Whole-molecule disorder of the Schiff base compound 4-chloro-N-(4-nitrobenzylidene)aniline: crystal structure and Hirshfeld surface analysis

In the solid state the title compound shows full-molecule disorder (occupancy ratio 0.65: 0.35), generated by a twofold rotation about the shorter axis of the molecule.


Chemical context
A number of benzylideneaniline derivatives crystallize in noncentrosymmetric space groups and are therefore of interest for their non-linear optical properties (Bar & Bernstein, 1977;Batra et al., 2004). In 1970, Bü rgi & Dunitz (1970 analysed a number of N-benzylanilines and found that they were twisted about the N C bond unlike trans-stilbenes (see for example: Behrnd et al., 2010;De Borger et al., 2005) or trans-azobenzenes (see for example : Huang et al., 2002;Bushuyev et al., 2016), which are almost planar. ISSN 2056-9890 Benzylideneaniline derivatives are known to exhibit disorder and Bernstein and collaborators (Bar & Bernstein, 1983;Kluge et al., 2003) have defined the different types of orientational disorder of these compounds, where the molecules may be oriented in different ways but in the two or more dispositions each atom is essentially superimposed on another at any one crystallographic site. Static disorder around the C N bond is also responsible for the apparent shortening of the C N bond at room temperature (Bar & Bernstein, 1984). This phenomenon has also been studied by Harada et al. (2004a), who, by means of a variable temperature study, concluded that the shortening depends on temperature and is due to a torsional vibration of the C-phenyl and Nphenyl bonds in the crystals.
The crystal structures of a number of disordered benzylideneaniline compounds have been reported on and various forms of the disorder have been analysed (Bar & Bernstein, 1977, 1984Harada et al., 2004a,b). The disorder appears to fall into three categories ( Fig. 1): D1 -twofold rotation about the longer axis of the molecule, D2 -the molecule is located about a crystallographic center of symmetry, and D3 -twofold rotation about the shorter axis of the molecule.
Three forms of p-methyl-N-(p-methylbenzylidene)aniline (Bernstein, Bar & Christensen, 1976;Bar & Bernstein, 1982;Bar & Bernstein, 1977) have been shown to exist: Form I (Bar & Bernstein, 1982), crystallizes in space group P2 1 /c and the C N bond of the molecule is located about a center of symmetry, hence the molecule has type D2 disorder; form II (Bar & Bernstein, 1977) crystallizes in space group P2 1 and the molecule is not disordered; form III (Bar & Bernstein, 1977;Harada et al., 2004b), has a fourfold disorder with the mol-ecule being located about a center of symmetry and has a twofold rotation about the longer axis of the molecule (D1 + D2).
To continue the series of 4-halogen species, we report herein on the crystal structure of 4-chloro-4 0 -nitro-benzylideneaniline (CNBA). It was previously synthesized by Batra et al. (2004), who found that the crystals they obtained showed good second harmonic generation (SHG) of 1.064 micron wavelength radiation. The crystal structure analysis carried out for CNBA in this work shows that it crystallizes in the centrosymmetric space group P2 1 /c, and that the molecule has positional disorder (type D3), hence no SHG properties are expected for this particular sample. It is interesting to note that the structure of 4-bromo-4 0 -nitrobenzylideneaniline (BNBA) crystallizes in a non-centrosymmetric space group (A2), while the title compound and 4-fluoro-4 0 -nitrobenzylidene aniline (FNBA; Subashini et al., 2013b) both crystallize in space group P2 1 /c.

Structural commentary
The molecular structure of CNBA is shown in Fig. 2. It crystallizes in the centrosymmetric monoclinic space group P2 1 /c, and is disordered with a twofold rotation about the shorter axis of the molecule -type D3. The twofold axis almost bisects the central C N bond, so that the two component molecules are superimposed head-to-tail, as shown clearly in a difference-Fourier map (Fig. 3). They have an occupancy ratio that, after initial refinement, was fixed at 0.649:0.351. As mentioned above, this type of disorder (D3) has been observed previously for related phases.
The configuration about the C N bond is E in both components. The dihedral angle between the benzene rings of the major component CNBA_1 (C1-C6 and C8-C13) is 38.6 (2) , and that between rings C21-C26 and C28-C33 of the Molecular structure of CNBA, with atom labelling. The displacement ellipsoids are drawn at the 50% probability level. The major component is shown with solids bonds, while the minor component is shown with dashed bonds.

