Epoxide/CO₂ Cycloaddition Reaction Catalyzed by Indium Chloride Complexes Supported by Constrained Inden Schiff-Base...

Cyclic carbonates have received significant interests for uses as reagents, solvents, and monomers. The coupling reaction of epoxides with carbon dioxide (CO₂ ) 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/CO₂ 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 CO₂ at room temperature. The catalyst system can be applied to various internal and terminal epoxide substrates to exclusively produce the corresponding cyclic carbonates.