Cooperative intramolecular interaction of diazacrown ether bearing β-diketone fragments on an ionic liquid extraction system

Article abstract of DOI:10.1039/b904596c

A novel extractant β-diketone-substituted diaza-18-crown-6 demonstrated very efficient extraction of Sr²⁺ due to an intramolecular synergistic effect on the ionic liquid extraction system and recovery of Sr²⁺ from the ionic liquid was successfully achieved under acidic conditions. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b904596c

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www.rsc.org/dalton | Dalton Transactions
Cooperative intramolecular interaction of diazacrown ether bearing
b-diketone fragments on an ionic liquid extraction system†
a
a,b
b
c
b
Kojiro Shimojo,* Hiroyuki Okamura, Naoki Hirayama, Shigeo Umetani, Hisanori Imura and
a
Hirochika Naganawa
Received 5th March 2009, Accepted 30th April 2009
First published as an Advance Article on the web 7th May 2009
DOI: 10.1039/b904596c
9
A novel extractant b-diketone-substituted diaza-18-crown-
such as HTTA, show comparable extraction performance and
a similar extraction mechanism between IL and organic solvent
2+
6
demonstrated very efficient extraction of Sr due to an
10
intramolecular synergistic effect on the ionic liquid extraction
systems. More recently, synergistic extraction systems in ILs were
achieved by the combination of crown ethers and other ligands.
2+
11
system and recovery of Sr from the ionic liquid was
successfully achieved under acidic conditions.
We have been interested in the development of novel extractants
suitable for IL-based extraction systems. The extractant plays a
key role in the extraction efficiency and selectivity of metal ions.
Unfortunately, there has been little progress in the development
of novel extractants, and commercially available extractants are
employed in most studies on IL-based extraction systems. In the
present study, we have synthesized diaza-18-crown-6 bearing two
-acyl-5-pyrazolones (H2bDA18C6) as a novel extractant for IL-
based extraction systems (synthetic procedure in ESI†). 4-Acyl-
-pyrazolone derivatives are powerful b-diketone-type chelating
reagents and are soluble in ILs. The compounds are expected
to improve the extraction performance of diazacrown ethers in
ILs. Furthermore, nitrogen atoms in a diazacrown ether may
be capable of back extraction of metal ions.
extraction performance of H2bDA18C6, the extraction behavior
Ionic liquids (ILs) have recently received growing attention as a
new class of solvents. ILs, which are composed of an organic
cation and either an organic or an inorganic anion, have different
12
1
properties from those of molecular liquids. Not only are ILs
environmentally more attractive than organic solvents, but they
possess tunable solvent properties such as polarity, hydrophobicity
and solvent miscibility. These solvent properties are dependent on
the combination of cations and anions. For example, ILs can be
made hydrophobic while retaining ionicity. This dual nature has
led to their use as novel extracting media for the solvent extraction
4
13
5
14
2
of ionic species.
12,15
To assess the
Solvent extraction is one of the most effective analytical
methods for the separation and purification of target substances.
Currently, IL-based extraction systems of a variety of substances
2
+
of Sr using H2bDA18C6 into 1-ethyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide ([C mim][Tf N]) was com-
3
4
5
(
e.g. metal ions, organic compounds and proteins ) have been
2
2
pared with those using 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone
HPMBP), N,N¢-dibenzyl-4,13-diaza-18-crown-6 (DBzDA18C6)
and the mixture of HPMBP and DBzDA18C6. In addition,
the differences in the extraction behavior between the IL and
chloroform systems are discussed.‡ The molecular structures of
extractants and the IL employed in this study are shown in Fig. 1.
We describe herein that H2bDA18C6 provides a remarkably high
performance for Sr extraction due to a cooperative “intramolecu-
lar” interaction when compared with the “intermolecular” synergy
extraction system of the mixture of HPMBP and DBzDA18C6.
The extraction performance of H2bDA18C6 is greatly enhanced
widely investigated. In comparison to organic solvents, ILs show
remarkably high performance for the extraction of strontium ions
(
2
+
6
(
Sr ) when using crown ethers as the extractant. Furthermore,
the extraction mechanism changes from an ion pair extraction in
organic solvent systems to a cation-exchange extraction process,
2
+
7
in which Sr is exchanged for the cationic constituent of the ILs.
(
Specifically, the predominant partitioning mode is dependent on
2
+
8
the hydrophobicity of the ILs. ) However, this system has difficulty
in the back extraction of the extracted metal ions because this
system provides high extraction performance which is independent
of the acid and salt concentrations in the aqueous phases. A similar
phenomenon is also observed in extraction systems using other
3
a,c
neutral extractants. In contrast, anionic extractants enable back
extraction of metal ions from ILs into acidic aqueous solutions.
