Expanding the Electrochemical Window: New Tunable Aryl Alkyl Ionic Liquids (TAAILs) with Dicyanamide Anions

Article abstract of DOI:10.1002/chem.201902797

A set of new tunable aryl alkyl ionic liquids (TAAILs) based on the 1-aryl-3-alkyl imidazolium motif has been synthesized, in which the following variables were systematically changed: alkyl chain length, aryl substitution (group and position), and counte

Full text of DOI:10.1002/chem.201902797

DOI: 10.1002/chem.201902797
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Ionic Liquids
Expanding the Electrochemical Window: New Tunable Aryl Alkyl
Ionic Liquids (TAAILs) with Dicyanamide Anions
Swantje Lerch and Thomas Strassner*[a]
Chem. Eur. J. 2019, 25, 1 – 7
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Abstract: A set of new tunable aryl alkyl ionic liquids
(TAAILs) based on the 1-aryl-3-alkyl imidazolium motif has
been synthesized, in which the following variables were sys-
tematically changed: alkyl chain length, aryl substitution
(group and position), and counter ion. TAAILs with dicyan-
anion increased the electrochemical stability leading to
TAAILs with an extremely wide electrochemical window of
up to 7.17 V. Additionally, both classes of TAAILs extract tran-
sition metals from aqueous solutions: TAAILs with the DCA
anion extract both platinum and copper while TAAILs with
the NTf2 anion are selective towards platinum. This demon-
strates that minor changes of the molecular structure lead
to TAAILs with drastically changed physicochemical proper-
ties.
amide
(DCA)
and
bis(trifluoromethylsulfonyl)imide
(N(SO2CF3)2, NTf2) anions showed remarkable differences of
their physical properties: NTf2 ionic liquids were found to
have high decomposition temperatures and viscosities well
below those of the DCA TAAILs. In contrast, the dicyanamide
Introduction
Over the past decades, ionic liquids (ILs) have attracted atten-
tion due to their unique physicochemical properties. They can
be used as solvents in organic synthesis,[1,2] as a catalyst in ho-
mogeneous catalysis,[3–8] in dye-sensitized solar cells,[9,10] as
electrolytes in batteries or supercapacitors,[11–17] for the extrac-
tion[18–26] and deposition[27–31] of metals, and for the stabiliza-
tion of nanoparticles.[32–35] They have also been used as lubri-
cants[36,37] and in nanoelectronics.[38] Per definition, these salts
have a melting point below 1008C, are conductive, have a
negligible vapour pressure and show an extremely high ther-
mal stability.
Figure 1. General structure of TAAILs and recent applications featuring dif-
ferent anions.
ILs are composed of organic cations and organic or inorgan-
ic anions, which allows for a great variability of structural
motifs and hence of their physical properties.[39,40] Common
cations used are all-alkyl ammonium-, phosphonium- or imida-
zolium-based moieties.
introducing palladium-containing anions, we extended the
range of catalytically active ionic liquids.[48]
Recently, it was reported that the dicyanamide (DCA) anion
led to remarkable improvements of the viscosity and electro-
chemical window (EW) of ammonium-, phosphonium- and imi-
dazolium-based ionic liquids leading to be promising candi-
dates for the applications mentioned above.[54–57]
In 2009, we introduced a new class of imidazolium-based
ionic liquids, the so called tunable aryl alkyl ionic liquids
(TAAILs, Figure 1).[41,42] Over the past years, we demonstrated
that the phenyl ring not only alters melting points and thermal
stabilities, but also allows to fine tune the physicochemical
properties according to one’s requirements.[43] In collaboration
with the Krossing group, we combined the weakly coor-
dinating tetrakis- (1,1,1,3,3,3-hexafluoropropan-2-yl)oxy)borate
(B(CH2(CF3)2)4, B(hfip)4) anion with TAAIL cations, yielding ionic
liquids with remarkable low glass transition points.[44] The
Janiak group used TAAILs for the synthesis of ruthenium and
iridium nanoparticles.[45] They observed that both nanoparticle
size and size distribution are drastically influenced by the aryl
substitution of the TAAIL cation. TAAILs also proved to be suit-
able solvents in transition metal catalysis, for example, the hy-
drosilylation[46] or the hydroamination and hydroarylation.[47] By
Therefore, we were interested in the combination of the di-
cyanamide anion with different TAAIL cations and its influence
on the physicochemical properties of the resulting materials.
