Tartrate-based ionic liquids: Unified synthesis and characterisation

Article abstract of DOI:10.1039/c2ra21637j

A convenient and general preparative approach for tartrate-derived organic salts with bulky non-coordinating cations is described. This route is based on neutralisation of tartaric acid with cation hydroxides in aqueous solution. A series of 24 tartrate salts was prepared by systematic variation of the hemi- or bis-tartrate anion and the nature of the organic cation. Chirality of the anion was also explored as a vector for structural modification. Complete characterisation, including X-ray crystallography, was carried out. This comparative study of the physicochemical properties of these salts led to the identification of several informative trends useful for designing proper tartrate-based chiral ionic liquids.

Full text of DOI:10.1039/c2ra21637j

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PAPER
Tartrate-based ionic liquids: unified synthesis and
characterisation3
Cite this: RSC Advances, 2013, 3, 413
Anne Rouch,a Tessa Castellan,a Isabelle Fabing,a Nathalie Saffon,b Jean Rodriguez,c
c
a
a
´
Thierry Constantieux, Jean-Christophe Plaquevent and Yves Genisson*
A convenient and general preparative approach for tartrate-derived organic salts with bulky non-
coordinating cations is described. This route is based on neutralisation of tartaric acid with cation
hydroxides in aqueous solution. A series of 24 tartrate salts was prepared by systematic variation of the
hemi- or bis-tartrate anion and the nature of the organic cation. Chirality of the anion was also explored as
a vector for structural modification. Complete characterisation, including X-ray crystallography, was carried
out. This comparative study of the physicochemical properties of these salts led to the identification of
several informative trends useful for designing proper tartrate-based chiral ionic liquids.
Received 1st August 2012,
Accepted 1st October 2012
DOI: 10.1039/c2ra21637j
www.rsc.org/advances
a certain sophistication in the structure of ILs, well illustrated
by the concept of task specific ionic liquids (TSILs).9
Therefore, the impact of such approaches would be maximised
using simple chiral pool-derived salts as CILs, and there is a
need to develop new series of readily available biorenewable
CILs with potential catalytic applicability.10
Introduction
Room temperature ionic liquids (RTILs) have received ever-
growing attention during the last two decades as a unique
alternative to standard molecular solvents, with widespread
innovative applications in the fields of analysis, separation
and extraction. Thanks to several key properties, such as their
negligible vapour pressure, good thermal stability, non-
flammability and tuneable miscibility, they have also strongly
contributed to the development of sustainable chemistry as
safe and recyclable reaction media.1 Yet, one of the most
fascinating properties of ionic liquids (ILs) for the synthetic
chemistry and catalysis community may be their capacity to
influence the course of chemical processes, either by interact-
ing with or acting as the catalytic species.2 One may even
notice that, in the chemical literature vocabulary, ILs have
progressively moved from the status of ‘‘non-innocent’’
solvent3 to that of true ‘‘catalyst’’.4 In this regard, a general
picture may be drawn wherein the cation plays the role of a
Lewis (or Brønsted, for protic ionic liquids) acid,5 whereas the
anion acts as a strong (for hydroxide or carbonate) or weaker
(for acetate) base.6 In this context, chiral ionic liquids (CILs)7
have begun to offer some additional innovation opportunities
in successfully merging the fields of ILs and asymmetric
organocatalysis.8 However, this trend has been associated with
From Pasteur’s seminal studies in the 19th century to the
Sharpless’ asymmetric epoxidation reaction, tartrate deriva-
tives have remained almost ubiquitous in the quest of
chirality. Its availability from renewable bioresources com-
bined with its simple symmetrical, yet densely functionalised,
structure gives tartaric acid its versatility (Fig. 1). The latter is
illustrated by the widespread use of tartrate derivatives either
as a starting material or as a chiral agent for asymmetric
synthesis.
Quite naturally, the field of CILs also recently began to draw
inspiration from this prestigious heritage. Tartrate-derived
ionic liquids have been sporadically reported in the literature
during the last six years. Maschmeyer first described in 2006
tetrabutylammonium tartrates as part of a large family of ionic
liquids possessing chiral carboxylates.11 The same year, Yuan
proposed a 1-butyl-3-methyl imidazolium-derived ionic liquid
with buffering properties, which embedded a hemi-tartrate
anion.12 In 2009, Petrich described a tetrabutylphosphonium
hemi-tartrate in the context of a study on stereoselective
a
´
LSPCMIB, UMR CNRS 5068, Universite Paul Sabatier – Toulouse III, Toulouse,
France. E-mail: genisson@chimie.ups-tlse.fr; Fax: +33 5 61 55 60 11; Tel: +33 5 61 55
62 99
b
´
Institut de Chimie de Toulouse, ICT FR 2599, Universite Paul Sabatier—Toulouse
III, Toulouse, France
c
´
Aix Marseille Universite, CNRS, iSm2 UMR 7313, 13397 Marseille cedex 20, France
Electronic Supplementary Information (ESI) available: Copies of 1H and 13C
3
NMR spectra of 19 different representative hydroxides, hemi- and bistartrate
salts. CCDC 887928. See DOI: 10.1039/c2ra21637j
Fig. 1 Structures of tartaric acid.
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fluorescence quenching13 and Jing reported several dialkylpyr-
idinium tartrates as chiral additives in the asymmetric
cycloaddition of CO2 to epoxides.14 While the tartrate anion
represents a priori a relevant platform for designing CILs,
analysis of these precedents revealed several features. First,
the reported tartrate entities were not systematically described,
either in terms of preparation or characterisation, therefore
hampering their proper designation as ILs. Second, there was
a lack of a general preparative approach for these tartrate salts,
although it would offer an entry to their systematic study.
commercially available, the cation hydroxide precursor could
be prepared from the corresponding halide salt. Since the
independent contributions of Ohno18 and Ranu19 in 2005, the
attention of the community has focused on 1-alkyl-3-methyli-
midazolium hydroxides. The latter can thus be obtained by
treatment of the halide precursor with KOH in a chlorinated
solvent.20 However, the known propensity of the imidazolium
ring to generate highly reactive diaminocarbenes under basic
conditions, in addition to the eventuality of a Hofmann
elimination, hampers access to a clean concentrated mate-
rial.21 On the other hand, the use of an anion exchange resin22
developed by Ohno18 has been essentially limited to 1-alkyl-3-
methylimidazolium, maybe in part due to the individual
optimisation required for its generalisation to different cations
of varying hydrophilicity.23
With the aim of developing a unified approach to tartrate-
based ionic liquids, taking advantage of the straightforward
acid/base ion metathesis strategy, we first sought general
access to the required cation hydroxides. For this purpose, we
turned our attention to the use of halophilic silver salts which
are likely to ensure complete sequestration of any halide
anion, according to the general equation in Scheme 2.
These observations prompted us to propose
methodology and systematic description of
a
unified
a
series of
tartrate-derived ILs. This work was further motivated by the
promising results recently obtained with a tetrabutylpho-
sphonium tartrate in the enantioselective water–ionic liquid
extraction process.15
Results and discussion
Preparation of the tartrate species
IL preparation traditionally includes a quaternarisation step
followed by an anion metathesis.16 Among the different
approaches available for the exchange of an anion, neutralisa-
tion of a basic precursor, such as an hydroxide salt, by the
corresponding acid presents several attractive features.17
Unlike the approach based on the use of an alkaline metal
salt (Scheme 1, eqn. a), implying the sluggish formation and
separation of the metal halide by-product from the final ionic
liquid, the acid/base strategy relies on a highly favourable,
water-producing transformation carried out in a homogenous
aqueous media (Scheme 1, eqn. b). However, this approach
also presents limitations mainly related to the availability of
the cation hydroxide precursor. The latter might, if available,
be obtained from a commercial material, provided that
accurate information on its halide content is indicated.
