Task-specific ionic liquids bearing 2-hydroxybenzylamine units: Synthesis and americium-extraction studies

Article abstract of DOI:10.1002/chem.200500741

The synthesis of two task-specific ionic liquids (TSILs) bearing 2-hydroxybenzylamine entities is described. These compounds are based on an imidazolium substructure onto which one hydrobenzylamine-complexing moiety is grafted. We have demonstrated that,

Full text of DOI:10.1002/chem.200500741

DOI: 10.1002/chem.200500741
Task-Specific Ionic Liquids Bearing 2-Hydroxybenzylamine Units: Synthesis
and Americium-Extraction Studies
Ali Ouadi,*[a] Benoît Gadenne,[b] Peter Hesemann,[b] Joel J. E. Moreau,[b]
Isabelle Billard,[a] Clotilde Gaillard,[a] Soufiane Mekki,[a, c] and Gilles Moutiers[c]
Abstract: The synthesis of two task-specific ionic liquids (TSILs) bearing 2-hydrox-
ybenzylamine entities is described. These compounds are based on an imidazolium
substructure onto which one hydrobenzylamine-complexing moiety is grafted. We
have demonstrated that, whether pure or diluted, TSIL is able to extract americi-
um ions. Furthermore, recovery of americium from the IL phase into a receiving
phase can be achieved under acidic conditions. A possible mechanism for the
metal-ion partitioning is presented, in which the extraction system is driven by an
ion-exchange mechanism.
Keywords: americium
chemistry · ion exchange · ionic
liquids · solvent extraction
·
green
Introduction
are noncoordinating, and the highly hydrated metal ions
remain in the aqueous phase.[7] Improvements in extraction
efficiencies have been achieved by the addition of organic
coordinating compounds, which significantly increase the
distribution ratios of metal ions between the ionic-liquid
and aqueous phases.[8–11] The extraction of various metallic
species by using well-known ligands, such as crown ethers[12]
or phosphates[13] dissolved in RTILs, such as 1-butyl-3-meth-
ylimidazolium bis-trifluorosulfonimide ([Bmim][Tf2N]) 1a
Room-temperature ionic liquids (RTILs) are promising sol-
vent alternatives for organic synthesis, catalysis, and electro-
chemistry.[1] In the field of separation processes, the useful-
ness of RTILs for the extraction of simple, neutral organic
compounds, such as naphthalene,[2–4] has already been re-
ported. Among industrial extraction processes, extraction
and separation of actinides and lanthanides from nuclear
waste is one of the most challenging fields, owing to the
high activity of the solutions to be treated. Ionic-liquid
phases are promising reception media for metallic species in
liquid/liquid extraction processes as they show high stability
under a- and g-irradiation[5] and enhanced safety towards
criticality.[6] However, the partitioning of charged metallic
species is largely limited by the unfavorable complexation
properties of hydrophobic ionic liquids: in general, RTILs
or
1-butyl-3-methylimidazolium
hexafluorophosphate
([Bmim][PF6]) 1b, highlights the potential of these new sol-
vents for extraction.
The introduction of RTILs into the nuclear-fuel cycle can
be envisioned through two different approaches. Classically,
RTILs can be considered as alternative solvents that replace
the highly toxic and flammable kerosene mixtures currently
in use.[14] The extraction of actinides (UVI, AmIII) into RTIL
phases by using tributylphosphate (TBP) or octyl(phenyl)-
N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO)
has already been reported. Although such studies are scarce,
it is clear that the extraction mechanism in a biphasic water/
RTIL system is rather different from that observed in classi-
cal organic solvents. As demonstrated by Dietz et al., ion-
exchange processes seem to play a significant role.[12]
Another very promising approach for metal extraction is
the concept of task-specific ionic liquids (TSILs). These
compounds, consisting of extracting entities grafted onto the
cation of the ionic liquid, combine the properties of ionic
liquids (e.g., nonvolatility, nonflammability) with those of
classical extracting compounds. Upon grafting complexing
[a]Dr. A. Ouadi, Dr. I. Billard, Dr. C. Gaillard, S. Mekki
IReS, CNRS/IN2P3-ULP Chimie NuclØaire
B.P. 28, 67037, Strasbourg Cedex 2 (France)
Fax : (+33)388-106-431
E-mail: ali.ouadi@ires.in2p3.fr
[b]Dr. B. Gadenne, Dr. P. Hesemann, Prof. J. J. E. Moreau
UMR 5076 CNRS/ENSCM
HØtØrochimie MolØculaire et MacromolØculaire
8, rue de lꢀEcole Normale
34296 Montpellier Cedex 05 (France)
[c]S. Mekki, Dr. G. Moutiers
CEA Saclay, DEN/DPC/SCP/DIR
91191 Gif Sur Yvette Cedex (France)
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FULL PAPER
substructures onto the organic cation of RTILs, the resulting
TSILs would behave as both the organic phase and the ex-
tracting agent, suppressing the problems encountered
through extractant/solvent miscibility and facilitating species
extraction and solvent recovery. This concept has been the
subject of very few studies in the field of separation. Howev-
er, it has already been shown that TSILs bearing urea, thio-
urea, thioether,[7] or ethylene glycol[15,16] groups allow the
partitioning of metal ions, such as HgII and CdII, from water.
