3) Mn(9)-O(8)-Mn(6) 134.2(3) O(48)-Mn(11)-O(45) 169.1(3) Mn(9)-O(8)-Mn(5) 130.2(4) O(11)-Mn(11)-O(47) 90.7(4) Mn(6)-O(8)-Mn(5) 95.0(3) O(46)-Mn(11)-O(47) 85.4(4) Mn(6)-O(9)-Mn(3) 96.4(3) O(2)-Mn(11)-O(47) 174.1(4) Mn(6)-O(9)-Mn(7) 100.8(3) O(48)-Mn(11)-O(47) 89.7(4) Mn(3)-O(9)-Mn(7) 94.6(3) O(45)-Mn(11)-O(47) 81.3(3) Mn(2)-O(10)-Mn(7) 96.2(3) O(12)-Mn(12)-O(6) 92.9(3) Mn(2)-O(10)-Mn(6) 94.8(3) O(12)-Mn(12)-O(42) 92.0(4) Mn(7)-O(10)-Mn(6) 100.4(3) O(6)-Mn(12)-O(42) 173.8(4) Mn(7)-O(11)-Mn(11) 134.2(4) O(12)-Mn(12)-O(43) 173.9(3) Mn(7)-O(11)-Mn(8) 95.7(3) O(6)-Mn(12)-O(43) 93.2(3) Mn (11)-O(11)-Mn(8) 122.9(3) O(42)-Mn(12)-O(43) 81.9(4) Mn(7)-O(12)-Mn(12) 134.1(4) O(12)-Mn(12)-O(44) 94.5(3) Mn(7)-O(12)-Mn(8) 96.0(3) O(6)-Mn(12)-O(44) 94.7(3) Mn (12)-O(12)-Mn(8) 128.4(4)

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339 Table A-14. Selected interatomic distances () and angles () for (NMe4)[Mn12O12(O2CC6F5)16(H2O)4]4.5CH2Cl2H2O (214.5CH2Cl2H2O). Mn(1)-O(1) 1.874(3) Mn(6)-O(7) 1.915(3) Mn(1)-O(5) 1.882(3) Mn(6)-O(24) 1.992(4) Mn(1)-O(6) 1.890(3) Mn(6)-O(20) 2.031(4) Mn(1)-O(2) 1.907(3) Mn(6)-O(22) 2.069(4) Mn(1)-O(4) 1.911(3) Mn(6)-O(21) 2.072(4) Mn(1)-O(14) 1.924(3) Mn(7)-O(8) 1.856(3) Mn(1)-Mn(4) 2.7972(9) Mn(7)-O(7) 1.885(3) Mn(1)-Mn(5) 2.8012(10) Mn(7)-O(28) 1.947(3) Mn(1)-Mn(2) 2.8253(10) Mn(7)-O(25) 1.981(3) Mn(1)-Mn(3) 2.9486(9) Mn(7)-O(23) 2.199(3) Mn(2)-O(8) 1.832(3) Mn(7)-O(27) 2.236(4) Mn(2)-O(7) 1.856(3) Mn(8)-O(9) 2.080(3) Mn(2)-O(2) 1.915(3) Mn(8)-O(29) 2.135(3) Mn(2)-O(26) 1.927(3) Mn(8)-O(8) 2.153(3) Mn(2)-O(3) 1.943(3) Mn(8)-O(30) 2.164(3) Mn(2)-O(1) 1.967(3) Mn(8)-O(48) 2.179(4) Mn(2)-Mn(7) 2.7454(10) Mn(8)-O(47) 2.200(4) Mn(2)-Mn(3) 2.8654(10) Mn(9)-O(9) 1.842(3) Mn(2)-Mn(4) 2.9575(9) Mn(9)-O(10) 1.919(3) Mn(3)-O(9) 1.818(3) Mn(9)-O(31) 1.947(3) Mn(3)-O(10) 1.878(3) Mn(9)-O(36) 1.957(3) Mn(3)-O(3) 1.903(3) Mn(9)-O(32) 2.193(3) Mn(3)-O(2) 1.944(3) Mn(9)-O(34) 2.206(3) Mn(3)-O(4) 1.945(3) Mn(10)-O(10) 1.860(3) Mn(3)-O(33) 1.951(3) Mn(10)-O(11) 1.898(3) Mn(3)-Mn(9) 2.7645(10) Mn(10)-O(40) 1.947(3) Mn(3)-Mn(4) 2.8284(9) Mn(10)-O(37) 2.007(3) Mn(4)-O(11) 1.875(3) Mn(10)-O(38) 2.150(3) Mn(4)-O(4) 1.884(3) Mn(10)-O(35) 2.163(3) Mn(4)-O(12) 1.903(3) Mn(11)-O(11) 1.883(3) Mn(4)-O(1) 1.905(3) Mn(11)-O(12) 1.901(3) Mn(4)-O(3) 1.916(3) Mn(11)-O(42) 1.953(3) Mn(4)-O(45) 1.945(3) Mn(11)-O(41) 1.977(3) Mn(4)-Mn(11) 2.7957(9) Mn(11)-O(39) 2.164(3) Mn(5)-O(5) 1.880(3) Mn(11)-O(44) 2.179(3) Mn(5)-O(6) 1.895(3) Mn(12)-O(5) 1.868(3) Mn(5)-O(19) 1.919(3) Mn(12)-O(12) 1.881(3) Mn(5)-O(16) 1.957(3) Mn(12)-O(43) 1.948(3) Mn(5)-O(18) 2.204(4) Mn(12)-O(15) 1.971(3) Mn(5)-O(13) 2.222(3) Mn(12)-O(17) 2.207(4) Mn(6)-O(6) 1.899(3) Mn(12)-O(46) 2.221(4) O(1)-Mn(1)-O(5) 89.76(13) O(1)-Mn(1)-O(6) 92.08(13)

