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

A new mononuclear nickel(II) complex, [Ni super(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 super(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 super(IV)-oxo (4) species, based on the evidence from a PPAA (peroxyphenylacetic acid) probe, Hammett studies, H sub(2) super(18)O 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 CH sub(3)OH. These results suggest that possibly a Ni super(IV)-oxo species is a common reactive intermediate in protic solvents. The two active oxidants, namely Ni super(IV)-oxo (3) and Ni super(III)-oxo (4), which are responsible for stereospecific olefin epoxidation and radical-type oxidations, respectively, operate in aprotic solvents. Ni super(IV)-oxo versus Ni super(III)-oxo: A new mononuclear nickel(II) complex [Ni super(II)(dpaq)Cl] (dpaq=2-[bis(pyridin-2-ylmethyl)amino]-N-(quinolin -8-yl)acetamide) displayed efficient catalytic activity toward numerous olefin epoxidation reactions. Partitioning between the heterolytic and homolytic O-O bond cleavage of the Ni-acylperoxo intermediate was significantly affected by the nature of the solvent. An Ni super(IV)-oxo species was the common reactive intermediate in protic solvents, whereas the two active oxidants, Ni super(IV)-oxo and Ni super(III)-oxo, operated in aprotic solvents.