Benzene-1,2-dicarboxylic acid–pyridinium-2-olate (1/1)

The asymmetric unit of the title compound, C5H5NO·C8H6O4, contains one o-phthalate acid molecule and one pyridin-2-ol molecule, which exists in a zwitterionic form. In the o-phthalate acid molecule, the carboxylate groups are twisted from the benzene ring by dihedral angles of 13.6 (1)° and 73.1 (1)°; the hydroxy H atom in the latter group is disordered over two positons in a 1:1 ratio. In the crystal, O—H⋯O and N—H⋯O hydrogen bonds link the molecules into zigzag chains in [-101].

The asymmetric unit of the title compound, C 5 H 5 NOÁC 8 H 6 O 4 , contains one o-phthalate acid molecule and one pyridin-2-ol molecule, which exists in a zwitterionic form. In the ophthalate acid molecule, the carboxylate groups are twisted from the benzene ring by dihedral angles of 13.6 (1) and 73.1 (1) ; the hydroxy H atom in the latter group is disordered over two positons in a 1:1 ratio. In the crystal, O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds link the molecules into zigzag chains in [101].

Chua-Hua Yu Comment
The title compound was synthesized to find potential ferroelectric phase change materials via dielectric constant measurements of compounds on the basis of temperature (Zhang, Chen et al., 2009;Zhang, Ye et al., 2010;Zhang & Xiong, 2012), with reference to the compound C 5 H 9 N 2 + .C 8 H 5 O 4 - (Zhu & Yu, 2011). Regrettably, no dielectric anomaly was observed ranging from 120 K to 353 K near its melting point. Herewith we report the crystal structure of the title compound, (I).
The asymmetric unit of (I) contains one molecule of the o-phthalate acid and one pyridin-2-ol molecule, which exists in a zwitterionic form ( Fig. 1). In the o-phthalate acid molecule, atom H3A is disordered over two positions being attached either to O3 or to O4 in a ratio 1:1. Intermolecular N-H···O and O-H···O hydrogen bonds (Table 1)  Experimental 0.83 g (5 mmol) of phthalic acid and 10 ml water which were heated, then added with a few ethanol dropst, and 0.476 g (5 mmol) 2-hydroxypyridine was added to the solution. After stirring the mixture for minutes for the sake of achieving the ambient temperature, the liquid was filtered to give a clear solution. Colourless block crystals suitable for X-ray structure analysis were obtained, by the slow evaporation of the above solution after sever days at the ambient temperature.

Refinement
O-bound H atoms were located on a difference map and isotropically refined with restraint O-H = 0.85 (2) Å. The rest H atoms were placed in geometrically idealized positions (N-H = 0.86 Å; C-H = 0.93 Å) and refined as riding, with  A content of asymmetric unit, with displacement ellipsoids drawn at the 30% probability level. For the disordered atom H3A (attached either to O3 or to O4), only one position is shown. C-bound H atoms omitted for clarity.

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.