Crystal structure and Hirshfeld surface analysis of (E)-2-[1-hydroxy-2-(pyridin-2-yl)ethyl]-4-[2-(4-methoxyphenyl)diazen-1-yl]phenol

In the title compound, the configuration about the N=N bond is E, and the central benzene ring is inclined to the pyridine ring by 31. 43 (8)° and to the 4-methoxyphenyl ring by 4.73 (8)°. In the crystal, molecules are linked by pairs of O—H⋯N hydrogen bonds, forming inversion dimers with an (12) ring motif.


Chemical context
Azo compounds have received much attention in fundamental and applied chemistry (Nishihara, 2004;İspir, 2009). The wellknown applications of azo dyes in acid-base indicators and chemical sensors and as electron-transfer catalysts have attracted the interest of many investigators (Tunçel & Serin, 2006). The versatile applications of azo compounds in various fields include dyeing textile fibres, colouring different materials, plastics, biological medical studies, lasers, liquid crystalline displays, electro-optical devices and ink-jet printers in high-technology areas (Gregory, 1991). The conversion from the trans to the cis form in azo compounds can lead to photochromism. Photochromic compounds are of great interest for the control and measurement of radiation intensity, optical computers and display systems (Dü rr & Bouas-Laurent, 1990), and for potential applications in molecular electronic devices (Martin et al., 1995). Schiff bases often exhibit various biological activities, including antibacterial, anticancer, anti-inflammatory and antitoxic properties (Lozier et al., 1975). The present work is part of an ongoing structural study of heterocyclic compounds (Faizi et al., 2016(Faizi et al., , 2017 and excited state proton-transfer compounds and fluorescent chemosensors Kumar et al., 2018;Mukherjee et al., 2018). In the present work, we report the synthesis, crystal structure and Hirshfeld surface analysis of the title compound, (E)-2-[1-hydroxy-2-(pyridin-2-yl)ethyl]-4-[2-(4methoxyphenyl)diazen-1-yl]phenol.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The configuration about the azo N N bond is E, and the N2 N3 bond length is 1.256 (2) Å . The molecule is nonplanar, with the central benzene ring (C8-C13) being inclined to the pyridine ring (N1/C1-C5) by 31.43 (8) and to the outer 4-methoxyphenyl ring (C14-C19) by 4.73 (8) .

Supramolecular features
In the crystal, molecules are linked by pairs of O-HÁ Á ÁN hydrogen bonds, forming inversion dimers with an R 2 2 (12) ring motif (Table 1 and Fig. 2). The dimers are linked by O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, forming undulating layers lying parallel to the ac plane ( Fig. 3 and Table 1). There are C-HÁ Á Á interactions present within the layers and between the layers, leading to the formation of a supramolecular framework (Table 1 and  The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 40% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view of the inversion dimer forming an R 2 2 (12) ring motif; see Table 1 for details of the hydrogen-bonding (dashed lines) interactions involved.

Figure 3
A view along the c axis of the crystal packing of the title compound. For clarity, H atoms not involved in hydrogen bonding (dashed lines, see Table 1) have been omitted.

Figure 4
A view along the b axis of the crystal packing of the title compound. For clarity, H atoms not involved in hydrogen bonding (dashed lines, see Table 1) have been omitted. The C-HÁ Á Á interactions are represented by brown arrows and the offsetinteractions by blue double arrows. offsetinteractions, involving inversion-related 4-methoxyphenol rings, which strengthen the supramolecular framework [Cg3Á Á ÁCg3 vi = 3.584 (2) Å , interplanar distance = 3.416 (2) Å , offset = 1.085 Å ; Cg3 is the centroid of the C14-C19 ring; symmetry code: (vi) Àx + 1, Ày + 1, Àz + 1].  İskeleli et al., 2006). In all five compounds, the configuration about the N N bond is E, and the dihedral angles between the 4-methoxyphenyl ring and the other aryl ring are ca 3.04, 5.43, 11.61, 8.34 and 16.01 , respectively. In the title compound, this dihedral angle is 4.73 (8) , similar to that in AQIDIO and FUGYIP.

Hirshfeld surface analysis and two-dimensional fingerprint plots
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed with Crystal-Explorer17 (Turner et al., 2017). The reader is referred to a recent article by Tiekink and collaborators (Tan et al., 2019) who have published an excellent explanation of the use of Hirshfeld surface analysis and other calculations to study molecular packing.

Synthesis and crystallization
The title compound was prepared by adding n-butyllithium (4.91 ml, 12.29 mmol, 2.5 M in cyclohexane) to a solution of 2-picoline (1 ml, 10.24 mmol) in anhydrous THF (25 ml) cooled at 195 K. The orange mixture was left to warm up to 143 K and then 5-(4-methoxyphenylazo)salicyaldehyde (MPS) (2.00 g, 8.53 mmol) dissolved in THF (10 ml) was added, giving a yellow solution. The solution was then stirred for 2 h at room temperature. The reaction was quenched by the addition of an aqueous saturated solution of ammonium chloride (50 ml), and the product was extracted with diethyl ether. It was then dried over MgSO 4 and purified by column chromatography (cyclohexane/ethyl acetate 9/1) to give a yellow solid (1.10 g, 3.36 mmol, yield: 60%). Yellow needlelike crystals of the title compound were obtained by slow evaporation of a solution in methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The OH and C-bound H atoms were included in calculated positions and treated as riding atoms: O-H = 0.82 Å and C-H = 0.93-0.98 Å , with U iso (H) = 1.5U eq (O-hydroxyl and C-methyl) and 1.2U eq (C) for other H atoms.

(E)-2-[1-Hydroxy-2-(pyridin-2-yl)ethyl]-4-[2-(4-methoxyphenyl)diazen-1-yl]phenol
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.