However, most anionic extractants, except b-diketone type ligands
a
Division of Environment and Radiation Sciences, Nuclear Science and En-
gineering Directorate, Japan Atomic Energy Agency, Tokai-mura, Ibaraki,
3
19-1195, Japan. E-mail: shimojo.kojiro@jaea.go.jp; Fax: +81-29-282-
757; Tel: +81-29-282-5090
6
b
Division of Material Chemistry, Graduate School of Natural Science and
Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192,
Japan
c
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011,
Japan
†
Electronic supplementary information (ESI) available: Synthetic pro-
cedure for H2bDA18C6 and slope analysis, Fig. S1. See DOI:
0.1039/b904596c
Fig. 1 Molecular structures and abbreviations of the ionic liquid and the
extractants.
1
4
850 | Dalton Trans., 2009, 4850–4852
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2
+
2+
by dissolution in the IL compared with in chloroform. The
Sr . The transfer of Sr with DBzDA18C6 into [C
2
mim][Tf
2
N]
2
+
2+
back extraction of Sr from the IL into a receiving phase was
successfully achieved under acidic conditions.
proceeds via the exchange reaction of Sr for one proton and
one C mim . Consequently, this reaction maintains the charge
+
2
2
+
Fig. 2 shows the extraction behavior of Sr in the chloroform
balance during the partitioning process. Very similar extraction
plots were obtained in the HPMBP and DBzDA18C6 mixed
system when compared with those using only DBzDA18C6.
We confirmed that the extraction dependence of Sr on the
HPMBP concentration in the mixed system was negligible. This
result indicates that HPMBP cannot act as a synergistic reagent
(
a) and IL (b) systems with H2bDA18C6 as a function of pH
in the aqueous phase. The extraction behavior of these two
systems was compared with those with other extractants. The
concentration of DBzDA18C6 was equal to that of H2bDA18C6,
but the concentration of HPMBP was two-fold greater than
H2bDA18C6. This difference in concentrations was to ensure that
the same number of functional b-diketone groups were available.
In the chloroform system, neither HPMBP nor DBzDA18C6
2
+
2
+
in [C
2
mim][Tf
2
N] because transfer of Sr into [C
2
mim][Tf N]
2
2
+
was possible without neutralizing the cationic charge of Sr by
HPMBP. This observation contrasts with the results from the
chloroform system. Probably, the extraction mechanism of Sr in
the system mixed with HPMBP and DBzDA18C6 is identical to
that in the system with only DBzDA18C6 present. Interestingly,
H2bDA18C6 enables the transfer of Sr from the more acidic
aqueous solutions compared with the system mixed with HPMBP
and DBzDA18C, that is, H2bDA18C6 in [C
considerably high extraction performance for Sr . The covalent
attachment of two b-diketone groups to a diazacrown ether
2
+
2+
were very effective at Sr extraction. In contrast, when using
the mixture of HPMBP and DBzDA18C6, the extraction of
2
+
Sr was significantly enhanced with increasing pH. These results
clearly show that the combination of HPMBP and DBzDA18C6
2
+
2
+
generate a synergistic effect for Sr extraction. HPMBP probably
2
+
neutralizes the cationic charge of Sr whereas DBzDA18C6 most
2
mim][Tf N] possesses
2
2
+
2+
likely replaces the hydration waters of Sr , thereby yielding a
more organophilic metal complex. H2bDA18C6 provided a rather
powerful extraction performance when compared with the mixture
of HPMBP and DBzDA18C6, but was not able to transfer fully
2
+
leads to a dramatic increase of the extraction efficiency for Sr .
Although the structure of the extracted species is not exactly
known, it is justified to assume that the two b-diketone groups
2
+
the Sr .
2
+
are in close neighborhood to Sr bound by azacrown ether. In
that sense, the dramatic increase of the extraction performance
17
may be due to a kind of “intramolecular” synergistic effect.
Furthermore, a slope analysis gave linear plots with a slope of
, indicating that two protons from protonated H2bDA18C6 were
2
2
+
2+
released to extract Sr . Thus the transfer of Sr with H2bDA18C6
into [C mim][Tf N] proceeds via the proton exchange reaction.
To evaluate the difference in the extraction performance of
2
2
2
+
H2bDA18C6 for Sr between [C
2
mim][Tf
extractability of Sr with increasing H2bDA18C6 concentration
in [C mim][Tf N] or in chloroform was compared (Fig. 3). This
2
N] and chloroform, the
2
+
2
2
trial was performed under the same condition (pH 7.0) for
the IL and chloroform systems. The extraction performance of
2
+
H2bDA18C6 for Sr was significantly higher in [C
2
mim][Tf
2
N]
than in chloroform. Only 0.1 mM of H2bDA18C6 was required to
2
+
quantitatively extract Sr into [C
2
mim][Tf N]. In the chloroform
2
system, however, only modest extraction was observed at the
same H2bDA18C6 concentration. ILs can provide an appropriate
environment that contributes to the performance of H2bDA18C6.
