4-Thioxo-3,5-dithia-1,7-heptanedioic acid

The complete molecule of the title compound, C5H6O4S3, is generated by crystallographic twofold symmetry with the C=S group lying on the rotation axis. The molecules are linked through weak hydrogen-bond contacts by glide-plane operations to form R 2 2(20) rings and ladder-like C(4) chains along the c axis.

The complete molecule of the title compound, C 5 H 6 O 4 S 3 , is generated by crystallographic twofold symmetry with the C S group lying on the rotation axis. The molecules are linked through weak hydrogen-bond contacts by glide-plane operations to form R 2 2 (20) rings and ladder-like C(4) chains along the c axis.

Comment
Although the synthesis and the molecular structure of the title compound, also named as trithiocarbodiglycolic acid (TTCD), have been reported, to our knowledge, there is no report on the unit-cell parameters and the crystal structure of TTCD in the literature (Reid, 1962;Strube, 1963;El-Bindary et al., 1994). The crystal structure of a 1:1:1 cocrystal of TTCD and trithiocarbodiglycolate and bis(dicyclohexylammonium) (Ng, 1995) have been reported.
The molecule of the title compound occupies a crystallographic twofold rotation axis with one half-molecule in the asymmetric unit, the C 2 axis running through the C═S group (Fig. 1). The same molecular symmetry can be observed for the trithiocarbodiglycolate 2and the neutral TTCD molecules in the structure reported by Ng (1995), which crystallises in space group P2/a. Bond distances and angles (Table 1) are close to those in the complexes composed of TTCD reported in the literature (Ng, 1995).
The molecules are linked through weak hydrogen-bond contacts (Table 2) by glide-plane operations to form R 2 2 (20) rings and C(4) chains along the c axis (Fig. 2).

Experimental
A mixture of trithiocarbodiglycolic acid (0.25 mmol), CoCl 2 .6H 2 O (0.25 mmol) was dissolved in a 10 ml water in order to synthesize the Co complexes with trithiocarbodiglycolic acid as the ligand. After stirring for about 8 h, the mixed solution was filtered. The filtrate was allowed to stand at room temperature. Colorless crystals of the title complex but not the Co complex with the trithiocarbodiglycolic acid as the ligand were obtained over a period of 10 d.

Refinement
H atoms were allowed to ride on their respective parent atoms with C-H and O-H distances of 0.97 and 0.82 Å, respectively, and were included in the refinement with isotropic displacement parameters U iso (H) = 1.2U eq (C) and U iso (H) = 1.5U eq (O), respectively.  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 Rfactors(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.