Tunable aryl alkyl ionic liquids (TAAILs) based on 1-aryl-3,5-dimethyl-1H-pyrazoles

Journal of Molecular Liquids 248 (2017) 314–321

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Tunable aryl alkyl ionic liquids (TAAILs) based on 1-aryl-3,5-dimethyl-1H-pyrazoles Melek Canbulat Özdemir ⁎, Beytiye Özgün Department of Chemistry, Faculty of Science, Gazi University, 06500 Teknikokullar, Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 10 April 2017 Received in revised form 22 September 2017 Accepted 6 October 2017 Available online 10 October 2017 Keywords: Ionic liquids Pyrazolium cations [PhRC1pz]+ TAAILs Substituent effects

a b s t r a c t A series of fifteeen new 1-aryl-2,3,5-trimethylpyrazolium salts ([PhRC1pz][X], R: 4-Cl, 4-Br, 4-Me, 4-OMe, 4-NO2; − − C1: methyl, X: methanesulfonate [CH3SO− 3 ], tetrafluoroborate [BF4 ] and hexafluorophosphate [PF6 ]) were synthesized and characterized. Thermal properties (Tm/Tg and Td) of all the salts were investigated. Five of the salts can be classed as ionic liquids since they are liquid at rate time or melt below 100 °C three of which are room temperature ionic liquids (RTILs). The influence of the substituents at the para position of the aryl ring was investigated by studying the changes in the melting points of the corresponding methanesulfonate, tetrafluoroborate, and hexafluorophosphate salts. The results demonstrate that the electron-withdrawing substituents tend to higher melting points than electron-donating substituents. Thermophysical properties (density, viscosity, and refractive index) of the room temperature ionic liquids were measured as a function of temperature. The electrochemical windows (EW) of all the salts were determined experimentally. The EW values of the salts were calculated theoretically to make a comparison with the experimentally determined EW values. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquids (ILs) are defined as organic molten salts with melting points below 100 °C. In particular, salts which are even liquid at or below room temperature are called room temperature ionic liquids (RTILs) [1–5]. The interest in ILs increased significantly over the last decade, owing to their remarkable properties, such as negligible volatility, non-flammability, high ionic conductivity, high chemical, thermal, and electrochemical stability [6–13]. In order for ionic liquids to become simple and effective replacements for solvents in synthetic chemistry and also in other areas such as extraction processes, electrochemistry, lubricants, etc., it is very important that a wide range of salts becomes available. Thus, the design and synthesis of new ILs for numerous functional applications both in academia and industry is a dynamic area [14–20]. The physicochemical properties of ILs can be finely tuned for diverse scientific and technological applications by modifying the structure of both the cation and anion. In recent years, a new class of ILs called tunable aryl alkyl ionic liquids (TAAILs) were synthesized by Strassner and co-workers [21]. By introducing an aryl part to the imidazole or 1,2,4triazole core, researchers achieved to tune the properties of ILs with mesomeric (+ M, − M) and inductive effects (+ I, − I). In addition, the exchange of anion, modification of alkyl chain length and the type

(electron-withdrawing or -donating) and position of the substituents at the phenyl ring have an influence on the properties of TAAILs [21–24]. At recent years, it has also been shown that the quantum chemical calculations are an excellent tool to predict the electrochemical window of ionic liquids. The results obtained in some of these works could provide insights on which substituents can be attached to the phenyl ring of the cations in TAAILs for designing and developing TAAILs with excellent properties such that the electrochemical stability [25,26]. In our previous work, we focused on phenyl/alkyl-3,5dimethylpyrazolium based ILs which can be distinguished from the standard pyrazolium based ionic liquids by the phenyl ring substituted at the N-1 nitrogen atom of the pyrazole heterocycle. The combination of sp3-alkyl at the N-2 nitrogen atom, and sp2-phenyl substituent at the N-1 nitrogen atom on the pyrazole ring allows for additional mesomeric and inductive effects as well as π-π interactions [27]. In continuation of our studies, we investigated the influence of the electron withdrawing (-NO2, -Cl, -Br), and electron donating (− Me, − OMe) substituents at the para position of phenyl ring on the properties of 1aryl-2,3,5-trimethylpyrazolium salts by keeping alkyl part (methyl) − − constant for the counter anions [CH3SO− 3 ], [BF4 ] and [PF6 ]. 2. Results and discussion 2.1. Synthesis and characterization

⁎ Corresponding author at: Department of Environmental Engineering, Faculty of Engineering, Middle East Technical University, 06800 Ankara, Turkey. E-mail address: [email protected] (M.C. Özdemir).

https://doi.org/10.1016/j.molliq.2017.10.033 0167-7322/© 2017 Elsevier B.V. All rights reserved.

The application of MW energy is one of the eco-friendly methods to accelerate the course of many organic reactions, and having number of

M.C. Özdemir, B. Özgün / Journal of Molecular Liquids 248 (2017) 314–321

315

H3C O

O

C

C

H3C

+

R

NH-NH2.HCl

CH3COOH

N

MW, 120 oC

CH3

N

CH3

R

(1a-1e) H3C

R: -Cl, -Br, -CH3, -OCH3, -NO2 (a) (b) (c) (d) (e)

N

CH3

N

H3C

HBF4(aq)

H3C

O

CH3

(3a-3e) [PhRC1pz][BF4-]

RT H3CO

S

CH3 N

N N

CH3

R

O MW, 80 oC

R

R: -Cl, -Br, -CH3, -OCH3, -NO2 C1: methyl (a) (b) (c) (d) (e)

CH3

N

CH3SO3

-

CH3

(1a-1e)

BF4-

R

H3C

(2a-2e)

KPF6(aq)

[PhRC1pz][CH3SO3-]

N

R: -Cl, -Br, -CH3, -OCH3, -NO2 C1: methyl (a) (b) (c) (d) (e)

RT

N R

CH3

CH3

PF6-

(4a-4e)

[PhRC1pz][PF6-] R: -Cl, -Br, -CH3, -OCH3, -NO2 C1: methyl (a) (b) (c) (d) (e) Fig. 1. Synthetic pathway for the synthesis of 1-aryl-2,3,5-trimethylpyrazolium ILs/salts.

advantages such as shorter reaction times, enhancement in conversion and selectivity, higher yields of product, improved reproducibility, reduction of side reactions, and high efficiency of heating when compared to conventional methods [28–30]. Our interest in utilising microwave radiation is to minimize waste generation and to accelerate both the synthesis of 1-aryl-3,5-dimethyl-1H-pyrazole compounds and the quaternisation of these pyrazole compounds which is the first step in the synthesis of methanesulfonate ionic liquids. The synthesis of 1-aryl-2,3,5-trimethylpyrazolium methanesulfonate ionic liquids was carried out via two-step reaction (Fig. 1). The first step of the reaction is the synthesis of 1-aryl-3,5dimethyl-1H-pyrazole compounds. These compounds were synthesized from aryl hydrazinium hydrochloride derivatives and acetylacetone under MW irradiation (1a–1e). The second step of the reaction is alkylation of 1-aryl-3,5-dimethyl-1H-pyrazole compounds with methyl methanesulfonate in acetonitrile under MW irradiation. These reactions were yielded 1-aryl-2,3,5-trimethylpyrazolium methanesulfonate ionic liquids (2a–2e). To achieve the synthesis of 1aryl-2,3,5-trimethylpyrazolium tetrafluoroborate salts (3a–3e) and hexafluorophosphate salts (4a–4e), anion exchange reactions of the corresponding methanesulfonate salts were performed (Fig. 1).

