Zirconium complexes of amine-bis(phenolate) ligands as catalysts for 1-hexene polymerization: Peripheral structural parameters strongly affect reactivity

Novel amine bis(phenolate) zirconium dibenzyl complexes were synthesized in quantitative yields from a versatile family of chelating amine-bis((2-hydroxyaryl)methyl) ligand precursors, their X-ray structures solved, and their reactivity in the polymerization of 1-hexene in the presence of B(C₆F₅)₃ studied. Several minor peripheral structural modifications were studied and found to have a major influence on the catalyst performance. Thus, a variety of reactivities, ranging from extremely high to negligible, were obtained, demonstrating a unique structure-reactivity relationship. This relationship is partially revealed from the crystal structures of the precatalysts, indicating similar [ONO] ligand cores in all structures solved. A correlation between the solid and the solution structures is obtained from ¹H NMR spectra, which reveal a rigid binding of the ligand to the metal. The solid structures are therefore proposed to serve as reliable references when studying structure-reactivity relationships. The most significant structural parameter was found to be the existence of an extra donor located on a pendant arm. [ONO]-type pentacoordinate complexes lacking such an additional donor are rapidly deactivated and lead only to traces of oligomers. On the other hand, hexacoordinate complexes based on [ONNO]-type ligands, in which strong donation of a side donor to the metal is obtained through formation of a five-membered chelate, lead to extremely reactive polymerization catalysts. The nitrogen hybridization and aromatic ring substituents have a more subtle effect on reactivity. Increasing the chelate size results in either no binding of the side donor, yielding negligible reactivity, or strong binding yet moderate polymerization reactivity. Increasing the steric bulk on the donor results in weakening of the metal-donor bond, leading to a moderate oligomerization catalyst. The sidearm nitrogen is therefore proposed to play a crucial role in determining the propagation process rate, as well as the propagation/termination rate ratio.