Synthesis of mixed-ligand cobalt complexes and their applications in high cis-1,4-selective butadiene polymerization

Abstract

Incomplete oxidation of (N-di-tert-butylphosphino)-6-(2-methyl-2′H-benzoimidazole)-2-aminepyridine dichlorocobalt (PN3CoCl2) in DMF results in a unique co-crystal I formed with three parts including DMF, unit A and unit B complex with Co1 and Co2, respectively (PN3 ligand in unit A: (N-di-tert-butylphosphino)-6-(2′-methyl-2′H-benzoimidazole)-2-aminepyridine, and O=PN3 ligand in unit B: (N-di-tert-butylphosphinoxide)-6-(2′-methyl-2′H-benzoimidazole)-2-aminepyridine), with 1:1:1 molar ratio. Co1 and Co2 complexes both display a five-coordinated distorted-square-pyramidal geometry around the metal center. The Co1 center is coordinated with PN3 ligand via two N atoms from pyridine, benzoimidazole moiety as well as one P atom, and the Co2 center is coordinated with the oxidized ligand O=PN3 via two N atoms from pyridine, benzoimidazole moiety as well as one O atom from DMF molecule, while the oxidized phosphine moiety (O=P) being excluded from the coordination sphere. Activated with AlEt2Cl, the co-crystallized complexes I are able to actively convert butadiene to polybutadiene, affording cis-1,4 polybutadiene with cis-1,4 unit up to 95.5-97.8% and number average molecular weight of cal. 105 g/mol. The high cis-1,4 selectivity and monomodal GPC curve of resultant polymer imply that the identical active species generated from two distinctive cobalt centers.