Two polymorphs of trans-[3-(3-nitrophenyl)oxiran-2-yl](phenyl)methanone

The title compound, C15H11NO4, crystallizes in two polymorphic forms, centrosymmetric monoclinic and chiral orthorhombic. The geometry of the molecules in the two polymorphs is slightly different, possibly due to intermolecular interactions. A number of C—H⋯O intermolecular interactions, involving both O atoms of the nitro as well the benzoyl groups, stabilize the crystal structures.


Chemical context
The title compound is a substituted chalcone oxide, a representative of a large group of organic compounds which are precursors for pharmaceutically significant flavonoids (Marais et al., 2005). As for most biologically important molecules, chirality plays an important role in their reactions. These compounds can be also considered as isomers of substituted dibenzoylmethanes.
The simplest compound of this group, 1,3-diphenyl-2,3epoxypropan-1-one (2-benzoyl-3-phenyloxirane, benzalacetophenone oxide) was isolated by Widman (1916) using Darzens condensation of benzaldehyde and bromoacetophenone in the presence of sodium ethoxide. When m-nitrobenzaldehyde was employed in this reaction, the title compound was obtained (Bodforss, 1916). The original publication mentioned the possibility of two different types of colorless crystals, both having the same melting point of 391 K. Later, a number of alternative synthetic routes were developed, including Claisen condensation of m-nitrobenzaldehyde with acetophenone with subsequent oxidation (Roth & Schwarz, 1961). The authors described the title compound as pale-yellow needles. A one-pot version of this synthesis was reported recently (Ngo et al., 2014). Preparation of nitro- ISSN 2056-9890 chalcone oxides seems to be one of the simplest condensation reactions and therefore attractive for use in undergraduate organic chemistry teaching laboratories. The interesting observation of possible polymorphism in the original publication encouraged us to conduct a structural study, exactly one hundred years after the first preparation of this compound had been reported.

Structural commentary
The title compound, C 15 H 11 NO 4 , crystallizes in two polymorphic forms, centrosymmetric monoclinic (1) and chiral orthorhombic (2). Bond lengths and angles in the molecules of both polymorphs are very similar (Figs. 1 and 2). However, a molecular overlay (Fig. 3) reveals some difference in confor-mation, possibly because of different types of intermolecular interactions.
All atoms of the title polymorphs, except the oxiran ring hydrogen atoms, are located close to one of three planes: the benzene ring mean plane of the nitro-phenyl group (A), the oxiran ring plane (B), and the benzene ring plane of the benzoyl group (C). The largest deviations from these planes are À0.2003 (14) and 0.0457 (15) for O3 and O4 (monoclinic polymorph), 0.091 (4) and À0.189 (3) for O3 and O4 (orthorhombic), and 0.3398 (14) and 0.065 (3) for atom O1 in the monoclinic and the orthorhombic forms, respectively. Planes A and B are almost perpendicular in both polymorphs ( Table 1). The angles between the two other planes differ significantly (Table 1).

Supramolecular features
There are no classical hydrogen bonds in these two polymorphs. In the molecules, areas of negative electrostatic potential are located in the vicinity of all four oxygen atoms. Areas near hydrogen atoms are obviously positive, providing a tool for intermolecular interactions. This expectation is supported by the packing data. In both polymorphs, the two oxygen atoms O3 and O4 of the nitro group and oxygen atom O1 of the carbonyl group act as acceptors for C-HÁ Á ÁO hydrogen bonds. Despite being relatively weak, such bonds play a significant role in intermolecular interactions (Desiraju & Steiner, 1999). Hydrogen atom H5 of the nitrophenyl group makes short contacts with the O1 oxygen of the carbonyl group in both cases. However, the short contacts involving the nitro group oxygen atoms O3 and O4 (Tables 2 and 3) are Numbering scheme of the title compound with 50% probability ellipsoids (monoclinic polymorph).
Plane A: mean plane of the m-nitrophenyl benzene ring; plane B: oxirane ring; plane C: mean plane of the benzoyl benzene ring.
Without strong intermolecular bonding, the close-packing principle directs the assembly of molecules in the crystal. A multi-step approach to assembling is sometimes referred as the Kitaigorodskii Aufbau Principle (KAP) and may consist of the following sequence (Kitaigorodskii, 1961;Perlstein, 1994): (a) a single molecule or a number of molecules forming a unit; (b) units join up to form a chain; (c) chains assemble to make a 2D surface and (d) surfaces are stacked to form a crystal.
This sequence can be traced in the structure of the orthorhombic polymorph. Molecules of the title compound are stacked to form a chain along [100] axis (Fig. 4). An oxiran group forms a 'wedge' that fits into a concave 'pocket' between two phenyl rings of the next molecule. The interatomic distances between oxiran oxygen atom O2 and the corresponding carbon atoms are unusually short: O2Á Á ÁC7(1 + x, y, z) = 3.113 (2), O2Á Á ÁC8(1 + x, y, z) = 2.960 (2) and O2Á Á ÁC9(1 + x, y, z) = 2.979 (2) Å . Two separate causes can make these short contacts possible: (i) dipoledipole attraction of consecutive oxiran groups and (ii) close 1056 Greenberg and Nazarenko C 15 H 11 NO 4 and C 15 H 11 NO 4 Acta Cryst. Packing diagram for the orthorhombic polymorph. One chain of molecules along the [100] axis is shown.