The reason why H2bDA18C6 gives rise to a cooperative
2
+
Fig. 2 Dependency of Sr extraction on the aqueous phase pH in
the chloroform system (a) and in the [C mim][Tf N] system (b). Aque-
ous phase: [Sr ] = 0.01 mM. Extracting phase: [HPMBP] = 2 mM,
DBzDA18C6] = 1 mM and [H2bDA18C6] = 1 mM. The HPMBP system
black symbols), the DBzDA18C6 system (blue symbols), the HPMBP +
2
2
2+
[
(
DBzDA18C6 system (green symbols) and the H2bDA18C6 system (red
symbols).
2
+
Although the partitioning of Sr using only HPMBP was
negligible in the [C mim][Tf N] system, the extraction perfor-
2
2
mance of DBzDA18C6 improved with increasing pH. This is
contrary to the results in the chloroform system. Slope analysis
as a function of the equilibrium pH (Fig. S1 in ESI†) showed
that the slope of the logarithmic plots of the distribution ratio
2
+
of Sr versus pH was 1. This result suggests that one proton
16
from protonated DBzDA18C6 (pK
1
= 7.5, pK
2
= 6.83) was
2+
Fig. 3 Dependency of Sr extraction on H2bDA18C6 concentration
2
+
released to extract the divalent cation Sr . It is noteworthy that
the number of released protons is not equivalent to the valence of
in chloroform (open symbols) and in [C mim][Tf N] (closed symbols).
Aqueous phase: [Sr ] = 0.01 mM, pH 7.0.
2
2
2+
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“
intramolecular” interaction only in the IL-based extraction
adjusted by the addition of HNO
3
or LiOH into HEPES buffer. Equal
volumes of the extracting and aqueous solutions were mixed and shaken
system is not fully understood and is currently under investigation.
◦
using a vortex mixer at 25 C for 30 min to attain equilibrium. These
2
+
Furthermore, the back extraction of Sr
H2bDA18C6 in [C mim][Tf N] was performed under the pH
control of the receiving phase. As shown in Fig. 4, the degree
of back extraction of Sr was enhanced as the acidity of the
receiving phase was increased. This is because H2bDA18C6 loses
its coordination ability under acidic conditions. The extraction
extracted by
mixtures were centrifuged for 3 min to promote phase separation. After
phase separation, Sr2+ in the extracting phase was back-extracted into
2
2
2+
3
a HNO solution (pH 2). The concentrations of Sr in the aqueous
2
+
phase and the receiving phase were determined using inductively coupled
plasma mass spectrometry (ICP-MS, Hewlett Packard HP 4500) to
2+
2+
obtain the extractability (= [Sr ]ext/[Sr ]ini ¥ 100), the distribution ratio
2
+
2+
2+
2+
(= [Sr ] /[Sr ] ) and the degree of back extraction (= [Sr ] /[Sr ]
ext
aq
rec
ext
¥
2
+
100). The subscripts ext, aq, rec and ini denote the extracting phase, the
aqueous phase, the receiving phase and the initial condition, respectively.
The equilibrium pH values in the aqueous phase were also measured.
and recovery of Sr using H2bDA18C6 in the IL was readily
switched by simply controlling the pH of the aqueous solutions.
1
2
3
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[
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mim][Tf
Sr ] = 0.01 mM. Extracting phase: [H2bDA18C6] = 1 mM. Receiving
phase: HEPES–HNO or HEPES–LiOH solutions.
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based system. The stripping of metal ions extracted in the IL was
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Acknowledgements
1
The authors would like to acknowledge financial assistance by
a Grant-in Aid for Scientific Research (No. 19760617) from the
Ministry of Education, Culture, Sports, Science, and Technology
of Japan (to K.S.).
1
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Notes and references
1
5 K. Shimojo, H. Naganawa, F. Kubota and M. Goto, Chem. Lett., 2006,
‡
Extracting phases were prepared by dissolving each extractant in
mim][Tf N]. For comparison with the performance of IL, a conven-
tional organic solvent, chloroform, containing each extractant was also
prepared using the same procedures. Aqueous phases were prepared by
35, 484–485.
[C
2
2
16 O. Murillo, S. Watanabe, A. Nakano and G. W. Gokel, J. Am. Chem.
Soc., 1995, 117, 7665–7679.
17 L. Mikul a´ sˇ ek, B. Gr u¨ ner, C. Danila, V. B o˝ hmer, J. Cˇ a´ slavsk y´ and P.
dissolving 0.01 mM Sr(NO
3
)
2
. The pH of the aqueous solutions was
Seluck y´ , Chem. Commun., 2006, 4001–4003.
4
852 | Dalton Trans., 2009, 4850–4852
This journal is © The Royal Society of Chemistry 2009