Crown-ether enclosure generated by ionic liquid components—synthesis, crystal structure and Raman spectra of compounds of imidazolium based salts and 18-crown-6

Abstract

Bis(1,3-dimethylimidazolium methylsulfate) [18-crown-6] (1), ([DMIm]₂[CH₃SO₄]₂[18-crown-6], bis(1-butyl-3-methylimidazolium methylsulfate) [18-crown-6] (2), ([BMIm]₂[CH₃SO₄]₂[18-crown-6]), bis(1-ethyl-3-methylimidazolium methanesulfonate) [18-crown-6] (3), ([EMIm]₂ [CH₃SO₃]₂[18-crown-6]) and bis(1-ethyl-3-methylimidazolium trifluoromethanesulfonate) [18-crown-6] (4), ([EMIm]₂[CF₃SO₃]₂[18-crown-6]) were prepared and characterized by single-crystal X-ray diffraction, NMR and Raman spectroscopy. Coulomb interactions between the ionic (liquid) components as well as hydrogen bonding are important. No significant close contacts are observed between the ionic components and the 18-crown-6 molecules. Crystal structures of all compounds consist of alternated layers of crown-ether molecules and respective ionic units compounds. Within the layers of the ionic components (compounds 1, 3 and 4), two cations are linked to each other by C–H⋯π interactions between one methyl carbon and the imidazolium ring of another cation.

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Introduction

Room temperature ionic liquids (ILs) have been an area of interest for fundamental reasons associated with their negligible vapour pressure, good electrical conductivity, high ionic mobility, excellent thermal and chemical stabilities and solvent capability.^1,2 These materials have been used as media for clean liquid–liquid extraction processes,³catalysts for organic⁴ and organometallic synthesis,⁵polymerisation and radical polymerisation,⁶ and as media for analytical and physical chemistry.⁷ Some of them possess liquid crystal⁸ and lubricant properties⁹ and can behave as sensors.¹⁰ One of the barriers to these applications is the relatively high viscosity compared to those of classical organic solvents.¹¹ After the discovery of the first air- and water-stable ionic liquid compounds such as [EMIm]BF₄ and [EMIm][CF₃SO₃], which are reported by Wilkes and Zaworotko, and Cooper and O'Sullivan,¹² respectively, much effort has been devoted to the basic understanding of their structure properties; especially those composed of 1-alkyl-3-methylimidazolium cation.^13–15

In this work, we report the co-crystallisation of imidazolium based salts with 18-crown-6. Comparable (inclusion) compounds have been synthesized very recently by Bates, Gale and coworkers^16–18 out of imidazolium salts and calixarene derivatives. These compounds show interesting structural architectures as well as the capacity to act as (selective) ion-pair receptors. Different kinds of interactions can also be studied thereby. Generally the complex formation of organic molecules via weak non-covalent interactions such as hydrogen bonds, π-stacking, charge-transfer interaction and electrostatic interaction is of great importance.¹⁹ These interactions and charge transfers have received increasing attention recently.²⁰ The investigated compounds were characterised by X-ray analysis, Raman spectroscopy and NMR.

Results and discussions

All the studied imidazolium based salts formed compounds with 18-crown-6 (Scheme 1).

Scheme 1 Molecular structure of 1-alkyl-3-methylimidazolium cation (DMIm: R = methyl, BMIm: R = n-butyl and EMIm: R = ethyl).

Four low melting alkylimidazolium salts, bis(1,3-dimethyl-imidazolium methylsulfate) [18-crown-6] (1), bis(1-butyl-3-methylimidazolium methylsulfate) [18-crown-6] (2), bis(1-ethyl-3-methylimidazolium methanesulfonate) [18-crown-6] (3), and bis(1-ethyl-3-methylimidazolium trifluoromethane-sulfonate)[18-crown-6] (4) have been prepared and their single-crystal structures determined.