Figure 5
Packing diagram for the orthorhombic polymorph. Chains of molecules with one direction are stacked in the (001) plane.
packing of recurrent flat benzoyl and nitrophenyl groups with the distances between their mean planes being 3.472 (2) and 3.493 (2), respectively. Because all these groups are parallel, there is no hydrogen bonding within the chain. At the next level, chains are packed in the (001) plane via a 2 1 symmetry operation, with all oxiran groups oriented in one direction (Fig. 5). Finally, chains are closely packed with the next 2 1 operation, forming a crystal with favorable hydrogen bonding (Fig. 6). The monoclinic form of the title compound is possible only if the starting solution contains a racemic mixture. In the first step, two molecules are -stacked via inversion centers via their nitrophenyl groups and two symmetric hydrogen bonds (Fig. 7). The distance between the parallel planes of these phenyl rings is 3.4115 (10), which is slightly longer than in polyaromatic hydrocarbons (3.38 Å ; Kitaigorodskii, 1961) and indicates very close packing. These centrosymmetric units are assembled in the (100)  Packing of the monoclinic polymorph. Two molecules are related by inversion.

Figure 6
Packing diagram for the orthorhombic polymorph. View along the a axis. Hirshfeld surface shown for one molecule (calculated using Crystal-Explorer; Wolff et al., 2012).

Figure 8
Packing diagram of the monoclinic polymorph. Molecules are assembled in the (100) plane.

Figure 9
Crystals of different polymorphs in solution. Large blocks are monoclinic, needles are orthorhombic. (Fig. 8). The stacking planar assemblies in the 3D crystal uses no additional hydrogen bonding.
The assembling sequence is mechanically more straightforward in the case of the chiral orthorhombic form, which results in favorable formation of the orthorhombic polymorph. The absence of an enantiomer requirement may also make it kinetically more favorable. These two factors can serve as a qualitative explanation of the preferred formation of the orthorhombic form upon crystallization from alcohols or from hexane. The monoclinic form has a slightly smaller cell volume (see Table 4) and, therefore, closer packing of molecules, an indication that the monoclinic form might be the thermodynamically slightly more stable of the polymorphs according to Burger and Ramberger's Density Rule (Burger & Ramberger, 1979a,b). Nevertheless, the packing of the two forms is significantly different and transition from one form to another requires dissolution of the crystal. This observation explains the kinetic stability of both forms at room temperature and at 173 K.

Database survey
There are sixteen reported chalcone oxide structures deposited in the Cambridge Structural Database (CSD Version 5.37; Groom et al., 2016). Of these structures, six report hydroxy-and methoxy-substituted molecules with strong intermolecular interactions. The closest to our study is [3-(4-nitrophenyl)oxiran-2-yl](phenyl)methanone (refcode COVKAB; Obregó n- Mendoza et al., 2014). In this case, the oxiran oxygen atom makes short contacts instead of the benzoyl group; the p-nitrophenyl ring is practically flat. The simplest unsubstituted chalcone oxide was recently reported (refcode TIBXIM; Zaidi et al., 2007). In this structure, like in our case, only the benzoyl group oxygen atom makes short intermolecular contacts. Chains similar to those in the orthorhombic form of the title molecule are present in the chiral P2 1 crystal of [3-(4-chlorophenyl)oxiran-2-yl](phenyl)methanone (refcode QECFAF; Bakó et al., 1999). However, the distances between oxiran oxygen atom and the subsequent carbon atoms are much longer than in the present case.

Synthesis and crystallization
The title compound was obtained via the classic route (Bodforss, 1916). Mass-spectrum (EI): 269 (M+, 20%), 105 (PhCO+, 100), 77 (Ph+, 60). Because all precursor compounds were non-chiral and synthetic conditions should not induce chirality, we expected to see a racemic product. Crystallization from hexane yielded colorless thin needles suitable for singlecrystal investigation. X-ray diffraction data revealed the chiral  orthorhombic space group P2 1 2 1 2 1 . Crystallization from ethanol produced better quality crystals of the same polymorph, one of which was used in this study. After two weeks of standing at 273 K, a number of large (up to 1 mm) crystals were observed in the remaining ethanol solution (Fig. 9). A suitable crystal was cut to dimensions appropriate for X-ray analysis. It turned out to be a monoclinic P2 1 /c polymorph of the same compound. Several crystals of different shape, also formed from the same solution, resulted to be of a benzoin admixture.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The structure of the orthorhombic polymorph was refined as a two-component inversion twin.
All hydrogen atoms in the monoclinic form were refined in isotropic approximation. In the orthorhombic form, the oxiran ring hydrogen atoms H7 and H8 were refined in isotropic approximation with U iso = 1.2U iso (C). All aromatic hydrogen atoms in this molecule were refined with riding coordinates and U iso = 1.2U iso (C). For the monoclinic polymorph structure, positive residual density was observed at all bonds between non-hydrogen atoms, demonstrating the limitations of the atom-in-molecule approach for high-resolution structures of organic molecules.

Special details
Experimental. SADABS-2014/5 (Bruker,2014/5) was used for absorption correction. wR2(int) was 0.0614 before and 0.0562 after correction. The Ratio of minimum to maximum transmission is 0.8403. The λ/2 correction factor is 0.00150. 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.