Intramolecular Hydrogen Bonding and Intermolecular Dimerization in the Crystal Structures of Imidazole-4,5-dicarboxylic Acid Derivatives

Diamide and amide-ester derivatives of imidazole-4,5-dicarboxylic acid form reliable hydrogen-bonding motifs in the solid state. The crystal structures of symmetrically substituted and dissymmetrically substituted diamides as well as amide-ester combinations were analyzed in order to identify the intermolecular hydrogen-bonding patterns. An intramolecular seven-membered hydrogen-bonded conformation forms in all derivatives where the possibility existed due to the functionality present. The motifs observed for the diamides include intermolecular NH···O and NH···N hydrogen-bonded dimers, with the exceptions to these motifs occurring in compounds having benzylamine substituents. The amines with a higher classification (i.e., 3° > 2° > 1°) in the dissymmetrically substituted diamides are the intramolecular hydrogen bond donors in the solid state, consistent with the capacity of the alkyl group to stabilize developing carbocation character resulting from bond polarization. The amide-ester derivatives also form an intramolecular hydrogen bond and an intermolecular motif based on NH···N and two different C2−H···O hydrogen bonds. A pyrrole amide-ester derivative forms an intramolecular NH···O hydrogen bond in the solid state and an intermolecular NH···O hydrogen-bonded chain. With the exception of the benzylamine-substituted diamides, the intermolecular hydrogen-bonded motifs appear reliable for these imidazole-4,5-dicarboxylic acid derivatives and will be useful in the design of analogues for specific applications.