Location of dopant dictates proton-coupled electron transfer mechanism in vanadium-substituted polyoxotungstates

Abstract

Heterometal doping in polyoxometalates (POMs) is a useful strategy to impart modular reactivity by leveraging control over the physicochemical properties of the resulting materials. The dopant can occupy different position(s) within the POM that may affect the mechanism and/or outcome of a desired reaction. In this work, we illustrate that substituting one tungsten atom with vanadium in [PVoutW11O40]4− (PVoutW11) modulates the basicity of a bridging μ2–O2− ligand, increasing the strength of the O–H bond formed upon addition of the first proton–electron pair to the cluster by > 20 kcal mol−1 over that of its homometallic congener. Reaction of PVoutW11 with an H-atom donor of weaker bond dissociation free energy (BDFE(E–H)) results in the successful isolation of singly-reduced, singly-protonated cluster 1e−/1H+-PVoutW11; kinetic analysis of the reaction of PVoutW11 with hydrazobenzene reveals that the H-atom uptake proceeds via a concerted proton–electron transfer mechanism. By contrast, the centrally-substituted [VinW12O40]3− (VinW12) decouples the proton from electron transfer, leading to differential reactivity of 5,10-hydrophenazine to give the products of electron transfer. These results highlight that the proton-coupled electron transfer reactivity of heterometal-substituted metal oxides critically depends on the physical accessibility of dopants to the hydrogen donor.