Oxalic acid–pyridine-4-carbonitrile (1/2)

In the title compound, 2C6H4N2·C2H2O4, the oxalic acid molecule lies about an inversion center. The pyridine ring of the pyridine-4-carbonitrile molecule is almost planar, the largest deviation from the least-squares plane being 0.006 (1) Å; the nitrile N atom deviates from this plane by 0.114 (1) Å. In the crystal, the oxalic acid molecules and the pyridine-4-carbonitrile molecules form stacks. Neighboring molecules within the stacks are related by translation in the a direction, with interplanar distances of 3.183 (1) and 3.331 (2) Å, respectively. Each oxalic acid molecule forms strong O—H⋯N hydrogen bonds with two molecules of pyridine-4-carbonitrile. Besides this, there are also weak C—H⋯O interactions.

In the title compound, 2C 6 H 4 N 2 ÁC 2 H 2 O 4 , the oxalic acid molecule lies about an inversion center. The pyridine ring of the pyridine-4-carbonitrile molecule is almost planar, the largest deviation from the least-squares plane being 0.006 (1) Å ; the nitrile N atom deviates from this plane by 0.114 (1) Å . In the crystal, the oxalic acid molecules and the pyridine-4-carbonitrile molecules form stacks. Neighboring molecules within the stacks are related by translation in the a direction, with interplanar distances of 3.183 (1) and 3.331 (2) Å , respectively. Each oxalic acid molecule forms strong O-HÁ Á ÁN hydrogen bonds with two molecules of pyridine-4-carbonitrile. Besides this, there are also weak C-HÁ Á ÁO interactions.

Wen-Ni Zheng Comment
Simple organic salts containing strong intermolecular H-bonds have attracted an attention as materials which display ferroelectric-paraelectric phase transitions (Fu et al., 2011a,b,c). With the purpose of obtaining crystals of organic salts exhibiting ferroelectric phase transitions, various organic compounds have been studied and the series of new materials have been elaborated (Dai & Chen, 2011;Xu et al., 2011;Zheng, 2011). Herewith we present the synthesis and crystal structure of the title molecular complex, pyridine-4-carbonitrile-oxalic acid (2/1).
All bond lengths and angles in the studied structure have expected values (Allen et al., 1987). The dihedral angle between the pyridine ring and the oxalic acid molecule is 10.29 (8)°. The H atoms of oxalic acid are involved in strong intramolecular O-H···N hydrogen bonds ( Fig. 1 and Table 1), with the O···N distance of 2.617 (3)Å. The weak intermolecular C-H···O interaction is also presented in this structure, with C4···O1 = 3.364 (2)Å. The crystal packing is further stabilized by the π···π interactions between the pyridine rings of the neighbouring pyridine-4-carbonitrile molecules (Fig. 2).

Experimental
The oxalic acid (10 mmol), pyridine-4-carbonitrile (20 mmol) and ethanol (50 mL) were put into a 100mL flask. The mixture was stirred at 60°C for 2 h, and then the precipitate was filtered off. Colourless crystals suitable for X-ray diffraction were obtained by slow evaporation of the solution.

Refinement
All the H atoms attached to C atoms were placed into the idealized positions and treated as riding with C-H = 0.93 Å and with U iso (H)=1.2U eq (C). The positional parameters of the H atom attached to oxygen were refined freely, and at the last stage of the refinement, they were restrained with the H-O = 0.82 (2)Å and with U iso (H)=1.5U eq (O).

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