Alkali metal oligoether carboxylates - A new class of ionic liquids

Article abstract of DOI:10.1002/chem.200801806

A new group of ionic liquids (IL) based on small inorganic cations and oligoether carboxylate anions were synthesized. The physicochemical properties of the salts were also investigated, and possible reasons for the formation of ionic liquids with alkali metal ions were proposed. It was found that complexation of the alkali metal cations by their combined oligoether- carboxylate counter-ion was sufficient to generate room-temperature liquid salts. It was also found that the use of simple tri- and tetraalkylammonium instead of alkali metal ions could also yielded ionic liquids at ambient temperature that appear to display additional desirable properties such as low viscosity. The result also showed that the combination of the 2,5,8,1 l-tetraoxatridecan-13-oate (TOTO) anion with biological cations such as choline could also lead to the development of even greener ILs.

Full text of DOI:10.1002/chem.200801806

COMMUNICATION
DOI: 10.1002/chem.200801806
Alkali Metal Oligoether Carboxylates—A New Class of Ionic Liquids
Oliver Zech, Matthias Kellermeier, Stefan Thomaier, Eva Maurer, Regina Klein,
Christian Schreiner, and Werner Kunz*[a]
Ionic liquids (ILs) have attracted more and more atten-
tion in recent years because of their unique properties and
the wide field of potential applications.[1,2] Generally, ionic
liquids are defined as salts with melting points below 1008C.
Conventional ILs typically contain bulky organic cations
with a low degree of symmetry such as imidazolium, pyrroli-
dinium, tetraalkylphosphonium, trialkylsulfonium, or quater-
nary ammonium. These cations hinder the regular packing
in a crystal lattice. Consequently, the solid crystalline state
becomes energetically less favorable, leading to low melting
points.[3a,b] This effect can be enhanced further by using an
anion with a delocalized charge, which results in decreased
interionic interactions.[4] The electrochemical reactivity in
ionic liquids[5] as well as their potential in biocatalysis,[6] cat-
alysis in general,[7] and synthesis,[8a,b] have been reviewed re-
cently.
cently, Justus et al. reported novel ionic liquids consisting of
trialkylammoniododecaborate anions and counter-ions in-
cluding lithium and potassium.[13] Imidazolium cation-based
ionic liquids with poly(ethyleneglycol) moieties[14] and fluo-
rinated anions or ether and alcohol functional groups[15] with
halide and fluorinated anions have also been discussed.
Pernak et al. described dialkoxymethyl-substituted imidazo-
lium cations combined with [BF4] and [NTf2] anions,[16] and
ether-derivatized imidazolium halides were reported by Fei
et al.[17]
In the present study, ionic liquids based on small inorgan-
ic cations and oligoether carboxylate anions were synthe-
sized. We present for the first time a new family of ILs com-
prising alkali metal cations and 2,5,8,11-tetraoxatridecan-13-
oate (TOTO) as anion, as shown exemplarily for the sodium
salt in Scheme 1. Herein, we investigate the physicochemical
properties of the salts, and propose possible reasons for the
formation of ionic liquids with alkali metal ions.
To date, little attention has been paid to systems of ionic
liquids involving small inorganic cations. Rees Jr. and
Moreno investigated monomeric barium bisalkoxides of the
general formula Ba[OACHTNUTRGNE(NUG CH2CH2O)nCH3]2 that were reported
to be liquid at ambient temperature.[9] For these compounds,
a solution structure with coordinative saturation at the cen-
tral metal atom with oxygen atoms was proposed, which im-
plied intramolecular complexation of the metal ions similar
to the coordination prevailing in macrocyclic polyethers.[10]
Other studies have described the synthesis and characteriza-
tion of polyether carboxylates of heavy alkaline-earth
metals,[11] which were found to be viscous oils that transform
into hygroscopic solids upon reduction of the water content.
Scheme 1. Sodium 2,5,8,11-tetraoxatridecan-13-oate [Na]ACHTNUTGRNEUNG[TOTO].
2,5,8,11-Tetraoxatridecan-13-oic acid (TOTOA) was pre-
pared in high purity (>99%, GC) according to a modified
procedure described by Matsushima et al.[18] Ionic liquids
were easily obtained by neutralizing the acid with an equi-
molar amount of alkali metal hydroxide or hydrogencarbon-
ate in aqueous solution. Water was first removed by lyophi-
lization, followed by drying in vacuum. Both the lithium and
the sodium salt were obtained as colorless or faint yellow
liquids at room temperature, whereas the potassium carbox-
ylate was a white solid.
