Neutral Ni(II) Catalysts based on Maple-Lactone Derived [N,O] Ligands for the Polymerization of Ethylene

This paper describes the synthesis of neutral nickel(II) catalysts for the polymerization of ethylene. A series of 2-(arylamino)-5-methylcyclopent-2-en-1-one (aryl = 2,6-diisopropylphenyl, 2,4,6-trimethylphenyl, 3,5-dimethylphenyl, and 4-methylphenyl) proligands, derived from the condensation of aryl amines with maple lactone, were synthesized as their keto-enamine tautomers. Deprotonation with NaH gave their respective sodium imino-enolate salts, which underwent salt metathesis with Ni(II) precursors to give square planar [Ni(Ar)(κ ²-imino-enolate)(PPh ₃)] (Ar = Ph, ortho-tolyl) complexes containing 5-membered [N,O] chelates. Precatalysts with N-Dipp (Dipp = 2,6-diisopropylphenyl) substituents were found to exhibit moderate activity in the polymerization of ethylene when using B(C ₆F ₅) ₃ or [Ni(cod) ₂] (cod = 1,4-cyclooctadiene) as coactivators, producing polymers with a high molecular weight, as determined by GPC. The reaction was found to be highly dependent on temperature, with the catalysts most active at 80 °C. Differences in polymer structure were also found when using B(C ₆F ₅) ₃ or [Ni(cod) ₂] as the cocatalyst, with [Ni(cod) ₂] producing a more highly branched polymer. Reducing the steric bulk at the N substituent of the ligand led to inactive species.