Monoclinic polymorph of poly[aqua(μ4-hydrogen tartrato)sodium]

A monoclinic polymorph of the title compound, [Na(C4H5O6)(H2O)]n, is reported and complements an orthorhombic form [Kubozono, Hirano, Nagasawa, Maeda & Kashino (1993 ▶). Bull. Chem. Soc. Jpn, 66, 2166–2173]. The asymmetric unit contains a hydrogen tartrate anion, an Na+ cation and a water molecule. The Na+ ion is surrounded by seven O atoms derived from one independent and three symmetry-related hydrogen tartrate anions, and a water molecule, forming a distorted pentagonal–bipyramidal geometry. Independent units are linked via a pair of intermolecular bifurcated O—H⋯O acceptor bonds, generating an R 2 1(6) ring motif to form polymeric two-dimensional arrays parallel to the (100) plane. In the crystal packing, the arrays are linked by adjacent ring motifs, together with additional intermolecular O—H⋯O interactions, into a three-dimensional network.

A monoclinic polymorph of the title compound, [Na(C 4 H 5 O 6 )(H 2 O)] n , is reported and complements an orthorhombic form [Kubozono, Hirano, Nagasawa, Maeda & Kashino (1993). Bull. Chem. Soc. Jpn, 66, 2166-2173. The asymmetric unit contains a hydrogen tartrate anion, an Na + cation and a water molecule. The Na + ion is surrounded by seven O atoms derived from one independent and three symmetry-related hydrogen tartrate anions, and a water molecule, forming a distorted pentagonal-bipyramidal geometry. Independent units are linked via a pair of intermolecular bifurcated O-HÁ Á ÁO acceptor bonds, generating an R 2 1 (6) ring motif to form polymeric two-dimensional arrays parallel to the (100) plane. In the crystal packing, the arrays are linked by adjacent ring motifs, together with additional intermolecular O-HÁ Á ÁO interactions, into a three-dimensional network.

Comment
Tartaric acid is found free throughout nature, especially in many fruits and in wine, and as salts with Ca 2+ , K + , and Na + . It has many applications such as in making silver mirrors, in the manufacture of soft drinks, to provide tartness to foods, in tanning leather, and in making blueprints. Tartaric acid has optical activity (Synoradzki et al., 2008). Kubozono et al. (1993) reported the structure of the title compound, in the orthorhombic space group P2 1 2 1 2 1 . Herein, a new polymorph of the title compound is reported which crystallizes in the monoclinic space group P2 1 /c. The asymmetric unit contains a hydrogen tartrate anion, a Na cation and a water molecule (Fig. 1). The independent unit forms polymeric two-dimensional networks parallel to the plane (100) (Fig. 2). Each Na + ion is surrounded by seven O atoms ( Fig. 3) derived from a independent and three symmetry related hydrogen tartrate anions; and a water molecule, forming a distorted pentagonal bipyramidal geometry with Na-O distances ranging from 2.3331 (12) to 2.6740 (12) Å which are comparable to those reported in (Wong et al., 2009), whereas the angles around the Na + ion range from 62.55 (4) to 151.93 (4)°. Bond lengths and angles are within normal ranges and comparable to the orthorhombic polymorph of C 4 H 5 O 6 Na.H 2 O (Kubozono et al., 1993).

Experimental
Anhydrous tartaric acid (1.5 g, 0.1 mmol) was dissolved in water in a flat bottom flask with magnetic stirrer. In a separating funnel, sodium bicarbonate (0.85g, 0.1 mmol) was dissolved in water. The sodium bicarbonate solution was added in small portions to the flask of tartaric acid with stirring. The reaction mixture was refluxed for 1 h. After cooling the reaction mixture to room temperature, it was left for overnight stirring. The colourless crystals that subsequently formed were filtered and washed with methanol and dried at 353 K.

Refinement
All H atoms were located in a difference Fourier map and fixed at these positions [C-H = 0.92-1.00 Å; O-H = 0.77-0.90 Å] and U iso (H) = 1.2 U eq (C) and 1.5 U eq (O). Fig. 1. The asymmetric unit of the title compound, showing 50% probability displacement ellipsoids for non-H atoms and the atom-numbering scheme.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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 > 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.