Synthesis, structure, and catalytic ethylene oligomerization of nickel(II) and cobalt(II) complexes with symmetrical and unsymmetrical 2,9-diaryl-1,10-phenanthroline ligands

A series of nickel(II) and cobalt(II) complexes, NiX2L (X = Cl, Br; 1–6) and CoCl2L (7–9), with 2,9-diaryl-1,10-phenanthroline ligands (L1–L3) have been synthesized and characterized by elemental analysis, UV–Vis, IR spectroscopy, and X-ray crystal structural study (for 1, 4–7, 9). The solid-state structures of 1, 5–7 and 9 show four-coordinate, slightly flattened tetrahedral geometry at the Ni(II) or Co(II) center, while 4 is five-coordinated (square-pyramidal), containing a THF molecule as an auxiliary ligand. The title complexes (1–9) display good catalytic activities in ethylene oligomerization when activated with methylaluminoxane (MAO). While the Co(II) precatalysts produce primarily C4 isomers, the Ni(II) complexes give ethylene dimers and trimers at normal pressure. The activities and yields of linear α-olefins increase with increasing ethylene pressure for the Ni(II) complexes, leading to more high-molar-mass products (C8–C18). Complex 6 displays the best catalytic activity among the complexes studied (up to 1518 kg/mol[Ni] h at 10 atm).