In situ crystallization of ionic liquids with melting points below −25 °C

Abstract

1-Ethyl-3-methylimidazolium-based ionic liquids, [emim]OTf, OTf = trifluoromethanesulfonate (mp = −25.7 °C), and [emim]NTf₂, NTf₂ = bis(trifluoromethanesulfonyl)amide (mp = −25.7 °C), have been crystallized by in situ cryo-crystallization using a zone-melting technique and found to pack via interionic C–H⋯O and F⋯F interactions.

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In situ crystallization of liquids and gases^1,2 has been applied to the growth and study of the crystal structures of a number of molecular liquids³ and gases⁴ and their mixtures.⁵ We have applied such methods to the crystallization of low-melting ionic liquids⁶ seeking means for their purification. Many ionic liquids tend to form glasses⁷ on cooling presenting, as a consequence, practical challenges for their recovery and recycling (as traditional methods, such as distillation,⁸ are not options). Zone-melting,⁹ the basis of the current study, is one approach to ionic liquid purification and focuses attention on their phase behaviour and crystallizability. We have been successful in crystallizing and determining the crystal and molecular structure⁶ of five representative materials ([emim]BF₄ (emim = 1-ethyl-3-methylimidazolium), [bmim]PF₆ (bmim = 1-butyl-3-methylimidazolium), [bmim]OTf (OTf = trifluoromethanesulfonate), [hexpy]NTf₂ (hexpy = N-hexylpyridinium and NTf₂ = bis(trifluoromethanesulfonyl)amide) and [bmpyr]NTf₂ (bmpyr = 1-butyl-1-methylpyrrolidinium)) with melting points ranging from +6.7 °C to −10.8 °C. Here we present the crystal structures of two ionic liquids, [emim]OTf, 1 and [emim]NTf₂, 2, both melting at −25.7 °C.

Differential scanning calorimetry (DSC) and in situ cryo-crystallization in combination have established conditions under which single crystals of very low melting ionic liquids can be grown.⁶ Using a modification of an in situ cryo-crystallization method, developed by Thalladi et al.,² we can avoid ionic liquid glass formation on cooling and successfully grow single crystals of as-supplied commercially available 1 (Merck KGaA) and 2 (kindly donated by AG Degussa, Germany). The liquid was first introduced into a Lindemann glass capillary (3 cm long and 0.3 mm diameter), sealed by rapidly setting glue and mounted vertically on a Bruker APEX CCD diffractometer in a stream of cold N₂ (Oxford Cryosystem) and cooled to ∼−120 °C. As can be seen from the DSC plots of the two materials [Fig. 1(a) and (b)], [emim]OTf crystallizes spontaneously on cooling below −60 °C whereas [emim]NTf₂ does not show any transition in the cooling cycle. Instead, it crystallizes on heating to −50 °C. Hence, [emim]NTf₂ was heated back to −40 °C to initiate crystallization in the capillary. Once the capillary was found to contain a polycrystalline column, the zone-melting technique was applied using the optical heating and crystallization device (OHCD)¹⁰ to melt and re-grow the crystals. A ∼1 mm long section of the lower portion of the capillary was heated with an IR laser to create a molten zone which was slowly moved along the length of the capillary, the laser power being slowly increased from zero to a value sufficient to melt the polycrystalline material. Then the laser beam was moved along a ∼2.5 cm length of the capillary from bottom to top in 60–90 min. The laser power was then slowly reduced to zero and the cycles repeated several times until a single crystal of acceptable quality was grown. Single crystal diffraction data on 1 (collected at −123 °C), and 2 (−43 °C) were obtained using 2 ω scans (one with ϕ = 0° and other with ϕ = 6°) in order to achieve higher percentage of completion and the crystal structures were solved and refined using standard methodology.^11,12 Since the data were collected using two ω scans with 0.5° width, the data : parameter ratio was low, not permitting H atom refinement. The H atoms were thus fixed at their geometrically ideal positions. Their thermal parameters were not refined.

Fig. 1 DSC plot of (a) [emim]OTf and (b) [emim]NTf₂.

[emim]OTf crystallizes in the orthorhombic Pbca space group with one ion pair in the asymmetric unit [Fig. 2(a)]. The ions are packed in the crystal lattice via weak interionic C–H⋯O hydrogen bonds as well as a significant F⋯F¹³ interaction. Three significant C–H⋯O hydrogen bonds involving H2, H4 and H5 connect the cations to the anions in the ‘ab’ plane forming a layered structure. A similar feature has been observed for [emim]NO₃, [emim]NO₂ and [emim]SO₄·H₂O.¹⁴ Interestingly, the imidazolium rings are not stacked one above the other but, instead, are offset in an ABAB... type arrangement with distances of 7.689(3) and 6.124(3) Å between the centroids of the rings of the adjacent layers [Fig. 2(c)]. The rings in layers A and B define intersecting, rather than parallel, planes.

