Bis{4-[(E)-2-(1H-indol-3-yl)ethenyl]-1-methylpyridinium} 4-chlorobenzenesulfonate nitrate

In the title mixed salt, 2C16H15N2 +·C6H4ClO3S−·NO3 −, one of the cations shows whole molecule disorder over two sets of sites in a 0.711 (7):0.289 (7) ratio. The 4-chorobenzenesulfonate anion is also disordered over two orientations in a 0.503 (6):0.497 (6) ratio. The cations are close to planar, the dihedral angles between the pyridinium and indole rings being 1.48 (3)° in the ordered cation, and 5.62 (3) and 2.45 (3)°, respectively, for the major and minor components of the disordered cation. In the crystal, the cations are stacked in an antiparallel manner approximately along the a-axis direction and linked with the anions via N—H⋯O hydrogen bonds and C—H⋯O interactions, generating a three-dimensional network. Weak C—H⋯π and π–π interactions [with centroid–centroid distances of 3.561 (2)–3.969 (7) Å] are also observed.


Comment
Organic molecules that exhibit second-order NLO properties usually consist of a framework with delocalized π system, end-capped with either a donor or acceptor substituent or both. Several pyridinium derivatives have been reported to exhibit second-order NLO properties such as single crystals of 1-methyl-4-(2-(4-(dimethylamino)phenyl)ethynyl)pyridinium p-toluenesulfonate (DAST) and its analogues (Dittrich et al., 2003;Sato et al., 1999). Based on the knowledge that the organic dipolar compounds with extended π systems having terminal donor and acceptor groups are likely to exhibit large hyperpolarizability (β) (Raimundo et al., 2002), we have synthesized several quinolinium derivatives which exhibit NLO properties (Ruanwas et al., 2010). In a similar manner, the title compound (I) was designed and synthesized in order to study for its NLO property. Unfortunately (I) crystallizes in a centrosymmetric P-1 space group which precluded the second-order NLO properties. Herein the crystal structure of (I) is reported.
In the crystal structure of (I), the asymmetric unit consists of two C 16 H 15 N 2 + cations, C 6 H 4 ClO 3 Sand NO 3anions ( Fig.   1). One cation [C23-C38/N3/N4] exhibits whole molecule disorder over two sets of sites with a refined site-occupancy ratio of 0.711 (7):0.289 (7). The molecule is disordered in such a way that the ethynyl unit in the major and minor (A) components are related by a 180° rotation. The two cations exist in the E conformation with respect to the ethenyl unit and the torsion angle C11-C12-C13-C14 = -178.8 (3)° for the non-disordered cation, and C27-C28-C29-C30 = 176.8 (6)° and -179.8 (15)° for the major and minor (A) components for the disordered cation. The cations are close to planar with the dihedral angles between the pyridinium and the indole rings being 1.28 (3)° for the non-disordered cation, and 5.62 (3) and 2.45 (3)° for the major and minor components respectively for the disordered cation. The 4-chlorobenzenesulfonate anion also shows whole molecule disorder ( Fig. 1) with a 0.503 (6):0.497 (6) site occupancy ratio.

Refinement
All H atoms were positioned geometrically and allowed to ride on their parent atoms, with N-H = 0.78, 0.81 and 0.86 Å, CH and C aryl -H = 0.93 Å and C methyl -H = 0.96 Å. The U iso values were constrained to be 1.5U eq of the carrier atom for methyl H atoms and 1.2U eq for the remaining H atoms. A rotating group model was used for the methyl groups. One cation is whole molecule disordered over two sites with refined site occupancies ratio 0.711 (7):0.289 (7), whereas the 4chlorobenzenesulfonate anion is disordered over two sites with refined site occupancies ratio 0.503 (6):0.497 (6).
Similarity and simulation restraints were applied. The displacement ellipsoids of each of the two pairs of atoms i.e. "CL1 C6" and "N3 C38" were restrained to be almost equal.

Figure 1
The structure of (I) showing 30% probability displacement ellipsoids. Open bonds show the minor disorder components.  The crystal packing (involving only the major components of the disordered ions) viewed along the c axis. Hydrogen bonds are drawn as dashed lines.

Bis{4-[(E)-2-(1H-indol-3-yl)ethenyl]-1-methylpyridinium} 4-chlorobenzenesulfonate nitrate
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 > σ(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.