Surprisingly complex supramolecular behaviour in the crystal structures of a family of mono-substituted salicylic acids

Crystal structure packing analyses of a family of monosubstituted salicylic acids, including some hydrates, show a surprisingly varied choice of supramolecular arrangements, even though the expected carboxyl-carboxyl dimer occurs in 17 of the 24 structures studied, and the 2-hydroxyl group hydrogen bonds with the carboxyl carbonyl oxygen in all molecules. In three of the four hydrates (3-MeO, 3NO(2) and 5-Cl), the water molecules disrupt the carboxyl-carboxyl dimer to form a centrosymmetric [carboxyl-water](2) cluster. In the fourth hydrate, of the 5-acetamido (ACM) derivative, one water molecule forms an (ACM) C=O center dot center dot center dot H-O(H)center dot center dot center dot H-O(carbox) single bridge. In the 4-ACM structure, the acetamido and carboxyl groups interact via (ACM) C=O center dot center dot center dot H-O(carbox) and (ACM-Me) C-H center dot center dot center dot O-C(carbox) double bridges, whilst the 6-MeO molecules, in which the carboxyl OH group is H-bonding intramolecularly to the MeO oxygen, forming catemers via bifurcating carboxyl OH and C=O functions. The remaining outlier is the 5-amino (5-NH2) derivative, which exists in a zwitterionic form, with one carboxyl oxygen forming a short O center dot center dot center dot H-N intermolecular bond to an -NH3+ group, whilst the second oxygen forms three longer O center dot center dot center dot H-N bonds to three different -NH3+ groups. The larger, "normal" group of 17 derivatives adopt 16 different structures, generally comprising stacks of molecules with varying relative shifts of the molecular planes, and interactions involving the substituents. Surprisingly, the family contains only two polymorphic pairs, 5-Br alpha, beta and 5-I alpha, beta, in each of which the forms show low similarities, but two isostructures, the 5-Br alpha and 5-I beta forms.