Preparation , Cation-Anion Interactions and Physicochemical Properties of Ether-Functionalized Imidazolium Ionic Liquids

A set of 1-alkyl ether (and 1-alkyl)-3-methylimidazolium ionic liquids 2-4 ([CxOyMIm]+[Anion]- or [CxMIm]+[Anion]-, where MIm = 3-methylimidazolium; CxOy = 1-alkyl ether, C7O3 = -(CH2)2O(CH2)2O(CH2)2OCH3 (A), C3O1 = -(CH2)2OCH3 (B); Cx = 1-alkyl, C10 = C10H21 (C), C4 = C4H9 (D); and [Anion]- = H3CSO3- (2), BF4- (3) or PF6- (4)) was prepared and characterized. The cation-anion hydrogen bonding strength showed to be mainly anion dependent and decreased in the order H3CSO3- > BF4- > PF6-. All methanesulfonate ionic liquids 2 possessed a strongly deshielded H2 imidazolium ring proton. 1-Alkyl ether functionalized ionic liquids showed higher densities in comparison to their 1-alkyl equivalents. The salts 2a-b, 3a-d and 4a-b are room-temperature ionic liquids. All 1-alkyl ether functionalized ionic liquids (except 4b) are completely amorphous. The widest liquid ranges were obtained with the tetrafluoroborate ionic liquids due to their late solidification and excellent thermal stability. These data provide important information for the understanding of their application scope and the preparation of task-specific ionic liquids.


Introduction
Room-temperature ionic liquids (RTILs), ionic salts that melt at and below ambient temperature, typically comprise of an organic cation and an organic or inorganic anion.The class of imidazolium RTILs is used in a wide variety of applications due to their attractive physical and chemical properties, [1][2][3] which include: air and moisture stability, low flammability, thermal stability, a neglectable vapor pressure, being liquid over a wide temperature range, wide electrochemical windows, high conductivities and ionic mobilities, easy recycling, tunable miscibility with water and organic solvents, and being a good solvent for a wide variety of organic and inorganic chemical compounds.As a result, imidazolium RTILs are applied in, for instance, synthetic chemistry, 1,3 (biphasic) catalysis, [2][3][4] extraction/separation processes, 1 material science 1,5 and electrochemistry. 6The number of imidazolium RTIL applications is expanding rapidly as structural modifications are easily introduced in both the imidazolium cation (especially the 1 and 3 positions of the imidazolium ring) and the anion, which promotes the development of taskspecific RTILs. 70][11][12] Applications of these are found in the fields of molecular-recognition, 13 transition-metal catalysis, 14 biocatalysis, 15 carbohydrate/nucleoside chemistry, 16,17 anti-microbial activities, 18 metal extraction, 19 liquid-phase organic synthesis, 20 polymer chemistry, 21 organic synthesis, 22 self-organization 23 and lubricants. 24Understanding how structural changes affect physicochemical properties is of great importance for the preparation of task-specific RTILs.We report herein the synthesis, cation-anion interactions and physicochemical properties of a complementary set of 1-alkyl ether (and 1-alkyl)-3-methylimidazolium ILs 2-4 (Figure 2).

Experimental Density
The densities of 2a-b, 3a-c and 4a-b were determined by measuring the weight of 5.0 mL IL at a constant temperature of 30 °C.A volumetric flask was calibrated with distilled water at 30 °C using a density of 0.995646 g mL -1 for water. 25

Differential Scanning Calorimetry (DSC)
The phase transitions of 2a-d, 3a-d and 4a-b were determined using a TA Instruments DSC 2010 differential scanning calorimeter, equipped with a manual cooling unit.The DSC instrument was calibrated using indium.An average sample weight of 10-20 mg was sealed in an aluminium pan.RTILs were cooled to -100 °C, heated to 70 °C, cooled to -100 °C and heated to 70 °C at a rate of 10 °C min -1 under a flow of nitrogen.Room-temperature ionic solids were heated to 100 °C, cooled to -100 °C and heated to 100 °C at a rate of 10 °C min -1 under a flow of nitrogen.The glass transition temperature (T g , determined at the onset of the heat capacity change), crystallization temperature (T c , determined at the onset of the exothermic peak), and melting point (T m , determined at the onset of the endothermic peak) were determined on heating in the second heating run.

