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

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.