A calix[4]arene strapped calix[4]pyrrole: An ion-pair receptor displaying three different cesium cation recognition modes

Article abstract of DOI:10.1021/ja100715e

An ion-pair receptor, the calix[4]pyrrole-calix[4]arene pseudodimer 2, bearing a strong anion-recognition site but not a weak cation-recognition site, has been synthesized and characterized by standard spectroscopic means and via single-crystal X-ray diffraction analysis. In 10% CD₃OD in CDCl ₃ (v/v), this new receptor binds neither the Cs⁺ cation nor the F^- anion when exposed to these species in the presence of other counterions; however, it forms a stable 1:1 solvent-separated CsF complex when exposed to these two ions in concert with one another in this same solvent mixture. In contrast to what is seen in the case of a previously reported crown ether strapped calixarene-calixpyrrole ion-pair receptor 1 (J. Am. Chem. Soc. 2008, 130, 13162 -13166), where Cs⁺ cation recognition takes place within the crown, in 2CsF cation recognition takes place within the receptor cavity itself, as inferred from both single-crystal X-ray diffraction analyses and ¹H NMR spectroscopic studies. This binding mode is supported by calculations carried out using the MMFF94 force field model. In 10% CD₃OD in CDCl₃ (v/v), receptor 2 shows selectivity for CsF over the Cs⁺ salts of Cl^-, Br^-, and NO ₃^- but will bind these other cesium salts in the absence of fluoride, both in solution and in the solid state. In the case of CsCl, an unprecedented 2:2 complex is observed in the solid state that is characterized by two different ion-pair binding modes. One of these consists of a contact ion pair with the cesium cation and chloride anion both being bound within the central binding pocket and in direct contact with one another. The other mode involves a chloride anion bound to the pyrrole NH protons of a calixpyrrole subunit and a cesium cation sandwiched between two cone shaped calix[4]pyrroles originating from separate receptor units. In contrast to what is seen for CsF and CsCl, single-crystal X-ray structural analyses and ¹H NMR spectroscopic studies reveal that receptor 2 forms a 1:1 complex with CsNO ₃, with the ions bound in the form of a contact ion pair. Thus, depending on the counteranion, receptor 2 is able to stabilize three different ion-pair binding modes with Cs⁺, namely solvent-bridged, contact, and host-separated.

Full text of DOI:10.1021/ja100715e

Published on Web 04/01/2010
A Calix[4]arene Strapped Calix[4]pyrrole: An Ion-Pair
Receptor Displaying Three Different Cesium Cation
Recognition Modes
Sung Kuk Kim,† Jonathan L. Sessler,*,† Dustin E. Gross,† Chang-Hee Lee,‡
Jong Seung Kim,§ Vincent M. Lynch,† Lætitia H. Delmau,⊥ and Benjamin P. Hay⊥
Department of Chemistry and Biochemistry, 1 UniVersity Station-A5300, The UniVersity of Texas at
Austin, Austin, Texas 78712-0165, Department of Chemistry, Kangwon National UniVersity,
Chun-Chon 200-701, Korea, Department of Chemistry, Korea UniVersity, Seoul 136-701, Korea, and
Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830-6119
Received January 26, 2010; E-mail: sessler@mail.utexas.edu
Abstract: An ion-pair receptor, the calix[4]pyrrole-calix[4]arene pseudodimer 2, bearing a strong anion-
recognition site but not a weak cation-recognition site, has been synthesized and characterized by standard
spectroscopic means and via single-crystal X-ray diffraction analysis. In 10% CD3OD in CDCl3 (v/v), this
new receptor binds neither the Cs+ cation nor the F- anion when exposed to these species in the presence
of other counterions; however, it forms a stable 1:1 solvent-separated CsF complex when exposed to these
two ions in concert with one another in this same solvent mixture. In contrast to what is seen in the case
of a previously reported crown ether “strapped” calixarene-calixpyrrole ion-pair receptor 1 (J. Am. Chem.
