Structural (at 100 K) and DFT studies of 2′-nitroflavone

The title molecule, C15H9NO4, is non-planar, with the unusually large dihedral angle between mean planes of phenyl ring and chromone moiety being 50.73 (5)°.

The geometry of the title molecule [systematic name: 2-(2-nitrophenyl)-4Hchromen-4-one], C 15 H 9 NO 4 , is determined by two dihedral angles formed by the mean plane of phenyl ring with the mean planes of chromone moiety and nitro group, being 50.73 (5) and 30.89 (7) , respectively. The crystal packing is determined byinteractions and C-HÁ Á ÁO contacts. The results of DFT calculations at the B3LYP/6-31G* level of theory provided an explanation of the unusually large dihedral angle between the chromone moiety and the phenyl group. The electrostatic potential map on the molecular surface was calculated in order to determine the potential binding sites to receptors.

Chemical context
The naturally occurring group of heterocyclic compounds known as flavonoids has received considerable attention over the past 15 years. The synthesis and applications of flavones and their derivatives have been studied extensively because of their diverse pharmaceutical properties. Besides their physiological role in plants (Agati et al., 2012), this class of compounds has demonstrated antiallergic, antiviral, anxiolytic and anti-inflammatory activities (Manthey et al., 2001). Several synthetic flavonoids and their nitro derivatives, including a few halogen-substituted compounds, have been found to act as highly competitive ligands for benzodiazepine receptors, suggesting a possible use as anxiolytic drugs (Marder et al., 1995). Most importantly, several nitro derivatives of flavones have been reported to possess antiproliferative properties against human and murine cancerous cells, by the mechanism of induced apoptosis (Blank et al., 2004). Moreover, some flavonoids have been found to be capable of restoring the viability of human vascular endothelial cells, thus providing both cytoprotective and cytotoxic effects on normal and cancerous cells, respectively (Ramos, 2008). The title compound, 2 0 -nitroflavone, has previously been shown to effectively inhibit human and murine tumor cell activity without affecting the non-tumor cells. Induced apoptosis molecular mechanisms have been studied in vitro for HeLa human cervix carcinoma (Cá rdenas et al., 2008) and in vivo in murine adenocarcinoma cells (Cá rdenas et al., 2009). Several haematological cancer cell lines were used in the cytotoxicity evaluation of the title compound, along with a culture of healthy peripheral blood mononuclear cells (PBMCs); the IC 50 values (drug concentrations needed to induce a 50% inhibition of cell growth) after 2 0 -nitroflavone treatment ranged from 1AE0.5 mmol L À1 to 9AE1.4 mmol/L for various neoplastic cells, while the healthy cells IC 50 was found to be over 80 mmol L À1 , effectively leaving the cells intact under the concentrations sufficient for cancerous cells (Cá rdenas et al., 2012). Despite the evident importance of nitroflavone derivatives, structural studies until now have been limited to only one reported nitroflavone-based compound (Kendi et al., 1996). In this work, a combined study consisting of X-ray diffraction (XRD) structural analysis and quantum-chemical DFT calculations was carried out in order to obtain insight into the structure-property relationship, and more specifically the effect of the nitro substituent in the ortho-position of the phenyl moiety of a flavone.

Structural commentary
The molecular structure of 2 0 -nitroflavone is presented in Fig. 1. The mean plane of the benzene ring makes dihedral angles of 50.73 (5) and 30.89 (7) with the mean planes of the chromone moiety and the nitro group, respectively. The dihedral angle between mean planes of the chromone and benzene groups is unusually large when compared to other ortho-substituted flavone derivatives. For instance, the mol-ecule of 2 0 -methoxyflavone was reported to be almost planar, with a dihedral angle of 2.9 (Wallet et al., 1990). Even in the flavonoid with a bulky carbazole substituent in the same position, this dihedral angle is only 29.2 (Zheng, 2018). The length of the single bond between the chromone and benzene moieties is 1.469 (2) Å , indicating someconjugation. The unusually large dihedral angle in the title molecule can be attributed to the steric tension between the nitro group and the chromone oxygen atom, whereas in the carbazole derivative this substituent is twisted far enough from the plane of the benzene ring to avoid it coming into close proximity with the flavone core.

