Hollow circular compound-based inclusion complexes of an ionic liquid

Abstract

Inclusion complex formation between hollow circular compounds, e.g. crown ethers, and an ionic liquid, 1-methyl-3-octylimidazolium tetrafluoroborate, in acetonitrile solvent is studied by means of conductivity measurements, IR spectra and NMR spectra. The results reveal the formation of 1 : 1 complexes between the crown ethers and ionic liquid molecules in acetonitrile. Crown ether complexes with electron-deficient imidazolium cations are formed by H-bond formation between the acidic protons of the imidazolium ring of the ionic liquid and the lone pair of electrons of the crown oxygen atom. In the case of dibenzo-18-crown-6, complexation is caused by H-bonding; however, π-stacking or charge-transfer interactions also appear to have minor contributions to the complex formation. Thus, hydrogen bonding is mainly responsible for the complexation, and ion–dipole interactions also may be responsible for complex formation between ionic liquid molecules and the crown ethers. The interactions in the complexation are analyzed and discussed.

---

1. Introduction

The crown ether (CE) family of macrocyclic compounds has attracted an enormous amount of interest since their discovery in 1967,^1,2 especially in the fields of host–guest and coordination chemistry. CEs can form complexes with a variety of guest species, such as metal cations, protonated species and neutral molecules, in their cavities via different types of interactions with multiple oxygen atoms.^3,4 Studies of applications of CEs, such as phase transfer catalysts,^5,6 photo-switching devices,⁷ and drug carriers,⁸ are in progress on the basis of this inclusion ability. Crown ethers have proved to be unique cyclic molecules for molecular recognition of suitable substrates by hydrogen bonds, ionic interactions and hydrophobic interactions. The study of the interactions involved in the complexation of different cations with crown ethers in mixtures of solvents is important to improve our understanding of the mechanisms of biological transport, molecular recognition, and other analytical applications.⁹

It is already known that imidazolium cations can form inclusion complexes with large crown-ether-type hosts via H-bonding.¹⁰ 1,3-Disubstituted imidazolium salts are known to form inclusion complexes with DB24C8 or its derivatives through intermolecular hydrogen-bond formation, as demonstrated by different research groups.^11–14 In 1,3-disubstituted imidazolium salts, all protons on the imidazolium ring are quite acidic, as the positive charge is delocalized over the entire imidazolium ring.¹⁵ Acidic protons are attractive in supramolecular chemistry because the acidic protons participate in stronger hydrogen-bond formation with the lone pair of electrons of the oxygen; this accounts for the stability of the adduct formed. Biologically important heterocyclic bases, such as imidazole, form planar cations and act as effective structural units at the active sites of various proteins and nucleic acids. However, during enzymatic reactions, imidazole can also exist as a protonated cation and may thus interact with the substrate by direct electrostatic or π–π interactions. Imidazolium salts have been and will be significant not only in organometallic chemistry as precursors of N-heterocyclic carbenes,^16,17 but also in organic chemistry and material science areas as ionic liquids due to their unique chemical, physical, and electrical properties.^18–21

In this work, we have studied the inclusion complex formation of an ionic liquid (IL), 1-methyl-3-octylimidazolium tetrafluoroborate, with hollow circular hosts, 18-crown-6 (18C6) [complex 1] and dibenzo-18-crown-6 (DB18C6) [complex 2], in acetonitrile (ACN). The complexes were characterised by conductance, IR and NMR studies. The formation constants and thermodynamic parameters of the above-specified interactions in solution are discussed here. The structures of the IL, 1-methyl-3-octylimidazolium tetrafluoroborate, and both crown ethers are shown in Scheme 1.

Scheme 1 Molecular structures of the crown ethers and the Ionic Liquid.

2. Experimental section

2.1 Reagents

The ionic liquid (97%) and crown ethers [18C6 (99%), DB18C6 (98%)] were bought from Sigma-Aldrich, Germany and were used as purchased.

2.2 Instrumentation

Prior to the start of the experimental work, the solubility of the chosen CEs and IL in ACN were precisely checked; it was observed that the selected IL salt was freely soluble in all proportions of the CE solutions.

The conductance measurements were carried out in a Systronics-308 conductivity bridge with an accuracy of ±0.01% using a dip-type immersion conductivity cell, CD-10, with a cell constant of approximately (0.1 ± 0.001) cm⁻¹.²² The measurements were performed in an auto-thermostated water bath while maintaining the experimental temperature. The cell was calibrated using 0.01 M aqueous KCl solution. The uncertainty in temperature was 0.01 K.

