Esterification of carboxylic acids with alkyl halides using imidazolium based dicationic ionic liquids containing bis-trifluoromethane sulfonimide anions at room temperature

Article abstract of DOI:10.1039/c5ra00802f

Task-specific room temperature ionic liquids (RTILs) composed of symmetrical N-methylimidazolium rings linked with a short oligo (ethylene glycol) chain (cationic part) and bis-trifluoromethane sulfonimide (NTf₂, anionic part) were successfully synthesized, and their physicochemical properties were determined by various modern analytical techniques. The catalytic activity of the synthesized RTILs was evaluated in the esterification reaction of acids with alkyl halides in solvent-free conditions at room temperature. From the screening test, all the synthesized RTILs showed a high yield with significant selectivity for respective esters in a very short reaction time. Especially, 0.1 equimolar of RTIL-1 ([tetraEG(mim)₂][NTf₂]₂) was found to be, the most efficient and reusable catalyst for this reaction. As a result, 100% conversion and up to a 94% yield of the respective ester product was obtained in a 30 min reaction time. This might be due to their synergetic effect of Lewis acidity, wide liquid range, and high miscibility compared to the other homogeneous and heterogeneous catalysts. Beside this, RTIL was easily separated from the reaction mixture and reused several times without any significant loss of catalytic activity and structural property. The present dicationic ionic liquids (ILs) under a solvent-free catalytic system were found to be kinetically fast, naturally benign, and achieved good yields for esterification of carboxylic acids with alkyl halides.

Full text of DOI:10.1039/c5ra00802f

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Esterification of carboxylic acids with alkyl halides
using imidazolium based dicationic ionic liquids
containing bis-trifluoromethane sulfonimide
anions at room temperature
Cite this: RSC Adv., 2015, 5, 26197
a
Arvind H. Jadhav,a Kyuyoung Lee,a Sangho Kooab and Jeong Gil Seo
*
Task-specific room temperature ionic liquids (RTILs) composed of symmetrical N-methylimidazolium rings
linked with a short oligo (ethylene glycol) chain (cationic part) and bis-trifluoromethane sulfonimide (NTf2,
anionic part) were successfully synthesized, and their physicochemical properties were determined by
various modern analytical techniques. The catalytic activity of the synthesized RTILs was evaluated in the
esterification reaction of acids with alkyl halides in solvent-free conditions at room temperature. From
the screening test, all the synthesized RTILs showed a high yield with significant selectivity for respective
esters in a very short reaction time. Especially, 0.1 equimolar of RTIL-1 ([tetraEG(mim)2][NTf2]2) was found
to be, the most efficient and reusable catalyst for this reaction. As a result, 100% conversion and up to a
94% yield of the respective ester product was obtained in a 30 min reaction time. This might be due to
their synergetic effect of Lewis acidity, wide liquid range, and high miscibility compared to the other
homogeneous and heterogeneous catalysts. Beside this, RTIL was easily separated from the reaction
mixture and reused several times without any significant loss of catalytic activity and structural property.
The present dicationic ionic liquids (ILs) under a solvent-free catalytic system were found to be
kinetically fast, naturally benign, and achieved good yields for esterification of carboxylic acids with alkyl
halides.
Received 14th January 2015
Accepted 4th March 2015
DOI: 10.1039/c5ra00802f
www.rsc.org/advances
solvents or catalyst” with tailored properties either by altering
their structures or by varying cations and anions.1–4,11,12
1. Introduction
Esters and their derivatives are substantial products or
intermediates in the chemical and pharmaceutical industries.13
Therefore, the Fischer ester synthesis method14 and various
protable esterication approaches catalysed by ion exchange
resins, heteropoly acids, Lewis acids, zeolite, Brønsted acids,
etc. have been reported.15–21 Today, eco-friendly organic
synthesis has become very signicant and widespread, directing
towards green chemistry. Especially, RTILs attracted great
interest as environmentally friendly reaction media for organic
synthesis.1–5 In this respect, RTILs have been successfully
utilized as catalyst or solvent in number of organic reactions
such as hydrogenation,22 Friedel–Cras reaction,23 Heck,
Suzuki, Sonogashira,24–26 and Diels–Alder reactions,27 oxida-
tion,28 Michael addition,29 nucleophilic substitution such as the
Williamson ether synthesis,30 and etc.31,32
With the rapid improvement in the eld of synthetic organic
chemistry, more and more naturally benign and eco-friendly
processes are coming up at tremendous rate.1 Powered by the
current scenario of environmental awareness and green chem-
istry, it seems that the ILs are here to stay in the current pattern
of scientic research.2 Especially, RTILs have attracted great
attention from the eld of science and engineering. RTILs were
initially introduced as a green reaction media for the substitu-
tion for organic solvents because of their unique chemical and
physical properties such as thermal stability, nonvolatility, and
nonammability, specic miscibility, and easy reusability.3–7
Nowadays, they have trailed far away from this edge, showing
their signicant role in directing the reaction as catalysts as well
as solvent.8–10 The properties of ILs are strongly reliant on their
cations and anions therefore, ILs can be referred to as “designer
Esterication of carboxylic acids with alcohol using
homogenous and heterogeneous organic or inorganic acid
catalysts is prominent.33 On the other hand, elimination of
water formed during the reaction and use of additional amount
of the reactants are generally required for reasonable conver-
sion with sufficient yield, and a bulky volume of volatile organic
solvents is regularly mandatory. Meanwhile, it is very
aDepartment of Energy Science and Technology, Energy and Environment Fusion
Technology Center, Myongji University, Nam-dong, Cheoin-gu, Yongin-si,
Gyeonggi-do 449-728, Republic of Korea. E-mail: jgseo@mju.ac.kr; Fax: +82-31-336-
6336
bDepartment of Chemistry, Myongji University, Nam-dong, Cheoin-gu, Yongin-si,
Gyeonggi-do 449-728, Republic of Korea
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problematic to recover the homogenous or soluble acid catalyst samples. All solvents were obtained from commercial sources
which should be neutralized subsequently aer the reaction. In and were distilled from appropriate agents prior to use it.
