Solid-State Supramolecular Chemistry of a Benzylpyridine-Functionalized Zwitterion: Polymorphism, Interconversion, and Porosity

The reaction between acetylenedicarboxylic acid and 4-benzylpyridine yields two products: a salt and a zwitterion. The relationship between these two products has been investigated, revealing that the salt is the kinetic product of the reaction and the zwitterion is the thermodynamic product. The zwitterion has four polymorphs and three solvates. Interconversion between these forms has been studied and was shown to be solvent-mediated when it occurs; conversion between most polymorphic forms cannot be achieved using thermal methods. The temperature at which the reaction to form the zwitterion is carried out and the solvent system used are crucial in determining which polymorphic form crystallizes from the reaction. In addition, polymorphic Form I is shown to be porous to dioxane.