2-[(E)-2-(1H-Indol-3-yl)ethenyl]-1-methylpyridinium 4-bromobenzenesulfonate1

In the title compound, C16H15N2 +·C6H4BrO3S−, the cation exists in the E configuration and is essentially planar with a dihedral angle of 3.10 (5)° between the pyridinium ring and the indole ring system. The π-conjugated planes of the cation and the anion are inclined to each other at a dihedral angle of 64.32 (4)°. In the crystal structure, the cations are stacked in an antiparallel manner along the a axis. The anions are linked into a chain along the a axis. The cations and the anions are linked into a three-dimensional network by N—H⋯O and weak C—H⋯O hydrogen bonds. The crystal structure is further stabilized by C—H⋯π interactions. A π–π interaction between the five-membered heterocyclic ring of the indole system and the pyridinium ring is also observed with a centroid–centroid distance of 3.5855 (7) Å.


Comment
Organic crystals with extensive conjugated π systems are attractive candidates for nonlinear optic (NLO) studies because of their large hyperpolariability (β) and ease of preparation (Coe et al., 2003;Dittrich et al., 2003;Ogawa et al., 2008;Otero et al., 2002;Weir et al., 2003;Yang et al., 2007). One strategy to enhance the hyperpolariability of the cations is by elongation of its π-conjugation system. Based on these studies, we have previously synthesized and reported the crystal structure of the pyridinium salts (Chanawanno et al., 2008;Chantrapromma et al., 2006Chantrapromma et al., , 2007Chantrapromma et al., , 2008Chantrapromma et al., , 2009Jindawong et al., 2005) in order to study for their NLO properties. We herein report the crystal structure of the title compound, (I), which is another pyridinium salt. Figure 1 shows the asymmetric unit of (I), which consists of a C 16 H 15 N 2 + cation and a C 6 H 4 BrO 3 Sanion. The cation exists in the E configuration with respect to the C6═C7 double bond [1.3568 (16) Å] and is essentially planar with a dihedral angle between the pyridinium and indole rings being 3.10 (5)°, the torsion angles C4-C5-C6-C7 = -2.13 (19)° and C6-C7-C8-C15 = 3.9 (2)°. The indole ring system is planar with the most deviation of -0.0137 (12) Å for atom C8. The π-conjugated planes of the cation and the anion are inclined to each other with the interplanar angle between them being 64.32 (4)°. The methyl group is co-planar with the attached N1/C1-C5 ring. The bond lengths in (I) are in normal ranges (Allen et al., 1987) and comparable with those in related structures (Chanawanno et al., 2008;Chantrapromma et al., 2006Chantrapromma et al., , 2007Chantrapromma et al., , 2008Chantrapromma et al., , 2009Jindawong et al., 2005).
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 and the anions are linked together into chains 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 ( Table 1). The crystal structure is further stabilized by C-H···π interactions (Table 1). A π-π interaction with a distance Cg1···Cg2 = 3.5855 (7) Å (symmetry code: 2-x, 2-y, 1-z) is observed; Cg1 and Cg2 are the centroids of the N2/C8-C10/C15 and N1/C1-C5 rings, respectively.

Experimental
A solution of indole-3-carboxaldehyde (2.47 g, 17.02 mmol) in methanol (25 ml) was added dropwise to a stirred solution of 1,2-dimethylpyridinium iodide (4.00 g, 17.02 mmol) in methanol (15 ml) in the presence of piperidine (1.68 ml, 17.02 mmol) over a period of 15 mins at room temperature. The mixture was then refluxed for 1 hr in the nitrogen atmosphere.
Silver(I) p-bromobenzenesulfonate (compound B) was synthesized according to our previously reported procedure (Chantrapromma et al., 2006). The title compound was synthesized by disolving compound B (0.20 g, 0.58 mmol) in 20 ml methanol which upon heating was added a solution of compound A (0.21 g, 0.58 mmol) in hot methanol (30 ml). The mixture turned yellow and cloudy immediately. After stirring for 0.5 hr, the precipitate of silver iodide was filtered and the supplementary materials sup-2 filtrate was evaporated to give an orange gum. Orange block-shaped single crystals of the title compound suitable for x-ray structure determination were recrystalized from methanol by slow evaporation of the solvent at room temperature after a few weeks (m.p. 508-510 K).

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 C-H = 0.93 Å and U iso (H) = 1.2U eq (C) for aromatic and CH, and with C-H = 0.96 Å and 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 0.36 Å from Br1.
Figures Fig. 1. The molecular structure of the title compound, with 50% probability displacement ellipsoids and the atom-numbering scheme.  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.