2-amino-3-hydroxy-6-phenylazopyridine

The solid-solid transformation of 2-amino-3-hydroxy-6-phenylazopyridine, 57a, to 57b proceeds through two intermediate phases (119). X-Ray and IR studies of the former, low-temperature, and the latter, high-temperature phase show that they are the phenolazo and quinone hydrazone forms, respectively. This solid-state tautomerism can be accounted for by a cooperative intermolecular shift of protons across the various hydrogen bonds. However, because of the complexity of the hydrogen-bond network, the actual pathway of the proton shift has not been uniquely defined. 

Desiraju, G. R. (1983).Intermolecular proton transfers in the solid state. Conversion of the hydroxyazo into the quinone hydrazone tautomer of 2-amino-3-hydroxy-6-phenylazopyridine. X-ray crystal structures of the two forms. J. Chem. Soc., Perkin Trans. 2,1983, 1025-30. [84]... 

When different configurations of the same molecule crystallize in separate crystalline forms, they are termed as configurational polymorphs. As a caveat, since configurational isomers can be considered to be separate chemical entities, these solids may not be strictly classified as polymorphs. Yet another classification of polymorphs that is closely related to confignrational polymorphism is tautomeric polymorphism which occurs when different tautomers of a compound exist in different polymorphs. There are cases where confignrational isomers or tautomers can interconvert in solution, and the fact that their relative population and stability depends on factors such as temperature, solvent, pH, and so on makes them appear as different crystalline polymorphs. In contrast to other, previously described classes of polymorphs, tautomeric polymorphs are rare. An early report of tautomeric polymorphism by Desiraju is the compound 2-amino-3-hydroxy-6-phenylazopyridine, which exists as hydroxyazo and quinonehydrazone crystals of different colors. Polymorphs of the antiulcer drug omeprazole (Figure 6) also represent tautomeric polymorphs that were characterized by SC-XRD. ... 

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