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Comproportionation Reactions as a Route to Single-Molecule Magnets and Synthetic Analogues of the Oxygen-Evolving Comple...

Permanent Link: http://ufdc.ufl.edu/UFE0043644/00001

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Title: Comproportionation Reactions as a Route to Single-Molecule Magnets and Synthetic Analogues of the Oxygen-Evolving Complex of Photosystem II
Physical Description: 1 online resource (282 p.)
Language: english
Creator: Mukherjee, Shreya
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: bioinorganic -- biomimic -- calcium -- cluster -- hysteresis -- magnetism -- manganese -- oec -- photosystem -- smm -- spin-frustration -- synthetic-analogues
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: This thesis describes the development of new synthetic strategies for synthesizing and characterizing polynuclear metal cluster compounds that are either nanoscale magnetic materials or biomimetic synthetic analogues of the oxygen-evolving complex (OEC) of photosystem II (PS II) in plants and cyanobacteria. An important development in molecular magnetism was the discovery of zero-dimensional magnets, more commonly known as single-molecule magnets (SMMs). SMMs display slow relaxation of magnetization, resulting in hysteresis loops below the blocking temperature (TB). On the other hand, the OEC catalyzes the thermodynamically demanding oxidation of water to oxygen gas. Elucidating the properties and mechanism of action of the OEC represents one of the holy grails of metallobiochemistry. Recent X-ray structure of PS II identified the OEC to be a Mn4CaOx cluster with primarily carboxylate ligation. The common theme that binds the work in this thesis together is the use of comproportionation reactions under acidic conditions to prepare the high oxidation state homo- and heterometallic clusters. Using this strategy, a Mn9 and two Mn8 complexes possessing unusual structures have been prepared. One of the Mn8 exhibits magnetization hysteresis below its TB, making it a new addition to the small family of SMMs possessing a half-integer spin (S = 15/2). The same reaction system in the bioinorganic project gave the first Mn3Ca cubane with an additional Ca attached, Mn3Ca2O4(O2CBut)8(HO2CBut)4. Magnetic characterization revealed the compound to be spin frustrated, and in an attempt to change the S = 9/2 ground state spin, structural perturbation of the Mn3Ca2 core was accomplished with 2,2'-bipyridine (bpy), giving Mn3Ca2O4(O2CBut)8(bpy)(HO2CBut)(MeCN). However, the perturbations were not enough to change the ground state. Initial attempts to replace the external Ca with Mn to obtain an exact structural correspondence to the OEC gave instead the new Mn6Ca2O9(O2CBut)9(H2O)6(NO3) cluster. Addition of chelates directly into the comproportionation reaction led to Mn4Ca2O6(O2CBut)6(phen)4(O2CBut), whose core is essentially that of the OEC with an additional Ca attached to it. This exciting complex possess an S = ½ ground state, the same as the OEC in its S2 Kok state. Mn4Ca2 is the only Mn/Ca cluster to mimic the OEC both structurally and magnetically. Replacement of the Ca in Mn4Ca2 with Sr was targeted and achieved to give isostructural Mn4Sr2O6(O2CBut)6(phen)4(O2CBut). This is the first example of analogous Mn/Ca vs Mn/Sr cluster chemistry. Finally, tuning the average Mn oxidation state in the reaction to +2.83 instead of +4.5 (as in the previous reaction schemes) allowed entry into (NBun4)Mn9Ca2O4(OH)4(O2CBut)16(H2O)2 and Mn11CaO8(OH)Cl2(O2CBut)16 clusters which mimic the S0 state of the Kok cycle. This work lists a library of Mn/Ca model complexes, which are the closest synthetic analogues of the OEC reported to date.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Shreya Mukherjee.
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 2014-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043644:00001

Permanent Link: http://ufdc.ufl.edu/UFE0043644/00001

Material Information

Title: Comproportionation Reactions as a Route to Single-Molecule Magnets and Synthetic Analogues of the Oxygen-Evolving Complex of Photosystem II
Physical Description: 1 online resource (282 p.)
Language: english
Creator: Mukherjee, Shreya
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: bioinorganic -- biomimic -- calcium -- cluster -- hysteresis -- magnetism -- manganese -- oec -- photosystem -- smm -- spin-frustration -- synthetic-analogues
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: This thesis describes the development of new synthetic strategies for synthesizing and characterizing polynuclear metal cluster compounds that are either nanoscale magnetic materials or biomimetic synthetic analogues of the oxygen-evolving complex (OEC) of photosystem II (PS II) in plants and cyanobacteria. An important development in molecular magnetism was the discovery of zero-dimensional magnets, more commonly known as single-molecule magnets (SMMs). SMMs display slow relaxation of magnetization, resulting in hysteresis loops below the blocking temperature (TB). On the other hand, the OEC catalyzes the thermodynamically demanding oxidation of water to oxygen gas. Elucidating the properties and mechanism of action of the OEC represents one of the holy grails of metallobiochemistry. Recent X-ray structure of PS II identified the OEC to be a Mn4CaOx cluster with primarily carboxylate ligation. The common theme that binds the work in this thesis together is the use of comproportionation reactions under acidic conditions to prepare the high oxidation state homo- and heterometallic clusters. Using this strategy, a Mn9 and two Mn8 complexes possessing unusual structures have been prepared. One of the Mn8 exhibits magnetization hysteresis below its TB, making it a new addition to the small family of SMMs possessing a half-integer spin (S = 15/2). The same reaction system in the bioinorganic project gave the first Mn3Ca cubane with an additional Ca attached, Mn3Ca2O4(O2CBut)8(HO2CBut)4. Magnetic characterization revealed the compound to be spin frustrated, and in an attempt to change the S = 9/2 ground state spin, structural perturbation of the Mn3Ca2 core was accomplished with 2,2'-bipyridine (bpy), giving Mn3Ca2O4(O2CBut)8(bpy)(HO2CBut)(MeCN). However, the perturbations were not enough to change the ground state. Initial attempts to replace the external Ca with Mn to obtain an exact structural correspondence to the OEC gave instead the new Mn6Ca2O9(O2CBut)9(H2O)6(NO3) cluster. Addition of chelates directly into the comproportionation reaction led to Mn4Ca2O6(O2CBut)6(phen)4(O2CBut), whose core is essentially that of the OEC with an additional Ca attached to it. This exciting complex possess an S = ½ ground state, the same as the OEC in its S2 Kok state. Mn4Ca2 is the only Mn/Ca cluster to mimic the OEC both structurally and magnetically. Replacement of the Ca in Mn4Ca2 with Sr was targeted and achieved to give isostructural Mn4Sr2O6(O2CBut)6(phen)4(O2CBut). This is the first example of analogous Mn/Ca vs Mn/Sr cluster chemistry. Finally, tuning the average Mn oxidation state in the reaction to +2.83 instead of +4.5 (as in the previous reaction schemes) allowed entry into (NBun4)Mn9Ca2O4(OH)4(O2CBut)16(H2O)2 and Mn11CaO8(OH)Cl2(O2CBut)16 clusters which mimic the S0 state of the Kok cycle. This work lists a library of Mn/Ca model complexes, which are the closest synthetic analogues of the OEC reported to date.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Shreya Mukherjee.
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 2014-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043644:00001


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1 COMPROPORTIONATION REACTIONS AS A ROUTE TO SINGLE MOLECULE MAGNETS AND SYNTHETIC ANALOGUES OF THE OXYGEN EVOLVING COMPLEX OF PHOTOSYSTEM II By SHREYA MUKHERJEE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERS ITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2011

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2 2011 Shreya Mukherjee

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3 To Ma, Baba and Bon for their unconditional love support and faith

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4 ACKNOWLEDGMENTS From the day I was admitted to this PhD program in August 2006, I have been looking forward and working towards this moment when I defend my doctoral dissertation and now that it is near I have a bitter sweet feeling regarding this. As Dr. been an amazing journey and I will not change a thing as it has made me who I am today. However, it would be impossible to reach this goal without the encouragement and support of the many p eople who has made this journey memorable. First and foremost, I would like to thank my mentor and research advisor Prof. George Christou, for his guidance, inspiration, encouragement and help over the last 5 years. His passion and enthusiasm for resear ch as well as teaching was motivational and allowed me to steer through even the toughest time during my PhD. I would like to thank him for giving me enough freedom to pursue my research and the much needed guidance when required. I am grateful to him for molding me into a more mature and confident scientist and I will forever cherish the values that he instilled in me. The thing that I really enjoyed and will miss the most are the long discussions that we had in his office about new research ideas, philos ophies of life as well as cricket. I would also like to take this opportunity to thank all my committee members: Dr. Daniel R. Talham, Dr. Adam S. Veige, Dr. Mark W. Meisel and Dr. Sukwon Hong for their suggestions and thoughtful insights that helped me throughout my PhD research. I would always treasure the conversations that I had with Dr. Talham and Dr. Veige whenever our paths crossed in the last five years I appreciate Dr. Meisel for always having the time to answer my questions about physics and th e SQUID. I would also like to thank Dr. Khalil A. Abboud and his staff (Dan, Patrick and Yusoon) for solving all the

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5 crystal structures in this thesis. I am grateful to Dr. Abboud for always being patient both with me and my crystals and never giving up on disordered pivalate groups. I would also like to acknowledge Dr. Wolfgang Wernsdorfer for providing essential single crystal measurements on the Mn 8 compound below 1.8 K using his micro SQUID apparatus. Throughout my PhD, I have been fortunate enough to w ork with many collaborators and I would like to thank all of them for their contribution in my research work. The EPR work reported in this thesis would not be possible without the help of Dr. R. David Britt and Jamie St ull at University of California I w ould also like to acknowledge Dr. Vittal K. Yachandra and his group members at Lawrence Berkeley National Laboratory for performing XAS measurements on the Mn/Ca complexes. Speaking of friends, I would first like to thank Soumya for being there for me no matter what. The patience with which you listen ed to me talk (or complain) for the past 7 years is a feat by itself. Your passion for chemistry is contagious and I have learned a lot from you. It has been an extraordinary journey and I cannot thank you en ough for supporting me and believing in me even when the going got tough. It would have been impossible to finish this journey without your friendship and love. Special thanks to Rashmi and Charis for helping me in get ting started in the Christou group an d answering all my questions. I am grateful to both of you for getting me excited in this research and I will always treasure the friendship that we have built throughout my life I would also like to thank Christos, Konstantina, Jennifer Yacoubian, Taketo Andy, Andrew, Tu, Yan and Galia for all the fun discussions that we had in the lab and innumerable stories about our teaching assignments Special thanks to Annaliese for proof reading this document and I had a great time working with you. I

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6 would always cherish the nice chats that we had in the lab and thank you for always cheering me up whenever I am feeling low. Life in the Christou group would not be as smooth flowing without the help from Melinda and Alice and I would like to thank them for taking ca re of all the small details. During my time in UF I have made some friends who need special mention. Firstly, I would like to thank Gary and Natalie for their warm friendship and making my stay in Gainesville so memorable. It was fun learning about America n culture from you and teaching you about my culture. Special thanks to Roxanne for the amazing time we had teaching 2045L and I am really inspired by your tension free attitude to life. Finally I would like to thank my parents and my sister Simita for bei ng an integral part of my existence. There are not enough words to thank you for your unconditional love, trust, pride and encouragement and for keeping all the adversities of life away from me as I pursued my dream. Simita thank you for being my best fri end, my biggest critique and my strongest support system. I could not have asked for a better family to be born into and would like to thank God for giving me this life. My final gratitude goes to the Almighty for his blessings and for giving me the streng th to believe in myself and pursue my dreams.

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7 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ .......... 10 LIST OF FIGURES ................................ ................................ ................................ ........ 13 LIST OF ABBREVIATIONS ................................ ................................ ........................... 21 ABSTRACT ................................ ................................ ................................ ................... 22 CHAPTER 1 BACKGROUND INFORMATION ................................ ................................ ............ 24 2 COMPROPORTIONATION REACTIONS AS A ROUTE TO NEW HIGH OXIDATION STATE MANGANESE CLUSTERS AND SINGLE MOLECULE MAGNET ................................ ................................ ................................ ................ 40 2.1 Introduction ................................ ................................ ................................ ....... 40 2.2 Experimental Section ................................ ................................ ........................ 42 2.2.1 Syntheses ................................ ................................ ................................ 42 2.2.2 X ray Crystallography ................................ ................................ .............. 44 2.3 Results and Discussion ................................ ................................ ..................... 46 2.3.1 Syntheses ................................ ................................ ................................ 46 2.3.2 Description of Structures ................................ ................................ ......... 48 2.3.2.1 Structure of [Mn 8 O 6 (OH)(O 2 CBu t ) 9 Cl 3 (Bu t CO 2 H) 0.5 (MeCN) 0.5 ] ( 2 1 ) ................................ ................................ ................................ ............ 48 2.3.2.2 Structure of [Mn 8 O 9 (O 2 CBu t ) 12 ] ( 2 2 ) ................................ .............. 50 2.3.2.3 Structure of [Mn 9 O 7 (O 2 CBu t ) 13 (THF) 2 ] ( 2 3 ) ................................ ... 51 2.3.3 Magn etochemistry of Complexes 2 1 to 2 3 ................................ ............ 52 2.3.3.1 DC studies ................................ ................................ ..................... 52 2.3.3.2 AC studies ................................ ................................ ...................... 54 2.3.3.3 Single crystal hysteresis studies ................................ .................... 56 2.4 Conclusions ................................ ................................ ................................ ...... 57 3 SYNTHETIC MODEL OF THE OXYGEN EVOLVING COMPLEX (OEC) OF PHOTOSYSTEM II ISOLATION OF THE DISCRETE CUBANE UNIT ................ 68 3.1 Introduction ................................ ................................ ................................ ....... 68 3.2 Experimental Section ................................ ................................ ........................ 71 3.2.1 Syntheses ................................ ................................ ................................ 71 3.2.2 X ray Crystallography ................................ ................................ .............. 72 3.3 Results and Discussion ................................ ................................ ..................... 73 3.3.1 Syntheses ................................ ................................ ................................ 73

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8 3.3.2 Description of Structures ................................ ................................ ......... 75 3.3.2.1 Structu re of [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (HO 2 C t Bu) 4 ] ( 3 1 ) ....................... 75 3.3.2.2 Comparison of the structural features of 3 1 with the native OEC 76 3.3.2.3 S tructure of [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (bpy)(HO 2 C t Bu)(MeCN)] ( 3 2 ) .... 78 3.3.3 Mn and Ca X ray Absorption Spectroscopy (XAS) ................................ .. 79 3.3.4 Magnetoc hemistry of Complexes 3 1 and 3 2 ................................ ......... 82 3.3.4.1 DC studies ................................ ................................ ..................... 82 3.3.4.2 AC studies ................................ ................................ ...................... 87 3.3.5 EPR Spectroscopy ................................ ................................ .................. 88 3.3.6 Electrochemistry ................................ ................................ ...................... 91 3.4 Conclusions ................................ ................................ ................................ ...... 92 4 THE FIRST SYNTHETIC MODEL POSSESSING THE EXACT CUBANE CORE OF THE OEC AND MIMICKING THE S 2 STATE OF THE KOK CYCLE .............. 117 4.1 Introduction ................................ ................................ ................................ ..... 117 4.2 Experimental Section ................................ ................................ ...................... 118 4.2.1 Syntheses ................................ ................................ .............................. 118 4.2.2 X ray Crystallography ................................ ................................ ............ 120 4.3 Results and Discussion ................................ ................................ ................... 122 4.3.1 Syntheses ................................ ................................ .............................. 122 4.3.2 Description of Structures ................................ ................................ ....... 123 4.3.2.1 Structure of [Mn 8 Ca 4 O 14 (O 2 CBu t ) 10 (phen) 2 (H 2 O) 6 ]Cl 2 ( 4 1 ) .......... 123 4.3.2.2 Structure of [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) ( 4 2 ) ............. 125 4.3.2.3 Comparison of the structure of 4 2 and the native OEC ............... 127 4.3.3 Mn and Ca X ray Absorption Spectroscopy (XAS) ................................ 128 4.3.4 Magnetochemistry of Complexes 4 1 and 4 2 ................................ ....... 130 4.3.4.1 DC studies ................................ ................................ ................... 130 4.3.4.2 AC studies ................................ ................................ .................... 135 4.3.5 EPR Spectroscopy ................................ ................................ ................ 135 4.3.6 Electrochemistry ................................ ................................ .................... 137 4.4 Conclusions ................................ ................................ ................................ .... 138 5 SYNTHESIS OF THE FIRST ISOSTRUCTURAL MANGANESE STRONTIUM CLUSTER MIMICKING THE OXYGEN EVOLVING COMPLEX OF PHOTOSYSTEM II ................................ ................................ ............................... 158 5.1 Introduction ................................ ................................ ................................ ..... 158 5.2 Experimental Section ................................ ................................ ...................... 160 5.2.1 Syntheses ................................ ................................ .............................. 160 5.2.2 X ray Crystallography ................................ ................................ ............ 162 5.3 Results and Discussion ................................ ................................ ................... 163 5.3.1 Syntheses ................................ ................................ .............................. 163 5.3.2 Description of Structures ................................ ................................ ....... 165 5.3.2.1 Structure of [Mn 4 Sr 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) ( 5 1 ) .............. 165 5.3.2.2 Structure of [Mn 8 Sr 4 O 14 (O 2 CBu t ) 12 (phen) 4 (H 2 O) 2 ] ( 5 2 ) ................ 167 5.3.3 Mn and Sr X ray Absorption Spectroscopy ................................ ............ 168

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9 5.3.4 Magnetochemistry of Complexes 5 1 and 5 2 ................................ ....... 170 5.3.4.1 DC studies ................................ ................................ ................... 170 5.3.4.2 AC studies ................................ ................................ .................... 173 5.3.5 Electrochemistry ................................ ................................ .................... 174 5.4 Conclusions ................................ ................................ ................................ .... 174 6 REACTIVITY STUDIES ON PREFORMED MANGANESE /CALCIUM CLUSTERS AND ATTEMPTS TOWARD TARGETED SYNTHESIS OF THE EXACT OEC CUBANE UNIT ................................ ................................ ................ 190 6.1 Introduction ................................ ................................ ................................ ..... 190 6.2 Experimental Se ction ................................ ................................ ...................... 191 6.2.1 Syntheses ................................ ................................ .............................. 191 6.2.2 X ray Crystallography ................................ ................................ ............ 194 6.3 R esults and Discussion ................................ ................................ ................... 198 6.3.1 Syntheses ................................ ................................ .............................. 198 6.3.2 Description of Structures ................................ ................................ ....... 202 6.3.2.1 Structure of [Mn 6 Ca 2 O 9 (O 2 CBu t ) 9 (H 2 O) 6 ](NO 3 ) ( 6 1 ) .................... 202 6.3.2.2 Structure of [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (bpy) 2 (Et 2 O) 2 ][Ca 6 (O 2 CBu t ) 12 Cl] ( 6 2 ) ................... 203 6.3.2.3 Structure of [Mn 6 Ca 2 O 10 (O 2 CBu t ) 7 (phen) 3 (H 2 O)] ( 6 3 ) ................. 204 6.3.2.4 Structure of (NBu n 4 )[Mn 9 Ca 2 O 4 (OH) 4 (O 2 CBu t ) 16 (H 2 O) 2 ] ( 6 4 ) ...... 205 6.3.2.5 Structure of [Mn 11 CaO 8 (OH)Cl 2 (O 2 CBu t ) 16 ] ( 6 5 ) ......................... 207 6.3.3 Magnetochemistry of Complexes 6 1 to 6 5 ................................ .......... 208 6.3.3.1 DC studies ................................ ................................ ................... 208 6.3.3.2 AC studies ................................ ................................ .................... 213 6.4 Conclusions ................................ ................................ ................................ .... 214 APPENDIX A BOND DISTANCES AND ANGLES ................................ ................................ ...... 234 B LIST OF COMPOUNDS ................................ ................................ ........................ 248 C PHYSICAL MEASUREMENTS ................................ ................................ ............. 249 D VAN VLECK EQUATIONS ................................ ................................ .................... 253 LIST OF REFERENCES ................................ ................................ ............................. 260 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 282

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10 LIST OF TABLES Table page 1 1 Selected Manganese Enzymes and their Catalytic Functions ............................ 34 2 1 Crystallogra phic Data for 2 1 3MeCN, 2 2 MeCN and 2 3 1 / 3 THF 2 / 3 MeCN ...... 59 2 2 BVS for the Mn a and selected O atoms b in 2 1 ................................ .................. 59 2 3 BVS for the M n a and selected O atoms b in 2 2 ................................ .................. 60 2 4 BVS for the Mn a and selected O atoms b in 2 3 ................................ .................. 60 3 1 Crystallographic Data for Complexes 3 1 and 3 2 MeCN. ................................ .. 94 3 2 BVS for the Mn a and selected O atoms b in 3 1 ................................ .................. 95 3 3 BVS for the Mn a and selected O atoms b in 3 2 ................................ .................. 95 3 4 Comparison of MnMn and MnCa distances in 3 1 and 3 2 ......................... 95 3 5 Comparison of metal oxide bond distances in 3 1 and 3 2 ................................ 96 3 6 Comparison of metal oxo metal bond angle in 3 1 and 3 2 ............................... 96 3 7 Mn EXAFS curve fitting for complex 3 1 ................................ ............................ 96 3 8 Ca EXAFS curve fitting for complex 3 1 ................................ ............................ 97 3 9 Distribution of spin states for complex 3 1 ................................ ......................... 97 3 10 55 Mn hyperfine coupling tensors. ................................ ................................ ........ 98 4 1 Crystallographic Data for Complexes 4 1 t BuCO 2 H5.5MeCNH 2 O and 4 2 MeCN0.1H 2 O ................................ ................................ ............................... 139 4 2 BVS for the Mn a and selected O atoms b in 4 1 ................................ ................ 140 4 3 BVS for the Mn a and selected O atoms b in 4 2 ................................ ................ 140 4 4 Mn EXAFS curve fitting for complex 4 2 ................................ .......................... 140 4 5 Ca EXAFS curve fitting for complex 4 2 ................................ .......................... 141 4 6 Characteristic EPR signals obtained for dif ferent synthetic analogues of the OEC possessing an S = ground state ................................ ........................... 141 5 1 Metal ion activators and inhibitors of the OEC ................................ ................ 177

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11 5 2 Crystallographic Data for Complexes 5 2 MeCN0.2H 2 O and 5 1 ................... 177 5 3 BVS for the Mn a and selected oxygen atoms b in 5 1 ................................ ....... 178 5 4 Comparison of MnMn and MnCa/Sr distances in 5 1 and 4 2 .................. 178 5 5 Comparison of metal oxide bond distances in 5 1 and 4 2 .............................. 178 5 6 Comparison of metal oxo metal bond angle in 5 1 and 4 2 ............................. 179 5 7 BVS for the Mn a and selected oxygen atoms b in 5 2 ................................ ....... 179 5 8 Comparison of the exchange couplings in 5 1 and 4 2 ................................ .... 179 6 1 Crystallographic Data for 6 1 10THF, 6 2 2CH 2 Cl 2 H 2 O0.8MeCN, 6 3 8CH 2 Cl 2 6 4 1 / 2 MeCN 1 / 2 CHCl 3 and 6 5 3 / 2 CH 2 Cl 2 ................................ ....... 216 6 2 BVS for the Mn a and selected oxygen atoms b in 6 1 ................................ ....... 217 6 3 BVS for the Mn a and selected O atoms b in 6 2 ................................ ................ 217 6 4 Comparison of MnMn and MnCa distances in 6 2 and 4 2 ....................... 217 6 5 Comparison of Mn O Mn angles in 6 2 and 4 2 ................................ ............... 218 6 6 BVS for the Mn a and selected oxygen atoms b in 6 3 ................................ ....... 218 6 7 BVS for the Mn a and selected O atoms b in 6 4 ................................ ................ 218 6 8 Comparison of MnMn and MnCa distances in 6 4 3 1 and 4 2 ............... 219 6 9 BVS for the Mn a and selected O atoms b in 6 5 ................................ ................ 219 A 1 Selected interatomic distances ( ) and angles ( o ) for 2 1 3MeCN ................... 234 A 2 Selected interatomic distances ( ) and angles ( o ) for 2 2 MeCN ..................... 235 A 3 Selected interatomic distances ( ) and angles ( o ) for 2 3 1 / 3 THF 2 / 3 MeCN ...... 236 A 4 Selected interatomic distances () and angles ( o ) for 3 1 ................................ 237 A 5 Selected interatomic distances ( ) and angles ( o ) for 3 2 MeCN ..................... 238 A 6 Selected interatomic distances ( ) and angles ( o ) for 4 1 t BuCO 2 H5.5MeCNH 2 O ................................ ................................ ................ 239 A 7 Selected interatomic distances ( ) and angles ( o ) for 4 2 MeCN0.1H 2 O ......... 240 A 8 Selected interatomic distances ( ) and angles ( o ) for 5 2 MeCN0.2H 2 O ......... 241

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12 A 9 Selected interatomic distances ( ) and angles ( o ) for 5 1 ................................ 242 A 10 Selected interatomic distances ( ) and angles ( o ) for 6 1 10THF ..................... 243 A 11 Selected interatomic distances ( ) and angles ( o ) for 6 2 2CH 2 Cl 2 H 2 O0.8MeCN ................................ ................................ ................. 244 A 12 Se lected interatomic distances ( ) and angles ( o ) for 6 3 8CH 2 Cl 2 .................. 245 A 13 Selected interatomic distances ( ) and angles ( o ) for 6 4 1 / 2 MeCN 1 / 2 CHCl 3 ... 246 A 14 Selected interatomic distances ( ) and angles ( o ) for 6 5 3 / 2 CH 2 Cl 2 ................. 247

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13 LIST OF FIGURES Figure page 1 1 Relative frequenc ies of the catalytic metal ions in the six enzyme classes (Me stands for metal). ................................ ................................ ................................ 35 1 2 Z scheme of photosynthesis emphasizing the electron transfer achieved by three membrane spanning supramol ecular complexes PSII, the cytochrome b 6 /cytochrome f complex, and PSI. ................................ ................. 35 1 3 The Kok cycle showing advancement of the OEC from S 0 to the transient S 4 state and subsequent O 2 evolution.. ................................ ................................ ... 36 1 4 Possible metal topologies of the Mn 4 cluster of the OEC in PSII, as proposed from EXAFS studies. ................................ ................................ .......................... 36 1 5 Mn K edge XANES spectr a of flash illuminated PSII samples (top) and the S state XANES difference spectra.. ................................ ................................ ....... 37 1 6 Fourier transformed EXAFS spectra of the Mn complex in the semi stable S state (at 20 K). Solid lines represent simulations of the experimental spectra (dotted line). ................................ ................................ ................................ ........ 37 1 7 Comparison of the 55 Mn hyperfine resolved EPR signal associated with the S 0 S 1 and S 2 states of the OEC.. ................................ ................................ ....... 38 1 8 Crystallographically deduced proposals for the [Mn 4 CaO x ] core of the OEC differing in the means of attachment of the external Mn atom. ........................... 38 1 9 Crystallographically deduced structure of the [Mn 4 CaO x ] core of the OEC along with its peripheral ligation (a) Ferreira et al. ; (b) Loll et al. and (c) Umena et al... ................................ ................................ ................................ ..... 39 1 10 Proposed st ructures of the [Mn 4 CaO x ] core of the OEC from high resolution EXAFS and polarized EXAFS studies. ................................ ............................... 39 1 11 Proposed structures of the [Mn 4 CaO x ] core of the OEC from EXAFS studies by Dau et al .. ................................ ................................ ................................ ...... 39 2 1 (top) Labeled representation of the structure of 2 1 Hydrogen atoms have been omitted for clarity. (bottom) Fully labeled core of 2 1 ................................ 61 2 2 (top) Labeled representation of the structure of 2 2 (bottom) Fully labeled core of 2 2 emphasizing the two open faced cubane units. ................................ 62 2 3 Labeled representation of comple x 2 3 (top) and its core (bottom). Hydrogen atoms have been omitted for clarity. Color code: Mn III green; O red; C grey. ..... 63

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14 2 4 Plot of X M T vs T for complexes 2 1 2 2 2 3 ............................. 64 2 5 Plot of reduced magnetization ( M / B ) vs H / T for complex 2 1 The solid lines are the fit of the data; see text for the fit parameters. ................................ 64 2 6 AC susceptibility data of complex 2 1 in a 3.5 G field oscillating at the indicated frequencies: (top) in phase signal ( X' M ) plotted as X' M T vs T; and (bottom) out of phase signal ( X M ) vs T. ................................ ............................. 65 2 7 In phase AC magnetic susceptibility of complex 2 2 (top) and complex 2 3 (bottom) in a 3.5 G field oscillating at the indicated frequencies plotted as X' M T vs T ................................ ................................ ................................ ........... 66 2 8 Magnetization ( M ) vs dc field hysteresis loops for a single crystal of 2 1 at (top) the indicated field sweep rates and fixed temperature of 0.04 K and (bottom) at the indicated temperatures and fixed field sweep rate of 0.14 T/ s.. 67 3 1 Labeled representation of the structure of 3 1 The [Mn 3 CaO 4 ] cubane is emphasized with green bonds. ................................ ................................ ........... 98 3 2 Comparison of the core of 3 1 with crystallographically derived core of the native OEC. ................................ ................................ ................................ ........ 99 3 3 (a) Bond distances in 3 1 1.9 PSII structure by Umena et al. atom labels as in reference and (c) metal metal separations in 3 1 ................................ ....... 99 3 4 Labeled representation of the structure of 3 2 The [Mn 3 CaO 4 ] cubane is emphasized with g reen bonds. ................................ ................................ ......... 100 3 5 Mn XANES from spinach PS II in the S 0 S 1 S 2 and S 3 states compared with the spectrum from the Mn IV 3 Ca 2 complex 3 1 ................................ .................. 101 3 6 Mn XANES from spinach PS II in the S 1 state compared with the spectrum from the Mn IV 3 Ca 2 bpy complex 3 2 ................................ ................................ .. 101 3 7 a) and b) The Mn EXAFS and the Fourier transforms of the OEC in the S 1 state, compared with complex 3 1 c) and d) The Ca EXAFS and the Fourier transforms of the OEC in the S 1 state compared with complex 3 1 ................. 102 3 8 a) and c) are Mn and Ca EXAFS data from 3 1 and the best fits. b) and d) are the FTs of the Ca EXAFS from the complex and the best fits. The fit parameters are shown in Tables 3 7 and 3 8. ................................ .................. 103 3 9 a) and b) The Mn EXAFS and the Fouri er transforms of the OEC in the S 1 state, compared with complex 3 2 c) and d) The Ca EXAFS and the Fourier transforms of the OEC in the S 1 state compared with complex 3 2 ................. 104

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15 3 10 Plot of X M T vs T for complex 3 1 The solid line is the fit of the data; see the text for the fit parameters. ................................ ................................ ................. 105 3 11 (left) [Mn 3 Ca 2 O 4 ] core of 3 1 from a viewpoint emphasizing the isosceles Mn IV 3 triang ular unit; (right) the corresponding 2 J coupling scheme and ................................ ............... 105 3 12 Plot of all the S T states for complex 3 1 as a function of energy.. ..................... 106 3 13 The core of 3 1 emphasizing the exchange coupling model employed, and the spin alignments rationalizing the S T = 9 / 2 ground state. .............................. 106 3 14 (top) Plot of S T (bottom) Depictions of the indicated | S T S A > state as a function of varying ................................ ................................ ................................ ........... 107 3 15 (a) Plot of X M T vs T for compl ex 3 2 The solid line is the fit of the data; see text for fit parameters. (b) Plot of all the S T states for complex 3 2 as a function of energy. ................................ ................................ ............................ 108 3 16 (a) Plot of reduced magnetization ( M / B ) vs H / T for complex 3 1 (b) Two dimensional contour plot of the root mean square error surface vs D and g for the magnetization fit for 3 1 ................................ ................................ ........ 109 3 17 (a) Plot of reduced magnetization ( M / N B ) vs H / T for complex 3 2 (b) Two dimensional contour plot of the root mean square error surface vs D and g for the magnetization fit for 3 2 ................................ ................................ ........ 110 3 18 In phase AC magnetic susceptibility plo tted as X' M T vs T of complexes 3 1 (a) and 3 2 (b) in a 3.5 G field oscillating at the indicated frequencies. ............ 111 3 19 CW EPR spectra at X band (9.3752 GHz) and Q band (34.1877 GHz; derivative) of complex 3 1 .. ................................ ................................ .............. 112 3 20 CW EPR of complex 3 1 dissolved in MeCN/CH 2 Cl 2 (1:1, v/v) and the powder.. ................................ ................................ ................................ ............ 112 3 21 CW EPR temperature dependence of complex 3 1 at 5, 10, 15, 20, 25, 30, 40, 50, 60, 70 and 80 K; spectra have been scaled by the temperature.. ......... 113 3 22 Q band 55 Mn Davies ENDOR spectrum of complex 3 1 (black) coll ected at 1142.2 mT, and simulation (dashed).. ................................ .............................. 113 3 23 Q band 55 Mn Davies ENDOR of complex 3 1 taken across the EPR envelope, simulations shown in green.. ................................ ............................ 114 3 24 CW EPR spectra at X band (9.3805 GHz) and Q band (34.0943 GHz ; derivative) of complex 3 2 .. ................................ ................................ .............. 115

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16 3 25 Comparison of the CW EPR spectrum of 3 1 (blue) and 3 2 ( green). .............. 115 3 26 CW EPR temperature dependence of complex 3 2 at 8, 10, 20, 30, 50, 70, 90, 110, 130, 150 K; spectra have been scaled by the temperature.. ............... 116 3 27 Cyclic voltammogram at 100mV/s for complex 3 1 (a) and 3 2 (b) in MeCN and DCM respectively, containing 0.1 M N n Bu 4 PF 6 as supporting electrolyte. 116 4 1 Label ed representation of the structure of 4 1 Hydrogen atoms have been omitted for clarity. Color code: Mn IV blue; Ca II yellow; O red; N cyan; C grey. .. 142 4 2 (top) Core of complex 4 1 and (bottom ) core of 4 1 emphasizing the two [Mn IV 4 Ca] cubane moieties in the complex. Color code: Mn IV blue; Ca II yellow; O red. ................................ ................................ ................................ ............... 143 4 3 Labeled representation of the structure of 4 2 Hydrogen atoms hav e been omitted for clarity. Color code: Mn IV blue; Mn III green; Ca II yellow; O red; N cyan; C grey. ................................ ................................ ................................ .... 144 4 4 Comparison of the core of 4 2 with the crystallographically derived core of the nativ e OEC.. ................................ ................................ ............................... 145 4 5 (a) Bond distances in 4 2 since the molecule is centrosymmetric only the unique distances are reported; (b) Bond distances in the 1.9 PSII structure atom labels as in refer ence and (c) metal metal separation in 4 2 ............... 146 4 6 Mn XANES from spinach PSII in the S 0 S 1 S 2 and S 3 states compared with the spectrum from Mn III Mn IV 3 Ca 2 complex 4 2 Inset shows the pre edg e region. ................................ ................................ ................................ .............. 147 4 7 a) and b) The Mn EXAFS and the Fourier transforms of the OEC in the S 1 state, compared with complex 4 2 c) and d) The Ca EXAFS and the Fourier transforms of the OEC in the S 1 state compared with complex 4 2 ................. 147 4 8 a) and c) are Mn and Ca EXAFS data from 4 2 and the best fits. b) and d) are the FTs of the Ca EXAFS from the complex and the best fits. The fit parame ters are shown in Tables 4 4 and 4 5. ................................ .................. 148 4 9 Plot of X M T vs T for complex 4 1 ................................ ................................ ..... 149 4 10 Plot of X M T vs T for complex 4 2 The sol id line is the fit of the data; see the text for fit parameters. ................................ ................................ ....................... 149 4 11 (top) [Mn III Mn IV 3 Ca 2 O 6 ] core of 4 2 from a view point emphasizing the Mn III Mn IV 3 butterfly unit; (bottom) the correspon ding 2 J coupling scheme and definition of J wb and J bb exchange parameters. ................................ ................ 150 4 12 Plot of all the S T states for complex 4 2 as a function of energy.. ..................... 151

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17 4 13 The core of 4 2 emphasizing the exchange coupling model employed, and the spin alignments corresponding to the | 1 / 2 3, 7 / 2 > ground state i.e. S T = 1 / 2 S A = 3 and S B = 7 / 2 ................................ ................................ ........................... 151 4 14 (top) Plot of S T energies vs the J wb /J bb ratio, showing the change in the ground state. (bottom) Depictions of the indicated | S T S A S B > as a function of varying J wb /J bb ratio. ................................ ................................ ...................... 152 4 15 (top) [Mn III Mn IV 3 Ca 2 O 6 ] core of 4 2 from a view point emphasizing the Mn III Mn IV 3 butterfly unit; (bottom) the corresponding 3 J coupling scheme and definition of J 1 J 2 and J 3 exchange parameters. ................................ .............. 153 4 16 Plot of X M T vs T for complex 4 2 The solid line represents a simulation of the data in the temperature range 5 300K to the 3J spin Hamiltonian of eq. 4 8. ................................ ................................ ................................ ................... 153 4 17 In phase AC magnetic susceptibility of complex 4 1 (top) and complex 4 2 (bottom) in a 3.5 G field oscillating at the indicated frequencies plotted as X M T vs T. ................................ ................................ ................................ ......... 154 4 18 Comparison of the C W EPR spectrum of 4 2 dissolved in 1:1 ratio of DCM/DMF and the powder. ................................ ................................ .............. 155 4 19 CW EPR at X band of (A) complex 4 2 dissolved in acetonitrile and dichloromethane (1:1 ratio) (9.48 GHz, 30K) an d (B) S 2 state of PSII (9.65 GHz, 5K).. ................................ ................................ ................................ ......... 155 4 20 CW X band temperature dependent spectra of complex 4 2 dissolved in dimethylformamide and dichloromethane (1:1 ratio). Temperature varied from 5 to 80 K with 10 K increments.. ................................ ................................ ....... 156 4 21 CW X band temperature dependent spectra of complex 4 2 dissolved in acetonitrile and dichloromethane (1:1 ratio). Temperature varied from 5 to 80 K with 10 K increments.. ................................ ................................ ................... 156 4 22 CW X band temperature dependent spectra of the powder of complex 4 2 Temperature varied from 10 to 70 K with 10 K increments.. ............................. 157 4 23 Cyclic voltammogram at 100 mVs 1 for complex 4 2 in dichloromethane, containing 0.1 M N n Bu 4 PF 6 as supporting electrolyte. ................................ ...... 157 5 1 Labeled representation of th e structure of 5 1 Hydrogen atoms have been omitted for clarity. Color code: Mn IV blue; Mn III green; Sr II orange; O red; N cyan; C grey. ................................ ................................ ................................ .... 180 5 2 Comparison of [Mn 4 M 2 ] core 5 1 (left) and 4 2 (right). ................................ ... 180

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18 5 3 Labeled representation of the structure of 5 2 Color code: Mn IV blue; Sr II orange; O red; N cyan; C grey. ................................ ................................ ......... 181 5 4 (top) Core of complex 5 2 and (bottom) core of 4 1 emphasizing the four open faced [Mn IV 3 SrO 4 ] cubane moieties in the complex. Color code: Mn IV blue; Sr II orange; O red. ................................ ................................ .................... 182 5 5 Structural c omparison of the two [Mn 8 M 4 ] (M = Sr/Ca) core; (left) [Mn 8 Sr 4 ] core of 5 2 and (right) [Mn 8 Ca 4 ] core of 4 1 ................................ ..................... 183 5 6 Mn XANES from spinach PSII in the S 1 state compared with the spectrum from M n III Mn IV 3 Sr 2 complex 5 1 ................................ ................................ ........ 183 5 7 a) and b) The Mn EXAFS and the Fourier transforms of the Sr reactivated PSII sample, compared with 5 1 c) and d) The Sr EXAFS and the Fourier transforms of th e Sr reactivated PSII sample, compared with 5 1 ................... 184 5 8 (top) [Mn III Mn IV 3 Sr 2 O 6 ] core of 5 1 from a view point emphasizing the Mn III Mn IV 3 butterfly unit; (bottom) the corresponding 2 J coupling scheme and definition of J wb and J bb exchange parameters. ................................ ................ 185 5 9 Plot of X M T vs T for complex 5 1 The solid line is the fit of the data; see the text for the fit parameters. ................................ ................................ ................. 186 5 10 Plot of all the S T states for complex 5 1 as a function of energy.. ..................... 186 5 11 Plot of S T energies vs the J wb /J bb ratio, showing the change in the ground state.. ................................ ................................ ................................ ................ 187 5 12 Plot of X M T vs T for complex 5 2 ................................ ................................ ..... 187 5 13 In phase AC magnetic susceptibility of complex 5 1 (top ) and complex 5 2 (bottom) in a 3.5 G field oscillating at the indicated frequencies plotted as X M T vs T. ................................ ................................ ................................ ......... 188 5 14 Cyclic voltammogram at 100 mVs 1 for complex 5 1 in dichloromethane, containi ng 0.1 M N n Bu 4 PF 6 as supporting electrolyte. ................................ ...... 189 6 1 Labeled representation of the structure of 6 1 Color code: Mn IV blue; Ca II yellow; O red; and C grey. ................................ ................................ ................ 220 6 2 (top) Core of complex 6 1 emphasizing the fused [Mn IV 2 Ca 2 ] cubane moieties and (bottom) core of 6 1 emphasizing Mn IV 6 plane. Color code: Mn IV blue; Ca II yellow; O red. ................................ ................................ ............................. 221 6 3 Labeled representation of the anion (left) and cation (right) of 6 2 Color code: Mn IV blue; Mn III green; Ca II yellow; Cl purple; N cyan; O red; and C grey. ................................ ................................ ................................ ................. 222

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19 6 4 Labeled rep resentation of the cation of 6 2 Color code: Mn IV blue; Mn III green; Ca II yellow; N cyan; O red; and C grey. ................................ ................. 222 6 5 Comparison of [Mn 4 Ca 2 ] core 6 2 (left) and 4 2 (right). ................................ 223 6 6 Labeled representation of the anion of 6 2 The bold green lines emphasize the central [Ca 6 ( 6 Cl] 11+ unit. Color code: Ca II yellow; Cl purple; O red; and C grey. ................................ ................................ ................................ ................. 223 6 7 Labeled representation of the structure of 6 3 Color code: Mn IV blue; Mn III green; Ca II yellow; O red; N cyan and C grey. ................................ .................. 224 6 8 Core of complex 6 3 emphasizing the defective dicubane [Mn III Mn 4 IV Ca 2 ( 4 O)( 3 O) 6 ] 9+ unit. Color code: Mn IV blue; Mn III green; Ca II yellow and O red. .... 224 6 9 Labeled representation of the structure of 6 4 Color code: Mn III green; Mn II light blue; Ca II yellow; O red and C grey. ................................ .......................... 225 6 10 Core of complex 6 4 emphasizing the two fused [Mn 5 III Ca 2 ( 3 OH) 4 ( 4 O) 4 ] 7+ unit. Color code: Mn III green; Mn II light blue; Ca II yellow and O red. ................. 225 6 11 Comparison of the Mn 3 Ca cubane unit in (a) 6 4 (b) 3 1 and (c) 4 2 The manganese oxidation st ates in the three cubes are different. .......................... 226 6 12 Labeled representation of the structure of 6 5 Color code: Mn III green; Ca II yellow; O red; Cl purple and C grey. The JT axes in 6 5 are highlig hted as bold blue lines. ................................ ................................ ................................ .. 226 6 13 Core of complex 6 5 emphasizing the fused [Mn 3 III CaO] 9+ tetrahedral units. Color code: Mn III green; Ca II yellow and O red. ................................ ................ 227 6 14 Plot of X M T vs T for complex 6 1 6 2 ................................ ........... 227 6 15 (top) [Mn III Mn IV 3 Ca 2 O 6 ] core of 6 2 from a view point emphasizing the Mn III Mn IV 3 butterfly unit; ( bottom) the corresponding 2 J coupling scheme and definition of J wb and J bb exchange parameters. ................................ ................ 228 6 16 Plot of X M T vs T for complex 6 2 The solid line is the fit of the data; see text for f it parameters. ................................ ................................ ............................. 228 6 17 Plot of the S T states for complex 6 2 as a function of energy.. ......................... 229 6 18 The core of 6 2 emphasizing the e xchange coupling model employed, and the spin alignments in the S T = | 1 / 2 3, 5 / 2 > ground state. ................................ .. 229 6 19 Plot of S T energies vs the J wb /J bb ratio for complex 6 2 showing the variable ground s tate.. ................................ ................................ ................................ .... 230

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20 6 20 Plot of X M T vs T for complex 6 3 6 4 6 5 ............................... 230 6 21 Plot of reduced magnetization ( M / B ) vs H / T for complex 6 3 The solid lines are the fit of the data; see text for the fit parameters. ............................... 231 6 22 Two dimensional contour plot of the root mean square error surface vs D and g for the magnetization fit for 6 3 ................................ ................................ ..... 231 6 23 Plot of reduced magne tization ( M / B ) vs H / T for complex 6 4 The solid lines are the fit of the data; see text for the fit parameters. ............................... 232 6 24 Plot of X' M T vs T for 6 1 in a 3.5 G field oscillating at the indicated frequenci es. ................................ ................................ ................................ ...... 232 6 25 Plot of X' M T vs T for complexes 6 2 6 5 in a 3.5 G field oscillating at the indicated frequencies. ................................ ................................ ....................... 233

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21 LIST OF ABBREVIATION S bpy 2,2' Bipyridin e BVS Bond Valence Sum CV Cyclic Voltammetry ENDOR Electron Nuclear Double Resonance EPR Electron Paramagnetic Resonance EXAFS Extended X ray Absorption Fine Structure Spectroscopy FTIR Fourier Transform Infrared Spectroscopy HFI Hyperfine Interactions NHE Normal Hydrogen Electrode NQI Nuclear Quadrupole Interaction OEC Oxygen Evolving Complex phen Phenanthroline pheo Pheophytin PS II Photosystem II ps picosecond Q A Plastoquinone RIXS Resonant Inelastic X ray Scattering TIP Temperature Independent Paramagne tism Bu t tertiary butyl XANES X ray Absorption Near Edge Spectroscopy XAS X ray Absorption Spectroscopy ZFS Zero Field Splitting

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22 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 COMPROPORTIONATION REACTIONS AS A ROUTE TO SINGLE MOLECULE MAGNETS AND SYNTHETIC ANALOGUES OF THE OXYGEN EVOLVING COMPLEX OF PHOTOSYSTEM II By Shreya Mukherjee December 2011 Chair: George Chris tou Major: Chemistry This thesis describes the development of new synthetic strategies for synthesizing and characterizing polynuclear metal cluster compounds that are either nanoscale magnetic materials or biomimetic synthetic analogues of the oxygen evo lving complex (OEC) of photosystem II (PS II) in plants and cyanobacteria. An important development in molecular magnetism was the discovery of zero dimensional magnets, more commonly known as single molecule magnets (SMMs). SMMs display slow relaxation of magnetization, resulting in hysteresis loops below the blocking temperature ( T B ). On the other hand, the OEC catalyzes the thermodynamically demanding oxidation of water to oxygen gas. Elucidating the properties and mechanism of action of the OEC represen ts one of the holy grails of metallobiochemistry. Recent X ray structure of PS II identified the OEC to be a Mn 4 CaO x cluster with primarily carboxylate ligation. The common theme that binds the work in this thesis together is the use of comproportionation reactions under acidic conditions to prepare the high oxidation state homo and heterometallic clusters. Using this strategy, a Mn 9 and two Mn 8 complexes possessing unusual structures have been prepared. One of the Mn 8 exhibits

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23 magnetization hysteresis bel ow its T B making it a new addition to the small family of SMMs possessing a half integer spin ( S = 15 / 2 ). The same reaction system in the bioinorganic project gave the first Mn 3 Ca cubane with an additional Ca attached, [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (HO 2 CBu t ) 4 ] M agne tic characterization revealed the compound to be spin frustrated, and in an attempt to change the S = 9 / 2 ground state spin, structural perturbation of the Mn 3 Ca 2 core was accomplished with 2,2 bipyridine (bpy), giving [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (bpy)(HO 2 CBu t )(MeCN)] However, the perturbations were not enough to change the ground state. Initial attempts to replace the external Ca with Mn to obtain an exact structural correspondence to the OEC gave inst ead the new [Mn 6 Ca 2 O 9 (O 2 CBu t ) 9 (H 2 O) 6 ](NO 3 ) cluster Addition of chelates directly into the comproportionation reaction led to [Mn 4 Ca 2 O 6 ( O 2 CBu t ) 6 (phen) 4 ]( O 2 CBu t ), whose core is essentially that of the OEC with an additional Ca attached to it. This exciting complex possess an S = ground state, the same as the OEC in its S 2 Kok state. Mn 4 Ca 2 is the only Mn/Ca cluster to mimic the OEC both structurally and magnetically. Replacement of the Ca in Mn 4 Ca 2 with Sr was targeted and achieved to give isostructural [M n 4 Sr 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ). This is the first example of analogous Mn/Ca vs Mn/Sr cluster chemistry. Finally, tuning the average Mn oxidation state in the reaction to +2.83 instead of +4.5 (as in the previous reaction schemes) allowed entry into (NBu n 4 )[Mn 9 Ca 2 O 4 (OH) 4 (O 2 CBu t ) 16 (H 2 O) 2 ] and [Mn 11 CaO 8 (OH)Cl 2 (O 2 CBu t ) 16 ] clusters which mimic the S 0 state of the Kok cycle. This work lists a library of Mn/Ca model complexes, which are the closest synthetic analogues of the OEC reported to date.