Indeed, the use of a hydroxide salt contaminated with the
corresponding halide leads, after eventual filtration of the
excess of acid, to a useless final material with impaired
stoichiometry due to an irreparable default of the anion. We
experienced such difficulties during the preparation of
phosphonium and tetrabutylammonium tartrate salts and
found that the commercial starting hydroxide had to be either
purified before use or purchased on the basis of its low halide
content.15 Whether these observations should be considered
as implicit or not, it is surprising that no such warning
regarding the purity of the starting commercial tetrabutylpho-
sphonium or ammonium is made in the articles reporting the
preparation of tartrate salts by this approach.11,13 If not
Recently, a series of green amino acid ester-derived nitrate
ILs has, for example, been prepared by reacting the corre-
sponding hydrochloride with AgNO3.24 On the other hand,
treatment of a tetraalkylammonium halide with Ag2O is a long-
known procedure to prepare tetraalkylammonium hydro-
xide.25 We thus studied the production of several cation
hydroxides, according to this straightforward approach which
had only been sporadically applied to the synthesis of ILs.14,22
In order to establish a reproducible procedure we selected as
a basic reagent the Ag(I) species freshly obtained from aqueous
AgNO3 and NaOH that we will refer to as AgOH for a matter of
simplicity.26 Clean production of the cation hydroxide was first
checked with the representative 1-butyl-1-methylpiperidinium
bromide ([Bmpip]Br) by running the reaction in D2O. After 2 h
of stirring at room temperature in the presence of a slight
excess of AgOH, filtration of the insoluble inorganic salts
delivered a clean material, as ascertained by direct 1H NMR
analysis. No particular alteration of the spectra was observed
for [Bmpip]OH compared with its bromide precursor (see
ESI ). We then turned our attention to more challenging
3
aromatic cations, such as pyridinium and imidazolium,
known for their capacity to generate various neutral species
in the presence of basic anions.27 When the above-described
procedure was applied to 1-octylpyridinium bromide
([OctPyr]Br), there again, no sign of decomposition was
observed in 1H NMR for the expected [OctPyr]OH. However,
in this case, the intensity of the signal due to the H-2/H-6
protons of the pyridinium nucleus was strongly diminished
(Fig. 2).
Scheme 1
Scheme 2 General route to cation hydroxides.
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Fig. 3 Enlargements of the 1H NMR (300 MHz, D2O + H2O) spectrum of a
partially C-2/C-6 deuterated sample of [OctPyr]2D-Tar (2)-3b showing
progressive protonation (from top to bottom) upon heating with water.
Fig. 2 1H NMR (300 MHz, D2O) spectra of (from top to bottom) [Bmim]OH,
[Emim]OH, [Bmmim]OH and [OctPyr]OH.
hydroxide salts, already proposed on computational grounds,
was confirmed here by the rapid proton exchange in D2O.27,31
Gratifingly, an identical behaviour was also observed for
[Emim]I in D2O, leading to a clean solution of [Emim]OH
(Fig. 2). Finally, we studied the reaction of 1-butyl-2,3-
dimethylimidazolium bromide ([Bmmim]Br) with AgOH in
water. There again a clean formation of [Bmmim]OH was
observed from 1H NMR analysis of the crude D2O solution
(Fig. 2). As suggested by the theoretical study of Nyulaszi,27 the
2-methyl protons proved substantially acidic, leading to an
almost complete proton-deuterium exchange in the presence
of the hydroxide anion.
Before exploring the use of these aqueous cation hydroxide
solutions in the preparation of ILs, we verified their bromide
contents. Indeed, impurities in general, and halide contam-
ination in particular, are well known to alter the physico-
chemical properties of ILs.32 The halide content of ILs has
been assessed by various analytical techniques, like capillary
electrophoresis33 or ion chromatography.34 Another conveni-
ent method is based on the measurement of the potential
difference developed across a semi-permeable membrane.35 In
the present study, this was accomplished using ion-selective
electrodes (ISEs) that are specially designed to respond
selectively to a particular species in a solution. Bromide
contents in aqueous cation hydroxide solutions obtained after
a simple filtration over hydrophilic polypropylene 0.2 mm
membrane filters were found to be systematically inferior to 5
ppm.
Such a selective deuteration under the influence of the
strongly basic hydroxide anion clearly indicates the relative
acidity of the H-2/H-6 protons in the pyridinium cations. The
likely implication of transient C-2 carbene species is con-
cordant with the known occurrence of pyridin-2-ylidene metal
complexes.28 Interestingly, this C-2/C-6 deuteration of the
pyridinium ring proved somewhat reversible upon prolonged
heating of the corresponding tartrate salt [OctPyr]2D-Tar (2)-
3b (vide infra) in the presence of a large excess of H2O (Fig. 3).
We next focused on the imidazolinium series. The reaction
with 1-butyl-3-methylimidazolium bromide ([Bmim]Br) was
particularly challenging, due to the high reactivity of the C-2
position. In fact, treatment of 1-alkyl-3-methylimidazolium
halides with Ag2O in an organic solvent is a known procedure
to produce silver carbene complexes.29 In order to get a clear
picture, we first intentionally prepared the metal carbene from
[Bmim]Br and commercial Ag2O in CD3OD. After filtration, the
resulting solution displayed several characteristic points in the
NMR data. First, a total disappearance of the H-2 (which is
only slow and partial with [Bmim]Br in CD3OD) and a
shielding of ca. 0.2 ppm of the H-3 and H-4 were observed.
More characteristic of the silver carbene was the strong
downfield shift of the C-2, detected at 181.2 ppm by means
of 3J correlations, with both the protons of the methyl and the
proximal methylene of the butyl residue in 2D NMR HMBC
(heteronuclear multiple-bond correlation) experiments. To our
satisfaction, we found that, under optimised conditions, it was
possible to selectively form [Bmim]OH in water. As shown by
With this general route to the required hydroxide salts in
hand, we turned our attention to the preparation of the
1
the H NMR analysis of the reaction mixture in D2O, a clean
material was produced (Fig. 2). In this case a minor shielding
of ca. 0.1 ppm of the H-3 and H-4 was recorded. Importantly, 3J
correlations with the C-2 were observed at 134.0 ppm, as
expected for a Bmim cation.
Interestingly, not only was the C-2 position fully deuterated,
but also the protons at C-4 and C-5 were partially exchanged,
an observation in agreement with the experimental study of
Giernoth.30 Regarding the C-2 position, the contribution of a
transient carbene species in 1-alkyl-3-methylimidazolium
Scheme 3
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Characterisation of the tartrate-based ILs
All products proved analytically pure, as determined by
elementary analysis. Water contents of preparative batches,
determined by the Karl Fischer method, were routinely found
in the range of 2–4%, after simply standing in a desiccator over
phosphoric anhydride. Further drying of samples under
vacuum (overnight, room temperature, 1023 mbar) reduced
water content to 0.5–1%.{
Optical rotations were measured in water at 20 uC. Values
ranging from 8.5 to 13 were recorded, being positive in the L-
(R,R) series and negative in the D-(S,S) series, as expected. Data
obtained for [TBA]HTart and [TBA]2Tart were in agreement
with those reported in the literature.11 Overall, only a small
influence of the cation was observed. On the other hand, bis-
tartrates displayed optical rotations slightly smaller than their
hemi-tartrate counterparts, which is likely due to a small mass
dilution effect.
Scheme 4
targeted tartrate derivatives. Before exploiting the acid/base
strategy from the previously accessed hydroxide precursors, we
briefly explored an alternative approach. Indeed, it had
originally appeared to us that the use of a pivotal silver
carboxylate might represent a flexible route to the desired
series of ILs.
1H and 13C NMR spectroscopic characterisations in CD3OD
are collected in Table 2. As far as the chemical shifts of the
cationic moiety were concerned, only limited influence of the
nature of the anion (Tart vs. HTart, D/L vs. meso) was recorded.
On the other hand, several informative trends were observed
regarding the tartrate moiety. 1H and 13C chemical shifts were
indeed highly conserved within the bis-tartrate or the hemi-
tartrate series. For example, typical patterns of d 177.0/74.1
ppm (CO/HCOH) for the hemi-tartrates vs. d 179.0/75.1 ppm
for the bis-tartrates were recorded. Maschmeyer had previously
noticed the gradual upfield shift in 1H NMR in D2O of the C–H
protons of the tartaric acid moiety, going from the hemi- to the
bis-tartrate, concordant with an overall increase in electronic
density.11 Parallel behaviours were recorded both in CD3OD
and D2O with respective chemical shifts of d 4.40/4.50 ppm for
the hemi-tartrates relative to d 4.30 ppm for the bis-tartrates in
both solvents. Furthermore, we found that this particular
trend may also serve as a direct probe of the sample
homogeneity. Indeed, comparison of the 1H NMR in DMSO-
d6 of our analytically pure sample of [Bmim]2D-Tart with the
spectra reported by Yuan12 revealed a significant shielding of
ca. +0.3 ppm for the two protons of the hemi-tartrate anion in
our analysis, the rest of the spectra being otherwise identical.
Interestingly, we observed that, starting from our sample, the
chemical shift of the tartrate protons could be increased from
d 3.76 ppm to 4.02 ppm by addition of a tartaric acid solution
in DMSO-d6, eventually leading to a spectra identical to that in
Yuan’s publication (Fig. 4).12 This observation illustrates well
the difficulties encountered in assessing by NMR the homo-
geneity RTILs, in which mixtures of species lead to a single set
of signals of averaged chemical shifts. It also points out the
key importance of the relative chemical integrations of the two
ionic partners.