Likewise, the yttriumACTHNUTRGNE(NUG III) salt of 3,6,9-trioxadecanoic acid
(TOD) was characterized as an extremely hygroscopic vis-
cous oil that could only be isolated as a trihydrate.[12] Re-
[a] O. Zech, M. Kellermeier, S. Thomaier, E. Maurer, R. Klein,
C. Schreiner, Prof. Dr. W. Kunz
Institute of Physical and Theoretical Chemistry
University of Regensburg, 93040 Regensburg (Germany)
Fax : (+49)941-943-4532
The water content of the hygroscopic salts was deter-
mined by means of a Karl–Fischer titration and found to be
below 300 ppm for the room-temperature liquid salts. All
E-mail: werner.kunz@chemie.uni-regensburg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801806.
Chem. Eur. J. 2009, 15, 1341 – 1345
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1
ionic liquids were characterized by H and 13C NMR spec-
troscopy, mass spectrometry, and elemental analysis. Melting
and glass transition temperatures were measured by differ-
ential scanning calorimetry (DSC), decomposition tempera-
tures by thermogravimetric analysis (TGA) (for more de-
tails see the Supporting Information). [Li]- and [Na]ACTHUNTGRNEUNG[TOTO]
show glass transitions, the temperature of which was deter-
mined from the intersection of the curve with the half-way
line between the two baselines. The results are summarized
in Table 1. Generally, the thermal stability of ILs is very sen-
Table 1. Physical properties of the alkali metal salts of 2,5,8,11-tetraoxa-
tri-decan-13-oic acid.
[a]
[c]
Cation
H2O [ppm]
Tm or Tg[b] [8C]
Td [8C]
Figure 1. Specific conductivities
sodium salt as a function of temperature, and the corresponding fit ac-
cording to the VFT equation (line).
k (*) and viscosities h (*) of the
Li+
Na+
K+
103
211
1292
À53[b]
À57[b]
+60[a]
357
384
369
[a] Tm: melting point. [b] Tg: glass transition temperature. [c] Td: decom-
position temperature.
Table 2. VFT[a] equation parameters of conductivity and viscosity data
for [Na]ACTHNUTRGNEUG[N TOTO].
k0 [ScmÀ1
0.337
]
B [K]
1485
T0 [K]
193
sitive to the type of both the cation and the anion. For in-
stance, imidazolium cations tend to be thermally more
stable than tetraalkylammonium species. Regarding the
anions, a series of relative stability can be established rank-
ing from PF6 over BF4 to halides.[20] All TOTOA alkali
metal salts exhibit excellent thermal stability. The decompo-
sition temperatures of the three substances are very similar,
indicating that the nature of the cation plays a minor role in
this context.
[Na]
ACHTUNGTRNE[NUNG TOTO]
h0 [P]
B [K]
1140
T0 [K]
213
[Na]
A
0.0024
[a] VFT equation for conductivity k =k0exp
E
G
=h0exp(B/
AHCTUNGTRENNUNG
AHCTUNGTREG[NNNU TOTO] in acetonitrile without added inert salt. These data
Exemplarily, we discuss in the following the conductivity,
viscosity, and electrochemical stability for the sodium salt.
Conductivity measurements were carried out in a tempera-
ture range between 258C and 1458C. Viscosities were mea-
sured at defined temperatures between 258C and 658C,
which revealed Newtonian behavior over the whole range.
For the sodium salt, a characteristic increase in specific con-
ductivity and decrease in viscosity with increasing tempera-
ture was detected. Both the conductivity and viscosity data
were found to be well described by the empirical Vogel–
Fulcher–Tammann equation (VFT). The three parameters
were obtained by nonlinear fits. The corresponding plot and
the fit results are given in Figure 1 and Table 2, respectively.
The electrochemical stability was studied by using cyclic
voltammetry (CV) with Pt working and counter electrodes
versus an Ag/Ag+/Kryptofix reference, according to the
method employed by Izutsu and co-workers.[19] Generally
speaking, the electrochemical window of an ionic liquid is
defined by the reduction of the cation and oxidation of the
anion. The width of this window is quite high for many ILs,
often exceeding 4 V.[20] The cyclic voltammogram of [Na]-
gave rise to the appearance of traces of impurities such as
the small peak seen in the anodic branch. All experiments
were carried out in an inert gas atmosphere. For [Li]-
AHCTUNGTREG[NNNU TOTO], an outstandingly wide electrochemical window of
6.7 V was found, with a cathodic limit of about À3.3 V and
an anodic limit of about 3.4 V (for more details see the Sup-
porting Information, Figure S8). These findings indicate an
extraordinarily high electrochemical stability of the as-syn-
thesized alkali metal oligoether carboxylates.
