2-{(1E)-[(E)-2-(2,6-Dichlorobenzylidene)hydrazin-1-ylidene]methyl}phenol: crystal structure, Hirshfeld surface analysis and computational study

An E configuration about each of the two imine bonds is found in the title molecule which, to a first approximation, is planar. The main feature of the molecular packing is π–π stacking leading to helical, supramolecular chains.

The title Schiff base compound, C 14 H 10 Cl 2 N 2 O, features an E configuration about each of the C N imine bonds. Overall, the molecule is approximately planar with the dihedral angle between the central C 2 N 2 residue (r.m.s. deviation = 0.0371 Å ) and the peripheral hydroxybenzene and chlorobenzene rings being 4.9 (3) and 7.5 (3) , respectively. Nevertheless, a small twist is evident about the central N-N bond [the C-N-N-C torsion angle = À172.7 (2) ]. An intramolecular hydroxy-O-HÁ Á ÁN(imine) hydrogen bond closes an S(6) loop. In the crystal,stacking interactions between hydroxy-and chlorobenzene rings [inter-centroid separation = 3.6939 (13) Å ] lead to a helical supramolecular chain propagating along the b-axis direction; the chains pack without directional interactions between them. The calculated Hirshfeld surfaces point to the importance of HÁ Á ÁH and ClÁ Á ÁH/HÁ Á ÁCl contacts to the overall surface, each contributing approximately 29% of all contacts. However, of these only ClÁ Á ÁH contacts occur at separations less than the sum of the van der Waals radii. The aforementionedstacking interactions contribute 12.0% to the overall surface contacts. The calculation of the interaction energies in the crystal indicates significant contributions from the dispersion term.

Chemical context
Being deprotonable and readily substituted with various residues, Schiff base molecules are prominent as multidentate ligands for the generation of a wide variety of metal complexes. In our laboratory, a key motivation for studies in this area arises from our interest in the Schiff bases themselves and of their metal complexes, which are well-known to possess a wide spectrum of biological activity against disease-causing microorganisms (Tian et al., 2009;2011). Over and beyond biological considerations, Schiff bases are also suitable for the development of non-linear optical materials because of their solvato-chromaticity (Labidi, 2013).
As reported recently, the title compound, (I), a potentially multidentate ligand has anti-bacterial and anti-fungal action ISSN 2056-9890 against a range of microorganisms (Manawar et al., 2019). As a part of complementary structural studies on these molecules, the crystal and molecular structures of (I) are described herein together with a detailed analysis of the calculated Hirshfeld surfaces.

Supramolecular features
The most prominent supramolecular association in the crystal of (I) arestacking interactions. These occur between the hydroxy-and chlorobenzene rings with an inter-centroid separation = 3.6939 (13) Å and angle of inclination = 4.32 (11) [symmetry operation 3 2 À x, 1 2 + y, 1 2 À z]. As these interactions occur at both ends of the molecule and are propagated by screw-symmetry (2 1 ), the topology of the resultant chain is helical, Fig. 2 Table 1 Hydrogen-bond geometry (Å , ).  The molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 35% probability level.
incorporated in PLATON (Spek, 2009), there are no directional interactions connecting chains; a view of the unit-cell contents is shown in Fig. 2(b). The presence of other, weaker points of contact between atoms and between residues are noted -these are discussed in more detail in Hirshfeld surface analysis.

Hirshfeld surface analysis
The Hirshfeld surface calculations for (I) were performed employing Crystal Explorer 17 (Turner et al., 2017) and recently published protocols (Tan et al., 2019). On the Hirshfeld surface mapped over d norm in Fig. 3, the short interatomic contact between the hydroxyphenyl-C2 and chlorophenyl-C12 atoms (Table 2) is characterized as small red spots near them. The Cl1 and Cl2 atoms form short intra-layer ClÁ Á ÁH contacts with the H4 and H6 atoms of the hydroxyphenyl ring (Table 2) and are represented in Fig. 4, showing a reference molecule within the Hirshfeld surface mapped over the electrostatic potential. The Hirshfeld surface mapped with curvedness is shown in Fig. 5, which highlights the influence of the short interatomic CÁ Á ÁC contacts in the packing (Table 2) consistent with the edge-to-edgestacking between symmetry related molecules. The full two-dimensional fingerprint plot for (I), Fig. 6(a), and those decomposed into HÁ Á ÁH, OÁ Á ÁH/HÁ Á ÁO, ClÁ Á ÁH/ HÁ Á ÁCl, CÁ Á ÁC and CÁ Á ÁH/HÁ Á ÁC contacts are illustrated in Fig. 6(b)-(f), respectively. The percentage contributions from the different interatomic contacts to the Hirshfeld surface of (I) are quantitatively summarized in Table 3 Table 2 Summary of short interatomic contacts (Å ) in (I) a .

Contact
Distance Symmetry operation 3.399 (3) 1 À x, À y, 1 À z Note: (a) The interatomic distances were calculated using Crystal Explorer 17 (Turner et al., 2017) whereby the X-H bond lengths are adjusted to their neutron values.