addition, large volumes of volatile organic solvents and acidic
catalyst may result in contamination to the environment.34–37 It
2.2. Characterization
has been reported that esterication could be carried out using
1
All synthesized RTILs were characterized by H NMR and 13C
ILs, such as ([Hmim][HSO4]) and ([Heemim][HSO4]).38 Though,
NMR spectroscopy on Bruker spectrometer 400 MHz and
100 MHz respectively using CDCl3 or DMSO as a solvent. The
reported chemical shis were against TMS as reference for H
¨
the synthesis of these Bronsted acidic ILs were not appropriate,
and volatile organic solvents were inevitable for the preparation
and reuse. Additionally, the yield and conversions were lower
than average for most of the substrates.
1
and 13C NMR. Mass spectra were acquired using electro spray
ionization on Waters Micrimass ZQ LC/MS 2000 mass spec-
trometer. FT-IR spectra were obtained on Varian 2000 (Scimitar
series) spectrophotometer. TLC analysis was accomplished on
silica-gel Poly Gram SIL G/UV 254 plates to monitor the reaction
Esterication of acid with alkyl halide is a simple process,
nevertheless there are still many encounters to produce a
simple and clean route to produce esters from organic acids and
alkyl halides.39 We believe that the predominantly attractive
method uses hydrophobic mild acidic RTILs to carry out the
reaction.40 The homogeneous phase of RTILs formed more
interaction with the reactants, apparently when hydrophobic
RTILs are used as an alternative, poor solubility of the ester
products in the RTILs produced phase separation, shiing the
equilibrium towards the product.40 High yield could be achieved
with a simple green separation method due to the hydrophobic
nature, and RTILs could be reused several times beneath the
protection of green chemistry. The NTf2 containing RTILs are
hydrophobic and stable at atmospheric condition. They exhibit
a wide liquid range, even recent study indicates that decom-
position temperatures were overestimated in previous thermal
studies.2–5 Moreover, the relatively low cost and their high
chemical stability at atmospheric condition, the NTf2 contain-
ing RTILs are attracting more attention for their industrial
use.2–5 So far hydrophilic ILs with acidic counter anions and
protonated acidic ILs either solvent or catalyst have been used
in esterication reactions.38
growth. All ester products were also characterized by 1H and 13
C
NMR, IR spectroscopy and matched with the authentic samples.
Determination of acidity by pyridine adsorption investigates
with prepared RTILs were accomplished using dry pyridine with
RTIL by 1 : 5 weight ratio. The mixture of sample was speared
on CaF2 windows and these all experiments were performed in
dry condition and analysed at room temperature. Differential
Scanning Calorimetry (DSC) results were obtained using
aluminium sample pan of diameter 7 mm and between
10–15 mg of the RTIL was measured into the aluminium sample
pan. A minor incision was made in the tops of both the refer-
ence pan and sample pan. Samples were measured at atmo-
ꢁ
spheric pressure between ꢀ30 and +80 C temperature with a
heating (cooling) rate 5 ꢁC minꢀ1 in a nitrogen atmosphere.
Thermogravimatric analyses (TGA) were performed on Scinco
TGA N-100 instrument with heating rate 5 ꢁC minꢀ1 in a
nitrogenꢁatmosphere. The RTILs samples were measured in 30
and 800 C temperature range. Viscosity determination experi-
ments were performed on Brook-eld DV-II+ model Program-
mable viscometer associated with temperature measured
heating bath at atmospheric pressure.
Herein, we report the synthesis of a series of hydrophobic
imidazolium based RTILs functionalized with oligo ethylene
glycol chain between two cations and associated with NTf2
anions. Particularly, we concentrated on imidazolium based
RTILs as catalysts for biodegradability and toxicity reasons.2–5
The prepared hydrophobic RTILs were achieved fascinating
physicochemical properties such as viscosity, wide liquid range,
2.3. Synthesis of dicationic room temperature ionic liquids
(RTILs)
2.3.1. Procedure for synthesis of mesylate anion contain-
and high thermal stability. The catalytic activity was tested for ing dicationic RTILs precursors. All short oligo ethylene glycol
the esterication of carboxylic acids with alkyl halides. The dimesylate precursors and mesylate anions containing RTILs
obtained results were compared with the other homogenous as were synthesized and characterized according previous method
well as heterogeneous catalysts. In addition, recyclability test and detail characterization data were also conrmed by corre-
was also performed for these RTILs.
sponding data with the preceding report.41
2.3.2. Procedure for synthesis of imidazolium based bis-
(triuoromethanesulfonyl)imide anions RTILs
2. Experimental
2.1. Materials and methods
2.3.2.1. Tetraethylene
glycol-bis(3-methylimidazolium)bis-
(triuoromethanesulfonyl)imide ([tetraEG(mim)2][NTf2]2) (RTIL-1).