All compounds consist of alternating layers of crown-ether molecules and the respective ionic components [DMIm]⁺, [CH₃SO₄]^– for 1, [BMIm]⁺, [CH₃SO₄]^– for 2, [EMIm]⁺, [CH₃SO₃]^– for 3 and [EMIm]⁺, [CF₃SO₃]^– for 4. The layers of crown ether are surrounded above and below from methyl groups of imidazolium with a distance of 1.976 Å for 1, 2.001 Å for 2, 2.011 Å for 3 and 2.090 Å for 4. The crystallographic data and details of the single crystal structure determinations are given in Table 1. Selected interatomic distances and angles for compounds 1–4 are collected in Table 2.

Table 1 Crystal data and refinement parameters for 1, 2, 3 and 4

	1	2	3	4
a R = Σ| |Fo| – |Fc| |/Σ||Fo|. wR2 = {Σ[w(|Fo|^2 – |Fc|^2)^2]/Σ[w(|Fo|^2)^2]}^1^/2.
Empirical formula	C24H48N4O14S2	C30H60N4O14S2	C26H52N4O12S2	C26H46F6N4O12S2
Formula weight/g mol^–1	680.78	764.94	676.84	784.79
Temperature/K	223	223	223	223
Wavelength/Å	0.71073	0.71073	0.71073	0.71073
Crystal system	Triclinic	Monoclinic	Triclinic	Triclinic
Space group	P1̄ (no. 2)	P21/n (no. 14)	P1̄ (no. 2)	P1̄ (no. 2)
a/Å	8.551(2)	9.123(11)	8.751(5)	8.946(2)
b/Å	9.298(2)	16.188(1)	9.126(4)	9.165(2)
c/Å	10.774(3)	13.939(2)	11.357(6)	11.469(3)
α/°	86.93(2)		97.26(4)	92.60(2)
β /°	80.09(2)	98.03(1)	97.99(4)	98.72(2)
γ/°	82.06(2)		97.76(4)	97.86(2)
Volume/Å^3	835.3(3)	2038.2(4)	880.1(7)	918.6(4)
Z	1	2	1	1
Density (calc.)/g cm^–3	1.353	1.246	1.277	1.419
Abs. coefficient/mm^–1	0.228	0.194	0.212	0.236
F(000)	364	824	364	412
θ min, max/°	2.21, 25.02	1.94, 25.02	2.25, 25.03	2.25, 25.02
Index ranges	–9 ≤ h ≤ 10	–10 ≤ h ≤ 10	–10 ≤ h ≤ 9	–10 ≤ h ≤ 9
	–11 ≤ k ≤ 11	–19 ≤ k ≤ 18	–10 ≤ k ≤ 10	–10 ≤ k ≤ 10
	–12 ≤ l ≤ 12	–16 ≤ l ≤ 15	–13 ≤ l ≤ 13	–13 ≤ l ≤ 13
Unique reflections	2903	3598	3085	3202
Data/restrains/parameters	2903/0/293	3598/0/336	3085/0/303	3202/0/319
Goodness-of-fit on F^2	1.030	1.090	1.038	1.080
R indexes [I > 2σI]	R1 = 0.0621	R1 = 0.0612	R1 = 0.0725	R1 = 0.0612
	wR2 = 0.1542	wR2 = 0.1484	wR2 = 0.1454	wR2 = 0.1080
R indexes (all data)	R1 = 0.0845	R1 = 0.0779	R1 = 0.1172	R1 = 0.1016
	wR2 = 0.1682	wR2 = 0.1591	wR2 = 0.1638	wR2 = 0.1217
Largest diff. peak and hole/e Å^3	0.576 and –0.295	0.418 and –0.289	0.365 and –0.261	0.261 and –0.232

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Table 2 Selected bond lengths (Å) and angles (°) for compounds 1, 2, 3 and 4

