Syntheses and physicochemical properties of new ionic liquids based on the hexafluorouranate anion

Article abstract of DOI:10.1246/cl.2009.714

The first syntheses of a series of ionic liquids based on the hexafluorouranate(V) anion are described along with their physicochemical and electrochemical properties. The green roomtemperature ionic liquid, 1-ethyl-3-methylimidazolium hexafluorouranate (EMImUF₆), exhibits a conductivity of 7.9 mS cm^-1, viscosity of 59 cP, and electrochemical window of 2.3 V at 298 K. Copyright

Full text of DOI:10.1246/cl.2009.714

714
Chemistry Letters Vol.38, No.7 (2009)
Syntheses and Physicochemical Properties of New Ionic Liquids
Based on the Hexafluorouranate Anion
Takatsugu Kanatani, Kazuhiko Matsumoto, and Rika Hagiwaraꢀ
Graduate School of Energy Science, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501
(Received April 9, 2009; CL-090350; E-mail: hagiwara@energy.kyoto-u.ac.jp)
The first syntheses of a series of ionic liquids based on the
UF5
Cat+(HF)nF−
Cat+UF6
N
−
hexafluorouranate(V) anion are described along with their phys-
icochemical and electrochemical properties. The green room-
temperature ionic liquid, 1-ethyl-3-methylimidazolium hexa-
fluorouranate (EMImUF6), exhibits a conductivity of 7.9
mS cmꢁ1, viscosity of 59 cP, and electrochemical window of
2.3 V at 298 K.
- HF
N
N
N
1-ethyl-3-methylimidazolium
1-butyl-3-methylimidazolium
N+
Ionic liquids (ILs) are liquid composed of ionic species.
These species have been extensively investigated for uses as
electrolytes and reaction media. These liquids are of interest
due to their favorable properties including; nonflammability,
negligible vapor pressure, wide liquid-phase temperature range,
high ionic conductivity, high chemical stability, and/or electro-
chemical stability.1 Reports on the syntheses of moisture-stable
ILs containing fluoro anions (such as BF4ꢁ, PF6ꢁ, and
N(SO2CF3)2ꢁ) have increased as well as their applications in
various fields, replacing conventional aqueous or organic sol-
vents.2 Another attractive feature of ILs is that their functional-
ities are tunable by substituting the organic component, func-
tional group, and counter ion. Recently, ILs have attracted inter-
est in the field of nuclear chemistry. This interest is because
isotope enrichment using extraction, electrodeposition, or elec-
trophoresis can be achieved with these materials. Previous stud-
ies on the radiochemical stability of ILs showed that ILs based
on the imidazolium cation are stable enough against radiation
doses to be used for such purposes.3 This letter reports the syn-
theses and physicochemical properties of a series of novel ILs
based on the pentavalent hexafluorouranate(V) anion (UF6ꢁ).
N
1-butylpyridinium
1-butyl-1-methylpyrrolidinium
ꢁ
Scheme 1. Preparation of ILs based on UF6 along with the
structures of the cations (Catþ) used in the current work.
Table 1. Thermal and physicochemical propertiesa of the pres-
ent UF6 salts measured
EMImUF6 BMImUF6 BPyUF6 BMPyrUF6
MW
ꢁ/g cmꢁ3
Tm/K
463.2
2.43
284
n.d.
576
59
491.2
2.38
n.d.
189
577
110
3.3
488.2
—
335
n.d.
511
—
494.3
—
n.d.
n.d.
613
—
Tg/K
Td/K
ꢂ/cP
ꢃ/mS cmꢁ1
7.9
—
—
aMW: molecular weight, Tm: melting point, Tg: glass-transition
temperature, Td: decomposition temperature, ꢁ: density at
298 K, ꢂ: viscosity at 298 K, ꢃ: ionic conductivity at 298 K,
n.d.: not detected.
ꢁ
Scheme 1 shows the synthetic route for UF6 salts and the
structures of the cations used in the current work: 1-ethyl-3-
methylimidazolium (EMIm), 1-butyl-3-methylimidazolium
(BMIm), 1-butylpyridinium (BPy), and 1-butyl-1-methylpyrroli-
dinium (BMPyr). The present synthetic method shows the fluoro
acid–base reaction between fluorohydrogenate anions and UF5
(cf. Supporting Information, SI),4 which has been previously re-
ported for several fluoro complex salts using a fluorohydrogen-
ate IL as a precursor.5,6 A low halide ion or water content is re-
alized using this method. In this reaction, no sign of decomposi-
tion of the organic cations was observed, although UF5 is a mod-
erate oxidizer. The resulting salts are a green liquid (EMImUF6
and BMImUF6) or green solid (BPyUF6 and BMPyrUF6) at
room temperature. The room-temperature ionic liquid,
EMImUF6, formed solid, indicating its unstability against hy-
drolysis. Raman and IR spectroscopy for the obtained salts iden-
that there is no strong hydrogen bonding between the imidazoli-
um ring proton and the anion.
Table 1 summarizes the thermal and physicochemical prop-
erties at 298 K of EMImUF6, BMImUF6, BPyUF6, and
