Bidentate forms of β-triketimines: syntheses, characterization and outstanding performance of enamine–diimine cobalt complexes in isoprene polymerization. Issue 25 (6th June 2016)

• Record Type: Journal Article
• Authors: Alnajrani, Mohammed N.; Mair, Francis S.
• Is Part Of: Dalton transactions. Volume 45:Issue 25(2016)
• Page Start: 10435
• Page End: 10446
• Publication Date: 2016-06-06
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c6dt01064d
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : 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. Abstract : New cationic enamine-β-diimine cobalt complex [LCoBr·THF][BArF] (I ) and its neutral analogue [LCoBr2 ] (II ) where L = [(2, 4, 6-Me3 -C6 H2 )NHCMeC{CMe( N -2, 4, 6-Me3 (C6 H2 ))}2 ] and BArF − = [{3, 5-(CF3 )2 C6 H3 }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 inI strongly improved activity in comparison toII . BothI andII 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 bothI andII demonstrated that the polymerization was first-order in monomer and that approximately 46% and 50% of cobalt formed active centres forI andII respectively. EASC was the most active of a range of organoaluminium compounds screened for bothI andII . 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.