Novel dendritic nickel complexes: Synthesis, characterization and oligomerization properties of ethylene

Two novel dendritic nickel complexes were synthesized via Schiff base formation and complexation reactions using 1.0-generation dendritic polyamine (1.0 g), salicylaldehyde, and anhydrous nickel chloride. The structures of the dendritic ligands and their corresponding nickel complexes were characterized using FT-IR, ¹H NMR, ESI-MS, and elemental analysis.These dendritic nickel complexes were evaluated as catalyst precursors for ethylene oligomerization in the presence of co-catalysts, including EtAlCl₂, Et₂AlCl, AlEt₃, and IBAL. Among them, the combination of the new dendritic nickel complex (as the pre-catalyst) and Et₂AlCl (as the co-catalyst) exhibited notable catalytic performance. The effects of key reaction parameters—such as the Al/Ni molar ratio, temperature, and ethylene pressure—on oligomerization efficiency were systematically investigated.The catalytic activities of the two dendritic nickel complexes, featuring 5-bromosalicylaldehyde (5Br-cat) and 3,5-dibromosalicylaldehyde (3,5br-cat) in their branch chains, were determined to be 9.44 × 10⁵ g/(mol Ni·h) and 7.34 × 10⁵ g/(mol Ni·h), respectively. The results indicate that both catalytic activity and product distribution are strongly influenced by the nature of the co-catalyst and the structural features of the nickel complex. Notably, the catalytic activity of the dendritic nickel complex decreases with increasing steric hindrance in the branching structure.