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] () 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.

Iminoimidazole‐based Co(II) and Fe(II) complexes: Syntheses, characterization, and catalytic behaviors for isoprene polymerization

“…Very recently, Huang and other groups reported Co(II) complexes for isoprene polymerization that offer high activity, precise control over the molecular weight and high cis ‐1,4‐alt‐3,4 microstructures . Alnajrani group reported β‐triketimine coordinated Co(II) complexes that gave the highest Co(II) activity (6 × 10 5 g·mol −1 ·h −1 ) for isoprene polymerization and generated predominantly cis ‐1,4 polymer with variable degree of 3,4‐regioisomers (10–20%) …”

“…Electron‐withdrawing substituents of N ‐aryl groups not only enhance the activity but also improve the cis ‐1,4 selectivity ( η 4 coordination) . To control the regio−/stereoselectivity of polyisoprene, less sterically crowded N,N‐bidentate ligands showed more potential than the N,N,N‐tridentate ligands such as β‐triimines …”

“…58,59 Alnajrani group reported β-triketimine coordinated Co(II) complexes that gave the highest Co(II) activity (6 × 10 5 gÁmol −1 Áh −1 ) for isoprene polymerization and generated predominantly cis-1,4 polymer with variable degree of 3,4-regioisomers (10-20%). 60 In general, the steric and electronic substituents of the ligands play a key role in the catalytic performance and properties of the resultant polyisoprene. Particularly, the coordination mode of the ligand is the most important to control the streoselectivities of the isoprene.…”

“…68 To control the regio−/stereoselectivity of polyisoprene, less sterically crowded N,N-bidentate ligands showed more potential than the N,N,N-tridentate ligands such as β-triimines. 60 Herein, we described the synthesis and characterization of a series of iminoimidazole Co(II) and Fe(II) complexes and tailored the catalysts with different substituents ranging from electron donating to the electron withdrawing to find the best couple of steric and electronic influences for the high productivities. To ascertain the effects of different reaction parameters such as co-catalyst effect, Al/Co ratio.…”

1,2 Enriched polymerization of isoprene by cobalt complex carrying aminophosphory fused (PN3) ligand

“…Metal‐catalyzed selective isoprene polymerization has been a major entry toward cis ‐1,4, trans ‐1,4, and 3,4 isomers (Scheme ). Ziegler‐Natta type of titanium and neodymium, metallocenes (Cp: cyclopentadienyl and its derivatives) type of titanium and rare earth metal catalysts, non‐Cp ligated lanthanide alkylaluminates, multi‐dentated PNP, or NCN ligand chelated rare earth metal complexes, and cobalt complexes supported by multi‐dentate ligand have been extensively explored for cis ‐1,4 polymerization. The resultant polymer could be a promising alternative to natural rubber for tire industry.…”