Pyridinium tosylate

In the crystal, classical N—H⋯O hydrogen bonds as well as C—H⋯O contacts connect the cationic and anionic entities into sheets lying parallel to the ab plane.

The title compound (systematic name: pyridinium 4-methylbenzenesulfonate), C 5 H 6 N + •C 7 H 7 O 3 S À , is the pyridinium salt of para-toluenesulfonic acid.In the crystal, classical N-H� � �O hydrogen bonds as well as C-H� � �O contacts connect the cationic and anionic entities into sheets lying parallel to the ab plane.

Structure description
Many fundamental synthesis reactions in preparative organic chemistry make use of activated reagents to allow for the faster and easier production of certain key compounds or to avoid the presence of cumbersome equilibrium reactions.A prime example for this finding is a series of derivatives of carboxylic acids such as esters and amides thatinstead of employing the free acid as staring material -are often more conveniently obtained by using the pertaining carboxylic anhydride or acyl chloride or bromide as starting materials (Becker et al., 2000).One downside of this increased reactivity is the frequent need to use auxiliary reagents that can mitigate potential side effects of the byproducts produced, most notably basic reagents that can act as acid scavengers to prevent undesired hydrolysis effects.Among the more common ingredients used in the latter context are amines such as triethylamine or pyridine whose onium salts can often conveniently be removed from reaction mixtures in organic solvents by means of simple filtration.Occasionally, however, some of the material tenaciously migrates through many steps of purification procedures and can manifest as lingering impurity in the assumed final product.To prevent the waste of valuable data-collection time on diffractometers for future researchers, it is of importance to report the structures even of such undesired compounds as a reference point for the broader scientific community, as done previously by us for ammonium formate (Hosten & Betz, 2014), ammonium phenyl glyoxylate (Hosten & Betz, 2015) as well as the chlorides (Maritz et al., 2021;Muller et al., 2021a,b,c) and tosylate salts (Moleko et al., 2015) of a number of protonated amines.Furthermore, the molecular and crystal structures of the non-radioactive halogenide salts of the pyridinium cation are apparent in the literature (Boenigk & Mootz, 1988;Mootz & Hocken, 1989;Klooster et al., 2019;Owczarek et al., 2012).
The asymmetric unit of the title compound, C 5 H 6 N + •-C 7 H 7 O 3 S À , is shown in Fig. 1 and consists of one complete ion pair.The S-O bond lengths in the anion are found in the narrow range of 1.4525 ( 14)-1.4682(14) A ˚, which is in agreement with full resonant delocalization of the anionic charge over all three oxygen atoms.All other bond lengths and angles are found in good agreement with other tosylates whose molecular and crystal structures were determined on grounds of diffraction studies conducted on single crystals and whose metrical parameters have been deposited with the Cambridge Structural Database (Allen, 2002).The leastsquares planes as defined by the non-hydrogen atoms of the cation as well as the intracyclic carbon atoms of the tosylate anion intersect at an angle of 74.44 (10) � , i.e. the two separate aromatic systems in the asymmetric unit are orientated almost perpendicular to one another.
In the crystal, classical N-H� � �O hydrogen bonds are observed as well as C-H� � �O contacts whose range falls by more than 0.1 A ˚below the sum of van der Waals radii of the atoms participating in them (Table 1).While the classical hydrogen bonds are established by the pnictogen-bonded hydrogen atom as donor and one of the oxygen atoms of the sulfato group as acceptor, the C-H� � �O contacts are supported by each of the aromatic hydrogen atoms of the cation except for the one in para position to the protonated nitrogen atom.All three sulfur-bonded oxygen atoms act as acceptors in for the latter contacts.In terms of graph-set analysis (Etter et al., 1990;Bernstein et al., 1995), the descriptor for the classical hydrogen bonds is D on the unary level while the C-H� � �O contacts require a DDDD descriptor on the same level.Overall, the intermolecular contacts connect the ions of the title compound into sheets lying parallel the the ab plane.A depiction of the pattern is shown in Fig. 2. Aromatic �-� stacking is not a prominent feature in the crystal structure of the title compound with the shortest intercentroid distance between two aromatic systems measuring 4.9276 (12) A ˚for the anion and its symmetrygenerated equivalent.

Figure 1
The molecular structure of the title compound, with anisotropic displacement ellipsoids drawn at 50% probability level.

Synthesis and crystallization
After an initial unintentional isolation of the crystalline compound from a different synthesis product the compound was targeted by reacting a slight excess of liquid pyridine with solid tosylic acid in solvent-free conditions.Crystals of the title compound in the form of colourless blocks suitable for the diffraction study were obtained upon free evaporation of the reaction mixture at room temperature.

Refinement
Data collection and crystallographic data are summarized in Table 2.The crystal used for data collection was found to be an an inversion twin with a volume ratio of 79.3:20.7.

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.

data reports data-2
IUCrData (2024).9, x240831 Refinement.Refined as a 2-component inversion twin.The N-bonded H atom was located in a difference map and refined freely.The aromatic carbon-bound H atoms were placed in calculated positions (C-H = 0.95 Å) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C).The H atoms of the methyl group were allowed to rotate but not to tip around the C-C bond to best fit the experimental electron density with U(H) set to 1.5U eq (C).

Table 2
Experimental details.