Zwitterionic 1-{(1E)-[(4-hydroxyphenyl)iminio]methyl}naphthalen-2-olate: crystal structure and Hirshfeld surface analysis

The title zwitterion exists in the iminium/phenoxide form. The molecule is twisted around the N—C(benzene) bond with the C=N—C—C torsion angle being 39.42 (8)°. In the crystal, a zigzag supramolecular chain is sustained by charge-assisted hydroxy-O—H⋯O(phenoxide) hydrogen bonding.


Chemical context
Schiff bases derived from o-hydroxynaphthalehyde have attracted significant attention owing to their biological properties, such as anti-tumour activity (Richardson & Bernhardt, 1999;Gou et al., 2015), and their photophysical properties, such as thermo-and photochromism (Matijević-Sosa et al., 2006). Furthermore, the physical properties of these molecules led to their application in various areas of materials science, such as in the control and measurement of radiation intensity, display systems and optical memory devices (Dü rr, 1989;Hadjoudis & Mavridis, 2004). These Schiff bases have also been used as tools for assessing the nature of hydrogen bonding (Richardson & Bernhardt, 1999), as well as ketoamine and phenol-imine tautomerism (Ü nver et al., 2000) in related molecules. In view of these various applications, our recent investigations have focused on the structure determination of Schiff bases of this type, e.g. of (E)-N-[(2-methoxynaphthalen-1-yl)methylidene]-3-nitroaniline (Bhai et al., 2015). As a continuation of these studies, the crystal and molecular structures of the title compound, (I), are described herein along with an analysis of the Hirshfeld surface, performed in order to gain more information on the nature of the molecular packing.

Structural commentary
The molecular structure of (I) is shown in Fig. 1. Crystallography established the molecule to exist in a zwitterionic ISSN 2056-9890 form with the putative H atom of the naphthyl-hydroxy group being located on the imine-N atom. This assignment is supported by the short C9-O2 bond length of 1.283 (2) Å . The molecule features two planar regions connected by an imine (iminiumyl) bridge; the configuration about the imine bond [C1 N = 1.308 (2) Å ] is E. The twist in the molecule occurs around the N1-C2 bond, is seen in the value of the C1-N1-C2-C7 torsion angle of 31.1 (3) . The dihedral angle between the two aromatic regions is 39.42 (8) . The coplanar relationship between the imine and naphthyl residues is stabilized by an intramolecular charge-assisted N + -HÁ Á ÁO À hydrogen bond, Table 1.

Analysis of the Hirshfeld surface
The Hirshfeld surface was calculated for (I) according to earlier work on organic molecules (Tan et al., 2017) and provides more detailed information on the intermolecular interactions influential in the crystal. In addition to the bright-red spots near those atoms participating in charge-assisted O1-H1OÁ Á ÁO2 and C7-H7Á Á ÁO1 interactions on the Hirshfeld surface mapped over d norm , Fig. 3, the bright-red spots appearing near the benzene-C4, -C5 and -H7, and naphthyl-H13 atoms are indicative of short interatomic CÁ Á ÁH/ HÁ Á ÁC contacts significant in the crystal,    Views of the Hirshfeld surface for (I) mapped over the electrostatic potential in the range AE0.116 a.u.

Table 3
Percentage contributions of interatomic contacts to the Hirshfeld surface for (I).

