Controlling cesium cation recognition via cation metathesis within an ion pair receptor

Article abstract of DOI:10.1021/ja209706x

Ion pair receptor 3 bearing an anion binding site and multiple cation binding sites has been synthesized and shown to function in a novel binding-release cycle that does not necessarily require displacement to effect release. The receptor forms stable com

Full text of DOI:10.1021/ja209706x

Article
pubs.acs.org/JACS
Controlling Cesium Cation Recognition via Cation Metathesis within
an Ion Pair Receptor
Sung Kuk Kim,† Gabriela I. Vargas-Zuniga,† Benjamin P. Hay,*,∥ Neil J. Young,∥ Lætitia H. Delmau,∥
́
̃
Charles Masselin,∥ Chang-Hee Lee,*,‡ Jong Seung Kim,*,§ Vincent M. Lynch,† Bruce A. Moyer,*,∥
and Jonathan L. Sessler*,†,⊥
†Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station, A5300, Austin, Texas
78712-0165, United States
‡Department of Chemistry, Kangwon National University, Chun-Chon 200-701, Korea
§Department of Chemistry, Korea University, Seoul 136-701, Korea
∥Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830-6119, United States
⊥Department of Chemistry, Yonsei University, Seoul 120-749, Korea
S
* Supporting Information
ABSTRACT: Ion pair receptor 3 bearing an anion binding site
and multiple cation binding sites has been synthesized and
shown to function in a novel binding−release cycle that does
not necessarily require displacement to effect release. The
receptor forms stable complexes with the test cesium salts,
CsCl and CsNO3, in solution (10% methanol-d4 in chloroform-
1
d) as inferred from H NMR spectroscopic analyses. The
addition of KClO4 to these cesium salt complexes leads to a
novel type of cation metathesis in which the “exchanged”
cations occupy different binding sites. Specifically, K+ becomes
bound at the expense of the Cs+ cation initially present in the complex. Under liquid−liquid conditions, receptor 3 is able to
extract CsNO3 and CsCl from an aqueous D2O layer into nitrobenzene-d5 as inferred from 1H NMR spectroscopic analyses and
radiotracer measurements. The Cs+ cation of the CsNO3 extracted into the nitrobenzene phase by receptor 3 may be released
into the aqueous phase by contacting the loaded nitrobenzene phase with an aqueous KClO4 solution. Additional exposure of the
nitrobenzene layer to chloroform and water gives 3 in its uncomplexed, ion-free form. This allows receptor 3 to be recovered for
subsequent use. Support for the underlying complexation chemistry came from single-crystal X-ray diffraction analyses and gas-
phase energy-minimization studies.
actual practice, the latter part of the recognition event often
constitutes the greater challenge. Traditionally, release
strategies have centered around simple mass-action equilibrium
shifts driven by concentration differences and simple displace-
ment reactions. However, the accessible range of gradients is
usually modest. This has the consequence that many stripping
stages are generally required, as is the case for the caustic-side
solvent extraction (CSSX) process currently operating at the
pilot scale for the removal of 137Cs from tank waste.1,3 By
contrast, a binding−release cycle effected by an on−off
switching mechanism could in principle reduce the separation
process to just two ideal stages, namely, initial binding and
subsequent controlled release. Examples of synthetic recog-
nition systems displaying such efficiency remain elusive. Among
the concepts explored, albeit not in the context of Cs+
complexation, are photochemical,4 electrochemical,5 and bind-
ing-induced conformational switching mechanisms.6 Here we
INTRODUCTION
■
The March 2011 Tohoku earthquake and tsunami and
subsequent release of radioactive material, including 137Cs
(half-life 30.2 y), from the Fukushima Daiichi Nuclear Power
Plant has added new urgency to the long-standing need to
develop synthetic receptors for Cs+. Precedent for use of metal
ion receptors in nuclear applications has been growing rapidly
in the past 10 years, with first simple crown ethers and more
recently more powerful calixarenes being applied to the
problem of Cs+ recognition.1 To be effective, these receptors
must recognize the cesium cation selectively under a variety of
conditions and in the presence of various potentially competing
cations, including notably Na+ (as found in seawater and so-
called tank waste). However, selective binding is only the first
part of recognition as defined by Lehn.2 Specifically,
recognition in a supramolecular sense implies control of
function. For most applications involving the cesium cation
(e.g., sensor development, solid-phase separations, liquid−
liquid extraction), this translates, at a minimum, into an ability
to control both initial Cs+ binding and subsequent release. In
Received: October 25, 2011
Published: December 19, 2011
© 2011 American Chemical Society
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report a new approach involving cation metathesis within an
ion pair receptor. As detailed below, it permits the complex-
ation and controlled release of various cesium salts (including
the nitrate and chloride anion forms relevant to studies
involving tank waste and seawater, respectively) both in organic
media (10% methanol-d4/chloroform-d) and under conditions
of liquid−liquid (aqueous−nitrobenzene) extraction. The
receptor itself can be regenerated in its ion-free form through
a simple sequence of liquid−liquid contacts with chloroform
and neutral aqueous phases. To the best of our knowledge, a
binding−release cycle employing release stimulated by remote
binding has not been previously observed using either ion pair
receptors or combinations of simple anion or cation receptors.
