A new crystal form and antimicrobial activity of (E)-1-[3-(2-hydroxybenzylideneamino)phenyl]ethanone

In the orthorhombic crystal form, the molecules are linked by weak C—H⋯O hydrogen bonds into supramolecular chains propagating along the b-axis direction.


Chemical context
Schiff bases (Wang et al., 2008) are versatile ligands synthesized from the condensation of primary amines with carbonyl groups. These compounds have been shown to exhibit a broad range of biological activities, including antifungal, antibacterial, anti-malarial, antiproliferative, anti-inflammatory, antiviral and antipyretic properties (Dhar & Taploo, 1982;Przybylski et al., 2009). Imine or azomethine groups are present in various natural, natural-derived and non-natural compounds and have been shown to be critical for their biological activity (Bringmann et al., 2004;de Souza et al., 2007).
In this paper, we report the structural characterization using X-ray diffraction of the title Schiff base derived from salicylaldehyde, including an investigation of the Hirshfeld surfaces and its antimicrobial activity against two gram-negative (Escherichia coli and Salmonella typhimurium) and one grampositive (Staphyloccus aureus) bacteria. Schiff bases derived from this benzaldehyde are members of one of the most commonly investigated classes of compound, and have attracted the interest of chemists and physicists because they show photochromism and thermochromism in the solid state. ISSN 2056-9890 These photo-and thermochromic features are caused by proton transfer to the N atom from the O atom under the influence of light or temperature, respectively. It has been proposed that molecules showing thermochromism are planar and those showing photochromism are non-planar (Moustakali-Mavridis et al., 1980;Hadjoudis et al., 1987).

Structural commentary
The title Schiff base ( Fig. 1) consists of two aromatic phenyl rings linked via an azomethine (C N) group. The two phenyl rings are monosubstituted by a hydroxyl group on the same side as the azomethine carbon atom and by an aceto group on the other side. Relevant bond distances and angles are in good agreement with those reported in similar Schiff base compounds (Benarous et al., 2016;Chen et al., 2011). The C1-N1-C7-C8 torsion angle of À179.4 (2) indicates an almost planar E configuration with respect to the imine C N bond, as expected for a compound having an azomethine HC N bond as this is the most thermodynamically stable configuration (Ciciani et al., 2008). The N1 C7 [1.293 (3) Å ] and C9-C10 [1.394 (3) Å ] bond distances indicate that the compound adopts the phenol-imine tautomeric form with an N C double bond and a C-C single bond (Table 1). Comparable values are observed in Schiff bases obtained from the same salicylaldehyde derivative with phenol-imine tautomeric form (Albayrak et al., 2010;Ş ahin et al., 2009).
An intramolecular O-HÁ Á ÁN hydrogen bond (Table 2) occurs between the O-hydroxyl and N azomethine atoms, forming an S(6) ring motif. Such a hydrogen bond is frequently observed in Schiff bases derived from salicylaldehyde (Alpaslan et al., 2011).

Supramolecular features
In the crystal, the molecules are linked via C-HÁ Á ÁO hydrogen bonds between the carbon atom of azomethine group and the oxygen atom of the methoxy substituent, generating infinite chains with graph-set motif C(8) along the b-axis direction ( Table 2, Fig. 2). The chains are linked viainteractions (3.535 Å ) between the C8-C13 benzene ring and the C N double bond (Fig. 3).

Hirshfeld surfaces analysis
Hirshfeld surfaces and two-dimensional fingerprint plots were generated by CrystalExplorer 3.1 (Wolff et al., 2012) to visualize and explore the intermolecular interactions. These molecular surfaces reflect intermolecular contacts based on colour coding distances from the surface to the nearest atom exterior (d e ) or interior (d i ) to the surface. In the Hirshfeld  Symmetry code: (i) Àx þ 1; y À 1 2 ; Àz þ 1 2 .

