PNNL

Abstract

In the title paper, Vetere et al. reported a computational investigation of the mechanism of oxidation of H2 / proton reduction using a model nickel complex for nickel-based electrocatalysts with cyclic phosphorous ligands incorporating pendant amines. These catalysts are attracting considerable attention owing to their high turn-over rates and relatively low overpotentials. These authors interpreted the results of their calculations as evidence for a symmetric bond breaking (forming) of H2 directly to (from) two protonated amines in concert with a 2-electron reduction of the Ni(II) site to form a Ni(0) di-proton state. We show here that this interpretation is erroneous as we report the structure of an heterolytic cleavage transition state consistent with the presence of the Ni(II) center acting as a Lewis acid and of the pendant amines acting as Lewis bases. We determined the associated intrinsic reaction coordinate (IRC) pathway connecting the di-hydrogen (?2-H2) adduct and a hydride-proton state. We also characterize differently the nature of the transition state reported by these authors. H2 oxidation / proton reduction with this class of catalysts is a heterolytic process.