Defect engineering: an effective tool for enhancing the catalytic performance of copper-MOFs for the click reaction and the A³coupling

A series of Cu(i)-enriched metal-organic frameworks (MOF) of the type CuBTC (BTC = benzene-1,3,5-tricarboxylate) was prepared by a mixed-linker defect engineering technique, namely substituting a portion of a parent linker with truncated pyridine-3,5-dicarboxylate (PyDC) in the synthesis process. The reduced carboxyl coordination sites and the emerged Lewis basic pyridyl sites of PyDC spawned mixed-valence Cu(i)-Cu(ii) paddlewheels (PWs) in the defect-engineered CuBTC (DE-CuBTC) structure. Cu(i)-enriched DE-CuBTC shows significantly enhanced catalytic performance for the click reaction of azide-alkyne cycloaddition by accelerating the rate determining step of Cu(i)-acetylide intermediate formation. To further evaluate the catalytic activity of Cu(i)-enriched DE-CuBTC for reactions involving a Cu(i)-acetylide intermediate, the A³coupling reaction of phenylacetylene, paraformaldehyde and piperidine was studied as well. This study shows that defect engineering is an effective editing tool of catalytic active sites in catalysts.