Fluoride-Enabled Assembly of Iron-Hydrogen-Bonded Frameworks as Bifunctional Lewis Acid-Base Catalysts for Knoevenagel Condensation.

Abstract

The development of recyclable heterogeneous catalysts that enable efficient organic transformations without solvents is important for sustainable chemical synthesis. In this work, two new fluoride-mediated metal-hydrogen-bonded organic frameworks (M-HOFs), labeled as JOU-40 and JOU-41, were successfully synthesized. Single-crystal X-ray diffraction reveals that both M-HOFs feature [Fe3F11]n chains that are extended into three-dimensional supramolecular networks through O-H···F hydrogen bonds. The resulting frameworks integrate Lewis-acidic iron centers and basic fluoride sites within a confined porous environment, enabling efficient Lewis acid-base catalysis. As heterogeneous catalysts, JOU-40 and JOU-41 efficiently promote the solvent-free Knoevenagel condensation of benzaldehyde with malononitrile, affording >99.00% conversion and selectivity. The catalyst also exhibits good substrate compatibility across a range of substituted aldehydes. More fascinatingly, kinetic analyses reveal that the reaction can be described by the Avrami nucleation-growth model, highlighting the coupled contributions of chemical conversion and product crystallization. Density functional theory (DFT) calculations demonstrate that the adjacent Fe3+ and F- sites activate the two reactants and facilitate C═C bond formation, providing a molecular-level basis for the observed catalytic activity. This work offers new design insights for developing highly efficient, stable, and recyclable catalytic systems for solvent-free Knoevenagel condensation.

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