Here, we report the synthesis of functionalized hydropho-
bic ionic liquids bearing the 2-hydroxybenzylamine substruc-
ture, and their application in the liquid/liquid extraction of
AmIII. The extraction abilities of these ionic compounds
were compared with the molecular counterpart of the ionic
liquids, the 2-propylaminomethyl-phenol. Finally, we pro-
pose a mechanism for the extraction of AmIII in biphasic
ionic-liquid/water systems.
with sodium borohydride. The water-soluble imidazolium
bromide was finally transformed into the bis(trifluorometh-
anesulfonimide) 5a and the hexafluorophosphate 5b by
using lithium bis(trifluoromethane)sulfonimide or potassium
hexafluorophosphate, respectively. The water-immiscible
imidazolium salts were isolated as viscous orange oils. The
hexafluorophosphate 5b, though much too viscous to be
used in liquid/liquid extraction experiments (h=257000 cP),
has good solubility in the 1-butyl-3-methyl-imidazolium hex-
afluorophosphate. Notably, the compound 5a shows a suffi-
ciently low viscosity (h=2070 cP) to be directly employed in
liquid/liquid extraction experiments.
Determination of pKa: In water, both TSILs 5a and 5b are
pH sensitive, owing to their functionalization by an amino
and a phenolic group, respectively. An understanding of the
acid-base behavior of these compounds is important in
order to rationalize the chemical properties in separation
processes. Hereafter, for the sake of simplicity, the TSILs
will also be denoted as LH2, with no mention of the counter-
ion, unless necessary. The TSILs shown in Figure 1 may
Results and Discussion
Synthesis of the functionalized ionic liquids 5a and 5b: 2-
Hydroxy-benzylamine groups were grafted onto imidazoli-
um substructures by using a three-step synthesis starting
from
salicylaldehyde
and
3-aminopropylimidazole
(Scheme 1). The product imine 3 was first alkylated by using
1-bromobutane, and the resulting ionic imine 4 was reduced
Figure 1. UV/Vis spectra of TSIL 5a as a function of pH. The pH values
are indicated on the spectra.
accept one proton at the nitrogen group to give H3(L)+. Be-
cause the complexing substructure contains two acidic pro-
tons, two deprotonation reactions need to be considered.
However, as the pKa values calculated for compounds 5a
and 5b are very similar, we were not able to attribute them
to the ammonium and phenol groups. The deprotonation
steps are expressed for H3(L)+ in the equilibria below:
LH3 Ð LH2 þ Hþ, with Ka1 ¼ ½LH2Š½HþŠ=½LH3
Š
þ
þ
LH2 Ð LHÀ þ Hþ, with Ka2 ¼ ½LHÀŠ½HþŠ=½LH2Š
Figure 1 displays the absorption spectra of 5a within the
range 190–330 nm as a function of pH range 2–12. Figure 2
shows the variation in absorption at two given wavelengths
(l=237 and 290 nm) for the same pH range. The corre-
Scheme 1. Synthetic route to TSILs. a) EtOH; b) C4H9Br; c) NaBH4,
EtOH; d) LiN(CF3SO2)2 or KPF6.
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A. Ouadi et al.
Figure 2. Variation in the absorbance of TSIL 5a as a function of pH in
*
*
water. : l=237 nm; : l=290 nm; the solid line represents the fit (see
text).
sponding data (not shown) for TSIL 5b are very similar. In
the case of the nonionic extractant molecule 2, the wave-
length range investigated is not large enough to reach ab-
sorbance saturation. The changes in the absorption spectra
in response to pH variation are consistent with two succes-
sive deprotonations, which is in agreement with the chemical
structures of the compounds 2, 5a, and 5b.
+
Figure 3. Calculated absorption spectra of the fully protonated LH3
(b), neutral LH2 (c), and deprotonated LHÀ (+++) forms of TSIL
5a in water.
Table 1. The pKa values derived for the three compounds investigated in
this work.
