The crystal structure of (E)-2-ethyl-N-(4-nitrobenzylidene)aniline: three-dimensional supramolecular assembly mediated by C—H⋯O hydrogen bonds and nitro⋯π(arene) interactions

The 2-ethylphenyl group in the title compound is disordered over two sets of atomic sites and the molecules are linked into a three-dimensional array by a combination of C—H⋯O hydrogen bonds and nitro⋯π(arene) interactions.

In the molecule of the title compound, C 15 H 14 N 2 O 2 , the 2-ethylphenyl group is disordered over two sets of atomic sites having occupancies of 0.515 (19) and 0.485 (19), and the dihedral angle between the two partial-occupancy aryl rings is 6(2) . A combination of C-HÁ Á ÁO hydrogen bonds and nitroÁ Á Á(arene) interactions links the molecules into a continuous three-dimensional framework structure. Comparisons are made with the structures of some related compounds.

Chemical context
Schiff bases exhibit a very wide range of biological activities (da Silva et al., 2011) and are also of interest because of their photochromic and thermochromic properties (Hadjoudis & Mavridis, 2004;Minkin et al., 2011). In view of the general importance of Schiff bases, and in a continuation of our own structural study of compounds of this type (Girisha et al., 2017(Girisha et al., , 2018 we report here the molecular and supramoleuclar structure of (E)-2-ethyl-N-(4-nitrobenzylidene)aniline (I) (Fig. 1), where the ethyl group turns out to be disordered over two sets of atomic sites and where the molecules are linked into a three-dimensional supramolecular array.

Structural commentary
The 2-ethylphenyl group in compound (I) is disordered over two sets of atomic sites having occupancies of 0.515 (19) and 0.485 (19) and it is possible that the ethyl group is simply making full use of an available space within the structure: the dihedral angle between the two components of the disordered aryl ring is 6(2) . The nitro group is almost coplanar with the ISSN 2056-9890 adjacent aryl ring, with a dihedral angle of only 8.3 (2) between these two units; on the other hand, the dihedral angles between the nitrated aryl ring and the major and minor components of the disordered ring are 36.7 (10) and 42.6 (11) , respectively. The molecules of (I) are therefore conformationally chiral but, in the absence of significant resonant scattering, it was not possible to determine the absolute configuration of the molecules in the crystal selected for data collection. It is reasonable to assume that, in solution, the two conformational enantiomers will be in rapid equilibrium.

Figure 1
The molecular structure of compound (I) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level, and for the disordered 2-ethylphenyl group, the major component is drawn using solid lines and the minor component is drawn using dashed lines.

Figure 2
Part of the crystal structure of compound (I) showing the formation of a C(6) hydrogen-bonded chain along [001]. For the sake of clarity, the H atoms not involved in the motif shown have been omitted. The atoms marked with an asterisk (*), a hash (#) or a dollar sign ($) are at the symmetry positions ( 3 2 À x, 1 À y, 1 2 + z), (x, y, 1 + z) and ( 3 2 À x, 1 À y, À 1 2 + z), respectively. For the disordered 2-ethylphenyl group, the major component is drawn using solid lines and the minor component is drawn using dashed lines. chain running parallel to the [100] direction ( Fig. 3), while the combined action of the hydrogen bond and the nitroÁ Á Á(arene) interactions links the molecules into a chain running parallel to the [010] direction (Fig. 4)

Database survey
It is of interest to briefly compare the three-dimensional supramolecular assembly in compound (I), with the patterns of aggregation found in related compounds (II)-(V). In compound (II), two independent aromaticstacking interactions combine to link the molecules into chains . The structure of compound (III) (Akkurt et al., 2008) contains three short C-HÁ Á ÁO contacts, but two of these involve an H atom in a methyl group, while for the third the C-HÁ Á ÁO angle is only 131 , so that none of these contacts is likely to be structurally significant (Wood et al., 2009). The molecules of compound (IV) (Madhuprasad et al., 2014) are linked into centrosymmetric dimers by inversionrelated O-HÁ Á ÁO hydrogen bonds, while those of compound (V) are linked into a three-dimensional framework structure by a combination of C-HÁ Á ÁO and C-HÁ Á Á(arene) hydrogen bonds and an aromaticstacking interaction (Girisha et al., 2018).

Synthesis and crystallization
Solutions of 2-ethylaniline (100 mg, 0.826 mmol) and 4-nitrobenzaldehyde (124 mg, 0.826 mmol), each in ethanol (15 ml). were mixed and a catalytic amount of glacial acetic acid was added. The resulting mixture was heated under reflux for 3 h, when completion of the reaction was confirmed using thin layer chromatography. The solid product was collected by filtration and recrystallized from acetonitrile to give crystals of (I) suitable for single crystal X-ray diffraction; yield 150mg, 0.590 mmol, 71%; m.p. 369-373 K.

Refinement
It was apparent from an early stage in the refinement that the methyl group of the ethyl substituent was disordered over two sets of atomic sites having unequal occupancies, and satisfactory resolution of the disorder required a model in which the whole 2-ethylphenyl unit was disordered over two sets of atomic sites. For the minor disorder component, the bonded distances and the 1,3 non-bonded distances were restrained to be the same as the corresponding distances in the major Part of the crystal structure of compound (I) showing the formation of a chain parallel to the [010] direction built from alternating C-HÁ Á ÁO hydrogen bonds and nitroÁ Á Á(arene) interactions. For the sake of clarity, the H atoms not involved in the motif shown have been omitted. The atoms marked with an asterisk (*), a hash (#), a dollar sign ($) or an ampersand (&) are at the symmetry positions ( 3 2 À x, 1 À y, 1 2 + z), (1 À x, 1 2 + y, 3 2 À z), (À 1 2 + x, 3 2 À y, 1 À z) and (x, 1 + y, z), respectively. For the disordered 2-ethylphenyl group, the major component is drawn using solid lines and the minor component is drawn using dashed lines.

Figure 3
Part of the crystal structure of compound (I) showing the formation of a chain along [100] built from nitroÁ Á Á(arene) interactions. For the sake of clarity, the H atoms have all been omitted. The atoms marked with an asterisk (*), a hash (#) or a dollar sign ($) are at the symmetry positions ( 1 2 + x, 1 2 À y, 1 À z), (À 1 2 + x, 1 disorder component, subject to s.u. values of 0.01 and 0.02 Å , respectively. In addition, the anisotropic displacement parameters for the corresponding pairs of C atoms in the disordered ring were constrained to be identical. All H atoms apart from those in the ethyl unit were located in difference maps and then treated as riding atoms with C-H 0.93 Å and U iso (H) = 1.2U eq (C); the H atoms of the ethyl unit were included in calculated positions with C-H distances of 0.96 Å (CH 3 ) or 0.97 Å (CH 2 ) and with U iso (H) = kU eq (C), where k = 1.5 for the methyl groups, which were permitted to rotate but not to tilt, and 1.2 for the CH 2 groups. Subject to these conditions, the occupancies of the two disorder components refined to 0.515 (19) and 0.485 (19). Although the coverage of Friedel pairs was 98%, it was not possible to determine the absolute configuration of the molecules in the crystal selected for study, as the value of the Flack x parameter (Flack, 1983), calculated using 484 quotients of the type [(I + ) À (I À )]/[(I + )+(I À )] (Parsons et al., 2013), was À0.5 (7), and value calculated for the Hooft y parameter (Hooft et al., 2008) was À0.4 (7). Crystal data, data collection and structure refinement details are summarized in Table 3.  Computer programs: APEX2 and SAINT (Bruker, 2012), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) 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.