To further investigate the structure–property correlation, we
synthesized a set of TAAILs with different aryl substituents in
the ortho- and para-position and different alkyl chain lengths
(butyl, octyl and dodecyl) with both the DCA anion and the
bis(trifluoro-methylsulfonyl)imide (N(SO2CF3)2, NTf2) anion for
comparison. We examined the thermal properties, electro-
chemical stability, viscosities and metal extraction performance
of these two classes of ionic liquids.
Results and Discussion
Synthesis
[a] S. Lerch, Prof. Dr. T. Strassner
TAAILs are accessible by a three-step synthesis starting from
commercially available aniline derivates (see Scheme 1). In the
first step, a ring-closing reaction is performed using aniline,
formaldehyde, glyoxal and ammonium chloride. The reaction
can be done in a 6 L batch reactor on a scale of up to 0.5 kg
of aniline, yielding substituted aryl imidazoles 1–4. The follow-
Physikalische Organische Chemie
Technische Universitꢀt Dresden
Bergstraße 66, 01069 Dresden (Germany)
E-mail: thomas.strassner@chemie.tu-dresden.de
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201902797.
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Scheme 1. Synthesis of ionic liquids: i) 1.1 equiv. glyoxal, 2.1 equiv. formaldehyde, 2 equiv. NH4Cl, cat. H3PO4, MeOH, 608C, 24 h; ii) 1.1 equiv. alkyl bromide
(CnH2n+1Br), THF, 708C, 2–7 d; iii) 1.1 equiv. LiNTf2, DCM/MeOH/H2O, rt, 24 h; vi) 1.5 equiv. NaN(CN)2, H2O, rt, 24 h.
ing alkylation with different alkyl bromides gives TAAILs 5–16
with bromide counterion, synthesized as previously reported
by our group in multi-molar scale.[47] Anion exchange with lithi-
um bis(trifluoromethylsulfonyl)imide (LiNTf2) afforded the cor-
responding TAAILs 17–28.
Table 1. Physicochemical data of TAAILs 17–40. Tg(h/c): glass transition
point upon heating/cooling in [8C], td: thermal decomposition tempera-
ture in [8C], [a] crystallization temperature.
TAAIL
Tg(h/c)
Td
TAAIL
Tg(h/c)
Td
17
18
19
20
21
22
23
24
25
26
27
28
ꢀ61/ꢀ61
ꢀ63/ꢀ60
ꢀ13[a]/<ꢀ80
ꢀ61/ꢀ62
ꢀ61/ꢀ61
ꢀ9[a]/<ꢀ80
ꢀ66/ꢀ67
ꢀ71/ꢀ71
< ꢀ80
372
367
335
369
314
333
402
390
311
334
367
375
29
30
31
32
33
34
35
36
37
38
39
40
ꢀ57/ꢀ59
ꢀ59/ꢀ60
ꢀ24[a]/<ꢀ80
ꢀ58/ꢀ60
ꢀ61/ꢀ60
< ꢀ80
265
270
264
270
256
279
279
270
267
267
277
264
Dicyanamide ILs are generally obtained by the reaction of
the chloride or bromide salt with AgN(CN)2.[49,50,53] To avoid any
silver contamination, TAAILs 29–40 were synthesized following
a modified protocol of Boudesocque and co-workers.[54] The
corresponding bromide salt is suspended in sodium dicyana-
mide solution in water (1.5 equivalents) and stirred at room
temperature for 24 hours. The crude product is then extracted
with dichloromethane three times. For complete anion ex-
change, the organic layer is washed with sodium dicyanamide
solution (1 equivalent) and twice with water. After removal of
the solvent, the ionic liquids were dried in high vacuum for
16 hours. All TAAILs described were obtained as liquids at
room temperature, thus belonging to the class of room tem-
perature ionic liquids (RTILs). The synthetic procedures, analyti-
cal data, and NMR spectra of TAAILs 5–40 are given in the Sup-
porting Information.
