Pharmaceutical Co-crystals with Isonicotinamide-Vitamin B3, Clofibric Acid, and Diclofenac-and Two Isonicotinamide Hydrates

The co-crystals of three industrially important pharmaceutical molecules, the Vitamin B group member nicotinamide(1), the antihyperlipidemic drug clofibric acid (2), and the nonsteroidal anti-inflammatory drug diclofenac (3), are synthesized with the co-crystal former isonicotinamide and characterized by thermal analysis and single crystal X-ray diffraction. Two dimorphic hydrates of isonicotinamide were obtained during the course of these experiments: hydrate 4 (form I) has been reported recently, and hydrate 5 (form II) is new. Both are monohydrates but differ in the number of independent molecules in the asymmetric unit, Z' = 2 and 8, respectively. Form II is metastable compared to I and converts to form 1 in the solid state. In all three pharmaceutical co-crystals, it is the pyridine N atom of either the nicotinamide molecule in I or the N atom of the isonicotinamide molecule in 2 and 3 that is used in connecting the different molecules together, as a hydrogen bond acceptor from the amine of the isonicotinamide in I and the carboxylic acid protons in 2 and 3. The carboxylic acid center dot center dot center dot pyridine hydrogen bond is an often used supramolecular synthon. A survey of relevant structures in the Cambridge Structural Database of isonicotinamide and nicotinamide co-crystals is given for completeness, and the co-crystal former ability of isonicotinamide and nicotinamide was investigated by performing density functional theory calculations.