Probing the structure-property relationship of regioisomeric ionic liquids with click chemistry

Article abstract of DOI:10.1039/c1gc16067b

Understanding the structure-property relationship of ionic liquids is a key to the development of optimized materials for specific applications, such as CO₂ capture, battery electrolytes, or lubricants. Using Cu(i)-catalyzed click chemistry as

Full text of DOI:10.1039/c1gc16067b

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Probing the structure-property relationship of regioisomeric ionic liquids
with click chemistry†
Hunaid B. Nulwala,*a,d Chau N. Tang,*a,b Brian W. Kail,c Krishnan Damodaran,a Palwinder Kaur,a
Shan Wickramanayake,c Wei Shic and David R. Luebke*a
Received 29th August 2011, Accepted 27th September 2011
DOI: 10.1039/c1gc16067b
Understanding the structure-property relationship of ionic
liquids is a key to the development of optimized mate-
rials for specific applications, such as CO2 capture, bat-
tery electrolytes, or lubricants. Using Cu(I)-catalyzed click
chemistry as a synthetic platform, a library of triazolium-
based ionic liquids has been accessed for the first time.
These regioisomers of ionic liquids give insight into the
structure property of these important materials. The effect
of substituents and the effect of regioisomerization on
thermal stability, CO2 solubility and inductive effects have
been correlated.
choices of cations, the substituents on the cation, and the anion
can enhance or suppress the properties of the ILs.2,10 Structure-
property relationships for ILs can be developed based on similar
concepts developed in the area of polymer sciences.11
One such approach is to establish key parameters and
structure-property trends to make the selection of ILs for a
particular technological need easier. Watanabe and MacFarlane
have established key characteristic parameters using the ionic
character of ILs.10a,10c,12 However, there is a need to develop
empirical structure-property relationships similar to polymer
chemistry due to the large number of possible ionic liquids
which can be accessed. Therefore, we have employed the Cu(I)-
catalyzed click13 reaction to develop structure-property trends
for ILs based on a triazolium14 core (Fig. 1) and study the
CO2 solubility for selected ionic liquids. The Cu(I)-catalyzed
click reaction is an ideal synthesis platform to systematically
probe properties of ILs due to its excellent molecular control,
ease of synthesis, benign reaction conditions, and fidelity.15 In
short, the Cu(I)-catalyzed click reaction allows easy access to
regioisomers and simple variation of functional groups, which
enables us to study the effect of structural changes on properties
(Fig. 2). Herein, we report the synthesis and characterization
of 1,4-disubstitued triazolium-based ILs and elucidate the effect
of structural changes on their physical properties. Five pairs
of 1,2,3-triazolium-based ILs (1-5 and 1p-5p in Fig. 2) were
prepared with bis[(trifluoromethyl)sulfonyl]amide (Tf2N) and
iodide as anions. A phenyl or a propyl group is attached at the
Ionic liquids (ILs) are salts that are liquids below 100 ◦C
and are largely comprised of ions and short-lived ion pairs.1
The number of potential ILs is estimated to be as great as
1018.2 Recently, interest in room-temperature ILs has been
revitalized due to their wide liquid ranges, non-flammability,
good electrolytic properties unique solubility, negligible vapor
pressure, and good thermal stability. These attractive char-
acteristics have promoted IL use in various applications as
green alternatives to normal solvents – including synthesis – as
both solvents3 and catalysts;4 as energy storage, as electrolytes;5
for extraction of radioactive materials,6 metals,6,7 and organic
liquids;7b for polymer processing;8 and for gas separation.9 The
above mentioned attributes make ILs an attractive green re-
placement solvents/membranes for CO2 capture and separation
from post/pre-combustion gases from coal-fired power plants.9
However, selecting ILs with the appropriate attributes for a
technological application is a daunting task, due to available
structural possibilities. It is clear from the published data that the
aNational Energy Technology Laboratory, P.O. Box 10940, Pittsburgh,
PA, 15236, United States.
