Aminopropylimidazole and its zinc complex: CO2 chemistry and catalytic synthesis of cyclic carbonates†

Abstract

The biomimicry of carbonic anhydrase as the interaction between zinc and imidazole serves as an inspiration to engineer synthetic systems for CO₂ capture and utilization. In this research, we developed a zinc–aminopropylimidazole (Zn–api) complex to achieve CO₂ insertion and subsequently catalyze its cycloaddition reaction with a variety of epoxides. We investigated the complexation and carbamation reactions of both the unbound ligand and its zinc complex in aqueous and DMSO solutions. In D₂O, unbound api reacts with CO₂ through a 1 : 2 mechanism, forming ammonium carbamate. However, in DMSO-d₆, the reaction follows a 1 : 1 pathway and leads to carbamic acid. Interestingly, the Zn–api complex captures CO₂ differently depending on the solvent, forming an uncharged species (–NHCO₂–Zn) in water and a zwitterionic carbamate (–NH₂⁺Br⁻–CO₂–Zn–Im) in DMSO. To our knowledge, a first time zwitterionic carbamate coordination mode via CO₂ insertion is verified by in situ ATR-FTIR with a peak centered at 1704 cm⁻¹ and further supported by quantum chemical calculations. The latter complex exhibits excellent catalytic performance for cyclic carbonates synthesis, achieving 94% and 96% conversion for epichlorohydrin carbonate and glycidol carbonate, respectively, under ambient reaction conditions using a CO₂ balloon. Notably, it demonstrates remarkable stability over five consecutive catalytic cycles for the coupling of epichlorohydrin and CO₂ without a discernible decrease in activity.