Nickel complexes based on tridentate pyrazolyl ligands for highly efficient dimerization of ethylene to 1-butene

A series of new pentacoordinated nickel complexes NiCl2(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 NiCl2{bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine} (1). Upon activation with methylaluminoxane (MAO), these complexes show high activity in ethylene oligomerization (TOF = 7-80 x 10(3) h(-1)), which varies according to the ligand environment. Up to 92% selectivity for 1-butene, combined with a TOF of 60 x 103 h(-1), has been obtained under moderate conditions (30 degrees C, 40 bar, MAO-to-Ni = 250) using NiCl2{bis[2-(5-phenylpyrazolyl)ethyl)]ether} (3) as the catalyst precursor.