Generation and Reactivity of Nickel(III)–Oxyl Complex

Transition-metal terminal-oxido species are important reactive-oxygen intermediates involved in many biological and abiological oxidation reactions. In contrast to the large number of transition-metal oxido complexes of Group 5 to 8 elements in the periodic table, terminal oxygen complexes of Group 9 to 11 elements are very rare due to the existence of the “oxo-wall” between Group 8 and 9 elements. Herein, we demonstrate the generation of a nickel(III)-oxyl complex 2Cl using a diphenol supporting ligand via the reaction of the nickel(II) precursor complex 1Cl with m-chloroperbenzoic acid in CH2Cl2 at a low temperature. The terminal oxygen atom of 2Cl was confirmed to originate from m-CPBA using an 18O-labeled derivative. 2Cl exhibited a paramagnetic 1H NMR spectrum, indicating a triplet ground state (S = 1). DFT calculations supported the electronic structure of 2Cl as a nickel(III)-oxyl structure having a triplet ground state. 2Cl also exhibited an intense near-IR absorption band at 1090 nm, which was assigned to a charge transfer from the phenolate moiety of the supporting ligand to the Ni–O moiety based on TD-DFT calculations. Furthermore, 2Cl showed electrophilic reactivity in the sulfoxidation of thioanisole derivatives and induced C–H bond activation via a hydrogen atom abstraction mechanism.