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] ( I ) and its neutral analogue [LCoBr 2 ] ( II ) where L = [(2,4,6-Me 3 -C 6 H 2 )NHCMe&z.dbd;C{CMe&z.dbd;( N -2,4,6-Me 3 (C 6 H 2 ))} 2 ] and BArF − = [{3,5-(CF 3 ) 2 C 6 H 3 } 4 B] − , 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 × 10 5 mol isoprene per mol Co per h are the highest yet recorded for catalysts selective for cis -1,4 enchained polyisoprene. When a normally tridentate β-triketimine ligand binds to Co( ii ) in a bidentate enamine-diimine form, is partnered with the weakly co-ordinating BArF anion and activated with ethylaluminiumsesquichoride, the most active catalyst yet found for cis -1,4-selective polymerization of isoprene results.