Bidentate forms of β-triketimines: syntheses, characterization and outstanding performance of enamine-diimine cobalt complexes in isoprene polymerization.

New cationic enamine-β-diimine cobalt complex [LCoBr·THF][BArF] () and its neutral analogue [LCoBr2] () where L = [(2,4,6-Me3-C6H2)NHCMe[double bond, length as m-dash]C{CMe[double bond, length as m-dash](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 strongly improved activity in comparison to . Both and 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 and demonstrated that the polymerization was first-order in monomer and that approximately 46% and 50% of cobalt formed active centres for and respectively. EASC was the most active of a range of organoaluminium compounds screened for both and . The resulting activities of up to 6 × 10(5) mol isoprene per mol Co per h are the highest yet recorded for catalysts selective for cis-1,4 enchained polyisoprene.