Crystal structure of (E)-3-[(2,6-dimethylphenyl)diazenyl]-7-methyl-1H-indazole

The title compound is composed of a benzene ring linked to an indazole unit by an N=N bond and has high planarity. The diazenyl group adopts a trans (E) conformation.


Chemical context
Azobenzene derivatives are known to be photochromic compounds and numerous studies have been reported (Aritake et al., 2011;Bobrovsky et al., 2016;Li et al., 2017). As an example of this, our group has reported the crystal structures of several azobenzene derivatives (Moriwaki & Akitsu, 2015;Moriwaki et al., 2017).
Pyrazole is an aromatic compound comprising a fivemembered ring with two adjacent N atoms. Pyrazole derivatives are biologically active and have attracted attention for the synthesis of new medicinal products (Ansari et al., 2017).

Structural commentary
The molecular structure of the title compound is composed of a benzene ring linked to an indazole unit by an N N bond. In the azobenzene moiety, the azo N N double bond adopts an E configuration, with an N N bond length of 1.265 (4) Å and a corresponding C9-N3-N4-C15 torsion angle of 0.7 (4) .

Supramolecular features
In the crystal, molecules are helically connected along the baxis direction by N-HÁ Á ÁN hydrogen bonds (Table 1 and Fig. 2). As a result, chiral crystals of achiral molecules are generated. The angles between the planes of neighbouring molecules in the hydrogen-bonded chains is 82.6 (2) . Many examples of such achiral molecules forming chiral crystals have been reported, but the prediction of chiral crystallization is still not possible (Koshima & Matsuura, 1998;Matsuura & Koshima, 2005).

Database survey
A similar compound, i.e. 7-methyl-1H-indazole (CCDC refcode 263698;Foces-Foces, 2005), has already been reported and shows a structure comparable with that of the title compound. However, surveys of the Cambridge Structural Database (CSD, Version 5.38; Groom et al., 2016) for the title compound revealed no hits. To our knowledge, this is the first crystal structure reported for indazole-type azo dyes.

Synthesis and crystallization
A mixture of 2,6-dimethylaniline (0.4847 g, 4.000 mmol), concentrated hydrochloric acid (37%, 1 ml) and water was heated and completely dissolved. The mixture was cooled in an ice bath and NaNO 2 (0.2967 g, 4.300 mmol) in 4.5 ml water was added. The reaction mixture was stirred at 273 K for 30 min and then salicylaldehyde (0.4885 g, 4.000 mmol) in 10 ml of a 10% NaOH aqueous solution was added dropwise and allowed to stir for an additional 1 h. The obtained orange precipitate was filtered off, washed with water and dried in a desiccator for several days (yield 0.2650 g, 26.06%). This crude orange compound was recrystallized by slow evaporation from acetone to give orange prismatic single crystals. IR (KBr, cm  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view of the N-HÁ Á ÁN hydrogen bonds (blue dashed lines) present in the crystal lattice of the title compound.

Figure 3
A view of the various C-HÁ Á Á interactions (blue dashed lines) present in the crystal lattice of the title compound.

Figure 1
The structure of the title compound shown with 50% probability displacement ellipsoids.

(E)-3-[(2,6-Dimethylphenyl)diazenyl]-7-methyl-1H-indazole
Crystal data Special details 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )