Nickel Complexes Bearing Bidentate Schiff Base as Ethylene Oligomerization Catalysts

Several nickel complexes [N,N]NiBr2, in which [N,N] indicates bidentate nitrogencontaining ligands (1: [N,N]=N-(2,6-diisopropylphenyl)pyridine-2-carboxaldimine (C1sH22N2); 2: N-(2,6-diisopropylphenyl)-6-methylpyridine-2-carboxaldiminc (C19H24N2); 3: N-(2,4,6-trimethylphenyl)pyridine2-carboxaldimine(C15H16N2); 4: N-(2,4,6-trimethylphenyl)-6-methylpyridine-2-carboxaldimine (C16H18-N2) were synthesized. Some of the nickel complexes exhibit high activity for ethylene oligomerization in the presence of an organoaluminum activator. The main factor affecting the activity and the structure of oligomers is the steric effect of substituents on [N,N] ligands. Methylaluminoxane (MAO) -activated catalysts showed higher activities and produced oligomers with higher molecular weight than Et2AlCl-activated ones. The oligomerization in toluene rather than hexane results in much higher activity, and the oligomers produced in toluene have relatively high molecular weight. With activation of MAO or Et2AlCl,the [N,N]NiBr2 system tended to produce highly branched oligomers with low α-olefin content, but the α-olefin content could be increased by changing the reaction conditions.