Tuning Reactivity of Bioinspired [NiFe]-Hydrogenase Models by Ligand Design and Modeling the CO Inhibition Process

Despite the report of several structural and functional models of the [NiFe]-hydrogenases, it is still unclear how the succession of electron and proton transfers during H2 production catalysis are controlled in terms of both sequence (order of the chemical or redox steps) and sites (metal and/or ligand). To address this issue, the structure of the previously described bioinspired [NiFe]-hydrogenase complex [LN2S2NiIIFeIICp­(CO)]+ (LNiIIFeIICp, with LN2S2 = 2,2′-(2,2′-bipyridine-6,6′-diyl)­bis­(1,1′-diphenylethanethiolate) and Cp = cyclopentadienyl) has been fine-tuned by modifying exclusively the Fe site. In [LN2S2NiIIFeIICp*­(CO)]+ (LNiIIFeIICp*, with Cp* = pentamethylcyclopentadienyl), the Cp– ligand has been replaced by Cp*– to change both the redox and structural properties of the overall complex as a consequence of the steric hindrance of Cp*–. The LNiIIFeIICp* complex acts as an efficient electrocatalyst to produce H2. Density functional theory (DFT) calculations support a CEEC cycle, following an initial reduction. The initial protonation leads to the cleavage of one thiolate–iron bond and the next reduction to the generation of a bridging Fe-based hydride moiety. Interestingly, the second protonation step generates a species containing a terminal Ni-based thiol and a bridging hydride. In the presence of CO, the electrocatalytic activity of LNiIIFeIICp* for H2 production is markedly inhibited (about 90% of loss), while only a partial inhibition (about 30% of loss) is observed in the case of LNiIIFeIICp. DFT calculations rationalized this effect by predicting that interactions of the one- and two-electron-reduced species for LNiIIFeIICp* with CO are thermodynamically more favorable in comparison to those for LNiIIFeIICp