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

Abstract

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 (peroxy-phenylacetic acid) probe, Hammett studies, (H2O)-O-18 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.