The relationship between fluidity and conductance can be
considered in terms of a Walden plot of the data, as de-
scribed by Angell et al.[21a–c] The Walden rule relates the ion
mobilities to the fluidity of the medium: if a liquid substan-
tially consists of independent ions only, then the Walden
plot will be close to an ideal line, which is typically repre-
sented by potassium chloride.[21b] Substances whose plot lies
more than one order of magnitude below the ideal line can
in this context be classified as “poor” ionic liquids. On that
basis, MacFarlane et al. demonstrated recently that
a
number of phosphonium-based ILs appear to exhibit strong
ion pairing.[22] The Walden plot obtained for the sodium salt
over a temperature range of 25 to 658C is shown in
ACHTUNGTRENNUNG[TOTO] was recorded first in the anodic direction with a
scan rate of 10 mVsÀ1. The cathodic and anodic limits are
about À2.0 V and 2.7 V versus Ag/Ag+, respectively, result-
ing in an electrochemical window of 4.7 V (more details are
given in the Supporting Information, Figure S7). The CV
data were recorded by using a 0.55m solution of [Na]-
Figure 2. It is obvious that the curve found for [Na]ACHTUNGTRENNUNG[TOTO]
lies significantly below the ideal KCl line. MacFarlane
et al.[22] proposed to term systems exhibiting such behavior
“liquid ion pairs”. The divergence of the single values ob-
tained for [Na]ACHTUNTRGNEUNG[TOTO] at different temperatures from the
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Alkali Metal Oligoether Carboxylates
COMMUNICATION
Based on this, we suggest that in the present case a com-
plexation of the alkali metal cations by their combined oli-
goether–carboxylate counter-ion is sufficient to generate
room-temperature liquid salts. Simultaneous “intramolecu-
lar” charge neutralization and complexation apparently di-
minishes the salt character of the substance, thereby shifting
it to the state of a neutral “molecule” and correspondingly
lowering the melting point. Such a hypothesis is supported
by the recorded mass spectra, which indicate the presence
of associated ion pairs in the gas phase (see the Supporting
Information, Figures S8–S10), and by the Walden plot,
which gives evidence for the formation of liquid ion pairs.
Apart from that, there is experimental evidence suggesting
that the net interactions between the alkali metal cations
and the TOTO anion are stronger in the case of Li+ than
for Na+. This becomes manifest in the extension of the elec-
trochemical window both in cathodic and anodic direction
when sodium is replaced by lithium. More thorough NMR
and IR analyses further confirm the notion of a pronounced
complexation in the present systems (data not shown). De-
tailed studies on the structure and properties of such com-
plexes will be presented in a forthcoming publication.
Figure 2. Walden plot for [Na]ACTHUNRTGNEUNG[TOTO] over a temperature range between
25 and 658C in comparison to the ideal KCl line.
ideal line is summarized in Table 3, where DW is the vertical
deviation. These findings clearly suggest the presence of
strong ion pairing in the liquid sodium oligoether carboxyl-
ate.
To gain a better insight into potential applications, the cy-
totoxicity of the new ionic liquids was studied by using a
MTT assay with Hela cells (for details see the Supporting
Information). As a reference, several common ILs were
tested, namely ethylammonium nitrate (EAN) as well as the
imidazolium-based cations [emim] (1-ethyl-3-methylimida-
zolium) and [bmim] (1-butyl-3-methyl-imidazolium) with
the anions tetrafluoroborate and ethyl sulfate. For each
sample, the IC50 value was determined, which represents the
concentration of the test substance that reduces cell viability
by 50% compared to the untreated control. Thus, the higher
the IC50 value, the less toxic is the substance. All experi-
ments were repeated four times; the resulting averaged IC50
values are plotted in Figure 3. From these data, two impor-
tant conclusions can be drawn. First, the cytotoxicity of the
alkali metal TOTO salts depends on the nature of the alkali
Table 3. Deviation DW of [NA]
Walden plot at different temperatures.
