1,3-di(alkoxy)imidazolium-based ionic liquids: Improved synthesis and crystal structures

Article abstract of DOI:10.1071/CH12485

A new and convenient synthetic pathway to 1,3-di(alkoxy)imidazolium bis(trifluoromethylsulfonyl)amides and novel 1,3-di(alkoxy)imidazolium tetrachloroferrates was developed. As an intermediate isolation step of the respective hexafluorophosphates was required in previously reported preparations, they suffered from low overall yields and additional expense. The use of FeCl3/HCl resulted in substantially improved yields and allows one-pot preparations with good scalability. Results of single-crystal X-ray structure determination of the new tetrachloroferrate salts are discussed.

Full text of DOI:10.1071/CH12485

CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 391–395
Communication
http://dx.doi.org/10.1071/CH12485
1,3-Di(alkoxy)imidazolium-based Ionic Liquids:
Improved Synthesis and Crystal Structures
A B D
, ,
A
A
Carmen Froschauer,
Robert Salchner, Gerhard Laus,
B
C
A
A
Hedda K. Weber, Richard Tessadri, Ulrich Griesser, Klaus Wurst,
C
A
Volker Kahlenberg, and Herwig Schottenberger
A
Faculty of Chemistry and Pharmacy, University of Innsbruck, Innrain 80–82,
6020 Innsbruck, Austria.
B
Competence Center of Wood Composites and Wood Chemistry K-Plus, Altenberger
Strasse 69, 4021 Linz, Austria.
C
Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52,
6020 Innsbruck, Austria.
D
Corresponding author. Email: carmen.froschauer@uibk.ac.at
A new and convenient synthetic pathway to 1,3-di(alkoxy)imidazolium bis(trifluoromethylsulfonyl)amides and novel
1,3-di(alkoxy)imidazolium tetrachloroferrates was developed. As an intermediate isolation step of the respective
hexafluorophosphates was required in previously reported preparations, they suffered from low overall yields and
additional expense. The use of FeCl3/HCl resulted in substantially improved yields and allows one-pot preparations with
good scalability. Results of single-crystal X-ray structure determination of the new tetrachloroferrate salts are discussed.
Manuscript received: 24 October 2012.
Manuscript accepted: 19 January 2013.
Published online: 22 February 2013.
Ionic liquids (IL) are defined as salts melting below 1008C. Over
the past decades, their development has been fast-paced and
offers unexpected opportunities in numerous fields ranging
from electrochemistry to biology.[1] Their recognition as aca-
demic curiosities has shifted meanwhile to rank them as indis-
pensable technical tools for process design and engineering. The
large number of organic cations and anions that can be combined
to form different IL is one of the most important conceptual
features of these compounds, allowing a wide range of tailorable
physicochemical properties (conductivity, density, viscosity,
polarity, solubility).[2] In particular, the highly versatile imid-
azole parent structure gives access to an exceptional variability
of substitution patterns with extreme synthetic ease, thus even
impacting the related chemical realm of nitrogen heterocyclic
(NHC) carbenes.[3] Regarding anions, special chemical and
theoretical interest has emerged in hydrophobic IL, mainly
based on the bis(trifluoromethylsulfonyl)amide (‘triflimide’)
anion.[4] Owing to their low residual water content, their
hydrolytic and chemical stability and low viscosity, all these
classes of IL are valuable sorption media for greenhouse gas-
es,[5,6] electrolytes,[7] agents for NHC-mediated transition metal
catalysis,[8] and sacrificial substances in radiation-induced
fragmentations.[9]
IL degradation is still in its infancy. Comparative toxicity
and biodegradability studies of IL based on 1-alkyl-3-
methylimidazolium cations with or without different oxygenated
functional groups[2,16] (hydroxyl, ester, and ether) revealed
that the presence of oxygen groups on the imidazolium cation
reduces the toxicity of the respective triflimides or octyl sul-
fates.[17] Furthermore, the presence of ester functions, especially
with long alkyl side chains, renders the IL more easily (but only
partially) biodegradable. Unfortunately, the (quaternized) imi-
dazolium core was reported to be resistant to biological and
abiotic degradation.[17,18] Principally, a partial biodegradation
of imidazolium salts by alteration or removal of a side chain
functionality does not justify the presumption of overall reduced
toxicity.[19] A successful tool for effective cation degradability
is the attachment of side chains by weakly bonded links, offering
intentional chemical scission. This can be conveniently realized
by the incorporation of N–O single bonds, as already described
for alkoxypyridinium, alkoxymorpholinium,[19] 1-alkyl-3-
alkoxyimidazolium,[20] and di(alkoxy)imidazolium[21–23] moi-
eties, which bear the fundamental advantage of cationic core
degradability by complete removal of the entire side
chains.[24,25] On fragmentation (e.g. by stepwise hydrogena-
tion),[22] such 1,3-di(alkoxy)imidazolium-based IL are convert-
ible to the parent structure of imidazole,[26,27] which has been
identified as a non-toxic natural compound, abundant in edible
lentils.[28] Another mode of (partial) core degradation was
recently demonstrated by mild alkaline hydrolysis.[29,30] In such
degradation steps, the essential triflimide anion can be recycled
unchanged.
