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

Abstract

A series of nickel(II) and cobalt(II) complexes, NiX 2L (X = Cl, Br; 1– 6) and CoCl 2L ( 7– 9), with 2,9-diaryl-1,10-phenanthroline ligands ( L 1– L 3) 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 C 4 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 (C 8–C 18). Complex 6 displays the best catalytic activity among the complexes studied (up to 1518 kg/mol[Ni] h at 10 atm).