The structures of 1-aryl-2,3,5-trimethylpyrazolium salts were characterized by FTIR, 1H NMR, 13C NMR, 19F NMR (3a–3e and 4a–4e), TOF MS and elemental analysis. All the spectral data elucidated the structure of the synthesized pyrazole salts. The FTIR spectra of all the salts show the characteristic bands of the pyrazolium moiety as well as those of the corresponding counterions, where appropriate. In particular, the ν(C = N) and ν(C = C) absorption bands from the pyrazolium cation appear at ca. 1595–1560 cm−1. Additionally, characteristic bands of the counterions − − are also observed. In particular, for [CH3SO− 3 ], [BF4 ], and [PF6 ] salts bands at ca. 1036–1039 cm−1, 1034–1037 cm−1, and 821–832 cm−1, corresponding to νs(SO3), ν(BF) and ν(PF) from the corresponding anions [31,32], appear at values that agree with the ionic nature of the salts. The pyrazole proton was also verified with 1H NMR spectra by emergence of one singlet around δ 6.43–6.85. In addition, doublets around δ 7.10–8.59 substantiated the presence of aromatic protons of aryl ring at the N-1 nitrogen atom of the pyrazole heterocycle. For the salts 2a–2e one singlet at ca. δ 2.68–2.76 proved three aliphatic protons

Table 1 Thermal properties of the 1-aryl-2,3,5-trimethylpyrazolium ILs/salts. Entry salts 1 2 3 4 5 6 7 8 a b c

2a 2b 2c 2d 2e 3a 3b 3c

Tma (°C)

Tgb (°C)

Tdc (°C)

Entry

Salts

Tma (°C)

Tgb (°C)

Tdc (°C)

– 76.3 – – 183.3 133.2 161.2 147.9

−38.8 – −51.9 −59.7 – – – –

271.1 269.8 291.6 284.2 247.5 368.6 364.5 363.6

9 10 11 12 13 14 15

3d 3e 4a 4b 4c 4d 4e

75.6 166.5 153.9 179.8 138.3 115.6 222.0

– – – – – – –

387.5 328.0 359.8 291.0 305.3 336.4 316.7

Tm – melting point. Tg – glass transition temperature. Td – decomposition temperature.

Fig. 2. Melting points of the 1-aryl-2,3,5-trimethylpyrazolium salts.

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Table 2 Cathodic and anodic potentials vs. Ag/Ag+ for EW values of the 1-aryl-2,3,5-trimethylpyrazolium ILs/salts at cut-off current density 1.0 mA/cm2 using GC macro-electrode as a working electrode at 25 °C. Entry

Salts

Ecathodic(V)

Eanodic(V)

EW(V)

Entry

Salts

Ecathodic(V)

Eanodic(V)

EW(V)

1 2 3 4 5 6 7 8

2a 2b 2c 2d 2e 3a 3b 3c

−1.37 −0.53 −0.86 −0.71 −1.07 −2.02 −1.60 −2.00

2.44 2.31 2.32 1.73 2.15 2.31 2.65 2.31

3.81 2.84 3.18 2.44 3.22 4.33 4.25 4.31

9 10 11 12 13 14 15

3d 3e 4a 4b 4c 4d 4e

−2.00 −1.05 −1.69 −1.61 −1.71 −1.75 −1.04

1.83 2.51 2.68 2.62 2.68 2.30 2.64

3.83 3.56 4.37 4.23 4.39 4.05 3.68

of methanesulfonate anion. 19F NMR spectra of 3a–3e show two signals at ca. δ (−148.30)–(−153.94) due to coupling of 19F nucleus with the two different boron isotopes, 10B and 11B, present in the [BF− 4 ] anion, and the 19F NMR spectra of 4a–4e show typical doublets of [PF− 6 ] octahedral structure at ca. δ (−68.90)–(−74.93). 2.2. Thermal properties Thermal transitions (Tm or Tg) of the 1-aryl-2,3,5trimethylpyrazolium salts were determined by differential scanning calorimetry (DSC), the results of which are summarized in Table 1. Five of the salts (2a–2d, 3d) can be classed as ionic liquids since they are liquid at rate time or melt below 100 °C. All of the 1-aryl-2,3,5− trimethylpyrazolium [PF− 6 ] and [BF4 ] salts (except 3d) and one of the [CH3SO− 3 ] salt (2e) have melting points above 100 °C (Fig. 2, Table 1). Variation of the substituent (R) on the phenyl ring of the 1-aryl2,3,5-trimethylpyrazolium salts leads to different melting points/glass transitions using the same alkyl chain length and anion depending on the mesomeric or inductive effects of the substituent. As seen in Fig. 2 and Table 1 electron withdrawing substituents tend to higher melting points than electron donating substituents at the para position of the phenyl ring. The electron donating substituents like 4-Me and 4-OMe have a significant influence on the electronic properties of the aryl ring. The inductive effect of the methyl group at the para position is very different compared to the mesomeric effect of the methoxy substituent, which interacts via the π-system. Thus, one can tune the properties of the salts through the introduction of an electronic variation through para substitution on the aryl ring. Also, the effect of the counterion can be shown by comparing the pyrazolium salts which have the same cationic structure (Fig. 2, Table 1). The asymmetric [CH3SO− 3 ]

anion generally shows a much weaker coordination to the cation in − comparison to [BF− 4 ] and [PF6 ] anions and has more available conformations [33,34]. Thus, the methanesulfonate salts (except 2b and 2e) are in liquid state at room temperature, while all hexafluorophosphate and tetrafluoroborate salts are in solid state. The thermal stabilities of the 1-aryl-2,3,5-trimethylpyrazolium salts were measured by thermogravimetry analysis (TGA). The thermal decomposition temperatures of the salts are presented in Table 1. All of the 1-aryl-2,3,5-trimethylpyrazolium salts proved to be stable up to temperatures between 247.5 and 387.5 °C. The difference between the − thermal stabilities of the [BF− 4 ] and [PF6 ] containing salts is not notable, but [CH3SO− ] containing salts exhibited much lower thermal stabilities 3 − than the corresponding salts with [BF− 4 ] and [PF6 ] counterions with almost 50 °C difference in the onset of decomposition curves. It was observed that the thermal stability of these salts is significantly influenced by the nature of the anion similar to that reported previously [27]. Altogether, the new 1-aryl-2,3,5-trimethylpyrazolium salts are sufficiently stable for most applications, although no long term TGA measurements have yet been performed.

