7-Methyl-5-[(4-methylbenzene)sulfonyl]-2H,5H-[1,3]dioxolo[4,5-f]indole: crystal structure and Hirshfeld analysis

The molecule in the title compound has the shape of the letter L. In the crystal, weak 4-tolyl-C—H⋯π(C6-ring of indole) and S—O⋯π(1,3-dioxole) contacts link the molecules into a supramolecular layer in the ab plane.

In the title indole derivative, C 17 H 15 NO 4 S, the fused dioxolo-indole system is essentially planar [r.m.s. deviation of the 12 fitted atoms = 0.0249 Å ] and is effectively perpendicular to the appended 4-tolyl ring, forming a dihedral angle of 89.95 (6) . Overall, the molecule has the shape of the letter L. In the crystal, supramolecular layers in the ab plane are formed via weak 4-tolyl-C-HÁ Á Á(C 6ring of indole) and S-OÁ Á Á(1,3-dioxole) contacts. The aforementioned interactions along with interatomic HÁ Á ÁH and HÁ Á ÁO contacts are all shown to make significant contributions to the calculated Hirshfeld surfaces.

Chemical context
Nitrogen-based heterocycles comprise a class of compounds with significant biological importance that are crucial in organic synthesis (Trofimov et al., 2004). In particular, indole and oxindole derivatives continue to receive significant attention in both contexts as these residues are found in both natural products as well as in synthetic drugs (Dalpozzo, 2015). Not surprisingly, considerable effort is continually being made to develop new and efficient methods for their synthesis. Recently, the development of a useful method for the synthesis of indoles and oxindoles was described (da Silva et al., 2015). The protocol was based on a combination of tris(trimethylsilyl)silane, as the hydride source, and visible light to promote intramolecular reductive cyclization of suitable precursors. Among the compounds synthesized in this study was the title compound (I), which features an indole residue N-bound to a (4-methylbenzene)sulfonyl, i.e. tosyl, residue and fused to a 1,3-dioxole ring at the benzene ring. Herein, the crystal and molecular structures of (I) are described along with an analysis of the calculated Hirshfeld surfaces.

Structural commentary
The molecular structure of (I), Fig. 1, comprises two essentially planar residues, viz. 4-tolyl and the fused dioxolo-indole system, hinged at the SO 2 group. The r.m.s. deviation of the five non-hydrogen atoms comprising the 1,3-dioxole ring is 0.0158 Å with the maximum deviations above and below this plane being 0.022 (14) and 0.021 (14) Å for the C1 and O2 atoms, respectively. This planarity extends over the entire dioxolo-indole residue, which exhibits an r.m.s. deviation of 0.0249 Å for the 12 constituent atoms with maximum deviations of 0.058 (2) and 0.0284 (14) Å for the C1 and C8 atoms, respectively. The dihedral angle between the residues linked at the S atom is 89.95 (6) , i.e. indicating a perpendicular relationship consistent with the shape of the letter L. The CNO 2 atoms about the S atom define a tetrahedron with widest angle being subtended by the doubly bonded O3 and O4 atoms, i.e. O3-S-O4, is 120.32 (10) .

Supramolecular features
The molecular packing of (I) features a number of weak intermolecular contacts with the weaker ones discussed below in Analysis of the Hirshfeld surface (x4). Three specific points of contact between molecules are highlighted here, i.e. within the standard distance criteria in PLATON (Spek, 2009). These are: a 4-tolyl-C11-H11Á Á Á(C2-C4,C7-C9) contact and a pair of S-OÁ Á Á(1,3-dioxole) contacts, Table 1, implying the 1,3-dioxole ring serves as a bridge between two symmetry-related molecules. These interactions cooperate to form a supramolecular layer in the ab plane as shown in Fig. 2a. Layers stack along the c axis with no directional interactions between them, Fig. 2b.

Hirshfeld surface analysis
The Hirshfeld surfaces calculated for (I) were performed in accord with a recent report on a related organic molecule (Zukerman-Schpector et al., 2017) and provide an explanation of the influence of short interatomic contacts upon the molecular packing in the absence of conventional hydrogen bonding. The donor and acceptor of the relatively weak interatomic C-HÁ Á ÁO interaction, summarized in Table 2 The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 35% probability level.

Figure 2
Molecular packing in (I): (a) view of the supramolecular layer in the ab plane and (b) the unit-cell contents shown in projection down the a axis; one layer is highlighted in space-filling mode. The C-HÁ Á Á and S-OÁ Á Á contacts are shown as purple and orange dashed lines, respectively. Table 1 Hydrogen-bond geometry (Å , ).

Contact
Distance Symmetry operation Two views of the Hirshfeld surface mapped over d norm for (I) in the range À0.039 to +1.643 au.

Figure 4
Two views of the Hirshfeld surface mapped over the electrostatic potential for (I) in the range AE0.075 au.

Figure 5
Two views of the Hirshfeld surface about reference molecule of (I) mapped with the shape-index property highlighting (a) HÁ Á ÁH, OÁ Á ÁH/ HÁ Á ÁO and CÁ Á ÁH/HÁ Á ÁC contacts by sky-blue, red and yellow dashed lines, respectively, and (b) C-HÁ Á Á/Á Á ÁH-C contacts by red dashed, S-OÁ Á Á and its reciprocal, i.e. Á Á ÁO-S, contacts by black and white dashed lines, respectively. neated fingerprint plots, Fig. 6b-d. The intermolecular C-HÁ Á Á contact involving the tolyl-C11 atom and the fused (C2-C4,C7-C9) ring is viewed as the pair of characteristic wings in the fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC contact, Fig. 6d. The presence of a pair of intermolecular S-OÁ Á Á contacts in the crystal is also indicated by small but significant contributions from CÁ Á ÁO/OÁ Á ÁC and OÁ Á ÁO contacts to the Hirshfeld surface, Table 3. The contribution from CÁ Á ÁC and NÁ Á ÁH/HÁ Á ÁN contacts do not have a great influence on the molecular packing as their interatomic separations are greater than sum of their respective van der Waals radii.

Database survey
The N-bound tosyl and methyl group substitution pattern, flanking the central hydrogen atom, in the five-membered ring of the indole residue, as in (I), has one precedent in the literature, namely, a derivative with a benzoyloxy substituent in the indole-benzene ring, i.e. 5-benzyloxy-3-methyl-1-tosyl-1H-indole (Pozza Silveira et al., 2013). On the other hand, there are several more examples where a 1,3-dioxole ring has been fused to the indole-benzene ring. A closely related species to (I) has two such fused ring systems linked via a C( O)-C( O) bridge and with each nitrogen bound to a benzyl group, i.e. 1,2-bis[5-benzyl-5H-(1,3)dioxolo(4,5-f)indole-6-yl]ethane (Lindsay et al., 2007); the molecule has twofold symmetry. To a first approximation, the conformations of the ring systems in the cited literature structures matches that observed in (I).

Synthesis and crystallization
The compound was prepared and characterized as described in the literature (da Silva et al., 2015). Irregular, colourless, crystals of (I) for the X-ray study were obtained by slow evaporation from its ethanol solution.

7-Methyl-5-[(4-methylbenzene)sulfonyl]-2H,5H-[1,3]dioxolo[4,5-f]indole
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.