ACHTUNGTRNE[NUNG TOTO] from the ideal KCl line in the
T [8C]
DW
25
35
45
55
65
0.8
1.0
1.1
1.2
1.3
It is evident that these new ionic liquids display interest-
ing physicochemical properties, including remarkably high
thermal and electrochemical stability. Noteworthy is that the
lithium and sodium TOTO salts are liquids at room temper-
ature. So far room-temperature ILs based on alkali metal
cations have not been described in literature. Nonetheless,
the reason for these substances being liquid at ambient tem-
perature remains to be discussed.
The interactions of alkali and alkaline-earth metal ions
with the CH2CH2O unit in solution have been studied exten-
sively. The complexation of alkali metal ions by cyclic and
acyclic polyether ligands has been reviewed.[23] It is further-
more well known that the so-called “glymes” (polyethylen-
glycol dimethyl ethers) with the general structure CH3O-
metal cation, with [Na]ACTHUNGTERNNU[G TOTO] exhibiting the highest IC50
value and therefore being least toxic. Second, the IC50
values of the TOTO salts are significantly higher than those
ACHTUNGTRENNUNG(CH2CH2O)nCH3 possess a high affinity to alkali metal cat-
ions. Interestingly, in particular the tetraglyme (TeG, n=4)
is structurally very similar to the TOTO anion. Grobelny
et al. conducted a comparative study on solutions of potassi-
um ions in tetraglyme in the presence and absence of
[18]crown-6.[24] It was claimed that tetraglyme is a strong
complexing agent for K+, the latter being surrounded by the
tetraglyme molecules in a pseudo-crown ether fashion.
Other studies reported significant interactions of glymes
with Li+ in solution.[25] Ab initio calculations for the tetra-,
penta-, and hexaglyme complexes of lithium led to the con-
clusion that the coordination number for Li+ with respect to
the solvent oxygen atoms varies between 4 and 6.[26]
Figure 3. Cytotoxicity of the TOTO alkali metal salts as compared to
common ILs.
Chem. Eur. J. 2009, 15, 1341 – 1345
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W. Kunz et al.
were measured at frequencies between 10 kHz and 240 Hz and extrapo-
lated to infinite frequency with cell constants in the range from 0.54 to
11.6 cmÀ1. Viscosity measurements for the sodium salt were carried out
on a Bohlin rheometer (type CVO 120 High Resolution) in an argon at-
mosphere at controlled temperature in the range 25–658C, working with
a CP40/48 cone. Samples were studied at shear rates ranging from 0.25 to
200 sÀ1, except for the measurement at 258C, which had to be stopped at
of the widely studied imidazolium-based ILs. In this sense,
[Na][TOTO] is, for example, fifty times less cytotoxic than
[bmim][BF4]. In fact, the cytotoxicity of the alkali metal
TOTO salts was found to be comparable to that of EAN.
This finding underlines the potential of these new ionic liq-
uids for many applications.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
50 sÀ1 due to the high viscosity. Densities of [Na]
ACTHNUTRGNEUNG[TOTO], required for
In conclusion, a new family of ionic liquids was intro-
duced based on the combination of simple alkali metal ions
and oligoether carboxylates. The described substances are
promising materials due to their pronounced electrochemi-
cal and thermal stability. The concept of the ionicity plot
was successfully applied to the sodium salt for which strong
ion pairing was observed. Furthermore, it was shown that
the cytotoxicity of such “simple” alkali metal carboxylate
ionic liquids is very low. The physicochemical properties and
the design of additional new room-temperature liquid
TOTOA derivatives are currently under investigation. In
fact, the use of simple tri- and tetraalkylammonium instead
of alkali metal ions has also yielded ionic liquids at ambient
temperature that appear to display additional desirable
properties, such as low viscosity. The combination of the
TOTO anion with “biological” cations such as choline might
pave the way to even “greener” ILs.
the calculation of the molar conductivities were measured by using a vi-
brating-tube densimeter (Anton Paar DMA60/601 HT) in a temperature
range between 25 and 658C. The linear density equation for [Na]ACHTUNGTRENNUNG[TOTO]
was found to be 1[Na][TOTO] =1.26ꢁ10À4 to 4.76ꢁ10À4 t [gcmÀ3] (t in 8C).
