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Synthesis of Redox-Active Ligands and Their Use in Hydrogenase Modeling

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Synthesis of Redox-Active Ligands and Their Use in Hydrogenase Modeling SYNTHESIS OF REDOX-ACTIVE LIGANDS AND THEIR USE IN HYDROGENASE MODELING BY JAMES CHRISTOPHER LANSING DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry in the Graduate College of the University of Illinois at Urbana-Champaign, 2015 Urbana, Illinois Doctoral Committee: Professor Thomas B. Rauchfuss, Chair Professor Gregory S. Girolami Professor Andrew A. Gewirth Professor Douglas A. Mitchell Abstract In Nature, hydrogen (H2) is used both as a fuel source and as a facile way to control the pH to biological systems. Hydrogen possesses many characteristics that make it desirable as a clean fuel: H2 can ideally be derived from water (or mildly acidic aqueous conditions), and the combustion of hydrogen with air yields water as a byproduct, rather than carbon based wastes (C, CO, CO2, etc.). Thus the storage of energy in the form of hydrogen is a desirable goal. Nature uses the enzymatic system of hydrogenases (H2ases) to reversibly convert protons and electrons into H2. The two primary H2ases are the [FeFe] and [NiFe]-H2ases, aptly named for the metals present at the active sites. Both H2ases possess numerous unique features, including FeS cluster to relay electrons, CN and CO ligands to maintain low spin metal centers, and thiolate ligands and internal bases near the metal active sites. The inclusion of ferrodoxin clusters in both families of H2ases, as well as the fact that a ferredoxin cluster is present in the H-cluster of [FeFe]-H2ase, suggests that the utilization of redox-active ligands may be able to stimulate novel reactivity in model systems. Chapter 1 of this thesis introduces the concept of redox-active ligands, as well as presenting a brief overview of substitutional changes at the cyclopentadiene rings that lead to changes in electrochemical response of ferrocene (Fc). Chapter 1 also provides an overview of current model systems for H2 generation and oxidation derived from H2ase model systems. In Chapter 2, synthetic considerations are taken into account for the design of redox- active ligands. The beauty of the Fc core lies in both its stability and ease of functionalization. Synthetic routes to ferrocenyl phosphine complexes are presented. Routes allow for variability in ligand synthesis, allowing the Fc core electronics to be altered by both the number of methyl groups and the identity of the phosphine linker atom. Furthermore, exploration of these redox active ligands in metal systems was explored. Both Mo and Ir systems were studied to gauge the effect of metal binding on the ligand redox potential. The effect of stereochemistry and the presence of multiple ligands were also explored. Iridium systems were constructed reminiscent of Vaska’s complex, and it was found that the presence of electron holes on the ligands lead to unique oxidative addition reactions. ii Chapter 3 describes the implementation of highly reducing ferrocenyl phosphine ligands in FeFe models. Addition of ferrocenyl phosphine ligands to [FeFe]-H2ase model systems yielded a complex containing two accessible electrons and an internal base, all of which in turn were able to produce H2 in the presence of excess acid. When additional reductant was present, these [FeFe]-H2ase systems became catalytic. Variations in the catalytic system were implemented and explored. Catalytic behavior was also observed when the redox active ligand was not covalently detached, but no reactivity was observed when the internal base was removed from the system. While catalysis was slow, multiple turnovers were observed, and the system features low overpotentials in catalysis. Chapter 4 describes the synthesis of Fe(dithiolate)(diphosphine)(CO)2 complexes as building blocks to yield bimetallic model systems. When models are constructed with non-rigid disphosphine ligands, the synthesis and isolation of FeFe systems proves challenging. However, 0 the comproportionation reaction of Fe(dithiolate)(diphosphine)(CO)2 with an Fe source proved to be a high yielding synthetic route to unsymmetrical FeFe complexes. The utilization of Fe(dithiolate)(diphosphine)(CO)2 units also led to the synthesis of MnFe and CoFe systems. MnFe and CoFe systems were also studied as possible [FeFe]-H2ase models. Chapter 5 describes work with cobalt systems with respect to hydrogen generation. Implementation of redox active ferrocenyl ligands into cyclopentadienylcobalt complexes was I investigated, attempting to utilize Co platforms for hydride formation and subsequent H2 generation. Chapter 5 also describes the synthesis of cobalt systems designed to mimic [FeFe]- H2ase. The Fe(CO)3 moiety of Fe2(xdt)(CO)6 was replaced with CpCo, and in some cases Cp′Co. These CpCo systems were found to reversibly protonate, allowing for the determination of pKa values. The effect of changing the Cp unit and dithiolate were also explored with respect to pKa and oxidation potentials. Furthermore, a one-electron oxidized system was synthesized and investigated by