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A Molecular Approach to Nanoscale Magnetic Materials

Permanent Link: http://ufdc.ufl.edu/UFE0041194/00001

Material Information

Title: A Molecular Approach to Nanoscale Magnetic Materials New Iron and Manganese Clusters from the Use of Pyridyl Alcohols
Physical Description: 1 online resource (245 p.)
Language: english
Creator: Taguchi, Taketo
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: iron, magnetism, manganese
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Polynuclear transition metal clusters have been of great interest primarily due to their relevance in molecular magnetism and bioinorganic chemistry, as well as their aesthetically pleasing structures. A particularly appealing area in molecular magnetism is the study of singlemolecule magnets (SMMs) which possess a significant energy barrier to relaxation of their magnetization vector and thus function as monodisperse, nanoscale magnetic particles below their blocking temperature. This work focuses on the preparation and characterization of polynuclear iron and manganese clusters as new nanoscale magnetic materials. Non-carboxylate FeIII cluster chemistry has been explored in the presence of the bidentate N,O ligand, the anion of 2-hydroxymethylpyridine (hmp-), and it has proven to be a useful route to new FeIII clusters spanning Fe4 and Fe6 nuclearities and topologies that are both very rare. Of these, the Fe6 complex displayed a counterintuitive S = 3 ground state spin value, and it was rationalized by semiemperical theoretical calculations using ZILSH methods. A non-carboxylate approach for FeIII cluster synthesis was further explored in the presence of the tridentate O,N,O group, 2,6-pyridinedimethanol (pdmH2), and it has led to the isolation of new Fe8 non-carboxylate cluster products with an unprecedented topology, and with an S = 0 ground state spin. The reactivity of these Fe8 clusters was explored, and this revealed that they transform to an Fe18 product under mild hydrolysis. The Fe18 product shows an unprecedented metal topology and an S = 4 ground state spin, and is the highest nuclearity non-carboxylate FeIII cluster discovered to date. Apart from the non-carboxylate FeIII chemistry, the reactions of pdmH2 in the presence of carboxylate were also explored, and led to the isolation of a new Fe9 product with a prototypical structure, and with a ground state spin of 5/2. The latter was rationalized on the basis of the exchange interactions predicted from magnetostructural correlations. The combined results demonstrate the versatility of pdmH2 to give new high nuclearity products, and show that the presence and absence of carboxylates can have a marked effect on the obtained products. In the development of new synthetic routes to polynuclear metal clusters, the choice of the ligands has always been a key issue. In order to provide a new ligand design strategy for the isolation of novel clusters, the introduction of two bulky phenyl or methyl groups onto the CH2 group of hmpH was investigated. The use of diphenyl-hmpH (dphmpH) has led to the isolation of Mn4, Mn6, and Mn11 products with very rare or prototypical structures, and with S = 0, 3, and 5/2 ground state spin values, respectively. These clusters are distinctly different from those obtained previously with hmpH itself, and it is also seen that dphmp- prefers to bind as a bidentate chelate, disfavoring the bridging modes favored by hmp-. It was concluded that the two phenyl groups had almost completely removed the ability of the alkoxide O atom to bridge, and thus dphmp- primarily functions as a bidentate chelate. This was rationalized as a combination of the steric bulk of the two phenyl groups, as well as their electron-withdrawing effect on the O atom. The less-bulky dimethyl-hmpH (dmhmpH) was then employed in order to switch back on the bridging modes as a result of both the smaller size of methyl groups and their electrondonating rather than electron-withdrawing character, while still hopefully providing sufficient steric differences with hmp- to lead to new products. This hope was fulfilled, with dmhmp- acting as a perturbed hmp-; that is, it functioned as a chelating and bridging ligand, as does hmp-, but it yielded products that are structurally distinct from any seen previously in Mn/hmp- chemistry. The resulting Mn/dmhmp- products, NaMn6, Mn7, and Mn12, show very rare or prototypical structures and significant ground state spin values of S = 12, 7/2, and 13/2, respectively. In addition, the Mn12 proved to be a new SMM, as confirmed by single-crystal micro-SQUID studies. As an expansion of this ligand design strategy, a potentially tridentate ligand phenyldipyridine-2-ylmethanol (pdpmH), in which one phenyl and one pyridyl groups were added onto the CH2 unit of hmpH, was employed and successfully led to novel Mn4 and Mn7 clusters. Interestingly, the potentially tridentate ligand pdpmH mainly acts as a bidentate ligand. The reluctance of pdpm- to serve as a tridentate ligand is found to be due to the nearby steric bulk of the phenyl and pyridyl groups. Magnetochemical characterization revealed that the Mn4 and Mn7 have ground state spin values of S = 0 and 29/2, respectively. The combined results demonstrate the usefulness and potential of a ligand design strategy in the synthesis of a variety of new metal clusters.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Taketo Taguchi.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Christou, George.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0041194:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041194/00001

Material Information

Title: A Molecular Approach to Nanoscale Magnetic Materials New Iron and Manganese Clusters from the Use of Pyridyl Alcohols
Physical Description: 1 online resource (245 p.)
Language: english
Creator: Taguchi, Taketo
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: iron, magnetism, manganese
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Polynuclear transition metal clusters have been of great interest primarily due to their relevance in molecular magnetism and bioinorganic chemistry, as well as their aesthetically pleasing structures. A particularly appealing area in molecular magnetism is the study of singlemolecule magnets (SMMs) which possess a significant energy barrier to relaxation of their magnetization vector and thus function as monodisperse, nanoscale magnetic particles below their blocking temperature. This work focuses on the preparation and characterization of polynuclear iron and manganese clusters as new nanoscale magnetic materials. Non-carboxylate FeIII cluster chemistry has been explored in the presence of the bidentate N,O ligand, the anion of 2-hydroxymethylpyridine (hmp-), and it has proven to be a useful route to new FeIII clusters spanning Fe4 and Fe6 nuclearities and topologies that are both very rare. Of these, the Fe6 complex displayed a counterintuitive S = 3 ground state spin value, and it was rationalized by semiemperical theoretical calculations using ZILSH methods. A non-carboxylate approach for FeIII cluster synthesis was further explored in the presence of the tridentate O,N,O group, 2,6-pyridinedimethanol (pdmH2), and it has led to the isolation of new Fe8 non-carboxylate cluster products with an unprecedented topology, and with an S = 0 ground state spin. The reactivity of these Fe8 clusters was explored, and this revealed that they transform to an Fe18 product under mild hydrolysis. The Fe18 product shows an unprecedented metal topology and an S = 4 ground state spin, and is the highest nuclearity non-carboxylate FeIII cluster discovered to date. Apart from the non-carboxylate FeIII chemistry, the reactions of pdmH2 in the presence of carboxylate were also explored, and led to the isolation of a new Fe9 product with a prototypical structure, and with a ground state spin of 5/2. The latter was rationalized on the basis of the exchange interactions predicted from magnetostructural correlations. The combined results demonstrate the versatility of pdmH2 to give new high nuclearity products, and show that the presence and absence of carboxylates can have a marked effect on the obtained products. In the development of new synthetic routes to polynuclear metal clusters, the choice of the ligands has always been a key issue. In order to provide a new ligand design strategy for the isolation of novel clusters, the introduction of two bulky phenyl or methyl groups onto the CH2 group of hmpH was investigated. The use of diphenyl-hmpH (dphmpH) has led to the isolation of Mn4, Mn6, and Mn11 products with very rare or prototypical structures, and with S = 0, 3, and 5/2 ground state spin values, respectively. These clusters are distinctly different from those obtained previously with hmpH itself, and it is also seen that dphmp- prefers to bind as a bidentate chelate, disfavoring the bridging modes favored by hmp-. It was concluded that the two phenyl groups had almost completely removed the ability of the alkoxide O atom to bridge, and thus dphmp- primarily functions as a bidentate chelate. This was rationalized as a combination of the steric bulk of the two phenyl groups, as well as their electron-withdrawing effect on the O atom. The less-bulky dimethyl-hmpH (dmhmpH) was then employed in order to switch back on the bridging modes as a result of both the smaller size of methyl groups and their electrondonating rather than electron-withdrawing character, while still hopefully providing sufficient steric differences with hmp- to lead to new products. This hope was fulfilled, with dmhmp- acting as a perturbed hmp-; that is, it functioned as a chelating and bridging ligand, as does hmp-, but it yielded products that are structurally distinct from any seen previously in Mn/hmp- chemistry. The resulting Mn/dmhmp- products, NaMn6, Mn7, and Mn12, show very rare or prototypical structures and significant ground state spin values of S = 12, 7/2, and 13/2, respectively. In addition, the Mn12 proved to be a new SMM, as confirmed by single-crystal micro-SQUID studies. As an expansion of this ligand design strategy, a potentially tridentate ligand phenyldipyridine-2-ylmethanol (pdpmH), in which one phenyl and one pyridyl groups were added onto the CH2 unit of hmpH, was employed and successfully led to novel Mn4 and Mn7 clusters. Interestingly, the potentially tridentate ligand pdpmH mainly acts as a bidentate ligand. The reluctance of pdpm- to serve as a tridentate ligand is found to be due to the nearby steric bulk of the phenyl and pyridyl groups. Magnetochemical characterization revealed that the Mn4 and Mn7 have ground state spin values of S = 0 and 29/2, respectively. The combined results demonstrate the usefulness and potential of a ligand design strategy in the synthesis of a variety of new metal clusters.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Taketo Taguchi.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Christou, George.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0041194:00001


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1 A MOLECULAR APPROACH TO NANOSCALE MAGNETIC MATERIALS: NEW IRON AND MANGANESE CLUSTERS FROM THE USE OF PYRIDYL ALCOHOLS By TAKETO TAGUCHI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PART IAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2009

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2 2009 Taketo Taguchi

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3 To my family

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4 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, insightful advice and constant encouragement have been invaluable in attaining my goal. He was always available for me to ask questions and seek advice He was always open to the bizarre ideas I often came up with and brought to him. He always encouraged me to do whatever I was interested in trying. His comments on my research projects or chemistry in general were always eye -opening, and I have said the phrase Oh, Ive never thought of that! a million times in the 5 years of my PhD. stud ies Additionally, his advice o n writing scientific papers and making effective presentations has been truly enlightening and I sincerely appreciate it. I learn ed from him to look at data from different perspectives, to speculate deeply about them, and to organize given information with comprehensive views. Furthermore, he often spent hours with me in person in the lab to teach me chemistry and techniques. After 5 years of his guidance, I finally feel that I started to understand chemistry slowly but surely. The f un I have now seems to be increasing exponentially. Thus it is indeed sad that it is time to graduate and leave his group, but wha t he taught me here, what he helped me to accomplish here, and all what we talked about here are definitely my treasures in my future endeavors. I would also like to express my gratitude to the other members of my committee, Professor Daniel. R Talham, Pr ofessor Nicolo Omenetto, Professor Adam S. Veige, and Professor Mark W. Meisel. I often asked Professor Veige about organic synthesis and techniques. He also gave me invaluable advice about applications for postdoctoral fellow positions. I truly appreciate h is time and thoughtful advice for me. I would also like to thank Dr. Khalil A. Abboud and his staff at University of Florida Center for X -Ray Crystallography for solving my crystal structures. I would like to acknowledge Dr. Wolfgang Wernsdorfer for prov iding essential single crystal

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5 measurements on the Mn12 compound below 1.8 K using his micro-SQUID apparatus, and Professor Stephen Hill and Changhyun Koo at the UF Physics Department for the HFEPR measurements. Special gratitude goes to Dr. Ted A. OBrien for performing semiempi rical calculations on my Fe compounds. I would also like to thank Dr. Ogawa who taught me organic synthesis. Thanks to his assis tance in the early stage of my P h D study, I became able to carry out multistep organic reactions witho ut much hesitation. I also express my sincerest gratitude to Dr. Mukaibo for her constant encouragement and insightful discussions. Thanks to her encouragement and support, I very much enjoyed my study at UF and my life in Gainesville. I had a really good time in Christou group thanks to all of its members past and present. I thank all secretaries, Sondra, Melinda, and Alice, who greatly assisted my research I thank all post docs past and present, Dr. Murugesu, Dr. King, Dr. Stamatatos, Dr. Liu, Dr. Papa triantafyllopoulou, Dr. Ethymiou, Dr. Maayan, and Dr. Singh for th eir great assistance and advice Special appreciation is given to Dr. Stamatatos. When I was trying to get initial results, he guided me and assisted me greatly. When I was preparing present ations and writing scientific reports, he spent a tremendous amount of time and energy to help me. Thanks to his great assistance in the early stage of my P h.D. study, I was able to get my study on track. I would also like to thank the graduate students in Christou group, past and present, John, Nicole, Abhu, Dolos, Alina, Rashmi, Chris, Antonio, Arpita, Shreya, Jennifer, Dina, Jenn, Andy, Emir, Katye, and Xun Gao. My appreciation also goes to the high school and undergraduate students with who m I worked G arrett, Mike, Matt, Alex, and Radhika. When they were struggling with their chemistry, I found myself struggling far harder for their success than my own. When they got their first crystals, it was more pleasing to me than getting results for myself. I was really fortunate to work with these wonderful people throughout m y study here. In this regard, I would

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6 also like to thank Professor Christou for giving me such w onderful opportunities. I learned really a lot from all Christou group members not only about chemistry but also about life in general. Everyone is in my treasured memory, and every minute I spent with them consists of my entire life. Finally, I would like to express my sincerest gratitude to my family, my father, mother, sister, grandfather, and grandmother for being always understanding and supportive. Their faith in me has been invaluable in every aspect of my life. I really appreciate that they raised me in a warm and lov ing family. Thanks to the same hearty and relaxing atmosphere, I always f elt happy and greatly energized when I came back to Japan during holidays. Because of my familys support, I was able to come this far. I am forever grateful for their love and support.

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7 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................................... 4 LIST OF TABLES .............................................................................................................................. 10 LIST OF FIGURES ............................................................................................................................ 12 LIST OF ABBREVIATIONS ............................................................................................................ 19 ABSTRACT ........................................................................................................................................ 20 CHAPT ER 1 GENERAL INTRODUCTION .................................................................................................. 23 2 NEW TETRA AND HEXANUCLE AR IRON CLUSTERS FROM THE USE OF 2 (HYDROXYMETHYL)PYRIDINE .......................................................................................... 32 2.1 Introduction ....................................................................................................................... 32 2.2 Experimental Section ........................................................................................................ 34 2.2.1 Syntheses ............................................................................................................... 34 2.2.2 X Ray Crystallography ......................................................................................... 35 2.2.3 Other Studies ......................................................................................................... 37 2.2.4 Theoretical Calculations ....................................................................................... 37 2.3 Results and Discussion ..................................................................................................... 38 2.3.1 Syntheses ............................................................................................................... 38 2.3.2 Description of Structures ...................................................................................... 40 2.3.3 Magnetochemistry ................................................................................................ 43 2.3.3.1 Direct current magnetic susceptibility studies ............................................ 43 2.3.3.2 Alternating current magnetic susceptibility studies .................................... 46 2.3.3.3 Rationalization of the S = 3 ground state of Complex 2 1 ......................... 48 2.4 Conclusions ....................................................................................................................... 51 3 UNUSUAL FE8, FE9, AND FE18 STRUCTURAL TYPES FROM THE USE OF 2,6 PYRIDINEDIMETHANOL ....................................................................................................... 67 3.1 Introduction ....................................................................................................................... 67 3.2 Experimental Section ........................................................................................................ 69 3.2.1 Syntheses ............................................................................................................... 69 3.2.2 X ray Crystallography .......................................................................................... 71 3.2.3 Other Studies ......................................................................................................... 72 3.3 Results and Discussion ..................................................................................................... 73 3.3.1 Syntheses ............................................................................................................... 73 3.3.2 Description of Structures ...................................................................................... 76 3.3.3 Magnetochemistry ................................................................................................ 79

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8 3.3.3.1 Direct current magnetic susceptibility studies ............................................ 79 3.3.3.2 Alternating current magnetic susceptibility studies .................................... 81 3.3.3.3 Rationalization of the S = 5/2 Ground State of Complex 3 4 .................... 82 3.3.3.4 Single -Crystal, High Frequency EPR (HFEPR) Spectroscopy .................. 84 3.4 Conclusions ....................................................................................................................... 87 4 MN CLUSTERS FROM THE USE OF BULKY PYRIDYLALCOHOLS: STRUCTURAL AND MAGNETIC STUDIES ..................................................................... 105 4.1 Introduction ..................................................................................................................... 105 4.2 Experimental Section ...................................................................................................... 107 4.2.1 Syntheses ............................................................................................................. 107 4.2.2 X ray Crystallography ........................................................................................ 111 4.2.3 Other Studies ....................................................................................................... 113 4.3 Results and Discussion ................................................................................................... 114 4.3.1 Syntheses ............................................................................................................. 114 4.3.1.1 Reactions with dphmpH ............................................................................. 114 4.3.1.2 Reactions with dmhmpH ............................................................................ 116 4.3.2 Description of Structures .................................................................................... 117 4.3.3 Magnetochemist ry .............................................................................................. 124 4.3.3.1 Direct current magnetic susceptibility studies .......................................... 124 4.3.3.2 Alternating current magnetic susceptibility studies .................................. 131 4.3.3.3 Hysteresis Studies below 1.8 K. ................................................................. 134 4.3.4 Struct ural Comparison of hmp-, dmhmp-, and dphmpMnx Products. ............ 135 4.4 Conclusions ..................................................................................................................... 137 5 NEW MN4 AND MN7 CLUSTERS FROM THE USE OF PHENYLDIPYRIDIN 2 YLMETHANOL ....................................................................................................................... 168 5.1 Introduction ..................................................................................................................... 168 5.2 Experimental Section ...................................................................................................... 170 5.2.1 Syntheses ............................................................................................................. 170 5.2.2 X Ray Crystallography ....................................................................................... 172 5.2.3 Other Studies ....................................................................................................... 173 5.3 Results and Discussion ................................................................................................... 174 5.3.1 Syntheses ............................................................................................................. 174 5.3.2 Description of Structures .................................................................................... 176 5.3.3 Electrochemistry ................................................................................................. 178 5.3.4 Magnetochemistry ................................................................................................. 179 5.3.4.1 Direct Current Magnetic susceptibility studies ............................................ 179 5.3.4.2 Alternating Current Magnetic susceptibility studies ................................... 181 5.4 Conclusions ......................................................................................................................... 182 APPENDIX A BOND DISTANCES AND ANGLES ..................................................................................... 196

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9 B LIST OF COMPOUNDS .......................................................................................................... 212 C VAN VLECK EQUATIONS ................................................................................................... 213 LIST OF REFERENCES ................................................................................................................. 225 BIOGRAPHICAL SKETCH ........................................................................................................... 245

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10 LIST OF TABLES Table page 2 1 Crystallographic Data for 2 -1 MeCN, 2 -2 6MeCN and 2 -3 2MeOH. ............................... 53 2 2 Calculated ground states ST of complex 2 -1 with various J23 values. ................................ 54 3 1 Crystallographic Data for 3 -1 7MeOH, 3 -3 7MeCNH2O and 3 -4 7CH2Cl2. .................... 89 3 2 Bond Valence Sums for Selected O Atoms in Complex 3 -3 .............................................. 90 3 3 Bond Valence Sums for Selected O Atoms in Complex 3 -4 .............................................. 90 4 1 Crystallographic Data for 4 -1 MeCN, 4 -2 3MeCN and 4 -3 4MeCN. ............................ 138 4 2 Crystallographic Data for 4 -4 7MeCN, 4 -5 3CH2Cl2 and 4 -6 Et2O. ................................. 139 4 3 Bond Valence Sums for the Mn Atoms in Complex 4 -1 ................................................... 140 4 4 Bond Valence Sums for the Mn Atoms in Complex 4 -2 ................................................... 140 4 5 Bond Valence Sums for the Mn Atoms in Complex 4 -3 ................................................... 140 4 6 Bond Valence Sums for the Mn Atoms in Complex 4 -4 ................................................... 141 4 7 Bond Valence Sums for the Inorga nic Oxygen Atoms in Complex 4 -4 .......................... 141 4 8 Bond Valence Sums for the Mn Atoms in Complex 4 -5 ................................................... 142 4 9 Bond Valence Sums for the Inorga nic Oxygen Atoms in Complex 4 -5 .......................... 142 4 10 Complexes with hmp-, dmhmp-, or dphmp-, and the Alkoxide O Atom Binding Mode 143 5 1 Crystallographic Data for 5 -1 0.5MeCN and 5 -2 2MeCN. ............................................... 184 5 2 Bond Valence Sums for the Mn Atoms in Complex 5 -1 ................................................... 185 5 3 BVS for Selected O Atoms in 5 -1 ....................................................................................... 185 A 1 Selected interatomic distances () and angles () for [Fe6O2(hmp)10(H2O)2](NO3)48MeCN ( 2 -1 8MeCN) ........................................................ 196 A 2 Selected interatomic distances () and angles () for [Fe6O2(hmp)8(NO3)4(H2O)2](NO3)26M eCN (2 -2 6MeCN) .............................................. 197 A 3 Selected interatomic distances () and angles () for [Fe4(N3)6(hmp)6]2MeOH ( 2 3 2MeOH) ................................................................ ............................................................. 198

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11 A 4 Selected inte ratomic distances () and angles () for [Fe8O3(OMe)(pdm)4(pdmH)4(MeOH)2](ClO4)5 7MeOH ( 3 -1 7MeOH) .......................... 199 A 5 Selected interatomic distances () and angles () for [Fe18O6(OH)8(pdm)10(pdmH)4(H2O)4]( ClO4)107MeCN3H2O ( 3 -3 7MeCN3H2O) ...... 200 A 6 Selected interatomic distances () and angles () for [Fe9O4(OH)2(O2CMe)10(pdm)(pdmH)4](NO3)7CH2Cl2 (3 -4 7CH2Cl2) .......................... 202 A 7 Selected interatomic distances () and angles () for [Mn4O2(O2CBut)5(dphmp)3] 2MeCN ( 4 -1 2MeCN) ......................................................... 203 A 8 Selected interatomic distances () and angles () for [Mn6O4(OMe)2(O2CPh)4(dphmp)4] 3MeCN ( 4 -2 3MeCN). ............................................. 204 A 9 Selected interatomic distances () and angles () for [Mn11O7(OMe)7(O2CPh)7(dphmp)4(MeOH)2] 4MeCN ( 4 -3 4MeCN). ............................ 205 A 10 Selected interatomic distances () and angles () for [Mn7O3(OH)3(O2CBut)7(dmhmp)4] 7MeCN (4 -4 7MeCN ) .............................................. 206 A 11 Selected interatomic distances () and angles () for [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] 3CH2Cl2 (4 -5 3CH2Cl2) ..................... 207 A 12 Selected interatomic distances () and angles () for (HNEt3)[NaMn6O4(dmhmp)4(N3)4](ClO4)2 Et2O ( 4 -6 Et2O). ........................................... 209 A 13 Selected interatomic distances () and angle s () for [Mn4O4(O2CMe)3(pdpm)3] 0.5MeCN ( 5 -1 0.5MeCN) ...................................................... 210 A 14 Selected interatomic distances () and angles () for [Mn7O4(pdpm)6(N3)4](ClO4)2 2MeCN ( 5 -2 2MeCN) ........................................................ 211

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12 LIST OF FIGURES Figure page 1 1 Representations of magnetic dipole arrangements in (i) paramagnetic, (ii) ferromagnetic, (iii) antiferromagnetic, and (iv) ferrimagnetic materials. ........................... 29 1 2 Typical hysteresis loop of a magnet, where M is magnetization, H is the applied magnetic field and Ms is the saturation value of the magnetization. .................................. 29 1 3 Representation of (a) the [Mn12O12]16+ core and (b) the [Mn12O12(O2CMe)16(H2O)4] complex with peripheral ligation. MnIV orange; MnIII green; O red; C gray. .................... 29 1 4 Rep resentative plots of the potential energy versus (a) the orientation of the ms 12 complex with an S = 10 ground state, experiencing zero-field splitting. ............................................................... 30 1 5 In -MT) and out -of M microcrystalline sample of [Mn12O12(O2CR)16(H2O)4] at the indicated oscillation frequencies. ............................................................................................................................. 30 1 6 Magnetization hysteresis 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. .............................................................................................................. 31 1 7 Representation of the change in energy of ms sublevels as the magnetic field is swept from zero to a non-zero value. Resonant magnetization tunneling occurs when the ms sublevels are aligned between the two halves of the diagram. ............................................ 31 2 1 The structure of 2 (hydroxymethyl)pyridine (hmpH). ......................................................... 54 2 2 The structure of complex 2 -1 (top) and a stereopair (bottom). Hydrogen atoms hav e been omitted for clarity. Color code: FeIII green; O red; N, blue; C grey. ......................... 55 2 3 The fully labeled core of complex 2 -1 Color code: FeIII green; O red. ............................. 56 2 4 The structure of complex 2 -2 (top) and a stereopair (bottom). Hydrogen atoms have been omitted for clarity. Color code: FeIII green; O red; N, blue; C grey. ......................... 57 2 5 The structure of complex 2 -3 (top), a stereopair (bottom), and the labeled core. Hydrogen atoms have been omitted for clarity. Color code: FeIII green; O red; N, blue; C grey. ........................................................................................................................... 58 2 6 Plot of MT vs T for complex 2 -1 ........................................................................................ 59 2 7 Plot of MT vs T for complex 2 -2 ........................................................................................ 59

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13 2 8 Plot of MT vs T for complex 2 -3 The solid line is the fit of the data; see the text for the fit parameters. ................................................................................................................... 60 2 9 B) vs H/T plot for complex 2 -1 The solid lines are the fit of the data; see the text for the fit parameters. ................................................................. 61 2 10 Two -dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 2 -1 ................................................................................................................ 61 2 11 Reduced magnetiz B) vs H/T plot for complex 2 -2 The solid lines are the fit of the data; see the text for the fit parameters. ................................................................. 62 2 12 Two -dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 2 -2 ................................................................................................................ 62 2 13 In -phase ac susceptibility ( M M2 -1 in a 3.5 Oe ac field oscillating at the indicated frequencies. ....................................................................... 63 2 14 In -phase ac susceptibility ( M M2 -2 in a 3.5 Oe ac field oscillating at the indicated frequencies. ....................................................................... 63 2 15 Sc heme presenting a continuum of spin coupling values and spin alignments between completely satisfied situation and completely frustrated situation for an antiferromagnetically coupled pathway: see the text for details. ........................................ 64 2 16 Diagrammatic scheme of the core structures of complex 2 -1 presenting the ZILSH exchange constants J and the spin coupling A B for each Fe -Fe pathway and rationalizing the S =3 ground state of 2 -1 : see the text for details. ..................................... 65 2 17 Mapping of the possible ST values of 2 -1 with various J13 and J23 values, presenting the influence of lowering of the symmetry of the triangular unit on the spin of the ground state. ............................................................................................................................ 66 3 1 Structure of ligand: 2,6 -pyridinedimethanol (pdmH2). ........................................................ 90 3 2 Summary of the reactions concerning 3 -1 3 -2 and 3 -3 ...................................................... 91 3 3 The structure of complex 3 -1 (top), a stereopair (middle), and the labeled core. Hydrogen atoms have b een omitted for clarity. Color code: FeIII green; O red; N blue; C gray. ........................................................................................................................... 92 3 4 The structure of complex 3 -3 (top), a stereopair (middle), and the labeled core. Hydrogen atoms have been omitted f or clarity. Color code: FeIII green; O red; N blue; C gray. ........................................................................................................................... 93 3 5 The structure of complex 3 -4 (top), and a stereopair (bottom), viewed along the ab plane. Hydrogen atoms have been omitted for c larity. Color code: FeIII green; O red; N blue; C grey. ....................................................................................................................... 94

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14 3 6 The structure of complex 3 -4 (top), and its labeled core (bottom), viewed along the b axis. Hydrogen atoms have been omitted for cla rity. Color code: FeIII green; O red; N blue; C grey. ....................................................................................................................... 95 3 7 Plot of MT vs T for complex 3 -1 ........................................................................................ 96 3 8 Plot of MT vs T for complex 3 -3 ........................................................................................ 96 3 9 Plot of MT vs T for complex 3 -4 ........................................................................................ 97 3 10 B) vs H/T plot for complex 3 -3 The solid lines are the fit of the data; see the text for the fit parameters. ................................................................. 97 3 11 Two -dim ensional contour plot of the root -mean -square error surface for the D vs g fit for complex 3 -3 ................................................................................................................ 98 3 12 3 -4 The solid lines are the fit of t he data; see the text for the fit parameters. ........................................................... 98 3 13 Two -dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 3 -4 ..................................................................................................................... 99 3 14 In -phase ac susceptibility ( M M3 -3 in a 3.5 Oe ac field oscillating at the indicated frequencies. ....................................................................... 99 3 15 In -phase ac susceptibility ( M M3 -4 in a 3.5 Oe ac field oscillating at the indicated frequencies. ..................................................................... 100 3 16 Rationalization of the S = 5/2 ground state of 3 -4 on the basis of the predicted magnitudes of the various pairwise Jij exchange const ants and the resulting spin frustration effects; frustrated interactions are shown in blue. The viewpoint and atom labels are those of Figure 3 6 (bottom). .............................................................................. 101 3 17 (a) Angle -dependent HFEPR spec tra (2 steps) for complex 3 -4 obtained at 8 K and 91.3 GHz. The red traces correspond to the hard planes of two crystals in the twinned sample (see main text). (b) Plot of the peak positions associated with the highest field doublet observed in (a); see m ain text for explanation. ..................................................... 102 3 18 91.3 GHz temperature dependent HFEPR spectra for complex 3 -4 with the applied field aligned closer to the easy axes of the two crystals in comparison to the dat a in Figure 3 crystals in the twinned sample; for some of the peaks, subscripts are included with the label indicating the approximate magnitude of ms associated with the level from w hich the transition is excited. ............................................................................................ 103 3 19 Frequency dependence of the HFEPR peak positions for complex 3 -4 corresponding to the same field orientation as the spectra displayed in Figure 3 17. The red and blue

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15 blue curves represent the corresponding best simulations of the frequency dependence; see text for further explanation. ..................................................................... 104 4 1 Structure of ligands: 2 (hydroxymethyl)pyridine (hmpH), 2 -(pyridine 2 -yl)propan 2 ol (dmhmpH), and diphenyl (pyridine 2 -yl)methanol (dphmpH). .................................... 143 4 2 A picture of crysta ls of 4 -1 ................................................................................................. 144 4 3 A picture of crystals of 4 -2 ................................................................................................. 144 4 4 A picture of crystals of 4 -3 ................................................................................................. 144 4 5 A picture of crystals of 4 -4 ................................................................................................. 145 4 6 A picture of crystals of 4 -5 ................................................................................................. 145 4 7 A picture of crystals of 4 -6 ................................................................................................. 145 4 8 The structure of complex 4 -1 with core Mn O bonds shown in purple. Hydrogen atoms and methyl groups on pivalates have been omitted for clarity. Color code: MnIII green; O red; N blue; C gray. ..................................................................................... 146 4 9 The structure of complex 4 -8 with core Mn O bonds shown in purple. Hydrogen atoms and methyl groups on pivalates have been omitted for clarity. Color code: MnIII green; O red; N blue; C gray. ..................................................................................... 146 4 10 The structure of complex 4 -2 with the MnIII Jahn Teller elongation axes shown as yellow bonds (top), and a stereopair (bottom). Hydrogen atoms have been omitted for clarity. Color code: MnIII green; O red; N blue; C gray. .............................................. 147 4 11 The structure of complex 4 -3 with intramolecular hydrogen-bonds shown as dashed lines (top), and a stereopair (bottom). Hydrogen atoms and benzo ate phenyl rings (except for the ipso carbon atoms) have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; N blue; C gray. ....................................................................... 148 4 12 The fully labeled core of complex 4 -3 Col or code: MnII yellow; MnIII green; O red; C gray. ................................................................................................................................... 149 4 13 The structure of complex 4 -4 with intramolecular hydrogen-bonds shown as dashed lines (top), and a stereopair The thicker orange bonds i ndicate the positions of the MnIII Jahn Teller elongation axes. Hydrogen atoms and methyl groups on pivalate groups have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; N blue; C grey. ......................................................................................................................... 150 4 14 The fully labeled core of complex 4 -4 Color code: MnII yellow; MnIII green; O red. .... 151 4 15 The structure of complex 4 -5 with intramolecular hydrogen-bonds shown as dashed lines (top), and a stereopair (bottom). Hydrogen atoms and phenyl rings (except for

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16 the ipso carbon atoms) on benzoate groups have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; C gray; N blue. ............................................................... 152 4 16 The fully labeled core of complex 4 -5 Color code: MnII yellow; MnIII green; O red; C gray. ................................................................................................................................... 153 4 17 The structure of complex 4 -6 (top), a stereopair (middle), and the labeled core (bottom). The thicker orange bonds indicate the positions of the MnIII Jahn Teller elongation axes. Hydrogen atoms have been omitted for clarity. Color code: MnIII green; O red; C gray; N blue; Na Purple. ........................................................................... 154 4 18 1 D chain of complex 4 -6 viewed perpendicular (top) and parallel (bottom) to c axis. Hydrogen atoms have been omitted for clarity. MnIII green; O red; C gray; N blue; Na Purple .............................................................................................................................. 155 4 19 Plots of MT vs. T for complexes 4 -1 ................................................................................ 156 4 20 Plots of MT vs. T for complexes 4 -2 ................................................................................ 156 4 21 Plots of MT vs. T for complexes 4 -3 ................................................................................ 157 4 22 Plots of MT vs. T for complexes 4 -4 ................................................................................ 157 4 23 Plots of MT vs. T for complexes 4 -5 ............................................................................... 158 4 24 Plots of MT vs. T for complexes 4 -6 The solid line is the fit of the data; see the text for the fit parameters ............................................................................................................ 158 4 25 Mn labeling scheme employed in eq.4 7. ........................................................................... 159 4 26 Plots of reduced magnetization (M/N B) vs H/T for complex 4 -5 The solid lines are the fit of the data; see the text for the fit parameters. ......................................................... 160 4 27 Two -dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 4 -5 .............................................................................................................. 160 4 28 Plots of reduced magnetization (M/N B) vs H/T for complex 4 -6 The solid lines are the fit of the data; see the text for the fit parameters. ......................................................... 161 4 29 Two -dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 4 -6 .............................................................................................................. 161 4 30 AC susceptibility of complex 4 -2 in a 3.5 G field oscillating at the indicated frequencies: (top) in -phase signal ( M M -of phase signal M vs T. ....................................................................................................... 162

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17 4 31 AC susceptibility of complex 4 -3 in a 3.5 G field oscillating at the indicated frequencies: (top) in -phase signal ( M M -of phase signal M vs T. ....................................................................................................... 163 4 32 In -phase ac susceptibility ( M M4 -4 in a 3.5 G ac field oscillating at the indicated frequencies. ..................................................................... 164 4 33 AC susceptibility of complex 4 -5 in a 3.5 G field oscillating at the indicated frequencies: (top) in -phase signal ( M M -of phase signal M vs T. ....................................................................................................... 165 4 34 In -phase ac susceptibility ( M M4 -6 in a 3.5 G ac field oscillating at the indicated frequencies. ..................................................................... 166 4 35 Single -crystal magnetization (M) vs dc field (H) hysteresis loops for a single crystal of 4 -5 3CH2Cl2 at different scan rates (top) and temperatures (bottom). ......................... 167 5 1 Structure of ligands: 2 (hydroxymethyl)pyridine (hmpH) and phenyldipyridin 2 ylmethanol (pdpmH). ........................................................................................................... 185 5 2 The structure of complex 5 -1 (top), and a stereopair (bottom). The thicker yellow bonds i ndicate the positions of the MnIII Jahn Teller elongation axes. Hydrogen atoms have been omitted for clarity. Color code: MnIII green; MnIV orange; O red; N blue; C grey. ......................................................................................................................... 186 5 3 Space -filling diagram of 5 -1 ; sideview (top), topview (middle), and a stereoview (bottom), emphasizing the sterically hindered positioning of the unbound pyridine groups. Color code: MnIII green; MnIV orange; O red; N blue; C grey. ........................... 187 5 4 The structure of complex 5 -2 (top), and a stereopair (bottom). Hydrogen atoms have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; N blue; C grey. 188 5 5 The labeled core of complex 5 -2 Color code: MnII yellow; MnIII green; O red; C grey. ....................................................................................................................................... 189 5 6 Cyclic voltammogram at glassy carbon electrode of complex 5 -1 in CH2Cl2 containing 0.1M N Bun 4PF6 and ferrocene as an internal standard. ................................... 190 5 7 Cyclic voltammogram of 5 -1 at different scan rates. ......................................................... 191 5 8 Plot of the square root of the scan rate, 1/2 vs the anodic peak current, ia for the oxidation process in Figure 5 -7 .......................................................................................... 191 5 9 Plot of MT vs T for complex 5 -1 The solid line is the fit of the data; see the text for the fit parameters. ................................................................................................................. 192 5 10 Plot of MT vs T for complex 5 -2 ...................................................................................... 192

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18 5 11 Plots of reduced magnetization (M/N B) vs H/T for complex 5 -2 The solid lines are the fit of the data; see the text for the fit parameters .......................................................... 193 5 12 Two -dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 5 -2 .............................................................................................................. 194 5 13 In -phase ac susceptibility ( M M5 -2 in a 3.5 Oe ac field oscillating at the indicated frequencies. ..................................................................... 195

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19 LIST OF ABBREVIATION S But tertiary butyl BVS bond valence sun CV cyclic voltammogram dphmpH diphenyl ( pyridine 2 -yl)methanol dmhmpH 2 (pyridine 2 yl)propan2 -ol hmpH 2 hydroxymethyl pyridine HFEPR high Frequency electron paramagnetic resonance pdmH2 2,6 pyridine dimethanol pdpmH phenyldipyridin 2 ylmethanol Py pyridine TIP temperature indepen dent paramagnetism ZFS zero -field splitting

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20 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy A MOLECULAR APPROACH TO NANOSCALE MA GNETIC MATERIALS: NEW IRON AND MANGANESE CLUSTERS FROM THE USE OF PYRIDYL ALCOHOLS By Taketo Taguchi December 2009 Chair: George Christou Major: Chemistry Polynuclear transition metal clusters have been of great interest primarily due to their relevanc e in molecular magnetism and bio inorganic chemistry as well as their aesthetically pleasing structures. A particularly appealing area in molecular magnetism is the study of single molecule magnets (SMMs) which possess a significant energy barrier to relaxation of their magnetization vector and thus function as monodisperse, nanoscale magnetic particles below their blocking temperature. This work focuses on the preparation and characterization of polynuclear iron and manganese clusters as new nanoscale magn etic materials. Non -carboxylate FeIII cluster chemistry has been explored in the presence of the bidentate N,O ligand, the anion of 2 -hydroxymethylpyridine (hmp-), and it has proven to be a useful route to new FeIII clusters spanning Fe4 and Fe6 nuclearit ies and topologies that are both very rare. Of these, the Fe6 complex displayed a counterintuitive S = 3 ground state spin value, and it was rationalized by semiemperical theoretical calculations using ZILSH methods A non -carboxylate approach for FeIII c luster synthesis was further explored in the presence of the tridentate O,N,O group, 2,6 pyridinedimethanol (pdmH2), and it has led to the isolation of new Fe8 non-carboxylate cluster products with an unprecedented topology, and with an S = 0 ground state spin. The reactivity of these Fe8 clusters was explored, and this revealed that they

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21 transform to an Fe18 product under mild hydrolysis. The Fe18 product shows an unprecedented metal topology and an S = 4 ground state spin and is the highest nucl e arity no n -carboxylate FeIII cluster discovered to date. Apart from the non-carboxylate FeIII chemistry, the reactions of pdmH2 in the presence of carboxylate were also explored, and led to the isolation of a new Fe9 product with a prototypical structure, and with a ground state spin of 5/2. The latter was rationalized on the basis of the exchange interactions predicted from magnetostructural correlations. The combined results demonstrate the versatility of pdmH2 to give new high nuclearity products, and show that t he presence and absence of carboxylates can have a marked effect on the obtained products. In the development of new synthetic routes to polynuclear metal cluste rs, the choice of the ligands has always been a key issue. In order to provide a new ligand des ign strategy for the isolation of novel clusters, the introduction of two bulky phenyl or methyl groups onto the CH2 group of hmpH was investigated. T he use of diphenyl -hmpH (dphmpH) has led to the isolation of Mn4, Mn6, and Mn11 products with very rare or prototypical structures, and with S = 0, 3, and 5/2 ground state spin values, respectively. These clusters ar e distinctly different from those obtained previously with hmpH itself and it is also seen that dphmpprefers to bind as a bidentate chelate, di sfavoring the bridging modes favored by hmp-. It was concluded that the two phenyl groups had almost completely removed the ability of the alkoxide O atom to bridge, and thus dphmpprimarily functions as a bidentate chelate. This was rationalized as a com bination of the steric bulk of the two phenyl groups, as well as their electron -withdrawing effect on the O atom. The less -bulky dimethyl -hmpH (dmhmpH) was then employed in order to switch back on the bridging modes as a result of both the smaller size of methyl groups and their electrondonating rather than electron -withdrawing character, while still hopefully providing sufficient

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22 steric differences with hmpto lead to new products. This hope was fulfilled, with dmhmpacting as a perturbed hmp-; that is, it function ed as a chelating a nd bridging ligand, as does hmp-, but it yielded products that are structurally distinct from any seen previously in Mn/hmpchemistry. The resulting Mn/dmhmpproducts, NaMn6, Mn7, and Mn12, show very rare or prototypical st ructures and significant ground state spin values of S = 12, 7/2, and 13/2, respectively In addition, the Mn12 proved to be a new SMM, as confirmed by single -crystal micro SQUID studies. As an expansion of this ligand design strategy, a potentially tridentate ligand phenyldipyridine 2 ylmethanol (pdpmH), in which one phenyl and one pyridyl groups were added onto the CH2 unit of hmpH, was employed and successfully led to novel Mn4 and Mn7 clusters Interestingly, the potentially tridentate ligand pdpmH main ly acts as a bidentate ligand. The reluctance of pdpmto serve as a tridentate ligand is found to be due to the nearby steric bulk of the phenyl and pyridyl groups. Magnetochemical characterization revealed that the Mn4 and Mn7 have ground state spin valu es of S = 0 and 29/2, respectively. The combined results demonstrate the usefulness and potential of a ligand design strategy in the synthesis of a variety of new metal clusters.

