Solvent-Free Coupling Reaction of Carbon Dioxide and Epoxides Catalyzed by Quaternary Ammonium Functionalized Schiff Base Metal Complexes under Mild Conditions

A series of bifunctional Schiff base metal catalysts (Zn-NPClR, Zn-NPXH, and M-NPClH) with two quaternary ammonium groups were prepared for carbon dioxide (CO2) and epoxide coupling reactions. The effects of the reaction variables on the catalytic activity were systematically investigated, and the optimal reaction conditions (120 degrees C, 1 MPa CO2, 3 h) were screened. The performances of different metal-centered catalysts were evaluated, and Co-NPClH showed excellent activity. This kind of bifunctional catalyst has a wide range of substrate applicability, excellent stability, and can be reused for more than five runs. A relatively high TOF could reach up to 1416 h(-1) with Zn-NPClH as catalyst by adjusting reaction factors. In addition, the kinetic study of the coupling reaction catalyzed by three catalysts (Zn, Co, and Ni) was carried out to obtain the activation energy (E-a) for the formation of cyclic carbonates. Finally, a possible mechanism for this cyclization reaction was proposed.

Automated summary

A series of bifunctional Schiff base metal catalysts were synthesized for the coupling reactions of carbon dioxide and epoxides. The reaction variables were studied, and the optimal reaction conditions were determined. The performance of different metal-centered catalysts was evaluated, and a bifunctional catalyst with excellent activity and stability was identified. The kinetic study and proposed mechanism for the cyclization reaction were also provided.