N,N′-(Ethane-1,2-diyl)dibenzenecarbothioamide

The title compound, C16H16N2S2, adopts a gauche +–gauche +–gauche + (g+g+g+) conformation in the NH—CH2—CH2—NH bond sequence. In the crystal, molecules are connected by pairs of N—H⋯S=C hydrogen bonds and C—H⋯π interactions, forming a tape structure along the c-axis direction.

The title compound, C 16 H 16 N 2 S 2 , adopts a gauche + -gauche +gauche + (g + g + g + ) conformation in the NH-CH 2 -CH 2 -NH bond sequence. In the crystal, molecules are connected by pairs of N-HÁ Á ÁS C hydrogen bonds and C-HÁ Á Á interactions, forming a tape structure along the c-axis direction.
Supporting information for this paper is available from the IUCr electronic archives (Reference: IS5357).
The MO calculations at the B3LYP/6-311+G(2d,p)//B3LYP/6-311+G(2d,p) level including the solvent effect of dimethyl sulfoxide have predicted conformational preferences of EDBTA; the first and second most stable conformers are tg + g -(-0.99) and g + g + g + (-0.76), respectively, where the values in the parentheses are Gibbs free energies in kcal mol -1 relative to that of the all-trans state.
According to the MO calculations, the tg + gconformer of EDBTA seems to form intramolecular C=S···H-N and C=S···C-H attractions. As shown in Figure 2, the crystallized EDBTA molecule, lying in the g + g + g + conformation, forms intermolecular C=S···H-N and C-H..π interactions. Probably, the crystalline EDBTA chooses the intermolecular C=S···H-N interaction rather than the intramolecular one to acquire a larger energy stability. The MO calculations predicted that stable conformers of N,N′-(ethane-1,2-diyl)dibenzamide (X = O and m =2, abbreviated as EDBA), the model compound of poly(ethylene terephthalamide), are, in the ascending order of free energy, tg + g -, g + tg + , g + g + g + , g + tg -,···; the energy difference between g + tgand tg + gwas estimated as 0.89 kcal mol -1 . Nevertheless, the EDBA molecule crystallizes to adopt the fourth stable conformation, g + tg - (Palmer & Brisse, 1980). In contrast with models of the polythioester (X = O, Y = S, and m = 2) and polydithioester (X = Y = S and m = 2) (Abe et al., 2011;Abe & Sasanuma, 2012), EDBA and EDBTA do not crystallize in the most stable conformation suggested by the MO calculations probably because of the significant stabilization of the intermolecular C=O···H-N and C=S···H-N hydrogen bonds.

Experimental
Benzoyl chloride (4.6 ml, 40 mmol), dissolved in 1,2-dichloroethane (100 ml), was added dropwise to ethylenediamine (14 ml, 210 mmol) and 1,2-dichloroethane (300 ml) stirred by a magnetic stirrer in a three-necked flask equipped with a dropping funnel and a calcium-chloride drying tube, with the flask being bathed in ice water. The mixture was stirred at room temperature for 8 h to yield white precipitate. The precipitate was collected by suction filtration, washed with water, and dried. The crude product was recrystallized from methanol and dried at 40 °C under reduced pressure to yield EDBA (yield 55%). In principle, this synthesis is based on the procedure of Jacobson et al. (1987).
Lawesson's reagent (1.8 g, 4.6 mmol) and EDBA (1.0 g, 3.7 mmol) were dissolved in toluene (20 ml) stirred in a threenecked flask equipped with a reflux condenser connected to a calcium-chloride drying tube. The solution was refluxed under dry nitrogen at ca 110 °C for 8 h to yield yellow precipitate. The precipitate was collected, washed with toluene, recrystallized from ethanol, and dried at 40 °C under reduced pressure to yield EDBTA (yield 79%).
A small quantity of EDBTA was dissolved in chloroform in a glass tube, whose top was sealed with a thin Teflon film.
The tube was placed in a vial container including a small amount of n-hexane, and the container was capped and left to stand still in a dark place. After a day, its crystals were found to be formed in the inner tube.