Regio- and stereo-selective polymerization of 1,3-butadiene catalyzed by phosphorus–nitrogen PN3-pincer cobalt(ii) complexes

Abstract

A 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.