Crystal structures and thermodynamic properties of polymorphs and hydrates of selected 2-pyridinecarboxaldehyde hydrazones

Synthesis, crystal structure, and thermal behavior studies of different solid-state forms of two new 2-pyridinecarboxaldehyde N-acylhydrazones are reported together with the corresponding computational analyses. Both compounds exist in two anhydrous polymorphic forms and crystallize also as hydrates. All of the studied crystals were obtained via solvent evaporation from solutions. Structural features were determined using single-crystal XRD (employing the transferred aspherical atom model, TAAM) supported by solid-state NMR. The relative stability of different crystal forms was examined experimentally using the TGA-DSC methods and supplemented with extended lattice and interaction energy calculations and crystal entropy estimation. The results confirmed the prevalent role of strong hydrogen bonds of N–H···O and N–H···N type in stabilization of anhydrous crystals. In the case of hydrates, water molecules incorporated into the crystal network are involved in the most efficient hydrogen bonds contributing si...