A series of aluminium complexes based on a β-diketiminate ligand: Synthesis, structures and their application to ring-opening polymerization of ε-caprolactone

Abstract

A series of aluminum complexes based on a β-diketiminate ligand HL (HL = N{-4-[(2,6-diisopropyl-phenylamino)pent-3-en-2-ylidene]-6-methyl}pyridin-2- amine) have been prepared and characterized spectroscopically and structurally. Reactions of the HL ligand with one equivalent of AlMe3, AlEt3 and AlEt2Cl, respectively, in toluene, gave complexes [LAlMe2] (1), [LAlEt2] (2) and [LAlEtCl] (3) in high yields. Complexes 1 and 2 contain four-coordinated mononuclear aluminum center. The aluminum center of 3 is five-coordinated. Complexes 1–3 were used to catalyze the ring-opening polymerization of ε-caprolactone. Complexes 1–3 are very rare cases of aluminum alkyl compounds supported by β-diketiminate ligands, which show moderate activity toward the ring-opening polymerization (ROP) of ε-caprolactone (CL) in the absence of benzyl alcohol. However, in the presence of benzyl alcohol, the polymerization rate is remarkably accelerated and the conversions are greatly improved. The monomer conversion and number average molecular weight of poly(ε-caprolactone) (PCL) catalyzed by 1–3 shows a linear relationship. This result indicates that the polymerizations mediated by 1–3 exhibited controllable manner.