No abstract

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.

A series of iminopyridine ligated Co(II) (1a-7a) and Ni(II) (1b-7b) complexes were synthesized. The structures of complexes 3a, 4a, 5a, 7a, 5b, and 6b were determined by X-ray crystallographic analyses. Complex 3a formed a chloro-bridged dimer, whereas 4a, 5a, and 7a, having a substituent (4a, 5a: CH 3 ; 7a: Br) at the 6-position of pyridine, producing the solid structures with a single ligand coordinated to the central metal. The nickel atom in complex 5b features distorted trigonal-bipyramidal geometry with one THF molecule ligating to the metal center. All the complexes activated by ethylaluminum sesquichloride (EASC) were evaluated in 1,3-butadiene polymerization. The catalytic activity and selectivity were significantly influenced by the ligand structure and central metal. Comparing with the nickel complexes, the cobalt complexes exhibited higher catalytic activity and cis-1,4-selectivity. For both the cobalt and nickel complexes, the aldimine-based complexes showed higher catalyst activity than their ketimine counterparts.