An acetonitrile-solvated cocrystal of piroxicam and succinic acid with co-existing zwitterionic and non-ionized piroxicam molecules

This work reports a new acetonitrile (ACN)-solvated cocrystal of piroxicam (PRX) and succinic acid (SA), 2C₁₅H₁₃N₃O₄S·0.5C₄H₆O₄·C₂H₃N or PRX:SA:ACN (4:1:2), which adopts the triclinic space group P\overline{1}. The outcome of crystallization from ACN solution can be controlled by varying only the PRX:SA ratio, with a higher PRX:SA ratio in solution unexpectedly favouring a lower stoichiometric ratio in the solid product. In the new solvate, zwitterionic (Z) and non-ionized (NI) PRX molecules co-exist in the asymmetric unit. In contrast, the nonsolvated PRX–SA cocrystal contains only NI-type PRX molecules. The ACN molecule entrapped in PRX–SA·ACN does not form any hydrogen bonds with the surrounding molecules. In the solvated cocrystal, Z-type molecules form dimers linked by intermolecular N—H...O hydrogen bonds, whereas every pair of NI-type molecules is linked to SA via N—H...O and O—H...N hydrogen bonds. Thermogravimetry and differential scanning calorimetry suggest that thermal desolvation of the solvate sample occurs at 148 °C, and is followed by recrystallization, presumably of a multicomponent PRX–SA structure. Vibrational spectra (IR and Raman spectroscopy) of PRX–SA·ACN and PRX–SA are also used to demonstrate the ability of spectroscopic techniques to distinguish between NI- and Z-type PRX molecules in the solid state. Hence, vibrational spectroscopy can be used to distinguish the PRX–SA cocrystal and its ACN solvate.