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23 CHAPTER 1 GENERAL INTRODUCTION Coordination chemistry is of interest from a variety of disciplines, including bioinorganic chemistry, supramolecular chemistry, and material sciences. This thesis work focuses on the preparation and characterization of new iron and manganese complexes especially with respect to their relevance in magnetic materials research. Magnetism is one of the fundamental properties of matter, and the utilization of magnetic materials has been central to technological development.1 The magnetic field associated with a magnetic substance is the result of an elec trical charge in motion, specifically, 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 substa nces, 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 m oment are called diamagnets.2, 3 Hence, the crucial element that distinguishes a magne tic substance, or a 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 are grouped according to their response or etic field. The electron pairs of a diamagnet interact with an applied field, generating a repulsive field that weakly repels the diamagnet from the applied ld; unpaired electrons in the material as well as the nature of the interactions of its electron spins.1, 46 Both the temperature dependence as well as the absolut measures of the various types of paramagnetism.6 Simple paramagnetic behavior is observed in

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24 substances in which the magnetic moments of unpaired electrons are independent of each other. In the absence of a magnetic fiel d, individual magnetic moments are randomly oriented. As a field is applied, the moments align parallel, albeit weakly, to the field; this alignment is opposed by the randomizing effect of thermal energy (Figure 1 1). The susceptibility of these materials is here C is the Curie constant.27 There are other paramagnetic materials, however, that display a temperature dependence unlike that of a simple paramagnet. In these substances the individual magnetic moments interact so that all the unpaired electrons align in the same direction: such species are called ferromagnets In contrast, some materials contain unpaired electrons interacting in such a way that the spins align in an ant iparallel fashion (i.e., pointing in opposite directions): these materials are called antiferromagnets, if the individual spins all have the same magnitude and there is no net magnetic moment, or ferrimagnets, if the interacting spins have different magnit udes and thus there is a net magnetic moment.1 7 Examples of ferromagnets include iron, cobalt, nickel, and several rare earth metals and their alloys, while magnetite, Fe3O4, is a ferrimagnet. Ferro -, antiferro and ferrimagnetic ordering occurs below a critical temperature, T c. Below T c, the magnetic moments for ferro and ferrimagnets align in small domains. In the absence of an applied magnetic field, despite the nature of interactions, a net zero magnetization is thermodynamically favored, as differe nt domains have their net magnetizations randomly oriented. The application of a strong magnetic 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 interaction of spins becomes sufficiently strong to overcome dipole interactions and entropy considerations that maintain the random alignment of the domains.5, 7

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25 When a magnetic field is applied and then removed at a temperature below T c, the magnet ization induced by the field does not entirely disappear and in some cases can remain equal to the field induced magnetization. This is in contrast to the behavior observed for paramagnetic systems, in which the spins immediately randomly reorient followin g removal of the field. Because an additional field is required to reverse the direction of the magnetization, magnetic storage of information is possible in particles of ferromagnetic or ferrimagnetic materials. This behavior is observed in the classical hysteresis behavior of magnets (Figure 1 2).4, 6, 7 In order to achieve the storage of greater quantities of digital information on smaller surface areas, the development of magnetic particles of nanoscale dimensions is necessary. One approach towards thi s 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 particl es 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 remnant m agnetization. The reversal of the magnetization direction of a single domain requires energy to overcome the crystal field anisotropy. Hence, slow magnetization relaxation is not related to domain formation as with a traditional ferromagnet but rather invo lves an energy barrier that arises in part from the magnetic anisotropy associated with the shape of the particles. Providing no control in size versus properties and a nonuniform response to an applied field, the major drawback of this approach is the wi de distribution of particle sizes that results from fragmentation. Another approach toward the preparation of nanoscale magnetic materials is called bottom up approach, which is based on the syntheses of molecule -based magnets. In 1967, it was

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26 discovered that the compound [FeIII(S2CNEt2)Cl] behaves as a ferromagnet at 2.46 K due to intermolecular interactions.79 Since then, chemists have been synthesizing a wide variety of such molecule -based magnets using paramagnetic molecular units. They are 1 2 or 3 D lattices of interacting molecular unit s, which are synthesized from single molecules and selected bridging groups. Advantages of this approach include low density, low temperature processability, solubility, and others.1011 A very specific case with in the area of molecule -based magnets is the zero -dimensional nanoscale magnets, called single-molecule magnets (SMMs), which are individual molecules capable of functioning as na noscale magnetic particles .12, 13 Such molecules behave as magnets below thei r blocking temperature ( TB), exhibiting hysteresis in magnetization versus dc field scans. The first molecule proven to show such behavior was [Mn12O12(O2CMe)16(H2O)4] ( 1 -1 ) with a ground state spin value ( ST) equal to 10 (Figure 1 3).1417 While the prope rties of common molecule -based magnets are due to long range interaction between individual units, the behavior of an SMM is intrinsic to the molecule. To be a single -molecule magnet, a molecule must have a large ground -state spin ( ST) and a significant anisotropy of the easy axis (Ising) type, which is reflected in a large and negative zero -field splitting parameter ( D ). In the dodecanuclear complex 1 -1 t he large ground state spin arises from exchange interactions between the S = 3/2 spins of the MnIV ions and the S = 2 spins of the MnIII ions. The anisotropy of a cluster is primarily a consequence of the single-ion anisotropies of the constituent ions within the cluster and of the relative orientations of the magnetic axes of these ions with respect to ea ch other. The significant anisotropy of the complex 1 -1 is resulted from the approximately parallel alignment of the Jahn Teller elongation axes of the eight MnI II ions

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27 The combination of a negative easy axis type magnetic anisotropy along with a large gr ound -state spin results in the splitting of the ground state spin into 2ST+1 sublevels. Since D is negative, the larger s 4). To reverse the spin of the molecule from along the z (spin up) to the + z (spin down) axis of the molecule, a potential magnetization direction is given by S221/4) integer spins. Experimental evidence for this beh avior is supported by the appearance of frequencyM -of -phase AC magnetic susceptibility measurements, as shown in Figure 1 5, and of hysteresis loops in magnetization versus DC field scans (Figure 1 6). One of the uniq ue magnetic behaviors of SMMs is seen in the appearances of steps in the hysteresis loops. Instead of the the above -mention ed thermal activation over the energy barrier (classical mechanism), the spin reversal can also occur by tunneling through the energy barrier Such tunneling, called quantum tunneling of the magnetization (QTM) was first reported in 1996 for molecules of 1 -1 .18 The observed steps correspond to an increase in the relaxation rate of magnetization that occurs when there is an energy coinc idence of ms sublevels on the opposite sides of the potential energy barrier (Figure 1 7). For these critical field values, H = n D/ g B, at which steps occur, QTM is allowed, resulting in an increase in the relaxation rate of the molecule.1821 For the cont inuing interest in understanding this new magnetic phenomenon of single molecule magnetism, and in finding other compounds that exhibit similar properties, numerous synthetic strategies aimed at the improvement of these materials have been considered. Howe ver, in contrast to the relative ease with which synthetic routes are developed in other fields of chemistry, the preparation of polynuclear metal complexes presents a considerable challenge.

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28 One of the primary goals of this research is the development of new synthetic methods aimed at the preparation of new polynuclear metal complexes In the following chapters, several new synthetic strategies to prepare polynuclear transition metal clusters, as well as a new concept in the ligand design, are described. T hese preparative methodologies have led to the isolation of novel complexes that exhibit very rare or unprecedented metal topologies and interesting magnetic properties. The synthesis, structure, and characterization of these compounds will be discussed.

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29 Figure1 1. Representations of magnetic dipole arrangements in (i) paramagnetic, (ii) ferromagnetic, (iii) antiferromagnetic, and (iv) ferrimagnetic materials. Figure 1 2. Typical hysteresis loop of a magnet, where M is magnetization, H is the applied magnetic field and Ms is the saturation value of the magnetization Figure 1 3. Representation of (a) the [Mn12O12]16+ core and (b) the [Mn12O12(O2CMe)16(H2O)4] complex with peripheral ligation. MnIV orange; MnIII green; O red; C gray. M H Ms Ms Paramagnetic Ferromagnetic Antiferromagnetic Ferrimagnetic

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30 Temperature (K) 2 4 6 8 10 0 10 20 30 40 50 Temperature (K) 2 4 6 8 10 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 M T (cm3K mol1) M (cm3mol1)(a) (b) Figure 1 4. Representative plots of the potential energy versus (a) the orientation of the ms 12 complex with an S = 10 ground state, experiencing zero-field splitting. Figure 1 5. In -MT) an d out -of -M microcrystalline sample of [Mn12O12(O2CR)16(H2O)4] at the indicated oscillation frequencies.

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31 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 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 6. Magnetization hysteresis 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. Figure 1 7. Representation of the change in energy of ms sublevels as the magnetic field is swept from zero to a non-zero value. Resonant magnetizat ion tunneling occurs when the ms sublevels are aligned between the two halves of the diagram. H = 0 H = 0 H = 0

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32 CHAPTER 2 NEW TETRA AND HEXANU CLEAR IRON CLUSTERS FROM THE USE OF 2 (HYDROXYMETHYL)PYRIDINE 2.1 Introduction The last two decades have witnessed an explosive growth in the interest in polynuclear ironIII compounds with primarily oxygen-based ligation. This has been mainly due to their relevance to two fields, bioinorganic chemistry and molecular magnetism, as well as the intrinsic architectural beauty and aestheti cally pleasing structures they possess. Iron -oxo centers are found in several non -heme metalloproteins. Hemerythrin, ribonucleotide reductase, and methane monooxygenase are examples of enzymes with diiron metallosites,22 24 whereas the protein ferritin, wh ich is responsible for iron storage, can accommodate up to ~4500 iron ions in an iro n/oxide/hydroxide core.2529 A number of polynuclear iron complexes have thus been synthesized and studied as possible models for ferritin to gain insights into the biomine ralization process involved in the formation of its metal core.30 32 In the area of magnetism, high -spin ironIII ions have a relatively large number of unpaired electrons (d5, S = 5/2) and normally undergo strong, antiferromagnetic exchange interactions. W ith high enough Fex nuclearities and the appropriate topologies, these compounds can sometimes possess large ground-state spin ( S ) values and can even occasionally function as single -molecule magnets (SMMs).13, 33, 34 The latter are molecules that display slow magnetization relaxation rates and which, below a certain (blocking) temperature (TB), can function as single domain magnetic particles of nanoscale dimensions.35, 36 Such SMMs thus represent a molecular, bottom up approach to nanomagnetism.3 5 Altho ugh exchange interactions between FeIII centers are essentially always antiferromagnetic, certain Fex topologies can nevertheless result in large spin ground states because of spin frustration effects. Spin frustration is defined here in its general sense as the occurrence of competing exchange interactions of comparable magnitude that prevent (frustrate)

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33 the preferred spin alignments.37 39 For example, in certain topologies the spins of two antiferromagnetically coupled metal ions (or other spin carriers) may be forced into a parallel alignment by other, stronger interactions; thus, the intrinsic preference of the spins to align antiparallel is frustrated. A sufficient quantity and distribution of frustrated exchange pathways in some Fex topologies can lead to the significantly large values of the total molecular spin mentioned above, even when all the pairwise Fe2 exchange interactions are antiferromagnetic. Thus, we continue to have a great interest in rationalizing and understanding the exchange interacti ons and the resulting ground state S of polynuclear FeIII molecules. For the above reasons, we continue to seek synthetic methods to new Fex complexes. One approach that has proven successful is the use of alcohol -containing chelates. On deprotonation, th ese will provide alkoxide groups, which are excellent bridging units that can foster formation of high nuclearity products.4 043 We have a particular fondness for pyridyl alcohols, especially 2 (hydroxymethyl)pyridine (hmpH, Figure 2 1) which have proven to be versatile chelating and bridging groups that have yielded several polynuclear 3 d metal clusters with large S values44, 45 and SMM behavior.4653 However, in FeIII chemistry there has been o nly very limited use of hmpH.54, 5 5 Even for the previously r eported FeIII clusters with pyridyl alkoxide groups, the majority also contain carboxylate groups as a result of the use of triangular [Fe3O(O2CR)6(L)3]+ compounds as reagents, a common strategy in both FeIII and MnIII chemistry.5662 While carboxylates (R CO2 -) are excellent bridging groups in FeIII chemistry,63 in the present chapter we report some results from a recent investigation of non carboxylate FeIII cluster chemistry, which have led to new Fe4 and Fe6 products. We describe the syntheses, structure s, and magnetochemical characterization of these complexes, as well as theoretical rationalization of the experimental observations.

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34 2.2 Experimental Section 2.2.1 Syntheses All manipulations were performed under aerobic conditions using chemicals and solvents as received unless otherwise stated. Safety note: Azide salts are potentially explosive; such compounds should be synthesized and used in small quantities and treated with utmost care at all times. [Fe6O2(hmp)10(H2O)2](NO3)4 (2 -1) To a stirred soluti on of hmpH (0.48 mL, 5.0 mmol) and NEt3 (0.14 mL, 1.0 mmol) in MeCN (30 mL) was added solid Fe(NO3)3 9H2O (0.40 g, 1.0 mmol). The resulting brown solution was stirred for 1 h and filtered, and the filtrate was layered with Et2O/ hexanes (1:1, v/v). After 2 days, large brown crystals of 2 -1 8MeCN were collected by filtration, washed with cold MeCN (2 5 mL) and Et2O (2 5 mL), and dried under vacuum; the yield was ~55%. Anal. Calcd (Found) for 2 -1 1MeCN: C, 41.99 (41.63); H, 3.81 (3.73); N, 11.85 (12.11). Selected IR data (KBr, cm1): 1608(m), 1570(w) 1483(w), 1483(m), 1439(m), 1383(s), 1355(s), 1285(m), 1221(w), 1156(w), 1079(m), 1050(m), 1019(w), 828(w), 764(m), 718(m), 675(m), 646(m), 528(m), 444(w), 410(w). [Fe6O2(hmp)8(NO3)4(H2O)2](NO3)2 (2 -2) To a stirred solution of hmpH (0.29 mL, 3.0 mmol) and NEt3 (0.14 mL, 1.0 mmol) in MeCN (30 mL) was added so lid Fe(NO3)3 9H2O (0.40 g, 1.0 mmol). The resulting brown solution was stirred for 1 h and filtered, and the filtrate was layered with Et2O (30 mL). After 2 days, large brown crystals of 2 -2 6MeCN were collected by filtration, washed with cold MeCN (2 5 mL) and Et2O (2 5 mL), and dried under vacuum; the yield was ~60%. Anal. Calcd (Found) for 2 -2 (solvent -free): C, 35.15 (35.03); H, 3.20 (3.05); N, 11.96 (11.68). Selected IR data (cm1): 3418 (mb), 1608 (m), 1570 (w),1483 (w), 1439 (m), 1383 (s), 1285 ( m), 1221 (w), 1156 (w), 1075 (m), 1049 (m), 1022 (w), 826 (w), 763 (m), 719 (m), 677 (m), 647 (m), 530 (m), 459 (w), 412 (w).

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35 [Fe4(N3)6(hmp)6] (2 -3) To a stirred solution of hmpH (0.29 mL, 3.0 mmol) and NEt3 (0.14 mL, 1.0 mmol) in a solvent mixture compri sed of MeCN/MeOH (30 mL, 5:1 v/v) was added solid Fe(NO3)3 9H2O (0.40 g, 1.0 mmol). The resulting brown solution was stirred for 1 h, during which time solid NaN3 (0.20 g, 3.0 mmol) was added in small portions. The resulting dark red solution was stirred for a further 3 h and filtered, and the filtrate was layere d with Et2O (30 mL). After 6 days, dark red crystals of 2 -3 2MeOH were collected by filtration, washed with cold MeCN (2 5 mL) and Et2O (2 5 mL), and dried under vacuum; the yield was ~50%. Anal. Calcd (Found) for 2 -3 (solvent -free): C, 38.46 (38.73); H, 3.23 (3.07); N, 29.90 (29.49). Selected IR data (cm1): 3419 (mb), 2077 (s), 2052 (s), 1607 (m), 1568 (w), 1482 (m), 1433 (m), 1355 (m), 1287 (m), 1220 (w), 1154 (w), 1085 (m), 1060 (m), 1047 (m), 821 (w), 764 (m), 721 (m), 667 (m), 647 (m), 513 (m), 474 (w), 420 (m). 2.2.2 X -Ray Crystallography Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphi te monochromator utilizing Mo K ). Suitable crystals of 2 -1 8MeCN, 2 -2 6MeCN, and 2 -3 2MeOH w ere attached to glass fibers using silicone grease and transferred to a goniostat where they were cooled to 173 K for data collection. An initial search of reciprocal space revealed a monoclinic cell for 2 -1 8MeCN and 2 -3 2MeOH, and a triclinic cell for 2 -2 6MeCN; the choices of space groups C2/c (for 2 -1 8MeCN), 1 P (for 2 2 6MeCN), and P21/c (for 2 -3 2MeOH) were confirmed by the subsequent solution and refinement of the structures. Cell parameters were refined using up to 8192 reflections. A full sphere of d ata (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was <1%). Absorption corrections by integrati on were

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36 applied based on measured indexed crystal faces. The structures were solved by direct methods in SHELXTL6,64 and refined on F2 using full -matrix least -squares. The non -H atoms were treated anisotropically, whereas the H atoms were placed in calculated, ideal positions and refined as riding on their respective C atoms. Unit cell parameters and structure solution and refinement data are listed in Table 2 1. For 2 -1 8MeCN, the asymmetric unit consists of a half Fe3 cluster, two NO3 anions, four MeCN molecules one of which is disordered over two positions. The pyridyl group is disordered and was refined in two parts with their site occupation factors dependently refined. The coordinated protons w ere obtained from a difference F ourier map and refined fr eely but with their thermal parameter s fixed at 1.5 that of the o xygen atom. Atom O3 did not refine properly anisotropically thus its anisotropic parameters were constrained to be equivalent to O1. A total of 611 parameters were refined in the final cycle of refinement using 19774 reflections with I > 2 (I) to yield R1 and wR2 of 7.33 and 17.75%, respectively. For 2 -2 6MeCN, the asymmetric unit consists of half of the Fe6 cation, one NO3 anion, and three disordered MeCN molecules of crystallization. The la tter could not be modeled prop erly; thus the program SQUEEZE,65 a part of the PLATON package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. One coordinated NO3 (mono dentate) has all three O atoms disordered while the second (also monodentate) has only two O atoms disordered, with the third O being common to both parts. A total of 447 parameters were included in the structure refinement using 8212 reflections with I > 2 (I ) to yield R1 and wR2 of 4.11 and 10.05%, respectively. For 2 -3 2MeOH, the asymmetric unit consists of half of the Fe4 cluster and a MeOH molecule of crystallization disordered near an inversion center. The azide ligand consisting of

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37 atoms N10-N11 N12 is disordered, and the N11 N12 segment was refined in two parts with their site occupation factors dependently refined. A total of 332 parameters were included in the structure refinement using 5769 reflections with I > 2 ( I) to yield R1 and wR2 of 4.82 and 11.56%, respectively. 2.2.3 Other Studies Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 400 4000 cm1 range. Elemental analyses (C, H, and N) were performed by the in -house facilities of t he University of Florida Chemistry Department. Variable temperature dc and ac magnetic susceptibility data were collected at the University of Florida using a Quantum Design MPMS -XL SQUID susceptometer equipped with a 7 T magnet and operating in the 1.8 300 K range. Samples were embedded in solid eicosane to prevent torquing. Magnetization versus field and temperature data wer e fit using the program MAGNET.66 Pascals constants were used to estimate the diamagnetic corrections, which were subtracted from th e experimental susceptibilities to give the molar paramagnetic susceptibilities (M). The exchange interactions in 2 -3 were calculated by Dr. Ted A. OBrien at Department of Chemistry and Chemical Biology, Indiana University Purdue University Indianapolis (IUPUI), using matrix diagonalization methods described elsewhere.67 2.2.4 Theor etical Calculations Semiempirical calculations were performed at IUPUI using the ZILSH method67, 68 to analyze the exchange interactions underlying the magnetic properties observed for complex 2 -1 These calculations take place in several stages. First, e nergies and spin couplings are computed for a set of spin component wavefunctions using the intermediate neglect of differential overlap spectroscopic (INDO/S) model of Zerner69 7 0 and the local spin operator of Davidson.71, 72

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38 Estimates of the exchange c onstants JAB that appear in the Heisenberg spin Hamiltonian (HSH) of eq. 2 1, where 0 contains all spin independent terms, = 0 2 JAB A B < (2 1) are then obtained from these quantities by simultaneous solution of eq. 2 2, = 0 2 JAB A B < (2 2) which to a good approximation, give the energies of the spin components and the actual spin states of th e complex.73 In the second stage, the exchange constants are adjusted using a g enetic algorithm fitting method68 to more closely reproduce the experimentally measured variable temperature magnetic susceptibility of the complex. In the third and final stage, the exchange constants found in the second stage are substituted into the He isenberg spin Hamiltonian, which is diagonalized in a basis of spin component to yield the final energies and wavefunctions of the spin states. One important quantity that can be calculated from the wave functions is the spin coupling A B for each pair of metal ions. These values are useful for identifying exchange pathways that are spin frustrated.73 7 4 The spin coupling indicates the actual alignment of z components Sz,A and Sz,B in the state, while the exchange constant JAB indicates the preferred alignment. Any pathway with A B and JAB of different signs is thus frustrated under the 2 J convention. This is used to describe the spin interactions in the ground state of 2 -1 (vide infra). 2.3 Results and Discussio n 2.3.1 Syntheses As stated earlier, the present study arose as part of our investigation of the reactions of hmpH with FeIII sources in the absence of carboxylate groups. We have in some cases also added

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39 a source of azides, which are also excellent bridgi ng ligands and can foster the formation of high nuclearity products.75 Various reactions have been systematically explored by differing reagent ratios, reaction solvents, and other conditions before the following successful procedures were identified. The reaction of Fe(NO3)3 9H2O with hmpH and NEt3 in a 1:5:1 molar ratio in MeCN gave a brown solution and the subsequent isolation of well -formed brown crystals of [Fe6O2(hmp)10(H2O)2](NO3)4 8MeCN ( 2 -1 8MeCN) in high yields (~55%). The formation of 2 -1 is sum marized in eq 2 3. Note that the NEt3 has the role of proton acceptor to f acilitate both the 6Fe3+ + 10hmpH + 14NEt3 + 4H2O [Fe6O2(hmp)10(H2O)2]4+ +14NHEt3 + (2 3) deprotonation of the hmpH groups and H2O molecules as a source of the bridging O2 ions; although the excess of hmpH employed could in principle also carry out these roles, the yield of complex 2 -1 was only ~5% in the absence of NEt3. However, when more than 1 equiv of NEt3 was used, insoluble amorphous precipitates that were probably polymeri c were rapidly formed. In contrast a decrease in the amount of hmpH from 5 to 3 equiv (or less) did not give 2 -1 but the similar product [Fe6O2(hmp)8(NO3)4(H2O)2](NO3)2 (2 -2 6MeCN) in high yields (~60%). The formation of 2 -2 is summarized in eq 2 4. 6F e3+ + 8hmpH + 12NEt3 + 4NO3 + 4H2O [Fe6O2(hmp)8(NO3)4(H2O)2]2+ +12NHEt3 + (2 4) Complexes 2 -1 and 2 -2 were also obtained using other reaction solvents (i.e., MeOH, CH2Cl2), but the yields were appreciably lower and the crystalline precipitate was found to be contaminated with some other solid products.

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40 The addition of 3 equiv of NaN3 to the reaction mixture that yields 2 -2 but in a MeCN/MeOH solvent mixture to aid solubility, gave a deep red solution and subsequent isolation of [Fe4(N3)6(hmp)6] 2 MeOH ( 2 -3 2MeOH) in yields of ~50%. The formation of 2 -3 is summarized in eq 2 5. Increasing the amount of sodium azide still gave complex 2 -3 but the 4Fe3+ + 6hmpH + 6NEt3 + 6N3 [Fe4(N3)6(hmp)6] + 6NHEt3 + (2 5) reaction was not so clean and the yield was appreciably lower. With the identity of 2 -3 established, we also tried several other Fe3+/hmpH/NEt3/N3 ratios, and particularly with a large excess of FeX3 (X = Cl-, NO3 -, ClO4 -), to see if higher nuclearity azide -containing products might be obtained, but in all cases complex 2 -3 was the isolated product, in varying yields. As for 2 -1 and 2 -2 the NEt3 was again essential to obtain 2 -3 in good yields and too much NEt3 also again gave insoluble powders, which formed rapidly from the reaction solution. 2.3.2 Description of Structures The partially labeled structure and a stereoview of the cation of [Fe6O2(hmp)10(H2O)2] (NO3)4 (2 -1 ) are shown in Figure 2 2, and its labeled c ore in Figure 2 3; selected interatomic distances and angles are listed in Table A 1. Complex 2 -1 crystallizes in monoclinic space group C2/c and displays crystallographic Ci symmetry; the asymmetric unit therefore contains only half of the Fe6 cation, tw o NO3 anions, and four lattice MeCN molecules. The structure comprises six Fe atoms in a chair conformation. This can be described as a central Fe4 above and below the central Fe4 plane. However, a better description of the structure is as two triangular [Fe33O2 -)] units joined together a alkoxo groups. Each [Fe33O2 -)] triangular unit is essentially isosceles (Fe1Fe3 = 3.064(2)

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41 Fe2Fe3 = 3.030(2) Fe1Fe2 = 3.671(2) ) and essentially planar (the oxide is only 0.004 3-oxide has Y -shaped geometry with the largest angle Fe1 O3 -Fe2 being 153.7(3). The two equivalent sides of each isosceles triangle are bridged by an alkoxide O atom of an hmpgroup that chelates t o a basal Fe atom. The ligation is completed by a chelating hmpH2O ligand. All the Fe atoms are six -coordinate with distorted octahedral geometries. The distortions at the four central Fe atoms are particularly pronounced, with angles at cis and trans ligands ranging from 73.2(2) to 111.6(2), and 148.7(2) to 167.6(2), respectively. The NO3 counterions are hydrogen (O7O63 = 2.668(8) O7O73 = 2.685(8) The partially labeled structure and a stereoview of the cation of [Fe6O2(hmp)8(NO3)4(H2O)2](NO3)2 (2 -2 ) are shown in Figure 2 4; selected interatomic distances and angles are listed in Table A 2. The struct ure of 2 -2 is very similar to 2 -1 with the difference being the two chelating hmpgroups in 2 -1 have been replaced by four terminal NO3 groups in 2 -2 Complex 2 -2 crystallizes in triclinic space group 1 P and displays crystallographic Ci symmetry. The cor e of the molecule again comprises two triangular [Fe33-O2-)] units joined together at two of their alkoxo groups. Each [Fe33O2-)] triangular unit is essentially isosceles (Fe1 Fe3 = 3.127(5) Fe2Fe3 = 3.095(3) Fe1 Fe2 = 3.648(3) ) and essentially planar (the oxide is only 0.004 from the Fe3 plane), 3oxide has Y -shaped geometry with the largest angle Fe1O3 -Fe2 being 149.1(1). The two equivalent sides of each isosceles triangle are bridged by an alkoxide O atom of an hmpgroup that chelates to a basal Fe atom. The ligation is completed by two monodentate NO3 groups on each apical Fe atom Fe3 and Fe 3 each of the latter also possessing a terminal H2O

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42 ligand. All the Fe atoms are six -coordinate with distorted o cta hedral geometries. The distortions at the four central Fe atoms are particularly pronounced, with angles at cis and trans ligands ranging from 72.6(8) to 114.9(8) and 147.5(8) to 173.3(8), respectively. The NO3 counterions are hydrogen -bonded to the ter minal water groups on Fe3 and Fe 3 O6O14 = 2.672(2) ) and serve to bridge separate Fe6 molecules in the crystal. There are several structural types of FeIII 6 clusters previously reported in the literature, differing in the Fe6 topology. These have been conven iently referred to as (a) planar,55 67, 7679 (b) twisted -boat,57, 8084 (c) chair like,55 (d) parallel triangles,85 88 (e) octahedral,89 92 (f) fused or extended butterflies,60, 61, 73 93100 (g) cyclic,101106 and (h) linked triangles.107 109 As can be anticipated, these different Fe6 topologies have led to a variety of ground -state spin S values among these complexes, spanning S = 0, 1, 3, and 5. There is only one known example of an FeIII 6 complex of class (c), i.e. possessing a chair like Fe6 topology; [Fe6O2Cl4(hmp)8](ClO4)2 (2 4 ).55 The structure of complex 2 -4 is very similar to that of 2 -1 except that the end Fe atoms Fe3 2 -1 each possess a chelating hmpand a terminal H2O, whereas these atoms in 2 -4 each possess only two terminal Clions and are thus five -coordinate. The partially labeled structure, a stereoview, and the labled core of [Fe4(N3)6(hmp)6] (2 -3 ) are shown in Figure 2 5; selected interatomic distances and angles are given in Table A 3. Complex 2 -3 2MeOH crystallizes in the monoclinic space group P 21/c with the Fe4 molecule lying on an inversion center. The molec ule comprises a nonlinear array of four FeIII atoms (Fe2 Fe1 -Fe1 Fe2 pair bridged by the alkoxide arms of two chelating hmpgroups. There is thus 12groups, and peripheral ligation is completed by six terminal azide groups, three each on the two end Fe atoms Fe2 and Fe2 O2 Fe1 O2

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43 are nearly so, with the Fe1 O1 -Fe2 O3 torsion angle being 5.2. The FeIII atoms are both six coordinate with distorted octahedral geometry, with Fe1 having the greater distortion from ideal geometry. The Fe N and Fe O bond lengths are as expected for high-spin ironIII.98, 110, 111 There have been a large number of Fe4 complexes reported in the literature, and these possess a wide variety of metal topologies such as rectangles, rhombs, butterflies, etc.112, 113 However, the only previous Fe4 compounds with a similar kind of extended, chainlike topology as in 2 -3 and an [Fe4 OR)6]6+ core are [Fe4{(py)2C(OMe)O}2{(Hpy )(py)C(OMe)O}2(dbcat)4]2+ (2 -5 )114 (dbcat2is 3,5 di t -butylcatecholate) an d [Fe4(OH)2(rac arabitol)4]6 -.115 2.3.3 Magnetochemistry 2.3.3.1 Direct current magnetic susceptibility studies Variable temperature mag netic susceptibility measurements were performed on dried polycrystalline samples of complex 2 -1 MeCN, 2 -2 and 2 -3 restrained in eicosane to prevent torquing, in a 1 kG (0.1 T) field and in the 5.0 300 K range. For 2 -1 T steadily decreases from 9.16 c m3mol1K at 300 K to a near -plateau value of ~6.10 cm3mol1K at 45 -20 K and then further decreases to 5.64 cm3mol1K at 5.0 K (Figure 2 6) The 300 K value is much less than the spinonly ( g = 2) value of 26.25 cm3mol1K for six noninteracting FeIII ions, indicating the presence of strong antiferromagnetic interactions, as expected for oxo -bridged FeIII systems. The T near -plateau value in the 20 45 K range appears to be heading for a final value of ~6 cm3mol1K, the spin only ( g = 2) value of a species with an S = 3 ground state, before exhibiting the final decrease at temperatures below 10 K. The latter decrease i s likely due to a combination of Zeeman effects from the applied dc field, zero -field splitting (ZFS), and any weak intermolecular antiferromagnetic exchange interactions. For 2 -2 T steadily decreases from 9.79 cm3mol1K at 300 K to a near -plateau valu e of ~6.80 cm3mol1K at 50 15 K and then slightly decreases to6.43 cm3mol1K at 5.0 K (Figure 2 7).

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44 The 300 K value is again much less than the spin -only ( g = 2) value of 26.25 cm3mol1K for six non interacting FeIII ions. The 5.0 K value is close to the s pin -only ( g = 2) value of a complex with an S = 3 ground state. For 2 -3 T steadily decreases from 12.56 cm3Kmol1 at 300 K to 0.56 cm3Kmol1 at 5.0 K (Figure 2 8 ). Again, the 300 K value is much less than the spin-only ( g = 2) value of 17.50 cm3Kmol1 for four noninteracting FeIII ions, indicating the presence of strong ant iferromagnetic exchange interactions and the 5.0 K value indicates an S = 0 ground state. To confirm the indicated ST = 3 ground state of complex 2 -1 and 2 -2 and to estimate the magnitude of the zero-field splitting parameter D magnetization vs dc fiel d measurements were made on restrained samples at applied magnetic fields and temperatures in the 1 70 kG and 1.8 10.0 K ranges, respectively. The resulting data for 2 -1 are shown in Figure 2 9 as a reduced magnetization ( M/N B) vs H/T plot, where M is the magnetization, N is Avogadros number, B is the Bohr magneton, and H is the magnetic field. The data were fit u sing the program MAGNET,66 by diagonalization of the spin Hamiltonian matrix assuming only the ground state is p opulated, incorporating axial anisotropy ( D z 2) and Zeeman terms, and employing a full powder average. The corresponding spin Hamiltonian is given by eq. 2 6, where z is the easy axis spin operator, g is the Land g factor and 0 is the vacuum permeabilit y. H = D z 2 + B0 H (2 6) The best fit for 2 -1 MeCN is shown as the solid lines in Figure 2 9 and was obtained with S = 3 and either of the two sets of parameters: g = 1.94 and D = +0.78 cm1, or g = 1.96 and D = 0.62 cm1. Alternative f its with S = 2 or 4 were rejected because they gave unreasonable values of g It is common to obtain two acceptable fits of magnetization data for a given S value, one with D > 0 and the other with D < 0, since magnetization fits are not very sensitive to the sign of

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45 D In order to assess which is the superior fit and also to ensure that the true global minimum had been located, we calculated the root -mean -square D vs g error surface for t he fits using the program GRID,116 which calculates the relative diff erence between the experimental M / B data and those calculated for various combinations of D and g The error surface is shown as a twodimens ional contour plot in Figure 2 10 and it clearly shows only two minima, with positive and negative D values, with the latter being superior quality and thus suggesting the true sign of D for 2 -1 MeCN is negative, but a more reliable and accurate determination of the sign and magnitude of D requires use of a more sensitive technique such as EPR spectroscopy. The best fit for 2 -2 is shown as the solid lines in Figure 2 11 and was obtained with S = 3 and either of the two sets of parameters: g = 2.07, D = 0.57 cm1 and g = 2.08, D = 0.44 cm1. Alternative fits with S = 2 or 4 were rejected because they gave unreasonable values of g The error surface for 2 -2 is shown as a two dimensional contour plot in Figure 2 -12, and it shows only the two minima with positive and negative D values, with the latter being of superior quality and thus suggesting the true sign of D is neg ative. Given the small size of complex 2 -3 the susceptibility data to 300 K were fit by a matrix diagonalizat ion method described elsewhere67 to obtain the individual pairwise exchange constants Jij between Fe atoms Fei and Fej. The isotropic Heisenberg spin Hamiltonian that is appropriate for centrosymmetric complex 2 -3 is given in eq. 2 7. Three parameters were employed in the fit: exchange constants J1 and J2 for the two outer (Fe1 -Fe2 and Fe1 -Fe2 H = 2 J1(1 2 + 1' 2') 2 J2( 1 1') 2 J3( 1 1' + 2 2') (2 7) one central (Fe1 Fe1 g factor. The next nearest neighbor interactions J3 were assumed to be zero. A temperature independent paramagnetism (TIP) term was kept

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46 constant at 800 106 cm3Kmol1. T he fit (solid lin e in Figure 2 8 ) gave J1 = 9.2 cm1, J2 = 12.5 cm1, and g = 2.079. Similar values of J1 = 4.8 cm1 and J2 = 13.0 cm1 were reported for 2 -5 .114 As expected, the interactions between the FeIII centers in 2 -3 are strongly anti ferromagnetic, resulting in a ground -state spin of S = 0. The latter is again as expected for a linear array of four FeIII atoms, since the constituent exchange interactions J1 and J2 are not competing, and J3 2.3.3.2 Alternating current magnetic susceptibility studies As an additional, independent assessment of the ground state S value, we collected ac susceptibility data on 2 -1 MeCN and 2 -2 in the 1.8 10 K range using a 3.5 G ac field oscillating at frequencies in the 50 1000 Hz range. If the magnetization vector can relax fast enough to keep up with the oscillating field, then there is no imaginary (out -of -phase) susceptibility signal ( M), a nd the real (in -phase) susceptibility ( M) is equal to the dc susceptibility. However, if the barrier to magnetization relaxation is significant compared to thermal energy ( kT ), then there is a non -zero M signal and the in -phase signal decreases. In addition, the M and M signals will be frequency -dependent. The main advantage of ac studies in the present case is that no dc field is used. This precludes problems arising from a dc field, such as the stabilization of MS levels of low lying excited spi n S states with an S greater than that of the ground state, and thus their approach (and even crossing) in energy with ground state MS levels. Since our fits of dc magnetization data assumed that only the ground state MS levels are populated, this Zeeman e ffect involving excited states could give erroneous estimates of the ground state. The obtained in -phase M signal for 2 -1 MeCN is plotted as MT in Figure 2 1 3 and can be seen to be almost temperatureindependent, decreasing only slightly with temperature below 10 K, before decreasing a bit more rapidly below ~4 K. Extrapolating to 0 K the data from above 4 K (to avoid lower temperature effects from the slight anisotropy and weak intermolecular interactions)

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47 gives a value of 5.8 5.9 cm3K mol1 range, which is consistent with an S = 3 ground state and g < 2, in excellent agreement with the dc magnetization f its. We conclude that complex 2 -1 does indeed have an S = 3 ground state. There is no out of -phase ac susceptibility signal down to 1.8 K, the operating limit of our SQUID magnetometer. For complex 2 -2 the in -phase MT signal below 10 K is almost te mperature independent (Figure 2 1 4 ), and extrapolation of the plot to 0 K from above 4 K gives a value of ~ 6.4 cm3K mol1 range. This indicates an S = 3 ground state and g ~ 2.07, in excellent agreement with the dc magneti zation fit. There is no out -of -phase ac susceptibility signal down to 1.8 K, the operating limit of our SQUID magnetometer. Complex 2 -1 and 2 -2 are thus confirmed to possess an S = 3 ground-state, which is an unusual ground state for an FeIII 6 complex. Mos t of the FeIII 6 complexes for which the ground state spin has been determined have an S = 0 or S = 5 ground state. In fact, it is not intuitively obvious how an S = 3 ground state could arise for an FeIII 6 complex, since it is clearly not the resultant of simple considerations of spin -up and spin -down alignment pictures. The usual qualitative rationalization in such cases is to say that spin frustration effects must be operative within the Fe3 triangular sub units. Spin frustration is here defined in the more general, chemical sense as the presence of competing exchange interactions of comparable magnitude that prevent (frustrate) the spin alignments preferred from the nature (ferro or antiferromagnetic) of the exchange interactions between them. Thus, the qualitative argument would say that intermediate spin alignments ( MS = 3/2, 1/2 for high-spin FeIII) are present at some number of the Fe atoms and this gives the observed S = 3 ground state. While such qualitative arguments are undoubtedly correct, they are nevertheless less than satisfying. As mentioned earlier, there is the other FeIII 6 complex in class (c), [Fe6O2Cl4(hmp)8](ClO4)2 (2 -4 ), and 2 -4 was also found to have an S = 3

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48 ground state.55 Seeking to understand the origin of this ground state for 2 -4 the authors carried out computational studies using irreducible tensor methods to obtain the various pairwise Jij exchange parameters for each FeiFej pair and thus rationalize the S = 3 ground state. However, these calculations led to a predicted S = 0 ground state for 2 -4 in conflict with the experimental data. The authors suggested several reasons for this discrepancy, but the bottom line is that the origin of the unusual S = 3 ground state for this class of FeIII 6 complex has yet to be satisfactori ly explained at a quantitative level. In order to do so for our present complex 2 -1 and 2 -2 and by extension for 2 -4 we carried out computational studies on complex 2 -1 using the ZILSH method. 2.3.3.3 Rationalization of the S = 3 ground state of Complex 2 -1 The spin alignments giving rise to the S = 3 ground state of the FeIII 6 complex 2 -1 are not obvious because it is not a simple sum of spin -up and spin -down of S = 5/2. It is clear that spin frustration is likely taking place in the compound, and we nee d to know the exact spin alignments at each Fe ion. In such cases it is useful to consider the spin coupling A B which can provide a direct probe of spin frustration: if two FeIII (S = 5/2) ions couple ferromagnetically, each has a z component Sz of +5/2, and the total spin ST is 5. The spin coupling A B between them, which can be calculated from eq. 2 8, has a value of +6.25. Similarly, if they couple A B = 1 2 [ ST( ST+ 1 ) SA( SA+ 1 ) SB( SB+ 1 ) ] (2 8) antiferromagnetically, one has Sz = +5/2, the other has Sz = 5/2, and the total spin ST is 0. The spin coupling A B between them has a value of 8.75. The spin coupling indicates the actual alignment of z components Sz, A and Sz, B in the state, while the exchange constant JAB indicates the preferred alignment.

