New hydrogen carbonate precursors for efficient and byproduct-free syntheses of ionic liquids based on 1,2,3-trimethylimidazolium and N,N-dimethylpyrrolidinium cores

Article abstract of DOI:10.1039/b920003g

Two new hydrogen carbonate IL precursors, 1,2,3-trimethylimidazolium and N,N-dimethylpyrrolidinium hydrogen carbonate salts, were synthesized and their structures confirmed by NMR and single-crystal X-ray diffraction. These salts were also evaluated for application in the syntheses of ILs by reacting them with a variety of acids and [NH₄][ClO₄], which resulted in the clean and quantitative formation of a family of 1,2,3-trimethylimidazolium- and N,N-dimethylpyrrolidinium-based salts. Synthetic protocols for the formation of the hydrogen carbonate salts involved simple alkylation reactions of the chosen neutral amines with dimethyl carbonate, and later conversion of the formed methyl carbonate anion-based salts to hydrogen carbonate salts. The reactions proceed in one step at temperatures close to room temperature using only water. The new organic salts with the chosen anions are formed with only gaseous byproducts (CO₂, H₂O, and in the case of [NH ₄][ClO₄], NH₃), thus eliminating further purification steps. This generalized synthetic protocol for the formation of hydrogen carbonate IL precursors may be used as a cleaner, contaminant-free (halides and metal ions) route to many classes of ILs.

Full text of DOI:10.1039/b920003g

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PAPER
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New hydrogen carbonate precursors for efficient and byproduct-free
syntheses of ionic liquids based on 1,2,3-trimethylimidazolium and
N,N-dimethylpyrrolidinium cores†
Marcin Smiglak, C. Corey Hines‡ and Robin D. Rogers*
Received 29th September 2009, Accepted 14th December 2009
First published as an Advance Article on the web 29th January 2010
DOI: 10.1039/b920003g
Two new hydrogen carbonate IL precursors, 1,2,3-trimethylimidazolium and
N,N-dimethylpyrrolidinium hydrogen carbonate salts, were synthesized and their structures
confirmed by NMR and single-crystal X-ray diffraction. These salts were also evaluated for
application in the syntheses of ILs by reacting them with a variety of acids and [NH4][ClO4], which
resulted in the clean and quantitative formation of a family of 1,2,3-trimethylimidazolium- and
N,N-dimethylpyrrolidinium-based salts. Synthetic protocols for the formation of the hydrogen
carbonate salts involved simple alkylation reactions of the chosen neutral amines with dimethyl
carbonate, and later conversion of the formed methyl carbonate anion-based salts to hydrogen
carbonate salts. The reactions proceed in one step at temperatures close to room temperature
using only water. The new organic salts with the chosen anions are formed with only gaseous
byproducts (CO2, H2O, and in the case of [NH4][ClO4], NH3), thus eliminating further purification
steps. This generalized synthetic protocol for the formation of hydrogen carbonate IL precursors
may be used as a cleaner, contaminant-free (halides and metal ions) route to many classes of ILs.
reactions. Ideally, in order to obtain pure IL salts, the synthesis
Introduction
would have to be carried out directly with routes that are
One of the most challenging problems faced by researchers
byproduct-free or that allow facile removal of byproducts by
working with ionic liquids (ILs, defined as salts which melt below
their evaporation, rather than by extraction.
100 ◦C1), is difficulty in the reliable and reproducible preparation
The preparation of ILs via simple methods that result in
of pure compounds. Unfortunately, current methods for prepa-
formation of the desired salts and easily separated byproducts
ration of various ILs often involve multi-step synthesis, complex
has been investigated for quite some time. Ohno and coworkers11
purification, and very often result in formation of undesirable
have suggested using an ion exchange column in order to prepare
and difficult to remove halide- or metal-containing species,
hydroxide derivatives of ILs that later could be reacted with acid,
including byproducts from metathesis reactions, incomplete
forming the desired product with little but H2O as a byproduct.
ion exchange, or contamination from clean-up columns.2–5
In our work we have suggested,12 along with Tommasi and
Thus, improved techniques for IL synthesis and purification
Sorrentino,13 the use of dialkylimidazolium-2-carboxylates and
would drive the end product cost down and provide higher
decarboxylation reactions in the presence of acids as robust
purity materials and consistency between batches, even between
IL precursors for the formation of pure, byproduct-free ILs.
different suppliers.
Utilizing this methodology, our laboratory successfully prepared
Limited examples of imidazolium- or other cation-based
the first proof of a stable dialkylimidazolium [HCO3]- salt that
halide-free synthesis of ILs are present in the literature; the most
was then shown to easily react with acids.14 Parallel to this
commonly known are replacements for haloalkyl alkylating
work, recent patent literature has also proposed the usefulness
agents like dialkyl sulfates6,7 and dialkyl carbonates.8 More
of a [HCO3]--based IL for the halide-free synthesis of many
recently, the use of alkane sulfonates9 and alcohols in Mitsunobu
ILs through titration with a Brønsted acid,15 and these salts
alkylation reactions10 for the synthesis of halide-free alkylated
have also recently appeared in the Sigma-Aldrich catalog.16
IL materials were reported. These salts, though halide-free,
were then typically used as precursors for further ion exchange
Moreover, Zheng et al. report the formation of carbonate-
based tetraalkylammonium salts via the reaction of ammonium
carbonate with two equivalents of dimethyl carbonate, but with
greatly depressed yields in the area of 2–45%.17
Center for Green Manufacturing and Department of Chemistry, The
Although the utilization of zwitterionic carboxylate precur-
sors and their use in halide-free synthesis of ILs as presented
in our previous report appears attractive,18 certain drawbacks
of the system suggested that further research was needed
toward other “green” IL precursors. While working on the
imidazolium carboxylate zwitterionic systems, a few limitations
University of Alabama, Tuscaloosa, AL, 35487, USA
† Electronic supplementary information (ESI) available: Tables S1
and S2. CCDC reference numbers 752754–752756. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b920003g
‡ Current address: Nuclear Radiation Center, Washington State Univer-
sity, Pullman, WA 99164, USA.
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were recognized, including: (i) the necessity to use heterocyclic
cation cores for formation of zwitterionic carboxylates, (ii)
synthesis of the zwitterionic imidazolium carbonate precursor
is a two-step reaction, where the second step (carboxylation) is
rate-limiting, and often low-yielding, (iii) possible formation of
regioisomeric products during the zwitterion synthesis having
different reactivities in subsequent reactions, and (iv) need for
use of low-volatile polar aprotic solvents such as DMSO for
decarboxylation, which is very hygroscopic and hard to remove
from the reaction mixture.
In our recent publication,14 we focused on the synthe-
sis of a hydrogen carbonate IL precursor by using the
previously described conditions for the decarboxylation of
the 1,3-dimethylimidazolium-2-carboxylate ([1,3-diMeIM-2-
COO]) zwitterion. We have utilized the ability of [1,3-diMeIM-
2-COO] to react with acids in polar solvents, followed by the
zwitterionic decarboxylation, resulting in the formation of a
new salt with a counterion from the acid used in the process.