Thermal Gravimetric Analysis (TGA)
TGA measurements of 2a-c, 3a-c and 4a-b were performed on a TA Instruments Q50 thermogravimetric analyzer.The TGA instrument was calibrated using nickel.An average sample weight of 8-12 mg was placed in a  platinum pan and heated at 20 °C min -1 from ca. 30 to 800 °C under a flow of nitrogen.The onset of decomposition was defined as the decomposition temperature (T d ).

Synthesis
A straightforward synthetic sequence (Scheme 1) was chosen for the preparation of halide-free imidazolium ILs 2-4 (Figure 2). 11The methanesulfonate ester alkylating agents 1 are easily accessible by the treatment of the corresponding alcohols with methanesulfonyl chloride in the presence of triethylamine.In contrast to the lower molecular weight methanesulfonate esters 1b and 1d, 1a and 1c decomposed at elevated temperatures before distillation started.A treatment with activated carbon completely removed the small quantity of yellow impurities from 1c, but not from 1a.All methanesulfonate esters 1 were isolated in high yields (> 90%).
The alkylations of 1-methylimidazole with the methanesulfonate esters 1 were performed under solventless conditions at 60 °C for 24 hours (Scheme 1).Roomtemperature ionic solids were obtained in the reactions with the alkyl methanesulfonate esters 1c and 1d, which allowed a facile re-crystallization for the purification of 2c and 2d.The 1-alkyl-3-methylimidazolium methanesulfonate salts 2c and 2d were obtained as ultra pure and white crystals in high yields (>90 %).In contrast, the 1-alkyl ether-3-methylimidazolium methanesulfonate salts 2a and 2b are RTILs and recrystallization attempts were unsuccessful for both.An acidbase purification procedure with methanesulfonic acid was applied as alternative to remove the slight excess of 1-methylimidazole.According to NMR spectral data, RTILs 2a and 2b were isolated as analytical pure yellow liquids in yields of > 90% after an activated carbon treatment.Undoubtly, re-crystallization was the only suitable purification procedure for the removal of the small quantities of yellow impurities.All methanesulfonate ILs 2 are extremely hygroscopic.
Preparation of the tetrafluoroborate 3 and hexafluorophosphate 4 ILs in high yields of > 90% was achieved by an anion metathesis of 2 in the presence of either NaBF 4 or KPF 6 at room-temperature for 2 hours. 9The 1-alkyl RTILs 3c and 3d were isolated as colorless liquids and the 1-alkyl ether RTILs 3a-b and 4a-b were obtained as yellow liquids.
This complementary set of ILs 2-4 made it possible to study the correlation between structural modifications and physicochemical properties:

Hydrogen bonding strength
7][28] Each imidazolium cation is surrounded by anions and each anion is surrounded by imidazolium cations in an extended network due to the formation of hydrogen bonds between the imidazolium ring protons and anions.It is especially this feature what distinguishes the imidazolium ILs from other types of ILs that form neutral ion pairs.Knowledge of the hydrogen bonding strengths could result in a better understanding of the correlation between structural modifications and physicochemical properties.][30] The imidazolium ring proton numbers used in this work are shown in Figure 3.
The IR spectral data of the aromatic C-H stretching region between 3200 and 3000 cm -1 are presented in Table 1.Methanesulfonate RTIL 2a showed three stretching frequencies at 3147, 3096 and 3006 cm -1 , respectively.A shift to lower frequencies, corresponding to a reduced strength of the aromatic C-H bond, is the consequence of an increased hydrogen bonding participation. 28,29The value of 3006 cm -1 for [C-H 2 ] corresponds to a very strong hydrogen bonding interaction.Theoretical calculations show that the H 2 proton bears the largest positive charge of the aromatic imidazolium protons and is, therefore, the strongest hydrogen bond donor. 30,31The hydrogen bonding strengths are mainly anion dependent and decrease in the order H 3 CSO 3 > BF 4 > PF 6 .Noticeable is the drastically reduced hydrogen bond interaction between the PF 6 anion and H 2 proton of RTIL 4a.The influence of the 1-alkyl ether and 1-alkyl imidazolium cation substituents is much less pronounced.

