Polymerization of 1,3-butadiene catalyzed by cobalt(II) and nickel(II) complexes bearing imino- or amino-pyridyl alcohol ligands in combination with ethylaluminum sesquichloride

A series of cobalt(II) (1a–6a) and nickel(II) (1b–6b) complexes supported by imino- or amino-pyridyl alcohol ligands (tridentate [NNO]), 2-(2,6-R12C6H3N=CMe)-6-{(HO)CR2}C5H3N (L1–L4) and 2-(2,6-R12C6H3NHCHMe)-6-(CH2OH)C5H3N (L5 and L6), were synthesized and characterized by elemental and spectroscopic analysis along with X-ray diffraction analysis. The X-ray diffraction demonstrated that all the complexes adopted distorted trigonal bipyramidal configuration with the equatorial plane formed by the pyridyl nitrogen atom and two chlorine atoms. On activation with ethylaluminum sesquichloride (EASC), the cobalt complexes (1a–6a) displayed high catalytic activity for the polymerization of 1,3-butadiene to yield cis-1,4-polybutadiene with high selectivity (>96%) under the Al/Co molar ratio of 40 at 25 °C. The conversion of butadiene, microstructure and molecular weight of the resulting polymers were affected by the reaction parameters and ligand environment. However, in comparison with the corresponding cobalt complexes, the nickel complexes (1b–6b) obtained relatively lower catalytic activity, cis-1,4 content and molecular weight under the similar reaction conditions.