Formation and Transformations of Organic Salt Hydrates: Four Encenicline Hydrochloride Monohydrates and Respective Isostructural Desolvates

Encenicline hydrochloride (Enc-HCl) crystallizes in four different monohydrate phases, but at the same time crystallization in a nonsolvated phase is not observed, indicating that water plays a crucial role in guiding the crystallization process and ensuring structure stability. All monohydrate phases show exceptionally high stability, and the main structural motif stays intact even after dehydration, leading to isostructural (for I and II) or isomorphic (for III) desolvates. Three monohydrate phases with determined crystal structure information consists of Enc-HCl-water hexamers that are stacked into similar slabs, that are further packed identically in monohydrates I, II, and III. The features of these hexamer slabs determine the properties of the Enc-HCl monohydrates and dehydrates, the dehydration mechanism, and stability of each phase. It was justified that in the Enc-HCl system efficient intermolecular interactions provided by the incorporation of water in the crystal structure play a crucial role in stabilization of the structures.