A general post-synthetic modification approach of amino-tagged metal-organic frameworks to access efficient catalysts for the Knoevenagel condensation reaction

In this manuscript, four common transition-metal derived metal-organic frameworks have been extensively investigated as heterogeneous catalyst supports for Knoevenagel condensation reactions. A simple post-synthetic modification strategy was employed for the rapid and facile introduction of a primary alkyl amino group. The resulting novel MOF-RNH2 catalysts showed greatly enhanced Knoevenagel condensation reactivities towards a variety of aldehyde electrophiles. IRMOF-3 proved to be an unsuitable heterogeneous catalyst support due to its fragile nature upon treatment with bases. The novel zirconium based UiO-66-NH-RNH2 and chromium based Cr-MIL-101-NH-RNH2 materials showed excellent catalytic reactivities, while being highly convenient to synthesize. The basic catalytic activity was further extended to the Henry reaction, and excellent catalytic reactivity was achieved. The size-selectivity was also studied to show that the Knoevenagel condensation occurred inside of the porous structure of the MOF catalyst. The recycling properties of zirconium, aluminum and chromium derived MOFs were evaluated and zirconium based UiO-66 and chromium based Cr-MIL-101 showed excellent catalytic efficiency after five reaction cycles.