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

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 MCl 2 ·nH 2 O (M   =   Fe, Co, Ni) to form three types of complexes (C1-C5). Both L1-L3 and C1-C5 were fully characterized by FT-IR, 1 H 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   ×   10 6   g   mol −1   h −1 for ethylene oligomerization and the oligomers showed Schulz-Flory distribution. However, on activation with EtAlCl 2 , the nickel complex C1 exhibited higher catalytic activity up to 8.94   ×   10 6   g   mol −1   h −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 (C 4 and C 6 ) obtained in this work possessed relatively higher.