8,10-Diiodo-2,6-dioxo-4λ3-ioda-3,5-dioxatricyclo[5.3.1.04,11]undeca-1(11),7,9-triene-9-carboxylic acid

In the title compound, C9HI3O6·2H2O, the molecule is located on a twofold axis that gives rise to disorder of the carboxyl group. This disorder is correlated with the disorder of one of the H atoms of the water molecule. The carboxyl group is twisted relative to the attached benzene ring by 75.1 (4)°. The intramolecular I⋯O distance is 2.112 (6) Å. Molecules are linked via O—H⋯O hydrogen bonding, C—I⋯O halogen bonding, with I⋯O distances in the range 3.156 (5)–3.274 (6) Å, and dipolar C=O⋯C=O interactions between the carboxyl and carboxylate groups, with an O⋯C distance of 2.944 (10) Å.

In the title compound, C 9 HI 3 O 6 Á2H 2 O, the molecule is located on a twofold axis that gives rise to disorder of the carboxyl group. This disorder is correlated with the disorder of one of the H atoms of the water molecule. The carboxyl group is twisted relative to the attached benzene ring by 75.1 (4) . The intramolecular IÁ Á ÁO distance is 2.112 (6) Å . Molecules are linked via O-HÁ Á ÁO hydrogen bonding, C-IÁ Á ÁO halogen bonding, with IÁ Á ÁO distances in the range 3.156 (5)-3.274 (6) Å , and dipolar C OÁ Á ÁC O interactions between the carboxyl and carboxylate groups, with an OÁ Á ÁC distance of 2.944 (10) Å .
In the title compound the organic molecule is located on a twofold axis what results in disorder of the carboxylic group ( Fig. 1). In the crystal structure, there are hydrogen bonds between symmetry related water molecules as well as between the water molecule and the carboxylic group. It indicates that one of the hydrogen atoms of the water molecule has to be disordered and this disorder is evidently correlated with the disorder of the carboxylic group. The hydrogen atom and the oxygen atom forming hydrogen bond between the water molecule and the carboxylic group are either from the water molecule or the carboxylic group. There is also a hydrogen bond between the water molecule and the carboxylate O1 atom. Hydrogen atom involved in this interaction has full occupancy. The dihedral angle between the plane of the carboxyl group and the benzene ring is 75.1 (4)°.
In addition to hydrogen bond, the structure is stabilized by halogen bonding between the I2 atom and the carboxylate group O1 and O2 atoms. There is also a halogen bond between the water molecule and I1 atom (Fig. 2). A dipolar interaction between carboxyl C6-O3 and carboxylate C4 also is observed (C···O 2.95 Å) (Fig. 3).

Experimental
A mixture of 1,3,5-triiodo-2,4,6-trimethylbenzene (5 g) and excess of potassium permanganate (80 g) was dissolved in pyridine (60 ml) and heated under reflux for 24 h to produce the title compound (m.p. 573 K, decompose). Crystallization was carried out from a mixture of water and methanol (v/v 1:2). Colorless crystals suitable for X-ray single-crystal diffraction were obtained by slow evaporation method.

Refinement
H atom of the carboxylic group was placed in geometrically calculated position and refined using a riding model the the occupantion factor of 0.5. Positions of H atoms from the water molecule were calculated after analysis of possible hydrogen-bond interactions. The occupantion factors of H1W and H2W were assigned as 0.5.
The isotropic displacement parameters of all H atoms were set to 1.5 times the equivalent displacement parameter of their parent O atoms.  (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Figure 1
Molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Symmetry code: (i) 1 -x, y, 1.5 + z.   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 2.36 e Å −3 Δρ min = −2.23 e Å −3 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 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.