Exploring the Coordination Chemistry of N, N, N', N'-Tetrakis(2-Hydroxyethyl)Ethylenediamine in Polynuclear Manganese Cl...

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Title:
Exploring the Coordination Chemistry of N, N, N', N'-Tetrakis(2-Hydroxyethyl)Ethylenediamine in Polynuclear Manganese Cluster Chemistry
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english
Creator:
Saha,Arpita
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University of Florida
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Gainesville, Fla.
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Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Chemistry
Committee Chair:
Christou, George
Committee Members:
Talham, Daniel R
Castellano, Ronald K
Murray, Leslie Justin
Meisel, Mark W

Subjects

Subjects / Keywords:
dft -- edte -- electrochemistry -- ins -- magnetochemistry -- manganese -- smm -- synthesis
Chemistry -- Dissertations, Academic -- UF
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Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract:
The research focuses on the investigation of new synthetic routes towards the preparation and subsequent characterization of novel multinuclear transition metal/lanthanide complexes that can function as molecular nanomagnets, better known as single-molecule magnets (SMMs). The major compelling property of these metal clusters is that they behave like tiny magnets i.e. they show slow relaxation of magnetization at low temperature. In addition these molecular systems often exhibit quantum properties which make them interesting candidates to use as qubits in quantum computation. In 1993, the first SMM was discovered, Mn12O12(O2CCH3)16(H2O)4, better known as Mn12Ac, which was a breakthrough in the field of molecular magnetism. In the subsequent years, there has been massive amount of research in this area, and the SMM database has greatly expanded as research groups around the world have made new ones. The synthesis of such polynuclear metal clusters involves the incorporation of multiple metal atoms supported by organic ligands. In this regard, alkoxide-based ligands play a pivotal role since this functionally is an excellent bridging group that fosters higher nuclearity product formation. Such polydentate ligands have led to the discovery of many interesting 3d clusters, some of which display SMM behavior. The dissertation is on the investigation of new synthetic methods, combining an alkoxide-based ligand with various carboxylates and azides. In this regard, potentially hexadentate (O,O,O,O,N,N) N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (edteH4) ligand has been used. Herein, the synthesis, structure and magnetic properties of various nuclearity Mnx clusters, where x = 3, 4, 6, 9, 10, 12, 18, 20, have been reported. Among them, the electrochemical behavior (cyclic voltammetry and differential pulse voltammetry) and magnetic susceptibilities (using SQUID magnetometer) of Mn9 and a family of Mn12 SMMs have been studied in detail. Single-crystal hysteresis loops using micro-SQUID apparatus were obtained and further proved the SMM behavior of the same. Mn9 is a new, rare half-integer spin SMM with a spin barrier of 49 K, the highest in mixed-valent Mn2+/Mn3+ chemistry. Inelastic neutron scattering (INS) and computational characterization (using Density Functional Theory) were further performed to assess the zero-field splitting and higher order anisotropic parameters, the energy barrier, the spin-ground state, the Heisenberg exchange coupling parameters (J) and the bonding criterion in Mn9 complex. In addition, a family of isostructural heterometallic Mn-Ln clusters using the same ligand with a MnII2MnIII2LnIII2 core (Ln = Gd, Tb, Dy, Ho) has been synthesized as well as a MnII2MnIII2YIII2 analog with diamagnetic Y3+ to assist the magnetic studies by assessing the nature of the Mn?Ln exchange interactions. Among them the Tb analog exhibits frequency-dependent out-of-phase ac susceptibility signals characteristic of SMMs, which was further confirmed by the observation of magnetization hysteresis.
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In the series University of Florida Digital Collections.
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Includes vita.
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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 Arpita Saha.
Thesis:
Thesis (Ph.D.)--University of Florida, 2011.
Local:
Adviser: Christou, George.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2013-08-31

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1 EXPLORING THE COORDI NATION CHEMISTRY OF N, N, N', N' TETRAKIS(2 HYDROXYETHYL)ETHYLEN EDIAMINE IN POLYNUCL EAR MANGANESE CLUSTE R CHEMISTRY By ARPITA SAHA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLO RIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2011

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2 2011 Arpita Saha

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3

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4 ACKNOWLEDGMENTS It feels like a life time achie vement to reach to the final stage of writing m y dissertation thesis. T his is my own journey, but I could never see the end of this long, winding road without the help of some people. I take this opportunity to thank them. I express my deep gratitude to my doctoral supervisor Dr. George Christou for his excellent guidance, assistance and patience during the course of my graduate studies. independent thinker and a chemist. Not only t he insightful discussions, I immensely value all his suggestions, criticism to improve my oral and written communication skills in a scientific community, which in turn made me a strong and confident person. Above all he introduced me to the edteH 4 chemist ry which is the soul of this dissertation and I sincerely acknowledge his faith in me This is an honor and privilege to be a part of the Christou group and I will carry the pride lifelong. I would like to thank my committee member Dr. Daniel R. Talham for his encouragement and kind words throughout my graduate career. I extend my sincere gratitude to Dr. Mark Meisel, Dr. Ronald Castellano and Dr. Leslie J. Murray for serving as my committee. I greatly appreciate all the scientific discussions with Dr. Dani el R Talham, Dr. Mark Miesel and Dr. Stephen Hill who helped me to become a physical chemist. I acknowledge gratefulness to all my collaborators, especially Dr. Wolfgang Wernsderfer for micro SQUID measurements, Dr. Oliver Waldmann for INS measurements, an d Dr. Artm E. Masunov and Shruba for the DFT calculations. I deeply thank to Dr. Khalil A. Abboud for all the X ray crystal structure analyses throughout my graduate study. I express my deep thankfulness to Dr. Sabyasachi

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5 Sarkar, my undergraduate research advisor, who strongly motivated and supported my decision to continue higher studies in the United States. I will never forget all my colleagues, especially Rashmi, Antonio, Taketo, Shreya, Tu, Andy, Lynn, Annalease, Amber and Mike for all the fun time s, help and understanding. Special thanks to Mike and Matthew to proofread my thesis. I want to thank Alice Maribel a y to day life. Needless to mention the warm friendships from Debapriya, Swati, Ruma, June, Jaya, Smita, Srishti, Marie and Anna for making my entire stay in Florida so memorable. Special thanks to Dola Kaki, Shampa Kaki and Seba Masi for their motherly support in a Wo rds are not enough to convey my forever indebtedness to my parents (Mr. Supriya K. Saha and Mrs. Kalyani Saha) for their unending love, faith and encouragement. I am also thankful to my father in law (Mr. Subhrangshu N. Saha) and mother in law (Mrs. Aloka Saha) and my sisters (Didi and Daisy di) & their families for the love and support. Finally, I express my indebtedness to my beloved husband Dr. Subhrajit Saha for h is unconditional love, support, motivation, trust, advice, zeal and strong sense of humor that brings fresh air to all my good and bad times. He is my greatest critic who motivates me to continue exploring myself and deliver ing possibly be so composed and confident without him and I thank him for believing in me and m aking me feel so special. My acknowledgement is incomplete without mentioning blessings of the almighty and the invisible presence is behind my existence and strength. I, therefore, humbly express my deepest gratitude by all my life.

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6 TABLE OF CONTENTS pa ge ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 11 LIST OF ABBREVIATIONS ................................ ................................ ........................... 17 ABSTRACT ................................ ................................ ................................ ................... 18 CHAPTER 1 GENERAL INTRODUCTION ................................ ................................ .................. 20 2 SYNTHESIS, STRUCTURE AND ELECTrOCHEMICAL AND MAGNETOCHEMICAL PROPERTIES OF A FAMILY OF Mn 12 COMPLEXES CONTAINING THE ANION OF N,N,N',N' TETRAKIS(2 HYDROXYETHYL) ETHYLENEDIAMINE ................................ ................................ .............................. 34 Experimental Section ................................ ................................ .............................. 35 Syntheses ................................ ................................ ................................ ......... 35 X ray Crystallography ................................ ................................ ....................... 37 Physical Measurements ................................ ................................ .................... 39 Results and Discussion ................................ ................................ ........................... 39 Syntheses ................................ ................................ ................................ ......... 39 Description of Structures ................................ ................................ .................. 41 Structure of [Mn 12 O 4 (OMe) 2 (edte) 4 (N 3 ) 8 ](ClO 4 )(N 3 ) (2 1) ........................... 41 Structure of [Mn 12 O 4 (OH)(edte) 4 (N 3 ) 9 ] (2 2) ................................ ............... 42 Structure of [Mn 12 O 4 (OH)(edte) 4 (N 3 ) 9 ](ClO 4 )(N 3 ) (2 3) ............................... 43 Electrochemistry ................................ ................................ ............................... 45 Magnetochemistry ................................ ................................ ............................ 46 Dc magnetic susceptibility studies of 2 1, 2 2 and 2 3 ............................... 46 Alternating current (ac) mag netic susceptibility studies for 2 1, 2 2 and 2 3 ................................ ................................ ................................ .......... 48 Magnetization hysteresis studies below 1.8 K for 2 1 ................................ 49 Concluding Remarks ................................ ................................ .............................. 50 3 EXPERIMENTAL AND COMPUTATIONAL STUDIES OF A NEW Mn 9 SINGLE MOLECULE MAGNET WITH A HALF INTEGER SPIN OF S = 21/2 ..................... 69 Experimental Section ................................ ................................ .............................. 71 Synthesis ................................ ................................ ................................ .......... 71 X ray Crystallography ................................ ................................ ....................... 71 Physical Measurem ents ................................ ................................ ................... 72

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7 Computational Details ................................ ................................ ...................... 74 Results and Discussion ................................ ................................ ........................... 74 Synthese s ................................ ................................ ................................ ......... 74 Description of the Structure [Mn 9 O 3 (OMe)(O 2 CBu t ) 7 (edte)(edteH) 2 (N 3 ) 2 ] ......... 75 Magnetochemistry ................................ ................................ ............................ 77 Dc magnetic susceptibility studies of 3 1 ................................ ................... 77 Alternating current (ac) magnetic susceptibility studies of 3 1 ................... 78 Single crystal hysteresis studies for 3 1 below 1.8K ................................ .. 81 Electrochemistry ................................ ................................ ............................... 83 Inelastic Neutron Scattering (INS) ................................ ................................ .... 84 Density Functional Theory ................................ ................................ ................ 87 Determination of ground spin state of 3 1 ................................ .................. 89 Heisenberg exchange constant in Mn 9 system ................................ .......... 89 Bonding analysis ................................ ................................ ........................ 91 Concluding Remarks ................................ ................................ .............................. 91 4 A RICHN ESS OF NEW MIXED VALENT Mn and Mn/C a CLUSTERS FROM THE USE OF N, N, N', N' TETRAKIS(2 HYDROXYETHYL)ETHYLENEDIAMINE: Mn 3 Mn 4 Mn 6 Mn 10 Mn 20 AND Mn 18 Ca 2 ................................ ................................ ................................ ................ 111 E xperimental Section ................................ ................................ ............................ 112 Syntheses ................................ ................................ ................................ ....... 112 X ray Crystallography ................................ ................................ ..................... 116 Phy sical Measurements ................................ ................................ ................. 120 Result and Discussion ................................ ................................ .......................... 120 Syntheses ................................ ................................ ................................ ....... 120 Desc ription of Structures ................................ ................................ ................ 125 Structure of [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](O 2 CMe) (4 1) ................................ 1 25 Structure of [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](ClO 4 ) (4 2) ................................ ..... 126 Structure of [Mn 4 (edteH 2 ) 2 (hmp) 2 Cl 2 ][Mn II Cl 4 ] (4 3) ................................ .. 127 Structure of [Mn 6 O 2 (O 2 CBu t ) 6 (edteH) 2 (N 3 ) 2 ] (4 4) ................................ .... 128 Structure of Na 2 [Mn 10 O 4 (OMe) 2 (O 2 CEt) 6 ( edte) 2 (N 3 ) 6 ] (4 5) ...................... 129 Structure of (NEt 4 ) 2 [Mn 10 O 4 (OH) 2 (O 2 CEt) 6 (edte) 2 (N 3 ) 6 ] (4 6) ................... 130 Structure of [Mn 20 O 8 (OH) 6 (O 2 CEt) 6 (edte) 4 (edteH) 2 ](ClO 4 ) 4 (4 7) ............. 130 Structure of [Ca 2 Mn 18 O 8 (OH) 5 (O 2 CBu t ) 4.5 (edte) 5 (NO 3 ) 3.5 ](O 2 CBu t ) 0.5 (NO 3 ) 0.5 (4 8) .. 132 Magnetochemistry ................................ ................................ .......................... 133 Dc and ac magnetic susceptibility studies of 4 1 and 4 2 ........................ 133 Dc and ac magnetic susceptibility studies of 4 3 ................................ ...... 137 Dc and ac magnetic susceptibility studies of 4 4 ................................ ...... 139 Dc and ac ma gnetic susceptibility studies of 4 5 and 4 6 ........................ 141 Dc and ac magnetic susceptibility studies of 4 7 ................................ ...... 142 Dc and ac magnetic susceptib ility studies of 4 8 ................................ ...... 143 Concluding Remarks ................................ ................................ ............................ 144

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8 5 A FAMILY OF RARE FUSED DOUBLE CUBANE Mn 4 Ln 2 (Ln = Gd, Tb, Dy, Ho) AND Mn 4 Y 2 COMP LEXES CONTAINING SINGLE MOLECULE MAGNET ......... 177 Experimental Section ................................ ................................ ............................ 179 Syntheses ................................ ................................ ................................ ....... 179 X ray Crystallography ................................ ................................ ..................... 180 Physical Measurements ................................ ................................ ................. 181 Results and Discussion ................................ ................................ ......................... 182 Syntheses ................................ ................................ ................................ ....... 182 Description of Structure Mn 4 Gd 2 O 2 (O 2 CBu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ........................ 184 Magnetochemistry ................................ ................................ .......................... 185 Dc magnetic susceptibility studies ................................ ........................... 185 Alternating current (ac) magnetic susceptibility studies ........................... 189 Single crystal hysteresis studies of 5 2 below 1.8 K. ............................... 190 Concluding Remarks ................................ ................................ ............................ 191 APPENDIX A BOND DIST ANCES AND ANGLES ................................ ................................ ...... 203 B LIST OF COMPOUNDS ................................ ................................ ........................ 213 C ELECTROCHEMICAL REVERSIBLE PROCESSES ................................ ............ 214 D DFT CALCULATIONS ................................ ................................ .......................... 216 E VAN VLECK EQUATIONS ................................ ................................ .................... 217 LIST OF REFERENCES ................................ ................................ ............................. 228 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 243

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9 LIST OF TABLES Table page 2 1 Crystallographic data for 2 1 2 2 and 2 3 ................................ .......................... 52 2 2 Bond valence sums for the Mn atoms of complex 2 1 2 2 and 2 3 a .................. 53 2 3 Bond valence sums for the O atoms of complex 2 1 a ................................ ......... 53 2 4 Bond valence sums for the O atoms of complex 2 2 a ................................ ......... 53 2 5 Bond valence sums for the O atoms of complex 2 3 a ................................ ......... 54 2 6 Structural types and ground state S values for known Mn 12 clusters .................. 54 3 1 Crystallographic data for 3 1 ................................ ................................ 93 3 2 BVS calculations for the Mn atoms of complex 3 1 a ................................ ........... 93 3 3 BVS calculations for the O atoms of complex 3 1 a ................................ ............. 94 3 4 Fitting parameters for plots of vs. angular frequency and '' vs. angular frequency for complex 3 1 ................................ ................................ .................. 94 3 5 Computational calculati on scheme for seven Heisenberg exchange constants in Mn 9 complex ( 3 1 ) ................................ ................................ ......................... 95 3 6 The distance between Mn i and Mn j Heisenberg exchange constants for two adjacent metal centers and calculated and ideal spin coupling .......................... 96 3 7 Lwdin Population analysis for spin densities of complex 3 1 ........................... 96 4 1 Crystallographic data for 4 1 2 Cl 2 4 2 4 3 .............................. 146 4 2 Crystallographic data for 4 4 4 5 and 4 6 .............................. 147 4 3 Crystallographic data for 4 7 a nd 4 8 ................................ ............................... 148 4 4 Bond valence sums for the Mn atoms of complexes 4 1 4 2 a .......................... 148 4 5 Bond valence sums for the Mn atoms of complexes 4 3 4 4 a .......................... 149 4 6 Bond valence sums for the Mn atoms of complexes 4 5 4 6 a .......................... 149 4 7 Bond valence sums for the Mn atoms of complex 4 7 and 4 8 a ....................... 149 4 8 Bond valence sums for the O atoms of complexes 4 1 and 4 2 a ...................... 150

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10 4 9 Bond valence sums for the O atoms of comp lexes 4 3 and 4 4 a ...................... 150 4 10 Bond valence sums for the O atoms of complexes 4 5 and 4 6 a ...................... 151 4 11 Bond valence sums for the O ato ms of complex 4 7 a ................................ ....... 151 4 12 Bond valence sums for the O atoms of complex 4 8 a ................................ ....... 152 5 1 Crystallographic data for 5 1 5 2 ................................ .. 192 5 2 Bond valence sums for the O atoms of complex 5 1 a ................................ ....... 192 5 3 Bond valence sums for the Mn atoms of complex 5 1 an d 5 2 a ....................... 192 5 4 Magnetic data summarized from the dc measurements ................................ ... 193 A 1 Selected interatomic distances () and angles (deg) for 2 1 ................. 203 A 2 Selected interatomic distances () and angles (deg) for 2 2 ............... 203 A 3 Selected interatomic distances () a nd angles (deg) for 2 3 ................. 204 A 4 Selected interatomic distances () and angles (deg) for 3 1 4MeCN ............... 204 A 5 Selected interato mic distances () and angles (deg) for 4 1 4CH 2 Cl 2 and 4 2 ................................ ................................ ................................ ........... 205 A 6 Selected interatomic distances () and angles (deg) for 4 3 and 4 4 ............... 205 A 7 Selected interatomic distances () an d angles (deg) for 4 5 ............... 206 A 8 Selected interatomic distances () and angles (deg) for 4 6 ............... 207 A 9 Selected interato mic distances () and angles (deg) for 4 7 x(Solv) ................ 207 A 10 Selected interatomic distances () and angles (deg) for 4 8 ............... 209 A 11 Selected interatomic distances () and angles (deg) for 5 1 ............... 211 A 12 Selected interatomic distances () and angles (deg) for 5 2 ............ 211 D 1 Spin of nine Mn atoms are shown in column S1 to S9 indicating different spin orientations. ................................ ................................ ................................ ...... 216

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11 LIST OF FIGURES Figure page 1 1 Representation of magnetic field lines of flux (A) for a diamagnetic substance in a magnetic field and (B) for a paramagnetic substance in a magnetic field .... 29 1 2 Magnetic dipole arr angements in different types of materials. ............................ 29 1 3 Typical hysteresis loop of a magnet, where M is magnetization, H is the applied magnetic field and M s is the saturation value of the magnetiza tion ........ 29 1 4 Representation of the [Mn III 8 Mn IV 4 ( 3 O 2 ) 12 ] 16+ core (A) and the [Mn 12 O 12 (O 2 CMe) 16 (H 2 O) 4 ] complex ................................ ................................ ... 30 1 5 Representa tive plots of the pote ntial energy barrier for an SMM ....................... 30 1 6 Magnetization hysteresis loops for a typical [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] complex ................................ ................................ ................................ .............. 31 1 7 Schematic representation of the change in energy of m s sublevels as the magnetic field is swept from zero to a non zero value. ................................ ....... 31 1 8 Representative examples of pyridyl alcoh ols, pyridyl oximes and non pyridyl oxime ligands ................................ ................................ ................................ ...... 32 1 9 Representative examples of non pyridyl alcohols ................................ ............... 33 1 10 N N N' N' tetrak is(2 hydroxyethyl) ethylenediamine ................................ ........... 33 2 1 Labeled representation (A) and a stereopair (B) of complex 2 1 ....................... 56 2 2 Labeled represen tation (A) and a stereopair (B) of complex 2 2 ....................... 57 2 3 Labeled representation (A) and a stereopair (B) of complex 2 3 ....................... 58 2 4 Th e core of 2 1 (A), 2 2 (B), 2 3 (C) viewed along the c axis.. ........................... 59 2 5 The core of 2 1 (A), 2 2 (B), and 2 3 (C) viewed along the b axis.. .................... 60 2 6 Coordination mode of edte found in complexes 2 1 to 2 3 .............................. 60 2 7 A full CV diagram of complex 2 1 ................................ ................................ ...... 61 2 8 A ful l DPV diagram for complex 2 1 ................................ ................................ ... 61 2 9 The CV diagram at the indicated scan rates for the reduction wave of 2 1 ....... 62

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12 2 10 Plots of vs. square root of scan rate ( 1/2 ) for 0.24 V (A) and 0.47 V (B) for complex 2 1 .............. 62 2 11 Plot of M T vs. T for complexes 2 1 ( ) 2 6H 2 ) and 2 3 ................................ ................................ ................................ ....... 63 2 12 Plots of reduced magnetization ( B ) vs. H/T for complexes 2 1 (A), 2 2 (B) and 2 3 (C). ................................ ................................ ................................ ... 63 2 13 Plot of reduced magnetization ( B ) vs. H/T for complex 2 3 ........................ 64 2 14 Two dimensional contour plot for the rms error surface vs. D and g for the reduced magnetization fit for complex 2 3 ................................ ......................... 64 2 15 Plots of in phase M (as M T ) vs. T and out of phase M vs. T ac signals for complex 2 1 ................................ ................................ ................................ ....... 65 2 16 Plots of in p hase M (as M T ) vs. T and out of phase M vs. T ac signals for complex 2 2 ................................ ................................ ................................ ....... 66 2 17 Plots of in phase M (as M T ) vs. T and out of phase M vs. T ac signals for complex 2 3 ................................ ................................ ................................ ....... 67 2 18 Magnetization (M) vs. dc field scans for a single crystal of complex 2 1 ........... 68 3 1 A partially la beled representation and a stereopai r of complex 3 1 .................... 97 3 2 The core of complex 3 1 (A) and the core with an emphasis on the triangular sub units (B).. ................................ ................................ ................................ ..... 98 3 3 The c oordination modes of edte and edteH found in complex 3 1 ............... 98 3 4 Plot of M T vs. T for complex 3 1 ................................ ................................ ....... 99 3 5 Plot of reduced m agnetization ( B ) vs. H/T for complex 3 1 (A) 2D contour plot for the rms error surface vs. D and g for complex 3 1 (B). .............. 99 3 6 Plots of in phase M (as M ) vs. T and out of phase M v s. T ac signals for complex 3 1 ................................ ................................ ................................ ...... 100 3 7 Plot of the ln(1/ ), vs. 1/ T for complex 3 1 (A). Arrhenius plot obtained from the magnetization vs. time decay study for complex 3 1 (B). ........................... 101 3 8 Plots of the in phase ( ) vs. angular frequency (A), out of phase ( '' ) vs. angular frequency (A) and '' vs. (B) at 3.0 K for complex 3 1 .. .................... 102 3 9 Argand plots of '' vs. for complex 3 1 at the indicated temperatures (A). The change in with temperature in the Argand plot for complex 3 1 (B). ....... 103

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13 3 10 Magnetizatio n (M) vs. dc field hysteresis loops for a single crystal of 3 1 vs. time decay data for complex 3 1 (C). .................... 104 3 11 A full CV diagram at 100 mV/s (A) and full DPV diagram for complex 3 1 ...... 105 3 12 The CV diagram at 50 500 mV/s (A). peak current dependence vs. square root of scan rate ( 1/2 ) (B). ..................... 106 3 13 INS spectra of 3 1 recorded with an incoming neutron wavelength of 6.5 at the indicated temperatures. ................................ ................................ .............. 107 3 14 INS spectra of 3 1 recorded with wavel ength 7.5 . ................................ ......... 107 3 15 S(Q,E) plot of the INS spectrum of 3 1 measured with wavelength 7.5 . ....... 108 3 16 INS spectra of 3 1 measure d with wavelength 3.2 . (A) S(Q,E) plots at 1.5 K. (B) ................................ ................................ ................................ ................ 108 3 17 (A) Compariso n of the experimental and (lines) INS spectra recorded with wavelength 7.5 . (B) Calculated energy spectrum of 3 1 ............................... 109 3 18 Depiction of the spin alignments in the S = 21/2 ground state of complex 3 1 110 4 1 Labeled representation of th e cation of complex 4 1 (A). A section of the 1D polymeric chain (B) of complex 4 1 ................................ ................................ 153 4 2 Labeled representation of the cation of complex 4 2 ................................ ....... 154 4 3 Labeled representation of the cation of complex 4 3 ................................ ....... 154 4 4 The core of complex 4 3 (A). A section of the 1D polymeric chains of complex 4 3 ................................ ................................ ................................ ...... 155 4 5 The labeled representation of complex 4 4 (A). a stereoview (B) and the core (C) of complex 4 4 . ................................ ................................ .......................... 156 4 6 Labeled representation of the part of (Na 2 Mn 10 ) polymer, 4 5 (A) and a stereopair (B).. ................................ ................................ ................................ .. 157 4 7 The core of complex 4 5 (A) and a section of the 1D polymeric chain of complex 4 5 (B). ................................ ................................ ............................... 158 4 8 Labeled representation of the cation of complex 4 6 (A) and a stereopair (B) .. 159 4 9 Structural representation of the cation of complex 4 7 (A) and a stereopair (B).. ................................ ................................ ................................ ................... 160 4 10 Labeled representation of the core of complex 4 7 ................................ .......... 161

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14 4 11 Structural representation of the cation of complex 4 8 ................................ .... 161 4 12 Structural representation of the stereopair of complex 4 8 .............................. 162 4 1 3 Labeled representation of the core of complex 4 8 (A) and a part of the core of complex 4 8 (B). ................................ ................................ ........................... 162 4 14 Coordination modes of edteH 2 edteH and edte in complexes 4 1 to 4 8 163 4 15 Schematic diagram for the linear Mn 3 core for complexes 4 1 and 4 2 with two exchange coupling parameters, J and J' ................................ .................... 164 4 16 Plot of M vs. T for complex 4 1 2 O. ................................ ............................... 164 4 17 Plot of M T vs. T for complex 4 2 2 O. ................................ ............................. 165 4 18 Plot of all possible energy states for complex 4 1 using the J value, obtained from the fit to the experimental M data. ................................ ........................... 165 4 19 Plot of all possible energy states for c omplex 4 2 using the J values, obtained from the fit to the experimental M T data. ................................ ......................... 166 4 20 Plot of reduced magnetization ( B ) vs. H/T for complex 4 2 2 O. .............. 166 4 21 Two dimensional contour plot for the rms error surface vs. D and g for the reduced magnetization fit for complex 4 2 ................................ ....................... 167 4 22 Plot of in phase M (as M ) vs. T ac signals at 1000 Hz for complexes 4 1 2 4 2 2 ................................ ................................ .............. 167 4 23 Plot of M T vs. T for complex 4 3 ................................ ................................ ..... 168 4 24 Schematic diagram for a Mn 4 core with two exchange coupling parameters, J wb and J bb ................................ ................................ ................................ ........ 168 4 25 (A) Plot of energies of various spin states vs. the ratio of J wb /J bb for complex 4 3 (B) Plot of all possible energy states for complex 4 3 ............................... 169 4 26 The plots of in phase M (as M T ) vs. T ac signals for complex 4 3 at the indicated frequencies. ................................ ................................ ....................... 170 4 27 Plot of M T vs. T for complex 4 4 ................................ ................................ ..... 170 4 28 P lot of reduced magnetization ( B ) vs. H/T for complex 4 4 ...................... 171 4 29 Two dimensional contour plot for the rms error surface vs. D and g for the reduced magnetization fit for complex 4 4 ................................ ....................... 171

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15 4 30 Plot of in phase M (as M T ) vs. T ac signals for complex 4 4 ........................ 172 4 31 Plot of M T vs. T for complexes 4 5 2 and 4 6 2 ...................... 172 4 32 Plot of in phase M (as M T ) vs. T ac signals for complexes 4 5 2 and 4 6 2 at 1000Hz. ................................ ................................ ............ 173 4 33 Plot of M T vs. T for complex 4 7 6 H 14 ................................ ...................... 173 4 34 Plot of reduced magnetization ( B ) vs. H/T for complex 4 7 (A). 2D contour plot for the rms error surface vs. D and g for compl ex 4 7 (B). ............ 174 4 35 Plots of in phase M (as M T ) vs. T and out of phase M vs. T ac signals for complex 4 7 6 H 14 ................................ ................................ ...................... 175 4 36 Plots of M T vs. T (A) and in phase M (as M T ) vs. T ac signals for complex 4 8 2 O (B). ................................ ................................ ................................ .... 176 5 1 The structure of 5 1 ................................ ................................ .......................... 193 5 2 The stereopair of 5 1 ................................ ................................ ....................... 194 5 3 The core of 5 1 ................................ ................................ ................................ 194 5 4 The coordination mode of edteH 2 found in 5 1 ................................ .............. 195 5 5 A partially labeled representation of complex 5 1 ................................ ............. 195 5 6 A partially labeled structure and a stereopair of complex 5 2 .......................... 196 5 7 Plot of M T vs. T for complexes 5 1 to 5 5 ................................ ........................ 197 5 8 Schematic diagram for Mn 4 core with two exchange coupling parameters, J wb and J bb ................................ ................................ ................................ .............. 197 5 9 Plot of M T vs. T for complex 5 5 ................................ ................................ ..... 198 5 10 Plot of all possible energy states for complex 5 5 using the J values, obtained from the fit to the experimental M T data. ................................ ......................... 198 5 11 Plot of energies of various spin states vs. the ratio of J wb /J bb .......................... 199 5 12 Plot of reduced magnetization ( B ) vs. H/ T for complex 5 1 ...................... 199 5 13 Plots of reduced magnetization ( B ) vs. H/ T for complex 5 5 .................... 200 5 14 Plot of in phase M (as M T ) vs. T ac signals at 250 Hz for complexes 5 1 to 5 5 ................................ ................................ ................................ .................... 200

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16 5 15 Plots of in phase M (as M T ) vs. T and out of phase M vs. T ac signals for complex 5 2 ................................ ................................ ................................ ...... 201 5 16 Magnetization ( M ) vs. dc field hysteresis loops for a single crystal of 5 2 ....... 202 C 1 A typical cyclic voltammogram obtained for a reversible one electron reduction process. ................................ ................................ ............................ 215

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17 LIST OF ABBREVIATION S AF antiferromagnetic bpy 2, 2' bipyridine t Bu tertiary butyl C(CH 3 ) 3 BVS bond valence sum CV cyclic voltammogram DPV differential pulse voltammogram DFT density functional theory DMF N, N' dim ethylformamide edteH 4 N, N, N', N' tetrakis(2 hydroxyethyl ) ethylenediamine Et Ethyl CH 2 CH 3 FM ferromagnetic hmpH 2 hydroxymethyl pyridine INS inelastic neutron scattering IR infrared spectroscopy JT Jahn Teller Me Methyl CH 3 PS II photosystem II QTM qu antum tunneling of magnetization SMM single molecule magnet SQUID superconducting quantum interference device THF tetrahydrofuran TIP temperature independent paramagnetism ZFS zero field splitting

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18 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 DIVERSITY OF STRUCTURAL TYPES IN M A N GANESE CLUSTER CHEMISTRY INCLUDING SINGLE MOLECULE MAGNETS FROM THE USE OF N, N, N', N' TET RAKIS(2 HYDROXYETHYL)ETHYLENEDIAMINE By Arpita Saha August 2011 Chair: George Christou Major: Chemistry The research focuses on the investigation of new synthetic routes towards the preparation and subsequent characterization of novel multinuclear tran sition metal/lanthanide complexes that can function as molecular nanomagnets better known as single molecule magnets (SMMs). The major compelling property of these metal clusters is that they behave like tiny magnets i.e. they show slow relaxation of magn etization at low temperature. In addition these molecular systems often exhibi t quantum properties which make them interesting candidate s to use as qubits in quantum computation. In 1993 the first SMM was discovered, [Mn 12 O 12 (O 2 CCH 3 ) 16 (H 2 O) 4 ], better know n as Mn 12 Ac, which was a breakthrough in the field of molecular magnetism. In the subsequent years, there has been massive amount of research in this area, and the SMM database has greatly expanded as research groups around the world have made new ones. Th e synthesis of such polynuclear metal clusters involves the incorporation of multiple metal atoms supported by organic ligand s In this regard alkoxide based ligands play a pivotal role since this functionally is an excellent bridging group that fosters h igher nuclearity product formation. Such

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19 polydentate ligands have led to the discovery of many interesting 3 d clusters, some of which display SMM behavior. T he dissertation is on the investigation of new synthetic methods, combining an alkoxide based ligan d with various carboxylate s and azides. In this regard potentially hexadentate (O,O,O,O,N,N) N,N,N',N' tetrakis(2 hydroxyethyl)ethylenediamine (edteH 4 ) ligand has been used Herein, the synthesis, structure and magnetic properties of various nuclearity Mn x clusters where x = 3, 4, 6, 9, 10, 12, 18, 20 have been reported. Among them, the electrochemical behavior (cyclic voltammetry and differential pulse voltammetry) and magnetic susceptibilities (using SQUID magnetometer) of Mn 9 and a family of Mn 12 SM Ms have been studied in detail. Single crystal hysteresis loops using micro SQUID apparatus were obtained and further proved the SMM behavior of the same. Mn 9 is a new, rare half integer spin SMM with a spin barrier of 49 K, the highest in mixed valent Mn 2 + / Mn 3+ chemistry. Inelastic neutron scattering (INS) and computational characterization (using Density Functional Theory) were further performed to assess the zero field splitting and higher order anisotropic parameters, the energy barrier, the spin ground state, the Heisenberg exchange coupling parameters ( J ) and the bonding criterion in Mn 9 complex. In addition a family of isostructural heterometallic Mn Ln clusters using the same ligand with a [Mn II 2 Mn III 2 Ln III 2 ] core (Ln = Gd, Tb, Dy, Ho) ha s been synt hesized as well as a [Mn II 2 Mn III 2 Y III 2 ] analog with diamagnetic Y 3+ to assist the magnetic studies by assessing the nature of the exchange interactions. Among them the Tb analog exhibits frequency dependent out of phase ac susceptibility signals char acteristic of SMMs which was further confirmed by the observation of magnetization hysteresis.

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20 CHAPTER 1 GENERAL INTRODUCTION The most distinctive aspect of transition metal chemistry is the formation of coordi nation complexes. These are species that cont ain metal ions coordinated to molecules and/or anions as ligands. Any ion or molecule with a pair of non bonding electrons can be a ligand e.g. NH 3 Cl and H 2 O. Ligands are classified in many ways e.g. charge, size (bulk), the identity of the coordinating atom(s), or the number of electrons donated to the metal (hapticity ). Ethylenediamine (en) is an example of a bidentate ligand since each end contains a pair of non bonding electrons that can make a covalent bond with a metal ion. E n is also a typical ex ample of a chelating ligand. The term chelate originates fr om the Greek word claw that is it grab s the metal in two or more places. Alfred Stock, a German inorganic chemist. He did p ioneering research on borohydrides, 1,2 silicohydrides 3 and coordination chemistry but coordination complexes have been known since the early eighteenth century e.g. P russian blue and coppe r vitriol. The real breakthrough in modern coordination chemistry occurred by a Swiss chemist, Alfred Werner who won the Nobel Prize in 1913 for proposing the octahedral configuration of transition metal complexes 4 However, it took se veral years to develop metal alkox ide chemistry. In early 19 5 did some ground breaking rese arch on synthesizing metal alkoxide e.g. Monomeric zircon ium tetra tert butoxide, Zr(OBu t ) 4 5 Since then, the field of inorganic metal alkoxide chemistry has developed and continued to be an important par t of the frontier research that for example, involves exploring complicated polydentate ligands e.g. crown ethers, cryptands poly alcohols, Schiff based ligands and so on designed to

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21 achieve specific purposes. One of the important a pplications of polyden tate ligands of relevance to these is to make polynuclear transition metal complexes or clusters containing paramagnetic metal ions. Mimic k ing the metal site s of biomolecules is one purpose of ma king oxide based metal clusters For example this synthetic modeling approach can unfold the nature and mechanism of the active site of the water oxidizing complex, a CaMn 4 cluster in photosystem II (PS II) 6,7 of green plants and cy an obacteria as well as illuminating the means of assembly of the multinuclear Fe 3+ /O 2 core of the iron storage protein, ferritin 8,9 which can store up to 4500 iron atoms. In addition ferritin is also considered a nanosize magnetic particle and has been investigated for its various magnetic properties. Simila rly, many 3d and mixed 3d/4f metal clusters often display interesting and sometimes novel magnetic properties, including high ground state spin values, 10 currently up to S = 83/2 and single molecule magnetism. 11 15 These metal based tiny magnets are individual molecules that can act as nanoscale magnetic particles below a certain temperature The first s ingle molecule magnet (SMM) [Mn 12 O 12 (O 2 CCH 3 ) 16 (H 2 O) 4 ] (better known as Mn 12 Ac ) 16 was discovered in 1993 w hich was a breakthrough in the field of molecular magnetism Over the years many research groups have enriched the database by making new SMMs to improve understanding and control over the synthesis and physiochemical properties of these tiny yet powerful magnets. For unders tanding the origin of SMMs, knowledge of magnetism is i mperative. The fascinating properties and uses of magnets have captivated human s eve r since Thales of Miletus (ca. 634 546 BC) first described the phenomenon as the attraction of i ron

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22 17 Over the last 2500 years, magnetism has played a pivotal role in the development of human civilization. Huma ns have used magnets for navigation, power production and various other technological application s, while at the same time exploring the origin of the effect. The popularly know n lod e stone is a magnetized piece of the mineral magnetite (F e 3 O 4 ), wh ich the G reeks observed In the modern technological society, magnetic materials are ubiquitous in various industrial and other applications, such as manufacturing of switches, compu ter hard drives, credit/debit/ATM cards, televisions, audio devices, motors, and highly specialized instruments like medical MRI equipment among many others. Modern day magnetic materials include magnetic alloys and oxides, particularly ferrites such as M gFe 2 O 4 which can function in transformer cores, and magnetic recording or information storage devices. Today magnetism is a multi billion dollar annual industr y. The behavior of any magnetic material is dependent on the presence of unpaired electrons and how they interact with each other. 18 To be more precise any moving electrical charge with spin and orbital angular momentum generates a magnetic field in a system. The qu antitative measurement of the magnetic response of a material to an applied magnetic field is known as susceptibility ( ). The magnetic materials are broadly classified into two categories which are dia and paramagnetic materials Diamagnetism arises from the interact ion of paired electrons with a magnetic field whereas paramagnetism comes from the presence of unpaired e lectrons in the syst em. A diamagnet with a small negative value ( 10 5 to 10 6 cm 3 mol 1 ) is slightly repelled by the magnetic field where as a paramagnet with a small positive value (10 3 to 10 5 ) is

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23 attracted towards the applied field (Figure 1 1). Dia magnetic susceptibilities are independent of field and temperature whereas paramagnetic susceptibilities of individual isolated paramagnets are inversely proportional with temperature, = C/T, where C is Curie constant. 19 However when the spins come within weak interactions with each other, the relationship is modified using the Curie Weiss expression, = C/( T where is proportional to the strength of coupling between adjacent spins. The type of spin coupling of adjacent spin pair s further categorizes the paramagnetic materials. For example, ferromagnetic orderi n g is achieved when the e spin pair (Figure 1 2). The commonly known ferromagnetic solids are the materials made of iron, cobalt, nickel and several rare earth metals and their alloys. 20 Their susceptibilities va ry from 50 to 10,000. Ferrimagnets (e.g. magnetite, Fe 3 O 4 ) also arise from antiferromagnetic coupling ; however a net magnetic moment is achieved du e to only partial cancellation of unequal spins (Figure 1 2). Ferro and ferrimagnetic ordering occurs below a critical temperature, T c and that for antiferromagnets is below the Neel temperature, T N when the magnetic moments align in small domains. In the absence of an applied field, the magnetization of the different domains orient randomly and cancel out each other to give a net zero magnetization, thermodynamically (entropically) favored, regardless of the nature of interactions. The situation changes in the presence of an external field when all the domains tend to line up in the direction of the field. At a certain field, magnetization saturation is achieved with a net magnetic moment when all the spins are aligned into one giant domain. The system retains the condition unless an external force is provided to overcome the

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24 energy barrier for domain formation This property is characteristic of any magnet and can be monitored by the magnetization vs. field study commonly known as hysteresis loop measurement (Figure 1 3). 18 The term hysteresis comes from the fact that the molecule can retain its history as long as no additional field is applied This makes magnetic data storage possible in ferro and ferrimagnetic materials. Other magnetic ordering phenomena include spin glass, m etamagnetic and canted ferro/antiferromagnetic behavior. 21 Metal and metal oxide based traditional magn ets or molecular magnets with three dimensional arrays of linked interacting molecules are well reviewed and discussed elsewhere. 19,22,23 The newest addition to this category is SMMs where magnetism is zero dimens ional. The necessity of finding new magnets can be understood by analyzing the need of modern technology which is to develop miniaturized devices In information storage, this would lead to a higher capacity to store digital information in a given area He nce, the essential need for nanoscale magnets of identical size and behavior. The so deriving smaller pieces of a magnet from larger chunks is unable to achieve the required demand of nanoscale magnets of both ide ntical size and shape In this regard SMMs have drawn significant attention in the f ield of nanomagnetism because they represent nanoscale magnetic materials. The approach has significant control over the s ize and magnetic properties of the materials by varying the protective organic shell around the metal core and hence, single size (monodispersity) crystallinity and true solubility (organic s olvents) can be achieved for SMM s. They are unique types of mag nets where individual molecule s possess a significant barrier ( vs.