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340 Table A-14. Continued. O(5)-Mn(1)-O(6) 83.36(13) O(7)-Mn(2)-O(2) 93.03(13) O(1)-Mn(1)-O(2) 85.41(12) O( 8)-Mn(2)-O(26) 94.73(14) O(5)-Mn(1)-O(2) 174.71(13) O( 7)-Mn(2)-O(26) 92.72(14) O(6)-Mn(1)-O(2) 98.95(12) O( 2)-Mn(2)-O(26) 171.79(13) O(1)-Mn(1)-O(4) 84.20(12) O(8)-Mn(2)-O(3) 101.98(13) O(5)-Mn(1)-O(4) 96.38(13) O(7)-Mn(2)-O(3) 172.34(13) O(6)-Mn(1)-O(4) 176.28(13) O(2)-Mn(2)-O(3) 82.73(12) O(2)-Mn(1)-O(4) 81.01(12) O(26)-Mn(2)-O(3) 90.88(13) O(1)-Mn(1)-O(14) 174.93(13) O(8)-Mn(2)-O(1) 174.10(13) O(5)-Mn(1)-O(14) 91.57(14) O(7)-Mn(2)-O(1) 94.15(13) O(6)-Mn(1)-O(14) 92.93(13) O(2)-Mn(2)-O(1) 82.69(12) O(2)-Mn(1)-O(14) 93.06(13) O(26)-Mn(2)-O(1) 91.06(13) O(4)-Mn(1)-O(14) 90.78(13) O(3)-Mn(2)-O(1) 79.02(12) O(1)-Mn(1)-Mn(4) 42.68(9) O(8)-Mn(2)-Mn(7) 42.24(9) O(5)-Mn(1)-Mn(4) 88.52(9) O(7)-Mn(2)-Mn(7) 43.21(9) O(6)-Mn(1)-Mn(4) 134.16(10) O(2)-Mn(2)-Mn(7) 100.34(9) O(2)-Mn(1)-Mn(4) 86.47(9) O(26)-Mn(2)-Mn(7) 87.86(10) O(4)-Mn(1)-Mn(4) 42.14(8) O(3)-Mn(2)-Mn(7) 143.80(9) O(14)-Mn(1)-Mn(4) 132.45(10) O(1)-Mn(2)-Mn(7) 137.16(9) O(1)-Mn(1)-Mn(5) 97.72(9) O(8)-Mn(2)-Mn(1) 132.73(10) O(5)-Mn(1)-Mn(5) 41.84(9) O(7)-Mn(2)-Mn(1) 87.96(10) O(6)-Mn(1)-Mn(5) 42.34(9) O(2)-Mn(2)-Mn(1) 42.21(9) O(2)-Mn(1)-Mn(5) 141.07(9) O(26)-Mn(2)-Mn(1) 132.26(10) O(4)-Mn(1)-Mn(5) 137.91(9) O(3)-Mn(2)-Mn(1) 84.61(9) O(14)-Mn(1)-Mn(5) 86.51(10) O(1)-Mn(2)-Mn(1) 41.39(8) Mn(4)-Mn(1)-Mn(5) 121.60(3) Mn(7)-Mn(2)-Mn(1) 121.77(3) O(1)-Mn(1)-Mn(2) 43.94(9) O(8)-Mn(2)-Mn(3) 92.05(10) O(5)-Mn(1)-Mn(2) 133.15(10) O(7)-Mn(2)-Mn(3) 135.31(10) O(6)-Mn(1)-Mn(2) 90.44(9) O(2)-Mn(2)-Mn(3) 42.43(9) O(2)-Mn(1)-Mn(2) 42.43(9) O(26)-Mn(2)-Mn(3) 131.94(10) O(4)-Mn(1)-Mn(2) 87.04(9) O(3)-Mn(2)-Mn(3) 41.31(9) O(14)-Mn(1)-Mn(2) 135.21(10) O(1)-Mn(2)-Mn(3) 84.90(9) Mn(4)-Mn(1)-Mn(2) 63.47(2) Mn(7)-Mn(2)-Mn(3) 125.72(3) Mn(5)-Mn(1)-Mn(2) 122.98(3) Mn(1)-Mn(2)-Mn(3) 62.41(2) O(1)-Mn(1)-Mn(3) 84.13(9) O(8)-Mn(2)-Mn(4) 141.47(10) O(5)-Mn(1)-Mn(3) 136.84(9) O(7)-Mn(2)-Mn(4) 133.56(10) O(6)-Mn(1)-Mn(3) 139.39(9) O(2)-Mn(2)-Mn(4) 81.83(9) O(2)-Mn(1)-Mn(3) 40.48(9) O(26)-Mn(2)-Mn(4) 89.96(10) O(4)-Mn(1)-Mn(3) 40.54(9) O(3)-Mn(2)-Mn(4) 39.63(9) O(14)-Mn(1)-Mn(3) 91.56(10) O(1)-Mn(2)-Mn(4) 39.43(8) Mn(4)-Mn(1)-Mn(3) 58.91(2) Mn(7)-Mn(2)-Mn(4) 175.94(3) Mn(5)-Mn(1)-Mn(3) 177.56(3) Mn(1)-Mn(2)-Mn(4) 57.80(2) Mn(2)-Mn(1)-Mn(3) 59.46(2) Mn(3)-Mn(2)-Mn(4) 58.09(2) O(8)-Mn(2)-O(7) 84.46(13) O( 9)-Mn(3)-O(10) 84.34(13) O(8)-Mn(2)-O(2) 91.65(13) O(9)-Mn(3)-O(3) 92.45(13)