We consider the use of an ion-pairing strategy appealing as a
means of controlling the binding and subsequent release of the
cesium cation. Broadly speaking, ion pair receptors are species
that have two or more different ion recognition sites and which
are able to bind both cations and anions.7,8 In early pioneering
work Smith and Beer reported ditopic systems that displayed
enhanced affinities for targeted ion pairs as compared to
appropriately chosen monotopic or single site receptor
controls.7,8 More broadly, the putative improvements in affinity
and selectivity that can be achieved relative to single-site
receptors has made ion pair receptors attractive for use in salt
solublization, ion extraction, transmembrane ion transport, and
ion sensing and as logic gates.8−17 However, in spite of their
potential advantages in functional recognition, to our knowl-
edge no ion pair receptor has been successfully applied to the
problem of controlled cesium cation binding, extraction, and
release.
It was our thought that ion pair receptors could function as
on−off switches if they contain two binding sites, each selective
for a different metal cation, within the same molecule. A
fundamental problem with displacement-type release mecha-
nisms is that they require a binding site that is able to
accommodate different cations, which paradoxically tends to
defeat the goal of high binding selectivity. However, if two
separate binding sites are present, each with high selectivity for
different cations and whose binding affinities are codependent,
then a switched binding−release cycle would become possible
via a novel process in which the exchange takes place at remote
sites. As detailed below, we have now demonstrated the viability
of this approach.
and CsNO3.14 Depending on the salt in question, the Cs+
cations were found to bind to the ethylene glycol moieties
between the calix[4]pyrrole subunit and the calix[4]arene
“cap”. In all cases, however, preference for Cs+ relative to other
alkali-metal cations was seen. Thus, neither 1 nor 2 acts as a
switchable functional receptor, wherein release of the bound
cation can be achieved by means other than mass action or
displacement. With the goal of obtaining an ion pair receptor
that might allow for the controlled recognition and release of
the cesium cation, we have now prepared receptor 3 (cf.
Scheme 1). While retaining the calix[4]pyrrole anion binding
site, receptor 3 differs from 1 in that it has one fewer oxygen
atom in the calix[4]arene crown ring (i.e., a crown-5, rather
than crown-6, strapping moiety). This change was expected to
provide a system with a dedicated K+ binding site and thus an
inherent preference for K+ relative to Cs+ at that site.19,20 To
the extent this proved true, it was expected to give rise to a
system that would allow for the initial complexation of the
cesium cation (in any of several possible ion pair binding
modes14,18,21) in the absence of K+ and then its subsequent
release by exposure to the K+ cation. In the limit, these
thermodynamic differences could be exploited to achieve
cesium cation extraction and stripping under conditions of
liquid−liquid extraction. The validity of this strategy, shown
schematically in Figure 1, has now been demonstrated in the
case of both cesium nitrate and cesium chloride salts (as
present in tank waste and seawater, respectively). Furthermore,
under conditions of aqueous nitrobenzene liquid−liquid
extraction, interference from the sodium cation was found to
to be nil. Finally, a simple sequential liquid−liquid contacting
strategy has been discovered that demonstrates the principle of
regeneration of the metal-free form of the receptor.
RESULTS AND DISCUSSION
■
The synthesis of ion pair receptor 3 is summarized in Scheme
1. Briefly, the hydroxyl group of 1-[4-(2-hydroxyethoxy)-
phenyl]ethanone (4) was tosylated using NaOH and TsCl in
THF to give tolsylate 5 in high yield. This intermediate was
subsequently condensed with pyrrole in the presence of 20
equiv of trifluoroacetic acid (TFA) at 65 °C to produce
dipyrromethane 6 in 62% yield. The dipyrromethane tosylate 6
was reacted with the calix[4]arene monocrown-5 (7)22 in the
presence of 3.0 equiv of Cs2CO3 in acetonitrile under reflux to
afford the calix[4]arene crown-5 dipyrromethane 8 in the 1,3-
alternate conformation in 61% yield. Further condensation of
compound 8 with acetone in the presence of a catalytic amount
of BF3·OEt2 gave ion pair receptor 3 in 18% yield.14,18
Compound 3 was fully characterized by standard spectroscopic
means, as well as by single-crystal X-ray diffraction analysis
(Figure S1, Supporting Information). The resulting crystal
structure revealed that the calix[4]arene subunit adopts the
expected 1,3-alternate conformation.
Recently, we reported the ion pair receptor 1 bearing two
strong ion binding sites (a calix[4]arene crown-6 for the Cs+
cation and a calix[4]pyrrole for anions) and demonstrated its
ability to stabilize a solvent-separated cobound CsF ion pair
complex.18 Subsequently, we found that the crown-free ion pair
receptor 2 formed ion pair complexes with CsF, CsCl, CsBr,
Theoretical support for the suggestion that receptor 3 would
prove selective for K+ over Cs+ came from molecular mechanics
calculations carried out for the gas phase in the absence of
solvent (cf. Supporting Information). Figure 2 shows four
limiting binding modes for the complexation of cesium salts.
These distinct modes, which are also seen for the
corresponding potassium salts, are designated crown/crown,
crown/pyrrole, glycol/pyrrole, and pyrrole/pyrrole. Here, the
first part of the name indicates the location of the cation and
the latter part of the name indicates the location of the anion.
