Supramolecular interactions in the 1:2 co-crystal of 4,4′-bipyridine and 3-chlorothiophene-2-carboxylic acid

The asymmetric unit comprises of one 3-chlorothiophene-2-carboxylic acid (3TPC) and one half of a 4,4′-bipyridine (BPY) molecule linked together via an O–H⋯N hydrogen bond.


Chemical context
Structurally homogeneous crystalline solids in well defined stochiometry are called co-crystals. In recent years, the physicochemical properties of active pharmaceutical ingredients have been improved widely with the use of co-crystals (Lemmerer & Bernstein, 2010). Supramolecular synthons -modular representation of primary recognition between functional groups -are of great importance in providing an effective strategy for designing solids in crystal engineering. All geometrical and chemical information of molecular recognition is contained in the structural units called synthons. In the context of co-crystal formation, heterosynthons provide a predictive justification in terms of unique intermolecular interactions (Mukherjee et al., 2011(Mukherjee et al., , 2013. There are many literature cases of O-HÁ Á ÁN-bonded interactions between acid and pyridine-based systems (Shattock et al., 2008;Lemmerer et al., 2015). 4,4 0 -Bipyridine (BPY) is a weak bidentate base commonly used in crystal engineering on account of its bridging abilities. It also acts as the co-crystal former in the present study because it readily participates in hydrogen bonds with carboxyl-attached organic molecules (Pan et al., 2008).
Intermolecular interactions involving halogen substituents, particularly chlorides, play an important role in molecular selfassembly in supra-and biomolecular systems to prepare highly stereoregular organic polymers. It has been observed that these interactions act as a tool in crystal engineering to ISSN 2056-9890 enhance crystal formation and for the design of supramolecular aggregates (Cavallo et al., 2016). In this context, the study of the effect of various halogens on the molecular packing and crystalline architecture of solids has attracted great attention (Csö regh et al., 2001). The structure-forming ability of ClÁ Á ÁCl interactions in assembling chains, ladders, two-dimensional sheets, etc. has been studied extensively (Navon et al., 1997;Metrangolo & Resnati, 2014). It is based on the values of the two C-HalÁ Á ÁHal angles, 1 and 2 (Vener et al., 2013).

Database survey
In the title compound, the most dominant interaction is the O-HÁ Á ÁN hydrogen bond formed between a carboxyl group and a pyridine N atom (Fig. 1). The length of this hydrogen bond [OÁ Á ÁN = 2.659 (4) Å ] is very close to those of O-HÁ Á ÁN bonds found in similar reported co-crystals, such as in the adduct of 2,5-dihydroxy-1,4-benzoquinone and BPY (Cowan et al., 2001) and in the co-crystal of BPY with N,N 0 -dioxide-3hydroxy-2-naphthoic acid (1/2) (Lou & Huang, 2007) and in a series of nine co-crystals involving acridine and benzoic acids (Kowalska et al., 2015). The angle of the hydrogen bond formed between the 3CTPC and BPY molecules is 178 (5) . A similar value is found in the co-crystal of BPY with 3,5-dinitro benzoic acid for which the OÁ Á ÁN distance is 2.547 (2) Å (Thomas et al., 2010). In the crystal structure of the co-crystal of adamantane-1,3-dicarboxylic acid and 4,4 0 -bipyridine,interactions connect the O-HÁ Á ÁN hydrogen-bonded zigzag chains, supporting a two-dimensional network (Pan et al., 2008).

Synthesis and crystallization
To 10 ml of a hot methanol solution of 3TPC (40.6 mg, 25 mmol), 10 ml of a hot methanolic solution of BPY (39.0 mg, 25 mmol) was added. The resulting solution was warmed over a water bath for half an hour and then kept at room temperature for crystallization. After a week, clear yellow plates were obtained. The crystal used for X-ray diffraction data collection was cut from a larger crystal.

bis(3-Chlorothiophene-2-carboxylic acid); 4,4′-bipyridine
Crystal data 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.