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

Abstract

New cationic enamine-β-diimine cobalt complex [LCoBr·THF][BArF] (I) and its neutral analogue [LCoBr₂] (II) where L = [(2,4,6-Me₃-C₆H₂)NHCMeC{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.