Compound 1
S–O(1)	1.397(4)	O(1)–S–O(2)	116.2(3)
S–O(2)	1.406(4)	O(1)–S–O(3)	111.3(3)
S–O(3)	1.436(4)	O(1)–S–O(4)	107.3(3)
S–O(4)	1.573(3)	O(2)–S–O(3)	110.0(3)
O(4)–C(8)	1.470(7)	C(8)–O(4)–S	114.0(4)
Compound 2
S–O(1)	1.400(3)	O(1)–S–O(2)	113.3(2)
S–O(2)	1.400(3)	O(1)–S–O(3)	113.4(2)
S–O(3)	1.434(3)	O(1)–S–O(4)	107.0(2)
S–O(4)	1.565(3)	O(2)–S–O(3)	112.7(2)
O(4)–C(11)	1.466(9)	C(11)–O(4)–S	114.2(5)
Compound 3
S–O(1)	1.429(4)	O(1)–S–O(2)	112.6(3)
S–O(2)	1.432(4)	O(1)–S–O(3)	110.3(3)
S–O(3)	1.440(4)	O(2)–S–O(3)	113.9(3)
S–C(9)	1.749(6)	O(1)–S–C(9)	106.9(3)
		O(2)–S–C(9)	106.8(3)
		O(3)–S–C(9)	105.6(3)
Compound 4
S–O(1)	1.406(3)	O(1)–S–O(2)	115.9(3)
S–O(2)	1.428(3)	O(1)–S–O(3)	115.0(3)
S–O(3)	1.430(3)	O(2)–S–O(3)	113.1(2)
S–C(9)	1.807(4)	O(1)–S–C(9)	104.0(2)
F(1)–C(9)	1.314(5)	O(2)–S–C(9)	103.4(2)
F(2)–C(9)	1.308(5)	O(3)–S–C(9)	103.3(2)
F(3)–C(9)	1.297(5)	F(1)–C(9)–S	112.0(3)
		F(2)–C(9)–S	111.3(3)
		F(3)–C(9)–S	116.6(3)

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Crystal structures

Bis(1, 3-dimethylimidazolium methylsulfate) [18-crown-6] (1) crystallizes in the triclinic P1̄ space group, consisting of alternating layers of crown-ether molecules and the respective ionic components [DMIm]⁺ and [CH₃SO₄]^– (Fig. 1).

Fig. 1 Perspective view of the unit cell of compound 1 along the c-axis and H atoms of crown ethers have been omitted for clarity.

Within the layer of the ionic components, two cations are linked to each other by C–H⋯π interactions between one methyl carbon C(7) and the imidazolium ring of another cation (Fig. 2). The distance between the methyl carbon and the center of the imidazolium ring is estimated to be 3.682 Å, which is in excellent agreement with the values expected for the lowest benzene⋯methane interactions.²¹ The only significant hydrogen bonding are contacts between the two (acidic) ring-hydrogens C(2) and C(5), and the two terminal oxygen atoms of the sulfate anion. Each imidazolium cation participates in two hydrogen bonds, forming a chain with C–H⋯O distances 2.351 and 2.730 Å and C–H⋯O angles 150.0 and 175.8° (Table 3). The hydrogen bonding and the C–H⋯π stacking contacts help stabilize the extended crystal lattice and allow a close packing in the crystal structure.

Fig. 2 Interionic contacts between [DMIm]⁺ cation and [CH₃SO₄]^– anion in compound 1; hydrogen bonding and C–H⋯π interactions are shown by dotted lines.

Table 3 Hydrogen-bond distances (Å) and angles (°) for compounds 1, 2, 3 and 4

Donor–H⋯Acceptor	D–H/Å	H⋯A/Å	D⋯A/Å	D–H⋯A/°
a x, y – 1, z. b x–1/2, –y + 1/2, z + 1/2. c x – 1, y, z. d –x + 1, –y + 1, –z + 2. e –x + 1, –y, –z + 2. f x, y + 1, z.
Compound 1
C(2)–H(2)⋯O(2)	0.824	2.738	3.476	150.0
C(5)–H(5)⋯O(1)^a	0.808	2.351	3.157	175.8
Compound 2
C(2)–H(2)⋯O(4)	0.918	2.771	3.620	154.4
C(4)–H(4)⋯O(1)^b	0.902	2.448	3.290	155.4
C(5)–H(5)⋯O(2)^c	0.906	2.521	3.352	152.8
Compound 3
C(2)–H(2)⋯O(3)^e	0.925	2.410	3.283	157.4
C(5)–H(5)⋯O(2)^d	0.911	2.286	3.169	163.1
Compound 4
C(2)–H(2)⋯O(2)	0.918	2.533	3.292	140.4
C(5)–H(5)⋯O(1)^f	0.830	2.373	3.180	164.2

