Visible light generation of high-valent metal-oxo intermediates and mechanistic insights into catalytic oxidations.

High-valent metal-oxo complexes play central roles as active oxygen atom transfer (OAT) agents in many enzymatic and synthetic oxidation catalysis. This review focuses on our recent advances in application of photochemical approaches to probe the oxidizing metal-oxo species with different metals and macrocyclic ligands. Under visible light irradiation, a variety of important metal-oxo species including iron-oxo porphyrins, manganese-oxo porphyrin/corroles, ruthenium-oxo porphyrins, and chromium-oxo salens have been successfully generated. Kinetical studies in real time have provided mechanistic insights as to the reactivity and reaction pathways of the metal-oxo intermediates in their oxidation reactions. In photo-induced ligand cleavage reactions, metals in n oxidation state with the oxygen-containing ligands bromate, chlorate, or nitrites were photolyzed. Homolytic cleavage of the O-X bond in the ligand gives (n + 1) oxidation state metal-oxo species, and heterolytic cleavage gives (n + 2) oxidation state metal-oxo species. In photo-disproportionation reactions, reactive M-oxo species can be formed by photolysis of μ-oxo dimeric M complexes with the concomitant formation of M products. Importantly, the oxidation of M products by molecular oxygen (O) to regenerate the μ-oxo dimeric M complexes in photo-disproportionation reactions represents an attractive and green catalytic cycle for the development of photocatalytic aerobic oxidations.