Benzo[1,2-b:4,5-b′]dithiophene-4,8-dione

The title molecule, C10H4O2S2, is situated on a crystallographic center of inversion. In the crystal, weak hydrogen bonding contributes to the packing of the molecules.

Given our interest in this field, we have isolated single crystals of this compound for structural determination. This structure is important in that it appears as crystalline products in poorly purified materials.
A thermal ellipsoid plot ( Fig. 1) displays the molecular structure of the title compound. Figure 2 shows a packing diagram of the crystal structure. Weak intermolecular interactions attribute to the packing of this compound, see: Sinnokrot et al. (2002); Janiak (2000). The closest CH···centroid distance is C1-H1···Cg1 at 3.715 (4) Å between a hydrogen atom and the center of a neighboring thiophene ring. No classic hydrogen bonds were found, but a weak hydrogen bond from C5-H5···O1 is present at 3.319 (4) Å D-A distance.

Experimental
The title compound was prepared according to a modified literature procedure (Beimlung & Kossmehl, 1986). In a typical reaction, 2 g of 3-theonic acid was reacted with excess thionyl chloride (50 ml) at reflux temperature overnight to produce the resulting acid chloride. Upon removal of the thionyl chloride, the acid chloride was dissolved in toluene (minimum amount). The acid chloride was then added to excess diethylamine (approximately 42 ml) to produce the thiophene amide. The product was isolated in diethyl ether, concentrated, and then re-dissolved in ether (30 ml). The amide was then cyclized with excess n-butyl lithium (1.6 M in hexane, approximately 7 ml) added dropwise and allowed to stir overnight. The reaction was quenched with water, filtered, and recrystallized from glacial acetic acid in a 31-45% yield depending on reaction.
Upon isolation of the final compound, crystals suitable for X-ray diffraction were obtained from slow evaporation of approximately 20 mg of product in approximately 5 ml of chloroform. Clear, yellow block crystals formed overnight.

Refinement
Crystallography. Hydrogen atom positions were placed in calculated positions and allowed to ride on the coordinates of the parent atom [C-H distances at 0.93 Å and U iso (H)=1.2U iso (C)].

Benzo[1,2-b:4,5-b′]dithiophene-4,8-dione
Crystal data C 10 H 4 O 2 S 2 M r = 220.25 Monoclinic, P2 1 /n a = 5.6402 (5)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.78 e Å −3 Δρ min = −0.57 e Å −3 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.