Synthesis and crystal structures of two 1,3-di(alkyloxy)-2-(methylsulfanyl)imidazolium tetrafluoridoborates

In both structures, the alkyloxy and methylsulfanyl groups are rotated out of the plane of the respective heterocyclic ring.


Chemical context
2-(Methylthio)imidazolium salts have attracted great interest because of their reactive properties. Compounds belonging to this class can be converted into important derivatives with useful biological activity, i.e. as anti-filarial agents (Link et al., 1990). Furthermore, they have been used as precursors for the synthesis of remote N-heterocyclic carbene complexes (rNHC) (Patel et al., 2018), as tunable alkylating reagents (Guterman et al., 2018) or as coupling reagents for the formation of bis(2-imidazolyl)methylium salts (Kuhn et al., 1993;Fü rstner et al., 2008).

Structural commentary
In the organic cation of 1, the two methoxy groups adopt a syn conformation relative to each other, and the methylsulfanyl group is anti to each of the methoxy groups (Fig. 1). In contrast, the structurally related molecule of 1,3-dimethoxy-imidazoline-2-thione displays an anti conformation of its methoxy groups (Laus et al., 2013). The two N-OMe fragments of 1 form dihedral angles with the mean plane of the imidazole ring of 82.3 (2) (for the ring involving O1 and C4) and of 76.8 (1) (for the ring involving O2 and C5). The methylsulfanyl group (S1-C6) is rotated out of the heterocyclic plane and forms a dihedral angle of 62.5 (1) with the mean plane of the heterocycle defined by atoms N1, C1, N2, C2, and C3. Similar to 1, the methylsulfanyl group (S1-C18) of 2 is rotated out of the plane of the heterocycle and forms a dihedral angle of 78.6 (1) with the mean plane defined by the imidazole ring atoms (N1, C1, N2, C2, C3). The arrangement of the two benzyloxy moieties in the cation of 2 relative to each other is anti (Fig. 2). They adopt distinct conformations, which is illustrated by the different values of the torsion angles N1-O1-C4-C5 = À174.2 (2) and N2-O2-C11-C12 = 95.5 (2) . The two benzene ring planes are inclined by 17.34 (9) for C5-C10 and by 30.6 (1) for C12-C17 relative to the plane of the central heterocycle. The tetrafluoridoborate counter-ion of 2 is disordered over three orientations (occu-

Figure 2
The ion pair structure of methylsulfanyl salt 2, showing displacement ellipsoids drawn at the 50% probability level and hydrogen atoms drawn as spheres of arbitrary size. Only one of the three different orientations of the disordered BF 4 À anion is shown.

Figure 1
The ion pair structure of methylsulfanyl salt 1, showing displacement ellipsoids drawn at the 50% probability level and hydrogen atoms drawn as spheres of arbitrary size. pancy ratio 0.42:0.34:0.24), which are related by a rotation about the B1-F1 bond (Fig. 3). The heterocycle C-S bond lengths [1.722 (2) and 1.721 (3) Å for 1 and 2, respectively] determined in this study are in good agreement with the mean value (1.735 Å ) calculated from 82 pertinent C-S distances compiled in the Cambridge Structural Database (selection criterion R 1 < 0.10; Groom et al., 2016).

Database survey
In addition to the classic 1,3-dimethylimidazolium-2-methylsulfanylimidazolium iodide (Williams et al., 1994), the Cambridge Structural Database (Version 5.41 November 2019; Groom et al., 2016) comprises a number of more unusual representatives such as very bulky 1,3-diaryl-2-phenylthioimidazolium (Iné s et al., 2010) and 1,3-diaryl-2-methylsulfanylimidazolium salts (Liu et al., 2017). These compounds are suitable precursors for the generation of exotic N-heterocyclic carbene-chalcogen cations. Noteworthy is also the structure of a stabilized imidazoline-2-thione methylide (Arduengo & Burgess, 1976). The attachment of a fluorine-containing group to a given molecule may enhance certain properties and therefore widen the range of potential applications. For example, salts bearing S-CF 3 groups (Mizuta et al., 2016) have been found to be effective electrophilic phase-transfer catalysts. Additionally, the intro-duction of perfluoroalkylthio groups (Hummel et al., 2017) resulted in improved surfactant properties.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms were identified in difference maps. Methyl H atoms were idealized and included as rigid groups allowed to rotate but not tip (C-H = 0.98 Å ), and their U iso parameters were set to 1.5 U eq (C) of the parent carbon atom. H atoms bonded to secondary carbon atoms (C-H = 0.99 Å ), and H atoms bonded to C atoms in aromatic rings (C-H = 0.95 Å ) were positioned geometrically and refined with U iso set to 1.2 U eq (C) of the parent carbon atom.
The structure of 2 displays disorder of the tetrafluoridoborate ion involving three distinct components. Therefore, distance restraints were applied for all chemically equivalent B-F and FÁ Á ÁF distances and restraints on displacement parameters of the F atoms affected by disorder were applied.

1,3-Dimethoxy-2-(methylsulfanyl)imidazolium tetrafluoridoborate (1)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.26 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.

1,3-Dibenzyloxy-2-(methylsulfanyl)imidazolium tetrafluoridoborate (2)
Crystal data 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 )