Fig. 2 (a) ORTEP of [emim]OTf drawn with 50% ellipsoidal probability, H atoms are excluded for clarity, (b) packing diagram of [emim]OTf, intermolecular interactions are shown as dotted lines and (c) stacking of imidazolium rings in [emim]OTf.²⁶

The –CF₃ group of the anion points towards the cationic imidazolium ring with a closest F…C distance of 3.352(4) Å, between F2 and C2. Two other F atoms, F1 and F3, point towards C4 and C5, respectively, with distances of 3.466(4) and 3.362(5) Å. This suggests a possible interaction between the positively charged imidazolium ring and the strongly electronegative –CF₃ group. The anions of the adjacent layers are connected to each other via F⋯F interaction as shown in the Fig. 2(b) [Table 1].

Table 1 List of intermolecular interactions in [emim]OTf

D-H⋯A	D⋯A/Å	H⋯A/Å	∠D-H⋯A/°	Symmetry
C2-H2⋯O3	3.079(5)	2.24	149	x, y, z
C4-H4⋯O1	3.248(5)	2.37	157	½ − x, y − ½, z
C5-H5⋯O1	3.359(5)	2.63	136	x + 1, y, z
C5-H5⋯O2	3.548(5)	2.64	164	x + 1, y, z
F1⋯F1		2.885(4)		−x, 1 − y, −z

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A short F⋯F (<2.96 Å) contact has also been observed with θ₁ = θ₂, where θ₁ = ∠C–F⋯F′ and θ₂ = ∠F⋯F′–C′, similar to observations for other halogens as reported by Desiraju and Parthasarathy.¹⁵ The –CH₃ group of the ethyl substituent points toward the inner core where the –CF₃ groups are located.

[emim]NTf₂ crystallizes in the non-centrosymmetric space group Pca2₁ with two ion pairs in the asymmetric unit. The torsion angles C9–S1–S2–C10 and C19–S3–S4–C20 are 26.8(12) and 27.0(11)°, indicating a cisoid conformation of the anion [Fig. 3(a)]. Ion packing involves several C–H⋯O hydrogen bonds and C–F⋯F interactions. As can be seen from Fig. 3(b), the imidazolium rings of the cations are parallel to the ‘ab’ plane and they are associated with the >SO₂ moieties via C–H⋯O hydrogen bonds in the same plane. Once again, the imidazolium rings are stacked offset from one layer to the next. The stacking is also of an ABAB⋯ type with a minimum centroid to centroid distance between A⋯B of 5.334(2) Å and between B⋯A of 8.079(3) Å [Fig. 3(c)]. Once again, the planes defined by the rings in layers A and B are not parallel to one another. Two anions from the two parallel layers of anions are connected to each other via two different F⋯F interactions [Fig. 3(b), Table 2]. Three different short F⋯F contacts <2.96 Å have been observed in this structure where θ₁ ≠ θ₂, also θ₁ ≠ 180° and θ₂ ≠ 90°, as is generally observed for other halogens, Cl, Br and I. This indicates that fluorine, too, displays short intermolecular F⋯F contacts but does not strictly follow the patterns displayed by heavier halogens.¹⁵

Fig. 3 (a) ORTEP of [emim]NTf₂ drawn with 50% ellipsoidal probability, H atoms are excluded for clarity, (b) packing diagram of [emim]NTf₂, intermolecular interactions are shown as dotted lines, and (c) stacking of imidazolium rings in [emim]NTf₂.²⁶

Table 2 List of intermolecular interactions in [emim]NTf₂

D-H⋯A	D⋯A/Å	H⋯A/Å	∠D-H⋯A/°	Symmetry
C12-H12⋯O2	3.29(2)	2.53	139	x, y, z
C12-H12⋯O3	3.30(3)	2.46	149	x, y, z
C2-H2⋯O7	3.29(2)	2.49	144	2 − x, −y, z − ½
C2-H2⋯O5	3.26(3)	2.47	143	2 − x, −y, z − ½
C4-H4⋯O6	3.16(3)	2.25	163	3/2 − x, y, z −½
C14-H14⋯O4	3.26(3)	2.37	159	x + ½, 1 − y, z
C15-H15⋯O1	3.53(3)	2.60	177	x + ½, −y, z
F4⋯F12		2.80(2)		x − ½, 1 − y, z
F7⋯F12		2.91(2)		x, y − 1, z
F3⋯F11		2.79(2)		x, y − 1, z

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It is noteworthy that the patterns of packing of the [emim] salts 1 and 2 are similar to one another. The imidazolium rings are parallel to the ‘ab’ plane and are hydrogen bonded to the available O donors in the same plane. On the other hand, fluorines interact between these planes. While this is so, similarities can be seen within the group of DSC plots of the ionic liquids containing OTf⁻ anions with different cations, such as [emim], [bmim],⁶ and within those of ionic liquids involving NTf₂⁻ with [emim], [bmpyr],⁶ [hexpy].⁶ However, major differences are evident in the phase behaviour of these two groups.

