Insight into the Mechanism of CO2 Chemical Fixation into Epoxides by Windmill-Shaped Polyoxovanadate and n-Bu4NX (X = Br, I).

Abstract

Carbon dioxide (CO₂) coupled with epoxide to generate cyclic carbonate stands out in carbon neutrality due to its 100% atom utilization. In this work, the mechanism of CO₂ cycloaddition with propylene oxide (PO) cocatalyzed by windmill-shaped polyoxovanadate, [(C₂N₂H₈)₄(CH₃O)₄V^IV₄V^V₄O₁₆]·4CH₃OH (V₈-1), and n-Bu₄NX (X = Br, I) was thoroughly investigated using density functional theory (DFT) calculations. The ring-opening, CO₂-insertion, and ring-closing steps of the process were extensively studied. Our work emphasizes the synergistic effect between V₈-1 and n-Bu₄NX (X = Br, I). Through the analysis of an independent gradient model based on Hirshfeld partition (IGMH), it was found that the attack of n-Bu₄NX (X = Br, I) on C_β of PO triggers a distinct attractive interaction between the active fragment and the surrounding framework, serving as the primary driving force for the ring opening of PO. Furthermore, the effect of different cocatalysts was explored, with n-Bu₄NI being more favorable than n-Bu₄NBr. Moreover, the role of V₈-1 in the CO₂ cycloaddition reaction was clarified as not only acting as Lewis acid active sites but also serving as "electron sponges". This work is expected to advance the development of novel catalysts for organic carbonate formation.