Density
A variety of physicochemical properties of the ILs 2-4 were determined and are presented in Table 2.The densities of the RTILs 2a-b, 3a-c and 4a-b were measured at 30 °C.Imidazolium cation-anion interactions and molecular packing are the parameters that determine the density.The anion exerts a strong influence on the density, which decreases in the order PF 6 > BF 4 > H 3 CSO 3 .This shows that the more bulky PF 6 anion is responsible for a more dense molecular packing.Apparently, the hydrogen bonding strength is not a decisive factor.Other trends are seen for the 1-alkyl ether and 1-alkyl imidazolium cation substituents.Ether-functionalized RTILs with the longer 1-triethylene glycol monomethyl ether substituent are less dense.Furthermore, the 1-alkyl ether functionalized RTIL 3a has a higher density compared to 1-alkyl functionalized RTIL 3c.These trends are consistent with previously reported data and enforce that it is possible to fine-tune the density of RTILs by structural modifications in the cation and anion. 12

Phase transitions
Differential scanning calorimetry experiments were performed to gain information about the phase transitions of the imidazolium salts 2-4 and the results are presented in Table 2.
Most of the salts reported herein are RTILs, only the 1-alkyl methanesulfonate salts 2c and 2d are solid at roomtemperature.The 1-alkyl ether RTILs were obtained as yellow liquids due to a tiny amount of a yellow impurity.][3] Interestingly, only a glass transition was observed for the 1-alkyl ether RTILs 2a-b, 3a-b and 4a, and 1-alkyl  RTIL 3d. 9,28In other words, crystallization does not occur upon cooling of these RTILs to low temperatures.This indicates that these RTILs are completely amorphous (0% crystallinity).In contrast, only a crystallization exotherm and melting endotherm were observed for methanesulfonate salt 2c, which is related to a 100% crystallinity.The ILs 2d, 3c and 4b have both crystalline and amorphous domains as glass transition endotherms as well as crystallization exotherms and melting endotherms were detected.A consistent correlation between crystallinity and structural modifications does not exist for the ILs 2-4.However, a few isolated trends are visible: (i) A higher crystallinity is observed when the imidazolium cation is functionalized with a longer 1-alkyl chain (2c vs 2d and 3c vs 3d); (ii) Crystallinity disappears completely upon substitution of the 1-decyl chain with the 1-triethylene glycol monomethyl ether functionality (2c vs 2a and 3c vs 3a), which could be due to the higher rotational freedom of the 1-alkyl ether chain; (iii) The 1-alkyl tetrafluoroborate RTILs 3c and 3d have lower crystallinities compared to their methanesulfonate equivalents 2c and 2d.
One of the most important parameters of ILs is their liquid range.The lower limit of the liquid range is either determined by the glass transition temperature for completely amorphous ILs or by the melting point for ILs with crystallinity.Alfonso and co-workers 9 previously reported the synthesis and phase transitions (Table 2) of the 1-monoethylene glycol monomethyl ether-3-methylimidazolium BF 4 and PF 6 ILs 3b and 4b, and the 1-diethylene glycol monomethyl ether-3-methylimidazolium BF 4 and PF 6 ILs. Figure 4 shows the DSC traces of the cation A [C 7 O 3 MIm] based RTILs 2a, 3a and 4a.For all cations A-D (Figure 1), modification of the anion results in an increase of the solid-liquid transition temperature in the order BF 4 < PF 6 < H 3 CSO 3 .However, other general trends upon modification of the imidazolium cation are not visible.

Decomposition temperature
Thermal gravimetric analysis experiments were used to determine the decomposition temperature (upper limit of the liquid range) of the ILs 2-4.The results are shown in Table 2 and the TGA traces of the RTILs 2b, 3b and 4a-b are presented in Figure 5. Interestingly, the BF 4 RTILs have both the highest thermal stability and lowest solid-liquid transition temperatures.Table 3 shows a highly anion dependent thermal stability, which decreases in the order BF 4 > H 3 CSO 3 > PF 6 .Furthermore, the 1-alkyl ether functionalized RTILs 2a and 3a have lower decomposition temperatures in comparison to their 1-alkyl equivalents 2c and 3c.Hexafluorophosphate RTIL 4b has the lowest thermal stability.

