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{κ2-mbpzbdmape}]I2 (X = Me (5), Et (6)), [AlX2{κ2-mbpzbdeape}]I2 (X = Me (7), Et (8)), [{AlX2(κ2-mbpzbdmape)}(μ-O){AlX3}]I2 (X = Me (9), Et (10)), and [{AlX2(κ2-mbpzbdeape)}(μ-O){AlX3}]I2 (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.