3 , 4-Diiodo-2 , 5-dimethylthiophene

The molecule of compound (I) possesses normal geometric parameters and is essentially planar. Intermolecular S I interactions are observed in the structure with a distance of 3.641 (4) Å (Fig. 2). Such intermolecular interactions are also observed in the structure of tetraiodothiophene cocrystallized with tetrabutylammonium iodide (Bock & Holl, 2002), with S I distances of 3.58 and 3.59 Å, and other work involving I2 interactions with thioethers suggest the distances are typical e.g. 3.70 Å in the formation of extended structural networks (Blake et al., 1997 and 1998). Weak intermolecular C—H I interactions are not observed in this structure.

In the crystal structure of the title compound, C 6 H 6 I 2 S, the molecules pack to form one-dimensional chains connected by intermolecular SÁ Á ÁI interactions.

Comment
The title compound, (I) (Fig. 1), was prepared as part of a study aimed at producing new photochromic materials. Thiophene derivatives are important intermediates in the synthesis of photochromic compounds, organic light emitting diodes (OLED) and organic conductors. For photochromic compounds, potential applications are in the areas of optical recording, full-colour display and photoswitches.
The molecule of compound (I) possesses normal geometric parameters and is essentially planar. Intermolecular SÁ Á ÁI interactions are observed in the structure with a distance of 3.641 (4) Å (Fig. 2). Such intermolecular interactions are also observed in the structure of tetraiodothiophene cocrystallized with tetrabutylammonium iodide (Bock & Holl, 2002), with SÁ Á ÁI distances of 3.58 and 3.59 Å , and other work involving I 2 interactions with thioethers suggest the distances are typical e.g. 3.70 Å in the formation of extended structural networks (Blake et al., 1997 and1998). Weak intermolecular C-HÁ Á ÁI interactions are not observed in this structure.

Figure 1
The molecular structure of (I), showing 50% probability ellipsoids for non-H atoms.
The solution was stirred at 323 K for 3 h and saturated aqueous sodium thiosulfate (150 ml) was added. The organic phase was extracted with diethyl ether (4 Â 50 ml), dried over MgSO 4 and the solvent was removed under reduced pressure. The crude product was dissolved in dichloromethane and the solution was passed through a column of silica gel to remove the coloured material (9.15 g, 57%). The compound was recrystallized from dichloromethane at room temperature, giving crystals of (I) suitable for X-ray analysis (yield 9.15 g, 57%). Mo K radiation = 7.14 mm À1 T = 150 (2)  H atoms were placed in idealized positions (C-H = 0.98 Å ) and refined as riding atoms, with U iso (H) = 1.2 U eq (C). The highest residual electron-density peak is located 1.05 Å from atom I1 and the deepest hole is located 0.67 Å from the same atom.
We thank the EPSRC for funds which enabled the purchase of the Stoe IPDSII diffractometer. We acknowledge the use of the EPSRC's Chemical Database Service at Daresbury (Fletcher et al., 1996).

Figure 3
The unit-cell contents of (I), viewed along [100] (50% probability ellipsoids). The dashed lines indicate the closest intermolecular SÁ Á ÁI contacts. 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )