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Understanding Metal-Metal Bonds in Heterobimetallic Complexes and Their Use As Catalysts for Dinitrogen Conversion

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Understanding Metal-Metal Bonds in Heterobimetallic Complexes and Their Use As Catalysts for Dinitrogen Conversion Understanding Metal-Metal Bonds in Heterobimetallic Complexes and Their Use as Catalysts for Dinitrogen Conversion A DISSERTATION SUBMITTED TO THE FACULTY OF THE UNIVERSITY OF MINNESOTA BY Laura J. Clouston IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Connie C. Lu, Advisor July 2016 © Laura J Clouston 2016 Acknowledgments I am grateful to obtain my degree from the Chemistry Department at the University of Minnesota, were I could not be surrounded by better people. I was lucky to be a part of the Lu group, where I could always find someone to talk science or have a good laugh. I would like to thank Connie for welcoming me into the group and providing me with numerous opportunities to grow as a scientist. Thanks to the rest of the Lu group, past and present, you have all taught me something valuable that I will keep with me. Thanks to Deanna for guiding me through my first two years, and to Steve, Reed and Ryan for always keeping my expectations high. Thanks to Dave, the second year crew- Matt, Bianca, JT and Sai, and Azim for keeping the lab to be a fun place to spend the majority of your time. A special thanks to the Gagliardi group for a great collaboration, especially Varinia, who has calculated almost everything I have made or wanted to make, I have truly enjoyed working together. I am also thankful for the groups in the department that I have worked closely with, the Tonks, Tolman, Que, Ellis and Hoye groups, who have provided a wealth of advice, resources and chemicals throughout my graduate work. Thank you to Victor and Greg, who brought me in to the X-ray lab and gave me some great experiences. Thanks to Victor and Yu-Sheng and anyone else that had the pleasure of traveling to APS with me to work five straight shifts. I am also thankful to have been a part of WISE during the last five years, where I was able to make great friends. Thanks to Letitia for being the fearless leader of WISE and a great NMR director. The staff in the chemistry department have made things so easy for me- Ben, Nick, and Nancy to name a few, we are so lucky to have you. Again, thanks to everyone at Minnesota that I had the pleasure to work with. Finally, thanks to my friends and family, some of you I have already thanked, for supporting me over the past five years. I am lucky to be surrounded by amazing people. And thanks to Chris, who joined me right at the start of this adventure, and will continue on with me to the next. i Table of Contents Acknowledgments................................................................................................................ i List of Figures .................................................................................................................... vi List of Tables .................................................................................................................... xii List of Schemes ................................................................................................................ xvi List of Abbreviations ...................................................................................................... xvii Chapter 1 ........................................................................................................................... 1 1.1 Introduction ............................................................................................................... 2 1.2 Modular Synthesis of Metal-Metal Compounds ....................................................... 3 1.3 Metal-Metal Bonding ................................................................................................ 6 1.3.1 Bonding Trends .................................................................................................. 8 1.3.2 Understanding Bonding through Theory ......................................................... 10 1.3.3 Spectroscopic Evidence of Multiple Bonding ................................................. 13 1.4 In Search of Synergistic Redox and Magnetic Properties ...................................... 14 1.5 Small molecule reactivity using multiple metals .................................................... 17 1.6 Literature examples of N-N bond cleavage with multimetallic complexes ............ 21 1.6.1 Reduction of small molecules by trigonal trimetallic clusters ......................... 21 1.6.2 Dinitrogen activation with heterobimetallic compounds ................................. 22 1.6.3 Tuning N2 reduction by Co and Fe by apical ligand selection ........................ 25 1.7 Scope of thesis ........................................................................................................ 27 Chapter 2 ......................................................................................................................... 29 Heterobimetallic Complexes that Bond Vanadium to Iron, Cobalt and Nickel 2.1 Overview of M-V Complexes ................................................................................. 30 2.2 Introduction ............................................................................................................. 31 2.3 Results and Discussion ........................................................................................... 31 ii 2.3.1 Synthesis .......................................................................................................... 31 2.3.2 Electrochemical and Electronic Absorption Characterization ......................... 33 2.3.3 X-ray Crystallography ..................................................................................... 37 2.3.4 NMR Spectroscopy .......................................................................................... 42 2.3.5 Magnetic Susceptibility and EPR Spectroscopy .............................................. 44 2.3.6 Mössbauer Spectroscopy ................................................................................. 47 2.3.7 Electronic Structure Calculations .................................................................... 48 2.4 Conclusion of M-V complexes ............................................................................... 52 2.5 Experimental Section .............................................................................................. 52 Chapter 3 ......................................................................................................................... 60 Dative vs. Covalent Bonding in Trigonal Ti-M Complexes 3.1 Overview ................................................................................................................. 61 3.2 Introduction ............................................................................................................. 61 3.3 Results and Discussion ........................................................................................... 63 3.3.1 Synthesis .......................................................................................................... 63 3.3.2 X-ray Crystallography ..................................................................................... 64 3.3.3 Cyclic Voltammetry ......................................................................................... 68 3.3.4 EPR Spectroscopy ............................................................................................ 71 3.3.5 Mössbauer Spectroscopy ................................................................................. 74 3.3.6 Discussion ........................................................................................................ 77 3.4 Conclusion .............................................................................................................. 78 3.5 Experimental Section .............................................................................................. 79 3.5.1 General Considerations .................................................................................... 79 3.5.2 Physical Methods ............................................................................................. 79 3.5.3 Synthetic Procedures ........................................................................................ 80 iii 3.5.4 X-ray Crystallography ..................................................................................... 82 Chapter 4 ......................................................................................................................... 84 Bimetallic cobalt-dinitrogen complexes: impact of the supporting metal on N2 activation 4.1 Overview ................................................................................................................. 85 4.2 Introduction ............................................................................................................. 86 4.3 Results and Discussion ........................................................................................... 87 4.3.1 Synthesis and characterization of CoTiL (1) ................................................... 87 4.3.2 Electrochemistry of CoML series .................................................................... 90 4.3.3 Synthesis and characterization of dinitrogen adducts 2 and 3 ......................... 91 4.3.4 Theory .............................................................................................................. 94 4.4 Conclusion .............................................................................................................. 97 4.5 Experimental Section .............................................................................................
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