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S–O bond distances in [CH₃SO₄]^– range from 1.396 to 1.572 Å, the O–S–O bond angles from 100.2 to 116.2° and C–O bond length in the [CH₃SO₄]^– amounts to 1.473 Å (Table 2).

These data correspond well to those previously reported for the methylsulfate anion in [DMIm] [CH₃SO₄]²² and [NH₄]₆[calix][4] [arenesulfonate] [MeOSO₃]·(H₂O)₂.²³

Bis(1-butyl-3-methylimidazolium methylsulfate) [18-crown-6] (2) crystallizes in the monoclinic P2₁/n space group. Similar to compound 1 the structure consists of alternate layers of 18-crown-6 and the ionic units ([BMIm]⁺ and [CH₃SO₄]^–). In the 1-butyl-3-methylimidazolium cation all C–C bonds of the alkyl chain are in a trans–trans conformation which projects the chain away from the imidazolium ring, with a torsion angle around C(7)–C(8) of 177.3° (Fig. 3).

Fig. 3 Trans–trans conformation of [BMIm]⁺ cation in compound 2.

Distinguished by a trans–trans conformation of the butyl chain, each cation in the lattice has three neighbouring counter ions in contact at the van der Waals distance. The shortest hydrogen bonds are between C(4) and C(5) of the imidazolium ring and two oxygen atoms of the sulfate anion O(1) and O(2), with C–H⋯O distances of 2.448 and 2.521 Å, and C–H⋯O angles of 152.8 and 155.4° (Table 3). The next interaction is between C(2) and O(4) with C–H⋯O bond distance of 2.771 Å and C–H⋯O angle of 154.4° (Fig. 4). In compound 2, the orientation of the butyl chain away from the imidazolium ring allows no other significant van der Waals contacts or C–H⋯π interactions.²⁴

Fig. 4 Interionic contacts between [BMIm]⁺ cation and [CH₃SO₄]^– anion in compound 2; hydrogen bondings are shown by dotted lines.

S–O bond distances in the [CH₃SO₄]^– range from 1.400 to 1.564 Å, the O–S–O bond angles from 99.8 to 113.3° and C–O bond length is 1.469 Å. These data correspond well with those found in 1 (Table 2).

Both imidazolium salts 3 and 4 crystallized in the triclinic space groupP1̄. The crystal structures contain one crystallographically independent [EMIm]⁺ cation, [CH₃SO₃]^– and crown ether for 3 and one crystallographically independent [EMIm]⁺ cation, [CF₃SO₃]^– and crown ether for 4. The crystallographic data for 3 and 4 are given in Table 1. The ions are packed in the crystal lattice of compounds 3 and 4via weak interionic C–H⋯O hydrogen bonds as well as significant C–H⋯π interactions.

Comparable to compound 1, two [EMIm]⁺ cations in both compounds 3 and 4 are linked to each other by C–H⋯π interactions between one methyl carbon (C(6) for 3 and C(6) for 4) and the imidazolium ring of another cation (Fig. 5 and 6). The distance between the methyl carbon and the center of the imidazolium ring is estimated to be 3.717 Å for 3 and 3.990 Å for 4, which agrees very well with the value expected for the lowest benzene⋯methane interactions.

Fig. 5 Interionic contacts between [EMIm]⁺ cation and [CH₃SO₃]^– anion in compound 3; hydrogen bonding and C–H⋯π interactions are shown by dotted lines.