Single wavelength fit
Chemometric analysis
The corresponding pKa values can be determined from
the change in absorption at a given wavelength (see
Figure 2, data for TSIL 5a). However, only the molecular
absorbances of the three forms involved (LH3+, LH2, and
LHÀ) at this particular wavelength can be derived in this
way. Chemometrics[17] is a more complex mathematical
treatment of the data that enables determination of 1) the
number of independent species present in solution, 2) the
pKa values, and 3) the individual absorption spectra of each
form (LH3+, LH2, and LHÀ) within the whole range investi-
gated. This provides a much more complete set of informa-
tion from the same set of experimental data.
The principal component analysis (PCA), which is the
first step of a chemometric analysis, performed on the three
sets of data confirms the presence of three independent spe-
cies for TSILs 5a and 5b, however, the result is less clear
for 2. Table 1 presents the pKa values derived from both
mathematical treatments (chemometrics and “classical” fit)
for the two TSILs and 2, on the basis of a double deprotona-
tion. Figure 3 displays the absorption spectra derived for the
three independent species in the case of TSIL 5a as an ex-
ample (chemometrics analysis). The individual spectra de-
rived for TSIL 5b are very similar to those obtained for
TSIL 5a, however, in the case of 2, the spectrum of the
third species is mostly background. This phenomenon is at-
tributed to the limited range of pH investigated in the case
of 2, which does not allow us to obtain the fully deprotonat-
ed form as a major component of the solution.
TSIL 5b
pKa1 =8.3Æ0.1
pKa2 =10.4Æ0.1
pKa1 =8.1Æ0.1
pKa2 =10.3Æ0.1
pKa1 =8.4Æ0.1
pKa2 =11.3Æ0.1
pKa1 =8.32Æ0.01
pKa2 =10.41Æ0.01
pKa1 =8.1Æ0.1
TSIL 5a
pKa2 =10.3Æ0.1
pKa1 =8.41Æ0.01
pKa2 =11.15Æ0.04
compound 2
lium ring, which is more strongly electron withdrawing than
the aliphatic propyl chain in 2.
Solubility of TSIL in water as a function of water pH: A
known concentration of TSIL diluted in the corresponding
RTIL (0.34m) was introduced into water and the solubility
of the TSIL in water was studied at different pH values. The
absorbance was measured by using a UV-visible spectropho-
tometer; the TSIL concentration in water was then derived
by using the Beerꢀs law plots that were obtained previously
for aqueous TSIL solutions of known concentrations. As
shown in Figure 4, the solubility of TSIL increases as the pH
of the aqueous phase increases.
The values in Table 1 reveal very good agreement be-
tween the two methods of analysing the data. The pKa1
values, ranging from 8.10 to 8.44, are very similar for all
three compounds, whereas the pKa2 values indicate that
grafting the complexing pattern onto the IL skeleton indu-
ces a shift of almost one pKa unit. This difference can be as-
cribed to the presence of the positive charge on the imidazo-
Figure 4. Solubilization of TSIL 5b in aqueous phase at basic pH.
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Task-Specific Ionic Liquids and Extraction of Am3+
FULL PAPER
Distribution ratios for AmIII as a function of pH, TSIL con-
centration, and ionic strength: To determine the optimum
conditions, the extraction of AmIII was performed at various
pH values (Figure 5 shows the data for pure TSIL 5a as an
Figure 6. Comparison of DAm for TSIL 5a, TSIL 5b, and 2 as a function
of pH. Organic phase, 0.34m: TSIL 5a diluted in [Bmim][Tf2N] 1a, TSIL
5b and compound 2 diluted in [Bmim][PF6] 1b.
Figure 5. Variation in DAm as a function of pH. Organic phase: pure TSIL
5a.
example). The variation in the distribution ratio D indicates
that the coordinating and complexing abilities of pure TSIL
5a are dependent on pH. At low pH values, the TSIL is in
+
mainly its LH3 (acidic) form, leading to a very low concen-
tration of the Am–TSIL complex; at higher pH values,
higher extraction efficiencies are obtained and the maxi-
mum extraction is obtained at around pH 10. The extraction
efficiency decreases above pH 10, owing to the solubilization
of the TSIL in the aqueous phase.
Figure 7. Plot of the logarithmic distribution ratios (logDAm) for americi-
um as a function of the logarithmic concentration of TSIL 5b in [Bmim]-
[PF6] 1b.
In contrast to TSIL 5a, which is a liquid at room tempera-
ture and has a moderate viscosity, TSIL 5b is highly viscous.
