Study on the synthesis of salicylaldimine nickel catalysts and their catalytic performance in ethylene oligomerization

A series of salicylaldimine ligands with different electronic effects and their corresponding nickel complexes were synthesized and characterized by FT-IR, UV-vis,1H NMR, TG, and MS. The ethylene oligomerization performances of the nickel catalysts were evaluated in cyclohexane. Under optimized conditions, catalyst C5 exhibited the highest activity of 1.25 × 106 g/(mol Ni·h), whereas C3 showed the lowest activity of 4.58 × 105 g/(mol Ni·h). The products obtained from catalysts C1–C5 were predominantly C4 fractions, accompanied by minor amounts of C6 and C8. Density functional theory (DFT) calculations were performed to investigate the structure–activity relationship between the catalyst structure and the oligomerization performance. The results indicated that C5 possessed the largest HOMO-LUMO gap (3.238 eV) and the most stable molecular configuration, which was more favorable for ethylene oligomerization. Furthermore, to elucidate the microscopic mechanism, the catalytic cycle of C5 was simulated. The structures and energies of key intermediates and transition states were calculated, explaining why the oligomerization products followed the Flory distribution from a thermodynamic perspective.