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Crystal structure of 4-[(3-methoxy-2-oxidobenzylidene)azaniumyl]benzoic acid methanol monosolvate

In the crystal, the Schiff base molecule exists in the zwitterionic form and an intramolecular N—H⋯O hydrogen bond stabilizes the molecular structure.


Chemical context
Vanillin and o-vanillin are natural compounds that have both a phenolic OH and an aldehyde group. They are positional isomers, in which o-vanillin shows contradictory effects. There are several reports indicating that o-vanillin induces mutations and it has also been found to enhance chromosomal aberrations in in vitro systems (Barik et al., 2004;Takahashi et al., 1989). Vanillin is also the primary component of the extract of the vanilla bean. Synthetic vanillin rather than natural vanilla extract is now more often used as a flavouring agent in foods, beverages and pharmaceuticals. Schiff bases containing ovanillin possess antifungal and antibacterial properties (Thorat et al., 2012). 4-Aminobenzoic acid (PABA) is an important biological molecule, being an essential bacterial cofactor involved in the synthesis of folic acid (Robinson, 1966). PABA shows polymorphism and so far four polymorphs of PABA are known, all of which are centrosymmetric; a noncentrosymmetric polymorph of 4-aminobenzoic acid has also been reported (Benali-Cherif et al., 2014). Schiff bases derived from 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) and PABA have not been investigated so thoroughly. Our research interest focuses on the study of Schiff bases derived from salicylaldehyde. It is well known that Schiff bases of salicylaldehyde derivatives may exhibit thermochromism or photochromism, depending on the planarity or non-planarity of the molecule (Cohen & Schmidt, 1964;Amimoto & Kawato, 2005). Schiff bases often exhibit various biological activities and in many cases have been shown to possess antibacterial, anticancer, anti-inflammatory and antitoxic properties (Lozier et al., 1975). They are used as anion sensors (Dalapati et al., 2011), as non-linear optical compounds (Sun et al., 2012) and as versatile polynuclear ligands for multinuclear magnetic exchange clusters (Moroz et al., 2012). New salicylaldehyde- ISSN 2056-9890 based Schiff bases have also been synthesized and reported (Faizi et al., 2015a,b;2016b;2017a,b,c). The present work is a part of an ongoing structural study of Schiff bases and their utilization in the synthesis of new organic, excited state proton-transfer compounds and fluorescent chemosensors (Faizi et al., 2016a;Faizi et al., 2018;Kumar et al., 2018;Mukherjee et al., 2018). We report herein the crystal structure of the title compound synthesized by the condensation reaction of 2-hydroxy-3-methoxybenzaldehyde and PABA.

Structural commentary
The asymmetric unit of the title compound contains a Schiff base molecule and a methanol molecule of crystallization. In the solid state, the Schiff base molecule (Fig. 1) exists in the zwitterionic form. An intramolecular N-HÁ Á ÁO hydrogen bond stabilizes the molecular structure (Table 1). The imine group, which displays a C9-C8-N1-C5 torsion angle of 177.6 (3) , contributes to the general planarity of the molecule. The Schiff base molecule displays a trans configuration with respect to the C N and C-N bonds. The vanillin ring (C9-C14) is inclined to the central benzene ring (C2-C7) by 5.34 (2) . A similar value of 5.3 (2) is observed in 4-chloro-N 0 -(2-hydroxy-4-methoxybenzylidene)benzohydrazide methanol monosolvate (Zhi et al., 2011). All bond lengths are in normal ranges. The O4-C15 bond length is 1.432 (2) Å and similar value of 1.432 (2) Å is observed in (E)-2-hydroxy-3-methoxy-5-[(3-methoxyphenyl)diazenyl]benzaldehyde (Karadayı et al., 2006). The methoxy group of the 2-hydroxy-3-methoxyphenyl is almost coplanar with its bound benzene ring, as seen by the C methyl -O-C-C torsion angle of 178.1 (2) .

Supramolecular features
In the crystal, the hydroxyl group of the methanol solvent molecule is linked to the carboxylate group of the neighboring Schiff base molecule and the deprotonated hydroxyl group of the other Schiff base molecule via classical O-HÁ Á ÁO hydrogen bonds, forming supramolecular chains propagating along the b-axis direction (Fig. 2). Weak C-HÁ Á ÁO hydrogen bonds further link the chains into a three-dimensional supramolecular architecture.  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view of the hydrogen-bonded chain extending along the b-axis direction. Hydrogen bonds are shown as dashed lines.

Synthesis and crystallization
To a hot stirred solution of 4-aminobenzoic acid (PABA) (1.00 g, 7.2 mmol) in methanol (15 ml) was added vanillin (1.11 g, 7.2 mmol)). The resulting mixture was then heated under reflux. After an hour, a precipitate formed. The reaction mixture was heated for about another 30 min until the completion of the reaction, which was monitored by TLC. The reaction mixture was cooled to room temperature, filtered and washed with hot methanol. It was then dried under vacuum to give the pure compound in 78% yield. Prismatic colourless single crystals of the title compound suitable for X-ray analysis were obtained by slow evaporation of a methanol solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Zwitterionic forms of some closely related compounds. Refined as a perfect inversion twin. Absolute structure parameter 0.5 located in a difference-Fourier map. Their positional and isotropic thermal parameters were included in further stages of the refinement. All C-bound H atoms were positioned geometrically and refined using a riding model with C-H = 0.93-0.97 Å and with U iso (H) = 1.2-1.5U eq (C). SHELXL2017 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009).  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.