β-Diketiminate aluminium complexes: synthesis, characterization and ring-opening polymerization of cyclic esters

Abstract

A series of aluminium alkyl complexes (BDI)AlEt₂ (3a–m) bearing symmetrical or unsymmetrical β-diketiminate ligand (BDI) frameworks were obtained from the reaction of triethyl aluminium and the corresponding β-diketimine. The monomeric structure of the aluminium complex 3k was confirmed by an X-ray diffraction study, which shows that the aluminium center is coordinated by both of the nitrogen donors of the chelating diketiminate ligand and the two ethyl groups in a distorted tetrahedral geometry. Attempt to synthesize β-diketiminate aluminium alkoxide complexes by the reactions of monochloride complex “(BDI-2a)AlMeCl” (4) with alkali salts of 2-propanol gave unexpectedly an aluminoxane [(BDI-2a)AlMe]₂(μ-O) (7) as characterized by X-ray diffraction methods. Complexes 3a–m and [(2,6-ⁱPr₂C₆H₃NCMe)₂HC]AlEt₂ (8) were found to catalyze the ring-opening polymerization (ROP) of ε-caprolactone with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-donating substituents at the para-positions of the aryl rings in the ligand resulted in an apparent decrease in catalytic activity. Complex 3h showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the meta-trifluoromethyl substituents on the aryl rings. The increase of steric hindrance of the ligand by introducing ortho-substituents onto the phenyl moieties also resulted in a decrease in the catalytic activity. Although the viscosity average molecular weights (M_η) of the obtained poly(caprolactone)s increased with the enhancement of monomer conversion, the ROPs of ε-caprolactone initiated by complexes 3a–m and 8 were not well-controlled, as judged from the broad molecular weight distributions (PDI = 1.66–3.74, M_w/M_n) of the obtained polymers and the nonlinear relationship of molecular weight versus monomer conversion.