Evidence for Site-Specific Tunneling in H Atom Uptake at the Surface of Polyoxovanadate-Alkoxides.

Lindqvist-type polyoxovanadate-alkoxides are used as atomically precise, soluble analogs of metal oxides to interrogate site-dependent (e.g., μ2-oxido vs terminal oxido) tunneling in proton-coupled electron transfer (PCET). Variable-temperature kinetic isotope effect (VT-KIE) analysis reveals that H atom uptake at the μ2-oxido site of [TBA]2[V6O8(OCH3)11] proceeds via quantum mechanical tunneling in acetonitrile, evidenced by a temperature-independent KIE, a small isotopic effect on activation energy, and positive Arrhenius prefactor ratio. This behavior is in contrast to PCET at the terminal vanadyl site of [TBA]-[V6O7(OCH3)12], which follows a semiclassical, concerted proton-electron transfer mechanism. Tunneling effects are solvent dependent; switching to dimethylformamide, a more basic solvent, results in a reduction in H-bonding interactions with the acceptor, lowering the observed rate of H atom uptake at the bridging site. This work highlights the role of site-specific basicity on tunneling in metal oxides, where competitive H-bonding with solvent disrupts tunneling.