Soc. 2008, 130, 13162-13166), where Cs+ cation recognition takes place within the crown, in 2·CsF cation
recognition takes place within the receptor cavity itself, as inferred from both single-crystal X-ray diffraction
1
analyses and H NMR spectroscopic studies. This binding mode is supported by calculations carried out
using the MMFF94 force field model. In 10% CD3OD in CDCl3 (v/v), receptor 2 shows selectivity for CsF
over the Cs+ salts of Cl-, Br-, and NO3 but will bind these other cesium salts in the absence of fluoride,
-
both in solution and in the solid state. In the case of CsCl, an unprecedented 2:2 complex is observed in
the solid state that is characterized by two different ion-pair binding modes. One of these consists of a
contact ion pair with the cesium cation and chloride anion both being bound within the central binding
pocket and in direct contact with one another. The other mode involves a chloride anion bound to the
pyrrole NH protons of a calixpyrrole subunit and a cesium cation sandwiched between two cone shaped
calix[4]pyrroles originating from separate receptor units. In contrast to what is seen for CsF and CsCl,
1
single-crystal X-ray structural analyses and H NMR spectroscopic studies reveal that receptor 2 forms a
1:1 complex with CsNO3, with the ions bound in the form of a contact ion pair. Thus, depending on the
counteranion, receptor 2 is able to stabilize three different ion-pair binding modes with Cs+, namely solvent-
bridged, contact, and host-separated.
Introduction
appreciated, attention has turned to the synthesis of so-called
ion-pair receptors that permit the concurrent complexation
The importance of supramolecular interactions in nature
has been increasingly recognized in recent years, as has the
utility of artificial receptors capable of recognizing selectively
ions or neutral substrates.1-3 Most ion-pair receptors studied
thus far have been designed to recognize selectively either
cations or anions, but not both.2-4 However, as the impor-
tance of counterion effects have come to be increasingly
of both cations and anions.5 Compared to simple ion
receptors, ion-pair receptors generally display significantly
enhanced affinities for ions as the result of, e.g., allosteric
effects and enhanced electrostatic interactions between the
cobound ions. As such, they are potentially attractive for use
(3) Gokel, G. W. In ComprehensiVe Supramolecular Chemistry: Molecular
Recognition, Receptors for Cationic Guests; Lehn, J.-M.; Atwood,
J. L.; Davies, J. E. D.; MacNicol, D. D.; Vo¨gtle, F., Eds.; Pergamon:
Oxford, U.K., 1996; Vol. 1.
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516. (b) Gale, P. A. Coord. Chem. ReV. 2003, 240, 191–221. (c) Gale,
P. A.; Quesada, R. Coord. Chem. ReV. 2006, 250, 3219–3244. (d)
Gale, P. A.; Garc´ıa-Garrido, S. E.; Garric, J. Chem. Soc. ReV. 2008,
37, 151–190.
† The University of Texas at Austin.
‡ Kangwon National University.
§ Korea University.
⊥ Oak Ridge National Laboratory.
(1) (a) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes;
VCH: Weinheim, Germany, 1995. (b) Steed, J. W.; Atwood, J. L.
Supramolecular Chemistry: An Introduction, Wiley, Chichester, U.K.,
2000. (c) Beer, P. D.; Gale, P. A.; Smith, D. K. Supramolecular
Chemistry; Oxford University Press: Oxford, U.K., 1999.
(2) Sessler, J. L.; Gale, P. A.; Cho, W.-S. Anion Receptor Chemistry;
Monographs in Supramolecular Chemistry, Stoddart, J. F., Ed.; RSC
Publishing: Cambridge, U.K., 2006.
(5) (a) Smith, B. D. In Ion-pair Recognition by Ditopic Receptors,
Macrocyclic Chemistry: Current Trends and Future; Gloe, K.;
Antonioli, B., Eds.; Kluwer, London, U.K., 2005; pp 137-152. (b)
Kirkovits, G. J.; Shriver, J. A.; Gale, P. A.; Sessler, J. L. J. Incl.