N-Methylimidazole (99.0%), methanesulfonyl chloride (99.0%), A mixture of tetraethylene glycol-bis(3-methylimidazolium)
triethyl amine (99.0%), tetraethylene glycol (99.0%), bis(tri- dimesylate ([tetraEG(mim)2][OMs]2) (1.0 mmol) RTIL and bis-
uoromethane)sulphonamide lithium salt (99.0%), triethylene triuoromethane sulfonimide (LiNTf2) (2.0 mmol) in double
glycol (99.0%), anhydrous pyridine (99.8%), di-ethylene glycol distilled water was stirred magnetically for 2 days in a single
(99.0%), acetonitrile (HPLC grade), ethyl acetate, anhydrous neck round bottom ask at room temperature. Aer completion
sodium sulfate, and all substrate for ester derivatives were of reaction, the reaction mixture showed two phases in which
purchased from Sigma-Aldrich and used without further puri- hydrophobic IL was separated out from the aqueous layer. The
cation. Ester derivatives were characterized by using TLC, aqueous layer was decanted and remaining IL was washed three
different spectroscopic methods and compared with authentic times with double distilled water to remove unreacted starting
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materials and nally heated at 80 ꢁC under reduced pressure 2H), 7.57 (m, 2H), 7.41 (m, 3H), 5.29 (s, 2H), 13C NMR (100 MHz,
on rotary evaporator and afforded oily liquid as resulting CDCl3): d 165.9, 136.1, 133.0, 130.1, 129.9, 128.6, 128.9, 127.6,
product tetraethylene glycol-bis(3-methylimidazolium)(bis- 127.1, 65.7.
triuoromethanesulfonyl)imide ([tetraEG(mim)2][NTf2]2) (1):
2.4.2. 4-Methylbenzyl 4-nitrobenzoate (Table 2, entry 4).
yield 88%; oily liquid; 1H NMR (400 MHz, CDCl3): d 9.41 (s, 2 ꢂ Yield 90%; 1H NMR (400 MHz, CDCl3): d 8.37 (m, 2H), 8.31 (m,
H), 7.72 (s, 2 ꢂ H), 7.14 (s, 2 ꢂ H), 4.41 (t, J ¼ 4.8 Hz, 2 ꢂ 2H), 2H), 7.16 (m, 2H), 7.12 (m, 2H), 5.31 (s, 3H), 2.29 (s, 3H), 13C
3.91 (s, 2 ꢂ 3H), 3.78 (t, 2 ꢂ 2H), 3.61 (t, 2 ꢂ 2H); 3.46 (s, 2 ꢂ NMR (100 MHz, CDCl3): d 164.9, 152.4, 137.1, 136.2, 133.0,
2H); 13C NMR (100 MHz, CDCl3): d 149.53, 137.42, 122.89, 131.1, 129.3, 127.1, 123.9, 64.9, 22.1.
123.09, 70.41, 70.11, 66.94, 56.26, 37.13; FT-IR (500–4000 cmꢀ1):
2.4.3. Butane-1,4-diyl dibenzoate (Table 2, entry 6). Yield
3158, 3123, 2968, 2884, 1569, 1453, 1348, 1180, 1134, 1053, 934, 87%; 1H NMR (400 MHz, CDCl3): d 8.1 (m, 2 ꢂ 2H), 7.61 (m, 2 ꢂ
838. MS-ESI: m/z [M ꢀ NTf2]+ calcd: 604.13; found: 604.11. Elem. 2H), 7.63 (m, 2 ꢂ H), 4.28 (t, 2 ꢂ 2H), 1.76 (t, 2 ꢂ 2H), 13C NMR
anal. calc. (%) for C20H28F12N6O11S4: C, 27.15; H, 3.19; N, 9.50; (100 MHz, CDCl3): d 166.1, 132.9, 130.02, 129.3, 128.9, 65.3,
found: C, 27.17; H, 3.12; N, 9.59.
2.3.2.2. Triethylene glycol-bis(3-methylimidazolium)(bis-
24.9.
2.4.4. Butyl 4-nitrobenzoate (Table 2, entry 8). Yield 88%;
triuoromethanesulfonyl)imide ([triEG(mim)2][NTf2]2) (RTIL-2). 1H NMR (400 MHz, CDCl3): d 8.4 (m, 2H), 8.29 (m, 2H), 4.29 (t,
Yield 86%; oily liquid; 1H NMR (400 MHz, CDCl3): d 9.38 (s, 2 ꢂ 2H), 1.78 (q, 2H), 1.49 (q, 2H), 0.90 (t, 3H), 13C NMR (100 MHz,
H), 7.71 (s, 2 ꢂ H), 7.32 (s, 2 ꢂ H), 4.21 (t, J ¼ 4.8 Hz, 2 ꢂ 2H), CDCl3): d 166.2, 151.9, 135.8, 131.3, 124.3, 63.9, 31.6, 19.2, 14.1.
3.87 (s, 2 ꢂ 3H), 3.73 (t, 2 ꢂ 2H), 3.58 (t, 2 ꢂ 2H); 3.42 (s, 2 ꢂ
2H); 13C NMR (100 MHz, CDCl3): d 149.46, 137.39, 122.92,
123.16, 71.04, 71.09, 66.86, 56.39, 37.27; FT-IR (500–4000 cmꢀ1):
3. Results and discussion
3159, 3122, 2958, 2880, 1569, 1453, 1347, 1180, 1134, 1053, 929,
833. MS-ESI: m/z [M ꢀ NTf2]+ calcd: 560.11; found: 560.19. Elem.
The imidazolium based dicationic RTILs with NTf2 anions were
developed using straightforward synthesis technique as pre-
sented in Scheme 1. In short, tetraethylene glycol (a) reacted
with methanesulfonyl chloride utilizing triethyl amine as base
in dichloromethane to obtain tetraethylene glycol dimesylate
(b). In succeeding step, addition of N-methylimidazole was done
in b to afford tetraethylene glycol-bis(3-methylimidazolium)
dimesylate ([tetraEG(mim)2][OMs]2) (c) as a thick oily liquid.
Followed by the metathesis reaction was done in double
distilled water with Li(NTf2)2 to obtain ([tetraEG(mim)2][NTf2]2)
(d) as thick oily liquid. All produced new dicationic RTILs
catalysts aer totally characterized by analytical and spectro-
scopic approaches later on, estimation of their physicochemical
properties such as acidity, viscosity, DSC, TGA, and etc. were
done.
anal. calc. (%) for C18H24F12N6O10S4: C, 25.72; H, 2.88; N, 10.00;
found: C, 25.77; H, 2.83; N, 10.29.
