2,2′-({4-[(4-Nitrophenyl)diazenyl]phenyl}imino)diethanol

In the title compound, C16H18N4O4, the molecule assumes an E conformation with respect to the N=N double bond. The aromatic rings are not coplanar, with a dihedral angle of 7.51 (8)°. The nitro group is tilted by 4.71 (11)° relative to the attached benzene ring. In the crystal, molecules are connected through O—H⋯O hydrogen bonds forming a double-stranded chain parallel to the b axis.

In the title compound, C 16 H 18 N 4 O 4 , the molecule assumes an E conformation with respect to the N N double bond. The aromatic rings are not coplanar, with a dihedral angle of 7.51 (8) . The nitro group is tilted by 4.71 (11) relative to the attached benzene ring. In the crystal, molecules are connected through O-HÁ Á ÁO hydrogen bonds forming a doublestranded chain parallel to the b axis. Mo K radiation = 0.10 mm À1 T = 296 K 0.24 Â 0.16 Â 0.04 mm

2,2′-({4-[(4-Nitrophenyl)diazenyl]phenyl}imino)diethanol
Tanwawan Duangthongyou, Potjanart Suwanruji, Jantip Suesat and Supakit Achiwawanich Comment A series of azo disperse dyes was recently synthesized by our group in order to study the influence of substituents on the chromatic properties of the dyes (Suesat et al., 2011). We report herein the crystal structure of one of these dyes.

Experimental
The azo disperse dye was prepared by dissolving 4-nitroaniline (0.01 mol) in 50 ml of an acetic acid/propionic acid (43:7 v/v) mixture. The solution was stirred and the temperature was kept in the range of 0-5°C. Diazotization took place when nitrosyl sulfuric acid (HNO 5 S) was added to the solution and stirred at 0-5°C for 30-60 minutes. The coupling component N-bis-β-hydroxyethyl aniline (0.01 mol) was then dissolved in 40 ml acetone, distilled water was added to make the total volume of 200 ml and sulfamic acid (0.5 g) was added. The coupling reaction was performed by slow addition of the diazonium salt solution to the coupling solution at 0-5°C. The reaction continued for 2 h with stirring and was monitored using TLC. On completion of the coupling reaction, the dye precipitate was collected by filtration and dried at room temperature. The dye was purified by recrystallization in n-propanol. Single crystals suitable for X-ray analysis were obtained by slow evaporation of a purified dye solution in n-propanol.

Refinement
All H atoms of the compound were placed in the calculated positions with C-H = 0.93 and 0.97 Å, O-H = 0.82 Å and included in the cycles of refinement in a rigid model, with U iso (H) = 1.2 U eq (C) and 1.5 U eq (O).  The molecular structure of the title compound showing displacement ellipsoids drawn at the 50% probability level.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 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.