Distillable ionic liquids: Reversible amide OAlkylation

Article abstract of DOI:10.1002/anie.201306476

Put it in reverse: The recycling of ionic liquids (ILs) by distillation of the regenerated volatile precursors was demonstrated to be feasible by using low-cost amide-cation-derived aprotic ionic liquids prepared from reversible Oalkylation. The low visco

Full text of DOI:10.1002/anie.201306476

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Angewandte
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DOI: 10.1002/anie.201306476
Ionic Liquids
Distillable Ionic Liquids: Reversible Amide O Alkylation**
Zheng-Jian Chen, Hong-Wei Xi, Kok Hwa Lim, and Jong-Min Lee*
Over the past three decades, ionic liquids (ILs) have
stimulated significant scientific interest because of their
appealing properties, structural diversity, and functional
versatility, as well as an extensive range of existing and
potential applications, such as reaction solvents, supporting
In contrast, ILs are characterized by the flexibility in their
design in relation to structure and function. The total number
8
of possible ILs was estimated to be on the order of 10 , and
their properties can be tailored by judicious selection of
cation and anion species, and functional groups, to serve
[1]
[11]
electrolytes, functional materials, and many more. One of
the main attractive features of ILs is their extremely low
vapor pressure, which makes them distinct from and superior
to the traditional volatile organic compounds (VOCs). This
feature not only enables the applications that require high
useful purposes. So far, several thousand ILs have been
developed and studied, and most of the cation species are
derived from the nucleophiles, such as imidazolium, quater-
nary phosphonium, and tertiary sulfonium, with electron-rich
N, P, and S atoms as a strong nucleophilic centers. For the
development of science and technology, there is always a need
to design and create novel ILs. Recently, low-cost amide-
cation-based ILs were developed and applied in liquid
[2]
vacuum conditions, such as in situ XPS investigation, but
also decreases the loss of solvents into the atmosphere and the
risk of exposure, thus making it an environmentally pref-
erable alternative to VOCs. However, an associated challenge
is that the ILs cannot be easily recycled by means of simple
distillation as in the case of VOCs, and thus severely limits
[
12]
[13]
[14]
crystals, electrolytes, solvent extraction of metal ions,
[
15]
ionothermal carbonization of sugars, and more. Most of
these amide-cation-derived ILs are Brønsted acidic, and the
cationic precursors mainly include aliphatic amides and
lactams (cyclic amides, e.g., caprolactam ). However, there
is still debate on whether the reaction of amides with H or
[3]
their large-scale industrial applications. Up to now, efforts
have been devoted to the study of the volatility or distillability
[12]
[
4–10]
[4]
+
of ILs.
P. G. Jessop et al. developed a class of CO2-
switchable solvents, which can be switch back and forth
between ionic and molecular forms by bubbling CO and N ,
other electrophilic reagents occurs at the amide nitrogen
[
12,13a–c]
[13d,14,15]
atom
or oxygen atom,
since the electron-with-
2
2
alternately, through the solution. The molecular forms enable
facile separations of the high-boiling-point species from
drawing carbonyl group conjugated to the amide N tends to
make the N less nucleophilic and the O more nucleophilic.
At the beginning of the present study, an attempt was
made to answer the above question using the DFT/B3LYP
[
4c]
nonvolatile solvents.
Recent studies have revealed the
presence of neutral ion pairs and ion aggregates in most ILs,
and a measurable vapor pressure could be detected under
[16]
method with the Pople 6-311 + G(d,p) basis set
optimize the amide DMF [(CH ) NCHO] and its O- or N-
to fully
[5]
[6]
vacuum and at elevated temperatures. Earle et al. first
demonstrated the feasibility of vacuum distillation of aprotic
ILs through vaporization of ion pairs at high temperature
3
2
alkylated products. The simulation results are summarized in
Figures 1 and S1, and Tables S1 and S2 (see the Supporting
Information). Since the n–p* electron delocalization in the
NÀC=O fragment (Figures 1, S1, and Table S1), DMF is
without decomposition. However, the low distillation rate, for
À1
example, 0.120 gh
0
for [EMIm]NTf2 at 3008C and
[
6]
[7]
.1 mbar, makes it very difficult for further application.
essentially planar and the O (À0.62215 e) carries more
negative charge than the N (À0.50138 e), thus indicting
a higher nucleophilicity of the former. Additionally, the O-
alkylated products are more stable than their N-alkylated
Actually, the quaternization reaction for the synthesis of ILs is
reversible with temperature, and a portion of the neutral
[
8]
precursors can be collected by thermal decomposition.
