Superbase-derived protic ionic liquids with chelating fluorinated anions

Article abstract of DOI:10.1016/j.tetlet.2011.05.037

Eighteen new protic ionic liquids were synthesized in one step from five organic superbases and five commercially available fluorinated β-diketones. Physical properties of the ionic liquids, including thermal decomposition temperature were determined. Nin

Full text of DOI:10.1016/j.tetlet.2011.05.037

Tetrahedron Letters 52 (2011) 3723–3725
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Superbase-derived protic ionic liquids with chelating fluorinated anions
Jason R. Bell a, Huimin Luo a, , Sheng Dai b
⇑
a Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181, United States
b Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6201, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Eighteen new protic ionic liquids were synthesized in one step from five organic superbases and five
commercially available fluorinated b-diketones. Physical properties of the ionic liquids, including thermal
decomposition temperature were determined. Nine of the ionic liquids were examined as extraction
media for La3+, with some very large distribution coefficients obtained.
Received 18 March 2011
Revised 4 May 2011
Accepted 9 May 2011
Available online 20 May 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Ionic liquid
Solvent extraction
Superbase
b-Diketone
Interest in ionic liquids for separation processes has grown rap-
idly in the last two decades. Ionic liquids possess properties that
make them excellent candidates for separations, including tunable
polarity, hydrophobicity or hydrophilicity, low vapor pressure, and
high thermal stability.1 Specifically, the use of ionic liquids for the
extraction of metal ions from aqueous solutions has been of signifi-
cant interest.2 Hydrophobic ionic liquids with dissolved neutral
ligands such as crown ethers,3 diamides,4 alkylphosphates and
phosphine oxides,5 and b-diketones6 have been studied, and it has
been shown that ionic liquids can improve metal ion separations
by several orders of magnitude over conventional solvents.3a,7 Due
to the ease of functionalizing the imidazolium cation, task-specific
ionic liquids with diverse and complex chelating functionality have
been synthesized.8 Examples of the functional groups incorporated
include urea,9 acetylamine,10 phosphoryl,11 polyethers,12 crown
ether,13 hydroxybenzylamine.14 Though a wide array of ILs with
functionalized cations has been synthesized, none have proven to
be exceptionally better than the conventional ILs with dissolved
neutral extractants. Task-specific ionic liquids with functional an-
ions are quite less common, with salicylate-type anions,15 and fluo-
rinated b-diketones16 being the most common for extraction
applications.
structure for ligating the metal cation. Therefore, the most efficient
b-diketones for extraction are also the best candidates for a protic
ionic liquid system,18,19 in which the b-diketone conjugate base
would perform as the functional IL anion. An aprotic IL containing
the hexafluoroacetylacetonate anion (hfac) was reported to be
immiscible with water, and a qualitative test showed the IL to be
highly efficient at extracting Nd3+, Cu2+, and Co2+ 16
The IL was,
.
however, synthesized by a multistep process involving anion
metathesis of an IL halide salt and ammonium hexafluoroacetyl-
acetonate. Protic ILs utilizing fluorinated b-diketones and tertiary
amines have also been reported,20 synthesized through a simple
one pot neutralization reaction, though none were examined for
extraction capabilities.
Protic ionic liquids utilizing strong bases (i.e., organic ‘superbas-
es’21) have been shown to have lower vapor pressures and superior
thermal stabilities compared to typical protic ionic liquids using
amine bases.18 This is in accord with Angell and co-workers
who found the vapor pressure of a protic IL varies inversely with
the pKa difference of the acid/base components.22 Therefore we
sought to investigate task-specific protic ionic liquids, utilizing
organic superbases and fluorinated b-diketones as the cationic
and anionic components, respectively. This would allow for a facile
synthesis of a functionalized IL with high thermal stability.18 Three
commercially available superbases were used, two phosphazene
b-Diketones have been utilized extensively as ligands for the
extraction of metal cations, including lanthanides and actinides,
into conventional organic solvents. Batzar found that the extrac-
tion efficiency of these ligands was inversely related to the pKa of
the b-diketone.17 This is due to the more acidic diketone exhibiting
a higher degree of enolate character, which is the necessary
bases:
and
tert-butylimino-tri(pyrrolidino)phosphorane
tetramethyl(tris(dimethylamino)phosphoranylidene)phos-
(P1-t-Bu)
phorictriamid-ethyl-imin (P2-Et), and the bicyclic guanidine
base 7-methyl-1,5,7-triazabicyclo[4,4,0]dec-5-ene (MTBD). Two
other organic superbases, 7-butyl-1,5,7-triazabicyclo[4,4,0]dec-5-
ene (BTBD)23 and 1,3-dimethyl-2-(1-butylimino)imidazolidine
(BuG5)24 were synthesized and used. The fluorinated b-diketones
⇑
Corresponding author.
E-mail address: luoh@ornl.gov (H. Luo).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.037