Infrared spectra were recorded on an 8300 FT-IR spectrometer (Shimadzu, Japan). The details of the instrument have been described previously.²³

¹H NMR spectra were recorded in CD₃CN at 300 MHz using a Bruker AVANCE 300 MHz instrument. Signals are quoted as δ values in ppm using residual protonated solvent signals as the internal standard (CD₃CN: δ 1.98 ppm). Data are reported as chemical shifts.

3. Results and discussion

3.1 Conductance

The advantage of conductometry is that measurements can be carried out with high precision at very low concentrations in solution systems. Conductance measurements of a solution of IL in the presence of a crown ether provide information about the stability and transport phenomena of the cation-crown ether complex in solution. Also, conductometry is one of the most reliable methods for obtaining the formation constants of cation–macrocyclic complexes (Takeda et al., 1991).²⁴

Conductance studies of the interactions between the imidazolium cation of the IL and 18C6 and DB18C6 in ACN solution were conducted at different temperatures, and the values are presented in Table 1. The stability of these complexes depends mainly on the strength of the bonds between the acidic protons of the imidazolium ring and the oxygen atoms of the crown ethers (Scheme 2). The formation constants (log K_f) of the 1 : 1 complexes at different temperatures varied in the order 18C6 > DB18C6 for the IL. The formation constants determined by the conductivity studies and the thermodynamic values for complex formation between the crown ethers and the imidazolium cation in acetonitrile solution are summarized in Table 2.

Table 1 Values of observed molar conductivities, Λ, at various mole ratios for the IL-18C6 (complex 1) and IL-DB186 (complex 2) systems at different temperatures

Mole ratio	Λ (S cm^2 mol^−1)
	DB18C6			18C6
	293.15 K	298.15 K	303.15 K	293.15 K	298.15 K	303.15 K
0	135.80	143.72	152.21	154.00	162.58	168.36
0.099	132.10	138.34	147.60	149.10	156.50	163.84
0.196	128.50	133.80	143.12	144.60	151.68	159.56
0.291	125.07	130.60	139.72	140.76	147.20	154.72
0.385	121.61	127.82	135.80	137.88	143.12	150.50
0.476	117.82	124.92	132.24	134.10	138.34	145.42
0.566	114.24	121.52	128.56	130.18	134.80	141.64
0.654	110.12	117.92	125.14	126.84	130.60	137.68
0.740	107.30	115.60	122.46	123.18	127.82	134.54
0.825	105.20	112.32	119.32	120.24	124.50	131.50
0.909	102.30	109.50	116.22	117.46	121.92	128.96
1.071	100.14	106.44	113.6	113.38	118.06	124.58
1.228	99.06	105.46	111.52	112.14	116.82	121.80
1.379	98.90	104.14	110.72	111.70	116.22	120.62
1.667	98.20	103.56	109.28	111.22	115.54	120.04
1.935	97.70	102.12	108.14	110.82	114.92	119.38
2.187	96.80	101.30	107.58	110.34	114.34	118.92
2.424	95.50	100.28	107.02	109.68	113.62	118.46
2.647	95.00	99.88	106.66	109.06	113.02	117.70
2.857	94.40	99.08	106.16	108.52	112.44	117.32
3.333	94.02	98.52	104.08	108.24	112.06	116.84
3.750	93.36	97.44	103.42	108.58	111.42	115.46

---

Scheme 2 Plausible schematic of complex formation between the imidazolium cation and the crown ethers.

Table 2 Formation constant, enthalpy, entropy and free energy change values of the crown ether complexes in ACN solution

Crown	log Kf (M^−1)			ΔH° (kJ mol^−1)	ΔS° (J mol^−1 K^−1)	ΔG° (kJ mol^−1)
	298.15 K	303.15 K	308.15 K
18C6	3.35	3.14	2.97	−65.02	−157.67	−18.01
DB18C6	3.05	2.96	2.87	−29.90	−43.57	−16.91