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341 Table A-14. Continued. O(10)-Mn(3)-O(3) 92.47(13) O( 4)-Mn(4)-O(12) 87.99(12) O(9)-Mn(3)-O(2) 98.30(13) O(11)-Mn(4)-O(1) 174.91(13) O(10)-Mn(3)-O(2) 174.84(13) O(4)-Mn(4)-O(1) 84.13(12) O(3)-Mn(3)-O(2) 83.01(12) O(12)-Mn(4)-O(1) 99.56(13) O(9)-Mn(3)-O(4) 175.06(13) O(11)-Mn(4)-O(3) 94.66(12) O(10)-Mn(3)-O(4) 97.78(12) O(4)-Mn(4)-O(3) 84.32(12) O(3)-Mn(3)-O(4) 83.02(12) O(12)-Mn(4)-O(3) 172.16(13) O(2)-Mn(3)-O(4) 79.25(12) O(1)-Mn(4)-O(3) 81.25(12) O(9)-Mn(3)-O(33) 91.97(14) O(11)-Mn(4)-O(45) 91.87(13) O(10)-Mn(3)-O(33) 92.62(13) O(4)-Mn(4)-O(45) 175.36(14) O(3)-Mn(3)-O(33) 173.57(13) O(12)-Mn(4)-O(45) 90.85(13) O(2)-Mn(3)-O(33) 91.72(12) O(1)-Mn(4)-O(45) 91.64(13) O(4)-Mn(3)-O(33) 92.39(13) O(3)-Mn(4)-O(45) 96.93(13) O(9)-Mn(3)-Mn(9) 41.29(9) O(11)-Mn(4)-Mn(11) 42.05(9) O(10)-Mn(3)-Mn(9) 43.87(9) O(4)-Mn(4)-Mn(11) 96.14(9) O(3)-Mn(3)-Mn(9) 99.85(9) O(12)-Mn(4)-Mn(11) 42.68(9) O(2)-Mn(3)-Mn(9) 139.32(9) O(1)-Mn(4)-Mn(11) 142.00(9) O(4)-Mn(3)-Mn(9) 141.41(9) O(3)-Mn(4)-Mn(11) 136.70(9) O(33)-Mn(3)-Mn(9) 86.55(9) O(45)-Mn(4)-Mn(11) 86.02(10) O(9)-Mn(3)-Mn(4) 134.28(10) O(11)-Mn(4)-Mn(1) 135.21(10) O(10)-Mn(3)-Mn(4) 90.07(9) O(4)-Mn(4)-Mn(1) 42.91(9) O(3)-Mn(3)-Mn(4) 42.39(9) O(12)-Mn(4)-Mn(1) 89.56(9) O(2)-Mn(3)-Mn(4) 84.91(9) O(1)-Mn(4)-Mn(1) 41.84(9) O(4)-Mn(3)-Mn(4) 41.54(8) O(3)-Mn(4)-Mn(1) 85.89(9) O(33)-Mn(3)-Mn(4) 133.67(10) O(45)-Mn(4)-Mn(1) 132.62(10) Mn(9)-Mn(3)-Mn(4) 124.16(3) Mn(11)-Mn(4)-Mn(1) 122.95(3) O(9)-Mn(3)-Mn(2) 90.03(10) O(11)-Mn(4)-Mn(3) 87.86(9) O(10)-Mn(3)-Mn(2) 134.29(10) O(4)-Mn(4)-Mn(3) 43.21(9) O(3)-Mn(3)-Mn(2) 42.37(9) O(12)-Mn(4)-Mn(3) 130.11(9) O(2)-Mn(3)-Mn(2) 41.65(9) O(1)-Mn(4)-Mn(3) 87.07(9) O(4)-Mn(3)-Mn(2) 85.30(9) O(3)-Mn(4)-Mn(3) 42.05(9) O(33)-Mn(3)-Mn(2) 132.97(9) O(45)-Mn(4)-Mn(3) 138.67(10) Mn(9)-Mn(3)-Mn(2) 122.51(3) Mn(11)-Mn(4)-Mn(3) 118.71(3) Mn(4)-Mn(3)-Mn(2) 62.58(2) Mn(1)-Mn(4)-Mn(3) 63.22(2) O(9)-Mn(3)-Mn(1) 137.79(10) O(11)-Mn(4)-Mn(2) 134.95(9) O(10)-Mn(3)-Mn(1) 137.43(9) O(4)-Mn(4)-Mn(2) 83.73(9) O(3)-Mn(3)-Mn(1) 81.86(9) O(12)-Mn(4)-Mn(2) 140.25(9) O(2)-Mn(3)-Mn(1) 39.56(9) O(1)-Mn(4)-Mn(2) 40.98(9) O(4)-Mn(3)-Mn(1) 39.71(8) O(3)-Mn(4)-Mn(2) 40.30(9) O(33)-Mn(3)-Mn(1) 91.73(9) O(45)-Mn(4)-Mn(2) 94.33(10) Mn(9)-Mn(3)-Mn(1) 177.95(3) Mn(11)-Mn(4)-Mn(2) 177.00(3) Mn(4)-Mn(3)-Mn(1) 57.88(2) Mn(1)-Mn(4)-Mn(2) 58.73(2) Mn(2)-Mn(3)-Mn(1) 58.13(2) Mn(3)-Mn(4)-Mn(2) 59.32(2) O(11)-Mn(4)-O(4) 92.48(13) O(5)-Mn(5)-O(6) 83.27(13) O(11)-Mn(4)-O(12) 84.08(13) O(5)-Mn(5)-O(19) 174.88(15)

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342 Table A-14. Continued. O(6)-Mn(5)-O(19) 95.81(14) O(7)-Mn(7)-O(27) 86.70(13) O(5)-Mn(5)-O(16) 95.08(13) O(28)-Mn(7)-O(27) 88.62(14) O(6)-Mn(5)-O(16) 177.22(14) O(25)-Mn(7)-O(27) 83.14(14) O(19)-Mn(5)-O(16) 85.64(14) O(23)-Mn(7)-O(27) 174.13(13) O(5)-Mn(5)-O(18) 93.79(14) O(8)-Mn(7)-Mn(2) 41.56(9) O(6)-Mn(5)-O(18) 93.54(14) O(7)-Mn(7)-Mn(2) 42.39(9) O(19)-Mn(5)-O(18) 91.28(15) O(28)-Mn(7)-Mn(2) 135.46(10) O(16)-Mn(5)-O(18) 88.79(14) O(25)-Mn(7)-Mn(2) 134.07(10) O(5)-Mn(5)-O(13) 85.63(13) O(23)-Mn(7)-Mn(2) 103.83(9) O(6)-Mn(5)-O(13) 87.20(13) O(27)-Mn(7)-Mn(2) 78.77(9) O(19)-Mn(5)-O(13) 89.30(14) O(9)-Mn(8)-O(29) 177.55(13) O(16)-Mn(5)-O(13) 90.45(14) O(9)-Mn(8)-O(8) 88.63(12) O(18)-Mn(5)-O(13) 179.01(14) O(29)-Mn(8)-O(8) 89.01(13) O(5)-Mn(5)-Mn(1) 41.89(9) O(9)-Mn(8)-O(30) 90.21(13) O(6)-Mn(5)-Mn(1) 42.20(9) O(29)-Mn(8)-O(30) 92.15(13) O(19)-Mn(5)-Mn(1) 136.15(11) O(8)-Mn(8)-O(30) 178.78(13) O(16)-Mn(5)-Mn(1) 135.76(10) O(9)-Mn(8)-O(48) 91.41(13) O(18)-Mn(5)-Mn(1) 101.40(10) O(29)-Mn(8)-O(48) 89.36(14) O(13)-Mn(5)-Mn(1) 78.71(9) O(8)-Mn(8)-O(48) 92.85(14) O(6)-Mn(6)-O(7) 93.64(13) O(30)-Mn(8)-O(48) 86.80(15) O(6)-Mn(6)-O(24) 174.33(14) O(9)-Mn(8)-O(47) 86.21(13) O(7)-Mn(6)-O(24) 90.74(14) O(29)-Mn(8)-O(47) 93.23(14) O(6)-Mn(6)-O(20) 92.76(14) O(8)-Mn(8)-O(47) 92.08(13) O(7)-Mn(6)-O(20) 173.61(14) O(30)-Mn(8)-O(47) 88.21(14) O(24)-Mn(6)-O(20) 82.88(14) O(48)-Mn(8)-O(47) 174.47(15) O(6)-Mn(6)-O(22) 93.50(13) O(9)-Mn(9)-O(10) 82.53(13) O(7)-Mn(6)-O(22) 93.99(13) O(9)-Mn(9)-O(31) 96.36(13) O(24)-Mn(6)-O(22) 89.78(15) O(10)-Mn(9)-O(31) 175.81(14) O(20)-Mn(6)-O(22) 85.66(14) O(9)-Mn(9)-O(36) 174.82(14) O(6)-Mn(6)-O(21) 95.27(14) O(10)-Mn(9)-O(36) 95.18(13) O(7)-Mn(6)-O(21) 91.72(15) O(31)-Mn(9)-O(36) 85.59(14) O(24)-Mn(6)-O(21) 80.99(16) O(9)-Mn(9)-O(32) 87.32(13) O(20)-Mn(6)-O(21) 87.65(16) O(10)-Mn(9)-O(32) 86.58(12) O(22)-Mn(6)-O(21) 169.21(15) O(31)-Mn(9)-O(32) 89.33(13) O(8)-Mn(7)-O(7) 82.99(13) O(36)-Mn(9)-O(32) 87.90(13) O(8)-Mn(7)-O(28) 95.83(13) O(9)-Mn(9)-O(34) 94.24(13) O(7)-Mn(7)-O(28) 175.22(14) O(10)-Mn(9)-O(34) 95.68(13) O(8)-Mn(7)-O(25) 170.03(14) O(31)-Mn(9)-O(34) 88.43(14) O(7)-Mn(7)-O(25) 95.10(13) O(36)-Mn(9)-O(34) 90.61(14) O(28)-Mn(7)-O(25) 85.27(14) O(32)-Mn(9)-O(34) 177.40(13) O(8)-Mn(7)-O(23) 98.46(13) O(9)-Mn(9)-Mn(3) 40.64(9) O(7)-Mn(7)-O(23) 91.76(13) O(10)-Mn(9)-Mn(3) 42.69(9) O(28)-Mn(7)-O(23) 93.00(14) O(31)-Mn(9)-Mn(3) 135.44(10) O(25)-Mn(7)-O(23) 91.37(14) O(36)-Mn(9)-Mn(3) 136.17(10) O(8)-Mn(7)-O(27) 86.98(13) O(32)-Mn(9)-Mn(3) 79.62(9)

