Crystal structure and DFT study of (E)-2-chloro-4-{[2-(2,4-dinitrophenyl)hydrazin-1-ylidene]methyl}phenol acetonitrile hemisolvate

The title Schiff base compound was obtained from a condensation reaction of 4-chloro-3-hydroxybenzaldehyde and 2,4-dinitrophenylhydrazine. The molecule is almost planar with the dihedral angle between the benzene rings being 3.70 (17)°.

The title Schiff base compound, C 13 H 9 ClN 4 O 5 Á0.5CH 3 CN, crystallizes as an acetonitrile hemisolvate; the solvent molecule being located on a twofold rotation axis. The molecule is nearly planar, with a dihedral angle between the two benzene rings of 3.7 (2) . The configuration about the C N bond is E, and there is an intramolecular N-HÁ Á ÁO nitro hydrogen bond present forming an S(6) ring motif. In the crystal, molecules are linked by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds, forming layers lying parallel to (101). The layers are linked by C-HÁ Á ÁCl hydrogen bonds, forming a supramolecular framework. Within the framework there are offsetstacking interactions [intercentroid distance = 3.833 (2) Å ] present involving inversion-related molecules. The DFT study shows that the HOMO and LUMO are localized in the plane extending from the phenol ring to the 2,4-dinitrobenzene ring, and the HOMO-LUMO gap is found to be 0.13061 a.u.

Chemical context
Over the past 25 years, extensive research has surrounded the synthesis and use of Schiff base compounds in organic and inorganic chemistry, as they have important medicinal and pharmaceutical applications. These compounds show biological activities including antibacterial, antifungal, anticancer and herbicidal activities (Desai et al., 2001;Singh & Dash, 1988;Karia & Parsania, 1999). Schiff bases are also becoming increasingly important in the dye and plastics industries as well as for liquid-crystal technology and the mechanistic investigation of drugs used in pharmacology, biochemistry and physiology (Sheikhshoaie & Sharif, 2006). 2,4-Dinitrophenylhydrazine is frequently used as a reagent for the characterization of aldehydes and ketones (Furniss et al., 1999). Its derivatives are widely used as dyes (Guillaumont & Nakamura, 2000). They are also found to have versatile coordinating abilities towards different metal ions (Raj & Kurup, 2007). The present work is a part of an ongoing structural study of Schiff bases and their utilization in the synthesis of quinoxaline derivatives (Faizi et al., 2016a), fluorescence sensors (Faizi et al., 2016b) and coordination compounds (Faizi & Prisyazhnaya, 2015). We report herein on the synthesis, crystal structure and DFT computational calculations of the title new Schiff base compound. The results of calculations by density functional theory (DFT) carried out at the B3LYP/6-311 G(d,p) level are compared with the experimentally determined molecular structure in the solid state.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The configuration about the C7 N1 bond is E, and there is intramolecular N-HÁ Á ÁO nitro hydrogen bond that generates an S(6) ring motif ( Fig. 1 and Table 1). The N1-N2 bond length is 1.380 (3) Å and the N1 C7 bond length is 1.275 (4) Å . These bond lengths are comparable with those of some closely related compounds (Fun et al., 2013;Faizi et al., 2017;Ghosh et al., 2016). The C8-C9 and C8-C13 bonds [1.411 (5) and 1.414 (4) Å , respectively], which are adjacent to the imino N2 atom, are significantly longer than the average distance of 1.375 (3) Å for the other C-C bonds in the same benzene ring. This same pattern of bond lengths has been observed previously in some 2,4-dinitrophenylhydrazone derivatives (Ohba, 1996;Borwick et al., 1997). The title molecule is almost planar with the dihedral angle between the benzene rings being 3.70 (17) . The nitro groups of the 2,4dinitrophenyl unit are twisted slightly with respect to the C8-C13 benzene ring to which they are attached: nitro group N2/ O4/O5 is inclined to the benzene ring by 2.1 (4) , while nitro group N3/O2/O3 is inclined to it by 6.5 (5) .

Figure 2
A view along the a axis of the crystal packing of the title compound. Hydrogen bonds (see Table 1) are shown as dashed lines. For clarity, the acetonitrile solvent molecules have been omitted and only hydrogen atoms H1 and H2 have been included.

Figure 1
The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 40% probability level. The intramolecular N-HÁ Á ÁO hydrogen bond (see Table 1), forming an S(6) ring motif, is shown as a dashed line.

Figure 3
A view normal to plane (110) of the crystal packing of the title compound. Hydrogen bonds (see Table 1) are shown as dashed lines, and, for clarity, only hydrogen atoms H1, H2 and H9 have been included.

DFT study
The DFT quantum-chemical calculations were performed at the B3LYP/6-311 G(d,p) level (Becke, 1993) as implemented in GAUSSIAN09 (Frisch et al., 2009). The DFT structure optimization of the title compound was performed starting from the X-ray geometry, with experimental values of bond lengths and bond angles matching with theoretical values. The 6-311 G(d,p) basis set is well suited in its approach to the experimental data. The DFT study shows that the HOMO and LUMO are localized in the plane extending from the whole phenol ring to the 2,4-dinitrobenzene ring. The electron distribution of the HOMO-1, HOMO, LUMO and the LUMO+1 energy levels are shown in Fig. 4. The HOMO molecular orbital exhibits both and character, whereas HOMO-1 is dominated by -orbital density. The LUMO is mainly composed of -density while LUMO+1 has both and electronic density. The HOMO-LUMO gap was found to be 0.13061 a.u. and the frontier molecular orbital energies, E HOMO and E LUMO are À0.24019 and À0.10958 a.u., respectively. Here the benzene rings are inclined to each other by 3.40 (9) , compared to 3.70 (17) in the title compound, and again there is an intramolecular N-HÁ Á ÁO nitro hydrogen bond present forming an S(6) ring motif. In fact, in all 71 structures (see supporting information) there is an intramolecular N-HÁ Á ÁO nitro hydrogen bond present forming an S(6) ring motif, and in the majority of the compounds the two benzene rings are almost coplanar with the dihedral angle varying between ca 0 to 8 , with a few exceptions. Electron distribution of the HOMO-1, HOMO, LUMO and the LUMO+1 energy levels for the title compound.

Synthesis and crystallization
The title compound was prepared by refluxing a mixture of 4chloro-3-hydroxybenzaldehyde (39.1 mg, 0.25 mmol) in ethanol (15 ml) and 2,4-dinitrophenylhydrazine (49.5 mg, 0.25 mmol) in ethanol (15 ml). The reaction mixture was stirred for 5 h under reflux. Orange plate-like crystals of the title compound were obtained by slow evaporation of a solution in ethanol (yield 68%, m.p. 542-544K).

Refinement
Crystal data, data collection and structure refinement details are summarized in  (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2018 (Sheldrick, 2015b), 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.