---

The molar conductance (Λ) of the imidazolium salt (5 × 10⁻⁴ M) in ACN solution was monitored as a function of the crown ether to imidazolium cation mole ratio at various temperatures. The resulting molar conductance vs. crown/cation mole ratio plots at 298.15, 303.15, and 308.15 K are shown in Fig. 1 and 2. In both cases, there is a gradual decrease in the molar conductance with increasing crown ether concentration. This behavior indicates that the complexed imidazolium cation is less mobile than the corresponding free imidazolium cation in ACN; because the imidazolium salt is a strong electrolyte in acetonitrile, the changes are not due to ion pairing, unless the complexation of the cation causes the imidazolium salt to associate. Both Fig. 1 and 2 show that in the complexation of imidazolium cation with both crown ethers, addition of the crown solution to the imidazolium salt solution causes a continuous decrease in the molar conductance, which begins to level off at a mole ratio greater than one, indicating the formation of a stable 1 : 1 complex.^25,26 By comparison of the molar conductance-mole ratio plot for imidazolium cation–crown ether systems obtained at different temperatures (Fig. 1 and 2), it can be observed that the corresponding molar conductance increased rapidly with temperature due to the decreased viscosity of the solvent and, consequently, the enhanced mobility of the charged species present.

Fig. 1 Molar conductance vs. [18C6]/[cation] at 298.15 K (), 303.15 K (), and 308.15 K ().

Fig. 2 Molar conductance vs. [DB18C6]/[cation] at 298.15 K (), 303.15 K (), and 308.15 K ().

The stability of these complexes depends mainly on the strength of the bonds between the acidic protons of the imidazolium ring and the crown ether oxygen atoms (Scheme 2). The formation constants (log K_f) of the 1 : 1 complexes at different temperatures varied in the order 18C6 > DB18C6 for the IL. Thus, a decrease in the net charge on the oxygen atoms during the introduction of two benzo groups into the macrocycle destabilizes the obtained complex.

3.2 Association constants and thermodynamic parameters

The following mathematical treatment to calculate the formation constant is based on Evans et al. (1972).²⁷

The 1 : 1 complexation of IL with 18C6 crown ether can be expressed by the following equilibrium:

The corresponding equilibrium constant, K_f is given by

where [MC⁺], [M⁺], [C] and f represent the equilibrium molar concentrations of the complex, free cation, and free ligand (crown ether) and the activity coefficients of the species indicated, respectively. Under the dilute conditions used, the activity coefficient of the uncharged macrocycle, f(C), can be reasonably assumed as unity.²⁸ The use of the Debye–Hückel limiting law²⁹ leads to the conclusion that f(M⁺) ∼ f(MC⁺); therefore, the activity coefficients in eqn (2) cancel. The complex formation constant in terms of the molar conductances, Λ, can be expressed as:^25,28wherehere, Λ_M is the molar conductance of the metal ion before the addition of ligand, Λ_MC is the molar conductance of the complexed ion, Λ_obs is the molar conductance of the solution during titration, C_C is the analytical concentration of the macrocycle added and C_M is the analytical concentration of the salt. The complex formation constant, K_f, and the molar conductance of the complex, Λ_MC, were evaluated using eqn (3) and (4).

Complexation enthalpy changes are mainly related to (i) cation-crown interactions, (ii) solvation energies of the species in solvent systems involved in the complexation reactions, (iii) repulsion between neighboring donor atoms, (iv) steric deformation of the crown, and (v) the number of H-bonds present for H-bonding. Entropy changes are linked to (i) changes in the number of particles involved in the complexation process and (ii) conformational changes of the crown ether accompanying the complexation.

In order to better understand the thermodynamics of the complexation reactions of imidazolium cation with crown ethers, it is useful to consider the enthalpic and entropic contributions to these reactions. The ΔH° and ΔS° values for the complexation reactions were evaluated from the corresponding log K_f and temperature data by applying linear least-squares analysis according to the equation:

The plots of log K_f vs. for both complexes (complex 1 and complex 2) are linear (Fig. 3 and 4).

Fig. 3 The linear relationships of log K_f vs. 1/T for the interaction of IL with 18C6 () and DB18C6 ().

Fig. 4 FTIR spectra of free IL (black), 18-crown-6 (blue) and their complex (red).

The enthalpy (ΔH°) and entropy (ΔS°) of complexation were determined from the slopes and intercepts of the plots, and the results are also listed in Table 2. Both of these parameters have negative values. True molecular recognition and physical attraction between host and guest should result in a favorable enthalpy change (ΔH) on complexation. The negative values of enthalpy confirm that when the imidazolium cation interacts with the crown ether molecules, the overall energy of the system is decreased, i.e., there is some stabilizing interaction in the system, whereas negative values of the entropy factor indicate that there is an ordered arrangement, i.e., complex formation takes place between the imidazolium and the crown molecules. Other investigators^30–32 established that the binding of free amino acids with 18C6 has negative enthalpy and negative entropy, which indicates that the process is driven by a favorable enthalpy change only.

