5-Methyl-1,3-phenylene bis[5-(dimethylamino)naphthalene-1-sulfonate]: crystal structure and DFT calculations

The title compound possesses crystallographically imposed twofold symmetry with the two C atoms of the central benzene ring and the C atom of its methyl substituent lying on the twofold rotation axis. The two dansyl groups are twisted away from the plane of methylphenyl bridging unit in opposite directions. The crystal packing features weak C—H⋯O hydrogen bonds.


Chemical context
Dansyl probes play important roles in many fields, including their use as industrial tracers and labelled biological tags (Tondi et al., 2005;Li et al., 2006;Liu et al., 2016). Dansyl derivatives have been employed to identify some diseases within cells and to detect DNA-duplex sequences. For example, modified oligonucleotides that contain a dansyl fluorophore and (S)-2, 3-dihydroxy propyl carbamates linked to guanine residues result in an enhancement of the fluorescence. Such modified oligonucleotides can be used to prepare and detect the sequence of fluorogenic probes in DNA (Suzuki et al., 2013). Cu-labelled dansyl molecules have been designed and synthesized as fluorescence probes for membrane tags on apoptosis cells. These compounds can also be used for PET imaging of the apoptosis in vivo (Han et al., 2016). Furthermore, the development of dansyl fluorogenic receptors for cations, anions and neutral molecules has attracted much attention because of their ability to turn fluorescence 'on' or 'off' through a number of mechanisms including ICT, PET and ET processes (Chen & Chen, 2005;Praveen et al., 2010;Dinake et al. 2012;Jeong et al. 2016). In this paper, we report the synthesis, molecular structure and crystal packing of 5-methyl-1,3-phenylene bis[5-(dimethylamino)naphthalene-1-sulfonate]. The results of DFT calculations on the molecule are also reported. ISSN 2056-9890

Structural commentary
The title compound crystallizes in the space group C2/c. The molecule lies on a crystallographic twofold axis running through atoms C1, C2 and C5 of the methylphenyl unit so that the asymmetric unit comprises one half-molecule (Fig. 1). The hydrogen atoms of the C1 methyl group are therefore disordered over two equivalent positions. Intramolecular C14-H14-O3 hydrogen bonds enclose S(6) rings, Fig. 1. The molecular structure comprises two O-dansyl groups on either side of a bridging methylphenyl ring that is essentially planar. The S1-O1-C4-C3 torsion angle is 72.98 (16) with the methylphenyl ring plane. The S1 sulfur atoms have distorted tetrahedral geometries, with an O2-S1-C6 bond angle of 109.18 (8) . The two naphthalene units in each dansyl group are inclined to one another at an angle of 52.29 (6) ; however, no stacking of the naphthalene units is observed.

Supramolecular features
In the crystal structure, the supramolecular packing is dominated by weak C-HÁ Á ÁO hydrogen bonds, Table 1. C9-H9-O1 contacts form dimers enclosing R 2 2 (22) rings and generate chains of molecules along the c-axis direction, Fig. 2. C1-H1B-O3 and C16-H16C-O2 contacts further link the molecules into sheets in the ab plane, Fig. 3. These contacts combine to stack rows of molecules arranged in an obverse fashion along the a-axis direction, Fig. 4.

Figure 2
Chains of dimers of the title compound along the c axis. Dashed lines represent the C-HÁ Á ÁO hydrogen bonds.

Figure 3
Sheets of molecules of the title compound formed in the ab plane.
performed starting from the X-ray geometry. The experimental values of the bond lengths and bond angles match reasonably well with the theoretical values in most cases. However, the lengths of bonds to O atoms involved in hydrogen bonding fit less well, Table 2. The important features such as conjugation and aromaticity are well illustrated by frontier molecular orbitals. The ionization potential of the molecule is determined from the energy of the highest occupied molecular orbital (HOMO) and the electron affinity is calculated from the energy of the lowest unoccupied molecular orbital (LUMO). The frontier molecular orbital ener-gies, E HOMO and E LUMO are À8.24 and À5.25 eV, respectively. Insights into the kinetic stability and chemical reactivity of a molecule can be determined from the energy difference between the HOMO and LUMO orbitals, the so-called HOMO-LUMO energy gap. This gap was found here to be 2.99 eV. The HOMO-LUMO energy levels indicate n!* and !* transitions and are shown in Fig. 5. The HOMO is mainly localized on the nitrogen atom of dimethylamine group as well as on the C C segments of the naphthalene ring systems while the LUMO is located again on the dimethylamine substituent and also on the aromatic rings of the naphthalene systems. In Fig. 5, the negative and positive phases are represented by green and red colours, respectively.

Database survey
There are many crystal structures of dansyl derivatives that are similar to the title compound. Two categories of crystal structures of dansyl derivatives are found. The first types are simple organic molecules that pack in the solid state through the many types of intermolecular interactions. For example, (Groom et al., 2016) refcode NUQDOU; Chainok et al., 2015), there are two dansyl units connecting to the amine and hydroxyl groups of a 2-aminophenol, while weak C-HÁ Á ÁO hydrogen bonds stabilize the crystal structure. In N-cyclododecyl-5-(dimethylamino)naphthalene-1-sulfonamide (HODDOU; Fischer et al., 2008) a cyclododecylamine linked to the dansyl substituent adopts a U-shaped conformation, and the crystal packing is stabilized by N-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions between neighbouring molecules. In 8-quinolyl  Table 2 Comparison of selected experimental (XRD) bond lengths and angles (Å , ) with those from DFT calculations.

Figure 5
Frontier molecular orbitals of the title compound.

Figure 4
The overall packing of the title compound viewed along the a-axis direction.  Buie et al., 2008), where the calix[4]arene bears two dansylcarboxamide groups, was found to be highly selective and sensitive for the recognition of and coordination to the Pb 2+ ion.

Synthesis and crystallization
The title compound was synthesized by mixing 3,5-dihydroxytoluene (1.05 g, 8.46 mmol) and dansyl chloride (4.55g, 17 mmol) using potassium carbonate(2.34g, 17 mmol) as a base in acetonitrile solvent (40 ml). The reaction mixture was heated at 363 K and stirred under an N 2 atmosphere for 24 h. The solvent was removed with a rotary evaporator. The residue was added to water (15 ml) and extracted with dichloromethane (3 Â 25ml). The organic layer was dried with anhydrous Na 2 SO 4 and the product was purified by column chromatography using CH 2 Cl 2 as the eluent. The dichloromethane was slowly evaporated to afford a green solid in 65% yield. Light-green block-like crystals were grown in chloroform at room temperature.

5-Methyl-1,3-phenylene bis[5-(dimethylamino)naphthalene-1-sulfonate]
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.