Post-synthetic ligand exchange in zirconium-based metal-organic frameworks: beware of the defects!

Post-synthetic ligand exchange is a valuable synthetic tool for functionalisation of zirconium-based metal-organic frameworks. It is generally assumed that the exchange involves organic linkers, however these materials are known to contain large amounts of defects, which are likely to play a role during the exchange process. This can eventually lead to unexpected effects on the physical-chemical properties of the material. Post-synthetic ligand exchange in the prototypical zirconium-based metal-organic framework UiO-66 was investigated by in situ solution nuclear magnetic resonance. Samples of UiO-66 prepared either with or without modulators (formic acid or benzoic acid) and having different degrees of defectivity were exchanged using solutions of 2,5-pyridinedicarboxylic acid at different temperatures. Linker exchange only occurred in a defect-free UiO-66, whereas monocarboxylates grafted at defective 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 2,5-pyridinedicarboxylic acid and modulator was observed, providing evidence that the defects had missing-cluster nature. These findings were corroborated by ex situ characterisation of exchanged samples.