Recurrence of Carboxylic Acid−Pyridine Supramolecular Synthon in the Crystal Structures of Some Pyrazinecarboxylic Acids

Abstract

X-ray crystal structures of pyrazinic acid 1 and isomeric methylpyrazine carboxylic acids 2−4 are analyzed to examine the occurrence of carboxylic acid−pyridine supramolecular synthon V in these heterocyclic acids. Synthon V, assembled by (carboxyl)O−H···N(pyridine) and (pyridine)C−H···O(carbonyl) hydrogen bonds, controls self-assembly in the crystal structures of pyridine and pyrazine monocarboxylic acids. The recurrence of acid−pyridine heterodimer V compared to the more common acid−acid homodimer I in the crystal structures of pyridine and pyrazine monocarboxylic acids is explained by energy computations in the RHF 6-31G* basis set. Both the O−H···N and the C−H···O hydrogen bonds in synthon V result from activated acidic donor and basic acceptor atoms in 1−4. Pyrazine 2,3- and 2,5-dicarboxylic acids 10 and 11 crystallize as dihydrates with a (carboxyl)O−H···O(water) hydrogen bond in synthon VII, a recurring pattern in the diacid structures. In summary, the carboxylic acid group forms an O−H···N hydrogen bond in pyrazine monocarboxylic acids and an O−H···O hydrogen bond in pyrazine dicarboxylic acids. This structural analysis correlates molecular features with supramolecular synthons in pyridine and pyrazine carboxylic acids for future crystal engineering strategies