Synthesis and structural characterization of aluminum complexes supported by NNO-tridentate ketiminate ligands: Efficient catalysts for ring-opening polymerization of l-lactide

Four NNO-tridentate ketiminate derivatives (HL-H, FL-H, OMeL-H and tBuL-H) were prepared through the condensation reaction of 1-(4-chlorophenyl)-4-(4-X-benzoyl)-3-methyl-1H-pyrazol-5(4H)-one (X = H, F, OMe, tBu) with N,N-dimethylenediamine (1.1 molar equiv) under reflux condition. Further reaction of AlMe3 (1.2 molar equiv) with HL-H, FL-H, OMeL-H and tBuL-H, respectively, affords penta-coordinated mono-adduct aluminum complexes [(HL)AlMe2] (1), [(FL)AlMe2] (2), [(OMeL)AlMe2] (3), and [(tBuL)AlMe2] (4) in high yield. Experimental results indicate complexes 1–4 are active catalysts for ring-opening polymerization of l-lactide (l-LA) in the presence of benzyl alcohol (BnOH). Al complex 2 catalyzes efficiently not only in a “living” fashion but also an “immortal” manner, giving polymers with the expected molecular weights and narrow PDIs.