Theoretical Analysis of the Sequential Proton-Coupled Electron Transfer Mechanisms for H2 Oxidation and Production Pathways Catalyzed by Nickel Molecular Electrocatalysts

The design of electrocatalysts for the oxidation and production of H-2 is important for the development of alternative energy sources. This Article focuses on the [Ni((P2N2R')-N-R)(2)](2+) electrocatalysts, where (P2N2R')-N-R denotes 1,S-diaza-3,7-diphosphacyclooctane ligands with substituent groups R and R' covalently bound to the phosphorus and nitrogen atoms, respectively. Theoretical methods are used to investigate the mechanism of the step in the catalytic cycle corresponding to [HNiII(P2N2)(2)](+) - e(-) --> [Ni-I(P2HN2)(P2N2)](2+) for H-2 oxidation and the reverse reaction for H-2 production. This step involves electron transfer (ET) between the Ni complex and the electrode as well as proton transfer (PT) between the Ni and the N. The sequential mechanisms, PT-ET and ET-PT, are investigated for the following (R,R') substituents: (Me,Me), (Ph,Ph), and (Ph,Bz), where Me, Ph, and Bz denote methyl, phenyl, and benzyl substituents. Density functional theory is used to calculate reduction potentials, pK(a) values, and PT pathways, and the inner- and outer-sphere reorganization energies for electrochemical ET are calculated within the framework of Marcus theory. For the (Ph,Ph) and (Ph,Bz) systems, the sequential PT-ET mechanism for H-2 production would require surmounting a large free energy barrier for the initial PT step, followed by thermodynamically favorable ET. The sequential ET-PT mechanism for these systems would require a moderate initial applied overpotential, followed by a PT reaction with a relatively low free energy barrier. Consistent with experimental data, the calculated overpotential required for the initial reduction in the ET-PT mechanism is lower for the (Ph,Bz) system than for the (Ph,Ph) system for H-2 production. The concerted mechanism, in which the electron and proton transfer simultaneously without a stable intermediate, may be thermodynamically favorable and is a direction of Future research.