Ethylene oligomerization studies utilizing nickel complexes bearing pyridine-imine ligands

Abstract

Two pyridine-imine ligands (6E,7E)-N1,N4-bis((pyridin-2-yl)methylene)benzene-1,4-diamine (L1) and (6E,13E)-N1,N4-bis(1-(pyridin-2-yl)ethylidene)benzene-1,4-diamine (L2) were synthesized in good yields. Reactions with L1 and L2 with Ni salts afforded nickel complexes NiCl2{(6E,7E)-N1,N4-bis((pyridin-2-yl)methylene)benzene-1,4-diamine}NiCl2 (C1) and NiCl2{(6E,13E)-N1,N4-bis(1-(pyridin-2-yl)ethylidene)benzene-1,4-diamine}NiCl2 (C2). The compounds were characterized through NMR (1H NMR and 13C NMR), IR, UV–Vis, MS and elemental analysis. Upon activation with a co-catalyst, C1 and C2 both displayed good catalytic activity (0.63 × 104–16.66 × 104 g/mol Ni h and 0.11 × 105–23.70 × 105 g/mol Ni h, respectively) in ethylene oligomerization experiments, the oligomer products followed the classic Schulz-Flory distribution with mostly 1-C4= oligomers. The catalytic performance was obviously affected by the reaction parameters including reaction temperature, reaction pressure, Al/Ni molar ratio, reaction time and co-catalyst type. The experimental data suggested that C1 and C2 generally had quite different catalytic performance under the same reaction conditions indicating the catalyst structure had a significant influence on the catalytic performance. The density functional theory (DFT) calculation was utilized to optimize the geometry structure for exploring the relationship between structure and catalytic properties. Combined with a proposed mechanism, reaction intermediates and frontier molecular orbitals were simulated for further studying the intermediates structure and energy variation in ethylene oligomerization process.