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49 When JAB is negative but small, and a preferred antiparallel alignment is overwhelmed completely by competing interactions, then the two spins align parallel, each with Sz = +5/2, and A B large and positive, on the order of the formal value of +6.25. Such a pathway is called frustrated, and the situation is called spin frustration: in general, if JAB and A B carry different signs, the A -B pathway is frustrated. Wh en the competing exchange interactions are not strong enough to totally frustrate the pathway under consideration, but also not weak enough to allow it to be in a preferred way, the spins do not point either straight up or straight down, and are in some intermediate z component such as 3/2 or 1/2. The spin coupling A B is then quite reduced in magnitude from the formal value of +6.25 or 8.75, but it can still be positive or negative, and thus the pathway might not be completely frustrated or satisfied Therefore, for an antiferromagnetically coupled pathw ay ( JAB < 0), there is a continuum ranging from completely frustrated on one end, A B large and positive, Sz,A = +5/2, Sz,B = +5/2, to completely satisfied on the other end, A B large and negative, Sz,A = +5/2, Sz,B = 5/2. In bet ween these extremes are smaller values of A B and Sz,A and Sz,B 3/2 or 1/2 (Figure 2 15). Note that the two spins can still be aligned parallel if not completely frustrated (incomplete frustration, A B positive but small), or antiparallel (incomplete satisfaction, A B negative but small in magnitude). Both of these scenarios are taking place in complex 2 -1 J values for complex 2 -1 calculated by ZILSH method, are presented in Figure 2 16. In the triangular unit F e1 -Fe2 -Fe3, J13 = 22cm1, J23 = 17cm1, and J12 = 60 cm1. The spin coupling A B values are also given in Figure 2 16. Considering both Figures 2 1 5 and 2 16, we can tell which pathways in the Fe6 complex are frustrated/satisfied and by how much; for example, the spin coupling A B of the Fe1 -F e3 pathway is 3.69 (Figure 2 16), and the range where t his value resides in Figure 2 15 shows that

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50 the Fe1 -Fe3 pathway is partially satisfied with actual spin alignments of Sz,1 = +3/2 and Sz,2 = 3/2. With the same argument, spin alignments on each Fe ion were assigned as in Figure 2 16, with a net result of Sz,1 = +3/2, Sz,2 = 3/2, and Sz,3 = +3/2. Based on the symmetry of the complex, there are four Fe ions with Sz = +3/2 and two with Sz = 3/2, and therefore in total ST = 3. The two J values J13 and J23 in each triangular unit are similar enough that the pathway with a slightly weaker J is frustrated, but only partially, and the one with a slightly stronger J is satisfied, but also only partially. As stated above, the other complex in class (c), [Fe6O2Cl4(hmp)8](ClO4)2 (2 4 ), was also experimentally found to have an S = 3 ground state,55 however, the theoretical calculations the authors carried out to rationalize the S = 3 ground state a ctually predicted an S = 0 ground state for the compound. This discrepancy results from the authors assumption that each triangle is isosceles, and their calculation based on this assumption predicted an S = 0 ground state for the FeIII 6 complex. In contr ast, we treated each triangle as scalene and obtained J13 = 22cm1, J23 = 17cm1, J12 = 60cm1, and thus predicted an S = 3 ground sate. We therefore performed our calculation of the ground state again, using the assumption that each triangle is isoscele s ( J13 = J23 = 19.5cm1, J12 = 60cm1), and obtained an S = 0 ground state for complex 2 -1 Therefore, in the literature, the experimentally observed S = 3 ground state of complex 2 4 was not able to be rationalized because of the incorrect assumption tha t each triangle is isosceles. Although it seems like a valid assumption, only a small difference in the structure and the resulting small differences in J values change the ground state of the Fe6 complexes. We further investigated how lowering the symmetr y of the triangular units below isosceles influences the ground state of the FeIII 6 molecule; we kept all of the J values constant except for J23 (Figure 2 16) and calculated the ground state with various J23 values. Interestingly, the

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51 ground state of the FeIII 6 molecule can be ST = 0, 1, 2, 3, 4, and 5, by successively reducing the J23 value in increments of 0.5 cm1; therefore, by lowering the symmetry of triangular units below isosceles, the ground state of the FeIII 6 complex can change from S = 0 to S = 5 in succession (Table 2 2). By thoroughly examining the influence of the relative strength of J values on the side pathways of its triangular subunits ( J13 and J23 in Figure 2 16) and the ground state ST of the FeIII 6 complex, the possible ST values with various J13 and J23 values wer e mapped, as shown in Figure 2 17. When the triangular unit (Fe1 -Fe2 -Fe3) is isosceles and the J values on the side pathways ( J13 and J23) are equivalent, the FeIII 6 complex has an S = 0 ground state. When J13 and J23 are dif ferent to a large extent, the FeIII 6 complex has an S = 5 ground state. However, when J13 and J23 differ to a small extent, the FeIII 6 complex can have S values ranging from 1 to 4. In the case of complex 2 -1 J13 and J23 are at the right strength to give an ST= 3 ground state. Figure 2 1 7 shows that a minute deviation from the isosceles sit u ation can lead to drastic changes in the ground states. 2.4 Conclusions The bidentate N,O ligand hmpin noncarboxylate FeIII chemistry has proven to be a useful route to new FeIII clusters spanning Fe4 and Fe6 nuclearities and topologies that are both very rare. In particular, the reaction between Fe(NO3)39H2O and hmpH in basic media has led to [Fe6O2(hmp)10(H2O)2](NO3)4( 2 -1 ) and [Fe6O2(hmp)8(NO3)4(H2O)2](NO3)2 ( 2 -2 ), whereas a similar reaction in the presence of NaN3 gives the tetranuclear cluster [Fe4(N3)6(hmp)6] (2 -3 ). It is interesting that the azide ligands in 2 -3 are terminal rather than bridging but, nevertheless, fostered formation of a product completely diff erent from that of the nonazide products 2 -1 and 2 -2 Magnetochemical characterization of 2 -1 and 2 -2 revealed that the FeIII 6 complex has an S = 3 ground state, and this counterintuitive result was rationalized by ZILSH theoretical methods. Interestingly, we found that the ground state of the FeIII 6 complex is very sensitive to the relative

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52 strength of the J values on the side pathways of its triangular subunits, and with a lowered symmetry of triangular units below isosceles, the molecule can present grou nd states of ST = 0, 1, 2, 3, 4, and 5. In complex 2 -1 the relative strength of the J values on the side pathways of its triangular subunits are found to be at the right strength to allow ST= 3 ground state. It will be interesting to see in the future wor k whether the ground state of the FeIII 6 complex can be altered by distorting its triangular units chemically or physically.

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53 Table 2 1. Crystallographic Data for 2 -1 8MeCN, 2 -2 6MeCN and 2 -3 2MeOH. parameter 2 1 2 2 2 3 formula C 76 H 88 N 22 O 26 Fe 6 C 60 H 70 N 20 O 30 Fe 6 C 38 H 44 N 24 O 8 Fe 4 fw, g mol 1 2060.78 1886.46 1188.37 crystal system monoclinic triclinic monoclinic space group C2/c P2 1 /c a 29.655(3) 11.8677(8) 13.6914(10) b 13.0398(12) 13.0369(9) 14.1350(11) c 23.822(2) 13.9403(10) 13.6611(10) deg 90 64.628(2) 90 deg 91.672(2) 82.289(2) 104.942(1) deg 90 73.042(2) 90 V 3 9207.7(15) 1863.9(2) 2554.4(3) Z 4 1 2 T C 173(2) 173(2) 173(2) radiation, a 0.71073 0.71073 0.71073 calc g cm 3 1.487 1.681 1.545 mm 1 1.005 1.236 1.187 R 1 b,c 0.0733 0.0411 0.0482 wR 2 d 0.1775 0.1005 0.1156 a Graphite monochromator. b I > 2 (I). c R 1 = 100 (|| Fo| | Fc|| )/ | Fo|. d wR 2 = 100[ [ w ( Fo 2 Fc 2)2]/ [ w ( Fo 2)2]]1/2, w = 1/[ 2( Fo 2) + [(ap)2 + bp], where p = [max( Fo 2, O) + 2 Fc 2]/3.

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54 Table 2 2. Calculated ground states ST of c omplex 2 -1 with various J23 values: see the text for details. Figure 2 1. The structure of 2(hydroxymethyl)pyridine (hmpH). N O H J 23 (cm 1 ) J 13 (cm 1 ) J 12 (cm 1 ) S T 18.5 22.0 60.0 0 18.0 22.0 60.0 1 17.5 22.0 60.0 2 17.0 22.0 60.0 3 16.5 22.0 60.0 4 16.0 22.0 60.0 5

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55 Figure 2 2. The structure of complex 2 -1 (top) and a stereopair (bottom). Hydrogen atoms have been omitted for clarity. Color code: FeIII green; O red; N, blue; C grey.

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56 Figure 2 3. The fully labeled core of complex 2 -1 Color code: FeIII green; O red.

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57 Figure 2 4. The structure of complex 2 -2 (top) and a stereopair (bottom). Hydrogen atoms have been omitted for clarity. Color code: FeIII green; O red; N, blue; C grey.

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58 Figure 2 5. The structure of complex 2 -3 (top), a stereopair (bottom), and the labeled core. Hydrogen atoms have been o mitted for clarity. Color code: FeIII green; O red; N, blue; C grey.

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59 Temperature (K) 0 50 100 150 200 250 300 MT (cm3Kmol-1) 0 2 4 6 8 10 12 Figure 2 6. Plot of MT vs T for complex 2 -1 Temperature (K) 0 50 100 150 200 250 300 350 MT (cm3Kmol-1) 0 2 4 6 8 10 12 Figure 2 7. Plot of MT vs T for complex 2 -2

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60 Figure 2 8. Plot of MT vs T for complex 2 -3 The solid line is the fit of the data; see the text for the fit parameters. Temperature (K) 0 50 100 150 200 250 300 350 MT (cm3Kmol-1) 0 2 4 6 8 10 12 14 experimental fitting

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61 g 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 D (cm-1) -1.0 -0.5 0.0 0.5 1.0 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66 0.68 0.70 0.76 0.74 0.72 0.70 0.68 0.66 0.64 0.62 0.60 0.58 0.56 0.54 0.52 0.50 0.48 0.46 0.44 0.42 0.40 0.38 0.36 0.34 0.32 0.30 0.28 0.46 0.26 0.44 0.24 0.42 0.22 0.40 0.38 0.20 0.64 0.36 0.18 0.62 0.34 0.60 0.16 0.32 0.30 0.28 0.14 0.26 0.24 0.12 0.22 0.20 0.14 0.18 0.10 0.16 0.16 0.14 0.18 0.12 0.08 0.20 0.32 0.22 0.30 0.24 0.26 0.24 0.28 0.12 0.30 0.10 0.20 0.14 0.32 0.16 0.18 0.34 0.18 0.36 0.16 0.12 0.38 0.20 0.14 0.14 0.22 0.40 0.14 0.24 0.42 0.44 0.26 0.16 0.28 0.46 0.30 0.48 0.16 0.18 Figure 2 9. Reduce B) vs H/T plot for complex 2 -1 The solid lines are the fit of the data; see the text for the fit parameters. Figure 2 10. Two-dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 2 -1 H/T 0 2 4 6 8 10 12 14 16 18 M/N B 0 1 2 3 4 5 6 0.1 T 0.5 T 1 T 2 T 3 T fitting

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62 g 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 D (cm-1) -1.0 -0.5 0.0 0.5 1.0 0.14 0.14 0.16 0.16 0.18 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.36 0.34 0.32 0.30 0.28 0.26 0.24 0.22 0.20 0.20 0.18 0.18 0.16 0.16 0.14 0.14 0.12 0.12 0.12 0.12 0.12 0.14 0.14 0.14 0.16 0.16 0.16 0.18 0.18 0.18 0.34 0.12 0.12 0.20 0.20 0.20 0.10 0.10 0.10 0.10 0.10 0.10 0.22 0.22 0.22 0.08 0.08 0.08 0.24 0.24 0.24 0.24 0.08 0.08 0.08 0.26 0.26 0.26 0.06 0.06 0.28 0.28 0.28 0.10 0.18 0.30 0.30 0.30 0.12 0.32 0.32 0.32 0.14 0.14 0.14 0.34 0.34 0.34 0.12 0.16 0.16 0.36 0.36 0.36 0.08 0.18 0.18 0.38 0.38 0.38 0.10 0.20 0.20 0.10 0.40 0.40 0.40 0.22 0.22 0.42 0.42 0.42 0.24 0.24 0.44 0.44 0.12 0.10 0.26 0.26 0.46 0.46 0.28 0.28 0.14 0.30 0.30 0.48 0.48 0.50 0.50 0.16 0.32 0.32 0.34 0.34 0.52 0.52 0.18 0.36 0.36 0.54 0.54 0.20 0.38 0.38 0.56 0.56 0.22 0.40 0.40 0.58 0.24 0.42 0.42 0.26 0.60 0.44 0.44 0.28 0.62 0.46 0.30 0.64 0.48 0.32 0.50 0.66 0.34 0.68 0.52 0.36 0.54 0.70 0.38 0.56 Figure 2 B) vs H/T plot for complex 2 -2 The solid lines are the fit of the data; see the text for the fit parameters. Figure 2 12. Two-dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 2 -2 H/T 0 2 4 6 8 10 12 M/N B 0 1 2 3 4 5 6 0.1T 0.5T 1T 2T fitting

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63 Figure 2 13. In-phase ac susceptibility ( M M 2 -1 in a 3.5 Oe ac field oscillating at the indicated frequencies. Figure 2 14. In-phase ac susceptibility ( M M lex 2 -2 in a 3.5 Oe ac field oscillating at the indicated frequencies. Temperature (K) 0 2 4 6 8 10 12 M'T (cm3Kmol-1) 0 2 4 6 8 10 12 1000 Hz 250 Hz 50 Hz Temperature (K) 0 2 4 6 8 10 12 M'T (cm3Kmol-1) 0 2 4 6 8 10 12 1000 Hz 250 Hz 50 Hz

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64 S1z= +5/2 S1z= +5/2 S1z= +5/2 S1z= +5/2 S1z= +5/2 S1z= +5/2 S2z= 3/2 S2z= 1/2 S2z= +3/2 S2z= +1/2 S2z= +5/2 S2z= 5/2 S1 S2= 8.75 S1 S2= 5.25 S1 S2 = 1.75 S1 S2 = 1.25 S1 S2 = 3.75 S1 S2= 6.25 ST= 0 ST= 1 ST= 2 ST= 3 ST= 4 ST= 5 S1z= +1/2 S2z= 1/2 S1 S2 = 0.75 ST= 0 S1z= +1/2 S2z= +1/2 S1 S2 = 0.25 ST= 1 S1z= +3/2 S1z= +3/2 S1z= +3/2 S1z= +3/2 S2z= 3/2 S2z= 1/2 S2z= +3/2 S2z= +1/2 S1 S2 = 3.75 S1 S2 = 1.25 S1 S2 = 0.75 S1 S2 = 2.25 ST= 0 ST= 1 ST= 2 ST= 3 complete spin satisfaction spin satisfied spin frustrated complete spin frustration J<0 Z Figure 2 15. Scheme presenting a continuum of spin coupling values and spin alignments between completely satisfied situation and completely frustrated situation for an an tiferromagnetically coupled pathway: see the text for details.

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65 Fe3' Fe1' Fe2' Fe3 Fe2 Fe1 17 60 22 33< S12>22 17 60 33J (cm1)+2.23 +2.23 3.69 3.69 3/2 3/2 3/2 3/2 3/2 3/2 3.69 3.69 3.69 3.69 Z Figure 2 16. Diagrammatic scheme of the core structures of complex 2 -1 presenting the ZILSH exchange constants J and the spin coupling A B for each Fe -Fe pathway and rationalizing the S =3 ground state of 2 -1 : see the text for details.

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66 Figure 2-17. Mapping of the possible ST values of 2-1 with various J13 and J23 values, presenting the influence of lowering of the symmetry of the triangular unit on the spin of the ground state. 0 -2.5 -5 -7.5 -10 -12.5 -15 -17.5 -20 -22.5 -25 -27.5 -30 -32.5 -35 -37.5 -40 -42.5 -45 -47.5 -50 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22 -24 -26 -28 -30 -32 -34 -36 -38 -40 -42 -44 -46 -48 -50 J13(cm-1) J13 = J23 J23= 17 cm 1J13= 22 cm 1 S = 5 S = 4 S = 3 S = 2 S = 1 S = 0 J23(cm-1) S = 3

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67 CHAPTER 3 UNUSUAL FE8, FE9, AN D FE18 STRUCTURAL TY PES FROM THE USE OF 2,6 PYRIDINEDIMETHANOL 3.1 Introduction The systematic development of iron cluster chemistry over many years has led to a remarkable variety of species that have been o f interest from several viewpoints, including structural aesthetics and spectroscopic and physical properties. From a biological viewpoint, the active sites of a variety of proteins such as hemerythrin, methane monooxygenase and ribonucleotide reductase ha ve been shown to contain di -iron cores bridged by oxo or hydroxo ligands.2224 The protein ferritin has also received attention, owing to the biological importance of the role of this protein in the storage and recycling of iron, a nd a variety of compounds of high metal nuclearity that model the storage of iron in the protein have been synthesized.25 29 Another interesting aspect of large polynuclear iron clusters is the potential for these clusters to possess large spin ( S ) values in their ground states du e to a relatively large number of unpaired electrons of high -spin FeIII ions: The exchange interactions between these FeIII ions are normally antiferromagnetic, but with high enough Fex nuclearities and appropriate topologies, such compounds can sometimes possess large ground-state spin values as a result of spin frustration effects among the various Fe2 pairwise exchange pathways,40, 43, 117123 and can even occasionally function as single -molecule magnets (SMMs).12, 13, 19, 102, 124 129 The latter are mol ecules that display slow magnetization relaxation rates and which, below a certain (blocking) temperature (TB), can function as single -domain magnetic particles of nanoscale dimensions. For the above reasons, we have long been interested in the synthesis o f polynuclear Fex clusters. One approach to their synthesis is the hydrolysis or alcoholysis of either a ferric salt in the presence of carboxylate groups, or of a preformed small nuclearity Fex carboxylate cluster, with or without other potentially chelating/bridging ligands. In line with this methodology, a

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68 wide variety of potential ligands have been explored, and a large number of high nuclearity products, with nuclearities up to 64,130 have been isolated.41, 112, 131139 The hydrolysis and alcoholysis r eactions of ferric salts with appropriately chosen ligands in the absence of carboxylates are also a common approach to the synthesis of oxide/hydroxide/alkoxide containing iron complexes, and have yielded various Fex clusters, with nuclearities up to 17.121, 140146 We have been recently exploring the extension of this non -carboxylate FeIII chemistry as a synthetic route to high nuclearity products, with a particular interest in using pyridyl alcohols as potential chelates. Pyridyl alcohols have proven to be extremely versatile chelating and bridging groups that have yielded a number of 3d metal clusters with various structural motifs, large S values, and SMM behavior.44 47, 147152 In a previous chapter, we described the synthesis and properties of Fe4 and Fe6 complexes obtained from the employment of the pyridyl alcohol, 2 (hydroxymethyl)pyridine (hmpH) in non -carboxylate FeIII chemistry.153 We deliberately targeted non -carboxylate FeIII cluster products, and have successfully isolated new complexes which have not been isolated in the presence of carboxylate groups.54 This initial success and the interesting products obtained encouraged us to extend the use of a different pyridyl alcohol, 2,6 pyridinedimethanol (pdmH2) in FeIII cluster chemistry (Figure 3 1 ). The ligand pdmH2 has also proven to be very successful in Mn cluster chemistry, but surprisingly, there is no report of the use of pdmH2 in FeIII cluster chemistry except for the heterometallic complexes.154 We thus explored a number of reactions of FeI II and pdmH2 in both the presence and the absence of carboxylate groups, and these efforts have led us to high nuclearity Fe8, Fe9 and Fe18 products, and ones that have particularly interesting structural features, especially the Fe18 cluster. In the prese nt chapter, we describe the syntheses, structures,

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69 magnetochemical characterization, and high -frequency EPR spectra for these new Fe8, Fe9 and Fe18 clusters obtained from the use of pdmH2. 3.2 Experimental Section 3.2.1 Syntheses All manipulations were pe rformed under aerobic conditions using chemicals and solvents as received, unless otherwise stated. [Fe3O(O2CMe)6(py)3](NO3) w as prepared as reported elsewhere.15515 7 Safety note: Perchlorate salts are potentially explosive; such compounds should be synth esized and used in small quantities, and treated with utmost care at all times [Fe8O3(OMe)(pdm)4(pdmH)4(MeOH)2](ClO4)5 (3 -1). To a stirred solution of pdmH2 (0.14 g, 1.0 mmol) and NEt3 (0.14 mL, 1.0 mmol) in MeOH (30 mL) was added solid Fe(ClO4)36H2O (0. 46 g, 1.0 mmol). The resulting brown solution was stirred for 1 h and filtered, and the filtrate was layered with Et2O (30 mL). After 2 days, brown crystals of 3 -1 7MeOH were collected by filtration, washed with cold MeOH (2 5 mL) and Et2O (2 5 mL), an d dried under vacuum; the yield was ~50%. Anal. Calcd (Found) for 3 -1 (solvent -free): C, 32.38 (32.53); H, 3.27 (3.09); N, 5.12 (5.09). Selected IR data (cm1): 3428 (mb), 1606 (m), 1582 (w), 1469 (w), 1438 (w), 1345 (w), 1265 (w), 1218 (w), 1144 (s), 1118 (s), 1089 (s), 786 (m), 720 (m), 676 (m), 628 (m), 592 (m), 510 (m), 467 (w), 430 (w). [Fe8O3(OEt)(pdm)4(pdmH)4(EtOH)2](ClO4)5 (3 -2). This compound was made by a slight modification to the procedure for the methoxide analogue 3 -1 To a stirred solution o f pdmH2 (0.14 g, 1.0 mmol) and NEt3 (0.14 mL, 1.0 mmol) in EtOH (30 mL) was added solid Fe(ClO4)36H2O (0.46 g, 1.0 mmol). The resulting orange solution was left stirring overnight, during which time a light brown -orange precipitate was obtained. The latte r was collected by filtration, washed with the copious amount of EtOH and Et2O, and dried under vacuum ; the yield

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70 was ~65%. Anal. Calcd (Found) for 3 -2 H2O : C, 32.38 (32.53); H, 3.27 (3.09); N, 5.12 (5.09). Selected IR data (cm1): 3405 (mb), 1607 (m), 1582 (w), 1470 (w), 1438 (w), 1346 (w), 1266 (w), 1218 (w), 1145 (s), 1111(s), 1088 (s), 786 (m), 754 (w), 723 (m), 677 (m), 627 (m), 592 (m), 541 (m), 508 (m), 470 (w), 436 (w). [Fe18O6(OH)8(pdm)10(pdmH)4(H2O)4](ClO4)10 (3 -3). Complex 3 -2 prepared and drie d as described above, was then dissolved in MeCN (30 mL). The resulting brown solution was filtered, and the filtrate was carefully layered with Et2O/hexanes (1:1 v/v). After several days, brown crystals had grown and were collected by filtration, washed w ith cold MeCN (2 5 mL) and Et2O (2 5 mL), and dried under vacuum; the yield was ~10 %. Anal. Calcd (Found) for 3 3 (solvent -free): C, 27.84 (27.66); H, 2.81 (2.79); N, 4.64 (4.46). Selected IR data (cm1): 3420 (mb), 1609 (m), 1582 (w), 1472 (w), 1438 (w), 1361 (w), 1346 (w), 1266 (w), 1216 (w), 1144 (s), 1190 (s), 1041 (s), 786 (w), 721 (m), 677 (m), 627 (m), 550 (m), 520 (m), 474 (m), 419 (m). [Fe9O4(OH)2(O2CMe)10(pdm)(pdmH)4](NO3) (3 -4). To a stirred solution of pdmH2 (0.035 g, 0.25 mmol) in CH2Cl2 (30 mL) was added [Fe3O( O2CMe )6(py)3](NO3) (0.46 g, 0.25 mmol). The resulting brown solution was stirred for 2 h and filtered, and the filtrate was left to slowly concentrate by evaporation. X ray quality crystals of 3 -4 7CH2Cl2 slowly formed over a week. T hese were collected by filtration, washed with cold CH2Cl2 and Et2O, and dried under vacuum; the yield was ~40%. Anal. Calcd (Found) for 3 -4 H2O: C, 33.69 (33.40); H, 3.75 (3.58); N, 4.29 (4.06). Selected IR data (cm1): 3417 (mb), 1584 (s), 1541 (s), 1435 (s), 1385 (s), 1348 (m), 1268 (w), 1236 (w), 1219 (w), 1162 (w), 1114 (m), 1086 (m), 1050 (m), 1021 (m), 981 (w), 790 (w), 713 (m), 657 (s) 617 (m), 534 (m), 503 (m), 448 (m).

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71 3.2.2 X -ray Crystallography Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo = 0.71073 ). Suitable crystals of 3 -1 7MeOH 3 -3 7 MeCN 3H2O and 3 -4 7 CH2Cl2 w ere attached to glass fibers using silicone grease and transferred to a goniostat where they were cooled to 173 K for data collection. An initial search of reciprocal space revealed a monoclinic cell for 3 -1 7MeOH 3 3 7 MeCN 3 H2O and 3 -4 7 CH2Cl2; the choices of space groups 1 P P 21/ n and C2 were confirmed by the subsequent solution and refinement of the structures. 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 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 appli ed based on measured indexed crystal faces. The structures were solved by direct methods in SHELXTL6 ,158, 159 and refined on F2 using full matrix least -squares. The nonH atoms were treated anisotropically, whereas the H atoms were calculated in ideal posi tions and were riding on their respective carbon atoms. Unit cell parameters and structure solution and refinement data are listed in Table 3 1. For 3 -1 7MeOH, the asymmetric unit consists of the complete Fe8 cluster, five ClO4 anions, and seven MeOH mole cules of crystallization. A total of 1225 parameters were included in the structure refinement using 30354 reflections with I > 2 ( I) to yield R1 and wR2 of 7.02 and 16.66%, respectively. For 3 -3 7 MeCN 3H2O the asymmetric unit consists of a half Fe1 8 clus ter (located on an inversion center), five perchlorate anions, three and a half acetonitrile solvent molecules and one and a half water molecules. Three half acetonitrile molecules are disordered against perchlorate

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72 anions. Two perchlorate anions are fully disordered and each was refined in two parts. The third had its oxygen atoms disordered and also was refined in two parts. All hydroxyl and water protons were obtained from a Difference Fourier map and were riding on their parent atoms. A total of 1100 pa rameters were refined in the final cycle of refinement using 8269 reflections with I > 2(I) to yield R1 and wR2 of 8.50 and 21.96%, respectively. For 3 -4 7 CH2Cl2, the asymmetric unit consists of a half Fe9 cluster cation, a half nitrate anion, three and a half dichloromethane solvent molecules. All half fragments are located on 2 fold rotation axes. The nitrate and the half dichlormethane as well as one of the latter located in general position are all disordered and each was refined in two parts. A total of 628 parameters were refined in the final cycle of refinement using 10062 reflections with I > 2(I) to yield R1 and wR2 of 4.22 and 11.61%, respectively. 3.2.3 Other Studies Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 4004000 cm1 range. Elemental analyses (C, H, and N) were perfo rmed by the in -house facilities of the University of Florida Chemistry Department. Variable temperature dc and ac magnetic susceptibility data were collected on a Quantum Design MPMS-XL SQUID susceptometer equipped with a 7 Tesla magnet and operating in the 1.8 300 K range. Samples were embedded in solid eicosane to prevent torquing. Magnetization vs field and temperature data were fit using the program MAGNET.66 Pascal's constants were used to estimate the diamagnetic corrections, which were subtracted fro m the experimental susceptibilities to give the molar paramagnetic susceptibilities (M). High -Frequency Electron Paramagnetic Resonance (HFEPR) measurements were carried out by the Hill group in the UF Physics Department, and were conducted on a crystal o f complex 3 -4 7CH2Cl2 in the 50 to 150

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73 GHz frequency range, and at temperatures between 2 and 20 K, using a Millimeter -wave Vector Network Analyzer (MVNA) and a sensitive cavity perturbation technique described elsewhere.160, 1 61 A dc magnetic field was pr ovided by a 7 T horizontal field, split -pair superconducting magnet associated with a Quantum Design Physical Property Measurement System (PPMS); temperature control was achieved using an associated 4He gas flow cryostat. Angle -dependent measurements were performed using a computer controlled stepper motor to rotate the EPR probe within the vertical split associated with the magnet. 3.3 Results and Discussion 3.3.1 Syntheses Various reactions have been systematically explored with differing reagent ratios, reaction solvents, and other conditions before the following successful procedures were identified The reaction of Fe(ClO4)36H2O and pdmH2 in MeOH gave a brown solution from which were subsequently isolated large brown crystals of [Fe8O3(OMe)(pdm)4(pdmH )4(MeOH)2](ClO4)5 (3 1 ) in ~50% yield. The formation of the cation of 3 -1 is summarized in eq 3 -1. An increase of the NEt3:pdmH2 ratio up to 3:1 gave comparable yields of complex 3 -1 rather than a hoped for higher nuclearity product as a result of complet e deprotonation of pdmHgroups. Further increases in the amount of NEt3 led to amorphous, insoluble precipitates. Complex 3 -1 was also obtained, but in lower yields (<30%), from reactions with FeIII:pdmH2 ratios of 2:1, 3:1, and 1:2 in MeOH. Clearly, comp lex 3 -1 is the preferred product of these reaction components under these conditions. 8Fe3+ + 8pdmH2 + 19NEt3 + 3H2O+3MeOH [Fe8O3(OMe)(pdm)4(pdmH)4(MeOH)2]5+ + 19NHEt3 + (3 1) T he corresponding reaction in EtOH solvent was then investigated and brown pr ecipitate was obtained that was at first thought to be a different type of product from the very soluble 3 -1 ;

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74 however, IR spectral similarities with 3 -1 and elemental analysis data soon established that the product was indeed the analogous [Fe8O3(OEt)(pdm)4(pdmH)4(EtOH)2](ClO4)5 (3 -2 ), and a crystal structure was not pursued. The formation of the cation is summarized in eq 3 2. 8Fe3+ + 8pdmH2 + 19NEt3 + 3H2O + 3EtOH [Fe8O3(OEt)(pdm)4(pdmH)4(EtOH)2]5+ + 19 NHEt3 + (3 2) We have carried out a variety of investigations of the reaction system that leads to 3 -1 and 3 -2 When the reaction mixtures were heated to reflux, light brown precipitates were formed in both MeOH and EtOH. Infrared spectra and elemental analysis data indicated the solids to contain pdmHand/or pdm2 groups and to be different from 3 -1 or 3 -2 We have not been able to characterize these products further, but we believe their insolubility indicates a polymeric structure. When the reaction medium was changed to MeCN, MeCN/alcohol or MeCN/water (without reflux), we were unable to isolate any pure, crystalline products. We therefore turned to exploring the reactivity properties of preformed 3 -1 and 3 -2 and found them to be very sensitive to hydrolysis, which is c onsistent with their bridging alkoxide groups. Thus, dissolution of complex 3 -2 in MeCN, and layering with Et2O/hexanes gave well -formed brown crystals of [Fe18O6(OH)8(pdm)10(pdmH)4(H2O)4](ClO4)10 (3 -3 ) in low (10%) yield. This is clearly a complicated hydrolysis and rearrangement reaction caused by the small amount of water in the MeCN; deliberate addition of more water did not give 3 -3 perhaps due to further hydrolysis. Nor, as impl ied above, did we obtain 3 -3 directly by carrying out the preparative reaction for 3 -2 in MeCN, MeCN/alcohol or MeCN/water. We thus settled for the low but reproducible yield of 3 -3 obtained from preformed 3 -2 The overall transformation is summarized in e q 3 3. 9[Fe8O3(OEt)(pdm)4(pdmH)4(EtOH)2]5+ + 45H2O 4[Fe18O6(OH)8(pdm)10(pdmH)4(H2O)4]10+ + 5H+ + 9EtOH + 16pdmH2 (3 3)

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75 Complex 3 -1 in MeCN was found to undergo the same transformation to 3 3 as confirmed by IR spectral comparisons. The above reactions are summarized in Figure 32. Other sol vents were also explored for the transformation of preformed 3 -2 and acetone was found to give complex 3 -3 in comparable yields. In earlier studies of the reactions of FeIII sources with pyridyl mono alcohol chelate, 2 hydroxymethylpyridine (hmpH),182, 183 the presence or absence of carboxylate groups has a profound effect on the identity of the product, giving structurally very different [Fe6O2(O2CBut)6(hmp)6](NO3)2 and [Fe6O2(NO3)2(hmp)8(H2O)2](NO3)2 products, for example, from the reaction in MeCN of h mpH with [Fe3O(O2CBut)6(H2O)3](NO3) or simple FeIII nitrate, respectively. We thus investigated this point in the present FeIII/pdmH2 chemistry. Indeed, after investigating several reaction conditions and solvents, we found that the reaction of pdmH2 with [Fe3O(O2CMe)6(py)3](NO3) in CH2Cl2 gave the enneanuclear FeIII cluster [Fe9O4(OH)2(O2CMe)10(pdm)(pdmH)4](NO3) (3 -4 ). Complex 3 -4 was the only isolable product, in lower or comparable yield, from a number of reactions in which we varied the solvent, the FeI II starting material, and the reagent ratio. The formation of the cation of 3 -4 is summarized in eq 3 4. 3[Fe3O(O2CMe)6(py)3]+ + 5pdmH2 + 3H2O [Fe9O4(OH)2(O2CMe)10(pdm)(pdmH)4]+ + 8MeCO2 + 9py + 10H+ (3 4) Finally, since 3 -3 and 3 -4 both contain bridging hydroxide ions, we explored the analogous reactions also containing azide to see if azide groups might be incorporated in their place, as seen by Perlepes162, 163 and coworkers for some Fe, Ni and Co clusters where bridging hydroxide groups could be replaced with end -on bridging azide. However, under our conditions, the only isolable products were again 3 -3 and 3 -4

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76 3 .3.2 Description of Structures The partially labeled structure and a stereoview of the cation of complex [Fe8O3(OMe)(pdm)4(pdmH)4(Me OH)2](ClO4)5 (3 -1) are presented in Figure 3 3, together with its labeled core. Selected interatomic distances and angles are listed in Table A 4. Complex 3 -1 possesses a central [Fe4(4O2-)] tetrahedral subunit (Fe3, Fe6, Fe7, Fe8) fused to two [Fe3(3O2 -)] triangular subunits (Fe4, Fe5, Fe6, and Fe1, Fe2, Fe8) at common atoms Fe6 and Fe8. The Fe 4O2 -Fe angles range from 98.1(2) to 131.3(2), deviating significantly from the 109.5 ideal values of a tetrahedron. The Fe atoms are additionally bridged by the alkoxide arms of four pdm2 and four pdmHgroups. The pdm2 -groups are doubly deprotonated and tridentate -chelating to an Fe atom, with each of their alkoxide arms also bridging to adjacent Fe atoms; these groups are thus 1: 3: 1: 3. The pdmHgroup s are singly deprotonated and again tridentate -chelating to an Fe atom, but only the deprotonated alkoxide arm bridges to an adjacent Fe atom; these groups are thus 1: 3: In addition, there is a single MeOgroup bridging Fe6 and Fe8. The complex theref ore contains a [Fe8(4O)(3O)2( OMe)( OR)12] core. The ligation is completed by a terminal MeOH group on each of Fe atoms Fe1 and Fe5. The complete cation has only C1 crystallographic symmetry, but virtual C2 symmetry, the C2 axis passing through metho xide O atom O18 and central 4O2 atom O19. The two [Fe3(3O2-)]7+ triangular units are essentially isosceles (Fe1Fe2 = 3.133(2) Fe1Fe8 = 3.101(4) Fe2Fe8 = 3.529(1) and Fe4 Fe5 = 3.129(2) Fe5Fe6 = 3.094(8) Fe4 Fe6 = 3.541(4) ), the long separation corresponding to the one not bridged by a pdm2or pdmHalkoxide group. This is also reflected in the geometry at the 3O2 ions, O22 and O15, which have Y -shaped geometry (largest Fe O Fe angles of 138.1(2) and 139.4(2), respe ctively) rather than the trigonal planar geometry usually seen in triangular metal carboxylates;164, 1 65 O22 and O15 are also 0.285 and 0.256 respectively, above their Fe3 planes. Six of the Fe atoms (Fe2, Fe3, Fe4, Fe6, Fe7, and Fe8) are

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77 six -coordinate with distorted octahedral geometries, whereas Fe1 and Fe5 are seven -coordinate with distorted pentagonal bipyramidal geometries. The FeN and Fe O bond lengths are as expected for high -spin ironIII.98, 110, 111 Complex 3 -1 p resents a new structural type i n a relatively small family of FeIII clusters with nuclearity of eight,73, 127, 166 174 as well as being the first homometallic FeIII cluster with pdm2and/or pdmHgroups. The s tructure and a stereoview of [Fe18O6(OH)8(pdm)10(pdmH)4(H2O)4](ClO4)10 (3 -3 ) are presented in Figure 3 4, together with its labeled core. Selected interatomic distances and angles are listed in Table A 5. Complex 3 -3 possesses a centrosymmetric [Fe184-O)43-O)23OH)2 OH)6 OR)24] core containing four 4O2(O1, O2), two 3-O2(O3), two OH(O23) and six OH(O18, O19, O20) ions, whose protonation levels were confirmed by bondvalence -sum (BVS) calculations (Table 3 2). The Fe atoms are additionally bridged by the alkoxide arms of ten pdm2and four pdmHgroups. The pdm2groups are doubly-deprotonated and tridentate -chelating to an Fe atom, with each of their alkoxide arms also bridging to adjacent 13: 13. The pdmHgroups are singly -deprotonated and again tridentate -chelating to an Fe atom, but only the deprotonated alkoxide arm bridges to an adjacent 132O groups on Fe6 -coordinate with distorted octahedral geometries. The Fe -N and Fe O bond lengths are as expected for high-spin FeIII.98 110, 111 The core of 3 -3 can be described as a central [Fe4O6] defective dicubane -like sub unit (Fe1, 3OH3O2alkoxide arms of 3pdm24, bottom). On both sides of the central unit, the 3O2is linked to [Fe7O11] units, which can be described as two [Fe4(4O)] tetrahedra (O1, Fe2, Fe4, Fe5, Fe8, and O2, Fe6, Fe7, Fe8, Fe9) fused at Fe8 The Fe 4O2--Fe angles within these

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78 tetrahedra range from 96.1 to 132.8 values of a tetrahedron. The two [Fe33O)] triangular units (Fe1, Fe2, Fe3) are essentially isosceles (Fe1 Fe2 = 3.066 Fe1 Fe3 = 2.996 Fe2 Fe3 = 3.555 ), the long separation corresponding to the one not bridged by a pdm2or OHgroup. This is also reflected in the 3O2ions, O3, which have Y -shaped geometry (largest Fe -O -Fe angle of 141.7) rather than the trigonal planar geometry usually seen in triangular metal carboxylates;164, 1 65 O3 is also 0.272 above its Fe3 plane. Complex 3 -3 joins a small family of only thre e previous Fe18 clusters, two of which are molecular wheel complexes and one a molecular chain, i.e. with a discrete, serpentine -like extended structure.1751 77 Complex 3 -3 is also the highest nuclearity non-carboxylate FeIII cluster discovered to date. T he structure and a stereoview of the cation of [Fe9O4(OH)2(O2CMe)10(pdm)(pdmH)4] (NO3) (3 -4 ) viewed along the ac plane are presented in Figure 3 5. A labeled view along the b axis of the structure and its core are shown in Figure 3 6. S elected interatomi c distances and angles are listed in Table A 6. Complex 3 -4 has imposed C2 symmetry and contains a [Fe93O)4 OH)2 OR)6] core held together by four 3O2 OH(O1 3). The Fe atoms are additionally bridged by the alkoxide arms of one pdm2and four pdmHgroups, as wel l as ten acetates in the 11-fused [Fe3O] triangular units. Two of these (Fe1 Fe2 -Fe5, and its symmetry partner) are scalene (Fe1 Fe2 = 3.507(1) Fe1 Fe5 = 3.364(1) and Fe2 Fe5 = 2.980(1) ), and their central O2 ) in the Fe3 plane. The other two triangular units (Fe2 Fe3 -Fe4, and their symmetry partner) are also scalene (Fe2 Fe3 = 3.508(1) Fe2 Fe4 =

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79 3.284(1) Fe3 Fe4 = 2.939(1) ), but with their O2atoms distinctly out (0.206 ) of their Fe3 plane. In each case, the shortest Fe Fe separation is the bridged by both a 3O2 and a deprotonated pdmHalkoxide arm. All Fe atoms are six -coordinate with octahedral geometry except for the Fe atom in the center (Fe1), which is five -coordinate with distorted trigonal bipyramidal geometry, pdm2 N and Fe O bond lengths in complex 3 -4 are as expected for high -spin FeIII.98 110, 111 There are several Fe9 clusters known in the literature,119, 120 136, 178 182 but none of them have the same Fe9 topology as 3 -4 3.3.3 Magnetochemistry 3.3.3.1 Direct current magnetic susceptibility studies Variable temperature magnetic susceptibility m easurements were performed on powdered polycrystalline samples of 3 -1, 3 -3 and 3 -4 H2O, restrained in eicosane to prevent torquing, in a 1 kG (0.1 T) field and in the 5.0 300 K range. For 3 -1 MT decreases steeply from 11.77 cm3Kmol1 at 300 K to 0.67 cm3Kmol1 at 5.0 K (Figure 3 7). The 300 K value is much less than the spin only ( g = 2) value of 35.00 cm3Kmol1 for eight non -interacting FeIII ions, indicating the presence of strong antiferromagnetic interactions, as expected for oxo -bridged FeIII systems, and an S = 0 ground state. For 3 -3 MT decreases with decreasing temperature from 26.23 cm3Kmol1 at 300 K to 8.47 cm3Kmol1 at 2 5 K and then in creases slightly to 9.05 cm3Kmol1 at 5.0 K (Figure 3 8). The 300 K value is much less than the spin -only ( g = 2) value of 78.75 cm3Kmol1 for eighteen non interacting FeIII ions, indicating dominant antiferromagnetic exchange interactions. The 5.0 K value is close to the spin -only ( g = 2) value of a complex with an S = 4 ground state ( 10.00 cm3Kmol1).

