Tethered Zn(II) Porphyrin-Quaternary Ammonium Catalysts for Efficient CO₂ Cycloaddition with Epoxides under Atmospheric Pressure

A series of tethered metalloporphyrin–quaternary ammonium bifunctional catalysts were designed and synthesized for the cycloaddition of CO₂ with epoxides under solvent-free conditions. In these catalysts, the Zn porphyrin center acts as a Lewis acidic site for epoxide activation, while the pendant quaternary ammonium bromide unit provides a proximal nucleophilic co-catalytic site and helps enrich CO₂ in the local reaction environment. Systematic variation of the metal center, halide anion, tertiary amine fragment, and linker length identified Zn-TPP-QA-1 as the optimal catalyst. Under the optimized conditions, 1.0 mmol of epoxide afforded 0.97 mmol of cyclic carbonate at 1 atm CO₂ using 0.02 mol% catalyst, indicating efficient CO₂ conversion. The catalyst could be readily recovered after reaction and reused without significant loss of activity. Kinetic analysis showed that the tethered bifunctional catalyst exhibited a lower apparent activation energy than ZnTPP alone and the untethered ZnTPP/TBAB system, supporting a proximity-enhanced cooperative effect. This work provides a simple molecular design strategy for efficient CO₂ fixation through integrated Lewis acid/nucleophile catalysis. This catalytic strategy contributes to the ongoing development of efficient molecular approaches for sustainable CO₂ valorization, addressing the need for mild and practical fixation methods.