Halogen free 1,2,3- and 1,2,4-triazolide based ionic liquids: Synthesis and properties

Article abstract of DOI:10.1039/c7cc05770a

Triazoles have been successfully used as building blocks to create "fully organic" ILs featuring on both sides organic ions, i.e., 1,2,3- or 1,2,4-triazolide anions and 1,2,4-triazolium or imidazolium cations. Glass transition temperatures, densities and viscosities of these ILs were determined. Their electrochemical and thermal stability, and also conductivity, are higher than those for known ILs.

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Reactivity differences of Sc
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Halogen free 1,2,3- and 1,2,4-triazolide based ionic liquids:
synthesis and properties
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
Aleksandr Savateev,* Clemens Liedel, Steffen Tröger-Müller, Alberto S. de León, Markus
a
a
Antonietti and Dariya Dontsova
DOI: 10.1039/x0xx00000x
www.rsc.org/
Triazoles have been successfully used as building blocks to create similibus solvuntur”.
fully organic” ILs featuring on both sides organic ions, i.e., 1,2,3- Triazolium cations (A, Fig. 1) are stabilized via conjugation with
“
or 1,2,4-triazolide anion and 1,2,4-triazolium or imidazolium nitrogen lone pairs as well as through the inductive effect of
cations. Glass transition temperatures, densities and viscosities of electron donating functional groups, which additionally may
these ILs were determined. Their electrochemical and thermal affect the charge distribution within the molecule affording
1
0
stability, and also conductivity are higher than for known ILs.
task specific ILs for targeted application. The advantage of
triazolium ILs compared to widely used imidazolium ones is the
reduction in number or even suppression of side-reactions
caused by deprotonation of the ring carbon atoms of
Ionic liquids (ILs) are in the focus of modern materials
chemistry and, for instance, used as a medium to perform
chemical transformation as well as for electrochemical and
1
1
heterocyclic cations.
Some 1,2,3-triazolium ILs were
1
1
,2,3
1
even biological applications.
The vast majority of ILs are
successfully used as a medium for Baylis-Hillman reaction
1
2
represented by an organic cation, typically imidazolium, and
inorganic or halogenated counterion. The pathways of anions
decompositions were explored both theoretically and
and for (±)-centrolobine synthesis while bicyclic triazolium ILs
were applied as molecular solvents for rutaecarpine
1
synthesis. Besides, several 1,2,3-triazolium ILs with tosylate
3
4
experimentally. For example, reduction of triflimidate anion,
⊖
and TfO⊖ anions were synthesized and characterized.14
Conversely, in the triazolide anion (B, Fig. 1) three nitrogen
+
Tf N , starts at -2.0 V vs. Fc /Fc and includes cleavage of S–N
2
5
bond followed by formation of CF ⊖. On the contrary, PF is
⊖
3
6
atoms can effectively stabilize negative charges and lower the
1
IL viscosity. In addition, due to the smaller size of the organic
⊖
anion compared, for example, to Tf N , the ion mobility of the
5
slowly oxidized on anode producing toxic gaseous PF and F·
5
6
radical, which upon hydrogen abstraction is converted to HF.
On the other hand, B–F bond cleavage in BF ⊖ occurs upon
2
4
IL is expected to be higher. As a result of the low acidity of
7
contact with water even under very mild conditions.
1
,2,4- and 1,2,3-triazoles, their corresponding anions are
1
considered to be superbases, and several 1,2,3-triazolide ILs
6
Considering potential risks that fluorinated ILs might carry, a
good alternative could be halogen free ILs.
1
7
were used for CO capture
2
and CO2 electrochemical
1
8-20
“
Fully organic” ILs, having both organic cation and anion, are
reduction.
part of highly energetic ILs as well.
electrochemical properties remain unexplored.
The triazole-1,2,4-ide anion was employed as a
8
less common and usually comprise simple anions like acetate,
2
1
However, their
9
bringing however obvious restrictions in use. In this context,
1
,2,3- and 1,2,4-triazoles are an interesting choice of
Triazoles could be an excellent platform to synthesize “fully
organic” ILs, comprising triazolium cations and triazolide
anions. However, throughout the challenging design a number
heterocyles which form both stable cations and anions. This
enables to set-up a new class of ILs. The advantage of “fully
organic” triazole-based ILs is their chemical stability, but also
that they possess many nitrogen atoms which can participate
in H-bonding with the solute. This makes them potentially
more compatible with many organic substances since “Similia
Fig. 1 Cationic triazole-1,2,4-based IL (A) and anionic triazole-1,2,4- (B, left) or triazole-
1
,2,3-based ILs (B, right). FG stands for ‘functional group’.