Despite growing ecological and toxicological con-
cerns,[10–12] IL may, under strict measures of eco-containment,
even be considered environmentally friendly. This predicate is
attributable to their mostly non-volatile nature,[13,14] and qua-
lifies them as greener alternatives to traditional volatile organic
solvents.[15] In general, the concept of deliberate chemical
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

392
C. Froschauer et al.
R
O
N
ϩ
N
Ϫ
A
FeCl4
1) NaHCO3
2) FeCl3/HCl
R
O
R
O
OH
1a: R CH3
Ϫ
R
O
O
N
O
S
1b: R CH2CH3
N
ϩ
N
ϩ
N
LiNTf2
S
F
F
F
S
O
O
ϩ
O O
F
F
N
O
F
R
R
O
O
R
O
3a: R CH3
3b: R CH2CH3
,
A: 74 %, B: 69 %
A: 74 %, B: 74 %
,
N
B
Ϫ
Cl
ϩ
1) NaHCO3
2) HCl
N
R
O
(not isolated)
2a: R CH3
2b: R CH2CH3
Scheme 1. Improved synthesis of 1,3-di(alkoxy)imidazolium triflimides.
Besides physical property profiles and safety requirements,
good accessibility in terms of preparative efficiency and afford-
ability are relevant for both laboratory and larger scales. As a
consequence, aqueous reaction media are frequently used, espe-
cially when alkyl sulfates are used for alkylation[31] as is the case
with the synthesis of 1,3-di(alkoxy)imidazolium salts. After the
starting compound 1-hydroxyimidazole-3-oxide is prepared by
well-proved literature procedures,[26,32,33] alkylation is carried
out using dialkyl sulfates to give the respective 1,3-di(alkyloxy)
imidazolium alkyl sulfate intermediates, which are difficult to
isolate. A change of the alkylation medium would not really be
practicable, becausehydroxyimidazole N-oxidesareonly readily
soluble in water. Hitherto, the cation prepared had to be isolated
by precipitation as a hexafluorophosphate from the aqueous
reaction mixtures (yield of this step 73 %). Ion metathesis using
lithium triflimide and extraction with dichloromethane yielded
the hydrophobic triflimides (yield for this step 78 %, resulting in
an overall yield of 57 %).[24] In order to skip the elaborate and
expensive intermediate hexafluorophosphate-based isolation
step, convenient procedures to 1,3-di(alkyloxy)imidazolium tri-
flimides were considered. For instance, the direct ion metathesis
of 1,3-di(alkyl)imidazolium alkyl sulfates using lithium trifli-
mide yields up to 81 %.[34] Unfortunately, this method is not
suitable for similar 1,3-di(alkoxy)imidazolium alkyl sulfates
(yield ,40 %), because too much of the residual alkyl sulfate
is soluble in the triflimide-containing organic phase. Therefore,
the respective direct metathesis to analytically pure triflimides
would require the use of considerably more than stoichiometric
equivalents of lithium triflimide and this consequently renders
the approach economically and ecologically unacceptable.