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9
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A R T I C L E S
Kim et al.
of the binding modes shown in Figure 1.8a-c,10c To the best of
our knowledge, we are unaware of any synthetic receptor with
which it has proved possible to stabilize all three limiting
interactions using a single molecular framework. Here, we report
the first receptor that binds ion pairs in accord with all three
limiting modes shown in Figure 1. This flexibility in binding,
which has been fully documented Via single-crystal X-ray
diffraction analyses, is of particular interest because the underly-
ing ion-pair recognition behavior can be modified by simply
changing the counteranion.
We recently reported the synthesis of the ion-pair receptor
1, a system that is characterized by the presence of two strong
ion-binding sites (a calix[4]pyrrole and calix[4]arene crown-6
subunits for anion and cation recognition, respectively).12 In
the solid state, X-ray diffraction analysis revealed the presence
of a 1:1 CsF complex that was characterized by a large (>10
Å) Cs+ and F- separation. In spite of this large separation, in
10% methanol-chloroform (v/v), compound 1 was found to
bind both the F- anion and Cs+ cation strongly, and to do so in
a sequential manner.12 In order to gain greater insights into the
nature of the binding interactions in 1, we have now prepared,
and report here, the crown-free ion-pair receptor (2). This new
system, which lacks the calix[4]-crown-6 cation recognition
site,13 displays anion-dependent ion-pair binding that differs
dramatically from that observed in the case of 1 or, indeed,
any other synthetic receptor system of which we are aware. As
will be discussed further below, we have found that, in 10%
methanol-chloroform, receptor 2 fails to bind either Cs+ or
F- when exposed to these ions in the form of salts containing
other counterions (i.e., ClO4- and tetrabutylammonium (TBA+),
for Cs+ and F-, respectively). However, it forms a very strong
and selective complex with CsF, when mixed with CsF or with
various combinations of salts that provide a source of CsF in
situ. Further, in contrast to what is true for 1, the binding of
the Cs+ and F- ions takes place concurrently, rather than
sequentially, on the NMR time scale in 10% CD3OD:CDCl3.
Also noteworthy is that in 1 the Cs+ cation is bound in the
crown ether strap, whereas in 2 it is bound closer to the F-
anion and within the receptor pocket. The result is a solvent-
bridged ion-pair structure in the case of 2 wherein the cation
and anion are separated by roughly 5.6 Å in the solid state.
This binding mode is protean and can be modified by the choice
of counteranion. For instance, no solvent bridging is seen in
the case of the CsNO3 complex; here, solid-state structural
analysis confirms the existence of a contact ion pair within the
receptor cavity. Finally, in the case of CsCl, single-crystal X-ray
structural analysis reveals the existence of an unprecedented
2:2 complex characterized by two different ion-pair binding
modes, including one that is best described as host-separated.
Such host-separated ion pairing has been observed recently in
Figure 1. Limiting ion-pair interactions relevant to receptor-mediated ion-
pair recognition: (a) Contact, (b) solvent-bridged, and (c) host-separated.
In this schematic, the anion is shown as “A-”, the cation as “C+”, and the
solvent as “S”.
in such areas as salt solublization, ion extraction, and through-
membrane transport.6-9
Ion-pair receptors generally bind cations and anions in three
limiting modes depending on the size of the ions, distance
between the anion and cobound cation, the nature of the
constituent recognition sites, and solvent, among other effects.