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5
of criteria should be kept in mind. These include acidity of the methyl group in second position of imidazole ring.
target triazole, which functional groups to use to adjust the Interestingly, due to the compact size of 1,2,4-triazolium
triazole properties, but also the synthetic approach to employ. cation, one could expect a high melting point of IL-2. However,
Herein we report on the synthesis of a set of “fully organic” this is not the case, as IL-2 still possesses a relatively low T
triazole based ILs and their properties. 39.0 °C, probably due to the influence of the anion. IL-3
Triazoles, bearing NO and CN functional groups, were demonstrates quite low T = -55.4 °C that could be explained,
g
= -
2
g
selected as anion precursors due to the ability of these groups at least partially, by the lower symmetry of the anion
to stabilize efficiently negative charge via a (–M) mesomeric compared, for example, to 1,2,3-triazolide.
2
effect. ILs precursors 2H-1,2,3-triazole-4,5-dicarbonitrile, 3- The thermal stability and thermal decomposition pathways of
2
2
3
nitro-1H-1,2,4-triazole, 1-Methyl-4-amino-4H-1,2,4-triazol-1- the synthesized ILs were explored with thermogravimetric
2
4
ium iodide
1
were synthesized by us according to the analysis mass spectroscopy (TGA-MS). Onset of decomposition
(Tdec) extracted from the TGA curves (Fig. 3b) are summarized
procedures given in the literature.
The ILs studied in the current work were obtained in high in Table 1. Tdec values could be used as a descriptor of ILs
yields via ion metathesis reactions as depicted in Fig. 2. The thermal stability upon short exposure to high temperatures.
method provides ILs with high purity, as shown below, and We have monitored charged species with m/z in the range
without laborious purification procedure. The chemical from 1 to 200 that are formed during the ILs heating up to
composition of the ILs is very close to the calculated one (Table 600 °C. The ion current curves where change of signal was
S1). The IL-1, IL-2 and IL-3 are free of any foreign elements, e.g. detected are shown in Fig. S29, S31, S33.
Ag, Cl, I, that was unambiguously proved by EDX analysis (Fig. IL-1 starts decomposing at 268 °C and complete decomposition
5,26
2
S2, Table S1).
The water mass fraction (w ) in ILs after occurs at 400 °C suggesting that only gaseous products are
w
drying in vacuum (0.1 mbar, 65 °C, 4 h) determined using Karl- formed. Importantly, that Tdec of IL-1 is higher than for some of
2
7,28
Fischer titration in IL-1 was ww = 0.00832±0.0002, IL-2 ww
.0144±0.0008 and IL-3 w = 0.0102±0.0004 (Table S1).
=
the known imidazolium ILs with organic anions.
The
0
decomposition of 4,5-dicyano-1,2,3-triazol-2-ide anion involves
1
The structures of ILs are consistent with H, C NMR spectra formation of C N (m/z 52), N2 (m/z 28), CN (m/z 26) and
w
13
•+
•+
+
2
2
+
including two-dimensional techniques (Fig. S3-23). The full CH (m/z 13) species (Fig. S29). Decomposition of 3-butyl-1,2-
assignment of peaks in NMR spectra to specific nuclei was dimethyl-1H-imidazol-3-ium cation probably involves
performed (Fig. S24). In HR-MS spectra the masses of cation elimination of n-butyl radical and its subsequent
and anion were observed independently in the positive and fragmentation along with dehydration as supported by a series
negative ion modes proving that all these compounds can give of charged species with m/z 54, 53, 52, 51, 50 (C fragments:
4
1
•+
+
•+
+
•+
stable charged species (Fig. S25-27). Combining H NMR data C4H6 , C H , C H , C H , C H ), 44, 43, 42, 41, 40, 39 (C
3
4
5
4
•
4
4
+
3
4
•+
2
+
+
•+
with elemental analysis and water content the purity of the ILs fragments: C H , C H , C H , C H , C H , C H ) and 30,
•+
in mass fraction) was determined to be 0.968-0.982 (Table 29, 28, 27, 26 (C fragments C H , C H , C H , C H ,
2 2 6 2 5 2 4 2 3
+
3
8
3
7
3
6
3
5
•
3
4
3 3
+
+
+
(
•
+
S1).
