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 has a five-coordinate distorted square pyramidal geometry, in which the nickel is coordinated with three nitrogen atoms of the 9-imino-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 and high selectivity for butenes (95.9%) and 1-butene (99.0%) could be achieved using Ni1 as a catalyst. A higher activity of up to 2.06 × 10⁸ g mol⁻¹(Ni) h⁻¹ was observed in the Ni1/Et₂AlCl system with the addition of 10 equiv. of PPh₃ as an auxiliary ligand. Isolation and structural characterization of a key Ni–Al intermediate provided crystallographic evidence of the role of a co-catalyst in the activation of the Ni precatalyst.