Fig. 6 Interionic contacts between [EMIm]⁺ cation and [CF₃SO₃]^– anion in compound 4. Hydrogen bonding and the C–H⋯π interactions are shown by dotted lines.

Two significant C–H⋯O hydrogen bonds involving H(2) and H(5) connect the cations and anions forming layered structures for compounds 3 and 4. C–H⋯O bond distances of 2.286 and 2.410 Å and C–H⋯O bond angles of 157.4 and 163.1° are found for compound 3, and two [CF₃SO₃]^– anions with C–H⋯O bond distances of 2.373 and 2.533 Å, and C–H⋯O bond angles of 140.3 and 164.2° for compound 4 (Fig. 6 and Table 3).

In 3 the methanesulfonate ion is in a staggered conformation with C_3v symmetry. The sulfonate group has an average S–O bond distance of 1.434 Å, an average O–S–O bond angle of 112.3°, and a C–S bond distance of 1.750 Å (Table 2).

The arrangement of the [CF₃SO₃]^– anion in 4 agrees very well with this in several dehydrated salts, such as LiCF₃SO₃ and [EMIm] [SO₃CF₃].²⁵ It also has a staggered conformation with C_3v symmetry. The S–C (1.808 Å), average C–F (1.306 Å), and average S–O (1.421 Å) bond lengths for compound 4 (Table 2) are in best agreement with those measured by Dehnicke et al.²⁶ and Jansen et al.²⁷

Raman spectroscopic characterization

Solid title compounds are composed by the respective cationic and anionic units as well as by 18-crown-6 molecules. The dominant interactions are Coulombic. H-bridge bonding of the ionic units to 18-crown-6 molecules is not found. Consequently, the Raman spectra of the solid title compounds are superpositions of those from 18-crown-6 and the corresponding ionic components. Usually, asymmetrically alkyl substituted imidazolium cations are employed for prototype ILs. In this context the 1,3-dimethylimidazolium cation (in 1) is here exceptional, and unfortunately, no corresponding vibrational spectral data were available. Both the other cations, [BMIm]⁺ (in 2) and [EMIm]⁺ (in 3 and 4), have been investigated intensely by vibrational spectroscopy as relevant units in ILs.^26–33 Especially, the rotational and vibrational freedom of the butyl group in [BMIm]⁺ can lead to at least two stable conformers. For [BMIm]⁺ Cl^– diagnostic Raman modes allow to distinguish between these conformers being in equilibrium.^28–30 For the trans–trans conformer (with a torsional angle t_λ ≈ 180°) the Raman bands at 625, 730, and 790 cm^–1 are characteristic.^28,30

Raman frequencies of the respective anions of the studied compounds are, more or less, known in detail. The Raman frequencies of the crystalline compounds are summarized along with their assignments in groups in Table S1 (ESI)† along with these of 18-crown-6. For the crystalline compound 2Raman spectra at –60 and –150 °C were recorded to extend the temperature range of the temperature dependence of the equilibrium between the trans–trans and trans–gauche[BMIm]⁺ conformers, demonstrated hitherto only between 50 and 110 °C.^28,30 Unexpectedly, no significant change in the intensity relation of the 730 cm^–1 Raman mode (trans–trans conformer) to the 602 cm^–1 band (trans–gauche) could be determined for all temperatures (20, –60, and –150 °C). This can be understood by the assumption that the mentioned equilibrium is frozen and the rotational formation of the different conformers is hindered.

The crystal-structure determination of 2 was carried out at –50 °C (Table S1),† resulting in the monoclinic space groupP2₁/n (trans–trans conformer), without any indication of a structural disorder. So, the low temperature Raman frequencies of 2 are not in complete accordance with the crystal structure (determination). An explanation could be that the X-ray radiation during the measurement overcomes the rotational hindering from the trans–gauche to the trans–trans conformers of [BMIm]⁺. In Fig. 7 the Raman spectra of compound 2 at 20 and –60 °C are shown.

