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

A new family of cobalt complexes (CoCl2-H, CoCl2-Me, CoCl2-Pr-i, CoBr2-H, CoBr2-Me, CoBr2-Pr-i, CoI2-H, CoI2-Me, and CoI2-Pr-i) 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-Pr-i 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](-)/(AlBu3)-Bu-i or (AlBu3)-Bu-i, 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-Pr-i > 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 approximate to 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.