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80 For 3 -4 MT steadily decreases with decreasing temperature from 11.97 cm3Kmol1 at 300 K to 4.17 cm3Kmol1 at 5.0 K (Figure 3 9). The 300 K value is much less than the spin-only ( g = 2) value of 39.38 cm3Kmol1 for nine non interacting FeIII ions, indicat ing domi nant antiferromagnetic exchange interactions The 5.0 K value is close to the spin-only ( g = 2) value of a complex with an S = 5/2 ground state (4.38 cm3Kmol1). To confirm the indicated S = 4 and S = 5/2 ground states of 3 -3 and 3 -4 respectively, and to estimate the magnitude of the zero -field splitting parameter D magnetization vs dc field measurements were made on restrained samples at applied magnetic fields and temperatures in the 1 70 kG and 1.8 10.0 K ranges, respectively. The obtained magnetizatio n ( M ) data for 3 -3 are plotted as reduced magnetization ( M / B) vs H / T in Figure 3 10, where N is Avogadros number and B is the Bohr magneton. They were fit using the program MAGNET66 to a model that assumes only the ground state is populated at these te mperatures and magnetic fields, includes axial zero -field splitting ( D z 2) and the Zeeman interaction, and incorporates a full powder average. The corresponding spin Hamiltonian is given by eq. 3 5, where z is the easy axis spin operator, g is the Land g factor, and 0 is the vacuum permeability. Only data H = D z 2 + B0 H (3 5) collected at fields up to 2 T were employed in the final fit, because satisfactory fits could not be obtained using data collected at higher fields. Such problem s are typical for high nuclearity complexes that have low -lying excited states with S greater than that of the ground state and whose MS levels thus approach those of the ground state with increasing applied fields. The resulting best fit is shown as the s olid lines in Figure 3 10 and was obtained with S = 4, g = 1.94, and D = 0.24 cm1. Alternative fits with S = 3 or 5 were rejected because they gave unreasonable values of g In order to examine the obtained fit quality, we calculated the root -mean -square D vs

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81 g error surface using the program GRID,116 which calculates the relative difference between the experimental M / NB data and those calculated for various combinations of D and g The obtained error surface, plotted as a two -dimensional contour plot in Figure 3 11, in fact exhibits two minima, one with positive D and the other with a negative one. This is as is typically found in such fits, which are not very sensitive to the sign of D Nevertheless, the fit with D < 0 clearly has the smaller relative error, suggesting the true sign of D is negative. Examination of the stretched nature of the contour lines around the best -fit position, corresponding to a somewhat soft minimum, allows the reliability in the fit parameters to be estimated as S = 4, g = 1.9 4(1), and D = 0.24(3) cm1. The M / B vs H / T plot for 3 -4 is shown in Figure 3 12, and we were able to obtain an excellent fit with the program MAGNET using all the data collected up to 7 tesla. This suggests that the ground state of 3 -4 is relatively wel l isolated from the nearest excited states. The obtained error surface for the fit, plotted as a two -dimensional contour plot in Figure 3 13, again exhibits two minima, both of which are much softer (more poorly defined) than those for 3 -3 : one has fit par ameters of S = 5/2, g = 1.94(3) and D = 0.63(9) cm1 (solid lines in Figure 3 12), and another with positive D and fit parameters S = 5/2, g = 1.96(3) and D = +0.89(12) cm1. However, unlike for 3 -3 the two fits for 3 -4 are essentially of equal quality, and it is thus not possible, on the basis of these magnetization fits, to conclude the more likely sign of the axial anisotropy parameter D for 3 -4 3.3.3.2 Alternating current magnetic susceptibility studies As we have described before on multiple occasio ns,152, 183186 ac susceptibility studies are a powerful complement to dc studies for determining the ground state of a system, because they preclude any complications arising from the presence of a dc field. In an ac experiment, a weak magnetic field (typ ically in the 1 5 Oe range) oscillating at some ac frequency is applied to a

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82 sample to investigate its magnetization relaxation dynamics. If the magnetization vector can relax fast enough to keep up with the oscillating field, then there is no out -of -phase susceptibility signal (M phase susceptibility (M ) is equal to the dc susceptibility. If a significant barrier (vs kT ) to magnetization relaxation is present, however, then there is a nonzero M signal and the in -phase signal decreases; such frequency dependent M signals are suggestive of a SMM. The ac data for complex 3 -3 were collected in the 1.8 15 K range in a zero dc field and a 3.5 Oe ac field oscillating at frequencies in the 50 1000 Hz range. The inphase ac susceptibility, plotted as M vs T is shown in Figure 3 14 (there were no out -of phase ac signals down to 1.8 K, the operating limit of our SQUID magnetometer). The M of 3 -3 is 8.87 cm3Kmol1 at 15 K, increases steadily to a plateau of 10.0 cm3Kmol1 at 1.8 K as excited states are depopulat ed, and the 1.8 K value is as expected for an S = 4 ground state with g = 2.0. Complex 3 -3 is thus confirmed to possess an S = 4 ground state. The in -phase M vs T plot for complex 3 -4 is shown in Figure 3 15, and it is essentially temperatureindependent at ~4.3 cm3Kmol1, except for a tiny decrease at the lowest temperatures assignable to very weak intermolecular interactions. The M vs T plot thus supports the conclusion from the dc reduced magnetization fit of a well isolated ground state for 3 -4 The value of ~4.3 cm3Kmol1 is as expected for an S = 5/2 ground state with g ~ 2; S = 3/2 and 7/2 would give M = 2.63 and 7.88 cm3Kmol1, respectively. 3.3.3.3 Rationalization of the S = 5 /2 Ground State of Complex 3-4 It is of interest to try to rati onalize the observed S = 4 and 5/2 ground states of 3 -3 and 3 -4 respectively. However, the high nuclearity and low symmetry of 3 -3 make it impossible for this compound. An S = 4 ground state cannot result from simple spinup/spin -down alignment of

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83 spins, and it is clear that many intermediate spin alignments must be present as a result of extensive spin frustration effects present in the many Fe3 triangular sub units in the core. The smaller size and C2 symmetry of 3 -4 allow more satisfying conclusions to be reached. The nuclearity is still too high to allow the experimental M vs T data to be fit by matrix diagonalization of the appropriate spin Hamiltonian involving the eight symmetry independent nearest -neighbor Fe exchange interactions, Jij; this wou ld involve diagonalizing a matrix of dimensions slightly greater than 1 x 107 by 1 x 107. We thus estimated the Jij values using the magnetostructural correlation of Gorun and Lippard,187 which is based on the bridging Fe O -Fe bond distances. The resulting Jij values ( H = 2 Jiji j convention) calculated for each Fei-Fej interaction are shown in Figure 3 16, where the viewpoint is that of Figure 3 6 (bottom). As expected, all interactions are antiferromagnetic and span a range from 6.0 to 41.2 cm1. Consideration of spin frus tration ( competing exchange interactions) within each Fe3 triangular sub unit leads to the qualitative conclusions of the individual spin alignments shown in Figure 3 16. In almost all cases, one of the interactions is significantly weaker (by a factor of two or more) than the other two interactions, allowing us to conclude that this interaction will clearly be frustrated and the spin alignments therefore determined by the other two. The weaker, frustrated interactions are shown in blue. The exception is J34 ( 25.4 cm1), which is within ~4 cm1 of J23 ( 29.6 cm1) but we propose the Fe3 spin is nevertheless locked parallel to the Fe4 spin by the combined stronger J23 and J35 interactions. We thus offer Figure 3 16 as a rationalization of the S = 5/2 ground state of 3 -4 which can be summarized as an outer S = 0 loop of four antiparallel aligned Fe2 pairs of S = 5 spins, and an inner S = 5/2 Fe1 spin, giving the overall S = 5/2 spin of the complete molecule. No doubt other spin alignments involving some spins in intermediate alignments also significantly contribute to the ground state spin wavefunction of the molecule,

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84 but we believe Figure 3 16 describes the main component, on the basis of the calculated Jij values at least. 3.3.3.4 Single -Crystal, High Freq uency EPR (HFEPR) Spectroscopy HFEPR studies were carried out by the Hill group in the UF Physics Department. For an independent means of confirming the ground state spin, S as well as the sign and magnitude of the axial ZFS parameter D HFEPR measurement s were carried out on a single crystal of 3 4 7CH2Cl2. As shown in the previous section, ac susceptibility and dc magnetization measurements can provide good estimates of S and D but HFEPR measurements provide much more accurate and reliable values, parti cularly in the case of D Furthermore, the sign of D can be determined unambiguously from EPR studies, which is not easy to do from fits of powder magnetization data, as stated.188 Sample alignment was first achieved by performing in-situ rotation of the s ample in order to locate extrema (easy/hard directions) among plots of the angle -dependent EPR peak positions. Once aligned, measurements were performed as a function of frequency and temperature so as to provide data sets that maximally constrain the ZFS parameters. Figure 3 17(a) displays a series of angle -dependent HFEPR spectra at 91.3 GHz; the data in the figure focus on a narrow angle interval either side of one of the extrema. Based on previous studies of similar FeIII complexes,60, 188 we identify this particular orientation as the hard plane. Upon closer inspection of the data, it is apparent that most of the peaks are split into doublets (see e.g. the two vertical arrows between 4.5 and 5 T). Furthermore, the angle -dependence of the two peaks in ea ch doublet appears to be slightly different. This is most clearly illustrated in Figure 3 17(b), which plots the locations of the two peaks associated with the highest field resonance observed above 4.5 T. From these plots, we conclude that the crystal emp loyed in this study possesses two hard planes. The X ray crystal structure shows only one orientation of molecules in the unit cell of 3 -4 7CH2Cl2, ruling out

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85 different orientations as the cause of the HFEPR peak doubling. We thus conclude (a posteriori) t hat the sample used for the HFEPR investigation was twinned, leading to the two rotation patterns observed in Figure 3 17(b). Although this complicates the analysis slightly, it does not preclude an accurate determination of S and D Indeed, the procedure s employed in this investigation have been applied successfully to many other samples for which two or more molecular orientations exist.227, 228 The angle -dependent plots in Figure 3 17(a) indicate that the field intersects the hard planes associated with the twinned crystal roughly 5 apart. Furthermore, the fact that the maxima occur at different magnetic field values is indicative of the presence of transverse ZFS interactions, i.e. the field intersects the two hard planes at different orientations with respect to the hard/medium axes associated with the twinned crystal.189 190 Thus, in some sense, more information is obtained than would otherwise have been the case for a single crystal. However, characterization of the transverse ZFS terms would requir e very time consuming two axis rotation studies,191 192 which is well beyond the scope of the present investigation. Given that only a single axis rotation capability was employed for these experiments, perfect easy axis alignment is never guaranteed. How ever, by rotating the sample 90o from one of the hard planes, one can usually achieve a sufficient parallel field component ( Bz) for accessing the z -component of the HFEPR spectrum from which the axial ZFS is easily extracted. Indeed, extrapolation of simu lations to zero -field enables very tight constraints on the axial ZFS parameters D and B4 0 (see below). However, the obtained g value is not so reliable. For these investigations, we rotated 90o away from the hard plane located at 7 in Figure 3 17(b) due to the fact that the corresponding peak is stronger than for the other hard plane. Figure 3 18 displays a series of HFEPR spectra for 3 -4 7CH2Cl2 at different temperatures in the 2 to 20 K range, and with the field aligned closer to the easyaxes of the twinned crystal

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86 than for the data in Figure 3 17. Two series of resonances can be seen (labeled and ), corresponding to the two different orientations within the twinned sample. The various peaks can be identified by virtue of the fact that they occur in pairs exhibiting essentially the same temperature dependence, and from the subsequent simulati ons (see below). For some of the peak assignments, subscripts are included with the label indicating the approximate magnitude of MS associated with the level from which the transition is excited. At the lowest temperature, the strongest peaks in each seri es ( and ) are observed at the lowest field, which indicates that they correspond to excitations from the lowest levels within the ground state S multiplet. This observation confirms the negative sign of D .60 However, even at the lowest temperature of 2 K, significant populations exist among excited MS levels due to the relatively weak ZFS in this complex. Figure 3 19 displays the frequency dependence of HFEPR peak positions for 3 -4 7CH2Cl2 corresponding to the same field orientation as the spectra dis played in Figure 3 18. The red and the corresponding best simulations of the frequency dependence, obtained via exact diagonalization of eq. 3 5. The primary adjustable parameters used for these simulations were the spin, S = 5/2, a single axial ZFS parameter D = 0.48(1) cm1, and the field orientation relative to the two easy axes of the twinned crystal (16o and 38o, see Figure 3 19). The g value was held co nstant and assumed to be isotropic (2.00), as expected for FeIII, and inclusion of a rhombic ZFS parameter E (~0.07 cm1) slightly improved agreement among the weak higher field resonances; inclusion of a fourth -order axial B4 0 term did not, however, impr ove the simulations. The uncertainty associated with the rhombic parameter is considerable, because the plane of field rotation relative to the two crystals is not known. Nevertheless, the improvement in the

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87 simulations in Figure 319 upon inclusion of a rhombic term is consistent with indications from the hard plane measurements that complex 3 -4 possesses a significant rhombic anisotropy. In contrast, the obtained D value is very reliable, as evidenced by the excellent agreement between the lower field pe ak positions and the corresponding simulations. We note the different field misalignment angles (relative to the two easy axes) found from the simulations in Figure 3 19. This indicates that the two components of the twinned crystal are significantly misal igned, and that it was fortuitous that the field intersected the two hard planes in relatively close proximity; see Figure 3 17(b). Note that the D = 0.48(1) cm1 obtained from HFEPR is significantly different from the 0.63(9) cm1 value obtained from th e fits of powder magnetization data, emphasizing again the need for studies by a technique such as EPR when more reliable and accurate values are required. Finally, we comment on the S = 5/2 ground state spin assignment based on HFEPR. The spectra in Figure 3 18 exhibit significant MSdependent peak broadening. This behavior may be attributed to D -strain, most likely resulting from solvent or ligand disorder that, in turn, causes inhomogeneous broadening of the transitions.193 195 Within this picture, one can understand the fact that the sharpest resonance corresponds to the MS = to MS = + transition ( 1/2), since there is no ZFS associated with these two Kramers levels.102 In fact, the 1/2 resonance becomes very sharp at higher fields/frequencies ( not shown), presumably due to reduced dipolar field fluctuations. On the basis that the MS = to MS = + assignment can be made with near 100% certainty, our confidence in the spin S = 5/2 ground state assignment is equally robust. 3 .4 Conclusions The first ever employment of the tridentate O,N,O ligands pdm2 -/pdmHin homometallic FeIII chemistry has led to three interesting new Fe8 (3 -1 ), Fe18 (3 -3 ) and Fe9 ( 3 -4 ) clusters, all of

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88 which are of an unprecedented structural type. The Fe18 complex 3 -3 is t he highest nuclearity complex to date in non -carboxylate FeIII chemistry and can be obtained readily from hydrolysis of [Fe8O3(OEt)(pdm)4(pdmH)4(EtOH)2](ClO4)5 (3 -2 ) or its MeO /MeOH analogue ( 3 -1 ), but we have not been able to prepare it directly from sim ple FeIII salts. It has an unusual cigar like core, and we see no reason why longer, higher nuclearity analogues might not be accessible. The complexes have S = 0 ( 3 -1 ), 4 ( 3 -3 ) and 5/2 ( 3 -4 ) ground states, the latter two clearly arising from spin frustrat ion effects within the many Fe3 triangular sub units; the S value for 3 -3 is impossible to rationalize but that for 3 -4 can be rationalized on the basis of the exchange interactions predicted from published magnetostructural correlations. The magnetization fits for 3 -3 gave a value for D of 0.24(3) cm1, whereas for 3 -4 equally good f its were obtained with positive and negative D values and with large uncertainties in D emphasizing the difficulty of determining the sign of D for FeIII clusters from such measurements. However, use of the powerful HFEPR spectroscopic technique both identified the true D value of 3 -4 to be negative, and provided a reliable value of 0.48(1) cm1. The combined results demonstrate the usefulness of pdmH2, which is poorly exp lored in FeIII chemistry, to give interesting new high nuclearity products, and shows once again that the presence or absence of carboxylates can have a marked effect on the obtained products. We also find it of interest that preformed alkoxide containing clusters can undergo mild hydrolysis to yield clusters not accessible directly from simple starting materials, suggesting a possible means of targeted nuclearity increase of known compounds.

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89 Table 3 1. Crystallographic Data for 3 -1 7MeOH, 3 -3 7MeCNH2O a nd 3 -4 7CH2Cl2. parameter 3 1 3 3 3 4 formula C 66 H 99 N 8 O 42 Cl 5 Fe 8 C 112 H 145 Cl 10 Fe 18 N 21 O 89 C 62 H 79 C l14 Fe 9 N 6 O 39 fw, g mol 1 2300.58 4569.29 2531.26 crystal system triclinic monoclinic monoclinic space group 1 P P 2 1 / n C 2 a 13.6178(14) 26.3129(17) 25.150(2) b 18.6824(19) 13.9984(9) 15.3942(13) c 19.069(2) 27.3080(18) 14.1502(12) deg 78.547(2) 90 90 deg 76.319(2) 116.9220(10) 115.8900(10) deg 87.39(2) 90 90 V 3 4619.9(8) 8968.5(10) 4928.7(7) Z 2 2 2 T C 173(2) 173(2) 173(2) radiation, a 0.71073 0.71073 0.71073 calc g cm 3 1.654 1.692 1.706 mm 1 1.457 1.6 57 1.747 R 1 b,c 0.0702 0.0850 0.0422 wR 2 d 0.1666 0.2196 0.1161 a Graphite monochromator. b I > 2 (I). c R 1 = 100 (|| Fo| | Fc||)/ | Fo|. d wR 2 = 100[ [ w ( Fo 2 Fc 2)2]/ [ w ( Fo 2)2]]1/2, w = 1/[ 2( Fo 2) + [(ap)2 + bp], where p = [max( Fo 2, O) + 2 Fc 2]/3.

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90 N O H O H Table 3 2. Bond Valence Sums for Selected O Atoms in Complex 3 -3 .a atom BVS assgt. atom BVS assgt. O1 1.79 O 2 O20 1.19 OH O2 1.82 O 2 O21 0.39 H 2 O O3 2.11 O 2 O22 0.34 H 2 O O18 1.11 OH O23 1.16 OH O19 1.14 OH a The oxygen atom is an O2if the BVS is ~1.8 2.0, an OHif the BVS is ~1.0 1.2, and an H2O if the BVS is ~0.2 0.4. Table 3 3. Bond Valence Sums for Selected O Atoms in Complex 3 -4 .a atom BVS assgt. O1 1.17 OH O2 2.08 O 2 O3 2.02 O 2 a The oxygen atom is an O2if the BVS is ~1.8 2.0, a n OHif the BVS is ~1.0 1.2, and an H2O if the BVS is ~0.2 0.4. pdmH2 Figure 3 1. Structure of ligand: 2,6 pyridinedimethanol (pdmH2).

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91 Fe3+ + pdmH2 + NEt3 1:1:1 in MeOH 1:1:1 in EtOH [Fe8O3(OMe)(pdm)4(pdmH)4(MeO H)2]5+ [Fe8O3(OEt)(pdm)4(pdmH)4(EtOH)2]5+ (cation of 3 -1 ) (cation of 3 -2 ) MeCN MeCN [Fe18O6(OH)8(pdm)10(pdmH)4(H2O)4]10+ (cation of 3 -3 ) Figure 3 2. Summary of the reactions concerning 3 -1 3 -2 and 3 -3

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92 Figure 3 3. The structure of complex 3 -1 (top), a stereopair (middle), and the labeled core. Hydrogen atoms have been omitted for clarity. Color code: FeIII green; O red; N blue; C gray.

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93 Figure 3 4. The structure of comp lex 3 -3 (top), a stereopair (middle), and the labeled core. Hydrogen atoms have been omitted for clarity. Color code: FeIII green; O red; N blue; C gray.

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94 Figure 3 5. The structure of complex 3 -4 (top), and a stereopair (bottom), viewed along t he ab plane. Hydrogen atoms have been omitted for clarity. Color code: FeIII green; O red; N blue; C grey.

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95 Figure 3 6. The structure of complex 3 -4 (top), and its labeled core (bottom), viewed along the baxis. Hydrogen atoms have been omitted for c larity. Color code: FeIII green; O red; N blue; C grey.

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96 Figure 3 7. Plot of MT vs T for complex 3 -1 Figure 3 8. Plot of MT vs T for complex 3 -3 Temperature (K) 0 50 100 150 200 250 300 350 MT (cm 3 Kmol -1 ) 0 2 4 6 8 10 12 14 Temperature (K) 0 50 100 150 200 250 300 MT (cm3Kmol-1) 0 5 10 15 20 25 30

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97 H/T (kG/K) 0 2 4 6 8 10 12 M/N B 0 2 4 6 8 0.1 T 0.5 T 1 T 2 T fitting Figure 3 9. Plot of MT vs T for complex 3 -4 Figure 3 B) vs H/T plot for complex 3 -3 The solid lines are the fit of the data; see the text for the fit parameters. Temperature (K) 0 50 100 150 200 250 300 MT (cm3Kmol-1) 0 2 4 6 8 10 12 14

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98 Figure 3 11. Two-dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 3 -3 Figure 3 3 -4 The solid lines are the fit of the data; see the text for the fit parameters. g 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 D (cm-1) -0.4 -0.2 0.0 0.2 0.4 0.60 0.65 0.70 0.80 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.75 0.55 0.35 0.70 0.50 0.30 0.65 0.45 0.60 0.40 0.25 0.55 0.35 0.50 0.20 0.30 0.45 0.25 0.40 0.20 0.15 0.35 0.20 0.15 0.30 0.10 0.25 0.10 0.25 0.20 0.05 0.30 0.15 0.10 0.35 0.10 0.10 0.15 0.40 0.20 0.45 0.15 0.10 0.25 0.50 0.30 0.20 0.55 0.35 0.25 0.60 0.40 0.30 H/T (kG/K) 0 10 20 30 40 M/N B 0 1 2 3 4 5 0.1 T 0.5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T fitting

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99 Figure 3 13. Two-dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 3 -4 Figure 3 14. In-phase ac susceptibility ( M M 3 -3 in a 3.5 Oe ac field oscillating at the indicated frequencies. g 1.80 1.85 1.90 1.95 2.00 2.05 D (cm-1) -1.0 -0.5 0.0 0.5 1.0 0.30 0.35 0.40 0.40 0.35 0.35 0.30 0.30 0.25 0.25 0.20 0.20 0.20 0.25 0.35 0.20 0.15 0.15 0.15 0.30 0.15 0.10 0.10 0.25 0.10 0.20 0.05 0.05 0.05 0.05 0.15 0.10 0.10 0.10 0.10 0.05 0.15 0.15 0.15 0.10 0.05 0.20 0.20 0.20 0.10 0.10 0.15 0.25 0.25 0.20 0.15 0.30 0.25 0.35 0.20 0.30 Temperature (K) 0 2 4 6 8 10 12 14 16 M'T (cm3Kmol-1) 0 2 4 6 8 10 12 1000Hz 250Hz 50Hz

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100 Temperature (K) 0 2 4 6 8 10 12 14 16 M'T (cm3Kmol-1) 0 1 2 3 4 5 6 1000Hz 250Hz 50Hz Figure 3 15. In-phase ac susceptibility ( M M 3 -4 in a 3.5 Oe ac field oscillating at the indicated frequencies.

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101 F e 5 F e 1 F e 5 O O F e 2 F e 2 F e 3 F e 3 O O F e 4 O O O O O O O O F e 4 3 0 4 3 5 6 1 7 1 6 0 2 9 6 2 5 4 4 1 2 1 5 5 1 5 5 4 1 2 3 5 6 2 5 4 3 0 4 2 9 6 6 0 1 7 1 Figure 3 16. Rationalization of the S = 5/2 ground state of 3 -4 on the basis of the predicted magnitudes of the various pairwise Jij exchange constants and the resulting spin frustration effects; frustrated interactions are sho wn in blue. The viewpoint and atom labels are those of Figure 3 6 (bottom).

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102 Figure 3 17. (a) Angle -dependent HFEPR spectra (2 steps) for complex 3 -4 obtained at 8 K and 91.3 GHz. The red traces correspond to the hard planes of two crystals in the twi nned sample (see main text). (b) Plot of the peak positions associated with the highest field doublet observed in (a); see main text for explanation.

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103 0 1 2 3 4 5 6 1 / 2f = 9 1 3 G H z Transmission (arb. units of 2 0 K 1 5 K 1 0 K 8 K 6 K 4 K 2 K5 / 25 / 23 / 23 / 2 Figure 3 18. 91.3 GHz temperature dependent HFEPR spectra for complex 3 -4 with the applie d field aligned closer to the easy axes of the two crystals in comparison to the data in Figure 3 crystals in the twinned sample; for some of the peaks, subscripts are included with t he label indicating the approximate magnitude of ms associated with the level from which the transition is excited.

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104 0 1 2 3 4 5 6 7 40 60 80 100 120 140 Frequency (GHz)Magnetic field (tesla)B inclined to the easy axis by 16o and 38o Figure 3 19. Frequency dependence of the HFEPR peak positions for complex 3 -4 corresponding to the same field orientation as the spectra displayed in Figure 3 17. The red and blue curves represent the corresponding best simulations of the frequency dependence; see text for furthe r explanation.

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105 CHAPTER 4 MN CLUSTERS FROM THE USE OF BULKY PYRIDYL ALCOHOLS : STRUCTURAL AND MAGNETIC STUDIES 4.1 Introduction The synthesis and characterization of 3 d transition metal clusters with various nuclearities and metal topologies have been of great interest due to their interesting physical properties a s well as the intrinsic architectural beauty and aesthetically pleasing structures they possess.196 In particular, they often have large ground -state spin and easy axis -type magnetic anisotropy, which provides a significant energy barrier to reversal of the magnetization vector. Thus, at sufficiently low temperatures they function as nanoscale magnetic particles, and they therefore represent a molecular approach to nanomagnetism.12, 13 3335 197 Such single -molecule magnets (SMMs) not only exhibit hysteresis in magnetization versus dc field scans, the diagnostic property of a classical magnet, but also fascinating quantum mechanical properties such as quantum tunneling of magnetization (QTM)18, 1 98 and quantum phase interference.199201 The first SMM discovered was [Mn12O12(O2CMe)16(H2O)4],15, 17, 18 and its synthetic manipulation has provided a very well studied family of related complexes.12 Since the discovery of Mn12 complexes, many polynucle ar complexes containing 3 d transition metals have been reported to be SMMs,124, 153, 154, 202, 203 with most of them containing primarily MnIII ions. This is because Mn clusters often display relatively large ground -state S values, as well as negative D va lues (easy axis anisotropy) associated with the presence of JahnTeller (JT) distorted MnIII atoms. In addition, some of these molecules have unique structures, such as the giant Mn84 SMM with a torus structure.204 For the above reasons, we and others hav e explored and successfully developed many new routes for the synthesis of polynuclear Mn complexes, with nuclearities as high as 84.204 These procedures have included comproportionation reactions of simple starting materials,46, 205, 206

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106 reductive aggrega tion of permanganate ions,207209 aggregation of clusters having smaller nuclearities,186, 210, 211 reductive aggregation or fragmentation of preformed clusters,212, 2 13 electrochemical oxidation,214216 and ligand substitution of preformed species,217219 among others. As part of this work, a wide variety of potentially chelating and/or bridging ligands were explored in order to foster the formation of high nuclearity products.44, 45, 147, 151,190, 220 238 Among these are pyridyl alcohols, especially 2 (hy droxymethyl)pyridine (hmpH, Figure 4 1), 44, 45, 147, 151,190 which has proven to be an extremely versatile chelating and bridging group that has yielded a number of 3 d metal clusters with various structural motifs, large S values, and SMM behavior. In ord er to explore new Mn cluster chemistry from such ligands, we have initiated a project in which the steric bulk of chelates such as hmpH has been increased by the addition of bulky groups at positions that we expect to influence the identity of obtained clu ster products. In the present chapter, we describe the use in Mn cluster chemistry of an hmpH derivative in which two bulky phenyl or methyl groups have been added onto the CH2 unit. The resulting molecules, diphenyl hmpH (dphmpH; IUPAC name is diphenyl (p yridine 2 -yl)methanol) and dimethyl hmpH (dmhmpH, IUPAC name is 2 -(pyridine 2 yl)propan 2 -ol) are shown in Figure 4 1. We anticipated that the use of dphmpH and dm hmpH in metal cluster chemistry would give products distinctly different from those with hmp H, and have therefore explored their use initially in Mn chemistry. Note that dphmpH has only been used for the synthesis of mononuclear Zr, W, and Mo complexes;239241 dmhmpH has only been employed for the synthesis of mononuclear complexes of V, Zr, Mo, and W, and dinuclear complexes of Ni and W .242 248 In this work, we have deliberately targeted higher nuclearity Mn products by exploring reactions of dphmpH and dm hmpH with MnII salts under slightly basic conditions. This has

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107 successfully led to new Mn4, Mn6, Mn7, Mn11, and Mn12 clusters, and their syntheses, structures, and magnetochemical characterization are described in this chapter. 4.2 Experimental Section 4.2.1 Syntheses All preparations were performed under aerobic conditions except for the synthe sis of dp hmpH and dm hmpH, which were carried out as previously reported.249, 250 All chemicals were used as received. Safety note: Perchlorate salts are potentially explosive; such compounds should be synthesized and used in small quantities, and treated w ith utmost care at all times [Mn4O2(O2CBut)5(dphmp)3] (4 -1 ). To a stirred solution of dphmpH (0.26 g, 1.0 mmol) and NEt3 (0.42 mL, 3.0 mmol) in MeCN/MeOH (30 mL, 5:1 v/v) was added solid MnCl2 4H2O (0.20 g, 1.0 mmol) and NaO2CButH2O (0.25 g, 2.0 mmol). T he mixture was stirred overnight, filtered to remove NaCl, and the filtrate layered with Et2O (60 mL). X ray quality crystals of 4 1 2MeCN slowly grew over 5 days in 55% yield. These were collected by filtration, washed with cold MeCN (2 3 mL) and Et2O ( 2 5 mL), and dried under vacuum. Anal. Calc. (Found) for 4 -1 2MeCN H2O (C83H93N5Mn4O15): C, 61.52 (61.10); H, 5.78 (5.99); N, 4.32 (3.93). Selected IR data (cm1): 3446(mb), 3059(w), 3020(w), 2956(m), 2926(m), 2896(w), 2867(w), 2361(m), 2337(m), 1603(s), 1578(m), 1564(s), 1481(s), 1445(w), 1406(s), 1369(m), 1356(m), 1224(m), 1168(w), 1111(w), 1082(w), 1402(m), 1024(m), 953(w), 930(w), 907(w), 891(w), 778(m), 750(w), 703(m), 683(m), 659(m), 648(m), 635(m), 607(m), 597(m), 541(w), 525(w), 492(w), 463(w), 434(m). [Mn6O4(OMe)2(O2CPh)4(dphmp)4] (4 -2 ). Method A. To a stirred solution of dphmpH (0.26 g, 1.0 mmol) and NEt3 (0.42 mL, 3.0 mmol) in MeCN/MeOH (30 mL, 5:1 v/v) was added solid Mn(O2CPh)2 (0.33 g, 1.0 mmol). The resulting dark brown solution was stirre d overnight, filtered, and the filtrate left undisturbed to concentrate slowly by evaporation. X -ray quality

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108 crystals of 4 -2 3MeCN slowly grew over 2 weeks in 20% yield. These were collected by filtration, washed with cold MeCN (2 3 mL) and Et2O (2 5 m L), and dried under vacuum. Anal. Calc. (Found) for 4 -2 H2O (C120H86N4Mn6O20): C, 60.73 (60.51); H, 4.30 (3.96); N, 2.78 (3.09). Selected IR data (cm1): 3415(mb), 3057(w), 2935(w), 2739(w), 2677(w), 2491(w), 1598(s), 1561(s), 1489(w), 1475(w), 1445(w), 1389(s), 1251(w), 1170(m), 1044(m), 1024(m), 951(w), 928(w), 910(w), 841(w), 774(m), 707(m), 681(m), 661(m), 633(m), 607(m), 582(m), 529(m), 461(w), 433(w). Method B. To a stirred solution of dphmpH (0.26 g, 1.0 mmol) and NEt3 (0.42 mL, 3.0 mmol) in MeCN/Me OH (30 mL, 5:1 v/v) was added solid MnCl2 4H2O (0.20 g, 1.00 mmol) and NaO2CPh (0.29 g, 2.00 mmol). The mixture was stirred overnight, filtered to remove NaCl, and the filtrate left undisturbed to concentrate slowly by evaporation. X ray quality crystals of 4 2 3MeCN slowly grew over 2 weeks in 15% yield. These were collected by filtration, washed with cold MeCN (2 3 mL) and Et2O (2 5 mL), and dried under vacuum; the product was identified by IR spectral comparison as identical with material from Method A. [Mn11O7(OMe)7(O2CPh)7(dphmp)4(MeOH)2] (4 -3 ). Method A. To a stirred solution of dphmpH (0.26 g, 1.0 mmol) and NEt3 (0.42 mL, 3.0 mmol) in MeCN/MeOH (30 mL, 1:29 v/v) was added solid Mn(O2CPh)2 (0.33 g, 1.0 mmol). The resulting dark brown solution was s tirred overnight, filtered, and the filtrate left undisturbed to concentrate slowly by evaporation. X ray quality crystals of 4 -3 4MeCN slowly grew over 2 weeks in 25% yield. These were collected by filtration, washed with cold MeCN (2 3 mL) and Et2O (2 5 mL), and dried under vacuum. Dried solid analyzed as solvent -free. Anal. Calc. (Found) for 4 -3 (solvent -free) (C130H120N4Mn11O34): C, 54.09 (53.89); H, 4.19 (3.98); N, 1.94 (1.91). Selected IR data (cm1): 3417(mb), 3059(w), 2919(w), 2809(w), 1598(s), 1557(s), 1490(w), 1474(w), 1445(w), 1390(s),

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109 1252(w), 1171(w), 1048(m), 953(w), 930(w), 910(w), 836(w), 773(w), 706(m), 680(m), 680(m), 657(w), 634(m), 592(m), 565(m), 468(w), 425(w). Method B. To a stirred solution of dphmpH (0.26 g, 1.0 mmol) and NEt3 (0.42 mL, 3.0 mmol) in MeCN/MeOH (30 mL, 1:29 v/v) was added solid MnCl2 4H2O (0.20 g, 1.0 mmol) and NaO2CPh (0.29 g, 2.0 mmol). The mixture was stirred overnight, filtered to remove NaCl, and the filtrate left undisturbed to concentrate slowly by evaporation. X ray quality crystals of 4 3 3MeCN slowly grew over 2 weeks in 20% yield. These were collected by filtration, washed with cold MeCN (2 3 mL) and Et2O (2 5 mL), and dried under vacuum; the product was identified by IR spectral comparison as identical with material from Method A. [Mn7O3(OH)3(O2CBut)7(dmhmp)4] (4 -4 ). To a stirred solution of dmhmpH (0.14 g, 1.0 mmol) and NEt3 (0.14 mL, 1.0 mmol) in a solvent mixture comprised of MeCN/MeOH (27 mL, 25:2 v/v) was added solid MnCl2 4H2O (0.20 g, 1.0 mmol) and NaO2CBut H2O (0.25 g, 2.0 mmol). The mixture was stirred overnight and filtered to remove NaCl, and the filtrate was left undisturbed to concentrate slowly by evaporation. X ray quality crystals of 4 -4 7MeCN slowly grew over 2 weeks in 50% yield. These were collected by filtration, washed with cold MeCN (2 3 mL), and d ried under vacuum. Anal. Calc. (Found) for 4 -4 MeCN (C69H113N5Mn7O24): C, 46.53 (46.93); H, 6.39 (6.32); N, 3.93 (3.51). Selected IR data (cm1): 3423(wb), 2956(m), 2926(w), 2868(w), 1595(s), 1545(s), 1482(s), 1458(w), 1415(s), 1358(s), 1277(w), 1225(w), 1 181(m), 1123(m), 1101(w), 1502(w), 1032(w), 982(m), 889(w), 782(m), 757(w), 741(w), 661(s), 611(m), 563(m), 516(w), 436(w). [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] (4 -5 ). Method A. To a stirred solution of dmhmpH (0.14 g, 1.0 mmol) and NEt3 (0.42 mL, 3.0 mmol) in a solvent mixture comprised of CH2Cl2/MeOH (31 mL, 30:1 v/v) was added solid Mn(O2CPh)2 (0.33 g, 1.0 mmol). The

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110 resulting dark brown solution was stirred overnight and filtered, and the filtrate was layered with Et2O (60 mL). X ray quality crysta ls of 4 -5 3CH2Cl2 slowly grew over 2 weeks in 55% yield. These were collected by filtration, washed with Et2O (2 5 mL), and dried under vacuum. Anal. Calc. (Found) for 4 -5 3H2O (C118H119N4Mn12O42): C, 48.46 (48.34); H, 4.10 (3.84); N, 1.92 (1.83). Select ed IR data (cm1): 3434(mb), 3063(w), 2974(m), 2922(m), 2813(w), 1603(s), 1565(s), 1483(m), 1401(s), 1305(w), 1278(w), 1252(w), 1180(m), 1123(m), 1101(w), 1067(w), 1052(m), 1027(m), 978(m), 936(w), 885(w), 839(w), 816(w), 782(m), 717(s), 667(s), 620(s), 557(s), 463(m), 433(m). Method B. To a stirred solution of dmhmpH (0.14 g, 1.0 mmol) and NEt3 (0.42 mL, 3.0 mmol) in a solvent mixture comprised of MeCN/MeOH (30 mL, 5:1 v/v) was added solid Mn(O2CPh)2 (0.33 g, 1.0 mmol). The resulting dark brown solution w as stirred for 3 hours and filtered, and the filtrate was left undisturbed. Brown crystals were obtained after 3 days in 45 % yeild. These were collected by filtration, washed with cold MeCN (2 3 mL) and Et2O (2 5 mL), and dried under vacuum; the produ ct was identified by IR spectral comparison with material from Method A. (HNEt3)[Na Mn6O4(dmhmp)6(N3)4](ClO4)2 (4 -6 ). To a stirred solution of dmhmpH (0.28 g, 2.0 mmol), NaN3 (0.13 g, 2.0 mmol) and NEt3 (0.28 mL, 2.0 mmol) in a solvent mixture comprised of MeCN/MeOH (21 mL, 20:1 v/v) was added solid Mn(ClO4)2 6H2O (0.72g, 2.0 mmol). The obtained solution was stirred for 2 hours, filtered, and the filtrate layered with Et2O. Red crystals of 4 -6 Et2O grew over a week in 50% yield. These were collected by filtration, washed with cold MeCN (2 3 mL) and Et2O (2 5 mL), and dried under vacuum. Anal. Calc. (Fo und) for 4 -6 Et2O (C58H92Cl2N19Mn6NaO19): C, 39.07 (39.29); H, 5.20 (4.82); N, 14.93 (15.14). Selected IR data (cm1): 3438(wb), 2970(m), 2678(w), 2058(s), 1610(m), 1485(w),

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111 1437(w), 1322(m), 1281(w), 1253(w), 1181(m), 1122(s), 1092(s), 1055(m), 1030(m), 986(m), 883(w), 790(w), 760(w), 740(w), 675(m), 662(s), 626(s), 603(m), 563(w), 530(w), 500(w), 425(w). 4 .2.2 X -ray Crystallography Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo -K radiation ( = 0.71073 ). Suitable crystals of 4 -1 2MeCN, 4 -2 3MeCN, 4 -3 4MeCN, 4 -4 7MeCN, 4 -5 3CH2Cl2, and 4 -6 Et2O were attached to glass fibers using silicone grease and transferred to a goniostat where they were cooled to 173 K for data collection. 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 were remeasured at the end of data collection to monitor instrument and crystal sta bility (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by Direct Methods in SHELXTL6 ,64 and refined on F2 using full -matrix least squares. The nonH a toms were treated anisotropically, whereas the H atoms were calculated in ideal positions and refined as riding on their respective C atoms. For 4 -1 2MeCN the asymmetric unit contains the Mn4 cluster and two MeCN molecules. A total of 979 parameters were refined in the final least -squares cycle using 14826 reflections with (I) to yield R1 and wR2 of 6.28 and 15.09%, respectively. For 4 -2 3MeC N the asymmetric unit contains two half Mn6 clusters, and three MeCN molecules. The latter were disordered and could not be modeled properly, thus program SQUEEZE,65 a part of the PLATON package of crystallographic software, was used to calculate the solv ent disorder area and remove its contribution to the overall intensity data. A total of 1171

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112 parameters were refined in the final least -squares cycle using 6680 reflections with I > 2(I) to yield R1 and wR2 of 8.07 and 18.53%, respectively. For 4 -3 4MeCN the asymmetric unit contains the Mn11 cluster and four MeCN molecules. The latter were disordered and could not be modeled properly, thus program SQUEEZE was again used to calculate the solvent disorder area and remove its contribution to the overall int ensity data. The O22 position of the Mn11 cluster is disordered between an OH and a methoxy group. The protons of the two coordinated MeOH groups were obtained from a difference Fourier map and refined freely. A total of 1637 parameters were refined in the final least-squares cycle using 12365 reflections with I > 2(I) to yield R1 and wR2 of 5.66 and 10.18%, respectively. For 4 -4 7MeCN the asymmetric unit consists of a Mn7 cluster and seven acetonitrile solvent molecules. The latter were disordered and c ould not be modeled properly, thus program SQUEEZE was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. The three protons H4, 5 and 6, on O4, 5 and 6 were obtained from a Difference Fourier map and H4 a nd H6 were refined freely but H5 was refined riding on O5. A total of 915 parameters were refined in the final cycle of refinement using 14701 reflections with I > 2(I) to yield R1 and wR2 of 8.59 and 16.20%, respectively. For 4 -5 3CH2Cl2, the asymmetric unit consists of a Mn12 cluster and three disordered dichloromethane solvent molecules. The latter were disordered and could not be modeled properly, thus program S QUEEZE was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. The benzoate ligands at Carbon atoms 81 and 91 are disordered and each was refined in two positions with their site occupation factors fixed a t 0.5 (after refining to near 50%). The phenyl rings of the disordered benzoate ligands were idealized

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113 to a hexagonal shape. A total of 30912 parameters were refined in the final cycle of refinement using 16954 reflections with I > 2(I) to yield R1 and wR2 of 4.19 and 11.64%, respectively. For 4 -6 Et2O the asymmetric unit consists of 1/3 Mn6Na clusters, two 1/3 perchlorate anions, 1/3 triethylamino cation and a 1/3 ether molecule also located on a 3-fold rotation axis. Both anions, ether and the amino cation are disordered and each was refined in two parts with their site occupation factors tied to a total on 1. Each species lies on a 3 -fold rotation axis. A total of 343 parameters were refined in the final cycle of refinement usi ng 3565 reflections with I > 2(I) to yield R1 and wR2 of 2.93 and 7.30%, respectively. Unit cell data and details of the structure refinements for 4 -1 to 4 -3 are collected in Table 4 1, 4 -4 to 4 -6 in Table 4 2. 4.2.3 Other Studies Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 4004000 cm1 range. Elemental analyses (C, H and N) were performed by the in house facilities of the University of Florida, Chemistry Department. Variable temperature DC and AC magnetic susceptibi lity data were collected on a Quantum Design MPMS -XL SQUID susceptometer equipped with a 7 T magnet and operating in the 1. 8 300 K range. Samples were embedded in solid eicosane to prevent torquing. Magnetization vs. field and temperature data was fit usin g the program MAGNET. Pascal's constants251 were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibilities to give the mola r paramagnetic susceptibility ( M). Magnetic studies below 1.8 K were carried out on single crystals using a micro-SQUID apparatus operating down to 0.04 K.

