Synthesis, Characterization, and Catalytic Activities of A Nickel(II) Monoamido-Tetradentate Complex: Evidence For Ni^III–Oxo and Ni^IV–Oxo Species

A new mononuclear nickel(II) complex, [Ni^II(dpaq)Cl] (1), containing a tetradentate monoamido ligand, dpaq (dpaq=2-[bis(pyridin-2-ylmethyl)amino]-N-(quinolin-8-yl)acetamide), has been synthesized and characterized by IR spectroscopy, elemental analysis, and UV/Vis spectroscopy. The structure of the nickel complex has been determined by X-ray crystallography. This nonheme Ni^II complex 1 catalyzed the epoxidation reaction of a wide range of olefins with meta-chloroperoxybenzoic acid (m-CPBA) under mild conditions. Olefin epoxidation using this catalytic system has been proposed to involve a new reactive Ni^IV–oxo (4) species, based on the evidence from a PPAA (peroxyphenylacetic acid) probe, Hammett studies, H₂¹⁸O exchange experiments, and ESI mass spectroscopic analysis. Moreover, the nature of solvent significantly influenced partitioning between heterolytic and homolytic O−O bond cleavage of the Ni–acylperoxo intermediate (2). The O−O bond of 2 proceeded predominantly through heterolytic cleavage in a protic solvent, such as CH₃OH. These results suggest that possibly a Ni^IV–oxo species is a common reactive intermediate in protic solvents. The two active oxidants, namely Ni^IV–oxo (3) and Ni^III–oxo (4), which are responsible for stereospecific olefin epoxidation and radical-type oxidations, respectively, operate in aprotic solvents.