2.3. Electrochemical window (EW) The electrochemical window of the 1-aryl-2,3,5trimethylpyrazolium salts was assessed by cyclic voltammetry (CV) at 50 mV/s sweep rate, starting from anodic to cathodic potentials and reversing back to the initial value. The electrochemical windows of the − − salts containing [PF− 6 ] and [BF4 ] anions are higher than [CH3SO3 ] − − anion containing salts and follow the order of stability [PF6 ] N [BF4 ] N [CH3SO− 3 ] in comparision to the salts having the same cation (Table 2).

Table 3 ELUMO and EHOMO (eV) of [PhRC1pz][PF6] salts, ELUMO, EHOMO (eV), potentials of cathodic (VCL) and anodic (VAL) limits of individual ions and, electrochemical window (EW) of 1-aryl-2,3,5− − trimethylpyrazole based [CH3SO− 3 ], [BF4 ], and [PF6 ] salts calculated by density functional theory (DFT) calculations at the B3LYP/6–311 + G(d,p) level in gas phase and in solvent acetonitrile. In gas phase

In solvent acetonitrile

[PhRC1pz][PF6] ELUMO EHOMO

R = Cl −1.99 −7.91

Br −1.99 −7.73

CH3 −1.71 −7.80

OCH3 −1.66 −7.17

NO2 −3.64 −8.48

Cl −1.57 −7.53

Br −1.56 −7.39

CH3 −1.39 −7.38

OCH3 −1.35 −6.82

NO2 −3.42 −7.92

[PhRC1pz]+ ELUMO EHOMO VCL/V VAL/V

−5.07 −10.51 5.07 10.51

−5.06 −10.22 5.06 10.22

−4.88 −10.52 4.88 10.52

−4.83 −9.78 4.83 9.78

−6.08 −11.29 6.08 11.29

−1.67 −7.68 1.67 7.68

−1.70 −7.53 1.70 7.53

−1.52 −7.55 1.52 7.55

−1.52 −6.97 1.52 6.97

−3.48 −7.07 3.48 7.07

Anions ELUMO EHOMO VCL/V VAL/V

[CH3SO− 3 ] 3.39 −1.97 −3.39 1.97

[BF− 4 ] 4.45 −4.55 −4.45 4.55

[PF− 6 ] 4.06 −5.54 −4.06 5.54

[CH3SO− 3 ] 0.20 −6.97 −0.20 6.97

[BF− 4 ] 0.29 −9.79 −0.29 9.79

[PF− 6 ] −0.04 −10.28 0.04 10.28

EW/V [PhRC1pz][CH3SO− 3 ] [PhRC1pz][BF− 4 ] [PhRC1pz][PF− 6 ]

−3.1 −0.52 0.47

−3.09 −0.51 0.48

−2.91 −0.33 0.66

5.29 6.01 6.01

5.27 5.83 5.83

5.44 6.03 6.03

5.45 5.45 5.45

3.49 3.59 3.59

−2.86 −0.28 0.71

−4.11 −1.53 −0.54

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Table 4 Experimental densities (ρ), dynamic viscosities (η) and refractive indices (nr) of the RTILs as a function of temperature at atmospheric pressurea. Aryl alkyl pyrazolium RTILs

T (K)

ρ (g/cm3)

η (mPa.s)

nr

2a

298 308 313 333 298 308 313 333 298 308 313 333

1.3046 1.2985 1.2925 1.2776 1.2236 1.2217 1.2066 1.1923 1.2596 1.2561 1.2321 1.2171

–b –b 2138.88 287.99 –b –b 821.74 148.52 –b –b 1096.01 192.48

1.5460 1.5430 1.5414 1.5354 1.5339 1.5305 1.5288 1.5237 1.5359 1.5333 1.5320 1.5266

2c

2d

a

Data were measured on samples which had a water content (wt%) in the range of 0.2–0.32 according to coulometric Karl-Fischer titration. b Not determined by a viscometer.

Electrochemical window of the salts, which can be correlated to the calculated difference in energy level of the lowest unoccupied molecular orbital (LUMO) of the cation (reductive stability) and the highest occupied molecular orbital (HOMO) of the anion (oxidative stability) were also calculated theoretically, both in gas phase and in solvent acetonitrile for comparison with the experimentally determined EW values. The HOMO and LUMO energy levels, the potentials of cathodic (VCL) and anodic limits (VAL) of the salts were calculated by density functional theory (DFT) at the B3LYP/6–311 + G(d,p) level in gas phase. All the computations in solvent acetonitrile were carried out using the SelfConsistent Reaction Field (SCRF) under the integral equation formalism polarizable continuum model (IEFPCM). The results are given in Table 3. EW is estimated from the following expressions; EW ¼ VAL −VCL ¼

−εHOMO −εLUMO εLUMO −εHOMO − ¼ e e e

The LUMO energy levels of the 1-aryl-2,3,5-trimethylpyrazolium hexafluorophosphate salts and those of their corresponding cations were calculated in gas phase and in solvent acetonitrile to test the correlation between these two LUMO energy levels. The results are shown in Table 3. Correlation between LUMO energy of cations and LUMO energy of [PhRC1pz][PF6] salts in gas phase and in solvent acetonitrile are shown in Fig. 3a and Fig. 3b, respectively, and clearly indicate that both LUMO energy levels are well correlated. Therefore, it can be assumed that the LUMO energy level of the cations solely determines the resistance of these salts against reduction. The cathodic stability of the salts against reduction is ordered as R = OMe N Me N Br N Cl N NO2, in gas phase. Similar to the results obtained in gas phase (with

Fig. 3. Linear correlation between LUMO energy of cations versus LUMO energy of 1-aryl2,3,5-trimethylpyrazolium hexafluorophosphate salts (4a–4e) in gas phase (a), in solvent acetonitrile (b).

Fig. 4. Correlation between calculated and experimental electrochemical windows (units: − − V) of 1-aryl-2,3,5-trimethylpyrazolium salts containing [CH3SO− 3 ], [BF4 ] and [PF6 ] anions.