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343 Table A-14. Continued. O(34)-Mn(9)-Mn(3) 102.92(9) O(12)-Mn(12)-O(17) 95.74(14) O(10)-Mn(10)-O(11) 94.83(13) O(43)-Mn(12)-O(17) 84.96(15) O(10)-Mn(10)-O(40) 174.45(14) O(15)-Mn(12)-O(17) 86.83(16) O(11)-Mn(10)-O(40) 89.57(13) O(5)-Mn(12)-O(46) 89.06(14) O(10)-Mn(10)-O(37) 92.49(13) O(12)-Mn(12)-O(46) 93.79(13) O(11)-Mn(10)-O(37) 170.95(13) O(43)-Mn(12)-O(46) 91.78(15) O(40)-Mn(10)-O(37) 83.46(14) O(15)-Mn(12)-O(46) 83.32(15) O(10)-Mn(10)-O(38) 94.45(13) O(17)-Mn(12)-O(46) 169.98(15) O(11)-Mn(10)-O(38) 93.09(13) Mn(1)-O(1)-Mn(4) 95.48(13) O(40)-Mn(10)-O(38) 88.67(14) Mn(1)-O(1)-Mn(2) 94.66(13) O(37)-Mn(10)-O(38) 81.04(14) Mn(4)-O(1)-Mn(2) 99.59(13) O(10)-Mn(10)-O(35) 94.98(13) Mn(1)-O(2)-Mn(2) 95.36(13) O(11)-Mn(10)-O(35) 97.76(14) Mn(1)-O(2)-Mn(3) 99.96(13) O(40)-Mn(10)-O(35) 81.06(14) Mn(2)-O(2)-Mn(3) 95.92(12) O(37)-Mn(10)-O(35) 86.87(14) Mn(3)-O(3)-Mn(4) 95.57(13) O(38)-Mn(10)-O(35) 164.96(13) Mn(3)-O(3)-Mn(2) 96.32(13) O(11)-Mn(11)-O(12) 83.91(13) Mn(4)-O(3)-Mn(2) 100.07(13) O(11)-Mn(11)-O(42) 178.84(14) Mn(4)-O(4)-Mn(1) 94.96(13) O(12)-Mn(11)-O(42) 95.11(13) Mn(4)-O(4)-Mn(3) 95.25(13) O(11)-Mn(11)-O(41) 94.39(13) Mn(1)-O(4)-Mn(3) 99.75(13) O(12)-Mn(11)-O(41) 171.53(14) Mn(12)-O(5)-Mn(5) 129.49(17) O(42)-Mn(11)-O(41) 86.48(13) Mn(12)-O(5)-Mn(1) 132.24(16) O(11)-Mn(11)-O(39) 94.51(13) Mn(5)-O(5)-Mn(1) 96.27(13) O(12)-Mn(11)-O(39) 95.77(12) Mn(1)-O(6)-Mn(5) 95.46(13) O(42)-Mn(11)-O(39) 86.21(13) Mn(1)-O(6)-Mn(6) 132.03(16) O(41)-Mn(11)-O(39) 92.64(14) Mn(5)-O(6)-Mn(6) 123.72(16) O(11)-Mn(11)-O(44) 87.91(13) Mn(2)-O(7)-Mn(7) 94.40(14) O(12)-Mn(11)-O(44) 84.64(13) Mn(2)-O(7)-Mn(6) 135.14(17) O(42)-Mn(11)-O(44) 91.38(13) Mn(7)-O(7)-Mn(6) 125.39(16) O(41)-Mn(11)-O(44) 87.00(14) Mn(2)-O(8)-Mn(7) 96.20(14) O(39)-Mn(11)-O(44) 177.58(13) Mn(2)-O(8)-Mn(8) 131.32(16) O(11)-Mn(11)-Mn(4) 41.83(9) Mn(7)-O(8)-Mn(8) 124.25(15) O(12)-Mn(11)-Mn(4) 42.73(9) Mn(3)-O(9)-Mn(9) 98.07(14) O(42)-Mn(11)-Mn(4) 137.11(10) Mn(3)-O(9)-Mn(8) 133.29(16) O(41)-Mn(11)-Mn(4) 133.86(10) Mn(9)-O(9)-Mn(8) 127.81(16) O(39)-Mn(11)-Mn(4) 102.70(9) Mn(10)-O(10)-Mn(3) 132.66(16) O(44)-Mn(11)-Mn(4) 79.22(9) Mn(10)-O(10)-Mn(9) 123.74(16) O(5)-Mn(12)-O(12) 95.31(13) Mn(3)-O(10)-Mn(9) 93.45(12) O(5)-Mn(12)-O(43) 173.48(14) Mn(4)-O(11)-Mn(11) 96.11(13) O(12)-Mn(12)-O(43) 91.09(13) Mn(4)-O(11)-Mn(10) 133.28(16) O(5)-Mn(12)-O(15) 91.76(14) Mn (11)-O(11)-Mn(10) 124.60(16) O(12)-Mn(12)-O(15) 172.32(14) Mn(12)-O(12)-Mn(11) 129.12(16) O(43)-Mn(12)-O(15) 81.92(14) Mn(12)-O(12)-Mn(4) 131.00(16) O(5)-Mn(12)-O(17) 93.12(14) Mn (11)-O(12)-Mn(4) 94.60(13)

