2-[(E)-2-(4-Ethoxyphenyl)ethenyl]-1-methylpyridinium 4-bromobenzenesulfonate monohydrate

In the title compound, C16H18NO+·C6H4BrO3S−·H2O, the cation exists in an E configuration with respect to the ethenyl bond and is slightly twisted with a dihedral angle of 8.5 (2)° between pyridinium and benzene rings. In the crystal, the cations are arranged in layers parallel to (100), with π–π interactions between pyridinium and benzene rings [centroid–centroid distances = 3.651 (3) and 3.613 (3) Å]. The anions and water molecules are located between the cationic layers. The ions and water molecules are linked into a three-dimensional framework by O—H⋯O and C—H⋯O hydrogen bonds.


Comment
Ionic organic crystals are of special interest due to their high second order optical nonlinearities (Coe et al., 2002). The orientation of ionic chromophores can be arranged simply by changing the counter-ions (Pan et al., 1996). During the course of our NLO materials research, we have previously synthesized and reported crystal structures of related pyridinium salts containing the 2-[(E)-2-(4-ethoxyphenyl)ethenyl]-1-methylpyridinium cationic part Laksana et al., 2008). The title compound was synthesized by retaining the same cationic part but changing the anion counter part to 4-bromobenzenesulfonate in order to investigate the influence of the counter-ions on the NLO properties. However, it was found that the title compound crystallized in a centrosymmetric space group P2 1 /c and hence no second-order nonlinear optical properties are observed.
In the title compound ( Fig. 1), the cation exists in an E configuration with respect to the ethenyl bond [C5-C6-C7-C8 = -179.9 (5)°]. The cation is slightly twisted with a dihedral angle between the pyridinium and benzene rings of 8.5 (2)°. The pyridinium and benzene rings of the cation form dihedral angles of 79.2 (2) and 71.0 (2)°, respectively, with the benzene ring of the anion. Bond distances in both cation and anion have normal values (Allen et al., 1987) and are comparable to those observed in related structures Chantrapromma et al., 2009;Laksana et al., 2008).
In the crystal, the cations are stacked along the b axis and are arranged in layers parallel to the (100) with π-π interactions involving pyridinium (centroid Cg1) and benzene (centroid Cg2) rings [Cg1···Cg1 ii = 3.651 (3) Å and Cg1···Cg2 iii = 3.613 (3) Å; symmetry codes as in Table 1]. The anions and water molecules are located between the cationic layers. The cations are linked with the water molecules and anions by C-H···O weak interactions (Table 1), whereas the anions are linked with water molecules by O-H···O hydrogen bonds (Table 1). These interactions connect the ionic units and water molecules into a three-dimensional network (Fig. 2).
Experimental 2-[(E)-2-(4-Ethoxyphenyl)ethenyl]-1-methylpyridinium iodide (0.21 g, 0.58 mmol) which was prepared according to the previous method (Laksana et al., 2008) was mixed with silver 4-bromobenzenesulfonate (Chantrapromma et al., 2006) (0.20 g, 0.58 mmol) in methanol (100 ml) and stirred for 0.5 h. The precipitate of silver iodide which formed was filtered and the filtrate was evaporated to give the title compound as a yellow solid. Yellow block-shaped single crystals of the title compound suitable for X-ray structure determination were recrystallized from methanol by slow evaporation at room temperature over a few weeks (m.p. 463-465 K).

Refinement
H atoms were positioned geometrically and allowed to ride on their parent atoms, with O-H = 0.85 Å and C-H = 0.93-0.97 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 supplementary materials sup-2 atoms. A rotating group model was used for the methyl groups. The highest residual electron density peak is located at 0.81 Å from Br1 and the deepest hole is located at 1.90 Å from Br1. Fig. 1. The asymmetric unit 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Br1 0.61259 (