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114 4.3 Results and Discussion 4.3.1 Syntheses One strategy to make clusters containing MnIII ions is to oxidize simple MnII salts by atomospheric O2 under prevailing basic conditions i n the presence of potentially chelating ligands. In Mn/hmpH chemistry, this strategy has led to the isolation of a series of clusters possessing an [Mn4(hmp)6]4+ core.148150, 252 254 Therefore, we decided to also employ this strategy with the bulkier deri vatives dmhmpH and dphmpH, and a variety of reaction ratios, reagents, and other conditions were investigated before the following procedures were developed. 4.3.1.1 Reactions with dphmpH The reaction of dphmpH with MnCl2 4H2O, NaO2CButH2O and NEt3 in a 1:1:2:3 ratio in MeCN/MeOH (5:1) afforded a brown solution from which was subsequently obtained the tetranuclear complex [Mn4O2(O2CBut)5(dphmp)3] (4 -1 ) in 55% yield; t he mixed solvent system was used to ensure adequate solubility of all reagents. A picture of crystals of 4 -1 is shown in Figure 4 2. The formation of 4 -1 is summarized in eq. 4 1 assuming atmospheric O2 as the oxidizing agent. Small variations in the Mn/ dphmpH / ButCO2 ratio still gave complex 4 -1 which clearly is a preferred product of these components and with pivalate. We also employed other 4 Mn2+ + 5 ButCO2 + 3 dphmpH + 3 NEt3 + O2 [Mn4O2(O2CBut)5(dphmp)3] + 3 NHEt3 + (4 1 ) Mn salts, e.g., NO3 or ClO4 -, but again obtained complex 4 -1 in every case. We then investigated the identity of Mn/dphmpproducts as a function of carboxylate groups. The reaction of dphmpH with Mn(O2CPh)2 and NEt3 in a 1:1:3 ratio in MeCN/MeOH (5:1) gave a brown solution from which was subsequently isolated the hexanuclear complex [Mn6O4(OMe)2(O2CPh)4(dphmp)4] (4 -2 ) in 20% yield (Method A). A picture of crystals of 4 -2 is

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115 shown in Figure 4 3 The similar reaction of dphmpH with MnCl2 4H2O, NaO2CPh and NEt3 in a 1:1:2:3 ratio in MeCN/MeOH (5:1) also gave 4 -2 in a slightly lower yield of 15% (Method B). Its formation via Method A is summarized in eq. 4 2 Increasing or decreasing the amount of dphmpH also gave complex 4 -2 but at a low er purity. The low yield of the reaction clearly 6 Mn(O2CPh)2 + 4 dphmpH + H2O + 2 MeOH + 3/2 O2 [Mn6O4(OMe)2(O2CPh)4(dphmp)4] + 8 PhCO2H (4 2 ) indicates as with many other reactions in MnIII chemistry, that the reaction solution probably contains a complicated mixture of several species in equilibrium, with factors such as relative solubility, lattice energies, crystallization kinetics, and others determining the identity of the product that crystallizes. One (or more) of these factors is undoubtedly the reason that changing the carboxylate used from pivalate to benzoate causes a major change in the identity of the product from 4 -1 to 4 -2 Along these lines, it is worth noting that we were unable to isolate any pure product from the use of Mn(O2CMe)2 4H2O in an otherwise identical reaction system. We also investigated the identity of the product as a function of the solvent composition. When the reaction that gives 4 -2 was carried out with less MeOH, the product was still 4 -2 but in an a ppreciably lower yield. However, at the other extreme, when the reaction was performed in predominantly MeOH, namely MeCN/MeOH (1:29), the undecanuclear complex [Mn11O7(OMe)7(O2CPh)7(dphmp)4(MeOH)2] ( 4 -3 ) was obtained in 25% yield. The small amount of MeCN was found beneficial in obtaining well -formed, X ray quality crystals. A picture of crystals of 4 -3 is shown in Figure 4 4 Its formation is summarized in eq. 4 -3. 11 Mn(O2CPh)2 + 4 dphmpH + 15 NEt3 + 9 MeOH + 2 H2O + 5/2 O2 [Mn11O7(OMe)7(O2CPh)7(dphmp)4(MeOH)2] + 15 PhCO2 + 15 NHEt3 + (4 3 )

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116 4.3.1.2 Reactions with dmhmpH The reaction of dmhmpH with MnCl2 4H2O, NaO2CBut H2O and NEt3 in a 1:1:2:1 ratio in MeCN/MeOH afforded a brown solution from which was subsequently obt ained the heptanuclear complex [Mn7O3(OH)3(O2CBut)7(dmhmp)4] (4 -4 ) in 50%. A picture of crystals of 4 -4 is shown in Figure 4 5. Its formation is summarized in eq. 4 4, assuming atmospheric O2 as the oxidizing agent. The mixed solvent system was needed to e nsure adequate solubility for all reagents. Small variations in the Mn/dmhmpH/ButCO2 ratio also gave complex 4 -4 which 7 Mn2+ +7 ButCO2 + 4 dmhmpH + 7 NEt3 + 1/2 H2O + 9/4 O2 [Mn7O3(OH)3(O2CBut)7(dmhmp)4] + 7 NHEt3 + (4 4) clearly is a preferred product of these components and with pivalate. We also employed other Mn salts, e.g., NO3 or ClO4 -, but again obtained complex 4 -4 in every case. We then investigated the identit y of product as a function of carboxylate groups. The reaction of dmhmpH with Mn(O2CPh)2 and NEt3 in a 1:1:3 ratio in CH2Cl2/MeOH (30/1) gave a brown solution from which was isolated the dodecanuclear complex [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] (4 -5 ) in 55% yield (Method A). A picture of crystals of 4 -5 is shown in Figure 4 6 The same reaction in MeCN/MeOH (25/5) also gave 4 -5 in a slightly lower yield of 45 % (Method B). Its formation is summarized in eq. 4 5 12 Mn(O2CPh)2 + 4 dmhmpH + 17 NEt3 + 18 H2O + 2 MeOH + 5/4 O2 [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] + 12 PhCO2 + 17 NHEt3 + (4 5 ) In our previous work in Mn/hmpH chemistry, we often observed the strong tendency of the hmpalkoxide arm to bridge two or even three Mn atoms, without the need for other chelatin g/bridging groups to form metal clusters. For example, a series of Mn clusters possessing an [Mn4(hmp)6]4+ core were isolated from the reactions of MnX2 (X = Cl-, NO3 -, or ClO4 -) and hmpH under basic conditions. 148150, 252254 Among the six hmpalkoxide arms in the

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117 [Mn4(hmp)6]4+ core, two are in the 3-bridging modes and four are in the -bridging mode. We therefore tried non -carboxylate reactions using dmhmpH to see whether identical or similar noncarboxylate clusters with / 3alkoxide arms of dmhmpare obtained. We have attempted a variety of d mhmpH reactions by differing Mn starting materials, reagent ratios, solvents, and other conditions; however, we were unable to isolate any pure crystalline materials from the se non -carboxylate reactions. Pure, crystalline materials were only isolated after the addition of carboxylate groups to the reactions as stated above. The use of azide is an exception to this statement ; when NaN3 was added to the noncarboxylate reactions, a Mn/dmhmp-/N3 product was successfully isolated. The reaction of dmhmpH with Mn(ClO4)2 6H2O, NaN3, and NEt3 in a 1:1:1:1 ratio in MeCN/MeOH (20:1) afforded a reddish brown solution from which was subsequently obtained the hexanuclear complex (HNEt3)[NaMn6O4(dmhmp)4(N3)4](ClO4)2 (4 -6 ) in 50% yield; the mixed solvent system was used to ensure the adequate solubility of all reagents. A picture of crystals of 4 -6 is shown in Figure 4 7 The formation of 4 -6 is summarized in eq. 4 6 6 Mn2+ + 6 dmhmpH + 4 NaN3 + 8 NEt3 + 3/2 O2 + H2O [Na Mn6O4(dmhmp)6(N3)4]+ + 3 Na+ + 8 NHEt3 + (4 6 ) Interestingly, the use of hmpH in an otherwise identical reaction system has led to the isolation of [Mn10O4(N3)4(hmp)12](ClO4)2MeCN ( 4 -7 ).44 Both 4 -6 and 4 -7 present very symmetric structures, and 4 -6 is the inner core of 4 -7 (vide infra); i.e., by using the bulky derivative of hmpH, part of the core of the known structure was isolated. 4.3.2 Description of Structures The partially labeled structure of [Mn4O2(O2CBut)5(dphmp)3] (4 -1 ) is shown in Figure 4 8 ; s elected interatomic distances and angles are lis ted in Table A 7

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118 Complex 4 -1 crystallizes in the rare triclinic space group P 1 and possesses a near planar Mn4 unit bridged by 3O2 ions O1 and O2 above and below the Mn4 plane. A [Mn4(3O)2]8+ core containing an exactly planar Mn4 rhombus is what we have described as a planar butterfly unit in the past,38, 255, 256 to emphasize its relationship to the related [Mn4O2]8+ complexes with a true butterfly (i.e. V -shaped) Mn4 topology, such as that in the hmpcluster anion [Mn4O2(O2CPh)7(hmp)2](4 -8 Fi gure 4 9 ) with virtual C2 symmetry.257 In both the butterfly and planar -butterfly complexes obtained previously, including 4 -8 each Mn2 edge of the [Mn4(3O)2] core is bridged by one or two carboxylate groups and each wingtip Mn atom is chelated by bidentate ligand. The butterfly vs planar butterfly difference is caused by the presence or absence, respectively, of a 1: 1: carboxylate group bridging t he two central (body) Mn atoms. Complex 4 -1 is overall similar to these previous complexes in possessing a dphmpchelate on wingtip atoms Mn1 and Mn2, and bridging ButCO2 groups, but differs in having an unusually low symmetry as a result of (i) posses sing a third dphmpgroup in a 1: 2: bridging mode chelating body atom Mn3 and bridging to Mn2 with its deprotonated alkoxide arm in a very asymmetric manner (Mn3 O5 = 1.887(3) vs Mn2 O5 = 2.505(3) ); and (ii) the carboxylate group bridging Mn2 and Mn4 also bonds weakly to Mn3 (M n3 -O12 = 2.784(3) vs Mn4 O12 = 2.282(3) ) and is thus in an approximately 2: 1: binding mode. As a result, the Mn4 topology of 4 -1 is intermediate between the true planar butterfly and V -shaped butterfly topologies observed previously. Charge conside rations and the metric parameters indicate all Mn atoms to be MnIII, as confirmed by BVS calculations258, 259 (Table 4 3 ). All Mn atoms are six coordinate with near -octahedral geometries, and undergo the expected Jahn -Teller (JT) axial elongation; the elon gation axes at Mn2 and Mn3 contain the long bonds mentioned above, with O12 also lying on the JT elongation axis of Mn4. For Mn1, the JT axis is O6 -Mn1 O8.

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119 The partially labeled structure and a stereoview of [Mn6O4(OMe)2(O2CPh)4(dphmp)4] (4 2 ) are present ed in Figure 4 10; selected interatomic distances and angles are listed in Table A 8 Complex 4 -2 crystallizes in the triclinic space group 1 P with two essentially superimposable independent molecules in the unit cell, both lying on inv ersion centers; only one will therefore be discessued below. The centrosymmetric molecule consists of a [Mn6O4(OMe)2]8+ core with peripheral ligation provided by six bridging PhCO2 and four bidentate, chelating dphmpgroups. The core has a face -sharing d ouble -cubane structure, containing two 3O2-, two 4O2-, and two 3-MeOions bridging the six Mn atoms. This is a very unusual and very rare structural type, and there has been only one previous report of a complex with this dicubane like Mn6 core, name ly [Mn6O4(OMe)2(OAc)4(Mesalim)4] (Mesalim= methyl salicylimidate anion).260 The Mn atoms in 4 -2 are all six coordinate with near octahedral geometry, and charge considerations and the metric parameters indicate they are all MnIII, as confirmed by BVS cal culations (Table 4 4). The MnIII atoms thus all display JT axial elongations. Normally, the latter avoid Mn -oxide bonds, almost always the strongest and shortest in the molecule, but this is not possible for every Mn atom in 4 -2 because of the double -cuban e topology: for Mn2 and Mn3 (and their symmetry related partners), the JT axes are those containing the O atoms of the 3MeOions, but for Mn1 and Mn1 the JT elongation axes contain a 4O2 ion O2 or O2, respectively; the location of all JT axes are indicated as yellow bonds in Figure 4 4. The JT axes of Mn1, Mn1 Mn3, and Mn3 are oriented nearly parallel to each other, and perpendicular to those of Mn2 and Mn2 ; they are thus all oriented in the xy planes, if the z axis is defined as the long axis of the double -cubane. There are no significant intermolecular interactions. The complete structure and a stereoview of [Mn11O7(OMe)7(O2CPh)7(dphmp)4(MeOH)2] (4 -3 ) are shown in Figure 4 11, and its labeled core in Figure 4 12; selected interatomic distances

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120 and angles are listed in Table A 9 Complex 4 -3 possesses a [Mn11(4O)4( 3-O)5(-OR)6] core containing four 4O2 (O30, O31, O33, and O34), three 3-O2(O28, O29 and O32), two 3MeO(O17 and O19), five -MeO(O18, O20, O21, O22, and O25), and one -ROalkoxide arm (O1) of a 1: 2: -dphmpgroup that is chelating on Mn2 and bridging to Mn3 in a very asymmetric manner (Mn2 O1 = 1.879(1) vs Mn3 O1 = 2.415(2) ). The other three dphmpgroups are bound as 2 chelates. Six of the seven benzoates are br idging two MnIII atoms in the common 1: 1: -bridging mode, and the seventh is bound terminally to MnII atom Mn11 and hydrogen -bonds to one of the two terminal MeOH groups on Mn11 that complete the ligation. The Mn oxidation states were deduced from charge considerations and the metric parameters, and confirmed by BVS calculations (Table 4 5). The core of 4 -3 can be described as consisting of a [Mn4O3(OMe) ] cubane (Mn4, Mn7, Mn8, and Mn9) linked on one side to the MnII atom (Mn11) by a cubane O2and two -MeOgroups. On the other side, the cubane face is linked to a [Mn5O7] unit which can be described as a central [Mn4O3] defective cubane ( Mn2, Mn3 and Mn5 ) sharing two of its faces with two other defective cubanes ( Mn1, Mn2, Mn5 and Mn3, Mn5, Mn6 ), the la tter of which is linked by 3-O2 -ion O28 to another Mn atom (Mn10) that is also attached to the cubane by a -MeOgroup. It is thus interesting that 4 -3 bears some structural similarity to 4 -2 in containing a cubane unit. Complex 4 -3 is the first example of this low symmetry Mn11 topology, and in fact joins only a very small family of Mn/O clusters with a nuclearity of eleven.50, 261 The structure and a stereoview of [Mn7O3(OH)3(O2CBut)7(dmhmp)4] (4 -4 ) are shown in Figure 4 13, and its core in Figure 4 14. Selected interatomic distances and angles are listed in Table A 10. The complex contains an [Mn7(4O)(3-O)2( O)5] core comprising a [Mn4(3O)2] butterfly like unit and a [Mn4(4O)] tetrahedral unit fused at shared Mn atom Mn1 (Figure 4 -

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121 14). Note that the butterfly unit is related to [Mn4O2(O2CPh)7(hmp)2](4 -8 Figure 4 9 ), which has an hmpgrou p chelating to each wing-tip MnIII atom, as do two of the dmhmpgroups in 4 4 One of the latter and the two other dmhmpgroups each bound in a 1: 2: mode, chelating to a MnIII of the tetrahedral unit and bridging with their O atom to the MnII atom, Mn7. Two additional OHions, O4 and O5, bridge between the body Mn atoms of the butterfly unit and Mn5 and Mn2, respectively. Ligation is completed by six 1: 1: pivalate groups bridging MnIIIMnIII pairs, a chelating pivalate on MnII atom Mn7, and ater minal OHion (O6) on Mn6. Charge considerations, the metric parameters, and the presence of MnIII Jahn Teller ( JT) distortions (axial elongations) indicate a MnIIMnIII 6 description, as confirmed by bond valence sum (BVS) calculations (Table 4 6). BVS cal culations were also performed on the inorganic O atoms to identify their degree of protonation and thus distinguish O2-, OH-, and H2O situations (Table 4 7). These confirm three O2 ions (one 4O2 (O2) and two 3O2 (O1 and O3) ions), and three OHions (a terminal OH(O6) and two OHO4 and O5 ions). The BVS value of 0.63 for O6 is a little lower than expected for a OHion, but is consistent with it acting as an acceptor atom for two hydrogenbonds with -OHions O4 and O5 (O4O6 = 2.711(5) O5O6 = 2.767(5) ). All the Mn atoms are six-coordinate with distorted octahedral geometry; Mn2 and Mn5 are very distorted, with one JT elongated bond being particularly long (Mn2 O15 = 2.588(3) Mn5 O13 = 2.522(3) ). The structure and a stereoview of [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] (4 -5 ) are shown in Figure 4 15, and its core in Figure 4 1 6 Selected interatomic distances and angles are listed in Table A 11. The complex is mixed -valen t MnII 3MnIII 9, as indicated by the metric parameters and confirmed by BVS calculations (Table 4 8 ), and contains an [Mn12(4O)4(3O)5( O)7] core consisting of a central [Mn4(4O)2(3O)4] face -sharing incomplete dicubane

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122 (Mn1, Mn2, Mn3, Mn4), and on either side of this is attached a [Mn4(4O)] tetrahedral unit. There is an interesting asymmetry in that the tetrahedral un it is MnIIMnIII 3 on one side and MnII 2MnIII 2 on the other. BVS calculations were performed on the inorganic O atoms to assess their degree of protonation (Table 4 9 ), and there are four 4O2(O1, O6, O7, O8), three 3O2 (O2, O4, O5), one 3OH(O3), o ne 3-MeO(O38), one MeO(O39), four -ROalkoxide arms (O9, O10, O11, O12) of 1: 2: -dmhmpgroups, and a terminal water molecule (O37) Two additional O atoms are provided by benzoate groups bridging in 3 modes (O22 and O16). Peripheral ligation is provided by twelve benzoate groups exhibiting four binding modes: eight bridge t wo Mn atoms in the common 1: 1: mode; two bridge three Mn atoms in the rarer 2: 1: mode; one chelates MnII atom Mn12; and one is terminally bound to MnII atom Mn6, with its inbound O atom (O30) statically disordered between forming a H -bond to the nea rby water (O37) (O30O37 = 2.654(5) 3OH(O3) group (O30O3 = 2.713(3) The four dmhmpgroups all bind in a 1: 2: mode, as in 4 -4 The Mn atoms are six -coordinate with distorted octahedral geometry, except for Mn11, which is five coordina te unless a long sixth bond (Mn11 O24 = 2.842(3) is included. It is interesting to note that the Mnx topologies of 4 -4 and 4 -5 are related, consisting of one (4 -4 ) or two ( 4 -5 ) [Mn4(4O)] tetrahedral units attached to a second type of unit: in 4 -4 the latter is an [Mn3(3O)] triangular unit (which forms a butterfly with one of the Mn atoms of the tetrahedral unit), whereas in 4 -5 it is two edge -sharing [Mn3(3-O)] triangular units (the core of the incomplete face -sharing dicubane). It is also of i nterest that Mn12 benzoate complex 4 -5 is similar to a reported Mn12 benzoate complex with hmp-, [Mn12O8Cl4(O2CPh)8(hmp)6] (4 -9 ).151 Disregarding differences such as MeOin 4 -5 vs Clin 4 -9 and looking instead at the overall structures, the two complexe s are

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123 similar in that both consist of a central face -sharing incomplete dicubane on either side of which are two tetrahedral units attached to the oxide ions of the former. However, there is a major difference in that 4 -9 is centrosymmetric whereas 4 -5 is unusually asymmetric, in both the distribution and binding modes of many ligands, and in the Mn oxidation state distribution, being [MnII 3MnIII 9] in 4 -5 and [MnII 2MnIII 10] in 4 -9 In both cases, however, the dmhmpand hmpgroups are all bound in the same 1: 2: modes. The structure and a stereoview of [Na Mn6O4(dmhmp)4(N3)4](ClO4) (4 -6 ) are shown in Figure 4 17, together with its labeled core. Selected interatomic distances and angles are listed in Table A 12. Complex 4 -6 crystallizes in the trigonal spac e group P 31c The structure of 4 -6 consists of a Mn6 octahedron with four nonadjacent faces bridged by 3N3 ions, and among the other four faces, three are bridged by 3O2 ions (O1), with the other one bridged by the 4-O2ion (O3), the latter being b ound to a Na ion. Alternatively, the [Mn6O4(N3)4]6+ core can be described as a tetrahedron with a N3 at each vertex, a Mn at the midpoint of each edge, and an O2bridging each face. The O2ions lie 0.44 (O1) and 0.35 (O3) above the Mn3 plane s they br idge Charge consideration and qualitative inspection of the metric parameters indicate a MnIII 6 metal oxidation -state description. All Mn ions are six -coordinate with distorted octahedral geometry. As expected for the high-spin MnIII (d4) in near -octahedr al geometry, there is a Jahn Teller (JT) distortion, and it takes the form of an axial elongation of the two trans Mn -N3 bonds; the JT axes form the edge in the tetrahedral description of [Mn6O4(N3)4]6+. L igation is completed by six dmhmpgroups. Three of them chelate to Mn atoms (Mn2), and the other three both chelate to Mn atoms (Mn1) and bridge with the ir alkoxide arms to the Na atom; these groups are thus 1: 2: The Na ion is further bound to a nitrogen atom (N6) of an azide group of a neighboring molecule, thus linking Mn6 units to form a one -dimentional chain (Figure 4 1 8 ). The

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124 molecule has a crystallographic C3 rotation axis passing thorough the N3 Na+ 4O2(O3) bonds. There are two previous examples of Mn6 complexes with a [Mn6O4( 3X)4] face capped octahedral topology in the literature, with X being Cl or Br.262 It is interesting that the Mn6 topology of 4 -6 is related to the previously report ed decanuclear Mn/hmp-/N3 cluster, [Mn10O4(N3)4(hmp)12](ClO4)2MeCN ( 4 -7 ),44 in that both consist of a Mn6 octahedron with four nonadjacent faces bridged by 3N3 ions: in 4 -6 among the other four faces, three are bridged by 3O2 ions, and the other one is bridged by a 4O2 ion, the latter of which is bound to a Na ion. I n 4 -7 the four remaining faces of the Mn6 octahedron are bridged by four 4-O2ions, which also bridge to four external MnII ions that thus cap the four non adjacent faces of the o ctahedron. Therefore, we found that the use of bulky hmpH, dmhmpH, led to the isolation of the inner core of th is known hmpH product. 4.3.3 Magnetochemistry 4.3.3.1 Direct current magnetic susceptibility studies Variable temperature magnetic susceptibilit y measurements were performed on microcrystalline powder samples of complexes 4 -1 2MeCN H2O, 4 -2 2H2O 4 -3 4 -4 MeCN, 4 5 3H2O, and 4 -6 Et2O in a 1 kG (0.1 T) dc field and in the 5.0 300 K range. The samples were restrained in eicosane to prevent torquing in the applied field. The obtained data for complex 4 -1 2MeCN H2O are shown as a MT versus T plot in Figure 4 1 9 MT gradually decreases from 7.57 cm3Kmol1 at 300 K to 4.99 cm3Kmol1 at 40.0 K and then decreases more rapidly to 1.72 cm3Kmol1 at 5.0 K. The 300 K value is much smaller than the spin -only ( g = 2) value of 12 cm3Kmol1 for four noninteracting MnIII atoms, indicating the presence of dominant antiferromagnetic exchange interactions within the molecule. MT is clearly heading for zero at the lowest temperatures, indicating that complex 4 -1 possesses an S =

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125 0 ground state. Because of this and the low symmetry of the molecule, which requires five separate Mn2 pairwise exchange parameters ( Jij) to describe its exchange coupling (and more if interactions with next -nearest neighbors are included), we did not pursue fits of the MT vs T data by matrix diagonalization to determine the values of the exchange interactions. Unfortunately, the more convenient Kambe vector coupling method cannot be applied to such a low symmetry molecule. In any case, the exchange interactions are expected to be weak, based on the many previous magnetic studies of such molecules that we have reported, making accurate determinations of so many independent J values problematic, to say the least. For complex 4 -2 2H2O ,MT gradually decreases from 13.8 cm3Kmol1 at 300 K to 6.64 cm3Kmol1 at 5.0 K (Figure 4 20). The 300 K value is again much smaller than the spin -only ( g = 2) value of 18 cm3Kmol1 for six noninteracting MnIII atoms, indicating the presence of at least s ome strong antiferromagnetic interactions. However, the plot is clearly not heading to zero at 0 K, indicating that 4 -2 has a ground state with S > 0, and the 5.0 K value is close to the spinonly value of 6.00 cm3Kmol1 expected for an S = 3 ground state. Although complex 4 -2 is more symmetric than 4 -1 it is still not possible to apply the Kambe method for fitting the MT vs T data. We thus chose to confirm the ground state by ac studies, as will be described below (vide infra). For complex 4 -3 ,MT steadily decreases from 29.4 cm3Kmol1 at 300 K to 19.9 cm3Kmol1 at 50.0 K, and then decreases more rapidly to 8.14 cm3K mol1 at 5.0 K (Figure 4 21). The 300 K value is much smaller than the spin -only value of 34.4 cm3Kmol1 for one MnII and ten MnIII noninteracting atoms indicating the presence of dominant antiferromagnetic interactions. As for 4 -2 2H2O, the MT at 5.0 K of 4 -3 is not heading for zero at 0 K, indicating an S > 0 ground state,

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126 and the value at 5 K can be compared with the spin -only ( g = 2) values of 4.38 and 7.88 cm3Kmol1 expected for S = 5/2 and 7/2 states, respectively. For complex 4 -4 MeCN, MT graduall y decreases from 16.22 cm3Kmol1 at 300 K to a value of 13.44 cm3Kmol1 at 25.0 K and then rapidly decreases to 10.11 cm3Kmol1 at 5.0 K (Figure 4 22). The 300 K value is much less than the spin-only ( g = 2) value of 22.38 cm3Kmol1 for six MnIII and one M nII noninteracting ions, indicating the presence of dominant antiferromagnetic exchange interactions. However, the plot is clearly not heading to zero at 0 K, indicating that 4 -4 has a ground state with S > 0, and the value at 5 K can be compared with the spin only ( g = 2) values of 7.88 cm3Kmol1 expected for S = 7/2 state. For complex 4 -5 3H2O,MT gradually decreases from 29.94 cm3Kmol1 at 300 K to a minimum of 20.69 cm3Kmol1 at 40.0 K, and then increases to 23.75 cm3Kmol1 at 5 K (Figure 4 23). The 3 00 K value is much less than the spin -only ( g = 2) value of 40.13 cm3Kmol1 for three MnII and nine MnIII non -interacting ions, indicating the presence of antiferromagnetic interactions, but the MT versus T profile suggests there may also be significant f erromagnetic interactions as well. The 5.0 K value is indicative of an S = 13/2 ground state (expected MT = 24.38 cm3Kmol1 for g = 2). For complex 4 -6 Et2O, MT gradually increases from 26.32 cm3Kmol1 at 300 K to a near plateau value of ~77 cm3mol1K at 4020 K and then slightly decreases to 74.23 cm3mol1K at 5.0 K (Figure 4 24). The 300 K value is larger than the spin-only ( g = 2) value of 18 cm3Kmol1 for six MnIII noninteracting ions, indicating the presence of dominant ferromagnetic exchange interactions. The T near -plateau value in the 20 40 K range appears to be heading for a final value of ~78 cm3mol1K, the spin -only ( g = 2) value of a species with an S = 12 ground state, before exhibiting the final decrease at temperatures below 10 K. The latter decrease is likely due

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127 to a combination of zero -field splitting (ZFS), Zeeman effects, and any weak intermolecular interactions. The metal ions are thus clearly involved in magnetic exchange interactions, and the data were thus fit to the theoretical MT vs T expr ession derived from the spin Hamiltonian appropriate for a Mn6 octahedron; this is given in eq. 4 7, where Si refers to the spin of metal = 2 Jcis(1 2 + 1 3 + 1 4 + 1 6 + 2 3 + 2 5 + 2 6 + 3 4 + 3 5 + 4 5 + 4 6 + 5 6) 2 Jtrans(1 5 + 2 4 + 3 6) (4 7) Mni, and Jcis and Jtrans are the pairwise exchange parameters for adjacent and opposite metals of the octahedron, respectively; the M n labeling scheme of Figure 4 25 was employed. Note that we assume that the interaction with the Na+ ion causes minimal change to the J values. This Hamiltonian can be transformed into an equivalent form (eq 4 8) by use of the Kambe vector coupling method263 and the substitutions A = 1 + 5, B = 2 + 4, C = 3 + 6 and T = A + B + C where ST is the resultant spin of the complete molecule. = Jcis(T 2 A 2 B 2 C 2) Jtrans(A 2+ B 2 + C 2 1 2 2 2 3 2 4 2 5 2 6 2) (4 8) From eq 4 8 can be obtained the energy expression (eq. 4 9) for the energies, E (ST), of each ST state ; constant terms contributing equally to all states have been omitted from eq 4 9. E (ST) = Jcis[ST(ST+1) SA(SA+1) SB (SB +1) SC(SC+1)] Jtrans[A(A+1) + B(B+1) + C(C +1)] (4 9) An expression for the molar paramagnetic susceptibility, M, was derived using the above and the Van Vleck equation,264 and assuming a n isotropic g tensor (Appendix C 1). This was then used to fit the experimental MT vs T data in Figure 4 1 8 as a function of the two exchange parameters Jcis and Jtrans, and the g facto r. Only data for the 20.0 300 K range were used, since the model does not incorporate ZFS and other minor effects and therefore cannot reproduce the

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128 decrease at lower temperatures. Good fits were obtained with fit parameters of Jcis = 5.75 cm1, Jtrans cm1, and g = 1.99, with temperature independent paramagnetism (TIP) held constant at 600 106 cm3mol1. The fit indicate s that Mn6 complex 4 -6 has an ST = 12 ground state In the notation ST, SA, SB, SC> this is the 12, 4, 4, 4 > state in which all s ix MnIII spins are aligned parallel. The first excited state is a triply degenerate set of ST = 11 states comprising the 4, 4 > > and > states at 92 cm1 above the ground state. The S = 12 ground state in the Mn6 complexe s is thus well isolated from the nearest excited state. To confirm the indicated S = 3, 5/2, 7/2, 13/2, and 12 ground states of complex 4 -2 4 -3 4 4 4 -5, and 4 -6 respectively, and to estimate the magnitude of the zero -field splitting parameter D magnet ization vs dc field measurements were made on restrained samples at applied magnetic fields and temperatures in the 1 70 kG and 1.8 10.0 K ranges, respectively. W e then attempted to fit the data, using the program MAGNET,66 by diagonalization of the spin H amiltonian matrix assuming only the ground state is populated, incorporating axial anisotropy ( D z 2) and Zeeman terms, and employing a full powder average. The corresponding spin Hamiltonian is given by eq 4 10, where z is the easy axis spin operator, g is the Land g factor, B is the Bohr magneton, and 0 is the vacuum permeability. However, for complex 4 -2 4 -3 and 4 -4 we could not get an H = D z 2 + B0 H (4 10) acceptable fit using data collected over the whole field range. This is com monly due to the presence of low lying excited states because (i) the excited states are close enough to the ground state that they are populated even at very low temperatures, and/or (ii) even higher -lying excited states whose S is greater than the ground -state become populated as their larger MS levels rapidly decrease in energy in the applied dc field and approach (or even cross) those of the ground state. Either (or both) of these two effects will lead to poor fits because the fitting program assumes

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129 po pulation of only the ground state. A common solution is to use only data collected at low fields (-valence MnII/MnIII clusters,183, 184, 265 but for 4 -2 4 -3 and 4 -4 it was still not possible to obtain a satisfactory fit assuming only the ground state is populated in this temperature range, suggesting particularly low -lying excited states. Thus, we turned to ac susceptibility measurements, which are a powerful complement to dc studies for determining the ground state of a system, because they preclude complications from a dc field (vide infra ). 152, 185, 186 For complex4 -5 3H2O, a gain we could not obtain a satisfactory fit using all data up to 7 T, but this time we were abl e to get around the problem from excited states by using only data collected up to 2 T. These are shown as a reduced magnetization ( M / NB) vs H / T plot in Figure 4 20, where N is Avogadros number, and the fit (solid lines in Figure 4 26) gave S = 13/2, D = 0.18 cm1, and g = 1.97. Alternative fits with S = 11/2 or 15/2 were rejected because they gave unreasonable values of g The root -mean -square D vs g e rror surface for the fit was generated using the program GRID,116 and is shown as a 2 D contour plot in Figure 4 -2 7 for the D = 0.3 to 0.3 cm1 and g = 1.8 to 2.2 ranges. Two soft fitting minima are observed with positive and negative D values, with the l atter being clearly superior and indicating D to be negative. From the shape and orientation of the contour describing the region of minimum error, we estimate the uncertainties in the fit parameters as S = 13/2, D = 0.18(2) cm1 and g = 1.97(2). The M / NB vs H / T plot for complex 4 -6 MeCN is shown in Figure 4 2 8 and we were able to obtain an excellent fit with the program MAGNET using all the data collected up to 7 tesla. This suggests that the ground state of 4 -6 is relatively well isolated from the near est excited states, as suggested also from the obtained J values (vide supra). The best fit is shown as the solid lines in Figure 4 2 8 and was obtained with S = 12, g = 2.00, and D = 0.05 cm1. An e qually

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130 good fit was also obtained with S = 12 g = 2.00 and D = 0.03 cm1. It is common to obtain two acceptable fits of magnetization data for a given S value, one with D > 0 and the other with D < 0, since magnetization fits are not very sensitive to the sign of D Alternative fits with S = 11 were rejected be cause they gave unreasonable values of g and D The obtained error surface for the fit, plotted as a 2 -dimensional contour plot in Figure 4 2 9 exhibits the two minima, one with positive D and the other with a negative one, but this time the two fits are essentially of equal quality, and it is thus not possible on the basis of these magnetization fits to conclude the more likely sign of the axial anisotropy parameter D for 4 -6 The ZFS of the ground state of a polynuclear MnIII complex is largely a conseq uence of the vectorial addition of the single -ion ZFS tensors. A large JahnTeller (JT) distortion is typically seen in an octahedral MnIII ion, in which a unique axis is formed by two noticeably longer Mn -L bond distances. This axis often defines the unique axis of the magnetic structure of the MnIII ion. Single -ion ZFS in MnIII ions can be very large; for example, high-frequency EPR experiments have shown that D = 4.52 cm1 for Mn(acac)3, where acac is the anion of 2,4pentanedione, and D = 5.90 cm1 fo r Mn(taa), where taa is the trianion of tris(1 (2 azolyl) 2 azabuten 4 yl)amine. When all JT axes are oriented parallel, the resultant ZFS of a MnIII x complex will be quite large; for example, in [Mn12O12(O2CR)16(H2O)4] complexes (8 MnIII, 4 MnIV), the 8 M nIII ions have their JT axes essentially parallel, and the net ZFS in the molecule is fairly large ( D ~ 0.5 cm1).217, 266 On the other hand, when the JT axes do not all point in the same direction, the resultant ZFS will be small, as the contributions fr om the individual sites partially or completely cancel out. In the Mn6 complexes 4 -6 because of the high symmetry of the molecule, the vectorial addition of single ion anisotropies sums to zero. This clearly explains the experimental observation of D ~ 0 for 4 -6

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131 4.3.3.2 Alternating current magnetic susceptibility studies Ac susceptibility studies are a powerful complement to dc studies for determining the ground state spin of a system, because they preclude complications arising from the presence of a dc field. They were performed for complex 4 -2 2H2O 4 -3 4 -4 MeCN, 4 -5 3H2O, and 4 -6 Et2O, respectively, in the 1.8 15 K range using a 3.5 G ac field oscillating at frequencies in the 501000 Hz range. For 4 -2 2H2O the obtained in -phase M T and out -of -phas e M ac susceptibility data are shown in Figure 4 30. M T decreases significantly with decreasing temperature indicating population of one or more excited states and rationalizing the problematic fits of dc magnetization data. In particular, a decreasing M T vs T plot with decreasing T is indicative of the population of low lying excited states with S values greater than the ground -state S and this rationalizing the problematic fits of dc magnetization data. Extrapolation of the plot from above 4 K to 0 K, where only the ground state will be populated, gives a M T value of ~ 6 cm3Kmol1, which is the value expected for an S = 3 state with g ~ 2.0. This conclusion is the same as that reached from the dc study, and provides an independent confirmation that complex 4 -2 possesses an S = 3 ground state. The out of -phase (M ) susceptibility in Figure 4 30 (bottom) is zero until ~ 3 K, and then shows a small frequency dependent rise. The M signal in Figure 4 24 is weak and parallels a tiny drop in the in -pha se M T ; nevertheless, the M :M T ratio is small (~1.5 %), and the M signal is clearly just the small tail of a much stronger M peak whose maximum is below the 1.8 K operating limit of our SQUID magnetometer. The data thus suggest that complex 4 -2 ex hibits the slow magnetization relaxation dynamics of a SMM, but with only a very small barrier. It would thus require single -crystal micro SQUID studies down to 40 mK to better study the SMM

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132 properties, but these were not pursued given the only small barri er evident and the many such studies on other SMMs with small barriers already reported. For 4 -3 the in phase M T and out -of phase M data are shown in Figure 4 -31. The M T vs T plot in Figure 42 5 (top) decreases very rapidly with decreasing temperature, supporting a very high density of low lying excited states consistent with the higher nuclearity of this co mplex and again rationalizing the unsatisfactory fits of dc magnetization data. The steeply decreasing and curving plot makes extrapolation to 0 K trickier, but it appears to be heading for a M T value in the 4 5 cm3Kmol1 range consistent with an S = 5 /2 ground state (4.38 cm3Kmol1 for g = 2) suggesting this is the ground state of the molecule; the spin-only M T values S = 3/2 and 7/2 states are 1.88 and 7.88 cm3Kmol1, significantly different than the experimental value. We conclude that complex 4 -3 has an S = 5/2 ground state. Below ~ 3 K, there is a small dip in the M T signal concomitant with the appearance of a frequency -dependent M signal (Figure 4 31, bottom). As for 4 -2 it appears that 4 -3 has a very small magnetization relaxation barrier t hat leads to M signals whose peaks lie well below 1.8 K. This suggests that 4 3 is also a SMM, but again one with only a very small barrier, and we thus did not pursue further characterization For 4 -4 the obtained in -phase ac susceptibility data (M plotted as M T ) are shown in Figure 4 32. The M T vs T plot of Figure 4 32 decreases significantly with decreasing temperature, again indicating population of one or more excited states with S greater than the ground state S rationalizing the problemati c fits of dc magnetization data. Extrapolation of the plot to 0 K, where only the ground state will be populated, gives a M T value of ~7 cm3Kmol1, which is the value expected for an S = 7/2 state with g slightly less than 2, as expected for a MnII/MnIII complex Extrapolation of a steeply sloping plot to 0 K can be fairly unreliable, especially for large S ground states and even in the absence (as here) of significant

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133 intermolecular interactions, but the data for 4 -4 clearly distinguish the S = 5/2, 7/2, and 9/2 possibilities, whose M T values are 4.38, 7.35, and 12.38 cm3Kmol1, respectively, for g = 2. We thus feel confident in our conclusion that 4 -4 has an S = 7/2 ground state. C omplex 4 -4 exhibited no out -of -phase (M ) ac signal down to 1.8 K, indicating that it does not exhibit a significant barrier (vs kT ) to magnetization relaxation, i.e., it is not an SMM. T he in -phase (MT ) and out -of -phase (M) ac susceptibilities for 4 -5 are shown in Figure 4 33. M T increases with decreasing temperature below 15 K to a plateau of ~24.5 cm3Kmol1 in the 4 5 K region, and then exhibits a frequency -dependent decrease below 3 K. The plateau value is clearly indicative of an S = 13/2 ground state with g ~ 2.0, in agreement with the dc magnetization fit. S = 11/2 and 15/2 ground states woul d be expected to give M T values of 17.9 and 31.9 cm3Kmol1, respectively, clearly very different from the experimental value. We conclude that complex 4 -5 has an S = 13/2 ground state. The frequency -dependent decrease in M T below 3 K is accompanied by the appearance of freq uency -dependent out of -phase M signals below 3 K, clearly the tails of peaks whose maxima are at <1.8 K. If the magnetization vector relaxes fast enough to keep up with the oscillating ac field, there will be no M signal, but if the relaxation barrier is significant compared to thermal energy ( kT ), then there is a nonzero M signal and the in -phase signal decreases. Such frequency -dependent ac signals are an indication of the superparamagnet -like slow relaxation of a single -molecule magnet (SMM). To co nfirm whether 4 -5 is an SMM, studies were carried out on a single crystal down to 0.04 K (vide infra ). The obtained in -phase M signal for 4 -6 is plotted as MT in Figure 4 34, and can be seen to be almost temperature -independent, confirming a well isol ated ground state, before decreasing slightly below ~5 K. Extrapolating to 0 K the data from above 5 K (to avoid lower temperature

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134 effects from the slight anisotropy and weak intermolecular interactions) gives a value of ~ 78 cm3K mol1, which is consistent with an S = 12 ground state and g ~ 2, in excellent agreement with the reduced magnetization fit We conclude that complex 4 -6 does indeed have an S = 12 ground state. There is no out -of -phase ac susceptibility signal down to 1.8 K, the operating limit of our SQUID magnetometer. 4.3.3.3 Hysteresis Studies below 1.8 K. Magnetization vs applied dc field studies down to 0.04 K were carried out using a micro SQUID apparatus and single crystals of 4 -5 3CH2Cl2 that had been kept in contact with their mother li quor.3267 Magnetization vs field hysteresis, the diagnostic property of a magnet, was o bserved below 0.7 K (Figure 4 35). The hysteresis loops exhibit increasing coercivity with increasing field sweep rate at a c onstant temperature (Figure 4 35, top), and increasing coercivity with decreasing temperature at a constant sweep rate (Figure 4 35, bottom), as expected for the superparamagnet like properties of a SMM. These loops thus confirm complex 4 -5 3CH2Cl2 to be a new addition to the family of SMMs. The most dominating feature of the hysteresis loops in Figure 4 35 is the large step at zero field due to quantum tunneling of the magnetization (QTM) through the anisotropy barrier. The large zero -fiel d step indicates that QTM in zero field is fast, and this is consistent with the low symmetry of the molecule, which introduces a significant rhombic (transverse) anisotropy into the spin Hamiltonian, i.e. E (x 2y 2), where E is the rhombic zero -field spli tting parameter. The greater is the transverse anisotropy, the greater will be the mixing of levels on either of the anisotropy barrier, leading to increased rates of QTM. The fast relaxation at zero field precludes magnetization decay vs time studies to p rovide relaxation rate vs T kinetic data; we cannot therefore construct an Arrhenius plot from which could be determined the effective barrier to magnetization relaxation, Ueff.

