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 ([XVW 11 O 39 (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 (Δ G PT and Δ G ET ) thereby dictating the operative PCET mechanism.