A synthetic strategy towards single crystals of Zr₆ cluster and phosphonate-based metal-organic frameworks

Although metal-organic frameworks (MOFs) composed of Zr-based nodes and phosphonate-based linkers show promise in terms of their excellent stability, synthetic strategies to access them are underdeveloped due to the less reversible coordination between Zr-based nodes and phosphonate groups. In this work, we developed a generalizable strategy to access Zr-phosphonate MOFs in two steps via post-synthesis modification which allows for retaining the topology of parent Zr MOFs. Solvent-assisted ligand exchange (SALE) replaces carboxylate linkers with phosphinate linkers, and post-synthetic oxidation affords phosphonate-based linkers. We confirmed the structures through single-crystal X-ray diffraction (SCXRD) studies and fully characterized each MOF through a variety of other techniques. Water and ammonia sorption isotherms reveal that incorporation of phosphonate groups yields a more hydrophilic pore environment with increased Brønsted acidity relative to those of the carboxylate-based analogues. Overall, this strategy provides a new avenue to tune the physical and chemical properties of Zr-MOFs and further enriches the reticular chemistry of this class of materials.