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 H2 is important for the development of alternative energy sources. This paper focuses on the (Ni(P2RN2R')2)2+ electrocatalysts, where P2RN2R' denotes 1,5-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- → (NiI(P2HN2)(P2N2))2+ for H2 oxidation and the reverse reaction for H2 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, pKa values, and PT pathways, and Marcus theory is used to describe the electrochemical electron transfer, including the effects of solute and solvent reorganization energies. For the (Ph,Ph) and (Ph,Bz) systems, the sequential PT-ET mechanism would require surmounting a large free energy barrier for the initial PT step, followed by thermodynamically favorable or thermoneutral ET. The sequential ET-PT mechanism for these systems would require a relatively large 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. 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.