1-Benzothiophene-2-carbaldehyde 4-ethylthiosemicarbazone

The title compound, C13H15N3S2, crystallizes with two unique molecules, A and B, in the asymmetric unit. These differ principally in that the methyl group of the 4-ethylthiosemicarbazone moiety is ordered in molecule A but disordered over two positions with equal occupancies in molecule B. The benzothiophene group and the semicarbazone unit are inclined at dihedral angles of 11.78 (8)° for molecule A and 8.18 (13)° for molecule B. Weak intramolecular N—H⋯N interactions contribute to the planarity of the semicarbazone units in both molecules and each molecule adopts an E configuration with respect to the C=N bonds. In the crystal structure, molecules form centrosymmetric dimers as a result of N—H⋯S hydrogen bonds, augmented by C—H⋯S interactions for molecule A and C—H⋯S interactions for molecule B. Weak C—H⋯π interactions stack the dimers of both molecules into columns down the a axis.


Comment
Thiosemicarbazones are a class of compounds that have been investigated because of their biological activity (de Sousa et al., 2007). As a continuation of our work on thiosemicarbazone compounds as potential ligands in transition metal chemistry (Kayed et al., 2008;) we report here the structure of the title compound ( Fig. 1), which crystallizes with two unique molecules, A and B, in the asymmetric unit. The two molecules are closely similar with the exception of the methyl C atom of the ethyl group which is disordered over two positions C131 and C132, with equal occupancies. The similarities of the remainder of the two molecules are demonstrated by the fact that the non-hydrogen atoms of molecules A and B overlay in Mercury (Macrae et al., 2006)  interactions. The benzothiophene groups and the semicarbazone groups are inclined at dihedral angles of 11.78 (8)° for molecule A and 8.18 (13)° for molecule B. Both molecules adopt an E configuration with respect to the C═N bonds, bond distances are normal (Allen et al., 1987) and comparable to those in similar structures (Chuev et al. 1992;de Lima et al. 2002;Isik et al. 2006;Kayed et al. 2008).
In the crystal structure, a centrosymmetric dimer with an R 2 2 (8) ring motif (Bernstein et al., 1995) is formed by through N2A-H2NA···S2A hydrogen bonds strengthened by additional C10A-H10A···S2A interactions for molecule A. A second dimer forms via C10B-H10D···S2B interactions for molecule B (Table 1 and Fig. 2). The dimers are further aggregated into columns down the a axis by weak C-H···π interactions involving the C10A and C10B methyl groups and the thiophene rings of adjacent molecules, Fig. 3.

Experimental
The title compound was prepared by heating 35 ml of an ethanolic solution of 2-acetylbenzothiophene (1.76 g, 10 mmol) and 4-ethyl-3-thiosemicarbazide (1.2 g, 10 mmol) under reflux for 2 h. Three drops of concentrated H 2 SO 4 were added.
The resulting product was isolated and recrystallized from acetonitrile to afford yellow needles of the title compound in 63.5% yield (m.p. 448-450 K).

Refinement
H atoms bound to N2A, N2B, N3A and N3B were located in a difference electron density map and refined freely with U iso = 1.2U eq (N). All other H-atoms were refined using a riding model with d(C-H) = 0.95 Å, U iso = 1.2U eq (C) for aromatic, and 0.98 Å and U iso = 1.5U eq (C) for CH 3 H atoms. The methyl C atom of the ethyl group in molecule B is disordered over two positions, C131 and C132, each with occupancies of approximately 0.5. These occupancies were fixed at 0.5 in the final refinement cycles.  Fig. 1. The asymmetric unit of the title compound, with atom labels and 50% probability displacement ellipsoids for non-H atoms. For clarity, only one disorder component is shown.

Special details
Geometry. All e.s. 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.