Proton transfer versus nontransfer in compounds of the diazo‐dye precursor 4‐(phenyldiazenyl)aniline (aniline yellow) with strong organic acids: the 5‐sulfosalicylate and the dichroic benzenesulfonate 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‐(phenyldiazenyl)aniline] with 5‐sulfosalicylic acid and benzenesulfonic acid, and a 1:2 nontransfer adduct compound with 3,5‐dinitrobenzoic acid have been determined at either 130 or 200 K. The compounds are 2‐(4‐aminophenyl)‐1‐phenylhydrazin‐1‐ium 3‐carboxy‐4‐hydroxybenzenesulfonate methanol solvate, C 12 H 12 N 3 + ·C 7 H 5 O 6 S − ·CH 3 OH, (I), 2‐(4‐aminophenyl)‐1‐phenylhydrazin‐1‐ium 4‐(phenyldiazenyl)anilinium bis(benzenesulfonate), 2C 12 H 12 N 3 + ·2C 6 H 5 O 3 S − , (II), and 4‐(phenyldiazenyl)aniline–3,5‐dinitrobenzoic acid (1/2), C 12 H 11 N 3 ·2C 7 H 4 N 2 O 6 , (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 interaction 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 R 1 2 (6) R 1 2 (7) R 2 1 (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 R 2 2 (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.