Comment on "New insights in the electrocatalytic proton reduction and hydrogen oxidation by bioinspired catalysts: a DFT investigation".

In the title paper, Vetere et al. reported a computational investigation of the mechanism of H(2) oxidation/proton reduction using a model of nickel-based electrocatalysts that incorporates pendant amines in cyclic phosphorus ligands. These catalysts are attracting considerable attention owing to their high turnover rates and relatively low overpotentials. These authors interpreted the results of their calculations as evidence for a symmetric bond cleavage of H(2) leading directly to two protonated amines in concert with a two-electron reduction of the Ni(II) site to form a Ni(0) diproton state. Proton reduction would involve a reverse symmetric bond formation. We report here an analysis that refutes the interpretation by these authors. We give, for the same model system, the structure of a heterolytic cleavage transition state consistent with the presence of the Ni(II) center acting as a Lewis acid and the pendant amines acting as Lewis bases. We present the associated intrinsic reaction coordinate (IRC) pathway connecting the dihydrogen (η(2)-H(2)) adduct and a hydride-proton state. We report also the transition state and associated IRC for the proton rearrangement from a hydride-proton state to a diproton state. Finally, we complete the characterization of the transition state reported by Vetere et al. through a determination of the corresponding IRC. In summary, H(2) oxidation/proton reduction with this class of catalysts involves a heterolytic bond breaking/formation.