Synthesis, characterization and 1,3-butadiene polymerization studies of cobalt dichloride complexes bearing pyridine bisoxazoline ligands

A series of ion-pair cobalt complexes bearing pyridine bisoxazoline ligands were successfully synthesized and characterized by IR spectroscopy and elemental analysis. Determined by X-ray crystallographic analysis, complexes 4a, 4b, 4e, and 4f existed as ion pairs comprised of [CoL₂]²⁺ and [CoCl₄]^2-. In the complex, the cobalt atom was coordinated by six nitrogen atoms from two ligands, and its coordination geometry at the cobalt center of [CoL₂]²⁺ can be described as distorted octahedron. In combination with ethylaluminum sesquichloride, these cobalt complexes displayed high catalytic activity and cis-1,4-selectivity towards 1,3-butadiene polymerization. The selectivity of the catalytic system can be tuned to predominantly 1,2-fashion via addition of PPh₃. The effects of reaction parameters and the ligand environment on the polymerization were also investigated.