Probing Structural Changes in a Phosphonate-based Metal–Organic Framework Exhibiting Reversible Dehydration

A one-step hydrothermal synthesis with small amines and 1,3,5-benzenetriphosphonic acid was used to prepare single crystals of isostructural anionic metal–organic frameworks (MOF): Zn2.5(H)0.4–0.5(C6H3O9P3)­(H2O)1.9–2(NH4)0.5–0.6 and Zn2.5(H)0.75(C6H3O9P3)­(H2O)2(CH3NH3)0.25. The ammonium ions are exchangeable with lithium ions. The MOF exhibits reversible dehydration, and the process was studied by two complementary methods: solid state NMR and in situ X-ray diffraction. These experiments revealed three different phases. The crystal structures of all phases have been determined, showing loss in volume of the structure due to a phase change. The ammonium ions remain in the structure and are forced to occupy the larger pores due to a reduction in free volume. The change in positions of the guest molecules in the framework has an effect on the potential conductivity properties of the materials. Changes in framework and guest molecules due to negative expansion have an effect on other physical and chemical properties and need to be explored.