Supramolecular interactions in a 1:1 co-crystal of acridine and 3-chlorothiophene-2-carboxylic acid

The asymmetric unit comprises one 3-chlorothiophene-2-carboxylic acid (3TPC) and one acridine molecule linked together via an O—H⋯N hydrogen bond.


Chemical context
Co-crystals are solids in which two or more molecules crystallize together and interact through non-covalent interactions (Odiase et al., 2015). The study of non-covalent interactions in co-crystals not only adds to our knowledge but also has an undeniable relevance in the context of their pharmaceutical and biological interest (Chakraborty et al., 2014;Desiraju, 1989). The main interactions concerned are various hydrogen bonding,and C-HÁ Á Á interactions (Aakerö y et al., 2010). The acridine molecule is a component present in antihelminthic agents which are used in animals (Durchheimer et al., 1980). Acridine derivatives also show in vitro activity against protozoa (Ngadi et al., 1993). The acridine group is a well known intercalator interacting with nucleobase pairs (Raju et al., 2016;Nafisi et al., 2007;Sazhnikov et al., 2013). Acridine dyes are also widely used (Solovyeva et al., 2014, Yasarawan et al., 2011. Halogenated thiophene carboxylic acid derivatives are the building blocks of many commercially available insecticides (Hull et al., 2007). We extended our study on supramolecular architectures in acridine molecules with the investigation of the title co-crystal with 3-chlorothiophene-2-carboxylic acid (3TPC).

Structural commentary
The compound (1) is a 1:1 co-crystal of 3TPC and acridine. The internal angle at N1 [C6-N1-C18 = 119.30 (15) ] and ISSN 2056-9890 bond lengths [C18-N1 = 1.346 (2) and C6-N1 = 1.354 (2) Å ] agree with those reported for neutral acridine structures (Aghabozorg et al., 2011;Binder et al., 1982;Goeta et al., 2002). The two external bond angles at the carbon atom of the carboxyl group are 124.13 (17) and 110.75 (15) . The high discrepancy between these two angles is typical of an unionized carboxyl group. The C O distance of 1.316 (2) Å and C-OH distance of 1.199 (2) Å are also typical of the carboxyl group. These values also agree with the carboxylic acids reported in the literature (Kowalska et al., 2015;Sienkiewicz-Gromiuk et al., 2016). The dihedral angle between the carboxylic acid group and the thiophene ring is 9.01 (13) . The bond distances and angles involving the thiophene ring agree with those in structures reported earlier (Zhang et al., 2014).

Figure 1
The asymmetric unit of the title compound, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. The dashed line represents the O-HÁ Á ÁN hydrogen bond.

Figure 2
A view of the O-HÁ Á ÁN hydrogen bonds (purple dashed lines),stacking (acridine-acridine and thiophene-thiophene; red dashed lines) and C-HÁ Á Á interactions between the acridine C-H group and the -system of thiophene (green dashed lines).

Synthesis and crystallization
To 10 ml of a hot methanol solution of 3TPC (40.6 mg, 25 mmol) were added 10 ml of a hot methanolic solution of acridine (44.8 mg, 25 mmol). The resulting solution was warmed over a water bath for half an hour and then kept at room temperature for crystallization. After a week yellow plate-like crystals of (1) were obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were readily located in difference Fourier maps and were subsequently treated as riding atoms in geometrically idealized positions, with C-H = 0.93 and O-H = 0.82 Å , and with U iso (H) = kU eq (C, O), where k = 1.5 for hydroxy and 1.2 for all other H atoms.  (Spek, 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: PLATON (Spek, 2009) and publCIF (Westrip, 2010). 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.