Bis(1,3-dimethyl-1H-imidazolium) hexafluorosilicate methanol 0.33-solvate

The title compound, 6C5H9N2 +·3SiF6 2−·CH3OH, (I), was prepared by recrystallization of the crude salt from methanol along with solvent-free 2C5H9N2 +·SiF6 2− (II). Crystals of these solvatomorphs can be separated manually. The solvate (I) crystallizes in a rare hexagonal space group P6/mcc. Its asymmetric unit comprises one half of an imidazolium cation bisected by the crystallographic m-plane, one-sixth and one-twelfth of two crystallographically independent SiF6 2– dianions (Si atoms are located on the 3.2 and 6/m inversion centres), and one-twelfth of a methanol molecule (C atoms are situated on the 622 inversion centres, other atoms are disordered between general positions). In (I), all F atoms of 3.2-located SiF6 2– dianions participate in the formation of symmetry-equivalent contacts to the H atoms of imidazolium fragments, thus forming rod-type ensembles positioned on the -6 axes. These ‘pillar’ rods are, in turn, F⋯H interlinked through SiF6 2– dianions disordered around the 6/m centres. The twelvefold disordered methanol molecules are appended to this array by O—H⋯F hydrogen bonds to the 6/m located SiF6 2– dianions. In terms of graph-set notation, the first and second level networks in (I) are N 1 = C 2 2(7)[3R 4 4(14)]D 2 2(4) and N 2 = D 2 2(5) (C—H⋯O hydrogen bonds are not considered). After locating all symmetrically independent atoms in the cation and anions, there remained a strong (> 3 e Å−3) residual electron density peak located at the 622 inversion centre. Treatment of this pre-refined model with the SQUEEZE procedure in PLATON [Spek (2009). Acta Cryst. D65, 148–155] revealed two voids per unit cell, indicative of the presence of the solvent methanol molecule disordered about the 622 inversion centre.

The title compound, 6C 5 H 9 N 2 + Á3SiF 6 2À ÁCH 3 OH, (I), was prepared by recrystallization of the crude salt from methanol along with solvent-free 2C 5 H 9 N 2 + ÁSiF 6 2À (II). Crystals of these solvatomorphs can be separated manually. The solvate (I) crystallizes in a rare hexagonal space group P6/mcc. Its asymmetric unit comprises one half of an imidazolium cation bisected by the crystallographic m-plane, one-sixth and onetwelfth of two crystallographically independent SiF 6 2dianions (Si atoms are located on the 3.2 and 6/m inversion centres), and one-twelfth of a methanol molecule (C atoms are situated on the 622 inversion centres, other atoms are disordered between general positions). In (I), all F atoms of 3.2-located SiF 6 2dianions participate in the formation of symmetry-equivalent contacts to the H atoms of imidazolium fragments, thus forming rod-type ensembles positioned on the 6 axes. These 'pillar' rods are, in turn, FÁ Á ÁH interlinked through SiF 6 2dianions disordered around the 6/m centres. The twelvefold disordered methanol molecules are appended to this array by O-HÁ Á ÁF hydrogen bonds to the 6/m located SiF 6 2dianions. In terms of graph-set notation, the first and second level networks in (I) are N 1 = C 2 2 (7)[3R 4 4 (14)]D 2 2 (4) and N 2 = D 2 2 (5) (C-HÁ Á ÁO hydrogen bonds are not considered). After locating all symmetrically independent atoms in the cation and anions, there remained a strong (> 3 e Å À3 ) residual electron density peak located at the 622 inversion centre. Treatment of this pre-refined model with the SQUEEZE procedure in PLATON [Spek (2009). Acta Cryst. D65, 148-155] revealed two voids per unit cell, indicative of the presence of the solvent methanol molecule disordered about the 622 inversion centre.

Related literature
For solvatomorphs of (I), see: Light et al. (2007); Tian et al. (2013). For solvatomorphism of (1,3-dimethyl-1H-imidazolium) hexafluorophosphate, C 5 H 9 N 2 + ÁPF 6 À , see: Holbrey et al. (2003). For the practical utility of sterically non-hindered 1,3dialkyl-1H-imidazolium salts with BF 4 À and PF 6 À anions for the preparation of Arduengo carbene adducts with BF 3 and PF 5 , see: Tian et al. (2012). For graph-set notation, see: Etter et al. (1990);Bernstein et al. (1995);Grell et al. (1999) Table 1 Hydrogen-bond geometry (Å , ). vessel under the layer of the mother liquor and the solvent-free salt, [C 5 H 9 N 2 + ] 2 [SiF 6 2-], (II) (crystals grew on the walls of a vessel above the solution surface during its gradual evaporation into air). Single crystals of (I) and (II) could be easily separated manually. Interestingly, crystallization from ethanol afforded only the solvent-free (II). Identity of the single crystals of (I) prepared from EtOH and MeOH was proved by the unit cell measurements. Details of the structural investigation of (II) can be found in a parallel publication (Tian et al., 2013). In crystal lattice of (I), O-H···F hydrogen bonds and an extended three-dimensional-network of C-H···F contacts are both present (see Table 1). All F-atoms of a 3.2-located [SiF 6 2-] dianion participate in C2-H2···F2 xxii and symmetryrelated contacts [symmetry code: (xxii) -x + 1, -y + 1, z]. These contacts are responsible for formation of rod-type ensembles positioned on the 6 axes which present the "pillar" elements of the entire lattice (see Fig. 2a). In their turn, these rods are F···H interlinked through [SiF 6 2-] dianions disordered around the 6/m centres. Each 6/m located dianion links six different rod-type structures (see Fig. 2 b). The vacancies at the 622 inversion centres are occupied by twelvefold disordered methanol molecules which are appended to the entire array by O1-H1A···F1 H-bonds (see Fig. 2c). The entire packing diagram is provided in Fig. 3.
Crystals of (I) exhibit remarkable stability in air. They do not loose methanol even if evacuated at ambient temperature during a prolonged time (296 K, 0.133 Pa, 5 h). However, when gradually heated in air in a microscopic melting point apparatus, crystals of (I) behave themselves unlike crystals of solvent-free (II) do [m.p. 550 K for (II); Tian et al. (2013)].
Within the temperature range of 463-466 K, crystals of (I) start to break down to a powder material which further melts at the same temperature as compound (II) does. These empirical observations explicitly point out the fact that interconversions between the solvatomorphs (I) and (II) are impossible without complete destruction of their crystal lattices.

Experimental
Crude 1,3-dimethyl-1H-imidazolium hexafluorosilicate was prepared by a reaction of 1,3-dimethyl-1H-imidazolium iodide and disilver hexafluorosilicate (molar ratio 2:1) in distilled water. Concentrating of the filtrate till dryness followed by re-crystallization from methanol gave both (I) and (II) in an over-all almost quantitative yield. If ethanol is used for recrystallization, only crystals of (II) are formed. A single crystal of (I) suitable for X-ray diffraction analysis was picked up directly from the material from the bottom of the crystallization vessel. Identity of the single crystals of (II) grown from EtOH and MeOH was proved by unit cell measurements. Melting point measurements were performed with a Microscopic Melting Point X4 apparatus (Beijing MAISIQI High-Tech Co., Ltd.)

Refinement
The straightforward solution of the structure of (I) in an actual centrosymmetric space group P6/mcc suggested by

sup-4
Acta Cryst. (2013). E69, o1216-o1217  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 > σ(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.