Influence of the Counterion on the Synthesis of Cyclic Carbonates Catalyzed by Bifunctional Aluminum Complexes.

New bifunctional aluminum complexes have been prepared with the aim of studying the effect of a counterion on the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). Neutral ligand 1 was used as a precursor to obtain four novel mesylate, chloride, bromide, and iodide zwitterionic NNO ligands (2-5). The reaction of these ligands with 1 or 2 equiv of AlR3 (R = Me, Et) allowed the synthesis of mono- and bimetallic bifunctional aluminum complexes [AlR2(κ2-mbpzappe)]X [X = Cl, R = Me (6), Et (7); X = Br, R = Me (8), Et (9); X = I, R = Me (10), Et (11)] and [{AlR2(κ2-mbpzappe)}(μ-O){AlR3}]X [X = MeSO3, R = Me (12), Et (13); X = Cl, R = Me (14), Et (15); X = Br, R = Me (16), Et (17); X = I, R = Me (18), Et (19)] via alkane elimination. These complexes were studied as catalysts for the synthesis of cyclic carbonates from epoxides and CO2. Iodide complex 11 showed to be the most active catalyst for terminal epoxides, whereas bromide complex 9 was found to be the optimal catalyst when internal epoxides were used, showing the importance of the nucleophile cocatalyst on the catalytic activity.