2.3.2.3. Diethylene
glycol-bis(3-methylimidazolium)(bis-
triuoromethanesulfonyl)imide ([diEG(mim)2][NTf2]2) (RTIL-3).
Yield 84%; oily liquid; 1H NMR (400 MHz, CDCl3): d 9.36 (s, 2 ꢂ
H), 7.68 (s, 2 ꢂ H), 7.29 (s, 2 ꢂ H), 4.19 (t, J ¼ 4.8 Hz, 2 ꢂ 2H),
3.89 (s, 2 ꢂ 3H), 3.71 (t, 2 ꢂ 2H), 3.61 (t, 2 ꢂ 2H); 3.41 (s, 2 ꢂ
2H); 13C NMR (100 MHz, CDCl3): d 148.91, 136.81, 123.02,
122.76, 71.00, 71.11, 67.01, 55.92, 38.01; FT-IR (500–4000 cmꢀ1):
3148, 3121, 2968, 2884, 1556, 1456, 1342, 1182, 1133, 1057, 921,
840. MS-ESI: m/z [M ꢀ NTf2]+ calcd: 516.08; found: 516.11, Elem.
anal. calc. (%) for C16H20F12N6O9S4: C, 24.12; H, 2.53; N, 10.55;
found: C, 24.19; H, 2.60; N, 10.51.
2.4. Typical esterication reaction procedure in presence of
RTILs
3.1. Lewis acidity determination of RTILs by FT-IR
spectroscopy
In a two necked round bottom ask containing a magnetic
needle, 1.0 mmol of organic acid and 1.0 mmol of alkyl halide The results of FT-IR spectra of pyridine adsorbed RTILs and pure
was added into it. 0.1 mmol of synthesized RTILs as catalyst was pyridine are shown in Fig. 1. Generally, pyridine adsorbed
added and stirred at room temperature. Reaction progress was complexes shows two major absorption peaks at 1438 and
monitored by thin layer chromatography (TLC). Aer comple- 1548 cmꢀ1 analogous to Lewis and Brønsted acidity, respec-
tion of the reaction, the mixture was washed three times by tively.42 Consequently, to dene both of Lewis and Brønsted
diethyl ether to extract reaction product and collected the ether acidity, pyridine adsorption study by FT-IR spectroscopy is
layer. The synthesized RTILs are insoluble in diethyl ether. The appropriate tool. It is reported, the presence of band for indi-
collected ether layer was dried on anhydrous sodium sulphate vidual N–H bending and C–C stretching modes of protonated
and concentrated on rotary evaporator to obtained crude ester pyridine experience upward shis or increased intensity of peaks
product. The residue containing RTIL was washed three times upon synchronization of pyridine molecule with both acid sites.42
with double distilled water to remove by-product (hydrogen
The IR peak detected at 1438 cmꢀ1 is allocated to character-
halide) formed in the reaction and obtained pure hydrophobic istic protonation of pyridine molecule onto Lewis acid sites and
RTIL at the bottom of round bottom ask. Later on, collected the peak at 1578 cmꢀ1 is illustrative peak for chemisorbed pyri-
RTIL dried at 80 ꢁC in vacuum oven for 2 h and reused it for next dine onto Lewis acid sites and both of these peaks were observed
attempt of esterication reaction.
in the spectra of prepared RTILs with pyridine in our study.42
2.4.1. 2-Phenylethyl benzoate (Table 2, entry 1). Yield 94%; When pyridine was mixed with all three types of RTILs respec-
1H NMR (400 MHz, CDCl3): d 8.08 (d, 2H), 7.62 (m, H), 7.51 (d, tively, could not nd any peak at 1548 cmꢀ1 corresponding to the
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Scheme 1 Synthesis of imidazolium based dicationic RTILs used for the esterification reaction of acids with alkyl halides.
3.2. Thermogravimetric analysis (TGA) of dicationic RTILs
NTf2 containing RTILs have a very wide stable liquid range for
example monocationic [C4mim][NTf2] as compared to organic
compounds.43 This liquid range is organized by their glass
forming (Tg) or melting temperature (Tm), which are both usually
below ambient temperature, moreover, leading to the new
generation tailored RTILs, and oen even below 0 ꢁC. Because of
their non-boiling character, the higher temperature characteris-
tics limit is given by their thermal decomposition (Td) tempera-
ture.2–5 In order to explain the thermal performance and the
temperature which permit for a perfect and harmless procedure,
it is important to study the exhibition of RTILs at elevated
temperatures and to study the degradation mechanisms.2,3
In the present study, conducted thermal analysis of all
Fig. 1 Acidity determination of prepared imidazolium based dicationic
RTILs containing NTf2 anions by FT-IR spectroscopy.
ꢁ
prepared RTILs from room temperature to 800 C in nitrogen
atmosphere and gas ow rate was 5 ꢁC minꢀ1. Prior to thermal
analysis, all RTILs were kept at 70 ꢁC for 12 h in vacuum oven.
The thermal decomposition proles are shown in the Fig. 2. Td
of all the RTILs estimated from these curves was compared with
reported values of monocationic imidazolium based ILs which
contains the conventional anions, as summarized in the Table
1. It was observed that the thermal stability of prepared RTILs
was signicantly higher than that of other conventional imi-
dazolium based monocationic and dicationic RTILs.43 This
might be due to high molecular weight of NTf2 anions conjugate
with two imidazolium rings separated with oligo ethylene glycol
chains and strong intermolecular interaction between them.
Moreover, the length of linker chain separating two imidazo-
lium cations was partially responsible for their high Td.3,4
In this study, the nature of linker chains and anions are
effective factors in facilitating excellent thermal property of
Brønsted (protic) acid sites. The intense peak at 1440 cmꢀ1
proved that Lewis acidity present in the RTIL-1.
