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 Fe-4{Me(CH2S)(3)}(2)(CO)(8) (4Fe6S) in the v(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 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 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 (FeFe11)-Fe-0 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 4Fe6S2-, where the rate of dihydrogen elimination from the (FeFe11)-Fe-11 core of 4Fe6S is ca. 500 times faster than that from the (FeFe1)-Fe-1 core of Fe-2(mu-S(CH2)(3)S)(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 ((FeFe11)-Fe-1 and (FeFe11)-Fe-0), the presence of an open site on the outer Fe atom of the (FeFe11)-Fe-0 unit, and the thiolate-bridge to a second one-electron redox unit.