Thieno[3,4-d][1,3]dithiole-2-thione

In the title compound, C5H2S4, the terminal monocyclic S atom deviates by 0.117 (1) Å from the mean plane of the other non-H atoms (r.m.s. deviation = 0.001 Å). All six C—S bonds and the central C—C bond in the rings are characterized by π-conjugated lengths, endowing the molecule with high π-conjugation. In the crystal, the molecules are parallel packed, forming columnar stacks along the a axis. Short intermolecular S⋯S contacts [3.397 (1) and 3.486 (1) Å], are observed.

In the title compound, C 5 H 2 S 4 , the terminal monocyclic S atom deviates by 0.117 (1) Å from the mean plane of the other non-H atoms (r.m.s. deviation = 0.001 Å ). All six C-S bonds and the central C-C bond in the rings are characterized byconjugated lengths, endowing the molecule with highconjugation. In the crystal, the molecules are parallel packed, forming columnar stacks along the a axis. Short intermolecular SÁ Á ÁS contacts [3.397 (1) and 3.486 (1) Å ], are observed.

Comment
Although the synthesis of the title compound was reported early by Gronowitz & Moses, 1962, there is no report about its crystal structure. We have recently re-synthesized this compound and determined its X-ray structure.
The molecule adopts a nearly planar C 2v conformation (see below for details). All the C-S bond lengths, from the longest 1.744 (2)Å to the shortest 1.635 (2) Å, are between the lengths of C-S single bond and C═S double bond. The central C≐C bond, shared by the two fused five-member rings, also features a delocalized bond with its length being 1.430 (2) Å.
Thus the molecule is characterized by a highly planar π-conjugation.
The terminal S4 atom deviates by 0.117 (1)Å from the least-squares plane through the other eight non-H atoms (C1, C2, C3, C4, C5, S1, S2, S3). We suppose that this deviation may be the result of the above S···S intermolecular interactions. To support this assumption, we carried out an optimization procedure for the molecular conformation by using the Gaussian-03 programs (Frisch et al., 2003) within the framework of the DFT at the B3LYP/6-311(d) level. The optimized "free" molecule indeed adopts a perfect planar conformation with a strict C 2v symmetry. All the theoretical bond parameters are in good agreement with those of the X-ray results.

Experimental
The title compound was synthesized by a similar procedure to Chiang et al. (1983). 3,4-Dibromothiophene (2.18 g, 9.0 mmol) was dissolved in 30 ml anhydrous diethyl ether and stirred in the presence of N 2 at 195 K while n-butyllithium (5.6 ml, 9.0 mmol, 1.6 M in hexane) was added via syringe. Stirring was continued for 0.5 h, then sulfur (0.288 g, 9.0 mmol) was added. The reaction mixture was stirred for 1 h, and n -butyllithium (5.6 ml, 9.0 mmol) was added via syringe. After being stirred for 0.5 h, sulfur (0.288 g, 9.0 mmol) was added to the yellow solution. After 1 h, the reaction mixture was allowed to come to r.t. and was dried in vacuo. After removal of the solvent, 2 M sodium hydroxide solution (20 ml) and carbon disulfide (12 ml) were added. The mixture was refluxed under N 2 at 363 K for 4 h. And then the solution was stirred overnight at r.t.. The excess of carbon disulfide was removed in vacuo. Filtration of the mixture gave a yellow solid.
Recrytallization of the solid from dichloromethane-hexane (1:5, v/v) gave 0.26 g (15% yield) of the compound. Crystals were grown by slow evaporation of a dichloromethane solution.