Arene-Bridged Salicylaldimine-Based Binuclear Neutral Nickel(II) Complexes: Synthesis and Ethylene Polymerization Activities

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Abstract

A series of novel m-arene-bridged salicylaldimine-based binuclear neutral nickel(II) complexes (2,4,6-(R2)3C6H-1,3-[NCH-4-X-6-R1C6H2ONi(Ph)(PPh3)]2 (3a−i: R1, R2, X = Me, Me, H (3a), tBu, Me, H (3b), Me, Et, H (3c), tBu, Et, H (3d), H, iPr, H (3e), Me, iPr, H (3f), tBu, iPr, H (3g), Ph, iPr, H (3h), NO2, iPr, NO2 (3i), H, iPr, NO2 (3j)) are synthesized. The structure of complex 3h is further confirmed by an X-ray diffraction study showing that the two Ni centers adopt an unsymmetrical, distorted square planar geometry. In the presence or absence of the phosphine scavenger Ni(COD)2, complexes 3a−j show high catalytic activities for ethylene polymerization. The effect of substituents on ethylene polymerization is significant. The introduction of an electron-withdrawing group to the ligand framework improves the catalytic activity significantly. The influence of polymerization temperature, polymerization time, and catalyst concentration on the activities of ethylene polymerization is also investigated. Highly branched polyethylenes (46−127 branches per 1000 carbon atoms) with moderate molecular weights (Mη = (1.0−169) × 104) and narrow molecular weight distributions (Mw/Mn = 2.3−2.4) are obtained by using complexes 3a−j as catalysts with or without a phosphine scavenger. In the presence of a polar solvent such as THF or Et2O, complex 3h polymerizes ethylene as a single-component catalyst. In comparison to the corresponding mononuclear Ni catalysts, the binuclear Ni catalysts generally show higher thermal stability, and complexes 3a, 3c, 3e, and 3f, which feature small R1 substituents, are capable of acting as single-component ethylene polymerization catalysts.

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