BMPyrUF6. The two imidazolium-based salts, EMImUF6 and
BMImUF6, are room-temperature ILs, whereas the pyridini-
um-based salt, BPyUF6, melts at 335 K. Although the nonaro-
matic cation-based BMPyrUF6 does not exhibit a melting point,
it shows a crystal–plastic crystal phase transition at 340 K with a
small enthalpy change, followed by decomposition at 613 K
without melting (cf. SI).4 As shown in previous studies,9 the
plastic crystal phase often appears for alkylpyrrolidinium cations
due to the high rotational freedom of the cation.
Figure 1 shows Arrhenius plots of the viscosity and ionic
conductivity of EMImUF6. The ionic conductivity increases
with decrease in viscosity. In the narrow temperature range
(298–328 K), both the viscosity and ionic conductivity have a
linear relationship. The Walden plot where the logarithmic re-
ꢁ
tified the octahedral UF6 ion in the region between 606–
610 cmꢁ1 for the Raman active ꢀ1 mode, 192–194 cmꢁ1 for
the Raman active ꢀ5 mode and in the region between 515–
517 cmꢁ1 for the IR active ꢀ3 mode (cf. SI).4,7,8 The absence
of an absorption band between 3000 and 3100 cmꢁ1 suggests
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.7 (2009)
715
1.8
1.7
1.6
3
2
1
η
1.5
0
1.4
-1
-2
1.3
3.0
1.3
3.1
3.2
3.3
3.4
-2
-1
0
1
2
Potential / V vs. Ag+/Ag
Figure 2. Cyclic voltammogram of a glassy carbon electrode in
EMImUF6 at 298 K. Counter electrode: glassy carbon, reference
electrode: Ag wire immersed in EMImBF4 containing 0.05 M
AgBF4.
1.2
1.1
1.0
0.9
0.8
σ
References and Notes
1
a) P. Wasserscheid, W. Keim, Angew. Chem., Int. Ed. 2000,
39, 3772. b) T. Welton, Chem. Rev. 1999, 99, 2071. c) R. D.
Rogers, K. R. Seddon, Science 2003, 302, 792. d) K. R.
Seddon, Nat. Mater. 2003, 2, 363. e) H. Ohno, Electrochem-
ical Aspects of Ionic Liquids, John Wiley and Sons, Inc.,
New Jersey, 2005.
3.0
3.1
3.2
3.3
3.4
103 × T −1 / K−1
2
a) H. Xue, R. Verma, J. M. Shreeve, J. Fluorine Chem. 2006,
127, 159. b) R. Hagiwara, Y. Ito, J. Fluorine Chem. 2000,
105, 221. c) K. Matsumoto, R. Hagiwara, J. Fluorine Chem.
2007, 128, 317.
V. A. Cocalia, K. E. Gutowski, R. D. Rogers, Cood. Chem.
Rev. 2006, 250, 755.
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
a) R. Hagiwara, K. Matsumoto, Y. Nakamori, T. Tsuda, Y.
Ito, H. Matsumoto, K. Momota, J. Electrochem. Soc. 2003,
150, D195. b) K. Matsumoto, R. Hagiwara, Y. Ito, Electro-
chem. Solid-State Lett. 2004, 7, E41.
Figure 1. Temperature dependence of viscosity (ꢂ) and ionic
conductivity (ꢃ) for EMImUF6.
ciprocal molar conductivities of ILs composed of dialkylimida-
zolium cations and fluoro anions are plotted against their loga-
rithmic viscosities, and the value for EMImUF6 does not deviate
from the general trend. This suggests that the mechanism of dif-
fusion of the ionic species in EMImUF6 is similar to the other
ILs and that the viscosity dominates the ionic conductivity
(cf. SI).4
3
4
5
The electrochemical window of EMImUF6 measured using
a glassy carbon working electrode is shown in Figure 2. The
anode and cathode limits of EMImUF6 are at +1.4 and
ꢁ0:9 V (vs. Agþ/Ag), respectively. The anode limit of
EMImUF6 is close to those for the other EMImMFmþ1 ionic
liquids (+1.4 V for EMImBF4, +1.5 V for EMImNbF6,
EMImTaF6, and EMImWF7),6 which results from oxidative de-
composition of EMIm. The cathodeꢁlimit for EMImUF6 can be
assigned to the reduction of the UF6 anion to the lower oxida-
tion state, since the EMIm cation is reductively decomposed at
ca. ꢁ3 V vs. Agþ/Ag.10
6
7
K. Matsumoto, R. Hagiwara, R. Yoshida, Y. Ito, Z. Mazej,
ˇ
ˇ
P. Benkic, B. Zemva, O. Tamada, H. Yoshino, S. Matsubara,
Dalton Trans. 2004, 144.
K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds. Part A. Theory and Application
in Inorganic Chemistry, 5th ed., Wiley Interscience, New
York, 1997, p. 218.
a) J. Selbin, J. D. Ortego, Chem. Rev. 1969, 69, 657. b) B.
Frlec, H. H. Hyman, Inorg. Chem. 1967, 6, 2233.
a) J. Adebahr, M. Forsyth, D. R. MacFarlane, Electrochim.
Acta 2005, 50, 3853. b) S. Forsyth, J. Golding, D. R.
MacFarlane, M. Forsyth, Electrochim. Acta 2001, 46, 1753.
8
9
In summary, the synthꢁeses and physicochemical properties
of new ILs based on UF6 have been reported. The physico-
chemical properties observed for these ILs are sufficient for elec-
trochemical and chemical processes in the field of nuclear chem-
istry. Another interesting appꢁlication may be isotope enrichment
using electrophoresis in UF6 based ILs.
10 Y. Katayama, S. Dan, T. Miura, T. Kishi, J. Electrochem.
Soc. 2001, 148, C102.
Published on the web (Advance View) June 13, 2009; doi:10.1246/cl.2009.714