To investigate its metal-extraction efficiency, we dissolved
this compound in [Bmim][PF6] 1b. The influence of pH on
Am extraction upon using both TSILs and extractant 2, dis-
solved in either [Bmim][Tf2N](TSIL 5a) or [Bmim][PF6]
(TSIL 5b and 2), is shown in Figure 6. The extraction of tri-
valent americium with the different TSILs 5a and 5b dis-
plays the same behavior as previously presented for the
pure TSIL 5a. At identical concentrations, the functional-
ized RTILs extract americium more effectively than extrac-
tant 2. Furthermore, the anionic part of the TSIL is shown
to have an effect on the extraction efficiency. This can be as-
species (metal:extractant) is 1:2. Furthermore, the slope of
the (logD+logaAm) versus pH plot indicates that two hydro-
gen ions participate in the extraction of metal ions
(Figure 8).
Finally, the performance of TSIL 5a in extracting americi-
um was evaluated in NaClO4 medium (data not shown). The
distribution coefficient of AmIII was determined at various
NaClO4 concentrations (0.05–1m) at a fixed pH of 10. The
distribution ratios increase linearly in this log/log plot,
À
which suggests that the ClO4 ions participate in the extrac-
tion mechanism.
cribed to Tf2NÀ having a more hydrophobic character than
Determination of the extraction equilibrium equation: We
attempted to fit the data of D versus pH (Figures 5 and 6)
based on a chemical scheme describing the extraction pro-
cess. The model we propose was conceived to reproduce as
best as possible the experimental findings described above.
Therefore, the proposed scheme takes into account 1) the
first americium hydrolysis,[18] which is known to occur above
pH 7; 2) the pH dependence of the variation in D, as dis-
played in Figure 6; and 3) the TSIL dependence (Figure 7)
together with 4) the experimentally observed increased solu-
À
PF6
.
The number of TSIL molecules that are present in the ex-
tracted moieties can be determined by plotting the distribu-
tion ratio (D) of the metal (at a constant pH value) as a
function of the TSIL concentration; in a log versus log plot,
the gradient of the slope is approximately equal to the
number of TSIL molecules. At the constant pH value of 10,
the graph of logD against logC is linear, with a gradient of 2
(Figure 7). Therefore, the stoichiometry of the extracted
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A. Ouadi et al.
of hydrolyzed forms of americium, the experimental effect
of ClO4À, and is consistent with both of the slopes observed
in Figures 7 and 8.
At equilibrium, the net charge of both the organic and
aqueous phases must be zero, the negative charge of the
complex anion in the RTIL phase must be balanced by
transferring an anion from the organic phase to the aqueous
phase.
To confirm this hypothesis of an anion exchange, spectro-
scopic studies of the extraction of europium at macro con-
centrations under the same conditions were carried out. The
aim was to demonstrate the relationship between the Tf2N
concentration in the aqueous phase and the formation of a
complex anion of europium in the RTIL phase. Unfortu-
nately, at pH 10 and macro concentrations, Eu is completely
hydrolyzed and precipitates, which prevents further reaction.
Despite this problem, our work is consistent with the find-
ings of Dietz et al.[12] and Jensen et al.,[19] who showed that
an ion-exchange mechanism can function if TSIL is used as
the organic phase together with a “classical” extracting
moiety.
Figure 8. Plots of the logarithmic distribution ratios (logD+logaAm) for
americium as a function of the aqueous pH. aAm is the complexation co-
efficient of americium in the aqueous phase.
bilization of the TSIL in the aqueous phase at high pH. It
À
also considers the effect of the ClO4 ions on the value of
D. However, because we encountered some experimental
difficulties (see below), the experimental indications are in-
complete, and so many models could, in principle, reproduce
the D variation. During the course of this work, we attempt-
ed fits of the data by using numerous models (data not
shown), however, we restrict the following discussion to the
best model that we tested.
By assuming that the predominant americium species in
the perchlorate medium are Am3+ and Am(OH)2+, the ex-
traction equilibrium of AmIII by TSILs can be given as
Equations (1)–(5),
Finally, Equations (4) and (5) take into account the en-
hanced solubilization of the TSIL in water at high pH
values, and the subsequent deprotonation that we observed.
By definition, pK4 =pKa2. Owing to the limited pH range
employed in this study, the presence of the additional aque-
+
ous species LH3 was not considered; the other hydrolyzed
Am products and carbonato–americium complexes were
also neglected. Therefore, the mathematical model contains
four unknown parameters. The mathematical treatment of
the scheme is based on the classical mass action law, with no
correction for ionic strength effect. The main hypothesis is
that Am is present at the trace level.