Carbonic acid (H2CO3) was used to promote the protona-
tion of the 2-carboxylate moiety, initially forming 2-carboxy-
1,3-dimethylimidazolium hydrogen carbonate ([2-(COOH)-1,3-
diMeIM][HCO3]), followed by the Krapcho decarboxylation re-
action, with the [HCO3]- anion serving as the weak nucleophile.
This reaction yielded formation of 1,3-dimethylimidazolium
hydrogen carbonate ([1,3-diMeIM][HCO3]).
We have also noted that that when [1,3-diMeIM][HCO3]
is dissolved in pure MeOH, it slowly converts into [1,3-
diMeIM][MeCO3] (Fig. 1a). As expected, this methyl carbonate
anion in the presence of a strong protic acid could still undergo
the decomposition reaction, forming MeOH, H2O, CO2, and
a new salt. Similarly, Tommasi and Sorrentino19 described
the utilization of 1,3-dialkylimidazolium-2-carboxylates as CO2
carriers for transcarboxylation reactions; for instance, the zwit-
terionic carboxylate was reacted with dry methanol and NaBF4
or NaPF6 to form dialkylimidazolium [BF4]- or [PF6]- salts and
sodium methyl carbonate (Fig. 1b).
the [HCO3]- anion, or alcohol and CO2, depending on reaction
conditions.
Pocker et al.20 investigated the water- and acid-catalyzed
decarboxylation of monosubstituted derivatives of carbonic acid
(Fig. 1c), and concluded that the alkyl carbonate anions behave
similarly to the [HCO3]- anion in the presence of an acid and
decompose to CO2 gas and alcohol, as opposed to CO2 and
H2O as in case of the [HCO3]- anion. Another interesting result
of this investigation was that the reaction of alkyl carbonate
anions with water results in the decomposition of the alkyl
carbonate anion with formation of an alcohol and [HCO3]-
anion. Similar findings have been reported by Mori et al.,21 where
the authors describe the conversion of tetraalkylammonium
methyl carbonate to the corresponding hydrogen carbonate salt
by addition of water. Unfortunately, no experimental data was
presented to support this hypothesis in the patent.
The results from the literature on the behavior of alkyl
carbonates in water or in acids point to the conclusion that
the reactions presented earlier by our group (Fig. 1a) might be
reversible, depending on the solvent used. Thus it is anticipated
that based on the solvent used for the dissolution of [MeCO3]--
or [HCO3]--containing salts, interconversion reactions may be
seen between both anions, with both potentially present at the
same time in equilibrium.
In this work, we have extended our research on the utility
of dialkyl carbonates as alkylating agents to form new [HCO3]-
salt precursors for further halide-free syntheses of other ILs. In
order to obtain [MeCO3]--based salts, we targeted two amines
as prototypes: (i) 1,2-dimethylimidazole, since the carboxylation
reaction at the C2 carbon during the imidazole alkylation
process is prevented due to methyl group substitution (the
carboxylation at C4/C5 position requires higher temperatures
and should not occur with appropriate reaction conditions),
and (ii) N-methylpyrrolidine, which due to lack of aromaticity
or other conjugation, is less likely to undergo any carboxylation
reaction to form a zwitterionic compound.
Both reactions presented in Fig. 1 suggest the possibility of
forming the [MeCO3]- anion directly from both the zwitterionic
carboxylate precursor, as well as from the [HCO3]- salt in the
presence of dry MeOH. On the other hand, even though limited
to only two literature examples, studies by Pocker et al.20 and
Mori et al.21 helped to clarify the mechanism of the reaction
as outlined in Fig. 1c, and showed that the reverse reaction is
also possible. In the reverse pathway, the [MeCO3]- anion reacts
with H2O and acids, with the consecutive formation of either
We report (Fig. 2) the formation of two new examples of
[HCO3]- IL precursors, 1,2,3-trimethylimidazolium hydrogen
carbonate ([1,2,3-triMeIM][HCO3]) and N,N-dimethylpyrro-
lidinium hydrogen carbonate ([N,N-diMePyr][HCO3]) where
the [HCO3]- anion is generated by conversion of the
[MeCO3]- anion in the presence of excess water, and [1,2,3-
triMeIM][MeCO3]. The crystallographic characterization of
highly deliquescent crystals of [1,2,3-triMeIM][HCO3]·H2O and
[N,N-diMePyr][HCO3] was performed in order to further
Fig. 1 Literature examples of possible conversions between [HCO3]- and [MeCO3]- anions depending on the reaction conditions and starting
materials: a) conversion of [HCO3]- anion to [MeCO3]- in the presence of excess MeOH; b) conversion of dialkylimidazolium-2-carboxylate to
dialkylimidazolium [BF4]- salt in the presence of MeOH and inorganic salt; c) conversion of [MeCO3]- anion to [HCO3]- anion in the presence of an
excess amount of H2O.
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Fig. 2 Efficient, three-step synthesis of halide-free imidazolium- and pyrrolidinium-based organic salts; a) alkylation reaction with formation of
[MeCO3]- salts; b) conversion of [MeCO3]- into [HCO3]-; c) reaction of [HCO3]- salt precursor with acid and formation of final product.
validate product formation of the three new IL precursors.
The resulting salts were later used in reactions with several
acids to prove their utility as IL precursors for the fast and
efficient synthesis of imidazolium- and pyrrolidinium-based
organic salts.
Results and discussion
Reactivity of 1,2,3-trimethylimidazolium methyl carbonate
([1,2,3-triMeIM][MeCO3])
Fig. 4 Possible products from the reaction of 1,2-diMeIM with DMC
(highlighted area) and their expected reactivities in the presence of
different deuterated solvents: a) polar aprotic solvent, trace H2O,
atmospheric CO2; b) MeOD; c) D2O.
The salt 1,2,3-trimethylimidazolium methyl carbonate ([1,2,3-
triMeIM][MeCO3]) was synthesized by alkylation of 1,2-
dimethylimidazole with dimethyl carbonate, at 70 ◦C for
10 days in a sealed pressure tube (Fig. 3a).18 Because of
the possible reaction outcomes (i.e., formation of the desired
[1,2,3-triMeIM][MeCO3] or the 1,2,3-trimethylimidazolium-4-
carboxylate ([1,2,3-triMeIM-4-COO]) byproduct (Fig. 4)), the
correct NMR solvent had to be used in order to provide reliable
proof for the structure of the final product.
and formation of [1,2,3-triMeIM][MeCO3] could occur (Fig. 4,
conditions b). Such reactions would interfere with the NMR
results if the real product of the reaction was actually the
anticipated [1,2,3-triMeIM][MeCO3]. Thus, the use of MeOD
as the NMR solvent would prove the formation of [1,2,3-
triMeIM][MeCO3], but could not rule out the possibility of its
formation from the reaction of [1,2,3-triMeIM-4-COO] with the
solvent. Also, the use of deuterated H2O as the NMR solvent
could greatly influence the experiment by possible conversion of
[1,2,3-triMeIM][MeCO3] into [1,2,3-triMeIM][HCO3], similar
to work by Pocker et al.20 In that case, the existence of the
[HCO3]- salt could be proven, but the starting material from
which it came could not (Fig. 4, conditions c). We therefore
1
analyzed our product using H and 13C NMR in deuterated
CHCl3 (chosen for its lower polarity than DMSO), relative
immiscibility with water, low hygroscopicity, and inert character
toward the product.