Based on the experience that even on the combination of
aqueous solutions of 1,3-di(alkyl)imidazolium chlorides with
aqueous solutions of ferric chloride,[35] the resulting paramag-
netic tetrachloroferrates form binary phase systems, we were
curious if this low affinity for H2O or, preferably, H2O/HCl
could be exploited for workup purposes from aqueous dialkyl
sulfate-based alkylation mixtures, as the new tetrachloroferrates
are interesting IL in their own right.[36]
Scheme 1). Fortunately, the entire reaction sequence from
the starting 1-hydroxyimidazolium-3-oxide to 1,3-di(alkoxy)
imidazolium triflimides (3a, 3b) can be conducted in a one-
pot synthesis, as the desired triflimide product can be extracted
in a final step with dichloromethane, resulting in 74 % overall
yield. Although methyl sulfates are known for their tendency to
migrate partially into the organic phase, none was detectable in
the resulting products. Products from pathway A show a fairly
low iron content of ,3 ppm and a lower chloride content
(0.04 mg Cl gꢀ1 IL) than their analogues from pathway B
(2.3 mg Cl gꢀ1 IL).
Consequently, two affordable methods led to the same target
product. Pathway A, using FeCl3/HCl, allows the isolation of the
two solid intermediates 1,3-dimethoxyimidazolium tetrachloro-
ferrate (1a) and 1,3-diethoxyimidazolium tetrachloroferrate
(1b). Single crystals suitable for X-ray measurements of 1a
were directly harvested from the mother liquor. Single-crystal
X-ray diffraction analysis of the salts revealed interesting
information about interionic interactions and conformations of
the ions involved. Crystal data and structure refinement details
are summarized in Table 1. The respective distances and angles
are given in Table 2. The asymmetric unit of 1a contains two
independent ion pairs. The tetrachloroferrate(III) ions deviate
only slightly from ideal tetrahedral geometry. One cation adopts
a syn and the other an anti conformation with respect to the
imidazole plane. The methoxy groups are twisted out of the
imidazole plane by 888 in the syn and by 908 and 778 in the anti
conformers. Interplanar angles are calculated from the least-
squares plane N1–C1–N2–C2–C3 and the pertinent N–O–C
planes. Hydrogen atoms of the imidazolium ring and of the
methoxy groups are engaged in short contacts with the chlorine
atoms of the anions, as shown in Fig. 1a. In a previous report on
1,3-di(alkoxy)imidazolium salts,[24] two distinct conformations,
syn and anti, of the alkoxy groups with respect to the imidazo-
lium ring plane were described only for two polymorphs of a
hexafluorophosphate. In the crystal structure of 1b, the imida-
zolium cation displays several short C–H?Cl contacts to four
surrounding tetrachloroferrate(III) anions (Fig. 1b). The ethoxy
groups are arranged in anti conformation, rotated out of the ring
plane by 898 and 658 respectively.
The replacement of expensive ammonium hexafluorophos-
phate by ferric chloride/hydrochloric acid[24] resulted in a cost-
effective, improved synthesis via tetrachloroferrates (1a, 1b,
pathway A, Scheme 1) and chlorides (2a, 2b, pathway B,
It was not possible to isolate the intermediate chlorides
(2a, 2b) of pathway B. Nevertheless, this did not affect the

1,3-Di(alkoxy)imidazolium-based Ionic Liquids
393
Table 1. Crystal data and structure refinement details
Compound
1a
1b
3b,A
CCDC no.
