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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.

The synthesis of alkoxotitanium(IV) and -zirconium(IV) complexes of seven chelating tetradentate di- or trianionic amine-phenolate ligands belonging to three families and their application in L-lactide polymerization are described. The isopropoxotitanium complexes were synthesized by a direct reaction between the ligand precursors and titanium tetraisopropoxide, whereas the zirconium complexes were synthesized by various routes. For titanium, complexes of all seven ligands could be synthesized. For zirconium, the hexacoordinate complexes derived from all dianionic ligands were synthesized; however, the only pentacoordinate complex that could be produced was the one derived from the bulky trianionic ligand. X-ray structures of zirconium complexes of the three families indicated a substantial pi donation from the alkoxo ligand to the metal. All complexes were found to be active lactide polymerization catalysts, and their activity was found to depend strongly on the metal, the coordination number around the metal, and the phenolate substituents but not on the ligand backbone.

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