The results of the NMR analyses in CDCl3 confirmed the
anticipated reaction outcome, the symmetric [1,2,3-triMeIM]+
cation core, by characteristic signals at 2.74 (C2-CH3), 3.94 (N-
CH3), and 7.74 ppm (C4/C5-H) (Table S1a†). Moreover, the
signal at 3.45 ppm has been identified as the CH3O- group of the
[MeCO3]- anion, which is highly suggestive of the formation
of the desired [1,2,3-triMeIM][MeCO3]. The 13C NMR also
confirmed formation of pure [1,2,3-triMeIM][MeCO3], with
[MeCO3]- anion signals appearing at 52.0 and 158.0 ppm
(Table S1b). The other possibility for the assignment of these
signals would be formation of [1,2,3-triMeIM][HCO3] and the
presence of MeOH formed during the decomposition of the
[MeCO3]- anion. However, the signal at 52.0 ppm, due to
its position slightly downfield from the expected position for
MeOH (49.8 ppm), was assigned to the [MeCO3]- anion. A
Fig. 3 General protocol for the synthesis of a) 1,2,3-trimethyl-
imidazolium methyl carbonate, b) 1,2,3-trimethylimidazolium hydrogen
carbonate, and c) N,N-dimethylpyrrolidinium hydrogen carbonate.
From our experience, we knew that in the case where the
product formed was the [1,2,3-triMeIM-4-COO] zwitterionic
salt, the addition of a strongly polar aprotic solvent in the
presence of trace amounts of water and atmospheric CO2 could
cause the partial decarboxylation reaction and formation of
[1,2,3-triMeIM][HCO3] (Fig. 4, conditions a).14 On the other
hand, the use of pure deuterated MeOD as the NMR solvent
could cause a reaction similar to the one reported by Tommasi
and Sorrentino,19 where transcarboxylation from the zwitterion
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shift in the position of the CH3O- signal was expected due
to the electron-withdrawing effect of the carbonate group in
1
the [MeCO3]- anion. Additionally, in the H NMR of [1,2,3-
triMeIM][MeCO3] (Table S1a), a small signal at 3.34 ppm was
present. This was assigned to trace amounts of MeOH, possibly
formed during the slow conversion process of the [MeCO3]-
anion into MeOH and [HCO3]-.
Fig. 5 Expected cross-peak correlation for the [MeCO3]- anion utilizing
an HMBC NMR experiment.
Additional analysis of the same [1,2,3-triMeIM][MeCO3]
sample dissolved in CDCl3, 24 h after the initial NMR exper-
iment confirmed the earlier supposition of slow conversion of
the [MeCO3]- anion into MeOH and [HCO3]-. In addition to
the peaks observed during initial analysis, the signal suspected
to be indicative of MeOH at 3.34 ppm appeared to be much
more distinct (Table S1c). Additionally, 13C NMR of the same
sample (after 24 h) (Table S1d) revealed the appearance of an
additional peak at 49.63 ppm, slightly upfield from the signal
of the methoxy group on the anion, thus again confirming slow
conversion of the [MeCO3]- anion into MeOH and [HCO3]-.
24 h after NMR sample preparation, the ratio of the peak for
MeOH (3.35 ppm) to the peak for [MeCO3]- (3.45 ppm) was
1 : 4.
species in the NMR solution. It was concluded from this NMR
experiment that the starting material, before dissolution in
CDCl3, was pure [1,2,3-triMeIM][MeCO3], which after 24 h
converted approximately 25% of the [MeCO3]- anion to [HCO3]-
and MeOH. This finding is consistent with experimental results
from the NMR experiment carried out in deuterated H2O as
solvent, and as discussed below.
To further investigate [1,2,3-triMeIM][MeCO3] and its pos-
sible conversion to [1,2,3-triMeIM][HCO3] in the presence of
water, as reported by Mori et al.,21 the NMR spectra of the
raw sample, dissolved in deuterated H2O, was analyzed (Table
1
S1e–g). The first H NMR spectrum was taken ~10 min after
dissolution in the NMR solvent and revealed two sets of signals
at 3.37 and 3.54 ppm in a 3 : 1 ratio. As previously rationalized,
the signal at 3.37 ppm was preliminarily assigned to the MeOH
formed from decomposition of the [MeCO3]- anion, and a
signal at 3.54 ppm was assigned to the remaining [MeCO3]-
anion. The same sample was analyzed again after 1 h using
the same NMR protocol and revealed the disappearance of
the signal at 3.54 ppm and only the presence of a signal at
3.37 ppm, corresponding to MeOH being formed during the
decomposition of [MeCO3]- in the presence of D2O. A 13C
NMR of the sample taken after 1 h (Table S1g) clearly shows
(i) all signals present for the imidazolium cation, (ii) a signal at
160.9 ppm assigned to the carbonate group from the anion, and
(iii) a signal at 49.6 ppm assigned to the formed MeOD.
A heteronuclear multiple bond correlation (HMBC) NMR
experiment was performed to confirm the assumptions about
the presence of the [HCO3]- anion in the product. The goal
of this experiment was to correlate one of the CH3O- signals
(at 3.45 or 3.35 ppm in the 1H NMR experiment) with the
carbon atom of the carbonate group in [MeCO3]-, since only
3
the [MeCO3]- could result in the appearance of the J cross-
peak. In contrast, the hydrogen signal of MeOH was anticipated
3
to show no J-coupling correlation with the carbonate signals
(Fig. 5).
Of the two analyzed peaks at 3.45 and 3.35 ppm (NMR
peaks 4 and 5, Fig. 6) only the peak at 3.45 ppm showed a
cross-peak with the carbonate peak on the 13C NMR scale at
158.03 ppm. This directly indicated the presence of the [MeCO3]-
Fig. 6 HMBC NMR (ppm) of [1,2,3-triMeIM][MeCO3] in CDCl3 24 h after initial sample preparation.
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Fig. 7 HMBC NMR (ppm) of [1,2,3-triMeIM][MeCO3] in D2O 10 min after initial sample preparation.
Lastly, one more HMBC NMR experiment (Fig. 7) was
performed to confirm the conversion reaction from [MeCO3]-
to [HCO3]- in the presence of water, and to allow definite
assignment of the observed NMR signals in D2O to the
hypothesized products. Even though the HMBC experiment
does not require a long time for data collection, it was
noticed that over the time of the experiment, the signal at
3.54 ppm almost completely disappeared due to conversion
of [MeCO3]- to MeOH and [HCO3]-. Fortunately, enough
evidence of the cross-peak was found for this signal, and
allowed for its successful correlation with the carbonate peak at
160.9 ppm.