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344 Table A-15. Selected interatomic distances () and angles () for (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4]6C7H8 (226C7H8). Mn(1)-O(4) 1.832(4) Mn(3)-O(10) 1.949(4) Mn(1)-O(3) 1.869(4) Mn(3)-O(11) 2.197(4) Mn(1)-O(2) 1.889(3) Mn(3)-O(9) 2.268(4) Mn(1)-O(1a) 1.921(3) Mn(4)-O(6) 1.878(4) Mn(1)-O(1) 1.945(4) Mn(4)-O(3) 1.882(4) Mn(1)-O(5) 1.947(3) Mn(4)-O(14) 1.967(4) Mn(1)-Mn(5) 2.7717(12) Mn(4)-O(13) 1.979(4) Mn(1)-Mn(2) 2.8206(12) Mn(4)-O(15) 2.151(4) Mn(1)-Mn(2a) 2.8582(12) Mn(4)-O(12) 2.180(4) Mn(1)-Mn(1a) 2.9588(17) Mn(5)-O(4) 1.862(4) Mn(2)-O(7) 1.830(4) Mn(5)-O(3) 1.904(4) Mn(2)-O(6) 1.877(4) Mn(5)-O(17) 1.955(4) Mn(2)-O(1) 1.893(3) Mn(5)-O(18) 1.980(4) Mn(2)-O(2a) 1.921(4) Mn(5)-O(19) 2.171(4) Mn(2)-O(8) 1.940(4) Mn(5)-O(16) 2.193(4) Mn(2)-O(2) 1.942(4) Mn(6)-O(7a) 2.075(4) Mn(2)-Mn(3) 2.7638(13) Mn(6)-O(4) 2.117(4) Mn(2)-Mn(1a) 2.8582(12) Mn(6)-O(21) 2.126(5) Mn(2)-Mn(2a) 2.9392(18) Mn(6)-O(22) 2.139(5) Mn(3)-O(7) 1.841(4) Mn(6)-O(20) 2.219(4) Mn(3)-O(6) 1.901(4) Mn(6)-O(23) 2.281(4) Mn(3)-O(24a) 1.944(5) O(4)-Mn(1)-O(3) 84.26(16) O( 4)-Mn(1)-Mn(2) 133.43(11) O(4)-Mn(1)-O(2) 91.34(15) O( 3)-Mn(1)-Mn(2) 87.87(11) O(3)-Mn(1)-O(2) 93.23(15) O( 2)-Mn(1)-Mn(2) 43.33(11) O(4)-Mn(1)-O(1a) 100.12(15) O(1a)-Mn(1)-Mn(2) 85.99(11) O(3)-Mn(1)-O(1a) 173.85(16) O(1)-Mn(1)-Mn(2) 42.00(10) O(2)-Mn(1)-O(1a) 82.40(14) O(5)-Mn(1)-Mn(2) 134.36(12) O(4)-Mn(1)-O(1) 175.42(15) Mn(5)-Mn(1)-Mn(2) 121.34(4) O(3)-Mn(1)-O(1) 95.23(15) O( 4)-Mn(1)-Mn(2a) 92.62(11) O(2)-Mn(1)-O(1) 84.14(15) O( 3)-Mn(1)-Mn(2a) 134.95(11) O(1a)-Mn(1)-O(1) 80.06(16) O( 2)-Mn(1)-Mn(2a) 41.81(11) O(4)-Mn(1)-O(5) 91.76(15) O( 1a)-Mn(1)-Mn(2a) 41.10(10) O(3)-Mn(1)-O(5) 91.45(15) O( 1)-Mn(1)-Mn(2a) 84.52(10) O(2)-Mn(1)-O(5) 174.62(16) O( 5)-Mn(1)-Mn(2a) 133.59(11) O(1a)-Mn(1)-O(5) 92.72(15) Mn(5)-Mn(1)-Mn(2a) 125.05(4) O(1)-Mn(1)-O(5) 92.80(15) Mn(2)-Mn(1)-Mn(2a) 62.34(4) O(4)-Mn(1)-Mn(5) 41.78(11) O(4)-Mn(1)-Mn(1a) 140.45(12) O(3)-Mn(1)-Mn(5) 43.21(11) O(3)-Mn(1)-Mn(1a) 135.00(12) O(2)-Mn(1)-Mn(5) 99.26(11) O(2)-Mn(1)-Mn(1a) 83.07(11) O(1a)-Mn(1)-Mn(5) 141.68(11) O(1a)-Mn(1)-Mn(1a) 40.36(11) O(1)-Mn(1)-Mn(5) 138.26(11) O(1)-Mn(1)-Mn(1a) 39.78(10) O(5)-Mn(1)-Mn(5) 85.97(11) O(5)-Mn(1)-Mn(1a) 91.74(11)

