3,5-Dicarboxy-2,6-dimethylpyridinium chloride dihydrate

In the title compound, C9H10NO4 +·Cl−·2H2O, both the cation and the anion have crystallographic twofold rotation symmetry; in the former, one N and one C atom lie on the rotation axis. In the crystal structure, the ions and water molecules are linked via O—H⋯O, O—H⋯Cl and N—H⋯Cl hydrogen bonds into layers parallel to (101).

In the title compound, C 9 H 10 NO 4 + ÁCl À Á2H 2 O, both the cation and the anion have crystallographic twofold rotation symmetry; in the former, one N and one C atom lie on the rotation axis. In the crystal structure, the ions and water molecules are linked via O-HÁ Á ÁO, O-HÁ Á ÁCl and N-HÁ Á ÁCl hydrogen bonds into layers parallel to (101).

Structure Reports Online
The asymmetric unit of the title compound is composed of a half of a 3,5-dicarboxy-2,6-dimethylpyridinium cation, a half of a chloride anion and a water molecule. Both cation and anion have crystallographically imposed twofold rotation symmetry ( Fig. 1). In the cation, the C-O bond lengths in the carboxylic group (C1-O1 = 1.300 (2) Å; C1-O2 = 1.218 (2) Å) conform to the expected values (Allen, 2002). The C3-N1-C3 angle of 126.6 (2) ° corresponds closely to the average value found in protonated pyridinium ions (122.0 (2) °). In the crystal structure ( Fig. 2), the 3,5-dicarboxy-2,6-dimethylpyridinium cations, the chloride anions and the water molecules are linked via O-H···O, O-H···Cl and N-H···Cl hydrogen bonds (Table 1) to form two-dimensional layers parallel to the (101) plane. Dielectric studies (capacitance and dielectric loss measurements) were performed on powder samples of the title compound pressed into tablets on the surfaces of which a conducting carbon glue was deposited. The automatic impedance TongHui 2828 Analyzer has been used. In the measured temperature ranges (80 to 480 K, m.p. > 480 K), the structure showed no dielectric anomaly.
Experimental 2,6-Dimethylpyridine-3,5-dicarboxylic acid (1.95 g , 10 mmol) and concentrated hydrochloric acid (10 mmol) were dissolved in methanol (25 ml). The solution was filtered and left at room temperature for 5 days. Colourless crystals suitable for X-ray analysis were obtained by slow evaporation of the solvent.

Refinement
The pyridinium and water H atoms were located in a difference Fourier map and refined freely. All other H atoms were calculated geometrically and allowed to ride on their parent atoms, with C-H = 0.93-0.97 Å, O-H = 0.82 Å, and with U iso (H) = 1.5 U eq (C, O) or 1.2 U eq (C) for the aromatic H atom.
supplementary materials sup-2 Figures   Fig. 1. The molecular structure of the title compound, showing the atomic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Atoms with suffix A are generated by the symmetry operation (2-x, y, 0.5-z).

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 > σ(F 2 ) is used only for calculating Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq