1-Alkyl-3-methyl-1,2,3-triazolium [NTf2] ionic liquids: synthesis and properties

Article abstract of DOI:10.1016/j.tetlet.2015.11.107

A series of 1-alkyl-3-methyl-1,2,3-triazolium bis(trifluoromethylsulfonyl)imide [NTf₂] ionic liquids were prepared synthetically using azide-alkyne 'click' cyclization chemistry and their physicochemical and thermal properties were determined. Increasing the alkyl chain length resulted in increased viscosities and higher thermal transitions (T_g or T_m) with depressed molar conductivities. Walden plot analysis indicated that the ionicity of 1-butyl-3-methyl-1,2,3-triazolium [NTf₂] was comparable to the analogous imidazolium system and higher than the 1,2,4-triazolium equivalent.

Full text of DOI:10.1016/j.tetlet.2015.11.107

Accepted Manuscript
1
-Alkyl-3-methyl-1,2,3-triazolium [NTf ] ionic liquids: synthesis and proper-
2
ties
Zachary K. Reeder, Abigail M. Adler, Kevin M. Miller
PII:
S0040-4039(15)30415-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.11.107
TETL 47047
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
2 November 2015
23 November 2015
30 November 2015
Please cite this article as: Reeder, Z.K., Adler, A.M., Miller, K.M., 1-Alkyl-3-methyl-1,2,3-triazolium [NTf ] ionic
2
liquids: synthesis and properties, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.11.107
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1
-Alkyl-3-methyl-1,2,3-triazolium [NTf
ionic liquids: synthesis and properties.
Zachary K. Reeder, Abigail M. Adler, Kevin M. Miller*
2
]
Leave this area blank for abstract info.

1
Tetrahedron Letters
journal homepage: www.els evier.com
1
-Alkyl-3-methyl-1,2,3-triazolium [NTf ] ionic liquids: synthesis and properties.
2
Zachary K. Reeder, Abigail M. Adler and Kevin M. Miller∗
Department of Chemistry, Murray State University, Murray, KY 42071
ARTICLE INFO
ABSTRACT
Article history:
Received
2
A series of 1-alkyl-3-methyl-1,2,3-triazolium bis(trifluoromethylsulfonyl)imide [NTf ] ionic
liquids were prepared synthetically using azide-alkyne ‘click’ cyclization chemistry and their
physicochemical and thermal properties were determined. Increasing the alkyl chain length
Received in revised form
Accepted
resulted in increased viscosities and higher thermal transitions (T
conductivities. Walden plot analysis indicated that the ionicity of 1-butyl-3-methyl-1,2,3-
triazolium [NTf ] was comparable to the analogous imidazolium system and higher than the
1,2,4-triazolium equivalent.
g m
or T ) with depressed molar
Available online
2
Keywords:
ionic liquid
1,2,3-triazolium
Walden plot
2
009 Elsevier Ltd. All rights reserved.
thermal stability
—
——
∗
Corresponding author. Tel.: +1-270-809-3543; fax: +0-000-000-0000; e-mail: kmiller38@murraystate.edu