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24 CHAPTER 1 BAC KGROUND INFORMATION bioinorganic chemistry and, with the rapid development in this field, the pivotal role played by metal ions in many biological systems, including their interplay with proteins, bec ame evident. 1, 2 It is well known that metal ions play a dual role in the physi ology of organisms: a few are vital for normal life, whereas most are toxic at elevated concentrations. 3, 4 an essential metal has been known since the 18 th century, and the role of other elements such as magnesium, cobalt, zinc, copper and manganese have been understood and well studied for more than half a century. Recent surveys o f three dimensional structures of enzymes indicate that 47% of enzymes require metals and 41% contain metals at their catalytic centers. 5, 6 Manganese is the 12 th most abundant element and the 3 rd most abundant t ransition metal on the planet. 7 It is an essential trace element for all life forms, which accumulates in and is essential for mitochondrial function. In biological systems, manganese adopts a wide range of coordination geometries and can acces s different oxidation states, mainly +2, +3 and +4. One of the most fascinating features of enzymes containing manganese is their varied functionality (Table 1 1). In addition to these enzymes, there are some Mn dependent enzymes which have yet to be compl etely structurally characterized. The oxygen evolving complex (OEC), present near photosystem II (PSII) in plants, cyanobacteria and algae, represents one such enzyme where structural elucidation is still in process. Before further details on the structura l and functional details of the OEC are described, it is important to trace the origin of this amazing enzyme and its impact on our planet.

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25 that existed were anaerobic, rel ying on reducing agents like H 2 S, H 2 or NH 3 to supply their metabolic energy. Any trace amounts of O 2 produced by UV radiation of the upper atmosphere were quickly consumed by these reducing conditions. 8 10 Then, approximately 2.5 2.7 billion years ago, a machinery was developed by cyanobacteria that allowed them to start taking advantage of the abundance of water on the planet by using it as a source of protons and electrons (eq. 1 1). 2H 2 O O 2 + 4H + + 4e (1 1) The O 2 produced by cyanobacteria first started reacting with the metals in seawater (mainly Fe 2+ ) generating insoluble Fe 3+ oxides. As the O 2 concentration started building up, it gradually oxidized the reducing atmosphere to its present oxidizing nature comprising primarily of CO 2 H 2 O and N 2 The free O 2 was poisonous to anaerobic life forms that existed at that time, and most of the life on e arth vanished; this is commonly 11 This increased level of O 2 on the planet also triggered photochemical reactions in the upper atmosphere leading to the formation of a protect ive ozone blanket that shielded the fragile life forms from harmful UV radiation and allowed them to flourish. The machinery developed as a part of photosynthesis by cyanobacteria is referred to as the oxygen evolving complex (OEC). Photosynthesis is a me tabolic process that converts light energy into chemical energy. It uses sunlight as an energy source to fix carbon dioxide and water to produce O 2 and carbohydrates like sucrose, glucose and starch. CO 2 + 4H + + 4e (CH 2 O) n + H 2 O (1 2)

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26 Photosynthesis thus transfers electrons from water to CO 2 using light energy to form sugar molecules. This electron transfer represents a redox process involving oxidation of water and reducti on of CO 2 Photosynthesis can be broadly divided into the following steps: (1) photoelectric charge separation using photons (light electric energy conversion), (2) fixation of electrical energy in the form of chemical energy (ATP synthesis) and (3) redu ctive biosynthesis of glucose from CO 2 using NADPH as the reducing agent. The first path of photosynthetic process is represented in the well known Z scheme as shown in Figure 1 2. It is a schematic representation of the light reactions of photosynthesis, where the photosynthetic reaction centers and electron carriers are arranged according to their Gibbs free energy in one dimension and their reaction sequence in the second dimension. This arrangement results in the formation of a Z shape which links the t wo reaction centers of PSI and PSII via a photosynthetic electron transport chain. 12 17 On the donor side of the Z scheme (PSII), solar photons are absorbed by several antennae proteins composed of ~ 400 chlorophyl l molecules, which funnel this energy to the reaction center chlorophyll P 680 in PSII. This photon absorption generates a strongly reducing excited state (P680*) that transfers an electron within a few ps t o + ) charge separated pair. 18, 19 Within a few hundred ps, this electron is transferred to plastoquinone (QA) 20 and the other members in the electron transport chain (Figure 1 2) before reachin + is a strong oxidizing agent (~ 1.25 V vs NHE) 21 and abstracts an electron from a redox active tyrosine (Tyr Z ) to generate Tyr Z Z during this process is replenished by the OEC. 22, 23 Four successive light driven

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27 abstractions of electrons from the OEC result in the oxidation of two water mol ecules to form one molecule of O 2 and four electrons (eq. 1 1), which replenish the electrons lost from the OEC. The sequential accumulation of four oxidizing equivalents at the OEC is summarized in the S state advancement of the Kok cycle. 24 The cycle starts with the dark adapte d S 1 state where absorption of a photon at P680 generates Tyr Z subsequently the S 2 state is generated. The second photon absorption induces the S 2 S 3 transition. Following the absorption of the third proton, a transient S 4 state is generated that then spontaneously releases O 2 from water oxidation, and the fou r electrons produced reset the OEC to the S 0 state. The last photon completes the cycle by forming the S 1 state. Although proton release was not an integral part of the original model proposed by Kok, recent studies by Haumann et al. have extended the conc ept of the Kok cycle to a reaction cycle comprising an alternative sequence of removal of electrons and protons from the OEC. 23, 25 The activation energies for the four steps in the Kok cycle range from E a = 0.05 t o 0.4 eV, (1 9 kcal/mol) with the rate constants varying in the range 10 3 10 4 s 1 26, 27 Elucidating the structure of the OEC and exactly how it binds and oxidizes water molecules has been one of the holy grail s of metallobiochemistry. In 1937, Pirson demonstrated for the first time that Mn deficiency in fresh water algae inhibits photosynthesis. 28 Photosynthetic activity could be restored by adding Mn 2+ to algal cells depleted of this nutrition. 29 Later Kessler et al. established that Mn was specifically involved in water oxidation and not in the light induced electron transport through PSI. 30 It was during this time that the concept of two photosystems in series began emergi ng. 12

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28 Initially the OEC was considered to be a dinuclear Mn cluster, 31 but the presence of four Mn is now universally accepted. Initially two techniques that played crucial role s in OEC structural elucidation were X ray absorption spectroscopy (XAS) and electron paramagnetic resonance (EPR) spectros copy Both XAS and EPR techniques allowed scientists to probe the Mn centers in the OEC without significant interference from the large protein milieu within which the OEC is embedded. Extended X ray absorption fine structure (EXAFS) spectra provide inform ation about nearest neighbor bond lengths with an accuracy ~ 0.02 and coordination numbers with an uncertainty of 25%. 32, 33 X ray absorption near edge structure (XANES) spectra, on the other hand, provide information about the metal site geometry and oxidation state. The Mn c oordination sphere was established to be composed of O and/or N based ligands by the early EXAFS studies. 34 36 The topology of the Mn cluster was proposed, based on the detection of short Mn Mn distances at 2.7 and a longer Mn Mn distance at 3.3 ; 37 39 in fact, a wide number of Mn 4 topologies were considered possible candidates for the OEC. 35 XANES alo ng with the EXAFS studies allowed scientists to probe the change in the Mn oxidation state and the structural changes of the Mn core through the course of the Kok cycle. The edge position of the X ray edge spectra is known to be sensitive to the metal oxid ation state, and generally shifts to higher energies upon oxidation. 40 43 XANES spectra along with EPR studies have been able to assign the Mn oxidation states in the different S n states of the Kok cycle. Although there still are some discrepancies in the literature, the following assignment of the Mn oxidations are widely accepted; S 0 Mn 3 III Mn IV S 1 Mn 2 III Mn 2 IV

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29 S 2 Mn III Mn 3 IV 35, 41 A significant increase in the edge position is observed from S 0 S 1 S 2 indicating that Mn atoms are oxidized during these transitions. 43, 44 A Mn III is oxidized to Mn IV in the S 0 S 1 transition and the short Mn Mn distance shortens from ~2.8 to 2.7 25 The S 1 S 2 transition is accompanied by a 0.7V shift in the X ray edge indicating the oxidation of a Mn III to Mn IV 25, 43, 45 Although it is accepted that a Mn based oxidation occurs in S 0 S 1 S 2 transition, occurrence of a similar situation in the S 2 S 3 transition is still debated. The following possibilities have been suggested: (i) Mn centered oxidation (Mn III Mn IV ); 41 (ii) oxidation of a ligating amino acid (histidine) leading to radical formation; 46 (iii) formation of an oxygen radical (O or OH ) on a direct Mn ligand; 45, 47 and (iv) delocalization of the oxidation equivalent. 48 Finally in the S 3 [S 4 ] S 0 transition an overall reduction in the Mn oxidation state is observed. Various possible pathways for this transition have been proposed, but the exact mechanism is still unknown. 23, 49, 50 EPR spect roscopy played a pioneering role in predicting the oxidation states of the Mn centers in the OEC. The light induced S 2 state is one of the most well studied states g ~ 2 arising from 55 Mn hyperfine coupling (Figure 1 7). 51 The S 2 state has an S = ground state and these d ata provided the first indication of an electronically coupled tetranuclear Mn 4 core for the OEC. 52 The historical popularity of the S 2 state arises in part from its simple 1 flash or continuous 195 K illumination generation starting from the dark stable S 1 state and the fact that it is a half integral spin state. 53 Apart from the g ~ 2 signal another g ~ 4.1 signal is also observed in the S 2 state. 54 Different theories have been proposed for the origin of this signal and the position of the signal is also sensitive to the method used to

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30 gene rate the S 2 state. 55, 56 EPR characterization of the more reduced S 1 and S 0 states has been difficult to attain. Since the S 2 state has been confirmed to be a half integer system, the first reduced state S 1 would b e an integer spin system. To probe this parallel mode EPR spectroscopy was utilized. A broad band at g = 4.8 was obtained, which was assigned to an S = 1 state (Figure 1 7). 57 However, no 55 Mn hyperfine structure was observed at first, but Cambell et al. were later able to obtain a highly structured EPR signal. 58 The S 0 state is further reduced from the S 1 state and is expected to be an odd EPR. In spite of this, the S 0 state h as been difficult to probe by EPR. Under normal physiological conditions, three separate illuminations of the PSII are required to access the S 0 state, which often leads to scrambling of the S states and makes signal assignment difficult. 59 To avoid this, hydrazine and hydroxylamine are used to generate an S 0 state by reduction of the S 1 state. 60 Irrespective of the gen eration condition, a multiline signal with 24 26 hyperfine lines is obtained and assigned as originating from an S = state (Figure 1 7). The S 3 state is also an integer spin system, and both parallel and perpendicular mode EPR spectroscopy have been used to probe this state. A multiline signal is observed which has been proposed to originate from either an S = 1 or S = 3 state. 61, 62 The S 4 state is least studied due to its transient nature and complications arisi ng from interaction with Tyr z Other techniques such as site directed mutagenesis 63 in combination with Fourier transform infrared (FTIR) 64 time resolved mass spectrometry 65 and fluorescence 66 spectroscopy, have also provided information about the surrounding ligation of this OEC Over the last 10 years X ray crystal structures of the PSII have begun to play a

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31 key role in structural elucidation of the OEC. 67 Although the first X ray struct ures by Zouni et al 68 and Kamiya et al 69 a t 3.8 and 3.7 confirmed the location of the OEC Mn 4 cluster; its exact structure and its environment were not well resolved. Finally, in 2004, a higher resolution X structure at 3.5 was reported by Ferreira et al. 70 The most striking feature of this structure was the detection of Ca 2+ intimately associated with the tetranuclear Mn cluster. The electron density attributed to the four Mn ion ray structures. Although it had been known for a long time that Ca 2+ was an essential cofactor for OEC activity, 71, 72 t his was the first time that the heterometallic nature of the OEC was revealed. Earlier Ca EXAFS studies had revealed a Mn Ca separation of ~ 3.4 which is consistent with this new crystal structure. 7 3 A modified version of this structure was also later reported by Loll et al. which supported the metal positioning, but there were some differences in the ligation around the pentanuclear Mn 4 Ca cluster. 74 At this time a more distorted version of the OEC c et al. based on high resolution EXAFS data along with polarized EXAFS data (Figure 1 10). 75, 76 It was argued that significant X ray damage occurred during data collection of X ray structure determination. 77 Dau et al. proposed a r to that of the 11). 78 Thus, there is an ongoing debate in the literature regarding the exact Mn 4 Ca core in the native OEC. Very recently, Umena et al. reported an X ray structure of the PSII at 1.9 resolution which has provided strong suppor t for the existence of a distorted [Mn 4 CaO 5 ] with an additional Mn attached to the cubane. 79

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32 Another ap proach to study the OEC has been synthesis of model complexes, which provides invaluable structural, spectroscopic and mechanistic insights into the active site. The last 40 years have seen the synthesis of a vast array of Mn clusters, which have helped in our understanding of the OEC. 80 82 However, in order to mimic the active site more accurately synthetic methodologies need to be developed to isolate heteronuclear Mn/Ca complexes. This widespread interest in the OE C has also been driven by motives beyond biochemistry. An alarming rise in the atmospheric CO 2 level, dwindling supplies of global oil and fossil fuels, and an ever growing global energy demand have made the development of clean alternative and renewabl e sources of energy essential. 83 Today, hydrogen gas is regarded as one of the most promising and clean forms of fuel for the future. It would be ideal to obtain hydrogen from water oxidation instea d of from petroleum. The hydrogen produced can then be used directly as a fuel, which produc es water when burned in air, or as a raw material for hydrocarbon production. Unfortunately, c urrent methods of producing H 2 are both expensive and unsustainable fo r mass worldwide use. Thus, finding out exactly how Nature has been carrying out water oxidation to O 2 2 + plus 4e kept and used separately by plants) for 2.5 Gy would be invaluable to guide our efforts in designing industrial catalysts for wat er oxidation driven by solar energy, e specially with the photovoltaic industry becoming increasingly efficient at harnessing sunlight as a source of energy. 84, 85 Apart from the bioinorganic research, Mn clusters ( both homo as well as hetero metallic) are also relevant to the field of nanoscale magnets. These are zero dimensional magnets, more commonly known as single molecule magnets. They exhibit

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33 slow relaxation of magnetization, giving rise to hysteresis plots be low a blocking temperature (T B ). 86 These nanoscale materials exhibit interesting physical properties as they display both classical and quantum properties. Thus, there is a continuous need to synthesize and characterize new SMMs. The primary motivation of the research featured in this dissertation is the development of new synthetic strategies for isolating novel metal clusters incorporating Mn which are relevant to the field of bioinorganic as well as magnetic materials research. In Chapter 2 synthesi s and magnetic characterization of Mn clusters are reported; one of which functions as an SMM. The remaining chapters in the thesis (Chapter 3 6) are dedicated to the ongoing pursuit to isolate the exact synthetic analog of the OEC.

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34 Table 1 1. Selecte d Manganese Enzymes and their Catalytic Functions Enzyme Metabolic Reaction Example Dehydrogenase s a Reduction of substrate molecule with NAD + as the reducing coenzyme Isocitric dehydrogenase 87 Oxidases a Oxidizes various s ubstrates Cytochrome c oxidase 88 Superoxide Dismutase a Conversion of superoxide to peroxide and oxygen Mn Superoxide Dismutase 89 Catalases a Conversion of peroxide to oxygen and water Mn Catalase 90 Peroxidases a Oxidize different substrates using peroxide as the oxidizing agent Chloroperoxidase 91 Kinases b Trans fer of phosphate group from ATP to an amino acid residue of proteins Pyruvate kinase 92 Nucleases b Hydrolysis reaction of phosphate esters DNA polymerase 93 Carboxylases b Carboxylation of substrate P hosphoenolpyruvate carboxylase 94 Peptidases c Breaking down of pepides Aminopeptidases 95 Phosphatases c Remove phosphate residue from phosphorylated pr oteins and sugars Inorganic pyrophosphatase 96 Arginase c Hydrolysis of arginine to urea Arginase 97 Carboxykinases d Cleavage of carboxyl bond Phosphoenolpyruvate carboxykinase 98 Keto Aldo Isomerase e Interconversion of keto sugars and aldo sugars Xylose isomerase 9 9 Cycloisomerase e Isomerization between cyclic and open structure Muconate cycloisomerase 100 Racemases e Racemization reaction Mandelate racemase 101 Synthetase f Formation of new chemical bond to j oin to substrates Glutamine synthetase 102 a Oxidoreductases b Transferases, c Hydrolases, d Lyases, e Isomerases and f Ligases

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35 Figure 1 1. Relative frequencies of the catalytic metal ions in the six enzyme classes (Me stands for metal). 5 Figure 1 2. Z scheme of photosynthe sis emphasizing the electron transfer achieved by three membrane spanning supramolecular complexes PSII, the cytochrome b 6 /cytochrome f complex, and PSI.

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36 Figure 1 3. The Kok cycle showing advancement of the OEC from S 0 to the transient S 4 state and su bsequent O 2 evolution. It also represents the cyclic release of protons and electrons along the Kok cycle. Figure 1 4. Possible metal topologies of the Mn 4 cluster of the OEC in PSII, as proposed from EXAFS studies.

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37 Figure 1 5. Mn K edge XANES spect ra of flash illuminated PSII samples (top) and the S state XANES difference spectra. Reprinted with permission from Messinger et al. 45 Copyright (2001) American Chemical Society. Figure 1 6. Fourier transformed EXAFS spectra of the Mn complex in the semi stable S state (at 20 K). Solid lines represent simulations of the experimental spectra (dotted line). Reprinted with permission from Dau et al. 103

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38 Figure 1 7. Compar ison of the 55 Mn hyperfine resolved EPR signal associated with the S 0 S 1 and S 2 states of the OEC. Reprinted with permission from Peloquin et al. 104 Copyright (2000) American Chemical Society. Figure 1 8. Cr ystallographically deduced proposals for the [Mn 4 CaO x ] core of the OEC differing in the means of attachment of the external Mn atom. (a) [Mn 4 CaO 4 ] core predicted by Ferreira et al 70 ; (b) a related [Mn 4 CaO 5 ] structure 67 and (c) the [Mn 4 CaO 5 ] core from Umena et al. structure. 79

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39 Figure 1 9. Crystallographically deduced structure of the [Mn 4 CaO x ] core of the OEC along with its peripheral ligation (a) Ferreira et al. ; 70 (b) Loll et al. 74 and (c) Umena et al. 79 Reprinted with permission from AAAS and Nature; Ferreira et al. Loll et al. and Umena et al. Figure 1 10. Proposed structure s of the [Mn 4 CaO x ] core of the OEC from high resolution EXAFS and polarized EXAFS studies. Reprinted with permission from Yano et al. 76 Figure 1 11. Proposed structures of the [Mn 4 CaO x ] core of the OEC from EXAFS studies by Dau et al .. Reprinted with permission from Dau et al. 78

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40 CHAPTER 2 COMPROPORTIONATION R EACTIONS AS A ROUTE TO NEW HIGH OXIDATION STATE MANGANESE CLUS TERS AND SINGLE MOLECULE MAGNET 2.1 Introduction Paramagnetic 3d metal clusters attract a great deal of attention primarily due to their relevance to the field of bioinorganic chemistry and nanoscale magne tic materials. 105 Among th ese, manganese clusters receive special attention for the following reasons: f irstly, manganese can access a range of oxidation states (II IV) under normal conditions Manganese compounds with oxidation states III VII have a long history as oxidizing agents for a vast variety of organic compounds. 106 107 C onsequently it is present in the active site of several redox enzymes 108 including the oxygen evolving complex ( OE C) in photosy stem II (PSII) of cyanobacteria and green plants which is responsible for the light driven oxidation of water to oxygen 109 Secondly, polynuclear manganese clusters containing Mn 3+ ions often possess a large ground state spin ( S ) due to ferromagnetic interactions and/or spin frustration effect s. 110 The combination of a large negative magnetic anisotropy ( D ) in addition to a high S value allows these species to behave as single molecule magnets (SMMs). 111 114 These are individual molecules that exhibit a signi ficant barrier ( vs kT ) to magnetization relaxation and hence function as nanoscale magnets below their blocking temperature ( T B ). 115 116 These nanoscale materials have also been found to straddle the classical/quantum interface by displaying not just the classical property of magnetization hysteresis but also the quantum properti es of quantum tunneling of the magnetization (QTM) through the anisotropy barrier, 117 119 and quantum phase interference (QPI) 120 121

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41 The first SMM to be described and studied wa s the [Mn 12 O 12 (O 2 CCH 3 ) 16 (H 2 O) 4 ] (Mn 12 Ac) 122 86 S ince then a large variety of manganese containing SMMs of various metal topologies 123 125 nuclearity 126 130 and spin ground states 131 133 ha ve been synthesized. Lastly, manganese compounds with oxidation states III VII have a long history as oxidizing agents for a vast variety of organic compounds. 106 107 For the above reasons, there is a continuing interest in developing synthetic strategies t o new high oxidation state polynuclear Mn/O clusters. T he reaction of chelating ligands with simple Mn carboxylate sources or with preformed Mn carboxylate cluster s that do not already contain any chelating ligands, 134 136 are strategies that have been employed successfully in the past. An alternative strategy is the use of potentially bridging ligands such as alcohols, to foster the formation of high nuclearity manganese products. 124 131, 132, 137 146 O ther synthetic routes that have been employed include comproportionation of simple starting materials, 147 149 aggregation of small nuclearity clusters, 150 151 fragmentation of higher nuclearity clusters, 152 153 reductive aggregation, 154 electrochemical oxidation, 155 ligand substitution of preformed species 156 157 etc. In this work, we have further explored comproportionation reactions which in the past have led to the isolation of various homo 148, 149, 158 160 and heterometallic Mn clusters, 161 162 including the well known Mn 12 Ac 122 86 In comproportionation reaction two species with the same element in different oxidation states form a product in which the element is in the intermediate oxidation state. M n+ + M (n+2)+ 2 Mn (n+1)+ (2 1) A salient feature of this strategy is that it allows for so me level of control o f the average manganese oxidation state in the reaction and consequently the final product isolated. This allows the targeti ng of high oxidation states in the cluster like Mn 3+ and

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42 Mn 4+ which is not easy to attain by other means such as aerial oxidation. T he stronger magnetic couplings promoted by these metal ions usually prevent complications from low lying excited states wh ich are frequently encountered in Mn 2+ /Mn 3+ systems due to weaker coupling s 150 163 Thus far most of the prior work in this area has been limited to the use of acetic acid, propionic acid and benzoic acid as the carboxylate source. 164 167 T h us the exploration of comproportionation reactions between Mn 2+ salts and Mn 7+ in the presence of an excess of pivalic acid seemed attractive It was anticipated that the difference in bulk and the higher pK a of pivalic acid ( pK a = 5.03 ) compared to that of acetic acid ( pK a = 4.75) would allow access to different manganes e complexes. The excess acid would serve the following purposes: (1) maintain ing an acidic pH thus preventing the formation of manganese oxide s and (2) act ing as a ligand to complete the c oordination of the manganese centers and assist in the formation of polynuclear clusters. Indeed this strategy proved successful and led to the isolation of two Mn 8 and one Mn 9 cluster The syntheses, structures and magnetochemical characterization s of the se complexes are described in this chapter 2.2 Experimental Section 2.2.1 Syntheses All manipulations were performed under aerobic conditions using chemicals (reagent grade) and solvents as received. NBu n 4 MnO 4 and Mn(O 2 CBu t ) 2 were prepared as previously reported. 168 169 Safety note : Perchlorate salts are potentially explosive; such compounds should be synthesized and used in small quantities, and treated with utmost care at all time s.

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43 [Mn 8 O 6 (OH)(O 2 CBu t ) 9 Cl 3 (Bu t CO 2 H) 0.5 (MeCN) 0.5 ] ( 2 1): To a stirred solution of Mn(O 2 CBu t ) 2 2H 2 O (0.16 g, 0.5 0 mmol) and Bu t CO 2 H (2.8 3 mL, 24.6 0 mmol) in hot MeCN (20 mL) was added solid Mn(ClO 4 ) 2 6H 2 O (0.18 g, 0.5 0 mmol) and NBu n 4 Cl xH 2 O (0.14 g, 0.5 0 m mol). Th e light pink slurry was stirred for 5 min followed by the slow addition of NBu n 4 MnO 4 (0.18 g, 0.5 0 mmol) which resulted in the formation of a dark brown black solution. The sol ution was stirred for 15 min and then filtered after cooling. The filtra te was allowed to stand undisturbed at room temperature. After 2 weeks, X ray quality dark brown crystals of 2 1 3 MeCN were collected by filtration and dried in vacuo The yield was 25%. Anal. Calcd (Found) for 2 1 MeCN (C 50.5 H 91.5 N 1.5 Mn 8 O 26 Cl 3 ): C, 36.06 (35.77); H, 5.48 (5.33); N, 1.25 (1.31). Selected IR data (cm 1 ): 3392 (w), 2971 (m), 2874 (s), 1671 (s), 1533 (s), 1484 (s), 1458 (m), 1421 (s), 1378 (s), 1364 (s), 1226 (s), 1032 (m), 938 (m), 896 (m), 656 (s), 621 (s), 597 (s), 480 (m), 458 (m). [Mn 8 O 9 (O 2 CBu t ) 12 ] ( 2 2): To a stirred solution of Mn(NO 3 ) 2 xH 2 O (0.09 g, 0.5 0 mmol) and Bu t CO 2 H (1.8 9 mL, 16.4 0 mmol) in hot MeCN (25 mL), was slowly added NBu n 4 MnO 4 (0.27 g, 0.75 mmol). The solution was stirred for 15 min during which the pink slurry changed t o a dark red solution. The solution was cooled, filtered and the filtrate allowed to stand undisturbed at room temperature. X ray quality deep red crystals of 2 2 MeCN slowly grew over a week and they were collected by filtration and dried in vacuo. The y ield was 36%. Anal. Calcd (Found) for 2 2 1 / 4 MeCN (C 60.5 H 1 08.75 N 0.25 Mn 8 O 33 ): C, 40. 2 1 (40.37); H, 6.0 7 (6.14); N, 0. 19 (0.21). Selected IR data (cm 1 ): 3442 (w), 2965 (m), 2930 (m), 1599 (s), 1529 (m), 1482 (s), 1416 (s), 1362 (m), 1223 (s), 1031 (m), 937 (m), 893 (m), 785(m), 721 (m), 621 (s), 570 (m), 453 (m).

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44 [Mn 9 O 7 (O 2 CBu t ) 13 (THF) 2 ] ( 2 3): To a stirred solution of Mn(O 2 CBu t ) 2 2H 2 O (0.16 g, 0.5 0 mmol) and Bu t CO 2 H (1.8 9 mL, 16.4 0 mmol) in hot MeCN/THF (20 mL, 2:1 v/v), was slowly added NBu n 4 MnO 4 (0.18 g, 0 .5 0 mmol). The solution was stirred for 15 min during which the pink slurry changed to a warm dark brown solution. This was cooled, filtered and the filtrate was allowed to stand undisturbed at room temperature. X ray quality black crystals of 2 3 1 / 3 THF 2 / 3 MeCN slowly grew over 3 d and were collected by filtration and dried in vacuo. Dried solid analyzed as solvent free. The yield was 35%. Anal. Calcd (Found) for 2 3 (C 73 H 133 Mn 9 O 35 ): C, 42.45 (42.81); H, 6.50 (6.90); N, 0.00 (0.00). Selected IR data (cm 1 ) : 2964 (m), 2928 (m), 1565 (s), 1484 (s), 1424 (s), 1375 (m), 1360 (m), 1228 (s), 1045 (m), 893 (m), 787 (m), 699 (m), 674 (m), 625 (s), 453 (m), 471 (m). 2.2.2 X ray Crystallography Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo K Suitable crystals of 2 1 3MeCN 2 2 MeCN and 2 3 1 / 3 THF 2 / 3 MeCN were attached to glass fibers using paratone oil and transferred to a goniostat where they were cooled to 173 K for data collection. Data collection and unit cell parameters are listed in Table 2 1 Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 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 the d irect m ethods in SHELXTL6 170 and refined on F 2 using full matrix least square cycles The non H atoms were treated anisotropically, whereas the H atoms were

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45 placed in calculated ideal positions and were refined as riding on their respective C atoms. Fo r 2 1 3MeCN the asymmetric unit consists of a Mn 8 cluster, one acetonitrile in a general position, and another acetonitrile disordered and in general position. One of the pivalate ligands is disordered against a coordinated acetonitrile ligand and two pa rtial acetonitrile molecules nearby in the lattice. Five of the coordinated pivalate ligands are disordered and their methyl groups were refined in two sites each. The disordered atoms of the solvent molecules were constrained to remain equivalent during the refinement. A total of 814 parameters were refined in the final cycle of refinement using 10979 I ) to yield R 1 and wR 2 of 5.77 and 15.70%, respectively. For 2 2 MeCN the asymmetric unit consists of a Mn 8 cluster and an acetonitrile solvent molecule The lat er is too disordered to be modeled properly, thus program SQUEEZE 171 a part of the PLATON package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall intensi ty data. A total of 950 parameters were refined in the final cycle of refinement using 15351 I ) to yield R 1 and wR 2 of 3.69 and 9.47%, respectively. Refinement was done using F 2 For 2 3 1 / 3 THF 2 / 3 MeCN the asymmetric unit consists of a Mn 9 cluster in a general position, a half Mn 9 cluster (located on a 2 fold rotation a xis), a THF solvent molecule disordered around a 2 fold rotation axis, and an MeCN solvent molecule in general position. The solvent molecule again were too disordered to be modeled properly, thus program SQUEEZE was again used to calculate the solvent d isorder area and remove its contribution to the overall intensity data. The pivalate ligand of C102 has its methyl

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46 group disordered because of symmetry. Each of the disordered atoms was given a 0.5 occupancy value due to symmetry. The cluster in a genera l position has five disordered pivalate ligands ( C11, C20, C39, C44 and C65 ) Each disordered pivalate was refined in two parts with their site occupation factors independently refined. A total of 1586 parameters were refined in the final cycle of refine ment using 28402 reflections with I > I ) to yield R 1 and wR 2 of 3.88 and 8.46%, respectively. 2.3 Results and Discussion 2.3.1 Syntheses The reaction of Mn(O 2 CBu t ) 2 Mn(ClO 4 ) 2 NBu n 4 Cl and NBu n 4 MnO 4 in 1:1:1:1 ratio with excess pivalic acid in hot acetonitrile afforded a dark brown solution which subsequently led to the isolation of the octanuclear complex, [Mn 8 O 6 (OH)(O 2 CBu t ) 9 Cl 3 (Bu t CO 2 H) 0.5 (MeCN) 0.5 ] ( 2 1 ) in 25% yield The formation of 2 1 has been summarized in eq 2 2 where NBu n 4 MnO 4 assist the oxidation of the Mn I I salts to yield the mixed valence 7Mn III 1Mn IV product. 5 Mn 2+ + 3 MnO 4 + 9.5 Bu t CO 2 H + 3 Cl + 0.5 MeCN + 2 H + + 6 e [Mn 8 O 6 (OH)(O 2 CBu t ) 9 Cl 3 (Bu t CO 2 H) 0.5 (MeCN) 0.5 ] + 5H 2 O ( 2 2 ) Although the pivalic acid is pre sent in excess in the above reaction, the total Mn:acid ratio is pivotal in controlling the crystallinity of the final product. The use of Mn(NO 3 ) 2 and MnCl 2 in place of Mn(ClO 4 ) 2 also leads to the isolation of the same Mn 8 complex but in much lower yields The isolation of 2 1 indicates the reaction condition to be conducive for isolating high oxidation state Mn clusters and we wanted to explore the system more by varying the Mn 2+ : Mn 7+ ratio to affect the average Mn oxidation state (in the reaction and t he final product). Thus in an attempt to incorporate more Mn 4+ in the clusters we tried

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47 increasing the amount of MnO 4 in the above reaction. Unfortunately, all our attempts led to formation of a mixture of products which were difficult to separate. We thu s changed our strategy to use a single Mn 2+ precursor instead of two. The reaction of Mn(NO 3 ) 2 with a higher equivalent of NBu n 4 MnO 4 ( 2 : 3 ) in hot acetonitrile gave [Mn 8 O 9 (O 2 CBu t ) 12 ] ( 2 2 ) in 36 % yield. The average Mn oxidation state in the reaction is now +5 instead of +3.67 in eq. 2 2. The formation of 2 2 is summarized in eq. 2 3. 3 Mn 2+ + 5 MnO 4 + 12 Bu t CO 2 H + 10 H + + 11 e [Mn 8 O 9 (O 2 CBu t ) 12 ] + 11H 2 O (2 3) This reaction is sensitive to the identity of the starting materials. The use of MnCl 2 and Mn(ClO 4 ) 2 instead of Mn(NO 3 ) 2 led to the formation of brown solids whose IR were different from 2 2 Significant efforts were employed t o crystallize the solids but they were unsuccessful. Changing the acid to benzoic acid or propionic acid under similar reaction conditions led to the isolation of the benzoate and propionate analog of Mn 12 Ac. 86 This indicates that the pKa of the pivalic ac id and its bulk is crucial in div erting these reactions. Using the same strategy of single Mn 2+ precursor in a mixed solvent system with the average Mn oxidation state of +4.5 (instead +5 as in eq. 2 3) led to the isolation of a nonanuclear cluster ( 2 3 ). The reaction of Mn(O 2 CBu t ) 2 and NBu n 4 MnO 4 in a 1:1 ratio with excess pivalic acid in hot acetonitrile/tetrahydrofuran (2:1) led to the isolation of [Mn 9 O 7 (O 2 CBu t ) 13 (THF) 2 ] ( 2 3 ) in 35% yield. The formation of 2 3 is summarized in eq 2 4 5 Mn 2+ + 4 Mn O 4 + 113 Bu t CO 2 H + 2 THF + 5 H + + 11 e [Mn 9 O 7 (O 2 CBu t ) 13 (THF) 2 ] + 9H 2 O ( 2 4 )

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48 Unlike 2 1 and 2 2 2 3 is no longer a mixed valent Mn cluster. Although the av erage Mn oxidation in the reaction was +4.5 the final product isolated had a +3 average Mn oxidation state. It can be deduced that the overall reactions are complicated and the reaction contains a mixture of several species in equilibrium. The identity of the product isolated depends on factors such as relative solubility, lattice energies, crystallization kinetics and other factors. It is important to have THF in the solvent system to obtain good quality crystals. If the reaction is done in just acetonitri le we still obtain the same Mn 9 complex (as evident from the inspection of elemental analysis and IR spectrum) but none of the samples were single crystals. 2.3.2 Description of Stru ctures 2 .3.2.1 Structure of [Mn 8 O 6 (OH)(O 2 CBu t ) 9 Cl 3 (Bu t CO 2 H) 0.5 (MeCN) 0.5 ] ( 2 1) The partially labeled structure of 2 1 is shown in Figure 2 1, and selected interatomic distances and angles are summarized in Table A 1 Complex 2 1 crystallizes in the triclinic space group P 1 The oxidation states of the manganese atoms and the protonation levels of the O 2 OH and the oxygen atoms of the carboxylate groups were determined from a combination of charge balance consideration, inspection of bond lengths and bond valence sum (BVS) calculations (Table 2 2 ). The complex contains a [Mn 7 III Mn IV ( 3 O) 4 ( 4 O) 2 ( 3 OH)( 4 Cl)( 2 Cl)] 8+ core (Figure 2 1 ). The core is mixed valent, comprising seven Mn III and one Mn IV ions. All the manganese centers have a distorted octahedr al geometry except for Mn5 which is heptacoordinated. The Mn III atoms (except Mn5) display JT elongated axis which are oriented towards the Mn Cl, Mn OH and Mn O (carboxylate) bonds in order to avoid Mn oxide bonds which are almost always the shortest a nd the strongest bond in the molecule. The core can be described as containing two [Mn 4 O 2 ] butterfly units

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49 (Mn5). The Mn 4 butterfly units are more closed up i.e. more acute V shape d compared to regular Mn 4 butterfly complexes, which is evident from the upward disposition of the 2 O 2 rhombus. The resulting Mn 3 O 4 basal unit arising from these fused butterfly units is almost planar with M n2 Mn5 Mn6 angle being 176.5 (4). The remaining manganese center, Mn4 is connected to the atoms of the butterflies via two 4 oxides (O3/O7) These 4 oxides bridge which mixed atom, Mn6 via a 3 OH (O5). This is quite unusual as Mn6 is in +4 oxidat ion state which prefers harder ligands like O 2 T here are only a few examples where Mn IV centers are bridged by hydroxide group. 172 176 This 3 OH (O5) is also weakly hydrogen bonded to an acetonitrile solvent mol ecule which justifies the lower BVS number observed for the oxygen (Table 2 2). Another interesting feature of this molecule is the presence of a 4 Cl (Cl1) which is quite rare. 151, 177, 178 It not only bridges t like O5, but is also connected to Mn4. Further asymmetry in this core is induced by a Cl (Cl2) Mn (Mn1/Mn3) of two different butterfly un its on one side of the molecule only. The Mn atoms on the other side are bridged by a pivalate group instead of chloride. The peripheral ligation to the Mn 8 core is provided by nine 1 : 1 : bridging pivalate groups, one terminal chloride on Mn3 and a 1 terminal pivalic acid on Mn2. There is a

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50 disorder between this pivalic acid and a solvent acetonitrile molecule (which has been removed for clarity). The OH proton of the terminal piva lic acid is hydrogen bonded to O2 in the Mn 8 core. There are quite a few Mn 8 clusters reported in the literature, possessing a wide variety of metal topologies ranging from rodlike, serpentine, rectangular, linked Mn 4 butterfly units, linked tetrahedral, r ings etc. 128, 179 186 There is only one Mn 8 complex known in the literature which is remotely related to the structure of 2 1 but differences are still significant 151 The unique feature of 2 1 is its unusual manganese oxidation states. All the Mn 8 complexes in the literature contain the metal oxidation states Mn 8 II 187 Mn 6 II Mn 2 III 188 189 Mn 4 II Mn 4 III 190 Mn 2 II Mn 6 III 191 192 Mn 8 III 138 151 193 or Mn 2 III Mn 6 IV 147 making 2 1 the only complex at the Mn 7 III Mn IV oxidation state. 2.3.2.2 Structure o f [Mn 8 O 9 (O 2 CBu t ) 12 ] (2 2 ) The partially labeled structure of 2 2 is shown in Figure 2 2 and selected interatomic distances and angles are summarized in Table A 2 Complex 2 2 crystall izes in monoclinic P 2 1 / n space group. Charge consideration and inspection of the metric parameters indicate a 2Mn III and 6Mn IV description for 2 2 This assignment was further confirmed from the BVS calculations (Table 2 3 ). The complex consist s of a [Mn 6 I V Mn 2 III ( 3 O) 6 ( O) 3 ] 12+ core (Figure 2 2 ). This Mn 8 core is also mixed valent like 2 1 but now comprises six Mn IV and two Mn III The core is composed of two Mn 3 O 4 defective or partial cubane units on either side linked by a bridging oxide in the middle. These cubane units are further linked by two Mn 3+ ions on top and bottom via four 3 oxide from the open faces of the cubane. T he manganese centers in the defective cubane are in +4 oxidation state. All t he manganese atoms possess a distorted octahedral ge ometry. The high spin (d 4 ) Mn 3+ ions undergo a Jahn

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51 Teller distortion ; which is evident from the elongated bonds in the crystal structure. The JT axes on the Mn 3+ ions ; which are oriented towards the Mn O (carboxylate) bonds (O23 Mn6 O28 and O32 Mn8 O10) align almost parallel to each other. The peripheral ligation is provided by twelve pivalate groups which are bridging in a 1 : 1 : bridging mode. Although there are quite a few Mn 8 clusters reported in the literature, no precedence for a metal topology l ike 2 2 is reported. Another interesting feature is the relatively high manganese oxidation states and this is only the second example of a Mn 2 III Mn 6 IV core. 147 It is also fascinating to note that the average manganese oxidation state in 2 2 is +3.75 which is the highest for any polynuclear Mn x cluster. This average manganese oxidation state is even higher than that for the well know Mn 12 family of clusters of +3.33. 2.3.2.3 Structure of [Mn 9 O 7 (O 2 CBu t ) 13 (THF) 2 ] (2 3 ) The partially labeled structure of 2 3 is shown in F igure 2 3 and selected interatomic distances and angles are summarized in Table A 3 Complex 2 3 crystallizes in orthorhombic Aba 2 space group. The oxidation state of the manganese centers as well as the protonation levels of the O 2 were determined from a combination of metric parameters and BVS calculations (Table 2 4 ). The complex contains a [Mn 9 III ( 3 O) 7 ] 13+ core (Figure 2 3 ). The structure can again be described as being composed of two body fused Mn 4 butterfly units [Mn9/M n3/Mn2/Mn6 and Mn8/Mn2/Mn 1/Mn5] The hinged Mn atom (Mn2) is lifted above the Mn 2 O 4 [Mn3/O7/O9/O6/O1/Mn1] basal plane by the 3 oxide which connects Mn2 to the remaining Mn atoms [Mn4/Mn7] of the Mn 9 core. These manganese atoms butterfly units via two more 3

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52 oxides [O29/O18]. Although the metal nuclearity is different, there are some structural similarities between 2 1 and 2 3 The Mn 4 butterfly units in 2 3 show a less acute V shape in comparison to 2 1 This is evident from t he comparison of the dihedral angles between the two Mn 3 planes which is 137 in 2 3 compared to 94 in 2 1 Furthermore, the resulting Mn 3 O 4 basal unit is no longer planar (143) as compared to 2 1 All the manganese ions possess a distorted octahedral g eometry which is typical for a d 4 metal undergoing Jahn Teller distortion. The peripheral coordination of 2 3 is completed by eleven pivalate groups bridging in the normal 1 1 mode and two pivalate groups in 3 1 2 binding mode. The coordination of the two body centers [Mn1/Mn3] are completed by two terminal THF solvent molecules. Compounds possessing a core similar to 2 3 have been previously reported in the literature. 151 194 195 2.3.3 Magnetochemistry of Complexes 2 1 to 2 3 2.3.3.1 DC s tudies Solid state variable temperature dc magnetic susceptibility data in 0.1 T field was collected on powdered microcrystalline samples of 2 1 2 3 restrained in eicosane in 5.0 300 .0 K range. The obtained data is plotted as X M T vs T in Figure 2 4 For 2 1 the 300 K value is 22.0 cm 3 Kmol 1 which is comparable to the spin only ( g = 2) value of 22.88 cm 3 Kmol 1 as expected for seven Mn III and one Mn IV non interacting ions. The X M T value slowly increases with decreasing temperature to a maximum value of 27.27 cm 3 Km ol 1 at 15 K, and then slightly drops to 26.06 cm 3 Kmol 1 at 5.0 K. The profile indicates that at least some of the interactions are ferromagnetic, and that the molecule has a large spin ground state. The X M T value at the lowest temperatures

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53 suggests an S = 15 / 2 ground state (spin only value = 31.88 cm 3 Kmol 1 ) with g slightly less than 2, as expected for a Mn III /Mn IV complex. For 2 2 t he 300 K X M T value of 13.75 cm 3 Kmol 1 is lower than the spin only value of 17.25 cm 3 Kmol 1 ( g = 2.0) expected for six Mn IV ( S = 3/2) and two Mn III ( S = 2) non interacting ions, indicating the presence of strong antiferromagnetic interactions. This is consistent with the observed decrease in X M T value with decreasing temperature and the low temperature data suggests an S = 2 gr ound state for 2 2 The 300 K X M T value of 19.81 cm 3 Kmol 1 for 2 3 is much lower than the spin only value of 27.00 cm 3 Kmol 1 ( g = 2.0) expected for nine non interacting Mn III ( S = 2) indicating the presence of strong antiferromagnetic interactions. The X M T value shows a steady decrease with the decrease in temperature from 300 K which also indicates the presence of antiferromagnetic interactions. The 5.0 K X M T value of 27.0 cm 3 Kmol 1 indicates a low, but possibly non zero ground state S value for 2 3 To p robe the ground state of 2 1 2 3 further, and to determine the zero field splitting parameter ( D ), magnetization ( M ) vs field ( H ) data were collected in the magnetic field and temperature ranges of 0.1 7 T and 1.8 10 K, respectively. The resulting da ta for 2 1 are plotted in Figure 2 5 as reduced magnetization ( M / N B ) vs H / T where N B is the Bohr magneton. The data were fit by diagonalization of the spin Hamiltonian matrix using the program MAGNET 196 which a ssumes only the ground state is populated at these temperatures and fields, includes the Zeeman interaction and axial zero field splitting ( z 2 ), and incorporates a full powder average. The corresponding spin Hamiltonian is given by eq 2 5 where z is the easy axis spin operator, g is the Land g factor, and 0 is the vacuum permeability.