ꢀ62/ꢀ63
ꢀ65/ꢀ67
ꢀ36/ꢀ27[a]
ꢀ60/ꢀ62
ꢀ60/ꢀ61
ꢀ13/ꢀ41[a]
ꢀ56/ꢀ58
ꢀ61/ꢀ61
< ꢀ80
Thermal decomposition occurs between 3118C and 4028C
for NTf2 salts and between 2568C and 2798C for DCA salts.
Thus, ILs with the NTf2 anion are thermally more stable than
their dicyanamide analogues. Still, TAAILs with the DCA anion
show a higher thermal stability than other unsymmetrical 1,3-
dialkyl imidazolium salts where the decomposition tempera-
ture is between 2208C and 2408C.[51] For most ILs with the
NTf2 anion, the decomposition temperature decreases with
longer alkyl chains. This trend is not seen in DCA ILs where the
decomposition temperature does not change significantly with
either aryl substitution or alkyl chain length.
Thermal properties
Glass transition points of TAAILs 17–40 were measured via dif-
ferential scanning calorimetry (DSC) in the range of 208C to
ꢀ808C with a scanning rate of 1 Kminꢀ1 using a Mettler
Toledo DSC1. Thermal decompositions (Td) were measured via
thermogravimetric analysis (TGA) using a Netzsch STA 409 PC/
PG measuring between 308C and 5008C with a heating rate of
5 Kminꢀ1. The Td is given at 5% mass loss. The results are
given in Table 1, TG and DSC plots are given in the Supporting
Information.
Electrochemical behavior
The electrochemical properties of ionic liquids depend strongly
on the molecular structure of the salt. The cathodic limit is de-
pendent on the cation and the anodic limit is determined by
the anion.[55–59] For imidazolium cations, the alkyl chain length
has only a minor influence on the electrochemical stability.[59]
Therefore, all eight TAAILs with the butyl chain were chosen
for electrochemical measurements as their properties can be
compared to the well investigated and commercially available
1-butyl-3-methyl-imidazolium analogues. Cyclic voltammo-
grams were measured using a three-electrode set up (glassy
carbon working electrode, Pt counter electrode and an Ag/Ag+
pseudo reference electrode). Table 2 shows the cathodic and
For ILs with a short or medium alkyl chain, very low glass
transition points were observed ranging between ꢀ508C and
ꢀ718C which is in the same order as those reported for TAAILs
with the B(hfip)4 anion.[44] TAAILs with the long dodecyl chain
show different solidification characteristics: for DCA TAAILs 37
and 40 crystallization temperatures were found at ꢀ368C and
ꢀ138C upon heating, respectively. For TAAILs 19, 22 and 31,
no solidification could be observed upon cooling, but upon
heating, a crystallization was observed. TAAILs 25, 28 and 34
show no solidification at temperatures above ꢀ808C.
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Figure 3 shows the viscosity of TAAILs 17–28 with the NTf2
anion between 208C and 808C. The viscosities range between
419 mPasꢀ1 for IL 23 and 884 mPasꢀ1 for compound 28 at
208C. The substitution pattern of the methoxy-substituted ILs
has only a minor influence on the viscosity, whereas the ortho-
methyl substituted ILs 23–25 have a lower viscosity in compar-
ison to their para-substituted isomers 26–28. More pro-
nounced, the increase of the alkyl chain length results in an in-
crease of viscosity which is in agreement with literature find-
ings and is a result of the increasing molar mass and of the
van der Waals interactions of the alkyl chains.[40,65] At 808C, the
Table 2. Electrochemical windows (EW) of TAAILs with different aryl sub-
stitutions and anions at a sweep rate of 100 mVsꢀ1 and a cut-off current
density of 1 mAcmꢀ2, vs. the Fc/Fc+ redox pair.
