Synthesis and characterization of novel hexafluorophosphate salts with tropine-type cations

Journal of Molecular Liquids 209 (2015) 648–656

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Synthesis and characterization of novel hexafluorophosphate salts with tropine-type cations Jing Lu, Hang Song, Yang Yang, Guofei Qian, Lirong Nie, Shun Yao ⁎ College of Chemical Engineering, Sichuan University, Chengdu 610065, China

a r t i c l e

i n f o

Article history: Received 20 April 2015 Received in revised form 4 June 2015 Accepted 8 June 2015 Available online 19 June 2015 Keywords: Ionic liquids Tropine Synthesis Polarity Hammett function Conductivity

a b s t r a c t In this study, a series of new ionic liquids (ILs) with tropine as cationic nucleus and hexafluorophosphate as anion were synthesized. As two types of functional groups, aliphatic chains and benzyl with different substituents have been linked on the tropine nucleus in order to modulate the physicochemical properties of the ILs. The whole family of ILs was comprehensively identified by FT-IR, NMR and MS. Furthermore, the thermal properties of these ILs were evaluated by different scanning calorimetry (DSC) together with their melting points and enthalpy of fusion. Then the polarities of these tropine-type ILs were investigated and discussed. Resultantly, it was proved that their polarity was comparable with that of short-chain alcohols. Moreover, the Hammett function values of these involved ILs were determined to describe their Brønsted acidities. Finally, molar conductivities (Λ) of the tropine-type ILs in dilute solutions at 298.15 K were determined. The data were correlated with the Arrhenius–Ostwald model to obtain limiting molar conductivities (Λ∞) and association constants (KA). The influences of the branch chain of ILs on the ion transfer and association were also discussed. It was found that the size of cation was the main factor influencing the ion transfer. And the weak ion association of tropinetype ILs in water was proved by KA, which was less than 7 L·mol−1. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquids have been described as molten salts with various cationic and anionic species. They are organic–inorganic multicomponent materials whose physicochemical properties can be tailored by modifying the cations and anions [1]. Based on the promising designability and diversity of ILs, a large scale of ILs have been continuously developed including imidazolium, ephedrinium, pyrrolidinium, pyridinium, pyrazinium, piperidinium, quinolinium, isoquinolinium and morpholinium (as shown in Table 1 [2–28]). It is easy to find that some ILs originated from alkaloid skeleton with various nitrogenous heterocycles. As known to all, tropane alkaloid is an important representative of natural bioactive products, which exists extensively in many advanced plants and can act as anticholinergics (e.g. atropine, hyoscyamine, scopolamine) or stimulants (e.g. cocaine, hydroxytropacocaine) [29]. As the hydrolyzate of tropane alkaloids, tropine has been applied to synthesize atropine as a kind of pharmaceutical intermediate. Recently, one tropine-type IL and its X-ray powder diffraction information have been reported in our work [30]. However, to the best of our knowledge, further study of synthesis and characterization of more similar tropinetype ILs has not been reported. On the other side, the potential application as a solvent to extract target substances from their solution

⁎ Corresponding author. E-mail address: [email protected] (S. Yao).

http://dx.doi.org/10.1016/j.molliq.2015.06.027 0167-7322/© 2015 Elsevier B.V. All rights reserved.

was considered in the design of tropine-type ionic liquid. Hexafluorophosphate, as a hydrophobic anion, was very popularly applied in most extraction process for its advantage of easy recycling from an aqueous solution and strong intermolecular interaction. And the potential ability of six fluorine atoms to form hydrogen bond between ionic liquid and solute moleculars would benefit the selectivity of separation. Polarity, as one of the most fundamental parameters for ILs, is of great importance for solvation and related applications of ILs, especially for polarity-sensitive reaction and separation process [31,32]. For example, Diels–Alder reaction processed in highly polar ILs could produce a much higher endo/exo ratio as compared with others [33]. It was also found that the polarity played a crucial role in the design of ILs as solvents for cellulose dissolution [34]. Moreover, the high polarity of a Brønsted acidic pyrrolidinium IL was considered to be an important factor for the oxidative desulfurization of diesel fuel in the presence of H2O2 [35]. To characterize the polarity of IL, the common approach was developed with the aid of some solvatochromic dyes, and among them the Reichardt's dye is the most widely used probe (see Fig. 1) [36]. Now, a lot of data on the polarity of ionic liquids have been reported with this kind of dye [37]. Some fundamental studies have demonstrated that common ILs could exhibit similar polarity as short chain alcohols, and the polarities of ILs were mainly dominated by the nature of their cations [38]. Acidity, as another important property for ILs, is also vital for their widespread applications especially in the field of catalysis [39,40]. It has been found that the acidity order of ILs was consistent with their

J. Lu et al. / Journal of Molecular Liquids 209 (2015) 648–656 Table 1 Ionic liquids based on similar alkaloid parent nucleus and their applications. Cation

Anion

Application

Reference

Ephedrinium

− PF− 6 , OTf , (CF3SO2)2N−

Catalyst

[2]

Pyrrole

− − PF− 6 , Tos , Mes , − NTf− 2 , TFSI

Electrolyte

[3–10]

Piperidine

− − BF− 4 , OTs , TFSI

Catalyst Electrolyte

[11–17]

Pyrazine

− NTf− 2 , BF4

Catalyst

[18,19]

Pyridine

− − − NO− 3 , BF4 , Ac , HSO4 , Separation − − − PF− Catalyst 6 , Cl , OH , DCA , − Extraction NTf− 2 , CF3SO3

Quinoline

Br−, Cl−, BF4−, PF6−, DOS−

Isoquinoline

(CF3SO2)2N− − − − NTf− 2 , PF6 , Br , BF4 , DOS−, BETA− TFSI−

Morpholine

[20–22]

[23–25] Separation Catalyst Antimicrobial Extraction [26,27]

