Crystal structure of N-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)-2-(thiophen-3-yl)acetamide

The synthesis and crystal structure of a new thiophene monomer containing an additional rhodanine heterocycle are reported. The crystal packing is sustained by N—H⋯O, C—H⋯O, C—H⋯S and C—H⋯π interactions.

The title compound, C 9 H 8 N 2 O 2 S 3 , crystallizes with two molecules (A and B) in the asymmetric unit. Both have similar conformations (overlay r.m.s. deviation = 0.209 Å ) and are linked by an N-HÁ Á ÁO hydrogen bond. In both molecules, the thiophene rings show orientational disorder, with occupancy factors of 0.6727 (17) and 0.3273 (17) for molecule A, and 0.7916 (19) and 0.2084 (19) for molecule B. The five-membered rings make an angle of 79.7 (2) in molecule A and an angle of 66.8 (2) in molecule B. In the crystal, chains of molecules running along the a-axis direction are linked by N-HÁ Á ÁO hydrogen bonds. The interaction of adjacent chains through N-HÁ Á ÁO hydrogen bonds leads to two types of ring structures containing four molecules and described by the graphset motifs R 4 4 (18) and R 4 2 (14).

Chemical context
Thiophene, C 4 H 4 S, belongs to a class of aromatic fivemembered heterocycles containing one S heteroatom. Thiophene and its derivatives occur in petroleum or coal (Orr & White, 1990). Thiophene-based compounds have applications in modern drug design (Santagati et al., 1994), electronic and optoelectronic devices (Barbarella et al., 2005), and conductive and electroluminescent polymers (Friend et al., 1999). Also, several reviews of various aspects of thiophene coordination and reactivity in transition-metal complexes have been reported (Barbarella et al., 2005).
Derivatives of rhodanine (or 2-thioxo-1,3-thiazolidin-4one) have interesting pharmacological properties, such as the drug Epalrestat, which is an aldose reductase inhibitor used to treat diabetic neuropathy (Tomašić & Mašič, 2012). Some other rhodanine derivatives were designed and synthesized for detecting tau pathology in the brains of patients with Alzheimer's disease (Ono et al., 2011).
As a continuation of our research  on the chemical, physical and biological properties of new polythiophenes, a new thiophene monomer containing ISSN 2056-9890 rhodanine has been prepared. In the presence of FeCl 3 , thiophene monomers can be polymerized by C-C bond formation between the 2-and 5-positions of two subsequent thiophene monomers, resulting in an extended -conjugated system. We present here the synthesis and crystal structure of N-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)-2-(thiophen-3yl)acetamide, 3.

Figure 1
View of the asymmetric unit of the title compound, showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as small circles of arbitrary radii. The minor component of the disordered thiophene rings is shown in pale yellow.

Figure 2
Part of the crystal packing of the title compound, showing a chain of molecules along the a axis formed by N-HÁ Á ÁO hydrogen-bond interactions a and b [see Table 1; symmetry codes:

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.38, last update February 2017; Groom et al., 2016) for structures containing an N-substituted 2-thioxo-1,3-thiazolidin-4-one ring gave 26 hits (169 hits when substituents at the 5-position are also allowed). In all cases, the 1,3-thiazolidine ring can be considered to be planar, as the largest deviation from the best plane through the ring atoms was only 0.070 Å [for the complex bis(rhodanine)copper(I) iodide; refcode VICJUM; Moers et al., 1986]. The substituent at the N3 position is situated in the 1,3-thiazolidine plane, with a largest deviation of 0.174 Å for the case with -NH 2 as substituent (refcode EDEPUZ01; Jabeen et al., 2007). Rotational disorder in 3-CH 2 -thiophene fragments is frequently observed (25 structures of the 67 fragments present in the CSD).

Synthesis and crystallization
The reaction scheme to synthesize the title compound, 3, is given in Fig. 5.

Synthesis of methyl 2-(thiophen-3-yl)acetate, 1
Methyl thiophene-2-acetate, 1 (5 mmol), was added to an excess of hydrazine hydrate (40 mmol) in ethanol (20 ml). The mixture was refluxed for 6 h. The reaction mixture was allowed to cool. The resulting precipitate was filtered and recrystallized from ethanol solution to give 0.57 g (yield 74%) of hydrazide 2 in the form of white crystals (m.p. 343 K). IR

Synthesis of 2-(thiophen-3-yl)acetohydrazide, 2
Methyl thiophene-2-acetate, 1 (5 mmol), was added to an excess of hydrazine hydrate (40 mmol) in ethanol (20 ml). The mixture was refluxed for 6 h. The reaction mixture was allowed to cool. The resulting precipitate was filtered and recrystallized from ethanol solution to give 0.57 g (yield 74%) of hydrazide 2 in the form of white crystals (m.p. 343 K). IR Reaction scheme for the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Both thiophene rings are disordered over two positions by a rotation of approximately 180 around the C5-C3 or C15-C13 bond for molecules A and B, respectively. The final occupancy factors are 0.6727 (17) and 0.3273 (17) for molecule A, and 0.7916 (19) and 0.2084 (19) for molecule B. Bond lengths and angles in the disordered thiophene rings were restrained to target values derived from mean values observed in 3-CH 2 -thiophene fragments in the CSD (Groom et al., 2016). The same anisotropic displacement parameters were used for equivalent atoms in the disordered thiophene rings (e.g. EADP C1A C1B). The H atoms attached to atoms N1 and N11 were found in the difference density Fourier map and refined freely. The other H atoms were placed in idealized positions and refined in riding mode, with U iso (H) values assigned as 1.2U eq of the parent atoms, with C-H distances of 0.95 (aromatic) and 0.99 Å (CH 2 ). In the final cycles of refinement, four outliers were omitted.

N-(4-Oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)-2-(thiophen-3-yl)acetamide
Crystal data 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (