Two trans-4-aminoazoxybenzenes

Two isomeric trans-4-amino­azoxy­benzenes, trans-1-(4-amino­phenyl)-2-phenyl­diazene 2-oxide (α, C₁₂H₁₁N₃O) and trans-2-(4-amino­phenyl)-1-phenyl­diazene 2-oxide (β, C₁₂H₁₁N₃O), have been characterized by X-ray diffraction. The α isomer is almost planar, having torsion angles along the C_aryl—N bonds of only 4.9 (2) and 8.0 (2)°. The relatively short C_aryl—N bond to the non-oxidized site of the azoxy group [1.401 (2) Å], together with the significant quinoid deformation of the respective phenyl ring, is evidence of conjugation between the aromatic sextet and the π-electron system of the azoxy group. The geometry of the β isomer is different. The non-substituted phenyl ring is twisted with respect to the NNO plane by ca 50°, whereas the substituted ring is almost coplanar with the NNO plane. The non-oxidized N atom in the β isomer has increased sp³ character, which leads to a decrease in the N—N—C bond angle to 116.8 (2)°, in contrast with 120.9 (1)° for the α isomer. The deformation of the C—C—C angles (1–2°) in the phenyl rings at the substitution positions is evidence of the different character of the oxidized and non-oxidized N atoms of the azoxy group. In the crystal structures, mol­ecules of both isomers are arranged in chains connected by weak N—H⃛O (α and β) and N—H⃛N (β) hydrogen bonds.