ONNO]-type oxovanadium(V) complexes containing amine pyridine bis(phenolate) ligands: synthesis, characterization and catalytic behavior for ethylene (co)polymerization

Abstract

A series of oxovanadium(V) complexes bearing dianionic [ONNO] chelate ligands 2-[bis(3-R1-5-R2-2 -hydroxybenzyl) aminomethyl]pyridine (2a: R1=tBu, R2=H; 2b: R1=CF3, R2=H; 2c: R1=OCH3, R2=H; 2d: R1=R2=tBu) and 2-[bis(3-R1-5-R2-2 -hydroxybenzyl) aminoethyl]pyridine (2e: R1=R2=tBu) have been synthesized by reacting VO(OnPr)3 with 1.0 equiv. of the ligands in CH2Cl2. All these complexes were characterized by 1H, 13C, 51V NMR spectra and elemental analysis. X-ray structural analysis for 2d revealed a six-coordinate distorted octahedral geometry around the vanadium center in the solid state. It was observed that these complexes existed as a mixture of two isomers, and the main isomer had the oxo moiety in trans configuration to the tripodal nitrogen atom. In the presence of Et2AlCl and CCl3COOEt, these complexes displayed high catalytic activities for ethylene polymerization even at elevated reaction temperature, depending on ligand structures. The resultant polymers possessed high molecular weights and unimodal molecular weight distributions, indicative of a single active site nature. In addition, copolymerizations of ethylene and norbornene using precatalysts 2a–e were also investigated, and the observed catalytic activity was nearly comparable with that for ethylene homopolymerization. When the concentration of comonomer in the feed amounted to 3.0mol/L, a NBE incorporation up to 41.5% could be achieved. Other reaction parameters that influenced the polymerization behavior, such as reaction temperature and Al/V (molar ratio), are also examined in detail.