In addition, this peak is shied to higher wavenumber from
1438 cmꢀ1 to 1444 cmꢀ1 and the intensity increased compared
with the pure pyridine. Subsequently, RTIL-2 and RTIL-3 also
showed peaks at higher wavenumber shied at 1440 cmꢀ1 and
1442 cmꢀ1, respectively. These results conrmed that all the
prepared RTILs have Lewis acidity and the order is RTIL-1 >
RTIL-2 > RTIL-3. The highest acidity was observed in RTIL-1
([tetraEG(mim)2][NTf2]2), which has the tetraethylene glycol
linker between the imidazolium cations. We believed that, the
variation between these Lewis acidities of RTILs appeared
because of their linker in between symmetrical imidazolium
cations and synergetic effect of NTf2 anions present in RTILs.
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suitable of manipulation, exibility, and quick measurement
with exactness of temperature control. DSC is also very suitable
for the study of physicochemical progresses, such as the
determination of the kinetics and thermodynamics of organic
compounds due to the fact that these kinds of reactions are
exothermic and are easily followed dynamically and isother-
mally by DSC.
In the present study, analysed all the prepared RTILs by DSC
analysis and heating traces results are collected in the Fig. 3. As
RTILs were stable at atmospheric condition, before conducting
the DSC analysis, all RTILs were kept for drying at 70 ꢁC for 24 h
in vacuum oven.
Typically, 6–15 mg of RTIL was kept in a standard 40 mL
aluminium DSC pan. Air exibility alterations were made by
using known densities of resources. The DSC pan was hermet-
ically wrapped with a cover to eliminate any evaporation of the
substances. The pan was weighed before and aer each exper-
iment to detect any mass loss during the investigation.
Measurements were accomplished in two cycles and tempeꢀra1-
Fig. 2 Thermogravimetric analysis (TGA) of prepared imidazolium
based dicationic RTILs containing NTf2 anions.
prepared RTILs. All the prepared RTILs did not lose their weight
as heated up to 435 ꢁC. This is an outstanding thermal stability
of RTILs containing the organic moieties, suggesting that
prepared RTILs could be utilized in high temperature organic
transformation reaction. Among the RTILs tested, RTIL-1
having tetraethylene glycol chain with NTf2 anions showed
the most thermally stable (Td ¼ 455 ꢁC) characteristic. Ther-
mograms of RTIL-2 and RTIL-3 showed Td at 442.92 ꢁC and
437.42 ꢁC, respectively and these slight reductions in Td of RTIL-
2 and RTIL-3 were might be due to the association of tri and di-
ethylene glycol separating two imidazolium rings and their
synergetic effect on RTILs (insight Fig. 2). However, the results
describes that well-constructed RTILs showed great thermal
stability compared with traditional monocationic ILs composed
of alkyl imidazolium cations and conventional halide anions.36
ꢁ
ꢁ
ture range was ꢀ30–80 C with scanning rates of 5 C min
.
From all the DSC analysis results of RTILs, we could observe
that RTILs did not experience any phase change behaviour
3.3. Differential scanning calorimetry (DSC) analysis of
dicationic RTILs
Differential scanning calorimetry (DSC) is normally used for
studies of heat capacities, phase transitions, enthalpies, and
chemical reactions. This method offers signicant benets,
Fig. 3 Differential scanning calorimetry (DSC) analysis of prepared
such as the minor amount of sample needed for the experiment, imidazolium based dicationic RTILs containing NTf2 anions.
Table 1 Comparison of Td of reported monocationic and dicationic ILs with prepared imidazolium based NTf2 containing dicationic RTILs
Sr. no.
IL
Type of IL
Cations
Anions
Td (ꢁC)
Reference
1
2
3
4
5
6
7
8
([C4mim][Cl])
([C4mim][NTf2])
([N2222][Cl])
([N2222][NTf2])
[C9(mim)2][NTf2]2
([C12(benzim)2][NTf2]2)
([C9(mpy)2][NTf2]2)
([C9(bpy)2][NTf2]2)
([TetraEG(mim)2][NTf2]2)
([TriEG(mim)2][NTf2]2)
([DiEG(mim)2][NTf2]2)
Monocationic
Monocationic
Monocationic
Monocationic
Dicationic
Dicationic
Dicationic
Dicationic
Dicationic
Imidazolium
Imidazolium
Ammonium
Ammonium
Imidazolium
Imidazolium
Pyrrolidinium
Pyrrolidinium
Imidazolium
Imidazolium
Imidazolium
Cl-
NTf2
Cl-
125
409
264
403
350
350
>400
330
455
442
437
44
43
45
45
46
46
46
46
NTf2
NTf2
NTf2
NTf2
NTf2
NTf2
NTf2
NTf2
9a
10a
11a
In present study
In present study
In present study
Dicationic
Dicationic
a
TGA condition: 25–800 ꢁC in nitrogen atmosphere and gas ow was 5 ꢁC minꢀ1
.
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ꢁ
throughout the cycles down to ꢀ30 ꢁC as well as no other viscosity among the prepared RTILs. At 25 C, RTIL-1 specied
transitions detected. This means that pure RTILs could not be 330 cP viscosity and gradually it is decreased while increased
crystallized with no glass transition temperature in the DSC the temperature. Finally, at 100 ꢁC it displayed 160 cP viscosity,
pans and remain liquid even at very low temperature (ꢀ30 ꢁC). which is very high compared to other RTILs studied in this
This is likely attributed to the largely unfavourable packing of report.2–5 The higher viscosity may be attributed due to the
the short oligo ethylene glycol linked with imidazolium based tetraethylene glycol chain length between the imidazolium
cations associated with the bulky NTf2 anions despite repeated cations. The length of linker chain is sufficient to make inter-
cycling at low temperatures. Such an analogous behaviour was action between two imidazolium cations, resulting in a forma-
earlier detected in similar mixed anion and cations ILs systems tion of cage-like structure with the bulky NTf2 cations which
which were not able to be crystallized and did not showed any make RTIL more viscous. On the other hand RTIL-2 showed
glass transition temperature.47 These extraordinary and inter- viscosity range between 280–110 cP at 25 C to 100 C respec-
esting features of RTILs conrm that the use of these RTILs in tively, which is quite favourable viscosity compared to the
extensive selection of applications, particularly liquid range at reported NTf2 containing RTILs.48 This might be due to that
ꢁ
ꢁ
high as well as low temperature can be utilized.
