Synthesis and crystal structure of a new pyridinium bromide salt: 4-methyl-1-(3-phenoxypropyl)pyridinium bromide

The simple synthesis and crystal structure of a new pyridinium bromide salt, 4-methyl-1-(3-phenoxy propyl)pyridinium bromide, are reported. The C–H⋯Br− interactions have an effect on the NMR signals of the ortho- and meta-pyridinium protons.


Chemical context
In the last two decades, ionic liquids (ILs) have gained considerable interest as excellent alternatives to volatile organic compounds (VOCs) because of their unusual range of properties such as negligible vapour pressure, excellent thermal stability in a wide temperature range, no flammability, high ionic conductivity and solvation ability (Davis, 2004).
In view of the above mentioned, and of our ongoing research interest in the synthesis of ionic liquids (Messali, 2016(Messali, , 2015Messali et al., 2014), we present in this study the preparation and the crystal structure of the novel title pyridinium halide salt, 4-methyl-1-(3-phenoxypropyl)pyridinium bromide. ISSN 2056-9890

Structural commentary
The molecular structure of the title pyridinium bromide salt is illustrated in Fig. 1. There is a weak intramolecular C-HÁ Á ÁO contact present, with an HÁ Á ÁO distance of 2.52 Å and a C-HÁ Á ÁO angle of only 100 (see Table 1). The cation consists of two planar groups, a pyridinium ring (N11/C12-C16) and a phenyl group (C1-C6); atom N11 has the expected planartrigonal conformation. The two aromatic rings are inclined to one another by 11.80 (8) and there is a step of ca 1.35 Å between the two groups along the C9-C10 bond, see Fig. 2. The C1-O7, C8-C9, C10-N11 and C14-C17 bonds are roughly parallel, so that the two aromatic groups are at opposite ends of an approximately linear cation. This is in contrast to the alignment found in 1-[2-(5-nitro-1H-indol-2yl)phenyl]methylpyridinium chloride where the cation is Ushaped with the pyridinium ring lying over the pyrrolo ring of the indole moiety (Bremner et al., 2011), possibly as a result of electronic interactions between the two rings.

Supramolecular features
In the crystal, the bromide anion is linked to the cation by a C10-H10BÁ Á ÁBr1 i hydrogen bond (Table 1). The anion is surrounded by three other cations with the most significant C--HÁ Á ÁBr short contacts varying from ca 3.07 to 3.11 Å ( Table 1). The bromide ions are aligned approximately in the planes of the aromatic rings, which is similar to the arrangement found in N-benzylpyridinium bromide (Anders et al., 1990), and in contrast to those in a series of N-(pentafluorobenzyl)pyridinium salts where the anion faces the aromatic rings with formation of anion-interactions (Giese et al., 2014).

Figure 2
A view of the cation showing the step formation about bond C9-C10 and the approximately parallel ring planes.

Figure 3
Crystal packing viewed along the b axis, showing the stacking of the phenyl and pyridinium groups along that axis.

Figure 1
The molecular structure of the component ions of the title salt, indicating the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

C-HÁ Á Áanion interactions in the 1 H NMR spectrum
The C-HÁ Á Áanion interactions are clearly manifested in the 1 H NMR spectrum (see Section 5. Synthesis and crystallization). Such an effect has previously been shown by a solution study of the C-HÁ Á ÁBr interaction on the signals of the ortho-and meta-pyridinium protons in the 1 H NMR spectra of a series of N-(pentafluorobenzyl)pyridinium salts (Giese et al., 2014). The present study in D 2 O solvent involves only the pyridinium protons (Ha and Hb) of a series of 4-methyl-1-(4-phenoxybutyl)pyridin-1-ium X À ionic liquids, and the title compound, shown in Fig. 4. The results, given in Table 2, reveal significant shifts for the hydrogen atom Ha in various pyridinium salts, whereas hydrogen atom Hb is only slightly affected by the different counter-anions (Messali, 2015); viz. the study reveals a range of 0.75 p.p.m. for the signals of the ortho-pyridinium protons (Ha) and a shorter range of 0.29 p.p.m. for meta-pyridinium protons (Hb). The determination of the causes behind this variation remains a challenging task for our research group.

Synthesis and crystallization
The synthesis of the title compound is illustrated in Fig. 5. To a solution of 1 g of 4-picoline (10.7 mmol) in 20 ml of toluene, were added 2.53 g of (3-bromopropoxy)benzene (118 mmol) at room temperature, followed by stirring at 355 K for 18 h.
The completion of the reaction was marked by the separation of a solid from the initially obtained clear and homogeneous mixture of the starting materials. The product was isolated by filtration to remove the unreacted starting materials and solvent. Subsequently, the title picolinium salt was washed with ethyl acetate. The product was finally dried at reduced pressure to remove all volatile organic compounds. The title compound was obtained as a white solid. Colourless prismatic crystals were obtained by slow evaporation of a solution in dichloromethane. Synthesis of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms were included in idealized positions and treated as riding atoms: C-H = 0.93-0.97 Å with U iso (H) = 1.5U eq (C-methyl) and = 1.2U eq (C) for other H atoms. PRO (Agilent, 2014); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015) and WinGX (Farrugia, 2012). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.27 e Å −3 Δρ min = −0.25 e Å −3 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.

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