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25 k T ) to magnetization relaxation, and thus exhibit the properties of a magnet below the blocking temperature ( T B ). Here the magnetism is intrinsic to the molecule and does not occur from the intermolecular long range ordering as observed in traditional magnets This interesting magnetic property comes fr om a combination of a high ground state spin ( S ) value (i.e. several unpaired electrons) and magnetic anisotropy (negative zero field paramet er, D ). Even after the discovery of various SMMs in transition metal chemistry, the aforementioned Mn 12 Ac (Figure 1 4) 24 and other subsequent members of the [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] (R = Et, Ph, CHCl 2 etc.) family 11 remain the most popular choice for study by chemists and physicists due to their ease of preparation, crystallinity, high S and D values and high molecular and crystallographic symmetry, which simplifies the s pin Hamiltonian and reduces the complexity in various theoretical calculations. Structurally, this well explored family consists of an external belt of eight Mn 3+ ions ( S = 2) anti ferromagnetically coupled to an inner core of four ferromagnetically coupl ed Mn 4+ ions ( S = 3/2, Figure 1 4) to give an S = 10 ground state spin. 25 The n egative zero field parameter D is responsible for the loss of degeneracy of the associated m s levels where m s = +10 and m s = 10 are the lowest in energy. Thus, there exists an energy barrier for the conversion of the the situatio n (Figure 1 5). The slow relaxation of the magnet ization vector is a result of this energy barrier calculated as S 2 S 2 for a half integer spin system. 12 This spin barrier is the reason behind the interesting magnetic behavior of these individual molecules and the barrier results in a hysteresis loop as also observed in traditional magnets (Figure 1 5). The most unique feature of these magnets is not only the ca pacity of storing information at a molecular level but the

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26 quantu m effects which are manifested in the step like hysteresis loop (Figure 1 6). 19 Th e observed steps are the result of faster rate of relaxation of the magnetization vector from one side of the potential energy barrier to the other side when there is an accidental degeneracy in the m s levels (Figure 1 7). The phenomenon is known as the quantum tunneling of magnetization (QTM) 26 which indicates the relaxation in an SMM is not just due to the thermal activation barrier Transverse anisotropy promote s the tunneling through th e energy barrier which can be achieved by the low symmetry components of the crystal field and by a magnetic field generated by magnetic nuclei. This quantum feature in an SMM makes them interesting potential candidates to be used as quantum bits ( qubits) in quantum computation. 27 32 A q uantum computer is a device for computation that is based on quantum mechanics such as superposition and entanglement. Information in traditional computer is stored by strings of called bits. Q uantum information uses qubits, where the two distinguishable configurations (bits, 0 & 1) can have superposition or any combination to give coupled states (00, 11, 01, 10). 28 The QTM and quantum phase interference obser ved in an SMM surely make them attractive to be a part of the designing of new quantum information processing s ystem In order to achieve the dream goal of using SMMs in real world application, extensive research is necessary to synthesize the appropriate systems and a thorough investigation of various syn thetic schemes is the key to this success. The criteria for making nanomaterials at a molecular level depend mainly on the choice of metal ions and the ligands needed to hold several metals together. Manga nese has been the transition metal of choice for SMMs mainly due to the

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27 availability of the anisotropic Mn 3+ ion 10,33 45 Large S values of the resulting molecule can result from (a) ferro or ferrimagnetic spin al ignments and/or (b) competing interactions (spin frustration) in certain M x topologies that prevent (frustrate) the preferred spin align ment. In case (a), ferromagnetic interactions can result from ligands or structural characteristics known to give ferrom agnetic coupling between metal centers. In this regard various RCO 2 (R = Me, Et etc.) groups and end on azide bridges are known to promote ferromagnetic coupling between bridging metal ions. T o facilitate the formation of polynuclear manganese clusters many researchers have employed alkoxide based ligands (Figure 1 8 an d 1 9) since this functionally is an excellent bridging group that fosters higher nuclearity product formation. 46 52 Such polydentate ligands have led to the discovery of many interesting 3 d clusters, some of which display SMM behavior. The successful alkoxide based ligands so far used in making SMMs in 3d cluster chemistry can be broadly classified into four categorie s such as pyridyl alcohols, non pyridyl a lcohols, pyridyl oximes and non pyridyl oximes ligands (Fi gure 1 8 and Figure 1 9). 53 Variation of t he bulki ness and the electronic properties of the ligand s esp ecially near the bridging O atoms, make th em quite interesting to study. It is important to note that many of t he ligands are comprised of an ethylene amine backbone These N, O based ligands have proven to be quite useful in link ing higher oxidation state Mn 3+ or Mn 4+ to make polynuclear metal clusters. Following the trail of the ongoing research on making polynu clear Mn based metal clusters, work in this thesis has employed the potentially hexadentate (O,O,O,O,N,N) N,N,N',N' tetrakis(2 hydroxyethyl) ethylenediamine (edteH 4 Figure 1 10) ligand. Once deprotonated t he edteH 4 contains four flexible alkoxide arms th at can bridge two or

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28 more metal ions and give high nuclearity products. 54 The following chapters will show how small changes to the reaction conditions affect the product identity in edteH 4 c oordination chemistry. C hapter 2 presents the use of simple Mn salts in making a family of Mn 12 SMMs. The electrochemical and magnetochemical properties are discussed. C hapter 3 elucidates how a bulky carboxylate can affect the identity and physiochemical properties of the product which leads to the isolation of Mn 9 .The obtained complex is a rare example of new half integer spin SMM and various experime ntal and computational analyse s have been performed. Chapter 4 shows the richness and structural variety encountered in Mn x and Mn/Ca cluster chemistry (where x = 3, 4, 6, 10 and 20 ) using the same ligand. Chapter 5 brings new flavor by introducing heterom etallic Mn Ln SMMs (Mn 4 Ln 2 ) using the same ligand. Hence, edteH 4 has proved to be a rich source of many unprecedented metal clusters and the following chapters will unfold the story of this polydentate ligand edteH 4 in Mn Mn Ca and Mn Ln cluster chemistry

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29 A B Figure 1 1 Representation of magnetic field lines of flux (A ) for a diamagnetic s ubstance in a magnetic field and (B ) for a paramagnetic substance in a magnetic field Figure 1 2 Magnetic dipole arrangements in d if ferent types of ma terials. (A) ferromagnetic, (B) ferri magnetic, (C) antiferromagnetic, and (D ) para magnetic materials Figure 1 3 Typical hysteresis loop of a magnet, where M is magnetization, H is the applied magnetic field and M s is the saturation value of the magnet ization

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30 Figure 1 4 R epresentation of the [Mn III 8 M n IV 4 ( 3 O 2 ) 12 ] 16+ core (A) and the [Mn 12 O 12 (O 2 CMe) 16 (H 2 O) 4 ] complex with acetates as peripheral ligands (B) Mn 4+ green; Mn 3+ blue; O red; C gray Figure 1 5. Representative plots of the potent ial en ergy ba r rier for an SMM. (A ) Plot of the energy versus the m s sublevel for a Mn 12 complex with an S = 10 ground state, experiencing zero field splitting, D 2 z and (B) p lot of the energy versus the orientation of the m s vector ( ) along the z axis.

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31 Figure 1 6. Magnetization hysteresis loops for a typical [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 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, M s Fugure 1 7. Schematic representation of the change in ener gy of m s sublevels as the magnetic field is swept from zero to a non zero value. Resonant magnetization tunneling occurs when the m s sublevels are aligned between the two halves of the diagram

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32 Figures 1 8. Representative examples of pyridyl alcohols pyridyl oximes and n on pyridyl oxime ligands

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33 Figure 1 9. Representative examples of non pyridyl alcohols Figure 1 10. N N N' N' tetrakis(2 hydr oxyethyl) ethylenediamine

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34 CHAPTER 2 SYNTHESIS, STRUCTURE AND ELECTR OCHEMICAL AND MAGNET OCHEMICAL PR OPERTIES OF A FAMILY OF Mn 12 COMPLEXES CONTAINING THE ANION OF N,N,N',N' TETRAKIS(2 HYDROXYETHYL) ETHYLE NEDIAMINE The synthesis of high spin transition metal clusters has become an important sub discipline of coordination chemistry, especially since the discovery of single molecule magnet s (SMMs) 11 in the early nineties. These are individual molecules that possess a significant ba rrier ( vs k T ) to magnetization relaxation, and thus exhibit the properties of a magnet below their blocking temperature ( T B ). The SMM property results from a combination of a high spin ground state ( S ) value and an easy axis type magnetoanisotropy (negativ e zero field parameter, D ) 55 58 Manganese has been the transition metal of choice for SMMs mainly due to the availability of the anisotropic Mn 3+ (Jahn Teller distorted ion ) 16,25,26,33 Large S values can result from (a) ferro or ferrimagnetic spin alignments and/or (b) competing interactions (spin frustration) in certain M x topologies that prevent (frustrate) the preferred spin alignment. In case (a), ferromagnetic intera ctions can result from ligands or structural characteristics known to give ferromagnetic coupling between metal centers. One of the best ferromagnetic couplers is the azide (N 3 ) group when it bridges metals in the 1,1 (end on) fashion 59 Following the trail of ongoing research, we have been recently exploring new synthetic methodologies by co mbining a potentially hexad entate, alcohol based chelate (O,O,O,O,N,N) N,N,N',N' tetrakis(2 hydroxyethyl)ethylenediamine (edteH 4 Figure 1 10 ) with azides This chelate is comprised of an ethylenediamine backbone with hydroxyethyl arms, which upon deprotonation act as bridging group s to foster formation of high nuclearity clusters. EdteH 4 has been reported by our group to produce Mn 8 Mn 12 and Mn 20 clusters with novel metal topologies. 60 In addition, edteH 4 has also

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35 produced various Fe x (x = 5, 6, 12) 54 complexes recently for our group. Previous use of edteH 4 in the literature with other metals has been li mited to the preparatio n of only mononuclear Ca and dinuclear Ba, Cu and V complexes. 61,62 It should also be mentioned that during the progress of the ongoing research in our group two Mn 12 complexes similar to complexes 2 2 and 2 3 were reported by another research group, Zhou et al. 63 However, a thorough study of a much cleaner synthesis and a detailed magnetic study have been pe rformed in our work. For the first time, low temperature hysteresis measurements have been performed for complex 2 1 of this newly found family of Mn 12 complexes which differ a lot from the well explored family of novel [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] complexes ( R = Me, Et etc) known for many years. 11 Herein, we report that the use of edteH 4 and azide together in a variety of reactions wit h manganese salts has resulted in a family of Mn 12 SMMs at different oxidation states. The syntheses, crystal structures and electrochemical and magnetic characterization of this Mn 12 family will be described. Experimental Section Syntheses All preparation s were performed under aerobic conditions using reagents and solvents as received. Caution! Although no such behavior was observed during the present work, perchlorate and azide salts are potentially explosive; such compounds should be synthesized and used in small quantities, and treated with utmost care at all times. [ Mn 12 O 4 (OMe) 2 (edte) 4 (N 3 ) 8 ](ClO 4 )(N 3 ) ( 2 1) To a stirred solution of edteH 4 (0.15 g, 0.64 mmol) and NMe 4 OH (0.12 g, 0.64 mmol) in MeCN/MeOH (10/1, v/v) was added

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36 Mn(Cl O 4 ) 2 (0.46 g, 1.28 mmol) The resulting dark brown solution was stirred for 15 minutes and then NaN 3 (0.08 g, 1.28 mmol) was added to the solution and stirred for further 3 hours. The solution was then filtered and the filtrate left undisturbed at room temperature. After 5 days from slow evaporation, X ray quality, dark brown plate like crystals of 2 1 MeCN slowly grew in a yield of 30%. The crystals were collected by filtration, washed with Et 2 O and dried in vacuum. Anal. Calc. (found) for 2 1 (solvent free) : C, 23.01 (23.03); H, 3.95 (3.90); N, 22.36 (22.14) %. Selected IR data (cm 1 ): 2858 (m), 2066 (vs), 1636 (w), 1464 (w), 1337 (w), 1086 (s), 927 (m), 899 (m), 676 (s), 619 (s), 559 (s). [Mn 1 2 O 4 (OH)(edte) 4 (N 3 ) 9 ] ( 2 2) To a stirred solution of edteH 4 (0.15 g, 0.64 mmol) and NMe 4 OH (0.12 g, 0.64 mmol) in MeCN/MeOH (10/1, v/v) was added MnCl 2 (0.25 g, 1.28 mmol). The resulting dark brown solution was stirred for 15 minutes and then NaN 3 (0.16 g, 2.56 mmol) was added to the solution and stirred for a further 3 hours. The solution was then filtered and the filtrate was layered with diethyl ether. After 10 days, X ray quality, dark brown plate like crystals of 2 2 x(solvent) grew in a yield of 13%. The crystals were collected by filtration, washed with Et 2 O and dried in vacuum. Anal. Calc. (found) for 2 2 H 2 2 MeCN : C, 23.62 (24.17); H, 4.4 6 (4.14); N, 23.16 (22.66 ) % Selected IR data (cm 1 ): 2855 (s), 2056 (vs), 1634 (w), 1459 (w), 1336 (w), 1086 (s), 926 (m), 900 (m), 657 (m), 606 (m), 567 (s). [Mn 12 O 4 (OH)(edte) 4 (N 3 ) 9 ](ClO 4 )(N 3 ) ( 2 3) To a stirred solution of edteH 4 (0.15 g, 0.64 mmol) and NEt 3 (0 .09 g, 0.64 mmol) in MeCN (10mL) was added Mn(Cl O 4 ) 2 (0.46 g, 1.28 mmol). The resulting dark brown solution was stirred for 15 minutes and then NaN 3 (0.08 g, 1.28 mmol) was added to the solution and stirred for a further 3 hours.

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37 The solution was then filt ered and the filtrate left undisturbed at room temperature. After 7 days from slow evaporation X ray quality, dark brown plate like crystals of 2 3 MeCN grew in a yield of 36%. The crystals were collected by filtration, washed with Et 2 O and dried in vacu um. Anal. Calc. (found) for [Mn III 12 O 4 (OH)(edte) 4 (N 3 ) 9 ](ClO 4 )(OH ) : C, 23.53 ( 23.60); H, 3.95 (3.90); N, 23.07 (23.1 1 ) % Selected IR data (cm 1 ): 2867 (w), 2066 (m), 1636 (m), 1458 (w), 1089 (vs), 924 (m), 628 (s), 552 (w). X ray Crystallography Dat a were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo Suitable crystals of 2 1 MeCN, 2 2 x(solvent) and 2 3 MeCN were attached to glass fibers using silicone g rease 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 scan method (0.3 frame width). The first 50 frames were re measured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by d irect methods in SHELXTL6, and refined on F 2 using full matrix least squares. The non H atoms were treated anisotropically, whereas the hydrogen atoms were placed in ideal, calculated positions and were refined as riding on their respective C atoms. For 2 1 he asymmetric unit consists of a Mn 12 cluster dication, a perchlorate anion, and a azide anion diso rdered against a acetonitrile The latter would imply that the acetonitrile or the azide anion should ha ve a 0.5 site occupation factor but because of the disorder, each fragment has a site occupation factor of 0.25

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38 The perchlorate anion is disordered in its oxygen atoms which were refined in two parts. One of the cluster azides, N6 7 8, is disordered and the latter two were refined in two parts. A total of 282 parameters were included in the final cycle of refinement using 4718 reflections with I > 2 (I) to yield R 1 and wR 2 of 5.06 and 15.97%, respectively. For 2 2 x(solvent) t he asymmetric unit consists of the Mn 12 cluster and disordered solvent molecules. Program SQUEEZE, 64 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. H1, the O18 hydroxyl proton was obtained from a Difference Fourier map and refined as riding on its parent atom. A total of 972 parameters were included in the final cycle of refinement using 6671 reflections w ith I > 2 (I) to yield R 1 and wR 2 of 5.54 and 12.44%, respectively. For 2 3 MeCN, t he asymmetric unit consists of a Mn 12 cluster di cation, a perchlorate anion, and a azide anion diso rdered against a acetonitrile, located on the 2 fold rotation ax is. The latter would imply that the acetonitrile or the azide anion should have a 0.5 site occupation factors but because of the disorder, each fragment has a site occupation factor of 0.25 The cluster cation has one azide, N9 10 11, disordered and refi ned in two parts. There is also a disorder between the N6 7 8 and the O6 hydroxyl group. Due to symmetry, each fragment is assigned a site occupation f actor of 0.5 A tota l of 282 parameters were included in the final cycle of refinement using 3060 ref lections with I > 2 (I) to yield R 1 and wR 2 of 5.68 and 16.47%, respectively. The crystallographic data and structure refinement details for the three compounds are listed in Table 2 1.

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39 Physical Measurements Infrared spectra were recorded in the solid sta te (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 400 4000 cm 1 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 und er 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) with 0.1 M NBu n 4 PF 6 as supporting electrolyte. Quoted potentials are vs the ferrocene/ferroce nium couple, used as an internal standard. 65 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 m agnet and operating in the 1.8 300 K rang e. Samples were embedded in solid eicosane to prevent torquing. Magnetization vs field and temperature data was fit using the program MAGNET 66 Pascal' s constants were used to estimate the diamagnetic correction, which was subtracted from the M ). Low temperature (<1.8 K) hysteresis loop and dc relaxation measurements were performed in Grenoble using an a rray of micro SQUIDs 67 The high sensitivity of this magnetometer allows the study of single crystals of the order of 10 to 500 m. The field can be applied in any direction by separately driving three orthogonal coils. Result s and Discussion Syntheses Recently, end on bridging azide groups have proved to be ferromagnetic couplers, and they have been widely employed in manganese cl uster chemistry 59 In order to make clusters containing Mn 3+ ions, it is generally necessary to oxidize simple Mn 2+

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40 salts. Here, t he combination of N N N' N tetrakis(2 hydroxyethyl)ethylenediamine (edteH 4 ), and azides with various Mn salts has been employed and has afforded three Mn 12 complexes which have similar but not identical structures The reaction of edteH 4 wi th Mn( ClO 4 ) 2 NEt 4 OH and NaN 3 in a 1:2:1:2 molar ratio in MeCN/MeOH (10/1, v/v) afforded a reddish brown solution from which was subse quently obtained the dodecanuclear complex [Mn 12 O 4 (OMe) 2 (edte) 4 (N 3 ) 8 ](ClO 4 )(N 3 ) ] ( 2 1 ) in 3 0% yield Its formation is summ arized in eq. 2 1, where atmospheric oxygen gas is assumed to provide the oxidizing equivalents. 12Mn(ClO 4 ) 2 + 4edteH 4 + 9NaN 3 + 2MeOH + 3O 2 [Mn 12 O 4 (OMe) 2 (edte) 4 (N 3 ) 8 ](ClO 4 )(N 3 ) + 9NaClO 4 + 14HClO 4 + 2H 2 O (2 1) The same product in comparable yiel d can be obtained by using a Mn(NO 3 ) 2 as a metal source or different base like LiOH, Et 3 N or by varying the amount of NaN 3 or by using NBu n 4 N 3 as an azide source However, the above ratio gives the purest product in the highest yield. A slight variation in the manganese salt and with a higher amount of NaN 3 resulted in a mixed valent Mn 12 complex with Mn 2+ and Mn 3+ ions. The reaction of edteH 4 with Mn Cl 2, NEt 4 OH and NaN 3 in a 1:2:1:4 molar ratio in MeCN/MeOH (10/1, v/v) afforded a reddish brown solution f rom which was subse quently obtained the dodecanuclear complex [Mn 3+ 10 Mn 2+ 2 O 4 (OH )(edte) 4 (N 3 ) 9 ] ( 2 2 ) in 13% yield (eq 2 2 ) The same product with a comparable yield was obtained by using a different solvent combination of MeCN/DMF (10/1, v/v). Complex 2 2 contains five bridging azide ligand whereas compound 2 1 contains four ( Figure 2 5 ). 12MnCl 2 + 4edteH 4 + 9NaN 3 + 5/2O 2 [Mn 1 2 O 4 (OH)(edte) 4 (N 3 ) 9 ] + 9NaCl + 15HCl (2 2)

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41 It is important to mention that the above reaction scheme in the absence of azi de gives a known Mn 12 complex 60 containing bridging and terminal chlorides. The use of azide promotes preferential binding of azide to high oxidation state Mn ions owing the hard acid hard b ase interaction. When only MeCN was used as the solvent rather than the mi xed solvent, yet another Mn 12 complex was isolated. The reaction of edteH 4 with Mn( ClO 4 ) 2 NEt 4 OH and NaN 3 in a 1:2:1:2 molar ratio in MeCN afforded a reddish brown solution from which was subse quently obtained the dodecanuclear complex [Mn 12 O 4 (OH)(edte) 4 (N 3 ) 9 ](ClO 4 )(N 3 ) ( 2 3 ) in 36% yield (eq 2 3 ) 12Mn(ClO 4 ) 2 + 4edteH 4 + 10NaN 3 + 3O 2 [Mn 12 O 4 (OH)(edte) 4 (N 3 ) 9 ](ClO 4 )(N 3 ) + 10NaClO 4 + 13HClO 4 + H 2 O (2 3) It is clear that the described reactions to complex 2 1 to 2 3 are complicated and involve acid ba se (water deprotonation to O /OH or methanol to OMe ) and redox chemistry (Mn 2+ oxidation) as well as structural rearrangements. It is interesting to note that a slight variation in the identity of the starting materials including Mn salt, base or azide and solvent and their ratio leads to three new Mn 12 complexes. Similarity in the reaction scheme has influenced certain structural resemblance results in Mn/O /edte ratio as 3:1:1 in all three complexes. Description of S tructures Structure of [Mn 12 O 4 (O Me) 2 (edte) 4 (N 3 ) 8 ](ClO 4 )(N 3 ) (2 1) A labeled representation and a stereopair of the cation of 2 1 are shown in Figure 2 1 and selected interatomic distances and angles are listed in Table A 1 Complex 2 1 crystallizes in the tetragonal space group P 42 1 c wi th the Mn 12 molecule lying on an S 4 symmetry axis and thus only one quarter of this is in the asymmetric unit. The core of 2

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42 1 consists of a [Mn 12 ( 4 O) 4 ] 28 + unit (Figure 2 4, A ) where each [Mn 4 ( 4 O) ] 10+ subunit is connected to a neighboring one by sharing a common Mn 3+ ion. For the sake of brevity, reference to specific atoms in the following discussion includes their symmetry related partners. The four 4 O 2 ions (O1) thus serve to connect all twelve Mn atoms. A BVS calcul ation 68 70 for the Mn atoms (Table 2 2 ) identi fied Mn1, Mn2 and Mn3 as Mn 3+ ions. Mn1 and Mn2 are six coordinate while Mn3 is seven coo rdinate. Each edte 4 group is hexadentate chelating on Mn3, with each of its deprotonated alkoxide arms bridging to either one (O2, O3, O5) or two (O4) additional Mn atoms. Thus, the edte 4 groups are overall 2 2 2 3 : 5 bridging, as shown in Figure 2 6 The core of 2 1 is additionally bridged by two Mn1 OMe Mn1 bridges and four end on azide bridges (Mn1 N3 Mn2), as shown in Figure 2 5, A Along with the end on azide bridges, there are four terminal azide groups complete the coordination of Mn2 atom. Cha rge balance consideration requires 12 Mn 3 + 4 O 2 2 OMe 4 edte 4 8 N 3 and two additional negative charges from the counter anions. T he protonation levels of all O atoms were confirmed in 2 1 by BVS calculations, and the results are listed in Table 2 3 The oxide MeO and edte 4 oxygen atoms have BVS values of > 1.87 confirming them as completely deprotonated, as suggested from their bridging modes. Structure of [Mn 1 2 O 4 (OH)(edte) 4 (N 3 ) 9 ] (2 2) A labeled representation and a stereopair of 2 2 are shown in Figure 2 2 and selected interatomic distances and angles are listed in Table A 2 Complex 2 2 crystallizes in the monoclinic space group P 2 1 /n. The core of 2 2 consists of a [Mn 12 ( 4 O) 4 ] 26 + unit (Figure 2 4, B ) where each [Mn 4 ( 4 O)] n+ (n = 10, 9) subunit is connected to a neighboring one by sharing a common Mn 3+ ion. The four 4 O 2 ions (O1) together serve to connect all twelve Mn atoms. BVS calculations for the Mn atoms (Table 2 2 )

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43 identi fied Mn1, Mn3, Mn4, Mn5, Mn6, Mn7, Mn9, Mn10, Mn11 and Mn12 as Mn 3+ ions, and Mn2 and Mn8 as Mn 2+ ions. Mn9, Mn3, Mn6, Mn5, Mn2, Mn8, Mn11 and Mn12 are six coordinate while Mn1, Mn4, Mn7 and Mn10 are seven coordinate. Each edte 4 group is hexaden tate chelating on a Mn 3+ atom with each of its deprotonated alkoxide arms bridging to either one or two additional Mn atoms. Thus, the edte 4 groups are overall 2 2 2 3 : 5 bridging, as shown in Figure 2 6 The core of 2 2 also contains one Mn3 OH18 Mn 9 and one Mn6 N30 Mn12 bridging units, and four end on azid e bridges as shown in Figure 2 5 (B) Along with the end on azide bridges, there are four terminal azide groups complete the coordination of Mn2, Mn5, Mn8 and Mn11 atoms. Charge b alance considerati on and comparison with 2 1 suggests 10 Mn 3 + 2Mn 2+ 4O 2 1OH 4edte 4 and 9N 3 T he protonation levels of all O atoms in 2 2 were confirmed by BVS calculations, and the results are listed in Table 2 4 The oxide and edte 4 O atoms have BVS values of > 1.85 confirming them as completely deprotonated, and BVS value for O18 is 1.01 as expected for OH group in the complex. Structure of [Mn 12 O 4 (OH)(edte) 4 (N 3 ) 9 ](ClO 4 )(N 3 ) ( 2 3) A labeled representation and a stereopair of the cation of 2 3 is shown in Figure 2 3 and selected interatomic distances and angles are li sted in Table A 3 Complex 2 3 crystallizes in the tetragonal space group P 42 1 c with the Mn 12 molecule lying on an S 4 symmetry axis and thus only one quarter of it is in the asymmetric unit. The core of 2 3 consists of a [Mn 12 ( 4 O) 4 ] 28 + unit where each [ Mn 4 ( 4 O) ] 10+ subunit is connected to a neighboring one by sharing a common Mn 3+ ion. (Figure 2 4, C ). For the sake of brevity, reference to specific atoms in the following discussion includes their symmetry related partners. The four 4 O 2 ions (O1) thus serve to connect all twelve Mn atoms. BVS calculations for the Mn atoms ( Table 2 2 ) identi fied Mn1, Mn2 and Mn3 as Mn 3+ ions.

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44 Mn1 and Mn2 are six coordinate while Mn3 is seven coordinate. Each edte 4 group is hexadentate chelating Mn3, with each of its de protonated alkoxide arms bridging to either one (O2, O3, O5) or two (O4) additional Mn atoms. Thus, the edte 4 groups are overall 2 2 2 3 : 5 bridging, as shown in Figure 2 6 There is a disorder between the N6 7 8 and the O6 hydroxyl group. The Mn1 O6 bond length of 2.049(9) is typical of Mn 3+ OH and Mn1 N6 bond length of 2.151(15) is also indicative of the presence of Mn 3+ N moiety in the cluster. 71 73 Due to the symmetry, each fragment is assign ed a site occupation factor of 0.5 Hence, the molecular formula contains of one Mn1 OH Mn1 and one Mn1 N6 Mn1 bridging moieties and four end on azide bridges (Mn 1 N3 Mn2) as shown in Figure 2 5 (C) Along with the end on azide bridges, there are four terminal azide groups complete the coordination of Mn2 atom. Charge ba lance consideration and comparison with 2 1 suggests 12 Mn 3 + 4O 2 1OH 4edte 4 9N 3 and two additional negative charges come from counteranions. T he protonation levels of all O atoms in 2 3 were confirmed by BVS calculations, and the results are listed in Table 2 4 The oxide and edte 4 O atoms have BVS values of > 1 .85 confirming them as completely deprotonated, as concluded above from their bridging modes. All the three Mn 12 complexes possess similar metal topology however; the difference comes from the number of end on azides bridges in the core of the complex. Th ere are many other structural types of Mn 12 complexes known in the literature (Table 2 6 ), the most well explored being the [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] family, which has been extended over the years to include the 1e 2e and 3e reduced versions of [Mn 12 ] family. 11,74 80 Another Mn 12 family was isolated through the reductive aggregation of MnO 4 in MeOH containing media; this family differs from the previous

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45 one in having a central Mn 4+ 4 rhombus rather than a Mn 4+ 4 tetrahedron. 81,82 The rest of the Mn 12 complexes cover a wide variety of other structural motifs, including loops, more complicated face sharing cuboidal units, and wheel s amongst others. 46,83 92 Electrochemistry The scan rates for cyclic voltammetry (CV) and diffe rentia l pulse voltammetry (DPV) were 2 0 500 mV/s and 20 mV/s respectively. Dry DMF solvent was used, and the concentrat ion of the complex 2 1 w as approximately 1 mM and the compound is partially soluble in the electrochemical solvent Extensive electrochemical studies on the 2 1 have revealed a rich redox chemistry involving more than one peak in reduction processes measur ed at 100 mV/s ( Figure 2 7 ) Two peaks at 0.24 V and 0.47 V are partially reversible by the usual electrochemical criteria ( see Appe ndix C ) suggesting that the Mn 12 complexes are stable in multiple oxidation levels. The two reduction processes were mea su red at several scan rates of 2 0 500 mV/s ranges (Fig ure 2 9 ). The two observed partially reversible process es can be assigned as the one electron reduction of Mn 3+ to corresponding Mn 2+ ion. 93 The isolation of a mixed valent Mn II /Mn III complex 2 2 with a similar metal topology suggests the existence of the reduced version of complex 2 1 The observed CV behavior suggests the electron tra nsfer series shown i n equation 2 4 The reported potentials are v s ferrocene (ferrocene couple was measured at + 89 mV under the same conditions). (2 4) The DPV scans (Figure 2 8 ) for 2 1 comple ment the CV measurements. The sharpness of the two peaks at 0.24 and 0.47 V in DPV plot suggests the reversibility of the process. There is another irreversible peak observed at 0.005 V which is likely the

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46 overlap of two oxidation processes. In the latter, a study of the scan rate dependence for the two reduction processes sho wed a linear dependence of the peak current with square root of the scan rates ( 1/2 ) indicating that the reductions are diffusion controlled processes (Figure 2 10 ). Magnetochemistry Dc magnetic susceptibility s tudies of 2 1, 2 2 and 2 3 Solid state, variable temperature dc magnetic susceptibility data in a 0.1 T field and in the 5. 0 300 K range were collected on powdered microcrystalline samples of 2 1 2 2 H 2 2 MeCN and 2 3 MeCN The obtained data are plotted as M T v s. T in Figure 2 11 The M T values of 42.09, 42.44, and 43.19 cm 3 Kmol 1 are little smaller than the spin only v alues ( g = 2) as expected for non interacting Mn 3+ 12 and Mn 3+ 10 Mn 2+ 2 mixed valence situations of 2 1 to 2 3 The behavior is indicative of predominant antiferromagnetic interactions between the metal ions in the molecule For all three complex es M T valu es stay fairly constant up to 100 K and decrea se smoothly down to 20.31, 19.09 a nd 17.42 cm 3 Kmol 1 suggestive of an S = 6 spin ground state for all three of them. To confirm the above initial estimates of the ground state spin of the three compounds, va riable field ( H ) and temperature magnetization ( M ) data were collected in the 0.1 3 T and 1.8 10 K ranges. The resulting data for all three complexes are plotted as reduced magnetization ( B ) v s. H / T where N is Avogadro's number and B is the Bohr magneton shown in Figure 2 12 The data were further fitted using the program MAGNET, 66 by diagonalization of the spin Hamiltonian matrix assuming only the ground state is populated, incorporating axial anisotropy ( z 2 ) and Zeeman terms,

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47 and employing the full powder average. The corresponding spin Hamiltonian is given by the equation 2 5 where z is the easy axis spin operator, g is the Land g factor, and 0 is the vacuum perm eability. The last term in eq. 2 5 is the Zeeman energy associated with the applied magnetic field. z 2 B 0 (2 5) The best fit data can be obtai ned for a certain value of S g and D Hence, the fitting verifies the spin ground state as well as gives an estimation of g and D However, for all the complexes, a ttempts to fit the data (using the procedure as above) resulted in poor quality and unrelia ble fits. The reason could be attributed to the presence of low lying excited states because (i) the excited states are close enough to the ground state and they have a non zero Boltzmann population even at the low temperatures used in the magnetization da ta collection, and/or (ii) even excited states that are more separated from the ground state but have an S value greater than that of the ground state become populated as their larger M S levels rapidly decrease in energy in the applied dc magnetic field an d 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 popu lation of only the ground state. 94 96 A large density of lo w lying excited states is expected for higher nuclearity complexes and the three complexes discussed in this chapter are no exceptions However, fitting the data for complex 2 3 by fixing S = 6 gave a rough estimation of g (1.99) and D ( 0.23 cm 1 ) values. The obtained g value is quite reasonable for a system containing Mn 3+ ions. One way to avoid the complications and to get a more reliable fitting parameters is to collect the data at lower fields and hence data were collected further in the 0.5 0.8 T an d 1.8 10 K ranges. Due to the

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48 similarity in the magnetic properties as established from the dc magnetic susceptibility studies, the results are mentioned for the complex 2 3 only. The best fit to the data is shown as the solid lines in Figure 2 13 and w as obtained with S = 6 D = 0.27 cm 1 and g = 1.96 Fitting with the positive D value gave an unreasonable g value of 2.15 for a system containing all Mn 3+ ions and is discarded. Alternative fits with S = 7 and S = 5 gave g = 1.74 and g = 2.39, respectiv ely. In order to ensure that the true global minimum was obtained and to assess the hardness of the fit, a root mean square D vs g error surface for the fit was generated using the program GRID 97 which calculates the relative difference between the experimental M/N B data and those calculated for various combinations of D and g This is shown as 2 D contour plot in Figure 2 14 Three local minima are observed The one at D = 0.35 cm 1 exhibits the largest error and is hence discarded. T wo global minima were obtaine d for D = 0.2 5 cm 1 g = 1.95 and D = 0.3 0 cm 1 g = 1.98 The minima of the other parameter set is shallow, and the fit uncertainties are thus estimated as D = 0.2 7(2) cm 1 and g = 1.96 (2). Alternating current (ac) magnetic susceptibility studies for 2 1, 2 2 and 2 3 In an ac magnetic susceptibility experiment, a weak field of 3.5 G, oscillating at a particular frequency ( ) is applied to a sample to probe the magnetization relaxation dynamics. At a higher temperature (above T b ) the magnetization relaxation vector can keep in phase with the oscillating ac field and the magnetic moment is same as dc susceptibility. The data is plotted as a real part of the ac susceptibility signal which is M However, at low enough temperature (below T b ) the thermal energy is lower compared to the barrier of magnetization relaxation and the molecule cannot keep in phase with the oscillating ac field. As a result the molecule exhibit s an out of phase ac

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49 susceptibility signal ( M ), imaginary part of the real ac signal and the rate at which the magnetization of a molecule relaxes is related to the operating frequency of the ac field. Ac study also plays a pivotal role in assessing the spin ground state of a molecule without the interference of a dc field and these signals for complexes 2 1 2 2 H 2 2 MeCN and 2 3 MeCN at the range of 5 1500 Hz freq uencies are plotted as M 'T vs T in Figure 2 15, 2 16 and 2 17 respectively at the te mperature spanning over 1.8 10 K range Extrapolation of the plots to 0 K, from temperatures above ~5 K to avoid the effect of weak intermolecular interactions (dipolar and supe rexchange), gives values of ~ 19.5 ~1 6.2 and ~17.0 cm 3 K m ol 1 confirming the presence of an S = 6 spin ground state for all three complexes. The slopes of all the plots decrease rapidly with a decrease in the temperature, revealing the presence of several spin states of larger S values lyin g very close to the ground state. Frequency dependent in phase ac signals are observed below 2.5 K which is concomitant with appearance of ac out of phase M signal s as shown in Figure 2 15, 2 16 and 2 17 However, only tail of the peaks are observed; i.e ., the peak maxima lie below 1.8 K minimum operating temperature of the SQUID magnetometer This behavior is suggestive of the superparamagnet like properties of an SMM and single crystal hysteresis studies were further performed. Magnetization hysteresis s tudies below 1.8 K for 2 1 The ac studies strongly suggest an SMM behavior for all three complexes. However, due to the similarity in the dc and ac magnetic susceptibility measurements, the magnetization hysteresis studies were performed for the complex 2 1 only as a rep resentative member of the family SMM behavior was confirmed by the appearance of hysteresis loops in magnetization vs. dc field scans, measured on a single crystal of

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50 2 1 using the micro SQUID apparatus. The applied field was aligned parallel to the easy axis ( z axis) of the molecules using the recently reported method. 67 The temperature dependence at 0. 14 T s 1 and the scan rate dependence at 0.0 4 K of the hysteresis loops are shown in Figure 2 18 The coercivities clearly increase with decreasing temperature and increasing scan rate, as expected for the superparamagnet like behavior of SMMs The d ata thus confirm that complex 2 1 is a new addition to the family of SMMs 42,98 with a blocking temperature ( T B ) of 1.0 K. However, the hysteresis loops do not show steps characteristic of the quantum tunneling of magnetization (QTM); this behavior is typical for large SMMs, which are more susceptible to step broadening effects associated with low lying excited states, intermolecular interactions, and distributions of local environments due to ligand and solvent dis order 99,100 101 For 2 1 the crystal structure shows the presence of disordered solvate molecules in the gaps betwee n molecules and the magnetism data indicates the presence of low lying excited states in the molecule. Hence, step broadening effect can be rationalized. Concluding Remarks The use of edteH 4 and NaN 3 in the manganese cluster chemistry resulted in a new fam ily of Mn 12 complexes with spin ground state of S = 6. After isolating three initial Mn x complexes of the nuclearity 8, 12 and 20 with edteH 4 this Mn 12 family of SMMs adds a new dimension in the study of alkoxide containing polymetallic Mn cluster chemist ry. The initial findings bring hope to further scrutinize the system to gain more access to various other nuclearity Mn metal clusters with possible new SMMs. The initial use of azide in this synthetic scheme brings successful incorporation of end on azide s, however, dominant antiferromagnetic interactions are still present in these Mn 12

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51 clusters. In the following chapters more use of azide will be studied in the edteH 4 chemistry to tune the magnetic exchange interactions among the Mn ions. Nevertheless, al l the complexes show significant barrier of magnetization and display SMM behavior (in phase and out of phase ac magnetic susceptibility signals for all three complexes and hysteresis loops in scans of magnetization v s. dc field for complex 2 1 ). Given thi s success, the use of edteH 4 ligands in Mn cluster chemistry will continue to be investigated.

