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

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 [PV_outW₁₁O₄₀]^4- (PV_outW₁₁) modulates the basicity of a bridging μ₂-O^2- 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. The reaction of PV_outW₁₁ with an H-atom donor of weaker bond dissociation free energy results in the successful isolation of singly reduced, singly protonated cluster 1e^-/1H⁺-PV_outW₁₁; kinetic analysis of the reaction of PV_outW₁₁ with hydrazobenzene reveals that H-atom uptake proceeds via a concerted proton-electron transfer mechanism. By contrast, the centrally substituted [V_inW₁₂O₄₀]^3- (V_inW₁₂) 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.