Controlled Photocatalytic Hydrocarbon Oxidation by Uranyl Complexes

Controlled, photocatalytic C-H bond activations are key reactions in the toolkits of the modern synthetic chemist. While it is known that the uranyl(VI) ion, [(UO2)-O-VI](2+), the environmentally dominant form of uranium, is photoactive, most literature examines its luminescent properties, neglecting its potential synthetic utility for photocatalytic C-H bond cleavage. Here, we synthesise and fully characterise an air-stable and hydrocarbon-soluble uranyl phenanthroline complex, [(UO2)-O-VI(NO3)(2)(Ph(2)phen)], U-Ph2phen, and demonstrate that it can catalytically abstract hydrogen atoms from a variety of organic substrates under visible light irradiation. We show that the commercially available parent complex, uranyl nitrate ([(UO2)-O-VI(NO3)(2)(OH2)(2)]center dot 4H(2)O; U-NO3), is also competent, but from electronic spectroscopy we attribute the higher rates and selectivity of U-Ph2phen to ligand-mediated electronic effects. Ketones are selectively formed over other oxygenated products (alcohols, etc.), and the catalytic oxidation of substrates containing a benzylic C-H position is particularly improved for U-Ph2phen. We also show uranyl-mediated photocatalytic C-C bond cleavage in a model lignin compound for the first time.