Robust cobalt catalysts with N,N-bidentate aldimine imidazolidine-2-imine/guanidine ancillary ligand for isoprene polymerization

A new family of well-defined cobalt complexes (Co1–Co7) bearing N,N-bidentate chelate aldimine imidazolidine-2-imine/guanidine ancillary ligands were designed and successfully synthesized. All of the target metal complexes were structurally characterized by single-crystal X-ray diffraction analysis. It revealed that the Co atom adopted distorted tetrahedral coordination geometry and formed a unique six-membered chelate ring which exhibited structural distinctions with traditional N^N-based α-diimine and β-diimine Co complexes. In combination with a very low amount of Et₂AlCl (as low as 50 equiv.), these Co complexes exhibited high activity (up to 99 % yield within 6 hrs) and good thermal stability (up to 80 °C) in the coordination polymerizations of isoprene to afford cis-1,4/3,4-polyisoprenes with adjustable molecular weights M_n (25.9–138.4 kg/mol) and cis-1,4 contents (64.7–73.3 %). The different substituents on these Co catalysts also exerted influence on catalytic behaviors, chain microstructures, and polymer properties. This study represents the first example of imidazolidine-imine/guanidine analog ligated Cobalt complexes for the polymerization of isoprene.