À1
More recently, the formation of the neutral acid and base
precursors was found to occur readily in protic ILs through
congeners, with an energy difference of 13.29 kcalmol for
[9]
[10]
a proton-transfer mechanism. King et al.
successfully
regenerated protic ILs, which dissolved cellulose, by vacuum
distillation of the neutral acid and base IL precursors at
elevated temperatures, with a recovery rate up to 99.4%.
[
*] Dr. Z. J. Chen, Dr. H. W. Xi, Prof. K. H. Lim, Prof. J.-M. Lee
School of Chemical and Biomedical Engineering
Nanyang Technological University
6
2 Nanyang Drive, Singapore 637459 (Singapore)
E-mail: jmlee@ntu.edu.sg
**] This work was supported by the Academic Research Fund (RG21/
9) of the Ministry of Education in Singapore.
Figure 1. The color-coded electron density surface of DMF, its two
possible methylated products and their relative energies, and the n–p*
delocalization of the O-methylated product. The calculations were
performed in the gas phase at the B3LYP/6-311 +G(d,p) level of
theory.
[
0
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306476.
1
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Chemie
the methylated DMF cation (Figure 1 and Table S2). In fact,
the O-alkylated compounds maintain the basic conjugated
chemical shift of d = 8.27 ppm, thus implying its acidity. The
quartet (d = 127.36, 123.12, 118.87, 114.63 ppm) in Figure 2b
system of amides, thus resulting in bond orders of 1.51 and
is characteristic of CF in a TfO anion. Besides, NTf -based
3
2
+
1
a
(
.22 for NÀC and CÀO, respectively, for N -C-O, and
amide ILs were also prepared by anion exchange (see the
Supporting Information for details) and characterized.
As a matter of fact, the amide O-alkylation reaction is
readily reversible with temperature, and the ILs can revert to
volatile molecular precursors (amide and alkyl triflate) at
elevated temperatures. According to the TG curves shown in
Figure 3, the decomposition temperatures of the TfO-based
À1
second-order stability energy of 64.30 kcalmol
in
+
CH ) N CHOCH (Table S2). As for the N-methylated
3 2 3
+
DMF, the bond orders are 0.76 and 2.04 for the N -C-O
unit, and no n–p* delocalization was observed (Table S2).
Similar results were also found for the ethylated DMF
compounds (Table S2). Based on the higher nucleophilicity
and more stable products, the amide O should be the position,
or at least is the more favorable position, for the nucleophilic
alkylation of amides with alkylating agents.
The above finding was further supported by our exper-
imental results. As shown in Scheme 1, a series of O-
alkylated-amide ILs were synthesized by the reaction of
Figure 3. TGA curves of five ILs.
Scheme 1. Structures and abbreviations of cations and anions used in
this study.
ILs (Td at 5% mass loss) are around 1708C, as listed in
Table 1. The recovery of the amide ILs by distillation of the
regenerated volatile precursors at elevated temperatures was
first conducted for [MDMF]TfO, as detailed in Table 1. In
a short-path vacuum (0.5 mbar) distillation apparatus, 8.0 g of
[MDMF]TfO was added, and then the oil bath temperature
was raised gradually. When heated to around 1458C, the
bubbles began to appear. At 2108C, the steam distillate was
collected in an ice/water bath within 10 minutes, thus giving
92 wt% of a liquid. NMR results revealed that the collected
distillate contains only 87 mol% of
amide with alkyl triflate (see the Supporting Information for
1
details). This proposed structure was confirmed by H/
1
3
C NMR spectroscopy. As illustrated for [MDMF]TfO
1
13
(
Figures 2a and b), both the H and C NMR signals assigned
to the three methyl groups do not coalesce but instead show
greater separation, which exactly matches the N(CH3)2
asymmetry imposed by the C=N bond and the OÀCH . The
3
proton of the conjugated group N=C(H)-O appears at a high
[
MDMF]TfO and 11 mol% of DMF (Fig-
ure 2c). The presence of DMF indicates that
the distillation of [MDMF]TfO undergoes
a process of amide O-dealkylation. After the
addition of an equivalent of CF SO CH , the
3
3
3
residual DMF could be completely converted
into [MDMF]TfO, thus affording a recovery of
9
9 wt%, with a purity of up to 98%. Since the
boiling point of CF SO CH is only 908C at
3
3
3
atmospheric pressure, the DMF impurity in the
distillate might arise from the volatility of
CF SO CH under vacuum. When the distillate
3
3
3
was trapped in liquid N (À1968C), 99 wt% of
2
[
MDMF]TfO with a purity of 97% could be
recovered by vacuum distillation. This result
demonstrates the feasibility of the direct
recycling and reuse of ILs by the rapid
distillation of the regenerated volatile precur-
sors at elevated temperatures with no need for
further processing.