To test this, we prepared silver tartrate by simply reacting
tartaric acid with 1 equiv. of Ag2CO3 in water (Scheme 3).
Further addition of a two fold excess of a halide salt, such as
[Bmim]Br, to the resulting suspension led, after filtration, to a
colourless sticky material of good purity in a highly practical
1
procedure. In concordance with our previous observation, H
NMR analysis of the thus prepared [Bmim]2Tart (2)-1b in D2O
showed a varying level of integration for H-2, demonstrating a
substantial deprotonation at C-2 by the otherwise weakly basic
carboxylate counter-anion (see ESI ). This procedure proved
3
readily usable for the preparation of tartrate salts from various
cations, such as [Bmim]2meso-Tart. However, contrary to our
initial expectations, unexpected difficulties in obtaining the
correct stoichiometric balance were encountered in our
attempts to use silver hemi-tartrate for the preparation of
the corresponding hydrogen tartrate ILs.
Keeping in mind the need for a unified route to tartrate-
based ILs, we thus considered a more secure two-step
procedure involving neutralisation of hydroxide salts.
Thus, a general protocol for the preparation of the cation
hydroxide was established by reacting the starting halide salt
in H2O for 2 h with 1.2 equiv. of freshly prepared AgOH
(Scheme 4). Filtration of the insoluble silver salts delivered an
aqueous solution of the desired cation hydroxide that was
used immediately. Simple addition of either 0.5 or 1.0 equiv. of
tartaric acid led to the bis- or hemi-tartrate salts respectively.
This procedure was smoothly applied to the following series of
6 different cations: 1-butyl-3-methylimidazolium, 1-butyl-2,3-
dimethylimidazolium, 1-octylpyridinium, tetrabutylammo-
nium, 1-butyl-1-metylpyrrolidinium and 1-butyl-1-methylpiper-
idinium.
Varying the stoichiometry and the stereoisomeric forms of
tartaric acid led to a total of 24 bis- or hemi-tartrate derivatives.
Yields in the range of 90–95% were routinely obtained on a 5
mmol scale. Table 1 gives a selected overview of 14 of the most
representative prepared structures 1–6. Compounds were
isolated either as viscous oils, waxes or solids. For examples,
[TBA]D-HTart (2)-4a, previously described as an oil,11 was
repeatedly obtained as a white solid. All salts were found to be
readily soluble in water, methanol and acetonitrile.
All the new tartrate salts prepared were analysed by ATR-
FTIR (attenuated total reflectance Fourier transform infrared
spectroscopy) using carefully dehydrated samples. In the bis-
tartrate series, the OH stretching frequencies (between 3399–
{ Although these water content values may be consider to be a little high, it
should be noted that they result only from smooth drying at room temperature
and are likely to reflect the regular state of these hygroscopic salts.
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Table 1 Overview of the most representative prepared tartrate salts
Cmpd
no.
Cmpd
Yieldsa no.
Symbol
Structure
Symbol
Structure
Yieldsa
(2)-1a [Bmim]D-HTart
95%
98%
(2)-1b [Bmim]2D-Tart
93% (92%)b
(2)-2a [Bmmim]D-HTart
(2)-2b [Bmmim]2D-Tart
95%
(2)-3a [OctPyr]D-HTart
meso-3a [OctPyr]meso-HTart
(2)-4a [TBA]D-HTart
95%
90%
95%
(2)-3b [OctPyr]2D-Tart
meso-3b [OctPyr]2meso-Tart
(2)-4b [TBA]2D-Tart
95% (85%)b
94%
95%
(2)-5a [Bmpyr]D-HTart
(2)-6a [Bmpip]D-HTart
95%
93%
(2)-5b [Bmpyr]2D-Tart
(2)-6b [Bmpip]2D-Tart
92%
94%
a
b
Yields in analytically pure isolated product according to method B: the cation hydroxide is neutralised with tartaric acid. Yields in isolated
product according to method A: the silver tartrate is treated with the cation halide.
3369 cm21) were blue-shifted compared to those observed in
potassium sodium tartrate salt (3237 cm21). This shift,
characteristic of more weakly bonded hydrogens, likely reflects
the poor coordinating properties of the ionic liquid cations.
This effect was more pronounced in pyridinium and imidazo-
lium aromatic cations (3399–3387 cm21) than in ammonium
derivatives (3370–3369 cm21). The same overall behaviour was
observed with hemi-tartrate salts. The OH stretching frequen-
cies (between 3418–3394 cm21) were found to be higher than
in potassium hydrogen tartrate (3309 cm21). In the imidazo-
lium series, the detailed study of Li also yielded interesting
observations.36 In this work, the authors assigned the different
C–H stretching vibrations of the imidazolium ring to a series
of anions and correlated the C-2–H stretching frequency to the
electron density of H-bonding between the cation and the
anion. Results from this study showed that the stronger the
cation–anion interaction, the lower the stretching frequency,
with the largest red shift being for [Bmim]OH (n 3057 cm21).
In our series, the C-2–H stretching frequencies were easy to
identify by comparing the Bmim species with their 2-methyl
congeners lacking this vibrational band (Fig. 5 and 6). A
frequency of 3109 cm21 was recorded for [Bmim]D-HTart,
whereas an even smaller value of 3102 cm21 was found for
[Bmim]2D-Tart containing the more coordinating bis-carbox-
ylate moiety. Noteworthy, this vibrational band was observed
at 3088 cm21 in the corresponding meso stereoisomeric bis-
tartrate, isolated as a solid (vide infra). These C-2–H stretching
frequencies are comparable with that of [Bmim]HSO4 (3107
cm21) and [Bmim]CF3CO2 (3105 cm21).36 Additionally, the
presence of a methyl in C-2 was accompanied by a lowering of
the stretching frequency of the C-4,5-H relative to the 2-H
derivatives (3141 vs. 3151 cm21 for D-HTart and 3138 vs. 3148
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Fig. 4 Enlargement of the 1H NMR spectra (300 MHz, DMSO-d6) of a pure
sample of [Bmim]2D-Tart (2)-1b (top) and of the same sample after addition of
D-tartaric acid (bottom) showing the downfield shift of the tartrate moiety
protons.
cm21 for D-Tart) coherent with an enhanced H-bonding of the
two remaining imidazolium hydrogens in the 2-methyl series.
The thermal behaviour of all hemi-tartrate and bis-tartrate
salts prepared was assessed by differential thermal analysis
(DTA) and differential scanning calorimetry (DSC) analyses
(Table 3). All species displayed fair thermal stability, showing
no apparent decomposition up to near 200 uC. Salts possessing
an ammonium cation generally displayed fast one-step
processes whereas aromatic cation, such as OctPyr, led to
multistep decompositions of the resulting salts. The thermal
decomposition temperatures (Td) determined from the onset
of weight loss, were found to be between 189 uC and 229 uC.
Hemi-tartrates generally were slightly less stable than their
tartrate analogues. All prepared hemi- and bis-tartrate salts
demonstrated sufficient stability to be employed over a large
range of temperatures.
Fig. 5 ATR-IR absorption spectra in the range 220024000 cm21 of [Bmim]D-
HTart (upper trace showing the nC-2–H at 3109 cm21) and [Bmmim]D-HTart
(lower trace missing this specific absorption band).
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Fig. 7 DSC plot recorded during heating (two upper traces) and cooling (lower
trace) for [TBA]2D-Tart.
Fig. 6 ATR-IR absorption spectra in the range 220024000 cm21 of [Bmim]2D-
Tart (upper trace showing the nC-2–H at 3102 cm21) and [Bmmim]2D-Tart
(lower trace missing this specific absorption band).
room temperature. Lower glass transition temperatures were
also recorded, for example with [Bmim]2D-Tart (Tg 215 uC),
[Bmim]D-HTart (Tg 223 uC) or [Bmim]2meso-Tart (Tg 247 uC).
When compared to those of their bis-tartrate analogues, Tg
values of the hemi-tartrate salts were found to be generally
higher. This trend may be correlated to the higher capacity of
the hemi-tartrate anion to establish highly cohesive hydrogen
bonds. When also observed on the first heating cycles, Tg were
found to be at far lower temperatures than during the second
cycle (261 uC§ vs. 215 uC for instance with [Bmim]2D-Tart),
which is likely due to the residual presence in the sample of
volatile contaminants, such as water or solvents, requiring a
high temperature to be evaporated off. These data indicated
that, with the exception of [Bmmim]D-HTart and [Bmim]2meso-
Tart, all hemi- and bis-tartrate salts reported here can formally
be considered as ILs melting below 100 uC. All bis-tartrate
derivatives were liquid at room temperature, with the excep-
tion of [Bmim]2meso-Tart. The same was true for several hemi-
tartrates, depending on the cation. Compared to TBA and
Bmmim, cations such as Bmim and OctPyr were revealed as
particularly favourable in conferring RTIL behaviour.
