A silver and water free metathesis reaction: A route to ionic liquids

Article abstract of DOI:10.1039/c3gc37034h

A versatile, cheaper, silver and water-free metathesis reaction was developed for imidazolium, phosphonium and pyrrolidinium based ionic liquids (ILs) associated with different anions such as dicyanamide, thiocyanate, tetrafluoroborate and bis(trifluoromethylsulfonyl)imide. This route, using the melt of amine chloride as a solvent and reagent, favours the ion exchange reaction using anion salts of Na or Li, yielding ionic liquids in high purity (≥99.5%) and high yields (≥90%). This route is particularly well adapted for water miscible ILs preparation.

Full text of DOI:10.1039/c3gc37034h

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Graphical abstract
A versatile, lowꢀcost, silver and waterꢀfree metathesis reaction using melt amine
chloride as solvent yields ionic liquids in high purity (≥ 99.5 %) and high yields
(≥ 90 %)

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Silver and water free metathesis reaction: a route to ionic liquids†
Hassan Srour,a,b Hélène Rouault,b Catherine C. Santini,a* Yves Chauvina*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
A versatile, cheaper, silver and waterꢀfree metathesis reaction was developed for imidazolium,
phosphonium and pyrrolidinium based ionic liquids (ILs) associated with different anions such as
dicyanamide, thiocyanate, tetrafluoroborate and Bis(trifluoromethylsulfonyl)imide. This route, using the
melt of amine chloride as a solvent and reagent, favours the ion exchange reaction using anion salts of Na
or Li, yielding ionic liquids in high purity (≥ 99.5%) and high yields (≥ 90%) This route is particularly
10 well adapted for water miscible ILs preparation.
ILs are needed to remove them. For industrial or large scale
quantities, it would be advantageous to use a silver and water free
route.
In this work, we reported an alternative synthetic silver and
water free route for ILs, in an attempt to reduce the wastes and
their cost. For this purpose, the metathesis reaction was performed
at the melting point temperature of the amine halide salts, playing
Introduction
During the last decade, great attention was paid to the synthesis
and application of ionic liquids (ILs) for distinct purposes, as
evidenced by the appearance of a large number of publications.1ꢀ4
15 ILs come to be solvents of choice in many applications, due to
their industrial accessibility and their tunable physicoꢀchemical
properties (e.g., separation technology,5ꢀ8 catalysis,9,10 biomass
treatment,11 lubrication,12 and energy storage13ꢀ15). Ionic liquids
are generally prepared through one of two routes. The halideꢀfree
20 ILs are formed via direct alkylation reaction of phosphine, amine,
pyridine, or azole with e.g., dialkyl sulfates, alkyl nitrate, dialkyl
phosphonate, or alkyl triflates yielding the corresponding alkyl
sulfate, dialkyl phosphonate, or triflate ionic liquids, etc.16,17
However, the range of ionic liquids accessible via this route is
25 limited. Most ionic liquids are produced in twoꢀsteps, whereby an
organic halide salt is formed by alkylation of a base by a
haloalkane. The quaternization reaction has been improved by the
use of microwaves,18,19 or ultrasound,20 no solvent, and small
excess of 1ꢀhaloalkane. Then, there is anion exchange by
30 metathesis or acidꢀbase reaction (Scheme 1). Hydrophobic ILs are
easily separated from the reaction media if the metathesis reaction
occurs in water. However, the slow decantation and water removal
require a large energy input. For the waterꢀmiscible ionic liquids,
the separation of the byꢀproducts from the desired ionic liquid
35 becomes challenging. To make feasible this separation, the
exchange reaction of the halide anion is generally performed in
the presence of a silver salt of the numerous anions e.g.,
dicyanamide, thiocyanate, or tetrafluoroborate.21ꢀ25 This method
has proven to be a reliable and effective method for smallꢀscale
40 synthesis. Unfortunately, for industrial or large scale quantities
this route is very expensive. Moreover, the large amount of
obtained silver halide precipitates slowly from water, leading to
silver contaminated ILs. Alternatively, the metathesis reaction can
be carried out in organic solvent (CH2Cl2, acetone, etc.)26,27
45 However, the silver halide byꢀproducts are slightly soluble in
organic solvents as in ILs. Therefore, several water washings of
50
the role of solvent, in the presence of the alkaline salt of the
28
55 chosen anion
.
Scheme 1 General procedure for the synthesis of imidazoliumꢀbased ionic
liquids.