To complete the NMR structural identification of crude
[1,2,3-triMeIM][MeCO3], one further experiment was per-
formed using deuterated MeOH to confirm its identity. In the
1H NMR spectrum, the characteristic peak at 3.24 ppm was
assigned to the CH3O- group of the [MeCO3]- anion. Also the
13C NMR showed the peak for CH3O- at 49.9 ppm adjacent to
the septet for MeOD. All chemical shifts for the sample run in
MeOD are included in Table S1h–i.
Confirmation of the synthesis of [1,2,3-triMeIM][MeCO3]
also came from single-crystal X-ray diffraction analysis (Fig. 8).
An examination of the closest contacts suggests six anions sur-
rounding each cation (and vice versa) in addition to one cation–
cation stacking contact. The closest anion–cation contacts are
the asymmetric and bifurcated contacts between the acidic ring
hydrogen atoms at C4 and C5 with the carboxylate oxygen
˚
atoms of the anions (2.30(2) to 2.40(2) A). The cation stacking
interactions consist of parallel head-to-tail alignment over the
˚
N1–C2–N3 bond with a separation of 3.60(4) A, calculated using
centroid-to-centroid distances, where the centroid represents the
center of the three atoms. Interestingly, the opposite face of each
stacked cation pair is oriented toward the methyl-substituted
˚
oxygen in the anions with an O3-to-C2 separation of 2.960(1) A.
Conversion of [1,2,3-triMeIM][MeCO3] to
[1,2,3-triMeIM][HCO3]
As suggested by the results above, the conversion reaction of
[MeCO3]- to [HCO3]- was attempted using EtOH as a solvent
Fig. 8 Asymmetric unit (left, 50% probability thermal ellipsoids; closest anion–cation contact noted) and packing diagram (right, hydrogen atoms
omitted for clarity) of [1,2,3-triMeIM][MeCO3].
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and H2O as a reagent (in large excess over any other reactant).
The reaction was stirred overnight at 40 ◦C, after which time
the solvent was evaporated using a rotary evaporator and the
product isolated (Fig. 3b). The NMR spectra of the product
confirmed quantitative conversion to [1,2,3-triMeIM][HCO3],
and the NMR signals were comparable with those for [1,2,3-
triMIM][MeCO3] analyzed in D2O 1 h after initial dissolution,
as described above (Table S1f–g).
and the water molecule. The strong hydrogen bonds between
the water and anions results in a corrugated 2D network with
the cations residing between the sheets, similar to the crystal
structure of [1,3-diMIM][HCO3]·H2O.14
Formation of N,N-dimethylpyrrolidinium hydrogen carbonate
([N,N-diMePyr][HCO3])
In an attempt to reduce the reaction steps and streamline
the process, the same overall reaction was carried out by
addition of wet acetone to [1,2,3-triMeIM][MeCO3] in EtOH
resulting in the formation of [1,2,3-triMeIM][HCO3] directly as
a solid precipitating out of solution. The solid was separated by
centrifugation, the extract concentrated, and another portion
of acetone added to the liquid phase, allowing more of the
product to precipitate. (An additional advantage of this route
is that colored impurities formed in the raw sample of [1,2,3-
triMeIM][MeCO3] are retained in the mother liquor during the
precipitation of pure [1,2,3-triMeIM][HCO3].) The total yield of
the process (after drying the product under high vacuum) was
calculated to be ~91%.
The pyrrolidinium core was chosen for study to represent
non-aromatic amines that would not undergo bonding of the
carbonate group to the pyrrolidinium core, and thus not form a
zwitterionic species. Another benefit of working with this cation
core is its wide electrochemical window, thus having potential
for use in electrochemical applications.22 It was anticipated that
in the reaction of N-alkylpyrrolidine and dialkyl carbonate,
only the N,N-dialkylpyrrolidinium alkyl carbonate would be
formed. This could then undergo an anion conversion process,
as described above, with clean formation of the [HCO3]--based
salt and alcohol as the byproduct.
Methylpyrrolidine and 200% excess of DMC were reacted at
90 ◦C for 14 days in a pressure tube. After this time, excess
DMC and any possible unreacted N-methylpyrrolidine were
removed using rotary evaporation; no solid precipitate was
observed. Different solvent systems were tried to separate the
product, [N,N-diMePyr][MeCO3], from the colored impurities
without success. As in the previous experiments, solvents like
DMSO, DMF, trialkylamine, acetone, water, and similar polar
solvents had to be generally avoided due to the possible direct
conversion of [MeCO3]- into [HCO3]- prior the isolation of
[N,N-diMePyr][MeCO3]. Unfortunately, all the tested solvent
systems failed to result in successful isolation of product from
the reaction mixture. The only solvent system that successfully
separated the product from the decomposition products was
found to be acetone. It was also found that the use of acetone
in product purification, as previously presented, facilitated the
conversion of [MeCO3]- to [HCO3]- and MeOH, thus isolation
of a pure [N,N-diMePyr][MeCO3] salt was not possible, and
only a [HCO3]--based salt could be isolated.
The NMR of [1,2,3-triMeIM][HCO3] was performed in D2O
and MeOD separately (Table S1j–m). The NMR analyses in
D2O confirmed the formation of pure [1,2,3-triMeIM][HCO3],
evidenced by the lack of the distinctive peak for the CH3O-
signal of [MeCO3]- at 59.9 ppm (13C NMR) and 3.2 ppm (1H
NMR). A trace signal for the [MeCO3]- anion at 3.24 ppm was
1
present in the H NMR in MeOD after 1 h, indicating that
back-conversion of [HCO3]- to [MeCO3]- was occurring,14 but
very slowly.
Slow precipitation of the product from solution produced
single-crystals of [1,2,3-triMeIM][HCO3]·H2O suitable for X-
ray diffraction analysis. In the crystal structure (Fig. 9), one
cation, one anion, and one water molecule complete the
asymmetric unit. The [HCO3]- anions form classic head-to-
head hydrogen bonded dimers around crystallographic centers
of inversion, with each anion in the dimers accepting additional
hydrogen bonds from the cation (the acidic proton on C5)
Fig. 9 Crystal structure of [1,2,3-triMeIM][HCO3]·H2O (left, 50% probability thermal ellipsoids; depicting the hydrogen bonded anion dimer and
closest cation contact) and packing diagram (right, hydrogen atoms removed for clarity) showing the 2D corrugated sheets interpenetrated with
stacked [1,2,3-triMeIM]+ cations.
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To the oily-red reaction mixture isolated from the rotary
evaporation step above, wet acetone was added, resulting in
the complete dissolution of the reaction mixture, from which,
after 1 h, crystalline product slowly started to appear (Fig. 3c).
This white, solid, crystalline, and very hygroscopic material was
separated from the liquor by centrifugation, and washed twice
with fresh acetone. After drying under high vacuum, NMR and
X-ray diffraction analyses were performed.
