The oxamate route, a versatile post-functionalization for metal incorporation in MIL-101(Cr): Catalytic applications of Cu, Pd, and Au

A new consecutive post-functionalization method has been developed for the inclusion of additional metal functionalities in Metal Organic Frameworks (MOFs) through oxamate as chelating agent. This may result in catalytic centers of metal-organic complexes or in controlled formation of metal nanoparticles, demonstrated for Cu, Pd and Au, in the highly stable MIL-101(Cr) framework. In a first post-synthesis step, reduction of the NO₂-MIL-101(Cr) leads to the formation of NH₂-MIL-101(Cr). The second functionalization consists of a straightforward condensation of the amino groups of the ligand with ethyl chloro-oxoacetate resulting in the formation of free oxamates attached to the linker. Two different copper complexes have been coordinated through the oxamate ligand, attaining loadings as high as 9 wt.% and showing similar performance as the best reported copper MOFs (HKUST-1, [Cu(2-pymo) ₂] and [Cu(im)₂]) in "click chemistry" catalysis (1,3-dipolar cycloaddition reactions) and multicomponent coupling reactions (propargylamine formation). Highly dispersed palladium and gold nanoparticles can be incorporated and stabilized in the MOF. The resultant Pd-catalysts are very active and selective in Suzuki-Miyaura condensation, although palladium leaching is observed. After reduction of the Au-catalysts, immobilized gold nanoparticles are formed, which maintain a good performance in BA oxidation to benzaldehyde for at least 4 consecutive runs. No side reactions (further oxidation and esterification) are observed, which are attributed to the absence of any cationic gold species. TEM analysis and DRIFTS analysis during CO adsorption show the very uniform size (∼2 nm) and metallic character of these gold particles encapsulated in the MIL-101 cages.