OH…O− and OH…O supramolecular hydrogen bonding interaction connecting oxalate, hydrogen oxalate and oxalic acid. Mirtazapine cations are connected via oxalate anions through N+H…O hydrogen bonds.
Pharmaceutically suitable non-sublimating salts and molecular salts of anti-depressant drug R/S-mirtazapine with one of several dicarboxilic acids were studied. The salts/salt molecules were characterized by powder X-ray diffraction, differential scanning calorimetry and thermogravimetric analysis and crystal structure of tartarate and oxalate molecular salt were determined by single crystal X-ray diffraction. The salts/salt molecules of mirtazapine do not show any sublimation at elevated temperature whereas sublimation of mirtazapine has been observed at ambient temperature. The aqueous solubility of the mirtazapine molecular salts was significantly improved with a maximum of citrate salt which was about 180 times more than the solubility of the parent mirtazapine at 35 °C.