1-Methyl-4-[(1E,3E)-4-phenylbuta-1,3-dienyl]pyridinium iodide monohydrate

The asymmetric unit of the title compound, C16H16N+·I−·H2O, contains two 1-methyl-4-{[(1E,3E)-4-phenylbuta-1,3-dienyl]}pyridinium cations, two iodide ions and two solvent water molecules. The cation is twisted slightly, the dihedral angle between the pyridinium and the phenyl rings being 10.68 (18)° in one molecule and 18.9 (3)° in the other. The two water molecules are disordered over three positions with site-occupancy ratio of 0.9/0.7/0.4. In the crystal packing, the cations are arranged into ribbons along the b axis with the iodide ions and water molecules located between adjacent cations. The cations are linked to the iodide ions and water molecules by weak C—H⋯I and C—H⋯O interactions, respectively. These interactions together with O—H⋯I hydrogen bonds link the molecules into a two-dimensional network parallel to the bc plane. π⋯π interactions with a centroid–centroid distance of 3.669 (2) Å are also observed.


Comment
It was known that the non-linear optic (NLO) materials require molecular first hyperpolarizability (β) and at the molecular level, compounds likely to exhibit large β values must have polarizable electrons (i.e. π-electrons) spread over a large distance. Thus, organic dipolar compounds with extended π systems having terminal donor and acceptor groups are likely to exhibit large π values (Raimundo et al., 2002). We have previous reported the crystal structures of the NLO-active compounds (Chantrapromma et al., 2009a, b;Fun et al., 2009) in which the cations consist of an ethenyl bridge between two rings. The title compound was designed and synthesized by extending the π-conjugate systems of the cation with an expectation for better NLO properties. However, the title compound crystallizes in centrosymmetric C2/c space group and does not exhibit second-order nonlinear optical properties.  (Allen et al., 1987) and comparable to those in related structures (Chantrapromma et al., 2009a, b, Fun et al., 2009).
In the crystal packing ( Fig. 2), the cations are arranged into ribbons along the b axis with the iodide ions and water molecules located between adjacent cations. The cations are linked to the iodide ions and water molecules by C-H···I and C-H···O weak interactions (Table 1), respectively whereas water molecules form O-H···I hydrogen bonds (Table 1) with iodide ions. These interactions linked the molecules into two-dimensional networks parallel to the bc plane. π···π interactions involving pyridinium and phenyl rings was also observed with the distance of Cg 1 ···Cg 2 = 3.669 (2) Å (symmetry code: 3/2-x, 1/2-y, -z); Cg 1 and Cg 2 are the centroids of N1A/C1A-C5A and C10A-C15A rings, respectively.

Experimental
The title compound was prepared by mixing 1:1:1 molar ratio solutions of 1,4-dimethylpyridinium iodide (2 g, 8.5 mmol), cinnamaldehyde (1.1 g, 8.5 mmol) and piperidine (0.84 ml, 8.5 mmol) in methanol (40 ml). The resulting solution was refluxed for 3 h under a nitrogen atmosphere. The yellow solid which formed was filtered, washed with diethylether and recrystallized from methanol by slow evaporation at room temperature to yield the yellow block-shaped single crystals suitable for x-ray diffraction analysis over a few weeks (Mp. 496-498 K).

sup-2 Refinement
All H atoms were positioned geometrically and allowed to ride on their parent atoms, with d(O-H) = 0.71-0.92 Å, d(C-H) = 0.93 Å for aromatic and CH and 0.96 Å for CH 3 atoms. 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. The two water molecules are disordered over three sites with occupancies 0.931 (9), 0.695 (9) and 0.354 (9), respectively. In the final refinement, this ratio was fixed as 0.90 : 0.70 : 0.40. The highest residual electron density peak is located at 0.84 Å from I1B and the deepest hole is located at 0.83 Å from I1B. 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 ) supplementary materials sup-9