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(P₂RN₂R′) ₂]²⁺ electrocatalysts, where P₂RN ₂R′ 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 [HNi ^II(P₂N₂)₂]⁺ - e ^- → [Ni^I(P₂HN₂)(P ₂N₂)]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, 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 H₂ 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 H₂ 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.