Am3þ þ 2 ClO4À þ LH2 Ð AmðClO4Þ2LH þ Hþ
Am3þ þ H2O Ð AmOH2þ þ Hþ
To fit the curves, we first focussed on the TSIL
5a/[Bmim][Tf2N] 1a series at either fixed or variable ionic
strength. In the fit, the values of all four parameters were
unfixed, however, it appears that fixing the value of K2 to
the value of the Am hydrolysis reported in the literature
(pKa =7.2) does not significantly modify the quality of the
fit, which is reasonable (not shown). In other words, the sen-
sitivity of the model to the exact value of K2 is poor. Simi-
larly, owing to the few points above pH 9.5, the precision of
K5 is low. Interestingly, the impact of ClO4À, as included in
the model, corresponds very well to that observed experi-
mentally on the D versus pH plots obtained at variable
ð1Þ
ð2Þ
AmOH2þ þ ClO4À þ ½BmimŠ½Tf2NŠ þ 2LH2 Ð
AmOHðLHÞ2ClO4 þ ½BmimŠ þ 2H þ Tf2N
ð3Þ
þ
À
À
þ
LH2 Ð LHÀ þ Hþ
ð4Þ
ð5Þ
LH2 Ð LHÀ þ Hþ
in which bars indicate species in the ionic-liquid phase. Each
of the five equations is characterized by an equilibrium con-
stant, K1 to K5. Equation (1) accounts for a possible extrac-
tion of americium below the hydrolysis pKa of AmIII. Nu-
merous numerical trials have shown that the shoulder ob-
served in the D variation within the pH range 6–8.5 is best
described by the introduction of such an extraction process
for free Am3+ that involves a single LH2 moiety. In fact,
both the introduction of two LH2 moieties in Equation (1)
and the suppression of Equation (1) (i.e., K1 =0) lead to a
steep increase in D from almost 0 to high values, without in-
volving any intermediate values. Equation (2) corresponds
to the Am hydrolysis. Equation (3) introduces the extraction
À
ionic strength, for which the ClO4 concentration varies by
almost four orders of magnitude (from 10À5 to 0.3m), de-
pending on the pH of the water phase before equilibration.
This is a good indication that the model proposed is appro-
priate. For the fits, the K4 value was fixed to the pKa2 value
given in Table 1.
Secondly, we attempted fits of the series in which TSIL
5a was used as a pure solvent. In the fitting procedure, the
concentration of TSIL 5a was changed from 0.34 to 2.5m,
which is the molar value of this solvent, and the ion-ex-
change mechanism of Equation (3), which is a crude approx-
imation, was kept. In this case, fixing K4 to its experimental
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Task-Specific Ionic Liquids and Extraction of Am3+
FULL PAPER
values leads to an imperfect fit, whereas fixing K4 to 0
allows satisfactory recovery of the data, as can be seen in
Figure 9, which displays the fit of the D data on the basis of
Conclusion
A solvent extraction system based on TSIL was investigated.
This system presents innovative properties; the organic
phase used is both the phase of reception for the actinide
and the extracting agent. We have demonstrated that,
whether pure or diluted, TSIL is able to extract americium
ions, and that recovery of americium from the IL phase into
a receiving phase can be achieved under acidic conditions.
A possible mechanism for the metal-ion partitioning is
presented, in which the extraction system is driven by an
ion-exchange mechanism.
Even though the TSILs developed in this work could be
used for americium extraction under special conditions at
basic pH, they are not well adapted for industrial processes
at acidic pH. However, this work has allowed us to establish
what a TSIL should not be with respect to actinide extrac-
tion. Firstly, we have confirmed the importance of the anion
in lowering the viscosity of the TSIL. In addition to the
problems of PF6-based RTIL degradation that were encoun-
tered in many studies,[9,13] it appears that this anion is not
suitable for fluid RTILs or TSILs. Although Tf2NÀ seems a
good choice in this respect, other anions should be tested in
the future so that the advantages of fluid and noncoordinat-
ing TSILs can be combined. Other possible weakly coordi-
nating anions[22,23] might also be tried. Secondly, the extrac-
tion pattern may be changed to lower the effective extrac-
tion pH from basic to acidic values, or a neutral extracting
pattern may be investigated. Finally, this complexing pattern
could be grafted onto an RTIL-basis other than imidazoli-
um, for example, tetraalkylammonium, which would interest
the radiochemistry community because of the enhanced sta-
bility of radiolysis relative to that of imidazolim-based
RTILs.
Figure 9. Fit of DAm as a function of pH. Organic phase: pure TSIL 5a.
the model described above. The quality of the fit is rather
good, although the ion-exchange mechanism [Eq. (3)]is
modified and other physicochemical phenomena occur that
are not properly described by the model. In particular, the
viscosity is changed.
Finally, we attempted fits of the TSIL 5b series. In this
case, no convincing adjustment of the data was possible (not
shown). AmIII, either free or hydrolyzed, may be liable to
complexation with PF6À, which would modify significantly
the proposed complexation scheme.