The two fundamental equations ΔG = −RT ln K and ΔG = ΔH − TΔS are useful in comparing the contributions of enthalpy and entropy to the stabilities of different complexes. A negative value of entropy is unfavourable for spontaneous complex formation; however, this effect is overcome by higher negative values of ΔH°. The values of ΔG° (Table 2) for complex formation were found to be negative, suggesting that the complex formation process proceeds spontaneously.

The data shown in Table 2 indicate that the formation constant log K_f for imidazolium cation with both crowns is highest at 298.15 K and decreases with increasing temperature, i.e. imidazolium cation forms stable complexes with the crowns at 298.15 K.

3.3 IR studies

The IR spectra of 18C6, IL and complex 1 are shown in Fig. 4 and the spectra of DB18C6, IL and complex 2 are shown in Fig. 5 in the 4000 to 500 cm⁻¹ region. The shift of the IR spectra of the crown ethers in ACN solution indicates that the specific interactions observed in the crown ether complexes are in fact typical hydrogen bonds of the imidazolium ring with the donor atoms of the crown ether. Compared with the spectra of the free crown ethers, most of these bands are shifted to lower energies, presumably due to less restriction on the coupling of some vibrational modes caused by bonding of the oxygen atoms of the polyether ring with the C–H protons of the imidazolium ring in both complexes. In the case of 18C6, a very strong and sharp IR band centered at 1102 cm⁻¹ is assigned to the characteristic absorption due to the C–O–C asymmetric stretching vibrational motion [ν(C–O–C)_aliph]. This sharp peak is shifted to a lower frequency of 1082 cm⁻¹ in complex 1 (Fig. 4). The ν(C–O–C)_arom stretching vibrations of DB18C6 are observed at 1126 cm⁻¹; this peak is also shifted to a lower frequency, 1108 cm⁻¹, in complex 2 (Fig. 5). The presence of benzene rings in DB18C6 make the IR spectra more difficult to assign because their characteristic bands may overlap with those of the ethylene glycol groups. In the IR spectra, the bands in the 2800 to 3000 cm⁻¹ region correspond to the CH stretching vibrations of the methylene groups of crown ethers. The CH stretching frequency of the methylene groups observed at 2895 cm⁻¹ in 18C6 is shifted to a higher frequency due to the perturbation of the methylene groups. Interaction of the O atoms of the crown with the protons of the imidazolium ring via hydrogen H-bonds are responsible for the perturbation. In the 1200 to 1300 cm⁻¹ range of the IR spectra of DB18C6 and its complex, there are two bands assignable to anisole ν_s(Ph–O–C) and ν_as(Ph–O–C) vibrations.³³ These anisole oxygens are involved in H-bond formation in complex 2, as indicated by the shifts of the ν_as(Ph–O–C) and ν_s(Ph–O–C) bands from 1216 and 1253 cm⁻¹ to 1198 and 1237 cm⁻¹, respectively. Selected IR data for the free compounds and their complexes and the corresponding changes in frequencies are listed in Table 3.

Fig. 5 FTIR spectra of free IL (black), dibenzo-18-crown-6 (blue) and their complex (red).

Table 3 Comparison between the frequency changes (cm⁻¹) of different functional groups of the free compounds and their complexes

Functional group	Wavenumber (cm^−1)		Change (cm^−1)
	18C6	Complex 1	Δδ
ν(C–O–C)aliph	1102	1082	20

---

Functional group	Wavenumber (cm^−1)		Change (cm^−1)
	DB18C6	Complex 2	Δδ
ν(C–O–C)arom	1126	1108	18
νas(Ph–O–C)	1216	1198	18
νs(Ph–O–C)	1253	1237	16

---

Functional group	Wavenumber (cm^−1)		Change (cm^−1)
	IL	Complex 1	Δδ
ν(C–H)	3082, 2930	3066, 2905	16, 25

---

Functional group	Wavenumber (cm^−1)		Change (cm^−1)
	IL	Complex 2	Δδ
ν(C–H)	3082, 2930	3069, 2921	13, 9