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54 H = z 2 + g B 0 H ( 2 5 ) The last term in eq 2 5 is the Zeeman energy associated with the applied magnetic field. The best fit for 2 1 is shown as the solid lines in Figure 2 5. A satisfactory fit could only be obta ined if data collected at fields above 4T were excluded, suggesting that some low lying excited states with S > 15 / 2 are being stabilized at these higher fields causing them to be significantly populated at these temperatures. The best fit for 2 1 was obta ined with S = 15 / 2 g = 1.98 (4) and D = 0. 2 2 (7) cm 1 and represented by the solid lines in Figure 2 5 Alternative fitting with positive D value was rejected because they gave unreasonable D and g values. For 2 2 no satisfactory fit could be obtained both with high as well as low field data. This is usually attributed to the presence of low lying excited states. Such low lying excited states can be present in high nuclearity spin frustrated systems like 2 2 where the spin frustration can be correlated to the presence of large number of triangular units within the core topology of 2 2 A similar problem was encountered for 2 3 where all our attempts to fit the magnetization data resulted in unreasonable D and g values. 2.3.3.2 AC s tudies An AC magnetic susceptibility study was performed to independently confirm the ground state and probe the dynamics of magnetization (magnetic moment) relaxation. The AC studies on 2 1 2 3 were performed in the 1.8 15 .0 K range using a weak 3.5 G ac field oscillating at frequencies of 50 1000 Hz. Figure 2 6 shows the in phase X M component of the ac magnetic susceptibility (plotted as X M T vs T ) for 2 1 in three

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55 frequencies 50, 250 and 997 Hz, which is invaluable for assessing S without any complication from a dc fi eld. The X M T signal shows a very slight drop in the temperature range indicating the ground state to be somewhat isolated from the first excited state. Extrapolation of the plot from 10 K to 0 K, gives a X M T value of 29 cm 3 Kmol 1 which is consistent wit h the value expected for S = 15 / 2 with g = 1.98 and this is in good agreement with the fits of the DC magnetization data and DC magnetic susceptibility data Although the X M T shows no obvious frequency dependence except perhaps at the lowest temperatures, the X M vs T plot for 2 1 (Figure 2 5 ) is more sensitive and clearly shows below 3 K frequency dependent tails of signals whose peaks lie at temperatures below the operating limit of our SQUID (1.8 K). Th ese data suggest that 2 1 may exhibit the slow magn etization relaxation characteristic of SMMs. To confirm the SMM behavior of 2 1 magnetization vs field studies were carried out on single crystals, to see whether they display hysteresis loops; the diagnostic behavior of a magnet. The in phase X M T vs T ac susceptibilities for 2 2 (Figure 2 7) show a steady decrease in X M T value with decreasing temperature which indicates the depopulation of one or more excited states with an S greater than the ground state S This rationalizes the unavailability of a reasonable fit for the magnetization vs field data. Extrapolation of the plot from above 3K to 0K (to avoid the effect of weak intermolecular interaction s ) gives a X M T value of ~ 3cm 3 Kmol 1 which indicates an S = 2 ground state with g ~ 2. An S = 1 or S = 3 ground state would give a X M T value of ~ 1 or ~ 6

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56 cm 3 Kmol 1 respectively, which clearly differ from the experimental data. We thus feel confident in our conclusion that 2 2 has an S = 2 ground state. The in phase X M T vs T AC susceptibilities for 2 3 (Figure 2 7) show a similar profile like 2 2 E xtrapolation of the data from 8 K to 0 K gives a value of 1.5 cm 3 Kmol 1 which is still high for an S = 1 ground state. It is thus reasonable to say that 2 3 possess a non zero ground state spin with extreme ly close S = 0, 1 and 2 states which complicates the unambiguous assignment of the ground state spin for this molecule. Although all the manganese in 2 3 are in +3 oxidation state there were no peaks observed in the out of phase ac signals down to 1.8 K. 2 .3.3.3 Single crystal h ysteresis studies To confirm that 2 1 is an SMM, magnetization vs applied dc field data down to 0.04 K were collected on single crystals that had been kept in contact with the mother liquor, using a micro SQUID apparatus. 197 The corresponding magnetization responses at different temperatures and a fixed field sweep rate of 0.14 T/s are shown in Figure 2 8 Hysteresis loops are observed in both the figures, and their coercivities increase with increasing field sweep rates (at a constant temperature) and decreasing temperature (at a constant sweep rate), as expected for the superparamagnet like properties o f a SMM below the blocking temperature ( T B ). These loops thus confirm 2 1 to be a new addition to the family of half integer spin SMMs. One characteristic feature of the hysteresis loops is the observance of a large step at zero field which arises from qua ntum tunneling of magnetization (QTM) through the anisotropy barrier via the lowest lying M s levels of the S = 15/2 manifold. QTM causes an increase in the magnetization relaxation rate, giving the jump (step) in the loop. The

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57 large zero field step is indi cative of fast QTM rates, as is typical of low symmetry molecules, due to the introduction of significant transverse anisotropy E ( x 2 y 2 ) into the spin Hamiltonian where E is the rhombic zero field splitting parameter. The step size decreases with increasing field scan rate, as expected from the standard Landau Zener model for tunneling between two states. 197 It should be noted that for half integer system transverse fields are essential for QTM E ven in the absence of external applied fields these a re provided by the transverse components of dipolar and exchange fields from neighboring molecules and hyperfine fields from 55 Mn ( I = 5 / 2 100%) nuclei and intermolecular exchange coupling. 120 198 Another key feature is the broadening of the steps arising from a distribution of molecular environments (disordered ligands, solvent of crystallization) that result in a significant distribution of step positions or to a high density of low lying excited states. Since the QTM steps are clearly observed in hysteresis loops, a direct measure of the D and is given by eq. 2 6. H = D / g B (2 6) Measurement of the step position in Figure 2 H of 0.2 T. This corresponds to a D /g value of 0.09 cm 1 Approximating g ~ 2 gives a D of 0.22 cm 1 which is very close t o the value obtained from the reduced magnetization fit complex 2 1 2.4 Conclusions The use of a comproportionation reaction between Mn II and Mn VII in the presence of excess pivalic acid has proven to be a fruitful new route to obtaining new high oxidat ion state manganese clusters Two Mn 8 and on Mn 9 species have been obtained by variation of the average manganese oxidation state and other reaction conditions

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58 such as the metal reagents and the reaction solvent The two octanuclear manganese clusters poss ess unprecedented Mn 8 topologies and manganese oxidation states: [Mn 7 III Mn IV ] ( 2 1 ) and [Mn 2 III Mn 6 IV ] ( 2 2 ). In fact, 2 2 is the second manganese cluster to possess such a high average manganese oxidation state of +3.67 for homo metallic Mn clusters Alth ough we were unable to assign the ground state for 2 3 due to complications from low lying excited states, complexes 2 1 and 2 2 were found to possess an S = 15 / 2 and S = 2 ground state respectively. Furthermore, complex 2 1 showed signals in the out of ph ase ac susceptibility measurements which are indicative of slow magnetization relaxation of SMMs. This was further verified from magnetization versus field study which showed the hysteresis loop, a diagnostic behavior of SMMs. Complex 2 1 is thus a new mem ber in the small family of half integer spin SMMs. T he isolation of complexes 2 1 2 3 emphasizes that comproportionation reaction using different carboxylic acids of varying pK a and steric bulk may allow us access to different manganese clusters

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59 Table 2 1. Crystallographic Data for 2 1 3MeCN, 2 2 MeCN and 2 3 1 / 3 THF 2 / 3 MeCN Parameter 2 1 2 2 2 3 formula a C 54.50 H 97 N 3.50 O 26 Cl 3 Mn 8 C 60 H 108 O 33 Mn 8 C 73 H 133 O 35 Mn 9 fw, g mol 1 a 1763.23 1796.98 2065.25 crystal system Triclinic Monoclinic Orthorhombic space group P P 2 1 / n Aba 2 a 14.045(4) 15.010(2) 20.555(3) b 14.666(4) 21.876(4) 68.895(9) c 22.284(6) 26.275(4) 21.549(3) deg 80.803(4) 90 90 deg 88.478(4) 98.750(8) 90 deg 64.989(4) 90 90 V 3 4101.8(19) 8527(2) 30516(7) Z 2 4 12 T C 173( 2) 173(2) 173(2) radiation, b 0.71073 0.71073 0.71073 calc mg/m 3 1.428 1.400 1.349 mm 1 1.355 1.219 1.150 R 1 c,d 0.0577 0.0369 0.0388 wR 2 e 0.1570 0.0947 0.0846 a Including solvate molecules. b Graphite monochromator. c I > 2 ( I ). d R 1 = (|| F o | | F c ||) / | F o |. e wR 2 = [ [ w ( F o 2 F c 2 ) 2 ] / [ w ( F o 2 ) 2 ]] 1/2 w = 1/[ 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O) + 2 F c 2 ]/3. Table 2 2. BVS for the Mn a and selected O atoms b in 2 1 Mn II Mn III Mn IV BVS Assignment Mn1 3.20 2.99 3.09 O1 2.03 O 2 Mn2 3.18 2.94 3.07 O2 1.98 O 2 Mn3 3.10 2.95 3.02 O3 2.00 O 2 Mn4 3.25 2.99 3.12 O4 2.04 O 2 Mn5 3.13 2.86 3.00 O5 1.12 OH Mn6 4.24 3.88 4.07 O6 2.02 O 2 Mn7 3.14 2.87 3.01 O7 2.04 O 2 Mn8 3.21 2.95 3.08 O21 1.02 OH (pivalic acid) a The bold value is t he 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 bold value. b A BVS in the ~ 1.8 2.0, ~1.0 1.2, and ~0.2 0.4 ranges for an O atom is indicative of non sing le and double protonation, respectively, but can be altered somewhat by hydrogen bonding.

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60 Table 2 3. BVS for the Mn a and selected O atoms b in 2 2 Mn II Mn III Mn IV BVS Assignment Mn1 4.31 3.94 4.14 O1 2.13 O 2 Mn2 4.20 3.84 4.03 O2 2.15 O 2 Mn3 4.25 3.90 4.08 O3 1.80 O 2 Mn4 4.29 3.92 4.11 O4 1.80 O 2 Mn5 4.35 3.98 4.18 O5 2.20 O 2 Mn6 3.31 3.03 3.18 O6 2.20 O 2 Mn7 4.20 3.84 4.03 O7 2.21 O 2 Mn8 3.32 3.04 3.19 O8 2.19 O 2 O9 2.04 O 2 a See footnote a of table 2 2 b See footnote b of tabl e 2 2 Table 2 4. BVS for the Mn a and selected O atoms b in 2 3 Mn II Mn III Mn IV BVS Assignment Mn1 3.29 3.02 3.17 O1 2.14 O 2 Mn2 2.99 2.73 2.87 O6 2.08 O 2 Mn3 3.27 2.99 3.14 O7 2.16 O 2 Mn4 3.59 2.89 3.48 O8 1.84 O 2 Mn5 3.29 3.01 3.16 O9 2.07 O 2 Mn6 3.20 2.93 3.08 O18 2.18 O 2 Mn7 3.28 2.91 3.05 O29 2.17 O 2 Mn8 3.18 2.91 3.05 Mn9 3.24 2.97 3.11 a See footnote a of table 2 2 b See footnote b of table 2 2

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61 Figure 2 1. (top) Labeled representation of the structure of 2 1 Hydro gen atoms have been omitted for clarity. Color code: Mn IV blue; Mn III green; O red; Cl purple; C grey.(bottom) Fully labeled core of 2 1

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62 Figure 2 2. (top) Labeled representation of the structure of 2 2 Hydrogen atoms have been omitted for clarity. C olor code: Mn IV blue; Mn III green; O red; Cl purple; C grey. (bottom) Fully labeled core of 2 2 emphasizing the two open faced cubane units.

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63 Figure 2 3. Labeled representation of complex 2 3 (top) and its core (bottom). Hydrogen atoms have been omitte d for clarity. Color code: Mn III green; O red; C grey.

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64 Figure 2 4. Plot of X M T vs T for complexes 2 1 ( ), 2 2 ( ) and 2 3 ( ). Figure 2 5. Plot of reduced magnetization ( M / B ) vs H / T for complex 2 1 The solid lines are the fit of the data; see tex t for the fit parameters.

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65 Figure 2 6. AC susceptibility data of complex 2 1 in a 3.5 G field oscillating at the indicated frequencies: (top) in phase signal ( X M ) plotted as X M T vs T ; and (bottom) out of phase signal ( X M ) vs T

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66 Figure 2 7. In p hase AC magnetic susceptibility of complex 2 2 (top) and complex 2 3 (bottom) in a 3.5 G field oscillating at the indicated frequencies plotted as X M T vs T

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67 Figure 2 8. Magnetization ( M ) vs dc field hysteresis loops for a single crystal of 2 1 at (to p) the indicated field sweep rates and fixed temperature of 0.04 K and (bottom) at the indicated temperatures and fixed field sweep rate of 0.14 T/s. The magnetization is normalized to its saturation value, M s

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68 CHAPTER 3 SYNTHETIC MODEL OF T HE OXYGEN EVOL VING COMPLEX (OEC) O F PHOTOSYSTEM II ISOLATION OF THE DIS CRETE CUBANE UNIT 3.1 Introduction The oxygen evolving complex ( OEC ) in photosystem II (PS II) present in the thylakoid membrane of plants and internal membranes of cyanobacteria is one of the most fascinating and influential enzyme s 35, 49, 67, 199, 200 It provides the electrons necessary for the sunlight driven reduction of plastoquinone at the start of the photosynthetic electron transfer chain. The elect rons are eventually replenished from the four electron oxidation of water to O 2 gas, which is essentially the source of almost all the oxygen gas on the planet. 201 Nature has figured out a way of coupling the one electron photochemistry of the reaction center with the four electron water oxidation process by sequentially storing the oxidizing equivalents in a series of S states (S n where n = 0 4) of the OEC in the well known Kok cycle. 24 Various spectroscopic and biochemical techniques have bee n employed over the years to determine the structure of the OEC and exactly how it carries out this thermodynamically challenging, water oxidation reaction. 49, 63 66, 202 Prior to the availability of the X ray cry stal structure of PSII, X ray absorption spectroscopy (XAS) and EPR spectroscopy played a pivotal role in deciphering the structural and electronic properties of the OEC. Starting with detection of Mn using XAS, extended X ray absorption fine structure (EX AFS) and X ray absorption near edge spectroscopies (XANES) provided insights into the metal topologies and the distance between the Mn centers. 35, 38, 39 EPR spectroscopy, on the other hand, provided crucial inform ation about the Mn oxidation states as the OEC cycles through the Kok cycle. 203 206 Based on this data, many groups, including ours have been involved in the synthesis

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69 of inorganic model complexes of the OEC, and a plethora of manganese clusters (dimers, trimers and tetramers) have been isolated. 80 82 In 2001, the first X ray structure of the PSII at 3.8 was reported by Zouni et al 68 followed by another at 3.7 in 2003. 69 Both these structures indicated the OEC to be composed fashion. The 3.5 X ray structure of the PSII reported by Ferreira et al in 2004 showed the OEC to be composed of a [Mn 3 CaO 4 ] cubane with a fourth Mn attached to one cubane oxo io n. 70 Although the incorporation of the Ca as an integral part of the OEC was generally accepted, there were still ambiguities about the precise met al topology and structure of this Mn 4 Ca cluster. These ambiguities were mainly due to the low structural resolution and suspected radiation damage to the OEC from X ray exposure. 207 However, the recent structure at 1.9 resolution of PSII by Umena et al in 2011, has provided strong support for the presence of a distorted [Mn 4 CaO 5 ] with an additional Mn attached to the cubane. 79 Despite our growing knowledge of the OEC being a heterometallic [Mn 4 CaO x ] cluster, there are only a handful of Mn/Ca cluster s known in the inorganic literature. In 2005, our group reported the first high oxidation Mn/Ca cluster. Although the nuclearity was much larger than that of the native OEC, it showed, for the first time, the feasibility of isolating such heteronuclear clu sters. 208 Following this, smaller Mn/Ca clusters like Mn 4 Ca 2 209 Mn 4 Ca 210, 211 and [Mn 6 Ca 2 ] n 212 were reported in the literature, but none approached the distorted cubane moiety of the native OEC It was not until recently that Kanady et al. reported the first discrete [Mn 3 Ca] cubane which approaches the cubane fragment of the OEC. 213 Although this work established that the discrete [Mn 3 CaO 4 ]

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70 cubane can be synthetically accessed, the multistep synthesis, lo w yield and the inability to attach another metal to the outside of the cubane indicates the continuing need to develop synthetic analogues of the OEC for detailed insights into the nature and mode of action of this crucial biological site. I nitial foray s into the synthesis of heterometallic Mn/Ca cluster s with primarily carboxylate ligation were quite promising H owever achieving the final target was challenging due to the boundary conditions that we set on our syntheses. Firstly, we avoided the use of N based chelate ligands like terpyridine, 1, 4, 7 triazacyclonane, tris (pyrazole)borate, tris(2 pyridyl)methane and other such ligands which have been previously used to ensure small Mn x we aimed for a totally carboxylate ligation which would mimic the asp/glu ligation of the OEC as closely as possible. Finally, we target ed products with Mn III and/or Mn IV ions to ensure the presence of O 2 bridging ions favored by these oxidation states. In order to satisfy all these criteria, comproportionation reactions between Mn 2+ and Mn 7+ in the presence of Ca 2+ salts under acidic conditions were explored This route in the past was successfully used in the synthesis of a variety of both homo and heterometallic clusters. 149, 161 One of the main advantages of this strategy is that it allows some level of control of the average manganese oxidation state in the reaction (by tuning the Mn 2+ :Mn 7+ ratio ) and consequently the products isolated. In the absence of any chelating ligands, we anticipated that choosing a bulky carboxylate might allow us access to lower nuclearity products. The excess acid would not only prevent the formation o f manganese oxide polymers ( the thermodynamically favored products under such conditions ), but also provide plentiful carboxylate ligands After much

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71 experimentation, the above strategy proved successful, and herein is described the development of syntheti c methodology to the [Mn 3 CaO 4 ] cubane with an external metal attached on the outside. Detailed structural, magnetic and electronic characterization of these complexes and its comparison with the native OEC is reported in this chapter. 3.2 Experimental Sect ion 3.2.1 Syntheses All manipulations were performed under aerobic conditions using chemicals and solvents as received, unless otherwise stated. Mn(O 2 CBu t ) 2 and NBu n 4 MnO 4 were prepared as previously reported in the literature. 168, 169 Warning : Appropriate care should be taken in the use of NBu n 4 MnO 4 and readers are referred to the detailed warning given elsewhere. [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (HO 2 CBu t ) 4 ] ( 3 1 ) : Mn(O 2 CBu t ) 2 2H 2 O ( 0.16 g, 0.5 0 mmol) was di ssolved in hot acetoni trile (25 mL), and the resulting pink slurry was treated with pivalic acid ( 1.89 mL, 16.4 0 mmol ) and CaCl 2 ( 0.06 g, 0.5 0 mmol), which caused the color to change to deep red. The solution was stirred at 80 o C for 15 min, and during this period solid NBu n 4 Mn O 4 ( 0.18 g, 0.5 0 mmol) was added in small portions. The resulting dark brown solution was cooled, filtered and the filtrate was left undisturbe d in a closed vial. After 4 d X ray quality dark brown plate like crystals of 3 1 had formed, and were collected by filtration and dried under vacuum. The yield was 64 %. Anal. Calcd (found) for 3 1 (C 60 H 112 O 28 Mn 3 Ca 2 ): C, 47.21 (47.04); H 7.39 (7.59); N 0.00 (0.00). Selected IR data (cm 1 KBr pellets): 3414 (br), 2973 (m), 2873 (m), 2575 (m), 2362 (m), 1698 (s), 164 5 (s), 1559 (s), 1482 (s), 1412 (s), 1363 (s), 1335 (s), 1220 (vs), 1031 (m), 937 (m), 895 (m), 871 (m), 784 (m), 621 (vs), 533 (m) and 444 (m).

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72 [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (bpy)(HO 2 CBu t )(MeCN)] ( 3 2 ) : To a stirred solution of complex 3 1 ( 0.38 g, 0.25 mmol) in acet onitrile (25 mL) was a dded solid 2,2 bipyridine (0.04 g, 0.25 mmol), and the resulting solution was stirred for 1 h. This was filtered, and the filtrate was allowed to stand undisturbed at 4 o C. X ray quality brown plate like crystals of 3 2 MeCN formed o ver a week, which were isolated by filtration and dried under vacuum The yield was 32 % yield. Anal. Calcd (found) for 3 2 H 2 O (C 57 H 9 5 N 3 O 2 3 Mn 3 Ca 2 ): C, 47.70 (47.72); H 6.6 7 (6.60); N 2.9 2 (2. 5 5). Selected IR data (cm 1 KBr pellets): 3454 (br), 2964 (m), 2927 (m), 2869 (m), 1695 (m), 1632 (vs), 1568 (m), 1511 (m), 1481 (s), 1458 (m), 1401 (s), 1353 (s), 1219 (vs), 1058 (m), 1030 (m),1006 (m), 937 (m), 894 (m), 7 83 (m), 765 (m), 740 (m), 621 (v s), 530 (m) and 443 (s). 3.2.2 X ray Crystallography Data were c ollected at 173 K on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo = 0.71073 ). Suitable crystal s of 3 1 and 3 2 MeCN were attached to glass fiber s using paratone oil and transferred t o a goniostat, where they were cooled to 173 K for data collection. Data collection and unit cell parameters are listed in Table 3 1. Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 scan method (0.3 o 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 index ed crystal faces. The structures were solved by d irect m ethods in SHELXTL6 170 and refined on F 2 us ing full matrix least squares. The

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73 non H atoms were treated anisotropically, whereas the hydrogen atoms were placed in calculated ideal positions and were refined as riding on their respective carbon atoms. Complex 3 1 crystallizes without any solvent molecule s in the lattice and hence the asymmetric unit contains just the Mn 3 Ca 2 cluster. However, there are eight pivalate ligands where the three methyl groups wer e refined in two parts each. One of those was refined in three parts and the last one was refined according to an imaginary mirror plane bisecting the OCO bridge. All four protons on the monodentate pivalic acid groups were calculated in idealized positi ons and were refined riding on their parent O atoms. A total of 807 parameters were refined in the final cycle of refinement using 10918 reflections with I I ) to yield R 1 and wR 2 of 6.35 and 15.75%, respectively. The asymmetric unit of 3 2 MeCN consists of a Mn 3 Ca 2 cluster and one acetonitrile solvent molecule. There are several disorders in the cluster, first of which is the three methyl groups on C7 There are two similar disorders on C27, C32 and C42. In each case the site occupation factors of the major and minor parts were dependently refined to 0.56(2)/0.44(2), 0.90/0.10 (fixed values), and 0.52(1)/0.48(1), 0.613(8)/0.387(8), respectively. The H8 proton was obtained from a d ifference Fourier map and was refined as riding on its parent atom (O8). A total of 820 parameters were refined in the final cycle of refinement using 11563 reflections with I > 2 (I) to yield R 1 and wR2 of 5.40 and 10.73%, respectively. 3.3 Results and Discussion 3.3.1 Syntheses The reaction of Mn(O 2 C t Bu) 2 CaCl 2 and Bu n 4 NMnO 4 in 1 : 1 : 1 ratio in hot acetonitrile in the presence of excess pivalic acid afforded a dark brown solut ion which subsequently led to the isolation of [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (HO 2 C t Bu) 4 ] ( 3 1 ) in 64 % yield

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74 (based on Ca) The formation of 3 1 is summarized in eq 3 1, where the oxidation of Mn 2+ salt by MnO 4 in presence of Ca 2+ and a large excess of acid allowed a ccess to the heteronuclear cl uster 3 Mn 2+ + 4 Ca 2+ + 3 MnO 4 + 24 t BuCO 2 H + 3 e 2 [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (HO 2 C t Bu) 4 ] + 8 H + + 4 H 2 O ( 3 1) The 1:1 ratio of Mn 2+ and Mn 7+ gives an average manganese oxid ation state of +4.5 in the reaction which is reflected in 3 1 where all the manganese are in a high ( +4 ) oxidation state. This reaction is very sensitive to the choice of solvent as well as the manganese and calcium salts. Although there is an excess of pivalic acid in t he reaction the total manganese : acid ratio is crucial for the crystallinity as well as the yield of the final product. When the reaction is performed at room temperature complex 3 1 can still be isolated, but in very low yields and often there is contamination from an intractable mixture of other compounds. For reasons that will be described in detail later in the section, we were interested in synthesizing structurally perturbed derivatives of 3 1 The preferred strategy to induce core distortion is the introduction of strongly coordinating chelates like 2,2' bipyridine (bpy) and 1,10 phenanthroline (phen) just to mention a few. Treating 3 1 with 1 equivalent of bpy in acetonitrile followed by slow evaporation at low temperature provide d us with the first such derivative of [ Mn 3 Ca 2 ] The amount of chelate (bpy) used in this reaction was crucial because we only wanted to impose a structural perturbation rather than a major disruption leading to core fragmentation due to the excess chelate The synthesis of 3 2 is summarized in eq. 3 2 The use of any other solvent in the above reaction has not been fruitful. The low temperature is crucial for crystallizing 3 2

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75 as in room temperature brown solid crashes out from the reaction mixture, the IR spectrum of which slightly different from that of 3 2 The successful preparation of 3 2 suggests that the use of other chelates in the future could be a productive route to other structurally perturbed derivatives. [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (HO 2 C t Bu) 4 ] + bpy + MeCN [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (bpy)(HO 2 C t Bu)(MeCN)] + t BuCO 2 H ( 3 2) 3.3.2 Description of Structures 3.3.2.1 Structure o f [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (HO 2 C t Bu) 4 ] ( 3 1 ) The partially labeled structure of [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (HO 2 C t Bu ) 4 ] ( 3 1 ) is shown in Figure 3 1 and selected interatomic distances and bond angles are summarized in Table A 4 Complex 3 1 crystallizes in the monoclinic space group C2/c. The oxidation state of the manganese atoms and the protonation levels of the O 2 and the oxygen atoms of the carboxylate groups were determined by a combination of charge balance consideration, inspection of bond lengths, and bond valence calculations (Table 3 2). The core of the molecule contains a distorted Mn 3 CaO 4 cubane with an add itional calcium atom attached through one of the oxides of the cube (O4) Thus for the first time the main Mn 3 CaO 4 unit of the OEC has been obtained synthetically in discrete form with an additional metal center attached to the cube atom attached to the cube is calcium (in 3 1 ) as opposed to manganese in the native OEC its mode of attachment to the cubane is the same as in one of the favored crystallographic structure s of the OEC 67 The Mn 3 CaO 4 cubane is not a symmetric unit primarily due the incorporation of Ca 2+ whi ch has a much larger ionic radius compared to Mn 4+ ( 0. 99 vs 0.53 ) This slight distortion is also reflected in the cube angles deviating from an ideal 90 o which range from 92 o 105 o in 3 1

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76 The peripheral ligation is provided by eight 1 : 1 : pivalate groups and four terminal pivalic acid groups; two on each of the calcium centers. All the manganese atoms are hexa coordinated and possess a distorted octahedral geometry, whereas the calcium centers are octa coordinated. 3.3.2.2 Comparison of the structural features of 3 1 with the native OEC The successf ul attainment of the discrete [ Mn 3 CaO 4 ] cubane unit allows testing of the current structural hypothesis for the OEC and also assists in the interpretation of the OEC data from a variety of XAS/EXAFS and other spectroscopic and physical techniques. The Mn Ca separations (3.394 (3) 3.454 (3) ) within the cubane of 3 1 (Figure 3 3) agree with the conclusion from Ca EXAFS studies on the OEC (~ 3.4 ). 73 The attachment of the external Ca to oxide O4 causes a si gnificant lowering of the threefold symmetry of a [ Mn 3 CaO 4 ] cubane. This occurs most notably in the Mn Mn separations, where Mn2 Mn3 (2.857 (1) ) is significantly longer than the others (2.730 (1) and 2.757 (1) ) as shown in Figure 3 3. This ~ 2.74 v s. 2.86 partition of Mn Mn separations in 2:1 ratio is consistent with high resolution Mn EXAFS data on the OEC in the S 2 state, whose short Mn Mn Fourier transform (FT) peaks can be fit to a 2:1 ratio of 2.73:2.82. 214 The lowering of symmetry also has an effect on the Mn Ca1 distances (3.4539 (10) and 3.4178 (10) vs. 3.3942 (10) ), but to a lesser extent. The remaining intermetallic separations and the Mn O and Ca O bond lengths are shown in Figure 3 3. For comparison, the corresponding distances in the recent PSII structure are shown in Figure 3 3 (b). It should be noted that differences in bond distances between these two structures are expected given the greater uncertainty in the distances in the crystal structure of a large PSII multi component assembly and structural perturbations caused by the polypeptide environment. Furthermore, the OEC in the PSII crystal

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77 structure will be in the S 1 state or lower of the Kok cycle. 79 Thus at least one of the cubane Mn atoms will be Mn III leading to slightly longer bond distances on average. Radiation damage of about ~25% ha s also been reported, which would lead to longer Mn ligand, Mn Mn and Mn Ca distances. 207 Apart from the close similarity in the overa ll structural topology and bond distances between 3 1 and the native OEC, the isolation of the discrete [Mn 3 CaO 4 ] cubane unit provides several pieces of information or insights that can help our understanding of this crucial biological site: A) 3 1 supports the feasibility of a [ Mn 3 CaO 4 ] cubane in the OEC which is consistent with the current X ray crystallographic data refinement of the PS II 67, 79 B) There has been a debate regarding the binding of the carboxylate gro ups in the native OEC It has been suggested that the carboxylates bind in a monodentate, terminal mode to maximize electron donation to Mn for stabilizing the cube in the Mn IV 3 Ca oxidation level. 215 However, c omp lex 3 1 shows that this is not necessary to stabilize Mn IV as a ll the carboxylate groups are binding in a 1 : 1 : bridging mode in 3 1 This supports a similar binding mode in the native OEC, although it can not by itself prove the exact binding mode of the asp/glu amino acid residues in the native OEC C) The identity of the ions connecting the metals in the OE C could not be determined in the previous low resolution X ray data of the PSII A lthough the likelihood of oxide/hydroxide ions has long been recognized (and recently strongly supported by high resolution EXAFS and X ray analysis), 67, 78 complex 3 1 supports this hypothesis and also provides precise bond distances and angles. It should be noted that the recent 1.9 PSII structure confirmed the bridging units to be oxo groups and also determined the exact distances between the individual atoms in the native OEC ; 79 however, it cannot distinguish oxide (O 2 ) from hydroxide ( OH ) since protein X ray crystal structures cannot observe H atoms. In conclusion, a lthough complex 3 1 may not reproduce all the structural features of the native OEC, it nevertheless provides the closest synthetic analog of the OEC reported in the litera ture to date.

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78 3.3.2.3 Structure of [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (bpy)(HO 2 C t Bu)(MeCN)] ( 3 2) The partially labeled structure of [Mn 3 Ca 2 O 4 (O 2 C t Bu) 8 (bpy)(HO 2 C t Bu)(MeCN)] ( 3 2 ) is shown in Figure 3 4 and selected interatomic distances and bond angles are summarized in T able A 5 Complex 3 2 crystallizes in monoclinic space group Cc. All the manganese atoms are again in the +4 oxidation state as shown by BVS calculations (Table 3 3) inspection of metric parameters and charge consideration. This complex again contains a M n 3 CaO 4 cubane unit with an additional calcium attached via one of the oxides of the cubane unit, similar to 3 1 The peripheral ligation is provided by eight pivalate groups that are bridging in 1 : 1 : mode s However, the two terminal pivalic acids on t he external calcium are now replaced by bpy and one of the pivalic acid s on the cube Ca has been replaced by an acetonitrile solvent molecule. As mentioned previously, the reaction of 3 1 with chelates like bpy was carried out to induce structural perturb ation in 3 1 and this is evident in the comparison of the metric parameters of the two cores (Tables 3 4 and 3 5) The substitution of the terminal pivalic acids on Ca2 in 3 1 does not affect the Mn Mn and Mn Ca distances (Table 3 4), and maintains th e partition of Mn Mn separations into a 2:1 ratio like 3 1 (2.74 vs. 2.83 ). 214 Instead, t he most significant effect of the bpy on the [ Mn 3 Ca 2 ] core is the elongation of the external Ca oxide (cubane) bond by ~ 0.2 This elongation also affects the Mn O / Ca O bond lengths in the core, either elongating or contracting some bonds by ~ 0.02 while others remain unaffected (Table 3 5) The other prominent effect of the bpy substitution is the distortion of the Ca1 O4/O20 Ca2 angle from 113 to 120 o This in turn affects all the angles centered about this bridging oxide (O4/O20) (Table 3 6) However, the Mn O Mn angles within the cube are not significantly affected by the bpy substitution. Since the bpy substitut ed on the external Ca, it is likely that

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79 replacing the labile pivalic acid groups on the cube Ca with chelates would impart a greater perturbation compared to that in 3 2 3.3. 3 Mn and Ca X ray Absorption Spectroscopy (XAS) The Mn and Ca XAS spectra were c ollected by the Yachandra group at the Lawrence Berkeley National Laboratory Detailed experimental procedure listed in Appendix C. Mn and Ca XAS ha s provided important insights into the nature of the OEC. The formal oxidation state of the S 0 S 1 and S 2 st ates are generally accepted as Mn III 3 Mn IV Mn III 2 Mn IV 2 and Mn III Mn IV 3 respectively. 35, 216 Figure 3 5 compares the Mn XANES (X ray absorption near edge spectroscopy) spectra of the OEC in the S 0 through S 3 states with the spectrum of the Mn 3 Ca 2 complex ( 3 1 ) The formal oxidation state of 3 1 is Mn IV 3 Ca 2 and its edge position falls between the S 2 and S 3 states. Interestingly, the shape of the edge of complex 3 1 is similar to that seen for the OEC. Of the many mul tinuclear Mn complexes that have been synthesized, this kind of similarity with the OEC has only been observed previously for the distorted Mn 4 cubane like complexes Mn III 4 Mn IV which like complex 3 1 also have mostly non aromatic O based ligands. 217 Both 3 1 and 3 2 have all the Mn in +4 oxidation state, which creates the similarity in the edge position in the XANE S spectrum. Substitution of the labile pivalic acid groups on the external Ca (Ca2) does not affect the edge position for the 3 2 as is evident from the Mn XANES spectrum in Figure 3 6. This is not unexpected as the edge position is sensitive to the Mn oxi dation state and the edge position shifts as the OEC cycles through the Kok cycle. Mn EXAFS (extended X ray absorption fine structure) studies of the OEC in the S 1 and S 2 states have established the presence of two Mn Mn distances at ~2.7 and

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80 one Mn Mn distance of ~2.8 which is characteristic of di oxide bridged Mn 2 pairs with mostly O based ligation at 1.8 2.0 The Mn EXAFS also shows a longer distance interaction, assigned to one Mn Mn distance at 3.3 and Mn Ca at ~3.4 Further supp ort for a Mn Ca at ~3.4 comes from Sr EXAFS studies on PSII in which the calcium was replaced with strontium. In that work, four Mn Sr(Ca) distances in the 3.4 3.9 range were detected 218 These Mn Mn an d Mn Ca distances are known to be particularly sensitive to the S state i n which the OEC is present. Figure 3 7 shows the comparison of the Mn and Ca EXAFS of the OEC in the S 1 state with Mn 3 Ca 2 complex 3 1 The comparison of the Mn EXAFS in Fig ure 3 7 a shows that there are similarities in the beat pattern but there are many differences in the phase, frequencies and intensity of the spectral features. The FT in Fig ure 3 7 b shows that there is substantial similarity in the features, except for the differe nce in the intensity of the FT peak at ~1.8 2.0 i.e. from Mn ligand distances. The similarity in the FT peak at 2.7 2.8 is because of the presence of three Mn Mn distances at 2.7 2.8 in both the OEC and complex 3 1 The FT peak at ~3.3 is also s imilar between the OEC and 3 1 because of the presence of Mn Mn/Ca distances of ~3.3 in both systems. The differences in the spectra are not surprising because of the presence of the extra (external) Ca atom in Mn 3 Ca 2 that is not present in the OEC (Mn 4 Ca). Fig ure s 3 7 c and 3 7 d show the Ca EXAFS and the FTs of the OEC in the S 1 state compared with that from 3 1 The Ca EXAFS spectra in Fig ure 3 7 c show s just as in the Mn EXAFS, that there are similarities between the [Mn 3 Ca 2 ] cluster and the OEC H owe ver, there are still differences in frequency and intensity. The FTs show that the OEC and 3 1 both have similar Ca Mn distances including the first FT peak from Ca O distances, and the

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81 second FT peak at ~3.3 from Ca Mn distances. The differences are not surprising because of the presence of the extra Ca atom in the complex. Overall, the similarities in both the Mn and Ca EXAFS and FTs show that complex 3 1 has many of the features, especially the Mn Mn and Mn Ca distances, that are present in the OEC. This degree of similarity in the spectra between the OEC and a model complex is unique and has not been seen in comparisons with the myriad of Mn complexes that have been synthesized to date. Mn and Ca EXAFS Fits : The EXAFS fitting was carried out us ing the information about the distances and the coordination numbers obtained from the crystal structure of the Mn 3 Ca 2 complex 3 1 The fits are shown in Fig ure 3 8. I n the Mn EXAFS, the fit quality is satisfactory even with a four shell fit where only the main interactions were used (Table 3 7 Fit#1). Addition of light atom contributors such as Mn O and Mn C for > 2 nd coordination sphere further improved the fit quality (Table 3 7 Fit#2). In the Ca EXAFS, the fit quality did not improve by the addition o f light atom contributors (Table 3 8 Fit#1 vs Fit#2). The overall profile of both the Mn and Ca EXAFS data of 3 2 is very similar to that of 3 1 as is evident in Figure 3 9. The Mn EXAFS of 3 2 exhibits two Mn Mn distances at ~ 2.7 and one Mn Mn dis tance at ~ 2.8 similar to 3 1 The longer Mn Mn distance at 3.3 and the Mn Ca distance at 3.4 are also shown in Figure 3 9. Thus the substitution of the labile pivalic acids on the external Ca does not affect the Mn Mn/Ca distances within the c ore significantly. It is now evident that in order to induce a higher distortion of the Mn 3 Ca 2 core, ligand substitution on the cube Ca is

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82 required. Although complex 3 2 did not display a strong structural perturbation, its isolation paved the way for isol ating other structural derivatives of 3 1 3.3. 4 Magnetochemistry of Complexes 3 1 and 3 2 3.3. 4 .1 DC s tudies Solid state, variable temperature DC magnetic susceptibility data in a 0.1 T field were collected on powdered microcrystalline samples of 3 1 and 3 2 restrained in eicosane, in the 5.0 300.0 K range. The 300 K X M T value of 8.01 cm 3 Kmol 1 is greater than the spin only ( g = 2) value of 5.63 cm 3 Kmol 1 expected for three non interacting Mn IV ions. The X M T value of 3 1 increase with decreasing temper ature a nd reaches a maximum of 11.94 cm 3 Kmol 1 at 15 K and then drops slightly to 11.72 cm 3 Kmol 1 at 5 K. The low temperature data suggests an S = 9 / 2 ground state with g < 2 as expected for Mn 4+ system. Th e increase of X M T with decreas ing temperature sug gests that the low lying excited states are of low spin values and that some or all of the exchange interactions between the manganese centers are ferromagnetic. The individual exchange parameters (J) between the Mn Mn pairs within the molecule w ere obtai ned by fitt ing the X M T vs T data of 3 1 to the appropriate theoretical expression for a Mn IV 3 triangle. On the basis of the structure, the Mn IV 3 triangle in 3 1 is isosceles, and thus requir es two different Mn 2 pairwise exchange interaction s (J and J') (Fi gure 3 11) The Heisenberg spin Hamiltonian for t his exchange scheme is given by eq. 3 3, where S i (i = 1 3) is the spin of metal M i (labeling as in Figure 3 1 ), and S 1 = S 2 = S 3 = 3/2 for Mn IV which is an isotropic t 2g 3 ion in octahedral symmetry. H = 2J( 1 2 + 1 3 ) 2 3 ) ( 3 3)

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83 The eigen values of the spin Hamiltonian of eq. 3 3 can now be determined analytically using the Kambe vector coupling method as described elsewhere. 219 Using the coupling scheme A = 2 + 3 and T = 1 + A the spin Hamiltonian can be transformed into the equivalent form given by eq. 3 4, where T is the total spin of the molecule. H = J( T 2 A 2 1 2 ) J'( A 2 2 2 3 2 ) ( 3 4) The eigen value of eq. 3 4 can be determined using the relationship i 2 = S i ( S i + 1) and is given in eq. 3 5, where E S T S A S T S A >, and the constant terms contributing equally to all the states have been omitted. E S T S A > = J[ S T ( S T + 1) S A ( S A + 1)] J'[ S A ( S A + 1)] ( 3 5) The overall multiplicity of the spin system is 64, and is made up of 12 individual spin states ranging from S T = 9 / 2 1 / 2 (Table 3 9) An expression for the molar magnetic susceptibility X M was derived using eq. 3 5, the Van Vleck equation (eq. 3 6) and by assuming an isotropic g tensor. 220 This equation was then used to fit the experimen tal X M T vs T (Figure 3 10 ), as a function of the two exchange parameters (J and J') and g A contribution from the temperature independent paramagnetism (TIP) was held constant at 300 x 10 6 cm 3 mol 1 (3 6) The fitting parameters were as follows: J = +40.5 (1.1) cm 1 J' = 10.8 (7) cm 1 and g = 1.960 (2). Using these values an energy ladder can be created (Figure 3 12 ) which confirms a | 9 / 2 3> ground state for 3 1 and the first excited state is | 7 / 2 2>, located 56.7 cm 1 above the ground state.

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84 Strong ferromagnetic co upling between oxo bridged Mn IV ions, while not unprecedented is extremely rare and can be assigned to the acute (< 97 o ) Mn O Mn angle in 3 1 86 On the other hand a slightly larger angle makes J' antiferromagnetic (Figure 3 13 ). These large differences i n coupling with such small changes in angle are dramatic, but J in oxo bridged metal complexes is indeed known to be sensitive to M O M bridge angle, particularly in the 90 100 o range where the J value can even change sign as seen in 3 1 110 The parallel spin alignments giving an S = 9 / 2 ground state are clearly a result of | J | >> | J' | which lea ves the weaker J' frustrated As has been previously observed in many spin frustrated systems, the ground state is known to be sensitive to slight structural distortions that affect the relative magnitude of competing interactions 136, 217, 221 We have thus computed a spin state energy manifold vs J/J' using eq. 3 5, to explore the change in the ground state ( S ) as a function of the J/J' ratio. The experimental J/J' = 3.75 (shown as a dashed line in Figure 3 14 ) sh ows an S = 9 / 2 ground state is expected at this ratio, as concluded above In spin frustrated systems, different ground state spin s are often possible depending on the exact ratio of the exchange interactions between the metal centers. 221 In Figure 3 14 which shows just the region for J > 0 and J' < 0, i t is clearly evident that substantial changes in ground state are possible from relatively minor changes in the strength of the various int eractions. For example, the ground state can vary from | 9 / 2 3> to | 3 / 2 0> by varying the J/J' ratio by only 4 units. When J >> J' and |J/J'| > 2, the ground state is the | 9 / 2 3> state, that arises from J completely overwhelming J', and aligning the spin s on the three MnIV parallel to each other. This is depicted in Figure 3 14b, where the antiferromagnetic J' is completely frustrated. As |J'| increases in comparison to |J|,

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85 the spins on Mn2 and Mn3 are no longer perfectly parallel, but, at some intermedi ate alignment which is determined by the relative magnitudes of J and J'. When 2 > |J/J'| > 1.3, the ground state is | 7 / 2 2 > and when ~ 1.3 > |J/J'| > ~ 0.6, the ground state is | 5 / 2 1 > (Figure 3 14). Finally, when J << J' and ~ 0.6 > |J/J'| > 0, the J interaction is completely frustrated, the spins on Mn2 and Mn3 are perfectly antiparallel, and the ground state is thus | 3 / 2 0 > (Figure 3 14). We were thus interested in synthesizing structurally perturbed derivatives of 3 1 to test if the ground state i n th is system can be flipped. For 3 2 t he 300 K X M T value of 8.43 cm 3 Kmol 1 is again greater than the spin only value expected for three non interacting Mn IV ions. Like 3 1 the X M T value increases with decreasing temperature until it reaches a maximum va lue of 11.55 cm 3 Kmol 1 at 25 K followed by a slight drop to 11.30 cm 3 Kmol 1 at 5 K (Figure 3 15 ). The low temperature data is consistent with an S = 9 / 2 ground state, with g < 2 as expected for Mn IV system. Since the magnetic core of 3 2 has the same Mn IV 3 triangu lar topology as 3 1 we used the same isosceles triangle model to obtain the exchange interactions (J and J'). As described in the previous section, the experimental X M T vs T data w ere fit using the Van Vleck equation to obtain the exchange parame ters in 3 2 220 The fit is shown as the solid line in Figure 3 15 which gave J = 43.32 (1.3) cm 1 J' = 3.12 (1.2) cm 1 g = 1.93(2) with the TIP kept constant at 300 x 10 6 cm 3 mol 1 The energy ladder sho ws that the first excited state is still | 7 / 2 2> but it is now located 111 cm 1 above the | 9 / 2 3> ground state (Figure 3 15 ) which is almost twice the value for 3 1 Thus, a lthough the exchange parameters in 3 2 are different from 3 1 we were not able to flip the ground

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86 state in this Mn 3 Ca 2 derivative. Substitution of the labile pivalic acid groups on the external Ca with bpy does induce some distortion to Mn 3 Ca 2 structure; however these are localized on one side of the cube only and the magnetic core is not affected to a large extent. Comparing the exchange pathways in 3 1 and 3 2 shows that in 3 2 J' pathway became more antiferromagnetic whereas, the J pathway remains the same. Thus, no flip in the ground state was observed due to this structural per turbation; however, had the J' become more antiferromagnetic, maybe the ground state would have flipped. Efforts to induce a greater structural distortion of the magnetic core via replacement of the labile groups on the cube Ca are clearly necessary. To pr obe the ground state of 3 1 and 3 2 further, and to determine the zero field splitting parameter ( D ), magnetization ( M ) vs field ( H ) data were collected in the magnetic fiel d and temperature ranges of 0.1 7 T and 1.8 10 K, respectively. The resulting data are plotted in Figure 3 16 as reduced magnetization ( M / N B ) vs H / T where N B is the Bohr magneton. The data were fit, by diagonalization of the spin Hamiltonian matrix, using the program MAGNET 196 which assumes only the ground state to be populated at these temperatures and fields, includes the Zeeman interactions and axial zero field splitting ( z 2 ), and incorporates a full powder average. The corresponding spin Hamiltonian in given by eq. 3 7 where z is the easy axis spin operator, g is the Land g factor and o is the vacuum permeability. H = z 2 + g B o H ( 3 7 ) The last term in eq. 3 7 is the Zeeman energy term associated with the applied magnetic field. The best fit is shown by the solid lines in Figure 3 16 and was obtained with S = 9 / 2 and ei ther of the two sets of parameters: g = 1.91 (2) and D = 0.15 (6)

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87 cm 1 ; and g = 1.91 (2) and D = 0.25 (4) cm 1 It is common to obtain two acceptable fits of magnetization data for a given S value, since magnetization fits are not very sensitive to th e sign of D To identify the superior fit and to ensure that the true global minimum ha d been located, we calculated the root mean square error surface for the fits as a function of D and g using the program GRID 222 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 two dimensional contour plot (Figure 3 16 ), and the fit with the positive D was fou nd to be superior, thereby suggesting this to be the true sign of D It should be noted that the error surface only indicates the precision of the fit. More a ccurate D and g values can be obtained from EPR studies, which are reported later in the section. The reduced magnetization plot saturates at 8.54 for 3 2 suggesting an S = 9 / 2 ground state and g < 2. The best fit is shown as the solid lines in Figure 3 17 and was obtained with S = 9/2 and either of the two sets of parameters: g = 1.90 (5) and D = 0.13 (3) cm 1 ; and g = 1.91 (5) and D = 0.22 (4) cm 1 To assess the superior fit, root mean square error surface for the fits were again calculated as a function of D and g The two dimensional contour plot (Figure 3 17 ) shows the fit with positive D t o be superior, confirming this as the true sign of D 3.3. 4 .2 AC studies An ac magnetic susceptibility study was performed to independently confirm the ground state and probe the dynamics of magnetization (magnetic moment) relaxation. The ac studies on 3 1 and 3 2 were performed in the 1.8 15.0 K range using a weak 3.5 G ac field oscillating at frequencies of 50 1000 Hz. The in phase ( X M ) ac

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88 susceptibility is invaluable for assessing the ground state without any complications from a dc field. For bot h 3 1 and 3 2 t he X M T essentially remains temperature independent with decreasing temperature (Figure 3 18 ), indicating the ground state to be well isolated. Extrapolation of the data to 0 K gives a X M T value ~ 12 cm 3 Kmol 1 which is consistent with an S = 9 / 2 ground state with g < 2 for both 3 1 and 3 2 3.3. 5 EPR Spectroscopy The X band and Q band EPR spectra were collected by the Britt group at the CalEPR center at the University of California at Davis. All EPR samples of complex es 3 1 and 3 2 were di ssolved in MeCN/CH 2 Cl 2 (1:1, v/v) to a concentration of 1 mM. Perpendicularly polarized CW X band (9 GHz) spectra were collected using a Bruker model ECS106 spectrometer equipped with a standard mode cavity. All CW X band spectra were collected at 10 K und er non saturating slow passage conditions. Temperature control was maintained with an Oxford Instruments model ESR900 helium flow cryostat with an Oxford ITC 503 temperature controller. Q band (34 GHz) pulsed EPR spectra were acquired with a Bruker EleXsys E580 spectrometer using either an EN 5107D2 Q band EPR/ENDOR probe or a laboratory built TE011 brass cavity and coupler following a standard design. 223 The laboratory built probe was also used to acquire the Q band field swept spectrum. The resonator was modified from previous applications, for an Oxford CF935 cryostat. 224 Electron spin echo (ESE) detected electron nuclear double resonance (ENDOR) spect ra were obtained using the Davies ENDOR sequence ( MW RF t /2 MW MW ) with stochastic sampling of the RF excitation frequencies, 225 and an Amplifier Research model 253 (250 W) RF amplifier. Additional spectrometer settings used were: = 230 ns; /2 MW = 32 ns; MW

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89 RF spectral simulations were performed using Matlab 7.8.0 and the EasySpin 3.1.7 package. 226 227 The low temperature EPR spectrum of complex 3 1 is complicated due to the zero field splitting (ZFS) of the five Kramers doublets of the S T = 9 / 2 spin system (Fig ure 3 19 ). The low temperature (5 K) CW EPR spectrum of a microcrystalline sample of c omplex 3 1 possesses almost identical spectral features to those from a sample that has been dissolved in a MeCN/CH 2 Cl 2 solvent mixture. This suggests that the complex retains its structure on dissolution, with little changes to its electronic properties ( Fig ure 3 20 ). The X (9.375 GHz) and Q band (34.188 GHz) EPR spectra at 5.0 K of a solution of 3 1 (Fig ure 3 21 ) are characterized by many resonances across a large field range. This is diagnostic of zero field splittings between the five Kramers doublets of the S T = 9 / 2 spin system that are on the order of the microwave excitation energy (h and 1.1 cm for X and Q band, respectively). The effective ZFS constants ( D = +0.073 cm 1 E = 0.005 cm 1 ) were determined by simultaneously fitting spectra acquired at X and Q band. Based on the temperature dependence of the EPR spectrum, the sign of D was found to be positive. 228 Additional splittings from the hyperfine interactions (HFI) of the three 55 Mn ( I = 5 / 2 ) nuclei are obscured in the inhomogeneously broadened EPR line. These 55 Mn HFI were probed using Davies electron nuclear double resonance (ENDOR) and the spectrum collected at 1144.2 mT (Fig ure 3 22 ) reveals 8 peaks in the 0 200 MHz frequency range. The relative peak intensities change as the field is varied

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90 due to each experiment being resonant to a different degree with some of the overlapping EPR transitions ( Figure 3 23 ). For each 55 Mn nucleus, a set of two ENDOR peaks ce ntered at A m S L ( 55 Mn) = 26.2 MHz at 1200 mT) is expected. Each of these peaks will be further split into sextets due to the nuclear quadrupole interaction (NQI). Although these individual transitions are unlikely to be resolved, they will manifest as an m S dependent broadening of each ENDOR line. 229 To model this behavior in our simulations, NQI parameters P and were respectively set to 2.5 MHz and 0.2, which are the intermediate of values found for Mn IV centers in other exchange coupled clusters. 212, 230 To further simplify the analysis of the ENDOR spectrum, the isosceles triangle model was applied. Two classes of hyperfine coupled Mn centers were considered, and the best fit of all the field dependent ENDOR data yielded the following set of effective hyperfine parameters: for the two equivalent Mn nuclei, A 2,3 = [61, 61, 63] MHz and A 1 = [61, 61, 57] MHz for the unique Mn center (numbering scheme same as that in Fig ure 3 1 ). Because the total effective spin ( S T = 9 / 2 ) of 3 1 is different from that of the isolated ions ( S of Mn IV is 3 / 2 ), the observed HFI must be scaled by an appropriate projection factor. With J >> D (i.e. strong exchange coupling limit), the projection factors for each Mn IV ion are simply 1 / 3 As a result, the magnitude of the site specific HFI are a 2,3 = [183, 183, 189] MHz and a 1 = [183, 183, 1 71] MHz These Mn IV hyperfine values in particular, the low value of the hyperfine anisotropy compare well to the literature (Table 3 10 ). 231 234 The slightly smaller values for the isotropic part of the hyperf ine c an be attributed to spin polarization effects 235 and strong covalent bonding with the

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91 carboxylates This complete s the coordination sphere of the Mn IV ions, leading to reduced metal hyperfine interactions and draw s spin densit y away from the Mn ions. The X and Q band spectra of complex 3 2 in Figure 3 24 reveal remarkable similarity to the low temperature CW spectra of 3 1 Simulations of the experimental data g ave a slight ly lower g value compared to 3 1 but the value was still lower than the free electron value. EPR simulations of the zero field splitting (ZFS) contributions responsible for the complex spectrum were found to be larger for 3 2 than complex 3 1 However, mu ch larger strain in the ZFS is needed to fit the spectra suggesting greater inhomogeneity in the sample. The temperature dependence reveals much less dramatic changes in the spectral features as the temperature is increased, signifying a much larger splitt ing between the S = 9 / 2 ground state and the excited S = 7 / 2 state. The population of the S = 7 / 2 excited state also suggests that D is positive. 3.3. 6 Electrochemistry Complex 3 1 and 3 2 have been investigated by cyclic voltammetry (CV) to probe the redox properties of these systems. The CV of complex 3 1 was recorded in acetonitrile and that of 3 2 was recorded in dichloromethane ; both solvents contain ed 0.1 M n Bu 4 PF 6 as supporting electrolyte, at 100 mVs 1 scan rate. The f errocene/ferroce nium couple was used as the internal standard. Glassy carbon was used as the working electrode, Ag wire served as the reference electrode and Pt wire was used as the auxiliary electrode. Both the complexes displayed broad peaks in the CV which are indicati ve of irreversible processes. Complex 3 1 shows two irreversible reduction peaks at 427 and 1125 mV. It was difficult to compare the effect of chelate binding on the redox property of the Mn 3 IV core due to the poor solubility of 3 2 in acetonitrile. Like 3 1 3 2 also displays two irreversible reduction peaks at 716 and

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92 1127 mV These peaks can be tentatively assigned to the reduction of Mn 4+ to Mn 3+ and the irreversibility in the peaks indicates the instability of these complexes at these oxidation st ates under the experimental conditions. 3.4 Conclusions This work demonstrated our initial entry and development of Mn/Ca cubane chemistry. Inspired by the recent X ray structure of PSII showing the OEC to be composed of a [Mn 4 CaO 5 ] unit, we were able to d evelop synthetic strategies to isolate an asymmetric [Mn 3 CaO 4 ] carboxylate cubane with an additional Ca attached to it. The isolation of this low nuclearity, high valent, heterometallic cubane unit using simple carboxylate ligation indicates that this Mn/C existence and stabilization to large polypeptide matrix within the PSII. Comparison of both EXAFS and XANES data of 3 1 with the native OEC has shown striking similarities establishing 3 1 as the closest synthet ic analog reported in the literature till date. Furthermore, the attachment of the neutral pivalic acid groups to each Ca atoms is intriguingly reminiscent of the terminal solvent derived ligands attached to the cubane Ca and external Mn atom seen in the r ecent 1.9 structure of the OEC. 79 The presence of the diamagnetic external Ca is advantageous in providing l ow symmetry structural analogues of the OEC and also allows probing of the intrinsic magnetic property of the Mn 3 Ca cube without interference from the external paramagnetic Mn ion. Detailed magnetic analysis of 3 1 revealed an S = 9 / 2 ground state arising from competing interactions between coupling constants. This results in a spin frustrated system where the ground state is sensitive to small structural perturbations. A similar situation may be operating in the native OEC where the observed ground state f rom EPR techniques of a particular S n state is sensitive to the

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93 conditions used to generate that particular state. 236, 237 Furthermore, recent EPR studies suggests an S = 3 ground state for the S 3 state of the OEC, 61 which could arise from the [Mn IV 3 CaO 4 ] cubane ( S = 9 / 2 ) antiferromagnetically coupled to an external Mn IV ( S = 3 / 2 ). In an attempt to flip the ground state of 3 1 structural perturbation of the Mn 3 Ca 2 core via the use of a chelating ligand (bpy) was targeted. Introduction of bpy affected some metric parameters but the distortions were not strong enough to flip the S = 9 / 2 ground state. Although our initial attempt was unsuccessful, this result paved the way for isolating other structural derivatives of 3 1 Both 3 1 and 3 2 represent the c losest syn thetic analog of the OEC This is an important breakthrough which has provided some crucial information about the OEC and provides deeper insights into the nature and mode of action of this crucial biological site.

