<p>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
<i>in situ</i> 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 <i>ex situ</i> characterisation of exchanged samples.</p>
Post-synthetic ligand exchange in zirconium-based metal-organic frameworks: beware of the defects!