These different ion-pair binding arrangements are shown
schematically in Figure 1. They are conveniently defined as (i)
contact ion pairs, where the anion and cation are held in close
proximity (Figure 1a);8 (ii) solvent-bridged ion pairs, in which
solvent molecules help link the anion to the cobound cation
(Figure 1b),8a-c,11 and (iii) host-separated ion pairs, an arrange-
ment characterized by ostensibly independent anion and cation
recognition sites (Figure 1c).10 Examples of all three host-guest
arrangements are now known. However, as a general rule, these
arrangements have been defined in the context of different
receptor systems. Indeed, most ion-pair receptors reported to
date operate on the basis of only one or, in very rare cases, two
(6) (a) Pfeifer, J. R.; Reiss, P.; Koert, U. Angew. Chem., Int. Ed. 2006,
45, 501–504. (b) Sisson, A. L.; Shah, M. R.; Bhosale, S.; Matile, S.
Chem. Soc. ReV. 2006, 35, 1269–1286. (c) Nakamura, T.; Akutagawa,
T.; Honda, K.; Underhill, A. E.; Coomber, A. T.; Friend, R. H. Nature
1998, 394, 159–162. (d) Gokel, G. W.; Leevy, W. M.; Weber, M. E.
Chem. ReV. 2004, 104, 2723–2750. (e) Davis, A. P.; Sheppard, D. N.;
Smith, B. D. Chem. Soc. ReV. 2007, 36, 348–357.
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Gazzola, L.; Casnati, A.; Ungaro, R. J. Am. Chem. Soc. 1999, 121,
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boom, W.; Ungaro, R.; Reinhoudt, D. N. J. Am. Chem. Soc. 1999,
117, 6124–6125. (c) Tong, C. C.; Quesada, R.; Sessler, J. L.; Gale,
P. A. Chem. Commun. 2008, 6321–6323.
(8) (a) Mahoney, J. M.; Stucker, K. A.; Jiang, H.; Carmichael, I.;
Brinkmann, N. R.; Beatty, A. M.; Noll, B. C.; Smith, B. D. J. Am.
Chem. Soc. 2005, 127, 2922–2928. (b) Deetz, M. J.; Shang, M.; Smith,
B. D. J. Am. Chem. Soc. 2000, 122, 6201–6207. (c) Mahoney, J. M.;
Beatty, A. M.; Smith, B. D. Inorg. Chem. 2004, 43, 7617–7621. (d)
Mahoney, J. M.; Davis, J. P.; Smith, B. D. J. Org. Chem. 2003, 68,
9819–6820. (e) Mahoney, J. M.; Beatty, A. M.; Smith, B. D. J. Am.
Chem. Soc. 2001, 123, 5847–5858. (f) Mahoney, J. M.; Nawaratna,
G. U.; Beatty, A. M.; Duggan, P. J.; Smith, B. D. Inorg. Chem. 2004,
43, 5902–5907. (g) Mahoney, J. M.; Marshall, R. A.; Beatty, A. M.;
Smith, B. D.; Camiolo, S.; Gale, P. A. J. Supramol. Chem. 2003, 1,
289–292.
(12) Sessler, J. L.; Kim, S. K.; Gross, D. E.; Lee, C.-H.; Kim, J. S.; Lynch,
V. M. J. Am. Chem. Soc. 2008, 130, 13162–13166.
(13) Note: calix[4]arenes in their 1,3-alternate are known to bind Cs+
weakly, whereas calix[4]crown-6 is known to complex Cs+ strongly.
See: (a) Iwamoto, K.; Araki, K.; Shinkai, S. Tetrahedron 1991, 47,
4325–4342. (b) Verboom, W.; Datta, S.; Asfari, Z.; Harkema, S.;
Reinhoudt, D. N. J. Org. Chem. 1992, 57, 5394–5398. (c) Meier, U. C.;
Detellier, C. J. Phys. Chem. A 1998, 102, 1888–1893.
(9) Reeske, G.; Bradtmo¨ller, G.; Schu¨rmann, M.; Jurkschat, K.
Chem.sEur. J. 2007, 13, 10239–10245.
