The combination of a chromium­(III) salophen bromide complex and tetrabutylammonium bromide is shown to catalyze the reaction between terminal epoxides and carbon dioxide at ambient temperature and 1 bar carbon dioxide pressure and between internal epoxides and carbon dioxide at 80 °C and 10 bar carbon dioxide pressure to form cyclic carbonates. The optimal conditions involve the use of 1.5–2.5 mol % of both the chromium­(III) salophen bromide complex and tetrabutylammonium bromide and result in the formation of cyclic carbonates in 57–92% isolated yields after a reaction time of 24 h. Under these conditions, no polycarbonate formation is observed except when cyclohexene oxide is used as substrate. The reactions were found to proceed with retention of epoxide stereochemistry. A study of the reaction kinetics revealed that the chromium­(III) complex and tetrabutylammonium bromide react together to form a six-coordinate anionic chromium complex which is the actual catalyst, and a catalytic cycle is proposed which explains the experimentally observed results.

The mechanism by which [Al(salen)]2 O complexes catalyse the synthesis of cyclic carbonates from epoxides and carbon dioxide in the absence of a halide cocatalyst has been investigated. Density functional theory (DFT) studies, mass spectrometry and (1) H NMR, (13) C NMR and infrared spectroscopies provide evidence for the formation of an unprecedented carbonato bridged bimetallic aluminium complex which is shown to be a key intermediate for the halide-free synthesis of cyclic carbonates from epoxides and carbon dioxide. Deuterated and enantiomerically-pure epoxides were used to study the reaction pathway. Based on the experimental and theoretical results, a catalytic cycle is proposed.

Chromium and aluminium salphen complexes have been found to display remarkable catalytic activity in the synthesis of cyclic carbonates from a range of epoxides and carbon dioxide. The Al(salphen) complex is more reactive towards terminal epoxides at ambient temperature and pressure, whereas the Cr(salphen) complex exhibits higher catalytic activity towards more challenging internal epoxides at elevated temperature and pressure.

New neutral and zwitterionic chiral NNO‐donor scorpionate ligands 1 and 2 were designed to obtain new mononuclear and dinuclear NNO‐heteroscorpionate aluminum complexes. Reaction of 1 with [AlR3] (R=Me, Et) in a 1:1 or 1:2 molar ratio afforded the neutral mononuclear alkyl complexes [AlR2(κ2‐bpzappe)] {R=Me (3), Et (4); bpzappeH=2,2‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1‐[4‐(dimethylamino)phenyl]‐1‐phenylethanol} and bimetallic complexes [{AlR2(κ2‐bpzappe)}(μ‐O){AlR3}] [R=Me (5), Et (6)]. By reaction of complexes 3–6 with PhCH2Br, mononuclear and dinuclear cationic aluminum complexes [AlR2(κ2‐bbpzappe)]Br {R=Me (7), Et (8); bbpzappeH=N‐benzyl‐4‐[2,2‐bis(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)‐1‐hydroxy‐1‐phenylethyl]‐N,N‐dimethylbenzenaminium bromide} and [{AlR2(κ2‐bbpzappe)}(μ‐O){AlR3}]Br [R=Me (9), Et (10)] were synthesized. Both neutral aluminum complexes in the presence of Bu4NBr and cationic aluminum complexes were investigated as catalysts for cyclic carbonate formation from epoxides and carbon dioxide. Amongst them, complex 10 was found to be an efficient one‐component catalyst for the synthesis of cyclic carbonates from both monosubstituted and internal epoxides and was shown to have broad substrate scope.

Kinetic studies of the synthesis of glycerol carbonate from glycidol and carbon dioxide have been carried out. These showed that under suitable reaction conditions, bimetallic aluminium(salen) complex 4 is able to catalyse the conversion of epoxides into the corresponding cyclic carbonates without the need for a co-catalyst.