Unwin, DG; (2012) Investigations into electrocatalytic reduction of protons to hydrogen by complexes inspired by the FeFe hydrogenase enzyme active site. Doctoral thesis , UCL (University College London).

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Abstract

FeFe hydrogenase enzymes efficiently catalyse the reduction of protons to dihydrogen. The active site (H-cluster) of the enzyme is Fe2(μ-SCH2XCH2S)(CO)3(CN)2(H2O)(S(cys)(Fe2S2)) (X = CH2, NH or O). Although the enzyme is highly catalytic and consists of abundant elements, it has several drawbacks; for example, sensitivity to oxygen. Thus it has been proposed that complexes with similar structure to the H-cluster could be strategically designed in order to alleviate these drawbacks, and generate cheap catalysts for hydrogen generation. This dissertation reports on eleven mimics of the H-cluster, each expanding on the simplest model in the literature: Fe2(μ-SCH2CH2CH2S)(CO)6. The aim of the research was to assess the electrocatalytic ability of these complexes, and interpret these results to assist in developing more efficient catalysts in the future. The first two complexes investigated (Fe2(μ-SC6F5)2(CO)6 and Fe2(μ-SC6F5)2(CO)4(Ph2PCH2PPh2)) had a highly electron withdrawing dithiolate bridge to decrease the electron density on the Fe centres. The influence of the bridge was found to have a significant benefit to the overpotential required for catalysis. The next four complexes analysed (Fe2(μ-X)(CO)3(μ,η2-Ph2PCH2CHP(Ph)CH2CH2PPh2); X = SCH2CH2CH2S, SCH2N(CH2C6H5)CH2S, (SCH3)2 or SCH2CH2S) used a triphos ligand to exert steric and electronic influence on the complexes. Although the complexes were found to be catalytic, the overpotential required for catalysis was large. As a sub-investigation, a range of electrolyte solutions were used, and found to have a significant influence on the electrocatalytic behaviour of the complexes. Three tri-iron complexes have been investigated (Fe3(μ-SCH2CH2S)2(CO)7-x(PPh3)x; x = 0, 1, 2). It was found that moving from a di- to a tri-iron system significantly improved the catalytic overpotential. Finally, two isomeric complexes exhibiting a ligand bound to the di-iron centres in both a bridging or chelating orientation (Fe2(μ-SCH2CH2CH2S)(CO)4(Ph2PN(CH2CHCH2)PPh2)) were analysed. The orientation of the ligand played a role in the susceptibility to protonation of the complexes, and therefore their catalytic activities.

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