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.