the exception of the positions of salts having -Br and -Cl substituents) it can be found that cathodic stability of the salts against reduction is ordered as R = OMe N Me N Cl N Br N NO2 in solvent acetonitrile. Thus, the substituents at the para position of the phenyl ring of our interested cations have influence on the EW values of the salts. The electron donating groups such as -OMe, and -Me groups generally stabilize the cation (leads to greater EW) while electron-withdrawing groups such as halogen -Br, -Cl and -NO2 groups destabilise the cation (decreases EW). These results are in well accordance with the results obtained for para-X-phenyl methylimidazolium ([X − PhMIM]+: X = OCH3, CH3, and NO2) cations [25]. From HOMO energy of the anions, it is resulted that anodic stabilities of the anions are in the following order [CH3SO− 3 − ] b [BF− 4 ] b [PF6 ], and are in good agreement with those of calculated by Ong et al. and Roohi et al. [26,35]. − The EW values calculated theoretically for the [BF− 4 ] and [PF6 ] salts in solvent acetonitrile, follow the same order, R = NO2 b OMe b Br b Cl b Me. Besides, the experimental EW values for the [PF− 6 ] salts are in the same order as their calculated EW values. For the [BF− 4 ] salts a similar order (R = NO2 b OMe b Br b Me b Cl) for their experimentally determined EW values was observed. Thus, there is a reasonably good correlation between the theoretical and the experimental EW values with correlation coefficients of 0.93 and 0.84 for the [PF− 6 ] and the [BF− 4 ] salts, respectively (Fig. 4). The calculated EW values for [CH3SO− 3 ] ILs are in the order of R = NO2 b Br b Cl b Me b OMe while the experimental EW values are found to be in the order of R = OMe b Br b Me b NO2 b Cl in solvent acetonitrile. So, there is no reasonable correlation between the calculated and the experimental EW values for the methanesulfonate ionic liquids (R2: 0.36). 2.4. Thermophysical properties The thermophysical properties such as density, viscosity, and refractive index of the three dried 1-aryl-2,3,5-trimethylpyrazolium methanesulfonate room temperature ionic liquids (2a, 2c, and 2d) were investigated as a function of temperature from 298 K to 333 K (Table 4). It is difficult to dry these RTILs rigorously. As a consequnce, the residual water content may have effect on their thermophysical properties. The methanesulfonate RTILs (2a, 2c and 2d) are highly viscous at room temperature and their viscosities can not be determined by viscometer. As can be seen in Table 4, in the temperature range studied, the viscosity of these RTILs decreases with increasing temperature. Within the 1-aryl-2,3,5-trimethylpyrazolium RTILs the ordering of the viscosities followed is 4-Me b 4-OMe b 4-Cl. This trend may be attributed to a combination of steric and electronic effects in TAAILs [36]. The experimental densities and refractive indices of 1-aryl-2,3,5trimethylpyrazolium methanesulfonate ionic liquids (2a, 2c, 2d) were plotted as a function of temperature, and are shown in Fig. 5 and

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Fig. 6, respectively. Both the densities and the refractive indices of methanesulfonate ionic liquids decrease with increasing temperature as expected. Consequently, the density, viscosity and refractive indices of the RTILs were found to depend on temperature. 3. Conclusions In conclusion, a series of 1-aryl-2,3,5-trimethylpyrazolium salts − − comprising three different counterions ([CH3SO− 3 ], [BF4 ], and [PF6 ]) were synthesized and characterized. Thermal properties, electrochemical windows of all the salts and thermophysical properties (density, viscosity, and refractive index) of the RTILs (2a, 2c, and 2d) were evaluated. Electrochemical windows of all the salts were calculated theoretically and compared with the experimentally determined EW values. All the synthesized 1-aryl-2,3,5-trimethylpyrazolium salts have thermal stabilities in the range of 247.5–387.5 °C. 1-Aryl-2,3,5-trimethylpyrazolium − salts with [BF− 4 ], and [PF6 ] counterions exhibit large electrochemical window values which make them attractive for their application in electrochemical devices (supercapacitors, batteries), electrochemical synthesis, metal deposition, etc. The RTILs (2a, 2c, and 2d) with high refractive indices (1.53–1.55) are more environmentally benign than the vast majority of existing high refractive index immersion fluids. Therefore, these ionic liquids may be promising alternatives for optical immersion fluids. 4. Experimental section 4.1. General methods All chemicals were supplied by Acros, Aldrich Fluka, and Merck as well as other common suppliers and used as received. Microwaveassisted syntheses were performed with a professional multimode Microsynth - Milestone oven. A Thermo Scientific Madison 6700 spectrometer with an ATR (Attenuated Total Reflectance) attachment was used to obtain all IR spectra. 1 H (300 MHz), 13 C (75 MHz), and 19 F (282 MHz) NMR spectra were recorded on Bruker-Avance-300 MHz spectrometer (in CDCl3 or DMSO-d6). Elemental analysis of the synthesized salts was carried out with a LECO CHNS-932 elemental analyzer. Mass spectrometry was performed on a Micromass LCT Premier XE (Waters, MS Technologies). Shimadzu DSC-60 was used to perform differential scanning calorimetry (DSC) experiments, with a ramp temperature of 10 °C min− 1 under a nitrogen atmosphere. The thermal stability of all the salts was investigated on a Shimadzu TA-60WS Thermal Analyzer at a heating rate 10 °C min− 1 with nitrogen as the purge gas. The density measurement of RTILs at ambient pressure was carried out using Anton Paar DMA-4500 M digital densimeter with an uncertanity of ±0.01 K. The DMA-4500 M densimeter protocol includes an automatic correction for the viscosity of the sample. The apparatus is precise within 1.0 × 10−5 g/cm3, and the overall uncertainty of the experimental density measurement at ambient pressure was better than ±1.0 × 10−4 g/cm3. Calibration of the densimeter was carried out at atmospheric pressure using distilled and degassed water. An Anton Paar Microviscometer (AMVn) was used for measurement of the dynamic viscosities of the RTILs which is based on falling-ball principle. Refractive index measurements of RTILs were conducted with a refractometry DR 6300-TF, A. Krüss Optronic GmbH equipped with a temperature control using a He\\Ne light source with a wavelength 633 nm. The water content of RTILs was determined using coulometric KarlFischer titrator, Cou-Lo Aquamax KF moisture meter. Cyclic Voltammetry measurements were conducted at 25 °C on Electrochemical Workstation CHI-660B instrument, using glassy carbon macro electrode (surface area: 7.065 × 10− 2 cm2) as a working electrode, Pt as a counter electrode and Ag/AgCl as a reference electrode. 0.1 M solution of the 1-aryl-2,3,5-trimethylpyrazolium salts