Fig. 7 FT-Raman spectra (λ_exc. = 1064 nm) of crystalline [BMIm]₂[CH₃SO₄]₂[18-crown-6] at –60 and 20 °C. The Raman intensity is given in arbitrary units.

Considering the Raman spectra of the investigated compounds, it can be noticed that the vibrational modes of the ionic units and the 18-crown-6 molecules superimpose as mentioned; also, the “molecular groups” in the relevant species (e.g., CH₃, CH₂, CH, imidazole ring, –SO₃) give rise to uncoupled vibrations and hence to Raman bands in typical frequency regions. In consequence, single component modes e.g. CH₃ from CH₃SO₄^–, CH₃SO₃^– or CH₃ (ring substituent) are superpositioned and not always at their original frequency value anymore. Therefore, the assignments are done to groups in Table S1 (ESI).†

The assignments of the Raman frequencies of 1–4 are first related to the ionic components (IL) and 18-crown-6 (cr). The noticeable modes of the respective anions, [CH₃SO₄]^–, [CH₃SO₃]^–, and [CF₃SO₃]^–, are marked in italics in Table S1 (ESI).† Secondly, the frequency assignments are proposed to characteristic group frequencies in comparison to literature data partly based on sophisticated ab initio as well as on DFT computations. In the following, some related comments are given (c.f. Table S1, ESI).†

The most Raman bands of the title compounds belong to the imidazolium cations. The quasi-aromatic imidazolium contains “aromatic” C–H units with distinctive stretching modes in the area 3070–3200 cm^–1, whereas the corresponding deformations are attributed to vibrations around 1300 cm^–1.^24,28–30 Furthermore, the imidazolium ring itself has characteristic bands in the region 1450–1570 cm^–1 and at about 1150–1370 cm^–1.^29,31,33 The intensities of these modes can be affected by the kind and number of the ring (alkyl) substituents. The frequencies in the region 2800–3000 cm^–1 are a result of the aliphatic CH₃ and CH₂ stretching vibrations (superpositioned by CH₃groups of the respective anions and CH₂ units of 18-crown-6). Corresponding CH₃ and CH₂ bending modes are found at 1350–1440 and around 1100–1200 cm^–1, respectively. Butyl substituent in 2 further contributes a number of vibrations in the region from about 500 to 800 cm^–1 caused by alkyl chain orientations. As mentioned, two different conformers, trans–trans and trans–gauche, could be identified by Raman spectroscopy at room temperature.^28–30

Recently, [BMIm][CH₃SO₄] was studied in relation to the formation of solutions with some n-alkanes and aromatic hydrocarbons.³⁴ Unfortunately, IR and Raman data of methylsulfate, [CH₃SO₄]^–, are daunting,³⁵ nevertheless the characteristic ν_s(SO₃) can be attributed to the strong Raman band at 1061 cm^–1 for 1 and 2, and ν_s(SO₂) is assigned to the medium bands at 1144 cm^–1 (1) and 1146 cm^–1 (2), respectively. The methanesulfonate ion, [CH₃SO₃]^–, in 3 exhibits C_3v symmetry. The vibrational spectrum for this anion has been published for several compounds.³⁶ In accordance with literature data, the characteristic Raman frequencies are assigned as ν_s(SO₃): 1042 cm^–1, ν_as(SO₃): 1203 cm^–1, δ_s(SO₃): 551 cm^–1 and δ_as(SO₃): 526 cm^–1.

