Polyoxometalate Electron-Sponge-Mediated Photooxidative Hydroxylation of Arylboronic Acids

Reducing costs while maintaining efficiency in photocatalytic systems remains a major pursuit in the field of solar photocatalysis. Herein, we explore a universal strategy for regulating electronic transfer by employing earth-abundant polyoxometalates (POMs) as electron sponges, dramatically promoting electron shuttling between photosensitizers (PSs) and substrates. This approach enables highly efficient photooxidative hydroxylation of arylboronic acids while significantly reducing the usage of noble-metal PSs. Typically, replacing 90% PS [Ir(bpy)(coumarin6)₂]⁺ (Ir-2) with an earth-abundant Co₇ POM ([{(B-α-PW₉O₃₄)Co₃(OH)(H₂O)₂(O₃PC(O)(C₃H₆NH₃)PO₃)}₂Co]^14-) can efficiently boost photo-oxidation efficiency, achieving complete substrate conversion within 3 h in air, ∼3.2 or 5.6 times higher than the system with 100% or 10% Ir-2, respectively. Notably, this strategy exhibits excellent compatibility with a wide range of substrates and POMs. Systematic kinetic studies and unique "heteropoly blue" characteristic of POMs clearly demonstrated their excellent capabilities for electron accepting, storing, and releasing in these photo-oxidation systems. These properties can significantly promote electron transfer and stabilize reduced PSs, resulting in over five-fold increase in O₂ ^•- generation under weak visible-light irradiation (λ > 420 nm, 50 mW cm^-2) compared to POM-free systems. This work not only opens an avenue for efficient photochemical synthesis but also highlights a promising new direction for POM application.