Novel non-chelated cobalt(II) benzimidazole complex catalysts: Synthesis, crystal structures and cocatalyst effect in vinyl polymerization of norbornene

Abstract

The novel non-chelated monodentate benzimidazole (BI) complexes CoCl 2(BI) 2 ( 1)–( 3), where BI = 1-(2-methoxybenzyl)- 2-(2-methoxyphenyl)-1 H-benzimidazole ( 1), BI = 2-(2,6-difluorophenyl)-1 H-benzimidazole ( 2) and 2-methyl-1 H-benzimidazole ( 3) were synthesized and characterized by single X-ray crystallography. Unexpectedly, in solid state these complexes show similar coordination behavior to their analogue nickel(II) benzimidazole complexes such as inter-molecular H-bonding pattern and presence of acetonitrile solvent molecules per unit of complex molecule. Moreover, among these cobalt catalysts 1– 3, similar trend to that of nickel catalysts is observed for metal-to-nitrogen (M–X) coordination bond length and halogen–metal–halogen (X–M–X) bond angle. But unlike nickel(II) benzimidazole complexes, these catalysts show very low activity for vinyl polymerization of norbornene (NB) upon activation with methylaluminoxane (MAO); however, the activity abruptly increased in modified methylaluminoxane (MMAO). The presence of a small amount of toluene strongly hampered the activity, and the use of dry methylaluminoxane (dMAO) as a cocatalyst did not result in a high activity. The use of toluene-free solid modified methylaluminoxane (sMMAO) is found to be the best cocatalyst, where the highest activity of value 3.9 × 10 7 g of PNB mol Co −1 h −1 was achieved for 3/sMMAO at 30 °C.