TAAIL Cathodic
limit
Anodic
limit
EW TAAIL Cathodic
limit
Anodic
limit
EW
17
20
23
26
ꢀ2.43
ꢀ2.33
ꢀ2.33
ꢀ2.13
+1.44
+0.96
+1.78
+1.26
3.87 29
3.28 32
4.11 35
3.39 38
ꢀ2.42
ꢀ2.47
ꢀ2.53
ꢀ2.45
+4.75
+3.46
+2.98
+2.96
7.17
5.93
5.51
5.41
anodic limits and the electrochemical window (EW) of these
compounds, complete cyclic voltammograms are shown in Fig-
ures S71–S78 in the Supporting Information.
viscosities decrease to values between 25 and 36 mPasꢀ1
.
The NTf2 salts 17, 20, 23 and 26 have an EW between 3.28 V
and 4.11 V at a cut-off current density of 1 mAcmꢀ2, a little
smaller than the EW of the corresponding 1-butyl-3-methylimi-
dazolium NTf2 IL (4.6 V).[60] The electrochemical stability of
TAAILs increases drastically with the DCA anion. TAAILs 29, 32,
35 and 38 have extremely wide EWs between 5.41 V and
7.17 V (for comparison, the well-known 1-butyl-3-methylimida-
zolium DCA IL only has an EW of 4.7 V).[29]
In Figure 2, cyclic voltammograms of ILs 17 and 29 are
shown. As both contain the same cation, their cathodic limit is
nearly the same with ꢀ2.43 V for IL 17 and ꢀ2.42 V for IL 29
against the ferrocene redox pair at a cut-off current density of
1 mAcmꢀ2. The NTf2 anion leads to an anodic limit of +1.44 V
against Fc/Fc+. But we were surprised to learn that the change
of the anion to DCA shifts this limit drastically by more than
3 V to +4.75 V vs. Fc/Fc+ leading to an EW of 7.17 V. To the
best of our knowledge, this is the widest electrochemical
window reported for ionic liquids on a glassy carbon electrode
so far.[61–64]
Figure 3. Viscosities of NTf2 TAAILs 17–28 between 20 and 808C.
The DCA anion generally leads to an increase of viscosity in
comparison to its NTf2 analogues. Graphic representations of
the viscosities of the dicyanamide TAAILs 29–40 are shown in
Figure S80 (see supporting information). This finding can be
explained by additional strong p–p interactions between the
phenyl ring and the p-bonds of the cyano groups present in
the DCA anion which is known from ILs with the C(CN)3
anion.[66]
Selective extraction of transition metals
The extraction percentage E% is used to quantify an extraction
process and can be calculated using Equation (1)[54] with Ci and
Cf defined as the initial and final metal concentration in the
aqueous layer.
Figure 2. Electrochemical windows of TAAILs 17 (top) and 29 (bottom) with
a sweep rate of 100 mVsꢀ1 and a cut-off current density of 1 mAcmꢀ2 (vs.
Fc/Fc+ redox pair).
Ci ꢀ Cf
*
ð1Þ
E% ¼
100
Ci
The extraction of metal ions from an aqueous solution or
waste waters is known for TAAILs with the NTf2 anion. There,
long alkyl chains proved to be beneficial for the extraction of
platinum.[66] Therefore, different dodecyl substituted NTf2 (22
and 28) and DCA TAAILs (34, 37 and 40) were tested on their
extraction behavior towards a 1 mm solution of three transition
metal salts (K2PtCl4, CuSO4, NiCl2) in water (see Supporting In-
formation for detailed procedure).
Viscosity measurements
Viscosities were measured between 208C and 808C in 5 K
steps (see Supporting Information for detailed procedure). In
general, viscosities of TAAILs are higher in comparison to
common all-alkyl ILs as for example, [EMIm]NTf2 or [EMIm]DCA.
This is a result of the higher molecular weight and of the addi-
tional p–p interactions of the aryl moieties in the cation.
