Synthesis of New Bis(amidine)–Cobalt Catalysts and Their Application to Styrene Polymerization

Two new bis­(amidine) cobalt­(II) complexes were rationally designed and efficiently synthesized. The first synthesized cobalt complex was based on a cyclic bis­(amidine) ligand with chiral vicinal diphenyl groups. The structure was verified by single-crystal X-ray crystallography. As expected from the ligand structure, the Co–N bonds in this complex were significantly shorter than those found in the corresponding 1,2-diimine cobalt complex. This indicates that the bis­(amidine) ligand has better electron-donating capability than the 1,2-diimine ligand. Upon test polymerization of styrene, the complex exhibited a moderate activity of 5.88 × 105 g PS/(mol Co h). On the basis of this encouraging result, a new 1,2-diaminobenzene-derived bis­(amidine) ligand was efficiently synthesized and used to make the corresponding cobalt­(II) complex. When subjected to styrene polymerization, the resulting complex showed unusually high polymerization activity [164 × 105 g PS/(mol Co h)] and high conversion (>99%). The resulting polymer was identified as atactic polystyrene by 13C NMR spectroscopy analysis. The significantly enhanced styrene polymerization activity of the 1,2-diaminobenzene-originated bis­(amidine) cobalt­(II) complex is attributed to the improved electron-donating capability of the ligand.