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Title:Synthesis, spectroscopy, and electrochemistry of hydrogenase mimics
Author(s):Ulloa, Olbelina Alexandra
Director of Research:Rauchfuss, Thomas B.
Doctoral Committee Chair(s):Rauchfuss, Thomas B.
Doctoral Committee Member(s):Girolami, Gregory S.; Zimmerman, Steven C.; Weitzel, Alison R
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Hydrogenases (H2ases) provide microorganisms with hydrogen to be used as fuel for processes such a methanogenesis. The reversible hydrogen evolution reaction (HER) is catalyzed by anaerobic bacteria to expel hydrogen as a byproduct of photosynthesis. Hydrogen does not accumulate in our atmosphere because microorganisms consume it quickly in both aerobic and anaerobic environments. Hydrogen is therefore a commodity fuel for humans that spend many quadrillion BTUs of energy yearly. Currently, hydrogen is produced in industry via steam reforming of methane catalyzed by nickel in a non-thermoneutral process. To make hydrogen a suitable commercial fuel, earth-abundant metal catalysts that can mimic nature’s photosynthetic production of hydrogen are necessary. The study of the structure-function relationships of the active sites of H2ases is of great interest to achieve this goal. The H2ases use earth abundant metals (Ni and Fe) to catalyze the HER at fast rates. The study of model complexes allows for better understanding of the mechanism by which H2ases operate because these complexes can be thoroughly investigated crystallographically, spectroscopically, and electrochemically. The NiFe-H2ases are proposed to catalyze hydrogen processing via paramagnetic and diamagnetic Ni(mu-H)Fe intermediates. The synthesis of Ni(mu-H)Fe tetraphosphine complexes that catalyze the HER is discussed here. The biomimetic catalysts are shown to operate via mixed-valence metal hydrides and non-hydride intermediates. Extensive NMR, EPR, IR, GC, and DFT analysis of the structure and reactivity of these Ni(mu-H)Fe is shown to understand the reactivity of these model complexes. Additionally, the involvement of non-hydride mixed-valence complexes in biomimetic HER, suggests that the NiFe-H2ase may involve these states in biological HER catalysis. The FeFe-H2ase has been shown to catalyze the HER via kinetic protonation of an azadithiolate cofactor. Extensive studies of model complexes and hybrid proteins have revealed the importance of the azadithiolate in catalyzing the HER with fast rates. Current synthesis of metal complexes containing azadithiolates gives low yields and requires difficult workups. Here, an improved method for the synthesis of these metal complexes is presented. Additionally, the study of S,N-heterocycles with Fe(0) carbonyls is investigated. These heterocycles are shown to be the decomposition of azadithiols, which elude isolation. This new methodology for incorporation of the azadithiolates is predicted to open a new series of optimizations to further improve the rates of the HER by H2ase model complexes. Lastly, the synthesis of CpFeNi complexes is investigated as a route to higher oxidation mixed-valence CpFe(III)Ni(II) complexes. Piano-stool FeCp(CO) complexes lead to stable mixed-valence CpFe(III)Ni(II). However, these complexes are coordinatively saturated and carbonyl-free FeCp reagents did not yield any CpFeNi complexes. Instead the thermodynamic products were always Ni dimers. These studies show the stabilizing effect of carbonyl ligands when other electron-rich ligands are present in bimetallic complexes. Multiple model complexes describing aspects of the active-sites of H2ases are presented here. Overall these studies demonstrate the importance of model complexes in understanding how H2ases function. Detailed spectroscopic, crystallographic, and electrochemical studies that cannot always be done on the enzymes are presented to better understand how the enzymes catalyze the HER.
Issue Date:2015-04-21
Rights Information:Copyright 2015 Olbelina Ulloa
Date Available in IDEALS:2015-07-22
Date Deposited:May 2015

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