The phase behaviour of the prepared hemi- and bis-tartrate
salts was determined by DSC from two consecutive heating
cycles (from 280 to 150 uC at 10 K min21) (Table 3). For several
samples, including in particular compounds isolated as waxes
or low melting solids, an endothermic peak (from 56 to 92 J
g
21) of melting was recorded on the first heating cycle. This is
the case for all the following hemi-tartrates [Bmmim]D-HTart
(Tm 117 uC), [Bmpyr]D-HTart (Tm 96 uC), [TBA]D-HTart (Tm 94
uC), [Bmpip]D-HTart (Tm 33 uC) or bis-tartrates [Bmim]2meso-
Tart (Tm 125 uC), [TBA]2D-Tart (Tm 50 uC) (Fig. 7). This
behaviour may be associated with the solidification of the
samples induced by their initial hydration (vide infra).
Only [Bmmim]2D-Tart and [Bmpip]2D-Tart displayed
a
weakly endothermic peak (from 5 to 8 J g21) of melting or of
solid–solid transition on the second heating cycle with Tm of
82 uC and 18 uC, respectively (Fig. 8).
Glass transition temperatures were recorded on the second
heating cycle and are expressed as the inflexion point. With
the exception of [Bmpyr]2D-Tart, all derivatives showed a glass
transition on the second heating cycle with Tg usually around
Finally, within the OctPyr series, the influence of the tartrate
anion stereochemistry was briefly considered. Tg values in the
achiral meso series were found to be significantly altered
Table 3 Physical aspect and thermal behaviour of prepared tartrate salts
Compound
Aspecta
Td (uC)
Tg (uC)
Tm (uC)
[Bmim]D-HTart
[Bmim]2D-Tart
viscous oil
viscous oil
solid
solid
solid
viscous oil
viscous oil
viscous oil
viscous oil
viscous oil
viscous oil
solid
wax
wax
viscous oil
solid
solid
200
229
234
193
227
210
216
200
209
202
211
211
189
205
214
193
214
223
215
247
7
22
13
28
22
218
22
22
19
212
24
—
—
—
125
117
82
—
—
—
—
—
[Bmim]2meso-Tart
[Bmmim]D-HTart
[Bmmim]2D-Tart
[OctPyr]D-HTart
[OctPyr]2D-Tart
OctPyr]DL-HTart
[OctPyr]2DL-Tart
[OctPyr]meso-HTart
[OctPyr]2meso-Tart
[TBA]D-HTart
—
Fig. 8 DSC plot recorded during heating (two upper traces) and cooling (lower
94
50
96
—
33
18
trace) for [Bmpip]2D-Tart.
[TBA]2D-Tart
[Bmpyr]D-HTart
[Bmpyr]2D-Tart
[Bmpip]D-HTart
210
234
[Bmpip] D-Tart
2
§ This Tg value of 261 uC is in agreement with the Tg of 258 uC reported
previously as measured on the first heating cycle.15
a
Aspect after drying and prolonged storage at 4 uC.
420 | RSC Adv., 2013, 3, 413–426
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Fig. 9 DSC plot recorded during heating (two upper traces) and cooling (lower
trace) for [OctPyr]D-HTart.
Fig. 11 Molecular view of [TBA]2L-Tart (+)-4b in the solid state (thermal
ellipsoids at 50% probability); hydrogens are omitted for clarity excepted on
oxygen atoms.
compared to those recorded with chiral enantiopure anions. In
the hemi-tartrate series for example, [OctPyr]D-HTart has a Tg
of 13 uC (Fig. 9) whereas a Tg of 22 uC was recorded for
[OctPyr]meso-HTart (Fig. 10).
suitable for X-ray diffraction (Fig. 11). The resulting analysis
gave interesting informations regarding the structuring effect
of water present in the lattice. The influence of traces of water
on the crystal properties of ionic liquids has been studied.38
Here, one can see that the tartrate anion is embedded in a
network of hydrogen bonds involving water molecules while,
on the other hand, the bulky tetrabutylammonium cations
seem to only loosely interact with the carboxylates.
Organisation of ILs in polar and apolar regions under the
influence of water has been modelled.39 To the best of our
knowledge, this is the first crystal structure obtained from an
organic anion-based IL.
The same observation can be made when comparing chiral
enantiopure and meso pyridinium tartrates (Tg 28 uC for
[OctPyr]2D-Tart vs Tg 22 uC for [OctPyr]2meso-Tart). This trend
was also observed between the imidazolium tartrates
[Bmim]2D-Tart (Tg 215 uC) and [Bmim]2meso-Tart (Tg 247
uC). Moreover, the latter spontaneously solidified upon
isolation, whereas the former is obtained as a viscous oil. A
strong modification of thermal behaviour was also encoun-
tered between chiral enantiopure salts and their correspond-
ing racemates. There again, this was true both in the hemi-
tartrate series (Tg 13 uC for [OctPyr]D-HTart vs Tg 22 uC for
[OctPyr]DL -HTart) and bis-tartrate series (Tg 28 uC for
[OctPyr]2D-Tart vs Tg 218 uC for [OctPyr]2DL-Tart). Gotor and
Luis have recently reported a related influence of the chirality
of the cationic component of a series of imidazolium ILs on
their final physical properties.37 However, to the best of our
knowledge, this is the first report of such an effect being
driven by the chirality of the anion of an IL.
Conclusions
In conclusion, we report the first systematic preparation and
characterisation of a series of organic cation-derived tartrate
salts. This study allowed identification of a series of structural
factors influencing the physical properties of these ionic
entities. In particular, the IL behaviour was influenced not
only by the nature of the organic cation, but also by the chiral
tartrate moiety, in particular through its chirality. The
synergistic benefit of ILs and multicomponent transforma-
tions in the development of environmentally benign proce-
dures has been recently highlighted.40 Interestingly, the use of
a low melting point tartaric acid–urea mixture as a green
reaction media for the multicomponent Biginelli reaction has
been described lately.41 The present work thus opens promis-
ing prospects for the development of alternative chiral
reaction media. Work along these lines is currently under
way in our laboratories.
Crystallographic analysis of a representative tartrate salt was
also achieved. It was observed that a waxy sample of the
hydroscopic [TBA]2L-Tart (+)-4b was able to grow crystals upon
prolonged storage at 4 uC." This material proved gratifyingly
Experimental
Fig. 10 DSC plot recorded during heating (two upper traces) and cooling (lower
trace) for [OctPyr]meso-HTart.
General information
NMR spectroscopic data were obtained with Bruker Advance
300. Chemical shifts are quoted in parts per million (ppm)
relative to residual solvent peak. J values are given in Hz.
" DSC analysis of this crystalline sample gave a Tm of 37 uC.
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3H), 1.86–1.71 (m, 2H), 1.34–1.18 (m, 2H), 0.86 (t, J = 7.4 Hz,
3H).
Infrared (IR) analyses were recorded with a Nexus-Thermo
Nicolet FT-IR using an ATR diamond Smart-ITR. Mass
spectrometry (MS) data were obtained on a ThermoQuest
TSQ 7000 spectrometer. Elementary analyses were realized at
the ‘‘Service Central d’Analyse’’ of the CNRS, (Solaize, France).
Optical rotations were measured on a Jasco P 2000 polari-
meter. Bromide quantifications were done using a Mettler
1-ETHYL-3-METHYLIMIDAZOLIUM HYDROXIDE [EMIM]OH:. Prepared
from [Emim]I (99.3 mg, 0.42 mmol) according to the general
procedure under diluted conditons (20 mL g21) and with
stirring at 10 uC. 1H-NMR (300 MHz, D2O) d 7.39 (d, J = 1.9 Hz,
0.85H), 7.33 (d, J = 1.85 Hz, 0.85H), 4.15 (q, J = 7.4 Hz, 2H), 3.80
(s, 3H), 1.41 (t, J = 7.4 Hz, 3H).
Toledo SevenMultiTM S50-K pH meter equiped with
a
1-BUTYL-2,3-DIMETHYLIMIDAZOLIUM HYDROXIDE [BMMIM]OH:.