1
13
1
Figure 2. a, b) H/ C NMR spectra of [MDMF]TfO in CD CN. c) H NMR spectrum of
3
the recycled [MDMF]TfO containing 11 mol% of DMF.
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Table 1: The recycling of the amide-cation-based ILs by vaccum distillation.
[
a]
[b]
[c]
+
[d]
Entry
IL
Td
8C]
Vacuum degree
[mbar]
Cold trap
T [8C]
Oil bath
T [8C]
Recovery rate
[wt%]
Purity
Amide content
[mol%]
[H ] amide content
[
[mol%]
[mol%]
1
2
3
4
5
6
7
8
9
[MDMF]TfO
[MDMF]TfO
[MDEA]TfO
[MDEA]TfO
[MEPyr]TfO
[EDMF]TfO
[EDEA]TfO
[EDEA]TfO
[EEPyr]TfO
[EMPyr]TfO
[EDMA]TfO
165
179
0.5
0.5
0.5
0.002
0.002
0.5
0.5
0.002
0.5
0
210
210
210
175
175
210
210
175
210
210
210
92
99
98
98
98
66
65
91
66
52
67
87
97
86
89
90
89
77
77
82
81
81
11
–
À196
À196
À196
À196
À196
À196
À196
À196
À196
À196
–
12
9
8
9
21
21
16
16
16
177
165
177
171
166
165
1
1
0
1
0.5
0.5
[
a] Decomposition temperature with 5% mass loss. [b] By a Buchi vacuum pump V-700 with ultimate vacuum of 0.5 mbar or an Edwards RV12
1
vacuum pump with ultimate vacuum of 0.002 mbar. [c] Calculated from H NMR spectra. [d] Content of the O-protonated amide based on NMR
anylysis.
The distillation recovery of other samples was also studied
Table 1). These data are not as good as that for [MDMF]TfO.
the same purity. As for [MDEA]TfO, with a decrease from 0.5
to 0.002 mbar, the recovery rates were almost same and the
purity was increased slightly from 86 mol% to 89 mol%. The
high recovery rate of the methylated ILs relative to the
ethylated ILs could also be observed from the TG results. As
shown in Figure 3, the weight of the methylated ILs fell off
gradually to about 0%, while the curves for the ethylated ILs
went through an inflection point at around 20 wt%.
(
For example, only 66 wt% of [EEPyr]TfO was distilled
entry 9), thus leaving 28 wt% residue of O-protonated N-
(
+
ethyl-2-pyrrolidone [H EPyr]TfO (see Figures S3 and S4).
+
And the distillate also contains 16 wt% of [H EPyr]TfO.
Similar results were also obtained for other ethylated ILs
(
entries 6–11). The existence of a protonated amide should be
+
related to the generation of H during the O-ethyl elimina-
tion. For [MDEA]TfO and [MEPyr]TfO, though 98 wt% can
be recovered, the crude distillates contain around 10 mol% of
O-protonated amides. After crystallization in a mixture of
diethyl ether and ethyl acetate, the overall recovery (purity:
For further and more extensive studies of these new ILs,
their fundamental properties, such as melting point (T ),
m
crystallization temperature (T ), density (1), surface tension
c
(g), conductivity (k), viscosity (h), and electrochemical
window (EW), were determined (Tables 2 and S3 in the
Supporting Information), where appropriate. For the purpose
of comparison, two 1-ethyl-3-methylimidazolium, [EMIm]-
based ILs, which are one of the most commonly used ILs with
low viscosity and high conductivity, were synthesized and
characterized (Table 2). According to the solid–liquid phase
transitions obtained from DSC measurements, all the amide-
based ILs were shown to undergo both crystallization and
melting processes, and many of them are liquids or subcooled
liquids at room temperature with a T or T value of less than
9
8%) was determined to be 70 wt% for [MDEA]TfO and 71
wt% for [MEPyr]TfO. Compared with other ILs, DMF
derivatives (entries 1, 2, and 6) show better recovery ability
with a smaller amount of impurities. This result may be
related to the acidity of the N=CHÀO proton, which prevents
+
the formation of [H ] from alkyl cations. When the vacuum
pressure was decreased from 0.5 to 0.002 mbar, the distillation
temperature could be reduced to 1758C, and the recyclability
could be enhanced to 91 wt% for [EDEA]TfO, with almost
m
c
Table 2: Basic properties of nine amide-cation-based ILs and two [EMIm]-based ILs.