The relative stabilities of these binding modes with several
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Figure 1. Design concept underlying ion pair receptor 3.
Scheme 1. Synthesis of Ion Pair Receptor 3
Table 1. Calculated Gas-Phase Binding Energy for Ion Pairs
in Different Binding Modes of 3
a
ΔE (kcal/mol)
ion pair crown/crown crown/pyrrole glycol/pyrrole pyrrole/pyrrole
KCl
−152.4
−132.4
−149.7
−129.6
−143.9
−126.8
−130.1
−115.8
−132.7
−116.7
−119.1
−104.1
−144.8
−135.5
−149.9
−135.8
−138.1
−121.3
−130.0
−117.6
−114.6
−103.5
−103.1
−91.7
CsCl
KNO3
CsNO3
KClO4
CsClO4
a
ΔE = E(complex) − E(ligand) − E(cation) − E(anion).
the preferred gas-phase binding modes are those that minimize
the distance between the two ions. Crown/crown and glycol/
pyrrole pairs are thus preferred, but they are not greatly
different in energy nor is one mode consistently more stable
than the other. As a consequence, the KClO4 and CsClO4 salts
exhibit a modest preference for the crown/crown mode,
whereas CsCl and CsNO3 exhibit a slight preference for the
glycol/pyrrole mode. In contrast to what is true for simple
calix[4]pyrroles,21 complexation via the pyrrole/pyrrole mode
is not favored.
Experimental support for the notion that receptor 3 can
complex the cesium cation came from a single-crystal X-ray
diffraction analysis of the 1:1 complex with CsCl. The resulting
structure revealed that the Cs+ and Cl− ions are bound to
calix[4]arene crown-5 and the calix[4]pyrrole moieties,
respectively (Figure 3). The N···Cl− and Cs+···O distances
cation−anion pairs are summarized in Table 1. These values are
consistent with the design expectation that, irrespective of the
binding mode, there is a strong preference for binding K+ over
Cs+. They also reveal decisively that, in the absence of solvation,
Figure 2. Views of the four limiting binding modes proposed for the interaction of receptor 3 with CsCl. Similar modes are considered in the case of
other salts; cf. Table 1.
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and the π−metal separation between the Cs+ ion and the
aromatic carbon atoms of the calix[4]arene core were found to
Figure 4. Two different views of the single-crystal X-ray structure of
3·CsNO3·CH3CH2OH. Solvent molecules not involved in the ion pair
complex have been removed for clarity.
Figure 3. Two different views of the single-crystal X-ray diffraction
structure of 3·CsCl·H2O. Solvent molecules not involved in the ion
pair complex have been removed for clarity.
be 3.23−3.31, 2.84−3.10, and 3.29−3.40 Å, respectively. One
water molecule also interacts with the bound Cl− anion through
what is assumed to be a hydrogen-bonding interaction. The
relevant O···Cl− distance is 3.22 Å. Although it may seem that
the X-ray structure “disobeys” the gas-phase prediction of a
crown−crown structure, the disparity presumably reflects the
proximity of neighboring ions in the solid state. This, in turn,
permits the apparent cation−anion separation within the ion
pair receptor in the X-ray structure.
The structure of the CsNO3 complex in the solid state was
also determined by X-ray diffraction analysis of single crystals
obtained via the slow evaporation of an ethanol/chloroform
solution of 3 in the presence of excess CsNO3. The resulting
crystal structure revealed that, in contrast to what was seen for
the CsCl complex, the Cs+ cation is coordinated by the oxygen
atoms of the ethylene glycol spacers but not by the crown-5
ring (Figure 4). Such anion-dependent structural differences in
complexation mode stands in contrast to what was seen in the
case of 1 and 2.14,18 The distances between the Cs+ cation and
the oxygen atoms of the ethylene glycolic spacers were found to
be 3.01−3.63 Å in the CsNO3 complex. In addition, the bound
Cs+ ion interacts closely with two oxygen atoms of the cobound
nitrate anion with Cs+···O distances of 3.19 and 3.50 Å, as well
as with an ethanol molecule. Interactions between the Cs+
cation and the aromatic carbon atoms of the inverted phenoxy
groups of the calix[4]arene moiety were also inferred from the
structural parameters (e.g., Cs+···C contacts of 3.50−3.66 Å).
One oxygen atom of the NO3− ion is also hydrogen-bonded to
the calix[4]pyrrole NH protons, with the relevant N···O
distances being 2.92−3.00 Å.
Figure 5. Two different views of the single-crystal X-ray diffraction
structure of 3·KNO3·H2O. Solvent molecules not involved in the ion
pair complex have been removed for clarity.
KNO3 complex occurs via cation metathesis involving displace-
ment of Cs+ by a K+ ion in what is a thermodynamically driven
process. The resulting crystal structure revealed that the K+
cation is bound to the crown-5 ring rather than to the ethylene
glycol moieties (crown/pyrrole vs glycol/pyrrole mode). The
relevant distances are 2.73−2.84 Å for the Cs+···O and 3.07−
3.31 Å for the π−metal interactions, respectively (Figure S18,
Supporting Information). As expected, the NO3− anion is
hydrogen-bonded to the calix[4]pyrrole moiety at a N···O−
distance of 2.93−3.01 Å. The anion is separated from the
cobound K+ cation by a distance of 8.32 Å.