E-mail: hunaid.nulwala@cm.netl.doe.gov, david.luebke@netl.doe.gov;
Fax: +412-386-4604; Tel: +412-386-7343, +412-386-4118
bDepartment of Chemistry, University of Pittsburgh, Pittsburgh, PA,
15260, United States. E-mail: chau.tang@or.netl.doe.gov
cURS, P.O. Box 618, South Park, Pennsylvania, 15219, United States
dDepartment of Chemistry, Carnegie Mellon University, Pittsburgh, PA,
15213
† Electronic supplementary information (ESI) available: Synthe-
sis details, characterization and computational data. See DOI:
10.1039/c1gc16067b
Fig. 1 General synthetic route to obtain triazolium-based ILs.
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1H and 13C NMR spectra showed a direct influence of the
cationic substituents and counter-anion on the electron density
of the triazolium core. The assignments were further confirmed
by 2D NMR (1H–13C HETCOR) spectroscopy; see Fig. S1 and
1
S2 in the ESI†. Fig. 3 shows the H and 13C NMR spectra
1
of 1-Tf2N, 1A-Tf2N and 1p-Tf2N. In the H NMR spectra, a
downfield shift occurs for the resonances of the triazolyl proton
(labeled as “a,” 8.40 ppm) and N-methyl group (labeled as “b,”
4.23 ppm) of 1-Tf2N compared to the chemical shifts of 1p-
Tf2N (8.15 and 4.15 ppm, respectively). When compared to the
regioisomeric ILs, these resonances for 1A-Tf2N (8.49 ppm for
CH and 4.26 ppm for CH3) appear further downfield than those
of 1-Tf2N. These results demonstrate that the electron deficiency
of the cation increases in the order 1pA ª 1p-Tf2N < 1-Tf2N <
1A-Tf2N.
Fig. 2 Regioisomeric triazolium-based ILs in boxes, and 4-phenyl and
4-propyl substituted triazolium-based ILs with Tf2N and iodide as
anions were synthesized.
“4” position of the triazolium ring. The substituent at the “1”
position on the cation is a hexyl, (trimethylsilyl)methyl (TMS),
CH3(OCH2CH2)3, benzyl, or pyridyl group. Isomers consisting
of phenyl/hexyl, propyl/hexyl, phenyl/TMS, and propyl/TMS
substituents were synthesized to help us understand effects
associated with these groups on the ILs. Ours is the first study
to examine the influence of regioisomerization on IL properties.
The effect of cationic substituents on thermal stability,
inductive effect, and CO2 solubility have been correlated. 1,2,3-
Triazolium-based ILs were characterized by mass spectroscopy,
nuclear magnetic resonance (NMR) spectroscopy, differential
scanning calorimetry (DSC), and thermogravimetric analysis
(TGA). All the samples were dried at 100 ◦C under vacuum.
TGA data showed no weight loss for all samples till 200 ◦C (See
Fig. 3 1H and 13C (zoomed region) NMR spectra of triazolium-based
ILs 1p-Tf2N (top), 1-Tf2N (middle), and 1A-Tf2N (bottom); * = CDCl3.
◦
ESI†). The CO2 solubility of these ILs was measured at 37 C
In the 13C NMR spectra, the NC = CH resonance for the
triazolium-based ILs with a 4-propyl substituent (i.e., 144.9 ppm
for 1p-Tf2N) shifts downfield as compared to the 4-phenyl-
1H-1,2,3-triazolium cation (i.e., 143.6 ppm for 1-Tf2N), but
no shifting occurs in the CF3 resonances of the anion when
the substituents are 4-propyl and 4-phenyl. In the case of the
under variable pressures. In this work, abbreviations for the ILs
such as “1-Tf2N” and “1-I” correspond to “1-hexyl-3-methyl-
4-phenyl-1H-1,2,3-triazol-3-ium bis[(trifluoro-methyl)sulfonyl]
amide” and “1-hexyl-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium
iodide,” respectively.
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regioisomeric ILs with N-phenyl and N-propyl substituents, the
electron delocalization of the NC = CH resonance (i.e., 146.1
ppm for 1A-Tf2N) of the cation are affected as depicted in
Fig. 3.
temperature than those with 4-phenyl substituted ILs (e.g., Td =
290 ◦C for 2-Tf2N). This thermal behavior is maintained when
the anions of the triazolium-based ILs are Tf2N and iodide (Fig.
S5 in the ESI†).
