Synthesis and characterization of N-(2-pyridyl)benzamide-based nickel complexes and their activity for ethylene oligomerization

A series of N-(2-pyridyl)benzamides (1)–(11) and their nickel complexes, [N-(2-pyridyl)benzamide]dinickel(II) di-μ-bromide dibromide (12)–(16) and (aryl)[N-(2-pyridyl)benzamido](triphenylphosphine)nickel(II) (17)–(24), were synthesized and characterized. The single-crystal X-ray analysis revealed that 12 and 14 are binuclear nickel complexes bridged by bromine atoms and each nickel atom adopts a distorted trigonal bipyramidal geometry. The key feature of the complexes 17, 19 and 23 is each has a six-membered nickel chelate ring including a deprotonated secondary nitrogen atom and an O-donor atom. The nickel complexes show moderate to high catalytic activity for ethylene oligomerization with methylaluminoxane (MAO) as cocatalyst. The activity of 12–16/MAO systems is up to 3.3 × 104 g mol−1 h−1 whereas for 17–24/MAO systems it is up to 4.94 × 105 g mol−1 atm−1 h−1. The influence of Al/Ni molar ratio, reaction temperature, reaction period and PPh3/Ni molar ratio on catalytic activity was investigated.