---

IR spectroscopy has been extensively used to analyse the interactions present in ILs. Shifts in the C–H stretching frequencies in imidazolium-based ILs provide information about the existence of H-bonding in complexes. The imidazolium based IL shows the presence of C–H stretching vibrations in the region of 3000 to 3100 cm⁻¹, which is the characteristic region for the ready identification of C–H stretching vibrations.^34,35 According to Grondin et al.,³⁶ the IR bands at 3160 ± 15 cm⁻¹ can be assigned to the more or less symmetric and anti-symmetric combination of the C(4)–H and C(5)–H stretching vibrations of the imidazolium ring. The two bands around 3120 ± 15 cm⁻¹ result from the C(2)–H stretching mode and Fermi resonances of the C–H stretching vibrations with overtones of in-plane ring deformations. In our investigation, the C–H vibrations at 3082 and 2930 cm⁻¹ in the FTIR spectrum are shifted to 3066 and 2905 cm⁻¹ in complex 1 (Fig. 4) and to 3069 and 2921 cm⁻¹ in complex 2, respectively (Fig. 5). In the IR spectra, the region between 2800 cm⁻¹ and 3000 cm⁻¹ shows the CH₂ and CH₃ stretching vibrations of the alkyl groups at the nitrogen atoms of the imidazolium ring.

3.4 NMR studies

The complexations of the imidazolium salt with the crown ethers were investigated by ¹H NMR spectroscopy in CD₃CN at 298.15 K. The ¹H NMR spectra of IL (imidazolium ion) was recorded in the absence and the presence of 18C6 (Fig. 6) and DB18C6 (Fig. 7) in CD₃CN. A comparison of the ¹H NMR spectra for complex 1 (Fig. 6) with free IL revealed that the signals for the hydrogen atoms of the imidazolium ion (H2, H3 and H4) were shifted downfield. This downfield shift of the imidazolium protons supports the formation of the complex through H-bond formation involving [(C–H)_Imidazolium⋯O_Crown]– interactions. Signals for the –OCH₂ protons of the crown ethers were found to be shifted slightly downfield relative to those signals for the free individual components (Fig. 6).

Fig. 6 ¹H NMR spectra of complex 1 (18C6.IL) (above) and the uncomplexed imidazolium cation (below) recorded at 300 MHz in CD₃CN at 298.15 K.

Fig. 7 ¹H NMR spectra of complex 2 (DB18C6.IL) (above) and the uncomplexed imidazolium cation (below) recorded at 300 MHz in CD₃CN at 298.15 K.

In the case of complex 2 (Fig. 7), i.e. the complex of DB18C6, a downfield shift for the H2 signal was observed, while small upfield shifts for the other two imidazolium protons (H3, H4) were observed.^12,37 This suggests an orientation for the imidazolium ring that allows H-bond formation of H2 and a weak π–π interaction involving H3 and H4. Two opposing influences, namely H-bonding and π–π interactions, are responsible for the small upfield shifts for H3 and H4.³⁷ The changes in the chemical shifts suggest that host–guest complexation between the crown ether and the imidazolium salt exists in both complexes.^38,39

Based on the different associated modes of interaction and the ¹H NMR chemical shift data for the two complexes, complex 1 and complex 2, a plausible interaction scheme has been proposed; a schematic representation of this interaction is shown in Scheme 2. DB18C6 is a bowl-like host with two possible sites for interaction with the guest: the minor site formed by the O–CH₂–CH₂–O chains and the major site located between the phenyl rings. The minor site may interact with the guest molecules only via hydrogen bonds, while the major site can complex both via H-bonding and π-interactions (Scheme 2). The inclusion of a guest capable of interacting with both sites (imidazolium cation) leads to an interesting structure.^11,12

¹H NMR studies revealed an apparent perpendicular orientation of the imidazolium moiety of IL in the crown cavity of complex 2; however, this seemed to be different for complex 1. The possibility of such an orientation for the imidazolium ion was confirmed by Rissianen and Pursiainen for analogous inclusion complex formation between imidazolium ion and dibenzo-18-crown-6.¹¹ The ¹H NMR results also suggested that the electron-deficient imidazolium ion may be wrapped by benzene-substituted crown ethers and that the imidazolium ions are oriented face-to-face, such that the phenyl ring(s) and the substituents in the 1,3 position point away from the cavities of the crown ethers. In the 1,3-disubstituted imidazolium salt, both the 1 and 3 positions are substituted by alkyl groups, which are electron donating groups relative to hydrogen atoms. Thus, the substituents decrease the positive charge on the imidazolium ring and reduce the π–π stacking between the dibenzo crown host and the imidazolium guest in complex 2. In complex 1, the imidazolium ring can penetrate into the hollow circular based cavity of the macrocycle 18C6 and form strong H-bonds; however, the substituents in the 1,3 position point away from the cavities of the crown ethers.

Thus, detailed ¹H NMR spectral studies indicate that hydrogen bonding interactions [(C–H)_Imidazolium⋯O_Crown] in addition to weaker π–π/arene–arene donor–acceptor interactions resulted in moderately strong inclusion complex formation, i.e. the results of the ¹H NMR spectral studies support the results obtained from the conductivity and IR measurements.

3.5 Selected ¹H NMR data

1-Methyl-3-octylimidazolium tetrafluoroborate (IL). ¹H NMR (CD₃CN, 298.15 K): δ 8.47 (s, N–CH–N, 2H), 7.40–7.36 (d, N–(CH)₂–N, 2H), 3.84–3.81 (s, NCH₃, 3H), 4.15–4.10 (t, CH₂, 2H), 1.31 (m, C₅H_10, 10H), 0.92–0.88 (t, oct-CH₃, 3H).

18-Crown-6. ¹H NMR (CD₃CN, 298.15 K): δ 3.59–3.52 (s, OCH₂, 24H).

Dibenzo 18-crown-6. ¹H NMR (CD₃CN, 298.15 K): δ 6.96–6.89 (s, aryl, 8H), 4.13–4.10 (m, OCH₂, 8H), 3.88–3.85 (m, OCH₂, 8H).

18C6-1-methyl-3-octylimidazolium tetrafluoroborate (complex 1). 3.64 (m, OCH₂) ¹H NMR (CD₃CN, 298.15 K): δ 8.83 (s, N–CH–N, 1H), 7.53–7.50 (s, N–(CH)₂–N, 2H), 3.64–3.58 (m, OCH₂, 24H).

DB18C6-1-methyl-3-octylimidazolium tetrafluoroborate (complex 2). ¹H NMR (CD₃CN, 298.15 K): δ 8.75 (s, N–CH–N, 1H), 7.27–7.24 (d, N–(CH)₂–N, 2H), 6.95–6.90 (s, aryl, 8H), 4.09–4.05 (m, OCH₂, 8H), 3.89–3.86 (m, OCH₂, 8H).

3.6 Typical features of specific interactions involved in the complexation

The formation of inclusion complexes of crown ethers with the imidazolium ion involved three possible modes of interaction. The most prominent mode is the hydrogen bonding interaction between the oxygen atoms of the crown ethers (O_Crown) and the acidic C–H protons of the imidazolium ion [(C–H)_Imidazolium] for [(C–H)_Imidazolium⋯O_Crown]– interaction. π–π stacking interactions between the electron poor imidazolium ring and the aryl groups of the crown ether-based host (DB18C6) is the second mode which is expected to contribute to the stability of the adduct formation. The possibility of such an interaction for an analogous system was reported earlier.^11,14 In addition to H-bonding and π–π stacking interactions, induced dipole–dipole interactions between the imidazolium ion and O_Crown, having −δ charges, could also contribute to the overall stability of the adduct formation; this proposition was made independently by Schmitzer et al. and Pursiainen et al.^{11–14,40–42} However, this induced dipole–dipole interaction is expected to be weaker compared to the two previous modes of interaction discussed.

In complex 2 (Scheme 2), hydrogen bonding seems to play a secondary role. Obviously, the π–π interaction is dominant in this complex (Scheme 2) because the benzene rings of DB18C6 decrease the negative charge of the oxygen atoms and hence decrease their ability to undergo hydrogen bonding; however, under favourable conditions, hydrogen bonds can enhance the stability of crown ether complexes. Also, the electrostatic interactions between the aromatic ring of the crown and the positive charge of the imidazolium ring play an important role to stabilise the complex. The negative charge on the benzene rings of the crown ether skeleton is enhanced by the ether oxygen atoms, and this negative face of the aromatic ring interacts with the positive charge of the imidazolium ring. The unsubstituted crown ether imidazolium complex [complex 1] is likely stabilized by hydrogen bonds formed between the acidic protons of the imidazolium ring and the ether oxygen atoms (C–H⋯O interactions).⁴³

The stability constants (log K_f) for 1 : 1 complexation were measured in ACN solution by conductance studies and are presented in Table 2. In both complexes [complex 1 and complex 2], H-bonding with the ether oxygen atoms is obviously responsible for the complexation. This can be shown by a suitable plausible mechanism (Scheme 2). Complexation is mainly caused by H-bonding; however, either π-stacking or charge-transfer interactions (Scheme 2) also seem to make minor contributions towards complexation, with the possibility of ion–dipole interactions between the positive N atom of the imidazolium cation and ether oxygen atoms. The stability constant for complex 2 is slightly lower than the corresponding value for complex 1 (Table 2). The aromatic rings of the crown ether decrease the electron density of the adjacent oxygen atoms, and this seems to decrease the strength of the H-bonding in complex 2, explaining the lower stability constant. Although complex 2 has potential π-stacking or charge transfer interactions which are absent in complex 1, these results indicate that H-bonding is dominant for the formation of these complexes.

4. Conclusion

Conductometric titration data support the different types of interactions responsible for the complex formation of crown ethers with IL molecules and are consistent with the IR and NMR spectra. The stabilities of complexes between planar, five-membered imidazolium cations and crown ethers were established by different types of non-covalent interactions. We have found that the studied complexes are mainly stabilised by hydrogen bonds, and π-stacking or cation-π interactions play only a secondary role in the case of complex 2. The larger formation constant value for complex 1 compared to complex 2 determined by conductivity studies indicates that the imidazolium cation forms a more stable complex with 18C6 than with DB18C6 in ACN solution. The 1 : 1 complexation of the imidazolium-based IL by different crown ethers is driven by favourable changes in enthalpy (ΔH° < 0) and proceeds spontaneously (ΔG° < 0). This study may also help to provide important information about other host–guest systems with crown ethers.

Here, our studies of the complexation of an imidazolium ion, similar to the complexation of pyridinium ions,⁴⁰ provide further information on the nature of the complexation between positively charged organic guests and macrocyclic polyethers. This study is also significant for understanding the vital role of imidazolium cations in the design and construction of supramolecular host–guest materials.

Acknowledgements

The authors are grateful to the Special Assistance Scheme, Department of Chemistry, NBU under the University Grants Commission, New Delhi (No. 540/27/DRS/2007, SAP-1) for financial sustenance and instrumental conveniences in order to carry out this research. Prof. M. N. Roy is also highly obliged to the University Grants Commission, New Delhi, Government of India for being awarded a one-time grant under Basic Scientific Research via Grant-in-Aid No. F.4-10/2010 (BSR) in recognition of his dynamic service for the augmenting of research facilities to expedite and advance research.

References

1. C. J. Pedersen, J. Am. Chem. Soc., 1967, 89, 7017–7036.
2. C. J. Pedersen, J. Am. Chem. Soc., 1967, 89, 2495–2496.
3. G. W. Gokel, Crown Ethers and Cryptands, Royal Society of Chemistry, Cambridge, UK, 1991.
4. M. Dobler, Ionophores and their Structures, Wiley-Inter science, New York, USA, 1981.
5. A. M. Stuart and J. A. Vidal, J. Org. Chem., 2007, 72, 3735–3740.
6. N. Jose, S. Sengupta and J. K. Basu, J. Mol. Catal. A: Chem., 2009, 309, 153–158.
7. J. Malval, I. Gosse, J. Morand and R. Lapouyade, J. Am. Chem. Soc., 2002, 124, 904–905.
8. L. Tavano, R. Muzzalupo, S. Trombino, I. Nicotera, C. O. Rossi and C. L. Mesa, Colloids Surf., B, 2008, 61, 30–38.
9. F. A. Christy and P. S. Shrivastav, Crit. Rev. Anal. Chem., 2011, 41, 236–269.
10. T. B. Stolwijk, E. J. R. Sudhölter, D. N. Reinhoudt and S. Harkema, J. Org. Chem., 1989, 54, 1000–1004.
11. S. Kiviniemi, A. Sillanpaa, M. Nissinen, K. Rissanen, M. T. Lamsa and J. Pursiainen, Chem. Commun., 1999, 10, 897–898.
12. S. Kiviniemi, M. Nissinen, M. T. Lamsa, J. Jalonen, K. Rissanen and J. Pursiainen, New J. Chem., 2000, 24, 47–52.
13. M. Lee, Z. Niu, D. V. Schoonover, C. Slebodnick and H. W. Gibson, Tetrahedron, 2010, 66, 7077–7082.
14. N. Noujeim, L. Leclercq and A. R. Schmitzer, J. Org. Chem., 2008, 73, 3784–3790.
15. E. Ennis and S. T. Handy, Curr. Org. Synth., 2007, 4, 381–389.
16. D. Tapu, C. Owens, D. VanDerveer and K. Gwaltney, Organometallics, 2009, 28, 270–276.
17. J. C. Walton, M. M. Brahmi, L. Fensterbank, E. Lacôte, M. Malacria, Q. Chu, S. H. Ueng, A. Solovyev and D. P. Curran, J. Am. Chem. Soc., 2010, 132, 2350–2358.
18. T. Welton, Chem. Rev., 1999, 99, 2071–2083.
19. W. Xu, E. I. Cooper and C. A. Angell, J. Phys. Chem. B, 2003, 107, 6170–6178.
20. S. G. Lee, Chem. Commun., 2006, 10, 1049–1063.
21. J. E. Bara, D. E. Camper, D. L. Gin and R. D. Noble, Acc. Chem. Res., 2010, 43, 152–159.
22. D. Ekka and M. N. Roy, J. Phys. Chem. B, 2012, 116, 11687–11694.
23. A. Sinha, A. Bhattacharjee and M. N. Roy, J. Dispersion Sci. Technol., 2009, 30, 1003.
24. Y. Takeda and T. Kimura, J. Inclusion Phenom. Mol. Recognit. Chem., 1991, 11, 159–170.
25. M. Shamsipur and G. Khayatian, J. Inclusion Phenom., 2001, 39, 109–113.
26. G. Khayatian and F. S. Karoonian, J. Chin. Chem. Soc., 2008, 55, 377–384.
27. D. F. Evans, S. L. Wellington, J. A. Nadi and E. R. Cussler, J. Solution Chem., 1972, 1, 499–506.
28. G. Khayatian, S. Shariati and M. Shamsipur, J. Inclusion Phenom., 2003, 45, 117–121.
29. P. Debye and H. Hückel, Phys. Z., 1928, 24, 305–307.
30. A. F. Danil de Namor, M. C. Ritt, M. J. Schwing-Weill, F. Arnaud–Neu and D. F. Lewis, J. Chem. Soc., Faraday Trans., 1991, 87, 3231–3239.
31. H. J. Buschman, E. Schollmeyer and L. Mutihae, J. Inclusion Phenom., 1998, 30, 21–28.
32. M. Czekalla, K. Geo, B. Haberman, T. Kruger, H. Stephan, F. P. Schmidtchen and R. Trultzsch, Poster PB 37, XXI International Symposium on Macrocyclic Chemistry, Montieatini Treme, Italy, 1996.
33. A. Yu. Tsivadze, A. Yu. Varnek and V. E. Khutorskoi, Koordinatsionnye soedineniya metallov s kraunligandami (Coordination Compounds of Metals with Crown Ligands), Nauka, Moscow, 1991.
34. H. G. Silver and J. L. Wood, Trans. Faraday Soc., 1964, 60, 5–9.
35. R. Ramasamy, J. Appl. Spectrosc., 2013, 80, 506–512.
36. J.-C. Lassègues, J. Grondin, D. Cavagnat and P. Johansson, J. Phys. Chem. A, 2009, 113, 6419–6421.
37. A. K. Mandal, M. Suresh and A. Das, Org. Biomol. Chem., 2011, 9, 4811–4817.
38. Z. Zhou, X. Yan, T. R. Cook, M. L. Saha and P. J. Stang, J. Am. Chem. Soc., 2016, 138, 806–809.
39. X. Yan, T. R. Cook, J. B. Pollock, P. Wei, Y. Zhang, Y. Yu, F. Huang and P. J. Stang, J. Am. Chem. Soc., 2014, 136, 4460–4463.
40. M. Lämsä, J. Huuskonen, K. Rissanen and J. Pursiainen, Chem.–Eur. J., 1998, 4, 84–92.
41. M. Lämsä, T. Suorsa, J. Pursiainen, J. Huuskonen and K. Rissanen, Chem. Commun., 1996, 1443–1444.
42. M. Lämsä, J. Pursiainen, K. Rissanen and J. Huuskonen, Acta Chem. Scand., 1998, 52, 563–570.
43. T. Steiner, Chem. Commun., 1997, 727–734.

---