triethylene glycol chain is enough long to keep both imidazo-
lium cation rings apart from each other and make RTIL struc-
ture straight. The lowest viscosity was observed in the RTIL-3,
which has the diethylene glycol linker between the two imida-
zolium rings and the range was 100–220 cP at 25 ꢁC to 100 ꢁC. In
this study, consequently all RTILs exhibited high viscosity
compared to their reported monocationic ILs. The higher
viscosities of these prepared RTILs may be attributed to
supplementary hydrogen bonds containing the ether functional
groups of the cations with other cations or anions present in the
RTILs. In addition, conducted few more experiments for the
determination of hydrophobicity of prepared RTILs and results
are displayed in a photograph of Fig. 5.
3.4. Viscosity determination of dicationic RTILs
The viscosity is an important property of RTILs, because it
intensely inuences the diffusion of molecules dispersed or
dissolved in the RTILs. In addition, it is the internal friction or
resistance to ow produced by intermolecular components and
is consequently very signicant in all physicochemical devel-
opments. For that purpose, any scientic unit operations
assigning with the transfer of uids or energy involve knowl-
edge of the viscosity of pure compounds and/or their mixtures.
General prediction of viscosity is normally problematic, and
exible predictive models and their values will require addi-
tional experimental data in order to attain a better under-
standing of viscosity property. The NTf2 containing ILs exhibit a
wide liquid range even if a recent study point out that,
decomposition temperatures were overestimated in previous
thermal studies.43 The viscosities of prepared RTILs were
measured from 25 ꢁC to 100 ꢁC temperature and results are
reported in Fig. 4.
In a certain amount of water and RTILs were mixed respec-
tively and stirred for 24 h and analysed separation layer. Aer
24 h results clearly showed that, there are still two layers cor-
responding to RTIL (bottom) and water (upper) which designate
the hydrophobicity of RTILs.
3.5. Catalytic activity of dicationic RTILs in esterication
reaction
Before performing the viscosity measurement all RTILs were
vacuum dried at 70 ꢁC for 12 h. The viscosity of these neat RTILs
decreases with the temperature rising from 25 to 100 ꢁC. In this
study, we found that RTIL-1 containing imidazolium rings
separated by tetraethylene glycol chain showed the highest
Table 2 show the results of esterication reactions of benzoic
acid (1) with benzyl bromide (2) to obtain benzyl benzoate (3) as
a model ester compound using various homogeneous catalysts
including all the synthesized RTILs under solvent-free condi-
tion. In the beginning, performed catalyst-free and solvent-free
synthesis of benzyl benzoate (3) at room temperature and
progress of the reaction was monitored continuously and there
was no reaction up to 24 h (entry 1).
Next step, conducted the same reaction of 1 (1 mmol) and 2
(1 mmol) in 0.5 equiv. of ([diEG(mim)2][NTf2]2) RTILs as catalyst
Fig. 4 Viscosity determination at various temperature of prepared Fig. 5 Miscibility of (a) mixture of water and RTILs (b) mixture of water
imidazolium based dicationic RTILs containing NTf2 anions.
and RTILs after 24 h.
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Table 2 Esterification of organic acids with alkyl halides in presence of dicationic and monocationic RTILsa
Entry
Catalyst
Catalyst amount
Time (min)
Conversion (%)
Yieldb (%)
1
2
3
4
5
6
7
8
—
—
24 h
120
60
10
20
20
30
30
120
30
30
30
30
30
30
30
—
—
81
86
94
93
94
94
22
55
36
8
61
34
42
26
23
([DiEG(mim)2][NTf2]2)
([TriEG(mim)2][NTf2]2)
([TetraEG(mim)2][NTf2]2)
([TetraEG(mim)2][NTf2]2)
([TetraEG(mim)2][NTf2]2)
([TetraEG(mim)2][NTf2]2)
[BMIm][NTf2]
0.5 equiv.
0.5 equiv.
0.5 equiv.
0.3 equiv.
0.2 equiv.
0.1 equiv.
0.1 equiv.
0.1 equiv.
0.1 equiv.
0.1 equiv.
0.1 equiv.
0.1 equiv.
0.1 equiv.
0.1 equiv.
0.1 equiv.
100
100
100
100
100
100
37
59
42
12
68
9
[BMIm][NTf2]
10
11
12
13
14
15
16
1-Methyl imidazole
Tetra ethylene glycol
Li[NTf2]2
[BMIm][Br]
[BMIm][Cl]
44
56
32
28
FeCl3
CuCl2$2H2O
a
All reactions were carried out on 1.0 mmol scale of substrate with catalyst and without addition of any co-catalyst and solvent. b Yield refers to the
isolated product. Products were characterized by NMR and IR spectroscopy.
and stirred at room temperature (entry 2). Aer the completion RTILs, ([tetraEG(mim)2][NTf2]2) showed 94% yield in 10 min
of reaction (120 min) the reaction mixture was extracted from reaction time was found the most active RTIL. We believe that
RTIL using diethyl ether to get crude reaction mass and sub- the tetraethylene glycol chain with dicationic imidazolium rings
jected to 1H NMR analysis. In 1H NMR spectrum of crude mass, of ([tetraEG(mim)2][NTf2]2) IL produced highly active structure
it was observed that the reaction of 1 with 2 using RTILs was associated with supplementary hydrogen bonding with reactant.
preceded smoothly and provide high yield (81%) of 3 within On the other hand, the rest of dicationic RTILs have less
120 min showing the signicant catalytic activity of prepared hydrogen bonding with the reactant and obtained less yield.