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345 Table A-15. Continued. Mn(5)-Mn(1)-Mn(1a) 176.96(4) O(2)-Mn(2)-Mn(2a) 40.18(10) Mn(2)-Mn(1)-Mn(1a) 59.22(3) Mn(3)-Mn(2)-Mn(2a) 175.90(3) Mn(2a)-Mn(1)-Mn(1a) 57.98(3) Mn(1)-Mn(2)-Mn(2a) 59.46(3) O(7)-Mn(2)-O(6) 84.07(16) Mn(1a)-Mn(2)-Mn(2a) 58.21(3) O(7)-Mn(2)-O(1) 94.19(16) O(7)-Mn(3)-O(6) 83.09(16) O(6)-Mn(2)-O(1) 94.06(15) O( 7)-Mn(3)-O(24a) 94.65(18) O(7)-Mn(2)-O(2a) 97.80(16) O(6)-Mn(3)-O(24a) 171.60(18) O(6)-Mn(2)-O(2a) 176.02(15) O(7)-Mn(3)-O(10) 179.34(19) O(1)-Mn(2)-O(2a) 82.31(14) O(6)-Mn(3)-O(10) 97.45(17) O(7)-Mn(2)-O(8) 90.73(16) O(24a)-Mn(3)-O(10) 84.77(19) O(6)-Mn(2)-O(8) 92.33(16) O( 7)-Mn(3)-O(11) 90.29(15) O(1)-Mn(2)-O(8) 172.30(17) O( 6)-Mn(3)-O(11) 93.80(15) O(2a)-Mn(2)-O(8) 91.17(16) O(24a)-Mn(3)-O(11) 94.30(17) O(7)-Mn(2)-O(2) 177.92(16) O(10)-Mn(3)-O(11) 90.06(17) O(6)-Mn(2)-O(2) 97.21(15) O(7)-Mn(3)-O(9) 86.53(16) O(1)-Mn(2)-O(2) 84.09(15) O(6)-Mn(3)-O(9) 85.64(16) O(2a)-Mn(2)-O(2) 80.83(16) O(24a)-Mn(3)-O(9) 86.16(17) O(8)-Mn(2)-O(2) 90.87(15) O(10)-Mn(3)-O(9) 93.12(17) O(7)-Mn(2)-Mn(3) 41.30(12) O(11)-Mn(3)-O(9) 176.82(16) O(6)-Mn(2)-Mn(3) 43.33(11) O(7)-Mn(3)-Mn(2) 41.00(12) O(1)-Mn(2)-Mn(3) 100.96(11) O(6)-Mn(3)-Mn(2) 42.64(11) O(2a)-Mn(2)-Mn(3) 138.91(12) O(24a)-Mn(3)-Mn(2) 133.55(14) O(8)-Mn(2)-Mn(3) 86.64(12) O(10)-Mn(3)-Mn(2) 139.47(14) O(2)-Mn(2)-Mn(3) 140.18(11) O(11)-Mn(3)-Mn(2) 97.99(11) O(2)-Mn(2)-Mn(3) 140.18(11) O(9)-Mn(3)-Mn(2) 79.47(12) O(7)-Mn(2)-Mn(1) 136.67(12) O(6)-Mn(4)-O(3) 93.88(15) O(6)-Mn(2)-Mn(1) 89.79(11) O(6)-Mn(4)-O(14) 173.18(18) O(1)-Mn(2)-Mn(1) 43.41(11) O(3)-Mn(4)-O(14) 90.99(17) O(2a)-Mn(2)-Mn(1) 86.45(11) O(6)-Mn(4)-O(13) 92.73(17) O(8)-Mn(2)-Mn(1) 132.44(12) O(3)-Mn(4)-O(13) 172.52(17) O(2)-Mn(2)-Mn(1) 41.86(10) O(14)-Mn(4)-O(13) 82.71(18) Mn(3)-Mn(2)-Mn(1) 123.97(4) O(6)-Mn(4)-O(15) 93.02(16) O(7)-Mn(2)-Mn(1a) 92.94(12) O(3)-Mn(4)-O(15) 96.73(16) O(6)-Mn(2)-Mn(1a) 135.62(11) O(14)-Mn(4)-O(15) 81.66(17) O(1)-Mn(2)-Mn(1a) 41.85(10) O(13)-Mn(4)-O(15) 86.38(17) O(2a)-Mn(2)-Mn(1a) 40.96(10) O(6)-Mn(4)-O(12) 97.24(16) O(8)-Mn(2)-Mn(1a) 132.03(12) O(3)-Mn(4)-O(12) 94.78(16) O(2)-Mn(2)-Mn(1a) 84.99(10) O(14)-Mn(4)-O(12) 87.10(16) Mn(3)-Mn(2)-Mn(1a) 124.77(4) O(13)-Mn(4)-O(12) 80.93(17) Mn(1)-Mn(2)-Mn(1a) 62.80(4) O(15)-Mn(4)-O(12) 164.02(16) O(7)-Mn(2)-Mn(2a) 138.51(13) O(4)-Mn(5)-O(3) 82.49(15) O(6)-Mn(2)-Mn(2a) 137.39(12) O(4)-Mn(5)-O(17) 173.93(16) O(1)-Mn(2)-Mn(2a) 83.11(11) O(3)-Mn(5)-O(17) 95.87(17) O(2a)-Mn(2)-Mn(2a) 40.74(11) O(4)-Mn(5)-O(18) 97.44(17) O(8)-Mn(2)-Mn(2a) 89.28(12) O(3)-Mn(5)-O(18) 177.46(16)

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346 Table A-15. Continued. O(17)-Mn(5)-O(18) 84.46(18) O(22)-Mn(6)-O(20) 98.7(2) O(4)-Mn(5)-O(19) 86.89(15) O(7a)-Mn(6)-O(23) 95.29(15) O(3)-Mn(5)-O(19) 87.04(15) O(4)-Mn(6)-O(23) 80.15(15) O(17)-Mn(5)-O(19) 87.18(16) O(21)-Mn(6)-O(23) 93.98(17) O(18)-Mn(5)-O(19) 95.49(16) O(22)-Mn(6)-O(23) 87.9(2) O(4)-Mn(5)-O(16) 94.64(15) O(20)-Mn(6)-O(23) 173.26(17) O(3)-Mn(5)-O(16) 93.13(15) Mn(2)-O(1)-Mn(1a) 97.05(15) O(17)-Mn(5)-O(16) 91.29(16) Mn(2)-O(1)-Mn(1) 94.59(15) O(18)-Mn(5)-O(16) 84.34(16) Mn(1a)-O(1)-Mn(1) 99.87(16) O(19)-Mn(5)-O(16) 178.47(15) Mn(1)-O(2)-Mn(2a) 97.23(15) O(4)-Mn(5)-Mn(1) 40.96(11) Mn(1)-O(2)-Mn(2) 94.81(16) O(3)-Mn(5)-Mn(1) 42.24(11) Mn(2a)-O(2)-Mn(2) 99.08(16) O(17)-Mn(5)-Mn(1) 136.23(13) Mn(1)-O(3)-Mn(4) 135.0(2) O(18)-Mn(5)-Mn(1) 138.03(13) Mn(1)-O(3)-Mn(5) 94.55(16) O(19)-Mn(5)-Mn(1) 79.95(11) Mn(4)-O(3)-Mn(5) 124.08(18) O(16)-Mn(5)-Mn(1) 101.19(11) Mn(1)-O(4)-Mn(5) 97.25(17) O(7a)-Mn(6)-O(4) 93.37(14) Mn(1)-O(4)-Mn(6) 129.64(18) O(7a)-Mn(6)-O(21) 170.29(17) Mn(5)-O(4)-Mn(6) 125.87(18) O(4)-Mn(6)-O(21) 91.03(16) Mn(2)-O(6)-Mn(4) 133.0(2) O(7a)-Mn(6)-O(22) 89.87(17) Mn(2)-O(6)-Mn(3) 94.03(17) O(4)-Mn(6)-O(22) 167.8(2) Mn(4)-O(6)-Mn(3) 124.2(2) O(21)-Mn(6)-O(22) 87.61(19) Mn(2)-O(7)-Mn(3) 97.70(18) O(7a)-Mn(6)-O(20) 83.05(16) Mn(2)-O(7)-Mn(6a) 130.03(19) O(4)-Mn(6)-O(20) 93.40(15) Mn(3)-O(7)-Mn(6a) 127.57(19) O(21)-Mn(6)-O(20) 88.06(18)