was prepared in anhydrous acetonitrile and then the solution was purged with nitrogen for at least 10 min to remove oxygen and water. 4.2. General procedure for the synthesis of 1-aryl-3,5-dimethyl-1Hpyrazole compounds (1a–1e) To a mixture of arylhydrazinium hydrochloride (5.0 mmol) in acetic acid (30.0 mL) acetylacetone (5.0 mmol) was added, and the mixture was heated at 120 °C under MW irradiation until all the starting materials were consumed (TLC, 20% EtOAc–hexane). Subsequent to completion of the reaction, acetic acid was removed by rotary evaporation. The residue was dissolved in ethyl acetate, and washed with diluted sodium hydrogen carbonate, water and saturated brine, respectively. The separated organic phase was dried over anhydrous sodium sulfate. After removing of ethyl acetate the remaining product was purified by column chromatography (silica gel: 20% EtOAc–hexane). 4.2.1. 1-(4-chlorophenyl)-3,5-dimethyl-1H-pyrazole (1a) Reaction time: 1.5 min., Yield: 95%, orange oil. 1H NMR (300 MHz, CDCl3, ppm) δ = 2.29 (s,3H,CH3); 2.30 (s,3H,CH3); 6.0 (s,1H,CH); 7.36–7.43 (m,4H,Ph). 13C NMR(75 MHz, CDCl3, ppm) δ = 12.24; 13.36; 107.41; 125.43; 128.94; 132.47; 138.43; 139.15; 149.05. 4.2.2. 1-(4-bromophenyl)-3,5-dimethyl-1H-pyrazole (1b) Reaction time: 2.0 min.,Yield: 78%, light brown oil. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.29 (s,3H,CH3); 2.30 (s,3H,CH3); 6.0 (s,1H,CH); 7.31–7.34 (d,2H,Ph,J: 8.8 Hz); 7.55–7.58 (d,2H,Ph,J: 8.8 Hz). 13 C NMR(75 MHz CDCl3, ppm) δ = 12.38; 13.46; 107.51; 120.43; 125.73; 131.95; 138.93; 139.13; 149.12. 4.2.3. 1-(4-methylphenyl)-3,5-dimethyl-1H-pyrazole (1c) Reaction time: 1.5 min., Yield: 90%, orange oil. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.28 (s,3H,CH3); 2.30 (s,3H,CH3); 2.40 (s,3H,CH3); 5.98 (s,1H,CH); 7.23–7.25 (d,2H,Ph, J: 8.4 Hz); 7.29–7.32 (d,2H,Ph, J: 8.4 Hz). 13C NMR(75 MHz CDCl3, ppm) δ = 12.27; 13.51; 21.04; 106.60; 124.66; 129.51; 137.07; 137.49; 139.30; 148.63. 4.2.4. 3,5-dimethyl-1-(4-methoxyphenyl)-1H-pyrazole (1d) Reaction time: 1.0 min.,Yield: 90%, brown oil. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.25 (s,3H,CH3); 2.29 (s,3H,CH3); 3.84 (s,3H,CH3); 6.0 (s,1H,CH); 6.94–6.97 (d,2H,Ph, J: 8.9 Hz); 7.31–7.34 (d,2H,Ph, J: 8.9 Hz). 13C NMR(75 MHz CDCl3, ppm) δ = 12.12; 13.49; 55.49; 106.24; 114.09; 126.34; 133.10; 139.44; 148.48; 158.76. 4.2.5. 3,5-dimethyl-1-(4-nitrophenyl)-1H-pyrazole (1e) Reaction time: 2.5 min.,Yield: 85%, yellow solid, m.p: 100.5 °C. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.32 (s,3H,CH3); 2.45 (s,3H,CH3); 6.1 (s,1H,CH); 7.68–7.71 (d,2H,Ph, J: 9.2 Hz); 8.32–8.35 (d,2H,Ph, J: 9.2 Hz). 13C NMR(75 MHz CDCl3, ppm) δ = 13.11; 13.48; 109.35; 123.42; 124.67; 139.87; 145.03; 145.51; 150.80. 4.3. General procedure for the synthesis trimethylpyrazolium methanesulfonates (2a–2e)

of

1-aryl-2,3,5-

To a solution of 1-aryl-3,5-dimethyl-1H-pyrazole (5.0 mmol) in acetonitrile (5.0 mL) methyl methanesulfonate (5.0 mmol) was added and the mixture was heated at 80 °C under MW irradiation for 30 min. The progress of the reaction was monitored by TLC (80% EtOAc–hexane). After completion of the reaction, acetonitrile was removed under reduced pressure. The resulting product was washed two times with 20.0 mL of hexane and then two times with 20.0 mL of diethyl ether. Then, active charcoal and 20.0 mL acetonitrile were added and stirred for 24 h. After filtration the filtrate was dried over anhydrous sodium

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319

4.3.3. 1-(4-methylphenyl)-2,3,5-trimethylpyrazolium methanesulfonate (2c) Yield: 97%, yellow liquid. IR (ATR, cm−1) νmax = 3041, 2970–2929, 1563, 1510, 1195, 1036, 1000–750. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.23 (s,3H,5-CH3); 2.63 (s,3H,3-CH3); 2.49 (s,3H, PhCH3); 2.76 (s,3H,S-CH3); 3.78 (s,3H,NCH3); 6.49 (s,1H,CH); 7.44–7.47 (d,2H,Ph,J: 8.3 Hz), 7.51–7.54(d,2H,Ph,J: 8.3 Hz). 13C NMR (75 MHz, CDCl3, ppm) δ = 12.27; 12.35; 21.40; 34.87; 39.30; 108.03; 128.40; 128.51; 131.36; 143.25; 146.62 and 147.78. Analysis: calcd. for C14H20N2O3S: C 56.73, H 6.80, N 9.45, S 10.82. Found: C 56.39, H 6.83, N 9.39, S 11.2. TOF MS (ES+) m/z calcd. for C13H17N2: 201.1392. Found: 201.1393. TOF MS (ES−) m/z calcd. for CH3O3S: 94.9803. Found: 94.9799.

Fig. 5. Experimental densities of RTILs (2a, 2c and 2d) as a function of temperature.

sulfate and concentrated. The salts thus obtained were set under vacuum for 48 h at 75 °C.

4.3.1. 1-(4-chlorophenyl)-2,3,5-trimethylpyrazolium methanesulfonate (2a) Yield: 94%, yellow liquid. IR (ATR, cm−1) νmax = 3054, 2971–2931, 1563, 1491, 1195,1036, 1000–750. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.24 (s,3H,5-CH3); 2.62 (s,3H,3-CH3); 2.68 (s,3H,S-CH3); 3.80 (s,3H,NCH3); 6.51 (s,1H,CH); 7.63–7.66 (d,2H,Ph,J: 8.4 Hz), 7.74–7.77(d,2H,Ph,J: 8.4 Hz). 13C NMR (75 MHz, CDCl3, ppm) δ = 12.32; 12.40; 35.07; 39.40; 108.24; 129.55; 130.68; 131.03; 138.82; 146.69 and 148.30. Analysis: calcd. for C13H17ClN2O3S: C 49.29, H 5.41, N 8.84, S 10.12. Found: C 49.01, H 5.44, N 8.79, S 10.42. TOF MS (ES+) m/z calcd. for C12H14ClN2: 221.0846. Found: 221.0844. TOF MS (ES−) m/z calcd. for CH3O3S: 94.9803. Found: 94.9802.

4.3.2. 1-(4-bromophenyl)-2,3,5-trimethylpyrazolium methanesulfonate (2b) Yield: 92%, yellow solid, m.p: 76.3 °C. IR (ATR, cm−1) νmax = 3056, 2970–2931, 1562, 1489, 1193, 1036, 1000–750. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.25 (s,3H,5-CH3); 2.62 (s,3H,3-CH3); 2.73 (s,3H,SCH3); 3.80 (s,3H,NCH3); 6.51 (s,1H,CH); 7.65–7.68 (d,2H,Ph,J: 8.7 Hz), 7.80–7.83(d,2H,Ph,J: 8.7 Hz). 13C NMR (75 MHz, CDCl3, ppm) δ = 12.31; 12.36; 35.05; 39.32; 108.24; 127.17; 130.03; 130.79; 134.05; 146.68 and 148.26. Analysis: calcd. for C13H17BrN2O3S: C 43.22, H 4.74, N 7.75, S 8.88. Found: C 43.01, H 4.78, N 7.72, S 9.08. TOF MS (ES+) m/z calcd. for C12H14BrN2: 265.0340. Found: 265.0338. TOF MS (ES−) m/z calcd. for CH3O3S: 94.9803. Found: 94.9804.

