Effect of Ligand Structure on Ethylene Oligomerization over Salicylaldehyde Imine Nickel Complexes: Experiments and Calculations

Salicylaldehyde imine nickel complexes Ni1-Ni10 containing different electron-donating and electron-withdrawing groups on benzene rings were synthesized, using ethylenediamine, salicylaldehyde derivatives with different substituents and nickel chloride hexahydrate as raw materials. Structure characterization, such as elemental analysis, Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR), ultraviolet–visible (UV–Vis), and electrospray ionization mass spectrometry (ESI-MS), confirmed that the structure of synthesized complexes was consistent with the theoretical. Salicylaldehyde imine nickel complexes exhibited good catalytic activity in ethylene oligomerization process, and complexes containing electron-donating groups on the benzene ring had higher catalytic activity than complexes containing electron-withdrawing groups on the benzene ring. With the increase of steric hindrance of phenol hydroxyl ortho-substituents, the catalytic activity decreased, as well as the catalytic selectivity toward olefins with higher carbon number. The relationship between structure and catalytic properties was further studied by density functional theory (DFT) calculations, and the possible mechanism of nickel complexes catalyzing ethylene oligomerization was proposed.