[
a]
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
Entry
IL
Tm
8C]
Tc
[8C]
1
g
h
k
EW
[V]
Ecathodic
[V]
Eanodic
[V]
À3
À1
À1
[
[gcm
]
[mNm
]
[cP]
[mScm
]
1
2
3
4
5
6
7
8
9
[MDMF]TfO
[EDMF]TfO
[MEPyr]TfO
[EEPyr]TfO
[MDMF]NTf2
[EDMF]NTf2
[EDMA]NTf2
[MDEA]NTf2
[EEPyr]NTf2
[EMIm]NTf2
[EMIm]TfO
23.2
À6.95
1.4126
1.3492
1.3675
1.3165
1.5405
1.4946
1.4688
1.4469
1.4460
41.3
35.8
40.1
35.1
36.2
34.9
34.5
35.1
35.1
29.7
22.4
85.1
52.3
29.3
21.6
40.8
66.9
33.7
32.0
43.6
15.45
14.46
4.83
4.44
4.41
4.67
4.62
4.41
4.47
4.64
4.63
4.62
4.56
4.43
À1.62
À1.61
À1.87
À1.85
À1.60
À1.61
À1.83
À1.81
À1.88
À2.18
À2.18
2.82
2.80
2.80
2.77
2.81
2.86
2.81
2.82
2.74
2.38
2.25
À32.5
14.7
34.1
37.6
2.9
33.1
28.3
29.0
(À64.0)
À27.4
4.8
6.15
À16.1
(À60.7)
11.3
4.0
10.0
10.96
11.29
6.59
3.48
6.10
8.96
[j]
[k]
[l]
[m]
1
0
1
1.5182
1.3850
39.0
[
o]
1
41.9
9.27
[a] Melting point. [b] Crystallization temperature (cold crystallization in brackets). [c] Density at 25.08C. [d] Surface tension at 25.08C. [e] Viscosity at
2
1
4
5.08C. [f] Conductivity at 25.08C. [g] Electrochemical windows. [h] Cathodic potential limit. [i] Anodic potential limit. [j] Literature value,
À3 [17]
À1 [18]
[17]
À1 [17]
.519 gcm
.
[k] Literature value: 41.62 mNm
.
[l] Literature value: 32.6 cP. [m] Literature value: 9.1 mScm
.
[o] Literature value:
À1 [18]
4.46 mNm
.
1
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2
58C. The cold-crystallization behaviors and lower T values
The electrochemical windows of the neat ILs were studied
m
(
entries 2 and 6) of the two [EDMF]-based ILs suggest that
by cyclic voltammetry (CV) on a glassy-carbon electrode, as
illustrated in Figure S2 in the Supporting Information. Table 2
summarizes the test results for the cathodic (Ecathodic) and
anodic (Eanodic) limits, and the overall electrochemical win-
dows (EW= EanodicÀEcathodic). As can been seen, all of the
amide-derived ILs possess a relatively wide electrochemical
this cation is less prone to form an ordered structure than
others. The densities (1) of the ILs fall in the range from
1
those of [EMIm]-based ILs. For a given anion, the amide-
derived ILs exhibit a lower surface tension (g, 34.5–
4
4
À3
.3165 to 1.5405 gcm at 258C, densities which are close to
À1
1.3 mNm , 258C) than [EMIm] ILs, for example,
window, ranging up to approximately 4.5 V, which is much
À1
[13a]
1.9 mNm for [EMIm]TfO.