Preprared from [Bmmim]Br (105 mg, 0.45 mmol) according
to the general procedure. 1H-NMR (300 MHz, D2O) d 7.29 (d, J =
2.1 Hz, 1H), 7.25 (d, J = 2.1 Hz, 1H), 4.05 (t, J = 7.3 Hz, 2H), 3.71
(s, 3H), 2.5322.48 (m, 0.6H), 1.8021.65 (m, 2H), 1.3421.19 (m,
2H), 0.87 (t, J = 7.4 Hz, 3H).
1-OCTYLPYRIDINIUM HYDROXIDE [OCTPYR]OH:. Prepared from
[OctPyr]Br (219 mg, 0.81 mmol) according to the general
procedure. 1H-NMR (300 MHz, D2O) d 8.78–8.70 (m, 0.1H),
8.46 (d, J = 7.9, 1H), 7.98 (d, J = 7.8 Hz, 2H), 4.51 (t, J = 7.3, 2H),
1.90 (quintuplet, J = 7.1 Hz, 2H), 1.30–1.03 (m, 10H), 0.71 (t, J =
6.7 Hz, 3H).
InLabReferencePro electrode and a bromide-selective elec-
trode. The Ionic Strength Adjuster (ISA) buffer solution used
was purchased from Metler Toledo. DTA were run on a Perkin
Elmer Diamond TG/DTA apparatus. DSC plots were recorded
with a Netzsch DSC 204 apparatus. All bromide salts were
purchased from Solvionic, Acros or Io-Li-Tec and used without
additional purification, except N-octylpyridinium bromide
which was prepared using the standard procedure. 1-Ethyl-3-
methylimidazolium iodide was prepared using the standard
procedure. Crystallographic data were collected at a tempera-
ture of 193(2)K on a Bruker-AXS APEX II diffractometer using a
30 W air-cooled microfocus source (ImS) with focusing
multilayer optics, with graphite-monochromated Mo-Ka radia-
tion (l = 0.71073 Å) by using phi- and omega-scans. The data
were integrated with SAINT, and an empirical absorption
correction with SADABS was applied.42,43 The structures was
solved by direct methods, using SHELXS-9744 and refined
using the least-squares method on F2.45 All non-H atoms were
treated anisotropically. The hydrogen atoms, except H on the
oxygen atoms, were placed at calculated positions, and refined
applying riding models. The OH hydrogen atoms were located
on difference Fourier maps and isotropically refined.
Bromide content determination in cation hydroxide solutions
The bromide quantification in the [C]OH solutions was
performed with a Br2 Ion selective electrode (ISE) in aqueous
solution. As the calibration curve is known to vary with the
cation,32 for every [C]OH solutions, a set of aqueous solutions
of the corresponding [C]Br ranging from 0.5 to 500 ppm were
systematically used as calibration standards. To quantify the
Br2 concentration, the analyzed [C]OH aqueous solutions were
first diluted with purified water to 0.1 g/50 ml and mixtured
with 1 mL ISA ionic buffer solution (ionic strength adjuster; 5
M
NaNO3 solution). Potential measurements were then
Preparation of silver hydroxide
performed. The response of halide ISEs at 298 K can be
described, for instance for [Bmmim]Br, by the equation y =
2253e20.035x where y is the bromide concentration and x is the
measured voltage (potential).
To a solution of sodium hydroxide (1.00 g, 25 mmol) in
distilled water (10 mL) was added silver nitrate (4.25 g, 25
mmol). The solution was stirred for 10 min at room
temperature and away from light. The resultant mixture was
filtered, and the solid obtained washed with water. Silver
hydroxide was dried in the dark in a vacuum desiccator over
phosphorus pentoxide. The compound was obtained as a fine
dark brown powder (2.81 g, 90%).
General procedure for the preparation of tartrate salts
METHOD A. To a solution of tartaric acid in ‘‘ultrapure’’ water
(10 mL g21) was added silver carbonate (1 equiv.). After 2 h at
room temperature and out of direct light, [C]Br (2 equiv.) was
added rapidly and the reaction stirred overnight at room
temperature and always away from light. The solution was
filtered over 47 mm hydrophilic polypropylene 0.2 mm
membrane filters and water removed by lyophilisation. The
compound was then dissolved in acetonitrile (10 mL) and
filtered on a 25 mm Acrodisc1 syringe filter with 0.2 mm PTFE
membrane to remove the unreacted tartaric acid. The solvent
was removed under vacuum and the compound dried under
reduced pressure.
METHOD B. To a solution of [C]Br (1.00 g) in ‘‘ultrapure’’ water
(10 or 20 mL) was added silver hydroxide (1.2 equiv). After 2 h
at room temperature or at 10 uC and out of direct light the
solution was filtered on a 47 mm hydrophilic polypropylene
0.2 mm membrane filter and washed with water (15 mL).
Tartaric acid was added rapidly (0.5 or 1 equiv.) and the
solution was stirred for 2 h at room temperature and still away
from light. Water was removed by lyophilisation, and the
compound was dissolved in acetonitrile or methanol (10 mL)
General procedure for the preparation of hydroxide salts
To a solution of the cation bromide in D2O (10 or 20 mL g21
)
was added freshly prepared silver oxide (1.2 equiv.). After 2 h at
room temperature or at 10 uC and out of direct light the
solution was filtered over 47 mm hydrophilic polypropylene
0.2 mm membrane filters. The D2O solution was then analyzed
directly.
1-BUTYL-1-METHYLPIPERIDINIUM HYDROXIDE [BMPIP]OH. Prepared
from [Bmpip]Br (130 mg, 0.55 mmol) according to the general
1
procedure. H-NMR (300 MHz, D2O) d 3.36–3.19 (m, 6H), 2.95
(s, 3H), 1.88–1.74 (m, 4H), 1.72–1.50 (m, 4H), 1.32 (sextuplet, J
= 7.4 Hz, 2H), 0.90 (t, J = 7.4 Hz, 3H).
1-BUTYL-3-METHYLIMIDAZOLIUM HYDROXIDE [BMIM]OH. Prepared
from [Bmim]Br (101 mg, 0.48 mmol) according to the general
procedure under diluted conditons (20 mL g21) and with
stirring at 10 uC. 1H-NMR (300 MHz, D2O) d 7.42 (d, J = 1.7 Hz,
0.5H), 7.37 (d, J = 1.4 Hz, 0.5H), 4.14 (t, J = 7.1 Hz, 2H), 3.84 (s,
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and filtered over a 25 mm Acrodisc1 syringe filter with 0.2 mm
PTFE membrane to remove the unreacted tartaric acid. The
solvent was removed under vacuum and the compound dried,
for several days, in a vacuum desiccator over phosphorus
pentoxide.
1-Octylpyridinium meso-hemitartrate, [OctPyr]meso-HTart,
meso-3a: prepared following method B using [OctPyr]Br (1.00
g, 3.67 mmol), AgOH (550 mg, 4.41 mmol) and meso-tartaric
acid (551 mg, 3.67 mmol). The salt meso-3a was obtained as a
viscous light brown oil (1.12 g, 90%). 1H-NMR (300 MHz,
CD3OD) d 9.02 (d, J = 5.5 Hz, 2H), 8.60 (tt, J = 7.8, 1.24 Hz, 1H),
8.13 (t, J = 7.0 Hz, 2H), 4.64 (t, J = 7.6 Hz, 2H), 4.30 (s, 2H), 2.03
(quintuplet, J = 7.0 Hz, 2H), 1.48–1.21 (m, 10H), 0.90 (t, J = 6.8
Hz, 3H). 13C-NMR (75 MHz, CD3OD) d 178.1, 148.7, 146.1,
129.5, 76.0, 63.0, 32.8, 32.5, 30.2, 30.1, 27.2, 23.6, 14.4. ATR-
FTIR n 3394, 3129, 3084, 3061, 2957, 2856, 1718, 1631, 1603,
1487, 1466, 1352, 1209, 1177, 1078, 1006 cm21. MS (ESI+) m/z
192; (ESI2) m/z 149. Anal calc. for C17H27NO6 + 1.25 H2O C
56.11 H 8.17 N 3.85%; found C 56.18 H 8.27 N 3.90%
Tetrabutylammonium D-hemitartrate, [TBA]D-HTart, (2)-4a:
prepared following method B using [TBA]Br (1.00 g, 3.10
mmol), AgOH (465 mg, 3.72 mmol) and D-tartaric acid (465
mg, 3.10 mmol). The salt (2)-4a was obtained as a white solid
(1.15 g, 95%). [a]D20 211 (c 1.0, H2O) (litt.11 [a]2D0 210.20, c 5,
H2O). 1H-NMR (300 MHz, CD3OD) d 4.39 (s, 2H), 3.27–3.19 (m,
8H), 1.74–1.59 (m, 8H), 1.42 (sextuplet, J = 7.4 Hz, 8H), 1.03 (t, J
= 7.3 Hz, 12H). 13C-NMR (75 MHz, D2O) d 175.9, 72.8, 57.9,
23.1, 19.1, 13.0. ATR-FTIR n 3415, 2960, 2936, 2875, 1712, 1663,
1611, 1486, 1462, 1381, 1348, 1112, 1068, 1028 cm21. MS (ESI+)
m/z 242; (ESI2) m/z 149. Anal calc. for C20H41NO6 + 0.75 H2O C
59.30 H 10.58 N 3.46%; found C 59.25 H 10.32 N 3.38%.
Tetrabutylammonium L-hemitartrate, [TBA]L-HTart, (+)-4a:
prepared as its enantiomer; it displayed analytical data
identical to that of the D-isomer. [a]2D0 +11 (c 0.9, H2O) (litt.11
[a]2D0 +10.42, c 5, H2O).
