Synthesis and crystal structure of (E)-1,2-bis[2-(methylsulfanyl)phenyl]diazene

In the crystal structure of the title compound, two half-molecules are found in the asymmetric unit. The completed molecules differ only slightly in bond lengths and torsion angles.


Chemical context
The molecular switch azobenzene can undergo isomerization from its thermodynamically stable trans form to the metastable cis form using external stimuli such as light, temperature or pressure. Azobenzenes are common motifs in dyes because of their high thermal and photochemical stability (Yesodha et al., 2004;Lagrasta et al., 1997). We recently presented methods to substitute azobenzenes in the ortho, meta and para-positions with trimethyltin as a novel functionalization method, giving rise to a dual tin-lithium exchange (Strü ben et al., 2014(Strü ben et al., , 2015Hoffmann et al., 2019). In particular, we described the effect on the diortho-substitution on azobenzenes with trimethyl-tetrels and the resulting effects on the switching properties (Hoffmann et al., 2019). In this context, we present here a novel diortho-substituted azobenzene, (C 7 H 7 NS) 2 , (I), bearing two methylsulfide groups.

Structural commentary
The asymmetric unit of the title compound consists of two half-molecules (Ia and Ib), the other halves being completed by application of inversion symmetry. The midpoints of the N N bonds are located on inversion centres, resulting in a trans-configuration for the central N N bonds (Fig. 1). As indicated by the C6A-C1A-N1A-N1A i and C6B-C1B-N1B-N1B ii [symmetry codes: (i) Àx, 1 À y, Àz; (ii) 1 À x, 1 À y, 1 À z] torsion angles of 13.2 (2) and À5.3 (2) , respectively, in both molecules the phenyl rings are twisted slightly with respect to the azo unit. A weak distortion is also found for the N1-C1-C2-S1 torsion angles of À3.06 (16) for Ia and À2.06 (15) for Ib. The N N bond lengths differ marginally [1.255 (2) Å for Ia, 1.264 (2) Å for Ib], as do comparable C-C bonds. For example, the C1-C2 bond in Ia is at 1.408 (2) Å slightly shorter than Ib [1.415 (2) Å ]. In comparison, this bond is longer than all other C-C distances in the ring because of repulsion of the nitrogen and the sulfur atoms attached to C1 and C2, respectively. In both molecules, the SÁ Á ÁN distances [2.8625 (13) Å for Ia, 2.8761 (11) Å for Ib] are too long to be considered as attractive interactions. Fig. 2 represents an overlay plot of the two molecules, showing there are only slight conformational differences.

Supramolecular features and Hirshfeld surface analysis
The packing of Ia and Ib in the crystal is shown in Fig. 3. Despite the presence of phenyl rings and a parallel arrange-ment of the molecules, only weak offsetinteractions are observed; the shortest centroid-to-centroid distance is Cg2Á Á ÁCg2(1 À x, 1 À y, Àz) = 3.7525 (8) Å with a slippage of 1.422 Å . To further investigate the intermolecular interactions, Hirshfeld surfaces (Hirshfeld, 1977) and fingerprint plots were generated for both molecules using     Molecular structures (Ia left, Ib right) of the title compound with labelling and displacement ellipsoids drawn at the 50% probability level.

Figure 4
Hirshfeld surface of Ia mapped with d norm (top) and shape index (bottom), displaying no significant intermolecular interactions. (McKinnon et al., 2004). Hirshfeld surface analysis depicts intermolecular interactions by different colors, representing short or long contacts and further the relative strength of the interaction. The generated Hirshfeld surfaces mapped over d norm and the shape index are shown in Fig. 4 for Ia and in Fig. 5 for Ib. Whereas in Ia a significant intermolecular interaction is not apparent, characteristic red spots near S1B and H5B indicate weak SÁ Á ÁH interactions in Ib. The respective supramolecular arrangement is shown in Fig. 6. The sulfur atom S1B interacts with a phenyl proton (H4B) of another molecule of Ib (SÁ Á ÁH distance = 2.811 Å ). The two-dimensional fingerprint plots for molecule Ib for quantification of the contributions of each type of non-covalent interaction to the Hirshfeld surface (McKinnon et al., 2007) are given in Fig. 7. The packing is dominated by HÁ Á ÁH contacts, representing van der Waals interactions (44.5% contribution to the surface), followed by CÁ Á ÁH and SÁ Á ÁH interactions, which contribute with 24.0% and 18.1%, respectively. The contri-butions of the NÁ Á ÁH (8.6%) and CÁ Á ÁC (4.8%) interactions are less significant.

Synthesis and crystallization
The synthesis of 2,2 0 -bis(trimethylstannyl)azobenzene was recently described (Hoffmann et al., 2019). For further details of a similar transmetallation of a stannylated azobenzene, see: Strü ben et al. (2015). Dimethyl disulfide (99%) was purchased from Acros Organics and was used without further purification. Methyl lithium (1.88 M in diethyl ether, titrated against 2,2 0 -bipyridine) was purchased from Acros Organics. THF was purchased from VWR and was dried and degassed with a solvent purification system by Inert Technology.
2,2 0 0 0 -bis(Methylthio)azobenzene In an inert reaction tube, 2,2-bis(trimethylstannyl)azostannyl)azobenzene (200 mg, 0.39 mmol) was dissolved under Schlenk conditions in THF (12.5 ml) and cooled to 195 K. Then MeLi (1.88 M in diethyl ether, 0.63 ml, 1.18 mmol) was added within 5 min and after 1.5 h at this temperature, dimethyl disulfide (0.35 ml, 3.94 mmol) was added in one ration. The reaction mixture was warmed to 298 K over 14 h and the solvent was removed under reduced pressure. The obtained orange solid was purified in a silica column (Merck, 0.015-0.40 mm) with a gradient of eluents from n-pentane to Hirshfeld surface of Ib mapped with d norm (left) and shape index (right), together with the interaction of a neighbouring molecule.    dichloromethane giving dark-orange crystals (31 mg, 0.11 mmol; yield 29%). Single crystals suitable for X-ray analysis were obtained by slow evaporation from a saturated n-heptane solution.

(E)-1,2-Bis[2-(methylsulfanyl)phenyl]diazene
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.