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than 0.5% as shown by high resolution mass spectra (MS QTOF).
Experimental
Due to the air and moisture sensitivity of most materials, all
experiments were undertaken with standard vacuum and Schlenk
techniques (under vacuum of 0.1 Pa), as well as a glove box with
an argon atmosphere (H2O and O2 <10 ppm). 1ꢀmethyl imidazole
(>99%, Aldrich), 1ꢀchlorohexane, and 1ꢀchlorobutane (>99%
Synthesis and characterization
60 1-butyl-3-methylimidazolium dicyanamide: [C1C4Im][N(CN)2]
5
NaN(CN)2 (9.79 g, 110 mmol) was added with a Bush tube into
melt [C1C4Im][Cl] (17.4 g; 100 mmol) at ∼ 70°C and kept under
stirring for 24 h. After cooling down at 25°C, THF (100 mL) was
added. The white precipitate of NaCl was filtered, and the filtrate
65 was kept at ꢀ5°C overnight to complete the precipitation of NaCl,
which was then filtered off; this work up was repeated until no
more NaCl precipitate was observed. Evaporation of THF
afforded [C1C4Im][N(CN)2] as a slightly yellow oil; (yield. 85%).
1HꢀNMR (300 MHz, d6ꢀDMSO): δ (ppm) : 9.10 (s, 1H, C2H) ;
70 7.74 (d, 1H, C4H) ; 7.67 (d, 1H, C5H) ; 4.16 (t, 2H, NCH2) ; 3.85
(s, 3H, NCH3) ; 1.75 (qt, 2H, CH2CH2CH2) ; 1.23 (st, 2H,
CH2CH2CH3) ; 0.85 (t, 3H, CH2CH3) ; 13C{1H}ꢀNMR (300MHz,
d6ꢀDMSO) : δ (ppm) : 135.3 (C2H) ;123.5 (C4H) ; 122.2 (C5H) ;
119.74 (NCN); 49.81 (NCH2) ; 35.8 (NCH3) ; 31.5 (CH2CH2CH2)
75 ; 18.9 (CH2CH2CH3) ; 12.8 (CH2CH3). Electrospray, MS (+ve):
m/z 139 (100% ꢀ C1C4Im+), MS (ꢀve): m/z 66 (100% dcaꢀ).
Aldrich)
were
distilled
prior
to
use.
Lithium
bis(trifluoromethylsulfonyl)imide (>99% Solvionic), NaN(CN)2
(>96%, Aldrich), NaBF4, (>98%, Aldrich) and NaSCN (>98%,
10 Aldrich) were dried under vacuum before use . All other reagents
and solvents were commercially available and were purified
following the standard procedures under argon: THF
(Na/benzophenone) and CH2Cl2 (P2O5).
Solution NMR spectra were recorded at room temperature on a
1
15 BRUKER AVANCE 300 spectrometer. H, 13C, 31P, 11B, and 19F
solution NMR data were collected at room temperature on a
Bruker AC 300 MHz spectrometer with the resonance frequency
at 300.130 MHz for 1H nucleus. The solvent used (d6ꢀDMSO) was
distilled and kept in a rotaflo with molecular sieves. Chemical
20 shifts are reported in ppm (singlet = s, doublet = d, doublet of
doublet = dd, and multiplet = m) and were measured relative to
1
1-butyl-3-methylimidazolium thiocyanate: [C1C4Im][SCN]
residual proton of the solvent to CHDCl2 for H, to CD2Cl2 for
13C.
Slightly yellow clear oil, 90% yield. 1HꢀNMR (300 MHz,
CD2Cl2):δ (ppm) : 9.15 (s, 1H, C2H) ; 7.55 (d, 1H, C4H) ; 7.53 (d,
80 1H, C5H) ; 4.31 (t, 2H, NCH2) ; 4.04 (s, 3H, NCH3) ; 1.90 (qt, 2H,
CH2CH2CH2) ; 1.38 (st, 2H, CH2CH2CH3) ; 0.92 (t, 3H, CH2CH3)
The high resolution mass spectra (MS QTOF) were recorded in
25 a positive and negative ion mode on a hybrid quadrupole timeꢀofꢀ
flight mass spectrometer (MicroTOFQꢀII, Bruker Daltonics,
Bremen) with an Electrospray Ionization (ESI) ion source. The
gas flow of spray gas is 0.6 bar and the capillary voltage is +/ꢀ
4.5 kV. The solutions are infused at 180 ꢁL/h. The mass range of
30 the analysis is 50ꢀ1000 m/z and the calibration was done with
sodium formate.
