4,4′-Bipyridine–2-hydroxypropane-1,2,3-tricarboxylic acid (3/2)

The combination of 2-hydroxypropane-1,2,3-tricarboxylic acid (H3hypta, also called citric acid) and 4,4′-bipyridine (4,4′-bipy) in a 1:1.5 molar ratio leads to the formation of the title molecular cocrystal, 1.5C10H8N2·C6H8O7. The asymmetric unit contains one and a half 4,4′-bipy units, with one lying across a centre of inversion, and one H3hypta molecule. The significant differences in the C—O bond distances support the existence of the un-ionized acid molecule and confirm the formation of a cocrystal. There are π–π and C—H⋯π stacking interactions between the aromatic rings of 4,4′-bipy [with interplanar distances of 3.7739 (8) and 3.7970 (8) Å] and between CH groups of H3hypta [with an H⋯π distance of 2.63 Å]. In the crystal structure, intermolecular O—H⋯N hydrogen bonds occur and an O—H⋯O hydrogen bond occurs within the citric acid moiety.

Financial support from Ilam University is gratefully acknowledged.

Comment
The creation of new functional materials through the control of intermolecular bonding is a key aim of crystal engineering (Desiraju, 1989). The synthesis of crystalline supramolecular structures mediated by hydrogen bonds is of considerable importance. Among all the non-bonded interactions, hydrogen bonding has proved to be the most useful and reliable, because of its strength and directional properties (Aakeroy & Seddon, 1993;Aghabozorg, Heidari et al., 2008).
In the case of cocrystals, these are generally formed by dissolution and recrystallization from a suitable solvent, although sublimation and growth from the melt are also used. Co-crystallization is a deliberate attempt at bringing together different molecular species in one crystalline lattice without making or breaking covalent bonds (Aghabozorg et al., 2006). Cocrystals are used to reveal specific recognition motifs, such as those proposed for rational drug design (Baures, 1999;Houk et al., 1999) and crystal engineering applications.
The asymmetric unit of the title cocrystal is shown in Fig. 1 The dihedral angle involving the aromatic rings, N1/C7-C9/C15/C16 (Cg1) and N2/C10-C14 (Cg2), of a 4,4'-bipy is 18.67°, which shows these units are not in the same plane, and also indicates the flexibility of the central C-C bond.
A remarkable feature in the crystal structure of the title compound is the presence of a large number of O-H···O, O-H···N and C-H···O hydrogen bonds (Table 1). There is an intramolecular O7-H7A···O4 hydrogen bond between the hydroxyl group and the carboxylate carbonyl group of the H 3 hypta unit, with distance D···A of 2.6538 (13) Å. Two 4,4'-bipy and H 3 hypta fragments are linked together by O-H···N and C-H···O hydrogen bonds and form chains (Fig. 4). C-H···O hydrogen bonding is widely accepted (Desiraju & Steiner, 1999;Biradha et al., 1993), and weak hydrogen bonding can be exploited in supramolecular chemistry and crystal structure design (Aghabozorg, Manteghi & Sheshmani, 2008). The crystal packing of the title compound is illustrated in Fig. 5. supplementary materials sup-2 Experimental An aqueous solution (50 ml) of 4,4'-bipyridine (100 mg, 6 mmol) and 84 mg (4 mmol) of 2-hydroxypropane-1,2,3-tricarboxylicacid, [H 3 hypta, also called citric acid] were refluxed for two hours. Yellow crystals of the title compound were obtained from the solution after a few weeks at room temperature.

Refinement
The H atoms were included in calculated positions and treated as riding atoms: O-H = 0.84 Å, C-H = 0.95-0.99 Å, with U iso (H) = 1.2U eq (parent O or C atom).

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.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness 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 threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) 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.