Stripping test: Wei et al.[20] reported that neutral extractants,
such as CMPO and crown ether, can extract metal ions
under acidic conditions, whereas the use of neutral extrac-
tants leads to some difficulties in recovering the extracted
metal. Subsequently, Luo et al.[21] reported the extraction of
various alkali metals with alkyl aza crown ether, whereby
the aza group provides a pH-switchable site and permits
back-extraction at acidic pH. In the same way, the TSILs de-
veloped in our laboratory are pH sensitive and can be used
for separation processes.
The americium loaded in the organic phase was back-ex-
tracted by contacting the organic phase with 1m perchloric
acid. At neutral or basic pH, the extraction efficiency is
high, but at low pH, the amine group is protonated, reduc-
ing the binding constant for Am and, thus, allowing Am to
be released into the aqueous phase. This extraction and
stripping process was repeated three times. More than 99%
of the Am was recovered in the aqueous phase each time,
indicating that a stripping process based on these TSILs is
feasible. The IL phase could be reused for further liquid/
liquid extraction of metal ions. As this process occurs at
high pH, it may also be suitable for the extraction of other
metal ions.
We are now attempting to apply these considerations to
the synthesis of a more suitable TSIL.
Experimental Section
General remarks: All reactions were performed under an argon atmos-
phere and by using Schlenk-tube techniques as necessary. 1H and
13C NMR spectra in solution were recorded by using Bruker AC-200 and
AC-250 spectrometers. CDCl3, CD3OD, and [D6]DMSO were used as
NMR solvents. Chemical shifts are reported as d values in ppm relative
to TMS. IR spectra were recorded by using a Perkin–Elmer SPECTRUM
1000 FTIR spectrometer. Mass spectra were measured by using a JEOL
MS-DX 300 mass spectrometer. All reagents were obtained from com-
mercial sources and were used without purification. For experiments re-
quiring dry solvents, THF, toluene, and diethyl ether were distilled over
sodium benzophenone, DMF was distilled over CaH2, dichloromethane
was distilled over P2O5, and alcohols were distilled over Mg. [Bmim]-
[24]
[Tf2N]was synthesized and purified as previously reported;
[Bmim]-
[PF6](purity =99%) was purchased from Solvionic (France).
2-[(3-Imidazol-1-yl-propylimino)-methyl]phenol (3): N-(3-aminopropyl)-
imidazole (2.05 g, 16.4 mmol) was added to a solution of salisaldehyde
(2.00 g, 16.4 mmol) in absolute ethanol (20 mL). The homogeneous solu-
tion was stirred at RT for 30 min and then heated to reflux for 2 h. After
cooling to RT, the solvents were pumped off. The resulting crystalline
solids were purified by recrystallization from ethanol (3.55 g, 15.5 mmol,
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A. Ouadi et al.
95%). 1H NMR (CDCl3): d=2.09 (quintet, J=6.6 Hz, 2H; CH2), 3.45
(pseudo-t, J=6.6 Hz, 2H; CH2), 3.97 (pseudo-t, J=6.6 Hz, 2H; CH2),
6.77–6.94 (m, 3H), 7.00 (s, 1H), 7.14–7.30 (m, 2H), 7.39 (s, 1H), 8.23 (s,
128.5, 158.3 ppm; IR (KBr): n˜max =3320, 3130, 3071, 2961, 2929, 2875,
1590, 1493, 1469, 1259, 754 cmÀ1
; HRMS (FAB+) m/z: calcd for
C10H16NO [M+H]+: 166.1230; found: 166.1232.
1H; CH N), 13.11 ppm (s, 1H; OH); 13C NMR (CDCl3): d=31.8, 44.3,
À
Liquid/liquid extraction of americium: Ultrapure water (Millipore) was
used in all cases. Aqueous solutions of 241Am(ClO4)3 at pH 3 were pre-
pared. The ionic strength and pH values were adjusted by using NaClO4,
HClO4, and NaOH (Aldrich, used as received). To avoid undesired metal
complexation, no buffers were used for pH control. The pH of all sam-
ples was measured by using an ORION pH meter with an electrode
(Orion). No attempts were made to dry any of the ionic liquids used, be-
cause of the equilibration with an aqueous phase during the course of the
extraction experiments. The concentration of the TSILs in the corre-
sponding ionic liquid was 0.34m. The extraction protocol in the case of
the pure TSIL phase was identical to that used for the ionic-liquid/TSIL
solutions.
Distribution ratios of 241Am were determined by mixing equal volumes
(0.7 mL) of IL and an aqueous phase, followed by shaking on a vortex
mixer apparatus at RT for 30 min, and centrifuging to equilibrate the
phases. Owing to the variable solubility of the TSIL in water and its sub-
sequent deprotonation, a large change in the pH value was eventually ob-
served (in some cases, from pH 3 to pH 10).
