Metal Hydrogen-Bonded Organic Frameworks with Synergistic Lewis Acid-Hydrogen-Bonding Dual Activation for Mild CO2 Cycloaddition with Epoxides.

Converting CO2 into value-added chemicals remains challenging due to its chemical inertness and the limited activity of conventional heterogeneous catalysts. Here, we report two novel metal-hydrogen-bonded organic frameworks (M-HOFs), Fe-HOF (1) and Ni-HOF (2), constructed from 2,2'-biimidazole (H2biim) and pyromellitic dianhydride (PMDA) via synergistic coordination and hydrogen bonds. These M-HOFs combine the flexibility of hydrogen-bonded frameworks with well-defined Lewis acidic metal sites. The hydrogen-bond networks finely tune the pore microenvironment, enhancing CO2 adsorption and providing a stable catalytic environment. Single-crystal X-ray diffraction confirms robust supramolecular structures and accessible active centers. Using tetrabutylammonium bromide (TBAB) as a cocatalyst, both M-HOFs efficiently catalyze CO2 cycloaddition with epoxides at 70 °C and 1 atm, achieving cyclic carbonates in up to 99% yield with broad substrate scope. Mechanistic studies, including DFT calculations, 1H NMR titration, and semi-in situ IR spectroscopy, reveal a Lewis acid-hydrogen-bond dual activation pathway, where framework N-H groups anchor and activate epoxides via hydrogen bonding. This cooperative dual activation, combining flexible hydrogen-bond networks with open metal Lewis acidic sites, significantly enhances CO2 conversion. These findings establish M-HOFs as a flexible, recyclable platform for sustainable CO2 utilization.