(10) (a) Scheerder, J.; van Duynhoven, J. P. M.; Engbersen, J. F. J.;
Reinhoudt, D. N. Angew. Chem., Int. Ed. 1996, 35, 1090–1093. (b)
Mele, A.; Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. J. Am.
Chem. Soc. 2005, 127, 14972–14973. (c) Custelcean, R.; Delmau,
L. H.; Moyer, B. A.; Sessler, J. L.; Cho, W.-S.; Gross, D.; Bates,
G. W.; Brooks, S. J.; Light, M. E.; Gale, P. A. Angew. Chem., Int.
Ed. 2005, 44, 2537–2542.
(14) Roelens, S.; Vacca, A.; Francesconi, O.; Venturi, C. Chem.sEur. J.
2009, 15, 8296–8302.
(15) (a) No, K.; Lee, H. J.; Park, K. M.; Lee, S. S.; Noh, K. H.; Kim,
S. K.; Lee, J. Y.; Kim, J. S. J. Heterocycl. Chem. 2004, 41 (2), 211–
219. (b) Kim, J. S.; Shon, O. J.; Ko, J. W.; Cho, M. H.; Yu, I. Y.;
Vicens, J. J. Org. Chem. 2000, 65, 2386–2392.
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41, 1757–1759.
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Calix[4]arene Strapped Calix[4]pyrrole: Ion-Pair Receptor
A R T I C L E S
the case of Cs+ salts of simple calix[4]pyrrole, 3.10c However,
the ability to “adjust” the ion-pair recognition as a function of
specific conditions is, to the best of our knowledge, a unique
feature of receptor 2 and could make this new system of interest
as a selective ion-recognition agent, in accord with very recent
literature suggestions.14
Figure 2. Two different views of the single-crystal structure of
2·(CH3OH)2. Displacement ellipsoids are scaled to the 30% probability
level. Most hydrogen atoms have been removed for clarity. Dashed lines
are indicative of H-bonding interactions.
were observed when the receptor was treated with fluoride anion
salts (e.g., tetrabutylammonium fluoride, TBAF), but not other
analogous halide anion salts (Figures S1 and S2, Supporting
Information). Such findings lead us to suggest that, like the
earlier system 1, receptor 2 is selective for the fluoride anion.
This stands in contrast to what is true for normal calix[4]pyrrole
3, which is able to bind other halide anions in CDCl3 (Figures
S3 and S4, Supporting Information) and other solvent systems.17
Results and Discussion
Receptor 2 was synthesized using a synthetic procedure
similar to that used to prepare compound 1.12 As shown in
Scheme 1, the reaction of calix[4]arene ditosylate 415 with 4′-
hydroxyacetophenone in the presence of excess K2CO3 in
acetonitrile under reflux gave the calix[4]arene diketone 5 in
its 1,3-alternate conformation in 80% yield; subsequent con-
densation with pyrrole in the presence of excess trifluoroacetic
acid (∼23 equiv) at 65 °C then gave the dipyrromethane 6 in
48% yield. Compound 6 was further condensed with acetone
in the presence of a catalytic amount of BF3 ·OEt2 to give the
target compound (2) in 12% yield.12,16
Compound 2 was fully characterized by standard spectro-
scopic means, as well as by single-crystal X-ray diffraction
analysis (Figure 2). The datum crystal used for this latter analysis
was obtained by subjecting 2 in its ion-free form to slow
evaporation from a chloroform-methanol (1:1, v/v) solvent
mixture. The resulting structure revealed that, in the solid state,
the calix[4]arene moiety adopts the expected 1,3-alternate
conformation, while the calix[4]pyrrole moiety is in a partial
cone conformation with two methanol molecules bound to the
four pyrrolic NH protons.
1
As shown in Figure S1, Supporting Information, the H NMR
spectrum of free 2 is characterized by a broad singlet at δ )
6.75 ppm for the NH protons, as well as two triplets, resonating
at about δ ) 6.03 ppm and δ ) 5.95 ppm, for the ꢀ-pyrrolic
protons, respectively.
