Crystal structure of a new phenyl(morpholino)methanethione derivative: 4-[(morpholin-4-yl)carbothioyl]benzoic acid

The first molecular compound synthesized and crystallized in Benin is reported. This work was carried out during the first crystallographic training session at the X-TechLab in Sèmè City, organized within the IUCr–UNESCO OpenLab framework.


4-[(Morpholin
carbothioyl]benzoic acid, C 12 H 13 NO 3 S, a novel phenyl-(morpholino)methanethione derivative, crystallizes in the monoclinic space group P2 1 /n. The morpholine ring adopts a chair conformation and the carboxylic acid group is bent out slightly from the benzene ring mean plane. The molecular geometry of the carboxylic group is characterized by similar C-O bond lengths [1.266 (2) and 1.268 (2) Å ] as the carboxylate H atom is disordered over two positions. This molecular arrangement leads to the formation of dimers through strong and centrosymmetric low barrier O-HÁ Á ÁO hydrogen bonds between the carboxylic groups. In addition to these intermolecular interactions, the crystal packing consists of two different molecular sheets with an angle between their mean planes of 64.4 (2) . The cohesion between the different layers is ensured by C-HÁ Á ÁS and C-HÁ Á ÁO interactions.

Structural commentary
The title compound ( Fig. 1) crystallizes in the monoclinic space group P2 1 /n with four molecules in the unit cell (Z = 4). The hydrogen-atom coordinates were located using the highquality residual electron density maps (Fig. 2), which also show the bonding electrons and oxygen lone pairs. The molecular structure is not planar, as shown in Fig. 1. The morpholine ring adopts a chair conformation. The torsion angle between the morpholine group and the phenyl ring around C5-C8 (C thioamide) is 3.49 (2) . Such a conformation of the morpholine ring was also observed in the crystal structure of 2-methoxy-N-(morpholin-4-ylcarbonothioyl) benzohydrazide hemihydrate (Singh et al., 2007). The carboxylic acid group is bent slightly [0.15 (2) Å ] out of the plane of the aromatic ring. The electron density deformation map calculated without the contribution of the carboxylic hydrogen ( Fig. 2a) shows that this carboxylic H atom is split over two positions H1A and H1B, linked respectively to atoms O1 and to O2 with a refined population of 0.54 (4)/0.46 (4). This disorder is confirmed by the resulting residual map (Fig. 2b) and by the equivalent C-O1 [1.266 (2) Å ] and C-O2 [1.268 (2) Å ] bond lengths. As expected, these distances are significantly longer than classical C O bonds [1.210 (8) Å ] and are shorter than conventional C-O-H [1.311 (2) Å ] bonds (Allen, 2002;Groom et al., 2016). Given the fact that the obtained results are averaged over the time scale and space of the experiments, the distribution of the electronic density reflects the superposition of the two configurations associated with the disorder (flipping) of the hydrogen atom of the carboxylic group. Thus, the hydrogen atom is shared via a double-well hydrogen bond, which leads to equivalent C-O bond lengths

Supramolecular features
The crystal packing (Fig. 3) consists of two different molecular sheets. The angle between the mean planes of the two sheets is 64.4 (2) and the intra-sheet distance is 3.031 (2) Å . The building block is a centrosymmetric dimer built from strong and centrosymmetric double-well low-barrier O-HÁ Á ÁO hydrogen bonds between two COOH groups. It is worth noting that the carboxylic groups are interconnected in a head-to-head fashion with significantly short O1Á Á ÁO2 interaction [2.666 (1) Å ]. Gilli & Gilli (2000)  A view of 4-(morpholine-4-carbonothioyl) benzoic acid with the atomnumbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
Residual electron density maps in the dimer COO plane calculated at the end of the independent atom refinement: (a) without the contribution of the hydrogen atom of the carboxylic group and (b) with the contribution of the hydrogen atom of the carboxylic group. The contour level is 0.05 e Å À3 .

Figure 3
A packing diagram for the title compound viewed along the [101] direction, showing the arrangement of two different molecular sheets. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
All reagents along with the used solvent were obtained from Sigma-Adrich, Prolabo and Acros Organic and used without further purification. To a mixture of 4-formylbenzoic acid (0.75 g; 5 mmol) and morpholine (0.63 ml, 7.5 mmol) in dimethylformamide (15 ml) under agitation was added montmorillonite K-10 (0.35 g) and sulfur S 8 (0.26 g, 8 mmol).
The brown mixture obtained was irradiated in a microwave for 10-15 minutes at 940 W. The temperature of the reaction mixture was in the range 411-416 K. After cooling to room temperature, the mixture was poured into a solution of ethyl acetate and hydrochloric acid (0.1 M, 100 ml) to eliminate the excess of sulfur and amine. It was then saturated with an NH 4 Cl solution and finally washed with distilled water (2 Â 100 ml); the organic phase obtained was dried over MgSO 4 before being concentrated by evaporation. Brown prismatic crystals suitable for single-crystal X-ray analysis were grown by slow evaporation from an ethanol solution at ambient temperature in the presence of air or in the freezer. The synthesized crystals were stable in air and highly soluble in polar organic solvent (e.g. ethyl acetate, dimethyl sulfoxide).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were clearly identified in difference-Fourier maps and their atomic coordinates and isotropic displacement parameters were refined. At the end of refinement, the hydrogen atom of the carboxylic group was localized in the Fourier maps and refined accordingly by splitting its position on two sites with a refined occupancy ratio of 0.54 (4)/0.46 (4). The quality of this room-temperature (298 K) crystal structure is also indicated by the experimental electron density   (Macrae et al., 2020); software used to prepare material for publication: enCIFer (Allen et al., 2004) and WinGX (Farrugia, 2012). 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.