Electrolyte

[28]

activity order in some acid-catalyzed reactions [41]. For example, the SO3H-functionalized 1-(4-sulfonic acid) butyl-3-methylimidazolium hydrogen sulfate with strong acidity exhibited an excellent catalytic activity and selectivity in the esterification of salicylic acid with isoamyl alcohol [42]. A strong Brønsted acidity of ionic liquids could contribute to the catalytic reaction of methylal with trioxane for the preparation of polyoxymethylene dimethyl ethers [43]. In addition, four novel Brønsted acidic ionic liquids were applied in Fisher esterification for the synthesis of aromatic esters under solvent-free micro-wave irradiation conditions [44]. Until now, many methods have been utilized to measure the acidity of IL. Among these methods, the most well-known

649

one is the Hammett method in which a range of closely related UV–vis probes are used to describe the Hammett acidity (H0). It has been successfully applied for imidazolium, pyridinium and benzothiazolium ILs [40,41,45]. Additionally, the ion–ion and IL–solvent interactions can play a great influence on potential applications of ILs. To provide important insights of ion–ion, IL–solvent and solvent–solvent interactions in the IL–solvent mixture, the ion association constant (KA) together with the limiting molar conductivity (Λ∞) was well established. Currently, conductivity study of IL in solvents was always applied to reveal KA and Λ∞. Through conductivity studies, a great amount of researches on the ion association and transfer have been reported. It has been revealed that the stability of the ion pair was significantly dependent on the substituent groups in the cation and the type of the anion [46]. Also the weak association of IL in water, with the low constant below 20 L·mol− 1, has been found compared with the other non-aqueous solvents [47]. Based on the above research status, a homologous series of N-alkytropine salts containing alkyl substituents from propyl to hexyl and aromatic substituents have been prepared with the hexafluorophosphate anion in this work for the first time (as shown in Fig. 2). This family of ILs has been fully characterized by FT-IR, 1H NMR, 13C NMR and MS. The thermal properties of ILs have also been evaluated by different scanning calorimetry (DSC). The influence of the branched chain in cation on the melting points and the enthalpy of fusion have been explored. Moreover, the polarity of each IL has been studied by Reichardt's dye and UV–vis diffuse reflection spectroscopy. The influence of branched chain on the polarities of these ILs has been discussed. Additionally, the Hammett acidity function has been used to determine the acidity of the tropine-type ILs. Furthermore, the conductivities of ILs in water at 298.15 K have been measured. From the experimental data, KA and Λ∞ have been determined to reveal the ion association and transfer, respectively. Finally, the influence of the structure of ILs on their dissolving behavior in water has also been discussed. This work is expected to expand the kinds and properties of tropine-type ILs as well as to provide fundamental information for their further research and application. 2. Experimental 2.1. Reagents and materials Tropine was purchased from Aladdin Industrial Co. Ltd (Shanghai, China). N-propyl bromide, n-butyl bromide, n-amyl bromide, n-hexyl bromide, benzyl chloride, m-methoxybenzyl chloride, p-nitrobenzyl chloride, p-nitroaniline and the other solvents were purchased from Kelong Chemical Co. Ltd (Chengdu, China). 2,6-Diphenyl-4-(2,4,6triphenyl-N-pyridino) phenolate was purchased from Santa Cruz Biotechnology Corporation (Delaware, USA). All reagents and solvents were of pure analytical grade and were used without further purification, if not stated otherwise. Deionized water was obtained by the UPR-I-5T water purification system from Ulupure Technology Co. Ltd (Chengdu, China). 2.2. Synthesis and characterization

Fig. 1. Chemical structure of polarity probe Reichardt's dye.

2.2.1. Spectroscopy measurement All the spectral measurements were carried out at room temperature. IR spectra were measured on a Perkin Elmer Fourier transform infrared spectrometer (Waltham, USA) in potassium bromide discs, which were scanned from 400 to 4000 cm−1 with 4 cm−1 resolution. 1 H and 13C NMR spectra were recorded on a Bruker Avance 400 MHz nuclear magnetic resonance spectrometer (Fällanden, Switzerland) equipped with a 5 mm probe with all shifts referred to internal tetramethylsilane (TMS). Besides N-(p-nitrobenzyl)-tropine hexafluorophosphate dissolved in deuterated dimethyl sulfoxide, other samples (each 15 mg) were dissolved by deuterated methanol in the sample

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Fig. 2. Structures of synthesized tropine-type ionic liquids.

tubes. Mass spectrometry was performed on a Finnigan TSQ quantum ultra mass spectrometer (Massachusetts, USA) under the following conditions: positive ion mode, drying gas N2 600 L·h−1, temperature 573 K, capillary voltage 2.8 kV, and capillary temperature 373 K. MS analysis was operated in the full-scan mode (100–600 μ). 2.2.2. Synthesis of N-propyl-tropine bromide ([C3tr][Br]) Tropine (1.41 g, 0.01 mol) was dissolved in 90 mL ethyl acetate, and then n-propyl bromide (1.23 g, 0.01 mol) was added into the solution. The reaction solution was refluxed at 323 K for 12 h. At the end of the reaction, the solvent and the residual alkyl bromide were removed under reduced pressure. The white solids were then dried under vacuum at 323 K after being washed with ethyl acetate for several times to give n-propyl-tropine bromide (2.38 g, 0.009 mol). The other reaction conditions are shown in Table 2. 2.2.3. Synthesis of N-propyl-tropine hexafluorophosphate ([C3tr][PF6]) [C3tr][Br] (2.64 g, 0.01 mol) was dissolved by an appropriate amount of deionized water in 150 mL one-neck flask and then potassium hexafluorophosphate (1.84 g, 0.01 mol) was added. The reaction occurred at room temperature with stirring for 2 h. Then the residual white solids were washed with plenty of deionized water and dried

under reduced pressure to obtain the product of [C3tr][PF6] (3.17 g, 0.0096 mol). 1 H NMR (400 MHz, CD3OD) δ ppm: 1.06 (t, J = 7.2 Hz, 3H), 1.74– 1.88 (m, 2H), 1.97 (m, 2H), 2.26–2.41 (m, 2H), 2.47–2.71 (m, 4H), 3.05–3.08 (m, 2H), 3.23 (m, 2H), 3.87 (s, 3H), 4.09 (m, 1H), 4.87 (s, 1H); 13C NMR (100 MHz, CD3OD) δ ppm: 10.19, 15.98, 25.2, 34.83, 40.19, 60.34, 63.10, 66.87; IR (KBr) cm−1: 3599 (O–H), 2986 (CH3), 2894 (CH2), 1471 (CH3), 1452 (CH2), 1232 (C–N), 1093 (C–C), 1055 (C–O), 840 and 558 (PF− 6 ); fragment ion peak (M-PF6) of m/z 184 was found as C11H22NO+ in MS.

2.2.4. Synthesis of N-butyl-tropine hexafluorophosphate ([C4tr][PF6]) Similar procedure as earlier was carried out between tropine and n-butyl bromide, and the bromine salts were changed to the product of [C4tr][PF6] through metathesis reaction with the yield of 78%, as shown in Fig. 3. 1 H NMR (400 MHz, CD3OD) δ ppm: 1.04 (t, J = 7.4 Hz, 3H), 1.34– 1.54 (m, 2H), 1.65–1.84 (m, 2H), 1.95–1.99 (m, 2H), 2.23–2.40 (m, 2H), 2.47–2.66 (m, 4H), 3.06–3.20 (m, 2H), 3.24–3.28 (m, 2H), 3.87 (s, 3H), 4.09 (t, J = 5.4 Hz, 1H), 4.87 (s, 1H); 13C NMR (100 MHz, CD3OD) δ ppm: 13.95, 20.84, 25.35, 26.12, 35.72, 41.03, 61.22, 62.36, 67.72; IR (KBr) cm−1: 3599 (O–H), 2986 (CH3), 2893 (CH2), 1452 (CH3), 1438

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−1 1241 (C–N), 1096 (C–C), 1057 (C–O), 839 and 558 (PF− ; frag6 ) cm ment ion peak (M-PF6) of m/z 226 was found as C14H28NO+ in MS.

Table 2 Reaction conditions for the synthesis of tropine-type bromides and chlorides. Alkyl side chain

Solvent

Temperature (°C)

Time (h)

Yield (%)

n-propyl n-butyl n-amyl n-hexyl benzyl m-methoxybenzyl p-nitrobenzyl

Ethyl acetate Ethyl acetate Toluene Toluene Toluene Toluene Toluene

50 75 80 90 85 100 75

12 24 24 24 24 24 24

90 88 85 79 84 89 79

(CH2), 1232 (C–N), 1093 (C–C), 1055 (C–O), 841 and 558 (PF− 6 ); fragment ion peak (M-PF6) of m/z 198 was found as C12H24NO+ in MS. 2.2.5. Synthesis of N-amyl-tropine hexafluorophosphate ([C5tr][PF6]) Similar procedure as earlier was carried out between tropine and n-amyl bromide, and the bromine salts were changed to [C5tr][PF6] through metathesis reaction with the yield of 83%, as shown in Fig. 3. 1 H NMR (400 MHz, CD3OD) δ ppm: 0.97 (t, J = 7.2 Hz, 3H), 1.30– 1.57 (m, 4H), 1.71–1.88 (m, 2H), 1.95–1.99 (m, 2H), 2.29–2.36 (m, 2H), 2.52–2.61 (m, 4H), 3.05–3.08 (m, 2H), 3.26 (m, 2H), 3.87 (s, 3H), 4.10 (t, J = 5.6 Hz, 1H), 4.88 (s, 1H); 13C NMR (100 MHz, CD3OD) δ ppm: 14.19, 23.10, 23.32, 26.12, 29.64, 35.72, 41.02, 61.22, 62.55, 67.71; IR (KBr) cm−1: 3598 (O-H), 2962 (CH3), 2875 (CH2), 1472 (CH3), 1438 (CH2), 1244 (C–N), 1095 (C–C), 1057 (C–O), 838 and 559 (PF− 6 ); fragment ion peak (M-PF6) of m/z 212 was found as C13H26NO+ in MS. 2.2.6. Synthesis of N-hexyl-tropine hexafluorophosphate ([C6tr][PF6]) Similar procedure as earlier was carried out between tropine and n-hexyl bromide, and the bromine salts were changed to the product of [C6tr][PF6] through metathesis reaction with the yield of 87%, as shown in Fig. 3. 1 H NMR (400 MHz, CD3OD) δ ppm: 0.95 (t, J = 6.6 Hz, 3H), 1.41 (s, 6H), 1.78 (s, 2H), 1.95–1.98 (m, 2H), 2.23–2.35 (m, 4H), 2.52–2.61 (m, 2H), 3.05–3.08 (m, 2H), 3.23 (m, 2H), 3.87 (s, 3H), 4.10 (t, J = 5.5 Hz, 1H), 4.87 (s, 1H); 13C NMR (100 MHz, CD3OD) δ ppm: 14.29, 23.36, 23.53, 26.13, 27.23, 32.45, 35.72, 41.04, 61.22, 62.57, 67.71; IR (KBr) cm−1: 3605 (O–H), 2959 (CH3), 2865 (CH2), 1473 (CH3), 1437 (CH2),

2.2.7. Synthesis of N-benzyl-tropine hexafluorophosphate ([Bntr][PF6]) Similar procedure as earlier was carried out between tropine and benzyl chloride, and the chloride salts were changed to [Bntr][PF6] through metathesis reaction with the yield of 91%, as shown in Fig. 3. 1 H NMR (400 MHz, CD3OD) δ ppm: 1.97–2.01 (m, 2H), 2.48–2.55 (m, 2H), 2.60–2.68 (m, 4H), 2.72–2.79 (m, 2H), 3.91 (s, 3H), 4.09 (t, J = 5.4 Hz, 1H), 4.49 (s, 2H), 4.88 (s, 1H), 7.55 (brd, 5H); 13C NMR (100 MHz, CD3OD) δ ppm: 26.35, 35.95, 40.81, 61.23, 65.21, 67.76, 129.23, 130.43, 131.78, 133.97; IR (KBr) cm− 1: 3619 (O–H), 2976 (CH3), 2910 (CH2), 1585 (C–H, benzene ring), 1471 (CH3), 1455 (CH2), 1239 (C–N), 1111 (C–C), 1050 (C–O), 840 and 559 (PF− 6 ); fragment ion peak (M-PF6) of m/z 232 was found as C15H22NO+ in MS. 2.2.8. Synthesis of N-(m-methoxybenzyl)-tropine hexafluorophosphate ([m-MBntr][PF6]) Similar procedure as earlier was carried out between tropine and m-methoxybenzyl chloride, and the chloride salts were changed to the product of [m-MBntr][PF6] through metathesis reaction with the yield of 85%, as shown in Fig. 3. 1 H NMR (400 MHz, CD3OD) δ ppm: 2.02–1.98 (m, 2H), 2.25–2.49 (m, 2H), 2.59–2.67 (m, 4H), 2.72–2.80 (m, 2H), 3.86 (s, 3H), 3.92 (s, 3H), 4.09 (t, J = 5.4 Hz, 1H), 4.46(s, 2H), 4.89 (s, 1H), 7.07–7.14 (m, 3H), 7.45 (t, J = 8.0 Hz, 1H); 13C NMR (100 MHz, CD3OD) δ ppm: 24.82, 34.45, 39.41, 54.44, 59.69, 63.69, 66.35, 115.58, 118.05, 124.33, 128.94, 129.97, 160.16; IR (KBr) cm−1: 3605 (O–H), 2967 (CH3), 2901 (CH2), 1605 (C–H, benzene ring), 1455 (CH3), 1438 (CH2), 1239 (C–N), 1178 (C–O, methoxy group), 1111 (C–C), 1049 (C–O), 840 and 559 (PF− 6 ); fragment ion peak (M-PF6) of m/z 262 was found as C16H24NO+ 2 in MS. 2.2.9. Synthesis of N-(p-nitrobenzyl)-tropine hexafluorophosphate ([p-NBntr][PF6]) Similar procedure as earlier was carried out between tropine and 4-nitrobenzyl chloride, and the chloride salts were changed to [p-NBntr][PF6] through metathesis reaction with the yield of 82%, as shown in Fig. 3. 1 H NMR (400 MHz, DMSO-d6) δ ppm: 1.83 (m, 2H), 2.35–2.39 (m, 2H), 2.49–2.53 (m, 4H), 2.73–2.91 (m, 2H), 3.35 (s, 2H), 3.83 (s, 3H),

Fig. 3. Two-step synthesis procedure of new ionic liquids.

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3.92 (t, J = 5.1 Hz, 1H), 5.04 (s, 1H), 7.82 (d, J = 8.8 Hz, 2H), 8.34 (d, J = 8.8 Hz, 2H); 13C NMR (100 MHz, DMSO-d6) δ ppm: 24.83, 34.50, 58.91, 61.57, 66.13, 123.70, 134.46, 135.29, 148.49; IR (KBr) cm−1: 3595 (O– H), 2977 (CH3), 2901 (CH2), 1608 (C–H, benzene ring), 1527 (NO2), 1474 (CH3), 1442 (CH2), 1101 (C–C), 1049 (C–O), 847 and 559 (PF− 6 ); fragment ion peak (M-PF6) of m/z 277 was found as C15H21N2O+ 3 in MS. 2.3. Thermal analysis Thermal properties of synthesized ILs were measured by TA DSC Q2000 instrument (Delaware, USA) within the temperature range of 313.15–673.15 K, heating rate of 10 K·min−1 (with a temperature accuracy better than 2 K) and under a nitrogen atmosphere at the rate of 20 mL·min−1. The dry samples were tightly sealed in the aluminum pans. Their melting points were determined from the differential scanning calorimetry thermograms during the programmed heating scans. 2.4. Measurement of IL polarity The process of polarity measurement is shown in Scheme 1. All the probe stock solutions were prepared in methanol and stored at 277.15 K. Required amount of probes was weighed using Longteng ESJ200-4A balance (Shenyang, China) with a precision of ± 0.1 mg. The samples of ionic liquids (2.5 g) were dissolved in methanol followed by adding Reichardt' dye. The ratio of dye to IL was 3:200 (w:w). Methanol was used as solvent for its good miscibility with the tropinetype ILs. Then methanol was removed from the mixture by reduced pressure distillation. The obtained mixture was dried at vacuum for 48 h to eliminate the disturbance from trace methanol. The absorption spectra of the resulting samples were measured between 300 and 800 nm in Shimadzu 3600 UV–vis diffuse reflection spectroscopy (Kyoto, Japan). 2.5. Brønsted acidity analysis of the ILs The Brønsted acid strengths of ILs were determined with a Soptop UV–vis 2800 spectroscopy (Beijing, China). The indicator of p-nitroaniline was dissolved in methanol to prepare the indicator solution with 0.1 mmol·L−1. Followed by the addition of ILs at the concentration of 20 mmol·L− 1 and then the Hammett function (H0) was calculated as the following: H 0 ¼ pKa þ logð½I=½HIÞ

ð1Þ

where pKa is the electrolytic constant of p-nitroaniline (pKa = 0.99), [I] and [HI] are the percentages (%) of unprotonated and protonated p-nitroaniline, respectively.

