Mechanism for CO2 Fixation with Aziridines Synergistically Catalyzed by HKUST‑1 and TBAB: A DFT Study

It was revealed in recent experimental studies that metal–organic frameworks (MOFs) show very attractive activities for the catalytic CO2 cycloaddition with aziridines in cooperation with tetrabutylammonium bromide (TBAB). However, the mechanistic details and the selectivity origin remain largely uncertain, rendering great difficulty for a logical improvement of the existing catalysts and the rational design of new catalysts. In this work, the HKUST-1/TBAB-catalyzed CO2–aziridine cycloaddition reaction mechanism was comparatively explored with the noncatalyzed and HKUST-1-catalyzed cases at the density functional theory (DFT) level. Our calculations showed that the HKUST-1/TBAB-catalyzed cycloaddition proceeds through a three-step mechanism involving the ring opening of aziridine, CO2 insertion, and intramolecular cyclization. Additionally, CO2 cycloaddition is much easier for the HKUST-1/TBAB catalytic systems (26.1 kcal mol–1) than for the noncatalytic (44.3 kcal mol–1) and HKUST-1-catalytic (33.6 kcal mol–1) systems. The preferential cleavage of the substituted over the unsubstituted C–N bond of aziridine in the step of ring opening is the origin of the preferential formation of 5-substituted oxazolidinone as observed in experiment. Additionally, the energy barriers for the steps of ring opening and ring closure depend greatly on the nucleophilicity of the cocatalysts. Tetrabutylammonium chloride (TBAF) and tetrabutylammonium chloride (TBAC) were predicted to be efficient cocatalysts that can overcome the existing TBAB used in experimental reports. The proton attachment energy (PAE) of the halogen anion is correlated as the function of the energy barrier for the key steps. The in silico investigations should pave the way for designing more powerful catalytic materials for the chemical conversion of CO2 by aziridine.