larger than the protic amide ILs (ca. 2.2 V),
and compa-
The viscosities (h) of the ILs were measured at 258C and
rable to [EMIm]-based ILs (ca. 4.5 V, Table 2). Compared to
the results are listed in Table 2. Accordingly, the amide-
derived ILs are generally of low viscosity, with values as low
[EMIm] ILs (Eanodic ꢀ 2.3 V for NTf and TfO), the anodic
2
limit, related to the oxidation of anions, of the amide-based
ILs is more positive (ca. 2.8 V). So although the cathodic limit
of the latter (ca. À1.7 V) is less negative than that of the
former (ca. À2.2 V), their overall electrochemical windows
as 21.6 cP for [EDMF]NTf , which is even less viscous than
2
[
EMIm]NTf (32.5 cP). Such low viscosity can be related to
2
the structures of the amide cations, which are of (quasi)planar
geometry, low symmetry, and contain an ether moiety
are almost equal. This difference in the E
value suggests
anodic
(
Scheme 1). These structural features are useful for decreas-
that counterions may exert a significant effect on the electro-
chemical stability of each other, although it was generally
assumed that the cathodic limit is set by the reduction of the
cations and the anodic limit is set by the oxidation of the
ing the ion packing efficiency and offer more free volumes for
[
19]
promoting mass transfer. The viscosity values are generally
governed by three major factors: the amide precursor, the O-
alkyl chain length, and anionic species. First of all, compared
with other ILs (e.g., [EDMA]NTf : 40.8 cP), DMF-derived
ILs are the least viscous (e.g., [EDMF]NTf : 21.6 cP). This
result can be attributed to the hydrogen atom on N=CHÀO,
with reference to imidazolium ILs, where the small C2-H
atom on the cation ring allows more rotational freedom of the
[
20]
anions. Besides, the cathodic limits of DMF derivatives are
a bit less negative (ca. À1.6 V), and should be related to the
acidity of the N=CHÀO proton, analogous to imidazolium
2
2
[
19a]
ILs.
The combination of the wide electrochemical window
and high conductivity implies a potential application in
electrochemistry.
[
19a]
N-alkyl chains for low viscosity.
Secondly, the amide O-
In conclusion, we present the synthesis of a new class of
low-cost amide-derived aprotic ILs from amide O-alkylation
with alkyl triflate. The amide O-alkylation reaction is readily
reversible with temperature, and the regenerated volatile
precursors can be easily vacuum distilled at about 2008C and
also revert to ILs at room temperature, with the maximum
recovery rate of up to 99 wt% and purity of 97%. To our
knowledge, this is the first case demonstrating the feasibility
of the easy recycling of aprotic ILs by the distillation of the
regenerated molecular precursors at elevated temperatures
without the need for further processing. We believe that this
synthesis strategy can be extended to designing and develop-
ing new ILs and other useful compounds, and the recycling
strategy can be applied to solving the recycling challenge of
the nonvolatile ILs in large-scale applications. Besides, the
amide-cation-based ILs have low viscosity (21.6 cP at 258C),
bonded alkyl chain result in an ether group. Ether moieties
are featured for their ability to reduce the liquid viscosity, and
the O-terminal rodlike CH CH tail is much more efficient in
reducing viscosity than the spherical CH tail. Our exper-
imental results are in complete accordance with the relevant
2
3
3
[
19b]
literature,
O-methyl ILs, for example, [EEPyr]TfO (52.3 cP) versus
MEPyr]TfO (85.1 cP). Thirdly, by keeping the anion con-
stant, most TfO ILs are much more viscous than NTf ILs, as
and the O-ethyl ILs are much less viscous than
[
2
[
17]
expected. However and interestingly, DMF-derived TfO
ILs are nearly as low in viscosity as NTf ILs, for example,
2
[
EDMF]TfO (22.4 cP) versus [EDMF]NTf2 (21.6 cP). In
addition, this unusual result can also be observed in the
following conductivity values.
Generally, low viscosity allows high conductivity for ILs
and thus they serve as better electrolytes in electrochemical
devices. The conductivities (k; Table 2) of the samples that are
À1
high conductivity (15.45 mScm at 258C), and wide electro-
chemical windows (ca. 4.5 V), thus indicating their potential
in electrochemical applications.
(
supercooled) liquids at 258C were investigated, and ranged
À1
from 3.48 to 15.45 mScm . Clearly, the overall relatively high
conductivity values benefit from the low viscosity. These
DMF-derived ILs are even more conductive than their
Received: July 25, 2013
Published online: October 31, 2013
[
(
EMIm]-based counterparts, for example, [EDMF]NTf2
11.29 mScm ) versus [EMIm]NTf (8.81 mScm ). Another
2
À1
À1
Keywords: alkylation · amides · computational chemistry ·
cyclic voltammetry · ionic liquids
.
major factor in determining conductivity is ionic size, of which
the small sizes are convenient for ion transfer. For a given
cation, although TfO-based ILs are generally more viscous
than NTf -based ILs, the former are more conductive than the
2
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[
(
MDMF]TfO
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versus
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À1
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[
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Angew. Chem. Int. Ed. 2013, 52, 13392 –13396
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www.angewandte.org 13395

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