The solution-phase anion and cation binding behavior of 3
1
was investigated via H NMR spectroscopy using a mixed
Initial evidence that the potassium cation would be bound in
preference to Cs+ came from an X-ray diffraction analysis of
single crystals of the KNO3 complex of 3 obtained by allowing a
chloroform/ethanol solution of the preformed 3·CsNO3
complex to undergo slow evaporation in the presence of 1
molar equivalent of KClO4 relative to the added CsNO3
(Figure 5). That a new complex, containing K+ instead of
Cs+, is formed leads us to suggest that the formation of the
solvent system consisting of CDCl3 and CD3OD (9:1, v/v).
This particular choice of solvents was dictated by the solubility
of the salts under study. In analogy to what was seen with
receptors 1 and 2 (which were also studied in this same solvent
system),14,18 no appreciable change in the H NMR spectrum
1
of 3 was seen upon treatment with 5 equiv of TBAClO4,
TBACl, or TBANO3 (Figure S2, Supporting Information). On
this basis, we conclude that the TBA+ cation is not bound and
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1
Figure 6. Partial H NMR spectra of (a) 3 (4 mM) only, (b) 3 with 4.0 equiv of KClO4, (c) 3 with 4.0 equiv of CsClO4, (d) 3 with 4.0 equiv of
CsCl, (e) 3 with 4.0 equiv of KNO3, and (f) 3 with 4.0 equiv of CsNO3 in CD3OD/CDCl3 (1:9, v/v).
that neither the chloride nor nitrate anion is complexed by
receptor 3 as a TBA+ salt in this mixed solvent system.
Supporting Information). This observation is consistent with
slow exchange and strong K+ complexation. Indeed, the binding
constant of receptor 3 for the K+ cation measured by isothermal
titration calorimetry (ITC) in acetonitrile using KTPB
(potassium tetraphenylborate) was found to be much higher
than that for the Cs+ cation (Ka = 6.5 × 106 M−1 for K+ vs Ka =
3.3 × 104 M−1 for Cs+) (Table S1 and Figures S5 and S6,
Supporting Information). These observations are consistent
with calculation results showing that receptor 3 exhibits an
intrinsic preference for the K+ cation over the Cs+ cation. They
also provide support for the suggestion that treatment with K+
could be used to induce the thermodynamically driven,
chemically induced release of a Cs+ cation prebound in
receptor 3.
In accord with design expectations, receptor 3 was found to
form a 1:1 ion pair complex with KNO3 in 10% CD3OD in
CDCl3. As inferred from the 1H NMR spectroscopic analyses, it
does so in a sequential manner. Specifically, upon addition of
KNO3 to a solution of receptor 3 in 10% CD3OD in CDCl3,
the proton signals of the calix[4]arene crown-5 were seen to
undergo a shift while those of the calix[4]pyrrole moiety were
largely unchanged (Figure 6e). Such observations are consistent
with receptor 3 coordinating the K+ cation first through the
calix[4]arene crown-5 ring without the NO3− anion being
bound to the calix[4]pyrrole moiety (crown/crown mode).
Once the K+ is bound to receptor 3 (to produce the potassium
complex [3·K+]), the binding of the NO3− counteranion
becomes thermodynamically favorable under these solution-
phase conditions (cf. Figures 6 and 7 and Figure S7 in the
Supporting Information). As the ion pair complex of receptor 3
with KNO3 ([3·KNO3]; crown/pyrrole mode) forms, it starts
to precipitate from solution (Figures 6−8; Figures S7 and S8,
Supporting Information). As a consequence, complexation of
these potassium salts by 3 becomes irreversible under these
experimental conditions.
Remarkably different spectral behavior was observed when
receptor 3 was treated with KClO4 and CsClO4. As shown in
Figure 6, the addition of either of these salts to solutions of 3 in
CDCl3 and CD3OD (9:1, v/v) led to a significant chemical shift
change in the signals for both the aromatic protons of the
calix[4]arene core and the aliphatic protons of the crown ether
ring (Figure 6; Figure S3, Supporting Information). These
changes are consistent with the cations being encapsulated by
the crown ether ring with the aid of the aromatic rings of the
calix[4]arene, perhaps via π−metal interactions. Notably, the
addition of these salts did not induce a substantial change in the
proton signals of the calix[4]pyrrole moiety. This lack of
change provides support for the notion that the perchlorate
anion is bound either very weakly or not at all by the
calix[4]pyrrole moiety. Therefore, taken in concert, these
findings are consistent with the expectation that the addition of
KClO4 and CsClO4 leads to the formation of the cation-bound
complexes ([3·K+]ClO4− and [3·Cs+]ClO4−), where the ClO4−
counteranion is not cobound (i.e., crown/crown mode; cf.
Figure 2). This finding is fully in line with the results of the
calculations, which revealed that both KClO4 and CsClO4
prefer this binding mode in the gas phase (see Table 1).