Figure 1
View of the title compound with the atom-numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level. interaction between a benzene ring and the C N double bond.
surface mapped over d norm (Fig. 4), red indicates the presence of short contacts and white represents contacts around the van der Waals separation, while the blue areas are completely devoid of close contacts. The intermolecular interactions were analysed by a combination of 3D Hirshfeld surfaces and 2D fingerprint plots, showing that the intermolecular HÁ Á ÁH contacts make the largest contribution, corresponding to 46% of the total Hirshfeld surface area (Fig. 5). The presence of short intermolecular HÁ Á ÁH contacts is observed in the vicinity of 2.30 Å . These contacts are manifested as white spots on the d norm surface and are considered to be weak interactions. In the fingerprint plots ( Fig. 6), the CÁ Á ÁH/HÁ Á ÁC contacts, representing 21.6% of the total Hirshfeld surface, appear as two short spikes. The red spots on the d norm surface in Fig. 4 are due to the CHÁ Á ÁO contacts corresponding to the C-HÁ Á ÁO hydrogen bond. The OÁ Á ÁH contacts (19.4% of the total Hirshfeld surface) show up as a sharp spike in the fingerprint plots at d e + d i ' 2.3 Å . Finally, the packing cohesion in this structure is also provided by CÁ Á ÁN and CÁ Á ÁC interactions, which correspond tostacking interactions.

Database survey
A search for the title compound in the Cambridge Structural Database (Version 2.39; Groom et al., 2016) revealed that the crystal structure of the title compound had been previously been reported in the monoclinic P2 1 /n space group (De et al., 2009). The latter differs from the title structure at position 3 of the aceto substituent, which is on the same side of the hydroxyl group in the title compound and in the opposite side in the reported one. This difference in position directly affects the hydrogen-bonding pattern. Similar infinite C-HÁ Á ÁO chains occur in both compounds but the angle between linked molecules is ca 67.99 in the title compound and 77.61 in the reported one. The CSD search also found seven hits for structures containing the title molecule but with additional substituents (a methoxy or an additional hydroxyl group).

Antimicrobial activity
The title compound was screened for its antibacterial activity against two gram-negative (Escherichia coli and Salmonella typhimurium) and one gram-positive (Staphyloccus aureus) bacterial strains by the agar-well diffusion method (Cruickshank,1970). The solvent DMSO was used as negative control. A 0.5 ml spore suspension (10 À6 -10 À7 spore ml À1 ) of each of Two-dimensional fingerprints of the compound, showing all interactions and HÁ Á ÁH, CÁ Á ÁH, OÁ Á ÁH, CÁ Á ÁC and CÁ Á ÁN contacts.

Figure 7
MICs (minimum inhibitory concentrations) antibacterial activity of the title compound and the standard Cefotaxime Hirshfeld surface of the title compound mapped over d norm (À0.60 to 0.90 a.u.).

Figure 5
Relative contributions of various interactions to the Hirshfeld surface area.
the investigated organisms was added to a sterile agar medium just before solidification, then poured into sterile petri dishes (9 cm in diameter) and left to solidify. Using a sterile cork borer (6 mm in diameter), five holes (wells) were made in each dish, and then 5 mL of the tested compound, dissolved in DMSO with different concentrations (C, C/2, C/4, C/8), was poured into these holes. Finally, the dishes were incubated at 310 K for 48 h. Clear or inhibition zones were detected around each hole. DMSO alone (0.5 mL) was used as a control under the same conditions for each organism by subtracting the diameter of inhibition zone resulting with DMSO from that obtained in the study compound. The antibacterial activity of Cefotaxime was also measured in comparison to the title compound and used as a standard to reveal the potency of synthesized derivative. The results of the antimicrobial screening indicate that the compound shows significant activity only against Salmonella typhimurium with an inhibition zone diameter of 15 mm. This value is close to that observed with the standard used (Cefotaxime) against the same bacterium (16 mm). For the two other bacteria, Escherichia coli and Staphyloccus aureus, the standard exhibits a higher activity than the study compound, for which the inhibition zone diameter is under 2 mm. These results are summarized in Fig. 7, which gives the MICs (minimum inhibitory concentrations) of the title Schiff base and Cefotaxime.

Synthesis and crystallization
The title Schiff base was synthesized by reacting 3-aminoacetophenone (0.13 g, 1 mmol) and salicylaldehyde (0.12 g, 1 mmol) in ethanol (20 ml). The resulting mixture was refluxed for 3 h. Yellow single crystals suitable for single crystal X-ray diffraction studies were obtained by slow evaporation of the solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were located in difference electron-density maps and treated as riding on their parent atoms, with C-H = 0.95-0.98 Å with U iso (H) = 1.2U eq (C) or 1.5U eq (Cmethyl) and O-H = 0.84 Å with U iso (H) = 1.5U eq (O).