Reversible Single-Crystal-to-Single-Crystal Polymorphic Transformation and Other Solid-State Adaptations in the...

3,5-Dinitrobenzoic acid (1) and 1,3-bis-(4-pyridyl)-propane (a), together in a 1:1 molar ratio, form a wide range of cocrystals in variable compositions, depending upon the crystallization process and the solvent of crystallization. For example, at room temperature (RT), CH₃OH or an aqueous solution of the coformers yields monohydrated cocrystals, 1a·H ₂ O. However, crystallization of the same in a desiccator produces anhydrous cocrystals, 1a, from a CH₃OH solution, while an aqueous solution gives sesquihydrate cocrystals, 1a·1.5H ₂ O (α-form). In fact, in all the three cocrystals (1a, 1a·H ₂ O, and α-) the stoichiometry of the coformers is 2:1. Further, upon exposure to atmosphere, 1a·H ₂ O transforms into anhydrous cocrystals, 1a', with the coformers in a 1:1 ratio, while 1a·1.5H ₂ O undergoes reversible polymorphic transformation to β-form with the variation of temperature in the range 295-100-295 K (Single-Crystal-to-Single-Crystal). Remarkably, the coformers form anhydrous cocrystals, 1a″, with three molecules in the asymmetric unit, upon heating in the solid state (sublimation crystallization). All structures are characterized by the single-crystal X-ray diffraction methods. Packing analysis reveals that all the structures form voids in a two-dimensional arrangement. However, in the structures of 1a and 1a″ the voids are masked by the molecules from the adjacent layers, while in 1a' and 1a·H ₂ O the catenation process effectively stabilizes the voids. Nevertheless, the polymorphs α and β yield host-guest network structures with bpypa being a guest in the hosts formed by the acid molecules. Furthermore, an intriguing feature is that both anhydrous and hydrated cocrystals are metastable at RT and transform into 1a″. The transformations are further authenticated by thermal analysis and variable-temperature X-ray diffraction methods.