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.近期实验研究表明，金属有机框架（metal–organic frameworks, MOFs）与四丁基溴化铵（tetrabutylammonium bromide, TBAB）协同催化二氧化碳与氮丙啶的环加成反应时，展现出优异的催化活性。然而，该反应的机理细节与选择性起源仍尚未明确，这为现有催化剂的理性优化与新型催化剂的合理设计带来了极大挑战。本研究基于密度泛函理论（density functional theory, DFT），对比探究了HKUST-1/TBAB催化的二氧化碳-氮丙啶环加成反应机理，并与非催化体系及纯HKUST-1催化体系进行了对照分析。计算结果显示，HKUST-1/TBAB催化的环加成反应遵循三步机理，依次为氮丙啶开环、二氧化碳插入以及分子内环化。此外，HKUST-1/TBAB催化体系下的二氧化碳环加成反应能垒为26.1 kcal mol⁻¹，远低于非催化体系的44.3 kcal mol⁻¹与纯HKUST-1催化体系的33.6 kcal mol⁻¹。在开环步骤中，氮丙啶取代位点的C-N键优先于未取代位点的C-N键发生断裂，这正是实验中观测到的5-取代恶唑烷酮优先生成的根本原因。此外，开环与闭环步骤的能垒极大程度上取决于助催化剂的亲核性。四丁基氟化铵（tetrabutylammonium fluoride, TBAF）与四丁基氯化铵（tetrabutylammonium chloride, TBAC）被预测为高效助催化剂，可替代现有实验报道中使用的TBAB。卤离子的质子亲和能（proton attachment energy, PAE）与关键步骤的反应能垒存在关联。本计算模拟研究将为设计更高效的催化材料、实现氮丙啶介导的二氧化碳化学转化铺平道路。