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94 Table 3 1. Crystallographic Data for Complexes 3 1 and 3 2 MeCN. Parameter 3 1 3 2 formula a C 60 H 112 O 28 Mn 3 Ca 2 C 59 H 96 N 4 O 22 Mn 3 Ca 2 fw, g mol 1 a 1526.48 1458.38 crystal system Monoclinic Monoclinic space group C 2/c C c a 47.903 (4) 15.5757 (7) b 15.4607 (11) 23.1355 (11) c 23.4133 (17) 20.3491 (9) deg 90 90 deg 111.8440 (10) 97.328 (1) deg 90 90 V 3 16095 (2) 7272.9 (6) Z 8 4 T C 173 (2) 173(2) radiation, b 0.71073 0.71073 calc mg/m 3 1.260 1.332 mm 1 0.662 0.724 R 1 c,d 0.0635 0.0540 wR 2 e 0.1575 0.1073 a Including solvate molecules. b Graphite monochromator. c I > 2 ( I ). d R 1 = (|| F o | | F c ||) / | F o |. e wR 2 = [ [ w ( F o 2 F c 2 ) 2 ] / [ w ( F o 2 ) 2 ]] 1/2 w = 1/[ 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O) + 2 F c 2 ]/3.

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95 Table 3 2. BVS for the Mn a and selected O atoms b in 3 1 Mn II Mn III Mn IV BVS Assignment Mn1 4.23 3.87 4.06 O1 2.13 O 2 ( 3 ) Mn2 4.18 3.82 4.01 O2 1.88 O 2 ( 3 ) Mn3 4.11 3.77 3.95 O3 2.00 O 2 ( 3 ) O4 2.01 O 2 ( 4 ) O14 1.57 OH (pivalic acid) O16 1.27 OH (pivalic acid) O26 1.20 OH (pivalic aci d) O28 1.16 OH (pivalic acid) a The bold 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 bold value. b A BVS in the ~ 1.8 2.0, ~1.0 1.2, and ~0.2 0.4 ranges for an O atom is indicative of non single and double protonation, respectively, but can be altered somewhat by hydrogen bonding. Table 3 3. BVS for the Mn a and selected O atoms b in 3 2 Mn II Mn III Mn IV BVS Assignment Mn1 4.15 3.80 3.99 O19 1.95 O 2 ( 3 ) Mn2 4.16 3.80 3.99 O20 1.93 O 2 ( 4 ) Mn3 4.17 3.81 4.00 O21 2.19 O 2 ( 3 ) O22 1.98 O 2 ( 3 ) O8 1.14 OH (pivalic acid) a See footnote a of table 3 2 b See footnote b of table 3 2 Table 3 4. Comparison of Mn Mn and Mn Ca distances in 3 1 and 3 2 3 1 [Mn 3 Ca 2 ] 3 2 [Mn 3 Ca 2 bpy] n a Mn Mn 2.743 (8) b 2.739 (8) b 2 Mn Mn 2.857 (8) 2.832 (9) 1 Mn Ca 3.422 (10) b 3.406 (10) b 3 a n is the number of such separations b average values

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96 Table 3 5. Compar ison of metal oxide bond distances in 3 1 and 3 2 [Mn 3 Ca 2 ] ( 3 1 ) [Mn 3 Ca 2 bpy] ( 3 2 ) Ca2 O4 2.361 (3) Ca1 O20 2.545 (3) Ca1 O4 2.660 (3) Ca2 O20 2.692 (3) Mn2 O4 1.866 (3) Mn2 O20 1.876 (3) Mn3 O4 1.889 (3) Mn3 O20 1.858 (3) Mn1 O3 1.820 (3) Mn1 O19 1. 830 (3) Mn1 O2 1.844 (3) Mn1 O22 1.821 (3) Mn2 O2 1.862 (3) Mn2 O22 1.847 (3) Mn3 O1 1.892 (3) Mn3 O21 1.876 (3) Ca1 O2 2.452 (3) Ca2 O22 2.432 (3) Table 3 6. Comparison of metal oxo metal bond angle in 3 1 and 3 2 [Mn 3 Ca 2 ] ( 3 1 ) [Mn 3 Ca 2 bpy] ( 3 2 ) Ca2 O4 Ca1 113.28 (10) Ca1 O20 Ca2 121.83 (11) Mn3 O4 Ca2 125.23 (12) Mn3 O20 Ca1 116.71 (14) Mn2 O4 Ca1 97.93 (10) Mn2 O20 Ca2 94.49 (11) Mn3 O4 Ca1 95.92 (10) Mn3 O20 Ca2 97.12 (11) Mn1 O3 Ca1 103.55 (12) Mn1 O19 Ca2 101.78 (13) Mn3 O4 Ca1 95.92 ( 10) Mn3 O20 Ca2 97.12 (11) Table 3 7. Mn EXAFS curve fitting for complex 3 1 Fit# Path R N 2 R (%) 1 MnO 1.89 6 .0 0.005 8.2 E 0 = 6.7 MnMn 2.78 2 .0 0.00 4 MnCa 3.36 1 .0 0.003 MnCa 3.78 0.7 0.00 3 2 MnO 1.89 6 .0 0.00 5 5.2 E 0 = 7.5 MnMn 2.75 1.3 0.002 MnMn 2.88 0.7 0.002 MnC 2.84 2.7 0.005 MnCa 3.3 6 1 .0 0.003 MnCa 3.77 0.7 0.003 MnO 3.0 5 3.3 0.00 3

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97 Table 3 8. Ca EXAFS curve fitting for complex 3 1 Fit# Path R N 2 R (%) 1 CaO 2.42 7 0.009 2.4 E 0 = 1.8 CaMn 3.3 9 1.5 0.006 CaC 3.14 5.5 0.016 CaMn 3.77 1 .0 0.018 2 CaO 2.41 7 .0 0.009 2.9 E 0 = 2.1 CaMn 3.39 1.5 0.007 CaC 3.29 5.5 0.045 CaMn 3.77 0.5 0.007 CaOC 3.56 10 .0 0.011 CaO 3.88 5 .0 0.014 Table 3 9. Distribution of spin stat es for complex 3 1 S T n a n (2 S T + 1) b 9 / 2 1 10 7 / 2 2 16 5 / 2 3 18 3 / 2 4 16 1 / 2 2 4 a number of individual states with a particular S T value b n (2 S T + 1)] = 64, the overall multiplicity of the spin system.

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98 Table 3 10. 55 Mn hyperfine coupling tensors. A iso [MHz] References A iso [MHz] References [Mn IV ] 185 This work [5] 207 234 [Mn IV' ] 179 This w ork [6] 213 234 [1] 190 232 [Mn cat] 238 234 [2] |183| 231 Mn 4 O x Ca 191 251 a 238 [3] 218 234 Mn 4 O x Sr 173 243 a 238 [4] 221 233 1 : [Mn IV H 3 buea(O)] [H 3 buea] = tris[ tert butylureaylato) N ethylene]aminato); 2 : O)MnL] 3+ L = N,N bis(2 pyr idylmethyl) salicyliden 1,2 diaminoethane); 3 : [bpy) 2 O) 2 Mn(bpy) 2 ] 3+ 4 : [(phen) 2 O) 2 Mn(phen) 2 ] 3+ 5 O) 2 OAc)Mn] 2+ dtne = 1,2 bis(1,4,7 triazacyclonon 1 yl)ethane); 6 : [(tacn)Mn 2 O) 2 OAc)(tacn)] 2+ tacn = 1,4,7 triaz acyclononane); [(tacn)Mn 2 OAc) 2 Mn(tacn)] 3+ ; Mn Cat : manganese catalase; Mn 4 O x Ca OEC from T. elongatus ; Mn 4 O x Sr OEC from T. elongatus a The range of effective isotropic hyperfine interaction reported for the three Mn IV ions in the S 2 state of the OEC determined using 55 Mn ENDOR at Q band. Figure 3 1. Labeled representation of the structure of 3 1 The CH 3 groups of the pivalate ligand have been omitted for clarity. Color code: Mn IV blue; Ca II yellow; O red; C grey. The [Mn 3 CaO 4 ] cubane is emphas ized with green bonds.

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99 Figure 3 2. Comparison of the core of 3 1 with crystallographically derived core of the native OEC. (a) Fully labeled core of 3 1 and (b) The 2004 [Mn 4 CaO 4 ] proposal by Ferreira et al. of a cubane with the external Mn attached to a cubane oxide ion. 70 Figure 3 3. (a) Bond distances in 3 1 1.9 PSII structure by Umena et a l. atom labels as in reference 79 and (c) metal metal separations in 3 1

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100 Figure 3 4. Labeled representation of the structure of 3 2 The CH 3 groups of the pivalate ligand has been omitted for clarity. Color code: Mn IV blue; Ca II yellow; N cyan; O red; C grey. The [Mn 3 CaO 4 ] cubane is emphasized with green bonds.

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101 Figure 3 5. Mn XANES from spinach PS II in the S 0 S 1 S 2 and S 3 states compared with the spectrum from the Mn IV 3 Ca 2 complex 3 1 Figure 3 6. Mn X ANES from spinach PS II in the S 1 state compared with the spectrum from the Mn IV 3 Ca 2 bpy complex 3 2

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102 Figure 3 7. a) and b) Th e Mn EXAFS and the Fourier transforms of the OEC in the S 1 state, compared with complex 3 1 c) and d) The Ca EXAFS and the Fourier transforms of the OEC in the S 1 state compared with complex 3 1

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103 Figure 3 8. a) and c) are Mn and Ca EXAFS data from 3 1 and the best fits. b) and d) are the FTs of the Ca EXAFS from the complex and the best fits. The fit parameters are shown i n Tables 3 7 and 3 8

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104 Figure 3 9. a) and b) The Mn EXAFS and the Fourier transforms of the OEC in the S 1 state, compared with complex 3 2 c) and d) The Ca EXAFS and the Fourier transforms of the OEC in the S 1 state compared with complex 3 2

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105 Figure 3 1 0. Plot of X M T vs T for complex 3 1 The solid line is the fit of the data; see the text for the fit parameters. Figure 3 11. (left) [Mn 3 Ca 2 O 4 ] core of 3 1 from a viewpoint emphasizing the isosceles Mn IV 3 triangular unit; (right) the corresponding 2 J coupling scheme and

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106 Figure 3 1 2. Plot of all the S T states for complex 3 1 as a function of energy. The ground state is 9 / 2 3> with the first excited state 7 / 2 2> ~ 56 cm 1 higher in energy than the ground state. Figure 3 1 3. The core of 3 1 emphasizing the exchange coupling model employed, and the spin alignments rationa lizing the S T = 9 / 2 ground state.

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1 07 Figure 3 1 4. (top) Plot of S T of 3.75 for complex 3 1 (bottom) Depictions of the indicated | S T S A > state as a function of varying

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108 F igure 3 1 5. (a) Plot of X M T vs T for complex 3 2 The solid line is the fit of the data; see text for fit parameters. The inset shows the exchange coupling model employed, and the spin alignments in the S T = 9 / 2 ground state. (b) Plot of all the S T states for complex 3 2 as a function of energy.

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109 Figure 3 1 6. (a) Plot of reduced magnetization ( M / B ) vs H / T for complex 3 1 The solid lines are the fit of the data; see the text for the fit parameters. (b) Two dimensional contour plot of the root mean square error surface vs D and g for the magnetization fit for 3 1

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110 Figure 3 1 7 (a) Plot of redu ced magnetization ( M / B ) vs H / T for complex 3 2 The solid lines are the fit of the data; see the text for the fit parameters. (b) Two dimensional contour plot of the root mean square error surface vs D and g for the magnetization fit for 3 2

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111 Figure 3 1 8 In phase AC magnetic susceptibility plotted as X' M T vs T of complex es 3 1 (a ) and 3 2 ( b ) in a 3.5 G field oscillating at the indicated frequencies.

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112 Figure 3 1 9. CW EPR spectra at X band (9.3752 GHz) and Q band (34.1877 GHz; derivative) of complex 3 1 All spectra were acquired at 5 K. Simulations ( dashed ) of data generated using the following parameters: spin S = 9 / 2 ; g = 1.975; zero field splitting D = +0.073 cm 1 E = 0.005 cm 1 ; line width 16 mT. Figure 3 20. CW EPR of complex 3 1 dissolved i n MeCN/CH 2 Cl 2 (1:1, v/v) and the powder. X band EPR parameters: MW = 9.4773 GHz (frozen solution); 9.4805 GHz (powder); microwave power = 6.3 mW; modulation amplitude = 0.8 mT; temperature = 5 K.

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113 Figure 3 21. CW EPR temperature dependence of complex 3 1 at 5, 10, 15, 20, 25, 30, 40, 50, 60, 70 and 80 K; spectra have been scaled by the temperature. Experimental parameters: MW = 9.3753 GHz; modulation amplitude =1 mT, microwave power = 1.6 mW. Figure 3 22. Q band 55 Mn Davies ENDOR spectrum of complex 3 1 (black) collected at 1142.2 mT, and simulation ( dashed ). Experimental parameters; tau of 230 ns; /2 pulse of 32 ns; MW 34.186 GHz; microwave power 1.920 mW; RF

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114 Figure 3 23. Q band 55 Mn Davies ENDOR of complex 3 1 taken across the EPR envelope, simulations shown in green. Experimental parameters; = 230 ns; /2 MW =32 ns; MW = 34.186 GHz ; microwave power = 1.920 mW; RF pulse

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115 Figure 3 24. CW EPR spectra at X band (9. 3805 GHz) and Q band ( 34.0943 GHz ; derivative) of complex 3 2 All spectra were acquired at 5 K. Simulations (black) of data generated using the following parameters: spin S = 9 / 2 ; g = 1.99; zero field splitting D = 0.073 cm 1 E = 0.0050 cm 1 ; zero field splitting strain D = 0.024 cm 1 E = 0.005 cm 1 ; line width 16 mT. Figure 3 25. Comparison of the CW EPR spectrum of 3 1 (blue) and 3 2 (green).

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116 Figure 3 26. CW EPR temperature dependence of complex 3 2 at 8, 10, 20, 30, 50, 70, 90, 110, 130, 150 K ; spectra have been scaled by the temperature. Experimental parameters: MW = 9.3805 GHz; modulation amplitude =1 mT, microwave power = 1.6 mW. Figure 3 27. Cyclic voltammogram at 100mV/s for complex 3 1 (a) and 3 2 (b) in MeCN and DCM respectively, con taining 0.1 M N n Bu 4 PF 6 as supporting electrolyte.

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117 CHAPTER 4 THE FIRST SYNTHETIC MODEL POSSESSING THE EXACT CUBANE CORE OF THE OEC AND MIMICKIN G THE S 2 STATE OF THE KOK CYC LE 4.1 Introduction Among the large variety of manganese containing enzymes exis ting in Nature, the oxygen evolving complex (OEC) present on the donor side of photosystem II (PSII) is one of the most fascinating and intriguing. 35, 49, 67, 199, 200 Nature has developed this enzyme to carry out the thermodynamically demanding water oxidation reaction, and the electrons released are coupled to the photochemistry occurring at the P680 chlorophyll dimer of PSII. 239 241 ency in catalytic water oxidation using inexpensive and abundant metals (manganese and calcium) is unmatched in any of the artificial systems known to date 201 Thus, this emphasizes the importance of elucidating the structure and understanding the mechanism of water oxidation in order to develop more efficient bio inspired catalysts for photolysis of water. 242 248 As mentioned in the previous chapters, various techniques have been util ized to unravel the mysteries of this fascinating enzyme. 35, 38, 63 66, 202, 205, 249 Our understanding of its structural, spectroscopic and functional properties is an amalgamation of the pieces of information tha t have been obtained from these different spectroscopic tools. Synthetic chemists continue to play a crucial role in this multidisciplinary quest for deciphering the structural and functional aspects of the OEC by synthesizing model complexes that mimic th e active site of this metalloprotein. 81, 250 The heteronuclear core of the OEC (i.e. the integral incorporation of calcium in the tetranuclear manganese cluster) has been known for some time, 68 70 but the exact topology of this Mn 4 Ca core has been debated fiercely over the years. 75, 207 It was not until recently that the crystal

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118 structure of the PSII at 1.9 resolution was reported, which unambiguously identified the OEC to be composed of a Mn 3 CaO 4 cube with a fourth Mn attached to it via one of the cubane oxide ions and a fifth external oxide ion giving rise to a Mn 4 CaO 5 cluster with a distorted chair topology. 79 Synthetic chemists have been targeting this illusive Mn/Ca core for quite some time, and in 2011, entry into [Mn 3 CaO 4 ] cubane chemistry was finally attained. 213, 251 However, the complete [Mn 4 CaO 5 ] unit has never been obtained. In the previous chapter, we demonstrated that using a comproportionation approach, entry into high valent [Mn 3 C aO 4 ] cubane chemistry can be achieved. Isolation of Mn 3 Ca 2 ( 3 1 ) proved that the discrete Mn 3 Ca cubane with an additional metal attached to the cube can be synthesized with only carboxylate ligation. 251 Although detailed spectroscopic characterization of 3 1 revealed striking similarity with the native OEC, isolation of the exact Mn 4 Ca unit has yet to be accomplished synthetically. In order to target this product, we further explored the above synthetic strategy (comproportionation reaction) aiming for products with a higher Mn:Ca ratio. This approach has now provided access to the [Mn 4 CaO 5 ] uni t for the very first time. Apart from the structural similarities that will be described, this complex reproduces very closely the electronic and magnetic properties of the native OEC in the S 2 state of the Kok cycle very closely. Herein, we report the d et ailed structural, magnetic and electronic characterization of several compounds, along with their compa rison with the native OEC. 4.2 Experimental Section 4.2.1 Syntheses All manipulations were performed under aerobic conditions using chemicals and solven ts as received, unless otherwise stated. Mn(O 2 CBu t ) 2 Ca (O 2 CBu t ) 2 and

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119 NBu n 4 MnO 4 were prepared as previously reported in the literature. 168, 169 W arning : Appropriate care should be taken in the use of NBu n 4 MnO 4 and readers are referred to the detailed warning given elsewhere [Mn 8 Ca 4 O 14 ( O 2 CBu t ) 10 (phen) 2 (H 2 O) 6 ]Cl 2 ( 4 1 ): Mn(O 2 CBu t ) 2 2H 2 O (0.16 g, 0.50 mmol) was dissolved in acetonitrile (2 0 mL), and the resulting pink slurry was treated with pivalic acid ( 0.14 mL, 1. 20 mmol) CaCl 2 (0.06 g, 0.50 mmol) and 1,10 phenanthroline (0.09 g, 0.50 mmol) which caused the color to change to pale yellow To this solution NBu n 4 MnO 4 (0.18 g, 0.50 mmol) was added dropwise as an acetonitrile solution (5 mL). The resulting reddish br own solution was stirred for 4 h, during which the solution color changed to deep brown. The solution was then filtered and the filtrate was left undisturbed in a closed vial. After 2 weeks, X ray quality brown crystals of 4 1 t BuCO 2 H 5.5MeCN H 2 O had formed, which were collected by filtration, washed with ether and dried under vacuum. The yield was 36%. Anal. Calcd (found) for 4 1 t BuCO 2 H H 2 O MeCN (C 81 H 133 O 43 N 5 Cl 2 Mn 8 Ca 4 ): C, 38.36 ( 37.96 ); H 5.28 ( 5.16 ); N 2.76 ( 2.72 ). Selected IR da ta (cm 1 KBr pellets): 3414 (br), 2973 (m), 2873 (m), 2575 (m), 2362 (m), 1612 (s), 1560 (s), 1518 (m), 1479 (s), 1423 (m), 1399 (s), 1343 (vs), 1219 (vs), 1144 (m), 1107 (m), 1028 (m), 939 (m), 829 (m), 874 (m), 846 (s), 782 (s), 721 (s), 654 (m) and 599 (br, s). [Mn 4 Ca 2 O 6 ( O 2 CBu t ) 6 (phen) 4 ]( O 2 CBu t ) ( 4 2 ): Mn(O 2 CBu t ) 2 2H 2 O (0.16 g, 0.50 mmol) was dissolved in acetonitrile (2 0 mL), and the resulting pink slurry was treated with pivalic acid ( 0.14 mL, 1.20 mmol) Ca (O 2 CBu t ) 2 2H 2 O (0. 14 g, 0.50 mmol) and 1,10 phenanthroline (0.18 g, 1.00 mmol) which caused the color to change to pale yellow To this solution NBu n 4 MnO 4 (0.18 g, 0.50 mmol) was added dropwise as an

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120 acetonitrile solution (5 mL). The resulting reddish brown solution was stirred for 4 h, during whi ch the solution color changed to deep brown. The solution was then filtered and the filtrate left undisturbed in a closed vial. X ray quality black crystals of 4 2 MeCN 0.1H 2 O grew over a week, which were collected by filtration, washed with ether and dried under vacuum. The yield was 46%. Anal. Calcd (found) for 4 2 MeCN H 2 O (C 85 H 1 00 O 21 N 9 Mn 4 Ca 2 ): C, 54.19 ( 54.10 ); H 5.35 ( 5.45 ); N 6.69 ( 6.68 ). Selected IR data (cm 1 KBr pellets): 3433 (br), 3074 (m), 2951 (s), 2865 (m), 2 361 (m), 2249 (m), 1702 (m), 1610 (vs), 1566 (s), 1514 (m), 1479 (s), 1455 (m), 1399 (vs), 1349 (vs), 1220 (vs),1144 (m), 1106 (m), 1028 (m), 890 (m), 856 (s), 785 (m), 767 (m), 725 (s), 664 (vs), 605 (vs), 515 (m) and 436 (s). 4.2.2 X ray Crystallography Data were collected at 173 K on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo 0.71073 ). Suitable crystals of 4 1 t BuCO 2 H 5.5MeCN H 2 O and 4 2 MeCN 0.1H 2 O were attached to glass fiber s using paratone oil and transferred to a goniostat, where they were cooled to 173 K for data collection. Data collection and unit cell parameters are listed in Table 4 1. Cell parameters were refined using up to 8192 reflections. A full sphere of data (18 scan method (0.3 o 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 w ere applied based on measured indexed crystal faces. The structures were solved by th e direct methods in SHELXTL6 170 and refined on F 2 using full matrix least squares. The non H atoms were treated anisotropically, whereas the H

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121 atoms were calculated in ideal po sitions and were refined as riding on their respective carbon atoms. The asymmetric unit of 4 1 t BuCO 2 H 5.5MeCN H 2 O consists of two half Mn 8 Ca 4 clusters, located on inversion centers, one pivalic acid in a general position, five and a half acetonitrile sol vent molecules (the N7 molecule refined with 50% occupancy), one chloride anion in a general position and another disordered in two positions, Cl2/Cl2 and a partial water existing only with the Cl2 part. There are four disordered t butyl methyl groups ( C2, C6, C12 and C32 ) and each was refined in two parts. The disorder at C6 was refined in three parts with all bond distances constrained to maintain optimal geometry using SADI command lines. Thus all methyl bond distances were kept equal while inter met hyl distances were also kept equal to each other. All of the water protons were obtained from a d ifference Fourier map and refined as riding on their parent O atoms. A total of 1438 parameters were included in the final cycle of refinement using 16647 re flections with I > 2 (I) to yield R 1 and wR 2 of 4.81 and 10.78%, respectively. The asymmetric unit of 4 2 MeCN 0.1H 2 O consists of a half Mn 4 Ca 2 cluster, located on an inversion center, a pivalic acid solvent molecule, an acetonitrile solvent molecule and a partial water mol ecule (10% occupancy). It was concluded from a BVS analysis that the cluster is positively charged, which implies that the solvent molecule is a pivalate anion. This conclusion is supported by the fact that the X ray intensity data could not confirm the pr esence of a hydroxyl proton. A total of 603 parameters were refined in the final cycle of refinement using 8746 reflections with I > 2 (I) to yield R 1 and wR 2 of 3.78 and 8.46%, respectively.

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122 4.3 Results and Discussion 4.3.1 Syntheses The isolation of 3 1 and 3 2 paved the way into high oxidation state Mn/Ca cubane chemistry. 251 The next logical step was to modify the reaction scheme to target the exact Mn 4 Ca cubane unit of the OEC. However, just increasing the Mn:Ca ratio in the preparation of 3 1 to 4:1 was insufficient, as it led to the formation of 3 1 in lower yields. We thus decided to introduce some chelate into the reaction system with the hope of diverting the reaction away from 3 1 We started by investigating the effect of varying amounts of the chelate 1,10 phenanthroline in the reaction scheme of 3 1 The first hurdle that we faced was the presence of excess acid in the synthetic scheme, which hindered the incorporation of the chelate into the isolated product. T hus, the ratio of acid:chelate was systematically explored to ensure the presence of the chelate in the final product. Although this optimized synthetic scheme ensured chelate incorporation, the presence of heat (as in the reaction of 3 1 ) always led to th e isolation of the well known [Mn 4 O 2 (O 2 C t Bu) 6 (phen) 2 ] butterfly complex. 81, 252 254 In an attempt to isolate a heterometallic cluster, the reaction was carried out in the absence of heat and the reaction time was v aried, and this finally led to the isolation of 4 1 The reaction of Mn(O 2 C t Bu) 2 CaCl 2 phenanthroline and NBu n 4 MnO 4 in a 1:1:1 ratio in acetonitrile in the presence of stoichiometric amount of pivalic acid afforded a dark brown solution from which [Mn 8 C a 4 O 14 (O 2 CBu t ) 10 (phen) 2 (H 2 O) 6 ]Cl 2 ( 4 1 ) was subsequently isolated in 36% yield based on Ca. The formation of 4 1 is summarized in eq. 4 1. 4 Mn 2+ + 4 Ca Cl 2 + 4 MnO 4 + 10 t BuCO 2 H + 2 phen + 4 H 2 O + 4 e [Mn 8 Ca 4 O 14 (O 2 CBu t ) 10 (phen) 2 (H 2 O) 6 ] Cl 2 + 6 H + + 6 Cl ( 4 1)

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123 The ratio of Mn 2+ and Mn 7+ in the reaction is 1:1, which gives an average manganese oxidation state of +4.5, as in the synthesis of 3 1 The reaction was found to be very sensitive to the choice of metal reagents as well as to the solvent. Use of different Mn 2+ and/or Ca 2+ salts in the above reaction always led to mixture of products that were difficult to separate. Decreasing the reaction time from 4 h led to the formation of 4 1 in much lower yi eld. Our strategy to avert the formation of 3 1 by incorporation of the chelate was successful, but it caused the nuclearity of the product to change from Mn 3 Ca 2 in 3 1 to Mn 8 Ca 4 in 4 1 In an attempt to achieve lower nuclearity products, the reaction was performed with varying amounts of phenanthroline. In one such reaction, where the amount of phenanthroline used was double that of eq. 4 1, a crystalline material was obtained whose IR was different from 4 1 Changing the Ca 2+ source to Ca (O 2 CBu t ) 2 instead of CaCl 2 led to the isolation of 4 2 in 48% yield. The formation of 4 2 is summarized in eq. 4 2. 2 Mn 2+ + 2 Ca 2+ + 2 MnO 4 + 7 t BuCO 2 H + 4 phen + 3 e [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) + 3 H + + 2 H 2 O ( 4 2 ) The average mangane se oxidation state in this reaction was again +4.5, but the final product has a lower average manganese oxidation state this is discussed in more detail in a later section. As for the preparation of 4 1 the reaction time is crucial to maximize the yield of 4 2 4.3.2 Description of Structures 4.3.2.1 Structure of [Mn 8 Ca 4 O 14 ( O 2 CBu t ) 10 (phen) 2 (H 2 O) 6 ]Cl 2 ( 4 1 ) The partially labeled structure of [Mn 8 Ca 4 O 14 (O 2 CBu t ) 10 (phen) 2 (H 2 O) 6 ]Cl 2 ( 4 1 ) is shown in Figure 4 1, and selected interatomic distances and bond ang les are

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124 summarized in Table A 6 Complex 4 1 crystallizes in the monoclinic space group P2 1 /c. The oxidation state of the manganese atoms and the protonation levels of the O 2 H 2 O, and carboxylate O atoms were determined by a combination of charge balance considerations, inspection of bond lengths, and bond valence sum calculations (Table 4 2). 4 1 is a homovalent complex with all the manganese being in +4 oxidation state. The complex contains a [Mn 8 IV Ca 4 ( O) 2 ( 3 O) 10 ( 4 O) 2 ( H 2 O) 2 ] 12+ core, which is sho wn Figure 4 2. All the manganese centers are hexa coordinated and possess a distorted octahedral geometry. Two of the calcium centers (Ca1/Ca1') are octa coordinated whereas the remaining two calcium (Ca2/Ca2') are hexa coordinated. Although the nuclearity of 4 1 is much larger than the native OEC, careful examination of the core reveals the presence of a [Mn 4 Ca] cubane plus external Mn, as found in the OEC (Figure 4 2). 67, 79 There are two [Mn 3 IV CaO 4 ] cubanes like those in 3 1 and they are connected to an external manganese atom (Mn4/Mn4') via a oxide (O14/O14') and a 3 oxide (O16/O16'), giving rise to the [Mn 4 IV CaO 6 ] unit. This structural subunit is very similar to the native OEC although the mode of attachment of the external Mn to the cube is slightly different from the ones deduced from X ray crystallography of the native OEC. 67, 79 Like 3 1 the angles in the cube deviate from ideal 90 o ranging from 90 107 o 251 Within this cubane unit the Mn Mn distances lie within the range 2.7813 (7) 2.911 (7) a nd the average Mn Ca distance is ~ 3.4 These values are comparable to 3 1 251 The two Mn 4 Ca cubane units are linked via one of the oxides from the cube (O18/O18') and another 3 oxide (O16/O16'). Two additional calcium units attach to this

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125 [Mn 8 IV Ca 2 O 12 ] moiety via a 3 oxide (O17/O17') to complete the core of 4 1 Another interesting feature about this structure is the presence of a terminal water molecule on each calcium cente r, which is similar to the recent 1.9 crystal structure of the OEC, where four water molecules have been detected on Mn and Ca centers. 79 Apart from the terminal water molecules the Ca centers in 4 1 are also linked by a bridging water molecule (O11/O11') which may be attributed to the low charge of Ca II Although such bridging water units are rare but it i s not unprecedented. The Ca1' Ca2 distance of 3.7982 (10) and Ca1' O11 Ca2 angle of 100.292 (9) o lie within the range reported in the literature for these parameters. 255 265 This bridging water is also strongly h ydrogen bonded to the oxide (O14) bridging Mn4/Mn2, which is evident from the O14 H11 distance of 1.667 (15) and low BVS value for O14. The peripheral ligation is provided by eight pivalates in 1 : 1 : bridging modes and two pivalates in 1 : 2 : bridging modes. Two phena nthroline moieties, one on each Mn4 and Mn4', complete the ligation of 4 1 4.3.2.2 Structure of [Mn 4 Ca 2 O 6 ( O 2 CBu t ) 6 (phen) 4 ]( O 2 CBu t ) ( 4 2) The partially labeled structure of 4 2 is shown in Figure 4 3, and selected interatomic distances and angles are summa rized in Table A 7 Complex 4 2 crystallizes in triclinic space group P 1 The oxidation state of the manganese atoms and the protonation levels of the O 2 and the oxygen atoms of the carboxylate groups were determined from a combination of charge considerations, inspection of bond lengths and bond valence sum (BVS) calculatio ns (Table 4 3). Based on BVS calculations, Mn1/Mn1' were assigned as Mn III and Mn2/Mn2' were assigned as Mn IV Since 4 2 is centrosymmetric, it was impossible to unambiguously assign the core to be either

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126 [Mn 2 III Mn 2 IV Ca 2 O 6 ] 6+ or [Mn III Mn 3 IV Ca 2 O 6 ] 7+ based on BVS calculations alone. The BVS value for Mn1 is higher than a typical Mn III and lower than Mn IV and it takes an average value of Mn 3.5+ This is due to the centrosymmetric core of 4 2 in which the Mn III and Mn IV are indistinguishable by X ray crysta llography. It should be noted that this cannot be attributed to the electron delocalization between Mn III and Mn IV which requires more energy due to the structural changes associated with the JT elongation of Mn III BVS calculations also assigned the latti ce solvent molecule to be a pivalate group and the lower BVS number for O11 is due to hydrogen bonding with lattice water molecule. Thus, by charge consideration, the manganese oxidation states were assigned as 1 Mn III and 3 Mn IV This oxidation state assi gnment was confirmed by magnetism data and EPR studies, which are explained in detail in a later section. All the manganese atoms are hexa coordinated and possess a distorted octahedral geometry, whereas the calcium centers are octa coordinated. The [ Mn III Mn 3 IV Ca 2 ( 4 O ) 2 ( 3 O) 4 ] 7+ core of 4 2 can be described as being composed of two Mn 3 Ca cubes fused together via a common face, giving rise to a dicubane topology. The cubes are connected via four oxides, and the common face between the cubes is composed of a [Mn 2 O 2 ] rhombus. Such dicubane topologies are rare; there are only a few examples of homometallic [M 6 O 6 ] cores, 266 269 and only one heterometallic core has been reported. 270 Complex 4 2 represents the first discrete [Mn 4 Ca 2 O 6 ] dicubane in Mn/Ca chemistry, and it also possesses the highest average manganese oxidation state of +3.7 5 reported for such a metal topology. The angles in the individual cubes deviate from the ideal value of 90 o which can be attributed to the fusion of the cubes, coupled with the incorporation of the larger ionic radius Ca II The

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127 peripheral ligation is pro vided by six 1 : 1 : pivalate bridges and four phenanthroline groups, one on each of the peripheral atoms of the dicubane. All the manganese atoms are hexa coordinated and possess a distorted octahedral geometry, whereas the calcium centers are octa coordinate. 4.3.2.3 Comparison of the structure of 4 2 and the native OEC Synthesis of 3 1 had paved our way into discrete Mn/Ca cubane chemistry, and isolation of 4 2 would now allow us to build on this interesting breakthrough. Along with 3 1 4 2 should aid in the unders tanding of structure and spectroscopic data of the native OEC. In complex 3 1 we were able to show the mode of attachment of an external metal to a Mn 3 Ca, which was similar to structure proposed by Ferreira et al. 70 but the external metal was calcium instead of manganese. 4 2 possesses a similar Mn 3 Ca cube as 3 1 and it is now attached to an external Mn via both a cube oxide (O1) and an external oxide (O3) which is similar to the recent 1.9 crystal structure of the OEC. 79 Figure 4 5 shows a compar ison of the intermetallic distances in 4 2 and this recent PSII structure. The bond distances are comparable to those in the native OEC. However, the longer Mn O distances arising from the asymmetric binding of the external Mn in the OEC are lost in 4 2 du e to its symmetric dicubane topology. Ca EXAFS studies on the native OEC have revealed the presence of two Mn Ca distances at 3.3 and 3.4 218, 271 The Mn Ca separation within the Mn 3 Ca cube lies in the range 3.925 3.414 which is very similar to the distances in the OEC. 67, 79 However, due to the dicubane topology the external Mn atom (Mn1) is pulled further away from the cube (compared to the OEC crystal structur e Figure 4 5) resulting in a much longer Mn Ca distance of 4.3724, which is absent in the native OEC. Mn EXAFS studies on OEC has revealed the presence of three short Mn Mn distances in the

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128 range 2.7 2.8 in a 2:1 ratio and a longer Mn Mn distan ce at 3.3 76, 272 The Mn Mn distances in 4 2 are slightly longer than those predicted by EXAFS studies. The dicubane topology gives rise to three distinct Mn Mn distances lying in the range ~2.8 3.0 Anot her interesting similarity between 4 2 and the native OEC is found in the manganese oxidation states. It has been well established that the formal oxidation state of the manganese centers in the S 2 state of the Kok cycle is Mn III Mn IV 3 the same as in 4 2 35, 56, 273 Although there are a few manganese clusters that possess the same Mn oxidation states as the S 2 state (Table 4 6), 4 2 is the first Mn/Ca cluster to exhibit such similarities. 4.3.3 Mn and Ca X ray Abso rption Spectroscopy (XAS) The Mn and Ca XAS spectra were collected by the Yachandra group at the Lawrence Berkeley National Laboratory Detailed experimental procedure listed in Appendix C. Mn and Ca XAS has provided crucial information about the metal top ology and metal metal distances in the native OEC. 35, 39, 273 The XANES technique has been extensively used to investigate the oxidation state of the Mn centers as the OEC cycles through the Kok cycle. The edge pos ition of the XANES spectra is known to be sensitive to the Mn oxidation state i.e. it shifts to higher energy with higher average Mn oxidation state. 274, 275 Figure 4 6 compares the Mn XANES spectra of the OEC in t he S 0 through S 3 states with the spectrum for the Mn 4 Ca 2 complex ( 4 2 ). The formal oxidation state of 4 2 is Mn III Mn IV 3 Ca 2 and its edge position is very similar to the S 2 state, as expected. This kind of similarity with the OEC has been observed previousl y only for 3 1 and Mn 4 cubane like complexes. It is interesting to see such striking similarities between 4 2 and the OEC, even in the presence of such aromatic ligands. 217

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129 Mn EXAFS studies of the OEC in the S 1 and S 2 states have established the presence of two Mn Mn distances at ~2.7 and one Mn Mn distance of ~2.8 which are characteristic of di O bridged M n 2 pairs and mostly O based ligation at 1.8 2.0 76, 272 The Mn EXAFS also shows a longer distance interaction, assigned to one Mn Mn distance at 3.3 and Mn Ca at ~3.4 218, 271 Figure 4 7 shows the comparison of the Mn and Ca EXAFS of the OEC in the S 1 state with 4 2 The comparison of the Mn EXAFS in Figure 4 7a shows that there are similarities in the beat pattern, but there are some differences in the phase and f requencies of the spectral features. The FT in Figure 4 7b shows that there are substantial similarities in the features and in contrast to 3 1 the intensities of the FT peaks are almost identical to those of the OEC. Although there are many structural sim ilarities between the [ Mn 4 Ca O 5 ] cluster in the OEC and the Mn 4 Ca 2 complex due to the presence of a distorted Mn 4 Ca cubane motif, the presence of the extra Ca makes the Mn and Ca EXAFS of 4 2 distinguishable from the spectra of the OEC. Mn Mn distances a re slightly elongated in the Mn 4 Ca 2 complex (~ 2.8 ) compared to the OEC S 1 state. 272 The 3 rd peak of the OEC S 1 spectrum originates from the mixture of the mono oxo b ridged Mn Mn (~ 3.3 ) and Mn Ca (3.4 ) interactions. 271 Since a mono oxo bridged Mn Mn interaction does not exist in the Mn 4 Ca 2 complex the intensity of the 3 rd peak arises mainly from the Mn Ca (3.3 3.4 ) interactions. Figures 4 7c and 4 7d show the Ca EXAFS and the FTs of the OEC in the S 1 state compared with those for 4 2 The Ca EXAFS spectra in Figure 4 7c show, just as in the Mn EXAFS, that there are similarities between the [Mn 4 Ca 2 ] cluster and the OEC. The FTs show that the OEC and 4 2 both have similar Ca Mn distances, including the first FT peak from Ca O

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130 distances and the second FT peak at ~3.3 from Ca Mn. However, there are still differences in frequency and intensity of the peaks. Furthermore, the presence of a longer Ca Mn interaction (~4.4 ) in the Mn 4 Ca 2 complex arising from the dicubane topology is noticeable only in the Ca EXAFS, and it is not clear in the Mn EXAFS. Such a long Mn Ca distance is not present in the OEC and can be attributed to the dicubane topology of 4 2 arising from th e presence of an additional Ca atom in the cluster compared to the OEC. Overall, the similarities in both the Mn and Ca EXAFS and FTs show that complex 4 2 has many of the features that are present in the OEC. Mn and Ca EXAFS Fits : The EXAFS fitting was ca rried out using the information about the distances and the coordination numbers obtained from the crystal structure of the Mn 4 Ca 2 complex 4 2 The fits are shown in Figure 4 8 Tables 4 4 and 4 5 summarize the EXAFS curve fitting parameters and the XRD di stances. The EXAFS curve fitting results for Mn and Ca in the Mn 4 Ca 2 complex both fit well with the crystal structure data. 4.3.4 Magnetochemistry of Complexes 4 1 and 4 2 4.3.4.1 DC studies Solid state variable temperature DC magnetic susceptibility data in a 0.1 T field w ere collected in 5.0 300.0 K range on powdered microcrystalline samples of 4 1 and 4 2 restrained in eicosane to avoid torqu ing For 4 1 the 300 K X M T value of 13.74 cm 3 Kmol 1 is slightly lower than the spin only ( g = 2) value of 15.0 0 cm 3 Kmol 1 expected for eight non interacting Mn IV ions, indicating the presence of intramolecular antiferromagnetic interactions. The X M T decreases with decreasing temperature to 1.45 cm 3 Kmol 1 at 5.0 K, and the low

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131 temperature value suggests an S = 0 gr ound state (Figure 4 9). It is not easy to rationalize this ground state, due to the presence of several competing interactions arising from the large number of triangular units within the core topology of 4 1 For 4 2 the X M T value of 5.68 cm 3 Kmol 1 at 3 00 K is again less than the spin only value of 8.63 cm 3 Kmol 1 ( g = 2.0) expected for three non interacting Mn IV ( S = 3 / 2 ) and one Mn III ( S = 2) ions, indicating the presence of antiferromagnetic exchange interactions within complex 4 2 The X M T value stead ily decreases with decreasing temperature to 0.51 cm 3 Kmol 1 at 5.0K, which is consistent with an S = ground state (Figure 4 10). A half integer ground state validates our manganese oxidation state assignment, as both Mn 4 IV Ca 2 and Mn 2 III Mn 2 IV Ca 2 cores wou ld lead to an integer spin system. The individual exchange parameters (J) between the Mn Mn pairs within the molecule was obtained by fitting the X M T vs T data for 4 2 to the appropriate theoretical expression for a Mn III Mn 3 IV butterfly (C 2v symmetry) mode l (Figure 4 11). A Heisenberg Dirac van Vleck (HDVV) spin Hamiltonian ( 2J i j ) for pairwise exchange interactions was employed. The spin Hamiltonian appropriate for a tetranuclear butterfly arrangement of metal ions assuming all the pairwise interaction s to be inequivalent is given by eq. 4 3. H = 2J 12 1 2 2J 23 2 3 2J 34 3 4 2J 41 4 1 2J 24 2 4 (4 3) However, based on the structure of 4 2 it is reasonable to simplify eq. 4 3 to eq. 4 4, where all of the butterfl wb ) has been approximated to be equal, in spite of the differences in the manganese oxidation states. H = 2J wb ( 1 2 + 2 3 + 3 4 + 4 1 ) 2J bb 2 4 (4 4)

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132 This approximation is justified as Mn III O and Mn IV O interactions are known from the literature to be very similar. 276, 277 to 24 is renamed J bb in a butterfly model. Within this framework of the model described by eq. 4 4, the eigen values of this spin Hamiltonian can be determined using the Kambe vector coupling method as described elsewhere. 219 Using the coupling scheme A = 2 + 4 B = 1 + 3 and T = A + B the spin Hamiltonian can be transformed into the equivalent form given by eq. 4 5, where T is the total spin of the molecule. H = 2J wb ( T 2 A 2 B 2 ) 2J bb A 2 (4 5) The eigen values for eq. 4 5 can be determined using the relationship i 2 = S i (S i + 1) and are given by eq. 4 6, where E S T S A S B > is the energy of state S T S A S B >, and the constant terms contributing equally to all states have been omitted. E S T S A S B > = J wb [ S T ( S T + 1) S A ( S A + 1) S B ( S B + 1)] J bb [ S A ( S A + 1)] (4 6) The overall multiplicity of the system is 320, comprising of 50 individual spin states ranging from S T = 11 / 2 1 / 2 An expression for the molar magnetic susceptibility X M was derive d using eq. 4 6 the Van Vleck equation and by assuming an isotropic g tensor (eq. 4 7) 220 This equation was then used to fit the experimental X M T vs T ( solid line in Figure 4 1 0 ), as a function of the two exchange parameters (J wb and J bb ) and g A contribution from temperature independent paramagnetism (TIP) was held constant at 600x 10 6 cm 3 mol 1 (4 7) The fitting parameters are as follows: J wb = 14.82 (1) cm 1 J bb = 20.13 ( 1.5 ) cm 1 and g = 2.00 (2). Using these values an energy ladder can be create d (Figure 4 1 2 ) which

