Asymmetric bis(formamidinate) group 4 complexes: Synthesis, structure and their reactivity in the polymerization of α-olefins.

A series of asymmetric formamidine ligands bearing different substituents with various steric and electronic properties on the nitrogen of the N-C-N motif were synthesized. Group 4 bis(formamidinate) dimethylamido, chloride, and benzyl complexes were studied using these asymmetric ligands and their solid-state structures and their behavior in solution were determined. These complexes were activated with MAO (methylalumoxane) or a combination of cocatalysts and tested in the polymerization of ethylene and propylene. A noticeable influence of the formamidine nitrogen substituents on the activity of the catalyst and properties of the obtained polymers was observed. Further, a plausible mechanism for the polymerization of propylene is presented derived from a combination of ESR-C₆₀ and MALDI-TOF trapping experiments which shed light on the nature of the active catalytic species.