Photochemically initiated oxidative carbon-carbon bond-cleavage reactivity in chlorodiketonate Ni ^II complexes

Three mononuclear Ni ^II complexes containing a 2-chloro-1,3-diketonate ligand and supported by the 6-Ph ₂TPA chelate, as well as analogues that lack the 2-chloro substituent on the β-diketonate ligand, have been prepared and characterized. Upon irradiation at 350 nm under aerobic conditions, complexes containing the 2-chloro-substituted ligands undergo reactions to generate products resulting from oxidative cleavage, α-cleavage, and radical-derived reactions involving the 2-chloro-1,3-diketonate ligand. Mechanistic studies suggest that the oxidative cleavage reactivity, which leads to the production of carboxylic acids, is a result of the formation of superoxide, which occurs through reaction of reduced nickel complexes with O ₂. The presence of the 2-chloro substituent was found to be a prerequisite for oxidative carbon-carbon bond-cleavage reactivity, as complexes lacking this functional group did not undergo these reactions following prolonged irradiation. The approach toward investigating the oxidative reactivity of metal β-diketonate species outlined herein has yielded results of relevance to the proposed mechanistic pathways of metalloenzyme-catalyzed β-diketonate oxidative cleavage reactions.