903893
903894
903895
Chemical formula
C5H9N2O2ꢁCl4Fe
326.79
C7H13N2O2ꢁCl4Fe
354.84
C7H13N2O2ꢁC2F6NO4S2
437.34
Mr
Crystal size [mm3]
Crystal system
Space group
0.40 ꢂ 0.25 ꢂ 0.22
Monoclinic
P21
0.30 ꢂ 0.20 ꢂ 0.12
Monoclinic
P21/c
0.31 ꢂ 0.18 ꢂ 0.15
Monoclinic
P21/n
˚
a [A]
˚
b [A]
˚
c [A]
7.1156(2)
14.3223(3)
12.6720(5)
91.846(2)
1290.76(7)
4
8.3306(3)
13.4364(4)
13.3154(4)
91.805(3)
1489.70(8)
4
12.1565(4)
11.5005(3)
12.8449(4)
98.905(2)
1774.15(9)
4
b [8]
3
˚
V [A ]
Z
Dx [g cmꢀ3
]
1.68
1.58
1.64
m [mmꢀ1
F(000) [e]
]
1.97
1.72
0.39
652
716
888
Diffractometer
Nonius Kappa CCD
f and v scans
233
Oxford Gemini Ultra
v scans
173
Nonius Kappa CCD
f and v scans
233
Data collection method
Temperature [K]
ymax [8]
h, k, l range
27.5
25.4
25.0
ꢀ9 # h # 8
ꢀ18 # k # 18
ꢀ15 # l # 16
8847
ꢀ10 # h # 7
ꢀ14 # k # 16
ꢀ14 # l # 16
8754
ꢀ14 # h # 14
ꢀ13 # k # 13
ꢀ15 # l # 13
9407
Measured reflections
Independent reflections (Rint
)
5626 (0.039)
5230
2714 (0.027)
2324
3121 (0.022)
2605
Observed reflections [I $ 2s(I)]
Restraints/parameters
R1/wR2 [I $ 2s(I)]
1/254
0/147
0/235
0.038/0.098
0.042/0.101
1.04
0.025/0.057
0.032/0.061
1.03
0.036/0.093
0.045/0.099
1.05
R1/wR2 (all data)
Goodness-of-fꢀit3
˚
Drmax/min [eA
]
0.75/–0.33
0.45/–0.40
0.27/ꢀ0.33
Table 2. Hydrogen bond parameters in the crystal structures of 1a, 1b, and 3b,A
˚
˚
Interaction
H?A [A]
D?A [A]
DH?A [8]
Symmetry operation A
1a
C3–H?Cl3
C9–H?Cl2
C1–H?Cl2
C4–H?Cl6
C6–H?Cl8
C10–H?Cl7
1b
2.802(1)
2.883(1)
2.801(1)
2.828(1)
2.733(1)
2.820(1)
3.725(5)
3.663(5)
3.567(4)
3.686(6)
3.575(5)
3.780(1)
167.6(3)
138.2(4)
139.5(3)
147.8(3)
149.5(3)
171.2(6)
x, y, z
x, y, z
1 ꢀ x, 1/2 þ y, 2 ꢀ z
ꢀ1 þ x, y, z
x, y, 1 þ z
2 ꢀ x, 1/2 þ y, 2 ꢀ z
C3–H?Cl3
C4–H?Cl3
C2–H?Cl1
C1–H?Cl4
3b,A
2.7540(6)
2.8315(6)
2.8788(6)
2.7360(6)
3.410(2)
3.664(3)
3.778(2)
3.621(3)
126.8(1)
142.2(1)
158.2(1)
155.2(1)
x, y, z
ꢀ1 þ x, y, z
1 ꢀ x, ꢀ1/2 þ y, 1.5 ꢀ z
ꢀ1 þ x, 1/2 ꢀ y, 1/2 þ z
C2–H?N3
C3–H?O5
C1–H?O4
C1–H?O6
2.491(2)
2.487(2)
2.247(2)
2.505(2)
3.296(3)
3.357(3)
3.137(3)
3.156(3)
143.7(1)
154.0(1)
157.5(1)
126.5(1)
x, y, z
1 ꢀ x, 1 ꢀ y, ꢀz
ꢀ1/2 þ x, 1/2 ꢀ y, 1/2 þ z
ꢀ1/2 þ x, 1/2 ꢀ y, 1/2 þ z
yield of the target triflimides. As the FeCl4-based IL are valuable
isolable intermediates for further ion metathesis, pathway A
may be more favourable for such purposes.