Subsequently, 241Am tracer (ca. 0.005 mCi, 2 mL) was added and extrac-
tion was performed by shaking and centrifuging to ensure that the phases
were fully mixed and separated. No significant pH changes were ob-
served after addition of the americium solution, owing to the very low
volume added. Afterwards, 0.5 mL aliquots were taken from each phase
and their g-radioactivities were measured by using a well-type NaI(Tl)
scintillation detector. The distribution ratio D was determined as the
ratio of radioactivity of the organic phase to that of the aqueous phases.
55.8, 116.9, 118.5, 118.7, 118.8, 129.7, 131.4, 132.5, 137.1, 160.9,
166.1 ppm; IR (KBr): n˜max =3435, 3090, 2928, 1628, 1604, 1571, 1485,
1457, 1438, 1390, 1271, 1223, 1195, 1147, 1076, 1023, 976, 885, 804, 757,
657, 623, 466 cmÀ1; HRMS (FAB+): m/z: calcd for C13H16N3O [M+H]+:
230.1293; found: 230.1291.
1-Butyl-3-(3-{[1-(2-hydroxyphenyl)meth-(E)-ylidene]amino}propyl)-3H-
imidazol-1-ium bromide (4): The imine (3.50 g, 15.3 mmol) was dissolved
in 1-bromobutane (10 mL) and the mixture was heated to 808C for 10 h.
After cooling to RT, the excess alkyl halide was eliminated by evapora-
tion under vacuum and then by washing with diethyl ether (3”20 mL),
yielding the product as a highly viscous orange oil (5.46 g, 14.9 mmol,
1
97%). H NMR (CD3OD): d=0.94 (t, J=7.3 Hz, 3H; CH3), 1.35 (septet,
J=8.1 Hz, 2H; CH2), 1.80 (quintet, J=7.6 Hz, 2H; CH2), 2.34 (quintet,
J=6.7 Hz, 2H; CH2), 3.75 (t, J=6.7 Hz, 2H; CH2), 4.14 (t, J=7.4 Hz,
2H; CH2), 4.40 (t, J=7.0 Hz, 2H; CH2), 4.80 (brs, 1H; OH), 6.81–6.91
(m, 2H), 7.26–7.40 (m, 2H), 7.64 (t, J=1.8 Hz, 1H), 7.74 (t, J=1.9 Hz,
1H), 8.52 (s, 1H; CH-N), 9.21 ppm (s, 1H); 13C NMR (CD3OD): d=13.9,
20.6, 32.0, 32.9, 49.4, 50.7, 57.3, 117.9, 119.9, 120.0, 123.9, 124.0, 133.2,
133.9, 137.3, 162.6, 168.1 ppm; IR (KBr): n˜max =3425, 3130, 3071, 2950,
2870, 1628, 1560, 1495, 1457, 1276, 1209, 1161, 1120, 1023, 976, 833, 761,
638, 557, 457 cmÀ1; HRMS (FAB+) m/z: calcd for C17H24N3O [M+]:
286.1919; found: 286.1915.
1-Butyl-3-[3-(2-hydroxybenzylamino)propyl]-3H-imidazol-1-ium bis(tri-
fluoromethane) sulfonimide (5a) and 1-butyl-3-[3-(2-hydroxybenzylami-
no)propyl]-3H-imidazol-1-ium hexafluorophosphate (5b): The alkylated
compound (3.00 g, 8.2 mmol) was dissolved in methanol (20 mL). The re-
sulting homogeneous solution was cooled to 08C and sodium borohy-
dride (310 mg, 8.2 mmol) was slowly added under vigorous stirring. The
resulting suspension was stirred for 3 h. The methanol was evaporated
and the crude product was dissolved in water (50 mL). The aqueous solu-
tion was washed with diethyl ether (2”50 mL). A solution of N-lithiotri-
fluoromethanesulfonimide (2.58 g, 9.0 mmol) or potassium hexafluoro-
phosphate (1.66 g, 9.0 mmol) in water (30 mL) was added to the aqueous
solution of the functional imidazolium bromide. Demixation of the hy-
drophobic ionic liquid occurred immediately. The products were extract-
ed with dichloromethane (50 mL). The organic layers were washed with
water (3”30 mL) and dried (Na2SO4). Evaporation of the solvent yielded
the title compounds as viscous brownish oils. Yields: 5a: 4.48 g
(7.9 mmol, 96%), 5b: 3.27 g (7.5 mmol, 92%).
Solubility of TSIL in water as a function of water pH: Each TSIL was
shaken with an equal volume of water on a vortex mixer at 258C for
30 min. This was performed for water phases with various pH values.