Supramolecular features
A view along the a axis of the crystal packing of CNBA is presented in Fig. 4, and details of the hydrogen bonding are given in Table 1. The crystal packing of the individual components, CBNA_1 and CBNA_2, are given in Fig. 5a and 5b, respectively. In Fig. 5a it can be seen that the molecular packing for CNBA_1 is influenced by two C-HÁ Á ÁO interactions: namely, C5-H5Á Á ÁO2 and C13-H13Á Á ÁO1. The first of these links the molecules into C(11) chains and the second generates C(6) chains. In Fig. 5b, it can be seen that for CNBA_2 the molecular packing features weak C-HÁ Á ÁCl interactions ( Table 1). As a result of these interactions, corrugated layers are formed, lying parallel to the ac plane.

Database survey
A search of the Cambridge Structural Database (Version 5.41, last update November 2019; Groom et al., 2016) for N,1-diphenylmethanimines gave 73 hits for 63 compounds, while a search for 1-(4-nitrophenyl)-N-phenylmethanimines gave 25 hits for six compounds. In these searches a number of compounds have multiple reports, or have been studied at different temperatures, or concern polymorphs. The most relevant compounds that concern us here include those reported above in x1 (Chemical context), viz. N-(4nitrobenzylidene)aniline (CSD refcodes QQQAIY01 A difference electron-density map showing the density peaks related to the minor disordered component.

Figure 5
A view along the a axis of the crystal packing of (a) the major disorder component and (b) the minor component. The hydrogen bonds (see Table 1) are shown as dashed lines.

D-HÁ
component, while at 90 K (QQQAIY03) no disorder was observed. For the 4-fluoro derivative measured at 173 K (MIMDUJ) no disorder was observed. For the 4-bromo derivative (FIBXIZ01), the crystals were incommensurate and twinned and the structure was refined in space group A2. A triclinic polymorph of the 4-methyl derivative (NMBYAN) with two independent molecules in the asymmetric unit was reported on by Filipenko et al. (1976). A monoclinic polymorph, with one molecule in the asymmetric unit, was reported on first by Filipenko et al. (1976) for NMBYAN22, and later a neutron diffraction study at 20 K was carried out by Cole et al. (2001) for NMBYAN01. The triclinic polymorph was also studied by Harada et al. (2004a), at 300 K (NMBYAN25) and at 90 K (NMBYAN26) and showed only disorder of the methyl hydrogen atoms at both temperatures. The 4-methoxy benzylidene derivative, measured at 300 K (NMBZYA01) and 90 K (NMBZYA02), showed no disorder at either temperature. Finally, the 4-hydroxy derivative, WOTQED, crystallizes with four independent molecules in the asymmetric unit, and one of the molecules has type D1 disorder. The N=C bond lengths vary from as short as ca.1.187 Å , in one of the four independent molecules of WOTQED, to ca 1.281 Å in NMBZYA02. In the title compound, the N1=C7 bond length in the major component is 1.291 (6) Å , while for the minor component the N21=C27 bond length is 1.234 (12) Å . In the above-mentioned compounds, the benzene rings are inclined to each other by dihedral angles varying from ca 2.24 in one of the independent molecules of WOTQED to ca 55.76 in one of the two independent molecules in NMBYAN26; thus the dihedral angles for the disorder components of the title compound fall roughly in the middle of this range.
The Hirshfeld surface of CNBA mapped over d norm is given in Fig. 6a, where short interatomic contacts are indicated by the red spots. The Hirshfeld surfaces of the individual components, CNBA_1 and CNBA_2, mapped over d norm are given in Fig. 6b and 6c, respectively.

Synthesis and crystallization
The commercially available organic compounds p-nitrobenzaldehyde and p-chloroaniline were used without further purification and the title compound was synthesized following reported procedures (Batra et al., 2004;Subashini et al., 2013a): the two reactants were taken in equimolar ratio and refluxed in ethanol for 6 h. On cooling, the synthesized compound was deposited at room temperature as a deepyellow microcrystalline powder. The material was purified by repeated recrystallization using ethanol at room temperature and the purity of the sample was confirmed by thin layer chromatography. A saturated solution of CNBA was prepared using mixed solvents of ethanol and ethylacetate (1:1, v:v) and single crystals were obtained as yellow rods by slow evaporation of the solvents at room temperature over a period of 18 days. The 1 H NMR spectrum of CNBA recorded in CDCl 3 is shown in the supporting information, Fig. 1S, and the FTIR and FT Raman spectra are shown in Fig. 2S.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-bound H atoms were included in calculated positions and refined as riding: C-H = 0.95 Å with U iso (H) = 1.2U eq (C). The molecule is disordered with an occupancy ratio that after refinement was fixed at 0.649: 0.351. The benzene rings in the two components were refined as rigid bodies and the anisotropic displacement parameters of corresponding C atoms were made equal.   (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2020) and Mercury (Macrae et al., 2020); software used to prepare material for publication: SHELXL2018 (Sheldrick, 2015), PLATON (Spek, 2020) and publCIF (Westrip, 2010).

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (