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New cationic enamine-β-diimine cobalt complex [LCoBr·THF][BArF] (I) and its neutral analogue [LCoBr2] (II) where L = [(2,4,6-Me3-C6H2)NHCMe=C{CMe=(N-2,4,6-Me3(C6H2))}2] and BArF− = [{3,5-(CF3)2C6H3}4B]−, were synthesised and then characterized by single-crystal X-ray diffraction, MALDI-MS, IR and elemental analysis. These complexes, the first examples reported where putatively tridentate β-triketimines prefer a bidentate coordination mode, were examined as catalysts for the polymerization of isoprene, activated by diethylaluminium chloride (DEAC) or ethylaluminium sesquichloride (EASC). The weakly coordinating BArF anion in I strongly improved activity in comparison to II. Both I and II produced polyisoprene of ca. 80% cis-1,4 and 20% 3,4 enchainment, with trace levels of trans-1,4 and no 1, 2 polymer. A kinetic study for both I and II demonstrated that the polymerization was first-order in monomer and that approximately 46% and 50% of cobalt formed active centres for I and II respectively. EASC was the most active of a range of organoaluminium compounds screened for both I and II. The resulting activities of up to 6 × 105 mol isoprene per mol Co per h are the highest yet recorded for catalysts selective for cis-1,4 enchained polyisoprene. [ABSTRACT FROM AUTHOR]

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 L1-3 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. [ABSTRACT FROM AUTHOR]