Crystal structure and Hirshfeld surface analysis of 2-(4-nitrophenyl)-2-oxoethyl benzoate

The title compound, 2-(4-nitrophenyl)-2-oxoethyl benzoate, is relatively planar with the two aromatic rings being inclined to each other by 3.09 (5)°.


Chemical context
Photoreleasable protecting groups have been of long-standing interest for their diverse applications in various multistep syntheses (Ruzicka et al., 2002;Literá k et al., 2006). The reaction between an acid and a phenacyl bromide yields the keto ester derivative. As a protecting group, the ester derivatives are well known as protecting groups for carboxylic acids in chemical synthesis (Rather & Reid, 1919;Literá k et al., 2006). They can easily be cleaved under completely neutral or mild conditions (Sheehan & Umezawa, 1973) and are therefore used for the identification of organic acids. Versatile applications of these compounds are seen in the field of synthetic chemistry, such as in the synthesis of oxazoles and imidazoles (Huang et al., 1996), as well as benzoxazepine (Gandhi et al., 1995), and they are also useful in peptide synthesis. Studies reveal an inhibitory activity against two isozymes of 11b-hydroxysteroid dehydrogenases (11b-HSD1 and 11b-HSD2), which catalyze the interconversion of active cortisol and inactive cortisone (Zhang et al., 2009). Researchers have reported the synthesis and photolysis studies of a number of phenacyl esters. The commercial importance of phenacyl benzoates arose due to their applications in various fields of chemistry. In continuation of our work on such molecules Chidan Kumar et al., 2014), we report herein on the crystal and molecular structure of 2-(4-nitrophenyl)-2-oxoethyl benzoate.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The compound is composed of two aromatic rings linked by a C-C( O)-O-C( O) bridge. The unique molecular conformation of this compound is characterized by three torsion angles, viz. 1 (C11-C10-C9-O3), 2 (C7-C8-O1-C9) and 3 (O2-C7-C8-O1), whereby the 1 value of 9.60 (16) signifies the apparent coplanarity between the mean planes of the phenyl ring and the adjacent carbonyl groups of the connecting bridge. The 2 value of 174.08 (9) between the two carbonyl groups indicates an antiperiplanar conformation. Likewise, owing to a substitution on the functional group, the title compound experiences steric repulsion between the substituent and adjacent carbonyl groups, influencing the torsion angle [ 3 = 1.88 (15) ], and it adopts a +synperiplanar conformation. The bond lengths and angles are normal and the molecular conformation is characterized by a dihedral angle of 3.09 (5) between the mean planes of the two aromatic rings indicating that they are coplanar. The nitro group lies almost in the plane of the phenyl ring, as indicated by the torsion angle values of 7.80 (15) and 8.46 (15) for C4-C3-N1-O4 and C2-C3-N1-O5, respectively.

Supramolecular features
In the crystal, there are no classical hydrogen bonds present. However, the structure is stabilized by weak intermolecular C-HÁ Á ÁO interactions. Specifically, a pair of intermolecular C5-H5Á Á ÁO3 i interactions stabilize the supramolecular architecture by forming inversion dimers with an R 2 2 (16) ring motif (Table 1 and Fig. 2). The dimers are linked by a further pair of C-HÁ Á ÁO hydrogen bonds, forming ribbons that enclose R 4 4 (26) ring motifs (Table 1 and Fig. 2). The ribbons are linked by a series of offsetinteractions (Table 2), forming layers that stack up the b-axis direction (Fig. 3).

Hirshfeld surface analysis and two-dimensional fingerprint plots
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed and created with CrystalExplorer17 (Turner et al., 2017). Hirshfeld surface analysis enables the visualization of intermolecular interactions by different colours and colour intensity, representing short or long contacts and indicating the relative strengths of the interactions. Figs. 4 and 5 show the Hirshfeld surfaces mapped over d norm (À0.195 to 1.091 a.u.) and shape-index (À1.0 to 1.0 a.u.), respectively.

Figure 1
The molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A partial view of the crystal packing of the title compound. The hydrogen bonds are shown as dashed lines (Table 1) and only H atoms H5 and H13 have been included.

Figure 3
The crystal packing of the title compound, viewed along the c axis. The hydrogen bonds are shown as dashed lines (Table 1) and only H atoms H5 and H13 have been included.
regions around the participating atoms, which correspond to positive and negative electrostatic potential, respectively. The shape-index surface clearly shows that the two sides of the molecules are involved in the same contacts with neighbouring molecules and the curvedness plots show flat surface patches characteristic of planar stacking.
The overall two-dimensional fingerprint plot for the title compound and those delineated into O-H/H-O, H-H, C-H/H-C and C-C contacts are illustrated in Fig. 6. The percentage contributions from the different interatomic contacts to the Hirshfeld surfaces are OÁ Á ÁH = 35.9%, HÁ Á ÁH = 29.7%, CÁ Á ÁH = 14.7% and CÁ Á ÁC = 10.3%, and are shown in the two-dimensional fingerprint plots in Fig. 6. The percentage contributions of other intermolecular contacts are less than 5% in the Hirshfeld surface mapping.  Table 2 contacts (Å , ) in the crystal of the title compound.

Figure 5
Hirshfeld surface of the title compound, mapped over (a) the shape-index and (b) the curvedness.

Figure 4
A view of the three-dimensional Hirshfeld surface of the title compound mapped over d norm .
In these structures, the two aromatic rings are inclined to each other by dihedral angles varying from ca 0 to 90 . There are seven structures with a nitro substituent on one of the aromatic rings (see supplementary information file S2). However, there is only one compound with the same skeleton as the title compound, i.e. 2-(biphenyl-4-yl)-2-oxoethyl 4-nitrobenzoate (CSD refcode CISSAB; Kwong et al., 2017).
Here, the two aromatic rings are inclined to each other by ca 70.96 , compared to an inclination of only 3.09 (5) in the title compound.

Synthesis and crystallization
The title compound was synthesized as per the procedure of Kumar et al. (2014). A mixture of 2-bromo-1-(4-nitrophenyl)ethanone (0.2 g, 0.5 mmol), potassium carbonate (0.087 g, 0.63 mmol) and benzoic acid (0.079 g, 0.65 mmol) in dimethylformamide (5 ml) was stirred at room temperature for 2 h. After completion of the reaction, the reaction mixture was poured into ice-cold water. The solid product obtained was filtered off, washed with water and recrystallized from ethanol to give colourless needle-like crystals (m.p. 386-390 K).

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The observed criterion of F 2 > 2sigma(F 2 ) is used only for calculating -R-factor-obs etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.