Thermodynamic Compensation of Surface Hydroxyl Moieties in Reduced Polyoxometalates Translates to Deviations in the Mechanism of Proton–coupled Electron Transfer

The design of H-atom transfer catalysts requires the control of redox potential and proton affinity to direct the mechanism, rate, and selectivity of proton-coupled electron transfer (PCET). In the present study, we demonstrate that a series of reduced and protonated vanadium-substituted Keggin-type polyoxotungstates ([XVW₁₁O₃₉(OH)]^n-, X = Si, n = 5; X = P, n = 4; X = S, n = 3), exhibit invariant bond dissociation free energies of surface O─H bonds (BDFE(O─H)s). Despite uniform driving forces for H-atom uptake, kinetic analysis with a variety of H-atom donors (5,10-dihydrophenazine, hydrazobenzene, and hydroquinones) reveals differences in rates of substrate oxidation, which are ascribed to disparate PCET mechanisms. Together, these findings show that substitution at the central heteroatom tunes the electron- and proton-transfer driving forces (ΔG_PT and ΔG_ET) thereby dictating the operative PCET mechanism.