Photochemically Initiated Oxidative Carbon-Carbon Bond-Cleavage Reactivity in Chlorodiketonate NiII Complexes

Three mononuclear NiII complexes containing a 2‐chloro‐1,3‐diketonate ligand and supported by the 6‐Ph2TPA 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 O2. 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. Turn on the light: NiII complexes containing a 2‐chloro‐1,3‐diketonate ligand undergo oxidative cleavage to produce carboxylic acids upon irradiation with UV light under aerobic conditions (see scheme). This reactivity, which involves the formation of reduced nickel species that activate dioxygen, has relevance to enzymatic systems that catalyze the oxidative cleavage of β‐diketone substrates.