Nickel Complexes Based on Tridentate Pyrazolyl Ligands for Highly Efficient Dimerization of Ethylene to 1-Butene

A series of new pentacoordinated nickel complexes NiCl 2 (NZN) based on nitrogen-, oxygen-, or sulfur-bridged bis(pyrazolyl) ligands (NZN) were synthesized and characterized by elemental analysis and an X-ray diffraction study for NiCl 2 {bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine} ( 1 ). Upon activation with methylaluminoxane (MAO), these complexes show high activity in ethylene oligomerization (TOF = 7−80 × 10 3 h - 1 ), which varies according to the ligand environment. Up to 92% selectivity for 1-butene, combined with a TOF of 60 × 10 3 h - 1, has been obtained under moderate conditions (30 °C, 40 bar, MAO-to-Ni = 250) using NiCl 2 {bis[2-(5-phenylpyrazolyl)ethyl)]ether} ( 3 ) as the catalyst precursor.