Regio- and stereo-selective polymerization of 1,3-butadiene catalyzed by phosphorus–nitrogen PN3-pincer cobalt(ii) complexes
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AbstractA new family of cobalt complexes (CoCl2-H, CoCl2-Me, CoCl2-iPr, CoBr2-H, CoBr2-Me, CoBr2-iPr, CoI2-H, CoI2-Me, and CoI2-iPr) supported by a PN3 ligand (6-(N,N′-di-t-butylphosphino)-2-pyrazol-yl-aminopyridine) have been prepared and fully characterized by FT-IR, elemental analysis, and X-ray analysis. The X-ray analysis reveals a trigonal bipyramidal conformation in the solid state for all representative complexes, CoCl2-H, CoBr2-H, CoBr2-iPr and CoI2-Me. The cobalt center is chelated by the PN3 ligand through the pyridinyl nitrogen, the pyrazol nitrogen and the phosphorus donor, with a long Co-P bond distance indicating a labile character. On activation with AlEt2Cl, Al2Et3Cl3, MAO, [Ph3C]+[B(C6F5)4]-/AliBu3 or AliBu3, cis-1,4 selective butadiene polymerization was achieved with up to 98.6% selectivity. The polymerization results show that the cis-1,4 selectivity is influenced by the steric hindrance, increasing with the bulkiness of the substituent groups (CoX2-iPr > CoX2-Me > CoX2-H) at the 3,5-positions of the pyrazole moiety, together with a slight decrease in activity. The activity changes in the order CoCl2L ≈ CoBr2L > CoI2L (for the same ligand L) when MAO is used as the activator, while the high level of cis-1,4 selectivity is maintained. It is possible to switch the selectivity from cis-1,4 to syndiotactic-1,2 by adding PPh3 © The Royal Society of Chemistry.
CitationGong D, Zhang X, Huang K-W (2016) Regio- and stereo-selective polymerization of 1,3-butadiene catalyzed by phosphorus–nitrogen PN3-pincer cobalt(ii) complexes. Dalton Trans 45: 19399–19407. Available: http://dx.doi.org/10.1039/c6dt03722d.
SponsorsThe authors thank KAUST for the generous financial support. This work is also supported by the Natural Science Foundation of China (Grant No. 21304050), the Natural Science Foundation of Ningbo (2014A610109) and the Open Research Fund of Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (KLSR1501).
PublisherRoyal Society of Chemistry (RSC)