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

Abstract

Four new b-triketimine cobalt complexes [(LCo-m-Br) 2 ][BArF] 2 where L ¼ N,N 0 ,N 00 -triaryl-b-diketimine were synthesised and then characterized by single-crystal X-ray diffraction, MALDI-MS and elemental analysis. In combination with three previously reported, they provided a series of seven subtly different complexes which were screened for activity in the catalysis of isoprene polymerization: the structure of the ligand (L) had strong effects on activity and stereoselectivity. The produced polyisoprene contained a mixture of cis-1,4, trans-1,4 and 3,4-enchained monomers; 4,1-errors followed 3,4 errors. The highest percentage of cis-1,4 content (80%) was obtained with complex 5 where two electron-withdrawing fluorine substituents occupied the 2-and 6-positions of two of the imino-aryl rings. This polymer was obtained in 98.5%conversion. Both cis-1,4 content and activity decreased when electron donor substituents were present on the aryl rings, so that complex 1, with three methyl substituents in positions 2, 4 and 6 gave the lowest cis-1,4 content (73.6%), with a conversion of only 58.5%, under identical conditions. The temperature of the polymerization, the type of aluminium co-catalyst used, and the Al/Co ratio strongly affected the activity and microstructure of the polyisoprene produced. Ethylaluminiumsesquichloride was the most active of a range of organoaluminiums screened. A kinetic study using complex 6 as catalyst demonstrated that the polymerization was first-order in monomer, and that approximately 12% of cobalt formed active centers. The combination of high molecular weight (>10 5 ) with moderately high activity at conveniently accessible temperatures to give predominantly cis-1,4-polyisoprene but with 3,4 units to promote efficient crosslinking is potentially attractive, and has not previously been attained with 3d elements.

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 …”

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

“…14,[18][19][20] These complexes mainly employed bis-imino-pyridine and analogous cases such as bis-benzimidazolylpyridine, exclusively planar mer-N,N,N-ligands on cobalt. In contrast, in our previous work on Cr(0), Mo(0), W(0), 21 Tl(I), 22 Ni(II) 23 and Co(II), 7,24 the triketimine N,N,N ligands were found exclusively to bind in a fac tridentate manner. In our mechanistic hypotheses, we reasoned on the basis of electron counts that it was likely that bidentate forms may be found at key points in the catalytic cycle.…”

“…In the several previous examples of reaction of enamine-diimines with Co(II) and other metals, in all cases the ligands converted exclusively to the tridentate β-triketimine forms. 7,[21][22][23][24] Here, uniquely, the enamine-diimine tautomer present in the proligand is retained, though the internal hydrogen bond is lost. This different behaviour may be ascribed to the presence of alkyl substituents in both ortho positions of each aryl, a degree of bulk which appears not be tolerated in the tridentate form.…”

“…6 A summary of the development of the various heterogeneous and homogeneous (ligated) metal catalysts, which can select for high cis-1,4 high trans-1,4 high 3,4-syndiotactic, alt cis-1,4/ 3,4 and other less regular microstructures, can be found in our previous paper in this series. 7 Most pertinent here are the results showing the control on activity and microstructure achievable by catalyst systems based on metal complexes with ligands having donor atoms such as nitrogen, oxygen and phosphorus. [8][9][10][11][12] Ligands coordinated to the metal have been found to affect both the activity and the form (syn or anti) of the allyl active centre during the polymerization.…”

Isoprene Polymerization with Pyrazolylimine Cobalt(II) Complexes: Manipulation of 3,4‐Selectivities by Ligand Design and Use of Triphenylphosphine

“…Compared to the lanthanide counterparts which are highly air‐/moisture‐sensitive and thus need delicate handling, late transition metal complexes display unique advantages, such as highly stable, high functional tolerance, cheap and earth abundant, etc., and therefore attract extensive investigations in the past few years Porri et al reported diethylbis(2,2′‐bipyridine)Fe/MAO system for highly 3,4‐regio‐polymerization to afford crystallizable 3,4‐PI;[4b] Ricci et al reported the preparation of cis ‐1,4‐ alt ‐3,4‐PIs from CoCl 2 (PRPh 2 ) 2 /MAO system; a similar breakthrough was recently achieved by Gong et al, in which aminophosphine fused bipyridine Co II complexes were reported as effective catalytic precursor for precise synthesis of cis ‐1,4‐ alt ‐3,4‐PIs . Besides the above chelating ligands, imine‐based ligands were also another important type of skeletons to support late transition metal for selective polymerization of 1,3‐dienes because of the ease manipulation and regulation of imine moiety . However, different from the high catalytic activity and high cis ‐1,4‐ selectivity displayed in 1,3‐butadiene polymerization, the imine‐based late transition metal complexes demonstrated some challenges in mediating isoprene polymerization, such as lower catalytic activities, formation of insoluble gels, etc ., therefore, up to date, the examples of imine‐based late transition metal in catalyzing isoprene polymerization are very limited,, and exploring new catalyzing systems with high activity and meanwhile the ability to afford cis ‐1,4‐/3,4‐PI is an obviously challenging and promising project.…”