Crystal structure of the tetragonal polymorph of bis(1-ethyl-3-methylimidazolium) tetrabromidocadmate

The title structure represents the tetragonal polymorph (the other known structure being monoclinic) and is isotypic with its [MBr4] analogues (M = Co, Ni, Zn).

Both unique Cd atoms in the tetragonal polymorph of bis(1-ethyl-3-methylimidazolium) tetrabromidocadmate, (C 6 H 11 N 2 ) 2 [CdBr 4 ], occupy special positions (site symmetry 4). The crystal structure consists of isolated tetrahedral [CdBr 4 ] 2À anions which are surrounded by 1-ethyl-3-methylimidazolium cations. The methyl and ethyl side chains of the cations show positional disorder in a 0.590 (11):0.410 (11) ratio. In the crystal, (C 6 H 11 N 2 ) + cations display three weak C-HÁ Á ÁBr hydrogen-bond interactions through the imidazolium ring H atoms with the Br À ligands of the surrounding complex anions. The alkyl groups of the side chains are not involved in hydrogen bonding.

Chemical context
Laboratories around the world have used ionic liquids to prepare many different types of solids, ranging from nanoparticles of different types, to semiconductors, and inorganic and organic solids (Morris, 2009). In an attempted synthesis of mineral-related arsenates, the ionic liquid 1-ethyl-3-methylimidazolium bromide (eminBr), C 6 H 11 BrN 2 , was tested as a solvent and template. C 6 H 11 BrN 2 has a wide liquid range (despite being a solid at room temperature, with a melting point of 356 K), low vapour pressure and has been used extensively for ionothermal synthesis because it is a relatively polar solvent.
The title compound, (C 6 H 11 N 2 ) 2 [CdBr 4 ], was obtained under ionothermal conditions using eminBr as the solvate. The SEM-EDS study of the title compound showed small amounts of a cadmium-manganese arsenate in the form of small needle-like crystals up to maximal 15 mm on the top of the plate-like crystals of the title compound (Fig. 1). This phase is present in very small amounts and therefore could not be identified using powder or single-crystal X-ray diffraction techniques. The powder pattern indicated the tetragonal polymorph of the title compound as the main phase and the monoclinic polymorph (Gou et al., 2016) as a minority phase.

Structural commentary
Emim, C 6 H 11 N 2 + , cations together with [CdBr 4 ] 2À anions as discrete tetrahedra are the main structural building units (Fig. 2). The imidazolium ring is, as expected, a planar, slightly distorted pentagon. The deviation of the ring atoms from the least-squares plane is smaller than 0.006 (7) Å . The bond lengths of 1.356 (8) and 1.297 (7) Å for the N1-C1 and C1-N2 bonds, respectively, indicate conjugated double-bond character, having one bond slightly longer than the usual C N double-bond length, 1.27 Å . The N1-C2 and N2-C3 bond lengths [1.360 (7) and 1.359 (8) Å ] are shorter than a typical C-N single bond (1.472 AE 6 Å ) and close to the shortened (partial double bond) in heterocyclic systems, 1.352 AE 5 Å , while the bond length of 1.373 (9) Å for C2-C3 is slightly longer than a typical C C double bond of 1.337 AE 6 Å (Macgillavry & Rieck, 1968). The alkyl groups of the side chains showed strong anisotropic atomic displacements during refinement, suggesting a statistical positional disorder that was taken into account for the final model (Fig. 2). The carbon atoms C4, C5, C6 and C7 from the disordered alkyl groups of side chains are also planar and the largest deviation from the least-squares plane through the imidazolium ring atoms is 0.163 (16) Å for C7 and À0.949 (19) Å for C6, while C5 and C4 are just À0.013 (1) and 0.039 (1) Å , respectively, out of plane.
Both unique Cd atoms occupy special positions (on a fourfold rotoinversion axis parallel to the c axis, site symmetry 4). Consequently both tetrabromidocadmate anions possess crystallographically imposed 4 symmetry and therefore, each Cd atom bonds to four symmetry-related Br atoms (Fig. 2). The Cd1-Br1 bond length of 2.5745 (6) Å in the almost regular tetrahedral configuration of the [Cd1Br 4 ] 2À anion is slightly shorter than 2.5806 (5) Å for the [Cd2Br 4 ] 2À anion. The Br-Cd-Br bond angles are 109.14 (3) and 109.64 (2) in [Cd1Br 4 ] 2À but 107.88 (1) and 112.71 (3) in the slightly more distorted [Cd2Br 4 ] 2anion. The angular range for both anions is comparable with those reported by Sharma et al. (2006).  A view of the molecular entities in the structure of (C 6 H 11 N 2 ) 2 [CdBr 4 ]. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented as small spheres of arbitrary radius. C-HÁ Á ÁBr hydrogen-bonding interactions are shown with dashed blue lines. Disordered alkyl groups are distinguished by solid and dotted bonds, together with the C and H atoms being shown in different colours.
The title compound shows characteristic bands of the imidazolium ring and the alkyl chains (Barbara, 2004;Nakamoto, 1978) (Fig. 3). The bands at 3134 and 3101 cm À1 can be attributed to aromatic C-H stretching (Tait & Osteryoung, 1984). Their relatively low values confirm the presence of weak hydrogen bonds. A higher wave number would indicate a diminution or absence of hydrogen bonds (Larsen et al., 2000). The band at 2985 cm À1 can be attributed to aliphatic C-H stretching (Tait & Osteryoung, 1984); aliphatic C-H bending vibrations [(CH 2 ), (CH 3 ), as (CH 3 )] are located between 1470 and 1380 cm À1 (Katsyuba et al., 2004) and mostly represented by the band at 1460 cm À1 . The band at 1578 cm À1 is assigned to the C C and C-N stretching vibrations of the imidazolium ring. Bands centred at 1342 and 1162 cm À1 , respectively, represent the stretching vibrations between the alkyl chains and N atoms (Katsyuba et al., 2004). All bands below 850 cm À1 can be attributed to the out-ofplane vibrations of the imidazolium cation (Katsyuba et al., 2004). The most intense bands are located at 854, 775 and 621 cm À1 . Even if there is no water in the structure of (C 6 H 11 N 2 ) 2 [CdBr 4 ], O-H vibrations may still be present because of the hygroscopic character of the ionic liquid.

