4,4′-Bipyridine–trans,trans-hexa-2,4-dienedioic acid (1/1)

The title cocrystal, C10H8N2·C6H6O4, crystallizes with half-molecules of 4,4′-bipyridine and trans,trans-hexa-2,4-dienedioic acid in the asymmetric unit, as each is located about a crystallographic inversion center. The bipyridine molecule is planar from symmetry. In the dicarboxylic acid molecule, the O—C—C—C torsion angle is −13.0 (2)°. In the crystal, O—H⋯N and C—H⋯O hydrogen bonds generate a three-dimensional network.


Suk-Hee Moon and Ki-Min Park Comment
Considerable effort has been devoted to form co-crystals made up of two or more components because of their potential applications in pharmaceutical chemisty (Weyna et al., 2009), supramolecular chemistry (Bhogala & Nangia, 2003;Gao et al., 2004) and materials chemistry (Hori et al., 2009). In particular, numerous studies have focused on hydrogen bonding between carboxylic acid and pyridine molecules (Bhogala & Nangia, 2003;Hou et al., 2008;Jiang & Hou, 2012). We report here the structure of a co-crystal of trans, trans-hexa-2,4-dienedioic acid with 4,4′-bipyridine in the solid state.
The title compound is shown in Fig. 1. The asymmetric unit contains half-molecules of 4,4′-bipyridine and trans,trans-1,3 -butadiene-1,4-dicarboxylic acid each located on crystallographic inversion centers. Both components are planar by symmetry and tilted by 32.02 (7)° with respect to each other.
In the crystal, the dicarboxylic acid molecules are arranged side by side by intermolecular C-H···O hydrogen bonds between the dicarboxylic acid molecules, leading to the formation of a one dimensional chain. Moreover, intermolecular O-H···N and C-H···O hydrogen bonds between dicarboxylic acid and 4,4′-bipyridine molecules generate a threedimensional network (Fig. 2, Table 1).

Experimental
A mixture of stoichiometric amounts of trans, trans-hexa-2,4-dienedioic acid and 4,4′-bipyridine in DMF (in a 1:1 volume ratio) was heated until the two components dissolved and was then kept at room temperature. Upon slow evaporation of the solvent, X-ray quality single crystals were obtained.

Refinement
The carboxyl-H atom was located in a difference Fourier map and refined isotropically. Csp 2 H atoms were positioned geometrically and refined using a riding model with d(C-H) = 0.95 Å and U iso (H) = 1.2U eq (C).  The molecular structure of the title compound, showing displacement ellipsoids drawn at the 50% probability level.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.