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133 further confirms a | 1 / 2 3 7 / 2 > ground state for this system T he first excited state is | 3 / 2 2 7 / 2 >, located ~ 12 cm 1 above the ground state. The antiferromagnetic J values obtained for 4 2 are in contrast with those of 3 1 whi ch exhibits strong ferromagnetic interactions between the Mn centers arising from the ~ 90 o angles between the metal centers. 110 All the interactions between the metal centers in 4 2 are antiferromagnetic which can be attributed to the slightly larger angles compared to those 3 1 Based on the obtained J values, and the identification of the ground state as | 1 / 2 3 7 / 2 > (i.e. S T = 1 / 2 S A = 3 and S B = 7 / 2 ), the S = ground state of complex 4 2 is shown by the spin alignments in Figure 4 13 Thus, this is a spin frustrated system with the J bb pathway losing out to the four J wb interactions. It is well kn own that the ground state of a spin frustrated system is very sensitive to slight structural perturbations. 136, 217, 276 Therefore, a spin state energy manifold vs the J wb /J bb ratio has been computed to explore th e variation of the ground state as a function of the relative magnitudes of J wb and J bb (Figure 4 14) The experimental J wb /J bb = 0.74 (shown as the dashed line in Figure 4 14 ) shows that the J wb /J bb ratio is close to the crossover point to an S = 3 / 2 grou nd state. This thus also indicates that a small change in the J wb /J bb ratio induced by minor structural changes would cause this system to flip its ground state For example; when ~ 0.45 < J wb /J bb < ~ 0.68, the ground state is | 3 / 2 2, 7 / 2 > when ~ 0.25 < J wb /J bb < ~ 0.45, the ground state is | 5 / 2 1, 7 / 2 > and for 0 < J wb /J bb < ~ 0.25, the ground state is | 1 / 2 0, 1 / 2 > (Figure 4 14). Although we were able to explain the observed ground state based on the exchange interactions obtained from the 2J (C 2v sy mmetry) model on a butterfly, we now wanted to test whether such an approximation was justified. The susceptibility data

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134 for 4 2 to between the Mn III O is different from the Mn IV O as shown in Figure 4 15. The corresponding spin Hamiltonian is given in eq. 4 8. Using the program MAGPACK 278, 279 afforded the parameters J 1 = 13.50 cm 1 J 2 = 14.82 cm 1 and J 3 = 19.33 cm 1 H = 2J 1 ( 1 2 + 1 4 ) 2J 2 ( 2 3 + 3 4 ) 2J 3 2 4 (4 8) The obtained J values indicate that our initial assumption was justified that a 2J model was reasonable, but in order to get the exact exchange interactions, DFT calculations or IN S measurements would be required. Complex 4 2 mimic s the S 2 state of the Kok cycle which has been studied extensively as it gives rise to EPR signals that can be detected using conventional EPR spectroscopy The first signal to be detected by EPR from PSI I was a multiline signal at g ~ 2 which was assigned to an S = ground state of the S 2 state. 280 283 Depending on the mode of generation and experimental condition s, another signal at g = 4.1 ( or 4.25 ) can be obta ined 284 286 which has been proposed to arise from an S = 3 / 2 283, 287 or S = 5 / 2 state. 288, 289 Shifting of the intensity from th e g = 4.1 to g = 2 signal has also been observed depending on the sample preparation conditions, freezing procedures, and the nature of the solvent used. 290 292 This indicates that the two signals arise from differ ent spin states of the same manganese cluster. 283, 286 There are examples of Mn 2 Mn 3 and Mn 4 clusters in the inorganic literature that have reproduced the S = 1 / 2 ground state and g = 2 multiline EPR spectrum of t he S 2 state (Table 4 6). However 4 2 is the only Mn/Ca cluster to mimic both the structural and magnetic properties of the S 2 state. Furthermore, detailed magnetic studies have revealed 4 2 to be a spin frustrated system where the ground state was found to be sensitive to slight structural

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135 perturbation. Based on th ese data a similar situation can be anticipated to occur in the native OEC which exhibits different ground state for a particular S n state depending on the generation conditions resulting from s light structural perturbations in the Mn 4 Ca complex. 4.3.4. 2 AC studies An AC magnetic susceptibility study was performed to independently confirm the ground state and probe the dynamics of magnetization (magnetic moment) relaxation. The AC studies on 4 1 and 4 2 were performed in the 1.8 15.0 K range using a weak 3.5 G ac field oscillating at frequencies of 50 1000 Hz. The in phase ( X M ) ac susceptibility measurement is invaluable for assessing the ground state without any complications from a dc field (Figure 4 17 ). For 4 1 the X M T value shows a steep decrease with decrease in temperature, which indicates the depopulation of one or more excited states with an S greater than the ground state S The lowest temperature data are consistent with an S = 0 g round state for 4 1 The in phase X M T vs T AC susceptibilities for 4 2 show a similar profile to 4 1 although the drop in the X M T value is not as steep as 4 1 Extrapolation of the data from 8 K to 0K gives a value of ~ 0.4 cm 3 Kmol 1 which is consisten t with the S = ground state deduced from the dc studies. 4.3.5 EPR Spectroscopy The X band EPR was collected at the CalEPR center at the University of California at Davis. The low temperature CW EPR spectrum of a microcrystalline sample of complex 4 2 po ssesses almost identical spectral features to those from a sample that has been dissolved in dichloromethane (DCM)/dimethylformamide (DMF) solvent

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136 mixture. This suggests that the electronic structure of the paramagnet is not greatly perturbed by dissolutio n of the complex (Figure 4 18). The l ow temperature X band EPR spectrum of 4 2 reveals a broad derivative shaped feature centered at g = 1.95 that spans 277 mT (Figure 4 19A ). Approximately 19 peaks with average spacing of 7.9 mT are observed on top of thi s central resonance. These features likely result from the hyperfine coupling of multip le exchanged coupled 55 Mn ions. 280, 281, 293 The obtained spectrum is remarkably similar to the multiline signal (MLS) which is evident when photosystem II (PSII) is poised in the S 2 sta te of the Kok cycle (Figure 4 19B). The MLS is attributed to a S = 1 / 2 ground state that results from ex change coupling of the three Mn IV ions and one Mn III ion of the OEC in the S 2 state. 294 For this state, 18 22 55 Mn hyperfine lines are observed centered at g = 1.98 that are spaced by 8.5 9.0 mT. A different exchange coupling scheme for the Mn ions in the S 2 state of the OEC leads to a S = 5 / 2 ground state that giv es rise to a g = 4.1 signal in the X band EPR spectrum (Figure 4 19 B). 295 For 4 2 dissolved in acetonitrile and dichloromethane (1:1 ratio), a feature at g = 4.8 is observed that also could be due to a higher spin state of the cluster. 296 Upon raising the temperature, this low field feature increases in intensity concurrent with diminution of the resonance at g = 2, indicating that the corresponding transition is betw een levels in a thermally excited spin manifold (Figure 4 20 ). Another spin manifold is populated above 40 K evidenced by the 55 Mn hyperfine split peaks centered at g = 2, which grow back in above this temperature. This high density of spin manifolds is di agnostic of the weak antiferromagneti c coupling in 4 2 resulting in a spin

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137 frustrated system (Figure 4 14). Slight perturbations in the structure could alter the J wb /J bb ratio and thus the accessibility of these low lying excited states. Evidence for such an effect is found in the X band CW EPR spectra of 4 2 dissolved into different solvents (Figure s 4 20 4 21 and 4 22 ). Significant solvent dependent shifts in the position and intensity of the low field features are observed at equivalent temperatures. Si milarly in PSII, the equilibrium between the MLS and the g = 4.1 signal can be altered with the addition of glassing agents or small molecules. For example, the MLS signal is favored after the addition of small alcohols such as methanol and ethanol (5% v/v ), 30% glycerol, or 30% polyethylene glycol to the buffer. 297, 298 Conversely, the addition of sucrose, certain amines, F or Cl inhibitors biases the spectrum toward the g = 4.1 signal. 4 2 represents the only Mn /Ca complex to reproduce all these interesting spectroscopic properties of the OEC for the first time. 286, 289, 291, 299, 300 Simulation of these data to get more information about this system is currently in prog ress. 4.3.6 Electrochemistry Complex 4 2 ha s been investigated by cyclic voltammetry (CV) to probe the redox properties of these systems. The CV of complex 4 2 was recorded in dichloromethane containing 0.1 M n Bu 4 PF 6 as supporting electrolyte, at 100 mVs 1 scan rate s A f errocene/ferrocenium couple was used as the standard. Glassy carbon was used as the working electrode, Ag wire served as the reference electrode and Pt wire was used as the auxiliary electrode. Complex 4 2 displays two irreversible reductio n peaks at 466 and 950 mV and one irreversible oxidation peak at 380 mV The reduction peaks can be tentatively assigned to the reduction of Mn IV and Mn III and the oxidation peak may be assigned to the oxidation of Mn III

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138 4.4 Conclusions A comproportio nation reaction in the presence of a calcium salt and chelate has proven to be a useful new route to novel Mn/Ca clusters. Our goal of targeting higher Mn:Ca ratio in the final product and diverting the reaction from Mn 3 Ca 2 was achieved by the incorporatio n of phenanthroline in the reaction system. Although the first product isolated from this strategy, Mn 8 Ca 4 ( 4 1 ), had a much higher nuclearity compared to the native OEC, it featured subunits within its structure that are similar to the OEC. Increasing the amount of chelate in a similar reaction led to the isolation of Mn 4 Ca 2 ( 4 2 ), where the metal nuclearity was halved and the amount of phenanthroline was doubled in comparison to 4 1 One of the striking features of 4 2 is the presence of the discrete [Mn 4 CaO 5 ] unit which is the exact metal topology of the OEC as obtained from the recent X ray structure at 1.9 Furthermore, the manganese oxidation state in 4 2 Mn III Mn 3 IV is the same as that of the OEC in the S 2 state of the Kok cycle. Detailed magnetic characterization of 4 2 revealed an S = ground state which is the same as the OEC in the S 2 state. EPR studies on 4 2 showed a multiline signal at g = 2 which is a characteristic feature of the S 2 state and another signal at g = 4.1. Temperature dependen ce study of the EPR signal also revealed a shift in the intensities of these two signals which has been attributed to the presence of easily accessible excited states in 4 2 arising from spin frustration. Although complex 4 2 possesses an additional Ca cen ter unlike the OEC, comparison of both EXAFS and XANES data of 4 2 with the native OEC has shown very close similarities establishing 4 2 to be a very close synthetic analog. Thus, 4 2 represents the only Mn/Ca in the literature to mimic the native OEC b oth structurally, electronically and magnetically.

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139 Table 4 1. Crystallographic Data for Complexes 4 1 t BuCO 2 H5.5MeCNH 2 O and 4 2 MeCN0.1H 2 O Parameter 4 1 4 2 formula a C 90 H 146.5 Ca 4 Cl 2 Mn 8 N 9.5 O 43 C 85 H 98.2 N 9 O 20.1 Mn 4 Ca 2 fw, g mol 1 a 2720.41 1867.47 crys tal system Monoclinic Triclinic space group P2 1 /c P 1 a 27.5305 (18) 13.217 (5) b 18.1039 (12) 13.495 (5) c 26.7466 (14) 14.571 (6) deg 90 77.380 (7) deg 111.541 (2) 76.653 (7) deg 90 87.050 (7) V 3 12399.7 (13) 2467.6 (17) Z 4 1 T C 173 (2) 100 (2) radiation, b 0.71073 0.71073 calc mg/m 3 1.457 1.355 mm 1 1.077 0.677 R 1 c,d 0.0481 0.0378 wR 2 e 0.1078 0.0846 a Including solvate molecules. b Graphite monochromator. c I > 2 ( I ). d R 1 = (|| F o | | F c ||) / | F o |. e wR 2 = [ [ w ( F o 2 F c 2 ) 2 ] / [ w ( F o 2 ) 2 ]] 1/2 w = 1/[ 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O) + 2 F c 2 ]/3.

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140 Table 4 2. BVS for the Mn a and selected O atoms b in 4 1 Mn II Mn III Mn IV BVS Assignment Mn1 4.17 3.81 4.00 O12 1.99 O 2 ( 3 ) Mn2 4.10 3.75 3.93 O13 1.94 O 2 ( 3 ) Mn3 4.16 3.80 3.99 O14 1.68 O 2 ( ) Mn4 4.14 3.85 3.95 O15 2.01 O 2 ( 3 ) O16 1.94 O 2 ( 3 ) O17 2.07 O 2 ( 3 ) O18 2.02 O 2 ( 4 ) O11 0.54 H 2 O ( ) O19 0.33 H 2 O (terminal) O20 0.34 H 2 O (terminal) a The bold 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 bold value. b A BVS in the ~ 1.8 2.0, ~1.0 1.2, and ~0.2 0.4 ranges for an O atom is indicative of non single and double protonation, respectively, but can be altered somewhat by hydrogen bonding. Table 4 3. BVS for the Mn a and selected O atoms b in 4 2 Mn II Mn III Mn IV BVS Assignment Mn1 3.62 3.31 3.47 O1 1.97 O 2 ( 4 ) Mn2 4.21 3.90 4.01 O2 1.93 O 2 ( 3 ) O3 1.89 O 2 ( 3 ) O11 1.62 t BuCO 2 a See footnote a of table 4 2. b See footnote b of table 4 2. Table 4 4. Mn EXAFS curve fitting for complex 4 2 Path R N 2 ( 2 ) R f (%) XRD () EXAFS () Mn O 1. 80 2.09 1.89 5 0.007 0.7 Mn N 2.11 2.15 1 0.003 Mn Mn 2.81 2.87 2.81 2 0.004 Mn C 2.9 3.1 2.97 3.5 0.004 Mn Mn 3.07 3.09 0.5 0.003 Mn Ca 3.29 3.41 3.40 1.5 0.007 Mn O 3.22 3.43 3.55 2.5 0.007 k=2.4 11.7 1 was used for the fit S 0 2 was fixed with 0.85, and E 0 of 6561.3 eV was used N value was fixed according to the crystal structure

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141 Table 4 5. Ca EXAFS curve fitting for complex 4 2 Path R N 2 ( 2 ) R f (%) XRD () EXAFS () Ca O 2.49 2.57 2.46 6.0 0.008 3.8 Ca N 2.62 2.65 2.0 0.003 Ca Mn 3.29 3.41 3.40 3.0 0.006 Ca C ~ 3.4 3.62 7.0 0.037 Ca Mn 4.37 4.37 1.0 0.004 Ca O Mn 4.39 4.39 2.0 0.004 k=2. 7 11. 5 1 was used for the fit S 0 2 was fixed with 1.0 and E 0 of 4050.0 eV was used N value was fixed according to the crystal structure Table 4 6. Characteristic EPR signals obtained for different synthetic analogues of the OEC possessing an S = ground state Mn Complex EPR Signals (ML = multiline) References [Mn III Mn IV ( O) 2 (bpy) 4 ] 3+ X band: ML with 16 peaks centered at g = 2, 6.6 K 280 [(CH 3 ) 4 (dtne)Mn III Mn IV ( O) ( OAc)](BPh 4 ) 2 X band: ML with 16 peaks centered at g = 2, 2.4 K 301 [(2 OH 3,5 Cl 2 salpn) 2 Mn III Mn VI ] X band: ML with 11 peaks centered at g = 2, 4 20 K, broad signal at g = 4.6, 110 K 302 [Mn III Mn II Mn III (5 Cl hsaladhp) 2 (sal) 4 ] X band: ML with 16 pea ks centered at g = 2 303 [Mn 3 O 4 ( bpy) 4 ( H 2 O ) 2 ] 4+ X band: ML with 35 peaks centered at g = 1.96, 4 K 304 [Mn 3 O 4 (bpy) 4 (Cl) 2 ] 4+ X band: ML with 35 peaks centered at g = 2, 4 K 305 [Mn 4 ( O) 3 ( 3 Cl)] 6+ X band: ML with 16 peaks centered at g = 2, broad signal at g = 5.2 and 12, 10 K 306, 307 [Mn 4 O 3 Cl 6 (OAc) 3 (HIm)] X band: ML with 16 peaks centered at g = 2, broad signal at g = 5.2 and 12, 10 K 276

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142 Fig ure 4 1. Labeled representation of the structure of 4 1 Hydrogen atoms have been omitted for clarity. Color code: Mn IV blue; Ca II yellow; O red; N cyan ; C grey.

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143 Figure 4 2. (top) Core of complex 4 1 and (bottom) core of 4 1 emphasizing the two [Mn IV 4 Ca] cubane moieties in the complex. Color code: Mn IV blue; Ca II yellow; O red

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144 Figure 4 3. Labeled representation of the structure of 4 2 Hydrogen atoms have been omitted for clarity. Color code: Mn IV blue; Mn III green; Ca II yellow; O red; N cyan ; C grey.

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145 Figure 4 4. Comparison of the core of 4 2 with the crystallographically derived core of the native OEC. (a) The 2011 [Mn 4 CaO 5 ] core of the OEC proposed by Umena et al. ; 79 (b) One of the proposed [Mn 4 CaO 4 ] cubane topologies with a single mono oxo bridge between Mn 3 Ca cube and the external Mn, 67 and (c) Fully labeled core of 4 2 emphasizing the Mn 4 CaO 5 unit.

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146 Figure 4 5. (a) Bond distances in 4 2 since the molecule is centrosymmetric only the unique distances are reported; (b) Bond distances in the 1.9 PSII structure atom labels as in reference 79 and (c) metal metal separation in 4 2

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147 Figure 4 6. Mn XANES from spinach PSII in the S 0 S 1 S 2 and S 3 states compared with the spectrum from Mn III Mn IV 3 Ca 2 complex 4 2 Inset shows the pre edge region. Figure 4 7. a) and b) The Mn EXAFS and the Fourier transforms of the OEC in the S 1 state, compared with complex 4 2 c) and d) The Ca EXAFS and the Fourier transforms of the OEC in the S 1 state compared with complex 4 2

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148 Figure 4 8. a) and c) are Mn and Ca EXAFS data from 4 2 and the best fits. b) and d) are the FTs of the Ca EXAFS from the complex and the best fits. The fit parameters are shown in Tables 4 4 and 4 5

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149 Figure 4 9. Plot of X M T vs T for complex 4 1 Figure 4 10. Plot of X M T vs T for complex 4 2 The solid line is the fit of the dat a; see the text for fit parameters.

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150 Figure 4 11. (top) [Mn III Mn IV 3 Ca 2 O 6 ] core of 4 2 from a view point emphasizing the Mn III Mn IV 3 butterfly unit; (bottom) the corresponding 2 J coupling scheme and definition of J wb and J bb exchange parameters.

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151 Fig ure 4 12. Plot of all the S T states for complex 4 2 as a function of energy. The ground state is | 1 / 2 3 7 / 2 > with the first excited state | 3 / 2 2 7 / 2 > ~ 12 cm 1 above the ground state. Figure 4 13. The core of 4 2 emphasizing the exchange coupling m odel employed, and the spin alignments corresponding to the | 1 / 2 3 7 / 2 > ground state i.e. S T = 1 / 2 S A = 3 and S B = 7 / 2

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152 Figure 4 14. (top) Plot of S T energies vs the J wb /J bb ratio, showing the change in the ground state. The dashed line corresponds to the experimentally determined J wb /J bb ratio of 0.74 for complex 4 2 (bottom) Depictions of the indicated | S T S A S B > as a function of varying J wb /J bb ratio.

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153 Figure 4 15. (top) [Mn III Mn IV 3 Ca 2 O 6 ] core of 4 2 from a view point emphasizing the Mn III Mn IV 3 butterfly unit; (bottom) the corresponding 3 J coupling scheme and definition of J 1 J 2 and J 3 exchange parameters. Figure 4 16. Plot of X M T vs T for complex 4 2 The solid line represents a simulation of the data in the temperature range 5 300K t o the 3J spin Hamiltonian of eq. 4 8.

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154 Figure 4 17. In phase AC magnetic susceptibility of complex 4 1 (top) and complex 4 2 (bottom) in a 3.5 G field oscillating at the indicated frequencies plotted as X M T vs T.

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155 Figure 4 18. Comparison of the CW E PR spectrum of 4 2 dissolved in 1:1 ratio of DCM/DMF and the powder. Figure 4 19. CW EPR at X band of (A) complex 4 2 dissolved in acetonitrile and dichloromethane (1:1 ratio) (9.48 GHz, 30K) and (B) S 2 state of PSII (9.65 GHz, 5K). Experimental paramet ers: power = 6mW; modulation amplitude = 8 G; modulation frequency = 100 kH.

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156 Figure 4 20. CW X band temperature dependent spectra of complex 4 2 dissolved in dimethylformamide and dichloromethane (1:1 ratio). Temperature varied from 5 to 80 K with 10 K increments. Experimental parameters: microwave frequency = 9.38 GHz; power = 2.5 mW; modulation amplitude = 10 G; modulation frequency = 100 kHz. Figure 4 21. CW X band temperature dependent spectra of complex 4 2 dissolved in acetonitrile and dichlorom ethane (1:1 ratio). Temperature varied from 5 to 80 K with 10 K increments. Experimental parameters: microwave frequency = 9.38 GHz; power = 2.5 mW; modulation amplitude = 10 G; modulation frequency = 100 kHz.

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157 Figure 4 22. CW X band temperature dependen t spectra of the powder of complex 4 2 Temperature varied from 10 to 70 K with 10 K increments. Experimental parameters: microwave frequency = 9.38 GHz; power = 2.5 mW; modulation amplitude = 10 G; modulation frequency = 100 kHz Figure 4 23. Cyclic vo ltammogram at 100 mVs 1 for complex 4 2 in dichloromethane, containing 0.1 M N n Bu 4 PF 6 as supporting electrolyte.

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158 CHAPTER 5 SYNTHESIS OF THE FIR ST ISOSTRUCTURAL MAN GANESE STRONTIUM CLUSTER MIMICKING TH E OXYGEN EVOLVING COMPLEX OF PHOTOSYSTEM II 5.1 Introdu ction The amazing mechanism developed by cyanobacteria ~ 2.7 billion years ago for oxidizing water using energy from sunlight represents one of the most challenging reactions that scientists would like to understand and mimic outside the biological site. 35, 49, 109, 199, 200, 308 The water oxidation catalyst developed by N ature is a heterometallic Mn/Ca cluster located at the beginning of the electron transport chain. This catalyst generates sufficient oxidizing po wer to extract electrons from water, which are then transferred to PSII. 239 241 As described in the previous chapters, various spectroscopic techniques have guided our understanding of this remarkable enzyme but there are still many unanswered questions. 35, 38, 63 66, 202, 309 Both Ca 2+ and Cl were known to be essential cofactors in the functioning of the OEC, even before the intimate association of calcium in the OEC wa s reported in the 3.5 X ray structure of the PS II. 310 314 Removing the calcium from PSII preparations suppresses O 2 evolution from the OEC, which can be restored (up to 90%) by replenishing with Ca 2+ 315 318 Although the calcium was initially attributed to only a structural role, blocking of the S state transitions arising from calcium extraction hints towards a functional role of calcium. 319, 320 In nearly all biological systems containing calcium, water has been known to complete the ligand environment around calcium. A similar situation has been proposed for the OEC, where the calcium center acts as a Lewis acid that activates t he bound water during the Kok cycle, leading to its oxidation to generate O 2 320 323

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159 One of the techniques that have been employed to investigate the function of Ca 2+ in the enzyme is to replace it with other meta l cations, and study the effect of this substitution on the structure and function of the enzyme. Among the various cations studied so far, Sr 2+ is the only metal that can functionally substitute for Ca 2+ to a significant extent ( ~ 40% of the activity is r estored). 324 Although Sr 2+ has a much larger ionic radius compared to Ca 2+ (1.12 vs 0.99 ) the similar pK a of water bound to Ca 2+ and Sr 2+ ions (12.80 vs 13.18) is the main reason for the activity in Sr substituted OEC. 320 Additionally, the lack of spectroscopic features for Ca 2+ has prompted the study of Sr substituted OEC to gain a better insight into the role of Ca 2+ in the OEC. 325, 326 Substitution of Ca 2+ by Mg 2+ which has a smaller ionic radius than Ca 2+ inhibits photoactivation 320, 324 whereas Ba 2+ acts as an inhibitor by competing with Mn 2+ in the formation of the firs t intermediate in construction of the OEC 327 Dy 3+ and Cd 2+ also inhibit the functioning of the OEC, although EXAFS studies on these substituted derivatives does not show many structural changes. 328, 329 Group I metal cations which have similar chemical properties to those of group II metal cations however, show lower affinit y for the calcium binding site due to their lower positive charge. The results are summarized in Table 5 1. It has been recently shown that thermophillic cyanobacteria can be photoautotrophically grown in either Ca 2+ or Sr 2+ containing media. 330 332 It has also been proposed that Nature may have tuned the active site to thermodynamically favor the binding of larger Sr 2+ with higher affinity. This selectivity would suppress the competition from smaller more abundan t ions like Mg 2+ Zn 2+ and Mn 2+ 327, 333, 334 As mentioned in Chapter s 3 and 4, the availability of synthetic analog ue s

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160 mimicking the OEC can play a crucial role in our understanding of the structure and function o f this enzyme. Although in the past couple of years, a handful of molecular Mn/Ca clusters have been reported, to date there is only one report of a Mn/Sr cluster a Mn 14 Sr cluster prepared in our group 335, 336 Ap art from the metal nuclearity being much higher than the OEC, an isostructural Ca analog could not be isolated. Thus, there is a need to develop synthetic methodology to isolate such isostructural complexes that would help test the current hypothesis in th e field as well as provide better understanding of the role of Ca 2+ in catalytic water oxidation. Our first attempt to achieve this goal was to replace the Ca 2+ with Sr 2+ in the synthesis of 3 1 Unfortunately, the isolated compound from this reaction crys tallized in a high space group and a unique solution could not be obtained. Metal analysis of this compound did show the incorporation of Sr in the final product. Extending this strategy in the synthesis of 4 2 has allowed us access to discrete Mn/Sr cuban e chemistry for the very first time. Moreover, one of the isolated complex es is isostructural with the Ca analog ue described in the previous chapter, which has allowed us to compare the spectroscopic and structural features of these related heterometallic cubane units. Detailed structural, magnetic and electronic characterization of these complexes and comparison s with the native OEC are reported in this chapter. 5.2 Experimental Section 5.2.1 Syntheses All manipulations were performed under aerobic condi tions using chemicals and solvents as received unless otherwise state. Mn(O 2 CBu t ) 2 Sr (O 2 CBu t ) 2 and NBu n 4 MnO 4 were prepared as previously reported in the literature. 168, 169

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161 W arning : Appropriate care should be take n in the use of NBu n 4 MnO 4 and readers are referred to the detailed warning given elsewhere. [ Mn 4 Sr 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) ( 5 1 ) : Mn(O 2 CBu t ) 2 2H 2 O (0.16 g, 0.50 mmol) was dissolved in a solvent mixture of acetonitrile /dichloromethane (20 mL 2:1 v/v ), and the resulting pink slurry was treated with pivalic acid (0.14 mL, 1.20 mmol), Sr (O 2 CBu t ) 2 2H 2 O (0.1 6 g, 0.50 mmol) and phenanthroline (0.18 g, 1.00 mmol), which caused the color to change to pale yellow. To this solution NBu n 4 MnO 4 (0.18 g, 0.50 mmol) was added drop wise as an acetonitrile solution (5 mL). The resulting reddish brown solution was stirred for 4 h, during which the solution color changed to deep brown. The solution was then filtered and the filtrate left undisturbed in a closed vial. X r ay quality black crystals of 5 1 MeCN0. 2 H 2 O grew over a week, which were collected by filtration, washed with ether and dried under vacuum. The yield was 4 4 %. Anal. Calcd (found) for 5 1 ( C 8 3 H 9 55 N 8 O 20 Mn 4 Sr 2 ): C, 52.07 ( 52.28 ); H 5.04 ( 5.32 ); N 6.43 ( 6.19 ). Selected IR data (cm 1 KBr pellets): 3432 (br), 3073 (m), 2951 (s), 2865 (m), 1700 (m), 1606 (s), 1565 (s), 1513 (m), 1479 (s), 1455 (s), 1426 (s), 1399 (s), 1348 (s), 1220 (s), 1144 (m), 1105 (m), 1028 (m), 889 (m), 855 (s), 785 (m), 725 (m), 659 (s), 603 (s), 578 (s), 511 (m) and 436 (m). [Mn 8 Sr 4 O 14 (O 2 CBu t ) 12 (phen) 4 (H 2 O) 2 ] (5 2) : Mn(O 2 CBu t ) 2 2H 2 O (0.16 g, 0.50 mmol) was dissolved in a mixture of acetonitrile /chloroform (20 mL 10:1 v/v ), and the resulting pink slurry was treated with pivalic acid (0. 14 mL, 1.20 mmol), Sr(ClO 4 ) 2 2H 2 O (0. 10 g, 0. 25 mmol) and phenanthroline (0.09 g, 0.50 mmol), which caused the color to change to pale yellow. To this solution solid NBu n 4 MnO 4 (0.18 g, 0.50 mmol) was slowly added The resulting reddish brown solution was s tirred for 4 h, during which the

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162 solution color changed to deep brown. This was then filtered and the filtrate left undisturbed in a closed vial. After 3 weeks, X ray quality brown cry stals of 5 2 had formed which were collected by filtration, washed with ether and dried under vacuum. The yield was 3 0 %. Anal. Calcd (found) for 5 2 ( C 108 H 144 N 8 O 40 Mn 8 Sr 4 ): C, 43.46 ( 43.09 ); H 4.86 ( 4.88 ); N 3.75 ( 3.32 ). Selected IR data (cm 1 KBr pellets): 3422 (br), 2954 (m), 2866 (m), 1627 (m), 1588 (s), 1555 (s), 1514 (s ), 1480 (s), 1456 (s), 1406 (s), 1342 (s), 1220 (s), 1142 (m), 1101 (m), 874 (m), 844 (m), 786 (m), 726 (m), 650 (m), 591 (s), 481 (m) and 442 (m). 5.2.2 X ray Crystallography Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detecto r and a graphite monochromator utilizing Mo Suitable crystals of 5 1 MeCN0. 2 H 2 O and 5 2 were attached to a glass fiber s using paratone oil and transferred to a goniostat for data collection. Data collection and unit cell para meters are listed in Table 5 2 Cell parameters were refined using up to 8192 scan method (0.3 o frame width). The first 50 frames were re measured at the end of data collection to m onitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. Raw data frames were read by program SAINT and integrated using 3D profiling algorithms The resulting data were reduced to produce hkl reflections and their intensities and estimated standard deviations. The data were corrected for Lorentz and polarization effects and numerical absorption corrections were applied based on indexed and mea sured faces. The structures were solved by d irect m ethods in SHELXTL6 170 and refined on F 2 using full matrix least squares. The non H atoms were

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163 treated anisotropically, whereas the H atoms were calculated in ideal positions and were riding on their respective C atoms. For 5 1 t he asymmetric unit consists of a half Mn 4 Sr 2 cluster, located on an inversion center, a pivalate anion an acetonitrile solvent molecule and a partial water molecule (20% occupancy ). It was concluded from a BVS analysis that the cluster is positively charged, which supports a pivalate anion rather than a pivalic acid This conclusion is supported by the fact that the X ray intensity data could not locate the presence of the hydroxyl proton on the pivalate anion The acetonitrile molecule is disordered and was refined in three parts with the displacement parameters of the N atom and the two C atoms held equivalent during the final refinement cycles (using the EADP command). A total of 615 parameters were refined in the final cycle of refi nement using 9413 reflections with I > 2 (I) to yield R 1 and wR 2 of 2.92 and 7.30%, respectively. For 5 2 t he asymmetric unit consists of a half Mn 8 Sr 4 cluster. The protons on the water ligand O14 were obtained from a d ifference Fourier map and refined fr eely. In the final cycle of refinement, 14486 reflections (of which 11043 are observed with I > 2 (I)) were used to refine 784 parameters and the resulting R 1 wR 2 and S (goodness of fit) were 3.10%, 7.62% and 0.949, respectively. 5.3 Results and Discu ssion 5.3.1 Syntheses Isolation of 4 1 and 4 2 demonstrated that incorporation of chelates and slight modification of the reaction condition s from the original comproportionation reaction scheme in Chapter 3 w ere fruitful in diverting the reaction from 3 1 The first entry to the Mn 4 CaO 5 cubane was achieved using this strategy. One of our long standing goals has

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164 been to isolate isostructural Mn/Sr clusters for comparative Ca vs Sr studies. Replacement of Ca 2+ in the synthesis of 4 2 was targeted to achieve this objective. This strategy did prove fruitful. When Ca(O 2 CBu t ) 2 was replaced with Sr(O 2 CBu t ) 2 a crystalline material was obtained whose IR spectrum showed close similarity with 4 2 Tuning of the solvent system in the reaction to an acetonitrile/dichl oromethane solvent mixture finally led to the isolation of 5 1 in 44% yield. 5 1 is the first isostructural Mn/Sr cluster to a known Mn/Ca cluster to be reported in the literature and detailed structural comparisons of the two are reported later in the se ction. The formation of 5 1 is illustrated in eq. 5 1. 2 Mn 2+ + 2 Sr 2+ + 2 MnO 4 + 7 t BuCO 2 H + 4 phen + 3 e [Mn 4 Sr 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) + 3 H + + 2 H 2 O ( 5 1 ) Like all the previous reactions, the choice of solvent, reaction time and s tarting material s play a crucial role in isolating pure 5 1 in high yields. Following this breakthrough into Mn/Sr cubane chemistry the next strategy was to tune the reaction logically for targeting higher Mn:Sr ratio in the final product. The goal was to isolate the exact OEC core with Sr instead of Ca. In order to meet these criteria the amount of Sr in eq. 5 1 was decreased while keeping all the other parameters constant. This procedure led to a mixture of products which were difficult to separate. Chan ging the Sr 2+ source to Sr(ClO 4 ) 2 proved promising, and fine tuning the reaction solvent finally led to the isolation of pure crystalline 5 2 in 30% yield. However, this did not give a Mn 4 Sr complex; instead a higher nuclearity Mn 8 Sr 4 complex formed This complex bears some structural similarities with 4 1 details of which are described in the later section. The formation of 5 2 is summarized in eq. 5 2.

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165 4 Mn 2+ + 4 Sr 2+ + 4 MnO 4 + 12 t BuCO 2 H + 4 phen + 4 e [Mn 8 Sr 4 O 14 (O 2 CBu t ) 1 2 (phen) 4 (H 2 O) 2 ] + 8 H + ( 5 2 ) Increasing the amount of phenanthroline to target lower nuclearity Mn/Sr product proved to be unsuccessful, and led to the formation of 5 2 in lower yields. 5.3.2 Description of Structures 5. 3.2.1 Structure of [ Mn 4 Sr 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) ( 5 1 ) The partially labeled structure of 5 1 is shown in Figure 5 1, and selected interatomic distances and angles are summarized in Table A 8 Complex 5 1 crystallizes in triclinic space group P 1 The oxidation state of the manganese atoms and the protonation levels of the O 2 and carboxylate O atoms were determined from the a combination of charge consideration, inspection of bond lengths and bond valence sum (BVS) calculations (Table 5 3). Like 4 2 complex 5 1 was also found to be mixed valent, comprising of 1 Mn III and three Mn IV ions. 5 1 is also centrosymmetric, like 4 2 which again prevented the unambiguous assignments of the metal oxidation states based on BVS calculations alone. The BVS value for Mn 1 is higher than a typical Mn III and lower than Mn IV and it takes an average value of Mn 3.5+ This is due to the centrosymmetric core of 5 1 in which the Mn III and Mn IV are indistinguishable by X ray crystallography It should be noted that this ca nnot be attributed to the electron d elocalization between Mn III and Mn IV which requires more energy due to the structural changes associated with the JT elongation of Mn III All the manganese atoms are hexa coordinated and possess a distorted octahedral ge ometry, whereas the strontium centers are octa coordinated. 5 1 again like 4 2 also possesses a [ Mn III Mn 3 IV Sr 2 ( 4 O ) 2 ( 3 O) 4 ] 7+ core which is composed of two Mn 3 Sr cubes fused together via a common face, giving rise to a dicubane topology (Figure 5 2) Th e cubes are connected via four

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166 oxides and the common face between the cubes is composed of a [Mn 2 O 2 ] rhombus. The peripheral ligation is provided by six bridging 1 : 1 : pivalate groups and four phenanthroline groups; one on each of the peripheral atoms of the dicubane. With the isolation of 5 1 entry into the discrete Mn 4 Sr unit has been achieved for the very first time. Apart from Mn 14 Sr complex this is the second molecular Mn/Sr cluster to be discovered 335, 336 Furthermore, 5 1 is the only example of an isostructural Mn/Ca vs Mn/Sr pair of cluster s, which sh ould help in test ing the current structural hypothes e s related to Sr substituted analog ue s. Initially, it was hypothesized that substitution of Ca 2+ with S r 2+ would cause major structural and electronic changes to the OEC. However, recent studies (DFT calculation, 337 339 polarized EXAFS 218, 340, 341 and 55 Mn ENDOR 342, 343 ) have confirmed that the overall geometry of the OEC cluster remains unaffected by this substitution. 344 Incorporation of Sr 2+ however elongates the Sr Mn distances by 0.06 when compared to Ca Mn distances. 218, 271, 344 A similar situation is observed when the Mn M distances (M = Ca / Sr ) are compared between 4 2 and 5 1 ( Table 5 4 ); an ~0.1 elongation in Mn M bonds is observed in 5 1 compared with 4 2 Comparison of the metal oxide bond distances shows slight elongation in Sr O bond lengths as opposed to Ca O which can be attributed to the larger ionic radi us of Sr 2+ compared with Ca 2+ A similar trend is also observed in the M O M bond angles, as shown in Table 5 6. Th ese data are consistent with the current hypothesis that substitution of Ca 2+ with Sr 2+ does not affect the Mn Mn and Mn O distances significantly, which emphasizes a functional instead of structural role of Ca 2+ in water splitting at the OEC. 311 345

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167 5.3.2.2 Structure of [Mn 8 Sr 4 O 14 (O 2 CBu t ) 12 (phen) 4 (H 2 O) 2 ] (5 2) The partially labeled structure of [Mn 8 Sr 4 O 14 (O 2 CBu t ) 12 (phen) 4 (H 2 O) 2 ] ( 5 2 ) is shown in Figure 5 3, and the selected interatomic distances and bond angles are summarized in Table A 9 C omplex 5 2 crystallizes in the triclinic space group P 1 The oxidation state of the manganese atoms and the protonation levels of the O 2 H 2 O and the carboxylate O atoms were determined by a combination of charge balance consideration, inspection of bond lengths and bond valence sum calculations (Table 5 7). The lower BVS value for O18 (a terminal pivalate group) is due to hydrogen bonding with the bridging water molecule. All the manganese centers in 5 2 are in +4 oxidation state, making it a homovalent complex in contrast to 5 1 The [Mn 8 IV Sr 4 ( O) 2 ( 3 O) 10 ( 4 O) 2 ( H 2 O) 2 ] 12+ core of 5 2 is shown in detail in Figure 5 4. All the manganese centers are hexa coordinated and possess a distorted octahedral geometry whereas the strontium centers are hepta coordinated. The metal nuclearity and the ove rall metal topology of 5 2 are similar to 4 1 but there are several key differences which may be attributed to the presence of more phenanthroline groups and the larger ionic radi us of Sr 2+ compared with Ca 2+ Unlike 4 1 which contained the [Mn 3 IV CaO 4 ] cubane units, 5 2 does not contain any cubane units. The core of 5 2 can be described as being composed of two open faced [Mn 3 IV SrO 4 ] cubane units resulting from O5/O5' bridging only two manganese centers, which is in contrast to 4 1 O5/O5' is hydrogen bonded to the bridging water molecule connecting Sr1 and Sr2, which results in a lower BVS number for O5/O5' (Table 5 7). These cubane units are further connected to an external manganese atom (Mn4/Mn4') via two 3 oxides (O2 and O8). This is slightly diff erent from 4 1 where the external Mn

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168 was connected via oxide and a 3 oxide (Figure 5 5). As a consequence the strontium atom s are further apart in 5 2 (4.0283 (3) ) in comparison to the calcium atoms in 4 1 (3.7982 (10) ). So, the overall core of 5 2 can be best described as being composed of four incomplete [Mn 3 IV SrO 4 ] units sharing a common edge (Mn O) and vertex (Mn center) with the neighboring units. Finally, the peripheral ligation is provided by eight pivalates in a 1 : 1 : bridging mode, two pivalates in 1 : 2 : bridging mode and two pivalates binding in a 1 terminal mode (hydrogen bonds with the bridging water molecule which lowers the BVS value). Four phenanthroline groups each on Mn4, Mn4', Sr2 and Sr2' complete the peripheral ligation of 5 2 The terminal water molecules present in 4 1 are now replaced by phenanthroline and terminal pivalate groups in 5 2 5.3.3 Mn and Sr X ray Absorption Spectroscopy The Mn and Sr XAS spectra were collected by the Yachandra group at the Lawrence Berkeley National Laboratory Detailed experimental procedure listed in Appendix C. Apart from Mn XAS measurements on the native OEC, a common strategy that has been employed for structural elucidation of the OEC is substitution of Ca with other metals (Sr 2+ Cd 2+ Dy 3+ etc ) and then the use of Mn EXAFS data to detect the changes in the cluster. 328, 346 However, due to the difficulty in isolating all Mn Ca/Sr com ponents of the Mn EXAFS spect r a the reverse experiment in wh ich the back scattering from the Mn centers is probed using Ca/Sr EXAFS is an elegant alternative. 271 Although Ca 2+ is present in the native OEC, Sr 2+ is a better candidate for XAS study due to the following reasons: (i) the X ray energies for the K edge (16 keV) are much more penetrating and less attenuated by air and (ii) strontium yields a higher

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169 X ray absorption cross section and fluorescence in comparison to calcium, which makes this experiment much more facile. 271 Figure 5 6 compares the Mn XANES spectrum of the OEC in the S 1 state with the sp ectrum of Mn 4 Sr 2 ( 5 1 ). The overall profile of the curve is very similar to that obtained from 4 2 which is expected as the edge position is sensitive to the Mn oxidation state. In both these complexes the formal oxidation state is Mn III Mn IV 3 M 2 [ M = Ca( 4 2 ) or Sr ( 5 1 )] Mn EXAFS studies on 5 1 were also carried out. Comparison s of the Mn K edge EXAFS (k 3 weighted) spectra with those of the OEC in the S 1 state are shown in Figure 5 7 (a), and the Fourier transforms are shown in Figure 5 7(b). As mentioned in the previous chapters, the three main peaks, labeled I III, have been assigned to the Mn ligand (peak I), di oxide bridged Mn Mn (peak II), and the Mn Mn and Mn Ca separations (peak III) in the native OEC. 218, 271, 272, 347 Like 4 2 5 1 also displayed a similarity in beat patterns along with minor differences in the phases and spectral features (Figure 5 7a). The corresponding FT in Figure 5 7b shows significant similarity to the spectral features obtained from the native OEC. However, the presence of the additional strontium center in 5 1 affects the Mn Mn dis tances which are slightly elongated in comparison to the OEC. Furthermore, peak III in 5 1 is attributed only to the Mn Sr distance (>3.3 ) as no mono oxo bridged Mn Mn interaction exists in 5 1 When the FT EXFAS of 5 1 is compared with 4 2 the Mn Mn interactions are very similar, but the Mn Sr are slightly elongated in comparison to the Mn Ca distance (in 4 2 ) which is consistent with the EXAFS data from Sr substituted OEC. 218

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170 The Sr K edge EXAFS ( k 3 weighted) spectra of 5 1 are Sr reactivated PSII preparations in the S 1 state are shown in Figure 5 7c. The Fourier transformed spectra are shown in Figure 5 7d. The first Fourier peak arises from the first shell of ligands arising around the Sr, and co mparison of peak I in Figure 5 7d indicates close similarity between 5 1 and the Sr substituted OEC. Peak II which arises from Sr Mn distances shows striking similarities, and this distance is slightly longer than the Ca Mn distances obtained in 4 2 Finally, the dicubane topology of 5 1 gives rise to the longer Mn Sr 218 interactions which are absent in the native OEC. Thus, 5 1 is the only Mn/Sr cluster in the literature to show such close similarity to Sr substituted OEC derivative. 5.3.4 Magnetochemistry of Complexes 5 1 and 5 2 5.3.4.1 DC studies Solid state variable temperature DC magnetic susceptibility data in a 0.1 T field w ere collected on powdered microcrystalline samples of 5 1 and 5 2 restrained in eicosane in the 5.0 300.0 K range. For 5 1 the X M T value of 5.49 cm 3 Kmol 1 at 300 K is less than the spin only value of 8.63 cm 3 Kmol 1 ( g = 2.0) expected for three non interacting Mn IV ( S = 3 / 2 ) and one Mn III ( S = 2) ions, indicating the presence o f dominant antiferromagnetic exchange interactions within complex 5 1 The X M T value steadily decreases with decreasing temperature to 0.44 cm 3 Kmol 1 at 5.0K, which is consistent with an S = ground state. The individual exchange parameters (J) between th e Mn Mn pairs within the molecule w ere obtained by fitting the X M T vs T data for 5 1 to the appropriate theoretical expression for a Mn III Mn 3 IV butterfly model (Figure 5 8). As described in detail in the previous chapter, the isotropic Heisenberg Dirac Van Vleck (HDVV) spin

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171 Hamiltonian describing the exchange interactions within this Mn 4 core is given by eq. 5 to to H = 2J wb ( 1 2 + 2 3 + 3 4 + 4 1 ) 2J bb 2 4 (5 3) The eigen values of the spin Hamiltonian can be determined using the Kambe vector coupling method 219 with the following coupling scheme: A = 2 + 4 B = 1 + 3 and T = A + B The spin Hamiltonian in eq. 5 3 can now be expressed in the equivalent form of H = 2J wb ( T 2 A 2 B 2 ) 2J bb B 2 (5 4) The energies of the spin states, which are eigen values o f the Hamiltonian in this coupling scheme, are given by eq. 5 5 where E S T S A S B > is the energy of the state S T S A S B >, and the constant terms contributing equally to all the states have been omitted. The overall multiplicity of the system is 320, an d is made of 50 individual spin states ranging from S T = 11 / 2 1 / 2 E S T S A S B > = J wb [ S T ( S T + 1) S A ( S A + 1) S B ( S B + 1)] J bb [ S B ( S B + 1)] (5 5 ) A theoretical expression for the molar paramagnetic susceptibility ( X M ) vs temperature was derived for 5 1 like 4 2 using the van Vleck equation 220 This expression was used to fit the experimental X M T vs T data for 5 1 ( solid line in Figure 5 9 ) and the parameters varied were J wb J bb g and TI P. The fitting parameters were, J wb = 17.95 ( 2 ) cm 1 J bb = 21.55 ( 3 ) cm 1 g = 2.03 (2) and TIP = 600x10 6 cm 3 mol 1 Using these values the S T state energy ladder can be created (Figure 5 10 ) which confirms a | 1 / 2 3 7 / 2 > ground state for this system with the first excited state being | 3 / 2 2 7 / 2 > at ~ 32 cm 1 above the ground state.

