Synthesis and characterization of β-triketimine cobalt complexes and their behaviour in the polymerization of 1,3-butadiene

Abstract

Three β-triketimine ligands, (L¹: [CH{CMeN(2,4,6-Me₃-C₆H₂)}₂C(CMe₃N(2-MeO-C₆H₄)], L²: [CH{CMeN(2,4-Me₂-C₆H₃)}₂C(CMe₃N(2-MeO-C₆H₄)] and L³: [CH{CMeN(2-Me-C₆H₄)}₂C(CMe₃N(2-MeO-C₆H₄)]), were synthesized and then characterized by ¹H and ¹³C{¹H} NMR spectroscopy, elemental analysis and electrospray (ES) MS. β-triketimine cobalt(II) complexes were then prepared by the interaction of cobalt(II) bromide with L^1–3 in the presence of NaBArF (BArF = [{3,5-(CF₃)₂-C₆H₃}₄B]⁻ ). Five-coordinate dimeric bromide-bridged species [(LCoμ-Br)₂][BArF]₂ were obtained. The geometry of the complexes was found by single-crystal X-ray diffraction to be slightly distorted square-pyramidal. The polymerization of 1,3-butadiene catalysed by these complexes upon activation with methylaluminoxane (MAO) in chlorobenzene yielded high cis-1,4-polybutadiene (>97% cis). The effect of steric and electronic properties of the ligands on the performance of the catalytic system was investigated: it was found that ligands with fewer methyl substituents gave more active catalytic systems. It was also found that increasing MAO: Co ratio resulted in higher activity. Stereoselectivity of all catalysts slightly decreased at higher temperature, whereas activities were maximised at 20 °C, where very high values of activity were recorded.