Enolic Schiff-base aluminum complexes and their application in lactide polymerization

A series of NNOO-tetradentate enolic Schiff-base ligands were prepared where ligand L1 = bis(benzoylacetone)propane-1,2-diimine, L2 = bis(acetylacetone)-propane-1,2-diimine, L3 = bis-(acetylacetone)cyclohexane-1,2-diimine. Their further reaction with aluminum tris(ethyl) formed complexes LAlEt (1a, 2a and 3a). The solid structure of complexes 1a, 2a and 3a confirmed by X-ray single crystal analysis manifested that these complexes were all monomeric and five-coordinated with an aluminum atom in the center. The configurations of these complexes varied from trigonal bipyramidal geometry (tbp) to square pyramidal geometry (sqp) due to their different auxiliary ligand architectures. 1H NMR spectra indicated that all these complexes retained their configuration in solution states. Their catalytic properties to polymerize racemic-lactide (rac-LA) in the presence of 2-propanol were also studied. The diimine bridging parts as well as the diketone segment substituents had very close relationship with their performance upon the polymerization process. All these complexes gave moderately isotactic polylactides with controlled molecular weight and very narrow molecular weight distributions.