Bis(1,3-dimethyl-1H-imidazolium) hexafluorosilicate: the second monoclinic polymorph

The title compound, 2C5H9N2 +·SiF6 2−, (I), crystallized as a new polymorph, different from the previously reported one (Ia) [Light et al. (2007 ▶) private communication (refcode: NIQFAV). CCDC, Cambridge, England]. The symmetry [space groups P21/n for (I) and C2/c for(Ia)] and crystal packing patterns are markedly different for this pair of polymorphs. In (I), all imidazolium cations in the lattice are nearly parallel to each other, whereas a herringbone arrangement can be found in (Ia). In (I), each SiF6 2– dianion forms four short C—H⋯F contacts with adjacent C5H9N2 + cations, resulting in the formation of layers parallel to the ac plane. In (Ia), the C—H⋯F contacts are generally longer and result in the formation of layers along the bc plane.

The title compound, 2C 5 H 9 N 2 + ÁSiF 6 2À , (I), crystallized as a new polymorph, different from the previously reported one (Ia) [Light et al. (2007) private communication (refcode: NIQFAV). CCDC, Cambridge, England]. The symmetry [space groups P2 1 /n for (I) and C2/c for(Ia)] and crystal packing patterns are markedly different for this pair of polymorphs. In (I), all imidazolium cations in the lattice are nearly parallel to each other, whereas a herringbone arrangement can be found in (Ia). In (I), each SiF 6 2dianion forms four short C-HÁ Á ÁF contacts with adjacent C 5 H 9 N 2 + cations, resulting in the formation of layers parallel to the ac plane. In (Ia), the C-HÁ Á ÁF contacts are generally longer and result in the formation of layers along the bc plane.

Experimental
Recently, being interested in preparation of a variety of sterically non-hindered 1,3-dialkyl-1H-imidazolium salts with main-group element perfluorato anions as potential precursors of Arduengo carbene adducts with the Group 13-15 element fluorides (Tian et al., 2012), we analyzed materials obtained by re-crystallization of crude bis(1,3-dimethyl-1Himidazolium) hexafluorosilicate, [C 5 H 9 N 2 + ] 2 [SiF 6 2-], from either ethanol or methanol solutions. While crystallization from ethanol afforded only the solvent-free [C 5 H 9 N 2 + ] 2 [SiF 6 2-], (I), the material obtained from methanol presented both (I) (crystals grow on the walls of a vessel above the solution surface during its gradual evaporation into air) and its adduct with methanol, {[C 5 H 9 N 2 + ] 2 [SiF 6 2-]} 3 (CH 3 OH), (II), crystals of which grow at the bottom of a vessel under the layer of the mother liquor. In the latter case, single crystals of (I) and (II) could be easily separated manually. Identity of the single crystals of (I) prepared from EtOH and MeOH was proved by the unit cell measurements. Due to a better quality of the sample of (I) grown from ethanol, only these data are provided and will be referred to further in the discussion.
The longer contacts in (Ia) also form non-interconnected layers in its crystal lattice. The neighbour pairs of layers in (Ia) are connected by the C-centering translation, C 2 rotation, n-glide reflection, and inversion operations. Moreover, these layers belong to the same layer group as it is observed in (I; p2 1 /b11), with its a′ and b′ parameters being comparable [9.7901 (9) and 12.0377 (9) Å for (I) and 11.988 and 11.258 Å for (Ia); see Fig. 3; priming is used to distinguish between unit cell axes a and b and layer-related axes a′ and b′]. Distances between adjacent inversion-related pairs of imidazolium rings are also close (the interplane distances are 3.418 (2) and 3.449 Å in (I) and (Ia), respectively]. Fig. 3 also illustrates that the layers within (I) and (Ia) can be converted one into another by a diffusionless transformation which can be best described as continuous rotations of the inversion-related pairs of imidazolium cations and SiF 6 2groups around the corresponding centers accompanied with dilations/contractions of the layers along the a′ and b′ directions.
Transformation of the entire lattice of (Ia) into that of (I) also requires a mutual (0, 1/2, 0) shear of adjacent layers in the b-direction what results in vanishing of centering translations and conversion of C 2 rotations into 2 1 screw ones. Being a continuous transformation, such a layer shift, however, can not be classified as a "diffusionless" one. Thus, any direct first-order phase transition between (I) and (Ia) is hardly believable. Unfortunately, the lack of the information about the sample crystal of (Ia) [in the corresponding Private communication to the CCDC (Light et al., 2007), no data on the crystallization conditions are provided] does not allow us to outline the actual reasons of the (I)/(Ia) polymorphism.

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. Concentration of the filtrate till dryness followed by re-crystallization from ethanol gave (I) in an almost quantitative yield. If methanol is used as a solvent, crystals of both (I) and (II) are formed. Single crystals of (I) suitable for X-ray diffraction analysis were picked up directly from the material (when methanol was used as a solvent, the crystals located on the vessel walls above the solution surface were selected). Identity of the single crystals of (I) 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
All non-H atoms were refined anisotropically. All H-atoms were found from the difference Fourier synthesis and refined isotropically.

Bis(1,3-dimethyl-1H-imidazolium) hexafluorosilicate
Crystal data   (7) Special details Experimental. A very tight closeness of the β-angle to 90° presented a certain difficulty in determination of the actual crystal system and the space group for (I). 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 > 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.  (7) 0.0036 (6) −0.0050 (7) Geometric parameters (Å, º)