Double Cationization Approach toward Ionic Metal-Organic Frameworks with a High Bromide Content for CO₂Cycloaddition to Epoxides

Solid catalysts for CO2 cycloaddition to epoxides often require homogeneous halide anions in large amounts, which sacrifice the heterogeneity and ecofriendliness. Here, we demonstrate a novel approach to ionic metal-organic frameworks (IMOFs) with a high bromide content, which utilizes two convenient postsynthetic "click"reactions (alkyne-azide cycloaddition and triazole N-alkylation) to install both pyridinium and triazolium groups and thus to include more bromides. The catalytic performance of various neutral and ionic MIL-101-based MOFs with different bromide-to-metal ratios and functional groups was compared. The doubly cationized IMOF with the high bromide-to-metal ratio can efficiently catalyze the cycloaddition reaction under relatively mild conditions with good recyclability. We also demonstrated the catalytic activity of the triazole group in the absence of nucleophilic anions. The results can not only provide a new approach to highly active and fully heterogeneous catalysts for the carbon fixation reaction but can also have important implications for development of IMOFs for other applications.