Insights into Thiourea-Based Bifunctional Catalysts for Efficient Conversion of CO₂ to Cyclic Carbonates

The bifunctional thiourea catalyst system with both electrophilic and nucleophilic centers has been certified to be effective for fixing CO₂ under mild reaction conditions; however, many questions remain, especially concerning the relationship between structure and performance. Herein, we systematically studied a series of such bifunctional catalysts with different chain lengths, nucleophilic anions, and substituents, which impact obvious influence on the catalytic performance. The activation energies of catalysts with different chain lengths are calculated via in situ IR. On this basis, we disclosed for the first time that the spacer length of tetramethylene -(CH₂)₆- is the optimal spatial effect for the coupling of epoxides and CO₂. Particularly, the single crystal X-ray diffraction analysis of the molecular structures of the bifunctional catalyst C8 indicated the discovery of the existence of interaction force between the sulfur atom on the thiourea group and one hydrogen atom on the benzene ring, as well as the intermolecular hydrogen bonding interaction of the bromide (Br^-) and two NH groups on the thiourea group. The catalyst structure performance, direct observation of the crystal structure, the thermodynamic study, and a wide range of substrates (12 examples) should be informative on the optimization of the existing catalysts or the design of new catalysts in the future.