Two azo pigments based on beta-naphthol.

There has been much discussion in the literature of the azo-hydrazone tautomerism of pigments. All commercial azo pigments with beta-naphthol as the coupling compound adopt the hydrazone tautomeric form (Ph-NH-N=C) in the solid state. In contrast, the red pigments 1-[4-(dimethylamino)phenyldiazenyl]-2-naphthol, C(18)H(17)N(3)O, (1a), and 1-[4-(diethylamino)phenyldiazenyl]-2-naphthol, C(20)H(21)N(3)O, (1b), have been reported to be azo tautomers or a mixture of azo and hydrazone tautomers in the solid state. To prove these observations, both compounds were synthesized, recrystallized and their crystal structures redetermined by single-crystal structure analysis. Difference electron-density maps show that the H atoms of the hydroxyl groups are indeed bonded to the O atoms. Nevertheless, a small amount of the hydrazone form seems to be present. Hence, the compounds are close to being ;real' azo compounds. Compound (1a) crystallizes with a herring-bone structure and compound (1b) forms a rare double herring-bone structure.