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172 This detailed magnetic characterization of 5 1 and its comparison with 4 2 reveals several key features that are consistent with the recent EPR and DFT studies on Sr substituted OEC. Firstly, substitution of Ca 2+ with Sr 2+ does not affect the overall electronic properties of the Mn 4 M 2 core, and both complexes display the same ground state ( S = 1 / 2 ). A similar situation is encountered in the Sr substituted OEC where th e same g = 2 multiline signal is observed. 344 Initially, slight differences in the hyperfine pattern of the EPR signal of Sr substituted OEC, were attributed to major structural changes in the OEC due to strontium substitution. 348 However, recent CW X band, field sweep ESE Q band and 55 Mn ENDOR spectra have shown surprising similarities between the Mn 4 O X Ca and Mn 4 O X Sr forms of the OEC. 326, 344 Furthermore, DFT calculations have also proposed that the exchange coupling between the metal centers should not be very sensitive to strontium substitution. 344 This is consistent with our magnetic analyses, which concluded that the exchange couplings in 5 1 and 4 2 are very similar (Table 5 8). This c an be rationalized by the very small changes in the Mn Mn, Mn O distances and Mn O Mn angles in the ma gnetic core (Table 5 4 to 5 6). Like 4 2 the S = ground state in 5 1 can also be rationalized based on the obtained exchange interactions as shown in Figure 4 13. 5 1 also demonstrates spin frustration where the J bb pathway loses out to the four J wb interactions. Thus, a spin state energy manifold vs the J wb /J bb ratio has been computed to explore the variation of the ground state as a function of the relative magnitudes of J wb and J bb (Figure 5 11) The experimental J wb /J bb = 0. 83 (shown as the dashed line in Figure 5 11 ) shows that the J wb /J bb ratio is close to the crossover point to an S = 3 / 2 ground state. The overall profile is similar to that of 4 2 but the energies of the spin states and the slope of the

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173 lines are slightly different due to the difference in the J values in 4 2 and 5 1 For 5 2 the 300 K X M T value of 14.29 cm 3 Kmol 1 is slightly lower than the spin only value of 15.00 cm 3 Kmol 1 ( g = 2) expected for eight non interacting Mn IV ions, indicating the presence of intramolecular antiferromagnetic interactions. The X M T value of 5 2 decreases with decreasing temperature and the plot does not appear to be heading to zero at 0 K, indicating 5 2 possesses a non zero ground state. The 5 K value can be compared with the spin only ( g = 2) value of 1.00 cm 3 Kmol 1 expected for an S = 1 state. However, other experiments need to be performed to unambiguously assign the ground state in 5 2 The core to pology of 5 2 is similar to 4 1 which has an S = 0 ground state. The presence of several competing interactions within these cores arising from a large number of triangular units, substitution of Ca 2+ with Sr 2+ may be some of the factors that affect the c oupling between the metal centers resulting in a different ground state for 5 2 and 4 1 5.3.4.2 AC studies An AC magnetic susceptibility study was performed to independently confirm the ground state and probe the dynamics of magnetization (magnetic momen t) relaxation. The AC studies on 5 1 and 5 2 were performed in the 1.8 15.0 K range using a weak 3.5 G ac field oscillating at frequencies of 50 1000 Hz. The in phase ( X M ) AC susceptibility measurement is invaluable for assessing the ground state with out any complications from a DC field (Figure 5 13 ). For 5 1 the X M T decreases from 0.69 cm 3 Kmol 1 at 15 K to 0.42 cm 3 Kmol 1 at 1.8 K. Extrapolation of the data from 8 K to 0 K gives a value of ~ 0.4 cm 3 Kmol 1 which indicates an S = ground state in a greement with the dc studies. For 5 2 the drop in the X M T value is much steeper compared to 5

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174 1 which is indicative of the presence of several low lying excited states. Extrapolation of the plot gives a X M T value ~ 1 cm 3 Kmol 1 which indicates an S = 1 ground state for 5 2 again in agreement with the dc studies. 5.3.5 Electrochemistry Complex 5 1 ha s been investigated by cyclic voltammetry (CV) to probe the redox properties of these systems. The CV of complex 5 1 was recorded in dichloromethane contain ing 0.1 M n Bu 4 PF 6 as supporting electrolyte, at 100 mVs 1 scan rate s A f errocene/ferrocenium couple was used as the standard. Glassy carbon was used as the working electrode, Ag wire served as the reference electrode and Pt wire was used as the auxiliary electrode. Complex 5 1 displays two irreversible reduction peaks at 496 and 981 mV and one irreversible oxidation peak at 662 mV The reduction peaks can be tentatively assigned to the reduction of Mn IV and Mn III and the oxidation peak may be assigned t o the oxidation of Mn III Since the electrochemical measurements for both 5 1 and 4 2 were carried out in the same solvent, a comparison of the redox properties of the two complexes is possible The substitution of the Ca 2+ with Sr 2+ in 5 1 does affect the redox potential in comparison to 4 2 It has been shown that the oxidation potential of the manganese cluster in the calcium deprived S 2 state is abnormally low. 312, 349 351 It can be hypothesized that replacemen t of the Sr 2+ may have some effect on the redox properties of the Sr substituted OEC in a manner similar to 5 1 5.4 Conclusions As part of our continuing interest in preparing synthetic analog ue s of the OEC and its modified form s Mn/Sr chemistry has been explored which has allowed entry into

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175 discrete Mn/Sr cubane chemistry. Replacing Ca 2+ with Sr 2+ in the synthesis of 4 2 along with some tweaking of the reaction condition s has led to the isolation of 5 1 which is isostructural with 4 2 5 1 is the secon d molecular Mn/Sr cluster to be isolated to date and the only one to possess the discrete Mn 3 Sr cubane with an additional Mn attached to it. Synthesis of this isostructural Mn 4 Sr 2 cluster has allowed detailed structural, magnetic and spectroscopic comparis on s with the calcium analog has helped provide a better understanding about the role of calcium in the OEC. It has been hypothesized that although strontium has a larger ionic radi us compared to calcium it can substitute Ca in the native OEC without much structural change to the cluster. A similar situation is encountered in 5 1 where there is a ~ 0.1 elongation in Mn Sr distances compared to 4 2 whereas the Mn Mn distances are not affected by this substitution. This is further verified by both Mn an d Sr EXAFS data on 5 1 which also bears striking similarity to the Sr substituted OEC analog. 5 1 is the only Mn/Sr complex to provide such a striking similarity to those of the Sr substituted OEC. The manganese oxidation state in 5 1 is similar to 4 2 (M n III Mn 3 IV ) making 5 1 the only Mn/Sr cluster to mimic the S 2 state of Kok cycle electronically and magnetically. Detailed magnetic characterization of 5 1 revealed an S = ground state, and the exchange coupling between the manganese centers are comparabl e to those of 4 2 Th e s e data is consistent with the current hypothesis based on EPR and DFT calculations that Sr substitution does not affect the electronic propert ies of the OEC. 344 Detailed EPR studies on 5 1 and comparison s with 4 2 are currently in progress. Attempts to synthesiz e the exact Mn 4 Sr core by increasing the Mn:Sr ratio in 5 1 synthesis were not successful, and led to the isolation of a higher nuclearity Mn 8 Sr 4 ( 5

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176 2 ) which displays some structural similarities to 4 1 However, the incorporation of larger Sr and the pre sence of excess phenanthroline induce some structural perturbation to the core in comparison to 4 1 These subtle changes also affect the coupling between the manganese centers resulting in an S = 1 ground state in 5 2 (as opposed to S = 0 in 4 1 ). Th ese d ata emphasize the difficulty in making Sr analogues of Mn/Ca clusters with identical structural and electronic properties. The isolation of 5 1 is an interesting breakthrough in this area which provides an additional tool in understanding the structure an d functioning of this critical biological site.

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177 Table 5 1. Metal ion activators and inhibitors of the OEC Metal Ionic radius () pK a of aqua ion Effect on O 2 evolution activity Ca 2+ 0.99 12.80 Activator 317 S r 2+ 1.12 13.18 Activator 324 Ba 2+ 1.35 13.36 Weak inhibitor 333 Mg 2+ 0.65 11.41 Inhbitor 352 Mn 2+ 0.80 10.6 Weak inhibitor 327 Cd 2+ 1.03 9.00 Competitive inhibitor 329 La 3+ 1.04 8.82 Competitive inhibitor 353 K + 1.33 16.00 Competitive inhibitor 354, 355 Cs + 1.65 > 17.00 Competitive inhibitor 354, 355 Table 5 2 Crystallographic Data for Complexes 5 2 MeCN 0.2H 2 O and 5 1 Parameter 5 1 5 2 formula a C 85 H 98 .4 N 9 O 20 .2 Mn 4 Sr 2 C 108 H 144 N 8 O 40 Mn 8 Sr 4 fw, g mol 1 a 1964.356 2984.31 crystal system Triclinic Triclinic space group P 1 P 1 a 13.3075 (2) 15.0436 (15) b 13.4069 (2) 15.0750 (8) c 14.6438 (2) 16.4407 (9) deg 77.564 (1) 114.030 (3) deg 76.870 (1) 98.288 (4) deg 88.576 (1) 105.075 (4) V 3 2483.84 (6) 3153.2 (4) Z 1 1 T C 100 (2) 173 (2) radiat ion, b 0.71073 0.71073 calc mg/m 3 1.412 1.572 mm 1 1.632 2.529 R 1 c,d 0.0292 0.0310 wR 2 e 0.0730 0.0762 a Including solvate molecules. b Graphite monochromator. c I > 2 ( I ). d R 1 = (|| F o | | F c ||) / | F o |. e wR 2 = [ [ w ( F o 2 F c 2 ) 2 ] / [ w ( F o 2 ) 2 ]] 1 /2 w = 1/[ 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O) + 2 F c 2 ]/3.

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178 Table 5 3 BVS for the Mn a and selected oxygen atoms b in 5 1 Mn II Mn III Mn IV BVS Assignment Mn1 3.62 3. 28 3.47 O1 1.99 O 2 ( 4 ) Mn2 4.21 3.90 4.01 O2 1.81 O 2 ( 3 ) O3 1.80 O 2 ( 3 ) O11 1.59 t BuCO 2 a The bold 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 bold value. b A BVS in the ~ 1.8 2.0, ~1.0 1.2, and ~0.2 0.4 ranges for an O atom is indicative of non single and double protonation, respectively, but can be altered somewhat by hydrogen bonding. Table 5 4 Comparison of Mn Mn and Mn Ca/Sr distances in 5 1 and 4 2 [Mn 4 Sr 2 ] ( 5 1 ) [Mn 4 Ca 2 ] ( 4 2 ) Mn III M Mn1' Sr1 3.3812 (3) Mn1' Ca1 3.2925 (12) Mn I V M Mn2 Sr1 3.5145 (3) Mn2 Ca1 3.4143 (12) Mn I V M Mn2 Sr1' 3.4992 (3) Mn2 Ca1' 3.4006 (9) Mn III Mn IV Mn1' Mn2 2.8571 (4) Mn1' Mn2 2.8716 (11) Mn I V Mn IV Mn2' Mn2 3.0816 (5) Mn2' M n2 3.0715 (11) Mn I V Mn IV Mn2' Mn1 2.8571 (4) Mn2' Mn1 2.8716 (11) M stands for Sr or Ca. Table 5 5 Comparison of metal oxide bond distances in 5 1 and 4 2 [Mn 4 Sr 2 ] ( 5 1 ) [Mn 4 Ca 2 ] ( 4 2 ) Mn1' O3' 1.7968 (12) Mn1' O3' 1.7970 (14) Mn1' O2' 1.8007 (11) Mn1' O2' 1.8034 (15) Mn1' O1' 1.8688 (15) Mn1' O1' 1.8869 (15) Mn2' O3' 1.8746 (13) Mn2' O3' 1.8783 (16) Mn2' O1' 2.0659 (13) Mn2' O1' 2.0921 (16) Mn2 O2' 1.8760 (13) Mn2 O2' 1.8877 (16) Sr1' O3' 2.6422 (11) Ca1' O3' 2.5657 (14) Sr1' O2' 2.6110 (12) Ca1' O2' 2.4897 (17) Sr1' O1 2.6225 (15) Ca1' O1 2.5198 (19)

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179 Table 5 6 Comparison of metal oxo metal bond angle in 5 1 and 4 2 [Mn 4 Sr 2 ] ( 5 1 ) [Mn 4 Ca 2 ] ( 4 2 ) Mn1' O3' Sr1' 97.39 (5) Mn1' O3' Ca1' 96.40 (6) Mn1' O3' Mn2' 102.17 (6) Mn1' O3' Mn 2' 102.74 (7) Mn1' O2' Sr1' 98.39 (5) Mn1' O2' Ca1' 98.90 (6) Mn1' O2' Mn2 100.10 (6) Mn1' O2' Mn2 99.35 (6) Mn1' O1' Mn2' 93.87 (6) Mn1' O1' Mn2' 93.63 (6) Mn1' O1' Mn2 92.99 (6) Mn1' O1' Mn2 92.47 (7) Mn2' O3' Sr1' 100.17 (6) Mn2' O3' Ca1' 98.68 (7) Mn2' O1 Sr1' 97.80 (5) Mn2' O1 Ca1' 97.66 (6) Mn2' O1' Mn2 98.93 (5) Mn2' O1' Mn2 98.12 (6) Mn1 O3 Mn2 102.17 (6) Mn1 O3 Mn2 102.74 (7) Table 5 7 BVS for the Mn a and selected oxygen atoms b in 5 2 Mn II Mn III Mn IV BVS Assignment Mn1 4.14 3.79 3.9 8 O1 1.92 O 2 ( 4 ) Mn2 4.17 3.87 3.97 O2 1.94 O 2 ( 3 ) Mn3 4.08 3.73 3.91 O3 1.98 O 2 ( 3 ) Mn4 4.17 3.82 4.01 O4 1.86 O 2 ( 3 ) O8 1.84 O 2 ( 3 ) O9 1.87 O 2 ( 3 ) O5 1.62 O 2 ( ) O14 0.57 H 2 O ( ) O18 1.44 t BuCO 2 (terminal) a See footnote a of table 5 2. b See footnote b of table 5 2. Table 5 8 Comparison of the exchange couplings in 5 1 and 4 2 [Mn 4 Sr 2 ] ( 5 1 ) [Mn 4 Ca 2 ] ( 4 2 ) J wb (cm 1 ) 17.95 (2) 14.82 (1) J bb (cm 1 ) 21.55 (3) 20.3 (1.5)

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180 Figure 5 1. Labeled representation of the s tructure of 5 1 Hydrogen atoms have been omitted for clarity. Color code: Mn IV blue; Mn III green; Sr II orange; O red; N cyan ; C grey. Figure 5 2. Comparison of [Mn 4 M 2 ] core 5 1 (left) and 4 2 (right).

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181 Figure 5 3. Labeled representation of the stru cture of 5 2 Hydrogen atoms and methyl groups on the pivalate ligand have been omitted for clarity. Color code: Mn IV blue; Sr II orange; O red; N cyan ; C grey.

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182 Figure 5 4. (top) Core of complex 5 2 and (bottom) core of 4 1 emphasizing the four open faced [Mn IV 3 SrO 4 ] cubane moieties in the complex. Color code: Mn IV blue; Sr II orange; O red

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183 Figure 5 5. Structural comparison of the two [Mn 8 M 4 ] (M = Sr/Ca) core; (left) [Mn 8 Sr 4 ] core of 5 2 and (right) [Mn 8 Ca 4 ] core of 4 1 Figure 5 6. Mn XANES f rom spinach PSII in the S 1 state compared with the spectrum from Mn III Mn IV 3 Sr 2 complex 5 1

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184 Figure 5 7. a) and b) The Mn EXAFS and the Fourier transforms of the Sr reactivated PSII sample in the S 1 state, compared with complex 5 1 c) and d) The Sr E XAFS and the Fourier transforms of the Sr reactivated PSII sample in the S 1 state compared with complex 5 1

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185 Figure 5 8. (top) [Mn III Mn IV 3 Sr 2 O 6 ] core of 5 1 from a view point emphasizing the Mn III Mn IV 3 butterfly unit; (bottom) the corresponding 2 J c oupling scheme and definition of J wb and J bb exchange parameters.

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186 Figure 5 9. Plot of X M T vs T for complex 5 1 The solid line is the fit of the data; see the text for the fit parameters. Figure 5 10. Plot of all the S T states for complex 5 1 as a function of energy. The ground state is | 1 / 2 3 7 / 2 > with the first excited state | 3 / 2 2 7 / 2 > ~ 32 cm 1 higher in energy than the ground state.

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187 Figure 5 11. Plot of S T energies vs the J wb /J bb ratio, showing the change in the ground state. The dashed line corresponds to the experimentally determined J wb /J bb ratio of 0.83 for complex 5 1 Figure 5 12. Plot of X M T vs T for complex 5 2

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188 Figure 5 13. In phase AC magnetic susceptibility of complex 5 1 (top) and complex 5 2 (bottom) in a 3.5 G field oscillating at the indicated frequencies plotted as X M T vs T.

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189 Figure 5 14. Cyclic voltammogram at 100 mVs 1 for complex 5 1 in dichloromethane, containing 0.1 M N n Bu 4 PF 6 as supporting electrolyte.

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190 CHAPTER 6 REACTIVITY STUDIES O N PREFORMED MANGANES E / CALCIUM CLUSTERS AND ATTEMPTS TOWARD TARG ETED SYNTHESIS OF TH E EXACT OEC CUBANE U NIT 6.1 Introduction Understanding the structure of the OEC and how it functions, have been of great interest to many scientists all over the world. 35, 49, 109, 199, 200, 308 Our current understanding of this amazing enzyme is an ensemble of different information obtained using various spectroscopic tools. 35 Although, our knowledge about the OEC has increased substantially over the past 40 years, several key features still remain unanswered. 38, 63 66, 202, 249 As synthetic inorganic chemists, our role in this multifaceted research h as been to develop synthetic strategies for isolating model complexes that mimic the active site of the OEC. 81, 242, 246, 248, 250, 356 Comparison of the spectroscopic properties of these model complexes with those from the native OEC has helped scientists to develop the structure of the OEC, as well as to predict the possible mechanism for water oxidation. 247, 357 361 The previous chapters have described in detail our ende avor in designing reaction schemes for isolating high oxidation state Mn/Ca clusters. The use of a comproportionation reaction under different conditions has allowed access to Mn/Ca cubane chemistry for the first time. This strategy has allowed successful synthesis of a variety of model complexes that mimic the OEC both structurally and electronically. Isolation of these complexes has been an invaluable breakthrough that will provide more insights into this biological site. Although these clusters are the c losest synthetic analogues of the OEC available in the literature till date, it is still not the exact Mn 4 Ca unit of the OEC.

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191 This chapter describes our efforts towards the targeted synthesis of the Mn 4 Ca cluster starting from preformed Mn/Ca clusters suc h as 3 1 and 4 2 Although this strategy was not fruitful, this reaction strategy led to the isolation of different Mn/Ca clusters that possess interesting features. Synthetic strategies for isolating lower oxidation state Mn/Ca clusters to mimic the S 0 S 1 state of the Kok cycle are also described in this chapter. This has led to the isolation of the first Mn/Ca cluster containing a Mn III 3 Ca cubane unit. In addition, incorporation of chloride in a Mn/Ca cluster has been achieved for the first time. Detail ed structural, magnetic and electronic characterization of these complexes and their comparison with the native OEC is reported in this chapter. 6.2 Experimental Section 6.2.1 Syntheses All manipulations were performed under aerobic conditions using chemic als and solvents as received unless otherwise stated. Mn(O 2 CBu t ) 2 Mn(bpy) 2 Cl 2 Ca (O 2 CBu t ) 2 and NBu n 4 MnO 4 were prepared as previously reported in the literature. 168, 169, 362 [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (HO 2 CBu t ) 4 ] ( 3 1 ) and [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) ( 4 2 ) were prepared as described in previous chapters. [Mn 6 Ca 2 O 9 (O 2 CBu t ) 9 (H 2 O) 6 ](NO 3 ) (6 1). Method A : Complex 3 1 (0.25 mmol, 0.38 g) was dissolved in acetonitrile (25 mL) to give a dark brown solution, which was treate d with 1,2 stirred for 1 h. This was filtered, and the filtrate was allowed to stand undisturbed at 4 o C. X ray quality deep red crystals of 6 1 formed over a week. The crystals were col lected by filtration and dried under vacuum. The yield was 26%. However, the presence of a large number of disordered pivalate groups and poor diffraction quality

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192 hindered the complete X ray characterization of this complex. An alternative synthetic route as described in Method B allowed for full characterization. Comparison of IR spectrum and elemental analysis confirmed that both methods lead to the isolation of the same product, differing only in the identity of the counterion (pivalate vs. NO 3 ). Method B : Mn(O 2 CBu t ) 2 2H 2 O (0.16 g 0.5 0 mmol) was dissolved in hot acetonitrile (25 mL), and the resulting pink slurry was treated wit h pivalic acid (1.89 mL 16.4 mmol ) and Ca (NO 3 ) 2 4H 2 O (0. 12 g 0.5 0 mmol,), which caused the color to change from pink to reddi sh brown The solution was stirred at 80 o C for 15 min, and during this period solid NBu n 4 MnO 4 ( 1.44 g 4.0 mmol) was added in small portions. The resulting brown precipitate was collected by filtration and dried in air. The precipitate was dissolved in te trahydrofuran and the resulting solution was left undisturbed at 4 o C. X ray quality red crystals of 6 1 10THF formed over a period of 2 weeks. The yield was 15% yield. The crystals were collected by filtration and dried under vacuum. Anal. Calcd (found) f or 6 1 (C 45 H 93 O 36 N 1 Mn 6 Ca 2 ): C, 33.07 ( 33.15 ); H 5.74 ( 5.91 ); N 0. 85 (0. 73 ). Selected IR data (cm 1 KBr pellets): 3420 (br), 2967 (m), 2930 (m), 1699 (m), 1644 (m), 1558 (s), 1509 (m), 1483 (s), 1458 (m), 1414 (s), 1374 (s), 1225 (s), 1031 (m), 938 (m), 89 6 (m), 866 (m), 751 (s), 621 (s), 530 (m), 449 (m) and 406 (m). [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (bpy) 2 (Et 2 O) 2 ][Ca 6 (O 2 CBu t ) 12 Cl] ( 6 2 ) : To a stirred solution of 3 1 (0.25 mmol, 0.38 g) in aceton itrile (15 mL) was added solid Mn(bpy) 2 Cl 2 (0.25 mmol, 0.11 g), and the resul ting solution was stirred for 3 h. During the course of the reaction a brown precipitate formed which was collected by filtration and dried in air. The precipitate was redissolved in dichloromethane (10 mL) and layered with an equal volume of ether, and th e solution was kept in refrigerator at 4 o C for a week. The

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193 resulting black crystals of 6 2 2CH 2 Cl 2 H 2 O0.8MeCN were collected by filtration, washed with ether and dried under vacuum. The yield was 35%. Anal. Calcd (found) for 6 2 (C 118 H 198 N 4 Cl 1 O 44 Mn 4 Ca 8 ): C, 48.00 (47. 92 ); H 6.76 (6. 77 ); N 1.89 ( 1.86 ). Selected IR data (cm 1 KBr pellets): 3423 (br), 2959 (m), 2871 (m), 1606 (m), 1555 (s), 1484 (s), 1422 (s), 1361 (s), 1224 (s), 1156 (m), 1099 (m), 1068 (m), 1029 (m), 937 (m), 898 (s), 806 (m), 794 (m), 77 1 (m), 732 (m), 663 (s), 604 (s), 538 (m) and 439 (m). [Mn 6 Ca 2 O 10 (O 2 CBu t ) 7 (phen) 3 (H 2 O)] ( 6 3 ): To a stirred solution of 4 2 (0.25 mmol, 0.47 g) in dichloromethane (20 mL) was added 1,2 dimethoxybenzene (0.25 mmol, 32 rred for 4 h. This was filtered, and the filtrate was allowed to stand undisturbed at 4 o C X ray quality brown plate like crystals of 6 3 8CH 2 Cl 2 formed within 4 d, and these were isolated by filtration and dried under vacuum. The yield was 66%. Anal. Cal cd (found) for 6 3 (C 71 H 89 N 6 O 25 Mn 6 Ca 2 ): C, 46.44 ( 46.04 ); H 4.88 ( 4.80 ); N 4.57 ( 4.36 ). Selected IR data (cm 1 KBr pellets): 3429 (br), 2955 (m), 2867 (m), 1608 (m), 1560 (s), 1516 (s), 1481 (s), 1457 (m), 1408 (s), 1351 (s), 1221 (s), 1144 (m), 1106 (m), 1029 (m), 893 (m), 872 (m), 786 (m), 723 (s), 632 (s), 600 (s) and 441 (m). (NBu n 4 )[Mn 9 Ca 2 O 4 (OH) 4 (O 2 CBu t ) 16 (H 2 O) 2 ] ( 6 4 ): Mn(O 2 CBu t ) 2 2H 2 O (0.16g, 0.50 mmol) was dissolved in a solvent mixture of acetonitrile /chloroform (20mL 10:1 v/v ), and the resultin g pink slurry was treated with Ca (O 2 CBu t ) 2 2H 2 O (0.07g, 0.25 mmol). To this solution solid NBu n 4 MnO 4 (0.04g, 0.1 mmol) was slowly added. The resulting purple solution was stirred for 90 min, during which the solution color changed from pink to reddish brow n. This was then filtered and the filtrate was left undisturbed in a closed vial. After 2 weeks, X ray quality reddish brown crystals of 6 4 1 / 2 MeCN 1 / 2 CHCl 3 had

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194 formed which were collected by filtration and dried under vacuum. The yield was 14%. Anal. Cal cd (found) for 6 4 (C 96 H 188 N 1 O 42 Mn 9 Ca 2 ): C, 44.30 ( 44.32 ); H 7.28 ( 7.36 ); N 0.54 ( 0.26 ). Selected IR data (cm 1 KBr pellets): 3428 (br), 2961 (m), 2929 (m), 2873 (m), 1621 (m), 1591 (s), 1572 (s), 1483 (s), 1460 (m), 1418 (s), 1372 (s), 1350 (s), 1225 (s) 1029 (m), 892 (m), 791 (m), 755 (m), 617 (s), 557 (m) and 430 (m). [Mn 11 CaO 8 (OH)Cl 2 (O 2 CBu t ) 16 ] ( 6 5 ) : Mn(O 2 CBu t ) 2 2H 2 O (0.16g, 0.50 mmol) was dissolved in a solvent mixture of acetonitrile /dichloromethane (20mL 1:1 v/v ), and the resulting pink slurry wa 2 (0.03g, 0.25 mmol). To this solution solid Bu n 4 NMnO 4 (0.04g, 0.1 mmol) was slowly added, and the resulting dark brown solution was stirred for 2 h. This was then filtered and the filtrate was left un disturbed in a closed vial. Black crystals were obtained in a week, which were redissolved in acetonitrile (10 mL). Layering this solution with dichloromethane (10 mL) formed X ray quality black crystals of 6 5 3 / 2 CH 2 Cl 2 in 2 d. The crystals were collected by filtration, and dried under vacuum. The yield was 25%. Anal. Calcd (found) for 6 5 4H 2 O (C 80 H 153 O 45 Cl 2 Mn 11 Ca): C, 37.67 ( 37.43 ); H 6.04 ( 5.61 ); N 0.00 ( 0.00 ). Selected IR data (cm 1 KBr pellets): 3430 (br), 2964 (m), 2930 (m), 2872 (m), 1593 (s), 1533 (s), 1485 (s), 1458 (m), 1421 (s), 1360 (s), 1227 (s), 1031 (m), 1005 (m), 938 (m), 895 (m), 871 (m), 786 (s), 761 (s), 619 (br, s), 555 (s) and 462 (s). 6.2.2 X ray Crystallography Data were collected on a Siemens SMART PLATFORM equipped with a CCD area detector and a graphite monochromator utilizing Mo = 0.71073 ). Suitable crystal s of 6 1 10THF 6 2 2CH 2 Cl 2 H 2 O0.8MeCN, 6 3 8CH 2 Cl 2 6 4 1 / 2 MeCN 1 / 2 CHCl 3 and 6 5 3 / 2 CH 2 Cl 2 were attached to glass fiber s using paratone oil and transferred to a goniostat, where they were cooled to 173 K (or 100 K) for data

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195 collection. Cell parameters were refined using up to 8192 reflections. A full sphere of scan method (0.3 o frame width). The first 50 frames were re measured at the end of data collection to mo nitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. Raw data frames were read by program SAINT and integrated using 3D profiling algorithms The resulting data were reduced to produce hkl reflections and their intensities and estimated standard deviations. The data were corrected for Lorentz and polarization effects and numerical absorption corrections were applied based on indexed and meas ured faces. The structures were solved by d irect m ethods in SHELXTL6 170 and refined on F 2 using full matrix least squares. The non H atoms were treated anisotropically, whereas the H atoms were calculated in ideal positions and refined as riding on their resp ective C atoms. In 6 1 10THF, t he asymmetric unit consists of a half Mn 6 Ca 2 cluster (located on a mirror plane ) and five THF solvent molecules which were disordered and could not be modeled properly The program SQUEEZE 171 a part of the PLATON package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall i ntensity data. T he mirror plane passes th rough three pivalate li gands rendering them disordered, and t hey were refined in two parts each. Two other pivalate ligands in general positions were also disordered and were also refined in two parts each. All site occupation facto rs were dependently re fined. The lat er had a total of 21940 parameters and were refined in the final cycle of refinement using 2986 reflections with I > 2 ( I ) to yield R 1 and wR 2 of 5 .55 and 16.11%,

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196 respectively. For 6 2 2CH 2 Cl 2 H 2 O0.8MeCN, t he asymmetric unit consists of hal f of a Mn 4 Ca 2 cation, half a Ca 6 anion, a dichloromethane solvent molecule, a 40% acetonitrile molecule (as a result o f the cation ether ligand disorder) and half a wate r molecule. Protons of the lat er could not be located from d ifference Fourier maps and thus were not included in the final refinement model. The cation has three disordered regions n amely the methyl groups on C22, C32 and the ether ligand on O10. The methyl groups on C22 were refined in two positions while those on C32 were refined in thr ee parts. The ether ligand has one ethyl group refined in two parts. The anion has two disordered sets of methyl groups on C72 and C92. Each was refined in two parts. In the final cycle of refinement, 18856 reflections (of which 10050 are observed with I > 2 (I)) were used to refine 855 parameters and the resulting R 1 wR 2 and S (goodness of fit) were 5.59%, 13.65% and 0.909, respectively. For 6 3 8CH 2 Cl 2 t he asymmetric unit consists of two chemically equivalent but crystallographically independent Mn 6 Ca 2 clusters and 16 dichloromet hane solvent molecules. The lat er 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. There were two disordered t butyl groups that were refined in two parts with their site occupation factors dependently refined. The protons of the coordinated water molecules were obtained from a d ifference Fourier map and were constrained to be rid ing on their parent O atoms. A total of 1979 parameters were refined in the final cycle of refinement using 28166 reflections with I > 2 (I) to yield R 1 and wR 2 of 5.04% and 11.45%, respectively.

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197 For 6 4 1 / 2 MeCN 1 / 2 CHCl 3 t he asymmetric unit consists of half a Mn 9 Ca 2 anionic cluster, half a tetrabutylammonium cation, of each of an acetonitrile and a chloroform solvent molecules. The cluster has two hydrox yl and one water ligands Their protons were located in the d ifference Fourier map and refined as riding on their parent atoms. There are also two disordered t butyl methyl groups on C2 and C42. The counterion is disordered around an inversion center thus all atom site occupation factors were fixed at 50%. Also, after several refinements, the whole cation was treated as a rigid body. The site occupation factors for the solvent molecules were also fixed at 25% after several refinements. In the final cycle of refinement, 16193 reflections (of which 10561 are observed with I > 2 (I)) were used to ref ine 681 parameters and the resulting R 1 wR 2 and S (goodness of fit) were 6.06%, 17.66% and 1.157, respectively. For 6 5 3 / 2 CH 2 Cl 2 t he a symmetric unit consists of two Mn 11 Ca clusters and three dichlorometh ane solvent molecules. The lat er 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. In cluster Ca1, the Cl2 ligand is disordered against the C311 pivalate ligand. A lso three pivalate methyl groups on C2, C12 and C112 are disordered and each w as refined in two parts. In the second cluster, the Ca2, the Cl4 ligand, along with two water molecules O336 and O337 are disordered against the C331 pivalate ligand. Nearby, the Cl3 ligand bridging Mn13, Mn15 and Mn17, is disordered against a hydroxide on Mn17 and a water molecule on Mn13. The protons here were calculated in idealized positions. The last disorder is in the methyl groups of C192 which were treated similar to o ther disorders. Disordered parts on C2, C311, C331, C112, C192 were constrained to maintain idealized

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198 geometries using DFIX in SHELX Displacement parameters of the disordered parts were restrained to maintain similar values using EADP in SHELX In the fi nal cycle of refinement, 52291 reflections (of which 27571 are observed with I > 2 (I)) were used to refine 2186 parameters and the resulting R 1 wR 2 and S (goodness of fit) were 7.30%, 15.12% and 0.911, respectively. 6.3 Results and Discussion 6.3.1 Syntheses Initial attempt towards targeted synthesis of a Mn 4 Ca complex involved the abstraction of the external Ca II ion from 3 1 by using a chelating ligand to isolate the discrete Mn 3 Ca unit, which could later be treated with Mn II /Mn III salt to isolate the desired Mn 4 Ca complex. We wanted to use a chelating ligand with ether linkages as it would favorably bind to the oxophillic Ca center and abstract it. Alcohols and carboxylate ligands were avoided to minimize complications from their reaction with 3 1 possibly causing structural changes. However, the reaction of 3 1 with one equivalen t of 1,2 dimethoxybenzene led to the isolation of Mn 6 Ca 2 cluster. Complete crystallographic characterization of the cluster from this synthetic route was not possible, but the product identity allowed the development of an alternate synthetic scheme, descr ibed below. The isolation of Mn 6 Ca 2 instead of the target Mn 3 Ca suggests that the abstraction of that external Ca from 3 1 makes the cube unstable, which then dimerizes to form Mn 6 Ca 2 ; however, this is just a hypothesis. A rational synthesis of the Mn 6 Ca 2 complex was developed using a comproportionation reaction, which has proven successful in isolating other high oxidation state Mn/Ca clusters, as shown in the previous chapters. T he same reaction scheme used for synthesizing 3 1 was tweak ed it to target sy nthesis of Mn 6 Ca 2 The

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199 Mn:Ca ratio to make 3 1 was 2:1, and this ratio was systematically increased to target Mn 6 Ca 2 After several attempts, a successful synthetic procedure was developed to prepare pure 6 1 The reaction of Mn(O 2 C t Bu) 2 Ca (NO 3 ) 2 and NBu n 4 MnO 4 in a 1 : 1 : 8 ratio in hot acetonitrile in the presence of excess pivalic acid led to the formation of a brown solid precipitate, which upon recrystallization from cold tetrahydrofuran led to the isolation of [Mn 6 Ca 2 O 9 (O 2 CBu t ) 9 (H 2 O) 6 ](NO 3 ) ( 6 1 ) in 15% yield. The crude product was confirmed as also being 6 1 but with additional pivalic acid, by comparing its IR spectrum and elemental analysis with those for the recrystallized material. Slow evaporation of the filtrate from the reaction mixture alwa ys led to the formation of a mixture of products, which was difficult to crystallize in pure form. The poor solubility of 6 1 in acetonitrile caused it to precipitate out from the reaction mixture as a pure product. The formation of 6 1 is summarized in eq 6 1. Mn 2+ + 5 MnO 4 + 2 Ca 2+ + 9 t BuCO 2 H + 13 H + + 13 e [Mn 6 Ca 2 O 9 (O 2 CBu t ) 9 (H 2 O) 6 ] + + 5 H 2 O (6 1) The next approach to obtaining a Mn 4 Ca complex involved the reaction of Mn(bpy) 2 Cl 2 with 3 1 in acetoni trile. The rational for choosing this Mn II complex was that the labile chloride groups on the Mn II would react with the external Ca II on 3 1 forming insoluble CaCl 2 and generating the [Mn 3 Ca] in situ. This would then hopefully react with the Mn(bpy) 2 2+ to form the Mn 4 Ca. The presence of the bypyridine group on the Mn II would prevent the formation of bigger clusters by blocking the coordination sites on the Mn II ion. Unfortunately, this strategy also proved unsuccessful and a Mn 4 Ca 2 complex 6

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200 2 was isolated from this reaction scheme. The formation of 6 2 is complicated and an attempt to summarize its formation is given in eq. 6 2. [Mn 3 Ca 2 O 4 ( O 2 CBu t ) 8 ( t BuCO 2 H) 4 ] + [Mn(bpy) 2 Cl 2 ] + 2H 2 O + 2 Et 2 O [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (bpy) 2 (Et 2 O) 2 ] + + 6 t BuCO 2 H + 2 H + + 2Cl + e (6 2) As attempts to isolate the Mn 4 Ca starting from the preformed 3 1 were unsuccessful, an alternate strategy involved the use of the Mn 4 Ca 2 complex 4 2 as the starting material. Although, the additional Ca atom in 4 2 is not as labile (or easily accessible) as that in 3 1 we still wanted to explore whether it was possible to abstract the Ca II using 1,2 dimethoxybenzene. It was hoped that the presence of the phenanthroline groups would prevent dimerization to 6 1 and allow access to a Mn 4 C a product. However, the reaction of 4 2 with one equivalent of 1,2 dimethoxybenzene led to the isolation of 6 3 in 66% yield. Performing this reaction in the presence of suitable counter ions for isolating other charged species that may be present in this reaction mixture, also led to isolation of Mn 6 Ca 2 complex 6 3 in lower yields. As for 6 2 the formation of 6 3 is complicated, and an attempt to summarize its formation is shown in eq. 6 3. 3 [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) + 4 C 8 H 10 O 2 + 4 H 2 O 2 [Mn 6 Ca 2 O 10 (O 2 CBu t ) 7 (phen) 3 (H 2 O)] + 2 [Ca (O 2 CBu t ) 2 (C 8 H 10 O 2 ) 2 ] + 3 t BuCO 2 H + 6 phen + H + + e (6 3) The other synthetic target wa s the synthesis of related Mn/Ca cubane clusters with lower manganese oxidation states. The reaction of Mn(O 2 C t Bu) 2 Ca (O 2 C t Bu) 2 and NBu n 4 MnO 4 in a 5 : 2 : 1 ratio in a solvent mixture of acetonitrile/chloroform afforded a reddish brown solution which subs equently led to the isolation of the

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201 [Mn 9 Ca 2 O 4 (OH) 4 (O 2 CBu t ) 16 (H 2 O) 2 ] ( 6 4 ) anion as the NBu n 4 + salt in 14% yield. The formation of 6 4 is summarized in eq. 6 4. The average manganese oxidation state in this scheme is +2.83, which is much lower than those used in the synthesis of 3 1 4 2 and 5 1 (+4.5 average Mn oxidation state). 6 Mn 2+ + 3 MnO 4 + 2 Ca 2+ + 16 t BuCO 2 H + 10 e [Mn 9 Ca 2 O 4 (OH) 4 (O 2 CBu t ) 16 (H 2 O) 2 ] + 2 H 2 O + 4 H + (6 4) Interestingly, the choic e of solvent, reaction time and starting materials play a crucial role in obtaining pure 6 4 Another key feature in the above reaction is the role of pivalic acid. It was observed that the presence of pivalic acid hinders the isolation of 6 4 the only pr oduct isolated being a Mn 9 cluster ( 2 3 ). We still do not have an explanation as to why this Mn 9 is formed under these conditions. Further exploration of this reaction scheme with a lower average Mn oxidation state was carried out in an attempt to isolat e a smaller nuclearity product, hopefully a Mn 4 Ca unit in low oxidation states. The reaction of Mn(O 2 C t Bu) 2 CaCl 2 and NBu n 4 MnO 4 in a 5 : 2 : 1 ratio in the presence of a small amount of pivalic acid in acetonitrile/dichloromethane afforded a dark brown so lution, from which was subsequently isolated [Mn 11 CaO 8 (OH)Cl 2 (O 2 CBu t ) 16 ] ( 6 5 ) in 25% yield. The average Mn oxidation state in this reaction is still +2.83, but the Mn oxidation in the final product 6 5 (Mn 11 Ca) is different from 6 4 (Mn 9 Ca 2 ). The formatio n of 6 5 is summarized in eq. 6 5. 9 Mn 2+ + 2 MnO 4 + CaCl 2 + 16 t BuCO 2 H + H 2 O [Mn 11 CaO 8 (OH)Cl 2 (O 2 CBu t ) 16 ] + 17 H + + e (6 5) Another interesting feature of this reaction is the incorporatio n of chloride into the final

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202 product. This result was not anticipated because although CaCl 2 was used in the synthesis of 3 1 no chloride incorporation was observed. It may be speculated that the lower concentration of pivalic acid in the synthesis of 6 5 compared to that to 3 1 may have led to this result. 6.3.2 Description of Structures 6.3.2.1 Structure of [Mn 6 Ca 2 O 9 (O 2 CBu t ) 9 (H 2 O) 6 ](NO 3 ) ( 6 1 ) The partially labeled structure of 6 1 is shown in Figure 6 1, and selected interatomic distances and angles ar e summarized in Table A 10 Complex 6 1 crystallizes in an orthorhombic space group Pnma. The oxidation state of the manganese atoms and protonation levels of the O 2 and the O atoms of the carboxylate groups were determined from a combination of charge co nsideration, inspection of bond lengths and bond valence sum (BVS) calculations (Table 6 2). All the manganese centers were concluded to be in +4 oxidation state. The core is composed of a [Mn 6 IV Ca 2 ( 4 O) 3 ( 3 O) 6 ] 10+ unit, which is depicted in Figure 6 2. All the manganese centers are hexa coordinated and possess a distorted octahedral geometry. The calcium centers on the other hand are nona coordinated. Three [Mn 2 IV Ca 2 ( 3 O) 4 ] distorted cubanes fuse one of their faces with the neighboring unit to give rise to the Mn 6 Ca 2 core. An alternative description is a Mn IV 6 wheel, with the Ca atoms situated 1.78 above and below this plane (Figure 6 2, bottom). The edges of this wheel are alternately bridged by [( 3 O) 2 ( O 2 CBu t ) ] and [ 4 O)( O 2 CBu t ) 2 ], giving rise to Mn Mn separations of 2.699 and 3.259 respectively. Furthermore, each of the Ca centers possesses three terminal waters. This is not surprising considering the high oxophillicity and low charge of Ca 2+ The recent crystal structure of the OEC has also revealed the presence of four water molecules bound to the Mn and

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203 Ca centers. 79 6 1 is the first Mn/Ca cluster to possess this Mn 6 M 2 core; however, this heterometallic topology has been recently reported in Mn/Bi chemistry. 363 Apart from the similarity in the manganese oxidation state and the metal topology, the Mn 6 Ca 2 and Mn 6 Bi 2 differ in peripheral ligation and metal metal distances arising from the incorporation of Ca 2+ instead of Bi 3+ 6.3.2.2 Structure of [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (bpy) 2 (Et 2 O) 2 ][Ca 6 (O 2 CBu t ) 12 Cl] ( 6 2 ) The partially labeled structure of 6 2 is shown in Figure 6 3, and selected interatomic distances an d angles are summarized in Table A 11 Complex 6 2 crystallizes in triclinic space group P 1 The oxidation state of the manganese atoms and the protonation levels of the O 2 and the O atoms of the carboxylate groups were determined from a combination of c harge consideration, inspection of bond lengths and bond valence sum (BVS) calculations (Table 6 3). Like 4 2 and 5 1 the cation of 6 2 is again centrosymmetric, which hindered the unambiguous assignment of the metal oxidation states based on BVS calculat ions alone. Unlike 4 2 and 5 1 the counterion for 6 2 is [ Ca 6 (O 2 CBu t ) 12 Cl ] instead of a pivalate group; this is a quite unusual anion. Thus, by charge consideration, the manganese oxidation states were assigned as Mn III and 3 Mn IV As described in previo us chapters, the higher BVS value for Mn2 is due to the crystallographic averaging of the Mn III and Mn IV atoms in the centrosymmetric core. All the manganese atoms are hexa coordinated and possess a distorted octahedral geometry, whereas the calcium atoms are hepta coordinated (both in the cation and anion). The cation contains a [ Mn III Mn 3 IV Ca 2 ( 4 O ) 2 ( 3 O) 4 ] 7+ core that can be described as a fusion of two Mn 3 Ca cubes via a common face giving rise to a dicubane topology. This

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204 is similar to 4 2 ; however, the most striking difference is the position of the Mn III atom. In 4 2 the Mn III is at an out er position of the dicubane topology, whereas in 6 2 it is located in the central [Mn 2 O 2 ] face linking the two cubes (Figure 6 5). The different positioning of the Mn III ion in 6 2 vs 4 2 causes slight changes in the metal metal distances, as shown in Tabl e 6 4. The peripheral ligation in 6 2 is provided by six bridging 1 : 1 : pivalate groups, two phenanthroline groups on the peripheral Mn atoms (Mn1/Mn1') and two ether molecules coordinated to the calcium centers. This is in contrast to 4 2 where the peripheral ligation consisted of four phenanthroline groups. The anio n in 6 2 is also very interesting and consists of a Ca 6 octahedron centered on a 6 Cl ion. There are a few Ca 6 clusters reported in the literature, but they almost all possess different metal topologies from 6 2 such as rod like and dicubane. 364 366 Only one Ca 6 octahedron has been reported in the literature previously [Ca 6 OI 2 (O CH=CH 2 ) 8 (THF) 4 ], but it differ from 6 2 in the central bridging unit ( 6 O vs 6 Cl) and the peripheral ligation. 364 The 6 bridging mode of the chloride ion is also very rare and there are only a few such examples reported in the literature. 178, 367 376 The Ca Ca distances in 6 2 vary between 4.0324 (13) 5.7508 (11) which ar e longer than those in the known Ca 6 octahedron. 364 This may be attributed to the presence of the bigger size and smaller charge of Cl compared with O 2 The Ca Cl Ca angles in 6 2 are close to 90 o All the calcium atoms are hepta coordinated and the peripheral ligation is provided by twelve pivalate groups binding in a 1 : 2 : bridging mode. 6.3.2.3 Structure of [Mn 6 Ca 2 O 10 (O 2 CBu t ) 7 (phen) 3 (H 2 O)] ( 6 3 ) The labeled structure of [Mn 6 Ca 2 O 10 (O 2 CBu t ) 7 (phen) 3 (H 2 O)] ( 6 3 ) is shown in Figure 6 7, and selected interatomic distances and bond angles are summarized in Table A 12 Com plex 6 3 crystallizes in the monoclinic space group P2 1 /c The

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205 oxidation state of the manganese atoms and the protonation levels of the O 2 H 2 O and the carboxylate O atoms were determined by a combination of charge balance consideration, inspection of bon d lengths and bond valence calculations (Table 6 6). 6 3 is a heterovalent cluster with 5 Mn IV and 1 Mn III All the manganese centers are hexa coordinated and possess a distorted octahedral geometry. Ca1 is octa coordinated whereas Ca2 is hepta coordinated in 6 3 The core of 6 3 contains a [Mn III Mn 5 IV ( 4 O)( 3 O) 7 ( O) 2 ( H 2 O)] 7+ unit which is shown in detail in Figure 6 8. The core consists of a [Mn III Mn 2 IV CaO 4 ] cubane unit that shares an edge with a neighboring open faced [Mn 3 IV CaO 4 ] unit. This gives ris e to a defective dicubane topology, comprising a [Mn III Mn 4 IV Ca 2 ( 4 O)( 3 O) 6 ] 9+ unit, with the cubanes sharing an edge as opposed to a face, as in 4 2 and 5 1 The JT axis on the Mn III center is oriented along the Mn oxide ( 4 ) bond and Mn O (carboxylate) bond. The remaining Mn IV center is connected to this dicubane via two oxide (O6 and O8) and a 3 oxide (O5), thus completing the core. A bridging water molecule (O11) between the two calcium centers also links the two cubes together. 255 265 This water molecule is hydrogen bonded to the oxide (O8) and a terminal pivalate group (O18) on Ca2, leading to lower BVS numbers for these oxygen atoms, as shown in Table 6 5. The peripheral ligation is provided by five piva lates in 1 : 1 : bridging modes, one pivalate in a 1 : 2 : bridging mode, and a terminal 1 pivalate group. Three phenanthrolines, one on each of Mn1, Mn2 and Ca2, complete the ligation. 6.3.2.4 Structure of (NBu n 4 )[Mn 9 Ca 2 O 4 (OH) 4 (O 2 CBu t ) 16 (H 2 O) 2 ] (6 4) The partially labeled structure of [Mn 9 Ca 2 O 4 (OH) 4 (O 2 CBu t ) 16 (H 2 O) 2 ] ( 6 4 ) is shown in Figure 6 9, and the selected interatomic distances and bond angles are shown in Table A 13 Complex 6 4 crystallizes in the triclinic space group P 1 The oxidation state

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206 of the manganese atoms and the protonation levels of the O 2 OH H 2 O and the carboxylate O atoms were determined by a combination of charge balance consideration, inspection of bond lengths and bond valence sum calculations (Table 6 7). 6 4 is heterovalent with five Mn III and four Mn II atoms. All the manganese centers are hexa coordinated and possess a distorted octahedral geometry, except Mn5/Mn5' which are tetra coordinated and displays a tetrahedral geometry, while the calcium centers are hepta coordinat ed. The core of 6 4 contains a [Mn 5 III Mn 4 II Ca 2 (( 3 OH) 4 ( 4 O) 4 ] 15+ unit, which is shown in detail in Figure 6 10. The core can be described as being composed of two [Mn 3 III Ca ( 3 OH) 2 ( 4 O) 2 ] 5+ units linked together via a common manganese center, Mn1. This also gives rise to a dicubane topology, but the mode of attachment is different from those in 4 2 5 1 (face sharing) and 6 3 (edge sharing). Complex 6 4 possesses several key features which are relevant to those of the native OEC. It is well known that t he manganese oxidation states in the S 0 state of the Kok cycle are Mn 3 III Mn IV 25, 377, 378 The core of 6 4 contains all Mn III atoms, which can be compared to the S 0 state. Isolation of 6 4 proves that synthetic acc ess to such Mn 3 Ca cubane units is possible for lower manganese oxidation states. All the low oxidation state Mn/Ca clusters reported in the literature to date do not possess such cubane units, making 6 4 novel. 209 2 12 Furthermore, it has been proposed that at lower S states in the Kok cycle the manganese centers in the Mn 4 Ca cluster may be linked via OH ligands instead of O 2 the latter a re more favored by higher Mn oxidation states such as Mn IV 322, 379 384 The presence of two 3 OH groups in the Mn 3 III Ca cubane unit in 6 4 is consistent with this idea. It can be hypothesized that due to the presence of higher Mn

