Effect of ligand structure and metal center on ethylene oligomerization with N, N′ Schiff base late transition complexes: experiments and calculations

Abstract

A series of N, N’ bis(pyridine-imine) Schiff base late transition metal (Fe, Co, and Ni) complexes with different ligand structures and catalytic active sites were synthesized. Ethylene oligomerization results showed that bis(pyridine-imine) iron complex had the highest catalytic activity (10.8 × 105 g/mol Fe h), and the catalytic activity of cobalt complex was the lowest (2.5 × 105 g/mol Co h); nickel complex had the highest selectivity toward high carbon olefins (28.6%), while the iron complex had the lowest (0.2%). The structure-activity relationship between the catalyst structure and catalytic properties for ethylene oligomerization was further studied by density functional theory (DFT) calculations. The bis(pyridine-imine) iron complex had the highest energy gap value and the most stable catalyst structure, which was more conducive to oligomerization; and the N–M–N bond of the nickel complex had the smallest bond angle, which was beneficial for chain growth. Based on the Cossee mechanism, a mechanism for ethylene oligomerization catalyzed by bis(pyridine-imine) metal complexes was proposed. To further study the micro reaction process of ethylene oligomerization, a molecular model of pyridine imine nickel complex and its catalytic ethylene oligomerization reaction path were proposed. The structure and energy of the reaction intermediate and transition state were calculated using DFT. The molecular model of the nickel complex had a lower ethylene insertion barrier and β-H elimination of energy barriers, indicating that the molecular model of the pyridine imine nickel complex had ethylene oligomerization active centers.