Conclusions
In summary, a complementary set of 1-alkyl ether and 1-alkyl functionalized imidazolium ILs was successfully prepared via a halide-free approach.Most of these salts are RTILs, but the 1-alkyl methanesulfonate ILs 2c-d are solid at room-temperature.Hydrogen bonding interactions between the aromatic heterocyclic protons and the IL anion are principally anion dependent and decrease in the order H 3 CSO 3 > BF 4 > PF 6 .In comparison, the acidity of the imidazolium ring H 2 proton is strongly enhanced for the methanesulfonate ILs 2. The density is easily fine-tuned within this set of RTILs   amorphous materials.Interestingly, the broadest liquid ranges were identified for the tetrafluoroborate RTILs.The lower limit (solid-liquid transition temperature) increases in the order BF 4 < PF 6 < H 3 CSO 3 .On the other side (upper limit), the tetrafluoroborate RTILs have the highest decomposition temperatures ((1) BF 4 > H 3 CSO 3 > PF 6 ; (2) 1-alkyl > 1-alkyl ether).Altogether, the presented IL characterization study is of importance to obtain a better understanding of their application scope.Besides, this knowledge could facilitate the development of new task-specific ILs.

General
All reactions were carried out under an argon atmosphere in dried glassware using standard Schlenk, syringe and septa techniques. 1 H and 13 C NMR spectra were recorded on a Varian Inova 300 MHz spectrometer in CDCl 3 or d6-acetone.Chemical shifts are reported relative to TMS in CDCl 3 ( 0.00 for 1 H), residual d5-acetone ( 2.05 for 1 H), CDCl 3 ( 77.00 for 13 C), and d6-acetone ( 30.5 for 13 C).Chemical shifts are quoted in parts per million (ppm), and coupling constants J are given in hertz (Hz).IR-spectra were recorded in a range of 4000-500 cm -1 using a Shimadzu FTIR-8300 Fourier Transform Infrared Spectrophotometer, and were measured as neat oil or as film (w = weak, m = medium, and s = strong).Mass spectra were recorded on a Waters Micromass Q-Tof micro quadrapole mass spectrometer in ESI-mode.

Materials
The solvents dichloromethane, acetone and diethyl ether were purchased from VETEC Química Fina LTDA and used without further purification.Triethylene glycol monomethyl ether, 2-methoxyethanol, 1-decanol, 1-butanol, methanesulfonic acid, sodium tetrafluoroborate and potassium hexafluorophosphate were used as purchased from Sigma-Aldrich.The reagents triethylamine, methanesulfonyl chloride and 1-methylimidazole were purchased from Sigma-Aldrich and distilled under argon prior to use.CDCl 3 and d6-acetone were purchased from Cambridge Isotope Laboratories.Activated carbon, celite, basic aluminum oxide and silica gel 60 (40-60 µm) were purchased from Merck.A procedure reported previously in the literature was used for the synthesis of 1d, 1 2d 1 and 3d, 1 and the spectral data were in accordance with the literature data.

General procedure for the synthesis of alkylating agents 1a-c
A modified literature procedure was used for the synthesis of 1a-c. 1 Alcohol (1.00 equiv.),methanesulfonyl chloride (1.05 equiv.),triethylamine (1.10 equiv.)and dichloromethane.Alkylating agents 1a and 1c were not purified by a vacuum distillation as decomposition occurred at elevated temperatures.Instead of this, 1a and 1c were dissolved in dichloromethane and treated with activated carbon.Filtration over a short silica column, solvent evaporation, and vacuum drying at 70 °C for 8 hours resulted in the isolation of 1a as a yellow liquid and 1c as a colorless liquid.Alkylating agent 1b was obtained as a colorless liquid after vacuum distillation.The alkylating agents 1a-c were obtained in high yields (>90 %).Spectral data of 1b 2 and 1c 3 were in agreement with those previously reported in the literature.

