Vanadium Substitution Dictates H Atom Uptake at Lindqvist-type Polyoxotungstates

Understanding how modification of molecular structures changes the thermochemistry of H atom uptake can provide design criteria for the formation of highly active catalysts for reductive transformations. Herein, we describe the effect of doping an atomically precise polyoxotungstate with vanadium on proton-coupled electron transfer (PCET) reactivity. The Lindqvist-type polyoxotungstate [W₆O₁₉]^2– displays reversible redox chemistry, which was found to be unchanged in the presence of acid, indicating an inability to couple reduction with protonation. However, the incorporation of a single vanadium center into the structure significantly changes the reactivity, and the potential required for one-electron reduction of [VW₅O₁₉]^3– was shown to vary with the strength of the acid added. Construction of a potential-pK_a diagram allowed assessment of the thermodynamics of H atom uptake, indicating BDFE(O–H) ≈ 64 kcal/mol, while chemical synthesis of the reduced/protonated derivative (TBA)₃[VW₅O₁₉H] was used to probe the position of protonation.

Electrochemical studies show that the potential required for the one-electron reduction of [VW₅O₁₉]³⁻ changes in the presence of acid. Performing the same experiments on [W₆O₁₉]²⁻ reveals that the redox chemistry of this polyoxometalate is acid-independent. This indicates that doping a Lindqvist-type polyoxotungstate with vanadium “switches on” proton-coupled electron transfer (PCET) reactivity. Performing experiments with a range of acids allows extraction of a BDFE(O−H) value of ≈64 kcal/mol⁻¹ for the reduced/protonated compound [VW₅O₁₈(OH)]³⁻.