C2H2 ). The charged species with m/z 68 and 66 could be
At room temperature IL-1, IL-2, and IL-3 are viscous liquids. assigned to the products of imidazole ring decomposition were
The glass transition temperatures (T ) of the ILs were also detected. The proposed mechanism of IL-1 thermal
g
determined from differential scanning calorimetry (DSC) decomposition is schematically depicted in Fig. S30.
curves (Fig. 3a) and are summarized in Table 1. The ILs are The IL-2 has the onset of decomposition at 219 °C. It loses 60%
stable in the range of temperatures -100…+150 °C as proved of initial mass at 350 °C. Decomposition of IL-2 on the first step
by the identity of T values during cyclic heating/cooling (Fig. could involve transfer of methyl cation from cation to anion
g
S28). IL-1 has T = -66.6 °C, which is higher than for 1-butyl-3- leading to the formation of 4H-1,2,4-triazol-4-amine and 2-
g
methyl-1H-imidazol-3-ium 4,5-dicyano-1,2,3-triazol-2-id, -87 °C methyl-2H-1,2,3-triazole-4,5-dicarbonitrile (Fig. S32). The latter
•+
-
1
+
(
heating rate 2 °C·min ), evidently due to the presence of
is decomposed to C N
2
(m/z 52), C N (m/z 38) and
2
2
+
methanediazonium (CH N , m/z 43) as initial fragments.
3
2
+
CH N , upon subsequent fragmentation, gives rise to other
3
2
•
pieces, such as CH N (m/z 42), CH N (m/z 40), diazene
+
•+
2
2
2 2
•
+
+
(
2 2 3
N H , m/z 30), CH N (m/z 29), etc. (Fig. S31). The
decomposition of 4H-1,2,4-triazol-4-amine, in turn, involves
•
evolution of nitrogen (N , m/z 28), ammonia (NH3 , m/z 17)
+
•+
2
•
+
•+
and C HN (m/z 39) (Fig. S31). The later could give HCN (m/z
2
+
7) and CN (m/z 26) charged species, while liberation of
2
+
ammonia is supported by the presence of N (m/z 14), NH
•+
+
m/z 15), NH2 (m/z 16) and NH4 (m/z 18).
+
(
Interestingly, introduction of a nitro group into the triazole
ring did not destabilize the respective anion significantly. The
IL-3 starts decomposing at 234 °C and loses 90% of the initial
mass at 300 °C. N-butylimidazolium cation follows the
decomposition pattern described for IL-1 above and includes
Fig. 2 Synthesis of ILs by ion metathesis reactions.
2
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Table 1. Physical properties of ILs.
[
,
a]
[b]
,
[c]
[d]
-3
[e]
-1
[f]
+
[f]
+
[g]
IL
T
g
°C
T
dec
°C
µ, cP
ρ, g·cm
κ, mS·cm
Cathodic limit, V vs. Fc/Fc
Anodic limit, V vs. Fc/Fc
ECW, V
IL-1
IL-2
IL-3
-66.6±0.1
-39.0±0.3
-55.4±0.2
268
219
234
85±2
1.107±0.001
1.325±0.001
1.181±0.001
2.122±0.185
1.180±0.008
0.731±0.110
-1.60
-1.64
-1.3
1.51
0.61
1.42
3.02
2.25
2.72
376±20
414±20
[
a] Glass transition temperatures determined by DSC; [b] onset of decomposition; [c] dynamic (shear) viscosity measured at 25 °C; [d] measured at 25 °C in
-
triplicate; [e] measured at 25 °С; [f] cathodic and anodic limits were determined from LSV (scan rate 1 mV·s ) using glassy carbon working electrode, Pt
1
+
-
-2
counter electrode and Ag/AgNO
3 4 2 4
/Bu N H PO reference electrode (Fig. 3f) applying current density cutoff at j = 20 µA·cm ; [g] electrochemical window
(ECW) = Anodic limit – Cathodic limit.