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52 Table 2 1. Crystallographic data for 2 1 2 2 and 2 3 parameter 2 1 MeCN 2 2 2 3 MeCN formula C 44 H 89 ClMn 12 N 36 O 26 C 4 0 H 81 Mn 12 N 3 5 O 21 C 42 H 84 ClMn 12 N 39 O 2 5 Fw, g mol 1 2233.24 2047.68 2230.21 crystal system tetragonal monoclinic tetragonal space group P 421c P21/n P 421c a, 14.2051(6) 11.3888(8) 14.1908(8) b, 14.2051(6) 27.1349(19) 14.1908(8) c, 22.2065(16) 26.1080(18) 22.114(2) 90, 90, 90 90, 91.681(2), 90 90, 90, 90 V, 3 4480.9(4) 8064.8(10) 4453.2(6) Z 2 4 2 T, K 173(2) 173(2) 173(2) radiation, a 0.71073 0.71073 0.71073 3 1.655 1.788 1.663 1 1.742 1.897 1.753 R1 b,c 0.0506 0.0554 0.0568 wR2 d 0.1597 0.1244 0 .1647 a Graphite monochromator. b I I ). c R F o | | F c F o |. d wR w ( F o 2 F c 2 ) 2 w ( F o 2 ) 2 ]] 1/2 w 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O) + 2F c 2 ]/3

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53 Table 2 2 Bond valence sums for the Mn atoms of com plex 2 1 2 2 and 2 3 a Atom 2 1 2 2 2 3 Mn II Mn III Mn IV Mn II Mn III Mn IV Mn II Mn III Mn IV Mn1 3.38 3.14 3.07 3.18 2.96 2.89 3.37 3.13 3.07 Mn2 2.86 2.70 2.63 2.24 2.10 2.05 2.89 2.72 2.65 Mn3 3.15 2.93 2.87 3.44 3.19 3.12 3.18 2.97 2.90 Mn4 3.23 3. 01 2.94 Mn5 3.05 2.87 2.79 Mn6 3.50 3.27 3.19 Mn7 3.11 2.89 2.83 Mn8 2.19 2.05 1.99 Mn9 3.47 3.22 3.15 Mn10 3.23 3.01 2.94 Mn11 2.84 2.67 2.59 MN12 3.65 3.41 3.33 a The underlined value is the one closest to the charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value Table 2 3 Bond valence sums for the O atoms of complex 2 1 a Atom BVS Assignment Group O1 1.89 O O O2 1.93 OR edte O3 1.97 OR edte O4 1.87 OR edte O5 2.07 OR edte O6 2.40 OR OMe a The BVS values for O atoms of O 2 OH and H 2 O groups are typically 1.8 2.0, 1.0 1.2 and 0.2 0.4, respectively, but can be affected somewhat by hydr ogen bonding. Table 2 4 Bond valence sums for the O atoms of complex 2 2 a Atom BVS Assignment Group O1 1.98 OR edte O2 1.85 OR edte O3 1.97 OR edte O4 1.92 OR edte O17 1.91 O O O18 1.01 OH OH a See T able 2 3

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54 Table 2 5 Bond valence sums for the O atoms of complex 2 3 a Atom BVS Assignment Group O1 1.89 O O O2 1.92 OR edte O3 1.99 OR edte O4 1.85 OR edte O5 2.16 OR edte O6 0.87 OH OH a See T able 2 3 Table 2 6. Structural types and ground s tate S v alues for known Mn 12 c lusters Complex a,b S Core Ref [Mn 12 O 12 (O 2 CMe) 16 (H 2 O ) 4 10 Mn III 8 Mn IV 4 16 Mn 12 O 12 (O 2 CCH 3 ) 16 (CH 3 OH) 4 10 Mn III 8 Mn IV 4 82 [Mn 12 O 12 (O 2 CPh) 16 (H 2 O ) 4 9 Mn III 8 Mn IV 4 16 [Mn l2 O 12 (O 2 CEt) 16 (H 2 O) 3 9 Mn III 8 Mn IV 4 74 [ Mn 12 O 12 ( O 2 CC 6 H 4 p Me ) 16 ( H 2 O ) 4 ] 10 Mn III 8 Mn IV 4 102 [Mn 12 O 12 (O 2 CCHCl 2 ) 16 (H 2 O) 4 ] 10 Mn III 8 Mn IV 4 103 [Mn 12 O 12 (O 2 CCHCl 2 ) 8 (O 2 CCH 2 Bu t ) 8 (H 2 O) 3 ] 10 Mn III 8 Mn IV 4 103 [Mn 12 O 1 2 (O 2 CCHCl 2 ) 8 (O 2 CEt) 8 (H 2 O) 3 ] 10 Mn III 8 Mn IV 4 103 [Mn 12 O 12 (O 2 CCH 2 Bu t ) 16 (H 2 O) 4 ] 10 Mn III 8 Mn IV 4 104 Mn 12 O 12 (O 2 CCH 2 Bu t ) 16 (CH 3 OH) 4 10 Mn III 8 Mn IV 4 82 [Mn 12 O 12 (O 2 CCH 2 Bu t ) 16 (Bu t OH)(H 2 O) 3 ] 10 Mn III 8 Mn IV 4 105 [Mn 12 O 12 (O 2 CCH 2 Bu t ) 16 (C 5 H 11 OH) 4 ] 10 Mn III 8 Mn IV 4 105 [Mn 12 O 12 (O 2 CC 6 F 5 ) 16 (H 2 O) 4 ] 10 Mn III 8 Mn IV 4 76 Mn 12 O 12 (CH 2 BrCOO) 16 (H 2 O) 4 10 Mn III 8 Mn IV 4 106 (P Ph 4 )[ Mn 12 O 12 ( O 2 CEt) 16 ( H 2 O) 4 ] 19/2 Mn II Mn III 7 Mn IV 4 74 (NPr n 4 )[Mn 12 O 12 (O 2 CPh) 16 (H 2 O) 4 ] nr nr 74 (P Ph 4 )[Mn 12 O 12 ( O 2 CPh) 1 6 ( H 2 0)4] nr nr 74 (Ph 3 P) 2 N[Mn 12 O 12 ( O 2 CPh) 16 ( H 2 O) 4 ] nr nr 74 (PPh 4 )[Mn 12 O 12 (O 2 CCHCl 2 ) 16 (H 2 O) 4 ] 19/2 Mn II Mn III 7 Mn IV 4 75 (PPh 4 ) 2 [Mn 12 O 12 (O 2 CCHCl 2 ) 16 (H 2 O) 4 ] 10 MnII2MnIII6MnIV4 75 (NMe 4 )[Mn 12 O 12 (O 2 CC 6 F 5 ) 16 (H 2 O) 4 ] 19/2 Mn II Mn III 7 Mn IV 4 76 (NMe 4 ) 2 [Mn 12 O 12 (O 2 CC 6 F 5 ) 16 (H 2 O) 4 ] 10 Mn II 2 Mn III 6 Mn IV 4 76 (NPr n 4 )[Mn 12 O 12 (O 2 CCHCl 2 ) 16 (H 2 O )4 ] 19/2 Mn II Mn III 7 Mn IV 4 107 (NPr n 4 ) 2 [Mn 12 O 12 (O 2 CCHCl 2 ) 16 (H 2 O) 4 ] 10 Mn II 2 Mn III 6 Mn IV 4 107 (NPr n 4 ) 3 [Mn 12 O 12 (O 2 CCHCl 2 ) 16 (H 2 O) 4 ] 17/2 Mn II 3 Mn III 5 Mn IV 4 107 [Mn 12 O 12 (Z) 16 (H 2 O) 4 ][PF 6 ] 16 10 nr 108 [Fe(C 5 Me 5 ) 2 ] n [Mn 12 O 12 (O 2 CC 6 F 5 ) 16 (H 2 O) 4 ] 21/2 Mn II Mn III 7 Mn IV 4 109 [Mn 12 O 12 (bet) 16 (EtOH) 4 ] [PF 6 ] 14 11 Mn II 4 Mn III 4 Mn IV 4 79 [Mn 12 O 12 (bet) 16 (EtOH) 3 (H 2 O)] [PF 6 ] 13 [OH] 11 Mn II 4 Mn III 4 Mn IV 4 110 (NBu n 4 ) 2 [Mn 12 O 12 (O 2 C Ph) 16 (OMe) 2 (H 2 O) 2 ] 6 Mn III 8 Mn IV 4 81 [Mn 12 O 10 (OMe) 3 (OH)(O 2 CC 6 H 3 F 2 ) 16 (MeOH) 2 ] 5 Mn III 8 Mn IV 4 82 [Mn 12 O 10 (OMe) 4 (O 2 CBu t ) 16 (MeOH) 2 ] 9 Mn III 8 Mn IV 4 82 [Mn 12 O 4 (OH) 2 (PhCOO) 12 (thme) 4 (py) 2 ] 7 Mn II 2 Mn III 10 83 [Mn 12 O 2 ( OMe) 2 (Hpeol) 4 (O 2 CPh 2 ) 10 (H 2 O) 2 ] 3 Mn III 4 Mn II 8 84

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55 [Mn 12 O 2 (OMe) 2 (thme) 4 (OAc) 10 (H 2 O) 4 ] 3 Mn II 4 Mn III 4 Mn II 4 85 [Mn 12 (Adea) 8 (CH 3 COO) 14 ] 7 Mn I I 6 Mn III 6 91 [Mn 12 (dmbshz) 12 (EtOH) 6 ] 2 Mn III 12 92 [Mn 12 (O 2 CMe) 14 (mda) 8 ] 7 Mn II 6 Mn III 6 46 [Mn 12 O 4 (OH) 2 (edte) 4 Cl 6 (H 2 O) 2 ] 7 Mn II 4 Mn III 8 60 [Mn 12 O 8 Cl 4 (O 2 CPh) 8 (hmp) 6 ] 7 Mn II 2 Mn III 10 86 [Mn 12 O 8 Cl 4 (O 2 CPh) 8 (hep) 6 ] 0 Mn II 2 Mn III 10 87 [Mn 12 O 8 (O 2 CMe) 6 (O 3 PC 6 H 9 ) 7 (bipy) 3 ] 2 Mn III 12 89 [Mn 1 2 O 6 (O H) 4 ( OCH 3 ) 2 (pko) 12 ](OH)(ClO 4 ) 3 nr Mn II 4 Mn III 6 Mn IV 2 90 [Mn 12 O 7 (OH)(OMe) 2 (O 2 CPh) 12 (dmhmp) 4 (H 2 O)] 13/2 Mn II 3 Mn III 9 111 a Solvate molecules are omitted. b Abbreviations: nr = not reported, ZHPF 6 = (4 carboxybenzyl)tributylammonium hexafluorophosphate bet = betain e, mdaH 2 = N methyldiethanolami ne hmpH = 2 hydroxymethylpyridine H 4 peol = pentaerythritol hepH = 2 hydroxy ethylpyridine H 3 dmbshz = 2,6 dimethoxybenzoylsalicylhydrazide Hpko = di pyridyl ketone oxime, adeaH 2 = N allyl diethanolamine H 3 thme = 1,1,1 tris(hydroxymethyl) methane bipy = bipyridine, dmhmpH = 2 (Pyridine 2 yl)propan 2 ol edteH 4 = N,N,N',N' Tetrakis(2 hydroxyethyl)ethylenediamine

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56 A B Figure 2 1 Labeled representation (A) and a stereopair (B) of complex 2 1 H ydrogen atoms have been omitted for clarit y. Color code: Mn 3+ purple; O, red; N, blue; C, light grey.

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57 A B Figure 2 2 Labeled representation (A) and a stereopair (B) of complex 2 2 H ydrogen atoms have been omitted for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue; C, light grey.

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58 A B Figure 2 3 Labeled representation (A) and a stereopair (B) of complex 2 3 H ydrogen atoms have been omitted for clarity. Color code: Mn 3+ purple; O, red; N, blue; C, light grey.

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59 A B C Figure 2 4 T he core of 2 1 (A ) 2 2 (B ), 2 3 (C ) viewed along the c axis. Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue; C, light grey.

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60 Figure 2 5 The core of 2 1 (A ) 2 2 (B ) and 2 3 (C ) viewed along the b axis Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue; C, light grey 2 2 2 3 : 5 Figure 2 6 C oordination mode of edte 4 found in complexes 2 1 to 2 3

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61 Figure 2 7. A full CV diagram of complex 2 1 at 100 mV/s in DMF containing 0.1 M NBu n 4 PF 6 as the supporting electrolyte. The indic ated potentials are given vs. f errocene as an internal standard. Figure 2 8. A full DPV diagram for complex 2 1 at 20 mV/s The indic ated potentials are given vs. ferrocene as an internal standard

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62 Figure 2 9 The CV diagram at the indicated scan rates for the reduction wave of 2 1 The indic ated potentials are given vs. ferrocene as an internal standard Figure 2 10. Plots v s. square root of scan rate ( 1/2 ) for 0.24 V (A) and 0.47 V (B) reduction wave for complex 2 1

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63 Figure 2 11 Plot of M T vs T for complexes 2 1 ( ) 2 2 6 H 2 2 MeCN ( ) and 2 3 MeCN ( Figure 2 12. P lots of reduced magnetization ( B ) vs. H/T for complex es 2 1 (A), 2 2 (B) and 2 3 (C) at applied fields of 0.1 3.0 T and in the 1.8 10 K temperature range.

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64 Figure 2 13 P lot of reduced magnetization ( B ) vs. H/T for complex 2 3 at applied fields of 0.5 0.8 T and in the 1.8 10 K temperature range. The solid lines are the fit of the data; see the text for the fit parameters. Figure 2 14. Two dimensional contour plot for the rms error surface vs. D and g for the reduced magn etization fit for complex 2 3

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65 Figure 2 15 Plots of in phase M (as M T ) vs. T and out of phase M vs T ac signals for complex 2 1 at the indicated frequencies.

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66 Figure 2 16 Plots of in phase M (as M T ) vs. T and out of phase M vs T ac signals for complex 2 2 at the indicated frequencies.

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67 Figu re 2 17 Plots of in phase M (as M T ) vs. T and out of phase M vs T ac signals for complex 2 3 at the indicated frequencies.

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68 A B Figure 2 18 Magnetization (M) vs. dc field scans for a single crystal of complex 2 1 at the indicated field sweep rates (A ) and temperatures (B ). The magnetization is normalized to its saturation value, M S

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69 CHAPTER 3 EXPERIMENTAL AND C OMPUTATIONAL STUDIES OF A NEW M n 9 SINGLE MOLECULE MAGNET WITH A HALF INTEGER SPIN OF S = 21/2 Single molecule magnets (SMMs) co ntinue to attract a lot of attention for their potential applications to new technologies such as molecule based information storage, molecular spintronics, and quantum information processing. 11,25,30 SMMs represen t a molecular approach to nanoscale magnetism, where each individual molecule possesses a significant barrier ( vs. k T ) to magnetization relaxation, and thus exhibits properties such as magnetic hysteresis below a blocking temperature, T B 12 The SMM behavior results from a combination of a high ground state spin ( S ) value and an easy axis type magnetoanisotropy (negative zero field parameter D ), where the latter is the more important parameter for enhancing the SMM property. 112,113 SMMs also straddle the classical/quantum interface by displaying not only classical magnetic hysteresis but also quantum phase interference 114 and quantum tunneling of magnetization (QTM), 115 117 which makes SMMs potential candidates for use as quantum bits (qubits) in quantum computation. 28,30 Manganese has been the transition metal of choice to date for SMM s due to its common propensity to yield molecules with large S values, and significant anisotropy from the presence of Jahn Teller distorted Mn 3+ ions. 33 Such large S values in Mn x clusters can result from ferro or ferrimagnetic spin alignments and/or competing interactions (spin frustration) in certain M x topologies that prevent (frustrate) the preferred spin alignments. Ferromagnetic interactions can result from ligands or st ructural characteristics known to give ferromagnetic coupling between metal centers; for example, end on azide bridges are known to promote ferromagnetic coupling between bridging metal ions. 59 In addition, to facilitate the formation of polynuclear metal clusters, many researchers have employed alkoxide based ligands, since this

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70 functionally is an excellent bridging gr oup that fosters formation of higher nuclearity products. 47 52,118 These synthetic strategies have proved extremely useful in expanding the database of known Mn high spin molecules and/or SMMs in different directio ns, including large size (metal nuclearity), very high spin values, and relatively large ( vs. k T ) anisotropy barriers. 10,98 In the present work, we have employed the potentially N,N,O,O,O,O hexadentate chelate N ,N,N',N' tetrakis(2 hydroxyethyl)ethylenediamine (edteH 4 ) in Mn reactions in the presence of carboxylate s and azide as co ligands. The edteH 4 chelate has been little explored to date in the literature, having been employed in the preparation of mononuclear Ca and dinuclear Ba, Cu and V complexes. 61,62,119 More recently it began to be investigated in Mn cluster chemistry, and the compounds obtained with it so far have comprised Mn 8 Mn 12 and Mn 20 clusters with new an d interesting metal topologies. 60 We also reported recently its use in Fe cluster chemistry, where it again proved to be a source of interesting new compounds of various Fe x (x = 5, 6, 12) nuclearities. 54 We thus concluded we had merely scratched the surface of what it could deliver in polynuclear metal chemistry, believing it had the potential to pro vide further access to a variety of new structural types with important properties. Herein, we report a new and particularly interesting Mn 9 SMM obtained from edteH 4 that represents a very rare example of a half integer, high spin SMM, and the results of i ts detailed study with a wide range of physical (magnetic, electrochemical), spectroscopic (inelastic neutron scattering), and computational (DFT) methods.

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71 Experimental Section Synthesis All preparations were performed under aerobic conditions using reage nts and solvents as received. Mn(O 2 CBu t ) 2 was prepared as reported in the literature. 120 Caution! Although no such behavior was observed during the present work, azide salts are potentially expl osive; such compounds should be synthesized and used in small quantities, and treated with utmost care at all times. [Mn 9 O 3 (OMe)(O 2 CBu t ) 7 (edte)(edteH) 2 (N 3 ) 2 ] (3 1). To a stirred solution of edteH 4 (0.20 g, 0.84 mmol) and NEt 3 (0.25 mL, 1.68 mmol) in MeCN/M eOH (20/1, v/v) was added Mn(O 2 CBu t ) 2 (0.44 g, 1.68 mmol). The resulting dark brown solution was stirred for 15 minutes under mild heating (~60C) to dissolve all solids, and then NaN 3 (0.12 g, 1.84 mmol) was added. The solution was stirred for a further 2 hours, filtered, and the filtrate was allowed to slowly evaporate undisturbed at ambient temperature. X ray quality, dark brown, plate like crystals of 3 1 crystallographic sample was maintained in contact with mothe r liquor to prevent damage from exposure to the atmosphere. Otherwise, the crystals were collected by filtration, washed with Et 2 O, and dried in vacuum. The yield was 58%. Anal. Calc (Found) for 3 1 (solvent free): C, 38.40 (38.73); H, 6.25 (6.12); N, 8.14 (7.93) %. Selected IR data (cm 1 ): 3427 (br), 2957 (s), 2869 (s), 2056 (vs), 1587 (vs), 1482 (m), 1457 (w), 1361 (m), 1289 (w), 1225 (m), 1075 (s), 904 (m), 662 (s), 606 (s), 512 (m), 436 (m). X ray Crystallography Data were collected for 3 1 0 K on a Bruker DUO system equipped

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72 = 0.71073 ). Cell parameters were refined using 9999 reflections. A hemisphere of scan method (0.5 f rame width). Absorption corrections by integration were applied based on measured indexed crystal face s. The structure was solved by direct m ethods in SHELXTL6, and refined on F 2 using full matrix least squares. The non H atoms were treated anisotropically whereas H atoms were placed in calculated positions and refined as riding on their respective C atoms. Crystallographic parameters are listed in Table 3 1. The asymmetric unit consists of two chemically equivalent but crystallographically independent Mn 9 clusters, and eight MeCN molecules. The latter were too disordered to be modeled properly, and program SQUEEZE, 64 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 cluster has both of its hydroxyl groups disordered, and they were refined in two parts each. The Me groups of t BuCO 2 on C12 and C42 were also disordered and refined in two parts each. The second cluster has a similar hydroxyl group disorder, but no Me group disorder. For each case of disorder, site occupancy factors were dependently refined. A total of 2103 parameters were 1 and wR 2 of 4.74 and 12.45 %, respectively. Physical Measurements Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 400 4000 cm 1 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 in dry N, N dimethylformamide (DMF) under argon using a BAS model CV 50W voltammetric analyzer and a standard

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73 three electrode assembly (glassy carbon working, Pt wire auxiliary, and Ag wire reference) with 0.1 M NBu n 4 PF 6 as supporting electrolyte. The concentration of 3 1 was ~1 mM. The scan rates for cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were 50 500 mV/s and 20 mV/s, respectively. Quoted potentials are vs. the ferrocene/ferrocenium couple, used as an internal standard. 65 Solid state, variable temperature dc and ac magnetic suscepti bility data were collected on powdered microcrystalline samples of 3 1 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 eicosan e to prevent torquing. Magnetization vs. field and temperature data were fitted using the program MAGNET. 66 Pascal's constants were used to estimate the diamagnetic correction, which was subtracted from the experimental susceptibility to give the mola r paramagnetic susceptibility ( M ). Low temperature (<1.8 K) hysteresis loop and dc relaxation measurements were performed in Grenoble using an array of micr o SQUIDs. 67 The field can be applied in any direction by separately driving three orthogonal coils. The applied field was aligned parallel to the easy axis (z axis) of the molecules using a published method. 121 Inelastic neutron scattering (INS) experiments were done at the direct time of flight instrument IN5 at the ILL, Grenoble. The powdered, microcrystalline, Mn 9 sample was placed in a doubl e walled hollow aluminum cylinder (diameter 21 mm, 2 mm cartridge); the sample weight was 2.01 g. The measurements were done at wavelengths of 3.2, 5.0, 6.5, 7.0, 7.5, and 8.0 for temperatures in the 1.5 to 40 K range. Only the 3.2, 6.5 and 7.5 data ar e presented here; the resolutions at the elastic

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74 vanadium standard. The low angle part of the spectrum is neglected in the analysis, because of the significant instrumenta l features near the elastic line. Computational Details All the reported calculations were performed with the Quantum ESPRESSO 4.1 program, 122 using PBE exchange correlation functional, Vanderb ilt ultrasoft pseudopotentials, 123 and a plane wave basis set at the University of Central Florida. The energy cutoffs for the wave functions and charge densities were set at 25 and 250 Ry, respectively. To ensure total energy convergence, all calculations used the spin polarized approach with the Marzari Vanderbilt 124 cold smearing (smearing factor 0.0008), and local Thomas Fermi mixing mode to improve SCF convergence. To calculate the Heisenberg exchange constants, the FM state (multiplicity 38) with all Mn spins parall el, was first optimized. In order to obtain desirable spin states, various different spin alignments were considered and verified with Lwdin spin densities after SCF convergence. The optimization was previously found to be important for the accuracy of th e final results, presumably due to inaccuracies introduced in some of X ray structures by partial disorder. In the application of the DFT+U method, we simplified rotationally invariant formulation, implemented by Cococcioni. 125 The following values of the U parameter were used: 2.10 eV for Mn, 1.00 eV for O and 0.20 eV as previous ly suggested. 126 Results and Discussion Syntheses The reaction of edteH 4 with Mn(O 2 CBu t ) 2 NEt 3 and NaN 3 in a 1:2:2:2.2 molar ratio in MeCN/MeOH (20/1, v/v) afforded a reddish brown solution from which [Mn 9 O 3 (OMe)(O 2 CBu t ) 7 (edte)( edteH) 2 (N 3 ) 2 ] ( 3 1 ; Mn 2+ 8Mn 3+ ) was obtained in 58%

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75 isolated yield. The same produc t was also obtained by using a 1:3 ligand to metal ratio ; however, the best yield was obtained from the above ratio. The little excess of t BuCO 2 helps to facilitate the deprotonation of the edteH 4 ligand along with NEt 3 The formation of 3 1 is summarized in equation 3 1, where atmospheric O 2 gas is assumed to provide the oxidizing equivalents to generate Mn 3+ from Mn 2+ and the equation is 9Mn(O 2 CBu t ) 2 + 3edteH 4 + 2N 3 + 2O 2 [Mn 9 O 3 (OMe)(O 2 CBut) 7 (edte)(edteH) 2 (N 3 ) 2 ] + 11 t BuCO 2 + 9H + + H 2 O (3 1) balanced accordingly. The specified combination of mixed solvent MeCN/MeOH is crucial to obtain single crystals of 3 1 Similar microcrystallin e products were obtained by changing the second solvent from MeOH to EtOH, DMF, tetrahydrofuran, water or dichloroethane, but the crystal quality was poor. The described procedure in the Experimental Section is the optimized one resulting from an investiga tion of how various reagent ratios and solvent compositions affected the yield and purity of the obtained complex 3 1 Description of the S tructure [Mn 9 O 3 (OMe)(O 2 CBu t ) 7 (edte)(edteH) 2 (N 3 ) 2 ] A partially labeled representation (A) and a stereopair (B) of 3 1 are shown in Figure 3 1, and selected inter atomic distances and angles are summarized in Table A 4. Complex 3 1 crystallizes in the monoclinic space group P2 1 /n. The compound has apparent C 2v symmetry and h ence, it can be considered as if two Mn 5 subunits are sharing a common Mn 3 3 OMe) unit (Figure 3 2, A ). Each [Mn 5 3 3 OR) 4 ] subunit is comprised of a triangular Mn 3 3 O) and a butterfly unit of Mn 4 3 OR) 2 fused together (highlighted by solid lines in Figure 3 2, A ). In other words, the core is comprised of a series of fused triangular subunits to give a molecular ladder (Figure 3 2, B ). BVS calculations 68 70 for the Mn atoms (Table 3 2) identified Mn5 as Mn 2+ and the

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76 rest are Mn 3+ ions. Mn1, Mn3, Mn4, Mn6, Mn7, Mn9 are six c oordinate while Mn2, Mn5 and Mn8 are seven coordinate. There are two types of ligand bridging modes present in the cluster, singly deprotonated edteH and completely deprotonated edte group (Figure 3 3). The edte group is hexadentate chelating on Mn5 atom, with each of its deprotonated alkoxide arms bridging to either one (O20, O22) or two (O19, O21) additional Mn atoms. Thus, the edte group is 2 2 3 3 5 bridging, as shown in Figure 3 3. The other two edteH 3 groups are pentadentate chelating on Mn2 and Mn8 atoms, with each of its deprotonated alkoxide arms (O15, O17, O18) bridging to one additional Mn atom. Hence, these two edte H 3 2 2 3 4 bridging, as shown in Figure 3 3. The peripheral coordination on all Mn 3+ ions are 1 1 bridging t BuCO 2 groups. Charge balance consideration requires 8Mn 3+ Mn 2+ 3O OMe 7 t BuCO 2 edte edteH 3 and 2N 3 The protonation levels of all O atoms in 3 1 have been determined by BVS calculations, and the results are listed in Table 3 3. The oxides, MeO and edte O atoms have BVS values of >1.85, confirming them as completely deprotonated, as conclud ed above from their bridging modes. There are six significant parallel JT elongations axes (Figure 3 1) present for six, near octahedral Mn 3+ (Mn1, Mn3, Mn4, Mn6, Mn7, Mn9) ions. The elongated Mn 3+ O bonds are ~0.2 0.4 longer than the other Mn 3+ O bond s and are being considered as the primary source of anisotropy in the molecule. There are only a few examples of Mn 9 complexes in the literature of structural motifs including molecular cage, and molecular rod. 127 1 32 However, complex 3 1 is structurally unprecedented in the Mn 9 literature.

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77 Magnetochemistry Dc magnetic susceptibility s tudies of 3 1 The variable temperature dc magnetic susceptibility data on compound 3 1 in a 0.1 T field are shown as a M T vs. T plot in Figure 3 4. The M T value at 300 K of 25.92 cm 3 Kmol 1 is slightly smaller than the spin only ( g = 2) value expected for eight non interacting Mn 3+ ions and one Mn 2+ ion With decreasing temperature M T stays fairly constant to ca. 70 K, then increases markedly to a maximum value of 59.41 cm 3 Kmol 1 at ca. 10 K, and then decreases slightly to 58.33 cm 3 Kmol 1 at 5 K, supporting an S = 21/2 spin gro und state. The ove rall behavior of M T is indicative of predominant ferromagnetic interactions between the met al ions in the molecule The slight drop in M T at the lowest temperatures indicates the presence of zero field splitting and/or intermolecular interactions. To confirm the above estimates of the ground state spin of 3 1, variable field (H) and temperatur e magnetization ( M ) data were collected in the 0.1 7 T and 1.8 10 K ranges. The resulting data are plotted as reduced magnetization ( B ) vs. H/T, where N is Avogadro's number and B is the Bohr magneton. The data were fit using the program MAGNET, 66 which is based on diagonalizing the giant spin Hamiltonian given by equation 3 2, which can be assumed to be valid at low temperatures where only the ground state is thermally populated, and employs a full powder average. z 2 B 0 (3 2) The best fit to the data is shown as the solid lines in Figure 3 5, A and was obtained with S = 21/2, D = 0.37 cm 1 and g = 2.03. Fairly good fits were also obtained for D = 0.35 cm 1 and g = 1.81. In order to ensure tha t the true global minimum had

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78 been located and to assess the robustness of the fit, a root mean square D vs. g error surface was g enerated using the program GRID, 97 which calculates the relative differen ce between the experimental B data and those calculated for various combinations of D and g This is shown as 2 D contour plot in Figure 3 5, B Six local minima are observed. The one at D = 0.35 cm 1 exhibits the largest error and is hence discarded. Among the other five local m inima, two g lobal minima were obtained for D = 0.35cm 1 g = 2.00 and D = 0.40 cm 1 g = 2.05, where the latter is again discarded based on the g value. The minima of the other parameter set is shallow, and the fit uncertainties are thus estimated as D = 0.37(2) cm 1 and g = 2.03(2). The reduced magnetization fit also provides an alternative spin ground state of S = 23/2 with D = 0.31 cm 1 and g = 1.85. Further confirmation of the ground state spin comes from the INS and computational studies. Alternati ng current (ac) magnetic susceptibility s tudies of 3 1 A c susceptibilities were measured on microcrystalline sample s of 3 1 in a 3.5 G ac field. Frequency dependent ac susceptibility is an excellent tool to look for slow relaxation of the magnetization vec tor at low temperature which is a characteristic feature of an SM M. In addition, the in phase ( M ) ac susceptibility signal is invaluable for assessing S without any complications from a dc field, and these signals for complexes 3 1 were measured in the 1.8 10 K range at 5 1500 Hz. The data are presented as M 'T vs. T in Figure 3 6. Extrapolat ion of the plots to 0 K, from temperatures above ~5 K to avoid the effect of weak intermolecular interactions (dipolar and superexchange), gives values of ~ 62 cm 3 Kmol 1 for 3 1 which confirms the presence of S = 21/2 spin ground state for g ~ 2.03 At low er temperature, a fr equency

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79 dependent decrease in M 'T and a co ncomitant rise in out of phase M signal were seen (Figure 3 6) which are indicative of a significant barrier to magnetization relaxation. To quantify the effective barrier, construction of an Arrhenius plot is necessary. Ac susceptibility s tudies at several oscillation frequencies are important tools for determining the true or effective energy barrier ( U eff ) to magnetizati on relaxation, because at the M '' peak maxima, the ma gnetization relaxation rate (1/ where is the relaxation time) is equ al to the angular frequency (2 ) of the oscillating ac field the data can be fitted to Arrhenius equation 3 3. Hence, the plot was constructed and shown in Figure 3 7 ( A ) and it gave U eff = 49 K and 0 = 3.09 x 10 s, where 0 is the pre exponen tial factor. The U eff value of 49 K is the highest yet observed for a high nuclearity Mn 2+ / 3+ mixed valent complex, although still significantly smaller than for the Mn 3+ 6 (86 K) 42 and Mn 3+/4+ 12 (74 K) complexes. 105 1/ = (1/ 0 U eff /k T ) (3 3) T he upper limit to the energy barrier ( U ) for a half integer spin system of S = 21/2 and D = 0.37 cm 1 should be (S 2 1/4)| D | which is 59 K and higher than the ef fective energy barrier ( U eff ) determined for 3 1 Thus, U eff is less than U which is as expected due to the presence of QTM. For a single relaxation process, as would be expected in a crystalline ensemble of molecules in identical environment s with identi cal barriers, the and '' behavior as a function of angular frequency ( 2 ) is given by equations (3 4) and (3 5), respectively, where S ( the adiabatic susceptibility, T ( isothermal susceptibility, and is the magnetization relaxation time. For paramagnets obeying the Curie law the isothermal susceptibility corresponds to the dc susceptibility.

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80 (3 4) (3 5) In the case of a distribution of relaxation processes which results fr om a distribution of molecular environments in the crystal and an associated range of U eff barrie r heights, the expressions for and '' are given by eq uations (3 6) and (3 7), where gauges the width of the distribution and has a value between 0 and 1. (3 6) (3 7) When = 0, equations (3 6) and (3 7) reduce to equations ( 3 4) and (3 5), respectively, describing a single relaxation process. Magnetization relaxation data were collected at several temperatures of 2.0 4.2 K on a vacuum dried, microcrystalline sample of 3 1 in a 3.5 G ac field while the oscillating frequenci es were in the range of 0.1 to 1500 Hz. The data in the form of and '' were fitted to see if they followed a single relaxation process or a distribution of single relaxation processes. The data for 3.0 K are presented in Figure 3 8 ( A ) as vs. (2 ) and '' vs. (2 ) The solid lines result from a least squares fitting of the data to a single relaxation process, as described by equation 3 4 and 3 5. Better fits were obtained when the data were fitted for a distribution of single relaxation processes (equation 3 6 and 3 7) and are presented in the same fi gure as dashed lines. A complete list of all the least squares fitting parameters of the data for the single

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81 relaxation process and the distribution of a single relaxation processes are presented in Table 3 4. The a c data are also presented as a vs. '' plot, which is known as Cole Cole or Argand plot, 11,133 as shown in Figure 3 8 ( B ). The symmetrical shape of the plot suggests that a single type of species is present. The solid line represents a least squares fit of the data to a single relaxation process as described by equations (3 4) and (3 5) in Figure 3 8. The dashed line represents a fit to a distribution of relaxation processes as described by equations (3 6) and (3 7), yielding the values 0.32 9(2) ( for ) and 0.237(2) (for '' ), respectively. The latter fit is clearly superior and indicates a small range of molecular environments. The temperature dependence of the distribution of relaxation processes in the range of 2.0 to 4.2 K is shown in Fi gure 3 9 ( A ). Interestingly, increases with d ecreasing temperature causing a wider distribution of relaxation rates at lower temperature (Figure 3 9, B ). Such a distribution of molecular environments in Mn 9 molecule would lead to a distribution of ZFS pa rameter D which affects the potential energy barrier height. A distribution in transverse zero fie l d interactions could also affect the rate of magnetization quantum tunneling. Single crystal hysteresis s tudies f or 3 1 below 1.8K The ac measurements sugge st strongly that 3 1 behaves as an SMM, which was confirmed by the observation of hysteresis loops in magnetization vs. dc field scans, measured on a single crystal of 3 1 SQUID apparatus. 67 The temperature dependence at 0.14 T s 1 and the scan rate dependence at 0.0 4 K of the hysteresis loops are shown in Figure 3 10. The coercivities clearly increase with decreasing temperature and increasing scan rate, as expected for the

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82 superparamagnet like behavior of SMMs. 11,19,59 The d ata thus confirm that complex 3 1 is a new addition to the family of SMMs, with a blocking temperature ( T B ) of 2.5 K. The hysteresis loops also clearly show steps at periodic values of applied field due to quantum tunneling of magnetization (QTM), which ca uses a surge in the relaxation as the M S levels on the opposite sides of the energy barrier in the S = 21/2 double well between the steps is proportional to D and is given b y equation 3 8. D g B (3 8) Measurement of the step positions in Figure 3 f 0.375 T D g B value of 0.17 cm 1 (where D = 0.34 cm 1 for g = 2.0 ), which agrees closely with both the values obtained fro m the fits to the magnetization data and the INS studies (0.343 cm 1 vide infra). It should be noted that 3 1 has a half integer spin ground state and hence, it shows no step at zero applied field. The reason behind the suppression of QTM at zero applied field is the spin parity effect in a half integer spin system. 76 However the omission of QTM at zero applied field is less likely due to the fact that it is impossible to ensure absolutely zero external fields. The dipolar fields of neighboring molecules and the hyperfine fields from the 55 Mn nuclei ( I = 5/2, ~ 100% natural abundance) provide a m eans for QTM to occur at apparently zero applied fields. 11 5 Thus, the expected suppression of QTM for half integer spin at zero external field is actually quite surprising. Magnetization vs. time decay data were collected on a single crystal of 3 1 in order to obtain a more quantitative assessment of the magnetization relaxation

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83 dynamics (Figure 3 10, C direction at ~5 K with a large applied dc field, the temperature was then decreased to a chosen value, and then the field was removed and the magneti zation decay monitored with time. The results are shown in Figure 3 10 ( C ) for the 1.3 2.5 K range. From these measurements the Arrhenius plot was con structed and fitted based on equation 3 3 (Figure 3 7, B ). These data complement the ac out of phase dat a and confirm the effective energy barrier of 49K, which is the highest in a mixed valent Mn 2+/ Mn 3+ system. Electrochemistry E lectrochemical studies on 3 1 in DMF have revealed a rich redox chem istry involving multiple peaks on the reduction side and one p eak in the oxidation side (Figure 3 11, A ). Two reduction peaks are irreversible and the peak in oxidation is reversible. The observed CV behavior suggests the electron transfer series shown in equation 3 9. The reported potentials are vs. ferrocene. (3 9) A study of s can rate dependence for the 0.17 V oxidation process has been performed at 50 500 mV/s scan rate which shows reversibility of the peak. T he cathodic to anodic peak current separation increases with the scan rate (Figure 3 12, A ). In addition, a linear dependence of the peak current with 1/2 ( = scan rate) is shown in Figure 3 12 ( B ) indicating that the oxidation process at the electrode is a diffusion controlled process. The observed reversible process can be assigned as the one electron oxidation of Mn 2+ to corresponding Mn 3+ ion. 52 The irreversible processes ( 0.62 V and 1.18 V) can be attributed to the reduction of Mn 3+ to Mn 2+ The DPV scans

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84 at 20 mV/s for 3 1 are shown in the Figure 3 11 ( B ). The data obtained from DPV measurement complement the CV data and the peak broadness in DPV study justifies the irreversibility of the reduction peak The overall result indicates the possibility of isolating a new Mn 9 complex very similar to 3 1 but with all Mn ions in the +3 oxidation state, possibly improving the anisotropy of the molecule by having seven significant JT elongation axes rather than the six JT axes currently seen. Inelastic Neutron Scattering (INS) In Figure 3 13 the spectra measured with an incoming neutron wavelength of 6.5 are presented. At 1.5 K a strong feature I at ca 0.85 meV and a small feature at ca 0.4 meV, which is marked by an asterisk, are observed. With increasing temperature, featur e I decreases and the corresponding anti Stokes feature I' on the neutron energy gain side appears. Feature I is hence assigned to a cold transition. The feature at 0.4 meV appears to be temperature independent between 1.5 and 5.0 K and is not observed on the energy gain side. It is hence assigned to a spurious feature. With further increasing temperature, hot features ii to v appear between 0.5 and 0.8 meV. In order to observe the peaks with higher resolution, data were recorded with a wavelength of 7.5 and are shown in Figures 3 14. At 1.5 K, feature I at 0.847(1) meV is very intense. Its intensity decreases on the neutron energy loss side and increases on the energy gain side with increasing temperature, as expected for a cold magnetic transition. With increasing temperature more and more features related to hot magnetic transitions evolve at 0.778(1), 0.706(3), 0.625(4), 0.530(8), 0.41(3), and 0.34(7) meV, which are labeled ii to vii. A feature similar to the one observed in the 6.5 data at ca 0.4 meV (marked by the asterisk in Figure 3 13) is not observed in the 7.5 data, which confirms our earlier assignment to a spurion. In Figure 3

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85 spectrum measured at 10.0 K is shown, where Q is the momentum transfer. Features I to vi can clearly be observed, which is impressive and demonstrates the high quality of the recorded data. These peaks all show significant intensity over the whole Q range, in particular at the lower Q values, which unambiguously confirms their magnetic o rigin. 134 136 Figure 3 16 ( A ) presents the INS spectra measured with a wavelength of 3.2 at the temperatures 1.5 and 40 K. The 1.5 K spectrum shows some peak like features while the 40 K spectrum is comparatively featureless. Hence, the scattering intensity due to magnetic transitions is apparently swamped out already at 40 K such that these data reflect the lattice contribution. This affords to use them for estimating the lattice contribution at 1.5 K by applying the Bose factor to the 40 K data. 137 139 The resulting curve is shown as green dots in Figure 3 16 ( A ). The measured 1.5 K data can then (approximately) be corrected for the lattice contributions by subtracting fr om them the Bose scaled curve, which yields the data shown as blue dots. The Bose corrected 1.5 K spectrum shows feature I in the tail of the elastic line. At higher energies several further features are observed: feature A at ca. 2.0 meV, feature VIII at ca. 2.7 meV, feature B at ca. 3.1 meV, feature IX at ca. 3.9 meV, and feature C at ca. 4.6 meV. Figure 3 16 ( B ) correction on the right, respectively. In the whole energy range a large phononic scattering contribution is observed (left panel ), which is well accounted by the Bose corr ection (right panel). It is noted that a large phononic scattering intensity above energies of 2 meV is commonly observed in non deuterated samples of molecular nanomagnets. The Q dependence of features VIII and IX suggests a magnetic origin.