Temperature (K)

Fig. 6. Experimental refractive indices of RTILs (2a, 2c, and 2d) as a function of temperature.

4.3.4. 2,3,5-trimethyl-1-(4-methoxyphenyl)-pyrazolium methanesulfonate (2d) Yield: 98%, light brown liquid. IR (ATR, cm− 1) νmax = 3063, 2970–2842, 1604, 1563, 1509, 1254, 1194, 1036, 1000–750. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.23 (s,3H,5-CH3); 2.61 (s,3H,3-CH3); 2.75 (s,3H,S-CH3); 3.77 (s,3H,NCH3); 3.90 (s,3H, PhOCH3); 6.47 (s,1H,CH); 7.12–7.14 (d,2H,Ph,J: 8.3 Hz), 7.58–7.60(d,2H,Ph,J: 8.3 Hz). 13 C NMR (75 MHz, CDCl3, ppm) δ = 11,91; 12.18; 34.56; 39.26; 55.74; 107.76; 115.74; 122.98; 130.15; 146.72; 147.37 and 162.12. Analysis: calcd. for C14H20N2O4S: C 53.83, H 6.45, N 8.97, S 10.26. Found: C 53.52, H 6.48, N 98.92, S 10.59. TOF MS (ES+) m/z calcd. for C13H17N2O: 217.1341. Found: 217.1338. TOF MS (ES−) m/z calcd. for CH3O3S: 94.9803. Found: 94.9802.

4.3.5. 2,3,5-trimethyl-1-(4-nitrophenyl)-pyrazolium methanesulfonate (2e) Yield: 90%, yellow solid, m.p: 183.3 °C. IR (ATR, cm−1) νmax = 3066, 2970–2930, 1613, 1564, 1532, 1492, 1358, 1199, 1039, 1000–750. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.29 (s,3H,5-CH3); 2.65 (s,3H,3CH3); 2.72 (s,3H,S-CH3); 3.85 (s,3H,NCH3); 6.57 (s,1H,CH); 8.14–8.17 (d,2H,Ph,J: 8.8 Hz), 8.53–8.56(d,2H,Ph,J: 8.8 Hz). 13C NMR (75 MHz, CDCl3, ppm) δ = 12.43; 12.49; 35.51; 39.36; 108.73; 125.93; 131.18; 136.13; 146.99; 149.19 and 149.82. Analysis: calcd. for C13H17N3O5S: C 47.70, H 5.23, N 12.84, S 9.80. Found: C 47.44, H 5.27, N 12.77, S 10.08. TOF MS (ES+) m/z calcd. for C12H17N3O2: 232.1086. Found: 232.1081. TOF MS (ES−) m/z calcd. for CH3O3S: 94.9803. Found: 94.9800.

4.4. General synthesis procedure for 1-aryl-2,3,5-trimethylpyrazolium tetrafluoroborate salts (3a–3e) To a stirred solution of appropriate 1-aryl-2,3,5trimethylpyrazolium methanesulfonate salt (5.0 mmol) in water (10 mL) was added tetrafluoroboric acid solution (5.0 mmol, 48% in water) dropwise over 10–15 min at room temperature. Due to the anion exchange, the product precipitated after a few minutes. The reaction mixture was allowed to stir for 2 h to ensure that the conversion was complete. Precipitated solid was filtered, recrystallized from ethanol and dried under reduced pressure at 75 °C for 48 h.

4.4.1. 1-(4-chlorophenyl)-2,3,5-trimethylpyrazolium tetrafluoroborate (3a) Yield: 95%, light yellow solid, m.p: 133.2 °C. IR (ATR, cm−1) νmax = 3069, 2970–2937, 1561, 1489, 1035, 851. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.23 (s,3H,5-CH3); 2.56 (s,3H,3-CH3); 3.69 (s,3H,NCH3); 6.49 (s,1H,CH); 7.57–7.60 (d,2H,Ph,J: 8.9 Hz), 7.65–7.68(d,2H,Ph,J: 8.9 Hz). 13C NMR (75 MHz, CDCl3, ppm) δ = 11.83; 12.03; 34.35; 108.19; 129.55; 130.41; 131.05; 138.83; 146.86 and 148.25. 19F NMR (282 MHz, CDCl3, ppm): δ = − 153.88, − 153.94. Analysis: calcd. for C12H14ClN2BF4: C 46.72, H 4.57, N 9.08. Found: C 46.23, H 4.47, N 8.90. TOF MS (ES+) m/z calcd. for C12H14ClN2: 221.0846. Found: 221.0851.

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4.4.2. 1-(4-bromophenyl)-2,3,5-trimethylpyrazolium tetrafluoroborate (3b) Yield: 86%, yellow solid, m.p: 161.2 °C. IR (ATR, cm − 1) νmax = 3093, 2971–2932, 1561, 1486, 1035, 850. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.22 (s,3H,5-CH3); 2.54 (s,3H,3-CH3); 3.67 (s,3H,NCH3); 6.48 (s,1H,CH); 7.49–7.52 (d,2H,Ph,J: 8.5 Hz), 7.79–7.82 (d,2H,Ph,J: 8.5 Hz). 13 C NMR (75 MHz, CDCl 3, ppm) δ = 11.98; 12.16; 34.47; 108.19; 127.32; 130.08; 130.61; 134.13; 146.71 and 148.27. 19 F NMR (282 MHz, CDCl3 , ppm): δ = − 153.85, − 153.80. Analysis: calcd. for C12H14BrN2BF4: C 40.83, H 4.00, N 7.94. Found: C 41.14, H 3.91, N 7.97. TOF MS (ES+) m/z calcd. for C12 H14BrN 2 : 265.0340. Found: 265.0340.

4.5.1. 1-(4-chlorophenyl)-2,3,5-trimethylpyrazolium hexafluorophosphate (4a) Yield: 87%, white solid, m.p: 153.9 °C. IR (ATR, cm−1) νmax = 3067, 2973–2933, 1560, 1489, 831, 748. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.22 (s,3H,5-CH3); 2.52 (s,3H,3-CH3); 3.64 (s,3H,NCH3); 6.47 (s,1H,CH); 7.49–7.51 (d,2H,Ph,J: 8.8 Hz), 7.65–7.68(d,2H,Ph,J: 8.8 Hz). 13C NMR (75 MHz, DMSO-d6, ppm) δ = 11.93; 12.13; 34.85; 108.27; 130.35; 131.12; 131.24; 137.75; 147.16 and 147.96. 19F NMR (282 MHz, DMSO-d6, ppm): δ = − 68.90, − 71.41. Analysis: calcd. for C12H14ClN2PF6: C 39.31, H 3.85, N 7.64. Found: C 39.61, H 3.94, N 7.71. TOF MS (ES+) m/z calcd. for C12H14ClN2: 221.0846. Found: 221.0843. TOF MS (ES−) m/z calcd. for PF6: 144.9642. Found: 144.9641.

