Salts and Co-Crystals of Gentisic Acid with Pyridine Derivatives: The Effect of Proton Transfer on the Crystal Packing (and Vice Versa)

Abstract

A series of 22 salts and co-crystals of gentisic acid and pyridine derivatives were synthesized and their crystal structures were studied by X-ray diffraction. The proton transfer was found to to be absent when the difference in pKa values was below 2, and it always occurred when it was above 2.5. The hydrogen bonds between the carboxylic group and pyridine nitrogen atom were found in all structures but one. These hydrogen bonds are shorter and apparently stronger in co-crystals than in salts. In the structures of salts the most significant intermolecular interaction is hydrogen bonding between anions, usually leading to chains or dimers, whereas in co-crystals the direct bonding between neutral gentisic acid molecules is mostly absent. Using 4,4′-bipyridine, two desmotropes were obtained in which the difference in hydrogen bonding of gentisic acid molecules leads to different protonation in two structures.