Co-crystals of caffeine and hydroxy-2-naphthoic acids: unusual formation of the carboxylic acid dimer in the presence of a heterosynthon.

A group of caffeine-containing co-crystals of hydroxy-2-naphthoic acids were synthesized and analyzed via single-crystal X-ray diffraction and IR analysis. The imidazole-carboxylic acid synthon was observed in co-crystals involving 1-hydroxy-2-naphthoic and 3-hydroxy-2-naphthoic acid. In the case of 6-hydroxy-2-naphthoic acid, the co-crystal exhibits a hydrogen-bonded carboxylic acid dimer in the presence of a hydroxyl-caffeine heterosynthon.