1-(2,3-Dihydroxypropyl)-4-{2-[4-(dimethylamino)phenyl]vinyl}pyridinium chloride

The title compound, C18H23N2O2 +·Cl−, crystallizes with two independent cations and anions per cell. Each cation has twofold rotational disorder about the linking vinyl groups but with unequal occupancies [0.963 (5):0.037 (5) and 0.860 (8):0.140 (8)]. The two independent cations are close to being related by an inversion centre but the data does not support the expected centrosymmetric space-group assignment. The conclusion is that the differing rotational disorder has lead to an overall non-centrosymmetric lattice. In the crystal, the molecules pack in layers parallel to (133) and (-13-3), chain-linked with motif C 1 2(7) by the dihydroxypropyl O–H⋯Cl⋯H–O hydrogen bonds. Other lattice binding is provided by O—H⋯Cl, C—H⋯Cl and C—H⋯N interactions.


Comment
Organic push-pull chromophores with large second-order nonlinear optical (NLO) properties are in demand due to their potential applications in photonic devices and optical information processing (Kay et al., 2004;Bass et al., 2001;Prasad et al., 1988). A significant number of organic compounds available in the literature with large molecular NLO responses contain N,N-disubstituted anilines as this nucleus is an excellent electron donor. Due to the lack of tethering functionality of these chromophores, the possibility of synthesizing polymer containing chromophores is restricted. In this work, we have synthesized a new NLO chromophore containing an N,N-dimethyl aniline donor and an acceptor based on the dihydroxypropyl pyridinium chloride. The dihydroxypropyl substituent on the acceptor pyridinium nucleus will allow for covalent attachment of the molecule to a polymer backbone.
The asymmetric unit of the title compound (I) contains two independent 1-(2,3-dihydroxy-propyl)-4-[2-(4-dimethylamino-phenyl)-vinyl]-pyridinium cations (with primed and unprimed labels) and chloride anions almost related by an inversion centre ( Fig. 1) in space group P2 1 . The screw axis and the c glide defining data number, average intensities and ratio of intensity/standard deviations were 61, 1/5, 0.4 and 906, 1.9, 3.1 respectively. The corresponding centosymmetric, and more usual, space group found for these compounds, P2 1 /c was rejected through the small but significant presence of the required glide plane absences; this was confirmed in attempted least squares refinements. At the conclusion of both space group refinements, the difference Fourier maps show the presence of two partially occupied rotational conformers about the alkene atoms (C9═C10, C9′═ C10′) apparently confined to the vinyl linking atoms, also related by the same inversion centre (Fig. 2). Inclusion of the carbon atoms located on the Fourier difference map (see experimental) The molecules pack in layers parallel to (133) and (133 planes), chain-linked by dihydroxypropyl O-H···Cl···H-O hydrogen bonds with motif C 1 2 (7) (Bernstein et al., 1995). Other lattice binding is provided by O-H···Cl and C-H···Cl, C-H···N interactions (Table 1).

Experimental
We synthesized the title compound by following the procedure in Kay et al. (2001). Single crystals were grown by slow ethyl acetate diffusion into a methanol solution of the compound.

Refinement
Indications from the final solution were that the structure could be refined in the centrosymmetric space group P21/c (viz. 95% in agreeement according to a PLATON analysis (Spek, 2009), with each molecule except atoms O1 & O1′ related by inversion symmetry). The dataset does not support this with 984 glide plane systematic absences found to be weakly but significantly present out of the total set of 38305. Our experience with these planar NLO molecules is that they frequently form crystals with centrosymmetrically related molecules. In addition at the R1 value of 0.049, residual peaks in the same plane as the target molecules formed recognizable partial occupancy two fold rotational cation atoms, about the C9═C10 & C9′═C10′ linkages (Figure 2).
The defined atoms were paired with the corresponding two major conformer cation atoms (a & b labels) and corresponding H atoms added in calculated positions where observed on the difference maps. H atoms on the located phenyl pyridinium C atoms were not resolved and so were fixed in calculated positions. All non-hydrogen atoms in the partially occupied minor rotamers were given a single group isotropic thermal parameter, which refined to 0.005 (3) A 2 . It was possible to model a chemically reasonable model using the SHELXL SAME controlling parameters (but only) with the pyridinium and phenyl rings treated independently & occupancies fixed. We elected to remain with the linked group occupancy refinement model presented here. Using  Six reflections with F o <<Fc at low angle were omitted on the basis of background scatter and 2 were OMITted as outliers with Δ|(F o 2 -F c 2 )|/σ(F o 2 ) > 4.5. A l l carbon-bound H atoms were constrained to their expected geometries [C-H 0.95,0.98, 0.99 Å] and refined with U iso 1.2 times the U eq of their parent atom except for the minor rotamer(b) H atoms with U iso =1.5U eq of their parent atom. All other non-hydrogen atoms were refined with anisotropic thermal parameters.

Figure 2
The total contents of the asymmetric unit, as for