2-[(E)-2-(1H-Indol-3-yl)ethenyl]-1-methylpyridinium 4-chlorobenzenesulfonate1

In the title compound, C16H15N2 +·C6H4ClO3S−, the cation exists in an E configuration with respect to the central C=C bond and is approximately planar, with a dihedral angle of 2.95 (5)° between the pyridinium and indole rings. The mean plane of the π-conjugated system of the cation and the benzene ring of the anion are inclined to each other at a dihedral angle of 69.65 (4)°. In the crystal packing, the cations are stacked in an antiparallel manner along the a axis, resulting in a π–π interaction with a centroid–centroid distance of 3.5889 (7) Å. The anions are linked into a chain along the a axis by weak C—H⋯O interactions. The cations are linked with the anions into a three-dimensional network by N—H⋯O hydrogen bonds and weak C—H⋯O interactions. There are also short O⋯Cl [3.1272 (10) Å] and C⋯O [3.1432 (14)–3.3753 (14) Å] contacts. The crystal structure is further stabilized by C—H⋯π interactions.


Comment
Molecules with extensive conjugated π systems are attractive candidates for non-linear optical (NLO) studies (Ogawa et al., 2008;Weir et al., 2003;Yang et al., 2007). However a molecule with extensive conjugated π systems does not always exhibit second order NLO properties unless the alignment of these molecules is in a noncentrosymmetric space group in the crystal. In our NLO research we have solved a number of crystal structures of pyridinium salt derivatives (Chanawanno et al., 2008;Chantrapromma et al., 2006Chantrapromma et al., , 2007Chantrapromma et al., , 2008Chantrapromma et al., , 2009) which we attempt to examine in details of the relationship between their crystal packings and the NLO properties. We herein report the crystal structure of the title compound (I) which is iso-structure and iso-packing with 2-[(E)-2(1H-Indol-3-yl)ethenyl]-1-methylpyridinium 4-bromobenzenesulfonate (Chantrapromma et al., 2009).  (Allen et al., 1987) and are comparable with those in related structures (Chanawanno et al., 2008;Chantrapromma et al., 2006Chantrapromma et al., , 2007Chantrapromma et al., , 2008Chantrapromma et al., , 2009).
In the crystal packing ( Fig. 2), all O atoms of the sulfonate group are involved in weak C-H···O interactions ( Table   1). The arrangement of the cations and anions is interesting (Fig. 2). The cations are stacked in an antiparallel manner along the a axis resulting in a π-π interaction with the distance Cg 1 ···Cg 2 = 3.5889 (7) Å (symmetry code: -x, -y, -z). The anions are linked together into chains by weak C-H···O interactions along the same direction. The cations are linked to the anions into a three dimensional network by N-H···O hydrogen bonds and weak C-H···O interactions (  (14) Å] short contacts. The crystal structure is further stabilized by C-H···π interactions (Table 1); Cg 1 , Cg 2 and Cg 3 are the centroids of the N2/C8-C9/C10/C15, N1/C1-C5 and C10-C15 rings, respectively.

Refinement
H atom attached to N was located from the difference map and refined isotropically. The remaining H atoms were placed in calculated positions with d(C-H) = 0.93 Å, U iso (H) = 1.2U eq (C) for aromatic and CH and 0.96 Å, U iso (H) = 1.5U eq (C) for CH 3 atoms. A rotating group model was used for the methyl groups. The highest residual electron density peak is located at 0.59 Å from S1 and the deepest hole is located at 0.65 Å from S1. Fig. 1. The molecular structure of the title compound, with 50% probability displacement ellipsoids and the atom-numbering scheme.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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. 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 > 2sigma(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.