Pyridine bis(imino) iron and cobalt complexes for ethylene polymerization: influence of the aryl imino substituents

In the present paper, the synthesis of new pyridine bis(imine) ligands modified with halogens (Cl, Br, CF3) or alkyl groups (Heptyl, tert-butyl, Phenyl, …) is reported. When coordinated with iron or cobalt dichloride, they yielded complexes which were associated to methylaluminoxane (MAO) to achieve the polymerization of ethylene. It was shown that cobalt catalysts are generally more sensitive to the ligand substitutions than the iron ones. The addition of a chlorine atom on the ligand frame is generally unfavorable. On the contrary, the presence of a bromine atom seems more favorable. Phenyl rings lead to almost completely inactive catalysts, probably because of a too weak coordination to the metal. It was also demonstrated that a mono-substitution of the aryl groups with an electron-withdrawing group (-CF3) is sufficient to yield polymers, whereas, considering the bulkiness of this substituent only, oligomers would have been expected.