General procedure for the synthesis of methanesulfonate ILs 2a-c
A modified literature procedure was used for the synthesis of 2a-c. 1 The mixture of 1a-c (1.000 equiv.)and 1-methylimidazole (1.001 equiv.) was heated at 60 °C for 24 hours.ILs 2a-b were dissolved in dichloromethane.Methanesulfonic acid was added in order to remove excess 1-methylimidazole and the reaction mixture was stirred for 15 minutes at 25 °C.Filtration over a short basic aluminum/celite column was followed by the removal of the dichloromethane.Consecutively, 2a and 2b were dissolved in acetone and treated with activated carbon.Filtration over a short celite column, solvent evaporation, and vacuum dry-Preparation, Cation-Anion Interactions and Physicochemical Properties J. Braz.Chem.Soc.S2 ing at 100-120 °C for 8 hours resulted in the isolation of 2a and 2b as yellow liquids.Room-temperature ionic salt 2c was dissolved in hot acetone.White crystals of 2c formed upon cooling, and diethyl ether was added to promote the crystallization.Ionic salt 2c was vacuum dried at 30 °C for 8 hours.The very hygroscopic ILs 2a-c were obtained in high yields (>90 %).Cyclic voltammetry experiments indicated the absence of a significant amount of water in the ILs.Spectral data of 2b 4 were in agreement with those previously reported in the literature.

General procedure for the synthesis of tetrafluoroborate ILs 3a-c and hexafluorophosphate ILs 4a-b
A modified literature procedure was used for the synthesis of 3a-c and 4a-b. 5NaBF 4 (1.12 equiv.)or KPF 6 (1.12 equiv.)were added to a solution of IL 2a-c (1.0 equiv.) in acetone at 25 °C.The NaO 3 SCH 3 and KO 3 SCH 3 salts were removed by filtration after 2 hours at 25 °C.Activated carbon was added to the acetone solution and the reaction mixture was stirred for 24 hours at 25 °C.Removal of the activated carbon by filtration was followed by the removal of the acetone.Consecutively, the ILs were dissolved in dichloromethane.Filtration over a short celite column, solvent evaporation and vacuum drying at 100-120 °C (3a-c and 4a) or 75 °C (4b) for 8 hours resulted in the isolation of 3a-c and 4a-b in high yields (>90 %).The ILs 3a-b and 4a-b were isolated as yellow liquids and ionic liquid 3c as a colorless liquid.Cyclic voltammetry experiments indicated the absence of a significant amount of water in the ILs.Spectral data of 3b-c 5 and 4b 5 were in agreement with those previously reported in the literature.

Figure 2 .
Figure 2. Imidazolium ILs prepared and studied in this work.
(i) Imidazolium cations -Comparison of the 1-alkyl ether functional groups with the 1-alkyl chains due to the same number of atoms (C 7 O 3 a versus C 10 c and C 3 O 1 b versus C 4 d); (ii) Imidazolium cations -Comparison of the length of the 1-alkyl ether and 1-alkyl chains (C 7 O 3 a versus C 3 O 1 b and C 10 c versus C 4 d); (iii) IL anions -Comparison of methanesulfonate 2 versus tetrafluoroborate 3 versus hexafluorophosphate 4. 1 H NMR, 13 C NMR, IR, MS-ESI, DSC, TGA and density measurements were performed for the characterization of the ILs 2-4, and to study the imidazolium cation-anion interactions and physicochemical properties.

2 - 4
due to a linear response upon structural modifications: (i) PF 6 > BF 4 > H 3 CSO 3 ; (ii) C 3 O 1 MIm > C 7 O 3 MIm; (iii) C 7 O 3 MIm > C 10 MIm.In contrast, physicochemical properties like phase transitions and decomposition temperature are tuneable by structural modifications, but, although some trends are observed, their correlation is less predictive.Almost all 1-alkyl ether functionalized RTILs only have a glass transition and are completely

Table 1 .
Infrared spectral data for the aromatic stretching region (3200-3000 cm -1 ), corresponding with C-H stretching frequencies of the imidazolium cation a Additional stretching frequency observed at 3053 cm -1 .b Not observed.

Table 2 .
Physicochemical properties of the imidazolium ionic liquids 2

-4 IL 2-4
a Density at 30 °C.b Glass transition temperature determined by DSC on heating.c Crystallization temperature determined by DSC on heating.d Melting point determined by DSC on heating.e Decomposition temperature determined by TGA.f Solid at 30 °C.g Crystallization temperature determined by DSC on cooling.h Not determined.i Reported values in reference 9.