Crystal structure of bis(N-methyl-N-phenylamino)trisulfane

The title compound was obtained in crystalline form after preparative HPLC. Conformation of the proposed molecular structure was obtained by single-crystal X-ray analysis at 173 K. The molecules do not take advantage of the twofold axis provided as an available symmetry option by the Fdd2 space group. Instead, there are two molecules in the asymmetric unit, and both of them display a pseudo-trans conformation.


Chemical context
The reactions of substrates with one or two sulfanyl chloride, acid chloride, and/or (alkoxydichloromethyl)sulfanyl moieties have been of interest to our laboratory for some time (Barany et al., 1983;Barany & Mott, 1984;Schroll & Barany, 1986;Schroll et al., 1990;Schroll et al., 2012). In some of these experiments, bis[methyl(phenyl)amino]trisulfane was a component of more complicated mixtures of polysulfanes with varying numbers of S atoms. One such mixture was separated by preparative HPLC at 298 K, eluting with methanol-water (17:3). The fraction containing the title compound (dissolved in the eluting solvent) was cooled to 277 K, after which the trisulfane was obtained directly in crystalline form.

Structural commentary
The title compound, (1), was obtained in crystalline form after preparative HPLC, as described by Schroll & Barany (1986). The proposed molecular structure of (1) was confirmed by single-crystal X-ray analysis at 173 K. The molecules do not take advantage of the twofold axis provided as an available symmetry option by the Fdd2 space group. Instead, there are two molecules, (1a) and (1b), in the asymmetric unit (Fig. 1), and both of them display a pseudo-trans conformation (see ISSN 2056-9890 later). All bond distances and angles in both molecules are within expected ranges. Selected geometric parameters for compound (1) are given in Table 1. The two consecutive S-S bond lengths (comprising the trisulfane) of molecule (1a) are 2.064 (3) and 2.078 (3) Å , and for molecule (1b) are 2.076 (3) and 2.067 (2) Å . These values are similar to the value of 2.07 Å reported for the S-S bond length in elemental sulfur (S 8 ). Torsion angles about each of the two S-S bonds (comprising the trisulfane) are, respectively, 86.6 (2) and 87.0 (2) for (1a), and À84.6 (2) and À85.9 (2) for (1b). The core atoms, viz. the N-S-S-S-N moiety, of the two units superimpose well if one is inverted on the other, but the phenyl groups do not. Thus, the two units are essentially conformational enantiomers. Moreover, with respect to the four measured torsion angles, which range in absolute value from 84.6 (2) to 87.0 (2) , these are slightly smaller than the theoretical optimum of 90.0 (Pauling, 1949;Torrico-Vallejos et al., 2010). Finally, given the presence of three consecutive linearly connected sulfur atoms, representing two dihedral angles close to 90 , it is noteworthy that both of the molecules in the asymmetric unit display a pseudo-trans conformation (torsion angles +,+ or -,-across the two S-S bonds). The theoretically possible pseudo-cis (torsion angles +,-or -,+) conformation (Meyer, 1976) was not observed for these structures.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.36, February 2015;Groom & Allen, 2014) revealed the presence of two compounds (see Fig. 3) that also have an N-S-S-S-N moiety, viz. bis(oxamido)trisulfane, (2) (CSD refcode GEHPUE; Brunn et al., 1988), and bis [tert-butyl(di- The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A view along the b axis of the crystal packing of the title compound. The dashed lines indicate the C-HÁ Á Á interactions (see Table 2 for details).
Only the H atoms involved in these interactions have been included for clarity.

Synthesis and crystallization
The title compound, (1), was synthesized and obtained in crystalline form after preparative HPLC, as described by Schroll & Barany (1986): compound (37) in that publication.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms were positioned geometrically and refined using a riding model, with C-H = 0.95-0.98 Å and U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for other H atoms.

Special details
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 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 > 2σ(F 2 ) is used only for calculating R-factors(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.