EPR. iii Acknowledgements I would like to use this potion of my thesis to thank the many influential people in my life that have aided in the obtaining of this thesis. First off, I want to thank Jesus Christ for his salvation, and the daily reminder that I can do all things through His strength (Phillipians 4:13). Additionally, I want to thank my wife, Jennifer, for her support, especially her emotional support, through the arduous process that is graduate school. I can say for a fact that I would not be the person I am today without her love and support. I also want to thank my extended family, including the newest addition of my son Elijah for all of their love. Additionally I want to thank my church family at Stratford Park Bible Chapel for all the different ways they have supported not only me, but also my family. The friendships I have made there I know will last a lifetime, and because of Stratford Park I really desired to stay in Urbana-Champaign for my career. I also want to express my gratitude to my advisor, Tom Rauchfuss, for his advice throughout my graduate studies. The knowledge I have gained through talking with him, as well as exploring problems that he has presented me with, has allowed me to grow as a scientist and scholar, for which I am truly grateful. I know that the synthetic skill I have gained is important, but the mental research skills honed by Tom will also serve me well in my coming career. No lab member is able to function without the rest of the lab, and I would like to thank the Rauchfuss lab members, both current and graduated, for their advice and support. I would like to especially thank Dr. Chris Letko for initially training me and assisting me in getting my feet wet in the lab. I also want to thank Professor James Camara, who I collaborated with quite extensively on, for his advice and support throughout my time here. Lastly, I want to make special recognition of Dr. Maria Carroll, who provided many insights and advice with both chemical systems and research equipment. I also want to thank my many chemistry friends here at the university for their friendship and camaraderie during a difficult portion of my life. I want to make special mention of Dr. Brain Manor, a co-worker in the Rauchfuss lab, who kept me sane with ample opportunities to talk football and current events, rather than solely chemistry. I also want to thank those on the ultimate Frisbee team and those who met up on Sundays who gave me a chance to get some exercise before my knees decided to prevent me from playing. iv Lastly, I would like to thank the excellent support staff we have here in the Chemistry department, especially, the secretaries of the IMP office Connie Knight, Beth Myler, Theresa Struss, and Karen Neuman, all of whom possess the ability to put a smile on your face with either a kind word, a yummy treat, or both! v Table of Contents Terms Commonly Used Within This Thesis………………………………………...…………..vii Chapter 1: Introduction: Overview of Redox-Active Ligands and Hydrogen Production.……….1 Chapter 2: Synthesis of Redox-Active Ferrocenyl Ligands and Applications in Metal Complexes……………………………………………………………………………33 Chapter 3: Applications of Redox-Active Ligands towards Hydrogen Production and Bidirectional [FeFe]-H2ase Models...…………………………………………..……73 Chapter 4: Synthesis and Usage of Monometallic Iron Sources to Synthesize Bimetallic Complexes for Modeling [FeFe]-H2ase…………………….………………………106 Chapter 5: Utilization of Cobalt Complexes for Hydrogen Generation and [FeFe]-H2ase Modeling……………………………………………………………………………125 vi Terms Commonly Used Within This Thesis NH 2- adt azadithiolate, [(SCH2)2(NH)] NBn 2- adt benzylazadithiolate, [(SCH2)2(NCH2Ph)] F - BAr 20 tetrakis(pentafluorophenyl)borate, [B(C6F5)4] F - BAr 4 tetrakis(3,5-bis(trifluoromethyl)phenyl)borate, [B(C6H3(CF3)2)4] - Cp cyclopentadidide, [C5H5] - Cp′ methylcyclopentadide, [C5H4(CH3)] # - Cp tetramethylcyclopentadide, [C5(CH3)4H] - Cp* pentamethylcyclopentadide, [C5(CH3)5] CV cyclic voltammetry Cy cyclohexyl, C6H11 dmg dimethylglyoxime, CH3C(NOH)C(NOH)CH3 dpg diphenylglyoxime, PhC(NOH)C(NOH)Ph dppc 1,1′-bis(diphenylphosphino)cobaltocene dppe 1,2-bis(diphenylphosphino)ethane, cis-PPh2C2H4PPh2 dppf 1,1′-bis(diphenylphosphino)ferrocene dppv cis-1,2-bis(diphenylphosphino)ethene, cis-PPh2C2H2PPh2 2- edt 1,2-ethanedithiolate, [S2C2H4] Et ethyl, C2H5 0/+ 0/+ Fc ferrocene/ferrocenium couple, [(C5H5)2Fe] vii Fc′ 1,1′-dimethylferrocene Fc# 1,1′,2,2′,3,3′,4,4′-octamethylferrocene Fc* decamethylferrocene H2ase Hydrogenase H-Cluster Fe6S6 cluster active site of [FeFe]-H2ase HER Hydrogen Evolution Reaction Hox one-electron oxidized EPR active site of [FeFe]-hydrogenase Hred fully reduced active site of [FeFe]-hydrogenase i Pr iso-propyl, CH(CH3)2 Me methyl, CH3 n Bu normal butyl, -CH2CH2CH2CH3 NHC n-heterocyclic carbene ORR Oxygen Reduction Reaction 2- pdt 1,3-propanedithiolate, [S2C3H6] Ph phenyl , C6H5 t Bu tertiatybutyl, -C(CH3)3 2- tdt 3,4-toluenedithiolate, [S2(C6H3(CH3)] 2- xdt generic dithiolate, [S2(R)] viii Chapter 1: Introduction: Overview of Redox-Active Ligands and Hydrogen Production 1.1 Overview of Redox Active Ligands.
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