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The selectivity and extraction efficiency are strongly depen-
dent on the anion (see Table 3). Without further additives, all
ILs bearing the NTf2 anion (22, 28) selectively extract platinum
(29% and 90%, respectively) and show no extraction of neither
copper nor nickel. The DCA ILs 34, 37 and 40 extract platinum
quantitatively and copper with an efficiency of up to 51% for
IL 37. The extraction of nickel is negligible for all TAAILs under
investigation.
window is extraordinary wide for DCA TAAILs with an EW of up
to 7.17 V. The selective extraction of transition metal cations
from an aqueous solution was feasible by the right choice of
ionic liquid, depending both on the anion and the aryl substi-
tution. This comparative study emphasizes the versatility of
TAAILs: by changing either the anion, the aryl substitution or
the alkyl chain length, physicochemical properties as thermal
stability, electrochemical window or viscosity, can be modulat-
ed as well as the metal extraction behavior can be adjusted.
Table 3. Extraction efficiency (in %) of TAAILs towards aqueous solutions
of the PtII, CuII and NiII salts.
Acknowledgements
TAAIL
22
28
34
37
40
We would like to thank the Bundesministerium fꢁr Bildung
und Forschung (MinSEM, 033R141G) and the Deutsche For-
schungsgesellschaft (SPP 1708, STR 526/20-1/2) for funding.
We would like to thank A.-L. Bachmann (Fraunhofer ISC) for
ICP/OES measurements and L. Hillebrand for the synthesis of
aryl imidazoles.
Pt
Cu
Ni
29
0
0
90
0
0
100
35
3
100
51
0
100
19
0
ILs 34 (R=4-OMe, n=12, X=DCA) and 37 (R=2-Me, n=12,
X=DCA) were chosen for successive extraction as they both
extract platinum quantitatively. Additionally, TAAIL 37 was
tested on the extraction of copper (see Figure 4).
Conflict of interest
TAAIL 34 extracts platinum quantitatively for the first two
runs. Then, the extraction efficiency decreases slowly to 80%
at the 6th run. TAAIL 37 performs slightly better with a com-
plete extraction of platinum for the first three runs and a de-
crease to 83% over the following three runs. 72% of copper is
extracted by TAAIL 37 from a 0.5 mm solution of copper(II)
chloride, but the extraction efficiency drops to 13% for the
second run and stays almost constant in the following two
runs (12% and 11%, respectively).
The authors declare no conflict of interest.
Keywords: dicyanamide anion · electrochemical window ·
ionic liquids · metal extraction · viscosity
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Manuscript received: June 18, 2019
Revised manuscript received: August 15, 2019
Version of record online: && &&, 0000
&
&
Chem. Eur. J. 2019, 25, 1 – 7
www.chemeurj.org
6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Broadening horizons: Different 1-aryl-3-
alkyl imidazolium ionic liquids (tunable
aryl alkyl ionic liquids, TAAILs) with
spectacular electrochemical properties
are presented. By changing the molecu-
lar system, ILs with thermal stability of
up to 4028C, an electrochemical
window of up to 7.17 V and glass transi-
tion points below ꢀ608C could be ob-
tained. Depending on the anion, transi-
tion metals were extracted selectively
from aqueous solution using different
TAAILs.
&
Ionic Liquids
S. Lerch, T. Strassner*
&& – &&
Expanding the Electrochemical
Window: New Tunable Aryl Alkyl Ionic
Liquids (TAAILs) with Dicyanamide
Anions
Like two dancers on stage, the anion and cation of an ionic liquid (IL) interact with
each other, each one contributing its own properties. With the combination of a 1-
aryl-3-alkyl-imidazolium-based cation (TAAIL) and the dicyanamide anion, the authors
were able to expand the electrochemical window of these ILs. Furthermore, a
compound was synthetized that is able to repeatedly extract transition metals from
aqueous solutions. For more details see the Full Paper by T. Strassner et al. on page &
& ff.
Chem. Eur. J. 2019, 25, 1 – 7
www.chemeurj.org
7
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