Bifunctional Aluminum Catalysts for the Chemical Fixation of Carbon Dioxide into Cyclic Carbonates

Bifunctional aluminum complexes supported by novel zwitterionic NNO-donor scorpionate ligands were found to be efficient bifunctional catalysts for cyclic carbonate synthesis from terminal and internal epoxides in good yields and with broad substrate scope. Neutral scorpionate ligands (1-2) were designed and used as precursors to obtain two novel zwitterionic NNO-heteroscorpionate ligands (3-4). Reaction of 3 or 4 with [AlX3] (X = Me, Et) in a 1:1 or 1:2 molar ratio afforded the mononuclear and dinuclear cationic aluminum complexes [AlX2{kappa(2)-mbpzbdmape}]I-2 (X = Me (5), Et (6)), [AlX2{kappa(2)-mbpzbdmape}]I-2 (X = Me (7), Et (8)), [{AlX2(kappa(2)-mbpzbdmape)}(mu-O){AlX3}]I-2 (X = Me (9), Et (10)), and [{AlX2(kappa(2)-mbpzbdmape)}(mu-O){AlX3}]I-2 (X.= Me (11), Et (12)) with elimination of the corresponding alkane. These complexes were investigated as catalysts for cyclic carbonate formation from epoxides and carbon dioxide in the absence of a co-catalyst. Complex 7 was found to be the most active catalyst for cyclic carbonate formation from various epoxides and carbon dioxide.