Synthesis and structural characterization of monomeric aluminum keto-imine complexes: Catalytic activity toward CO2/styrene oxide coupling and ε-caprolactone ring opening polymerization

A series of benzoylacetone-imine ligands, PhCOCHCMeNHR ((LH)-H-1, R = CH2Ph; (LH)-H-2, R = CH2Ph-4-Me; and (LH)-H-3, R = CH2Ph-4-OMe), are first synthesized following the condensation of benzoylacetone and 4-substituted benzylamine. Later, reacting (LH)-H-1-(LH)-H-3 with one equivalent of AlMe3 in toluene, the monomeric Al derivatives are generated, (PhCOCHCMeNR)AlMe2 (1, R = CH2Ph; 2, R = CH2Ph-4-Me; 3, R = CH2Ph-4-OMe). All the ligands and metal derivatives are characterized by H-1 and C-13 NMR spectroscopy. Single crystal X-ray diffraction analysis reveals the geometry of all ligands and complex 2. As supporting catalysts, the Al derivatives, 1-3, are effective to initiate the coupling of CO2 and styrene oxide to form cyclic carbonates. Although employing TBAI (tetra-n-butylammonium iodide), TBAC, tetrabutylammonium chloride, and TBAB (tetra butyl ammonium bromide) as cocatalyst, the optimized catalytic system exhibits the best conversion of epoxide in presence of TBAI under the reaction temperature and time, respectively, 90 degrees C and 6 hr, respectively. The ring opening polymerization of epsilon-caprolactone is also appraised subjecting the Al derivatives, 1-3, as catalysts in presence of benzyl alcohol and all the conversions of epsilon-caprolactone reach over 95% at the reaction condition of 100 degrees C and 1 hr.

Automated summary

A series of benzoylacetone-imine ligands were synthesized by condensation reaction. Monomeric Al derivatives were obtained through the reaction with AlMe3. The ligands and metal derivatives were characterized by spectroscopy. The Al derivatives showed effectiveness as catalysts in the coupling reaction of CO2 and styrene oxide, as well as the ring opening polymerization of epsilon-caprolactone.