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135 4.3.4 Structural Comparison of hmp-, dmhmp-, and dphmpMnx Products. The init ial objective of this work was to explore whether the introduction of bulky substituents at the CH2 position of hmpH might lead to products distinctly different from those previously obtained from hmpH, and this has been found to be the case. None of the p roductsobtained to date containing bulky dphmpor dmhmpare structurally the same as those obtained previously with hmp-. Instead, all six products of the present study have been found to be prototypical or very rare structural types in Mn cluster chemi stry. However, Mn chemistry is already known to be amazingly fickle, with obtained products often changing dramatically when reaction conditions such as the identity of the solvent or the carboxylate are changed (for reasons discussed above). Thus, the sim ple fact that dphmpand dm hmpgive different products from hmpis not in itself so surprising. A much more pertinent and useful question to ask is whether there is anything systematically different between the products with hmpand its bulky derivative s? The answer is yes the bulky derivatives, dmhmpand especially dphmp-, are exhibiting distinctly different binding modes that can be directly assigned to the presence of the Ph and Me groups and which consequently lead to the different structural type s of products. The various types of known Mn clusters containing hmpare listed in Table 4 10, together with complexes 4 -1 4 -6 from this work, and the binding modes of the alkoxide O atom. Immediately apparent is a strong tendency of the hmpalkoxide a rm to bridge two and even three Mn atoms in most of the [Mn4(hmp)6]4+and [Mn4(hmp)4]6+-containing complexes as well as [Mn7(OH)3(hmp)9Cl3]2+ and [Mn10O4(OH)2(O2CMe)8(hmp)8]4+.44, 51, 147 150, 252254 The carboxylate -rich anion [Mn4O2(O2CPh)7(hmp)2]is the exception that proves the rule in containing only chelating hmpgroups. This preference of the numerous hmpgroups in these many complexes to almost all favor a bridging mode for their alkoxide group is as expected for mid and late transition metal s. In contrast, for Mn/dphmpcomplexes 4 -1 4 -3 nine of the

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136 eleven dphmpgroups bind in non-bridging, chelate modes. Even the tenth and eleventh, the ostensibly bridging dphmpgroup bridges very asymmetrically, as described above (complex 4 1 : Mn3 O5 = 1.887(3) vs Mn2 O5 = 2.505(3) ; complex 4 -3 : Mn2 O1 = 1.879(1) vs Mn3 O1 = 2.415(2) and perhaps are best described as semi -bridging. Thus, although the number of dphmpproducts is admittedly limited to date, we believe the trend is nevertheless clear: the influence of the two bulky Ph groups next to the alkoxide O atom disfavors the adoption of a bridging mode. We believe this is primarily due to steric effects, but there will also be an electronic effect of the electron -withdrawing Ph groups that lowers the basicity of the O atom. Thus, w ith dphmpfavoring a chel ating mode, it is not surprising that its Mn clusters are distinctly different from those obtained with hmp-. It now also makes sense that the Mn6 core of complex 4 -2 is the same as that found previously in [Mn6O4(OMe)2(OAc)4(Mesalim)4], where the Mesalimanion is a bidentate chelate. On the other hand, the dmhmpproducts show an increased preference for dmhmpbridging mode s as in the hmpproducts; in the three Mn/dmhmpcomplexes 4 -4 4 -6 ten of the fourteen dmhmpgroups exhibit bridging ( ) modes a s a result of both the smaller size of Me (vs Ph) groups and their electrondonating rather than withdrawing character. Although we have not observed the 3-bridging mode in dmhmpproducts, it is apparent that the tendency of the alkoxide oxyge n to bridge metal ions, which was lost when adding Ph groups to the alcohol arm, was restored in dmhmp-. However, t he two Me groups at the CH2 position of hmpH still provide sufficient steric differences with hmpto lead to new products. It is thus interesting that Mn/dmhmpproducts bear some structural similarity to Mn/hmpproducts, as a result of dmhmpacting as a perturbed hmp-.

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137 4.4 Conclusions The introduction of two bulky phenyl or methyl groups onto the CH2 group of hmphas proven to be a route to new Mn clusters not accessible with hmpitself, and with prototypical or very rare structures. In contrast to hmp-, the bulkier dphmpfavors a chelating, non -bridging binding mode, and as a result, products with alternative bridging ligands such as carboxylates filling up coordination sites are readily obtained. This is in contrast to hmp-, which favors chelating/bridging binding modes and whose higher denticity will often preclude the incorporation of additional bridging groups such as carboxylates. The relucta nce of dphmpto adopt a bridging mode is clearly due to the nearby steric bulk of the two Ph groups, and perhaps their electron -withdrawing effect on the O atom. On the other hand, the less bulky dmhmpfunctions as a chelating and bridging ligand, as doe s hmp-, but yields products that are structurally distinct from any seen in Mn/hmpchemistry as a result of sufficient steric differences with hmp-. The combined results support our initial idea that (i) the addition of bulky groups near the functional gr oups of known ligands could lead to the isolation of new cluster products, and (ii) the use of different bulky group sizes leads to the formation of different clusters. It would be interesting to investigate whether the binding mode, and thus the identity of the obtained Mn clusters, can be altered in a targeted way by modifying the size and electronic effects of the two substituents.

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138 Table 4 1. Crystallographic Data for 4 -1 2MeCN, 4 -2 3MeCN and 4 -3 4MeCN. 4 1 4 2 4 3 formula a C 83 H 93 Mn 4 N 5 O 15 C 108 H 96 Mn 6 N 7 O 18 C 138 H 133 Mn 11 N 8 O 34 fw, g mol 1 a 1620.38 2019.56 3051.86 crystal system Triclinic Triclinic Triclinic space group 1 P 1 P 1 P a 13.1020(16) 13.5311(19) 17.0807(15) b 13.6590(17) 13.6 589(18) 18.5090(16) c 14.6292(18) 27.034(4) 21.0431(18) deg 63.067(2) 98.178(3) 94.9260(10) deg 63.803(2) 90.914(3) 93.104(2) deg 65.072(2) 105.032(3) 96.2410(10) V 3 2012.4(4) 4769.1(11) 6575.3(10) Z 1 2 2 T K 173(2) 173(2) 173(2) radiation, b 0.71073 0.71073 0.71073 calc g cm 3 1.337 1. 469 1.541 mm 1 0.680 0.845 1.100 R 1 c,d 0.0628 0.0807 0.0566 wR 2 e 0.1509 0.1853 0.1018 a Graphite monochromator. b I > 2 (I). c R 1 = 100 (|| Fo| | Fc||)/ | Fo|. d wR 2 = 100[ [ w ( Fo 2 Fc 2)2]/ [ w ( Fo 2)2]]1/2, w = 1/[ 2( Fo 2) + [(ap)2 + bp], where p = [ma x( Fo 2, O) + 2 Fc 2]/3.

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139 Table 4 2. Cry stallographic Data for 4 -4 7MeCN 4 -5 3CH2Cl2 and 4 -6 Et2O. parameter 4 4 4 5 4 6 formula C 67 H 106 Mn 7 N 4 O 24 C 118 H 108 Mn 12 N 4 O 39 C 58 H 86 Cl 2 Mn 6 N 19 NaO 19 fw, g mol 1 1736.14 2865.36 1776.99 crystal system Triclinic Triclinic T rigonal space group 1 P 1 P P31c a 14.396(3) 15.9055(12) 14.0257(7) b 15.169(3) 17.0160(13) 14.0257(7) c 25.551(6) 27.771(2) 23.328(2) deg 77.295(4) 107.1080(10) 90 deg 88.592(4) 99.8710(10) 90 deg 63.557(3) 108.4790(10) 120 V 3 4856.3(18) 6516.3(9) 3974.3(5) Z 2 2 2 T C 173(2) 173(2) 173(2) radiation, a 0.71073 0.71073 0.71073 calc g cm 3 1.187 1.460 1.485 mm 1 0.940 1.197 1.072 R 1 b,c 0.0610 0.0419 0.0293 wR 2 d 0.1538 0.1164 0.0730 a Graphite monochromator. b I > 2 (I). c R 1 = 100 (|| Fo| | Fc||)/ | Fo|. d wR 2 = 100[ [ w ( Fo 2 Fc 2)2]/ [ w ( Fo 2)2]]1/2, w = 1/[ 2( Fo 2) + [(ap)2 + bp], where p = [max( Fo 2, O) + 2 Fc 2]/3.

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140 Table 4 3. Bond Valence Sums for the Mn Atoms in Complex 4 -1a atom Mn II Mn III Mn IV Mn1 3.22 2.98 3.08 Mn2 3.13 2.89 2.99 Mn3 3.23 2.99 3.08 Mn4 3.20 2.92 3.07 a The underlined value is the one closest to the charge for which it was calculated, and the nearest whole number can be taken as the oxidation state of that atom. Table 4 4. Bond Valence Sums for the Mn Atoms in Complex 4 -2a atom Mn II Mn III Mn IV Mn1 3.15 2.88 3.02 Mn2 3.15 2.91 3.00 Mn3 3.15 2.92 3.01 a The underlined value is the one closest to the charge for which it was calculated, and the nearest whole number can be taken as the oxidation state of that atom. Table 4 5. Bond Valence Sums for the Mn Atoms in Complex 4 -3a atom Mn II Mn III Mn IV Mn1 3.18 2.95 3.04 Mn2 3.28 3.03 3.13 Mn3 3.13 2.89 2.98 Mn4 3.12 2.86 3.00 Mn5 3.23 2.96 3.11 Mn6 3.15 2.88 3.02 Mn7 3.16 2.89 3.03 Mn8 3.14 2.87 3.01 Mn9 3.17 2.90 3.05 Mn10 3.26 3.02 3.12 Mn11 1.98 1.81 1.90 a The underlined value is the one closest to the charge for which it was calculated, and the nearest whole number can be taken as the oxidation state of that atom.

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141 Table 4 6. Bond Valence Sums for the Mn Atoms in Complex 4 -4a Mn II Mn III Mn IV Mn1 3.32 3.07 3.06 Mn2 3.13 2.90 2.89 Mn3 3.13 2.86 3.01 Mn4 3.12 2.85 2.99 Mn5 3.16 2.93 2.92 Mn6 3.25 3.00 3.00 Mn7 1.80 1.65 1.73 a The underlined value is the one closest to the charge for which it was calculated, and the nearest whole number can be taken as the oxidation state of that atom. Table 4 7. Bond Valence Sums for the Inorganic Oxygen Atoms in Complex 4 -4a atom BVS assgt a O1 1.85 O 2 O2 1.98 O 2 O3 1.79 O 2 O4 0.93 OH O5 0.93 OH O6 0.63 OH a The oxygen atom is an O2if the BVS is ~ 1.8 2 .0 an OHif the BVS is ~ 0.91.2 and an H2O if the BVS i s ~0 .2 0.4, although these numbers may vary a little due to hydrogen -bonding.

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142 Table 4 8. Bond Valence Sums for the Mn Atoms in Complex 4 -5a Mn II Mn III Mn IV Mn1 3.13 2.86 3.01 Mn2 3.17 2.90 3.05 Mn3 3.12 2.85 3.00 Mn4 3.20 2.93 3.07 Mn5 3.22 2.97 2. 97 Mn6 1.87 1.71 1.80 Mn7 1.81 1.66 1.74 Mn8 3.27 3.02 3.02 Mn9 3.32 3.07 3.07 Mn10 3.26 3.01 3.01 Mn11 3.14 2.87 3.01 Mn12 1.92 1.76 1.84 a The oxygen atom is an O2if the BVS is ~1.8 2.0, an OHif the BVS is ~0.9 1.2, and an H2O if the BVS is ~ 0.2 0.4, although these numbers may vary a little due to hydrogen -bonding. Table 4 9. Bond Valence Sums for the Inorganic Oxygen Atoms in Complex 4 -5a atom BVS assgt a O1 1.73 O 2 O2 1.88 O 2 O3 1.07 OH O4 1.85 O 2 O5 1.67 O 2 O6 1.73 O 2 O7 1.73 O 2 O8 1.93 O 2 O37 0.27 H 2 O a The oxygen atom is an O2if the BVS is ~1.8 2.0, an OHif the BVS is ~0.9 1.2, and an H2O if the BVS is ~0.2 0.4, although these numbers may vary a little due to hydrogen -bonding.

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1 43 Table 4 10. Complexes with hmp-, dmhmp or dphmp -, and the Alkoxide O Atom Binding Mode Complex a n O) b Ref [Mn 4 O 2 (O 2 CPh) 7 (hmp) 2 ] 1 257 [Mn 4 (hmp) 6 X 4 x (solv) x ] z+ c 3 148 150, 252 254 [Mn 4 (6 Me hmp) 6 Cl 4 ] 3 148 [Mn 4 (hmp) 4 Br 2 (OMe) 2 (dcn) 2 ] 3 150 [Mn 4 (hmp) 4 (acac) 2 (MeO) 2 ] 2+ 2 51 [Mn 7 (OH) 3 (hmp) 9 Cl 3 ] 2+ 2, 3 45 147 [Mn 10 O 4 (OH) 2 (O 2 CMe) 8 (hmp) 8 ] 4 + 2, 3 45 [Mn10O4(N3)4(hmp)12] 2+ 2 4 4 [Mn 12 O 8 Cl 4 (O 2 CPh) 8 (hmp) 6 ] 2 151 [Mn 21 O 14 (OH) 2 (O 2 CMe) 16 (hmp) 8 (pic) 2 (py)(H 2 O)] 4+ 2 152 [Mn 7 O 3 (OH) 3 (O 2 CBu t ) 7 (dmhmp) 4 ] ( 4 4 ) 1, 2 t.w. [Mn 12 O 7 (OH)(OMe) 2 (O 2 CPh) 12 (dmhmp) 4 (H 2 O)] ( 4 5 ) 2 t.w. [Mn 6 O 4 (dmhmp) 6 (N 3 ) 4 ] ( 4 6 ) 1, 2 t.w. [Mn 4 O 2 (O 2 CBu t ) 5 (dphmp) 3 ] ( 4 1 ) 1, 2 t.w. [Mn6O4(OMe)2(O2CPh)4(dphmp)4] ( 4 2 ) 1 t.w. [Mn 11 O 7 (OMe) 7 (O 2 CPh) 7 (dphmp) 4 (MeOH) 2 ] ( 4 3 ) 1, 2 t.w. a Counterions omitted. b bridging mode of the hmp-, dmhmp-, or dphmp1-O indicates a non -bridging (terminal) mode. c Many complexes, varying in the content of monodentate anionic ligand Xand solvent molecules (solv). t.w. = this work. Figure 4 1. Structure of ligands: 2 (hydroxymethyl)pyridine (hmpH), 2 (pyridine 2 yl)propan 2 ol (dmhmpH), and diphenyl (pyridine 2 -yl)methanol (dphmpH). hmpH dphmpH dmhmpH C H3 C H3 O H N

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144 Figure 4 2. A picture of crystals of 4 -1 Figure 4 3. A picture of crystals of 4 -2 Figure 4 4. A picture of crystals of 4 -3

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145 Figure 4 5 A picture of crystals of 4 -4 Figure 4 6 A picture of crystals of 4 -5 Figure 4 7 A picture of crystals of 4 -6

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146 Figure 4 8 The structure of complex 4 -1 with core Mn O bonds shown in purple. Hydrogen atoms and methyl groups on pivalates have been omitted for clarity. Color code: MnIII green; O red; N blue; C gray. Figure 4 9 The structure of complex 4 -8 with core Mn O bonds shown in purple. Hydrogen atoms and methyl groups on pivalates have been omitted for clarity. Color code: MnIII green; O red; N blue ; C gray.

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147 Figure 4 10. The structure of complex 4 -2 with the MnIII Jahn Teller elongation axes shown as yellow bonds (top), and a stereopair (bottom). Hydrogen atoms have been omitted for clarity. Color code: MnIII green; O red; N blue; C gray.

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148 F igure 4 11. The structure of complex 4 -3 with intramolecular hydrogen-bonds shown as dashed lines (top), and a stereopair (bottom). Hydrogen atoms and benzoate phenyl rings (except for the ipso carbon atoms) have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; N blue; C gray.

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149 Figure 4 12. The fully labeled core of complex 4 -3 Color code: MnII yellow; MnIII green; O red; C gray.

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150 Figure 4 13. The structure of complex 4 -4 with intramolecular hydrogen-bonds shown as dashed lin es (top), and a stereopair The thicker orange bonds indicate the positions of the MnIII Jahn Teller elongation axes. Hydrogen atoms and methyl groups on pivalate groups have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; N blue; C g rey.

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151 Figure 4 14. The fully labeled core of complex 4 -4 Color code: MnII yellow; MnIII green; O red.

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152 Figure 4 15. The structure of complex 4 -5 with intramolecular hydrogen-bonds shown as dashed lines (top), and a stereopair (bottom). Hydrogen atoms and phenyl rings (except for the ipso carbon atoms) on benzoate groups have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; C gray; N blue.

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153 Figure 4 16. The fully labeled core of complex 4 -5 Color code: MnII yellow; MnIII green; O red; C gray.

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154 Figure 4 1 7 The structure of complex 4 -6 (top), a stereopair (middle), and the labeled core (bottom). The thicker orange bonds indicate the positions of the MnIII Jahn Teller elongation axes. Hydrogen atoms have been omitted for clarity. Color code: MnIII green; O red; C gray; N blue; Na Purple.

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155 Figure 4 18. 1 D chain of complex 4 -6 viewed perpendicular (top) and parallel (bottom) to c axis. Hydrogen atoms have been omitted for clarity. MnIII green; O red; C gray; N blue; Na Purple

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156 Figure 4 19. Plots of MT vs. T for complexes 4 -1 Figure 4 20. Plots of MT vs. T for complexes 4 -2 Temperature (K) 0 50 100 150 200 250 300 MT (cm 3 Kmol -1 ) 0 2 4 6 8 10 Temperature (K) 0 50 100 150 200 250 300 MT (cm 3 Kmol -1 ) 0 2 4 6 8 10 12 14 16

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157 Figure 4 21. Plots of MT vs. T for complexes 4 -3 Figure 4 22. Plots of MT vs. T for complexes 4 -4 Temperature (K) 0 50 100 150 200 250 300 MT (cm 3 Kmol -1 ) 0 5 10 15 20 25 30 35 Temperature (K) 0 100 200 300 MT (cm 3 Kmol -1 ) 0 5 10 15 20

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158 Figure 4 23. Plots of MT vs. T for complexes 4 -5 Figure 4 24. Plots of MT vs. T for complexes 4 -6 The solid line is the fit of the data; see the text for the fit parameters Temperature (K) 0 100 200 300 MT (cm 3 Kmol -1 ) 0 5 10 15 20 25 30 35 Temperature (K) 0 50 100 150 200 250 300 MT (cm3Kmol-1) 0 20 40 60 80 100 fitting

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159 Figure 4 25. Mn labeling scheme employed in eq. 4 7. Mn1 Mn4 Mn2 Mn5 Mn3 Mn6

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160 Figure 4 2 6 Plots of reduced magnetization (M/N B) vs H/T for complex 4 5 The solid lines are the fit of the data; see the text for the fit parameters. Figure 4 27. Two -dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 4 -5 Temperature (K) 0 2 4 6 8 10 12 M/N B 0 2 4 6 8 10 12 14 0.1 T 0.5 T 1 T 2 T fitting g 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 D (cm -1 ) -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 1.2 1.3 1.4 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 1.5 1.1 0.5 1.0 1.4 0.9 1.3 0.4 0.8 1.2 1.1 0.7 0.3 1.0 0.6 0.9 0.4 0.5 0.8 0.2 0.4 0.5 0.7 0.6 0.6 0.5 0.7 0.3 0.8 0.4 0.4 0.9 0.4 0.5 1.0 0.5 0.6 1.1 0.7

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161 Figure 4 2 8 Plots of reduced magnetization (M/N B) vs H/T fo r complex 4 6 The solid lines are the fit of the data; see the text for the fit parameters. Figure 4 2 9 Two -dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 4 -6 H/T (kG/K) 0 10 20 30 40 50 M/N B 0 5 10 15 20 25 0.1 T 0.5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T fitting g 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 D (cm-1) -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.3 2.3 2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.5 1.6 1.7 1.2 1.1 1.4 1.8 1.0 0.9 1.3 0.8 1.2 1.9 1.7 1.6 0.7 1.1 2.0 1.5 0.6 1.0 1.4 2.1 1.3 0.9 1.2 0.5 0.8 2.2 1.1 0.7 2.3 1.0 0.6 0.4 0.6 2.4 0.7 0.5 2.5 0.6 0.8 0.3 0.4 2.6 0.9 0.3 0.2 2.7 1.0 0.2 2.8 0.3 1.1 2.9 0.3 1.2 3.0 1.3 0.3 1.4 3.1 1.5 3.2 1.6 3.3 0.7 1.7 3.4 1.8 3.5 0.8 1.9 3.6 0.9 2.0 3.7 1.0 2.1 3.8 1.2 2.2 1.3 2.3 3.9 1.4 2.4 4.0 1.5 2.5 4.1 1.6 2.6 4.2 1.7 2.8 2.7 4.3 1.8 2.9 4.4 1.9 3.0 4.5 2.0

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162 Figure 4 30. AC susceptibility of complex 4 -2 in a 3.5 G field oscillating at the indicated frequencies: (top) in -phase signal ( M M -of phase signal M

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163 Figure 4 31. AC susceptibility of complex 4 -3 in a 3.5 G field oscillating at the indicated frequencies: (top) in -phase signal ( M M -of phase s ignal M

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164 Figure 4 32. In -phase ac susceptibility ( M M 4 -4 in a 3.5 G ac field oscillating at the indicated frequencies.

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165 Figure 4 33. AC susceptibility of complex 4 -5 in a 3.5 G field os cillating at the indicated frequencies: (top) in -phase signal ( M M -of phase signal M

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166 Figur e 4 34. In -phase ac susceptibility ( M M 4 -6 in a 3.5 G ac field oscillat ing at the indicated frequencies.

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167 Figure 4 35. Single -crystal magnetization (M) vs dc field (H) hysteresis loops for a single crystal of 4 -5 3CH2Cl2 at different scan rates (top) and temperatures (bottom). -1 -0.5 0 0.5 1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 0.070 T/s 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s 0.002 T/s 0.001 T/s M/Ms 0H (T) 0.04 K -1 -0.5 0 0.5 1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 0.04 K 0.1 K 0.2 K 0.3 K 0.4 K 0.5 K 0.6 K 0.7 K M/Ms 0H (T) 0.035 T/s

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168 CHAPTER 5 NEW MN4 AND MN7 CLUS TERS FROM T HE USE OF PHENYLDIPY RIDIN 2 YLMETHANOL 5 .1 Introduction There continues to be great interest in the synthesis and characterization of high nuclearity 3 d transition metal clusters. In some cases, this is because of their relevance to b ioinorganic chemistry as models for the metallosites of various proteins and enzymes, whereas in other cases it is their importance to molecular magnetism that drives this interest. And in many instances, the intrinsic architectural beauty and aesthetically pleasing structures of metal clusters is itself the primary interest within the field of supramolecular chemistry.196 In manganese chemistry, for example, Mn carboxylate clusters are of bioinorganic interest because of their relevance to elucidating the nature and mechanism of action of the water oxidizing complex (WOC) on the donor side of photosystem II in green plants and cyanobacteria.269 272 The WOC comprises a pentanuclear Mn4Ca cluster, the exact structure of which is still unclear, and which is responsible for the lig ht -driven, oxidative coupling of two molecules of water into dioxygen.273, 274 Secondly, some polynuclear Mn compounds have been found to be single -molecule magnets (SMMs) which are individual molecules capable of functioning as nanoscale magnetic particl es and thus represent a molecular approach to nanomagnetism.12 13 Such molecules behave as magnets below their blocking temperature ( TB), exhibiting hysteresis in magnetization versus dc field scans. This magnetic behavior of SMMs results from the combina tion of a large ground spin state ( S ) with a large and negative Ising (or easy axis) type of magnetoanisotropy, as measured by the axial zero -field splitting parameter D As a result of the above, there is a continuing search for new synthetic methods tha t can yield new polynuclear Mn/O complexes. In the design of potentially new synthetic routes to polynuclear metal complexes, the choice of the ligands and bridging groups is always a key

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169 issue. Accordingly, we and others have explored a wide variety of potentially chelating and/or bridging ligands that might foster formation of high nuclearity products, and have successfully isolated and studied new cluster products. Examples include aliphatic or aromatic alcohols, 185, 186, 212, 220 222, 232 234, 275287 alcohol amines,226231, 288 293 di 2 -pyridylketone,236238, 294301 pyridyl alcohols,4446, 49, 151,302 pyridyl ketone oximes,190, 235, 303313 salicylaldoximes,206, 223225, 314325 and others.326 334 As a new ligand design strategy to isolate new polynuc lear metal complexes, i n the previous chapter we reported the use of derivatives of a pyridyl alcohol, 2 (hydroxymethyl) pyridine (hmpH, Figure 5 1), in which two phenyl or two methyl or groups have been added onto the CH2 unit. The resulting molecules, d iphenyl -hmpH (dphmpH; IUPAC name is diphenyl (pyridine 2 -yl)methanol) and dimethyl -hmpH (dmhmpH, IUPAC name is 2 -(pyridine 2 yl)propan 2 -ol) have given distinctly different products from those obtained from hmpH.335 As a further expansion of our ligand des ign strategy, in this chapter we describe the use in Mn cluster chemistry of an hmpH derivative in which one phenyl and one pyridyl groups have been added onto the CH2 unit of hmpH. The resulting molecule, phenyldipyridin2 -ylmethanol (pdpmH), is shown in Figure 5 1, and is now a potentially tridentate chelating/bridging group, in contrast to hmpH, dmhmpH, and dphmpH. It was thus anticipated that pdpmH would thus give distinctly different products than obtained previously with other chelates. Note that pdp mH has been employed to date in the literature only for the synthesis of a mononuclear complex of In.336 In the present investigation, we have deliberately targeted higher nuclearity Mn products by exploring the reactions between pdpmH and Mn starting mate rials under basic conditions. This has successfully led to Mn4 and Mn7 cluster products containing chelating and bridging pdpm-. The syntheses, structures, and electrochemical and magnetochemical characterization of these complexes are described in this ch apter.

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170 5.2 Experimental Section 5.2.1 Syntheses All preparations were performed under aerobic conditions using chemicals as received, except for the synthesis of pdpmH, which was carried out as previously reported.337 [Mn12O12(O2CMe)16(H2O)4]2MeCO2H4H2O and NBun 4MnO4 were prepared as previously reported.17, 338, 339 Safety note: Perchlorate salts are potentially explosive; such compounds should be synthesized and used in small quantities, and treated with utmost care at all times [Mn4O4(O2CMe)3(pdpm)3] (5 -1 ). Method A. To a stirred solution of pdpmH (0.17 g, 0.65 mmol) in MeCN (20 mL) was added [Mn12O12(O2CMe)16(H2O)4] 2MeCO2H 4H2O (0.20 g, 0.11 mmol). The dark brown solution was stirred for 3 hours, and filtered to remove undissolved solid. Vapor diffusion of Et2O into the filtrate gave X ray quality crystals of 5 -1 0.5MeCN after 2 weeks. These were collected by filtr ation, washed with Et2O, and dried in vacuo; the yield was ~5%. Anal. Calc. (Found) for 5 -1 (solvent -free): C, 55.00 (54.74); H, 3.89 (3.74); N, 6.75 (6.55). Selected IR data (cm1): 3446 (m), 3054 (w), 1585 (s), 1569 (s), 1491 (m), 1473 (m), 1428 (s), 1410 (s), 1338 (m), 1296 (w), 1254 (w), 1179 (w), 1098 (m), 1053 (s), 1034 (m), 995 (w), 947 (w), 928 (w), 776 (m), 730 (w), 683 (s), 665 (s), 619 (s), 581 (s), 548 (m). Method B. To a stirred solution of pdpmH (0.20 g, 0.75 mmol), Mn(O2CMe)2 4H2O (0.17 g, 0. 70 mmol) and acetic acid (0.07 mL, 1.2 mmol) in MeCN (25 mL) was added NBun 4MnO4 (0.11 g, 0.3 mmol) in small portions, resulting in a dark purple solution that quickly turned dark brown. After being stirred at 60 C for 15 min, the solution was cooled to r oom temperature, filtered, and the filtrate left undisturbed to concentrate slowly by evaporation. X -ray quality crystals of 5 -1 0.5 MeCN slowly grew over 2 weeks in 20% yield. These were collected by filtration, washed with cold MeCN (2 3 mL) and Et2O (2 5 mL), and dried under vacuum ; the product was identified by IR spectral comparison and elemental analysis as identical with

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171 material from Method A. Anal. Calc. (Found) for 5 -1 H2O: C, 54.55 (54.56); H, 3.9 9 ( 3.82); N, 6.66 (6.95). Method C To a stirred solution of pdpmH (0.20 g, 0.75 mmol), Mn(O2CMe)2 4H2O (0.15 g, 0.60 mmol) and acetic acid (0.09 mL, 1.6 mmol) in MeCN (25 mL) was added NBun 4MnO4 (0.14 g, 0.4 mmol) in small portions, resulting in a dark purple solution that quickly turned dark brown. After being stirred at 60 C for 15 min, the solution was cooled to room temperature, during which time dark brown microcrystalli ne solid began to deposit. When crystallization was judged complete the solid was collected by filtration, washed with MeCN (2 5 mL), and dried under vacuum; the yield was ~50%. The product was identified by IR spectral comparison and elemental analysis as identical with material from Method A. Anal. Calc. (Found) for 5 -1 H2O: C, 54.21 (53.92); H, 3.99 (3.96); N, 6.66 (6.58). [Mn7O4(pdpm)6(N3)4](ClO4)2 (5 -2 ). Solid NaN3 (0.13 g, 2.0 mmol) was added to a stirred, pale yellow solution of pdpmH (0.52 g, 2.0 mmol) and NEt3 (0.28 mL, 2.0 mmol) in MeCN/MeOH (21 mL, 20:1 v/v). To this solution was added solid Mn(ClO4)26H2O (0.50 g, 2.0 mmol), which caused a rapid color change to dark brown. The resulting dark brown solution was stirred for a further 2 h and filt ered, and the filtrate was layered with Et2O. After 3 days, X -ray quality dark red crystals of 5 -2 2MeCN appeared and were collected by filtration, washed with MeCN (2 5 mL) and Et2O (2 5 mL), and dried in vacuo; the yield was ~50%. Anal. Calc. (Found) for 5 -2 5H2O : C, 49.41 (49.02); H, 3.82 (3.42); N, 13.56 (13.24). Selected IR data (cm1): 3424(w b), 3057(wb), 2060(s), 1603(m), 1567(w), 1490(w), 1467(m), 1446(w), 1432(m), 1312(m), 1229(m), 1253(w), 1229(w), 1202(w), 1156(w), 1089(s), 1049(s), 1009(m), 958(w), 931(w), 907(w), 778(m), 750(w), 731(w), 703(s), 663(s), 637(s), 624(s), 588(m), 542(m), 496(w), 470(w) 426(w), 409(w).

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172 5.2.2 X -Ray Crystallography Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo -K radiation ( = 0.71073 ). Suitable crystals of 5 -1 0.5MeCN and 5 -2 2MeCN were attached to glass fibers using silicone grease and transferred to a goniostat where they were cooled to 173 K for data collection. 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 were remeasured 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. The structure was solved by the direct methods in SHELXTL6 ,64 and refined using full -matrix least squares. The non H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal positions and were riding on their respective carbon atoms. Refinement was done using F2. For 5 -1 0.5MeCN, the asymmetric unit consists of a Mn4 cluster and a half acetonitrile solvent molecule. A total of 39377 pa rameters were refined in the final cycle of refinement using 12489 reflections with I > 2 (I) to yield R1 and wR2 of 3 .75 and 8.35%, respectively. For 5 -2 2MeCN, the structure is twinned with a twin law being a 2 -fold rotation around the c axis with a bas f value of 0.127. The asymmetric unit consists of a 1/3 Mn7 cluster, two 1/3 perchlorate anions (disordered around a 3 -fold axis), a 1/3 acetonitrile solvent molecule and another disordered in two parts with a common methyl group. The cluster itself has tw o disordered regions. In one, the terminal nitrogen of an azide, N6/N6 was refined in two parts with a fixed site occupation distribution of 0.2/0.133, and the second is where an aryl group is disordered and refined in two parts with equal site occupation factors fixed at 50%. There were

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173 several constraints applied namely k eeping all distances in the N4 N5 N6/N6 equivalent using the SADI command. The two acetonitrile solvent molecules also constrained to maintain equivalent bond lengths in each anion. Also, the displacement parameters of the oxygen atoms of each anion were set to be equivalent by EADP. A total of 485 parameters were refined in the final cycle of refinement using 4738 reflections with I > 2 (I ) to yield R1 and wR2 of 7.47 and 21.97%, respectively. Unit cell data and details of the structure refinements for the two complexes are listed in Table 5 1. 5.2.3 Other Studies Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 4004000 cm1 range. Elemental analyses (C, H and N) were performed by 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 wire reference) in 0.1 M NBun 4PF6 CH2Cl2 solution. Quoted potentials are vs the ferrocene / ferrocenium couple, used as an internal standard. Variable -temperature DC and AC magnetic susceptibility data were collected at the University of Florida using a Quantu m Design MPMSXL SQUID susceptometer equipped with a 7 T magnet and operating in the 1.8 300 K range. Samples were embedded in solid eicosane to prevent torquing. Magnetization vs. field and temperature data was fit using the program MAGNET. Pascal's const ants251 were used to estimate the diamagnetic correction, which was subtracted from the experimental susceptibility to give the molar paramagnetic susceptibility ( M).

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174 5.3 Results and Discussion 5.3.1 Syntheses Our initial attempts to use pdpmH in polynuc lear Mn cluster syntheses involved the oxidation reactions of simple MnII salts by atomospheric O2 under basic conditions in the presence of carboxylate groups. This approach has proven to be successful in Mn/dphmpH and Mn/dm hmpH chemistry resulting in var ious Mn clusters with nuclearities ranging from 4 to 12. Therefore, a variety of reaction ratios, reagents, solvents and other conditions were investigated; however, despite many attempts, we were unable to isolate any pure, crystalline products. We thus t urned our attention to the reactions with preformed Mn clusters, and subsequently developed the following procedure. The reaction of [Mn12O12(O2CMe)16(H2O)4]2MeCO2H 4H2O in MeCN with 6 equiv of pdpmH afforded a dark brown solution from which was subsequen tly obtained the new mixed valent [MnIII 2MnIV 2] complex [Mn4O4(O2CMe)3(pdpm)3] (5 -1 ). Its formation is summarized in eq. 5 1. 0.5 [Mn12O12(O2CMe)16(H2O)4] + 4 pdpmH 4O4(O2CMe)3(pdpm)3] + 2 Mn3+ + pdpm+ 5 MeCO2 + 4 H2O ( 5 1) However, the yield of complex 5 -1 was only ~5% and attempts to increase the yield were unsuccessful; thus, a better route to this compound was sought and subsequently developed. Complex 5 -1 is composed of two MnIII and two MnIV ions. It is generally not easy to obtain the MnIV oxidation state by simple aerobic oxidation reactions starting with MnII sources. A commonly used synthetic procedure for the preparation of high oxidation state manganese clusters is the comproportionation reaction of a MnII source with MnO4 in the prese nce of appropriate ligand groups. This is a convenient general procedure involving oxidation of MnII ions and concomitant reduction of MnVII ions, producing a product at the MnIII and/or MnIV oxidation level. The MnII/MnVII reaction ratio can readily be va ried, and mixed -valent

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175 MnIV/MnIII and MnIII/MnII complexes can also be obtained.51, 148, 340 In line with this approach, we investigated the comproportionation reactions of MnII and MnV II sources in the presence of pdpmH, and successfully developed the fol lowing procedure s for 5 -1 with a higher yield. The reaction of Mn(O2CMe)2 4H2O, NBun 4MnO4, and pdp mH in the molar ratio of 0.7:0.3:0.75 in MeCN/MeCO2H afforded a dark brown solution from which dark brown crystals of complex 5 -1 were isolated in ~20% yield (Method B). This ratio of reagents is that calculated to give a Mn4 complex in the MnIII 2MnIV 2 oxidation state and with three pdpmH ligands. After s ubsequent reactions exploring the effect of varying the reagents ratio the increased ratio of MnVII/MnII wa s fornd to afford a better yield of ~50%; Mn(O2CMe)2 4H2O, NBun 4MnO4, and pdpmH in the molar ratio of 0.6:0.4:0.75 in MeCN/MeCO2H (Method C). The overall reaction is summarized in eq. 5 2. 2.8 Mn(O2CMe)2 + 1.2 MnO4 + 3 pdpmH 4O4(O2CMe)3(pdpm)3] + 1.2 MeCO2 + 1.4 MeCO2H + 1.6 H2O (5 2) The ligand pdpm H was also explored in non -carboxylate Mn chemistry in the presence of N3 -. The reaction of Mn(ClO4)26H2O, NaN3, pdpmH, and NEt3 in MeCN/MeOH afforded a dark red solution and subsequent isolation of 5 -2 in 50% yield; its formation is summarized in eq. 5 3 7 Mn2+ + 6 pdpmH + 4 NaN3 + 8 NEt3 + 1.5 O2 + H2O 7O4(pdpm)6(N3)4]2+ + 8 HNEt3 + + 4 Na+ (5 3) This reaction is an oxidation, undoubtedly by O2 under the prevailing basic conditions, and has been balanced accordingly. With the identity of 5 -2 established, we also tried several other MnII/pdpmH/NEt3/N3 ratios, particularly with a large excess of MnX2 (X = Cl-, NO3 -, ClO4 -), to see if higher nuclearity azide -containing products might be obtained, but in all cases complex 5 -2 was the isolated product. It is of interest that t his reaction procedure is same as the ones for

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176 [Mn10O4(N3)4(hmp)12](ClO4)2MeCN ( 4 -7 ) and (HNEt3)[NaMn6O4(dmhmp)6(N3)4] (ClO4)2 (4 -6 ), and these complexes all presents [Mn6O4(N3)4]6+ tetrahedral units, and they all possess their highest possible ground state spin (vide infra). 5.3.2 Description of Structures The partially labeled structure and a stereoview of [Mn4O4(O2CMe )3(pdpm)3] (5 -1 ) are shown in Figure 5 2; selected interatomic distances and angles are listed in Table A 13. Complex 5 -1 crystallizes in orthorhombic space group P 212121. The molecule consists of a [Mn4O4] core with peripheral ligation provided by three c helating pdpmand three -bridging MeCO2 groups. The core has a cubane structure consisting of four Mn atoms and four 3O2ions. Charge considerations and the metric parameters indicate a 2MnIII, 2MnIV metal oxidationstate description, as confirmed by BVS calculations (Table 5 2), which identified Mn2 and Mn4 as the MnIV atoms and the others as MnIII. The latter was con sistent with the Jahn Teller (JT) axial elongations at Mn1 and Mn3. Normally, the JT axial elongations avoid Mn-oxide bonds, almost always the strongest and shortest in the molecule, but this is not possible for every Mn atom in 5 1 because of the [Mn4O4] cubane topology. The locations of two JT axes are indicated as yellow bonds in Figure 52. The JT axes of Mn1 and Mn3 are oriented nearly perpendicular to each other. BVS calculations were also performed on the inorganic O atoms to identify their degree of protonation and thus distinguish O2 -, OH-, and H2O situations (Table 5 3), and these confirm four O2ions. The BVS value of 1.53 for O1 is slightly lower than expected for an O2 ion, but is consistent with it being positioned on the JT elongation axis of Mn3, which presents unusually long MnO2 distance (Mn3 O1 = 2.297(2) ). All Mn atoms are six -coordinate with distorted octahedral geometry. Complex 5 -1 contains three pdpmligands, and one of two pyridine ring s in each pdpmis unbound, i.e., pdpmis serving as a bidentate chelate. Interestingly, the unbound pyridine rings

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177 are not protonated either. This is because they are sterically hindered by the neighboring phenyl and p yridyl rings of the same ligand, as shown in the spacefilling diagram in Fig ure 5 3. The structure of 5 -1 is similar to those previously reported for other [Mn4O3X]6+(X = Cl-, Br-) containing complexes that also present a cubane arrangement of four Mn atoms (1MnIV and 3MnIII), with three 3O2ions and one 3-X ion, 341 342 with the exception that complex 5 -1 comprises 2MnIV and 2MnIII. There has been only one previous report of a Mn4 cubane complex with a MnIII 2MnIV 2 oxidation state description, namely [Mn4O4(O2PPh2)6].343345 The complete structure and a stereoview of the cati on of [Mn7O4(pdpm)6(N3)4](ClO4)2 (5 2 ) are shown in Figure 5 4 an d its labeled core in Figure 5 5 ; selected interatomic distances and angles are listed in Table A 14. Complex 5 -2 crystallizes in rhombohedral space group 3 R The core of the cluster consists of an [Mn6O4(N3)4]6+ super -tetrahedron with an N3 at each vertex, a Mn at the midpoint of each edge, and an O2 bridging each face; to one of the latter O2ions an additional MnII atom is attached, making the O2ion 4. The MnII ato m is additionally bridged to the MnIII 3O face by three alkoxide arms of pdpmgroups; thus overall, the complex contains a [Mn7(4O)(3O)3(3-N3)4( OR)3]5+ core. Complex 5 -2 contains six pdpmgroups; t hree of them ch e late to MnIII ions (Mn2), with one of two pyridine groups of the ligand being unbound, and thus these pdpmgroups are serving as bidentate chalate s T he other three pdpmboth chelate to MnIII ions (Mn1) and bridge with thir alkoxide arm to MnII ion (Mn3) which is also coordinated by the p yridine rings of the pdpmgroups, thus these pdpmgroups are serving as tridentate ligand. The molecule has a crystallographic C3 rotation axis passing thorough the MnII ion, 4O2ion bridging the Mn1 atoms (O4), and 3N atom of azide ion bridging the Mn2 atoms (N4).