;
13C{1H}ꢀNMR (300 MHz, CD2Cl2) : δ (ppm) : 136.48 (SCN);
131.58 (C2H) ;123.80 (C4H) ; 122.38 (C5H) ; 49.54 (NCH2) ;
36.54 (NCH3) ; 32.01 (CH2CH2CH2) ; 19.8 (CH2CH2CH3) ; 13.6
85 (CH2CH3). Electrospray, MS (+ve): m/z 139 (100% ꢀ C1C4Im+),
MS (ꢀve): m/z 58 (100% ꢀSCN).
The quaternized chloride salts of 1ꢀmethylimidazole were
synthesized according to literature methods.29,30 The other ionic
liquids were used as received: 1ꢀhexylꢀ3ꢀmethylimidazolium
35 dicyanamide (Solvionic, 99.5%), NꢀbutylꢀNꢀmetylpyrrolidinium
chloride (Solvionic), and tetrabutylphosphonium bromide (sigma
Aldrich).
1-butyl-3-methylimidazoliumtetrafluoroborate:[C1C4Im][BF4]
Slightly yellow clear oil, 90% yield. 1HꢀNMR (300 MHz,
CD2Cl2):δ (ppm) : 8.87 (s, 1H, C2H) ; 7.43 (d, 1H, C4H) ; 7.42 (d,
90 1H, C5H) ; 4.15 (t, 2H, NCH2) ; 3.92 (s, 3H, NCH3) ; 1.85 (qt, 2H,
CH2CH2CH2) ; 1.32 (st, 2H, CH2CH2CH3) ; 0.91 (t, 3H, CH2CH3)
;
13C{1H}ꢀNMR (300 MHz, CD2Cl2) : δ (ppm) : 136.28 (C2H)
Metathesis reaction: General synthetic method
;123.66 (C4H) ; 122.25 (C5H) ; 49.69 (NCH2) ; 36.10 (NCH3) ;
31.84 (CH2CH2CH2) ; 19.27 (CH2CH2CH3) ; 13.05 (CH2CH3).
95 19FꢀNMR (300 MHz, CD2Cl2); δ (ppm): ꢀ150.7 (s, BF4). 11BꢀNMR
(300 MHz, CD2Cl2): δ (ppm): ꢀ0.96 (s, BF4). Electrospray, MS
(+ve): m/z 139 (100% ꢀ C1C4Im+), MS (ꢀve): m/z 87 (100% ꢀBF4).
1ꢀbutylꢀ3ꢀmethylꢀimidazolium chloride was heated gently at
40 70°C. The salt of lithium or sodium of each anion were added to
the melt under argon and stirred for 24 h. The resulting product
was dissolved in CH2Cl2 (THF is used for dicyanamide anion) and
stirred for 20 min to precipitate the resulting halide salt (NaCl or
LiCl) which was filtered off (0.2 ꢁm filter). The resulting filtrate
45 was placed in the fridge (ꢀ5°C) for one night for complete
precipitation of the halide salt, in case of salt precipitation another
filtration was performed and the resulting filtrate evaporated to
dryness.
Note that [C1C4Im][Cl] is soluble in CH2Cl2 but not in THF at the
50 end of the reaction, THF was used to easily separate
[C1C4Im][N(CN)2] from the residual chloride content. For Nꢀ
butylꢀNꢀmetylpyrrolidinium chloride and tetrabutylphosphonium
bromide melting arournd ∼120°C, the exchange reactions were
performed at this temperature. ILs were kept under high vacuum
55 (10ꢀ5 mbar) at room temperature for 24 h then stocked under argon
in glove box. Their water content was lower than 100 ppm, as
assessed by Karl Fisher titration. The chloride content was lower
1-butyl-3-methylimidazolium
sulfonimide: [C1C4Im][NTf2]
bis
(trifluoromethane)-
1
100 Slightly yellow clear oil, 90% yield. HꢀNMR (CD2Cl2): δ (ppm) :
8.73 (s, 1H, C2H) ; 7.28 (d, 1H, C4H) ; 7.24 (d, 1H, C5H) ; 4.15
(t, 2H, NCH2) ; 3.90 (s, 3H, NCH3) ; 1.83 (qt, 2H, CH2CH2CH2) ;
1.32 (st, 2H, CH2CH2CH3) ; 0.94 (t, 3H, CH2CH3) ; 13C{1H}ꢀ
NMR (CD2Cl2) : δ (ppm) : 134.3 (C2H) ; 124.3 (C4H) ; 122.4
105 (C5H) ; 118.2 (CF3) ; 50.3 (NCH2) ; 37.1 (NCH3) ; 33.2
(CH2CH2CH2)
;
19.8 (CH2CH2CH3)
;
13.5 (CH2CH3).
