Accessible, Highly Active Single-Component β-Ketiminato Neutral Nickel(II) Catalysts for Ethylene Polymerization

Abstract

A series of novel neutral nickel complexes based on cyclic β-ketiminato ligands, [(2,6-iPr2C6H3)N═CHC6H8O]Ni(Ph)(PPh3) (3b), [(2,6-iPr2C6H3)N═CHCn+3H2n+2O(C6H4)]Ni(Ph)(PPh3) (6a−c), and [(2,6-iPr2C6H3)N═CHCn+3H2n+2O(C6H4)]Ni(CH3)(Py) (7a−c: a, n = 0; b, n = 1; c, n = 2) have been synthesized and characterized. These conveniently accessible complexes proved to be highly active catalysts for ethylene polymerization without an activator. Under the optimized conditions, an activity of 71.4 kg of PE/((mol of Ni) h atm) was observed using 6c as a catalyst. Particularly, it is of great interest that a bulky substituent proximate to the oxygen atom of the β-ketiminato complex is no longer a prerequisite to attain high catalytic efficiency, which is much different from the case for salicylaldiminato neutral nickel catalysts. This is effectively supported by the lower activation enthalpy changes of complexes 3b and 6a−c relative to values for the classic salicylaldiminato complexes. Moreover, for complexes 6a−c and 7a−c, the degree of conjugation between the phenylene ring and the corresponding nickel chelate of the complex can be tuned via changes in the ligand structure, which remarkably influence the molecular weights and the microstructures of the resulting polyethylenes. In comparison with the highly conjugated complexes 6a and 7a, complexes 6b,c and 7b,c, with a low degree of conjugation, produced polyethylenes with much higher molecular weights and lower branch contents. X-ray crystallographic analysis of 3b and 6a−c provides detailed information about the differences among these structures, and various typical angles and bond distances produce evidence of the conjugation modulation effect.