2
Tetrahedron
targeted 1-alkyl-3-methyl-1,2,3-triazolium [NTf
3
NMR spectroscopy, as well as elemental analysis, and residual
iodide content was determined to be less than 0.01 % w/w by ion
chromatography.
2
] ionic liquids
1 13
Introduction
Ionic liquids (ILs) continue to be explored as solvents in a
a-e. Characterization and purity were determined by H and
C
number of organic reactions due to their low vapor pressures,
reduced flammability and tunable viscosities. Some examples of
reactions employing ILs have included S 1 and S 2 substitution
N
N
1
,2
3,4
reactions,
Diels-Alder cycloadditions and organometallic
5
,6
carbon-carbon bond forming reactions. While the majority of
these efforts have focused on ILs that utilize the ammonium,
imidazolium or phosphonium cation, there remains a need to
explore other cations in order to achieve a more diversified IL
portfolio for application-specific needs. Nitrogen-rich
heterocyclic cations (1,2,3- and 1,2,4-triazolium) have been
employed in ILs for energy-rich applications such as explosive
and propellant fuels due to their large, positive heats of formation
(
densities and good thermal stabilities.
compared to the imidazolium cation), low melting points, high
7
-11
Despite their growing use, there remains a lack of
experimental data on more ‘fundamental’ triazolium ionic liquid
structures which would allow for direct correlation with
imidazolium and other IL systems. We recently reported the
synthesis and properties of a series of 1-alkyl-4-methyl-1,2,4-
2
Scheme 1. Synthesis of 1,2,3-triazolium [NTf ] ILs 3a-e.
Temperature-dependent viscosities (η), densities (ρ) and
molar conductivities (Λ ) were determined for each of the 1,2,3-
triazolium [NTf ] ionic liquids 3a-e over a range of 25-80 °C.
m
2
triazolium
bis(trifluoromethylsulfonyl)imide [NTf
ionic
liquids
which
]
utilized
anion.
the
It was
1
2,13
The results obtained at 30 °C are shown in Table 1. For
comparison, data obtained from freshly prepared samples of 1-
2
determined that increasing the alkyl chain length led to increased
viscosities, decreased molar conductivities, and improved long-
term thermal stabilities. Ultimately, the ‘ionicities’ of these ILs
butyl-3-methylimidazolium [NTf
2
] ([bmim][NTf
] ([bm124tri][NTf
Complete viscosity, density and molar conductivity
2
]) and 1-butyl-
4
included.
-methyl-1,2,4-triazolium [NTf
2
2
]) have been
12,25
(
i.e. the ability of a material to conduct a charge) were found to
profiles for 3a-e can be found in the supplementary data section.
Structure-activity trends for the 1,2,3-triazolium [NTf ] ILs
were found to be similar to previously reported imidazolium and
be poorer than their imidazolium analogs due to an increase in
cation-anion interactions from the enhanced Lewis acidity of the
2
1
4,15
1
,2,4-triazolium ring.
We have now turned our attention to the analogous 1,2,3-
triazolium-containing [NTf ] ILs. Recent publications have
13,24-26
1
,2,4-triazolium systems.
An increase in the alkyl chain
length leads to an increase in viscosity and a decrease in molar
conductivity due to an increase in van der Waals interactions
between the longer chains. 1-Dodecyl-3-methyl-1,2,3-triazolium
2
highlighted the utility of 1,2,3-triazolium ionic liquids beyond
energy rich applications, examples of which include highly-stable
dye-sensitized solar cells, liquid crystalline materials and
1
6
17
[NTf ] 3e was found to exhibit the highest viscosity (136.9 mP-s
2
2
-1
1
8,19
at 30 °C) and the lowest molar conductivity (0.23 S-cm mol ).
Density was found to decrease with increasing alkyl chain length.
Of greater interest than the variation in alkyl chain length was the
influence of the heterocyclic ring on the observed
physicochemical properties. 1-Butyl-3-methyl-1,2,3-triazolium
solvents in Baylis-Hillman reactions.
to report the synthesis of a series of 1-alkyl-3-methyl-1,2,3-
triazolium bis(trifluoromethylsulfonyl)imide [NTf ] ionic liquids
In this Letter, we wish
2
and compare the data with their imidazolium and 1,2,4-triazolium
counterparts.
[
and higher molar conductivity than [bm124tri][NTf
NTf
2
] 3a was found to display a significantly lower viscosity
], but a
2
Results and Discussion:
slightly higher viscosity and lower molar conductivity than the
[bmim][NTf2]. These observations correlate reasonably well
with the Lewis acidities of the heterocyclic ring (1,2,4-triazolium
The preparation of 1,2,3-triazoles and 1,2,3-triazolium ionic
liquids has become much more synthetically manageable with the
advent of the copper-catalyzed, Huisgen 1,3-dipolar
cycloaddition between an alkyne (typically terminal) and an
14,15
> 1,2,3-triazolium > imidazolium).
An increased Lewis
acidity corresponds to stronger cation-anion interactions, leading
to a decrease in ion mobility (i.e. higher viscosity and lower
molar conductivity).
20,21
azide.
This ‘click’ reaction has been modified in order to
prepare monosubstituted 1,2,3-triazoles through the use of
trimethylsilylacetylene as the alkyne source, followed by removal
of the 1 TMS group with tetrabutylammonium fluoride
Table 1. Physicochemical properties of ionic liquids at 30°C.
8,22
(
TBAF).
Thus, our preparation of the desired 1-alkyl-3-
2
Λ (S-cm mol )
-1
Ionic Liquid
bmim][NTf
[bm124tri][NTf
a (butyl)
b (hexyl)
3c (octyl)
η (mPa-s)
41.3
ρ (g/mL)
1.43
m
2
methyl-1,2,3-triazolium [NTf ] ionic liquids 3a-e began by
[
2
]
1.28
0.44
0.96
0.62
0.46
0.30
0.23
reacting the appropriate alkyl bromide with sodium azide,
followed by the addition of trimethylsilylacetylene and catalytic
copper(I) iodide, without isolation of the alkyl azide intermediate
2
]
79.2
1.48
3
45.0
1.46
(
Scheme 1). After removal of the DMF solvent, a solution of
3
54.7
1.41
TBAF was added. Workup and purification led to the isolation of
a series of 1-alkyl-1,2,3-triazoles 1a-e. (NOTE: We did not
encounter any issues with this process since the alkyl azide
intermediates were never isolated, however such azides are
72.3
1.33
3
d (decyl)
89.2
1.29
2
3
3e (dodecyl)
136.9
1.27
potentially explosive and proper precautions should be taken. )
Quaternization at the N-3 position was then accomplished with
iodomethane to provide 1-alkyl-3-methyl-1,2,3-triazolium
In addition to the individual physicochemical properties
explored above, it was of interest to determine the ‘ionicity’ of
ILs 3a-e. One multi-parameter scale that has been frequently
iodides
bis(trifluoromethylsulfonyl)imide (LiNTf
2a-e.
Anion
exchange
with lithium
resulted in the
2
)