Electrospray, MS (+ve): m/z 139 (100% ꢀ C1C4Im+), MS (ꢀve):
ꢀ
m/z 280 (100% NTf2 ).
1-hexyl-3-methylimidazolium dicyanamide: [C1C6Im][N(CN)2]
110 Slightly yellow clear oil, >90% yield. 1HꢀNMR (300 MHz, d6ꢀ
2
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DMSO): δ (ppm) : 8.68 (s, 1H, C2H) ; 7.45 (d, 1H, C4H) ; 7.41
(d, 1H, C5H) ; 4.16 (t, 2H, NCH2) ; 3.86 (s, 3H, NCH3) ; 1.84 (qt,
6H,CH2CH2CH2CH2CH2) ; 1.27 (st, 2H, CH2CH2CH3); 0.81 (t,
3H, CH2CH3) ; 13C{1H}ꢀNMR (300 MHz, d6ꢀDMSO) : δ (ppm)
(CH2CH2CH2CH2CH2CH2); 28.98 (CH2CH2CH2CH2CH2); 28.86
(CH2CH2CH2CH2); 26.11 (CH2CH2CH2); 22.55 (CH2CH2CH3);
13.80 (CH2CH3). 19FꢀNMR (300 MHz, CD2Cl2); δ (ppm): ꢀ150.96
(s, BF4). 11BꢀNMR (300 MHz, CD2Cl2): δ (ppm): ꢀ1.05 (s, BF4).
5
:135.82 (C2H) ;123.62 (C4H) ; 122.28 (C5H) ; 119.74 (NCN), 60 Electrospray, MS (+ve): m/z 195 (100% ꢀ C1C8Im+), MS (ꢀve):
49.68 (NCH2) ; 35.73 (NCH3) ; 30.56 (CH2CH2CH2CH2CH2) ;
29.37 (CH2CH2CH2CH2); 25.21 (CH2CH2CH2);
m/z 87 (100% ꢀBF4).
N- butyl- N- metylpyrrolidinium dicyanamide: [Py14][N(CN)2]
21.96(CH2CH2CH3); 13.40 (CH2CH3). Electrospray, MS (+ve):
Slightly yellow clear oil, 90% yield. 1H NMR (300 MHz, d6ꢀ
DMSO): δ (ppm): 3.45 (m, 4H, C2H, C5H), 3.24 (t, NCH2), 2.98
65 (s, CH3), 2.08 (m, 4H, C3H, C4H), 1.68 (m, CH2CH2CH2), 1.31
(m, CH2CH2CH3), d 0.93 (t, CH2CH3). 13C NMR (300 MHz d6ꢀ
DMSO): δ (ppm): 119.56 (NCN); 63.93 (C2H); 63.89 (C5H);
63.85 (C3H); 63.39 (C4H); 47.96 (NCH3); 25.39 (NCH2); 21.56
(CH2CH2CH2); 19.78 (CH2CH2CH3); 13.92 (CH2CH3).
m/z 167 (100% ꢀ C1C6Im+), MS (ꢀve): m/z 66 (100% dcaꢀ).
10 1-hexyl-3-methylimidazolium thiocyanate: [C1C6Im][SCN]
Slightly yellow clear oil, 90% yield. 1HꢀNMR (300 MHz,
CD2Cl2): δ (ppm) : 9.32 (s, 1H, C2H) ; 7.51 (d, 1H, C4H) ; 7.46
(d, 1H, C5H) ; 4.29 (t, 2H, NCH2) ; 4.07 (s, 3H, NCH3) ; 1.92 (qt,
6H,CH2CH2CH2CH2CH2) ; 1.33 (st, 2H, CH2CH2CH3); 0.88 (t,
15 3H, CH2CH3) ; 13C{1H}ꢀNMR (300 MHz, CD2Cl2) : δ (ppm) : 70 Electrospray, MS (+ve): m/z 142.1 (100% ꢀ P14+), MS (ꢀve): m/z
136.75 (SCN); 131.12 (C2H) ;123.69 (C4H) ; 122.26 (C5H) ;
50.57 (NCH2) ; 36.57 (NCH3) ; 31.03 (CH2CH2CH2CH2CH2) ;
66.0 (100% ꢀ ꢀN(CN)2).
N-butyl-N-metylpyrrolidinium
bis(trifluoromethanesulfonyl)amide: [Py14][NTf2]
30.08
(CH2CH2CH2CH2); 25.81
(CH2CH2CH2); 22.36
(CH2CH2CH3); 13.70 (CH2CH3). Electrospray, MS (+ve): m/z 167
Colourless oil, 90% yield. 1H NMR (300 MHz, d6ꢀDMSO): δ
75 (ppm): 3.45 (m, 4H, C2H, C5H), 3.24 (t, NCH2), 2.98 (s, CH3),
2.08 (m, 4H, C3H, C4H), 1.68 (m, CH2CH2CH2), 1.31 (m,
CH2CH2CH3), d 0.93 (t, CH2CH3). 13C NMR (300 MHz d6ꢀ
DMSO): δ (ppm): 119.56 (NCN); 63.93 (C2H); 63.89 (C5H);
63.85 (C3H); 63.39 (C4H); 47.96 (NCH3); 25.39 (NCH2); 21.56
80 (CH2CH2CH2); 19.78 (CH2CH2CH3); 13.92 (CH2CH3).13C NMR
(300 MHz d6ꢀDMSO): δ (ppm): 119.56 (NCN); Electroꢀspray,
MS(+ve): m/z 142.1 (P14+), MS (ꢀve): m/z 279.9 (ꢀNTf2).
20 (100% ꢀ C1C6Im+), MS (ꢀve): m/z 58 (100% ꢀSCN).
1-hexyl-3-methylimidazoliumtetrafluoroborate:[C1C6Im][BF4]
Slightly yellow clear oil, 90% yield. 1HꢀNMR (300 MHz,
CD2Cl2): δ (ppm) : 8.78 (s, 1H, C2H) ; 7.39 (d, 1H, C4H) ; 7.36
(d, 1H, C5H) ; 4.18 (t, 2H, NCH2) ; 3.95 (s, 3H, NCH3) ; 1.89 (qt,
25 6H,CH2CH2CH2CH2CH2) ; 1.34 (st, 2H, CH2CH2CH3); 0.90 (t,
3H, CH2CH3) ; 13C{1H}ꢀNMR (CD2Cl2) : δ (ppm) : 136.20 (C2H)
;123.67 (C4H) ; 122.18 (C5H) ; 50.07 (NCH2) ; 36.17 (NCH3) ;
30.98 (CH2CH2CH2CH2CH2) ; 29.91 (CH2CH2CH2CH2); 25.72
(CH2CH2CH2); 22.31 (CH2CH2CH3); 13.64 (CH2CH3). 19FꢀNMR
30 (300 MHz, CD2Cl2); δ (ppm): ꢀ151.16 (s, BF4). 11BꢀNMR (300
MHz, CD2Cl2): δ (ppm): ꢀ1.02 (s, BF4). Electrospray, MS (+ve):
m/z 167 (100% ꢀ C1C6Im+), MS (ꢀve): m/z 87 (100% ꢀBF4).
Tetrabutylphosphonium bis(trifluoromethanesulfonyl)amide:
[P4444][NTf2]
85 White solid; (yield 90%). 31P NMR (300 MHz CD2Cl2): δ (ppm):
ꢀ
33.18. MS (+ve): m/z 259 (+P4444), MS (ꢀve): m/z 280 (NTf2 ).
Tetrabutylphosphonium tetrafluoroborate: [P4444][BF4]
1-octyl-3-methylimidazolium dicyanamide: [C1C8Im][N(CN)2]
White solid; (yield 90%). 31P NMR (300 MHz CD2Cl2): δ (ppm):
33.18. MS (+ve): m/z 259 (+P4444), MS (ꢀve): m/z 280 (BF4ꢀ).
Slightly yellow clear oil, 90% yield. 1HꢀNMR (300 MHz, d6ꢀ
35 DMSO): δ (ppm) : 8.68 (s, 1H, C2H) ; 7.41 (d, 1H, C4H) ; 7.39
(d, 1H, C5H) ; 4.11 (t, 2H, NCH2) ; 3.83 (s, 3H, NCH3) ; 1.79 (qt,
6H,CH2CH2CH2CH2CH2) ; 1.20 (st, 2H, CH2CH2CH3); 0.75 (t,
3H, CH2CH3) ; 13C{1H}ꢀNMR (300 MHz, d6ꢀDMSO) : δ (ppm) :
135.87 (C2H) ;123.72 (C4H) ; 122.13 (C5H) ; 119.65 (NCN);
90 Results and discussion
Water miscible ionic liquids are a difficult task, since they require
separation of the byꢀproducts from the desired ionic liquid. Purity
and yield are very dependent on both cation and anion nature.