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86 For features A, B, and C the situation is less clear, because of their significantly lower intensity. From carefully comparing the 40 K data, the 1.5 K data, and the Bose corrected 1.5 K data, features A and B seem to appear from an incomplete estimation of the backgro und scattering intensity in the range of 1.5 to 3.5 meV, which is also present at 40 K. This suggests that these features are of phononic origin. Also feature C appears to be of phononic origin. However, while the features I, VIII, and IX are safely assign ed as magnetic, the conclusions as regards A, B, and C have to be taken with care in view of the experimental data. The seven magnetic INS peaks at energies below 1 meV show the typical temperature dependence and picket fence structure observed in cluster s with a zero field split spin ground state, which is well separated from higher multiplets, as it is characteristic for SMMs described in the giant spin Hamiltonian approach. 15,140 From the temperature dependence of these peaks the anisotropy has to be of the easy axis type, which is consistent with the observation of an out of phase magnetization, magnetic hysteresis, and QTM steps. The peaks observed at higher energies correspond then to transitions to higher lyi ng spin multiplets, and their large energy supports the validity of the giant spin Hamiltonian picture. However, in view of the many exchange paths and complicated exchange topology, their interpretation is clearly a challenging task, and is not attempted here. Fitting the energies of the peaks below 1 meV to a rigid spin S = 21/2 model with only a second order easy axis anisotropy ( D term) incorporated did not work well. Hence, higher order terms 141,142 were systematically included and the fit results carefully checked for the statistical significance of the terms. Inclusion of a B 4 0 term provide d a

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87 nearly perfect description of peaks I and ii, but significant deviations for the lower lying peaks remained, hence a B 6 0 term had additionally to be added. The best fit values were D = 0.343(1) cm 1 B 4 0 = 0.03( 1) 10 4 cm 1 and B 6 0 = 0.017(2) 10 6 c m 1 which reproduce all seven peaks very well, as shown by the INS simulations presented in Figure 3 17 ( A ). Some minor discrepancies between experimental and simulated peak positions remain, which in principle could be accounted for, by including further anisotropy terms, however, they could not be modeled properly within statistical significance. The peak intensities are less well simulated at temperatures above 15 K, which indicates the onset of the transitions to and within higher lying spin multiplets swamping out magnetic intensity at 28 K. The derived energy spectrum and observed transitions are shown in Figure 3 17 ( B ). The D value as determined from INS is slightly smaller than that determined from the magnetic data, but within experimental error The calculated height of the energy barrier is 54.5 K, which is close to the calculated height of 54.8 K with neglecting the higher order terms, i.e., these have little effect on the barrier height. Density Functional Theory Using first principle methods of electronic structure theory helps in understanding the intricate details of molecular magneti c systems and may provide comple mentary information on their behavior. The most common theoretical method for prediction of magnetic coupling parameter J is De nsity Functional Theory (DFT). DFT is the only ab initio method capable of describing large molecules, such as Mn 12 Ac, Fe 8 and V 15 and other molecular magnets. 143 148 In this method the antiferromagnetic (AF) stat e is described by the open shell single Slater determinant, where singly occupied magnetic

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88 orbitals are localized on the transition metal atoms (alpha electrons on some, beta electrons on others). Unlike the Slater determinant describing ferromagnetic stat e (FM), The difference in energy between AM and FM states (spin gap) is then related to J The pure DFT is known to artificially over delocalize electrons due to the s elf interaction error. As a result, the band gap and spin gap are strongly underestimated. In order to correct for this error a fraction of Hartree Fock (HF) exchange often replaces local or semi local exchange correlation functional, yielding the so calle d hybrid DFT functional. In the HF method the Slater determinant has the physical meaning of the wavefunction of the system, so that broken symmetry indicates the AF determinant is unphysical and does not correctly predict energy for the low spin system. I n a complete analogy, hybrid DFT approaches imply that AF energy needs corrections. Several correction formulas had been developed by Noodleman, 149 153 Yamaguchi, 154 157 R uiz, 158 Nishino, 159 and others. 160 While accurate in many cases, this approach tends to err in systems where magnetic orbitals are strongly delocalized (for example binuclear complexes with acetate bridges ). 148,161 Another way to alleviate the effects of self interaction error is DFT+U (where the onsite Coulomb repulsion term U is added to the effective Hamiltonian). DFT+U was introduced by Anisimov et al. 162 and simplified by Cococcioni e t al. 163 Recently it was shown 126,164 that both metal centers and ligand atoms need to be assigned a specific U value in order to accurately describe the J values. The inclusion of a Hubbard U term for both the M 3d and O 2p e lectrons greatly enhances the localization, and is essential in order to obtain the correct J value and, in some cases, correct ground state.

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89 Determination of ground s pin state of 3 1 The primary goal of the DFT calculation was to predict the correct grou nd spin arrangement in the molecule. Inelastic Neutron Scattering (INS) data shows the ground spin state has a multiplicity of 22. This value suggests that any two Mn 3+ atoms in this molecule are aligned as spin down and the other seven possess spin up ori entation. Starting from FM optimized geometry, energy for 29 hypothetical spin states were obtained as listed in Table D 1 (atoms are labeled according to Figure 3 18 ). Since Mn +3 Mn +3 and Mn +2 Mn +3 centers have relatively small magnetic exchange constants small energy gaps between these hypothetical spin states are expected. It can be noted that the lowest energy state corresponds to the minority spin localized on Mn1 and Mn9 atoms, with total multiplicity consistent with th e INS observation (Table D 1 ). Heisenberg exchange constant in Mn 9 system The following is the Heisenberg Hamiltonian written with the assumption of neglecting the second and higher neighbor magnetic interactions (equation 3 10): = J 13 S 1 S 3 J 12 S 1 S 2 J 24 S 2 S 4 J 35 S 3 S 5 J 34 S 3 S 4 J 45 S 4 S 5 J 23 S 2 S 3 J 46 S 4 S 6 J 13 S 9 S 7 J 12 S 9 S 8 J 24 S 8 S 6 J 35 S 7 S 5 J 34 S 7 S 6 J 45 S 6 S 5 J 23 S 8 S 7 ( 3 10 ) The molecule is composed of two symmetrically equivalent fragments such that Mn1, Mn2, Mn3, Mn4 are equivalent to Mn9, Mn8, Mn7, and Mn6 respectively. The arrangement indicates the presence of eight inequivalent exchange interactions. The J values (apart from J 46 ) were calculated as direct energy differences between the spin states shown in columns 2 and 3 in Table 3 5 by using the equa tion 3 11. J ij = (E SC1 SC2 )/ S i S j (3 11) In equation 3 11, E sc1 is the energy of (S) ij state from column 2, and is that from the column 3. For example to calculate the exchange constant between Mn1 Mn3 the

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90 energy difference between two hypothetical states have been taken, where in one Mn1 and Mn3 are spin down, and in another Mn1 and Mn7 are spin down. To predict J 46 the energy difference between two hypothetical spin states is taken where in one Mn2 and Mn4 are spin down, and in the other all Mn atoms are spin up. The energy difference is shown in equation 3 12. J 12 8 J 34 10 J 45 8 J 23 8 J 46 (3 12) The equation 3 12 was solved for J 46 using J values obtained previously. Mn 9 topology consists of fused Mn 3 triangles, known to be susceptible for spin frustration, caused by competing exchange pathways Th e ground spin state in Table D 1 is characterized by the presence of two AF couplings between Mn1/Mn3 and Mn1/Mn2, while the rest are FM. The calculated J 12 (Mn 1/Mn2) interactions in Table 3 6 are negative (AF) with spin couplings of c.a. 16.87 cm 1 as expected for antiparallel alignments. The rest of the spin couplings (ranging from 1.14 cm 1 to 27.32 cm 1 ) are FM. Coupling (equation 3 13) for two multi electron spin centers was expressed by Clark and Davidson as: 165 J ij = [ S T ( S T + 1) S i ( S i + 1) S j ( S j + 1)] (3 13) where S i and S j coupled together to give a total spin S T ( S T = S i + S j ). J predicted by equation 3 13 can be compared for two adjacent spins. For a Mn +2 /Mn +3 and Mn +3 / Mn +3 pairs aligned parallel, the spin couplings from equation 3 13 are 5 and 4, respectively. The calculated J 13 M n1/Mn3 interactions in Table 3 6 are positive (ferromagnetic) with spin couplings of 7.48 cm 1 However, the J 13 (Mn1/Mn3) interactions of the Mn1Mn3, pair in Table 3 6 are ferromagnetic, and yet their spin

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9 1 coupling value are very negative ( 3.84 cm 1 ). This means that these ferromagnetic interactions are completely frustrated, and the spins are aligned antiparallel. Hence, the data is rationali zed by the ligand induced spin frustration effect that dominates the spin alignment in Mn1, Mn2, Mn3 containing triangle. Bonding analysis The Lwdin population a nalysis is reported in Table 3 7 Th e oxide dianions serve as pure donors and have spin polarization opposite to that of the nearest Mn ion which is in agreement with th e superexchange mechanism. The m onodentate azide ion attached to M n1 is also acting as a pure donor as it known to be a weak field ligand. The azide n itrogen coordinated to Mn1 is showing spin polarization opposite to that of the metal ion. The O atoms of the acetates have the same spin polarization as the nearest Mn cations. This observation contradicts a simple superexchange picture and can be explain e d with a dative (also known as back bonding) mechanism. 166 The acetate has vacant orbital extended over three atoms, and can serve as acceptor for the d hange mechanism, developed for bonding metal ligand interactions, no longer holds. This is a suggested reason of limita tion of hybrid DFT for acetate bridging complex. The chelating etdeH 4 is also showing similar Lwdin spin densiti es as the acetate and making a back bond with manganese. Concluding Remarks The present work provides a further confirmation of the ability o f the edteH 4 ligand to yield interesting complexes. In the present case, it has given a novel, polynuclear, high spin, half integer spin Mn 9 SMM which also displays interesting magnetic

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92 properties. The Mn 9 cluster is comprised of a series of edge sharing t riangular units, which is a rare metal topology and allows the presence of six significant Jahn Teller elongation axes. The anisotropy of the molecule is considerably large with a relati vely high energy barrier of 49 K determined from the Arrhenius plot w hich is the highest known in a mixed valent Mn III / Mn II system. The energy spectrum for observed transitions is derived from the INS study. DFT computational analysis calculates all the exchange coupling parameters and rationalizes the spin ground state of Mn 9 SMM. The hysteresis plot reveals plenty of QTM steps, but no step at zero field. This is expected for a half integer spin SMM but a still rarely observed phenomenon. Further investigations are in progress in order to better understand the physics of this interesting feature.

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93 Table 3 1. Crystallographic data for 3 1 Parameter 3 1 Formula C 66 H 128 Mn 9 N 12 O 30 Formula weight, g/mol 2064.26 Crystal system Monoclinic Space group P2 1 /n a 20.965(10) b 33.819(13) c 2 8.485(12) 90, 92.859(8) 90 V 3 20172(15) Z 8 T,K 100(2) Radiation, a 0.71073 g/cm 3 1.359 mm 1 1.159 R1 b,c 0.0886 wR2 d 0.1329 a Graphite monochromator. b I I ). c R F o | | F c F o |. d wR w ( F o 2 F c 2 ) 2 w ( F o 2 ) 2 ]] 1/2 w 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O)+ 2F c 2 ]/3 Table 3 2. BVS calculations for the Mn atoms of complex 3 1 a Atom Mn 2+ Mn 3+ Mn 4+ Mn1 3.39 3.16 3.08 Mn2 3.15 2.94 2.87 Mn3 3.41 3.14 3.08 Mn4 3.47 3.20 3.14 M n5 1.99 1.86 1.81 Mn6 3.45 3.18 3.12 Mn7 3.42 3.15 3.09 Mn8 3.19 2.97 2.89 Mn9 3.41 3.17 3.10 a The underlined value is the one closest to the charge for which it was calculated. The oxidation state of a particular atom can be taken as the neares t whole number to the underlined value.

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94 Table 3 3. BVS calculations for the O atoms of complex 3 1 a Atom BVS Assignment Group O15 1.86 OR edte O17 1.99 OR edte O18 1.89 OR edte O19 1.98 OR edte O20 1.86 OR edte O27 2.07 O OMe O29 1.71 O O O28 2.03 O O O30 2.04 O O a The B VS values for O atoms of O 2 R OH and H 2 O groups are typically 1.8 2.0, 1.0 1. 2 and 0.2 0.4, respectively, but can be affected somewhat by hydrogen bonding. Table 3 4. F itting parameters for plots of vs. angular frequency and '' vs. angular frequency to a single relaxation process and a distribution of single relaxation pro cesses for complex 3 1 Compound Single relaxation process Mn 9 ( 3 1 ) '' S (cm 3 ) 1.2808*10 4 0.4996 T (cm 3 ) 9.2070*10 4 0.5004 (s) 4.4867*10 3 4.2759*10 3 Distribution of a single relaxation process '' S (cm 3 ) 8.2713*10 5 12.4997 T (cm 3 ) 9.4188*10 4 12.5003 (s) 4.2237*10 3 4.1966*10 3 0.3287 0.2365

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95 Table 3 5 Computational calculation scheme for seven Heisenberg exchange constants in Mn 9 complex ( 3 1 ). T he symbol 1, 2, 3, 4, 6, 7, 8, 9 are for Mn +3 and 5 is for Mn +2 having four and five unpaired electrons respectively. Here i is presenting spin up and i is showing spin down orientation.

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96 Table 3 6 The distance between Mn i and Mn j Heisenberg exchange constants for two adjacent metal centers and calculated and ideal spin coupling are presented (using equation 3 13) Mni Mnj J cm 1 J 13 3.35 7.48 3.84 4 J 12 2.95 16.87 3. 82 4 J 24 3.43 1.14 3.84 4 J 35 3.33 25.07 4.70 5 J 34 3.21 7.92 3.82 4 J 45 3.38 3.15 3.84 5 J 23 3.46 4.02 3.84 4 J 46 2.86 27.32 3.82 4 Table 3 7 Lwdin Population analysis for spin densities of complex 3 1 in its ground spin states and the magneti c states having highest possible multiplicity, all Heisenberg exchange constants set as ferromagnetic. The labels of all the indicated atoms refer to Figure 3 1 IS 19 F Mn1, Mn9 3.94 3.94 Mn2, Mn8 3.92 3.92 Mn3, Mn9 3.92 3.92 Mn4, Mn6 3.90 3.90 Mn5 4.83 4.83 N1 0.03 0.03 N2 0.05 0.05 N3 0.09 0.09 N8 0.04 0.04 N9 0.02 0.02 N10 0.02 0.02 O1 0.02 0.02 O15 0.05 0.05 O29 0.07 0.07 O20 0.04 0.04 O18 0.02 0.04

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97 A B Figure 3 1 A partially la beled representation of complex 3 1 (A Jahn Teller axes shown as green thick line s ) and a stereopair (B ). Hydrogen atoms have been omitte d for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue; C, light grey

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98 A B Figure 3 2 The core of complex 3 1 (A) and the core with an emphasis on the triangular sub units (B ). Color code: Mn 3+ purple; Mn 2+ cyan ; O, red Figure 3 3 The c oordination m odes of edte and edteH found in complex 3 1

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99 Figure 3 4 P lot of M T vs. T for complex 3 1 Figure 3 5 Plot of reduced magnetization ( B ) vs. H/T for complex 3 1 (A ) at applied fields of 0.1 7.0 T and in the 1.8 10 K temperature range. The solid lines are the fit of the data; see the text for the fit parameters Two dimensional contour plot for the rms error surface vs. D and g for the reduced m agnetization fit for complex 3 1 (B ).

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100 Figure 3 6 Plots of in phase M (as M ) vs. T and out of phase M vs. T ac signals for complex 3 1 at the indicated frequencies.

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101 A B Figure 3 7 Plot of the natural log arithm of relaxation rate ln(1/ ), vs. 1/ T for complex 3 1 using M '' vs. T da ta at different ac frequencies (A ). The solid line i s the fitting to the Arrhenius equation. Arrhenius plot obtained from the magnetization vs. time decay study for complex 3 1 (B ).

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102 A B Figure 3 8 Plots of the in phase ( ) vs. an gular frequency (A) out of phase ( '' ) vs. angular frequency (A) and ' vs. (B ) at 3.0 K for complex 3 1 The solid lines are the least squares fitting of the data to a single relaxation process using equations (3 4) and (3 5). The dashed lines are the least squares fitting of the data to a distribution of single relaxati on processes using equations (3 6) and (3 7). See Table 3 4 for fitting parameters.

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103 A B Figure 3 9 Argand plots of '' vs. for complex 3 1 at the indicated temperatures (A ). The dashed line s are the fit to a distribution of single relaxation pr ocess es as described by equati ons (3 6) and (3 7). The c hange in with temperature in the Argand plot for complex 3 1 (B ).

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104 A B C Figure 3 10 Magnetization (M) vs. dc field hysteresis loops for a single crystal of 3 1 t the indicated temperat ure s (A ) and field swe ep rate s (B ). The magnetization is normalized to its saturation value, M S Magnetization vs. time decay data for a single crystal of 3 1 e indicated temperatures (C ).

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105 A B Figure 3 1 1 A full CV diagram at 100 mV/s (A ) and full DPV diagram for complex 3 1 at 20 mV/s (B ) in DMF containing 0.1 M NBu n 4 PF 6 as the supporting electrolyte. The indicated potentials are given vs. ferrocene as an internal standard.

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106 A B Figure 3 1 2 T he CV diagram at the indicate d scan rates 50 500 mV/s (A ), for the oxidation wave of 3 1 in DMF containing 0.1 M NBu n 4 PF 6 as the supporting electrolyte. The indicated potentials are given vs. ferrocen e as an internal standard. Plot vs. square root of scan rate ( 1/2 ) f or 0.14 V oxidation wave (B ).

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107 Figure 3 13 INS spectra of 3 1 recorded with an incoming neutron wavelength of 6.5 at the indicated t emperatures. A B Figure 3 14 INS spectra of 3 1 recorded with wavelength 7.5 at the indi cated temperatures. Panel (A ) shows the energy g ain and loss sides, while (B ) shows the energy loss side of the spectra only.

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108 Figure 3 15 S(Q,E) plot of the INS spectrum of 3 1 measured with wavelength 7.5 at a temperature of 10 K. Intensity is color coded from blue (low) to red (high). A B Figure 3 16 INS spectra of 3 1 measu red with wavelength 3.2 . (A ) Energ y loss sides recorded at 40 and 1.5 K (open circles). The lattice approximation (green solid circles) was obtained by scaling the 40 K spectrum by the Bose factor. This approximation was then subtracted from 1.5 K spectrum, yielding the corrected 1.5 K spe ct rum (blue solid circles). (B ) S(Q,E) plots at 1.5 K. Intensity is color coded from blue (low) to red (high). The left panel shows the S(Q,E) plot for the uncorrected data and the right one the S(Q,E) plot for the Bose corrected data. The intensity scales are different for the plots.

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109 A B Figure 3 1 7 (A ) Comparison of the experimental (circles) and simulated (lines) INS spectra recorded with wavelength 7.5 . The simulated spectra were obtained with the best fit parameters as given in the text, and using an instrumental resolution of 45 eV for 1.5 and 5 K and 60 eV for 10K, 15K and 28K. Data are shown w ith offsets for clarity. (B ) Calculated energy spectrum of 3 1 as a function of the magnitude of the magnetic quantum number M.

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110 Figure 3 18 Depiction of the spin alignments in the S = 21/2 ground state of complex 3 1 as predicted by the DFT calculations, with the Mn1 Mn9 interaction being antiferromagnetic. The Mn labeling scheme is the same as that in Figure 3 1 (A). Color code: Mn 3+ purple; Mn 2+ cyan.

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111 C HAPTER 4 A RICHNESS OF NEW MIXED VALENT M n AND M n /C a C LUSTERS FROM THE USE OF N, N, N', N' TETRAKIS(2 HYDROXYETHYL ) ETHYLENEDIAMINE : M n 3 M n 4 M n 6 M n 10 M n 20 AND M n 1 8 C a 2 For the past two decades there has been a growing intere st in the field of polynuclea r m anganese complexes at intermediate oxidation states. For instance, the relevance of Mn carboxylate clusters as active site mimic for various metalloproteins and enzymes is profound For example synthetic chemists are trying to mimic the active site of t he water oxidizing complex (WOC) of photosystem II in green plants and cyanobacteria where the WOC comprises of a mixed metal, cubane subunit, Mn 4 Ca cluster. 6,7 As the name suggests, WOC calayses the light driven o xidation of water to dioxygen. In this regard inorganic Mn/Ca complexes can act as synthetic models of the WOC and they help to understand the magnetic and spectroscopic properties of the native site and the mechanism of its function. On the other hand, th e discovery of zero dimensional nanoscale magnets comprised of Mn clusters, which are better known as SMMs 11 grabs the attention in the field of molecular magnetism. An SMM behaves as a magnet below its blocking temperature ( T B ) and exhibits hysteresis in magnetization vs. dc field scans. This behavior requires the combination of a large ground state spin (S) with a large and negat ive Ising (or easy axis) type of magnetoanisotropy, as measured by the axial zero field splitting parameter D Apart from their bioinorganic relevance and magnetic properties, the intrinsic architectural beauty and the aesthetically pleasing structures of this plethora of polynuclear Mn metal clusters provide another area of research interest in the field of supramolecular chemistry. 167 V arious synthetic approaches have been developed in the past but there is still a continuing search for appropriate ligand precursor s to build new polynuc lear metal

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112 clusters. As a part of our continuing research on exploring new synthetic scheme s, the edteH 4 chelate with carboxylate and azide groups have been used The edteH 4 chelate is comprised of an ethylenediamine backbone with four hydroxyethyl arms wh ich, upon deprotonation act as bridging groups to promote formation of high nuclearity clusters. EdteH 4 has already been reported to produce Mn 8 Mn 9 Mn 20 and a family of Mn 12 complexes with novel metal topologies. 60 Among these, Mn 9 and one member of the Mn 12 fam ily are SMMs, proved by magnetization hysteresis studies. 168 Hence, the initial successful reactions with edteH 4 g ave us hope to further scrutinize the bridging capability of the same ligand under various reaction conditions. Herein, we report the synthesis, crystal structures and magnetic properties of an ensemble of mixed valent Mn x and Mn/Ca complexes containing edteH 4 ligand where x = 3, 4, 6, 10 and 2 0. Experimental Section Syntheses All preparations were performed under aerobic conditions using reagents and solvents as received. The synthese s of Mn 12 O 2 (O 2 CMe ) 16 (H 2 O) 4 16,24 [Mn 8 O 10 (O 2 CMe) 6 (H 2 O) 2 (bpy) 6 ] (ClO 4 ) 4 169 Mn(O 2 C Bu t ) 2 120 and Mn(O 2 C Et ) 2 170 w ere carried out as reported in the literature. Caution! Although no such behavior was observed during the present work, azide salts are potentially explosive; such compounds should be synthesized and used in small quantities, and treated with utmost care at all times. [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](O 2 CMe) ( 4 1 ). To a stirred solution of edteH 4 (0.22 g, 0.93 mmol) in MeCN (20 mL) was added Mn 12 O 2 (O 2 CMe ) 16 (H 2 O) 4 (Mn 12 Ac, 0.19 g, 0.10 mmol). The resulting dark brown solution was stirred overnight, filtered and the n the solvent was evaporated to dryness. T he solid was recrystallized from 15 mL CH 2 Cl 2

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113 layered with Et 2 O/C 6 H 14 X ray quality reddish brown cube like crystals of 4 1 2 Cl 2 grew over 5 days in a yield of 76 %. The crystals were collected by filtration, washed with Et 2 O and C 6 H 14 and dried under vacuum. Anal. Calc. (found) for 4 1 H 2 O: C, 37.69 (37.96); H, 6.69 (6.90); N, 6.76 (6.38). Selected IR data (cm 1 ): 2859 (m ), 1585 (s), 1413 (s), 1083 (s), 915 (m), 733 (w), 654 (s), 616 (s), 574 (s), 516 (m). [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](ClO 4 ) ( 4 2 ). To a stirred solution of edteH 4 (0.05 g, 0.21 mmol) in MeCN/MeOH (10/1, v/v) was added [Mn 8 O 10 (O 2 CMe) 6 (H 2 O) 2 (bpy) 6 ](ClO 4 ) 4 (Mn 8 Ac, 0. 10 g, 0.04 mmol). The resulting dark brown solution was stirr ed for six hours, filtered and the filtrate vapor diffused with Et 2 O. X ray quality reddish brown plate like crystals of 4 2 MeCN grew ove r 12 days in a yield of 51 % The crystals were collected by filtration, washed with Et 2 O and dried under vacuum. Anal. Calc. (found) for 4 2 2 O: C, 32.50 (32.35); H, 6.14 (5.76); N, 6.32 (6.45) Selected IR data (cm 1 ) : 2977 (m), 2857 (m), 1570 (s), 1405 (m), 1094 (vs), 912 (m), 733 (w), 650 (m), 625 (m), 586 (m), 543 (m), 508 (m). [Mn 4 (edteH 2 ) 2 (hmp) 2 Cl 2 ][MnCl 4 ] ( 4 3 ). To a stirred solution of edteH 4 (0.10 g, 0.42 mmol) and Et 3 N (0.06 mL, 0.42 mmol) in MeOH (12 mL) was added MnCl 2 (0.17 g, 0.84 mmol). The resulting dark brown solution was stirred for 30 minutes and then hmpH (0.20 mL, 2.1 mmol) was added and the solution was stirred for a further two hours. T he solution was then filtered and layered with Et 2 O. X ray quality, dark brown hexag onal plate like crystals of 4 3 grew ove r 15 days with a yield of 12%. The crystals were collected, washed with Et 2 O and dried under vacuum. Anal. Calc. (found) for 4 3 : C, 32. 7 9(33.24); H, 4.82 (4.88); N, 7.17 (6.87). Selected IR data (cm 1 ): 2937 (m), 2 677

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114 (m), 2492 (m), 1608 (s), 1475 (s), 1439 (s), 1397 (w), 1289 (w), 1171 (w), 1063 (vs), 925 (m), 771 (m), 731 (w), 665 (br), 519 (br). [Mn 6 O 2 (O 2 C Bu t ) 6 (edteH) 2 (N 3 ) 2 ] ( 4 4 ). To a stirred solution of edteH 4 (0.20 g, 0.84 mmol) and NEt 3 (0.24 mL, 1.68 mmol) in MeCN (20 mL) was added Mn(O 2 CBu t ) 2 ( 0.60 g, 2.29 mmol ). The resulting dark brown solution was stirred for 15 minutes under mild heating (~60C) to dissolve all the solids, and then Me 3 Si N 3 ( 0.4 mL, 3.01 mmol ) was added and solutio n was stirred for a fur ther two hours. T he solution was then filtered and left undisturbed to slowly evaporate X ray quality, dark brown plate like crystals of 4 4 grew over 10 days at 4C in a yield of 24 %. The crystals were collected, washed with Et 2 O and dried under vacuum Anal. Calc. (found) for 4 4 : C, 39.54 (39.71 ) ; H, 6.37 (6.68 ) ; N, 9.22 (9.45) Selected IR data (cm 1 ): 3395 (br), 2957 (vs), 2899 (s), 2863 (s), 2067 (vs), 1586 (vs), 1482 (s), 1415 (s), 1359 (s), 1225 (s), 1086 (vs), 919 (m), 897 (m), 787 (w), 695 (s), 587 (vs), 508 (m), 417 (w). Na 2 [Mn 10 O 4 (OMe) 2 (O 2 CEt) 6 ( edte) 2 (N 3 ) 6 ] ( 4 5 ). To a stirred solution of edteH 4 (0.15 g, 0.64 mmol) and LiOH (0.06 g, 0.64 mmol) in MeCN/MeOH (10/5, v/v) was added Mn(O 2 CEt) 2 (0.34 g, 1.28 mmol). The resulting dark brown solution was stirred for an hour and then NaN 3 (0.33 g, 5.12 mmol ) was added and the solution was stirred for a further three hours. T he solution was then filtered and left undisturbed to slowly evaporate X ray quality, dark brown plate like crystals of 4 5 grew over 14 days in a yield of 80 %. The crystals were collected by filtration, washed with Et 2 O and dried under vacuum. Anal. Calc. (found) for 4 5 2 O: C, 25.12 (25.05); H, 4.22 (4.06); N, 16.11 (16.14). Selected IR data (cm 1 ): 2980 (m), 2867 (m), 20 50 (vs), 1557 (s), 1412 (m), 1291 (m), 1062 (s), 911 (m), 558 (br).

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115 (NEt 4 ) 2 [Mn 10 (O) 4 (OH) 2 (O 2 CEt) 6 (Edte) 2 (N 3 ) 6 ] ( 4 6 ). To a stirred solution of 4 5 (0.10 g, 0.1 mmol) in 6 mL MeCN was added NEt 4 Cl (0.02 g, 0.2 mmol). The resulting dark brown solution was st irred for an hour and then filtered and the filtrate vapor diffused with Et 2 O X ray quality, dark brown, plate like crystals of 4 6 2MeCN grew over a few days in a yield of 40 % The crystals were collected, washed with Et 2 O and dried under vacuum. Anal. Calc. (found) for 4 6 H 2 O : C, 31.06 (30.91); H, 5.49 (5.51); N, 16.70 (16.54). Selected IR data (cm 1 ): 2980 (m), 2062 (s), 1635 (m) 1558 (m), 1396 (m), 1290 (m), 1072 (m), 911 (m), 558 (br). [Mn 20 O 8 (OH) 6 (O 2 CEt) 6 (edte) 4 (edteH) 2 ](ClO 4 ) 4 ( 4 7 ). To a stirred solution of edteH 4 (0.10 g, 0.42 mmol) and NEt 3 (0.12 mL, 0.84 mmol) in 10 mL EtOH was added Mn(O 2 CEt) 2 (0.23 g, 0.84 mmol) and NaC lO 4 (0.05 g, 0.42 mmol). The resulting dark brown solution was stirred for two hours then filtered. X ray quality brown plate like crystals of 4 7 grew from C 6 H 14 layering over 20 days in a yield of 30 % The crystallographic sample was kept in contact with mother liquor to prevent damage from exposure to the atmosphere. Otherwise, the crystals were collected by filtration, washed with Et 2 O and dried under vacuum. Anal. Calc. (found) for 4 7 1.5C 6 H 1 4 : C, 28.32 (28.41); H, 4.89 (4.81); N, 4.56 (4.78). Selected IR data (cm 1 ): 2972 (m), 2850 (m), 1567 (s), 1464 (m), 1421 (m), 1295 (m), 1088 (vs), 911 (m), 742 (w), 625 (s), 589 (s), 515 (s). [C a 2 Mn 18 O 8 (OH) 5 (O 2 CBu t ) 4.5 (edte) 5 (NO 3 ) 3.5 ](O 2 CBu t ) 0.5 (NO 3 ) 0.5 ( 4 8 ). To a stirred solution of edteH 4 (0.10g, 0.42mmol) and NEt 3 (0.16mL, 0.84 mmol) in MeCN/MeOH (10/1, v/v) was added Ca(NO 3 ) 2 (0.10g, 0.42 mmol) followed by Mn (O 2 CBu t ) 2 (0.12g, 0.42 mmol). Th e resulting dark brown solution was stirred for 15 minutes under mild

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116 heating (~60C) to dissolve all solids. Then the solution was stirred for a further two hours at room temperature, filtered, and the filtrate allowed to slowly evaporate undisturbed at a mbient temperature. X ray quality, dark brown, plate like crystals of 4 8 slowly grew over several days in a yield of 40 %. The crystals were collected by filtration, washed with Et 2 O and dried in vacuum. Anal. Calc. (found) for 4 8 3H 2 O : C, 30.00 ( 29.87); H, 5.21 (5.21); N, 6.58 (6.30 ). Selected IR data (cm 1 ): 3395 (br m), 2869 (m), 1605 (w), 1544 (m), 1480 (m), 1383 (s), 1207 (w), 1059 (m), 911 (w), 890 (w), 615 (m), 565 (m). X ray Crystallography Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo Suitable crystals of 4 1 2 Cl 2 4 2 4 3 4 4 4 5 2 MeCN, 4 6 4 7 and 4 8 MeCN were attached to glass fibers using silicone gre ase and transferred to a goniostat where they were cooled to 173 K for data collection. Cell parameters were refined using 8192 reflections. A full sphere of data (1850 frames) was scan method (0.3 frame width). The first 50 frames w ere re measured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by direct me thods in SHELXTL6, and refined on F 2 using full matrix least squares. The non H atoms were treated anisotropically, whereas the hydrogen atoms were placed in ideal, calculated positions and were refined as riding on their respective C atoms. For 4 1 2 Cl 2 t he asymmetric unit consists of two half Mn 3 cluster cations, an acetate anion and four dichloromethane solvent molecules. Three of the solvent

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117 molecules were disordered and could not be modeled properly, thus program SQUEEZE, 64 a part of the PLATON package of crystallographic software, was used to ca lculate all of the four solvent molecules disorder area and remove its contribution to the overall intensity data. Four hydroxyl protons, H2, H4, H8 and H10, were obtained from a d ifference Fourier map and refined freely. A total of 443 parameters were r efined in the final cycle of refinement using 6043 reflections with I > 2 (I) to yield R 1 and wR 2 of 6.41 and 16.42 %, respectively. For 4 2 MeCN, t he asymmetric unit consists of two half timers, a perchlorate anion and an acetonitrile solvent molecule. The latter molecule was disordered and could not be modeled properly, th us program SQUEEZE, 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. A total of 16312 parameters were refined in the final cycle of refine ment using 4676 reflections with I > 2 (I) to yield R 1 and wR 2 of 3.72 and 9.79 %, respectively. For 4 3 the asymmetric unit consists of one Mn 4 cluster cation and a MnCl 4 anion A total of 267 parameters were refined in the final cycle of refinement using 4499 reflections with I > 2 (I) to yield R 1 and wR 2 of 4.19 and 11.44 %, respectively. For 4 4 t he asymmetric unit consists of a half Mn 6 cluster located on an inversion center. The proton on O11, the uncoordinated hydroxyl group, was located from a d ifference Fourier map and was held r iding on its parent atom. A total of 407 parameters were refined in the final cycle of refinement using 2883 reflections with I > 2 (I) to yield R 1 and wR 2 of 5.30 and 11.24 %, respectively.

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118 For 4 5 he asymmetric unit consists of a half Mn 10 cluster and an acetonitrile solvent molecule. A total of 523 parameters were refined in the final cycle of refinement using 7541 reflect ions with I > 2 (I) to yield R 1 and wR 2 of 4.17 and 11.07 %, respectively. For 4 6 he asymmetric unit consists of two half Mn 10 cluster anions, two tetraethylammonium cations and two acetonitrile solvent molecules. Each half cluster has two diso rders. One disorder has a hydroxyl ligand disordered against an azide ligand. Another is where the edte ligand has the CH 2 around N1 and N1' are disordered and were refined in two parts each. A check for higher symmetry was performed but none exists. O ne acetonitrile molecule is disordered alongside the disorder in the OH / N 3 disorder. A total of 1086 parameters were refined in the final cycle of refinement using 14604 reflections with I > 2 (I) to yield R 1 and wR 2 of 4.03 and 8.96 %, respectively. F or 4 7 ( Sol v) the asymmetric unit consists of a half Mn 20 cluster, two perchlorate anions and thr ee ethanol solvent molecules. The solvent molecules were disordered and could not be modeled properly, thus program SQUEEZE, 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. Atom Mn10 has disorder between a monodentate propionate and a bidentate propionate ligand. Another disorder is observed i n the C13 C14/C13' C14' unit and in the C19 O15/C19' O15' unit. The l atter hydroxyl group is protonated but the proton was placed in a calculated idealized position. Similarly, oxygen atoms O1, O3, and O7 are protonated and their protons were placed in c alculated idealized positions. The perchlorate anions are also disordered

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119 an d each was refined in two parts; all partial site occupations factors were initially refined to near 50% values and thus were fixed at 50% in the final refinement cycles. A total of 41842 parameters were refined in the final cycle of refinement using 12607 (I) to yield R 1 and wR 2 of 6.2 and 18.67%, respectively. For 4 8 5MeCN, t he asymmetric unit consists of a M n 18 Ca 2 cluster cation, half of a nitrate anion, a half t BuCO 2 anion and five acetonitrile solvent molecules. The solvent molecules were disordered and could not be modeled properly; thus the program SQUEEZE was again used to calculate the solvent disorder area and remove its contribution to the overall intensity data. The cluster exhibits several disordered regions. The t B u group on C1, the methyl groups on C22 and C122 are disordered and each was refined in two parts. N14 of the nitrate anion is disordered an d refined in two parts (N14/N14') along with the O atoms. N8 of the nitrate ion is disordered against a t BuCO 2 ion (on C81). The final two disordered regions are on each Ca center. The edteH 4 ligand acts as a hexa dentat e ligand half of the time and as a penta dentate ligand in the other half. In the latter case, while one of the bra nches is not coordinated, an O H group occupies the bridging coordin ation position between the Ca a nd one of the adjacent Mn centers. The same disorder is also seen on the other side of the cluster cation with the second Ca center. This gives rise to a total of one (two half hydroxyl ligands) negative charge in addition to the charges coming from the edteH 4 ligands. The bridging O ligands O14, O16, O32 and O33 are believed to be hydroxyl groups and thus carry a proton each but the H atoms could not be located from d ifference Fourier maps. Th e protonation was deduced from the BVS calculations 171 which showed thos e O ato ms to be of the hydroxyl type. C onsidering all of the above,

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120 the cluster carries a +1 charge countered by a half nitrate and a half t BuCO 2 anions. In the final cycle of refinement on F 2 33638 reflections (of which 10946 are observed ( I)) were used to refine 1565 parameters and the resulting R1, wR2 and S (goodness of fit) were 7.68%, 2 0.23% and 0.873, respectively. R1 is calculated to provide a reference to the conventional R value but its function is not minimized. The crystallographi c data and structure refinement details for all the eight complexes are listed in Table s 4 1 4 2 and 4 3 respectively. Physical Measurements Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 6 70 FTIR spectrometer in the 4 00 4000 cm 1 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 susceptibility data were collected at the University of Florida usin g a Quantum Design MPMS XL SQUID susceptometer equipped with a 7 T m agnet and operating in the 1.8 300 K range. Samples were embedded in solid eicosane to prevent torquing. Magnetization v s. field and temperature data were fit using the program MAGNET 66 Pascal's constants were used to estimate the diamagnetic correction, which was subtracted from the experimental susceptibility to give the molar parama gnetic susceptibility ( M ) Result and Discussion Syntheses There are two ways to make polynuclear metal clusters with Mn 3+ ion s One is to start with a preformed higher oxidation state Mn x cluster 47,172 and the second is to oxidize sim ple Mn 2+ salts in the presence of a chelating ligand. 111 Both strategies have

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121 been efficiently employed in this chapter to obtain various Mn x cluste rs (x = 3, 4, 6 10 20) with the potentially hexadentate ligand edteH 4 Mn 12 Ac is a well known SMM with a ground state spin of S = 10, which comes from the combination of 8Mn III and 4Mn IV It also contains 16MeCO 2 and 4H 2 O molecules. There are many examples in the literature of the acetate group s being replaced partially or completely by other carboxylates via ligand substitution reactions. 11 The reactivity of Mn 12 Ac is further verified in presence of edteH 4 ligand. There are two possibilities: one is that the edteH 4 will replace carboxylate/water by keeping the core intact and the second is to decompose the Mn 12 Ac to form a completely new compound. In the reaction, Mn 12 Ac is decomposed to accommodate the hexadentate ligand edteH 4 and Mn 3.33+ (average oxidation state of Mn in Mn 12 Ac) is being reduced to Mn 2.33+ (average oxidation state of M n in 4 1 ) T he reaction of Mn 12 Ac with edteH 4 in a 1: 9 molar ratio led to a [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](O 2 CMe) complex. The yield of the reaction is as high as 76 %. No crystalline product was obtained by using a lesser quantity of the ligand. M icrocrystalline product was obtained from MeCN solvent which did not diffract under X ray and hence, several other techniques were investigated. After the initial reaction in MeCN, the solvent was evaporated to dryness and the solid product was recrystallized from a mixe d solvent system of CH 2 Cl 2 / Et 2 O/ C 6 H 1 4 to obtain an X ray quality single crystal of 4 1 A similar product was also obtained by using another preformed cluster of [Mn 8 O 10 (O 2 CMe) 6 (H 2 O) 2 (bpy) 6 ](ClO 4 ) 4 ( Mn 8 Ac ) The reason for choosing Mn 8 Ac as a starting mater ial is its high Mn 3.75+ average oxidation state, which seemed a good source for making polynuclear cluster containing Mn 3+ ion. However, the reaction between Mn 8 Ac and edteH 4 at a 1:5 molar ratio l ed to the

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122 isolation of a Mn 3 cluster similar to 4 1 with an average Mn oxidation state of +2.33. These two reactions, yielding Mn 3 clusters are summarized in equation 4 1 and 4 2. Mn 12 O 12 (O 2 CMe) 16 (H 2 O) 4 + 8edteH 4 + 12H + + 12e 4[Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](O 2 CMe) + 4MeCO 2 H + 16 H 2 O (4 1) 3[Mn 8 O 10 (O 2 CMe) 6 (H 2 O) 2 (b py) 6 ](ClO 4 ) 4 + 16edteH 4 + 34H + + 34e 8[Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](ClO 4 ) + 18bpy + 2MeCO 2 H + 4HClO 4 + 36 H 2 O (4 2) Going from preformed clusters to simple Mn 2+ salts as starting materials led to a variety of different products. The reaction of edteH 4 with Mn( ClO 4 ) 2 /MnCl 2 NEt 3 and NaN 3 in MeCN/MeOH gave a family of Mn 12 clusters. Both end on azides and N hydroxymethyl pyridine are known to be ferromagnetic couplers, and have been widely employed in Mn cluster chemistry 48,59 The above mentioned scheme is known to give Mn 12 clusters incorporating end on azides. However, the incorporation of the end on azides did not lead to the desired predominant ferromagnetic interaction in the system. Hence, to tune the magnetic properties of the system hmpH was employed rather than adding NaN 3 in the reaction and this time a Mn 4 cluster was isolated rather than Mn 12 The reaction of edteH 4 with Mn Cl 2 NEt 3 and hmpH in a 2:1:1: 5 molar ratio in MeOH afforded a reddish brown solut ion from which was subse quently obtained [Mn 2 II Mn III (O 2 CMe) 2 (edteH 2 ) 2 ]( ClO 4 ) ( 4 3 ) in 12 % yield (eq uation 4 3 ). Its formation is summarized in equation 4 3 where atmospheric oxygen gas is assumed to pr ovide the oxidizing equivalents and the oxidation is being facilitated in presence of NEt 3 5MnCl 2 + 2edteH 4 + 2hmpH + 1/2O 2 [Mn 4 (edteH 2 ) 2 (hmp) 2 Cl 2 ][MnCl 4 ] + 4HCl + H 2 O (4 3) The product can be obtained with a lesser quantity of hmpH; however the excess amount ensures the formation of single crystals of 4 3 No other product was isolated using this edteH 4 /hmpH, mixed chelate synthetic scheme.