Supramolecular features
There are no significant interactions between [Cd2Br 4 ] 2anions, except a short Br1Á Á ÁBr1 contact which amounts to 3.764 (2) Å . The crystal packing of the cations and anions in a three-dimensional network is realized through C-HÁ Á ÁBr interactions (Figs. 2 and 4, Table 1) involving the imidazolium ring H atoms (H1, H2 and H3), but not the H atoms of the alkyl side chains. Larsen et al. (2000) found that the imidazolium cation is often disordered whereby the disorder can take many different forms. They also have found that positional disorder of the cations in their crystal structures is a direct indicator of packing inefficiency, i.e. packing inefficiency becomes reflected in disorder when cation/anion interactions are reduced essentially to the level of van der Waals or very weak hydrogen-bonding-type forces. The resulting network in the title structure has a channel structure defined by the organization of the imidazolium cations, with the [CdBr 4 ] 2anions residing in the channels (Fig. 5). The packing of the structure of (C 6 H 11 N 2 ) 2 [CdBr 4 ], viewed down the a axis, showing the tetrahedral [CdBr 4 ] 2À anions linked to the emim, [C 6 H 11 N 2 ] + , cations by hydrogen-bonding interactions. C and N atoms are presented as black and blue spheres, respectively, and H atoms as grey small spheres. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1 2 ; Ày þ 3 2 ; Àz þ 1 2 ; (ii) Ày þ 3 4 ; x þ 1 4 ; z þ 1 4 .

Figure 5
The projection of the structure of (C 6 H 11 N 2 ) 2 [CdBr 4 ], viewed down the c axis, normal to the channels formed by the supramolecular organization of the imidazolium cations.

Synthesis and crystallization
A 1 g mixture of CdO, Mn(NO 3 ) 2 ÁH 2 O, As 2 O 5 in the molar ratio 2:2:1 was mixed with 2 g of molten emimBr and placed in a teflon container into a steel autoclave. A heating regime with three steps was chosen: the autoclaves were heated from 293 to 493 K (four h), held at 493 K for 72 h, and finally cooled to room temperature within 99 h. The obtained products were washed with ethanol, filtered and dried in the air at room temperature. The title compound crystallized as leafy-like crystals (yield ca 85%) together with crystals of the monoclinic polymorph (yield ca 10%) and small amounts of a yet unidentified Cd/Mn-arsenate (single-crystal size 10 mm). The crystals of tetragonal (C 6 H 11 N 2 ) 2 [CdBr 4 ]) are no longer than 0.15 mm in length.

Bis(1-ethyl-3-methylimidazolium) tetrabromidocadmate(II)
Crystal data (C 6  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 > 2sigma(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 )