elimination of butyl radical that upon further decay gives a set voltammetry (LSV) measurements were performed at low scan
36
-1
4 3 2
of characteristic C , C and C fragments (Fig. S34). On the rate 1 mV·s (Fig. 3f). The electrochemical window (ECW) of
other hand, anion decomposition could involve elimination of the IL naturally depends on current density cutoff used for
•+
+
NO (m/z 30) accompanied by OH ion (m/z 17), N
+
37
(m/z 28) calculations. We applied very low current density cutoff, j =
2
•+
-2
(m/z 39) (Fig. S33). The later, however, 20 µA·cm , which is considerably lower than generally used in
and presumably HC
2
N
+
overlaps with the signal of C
3
H
3
.
literature, in order to avoid overestimation of ECW (Table 1).
In order to estimate long term thermal stability of ILs we Thus, IL-1 has the widest electrochemical window (ECW)
performed isothermal TGA at 120 °C for 10 h. This among the studied compounds, 3.02 V. On the other hand, IL-2
temperature exceeds the typical operational temperature for has the highest stability on the cathodic side, -1.64 V. The ECW
the most common processes and could serve on a first of IL-3 is 2.72 V, and the anodic limit, 1.42 V, is higher than in
approximation a parameter describing the longstanding the cases of IL-1 and IL-2. This implies that 5-nitro-1,2,4-triazol-
2
9
stability of IL. Under the applied conditions IL-1 has lost 0.5% 1-ide is more stable against oxidation than 4,5-dicyanotriazole-
of the initial mass, while IL-2 and IL-3 about 1.3 % (Fig. 3c). 1,2,3-2-ide. In comparison with other ILs, the ECW of the
Densities of the synthesized ILs are in the range 1.107-1.325 electrolytes presented here are wider despite we have used
-
3
g·cm (Table 1). Typically the presence of water affects the
3
0
densities of ILs within 1-2 % range.
The novel ILs behave as Newtonian fluids in the range of shear
-1
.
rates 500-3000 s (Fig. 3d) IL-1 has the lowest dynamic
viscosity of 85 cP at 25 °C, among the ILs presented in current
work (Table 1). IL-2 and IL-3 have viscosities – 376 and 414 cP,
respectively. The viscosities of ILs gradually decrease with the
temperature and reach plateau at 70-80 °C (Fig. 3e). The
viscosities of ILs also depend on water content and for
[
C
4
mim][Tf
increases from 10 to 10480 ppm.
possesses lower viscosity compared, for example, to
mim][PF ] (397 cP, w = 0.0117, 25°C) and [C mim][PF
452 cP, w = 0.008837, 25°C) having nearly the same water
2
N], for example, is 33% lower when water content
3
1
Nevertheless, IL-1
[C
4
6
w
6
6
]
(
w
3
2
content.
The conductivities of ILs were determined using
electrochemical impedance spectroscopy (EIS) (Table 1).
Widegren and Magee have shown that conductivities of ILs
-
6 2 w
depend on water content and for dry [C mim][Tf N] (w = 10
5
-1
, for example, is 2.18 mS·cm , which is 30% lower compared
)
to the same IL, but measured under wet conditions (3.06
-
1
33
mS·cm , w
newly synthesized ILs are comparable, for example, to the
aforementioned [C mim][Tf N] and [C mim][Tf N]
0.00885, 25°C). In addition,
conductivities of reported here ILs are higher than that of
w
= 0.00898, 25°C). The conductivities of the
6
2
4
2
-1
34
(
5.42 mS·cm ,
w
w
=
-1
[
w
aP4443][Ala] (0.244 mS·cm , w = 0.0169), [aP4443][Val]
-1
-1
Fig. 3. DSC heating curve in the range of temperatures -100…+150 °C performed
(
0.116 mS·cm , w
w
= 0.0157) and [aP4443][Leu] (0.114 mS·cm ,
under flow of nitrogen. The fourth cycles are plotted (a); ramp TGA, heating rate
w
w
= 0.0151) measured at 25°C and having higher water
3
-
1
.5 K·min (b); isothermal TGA performed at 120 °C (c); shear stress – shear rate
2
5
content.
dependence measured at 25 °C (d); dependence of ILs dynamic viscosity from
In order to overcome the influence of viscosity, especially for the temperature (e); LSV curves of ILs (f).
