Crystal structure and Hirshfeld surface analysis of a pyridiniminium bromide salt: 1-[2-([1,1′-biphenyl]-4-yl)-2-oxoethyl]-3-methyl-1,4-dihydropyridin-4-iminium bromide

The Br− anion is linked to the cation by an N—H⋯Br hydrogen bond. C—H⋯O hydrogen bonds link adjacent pyridiniminium cations into inversion dimers with an (18) graph-set motif. These dimers are stacked in a phenyl–phenyl T-shaped geometry through C—H⋯π interactions.

In the cation of the title salt, C 20 H 19 N 2 O + ÁBr À , the phenyl rings are inclined to one another by 38.38 (8) , whereas the central phenyl ring and the pyridiniminium ring are almost perpendicular with a dihedral angle of 87.37 (9) . The N + C cationic double bond was verified by the shortened bond length of 1.337 (2) Å . In the crystal, the Br À anion is linked to the cation by an N-HÁ Á ÁBr hydrogen bond. C-HÁ Á ÁO hydrogen bonds link adjacent pyridiniminium cations into inversion dimers with an R 2 2 (18) graph-set motif. These dimers are stacked in a phenyl-phenyl T-shaped geometry through C-HÁ Á Á interactions. A Hirshfeld surface analysis was conducted to verify the contributions of the different intermolecular interactions.

Chemical context
Over the past decade, ionic liquids have been the subject of intense research as a customizable replacement for volatile organic solvents because of their negligible vapor pressure, excellent thermal stability, high ionic conductivity and solvation ability (Davis, 2004). A wide range of applications using ionic liquids has been reported in many areas, such as their use as homogeneous and heterogeneous catalysts (Dong et al., 2016) and biological reaction media (Lopes et al., 2017), and in nuclear waste treatment (Ha et al., 2010) and water purification (Fuerhacker et al., 2012;Wang & Wei, 2017).

Supramolecular features
In the crystal, the bromide anion is linked to the cation via an N2-H2N2Á Á ÁBr1 hydrogen bond ( Table 1). The bromide anion is surrounded by three other cations with short HÁ Á ÁBr contracts varying from 2.52 to 2.88 Å ( Table 1). Pairs of C1-H1AÁ Á ÁO1 hydrogen bonds link the pyridiniminium cations into inversion dimers with an R 2 2 (18) graph-set motif (Table 1, Fig. 2). The dimers are stacked in a phenyl-phenyl T-shaped geometry through C3-H3AÁ Á ÁCg1 interactions (Cg1 is the centroid of the C1-C6 phenyl ring).

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.  al., 2012), and comprised d norm surface plots, electrostatic potentials and 2D fingerprint plots (Spackman & McKinnon, 2002). The ball-and-stick model, d norm surface plots and electrostatic potentials of the title salt are shown in Fig. 3. Those plots were generated to quantify and visualize the intermolecular interactions and to explain the observed crystal packing. The dark-red spots on the d norm surface arise as a result of short interatomic contacts, while the other weak intermolecular interactions appear as light-red spots. Furthermore, negative electrostatic potential (red regions) in the electrostatic potential map indicates hydrogen-acceptor potential, whereas the hydrogen donors are represented by positive electrostatic potential (blue regions) (Spackman et al., 2008). The d norm surface of the title salt shows a dark-red spot on the N-H hydrogen atom and on the bromide atom, which is the result of the strong N2-H1N2Á Á ÁBr1 and N2-H2N2Á Á ÁBr1 hydrogen bonds present in the structure (Fig. 4a). These observations are further confirmed by the respective electrostatic potential maps, where the atoms involved in strong hydrogen bonds are seen as dark-blue and dark-red regions (Fig. 4b). Beside those two short intermolecular contacts, the C-HÁ Á ÁO and C-HÁ Á ÁBr interactions are shown as light-red spots on the d norm surface (Fig. 5). Finally, the C-HÁ Á Á interaction is shown as a light-red spot on the d norm surface (Fig. 6).
A quantitative analysis of the intermolecular interactions can be made by studying the fingerprint plots (FP). The FP is shown with characteristic pseudo-symmetry wings in the d e and d i diagonal axes represent the overall two-dimensional FP and those delineated into HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, HÁ Á ÁBr/ BrÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH contacts, respectively (Fig. 7). The most significant intermolecular interactions are the HÁ Á ÁH interaction (41.8%), which appear at the central region of the FP with d e = d i ' 2.2 Å (Fig. 7b). The reciprocal HÁ Á ÁC/CÁ Á ÁH interactions appear as two symmetrical broad wings with d e + d i ' 2.7 Å and contribute 29.2% to the Hirshfeld surface (Fig. 7c). The reciprocal HÁ Á ÁBr/BrÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH interactions with 16.7% and 7.3% contributions are present as sharp symmetrical spikes at diagonal axes d e + d i ' 2.3 and 2.4 Å , respectively (Fig. 7d-e). The percentage contributions for other intermolecular contacts are less than 5% in the Hirshfeld surface mapping.

Synthesis and crystallization
The synthesis of the title compound is illustrated in Fig. 8. A mixture of 1-([1,1 0 -biphenyl]-4-yl)-2-bromoethan-1-one (2.75 g, 10 mmol) and 3-methylpyridin-4-amine (0.11 g, 1mmol) was dissolved in 10 ml of toluene at room temperature, followed by stirring at 358 K for 18 h.     Visualization of C-HÁ Á Á interactions through the d norm maps. the reaction was marked by the amount of the separated solid from the initially clear and homogenous mixture of the starting materials. The solid was filtered from the unreacted starting materials and solvent, and subsequently washed with ethyl acetate. The final pyridiniminium salt was obtained after the solid was dried under reduced pressure to remove all volatile organic compounds (Said et al., 2017). Plate-like yellow crystals were obtained by slow evaporation of a solution in acetone.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were positioned geometrically [C-H = 0.95-0.99 Å ] and refined using a riding model with U iso (H) = 1.2 or 1.5U eq (C). All N-bound H atoms were located from a difference-Fourier map and freely refined.

Figure 8
Synthesis of the title compound.

Special details
Experimental. The following wavelength and cell were deduced by SADABS from the direction cosines etc. They are given here for emergency use only: CELL 0.71093 15.443 7.924 15.800 89.974 113.042 90.030 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.