ModelingFe-FeHydrogenase:Evidence for Bridging Carbonyl and Distal Iron Coordination Vacancy in an Electrocatalytically Competent Proton Reduction by an Iron Thiolate Assembly That Operates through Fe(0)-Fe(II)Levels

摘要

IR spectroelectrochemistry of Fe4{Me(CH2S)3}2(CO)8(4Fe6S)in the v(CO)region shows that the neutral and anion forms have all their CG groups terminally bound to the Fe atoms;however,for the dianion there is a switch of the coordination mode of at least one of the CO groups.The available structural and v(CO)spectra are closely reproduced by density-functional theory calculations.The calculated structure of 4Fe6S~(2-)closely mirrors that of the diiron subsite of theFe-Fehydrogenase H cluster with a bridging CO group and an open coordination site on the outer Fe atom of pairs of dithiolate-bridged Fe~0Fe~(II)subunits connected by two bridging thiolates.Geometry optimization based on the all-terminal CO isomer of 4Fe6S~(2-)does not give a stable structure but reveals a second-order saddle point ca.11.53 kcal mol~1 higher in energy than the CO-bridged form.Spectroelectrochemical studies of electrocatalytic proton reduction by 4Fe6S show that slow turnover from the primary reduction process(E_(1/2)'=-0.71 V vs Ag/AgCl)involves rate-limiting protonation of 4Fe6S~-followed by reduction to H:4Fe6S~-.Rapid electrocatalytic proton reduction is obtained at potentials sufficient to access 4Fe6S~(2-),where the rate of dihydrogen elimination from the Fe~(II)Fe~(II)core of 4Fe6S is ca.500 times faster than that from the Fe~IFe~I core of Fe2(mu-S(CH2)3S)(CO)6.The dramatically increased rate of electrocatalysis obtained from 4Fe6S over all previously identified model compounds appears to be related to the features uniquely common between it and the H-cluster,namely,that turnover involves the same formal redox states of the diiron unit(Fe~IFe~(II)and Fe~0Fe~(II)),the presence of an open site on the outer Fe atom of the Fe~0Fe~(II)unit,and,the thiolate-bridge to a second one-electron redox unit.