Halogen Interactions in 2,4,5-tribromoimidazolium Salts

The crystal structures of 2,4,5-tribromo-1-(prop-2-ynyl)imidazole and seven new 1,3-dialkyl-2,4,5-tribromoimidazolium salts (R 1 = propenyl, propynyl, dibromopropenyl; R 2 = Me, Et) with halogen-containing anions (tetrafluoroborate, hexafluorophosphate, triflimide) were determined. The structures revealed halogen ... halogen and anion ... π interactions. Contacts of the type Br. and O ... π were identified. Specific interactions were quantified by Hirshfeld surface analysis.


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derivatives are known to date.Here we present the structures of eight new compounds of this kind.The quaternary salts are intended for application as non-volatile fire retardants.The olefinic substituents allow the preparation of new materials by polymerization.

Results and Discussion
The synthesis (Figure 1) of the propenyl and propynyl derivatives 1 and 2 was carried out by known methods.The salts 3-6 were prepared by quaternization of 1 and 2. Compound 7 was obtained by ion metathesis from 4, and compounds 8 and 9 by bromination of 6.The choice of fluorine-containing anions such as tetrafluoroborate, hexafluorophosphate, and bis(trifluoromethanesulfonyl)imide ("triflimide") provided an opportunity to study non-covalent hetero-halogen, homo-halogen, hydrogen-halogen, and other interactions.Crystal data and structure refinement details are summarized in Table 1.Charge transfer complexes of halogens and electron-donor molecules have been the subject of the 1970 Nobel Prize lecture by Odd Hassel [20].Today it is known that halogen atoms can act as electron-acceptors (X-bond donors) and electron donors (X-bond acceptors).Theoretical calculations predicted negative ring and positive end cap domains of halogen atoms due to their polarizability [21], even for fluorine [22].In fact, hetero halogen-halogen interactions in both directions, i.e., Cl→F and F→Cl, have been observed recently [22].As expected, a classic Br ... N interaction was found in 2 (Table 2).Some typical examples of this interaction, displayed by brominated N-heterocycles, are found in the Cambridge Structural Database (CSD) [23][24][25][26].The role of fluorine, which exhibits the lowest polarizability and highest electronegativity of the halogens, in crystal structures of organic compounds has been reviewed [27].After some controversy, it has been concluded that short C-H ... F contacts between oppositely charged molecules are genuine interionic hydrogen bonds [28].Non-Linear allylic C-H ... F interactions [28] were observed in compound 8. Short interactions between covalent halogen atoms (Br ... F and Br ... Br) [21] were observed in compounds 3-9.Classic H ... Br − and Br ... Br − bonds were found in the bromide 9. Similar Br ... Br − interactions have been observed in the structures of 2-bromoimidazolium bromides [16,18,[29][30][31], a related Br ... F contact in a 2-bromoimidazolium hexafluorophosphate [30], and a typical Br ... Br interaction for example in pentabromoferrocene [32].Lately, attractive anion ... π interactions have been recognized as supramolecular forces between anions and electron-deficient heterocyclic ring system [33,34].We indeed observed interactions of the tetrafluoroborate, hexafluorophosphate, and triflimide anions with the tribromoimidazolium system.The cut-off for anion ... π interactions has not been clearly established [33].We judged such an interaction as existing when at least three interactions with the atoms of a five-membered ring shorter than the sum of van der Waals (vdW) radii were found, as in compounds 3-5 and 7 (Table 3).A few imidazolium tetrafluoroborates which also exhibit this type of interaction with short F ... centroid distances have been located in the CSD [35][36][37], although not described as such by the respective authors.Likewise, examples of PF 6 ... π [38][39][40] and S=O ... π [30,41,42] interactions in imidazolium hexafluorophosphates, triflimides, and triflates should be mentioned.

2,4,5-Tribromo-1-(prop-2-ynyl)imidazole (2)
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.

2,4,5-Tribromo-3-methyl-1-(prop-2-enyl)imidazolium Tetrafluoroborate (3)
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.

2,4,5-Tribromo-3-methyl-1-(prop-2-ynyl)imidazolium Tetrafluoroborate (4)
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.

2,4,5-Tribromo-3-ethyl-1-(prop-2-enyl)imidazolium Hexafluorophosphate (5)
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 PF 6 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.

2,4,5-Tribromo-3-ethyl-1-(prop-2-ynyl)imidazolium (6)
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.

2,4,5-Tribromo-3-methyl-1-(prop-2-ynyl)imidazolium Triflimide (7)
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.

2,4,5-Tribromo-1-(2,3-dibromoprop-2-enyl)-3-ethylimidazolium Bromide (9)
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]

General Procedure for the Alkylation of 1-Alkylimidazoles (3-6)
To a solution of trimethyloxonium tetrafluoroborate or triethyloxonium hexafluorophosphate (16 mmol) in anhydrous CH 2 Cl 2 (5 mL) the respective 2,4,5-tribromo-1-alkylimidazole (1.0 eq.) was added and stirred overnight at room temperature.Et 2 O was added, and the resulting colorless solid was filtered and dried under reduced pressure.Single crystals were obtained from MeOH.

X-ray Crystal Structure Determination
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.

Conclusions
The crystal structures of eight N-alkylated 2,4,5-tribromoimidazole compounds were determined.Halogen interactions and anion ... π interactions were detected.Hirshfeld surface analysis (Figure 9) yielded quantitative information about these interactions.These results are supposed to further our understanding of halogen interactions in the solid state.

Figure 2 .
Figure 2. Chains of molecules of 2 in the direction of the crystallographic a axis.

Figure 3 .
Figure 3. (a) Typical Br ... F interactions; and (b) the anion ... π interactions in the propenyl imidazolium tetrafluoroborate 3.Only one of the two independent cations is shown.The other one exhibits very similar Br ... F contacts.

Figure 4 .
Figure 4. (a) Typical Br ... F interactions; and (b) the anion ... π interactions in the propenyl imidazolium tetrafluoroborate 4.Only one of the two independent cations is shown.The other one exhibits very similar Br ... F contacts.
. 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 sp 2-H ... Br − interactions, followed by Br ... Br contacts.Some examples of similar situations are found in the Cambridge Structural Database [53-56].

Table 1 .
Crystal data and structure refinement details.

Table 4 .
Percent contributions of selected interactions relative to the whole Hirshfeld surface.Independent cations are termed A and B.