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 H-2 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 [(LNiFII)-Ni-N2S2-F-II Cp(CO)](+) ((LNiFeCp)-Fe-II-Cp-II, with L-N2S2 = 2,2'-(2,2'-bipyridine-6,6'-diy1)bis(1,1'-diphenylethanethiolate) and Cp = cyclopentadienyl) has been fine-tuned by modifying exclusively the Fe site. In [(LNiFeCp)-Ni-N2S2-Fe-II-Cp-II* (CO)](+) ((LNiFeCp)-Fe-II-Cp-II*, with Cp* = pentamethylcyclopentadienyl), the Cp- ligand has been replaced by Cp*- to change both the redox and structural properties of the overall complex as a consequence of the steric hindrance of Cp*-.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 (LNiFeCp)-Fe-II-Cp-II* complex acts as an efficient electrocatalyst to produce H-2. 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 (LNiFeCp)-Fe-II-Cp-II* for H-2 production is markedly inhibited (about 90% of loss), while only a partial inhibition (about 30% of loss) is observed in the case of (LNIFeCp)-I-II-Cp-II. DFT calculations rationalized this effect by predicting that interactions of the one- and two-electron-reduced species for (LNiFeCp)-Fe-II-Cp-II* with CO are thermodynamically more favorable in comparison to those for (LNiFeCp)-Fe-II-Cp-II.