The imidazole molecules are arranged in chains in the [1 0 0] direction by short Br...N interactions, as shown in Figure 2. Distance and angle of this directed interaction are in the typical range. Being the only neutral molecule in this study, electrostatic N...H interactions are found by Hirshfeld surface analysis, whereas the ionic compounds show zero interactions of this kind. On the other hand, H...H interactions are negligible in 2 compared to the salt structures 3–9.

Two independent ion pairs are found in the asymmetric unit. Short Br...F contacts are clearly detected between the tribromoimidazolium cations and the tetrafluoroborate anions (Figure 3). The C–Br...F angles are nearly linear, exceptions are probably due to packing effects. There are indisputable interactions of the anion with the aromatic π system observed, with a short anion...centroid distance. Evidently, the anion is located on top of the aromatic ring, fixed by four contacts shorter than the sum of van der Waals radii (Table 3). These weak interactions can be overridden by packing effects [49], and it has been remarked that it is still not possible to predict anion...π interactions [49]. Thus, it is not surprising that the π-system of the second cation does not exhibit perceptible interactions with an anion.

The unit cell is very similar to that of the previous compound 3. Again, two ion pairs are in the asymmetric unit. The Br...F contacts, with angles between 152 and 168° (Table 2) are slightly different, one bromine atom exhibiting a bifurcated Br...F interaction (Figure 4a). The π-system of one imidazolium cation is involved in four short contacts with a tetrafluoroborate anion (Table 3). In contrast to 3, a surprisingly high contribution of C...H interactions is revealed by Hirshfeld surface analysis which, everything else being equal, can only be attributed to the propynyl substituent.

In this structure, the two alkyl substituents of the imidazolium cation adopt an anti conformation. The pronounced prolate shape of the thermal ellipsoids of some of the fluorine atoms of the PF₆ anion indicated the presence of a disorder that was modeled by introducing split positions (A and B with occupancy factors of 0.50) for the F1, F3 and F4 atoms, which were refined isotropically. Since no temperature dependent measurements were performed the question remains whether the disorder is static or dynamic. Two short Br...F contacts with C–Br...F angles around 160° are observed. A skewed anion...π interaction (Figure 5a) is found with a short F...centroid distance (Table 3). Obviously, the anion is not located exactly over the center of the ring, only three contacts shorter than the sum of van der Waals radii are found in this case. The anion is interacting predominantly with the carbon atoms of the heterocyclic ring system.

Two ion pairs are located in the asymmetric unit. Again, the two alkyl substituents adopt the anti conformation. In this instance, no anion...π interaction is detected, but the typical Br...F contacts (Figure 5b) are found in both ion pairs. The C–Br...F angles range from 155 to 171° (Table 2) which is the typical angular preference [28]. One interaction (C–Br12...F21) exhibits a suspiciously small angle. However, this contact is three percent shorter than the sum of van der Waals radii and cannot be ignored. Once again, a surprisingly high contribution of C...H interactions is revealed by Hirshfeld surface analysis.

The triflimide ion adopts the anti conformation (C–S...S–C torsion angle of 170.6°); this conformation is preferred in metal-free triflimide structures [50]. The angle C–C≡C is found to be 175.8°. A “classic” C_sp–H...O hydrogen bond between H6 and O1 is observed, thus forming a cyclic dimer (Figure 6). Interestingly, in this structure no Br...Br or Br...F interactions are found, but Br...O contacts to the sulfonyl group, as noticed earlier in related structures [17,51,52], and a F...F contact across a symmetry center. However, an anion...π interaction is observed. This time one partially negatively charged oxygen atom of a sulfonyl group is involved in three contacts shorter than the sum of van der Waals radii. Again, the interacting atom of the anion is shifted from the center of the π system and is located over the rim of the heterocycle, exhibiting short contacts with N1, C2, and C3 of the imidazolium ring.

As before, there are almost linear Br...F contacts between the ions. The structure also exhibits Br2...Br2 interactions in the ring plane, forming layers which are connected by Br5...Br5 contacts to give a three-dimensional network (Figure 7). The Br...Br contacts are both across a symmetry center, deviate from linearity due to packing (Table 2), and are also found prominently in the Hirshfeld surface analysis (Table 4). The allylic C–H...F interactions are also detected in the Hirshfeld analysis.