The result indicates that the interaction between the phenyl
ring and the triazolyl ring is greater in the case of N-phenyl
substitution when compared to substitution at the 4-phenyl
position. This phenomenon is supported by computational
calculations which show less electron density in the phenyl ring
and a smaller angle between the phenyl ring and triazolyl ring
in the case of N-phenyl substitution (Fig. S3 in the ESI†). In
the 19F NMR spectra (Fig. S4 in the ESI†), the chemical shift
of the CF3 resonance of 1p-Tf2N (-81.66 ppm), 1-Tf2N (-81.60
ppm), and 1A-Tf2N (-81.63 ppm) are slightly different. These
observations provided an excellent insight into the inductive
effect of these groups, and how to tune the intermolecular
interaction between the cation and the anion by the development
of isomeric triazolium-based ILs.
For the series of ILs (excluding 5-Tf2N and 5p-Tf2N) in Fig.
2, those with a benzyl group at the “1” position of the triazolium
have lower thermal stability than those with CH3(OCH2CH2)3,
C6H13, and (CH3)3SiCH2 groups. The benzyl group is inherently
less stable due to its ability to form ionic or radical species at the
benzyl position. 2p-Tf2N has the highest thermal stability in this
set. An IL with basic groups, such as amino or pyridyl moieties,
tend to have lower thermal stability. We have observed this ther-
mal behavior in our lab for many ILs containing such groups, in
addition to the ILs listed here. The thermal decomposition tem-
peratures of the triazolium-based ILs are lower than that of 1-
hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide
([hmim][Tf2N], Td = 394 ◦C).
Regioisomers differ in thermal stability, viscosity, and the
physical state associated with the position of the substituents.
N-Phenyl substituted triazolium-based ILs have a higher de-
composition temperature (e.g., Td = 340 ◦C for 1A-Tf2N) than
those with 4-phenyl substitution (e.g., Td = 323 ◦C for 1-Tf2N).
This trend in thermal stability is the same when the triazolium-
based ILs are paired with either the Tf2N or iodide as anions.
1A-Tf2N is a solid (m.p. = 71 ◦C, r = 1.446 g cm-3), while 1-Tf2N
is a liquid (Tg < -20 ◦C, r = 1.335 g cm-3). In addition, melting
point of the 1A-I (m.p. = 123 ◦C) is higher than that of the 1-I
(m.p. = 92 ◦C), indicating that N-phenyl substitution is more
planar than 4-phenyl substitution, which was also confirmed by
computational studies (see Fig. S3 in the ESI†). The CH and
NC = CH resonances of 1A-Tf2N in the 1H and 13C NMR spectra
(Fig. 3), respectively, are shifted downfield compared to 1-Tf2N.
The result demonstrates the significance of regioisomerization
on the properties of the ILs.
In the case of the anion effect, the resonance for the proton
on the triazolyl ring in IL 1 with iodide as an anion (9.48 ppm)
shifts downfield compared to that for the 1-hexyl-4-phenyl-1H-
1
1,2,3-triazole (7.75 ppm) in H NMR spectra. This resonance
appeared upfield when the anion was changed from iodide to
Tf2N (8.40 ppm).
In Fig. 4, a trend appears that links cationic structure to
thermal stability in the ILs. The thermal decomposition temper-
ature (Td) was determined by TGA and evaluated at 5% mass
loss of the sample. Generally, the 4-propyl substituted ILs (e.g.,
Td = 357 ◦C for 2p-Tf2N) have a higher thermal decomposition
For N-propyl substituted triazolium-based ILs, the trend in
thermal behavior is the opposite of that for N-phenyl triazolium-
based ILs. While ILs 1p (Td = 180 ◦C) and 1pA (Td = 179 ◦C) with
an iodide anion have similar Td within experimental error, the
thermal stability of the former (Td = 335 ◦C) with Tf2N is higher
◦
than that of the latter (Td = 311 C) with Tf2N. The N-phenyl
substitution (Td = 340 ◦C for 1A-Tf2N) creates a more stabilize
cation than does the N-propyl substitution (Td = 311 ◦C for 1pA-
Tf2N), which is expected due to resonance and electron donation
from the N-phenyl substituent. In the 13C NMR spectra (Fig. S7
in the ESI†), the NC = CH resonance of 1A-Tf2N (146.1 ppm)
is shifted downfield compared to that of 1pA-Tf2N (145.1 ppm),
which indicates that the triazolyl ring of the former has greater
electron density than the latter.
