Synthesis of vanadium(iii) complexes bearing iminopyrrolyl ligands and their role as thermal robust ethylene (co)polymerization catalysts

Abstract

Bis(imino)pyrrolyl vanadium(III) complexes 2a–e [2,5-C₄H₂N(CHNR)₂]VCl₂(THF)₂ [R = C₆H₅ (2a), 2,6-Me₂C₆H₃ (2b), 2,6-ⁱPr₂C₆H₃ (2c), 2,4,6-Me₃C₆H₂ (2d), C₆F₅ (2e)] and bis(iminopyrrolyl) vanadium(III) complex 4f [C₄H₃N(CHN-2,6-ⁱPrC₆H₃)]₂VCl(THF) have been prepared in good yields from VCl₃(THF)₃ by treating with 1.0 and 2.0 equivalent deprotonated ligands in tetrahydrofuran (THF), respectively. These complexes were characterized by FTIR and mass spectra as well as elemental analysis. Structures of 2c and 4f were further confirmed by X-ray crystallographic analysis. DFT calculations indicated the configurations of 2a–e with two nitrogen atoms of the chelating ligand coordinating with vanadium metal centre were more stable in energy. These complexes were employed as catalysts for ethylene polymerization at various reaction conditions. On activation with Et₂AlCl, these complexes exhibited high catalytic activities (up to 22.2 kg mmol⁻¹_V h⁻¹bar⁻¹) even at high temperature, suggesting these catalysts possessed remarkable thermal stability. Moreover, high molecular weight polymer with unimodal molecular weight distributions can be obtained, indicating the polymerization took place in a single-site nature. The copolymerizations of ethylene and 1-hexene with precatalysts 2a–e and 4f were also explored in the presence of Et₂AlCl. Catalytic activity, comonomer incorporation, and properties of the resultant polymers can be controlled over a wide range by tuning catalyst structures and reaction parameters.