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

Three beta-triketimine ligands, (L-1: [CH{CMeN(2,4,6-Me-3-C6H2)}(2)C(CMe3N(2-MeO-C6H4)], L-2: [CH{CMeN-(2,4-Me-2-C6H3)}(2)C(CMe3N(2-MeO-C6H4)] and L-3: [CH{CMeN(2-Me-C6H4)}(2)C(CMe3N(2-MeO-C6H4)]), were synthesized and then characterized by H-1 and C-13{H-1) NMR spectroscopy, elemental analysis and electrospray (ES) MS. beta-triketimine cobalt(II) complexes were then prepared by the interaction of cobalt(II) bromide with L1-3 in the presence of NaBArF (BArF = [{3,5-(CF3)(2)-C6H3}(4)B](-)). Five-coordinate dimeric bromide-bridged species [(LCo mu-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-potybutadiene (>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 degrees C, where very high values of activity were recorded.