3724
J. R. Bell et al. / Tetrahedron Letters 52 (2011) 3723–3725
base was added slowly to the solid b-diketone in the reaction vial.
No purification was necessary for the neat ILs. For example, to a
5 mL conical vial under argon atmosphere, P2-Et (1.0326 g,
N
P
N
N
P
N
N
P
N
N
N
N
N
N
3.042 mmol) was added, Hfod (708
lL, 3.042 mmol) was then
added slowly via syringe while stirring.
NMR spectra of the ILs show an essentially independent behav-
ior of the cation and anion. For each b-diketonate anion, a single
proton resonance exists, independent of the cation. For example,
the [hfac] anion resonance occurs as a singlet at approximately
5.66 ppm for all ILs. The transferred H+ was observable in the
region between 8 and 10 ppm for all ILs except those utilizing
phosphazene bases, for which the transferred protons were not
observed.
P2-Et
P1-t-Bu
N
N
N
N
N
N
N
N
N
MTBD
BTBD
BuG5
The physical properties of the ILs studied are compiled in Table 1.
The thermal properties of ILs were examined by thermogravimetric
analysis(TGA) and differential scanning calorimetry (DSC). The ther-
mal decomposition temperatures range from 145 °C to 307 °C. This
was typically greater than other PILs based on fluorinated b-dike-
tones and tertiary amines.20b The [hfac] anion exhibited a distinctly
higher thermal stability than [fod], a trend which was observed in
the earlier work.20b The bicyclic guanidine cations, [MTBDH] and
[BTBDH], showed better stability than all other cations, except for
[P2-EtH] which was significantly higher. The melting points of the
ILs were strongly correlated to the anion. The [hfac] anion gave crys-
talline products with three of the five cations, whereas the [fod]
anion gave only one. Both of the solid b-diketones, Hbta and Htta,
gave solid salts with BuG5. [BuG5H][fod] was a liquid at all times
during this study, even after refrigerated storage, however DSC
shows a cold crystallization temperature at 26 °C, with a concomi-
tant melting at 34.5 °C.
The viscosities of these ILs, as expected, decreased drastically
with increasing temperature. ILs containing the bicyclic guanidi-
nium cations [BTBDH] and [MTBDH] were very viscous. The
[BuG5H] and [P2-EtH] cations gave moderate values for viscosity,
however one example, [BuG5H][hfac] gave a low viscosity, similar
to many aprotic ILs, of 66.1 cP.
All ILs in this study were immiscible with water, except for
[MTBDH] and [tfac] containing ILs. In order to study the extraction
efficiency of the ILs, a 100 lL sample of each of the liquid ILs was
Figure 1. Superbases used in synthesis of ILs.
O
O
O
O
F3C
CF3
H3C
O
CF3
Hhfac
Htfac
O
O
O
O
O
F2
C
CF3
CF3
C
CF3
F2
S
Htta
Hbta
Hfod
Figure 2. Fluorinated diketones used for synthesis of ILs.
were all commercially available and include 1,1,1,5,5,5-hexafluoro-
acetylacetone (Hhfac), 1,1,1-trifluoroacetylacetone (Htfac),
6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadione (Hfod), ben-
zoyl-1,1,1-trifluoroacetone (Hbta), and 2-thenoyltrifluoroacetone
(Htta). Their molecular structures are shown in Figures 1 and 2,
respectively. Though 25 acid/base combinations were possible,
only 18 ILs were synthesized. Further work with P1-t-Bu was
avoided due to the high melting points of initial ILs made with that
base. MTBD and Htfac were avoided due to the water miscibility of
their ILs.
The new task-specific protic ionic liquids were synthesized in
high yield in a one step neutralization process. The superbase
was cooled in an ice bath, and exactly one equivalent of fluorinated
b-diketone was added dropwise with stirring. In the case of solid
b-diketones, Hbta and Htta, the addition was reversed. The liquid
vortexed for 60 min with 1.00 mL of 500 ppm La(NO3)3 solution.
After the layers were separated by centrifugation for 5 min, the
metal concentration in the aqueous phase was measured by ion
chromatography (IC). Distribution coefficients can be calculated
from La3+ concentrations in the aqueous phase before and after
contacting with ILs and are listed in Table 2. For three of the nine
Table 1
Physical properties of ILs
IL
Tm (°C)
Tg (°C)
Tdec (°C)
Density (g/mL)
Viscosity 23 °C, cP
Viscosity 40 °C, cP
[MTBDH][hfac]
[MTBDH][fod]
[BTBDH][hfac]
[BTBDH][fod]
[BTBDH][tfac]
[BTBDH][bta]
[BTBDH][tta]
[BuG5H][hfac]
[BuG5H][fod]
[BuG5H][tfac]
[BuG5H][bta]
[BuG5H][tta]
[P2-EtH][hfac]
[P2-EtH][fod]
[P2-EtH][bta]
[P2-EtH][tta]
55
—
—
—
—
—
—
—
34.5
—
48
86
70
—
—
—
—
289
179
261
198
186
194
195
250
145
169
180
199
307
274
268
184
193
191
—
—
—
À44
À46
À41
À47
À40
À43
À68
À51
À66
—
—
—
À68
À57
À68
—
1.46
1.43
1.32
1.30
1.38
1.36
1.42
1.38
1.28
—
—
—
1.33
1.28
1.32
—
935
538
>1500
>1500
>1500
>1500
66.1
446
190
—
—
—
323
509
211
—
199
136
369
425
331
255
27.9
122
68.4
—
—
—
122
177
84.8
—
[P1-t-BuH][hfac]
[P1-t-BuH][fod]
52
69
—
—
—
—

