Crystal structure determination of two pyridine derivatives: 4-[(E)-2-(4-methoxyphenyl)ethenyl]-1-methylpyridin-1-ium hexafluoro-λ6-phosphane and 4-{(E)-2-[4-(dimethylamino)phenyl]ethenyl}-1-phenyl-1λ5-pyridin-1-ylium hexafluoro-λ6-phosphane

In both title pyridine derivatives, (I) and (II), the cation adopts an E configuration with respect to the C=C. In compound (I), the PF6 − anion is disordered with occupancy factors of 0.614 (7):0.386 (7). In both the compounds, the crystal packing is stabilized by C—H⋯F intermolecular interactions results into two-dimensional molecular sheets, which are formed by (14) ring motifs in compound (I), (40) ring motifs in compound (II). In addition to that, the crystal packing is further stabilized by P—F⋯π interactions in compound (I) and π–π in compound (II).

The title molecular salts, C 16 H 16 NO + ÁPF 6 À , (I), and C 21 H 21 N 2 + ÁPF 6 À , (II), are pyridine derivatives. In compound (I), the cation comprises a methyl N-substituted pyridine ring and a methoxy-substituted benzene ring connected by a C C double bond. The F atoms of the PF 6 À anion are disordered over two sets of sites with refined occupancy factors of 0.614 (7):0.386 (7). In compound (II), the cation comprises a pyridine ring attached to unsubstituted phenyl ring and a dimethylaniline ring, which are connected by a C C double bond. The anion is PF 6 À . In both salts, the cation adopts an E configuration with respect to the C C bond. The pyridine ring makes a dihedral angle of 9.86 (12) with the methoxy-substituted benzene ring in compound (I) and 11.2 (3) with the dimethylamine-substituted benzene ring in compound (II). In compound (I), the crystal packing is stabilized by weak C-HÁ Á ÁF intermolecular interactions which result in R 4 3 (14) ring motifs, forming molecular sheets running parallel to (103). These are further stabilized by weak P-FÁ Á Á interactions. In compound (II), the crystal packing is stabilized by C-HÁ Á ÁF interactions, which result in R 6 6 (40) ring motifs, forming molecular sheets running parallel to (101) and these are further connected byinteractions.

Chemical context
Stilbene-based compounds are the basic element for a number of biologically active natural and synthetic compounds. These compounds have a wide range of biological activities including anti-inflammatory, anticancer, antiviral, antioxidant and more recently neuroprotective effect (Giacomini et al., 2016). Pyridine and its derivatives play an important role in developing anticancer drugs (Ghattas et al., 2017) and show antibacterial activities (Chanawanno et al., 2010). Pyridine is the parent ring system of a large number of naturally occurring products and important industrial, pharmaceutical and agricultural chemicals. Pyridine derivatives have also shown antichagasic activity against Chagas disease, a parasitic infection caused by Trypanosoma cruzi, a parasite that is widely spread in central and South America (Dorigo et al., 1993). The title compounds have been tested for in vitro cytotoxicity and anticancer activity, using VERO and MCF-7 (breast cancer) cell lines, respectively. The cells were maintained in minimal essential ISSN 2056-9890 medium supplemented with 10% FBS, penicillin (100 U ml À1 ), and streptomycin (100 microgram ml À1 ) in a humidified atmosphere of 50 microgram ml À1 CO 2 at 310 K.
The pyridine ring (N1/C10-C14) makes a dihedral angle of 9.86 (12) with methoxy-substituted benzene ring (C2-C7) in compound (I) whereas in compound (II) the pyridine ring (N2/C7-C11) makes a dihedral angle of 11.2 (3) with dimethylamine-substituted benzene ring (C14-C19). The pyridine ring in compound (II) is inclined to the unsubstituted phenyl ring (C1-C6) by 54.9 (3) . The methoxy group oxygen atom O1 of compound (I) deviates from the benzene ring to which it is attached by 0.0317 (1) Å while the methyl group carbon atom C15 deviates from the benzene ring to which it is attached by 0.022 (3) Å . In compound (II), the methylamine group nitrogen atom (N1) deviates from the benzene ring to which it is attached by 0.017 (5) Å .

Compound (I)
A solution of N-phenyl-4-picolinium chloride (250 mg, 1.10 mmol), 4-(dimethylamino) benzaldehyde (363 mg, 2.4 mmol), and piperidine (4 drops) in methanol (20 ml) was heated under reflux for 4 h. The addition of diethyl ether to the deep-red solution yielded a dark precipitate, which was filtered, washed with diethyl ether and dried. This crude chloride salt was metathesized to dimethylamino N-phenyl stilbazolium hexafluoro phosphate (DAPSH) by precipitation from water/aqueous NH 4 PF 6 . A supersaturated solution of DAPSH was prepared using acetonitrile as solvent and the solution was filtered into the growth vessel for slow evaporation by covering the vessel with a perforated sheet. Good quality greenish crystals of compound (I) was grown in a period of 15-25 days.
Compound (II) Compound (II) was synthesized by the condensation of 1,4dimethylpyridinium iodide (2.35 g, 10 mmol), methanol (30 ml) and 4-methoxybenzaldehyde (1.36 g, 10 mmol) in the presence of piperidine (0.2 ml). The total mixture was taken in the round-bottom flask (1000 ml capacity) of a Dean-Stark apparatus and refluxed for 1 d and cooled to room temperature. The product 4-methoxy-N-methyl-4-stilbazolium iodide was filtered and recrystallized from methanol. This product (0.706 g, 2 mmol) was dissolved in 70 ml of millipore water and simultaneously sodium hexafluorophosphate (0.338 g, 2 mmol) was dissolved in 30 ml of millipore water by heating at 343 K. Both the solutions were stirred for 3 h and mixed. 4-Methoxy-N-methylstilbazolium hexafluorophosphate (MMSHP) was formed as a yellowish precipitate. A solution of MMSHP and aqueous acetone was prepared with 14.4 g of MMSHP in 200 ml of acetone-water mixed solvent (5:1) and stirred. The clear solution was collected in the growth vessel after filtering it by using 0.2 micrometer porosity millipore The crystal packing of the title compound (I), viewed along the a axis, showing C-HÁ Á ÁF intermolecular interactions, resulting in R 4 3 (14) ring motifs, which form two-dimensional molecular sheets running parallel to (103). Hydrogen atoms not involved in hydrogen bonding have been omitted for clarity.
filters and the solvent was allowed to evaporate slowly at room temperature. After three weeks, yellowish crystals of compound (II) were harvested.

Computing details
For both structures, data collection: APEX2 (Bruker, 2008); cell refinement: APEX2 (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.15 e Å −3 Δρ min = −0.14 e Å −3 Extinction correction: SHELXL, Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.016 (3) Absolute structure: Flack (1983), 1198 Friedel pairs Absolute structure parameter: 0.08 (11) Special details 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 Occ. (

4-{(E)-2-[4-(Dimethylamino)phenyl]ethenyl}-1-phenyl-1λ 5 -pyridin-\ 1-ylium hexafluoro-λ 6 -phosphane (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.49 e Å −3 Δρ min = −0.41 e Å −3 Absolute structure: Flack (1983), 1927 Fridel pairs Absolute structure parameter: 0.5 (2) Special details 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.