Synthesis and characterization of β-triketimine cobalt complexes and their behaviour in the polymerization of 1,3-butadiene

Abstract

Three β-triketimine ligands, (L(1): [CH{CMeN(2,4,6-Me3-C6H2)}2C(CMe3N(2-MeO-C6H4)], L(2): [CH{CMeN(2,4-Me2-C6H3)}2C(CMe3N(2-MeO-C6H4)] and L(3): [CH{CMeN(2-Me-C6H4)}2C(CMe3N(2-MeO-C6H4)]), were synthesized and then characterized by (1)H and (13)C{(1)H} NMR spectroscopy, elemental analysis and electrospray (ES) MS. β-triketimine cobalt(ii) complexes were then prepared by the interaction of cobalt(ii) bromide with L(1-3) in the presence of NaBArF (BArF = [{3,5-(CF3)2-C6H3}4B](-) ). Five-coordinate dimeric bromide-bridged species [(LCoμ-Br)2][BArF]2 were obtained. The geometry of the complexes was found by single-crystal X-ray diffraction to be slightly distorted square-pyramidal. The polymerization of 1,3-butadiene catalysed by these complexes upon activation with methylaluminoxane (MAO) in chlorobenzene yielded high cis-1,4-polybutadiene (>97% cis). The effect of steric and electronic properties of the ligands on the performance of the catalytic system was investigated: it was found that ligands with fewer methyl substituents gave more active catalytic systems. It was also found that increasing MAO: Co ratio resulted in higher activity. Stereoselectivity of all catalysts slightly decreased at higher temperature, whereas activities were maximised at 20 °C, where very high values of activity were recorded.

The behaviour of β-triketimine cobalt complexes in the polymerization of isoprene

“…These ligands exhibited extensive tautomerism in solution, as reported for earlier examples, 37 but as previously found when metal-bound, 36,37 the seven examples all adopted fac-triketimine ligated forms: b-triketimine cobalt complexes were prepared from the reaction of these ligands with cobalt bromide in the presence of NaBArF, Scheme 1. These ligands exhibited extensive tautomerism in solution, as reported for earlier examples, 37 but as previously found when metal-bound, 36,37 the seven examples all adopted fac-triketimine ligated forms: b-triketimine cobalt complexes were prepared from the reaction of these ligands with cobalt bromide in the presence of NaBArF, Scheme 1.…”

“…The structures of complexes 1, 2 and 4 (nomenclature: 36 where the coordination geometry at the cobalt centre was described as distorted square-pyramidal: one nitrogen atom (N2) occupied the axial site, whilst the other two nitrogen atoms (N1 and N3) and two bromine atoms (Br and Br 0 ) occupied basal sites. These complexes were all found to be vecoordinate centrosymmetric dimeric bromide-bridged species [(LCo-m-Br) 2 ][BArF] 2 .…”

“…This eld of diene polymerization, both of butadiene and isoprene, has received much academic study, which has been recently reviewed. 35 Here, we report our results on cobalt complexes of N,N,N facially capping ligands, recently employed to give high activity and selectivity in polymerization of butadiene, 36 and their performance in the polymerization of isoprene. A series of different catalytic systems have been found to be efficient in controlling such microstructure; ligands co-ordinated to the active metal centre were found to affect the form (syn or anti) of the putative allyl intermediate of the diene polymerization, and hence the conformation of the remaining double bond in the produced polymer, and also the activity of the propagating centre during the polymerization.…”

Controlled isoprene polymerization mediated by iminopyridine‐iron (II) acetylacetonate pre‐catalysts

“…While, for sterically less hindered complexes (such as Fe1–Fe4 ) based polymerization, both cis ‐ η 4 and cis ‐ η 2 or trans ‐ η 2 type of monomer coordination‐insertion is likely to be involved in the propagation step. Similar findings have been found in the literature …”

Influence of imine‐carbon substituent in 6‐bromo‐2‐iminopyridine‐based cobalt(II) complexes on 1,3‐butadiene polymerization

“…Until now, most imine‐based Co(II) complexes used for 1,3‐butadiene polymerization mainly focused on sites ‘2’ and ‘3’ using various aniline derivatives and employing several composing patterns . A variety of these kinds of complexes, including α ‐diimine Co(II), pyridine‐2‐imine Co(II), bis(imino)pyridine Co(II), β ‐triketimine Co(II), bis(salicylaldiminate) Co(II), 1,10‐phenanthroline‐2‐pyrazolyl Co(II), bis(oxazolinyl)pyridine Co(II), bis(imidazolyl)pyridine Co(II) and bis(benzimidazolyl)amine Co(II), have been reported as being capable of yielding polybutadienes, with microstructures and catalytic activities being tuned by changing the ligand substituents. In striking contrast, reports of modification on site ‘1’ are rather limited .…”