Proton transfer versus nontransfer in compounds of the diazo-dye precursor 4-(phenyl-diazenyl)aniline (aniline yellow) with strong organic acids: the 5-sulfosalicylate and the dichroic benzene-sulfonate salts, and the 1:2 adduct with 3,5-dinitrobenzoic acid

The structures of two 1:1 proton-transfer red-black dye compounds formed by reaction of aniline yellow [4-(phenyl-diazenyl)aniline] with 5-sulfosalicylic acid and benzene-sulfonic acid, and a 1:2 nontransfer adduct compound with 3,5-dinitro-benzoic acid have been determined at either 130 or 200 K. The compounds are 2-(4-aminophenyl)-1-phenylhydra-zin-1-ium 3-carboxy-4- hydroxybenzenesulfonate methanol solvate, C₁₂H₁₂N₃⁺·C₇H₅O₆S^-·CH₃OH, (I), 2-(4-aminophenyl)-1-phenylhydrazin-1- ium 4-(phenyldiazenyl)anilinium bis(benzenesulfonate), 2C₁₂H₁₂N₃⁺·2C₆H₅O₃S^-, (II), and 4-(phenyl-diazenyl)aniline-3,5-dinitrobenzoic acid (1/2), C₁₂H₁₁N₃·2C₇H₄N₂O₆, (III). In compound (I), the diazenyl rather than the aniline group of aniline yellow is protonated, and this group subsequently takes part in a primary hydrogen-bonding inter-action with a sulfonate O-atom acceptor, producing overall a three-dimensional framework structure. A feature of the hydrogen bonding in (I) is a peripheral edge-on cation-anion association also involving aromatic C - H⋯O hydrogen bonds, giving a conjoint R¹₂(6)R¹₂(7)R²₁(4) motif. In the dichroic crystals of (II), one of the two aniline yellow species in the asymmetric unit is diazenyl-group protonated, while in the other the aniline group is protonated. Both of these groups form hydrogen bonds with sulfonate O-atom acceptors and these, together with other associations, give a one-dimensional chain structure. In compound (III), rather than proton transfer, there is preferential formation of a classic R²₂(8) cyclic head-to-head hydrogen-bonded carboxylic acid homodimer between the two 3,5-dinitrobenzoic acid molecules, which, in association with the aniline yellow molecule that is disordered across a crystallographic inversion centre, results in an overall two-dimensional ribbon structure. This work has shown the correlation between structure and observed colour in crystalline aniline yellow compounds, illustrated graphically in the dichroic benzenesulfonate compound.