Polynuclear (α‐diimine) nickel(II) complex as catalyst for ethylene oligomerization

Late transition metal catalysts with single or multiple active centers have great attention in the ethylene oligomerization application owing to their unique properties. A new polynuclear (α‐diimine) nickel(II) complex (PS‐Ni complex) bearing the conjugated poly(Schiff‐base) ligand (PS‐ligand) was synthesized. The structures and the composition of the PS‐ligand and the PS‐Ni complex were characterized by Fourier transform infrared (FTIR) analysis, X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric (TG) analysis, nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). The PS‐Ni complex was used as precatalyst in ethylene oligomerization combined with methylaluminoxane (MAO) and has good catalytic activity for ethylene oligomerization under mild conditions. Compared with the corresponding mononuclear (α‐diimine) nickel(II) complex, the PS‐Ni complex showed higher activity of 17.62 × 105 g/(mol Ni•h), and the main products were C4 and C6 olefins in cyclohexane when the dosage of precatalyst was 2 μmol, the Al/Ni molar ratio was 500, the ethylene pressure was 0.5 MPa, the reaction time was 60 min, and the reaction temperature was 25°C. The main oligomers were α‐olefins, and the selectivity was up to 98.68%, due to the more electron‐deficient catalytic centers of the conjugated structure of poly(Schiff‐base) ligand. A novel polynuclear (α‐diimine) nickel(II) complex (PS‐Ni complex) was synthesized and applied in ethylene oligomerization. The catalytic activity of PS‐Ni complex (17.62 × 105 g/[mol Ni•h]) was more twice than that of the mononuclear counterpart complex (7.39 × 105 g/[mol Ni•h]) under the same conditions. The main oligomers of the PS‐Ni complex were C4 and C6 α‐olefins in cyclohexane because of the confinement effect and conjugated effect while the mononuclear (α‐diimine) nickel(II) complex produced more C8+α‐olefins.