Crystal Structures of Azo Pigments Derived from Acetoacetanilide

The crystal structures of ten azo pigments derived from acetoacetanilide have been determined by single crystal X-ray diffraction techniques. A review of these crystal structures indicates the existence of certain common features. It is found that, in the solid state the molecules do not exist in the azo form but in the form of the hydrazone tautomer. The hydrogen in the hydrazone group and the one in the CONH group both take part in intramolecular hydrogen bonding. In both cases the hydrogen bond may be difurcated (shared) between two acceptor atoms. These intramolecular hydrogen bonds hold the molecule more or less planar. In general the molecules stack in columns and these columns are linked by van der Waals forces, but in a minority of cases (three out of ten) intermolecular hydrogen bonds have been found. A relationship between molecular stacking and crystal morphology is pointed out