Analyzing the spectra in greater detail reveals that the peaks
of both the aromatic protons of the calix[4]arene moiety and
the crown ring are broadened in the presence of CsClO4, while
they remain sharp in the presence of KClO4 (Figure 6; Figure
S3, Supporting Information). Such findings are consistent with
the expectation that receptor 3 binds the K+ cation more
strongly than the Cs+ cation. This stronger affinity for the K+
cation was further evidenced by a 1H NMR spectrum measured
in the presence of 1.0 equiv of KClO4, where two sets of
distinguishable proton signals were seen corresponding to the
free receptor and its K+ complex, respectively (Figure S4,
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7 and Figures S8 and S9 in the Supporting Information).
However, they differ from what was observed in the case of 3
and KNO3. For instance, in the presence of 4.0 equiv of CsNO3
in CD3OD/CDCl3 (1:9, v/v), a new set of sharp peaks in the
1H NMR spectrum corresponding to the β-pyrrolic protons are
seen while other peaks corresponding to the aromatic protons
of the calix[4]arene moiety become broadened (Figure 6f).
This is consistent with two different kinds of binding
interactions involving receptor 3 and the CsNO3 ion pair. In
one set, only the Cs+ cation but not the NO3− anion is weakly
bound to the crown-5 ring to form a cesium complex
([3·Cs+]NO3−) where the NO3− counteranion is not cobound
(crown/crown binding mode). This complex exists in fast
equilibrium with the free receptor, as evidenced by peak
broadening seen for the aromatic protons of the calix[4]arene
moiety. By contrast, in the other binding mode, the Cs+ cation
and the NO3− anion are concurrently and strongly bound to the
receptor 3 being stabilized by the ethylene glycol spacers and
the calix[4]pyrrole moiety, respectively (i.e., [3·CsNO3];
glycol/pyrrole mode; cf. Figures 2 and 7 and Figures S7 and
S8 in the Supporting Information). This latter complexation
mode is similar to what was seen in the case of the CsNO3
complex of compound 2.14 These two species exist in
equilibrium, and over time, the ion pair complex [3·CsNO3]
is formed in preference, leading us to suggest that it represents
the thermodynamically favored species in this solvent system.
In contrast to what is seen with KClO4 and CsClO4, in the
presence of cesium chloride and potassium chloride, receptor 3
forms strong ion pair complexes wherein the cation is bound to
the calix[4]arene crown-5 moiety and the anion to the
calix[4]pyrrole cavity (crown/pyrrole mode; cf. Figures 3, 6,
and 7 and Figures S8 and S9 in the Supporting Information).
Support for this conclusion comes from NMR spectral studies
carried out in CD3OD/CDCl3 (1:9, v/v). For instance, upon
addition of CsCl to a solution of receptor 3 in 10% CD3OD in
CDCl3, the proton signals of the calix[4]arene crown-5
underwent a significant downfield shift (an observation
consistent with Cs+ complexation), while those of the
calix[4]pyrrole moiety were shifted upfield, presumably as the
result of chloride anion binding (Figure 6d; Figure S3d,
Supporting Information). A remarkable downfield shift (Δδ ≈
4.5 ppm) in the NH peak is also observed (Figure 6d). In
contrast, addition of KCl to a solution of receptor 3 in 10%
CD3OD in CDCl3 leads to precipitation of the KCl ion pair
complex, [3·KCl].
Figure 7. Proposed binding interactions involving receptor 3 and
various K+ and Cs+ ion pairs in 10% methanol in chloroform (10%
1
CD3OD in CDCl3 for H NMR spectral studies). Also shown are the
crystal structures of the various ion pair complexes in question grown
from mixtures of chloroform and methanol or ethanol.
1
To provide support for the inferences drawn from the H
NMR spectral measurements, namely, that receptor 3 binds
both cesium salts and potassium salts but displays high
selectivity for potassium salts over the cesium salts, we
investigated whether cation metathesis would occur when a
precomplexed Cs+ cation ion pair complex was exposed to K+.
These studies were carried out by adding KClO4 to a solution
of the [3·CsNO3] complex in 10% CD3OD/CDCl3 (Figures 7
and 8; Figure S7, Supporting Information). Under these
conditions, precipitation was observed. This is interpreted in
terms of the CsNO3 complex (glycol/pyrrole mode) being
converted into the KNO3 complex (crown/pyrrole mode) via
cation exchange as summarized in eqs 1 and 2. As noted above,
the latter complex is insoluble and precipitates from solution.
Presumably, this helps drive the initial removal of the cesium
cation from the CsNO3 complex.
Figure 8. Precipitation induced via cation metathesis observed upon
addition of the K+ cation (as KClO4) to the preformed CsNO3
complex of 3 in CD3OD/CDCl3 (1:9, v/v).
Spectral changes corresponding to different binding modes
1
are also seen in the H NMR spectrum when receptor 3 is
[3·CsNO ] + KClO → [3·KNO ] + CsClO
4
(2)
3
4
3
exposed to CsNO3 in 10% CD3OD in CDCl3 (cf. Figures 6 and
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was obtained, and neither was this species expected to be
significant.23
[3] + CsNO → [3·CsNO ]
(1)
3
3
An interesting feature of the system is the tendency for the
complex [32·CsNO3] to form at high concentrations of 3,
involving the participation of two molecules of 3 to
accommodate one ion pair. This complex becomes predom-
inant above 3 mM 3 at intermediate loading, as reflected in the
slope of 2 observed in the right side of the curve in Figure 12b.
