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The non-H atoms of the title compound, C 20 H 20 N 2 O 2 , is located on a mirror plane except two methyl groups of the tert-butyl group. Intramolecular N—HÁ Á ÁO hydrogen bonds exist between the hydroxy and diazenyl groups. In the crystal, molecules are linked by weak C—HÁ Á ÁO hydrogen bonds into supramolecular chains running along the a-axis direction. For general background to azo compounds and their use in dyes, pigments and advanced materials, see: Lee et al.

The non-H atoms of the title compound, C 20 H 20 N 2 O 2 , is located on a mirror plane except two methyl groups of the tertbutyl group. Intramolecular N-HÁ Á ÁO hydrogen bonds exist between the hydroxy and diazenyl groups. In the crystal, molecules are linked by weak C-HÁ Á ÁO hydrogen bonds into supramolecular chains running along the a-axis direction. 12 restraints H-atom parameters constrained Á max = 0.43 e Å À3 Á min = À0.23 e Å À3 Table 1 Hydrogen-bond geometry (Å , ).

Comment
Azo compounds are very important in the fields of dyes, pigments and advanced materials (Lee et al., 2004). Azo dyes are synthetic colours that contain an azo group, as part of the structure. We are involved in the color generation mechanism of azo pigments typically characterized by the chromophore of the azo group (-N=N-). However, some types of azo pigments are also known to possess the hydrazone structure (=N-NH-), often leading to the formation of intramolecular hydrogen bonds. The azo-hydrazone tautomerism in azo dyes has been known for more than a hundred years and is directly connected with the presence of at least one protic donor group in conjugation to the azo bridge (i.e. et al.., 1989). In particular, azo dyes that contain a naphtholic hydroxy group conjugated with the azo linkage exist in aqueous solution as an equilibrium mixture of two chemically distinct tautomers, the azo or hydrazone forms (Oakes, 2002). It is suggested that in a real azo compound the N=N double bond should have a length of 1.20-1.28 Å and the bond length of N-N single bonds, as in hydrazone tautomers, should be more than 1.4 Å. In the title compound, N-N bond lengths are 1.287 Å for N1-N2 , between the suggested N=N double bond and N-N single bond lengths. In the molecule, all bond lengths are in good agreement with those reported for other azo compounds (Yazıcı et al., 2010;Karadayı et al., 2006). We report here in the crystal structure of the title compound, obtained through the diazotization of 4-tert-butyl-2-hydroxy aniline followed by a coupling reaction with 2-naphthol.

2-naphthol) (Olivieri
The molecule of the title compound, with the atom numbering scheme, is shown in Fig. 1, crystallizes in the monoclinic space group P21/m. The molecular structure C 20 H 20 N 2 O 2 is shown in Figure 1 The molecule adopts an anti-configuration with the two aryl groups reside on the opposite side of azo-group. The intramolecular N-H···O hydrogen bond is found (Table 1). In the crystal molecules are linked by the weak C-H···O interactions into chains.

Experimental
The title compound was obtained through the diazotization of 4-tert-butyl-2-hydroxyaniline followed by a coupling reaction with 2-naphthol, according to the literature procedure used to synthesize other aromatic azo-compounds (Wang et al., 2003). Single crystals of the title compound were obtained by slow evaporation at room temperature of a solution in DMSO.

Refinement
H atoms, attached to carbon atoms have been placed in geometrically idealized positions and refined as riding, with C-H = 0.93 (aromatic) and 0.96 Å (methyl), and U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C). hydroxy H atoms were introduced in calculated positions and treated as riding on their parent atoms with O-H = 0.82 Å and U iso (H) = 1.5U eq (O).

Figure 1
The molecular structure of the title compound, displacement ellipsoids are drawn at 50% probability level.

1-[(E)-2-(5-tert-Butyl-2-hydroxyphenyl)diazen-1-yl]naphthalen-2-ol
Crystal data C 20 H 20 N 2 O 2 M r = 320.38 Monoclinic, P2 1 /m Hall symbol: -P 2yb a = 9.696 (5) Å b = 6.606 (5)  1767 reflections with I > 2σ(I) R int = 0.025 θ max = 30.5°, θ min = 3.2°h where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.43 e Å −3 Δρ min = −0.23 e Å −3 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 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 > 2sigma(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.