Crystal Structures of Azo Colorants Derived from Pyrazolone: A Review

The crystal structures of five azo colorants derived from pyrazolone have been determined by single crystal X-ray diffraction techniques. An examination of these crystals indicates the presence of certain common features. In the solid state the molecules do not exist in the azo form but in the form of the hydrazone tautomer. The hydrogen atom in the hydrazone group and one in one of the phenyl rings take part in intramolecular hydrogen bonding, which holds the molecule in a more or less planar configuration. The molecules stack in columns linked by van der Waals forces. Molecule dipole moments and shortest intermolecular distances in the crystal structures are tabulated in an attempt to explain the different solubilities of these solvent dyes and pigments