As the loading is increased, the relative concentration of
[32·CsNO3] diminishes as [3·CsNO3] becomes the sole
complex at saturation.
To examine the spectrum of only the ion pair complex
[3·CsNO3], we equilibrated 4 mM 3 in nitrobenzene-d5 with
0.5 M CsNO3 in D2O. Under these conditions, we obtained
exactly the same spectrum as the one shown in Figure 11d, a
finding that we interpret as confirming that the glycol/pyrrole
form of bound CsNO3 is the dominant species under these
conditions.
The fact that receptor 3 binds K+ selectively over Cs+ but
complexes the cesium cation in the absence of the potassium
salts led us to consider that this receptor could have use as an
extractant. In particular, it was thought that it could be used to
extract the Cs+ cation from aqueous media while permitting its
subsequent release by exposure to K+ as shown schematically in
Figure 9.
1
To test this possibility, H NMR spectroscopy was used in
conjunction with a two-phase system consisting of D2O and
nitrobenzene-d5. Upon exposure of receptor 3 in C6D5NO2 to
aqueous (D2O) solutions of NaNO3, KNO3, and CsNO3,
1
respectively, significant changes in the H NMR spectra were
observed in the case of KNO3 and CsNO3 but not NaNO3
(Figures 10 and 11; Figure S10, Supporting Information). This
is taken as evidence that receptor 3 is capable of extracting both
KNO3 and CsNO3 from an aqueous environment into a
nitrobenzene organic phase. Independent support for the
suggestion that receptor 3 was capable of effecting CsNO3
extraction came from radiotracer studies involving 137CsNO3.
Experiments were conducted with varying initial concentrations
of CsNO3 in the aqueous phase and 3 in the nitrobenzene
phase (Figure 12). The mass-action behavior was analyzed to
show the predominance of three species with stoichiometries
[3·Cs+], [3·CsNO3], and [32·CsNO3] (see the Supporting
Information). At concentrations of 3 less than approximately 3
mM, the ion pair complex [3·CsNO3] is the major species, but
at low loading, it dissociates to [3·Cs+] and free NO3− in the
nitrobenzene phase, with the apparent association constant for
this latter equilibrium being 1.2 × 105 M−1. This is a relatively
strong ion pair association for nitrobenzene and is in accord
with the previous finding that meso-octamethylcalix[4]pyrrole
itself can function as an ion pair receptor.21 The slope of
approximately 0.5 observed at the left side of the curve in
Figure 12b is consistent with the dissociated regime. No
evidence for the bound nitrate as the complex anion [3·NO3−]
Differences between KNO3 and CsNO3 were inferred from
1
the H NMR spectroscopic analyses of the nitrobenzene-d5
layer. For instance, in the case of KNO3, the proton signals of
both the aromatic ring of the calix[4]arene moiety and the
crown-5 ring were seen to shift toward lower field whereas the
peaks of the β-pyrrolic protons did not shift appreciably (Figure
10b). On this basis we propose that only the K+ cation of the
KNO3 ion pair is bound appreciably by receptor 3 to generate
[3·K+]NO3− in the organic phase (Figure S10, Supporting
Information). This crown/crown binding mode stands in
marked contrast with what is seen in the case of CsNO3. Here,
1
recording the H NMR spectrum after an analogous extraction
process reveals a significant downfield shift in the proton signals
of the calix[4]arene crown-5 protons relative to those of free
receptor 3. Upfield shifts in the signal of the β-pyrrolic protons
of the calix[4]pyrrole moiety were also observed (Figure 10c).
Such changes are attributable to the formation of an ion pair
complex between CsNO3 and receptor 3 to give [3·CsNO3]
(Figure 9). The conclusion that the NO3− anion is bound to the
calix[4]pyrrole cavity with the Cs+ cation bound to the ethylene
glycol moieties (i.e., glycol/pyrrole binding mode) was further
Figure 9. Schematic representation of the two-phase extraction and recovery of CsNO3 achieved using the ion pair receptor 3. The stages involved
include initial Cs+ complexation and K+-for-Cs+ cation exchange, followed by regeneration of the free receptor by contact with chloroform and water.
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Figure 10. Partial 1H NMR spectra of nitrobenzene-d5 solutions of 3 (4 mM) (a) after being contacted with D2O, (b) after being contacted with an
aqueous D2O/KNO3 solution (5 equiv), (c) after being contacted with an aqueous D2O/CsNO3 solution (5 equiv), (d) after the nitrobenzene phase
obtained from (c) was contacted with D2O and then with an aqueous D2O/KClO4 solution (5 equiv), and (e) after the organic phase obtained from
(d) was contacted with D2O and chloroform-d.