2.6. Conductivity measurement Electrical conductance measurement was carried out on a Hongyi DDS-12A digital conductivity meter (Shanghai, China) with platinized electrodes whose cell constant was 0.967 cm−1. The conductivity meter was calibrated using a standard KCl solution (0.01 mol·L− 1). The temperature was maintained constantly at 298 ± 0.05 K by using the thermostatic bath. The association constant of IL can be expressed as the following equation: Κ A ¼ lim Κ A ¼ lim c→0

c→0

ci cþ þ c−

ð2Þ

where ci, c+, and c− are the molar concentrations (mol·L−1) of ion pairs, free cations, and free anions at electrolyte concentration c, respectively. On the other hand, in order to express the transfer of ions in different solvents, the ion transfer number (ti) is introduced in the formula as the ratio of the ionic limiting molar conductivity and sum limiting molar conductivity. So it can be calculated according to the following equation: ti ¼

qi qþ þ q−

ð3Þ

where q+ and q− are the cation and anion transfer charges, respectively. 3. Results and discussion 3.1. Infrared spectroscopy For the synthesized tropine-type ILs with alkyl-chain, the IR characteristic peaks in the range from 2800 to 3000 cm−1 are attributed to the symmetric stretching and asymmetric stretching of C–H in the carbon chain of the cation, respectively [48]. Absorbance characteristics above 3000 cm−1 are related with the O–H vibrational mode on the tropine ring. The peaks at 1471 and 1452 cm− 1 are assigned to the bending vibrations of methyl and methylene. The absorptions at 1232 cm−1 and 1055 cm−1 are ascribed to C–N and C–O stretching vibrations on the tropine ring, respectively. The characteristic peaks at 840 cm− 1 and 558 cm−1 corresponded to the stretching and bending vibrations of PF− 6 , which indicate that anion exchange has been successfully achieved at the end of their synthesis process. Unlike the ILs discussed above, the assignment of peaks in IR shows some difference for those ILs containing benzene ring. Besides the absorbance caused by tropine ring, the typical peaks of benzene ring also appear clearly. For [Bntr][PF6], the peaks around 1585 cm−1 can prove that benzene ring has been attached on the tropine ring.

Scheme 1. The scheme of ionic liquid polarity measurement process.

J. Lu et al. / Journal of Molecular Liquids 209 (2015) 648–656

Moreover, those signals at 757 cm−1 and 714 cm−1, which originate from the C–H out-plane bending vibration, also could strongly indicate the emergence of mono-substituted benzene ring. For the ionic liquid [m-MBntr][PF6], the peak at 1279 cm− 1 is attributed to asymmetric stretching vibration of C–O–Ar. For the ionic liquid [p-NBntr][PF6], the signals at 1527 cm− 1 and 1351 cm−1 are assigned to the symmetric and asymmetric stretching modes of the nitro group at the paraposition. However, the negative inductive effect of the nitro group leads to the loss of characteristics at the region between 910 cm− 1 and 650 cm−1, which are supposed to be the evidence of the position of substituent group at benzene ring.

3.2. Assignment of signals for 13C and 1H NMR spectroscopy The IL structures are also confirmed on the basis of the chemical shifts and peak integrals of NMR spectrum. Besides the signals of tropine ring, more peaks could appear in the NMR spectrum for the introduction of branched chain bonded with nitrogen atom [49]. For instance, the 13C NMR spectra of ILs with aliphatic hydrocarbon exhibit more signals between 25 ppm and 15 ppm, which are assigned to the methyl and methylene carbon atoms attached on the nitrogen atoms. Moreover, the aromatic group of ILs could make the signals appear at downfield region above 120 ppm. Compared with the spectrum of [m-MBntr][PF6], the benzene rings of [Bntr][PF6] and [p-NBntr][PF6] have symmetry axis which makes their signals easier to be identified and assigned. However, the strong electron withdrawing group of –NO2 bonded on C13 results in the expected shielding and deshielding effects on the even and odd-numbered carbons, respectively. So the downfield signal at 135.29 ppm in the aromatic region of [p-NBntr][PF6] can be assigned to C13, the peak at 134.46 ppm which belongs to C11 and C15, and the signal at 123.70 ppm which belongs to C12 and C14. This order of aromatic carbon signals is reversed in the spectrum of [Bntr][PF6] because of the electron-donating methylene group bonded onto C10. For the [m-MBntr][PF6], the downfield signal at 160.16 ppm is assigned to C12, which is attached with the methoxy group. Based on the principle of chemical shift calculation about the substituted benzene, C13 and C11 are assigned to 115.58 ppm and 118.05 ppm for steric effect, respectively. Considering the conjugation effect from the electron-donating methoxy group, the downfield signals at 129.97 ppm and 128.94 ppm are assigned to C10 and C14, respectively. In 1H spectrum, the situation is similar with the 13C spectrum when the aliphatic hydrocarbon has been attached to nitrogen atom to obtain the quaternary ammonium salt. For these ILs, besides the peak of the hydroxyl group at 3.22 ppm, the signals assigned to methyl and methylene of alkyl chain can be found in the upfield region between 1.0 ppm and 2.0 ppm. However, for those ILs with benzene ring, the hydroxyl group of tropine ring is dishielded by the intermolecular hydrogen-bonding effect and removes toward downfield at 4.4 ppm as a single peak. For [Bntr][PF6] with mono-substituted benzen ring, there is only a group of downfield signals with five protons at 7.55 ppm. It is a characteristic for [p-NBntr][PF6] with two groups of symmetrical signals corresponding to H12, H14 and H11, H15, respectively. The chemical shift of H12 and H14 (8.35 ppm) is greater than that of H11 and H15 (7.83 ppm) for the strong electron-withdrawing effect of the nitro group. The coupling constant (J = 8.8 Hz) of the double peaks reveals the ortho-coupling of H11 and H12, H14 and H15. And the AA′BB′ system for these two groups of peaks (8.35 and 7.83 ppm) clearly indicates that the para-position of benzene ring is substituted. For [m-MBntr][PF6] with meta-substituted benzene ring, the chemical shift of 3.92 ppm is assigned to the signal of the methoxy group as a special single peak. The triplet peak at 7.45 ppm with the coupling constant of 8 Hz is assigned to the H11 which is doublecoupled with H13 and H15 at meta-position. The disubstituted groups make the chemical shifts of H13, H14 and H15 very similar, which are all assigned to the stacked peaks at 7.11 ppm.