1-Butyl-3-methylimidazolium D-hemitartrate, [Bmim]D-
HTart, (2)-1a: prepared following method B using [Bmim]Br
(1.00 g, 4.56 mmol), AgOH (0.684 g, 5.48 mmol) and D-tartaric
acid (685 mg, 4.56 mmol). The salt (2)-1a was obtained as a
viscous pale yellow oil (1.25 g, 95%). [a]2D0 211 (c 0.75, H2O).
1H-NMR (300 MHz, D2O) d 8.68 (s, 1H), 7,44 (t, J = 1.8 Hz, 1H),
7.40 (t, J = 1.8 Hz, 1H), 4.47 (s, 2H), 4.16 (t, J = 7.1 Hz, 2H), 3.85
(s, 3H), 1.88–1.74 (m, 2H), 1.36–1.20 (m, 2H), 0.88 (t, J = 7.4 Hz,
3H). 13C-NMR (75 MHz, CD3OD), d 177.1, 138.1, 124.9, 123.5,
74.3, 50.9, 36.5, 33.1, 20.4, 13.8. ATR-FTIR n 3404, 3151, 3110,
2962, 2876, 1721, 1601, 1573, 1465, 1352, 1234, 1168, 1123,
1073 cm21. MS (IC+, NH3) m/z 139; (IC2, NH3) m/z 149. Anal
calc. for C12H20N2O6 + 1.25 H2O C 46.37 H 7.30 N 9.01%; found
C 46.39 H 7.38 N 9.16%.
1-Butyl-3-methylimidazolium
L-hemitartrate,
[Bmim]L-
HTart, (+)-1a: prepared as its enantiomer, it displayed
analytical data identical to that of the D-isomer. [a]2D0 +11 (c
0.95, H2O).
1-Butyl-2,3-dimethylimidazolium D-hemitartrate, [Bmmim]D-
HTart, (2)-2a: prepared following method B using [Bmmim]Br
(1.00 g, 4.29 mmol), AgOH (643 mg, 5.15 mmol) and D-tartaric acid
(643 mg, 4.29 mmol). The salt (2)-2a was obtained as a white solid
(1.27 g, 98%). [a]2D0 211.5 (c 1.1, H2O). 1H-NMR (300 MHz, CD3OD)
d 7.51 (d, J = 2.1 Hz, 1H), 7.47 (d, J = 2.1 Hz, 1H), 4.38 (s, 2H), 4.15 (t,
J = 7.4 Hz, 2H), 3.81 (s, 3H), 2.62 (s, 3H), 1.87–1.73 (m, 2H), 1.48–
1.32 (m, 2H), 0.99 (t, J = 7.3 Hz, 3H). 13C-NMR (75 MHz, CD3OD) d
176.8, 145.7, 123.7, 122.1, 74.1, 49.2, 35.5, 32.7, 20.5, 13.9, 9.5. ATR-
FTIR n 3418, 3177, 3141, 2962, 2935, 2875, 1723, 1609, 1589, 1540,
1465, 1419, 1355, 1249, 1126, 1074 cm21. MS (IC+, NH3) m/z 153;
(IC2, NH3) m/z 149. Anal calc. for C13H22N2O6 + 0.25 H2O C 50.89 H
7.39 N 9.13%; found C 51.14 H 7.47 N 9.09%.
1-Butyl-1-methylpyrrolidinium D-hemitartrate, [Bmpyr]D-
HTart, (2)-5a: prepared following method B using [Bmpyr]Br
(1.00 g, 4.5 mmol), AgOH (674 mg, 5.4 mmol) and D-tartaric
acid (675 mg, 4.5 mmol). The salt (2)-5a was obtained as a
1
colourless wax (1.24 g, 95%). [a]2D0 213 (c 0.98, H2O). H-NMR
(300 MHz, CD3OD) d 4.39 (s, 2H), 3.61–3.43 (m, 4H), 3.37–3.31
(m, 2H), 3.05 (s, 3H), 2.30–2.15 (m, 4H), 1.86–1.71 (m, 2H), 1.43
(sextuplet, J = 7.5 Hz, 2H), 1.02 (t, J = 7.3 Hz, 3H). 13C-NMR (75
MHz, CD3OD) d 177.0, 74.2, 65.3 (2 peaks), 65.2, 49.8, 26.6,
22.5, 20.7, 14.0. ATR-FTIR n 3414, 3242, 3031, 2963, 2936, 2876,
1719, 1655, 1601, 1465, 1399, 1350, 1226, 1118, 1070, 1005
cm21. MS (IC+) m/z 142; (IC2) m/z 149. Anal calc. for
1-Butyl-2,3-dimethylimidazolium
L-hemitartrate,
[Bmmim]L-HTart, (+)-2a: prepared as its enantiomer; it
displayed analytical data identical to that of the D-isomer.
[a]2D0 +11.5 (c 0.99, H2O).
1-Octylpyridinium D-hemitartrate, [OctPyr]D-HTart, (2)-3a:
prepared following method B using [OctPyr]Br (1.00 g, 3.67
mmol), AgOH (550 mg, 4.41 mmol) and D-tartaric acid (551
mg, 3.67 mmol). The salt (2)-3a was obtained as a viscous pale
C
13H25NO6 + 2 H2O C 47.69 H 8.93 N 4.28%; found C 47.56
H 8.91 N 4.31%.
1-Butyl-1-methylpiperidunium D-hemitartrate, [Bmpip]D-
HTart, (2)-6a: prepared following method B using [Bmpip]Br
(1.00 g, 4.23 mmol), AgOH (634 mg, 5.08 mmol) and D-tartaric
acid (635 mg, 4.23 mmol). The salt (2)-6a was obtained as a
1
yellow oil (1.19 g, 95%). [a]2D0 210 (c 1.0, H2O). H-NMR (300
MHz, CD3OD) d 9.02 (d, J = 5.5 Hz, 2H), 8.60 (tt, J = 7.8, 1.2 Hz,
1H), 8.13 (t, J = 7.0 Hz, 2H), 4.64 (t, J = 7.6 Hz, 2H), 4.40 (s, 2H),
2.02 (quintuplet, J = 7.0 Hz, 2H), 1.4721.21 (m, 10H), 0.90 (t, J
= 6.8 Hz, 3H). 13C-NMR (75 MHz, CD3OD) d 176.7, 146.8, 146.0,
129.5, 74.1, 63.1, 32.8, 32.5, 30.2, 30.1, 27.2, 23.6, 14.4. ATR-
FTIR n 3381, 3130, 3086, 3064, 2953, 2928, 2858, 2827, 1730,
1634, 1610, 1489, 1467, 1355, 1221, 1178, 1126, 1077, 1033
cm21. MS (ESI+) m/z 192; (ESI2) m/z 149. Anal calc. for
C13H27NO6 + 1.5 H2O C 55.42 H 8.21 N 3.80%; found C 55.60 H
7.70 N 3.94%.
1
white solid (1.20 g, 93%). [a]2D0 211 (c 1.0, H2O). H-NMR (300
MHz, D2O) d 4.52 (s, 2H), 3.37–3.26 (m, 6H), 3.01 (s, 3H), 1.94–
1.80 (m, 4H), 1.80–1.56 (m, 4H), 1.38 (sextuplet, J = 7.4 Hz, 2H),
0.95 (t, J = 7.4 Hz, 3H). 13C-NMR (75 MHz, CD3OD) d 176.9,
74.2, 64.7, 62.1, 48.2, 24.7, 22.1, 20.9, 20.8, 14.0. ATR-FTIR n
3418, 3043, 2962, 2877, 1723, 1608, 1467, 1353, 1228, 1124,
1073 cm21. MS (IC+) m/z 156; (IC2) m/z 149. Anal calc. for
C
14H27NO6 + 0.25 H2O C 54.26 H 8.95 N 4.52%; found C 54.39
1-Octylpyridinium L-hemitartrate, [OctPyr]L-HTart, (+)-3a:
prepared as its enantiomer; it displayed analytical data
identical to that of the D-isomer. [a]2D0 +10 (c 1.5, H2O).
