Crystal structure of a methimazole-based ionic liquid

The structure of 1-methyl-2-(prop-2-en-1-ylsulfanyl)-1H-imidazol-3-ium bromide, C7H11N2S+·Br−, has monoclinic (P21/c) symmetry. In the crystal, the components are linked by N—H⋯Br and C—H⋯Br hydrogen bonds. The crystal structure of the title compound undeniably proves that methimazole reacts through the thione tautomer, rather than the thiol tautomer in this system.

Methimazole exists in two tautomeric forms, equilibrating between the 2-thiol 1a and 2-thione 1b, and both N-alkylation and S-alkylation reactions are possible, depending upon the reaction conditions and types of substrates employed, see Roy & Mugesh (2005). They reported that only S-alkylated methimazoles were formed. The product structures were established by NMR spectroscopy, which is elusive in terms of proving the exclusive formation of S-alkylated products over N-alkylated products. Herein, we report the crystal structure of S-allylated methimazolium bromide 2, which was prepared in quantitative yield (96%) via the reaction of methimazole with allyl bromide in refluxing acetonitrile (Scheme S1). The crystal structure of 2 undeniably proves that methimazole reacts through the 2-thione tautomer 1b.

S2. Synthesis and crystallization
2-Mercapto-1-methylimidazole (0.57 g, 5 mmol) and allyl bromide (0.85 g, 7 mmol) were dissolved in acetonitrile (5.0 mL) and the mixture refluxed for 48 hours. The solvent and excess allyl bromide were removed under vacuum to afford an off-white solid. The solid was washed with toluene (3 x 10 mL) and then recrystallized in acetonitrile to yield pure product 2 as an off-white solid in 96% isolated yield.

S3. Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The H-atom (H2) located on N2 was allowed to freely refine (isotropically). The remaining H-atoms were placed in calculated positions and allowed to ride during subsequent refinement, with U iso (H) = 1.5U eq (C) and C-H distances of 0.96 Å for methyl hydrogens, with U iso (H) = 1.2U eq (C) and C-H distances of 0.97 Å for the secondary hydrogens, and with U iso (H) = 1.2U eq (C) and C-H distances of 0.93 Å for all remaining hydrogen atoms. supporting information sup-2 Acta Cryst. (2015). E71, o1008-o1009

Figure 1
A thermal ellipsoid diagram of the structure of the title compound.

Special details
Experimental. CrysAlis Pro (Agilent, 2014) Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq