Thermally reversible solidification of novel ionic liquid [im]HSO₄ by self-nucleated rapid crystallization: investigations of ionic conductivity, thermal properties, and catalytic activity

Abstract

In recent years, the growing concern for the environment has triggered the search for new materials that meet the criteria for the preservation of natural habitats. Among these materials are ionic liquids, the new generation of which has contributed substantially to the rapid development of green chemistry. In this paper, we report the synthesis and characterization of an ionic liquid of imidazolium hydrogen sulphate ([im]HSO₄), which displays multifunctional properties. The [im]HSO₄ ionic liquid can be used either as an efficient catalyst in the preparation of hexahydroquinolines under green conditions or as a thermally reversible ionogel. Investigations of its catalytic properties showed that [im]HSO₄ can be reused at least four times without appreciable loss of activity, and the reaction yields are in the range of 92% to 98% referred to the isolated pure products. On the other hand, the ionogels formed by [im]HSO₄ showed high ionic conductivity (up to 25 mS cm⁻¹ in the solid phase) and melting points of approximately 54 °C. The process responsible for gelation was found to be solidification by self-nucleated crystallization. The structure of [im]HSO₄ was fully characterized using FT-IR, ¹H NMR, ¹³C NMR, XRD, SEM, TGA, and DTA. The physical properties important from an application point of view (e.g., ionic conductivity, thermal stability, phase transitions and microstructure) were investigated by thermal scanning conductometry (TSC), differential scanning calorimetry (DSC) and polarised optical microscopy (POM). The results of this work support the rational design, synthesis and application of task-specific ionic liquids for various purposes.

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Introduction

As green substances that consist entirely of ions, ionic liquids (ILs) display specific physical properties that significantly improve environmental protection (due to negligible vapor pressure and flammability) and allow them to be used in a wide range of applications. By appropriately changing the compositions of ILs by varying the respective ions, desirable properties like viscosity, ionic conductivity (σ), melting point, and catalytic activity can be tailored for to meet specific demands. Therefore, in recent years, the utilization of ILs has triggered exceptional possibilities in the design of advanced materials and physical–chemical systems.^1–11 The study of imidazolium ILs grew rapidly in the last 15 years as a result of growing research in the field, and because imidazolium salts can serve as N-heterocyclic carbene precursors, which are excellent ligands for transition metal-based catalysts.¹² The imidazolium cation is a flat, pentagonal ring. In combination with weakly coordinating anions, the ring bonds exhibit a pronounced aromatic bond character with a delocalized π-electron system.¹³ Two main reasons make these materials promising candidates to design adaptive, nanostructured, anisotropic (“low-dimensional”), ion-conductive materials for molecular electronics, batteries, fuel cells, and capacitors: high ionic conductivity and non-volatility. Compared to classical liquid electrolytes, ILs are far more stable in a wider temperature range. Nevertheless, the liquid state is still a disadvantage for a broad range of applications. Therefore, many strategies for combining the high conductivity of the liquid state with the stiffness and shape retention of the solid state have been developed. Four major approaches for the quasi-solidification of ILs can be distinguished: gelation with polymers,^4,14–22 gelation with inorganic substances (i.e., silica nanoparticles and carbon nanotubes),^23–26 gelation with low-molecular-mass gelators,^27–32 and mixing different ILs.³³ A huge effort in this field resulted in the design of polymer gel electrolytes (PGEs) and ionic gels (IGs) based on the supramolecular concept of noncovalent bonding in which the constituent ions act as mobile charge carriers.^34,35 In continuation of our studies on the knowledge-based development of task-specific ionic liquids (TSILs),³⁶ we now report the synthesis of imidazolium hydrogen sulfate ([im]HSO₄) as a nanocrystal ionic liquid (NCIL) for the first time and fully characterize it using FT-IR, ¹H NMR, ¹³C NMR, thermal gravimetric analysis (TGA), differential thermal analysis (DTA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal scanning conductometry (TSC) and polarizing optical microscopy (POM). The synthesized [im]HSO₄ displays multifunctional properties. Our investigations showed that the described IL can be used as a solid-state electrolyte (below dissolution temperature, T_s) and an efficient catalyst for the preparation of hexahydroquinolines under mild and solvent-free conditions. The described method is easy, and the products are obtained with high yields in short reaction times.

Results and discussion

To the best of our knowledge, the design, synthesis and application of TSILs provide great promise for various purposes such as catalysts and extractants,³⁶ extrication, solvents, gelators, buffers, ligands, and tags.⁴⁰ In this regard, we decided to introduce a novel nanocrystal IL, [im]HSO₄, as a multi-purpose IL with special properties. We first discuss the characterization of [im]HSO₄.

Characterization of the NCIL [im]HSO₄

To verify and characterize the synthesized [im]HSO₄, two complementary spectroscopic methods were used to identify the specific interactions characteristic of synthesized NCIL.

FT-IR studies

FT-IR (KBr, cm⁻¹) μ_max: 580, 887, 1176, 1284, 1413, 1576, 2615, 2842, 2971, 3130 cm⁻¹. The FT-IR spectral data of the NCIL [im]HSO₄ are presented in Fig. 1. The spectrum of the IL exhibits a broad peak at 3130 cm⁻¹, which is related to N–H stretching in the imidazolium ring. The strong absorptions at 580 and 887 cm⁻¹ were assigned to the stretching and bending S–O vibrations of hydrogen sulphate. The absorptions at 1176 and 1284 cm⁻¹ are related to O=S=O asymmetric and symmetric stretching and were absent in imidazole. In addition, C=N, N–H, and C=C vibrations were observed at 1643, 1576, and 1413 cm⁻¹, respectively. The broad and strong bands at 2400–3600 cm⁻¹ are attributed to the stretching of the hydroxyl group in the NCIL. These special IR peaks indicated that hydrogen sulphate was successfully assembled by the imidazole molecule as an anion within the corresponding molecular structure of the NCIL (Fig. 1).

Fig. 1 FT-IR spectra of imidazole (a), NCIL [im]HSO₄ (b), and the recycled NCIL sulphate [im]HSO₄ catalyst (c).

¹H NMR and ¹³C NMR studies

The ¹H and ¹³C NMR spectra of [im]HSO₄ are presented in Fig. 2. The important peak in the ¹H NMR spectrum of the IL catalyst is linked to the acidic hydrogens (N–H groups in the imidazolium ring and hydrogen sulphate) and is observed at δ = 10.71 ppm. To confirm that this peak (10.59 ppm) is really related to the N–H groups of the imidazolium ring and hydrogen sulphate and not the hydrogen of H₂SO₄ (unreacted starting material) H₂SO₄: the ¹H NMR spectrum (DMSO-d₆, 300 MHz, ppm): δ = 12.54 (s, 1H)⁴¹ was investigated. The difference between the peaks of the acidic hydrogens in NCIL [im]HSO₄ and H₂SO₄ (ref. 41) confirmed that the peak observed at 10.59 ppm in the ¹H NMR spectra of NCIL [im]HSO₄ is related to the acidic hydrogens of this compound. The structure of [im]HSO₄ indicates that this reagent can act as an efficient catalyst in reactions requiring mild acidic catalysts. Our results verified that this prediction is correct, and that NCIL [im]HSO₄ can act as a promoter for the preparation of hexahydroquinolines under mild and green conditions (Scheme 2). Spectral data of NCIL [im]HSO₄: ¹H NMR (400.22 MHz, DMSO-d₆): d (ppm) 7.64 (d, J = 1.2, 2H, =C–H), 8.98 (S, 1H, =C–H), 10.59 (S, 3H, 2N–H, HSO₄); ¹³C NMR (100.64 MHz, DMSO-d₆): (ppm) 119.8, 134.8.

Fig. 2 ¹H NMR (a) and ¹³C NMR (b) spectra of NCIL [im]HSO₄.

Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) of [im]HSO₄

The corresponding diagrams are shown in Fig. 3. As shown in Fig. 3a, the TGA and DTA profiles of the catalyst showed weight losses in one step; NCIL [im]HSO₄ was decomposed above 365 °C in one step. Notably, the number and appearance of the weight losses in TGA analysis are based on the nature of the specific components. Fig. 3b shows the DSC curve recorded for 2^nd heating and cooling cycle of the [im]HSO₄ sample in temperature range of 10 °C to 200 °C. This temperature range was chosen on the basis of the TGA results to avoid decomposition of the sample. During the heating stage, an endothermic peak at 78.3 °C occurred with a latent heat of −20.8 J g⁻¹. This transition is related to the melting of the [im]HSO₄ NCIL. During the cooling stage, two distinct exothermic peaks are observed at 68.9 °C and at 32 °C, with latent heats of 25.8 and 2.8 J g⁻¹, respectively. The large, sharp peak at 68.9 °C is related to the crystallization process, and the following peak at 32 °C is related to the polymorphic phase transition of enantiotropic nature. Such transitions are reversible and are normally observed as endothermic events in the heating cycle and exothermic events in the cooling cycle. As will be shown later, this reversible phase transition manifests itself in conductive properties revealed by TSC and the change in microstructure observed in POM investigations. Although the temperatures obtained by DSC and TSC differ, one should keep in mind that in TSC, the observed change in anomalies results from measuring the conductivity at different heating and cooling rates. Therefore as the thermal conditions in both experiments were different and the directly observed quantities were of different nature, we do not compare the temperatures obtained by both methods. We do however compare the number of observed anomalies in TSC and the number and order of peaks in DSC. The increased thermal stability of the investigated [im]HSO₄ can be understood on the basis of molecular self-assembly realized by intermolecular hydrogen bonds (Fig. S3 in ESI†). As a hydrogen-bond donor, the specific structure of the acidic NCIL [im]HSO₄ gives it the ability to produce molecular self-assembly (Fig. S2 in ESI†). Moreover, on the basis of the structure of NCIL [im]HSO₄, it can act as an efficient catalyst in reactions requiring acidic catalysts to accelerate the rate of reaction.

Fig. 3 TGA/DTG (a) and DSC (b) diagrams of NCIL [im]HSO₄.

SEM and XRD studies of [im]HSO₄

The morphology and particles size of [im]HSO₄ were investigated using scanning electron microscopy (SEM). The procedure can be described as follow. Before taking images, a sufficient amount of sample was dissolved in water and sprayed onto an SEM substrate. After evaporating the water, SEM images were taken at room temperature. All processes were carried out at room temperature. The obtained images are shown in Fig. 4. The obtained data confirmed that the particles size of the prepared NCIL was in the nanometer range. To study the purity and phase, the XRD pattern of the prepared NCIL [im]HSO₄ was obtained in the domain of 10–90 degrees (Fig. 5). The XRD pattern suggests that the synthesized NCIL [im]HSO₄ was crystalline in nature. Furthermore, the XRD data of the NCIL [im]HSO₄, including 2θ, peak width, particle size, and inter-planar distance were extracted using the Scherrer equation:where λ is the X-ray wavelength, K is the Scherrer constant, β is the peak width at half maximum, and θ is the Bragg diffraction angle (Table 1). The extracted XRD data indicate that the prepared material is nanometer-scale, consistent with the SEM images.

Fig. 4 SEM images of NCIL [im]HSO₄ at 20 KX magnification (a) and 40 KX magnification (b).

Fig. 5 X-ray diffraction pattern of NCIL [im]HSO₄.

Table 1 XRD data of NCIL [im]HSO₄

Entry	2θ	Full width at half maximum (degree)	Size (nm)	Inter-planar distance (nm)
1	11.5	0.15	46	0.7701
2	19.8	0.15	47	0.4479
3	20.5	0.13	59	0.4328
4	22.6	0.34	29	0.3931
5	57.2	0.10	69	0.1608

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Electrical properties of [im]HSO₄ gel electrolyte

The solidification of [im]HSO₄ occurs upon cooling from 90 °C to 20 °C (3 °C for a low-temperature study) and is driven by self-nucleated crystallization phenomena. As a result, a white, opaque ionogel is formed. The microstructure of the ionogel prevents bulk flow, changing the NCIL into a solid-like electrolyte. The observed gel–sol phase transition was reversible and repeatable. The [im]HSO₄ sample was stable over time at all of the studied temperatures and phases, and the registered ionic conductivities were repeatable. Thermal scanning conductometry (TSC) was applied to study the effect of electrode contact, which plays an important role in the production process; the efficiency of thermal reversibility; the renewability of the microstructure; and the properties of studied sample. From an application point of view, for every solid electrolyte (as well as for a liquid electrolyte), the ionic conductivity, σ, plays an important role. Many of the ionic gels prepared by different approaches^19,27,33,47 show σ values below 8 mS cm⁻¹ at room temperature, which are similar to the conductivity of the disclosed [im]HSO₄ gel electrolyte. Fig. 6 shows the temperature dependence of the direct current conductivity, σ_DC, for the studied sample, which was investigated during heating–cooling cycles at different rates of temperature change. Below the dissolution temperature (T_s), the investigated NCIL exists as an ionogel.

Fig. 6 Temperature dependence of the direct current conductivity σ_DC registered for the [im]HSO₄ sample during repeating heating cycles at 1.2 °C min⁻¹ (a) and cooling cycles at ∼7 °C min⁻¹ (b) and for different heating rates during heating cycles (c) and corresponding cooling cycles (d).

The observed difference in the conductivity dependence recorded for the 1^st and following thermal cycles (Fig. 6a) was caused by the electrode contact effect caused by placing the metal electrodes in the ionogel. Due to mechanical stress and the disruption of the gel matrix upon the insertion of the electrodes, the electrical performance of the ionogel decreased. This unfavourable effect occurring during the manufacturing stage makes the entire process much more difficult and expensive in order to obtain the best performance of the electrochemical device, based on the PGEs. This also shows that any disruption of the PGE's gel matrix leads to the gradual and irreversible degradation of the conductive properties. However, in the case of the studied NCIL [im]HSO₄, the gelation of which is mediated by noncovalent interactions, this problem can be easily solved by renewing the damaged microstructure using appropriate thermal treatment. Fig. 6a shows how the renewal process improves the conductive properties of the ionogel registered for the 1^st cycle (present electrode contact effect) and for the 2^nd and following cycles (removed electrode contact effect). Fig. 6b shows the reproducibility of the conductivity of the [im]HSO₄ ionogel registered for consecutive cooling stages with a transition to the solid electrolyte phase. The high reproducibility of the [im]HSO₄ conductive properties was also observed for different temperature change rates, simulating different operating conditions (Fig. 6c). The temperature change of the ionogel sample during heating–cooling cycles is presented in Fig. S4 (ESI†).

To determine whether the conductivity mechanism in the [im]HSO₄ IL/ionogel can be related to thermally activated processes, we analysed the 1^st derivative of the logarithm of σ_DC as a function of temperature. Fig. 7 shows the registered conductivity dependence for an extended temperature range and the corresponding 1^st derivative for the heating–cooling cycle. The data obtained for thermal cycles recorded at different rates of temperature change are presented in Fig. S5 in ESI.† The data of the corresponding activation energies are presented in Fig. S6.†

Fig. 7 Temperature dependence of the direct current conductivity σ_DC registered for the [im]HSO₄ sample with an extended low-temperature range in heating and cooling cycles (a). The 1^st derivative of the temperature dependence of the direct current conductivity σ_DC registered for the [im]HSO₄ sample for heating and cooling cycles (b).

Two different behaviours can be distinguished on the derivative course. During the heating stage of the thermal cycle, an anomaly in the derivative can be observed between 320 K and 333 K. This behaviour can be assigned to the gel–sol phase transition, when solidified [im]HSO₄ turns into the NCIL. The temperature region of the anomaly correlates well with the macroscopically observed gel–sol phase transition temperature in the sample. Furthermore, when looking closer at the heating stages (red points) in the high-temperature region (above T_s, observed as an anomaly on the derivative dependence), a continuous change in the derivative value can be observed. If the studied NCIL in the sol phase followed Arrhenius behaviour (as it does in the gel phase), we would observe a constant derivative value. The observed behaviour of the derivative in the sol phase indicates that the activation energy of the conductivity mechanism is also changing and cannot be described by a single value. However, by calculating the derivative of the natural logarithm of electrical conductivity over inverse temperature and multiplying it by the gas constant, it is possible to calculate the activation energy at each particular temperature (Fig. S6 in ESI†). During the cooling stage, two anomalies are observed, one in the region from 310 K to 319 K, and a second in the region from 292 K to 300 K. The origin of the 1^st anomaly in the cooling stage can be related to the gelation process of [im]HSO₄ caused by self-nucleated crystallization. The origin of the second anomaly is unknown, but we suspect that a structural change in the nanocrystal aggregates responsible for the creation of the gel phase. As can been seen in Fig. 7b, during the cooling stage, the derivative has a constant value outside the regions where the anomalies appear. This indicates that the conduction mechanism in all phases during cooling is dominated by thermally activated processes. A joint experimental and theoretical study has been carried out to determine the parameters characterizing the conduction mechanism in the [im]HSO₄ ionogel and sol phase; a theoretical fitting was performed for the experimental points. Fig. 7a shows that the electrical conductivity presents exponential behaviour with temperature during the heating stage in the ionogel phase; in contrast, above the gel–sol phase transition, non-exponential behaviour is observed. On the other hand, for all phases during the cooling stage, only exponential behaviour with temperature is observed. To explore this behavior, we plot the natural logarithm of σ_DC versus the inverse of absolute temperature during the heating and cooling stages in Fig. 8a and b, respectively.

Fig. 8 The temperature dependence of the direct current conductivity σ_DC registered for the [im]HSO₄ sample with an extended low-temperature range in heating (a) and cooling cycles (b). The solid lines represent the best fits of eqn (2), and the dashed lines are the best fits of eqn (3) to the experimental data recorded in the heating and cooling stages. The rectangles denote the regions where the anomalies were observed.

In the case of the exponential behaviour with temperature, we use an Arrhenius-type equation that can be written as⁴⁷

where E_a is the activation energy for electrical conduction, which indicates the energy needed for an ion to hop to a free hole, σ_∞ is the maximum electrical conductivity (that it would have at infinite temperature), and k_B is the Boltzmann constant. In the case where [im]HSO₄ follows the Arrhenius behaviour given by eqn (1), the data in Fig. 8a and b form a straight line. The non-exponential behaviour of the sol phase observed as curvature in Fig. 8a is accounted for by the Vogel–Fulcher–Tammann (VFT) equation. If we compare the Arrhenius equation (eqn (1)) with a general form of the VFT equation, it can be noted that the second is equal to the first if T_g = 0; thus, we can relate the fitting parameters of the VFT-type equation to the physical parameters of the Arrhenius equation and write the final version of the equation as follows:where T_g is the glass transition temperature.

Table 2 lists the values of the parameters σ_∞, E_a, and T_g obtained from fitting using eqn (1) and (2).

Table 2 Theoretical physical values extracted from the best fit of the Arrhenius-type (eqn (2)) and VFT-type (eqn (3)) equations to the experimental data

Heating rate	Heating stage
	Arrhenius		Vogel–Fulcher–Tammann (VFT)
	Ea (kJ mol^−1)	σ∞ (mS cm^−1)	Ea (kJ mol^−1)	σ∞ (mS cm^−1)	Tg (K)
1.5 °C min^−1	33.4	4.44 × 10^6	2.3	645	240.0

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Cooling rate	Cooling stage
	Arrhenius I		Arrhenius II		Arrhenius III
	Ea (kJ mol^−1)	σ∞ (mS cm^−1)	Ea (kJ mol^−1)	σ∞ (mS cm^−1)	Ea (kJ mol^−1)	σ∞ (mS cm^−1)
∼7 °C min^−1	39.7	7.95 × 10^7	33.7	6.48 × 10^6	25.9	3.19 × 10^5

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The results of the fitting procedure for other rates of temperature change are presented in Fig. S7 and Tables S1 and S2 in ESI.†

Microstructure of the studied [im]HSO₄ NCIL

The presence of the 2^nd anomaly in the cooling cycle was surprising. This intriguing observation pushed us to investigate the microstructure of the [im]HSO₄ as we suspected that the anomaly might be related to some structural change or reconstruction of the gel matrix. Fig. 9 shows the microstructures of the relevant phases of [im]HSO₄ observed under cross polarised light at different temperatures. The measurements were made during cooling from the isotropic phase of NCIL to the ionogel phase and back to the isotropic phase during heating stage.

Fig. 9 Microstructures of the [im]HSO₄, registered under cross polarised light, at different temperatures in the solid and liquid phases during heating–cooling cycles. The sequence and type of observed microstructures were repeatable for subsequent thermal cycles.

As shown in Fig. 9, the sol phase of [im]HSO₄ above 335 K was isotropic, resulting in no image under crossed polarizers. During cooling from the isotropic phase, the growth of molecular crystals via self-nucleated crystallization can be observed, and a distinct microstructure is revealed. The studied NCIL in the gel phase gains a birefringence properties causing the reflection and dispersion of polarised light and giving a colourful image of the crystalline phase. By further decreasing of the temperature, changes in the birefringence and the sizes and shapes of crystals were observed; moreover, a domain structure appeared. This low-temperature domain microstructure is preserved upon heating of the sample to the temperature where the sample becomes isotropic in phase. All of the observed phases of [im]HSO₄ are characterized by different activation energies of the conduction mechanism, which changes in steps when moving from one phase to another (see Table 2).

Catalytic activity study

Considering the reaction mechanism (Scheme 1), it can be proposed that in the first step of the reaction, dimedone is converted to its enol form by [im]HSO₄ and readily undergoes Knoevenagel condensation with benzaldehyde to generate a 2-benzylidenedimedone (1). On the other hand, the β-ketoester (activated by the catalyst) gives enamine (2) via the in situ liberation of ammonia from NH₄OAc. Afterwards, the 2-benzylidenedimedone (1) and enamine (2) react together by Michael addition to afford intermediate (3). The intermediate (3) is converted to intermediate (4) by tautomerization, and the intermediate (4) affords intermediate (5) through the intramolecular nucleophilic attack of the NH₂ group to the activated carbonyl group. Our suggested mechanism was confirmed by experimental data and also by data in the literature^41–49 (see Scheme 1).

Scheme 1 Plausible mechanism for the catalytic synthesis of ethyl 4-(phenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives catalyzed by NCIL [im]HSO₄.

Catalyst recycling study

An important aspect of the design, synthesis, and application of catalysts in large-scale production is the recyclability and reusability of the applied catalysts. In order to examine the recyclability of [im]HSO₄, the reaction between 4-methoxybenzaldehyde, dimedone, ethyl acetoacetate, and ammonium acetate was chosen (Fig. 10). For the recycling experiment, the reaction was carried out under the same conditions. At the end of the reaction, 3 mL of water was added to the reaction mixture. The IL was dissolved in water and filtered off to separate the crude product. The separated product was washed twice with water (2 × 3 mL). For recycling the catalysts, after washing the solid products with water, the water containing the NCIL, which is soluble in water, was evaporated under reduced pressure, and the NCIL was recovered and reused. The solid product was purified by recrystallization in ethanol. All of the desired product(s) were characterized by comparing their physical and spectral data (melting point, IR and ¹H NMR data) with those of known compounds.^42,49

Fig. 10 Results of the investigation of the reusability of [im]HSO₄ as a catalyst for the reaction described in the text.

Experimental section

Materials

All chemicals were purchased from Merck chemical company. The known products were characterized by comparing their physical properties and spectral data with reported authentic samples in the literature. The progress of the reaction and the purity of the products were inspected using TLC performed with silica gel SIL G/UV 254 plates.

Preparation of NCIL [im]HSO₄

Sulfuric acid (98%; 14.7 g, 150 mM) was added dropwise to a 500 mL round-bottomed flask containing imidazole (10.2 g, 150 mM) in dry CH₂Cl₂ (200 mL) over a period of 30 min at room temperature. After the addition was complete, the reaction mixture was stirred for 30 min and allowed to stand for 5 min. The CH₂Cl₂ was then decanted. The residue was washed with dry CH₂Cl₂ (3 × 100 mL) and dried under vacuum to give the NCIL [im]HSO₄ as a white crystal in 98% yield (24.4 g). The synthesis reaction is depicted in Scheme 2.

Scheme 2 Synthesis of the NCIL [im]HSO₄.

Catalytic activity

After the characterization of NCIL [im]HSO₄, we tested its catalytic activity for the preparation of hexahydroquinolines (Scheme 3) under mild and green conditions because the synthesis of 1,4-dihydropyridine compounds has received increasing interest in recent years due to their significant biological activities.³⁷ In particular, dihydropyridine drugs such as felodipine, amlodipine, and azelnidipine (see ESI, Fig. S1†) are effective cardiovascular agents for the treatment of hypertension and chest pain.³⁸

Scheme 3 Preparation of hexahydroquinolines under green conditions in the presence of NCIL [im]HSO₄.

General procedure for the synthesis of hexahydroquinolines under solvent-free conditions

A mixture of aldehydes (1 mM), dimedone (1 mM, 0.14 g), ethyl acetoacetate (1 mM, 0.13 g), ammonium acetate (1.1 mM, 0.09 g) and NCIL [im]HSO₄ (0.1 mol%, 0.166 g) as acidic catalyst was stirred at room temperature for a specific time. After reaction completion (as monitored by TLC), 3 mL of water was added to the mixture. The NCIL was dissolved in water and filtered to separate the crude product. The separated product was washed twice with water (2 × 3 mL). To recycle the catalyst, after washing the solid products with water, the water containing the NCIL was evaporated under reduced pressure, and the NCIL was recovered and reused. Please note that while extracting the catalyst with water, a small amount of ammonium acetate, which is unreacted in this reaction, may remain in the catalyst because both ammonium acetate and the NCIL are soluble in water. However, the amount of ammonium acetate is small and does not interfere with the following steps. The solid product was purified by recrystallization in ethanol. The results are summarized in Table 3.

Table 3 Synthesis of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives catalysed by NCIL [im]HSO₄

Entry	Product	Time (min)	Yield^a (%)	Mp/Mp [ref.] (°C)
a Yields refer to isolated pure products. The desired known pure products were characterized by comparing their physical data (melting points, ^1H NMR, ^13C NMR) with those of known reported compounds.
1		18	92	256–258/255–256 (ref. 39)
2		14	93	263–265/262–264 (ref. 43)
3		12	98	257–259/257–259 (ref. 44)
4		14	94	203–205/205–207 (ref. 43)
5		16	91	229–231/228–230 (ref. 44)
6		15	92	242–244/242–244 (ref. 44)
7		12	96	192–194/191–193 (ref. 45)
8		20	93	206–208/205–206 (ref. 43)
9		12	94	232–234/232–233 (ref. 43)
10		14	92	220–222/(220) (ref. 46)
11		18	94	196–198/196–198 (ref. 45)

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Ionogels

The solid-state electrolyte phase of [im]HSO₄ can be formed upon cooling from solution via self-nucleated crystallization, in which the previous crystallinity is incompletely melt or dissolved before cooling for recrystallization. As a result, we obtain a thermally reversible ionogel system composed of hydrogen-bonded aggregates of [im]HSO₄ ions. For ionic conductivity measurements using TSC, an appropriate amount of NCIL was placed in a tube covered with a rubber ring-sealed cap. After sealing, the samples were heated in a heating block at 373 K to form a clear solution and then cooled in the cooling block at 283 K to form aggregates. For self-nucleated gelation, the above samples were heated for specific lengths of time above dissolution temperature (T_s), which were in the vicinity of or just above the clearing temperature, T_cl, in a chamber with continuous nitrogen gas flow at given temperature. The samples were then cooled at gelation temperature and held for 15 min.

Verification and characterization of the synthesized [im]HSO₄

Melting points were measured using a Galen Kamp melting point apparatus. ¹H and ¹³C NMR spectra were recorded using a Bruker Avance 400 with DMSO-d₆ as the solvent and TMS as the internal standard. FT-IR spectra were recorded with an FT-IR spectroscope (JASCO, Japan). TG and DTA data were obtained using a PerkinElmer Pyris Diamond TG-DTA, DSC data were obtained by a Netzsch 200 F3 Maia (NETZSCH-Gerätebau GmbH, Germany) differential scanning calorimeter, XRD data were recorded using an APD 2000 PRO, and SEM images were recorded with a KYKY EM3200.

Ionic conductivity measurements

Electrical conductivity was characterized using a digital conductivity meter (S230 Seven Compact) with an InLab 710 four-electrode conductivity cell (Mettler-Toledo, USA). Thermal scanning conductometry (TSC) was carried out according to the method described previously.³⁹ The sample was loaded into a closed tube with a conductivity cell. The cell constant was calibrated using a 1.413 μS cm⁻¹ standard (aqueous solution of 0.01 mol L⁻¹ KCl). All experiments were carried out in a specially designed measurement chamber, and the heating medium was nitrogen gas at an initial temperature of 253 K. A purpose-built temperature controller was used to change the temperature of the ionogel sample. The measurements were carried out in heating–cooling cycles with set rates of temperature change and an accuracy of 0.1 K. The uncertainty in the conductivity measurements was estimated to be less than 0.5%. The temperature dependence of the conductivity of the gel electrolyte was measured in the temperature range of 285 K (275 K for one cycle to investigate the 2^nd anomaly) to 365 K.

Microscopic observation

The gel electrolyte microstructures were observed and registered at different temperatures with an Olympus BX35 microscope (Olympus, Japan) and Olympus Stream START software. The observations were done under cross polarised light, enabling the visualisation of molecular aggregates of [im]HSO₄ formed by self-nucleated crystallization. The microscopic images were obtained using an ionogel sample carefully cast on a microscopic slide covered with a 130 μm coverslip. The prepared sample was heated to 363 K (isotropic phase), cooled to 273 K, and heated again to the isotropic phase. Images were taken during the heating–cooling cycles.

Conclusions

In summary, a novel and efficient nanostructured IL, [im]HSO₄, was designed, synthesised and fully characterized by IR, ¹H NMR, ¹³C NMR, TGA, DTG, DSC, XRD, TSC, and POM. The catalytic ability of the NCIL [im]HSO₄ was studied for the synthesis of hexahydroquinolines via a one-pot multi-component reaction between various aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate under mild and solvent-free conditions. The synthesised catalyst showed excellent activity for the synthesis of hexahydroquinolines. Additional studies have indicated that nanostructured ILs can catalyse reactions requiring acidic catalysts. The NCIL reported herein provides the important advantages of a relatively short reaction time, high yield, clean reaction profile, low cost, simple product isolation, reusability of the nanostructured IL, and compliance with the principles of green chemistry. Moreover, the conductive properties, microstructure, and thermal reversibility of the solidification process of [im]HSO₄ used as a solid electrolyte were investigated. The studied NCIL showed high ionic conductivity up to 25 mS cm⁻¹ in the solid phase, complete reversibility of the solidification process by self-nucleated rapid crystallization, and excellent reproducibility of conductive properties over many heating–cooling cycles (i.e., going from a liquid-phase to a solid-phase electrolyte). Additionally, investigations of the temperature dependence of ionic conductivity showed Arrhenius behaviour in the gel phase and non-Arrhenius behaviour in the liquid phase above the dissolution temperature (T_s) in [im]HSO₄. The investigation of microstructure as a function of temperature revealed two distinct phases in the solid electrolyte state. The DSC investigations revealed anomalies in the TSC measurements and distinct microstructures caused by the enantiotropic polymorphic phase transition of [im]HSO₄. In our opinion, the present work provides new and promising insights for the rational design, synthesis and application of TSILs for various purposes.

Acknowledgements

The authors gratefully acknowledge the partial support of this work by the Polish Academy of Sciences and National Center for Science Poland (Grant No. DEC-2013/11/D/ST3/02694). Bu-Ali Sina University, Payame Noor University and Iran National Science Foundation (INSF) are acknowledged for the financial support (Grant of Allameh Tabataba'i's Award, Grant Number: BN093) of our research group.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21488f

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