([diEG(mim)2][NTf2]2) catalyst.
In the beginning, performed catalyst-free and solvent-free the advance study in esterication reaction at ambient condition.
synthesis of benzyl benzoate (3) at room temperature and In subsequent step, determined the minimum amount of
Therefore ([tetraEG(mim)2][NTf2]2) dicationic RTIL is selected for
progress of the reaction was monitored continuously and there ([tetraEG(mim)2][NTf2]2) RTIL as catalyst required for rapid
was no reaction up to 24 h (entry 1). Next step, conducted the conversion of reactants and formation of ester product in effi-
same reaction of 1 (1 mmol) and 2 (1 mmol) in 0.5 equiv. of cient yield by carrying out the several number of catalytic
([diEG(mim)2][NTf2]2) RTILs as catalyst and stirred at room reactions using 0.3, 0.2, 0.1 equiv. of ([tetraEG(mim)2][NTf2]2)
temperature (entry 2). Aer the completion of reaction RTIL in constant reaction condition (entries 5–7). These reac-
(120 min) the reaction mixture was extracted from RTIL using tions outcome showed that all reactions with ([tetraEG(mim)2]-
diethyl ether to get crude reaction mass and subjected to 1H [NTf2]2) were consummate efficiently well and deliver
NMR analysis. In analysis of 1H NMR spectrum of crude mass, it outstanding yield of 3 within 30 min reaction time. The amount
was observed that the reaction of 1 with 2 using RTILs was of catalyst dosage showed major inuence on the reaction time,
preceded smoothly and provide high yield (81%) of 3 within 120 0.5 equiv. of ([tetraEG(mim)2][NTf2]2) completed reaction within
min showing the signicant catalytic activity of prepared 10 min, successively when amount of catalyst decreased, reac-
([diEG(mim)2][NTf2]2) catalyst.
tions are sluggish and need more reaction time. 0.3 and
Later on, performed the same reaction using catalytic 0.2 equiv. of catalyst completed the reaction in 20 min under
amount of ([triEG(mim)2][NTf2]2) and ([tetraEG(mim)2][NTf2]2), same condition, respectively. 94% respective ester product yield
respectively (entries 3 and 4) and reactions were completed in was obtained in 30 min reaction time by using 0.1 equiv. of
very short reaction time with excellent yield of 3. These results ([tetraEG(mim)2][NTf2]2) as catalyst. Production an allowance
reveal that all prepared RTILs are highly reactive in esterica- for the mole economy, 0.1 equiv. of catalyst was nominated for
tion of acids with alkyl halides. Especially, among all prepared additional study with this protocol.
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Our next task was to compare and determine synergetic of several prepared ester derivatives using dicationic
effect of dicationic ([tetraEG(mim)2][NTf2]2) catalyst with other ([tetraEG(mim)2][NTf2]2) RTIL at ambient condition. Benzoic
mono-cationic ILs as well as homogenous and heterogeneous acid and benzyl bromide were selected as initial model reactant
catalyst in esterication reaction at room temperature. The and respective ester product as synthetic target on which to test
conventionally popular mono-cationic IL [BMIm][NTf2] was and support our design that synthesis of ester derivative could
selected for the comparison in esterication reaction at similar be promoted in these prepared dicationic RTILs and this reac-
condition. A 22% yield of respective ester product 3 was tion showed 94% yield in 30 min reaction period. In next step,
obtained using catalytic amount of monocationic IL under we performed substituted benzoic acid and substituted alkyl
same reaction condition (entry 8). In addition, increased reac- halides, these reactions were also showed promising results and
tion time up to 120 min with [BMIm][NTf2] was also fails to give reveals the catalytic activity of ([tetraEG(mim)2][NTf2]2) RTIL
the efficient respective ester product yield (entry 9). We also (entries 1–4).
carried out reaction with 1-methyl imidazole and tetraethylene
The straight chain mono and di-alkyl halides with
glycol as catalyst, these reactions are also unable to show the substituted and un-substituted acid were also exhibited effi-
efficient yield of relevant product in short reaction period cient yield of respective ester products at room temperature in
(entries 10 and 11). To rule out the potential promotional effect short reaction time (entries 5–10). Acetic acid reacted with
of the [NTf2]2 anions with prepared RTILs, we performed reac- benzyl bromide and substituted benzyl bromide or alkyl
tion with heterogeneous Li[NTf2]2 as catalysts. It is already bromide in presence of RTIL were also showed efficient yield of
reported and well known that, [NTf2]2 anions forms weak corresponding ester product in adequate time (entries 11 and
complexes with acidic moieties.4,5 Both uoro-alkyl groups of 12). Butane-1,4-diyl di-acetate were obtained in very good yield
[NTf2]2 with a large ionic size are known to increase the inter- by reacting with di-alkyl halides with acetic acid (entries 13–15).
action of the ILs and acidic groups as well as least co-ordination In addition, propionic acid were also showed ester products
with cations that is favourable for reaction kinetics compared to with benzyl bromide, substituted benzyl bromide, mono alkyl
traditional anions.1–4 It was found that ester product could be halide, di-alkyl halide correspondingly in efficient yield at
generated with this catalyst but reaction was sluggish dramat- ambient condition with ([tetraEG(mim)2][NTf2]2) RTIL as a
ically and obtained insufficient amount of respective product catalyst (entries 16–20). It is noteworthy to mention here, the
(entry 12). Meanwhile, synthesized RTILs were successfully present catalytic system is favourable for esterication of acid in
achieved efficient yield of respective ester in very short reaction presence of other acid sensitive functional groups (entries
time. This is a strong indication that, the imidazolium cation 21–24). We consider that, the present new approach for the
with [NTf2]2 anions signicantly inuences the reaction prog- synthesis of esters using organic acids with alkyl halides offers
ress of the esterication conversion. These results indicates numerous benets over traditional method. Whereas, present
that, there is a synergetic mutual action of short oligo ethylene method does not involve the use of alkali throughout the stages,
glycol functionalized imidazolium cation and [NTf2]2 anions, it provides high yield and conversion in short reaction time and
which plays a crucial role on the improvement of conversion of completed under mild and green condition. Currently, investi-
acids and alkyl halides into esters. These observations under- gating of mechanism of this esterication reaction with these
line the fact that the positive effect of ionic liquids in the RTILs and the extension of this method to multifunctional
esterication reaction is closely related to synergetic effect of molecules is in under way.
both counter ions “anions and cations” as well as organic
functional groups on the imidazolium rings, which determine
the acidic property of RTILs.