J. R. Bell et al. / Tetrahedron Letters 52 (2011) 3723–3725
3725
Table 2
article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.05.037.
Distribution coefficients of La3+ using ILs for
biphasic extraction
IL
DLa
130
References and notes
[BTBDH][hfac]
[BuG5H][hfac]
[BTBDH][fod]
[BuG5H][fod]
[P2-EtH][fod]
[BTBDH][bta]
[P2-EtH][bta]
[BTBDH][tta]
[P2-EtH][tta]
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limits of detection of the instrument, which yields a distribution
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for larger and more hydrophobic anions, such as [fod] and [tta],
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In conclusion, we have shown that task-specific protic ionic liq-
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liquids are hydrophobic, have a wide liquid temperature range,
have a thermal stability comparable to aprotic ionic liquids, and
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Acknowledgments
This research was supported by the Basic Energy Science Pro-
gram and Office of Nuclear Physics, Office of Science, U.S. Depart-
ment of Energy, under Contract DE-AC05-0096OR22725 with Oak
Ridge National Laboratory, managed by UT-Battelle, LLC. J.R.B.
acknowledges Oak Ridge Associated Universities (ORAU) for post-
doctoral fellowship.
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Supplementary data
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Supplementary data (experimental procedures for the synthesis
of BTBD, BuG5, ILs, as well as 1H NMR data) associated with this