Figure 3
A view of the Hirshfeld surface for (I) mapped over d norm in the range À0.001 to + 1.301 (arbitrary units), highlighting diminutive red spots near the C2 and C12 atoms owing to their participation in CÁ Á ÁC contacts. Table 3 Percentage contributions of interatomic contacts to the Hirshfeld surface for (I).

Contact
Percentage contribution

Figure 4
A view of the Hirshfeld surface mapped over the electrostatic potential (the red and blue regions represent negative and positive electrostatic potentials, respectively) in the range À0.065 to + 0.039 atomic units, with short interatomic ClÁ Á ÁH and OÁ Á ÁH contacts highlighted with red and black dashed dashed lines, respectively. the fingerprint plot delineated into HÁ Á ÁH contacts in Fig. 6(b) that their interatomic distances are equal to or greater than the sum of their respective van der Waals radii. The fingerprint plot delineated into OÁ Á ÁH/HÁ Á ÁO contacts in Fig. 6 Fig. 6(f) confirms the absence of significant C-HÁ Á Á and CÁ Á ÁH/HÁ Á ÁC contacts as the points in the respective delineated plot are distributed farther than sum of their respective van der Waals radii. The small contribution from other interatomic contacts to the Hirshfeld surfaces of (I) summarized in Table 3 have a negligible effect on the molecular packing.

Computational chemistry
In the present analysis, the pairwise interaction energies between the molecules in the crystal were calculated by summing up four different energy components (Turner et al., 2017). These comprise electrostatic (E ele ), polarization (E pol ), dispersion (E dis ) and exchange-repulsion (E rep ), and were obtained using the wave function calculated at the B3LYP/6-31G(d,p) level of theory. From the intermolecular interaction energies collated in Table 4, it is apparent that the dispersion energy component has a major influence in the formation of the supramolecular architecture of (I) as conventional hydrogen bonding is absent. The energy associated with thestacking interaction between symmetry-related hydroxyand chlorobenzene rings is greater than the energy calculated for the ClÁ Á ÁH/HÁ Á ÁCl and OÁ Á ÁH/HÁ Á ÁO contacts. The magnitudes of intermolecular energies were also represented graphically in Fig. 7 by energy frameworks whereby the cylinders join the centroids of molecular pairs using a red, green and blue colour scheme for the E ele , E disp and E tot components, respectively; the radius of the cylinder is proportional to the magnitude of interaction energy.

Database survey
Given the great interest in Schiff bases and their complexation to transition metals and other heavy elements, it is not surprising that there is a wealth of structural data for these compounds in the Cambridge Structural Database (CSD; Groom et al., 2016). Indeed, there are over 150 'hits' for the basic framework 2-OH-C 6 -C N-N C-C 6 featured in    The energy frameworks calculated for (I) showing the (a) electrostatic potential force, (b) dispersion force and (c) total energy. The energy frameworks were adjusted to the same scale factor of 50 with a cut-off value of 5 kJ mol À1 within 4 Â 4 Â 4 unit cells (I). This number is significantly reduced when H atoms are added to the imine-carbon atoms and examples where a second hydroxy substituent present in the 2-position of the phenyl ring is excluded. Thus, there are eight molecules in the CSD containing the fragment 2-OH-C 6 -C(H) N-N C(H)-C 6 , excluding two calix(4)arene derivatives. While the formation of the hydroxy-O-HÁ Á ÁN(imine) bond is common to all molecules, there is a certain degree of conformational flexibility in the molecules as seen in the relevant geometric data collated in Table 5. From the data in Table 5, the molecule reported herein, i.e. (I), exhibits the greatest twist about the central N-N bond, whereas virtually no twist is seen in the central C-N-N-C torsion angle for (V), i.e. À179.8 (2) . The dihedral angles between the central C 2 N 2 residue and the hydroxy-substituted benzene ring span a range 2.27 (9) , again in (V), to 10.58 (4) , for (IV). A significantly greater range is noted in the dihedral angles between C 2 N 2 and the second benzene ring, i.e. 2.32 (12) in (VII) to 29.03 (16) in (II). Accordingly, the greatest deviation from co-planarity among the nine molecules included in Table 5 is found in (II) where the dihedral angle between the outer rings is 31.35 (8) .

Synthesis and crystallization
Compound (I) was prepared as reported in the literature from the condensation reaction of 2,6-dichlorobenzaldehyde and hydrazine hydrate (Manawar et al., 2019). Crystals in the form of light-yellow blocks for the X-ray study were grown by the slow evaporation of its chloroform solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. Carbon-bound H-atoms were placed in calculated positions (C-H = 0.93 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C). The position of the O-bound H atom was refined with U iso (H) set to 1.5U eq (O).   Table 5 Geometric data ( ) for related 2-OH-C 6 -C(H) N-N C(H)-C 6 molecules, i.e. R 1 -C(H) N-N C(H)-R 2 .

2-{(1E)-[(E)-2-(2,6-Dichlorobenzylidene)hydrazin-1-\ ylidene]methyl}phenol
Crystal data C 14 H 10 Cl 2 N 2 O M r = 293.14 Monoclinic, P2 1 /n a = 8.5614 (8)  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.