A series of new zirconium complexes bearing bis(phenoxyketimine) ligands: Synthesis, characterization and ethylene polymerization

A series of new zirconium complexes bearing bis(phenoxyketimine) ligands, bis((3,5-di-tert-butyl-C₆H₂-2-O)R₁CN (2-R ₂-C₆H₄))ZrCl₂ {R₁ = Me, R₂ = H (2a); R₁ = Et, R₂ = H (2b); R ₁ = Ph, R₂ = H (2c); R₁ = 2-Me-Ph, R ₂ = H (2d); R₁ = 2-F-Ph, R₂ = H (2e); R ₁ = 2-Cl-Ph, R₂ = H (2f); R₁ = 2-Br-Ph, R ₂ = H (2g); R₁ = Ph, R₂ = Me (2h); R ₁ = Ph, R₂ = F (2i)}, have been prepared, characterized and tested as catalyst precursors for ethylene polymerization. Crystal structure analysis reveals that complex 2c has a six coordinate center in a distorted octahedral geometry with trans-O, cis-N, cis-Cl arrangement which possesses approximate C₂ symmetry. When activated with methylaluminoxane (MAO), complexes 2a-2i exhibited high ethylene polymerization activities of 10 ⁶-10⁸ g PE (mol M h)^-1. Compared with the bis(phenoxyimine) zirconium analogues bis((3,5-di-tert-butyl-C₆H ₂-2-O)CHNC₆H₅)ZrCl₂ (3), the introduction of substituent on the carbon atom of the imine double bond enhanced the catalytic activity and molecular weight of prepared polyethylene. Especially, when the H atom at the carbon atom of the imine double bond was replaced by 2-fluoro-phenyl with strong electronic-withdrawing property, complex 2e displayed the highest catalytic activity, and the polyethylene obtained possessed the highest molecular weight and melt point.