Nickel(II) Complexes Chelated by 2-Arylimino-6-benzoxazolylpyridine: Syntheses, Characterization, and Ethylene Oligomerization

2-(2-Benzoxazolyl)-6-(1-(arylimino)-ethyl)pyridines (L) were designed as ligands, and their nickel complexes LNiX2 (X = Cl, 1a-7a; X = Br, 1b-7b) were formed. All ligands were fully characterized by NMR, FT-IR spectra, and elemental analyses, while the nickel complexes were examined by FT-IR spectra and elemental analyses. The molecular structures of nickel complexes were established by single-crystal X-ray diffraction. Complex 6a possesses coordination number 6 by coordination of one ligand, two moles of methanol, and one chloride anion, whereas complexes 1b and 5b possess coordination number 5 with distinct distortions, for 1b toward trigonal-bipyramidal geometry and for 5b toward square-pyramidal geometry. The nickel complex la was evaluated in the catalytic oligomerization of ethylene with different alkylaluminums as cocatalysts, and diethylaluminum chloride (Et2AlCl) proved to be the most effective. Upon activation with Et2AlCl, all nickel complexes showed outstanding thermal stability and good catalytic activity toward ethylene dimerization with high selectivity for alpha-C-4.