N,N′-(Propane-1,3-diyl)dibenzothioamide

The title compound, C17H18N2S2, exhibits a trans–trans–trans–gauche+ (tttg +) conformation with regard to the NH–CH2–CH2–CH2–NH bond sequence. In the crystal, molecules are connected by N—H⋯S=C and C—H⋯S=C hydrogen bonds, forming a herringbone arrangement along the c-axis direction. The two thioamide groups make dihedral angles of 43.0 (2) and 33.1 (2)° with the adjacent phenyl rings.

The title compound, C 17 H 18 N 2 S 2 , exhibits a trans-trans-transgauche + (tttg + ) conformation with regard to the NH-CH 2 -CH 2 -CH 2 -NH bond sequence. In the crystal, molecules are connected by N-HÁ Á ÁS C and C-HÁ Á ÁS C hydrogen bonds, forming a herringbone arrangement along the c-axis direction. The two thioamide groups make dihedral angles of 43.0 (2) and 33.1 (2) with the adjacent phenyl rings.
Supporting information for this paper is available from the IUCr electronic archives (Reference: BV2233).
In this study, we have determined the crystal structure of its homologue, N,N′-(propane-1,3-diyl)dibenzothioamide (referred to here as PDBTA), a monomeric model compound of poly(trimethylene terephthalthioamide), [-(C=S)-C 6 H 4 -(C=S)-NH-(CH 2 ) 3 -NH-] n . Figure 1 shows the molecular structure of PDBTA, whose NH-CH 2 -CH 2 -CH 2 -NH bond sequence adopts the tttg + conformation. On the other hand, our molecular orbital (MO) calculations at the B3LYP/6-311+G(2 d,p)//B3LYP/6-311+G(2 d,p) level including the solvent effect of dimethyl sulfoxide have predicted that the alltrans form is the most stable of its possible conformations; the crystal conformation, tttg + , was suggested to have a free energy higher than tttt by as much as 1.19 kcal mol -1 . Figure 2 shows the molecular packing of the PDBTA crystal, in which two kinds of intermolecular hydrogen bonds, C=S···H-N and C=S···H-C, seem to be formed (not shown in the figure). For details, see Table 1. The intermolecular attractions may fully compensate the cost of conformational energy of 1.19 kcal mol -1 . The PDBTA molecules form a herringbone arrangement along the c-axis direction. Brisson & Brisse (1986) determined the crystal structure of N,N′-(propane-1,3-diyl)dibenzamide (PDBA), a model compound of poly(trimethylene terephthalamide). The crystalline PDBA molecule lies in the ttg + g + conformation and forms intermolecular C=O···H-N hydrogen bonds with the neighbors. According to our MO calculations, its most stable conformer is tttg + (-0.54 kcal mol -1 relative to that of the all-trans state), and the crystal conformation, ttg + g + , has a somewhat higher free energy (+0.32 kcal mol -1 ).

Experimental
Benzoyl chloride (12.0 ml, 103 mmol) was added dropwise to an aqueous solution of 1,3-diaminopropane (3.4 ml, 41 mmol) and sodium hydroxide (61.5 ml, 0.100 mol l -1 ) stirred by a mechanical stirrer in a three-necked flask equipped with a dropping funnel and a calcium-chloride drying tube, with the flask being bathed in water kept at 10 °C. The mixture was stirred at 10 °C for 1 h, diluted with water (100 ml), and stirred again at room temperature overnight to yield white precipitate. The precipitate was collected by suction filtration, washed with water, and dried. The crude product was recrystallized from a mixture of ethanol and toluene (1:1 in volume) and dried at 40 °C under reduced pressure to yield PDBA (yield 28%). In principle, this synthesis is based on the procedure of Hart & Brewbaker (1969).
PDBA (1.0 g, 3.6 mmol) and Lawesson's reagent (1.7 g, 4.2 mmol) (Cave & Levinson, 1985) were dissolved in toluene (20 ml) stirred in a three-necked flask equipped with a reflux condenser connected to a calcium-chloride drying tube. The mixture was refluxed under dry nitrogen at ca 110 °C for 8 h, and the completion of the reaction was confirmed by thinlayer chromatography. After toluene was removed under reduced pressure, the residual was subjected to column chromatography on silica gel with chloroform, and yellowish solution (retention factor R f = 0.3) was collected and condensed to yield yellow slurry, which was recrystallized twice from a mixture of ethyl acetate and diethyl ether (1:1 in supplementary materials sup-2 Acta Cryst. (2014). E70, o639 volume) and dried under reduced pressure to yield PDBTA (yield 48%).
A small quantity of PDBTA 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, crystals were found to be formed suitable for X-ray diffraction.

Refinement
All C-H hydrogen atoms were geometrically positioned with C-H = 0.95 and 0.99 Å for the aromatic and methylene groups respectively, and refined as riding by Uiso(H) = 1.2 Ueq(C). The N-H hydrogen atoms were located and fixed with N-H = 0.87 Å.

Special details
Experimental. SADABS (Sheldrick 1996) 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 was performed with all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 , while the R-factor on F. The threshold expression of F 2 > 2.0 σ(F 2 ) was used only for calculating R-factor.