Integrated experimental and theoretical insights into CO2 fixation: tetraazamacrocyclic catalysts in ionic liquids for cyclic carbonate formation

Abstract

The electrochemical cycloaddition of carbon dioxide to epoxides was investigated using tetraazamacrocyclic metal complexes as electrocatalysts in ionic liquids under mild conditions. The process was carried out in the absence of additional organic solvents, employing Ni(cyclam)Cl₂ and Co(cyclam)Cl₂Cl as catalysts, which facilitated the activation of CO₂. The electrosynthesis was conducted in 1-butyl-3-methylimidazolium-based ionic liquids, which not only acted as solvents but also played a crucial role in promoting epoxide ring opening and stabilizing reaction intermediates. Electrochemical experiments using propylene oxide, styrene oxide, and epichlorohydrin demonstrated that the nature of the epoxide substituent significantly impacts the formation of cyclic carbonates. The highest yields were obtained when BMImBr was used as the reaction medium, while other ionic liquids such as BMImBF₄ and BMImTFSI resulted in negligible conversion. Spectroelectrochemical studies provided additional insights into the reaction mechanism, confirming the role of halide anions in facilitating carbonate formation. Furthermore, density functional theory (DFT) calculations were performed to explore the interaction between Ni(cyclam) complexes and CO₂. Theoretical results indicate that the trans-I isomer of [Ni(cyclam)]⁺ favors CO₂ coordination and activation, which aligns with the experimental findings. Computational analysis also supported the importance of ionic liquid composition in stabilizing key reaction intermediates. This study highlights the potential of electrocatalytic methodologies for the sustainable conversion of CO₂ into high-value chemicals, contributing to the development of greener and more efficient synthetic strategies.