Supramolecular Gelation as the First Stage in Ostwald’s Rule

Ostwald’s rule of stages describes the observation that a metastable polymorph of a particular compound will crystallize before the thermodynamically stable form under the same conditions. While traditionally applied to crystalline systems, there have been recent examples in which noncrystalline supramolecular assemblies transform according to this rule. In this work we report the highly selective organogelation behavior of a monoiodinated 2,4,5-triphenylimidazole (lophine) derivative (I-TPI). The gel emerges as a kinetically trapped intermediate, prior to the crystallization of a series of increasingly stable methanol solvates. As such, we describe the supramolecular gel as a precrystalline phase, representing the first of Ostwald’s stages for this system. Similar behavior can be observed when the gel is used as a medium for pharmaceutical crystallization of diatrizoic acid (DTA). In this case, two solid forms are produced, both containing molecules of DTA, I-TPI, and methanol in different stoichiometries, and once again crystallizing in sequence according to Ostwald’s rule. These results demonstrate that supramolecular gelation may be considered the first stage in the stepwise crystallization of a gelator, as governed by Ostwald’s rule, and that this behavior also applies to multicomponent systems, facilitating the production of metastable pharmaceutical solid forms.