The oxidation of borohydrides by photoexcited [UO2(CO3)3]4−

Abstract

The carbonate ion is an effective quencher of uranyl(VI) luminescence and makes uranyl(VI) tricarbonate barely luminescent and photochemically inactive. We demonstrate here that photoexcited uranyl(VI) tricarbonate, *[U^VIO₂(CO₃)₃]⁴⁻, can however oxidize borohydrides (BH₃X⁻, X = H, CN) to give boric acid and H₂ gas, reducing itself to [U^VO₂(CO₃)₃]⁵⁻. This hypothesis was supported by UV-vis and NMR spectroscopy as well as quantum chemical calculations. The charge transfer states associated with photoreduction processes were modelled by density functional theory calculations. These results suggest that the mechanism of photoreduction of [U^VIO₂(CO₃)₃]⁴⁻ is similar to that of [U^VIO₂(H₂O)₅]²⁺ and that it occurs through a one-photon reduction process.