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, 27Al NMR spectra have demonstrated that they exist as both five-coordinate Al­(III) species in benzene-d6 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 °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.