Nickel(II) complexes bearing pyrazolylpyridines: synthesis, structures and ethylene oligomerization reactions

Reactions of 2-bromo-6-(3,5-dimethyl-1H-pyrazol-1-yl) pyridine (L1) and 2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl) pyridine (L2) with NiCl2 and NiBr2 led to the formation of their respective metal complexes [NiCl2(L1)] (1), [NiBr2(L1)] (2) and [NiBr2(L2)] (3) in moderate to high yields. The complexes were characterized using elemental analyses, mass spectrometry and single-crystal X-ray diffraction for 2. The solid-state structure of 2 confirmed the bidentate coordination mode of L1 and formation of a monometallic compound. Activation of the nickel(II) pre-catalysts with methylaluminoxane afforded active catalysts in the ethylene oligomerization reaction to produce mainly butenes (84-86%). In contrast, activation of nickel(II) pre-catalyst 2 with ethylaluminium dichloride resulted in partial Friedel-Crafts alkylation of the toluene solvent by the preformed oligomers. Complex structure, nature of co-catalyst employed, type of solvent and reaction conditions influenced the catalytic behaviour of these pre-catalysts. Copyright (C) 2015 John Wiley & Sons, Ltd.