Crystal water as the mol-ecular glue for obtaining different co-crystal ratios: the case of gallic acid tris-caffeine hexa-hydrate.

The crystal structure of the hexa-hydrate co-crystal of gallic acid and caffeine, C7H6O5·3C8H10N4O2·6H2O or GAL3CAF·6H2O, is a remarkable example of the importance of hydrate water acting as structural glue to facilitate the crystallization of two components of different stoichiometries and thus to compensate an imbalance of hydrogen-bond donors and acceptors. The water mol-ecules provide the additional hydrogen bonds required to form a crystalline solid. Whereas the majority of hydrogen bonds forming the inter-molecular network between gallic acid and caffeine are formed by crystal water, only one direct classical hydrogen bond between two mol-ecules is formed between the carb-oxy-lic oxygen of gallic acid and the carbonyl oxygen of caffeine with d(D⋯A) = 2.672 (2) Å. All other hydrogen bonds either involve crystal water or utilize protonated carbon atoms as donors.