Dinuclear Aluminum Complexes as Catalysts for Cycloaddition of CO2 to Epoxides

The synthesis and characterization of dinuclear aluminum complexes bearing multidentate aliphatic aminoethanol-based ligands are presented. Single-crystal X-ray analyses, NMR data, and mass data reveal that four aluminum complexes synthesized are all dimeric in the solid, solution, and gas states. Especially, Al-27 NMR spectra have demonstrated that they exist as both five-coordinate Al(III) species in benzene-d(6) solution. All aluminum complexes are effective catalysts for the cycloaddition of CO2 to propylene oxide in the presence of n-Bu4NI as a cocatalyst. Complexes 1 and 3, which have two methyl groups per aluminum center, are better catalytic systems than the corresponding complexes 2 and 4 with a mono methyl group per aluminum. In addition, complexes 3 and 4 containing a pendant -CH2CH2NMe2 group attached to nitrogen showed the higher activity than those 1 and 2 with a pendant -CH2CH2OMe group did. As expected, the catalytic activity for 3 increases as the reaction temperature increases up to 130 degrees C. In addition, compound 3 showed the highest activity for the cycloaddition of CO2 with propylene oxide in the presence of tetrabutylphosphonium bromide as a cocatalyst.