Disodium 4,5,6-trihydroxybenzene-1,3-disulfonate dihydrate

In the title compound, 2Na+·C6H4O9S2 2−·2H2O, the benzene rings of the 4,5,6-trihydroxybenzene-1,3-disulfonate ions, which are stacked parallel to each other forming rods parallel to the a axis, are slightly deformed (planarity, symmetry) mainly because of the high degree of substitution. The two sodium ions, located within pockets of the anion rods, are coordinated by six and seven O atoms, resulting in octahedral and pentagonal-bipyramidal coordinations, respectively. In addition to these coordinative bonds towards sodium, an extended network of intra- and intermolecular hydrogen bonds occurs.

In the title compound, 2Na + ÁC 6 H 4 O 9 S 2 2À Á2H 2 O, the benzene rings of the 4,5,6-trihydroxybenzene-1,3-disulfonate ions, which are stacked parallel to each other forming rods parallel to the a axis, are slightly deformed (planarity, symmetry) mainly because of the high degree of substitution. The two sodium ions, located within pockets of the anion rods, are coordinated by six and seven O atoms, resulting in octahedral and pentagonal-bipyramidal coordinations, respectively. In addition to these coordinative bonds towards sodium, an extended network of intra-and intermolecular hydrogen bonds occurs.

Comment
Cunitic compounds are amphiphilic substances consisting of wedge shaped molecules with the polarity of the wedges tip to be considerably different from the polarity of the wedges base (Beginn et al., 2008). In bulk and in solution of non -semi polar solvents the wedges selfassemble in form of cylindrical superstructures with the polar tip arranged in the centre of the cylinder, while the non-polar base is directed to the cylinders outer surface. In bulk the cylinders arrange in regular patterns and frequently form columnar mesophases, while in diluted solutions (< 1 -5 wt%) isolated cylinders as well as cylinder bundles tend to form three-dimensional networks that confine the solvent and cause the macroscopic gelation of the liquid (Percec et al., 2006).
Based on this peculiar structure formation, numerous investigations have been devoted to the subject. However, since the formation of cylindrical superstructures is a common phenomenon for cunitic molecules and since it occurs almost independently of the actual chemistry, i.e. the functional groups making the molecular tip of the compounds, the cylinders can be exploited to generate molecular defined channels to transport molecules, ions, photons or electrons (Percec et al., 2004).
Functional membranes containing self-organized supramolecular channels of cunitic molecules with crown-ether and carboxylate functional tips have been prepared. Presently there is ongoing research to generate supramolecular channels containing sulfonate groups, since these can be used as model materials to investigate the "superselectivity" effects of Nafion and other perfluoro -sulfonate membranes (Zhu et al., 2006(Zhu et al., , 2004. In research for new starting materials for cunitic compounds we tried to prepare 3,4,5-trihydroxybenzenesulfonic acid by sulfonation of 1,2,3-trihydroxybenzene. However, in applying the reaction conditions and work-up procedures described for this compound (Pješčić et al., 2000) only one compound could be isolated in very low yield. Subsequent spectroscopic measurements of this compound gave evidence that the product corresponds more to a "disulfonate" than to the desired "monosulfonate". Finaly these observations were confirmed by a single-crystal X-ray structure determination, the results of which we present here. The deviations from the geometry of a regular hexagon of the benzene ring itself are small (with respect to bond lengths and angles) those taking the substituents into account are considerable: neither are adjacent bonds parallel to each other nor are these bonds directed to the centre of the benzene ring. On the other side, carbon-oxygen [1.361 (3) -1.363 (2) Å, mean value 1.362 (1) Å] and carbon-sulfur [1.770 (2) Å for S1, 1.760 (2) Å for S3] bond lengths are in the range expected for single bonds between these atoms.
In the crystal structure, anions are stacked parallel [distances: 3.275 (1) Å and 3.411 (1) Å] to each other forming rods along the crystallographic a axis with a crystallographic centre of symmetry between each pair of anions. Therefore the anions within the rods change their orientation from one to another. Based on a mismatch of the anion centre from the a axis, the benzene rings are not completely congruent within these rods. Thus, possible π -π interactions between neighbouring benzene rings are restricted to only two carbon atoms (or one bond).
The sodium ions are located in pockets of the rods formed by the organic anions, whereas the two additional water molecules are situated between them (Fig. 2). In summary, the sodium ions are coordinated by 6 [Na1] and 7 [Na2] oxygen atoms of the water molecules, hydroxyl groups and SO 3 -groups resulting in an octahedral and pentagonal-bipyramidal coordination, respectively. The pentagonal-bipyramidal coordination is caused by a SO 3 -group coordinating as a bidental ligand to one of the sodium cations [Na2]. Both coordination polyhedrons are linked to each other via a common edge built up by two oxygen atoms of two different SO 3 -groups (Fig. 3).
Besides the already described interactions, there is an extended network of intra-and intermolecular hydrogen bonds.
Within 30 minutes the educt dissolved and the reaction mixture was stirred for another 48 h at 25°C. Subsequently the white precipitate was filtered over a P4 glass frit and the solid residue was redissolved in 20 ml water. After the solution was allowed to cool to ambient temperature it was rapidly mixed with a solution of 6 g sodium chloride in 17 ml water. After storing at 5°C over night in a refrigerator, the precipitated white crystals were collected by filtration over a P4 glass frit. In contrast to literature reports the crude product was hardly soluble in ethanol, but could be recrystallized twice from 10 ml of water to remove inorganic sodium salts. The final product 2 was dried in a desiccator at ambient temperature in vacuo over phosphorpentoxide until weight constancy. Yield: 1.15 g = 11.6% of theory, white crystals. After purification, the crystals changed their colour from white to a light red, which is caused by oxidation processes being typical for pyrogallol (Siegel & Siegel, 1950 120.8, 118.7. The 1 H-NMR spectrum was measured from a mixture of 11.9 mg 2 and 8.9 mg (0.101 mmol) 1,4-dioxane.