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3.3. Thermal property Thermal properties of these ILs are characterized by the DSC method at the heating rate of 10 K·min− 1. Both the melting point (Tm) and enthalpy of melting (ΔHm) are determined and summarized in Table 3. For the synthesized tropine-type hexafluorophosphates, the melting points are higher than 373.15 K and this phenomenon also occurs in some ILs as electrolyte [50]. The ILs with benzene ring exhibit higher melting temperature than those with carbon chain. The introduction of aromatic group increases van der Waals force through π–π stacking interaction between adjacent molecules, which could make the network structure of related ILs more stable and the melting temperature higher. Especially, Tm values of [m-MBntr][PF6] and [p-NBntr][PF6] are greatly higher than those of the other ILs. The methoxy group at meta-position and the nitro group at para-position, which both contain the oxygen atoms as hydrogen-bonding acceptor, can respectively form the intermolecular hydrogen bond with the hydroxyl group on the tropine ring of another molecular. It can be proved by infrared spectrum in which the assigned peaks of the hydroxyl group move toward low wavenumber region for the formation of hydrogen bonds. Another evidence is that the signals of the hydroxyl group in the two ILs have much higher intensity than the peak in the [Bntr][PF6]. On the other hand, [C6tr][PF6] shows lower Tm than those with amyl, butyl or propyl chain cations. Larger volume of the cation would hamper the intramolecular interaction between it and anion at some extent. So a longer chain attached to a tropine cation is speculated to help the ILs exhibit better liquid property than a shorter one. The tropine-type ILs exhibit different enthalpies of fusion which are not probably relevant with the difference in the chain length of cations. However, the enthalpy of fusion of ILs attached with aromatic groups, as the value of melting temperature, is generally higher than the other synthesized tropine ILs. The π–π stacking effect between benzene ring can make these ILs form a strong intermolecular force. 3.4. Polarity of tropine ILs The maximum absorption wavelengths and the calculated polarity values (ET(30), kcal·mol− 1) of the different ILs are summarized in Table 4, where the related values for some typical imidazolium-based ionic liquids are also included for comparison. The maximum wavelengths of Reichardt's dye in seven ILs are shown in Fig. 4. ET(30) is calculated by the following equation: ET ð30Þ ¼ 28591:5=λmax :

ð4Þ

It is obvious that the ET(30) values of [C5tr][PF6] and [C6tr][PF6] are similar to that of methanol (ET(30) = 55.1 kcal·mol−1), and they are slightly greater than the general results of imidazolium-based hexafluorophosphates [51,52]. Differently, the wavelengths of strongest absorbance bands in [C3tr][PF6] and [C4tr][PF6] are below 500 nm (493 and 498 nm, respectively), which result in higher polarity value close to 58 and make their polarity comparable with that of glycerol (ET(30) = 57 kcal·mol−1). Meanwhile, their polarity is much higher than that of most imidazolium ILs. It is also worth noting that, a shift Table 3 Thermal properties of tropine-type ionic liquids. Ionic liquid

Molecular weight M (g·mol−1)

Melting temperature T (K)

Enthalpy of fusion ΔHm (J·g−1)

[C3tr][PF6] [C4tr][PF6] [C5tr][PF6] [C6tr][PF6] [Bntr][PF6] [m-MBntr][PF6] [p-NBntr][PF6]

328 342 356 370 376 406 421

425.34 400.47 403.02 362.26 426.97 439.31 518.31

29.44 58.02 31.72 41.09 59.91 73.29 61.38

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J. Lu et al. / Journal of Molecular Liquids 209 (2015) 648–656

Table 4 Wavelengths of maximum adsorption (λmax) and ET(30) values of tropine-based and imidazolium-based ionic liquids. Ionic liquid

λmax (nm)

ET(30) (kcal·mol−1)

[C3tr][PF6] [C4tr][PF6] [C5tr][PF6] [C6tr][PF6] [Bntr][PF6] [m-MBntr][PF6] [p-NBntr][PF6] [BMIM][PF6] [EMIM][PF6]

493 498 512 519 525 529 511 545 543

57.99 57.41 55.84 55.09 54.46 54.05 55.95 52.50 52.60

of the absorbance band of the dye to longer wavelength can be observed with the increasing length of alkyl chain in the ILs, because the decrease of polarity of ILs will occur with the growing length of alkyl chains. After aromatic functionalization, the polarity of tropine-type ILs becomes weaker compared with that of those ILs with carbon chains. Among the tropine-type ILs modified with aromatic groups, the highest polarity can be observed in [p-NBntr][PF6]. On the contrary, [m-MBntr][PF6] possesses the weakest polarity of 54.05 kcal·mol−1, which is slightly higher than that of general imidazolium salt and comparable to ethanol (ET(30) = 52.4 kcal·mol−1). This result can be explained by the strong electron-withdrawing property of the nitro group at the para-position of electron-donating methylene group, which can offset the charge of gravity center. On the other hand, the weak polarity of [m-MBntr][PF6] can be attributed to the same electron-donating property of methoxy and methylene substituent groups, which can neutralize each other at some extent. The difference of ET(30) values has reflected the significant influence of cation on their polarity.