IL-3, and to achieve equilibrium conditions, linear sweep
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much lower current density cut off values. To support this the 15 S. Kitaoka, K. Nobuoka, N. Yoshiiwa, T. Harran and Y. Ishikawa,
Chem. Lett., 2010, 39, 1142-1143.
following ILs having similar water content could be mentioned:
-2
16 A. D. McNaugh, A. Wilkinson, M. Nic, J. Jirat, B. Kosata and A.
Jenkins, IUPAC. Compendium of Chemical Terminology XML on-
line corrected version: http://goldbook.iupac.org (2006-),
Blackwell Scientific Publications, Oxford, 2nd (the "Gold Book")
edn., 1997.
7 R. L. Thompson, W. Shi, E. Albenze, V. A. Kusuma, D. Hopkinson,
K. Damodaran, A. S. Lee, J. R. Kitchin, D. R. Luebke and H.
Nulwala, RSC Adv., 2014, 4, 12748-12755.
8 N. Hollingsworth, S. F. R. Taylor, M. T. Galante, J. Jacquemin, C.
Longo, K. B. Holt, N. H. d. Leeuwad and C. Hardacre, Faraday
Discuss., 2015, 183, 389-400.
9 S. F. R. Taylor, C. McCrellis, C. McStay, J. Jacquemin, C. Hardacre,
M. Mercy, R. G. Bell and N. H. d. Leeuw, J. Solution Chem., 2015,
[
C
4
mpyrr][Tf
mim][I] (2.0 V, w
N] (2.2 V, w
at +25°C using Pt microdisc as the working electrode and Pt
2
N] (2.0 V, w
w
= 0.011407, j = 1 mA·cm ),
-
2
[C
4
w
= 0.011349, j = 1 mA·cm ) and
-2
[N
6,2,2,2][Tf
2
w
= 0.0082, j = 1 mA·cm ), all measured
3
7
wire as quasi-reference electrode.
1
1
1
Conclusions
Three new “fully organic” ILs, which are built only from C, N, O,
H elements and comprise heterocycles as cation and anion
building blocks, have been synthesized. Their physical
properties were measured and compared to known ILs taking
into account water content. The obtained results suggest that
the reported here electrolytes have higher conductivity, wider
ECW and lower viscosities than that of some of the known ILs.
And this is in addition to good thermal stability. All these
4
4, 511-527.
2
2
0 J. J. Fillion, H. Xia, M. A. Desilva, M. Quiroz-Guzman and J. F.
Brennecke, J. Chem. Eng. Data, 2016, 61, 2897-2914.
1 H. Xue, Y. Gao, B. Twamley and J. n. M. Shreeve, Inorg. Chem.,
2005, 44, 5068–5072.
features make the reported here ILs suitable in energy storage 22 M.-J. Crawford, K. Karaghiosoff, T. M. Klapötke and F. A. Martin,
application. Moreover, due to the basic properties of the
anions and the presence of a large number of nitrogen atoms
these ILs could be used as a reaction media to accomplish
Inorg. Chem., 2009, 48, 1731-1743.
3 R. Petersen, J. F. Jensen and T. E. Nielsen, Org. Prep. Proced. Int.,
2
2
2
014, 46, 267-271.
4 G. Drake, T. Hawkins, K. Tollison, L. Hall, A. Vij and S. Sobaski, in
Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and
Opportunities, eds. R. D. Rogers and K. R. Seddon, American
Chemical Society, Washington, DC, 2005, vol. 902, ch. Chapter
desired chemical transformations, for example, CO
The work was supported by Deutsche Forschungsgemeinschaft
grant number DFG-An 156 13-1). We thank Antje Völkel,
2
reduction.
(
Ursula Lubahn and Sylvia Pirok for TGA-MS, DSC and elemental
analysis respectively.
2
0, pp. 259-302.
5 M. Holzweber, W. E. S. Unger and V.-D. Hodoroaba, Anal. Chem.,
016, 88, 6967-6970.
2
2
2
2
2
2
6 A. R. Santos, R. K. Blundell and P. Licence, Phys. Chem. Chem.
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7 M. T. Clough, K. Geyer, P. A. Hunt, J. Mertes and T. Welton, Phys.
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DOI: 10.1039/C7CC05770A
Synthesis and characterization of novel triazole based halogen free ionic liquids.