Pyridine-oxazoline and quinoline-oxazoline ligated cobalt complexes: Synthesis, characterization, and 1,3-butadiene polymerization behaviors

A series of cobalt complexes supported by pyridine-oxazoline (Pyox) and quinoline-oxazoline (Quox) were synthesized. Determined by single crystal X-ray crystallography, complexes 6a and 7c adopted distorted octahedron and trigonal bipyramid geometries, respectively, while complex 6b existed as an ion pair comprised of [CoL₂]²⁺ and [CoCl₄]^2-, in which the cationic and anionic moieties displayed distorted octahedron and tetrahedral geometries, respectively. Upon activation with ethylaluminium sesquichloride (EASC), these cobalt complexes exhibited high catalytic activity and cis-1,4-selectivity toward 1,3-butadiene polymerization. The selectivity of the catalytic system could be switched from cis-1,4 to 1,2-fashion via the addition of PPh₃. The effects of ligand environment, polymerization temperature, and [Al]/[Co] ratio on the polymerization were investigated in detail.