Epoxide/CO Cycloaddition Reaction Catalyzed by Indium Chloride Complexes Supported by Constrained Inden Schiff-Base Ligands.

Cyclic carbonates have received significant interests for uses as reagents, solvents, and monomers. The coupling reaction of epoxides with carbon dioxide (CO ) to produce cyclic carbonate is an attractive route which can significantly reduce greenhouse gas emissions and environmental hazards. Herein, a series of five indium chloride complexes supported by inden Schiff-base ligands were reported along with four X-ray crystal structures. The constrained five-membered rings were added to the ligands to enhance the coordination of epoxides to the In metal. From the catalyst screening, In inden complex having tert-butyl substituents and propylene backbone in combination with tetrabutylammonium bromide (TBAB) exhibited the highest catalytic activity (TON up to 1017) for propylene oxide/CO coupling reaction with >99 % selectivity for cyclic carbonate under solvent-free conditions. In addition, the catalyst was shown to be active at atmospheric pressure of CO at room temperature. The catalyst system can be applied to various internal and terminal epoxide substrates to exclusively produce the corresponding cyclic carbonates.