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207 content in the reaction, two Mn 2+ bind to the remaining oxid es in each of the cubes (instead of protons) giving rise to a Mn 9 Ca 2 core. The peripheral ligation is provided by fourteen pivalates in a 1 : 1 : bridging mode, two pivalates in the 1 : 2 : binding mode and two terminal water molecules on Mn4/Mn4', which are hydrogen bonded to the neighboring pivalate groups. The cluster has an overall negative charge, which is balanced by a N n Bu 4 + group. Table 6 8 shows a comparison of the Mn Mn and Mn Ca distances in the different Mn 3 Ca units, in which the oxidation states of the manganese atoms vary. Although the attachment of other metals to this cube affects the metal metal distances, the variation in the bond distances with the Mn oxidation state can be observed. 6.3.2.5 Structure of [Mn 11 CaO 8 (OH)Cl 2 (O 2 CBu t ) 16 ] ( 6 5 ) The partially labeled structure of [Mn 11 CaO 8 (OH)Cl 2 (O 2 CBu t ) 16 ] ( 6 5 ) is shown in Figure 6 12, and selected interatomic distances and bond angles are summarized in Table A 14 Complex 6 5 crystallizes in orthorhombic space group Pca2 1 The oxidation st ate of the manganese atoms and the protonation levels of the O 2 OH and the carboxylate O atoms were determined by a combination of charge balance consideration, inspection of bond lengths and bond valence sum calculations (Table 6 9). 6 5 is homovalent with all the manganese atoms in +3 oxidation state. All the manganese centers are hexa coordinated and possess a distorted octahedral geometry. All the Mn III atoms display JT elongated axis which usually tend to avoid Mn oxide bonds, which are almost alwa ys the shortest and the strongest bonds in the molecule. The JT axis in 6 5 (bold blue line in Figure 6 12) is randomly oriented in the

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208 cluster, which has an important consequence in the magnetic properties of this cluster. The calcium center on the other hand is nona coordinated. The core of 6 5 contains a [Mn 11 III Ca( 4 O) 6 ( 3 O) 2 ( 3 OH)] 18+ unit which is shown in detail in Figure 6 13. The core can be described as being composed of a non planar loop of 11 Mn III atoms which are attached to the central Ca I I via six 4 oxide (O34, O35, O37, O39, O40 and O41), two 3 oxide (O33 and O36) and one 3 hydroxide (O38). The core can also be visualized as six fused [Mn 3 III CaO] 9+ units each sharing one or more than one atoms. These are further linked together via two 3 oxide (O33 and O36) and a 3 hydroxide group (O38) to give rise to the complete Mn 11 Ca core of 6 5 (Figure 6 13). The peripheral ligation is provided by twelve pivalate groups, which bind in a 1 : 1 : mode, and four pivalate s binding in a 1 : 2 : 3 mo de. One 3 Cl and one Cl complete the ligation environment. As described in detail in section 6.2.2, there is a disorder between a chloride, pivalate and water ligands within the two Mn 11 Ca clusters in the asymmetric unit of 6 5 The structure describe d above is based on the weighted average of these disordered ligands. 6 5 displays a novel metal topology and it is the only Mn/Ca cluster to date to contain bridging chloride groups. 6.3.3 Magnetochemistry of Complexes 6 1 to 6 5 6.3.3.1 DC studies Solid state variable temperature dc magnetic susceptibility data were collected in a 0.1 T field on powdered microcrystalline samples of 6 1 6 5 restrained in eicosane to prevent torquing in the 5.0 300.0 K range. For 6 1 the 300 K X M T value of 10.59 cm 3 K mol 1 is slightly lower than the spin

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209 only value of 11.25 cm 3 Kmol 1 expected for six non interacting Mn IV ions, indicating the presence of dominant intramolecular antiferromagnetic interactions. The X M T value decreases with decreasing temperature to 1.56 c m 3 Kmol 1 and the low temperature data indicate an S = 0 or S = 1 ground state (Figure 6 14). The presence of several triangular units within the metal topology does not allow for easy rationalization of the observed ground state. For 6 2 the X M T value of 7.10 cm 3 Kmol 1 at 300 K is less than the spin only value of 8.63 cm 3 Kmol 1 ( g = 2.0) expected for three non interacting Mn IV ( S = 3 / 2 ) and one Mn III ( S = 2) ions, again indicating the presence of dominant antiferromagnetic exchange interactions. The X M T v alue steadily decreases with decreasing temperature to 0.54 cm 3 Kmol 1 at 5.0K, which is consistent with an S = ground state (Figure 6 14). This is the same ground state found for 4 2 and thus, the difference in the position of the Mn III ion does not af fect the ground state of the Mn 4 Ca 2 unit. The individual exchange parameters (J) between the Mn Mn pairs were obtained by fitting the X M T vs T data for 6 2 to the appropriate theoretical expression for a Mn III Mn 3 IV butterfly model (Figure 6 1 5 ). As describ ed in detail in the previous chapter, the isotropic Heisenberg Dirac Van Vleck (HDVV) spin Hamiltonian describing the exchange interactions within this Mn 4 core is given by eq 6 6, where J is the exchange to to H = 2J wb ( 1 2 + 2 3 + 3 4 + 4 1 ) 2J bb 2 4 (6 6) The eigen values of the spin Hamiltonian can be determined using the Kambe vector coupling method 219 with the following coupling scheme: A = 1 + 3 B = 2 + 4 and T = A + B The spin Hamiltonian in eq. 6 6 can now be expressed as follows:

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210 H = 2J wb ( T 2 A 2 B 2 ) 2J bb B 2 (6 7) The energies of the spin states, which are eigen values of the Hamilton ian in this coupling scheme, are given by eq 6 8, where E S T S A S B > is the energy of the state S T S A S B >, and the constant terms contributing equally to all the states have been omitted. The overall multiplicity of the system is 320, arising from 50 i ndividual spin states ranging from S T = 11 / 2 1 / 2 E S T S A S B > = J wb [ S T ( S T + 1) S A ( S A + 1) S B ( S B + 1)] J bb [ S B ( S B + 1)] (6 8) A theoretical expression for the molar paramagnetic susceptibility ( X M ) vs temperature was derived for 6 2 u sing the van Vleck equation 220 and assuming an isotropic g value. This expression was used to fit the experimental X M T vs T (Figure 6 1 6 ) for 6 2 with the parameters varied being J wb J bb g and TIP. The fi tting parameters were : J wb = 28.86 ( 6 ) cm 1 J bb = 39.38 ( 8 ) cm 1 g = 2.04 ( 6 ) and TIP = 600 x 10 6 cm 3 mol 1 Using these values the S T state energy ladder was created (Figure 6 17 ) which further validates a | 1 / 2 3 5 / 2 > ground state for this system w ith the first excited state being | 3 / 2 3, 3 / 2 > at ~ 34 cm 1 above the ground state. The J values obtained for 6 2 are significantly different from those obtained for 4 2 Both J wb and J bb are still negative, but their magnitudes are larger than those in 4 2 This can be attributed to the small changes in the Mn Mn distances and Mn O Mn angles in 6 2 compared to 4 2 (Table 6 4 and 6 5). Although the observed ground state is still S = the spin alignments on the individual Mn centers giving rise to this ground state are different from those in 4 2 (Figure 6 18). This is still a spin frustrated system, with the J bb pathway losing out to the four J wb interactions. As mentioned in the previous chapters, the ground state of spin frustrated systems is sensitiv e to slight structural

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211 perturbations. Therefore, we have computed the spin state energy manifold vs J wb /J bb to explore the variation of the ground state as a function of the J wb /J bb ratio. The experimental J wb /J bb = 0.7 3 ( shown as the dashed line in Figur e 6 19 ) shows that the J wb /J bb ratio is close to the crossover point to an S = 3 / 2 ground state. For 6 3 the 300 K X M T value of 8.66 cm 3 Kmol 1 is lower than the spin only value of 12.38 cm 3 Kmol 1 as expected for five non interacting Mn IV ( S = 3 / 2 ) and one Mn III ( S = 2) ions, again indicating the presence of dominant intramolecular antiferromagnetic interactions. The X M T value of 6 3 stays essentially constant with decreasing temperature that indicates the presence of a well isolated ground state. The low t emperature X M T value of 7.26 cm 3 Kmol 1 is consistent with an S = 7 / 2 ground state with g < 2 (Figure 6 20). The 300 K X M T value of 22.34 cm 3 Kmol 1 for 6 4 is again much lower than the spin only value of 32.50 cm 3 Kmol 1 as expected for five non interactin g Mn III ( S = 2 ) and four Mn II ( S = 5 / 2 ) ions. This along with the decrease in the X M T value with decreasing temperature up to 15K indicates the presence of dominant antiferromagnetic interactions. Below this temperature the X M T value increases slightly and the low temperature value of 16.01 cm 3 Kmol 1 indicates an S = 6 ground state (Figure 6 20). The increase in X M T value at low temperature could be attributed to the depopulation of excited states with lower S values, than the ground state. Finally, for 6 5 the 300 K X M T value of 30.35 cm 3 Kmol 1 is slightly lower than the spin only value of 33.0 cm 3 Kmol 1 as expected for eleven non interacting Mn I II ions. The steady decrease in X M T value with decreasing temperature indicates a low ground state

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212 spin for 6 5 The low temperature X M T value of 7.36 cm 3 Kmol 1 indicates an S = 3 ground state (Figure 6 20). To probe the ground states of 6 3 6 5 further, and to determine the zero field splitting parameter ( D ), magnetization ( M ) vs field ( H ) data were collected in the magnetic field and temperature ranges of 0.1 1 T and 1.8 10 K, respectively. The resulting data for 6 3 are plotted in Figure 6 21 as reduced magnetization ( M / N B ) vs H / T where N B is the Bohr magneton. The data were fit by diagonalization of the spin Hamiltonian matrix using the program MAGNET 196 as described elsewhere. The best fit for 6 3 is shown as the solid lines in Figure 6 21, and was obtained with S = 7 / 2 and either of two sets of parameters; g = 1.9 5 (4) and D = 0.65 (9) cm 1 or g = 1.93 (5) and D = 0.31 (11) cm 1 It should be noted that it is common to obtain two acceptable fits of magnetization data for a given S value, one with D > 0 and the other with D < 0. In order to assess which is the sup erior fit in this case, and also to ensure that the true global minimum had been located, we calculated the root mean square error surface for the fits as a function of D and g which is plotted as a two dimensional contour plot in Figure (6 22). 222 The error surface clearly shows that the fit with the positive D is superior, suggesting this to be the true sign of D The magnetization data for 6 4 are plotted in Figure 6 23 As for 6 3 lower field d ata were used for fitting the data in 6 4 due to the higher metal nuclearity and increased Mn II content. The resulting best fit line is shown as the solid lines in Figure 6 22, which was obtained with fit parameters S = 6, g = 1.85 (2) and D = 0.0 cm 1 A lternative fits with S = 5 and S = 7 gave unreasonable g values of 2.35 and 1.43 and hence were ignored.

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213 For 6 5 no satisfactory fit could be obtained both with high as well as low field data, as is typical in the presence of low lying excited states. Suc h low lying excited states are common in high nuclearity spin frustrated systems like 6 5 where the spin frustration can be correlated to the presence of a large number of triangular units within the core topology of 6 5 6.3.3.2 AC s tudies An ac magnetic susceptibility study was performed to independently to confirm the ground state (without any complications from the dc field) and probe the dynamics of magnetization (magnetic moment) relaxation. The AC studies on 6 1 6 5 were performed in the 1.8 15 0 K range using a weak 3.5 G ac field oscillating at frequencies of 50 1000 Hz. The in phase susceptibility signals for 6 1 6 5 are plotted as X M T vs T in Figures 6 24 and 6 25. For 6 1 the X M T value decreases steeply with decrease in temperature, i ndicating the presence of particularly low lying excited states. Extrapolation of the data to 0 K gives a X M T value of 0.0 cm 3 Kmol 1 which confirms an S = 0 ground state. The in phase X M T vs T AC susceptibility for 6 2 shows a similar profile as 6 1 alt hough the decrease with decreasing temperature is not as steep as for 6 1 Extrapolation of the data from 8 K to 0K gives a value of ~ 0.4 cm 3 Kmol 1 which is consistent with an S = ground state, in agreement with the conclusions of the dc studies. For 6 3 t he X M T remains essentially temperature independent with decreasing temperature, indicating the ground state to be well isolated. Extrapolation of the data to 0 K gives a X M T value ~ 7.2 cm 3 Kmol 1 which is consistent with an S = 9 / 2 ground state wit h g < 2 The X M T value of 6 4 shows a slight increase with decreasing temperature

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214 and the extrapolation of the plot to 0 K from 4 K gives a value ~ 21 cm 3 Kmol 1 which is consistent with an S = 6 ground state with g < 2 The in phase X' M T signal for 6 5 d ecreases with decreasing temperature. Extrapolation to 0K gives a value ~ 6 cm 3 Kmol 1 which indicates an S = 3 ground state with g ~ 2. Although 6 5 contains all Mn III ions, no out of phase signals were observed, which can be assigned as the random orient ation of the JT axis (Figure 6 12). 6.4 Conclusions As part of the ongoing quest to isolate the exact Mn 4 Ca unit of the native OEC, we have sought the synthesis of this elusive Mn 4 Ca complex by starting with preformed Mn/Ca clusters. The first idea was to abstract the external Ca atom in the Mn 3 Ca 2 complex ( 3 1 ) to synthesize the [Mn 3 Ca] cube and then treat the cube with Mn II /Mn III salt to form the Mn 4 Ca unit. However, this strategy led to the isolation of a bigger Mn 6 Ca 2 ( 6 1 ) complex. It can be hypothesiz ed that the abstraction of the external Ca makes the cube unstable, and it dimerizes to form 6 1 The next strategy involved generating the Mn 3 Ca unit in situ, which could then react with a Mn II precursor to generate the Mn 4 Ca cluster. The open co ordinati on site on the Mn II was minimized by using chelates to prevent the formation of bigger clusters like 6 1 This strategy worked on some levels, as it led to the formation a Mn 4 Ca 2 cluster from this scheme. Although it was not the Mn 4 Ca unit that we were see king, 6 2 possesses several interesting features. The core of 6 2 is similar to 4 2 but the positioning of the Mn III ion in the core is different in the two clusters. This, coupled with the small structural distortion, affected the exchange coupling betwe en the Mn centers, which is reflected in the spin orientation on the metal centers. It is logical to think that a similar situation might be operating in the native OEC,

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215 where different ground states or slightly different g values are obtained depending on the exact conditions by which the S 2 state is generated. 283, 284, 286 289 Since the use of 3 1 as the starting material did not allow access the Mn 4 Ca unit, use of 4 2 as the starting material was the next logical step. This already contains the exact Mn 4 Ca with an additional Ca attached to it. 4 2 had phenanthroline groups as the peripheral ligands and we hoped that it would prevent aggregation upon Ca abstraction. Unfortunately, this strategy also led to a bigger product, the Mn 6 Ca 2 phen 3 ( 6 3 ) complex. Although our efforts were not successful, they helped in understanding the system better and hopefully this will allow future efforts directed towards targeted synthesis of Mn 4 Ca to be successful. Another targeted objective of the project was the preparation of similar Mn/Ca cubane units, as obtained in the previous chapter, but containing lower Mn oxidation states to mimic the lower S n states of the Kok cycle. Isolation of 6 4 has shown that such cubanes can indeed be isolated at lower Mn oxidation state. Similarly 6 5 which does not contain any cubane units, is the only Mn/Ca cluster to contain bridging chloride ions in the core. It has been known that Cl is an essential co factor in the functioning of the OEC, b ut its exact role has still not been ellucidated. 332, 385 388 Although 6 4 and 6 5 have higher nuclearity than the OEC, isolation of all these clusters indicates that we are moving in the right direction, and effor ts are underway to achieve smaller Mn/Ca at these oxidation states too.

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216 Table 6 1. Crystallographic Data for 6 1 10THF 6 2 2CH 2 Cl 2 H 2 O0.8MeCN, 6 3 8CH 2 Cl 2 6 4 1 / 2 MeCN 1 / 2 CHCl 3 and 6 5 3 / 2 CH 2 Cl 2 Parameter 6 1 6 2 6 3 6 4 6 5 formula a C 45 H 93 N 1 O 36 Mn 6 C a 2 C 121.6 H 260.4 N 4.8 O 45 Cl 5 Mn 4 Ca 8 C 71 H 89 N 6 O 25 Mn 6 Ca 2 C 97.5 H 190 N 1.5 O 42 Cl 1.5 Mn 9 Ca 2 C 83.2 H 151.4 O 41 Cl 8.4 Mn 11 Ca fw, g mol 1 a 1634.00 3173.42 1836.29 2683.34 2750.09 crystal system Orthorhombic Triclinic Monoclinic Triclinic Orthorhombic space group Pnma P 1 P2 1 /c P 1 Pca2 1 a 27.109 (13) 14.842 (2) 24.8346 (9) 14.0305 (10) 27.8506 (19) b 18.043 (8) 14.951 (2) 33.1126 (12) 15.3209 (11) 17.0694 (13) c 22.097 (10) 19.842 (3) 26.8688 (10) 18.9818 (14) 50.479 (4) deg 90 93.872 (3) 90 87.474 (2) 90 deg 90 90.144 (3) 103.018 (1) 68.8140 (10) 90 deg 90 110.800 (3) 90 68.6450 (10) 90 V 3 10809 (9) 4105.1 (11) 21527.4 (14) 3525.3 (4) 23997 (3) Z 4 1 8 4 8 T C 173 (2) 173 (2) 100 (2) 100 (2) 100 (2) radiation, b 0.71073 0.71073 0.71073 0 .71073 0.71073 calc mg/m 3 0.956 1.277 1.133 1.264 1.376 mm 1 0.827 0.700 0.836 0.948 1.271 R 1 c,d 0.0555 0.0559 0.0504 0.06060 0.0730 wR 2 e 0.1611 0.1365 0.1145 0.1766 0.1512 a Including solvate molecules. b Graphite monochromator. c I > 2 ( I ). d R 1 = (|| F o | | F c ||) / | F o |. e wR 2 = [ [ w ( F o 2 F c 2 ) 2 ] / [ w ( F o 2 ) 2 ]] 1/2 w = 1/[ 2 ( F o 2 ) + [( ap ) 2 + bp ], where p = [max ( F o 2 O) + 2 F c 2 ]/3.

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217 Table 6 2 BVS for the Mn a and selected oxygen atoms b in 6 1 Mn II Mn III Mn IV BVS Assignment Mn1 4.35 3.98 4.17 O1 2.30 O 2 ( 4 ) Mn2 4.26 3.90 4.09 O2 2.34 O 2 ( 4 ) Mn3 4.33 3.96 4.16 O3 1.89 O 2 ( 3 ) O4 1.91 O 2 ( 3 ) O5 1.95 O 2 ( 3 ) O6 1.93 O 2 ( 3 ) O16 0.29 H 2 O (terminal) O17 0.30 H 2 O (terminal) O18 0.28 H 2 O (terminal) a The bold 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 bold value. b A BVS in the ~ 1.8 2.0, ~1.0 1.2, and ~0.2 0.4 ranges for an O atom is indicative of non single and double protonation, respectively, but can be altered somewhat by hydrogen bonding. Table 6 3. BVS for the Mn a and selected O atoms b in 6 2 Mn II Mn III Mn IV BVS Assignment Mn1 4.21 3.90 4.01 O1 1.95 O 2 ( 4 ) Mn2 3.62 3.31 3.48 O2 1.96 O 2 ( 3 ) O3 2.03 O 2 ( 3 ) a See footnote a of table 6 2. b See footnote b of table 6 2. Table 6 4 Comparison of Mn Mn and Mn Ca distances in 6 2 and 4 2 [Mn 4 Ca 2 ] ( 6 2 ) [Mn 4 Ca 2 ] ( 4 2 ) Mn III Ca Mn2 Ca1' 3.35 02 (13) Mn1' Ca1 3.2925 (12) Mn I V Ca Mn1' Ca1' 3.2109 (12) Mn2 Ca1 3.4143 (12) Mn I V Ca Mn2' Ca1' 3.3350 (13) Mn2 Ca1' 3.4006 (9) Mn III Mn IV Mn2 Mn1' 2.8185 (9) Mn1' Mn2 2.8716 (11) Mn I V Mn IV Mn1 Mn2' 2.8185 (9) Mn2' Mn2 3.0715 (11) Mn I V Mn IV Mn2' Mn1' 2.8562 (9) Mn2' Mn1 2.8716 (11)

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218 Table 6 5 Comparison of Mn O Mn angles in 6 2 and 4 2 [Mn 4 Ca 2 ] ( 6 2 ) [Mn 4 Ca 2 ] ( 4 2 ) Mn1' O9 Mn2' 102.34 (13) Mn1 O2 Mn2' 99.35 (6) Mn1' O8' Mn2 100.42 (12) Mn1 O3 Mn2 102.74 (7) Mn1' O 7 Mn2' 92.94 (12) Mn1 O1 Mn2' 93.63 (6) Mn1' O7 Mn2 93.87 (11) Mn1 O1 Mn2 92.24 (7) Mn2' O7 Mn2 98.90 (11) Mn2' O1 Mn2 98.12 (6) Table 6 6 BVS for the Mn a and selected oxygen atoms b in 6 3 Mn II Mn III Mn IV BVS Assignment Mn1 3.93 3.76 3.96 O1 2.08 O 2 ( 4 ) Mn2 4.00 3.82 4.02 O2 1.82 O 2 ( 3 ) Mn3 3.89 3.70 3.89 O3 1.98 O 2 ( 3 ) Mn4 4.10 3.90 4.10 O4 1.87 O 2 ( 3 ) Mn5 3.98 3.79 3.98 O5 1.99 O 2 ( 3 ) Mn6 3.29 3.12 3.29 O6 1.71 O 2 ( ) O7 2.11 O 2 ( 3 ) O8 1.64 O 2 ( ) O9 1.97 O 2 ( 3 ) O10 1.86 O 2 ( 3 ) O11 0.50 H 2 O ( ) O18 1.50 t BuCO 2 (terminal) a See footnote a of table 6 2. b See footnote b of table 6 2. Table 6 7 BVS for the Mn a and selected O atoms b in 6 4 Mn II Mn III Mn IV BVS Assignment Mn1 3.23 2.95 3.10 O1 1.20 OH ( 3 ) Mn2 3.13 2.86 3.00 O2 1.96 O 2 ( 4 ) Mn3 3.10 2.84 2.98 O3 1.20 OH ( 3 ) Mn4 1.94 1.78 1.87 O4 2.17 O 2 ( 4 ) Mn5 1.87 1.71 1.79 O19 0.32 H 2 O (terminal) a See footnote a of table 6 2. b See footnote b of table 6 2.

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219 Table 6 8 Comparison of Mn Mn and Mn Ca distances in 6 4 3 1 and 4 2 [Mn 9 Ca 2 ] ( 6 4 ) [Mn 3 Ca 2 ] ( 3 1 ) [Mn 4 Ca 2 ] ( 4 2 ) Mn1 Ca1' 3.5366 (8) Mn1 Ca1 3.3940 (13) Mn1' Ca1' 3.2925 (12) Mn3' Ca1' 3.4012 (8) Mn2 Ca1 3.4538 (10) Mn2' Ca1' 3.4006 (12) Mn2' Ca1' 3.3908 (11 ) Mn3 Ca1 3.4177 (11) Mn2 Ca1' 3.4144 (9) Mn1 Mn3' 3.1522 (7) Mn1 Mn2 2.7570 (1) Mn1' Mn2' 2.8716 (11) Mn3' Mn2' 2.8598 (7) Mn2 Mn3 2.8570 (8) Mn2' Mn2 3.0715 (11) Mn1 Mn2' 3.1439 (7) Mn1 Mn3 2.7295 (9) Mn2' Mn1 2.8146 (11) Table 6 9 BVS for the Mn a and selected O atoms b in 6 5 Mn II Mn III Mn IV BVS Assignment Mn1 3.40 3.11 3.26 O34 1.99 O 2 ( 4 ) Mn2 3.27 2.99 3.14 O35 2.27 O 2 ( 4 ) Mn3 3.22 2.95 3.10 O36 2.20 O 2 ( 3 ) Mn4 3.18 2.93 3.06 O37 2.21 O 2 ( 4 ) Mn5 3.14 2.87 3.0 1 O38 1.20 OH ( 3 ) Mn6 3.34 3.06 3.22 O39 2.13 O 2 ( 4 ) Mn7 3.39 3.11 3.25 O40 1.99 O 2 ( 4 ) Mn8 3.52 3.26 3.40 O41 2.10 O 2 ( 4 ) Mn9 3.11 2.87 3.00 O33 2.16 O 2 ( 3 ) Mn10 3.06 2.80 2.94 Mn11 3.30 3.05 3.18 a See footnote a of table 6 2. b Se e footnote b of table 6 2.

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220 Figure 6 1. Labeled representation of the structure of 6 1 Hydrogen atoms have been omitted for clarity. Color code: Mn IV blue; Ca II yellow; O red; and C grey.

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221 Figure 6 2. (top) Core of complex 6 1 emphasizing the fu sed [Mn IV 2 Ca 2 ] cubane moieties and (bottom) core of 6 1 emphasizing Mn IV 6 plane. Color code: Mn IV blue; Ca II yellow; O red

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222 Figure 6 3. Labeled representation of the anion (left) and cation (right) of 6 2 Hydrogen atoms have been omitted for clarity. C olor code: Mn IV blue; Mn III green; Ca II yellow; Cl purple; N cyan; O red; and C grey. Figure 6 4. Labeled representation of the cation of 6 2 Hydrogen atoms and methyl groups of the pivalate ligands have been removed for clarity Color code: Mn IV blue; Mn III green; Ca II yellow; N cyan; O red; and C grey.

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223 Figure 6 5. Comparison of [Mn 4 Ca 2 ] core 6 2 (left) and 4 2 (right). Figure 6 6. Labeled representation of the anion of 6 2 Hydrogen atoms and methyl groups of the pivalate groups have been remo ved for clarity The bold green lines emphasize the central [Ca 6 ( 6 Cl] 11+ unit. Color code: Ca II yellow; Cl purple; O red; and C grey.

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224 Figure 6 7. Labeled representation of the structure of 6 3 Hydrogen atoms and the pivalate methyl groups have been o mitted for clarity. Color code: Mn IV blue; Mn III green; Ca II yellow; O red; N cyan and C grey. Figure 6 8. Core of complex 6 3 emphasizing the defective dicubane [Mn III Mn 4 IV Ca 2 ( 4 O)( 3 O) 6 ] 9+ unit. Color code: Mn IV blue; Mn III green; Ca II yellow and O red

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225 Figure 6 9. Labeled representation of the structure of 6 4 Hydrogen atoms and pivalate methyl groups have been omitted for clarity. Color code: Mn III green; Mn II light blue; Ca II yellow; O red and C grey. Figure 6 10. Core of complex 6 4 emphas izing the two fused [Mn 5 III Ca 2 ( 3 OH) 4 ( 4 O) 4 ] 7 + unit. Color code: Mn III green; Mn II light blue; Ca II yellow and O red

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226 Figure 6 11. Comparison of the Mn 3 Ca cubane unit in (a) 6 4 (b) 3 1 and (c) 4 2 The manganese oxidation states in the three cubes are different. Figure 6 12. L abeled representation of the structure of 6 5 Hydrogen atoms and pivalate methyl groups have been omitted for clarity. Color code: Mn III green; Ca II yellow; O red ; Cl purple and C grey. The JT axes in 6 5 are highlighted as bold blue lines.

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227 Figure 6 13. Core of complex 6 5 emphasizing the fused [Mn 3 III CaO] 9+ tetrahedral units. Color code: Mn III green; Ca II yellow and O red Figure 6 14. Plot of X M T vs T for complex 6 1 ( ) and 6 2 ( ).

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228 Figure 6 15. (top) [Mn III Mn IV 3 Ca 2 O 6 ] core of 6 2 from a view point emphasizing the Mn III Mn IV 3 butterfly unit; (bottom) the corresponding 2 J coupling scheme and definition of J wb and J bb exchange parameters. Figure 6 16. Plot of X M T v s T for complex 6 2 The solid line is the fit of the data; see text for fit parameters.

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229 Figure 6 17. Plot of the S T states for complex 6 2 as a function of energy. The ground state is | 1 / 2 3 5 / 2 > with the first excited state being | 3 / 2 2 3 / 2 > Fi gure 6 18. The core of 6 2 emphasizing the exchange coupling model employed, and the spin alignments in the S T = | 1 / 2 3 5 / 2 > ground state.

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230 Figure 6 19. Plot of S T energies vs the J wb /J bb ratio for complex 6 2 showing the variable ground state. The d ashed line corresponds to the experimentally determined J wb /J bb ratio of 0.73. Figure 6 20. Plot of X M T vs T for complex 6 3 ( ) 6 4 ( ) and 6 5 ( )

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231 Figure 6 2 1. Plot of reduced magnetization ( M / B ) vs H / T for complex 6 3 The solid lines are the fit of the data; see text for the fit parameters. Figure 6 2 2. Two dimensional contour plot of the root mean square error surface vs D and g for the magnetization fit for 6 3

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232 Figure 6 2 3. Plot of reduced magnetization ( M / B ) vs H / T for complex 6 4 The solid lines are the fit of the data; see text for the fit parameters. Figure 6 2 4. Plot of X' M T vs T for 6 1 in a 3.5 G field oscillating at the indicated frequencies.

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233 Figure 6 2 5. Plot of X' M T vs T for complex es 6 2 6 5 in a 3.5 G fi eld oscillating at the indicated frequencies.

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234 APPENDIX A BOND DISTANCES AND A NGLES Table A 1. Selected interatomic distances ( ) and angles ( o ) for 2 1 3MeCN P arameters P arameters Mn1 O3 1.890(3) Mn8 O9 1.944(4) Mn1 O1 1.918(3) Mn8 Cl1 2.8003(15 ) Mn1 O12 1.940(3) Mn2 O1 Mn5 97.37(15) Mn1 O11 1.941(3) Mn2 O1 Mn1 111.11(16) Mn1 Cl2 2.4604(15) Mn5 O1 Mn1 111.84(16) Mn1 Cl1 2.7841(16) Mn8 O2 Mn2 112.28(17) Mn2 O1 1.869(3) Mn8 O2 Mn5 114.68(17) Mn2 O2 1.906(3) Mn2 O2 Mn5 96.11(15) Mn2 N3 2.359(17) Mn1 O3 Mn4 114.00(16) Mn2 Cl1 2.6586(15) Mn1 O3 Mn3 110.70(16) Mn3 O3 1.916(3) Mn4 O3 Mn3 132.40(17) Mn3 O4 1.946(3) Mn6 O4 Mn5 95.04(14) Mn3 Cl3 2.2305(14) Mn6 O4 Mn3 104.18(15) Mn3 O5 2.286(3) Mn5 O4 Mn3 113.84(16) Mn3 Cl2 2.5891(1 4) Mn6 O5 Mn3 90.46(13) Mn4 O7 1.882(3) Mn6 O5 Mn7 88.59(13) Mn4 O3 1.905(3) Mn3 O5 Mn7 122.39(16) Mn4 Cl1 2.7366(16) Mn6 O6 Mn7 106.32(16) Mn5 O2 1.915(3) Mn6 O6 Mn5 93.60(14) Mn5 O1 1.915(3) Mn7 O6 Mn5 116.21(16) Mn5 O4 1.920(3) Mn4 O7 Mn7 12 2.82(18) Mn5 O6 1.957(3) Mn4 O7 Mn8 112.40(17) Mn6 O4 1.840(3) Mn7 O7 Mn8 124.53(17) Mn6 O6 1.846(3) Mn2 Cl1 Mn4 110.31(5) Mn6 O5 1.906(3) Mn2 Cl1 Mn1 69.98(4) Mn7 O7 1.903(3) Mn4 Cl1 Mn1 70.41(4) Mn7 O6 1.921(3) Mn2 Cl1 Mn8 70.80(4) Mn7 O5 2 .385(4) Mn4 Cl1 Mn8 69.32(4) Mn8 O2 1.904(3) Mn1 Cl1 Mn8 106.98(5) Mn8 O7 1.908(4) Mn1 Cl2 Mn3 76.60(4)

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235 Table A 2. Selected interatomic distances ( ) and angles ( o ) for 2 2 MeCN P arameters P arameters Mn1 O9 1.7681(14) Mn8 O7 1.8760(13) Mn1 O5 1.8794(14) Mn3 O1 Mn2 92.50(6) Mn1 O6 1.8844(15) Mn3 O1 Mn1 94.17(6) Mn1 O1 1.9518(14) Mn2 O1 Mn1 98.73(7) Mn2 O4 1.8149(14) Mn5 O2 Mn7 92.67(6) Mn2 O1 1.8744(15) Mn5 O2 Mn4 94.62(6) Mn2 O6 1.8774(14) Mn7 O2 Mn4 99.69(7) Mn3 O4 1.8058(15) M n5 O3 Mn7 96.21(7) Mn3 O5 1.8401(14) Mn3 O4 Mn2 96.42(7) Mn3 O1 1.8625(14) Mn3 O5 Mn1 97.39(6) Mn4 O9 1.7642(15) Mn3 O5 Mn6 127.67(7) Mn4 O7 1.8817(14) Mn1 O5 Mn6 133.81(8) Mn4 O8 1.8893(14) Mn8 O6 Mn2 123.52(7) Mn4 O2 1.9520(16) Mn8 O6 Mn1 126 .39(7) Mn5 O3 1.8057(15) Mn2 O6 Mn1 101.07(7) Mn5 O7 1.8463(15) Mn5 O7 Mn8 126.84(8) Mn5 O2 1.8524(14) Mn5 O7 Mn4 97.23(6) Mn6 O8 1.8583(15) Mn8 O7 Mn4 134.69(8) Mn6 O5 1.9073(14) Mn6 O8 Mn4 125.86(8) Mn7 O3 1.8149(14) Mn6 O8 Mn7 123.49(8) Mn7 O2 1.8733(14) Mn4 O8 Mn7 101.32(7) Mn8 O6 1.8721(15) Mn4 O9 Mn1 135.00(8)

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236 Table A 3. Selected interatomic distances ( ) and angles ( o ) for 2 3 1 / 3 THF 2 / 3 MeCN P arameters P arameters Mn1 O1 1.865(2) Mn8 O1 1.873(2) Mn1 O6 1.866(2) Mn9 O29 1.8 82(2) Mn2 O7 1.898(2) Mn9 O9 1.883(2) Mn2 O1 1.902(2) Mn1 O6 Mn5 121.74(12) Mn2 O9 1.926(2) Mn1 O6 Mn2 97.25(10) Mn2 O6 1.938(2) Mn5 O6 Mn2 121.05(11) Mn2 O8 2.178(2) Mn3 O7 Mn6 129.14(12) Mn3 O7 1.861(2) Mn3 O7 Mn2 99.05(11) Mn3 O9 1.871(2) Mn6 O7 Mn2 122.00(12) Mn4 O8 1.861(2) Mn7 O8 Mn4 128.48(12) Mn4 O18 1.886(2) Mn7 O8 Mn2 116.01(11) Mn5 O6 1.871(2) Mn4 O8 Mn2 115.50(11) Mn5 O18 1.880(2) Mn3 O9 Mn9 121.59(12) Mn6 O18 1.858(2) Mn3 O9 Mn2 97.71(10) Mn6 O7 1.874(2) Mn9 O9 Mn2 12 0.17(12) Mn7 O8 1.857(2) Mn1 O1 Mn8 127.65(12) Mn7 O29 1.881(2) Mn1 O1 Mn2 98.56(10) Mn8 O29 1.864(2) Mn8 O1 Mn2 121.99(12)

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237 Table A 4. Selected interatomic distances () and angles ( o ) for 3 1 P arameter P arameter Mn1 O3 1.820(3) Ca1 O2 2.452 (3) Mn1 O2 1.844(3) Ca1 O6 2.454(3) Mn1 O5 1.885(3) Ca1 O15 2.470(3) Mn1 O1 1.899(3) Ca1 O3 2.470(3) Mn1 O9 1.964(3) Ca1 O4 2.660(3) Mn1 O7 1.979(3) Ca1 Ca2 4.1965(12) Mn1 Mn3 2.7296(8) Ca2 O4 2.361(3) Mn1 Mn2 2.7571(8) Ca2 O20 2.376(3) Mn1 Ca1 3.3942(10) Ca2 O18 2.385(3) Mn2 O2 1.862(3) Ca2 O27 2.394(3) Mn2 O4 1.866(3) Ca2 O24 2.424(5) Mn2 O1 1.891(3) Ca2 O22 2.435(4) Mn2 O17 1.901(3) Ca2 O25 2.483(4) Mn2 O19 1.907(3) Ca2 O23 2.935(5) Mn2 O8 1.977(3) Mn1 O1 Mn2 93.34(12) Mn2 Mn3 2.8570(8) Mn1 O1 Mn3 92.11(11) Mn2 Ca1 3.4539(10) Mn1 O2 Ca1 103.49(11) Mn2 Ca2 3.6672(11) Mn1 O2 Mn3 96.15(13) Mn3 O3 1.830(3) Mn1 O3 Ca1 103.55(12) Mn3 O4 1.889(3) Mn1 O3 Mn2 96.81(12) Mn3 O1 1.892(3) Mn2 O1 Mn3 98.08(12) Mn3 O11 1.9 14(3) Mn2 O3 Ca1 104.29(12) Mn3 O21 1.934(3) Mn2 O4 Mn3 99.05(12) Mn3 O10 1.988(3) Mn2 O4 Ca1 97.93(10) Mn3 Ca1 3.4178(10) Mn2 O4 Ca2 119.91(12) Mn3 Ca2 3.7798(10) Mn3 O2 Ca1 104.29(11) Ca1 O23 2.305(4) Mn3 O4 Ca1 95.92(10) Ca1 O12 2.379(3) Mn3 O4 Ca2 125.23(12) Ca1 O13 2.402(3) Ca1 O4 Ca2 113.28(10)

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238 Table A 5. Selected interatomic distances ( ) and angles ( o ) for 3 2 MeCN P arameter P arameter Mn1 O22 1.821(3) Ca1 O1 2.342(4) Mn1 O19 1.830(3) Ca1 O20 2.545(3) Mn1 O21 1.889(3) Ca1 N1 2.560(4) Mn1 O10 1.903(3) Ca1 N2 2.620(4) Mn1 O14 1.995(3) Ca1 C11 3.189(5) Mn1 O4 2.010(3) Ca2 O15 2.358(3) Mn1 Mn2 2.7362(8) Ca2 O9 2.391(3) Mn1 Mn3 2.7416(9) Ca2 O11 2.396(3) Mn1 Ca2 3.3639(11) Ca2 O22 2.432(3) Mn2 O22 1.847(3) Ca2 O19 2.474(3) Mn2 O20 1.876(3) Ca2 O7 2.483(3) Mn2 O21 1.888(3) Ca2 N4 2.604(5) Mn2 O16 1.888(3) Ca2 O20 2.692(3) Mn2 O5 1.933(3) Mn1 O19 Mn3 96.76(14) Mn2 O13 1.989(3) Mn1 O19 Ca2 101.78(13) Mn2 Mn3 2.8329(9) Mn3 O19 Ca2 105.59(12) Mn2 Ca2 3.3999(11) Mn3 O20 Mn2 98.70(13) Mn3 O19 1.838(3) Mn3 O20 Ca1 116.71(14) Mn3 O20 1.858(3) Mn2 O20 Ca1 122.43(13) Mn3 O21 1.876(3) Mn3 O20 Ca2 97.12(11) Mn3 O18 1.914(3) Mn2 O20 Ca2 94.49(11) Mn3 O2 1.930(3) Ca1 O20 Ca2 121.83(11) Mn3 O3 2 .008(3) Mn3 O21 Mn2 97.63(13) Mn3 Ca2 3.4553(11) Mn3 O21 Mn1 93.45(13) Mn3 Ca1 3.7651(10) Mn2 O21 Mn1 92.84(13) Ca1 O12 2.267(4) Mn1 O22 Mn2 96.49(14) Ca1 O6 2.295(3) Mn1 O22 Ca2 103.64(13) Ca1 O17 2.330(3) Mn2 O22 Ca2 104.41(13)

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239 Table A 6. Selected interatomic distances ( ) and angles ( o ) for 4 1 t BuCO 2 H5.5MeCNH 2 O Parameters Parameters Mn1 O13 1.838(2) Ca1 O1 2.324(2) Mn1 O12 1.846(2) Ca1 O19 2.379(2) Mn1 O15 1.910(2) Ca1 O10' 2.407(3) Mn1 O2 1.911(2) O13 Mn1 O12 89.77(10) Mn1 O3 1. 941(2) O13 Mn1 O15 83.62(10) Mn1 O5 1.968(2) O12 Mn1 O15 85.81(10) Mn2 O12 1.818(2) O13 Mn1 O2 97.59(10) Mn2 O14 1.842(2) O12 Mn1 O2 93.33(10) Mn2 O18' 1.920(2) O15 Mn1 O2 178.52(10) Mn2 O15 1.948(2) O13 Mn1 O3 95.21(10) Mn2 O16' 1.964(2) O12 Mn1 O3 174.66(10) Mn2 O6 1.972(2) O15 Mn1 O3 96.63(10) Mn2 Mn3 2.9111(7) O2 Mn1 O3 84.13(10) Mn3 O17 1.840(2) O13 Mn1 O5 170.73(10) Mn3 O13 1.841(2) O12 Mn1 O5 87.86(10) Mn3 O15 1.891(2) O15 Mn1 O5 87.28(10) Mn3 O16 1.923(2) O2 Mn1 O5 91.49(10) Mn3 O18' 1. 965(2) O3 Mn1 O5 87.51(10) Mn3 O18 1.967(2) O4 Ca2 O18 120.68(8) Mn4 O17 1.811(2) O9 Ca2 O18 64.81(7) Mn4 O14' 1.826(2) O17 Ca2 O18 56.83(7) Mn4 O16 1.903(2) O20 Ca2 O18 145.37(8) Mn4 O7 1.920(3) O8 Ca2 O18 112.24(8) Mn4 N2 2.079(3) O11' Ca2 O18 63.5 7(7) Mn4 N1 2.082(3) O15 Ca2 O18 54.35(6)

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240 Table A 7. Selected interatomic distances ( ) and angles ( o ) for 4 2 MeCN0.1H 2 O Parameters Parameters Mn1 O3 1.7970(14) O3 Mn1 O2 96.90(6) Mn1 O2 1.8034(15) O3 Mn1 O1 86.36(7) Mn1 O1 1.8869(15) O2 Mn 1 O1 85.80(6) Mn1 O7 1.9479(15) O3 Mn1 O7 93.45(7) Mn1 N1 2.1068(18) O2 Mn1 O7 96.20(6) Mn1 N2 2.1088(18) O1 Mn1 O7 178.01(6) Mn1 Ca1 3.2925(12) O3 Mn1 N1 93.08(7) Mn2 O3' 1.8783(15) O2 Mn1 N1 170.00(6) Mn2 O2 1.8878(15) O8 Ca1 O5 139.86(6) Mn 2 O1 1.9726(16) O8 Ca1 O6 93.96(6) Mn2 O4 1.9734(16) O5 Ca1 O6 122.18(6) Mn2 O9' 1.9975(18) O8 Ca1 O2 134.67(5) Mn2 O1' 2.0921(17) O5 Ca1 O2 73.34(5) Ca1 O8 2.3465(17) O6 Ca1 O2 81.04(5) Ca1 O5 2.3783(18) O8 Ca1 O1' 86.33(6) Ca1 O6 2.4170(16) O 5 Ca1 O1' 79.25(5) Ca1 O2 2.4897(17) O6 Ca1 O1' 136.04(5) Ca1 O1' 2.5195(16) O2 Ca1 O1' 68.54(5) Ca1 O3 2.5658(15) O8 Ca1 O3 71.17(5) Ca1 N3 2.6183(18) O5 Ca1 O3 132.61(5) Ca1 N4 2.6241(19) O6 Ca1 O3 72.16(5)

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241 Table A 8. Selected interatomic distances ( ) and angles ( o ) for 5 2 MeCN 0.2H 2 O Parameters Parameters Mn1 O3 1.7969(12) O3 Mn1 O2 98.01(6) Mn1 O2 1.8007(12) O3 Mn1 O1 86.12(6) Mn1 O1 1.8689(13) O2 Mn1 O1 85.64(6) Mn1 O7 1.9443(13) O3 Mn1 O7 94.65(6) Mn1 N1 2.1008(15) O2 Mn1 O7 96.66(6) Mn1 N2 2.1038(15) O1 Mn1 O7 177.44(5) Mn1 Mn2 2.8191(4) O3 Mn1 N1 92.30(6) Mn1 Sr1 3.3812(3) O2 Mn1 N1 169.68(6) Mn2 O3' 1.8745(13) O8 Sr1 O5 134.50(6) Mn2 O2 1.8760(13) O8 Sr1 O6 101.12(6) Mn2 O9' 1.9824(15) O5 Sr1 O6 122.60(5) Mn2 O4 1.9824(13) O8 Sr1 O2 132.47(5) Mn2 O1 1.9880(13) O5 Sr1 O2 73.30(4) Mn2 O1' 2.0659(14) O6 Sr1 O2 78.82(4) Sr1 O8 2.4565(16) O8 Sr1 O1' 83.08(6) Sr1 O5 2.4838(14) O5 Sr1 O1' 76.81(5) Sr1 O6 2.5411(13) O6 Sr1 O1' 132.31(4) Sr1 O2 2.6110(12) O 8 Sr1 O3 72.49(4) Sr1 O1' 2.6225(13) O5 Sr1 O3 129.52(4) Sr1 O3 2.6423(12) O6 Sr1 O3 72.01(4) Sr1 N3 2.7222(16) O2 Sr1 O3 62.24(4) Sr1 N4 2.7443(16) O1' Sr1 O3 64.06(4)

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242 Table A 9. Selected interatomic distances ( ) and angles ( o ) for 5 1 Param eters Parameters Mn1 O3 1.8418(16) O3 Mn1 O9' 93.28(7) Mn1 O9' 1.8488(15) O3 Mn1 O2 93.53(7) Mn1 O2 1.9040(15) O9' Mn1 O2 83.46(7) Mn1 O4 1.9147(15) O3 Mn1 O4 82.67(7) Mn2 O3 1.7981(15) O9' Mn1 O4 102.64(6) Mn2 O5 1.8202(16) O2 Mn1 O4 172.94(6 ) Mn2 O4 1.9034(16) O3 Mn1 O1 89.67(7) Mn2 N1 2.097(2) O9' Mn1 O1 172.98(7) Mn3 O8 1.8240(15) O2 Mn1 O1 90.01(6) Mn3 O5 1.8359(16) O4 Mn1 O1 84.04(6) Mn3 O1 1.9152(15) O3 Mn1 O1' 170.31(7) Mn3 O10 1.9843(15) O9' Mn1 O1' 94.59(7) Mn4 O9 1.8399 (15) O2 Mn1 O1' 81.77(6) Mn4 O8 1.8477(16) O4 Mn1 O1' 101.08(6) Mn4 O16' 1.9599(17) O3 Mn2 O5 100.74(7) Mn4 O11 1.9745(16) O3 Mn2 O4 84.15(7) Sr1 O19 2.4423(16) O5 Mn2 O4 84.24(7) Sr1 O17 2.4511(18) O8 Mn3 O5 97.77(7) Sr1 O15 2.4598(16) O8 Mn3 O 1 92.74(7) Sr1 O3 2.5387(15) O5 Mn3 O1 94.42(7) Sr1 O7 2.5910(17) O9 Mn4 O8 88.36(7) Sr1 O14 2.6317(17) O9 Mn4 O2' 84.35(6) Sr2 O9 2.5358(16) O9 Mn4 O11 171.86(7) Sr2 O14 2.5395(19) O8 Mn4 O11 88.01(7) Sr2 O19 2.5616(16) O2' Mn4 O11 88.13(6) S r2 O8 2.6188(15) O19 Sr1 O17 83.46(6) Sr2 O13 2.6399(17) O19 Sr1 O15 119.47(6) Sr2 N3 2.714(2) O17 Sr1 O15 94.79(6) Sr2 N4 2.721(2) O19 Sr1 O3 129.88(5) O9 Sr2 O8 59.78(5) O15 Sr1 O3 81.62(5) O14 Sr2 O8 75.15(6) O19 Sr1 O7 141.85(6) O19 Sr2 O8 11 5.84(5) O8 Sr2 O13 88.46(5)

