Synthesis and crystal structures of 2-bromo-1,3-dimethylimidazolium iodides

Short C—Br⋯I interactions and C—H⋯I hydrogen bonds are observed in the title compounds.


Chemical context
Salts containing 2-bromo-1,3-dimethylimidazolium (C 5 H 8 N 2 Br + ) cations are the objective of this work. They are presumed to be valuable precursors for substitution reactions. This cation, despite its simplicity, has not yet been described. Since brominations in the 1,3-dimethoxyimidazolium series (Laus et al., 2007) and also bromination of 1-hydroxyimidazole-3-oxide (Laus et al., 2012) gave the respective 2bromo derivatives, we hoped that in the present case bromination would also yield the desired 2-bromoimidazolium salts. However, on attempted bromination of 1,3-dimethylimidazolium hexafluoridophosphate (Holbrey et al., 2002), no substitution occurred in the 2-position as indicated by NMR. The absence of P-F vibrations in the infrared spectra suggested the formation of a different anion, which was confirmed by X-ray diffraction. Though direct bromination of the quaternary salt did not yield the desired product, it was discovered that an altered sequence of reaction was successful. Thus, the reaction between the 2-lithio derivative of 1-methylimidazole and an equimolar amount of CBr 4 (Boga et al., 2000) or Br 2 (El Borai et al., 1981) gave 2-bromo-1methylimidazole in good yield, followed by methylation using MeI to afford the desired quaternary salt as an iodide. Now that the elusive title cation has been secured, further modifications are envisioned, giving access to a plethora of new 2-substituted imidazolium derivatives.

Structural commentary
The 2-bromo-1,3-dimethylimidazolium cations and iodide counter-ions crystallize as a CHCl 3 1/3-solvate (1) (Fig. 1), a CH 2 Cl 2 monosolvate (2) (Fig. 2) and an I 2 adduct (3) (Fig. 3). ISSN 2056-9890 In every case, the cation is almost planar. In the asymmetric unit of 1, there are one and a half ion pairs, which are completed by mirror symmetry; the chloroform molecule also lies on a crystallographic mirror plane. In 2, there are two cations, two anions and two half-molecules of dichloromethane (both completed by crystallographic twofold symmetry) in the asymmetric unit. In 3, the iodine molecule is generated by crystallographic inversion symmetry.

Figure 1
The molecular structure of the chloroform solvate 1, showing the atom labels and 50% probability displacement ellipsoids for non-H atoms.

Figure 6
The crystal packing of compound 3 viewed along the a axis showing the C-HÁ Á ÁI hydrogen bonds (see Table 3) and BrÁ Á ÁI and IÁ Á ÁI short contacts as dashed lines.

Figure 5
The crystal packing of compound 2 viewed along the b axis showing the C-HÁ Á ÁI hydrogen bonds involving the solvent (see Table 2) and BrÁ Á ÁI short contacts as dashed lines.

Synthesis and crystallization
Compound 1: A solution of 2-bromo-1-methylimidazole (150 ml, 1.54 mmol) in CHCl 3 (1 ml) was carefully layered over a solution of CH 3 I (190 ml, 3.07 mmol) in CHCl 3 (2 ml). The mixture was kept at room temperature and protected from light. After 2 h, the formation of colourless crystals of 1 was observed. The product was collected after seven days at 278 K, yielding 252 mg (48%); m.p. 453 K (decomposition Compound 2: A solution of 2-bromo-1-methylimidazole (150 ml, 1.54 mmol) in CH 2 Cl 2 (1 ml) was carefully layered over a solution of CH 3 I (190 ml, 3.07 mmol) in CH 2 Cl 2 (2 ml). The mixture was kept at room temperature and protected from light. After 2 h, the formation of colourless crystals of 2 was observed. The product was collected after 18 h, yielding 145 mg (27%); m.p. 452-453 K (decomposition

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All H atoms were poisitioned geometrically (C-H = 0.95-1.0 Å ) and treated as riding with U iso (H) = 1.2-1.5U eq (C).

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. ( where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.76 e Å −3 Δρ min = −0.97 e Å −3 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.