4,4′-Bipyridine–terephthalic acid (1/1)

The asymmetric unit of the title compound, C10H8N2·C8H6O4, consists of one half-molecule of each moiety, 4,4′-bipyridine (bpy) and terephthalic acid (bdc), both being located on crystallographic inversion centers. They are linked together via strong intermolecular O—H⋯N hydrogen bonds, forming infinite chains propagating along [1-21]. The chains are further connected through C—H⋯O interactions giving sheets in (012). The sheets are linked via π–π interactions between the bpy rings and the bdc–bpy rings [centroid–centroid distances = 3.690 (2) and 3.869 (2) Å], resulting in the formation of a three-dimensional supramolecular layer-like structure.

The asymmetric unit of the title compound, C 10 H 8 N 2 ÁC 8 H 6 O 4 , consists of one half-molecule of each moiety, 4,4 0 -bipyridine (bpy) and terephthalic acid (bdc), both being located on crystallographic inversion centers. They are linked together via strong intermolecular O-HÁ Á ÁN hydrogen bonds, forming infinite chains propagating along [121]. The chains are further connected through C-HÁ Á ÁO interactions giving sheets in (012). The sheets are linked viainteractions between the bpy rings and the bdc-bpy rings [centroid-centroid distances = 3.690 (2) and 3.869 (2) Å ], resulting in the formation of a three-dimensional supramolecular layer-like structure.

Experimental
Data collection: APEX2 (Bruker, 2011); cell refinement: SAINT (Bruker, 2011); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXLE (Hü bschle et al., 2011) and SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: publCIF (Westrip, 2010). The title organic compound was obtained as a side-product during part of our investigations into the solvothermal synthesis of magnesium based metal organic framework compounds. Reaction of magnesium nitrate with terephthalic acid and 4,4′-bipyridine at 373 K did not yield an extended framework, but the high solubility of magnesium nitrate in DMF led to formation of the title organic compound in the form of colorless rod-like crystals. Similar dissolution of metal salts in organic solvents, leaving the organic compound as a side-product, have previously been observed (Karpova et al., 2004;Yao et al., 2008;Zhao et al., 2007).
The asymmetric unit of the title organic compound is composed of one half-molecule of each component, that is 4,4′-bipyridine (bpy) and terephthalic acid (bdc). The inversion symmetry within bpy and bdc generates the full molecules ( Fig.   1).

Strong inter-molecular O-H···N hydrogen bonds between the bpy and bdc molecules lead to the formation of infinite
one-dimensional chains propagating along [1 -2 1] (Table 1 and Fig. 2). The pyridine rings of the bpy molecule exhibit a planar configuration with a dihedral angle of 0° between the planes of the two pyridyl rings. The dihedral angle between the bpy and bdc molecules within the chains is 7.20 (4)°.
The bpy molecules and the bdc molecules in neighbouring chains are π-stacked above one another [centroid-centroid distance of 3.869 (2) Å; symmetry code: -x+1, -y, -z] in an ABAB···fashion resulting in a two-dimensional supramolecular layer-like structure (Fig. 2). The interplanar spacing between the chains, defined as the distance between the planes of neighbouring bpy rings is 3.2512 Å [centroid-centroid distance 3.690 (2) Å; slippage of 1.745 Å]. The layers, formed through π-stacking, are further connected through C-H···O interactions along (012) resulting in a three-dimensional supramolecular-like structure (Fig. 3). Thus the overall structure of the title compound is stabilized by strong hydrogen bonds and π-π interactions.

Experimental
The compound was synthesized under solvothermal conditions. In a typical synthesis, Mg(NO 3 ) 2 .6H 2 O (0.129 g, 1.0 mmol) and terepthalic acid (0.169 g, 2.0 mmol) were dissolved in DMF (5.0 ml). Then, 4,4′-bipyridine (0.239 g, 3.0 mmol) was added to the reaction mixture and stirred for one hour before transferring the mixture into a glass vial. The final mixture was heated to 373 K (100 °C) for 24 h. The vial was then slowly cooled to room temperature. Slow cooling of the reaction mixture yielded colorless rod-like crystals of the title compound as a minor product, along with an unidentified white powder.

Refinement
The carboxylic acid H atoms were located in difference electron density maps, but were placed in calculated positions 148 command in SHELXTL; Sheldrick, 2008). The C-bound H atoms were included in calculated positions: C-H = 0.95 Å; U iso (H) = k × U eq (O,C), where k = 1.5 for carboxylic acid H atoms, and = 1.2 for other H atoms.

Figure 1
View of the title organic compound with atom numbering. Displacement ellipsoids are drawn at the 50% probability level  A view along the a axis of the two-dimensional supramolecular layer-like structure of the title compound. Green and purple dots represent the hydrogen bonds within the chains and the π···π stacking interactions between the chains, respectively [see Table 1 for details].

Figure 3
View of the three-dimensional supramolecular-like structure. Green dots represent the C-H···O hydrogen bonds connecting parallel stacks of chains. The purple arrows represent the π-π interactions within the π-stacks [see Table 1 for details].

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.  (17)