Polymorphic packing arrangements in a class of engineered organic crystals

Polymorphism occurs in the family of engineered cocrystals that form from N,N'-bis(p-X-phenyl)melamines (X = H, F, Cl, Pr, I, CH3, and CF3) and barbital. X-ray powder diffraction indicates that only one crystalline phase formed for X = Cl, F, and CH3, that primarily one phase occurred for X = H and I, but that a small contribution from an alternative phase or phases was possibly also present, and that at least two different polymorphs exist for both X = Pr and CF3. Agreement of positions of peaks in experimental XPD traces and reflections calculated from single-crystal data is good, while agreement in intensities is good in some cases but only fair in others. Reasons for this disagreement are likely to include the physical design of the diffractometer and preferred orientation of crystallites in the sample. Two of the polymorphic forms of X = Br are converted to a third on annealing at 140 degrees C. Steric effects seem the most probable origin of the polymorphism: the bromine-substituted diphenylmelamine crystallizes in several polymorphs, and the bromine substituent is intermediate in size for the family of substituents examined. This cocrystal forms one polymorph that is formed by cocrystals having larger substituents (I and CH3) and one that packs similarly to a cocrystal having a smaller substituent (Cl).