Supramolecular features
In the crystal, the title molecules form a parquet-like structure, with alternating layers of coplanar chromone backbones (Fig. 2). The presence ofinteractions in the crystal packing can be suggested from the short intermolecular distance of 3.299 (4) Å between the overlapping C9 atoms from opposing molecules. Moreover, a short contact of 3.286 (3) Å between the carbonyl oxygen atom and the centroid of the opposing heterocyclic ring is found, which suggests an interaction of the oxygen atom with the -system ( Fig. 3). Such an intermolecular interaction was found in the crystal structure of chiral amino alcohol with a pentafluorophenyl group (Korenaga et al., 2003). Two short C-HÁ Á ÁO contacts also occur, indicating at additional structural stability (Table 1) Parquet-like molecular packing in the title structure. Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) Àx þ 1; y þ 1 2 ; Àz þ 5 2 .

Figure 1
A view of the molecular structure of the title compound with the atomlabeling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Database survey
A search of the Cambridge Crystallographic Database (CSD version 5.40, update of September 19; Groom et al., 2016) for the title molecule yielded no entries. A single nitroflavone entry, for 2 0 -methyl-3 0 -nitroflavone, was found (REZROD; Kendi et al., 1996). A search for flavone-core molecules with only an ortho-substituted phenyl ring returned a total of six entries, three of which correspond to the compound with a methoxy group in the 2 0 -position [KEPLAS (Wallet et al., 1990),

DFT calculations
In an attempt to get further insight into structure and properties of the title molecule (I), a DFT study was carried out at the B3LYP/6-31G* level of theory with GAUSSIAN 16 (Frisch et al., 2016) software. The geometry of the ground state was optimized, using the XRD data as a starting point. The optimized geometry was confirmed to be the minimum by vibrational frequency analysis. Two previously described flavonoids, with methoxy (II) and carbazole (III) substituents in the 2 0 -position, were also optimized and compared with the XRD data. Selected geometrical parameters are presented in Tables  2-4. The calculated parameters are in satisfactory agreement with those obtained experimentally. The range of calculated dihedral angles between the moieties comprising the flavone core is narrower than that observed in the crystal structures, but still demonstrates the same trend with the title compound having the largest angle.
Considering the importance of the biological functions of the title compound, including its ability to competitively bind to benzodiazepine receptors, the electrostatic potential on the van der Waals surface was calculated (Fig. 4). While initially we had expected the nitro group to be the negative charge concentration site, it turned out to be the oxygen of the  Table 2 Experimental (XRD) and calculated (DFT) dihedral angles ( ) between the phenyl and chromone moieties in the title molecule (I), the 2 0methoxy derivative (II) and the 2 0 -carbazole derivative (III).

Figure 4
Electrostatic potential on the van der Waals surface of the title compound. Table 3 Experimental (XRD) and calculated (DFT) lengths of single bonds (Å ) between the phenyl and chromone moieties in the title molecule (I), the 2 0 -methoxy derivative (II) and the 2 0 -carbazole derivative (III).  Table 4 Experimental (XRD) and calculated (DFT) dihedral/torsion angles ( ) between the phenyl group and the substituent in the 2 0 -position in the title molecule (I), the methoxy derivative (II) and the carbazole derivative (III).  Short intermolecular CÁ Á ÁC and C-OÁ Á Á contacts in the crystal of the title compound.
chromone moiety. We speculate that it could be the binding part in this molecule's interaction with benzodiazepine receptors.

Synthesis and crystallization
The synthesis of the title compound was performed as described in the literature (Barros & Silva, 2006). The obtained product was recrystallized by slow evaporation from ethanol solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Data collection was performed at 100 K. All hydrogen atoms were located from the difference-Fourier map and freely refined.

2-(2-Nitrophenyl)-4H-chromen-4-one
Crystal data C 15 H 9 NO 4 M r = 267.23 Monoclinic, P2 1 /c a = 8.079 (7)  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.