Nickel Complexes Supported by a New Class of Fused Imidazonaphthyridine Tridentate N,N,N-Ligands for Ethylene Oligomerization and Isolation of a Key Ni-Al Intermediate

Abstract

A series of 9-imino-imidazo[1,2-h][1,7]naphthyridine ligands and their nickel complexes were synthesized and structurally characterized by single-crystal X-ray crystallography. X-ray diffraction analysis revealed that 9-imino-imidazo[1,2-h][1,7]naphthyridines were used as tridentate ligands in most cases. Nickel complex Ni2 have a five-coordinate distorted square pyramidal geometry, in which the nickel was coordinated with three nitrogen atoms of imidazo[1,2-h][1,7]naphthyridine ligand and two chlorines. Nickel complexes Ni1, Ni3 and Ni5 displayed a six-coordinate distorted octahedral geometry upon coordination with a solvent molecule. Upon activation with diethylaluminum chloride (Et2AlCl), high catalytic activity up to 1.20×107 g·mol-1(Ni)·h-1 as well as high selectivity for butenes (95.9%) and 1-butene (99.0%) could be achieved by using Ni1 as catalyst. A higher activity up to 2.06×108 g·mol-1(Ni)·h-1 was observed in the Ni1/Et2AlCl system with addition of 10 equiv. of PPh3 as an auxiliary ligand. Isolation and structurally characterization of a key Ni-Al intermediate provided crystallographic evidence of the role of co-catalyst in the activation of the Ni precatalyst.