Iron, cobalt and nickel complexes bearing hyperbranched iminopyridyl ligands: Synthesis, characterization and evaluation as ethylene oligomerization catalysts

A series of hyperbranched iminopyridyl ligands and corresponding iron, cobalt and nickel complexes were synthesized, characterized and studied as catalyst for ethylene oligomerization. All complexes exhibited excellent catalytic properties for ethylene oligomerization. Meanwhile, the hyperbranched iminopyridyl nickel complex possessed better catalytic activity and selectivity of high carbon number olefins compared with iron and cobalt complexes. [Display omitted] •New hyperbranched iminopyridyl iron, cobalt and nickel complexes were synthesized and characterized structurally.•The oligomers showed Schulz-Flory distribution when nickel complexes combination with MAO.•Upon activation with EtAlCl2, the nickel complex exhibited higher catalytic activities, and the alkylated-toluenes were mainly products.•The hyperbranched iminopyridyl nickel complex exhibited better catalytic properties than iron and cobalt complexes for ethylene oligomerization. A series of hyperbranched iminopyridyl ligands [ethylenediamine as terminal groups (L1), 1,4-butanediamine as terminal groups (L2), 1,6-hexamethylenediamine as terminal groups (L3)] were prepared. Ligands L1-L3 were reacted with MCl2·nH2O (M=Fe, Co, Ni) to form three types of complexes (C1-C5). Both L1-L3 and C1-C5 were fully characterized by FT-IR, 1H NMR, UV and ESI-MS. The nickel complexes exhibited better catalytic properties than iron and cobalt complexes for ethylene oligomerization. On activation with methylaluminoxane (MAO), the nickel complex C1 exhibited moderate catalytic activities up to 1.56×106gmol−1h−1 for ethylene oligomerization and the oligomers showed Schulz-Flory distribution. However, on activation with EtAlCl2, the nickel complex C1 exhibited higher catalytic activity up to 8.94×106gmol−1h−1 for ethylene oligomerization, but the products were mainly alkylated-toluenes. The influence of the solvent and reaction parameters such as reaction temperature, ethylene pressure and Al/Ni molar ratio on the catalytic behavior of the nickel complex C1 was also investigated. Moreover, in comparison with the catalytic activity and the distribution of oligomers obtained with related hyperbranched salicylaldimine nickel complex and other non-hyperbranched iminopyridyl nickel complexes, the catalytic activity and the content of low carbon number olefins (C4 and C6) obtained in this work possessed relatively higher.