Figure 11. Partial 1H NMR spectra of nitrobenzene-d5 solutions of 3 (4 mM) (a) after being contacted with D2O, (b) after being contacted with an
aqueous D2O/NaCl solution (5 equiv), (c) after being contacted with an aqueous D2O/NaNO3 solution (5 equiv), (d) after being contacted with an
aqueous D2O/CsNO3 solution (5 equiv), and (e) after being contacted with an aqueous D2O solution consisting of a mixture of NaCl (5 equiv),
NaNO3 (5 equiv), and CsNO3 (5 equiv).
supported by the finding that the NH signal of the
calix[4]pyrrole moiety undergoes a downfield shift (Δδ ≈ 2.5
ppm) when receptor 3 is exposed to CsNO3 under these two
phase conditions (Figure 10c).
of this exchange process is evidenced by the emergence of a 1H
NMR spectrum for the nitrobenzene-d5 phase that is similar to
that recorded for the KNO3 complex (Figure 10d). In accord
with expectations, the 133Cs NMR spectrum of the water phase
measured after extraction of [3·CsNO3] with the aqueous
KClO4 solution revealed the presence of the Cs+ cation,
presumably reflecting the existence of a solubilized CsNO3 ion
pair in the water phase (Figure S11, Supporting Information).
While not established directly, the decomplexation of the
nitrate anion (and its transfer to the aqueous phase) was
inferred from the fact that, after contact with an aqueous KClO4
solution, the 1H NMR spectrum of 3 (recorded in the
nitrobenzene-d5 layer) matches that of complexes where an
anion is not cobound (i.e., the NH proton and β-pyrrolic
proton signals appear significantly upfield and downfield
When the nitrobenzene-d5 phase containing complex
[3·CsNO3] formed via extraction is contacted with an aqueous
D2O solution of KClO4, the binding of the K+ cation causes the
expulsion of the Cs+ cation originally cobound within 3. This
apparent exchange, which we propose involves an ion-
stimulated release, is ascribed to the destabilizing effect of the
incipient electrostatic repulsion between the two cations in
question, namely, the prebound Cs+ cation and the entering K+
cation.19 As a result of this metathesis, a new complex,
[3·K+]ClO4−, is produced in the organic phase (Figures 9 and
10) and CsNO3 is released into the aqueous phase. The efficacy
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Figure 12. Cesium distribution ratio (DCs) at 25 °C as a function of (a) increasing initial aqueous CsNO3 concentration with 10 mM 3 in
nitrobenzene and (b) increasing concentration of 3 in nitrobenzene with 3 mM initial aqueous CsNO3. The organic:aqueous volume ratio was 1:1.
The red squares represent a fit to the best mass-action equilibrium model (see the Supporting Information).
shifted, respectively, as compared to those of the initial
[3·CsNO3] complex).
first prototype of a novel type of binding−release cycle from
which new examples, perhaps more practical, can be designed.
The ability of receptor 3 to extract CsNO3 selectively over
These findings are in line with the standard molar Gibbs free
energies for hydration of the K+ cation (ΔhydG° = −295 kJ/
mol) and ClO4− (ΔhydG° = −214 kJ/mol) and those of the Cs+
cation (ΔhydG° = −250 kJ/mol) and NO3− (ΔhydG° = −300 kJ/
mol).24 In accord with its design features, receptor 3 extracts
KClO4 better than it does CsNO3, which is rationalized on the
basis of the stronger binding of K+ vs Cs+ and the lower
hydration of ClO4− vs NO3−. Stronger binding of K+ thus serves
to overcome the hydration energy bias that would otherwise
favor extraction of Cs+.24 As important, the lower hydration of
ClO4− drives the exchange for NO3−. Overall, the fact that
KClO4 is extracted well under these interfacial conditions
allows it to be used to effect Cs+ release from 3 using the simple
biphasic contacting procedure described above.
In a further step of note, it was found that contacting the
nitrobenzene layer containing [3·K+]ClO4− with chloroform-d
and D2O (twice) leaves receptor 3 in its free form in the
organic phase (Figures 8 and 9e). After separation of the
organic phase followed by removal of chloroform in vacuo,
receptor 3 can be recycled for further use.
The above experiments introduce a new paradigm for
devising a binding−release cycle in a separation system by
showing how a binding event in a remote part of the molecule
(that of K+ binding by the crown in the present instance) can
stimulate the release of prebound cation in a different location
(Cs+ in the glycol site in the case of receptor 3). Given this
proof of principle, it is worth considering the pathway to more
practical systems. Not only would one move to more acceptable
process diluents such as paraffinic hydrocarbons, as opposed to
toxic, volatile solvents such as nitrobenzene or chloroform, but
one would also design for an alternative to K+ as a metathesis
agent that would not be itself a waste constituent. If this
competing ion was also a preferred constituent of a final waste
form suitable for disposal, the strip solution could be routed
directly to waste-form production without concerns regarding
secondary waste production. This may be actually preferable to
stripping with pure water, which Figure 12 leads us to suggest
could be used directly to strip the loaded cesium, owing to the
low distribution ratios (DCs). An advantage of a metathesis
strategy for stripping over water stripping is that it is potentially
more efficient, and additionally, organic phases often do not
disengage from pure water cleanly. Thus, our system serves as a
1
sodium salts, such as NaCl and NaNO3, was also tested by H
NMR spectroscopy (Figure 11). No appreciable change was
observed in the 1H NMR spectrum of the nitrobenzene-d5 layer
after this layer (containing receptor 3) was contacted with an
aqueous D2O layer containing 5 equiv of NaCl or NaNO3
(relative to 3), leading us to suggest that such sodium salts are
not extracted efficiently (if at all) by receptor 3. In contrast,
contacting the nitrobenzene-d5 layer containing receptor 3 with
a mixture of NaCl, NaNO3, and CsNO3 (5 equiv of each) gave
rise to a 1H NMR spectrum identical to that obtained when the
same experiment was carried out using just 5 equiv of CsNO3.
