Crystal structure, Hirshfeld surface analysis and PIXEL calculations of a 1:1 epimeric mixture of 3-[(4-nitrobenzylidene)amino]-2(R,S)-(4-nitrophenyl)-5(S)-(propan-2-yl)imidazolidin-4-one

A 1:1 epimeric mixture of 2(R,S)-(4-nitrophenyl)-3-[(4-nitrobenzylidene)amino]-5(S)-(propan-2-yl)imidazolidin-4-one was derived from an initial reaction of 2(S)-amino-3-methyl-1-oxobutanehydrazine at its hydrazine moiety to provide a 4-nitrobenzylidine derivative, followed by a cyclization reaction with another molecule of 4-nitrobenzaldehyde to form the chiral five-membered imidazolidin-4-one ring.

In both molecules, the imidazoline rings are puckered, the puckers in each case being a twist at C12-N13 and C22-N23 in MolA and MolB, respectively. In the case of MolA, the Cremer & Pople puckering parameters (Cremer & Pople, 1975) are Q(2) of 0.287 (2)Å and '(2) of 54.7 (5) for reference bond N11-C12; for MolB, Q(2) is 0.103 (3)Å and '(2) is 230.3 (15) for reference bond N21-C22. In MolA, the dihedral angles between the mean planes of the imidazoline ring and the benzene ring (pivot atom C121) is 45.83 (18) , between the imidazoline ring and the benzene ring (pivot atom C131) is 28.04 (12) and between the two benzene rings is 69.86 (11) . In MolB, the dihedral angles between the mean planes of the imidazoline ring and the benzene ring (pivot atom C221) is 59.83 (13) , between the imidazoline ring and the benzene ring (pivot atom C131) is 6.86 (13) and between the two benzene rings is 66.38 (11).

Intermolecular interactions and contacts
As seen, each of the molecules of the asymmetric unit ( Fig. 1c) has two nitro groups, whose O atoms can act as acceptors for hydrogen bonding, and three rings that are able to participate instacking. Fig. 1(c) shows the two molecules labelled for the nitro O atom and the oxo atoms (O15 and O25), as well as the identification of ring A (benzene rings with pivot atoms C131 and C231), B (benzene rings with pivot atoms C121 and C221) and C (imidazoline rings).