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123 Modifying the reaction scheme by a bulkier carboxylate containing Mn 2+ salt gave a very different result. The reaction between Mn (O 2 CBu t ) 2 edteH 4 NEt 3 and Me 3 S i N 3 in ~ 3:1:2:4 molar ratio results in 4 4 with 24 % yield (equation 4 4). The reagent Me 3 Si N 3 serves a dual purpose by delivering azide to the cluster and abstracting carboxylate as Me 3 SiO 2 CR 173 It is interesting to note that the same reaction with NaN 3 as an azide source in MeCN/MeOH leads to a structurally unprecedented Mn 9 SMM. 168 6 Mn(O 2 C Bu t ) 2 + 2edteH 4 + 2SiMe 3 N 3 + 3/2O 2 [Mn 6 O 2 (O 2 C Bu t ) 6 (edteH) 2 (N 3 ) 2 ] + 2Me 3 SiO 2 CBu t 3 + 4Bu t CO 2 H + H 2 O (4 4) The react ion is solvent specific and single crystal s are obtained in MeCN s olvent only. Use of other solvents like MeOH or EtOH degrade the crystal quality and remove SiMe 3 N 3 as SiMe 3 OMe which hampers the formation of the desired product. The same product was obtained by using MnCl 2 NaO 2 CBu t edteH 4 and NaN 3 in a 2:4:1:3 molar ratio but the crystal quality was po or. The reaction was further explored by switching from a bulkier carboxylate, t BuCO 2 to a smaller carboxylate, EtCO 2 and as a result a larger cluster was obtained. The reaction between Mn (O 2 CEt) 2 edteH 4 LiOH and N aN 3 in 2:1:1:8 molar ratio in MeCN/MeOH gave the polymer 4 5 in a high yield of 80 % (equation 4 5). 10Mn(O 2 CEt) 2 + 2edteH 4 + 6NaN 3 + 2MeOH + 4H 2 O Na 2 Mn 10 O 4 (OMe) 2 (O 2 CEt) 6 (edte) 2 (N 3 ) 6 + 14EtCO 2 H + 4NaOH (4 5) The same product was obtained using NaOH a s the base. The use of only MeOH or the combination of MeCN/ DMF as the solvent can also give pure single crystals of 4 5 but the yield was highest from the above reaction The use of MeOH or DMF along with the main solvent MeCN ensures the solubility of a ll the components in the reaction mixture. An excess of ligand and NaN 3 helps to give well formed single crystals. The

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124 compound 4 5 is a [ Na 2 Mn 10 ] is a polymer where Mn 10 unit s are connected by end to end azide bridges through Na ions. In order to possibly obtain the discrete Mn 10 unit and assess its intrinsic magnetic properties, Cl ions were added to drive the precipitation of Na + as NaCl. Hence, one equivalent of Na 2 Mn 10 was treated with two equivalents of NEt 4 Cl salt in MeCN. NaCl was indeed formed and (NEt 4 ) 2 [Mn 10 ] was sucessfully obtained in discrete form The reaction is summa rized in equation 4 6. Na 2 Mn 10 O 4 (OMe) 2 (O 2 CEt) 6 (edte) 2 (N 3 ) 6 + 2NEt 4 Cl + 2H 2 O (NEt 4 ) 2 [Mn 10 O 4 (OH) 2 (O 2 CEt) 6 (edte) 2 (N 3 ) 6 ] + 2NaCl + 2MeOH (4 6) The reaction scheme further explored the effect of EtCO 2 edteH 4 and Mn 2+ in absence of NaN 3 The 4:2:4:1:2 m olar ratio of Mn(O 2 CEt) 2 edteH 4 NEt 3 and NaClO 4 in EtOH leads to the isolation of a new Mn 20 cluster where the atmospheric oxygen provided the oxidizing equivalents The reac tion is summarized in equation 4 7 20Mn(O 2 CEt) 2 + 6edteH 4 + 4NaClO 4 + 10H 2 O + 4 O 2 [Mn 20 O 8 (OH) 6 (O 2 CEt) 6 (edte) 4 (edteH) 2 ](ClO 4 ) 4 + 34EtCO 2 H + 4NaOH (4 7 ) The absence of azide clearly affects the reaction allowing a different product to be isolated. Changing the carboxylate from propionate to acetate gave another Mn 20 c luster with a s imilar metal core 60 The final reaction strategy explores the effect of t BuCO 2 in the above reaction scheme. The reaction of t BuCO 2 with edteH 4 and NaN 3 in presence of Mn 2+ salt led to the isolation of aforementioned Mn 9 168 and Mn 6 ( 4 4 ) clusters. Here, the reaction system is further modified by adding Ca 2+ salt to the reaction system. The 1:1:1:2 molar ratio of Mn(O 2 CBu t ) 2 Ca(NO 3 ) 2 edteH 4 and NEt 3 in MeCN/MeOH leads to the isolation of a new [ Ca 2 Mn 18 O 8 (OH) 5 (O 2 CBu t ) 4.5 (edte) 6 (NO 3 ) 3.5 ] + ( 4 8 ) cluster The reaction is summarized in equation 4 8.

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125 18 Mn(O 2 CBu t ) 2 + 2Ca(NO 3 ) 2 + 6edteH 4 + 6 H 2 O + 7/2 O 2 [ Ca 2 Mn 18 O 8 (OH) 5 (O 2 CBu t ) 4.5 (edte) 6 (NO 3 ) 3.5 ]( O 2 CBu t ) 0.5 ( NO 3 ) 0.5 + 31 t Bu CO 2 H (4 8 ) The product is obtained by using a large excess of the Ca 2+ salt whereas no product was isolated by using a 9:1 to 2:1 ratio of Mn 2+ to Ca 2+ salt An analogous Mn 18 Ca 2 cluster was isolated by switching t BuCO 2 to Me CO 2 as confirmed by IR and elemental analysis. Both reactions involve acid base (e.g. deprotonation of water) and redox chemistry (oxidation of Mn 2+ to Mn 3+ ) as well as structural rearrangements. It is clear that the described reactions to complex 4 1 to 4 8 are complicated and they involve acid base (e.g. deprotonation of water) and redox chemistry (oxidation of Mn 2+ to Mn 3+ ) as well as structural rearrangements. Description of S tructures Structure of [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ] (O 2 CMe) (4 1) A partiall y labeled representation of the [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ] + cation of 4 1 is shown in Figure 4 1(A ) and selected inter atomic distances and angles are summarized in Table A 5 Complex 4 1 crystallizes in the triclinic space group P 1 and has crystallographic inversion symmetry. The core of the complex is comprised of a linear [Mn 3+ Mn 2+ 2 (O R) 4 ] 3+ unit in which the central Mn 3+ ion (Mn1) is connected to the terminal Mn 2+ ion s (Mn2) by 1 1 acetate bridges The coordination of the two peripheral Mn ions is com pleted by two edteH 2 ligand s Each edte H 2 g roup is hexadentate chelating to the terminal Mn2 atom with two of its deprotonated alkoxide arms bridging to the central Mn1 (O1 O3 ) atom. Thus, the edte H 2 groups are overall 1 1 2 2 : bridging, as shown in Figure 4 14 The Mn oxidation s tates were determined by charge considerations and metric parameters and confirmed by BVS calculations 171 which are summarized in Table 4 4 Mn1 and Mn2 are six and seven coordinate respectively.

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126 Charge balance consideration s require 2Mn 2+ Mn 3+ 2 Me CO 2 2edteH 2 and one additional negative charge come s from the acetate counteranion Two Mn 3 units are connected through an acetate group with H bond and hence make a 1D polymeric chain (Figure 4 1, B ) The acetate ion is making H bond to the pro tonated OH group (O2) of the edteH 2 ligand. The bond distance between O2 and the acetate ion (O13) is 2.652. There are two additional H bonds between the protonated OH group (O4, O10) of the edteH 2 ligand in one Mn 3 unit and the acetate ion ( O12, O6) of the adjacent Mn 3 unit. The protonation levels of all the O atoms were determined by BVS calculations and the results are listed in Table 4 8 Two of t he edte H 2 O atoms have BVS values of > 1.8 confirming them as deprotonated whereas the othe r two have BVS values of around 1.1, confirming them as protonated. Structure of [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ] (ClO 4 ) (4 2) Complex 4 2 crystallizes in the monoclinic space group P2 1 /n The structure of 4 2 is very similar to 4 1 It comprises the same core as [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ] + with ClO 4 as a counterion (Figure 4 2) The selected interatomic distances and angles are summarized in Table A 5 The BVS calculations for the Mn ions and O atoms are shown in Table s 4 4 and Table 4 8 respectively. The linear Mn 2 2+ Mn 3+ core of complexes 4 1 and 4 2 is quite rare in the literature except for two examples where one is an SMM using tripodal alcohol ligand 174 and other is a trinuclear unit embedded in a macrocycle. 175 Th ere are few other examples of linear Mn 3 complexes known in the literature, which include Mn 2+ 3 176 Mn 4+ 3 177 Mn 2+ Mn 3+ Mn 4+ 178 and Mn 3+ 2 Mn 2+ 1 79 oxidation levels Apart from the linear topology a large family of triangular Mn 3+ 3 SMMs is also known in the literature. 180

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127 Structure of [Mn 4 (edteH 2 ) 2 (hmp) 2 Cl 2 ][Mn II Cl 4 ] (4 3 ) A partially labeled representation of the [Mn 4 (edteH 2 ) 2 (hmp) 2 Cl 2 ] 2 + cation of 4 3 is shown in Figure 4 3 and selected interatomic distances and angles are summarized in Table A 6 Complex 4 3 crystallizes in the monoclinic space group C2/c with the cation lying on an inversion center and consisting of a planer Mn 4 rhombus. The Mn 4 comprises of two bridg ing 3 O (O5) of hmp ligand above and below the Mn 4 plane. The [Mn 4 ( 3 O R ) 2 ] 6+ unit is commonly 4, A ) In this case 2+ ) is chelated by a hexadentate edteH 2 ligand and body is connect ed by two bridging 3 O (O5) of hmp ligand The Mn 2 edges of Mn 4 rhombus are comprised of O (O2, O4) of 1 1 2 2 : 3 bridging edteH 2 ligand ( Figure 4 14 ). Mn1 is seven coordinate and Mn2 is six coordinate and their oxidation states were determin ed by charge considerations and metric parameters, and confirmed by BVS calculations (Table 4 5 ) Mn2 undergoes Jahn Teller (JT) axial elongation as expected for near octahedral geometry with the elongated axes as ( green thick lines in Figure 4 3) The Mn2 is also ligated by a terminal Cl ion. Charge balance consideration s require 2 Mn 3+ 2 Mn 2+ 2 edte H 2 2 2hmp 2Cl and [ MnCl 4 ] The protonation levels of all O atoms in 4 3 have been de termined by BVS calculations, and the results are list ed in Table 4 9 hydrogen bonds between OH groups of edteH 2 and MnCl 4 These interactions serve to link the neighboring Mn 4 units in the crystal into a 1D pol ymeric chain (Figure 4 4, B ) There are a large number of Mn 4 clusters known in the literature 170 178 and th ey cover a wide range of metal topologies such as linear units, rectangles, rhomb i cubanes, b utterflies, and so on. The [Mn 4 ( 3 OR) 2 ( OR) 4 ] 4+ core

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128 has been seen before 181 but this is the first example with a mixed edteH 2 and hmp chelate system. Structure of [Mn 6 O 2 (O 2 C Bu t ) 6 (edteH) 2 (N 3 ) 2 ] (4 4) The structure (A) and a stereoview (B) of 4 4 are presented in Figure 4 5 and selected interatomic distances and an gles are summarized in Table A 6 The complex 4 4 crystallizes in the monoclinic space group C2 /c with the Mn 6 cluster located on an inversion center. The core (Figure 4 5, C ) comprises two [Mn 3 ( 3 O)] 7+ triangular subunits linked via bridging oxygen atoms (O9, O10) of 2 2 3 : edteH ligands ( Figure 4 14 ) Mn 1 and Mn2 are further bridged by a bridging oxygen atom (O8) from the edteH ligand. Coordination of the Mn center s is completed by six bridging t Bu CO 2 groups and two terminal azide groups. All Mn ions are six coordinate and in the +3 oxidation state, as determined by charge calculations, metric parameters and BVS calculations ( presented in Table 4 5 ) The near octahedral Mn 3+ ions undergo JT elongation and the elongated axes are shown as green thick line s in Figure 4 5 (A ) The JT elongated axes at Mn2 and Mn3 are oriented in a near parallel fashion to each other and perpendicular to that of Mn1. The c harge b alance consideration requires 6 Mn 3+ 2O 2edteH 6 t Bu CO 2 and 2 N 3 ; t he protonation levels of all O atoms in 4 4 have been confirmed by BVS calculations, and the results are listed in Table 4 9 There are a large number of substituted salicylaldoxime ligand containing Mn 6 clusters (R saoH 2 family) known in the literature. 182 The complex 4 4 has some structural resemblance with this series, but with an eth ylenediamine based alko xide containing ligand

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129 Structure of Na 2 [Mn 10 O 4 (OMe) 2 (O 2 CEt) 6 ( edte) 2 (N 3 ) 6 ] (4 5 ) A partially labeled representation (A) and a stereoview (B) of 4 5 are presented in Figure 4 6 S elected interatomic distances and angles are listed in Table A 9. Complex 4 5 cr ystallizes in the triclinic space g roup P 1 with the Mn 10 on an inversion center The core of 4 5 consists of a central Mn 4 rhombus, fused on each side by two distorted [Mn 4 O 2 (OR) 2 ] 5+ cubanes as shown in Figure 4 7 (A) Each distor ted cubane is further linked to Na1 by a bridging 4 O (O14) ion. Each cubane consists of three Mn 3+ ( Mn3, Mn4, Mn5), one Mn 2+ (Mn1) two bridging 4 O ( O13 & O14) and two oxygen atoms (O2, O4) from a 3 3 3 2 : 6 edte ligand ( Figure 4 14 ). The cubane is connected to the central Mn 2 dimer by t wo oxygens (O1, O3) from 3 3 3 2 edte ligand s and one O ( O7) from a OMe group. The coordination sites of the Mn ions are completed by six peripheral EtCO 2 groups and six terminal azide groups. The coordination sites of the Na ions are complete d by terminal MeCN, MeOH and end to end azide (N7 N8 N9) groups. Each Na ion is connected with the neighboring Na ion by two end to end azide bridges (N7 N8 N9), thus making a 1D polymer of Na 2 Mn 10 (Figure 4 7 B ). All Mn ions (Mn2, Mn3, Mn4, Mn5) are six coordinate and are in +3 oxidation state except Mn1 (seven coordinate and in +2 oxidation state ) as determined by charge calculations, metric parameters and BVS calculations ( Table 4 6 ) All eight near octahedral Mn 3+ ions undergo JT elongation roughly fo ur parallel and four perpendicular axes where all the elongated axes are shown as green thick line s in the Figure 4 6 (A) Charge balance considerations require 8Mn 3+ 2Mn 2+ 2Na + 4O 2MeO 6 EtCO 2 2edte and 6 N 3 The protonation levels of all O a toms in 4 5 have been determined by BVS calculations, and the results are listed in Table 4 10

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130 Structure of (NEt 4 ) 2 [Mn 10 O 4 (OH) 2 (O 2 CEt) 6 (edte) 2 (N 3 ) 6 ] (4 6 ) A partially labeled representation (A) and a stereoview (B) of [Mn 10 (O) 4 (OH) 2 (O 2 CEt) 6 (e dte) 2 (N 3 ) 6 ] 2 ( 4 6 ) are presented in Figure 4 8 and selected intera tomic distances and angles are listed in Table A 8 The cores of 4 5 and 4 6 are very similar (as described above). Complex 4 6 is a discrete Mn 10 cluster with two tetraethylammonium counter cations. These NEt 4 + cations replace the Na + ions of 4 5 but unlike 4 5 no end to end azide bridges, connecting the individual Mn 10 units exist in 4 6 The BVS calculations for all the Mn ions and O atoms have been listed in Table s 4 6 and Table 4 10 A number of other Mn 10 complexes have previously been reported 183 188 These possess a variety of metal topologies such as molecular cage, loop, wheel, and rod but none of them have possessed the core of the complex 4 5 / 4 6 which is unprecedented in the literature Structure of [Mn 20 O 8 (OH) 6 ( O 2 CEt ) 6 (edte) 4 (edteH) 2 ] (ClO 4 ) 4 (4 7 ) The structure of the cation of 4 7 (A) and a ster e opair (B) is shown in Figure 4 9 and selected inter atomic distances and angles are summarized in Ta ble A 9 Complex 4 7 cr ystallizes in the monoclinic space g roup P2 1 /c where the asymmetric unit c onsists of a half Mn 20 cluster and two perchlorate anions The cation of 4 7 holds a central face sharing dicubane unit [Mn 6 O 2 (OR) 4 ] fused with an adjacent ed ge sharing distorted dicubane unit on both sides (highlighted by thick pink lines in Figure 4 10 ). The central dicubane unit comprises of 4Mn 3+ (Mn1, Mn3), 2Mn 2+ (Mn2), 2 6 O (O2) and two bridging oxygen atoms (O9, O11) from 3 3 3 3 edte ligand. Each terminal dicubane unit is further linked with a butt erfly unit (highlighted by thick blue lines in F igu re 4 10 ), Mn 4 ( 3 O) 2 (Mn7, Mn8, Mn9, Mn10) shared by a common Mn 3+ ion (Mn7). This gives an overall tube like arrangement of twenty Mn atoms insid e of which are two

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131 6 O (O2) and two 4 O (O6) ions (Figure 4 10 ) There are four additional 3 O (O4, O5) ions and six OH (O1, O3, O7) ions helping to maintain this tubular structure There are three different bridging modes of the ligand in comp lex 4 7 All four edte groups bind as hexadentate chelates to Mn ions (Mn2, Mn9) and then bridge through its deprotonated alkoxide arms to various Mn atoms. Hence, two edte are overall 3 3 : 3 3 : 7 bridging ( Figure 4 14 ) and the other two are 2 : 2 2 3 : 5 bridging ( Figure 4 14 ). The remaining two edteH are pentadentate to Mn ions (Mn8) and are overall 2 : 3 2 : 4 bridging ( Figure 4 14 ). The remaining ligation of the molecule is provided by six propionate ligands, two of which are 1 1 bridging (Mn3, Mn7), two each are 2 chelating on Mn10, and 1 terminal on Mn6. Charge b alance considerations require 14 Mn 3+ 6 Mn 2+ 8 O 6 O H 6 Et CO 2 4 edte 2 edteH and 4ClO 4 The protonation levels of all Mn an d O atoms in 4 7 have been decid ed by B VS calculations, and the results are listed in Table s 4 7 and 4 11. The oxide s, OH and edte O atoms have B VS values of >1. 83 confirming them as completely deprotonated, as concluded above from their bridging modes. All Mn ions are six coordinate except, Mn3 (penta coordinate) and Mn2 (seven coordinate). The JT elongation axes on six coordinate M n 3+ atoms are shown as thick green bonds in Figure 4 9 It shows the presence of six parallel JT axes countered by six near perpendicular JT axes. There ar e only two other M n 20 cluster s available in the literature 60,189 Between them one has structural resemblance with complex 4 7. However, the core of complex 4 7 is different than that of the previously reported Mn 20 due to the oxidation state of the Mn ions and coordination modes of the surrounding ligands.

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132 Structure of [Ca 2 Mn 18 O 8 (OH) 5 (O 2 CBu t ) 4.5 (edte) 5 (NO 3 ) 3.5 ](O 2 CBu t ) 0.5 (NO 3 ) 0.5 (4 8 ) The structure of cation of 4 8 ( Figure 4 11 ) and a stereopair ( Figure 4 12 ) are s hown and selected interatomic distances and angles are listed in Table A 10 Complex 4 7 cr ystallizes in the monoclinic space g roup P2 1 /c where the asymmetric unit c onsists of a M n 18 Ca 2 clust er cation, a half nitrate group and a half t BuCO 2 anion The core of the structure is similar to that of 4 7 which means the cation of 4 8 also possesses a central face sharing dicubane unit [Mn 6 O 2 (OR) 4 ] fused with an adjacent edge sharing distorted dicubane unit on both sides (Figure 4 13 A) The cen tral dicubane unit comprises of 4Mn 3+ (Mn2, Mn7, Mn12, Mn16), 2Mn 2+ (Mn3, Mn17), 2 6 O (O14, O17) and four bridging oxygen atoms (O5, O6, O51, O52) from the 3 3 3 : 3 edte ligand (highlighted by thick pink lines in F igu re 4 13 B ). The two adjacen t distorted dicubane units consist of a set of 4Mn 3+ (Mn1, Mn2, Mn8, Mn18 and Mn4, Mn13, Mn15, Mn16), 2Mn 2+ (Mn11, Mn17 and Mn6, Mn3), 6 O (O14 and O17), 4 O (O31 and O37), 2 3 O (O1, O2 and O47, O48) and two bridging oxygen atoms (O53, O58 and O 7, O8) from the edte ligand. These two edte groups bind as hexadentate chelates to Mn ions (Mn3, Mn17) and then bridge through their deprotonated alkoxide arms to various Mn atoms. Hence, they are overall 3 3 3 3 : 7 bridging ( Figure 4 14 ). The edge of each distorted dicubane unit (Mn1, Mn18 and Mn15, Mn4) further connects to 1Ca 2+ (Ca1, Ca2) ion and 2Mn 3+ ions (Mn9, Mn10 and Mn5, Mn14) by two edte ligands. For example, Ca1 and Mn1 are connected by two 3 O atoms (O19, O27) of two edte ligand s. The total four terminal edte ligands are hexadentate chelated to Mn ions (Mn9, Mn10 and Mn5, Mn14) and are overall 2 2 2 : 3 : 5 bridging ( Figure 4 14 ) Mn4 and Mn14 are further connected by a bridging OH ion (O36'). The remaining ligation of the molecule is provided by four t BuCO 2 ligands

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133 and four nitrate groups. Two of the carboxylate ligands are 1 1 bridging (Mn1, Mn2 and Mn15 and Mn16), and two are 1 terminal on Mn4 and Mn18 atoms. The four nitrates are 2 chelating on Ca1 and Ca2 ions. One of the nitrate groups is disordered with a t BuCO 2 group by an occupancy factor of 0.5 at the Ca2 site. Charge b alance consideration requires 14 Mn 3+ 4 Mn 2+ 8 O 5 O H 5 t Bu CO 2 6 edte and 4ClO 4 The protonation levels of all Mn an d O atoms i n 4 7 have been decided by B VS calculations and the results are listed in Table s 4 7 and 4 12 The oxide s, OH and edte O atoms have B VS values of >1. 8 confirming them as completely deprotonated, as concluded above from their bridging modes. The BVS va lues for O9 and O31 are ~1.6, little smaller for edte O atoms and the reason can be attributed to the presence of H bonds in the respective O atoms. All Mn ions are six coordinate except, Mn4, Mn14, M10 and Mn18 (seven coordinate). The JT elongation axes on six coordinate M n 3+ atoms are shown as thick green bonds in Figure 4 11 Compound 4 8 is the largest Mn Ca cluster to date with a structurally unprecedented metal topology. There are only very few Mn/Ca complex es known in the literature 190 192 and the field is still in its infancy for mimicking the structural features and various physical properties of WOC in photosy stem II. Magnetochemistry Dc and a c m agneti c susceptibility s tudies of 4 1 and 4 2 Solid state, v ariable temperature dc magnetic susceptibility data in a 0.1 T field and in the 5.0 300 K range were collected on powdered microcrystalline samples of 4 1 H 2 O and 4 2 2 O The M T values for complexes 4 1 and 4 2 at 300 K are 9.13 and 10.98 cm 3 Kmol 1 lo wer than the spin only ( g = 2) value expected for three non interacting Mn ions ( Mn 3+ and 2 Mn 2+ ) which is 11.75 cm 3 Kmol 1 This is indicative of

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134 antiferromagnetic exchange interactions among the paramagnetic Mn ions in the system For both complexes M T stay s fairly constant down to 150 K and decreases smoothly to 5.75 and 5.79 cm 3 Kmol 1 at 5.0 K, which is suggestive of a S = 3 ground state. The data for complex 4 1 and 4 2 were fit to the theoretical M vs T and M T vs. T expression for a linear Mn II Mn I II Mn II core with two exchange coupling parameters, J and J' representing the Mn II Mn III and Mn III Mn III interactions, respectively. The corresponding Heisenberg s pin Hamiltonian is given by equation 4 8 (the atom labeling is that of Figure 4 15 ) and its eig envalues in equation 4 10 where A 2 3 and T A + 1 The corresponding Heisenberg spin Hamiltonian (equation 4 8) can be converted to an equivalent one in equation 4 9 by using the Kambe vector coupling method 193 J ( 1 2 + 1 3 J 2 3 (4 8) ( T 2 A 2 1 2 J ( A 2 2 2 3 2 ) (4 9) E ( S T ,S A J [ S T ( S T S A ( S A J [ S A ( S A +1)] (4 10) There are 24 possible S T states ranging in value from 0 to 7. The eigenvalue expression and the Van Vleck equation were used to derive the theoretical M vs. T and M T vs. T expression for complexes 4 1 and 4 2 and this was used to least squares fit the experimental data. The fit for complex 4 1 (solid line in Figure 4 16 ) gave J = 2.6(2) cm 1 = 0, and g = 1.99(1) and the fit for complex 4 2 (solid line in Figure 4 17 ) gave J = 1.4(1) cm 1 J' = 0.3(1) cm 1 and g = 1.98(1). These values identify the |S T S A > = |3, 5> state to be the ground state, as expected. For complex 4 1 the next excited state, |2, 4> lies 10.60 cm 1 above the ground state followed by two degenerate energy states of |4, 5> and |1, 3 > separated by 21.20 cm 1 from th e lowest energy state (Figure 4 18 ). For complex 4 2 the next two excited states, |2, 4> and |4, 5> lie 8.6 and 11.3 cm 1 a bove the ground state (Figure 4 19 ). The type of exchange interactions in both the

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135 molecules is sim ilar however in 4 1 the magnetic interaction between two Mn 2+ ions is practically zero. The spin configuration of the excited states is also different in both the complexes which show the effect of minor structural change in magnetic properties. To confirm the above initial estimates of the ground state spin of the compound 4 1 and 4 2 variable field ( H ) and temperature magnetization (M) data were collected in the 0.1 7 T and 1.8 10 K ranges. The resulting data for 4 1 and 4 2 are plotted as reduced m agnetization ( B ) vs. H/T where N is Avogadro's number and B is the Bohr magneton. The data were fit using the program MAGNET, 66 by diagonalization of the spin Hamiltonian matrix assuming only the ground state is populated, incorporating axial anisotropy ( z 2 ) and Zeeman terms, and employing a full powder average. The corresponding spin Hamiltonian is given by equation 4 11, where z is the easy axi s spin operator, g is the Land g factor, and 0 is the vacuum perm eability. The last term in equation 4 11 is the Zeeman energy associated with the applied magnetic field. H = z 2 + B 0 (4 11) However, for neither 4 1 nor 4 2 were we able to obtain a satisfactory fit. This is not unusual in Mn x cluster chemistry, and is almost always 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 t emperatures, and/or (ii) even higher lying excited states whose S is greater than the ground state become populated as their larger M S 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 popula tion of only the ground state 94 96 A large density of low lying excited states is expected for those with a significa nt content of

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136 Mn 2+ atoms, which give weak exchange couplings. One likely way to avoid the above mentioned complication is to collect the data at the lower fields Indeed, a satisfactory fit was achieved using data in fields up to 3 T for 4 2 with fit param eters S = 3, g = 2.03, and D = 0.38 cm 1 (Figure 4 20 ). Alternative fits were also obtained with S = 2, g = 2.93 and S = 4, g = 1.57, but were discarded because of the unreasonable g values. The fitting was also good for S = 3, g = 2.02, and D = 0.37 cm 1 In order to ensure that the true global minimum had been obtained and to assess the hardness of the fit, a root mean square D vs g error surface for the fit was generated using the program GRID, 97 which calculates the relative differen ce between the experimental M/N B data and those calculated for various combinations of D and g This is shown as a 2 D contour plot in Figure 4 21 True global minima were ob served. The fit uncertainties are quite hard and estimated as g = 2.03(1), D = 0.38(1) cm 1 The satisfactory fitting was not achieved for 4 1 even at lower fields. In all the previous chapters, it has been stated that ac magnetic susceptibility studies a re a powerful complement to dc studies. This is a useful technique for determining the ground state of a system, since they preclude any complications arising from the presence of a dc field. Hence, ac studies on microcrystalline complex 4 1 H 2 O and 4 2 2 O were carried out in the 1.8 15 K range using a 3.5 G ac field oscillating at 1000 Hz. Ac study can probe the relaxation of the magnetisation vector. If the magnetisation vector relaxes fast enough to keep in phase with the oscillating fi eld, then the suceptibility ( M ) will be equal to the dc suceptibility. However, if the barrier of magnetisation relaxation is significant compared to thermal energy (k T ), then the in phase signal can not keep up with the oscillating field and decreses wi th the

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137 concomitant rise of the out of phase signal. This behavior is suggestive of the superparamagnet like properties of an SMM. For both complexes the in phase M 'T signals below 15 K are almost temperature independent (F igure 4 22 ), and the extrapolati on of the plots to 0 K gives values of ~ 5.95 and 5.26 cm 3 K m ol 1 indicating an S = 3 ground state and g ~ 2.0. The dc magnetization fit for 4 1 gives a g value of 1.99(1) which closely agree with the ac data Both the complexes 4 1 and 4 2 did not exhibit a n out of phase ac magnetic suceptibility signals above 1.8 K, i.e., they are not SMMs. Dc and a c magnetic susceptibility s tudies of 4 3 Solid state, variable temperature dc magnetic susceptibility data in a 0.1 T field and in the 5.0 300 K range were c ollected on powdered microcrystalline samples of 4 3 The obtained data are plotted as M T vs T in Figure 4 23 The M T value at 300 K is 18.18 cm 3 Kmol 1 a little smaller than that expected for five non interacting Mn ions (19.13 cm 3 Kmol 1 for 3Mn 2+ and 2Mn 2+ ions). The magnetic susceptibility steadily rises to 41.58 cm 3 Kmol 1 at 6.5K, indicative of dominant ferromagnetic interaction s T he M T value decreases slightly to 41.01 cm 3 Kmol 1 at 5 K due to some intermolecular interactions. Since 4 3 has a MnC l 4 as counterion the spi n ground state of the cluster cation was contribution of 4.375 cm 3 Kmol 1 from the overall M T value The resulting value of 37.21 cm 3 Kmol 1 at 6.5 K suggests an S = 9 ground state spin for g < 2 F urther studies were carried out for investigating spin ground state as before. The M T vs T data were least squares fit to a theoretical expression derived for a Mn 4 core with two exchange coupling parameters, J wb and J bb (as shown in Figure 4 24 ). 181 The corresponding Heisenberg spin Hamiltonian (equation

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138 4 12) can be converted to an equivalent one in equation 4 13 by using the Kambe vector coupling method 193 A 1 3 B 2 4 T A B J wb ( 1 2 + 1 4 + 2 3 + 3 4 J bb 1 3 (4 12) wb ( T 2 A 2 B 2 J bb ( A 2 1 2 3 2 ) (4 13) E( S T S A S B J wb [ S T ( S T S A ( S A S B ( S B J bb [ S A ( S A +1)] (4 14) There are a total of 110 possible S T states ranging in value from 0 to 9, where S T is the total spin of the Mn 4 cluster. This eigenvalue expression (equation 4 1 4) and the Van Vleck equation were used to derive a theoretical M T vs T expression for 4 3 and this wa s used to least squares fit the experimental data. The fit (solid line in Figure 4 23 ) gave J bb = +7.20(3) cm 1 J wb = +1.34(3) cm 1 and g = 1.87(2) with temperature independent paramagnetism (TIP ) fixed at 6 x 10 4 cm 3 mol 1 The spin ground state is sensi tive to the ratio of J wb /J bb and the plots of E/J bb vs. J wb /J bb by using the equation 4 14, truly identify a | S T S A, S B > = |9, 4, 5> ground state for 4 3 as shown in Figure 4 25 (A) Since, the J values are positi ve, the system is not spin frustrated and the lowest energy state indicates only one spin state, independent of the J wb /J bb ratio. The first two excited states are the |8, 4, 4 > and |7, 4, 3 > states at 10.72 and 21.44 cm 1 above the ground state, respectively (Figure 4 25 B ). The ac susceptibili ty data were collected on microcrystalline samples in a 3.5 G ac field and these signals for complex 4 3 at 50 and 250 Hz are plotted as 'T vs T in Figure 4 26 The M 'T slowly rises from 40.49 cm 3 Kmol 1 at 15 K to 43.12 cm 3 Kmol 1 at ~ 7 K and then decreases sharply to 37.98 cm 3 Kmol 1 at 1.8 K for the complex 4 3 This decrease at low temperature is not frequency dependent and can be att ributed to intermolecular antiferromagnetic interaction s The behavior indicates that compound 4 3 is not an SMM. Due to the steep downfall of the ac signals, the determination of the

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139 spin ground state from in phase ac data is not accurate. There are other examples of Mn 4 clusters with similar butterfly core s of S = 9 spin ground state, which all exhibit characteristic properties of SMMs. However, the compound 4 3 does not show any slow relaxation at low temperature and the outcome could be explained by the presence of strong antiferromagnetic interaction s between Mn 4 units with MnCl 4 ion s in the cluster which prevails at very low temperature (Figure 4 4 B ) Dc and a c magnetic susceptibility s tudies of 4 4 Solid state, variable temperature dc magnetic sus ceptibility data in a 0.1 T field and in the 5.0 300 K range were collected on powdered microcrystalline samples of 4 4 The 300 K value 16.97 cm 3 Kmol 1 is slightly lower than the spin only ( g = 2) value of 18.0 cm 3 Kmol 1 for six non interacting Mn 3+ io ns, indicative of antiferromagnetic interaction s in the core The M T value smoothly decreases from 300 K down to 7.87 cm 3 Kmol 1 at 15K and then stays fairly constant down to 5K. This results in a plateau, observed at very low temperature which indicates a stable and well isolated ground state spin. The M T at 5 K sugge sts an S = 4 ground state (Figure 4 27 ). Confirmation of these came from the reduced magnetization fit. For this purpose, variable field ( H ) and temperature magnetization ( M ) data were collected i n the 0.1 7 T and 1.8 10 K ranges however, best fit da ta were obtained at l ower fields of 3.0 6.0 T. The resulting data are plotted as reduced magnetization ( B ) vs H / T and t he data were fit ted using the program MAGNE T 66 as explained before. A satisfactory fit was obtained with the fit parameters S = 4, D = 1.03 cm 1 and g = 1.86 (Figure 4 28 ). The obtained g value is reasonable for a system containing only Mn 3+ ions. The fit for an S = 4 gro und state spin considering a positive D gave an unreasonable value of D = +4.85 cm 1 and

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140 was therefore discarded. A lternative fits were also obtained with S = 3, g = 2.48 and S = 5, g = 1.52 but were discarded because of the unreasonable g values In order to ensure that the true global minimum had been obtained and to assess the hardness of the fit, a root mean squar e D vs g error surface for the fit was generated using the program GRID, 97 which calculates the relative difference between the experimental M/N B data and t hose calculated for various combinations of D and g This is shown as a 2 D contour plot in Figure 4 29 Five local minima were observed where the values of D are spanning over 0.95 to 1.15 cm 1 and the corresponding g varies from 1.83 to 1.90. T he fit uncertainties are quite soft and estimated as g = 1.86 +/ 0.04 D = 1.03 +/ 0.10 cm 1 The large negative D value indicates appreciable anisotropy in the system which would be consistent with the four near ly parallel JT elongated axes in the complex (Figure 4 5 A ). A c susceptibility data were colle cted on a microcrystalline sample of 4 4 in a 3.5 G ac field oscillating at 50 1000 Hz The data are plotted as 'T vs T in Figure 4 30 Below 10 K, the 'T value is almost temperature independent at a value of ~8.6 cm 3 Kmol 1 confirming an S = 4 ground state spin for 4 4 In spite of having significantly high negative D value, no frequency dependent ac signal was observed above 1.8 K. Such a notably high D value has been observed before in a family of substituted salicylaldoxime ligand containing Mn 6 and Mn 3 c lusters. 180,182 The absence of slow relaxation in 4 4 can be attributed to fast quantum tunneling of magnetization through the spin reversal barrier. 194 On a separate note, the D could be less than 1 cm 1 The reduced magnetization fit may not provide an accurate estimation of D value e.g., the

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141 magnetization saturation may have been affected by the presence of low lying excited states Dc and a c magnetic susceptibility s tudies of 4 5 and 4 6 Solid state, variable temperature dc magnetic susceptibility data in a 0.1 T field and in the 5.0 300 K range were collected on powdered microcrystalline samples of 4 5 2 O and 4 6 2 O The M T values at 300 K are 29.49 and 30.73 cm 3 K mol 1 a little lower than the spin only ( g = 2) value of 32.75 c m 3 Kmol 1 expected for a system containing ten non interacting Mn ions (8Mn 3+ and 2Mn 2+ ). The behavior is indicative of strong antiferromagnetic interactions among the paramagnetic metal centers within the cluster For both complex es M T decreases smoothly to 9.64 and 12.06 cm 3 Kmol 1 respectively, at 5.0 K suggesting ground state spin in the S = 4 or 5 region (Figure 4 3 1 ) The difference in the M T values at 5 K between the complexes 4 5 and 4 6 could be due to the effect of intermolecular interactions pr evails at a low temperature. Ac data were colle cted on microcrystalline sample s of 4 5 2 O and 4 6 2 O in a 3.5 G ac field oscillating at 1000 Hz and are plotted as 'T vs T in Figure 4 32 The extrapolation of in phase ( M 'T ) signal at ~7.4 and 9.8 cm 3 K m ol 1 confirms the S = 4 ground state spin for both complexes. The downward slope of the T plot in both the complexes indicates the presence of several low lying excited states coming from weak exchange coupling in Mn 2+ and/or spin frustration effects. This feature affects the dc reduced magnetization fit where the saturation of the magnetization could not be achieved at low temperature and high field. As a result, for both complexes, a ttempts to fit the data resulted in poor quality and unreliable fit and the reasons discussed in detail earlier for complexes 4 1 and 4 2

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142 Dc and ac m agneti c susceptibility s tudies of 4 7 Solid state, variable temperature dc magnetic susceptibility data in a 0.1 T field and in the 5.0 300 K range were collected on powde red microcrystalline samples of 4 7 6 H 14 The obtained data are plotted as M T vs T in Figure 4 33 for 4 7 The 300 K value is lower than the spin only ( g = 2) value of 68.25 cm 3 Kmol 1 indicative of strong antiferromagnetic interaction and the value c orresponds to twenty non interacting Mn ions (14 Mn 3+ and 6 Mn 2+ ) The M T reaches a plateau at 20 K and the value decreases down to 28.8 cm 3 Kmol 1 at 5 K suggesting S = 7 ground state spin. Further verification of spin ground state comes from the reduced magnetization fit. Variable field ( H ) and temperature magnetization ( M ) data were collected in the 0.1 0.8 T and 1.8 10 K ranges. The resulting data are plotted as reduced magnetization ( B ) vs H / T and t he data were fit ted using the program MAGNE T as explained before. The solid lines in Figure 4 34 (A ) are a fit of the experimental data with the fit parameters S = 7, D = 0.09 cm 1 and g = 2.00. Fitting is fairly good for D = 0.18 cm 1 and g = 1.99 An alternative fit is obtained with S = 8 and g = 1.76 where g is too small for a Mn 2+ / Mn 3+ system and hence this possibility is discarded. In order to ensure that the true global minimum had been obtained and to assess the hardness of the fit, a root mean square D vs g error surface for the fit was ge nerated using the program GRID, which calculates the relative difference between the experimental M/N B data and those calculated for various combinations of D and g This is shown as a 2 D contour plot in Figure 4 34 (B ) Two local minima were observed. T he one at D = 0.18 cm 1 exhibits the largest error and is hence discarded. A t rue global minimum was observed and the

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143 parameter set is shallow and the fit uncertainties are thus estimated as g = 1.98 (2), D = 0.08cm 1 (2). For further confirmation of the spin ground state in the system, ac susceptibility data were colle cted on a microcrystalline sample of 4 7 1.5C 6 H1 4 in a 3.5 G ac field oscillating at 50 1000 Hz The data are plotted as 'T vs T in Figure 4 35 over a te mperature range spanning 1.8 15 K. The extrapolation of the in phase ( M 'T ) signal (~28 cm 3 Kmol 1 ) to 0 K confirms the S = 7 ground stat e spin for compound 4 7 Despite the presence of small anisotropy in the molecule, no significant frequency dependent signal is observed in out of phase ac study which indicates that compound 4 7 is not an SMM (Figure 4 35) Dc and ac m agneti c susceptibili ty s tudies of 4 8 Solid state, variable temperature dc magnetic susceptibility data in a 0.1 T field and in the 5.0 300 K range were collected on powdered microcrystalline samples of 4 8 3 H 2 O The obtained data are plotted as M T vs T in Figure 4 36 (A ) for 4 8 The M T at 300 K is 39.67 cm 3 Kmol 1 significantly lower than the spin only ( g = 2) value of 59.50 cm 3 Kmol 1 expected for eighteen non interacting Mn ions (14 Mn 3+ and 4 Mn 2+ ) T his behavior indicates the presence of strong antiferromagnetic interactions among the Mn ions at 300 K. The M T at 5 K for 4 8 is 7.31 cm 3 K mol 1 suggesting an S = 4 ground state spin. Compound 4 8 is structurally similar to 4 7 where two terminal Mn 2+ (M n10) ions are replaced by two diamagnetic Ca 2+ ions. This structural difference causes a notable change in the interactions among the remaining Mn 2+ / Mn 3+ Mn 2+ /Mn 2+ and Mn 3+ /Mn 3+ ions reflecting the extent of the antiferromagnetic interactions among the pa ramagnetic centers along with the change in spin ground state. For further

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144 verification of the spin ground state variable field ( H ) and temperature magnetization ( M ) data were collected in the field ranges to both 0.1 1 T and 0.1 7 T field in the tem perature range spanning 1.8 10 K. The resulting data were plotted as reduced magnetization ( B ) vs H / T and t he data were fit ted using the program MAGNE T as explained before. Both possible solutions are discarded due to unreasonable D values (both pos itive and negative) assuming either S = 4 or S = 3 spin ground state. This can be attributed to the presence of low lying excited states as a result of weak interactions and/or spin frustration effects, which is evident from the in phase ac study. For conf irmation of spin ground state, ac susceptibility data were collected on a microcrystalline sample of 4 8 3 H 2 O in a 3.5 G ac field oscillating at 50 1000 Hz. The data are plotted as 'T vs. T in Figure 4 36 (B ) at the temperature range spanning 1.8 15 K. The 'T value at 15 K is 11.54 cm 3 Kmol 1 which then decreases sharply down to 5.94 cm 3 Kmol 1 at 1.8 K which is indicative of the depopulation of excited states of higher spin ground state values. The feature also suggests the presence of a large number of low lying excites states close to the ground state. This feature makes it difficult to extrapolate t he ac in phase data to 0 K to rationalize the spin ground state, however, a reasonable attempt justifies an S = 4 spin state for a value of ~ 7.0 cm 3 K mol 1 at 0 K. Concluding Remarks The present work provides an ensemble of various nuclearity oxide, carbox ylate rich Mn x and Mn/Ca clusters (x = 3, 4, 6, 10 20 ) containing edteH 4 ligand. The crystal structures, syntheses and magnetic properties of all these complexes have been discussed. This work once again proves the ability of the edteH 4 ligand to bridge t o

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145 many metal ions, e.g. one edte ligand links to six metal ions in 4 5 / 4 6 Mn 18 Ca 2 is the highest nuclearity Mn/Ca cl uster known to date and this establishes the gateway of making polynuclear Mn/Ca clusters relevant to WOC in photosystem II in bio inorganic research. This outcome open s up the possibility of isolating new higher nuclearity Mn clusters in polynuclear metal chemistry, perhaps mixed metal chemistry and will be explored in the following chapters. The current study shows the possibility of analyzing a new mixed chelate syste m using both the edteH 4 and hmpH ligands together for the first time in Mn cluster chemistry.