The triflate ion, [CF₃SO₃]^– in 4 also exhibits C_3v symmetry. All fundamental triflate vibrations are situated below 1400 cm^–1 (meaning also below most of the CH₃ and CH₂ bending modes).³⁷ Most of its vibrational modes can be ascribed to the SO₃ and CF₃ units, respectively. Very strong ν_s(SO₃) at 1033 cm^–1 dominates the Raman spectrum of 4 in the region below 1500 cm^–1, and ν_as(SO₃) comes out at 1246 cm^–1 with weak–medium intensity. The deformation δ_s(SO₃) is attributed to the weak–medium Raman band at 572 cm^–1. The Raman mode at 1226 cm^–1 is ascribed to the symmetric CF₃ stretching, and δ_s(CF₃) is the strong band at 756 cm^–1. For further details, see Table S1 (ESI).†

Experimental results

All reactions and manipulations were carried out under a dry argon atmosphere using standard Schlenk, vacuum-line and glove box techniques. NMR spectra were recorded on a BRUKER Digital FT-NMR ‘AVANCE 400’ spectrometer at room temperature. Chemical shifts are reported in ppm relative to the ¹H and ¹³C residue of the deuterated solvent (CDCl₃). Raman spectra obtained from the bulk solids were recorded at 200 mV using Bruker IFS-FRA-106. To obtain measurement, respective samples were sealed under an argon atmosphere in glass capillaries, and data were recorded at room temperature, if not stated otherwise. The compounds 1, 3-dimethylimidazolium methylsulfate, 1-butyl-3-methylimidazolium methyl-sulfate, 1-ethyl-3-methylimidazolium, methanesulfonate, and 1-ethyl-3-methylimidazolium trifluoromethane-sulfonate from Merck were dried for 140 h in Schlenk tubes at 150 °C under reduced pressure and rigorous stirring before use. 18-Crown-6 (Merck) was dried over activated 4 Å molecular sieves and re-crystallized from ether.

Syntheses

Bis(1,3-dimethylimidazolium methylsulfate) [18-crown-6] (1)

18-Crown-6 (1.135 mmol, 300 mg) was added to [DMIm] [CH₃SO₄] (1.135 mmol, 236 mg). The reaction mixture was heated at 150 °C for 2 h. After several days, colorless single crystals of compound 1 were formed. Mp 86 °C. ¹H NMR (CDCl₃), δ = 9.61 (s, 1H, H(2)), 7.25 (br s, 2H, H(4,5)), 4.02 (s, 3H, NCH₃), 3.76 (s, 3 H, CH₃OSO₃^–), 3.69 ([18-crown-6]).

Bis(1-butyl-3-methylimidazolium methylsulfate) [18-crown-6] (2)

18-Crown-6 (1.135 mmol, 300 mg) was added to [BMIm] [CH₃SO₄] (1.13 mmol, 284 mg). The reaction mixture was heated at 250 °C for 4 h. After two weeks, colorless single crystals of compound 2 were formed. Mp 39 °C. ¹H NMR (CDCl₃), δ 9.65 (s, 1H, H(2)), 7.31 (dd, ³J = 1.9 Hz, 1H, H(4)), 7.25 (dd, ³J = 1.9 Hz, 1H, H(5)), 4.25 (t, ³J = 7.4 Hz, 2H, NCH₂), 4.04 (s, 3H, NCH₃), 3.76 (s, 3H, CH₃OSO₃^–), 3.69 ([18-crown-6]), 1.88 (quint, ³J = 7.4 Hz, 2H, NCH₂CH₂), 1.39 (sext, ³J = 7.4 Hz, 2H, CH₂CH₃), 0.97 (t, ³J = 7.4 Hz, 3H, CH₂CH₃).

Bis(1-ethyl-3-methylimidazolium methanesulfonate) [18-crown-6] (3)

18-Crown-6 (1.135 mmol, 300 mg) was added to [EMIm] [CH₃SO₃] (1.135 mmol, 234 mg). The reaction mixture was heated at 180 °C for 2 h. Colorless single crystals of 3 were formed within a week. Mp 51 °C. ¹H NMR (CDCl₃), δ 10.08 (s, 1H, H(2)), 7.25 (br s, 2H, H(4,5)), 4.35 (q, ³J = 7.4 Hz, 2H, CH₂CH₃), 4.06 (s, 3H, NCH₃), 3.69 ([18-crown-6]), 2.82 (s, 3H, CH₃SO₃^–), 1.59 (t, ³J = 7.4 Hz, 3H, CH₂CH₃).

