Efficient ionic functionalization of metal-organic frameworks for efficient addition of carbon dioxide to epoxides

The use of homogeneous and polluting halides as nucleophilic cocatalysts lowers the economy and benignity of the CO₂-epoxide cycloaddition over heterogeneous Lewis-acid catalysts like metal-organic frameworks (MOFs). Creating net positive charge at frameworks is a reasonable strategy to heterogenize halide anions within MOFs, but it is still challenging to achieve high halide loading while keeping adequate porosity. In this article, we report a facile approach to ionic MOFs with high halide content and improved catalytic activity. The catalysts were prepared by reacting azide-tagged MIL-101 with alkyne-tagged organic bromide salts. The efficient azide–alkyne click reaction within the mesoporous MOF allows quantitative cationization of the linkers, and the resultant bifunctional ionic MOFs contain one halide ion per metal center to the benefit of the cooperative catalysis. Catalytic studies suggest that the effects of varying the cationic groups and halide anions in the confined ionic space are different from those observed for homogeneous organic halides. The highest catalytic activity was demonstrated for the MOF having the smallest cationic group (quaternized trimethyl ammonium) and the intermediately nucleophilic halide (Br⁻). The catalyst also outperforms the ionic MOF catalysts reported prior to this work.