4.4.3. 1-(4-methylphenyl)-2,3,5-trimethylpyrazolium tetrafluoroborate (3c) Yield: 90%, yellow solid, m.p: 147.9 °C. IR (ATR, cm−1) νmax = 3072, 2964–2927, 1566, 1512, 1034, 849. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.14 (s,3H,5-CH3); 2.46 (s,3H,3-CH3); 2.42 (s,3H, PhCH3); 3.58 (s,3H,NCH3); 6.43 (s,1H,CH); 7.34–7.40 (m,4H,Ph). 13C NMR (75 MHz, CDCl3, ppm) δ = 11.90; 12.11; 21.42; 34.28; 107.92; 128.27; 128.43; 131.37; 143.29; 146.61 and 147.68. 19F NMR (282 MHz, CDCl3, ppm): δ = − 153.82, − 153.88. Analysis: calcd. for C13H17N2BF4: C 54.20, H 5.95, N 9.72. Found: C 54.21, H 5.62, N 9.76. TOF MS (ES+) m/z calcd. for C13H17N2: 201.1392. Found: 201.1388.

4.5.2. 1-(4-bromophenyl)-2,3,5-trimethylpyrazolium hexafluorophosphate (4b) Yield: 79%, white solid, m.p: 179.8 °C. IR (ATR, cm− 1) νmax = 3056, 2973–2933, 1561, 1488, 832, 733. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.22 (s,3H,5-CH3); 2.52 (s,3H,3-CH3); 3.64 (s,3H,NCH3); 6.47 (s,1H,CH); 7.41–7.44 (d,2H,Ph,J: 8.5 Hz), 7.81–7.84 (d,2H,Ph,J: 8.5 Hz). 13 C NMR (75 MHz, DMSO-d 6 , ppm) δ = 11.93; 12.14; 34.88; 108.28; 126.56; 130.79; 131.40; 134.09; 147.11 and 147.98. 19 F NMR (282 MHz, DMSO-d6, ppm): δ = − 68.90, − 71.42. Analysis: calcd. for C12H14BrN2PF6: C 35.06, H 3.43, N 6.81. Found: C 35.45, H 3.48, N 6.91. TOF MS (ES +) m/z calcd. for C12 H14BrN 2 : 265.0340. Found: 265.0340. TOF MS (ES− ) m/z calcd. for PF6 : 144.9642. Found: 144.9648.

4.4.4. 2,3,5-trimethyl-1-(4-methoxyphenyl)-pyrazolium tetrafluoroborate (3d) Yield: 95%, yellow solid, m.p: 75.6 °C. IR (ATR, cm−1) νmax = 3067, 2970–2845, 1606, 1563, 1510, 1258, 1034, 850. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.20 (s,3H,5-CH3); 2.53 (s,3H,3-CH3); 3.65 (s,3H,NCH3); 3.90 (s,3H, PhOCH3); 6.45 (s,1H,CH); 7.10–7.13 (d,2H,Ph,J: 8.9 Hz), 7.45–7.48(d,2H,Ph,J: 8.9 Hz). 13C NMR (75 MHz, CDCl3, ppm) δ = 11.95; 12.15; 34.16; 55.86; 107.79; 115.87; 123.17; 130.18; 146.85; 147.51 and 162.34. 19F NMR (282 MHz, CDCl3, ppm): δ = − 153.82, − 153.87. Analysis: calcd. for C13H17N2OBF4:C 51.35, H 5.63, N 9.21. Found: C 50.82, H 5.35, N 9.15. TOF MS (ES+) m/z calcd. for C13H17N2O: 217.1341. Found: 217.1338.

4.5.3. 1-(4-methylphenyl)-2,3,5-trimethylpyrazolium hexafluorophosphate (4c) Yield: 80%, yellow solid, m.p: 138.3 °C. IR (ATR, cm− 1 ) νmax = 3080, 2970–2930, 1569, 1515, 830, 742. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.18 (s,3H,5-CH3 ); 2.48 (s,3H,3-CH 3); 2.49 (s,3H, PhCH3 ); 3.60 (s,3H,NCH 3); 6.44 (s,1H,CH); 7.34–7.37(d,2H,Ph,J: 8.4 Hz), 7.43–7.46 (d,2H,Ph,J: 8.4 Hz). 13 C NMR (75 MHz, CDCl3 , ppm) δ = 11.80; 12.08; 21.43; 34.13; 108.0; 128.33; 128.37; 131.48; 143.47; 146.76 and 147.69. 19 F NMR (282 MHz, CDCl3 , ppm): δ = − 72.41, − 74.93. Analysis: calcd. for C13H17 N2 PF 6 : C 45.09, H 4.95, N 8.09. Found: C 45.07, H 5.30, N 8.14. TOF MS (ES+) m/z calcd. for C13H17N2: 201.1392. Found: 201.1391. TOF MS (ES−) m/z calcd. for PF6: 144.9642. Found: 144.9643.

4.4.5. 2,3,5-trimethyl-1-(4-nitrophenyl)-pyrazolium tetrafluoroborate (3e) Yield: 80%, yellow solid, m.p: 166.5 °C. IR (ATR, cm−1) νmax = 3073, 2970–2871, 1614, 1570, 1527, 1496, 1356, 1037, 856. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.27 (s,3H,5-CH3); 2.60 (s,3H,3-CH3); 3.73 (s,3H,NCH3); 6.54 (s,1H,CH); 7.92–7.95 (d,2H,Ph,J: 8.9 Hz), 8.53–8.56(d,2H,Ph,J: 8.9 Hz). 13C NMR (75 MHz, DMSO-d6, ppm) δ = 12.02; 12.20; 35.24; 108.70; 126.12; 131.27; 136.46; 147.30; 148.69 and 149.97. 19F NMR (282 MHz, DMSO-d6, ppm): δ = − 148.30, − 148.36. Analysis: calcd. for C12H14N3O2BF4: C 45.17, H 4.42, N 13.17. Found: C 45.22, H 3.97, N 13.23. TOF MS (ES+) m/z calcd. for C12H17N3O2: 232.1086. Found: 232.1091.