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347 Table A-16. Selected interatomic distances () and angles () for [Mn12O12(O2CCH2Br)16(H2O)4]4CH2Cl2 (264CH2Cl2). Mn(1)-O(3a) 1.898(4) Mn(2)-O(9) 2.220(5) Mn(1)-O(2) 1.898(4) Mn(2)-Mn(3) 2.7938(12) Mn(1)-O(11) 1.959(4) Mn(3)-O(3) 1.865(4) Mn(1)-O(5) 1.970(5) Mn(3)-O(2) 1.878(4) Mn(1)-O(7) 2.114(5) Mn(3)-O(4b) 1.902(4) Mn(1)-O(1) 2.193(5) Mn(3)-O(4c) 1.922(4) Mn(2)-O(3) 1.871(4) Mn(3)-O(10) 1.923(4) Mn(2)-O(2) 1.902(4) Mn(3)-O(4) 1.934(4) Mn(2)-O(6) 1.932(4) Mn(3)-Mn(3a) 2.8180(14) Mn(2)-O(12b) 1.948(5) Mn(3)-Mn(3b) 2.8180(14) Mn(2)-O(8) 2.187(5) Mn(3)-Mn(3c) 2.9851(16) O(3a)-Mn(1)-O(2) 92.35(17) O( 6)-Mn(2)-Mn(3) 137.96(15) O(3a)-Mn(1)-O(11) 91.35(18) O(12b)-Mn(2)-Mn(3) 135.42(15) O(2)-Mn(1)-O(11) 175.94(19) O(8)-Mn(2)-Mn(3) 97.83(12) O(3a)-Mn(1)-O(5) 172.88(18) O( 9)-Mn(2)-Mn(3) 77.67(12) O(2)-Mn(1)-O(5) 93.06(18) O(3)-Mn(3)-O(2) 83.75(17) O(11)-Mn(1)-O(5) 83.4(2) O( 3)-Mn(3)-O(4b) 89.63(18) O(3a)-Mn(1)-O(7) 89.58(18) O( 2)-Mn(3)-O(4b) 91.05(17) O(2)-Mn(1)-O(7) 92.73(17) O(3)-Mn(3)-O(4c) 97.33(17) O(11)-Mn(1)-O(7) 85.63(18) O(2)-Mn(3)-O(4c) 175.16(18) O(5)-Mn(1)-O(7) 94.8(2) O(4b)-Mn(3)-O(4c) 84.25(18) O(3a)-Mn(1)-O(1) 93.34(18) O( 3)-Mn(3)-O(10) 93.56(18) O(2)-Mn(1)-O(1) 93.8(2) O( 2)-Mn(3)-O(10) 91.64(18) O(11)-Mn(1)-O(1) 87.7(2) O(4b)-Mn(3)-O(10) 176.03(17) O(5)-Mn(1)-O(1) 81.7(2) O(4c)-Mn(3)-O(10) 92.99(17) O(7)-Mn(1)-O(1) 172.74(19) O(3)-Mn(3)-O(4) 172.64(17) O(3)-Mn(2)-O(2) 82.91(17) O(2)-Mn(3)-O(4) 99.87(17) O(3)-Mn(2)-O(6) 176.0(2) O(4b)-Mn(3)-O(4) 83.91(18) O(2)-Mn(2)-O(6) 97.27(19) O(4c)-Mn(3)-O(4) 78.54(18) O(3)-Mn(2)-O(12b) 95.08(19) O(10)-Mn(3)-O(4) 92.75(17) O(2)-Mn(2)-O(12b) 175.9(2) O(3)-Mn(3)-Mn(2) 41.67(12) O(6)-Mn(2)-O(12b) 84.5(2) O(2)-Mn(3)-Mn(2) 42.69(12) O(3)-Mn(2)-O(8) 91.34(19) O(4b)-Mn(3)-Mn(2) 96.05(12) O(2)-Mn(2)-O(8) 92.13(17) O( 4c)-Mn(3)-Mn(2) 138.89(12) O(6)-Mn(2)-O(8) 92.6(2) O(10)-Mn(3)-Mn(2) 87.90(13) O(12b)-Mn(2)-O(8) 91.46(19) O(4)-Mn(3)-Mn(2) 142.52(12) O(3)-Mn(2)-O(9) 85.37(17) O( 3)-Mn(3)-Mn(3a) 131.93(14) O(2)-Mn(2)-O(9) 84.44(17) O( 2)-Mn(3)-Mn(3a) 89.65(13) O(6)-Mn(2)-O(9) 90.70(19) O( 4b)-Mn(3)-Mn(3a) 42.80(12) O(12b)-Mn(2)-O(9) 91.9(2) O( 4c)-Mn(3)-Mn(3a) 86.12(11) O(8)-Mn(2)-O(9) 175.50(17) O( 10)-Mn(3)-Mn(3a) 134.29(13) O(3)-Mn(2)-Mn(3) 41.51(12) O(4)-Mn(3)-Mn(3a) 42.28(11) O(2)-Mn(2)-Mn(3) 42.00(12) Mn(2)-Mn(3)-Mn(3a) 121.28(5)

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348 Table A-16. Continued. O(3)-Mn(3)-Mn(3b) 86.97(13) O(4)-Mn(3)-Mn(3c) 39.12(11) O(2)-Mn(3)-Mn(3b) 133.35(14) Mn(2)-Mn(3)-Mn(3c) 177.25(4) O(4b)-Mn(3)-Mn(3b) 43.17(12) Mn(3a)-Mn(3)-Mn(3c) 58.018(17) O(4c)-Mn(3)-Mn(3b) 42.25(12) Mn (3b)-Mn(3)-Mn(3c) 58.018(17) O(10)-Mn(3)-Mn(3b) 134.60(13) Mn(3)-O(2)-Mn(1) 132.8(2) O(4)-Mn(3)-Mn(3b) 85.89(12) Mn(3)-O(2)-Mn(2) 95.31(17) Mn(2)-Mn(3)-Mn(3b) 119.24(5) Mn(1)-O(2)-Mn(2) 122.5(2) Mn(3a)1-Mn(3)-Mn(3b) 63.96(3) Mn(3)-O(3)-Mn(2) 96.82(18) O(3)-Mn(3)-Mn(3c) 136.36(13) Mn(3)-O(3)-Mn(1b) 133.8(2) O(2)-Mn(3)-Mn(3c) 138.73(13) Mn(2)-O(3)-Mn(1b) 128.0(2) O(4b)-Mn(3)-Mn(3c) 81.77(12) Mn(3a)-O(4)-Mn(3c) 94.96(17) O(4c)-Mn(3)-Mn(3c) 39.42(11) Mn(3a)-O(4)-Mn(3) 94.55(17) O(10)-Mn(3)-Mn(3c) 94.29(12) Mn(3c)-O(4)-Mn(3) 101.46(18)

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349 APPENDIX B LIST OF COMPOUNDS [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O (1) [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2) [Mn12O12(O2CMe)12(O3SPh)4(H2O)4] (3) [Mn12O12(O2CMe)8(O2PPh2)8(H2O)4] (4) [Mn4O4(O2PPh2)6] (5) [Mn7O8(O2CMe)(O2SePh)8(H2O)] (6) [Mn7O8(O2SePh)9(H2O)] (7) [Mn12O12(O2CPh)16(H2O)4] (8) [Mn4O4(O2AsMe2)6] (9) {[Mn4O4(O2AsMe2)6](NO3)}2 (10) [Mn16O8Ca4(O2CPh)8(O2AsMe2)28(NO3)4] (11) [Mn16O8Sr4(O2CPh)16(O2AsMe2)24] (12) [Mn12O12(O2CCH2But)16(H2O)4] (13) [Mn12O12(O2CPet)16(H2O)4] (14) [Mn12O12(O2CPet)16(MeOH)4] (15) [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] (16) [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] (17) [Mn4O2(O2CPet)6(bpy)2] (18) [Mn9O7(O2CCH2But)13(THF)2] (19) [Mn12O12(O2CC6F5)16(H2O)4] (20)