Differential scanning calorimetry (DSC) data were recorded on a Mettler
DSC 30 in a nitrogen atmosphere using Al crucibles. The Li and Na salts
were investigated within a temperature range of À150 to 208C, whereas
the K salt was measured from À100 to 1008C. The heating rate in all
cases was 10 KminÀ1. Transition temperatures were generally obtained
from heating curves. Glass points were determined from the thermo-
grams using the half-step temperature of the transition. Thermal stability
was studied by using a thermogravimetric analyser from Perkin–Elmer
(model TGA 7). Samples were measured at a heating rate of 10 KminÀ1
,
applying a continuous nitrogen flow. Decomposition temperatures were
determined using onset points of mass loss, being defined as the intersec-
tion of the baseline before decomposition and the tangent to the mass
loss versus temperature plot in the following. The electrochemical stabili-
ty of the salts was investigated by cyclic voltammetry (CV) measure-
ments, employing platinum working and counter electrodes, and Ag/Ag+
(BAS) with Kryptofix 22 (Merck, for synthesis) as reference electrode.
For measurement, samples were dissolved in dry acetonitrile (Merck, for
DNA synthesis, ꢀ 10 ppm H2O). Scans were recorded first in the anodic
direction, with a rate of 10 mVsÀ1. Cytotoxicity tests were performed by
using Hela cells obtained from the American Type Culture Collection
(ATCC). MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bro-
mide) assays were conducted following a procedure proposed by Mos-
mann.[27]
Experimental Section
Synthesis of 2,5,8,11-tetraoxatridecan-13-oic acid (TOTOA): Sodium
(30.44 g, 1.32 mol) was added portionwise to triethylene glycol mono-
methyl ether (TEGME; 360 mL) under a nitrogen atmosphere. Dissolu-
tion of the sodium was achieved by vigorous stirring and gradual heating
to 1208C. The hydrogen that formed was removed by applying a slight N2
flow. The clear yellowish solution was subsequently cooled to 1008C, and
chloroacetic acid (62.91 g, 0.67 mol) dissolved in TEGME (110 mL) was
added dropwise within 20 min. Then, the reaction mixture was stirred at
1008C for 12 h. Excess TEGME was removed by distillation in vacuo,
leaving a brown suspension. Subsequent treatment with an aqueous solu-
tion of phosphoric acid (95.69 g, 0.98 mol) yielded a clear brown solution,
to which dichloromethane (300 mL) was added. The organic phase was
separated, and the aqueous phase was extracted repeatedly with di-
chloromethane (100 mL). The unified organic phases were dried over
magnesium sulfate. Filtration and solvent evaporation resulted in a slight-
ly yellow liquid. The crude 2,5,8,11-tetraoxatridecan-13-oic acid was puri-
fied by twofold distillation (b.p. 135–1458C at 10À7 mbar) to give a clear
colorless viscous liquid (120.91 g) in 81.2% yield.
Acknowledgements
The authors thank Dr. R. Mꢂller for help with the TGA measurements,
Prof. H. J. Gores for help with the acquisition of CV data, J. Kiermaier
for performing the MS analyses, S. Schroedle for advice concerning the
conducted syntheses, and B. Ramsauer for carrying out GC analyses. We
are further grateful to U. Schießl and Prof. A. Pfitzner (Institute of Inor-
ganic Chemistry, University of Regensburg) for performing the DSC
measurements. We thank Prof. J. Heilmann and Dr. B. Kraus (Institute of
Pharmaceutical Biology, University of Regensburg) for help with the cy-
totoxicity measurements. M. Kellermeier thanks the Fonds der Chemi-
schen Industrie for a scholarship. E. Maurer gratefully acknowledges fi-
nancial support by the Bayerische Elite Fçrderung of the Universitꢃt
Bayern e. V. We also thank Dr. H. Denzer from Kao Chemicals for val-
uable discussions.
Synthesis of TOTOA alkali metal salts: Alkali metal salts of 2,5,8,11-tet-
raoxatridecan-13-oic acid were prepared by direct neutralization of the
acid with alkali metal base. In the case of sodium and potassium, equimo-
lar amounts of the corresponding hydrogen carbonate and the acid were
dissolved in water and stirred for 1 h. Lyophilization and subsequent
drying in vacuo gave the desired salts in quantitative yields. Na-TOTO
was obtained as a faintly yellow viscous liquid, K-TOTO as white crys-
tals. The synthesis of the Li salt was carried out in a 5:1 (v:v) mixture of
ethanol and water using lithium hydroxide as base. After conversion, sol-
vents were removed by lyophilization, and the product was vacuum-
dried, resulting in a highly viscous colorless liquid.
Keywords: green chemistry
· ionic liquids · oligoether
carboxylate · toxicity · Walden plot
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Alkali Metal Oligoether Carboxylates
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Received: September 2, 2008
Revised: November 27, 2008
Published online: December 29, 2008
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