Cyclic Carbonates from CO2 and Epoxides Catalyzed by Tetra- and Pentacoordinate Amidinate Aluminum Complexes

A series of mono-, bi-, and trimetallic alkylaluminum complexes ((L1)AlMe2 (1), (L1)Al2Me5 (2), (L2)AlMe2 (3), (L2)Al2Me5 (4), (L2)2AlMe (5), (L2)2Al3Me7 (6)) supported by amidine ligand precursors ((E)-N-(2,6-diisopropylphenyl)-N′-(pyridin-2-yl)acetimidamide (L1H) and (E)-N-(2,6-diisopropylphenyl)-N′-(pyridin-3-yl)acetimidamide (L2H)) were obtained in excellent yields and characterized by spectroscopic and X-ray diffraction methods. These complexes were studied as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide in the presence of tetrabutylammonium iodide as cocatalyst. The reactions were carried out at 50 °C and 1 bar CO2 pressure in the absence of solvent, and we were able to achieve an extensive variety of terminal epoxides with moderate to excellent yields and high selectivities using aluminum complexes 2 and 6.