This finding is consistent with the suggestion that receptor 3 is
capable of extracting CsNO3 selectively in the presence of
excess NaCl and NaNO3 (Figure 11).
Chloride, rather than nitrate, is the dominant counteranion
in the case of 137Cs+ present in seawater. The chloride anion is
more highly hydrated than the nitrate anion (ΔhydG° = −340
kJ/mol for Cl−; ΔhydG° = −300 kJ/mol for NO3−). It thus
represents a greater challenge in terms of binding and
extraction. Gratifyingly, receptor 3 proved capable of extracting
CsCl from D2O into nitrobenzene-d5 (Figure S12, Supporting
Information). The 1H NMR spectrum shown in Figure S12b is
fully consistent with CsCl (20 mM in D2O) being extracted by
receptor 3 (4 mM in nitrobenzene-d5) in the form of an ion
pair complex, wherein the Cs+ cation and the Cl− anion are
cobound to 3 via the calix[4]arene crown-5 ring and
calix[4]pyrrole subunits, respectively (crown/pyrrole binding
mode; cf. Figure 2). As in the case of the extractions carried out
with the nitrate salt, further exposure to KClO4 led to spectral
changes consistent with decomplexation of the Cs+ cation
(Figure S12c). For example, after contact of the nitrobenzene
phase (C6D5NO2) obtained after the initial CsCl extraction
receptor 3 with an aqueous D2O solution containing 5 equiv of
1
KClO4, a H NMR spectrum was produced that was identical
to the one obtained when receptor 3 (in C6H5NO2) was
washed with an aqueous D2O solution of KCl (5 equiv). Thus,
as in the case of the nitrate salts, we conclude that Cs+ cation
release is being induced as the result of cation metathesis.
CONCLUSIONS
■
A new ion pair receptor, 3, that contains a dedicated
calix[4]pyrrole anion binding subunit and various sites suitable
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for K+ and Cs+ complexation has been synthesized and
characterized by standard spectroscopic means as well as by
single X-ray crystal diffraction analysis. The 1H NMR
spectroscopic analyses and the X-ray crystal structural data
reveal that both in the solid states and in mixed methanol/
chloroform solution, receptor 3 forms 1:1 ion pair complexes
Foundation (KRF) (2011-0000420 to J.S.K), and the Korean
World Class University (WCU) program (Grant R32-2010-
000-10217-0) administered through the National Research
Foundation of Korea funded by the Ministry of Education,
Science and Technology (MEST). B.P.H., B.A.M., L.H.D., and
N.J.Y. acknowledge support from the Division of Chemical
Sciences, Geosciences, and Biosciences, Office of Basic Energy
Sciences, U.S. DOE.
1
with potassium and cesium salts. As evidenced by H NMR
spectroscopic analyses, receptor 3 displays a higher affinity for
the K+ cation relative to the Cs+ cation. However, in the
absence of potassium salts, receptor 3 binds cesium salts. The
addition of potassium salts containing a noncoordinating anion,
such as perchlorate, to preformed cesium ion pair complexes of
3 induces an effective release of Cs+ by the binding of K+. This
produces new ion pair complexes containing the potassium
cation. This key feature enables receptor 3 to extract CsNO3
from an aqueous phase to an organic layer consisting of
nitrobenzene. By exploiting the dual cation binding features of
receptor 3, it is thus possible to stimulate the release of the Cs+
cation without displacement. Specifically, contacting a nitro-
benzene phase containing the 3·CsNO3 complex (produced by
extraction) with an aqueous KClO4 solution serves to release
CsNO3 into the aqueous phase. Further contacting the
nitrobenzene phase containing the newly formed KClO4
complex with chloroform and water serves to strip out the
KClO4 and regenerate the free form of receptor 3 in the organic
phase. This stepwise control of the thermodynamics appears
very efficient in terms of (i) the initial complexation of the Cs+
cation and (ii) its controlled release and (iii) subsequent
regeneration of the receptor. It is important to note, however,
that the present study was carried out under idealized
laboratory conditions using solvents and concentrations
selected to facilitate analysis. Thus, direct comparisons with
existing extraction-based methods for radioactive cesium
recovery are not realistic or useful. Nevertheless, we believe
that ion pair receptors such as the one described here could
serve as a useful model for liquid−liquid separations.
Furthermore, we think the new principle of cation metathesis
described here can be applied broadly, including to other
situations where functional recognition requires the careful
control of both initial substrate binding and subsequent release.
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Corresponding Author
chhlee@kangwon.ac.kr; jongskim@korea.ac.kr; moyerba@ornl.
gov; sessler@mail.utexas.edu
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ACKNOWLEDGMENTS
■
This work was supported by the Office of Basic Energy
Sciences, U.S. Department of Energy (DOE) (Grant DE-FG02-
01ER15186 to J.L.S.), a Korea National Research Foundation
(NRF) grant (MEST 2009-0087013 to C.-H.L.), the Creative
Research Initiatives (CRI) project of the Korea Research
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