Each mixture was then centrifuged for 3 min to promote phase separa-
tion. Once each phase was separated, the aqueous phase was diluted with
water by a factor of 10. The absorbance of this final solution at 291 nm
was measured by using a UV/Vis spectrophotometer. The TSIL concen-
tration in water was then derived by using the Beerꢀs law plots obtained
previously for TSIL aqueous solutions of known concentrations.
Determination of pKa values in water: Absorption spectra of aqueous
solutions of 2, 5a, and 5b of known concentrations (in the order of 1.8”
10À4 m) within the range 190–800 nm were acquired by using an Uvikon
930 spectrophotometer at RT. The pH was varied by addition of aliquots
of concentrated HClO4 or NaOH aqueous solutions.
Stripping test: Back-extraction of AmIII with TSIL was performed by fol-
lowing a procedure similar to that described for the water/TSIL extrac-
tion. After equal volumes of the aqueous and RTIL solutions were mixed
and shaken on the vortex mixer, so that AmIII was quantitatively transfer-
red to the RTIL phase, a fresh aqueous solution containing 1m perchloric
acid was added. Both phases were mixed and shaken on the vortex mixer
at 258C for 30 min. These mixtures were then centrifuged for 3 min to
promote phase separation.
Physical data for 5b: Viscosity: h=257000 cP; glass transition tempera-
1
ture: Tg =À398C; H NMR ([D6]DMSO): d=0.9 (t J=7.2 Hz, 3H; CH3),
1.24 (sextet, J=7.6 Hz, 2H; CH2), 1.77 (quintet, J=6.8 Hz, 2H; CH2),
À
1.99 (quintet, J=7.3 Hz, 2H; CH2), 2.45 (m, 2H), 3.76 (s, 2H; CH2 NH),
4.15 (m, J=7.1 Hz, 2H; CH2), 4.23 (m, J=7.2 Hz, 2H; CH2), 6.73 (m,
2H), 7.07 (m, 2H), 7.76 (s, 2H, Im+), 9.15 ppm (s, 1H, Im+);
13C NMR ([D6]DMSO): d=13.1, 18.7, 29.1, 31.1, 44.5, 46.9, 48.5, 49.7,
115.4, 118.4, 122.3 (2C), 124.5, 127.7, 128.6, 135.9, 156.7 ppm; IR (KBr):
n˜max =3645, 3320, 3157, 2937, 1585, 1561, 1461, 1252, 1161, 1104, 1033,
Fitting procedures and data analysis: The chemometric analysis was per-
formed by using a program written under the Maple format, based on a
program written previously by Pochon and co-workers.[25] The other fits
were performed by using a Fortran subroutine included in the CERN
package “MINUIT”, which uses simplex and migrad algorithms for con-
vergence.
938, 842, 757, 638, 557, 452 cmÀ1
; HRMS (FAB+): m/z: calcd for
C17H28N3O [M]+: 288.2076; found: 288.2072.
Physical data for 5a: Compound 5a shows similar spectroscopic proper-
ties to 5b, except: Viscosity: h=2070 cP; glass transition temperature:
Tg =À448C; 13C NMR: additional signal at 120.1 ppm (q, J=321 Hz;
À
CF3, N(SO2CF3)2 anion); IR (KBr): n˜max =3624, 3329, 3149, 3114, 2965,
2938, 1590, 1565, 1492, 1468, 1350, 1256, 1203, 1139, 1057, 740 cmÀ1
.
2-Propylaminomethylphenol (2): Compound 2 was synthesized by a reac-
tion sequence involving 1) the coupling of salicylaldehyde with propyla-
mine and 2) the reduction of the imine formed with sodium borohydride.
The crude product was purified by distillation under reduced pressure,
yielding the title compound as a viscous yellow oil in nearly quantitative
yield. 1H NMR (CDCl3): d=0.98 (t, J=7.4 Hz, 3H; CH3), 1.59 (sextet,
Acknowledgement
We thank the GdR Paris for financial support.
À
J=7.3 Hz, 2H), 2.67 (t, J=7.1 Hz, 2H), 4.01 (s, 2H; CH2 N), 6.78–6.89
(m, 2H; Ar), 7.00–7.03 (m, 2H; Ar), 7.17–7.24 ppm (m, 1H; Ar);
[1]P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH,
Weinheim, 2003, p. xvi, p. 364.
13C NMR (CDCl3): d=11.5, 22.6, 50.4, 52.5, 116.2, 118.8, 122.5, 128.1,
3080
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Chem. Eur. J. 2006, 12, 3074 – 3081

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Received: June 28, 2005
Revised: November 4, 2005
Published online: January 24, 2006
Chem. Eur. J. 2006, 12, 3074 – 3081
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3081