Upon subjecting compound 2 to titration with TBAF in
CDCl3, two sets of resonances were seen for all observable
1
proton signals in the various H NMR spectra recorded before
saturation was achieved. The deconvoluted spectra were found
to reflect, as expected, the presence of both the ion-free and
fluoride-bound forms of 2. As such, these studies serve to
indicate that the kinetics of fluoride anion binding/release is
slow on the NMR time scale (Figure S1, Supporting Informa-
tion). They also provide support for the notion that fluoride anion
is strongly bound by receptor 2 in this solvent medium.
The presumed strong complexation between receptor 2 and
fluoride anion in CDCl3 was further supported by the observation
of significant chemical shift changes in the ꢀ-pyrrolic proton
signals and, especially, the pyrrolic NH protons (Figure S1,
Supporting Information). Upon reaching saturation, the NH
As a first step toward analyzing the ion recognition properties
of 2, its ability to bind halide anion salts in CDCl3 solution
was investigated Via 1H NMR spectroscopy. In analogy to what
was found to be true in the case of the ion-pair receptor 1,12
significant chemical shift changes in the NMR spectrum of 2
1
proton signal in the H NMR spectrum was shifted downfield
Scheme 1. Synthesis of Compound 2
9
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A R T I C L E S
Kim et al.
Figure 3. Partial 1H NMR spectra of (a) 2 only, (b) 2 + 5 equiv of TBAF (tetrabutylammonium fluoride), (c) 2 + 5 equiv of CsClO4, and (d) 2 + 5 equiv
of CsF in CD3OD/CDCl3 (1:9, v/v).
to δ ≈ 13.0 ppm (∆δ ≈ 6.3 ppm). This resonance, initially a
singlet, was also split into a doublet (J ) 45.6 Hz), a finding
ascribed to coupling between the NH protons and the bound
fluoride anion (Figure S1, Supporting Information).18 In contrast,
the triplet peaks corresponding to the ꢀ-pyrrolic protons were
shifted upfield and appear as two singlets (at 5.85 ppm and 5.67
ppm, respectively; cf. Figure S1, Supporting Information) at
the end of the titration; this is as would be expected given the
anion-induced changes in the electronic features of the
calix[4]pyrrole portion of receptor 2.
Very different binding behavior was observed when experi-
ments analogous to those described above were carried out in
10% CD3OD in CDCl3 (v/v). In this more polar medium, no
appreciable change in any of the chemical shifts was seen in
the 1H NMR spectrum of receptor 2, even upon treatment with
5.0 equiv of TBAF. This result, interpreted in terms of receptor
2 not binding the fluoride anion under these particular experi-
mental conditions, is ascribed to the strong solvation of fluoride
anions by the CD3OD present in this solvent mixture (Figures
3, 4, and S5, Supporting Information).
combination of both ionic species is required to trigger both
Cs+ and F- complexation, the binding process mimics a
cooperative AND logic gate.19 Specifically, in accord with the
logic rules governing such devices, only the combined presence
of two inputs induces CsF binding by 2 (i.e., both the Cs+ cation
and the F- anion, but neither the Cs+ cation nor the F- anion
alone, triggers the change); presumably, this complexation leads
to formation of a solvent separated-ion pair, just as it does in
the solid state (cf. discussion below). As discussed later on,
this logic behavior was found to extend to other cesium salts.
A noteworthy feature of the spectra shown in Figure 3d, is
the CsF-induced downfield shift in all the proton signals
associated with the calix[4]arene subunit. Such a finding is fully
consistent with the 1,3-alternate calix[4]arene moiety, which is
known to be a weak Cs+ receptor,13 being involved in Cs+ cation
complexation. The fact that changes are seen in the signals
ascribed to protons Ha and Hb confirms that the oxygen atoms
of the calix[4]arene phenoxy groups participate in Cs+ cation
recognition. In contrast, the upfield shift seen for both the
ꢀ-pyrrolic and meso-aromatic proton signals of the calix[4]pyrrole
moiety, as well as the significant downfield shift and splitting
(into a doublet) observed for the pyrrolic NH resonance, are
taken as strong evidence that F- anion binding takes place within
the calix[4]pyrrole moiety (Figures 3d and S5d, Supporting
Information). These findings are consistent with the structure
of 2·CsF determined in the solid state Via X-ray diffraction
analysis (Vide infra).