Generally, water solubility decreases with increasing the alkyl
95 chain length on the cation. Following Wilkes and Zaworotko
report in 1992,21 the synthesis of ionic liquids can be achieved via
metathesis of amine halide with silver salt, but unfortunately, this
route is obviously limited by the relatively high cost of silver salts
and forms a large amount of silver halide as byꢀproduct. Complete
100 precipitation of silver halides from organic solvents can also be
quite slow, leading to silverꢀcontamination products. For these
reasons, the preferred and most common metathesis approach is
still to carry out the reaction in aqueous solution using either the
40 49.62
(NCH2)
;
35.78
(NCH3)
;
31.55
29.68
(CH2CH2CH2CH2CH2CH2CH2)
;
(CH2CH2CH2CH2CH2CH2); 28.82 (CH2CH2CH2CH2CH2); 28.82
(CH2CH2CH2CH2); 25.89 (CH2CH2CH2) ; 22.42 (CH2CH2CH3);
13.73 (CH2CH3). Electrospray, MS (+ve): m/z 195 (100% ꢀ
45 C1C8Im+), MS (ꢀve): m/z 66 (100% dcaꢀ).
1-octyl-3-methylimidazolium
[C1C8Im][BF4]
tetrafluoroborate:
Slightly yellow clear oil, 90% yield. 1HꢀNMR (300 MHz,
CD2Cl2): δ (ppm) : 9.42 (s, 1H, C2H) ; 7.45 (d, 1H, C4H) ; 7.39
50 (d, 1H, C5H) ; 4.22 (t, 2H, NCH2) ; 3.99 (s, 3H, NCH3) ; 1.88 (m,
free acid of the appropriate anion, or its ammonium or alkali metal
12H,CH2CH2CH2CH2CH2CH2 CH2CH3)
;
1.29 (st, 2H,
27
105 salt.26,
The resulting ionic liquids were extracted from the
CH2CH2CH3); 0.87 (t, 3H, CH2CH3) ; 13C{1H}ꢀNMR (300 MHz,
CD2Cl2): δ (ppm) : 136.82 (C2H) ;123.59 (C4H) ; 122.06 (C5H) ;
aqueous solution into CH2Cl2; however this leaves appreciable
amounts of chloride or bromide ions in the ionic liquid. This
approach clearly results in a lowering of the yield of the final
49.98
(NCH2)
;
36.17
(NCH3)
;
31.64
29.68
55 (CH2CH2CH2CH2CH2CH2CH2)
;
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product, so care must be taken that the volume of water used to 20 In these liquid phases, the appropriate alkaline salt was added. In
carry out the washing is low. Lowering of the temperature of the
water to near 0°C can also reduce the amount of ionic liquid lost.
This process was reported to give final yields in the region of 70–
80 %.
the exchange reaction (1), the choice of Na or Li associated to the
anions such as [A] – = [N(CN)2 ]–, [BF4]ꢀ, [NTf2]ꢀ afforded a total
exchange with [Cl]ꢀ associated to imidazolium, pyrrolidinium, or
phosphonium cations following Pearson’s principle of hard and
5
In our work on electrochemical applications of ionic liquids in 25 soft acids and bases, HSAB. 31
the field of lithium ion batteries33ꢀ35 we used the ionic liquids
[A]ꢀ[Na]+ + [Im]+[Cl]ꢀ → [A]ꢀ[ Im]+ + [Na]+[Cl]– (1)
synthesized using our route.28 High quality ionic liquids can be
obtained via water and silver free route. In the secondꢀstep for IL
10 synthesis, we have investigated alternative, general, and cheaper
approaches of the metathesis reaction to eliminate the use of silver
salt and water. Several ILs based on imidazolium, pyrrolidinium,
and phosphonium cations associated with dicyanamide,
Then, CH2Cl2 (THF is used for dicyanamide anion) was added to
the reaction media to dissolve the resulting IL and to precipitate
the resulting metal salt. For complete sodium chloride
30 precipitation, severalꢀfiltration cycles could be performed;
generally two with CH2Cl2 and at least three with THF. After
evaporation of the solvent, the ILs were kept under high vacuum
(10ꢀ5 mbar) at room temperature for 24 hours, at this step the yield
was c.a. 90%. The purity of the ionic liquids can be judged using
35 a variety of spectroscopic techniques.
thiocyanate,
tetrafluoroborate,
and
15 bis(trifluoromethylsulfonyl)imide anions have been formed in
high purity from the corresponding amine halides. The metathesis
reaction was performed using the melt of the amine halide reagent
as a solvent, 70°C for imidazolium chloride and 120°C for
pyrrolidinium and phosphonium chlorides (Table 1).