Two independent ion pairs are located in the asymmetric unit. The presence of the brominated side chain results in an increased number of Br...Br contacts such as short Br4...Br4 and weak Br1...Br1 and Br9...Br9 contacts across a symmetry center. Strong bromine...bromide contacts exist in the crystal which are significantly shorter than the sum of van der Waals radii, even without taking into account the fact that bromide has a larger van der Waals radius than covalently bound bromine [33]. The C–Br...Br⁻ angles are close to 180°, indicating linearity of the X-bonds. The coordination around the two independent bromide ions is shown in Figure 8. It is not unexpected that, due to the brominated side chain, contacts involving Br atoms are quite dominant in the Hirshfeld surface analysis. According to this analysis, C–H...Br contacts by far represent the majority of interactions, obviously due to the relatively strong C_sp2–H...Br⁻ interactions, followed by Br...Br contacts. Some examples of similar situations are found in the Cambridge Structural Database [53,54,55,56].

m.p. 123 °C. IR (neat): ν 3288 (m), 2129 (w), 1494 (m), 1433 (m), 1410 (m), 1380 (m), 1335 (m), 1214 (m), 1192 (m), 1121 (m), 985 (m), 937 (m), 758 (m), 682 (s), 651 (s)·cm⁻¹. ¹³C NMR (DMSO-d₆, 75 MHz): δ 37.7, 76.4, 76.8, 105.9, 116.0, 118.9 ppm. Single crystals were obtained from MeOH.

To a solution of trimethyloxonium tetrafluoroborate or triethyloxonium hexafluorophosphate (16 mmol) in anhydrous CH₂Cl₂ (5 mL) the respective 2,4,5-tribromo-1-alkylimidazole (1.0 eq.) was added and stirred overnight at room temperature. Et₂O was added, and the resulting colorless solid was filtered and dried under reduced pressure. Single crystals were obtained from MeOH.

2,4,5-Tribromo-3-methyl-1-(prop-2-enyl)imidazolium tetrafluoroborate (3): m.p. 179 °C. IR (neat): ν 1553 (w), 1501 (w), 1049 (s), 1035 (s), 946 (w), 669 (w)·cm⁻¹. ¹³C NMR (DMSO-d₆, 75 MHz): δ 37.8, 52.4, 110.8, 112.5, 118.9, 124.5, 129.1 ppm.

2,4,5-Tribromo-3-methyl-1-(prop-2-ynyl)imidazolium tetrafluoroborate (4): m.p. 203 °C. IR (neat): ν 2135 (w), 1554 (w), 1498 (w), 1050 (s), 1035 (s), 816 (w), 679 (w), 652 (w)·cm⁻¹.

2,4,5-Tribromo-3-ethyl-1-(prop-2-enyl)imidazolium hexafluorophosphate (5): m.p. 228 °C (dec). IR (neat): ν 1496 (w), 1122 (w), 936 (w), 831 (s), 669 (w)·cm⁻¹.

2,4,5-Tribromo-3-ethyl-1-(prop-2-ynyl)imidazolium hexafluorophosphate (6): m.p. 232 °C (dec). IR (neat): ν 2135 (w), 1559 (w), 1494 (w), 1118 (m), 1051 (s), 833 (s), 710 (s)·cm⁻¹.

A solution of 2,4,5-tribromo-3-methyl-1-(prop-2-ynyl)imidazolium tetrafluoroborate (4) and lithium triflimide (1.05 eq.) in MeOH was stirred for 18 h. After evaporation of the solvent, the residual solid was suspended in H₂O. The mixture was extracted twice with CH₂Cl₂. The organic phase was washed three times with H₂O and dried under reduced pressure to give a colorless solid. m.p. 122 °C. IR (neat): ν 2135 (w), 1556 (w), 1500 (w), 1351 (s), 1333 (m), 1190 (s), 1178 (s), 1137 (s), 1116 (m), 1046 (s), 1033 (s), 959 (w), 819 (w), 794 (w), 743 (w), 694 (w), 613 (s)·cm⁻¹. Single crystals were obtained from MeOH.

Bromine (1.0 eq.) was added to an ice-cooled suspension of 6 in CH₂Cl₂. The mixture was stirred at room temperature for 18 h, and the colorless precipitate was filtered, washed with CH₂Cl₂, and dried under reduced pressure. m.p. 206 °C. IR (neat): ν 837 (s), 673 (w)·cm⁻¹. Single crystals were obtained from MeOH.

Crystals of this minor byproduct were spotted in the major product 8.

Intensity data were recorded with an Oxford Diffraction Gemini-R Ultra diffractometer using Mo-Kα radiation at T = 173(2) K. For 6, Cu-Kα radiation was used. Experimental details are summarized in Table 1. Structure solution and refinement was performed with the programs SIR2002 (direct methods) [58] and SHELXL-97 [59]. A Flack parameter of 0.03(2) was obtained for the non-centrosymmetric structure of 2. The crystals of 8 were subject to non-merohedral twinning which was taken into consideration during data reduction, integration and refinement. Visualization of the structures and measurements of distances and angles was performed with the program Mercury [60]. CCDC reference numbers: 881951–881958. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.