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350 (NMe4)[Mn12O12(O2CC6F5)16(H2O)4] (21) (NMe4)2[Mn12O12(O2CC6F5)16(H2O)4] (22) [Mn12O12(O2CC6F5)16(D2O)4] (23) (NMe4)[Mn12O12(O2CC6F5)16(D2O)4] (24) (NMe4)2[Mn12O12(O2CC6F5)16(D2O)4] (25) [Mn12O12(O2CCH2Br)16(H2O)4] (26)

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351 APPENDIX C PHYSICAL MEASUREMENTS Infrared spectra were recorded in the so lid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrophotome ter in the 400-4000 cm-1 range. Elemental analyses (C, H, and N) were performed at the in-house facilities of the University of Florida Chemistry Department. Electrochemical studies were performed under argon using a BAS model CV-50W voltammetric analyzer and a standard three-electrode assembly (glassy carbon working, Pt wire auxiliary, and Ag / Ag3I4 reference) with 0.1 M NBun4PF6 as supporting electrolyte. No IR compensation was employed. Quoted potentials are vs the ferrocene / ferrocenium couple, used as an internal standard. The scan rates for cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were 100 and 20 mV/s, respectively. Distilled solvents were employed, and the concentration of the complex was approximately 1 mM. 1H NMR spectra were obtained at 300 MHz on a Varian VXR-300 spectrometer, using protio-solvent signals as internal references. 19F NMR spectra were obtained at 282 MHz on a Varian VXR-300 spectrometer, using CFCl3 as an internal standard. Variable-temperature DC magneti c susceptibility data down to 1.80 K were collected on a Quantum Design MPMS-XL SQUID magnetometer equipped with a 70 kG (7 T) DC magnet at the University of Florida. Pascal’s constants were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibility to give the mola r magnetic susceptibility ( M). AC magnetic susceptibility data were collected on the same instrume nt employing a 3.5 G field oscillating at frequencies up to 1500 Hz. Samples were embe dded in solid eicosane, unless otherwise

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352 stated, to prevent torquing. Magnetization vs fi eld and temperature data were fit using the program MAGNET, and contour plots were obtained using the program GRID, both written at Indiana University by E. R. D. Low temperature (< 1.8 K) hysteresis loop and DC relaxation measurements were performe d at Grenoble using an array of microSQUIDS.65 The high sensitivity of this magnetometer allows the study of single crystals of SMMs on the order of 10-500 m. The field can be applied in any direction by separately driving three orthogonal coils. INS experiments were performed on the ti me-of-flight spectrometer NEAT at the Hahn-Meitner Institute (HMI) in Berlin using cold neutrons of wavelength = 6.0 . This resulted in a re solution of about 70 eV (fwhm) at the elastic line. The polycrystalline powder samples were placed under helium into a rectagular flat aluminum container of dimensions 43 61 5 mm3, with an accessible sample volume of 30 50 3 mm3 for 23 and 25 and 30 50 2 mm3 for 24. Data were collected at several temperatures between 1.8 and 20 K and corrected for the detector efficiency by means of the spectrum of vanadium metal. Data reduc tion was performed using the program INX. The background of each spectrum was estimated from a polynomial fit to the baseline of the low temperature spectra for each com pound and then subtracted. The analysis is limited to the neutron energy loss side due to the experimental setup which was chosen to have its best resolution on the neutron ener gy loss side. Therefore, transitions on the neutron energy gain side could not be resolved and usefully analyzed. 55Mn NMR measurements were made using a home-built MagRes2000 Integrated Wideband NMR spectrometer with quadrature detection and a home-built probe.202 The crystal samples were prepared by removing the crystal from its mother liquor and

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353 immediately covering in 5-minute epoxy to prev ent interstitial solven t loss. Immediately after the epoxy was set, the sample was mount ed into the coil and cooled in a dewar of liquid helium. Aligned powder samples were made by crushing the crystals and then mixing with Stycast 1266 epoxy, which was allowed to cure overnight at room temperature in a field of 8.5 T. Data was acquired at ~ 2 K, below the blocking temperature of ~ 3 K as the signal deteriorat es rapidly above the bl ocking temperature of because of short T2 times. A Hahn echo pulse sequence was used while the frequency was scanned from 220-400 MHz, usually taking 0. 1-0.2 MHz steps. Frequency scans were necessary because of the large peak widths (~ 5-20 MHz). Pulse lengths were on the order of hundreds of nanoseconds giving a ba ndwidth of roughly ~ 2 MHz. After the scan, the data was processed using a Fast Fourier Transform Sum (FFT Sum). For angular measurements of 26 in its ab plane, the crystal was cooled to 2 K in a field of 5 T. Crystal orientation was possibl e with a relative accuracy of 0.2. High frequency electron paramagnetic re sonance (HFEPR) measurements were performed at various frequencies, ranging from ~50 GHz to ~350 GHz by sweeping the magnetic field at fixed microwave frequencies and temperatures. A millimeter-wave vector network analyzer (MVNA) was used as a microwave source a nd the use of a high sensitivity cavity perturbation technique203 allowed the detection of the EPR signals. The measurements were performed using a commer cial superconducting so lenoid capable of producing fields of up to 9 T. In all cases, the temperature was st abilized ( 0.01 K) relative to a calibrated CernoxTM resistance sensor. We have outfitted our cavities with rotatable endplates for samp le orientation with 0.18 resolution.204 The single crystal was optimally positioned and oriented in the cav ity driven at the fundamental TE01n modes

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354 (n = integer), and the DC field was aligned ei ther along the easy-axis or in the hard plane of the sample. The field orientation relative to the easy-axis (or hard plane) is determined by performing angle-dependent HFEPR spectra. The hard plane angle-dependent study is also done by rotating the DC field in the hard plane. In order to avoid so lvent loss in the EPR measurements, the sample was removed fr om the mother liquor, immediately sealed in silicone grease, and quickly transferred to the cryostat (ca. 5 minutes) where it was cooled under atmospheric pressure helium gas.

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371 BIOGRAPHICAL SKETCH Born in the USA on February 24, 1978, Nicole E. Chakov received a Bachelor of Science degree from the University of Al abama in May 2000. During her undergraduate studies, she performed research in the group of Professor John B. Vincent primarily on the synthesis and characterization of a bi ologically relevant chromium-containing oligopeptide. In addition to her undergraduate research, she participated in a National Science Foundation Research Experiences fo r Undergraduates (NSF-REU) program and also completed an internship sponsored by the Howard Hughes Medical Institute. After the completion of her undergraduate degree, sh e joined the Department of Chemistry at Indiana University in August 2000. She joined the research group of Professor George Christou and, with her group and research ad visor, transferred to the University of Florida in August 2001. Her doctoral research primarily involves the preparation and the physical and magnetic characte rization of polynuclear transition metal complexes that function as single-molecule magnets.


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