1-(4-Hydroxyphenyl)-2-(2-oxidonaphthalen-1-yl)diazen-1-ium methanol hemisolvate

In the title compound, C16H12N2O2·0.5CH3OH, the H atom of the –OH group has been transfered to the N atom in the azo group, forming a zwitterion. Hence, there is an intramolecular N—H⋯O, rather than an O—H⋯N, hydrogen bond in the molecule. The molecule is almost planar, the dihedral angle between the benzene ring and the mean plane of the naphthalene ring system being 4.51 (6)°. In the crystal, molecules are linked to and bridged by O—H⋯O hydrogen bonds involving the methanol molecule, which is located about a twofold rotation axis, and hence half-occupied, forming zigzag chains along [001]. Molecules are also linked via C—H⋯π and π–π interactions, the latter involving adjacent benzene and naphthalene rings and having a centroid–centroid distance of 3.6616 (13) Å, forming a three-dimensional network.

In the title compound, C 16 H 12 N 2 O 2 Á0.5CH 3 OH, the H atom of the -OH group has been transfered to the N atom in the azo group, forming a zwitterion. Hence, there is an intramolecular N-HÁ Á ÁO, rather than an O-HÁ Á ÁN, hydrogen bond in the molecule. The molecule is almost planar, the dihedral angle between the benzene ring and the mean plane of the naphthalene ring system being 4.51 (6) . In the crystal, molecules are linked to and bridged by O-HÁ Á ÁO hydrogen bonds involving the methanol molecule, which is located about a twofold rotation axis, and hence half-occupied, forming zigzag chains along [001]. Molecules are also linked via C-HÁ Á Á andinteractions, the latter involving adjacent benzene and naphthalene rings and having a centroidcentroid distance of 3.6616 (13) Å , forming a three-dimensional network.   Table 1 Hydrogen-bond geometry (Å , ).
Data collection: APEX2 (Bruker, 2006); cell refinement: SAINT (Bruker, 2006); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: WinGX publication routines (Farrugia, 2012). Azo compounds are very important in the fields of dyes, pigments and advanced materials (Lee et al., 2004;Oueslati et al., 2004). Characterized by the azo linkage (-N=N-). Many azo compounds have been synthesized by the diazotization and a diazo coupling reaction (Wang et al., 2003), followed by a coupling reaction with 2-naphthol. This entails an electrophilic substitution reaction where an aryl diazonium cation attacks another aryl ring. Since diazonium salts are often unstable near room temperature, the azo coupling reactions are typically conducted near ice temperatures. The pH of the solution is quite important; it must be mildly acidic or neutral, since no reaction takes place if the pH is too low.
We report herein on the crystal structure of the title compound, obtained through the diazotization of 4-hydroxyaniline followed by a coupling reaction with 2-naphthol. The synthesis and structure of a 4-methylaniline (Wang et al., 2003) and an aniline (Jin et al., 2008;Xu et al., 2010) analogue of the title compound have been described.
In the title molecule, Fig. 1, the bond lengths and angles are within normal ranges. Interestingly, the hydrogen atom of the OH group has been transfered to the N2 atom in the azo group to form a dipolar ion; the difference Fourier map indicated that the hydrogen atom site location is closer to the N atom of the azo group. Hence, there is an intramolecular N-H···O, rather than an O-H···N, hydrogen bond in the molecule ( Fig. 1 and Table 1). The molecule is relatively plane, with mean plane of the naphthalene ring system (C1-C10) oriented at a dihedral angle of 4.51 (6) ° with respect to the benzene ring (C11-C16).

Experimental
The title compound was prepared by the method of (Wang et al., 2003) for similar aromatic azo-compounds. Red prismatic crystals of the title compound were obtained by slow evaporation of a solution in methanol.

Refinement
The hydrogen atom of the OH group was located in a difference Fourier map and found to be near to the N atom, N2, of the azo group. In the final cycles of refinement it was included in a calculated position and treated as a riding atom: N-H = 0.86 Å with U iso (H) = 1.2U eq (N). The C-bound H atoms were positioned geometrically and refined as riding: C-H = 0.93 Å with U iso (H) = 1.2U eq (C). The disordered methanol solvent OH and CH 3 H atoms were located in a difference Fourier map and refined as riding atoms with U iso (H) = 1.5U eq (O,C). Two reflections (2 0 0

Figure 1
View of the molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level.   Table 1 for details). 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 esds are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness 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 threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (