Insights into the mechanism of electrocatalytic hydrogen evolution mediated by Fe2(S2C3H6)(CO)6: The simplest functional model of the Fe-hydrogenase active site

The di-iron complex Fe-2(S2C3H6)(CO)(6) (a), one of the simplest functional models of the Fe-hydrogenases active site, is able to electrocatalyze proton reduction. In the present study, the H-2 evolving path catalyzed by a has been characterized using density functional theory. It is showed that, in the early stages of the catalytic cycle, a neutral mu-H adduct is formed; monoelectron reduction and subsequent protonation can give rise to a diprotonated neutral species (a-mu H-SH), which is characterized by a mu-H group, a protonated sulfur atom, and a CO group bridging the two iron centers, in agreement with experimental IR data indicating the formation of a long-lived mu-CO species. H-2 release from a-mu H-SH, and its less stable isomer a-H-2 is kinetically unfavorable, while the corresponding monoanionic compounds (a-mu H-SH- and a-H-2(-)) are more reactive in terms of dihydrogen evolution, in agreement with experimental data. The key species involved in electrocatalysis have structural features different from the hypothetical intermediates recently proposed to be involved in the enzymatic process, an observation that is possibly correlated with the reduced catalytic efficiency of the biomimetic di-iron assembly.