Polymorphism of bis(1,3-benzothiazol-2-yl) trithiocarbonate

Bis(benzothiazol-2-yl)trithiocarbonate, C15H8N2S5, crystallizes in two visually distinguishable polymorphs, each containing a different conformer of the title compound and featuring different kinds of intra- and intermolecular S⋯S, S⋯N and π–π interactions.


Chemical context
Acyclic trithiocarbonates are important functional groups in several areas ranging from materials science and synthetic chemistry to pharmaceutics (Kazemi et al., 2018). Notably, their use as reagents in reversible addition-fragmentation chain-transfer (RAFT) free radical polymerization appears relevant since the relative stability of the conformers might have an influence on the stereochemistry of the obtained polymer (Huang et al., 2018). Earlier studies on the conformational properties of perfluorodimethyl trithiocarbonate based on gas electron diffraction and Raman spectroscopy (Hermann et al., 2000) show clear dependency of the solvent and aggregate state: The (syn,syn) conformer is predominant (84%) in the gas phase, as a liquid the distribution is almost equal [60% (syn,syn)], while in solution and with increasing polarity of the solvent, the ratio of the (syn,anti) conformer increases. The herein reported conformational polymorphism allows further structural comparison between trithiocarbonate conformers by X-ray diffraction analysis. ISSN 2056-9890

Structural commentary
The (syn,syn) conformer crystallizes from chloroform solution in space group Pbcn. The asymmetric unit contains half of the molecule, with a crystallographic twofold axis passing through S3-C8 generating the complete molecule. The molecule is slightly twisted in a propeller-like shape, the twist introduced by the C7-S2-C8-S3 torsion angle of 24.46 (12) , thus deviating from the idealized syn geometry of 0 (Fig. 1).
The bond lengths in both conformers show no significant differences that would correspond to the changed bond angles in respect of hyperconjugative effects.

Supramolecular features
To investigate the supramolecular features, the Hirshfeld surface (Spackman & Jayatilaka, 2009) was calculated for both conformers using CrystalExplorer17 (Turner et al., 2017). The resulting Hirshfeld surfaces mapped over d norm heatmaps for the (syn,syn) and (syn,anti) conformers are depicted in Fig. 2 and the corresponding fingerprint plots are shown in Fig. 3. Observation of the heatmap and the features of the fingerprint plots yields one apparent conclusion: the (syn,syn) conformer has no distinctive contacts while the surface for the (syn,anti) Table 1 Hydrogen-bond geometry (Å , ) for the (syn,anti) conformer. Symmetry codes: (i) x À 1; y; z; (ii) x þ 1; y; z.
conformer features in total five hot spots, which reappear as sharp features in the fingerprint plot (Fig. 3). Those contacts are identified as two C-HÁ Á ÁN hydrogen bonds (Table 1) between N1 and C6 and N2 and C15 with lengths of 3.467 (2) and 3.552 (2) Å , respectively. This is in the range of other C-HÁ Á ÁN hydrogen bonds reported previously (Mambanda et al. 2007;Pingali et al., 2014). The fifth contact is a symmetric SÁ Á ÁS interaction between S3 and its adjacent symmetry-equivalent clone, with a distance of 3.509 (1) Å . The relative contributions of various contacts to the Hirshfeld surface are given in Table 2. The Hirshfeld surface mapped over curvedness (Fig. 4) indicatesinteractions by wide flat areas on one side of each benzothiazol unit. Packing diagrams of the (syn,syn) ( Fig. 5) and (syn,anti) ( Fig. 6) conformers show the parallel arrangement of adjacent benzothiazol groups, which come in pairs (syn,anti) or in a continuous herringbone motif (syn,syn). The separation between the benzothiazol planes (defined by C1-C7/N1/S1 or C9-C15/N2/S5) are similar with distances of 3.54 Å in the (syn,syn) conformer and 3.43 and 3.58 Å in the (syn,anti) conformer.  Table 2 Relative element-element contributions to the Hirshfeld surface (in %). Asymmetric contacts include reciprocal contributions.

Figure 3
Full fingerprint plot (top) and decomposed plots (bottom) showing exclusive element element contacts.

Figure 5
Packing diagram for (syn,syn) displaying the herringbone motif.

Database survey
A search in the CSD (version 5.41, update of November 2019; Groom et al., 2016) for non-cyclic and non-oxidized trithiocarbonates produced 20 results, of which only one (refcode XUBNAJ; Sotofte & Senning, 2001) adopts a (syn,anti) conformation. There is one outlier neither close to a (syn,syn) nor a (syn,anti) conformation, which is chemically a thioanhydride (XISSAU; Weber et al., 2008). All other results are trithiocarbonates with a (syn,syn) conformation, which appears to be the predominant form. A substructure search for benzothiazoles and thiazoles yielded large numbers of hits (1500 and 2200, respectively). A comparable polymorphism with thiazolothiazols (Schneider et al., 2015) was reported with interplane separations around 3.45 Å , as well as arrangements in pairs ofinteractions for one polymorph and a herringbone motif in the other, closely matching our observations.

Synthesis and crystallization
The title compound was initially isolated in small amounts as a side product and crystallized from chloroform solution in an NMR tube, where two crystalline species could be identified visually: (syn,syn) in the form of orange needles and (syn,anti) as orange blocks. The synthesis of the title compound is based on a literature procedure (Runge et al., 1962). Benzothiazole-2-thiol (500 mg, 2.99 mmol, 1.0 eq.) and sodium hydroxide (179 mg, 4.48 mmol, 1.5 eq.) were dissolved in water (30 ml). Thiophosgene (165 mg, 1.44 mmol, 0.48 eq.) was added dropwise at room temperature. After complete addition, the solution was stirred for 15 minutes. Brine solution was added, the reaction mixture extracted with ethyl acetate and the combined organic phases dried over sodium sulfate. The solvent was removed in vacuo to yield the crude product as an orange solid (518 mg). After recrystallization from boiling benzene solution the pure product was obtained as orange crystals (432 mg, 1.15 mmol, 77%).
The melting range is 420-423 K, as measured on a Bü chi M-560.
NMR spectra recorded on a Bruker Avance III HD 300 and chemical shifts are given in parts per million. 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All aromatic hydrogen atoms were placed geometrically (C-H = 0.93 Å ) and refined using a riding model with U iso (H) = 1.2U iso (C).

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.

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. Refinement. Refined as a 2-component twin.