Hirshfeld surface and quantitative analyses of intermolecular interactions
Hirshfeld surfaces (Spackman & Jayatilaka, 2009) and twodimensional fingerprint (FP) plots (Spackman & McKinnon, 2002) provide complementary information concerning the intermolecular interactions deduced from the PLATON analysis. The Hirshfeld analysis, generated using Crystal-Explorer (Version 3.1; Wolff et al., 2012) and mapped over d norm (ranging from À0.329 to 1.708), indicated red areas related to specific intermolecular short contacts (see .
Briefly, the Hirshfeld surface analysis revealed that in MolA all the O atoms participate in hydrogen bonding, but in MolB only three do, the exception being O238 in ring A. A summary of these interactions is made in Interactions connecting molecule pairs I and II, and a view of the Hirsfeld surface.  (Gavezzotti, 2003(Gavezzotti, , 2008, were run in order to calculate the total stabilization energy of the crystal packing, E tot , distrib-uted as Coulombic, E Coul , polarization, E pol , dispersion, E disp , and repulsion, E rep , terms. Partial analysis of the PIXEL calculations have been made and the results obtained were used to identify pairs of molecules within the crystal network that most contribute to the total energy of the packing. The compound crystallized with two molecules (MolA and MolB) in the asymmetric unit and each has five O atoms that may be involved in the formation of hydrogen bonds, which are labelled in Fig. 1(c). In short, each molecule has two 4-NO 2 -phenyl substituents, one substituent connected to the imine C atom, ring A (pivot atoms C131 and C231 in MolA and MolB, respectively), and the other to the imidazoline ring, ring C (pivot atoms C121 and C221 in MolA and MolB, respectively). In addition, there is a carbonyl O atom in heterocyclic ring C (pivot atoms N11 and N21 in MolA and MolB, respectively), together with a potential donor, i.e. the -NH group on the same ring. The Hirshfeld surface mapped over d norm ranging from À0.329 to 1.708 for 1 show various red areas due to intramolecular short contacts (refer to Figs. 4-7). Briefly, the analysis revealed that in MolA all the O atoms participate in hydrogen bonds, while only one of the nitro O atoms of ring A of MolB establishes interactions. A summary of these interactions is made in Table 3. The carbonyl O atom of heterocyclic ring C and the nitro atoms O129 or O229 of ring B are involved in hydrogen bonding between two molecules with the same labels, that is AÁ Á ÁA or BÁ Á ÁB. These pairs interact in a similar way. In contrast, it seems that all the O atoms of MolA act as acceptors for H atoms of MolB. Some CÁ Á Á interactions that define some substructures are identified in Table 3. PIXEL energy calculations, as implemented in PIXEL3.1 (Gavezzotti, 2003(Gavezzotti, , 2008, give a total stabilization energy of À170.4 kJ mol À1 for the crystal packing, distributed as follows: E Coul = À78.4, E pol = À30.6, E disp = À199.51 and E rep = 138.2 kJ mol À1 for Coulombic, polarization, dispersion and repulsion energies, respectively. The polarization term is clearly less important than the Coulombic one. Partial analysis of the PIXEL calculations was also carried out to identify pairs of molecules within the crystal framework that contribute most to the total energy of the packing. Fig. 8 lists the symmetry operation, the specific close contacts and the individual energy components for each molecule pair. The identified molecule pairs, I to IX, are depicted in Figs. 4 to 7, together with appropriate views of the Hirshfeld surface. In the figures of the molecule pairs, the epimeric molecules are coloured green (MolA) and blue (MolB), the partner to the specific epimer in the molecular pair is coloured in standard element colours and any other relevant molecule is coloured grey.
Substructures I and II connect MolA with MolA (Table 3 and Fig. 4) and subtructures VIII and IX connect MolB with MolB (Table 3 and Fig. 7). There is a similarity between substructures I and VII, as well as between substructures II and IX. Pairs I and VII are made by C arom -HÁ Á ÁO oxo interactions that give two isoenergetic subsets for each pair (I a /I b and VII a /VII b ). These pairs relate MolAÁ Á ÁMolA and MolBÁ Á ÁMolB in chains, as can be visualized in Figs. 4 and 7. The total energies for the substructures of pairs I and pairs VII differ by about 5 kJ mol À1 (higher value for substructure I) and this may be due to the presence of an additional C-HÁ Á Á interaction in I that is not detected in VIII [VII?]. The similar substructures II a /II b and IX a /IX b , are built utilizing similar C-HÁ Á ÁO interactions, involving the O atom of the nitro group of ring B. Nevertheless, the total energies for those pairs also differ by about 5 kJ mol À1 , this time with a higher value for pairs IX due to a higher contribution of the dispersion term. Molecular pairs involved in substructures VI and VII, made by the green stick molecule at (x, y, z) with the [colour missing?] colour atoms molecules at (Àx, y + 1, z + 1) (VI) and (x, y À 1, z) (VII). The grey molecule in pair VI is considered to act as the conduit for electronic interactions, while in pair VII, the conduit is considered to be MolB (blue) of the asymmetric unit. The molecules that constitute the asymmetric unit form the nonsymmetric dimeric substructure III. In this substructure, the nitro O atoms of ring A act as acceptors in both molecules, but they interact with different H atoms, e.g. (i) a methyl H atom to form the O138 Á Á ÁH24D-C243 hydrogen bond in the MolAÁ Á ÁMolB contact and (ii) an H atom of the imidazoline ring thereby generating an O239Á Á ÁH12-C12 hydrogen bond in the MolBÁ Á ÁMolA contact (see Fig. 5).
In substructure IV, the N-H hydrogen of MolB makes a bifurcated hydrogen-bond interaction with both O atoms of the nitro group located in ring B of MolA, e.g. O129Á Á ÁH23-N23 and O128Á Á ÁH23-N23 (see pair IV in Fig. 5). This substructure, according to the model used for the calculation of interactions energies, contributes the highest amount of energy to the stabilization of the crystal packing. In the substructure made by pair V, atom O139 of MolA acts as an acceptor for atom H226 of MolB (see Fig. 7). This layout permits a supramolecular arrangement where aromatic rings appear to stack, but the Hirshfeld surface (HS) analysis did not reveal spots related to CÁ Á ÁC close contacts that are typical of theinteractions.
Finally, two more substructures have been identified as energetically important in the stabilization of the supramolecular structure for 1. Molecular pairs involved in substructures VI and VII, relate the molecule at (x, y, z) with the molecules at (Àx, y + 1, z + 1) (for VI) and (x, y + 1, z) (for VII). Although those molecules are not connected in a classical way, the pairs make a significant contribution to the lattice stabilization energy, i.e. À32.5 and À25.9 kJ mol À1 , respectively, for VI and VII. These pairs are depicted in Fig. 6 The molecular pairs involved in substructures VII and IX. The figure also depicts the Hirshfeld surface images.
with the grey molecule in pair VI shown in order to clarify a possible path explaining the electronic interactions, while in pair VII, the those interactions are made via molB of the asymmetric unit. Fig. 9 shows the fingerprint (FP) plots for MolA and MolB. The FP plots show two pairs of spikes pointing south-west and ending at (1.2; 0.9/0.9; 1.2) that are due to OÁ Á ÁH/HÁ Á ÁO close contacts, the light blue in the middle is due to the HÁ Á ÁH and CÁ Á ÁC close contacts. The percentages for atom-atom contacts were taken from the FP plots and are given in Table 4. These percentages are similar for both molecules with an exception made for the OÁ Á ÁH contacts that are smaller in MolB and the NÁ Á ÁH and HÁ Á ÁH contacts that are higher in MolA. Energies, close contacts and symmetry codes of the molecule pairs. AÁ Á ÁA stands for MolAÁ Á ÁMolA complexes, BÁ Á ÁB for MolBÁ Á ÁMolB complexes and AÁ Á ÁB for MolAÁ Á ÁMolB.

Figure 9
FP plots for MolA and MolB. The spikes are due to OÁ Á ÁH/HÁ Á ÁO contacts and the outer ones due to the NÁ Á ÁHÁ Á ÁN contacts.
To a stirred solution of 3 (1 mmol) in ethanol (10 ml) was added 4-nitrobenzaldehyde (2.2 mmol). The reaction mixture was stirred for 20 h at 351 K and rotary evaporated. The residue was purified by column chromatography using a mixture of 9.7:0.3 (v/v) dichloromethane-methanol as eluent. Further purification was achieved by crystallization from ethanol. The crystal of 1 used in the structure determination was obtained by slow evaporation of an ethanol solution at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. H atoms attached to C atoms were refined as riding atoms at calculated positions. That attached to the N atom was refined.

sup-1
Acta Cryst.   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.