Crystallographic studies:
For single-crystal X-Ray analysis a suitable single-crystal was selected under a polarization microscope and mounted on a 400/25 µm MicroMesh MiTeGen TM using FOMBLIN Y perfluropolyether (LVAC 16/6, Aldrich).

Refinement
Hydrogen atoms were clearly identified in difference Fourier syntheses. The positions of hydrogen atoms bonded to oxygen were refined with respect to two common O-H bond lengths (H 2 O: 0.81 Å; -OH: 0.80 Å), and an idealized H-O-H angle of 104.5°, before they were allowed to ride on the corresponding oxygen atoms. The hydrogen atom bonded to the carbon atom of the benzene ring was refined at a calculated position riding on the carbon atom with C-H = 0.95 Å. Two equivalent displacement factors were refined for hydrogen atoms: one for the four hydrogen atoms of the organic molecule and one for the four hydrogen atoms of the two water molecules. All other atoms were refined with anisotropic displacement parameters. Fig. 1. : Ball-and-stick model of the 4,5,6-trihydroxybenzene-1,3-disulfonate ion, with the atomic numbering scheme; with exception of the hydrogen atoms, which are shown as spheres with common isotropic radius, all other atoms are represented as thermal displacement ellipsoids showing 50% probability level of the corresponding atoms. Fig. 2. : Ball-and-stick model of the crystal packing structure of the title compound in direction of the a-axis with the atomic numbering scheme used for the sodium ions and the oxygen atoms of the water molecules; for clarity all atoms are shown as spheres with common isotropic radius. Fig. 3. : Ball-and-stick representation of the sodium environment; with exception of the hydrogen atoms, which are shown as spheres with common isotropic radius, all other atoms are represented as thermal displacement ellipsoids showing 50% probability level of the corresponding atom; covalent bonds as black rods, coordinative bonds as white ones; symmetry codes: (1) x, y, z; (2) -1 + x, y, z; (3) -1 + x, 1 + y, z; (4) -x, 1 -y, z; (5) -x, 1 -y, 1 -z; (6) x, 1 + y, z; (7) -x, 2 -y, -z. Fig. 4. : Ball-and-stick model of the hydrogen-bond arrangement the 4,5,6-trihydroxybenzene-1,3-disulfonate ion is involved in; with exception of the hydrogen atoms, which are shown as spheres with common isotropic radius, all other atoms are represented as thermal displacement ellipsoids showing 50% probability level of the corresponding atoms; symmetry codes: (1) 1 -x,1 -y,1 -z. Disodium 4,5,6-trihydroxybenzene-1,3-disulfonate dihydrate   (7) 0.0049 (7) −0.0081 (7)  Geometric parameters (Å, °)