Simple calix[4]pyrrole 3 is known to complex the cesium
cation weakly, if at all, in organic media.10c Therefore, as
expected, little evidence of an interaction between compound
2 and the Cs+ cation was seen when this receptor was treated
with 5.0 equiv of CsClO4 in 10% CD3OD in CDCl3 (v/v), as
1
evidenced by the absence of chemical shift changes in the H
NMR spectrum (Figures 3c and S5, Supporting Information).
In contrast to what is seen with TBAF or CsClO4, the addition
of CsF (5.0 equiv) to a solution of 2 in 10% CD3OD in CDCl3
causes significant changes in the proton signals of both the
Further evidence that receptor 2 forms a strong complex with
1
CsF came from H NMR spectroscopic titrations carried out
with CsF. Specifically, addition of 0.57-1.79 equiv of CsF to
a solution of 2 in CD3OD/CDCl3 (1:9, v/v) was found to give
rise to two sets of distinguishable resonances for all proton
signals. These sets of peaks are attributed to the ion-free and
1
calix[4]arene and the calix[4]pyrrole moiety in the H NMR
spectrum (Figure 3d). Such findings are consistent with the CsF
being bound to receptor 2 as an ion pair. Further, since the
(19) (a) de Silva, A. P.; Uchiyama, S. Nat. Nanotechnol. 2007, 2, 399–
410. (b) de Silva, A. P.; McClenaghan, N. D. Chem.sEur. J. 2004,
10, 574–586. (c) Magri, D. C.; Brown, G. J.; McClean, G. D.; de
Silva, A. P. J. Am. Chem. Soc. 2006, 128, 4950–4951. (d) de Silva,
A. P.; Gunaratne, H. Q. N.; McCoy, C. P. J. Am. Chem. Soc. 1997,
119, 7891–7892. (e) de Silva, R. A.; Gunaratne, H. Q. N.; McCoy,
C. P. Nature 1993, 364, 42–44.
(17) (a) Gale, P. A.; Sessler, J. L.; Kra´l, V.; Lynch, V. J. Am. Chem. Soc.
1996, 118, 5140–5141. (b) Sessler, J. L.; Gross, D. E.; Cho, W.-S.;
Lynch, V. M.; Schmidtchen, F. P.; Bates, G. W.; Light, M. E.; Gale,
P. A. J. Am. Chem. Soc. 2006, 128, 12281–12288.
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Calix[4]arene Strapped Calix[4]pyrrole: Ion-Pair Receptor
A R T I C L E S
Figure 4. Proposed AND logic gate binding behavior of 2 toward a CsF ion pair in CD3OD/CDCl3 (1:9, v/v).
CsF bound forms of 2, respectively. The presence of two
S1, Supporting Information). In 10% CH3OH in CHCl3 (v/v),
the titration of CsF [1.0 mM] with 2 [11.2 mM] is characterized
by a highly favorable enthalpic term (∆H ) -14.4 kcal/mol),
with a strong opposing entropy term (T∆S ) -8.7 kcal/mol).
The overall binding energy, ∆G ) -5.7 kcal/mol, corresponds
to a Ka ) 1.3 × 104 M-1, when fit to a 1:1 binding equation
(Figure S8, Supporting Information). This affinity is an order
of magnitude lower than what was observed for compound 1,
which contains a calix[4]arene crown-6 ring (∆H ) -16.2 kcal/
mol, T∆S ) -8.6 kcal/mol, ∆G ) -7.6 kcal/mol, and Ka )
3.77 × 105 M-1).12 The lower CsF affinity of 2 presumably
reflects the absence of the strong calix[4]crown-6 cesium binding
site, but may also be the result of reduced flexibility and
associated steric limitations to either anion or cation binding.