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243 Table A 10. Selected interatomic distances ( ) and angles ( o ) for 6 1 10THF Parameters Parameters Ca1 O17 2.433(9) O17 Ca1 O2 90.0(3) Ca1 O16 2.440(6) O16 Ca1 O2 137.54(19) Ca1 O16' 2.440(6) O16' Ca1 O2 137.54(19) Ca1 O1' 2.455(6) O1' Ca1 O2 73.1(2) Ca1 O1 2.456(6) O1 Ca1 O2 73.1(2) Ca1 O2 2.458(8) O1 Ca1 O5 60.7(2) Ca2 O2 2.440(9) O2 Ca1 O5 121.1(3) Ca2 O1' 2.451(6) O17 Ca1 O3 77.90(18) Ca2 O1 2.451(6) O16 Ca1 O3 76.8(2) Ca2 O18 2.456(7) O1 Mn1 O5 89.1( 3) Mn1 O1 1.821(6) O1 Mn1 O6 89.3(3) Mn1 O5 1.823(6) O5 Mn1 O6 84.2(3) Mn1 O6 1.826(6) O1 Mn1 O8 94.3(3) Mn1 O8 1.943(8) O1 Mn2 O11 94.6(3) Mn2 O1 1.821(6) O4 Mn2 O11 93.0(3) Mn2 O4 1.824(6) O3 Mn2 O11 176.1(3) Mn2 O3 1.835(6) O1 Mn2 O9 94.8( 3) Mn2 O11 1.958(7) O2 Mn3 O3 89.5(3) Mn3 O2 1.815(4) O2 Mn3 O4 89.9(3) Mn3 O3 1.829(6) O3 Mn3 O4 84.7(3) Mn3 O4 1.831(6) O2 Mn3 O14 94.4(3)

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244 Table A 11. Selected interatomic distances ( ) and angles ( o ) for 6 2 2CH 2 Cl 2 H 2 O0.8MeCN Parameters Parameters Mn1 O9 1.792(3) O9 Mn1 O8 96.84(12) Mn1 O8 1.795(2) O9 Mn1 O7 85.89(12) Mn1 O7 1.877(3) O8 Mn1 O7 85.29(11) Mn1 O4 1.950(3) O9 Mn1 O4 92.21(13) Mn1 N1 2.059(3) O8 Mn1 O4 96.64(12) Mn1 N2 2.073(3) O8 Mn2 O2 89.77(12) Mn2 O8 1.873(3 ) O9' Mn2 O2 97.70(12) Mn2 O9' 1.874(3) O8 Mn2 O7 80.41(11) Mn2 O2 1.968(3) O2 Mn2 O7 170.02(12) Mn2 O7 1.980(2) O8 Mn2 O5 100.17(11) Mn2 O5 2.004(3) O9' Mn2 O5 89.65(11) Mn2 O7' 2.058(3) O6' Ca1 O1 127.54(13) Ca1 O1 2.298(3) O6' Ca1 O3 106.49(13 ) Ca1 O3 2.316(4) O1 Ca1 O3 118.45(13) Ca1 O8 2.454(3) O6' Ca1 O8 138.98(10) Ca1 O9 2.464(3) O1 Ca1 O8 76.21(10) Ca1 O7' 2.480(3) O3 Ca1 O8 81.57(11) Ca2 O13 2.315(2) O13 Ca2 O22 158.37(9) Ca2 O22 2.315(2) O13 Ca2 O11 98.69(9) Ca2 O11 2.343(2 ) O22 Ca2 O11 95.55(9) Ca2 O18 2.351(3) O13 Ca2 O18 96.55(9) Ca2 O17 2.489(2) O22 Ca2 O18 97.02(9)

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245 Table A 12. Selected interatomic distances ( ) and angles ( o ) for 6 3 8CH 2 Cl 2 Parameters Parameters Mn1 O3 1.802(2) O3 Mn1 O4 98.69(10) Mn1 O4 1.814(2) O3 Mn1 O2 85.48(9) Mn1 O2 1.889(2) O4 Mn1 O2 85.40(9) Mn1 N3 2.076(3) O3 Mn1 O21 92.47(9) Mn2 O6 1.774(2) O6 Mn2 O8 98.36(10) Mn2 O8 1.833(2) O6 Mn2 O5 90.91(10) Mn2 O5 1.850(2) O8 Mn2 O5 85.17(10) Mn2 N5 2.052(3) O6 Mn2 O23 90.84(10 ) Mn3 O8 1.843(2) O8 Mn3 O9 99.95(10) Mn3 O9 1.853(2) O8 Mn3 O7 171.91(10) Mn3 O7 1.926(2) O9 Mn3 O7 84.88(9) Mn3 O1 1.936(2) O8 Mn3 O1 93.81(9) Mn4 O10 1.828(2) O7 Mn3 O5 92.42(9) Mn4 O9 1.829(2) O1 Mn3 O5 85.49(9) Mn4 O7 1.857(2) O8 Mn3 O16 99.12(10) Mn4 O25 1.888(2) O9 Mn3 O16 87.75(10) Mn5 O4 1.850(2) O3 Mn6 O22 99.86(10) Mn5 O10 1.854(2) O6 Mn6 O22 89.75(9) Mn5 O1 1.856(2) O5 Mn6 O22 88.8(1) Mn5 O7 1.914(2) O11 Ca1 O13 71.71(8) Mn6 O3 1.856(2) O12 Ca1 O13 52.91(8) Mn6 O6 1.8 86(2) O9 Ca1 O10 61.77(7) Mn6 O5 1.973(2) O24 Ca1 O10 80.49(7) Mn6 O2 1.982(2) O19 Ca2 O1 133.21(8) Ca1 O9 2.389(2) O20 Ca2 O1 136.30(8) Ca1 O13 2.489(2) O13 Ca2 O3 143.06(8) Ca1 O10 2.599(2) O1 Ca2 O3 74.35(7) Ca1 N2 2.611(3) O13 Ca2 O11 72.72 (8) Ca2 O1 2.379(2) O19 Ca2 O11 77.63(8) Ca2 O3 2.431(2) O1 Ca2 O4 61.76(7) Ca2 O11 2.570(3) O3 Ca2 O4 64.78(7) Ca2 O4 2.675(2) O11 Ca2 O4 119.10(8)

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246 Table A 13. Selected interatomic distances ( ) and angles ( o ) for 6 4 1 / 2 MeCN 1 / 2 CHCl 3 Paramet ers Parameters Mn1 O1 1.894(2) O1 Mn1 O1' 179.997(1) Mn1 O1' 1.894(2) O1 Mn1 O3' 95.33(11) Mn1 O3' 1.899(3) O1' Mn1 O3' 84.67(11) Mn1 O3 1.899(3) O1 Mn1 O3 84.67(11) Mn2 O4 1.900(3) O4 Mn2 O2 82.49(11) Mn2 O2 1.904(2) O4 Mn2 O9 94.13(11) Mn2 O 9 1.954(3) O2 Mn2 O9 175.13(11) Mn2 O5 1.960(3) O4 Mn2 O5 173.83(12) Mn3 O2 1.901(3) O4 Mn3 O15 94.13(12) Mn3 O4 1.905(3) O2 Mn3 O13 98.26(12) Mn3 O15 1.955(3) O2 Mn3 O4 82.42(11) Mn3 O11 1.966(3) O11 Mn3 O3 87.53(11) Mn4 O2 2.111(2) O12 Mn4 O 17 89.99(11) Mn4 O12 2.151(3) O6 Mn4 O17 89.53(11) Mn4 O6 2.151(3) O2 Mn4 O19 95.11(11) Mn4 O17 2.196(3) O17 Mn4 O18 58.39(11) Mn5 O20 2.024(3) O20 Mn5 O4 116.85(13) Mn5 O4 2.033(3) O20 Mn5 O8 112.04(14) Mn5 O8 2.064(3) O4 Mn5 O8 108.65(12) Mn 5 O14 2.068(3) O16 Ca1 O3 72.23(10) Ca1 O10 2.361(3) O10 Ca1 O3 132.03(10) Ca1 O4 2.376(3) O4 Ca1 O3 72.39(9) Ca1 O21 2.376(3) O4 Ca1 O1 72.54(9) Ca1 O3 2.487(3) O21 Ca1 O1 140.94(11) Ca1 O1 2.495(3) O3 Ca1 O1 61.68(8)

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247 Table A 14. Selected interatomic distances ( ) and angles ( o ) for 6 5 3 / 2 CH 2 Cl 2 Parameters Parameters Ca1 O33 2.284(6) O33 Ca1 O36 110.6(2) Ca1 O36 2.293(6) O33 Ca1 O37 123.2(2) Ca2 O81 2.570(7) O36 Ca1 O37 113.8(2) Ca2 O79 2.589(6) O33 Ca1 O39 112.8(2) Mn1 O1 1.94 0(6) O1 Mn1 O16 83.9(3) Mn1 O3 1.986(6) O3 Mn1 O16 80.5(3) Mn1 O16 2.137(9) O33 Mn1 O5 98.4(3) Mn2 O9 1.964(6) O34 Mn1 O5 91.1(3) Mn2 O11 2.138(6) O35 Mn2 O7 177.7(3) Mn2 O15 2.489(10) O36 Mn2 O9 173.9(3) Mn3 O34 1.853(6) O7 Mn2 O9 86.9(3) Mn3 O35 1.906(6) O36 Mn2 O11 102.2(2) Mn3 O4 1.937(6) O35 Mn3 O4 175.9(3) Mn4 O12 1.898(6) O34 Mn3 O10 174.0(3) Mn4 O17 1.932(6) O4 Mn3 O10 84.0(3) Mn5 O6 1.928(6) O34 Mn3 O15 95.6(3) Mn5 O34 2.175(6) O35 Mn4 O12 94.9(3) Mn6 O8 1.952(7) O35 Mn4 O 17 169.3(2) Mn6 O23 1.970(7) O17 Mn4 O37 93.2(3) Mn7 O40 1.960(7) O35 Mn4 O13 82.2(2) Mn7 O27 1.960(6) O14 Mn5 O37 92.2(2) Mn8 O40 1.901(6) O38 Mn5 O6 91.4(2) Mn8 O39 1.914(6) O37 Mn5 O34 87.4(2) Mn9 O41 1.938(6) O6 Mn5 O34 98.3(2) Mn9 O31 1. 967(7) O39 Mn6 O8 178.4(3) Mn10 O38 2.172(6) O36 Mn6 O23 171.2(3) Mn10 O30 2.178(6) O8 Mn6 O23 87.1(3) Mn11 O32 1.928(6) O36 Mn6 O21 103.7(2) Mn11 O18 1.964(6) O40 Mn7 O27 91.7(3) O40 Mn10 O20 172.5(2) O2 Mn7 O25 88.1(3) O26 Mn10 O38 95.3(2) O33 Mn7 O42 94.2(4) O20 Mn10 O38 88.2(2) O40 Mn8 O39 90.2(3) O32 Mn11 O18 83.8(2) O40 Mn8 O24 175.5(3)

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248 APPENDIX B LIST OF COMPOUNDS [Mn 8 O 6 (OH)(O 2 CBu t ) 9 Cl 3 (Bu t CO 2 H) 0.5 (MeCN) 0.5 ] ( 2 1 ) [Mn 8 O 9 (O 2 CBu t ) 12 ] ( 2 2 ) [Mn 9 O 7 (O 2 CBu t ) 13 (THF) 2 ] ( 2 3 ) [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (HO 2 CBu t ) 4 ] ( 3 1 ) [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (bpy)(HO 2 CBu t )(MeCN)] ( 3 2 ) [Mn 8 Ca 4 O 14 (O 2 CBu t ) 10 (phen) 2 (H 2 O) 6 ]Cl 2 ( 4 1 ) [Mn 4 Ca 2 O 6 ( O 2 CBu t ) 6 (phen) 4 ]( O 2 CBu t ) ( 4 2 ) [Mn 4 Sr 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) ( 5 1 ) [Mn 8 Sr 4 O 14 (O 2 CBu t ) 12 (phen) 4 (H 2 O) 2 ] ( 5 2 ) [Mn 6 Ca 2 O 9 (O 2 CBu t ) 9 (H 2 O) 6 ] (NO 3 ) ( 6 1 ) [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (bpy) 2 (Et 2 O) 2 ][Ca 6 (O 2 CBu t ) 12 Cl] ( 6 2 ) [Mn 6 Ca 2 O 10 (O 2 CBu t ) 7 (phen) 3 (H 2 O)] ( 6 3 ) (NBu n 4 )[Mn 9 Ca 2 O 4 (OH) 4 (O 2 CBu t ) 16 (H 2 O) 2 ] ( 6 4 ) [Mn 11 CaO 8 (OH)Cl 2 (O 2 CBu t ) 16 ] ( 6 5 )

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249 APPENDIX C PHYSICAL MEASUREMENT S Infrared spectra were recorded in t he 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. Variable temperature dc and a c magnetic susceptibility data were collected at the University of Florida using a Quantum Design MPMS XL SQUID magnetometer equipped with a 7 T magnet and operating in the 1.8 300 K range. Samples were embedded in solid eicosane to prevent torqueing. Ma gnetization vs field and temperature data were fit using the program MAGNET, 196 and contour plots were obtained using the program GRID. 222 Low temperature (< 1.8 K) hysteresis loop measur ements were performed at Grenoble using an array of micro SQUIDs. 197 The X band and Q band EPR was collected at the CalEPR center at the University of California at Davis. Perpendicularly polarized CW X band (9 GHz) spectra were collected using a Bruker model ECS106 spectrometer equipped with a standard mode cavity. All CW X band spectra were collected at 10 K under non satura ting slow passage conditions. Temperature control was maintained with an Oxford Instruments model ESR900 helium flow cryostat with an Oxford ITC 503 temperature controller. Q band (34 GHz) pulsed EPR spectra were acquired with a Bruker EleXsys E580 spectro meter using either an EN 5107D2 Q band EPR/ENDOR probe or a laboratory built TE011 brass cavity and coupler following a standard design. 229 The laboratory built probe was also used to acquire the Q band field swept spectrum. The resonator was modified from previous applications, for an Oxford CF935 cryo stat. 230 Electron spin

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250 echo (ESE) detected electron nucle ar double resonance (ENDOR) spectra were obtained using the Davies ENDOR sequence ( MW RF t /2 MW MW ) with stochastic sampling of the RF excitation frequencies, 231 and an Amplifier Research model 253 (250 W) RF amplifier. Additional spectrometer settings used were: = 230 ns; /2 MW = 32 ns ; MW RF pulses) = 1 s. All spectral simulations were performed using Matlab 7.8.0 and the EasySpin 3.1.7 package. 232, 233 Ca X ray absorpt ion spectra were measured at beam line 4 3 of the Stanford Synchrotron Light source operating at an electron energy of 3.0 GeV with an average current of 200 mA. The radiation was monochromatized by a Si(111) double crystal monochromator. Intensity of the incident X rays was monitored by a He filled ion chamber (I 0 ) in front of the sample. XAS samples were made by spreading a very thin layer of finely ground powdered compound on a Ca free Mylar tape covered with a 6 m thick polypropylene film. The polyprop ylene side was directed towards the X ray beam to reduce attenuation of the X rays. Samples were placed inside a helium gas filled bag and cooled with a liquid helium cryostream (Oxford) to ~40 K. Fluorescence spectra were recorded by using a four element Vortex detector. Energy was calibrated by the edge peak of calcium acetate (4050 eV). Each XANES and EXAFS scan required 17 and 30 minutes to complete, respectively. An X ray radiation damage study was conducted on each sample prior to data collection by m onitoring the Mn K edge before and after collection of data at the Ca edge. It was determined that it was safe to collect three scans per spot for XANES and two scans per spot for EXAFS with a incoming beam size of 10 (H) x 2 (V) mm.

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251 Mn X ray absorption sp ectra were collected at the SSRL beam line 7 3 at an electron energy of 3.0 GeV with an average current of 200 mA. The radiation was monochromatized by a Si(220) double crystal monochromator. The intensity of the incident X ray was monitored by an N 2 fille d ion chamber (I 0 ) in front of the sample. XAS samples were made by carefully grinding 5 10 mg of compound and diluting it with a 10 fold excess of boron nitride. The mixture was packed into 0.5 mm thick sample holders and sealed with Mylar tape. The data were collected as fluorescence excitation spectra with a PIPS detector. Energy was calibrated by the pre edge peak of KMnO 4 (6543.30 eV). The standard was placed between two N 2 filled ionization chambers (I 1 and I 2 ) after the sample. The X ray flux at 6.6 keV was between 2 and 5*10 9 photons s 1 mm 2 of the sample. Samples were kept at a temperature of 10 K in a liquid helium flow cryostat to minimize radiation damage. Data reduction of the EXAFS spectra was performed using EXAFSPAK (Drs. Graham George and I ngrid Pickering, SSRL). Pre edge and post edge backgrounds were subtracted from the XAS spectra, and the results were normalized with respect to edge height. Background removal in k space was achieved through a five domain cubic spline. Curve fitting was p erformed with Artemis and IFEFFIT software using ab initio calculated phases and amplitudes from the program FEFF 8.2. These ab initio phases and amplitudes were used in the EXAFS equation: The neighboring atoms to the central atom(s ) are divided into j shells, with all atoms with the same atomic number and distance from the central atom grouped into a single shell. Within each shell, the coordination number N j denotes the number of

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252 neighboring atoms in shell j at a distance of R j fro m the central atom. is the ab initio amplitude function for shell j and the Debye Waller term e accounts for damping due to static and thermal disorder in absorber backscatterer distances. The mean free path term e 2R j j ( k) reflects losses due to inelastic scattering, where j ( k ) is the electron mean free path. The oscillations in the EXAFS spectrum are reflected in the sinusoidal term, sin(2 kR j + ij ( k )) where ij ( k ) is the ab initio phase function for shell j S 0 2 is an amplitude reduction factor due to shake up/shake off processes at the central atom(s). The EXAFS equation was used to fit the experimental data using N R and the EXAFS Debye Waller factor ( 2 ) as variable parameters.

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253 APPENDIX D VAN VLECK EQUATIONS c = N B 2 /3k g = Land factor k = Boltzmann constant T = Temperature TIP = Temperature independent paramagnetism D 1. [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (HO 2 CBu t ) 4 ] ( 3 1 ) m = J/k/T n = J /k/T Numerator = + 15.0000 *exp( 3.7500 *m+ 0.0000 *n) + 1.5000 *exp( 1.2500 *m+ 2.0000 *n) + 15.0000 *exp( 1.7500 *m+ 2.0000 *n) + 52.5000 *exp( 6.7500 *m+ 2.0000 *n) + 1.5000 *exp( 5.2500 *m+ 6.0000 *n) + 15.0000 *exp( 2.2500 *m+ 6.0000 *n) + 52. 5000 *exp( 2.7500 *m+ 6.0000 *n) + 126.0000 *exp( 9.7500 *m+ 6.0000 *n) + 15.0000 *exp( 8.2500 *m+ 12.0000 *n) + 52.5000 *exp( 3.2500 *m+ 12.0000 *n) + 126.0000 *exp( 3.7500 *m+ 12.0000 *n) + 247.5000 *exp( 12.7500 *m+ 12.0000 *n) Denominator = + 4.000 0 *exp( 3.7500 *m+ 0.0000 *n) + 2.0000 *exp( 1.2500 *m+ 2.0000 *n) + 4.0000 *exp( 1.7500 *m+ 2.0000 *n) + 6.0000 *exp( 6.7500 *m+ 2.0000 *n) + 2.0000 *exp( 5.2500 *m+ 6.0000 *n) + 4.0000 *exp( 2.2500 *m+ 6.0000 *n) + 6.0000 *exp( 2.7500 *m+ 6.0000 *n) + 8.0000 *exp( 9.7500 *m+ 6.0000 *n) + 4.0000 *exp( 8.2500 *m+ 12.0000 *n) + 6.0000 *exp( 3.2500 *m+ 12.0000 *n) + 8.0000 *exp( 3.7500 *m+ 12.0000 *n) + 10.0000 *exp( 12.7500 *m+ 12.0000 *n)

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254 D 2. [Mn 3 Ca 2 O 4 (O 2 CBu t ) 8 (bpy)(HO 2 CBu t )(MeCN)] ( 3 2 ) m = J/k/T n = J /k/T Numerator = +15.0000*exp(3.7500*m +0.0000*n) +1.5000*exp( 1.2500*m +2.0000*n) +15.0000*exp(1.7500*m +2.0000*n) +52.5000*exp(6.7500*m +2.0000*n) +1.5000*exp( 5.2 500*m +6.0000*n) +15.0000*exp( 2.2500*m +6.0000*n) +52.5000*exp(2.7500*m +6.0000*n) +126.0000*exp(9.7500*m +6.0000*n) +15.0000*exp( 8.2500*m +12.0000*n) +52.5000*exp( 3.2500*m +12.0000*n) +126.0000*exp(3.7500*m +12.0000*n) +247.5000*exp(12.7500*m +1 2.0000*n) Denominator = +4.0000*exp(3.7500*m +0.0000*n) +2.0000*exp( 1.2500*m +2.0000*n) +4.0000*exp(1.7500*m +2.0000*n) +6.0000*exp(6.7500*m +2.0000*n) +2.0000*exp( 5.2500*m +6.0000*n) +4.0000*exp( 2.2500*m +6.0000*n) +6.0000*exp(2.7500*m +6.0000*n) +8.0000*exp(9.7500*m +6.0000*n) +4.0000*exp( 8.2500*m +12.0000*n) +6.0000*exp( 3.2500*m +12.0000*n) +8.0000*exp(3.7500*m +12.0000*n) +10.0000*exp(12.7500*m +12.0000*n) D 3. [Mn 4 Ca 2 O 6 ( O 2 CBu t ) 6 (phen) 4 ]( O 2 CBu t ) ( 4 2 )

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255 l = 0 m = J wb /k/T n = J bb /k/T Numerator = +1.5000*exp(0.7500*l+ 0.0000*m+ 0.0000*n) +15.0000*exp(3.7500*l+ 0.0000*m+ 0.0000*n) +52.5000*exp(8.7500*l+ 0.0000*m+ 0.0000*n) +252.0000*exp(15.7500*l+ 0.0000*m+ 0.0000*n) +1.500 0*exp(2.7500*l+ 2.0000*m+ 2.0000*n) +15.0000*exp(2.7500*l+ 1.0000*m+ 2.0000*n) +1.5000*exp(5.7500*l+ 5.0000*m+ 2.0000*n) +15.0000*exp(5.7500*l+ 2.0000*m+ 2.0000*n) +52.5000*exp(5.7500*l+ 3.0000*m+ 2.0000*n) +15.0000*exp(10.7500*l+ 7.0000*m+ 2.0000*n) + 52.5000*exp(10.7500*l+ 2.0000*m+ 2.0000*n) +126.0000*exp(10.7500*l+ 5.0000*m+ 2.0000*n) +52.5000*exp(17.7500*l+ 9.0000*m+ 2.0000*n) +126.0000*exp(17.7500*l+ 2.0000*m+ 2.0000*n) +247.5000*exp(17.7500*l+ 7.0000*m+ 2.0000*n) +15.0000*exp(6.7500*l+ 3.0000* m+ 6.0000*n) +52.5000*exp(6.7500*l+ 2.0000*m+ 6.0000*n) +1.5000*exp(9.7500*l+ 9.0000*m+ 6.0000*n) +15.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +52.5000*exp(9.7500*l+ 1.0000*m+ 6.0000*n) +126.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +1.5000*exp(14.7500*l+ 1 4.0000*m+ 6.0000*n) +15.0000*exp(14.7500*l+ 11.0000*m+ 6.0000*n) +52.5000*exp(14.7500*l+ 6.0000*m+ 6.0000*n) +126.0000*exp(14.7500*l+ 1.0000*m+ 6.0000*n) +247.5000*exp(14.7500*l+ 10.0000*m+ 6.0000*n) +15.0000*exp(21.7500*l+ 18.0000*m+ 6.0000*n) +52.5000 *exp(21.7500*l+ 13.0000*m+ 6.0000*n) +126.0000*exp(21.7500*l+ 6.0000*m+ 6.0000*n) +247.5000*exp(21.7500*l+ 3.0000*m+ 6.0000*n) +429.0000*exp(21.7500*l+ 14.0000*m+ 6.0000*n) +52.5000*exp(12.7500*l+ 4.0000*m+ 12.0000*n) +126.0000*exp(12.7500*l+ 3.0000*m+ 12.0000*n) +15.0000*exp(15.7500*l+ 12.0000*m+ 12.0000*n) +52.5000*exp(15.7500*l+ 7.0000*m+ 12.0000*n) +247.5000*exp(15.7500*l+ 9.0000*m+ 12.0000*n) +1.5000*exp(20.7500*l+ 20.0000*m+ 12.0000*n) +15.0000*exp(20.7500*l+ 17.0000*m+ 12.0000*n) +52.5000*exp( 20.7500*l+ 12.0000*m+ 12.0000*n) +126.0000*exp(20.7500*l+ 5.0000*m+ 12.0000*n) +247.5000*exp(20.7500*l+ 4.0000*m+ 12.0000*n) +429.0000*exp(20.7500*l+ 15.0000*m+ 12.0000*n) +1.5000*exp(27.7500*l+ 27.0000*m+ 12.0000*n) +15.0000*exp(27.7500*l+ 24.0000*m+ 12.0000*n) +52.5000*exp(27.7500*l+ 19.0000*m+ 12.0000*n) +126.0000*exp(27.7500*l+ 12.0000*m+ 12.0000*n) +247.5000*exp(27.7500*l+ 3.0000*m+ 12.0000*n) +429.0000*exp(27.7500*l+ 8.0000*m+ 12.0000*n) +682.5000*exp(27.7500*l+ 21.0000*m+ 12.0000*n) Denominat or = +2.0000*exp(0.7500*l+ 0.0000*m+ 0.0000*n) +4.0000*exp(3.7500*l+ 0.0000*m+ 0.0000*n) +6.0000*exp(8.7500*l+ 0.0000*m+ 0.0000*n) +16.0000*exp(15.7500*l+ 0.0000*m+ 0.0000*n) +2.0000*exp(2.7500*l+ 2.0000*m+ 2.0000*n) +4.0000*exp(2.7500*l+ 1.0000*m+ 2.000 0*n) +2.0000*exp(5.7500*l+ 5.0000*m+ 2.0000*n) +4.0000*exp(5.7500*l+ 2.0000*m+ 2.0000*n) +6.0000*exp(5.7500*l+ 3.0000*m+ 2.0000*n) +4.0000*exp(10.7500*l+ 7.0000*m+ 2.0000*n) +6.0000*exp(10.7500*l+ 2.0000*m+ 2.0000*n) +8.0000*exp(10.7500*l+ 5.0000*m+ 2. 0000*n) +6.0000*exp(17.7500*l+ 9.0000*m+ 2.0000*n)+8.0000*exp(17.7500*l+

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256 2.0000*m+ 2.0000*n) +10.0000*exp(17.7500*l+ 7.0000*m+ 2.0000*n) +4.0000*exp(6.7500*l+ 3.0000*m+ 6.0000*n) +6.0000*exp(6.7500*l+ 2.0000*m+ 6.0000*n) +2.0000*exp(9.7500*l+ 9.0000*m+ 6.0000*n) +4.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +6.0000*exp(9.7500*l+ 1.0000*m+ 6.0000*n) +8.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +2.0000*exp(14.7500*l+ 14.0000*m+ 6.0000*n) +4.0000*exp(14.7500*l+ 11.0000*m+ 6.0000*n) +6.0000*exp(14.7500*l+ 6.0 000*m+ 6.0000*n) +8.0000*exp(14.7500*l+ 1.0000*m+ 6.0000*n) +10.0000*exp(14.7500*l+ 10.0000*m+ 6.0000*n) +4.0000*exp(21.7500*l+ 18.0000*m+ 6.0000*n) +6.0000*exp(21.7500*l+ 13.0000*m+ 6.0000*n) +8.0000*exp(21.7500*l+ 6.0000*m+ 6.0000*n) +10.0000*exp(21.7 500*l+ 3.0000*m+ 6.0000*n) +12.0000*exp(21.7500*l+ 14.0000*m+ 6.0000*n) +6.0000*exp(12.7500*l+ 4.0000*m+ 12.0000*n) +8.0000*exp(12.7500*l+ 3.0000*m+ 12.0000*n) +4.0000*exp(15.7500*l+ 12.0000*m+ 12.0000*n) +6.0000*exp(15.7500*l+ 7.0000*m+ 12.0000*n) +10. 0000*exp(15.7500*l+ 9.0000*m+ 12.0000*n) +2.0000*exp(20.7500*l+ 20.0000*m+ 12.0000*n) +4.0000*exp(20.7500*l+ 17.0000*m+ 12.0000*n) +6.0000*exp(20.7500*l+ 12.0000*m+ 12.0000*n) +8.0000*exp(20.7500*l+ 5.0000*m+ 12.0000*n) +10.0000*exp(20.7500*l+ 4.0000*m + 12.0000*n) +12.0000*exp(20.7500*l+ 15.0000*m+ 12.0000*n) +2.0000*exp(27.7500*l+ 27.0000*m+ 12.0000*n) +4.0000*exp(27.7500*l+ 24.0000*m+ 12.0000*n) +6.0000*exp(27.7500*l+ 19.0000*m+ 12.0000*n) +8.0000*exp(27.7500*l+ 12.0000*m+ 12.0000*n) +10.0000*exp( 27.7500*l+ 3.0000*m+ 12.0000*n) +12.0000*exp(27.7500*l+ 8.0000*m+ 12.0000*n) +14.0000*exp(27.7500*l+ 21.0000*m+ 12.0000*n) D 4. [Mn 4 Sr 2 O 6 (O 2 CBu t ) 6 (phen) 4 ](O 2 CBu t ) ( 5 1 ) l = 0 m = J w b /k/T n = J bb /k/T Numerator = +1.5000*exp(0.7500*l+ 0.0000*m+ 0.0000*n) +15.0000*exp(3.7500*l+ 0.0000*m+ 0.0000*n) +52.5000*exp(8.7500*l+ 0.0000*m+ 0.0000*n) +252.0000*exp(15.7500*l+ 0.0000*m+ 0.0000*n) +1.5000*exp(2.7500*l+ 2.0000*m+ 2.0000*n) +15.0000 *exp(2.7500*l+ 1.0000*m+ 2.0000*n) +1.5000*exp(5.7500*l+ 5.0000*m+ 2.0000*n) +15.0000*exp(5.7500*l+ 2.0000*m+ 2.0000*n) +52.5000*exp(5.7500*l+ 3.0000*m+ 2.0000*n) +15.0000*exp(10.7500*l+ 7.0000*m+ 2.0000*n)

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257 +52.5000*exp(10.7500*l+ 2.0000*m+ 2.0000*n) + 126.0000*exp(10.7500*l+ 5.0000*m+ 2.0000*n) +52.5000*exp(17.7500*l+ 9.0000*m+ 2.0000*n) +126.0000*exp(17.7500*l+ 2.0000*m+ 2.0000*n) +247.5000*exp(17.7500*l+ 7.0000*m+ 2.0000*n) +15.0000*exp(6.7500*l+ 3.0000*m+ 6.0000*n) +52.5000*exp(6.7500*l+ 2.0000*m+ 6.0000*n) +1.5000*exp(9.7500*l+ 9.0000*m+ 6.0000*n) +15.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +52.5000*exp(9.7500*l+ 1.0000*m+ 6.0000*n) +126.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +1.5000*exp(14.7500*l+ 14.0000*m+ 6.0000*n) +15.0000*exp(14.7500*l+ 11.0000*m+ 6.0000*n) +52.5000*exp(14.7500*l+ 6.0000*m+ 6.0000*n) +126.0000*exp(14.7500*l+ 1.0000*m+ 6.0000*n) +247.5000*exp(14.7500*l+ 10.0000*m+ 6.0000*n) +15.0000*exp(21.7500*l+ 18.0000*m+ 6.0000*n) +52.5000*exp(21.7500*l+ 13.0000*m+ 6.0000*n) +126.00 00*exp(21.7500*l+ 6.0000*m+ 6.0000*n) +247.5000*exp(21.7500*l+ 3.0000*m+ 6.0000*n) +429.0000*exp(21.7500*l+ 14.0000*m+ 6.0000*n) +52.5000*exp(12.7500*l+ 4.0000*m+ 12.0000*n) +126.0000*exp(12.7500*l+ 3.0000*m+ 12.0000*n) +15.0000*exp(15.7500*l+ 12.0000*m + 12.0000*n) +52.5000*exp(15.7500*l+ 7.0000*m+ 12.0000*n) +247.5000*exp(15.7500*l+ 9.0000*m+ 12.0000*n) +1.5000*exp(20.7500*l+ 20.0000*m+ 12.0000*n) +15.0000*exp(20.7500*l+ 17.0000*m+ 12.0000*n) +52.5000*exp(20.7500*l+ 12.0000*m+ 12.0000*n) +126.0000*e xp(20.7500*l+ 5.0000*m+ 12.0000*n) +247.5000*exp(20.7500*l+ 4.0000*m+ 12.0000*n) +429.0000*exp(20.7500*l+ 15.0000*m+ 12.0000*n) +1.5000*exp(27.7500*l+ 27.0000*m+ 12.0000*n) +15.0000*exp(27.7500*l+ 24.0000*m+ 12.0000*n) +52.5000*exp(27.7500*l+ 19.0000*m + 12.0000*n) +126.0000*exp(27.7500*l+ 12.0000*m+ 12.0000*n) +247.5000*exp(27.7500*l+ 3.0000*m+ 12.0000*n) +429.0000*exp(27.7500*l+ 8.0000*m+ 12.0000*n) +682.5000*exp(27.7500*l+ 21.0000*m+ 12.0000*n) Denominator = +2.0000*exp(0.7500*l+ 0.0000*m+ 0.0000* n) +4.0000*exp(3.7500*l+ 0.0000*m+ 0.0000*n) +6.0000*exp(8.7500*l+ 0.0000*m+ 0.0000*n) +16.0000*exp(15.7500*l+ 0.0000*m+ 0.0000*n) +2.0000*exp(2.7500*l+ 2.0000*m+ 2.0000*n) +4.0000*exp(2.7500*l+ 1.0000*m+ 2.0000*n) +2.0000*exp(5.7500*l+ 5.0000*m+ 2.0000* n) +4.0000*exp(5.7500*l+ 2.0000*m+ 2.0000*n) +6.0000*exp(5.7500*l+ 3.0000*m+ 2.0000*n) +4.0000*exp(10.7500*l+ 7.0000*m+ 2.0000*n) +6.0000*exp(10.7500*l+ 2.0000*m+ 2.0000*n) +8.0000*exp(10.7500*l+ 5.0000*m+ 2.0000*n) +6.0000*exp(17.7500*l+ 9.0000*m+ 2.0 000*n)+8.0000*exp(17.7500*l+ 2.0000*m+ 2.0000*n) +10.0000*exp(17.7500*l+ 7.0000*m+ 2.0000*n) +4.0000*exp(6.7500*l+ 3.0000*m+ 6.0000*n) +6.0000*exp(6.7500*l+ 2.0000*m+ 6.0000*n) +2.0000*exp(9.7500*l+ 9.0000*m+ 6.0000*n) +4.0000*exp(9.7500*l+ 6.0000*m+ 6 .0000*n) +6.0000*exp(9.7500*l+ 1.0000*m+ 6.0000*n) +8.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +2.0000*exp(14.7500*l+ 14.0000*m+ 6.0000*n) +4.0000*exp(14.7500*l+ 11.0000*m+ 6.0000*n) +6.0000*exp(14.7500*l+ 6.0000*m+ 6.0000*n) +8.0000*exp(14.7500*l+ 1.000 0*m+ 6.0000*n) +10.0000*exp(14.7500*l+ 10.0000*m+ 6.0000*n) +4.0000*exp(21.7500*l+ 18.0000*m+ 6.0000*n) +6.0000*exp(21.7500*l+ 13.0000*m+ 6.0000*n) +8.0000*exp(21.7500*l+ 6.0000*m+ 6.0000*n) +10.0000*exp(21.7500*l+ 3.0000*m+ 6.0000*n) +12.0000*exp(21.75 00*l+ 14.0000*m+ 6.0000*n) +6.0000*exp(12.7500*l+ 4.0000*m+

PAGE 258

258 12.0000*n) +8.0000*exp(12.7500*l+ 3.0000*m+ 12.0000*n) +4.0000*exp(15.7500*l+ 12.0000*m+ 12.0000*n) +6.0000*exp(15.7500*l+ 7.0000*m+ 12.0000*n) +10.0000*exp(15.7500*l+ 9.0000*m+ 12.0000*n) +2.0 000*exp(20.7500*l+ 20.0000*m+ 12.0000*n) +4.0000*exp(20.7500*l+ 17.0000*m+ 12.0000*n) +6.0000*exp(20.7500*l+ 12.0000*m+ 12.0000*n) +8.0000*exp(20.7500*l+ 5.0000*m+ 12.0000*n) +10.0000*exp(20.7500*l+ 4.0000*m+ 12.0000*n) +12.0000*exp(20.7500*l+ 15.0000* m+ 12.0000*n) +2.0000*exp(27.7500*l+ 27.0000*m+ 12.0000*n) +4.0000*exp(27.7500*l+ 24.0000*m+ 12.0000*n) +6.0000*exp(27.7500*l+ 19.0000*m+ 12.0000*n) +8.0000*exp(27.7500*l+ 12.0000*m+ 12.0000*n) +10.0000*exp(27.7500*l+ 3.0000*m+ 12.0000*n) +12.0000*exp (27.7500*l+ 8.0000*m+ 12.0000*n) +14.0000*exp(27.7500*l+ 21.0000*m+ 12.0000*n) D 5. [Mn 4 Ca 2 O 6 (O 2 CBu t ) 6 (bpy) 2 (Et 2 O) 2 ][Ca 6 (O 2 CBu t ) 12 Cl] ( 6 2 ) l = 0 m = J wb /k/T n = J bb /k/T Numerator = +252.0000*exp(15.7500*l+ 0.0000*m+ 0.0000*n) +52.5000*exp(17.7500*l+ 9.0000*m+ 2.0000*n) +126.0000*exp(17.7500*l+ 2.0000*m+ 2.0000*n) +247.50 00*exp(17.7500*l+ 7.0000*m+ 2.0000*n) +15.0000*exp(21.7500*l+ 18.0000*m+ 6.0000*n) +52.5000*exp(21.7500*l+ 13.0000*m+ 6.0000*n) +126.0000*exp(21.7500*l+ 6.0000*m+ 6.0000*n) +247.5000*exp(21.7500*l+ 3.0000*m+ 6.0000*n) +429.0000*exp(21.7500*l+ 14.0000*m+ 6.0000*n) +1.5000*exp(27.7500*l+ 27.0000*m+ 12.0000*n) +15.0000*exp(27.7500*l+ 24.0000*m+ 12.0000*n) +52.5000*exp(27.7500*l+ 19.0000*m+ 12.0000*n) +126.0000*exp(27.7500*l+ 12.0000*m+ 12.0000*n) +247.5000*exp(27.7500*l+ 3.0000*m+ 12.0000*n) +429.0000* exp(27.7500*l+ 8.0000*m+ 12.0000*n) +682.5000*exp(27.7500*l+ 21.0000*m+ 12.0000*n) +52.5000*exp(8.7500*l+ 0.0000*m+ 0.0000*n) +15.0000*exp(10.7500*l+ 7.0000*m+ 2.0000*n) +52.5000*exp(10.7500*l+ 2.0000*m+ 2.0000*n) +126.0000*exp(10.7500*l+ 5.0000*m+ 2.000 0*n) +1.5000*exp(14.7500*l+ 14.0000*m+ 6.0000*n) +15.0000*exp(14.7500*l+ 11.0000*m+ 6.0000*n) +52.5000*exp(14.7500*l+ 6.0000*m+ 6.0000*n) +126.0000*exp(14.7500*l+ 1.0000*m+ 6.0000*n) +247.5000*exp(14.7500*l+ 10.0000*m+ 6.0000*n) +1.5000*exp(20.7500*l+ 20.0000*m+ 12.0000*n) +15.0000*exp(20.7500*l+ 17.0000*m+ 12.0000*n)

PAGE 259

259 +52.5000*exp(20.7500*l+ 12.0000*m+ 12.0000*n) +126.0000*exp(20.7500*l+ 5.0000*m+ 12.0000*n) +247.5000*exp(20.7500*l+ 4.0000*m+ 12.0000*n) +429.0000*exp(20.7500*l+ 15.0000*m+ 12.0000*n) +15.0000*exp(3.7500*l+ 0.0000*m+ 0.0000*n) +1.5000*exp(5.7500*l+ 5.0000*m+ 2.0000*n) +15.0000*exp(5.7500*l+ 2.0000*m+ 2.0000*n) +52.5000*exp(5.7500*l+ 3.0000*m+ 2.0000*n) +1.5000*exp(9.7500*l+ 9.0000*m+ 6.0000*n) +15.0000*exp(9.7500*l+ 6.0000*m+ 6.0000 *n) +52.5000*exp(9.7500*l+ 1.0000*m+ 6.0000*n) +126.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +15.0000*exp(15.7500*l+ 12.0000*m+ 12.0000*n) +52.5000*exp(15.7500*l+ 7.0000*m+ 12.0000*n) +247.5000*exp(15.7500*l+ 9.0000*m+ 12.0000*n) +1.5000*exp(0.7500*l+ 0.0 000*m+ 0.0000*n) +1.5000*exp(2.7500*l+ 2.0000*m+ 2.0000*n) +15.0000*exp(2.7500*l+ 1.0000*m+ 2.0000*n) +15.0000*exp(6.7500*l+ 3.0000*m+ 6.0000*n) +52.5000*exp(6.7500*l+ 2.0000*m+ 6.0000*n) +52.5000*exp(12.7500*l+ 4.0000*m+ 12.0000*n) +126.0000*exp(12.750 0*l+ 3.0000*m+ 12.0000*n) Denominator = +16.0000*exp(15.7500*l+ 0.0000*m+ 0.0000*n) +6.0000*exp(17.7500*l+ 9.0000*m+ 2.0000*n) +8.0000*exp(17.7500*l+ 2.0000*m+ 2.0000*n) +10.0000*exp(17.7500*l+ 7.0000*m+ 2.0000*n) +4.0000*exp(21.7500*l+ 18.0000*m+ 6.0 000*n) +6.0000*exp(21.7500*l+ 13.0000*m+ 6.0000*n) +8.0000*exp(21.7500*l+ 6.0000*m+ 6.0000*n) +10.0000*exp(21.7500*l+ 3.0000*m+ 6.0000*n) +12.0000*exp(21.7500*l+ 14.0000*m+ 6.0000*n) +2.0000*exp(27.7500*l+ 27.0000*m+ 12.0000*n) +4.0000*exp(27.7500*l+ 2 4.0000*m+ 12.0000*n) +6.0000*exp(27.7500*l+ 19.0000*m+ 12.0000*n) +8.0000*exp(27.7500*l+ 12.0000*m+ 12.0000*n) +10.0000*exp(27.7500*l+ 3.0000*m+ 12.0000*n) +12.0000*exp(27.7500*l+ 8.0000*m+ 12.0000*n) +14.0000*exp(27.7500*l+ 21.0000*m+ 12.0000*n) +6.000 0*exp(8.7500*l+ 0.0000*m+ 0.0000*n) +4.0000*exp(10.7500*l+ 7.0000*m+ 2.0000*n) +6.0000*exp(10.7500*l+ 2.0000*m+ 2.0000*n) +8.0000*exp(10.7500*l+ 5.0000*m+ 2.0000*n) +2.0000*exp(14.7500*l+ 14.0000*m+ 6.0000*n) +4.0000*exp(14.7500*l+ 11.0000*m+ 6.0000*n) +6.0000*exp(14.7500*l+ 6.0000*m+ 6.0000*n) +8.0000*exp(14.7500*l+ 1.0000*m+ 6.0000*n) +10.0000*exp(14.7500*l+ 10.0000*m+ 6.0000*n) +2.0000*exp(20.7500*l+ 20.0000*m+ 12.0000*n) +4.0000*exp(20.7500*l+ 17.0000*m+ 12.0000*n) +6.0000*exp(20.7500*l+ 12.0000 *m+ 12.0000*n) +8.0000*exp(20.7500*l+ 5.0000*m+ 12.0000*n) +10.0000*exp(20.7500*l+ 4.0000*m+ 12.0000*n) +12.0000*exp(20.7500*l+ 15.0000*m+ 12.0000*n) +4.0000*exp(3.7500*l+ 0.0000*m+ 0.0000*n) +2.0000*exp(5.7500*l+ 5.0000*m+ 2.0000*n) +4.0000*exp(5.7500*l + 2.0000*m+ 2.0000*n) +6.0000*exp(5.7500*l+ 3.0000*m+ 2.0000*n) +2.0000*exp(9.7500*l+ 9.0000*m+ 6.0000*n) +4.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +6.0000*exp(9.7500*l+ 1.0000*m+ 6.0000*n) +8.0000*exp(9.7500*l+ 6.0000*m+ 6.0000*n) +4.0000*exp(15.7500* l+ 12.0000*m+ 12.0000*n) +6.0000*exp(15.7500*l+ 7.0000*m+ 12.0000*n) +10.0000*exp(15.7500*l+ 9.0000*m+ 12.0000*n) +2.0000*exp(0.7500*l+ 0.0000*m+ 0.0000*n) +2.0000*exp(2.7500*l+ 2.0000*m+ 2.0000*n) +4.0000*exp(2.7500*l+ 1.0000*m+ 2.0000*n) +4.0000*exp(6 .7500*l+ 3.0000*m+ 6.0000*n) +6.0000*exp(6.7500*l+ 2.0000*m+ 6.0000*n) +6.0000*exp(12.7500*l+ 4.0000*m+ 12.0000*n) +8.0000*exp(12.7500*l+ 3.0000*m+ 12.0000*n)

PAGE 260

260 LIST OF REFERENCES 1. Lippard, S. J.; Berg, J. M., Principles of Bioinorganic Chemistry Mill Valley: CA, 1994. 2. Williams, R. J. P.; Silva, J. J. R. F. d., The Natural Selection of Chemical Elements Claredon press: Oxford, 1996. 3. Seiler, H. G.; Sigel, H.; Sigel, A., Handbook on Metals in Clinical and Analytical Chemistry Marcel Dekker: New Yo rk, 1994. 4. Seiler, H. G.; Sigel, H.; Sigel, A., Handbook on Toxicity of Inorganic Compounds Marcel Dekker: 1988. 5. Thornton, J. M.; Andreini, C.; Bertini, I.; Cavallaro, G.; Holliday, G. L., J. Biol. Inorg. Chem. 2008 13 1205. 6. Robinson, N. J.; Wal dron, K. J.; Rutherford, J. C.; Ford, D., Nature 2009 460 823. 7. Wiberg, N., Holleman Wiberg: lehrbuch der anorganischen chemie. English version 34 th ed.; Academic Press: San Diego, CA, 2001. 8. Lemmon, R. M., Chem. Rev. 1970 70 95. 9. Oparin, A. I., The Origin of Life on the Earth Academic Press: New York, 1957. 10. Calvin, M., Chemical Evolution Oxford University Press: New York, 1969. 11. Lane, N., Oxygen The molecule that made the world Oxford University press: Oxford, 2003. 12. Hill, R.; Ben dall, F., Nature 1960 187 417. 13. Boardman, N. K.; Anderson, J. M., Nature 1964 203 166. 14. Roger C, P., Trends Biochem. Sci. 1996 21 121. 15. David Alan, W., Trends Plant Sci. 2002 7 183. 16. Borrell, P.; Dixon, D. T., J. Chem. Educ. 1984 61 8 3. 17. Debus, R. J., Biochim. Biophys. Acta 1992 1102 269. 18. Groot, M. L.; Pawlowicz, N. P.; van Wilderen, L. J. G. W.; Breton, J.; van Stokkum, I. H. M.; van Grondelle, R., Proc. Natl. Acad. Sci. USA 2005 102 13087. 19. Holzwarth, A. R.; Mller, M. G.; Reus, M.; Nowaczyk, M.; Sander, J.; Rgner, M., Proc. Natl. Acad. Sci. USA 2006 103 6895.

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282 BIOGRAPHICAL SKETCH Shreya Mukherjee was born in Kolkata, India in 1982. After finishing her high College, Calcutta University in 2001. She graduated with a first class h onors degree in chemistry in 2004. She then successfully qualified in the joint entrance exam of the prestigious Indian Institute of Technology (IIT) India, and took admission at IIT Bombay in August 2004 to pursue Master of Science in chemistry. During th e summer of 2005, she did an internship for 3 months in the Material Science Department of Indian Association for the Cultivation of Science (IACS), Kolkata; under the guidance of Dr. Asim Bhaumik. Her project involved the synthesis of organic inorganic hy brid porous Ramaswamy Murugavel on the preparation metalophosphates and inorganic titanosiloxanes. After receiving her Master of Science degree in May 2004, she got selecte d in the graduate program at the University of Florida and moved to the United States of America in August 2004. She then joined the research group of Dr. George develop ment of new synthetic strategies for isolating metal clusters with relevance to the field of nanoscale magnetic materials and biomimetic synthetic analog ue s of the oxygen evolving complex (OEC) of Photosystem II.