3
used to quantify the ionicity of ILs is based upon the logarithmic
Walden Rule, which relates molar conductivity and inverse
2
4
viscosity:
-
1
log Λ
m
= log C + α log η
(1)
In equation (1), Λ
m
represents the molar conductivity of the IL, η
is the viscosity, and C is a constant that represents the deviation
from a standard set of parameters. The standard that is most
commonly used is a dilute solution of KCl, where the slope is α
N
N
N
=
its deviation from this ideal line, ILs can be classified as ‘good’,
1 and the intercept (log C) travels through origin. Based upon
NTf2
2
4
N
N
N
‘
poor’ or ‘superionic’.
A Walden plot for ILs 3a-e is shown in Figure 1. Overall,
the fitted data is grouped relatively close together, with an
NTf2
2
average standard deviation of ~ 0.1% and R > 0.998 for each
data set. As the alkyl chain length increases, so does the
deviation from the ideal KCl line, meaning that the ‘ionicity’
decreases as a function of increasing alkyl chain length. This
trend has been previously observed in analogous imidazolium
Figure 2. Walden plot comparison of butyl-substituted ILs.
1
3,24-26
Since Td5% has been shown to consistently overestimate the
thermal stability of ionic liquids since thermal decomposition can
and does occur well below this temperature, ‘isothermal’ TGA
and 1,2,4-triazolium systems.
1
1
0
0
.5
.0
.5
.0
KCl
line
experiments have become
consuming) tool to determine long-term thermal stability.
a more reliable (albeit time
27-29
The kinetics of decomposition in an isothermal TGA experiment
follows pseudo-zeroth order kinetics since the decomposition
process of the ionic liquid is assumed to lead to volatile
degradants, thus the concentration of cations and anions in the
bulk remains unchanged. A linear plot of weight % versus time
thus provides a slope equal to the rate constant kobs. Isothermal
TGA experiments were performed in duplicate for ILs 3a-e and
average zeroth-order rate constants kobs were determined at each
observed temperature. Using the Arrhenius equation for each set
butyl
3a
-
-
0.5
Λ
Λ
1.0
of isotherms, activation energies (E
a
) were determined for each
IL (R > 0.996), the results of which are provided in Table 2. A
rise in E was observed as the alkyl chain length was increased
2
-
1.0
-0.5
0.0 -1
0
-1.5-1
1.0
1.5
log (
η ) (P ·s )
a
from butyl 3a (113.7 kJ/mol) to dodecyl 3e (140.6 kJ/mol). For
comparison, the E for [bmim][NTf ] was found to be higher
(130.7 kJ/mol) while the E for [bm124tri][NTf ] was found to be
a 2
Figure 1. Walden plot for 1-alkyl-3-methyl-1,2,3-triazolium
NTf ] ILs 3a-e.
a
2
[
2
lower (110.3 kJ/mol).
A Walden plot comparison of 1-butyl-3-methyl-1,2,3-
triazolium [NTf ] 3a, [bmim][NTf ] and [bm124tri][NTf ] is
2
2
2
Table 2. Thermal properties of 1-alkyl-3-methyl-
1,2,3-triazolium [NTf ] ionic liquids 3a-e.
shown in Figure 2. It is clear from this plot that the 1,2,4-
triazolium analog has the largest deviation from the ideal KCl
line (i.e. lowest ionicity) of the group, which was expected
considering that the 1,2,4-triazolium ring exhibits the highest
Lewis acidity. What is interesting to note is that the ionicity of
the 1,2,3-triazolium IL was found to be nearly identical to that of
2
Tg
(°C)
Tm
(°C)
Td5%
Ea
T0.01/10
(°C)
Ionic Liquid
bmim][NTf
(°C) (kJ/mol)
[
2
]
-85.5 -3.5 385
130.7
110.3
113.7
124.7
127.1
133.7
140.6
196
175
187
193
199
202
205
[
bm124tri][NTf
2
]
-71.6
-81.0
-78.4
--
--
--
--
337
345
333
[
bmim][NTf
2
].
Thermogravimetric analysis (TGA) was used to determine
3a (butyl)
3
b (hexyl)
both the relative, short-term thermal stabilities (‘temperature
ramp’ experiments) and the long-term thermal stabilities
3
c (octyl)
-24.7 329
1.6 337
17.4 328
3
d (decyl)
--
(‘isothermal’ experiments) of ILs 3a-e. From each temperature-
ramped TGA experiment, Td5% values (temperature at which 5%
of the material has decomposed) were determined (Table 2;
reproducible within + 2 ˚C from duplicate runs). From the data
collected, there does not appear to be any relationship between
alkyl chain length and relative, short-term thermal stabilities,
although the butyl system 3a did exhibit the highest overall Td5%
3
e (dodecyl)
--
In an attempt to directly compare long-term thermal
stabilities from isothermal TGA experiments, the value “Tx/z” has
been developed, where x represents the extent of decomposition
(
h).
typically this is 0.01% or 1%) over a length of time z (e.g., 10
(
341 °C). As evident from the data in Table 2, the thermal
27-29
Essentially, such an exercise allows for the determination
stabilities of the 1,2,3-triazolium ILs were comparable to the
analogous 1,2,4-triazolium systems but lower than those
employing the imidazolium cation.
of the % decomposition of an IL over a period of time at a
specified operating temperature. To determine T0.01/10 specifically,
a plot of t99% versus temperature (K) is generated from each set of
IL isotherms, where t99% represents the time necessary for 1% of
the IL to decompose at each isothermal temperature.
Extrapolation of the resulting exponential curve to a time of 10h
provides the corresponding T0.01/10 value. A sample plot of t99%
2
5,26