3.6. Reusability test of ([tetraEG(mim)2][NTf2]2) RTIL
Furthermore, various traditional ILs were applied in the Progressive achievement of new eco-friendly, green, mild,
reaction system to gain further insights into the effect of the advance, cheap and reusable dicationic RTILs as a new catalytic
counter ions and the substituents of RTILs components on the coordination for the esterication reaction of acids to alkyl
esterication reaction. As elucidated in Table 2, the efficiency of halides under solvent free mild reaction condition is the major
production of ester was modulated strongly by the different type ambition of this study. Meanwhile, the catalytic recyclability
of counter ions examined. [BMIm][Br] and [BMIm][Cl] were and their strength aer utilization are very substantial factors in
tested for the esterication reaction under similar reaction catalysed organic reactions and it is essential to understand the
condition and resulted in 34% and 42% yields of corresponding stability of catalyst. To study the reuse of the ([tetraEG(mim)2]-
product (entries 13 and 14). Additionally, to determine the [NTf2]2) catalyst, the RTIL was collected as a thick oily liquid in
catalytic activity of heterogeneous catalyst in esterication, round bottom ask at the end of reaction and washed three
FeCl3, and CuCl2$H2O were also tested in similar reaction times with double distilled water and used it in next attempt.
condition and results provide 26 and 23% yield of 3 in 30 min Recyclability of ([tetraEG(mim)2][NTf2]2) catalyst was studied
reaction time (entries 15 and 16).
over eight cycles for the esterication reaction. There is no
The successful development of highly active dicationic signicant loss in the catalytic activity and selectivity over eight
([tetraEG(mim)2][NTf2]2) RTIL as catalyst for the esterication cycles (Fig. 6). Additionally, conversion of reactant was 100%
reaction prompted us to initiate a preliminary program to and the yield of product was sustained at 92% during recycling
investigate its usefulness as catalyst for synthesis of various tests. In addition, performed 1H and 13C NMR analysis of reused
ester derivatives at room temperature. Table 3 shows the results ([tetraEG(mim)2][NTf2]2) catalyst for the resolve of structural
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Table 3 Preparation of ester derivatives using dicationic ([tetraEG(mim)2][NTf2]2) RTIL at room temperaturea
Sr. no.
1
Acid
R–X
Time (min)
40
Product
Yieldb (%)
94
2
3
4
30
50
30
90
76
81
5
30
30
50
30
40
60
86
87
81
88
94
80
6
7
8
9
10
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Table 3 (Contd.)
Sr. no.
11
Acid
R–X
Time (min)
50
Product
Yieldb (%)
81
12
13
14
30
40
60
88
92
90
15
16
60
60
81
78
17
18
40
50
90
90
19
20
60
60
92
78
21
180
76
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Table 3 (Contd.)
Sr. no.
22
Acid
R–X
Time (min)
145
Product
Yieldb (%)
80
23
90
74
79
24
130
a
All reactions were carried out on 1.0 mmol scale of substrate with 0.1 equiv. of catalyst and without addition of any co-catalyst and solvent. b Yield
refers to the isolated product. Products were characterized by NMR and IR spectroscopy.
its property and catalytic activity in esterication reaction using
organic acids and alkyl halides. These RTILs possess excellent
physicochemical properties compared with tradition mono-
cationic ILs, such property can be utilized in various research
applications. The present hydrophobic catalytic system showed
100% consumption of reactants and achieved 94% yield of
respective ester products at room temperature in 30 min reac-
tion time using 0.1 equiv. of RTIL. Moreover, effect of catalyst
dosage and homogenous as well as heterogeneous catalyst were
also determined and discussed. The present catalyst showed
outstanding performance using very less amount of catalyst in
Fig.
6 Recyclability test of imidazolium based dicationic very short reaction period. At elevated reaction condition,
([tetraEG(mim)2][NTf2]2) RTILs in esterification reaction.
several ester derivatives were prepared and characterized. The
RTIL can be separated by a simple decantation method and
regenerated several times. Regenerated catalyst showed effi-
cient stability without considerable loss of physicochemical
property and catalytic activity. Such task-specic ILs may well
prove to be very signicant in transferring ILs from the labo-
ratory to production scale processes in esterication reaction.
changes in recycled catalyst. The results of NMR and IR analysis
revealed that, the protection of structural integrity, no remark-
able changes were found in the structure of recycled RTIL aer
being used eight cycles in esterication. 95% RTILs could be
achieved aer completion of reaction for the next cycle.
Therefore, the catalyst ([tetraEG(mim)2][NTf2]2) RTIL is
capable for recycle eight times without loss in its selectivity and
catalytic activity.
Acknowledgements
This work was supported by KCRC through the NRF funded by
Ministry of Science, ICT, and Future Planning (NRF-
2014M1A8A1049258) and National Research Foundation of
4. Conclusions
In conclusion, a series of symmetrical imidazolium based tailor- Korea (NRF) grant funded by the Ministry of Education (no.
made dicationic RTILs were prepared successfully and screened 2009-0093816).
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