H 9.02 N 4.60%.
1-Butyl-3-methylimidazolium D-tartrate, [Bmim]2D-Tart, (2)-
1b: prepared following method B using [Bmim]Br (1.00 g, 4.56
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mmol), AgOH (684 mg, 5.48 mmol) and D-tartaric acid (342
(IC2) m/z 149. Anal calc. for C30H48N2O6 + 3.5 H2O C 60.48 H
9.31 N 4.70%; found C 60.24 H 9.67 N 4.70%.
1-Octylpyridinium L-tartrate, [OctPyr]2L-Tart, (+)-3b: pre-
pared as its enantiomer; it displayed analytical data identical
to that of the D-isomer. [a]2D0 +9.5 (c 0.98, H2O).
1-Octylpyridinium DL-tartrate, [OctPyr]2DL-Tart, rac-3b: pre-
pared following method B using [OctPyr]Br (1.00 g, 3.67
mmol), AgOH (550 mg, 4.41 mmol) and DL-tartaric acid (276
mg, 1.84 mmol). The salt rac-3b was obtained as a viscous pale
yellow oil (931 mg, 95%). This compound displayed anaytical
data identical to that of its D- or L-stereoisomer. Anal calc. for
C30H48N2O6 + 4.25 H2O C 59.14 H 9.35 N 4.60%; found C 59.33
H 9.43 N 4.64%.
mg, 2.28 mmol). The salt (2)-1b was obtained as a viscous pale
1
yellow oil (904 mg, 93%). [a]2D0 210 (c 0.7, H2O). H-NMR (300
MHz, D2O) d 8.70 (s, 2H), 7.46 (d, J = 1.9 Hz, 2H), 7.42 (d, J = 1.9
Hz, 2H), 4.29 (s, 2H), 4.18 (t, J = 7.1 Hz, 4H), 3.88 (s, 6H), 1.90–
1.75 (m, 4H), 1.38–1.21 (m, 4H), 0.91 (t, J = 7.4 Hz, 6H). 13C-
NMR (75 MHz, D2O) d 178.3, C-2 non apparent, 123.4, 123.4,
122.2, 122.1, 73.8, 49.2, 35.5, 31.2, 18.7, 12.6. ATR-FTIR n 3389,
3148, 3102, 2962, 2936, 2875, 1603, 1466, 1352, 1170, 1115,
1062 cm21. MS (IC+) m/z 139; (IC2) m/z 149. Anal calc. for
C20H34N4O6 + 3 H2O C 49.99 H 8.39 N 11.66%; found C 49.71 H
8.52 N 11.67%.
1-Butyl-3-methylimidazolium L-tartrate, [Bmim]2L-Tart, (+)-
1b: prepared as its enantiomer; it displayed analytical data
identical to that of the D-isomer. [a]2D0 +10 (c 1.0, H2O) (litt.15
[a]2D0 +11.7 (c 1, H2O).
1-Butyl-3-methylimidazolium meso-tartrate, [Bmim]2meso-
Tart, meso-1b: prepared following method A using meso-
tartaric acid (1.12 g, 6.66 mmol), Ag2CO3 (1.84 g, 6.67 mmol)
and [Bmim]Br (2.92 g, 13.3 mmol). The salt meso-1b was
obtained as a white solid (2.61 g, 92%). 1H-NMR (300 MHz,
CD3OD) d 9.05 (s, nH), 7.62 (d, J = 1.9 Hz, 2H), 7.57 (d, J = 1.9
Hz, 2H), 4.23 (t, J = 7.3 Hz, 4H), 4.11 (s, 2H), 3.94 (s, 6H), 1.80–
1.94 (m, 4H), 1.45–1.32 (m, 4H), 0.99 (t, J = 7.3 Hz, 6H). 13C-
NMR (75 MHz, CD3OD) d 178.3, 138.1, 124.9, 123.5, 76.1, 50.5,
36.5, 33.1, 20.5, 13.8. ATR-FTIR n 3367, 3144, 3088, 2961, 2933,
2874, 1608, 1571, 1465, 1406, 1365, 1348, 1172, 1108, 1061
cm21. MS (IC+) m/z 139; (IC2) m/z 149. Anal calc. for
C20H34N4O6 + 3.5 H2O C 49.07 H 8.44 N 11.44%; found C
48.82 H 8.60 N 11.33%.
1-Octylpyridinium meso-tartrate, [OctPyr]2meso-Tart, meso-
3b: This compound was prepared following method B using
[OctPyr]Br (1.00 g, 3.67 mmol), AgOH (550 mg, 4.41 mmol) and
meso-tartaric acid (276 mg, 1.84 mmol). The salt meso-3b was
1
obtained as a viscous pale yellow oil (921 mg, 94%). H-NMR
(300 MHz, CD3OD) d 9.02 (d, J = 5.6 Hz, 4H), 8.60 (tt, J = 7.8, 1.1
Hz, 2H), 8.13 (t, J = 7.0 Hz, 4H), 4.64 (t, J = 7.6 Hz, 4H), 4.12 (s,
2H), 2.02 (quintuplet, J = 7.0 Hz, 4H), 1.49–1.21 (m, 20H), 0.90
(t, J = 7.2 Hz, 6H). 13C-NMR (75 MHz, CD3OD) d 178.2, 146.8,
145.1, 129.6, 76.0, 63.2, 63.1, 32.9, 32.6, 30.2, 30.1, 27.2, 23.7,
14.4. ATR-FTIR n 3391, 3126, 3059, 2955, 2927, 2856, 1608,
1488, 1467, 1358, 1223, 1177, 1109, 1072 cm21. MS (IC+) m/z
192; (IC2) m/z 149. Anal calc. for C30H48N2O6 + 5.25 H2O C
57.44 H 9.40 N 4.47%; found C 57.32 H 9.43 N 4.60%.
Tetrabutylammonium D-tartrate, [TBA]2D-Tart, (2)-4b: pre-
pared following method B using [TBA]Br (1.00 g, 3.10 mmol),
AgOH (465 mg, 3.72 mmol) and D-tartaric acid (233 mg, 1.55
mmol). The salt (2)-4b was obtained as colourless wax (932
mg, 95%). [a]2D0 28.5 (c 0.88, H2O). 1H-NMR (300 MHz, CD3OD)
d 4.31 (s, 2H), 3.27–3.21 (m, 16H), 1.72–1.60 (m, 16H), 1.42
(sextuplet, J = 7.3 Hz, 16H), 1.03 (t, J = 7.3 Hz, 24H). 13C-NMR
(75 MHz, CD3OD) d 179.0, 75.1, 59.5, 24.8, 20.7, 13.9. ATR-FTIR
n 3377, 2960, 2936, 2875, 1603, 1487, 1461, 1380, 1348, 1109,
1060, 1029 cm21. MS (IC+) m/z 242; (IC2) m/z 149. Anal calc.
for C36H76N2O6 + 2.5 H2O C 63.77 H 12.04 N 4.13%; found C
63.49 H 12.07 N 4.38%.
1-Butyl-2,3-dimethylimidazolium D-tartrate, [Bmmim]2D-
Tart, (2)-2b: prepared following method B using [Bmmim]Br
(1.00 g, 4.29 mmol), AgOH (643 mg, 5.15 mmol) and D-tartaric
acid (322 mg, 2.15 mmol). The salt (2)-2b was obtained as a
1
pale yellow solid (928 mg, 95%). [a]2D0 29.5 (c 0.95, H2O). H-
NMR (300 MHz, CD3OD) d 7.51 (d, J = 2.1 Hz, 2H), 7.48 (d, J =
2.1 Hz, 2H), 4.28 (s, 2H), 4.15 (t, J = 7.4 Hz, 4H), 3.82 (s, 6H),
2.62 (s, 6H), 1.88–1.73 (m, 4H), 1.48–1.31 (m, 4H), 0.99 (t, J =
7.3 Hz, 6H). 13C-NMR (75 MHz, CD3OD) d 179.0, 145.8, 123.7,
122.2, 75.1, 49.8, 35.4, 32.8, 20.5, 13.9, 9.5. ATR-FTIR n 3387,
3138, 2962, 2935, 2875, 1605, 1540, 1465, 1420, 1351, 1251,
1114, 1061 cm21. MS (IC+) m/z 153; (IC2) m/z 149. Anal calc.
for C22H38N4O6 + 2 H2O C 53.86 H 8.63 N 11.42%; found C
53.60 H 9.08 N 11.30%.
1-Butyl-2,3-dimethylimidazolium L-tartrate, [Bmmim]2L-
Tart, (+)-2b: prepared as its enantiomer; it displayed analytical
data identical to that of the D-isomer. [a]2D0 +9.5 (c 1.0, H2O).
