Crystal structure and Hirshfeld surface analysis of a pyridiniminium bromide salt: 1-[2-(adamantan-1-yl)-2-oxoethyl]pyridin-4-iminium bromide

The asymmetric unit of the title pyridiniminium halide salt comprise of one cation and one anion. In the crystal, molecules are linked by N—H⋯Br and C—H⋯O hydrogen bonds, C—H⋯π interactions, and π–π interactions into layers. The intermolecular interactions in the crystal structure are quantified by Hirshfeld surface analysis.


Chemical context
Adamantane derivatives have been shown to exhibit various biological activities such as antiviral (Zoidis et al., 2010), antidiabetic (Zettl et al., 2010), antimicrobial (Pié rard et al., 2009), anti-inflammatory (Lamanna et al., 2012), antioxidant (Priyanka et al., 2013) and central nervous system activities (Reisberg et al., 2003). Besides, adamantane-based chemotherapeutics have been developed for treating viral infections, for example influenza A, herpes simplex and HIV (Liu et al., 2011). There are a number of negatively charged enzymes and cofactors and many diseases, including cystic fibrosis, have been found to result from defects in the ion channel function (Ashcroft, 1999). The anion-non-covalent interaction has been explored both theoretically and experimentally and selective anion receptors and channels have been designed (Ballester, 2008;Schottel et al., 2008;Hay & Bryantsev, 2008;Frontera et al., 2011).
Ionic liquids (ILs) have attracted a lot of interest over the past decade because of their unusual range of properties such as negligible vapour pressure, excellent thermal stability in a wide temperature range, no flammability and high ionic conductivity (Davis, 2004). ILs are excellent alternatives to volatile organic compounds (VOCs). An ionic liquid has a strong solvation ability and can dissolve polar and non-polar species with efficient selectivity, which can be modified by changing the anion (Blanchard et al., 2001). ILs have been used successfully as solvents in several reactions such as isomerization, dimerization, hydrogenation, and Heck and Suzuki coupling reactions (Chauvin & Olivier-Bourbigou, 1995;Holbrey & Seddon, 1999). They have also performed well as solvents in bio-catalysed and homogeneous catalytic reactions, and can be used as lubricants to wet the surface of metals, polymers and inorganic materials (Crosthwaite et al., 2004). Fig. 1 shows the asymmetric unt of the title salt, which consists of a 1-[2-(adamantan-1-yl)-2-oxoethyl]pyridin-4-iminium cation and a bromide anion. The cation is constructed from an adamantyl moiety (C1-C10) and a pyridiniminium ring (N1/ C13-C17), which are connected by a ketone bridge [(C11 O1)-C12]. The bond angles formed by the quaternary carbon (C1) with the surrounding secondary carbons (C2, C6 and C7) are comparable with those reported for related structures which range from 107.40 (12) to 110.82 (13) (Rouchal et al., 2011). Both the adamantyl and pyridiniminium rings are twisted away from the ketone bridge to reduce repulsion, as indicated by the torsion angles C6-C1-C11 O1 [À78.1 (2) ] and C11-C12-N1-C13 [58.3 (2) ]. The ketone bridge is in an antiperiplanar conformation [C1-C11-C12-N1 = 174.80 (15) ]. The dihedral angle formed by the pyrimidinium ring with the ketone bridge is 59.77 (14) . Bond lengths and angles in the cation are within normal ranges (Allen, 2002). However, the N2-C15 bond length [1.325 (2) Å ] is shorter than expected for an NH 2 -C ar single bond [1.38 (3) Å ], indicating partial double-bond character. Similar bond lengths are found in related compounds with an N + C double bond (Chidan Kumar et al., 2017;Sharmila et al., 2014;Yue et al., 2013).