Figure 5
Views of the Hirshfeld surfaces about a reference molecule mapped over the shape-index property highlighting (a) intermolecular O-HÁ Á ÁO and C-HÁ Á ÁO interactions, and short interatomic CÁ Á ÁH/HÁ Á ÁC contacts by black, red and sky-blue dashed lines, respectively, and (b) short interatomic CÁ Á ÁC and HÁ Á ÁH contacts, andstacking interactions by red, sky-blue and black dashed lines, respectively.
short interatomic CÁ Á ÁC contacts between the naphthyl-C9 and -C17 atoms, Table 2, assigned tostacking interactions, appear as faint-red spots in Fig. 3. The donors and acceptors of the aforementioned interactions appear as blue and red regions, respectively, around the atoms on the Hirshfeld surface mapped over electrostatic potential in the views shown in Fig. 4. The short interatomic contacts together with the charge-assisted O-HÁ Á ÁO and C-HÁ Á ÁO interactions formed with the atoms of a reference molecule within shape-index mapped Hirshfeld surface are highlighted in the views of Fig. 5. The overall two-dimensional fingerprint plot, Fig. 6a, and those delineated into HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁC contacts (McKinnon et al., 2007) are illustrated in Figs. 6b-e, respectively; the relative contributions from different interatomic contacts to the Hirshfeld surfaces are summarized in Table 3. The presence of a small peak in the centre at d e + d i $ 2.3 Å in the fingerprint plot delineated into HÁ Á ÁH contacts, Fig. 6b, results from the short interatomic HÁ Á ÁH contact between symmetry related naphthyl-H15 and -H16 atoms, Table 2. In the fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC contacts, Fig. 6c, the short interatomic contacts summarized in Table 2 appear as the points distributed as the pair of thick spikes with tips at d e + d i $ 2.6 Å . The presence of charge-assisted O-HÁ Á ÁO hydrogen bonds in the structure are characterized by the distinctive spikes with tips at d e + d i $ 1.7 Å , Fig. 6d, whereas the points belong to intermolecular C-HÁ Á ÁO hydrogen bond are merged within the plot. The fingerprint plot delineated into CÁ Á ÁC contacts, Fig. 6e, indicate the presence of thestacking interactions between symmetry related naphthyl-(C8-C12/C17) and -(C12-C17) rings through the arrow-shaped distribution with the green points spread about d e = d i = 1.8 Å . The small contributions from other interatomic contacts summarized in Table 3 have negligible effect on the molecular packing.

Database survey
The most closely related structure to (I) in the crystallographic literature (Groom et al., 2016) is that of the ethanol hemisolvate of (I), i.e. (I)Á0.5EtOH (Safia et al., 2015). Here, there are two molecules in the asymmetric unit and each exists in the zwitterionic form with C-O À = 1.288 (4) and 1.2943 (19) Å . By contrast to (I), the zwitterions in (I)Á0.5EtOH are more planar than in (I), with the dihedral angles between the aromatic residues being 7.59 (4) in one of the independent zwitterions and 8.01 (4) in the other. The other structure deserving of comment is that of 2-{[(4-hydroxyphenyl)imino]-methyl}phenol, where the 2-oxidonaphthyl group of (I) has been replaced by a 2-oxidobenzene residue. This has been crystallized in two forms, viz. a P2 1 /c form with Z 0 = 2 (Ersanlı et al., 2004) and a C2/c form with Z 0 = 1 (Wang et al., 2011). The common feature of the three molecules is the formation of hydroxyl/imine tautomer, as opposed to zwitterionic (I) and (I)Á0.5EtOH (Safia et al., 2015). The three molecules have smaller deviations from planarity than (I), as seen in the dihedral angles between the aromatic rings of 10.43 (6) and 15.70 (6) for the P2 1 /c form, and 14.91 (9) for the C2/c form. Finally, a deprotonated form of (I), with the 4-hydroxy group intact, forms a six-membered {Pd-O-C . . . C-C N} chelate ring in its bis-complex with palladium(II) (Tardiff et al., 2007).

Synthesis and crystallization
4-Hydroxyaniline (0.00916 mol, 1.00 g) was added to a solution of 2-hydroxy-1-napthaldehyde (0.00916 mol, 1.58 g) in ethanol (25 ml). The resulting mixture was refluxed at 333 K and stirred for 2.5 h. The reaction mixture was cooled to room temperature and the resulting orange precipitate was filtered off and washed with cold ethanol to obtain the pure product in 65% yield. Crystals of (I) were grown from a mixture of chloroform and methanol (1:1 v/v) by slow evaporation.