Instead of relying on a lower pKa of the acidic reagent in
comparison to H2CO3, the kinetic equilibrium between [NH4]+ +
[HCO3]- and NH3 + H2CO3 was expected to produce volatile
products which could be removed from the system, shifting the
reaction toward products (Fig. 11, where [X]- = [ClO4]-).
The NMR spectra in D2O and DMSO (Table S2) clearly
confirmed that the only isolated product was the [N,N-
diMePyr][HCO3] due to the lack of the CH3O- singlet signal
of the [MeCO3]- anion expected at ~3.4–3.6 ppm. Additionally,
in the NMR spectra in D2O, no MeOD signal was observed,
which proved lack of conversion of [MeCO3]- to [HCO3]- and
MeOH during the NMR experiment. Finally, an HMBC NMR
experiment in D2O ruled out any possibility of the presence of
Fig. 11 Reactions of [1,2,3-triMeIM][HCO3] and [N,N-diMePyr]-
[HCO3] with hydrochloric, nitric, and picric acids, as well as ammonium
perchlorate.
1
[MeCO3]-, showing no cross-peaks between H NMR signals
and 13C NMR peaks corresponding to the carbonate peak at
~160 ppm; thus it was concluded that the isolated product is
pure [N,N-diMePyr][HCO3], as confirmed by X-ray diffraction.
The crystal structure of the anhydrous [N,N-diMePyr][HCO3]
(Fig. 10) consists of hydrogen-bonded hydrogen carbonate
dimers and discrete cations. Each anionic dimer is surrounded by
eight nearest-neighbor cations, and each cation has four anion
and two cation nearest neighbors.
All eight product salts were confirmed using 1H and 13C NMR.
In all cases, no remaining signal for [HCO3]- was recorded in the
13C NMR, confirming the anticipated quantitative conversion
of [HCO3]- IL precursors into new salt products.
Thermal analysis of synthesized salts
All synthesized compounds, along with their [HCO3]- IL
precursors, were thermally characterized. Melting points and
decomposition temperatures were analyzed (Table 1) using
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). The results indicated the thermal stabilities of
[MeCO3]- and [HCO3]- salts to be very low, with the lowest
thermal stability recorded for [1,2,3-triMeIM][MeCO3] (onset
temperature for 5% decomposition, T5%dec, = 104 ◦C). The
[HCO3]- salt of this cation exhibited a similar T5%dec of 112 ◦C.
In comparison, the thermal stability of [N,N-diMePyr][HCO3]
was substantially higher (T5%dec= 158 ◦C) than observed for the
imidazolium salts.
All of the analyzed salts formed by the reaction of the IL
precursors with acids exhibited thermal stabilities >200 ◦C.
This increase in stability was expected, since the new products
are composed of stable anions that do not undergo any simple
thermal decomposition, as normally found for [HCO3]- and
[MeCO3]-. The thermal stabilities of the [1,2,3-triMeIM]+-based
Utilization of new hydrogen carbonate-based IL precursors in the
synthesis of ILs and other organic salts
In order to show the possible synthetic applications of
[HCO3]--based salts as IL precursors, the newly formed [1,2,3-
triMeIM][HCO3] and [N,N-diMePyr][HCO3] salts were reacted
with a group of organic and inorganic acids (HCl, HNO3, picric
acid), and ammonium perchlorate ([NH4][ClO4]; considered
here as a very weak acid (pKa = 9.2)).23 In all cases but one,
[NH4][ClO4], the pKa values of the acids were lower than that of
H2CO3 (pKa1=6.35),24 thus complete conversion was expected
with the formation of H2CO3 as byproduct. H2CO3, due to its
low stability, was expected to readily decompose to H2O and
gaseous CO2, thus shifting the reaction equilibrium towards
product formation, and after solvent evaporation, to result in
byproduct-free salts.
In the case of the reaction of [HCO3]--based IL precursors
with [NH4][ClO4], a different reaction pathway was expected.
Fig. 10 Crystal structure of [N,N-diMePyr][HCO3] (left, 50% probability thermal ellipsoids; depicting the hydrogen-bonded anion dimer and closest
cation contact) and packing diagram (right, hydrogen atoms removed for clarity).
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Table 1 Melting point transitions and decomposition temperatures of
the [1,2,3-triMeIM]+ and [N,N-diMePyr]+-based saltsa
compound in the DCS cell and its contamination). Thus, visual
melting point analyses were performed, and revealed that the
analyzed salts remain solid until, or very close to, their decompo-
sition temperatures. In all cases the melting points were recorded
to be >220 ◦C, and the melting transition translated directly to
the beginning of the decomposition process. Literature reports,
although limited, confirm these findings.25,26,28
Cation
T5%dec
/
T5%dec
/
Anion
m.p./◦C
◦C
m.p./◦C
n/a
◦C
Conclusions
[MeCO3]- 79
104
112d
205
259
224
n/a
158
[HCO3]-
[Cl]-
—
71
63
—
c
c
In continuing the search toward novel halide- and metal-
free synthetic protocols for the synthesis of ILs, two new
[HCO3]- IL precursors ([1,2,3-triMeIM][HCO3] and [N,N-
diMePyr][HCO3]) were developed and their structures were
confirmed by NMR experiments and single-crystal X-ray
diffraction. These salts were also evaluated for future possible
applications in the syntheses of ILs by reacting them with a
variety of acids and [NH4][ClO4], which resulted in the clean
and quantitative formation of a family of [1,2,3-triMeIM]+- and
[N,N-diMePyr]+-based salts. In the course of development of
the synthetic protocols for the formation of [HCO3]- salts, the
resulting alkylation reactions of the chosen neutral amines (in
this case 1,2-dimethylimidazole and N-methlypyrrolidine) with
DMC, and later conversion of the formed [MeCO3]- salts to
[HCO3]- salts proceeded in one step at temperatures close to
room temperature, using only water as reagent.
Previously reported routes to [1,3-diMeIM][HCO3]14 were
limited to the use of zwitterionic [1,3-diMeIM-2-COO], which
upon reaction with carbonic acid, or water and CO2, led to the
formation of the desired [HCO3]- salt. Here, we have presented
a more generalized route to [HCO3]- salts which allows for more
efficient and faster syntheses of ILs in comparison with the
routes through zwitterionic feedstocks.
These new routes are advantageous for at least three reasons.
First, the formation of the [HCO3]- precursor using the new
synthetic protocol does not require prior formation of a zwit-
terionic salt, thus eliminating the need for a two-step reaction,
with the carboxylation step requiring elevated temperatures and
extended times. Second, the required formation of the [MeCO3]-
intermediate en route to the [HCO3]- salts does not limit the
range of cations to heterocycles that can undergo a carboxylation
reaction; the only limitation is the ability of the starting amine to
undergo alkylation with dialkyl carbonate. Finally, conversion
of [MeCO3]- to [HCO3]-, and then consecutive reaction with the
acid of choice, is simple in comparison with the requirements of
the Krapcho decarboxylation reactions of the zwitterions, which
often has to be performed in polar aprotic solvents, like DMSO,
and proceeds in two steps.