3.5. Brønsted acidity analysis of the ILs As shown in Table 5, the Brønsted acid strength of the ILs is in the order of [C6tr][PF6] N [C5tr][PF6] N [C4tr][PF6] ≈ [m-MBntr][PF6] N [Bntr][PF6] N [p-NBntr][PF6] N [C3tr][PF6]. In all the investigated tropine-type ILs, the anion is same and their acidities are only dependent on the cation. Among them, [C6tr][PF6] shows the strongest Brønsted acidity. The Brønsted acid strength of the ILs with a straight carbon chain is stronger than that of the ILs with benzene ring except for [C3Tr][PF6], whose acidity is the weakest. It is well known that benzene

Table 5 Brønsted acid strength of tropine-type ionic liquids. Object

Amax (AU)

[I] (%)

[HI] (%)

H0

p-nitroaniline [C3tr][PF6] [C4tr][PF6] [C5tr][PF6] [C6tr][PF6] [Bntr][PF6] [m-MBntr][PF6] [p-NBntr][PF6]

1.65 1.64 1.62 1.60 1.62 1.63 1.62 1.64

100 99.27 98.26 97.29 97.83 98.67 98.12 99.17

0 0.73 1.74 2.71 2.17 1.33 1.88 0.83

– 3.13 2.74 2.55 2.65 2.86 2.71 3.06

ring can withdraw electrons, while the straight carbon chain donates electrons. Therefore, the density of positive charge on the nitrogen of tropine ring in the ILs with benzene ring is greater than that in the ILs with a straight carbon chain. It is easier for the ILs with a straight carbon chain to dissociate proton than for the ILs with benzene ring, so the former has a stronger Brønsted acidity. Compared with [Bntr][PF6], [p-NBntr][PF6] has a weaker acidity for the introduction of the electronwithdrawing nitro group onto its benzene ring, which makes the density of positive charge lower. Similarly, [m-MBntr][PF6] shows a stronger acidity which can be attributed to the electron-donating methoxy group. The Brønsted acidity of ILs with different lengths of carbon chains will grow gradually with the increase of the number of carbon chains. It can prove that those ILs with a longer carbon chain tend to dissociate proton more easily, which can result in the stronger Brønsted acidity. 3.6. Conductivity The data about the measured molar conductivity of the IL aqueous solutions as a function of their molar concentration of IL are presented in Table 6. It is observed that the molar conductivity will decrease with the increasing concentration of IL solutions. At high concentration, an electrophoretic effect and association relaxation can occur between cation and anion of IL, which make more aggregates of the IL dispersed in water [53]. Accordingly, it should be noted that the salt-rich region could promote a way for displacement of ions by the presence of network of interconnected micellar aggregates [54]. Λ∞ can be determined by the extrapolation of molar conductivity vs the concentration of IL. The molar conductivity data are analyzed in the framework of the Arrhenius–Ostwald model as described by the following equation: 1 1 cΛΚ A ¼ þ Λ Λ ∞ ðΛ ∞ Þ2

ð5Þ

where Λ is the molar conductivity (S·cm2·mol−1) at the concentration c (mol·L−1), Λ∞ is the limiting molar conductivity (S·cm2·mol−1) at infinite dilution, and ΚA (L·mol−1) is the equilibrium constant of ion association. The obtained results for Λ∞ and ΚA are summarized in Table 7, and the fitting result is shown in Fig. 5. The molar conductivity + of cation (Λ∞ +) and transport number of positive ions (t ) can be calculated as the following. Λ ∞ ¼ Λ ∞þ þ Λ ∞− tþ ¼

Fig. 4. UV–vis absorption spectra of the Reichardt's dye in tropine-type ionic liquids: (1) [C3tr][PF6]; (2) [C4tr][PF6]; (3) [C5tr][PF6]; (4) [C6tr][PF6]; (5) [Bntr][PF6]; (6) [mMBntr][PF6]; and (7) [p-NBntr][PF6].

Λ ∞þ Λ∞

ð6Þ ð7Þ

which reflect ion mobility in the absence of solute–solute interactions and ion–solvent interactions. According to the reference data [55], the molar conductivity of hexafluorophosphate anion (Λ∞ −) for an aqueous solution at 298.15 K is 56.9 S·cm2·mol− 1. With the results of Λ∞ in Table 7, the corresponding values of t+ can be calculated. Association constants are determined by conductivity measurement for a series of dilute tropine-type IL aqueous solutions (c ≤ 0.001 mol·L− 1), which can provide benchmark data for cation–

J. Lu et al. / Journal of Molecular Liquids 209 (2015) 648–656

655

Table 6 Molar conductivities (Λ) of binary mixtures of ionic liquid + water at 293.15 K. 104c (mol·L−1) 10.0000 9.0909 8.3333 7.6923 7.1429 6.6667 6.0000 5.4545 5.0000 4.6154 4.0000 3.5294 3.1579 2.6087 2.2222 1.8182 1.3953

Λ (S·cm2·mol−1) [C3tr][PF6]

[C4tr][PF6]

[C5tr][PF6]

[C6tr][PF6]

[Bntr][PF6]

[m-MBntr][PF6]

[p-NBntr][PF6]

83.15 83.55 83.89 84.18 84.44 84.66 84.97 85.23 85.45 85.64 85.95 86.18 86.37 86.65 86.85 87.06 87.28

81.26 81.65 81.97 82.25 82.50 82.71 83.02 83.27 83.48 83.66 83.95 84.18 84.36 84.63 84.82 85.02 85.24

79.97 80.33 80.63 80.88 81.11 81.30 81.58 81.80 82.00 82.16 82.43 82.63 82.80 83.04 83.21 83.40 83.59

78.03 78.41 78.73 79.01 79.24 79.46 79.75 80.00 80.21 80.38 80.67 80.89 81.07 81.33 81.52 81.71 81.93

74.92 75.19 75.40 75.61 75.78 75.93 76.13 76.31 76.45 76.57 76.76 76.93 77.05 77.25 77.36 77.50 77.64

78.02 78.35 78.63 78.86 79.07 79.25 79.51 79.72 79.90 80.05 80.30 80.49 80.64 80.86 81.02 81.19 81.37