During the elaboration of these new methods, it surprisingly
turned out that one of our target IL, 1,3-diethoxyimidazolium
triflimide (3b), which was reported to be a liquid at room
temperature, wasactuallya subcooledmelt, whichnowsolidified
at room temperature (melting point ,318C). Single crystals of
3b,A and 3b,B were grown from dichloromethane at room
temperature. In the crystal structure of 3b,A, the imidazolium
ring protons are involved in quite short contacts with the N and O
atoms of three triflimide anions (Fig. 1c). The ethoxy groups are
rotated out of the ring plane by 788 and 898, adopting an anti
conformation. The triflimide anion adopts the trans conforma-
tion with a C8–S1?S2–C9 torsion angle of 1778. When we had
seed crystals of 3b,A at hand, it was even possible to crystallize
the formerly liquid 1,3-diethoxyimidazolium triflimide prepared
by the previous protocol (via the hexafluorophosphate

394
C. Froschauer et al.
(a)
of IL research and catalysis. By the development of the synthesis
described herein, these ILs are now more easily accessible in
bulk. Usually, when new compounds are introduced, their
application potential is not completely exploited, and also the
experimental procedures for their preparation are not fully
optimized. Therefore, this resumption and development of the
synthesis of these special classes of compounds may contribute
to the generation of further interest in the wider community and
propagate their application.[37]
Cl6
C2
Fe2
C4
O1
C1
N1
C3
N2
CI3
Fe1
CI2
O2
Fe1
CI7
Experimental
C9
CI2
Crystal data were recorded using an Oxford Diffraction
Gemini-R Ultra diffractometer and a Nonius Kappa Charges
Coupled Device diffractometer (radiation type for all structures:
MoKa). Crystallographic data reported in this manuscript have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC-903893,
CCDC-903894, CCDC-903895. Copies of the data can be
obtained free of charge from the Cambridge Crystallographic
Data Centre.
N4
O4
O3
N3
C6
C10
Fe2
CI8
Fe2
(b)
General Procedure for the Preparation of 1,3-Di(alkoxy)
imidazolium Tetrachloroferrates (1a, 1b) and
Bis(trifluoromethylsulfonyl)amides (3a, 3b)
Fe
CI3
CI1
A mixture of the respective dialkyl sulfate (2 equiv.) and
1-hydroxyimidazole 3-oxide (1 equiv.) was stirred at room
temperature for 1.5 h. Then NaHCO3 (1 equiv.) was added and
stirring was continued overnight. After the addition of H2O
and more stirring, two pathways were pursued.
In pathway A, HCl (37 wt-% aqueous solution, 1 equiv.) and
FeCl3 (1 equiv.) were added in succession to the clear solution,
which resulted in an additional FeCl4 phase. This magnetic
phase could be separated by the addition of a few millilitres of
dichloromethane in a separatory funnel. After removal of the
solvent, several analyses were conducted on these intermediate
tetrachloroferrate salts (1a, 1b).
C2
C3
N2
N1
O1
C4
C1
O2
CI4
CI3
(c)
In pathway B, the addition of HCl only (37 %, 1 equiv.)
resulted in a clear solution.
Lithium triflimide (1 equiv.) was added directly to the
respective aqueous solutions from both pathways in order to
obtain the hydrophobic 1,3-di(alkoxy)imidazolium triflimides
by ion metathesis. Stirring at room temperature overnight and
extraction with dichloromethane yielded the products 3a and 3b.
C9
O2
O5
O6
N2
S2
C3
Supplementary Material
C1
N1
O4
S1
C2
Further details about experimental procedures and analytical
data, including differential scanning calorimetry and thermo-
gravimetry, are available on the Journal’s website.
C8
O1
N3
Acknowledgements
Financial support was provided by the Austrian government, the provinces
of Lower Austria, Upper Austria, and Carinthia as well as by Lenzing AG.
We also express our gratitude to Johannes Kepler University, Linz, the
University of Natural Resources and Applied Life Sciences, Vienna,
Lenzing AG, and Sandoz AG for their in-kind contributions.
Fig. 1. Interionic hydrogen bonds in the crystal structures of (a) 1a; (b) 1b;
and (c) 3b,A.
intermediate). This is also true of liquid samples purchased from
commercial sources (Sigma–Aldrich). Further data (electro-
chemical window, dynamic viscosity, specific conductivity,
thermal stability) of the liquid triflimides are given elsewhere.[24]
In conclusion, there are various needs for hydrophobic, core-
degradable 1,3-di(alkyloxy)imidazolium triflimides in the fields
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