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178 Charge considerations and a qualitative inspection of the metric parameters indicate an [MnIIMnIII 6] metal oxidation -state description. As expected for high -spin MnIII (d4) in near octahedral geometry, there is a Jahn Teller (JT) distortio n, and it takes the form of an axial elongation of the two trans MnN3 bonds; the JT axes are the edges of the virtual tetrahedron [Mn6O4(N3)4]6+. Each Mn ion is six coordinate with distorted octahedral geometry except for the MnII ion which is seven coordinate. 5.3.3 Electrochemistry The electrochemical properties of the cubane like complex 5 -1 were investigated by cyclic voltammetry (CV) in 0.1M NBun 4PF6 solution in CH2Cl2. The obtained CV of 5 -1 is shown in Figure 5 6 where the potential is quoted vs F c/Fc+, which was used as an internal referenc e. The complex shows a reversible oxidat ion process at a potential of 660 m V with anodic vs cathodic peak separation of 3 00 m V at a scan rate of 100 mV/sec. In addition, one irreversible reductio n process was observed at 6 00 mV The reversible oxidation process observed for 5 -1 indicates the accessibility of the oxidation states outlined in eq. 5 4. [MnIII 2MnIV 2O4]6+ [MnIIIMnIV 3O4]7+ (5 4) To study the scan rate dependence of the reversible oxidation process of 5 -1 CV of complex 5 -1 was recorded in the appropriate potential range at 20 different scan rates between 30 and 600 mV/sec (Figure 5 7). From the linear dependence of anodic peak current, ia, vs the square root of the scan rate, 1/2 (Figure 5 8) it w as confirmed that the oxidation/reduction process in eq. 5 4 is reversible.

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179 5.3.4 Magnetochemistry 5.3.4.1 Direct current m agnetic susceptibility studies Variable temperature magnetic susceptibility measurements were performed on microcrystalline powder sa mples of complex 5 -1 H2O and 5 -2 5H2O, restrained in eicosane to prevent torquing, in a 1 kG (0.1 T) field and in the 5.0 300 K range. For 5 -1 H2O T steadily decreases from 6.28 cm3Kmol1 at 300 K to 0.63 cm3Kmol1 at 5.0 K (Figure 5 9). If there were no exchange interactions between the metal ions in a 2MnIII2MnIV complex, the spin -only ( g = 2) MT value would be 9.75 cm3Kmol1, and it would be temperature independent. The metal ions are clearly involved in antiferromagnetic exchange interactions, and the data were thus fit to the appropriate theoretical MT vs T expression. The latter was derived from the spin Hamiltonian of eq. 5 5, where J1 = J (MnIV MnIV), J2 = J (MnIVMnIII), J3 = J (MnIIIMnIII), S1 = S3 = S (MnI II) = 2 and S2 = S4 = S (MnI V) = 3/ 2, by employing an equivalent operator approach based on the Kambe vector coupling method.263 The substitutions employed were A = 2 + 4, B = 1 + 3 and T = A + B. = 2 J2(S1 S2 + S1 S4 + S2 S3 + S3 S4) 2 J1S2 S4 2 J3S1 S3 (5 5) This gives the equivalent form of eq. 5 5 that is given in eq. 5 6 = J2(ST 2 SA 2SB 2) J1(SA 2 S2 2 S4 2) J3(SB 2 S1 2 S3 2) (5 6) Eq 5 6 leads to the eigenvalue expression of eq 5 7, which gives the energy, E (ST), of each of the possible total spin stat es, ST, of the MnIII 2MnIV 2 complex. E( ST) = J 2[ST(ST+1) SA(SA+1) SB(SB+1)] J1[SA(SA+1)] J3[SB(SB+1)] (5 7) An expression for the molar paramagnetic susceptibility, M, was derived using the above and the Van Vleck equation,264 and assuming an is otropic g tensor (Appendix C 2 ). This equation was then used to fit the experimental MT vs T data in Figure 5 9 as a function of the

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180 three exchange parameters J1, J2 and J3 and the g factor. The contribution from temperature independent paramagnetism (TIP ) was held constant at 400 x 106 cm3mol1. The obtained fit is shown as the solid lines in Figure 59 : the fitting parameters are J1 = 16.1 cm1, J2 = 9.67 cm1, J3 = 8.26 cm1 and g = 2.01. These values yield an S = 0 ground state for complex 5 -1 I n ST, SA, SB > notation, the ground state is 0 0 0 > and the first and second excited states are 0 1 1 > and 1 0 1 > and 10 and 17 cm1 above the ground state, respectively. For 5 -2 5H2O, MT gradually increases from 30.59 cm3Kmol1 at 300 K to a value of 102.81 cm3mol1K at 8 K, and then slightly decreases to 102.09 at 5.0 K (Figure 5 10). The 300 K value is larger than the spin -only ( g = 2) value of 22.38 cm3Kmol1 for one MnII and six MnIII noninteracting ions, indicating the presence of domi nant ferromagnetic exchange interactions. The T value at 5.0 K is consistent with an S = 29/2 ground state and a g slightly less than 2.0; the spin -only value is ~112 cm3mol1K, and S = 29/2 is the maximum value for a MnIIMnIII 6 cluster, indicating a co mplete ferromagnetic system. In order to confirm the calculated S = 1 2 ground state of complex 5 -2 and to estimate the magnitude of the zero-field splitting parameter D magnetization vs dc field measurements were made on restrained samples at applied mag netic fields and temperatures in the 1 70 kG and 1.8 10.0 K ranges, respectively. The resulting data were fit, using the program MAGNET,66 by diagonalization of the spin Hamiltonian matrix assuming that only the ground state is populated, incorporating axi al anisotropy ( D z 2) and Zeeman terms, and employing a full powder average. The corresponding spin Hamiltonian is given by eq. 5 8 where z is the easy axis spin operator, g is the Land g factor, B is the Bohr magneton, and 0 is the vacuum permeability H = D z 2 + B0 H (5 8 )

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181 The result ing data are shown in Figure 5 11 as a reduced magnetization ( M / NB) versus H / T plot, where N is Avogadros number. The data show that the isofield lines almost superimpose, indicating that the ground state is only slightly zero -field split. The data were fit using the program MAGNET (vide supra) using all the data collected up to 7 tesla. This suggests that the ground state of 5 -2 is relatively well isolated from the nearest excited states. The best fit is shown as the solid lines in Figure 5 11 and was obtained with S = 29/2, g = 1.89, and D = 0.03 cm1. An equally good fit was also obtained with S = 29/2, g = 1. 89 and D = 0.02 cm1. Alternative fits with S = 27/2 were rejected because they gave unreasona ble values of g It is common to obtain two acceptable fits of magnetization data for a given S value, one with D > 0 and the other with D < 0, because magnetization fits are not very sensitive to the sign of D To assess which is the superior fit and als o to ensure that the true global minimum had been located, we calculated the root -mean -square D versus g error surface using the program GRID,116 which calculates the relative difference between the experimental M / B data and those calculated for various combinations of D and g The error surface, plotted as a two dimensional contour plot in Figure 5 12, shows the two minima with positive and negative D values, but with essentially equal quality; thus, it would require more sensitive techniques such as EPR spectroscopy or magnetization measurements on oriented single -crystals to confirm the sign of D However, D values for MnIII/MnIV clusters are essentially always negative, and thus very likely the situation here. 5.3.4.2 Alternating current m agnetic susceptibility studies We collected ac susceptibility data on 5 -2 5H2O in the 1.8 15 K range using a 3.5 G ac field oscillating at frequencies in the 50 1000 Hz range. The obtained in -phase M signal for 5 -2 is plotted as M T in Figure 5 13, and can be seen t o be almost temperature independent,

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182 indicating a well isolated ground state. Extrapolating the data to 0 K gives a value of ~103 cm3Kmol1, which is consistent with an S = 29/2 ground state and g ~ 1.91, in excellent agreement with the reduced magnetizati on fit. We conclude that complex 5 -2 does indeed have an S = 29/2 ground state, and that all exchange interactions in the molecule are ferromagnetic. It is worth noting that 5 -2 and (HNEt3)[NaMn6O4(dmhmp)6(N3)4] (ClO4)2 (4 -6 ) in Chapter 4, as well as two previous examples of Mn6 complexes with a [Mn6O4(3-X)4] face -capped octahedral topology with X being Cl or Br ,262 all present ferromagnetic exchange interactions within the molecules. There is no out of -phase ac susceptibility signal down to 1.8 K, the ope rating limit of our SQUID magnetometer. 5.4 Conclusions The tridentate N,N,O ligand pdpmin polynuclear Mn chemistry has provided new mixedvalent Mn clusters spanning Mn4 and Mn7 nuclearities and topologies that are either very rare or prototypical. In particular, the reaction of [Mn12O12(O2CMe)16(H2O)4] and pdpmH in MeCN has led to the isolation of the tetranuclear complex [Mn4O4(O2CMe)3(pdpm)3] (5 -1 ). The complex presents a cubane structure with rare Mn oxidation states: 2 MnIII and 2 MnIV. Subsequentl y, a better preparation of complex 5 -1 with a higher yield was developed by the comproportionation reaction between Mn(O2CMe)2 and MnO4 in the presence of pdpmH. Non-carboxylate Mn chemistry was also explored with pdpmH, and a new heptanuclear complex [Mn7O4(pdpm)6(N3)4](ClO4)2 (5 -2 ) was isolated. Interestingly in both 5 -1 and 5 -2 potentially tridentate ligand pdpmH mainly acts as bidentate chelate. The reluctance of pdpmto serve as a tridentate ligand is due to the nearby steric bulk of the Ph and pydine rings Magnetochemical characterization of these complexes revealed that 5 -1 and 5 -2 have ground state spin values of S = 0 and 29/2,

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183 respectively. The combined results emphasize the potential of our ligand design strategy, and it will be interesting to extend this idea to other alkoxide containing chelating/bridging ligands.

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184 Table 5 1. Crystallographic Data for 5 -1 0.5MeCN and 5 -2 2MeCN. parameter 5 1 5 2 formula C 58 H 49.5 Mn 4 N 6.5 O 13 C 106 H 84 Cl 2 Mn 7 N 26 O 18 fw, g mol 1 1265.30 2465.47 crystal system O rthorhombic Rhombohedral space group P2 1 2 1 2 1 R 3 a 12.440(3) 19.2878(8) b 19.761(4) 19.2878(8) c 22.399(5) 59.757(5) deg 90 90 deg 90 90 deg 90 120 V 3 5506.3(19) 19252.4(19) Z 4 6 T C 173(2) 100(2) radiation, a 0.71073 0 .71073 calc g cm 3 1.526 1.276 mm 1 0.968 0.775 R 1 b,c 0.0375 0.0747 wR 2 d 0.0835 0.2197 a Graphite monochromator. b I > 2 (I). c R 1 = 100 (|| Fo| | Fc||)/ | Fo|. d wR 2 = 100[ [ w ( Fo 2 Fc 2)2]/ [ w ( Fo 2)2]]1/2, w = 1/[ 2( Fo 2) + [(ap)2 + bp], where p = [ max( Fo 2, O) + 2 Fc 2]/3.

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185 Table 5 2. Bond Valence Sums for the Mn Atoms in Complex 5 -1a Mn II Mn III Mn IV Mn1 3.17 2.90 3.04 Mn2 4.02 3.71 3.84 Mn3 3.28 3.03 3.13 Mn4 4.16 3.84 3.98 a The underlined value is the one closest to the charge for which it wa s calculated, and the nearest whole number can be taken as the oxidation state of that atom. Table 5 3. BVS for Selected O Atoms in 5 -1a atom BVS assgt a O1 1.53 O 2 O2 1.70 O 2 O3 1.75 O 2 O4 1.85 O 2 a The O atom is an O2 if the BVS is ~1.8 2.0, an OHif it ~1.0 1.2, and an H2O if it is ~ 0.2 0.4, although the ranges can be affected slightly by H -bonding. hmpH pdpmH Figure 5 1. Structure of ligands: 2 (hydroxymethyl)pyridine (hmpH) and phenyldipyridin2 ylmethanol (pdpmH)

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186 Figure 5 2. The structure of complex 5 -1 (top), and a stereopair (bottom). The thicker yellow bonds indicate the positions of the MnIII Jahn Teller elongation axes. Hydrogen atoms have been omitted for clarity. Color code: MnIII green; MnIV orange ; O red; N blue; C grey.

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187 Figure 5 3. Space -filling diagram of 5 -1 ; sideview (top), topview (middle), and a stereoview (bottom), emphasizing the sterically hindered positioning of the unbound pyridine groups. Color code: MnIII green; MnIV orange; O r ed; N blue; C grey.

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188 Figure 5 4. The structure of complex 5 -2 (top), and a stereopair (bottom). Hydrogen atoms have been omitted for clarity. Color code: MnII yellow; MnIII green; O red; N blue; C grey.

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189 Figure 5 5. The labeled core of complex 5 -2 Color code: MnII yellow; MnIII green; O red; C grey.

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190 Figure 5 6. Cyclic voltammogram at glassy carbon electrode of complex 5 -1 in CH2Cl2 containing 0.1M NBun 4PF6 and ferrocene as an internal standard. Potential vs Fc/Fc+ (mV) -2000 -1000 0 1000 Current ( A) -150 -100 -50 0 50 100 > <

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191 Figure 5 7. Cyclic voltammogram of 5 -1 at di fferent scan rates. Figure 5 8. Plot of the square root of the scan rate, 1/2 vs the anodic peak current, ia for the oxidation process in Figure 5 -7 1/2 (mV1/2/sec1/2) 0 5 10 15 20 25 ia ( A) 0 50 100 150 200

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192 Temperature (K) 0 50 100 150 200 250 300 MT (cm3Kmol-1) 0 1 2 3 4 5 6 7 fitting Temperature (K) 0 50 100 150 200 250 300 MT (cm3Kmol-1) 0 20 40 60 80 100 120 Figure 5 9. Plot of MT vs T for complex 5 -1 The solid line is the fit of the data; see the text for the fit parameters. Figure 5 10. Plot of MT vs T for complex 5 -2

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193 H/T (kG/K) 0 10 20 30 40 50 M/N B 0 5 10 15 20 25 30 0.1 T 0.5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T fitting Figure 5 11. Plots of reduced magnetization (M/N B) vs H/T for complex 5 2 The solid lines are the fit of the data; see the text for the fit parameters

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194 g 1.80 1.85 1.90 1.95 2.00 2.05 D (cm-1) -0.10 -0.05 0.00 0.05 0.10 2.1 2.1 2.2 2.2 2.3 2.4 2.5 2.6 2.7 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.2 2.1 2.1 2.0 2.0 2.0 2.0 1.9 1.9 2.7 1.9 1.9 1.8 1.8 1.8 1.8 2.5 1.7 1.7 1.6 1.6 1.7 1.7 2.3 1.5 1.5 1.6 1.6 1.6 1.6 1.6 1.4 1.4 2.1 1.3 1.3 1.5 1.5 1.5 1.5 1.5 1.2 1.2 1.9 1.4 1.4 1.4 1.4 1.4 1.1 1.1 1.0 1.0 1.3 1.3 1.3 1.3 1.7 0.9 0.9 1.2 1.2 1.2 1.2 1.7 1.7 1.7 1.5 0.8 0.8 1.1 1.1 1.1 1.1 0.7 0.7 1.3 1.0 1.0 1.0 1.0 0.6 0.6 0.9 0.9 0.9 0.9 1.8 1.8 1.1 0.5 0.5 0.8 0.8 0.8 0.8 1.0 1.9 1.9 0.7 0.7 0.7 0.6 0.6 0.6 0.8 0.4 0.4 0.4 2.0 2.0 0.5 0.5 0.5 0.5 0.5 0.6 0.3 0.3 0.4 0.4 0.4 2.1 2.1 0.6 0.6 0.5 0.2 0.2 0.3 0.3 2.2 2.2 0.4 0.7 0.7 0.2 0.8 0.8 2.3 2.3 0.3 0.9 0.9 2.4 1.0 1.0 2.5 0.3 0.3 1.1 1.1 2.6 0.4 0.3 1.2 1.2 2.7 0.5 1.3 2.8 1.4 0.7 1.5 2.9 0.8 1.6 3.0 1.7 3.1 1.0 1.8 Figure 5 12. Two-dimensional contour plot of the root -mean -square error surface for the D vs g fit for complex 5 -2

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195 Figure 5 13. In-phase ac susceptibility ( M M 5 -2 in a 3.5 Oe ac field oscillating at t he indicated frequencies. Temperature (K) 0 2 4 6 8 10 12 14 16 M'T (cm3Kmol-1) 0 20 40 60 80 100 120 1000 Hz 250 Hz 50 Hz

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196 APPENDIX A BOND DISTANCES AND A NGLES Table A 1. Selected interatomic distances () and angles () for [Fe6O2(hmp)10(H2O)2](NO3)48MeCN ( 2 -1 8MeCN) atoms distances( ) atoms distances( ) Fe1 Fe2 3.671(1) Fe2 O4 2.001(4) Fe1 Fe3 3.604(3) Fe2 O5 2.016(5) Fe2 Fe3 3.030(3) Fe2 N4 2.148(6) Fe1 Fe2' 3.166(1) Fe2 N3 2.151(5) Fe1 O3 1.868(5) Fe3 O6 1.886(5) Fe1 O4' 1.993(4) Fe3 O1 1.989(5) Fe1 O2 2.004(4) Fe3 O5 2.009(5) Fe1 O1 2.027(4) Fe3 O3 2.054(4) Fe1 N2 2.158(5) Fe3 O7 2.059(5) Fe1 N1 2.186(6) Fe3 N5 2.188(11) Fe2 O3 1.880(5) O2 Fe2' 1.980(4) Fe2 O2' 1.980(4) O4 Fe1' 1.993(4) atoms angles( ) atoms angles( ) O3 Fe1 O4' 98.39(19) O2' Fe2 N3 149.35(19) O3 Fe1 O2 111.63(19) O4 Fe2 N3 76.54(19) O4' Fe1 O2 73.23(18) O5 Fe2 N3 107.69(19) O3 Fe1 O1 80.54(19) N4 Fe2 N3 83.4(2) O4' Fe1 O1 103.51(17) O6 Fe3 O1 97.8(2) O2 Fe1 O1 167.6(2) O6 Fe3 O5 96.3(2) O3 Fe1 N2 93.2(2) O1 Fe3 O5 153.5(2) O4' Fe1 N2 148.67(19) O6 Fe3 O3 99.99(19) O2 Fe1 N2 75.44(19) O1 F e3 O3 77.14(18) O1 Fe1 N2 107.08(18) O5 Fe3 O3 78.4(2) O3 Fe1 N1 154.9(2) O6 Fe3 O7 161.5(2) O4' Fe1 N1 94.7(2) O1 Fe3 O7 87.9(2) O2 Fe1 N1 92.6(2) O5 Fe3 O7 85.7(2) O1 Fe1 N1 75.7(2) O3 Fe3 O7 98.38(18) N2 Fe1 N1 86.4(2) O6 Fe3 N5 76.6(3) O3 Fe2 O2 101.29(19) O1 Fe3 N5 97.7(4) O3 Fe2 O4 104.68(19) O5 Fe3 N5 107.4(4) O2' Fe2 O4 73.54(18) O3 Fe3 N5 173.5(4) O3 Fe2 O5 82.4(2) O7 Fe3 N5 85.3(3) O2' Fe2 O5 101.23(19) Fe3 O1 Fe1 99.44(19) O4 Fe2 O5 171.8(2) Fe2' O2 Fe1 105.3(2) O3 Fe2 N4 155.5(2) F e1 O3 Fe2 156.7(3) O2' Fe2 N4 94.2(2) Fe1 O3 Fe3 102.6(2) O4 Fe2 N4 97.9(2) Fe2 O3 Fe3 100.7(2) O5 Fe2 N4 75.9(2) Fe1' O4 Fe2 104.9(2) O3 Fe2 N3 92.5(2) Fe3 O5 Fe2 97.7(2)

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197 Table A 2. Selected interatomic distances () and angles () for [Fe6O2(hmp)8(NO3)4(H2O)2](NO3)26MeCN ( 2 -2 6MeCN) atoms distances( ) atoms distances( ) Fe1 Fe2 3.648(3) Fe2 O3' 1.9750(19) Fe1 Fe3 3.127(5) Fe2 O5 1.9958(19) Fe2 Fe3 3.095(3) Fe2 O4 2.0052(18) Fe1 Fe2' 3.190(6) Fe2 N3 2.151(2) Fe1 O1 1.8903(17) Fe2 N4 2.159 (2) Fe1 O2 1.9974(19) Fe3 O5 1.9922(19) Fe1 O4' 1.9989(19) Fe3 O2 1.9959(19) Fe1 O3 2.0099(18) Fe3 O1 2.0184(18) Fe1 N2 2.180(2) Fe3 O6 2.034(2) Fe1 N1 2.194(2) Fe3 O7 2.044(4) Fe2 O1 1.8939(18) Fe3 O10 2.107(5) atoms angles( ) atoms ang les( ) O1 Fe1 O2 76.73(8) O5 Fe2 N4 75.66(8) O1 Fe1 O4' 99.13(8) O4 Fe2 N4 100.49(8) O2 Fe1 O4' 105.10(8) N3 Fe2 N4 84.94(9) O1 Fe1 O3 114.90(8) O5 Fe3 O2 149.01(8) O2 Fe1 O3 168.29(8) O5 Fe3 O1 75.40(7) O4' Fe1 O3 72.64(8) O2 Fe3 O1 73.94(7) O1 Fe1 N2 94.88(8) O5 Fe3 O6 90.02(9) O2 Fe1 N2 106.58(9) O2 Fe3 O6 90.92(9) O4' Fe1 N2 147.53(8) O1 Fe3 O6 99.95(8) O3 Fe1 N2 74.90(8) O5 Fe3 O7 81.01(14) O1 Fe1 N1 150.54(9) O2 Fe3 O7 106.25(14) O2 Fe1 N1 74.72(8) O1 Fe3 O7 95.56(13) O4' Fe1 N1 95.14(8) O6 Fe3 O7 159.52(13) O3 Fe1 N1 93.91(8) O5 Fe3 O10 127.70(13) N2 Fe1 N1 86.44(9) O2 Fe3 O10 83.27(13) O1 Fe2 O3' 101.59(8) O1 Fe3 O10 156.22(12) O1 Fe2 O5 78.13(8) O6 Fe3 O10 86.88(14) O3' Fe2 O5 101.30(8) O7 Fe3 O10 84.22(18) O1 Fe2 O4 106.60(8) Fe1 O1 Fe2 149.13(11) O3' Fe2 O4 73.24(8) Fe1 O1 Fe3 106.21(9) O5 Fe2 O4 173.26(8) Fe2 O1 Fe3 104.52(8) O1 Fe2 N3 93.31(8) Fe3 O2 Fe1 103.08(8) O3' Fe2 N3 148.93(9) Fe2' O3 Fe1 106.38(8) O5 Fe2 N3 108.43(9) Fe1' O4 Fe2 105.65(8) O4 Fe2 N3 76.43(8) Fe3 O 5 Fe2 101.80(8) O1 Fe2 N4 151.65(9) O5 Fe2 N4 75.66(8) O3' Fe2 N4 93.97(8) O4 Fe2 N4 100.49(8)

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198 Table A 3. Selected interatomic distances () and angles () for [Fe4(N3)6(hmp)6]2MeOH ( 2 3 2MeOH) atoms distances( ) atoms distances( ) Fe1 Fe2 3.170(1) F e1 N1 2.139(3) Fe1 Fe1' 3.165(1) Fe2 N7 1.988(3) Fe1 Fe2' 5.735(2) Fe2 N4 2.012(3) Fe1 O1 1.954(2) Fe2 N10 2.016(3) Fe1 O2' 1.988(2) Fe2 O1 2.058(2) Fe1 O3 1.989(2) Fe2 O3 2.072(2) Fe1 O2 1.997(2) Fe2 N3 2.191(3) Fe1 N2 2.135(3) O2 Fe1' 1.988(2) atoms angles( ) atoms angles( ) O1 Fe1 O2' 108.25(10) N4 Fe2 N10 92.06(13) O1 Fe1 O3 78.37(9) N7 Fe2 O1 90.96(11) O2' Fe1 O3 96.94(9) N4 Fe2 O1 90.28(11) O1 Fe1 O2 176.73(10) N10 Fe2 O1 169.37(12) O2' Fe1 O2 74.85(11) N7 Fe2 O3 160.78(11) O3 Fe1 O2 102.46(9) N4 Fe2 O3 93.15(11) O1 Fe1 N2 100.53(10) N10 Fe2 O3 95.31(11) O2' Fe1 N2 150.49(10) O1 Fe2 O3 74.20(9) O3 Fe1 N2 95.04(9) N7 Fe2 N3 93.72(13) O2 Fe1 N2 76.27(10) N4 Fe2 N3 166.86(12) O1 Fe1 N1 77.18(10) N10 Fe2 N3 83.48(12) O2' Fe 1 N1 88.65(10) O1 Fe2 N3 91.95(10) O3 Fe1 N1 155.44(10) O3 Fe2 N3 75.04(10) O2 Fe1 N1 102.08(10) Fe1 O1 Fe2 104.34(10) N2 Fe1 N1 91.54(11) Fe1' O2 Fe1 105.15(11) N7 Fe2 N4 99.19(14) Fe1 O3 Fe2 102.63(9) N7 Fe2 N10 98.89(13) Fe2 Fe1 Fe1' 129.73(2)

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199 T able A 4. Selected interatomic distances () and angles () for [Fe8O3(OMe)(pdm)4(pdmH)4(MeOH)2](ClO4)5 7MeOH ( 3 -1 7MeOH) atoms distances( ) atoms distances( ) Fe1 Fe2 3.133(23) Fe3 O7 2.072(4) Fe1 Fe8 3.101(4) Fe3 N3 2.084(5) Fe2 Fe3 3.499(2) Fe4 O15 1.856(4) Fe2 Fe7 3.246(14) Fe4 O12 1.993(4) Fe2 Fe8 3.529(14) Fe4 O20 2.005(4) Fe3 Fe4 3.425(27) Fe4 O10 2.026(4) Fe3 Fe5 3.645(9) Fe4 N5 2.109(5) Fe3 Fe6 3.365(33) Fe4 O11 2.157(4) Fe3 Fe7 3.562(5) Fe5 O15 1.973(4) Fe3 Fe8 3.091(10) Fe5 O7 2.034(4) Fe4 Fe5 3.129(15) Fe5 O14 2.074(5) Fe4 Fe6 3.541(4) Fe5 O12 2.078(4) Fe4 Fe7 3.484(3) Fe5 O13 2.145(5) Fe5 Fe6 3.094(8) Fe5 O16 2.160(5) Fe6 Fe7 3.072(3) Fe5 N6 2.240(5) Fe6 Fe8 3.160(7) Fe6 O15 1.920(4) Fe7 Fe8 3.366(35) Fe6 O18 1. 998(4) Fe1 O22 1.978(4) Fe6 O17 2.000(4) Fe1 O21 2.009(4) Fe6 O16 2.006(4) Fe1 O3 2.072(4) Fe6 N7 2.051(6) Fe1 O1 2.090(4) Fe6 O19 2.096(4) Fe1 O9 2.177(4) Fe7 O19 1.958(4) Fe1 O2 2.177(4) Fe7 O5 2.011(4) Fe1 N1 2.220(5) Fe7 O17 2.033( 4) Fe2 O22 1.865(4) Fe7 O20 2.053(4) Fe2 O5 2.005(4) Fe7 N8 2.060(5) Fe2 O3 2.010(4) Fe7 O21 2.088(4) Fe2 O6 2.020(4) Fe8 O22 1.915(4) Fe2 N2 2.106(6) Fe8 O18 1.948(4) Fe2 O4 2.147(5) Fe8 O8 2.003(4) Fe3 O19 1.952(4) Fe8 O9 2.015(4) F e3 O10 1.992(4) Fe8 N4 2.080(5) Fe3 O6 2.059(4) Fe8 O19 2.089(4) Fe3 O8 2.068(4) atoms angles( ) atoms angles( ) Fe2 O3 Fe1 100.25(18) Fe8 O18 Fe6 106.41(19) Fe2 O5 Fe7 117.11(19) Fe3 O19 Fe7 131.3(2) Fe2 O6 Fe3 118.1(2) Fe3 O19 Fe8 99.74(18) Fe5 O7 Fe3 125.2(2) Fe7 O19 Fe8 112.50(19) Fe8 O8 Fe3 98.78(17) Fe3 O19 Fe6 112.45(19) Fe8 O9 Fe1 95.31(18) Fe7 O19 Fe6 98.48(19) Fe3 O10 Fe4 117.0(2) Fe8 O19 Fe6 98.06(17) Fe4 O12 Fe5 100.40(19) Fe4 O20 Fe7 118.31(19)

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200 Table A 4. Continued. atoms ang les( ) atoms angles( ) Fe4 O15 Fe6 139.4(2) Fe1 O21 Fe7 126.3(2) Fe4 O15 Fe5 109.5(2) Fe2 O22 Fe8 138.1(2) Fe6 O15 Fe5 105.3(2) Fe2 O22 Fe1 109.2(2) Fe6 O16 Fe5 95.85(18) Fe8 O22 Fe1 105.6(2) Fe6 O17 Fe7 99.25(19) Table A 5. Selected interatomic d istances () and angles () for [Fe18O6(OH)8(pdm)10(pdmH)4(H2O)4](ClO4)107MeCN3H2O ( 3 -3 7MeCN3H2O) atoms distances( ) atoms distances( ) Fe1 O3 1.921(7) Fe5 N4 2.066(10) Fe1 O10 1.994(7) Fe5 O10 2.093(6) Fe1 O13 2.000(7) Fe6 O2 1.932(7) Fe1 O1 7 2.022(6) Fe6 O19 1.960(7) Fe1 O23 2.045(6) Fe6 O11 1.967(7) Fe1 O23 2.117(6) Fe6 O20 1.986(7) Fe2 O3 1.906(6) Fe6 O21 2.104(8) Fe2 O6 1.947(6) Fe6 O22 2.147(7) Fe2 O12 1.994(7) Fe7 O5 1.969(6) Fe2 O1 2.073(6) Fe7 O15 1.975(6) Fe2 N 5 2.074(10) Fe7 O8 1.981(6) Fe2 O13 2.130(6) Fe7 O2 1.981(7) Fe3 O3 1.857(6) Fe7 N6 2.131(9) Fe3 O4 1.987(7) Fe7 O14 2.162(7) Fe3 O16 1.998(7) Fe8 O18 1.937(7) Fe3 O17 2.068(6) Fe8 O15 1.979(7) Fe3 N7 2.072(8) Fe8 O6 2.060(7) Fe3 O23 2.162(6) Fe8 O7 2.126(6) Fe4 O19 1.970(7) Fe8 O2 2.170(6) Fe4 O1 1.981(7) Fe8 N2 2.175(8) Fe4 O12 2.034(7) Fe8 O1 2.254(6) Fe4 O4 2.069(6) Fe9 O7 1.908(6) Fe4 O5 2.086(6) Fe9 O20 1.918(7) Fe4 N1 2.100(9) Fe9 O8 1.985(8) Fe5 O1 1.965(7 ) Fe9 N3 2.111(8) Fe5 O16 2.004(6) Fe9 O2 2.169(6)

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201 Table A 5. Continued. Fe5 O18 2.013(6) Fe9 O9 2.177(8) Fe5 O11 2.054(7) atoms angles( ) atoms angles( ) Fe1 O3 Fe3 108.5(3) Fe4 O1 Fe8 121.9(3) Fe1 O3 Fe2 103.0(3) Fe4 O5 Fe7 121.7(4) Fe1 O10 Fe5 124.3(4) Fe4 O19 Fe6 123.7(3) Fe1 O13 Fe2 93.0(3) Fe5 O1 Fe8 96.1(3) Fe1 O23 Fe1 105.4(3) Fe5 O11 Fe6 119.1(4) Fe1 O23 Fe3 98.3(3) Fe5 O18 Fe8 105.5(3) O23 Fe3 91.5(3) Fe6 O2 Fe7 120.6(3) Fe1 O17 Fe3 102.2(3) Fe6 O2 Fe9 97.0(3) Fe2 O1 Fe5 110.5(3) Fe6 O2 Fe8 132.8(3) Fe2 O1 Fe4 96.9(3) Fe6 O20 Fe9 103.9(3) Fe2 O1 Fe8 99.2(3) Fe7 O2 Fe8 99.8(3) Fe2 O3 Fe3 141.7(3) Fe7 O2 Fe9 98.8(3) Fe2 O6 Fe8 110.8(3) Fe7 O8 Fe9 105.3(3) Fe2 O12 Fe4 97.7(4) Fe7 O15 Fe8 106.9(3) Fe3 O4 Fe4 119.9 (3) Fe8 O2 Fe9 100.0(2) Fe3 O16 Fe5 115.7(3) Fe8 O7 Fe9 110.9(3) Fe4 O1 Fe5 128.7(3)

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202 Table A 6. Selected interatomic distances () and angles () for [Fe9O4(OH)2(O2CMe)10(pdm)(pdmH)4](NO3)7CH2Cl2 (3 -4 7CH2Cl2) atoms distances( ) atoms distances( ) Fe1 O3 1.844(3) Fe3 O4 2.041(3) Fe1 O3 1.844(2) Fe3 O9 2.077(3) Fe1 O18 2.051(3) Fe4 O2 1.890(2) Fe1 O18 2.051(3) Fe4 O10 1.989(3) Fe1 N3 2.084(5) Fe4 O17 1.996(3) Fe2 O2 1.866(3) Fe4 O13 2.026(4) Fe2 O3 1.960(3) Fe4 O8 2.053(3) Fe 2 O15 2.034(3) Fe4 N2 2.208(3) Fe2 O11 2.065(3) Fe5 O3 1.935(3) Fe2 O6 2.074(3) Fe5 O1 1.948(3) Fe2 O12 2.086(3) Fe5 O15 1.982(3) Fe3 O2 1.942(3) Fe5 O5 2.029(3) Fe3 O1 1.962(3) Fe5 O7 2.051(3) Fe3 O18 2.008(3) Fe5 N1 2.253(3) Fe3 O 17 2.028(3) Fe5 Fe2 2.9796(7) atoms angles( ) atoms angles( ) Fe5 O1 Fe3 125.13(14) Fe1 O3 Fe2 134.41(14) Fe2 O2 Fe4 121.95(14) Fe5 O3 Fe2 99.79(11) Fe2 O2 Fe3 134.18(13) Fe5 O15 Fe2 95.76(11) Fe4 O2 Fe3 100.19(12) Fe4 O17 Fe3 93.84(11) Fe1 O3 Fe5 125.80(13) Fe3 O18 Fe1 124.44(13)

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203 Table A 7. Selected interatomic distances () and angles () for [Mn4O2(O2CBut)5(dphmp)3] 2MeCN ( 4 -1 2MeCN) atoms distances( ) atoms distances( ) Mn1 O3 1.867(3) Mn3 O2 1.883(3) Mn1 O1 1.899(3) Mn3 O5 1.88 7(3) Mn1 O14 1.965(3) Mn3 O1 1.904(3) Mn1 N1 2.053(4) Mn3 N3 2.021(4) Mn1 O8 2.193(3) Mn3 O9 2.101(3) Mn1 O6 2.219(3) Mn3Mn4 2.7412(9) Mn2 O4 1.861(3) Mn4 O1 1.898(3) Mn2 O2 1.887(3) Mn4 O2 1.903(3) Mn2 O13 1.974(3) Mn4 O7 1.931(3) M n2 N2 2.049(4) Mn4 O10 1.970(3) Mn2 O11 2.151(3) Mn4 O15 2.153(3) Mn2 O5 2.505(3) Mn4 O12 2.282(3) Mn2Mn3 3.1076(9) atoms angles( ) atoms angles( ) Mn4 O1 Mn1 120.15(16) Mn3 O2 Mn4 92.79(13) Mn4 O1 Mn3 92.25(13) Mn2 O2 Mn4 125.40(17) Mn1 O 1 Mn3 130.31(17) Mn3 O5 Mn2 88.93(12) Mn3 O2 Mn2 111.06(15)

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204 Table A 8. Selected interatomic distances () and angles () for [Mn6O4(OMe)2(O2CPh)4(dphmp)4] 3MeCN ( 4 -2 3MeCN). atoms distances( ) atoms distances( ) Mn1 O1 1.918(6) Mn2 O2 1.945(6) Mn1 O5 1.924(6) Mn2 N2 2.030(8) Mn1 O2 1.935(6) Mn2 O9 2.240(7) Mn1 O8 1.945(7) Mn2 O5 2.319(7) Mn1 O7 2.101(7) Mn2Mn1 3.084(2) Mn1 O2 2.390(7) Mn3 O3 1.839(6) Mn1Mn3 2.803(2) Mn3 O2 1.924(6) Mn1Mn2 2.934(2) Mn3 O1 1.925(6) Mn1M n2 3.084(2) Mn3 N1 2.037(8) Mn1Mn3 3.208(2) Mn3 O6 2.267(7) Mn2 O4 1.851(6) Mn3 O5 2.408(7) Mn2 O1 1.938(6) Mn3Mn1 3.208(2) atoms angles( ) atoms angles( ) Mn3 O2 Mn1 93.1(3) Mn2 O2 Mn1 84.5(2) Mn3 O2 Mn2 161.4(3) Mn1 O5 Mn2 92.7(3) Mn1 O2 Mn2 105.3(3) Mn1 O5 Mn3 94.9(3) Mn3 O2 Mn1 95.5(2) Mn2 O5 Mn3 88.2(2) Mn1 O2 Mn1 98.7(2)

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205 Table A 9. Selected interatomic distances () and angles () for [Mn11O7(OMe)7(O2CPh)7(dphmp)4(MeOH)2] 4MeCN ( 4 -3 4MeCN). atoms distances( ) atoms distances( ) Mn1 O2 1.850(3) Mn6 O13 1.996(3) Mn1 O32 1.892(3) Mn6 O33 2.160(3) Mn1 O29 1.948(3) Mn6 O26 2.279(3) Mn1 N2 2.044(4) Mn7 O21 1.881(3) Mn1 O17 2.185(3) Mn7 O31 1.903(3) Mn1 O31 2.442(3) Mn7 O8 1.993(3) Mn2 O33 1.873(3) Mn7 O 30 2.010(3) Mn2 O29 1.878(3) Mn7 O9 2.082(3) Mn2 O1 1.879(3) Mn7 O19 2.268(3) Mn2 N1 2.041(4) Mn8 O32 1.900(3) Mn2 O5 2.219(3) Mn8 O30 1.925(3) Mn2 O17 2.422(3) Mn8 O22 1.944(3) Mn3 O3 1.862(3) Mn8 O31 1.964(3) Mn3 O33 1.907(3) Mn8 O3 4 2.191(3) Mn3 O18 1.929(3) Mn8 O10 2.242(3) Mn3 N3 2.045(4) Mn9 O34 1.874(3) Mn3 O20 2.247(3) Mn9 O25 1.893(3) Mn3 O1 2.415(3) Mn9 O14 1.962(3) Mn4 O29 1.864(3) Mn9 O30 1.965(3) Mn4 O31 1.914(3) Mn9 O19 2.217(3) Mn4 O6 1.933(3) Mn9 O2 7 2.266(3) Mn4 O19 1.942(3) Mn10 O4 1.862(3) Mn4 O7 2.241(3) Mn10 O28 1.872(3) Mn4 O34 2.444(3) Mn10 O22 1.938(3) Mn5 O28 1.843(3) Mn10 N4 2.041(3) Mn5 O32 1.895(3) Mn10 O12 2.219(3) Mn5 O17 1.941(3) Mn10 O26 2.308(3) Mn5 O18 2.000(3) Mn11 O25 2.121(3) Mn5 O11 2.116(3) Mn11 O16 2.128(4) Mn5 O33 2.395(3) Mn11 O21 2.131(3) Mn6 O34 1.880(3) Mn11 O23 2.174(4) Mn6 O20 1.910(3) Mn11 O24 2.241(4) Mn6 O28 1.949(3) Mn11 O30 2.293(3)

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206 Table A 10. Selected interatomic distances () and angles () for [Mn7O3(OH)3(O2CBut)7(dmhmp)4] 7MeCN (4 -4 7MeCN ) atoms distances( ) atoms distances( ) Mn1 O1 1.866(2) Mn5 O23 2.127(3) Mn1 O7 1.887(2) Mn5 O13 2.522(3) Mn1 O2 1.895(2) Mn6 O10 1.871(3) Mn1 N1 2.015(3) Mn6 O6 1.882(3) Mn1 O15 2.263(3) Mn6 O3 1.920(3) Mn1 O13 2.303(3) Mn6 N4 2.040(4) Mn2 O5 1.867(2) Mn6 O20 2.258(3) Mn2 O8 1.899(3) Mn6 O22 2.273(3) Mn2 O2 1.910(3) Mn7 O8 2.141(3) Mn2 N2 2.031(3) Mn7 O9 2.148(3) Mn2 O24 2.110(3) Mn7 O7 2.191(3) Mn3 O3 1 .889(3) Mn7 O17 2.204(3) Mn3 O1 1.916(2) Mn7 O18 2.257(3) Mn3 O21 1.946(3) Mn7 O2 2.362(2) Mn3 O16 1.973(3) Mn7 C48 2.545(4) Mn3 O5 2.206(3) Mn1 Mn2 3.159(10) Mn3 O11 2.254(3) Mn1 Mn3 3.407(8) Mn4 O1 1.893(3) Mn1 Mn4 3.356(3) Mn4 O3 1.899(3) Mn1 Mn5 3.175(9) Mn4 O19 1.941(3) Mn1 Mn7 3.237(10) Mn4 O14 2.004(3) Mn2 Mn3 3.656(6) Mn4 O4 2.224(3) Mn2 Mn7 3.252(7) Mn4 O12 2.236(3) Mn3 Mn4 2.810(2) Mn5 O4 1.859(3) Mn3 Mn6 3.411(8) Mn5 O2 1.897(3) Mn4 Mn5 3.694(6) Mn5 O9 1.905(3) Mn4 Mn6 3.432(11) Mn5 N3 2.021(3) Mn5 Mn7 3.245(2) atoms angles( ) atoms angles( ) Mn1 O1 Mn3 128.58(13) Mn5 O2 Mn2 127.35(13) Mn1 O1 Mn4 126.46(13) Mn5 O2 Mn7 98.7(1) Mn1 O2 Mn2 112.28(12) Mn5 O4 Mn4 129.35(13) Mn1 O2 Mn5 113.73(13) Mn5 O9 Mn7 106.24(12) Mn1 O2 Mn7 98.4(1) Mn3 O3 Mn4 95.75(11) Mn1 O7 Mn7 104.84(11) Mn3 O3 Mn6 127.16(14) Mn2 O2 Mn7 98.59(10) Mn4 O3 Mn6 127.99(14) Mn2 O5 Mn3 127.47(12) Mn2 O8 Mn7 107.07(12)

