Multisite Proton–Coupled Electron Transfer at a Keggin-Type Polyoxotungstate

Proton-coupled electron transfer (PCET) governs many redox transformations, but is thermodynamically constrained when proton and electron transfer occur at a single site. Here, we introduce a new multisite PCET (MS-PCET) platform, based on the Keggin-type polyoxotungstate, [VW₁₂O₄₀]³⁻ (VW12). Pairing VW12 with either Brønsted bases or acids yields reagent pairs with tunable effective bond dissociation free energies (BDFEeff) over 15 kcal mol⁻¹, enabling both oxidative and reductive H atom transfer reactions. Kinetic studies on the oxidative pathway by using 2,4,6-tBu₃PhOH as a model hydrogen atom (H atom) donor reveal a product-like, entropy-dominated concerted proton-electron transfer (CPET) pathway from a preorganized hydrogen-bonded complex. By contrast, reductive H atom transfer reactions exhibit larger ΔH‡ values, measurable kinetic isotope effects, and balanced Brønsted slope, consistent with synchronous CPET-type mechanism. Extension to N-H, O-H, and C-H substrates demonstrates the versatility of the VW12 MS-PCET platform for tunable (de)hydrogenation.