Polymerization of 1,3-butadiene catalyzed by pincer cobalt(II) complexes derived from 2-(1-arylimino)-6-(pyrazol-1-yl)pyridine ligands

•Well-defined cobalt pincer complexes were synthesized.•The complex/MAO selectively catalyzed cis-1,4 butadiene polymerization.•The polymer yields and selectivity can be controlled by the ligation environment.•1,2 syndio-selectivity can be achieved by phosphine regulators.

A new class of air stable and structurally well-defined cobalt complexes with unsymmetrical pincer type ligands ([2-(ArNCMe)-6-(Py)C5H3N]CoCl2) (Ar = C6H5, Py = pyrazol-1-yl, 5a; Ar = 2,4,6-Me3C6H2, Py = pyrazol-1-yl, 5b; Ar = 2,6-iPr2C6H3, Py = pyrazol-1-yl, 5c; Ar = C6H5, Py = 3,5-Me2pyrazol-1-yl, 5d; Ar = 2,4,6-Me3C6H2, Py = 3,5-Me2pyrazol-1-yl, 5e; Ar = 2,6-iPr2C6H3, Py = 3,5-Me2pyrazol-1-yl, 5f; Ar = 2,6-iPr2C6H3, Py = 3,5-iPr2pyrazol-1-yl, 5g and [2-(OCMe)-6-(3,5-diphenylpyrazol-1-yl)C5H3N]CoCl25h) were prepared and the molecular structures of 5a, 5c and 5f were determined by single crystal X-ray crystallography. Upon activation by methylaluminoxane (MAO) in toluene at room temperature, all complexes initiate polymerization of 1,3-butadiene (polymer yields: 65–99%), affording polybutadiene with excellent cis-1,4 regularity (97.5–98.7%). The polymer yields and properties in terms of molecular weight and distribution are well controlled by the substituents on iminoaryl rings and pyrazole rings. Selectivity switch from cis-1,4 to syndio-1,2 was also achievable by adding phosphine as microstructure regulator.

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