Synthesis and characterization of iron, cobalt and nickel complexes bearing para-phenylene-linked pyridine imine ligand and their catalytic properties for ethylene oligomerization

The (6E,7E)-N1,N4-bis((pyridin-2-yl)methylene)benzene-1,4-diamine ligand L-0 was prepared in order to synthesize the corresponding pyridine imine metal (iron, cobalt and nickel) complexes Fe-1, Co-2 and Ni-3. The structures of the ligand and its corresponding metal complexes were characterized by element analysis, NMR, IR, UV–Vis and ESI–MS. Upon activation with cocatalyst, the metal complexes Fe-1, Co-2 and Ni-3 exhibited moderate catalytic activity, and the oligomer products were mainly 1-butene. The influence of reaction parameters, such as solvent type, Al/M (M = Fe, Co, Ni) molar ratio, reaction temperature, reaction pressure and cocatalyst type, on catalytic properties was investigated in detail. Ethylene oligomerization data suggested that metal center had an important impact on catalytic performance. In most cases, attributed to electron effects, Fe-1 had better catalytic activity than Co-2 and Ni-3 at the same reaction conditions, and Ni-3 had better catalytic selectivity toward high-carbon olefins (≥ 1-decene). The density functional theory calculations were utilized to optimize the geometry structure for exploring the relationship between structure and catalytic properties, which provides a possibility of predicting the catalytic selectivity in ethylene oligomerization. Moreover, the catalytic mechanism was further studied for better understanding of ethylene oligomerization process catalyzed by Fe-1, Co-2 and Ni-3. The para-phenylene-linked pyridine imine metal (iron, cobalt and nickel) complexes have the ability of catalyzing ethylene oligomerization. Attributed to electron effects, their catalytic properties including activity and selectivity are different.