Post‐Synthetic Ligand Exchange in Zirconium‐Based Metal–Organic Frameworks: Beware of The Defects!

Post-synthetic ligand exchange in the prototypical zirconium-based metal-organic framework (MOF) UiO-66 was investigated by in situ solution 1 H NMR spectroscopy. Samples of UiO-66 having different degrees of defectivity were exchanged using solutions of several terephthalic acid analogues in a range of conditions. Linker exchange only occurred in defect-free UiO-66, whereas monocarboxylates grafted at defect sites were found to be preferentially exchanged with respect to terephthalic acid over the whole range of conditions investigated. A 1:1 exchange ratio between the terephthalic acid analogue and modulator was observed, providing evidence that the defects had missing-cluster nature. Ex situ characterisation of the MOF powders after exchange corroborated these findings and showed that the physical-chemical properties of the MOF depend on whether the functionalisation occurs at defective sites or on the framework.