Modeling [Fe-Fe] hydrogenase: 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

IR spectroelectrochemistry of Fe₄{Me(CH₂S) ₃}₂(CO)₈ (4Fe6S) in the ν(CO) region shows that the neutral and anion forms have all their CO 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 ν(CO) spectra are closely reproduced by density-functional theory calculations. The calculated structure of 4Fe6S^2- closely mirrors that of the diiron subsite of the [Fe-Fe] hydrogenase H cluster with a bridging CO group and an open coordination site on the outer Fe atom of pairs of dithiolate-bridged Fe⁰Fe^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 ^IIFe^II core of 4Fe6S is ca. 500 times faster than that from the Fe^IFe^I core of Fe₂(μ-S(CH ₂)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 Fe⁰Fe ^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.