TWO Zn AND Hg BROMIDE SALTS BASED ON 1-ETHYL-3-METHYL IMIDAZOLIUM IONIC LIQUID: IONOTHERMAL SYNTHESIS, STRUCTURES AND SUPRAMOLECULAR ORGANIZATION

Two Zn(II) and Hg(II) bromide salts, [EMI]2[ZnBr4] (1) and [EMI][HgBr3] (2), have been synthesized under ionothermal conditions using 1-ethyl-3-methyl imidazolium bromide ([EMI]Br) as solvents. 1 consists of tetrahedral anion [ZnBr4] and 2 consists of 1D double chain locating in the cavities surrounded by [EMI] cations. Both compounds exhibit 3D supramolecular architectures organized by the C-H···Br hydrogen bondings and alkyl-alkyl interactions.


INTRODUCTION
In 2002, ionic liquid [BMIM][BF 4 ] (BMIM = 1-butyl-3-methylimidozolium) was used as reaction media for the first time in the synthesis of metal-organic frameworks [1].Ionothermal synthesis is now flourishing in many fields, such as nanomaterials [2][3][4], framework materials [5][6][7][8], organic synthesis, electrochemistry and catalysis [9][10][11].The principal feature of ionothermal synthesis is that the ionic liquids (ILs) act as both the "designed" green solvents and the structure-template providers, which have demonstrated their potential in novel material discovery.ILs consist of a bulky organic cation and an inorganic/organic anion [12].Bulky organic cations usually are quaternary ammonium, phosphonium, pyridinium, and imidazolium [13][14][15].Thus ILs are somewhat similar to polar solvents, suitable for dissolving the inorganic components required for the synthesis process [16][17].ILs have many unique properties: low melting point (< 100 o C), extremely low volatility, chemical and thermal stability, nonflammability, high ionic conductivity, high heat capacity, high thermal conductivity, and wide electrochemical potential window [18][19].For example, negligible vapor pressure will eliminate the safety concerns associated with high self-engendered pressures and lead to the integration with microwave synthesis [20].Compared with hydro(solvo)thermal synthesis, no other solvents added to the reaction system as space fillers, no competition in ionothermal synthesis between template-framework and solvent-framework interactions will offer significantly different reaction environment [21].Despite the difficulties in crystallization, we used 1-ethyl-3-methyl imidazolium bromide ([EMI]Br) as starting material reacting with Zn(NO 3 ) 2 /Hg(NO 3 ) 2 to obtain two new d 10

Materials and physical measurements
The reagents and solvents were used directly as supplied commercially without further purification except [EMI]Br.[EMI]Br was synthesized from the reaction of ethylbromide with 1-methylimidazole according to literature processes [22][23].The IR spectra were recorded on a Nicolet Magna 750 FT-IR spectrometer with KBr pellet in the range 4000-400 cm -1 .Elemental analyses of C, H and N were carried out on a Vario EL III elemental analyzer.The melting point of [EMI]Br was measured at X-4 microscopic melting point determinator.

Synthesis of 1-ethyl-3-methylimidazolium bromide
Under inert nitrogen atmosphere conditions, 100 mL degassed bromoethane (146.0 g, 1.34 mol) was added to 35.7 mL redistilled 1-methylimidazole (37.0 g, 0.45 mol) with constant stirring.The mixture was refluxed at 40 °C for 3 h and then cooled to room temperature.A pale yellow oil was produced.100 mL ethyl acetate was added, and the product crashed out of solution.The product was washed with ethyl acetate, and dried under a vacuum at 25 °C for one day to give 1-ethyl-3-methylimidazolium bromide as a white solid (m.p. = 80-83 o C, yield: 85%).