76.68 77.01 77.29 77.53 77.74 77.92 78.18 78.39 78.57 78.72 78.97 79.16 79.31 79.54 79.70 79.87 80.05

anion interaction in these systems. For the studied tropine-type ILs, the values of ΚA are very similar in the range of 5–7 L·mol−1. The results indicate that the ion association of these compounds in water is thus rather weaker than that of imidazolium ILs in the range of 12– 17 L·mol−1 [56]. It is thought that the low surface-charge density of ions can result in the weak ion pairing of tropine-type ILs in water. For the ILs with benzene ring, it is found that the values of ΚA are slightly smaller than those of other ILs with a straight carbon chain. The introduction of rigid benzene ring would increase the molecular volume, which leads to the lower surface-charge density of ions and weaker ion association. Moreover, the values of Λ∞ i are found closely related with the diffusion coefficients of the ions, and can be used to calculate effective hydrodynamic radii. These Λi values are essentially determined by solvent viscosity, which is the main frictional force influencing the ions, ion–solvent and non-specific ion–ion interactions. However, the value of Λ∞ i can be used to reflect the conductive capability which originated from ion–solvent interactions through the exclusion of the interaction between ions. For the synthesized tropine-type hexafluorophosphates, the difference of Λ∞ in water at 298.15 K is only influenced by the structure of cation and solvent. It can be found that the values of Λ∞ decrease with the elongation of carbon chain. Interestingly, the plot of Λ∞ vs the length of the carbon chains is linear for an aqueous solution of tropine ILs (as shown in Fig. 6). The correction fitting function between Λ∞ and n is determined as the following equation:
Λ ∞ ¼ 93:26−1:789n r 2 ¼ 0:9946

of carbon chain by the aromatic group can make the hydration ability of cation stronger and the active volume of cation greater, which results in the decrease of Λ∞. A general finding for tropine-type ILs is that the movement of anion is considerably faster than that of the cation because the former has the smaller size. It indicates that the size of IL is the main factor influencing its conductive ability. This finding is proved again by the transport number of ions, which decreases with the growth of the branch volume. 4. Conclusion A series of new hydrophobic ILs were successfully synthesized and identified by IR, NMR and MS, which were based on the tropine cation nucleus and hexafluorophosphate anion. The thermal investigation revealed that their melting points were higher than 373.15 K, and their thermal stability was good. ET(30) of these ILs showed that the polarity of tropine ILs was slightly higher than that of common imidazolium hexafluorophosphates, and was comparable with the polarity of short-chain alcohols. Moreover, the strength of Brønsted acidity indicated that their acidities were weak and nearly neutral, while no obvious difference existed among their acidity. At 298.15 K, the dilute-solution conductivity not only showed that tropine-type ILs

ð8Þ

where the slope represents the contribution of a methylene group for the Λ∞ of tropine ILs in water at 298.15 K and r2 is the correlation coefficient. As reflected by the corresponding values of Λ∞, the cations with benzene ring diffuse slower than the cations with carbon chain. The replacement Table 7 Limiting molar conductivities (Λ∞), association constants (ΚA), limiting molar conductivities of cation (Λ+) and transport number of positive ions (t+) for solutions of tropine-type ionic liquids in water at 298.15 K. Ionic liquid

Λ∞ (S·cm2·mol−1)

Λ+ (S·cm2·mol−1)

ΚA (L·mol−1)

t+

[C3tr][PF6] [C4tr][PF6] [C5tr][PF6] [C6tr][PF6] [Bntr][PF6] [m-MBntr][PF6] [p-NBntr][PF6]

88.03 85.96 84.24 82.64 78.13 81.97 80.65

22.51 20.44 18.72 17.12 12.61 16.45 15.13

6.21 6.12 5.62 6.15 4.46 5.32 5.44

0.26 0.24 0.22 0.21 0.16 0.20 0.19

Fig. 5. Molar conductivity fitting of different ionic liquids + water at 298.15 K: [C3tr][PF6]; [C4tr][PF6]; [C5tr][PF6]; [C6tr][PF6]; [Bntr][PF6]; [m-MBntr][PF6]; and [p-NBntr][PF6].

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J. Lu et al. / Journal of Molecular Liquids 209 (2015) 648–656

Fig. 6. The relationship between limiting molar conductivity and length of alky chain.

were weakly associated in water and thus comparable to common organic electrolytes. Furthermore, the limiting molar conductivity was very closely related with the length of carbon chain. In addition, the limiting molar conductivity in phenyl ring substituted ILs was less compared with the carbon chain substituted ILs. This can be attributed to the size of the former that was greater than that of the latter. Therefore, the molecular volume played an important role in ion transfer. As a part of the tropine-type IL family, their presentation could lay the foundation to design more new members by introducing more different functional groups. Acknowledgments Preparation of this paper was supported by the National Scientific Foundation of China (No. 81373284) and 2013 Scientific Research Foundation of Sichuan University for Outstanding Young Scholars (No. 2082604184184). References [1] G.H. Tao, L. He, W.S. Liu, L. Xu, W. Xiong, T. Wang, Y. Kou, Green Chem. 8 (2006) 639–646. [2] P. Wasserscheid, A. Bösmanna, C. Bolm, Chem. Commun. 3 (2002) 200–201. [3] G.B. Appetecchi, M. Montaninoa, D. Zaneb, M. Carewskaa, F. Alessandrinia, S. Passerini, Electrochim. Acta 54 (2009) 1325–1332. [4] J.L. Anderson, J. Ding, T. Welton, D.W. Armstrong, J. Am. Chem. Soc. 124 (2002) 14247–14254. [5] I. Osada, J.V. Zamory, E. Paillard, S. Passerini, J. Power Sources 271 (2014) 334–341. [6] N. Wongittharom, C.H. Wang, Y.C. Wang, C.H. Yang, J.K. Chang, Appl. Mater. Interfaces 6 (2004) 17564–17570. [7] J. Golding, S. Forsyth, D.R. MacFarlane, M. Forsythb, G.B. Deacon, Green Chem. 4 (2002) 223–229. [8] G. Annat, M. Forsyth, D.R. MacFarlane, J. Phys. Chem. B 116 (2012) 8251–8258. [9] A. Tsurumaki, M.A. Navarra, S. Panero, B. Scrosati, H. Ohno, J. Power Sources 233 (2013) 104–109.

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