Observations and Mechanistic Insights on Unusual Stability of Neutral Nickel Complexes with a Sterically Crowded Metal Center

Abstract

A series of novel nickel(II) methyl pyridine complexes based on beta-ketiminato and phenoxyiminato ligands, [(2,6-(i)Pr(2)C(6)H(3))N=CH(8-R(1)C(10)H(7))O]Ni(Me)(Py) (5a, R(1) = phenyl; 5b, R(1) = norbornyl), [(2,6-(i)Pr(2)C(6)H(3))N=CH(8-R(1)C(10)H(5))O]Ni(Me)(Py) (6a, R(1) = phenyl; 6b, R(1) = norbornyl), and [(2,6-(i)Pr(2)C(6)H(3))N=CHCHC(2'-R(2)C(6)H(4))O]Ni(Me)(Py) (7a, R(2) = H; 7b, R(2) = phenyl), have been synthesized and characterized. Molecular structures of 5a and 6b were further confirmed by X-ray crystallographic analysis. These complexes showed very different catalytic properties for ethylene polymerization. Design and construction of a special catalyst structure like 5a with a phenyl group directed toward the nickel(II) center proved to be an effective strategy for improving catalyst stability. Remarkably, according to (1)H NMR spectroscopy, bis-ligated deactivation of complex 5a was entirely avoided, while a rapid deactivation rate was observed with regard to the parent complex C without a phenyl group in the R(1) position. As a result, higher activities of complex 5a were obtained relative to complex C under the same polymerization conditions. In addition, the ligand backbone was found to have a great influence on polymerization behaviors. Specifically, the polyethylenes with greatly decreased molecular weights were produced by neutral nickel phenoxyiminato catalysts 6a,b in comparison with values for the corresponding beta-ketiminato catalysts 5a,b. This is best explained by DFT results that the ethylene insertion and chain termination barriers for complex 6a are very different from those of complex 5a.