Table 1 Metathesis reaction of ionic liquids.
Entry
Ionic Liquids
Yielda (%)
Conditions
Abreviations
1
2
3
4
1ꢀbutylꢀ3ꢀmethylimidazolium dicyanamide
1ꢀbutylꢀ3ꢀmethylimidazolium thiocyanate
1ꢀbutylꢀ3ꢀmethylimidazoliumtetrafluoroborate
1ꢀbutylꢀ3ꢀmethylimidazolium Bis
70°C, 24 h
70°C, 24 h
70°C, 24 h
70°C, 24 h
[C1C4Im][N(CN)2]
[C1C4Im][SCN]
[C1C4Im][BF4]
85
90
90
90
[C1C4Im][NTf2]
(trifluoromethane)ꢀsulfonimide
5
6
7
8
9
10
11
1ꢀhexylꢀ3ꢀmethylimidazolium dicyanamide
1ꢀhexylꢀ3ꢀmethylimidazolium thiocyanate
1ꢀhexylꢀ3ꢀmethylimidazoliumtetrafluoroborate
1ꢀoctylꢀ3ꢀmethylimidazolium dicyanamide
1ꢀoctylꢀ3ꢀmethylimidazolium tetrafluoroborate
Nꢀ butylꢀ Nꢀ metylpyrrolidinium dicyanamide
NꢀbutylꢀNꢀmetylpyrrolidinium
70°C, 24 h
70°C, 24 h
70°C, 24 h
70°C, 24 h
70°C, 24 h
120°C, 24 h
120°C, 24 h
[C1C6Im][N(CN)2]
[C1C6Im][SCN]
[C1C6Im][BF4]
[C1C8Im][N(CN)2]
[C1C8Im][BF4]
[Py14][N(CN)2]
[Py14][NTf2]
87
90
90
90
90
90
90
bis(trifluoromethanesulfonyl)amide
12
13
Tetrabutylphosphonium
bis(trifluoromethanesulfonyl)amide
Tetrabutylphosphonium tetrafluoroborate
120°C, 24 h
120°C, 24 h
[P4444][NTf2]
[P4444][BF4]
90
90
40 a Isolated yield after purification and drying.
50 proton (δC2ꢀH) in 1H NMR, which was attributed to the formation
Analysis
of stronger hydrogen bonds between C2ꢀH bond and [Clꢀ],
Nuclear Magnetic Resonance spectroscopy is the most common
method for characterizing ionic liquids, but it is not an appropriate
45 method for judging purity. Impurities and residual solvents have
dramatic effects on the physical properties of ILs.32 Seddon et al.
have showed the effects of water, chloride anions [Clꢀ], and
organic solvents on the chemical shift values of ILs in 1H NMR.24
High concentrations of [Clꢀ], led to downfield shifts of C2ꢀH
ꢀ
ꢀ
compared to the bond between C2ꢀH and [BF4 ] or [NTf2 ] anions.
A correlation between (δC2ꢀH) and the [Clꢀ] have been found,
which could indicate nearly the [Clꢀ] amount in ILs. Figure 1
55 compares the 1H spectra of 1ꢀhexylꢀ3ꢀmethylimidazolium
dicyanamide. The commercial [C1C6Im][N(CN)2] (99.5% pure,
(Fig 1b)) and [C1C6Im][N(CN)2] synthesized following our
procedure (Fig 1c) give identical chemical shift of C2ꢀH proton
4
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Green Chemistry
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(9.1 ppm, external d6ꢀDMSO), however the 1H NMR spectra (Fig
167.1
400.3
100
80
60
40
20
0
1a) of [C1C6Im][N(CN)2]/ [C1C6Im][Cl] (1 mol kgꢀ1) revealed that
the presence of chloride causes a downfield shift for the signal
(δC2ꢀH 9.4 ppm).32
a)
Purity: 99.5 %
a)
379.3
369.3
9.4
b)
100
200
300
400
500
m/z
400.3
100
80
60
40
20
0
9.1
b)
c)
167
Chloride peak
m/z 369.3
9.1
12
10
8
6
4
2
0
369.3
ppm
5
Fig 1 1H NMR spectra of [C1C6Im][N(CN)2], (a) contaminated with
100
200
300
400
500
[C1C6Im][Cl] (1 mol kgꢀ1) (b) commercial (c) prepared in our laboratory.
m/z
Fig 2 High resolution mass spectra of [C1C6Im][N(CN)2]. a) Commercial
product from Solvionic and b) from our lab.
