In Situ Engineering of Zwitterionic Betaine-Based Covalent Organic Frameworks Boost Cooperative CO₂ Catalytic...

The development of a privileged organocatalyst featuring well-defined catalytic sites to drive highly cooperative CO₂ conversion into high value-added fine chemicals is promising while challenging. Here in this work, three zwitterionic betaine-based covalent organic frameworks (COFs) bearing sulfonate, carboxylate, and phosphate moieties, namely, PS-TpBpy, BA-TpBpy, and PA-TpBpy, are in situ engineered through the post-synthetic quaternization of the bipyridine COF (TpBpy). Density functional theory-based Fukui function indicated that the pyridinium cation and the alkaline O anion in these betaine-based COFs serve as synergistic electrophilic and nucleophilic active sites, thus simultaneously activating o-phenylenediamine and CO₂ for their highly efficient conversion to benzimidazole. Meanwhile, such assembly endows the catalysts with enhanced stability, alkalinity, and hydrophobic microenvironment, facilitating the fast enrichment of phenylsilane and CO₂, and leading to largely enhanced reaction kinetic rates. In-depth mechanism investigations manifested that a cooperative double-catalytic cycle pathway is driven by the zwitterionic betaine-based COFs, which thermodynamically promote the carbamic acid formation and CO₂ reduction processes, thereby improving the overall catalytic efficiency. This work highlights the rational construction of unique zwitterionic betaine-based COFs as the bifunctional organocatalysts to boost the highly cooperative CO₂ catalytic conversion thermodynamically and kinetically.