All triazolium-based ILs were evaluated for glass transition
and melting temperatures using DSC. No glass transition a◦nd
melting temperatures were detected for these ILs from -20 C
to 100 ◦C, except for 1A-Tf2N, 4-Tf2N, and 5-Tf2N where Tm =
71 ◦C, 81 ◦C, and 146 ◦C, respectively. Aside from the noted solid
materials, the ILs in Fig. 2 are all liquid at room temperature.
The density of the triazolium-based ILs was measured at
25 ◦C and 37 ◦C, including those for [hmim][Tf2N] to ensure
the validity of our data. The values obtained in our laboratory
Fig. 4 Thermal decomposition temperature (Td) of 4-phenyl (blue) and
4-propyl (red) substituted triazolium-based ILs with Tf2N as an anion
(top), and the regioisomeric triazolium-based ILs with Tf2N as an anion
(bottom).
◦
(r = 1.3767 g cm-3 at 20 C; r = 1.3719 g cm-3 at 25 ◦C) and
the literature (r = 1.3765 g cm-3 at 20 ◦C) for [hmim][Tf2N]
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Table 1 Physicochemical properties of ionic liquids
Ionic liquida
Vm 25 ◦C (cm3 mol-1)
Density @ 25 ◦C (g cm-3)
Density @ 37 ◦C (g cm-3)
Td (◦C)a
Tm (◦C)
71
1-Tf2N
387.06
362.45
372.77
1.3551
1.4660
1.3158
1.3435
—
1.3039
323
340
335
311
290
324
357
282
305
325
276
273
184
190
394
427
1A-Tf2N
1p-Tf2N
1pA-Tf2N
2-Tf2N
2B-Tf2N
2p-Tf2N
2pB-Tf2N
3-Tf2N
371.25
1.3267
1.3148
421.78
398.43
332.37
341.53
1.3906
1.3867
1.5960
1.4536
1.3709
1.3767
—
3p-Tf2N
4-Tf2Nb
81
4p-Tf2N
5-Tf2N
5p-Tf2N
[hmim][Tf2N]c
[hmim][Tf2N]d
1.4413
146
326.44
1.3719
1.3765
1.3603
a Temperature at 5% mass loss of the sample. b Density measured at 20 ◦C with error 0.0007 g cm-3. c Density measured in-house using commercial
sample. d Literature reported values, density at 20 ◦C (ref. 16) and Td (ref. 17) first detection of decomposition.
are the same within experimental error. The density at room
temperature ranges from 1.316 to 1.454 g cm-3 (Table 1) for
the liquid triazolium-based IL samples. In this series, the ILs
with the highest and lowest molar volume are 3-Tf2N and 4p-
Tf2N, respectively. For the regioisomers 1-Tf2N and 1A-Tf2N,
the N-phenyl substituted triazolium IL has a smaller molar
volume.
The CO2 solubility of these triazolium-based ILs and
[hmim][Tf2N] was determined at 37 ◦C and various pressures
(Fig. 5). Except for 5-Tf2N, triazolium-based ILs have better
molar CO2 capacity than [hmim][Tf2N]. Emerging trends for
the CO2 solubility are (a) the 4-propyl substituted triazolium-
based IL dissolves more CO2 than those with the 4-phenyl group
and (b) the triazolium with the CH3(OCH2CH2)3 moiety has the
highest CO2 capacity.
was switched from iodide to Tf2N, indicating the relative stability
remains the same. This behavior holds true for both substituent
change and regioisomers. The structural manipulation affects
the electronic properties and plays a definitive role in dictating
the physical properties of the ILs. The ability to access regioi-
someric ILs allows us to tune the thermal properties and molar
volume of triazolium-based ILs by changing the position of
the substituent. This research underscores the importance of
structural configuration and its effect on the properties of the
ionic liquids. Currently detailed work is being carried out on
correlating solubility of CO2 with structural changes in ionic
liquids.
Acknowledgements
The authors thank Christina Myers of NETL for conducting
density measurements. Part of this technical effort was per-
formed in support of NETL’s ongoing research in CO2 capture
under the contract DE-FE0004000. Reference in this work to
any specific commercial product is to facilitate understanding
and does not necessarily imply endorsement by the United States
Department of Energy.
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In conclusion, Cu-catalyzed click chemistry, combined with
other efficient reactions, allowed the facile assembly of a
large number of unique but structurally-related materials. The
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