1-Octylpyridinium D-tartrate, [OctPyr]2D-Tart, (2)-3b: pre-
pared following method B using [OctPyr]Br (1.00 g, 3.67
mmol), AgOH (550 mg, 4.41 mmol) and D-tartaric acid (276
mg, 1.84 mmol). The salt (2)-3b was obtained as a viscous pale
yellow oil (931 mg, 95%). [a]2D0 29.5 (c 0.99, H2O). 1H-NMR (300
MHz, CD3OD) d 9.02 (d, J = 5.6 Hz, 4H), 8.60 (tt, J = 7.8, 1.1 Hz,
2H), 8.13 (t, J = 6.9 Hz, 4H), 4.64 (t, J = 7.6 Hz, 4H), 4.32 (s, 2H),
2.02 (quintuplet, J = 7.0 Hz, 4H), 1.48–1.21 (m, 20H), 0.90 (t, J =
7.0 Hz, 6H). 13C-NMR (75 MHz, CD3OD) d 179.0, 146.8, 146.0,
129.5, 75.1, 63.1, 32.9, 32.5, 30.2, 30.1, 27.2, 23.6, 14.4. ATR-
FTIR n 3388, 3129, 3090, 3063, 2955, 2927, 2857, 1634, 1604,
1489, 1467, 1353, 1177, 1118, 1067 cm21. MS (IC+) m/z 192;
Tetrabutylammonium L-tartrate, [TBA]2L-Tart, (+)-4b: pre-
pared as its enantiomer; it displayed analytical data identical
to that of that of the D-isomer. [a]2D0 +8.5 (c 1.2, H2O). Selected
crystal data for [TBA]2L-Tar (+)-4b: C36H90N2O13, M = 759.10,
Monoclinic, space group P21, a = 9.5730(5) Å, b = 15.2408(9) Å,
c = 16.3095(10) Å, V = 2378.1(2) Å3, Z = 2, crystal size 0.60 6
0.10 6 0.10 mm3, 18 337 reflections collected (8021 indepen-
dent, Rint = 0.0338), 534 parameters, 69 restraints, R1 [I . 2s(I)]
= 0.0625, wR2 [all data] = 0.1808, largest diff. peak and hole:
0.388 and 20.224 e Å23
.
1-Butyl-1-methyl pyrrolidinium D-tartrate, [Bmpyr]2D-Tart,
(2)-5b: prepared following method B using [Bmpyr]Br (1.00 g,
4.5 mmol), AgOH (674 mg, 5.4 mmol) and D-tartaric acid (338
mg, 2.25 mmol). The salt (2)-5b was obtained as a white solid
(895 mg, 92%). [a]2D0 211 (c 0.99, H2O). 1H-NMR (300 MHz,
D2O) d 4,31 (s, 2H), 3.59–3.42 (m, 8H), 3.39–3.22 (m, 4H), 3,02
(s, 6H), 2.25–2.12 (m, 8H), 1.84–1.65 (m, 4H), 1.38 (sextuplet, J
= 7.4 Hz, 4H), 0.94 (t, J = 7.4 Hz, 6H). 13C-NMR (75 MHz, D2O) d
178.0, 75.2, 65.3, 48.8, 26.6, 22.5, 20.7, 14.1. ATR-FTIR n 3334,
3021, 2963, 2933, 2874, 1607, 1469, 1357, 1124, 1061 cm21. MS
424 | RSC Adv., 2013, 3, 413–426
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Paper
11 C. R. Allen, P. L. Richard, A. J. Ward, L. G. A. van de Water,
A. F. Masters and T. Maschmeyer, Tetrahedron Lett., 2006,
47, 7367–7370.
12 G. N. Ou, M. X. Zhu, J. R. She and Y. Z. Yuan, Chem.
Commun., 2006, 4626–4628.
13 S. Bose, A. B. Wijeratne, A. Thite, G. A. Kraus, D.
W. Armstrong and J. W. Petrich, J. Phys. Chem. B, 2009,
113, 10825–10829.
14 S. L. Zhang, Y. Z. Huang, H. W. Jing, W. X. Yao and P. Yan,
Green Chem., 2009, 11, 935–938.
(IC+) m/z 142; (IC2) m/z 149. Anal calc. for C22H44N2O6 + 1.5
H2O C 57.49 H 10.31 N 6.09%; found C 57.50 H 9.94 N 5.89%.
1-Butyl-1-methyl piperidinium D-tartrate, [Bmpip]2D-Tart,
(2)-6b: prepared following method B using [Bmpip]Br (1.00
g, 4.23 mmol), AgOH (634 mg, 5.08 mmol) and D-tartaric acid
(318 mg, 2.12 mmol). The salt (2)-6b was obtained as a white
solid (0.918 g, 94%). [a]2D0 210 (c 1.0, H2O). 1H-NMR (300 MHz,
D2O) d 4.31 (s, 2H), 3.37–3.24 (m, 12H), 3.01 (s, 6H), 1.95–1.80
(m, 8H), 1.80–1.56 (m, 8H), 1.38 (sextuplet, J = 7.4 Hz, 4H), 0.95
(t, J = 7.4 Hz, 6H). 13C-NMR (75 MHz, CD3OD) d 179.00, 75.16,
64.75, 62.15, 48.23, 24.70, 22.14, 20.96, 20.80, 13.98. ATR-FTIR
n 3370, 2960, 2875, 1598, 1466, 1349, 1223, 1191, 1112, 1060,
1030 cm21. MS (IC+) m/z 156; (IC2) m/z 149. Anal calc. for
C24H46N2O6 + 1.75 H2O C 58.81 H 10.18 N 5.72%; found C
58.86 H 10.32 N 5.46%.
´
15 V. Zgonnik, C. Zedde, Y. Genisson, M. R. Mazieres and J.
C. Plaquevent, Chem. Commun., 2012, 48, 3185–3187.
16 Ionic liquids in synthesis, 2nd Ed., P. Wasserscheid and T.
Welton Edts, Wiley-VCH, 2008.
17 For an approach using carbonate salts as precursors see:
M. Fabris, V. Lucchini, M. Noe, A. Perosa and M. Selva,
Chem.–Eur. J., 2009, 15, 12273–12282.
18 K. Fukumoto, M. Yoshizawa and H. Ohno, J. Am. Chem.
Soc., 2005, 127, 2398–2399.
Acknowledgements
19 B. C. Ranu and S. Banerjee, Org. Lett., 2005, 7, 3049–3052.
20 (a) For an optimisation of this approach, see: Q. Peng, G.
Y. Li, J. G. Li and S. J. Yu, Tetrahedron Lett., 2009, 50,
4286–4288; (b) X. X. Q. Sun, Y. Ji, Y. Liu, J. Chen and D.
Q. Li, Aiche Journal, 2010, 56, 989–996.
21 For examples of the direct observation of diaminocarbenes
formation from 1-alkyl-3-methylimidazolium ionic liquids
and use in organocatalysis, see: (a) V. K. Aggarwal, I. Emme
and A. Mereu, Chem. Commun., 2002, 1612–1613; (b) L.
W. Xu, Y. Gao, J. J. Yin, L. Li and C. G. Xia, Tetrahedron
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J. Nagy and L. Nyulaszi, Org. Biomol. Chem., 2011, 9,
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22 For an example of precursory use of an anion exchange
resin in the preparation of ionic liquids through acid/base
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M. Forsyth and G. B. Deacon, Green Chem., 2002, 4,
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23 E. Alcalde, I. Dinares, A. Ibanez and N. Mesquida, Chem.
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24 G. H. Tao, L. He, W. S. Liu, L. Xu, W. Xiong, T. Wang and
Y. Kou, Green Chem., 2006, 8, 639–646.
We are grateful to the ‘‘Agence National de la Recherche’’
(ANR) for financial support (program ANR-07-CP2D-06). We
thank Jean-François Meunier (LCC, UPR 8241, CNRS,
Toulouse, France) for thermal DTA and DSC analyses and
Corinne Routaboul (ICT FR 2599, Universite Paul Sabatier -
Toulouse III, Toulouse, France) for IR analyses.
´
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