Pre-association enables visible-light induced proton-coupled electron-transfer in a titanium-functionalized polyoxotungstate.

Proton-coupled electron transfer (PCET) is central to many chemical transformations and catalytic processes. Heterometal-doped polyoxometalates (POMs) offer powerful model systems for bulk metal oxides to explore this fundamental reactivity. In this work we study the mechanism of the selective photochemical oxidation of benzyl alcohol to benzaldehyde facilitated by the titanium-doped Keggin polyoxotungstate [(TiOH)PW11O39]4-{TiOH}. In aqueous solution, reaction of this cluster with benzyl alcohol results in formation of an unusual, visible-light photoactive intermediate, [(TiOBn)PW11O39]4-{TiOBn}, which is capable of undergoing photoinduced PCET, resulting in the oxidation of benzyl alcohol to benzaldehyde, and simultaneous reduction/protonation of the POM. Experimental and theoretical studies are combined to provide comprehensive insights into the underlying mechanisms, to explain the unusual properties of the POM-alkoxide intermediate, and to probe the chemical and photophysical properties of the species involved in the photo-oxidation process. Non-aqueous Pourbaix analysis is used to provide further understanding of the PCET reactivity of the POM. This work provides unique insights into the design of noble metal-free, visible light-photoactive molecular metal oxides for selective organic photooxidation reactions in water.