The new [HCO3]--based IL precursors were also used in
reactions with several acids and [NH4][ClO4] to prove the
potential of this family of salts for future applications in halide-
and metal-free syntheses of ILs. As shown, it is possible to form
virtually any salt from the [HCO3]- precursor when the pKa
of the acidic reagent is lower than that of H2CO3. Also, the
use of ammonium salts results in formation of ammonia gas,
which can be evacuated from the system, shifting the equilibrium
towards product formation. This reaction can be manipulated
to accommodate a wide variety of anions by proper design of
the reaction conditions.
~260 a(lit. >350)25 231
[NO3]-
[Pic]-
~220b
276
104
~280 b(lit. >310)26 269
~280 b(lit. >330)28 254
[ClO4]-
~220 b(lit >250)27 220
a Salts melting below 100 ◦C are shown in bold. b Using a standard DSC
protocol, the m.p. was not observed; visual m.p. analysis confirmed
that the salt remains solid until its decomposition temperature, at
which point it melts and decomposes simultaneously, changing to
dark brown in color. c Due to the extreme hygroscopic nature of the
sample, the determination of the m.p. was not possible. d Hydrated salt
decomposition.
salts ranged from 205 ◦C for [1,2,3-triMeIM][Cl], to 258 ◦C for
[1,2,3-triMeIM][NO3]. In the case of the [N,N-diMePyr]+-based
salts, the thermal stabilities ranged between 231 ◦C for [N,N-
diMePyr][Cl] to 276 ◦C [N,N-diMePyr][NO3].
The melting points of the obtained salts were also analyzed;
however, due to the extreme hygroscopic nature of both [HCO3]--
based salts, the determination of the melting point using
DSC did not provide reliable results, and visual melting point
determination was attempted. In each of the [HCO3]--based
samples, placing them directly on a glass slide resulted in fast
moisture absorption and deliquescence of the compounds due
to the water absorbed from the atmosphere. When the glass slide
was placed on the hot-stage apparatus and heated to ~100 ◦C, the
absorbed water appeared to evaporate, leaving a white powder
on the slide. Continuous heating of the sample was carried out to
the decomposition temperature, at which point the compounds
simultaneously melted, evolved gasses, and turned brown in
color.
DSC analyses of the [1,2,3-triMeIM]+ salts, except for the
[HCO3]- and [ClO4]- analogs, exhibited a sharp melting tran-
sition (on heating) and a sharp crystallization transition (on
cooling). The melting points of the [1,2,3-triMeIM]+ salts were
surprisingly low, considering the high symmetry of the cation,
◦
◦
and were between 64 C for [1,2,3-triMeIM][NO3] and 104 C
for [1,2,3-triMeIM][Pic]. The melting transition for [1,2,3-
triMeIM][ClO4] was not detected on the DSC instrument due
to the safety limitations set for the DSC experimental protocol.
Visual melting point analysis revealed that the sample remained
◦
solid until ~220 C, at which point it melted and decomposed
simultaneously. This finding was supported by a literature report
where the melting point for [1,2,3-triMeIM][ClO4] was reported
to be >250 ◦C.27
DSC analysis of the melting points for the [N,N-diMePyr]+-
based salts did not result in determination of any melting point
transitions within the allowed DSC temperature ranges (up to
TDSCmax = T5%dec - 50 ◦C to avoid decomposition of the analyzed
498 | Green Chem., 2010, 12, 491–501
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until no precipitate was noticed. The combined solid was dried
under high vacuum at 40 ◦C for 12 h. A white, hygroscopic solid
(1.39 g), with a melting point at the decomposition temperature
was obtained with a total yield of 91%.
Experimental
Chemicals
All reagents were purchased from Sigma-Aldrich (St. Louis,
MO) and were used as received.
1,2,3-Trimethylimidazolium hydrogen carbonate monohydrate
([1,2,3-triMeIM][HCO3]·H2O). White solid, very hygroscopic,
91% yield, m.p. at T5%dec, T5%dec = 112 ◦C; 1H NMR (500 MHz,
D2O) d = 2.57 (s, 3H; C2-CH3), 3.78 (s, 6H, N-CH3), 7.30
(s, 2H, C4/C5-H); 13C NMR (125 MHz, D2O) d = 9.41 (C2-
CH3), 35.32 (N-CH3), 122.55 (C4/C5), 145.50 (C2), 161.03
(HCO3); 1H NMR (500 MHz, [D4]MeOH) d = 2.51 (s, 3H; C2-
CH3), 3.72 (s, 6H, N-CH3), 7.36 (s, 2H, C4/C5-H); 13C NMR
(125 MHz, [D4]MeOH) d = 9.37 (C2-CH3), 35.44 (N-CH3),
123.35 (C4/C5), 146.48 (C2), 161.32 (CH3CO3). Single-crystal
X-ray diffraction analysis of [1,2,3-triMeIM][HCO3]·H2O: col-
orless needles; C7H14N2O4; MW 190.20; T = 173 K; monoclinic;
Synthesis of 1,2,3-trimethylimidazolium methyl carbonate
([1,2,3-triMeIM][MeCO3])
The synthesis of [1,2,3-triMeIM][MeCO3] followed a previously
published method for the synthesis of [1,3-diMeIM-2-COO] via
an alkylation reaction of 1-methylimidazole with DMC.18 4.8 g
(50 mmol) of 1,2-dimethylimidazole and 9 g (100 mmol) of DMC
were placed in a 20 mL thick-walled glass pressure tube. The
tube was sealed, placed in an oven, and heated to 70 ◦C for
10 days (Fig. 3a). The resulting yellow liquor was cooled to
room temperature and unreacted substrates removed under high
vacuum, at which time the product (7.44 g) crystallized in the
form of slightly colored crystals.
˚
˚
˚
P21/n; a = 7.479(2) A; b = 13.289(4) A; c = 9.636(3) A; b =
◦
3
-3
˚
90.011(5) ; V = 954.0(5) A ; Z = 4; Dc = 1.324 g cm ; R1, wR2
[I > 2s(I)] = 0.0719, 0.1097; R1, wR2 (all data) = 0.1822, 0.1407.
