Regioselectivity of concerted proton-electron transfer at the surface of a polyoxovanadate cluster

Proton-coupled electron transfer (PCET) is an important process in the activation and reactivity of metal oxide surfaces. In this work, we study the electronic structure of a reduced polyoxovanadate-alkoxide cluster bearing a single bridging oxide moiety. The structural and electronic implications of the incorporation of bridging oxide sites are revealed, most notably resulting in the quenching of cluster-wide electron delocalization in the most reduced state of the molecule. We correlate this attribute to a change in regioselectivity of PCET to the cluster surface (e.g. reactivity at terminal vs. bridging oxide groups). Reactivity localized at the bridging oxide site enables reversible storage of a single H-atom equivalent, changing the stoichiometry of PCET from a 2e^-/2H⁺ process. Kinetic investigations indicate that the change in site of reactivity translates to an accelerated rate of e^-/H⁺ transfer to the cluster surface. Our work summarizes the role which electronic occupancy and ligand density play in the uptake of e^-/H⁺ pairs at metal oxide surfaces, providing design criteria for functional materials for energy storage and conversion processes.