Jones, Graham Joseph
(2018)
Synthetic analogues of the nickel-iron hydrogenase family of metalloenzymes.
PhD thesis, University of Nottingham.
Abstract
This thesis describes the syntheses and characterisations of heterobimetallic dithiolate complexes of NiII, PdII and CoII with FeII that derive inspiration from the chemistry and structures of the active sites of the [NiFe] hydrogenase family of metalloenzymes. These studies provide insight into the electronic structures of the Ni-C and Ni-L states of the [NiFe] hydrogenases and the complexes have potential as new catalysts for the production of dihydrogen.
Chapter One outlines the role of dihydrogen as a potential fuel and replacement for energy sources derived from fossil fuel feedstocks. The Chapter describes the roles offered by synthetic analogues of the active sites of the [NiFe] hydrogenases as routes to meet the demand for H2 production. An introduction to enzymes and the roles of transition metals in biology, together with a description of the structure and function of the [NiFe] hydrogenases, are presented. A proposed catalytic cycle for the [NiFe] hydrogenases and a review of the coordination chemistry relevant to the active site of the [NiFe] hydrogenases is presented. Key questions about the [NiFe] hydrogenases and the aims of the work described in this thesis are summarized.
Chapter Two describes the synthesis and characterisation of a series of bimetallic [NiIIFeII] complexes that incorporate a four coordinate NiII centre ligated by the N,N'-diethyl-3,7-diazanonane-1,9-dithiolate (L1)2- ligand bound to a pseudooctahedral FeII centre to give a Ni-(S2)-Fe bridging arrangement. The coordination sphere of the FeII centre is completed by a range of isocyanide ligands, R-NC (R = tBu, iPr, nBu, cyclohexyl, benzyl, 4-methoxyphenyl, 4-fluorophenyl) that act as surrogates for the π-acceptor CO and CN- found at the active site in the [NiFe] hydrogenases. Electrochemical and EPR studies on these complexes provide insight into the role of the isonitrile R group on the redox chemistry of the [NiIIFeII] centre. Our electrochemical and spectroscopic studies, supported by DFT calculations, show that these [NiFe] complexes possess a formal NiII/IFeII redox couple that is localized around the Ni unit. These complexes are compelling models of the Ni-L state of the [NiFe] hydrogenases and show promise as potential electrocatalysts for proton reduction.
Chapter Three describes the synthesis and characterisation of a pentacoordinate NiII complex in which NiII is bound by the 1,1'-(1,4,7-thiadiazonane)bis(2-methylpropane-2-thiol) (H2L3) ligand. We show that this unit can coordinate to a pseudo-octahedral FeII centre to generate a bridged Ni-(S2)-Fe structures in which the coordination sphere of the FeII centre is completed by four isonitrile ligands. These complexes we show that the heterobinuclear [NiIIFeII] system supports a formal [NiIII/IIFeII] redox couple. X-ray crystallographic, and electrochemical and spectroscopic studies, supported by DFT calculations, provide insights into the nature of the [NiIIIFeII] species that represents analogues of the Ni-C state of the [NiFe] hydrogenases.
Chapter Four describes the synthesis and characterisation of a range of novel four-coordinate [M(N2S2)] (M = Pd, Co, Zn) complexes in which the metal centre is bound by N,N'-diethyl-3,7-diazanonane-1,9-dithiolate (L1)2-. We describe attempts to coordinate these centres to a [FeII(CNtBu)4]2+ fragment to yield the corresponding heterobimetallic [MFe] complexes. We characterize these compounds by X-ray crystallography and investigate their electrochemical properties for direct comparisons with their [NiFe] counterpart described in Chapter Two to provide insight into the effect of substituting NiII for other transition metal centres.
Chapter Five provides a summary of the principal conclusions of the research described in this thesis.
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