Crystal structure and Hirshfeld surface analysis of 1-[(E)-2-(3-nitrophenyl)diazen-1-yl]naphthalen-2-ol

The hydrazone form is the predominant form in the solid state. The naphthol and benzene fragments attached to the –N=N– moiety adopt the s-trans conformation. There are only two types of intermolecular interactions in the crystal structure: strong hydrogen-bonding C—H⋯O interactions and π–π stacking interactions.


Chemical context
Azo compounds, which include the functional group R-N N-R 0 where R and R 0 can either be aryl or alkyl, aryl azo compounds being more common than aliphatic azo compounds (Christie, 2001), have striking colors. These colors, particularly reds, oranges, and yellows, are the result ofelectron delocalization through aromatic moieties (Debnath et al., 2015;Ferreira et al., 2013). They are therefore used as dyes, not only in textile colorants but in many other industrial fields for coloring different substrates, as printing inks, in biological reactions and in the cosmetics industry (Hunger, 2003;Ran et al., 2022;Mathieu-Denoncourt et al., 2014;Shi & Chen, 2014;Chudgar & Oakes, 2003;Benkhaya et al., 2020).
Detailed knowledge of molecular structures is essential for determining structure-function relationships and for a systematic approach to the design of new dyes. Structural information obtained from single-crystal X-ray diffraction analysis including conformation, stereochemistry, intra-and intermolecular interactions is related to the optical properties of azo dyes (Pavlović et al., 2009). In the case of 1-phenylazo-2-naphthol derivatives, a strong hydrogen bond enhanced by resonance is established, inducing the azo (OH) ! hydrazo (NH) tautomeric displacement (Benosmane et al., 2015;Bougueria et al., 2014). This is directly connected with the presence of at least one protic donor group in conjugation to the azo bridge (2-naphthol) (Antonov, 2016). As a part of our continuing interest in the synthesis and crystallography evaluation of azo-2 naphthol compounds, we embarked on the present crystallographic study and report herein the synthesis, molecular structure and Hirshfeld surface analysis of dye derived from 1-phenylazo-2naphtol: (E)-1-(3-nitrophenylazo)-2-naphtol.

Structural commentary
The molecular structure of the title compound ( Fig. 1) was solved in the orthorhombic space group P2 1 2 1 2 1 . The N1-N2, C1-N1, C7-N2 and C8-O1 bond lengths are 1.312 (4), 1.394 (5), 1.330 (5) and 1.276 (5) Å , respectively, indicating that the dye compound has crystallized in the hydrazone tautomeric form (i.e. proton transfer from the naphthol group to the azo group); bond lengths and angles are within normal ranges and are comparable to those reported for other azo compounds Bougueria, Benosmane et al., 2013;Mili et al., 2013;Xu et al., 2010). The molecule adopts an s-trans conformation, with the two aryl groups residing on the opposite side of the azo group. The naphthol and benzene rings attached to the hydrazo group are almost coplanar, subtending a dihedral angle of 2.63 (5) , indicating significant electron delocalization within the molecule. The molecular structure is stabilized by an intramolecular N-HÁ Á ÁO hydrogen bond involving hydrogen atoms from the hydrazo groups (Table 1).

Figure 3
-stacking interactions, view along the c axis of the stacked molecules.

Figure 1
The molecular structure with the atom-labeling scheme. Displacement ellipsoids drawn at the 30% probability level. Intramolecular hydrogen bonds are shown as dashed lines.

Figure 2
A view along the c axis of the crystal packing of the title compound. The C-HÁ Á ÁO hydrogen bonds are shown as dashed lines (see Table 1 for details).
tronic properties and theoretical investigation of a novel square-planar nickel (II) complex with an (o-tolyldiazenyl) naphthalen-2-ol ligand (Benosmane et al., 2023) that exhibits structural diversity with interesting optoelectronic properties.

Hirshfeld surface analysis
The supramolecular interactions in the title structure were investigated quantitatively and visualized with Crystal Explorer (Spackman & Jayatilaka, 2009;McKinnon et al., 2004). Fig. 4 shows the Hirshfeld surface mapped over d norm in the range À0.2344 (red) a.u. to 1.2354 (blue) a.u. The donors and acceptors of intermolecular C-HÁ Á ÁO closest interactions in the structure are seen as bright-red spots near the benzene-H2, naphthalene-H9, hydroxyl-O1 and nitro-O3 atoms. The Hirshfeld surface mapped over shape-index is shown in Fig. 5 where the triangles clearly illustrate thestacking interactions. The two-dimensional fingerprint plots are shown in Fig. 6. HÁ Á ÁO/OÁ Á ÁH interactions provide the largest contribution (28.5%) to the surface. The second largest contribution is from HÁ Á ÁH contacts (26.4%). The presence of CÁ Á ÁC interactions (6.1%), corresponding tostacking, is also important. Table 2 summarizes the percentage contributionsof different types of contacts to the Hirshfeld surface.

Synthesis and crystallization
The title compound was obtained through the diazotization of 3-nitroaniline followed by a coupling reaction with 2-naphthol.
A solution of hydrochloric acid (12 M) and 6 mL of water were added to 3-nitroaniline (0.02 mol) at 273 K. Sodium nitrite solution (0.02 mol, in 10 mL of water) was added dropwise to the cooled mixture and stirred for 15 min. To the formed diazonium salt was added dropwise an aqueous solution of 2-naphthol (0.02 mol in 100 mL of water) containing sodium hydroxide (16 mL). The mixture was then allowed to stir for 1 h at 273 K. The resulting red precipitate was filtered and washed several times with distilled water and dried in air.
Red needle-shaped crystals suitable for X-ray analysis were obtained by slow evaporation of an ethanol solution at room temperature (yield 85.4%).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3 Three-dimensional Hirshfeld surface mapped over shape-index.

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses 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 observed criterion of F 2 > 2sigma(F 2 ) is used only for calculating -R-factor-obs 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.