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146 Table 4 1. Crystallographic data for 4 1 4 CH 2 Cl 2 4 2 and 4 3 parameter 4 1 4 2 4 3 formula C 30 H 61 Cl 8 Mn 3 N 4 O 14 C 24 H 50 ClMn 3 N 4 O 16 C 32 H 52 Cl 6 Mn 5 N 6 O 10 fw, g/mol 1150.25 850.95 1168.2 crystal system Triclinic Monoclinic Monoclinic space group P 1 P2(1)/n C2/c a 11.571(2) 15.762(4 ) 26.757(2) b 13.783(2) 14.032(4) 10.7825(9) c 17.330(3) 17.582(5) 15.7930(13) 89.845(3) 88.967(3) 65.826(3) 90 113.187(4) 90 90 91.931(2) 90 V, 3 2521.0(8) 3574.7(16) 4553.9(6) Z 2 4 8 T,K 173(2) 100(2) K 173(2) radiati on, a 0.71073 0.71073 0.71073 1.515 1.581 1.704 1 1.222 1.191 1.758 R1b,c 0.0641 0.0372 0.0419 wR2d 0.1642 0.0979 0.1144 a Graphite monochromator. b I I ). c R F o | | F c F o |. d wR w ( F o 2 F c 2 ) 2 w ( F o 2 ) 2 ] ] 1/2 w 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [ max ( F o 2 O)+ 2F c 2 ]/3

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147 Table 4 2. Crystallographic data for 4 4 4 5 2 MeCN and 4 6 parameter 4 4 4 5 4 6 Formula C 50 H 94 Mn 6 N 10 O 22 C 50 H 94 Mn 10 N 26 Na 2 O 28 C 58 H 117.50 Mn 10 N 27.50 O 25.50 fw, g/mol 1 517.0 2102.9 2157.70 crystal system Monoclinic Triclinic Triclinic space group C2/c P 1 P 1 a 23.5084(17) 12.682(2) 13.058(3) b 14.4661(11) 12.716(2) 16.551(3) c 20.2570(15) 14.595(3) 22.339(5) 90 104.264(5) 90 90.132( 4) 96.501(3) 115.491(3) 82.451(4) 82.488(7) 66.784(7) V, 3 6676.5(9) 2107.5(7) 4382.3(16) Z 8 2 2 T,K 100(2) K 173(2) 100(2) K radiation, a 0.71073 0.71073 0.71073 1.509 1.657 1.635 1 9.620 1.541 1.474 R1b,c 0.0530 0.0417 0.0 403 wR2d 0.1124 0.1107 0.0896 a Graphite monochromator. b I I ). c R F o | | F c F o |. d wR w ( F o 2 F c 2 ) 2 w ( F o 2 ) 2 ]] 1/2 w 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O)+ 2F c 2 ]/3

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148 Table 4 3. Crystallographic data for 4 7 and 4 8 parameter 4 7 x(solv) 4 8 5MeCN formula C 78 H 158 Cl 4 Mn 20 N 12 O 66 C 85 H 170 Ca 2 Mn 18 N 16 O 59 fw, g/mol 3560.76 3429.45 crystal system monoclinic monoclinic space group P2(1)/c P2(1)/c a 17.5303(16) 21.56(2) b 25.922(2) 26.62(3) c 1 6.2321(15) 26.78(3) 90 103.134(2) 90 90, 107.625(18), 90 V, 3 7183.3(11) 14648(26) Z 4 4 T,K 173(2) 100(2) radiation, a 0.71073 0.71073 g/cm 3 1.646 1.555 mm 1 1.851 1.645 R1 b,c 0.0620 0.0768 wR2 d 0.1867 0.2023 a Graphite mon ochromator. b I I ). c R F o | | F c F o |. d wR w ( F o 2 F c 2 ) 2 w ( F o 2 ) 2 ]] 1/2 w 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O)+ 2F c 2 ]/3 Table 4 4 Bond valence sums for the Mn atoms of complex es 4 1 4 2 a Atoms 4 1 4 2 Mn 2+ Mn 3+ Mn 4+ Mn 2+ Mn 3+ Mn 4+ Mn1 3.43 3.16 3.10 1.62 1.51 1.48 Mn2 1.96 1.82 1.78 3.40 3.14 3.08 a The underlined value is the one closest to the charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest w hole number to the underlined value

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149 Table 4 5 Bond valence sums for the Mn atoms of complex es 4 3 4 4 a Atoms 4 3 4 4 Mn 2+ Mn 3+ Mn 4 + Mn 2 + Mn 3+ Mn 4 + Mn1 2.04 1.90 1.91 3.14 2.93 2.86 Mn2 3.33 3.11 3.05 3.44 3.21 3.13 Mn3 2.08 2.14 2.08 3.46 3.19 3 .13 a See the footnotes to Table 4 4 Table 4 6 Bond valence sums for the Mn atoms of complex es 4 5 4 6 a Atom 4 5 4 6 Mn 2+ Mn 3+ Mn 4 + Mn 2 + Mn 3+ Mn 4 + Mn1 2.03 1.89 1.85 2.02 1.88 1.84 Mn2 3.36 3.09 3.04 3.29 3.09 3.00 Mn3 3.44 3.17 3.11 3.35 3.09 3 .03 Mn4 3.36 3.14 3.06 3.31 3.05 2.99 Mn5 3.28 3.09 3.06 3.33 3.10 3.03 a See the footnotes to Table 4 4 Table 4 7 Bond valence sums for the Mn atoms of complex 4 7 and 4 8 a Atom 4 7 4 8 Mn 2+ Mn 3+ Mn 4+ Mn 2+ Mn 3+ Mn 4+ Mn1 3.34 3.09 3.03 3.39 3.13 3 .07 Mn2 1.92 1.80 1.75 3.36 3.10 3.04 Mn3 3.24 2.99 2.94 1.99 1.86 1.81 Mn4 3.18 2.93 2.87 3.28 3.15 2.97 Mn5 2.06 1.90 1.86 3.15 2.94 2.87 Mn6 3.37 3.11 3.06 2.81 2.59 2.54 Mn7 3.54 3.10 3.04 3.33 3.07 3.01 Mn8 3.30 3.08 3.00 2.81 2.60 2.54 Mn9 3. 14 2.93 2.86 3.20 2.98 2.91 Mn10 2.28 2.11 2.07 3.02 2.82 2.75 Mn11 2.34 2.19 2.15 Mn12 3.32 3.06 3.00 Mn13 2.38 2.19 2.15 Mn14 3.45 2.87 2.80 Mn15 3.28 3.03 2.97 Mn16 3.39 3.13 3.07 Mn17 1.95 1.82 1.79 Mn18 3.16 2.92 2.86 a See the footnotes to Table 4 4

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150 Table 4 8 Bond valence sums for the O atoms of complex es 4 1 and 4 2 a Compound Atom BVS Assignment Group 4 1 O1 1.82 OR edteH 2 O2 1.13 ROH edteH 2 O3 1.97 OR edteH 2 O4 1.18 ROH edteH 2 4 2 O1 1.15 ROH edteH 2 O2 1.12 ROH edteH 2 O3 1.92 OR edteH 2 O6 1.81 OR edteH 2 a The BVS values for O atoms of O 2 R OH and H 2 O groups are typically 1.8 2.0, 1.0 1.2 and 0.2 0.4, respectively, but can be affected somewhat by hydrogen bonding. Tab le 4 9 Bond valence sums for the O atoms of complex es 4 3 and 4 4 a Compound Atom BVS Assignment Group 4 3 O1 1.12 ROH edteH 2 O2 1.94 OR edteH 2 O3 1.19 ROH edteH 2 O4 1.98 OR edteH 2 O5 1.95 OR hmp 4 4 O1 2.06 O O O8 1.79 OR edt eH O9 1.77 OR edteH O10 2.03 OR edteH O11 0.88 ROH edteH a See the footnotes to Table 4 8

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151 Table 4 10 Bond valence sums for the O atoms of complex es 4 5 and 4 6 a Compound Atom BVS Assignment Group 4 5 O1 2.35 OR edte O2 1. 93 OR edte O3 1.93 OR edte O4 1.97 OR edte O7 2.05 OR OMe O13 1.87 O O O14 2.13 O O 4 6 O1 1.94 OR edte O2 1.89 OR edte O3 1.93 OR edte O10 1.89 OR edte O11 1.89 O O O12 1.92 O O O13 1.00 OH OH a See the footnotes to Table 4 8 Table 4 11 Bond valence sums for the O atoms of complex 4 7 a Compound Atom BVS Assignment Group 4 7 O1 1.13 OH OH O2 1.85 O O O3 1.15 OH OH O4 2.16 O O O5 1.84 O O O6 1.84 O O O7 1.11 OH OH O8 2.00 OR edte O9 1.96 OR edte O10 1.91 OR edte O11 1.97 OR edte O12 1.72 OR edteH O13 1.88 OR edteH O14 1.97 OR edteH O15 0.97 ROH edteH O16 1.83 OR edte O17 1.91 OR edte O18 1.83 OR edte O 19 1.97 OR edte a See the footnotes to Table 4 8

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152 Table 4 12 Bond valence sums for the O atoms of complex 4 8 a Compound Atom BVS Assignment Group 4 8 O1 1.85 O O O2 1.92 O O O5 1.91 OR edte O6 1.91 OR edte O7 1.93 OR edte 4 O8 1.89 OR edte O9 1.50 OR edte O12 2.12 OR edte O13 1.80 OR edte O14 1.18 OH OH O15 1.83 O O O16 1.19 OH OH O17 1.81 O O O31 1.73 O O O32 1.14 OH OH O33 1.17 OH OH O34 2.02 OR edte O36 1.57 OR edte O37 1.79 O O O38 2.03 OR edte O45 1.92 OR edte O46 1.94 OR edte O47 1.83 O O O48 1.89 O O a See the footnotes to Table 4 8

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153 A B Figure 4 1. Labeled representation of the cation of complex 4 1 ( A). Hydrogen atom s have been omitted for clarity. A section of the 1D polymeric chain (B) of complex 4 1 where the two Mn 3 unit s in a chain are conn ected through a MeCO 2 group by H bond s (shown as dashed lines ) Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue; C, light grey.

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154 Figure 4 2. Labeled representation of the cation of complex 4 2 Hydrogen atom s have been omitted for clarity Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue; C, light grey. Figure 4 3. Labeled representation of the cation of complex 4 3 with Jahn Teller elongation axes shown as green thick lines Hydrogen atom s have been omitted for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue; Cl, green; C, light grey.

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155 A B Figure 4 4. The core of complex 4 3 (A ) Hydrogen atom s have been omitted for clarity. A section of the 1D polymeric chain s of complex 4 3 where the two Mn 4 unit s in a chain are connected through a MnCl 4 group via H bond s ( shown as dashed lines ) (B) Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue ; Cl, green; C, light grey.

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156 A B C Figure 4 5 The labeled representation of complex 4 4 with Jahn Teller axes shown as green t hick line s (A ) a stereoview (B) and the core (C) of complex 4 4 H atoms have been removed for clarity. Color code: Mn 3+ purple; O, red; N, blue; C, light grey.

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157 A B Figure 4 6 Labeled representation of the part of ( Na 2 Mn 10 ) polymer, 4 5 with Jahn Teller elongation axes as green thick lines (A ) and a stereopair (B ). Hydrogen atoms have been omitted for clarity Color code: Mn 3+ purple; Mn 2+ cyan; Na, yellow; O, red; N, blue; C, light grey.

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158 A B Figure 4 7 T he core of complex 4 5 (A) and a section of the 1D polymeric chain of complex 4 5 (B ) Color code: Mn 3+ purple; Mn 2+ cyan; Na, yellow; O, red; N, blue; C, light grey.

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159 A B Figure 4 8 Labeled representation of the cation of complex 4 6 with Jahn Teller elongation axes shown as green thick lines (A ) and a stereopair (B). Hydrogen atom s have been omitted for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; Na, yellow; O, red; N, blue; C, light grey.

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160 A B Figure 4 9 Structural represe ntation of the cation of complex 4 7 with Jahn Teller elongation axes shown as green thick lines (A ) and a stereopair (B). Hydrogen atom s have been omitted for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; O, red; N, blue; C, light grey.

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161 Figure 4 10 Labeled representation of t he core of complex 4 7 Figure 4 11 Structural representation of the cation of complex 4 8 with Jahn Teller elongation axes shown as green thick lines. Hydrogen atom s have been omitted for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; Ca 2+ orange; O, red; N, blue; C, light grey.

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162 Figure 4 12 Structural representation of the stereopair of complex 4 8 A B Figure 4 13 Labeled representation of t he core of complex 4 8 (A) and a part of the core of complex 4 8 (B )

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163 Figure 4 14 Coordination modes of edteH 2 edteH and edte in complexes 4 1 to 4 8

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164 Figure 4 15 Schematic diagram for the linear Mn 3 core for complexes 4 1 and 4 2 with two exchange coupling parameters, J and J' Figure 4 16 P lot of M vs T for complex 4 1 H 2 O where solid line shows the fit to the experimental data. See the text for the fitting parameters.

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165 Figure 4 17. Plot of M T vs T for complex 4 2 H 2 O where solid line shows the fit to the experimental data. See the text for the fitting parameters. Figure 4 18 Plo t of all possible energy states for complex 4 1 using the J value, obtained from the fit to the experimental M data.

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166 Figure 4 19. Plot of all possible energy states for complex 4 2 using the J values, obtained from the fit to the experimental M T data. Figure 4 20 P lot of reduced magnetization ( B ) vs. H/T for complex 4 2 2 O at applied fields of 0.1 3 .0 T and in the 1.8 10 K temperature range The solid lines are the fit to the d ata; see the text for the fit parameters.

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167 Figure 4 21. Two d imensional contour plot for the rms error surface vs. D and g for the reduced magnetization fit for complex 4 2 Figure 4 22 Plot of in phase M (as M ) vs. T ac signals at 1000 Hz for complexes 4 1 H 2 O 4 2 2

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168 Figure 4 23 Plot of M T vs T for complex 4 3 (solid line indicates the fit to the experimental data) See the text for the fitting parameters. Figure 4 2 4 Schematic diagram for a Mn 4 core with two exchange coupling parameters, J wb and J bb

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169 A B Figure 4 25 ( A ) Pl ot of energies of various spin states vs. the ratio of J wb /J bb for complex 4 3 (B) Plot of all possible energy states for complex 4 3 using the J values, obtained from the fit to the experimental M T data

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170 Figure 4 26 The p lots of in phase M (as M T ) vs. T ac signals for complex 4 3 at the indicated frequencies. Figure 4 27 Plot of M T vs T for complex 4 4

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171 Figure 4 28 P lot of reduced magnetization ( B ) vs. H/T for complex 4 4 at applied fields of 3.0 6.0 T and in the 1.8 10 K temp erature range The solid lines are the fit to the da ta; see the text for the fit parameters. Figure 4 29 Two dimensional contour plot for the rms error surface vs. D and g for the reduced magnetization fit for complex 4 4

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172 Figure 4 30 Plot of in phase M (as M T ) vs. T ac signals for complex 4 4 at the indicated frequencies. Figure 4 31 Plot of M T vs T for complexes 4 5 2 O 4 6 2 O

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173 Figure 4 32 Plot of in phase M (as M T ) vs. T ac signals for complexes 4 5 2 O and 4 6 2 O at 1000Hz. Figur e 4 33 Plot of M T vs. T for complex 4 7 1.5C 6 H 1 4

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174 A B Figure 4 34 P lot of reduced magnetization ( B ) vs. H/T for complex 4 7 6 H 14 at applied fields of 0.1 0. 8 T and in the 1.8 10 K temperature range (A). The solid lines are the fit to the data; see the text for the fit parameters. Two dimensional contour plot for the rms error surface vs. D and g for the reduced magnetization fit for complex 4 7 6 H 14 (B )

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175 Figure 4 35 Plots of in phase M (as M T ) vs. T and out of phase M vs. T ac signals for complex 4 7 1.5C 6 H 1 4 at the indicated frequencies.

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176 A B Fi gure 4 36 Plots of M T vs. T (A) and in phase M (as M T ) vs. T ac signals for complex 4 8 3 H 2 O at the indicated frequencies (B )

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177 CHAPTER 5 A FAMIL Y OF RARE FUSED DOUB LE CUBANE M n 4 L n 2 (L n = G d T b D y H o ) AND M n 4 Y 2 COMPLEXES CONTAINING SINGLE MOLECULE MAGNET Polynuclear mixed metal oxide clusters are of great importance to chemist s and solid sta te physicist s interested in new metal alloys, intermetallics and perovskite type metal architectures 195 200 The h eterometallic nature of those materials influences many interesting properties such as ferromagnetis m and ferroeletricity. 201 203 In addition, polynuclear 3d/4f complexes have drawn significant interest in the field of molecular magnets, especially sin gle molecule magnets (SMMs) as promising substitute s to homom etallic transition metal complexes. 204 210 SMMs are individual molecules that possess a significant barrier to magnetization relaxation, and thus exhibit the properties o f a magnet below their blocking temperature ( T B ). These properties result from a combination of a high spin ground state ( S ) value (i.e. several unpaired electrons) and easy axis magnetic anisotrop y (negative zero field parameter, D ). SMMs also straddle the classical/quantum interface by displaying n ot just classical magnetization hysteresis but also quantum tunneling of magnetizatio n (QTM) and quantum phase interferenc e .These properties make SMMs potential candida tes to be used as qubits in quantum computatio n Besides, SMMs can be used for informati on storage due to their bulk magnetic properties, and other application s include molecular spintronics and biomedical applications (for example as MRI contrast agents ) 11,13 15,211 Ove r the past two decades there h ave been massive amount s of research in this area, and the SMM database has greatly expanded ; however, to date m anganese has been the transition metal of choice due to its tendencies to give molecules with large S value s and significant anisotropy from the presence of Jahn Teller (JT) distorted Mn 3+ ion s In the

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178 search for another metal source with large single ion anisotropy, trivalent lanthanides are wise alternatives. In particular Tb 3+ or Dy 3+ ions can provide large spin and high spin orbit coupling and have participated in making a new class of SMM s 212 220 A dditional advantage s of mixed metal clusters come from the often ferromagnetic couplin g between 3d and 4f ions resulting in a high spin ground stat e 221 In the past decade a number of Mn/Ln metal cluster s 118,222 234 have been made such as Mn 2 Ln 2 Mn 2 Ln 3 Mn 4 Ln 4 Mn 4 Ln 3 Mn 5 Ln 4 Mn 6 Dy 6 Mn 9 Dy 8 Mn 10 Ln 2 Mn 11 Dy 4 Mn 11 Gd 2 Mn 12 Gd, Mn 18 Dy and Mn 21 Dy. Formation of all these clusters incorporate s polydentate chelating ligands 223,225,226,233 N substituted diethanolamine s 118,224,227 carboxylate group s 222,224,227 232,234 and tripodal ligand s 222 All these ligands feature either alcohol or carboxyl ate arms which, upon deprotonation foster formation of high nuclearity products. Like the previous chapters, edteH 4 is the ligand of choice. The compounds so far obtained with edteH 4 are Mn x (x = 3, 4, 6, 8, 9, 10, 12, 20), Mn 18 Ca 2 and Fe x (x = 5, 6, 12) metal complexe s In the present work for the first time the reactivity of the edteH 4 group has been explored in the presence of carboxylate as co ligand in manganese/l anthanide cluster chemistry. Herein, we report the synthese s, crystal structure s, and m agnetic measurements on a family of 3d/4f, mixed metal, Mn 4 Ln 2 complexes ( where Ln = Y (diamagnetic), Gd ( isotropic) or Tb, Dy, Ho (anisotropic ) ) with an unprecedented metal topology T he Tb analogue is a new addition to the SMM family.

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17 9 Experimental Sect ion Syntheses All the preparations were performed under aerobic conditions using reagents and solvents as received. The s ynthesis of Mn(O 2 C Bu t ) 2 was carried out as reported in the literatur e 120 Mn 4 Gd 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 1) To a stirred solution of edteH 4 (0.20 g, 0.84 mmol) and NEt 3 (0.25 mL, 1.68 mmol ) in MeCN/MeOH (20/1, v/v) was added Mn(O 2 CBu t ) 2 ( 0.66 g, 2.52 mmol ). The resulting b rown solution was stirred for 20 minutes under mi ld heating and then Gd(NO 3 ) 3 ( 0.76 g, 1.68 mmol ) was added to the solution and stirred for further 2 hours. T he solution was then filtered and left undisturbed. X ray quality, orange plate like crystals of 5 1 from slow evaporati on with a yield of 12 %. The crystals were collected by filtration, washed with Et 2 O and dried in vacuum. Anal. Calc. (found) for 5 1 (solvent free): C: 34.01 (33.73); H: 5.59 (5.56); N: 4.76 (4.67) %. Selected IR data (cm 1 ): 3405 (br), 2967 (m), 2678 (w ), 1566 (s), 1484 (s), 1415 (br, s), 1308 (m), 1228 (s), 1072 (s), 1032 (m), 897 (m), 795 (w), 741(w), 594 (br, s), 530 (w), 454 (m). Mn 4 Tb 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 2) T he preparation is similar to that above but Tb(NO 3 ) 3 (0.73 mg, 1.68 mmol) was us ed as the Ln salt The yield was 15 %. Anal. Calc. (found) for 5 2 (solvent free): C: 33.95 (33.78); H: 5.58 (5.58); N: 4.75 (4.71) %. Selected IR data (cm 1 ): 3393 (br), 2967 (m), 2906 (m), 2679 (w), 1566 (s), 1484 (s), 1416 (br, s), 1309 (m), 1228 (s), 1 114 (w), 1072 (s), 1031 (m), 925 (w), 898 (m), 795 (m), 743 (w), 594 (br, s), 531 (w), 454 (m). Mn 4 Dy 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 3) T he preparation is similar to that above but Dy(NO 3 ) 3 (0.58 mg, 1.68 mmol) was used as the Ln metal ion. The yield was 15 %.

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180 Anal. Calc. (found) for 5 3 (solvent free) : C: 33.81 (33.61); H: 5.56 (5.58); N: 4.73 (4.98) %. Selected IR data (cm 1 ): 3392 (br), 2970 (w), 2739 (w), 2677 (w), 1567 (s), 1484 (s), 1412 (br, s), 1311 (m), 1228 (s), 1171 (w), 1072 (s), 1033 (s), 926 (w), 898 (s), 825 (m), 794 (w), 744 (w), 595 (br, s), 533 (w), 455 (m). Mn 4 Ho 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 4) T he preparation is similar to that above but Ho(NO 3 ) 3 (0.74 mg, 1.68 mmol) was used as the Ln salt The yield was 15 % Anal. Calc. (found) for 5 4 (solvent free): C: 33.72 (33.54); H: 5.55 (5.54); N: 4.72 (4.73) %. Selected IR data (cm 1 ): 3370 (br), 2967 (br), 2678 (w), 1567 (s), 1418 (br, s), 1312 (s), 1227 (s), 1114 (w), 1072 (s), 1031 (s), 897 (s), 820 (w), 795 (w), 746 (w), 595 (br, s), 533 (w), 455 (m). Mn 4 Y 2 O 2 (O 2 CBu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 5). The preparation is similar to that above but Y (NO 3 ) 3 (0.58 mg, 1.68 mmol) was used. The yield was 14 %. Anal. Calc. (found) for 5 5 (solvent free): C: 36.87 (36.58); H: 6.06 (6.01); N: 5.16 (5.18) % Se lected IR data (cm 1 ): 3396 (br), 2967 (s), 2905 (m), 1567 (s), 1484 (s), 1417 (s), 1374 (m), 1314 (m), 1228 (s), 1072 (vs), 1032 (s), 925 (m), 898 (m), 818 (w), 796 (w), 747 (w), 594 (br, s), 530 (w), 456 (m). X ray Crystallography Data were collected at 100 K on a Bruker DUO system equipped with an APEX II area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 9999 reflections. A hemisphere of data was collected using the scan met hod (0.5 frame width). Absorption corrections by integration were applied based on measured indexed crystal faces. T he structure was solved by direct m ethods in SHELXTL6, and refined on F 2 using full matrix least

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181 squares. The non H atoms were treated an isotropically, whereas the hydrogen atoms were calculated in ideal positions and refined as riding on their respective carbon atoms. Crystallographic parameters are listed in Table 5 1. For 5 1 3MeCN, t he asymmetric unit consists of a half Mn 4 Gd 2 cluster (located on an inversion center) and 1.5 acetonitrile solvent molecules (the half molecule is located on a 2 fold rotation axis). Fragment O2 C9 C10 was disordered and refined in two parts. A total of 448 parameters were refined in the final cycle of refinement using 5716 reflections with I > 2 (I) to yield R 1 and wR 2 of 3.68 % and 9.01 %, respectively. For 5 2 0.5MeCN, the asymmetric unit consists of a Mn 4 Tb 2 cluster and a half acetonitrile solvent molecule. The latter were disordered and could not be modeled properly, thus the program SQUEEZE, 64 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. There are two disordered region s in the cluster. The C10 O4 group of the edteH 4 ligand is disordered and refined in two parts with their site occupation factors dependently refined. The three methyl groups on atom C22 are disordered and treated in the same manner. In the final cycle o f refinement, 8260 reflections (of which 6090 are observed with I > 2 (I)) were used to refine 403 parameters and the resulting R 1 wR 2 and S (goodness of fit) were 3.00 %, 6.06 % and 0.902 respectively. The refinement was carried out by minimizing the wR 2 function using F 2 rather than F values. R 1 is calculated to provide a reference to the conventional R value but its function is not minimized. Physical Measurements Infrared spectra were recorded in the solid state (KBr pellets) on a Nicolet Nexus 670 FTIR spectrometer in the 400 4000 cm 1 range. Elemental analyses (C, H and N)

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182 were performed by the in house facilities of the 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 vs. field and temperature data were fit using the program MAGN ET. 66 Pascal's constants were used to estimate the diamagnetic correction, which was subtracted from the experimental susceptibility to give the molar p aramagnetic susceptibility ( M ). Low temperature (<1.8 K) hysteresis loop and dc relaxation measurements were performed in Grenoble using an array of micro SQUIDs. 67 The field can be applied in any direction by separately driving three orthogonal coils. The applied field was aligned parallel to the easy axis ( z axis) of the molecules using a published method. 121 The high sensitivity of this magn etometer allows the study of single crystals of the order of 10 to 500 m. Result s and Discussion Syntheses In this work eth ylenediamine based alkoxide containing edteH 4 ligand has been as the main ligand of choice The edteH 4 ligand has proven to be a go od chelating ligand on deprotonation mainly due to the presence of four flexible alkoxide arm s 54,60,168 In the previous chapter it has been noted that the reaction between Mn(O 2 CBu t ) 2 and edteH 4 gives a high spin high nuclearity Mn 9 SMM with a high spin barrie r .Therefore, it is interesting to further evaluate the synthetic scheme by adding isotropic, anisotropic and diamagnetic lanthanides ions to the system. The new reaction scheme has the potential to give new heterometallic clusters where the magnetism is now governed by both the Ln 3+ and Mn 3+ ions. So far there are only few examples of

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183 Mn/Ln metal clusters with N substituted alkoxide ligands in the literatur e 118,224,22 7 The following scheme will be a new addition to this family. Trivalent lanthanide ions show strong affinity towards oxygen and so the combination of t BuCO 2 edteH 4 Mn and Ln ions has been explored in this chapter The reaction of edteH 4 with Mn( O 2 CBu t ) 2 NEt 3 and Gd(NO 3 ) 3 in a 1:3:2 :2 molar ratio in MeCN/MeOH (10/1, v/v) afforded a red dish brown solution from which the Mn 2 III Mn 2 II Gd III 2 O 2 (O 2 CBu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 1 ) was obtained in 12 % yield Its formation is summarized in equation 5 1. The base NEt 3 facilitates the oxidation of Mn 2+ to Mn 3+ by atmospheric oxygen gas and the deprotonation of water to form more O Using MeCN only as solvent also leads to the same product but the use of MeCN/MeOH is imperative to obtain good single crystal s of 5 1 in the highest yield. 4Mn(O 2 C Bu t ) 2 + 2Gd(NO 3 ) 3 + 2edteH 4 + H 2 O + 1/2 O 2 Mn 4 Gd 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 + 2HO 2 C Bu t + 4HNO 3 (5 1) The same product is al so obtained by using 1:2 ligand to metal molar ratio ; however, the best yield is obtained from the above reaction The same product with comparable yield is obtained by using Mn(NO 3 ) 2 MnCl 2 or Mn(ClO 4 ) 2 and t BuCO 2 H in 1:2 molar ratio instead Mn(O 2 CBu t ) 2 Interestingly no product was isolated by switching the t BuCO 2 to MeCO 2 EtCO 2 or PhCO 2 groups in the original scheme However, the reaction of Mn(O 2 CMe) 2 /Mn(O 2 CEt) 2 Gd 3+ t BuCO 2 H, edteH 4 and NEt 3 gives the same product with no MeCO 2 /EtCO 2 in th e final product. Hence, the bulky t BuCO 2 plays a crucial role in trapping the Mn 4 Gd 2 cluster from the MeCN/MeOH solvent system. This reaction strategy was used to make the other family members of Mn 4 Ln 2 ( wh ere Ln = Tb, Dy, Ho) and Mn 4 Y 2 clusters The com plexes were characterized by IR and elemental analysis.

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184 Description of S tructure Mn 4 Gd 2 O 2 (O 2 CBu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 The complete structure and a stereoview of 5 1 are presented in Figure 5 1 and 5 2 ; selected interatomic distances and angles are listed in Ta ble A 11 This complex crystallizes in the monoc linic space group C2/c and has crystallographic inversion symmetry. The centrosymmetric molecule possesses a [Mn 4 Gd 2 O 2 (OR) 4 ] 8+ core. The core has a fused, face sharing double cubane structure, containing two 4 O (O5) and 3 OR (O1, O4) bridging to the four Mn and two Gd atoms (Figure 5 3 ). The coordination of the two peripheral Mn1 atoms is completed by two doubly deprotonated edteH 2 ligand s Each edteH 2 group is hexadentate chelating on to the te rminal Mn1 atom with two of its deprotonated alkoxide arms bridging to the two Mn2 atoms and Gd1 atom. Thus, the edteH 2 1 1 3 3 : 4 bridging, as shown in Figure 5 4 The determination of the protonation levels of all the O atoms was accomplished by BVS calculations 171 and the results are listed in Table 5 2 Two of the edteH 2 O atoms have BVS values of >1.9 confirming them as deprotonated whereas the other two have BVS values of aroun d 1.3 1.4, confirming them as protonated. Mn1 (Mn 2+ ) is seven coordinate and Mn2 (Mn 3+ ) is six coordinate with near octahedral geometry The oxidation states were confirmed by charge considerations, metric parameters and BVS calculations (Table 5 3 ). The two Mn 3+ atoms display JT axial elongation s, defined by the axis O1 Mn2 O4 and shown as green solid lines in Figure 5 5 T he JT axes are orien ted nearly parallel to each other and the elongated Mn 3+ O bonds are ~0.3 longer than the other Mn 3+ O bonds. The Gd atoms are nine coordinate with their geometries comp 1 1 : t BuCO 2 groups and one chelating NO 3 ion. Hence, the charge balance consideration requires 2Mn 2+ 2Mn 3+ 2Gd 3+ 2O 2 t BuCO 2 2edteH 2 and 2NO 3 The structure of 5 2

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185 (Figure 5 6) is similar to that of 5 1 where Gd 3+ is replaced by Tb 3+ The other family members ( 5 3 5 4 5 5 ) are isostructural to Gd 3+ /Tb 3+ analogues, proved by elemental and IR spectroscopic analyses. This face sharing double cubane core is a rare structural type. Ther e is no structur al precedence for the fused double cubane structure in 3d/4f cluster chemistry but two Mn III 6 fused double cubane clusters have been reported in the past. 235,236 Magnetochemistry Dc m agnet ic susceptibility s tudies Solid state, variable temperature magnetic susceptibility meas urements were performed on vacu um dried microcrystalline samples of complexes 5 1 to 5 5 The dc molar magnetic susceptibility ( M ) data were collected in the 5.0 300 K range in a 0.1 T (1000 Oe) magnetic field, and are shown in Figure 5 7 as M T versus T plots. At 300 K the experimental M T are a little smaller than the expected values for the four non interacting Mn ( S = 2 for Mn 3+ S = 5/2 for Mn 2+ g = 2) and two Ln 3+ /Y 3+ metal ions (as s ummarized in Table 5 4). The behavior suggests the presence of antiferromagnetic interaction at room temperature among the adjacent paramagnetic centers. Upon lowering the temperature M T stays fairly constant down to 50 K ( 5 1 ), 30 K ( 5 2 5 3 ), 20 K ( 5 4 ) and then increases markedly to reach a maximal value (Table 5 4). The M T value (63.93 cm 3 mol 1 K at 5 K) for complex 5 1 which contains isotropic Gd 3+ ion an ion devoid of any spin orbit coupling suggests the spin ground state as S = 12. For compound 5 5 with the diamagnetic Y 3+ ion M T decreases slowly from 11.36 cm 3 mol 1 K at 300 K to 8.02 cm 3 mol 1 K at 5 K and the M T value at 5 K suggests an S = 4 ground state spin C omplex 5 5 contains diamagnetic Y 3+ ions and hence, the

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186 corresponding magnetic beha vior is coming from Mn ions only. To further probe the type of exchange interactions within the paramagnetic Mn ions t he M T data for complex 5 5 were fit to the theoretical M T expression for a butterfly core of Mn II 2 Mn III 2 with two exchange coupling pa rameters, J wb and J bb representing the Mn II Mn III and Mn III Mn III interactions, respectively. The corresponding Heisenberg spin Hamiltonian is given by eq uation 5 2 (the a tom labeling is that of Figure 5 8 ) and its eigenvalues in eq uation 5 3 where A = 1 3 A = 2 4 T A B and S T is the total spin of the molecule. J wb ( + + + J b b (5 2) J wb ( T 2 A 2 B 2 J bb ( A 2 1 2 3 2 ) (5 3) E ( S T S A S B J wb [ S T (S T A (S A B (S B +1) ] bb [ S A (S A +1) ] (5 4) There are total of 110 possible S T states ranging in value s from 0 to 9, where S T is the total spin of the Mn 4 cluster. This e igenvalue expression (equation 5 4) and the Van Vleck equation were used to derive a theoretical M T vs T expression fo r 5 5 and this was used to least squares fit the experimental data. The fit (solid line in Figure 5 9) gave J bb = 32.5(4) cm 1 J wb = +1.0(1) cm 1 and g = 1.95(1 ) with temperature independen t paramagnetism (TIP) fixed at 4 x 10 4 cm 3 mol 1 The energies o f all possible spin states are shown in Figure 5 10 using the J values obtained from the fit. The plot indicates the existence of six degenerate ground states where | S T S A S B > = |0, 0, 0 > |1, 0, 1 > |2, 0, 2 > |3, 0, 3 > |4, 0, 4 > |5, 0, 5 > The large negative value for J bb makes the spin coupling S A is zero and hence S T is equal to S B The spin of Mn1 and Mn3 is S = 5/2 i.e. the S B can be of any value from 0 to 5 so as S T Considering the weighted average for six possible degenerate spin states the th eoretical M T value

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187 comes out as 8.32 cm 3 mol 1 K for g = 1.95 which is close to the experimental M T value at 5 K. The calculation is shown in equation 5 5 considering total 36 m s states where the spin states vary from 0 to 5. M T (theoretical) = (g 2 /8)[1/36 *0(0+1) + 3/36*1(1+2) + 5/36*2(2+1) + 7/36*3(3+1) + 9/36* 4(4 + 1) + 11/36*5(5+1)] (5 5) To further study the effect of exchange coupling parameters on ground state spin, energies of all possible spin states are plotted as a function of J wb / J bb (Figure 5 1 1). The value of the ratio below 0.2 indicates the presence six possible spin states as mentioned before. The ground state spin starts changing with the decrease in the J bb value as shown in the F igure 5 11 For example, t he lowest energy state becomes | 6, 1, 5 > for changing the ratio from 0.2 up to 0.4. Considering the structural similarities among complexes 5 1 to 5 5 it is fair to conclude that the interaction between Mn 3+ Mn 3+ is strongly antiferromagnetic and the interaction between Mn 2+ Mn 3+ is w eak Hence, t he overall behavior in dc magnet ic susceptibility study for complexes 5 1 to 5 4 suggests the presence of predominant ferromagnetic interaction s at low temperature coming from the interaction between Mn 3+ Ln 3+ (possibly from O1, O4, O5 and/or 1 1 : carboxylate ) and /or Mn 2+ Ln 3+ (possibly from O1, O4, O5) in addition to the intrinsic magnetic properties of Ln 3+ ions (Figure 5 7 ) It is interesting to note that previously reported homometallic, fused doub le cubane Mn 3+ 6 cluster s show predominantly antiferromagnetic interaction in dc magnetic susceptibility studies. 235,236 These findings indicate that the trivalent lanthanide ions play a pivotal role in overall magn etic exchange interactions between the paramagnetic centers in the Mn 4 Ln 2 complexes.