1,2,3-Trimethylimidazolium methyl carbonate ([1,2,3-
triMeIM][MeCO3]). White solid, very hygroscopic, 80% yield,
m.p. 79 ◦C, T5%dec= 104 ◦C; 1H NMR (500 MHz, CDCl3)
d = 2.74 (s, 3H, C2-CH3), 3.45 (s, 3H, CH3CO3), 3.94 (s,
6H, N-CH3), 7.74 (s, 2H, C4/C5-H); 13C NMR (125 MHz,
CDCl3) d = 9.52 (C2-CH3), 35.16 (N-CH3), 52.02 (CH3CO3),
122.69 (C4/C5), 143.99 (C2), 157.98 (CH3CO3); 1H NMR
(500 MHz, [D4]MeOH) d = 2.50 (s, 3H; C2-CH3), 3.23 (s,
3H; CH3CO3), 3.71 (s, 6H, N-CH3), 7.35 (s, 2H, C4/C5-H);
13C NMR (125 MHz, [D4]MeOH) d = 9.34 (C2-CH3), 35.42
(N-CH3), 49.90 (CH3CO3), 123.35 (C4/C5), 146.52 (C2), 161.37
(CH3CO3). Single-crystal X-ray diffraction analysis: colorless
plates; C8H14N2O3; MW 186.21; T = 173 K; Monoclinic; P21/n;
Synthesis of N,N-dimethylpyrrolidinium hydrogen carbonate
([N,N-diMePyr][HCO3])
4.0 g (50 mmol) of N-methypyrrolidine was initially dissolved
in 5 mL of EtOH and transferred into a 20 mL thick-walled
pressure tube. Next, 9 g (100 mmol) of DMC was added to
the solution at room temperature. The tube was sealed, stirred
vigorously, placed in the oven, and heated to 90 ◦C. The reaction
mixture was kept under those conditions for 14 days, and
then cooled to room temperature (Fig. 3c), giving a dark red
liquor. Excess unreacted substrates were removed using a rotary
evaporator at 90 ◦C, and later high vacuum at 70 ◦C. No product
crystallized from the mother liquor.
Due to problems with isolating the product from the red
liquor, the intermediate was converted in situ into N,N-
dimethylpyrrolidinium hydrogen carbonate. To the oily-red
concentrated reaction mixture 10 mL of wet acetone was added,
resulting in the complete dissolution of reaction mixture. After
1 h a crystalline product slowly started to appear. After 24 h
at room temperature, the vessel was placed in the refrigerator
and kept at this temperature for an additional 72 h. A white,
solid, very hygroscopic product was separated from the red
liquor by centrifugation, and washed twice with cold acetone.
The remaining extract was concentrated, more solid product
precipitated, and later was separated by centrifugation. After
combining the two precipitation batches, the product was dried
under high vacuum for 12 h at room temperature, resulting in a
white, hygroscopic solid (5.80 g; 72% total yield and 98% purity),
with a melting point at the decomposition temperature.
˚
˚
˚
a = 6.8805(6) A; b = 11.6254(10) A; c = 11.9614(11) A; b =
◦
3
-3
˚
96.229(2) ; V = 951.13(15) A ; Z = 4; Dc = 1.300 g cm ; R1,
wR2 [I > 2s(I)] = 0.0428, 0.1044; R1, wR2 (all data) = 0.0510,
0.1087.
Synthesis of 1,2,3-trimethylimidazolium hydrogen carbonate
([1,2,3-triMeIM][HCO3])
By reaction in EtOH–H2O. 0.5 g (2.7 mmol) of [1,2,3-
triMIM][MeCO3] was dissolved in 3 mL of EtOH and 1.5 mL
(83 mmol) DI H2O was added (Fig. 3b). The reaction was stirred
overnight at 40 ◦C, after which time the solvent was evaporated
on the rotary evaporator, and the sample was analyzed by
NMR. The NMR spectra confirmed the quantitative conversion
to [1,2,3-triMeIM][HCO3]. The slightly colored solid product
(0.46 g) was obtained in 99% yield.
By reaction in EtOH–acetone. 1.5 g (8.1 mmol) of [1,2,3-
triMeIM][MeCO3] was dissolved in a minimum amount of
EtOH (2 mL) and heated to 40 ◦C to allow complete dissolution
of the starting material. 10 mL of acetone, spiked with 0.3 mL
(16.7 mmol) DI H2O, was then added dropwise to the EtOH
solution, producing a precipitate which was separated by cen-
trifugation. The remaining solution was concentrated by partial
evaporation of the solvent on a rotary evaporator at 40 ◦C,
and another portion of acetone (10 mL) was added allowing
for more precipitation. The process was repeated 2 more times
N,N-Dimethylpyrrolidinium hydrogen carbonate ([N,N-
diMePry][HCO3]). White◦solid, very hygroscopic, 72% yield,
m.p. at T5%dec, T5%dec = 158 C; 1H NMR (500 MHz, D2O) d =
2.23 (t, 4H; CH2), 3.14 (s, 6H, N-CH3), 3.51 (t, 4H, N-CH2); 13
C
NMR (125 MHz, D2O) d = 21.65 (CH2), 51.67 (N-CH3), 65.82
(N-CH2), 160.17 (HCO3); 1H NMR (500 MHz, [D6]DMSO) d =
2.08 (t, 4H; CH2), 3.05 (s, 6H, N-CH3), 3.42 (t, 4H, N-CH2); 13
C
NMR (125 MHz, [D6]DMSO) d = 21.76 (CH2), 51.50 (N-CH3),
65.33 (N-CH2), 158.93 (HCO3). Single-crystal X-ray diffraction
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analysis: colorless plates; C7H15NO3; MW 161.20; monoclinic;
N,N-Dimethylpyrrolidinium nitrate ([N,N-diMePyr][NO3]).
White solid, 98% yield, m.p. at T5%dec (hot-stage apparatus)
~220 ◦C, T5%dec= 276 ◦C;1H NMR (500 MHz, [D4]MeOH) d =
2.26 (t, 4H, -CH2-), 3.18 (s, 6H, N-CH3), 3.55 (t, 4H, N-CH2);
13C NMR (125 MHz [D4]MeOH) d = 22.97 (-CH2-), 52.45 (t,
N-CH3), 66.93 (t, N-CH2).
˚
˚
˚
P21/n; a = 6.6199(6) A; b = 13.9451(12) A; c = 9.2587(8) A;
◦
3
-3
˚
b = 101.760(2) ; V = 836.8(1) A ; Z = 4; Dc = 1.280 g cm ; R1,
wR2 [I > 2s(I)] = 0.0356, 0.0890; R1, wR2 (all data) = 0.0445,
0.0937.
General procedure for the preparation of 1,2,3-trimethylimida-
zolium and N,N-dimethylpyrrolidinium chloride [Cl]-, nitrate
[NO3]-, picrate [Pic]-, and perchlorate [ClO4]- salts
N,N-Dimethylpyrrolidinium picrate ([N,N-diMePyr][Pic]).
Yellow crystalline solid, 98% yield, m.p. at T5%dec (hot-stage
apparatus) ~280 ◦C, T5%dec= 269 ◦C; 1H NMR (500 MHz,
[D4]MeOH) d = 2.28 (t, 4H, -CH2-), 3.19 (s, 6H, N-CH3), 3.56
(t, 4H, N-CH2), 8.87 (s, 2H, picrate); 13C NMR (125 MHz
[D4]MeOH) d = 22.79 (-CH2-), 52.69 (N-CH3), 66.96 (N-
CH2), 127.54 (picrate), 128.52 (picrate), 142.76 (picrate), 163.52
(picrate).
