(E)-2-Acetyl-4-[(3-methylphenyl)diazenyl]phenol: an X-ray and DFT study

The title compound, C15H14N2O2, an azo dye, displays a trans configuration with respect to the N=N bridge. The dihedral angle between the aromatic rings is 0.18 (14)°. There is a strong intramolecular O—H⋯O hydrogen bond. Geometrical parameters, determined using X-ray diffraction techniques, are compared with those calculated by density functional theory (DFT), using hybrid exchange–correlation functional, B3LYP and semi-empirical (PM3) methods.


Comment
Azo compounds are very important in the field of dyes, pigments and advanced materials (Klaus, 2003). It has been known for many years that the azo compounds are the most widely used class of dyes, due to their versatile applications in various fields such as the dyeing of textile fibers, the coloring of different materials, colored plastics and polymers, biological-medical studies and advanced applications in organic synthesis (Bahatti & Seshadri, 2004;Catino & Farris, 1985;Fadda et al., 1994;Taniike et al., 1996;Zollinger, 2003).
In the title compound, C 15 H 14 N 2 O 2 , the two aromatic groups atteched to the azo bridge are adopted (E) configuration.
The molecule is planar and the dihedral angle between the two aromatic rings is 0.18(0.14)°. All the bond lengths are in agreement with reported for other azo compounds (El-Ghamry et al., 2008). The title molecule (Fig. 1) has a strong intramolecular hydrogen bond between the hydroxyl group and the carbonyl O atom.
Density-functional theory (DFT) (Schmidt & Polik, 2007) and semi-empirical (PM3) calculations and full-geometry optimizations were performed by means of GAUSSIAN 03 W package (Frisch et al., 2004). The selected bond lengths and angles (Table 2.) obtained from semi-empirical and DFT/B3LYP (Becke, 1988;Becke 1993;Lee et al. 1988) are given in Table 2. As can be seen Table 2. the bond lenghts and angles achieved by DFT method are better than those values obtained from PM3 method.

Experimental
A mixture of 3-methylaniline (0.83 g, 7.8 mmol), water (20 ml) and concentrated hydrochloric acid (1.97 ml, 23.4 mmol) was stirred until a clear solution was obtained. This solution was cooled down to 0-5 °C and a solution of sodium nitrite (0.75 g 7.8 mmol) in water was added dropwise while the temperature was maintained below 5 °C. The resulting mixture was stirred for 30 min in an ice bath. 2-hydroxyacetophenone (1.067 g, 7.8 mmol solution (pH 9) was gradually added to a cooled solution of 3-methylbenzenediazonium chloride, prepared as described above, and the resulting mixture was stirred at 0-5 °C for 2 h in ice bath. The product was recrystallized from ethyl alcohol to obtain solid (E)-2-Acetyl-4-(3-methylphenyldiazenyl)phenol.

Refinement
All H atoms (except for H1) were placed in calculated positions and constrained to ride on their parents atoms, with C-H = 0.93-0.97 Å, O-H = 0.98 Å and U iso (H) = 1.2U eq (C) or 1.5U eq (C). The hydroxyl H atom was isotropically refined. Fig. 1. A view of the title compound with the atomic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. The dashed line indicates the intramolecular hydrogen bond.

Special details
Experimental. 330 frames, detector distance = 80 mm Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
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 > σ(F 2 ) is used only for calculating Rfactors(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.