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188 To confirm the e stimation of the ground state spin of 5 1 and 5 5 variable field ( H ) and temperature magnetization ( M ) data were collected at various field s up to 7 T and in the 1.8 10 K temperature ranges. The resulting data are plotted as reduced magnetization ( B ) vs H / T where N is Avogadro's number and B is the Bohr magneton in Figure 5 12 and 5 13 For Mn 4 Gd 2 complex t he saturation value at the highest fields and the lowest temperatures is ~24.0, as expected for an S = 12 spin state and g slightly less than 2. The saturation value should be gS in the absence of complications from low lying excited states or significant anisotropy. The data were fit ted using the program MAGNE T 66 which is based on diagonalizing the giant spin Hamiltonian given by equation 5 6 which can be assumed to be valid at low temperature s where only the ground state is thermally populated, and employs a full powder average. H = z 2 + B 0 H ( 5 6 ) The best fit to the data is shown as the solid lines in Figure 5 12 and was obtained with S = 12, D = 0 cm 1 and g for 5 1 The fitting is equally good for S = 11, D = 0 cm 1 and g the g value is little higher than 2 is unreasonable for a Mn 3+ /Gd 3+ system. The zero field splitting parameter D is zero which can be rationalized by the super imposable i sofield lines obtained from the reduced magnetization plot. Further confirmation of spin ground state comes from low temperature ac magnetic susceptibility measurements. For Mn 4 Y 2 complex, the best fit data was obtained at the lower fields 0.1 1. 0 T (Figure 5 13 ) This is often a case for polynuclear complexes where fitting is bad at higher fields due to the low lying excited states as a result of weak interactions and/or spin frustration

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189 effects. An acce ptable fit for 5 5 was obtained with S = 4, g = 1.81(1) and D = 0 cm 1 and the fitting is shown as the solid lines in Figure 5 13 The ZFS parameter is zero, same as the case for 5 1 The g value is littler smaller than usual and it indicates the fit is al so not the best even with the lower fields. Alternative fits with S = 3 or 5 were rejected because they gave unreasonable values of g of 2.34 and 1.48, respectively. Alternating current (ac) magnetic susceptibility s tudies T he temperature dependent ac sus ceptibilities of all compounds were measured on microcrystalline samples of 5 1 to 5 5 in a 3.5 G ac field. In addition, t he in phase ( M ) ac susceptibility signal is invaluable for assessing S without any complications from a dc field, and these signals for complexes 5 1 to 5 5 are measured in the 1.8 15 K range at 25 0 Hz (Figure 5 14 ) A plateau is observed at low temperature and extrapolation of the plot to 0 K, from temperature ~2 K, gives a value of ~ 76 cm 3 Kmol 1 for 5 1 which confirms the presenc e of S = 12 spin ground state for g < 2.00. The in phase ac signal for 5 5 is nearly constant at low temperature and the extrapolation of the M T at 0 K gives a value of 8.2 cm 3 Kmol 1 confirms an S = 4 ground state for a g value of ~1.8. The values of S and g compliment the dc magnetization fit data. Frequency dependent ac susceptibility measurement is an excellent tool to probe slow relaxat ion of the magnetization vector at low temperature. Only the Tb 3+ analogue showed such behavior in the present work and hence, data were collected for 5 2 in the 1.8 15 K range at 5 1500 Hz At lower temperature, below 2.5 K, a frequency dependent decr ease in M 'T and the concomitant rise in the out of phase M signal were seen (Figure 5 15 ) indicating the slow relaxation of a magnetization vector due to the presence of a spin barrier Howeve r, in the ac out of phase plots, no peaks were

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190 observed; i.e., the p eak maxima lie below 1.8 K the minimum operating temperature of the SQUID magnetometer Comparing the ac behaviors of the isotropic Gd 3+ and the anisotropic analogues, it can be concluded that the slow relaxation of magnetization originates from the addit ional magnetic anisotropy introduced by Tb 3+ Single crystal hysteresis s tudies of 5 2 below 1.8 K. The ac measurements strongly suggest that 5 2 behaves as an SMM which was confirmed by the observation of hysteresis loops in magnetization vs dc field s cans, measured on a single crystal of 5 2 using the micro SQUID apparatus. The temperature dependence at 0.035 T s 1 and the scan rate dependence at 0.0 4 K of the hysteresis loops are shown in Figure 5 16 The coercivities clearly increase with decreasing temperature and increasing scan rate, as expected for the superparamagnet like behavior of SMM s 19 The data thus confirm that complex 5 2 is a new addition to the family of 3d/4f SMM s with a blocking temperat ure ( T B ) of 0.9 K. Another interesting feature of the loops is to sho w s teps at periodic values of applied field positions. These steps are diagnostic of resonant QTM in several classes of SMMs in 3d famil y 14,76,83, 237 239 and only one in 3d/4f complexe s The first step in s weeping the field in Figure 5 16 occurs at the zero field, where the double well potential energy curve is symmetric and M s levels on one side of the barrier are in resonance with those on the ot her, allowing tunneling to occur through the barrier. Additional steps are also observed at the periodic fields when the M s levels are once again brought into resonance. H between the steps is proportional to D and is given by eq uati on 5 7 H = D / g B (5 7 )

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191 Measurement of the step positions in Figure 5 16 H of 0. 1 T and a resulting D / g B value of 0.046 cm 1 (where D 1 for g Concluding R emarks Five Mn 4 Ln 2 complexes of rare, fused double cubane metal topology containing isotropic, anisotropic and diamagnetic lanthanides have been reported with thorough syntheses, crystal structure s and magnetic analyse s. All five complexes follow a similar reaction scheme with a comparable yield. The Gd 3+ and Tb 3+ analogues were structurally characterized by X Ray crystallography and the rest of the family members were characterized by IR and elemental analysis. Spin orbit coupling is much larger for trivalent lanthanides than for a 3d metal ion which com plicates the magnetic investigation of the heterometallic complexes. In this regard, the isotropic Gd 3+ and diamagnetic Y 3+ analogue s of the Mn 4 Ln 2 complexes help to understand the magnetic properties of the cluster s in detail. The use of the diamagnetic Y 3+ analogue helps to nail down the type of exchange interactions between Mn and Ln metal ions in the core. The Tb 3+ analogue is a new member in the wide family of 3d/4f, heterometallic SMMs proved by single crystal hysteresis study.

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192 Table 5 1. Crys tallographic data for 5 1 and 5 2 MeCN parameter 5 1 5 2 Formula C 56 H 103 Gd 2 Mn 4 N 9 O 28 C 52 H 97 Mn 4 N 7 O 28 Tb 2 fw, g/mol 1884.73 1805.97 crystal system Monoclinic Monoclinic space group C2/c C2/c a 24.733(4) 24.2909(15) b 13.810(2) 14.4236(9) c 22.262(4) 21.8221(13) 90, 104.651(9), 90 90 109.987(1), 90 V, 3 7357(2) 7185.1(8) Z 4 4 T,K 100(2) 100(2) Radiation, a 0.71073 0.71073 1.702 1.669 1 2.527 2.705 R1b,c 0.0368 0.0300 wR2d 0.0901 0.0606 a Graphit e monochromator. b I I ). c R F o | | F c F o |. d wR w ( F o 2 F c 2 ) 2 w ( F o 2 ) 2 ]] 1/2 w 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O) + 2F c 2 ]/3 Table 5 2 Bond valence sums for the O atoms of complex 5 1 a Atom BVS Assignment Gro up O1 2.05 R O edteH 2 O2 1.45 R OH edteH 2 O3 1.29 R OH edteH 2 O4 1.99 R O edteH 2 O5 1.96 O O a The BVS values for O atoms of O (RO ) ROH and H 2 O groups are typically 1.8 2.0, 1.0 1.2 and 0.2 0.4, respectively, but can be affected s omewhat by hydrogen bonding. Table 5 3 Bond valence sums for the Mn atoms of complex 5 1 and 5 2 a Atom 5 1 5 2 Mn 2+ Mn 3+ Mn 4+ Mn 2+ Mn 3+ Mn 4+ Mn1 2.24 2.09 2.04 3.35 3 .09 3.04 Mn1 3.39 3.13 3.07 2.23 2.13 2.07 a The underlined value is the one close st to the charge for which it was calculated. The oxidation state of a particular atom can be taken as the nearest whole number to the underlined value

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193 Table 5 4. Magnetic data summarized from the dc measurements Compounds Ground state of the Ln 3+ ion M T (cm 3 mol 1 K) expected for non interacting ions per complex M T (cm 3 mol 1 K) measured at 300K per complex M T (cm 3 mol 1 K) measured at 5K per complex Mn 4 Gd 2 ( 5 1 ) Mn 4 Tb 2 ( 5 2 ) Mn 4 Dy 2 ( 5 3 ) Mn 4 Ho 2 ( 5 4 ) Mn 4 Y 2 ( 5 5 ) 8 S 7/2 7 F 6 6 H 15/2 5 I 8 1 S 0 30.51 38.39 43.09 42.89 14.75 27.71 35.57 37.42 39.52 11.36 63.93 70.39 59.24 48.76 8.02 Figure 5 1. The structure of 5 1 Hydrogen atoms have been omitted for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; Gd 3+ pink; O, red; N, blue; C, light grey

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194 Fi gure 5 2 The stereopair of 5 1 Hydrogen atoms have been omitted for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; Gd 3+ pink; O, red; N, blue; C, light grey Figure 5 3. The core of 5 1 Hydrogen atoms have been omitted for clarity. Color code: M n 3+ purple; Mn 2+ cyan; Gd 3+ pink; O, red.

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195 1 1 3 3 : 4 Figure 5 4. The coordination mode of edte H 2 2 found in 5 1 Figure 5 5 A partially labeled representation of complex 5 1 with JT axes shown as green thick lines M ethyl grou ps of the carboxylate moieties have been removed for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; Gd 3+ pink; O, red; N, blue; C, light grey

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196 Figure 5 6 A partially labeled structure and a stereopair of complex 5 2 JT axes are shown as green t hick lines. Hydrogen atoms have been omitted for clarity. Color code: Mn 3+ purple; Mn 2+ cyan; Tb 3+ green; O, red; N, blue; C, light grey

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197 Figure 5 7. Plot of M T vs T for complexes 5 1 to 5 5 Figure 5 8. Schematic diagram for Mn 4 core with two exchange coupling parameters, J wb and J bb

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198 Figure 5 9. Plot of M T vs. T for complex 5 5 where the solid line shows the fit of the experimental da ta. See the te xt for the fit parameters. Figure 5 10. Plot of all possible energy states for complex 5 5 using the J values, obtained from the fit to the experimental M T data.

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199 Figure 5 11. Plot of energies of various spin states vs. the ratio of J wb /J bb Fi gure 5 12 P lot of reduced magnetization ( B ) vs. H/ T for complex 5 1 at applied fields of 0.1 7.0 T in the 1.8 10 K temperature range. The solid lines are the fit of the data; see the text for the fit parameters.

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200 Figure 5 13 P lot s of reduced m agnetization ( B ) vs. H/ T for complex 5 5 at the applied fields of 0.1 1.0 T in the 1.8 10 K temperature range. The solid lines are the fit of the data; see the text for the fit parameters. Figure 5 14 Plot of in phase M (as M T ) vs. T ac si gnals at 250 Hz for complexes 5 1 to 5 5

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201 Figure 5 15 Plots of in phase M (as M T ) vs. T and out of phase M vs. T ac signals for complex 5 2 at the indicated frequencies

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202 A B Figure 5 16 Magnetization ( M ) vs. dc field hysteresis loops for a single crystal of 5 2 at the indicated temperature ( A ) and the field sweep rate ( B ). The magnetization is normalized to its saturation value, M S

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203 APPENDIX A BOND DISTANCES AND ANGLES Table A 1 Selected interatomic distances () and a ngles (deg) for 2 1 2 1 3.191(1) 3.224(1) 3.199(1) Mn1 O1' 1.880(3) Mn1 O2 1.896(4) Mn1 O1 2.039(4) Mn1 O4' 2.059(4) Mn1 O6 2.084(3) Mn2 O1 2.010(4) Mn2 O3 2.069(4) Mn2 O5' 2.078(4) Mn2 O4' 2.209(4) Mn3 O5 1.886(4) Mn 3 O3 1.898(4) Mn3 O2 2.124(4) Mn3 O1 2.227(4) Mn3 N1 2.257(5) Mn3 O4 2.264(4) Mn3 N2 2.280(5) Mn2 O1 Mn1 104.1(2) Mn2 O1 Mn3 97.9(1) Mn1 O1 Mn3 98.1(2) Mn1 O2 Mn3 106.5(2) Mn3 O3 Mn2 107.4(2) Mn2' O4 Mn3 96.5(2) Mn3 O5 Mn2' 114.6(2) Mn1'' O6 Mn1 128.8(3) a unp rimed, primed, and double primed atoms are related by the symmetry. Table A 2 Selected interatomic distances () and a ngles (deg) for 2 2 2 2 Mn3 3.196(1) 3.239(2) Mn12 3.153(1) Mn5 3.129(2) Mn6 3.236(2) Mn6 3.122(2) Mn8 3.166(1) Mn1 O1 1.885(4) Mn2 O1 2.148(5) Mn1 O2 2.397(5) Mn11 O2 2.111(5) Mn12 O2 2.107(5) Mn1 O3 2.137(4) Mn3 O3 1.881(4) Mn1 O4 1.890(4) Mn11 O4 2.120(5) Mn1 O17 2.109(4) Mn1 O1 Mn2 106.7(2) 3.193(2) 3.224(2) 3.149(1) 3.223(1) 3.231(1) 3.132(1) Mn2 O17 2.12 8(4) Mn3 O17 2.098(4) Mn12 O17 1.840(4) Mn3 O18 1.980(4) Mn9 O18 2.001(4) Mn6 N30 2.183(6) Mn12 N30 2.119(6) Mn2 N12 2.244(6) M n12 N12 2.082(6) Mn1 N2 2.259(6) Mn1 N1 2.289(6) Mn3 O17 Mn1 98.9(2)

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204 Mn1 O2 Mn11 96.3(2) Mn1 O3 Mn3 105.2(2) Mn1 O4 Mn11 113.9(2) Mn12 O17 Mn3 134.7( 2) Mn12 O17 Mn1 110.8(2) Mn12 O17 Mn2 105.0(2) Mn3 O17 Mn2 102.6(2) Mn1 O17 Mn2 99.8(2) Mn3 O18 Mn9 130.2(2) Mn2 N12 Mn12 93.5(2) Table A 3 Selected interatomic distances () and a ngles (deg) for 2 3 2 3 3.127(1) 3.197(2) 3.224(1) Mn1 O1 2.037(5) Mn2 O1 2.009(5) Mn3 O1 2.202(5) Mn1 O2 1.90 1(5) Mn3 O2 2.130(5) Mn2 O3 2.075(6) Mn3 O3 1.884(5) Mn3 O4 2.254(5) Mn1 O4' 2.061(5) Mn2 O4' 2.220(5) Mn1 O1 Mn2 106.4(3) Mn1' O1 Mn1 136.7(3) Mn2 O1 Mn1 104.4(2) Mn1' O1 Mn3 105.7(2) Mn2 O1 Mn3 98.6(2) Mn3 O5 1.900(6) Mn2 O5' 2.059(6) Mn1 O6 2.049(9) Mn1 N6 2.151(15) Mn1 N3 2.174(8) Mn2 N3' 2.185(8) Mn3 N1 2.245(7) Mn3 N2 2 .274(7) Mn1 O2 Mn3 106.1(2) Mn3 O3 Mn2 107.5(2) Mn1' O4 Mn2' 96.6(2) Mn1' O4 Mn3 98.5(2) Mn2 O4 Mn3 96.5(2) Mn3 O5 Mn2' 114.9(3) Mn1'' O6 Mn1 132.7(11) Table A 4 S elected interatomic distances () and a ngles (deg) for 3 1 4MeCN 3 1 4MeCN 3.349(1) Mn1 O28 1.871(2) Mn1 N1 1.987(3) Mn2 O15 1.913(3) Mn2 O17 1.917(2) M n3 O18 2.183(2) Mn3 O19 1.977(2) Mn3 O20 1.905(2) Mn3 O28 1.879(2) Mn4 O17 2.004(2)

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205 Mn2 O28 1.923(2) Mn2 N7 2 .144(3) Mn2 N8 2.371(3) Mn6 O21 2.191(2) Mn6 O23 2.008(2) Mn6 O24 1.914(2) Mn6 O27 2.045(2) Mn6 O29 1.81 3(2) Mn5 N10 2.304(3) Mn5 N9 2.361(3) Mn2 O15 Mn1 92.84(10) Mn2 O17 Mn4 121.20(12) Mn4 O18 Mn3 101.89(10) Mn3 O19 Mn4 99.54(10) Mn3 O19 Mn5 101.92(10) Mn4 O19 Mn5 96.52(9) Mn3 O20 Mn5 106.70(11) Mn4 O18 1.911(2) Mn4 O 19 2.188(2) Mn4 O27 2.050(2) Mn4 O29 1.807(2) Mn5 O20 2.222(2) Mn5 O27 2.230(2) Mn5 O22 2.246(2) Mn5 O21 2.269(2) Mn5 O19 2.285(2) Mn6 O21 Mn5 97.30(9) Mn6 O27 Mn4 87.22(10) Mn6 O27 Mn5 103.03(9) Mn4 O27 Mn5 102.45(9) Mn1 O28 Mn3 123. 36(13) Mn1 O28 Mn2 101.04(11) Mn3 O28 Mn2 129.80(12) Mn4 O29 Mn6 102.59(12 ) Table A 5. Selected interatomic distances () and a ngles (deg) for 4 1 4CH 2 Cl 2 and 4 2 4 1 4CH 2 Cl 2 4 2 3.131(1) Mn(1) O(3) 1.902(3) Mn(1) O(1) 1.933(3) Mn(2) O(3) 2.177(3) Mn(2) O(1) 2.213(3) Mn(2) O(4) 2.275(3) Mn(2) O(2) 2.297(3) Mn(2) N(2) 2.379(4) Mn(2) N(1) 2.427(3) Mn(1) O(1) Mn(2) 97.98(12) Mn(1) O(3) Mn(2) 100.21(12) Mn(1) O(3) Mn(2) 100.95(10) Mn(1) O(3) 2.163(2) Mn(1) O(6) 2.235(2) Mn(1) O(1) 2.334(2) Mn(1) O(2) 2.308(3) Mn(1) O(4) 2.160(2) Mn(1) N(1) 2.374(3) Mn(1) N(2) 2.386(3) Mn(2) O(3) 1.907(2) Mn(2) O(6) 1.931(2) Mn(2) O(5) 2.250(2) Mn(1) O(6) Mn(2) 97.73(9) Table A 6 Selected interatomic distances () and a ngles (deg) for 4 3 and 4 4 4 3 4 4 Mn(1) O(1) 2.311(2) Mn(1) O(2) 2.188(2) 3.320(1)

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206 Mn(1) O(3) 2.224(2) Mn(1) O(4) 2.174(2) Mn(1) O(5) 2.261(2) Mn(1) N(1) 2.335(2) Mn(1) N(2) 2.378(2) Mn(2) O(4) 1.863(2) Mn(2) O(5) 2.281(2) Mn(2) N(3) 2.093(2) Mn(2) Cl(1) 2.449(1) Mn(2) O(4) Mn(1) 110.19(9) Mn(1) O(5) Mn(2) 93.77(7) Mn(1) O(2) Mn(2') 107.24(9) Mn(1) O(1) 1.948(3) Mn(1) O(9) 2.671(4) Mn(1) O(8) 1.901(4) Mn(1) O(10) 1.953(4) Mn(2) O(1) 1.869(3) Mn(2) O(8) 2.239(4) Mn(3) O(9) 2.111(4) Mn(1) O(1) Mn(2) 104.31(15) Mn(2) O(1) Mn(3) 123.07(19) Mn(3) O(1) Mn(1) 122.03(18) Mn(1) O(8) Mn(2) 93.09(15) Mn(3) O (9) Mn(3') 102.24(15) Mn(3') O(10) Mn(1) 118.69(18) Table A 7 Selected interatomic distances () and a ngles (deg) for 4 5 4 5 Mn(1) O(3) 2.205(2) Mn(1) O(1) 2.237(2) Mn(1) O(4) 2.243(2) Mn(1) O(2) 2.364(2) Mn(1) O(13) 2.214(2) Mn(2) O(1) 1.938(2) Mn(2) O(7) 1.963(2) Mn(2) O(13) 1.902(2) Mn(3) O(2) 2.254(2) Mn(2) O(1) Mn(1) 100.65(8) Mn(2) O(1) Mn(2') 101.06(8) Mn(1) O(1) Mn(2') 90.29(7) Mn(4) O(2) Mn(1) 109.99(8) Mn(3) O(2) Mn(1) 88.53(6) Mn(2) O(3) Mn(1) 103.41(9) Mn(5) O(4) Mn(1) 102.14(8) Mn(5) O(4) Mn(4) 89.68(8) Mn(1) O(4) Mn(4) 101.86(8) Mn(5)...Na(1) 3.438(1) Mn(5)...Mn(1) 3.269(1) Mn(3) O(7) 1.943(2) Mn(3) O(13) 1.873(2) Mn(3) O(14) 1.9 16(2) Mn(4) O(2') 1.950(2) Mn(4) O(4') 2.318(2) Mn(4) O(14) 1.909(3) Mn(5) O(13) 2.345(2) Mn(5) O(14) 1.939(2) Mn(5) O(4') 1.953(2) Mn(3) O(13) Mn(2) 97.75(9) Mn(3) O(13) Mn(1) 103.83(8) Mn(2) O(13) Mn(1) 103.77(9) Mn(3) O(13) Mn(5) 93.51(8) Mn(2) O(13) Mn(5) 158.11(10) Mn(4) O(14) Mn(3) 101.16(9) Mn(4) O(14) Mn(5) 103.54(9) Mn(3) O(14) Mn(5) 106.54(9) Mn(4) O(14) Na(1) 141.69(10)

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207 Mn(3) O(7) Mn(2) 93.46(8) Mn(1) O(13) Mn(5) 91.58(7) Mn(3) O(14) Na(1) 97.05(8) Mn(5) O(14) Na(1) 103.11(8) Table A 8 Selected interatomic distances () and a ngles (deg) for 4 6 4 6 3.242(1) Mn(1) O(3) 2.190(2) Mn(1) O(11) 2.213(2) Mn(1) O(1) 2.237(2) Mn(1) O(10) 2.251(2) Mn(1) O(2) 2.350(2) Mn(2) O(1) 1.978(2) Mn(2) O(11) 2.356(2) Mn(2) O(12) 1.923(2) Mn(3) O(11) 1.872(2) Mn(2) O(1) Mn(1) 102.16(8) Mn(2) O(1) Mn(5) 89.33(8) Mn(1) O(1) Mn(5) 101.06(8) Mn(5) O(2) Mn(3) 88.84(7) Mn(5) O(2) Mn(1) 108.39(9) Mn(3) O(2) Mn(1) 88.95(7) Mn(4') O(3) Mn(1) 104.38(9) Mn(4') O(10) Mn(1) 99.87(8) Mn(4) O(13) Mn(3) 91 .69((9) Mn(1) O(10) Mn(4) 90.48(7) Mn(3) O(12) 1.907(2) Mn( 3) O(13) 1.995(2) Mn(4) O(3') 1.886(2) Mn(4) O(11) 1.892(2) Mn(4) O(10') 1.956(2) Mn(4) O(13) 1.993(2) Mn(4) O(10) 2.315(2) Mn(5) O(12) 1.89 7(2) Mn(5) O(2) 1.958(2) Mn(5) O(1) 2.297(2) Mn(3) O(11) Mn(4) 98.95(9) Mn(3) O(11) Mn(1) 105.29(9) Mn(4) O(11) Mn(1) 104.06(9) Mn(3) O(11) Mn(2) 92.77(8) Mn(4) O(11) Mn(2) 156.88(11) Mn (1) O(11) Mn(2) 91.83(7) Mn(5) O(12) Mn(3) 103.44(9) Mn(5) O(12) Mn(2) 104.16(10) Mn(3) O(12) Mn(2) 107.01(9) Table A 9. Selected interatomic distances () and a ngles (deg) for 4 7 x(Solv) 4 7 x(Solv) Mn ( 1 ) ( 1' ) 2.8792(17 ) Mn ( 1 ) ( 4 ) 3.1234(10) Mn ( 1 ) ( 5' ) 3.2149(10) Mn ( 1 ) ( 3 ) 3.2225(10) Mn ( 1 ) ( 3' ) 3.2366(12) Mn ( 1 ) Mn ( 2 ) 3.2988 (12) Mn ( 2 ) ( 3' ) 3.2171 (10) Mn ( 3 ) ( 5 ) 3.2186(13) Mn ( 3 ) ( 7 ) 2.7524(11) Mn ( 3 ) ( 4 ) 3.1561(11) Mn ( 4 ) ( 7 ) 3.0442(11) Mn ( 4 ) ( 6 ) 3.0613(13) Mn ( 5 ) ( 7 ) 3.0895(13) Mn ( 5 ) ( 6 ) 3.1213(12) Mn ( 5 ) ( 1' ) 3.2149(10) Mn ( 8 ) ( 10 ) 3.2311(15)

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208 Mn ( 3 ) ( 1' ) 3.2367(12) Mn ( 1 ) O ( 1 ) 1.897(4) Mn ( 1 ) O ( 3 ) 1.929(4) Mn ( 1 ) O ( 2 ) 1.932(4) Mn ( 1 ) O ( 2 ) 1.933(4) Mn ( 1 ) O ( 11 ) 2.257(4) Mn ( 1 ) O ( 9 ) 2.285(4) Mn ( 2 ) O ( 10 ) 2.240(4) Mn ( 2 ) O ( 9 ) 2.258(3) Mn ( 2 ) O ( 11 ) 2.284(4) Mn ( 2 ) O ( 8 ) 2.317(4) Mn ( 2 ) O ( 2 ) 2.324(4) Mn ( 3 ) O ( 4 ) 1.862(4) Mn ( 3 ) O ( 5 ) 1.907(4) M n ( 3 ) O ( 9 ) 1.937(4) Mn ( 3 ) O ( 11' ) 1.959(4) Mn ( 3 ) O ( 20 ) 2.142(4) Mn ( 3 ) O ( 2 ) 2.526(4) Mn ( 4 ) O ( 6 ) 1.915(4) Mn ( 4 ) O ( 5 ) 1.937(4) Mn ( 4 ) O ( 10 ) 1.942(4) Mn ( 4 ) O ( 3 ) 1.963(4) Mn ( 4 ) O ( 16 ) 2.186(4) Mn ( 4 ) O ( 2 ) 2.477(4) Mn ( 5 ) O ( 1 ) 2.051(4) Mn ( 5 ) O ( 4 ) 2.054(4) M n ( 5 ) O ( 12 ) 2.099(4) Mn ( 5 ) O ( 8 ) 2.131(4) Mn ( 5 ) O ( 6 ) 2.319(5) Mn ( 6 ) O ( 6 ) 1.870(4) Mn ( 6 ) O ( 22 ) 1.935(4) Mn ( 6 ) O ( 18 ) 1.955(4) Mn ( 6 ) O ( 8 ) 1.991(4) Mn ( 6 ) O ( 7 ) 2.085(4) Mn ( 6 ) O ( 10 ) 2.338(4) Mn ( 7 ) O ( 4 ) 1.886(4) Mn ( 7 ) O ( 5 ) 1.928(4) Mn ( 7 ) O ( 17 ) 1.971(4) Mn ( 7 ) O ( 13 ) 1.975(4) Mn ( 7 ) O ( 21 ) 2.138(4) Mn ( 7 ) O ( 6 ) 2.244(4) Mn ( 8 ) O ( 7 ) 1.869(4) Mn ( 8 ) O ( 12 ) 1.904(4) Mn ( 8 ) O ( 14 ) 1.909(4) Mn ( 8 ) O ( 13 ) 2.285(5) Mn ( 9 ) O ( 19 ) 1.879(4) Mn ( 9 ) Mn ( 10) 10 3.3222 (12) Mn ( 1 ) O ( 1 ) Mn ( 5 ) 109.0(2) Mn ( 1' ) O ( 2 ) Mn ( 1 ) 96.30(15) Mn ( 1' ) O ( 2 ) Mn ( 2 ) 99.12(16) Mn ( 1 ) O ( 2 ) Mn ( 2 ) 101.20(14) Mn ( 1' ) O ( 2 ) Mn ( 4 ) 168.99(18) Mn ( 1 ) O ( 2 ) Mn ( 4 ) 89.30(14) Mn ( 2 ) O ( 2 ) Mn ( 4 ) 89.03(12) Mn ( 1' ) O ( 2 ) Mn ( 3 ) 92.12(13) Mn ( 1 ) O ( 2 ) Mn ( 3 ) 91.56(15) Mn ( 2 ) O ( 2 ) Mn ( 3 ) 161.88(15) Mn ( 4 ) O ( 2 ) Mn ( 3 ) 78.22(11) Mn ( 1 ) O ( 3 ) Mn ( 4 ) 106.76(18) Mn ( 3 ) O ( 4 ) Mn ( 7 ) 94.50(19) Mn ( 3 ) O ( 4 ) Mn ( 5 ) 110.44(17) Mn ( 7 ) O ( 4 ) Mn ( 5 ) 103.20(17) Mn ( 3 ) O ( 5 ) Mn ( 7 ) 91.73(17) Mn ( 3 ) O ( 5 ) Mn ( 4 ) 110.38(19) Mn ( 7 ) O ( 5 ) Mn ( 4 ) 103.94(19) Mn ( 6 ) O ( 6 ) Mn ( 4 ) 107.95(19) Mn ( 6 ) O ( 6 ) Mn ( 7 ) 157.1(2) Mn ( 4 ) O ( 6 ) Mn ( 7 ) 93.77(18) Mn ( 6 ) O ( 6 ) Mn ( 5 ) 95.7(2) Mn ( 4 ) O ( 6 ) Mn ( 5 ) 104.98(17) Mn ( 7 ) O ( 6 ) Mn ( 5 ) 85.21(14) Mn ( 8 ) O ( 7 ) Mn ( 6 ) 138.3(2) Mn ( 6 ) O ( 8 ) Mn ( 5 ) 98.37(18) Mn ( 6 ) O ( 8 ) Mn ( 2 ) 108.36(19) Mn ( 5 ) O ( 8 ) Mn ( 2 ) 100.07(17) Mn ( 3' ) O ( 9 ) Mn ( 2 ) 104.71(16) Mn ( 3' ) O ( 9 ) Mn ( 1' ) 99.17(17) Mn ( 2 ) O ( 9 ) Mn ( 1' ) 91.32(13) Mn ( 4 ) O ( 10 ) Mn ( 2 ) 107.05(16) Mn ( 4 ) O ( 10 ) Mn ( 6 ) 90.86(15) Mn ( 2 ) O ( 10 ) Mn ( 6 ) 99.64(17) Mn ( 3' ) O ( 11 ) Mn ( 1 ) 100.06(15) Mn ( 3' ) O ( 11 ) Mn ( 2 ) 103.03(16) Mn ( 1 ) O ( 11 ) Mn ( 2 ) 93.18(13) Mn ( 7 ) O ( 13 ) Mn ( 10 ) 103.83(18) Mn ( 7 ) O ( 13 ) Mn ( 8 ) 122.69(18) Mn ( 10 ) O ( 13 ) Mn ( 8 ) 90.85(14) Mn ( 8 ) O ( 14 ) Mn ( 10 ) 106.30(19) Mn ( 9 ) O ( 16 ) Mn ( 4 ) 107.75(18) Mn ( 7 ) O ( 17 ) Mn ( 9 ) 116.98(1 9) Mn ( 7 ) O ( 17 ) Mn ( 10 ) 98.01(17) Mn ( 9 ) O ( 17 ) Mn ( 10 ) 93.34(15)

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209 Mn ( 9 ) O ( 16 ) 1.901(4) Mn ( 9 ) O ( 18 ) 2.012(5) Mn ( 9 ) O ( 17 ) 2.134(4) Mn ( 10 ) O ( 19 ) 2. 098(4) Mn ( 10 ) O ( 14 ) 2.125(5) Mn ( 10 ) O ( 25 ) 2.150(7) Mn ( 6 ) O ( 18 ) Mn ( 9 ) 118.8(2) Mn ( 9 ) O ( 19 ) Mn ( 10 ) 113.22(19) Mn ( 10 ) ( O24' ) 2.213(10) Mn ( 10 ) O ( 13 ) 2.251(4) Mn ( 10 ) O ( 24 ) 2.263(14) Mn ( 10 ) O ( 17 0 2.425(4) Tabl e A 10 Selected interatomic distances () and a ngles (deg) for 4 8 MeCN 4 8 5MeCN Mn ( 1 ) ( 2 ) 2.756(3) Mn ( 1 ) ( 8 ) 3.071(3) Mn ( 1 ) ( 11 ) 3.090(3) Mn ( 2 ) ( 11 ) 3.177(3) Mn ( 2 ) ( 8 ) 3.178(3) Mn ( 4 ) ( 6 ) 3.102(3) Mn ( 4 ) ( 13 ) 3.118(3) Mn ( 6 ) ( 15 ) 3.058(3) Mn ( 6 ) ( 7 ) 3.128(3) Mn ( 6 ) ( 16 ) 3.161(4) Ca ( 2 ) O ( 45 ) 2.555(6) Ca ( 2 ) O ( 46 ) 2.507(6) Ca ( 2 ) O ( 38 ) 2.384(7) Ca ( 2 ) O ( 12 ) 2.338(6) Mn ( 2 ) O ( 1 ) 1.895(5) Mn ( 1 ) O ( 1 ) 1.917(5) Mn ( 8 ) O ( 1 ) 1.986(5) Mn ( 1 ) O ( 2 ) 1.887(5) Mn ( 2 ) O ( 2 ) 1.861(5) Mn ( 11 ) O ( 2 ) 2.022(5) Mn ( 7 ) O ( 17 ) 1.960(5) Mn ( 12 ) O ( 17 ) 1.948(5) Mn ( 17 ) O ( 17 ) 2.280(5) Mn ( 1 ) O ( 31 ) 2.223(6) Mn ( 8 ) O ( 31 ) 2.058(7) Mn ( 11 ) O ( 31 ) 2.197(7) Mn ( 18 ) O ( 31 ) 1.870(6) Mn ( 6 ) O ( 14 ) 1.976(5) Mn ( 7 ) O ( 14 ) 1.902(5) Mn ( 7 ) O ( 16 ) 1.901(5) Mn ( 8 ) O ( 16 ) 1.973( 5) Mn ( 11 ) O ( 32 ) 2.026(5) Mn ( 12 ) O ( 33 ) 1.903(5) Mn ( 12 ) O ( 32 ) 1.907(5) Mn ( 13 ) O ( 33 ) 2.012(5) Mn ( 4 ) O ( 36' ) 1.998(9) Mn ( 7 ) ( 12 ) 2.901(3) Mn ( 7 ) ( 8 ) 3.120(3) Mn ( 7 ) ( 16 ) 3.213(3) Mn ( 8 ) ( 18 ) 3.088(3) Mn ( 11 ) ( 18 ) 3.138(3) Mn ( 11 ) ( 12 ) 3. 155(3) Mn ( 12 ) ( 13 ) 3.166(3) Mn ( 13 ) ( 15 ) 3.103(3) Mn ( 13 ) ( 16 ) 3.191(3) Mn ( 15 ) ( 16 ) 2.758(3) Ca ( 1 ) O ( 19 ) 2.552(6) Ca ( 1 ) O ( 20 ) 2.345(6) Ca ( 1 ) O ( 27 ) 2.477(5) Ca ( 1 ) O ( 28 ) 2.357(6) Mn ( 2 ) O ( 5 ) 1.960(5) Mn ( 3 ) O ( 5 ) 2.251(5) Mn ( 7 ) O ( 5 ) 2.267(5) Mn ( 2 ) O ( 6 ) 1.947(5) Mn ( 3 ) O ( 6 ) 2.271(5) Mn ( 12 ) O ( 6 ) 2.276(5) Mn ( 3 ) O ( 7 ) 2.224(6) Mn ( 4 ) O ( 7 ) 2.253(6) Mn ( 6 ) O ( 7 ) 2.037(5) Mn ( 3 ) O ( 8 ) 2.248(6) Mn ( 4 ) O ( 8 ) 2.105(7) Mn ( 13 ) O ( 8 ) 2.095(6) Mn ( 4 ) O ( 9 ) 1.988(6) Mn ( 5 ) O ( 9 ) 1.990(8) Mn ( 5 ) O ( 13 ) 1.909(5) Mn ( 6 ) O (13) 2.145(5) Mn ( 5 ) O ( 12 ) 1.876(6) Mn ( 5 ) O ( 45 ) 2.180(6) Mn ( 15 ) O ( 45 ) 1.976(5) Mn ( 13 ) O ( 34 ) 2.107(5) Mn ( 14 ) O ( 34 ) 1.903(6) Mn ( 14 ) O ( 38 ) 1.882(6)

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210 Mn ( 4 ) O ( 36 ) 2.097(13) Mn ( 14 ) O ( 36' ) 1.930(9) Mn ( 14 ) O ( 36 ) 2.001(13) Mn ( 2 ) O ( 1 ) Mn ( 1 ) 92.6(2) Mn ( 2 ) O ( 1 ) Mn ( 8 ) 1 09.9(2) Mn ( 1 ) O ( 1 ) Mn ( 8 ) 103.8(2) Mn ( 2 ) O ( 2 ) Mn ( 1 ) 94.7(2) Mn ( 2 ) O ( 2 ) Mn ( 11 ) 109.7(2) Mn ( 1 ) O ( 2 ) Mn ( 11 ) 104.4(2) Mn ( 2 ) O ( 5 ) Mn ( 3 ) 104.2(2) Mn ( 2 ) O ( 5 ) Mn ( 7 ) 100.2(2) Mn ( 3 ) O ( 5 ) Mn ( 7 ) 93.00(17) Mn ( 2 ) O ( 6 ) Mn ( 3 ) 103.9(2) Mn ( 2 ) O ( 6 ) Mn ( 12 ) 100.3(2) Mn ( 3 ) O ( 6 ) Mn ( 12 ) 91.18(19) Mn ( 6 ) O ( 7 ) Mn ( 3 ) 104.9(2) Mn ( 6 ) O ( 7 ) Mn ( 4 ) 92.5(2) Mn ( 3 ) O ( 7 ) Mn ( 4 ) 102.5(2) Mn ( 13 ) O ( 8 ) Mn ( 4 ) 95.9(3) Mn ( 13 ) O ( 8 ) Mn ( 3 ) 101.7(2) Mn ( 4 ) O ( 8 ) Mn ( 3 ) 106.7(2) Mn ( 4 ) O ( 9 ) Mn ( 5 ) 123.5(4) Mn ( 5 ) O ( 12 ) Ca ( 2 ) 114.6(3) Mn ( 5 ) O ( 13 ) Mn ( 6 ) 110.6(2) Mn ( 7 ) O ( 14 ) Mn ( 6 ) 107.5(2) Mn ( 12 ) O ( 15 ) Mn ( 7 ) 9 6.8(2) Mn ( 12 ) O ( 15 ) Mn ( 3 ) 99.4(2) Mn ( 7 ) O ( 15 ) Mn ( 3 ) 100.48(19) Mn ( 12 ) O ( 15 ) Mn ( 13 ) 89.50(18) Mn ( 7 ) O ( 15 ) Mn ( 13 ) 168.1(2) Mn ( 3 ) O ( 15 ) Mn ( 13 ) 88.37(15) Mn ( 12 ) O ( 15 ) Mn ( 6 ) 169.6(2) Mn ( 7 ) O ( 15 ) Mn ( 6 ) 87.97(18) Mn ( 3 ) O ( 15 ) Mn ( 6 ) 88.72(17) Mn ( 13 ) O ( 15 ) Mn ( 6 ) 84.28(15) Mn ( 12 ) O ( 15 ) Mn ( 16 ) 93.03(19) Mn ( 7 ) O ( 15 ) Mn ( 16 ) 91.07(18) Mn ( 3 ) O ( 15 ) Mn ( 16 ) 161.8(2) Mn ( 13 ) O ( 15 ) Mn ( 16 ) 78.48(14) Mn ( 6 ) O ( 15 ) Mn ( 16 ) 77.61(15) Mn ( 7 ) O ( 16 ) Mn ( 8 ) 107.3(2) Mn ( 12 ) O ( 17 ) Mn ( 7 ) 95.84(19) Mn ( 12 ) O ( 17 ) Mn ( 17 ) 100.8(2) Mn ( 7 ) O ( 17 ) Mn ( 17 ) 99.9(2) Mn ( 12 ) O ( 17 ) Mn ( 8 ) 168.7(2) Mn ( 7 ) O ( 17 ) Mn ( 8 ) 88.03(17) Mn ( 15 ) O ( 45 ) 1.976(5) Mn ( 5 ) O ( 45 ) 2.180(6) Mn ( 1 ) O ( 19 ) Mn ( 9 ) 117.8(3) Mn ( 1 ) O ( 19 ) Ca ( 1 ) 101.0(2) Mn ( 9 ) O ( 19 ) Ca ( 1 ) 96.7(2) Mn ( 9 ) O ( 20 ) Ca ( 1 ) 114.7(3) Mn ( 1 ) O ( 27 ) Mn ( 10 ) 119.1(2) Mn ( 1 ) O ( 27 ) Ca ( 1 ) 104.1(2) Mn ( 10 ) O ( 27 ) Ca ( 1 ) 96.38(18) Mn ( 10 ) O ( 28 ) Ca ( 1 ) 112.8(2) Mn ( 10 ) O ( 30 ) Mn ( 11 ) 112.6(3) Mn ( 18 ) O ( 31 ) Mn ( 8 ) 103.5(3) Mn ( 18 ) O ( 31 ) Mn ( 11 ) 100.7(3) Mn ( 8 ) O ( 31 ) Mn ( 11 ) 105.3(2) Mn ( 18 ) O ( 31 ) Mn ( 1 ) 159.3(3) Mn ( 8 ) O ( 31 ) Mn ( 1 ) 91.6(3) Mn ( 11 ) O ( 31 ) Mn ( 1 ) 88.7(2) Mn ( 12 ) O ( 32 ) Mn ( 11 ) 106.7(2) Mn ( 12 ) O ( 33 ) Mn ( 13 ) 107.9(2) Mn ( 14 ) O ( 34 ) Mn ( 13 ) 115.4(3) Mn ( 4 ) O ( 37 ) Mn ( 6 ) 106.9(3) Mn ( 4 ) O ( 37 ) Mn ( 13 ) 99.6(3) Mn ( 6 ) O ( 37 ) Mn ( 13 ) 106.2(2) Mn ( 4 ) O ( 37 ) Mn ( 15 ) 157. 4(3) Mn ( 6 ) O ( 37 ) Mn ( 15 ) 91.2(2) Mn ( 13 ) O ( 37 ) Mn ( 15 ) 87.6(2) Mn ( 14 ) O ( 38 ) Ca ( 2 ) 112.4(3) Mn ( 15 ) O ( 45 ) Mn ( 5 ) 117.7(3) Mn ( 15 ) O ( 45 ) Ca ( 2 ) 101.5(2) Mn ( 5 ) O ( 45 ) Ca ( 2 ) 96.99(19) Mn ( 15 ) O ( 46 ) Mn ( 14 ) 120.0(3) Mn ( 15 ) O ( 46 ) Ca ( 2 ) 103.2(2) Mn ( 14 ) O ( 46 ) Ca ( 2 ) 96.7(2) Mn ( 16 ) O ( 47 ) Mn ( 15 ) 92.8(2) Mn ( 16 ) O ( 47 ) Mn ( 6 ) 109.8(3) Mn ( 15 ) O ( 47 ) Mn ( 6 ) 103.7(2) Mn ( 16 ) O ( 48 ) Mn ( 15 ) 94.4(2) Mn ( 16 ) O ( 48 ) Mn ( 13 ) 110.4(2) Mn ( 15 ) O ( 48 ) Mn ( 13 ) 103.6(2) Mn ( 16 ) O ( 51 ) Mn ( 7 ) 98.6(2) Mn ( 16 ) O ( 51 ) Mn ( 17 ) 103.5(2) Mn ( 7 ) O ( 51 ) Mn ( 17 ) 91.81(19) Mn ( 16 ) O ( 52 ) Mn ( 17 ) 104.3(2) Mn ( 16 ) O ( 52 ) Mn ( 12 ) 100.5(2) Mn ( 17 ) O ( 52 ) Mn ( 12 ) 91.59(17) Mn ( 8 ) O ( 53 ) Mn ( 18 ) 93.9(2)