The high resolution mass spectroscopy (MS QTOF) is the most
10 sensitive method for determining the purity of ionic liquids. It
delivers both information about the mass and the isotope pattern
of a compound and can be used for the structural analysis upon
performance of MS/MS experiments. Therefore, it is a valuable
tool for identification and characterization of an analyte as well as
15 for the identification of impurities. Unwanted byꢀproducts formed
during the synthesis or by hydrolysis of components of the ILs can
be identified by this method, specifically the chloride
contamination. For convenience, it is possible to judge the level of
chloride up to 10 ppm using this technique. Fig. 2 shows the high
20 resolution mass spectra (MS QTOF) of the commercial
[C1C6Im][N(CN)2] (99.5% pure, Fig 2a) and a sample prepared by
our process [C1C6Im][N(CN)2] (Fig.2b). We have observed that it
is possible to reduce the chloride contamination to less than 0.5 %
following our route. The peak at m/z 369.26 corresponds to
25 [C1C6Im]2[Cl]+ due to the presence of chloride in the final
product. The intensity of the peak at m/z 369.26 in Fig b is much
lower than in Fig a, proving the higher puriy of our sample. All
the other synthesized ionic liquids were anaylsed using this
technique, and no chloride peak was observed for most of them
30 (see the supporting information).
35
The simple electrochemistry of ionic liquids could provide a
very quick indicator of the general impurity.30 We generally
employ a platinum working electrode, a platinum counter
electrode and a silver wire as the reference electrode. The effect of
presence of chloride anion in [C1C4Im][NTf2] and in
40 [C1C4Im][N(CN)2] onto their cyclic voltammogram was depicted
in Fig 3 and Fig 4, respectively. When [C1C4Im][Cl] has been
added into these ILs a broad peak appeared at ≈0 V in their
cyclic voltammogram and a substantially decrease of their
electrochemical stability was observed.
a)
b)
ꢀ2.0
ꢀ1.5
ꢀ1.0
ꢀ0.5
0.0
0.5
1.0
1.5
2.0
2.5
Potential / V
45
Fig. 3 Cyclic voltammogram of [C1C4Im][N(CN)2] (a) prepared in our lab
(b) contaminated with chloride (1 mol kgꢀ1).
This journal is © The Royal Society of Chemistry [year]
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Green Chemistry
View Article Online
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b)
ꢀ3
ꢀ2
ꢀ1
0
1
2
3
4
Potential / V
Fig 4 Cyclic voltammogram of [C1C4Im][NTf2] (a) prepared in our lab
(b) contaminated with chloride (1 mol kgꢀ1).
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An alternative synthetic silver and water free route for ILs, in an
attempt to reduce the wastes and their cost, has been developed.
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of the melting point of the amine halide salts, playing the role of
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thiocyanate,
tetrafluoroborate,
and
15 bis(trifluoromethylsulfonyl)imide were obtained in high purity (≥
99.5%) and high yields. This route is very well adapted for water
miscible ILs preparation.
85
90
Notes and references
a
Université de Lyon/UMR 5265 CNRSꢀC2P2, 43 Boulevard du 11
20 Novembre 1918, 69616 villeurbanne, France. Fax: +33(0)472431795; Tel:
+33(0)472431810;
Eꢀmail:
catherine.santini@univꢀlyon1.fr
and
yves.chauvin001@orange.fr
b
CEAꢀDRT/Liten/DEHT/LBA, 17 rue des Martyrs, Grenoble, France.
Fax: +33 (0) 4 38 78 51 98; Tel: +33 (0) 4 38 78 34 66; Eꢀmail:
25 helene.Rouault@cea.fr.
† Electronic Supplementary Information (ESI) available: [details of
experimental section, NMR analysis and mass spectra]. See
DOI: 10.1039/b000000x/.
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6
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]