Structural determination and refinement
Data collections of compounds 1 and 2 were performed on Rigaku Mercury CCD diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073 Å).Intensity data were collected by the narrow frame method at 293 K and corrected for Lorentz and polarization effects as well as for absorption by the ω scan technique and were reduced using CrystalClear program [24].The structures were solved by direct methods using SHELXTL TM package of crystallographic software and refined by full-matrix least-squares technique on F 2 [25].All nonhydrogen atoms were refined with anisotropic thermal parameters.Hydrogen atoms attached to C atoms were located at geometrically calculated positions and refined with isotropic thermal parameters included in the final stage of the refinement on calculated positions bonded to their carrier atoms.Because of bad crystal quality of 1, the not so good crystal diffraction data results in a large final wR 2 (0.1530).A summary of the structural determinations and refinements for 1 and 2 is listed in Table 1.Selected bond distances and angles of 1 and 2 are shown in Table 2

Structural description of [EMI][HgBr 3 ] (2)
The structural analysis indicates that compound 2 features a 1D inorganic chain anchoring in the channels made up of imidazolium cations.Compound 2 crystallizes in orthorhombic chiral space group P2  3), the 3D supramolecular architecture is formed and further stabilized (Figure 4a).The 1D double chains are surrounded by [EMI] + cations, thus to form channels of 5.90×5.75Ǻ 2 with [EMI] + cations as walls, which are fully occupied by the 1D chains (Figure 4b).Compounds 1 and 2 were obtained from similar ionothermal processes and derived from the same [EMI]Br ionic liquid.Compound 1 is to some degree like the reported [bpyr][AlCl 4 ] (bpyr = 1-butylpyridinium) [26] and Compound 2 has the same structure as [EMI]PbBr 3 [26], but contains richer interactions.Observing this type of the compounds [26,28,[29][30][31][32], we can find a phenomenon that [organic cation]X (X = halide or halide-containing anions) reacts with metal salts, specially the metal halides, X -tends to change into larger metal-containing polyatomic anion.This can be demonstrated in the compounds in this work and those reported in the literatures [26,31,32].

Figure 2 .
Figure 2. (a) The 3D supramolecular architecture in 1; (b) the channels along the a-direction in the 3D supramolecular architecture.

Figure 4 .
Figure 4. (a) The 3D supramolecular architecture in 2; (b) the channels along the a-direction in the 3D supramolecular architecture.
For example, [Im]Cl with FeCl 3 can produce [Im][FeCl 4 ] (Im = imidazolium) [32], [bpyr][AlCl 4 ] with V 2 O 5 to be [bpyr] 4 [V 4 O 4 Cl 12 ] [28] and [EMI][AlCl 4 ] with KZr 6 CCl 15 to be [EMI] 4 [Zr 6 CCl 18 ][33].The larger anions should be possible to promote the structural stability with the presence of the large organic cations.The metal-containing anions play a key role in the formations of the final 3D supramolecular architectures of 1, 2 and [EMI]PbBr 3[26].The anions result in the difference in the hydrogen bondings between the inorganic blocks and [EMI] + cations.Compound 1 has only two types of C-H•••Br hydrogen bondings per [EMI] + , both through the alkyl group in the imidazolium ring, none from the alkyl group of side chains.While in 2, more C-H•••Br hydrogen bondings exist around [EMI] + ; contrarily, all four types of C-H•••Br hydrogen bondings appear through the alkyl side chains, none from imidazolium ring.The alkyl-alkyl interactions in both compounds are very similar, showing little effect of anions on the [EMI] + itself.More interactions also induce the difference in sizes of cavities.The cavities in 2 are supposed to be bigger than those in 1, due to the bigger inorganic anions.Actually, the cavities in 1 are a little larger than those in 2, which reasonably relates with the interactions.CONCLUSIONSOur attempt in the investigation of alkyl-imidazolium ILs produced two d 10 dialkylimidazolium bromide salts, [EMI] 2 [ZnBr 4 ] (1) and [EMI][HgBr 3 ] (2), from ionothermal reactions with 1ethyl-3-methyl imidazolium bromide as solvents.1 consists of tetrahedral anion [ZnBr 4 ] 2− and 2 consists of 1D double chain, both locating in the cavities surrounded by [EMI] + cations.Compounds 1 and 2 are 3D supramolecular architectures based on the connections of C-H•••Br and alkyl-alkyl interactions.The structural analysis in C-H•••Br and alkyl-alkyl interactions, and cavity sizes suggests the anions in 1 and 2 play an important role in their 3D supramolecular structures.

Table 1 .
. Crystal data and structure refinement parameters for 1 and 2.