A number of [emim]⁺ salts are reported in the literature containing different anions such as Br⁻,¹⁶ I⁻,¹⁶ PF₆⁻,¹⁷ Cl⁻,¹⁸ HF₂⁻,¹⁹ and others containing metal atoms.²⁰ The structures of [emim]X (X = Cl, Br, I) are mainly governed by interionic C–H⋯X hydrogen bonds while, with PF₆⁻, the packing is believed to be due to cation–anion coulombic attraction and weak C–H⋯F interactions were not invoked. However, our earlier study⁶ has established that [emim]BF₄ and [bmim]PF₆ packing in the unit cell involves C–H⋯F interactions. In the case of [emim]HF₂,¹⁹ the packing was found to be governed by strong interionic C–H⋯F hydrogen bonding.

To our knowledge, there are six reported structures of ionic liquids containing the OTf anion and twelve with the NTf₂ anion.²⁰ It has generally been observed that IL structures containing OTf⁻ are packed via a series of C–H⋯O hydrogen bonds, as seen in the present study. The NTf₂ anion has been found to exist in both cisoid (ten examples) and transoid (two examples) forms. In a majority of (non-ionic liquid) structures NTf₂⁻ is observed to be in the transoid conformation with C–S⋯S–C ≈ 180°. The NTf₂ anion is also found²¹ to be transoid in the recently reported inclusion compound, [emim]NTf₂·C₆H₆. The centrosymmetric monoclinic packing may be contrasted with that of pure [emim]NTf₂ reported in the current study. In a few alkali and alkaline earth metal salts, NTf₂⁻ has been found to be in a cisoid conformation, with C–S⋯S–C ≈ 0°.²² Holbrey et al. first reported a cisoid conformation for NTf₂⁻ in the case of [dmim]NTf₂ (dmim = 1,3-dimethylimidazolium).²³ In our earlier communication, we reported [hexpy]NTf₂ in which the anion also has the cisoid conformation.⁶ In the current study, we illustrate the third cisoid conformation for a NTf₂-containing ionic liquid and observe that all cases in which the NTf₂ is associated with an organic cation, reported to date, and having cisoid conformation, crystallize with two ion-pairs in the asymmetric unit and the two anions marginally differ in C–S⋯S–C conformation. Very recently, Henderson et al.²⁴ have reported a disordered C₂ conformation for NTf₂ in a plastic crystalline phase of [Et₄N]NTf₂ super cooled to −173 °C. Simulations of both OTf⁻ and NTf₂⁻ containing ionic liquids, to which our structural studies are relevant, have been reported.²⁵

In the current study, we have demonstrated that [emim]OTf and [emim]NTf₂ form single crystals on in situ crystallization. The crystal structures feature C–H⋯O hydrogen bonds and short F⋯F contacts but do not show π⋯π interaction between the cations.

Acknowledgements

We thank the EPSRC for funding (Grant no. GR/S87089/01 (Speculative Research Leading to Green Chemistry)). ARC is a Crystal Faraday Associate.

Notes and references

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11. Crystal data: [emim]OTf, chemical formula C₇H₁₁SN₂O₃F₃, formula weight 260.2, orthorhombic, space group Pbca, a = 10.183(5), b = 12.384(7), c = 18.294(8) Å, V = 2307.0(2) ų, Z = 8, ρ_calc = 1.50 g cm⁻³, T = 150 K, μ = 0.316 mm⁻¹, reflections measured 10196, unique 2028, R1_obs = 0.052, wR2_obs = 0.130 (for reflections > 3σ(I)), δρ_min,max = −0.281, 0.353 e Å⁻³.
12. Crystal data: [emim]NTf₂, chemical formula C₈H₁₁N₃O₄S₂F₆, formula weight 391.3, orthorhombic, space group Pca2₁, a = 18.499(9), b = 8.626(8), c = 19.255(9) Å, V = 3072.6(4) ų, Z = 8, ρ_calc = 1.69 g cm⁻³, T = 230 K, μ = 0.432 mm⁻¹, reflections measured 25029, unique 2947, R1_obs = 0.082, wR2_obs = 0.189 (for reflections > 3σ(I)), δρ_min,max = −0.347, 0.424 e Å⁻³. CCDC reference numbers 614037 and 614038. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b609598d.
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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data for 1 and 2, details of the DSC measurements, interionic hydrogen-bonding interaction diagrams and additional relevant literature. See DOI: 10.1039/b609598d

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