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-hyd roxyaryl)methyl) ligand precursors, their X-ray structures solved, and thei r reactivity in the polymerization of l-hexene in the presence of B(C6F5)(3 ) studied. Several minor peripheral structural modifications were studied a nd found to have a major influence on the catalyst performance. Thus, a var iety of reactivities, ranging from extremely high to negligible, were obtai ned, demonstrating a unique structure-reactivity relationship. This relatio nship is partially revealed from the crystal structures of the precatalysts , indicating similar [ONO] ligand cores in all structures solved. A correla tion between the solid and the solution structures is obtained from H-1 NMR spectra, which reveal a rigid binding of the ligand to the metal. The soli d structures are therefore proposed to serve as reliable references when st udying structure-reactivity relationships. The most significant structural parameter was found to be the existence of an extra donor located on a pend ant arm. [ONO]-type pentacoordinate complexes lacking such an additional do nor are rapidly deactivated and lead only to traces of oligomers. On the ot her hand, hexacoordinate complexes based on [ONNO]-type ligands, in which s trong donation of a side donor to the metal is obtained through formation o f a five-membered chelate, lead to extremely reactive polymerization cataly sts. 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 bin ding yet moderate polymerization reactivity. Increasing the steric bulk on the donor results in weakening of the metal-donor bond, leading to a modera te 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.