1-(3-Acetylphenyl)-2-(2-oxidonaphthalen-1-yl)diazen-1-ium

The title compound, C18H14N2O2, crystallized with two independent zwitterion molecules (A and B) in the asymmetric unit. They are both close to planar, the dihedral angle between the benzene ring and naphthalene ring system being 4.30 (9)° in A and 4.69 (9)° in B. Each molecule has an E conformation with respect to the azo double bond. In each of the independent molecules, an intramolecular N—H⋯O hydrogen bond forms an S(6) ring motif. In the crystal, molecules are linked via C—H⋯O hydrogen bonds, forming –A—A—A– and –B—B—B– chains parallel to one another and propagating along the a-axis direction. There are also π–π interactions between adjacent molecules involving benzene and naphthalene rings [centroid–centroid distance of 3.626 (3) Å for adjacent A molecules and 3.652 (3) Å for adjacent B molecules].

The molecular structures of the two independent molecules (A and B) of the title compound are shown in Fig. 1. In both molecules the hydrogen atom of the OH group has been transfered to the N atom in the azo group to form a zwitterion.
There are no significant differences in their structures. The dihedral angle between the benzene ring and naphthalene ring system is 4.30 (9) ° in A and 4.69 (9) ° in the B. Each molecule has an E conformation with respect to the azo double bond. The torsion angle C11-N12-N13-C14 being 179.7 (2) ° in A, while in B the corresponding torsion angle C41-N42-N43-C44 is 179.2 (2)°. An intramolecular N-H···O hydrogen bond exists in each molecule (Table 1), forming an S(6) ring motif.

Experimental
The title compound was synthesized according to the literature procedure used for the synthesis of other aromatic azocompounds (Wang et al., 2003). Red prisms of the compound were obtained by slow evaporation at room temperature of a H 2 O/THF (1/1 v/v) solution of the title compound.

Refinement
The NH H atoms were located in difference Fourier maps. In the final cycles of refinement they and the C-bound H atoms were included in calculated positions and treated as riding atoms: N-H = 0.86 Å, C-H = 0.93 and 0.96 Å for CH and CH 3 H atoms, respectively, with U iso (H)= 1.5U eq (C-methyl) and = 1.2U eq (N,C) for other H atoms.

Figure 1
A view of the molecular structure of the two independent molecules (A and B) of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular N-H···O hydrogen bonds are shown as dashed lines (see Table 1 for details).   Table 1 for details).

1-(3-Acetylphenyl)-2-(2-oxidonaphthalen-1-yl)diazen-1-ium
Refinement Refinement on F 2 Least-squares matrix: full R[F 2 > 2σ(F 2 )] = 0.043 wR(F 2 ) = 0.120 S = 1.03 5097 reflections 399 parameters 1 restraint Primary atom site location: structure-invariant direct methods Secondary atom site location: difference Fourier map Hydrogen site location: inferred from neighbouring sites H-atom parameters constrained Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.