XAFS and DFT characterisation of protonated reduced fe hydrogenase analogues and their implications for electrocatalytic proton reduction

Abstract

The relationship between the structures of reduced, protonated diiron compounds relevant to the diiron subsite of the Fe-hydrogenase H cluster, [2Fe]H, and the rate of electrocatalytic proton reduction is explored by a combination of experimental and computational approaches. Analysis of the X-ray absorption fine structure (XAFS) of the two-electron, twoproton product of [Fe(CO)3]2(μ-PPh2)2 (DP) shows distortions of the primary coordination environment of the Fe centre that result from the trans influence of the terminally bound hydrido ligand. The difference in Fe-C(O) bond lengths for the CO groups cis and trans to the hydrido ligand is similarly predicted by density functional theory, although there is relatively poor agreement between the magnitude of the difference obtained by XAFS and DFT methods. The calculated energies of the cis and trans stereoisomers of DP-H2 with both hydrido ligands normal to the FeP2 plane (axial:axial) are within 0.5 kcalmol-1 of each other and lower, by approximately 4.5 kcalmol-1, than those of the axial:equatorial isomer. Similar trends in the relative energies are found for the hydrido forms of [Fe(CO)3]2[μ-PhP(CH2)3PPh] (3P). The rates of dihydrogen elimination from 3P-H2 and analogues with ethanedithiolate and propanedithiolate bridging ligands (2S-H2 and 3S-H2) have been re-evaluated by using recent DFT calculations of equilibrium constants and reduction potentials. The rate constants 4-8, 2-7 and 1.5-4 s-1 for dihydrogen elimination from the two-electron, two-proton analogues of 2S, 3S and 3P, respectively, for reactions conducted with p-toluenesulfonic acid fall within a narrow range and are consistent with reactions following a similar path but inconsistent with the different transition states proposed for dihydrogen elimination from 2S-H2 and 3S-H2.