(E)-1-(4-Methoxyanthracen-1-yl)-2-phenyldiazene

The title compound, C21H16N2O, has an E-conformation about the diazene N=N bond. It is reasonably planar with the phenyl ring being inclined to the mean plane of the anthracene moiety [planar to within 0.077 (3) Å] by 6.43 (10)°. The crystal structure is stabilized by C—H⋯π and weak π–π interactions [centroid–centroid distances of 3.7192 (16) and 3.8382 (15) Å], leading to the formation of two-dimensional networks stacking along [001] and lying parallel to (110).

The title compound, C 21 H 16 N 2 O, has an E-conformation about the diazene N N bond. It is reasonably planar with the phenyl ring being inclined to the mean plane of the anthracene moiety [planar to within 0.077 (3) Å ] by 6.43 (10) . The crystal structure is stabilized by C-HÁ Á Á and weakinteractions [centroid-centroid distances of 3.7192 (16) and 3.8382 (15) Å ], leading to the formation of two-dimensional networks stacking along [001] and lying parallel to (110).

Comment
More than 90% of the existing commercial azodyes are tautomeric ones, which makes the investigation of their tautomerism of substantial practical interest (Kelemen, 1981). However, in most of the tautomeric dyes the tautomeric equilibrium cannot be shifted to the pure, end tautomeric forms. In such cases model compounds, possessing the characteristics of the corresponding end-structures, are usually applied. As a part of our interest in sensing molecules based on tautomeric switches (Nedeltcheva et al., 2009, Antonov et al., 2009, 2010, the equilibrium in 4-phenylazo-antracene-1-ol was studied in the gas phase by mass spectrometry, and in solution by flash photolysis (Nedeltcheva et al., 2010). The corresponding O-methyl and N-methyl derivatives were used as model enol and keto tautomers, respectively, and the tautomeric constant was estimated.
Herein, we report on the crystal structure of the title compound, the model enol analogue of 4-phenylazo-antracene-1-ol.
The molecular structure of the title molecule is shown in Fig. 1. The molecule, which has the E-conformation about the diazene N1═N2 bond, is relatively planar, with phenyl ring (C1-C6) being inclined to the mean plane of the anthracene moiety (C7-C20) by 6.43 (10) °.
Dark red block-like crystals of the title compound, suitable for X-ray diffraction analysis, were grown by slow diffusion of hexane into a chloroform solution of the title compound.

Refinement
Because no heavy atoms are present the absolute structure and absolute configuration cannot be determined.Therefore, Friedel opposites were merged in the refinement. The C-bound H-atoms were included in calculated positions and treated as riding atoms: C-H = 0.95 and 0.98 Å for CH and CH 3 H-atoms, respectively, with U iso (H) = k × U eq (C), where k =  Fig. 1. A view of the molecular structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.  Table 1 for details; H-atoms not involved in these interactions have been omitted for clarity].