Engineering mechanisms of proton-coupled electron transfer to a titanium-substituted polyoxovanadate–alkoxide

Abstract

Metal oxides are promising catalysts for small molecule hydrogen chemistries, mediated by interfacial proton-coupled electron transfer (PCET) processes. Engineering the mechanism of PCET has been shown to control the selectivity of reduced products, providing an additional route for improving reductive catalysis with metal oxides. In this work, we present kinetic resolution of the rate determining proton-transfer step of PCET to a titanium-doped POV, TiV₅O₆(OCH₃)₁₃ with 9,10-dihydrophenazine by monitoring the loss of the cationic radical intermediate using stopped-flow analysis. For this reductant, a 5-fold enhanced rate (k_PT = 1.2 × 10⁴ M⁻¹ s⁻¹) is accredited to a halved activation barrier in comparison to the homometallic analogue, [V₆O₇(OCH₃)₁₂]¹⁻. By switching to hydrazobenzene as a reductant, a substrate where the electron transfer component of the PCET is thermodynamically unfavorable (ΔG_ET = +11 kcal mol⁻¹), the mechanism is found to be altered to a concerted PCET mechanism. Despite the similar mechanisms and driving forces for TiV₅O₆(OCH₃)₁₃ and [V₆O₇(OCH₃)₁₂]¹⁻, the rate of PCET is accellerated by 3-orders of magnitude (k_PCET = 0.3 M⁻¹ s⁻¹) by the presence of the Ti(IV) ion. Possible origins of the accelleration are considered, including the possibility of strong electronic coupling interactions between TiV₅O₆(OCH₃)₁₃ with hydrazobenzene. Overall, these results offer insight into the governing factors that control the mechanism of PCET in metal oxide systems.