Bis(1-ethyl-3-methylimidazolium trifluoromethanesulfonate) [18-crown-6] (4)

18-Crown-6 (1.135 mmol, 300 mg) was added to [EMIm] [CF₃SO₃] (1.135 mmol, 295 mg). The reaction mixture was heated at 200 °C for 4 h. After several days colorless single crystals of 4 as an insoluble product were formed. Mp 69 °C. ¹H NMR (CDCl₃), δ 9.41 (s, 1H, H(2)), 7.23 (dd, ³J = 1.8 Hz, 1H, H(4)), 7.21 (dd, ³J = 1.8 Hz, 1H, H(5)), 4.31 (q, ³J = 7.5 Hz, 2H, CH₂CH₃), 4.01 (s, 3H, NCH₃), 3.82 ([18-crown-6]), 1.61 (t, ³J = 7.5 Hz, 3H, CH₂CH₃).

X-Ray diffraction analysis

All data were collected on a Stoe IPDS-II single-crystal X-ray diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 223 K. Crystal structure solution by direct methods using SHELXS-97³⁸ yielded in all cases heavy atom positions. Subsequent difference Fourier analyses and least squares refinements with SHELXL-97³⁸ allowed localization of the remaining atom positions. In the case of compounds 1, 3 and 4, the hydrogen positions could be obtained from the difference Fourier map while for compound 2 the hydrogen positions in C(10) were calculated using the riding model. The crystal data and structure refinement parameters of the four structures are summarized in Table 1. For preparation of the structure drawings, the programs Diamond³⁹ and POV-Ray⁴⁰ were applied.

Further details on the crystal structure investigations may be obtained from the Cambridge Crystallographic Data Centre (CCDC, 12 Union Road, Cambridge CB2 1EZ, fax (+44)1223-336-033, e-mail deposit@ccdc.cam.ac.uk) on quoting the depository numbers CCDC-648112 for compound 1, CCDC-648113 for compound 2, CCDC-648115 for compound 3 and CCDC-648114 for compound 4. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b708724c

Summary and concluding remarks

Four 1-alkyl-3-methylimidazolium based compounds with anions [CH₃SO₄]^–, [CH₃SO₃]^– and [CF₃SO₃]^– co-crystallize with 18-crown-6 in 2 : 2 : 1 stoichiometry. Single-crystal structure for the title compounds could be obtained along with their NMR and Raman spectra. For the 1-butyl-3-methylimidazolium cation (compound 2), the distinctive Raman bands trans–trans and trans–gauche conformers²⁹ could be confirmed. The Raman spectra at –60 and –150 °C differ only marginally from those one at room temperature. This may be a hindering for the generation of different rotational conformers of [BMIm]⁺ at low temperatures, whereas the single-crystal structure determination (–50 °C) results in the monoclinic space groupP2₁/n (trans–trans conformer) without any indication of structural disorder.

One reason for the enclosure of 18-crown-6 in imidazolium based salts is obviously the formation of dense packing, or alternatively a space filling. The enclosure of 18-crown-6 stabilizes the new structures according to their melting temperatures in relation to these of the corresponding pure imidazolium based salts.

From the crystal structure conclusive information can be drawn for the relevant species in the liquid state and also for possible reaction mechanisms. The use of crown ethers in molten salt systems (ILs) can result in the synthesis of new compounds by exchange reactions or equilibrium shifting as, e.g.

AB + CD + crown → [A(crown)D]↓ + C

⁺

+ D

^–

.

Acknowledgements

The authors thank Karin Bode for the careful recording of the Raman spectra and Dr Viktor Zapolskii for the determination of the melting points.

References

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Footnote

† Electronic supplementary information (ESI) available :  Crystal structures of compounds 1–4 (Fig. S1–S6); spectroscopy data (Fig. S7–S14; Raman frequencies of the crystalline title compounds along with their estimated intensities and proposed assignments (Table S1). See DOI: 10.1039/b708724c

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