Crystal structure of 5-[2-(2,4,6-tribromophenyl)diazenyl]tropolone

The title compound is essentially planar, with an r.m.s. deviation of 0.054 Å. The molecular structure is fixed in the azo tautomer by intramolecular C—H⋯N interactions, with O—H⋯O hydrogen bonds creating linked dimers. Charge-transfer interactions are observed, with the segregated stacks linked by Br⋯Br interactions.


Chemical context
In modern times, dyes have become an enormous market (approx. $13.4 billion) with over one million tons of various dyes and pigments being produced each year, and their uses ranging from textiles, cosmetics, food coloring, printing inks to optical recording media. Azo dyes form one of the largest groups of synthetic chemical dyes with the chemical structure of these compounds featuring substituted aromatic rings that are joined by one or more azo groups (R-N N-R). The first of these dyes to be produced was aniline yellow, which contains an azo-substituted benzene group. Tropolone exhibits similar aromatic properties to benzene; as such, a study was undertaken to synthesize a series of azo-functionalized tropolones. A search of the Cambridge Structural Database (CSD; Groom et al., 2016) yielded twenty troponoid compounds with a mono-substituted 5-position. Of these, only eleven had the tropolone backbone, only two of which had an azo linking group. As part of this study, we report the crystal structure of the title compound, I (Fig. 1).

Structural commentary
The title compound, I, shows no signs of azo-hydrazone tautomerization, a phenomenon known to the phenylazoderivatives, because of the stabilizing intramolecular interaction of the hydrogen (H6) atom of the tropolone ring with the nitrogen (N2) atom of the azo group (Table 1, Fig. 2). Similar to 5-phenylazotropolone (Hill et al., 2012), I is essen- ISSN 2056-9890 tially planar, with the dihedral angle between the least-squares planes A (O1, O2, C1-C7, N1) and B (N2, C11-C16) of 5.07 (6) with an r.m.s. deviation of 0.054 Å . The largest variation from the molecular plane is for the phenyl carbon (C13) with a value of 0.096 (2) Å . However, this planarity does not extend to the other azotropolones: 5-(4-ethoxyphenylazo)tropolone (Kubo et al., 2008) was found to be twisted with an angle of 27.6 (1) .
Theinteractions that were observed for 5-phenylazotropolone and 5-(4-ethoxyphenylazo)tropolone are not observed for I. Instead, a ! * charge-transfer (CT) interaction between the aromatic phenyl and tropolone with the diazenyl group is seen (Fig. 2), with ring centroid to N N midpoint distances of 3.3463 (3) and 3.3415 (3) Å , respectively. Additionally, a BrÁ Á Á interaction is found with a Br to benzene ring centroid distance of 3.4563 (3) Å .
As a result of the observed O-HÁ Á ÁO, BrÁ Á ÁBr, BrÁ Á Á and ! * CT interactions, the assembly of I is seen as forming segregated stacks along the b-axis direction (Fig. 2).

Hirshfeld Surface Analysis
Hirshfeld surface plots were generated for tropolone (CSD refcode: TROPOL10; Shimanouchi & Sasada, 1973), 5-phenylazotropolone (CSD refcode: YDIVYZ; Hill et al., 2012) and I based on the crystallographic information file (CIF) using CrystalExplorer17.5 (Turner et al., 2017), to explore and compare the location of atom-to-atom short contacts along with the quantitative ratios of these interactions. Unfortunatly, the Hirshfeld surfaces for 5-(4-ethoxyphenylazo)tropolone could not be generated as the coordinates were not inputted into the CSD. The curvedness The packing of I as viewed along the b-axis. The top-right insert illustrates the BrÁ Á Á and ! * CT interactions, while the bottom-right insert illustrates the O-HÁ Á ÁO and C-HÁ Á ÁN interactions as dashed bonds. Only selected hydrogen atoms are shown for clarity.

Figure 3
The BrÁ Á ÁBr interactions of I illustrated with dashed bonds. Hydrogen atoms omitted for clarity.

Figure 1
View of I with 50% probability displacement ellipsoids Table 1 Hydrogen-bond geometry (Å , ). plots of tropolone and 5-phenyltropolone show large regions of green. This is attributed to a relatively flat surface area (planar), whilst the blue regions illustrate areas of curvature ( Fig. 4). For tropolone and 5-phenyltropolone, it can be seen that the molecules are essentially planar, as mentioned previously, with 5-phenyltropolone having a dihedral angle (between the least-squares planes of the tropolone and phenyl moieties) of 1.57 (8) . This is smaller than the corresponding angle found for I [5.07 (6) ], where the additional twist of the dihedral planes can clearly be seen in the curvedness plot, as there are additional blue regions which snake over the 'planar' surface of the molecule. The curvedness plots of the compounds show flat surface areas, which is consistent with the planar packing arrangement that has been observed for both tropolone and 5-phenylazotropolone. It is interesting to note that in the curvedness plots, the BrÁ Á ÁBr interactions are clearly visible as curved (red) regions and contribute 9% to the total surface area. These interactions are mirrored in the shape-index plots but are a little harder to observe. On the shape-index surface plots for tropolone, 5-phenylazotropolone and I (Fig. 4), convex blue regions represent donor groups, whilst the red concave regions are the acceptor groups. Theinteractions are generally indicated by adjacent red and blue triangles. These triangles are clearly observed for both tropolone (17.5% surface contribution) and 5-phenyltropolone (10.5% surface contribution), whereas this triangle formation was not found for I, further supporting the finding of nointeractions. As mentioned in the Supramolecular features section, I is seen to have both BrÁ Á Á (7.1% surface contribution) and CT (9.2% surface contribution) interactions (Fig. 2). This is further supported by the Hirshfeld shape-index surface illustrating the blue region of the donor azo group and the red accepting region of the tropolone moiety (Fig. 4).

Synthesis and crystallization
The reagents were commercially obtained and used without further purification.

2-Hydroxy-5-[2-(2,4,6-tribromophenyl)diazen-1-yl]cyclohepta-2,4,6-trien-1-one
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.