4.5.4. 2,3,5-trimethyl-1-(4-methoxyphenyl)-pyrazolium hexafluorophosphate (4d) Yield: 87%, white solid, m.p: 115.6 °C. IR (ATR, cm−1) νmax = 3021, 2958–2850, 1609, 1588, 1520, 1256, 831, 804. 1H NMR (300 MHz, CDCl3, ppm): δ = 2.19 (s,3H,5-CH3); 2.49 (s,3H,3-CH3); 3.60 (s,3H,NCH3); 3.90 (s,3H, PhOCH3); 6.43 (s,1H,CH); 7.10–7.13 (d,2H,Ph,J: 8.9 Hz), 7.39–7.42(d,2H,Ph,J: 8.9 Hz). 13C NMR (75 MHz, CDCl3, ppm) δ = 11.81; 12.08; 39.97; 55.87; 107.86; 115.95; 123.07; 130.06; 146.99; 147.49 and 162.41. 19F NMR (282 MHz, CDCl3, ppm): δ = − 72.41, − 74.93. Analysis: calcd. for C13H17N2OPF6: C 43.10, H 4.73, N 7.73. Found: C 43.30, H 4.56, N 7.81. TOF MS (ES+) m/z calcd. for C13H17N2O: 217.1341. Found: 217.1342. TOF MS (ES−) m/z calcd. for PF6: 144.9642. Found: 144.9643.

4.5. General synthesis procedure for 1-aryl-2,3,5-trimethylpyrazolium hexafluorophosphate salts(4a–4e)

4.5.5. 2,3,5-trimethyl-1-(4-nitrophenyl)-pyrazolium hexafluorophosphate (4e) Yield: 78%, yellow solid, m.p: 222.0 °C. IR (ATR, cm−1) νmax = 3080, 2970–2944, 1615, 1568, 1530, 1498, 1357, 821, 750. 1H NMR (300 MHz, DMSO-d6, ppm): δ = 2.24 (s,3H,5-CH3); 2.53 (s,3H,3-CH3); 3.66 (s,3H,NCH3); 6.85 (s,1H,CH); 8.0–8.03 (d,2H,Ph,J: 8.8 Hz), 8.56–8.59 (d,2H,Ph,J: 8.8 Hz). 13C NMR (75 MHz, DMSO-d6, ppm) δ = 11.98; 12.15; 35.19; 108.69; 126.13; 131.22; 136.46; 147.32; 148.72 and 149.97. 19F NMR (282 MHz, DMSO-d6, ppm): δ = − 68.91, − 71.43. Analysis: calcd. for C12H14N3O2PF6: C 38.21, H 3.74, N 11.14. Found: C 38.40, H 3.83, N 11.16. TOF MS (ES+) m/z calcd. for C12H17N3O2:

To a stirred solution of appropriate 1-aryl-2,3,5trimethylpyrazolium methanesulfonate salt (5.0 mmol) in water (10 mL) was added potassium hexafluorophosphate solution (prepared by dissolving 5.0 mmol KPF6 in 5 mL of water) dropwise over 10–15 min at room temperature. The solid formation was observed after a few minutes. For completion of the reaction, the reaction mixture was allowed to stir for 2 h. The resulting solid was filtered, recrystallized from ethanol, and dried in vacuo at 75 °C for 48 h.

M.C. Özdemir, B. Özgün / Journal of Molecular Liquids 248 (2017) 314–321

232.1086. Found: 232.1086. TOF MS (ES−) m/z calcd. for PF6: 144.9642. Found: 144.9645. Acknowledgements The authors are grateful to the Research Foundation of Gazi University (Project no. 05/2010-93) for supporting this study. Melek Canbulat Özdemir is also grateful for the support from TUBITAK (The Scientific and Technological Research Council of Turkey), namely, the BIDEP 2211 National Doctoral Scholarship Program. Appendix A. Supplementary data This material includes IR, 1H NMR, 13C NMR, 19F NMR, and TOF-MS spectra; TGA curves, DSC thermograms, Cyclic Voltammograms of all the 1-aryl-2,3,5-trimethylpyrazolium salts, and details of computational study. Supplementary data associated with this article can be found in the online version, at https://doi.org/10.1016/j.molliq.2017.10.033. References [1] C.C. Weber, A.F. Masters, T. Maschmeyer, Structural features of ionic liquids: consequences for material preparation and organic reactivity, Green Chem. 15 (2013) 2655–2679, https://doi.org/10.1039/C3GC41313F. [2] D. Rooney, J. Jacquemin, R. Gardas, Thermophysical properties of ionic liquids, Top. Curr. Chem. 290 (2009) 185–212, https://doi.org/10.1007/128_2008_32. [3] J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Room temperature ionic liquids as novel media for ‘clean’ liquid–liquid extraction, Chem. Commun. (1998) 1765–1766, https://doi.org/10.1039/A803999B. [4] S.T. Handy, Greener solvents: room temperature ionic liquids from biorenewable sources, Chem. Eur. J. 9 (2003) 2938–2944, https://doi.org/10.1002/chem. 200304799. [5] L.E. Barrosse-Antle, A.M. Bond, R.G. Compton, A.M. O'Mahony, E.I. Rogers, D.S. Silvester, Voltammetry in room temperature ionic liquids: comparisons and contrasts with conventional electrochemical solvents, Chem. Asian. 5 (2010) 202–230, https://doi.org/10.1002/asia.200900191. [6] M.J. Earle, J.M.S.S. Esperança, M.A. Gilea, J.N.C. Lopes, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, The distillation and volatility of ionic liquids, Nature 439 (2006) 831–834, https://doi.org/10.1038/nature04451. [7] K.R. Seddon, A. Stark, M.-J. Torres, Influence of chloride, water, and organic solvents on the physical properties of ionic liquids, Pure Appl. Chem. 72 (2000) 2275–2287, https://doi.org/10.1351/pac200072122275. [8] C. Maton, N.D. Vos, C.V. Stevens, Ionic liquid thermal stabilities: decomposition mechanisms and analysis tools, Chem. Soc. Rev. 42 (2013) 5963–5977, https://doi. org/10.1039/C3CS60071H. [9] A. Muhammad, M.I.A. Mutalib, C.D. Wilfred, T. Murugesan, A. Shafeeq, Thermophysical properties of 1-hexyl-3-methyl imidazolium based ionic liquids with tetrafluoroborate, hexafluorophosphate and bis(trifluoromethylsulfonyl)imide anions, J. Chem. Thermodyn. 40 (2008) 1433–1438, https://doi.org/10.1016/j.jct. 2008.04.016. [10] H.L. Ngo, K. LeCompte, L. Hargens, A.B. McEwen, Thermal properties of imidazolium ionic liquids, Thermochim. Acta 357–358 (2000) 97–102, https://doi.org/10.1016/ S0040-6031(00)00373-7. [11] O. Zech, A. Stoppa, R. Buchner, W. Kunz, The conductivity of imidazolium-based ionic liquids from (248 to 468) K. B.Variation of the anion, J. Chem. Eng. Data 55 (2010) 1774–1778, https://doi.org/10.1021/je900793r. [12] Y. Yoshida, O. Baba, G. Saito, Ionic liquids based on dicyanamide anion: influence of structural variations in cationic structures on ionic conductivity, J. Phys. Chem. B 111 (2007) 4742–4749, https://doi.org/10.1021/jp067055t. [13] M. Hayyan, F.S. Mjalli, M.A. Hashim, I.M. AlNashef, T.X. Mei, Investigating the electrochemical windows of ionic liquids, J. Ind. Eng. Chem. 19 (2013) 106–112, https://doi.org/10.1016/j.jiec.2012.07.011.

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