Hirshfeld Surface Analysis
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) of the title salt was performed using CrystalExplorer3.1 (Wolff et al., 2012), and comprises d norm surface plots, electrostatic potentials and two-dimensional fingerprint plots (Spackman & McKinnon, 2002). The ball-and-stick model, d norm surface and electrostatic potential plots of the title salt are shown in Fig. 3. Those plots were generated in order to quantify and give visual confirmation of the intermolecular interactions and to explain the observed crystal packing. The dark-red spots on the d norm surface arise because of short interatomic contacts, while the other weak intermolecular interactions appear as light-red spots. Furthermore, the negative electrostatic potential (red region) in the electrostatic potential map indicates hydrogen-acceptor potential, whereas the hydrogen donors are represented by positive electrostatic potential (blue region) (Spackman et al., 2008). Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Partial packing diagram of the the cations showing the C17-H17AÁ Á ÁO1 hydrogen bonds (blue dashed lines) and the C14-H14AÁ Á Á interactions (green dashed lines).

Figure 1
The molecular structure of the title salt with displacement ellipsoids drawn at the 50% probability level.
Dark-red spots that are close to atoms H1N2, H2N2, H12A and Br1 in the d norm surface mapping are the result of the N2-H1N2Á Á ÁBr1, N2-H2N2Á Á ÁBr1 and C12-H12AÁ Á ÁBr1 hydrogen bonds (Fig. 4a). This observation is further confirmed by the respective electrostatic potential maps where Br1 shows negative electrostatic potential as a hydrogen acceptor (red region, Fig. 4b). Beside those two short intermolecular contacts, the C-HÁ Á ÁO and C-HÁ Á Á interactions are shown as light-red spots on the d norm surface (Fig. 5).
A quantitative analysis of the intermolecular interactions can be made by studying the fingerprint plots (FP); characteristic pseudo-symmetry wings in the d e and d i diagonal axes can be seen in the overall two-dimensional FP (Fig. 6). The most significant intermolecular interactions are the HÁ Á ÁH interactions (63.5%), which appear in the central region of the FP with d e = d i ' 2.2 Å (Fig. 6b). The reciprocal HÁ Á ÁBr/ BrÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH interactions with 15.9% and 7.6% contributions, respectively are present as sharp symmetrical spikes at d e + d i ' 2.4 and 2.5 Å , respectively ( Fig. 6c and 6e). The reciprocal HÁ Á ÁC/CÁ Á ÁH interactions appear as two symmetrical narrow wings at d e + d i ' 2.5 Å and contribute 7.8% to the Hirshfeld surface (Fig. 6d). The reciprocal NÁ Á ÁH/ HÁ Á ÁN interactions appear as a symmetrical V-shaped wing in the FP map with d e + d i ' 2.7 Å and contribute 2.7% to the Hirshfeld surface (Fig. 6f). The percentage contributions for other intermolecular contacts are less than 2.6%.

Synthesis and crystallization
A mixture of 1-adamantly bromomethyl ketone (2.75 g, 10 mmol) and 4-aminopyridine (0.11 g, 1 mmol) was dissolved in 10 ml of toluene at room temperature, followed by stirring at 358 K for 18 h. The completion of the reaction was marked by the amount of the separated solid from the initially clear    Hirshfeld surfaces mapped over d norm and electrostatic potential to visualize the intermolecular contacts in the title salt. The molecule in the ball-and-stick model is in the same orientation shown in the Hirshfeld surface and electrostatic potential plots. and homogeneous mixture of the starting materials. The solid was filtered and washed by ethyl acetate. The final pyridiniminium salt was obtained after the solid had been dried under reduced pressure to remove all volatile organic compounds (Said et al., 2017;Sheshadri et al., 2018). Plate-like colourless crystals were obtained by slow evaporation of an acetone solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were positioned geometrically [C-H = 0.93-0.98 Å ] and refined using a riding model with U iso (H) = 1.2U eq (C). The N-bound H atoms were located in a difference-Fourier map and freely refined. One outlier (100) was omitted in the last cycles of refinement.

Funding information
HCK thanks the Malaysian Government for a MyBrain15 scholarship.

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 0.71078 12.009 7.162 18.799 89.983 98.202 90.025 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.