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207 Table A 11. Selected interatomi c distances () and angles () for [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] 3CH2Cl2 (4 -5 3CH2Cl2) atoms distances( ) atoms distances( ) Mn1 O4 1.880(3) Mn8 O10 1.894(3) Mn1 O1 1.918(3) Mn8 O7 1.896(3) Mn1 O3 1.930(3) Mn8 O14 1.943(3) Mn1 O23 1.9 60(3) Mn8 N2 2.041(3) Mn1 O33 2.214(3) Mn8 O27 2.120(3) Mn1 O6 2.343(3) Mn8 O9 2.264(3) Mn2 O4 1.902(3) Mn9 O5 1.844(3) Mn2 O20 1.942(3) Mn9 O11 1.908(3) Mn2 O1 1.958(3) Mn9 O8 1.908(3) Mn2 O21 1.974(3) Mn9 N3 2.034(4) Mn2 O34 2.140(3) Mn9 O18 2.118(3) Mn2 O5 2.165(3) Mn9 O22 2.447(3) Mn3 O5 1.866(3) Mn10 O12 1.879(3) Mn3 O2 1.901(3) Mn10 O4 1.895(3) Mn3 O17 1.973(3) Mn10 O8 1.909(3) Mn3 O1 2.004(3) Mn10 N4 2.022(4) Mn3 O16 2.205(3) Mn10 O24 2.217(3) Mn3 O6 2.251(3) Mn10 O22 2.321(3) Mn4 O2 1.880(3) Mn11 O6 1.857(3) Mn4 O6 1.890(3) Mn11 O39 1.891(3) Mn4 O38 1.942(3) Mn11 O8 1.925(3) Mn4 O35 1.988(3) Mn11 O36 2.003(3) Mn4 O25 2.153(3) Mn11 O26 2.093(3) Mn4 O3 2.302(3) Mn12 O39 2.085(3) Mn5 O2 1.87 9(3) Mn12 O11 2.143(3) Mn5 O9 1.883(3) Mn12 O12 2.160(4) Mn5 O7 1.905(3) Mn12 O32 2.225(10) Mn5 N1 2.038(3) Mn12 O31 2.227(11) Mn5 O15 2.231(3) Mn12 O8 2.326(3) Mn5 O38 2.387(3) Mn1 Mn2 2.8036(10) Mn6 O29 2.129(3) Mn1 Mn3 3.1568(9) Mn6 O10 2.130(3) Mn1 Mn4 3.1880(9) Mn6 O3 2.170(3) Mn2 Mn3 2.9905(9)

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208 Table A 11. Continued. Mn6 O37 2.226(3) Mn3 Mn4 3.0396(9) Mn6 O7 2.264(3) Mn3 Mn9 3.2389(9) Mn6 O38 2.284(3) Mn4 Mn5 3.1034(9) Mn7 O19 2.142(3) Mn5 Mn8 2.9 783(9) Mn7 O7 2.155(3) Mn6 Mn8 3.1335(9) Mn7 O28 2.168(3) Mn9 Mn10 3.1202(10) Mn7 O13 2.209(3) Mn9 Mn12 3.2111(10) Mn7 O16 2.296(3) Mn10 Mn12 3.2316(11) Mn7 O1 2.314(3) atoms angles( ) atoms angles( ) Mn1 O1 Mn2 92.65(12) Mn4 O2 M n5 111.26(14) Mn1 O1 Mn3 107.19(13) Mn4 O6 Mn11 119.97(15) Mn1 O1 Mn7 131.85(14) Mn4 O38 Mn5 90.98(11) Mn1 O3 Mn4 97.38(12) Mn4 O38 Mn6 102.66(12) Mn1 O3 Mn6 147.76(15) Mn5 O7 Mn7 115.82(13) Mn1 O4 Mn2 95.70(13) Mn5 O7 Mn6 103.96(12) Mn1 O4 Mn10 125. 59(16) Mn5 O7 Mn8 103.18(13) Mn1 O6 Mn3 86.78(10) Mn5 O9 Mn8 91.33(11) Mn1 O6 Mn4 97.15(12) Mn5 O38 Mn6 89.57(10) Mn1 O6 Mn11 127.49(14) Mn6 O7 Mn7 117.56(12) Mn2 O1 Mn3 98.03(13) Mn6 O7 Mn8 97.38(12) Mn2 O1 Mn7 118.86(13) Mn6 O10 Mn8 102.10(12) Mn2 O4 Mn10 125.90(17) Mn7 O7 Mn8 116.34(13) Mn2 O5 Mn3 95.50(12) Mn9 O8 Mn10 109.65(15) Mn2 O5 Mn9 124.13(14) Mn9 O8 Mn11 126.69(16) Mn3 O1 Mn7 103.42(11) Mn9 O8 Mn12 98.18(13) Mn3 O2 Mn4 107.00(13) Mn9 O11 Mn12 104.73(15) Mn3 O2 Mn5 137.65(15) Mn9 O22 M n10 81.69(10) Mn3 O5 Mn9 121.58(15) Mn10 O8 Mn11 116.58(16) Mn3 O6 Mn4 94.03(11) Mn10 O8 Mn12 98.99(12) Mn3 O6 Mn11 122.47(14) Mn10 O12 Mn12 106.04(14) Mn3 O16 Mn7 97.86(11) Mn11 O8 Mn12 99.37(12) Mn4 O3 Mn6 95.34(11) Mn11 O39 Mn12 109.62(15)

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209 Table A 12. Selected interatomic distances () and angles () for (HNEt3)[NaMn6O4(dmhmp)4(N3)4](ClO4)2 Et2O ( 4 -6 Et2O). atoms distances( ) atoms distances( ) Mn1 O2 1.862(2) Mn2 O1 1.897(2) Mn1 O3 1.8797(9) Mn2 O1 1.915(2) Mn1 O1 1.895(2) Mn2 N8 2.028 (3) Mn1 N7 2.045(3) Mn2 N1 2.354(3) Mn1 N1 2.394(3) Mn2 N4 2.418(3) Mn1 N1 2.423(3) Mn2 Mn2 3.2163(10) Mn1 Mn2 3.1731(8) Mn2 Mn1 3.2310(7) Mn1 Mn1 3.1985(10) Na1 O2 2.320(3) Mn1 Mn2 3.2310(7) Na1 O3 2.386(4) Mn1 Na1 3.302(2) Na1 N6 2.420(7) Mn2 O4 1.861(2) atoms angles() atoms angles() Mn1 O1 Mn2 113.61(11) Mn1 O3 Na1 100.76(11) Mn1 O1 Mn2 116.00(12) Mn2 N1 Mn1 85.78(10) Mn2 O1 Mn2 115.06(12) Mn2 N1 Mn1 83.25(10) Mn1 O2 Na1 103.73(12) Mn1 N1 Mn1 83.23(10) Mn1 O3 M n1 116.60(7) Mn2 N4 Mn2 83.39(13)

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210 Table A 13. Selected interatomic distances () and angles () for [Mn4O4(O2CMe)3(pdpm)3] 0.5MeCN ( 5 -1 0.5MeCN) atoms d istances ( ) atoms d istances ( ) Mn1 Mn2 2.7989(8) Mn2 O2 1.916(2) Mn1 Mn4 2.8548(7) Mn2 O6 2.019(2) Mn1 Mn3 2.8815(7) Mn2 N1 2.034(2) Mn2 Mn4 2.8543(7) Mn3 O12 1.846(2) Mn2 Mn3 3.0693(9) Mn3 O3 1.882(2) Mn3 Mn4 2.9579(7) Mn3 O2 1.968(2) Mn1 O2 1.913(2) Mn3 N3 2.029(3) Mn1 O9 1.966(2) Mn3 O8 2.188(2) Mn1 O4 1.976(2) Mn3 O1 2. 297(2) Mn1 O7 1.982(2) Mn4 O13 1.834(2) Mn1 O5 2.103(2) Mn4 O3 1.838(2) Mn1 O3 2.107(2) Mn4 O1 1.878(2) Mn2 O11 1.851(2) Mn4 O4 1.910(2) Mn2 O4 1.857(2) Mn4 O10 2.027(2) Mn2 O1 1.912(2) Mn4 N5 2.036(3) atoms angles ( ) atoms angles ( ) Mn4 O1 Mn2 97.73(10) Mn4 O3 Mn3 105.34(10) Mn4 O1 Mn3 89.63(9) Mn4 O3 Mn1 92.46(9) Mn2 O1 Mn3 93.19(8) Mn3 O3 Mn1 92.31(8) Mn1 O2 Mn2 93.92(9) Mn2 O4 Mn4 98.50(9) Mn1 O2 Mn3 95.88(9) Mn2 O4 Mn1 93.75(8) Mn2 O2 Mn3 104.40(9) Mn4 O4 Mn1 94.54(9)

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211 Tabl e A 14. Selected interatomic distances () and angles () for [Mn7O4(pdpm)6(N3)4](ClO4)2 2MeCN (5 2 2MeCN) atoms distances ( ) atoms distances ( ) Mn1 O4 1.8667(15) Mn2 O1 1.932(5) Mn1 O1 1.880(4) Mn2 O1 1.934(4) Mn1 O2 1.896(4) Mn2 N3 2.035(6) Mn1 N2 2.039(6) Mn2 N4 2.329(6) Mn1 N7 2.328(6) Mn2 N7 2.372(6) Mn1 N7 2.469(6) Mn2 Mn2 3.2150(18) Mn1 Mn1 3.1911(17) Mn3 O2 2.195(4) Mn1 Mn2 3.2016(15) Mn3 N1 2.329(6) Mn1 Mn2 3.2265(15) Mn3 O4 2.579(7) Mn2 O3 1.863(5) atoms angle s () atoms angles () Mn2 N4 Mn2 87.3(3) Mn1 O1 Mn2 115.6(2) Mn1 N7 Mn2 86.7(2) Mn2 O1 Mn2 112.6(2) Mn1 N7 Mn1 83.3(2) Mn1 O2 Mn3 113.1(2) Mn2 N7 Mn1 82.78(18) Mn1 O4 Mn1 117.46(12) Mn1 O1 Mn2 114.2(2)

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212 APPENDIX B LIST OF COMPOUNDS [Mn12O12(O2CMe)16(H2O)4] (1 1) [Fe6O2(hmp)10(H2O)2](NO3)4 (2 1). [Fe6O2(hmp)8(NO3)4(H2O)2](NO3)2 (2 2) [Fe4(N3)6(hmp)6] (2 3). [Fe8O3(OMe)(pdm)4(pdmH)4(MeOH)2](ClO4)5 (3 1). [Fe8O3(OEt)(pdm)4(pdmH)4(EtOH)2](ClO4)5 (3 2). [Fe18O6(OH)8(pdm)10(pdmH)4(H2O)4](ClO4)10 (3 3) [Fe9O4(OH)2(O2CMe)10(pdm)(pdmH)4](NO3) (3 4). [Mn4O2(O2CBut)5(dphmp)3] (4 1). [Mn6O4(OMe)2(O2CPh)4(dphmp)4] (4 2). [Mn11O7(OMe)7(O2CPh)7(dphmp)4(MeOH)2] (4 3 ). [Mn7O3(OH)3(O2CBut)7(dmhmp)4] (4 4). [Mn12O7(OH)(OMe)2(O2CPh)12(dmhmp)4(H2O)] (4 5). (HNEt3)[NaMn6O4(dmhmp)6(N3)4] (ClO4)2 (4 6). [Mn10O4(N3)4(hmp)12](ClO4)2MeCN (4 7) NBut 4[Mn4O2(O2CPh)7(hmp)2] (4 8) [Mn12O8Cl4(O2CPh)8(hmp)6] (4 9). [Mn4O4(O2CMe)3(pdpm)3] (5 1). [Mn7O4(pdpm)6(N3)4](ClO4)2 (5 2 ).

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213 APPENDIX C VAN VLECK EQUATIONS p = parama gnetic impurity B 2/3k N = Avogadro's number g = Lande's factor k = Boltzmann constant T = Temperature TIP = Temperature independent paramagnetism C1 (HNEt3)[NaMn6O4(dmhmp)4(N3)4](ClO4)2 (4 -6 ) M = (c g2)/T (Num/Den) + TIP m=Jcis/k/T n=Jtrans/k/T Num= +0.0000*exp(0.0000*m +0.0000*n) +18.0000*exp(0.0000*m +2.0000*n) +150.0000*exp(0.0000*m +6.0000*n) +504.0000*exp(0.0000*m +12.0000*n) +3780.0000*exp(0.0000*m +20.0000*n) +0.0000*exp( 4.0000*m +4.0000*n) +18.0000*exp( 2.0000*m +4.0000*n) +90.0000*exp(2.0000*m +4.0000*n) +36.0000*exp( 6.0000*m +8.0000*n) +180.0000*exp( 2.0000*m +8.0000*n) +504.0000*exp(4.0000*m +8.0000*n) +450.0000*exp( 8.0000*m +14.0000*n) +1260.0000*exp( 2.0000*m +14.0000*n) +2160.0000*exp(6.0000*m +14.0000*n) +504.0000*exp( 10.0000*m +22.0000*n) +1080.0000*exp( 2.0000*m +22.0000*n) +1980.0000*exp(8.0000*m +22.0000*n) +0.0000*exp( 12.0000*m +12.0000*n) +18.0000*exp( 10.0000*m +12.0000*n) +90.0000*exp( 6.0000*m +12.0000*n) +540.0000*exp(8.0000*m +12.0000*n) +54.0000*exp( -

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214 16.0000*m +18.0000*n) +330.0000*exp( 12.0000*m +18.0000*n) +840.0000*exp( 6.0000*m +18.0000*n) +1620.0000*exp(2.0000*m +18.0000*n) +2640.0000*exp(12.0000*m +18.0000*n) +450.0000*exp( 20.0000*m +26.0000*n) +1260.0000*exp( 14.0000*m +26.0000*n) +2700.0000*exp( 6.0000*m +26. 0000*n) +4950.0000*exp(4.0000*m +26.0000*n) +6552.0000*exp(16.0000*m +26.0000*n) +0.0000*exp( 24.0000*m +24.0000*n) +72.0000*exp( 22.0000*m +24.0000*n) +540.0000*exp( 18.0000*m +24.0000*n) +2016.0000*exp( 12.0000*m +24.0000*n) +4320.0000*exp( 4.0000*m +24. 0000*n) +5940.0000*exp(6.0000*m +24.0000*n) +6552.0000*exp(18.0000*m +24.0000*n) +72.0000*exp( 30.0000*m +32.0000*n) +450.0000*exp( 26.0000*m +32.0000*n) +1512.0000*exp( 20.0000*m +32.0000*n) +3780.0000*exp( 12.0000*m +32.0000*n) +5940.0000*exp( 2.0000*m + 32.0000*n) +8190.0000*exp(10.0000*m +32.0000*n) +10080.0000*exp(24.0000*m +32.0000*n) +0.0000*exp( 40.0000*m +40.0000*n) +18.0000*exp( 38.0000*m +40.0000*n) +90.0000*exp( 34.0000*m +40.0000*n) +252.0000*exp( 28.0000*m +40.0000*n) +540.0000*exp( 20.0000*m + 40.0000*n) +990.0000*exp( 10.0000*m +40.0000*n) +1638.0000*exp(2.0000*m +40.0000*n) +2520.0000*exp(16.0000*m +40.0000*n) +3672.0000*exp(32.0000*m +40.0000*n) +18.0000*exp( 4.0000*m +6.0000*n) +270.0000*exp( 4.0000*m +10.0000*n) +756.0000*exp( 4.0000*m +16. 0000*n) +0.0000*exp( 6.0000*m +6.0000*n) +36.0000*exp( 8.0000*m +10.0000*n) +504.0000*exp(2.0000*m +10.0000*n) +180.0000*exp( 10.0000*m +16.0000*n) +1080.0000*exp(4.0000*m +16.0000*n) +84.0000*exp(6.0000*m +6.0000*n) +0.0000*exp( 10.0000*m +10.0000*n) +540.0000*exp(10.0000*m +10.0000*n) +18.0000*exp( 14.0000*m +16.0000*n) +990.0000*exp(14.0000*m +16.0000*n) +54.0000*exp( 12.0000*m +14.0000*n) +540.0000*exp( 14.0000*m +20.0000*n)

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215 +1512.0000*exp( 8.0000*m +20.0000*n) +1512.0000*exp( 16.0000*m +28.0000*n) +324 0.0000*exp( 8.0000*m +28.0000*n) +5940.0000*exp(2.0000*m +28.0000*n) +0.0000*exp( 14.0000*m +14.0000*n) +72.0000*exp( 18.0000*m +20.0000*n) +3960.0000*exp(10.0000*m +20.0000*n) +360.0000*exp( 22.0000*m +28.0000*n) +6552.0000*exp(14.0000*m +28.0000*n) +990. 0000*exp(16.0000*m +14.0000*n) +0.0000*exp( 20.0000*m +20.0000*n) +3276.0000*exp(22.0000*m +20.0000*n) +36.0000*exp( 26.0000*m +28.0000*n) +5040.0000*exp(28.0000*m +28.0000*n) +54.0000*exp( 24.0000*m +26.0000*n) +540.0000*exp( 28.0000*m +34.0000*n) +1512.0000*exp( 22.0000*m +34.0000*n) +3240.0000*exp( 14.0000*m +34.0000*n) +5940.0000*exp( 4.0000*m +34.0000*n) +9828.0000*exp(8.0000*m +34.0000*n) +0.0000*exp( 26.0000*m +26.0000*n) +72.0000*exp( 32.0000*m +34.0000*n) +10080.0000*exp(22.0000*m +34.0000*n) +2520.0000*exp(30.0000*m +26.0000*n) +0.0000*exp( 34.0000*m +34.0000*n) +7344.0000*exp(38.0000*m +34.0000*n) +54.0000*exp( 40.0000*m +42.0000*n) +270.0000*exp( 36.0000*m +42.0000*n) +756.0000*exp( 30.0000*m +42.0000*n) +1620.0000*exp( 22.0000*m +42.0000*n) +297 0.0000*exp( 12.0000*m +42.0000*n) +4914.0000*exp(0.0000*m +42.0000*n) +7560.0000*exp(14.0000*m +42.0000*n) +0.0000*exp( 42.0000*m +42.0000*n) +7344.0000*exp(30.0000*m +42.0000*n) +5130.0000*exp(48.0000*m +42.0000*n) +0.0000*exp( 18.0000*m +18.0000*n) +546. 0000*exp(24.0000*m +18.0000*n) +2520.0000*exp(32.0000*m +24.0000*n) +0.0000*exp( 32.0000*m +32.0000*n) +3672.0000*exp(40.0000*m +32.0000*n) +450.0000*exp( 24.0000*m +30.0000*n) +1260.0000*exp( 18.0000*m +30.0000*n) +2700.0000*exp( 10.0000*m +30.0000*n) +25 20.0000*exp( 26.0000*m +38.0000*n) +5400.0000*exp( 18.0000*m +38.0000*n)

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216 +9900.0000*exp( 8.0000*m +38.0000*n) +54.0000*exp( 28.0000*m +30.0000*n) +3960.0000*exp(0.0000*m +30.0000*n) +720.0000*exp( 32.0000*m +38.0000*n) +13104.0000*exp(4.0000*m +38.0000*n) +0.0000*exp( 30.0000*m +30.0000*n) +4914.0000*exp(12.0000*m +30.0000*n) +108.0000*exp( 36.0000*m +38.0000*n) +15120.0000*exp(18.0000*m +38.0000*n) +5040.0000*exp(26.0000*m +30.0000*n) +0.0000*exp( 38.0000*m +38.0000*n) +14688.0000*exp(34.0000*m +38.0000*n) +3672.0000*exp(42.0000*m +30.0000*n) +10260.0000*exp(52.0000*m +38.0000*n) +450.0000*exp( 40.0000*m +46.0000*n) +1260.0000*exp( 34.0000*m +46.0000*n) +2700.0000*exp( 26.0000*m +46.0000*n) +4950.0000*exp( 16.0000*m +46.0000*n) +8190.0000*exp( 4.0000*m +46. 0000*n) +54.0000*exp( 44.0000*m +46.0000*n) +10080.0000*exp(10.0000*m +46.0000*n) +0.0000*exp( 46.0000*m +46.0000*n) +11016.0000*exp(26.0000*m +46.0000*n) +10260.0000*exp(44.0000*m +46.0000*n) +6930.0000*exp(64.0000*m +46.0000*n) +588.0000*exp( 24.0000*m + 36.0000*n) +3780.0000*exp( 24.0000*m +44.0000*n) +150.0000*exp( 30.0000*m +36.0000*n) +1080.0000*exp( 16.0000*m +36.0000*n) +1512.0000*exp( 32.0000*m +44.0000*n) +5940.0000*exp( 14.0000*m +44.0000*n) +18.0000*exp( 34.0000*m +36.0000*n) +1650.0000*exp( 6.0000*m +36.0000*n) +450.0000*exp( 38.0000*m +44.0000*n) +8190.0000*exp( 2.0000*m +44.0000*n) +0.0000*exp( 36.0000*m +36.0000*n) +2184.0000*exp(6.0000*m +36.0000*n) +54.0000*exp( 42.0000*m +44.0000*n) +10080.0000*exp(12.0000*m +44.0000*n) +2520.0000*exp(20.0000*m +36.0000*n) +0.0000*exp( 44.0000*m +44.0000*n) +11016.0000*exp(28.0000*m +44.0000*n) +2448.0000*exp(36.0000*m +36.0000*n) +10260.0000*exp(46.0000*m +44.0000*n) +1710.0000*exp(54.0000*m +36.0000*n) +6930.0000*exp(66.0000*m +44.0000*n)

PAGE 217

217 +1764.0000*exp( 4 0.0000*m +52.0000*n) +3780.0000*exp( 32.0000*m +52.0000*n) +6930.0000*exp( 22.0000*m +52.0000*n) +450.0000*exp( 46.0000*m +52.0000*n) +9828.0000*exp( 10.0000*m +52.0000*n) +54.0000*exp( 50.0000*m +52.0000*n) +12600.0000*exp(4.0000*m +52.0000*n) +0.0000*exp ( 52.0000*m +52.0000*n) +14688.0000*exp(20.0000*m +52.0000*n) +15390.0000*exp(38.0000*m +52.0000*n) +13860.0000*exp(58.0000*m +52.0000*n) +9108.0000*exp(80.0000*m +52.0000*n) +1620.0000*exp( 40.0000*m +60.0000*n) +588.0000*exp( 48.0000*m +60.0000*n) +2640. 0000*exp( 30.0000*m +60.0000*n) +150.0000*exp( 54.0000*m +60.0000*n) +3822.0000*exp( 18.0000*m +60.0000*n) +18.0000*exp( 58.0000*m +60.0000*n) +5040.0000*exp( 4.0000*m +60.0000*n) +0.0000*exp( 60.0000*m +60.0000*n) +6120.0000*exp(12.0000*m +60.0000*n) +6840.0000*exp(30.0000*m +60.0000*n) +6930.0000*exp(50.0000*m +60.0000*n) +6072.0000*exp(72.0000*m +60.0000*n) +3900.0000*exp(96.0000*m +60.0000*n) Den= +1.0000*exp(0.0000*m +0.0000*n) +9.0000*exp(0.0000*m +2.0000*n) +25.0000*exp(0.0000*m +6.0000*n) +42.0000*e xp(0.0000*m +12.0000*n) +189.0000*exp(0.0000*m +20.0000*n) +3.0000*exp( 4.0000*m +4.0000*n) +9.0000*exp( 2.0000*m +4.0000*n) +15.0000*exp(2.0000*m +4.0000*n) +18.0000*exp( 6.0000*m +8.0000*n) +30.0000*exp( 2.0000*m +8.0000*n) +42.0000*exp(4.0000*m +8.0000* n) +75.0000*exp( 8.0000*m +14.0000*n) +105.0000*exp( 2.0000*m +14.0000*n) +108.0000*exp(6.0000*m +14.0000*n) +42.0000*exp( 10.0000*m +22.0000*n) +54.0000*exp( 2.0000*m +22.0000*n) +66.0000*exp(8.0000*m +22.0000*n) +3.0000*exp( 12.0000*m +12.0000*n) +9.0000*exp( 10.0000*m +12.0000*n) +15.0000*exp( 6.0000*m +12.0000*n) +27.0000*exp(8.0000*m +12.0000*n) +27.0000*exp( 16.0000*m +18.0000*n)

PAGE 218

218 +55.0000*exp( 12.0000*m +18.0000*n) +70.0000*exp( 6.0000*m +18.0000*n) +81.0000*exp(2.0000*m +18.0000*n) +88.0000*exp(12.0000*m +18.0000*n) +75.0000*exp( 20.0000*m +26.0000*n) +105.0000*exp( 14.0000*m +26.0000*n) +135.0000*exp( 6.0000*m +26.0000*n) +165.0000*exp(4.0000*m +26.0000*n) +156.0000*exp(16.0000*m +26.0000*n) +6.0000*exp( 24.0000*m +24.0000*n) +36.0000*exp( 22.0000*m +24.0000*n) +90.0000*exp( 18.0000*m +24.0000*n) +168.0000*exp( 12.0000*m +24.0000*n) +216.0000*exp( 4.0000*m +24.0000*n) +198.0000*exp(6.0000*m +24.0000*n) +156.0000*exp(18.0000*m +24.0000*n) +36.0000*exp( 30.0000*m +32.0000*n) +75.0000*exp( 26.0000*m +32. 0000*n) +126.0000*exp( 20.0000*m +32.0000*n) +189.0000*exp( 12.0000*m +32.0000*n) +198.0000*exp( 2.0000*m +32.0000*n) +195.0000*exp(10.0000*m +32.0000*n) +180.0000*exp(24.0000*m +32.0000*n) +3.0000*exp( 40.0000*m +40.0000*n) +9.0000*exp( 38.0000*m +40.0000 *n) +15.0000*exp( 34.0000*m +40.0000*n) +21.0000*exp( 28.0000*m +40.0000*n) +27.0000*exp( 20.0000*m +40.0000*n) +33.0000*exp( 10.0000*m +40.0000*n) +39.0000*exp(2.0000*m +40.0000*n) +45.0000*exp(16.0000*m +40.0000*n) +51.0000*exp(32.0000*m +40.0000*n) +9.0000*exp( 4.0000*m +6.0000*n) +45.0000*exp( 4.0000*m +10.0000*n) +63.0000*exp( 4.0000*m +16.0000*n) +1.0000*exp( 6.0000*m +6.0000*n) +18.0000*exp( 8.0000*m +10.0000*n) +42.0000*exp(2.0000*m +10.0000*n) +30.0000*exp( 10.0000*m +16.0000*n) +54.0000*exp(4.0000*m +16.0000*n) +7.0000*exp(6.0000*m +6.0000*n) +3.0000*exp( 10.0000*m +10.0000*n) +27.0000*exp(10.0000*m +10.0000*n) +9.0000*exp( 14.0000*m +16.0000*n) +33.0000*exp(14.0000*m +16.0000*n) +27.0000*exp( 12.0000*m +14.0000*n) +90.0000*exp( 14.0000*m +20.0000* n) +126.0000*exp( 8.0000*m +20.0000*n) +126.0000*exp( 16.0000*m +28.0000*n) +162.0000*exp( 8.0000*m +28.0000*n)

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219 +198.0000*exp(2.0000*m +28.0000*n) +3.0000*exp( 14.0000*m +14.0000*n) +36.0000*exp( 18.0000*m +20.0000*n) +132.0000*exp(10.0000*m +20.0000*n) +60.0000*exp( 22.0000*m +28.0000*n) +156.0000*exp(14.0000*m +28.0000*n) +33.0000*exp(16.0000*m +14.0000*n) +6.0000*exp( 20.0000*m +20.0000*n) +78.0000*exp(22.0000*m +20.0000*n) +18.0000*exp( 26.0000*m +28.0000*n) +90.0000*exp(28.0000*m +28.0000*n) +27.0000*e xp( 24.0000*m +26.0000*n) +90.0000*exp( 28.0000*m +34.0000*n) +126.0000*exp( 22.0000*m +34.0000*n) +162.0000*exp( 14.0000*m +34.0000*n) +198.0000*exp( 4.0000*m +34.0000*n) +234.0000*exp(8.0000*m +34.0000*n) +3.0000*exp( 26.0000*m +26.0000*n) +36.0000*exp( 32.0000*m +34.0000*n) +180.0000*exp(22.0000*m +34.0000*n) +45.0000*exp(30.0000*m +26.0000*n) +6.0000*exp( 34.0000*m +34.0000*n) +102.0000*exp(38.0000*m +34.0000*n) +27.0000*exp( 40.0000*m +42.0000*n) +45.0000*exp( 36.0000*m +42.0000*n) +63.0000*exp( 30.0000*m +42.0000*n) +81.0000*exp( 22.0000*m +42.0000*n) +99.0000*exp( 12.0000*m +42.0000*n) +117.0000*exp(0.0000*m +42.0000*n) +135.0000*exp(14.0000*m +42.0000*n) +3.0000*exp( 42.0000*m +42.0000*n) +102.0000*exp(30.0000*m +42.0000*n) +57.0000*exp(48.0000*m +42.0000*n) +1.0000*exp( 18.0000*m +18.0000*n) +13.0000*exp(24.0000*m +18.0000*n) +45.0000*exp(32.0000*m +24.0000*n) +3.0000*exp( 32.0000*m +32.0000*n) +51.0000*exp(40.0000*m +32.0000*n) +75.0000*exp( 24.0000*m +30.0000*n) +105.0000*exp( 18.0000*m +30.0000*n) +135.0000*exp( 10.0000*m +30.0000*n) +210.0000*exp( 26.0000*m +38.0000*n) +270.0000*exp( 18.0000*m +38.0000*n) +330.0000*exp( 8.0000*m +38.0000*n) +27.0000*exp( 28.0000*m +30.0000*n) +132.0000*exp(0.0000*m +30.0000*n) +120.0000*exp( 32.0000*m +38.0000*n) +312.0000*exp(4.0000*m +38.0000*n) +3.0000*exp( 30.0000*m +30.0000*n) +117.0000*exp(12.0000*m +30.0000*n)

PAGE 220

220 +54.0000*exp( 36.0000*m +38.0000*n) +270.0000*exp(18.0000*m +38.0000*n) +90.0000*exp(26.0000*m +30.0000*n) +6.0000*exp( 38.0000*m +38.0000*n) +204.0000*exp(34.0000*m +38.0000*n) +51.0000*exp(42.0000*m +30.0000*n) +114.0000*exp(52.0000*m +38.0000*n) +75.0000*exp( 40.0000*m +46.0000*n) +105.0000*exp( 34.0000*m +46.0000*n) +135.0000*exp( 26.0000*m +46.0000*n) +165.0000*exp( 16.0000*m +46.0000*n) +195.0000*exp( 4.0000*m +46.0000*n) +27.0000*exp( 44.0000*m +46.0000*n) +180.0000*exp(10.0000*m +46.0000*n) +3.0000*exp( 46.0000*m +46.0000*n) +153.0000*exp(26.0000*m +46.0000*n) +114.0000*exp(44.0000*m +46.0000*n) +63.0000*exp(64.0000*m +46.0000*n) +49.0000*exp( 2 4.0000*m +36.0000*n) +189.0000*exp( 24.0000*m +44.0000*n) +25.0000*exp( 30.0000*m +36.0000*n) +54.0000*exp( 16.0000*m +36.0000*n) +126.0000*exp( 32.0000*m +44.0000*n) +198.0000*exp( 14.0000*m +44.0000*n) +9.0000*exp( 34.0000*m +36.0000*n) +55.0000*exp( 6.0 000*m +36.0000*n) +75.0000*exp( 38.0000*m +44.0000*n) +195.0000*exp( 2.0000*m +44.0000*n) +1.0000*exp( 36.0000*m +36.0000*n) +52.0000*exp(6.0000*m +36.0000*n) +27.0000*exp( 42.0000*m +44.0000*n) +180.0000*exp(12.0000*m +44.0000*n) +45.0000*exp(20.0000*m +36.0000*n) +3.0000*exp( 44.0000*m +44.0000*n) +153.0000*exp(28.0000*m +44.0000*n) +34.0000*exp(36.0000*m +36.0000*n) +114.0000*exp(46.0000*m +44.0000*n) +19.0000*exp(54.0000*m +36.0000*n) +63.0000*exp(66.0000*m +44.0000*n) +147.0000*exp( 40.0000*m +52.0000* n) +189.0000*exp( 32.0000*m +52.0000*n) +231.0000*exp( 22.0000*m +52.0000*n) +75.0000*exp( 46.0000*m +52.0000*n) +234.0000*exp( 10.0000*m +52.0000*n) +27.0000*exp( 50.0000*m +52.0000*n) +225.0000*exp(4.0000*m +52.0000*n) +3.0000*exp( 52.0000*m +52.0000*n) +204.0000*exp(20.0000*m +52.0000*n) +171.0000*exp(38.0000*m

PAGE 221

221 +52.0000*n) +126.0000*exp(58.0000*m +52.0000*n) +69.0000*exp(80.0000*m +52.0000*n) +81.0000*exp( 40.0000*m +60.0000*n) +49.0000*exp( 48.0000*m +60.0000*n) +88.0000*exp( 30.0000*m +60.0000*n) +25.0000*exp( 54.0000*m +60.0000*n) +91.0000*exp( 18.0000*m +60.0000*n) +9.0000*exp( 58.0000*m +60.0000*n) +90.0000*exp( 4.0000*m +60.0000*n) +1.0000*exp( 60.0000*m +60.0000*n) +85.0000*exp(12.0000*m +60.0000*n) +76.0000*exp(30.0000*m +60.0000*n) +63.0000*exp(50.0000*m +60.0000*n) +46.0000*exp(72.0000*m +60.0000*n) +25.0000*exp(96.0000*m +60.0000*n)

PAGE 222

222 C2 [Mn4O4(O2CMe)3(pdpm)] (5 -1 ) M = (c g2)/T (Num/Den) + TIP l=J1/k/T m=J2/k/T n=J3/k/T Num=+300.0000*exp(0.0000*l+0.0000*m+0.0000*n) +6.0000*exp(2.0000*l+0.0000*m+0.0000*n) +0.0000*exp(2.0000*l+ 4.0000*m+2.0000*n) +300.0000*exp(2.0000*l+ 2.0000*m+2.0000*n) +30.0000*exp(2.0000*l+2.0000*m+2.0000*n) +6.0000*exp(2.0000*l+ 6.0000*m+6.0000*n) +84.0000*exp(2.0000*l+4.0000*m+6.0000*n) +30.0000*exp(2.0000*l+ 8.0000*m+12.0000*n) +180.0000*exp(2.0000*l+6.0000*m+12.0000*n) +84.0000*exp(2.0000*l+ 10.0000*m+20.0000*n) +330.0000*exp(2.0000*l+8.0000*m+20.0000*n) +114.0000*exp(6.0000*l+0.0000*m+0.0000*n) +300.0000*exp(6.0000*l+ 6.0000*m+2.0000*n) +30.0000*exp(6.0000*l+ 2.0000*m+ 2.0000*n) +414.0000*exp(6.0000*l+4.0000*m+2.0000*n) +30.0000*exp(6.0000*l+ 12.0000*m+6.0000*n) +6.0000*exp(6.0000*l+ 10.0000*m+6.0000*n) +180.0000*exp(6.0000*l+8.0000*m+6.0000*n) +6.0000*exp(6.0000*l+ 16.0000*m+12.0000*n) +180.0000*exp(6.0000*l+2.0000*m+12.0000*n) +330.0000*exp(6.0000*l+12.0000*m+12.0000*n) +30.0000*exp(6.0000*l+ 20.0000*m+20.0000*n) +84.0000*exp(6.0000*l+ 14.0000*m+20.0000*n) +546.0000*exp(6.0000*l+16.0000*m+20.0000*n) +84.0000*exp(12.0000*l+0.0000*m+0.0000*n) +30.0000*exp(12.0000*l+ 8.0000*m+2.0000*n) +414.0000*exp(12.0000*l+ 2.0000*m+2.0000*n)

PAGE 223

223 +510.0000*exp(12.0000*l+6.0000*m+2.0000*n) +6.0000*exp(12.0000*l+ 16.0000*m+6.0000*n) +294.0000*exp(12.0000*l+ 12.0000*m+6.0000*n) +84.0000*exp(12.0000*l+ 6.0000*m+6.0000*n) +180.0000*exp(12.0000*l+ 2.0000*m+6.0000*n) +330.0000*exp(12.0000*l+12.0000*m+6.0000*n) +0.0000*exp(12.0000*l+ 24.0000*m+12.0000*n) +6.0000*exp(12.0000*l+ 22.0000*m+12.0000*n) +30.0000*exp(12.0000*l+ 18.0000*m+12.0000*n) +180.0000*exp(12.0000*l+ 4.0000*m+12.0000*n) +546.0000*exp(12.0000*l+18.0000*m+12.0000*n) +6.0000*exp(12.0000*l+ 30.0000*m+20.0000*n) +30.0000*exp(12.0000*l+ 26.0000*m+20.0000*n) +84.0000*exp(12.0000*l+ 20.0000*m+20.0000*n) +546.0000*exp(12.0000*l+10.0000*m+20.0000*n) +840.0000*exp(12.0000*l+24.0000*m+20.0000*n) D en=+25.0000*exp(0.0000*l+0.0000*m+0.0000*n) +3.0000*exp(2.0000*l+0.0000*m+0.0000*n) +1.0000*exp(2.0000*l+ 4.0000*m+2.0000*n) +24.0000*exp(2.0000*l+ 2.0000*m+2.0000*n) +5.0000*exp(2.0000*l+2.0000*m+2.0000*n) +3.0000*exp(2.0000*l+ 6.0000*m+6.0000*n) +7.0000* exp(2.0000*l+4.0000*m+6.0000*n) +5.0000*exp(2.0000*l+ 8.0000*m+12.0000*n) +9.0000*exp(2.0000*l+6.0000*m+12.0000*n) +7.0000*exp(2.0000*l+ 10.0000*m+20.0000*n) +11.0000*exp(2.0000*l+8.0000*m+20.0000*n) +12.0000*exp(6.0000*l+0.0000*m+0.0000*n) +24.0000*exp(6. 0000*l+ 6.0000*m+2.0000*n) +5.0000*exp(6.0000*l+ 2.0000*m+2.0000*n) +18.0000*exp(6.0000*l+4.0000*m+2.0000*n) +6.0000*exp(6.0000*l+ 12.0000*m+6.0000*n) +3.0000*exp(6.0000*l+ 10.0000*m+6.0000*n) +9.0000*exp(6.0000*l+8.0000*m+6.0000*n) +3.0000*exp(6.0000*l+ 1 6.0000*m+12.0000*n) +9.0000*exp(6.0000*l+2.0000*m+12.0000*n)

PAGE 224

224 +11.0000*exp(6.0000*l+12.0000*m+12.0000*n) +5.0000*exp(6.0000*l+ 20.0000*m+20.0000*n) +7.0000*exp(6.0000*l+ 14.0000*m+20.0000*n) +13.0000*exp(6.0000*l+16.0000*m+20.0000*n) +7.0000*exp(12.0000*l+0.0000*m+0.0000*n) +5.0000*exp(12.0000*l+ 8.0000*m+2.0000*n) +18.0000*exp(12.0000*l+ 2.0000*m+2.0000*n) +20.0000*exp(12.0000*l+6.0000*m+2.0000*n) +3.0000*exp(12.0000*l+ 16.0000*m+6.0000*n) +21.0000*exp(12.0000*l+ 12.0000*m+6.0000*n) +7.0000*exp(12.0000*l+ 6 .0000*m+6.0000*n) +9.0000*exp(12.0000*l+2.0000*m+6.0000*n) +11.0000*exp(12.0000*l+12.0000*m+6.0000*n) +1.0000*exp(12.0000*l+ 24.0000*m+12.0000*n) +3.0000*exp(12.0000*l+ 22.0000*m+12.0000*n) +5.0000*exp(12.0000*l+ 18.0000*m+12.0000*n) +9.0000*exp(12.0000*l+ 4.0000*m+12.0000*n) +13.0000*exp(12.0000*l+18.0000*m+12.0000*n) +3.0000*exp(12.0000*l+ 30.0000*m+20.0000*n) +5.0000*exp(12.0000*l+ 26.0000*m+20.0000*n) +7.0000*exp(12.0000*l+ 20.0000*m+20.0000*n) +13.0000*exp(12.0000*l+10.0000*m+20.0000*n) +15.0000*exp(12.0000*l+24.0000*m+20.0000*n)

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245 BIOGRAPHICAL SKETCH Taketo Taguchi was born in Shiga, Japan in 1978. He e ntered Osaka University, Japan in 1998 and received Bachelor of S cience. He did undergraduate research in the research group of Professor Mi chio Matsumura. T itle of his bachelors thesis is Chemical Etching of TiO2 Photocatalyst Particles. In 2002, he jo ined the research group of Professor Akira Fujishima at the University of Tokyo, and obtained his master s degree in engineering in 2004 T itle of his masters thesis is Solid -State Dye Sensitized Solar Cells. He then decided to pursue a doctoral degree in the U nited S tates and joined the research group of Professor George Christou in 2004. His doctoral research primarily involves in new ligand design for the preparation and magnetic characterization of polynuclear Mn and Fe clusters of unprecedented str uctural types.