All reactions were performed using the same procedure: 0.01 mol
of the appropriate acid (HPF6, HCl, H2SO4, picric acid) or salt
([NH4][ClO4]) was dissolved in 10 mL of H2O (ethanol in the case
of picric acid) and added dropwise, at room temperature (40 ◦C
in case of [NH4][ClO4]), to a stirred solution of 0.01 mol of [1,2,3-
triMeIM][HCO3] or [N,N-diMePyr][HCO3] in 50% aqueous
ethanol (v/v, 20 mL). The mixture was stirred for an additional
24 h in a closed flask (to avoid inorganic acid evaporation)
and the solvent and gaseous byproducts were evaporated on
a rotary evaporator under vacuum. All samples were then dried
under high vacuum at room temperature. All salts obtained were
N,N-Dimethylpyrrolidinium perchlorate ([N,N-diMePyr]-
[ClO4]). White solid, 97% yield, m.p. at T5%dec (hot-stage
apparatus) ~280 ◦C, T5%dec= 254 ◦C; 1H NMR (500 MHz,
[D4]MeOH) d = 2.26 (t, 4H, -CH2-), 3.17 (s, 6H, N-CH3), 3.55
(t, 4H, N-CH2); 13C NMR (125 MHz [D4]MeOH) d = 22.97
(-CH2-), 52.50 (N-CH3), 66.97 (N-CH2).
1
checked for the presence of starting material using H and 13C
NMR, and none of the spectra revealed any residual peaks for
the [HCO3]- anion.
X-Ray crystallographic studies
Solid [1,2,3-triMeIM][MeCO3] was crystallized out of the reac-
tion mixture after the excess of DMC was evaporated. The crys-
tals of [1,2,3-triMeIM][HCO3]·H2O and [N,N-diMePyr][HCO3]
were recrystallized from acetone, by dissolution of small amount
of the sample in warm acetone, and slow cooling of the solution
to room temperature.
Single crystals suitable for analysis were isolated in air,
mounted on fibers, and transferred to the goniometer. The
crystals were cooled to -100 ◦C with a stream of nitrogen
gas and data were collected on a Seimens SMART diffrac-
tometer equipped with a CCD area detector, using graphite-
monochromated MoKa radiation. The SHELXTL software
package was used for each solution and refinement.29 Absorp-
tion corrections were made with SADABS.30 Each structure was
refined by using full-matrix least-squares methods on F2. All
non-hydrogen atoms were readily located and their positions
refined anisotropically, while all hydrogen atoms were located
from difference Fourier maps and isotropically refined without
restraint.
1,2,3-Trimethylimidazolium chloride ([1,2,3-triMeIM][Cl]).
White solid, very hygroscopic, 99% yield, m.p. 71 ◦C, T5%dec
=
205 ◦C; 1H NMR (500 MHz, [D4]MeOH) d = 2.62 (s, 3H, C2-
CH3), 3.83 (s, 6H, N-CH3), 7.46 (s, 2H, C4/C5-H); 13C NMR
(125 MHz [D4]MeOH) d = 10.38 (C2-CH3), 35.57 (N-CH3),
123.26 (C4/C5), 146.45 (C2).
1,2,3-Trimethylimidazolium nitrate ([1,2,3-triMeIM][NO3]).
◦
◦
White crystalline solid, 98% yield, m.p. 63 C, T5%dec= 259 C;
1H NMR (500 MHz, [D4]MeOH) d = 2.60 (s, 3H, C2-CH3),
3.84 (s, 6H, N-CH3), 7.43 (s, 2H, C4/C5-H); 13C NMR
(125 MHz [D4]MeOH) d = 10.01 (C2-CH3), 35.34 (N-CH3),
123.26 (C4/C5), 146.40 (C2).
1,2,3-Trimethylimidazolium picrate ([1,2,3-triMeIM][Pic]).
Yellow crystalline solid, 98% yield, m.p. 104 ◦C, T5%dec= 224 ◦C;
1H NMR (500 MHz, [D4]MeOH) d = 2.56 (s, 3H, C2-CH3), 3.76
(s, 6H, N-CH3), 7.58 (s, 2H, C4/C5-H), 8.59 (s, 2H, picrate); 13
C
NMR (125 MHz [D4]MeOH) d = 9.48 (C2-CH3), 34.54 (N-
CH3), 121.82 (C4/C5), 124.03 (picrate), 125.07 (picrate), 141.73
(picrate), 144.59 (C2), 160.68 (picrate).
Thermal analysis
Thermal decomposition temperatures were measured in the
dynamic heating regime using a TGA 2950 TA Instrument under
dried air atmosphere. Samples between 5–15 mg were heated
1,2,3-Trimethylimidazolium perchlorate ([1,2,3-triMeIM]-
[ClO4]). White crystalline solid, 98% yield, m.p. at T5%dec (hot-
stage apparatus) ~220 ◦C, T5%dec= 220 ◦C; 1H NMR (500 MHz,
[D4]MeOH) d = 2.57 (s, 3H, C2-CH3), 3.77 (s, 6H, N-CH3), 7.50
(s, 2H, C4/C5-H); 13C NMR (125 MHz [D4]MeOH) d = 10.00
(C2-CH3), 35.81 (N-CH3), 123.36 (C4/C5), 146.35 (C2).
from 40–500 ◦C with an isocratic heating rate of 5 C min-1
◦
under air atmosphere. Decomposition temperatures (T5%dec
)
were determined from the onset to 5 wt% mass loss in an isocratic
TGA experiment, which provides a more realistic representation
of thermal stability at elevated temperatures.
N,N-Dimethylpyrrolidinium chloride ([N,N-diMePyr][Cl]):.
White solid, very hygroscopic, 99% yield, m.p. at T5%dec (hot-
stage apparatus) ~260 ◦C, T5%dec= 231 ◦C; 1H NMR (500 MHz,
[D4]MeOH) d = 2.26 (t, 4H, -CH2-), 3.20 (s, 6H, N-CH3), 3.58
(t, 4H, N-CH2); 13C NMR (125 MHz [D4]MeOH) d = 22.99
(-CH2-), 52.55 (t, N-CH3), 66.95 (t, N-CH2).
Melting points were determined by differential scanning
calorimetry (DSC) TA Instruments model 2920 Modulated DSC
(New Castle, DE) cooled with a liquid nitrogen cryostat. The
calorimeter was calibrated for temperature and cell constants
◦
using indium (mp 156.61 C, DH 28.71 J g-1). Data were col-
lected at constant atmospheric pressure, using samples between
500 | Green Chem., 2010, 12, 491–501
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10–40 mg in aluminium sample p◦ans. Experiments were per-
formed with heating at a rate of 5 C min-1 to the temperature
50 ◦C below the decomposition temperature (T5%dec) and then
cooled down to -100 ◦C. This heating and cooling cycle was
repeated twice for each sample. The DSC instrument was
adjusted so that zero heat flow was between 0 and -0.5 mW, and
the baseline drift was less than 0.1 mW over the temperature
range 0–180 ◦C. An empty sample pan was used as reference.
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Acknowledgements
This research was supported by the Air Force Office of Scientific
Research (Grant F49620-03-1-0357).
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