Synthesis of bis(N-arylcarboximidoylchloride)pyridine cobalt(II) complexes and their catalytic behavior for 1,3-butadiene polymerization.

A new family of bis(N-arylcarboximidoylchloride)pyridine cobalt(II) complexes with the general formula [2,6-(ArN=CCl)2C5H3N]CoCl2 (Ar = 2,4,6-Me3C6H2, 4a; 2,6-(i)Pr2C6H3, 4b; 2,6-Me2C6H3, 4c; C6H5, 4d; 4-Cl-2,6-Me2C6H2, 4e) and a typical Brookhart-Gibson-type reference complex [2,6-(2,4,6-Me3C6H2N=CMe)2C5H3N]CoCl2 (5a) were synthesized and characterized. Determined by X-ray crystallographic analysis, complexes 4a, 4c-e, and 5a adopted a trigonal bipyramidal configuration, and 4b adopted a distorted square pyramidal geometry. In combination with ethylaluminum sesquichloride (EASC), all the complexes were highly active towards 1,3-butadiene polymerization, affording polybutadiene with predominant cis-1,4 content (up to 96%). 4a with chlorine atoms at the imine groups exhibited higher catalytic activity than did 5a, indicating that the incorporation of chlorine atoms into the ligand improves the activity. The activity of the complexes in 1,3-butadiene polymerization was in the order of 4a > 4c ∼ 4e ∼ 4b > 4d, which is consistent with the trend of spatial opening degree around the metal center in the complexes as revealed by crystallographic data. Screening polymerization conditions proved that EASC was the most efficient among the cocatalysts examined.