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211 Mn ( 17 ) O ( 17 ) Mn ( 8 ) 88.90(15) Mn ( 12 ) O ( 17 ) Mn ( 11 ) 89.20(18) Mn ( 7 ) O ( 17 ) Mn ( 11 ) 167.8(2) Mn ( 17 ) O ( 17 ) Mn ( 11 ) 89.94(17) Mn ( 8 ) O ( 17 ) Mn ( 11 ) 85.05(15) Mn ( 9 ) O ( 18 ) Mn ( 8 ) 113.7(3) Mn ( 8 ) O ( 53 ) Mn ( 17 ) 103.6(3) Mn ( 18 ) O ( 53 ) Mn ( 17 ) 102.6(3) Mn ( 9 ) O ( 57 ) Mn ( 18 ) 124.7(3) Mn ( 18 ) O ( 58 ) Mn ( 11 ) 96.9(3) Mn ( 18 ) O ( 58 ) Mn ( 17 ) 106.4(3) Mn ( 11 ) O ( 58 ) Mn ( 17 ) 102.2(2) Table A 11 Selected inte ratomic distances () and a ngles (deg) for 5 1 MeCN 5 1 2.8813(14) 3.1442(9) 3.3195(8) Gd(1) O(5) 2.331(3) Gd(1) O(6) 2 .364(3) Gd(1) O(10) 2.415(3) Gd(1) O(4) 2.442(3) Gd(1) O(9) 2.445(4) Gd(1) O(1) 2.470(3) Gd(1) O(12) 2.495(3) Gd(1) O(13) 2.498(4) Gd(1) O(8) 2.586(3) Mn(1) O(4) 2.176(3) Mn(1) O(1) 2.192(3) Mn(2) O(1) Gd(1) 92.26(10) Mn(1) O(1) Gd(1) 107.89(12) Mn(1) O(4) Mn(2') 91.30(14) Mn(1) O(4) Gd(1) 109.42(13) Mn(2') O(4) Gd(1) 90.65(11) Mn(2') O(5) Mn(2) 98.01(16) 3.1442(9) 3.1673(11) 3.3195(8) Mn(1) O(5') 2.275(3) Mn(1) O(2) 2.296(9) Mn(1) N(2) 2.335(5) Mn(1) O(3) 2.344(4) Mn(1) N(1) 2.379(3) Mn(2) O(5') 1.902(3) Mn(2) O(5) 1.916(3) Mn(2) O(11') 1.954(4) M n(2) O(7) 1.964(4) Mn(2) O(1) 2.163(3) Mn(2) O(4') 2.221(3) Mn(2') O(5) Mn(1') 98.24(12) Mn(2) O(5) Mn(1') 6.88(15) Mn(2') O(5) Gd(1) 102.83(15) Mn(2) O(5) Gd(1) 103.61 (11) Mn(1') O(5) Gd(1) 148.03(16) Mn(2) O(1) Mn(1) 93.31(12) Table A 12 Selected interatomic distances () and a ngles (deg) for 5 2 MeCN 5 2 0.5 MeCN 3.3210(5) Tb(1) O(7 ) 2.318(2) Tb (1) O(6) 2.325(2) Tb (1) O(10) 2.367(2) Tb (1) O(8 ) 2.437(2) Tb (1) O(2 ) 2.459(2) Mn(1) O(2') 2.213(2) Mn(1) Mn(1') 2.8964(10) Mn(1) Mn(2') 3.1609(7) Mn ( 1 ) Mn ( 2 ) 3.1767(7) Mn ( 1 ) Tb ( 1' ) 3.3210(5)

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212 Tb (1) O(1) 2.460(2) Tb 1) O(12) 2.475(2) Tb (1) O(13) 2.475(2) Tb (1) O(9 ) 2.616 (2) Mn(1) O(7') 1.897(2) Mn ( 1 ) O ( 7 ) 1.918(2) Mn ( 1 ) O ( 5 ) 1.967(2) Mn(1) O(11') 1.973(2) Mn ( 1 ) O ( 1 ) 2.176(2) Mn ( 1 ) O ( 1 ) Mn ( 2 ) 93.01(8) Mn ( 1 ) O ( 1 ) Tb ( 1 ) 92.04(8) Mn ( 2 ) O ( 1 ) Tb ( 1 ) 108.61(9) Mn ( 2 ) O ( 2 ) Mn ( 1' ) 92.37(8) Mn ( 2 ) O ( 2 ) Tb ( 1 ) 109.85(9) Mn ( 1' ) O ( 2 ) Tb ( 1 ) 90.46(7) Mn ( 2 ) O ( 2 ) 2.168(2) Mn ( 2 ) O ( 4' ) 2.198(6) Mn ( 2 ) O ( 1 ) 2.203(2) Mn ( 2 ) O ( 7' ) 2.29 1(2) Mn ( 2 ) O ( 4 ) 2.330(6) Mn ( 2 ) O ( 3 ) 2.342(3) Mn ( 2 ) N ( 1 ) 2.343(3) Mn ( 2 ) N ( 2 ) 2.389(3) Mn ( 1' ) O ( 7 ) Mn ( 1 ) 98.8(1) Mn ( 1' ) O ( 7 ) Mn ( 2' ) 98.21(9) Mn ( 1 ) O ( 7 ) Mn ( 2' ) 96.95(9) Mn ( 1' ) O ( 7 ) Tb ( 1 ) 103.54(9) Mn ( 1 ) O ( 7 ) Tb ( 1 ) 103.82(9) Mn ( 2' ) O ( 7 ) Tb ( 1 ) 146.99(10)

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213 APPENDIX B LIST OF COMPOUNDS [Mn 12 O 4 (OMe) 2 (edte) 4 (N 3 ) 8 ](ClO 4 )(N 3 ) ( 2 1 ) [Mn 1 2 O 4 (OH)(edte) 4 (N 3 ) 9 ] ( 2 2 ) [Mn 12 O 4 (OH)(edte) 4 (N 3 ) 9 ](ClO 4 )(N 3 ) ( 2 3 ) [Mn 9 O 3 (OMe)(O 2 CBu t ) 7 ( edte)(edteH) 2 (N 3 ) 2 ] ( 3 1 ) [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](O 2 CMe) ( 4 1 ) [Mn 3 (O 2 CMe) 2 (edteH 2 ) 2 ](ClO 4 ) ( 4 2 ) [Mn 4 (edteH 2 ) 2 (hmp) 2 Cl 2 ][MnCl 4 ] ( 4 3 ) [Mn 6 O 2 (O 2 C Bu t ) 6 (edteH) 2 (N 3 ) 2 ] ( 4 4 ) Na 2 [Mn 10 O 4 (OMe) 2 (O 2 CEt) 6 (edte) 2 (N 3 ) 6 ] ( 4 5 ) (NEt 4 ) 2 [Mn 10 O 4 (OH) 2 (O 2 CEt) 6 (Edte) 2 (N 3 ) 6 ] ( 4 6 ) [Mn 20 O 8 (OH) 6 (O 2 CEt) 6 (edte) 4 (edteH) 2 ](ClO 4 ) 4 ( 4 7 ) [ Ca 2 Mn 18 O 8 (OH) 5 (O 2 CBu t ) 4.5 (edte) 5 (NO 3 ) 3.5 ](O 2 CBu t ) 0.5 (NO 3 ) 0.5 ( 4 8 ). Mn 4 Gd 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 1 ) Mn 4 Tb 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 2 ) Mn 4 Dy 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 3 ) Mn 4 Ho 2 O 2 (O 2 C Bu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 4 ) Mn 4 Y 2 O 2 (O 2 CBu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 5 )

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214 APPENDIX C ELECTROCHEMICAL REVERSIBLE PROCESSES Cyclic voltammetry is a simple, rapid and powerful method for characterizing the electrochemical behavior of analytes that can be electrochemically oxidized or reduce d. The reversibility of such processes can be determined by studying the voltammogram of the compound. The important accessible parameters from a CV are the magnitude of the anodic current ( i p ox ), the cathodic peak current ( i p red ) the anodic peak potentia l ( E p ox ), and the cathodic peak potential ( E p red ). The study is performed by sweeping the potential of the working electrode at a specific sweep rate (in volts/second) and by measuring the resulting current vs. time curve. Usually the sweep is reversed at a specific switching potential, hence the method is cyclic voltammetry. The standard protocol is to record current vs. applied potential. A typical cyclic voltammogram obtained for a reversible one electron reduction process is shown in Figure C 1. The fo rmal reduction potential ( E o ') for a reversible couple is ce ntered between E ox p and E red p E o = ( E ox p + E red p ) /2 (C 1 ) The peak current for a reversible system is described by the Randles Sevcik equation for the forward sweep of the first cycle: i p = (2.69 x 10 5 ) n 3/2 AD 1/2 1/2 C (C 2) where, i p = peak current, amperes n = electron stoichiometry, eq/mol A = electrode area, cm 2 C = analyte concentration, mol/cm 3 = scan rate, volts/second

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215 According to the equation C 2, i p increases with 1/2 and is directly proporti onal to analyte concentration. Following properties are true for a reversible couple: a) i ox p is approximately equal to i red p i.e. i ox p / i red p 1 b) E p is independent of the scan rate, E p = 59/n mV at 25 C c) i p 1/2 (diffusion controlled) For a quasi reversib le couple, a) E p > 59 mV and E p in creases with b) i p 1/2 (diffusion controlled) For an irreversible couple, a) chemically no return wave in CV Figure C 1. A typical cyclic voltammogram obtained for a reversible one electron reduction process.

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216 APPENDIX D DFT CALCULATIONS Table D 1. Spin of nine Mn atoms are shown in column S1 to S9 indicating different spin orientation s and the labels refer to Figure 3 1 8 .The multiplicity is shown in where all the orientations are ferromagnetic. IS is designated for the intermediate multiplicity of 28, 22, 20, 12. LS is designated for lowest multiplicity of 6. The suffix(s) are indicati ng the index of Mn according to Figure 3 18 S1 S2 S3 S4 S5 S6 S7 S8 S9 M IS 19 2 2 2 2 2.5 2 2 2 2 22 0.000 IS 29 2 2 2 2 2.5 2 2 2 2 22 0.069 IS 18 2 2 2 2 2.5 2 2 2 2 22 0.139 HS 2 2 2 2 2.5 2 2 2 2 38 0.188 IS 28 2 2 2 2 2.5 2 2 2 2 22 0.215 LS 2468 2 2 2 2 2.5 2 2 2 2 6 0.696 IS 46 2 2 2 2 2.5 2 2 2 2 22 0.891 IS 12 2 2 2 2 2.5 2 2 2 2 22 0.912 IS 13 2 2 2 2 2.5 2 2 2 2 22 1.128 IS 23 2 2 2 2 2.5 2 2 2 2 22 1.373 LS 1289 2 2 2 2 2.5 2 2 2 2 6 1.428 IS 39 2 2 2 2 2.5 2 2 2 2 22 1.448 IS 17 2 2 2 2 2.5 2 2 2 2 22 1.471 IS 24 2 2 2 2 2.5 2 2 2 2 22 1.54 4 IS 16 2 2 2 2 2.5 2 2 2 2 22 1.552 LS 2346 2 2 2 2 2.5 2 2 2 2 6 1.554 IS 27 2 2 2 2 2.5 2 2 2 2 22 1.557 IS 26 2 2 2 2 2.5 2 2 2 2 22 1.596 IS 25 2 2 2 2 2.5 2 2 2 2 20 1.720 IS 15 2 2 2 2 2.5 2 2 2 2 20 1.762 IS 5 2 2 2 2 2.5 2 2 2 2 28 1.885 IS 35 2 2 2 2 2.5 2 2 2 2 20 1.941 IS 357 2 2 2 2 2.5 2 2 2 2 12 1.983 IS 456 2 2 2 2 2.5 2 2 2 2 12 2.020 IS 459 2 2 2 2 2.5 2 2 2 2 12 2.578 IS 34 2 2 2 2 2.5 2 2 2 2 22 2.658 IS 156 2 2 2 2 2.5 2 2 2 2 12 2.987 IS 37 2 2 2 2 2.5 2 2 2 2 22 3.01 9 IS 36 2 2 2 2 2.5 2 2 2 2 22 3.020

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217 APPENDIX E VAN VLECK EQUATIONS c = N B 2 /3K N g k = Boltzmann constant T = Temperature TIP = Temperature independent paramagnetism E 1 [Mn 2 II Mn III (O 2 CMe) 2 (edteH 2 ) 2 ] (O 2 CMe) ( 4 1 ) (see Figure 4 15 ) m = J /k/ T n = J' /k/ T N um= + 30.0000 *exp( 6.0000 *m+ 0.000 0 *n) + 6.0000 *exp( 0.0000 *m+ 2.0000 *n) + 30.0000 *exp( 4.0000 *m+ 2.0000 *n) + 84.0000 *exp( 10.0000 *m+ 2.0000 *n) + 0.0000 *exp( 6.0000 *m+ 6.0000 *n) + 6.0000 *exp( 4.0000 *m+ 6.0000 *n) + 30.0000 *exp( 0.0000 *m+ 6.0000 *n) + 84.0000 *exp( 6.0000 *m+ 6.0000 *n) + 180.0000 *exp(14.0000*m+ 6.0000 *n) + 6.0000 *exp( 10.0000 *m+ 12.0000 *n) + 30.0000 *exp( 6.0000 *m+ 12.0000 *n) + 84.0000 *exp( 0.0000 *m+ 12.0000 *n) + 180.0000 *exp( 8.0000 *m+ 12.0000 *n) + 330.0000 *exp( 18.0000 *m+ 1 2.0000 *n) + 30.0000 *exp( 14.0000 *m+ 20.0000 *n) + 84.0000 *exp( 8.0000 *m+ 20.0000 *n) + 180.0000 *exp( 0.0000 *m+ 20.0000 *n) + 330.0000 *exp( 10.0000 *m+ 20.0000 *n) + 546.0000 *exp( 22.0000 *m+ 20.0000 *n) + 84.0000 *exp( 18.0000 *m+ 30.0000 *n) + 180.0000 *exp( 10.0000 *m+ 30.0000 *n) + 330.0000 *exp( 0.0000 *m+ 30.0000 *n) + 546.0000 *exp( 12.0000 *m+ 30.0000 *n) + 840.0000 *exp( 26.0000 *m+ 30.0000 *n)

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218 D en= + 5.0000 *exp( 6.0000 *m+ 0.0000 *n) + 3.0000 *exp( 0.0000 *m+ 2.0000 *n) + 5.0000 *exp( 4.0000 *m+ 2.0000 *n) + 7.0000 *exp( 10.0000 *m+ 2.0000 *n) + 1.0000 *exp( 6.0000 *m+ 6.0000 *n) + 3.0000 *exp( 4.0000 *m+ 6.0000 *n) + 5.0000 *exp( 0.0000 *m+ 6.0000 *n) + 7.0000 *exp( 6.0000 *m+ 6.0000 *n) + 9.0000 *exp( 14.0000 m+ 6.0000 *n) + 3.0000 *exp( 10.0000 *m+ 12.0000 *n) + 5.0000 *exp( 6.0000 *m+ 12.0000 *n) + 7.0000 *exp( 0.0000 *m+ 12.0000 *n) + 9.0000 *exp( 8.0000 *m+ 12.0000 *n) + 11.0000 *exp( 18.0000 *m+ 12.0000 *n) + 5.0000 *exp( 14.0000 *m+ 20.0000 *n) + 7.0000 *exp( 8.0000 *m+ 20.0000 *n) + 9.0000 *exp( 0.0000 *m+ 20.0000 *n) + 11.0000 *exp( 10.0000 *m+ 20.0000 *n) + 13.0000 *exp( 22.0000 *m+ 20.0000 *n) + 7.0000 *exp( 18.0000 *m+ 30.0000 *n) + 9.0000 *exp( 10.0000 *m+ 30.0000 *n) + 11.0000 *ex p( 0.0000 *m+ 30.0000 *n) + 13.0000 *exp( 12.0000 *m+ 30.0000 *n) + 15.0000 *exp( 26.0000 *m+ 30.0000 *n) Fitting equation: M = (c g 2 /T)(Num/D en ) Fitting parameters: J = 2.6 (2) J' = 0 g = 1.99(1) E 2 [Mn 2 II Mn III (O 2 CMe) 2 (edteH 2 ) 2 ] (ClO 4 ) ( 4 2 ) (see Figure 4 15 ) m = J /k/ T n = J' /k/ T N um = +30.0000*exp(6.0000*m+0.0000*n) +6.0000*exp(0.0000*m+2.0000*n) +30.0000*exp(4.0000*m+2.0 000*n)

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219 +84.0000*exp(10.0000*m+2.0000*n) +0.0000*exp( 6.0000*m+6.0000*n) +6.0000*exp( 4.0000*m+6.0000*n) +30.0000*exp(0.0000*m+6.0000*n) +84.0000*exp(6.0000*m+6.0000*n) +180.0000*exp(14.0000*m+6.0000*n) +6.0000*exp( 10.0000*m+12.0000*n) +30.0000*exp ( 6.0000*m+12.0000*n) +84.0000*exp(0.0000*m+12.0000*n) +180.0000*exp(8.0000*m+12.0000*n) +330.0000*exp(18.0000*m+12.0000*n) +30.0000*exp( 14.0000*m+20.0000*n) +84.0000*exp( 8.0000*m+20.0000*n) +180.0000*exp(0.0000*m+20.0000*n) +330.0000*exp(10.0000 *m+20.0000*n) +546.0000*exp(22.0000*m+20.0000*n) +84.0000*exp( 18.0000*m+30.0000*n) +180.0000*exp( 10.0000*m+30.0000*n) +330.0000*exp(0.0000*m+30.0000*n) +546.0000*exp(12.0000*m+30.0000*n) +840.0000*exp(26.0000*m+30.0000*n) D en = +5.0000*exp(6.0000* m+0.0000*n) +3.0000*exp(0.0000*m+2.0000*n) +5.0000*exp(4.0000*m+2.0000*n) +7.0000*exp(10.0000*m+2.0000*n) +1.0000*exp( 6.0000*m+6.0000*n) +3.0000*exp( 4.0000*m+6.0000*n) +5.0000*exp(0.0000*m+6.0000*n) +7.0000*exp(6.0000*m+6.0000*n) +9.0000*exp(14.0 000*m+6.0000*n) +3.0000*exp( 10.0000*m+12.0000*n) +5.0000*exp( 6.0000*m+12.0000*n) +7.0000*exp(0.0000*m+12.0000*n) +9.0000*exp(8.0000*m+12.0000*n) +11.0000*exp(18.0000*m+12.0000*n) +5.0000*exp( 14.0000*m+20.0000*n) +7.0000*exp( 8.0000*m+20.0000*n) +9.0000*exp(0.0000*m+20.0000*n) +11.0000*exp(10.0000*m+20.0000*n) +13.0000*exp(22.0000*m+20.0000*n) +7.0000*exp( 18.0000*m+30.0000*n) +9.0000*exp( 10.0000*m+30.0000*n) +11.0000*exp(0.0000*m+30.0000*n) +13.0000*exp(12.0000*m+30.0000*n) +15.0000*exp(2 6.0000*m+30.0000*n)

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220 Fitting equation : f = (c g 2 /T)*Num/D en Fitting parameters : J = 1.4(1) J' = 0.3(1) g = 1.98(1) E 3 [Mn II 2 Mn III 2 (edteH 2 ) 2 (hmp) 2 Cl 2 ][Mn II Cl 4 ] ( 4 3 ) (see Figure 4 24 ) l = J bb /k/ T m = J wb /k / T TIP = 600*10 6 Num= +630.0000*exp(0.0000*l+0.0 000*m) +6.0000*exp(2.0000*l+0.0000*m) +0.0000*exp(2.0000*l+ 4.0000*m) +630.0000*exp(2.0000*l+ 2.0000*m) +30.0000*exp(2.0000*l+2.0000*m) +6.0000*exp(2.0000*l+ 6.0000*m) +84.0000*exp(2.0000*l+4.0000*m) +30.0000*exp(2.0000*l+ 8.0000*m) +180.0000*exp(2.0000*l+ 6.0000*m) +84.0000*exp(2.0000*l+ 10.0000*m) +330.0000*exp(2.0000*l+8.0000*m) +180.0000*exp(2.0000*l+ 12.0000*m) +546.0000*exp(2.0000*l+10.0000*m) +114.0000*exp(6.0000*l+0.0000*m) +630.0000*exp(6.0000*l+ 6.0000*m) +30.0000*exp(6.0000*l+ 2.0000*m) +414.0000* exp(6.0000*l+4.0000*m) +30.0000*exp(6.0000*l+ 12.0000*m) +6.0000*exp(6.0000*l+ 10.0000*m) +180.0000*exp(6.0000*l+8.0000*m) +186.0000*exp(6.0000*l+ 16.0000*m) +180.0000*exp(6.0000*l+2.0000*m) +330.0000*exp(6.0000*l+12.0000*m) +30.0000*exp(6.0000*l+ 20.0000* m) +84.0000*exp(6.0000*l+ 14.0000*m) +546.0000*exp(6.0000*l+16.0000*m)

PAGE 221

221 +84.0000*exp(6.0000*l+ 24.0000*m) +546.0000*exp(6.0000*l+6.0000*m) +840.0000*exp(6.0000*l+20.0000*m) +630.0000*exp(12.0000*l+0.0000*m) +30.0000*exp(12.0000*l+ 8.0000*m) +414.0000*exp(12 .0000*l+ 2.0000*m) +510.0000*exp(12.0000*l+6.0000*m) +6.0000*exp(12.0000*l+ 16.0000*m) +624.0000*exp(12.0000*l+ 12.0000*m) +84.0000*exp(12.0000*l+ 6.0000*m) +180.0000*exp(12.0000*l+2.0000*m) +330.0000*exp(12.0000*l+12.0000*m) +0.0000*exp(12.0000*l+ 24.0000 *m) +186.0000*exp(12.0000*l+ 22.0000*m) +30.0000*exp(12.0000*l+ 18.0000*m) +180.0000*exp(12.0000*l+ 4.0000*m) +546.0000*exp(12.0000*l+18.0000*m) +90.0000*exp(12.0000*l+ 30.0000*m) +30.0000*exp(12.0000*l+ 26.0000*m) +84.0000*exp(12.0000*l+ 20.0000*m) +546.0 000*exp(12.0000*l+10.0000*m) +840.0000*exp(12.0000*l+24.0000*m) +30.0000*exp(12.0000*l+ 36.0000*m) +840.0000*exp(12.0000*l+14.0000*m) +1224.0000*exp(12.0000*l+30.0000*m) +180.0000*exp(20.0000*l+0.0000*m) +414.0000*exp(20.0000*l+ 10.0000*m) +510.0000*exp(20 .0000*l+ 2.0000*m) +330.0000*exp(20.0000*l+8.0000*m) +624.0000*exp(20.0000*l+ 20.0000*m) +84.0000*exp(20.0000*l+ 14.0000*m) +180.0000*exp(20.0000*l+ 6.0000*m) +330.0000*exp(20.0000*l+4.0000*m) +1386.0000*exp(20.0000*l+16.0000*m) +186.0000*exp(20.0000*l+ 30 .0000*m) +30.0000*exp(20.0000*l+ 26.0000*m) +180.0000*exp(20.0000*l+ 12.0000*m) +546.0000*exp(20.0000*l+10.0000*m) +840.0000*exp(20.0000*l+24.0000*m) +0.0000*exp(20.0000*l+ 40.0000*m) +90.0000*exp(20.0000*l+ 38.0000*m) +30.0000*exp(20.0000*l+ 34.0000*m) +8 4.0000*exp(20.0000*l+ 28.0000*m) +546.0000*exp(20.0000*l+2.0000*m) +1224.0000*exp(20.0000*l+32.0000*m) +6.0000*exp(20.0000*l+ 48.0000*m)

PAGE 222

222 +30.0000*exp(20.0000*l+ 44.0000*m) +546.0000*exp(20.0000*l+ 8.0000*m) +840.0000*exp(20.0000*l+6.0000*m) +1224.0000*exp( 20.0000*l+22.0000*m) +1710.0000*exp(20.0000*l+40.0000*m) Den = +36.0000*exp(0.0000*l+0.0000*m) +3.0000*exp(2.0000*l+0.0000*m) +1.0000*exp(2.0000*l+ 4.0000*m) +35.0000*exp(2.0000*l+ 2.0000*m) +5.0000*exp(2.0000*l+2.0000*m) +3.0000*exp(2.0000*l+ 6.0000*m) + 7.0000*exp(2.0000*l+4.0000*m) +5.0000*exp(2.0000*l+ 8.0000*m) +9.0000*exp(2.0000*l+6.0000*m) +7.0000*exp(2.0000*l+ 10.0000*m) +11.0000*exp(2.0000*l+8.0000*m) +9.0000*exp(2.0000*l+ 12.0000*m) +13.0000*exp(2.0000*l+10.0000*m) +12.0000*exp(6.0000*l+0.0000*m) +35.0000*exp(6.0000*l+ 6.0000*m) +5.0000*exp(6.0000*l+ 2.0000*m) +18.0000*exp(6.0000*l+4.0000*m) +6.0000*exp(6.0000*l+ 12.0000*m) +3.0000*exp(6.0000*l+ 10.0000*m) +9.0000*exp(6.0000*l+8.0000*m) +12.0000*exp(6.0000*l+ 16.0000*m) +9.0000*exp(6.0000*l+2.0000* m) +11.0000*exp(6.0000*l+12.0000*m) +5.0000*exp(6.0000*l+ 20.0000*m) +7.0000*exp(6.0000*l+ 14.0000*m) +13.0000*exp(6.0000*l+16.0000*m) +7.0000*exp(6.0000*l+ 24.0000*m) +13.0000*exp(6.0000*l+6.0000*m) +15.0000*exp(6.0000*l+20.0000*m) +20.0000*exp(12.0000*l+ 0.0000*m) +5.0000*exp(12.0000*l+ 8.0000*m) +18.0000*exp(12.0000*l+ 2.0000*m) +20.0000*exp(12.0000*l+6.0000*m) +3.0000*exp(12.0000*l+ 16.0000*m) +32.0000*exp(12.0000*l+ 12.0000*m) +7.0000*exp(12.0000*l+ 6.0000*m) +9.0000*exp(12.0000*l+2.0000*m) +11.0000*exp (12.0000*l+12.0000*m) +1.0000*exp(12.0000*l+ 24.0000*m) +12.0000*exp(12.0000*l+ 22.0000*m)

PAGE 223

223 +5.0000*exp(12.0000*l+ 18.0000*m) +9.0000*exp(12.0000*l+ 4.0000*m) +13.0000*exp(12.0000*l+18.0000*m) +10.0000*exp(12.0000*l+ 30.0000*m) +5.0000*exp(12.0000*l+ 26.000 0*m) +7.0000*exp(12.0000*l+ 20.0000*m) +13.0000*exp(12.0000*l+10.0000*m) +15.0000*exp(12.0000*l+24.0000*m) +5.0000*exp(12.0000*l+ 36.0000*m) +15.0000*exp(12.0000*l+14.0000*m) +17.0000*exp(12.0000*l+30.0000*m) +9.0000*exp(20.0000*l+0.0000*m) +18.0000*exp(20 .0000*l+ 10.0000*m) +20.0000*exp(20.0000*l+ 2.0000*m) +11.0000*exp(20.0000*l+8.0000*m) +32.0000*exp(20.0000*l+ 20.0000*m) +7.0000*exp(20.0000*l+ 14.0000*m) +9.0000*exp(20.0000*l+ 6.0000*m) +11.0000*exp(20.0000*l+4.0000*m) +28.0000*exp(20.0000*l+16.0000*m) +12.0000*exp(20.0000*l+ 30.0000*m) +5.0000*exp(20.0000*l+ 26.0000*m) +9.0000*exp(20.0000*l+ 12.0000*m) +13.0000*exp(20.0000*l+10.0000*m) +15.0000*exp(20.0000*l+24.0000*m) +1.0000*exp(20.0000*l+ 40.0000*m) +10.0000*exp(20.0000*l+ 38.0000*m) +5.0000*exp(20.0 000*l+ 34.0000*m) +7.0000*exp(20.0000*l+ 28.0000*m) +13.0000*exp(20.0000*l+2.0000*m) +17.0000*exp(20.0000*l+32.0000*m) +3.0000*exp(20.0000*l+ 48.0000*m) +5.0000*exp(20.0000*l+ 44.0000*m) +13.0000*exp(20.0000*l+ 8.0000*m) +15.0000*exp(20.0000*l+6.0000*m) +1 7.0000*exp(20.0000*l+22.0000*m) +19.0000*exp(20.0000*l+40.0000*m) Fitting equation: M = (c g 2 / T ) Num/Den + (c g 2 / T ) 8.75 + TIP

PAGE 224

224 Fitting parameters: J bb = +7.20(3) J wb = +1.34(3) g = 1.87(2) E 4 Mn II 2 Mn III 2 Y 2 O 2 (O 2 CBu t ) 6 (edteH 2 ) 2 (NO 3 ) 2 ( 5 5 ) (See Figure 5 8) l = J bb /k/ T m = J wb /k/ T TIP = 4 00*10 6 Num = +630.0000*exp(0.0000*l+0.0000*m) +6.00 00*exp(2.0000*l+0.0000*m) +0.0000*exp(2.0000*l+ 4.0000*m) +630.0000*exp(2.0000*l+ 2.0000*m) +30.0000*exp(2.0000*l+2.0000*m) +6.0000*exp(2.0000*l+ 6.0000*m) +84.0000*exp(2.0000*l+4.0000*m) +30.0000*exp(2.0000*l+ 8.0000*m) +180.0000*exp(2.0000*l+6.0000*m) +8 4.0000*exp(2.0000*l+ 10.0000*m) +330.0000*exp(2.0000*l+8.0000*m) +180.0000*exp(2.0000*l+ 12.0000*m) +546.0000*exp(2.0000*l+10.0000*m) +114.0000*exp(6.0000*l+0.0000*m) +630.0000*exp(6.0000*l+ 6.0000*m) +30.0000*exp(6.0000*l+ 2.0000*m) +414.0000*exp(6.0000*l +4.0000*m) +30.0000*exp(6.0000*l+ 12.0000*m) +6.0000*exp(6.0000*l+ 10.0000*m) +180.0000*exp(6.0000*l+8.0000*m) +186.0000*exp(6.0000*l+ 16.0000*m) +180.0000*exp(6.0000*l+2.0000*m) +330.0000*exp(6.0000*l+12.0000*m) +30.0000*exp(6.0000*l+ 20.0000*m) +84.0000* exp(6.0000*l+ 14.0000*m) +546.0000*exp(6.0000*l+16.0000*m) +84.0000*exp(6.0000*l+ 24.0000*m) +546.0000*exp(6.0000*l+6.0000*m) +840.0000*exp(6.0000*l+20.0000*m) +630.0000*exp(12.0000*l+0.0000*m)

PAGE 225

225 +30.0000*exp(12.0000*l+ 8.0000*m) +414.0000*exp(12.0000*l+ 2.0 000*m) +510.0000*exp(12.0000*l+6.0000*m) +6.0000*exp(12.0000*l+ 16.0000*m) +624.0000*exp(12.0000*l+ 12.0000*m) +84.0000*exp(12.0000*l+ 6.0000*m) +180.0000*exp(12.0000*l+2.0000*m) +330.0000*exp(12.0000*l+12.0000*m) +0.0000*exp(12.0000*l+ 24.0000*m) +186.000 0*exp(12.0000*l+ 22.0000*m) +30.0000*exp(12.0000*l+ 18.0000*m) +180.0000*exp(12.0000*l+ 4.0000*m) +546.0000*exp(12.0000*l+18.0000*m) +90.0000*exp(12.0000*l+ 30.0000*m) +30.0000*exp(12.0000*l+ 26.0000*m) +84.0000*exp(12.0000*l+ 20.0000*m) +546.0000*exp(12.0 000*l+10.0000*m) +840.0000*exp(12.0000*l+24.0000*m) +30.0000*exp(12.0000*l+ 36.0000*m) +840.0000*exp(12.0000*l+14.0000*m) +1224.0000*exp(12.0000*l+30.0000*m) +180.0000*exp(20.0000*l+0.0000*m) +414.0000*exp(20.0000*l+ 10.0000*m) +510.0000*exp(20.0000*l+ 2.0 000*m) +330.0000*exp(20.0000*l+8.0000*m) +624.0000*exp(20.0000*l+ 20.0000*m) +84.0000*exp(20.0000*l+ 14.0000*m) +180.0000*exp(20.0000*l+ 6.0000*m) +330.0000*exp(20.0000*l+4.0000*m) +1386.0000*exp(20.0000*l+16.0000*m) +186.0000*exp(20.0000*l+ 30.0000*m) +30 .0000*exp(20.0000*l+ 26.0000*m) +180.0000*exp(20.0000*l+ 12.0000*m) +546.0000*exp(20.0000*l+10.0000*m) +840.0000*exp(20.0000*l+24.0000*m) +0.0000*exp(20.0000*l+ 40.0000*m) +90.0000*exp(20.0000*l+ 38.0000*m) +30.0000*exp(20.0000*l+ 34.0000*m) +84.0000*exp(2 0.0000*l+ 28.0000*m) +546.0000*exp(20.0000*l+2.0000*m) +1224.0000*exp(20.0000*l+32.0000*m) +6.0000*exp(20.0000*l+ 48.0000*m) +30.0000*exp(20.0000*l+ 44.0000*m) +546.0000*exp(20.0000*l+ 8.0000*m) +840.0000*exp(20.0000*l+6.0000*m) +1224.0000*exp(20.0000*l+22 .0000*m)

PAGE 226

226 +1710.0000*exp(20.0000*l+40.0000*m) Den = +36.0000*exp(0.0000*l+0.0000*m) +3.0000*exp(2.0000*l+0.0000*m) +1.0000*exp(2.0000*l+ 4.0000*m) +35.0000*exp(2.0000*l+ 2.0000*m) +5.0000*exp(2.0000*l+2.0000*m) +3.0000*exp(2.0000*l+ 6.0000*m) +7.0000*exp(2 .0000*l+4.0000*m) +5.0000*exp(2.0000*l+ 8.0000*m) +9.0000*exp(2.0000*l+6.0000*m) +7.0000*exp(2.0000*l+ 10.0000*m) +11.0000*exp(2.0000*l+8.0000*m) +9.0000*exp(2.0000*l+ 12.0000*m) +13.0000*exp(2.0000*l+10.0000*m) +12.0000*exp(6.0000*l+0.0000*m) +35.0000*exp (6.0000*l+ 6.0000*m) +5.0000*exp(6.0000*l+ 2.0000*m) +18.0000*exp(6.0000*l+4.0000*m) +6.0000*exp(6.0000*l+ 12.0000*m) +3.0000*exp(6.0000*l+ 10.0000*m) +9.0000*exp(6.0000*l+8.0000*m) +12.0000*exp(6.0000*l+ 16.0000*m) +9.0000*exp(6.0000*l+2.0000*m) +11.0000* exp(6.0000*l+12.0000*m) +5.0000*exp(6.0000*l+ 20.0000*m) +7.0000*exp(6.0000*l+ 14.0000*m) +13.0000*exp(6.0000*l+16.0000*m) +7.0000*exp(6.0000*l+ 24.0000*m) +13.0000*exp(6.0000*l+6.0000*m) +15.0000*exp(6.0000*l+20.0000*m) +20.0000*exp(12.0000*l+0.0000*m) +5 .0000*exp(12.0000*l+ 8.0000*m) +18.0000*exp(12.0000*l+ 2.0000*m) +20.0000*exp(12.0000*l+6.0000*m) +3.0000*exp(12.0000*l+ 16.0000*m) +32.0000*exp(12.0000*l+ 12.0000*m) +7.0000*exp(12.0000*l+ 6.0000*m) +9.0000*exp(12.0000*l+2.0000*m) +11.0000*exp(12.0000*l+1 2.0000*m) +1.0000*exp(12.0000*l+ 24.0000*m) +12.0000*exp(12.0000*l+ 22.0000*m) +5.0000*exp(12.0000*l+ 18.0000*m) +9.0000*exp(12.0000*l+ 4.0000*m) +13.0000*exp(12.0000*l+18.0000*m) +10.0000*exp(12.0000*l+ 30.0000*m)

PAGE 227

227 +5.0000*exp(12.0000*l+ 26.0000*m) +7.0000 *exp(12.0000*l+ 20.0000*m) +13.0000*exp(12.0000*l+10.0000*m) +15.0000*exp(12.0000*l+24.0000*m) +5.0000*exp(12.0000*l+ 36.0000*m) +15.0000*exp(12.0000*l+14.0000*m) +17.0000*exp(12.0000*l+30.0000*m) +9.0000*exp(20.0000*l+0.0000*m) +18.0000*exp(20.0000*l+ 10. 0000*m) +20.0000*exp(20.0000*l+ 2.0000*m) +11.0000*exp(20.0000*l+8.0000*m) +32.0000*exp(20.0000*l+ 20.0000*m) +7.0000*exp(20.0000*l+ 14.0000*m) +9.0000*exp(20.0000*l+ 6.0000*m) +11.0000*exp(20.0000*l+4.0000*m) +28.0000*exp(20.0000*l+16.0000*m) +12.0000*exp (20.0000*l+ 30.0000*m) +5.0000*exp(20.0000*l+ 26.0000*m) +9.0000*exp(20.0000*l+ 12.0000*m) +13.0000*exp(20.0000*l+10.0000*m) +15.0000*exp(20.0000*l+24.0000*m) +1.0000*exp(20.0000*l+ 40.0000*m) +10.0000*exp(20.0000*l+ 38.0000*m) +5.0000*exp(20.0000*l+ 34.00 00*m) +7.0000*exp(20.0000*l+ 28.0000*m) +13.0000*exp(20.0000*l+2.0000*m) +17.0000*exp(20.0000*l+32.0000*m) +3.0000*exp(20.0000*l+ 48.0000*m) +5.0000*exp(20.0000*l+ 44.0000*m) +13.0000*exp(20.0000*l+ 8.0000*m) +15.0000*exp(20.0000*l+6.0000*m) +17.0000*exp(2 0.0000*l+22.0000*m) +19.0000*exp(20.0000*l+40.0000*m) Fitting equation: f = (c g 2 / T )* Num/Den + TIP Fitting parameters: J bb = 32.5(4) J wb = 1.0(1) g = 1.95(1)

PAGE 228

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243 BIOGRAPHICAL SKETCH Arpita Saha was born in West Bengal, India. She is the younger daughter of Mr. Supriya Kumar Saha and Mrs. Kalyani Saha. She did her undergraduate studies from Presidency College, Kolkata and received a Bachelor of Science degree in 2002. She successfully qualified the Joint Admission test for M.Sc joined the research group of Dr. Sabyasachi Sarakar and worked on the synthesis and characterization of carbon nanotubes. She studied the t ransport of water through the channel of water soluble carbon nanotubes and its interaction with amino acids and DNA In 2003 she did a summer internship from Indian Institute of Science, Bangalore under the supervision of Dr. A. R. Chakravarty and investi gated the modeling of the trinuclear active site of Ascorbate Oxidase (AO) with spectral and analytical characterization of three tri nuclear copper complexes Then s he came to United States of America for higher studies She started her graduate career in 2005 in University of Florida where she met a gator Subhrajit K. Saha and got married in 2006. She joined the research group of Dr. George Christou in the Department of Chemistry at UF. Her doctoral research primarily focuses on the s ynthesis, structure, electrochemical, magnetochemical and spectroscopic properties of several transition metals and lanthanide containing nano materials using various alcohol based ligands She received her Ph.D. from the University of Florida in the summer of 2011.