4
Tetrahedron
versus K for 1-butyl-3-methyl-1,2,3-triazolium [NTf
shown in Figure 3. The remaining plots for 1,2,3-triazolium ILs
b-e are provided in the Supplementary Data section.
2
] IL 3a is
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2
1-dodecyl-3-methyl-1,2,3-triazolium [NTf ] 3e exhibiting the
highest T0.01/10 value at 205 °C (Table 2). Such a trend may
simply be due to increasing the molecular weight of the IL. It is
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was found to be higher than [bm124tri][NTf
bmim][NTf ], which follows the relative Lewis acidities of the
heterocyclic rings.
In summary, a series of 1-alkyl-3-methyl-1,2,3-triazolium
NTf ] ionic liquids were prepared and their physicochemical and
2
] but less than
[
2
14,15
[
2
thermal properties determined. An increase in alkyl chain length
led to an increase in viscosity, a decrease in molar conductivity
and, as a result, a decrease in ionicity as determined from Walden
plot analysis. Long-term, isothermal TGA studies showed an
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increase in E
length. It was also found that the ionicity of 1-butyl-3-methyl-
,2,3-triazolium [NTf ] was similar to [bmim][NTf ] and a
significant improvement over [bm124tri][NTf ].
a
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1
13
(
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3
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Acknowledgements
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund (PRF# 53097-UNI7)
as well as to the Department of Chemistry and the Committee on
Institutional Studies and Research (Presidential Research Award
-
(
KMM) at Murray State University. All thermal measurements
DSC and TGA) were conducted in the Polymer and Materials
Characterization Laboratory (PMCL) at Murray State University.
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