syn and anti conformations in 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoic acid and two related salts

The crystal structures of 2‐hydroxy‐5‐(E)‐(4‐nitrophenyl)diazenyl]benzoic acid, C₁₃H₉N₃O₅, (I), ammonium 2‐hydroxy‐5‐(E)‐phenyldiazenyl]benzoate, NH₄⁺·C₁₃H₉N₂O₃^?, (II), and sodium 2‐hydroxy‐5‐(E)‐(4‐nitrophenyl)diazenyl]benzoate trihydrate, Na⁺·C₁₃H₈N₃O₅^?·3H₂O, (III), have been determined using single‐crystal X‐ray diffraction. In (I) and (III), the phenyldiazenyl and carboxylic acid/carboxylate groups are in an anti orientation with respect to each other, which is in accord with the results of density functional theory (DFT) calculations, whereas in (II), the anion adopts a syn conformation. In (I), molecules form slanted stacks along the 100] direction. In (II), anions form bilayers parallel to (010), the inner part of the bilayers being formed by the benzene rings, with the –OH and –COO^? substituents on the bilayer surface. The NH₄⁺ cations in (II) are located between the bilayers and are engaged in numerous N—H...O hydrogen bonds. In (III), anions form layers parallel to (001). Both Na⁺ cations have a distorted octahedral environment, with four octahedra edge‐shared by bridging water O atoms, forming Na₄(H₂O)₁₂]⁴⁺ units.