Nevertheless, even if the binding is reduced compared to 1, it
is to be appreciated that in absolute terms, compound 2 is a
highly effective receptor for CsF, at least in this moderately
polar solvent mixture.
The binding behavior of 2 toward the Cs+ cation and the F-
anion was further studied in the presence of various cations and
anions. The concurrent addition of both TBAF and CsClO4,
neither of which is individually bound to compound 2, gives
rise to 1H NMR spectral changes in 10% CD3OD in CDCl3 (v/
v) that are very similar to those seen upon the addition CsF
(Figures S5e, Supporting Information). Such observations
provide support for the conclusion that compound 2 is able to
capture selectively the Cs+ cation and the F- anion (as an ion
pair) through a process involving counterion exchange (eq 4).
By contrast, in the presence of perchlorate salts of other metal
ions (i.e., Li+, Na+, K+, Rb+ and NH4+, as opposed to Cs+), no
spectral changes are observed; this lends credence to the notion
that the F- anion is bound by compound 2, but only in the
presence of Cs+.
separate signals is consistent with the ion-pair binding/decom-
1
plexation equilibrium being slow on the H NMR time scale,
as noted above. As proved true after the titration was complete
(cf. discussion of Figure 3d above), a detailed analysis of the
signals associated with the CsF complex (Figure S6, Supporting
Information) revealed that shifts are seen for both the pyrrolic
NH and protons Ha and Hb of the calix[4]arene. This is
consistent with the Cs+ cation and F- anion being bound
1
concurrently, at least on the H NMR time scale (eq 1).
The binding behavior for the new receptor 2 is very different
for what is seen for compound 1. In the case of the latter species,
addition of 0.57-1.79 equiv of CsF gives rise at first to changes
in the proton signals of the calix[4]arene moiety and the crown-6
ring, but not those of the calix[4]pyrrole moiety (Figure S7,
Supporting Information). Such a finding is consistent with the
Cs+ cation being bound to the calix[4]crown-6 ring before the
F- anion is bound to the calix[4]pyrrole (eq 2). Only after the
Cs+ cation is bound to the calix[4]arene crown-6 ring of 1 does
complexation of the F- counteranion occur;12 as expected, this
gives rise to two distinct (and distinguishable) sets of pyrrolic
ꢀ-proton signals during the course of the titration (Figure S7,
Supporting Information, and eq 3).
[2] + CsF f [2·CsF]
[1] + CsF f [1·Cs]F
(1)
(2)
(3)
[1·Cs]F + CsF f [1·CsF] + CsF
ITC was utilized to quantify the CsF affinity of receptor 2.
To maintain consistency with the prior 1H NMR spectroscopic
analyses, a solvent mixture consisting of 10% MeOH in CHCl3
(v/v) was employed (Figure S8, Supporting Information). This
choice reflects the fact that no other solvent systems allowed
for interpretable data to be obtained for either this salt or other
combinations of F- and Cs+ and various counterions (see Table
[2] + CsClO4 + TBAF f [2·CsF] + TBAClO4
(4)
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A R T I C L E S
Kim et al.
In the presence of TBA salts of other anions (Cl-, Br-, and
NO3-), the Cs+ cation is strongly complexed by receptor 2, as
inferred from 1H NMR spectral measurements (cf. Figures S5,
S9, S10, and S11, Supporting Information). However, even
though such complexes are formed when CsCl, CsBr, and
CsNO3 are added independently (Figure S9, Supporting Infor-
mation), receptor 2 binds only CsF when a mixture of cesium
salts (CsF, CsCl, CsBr, and CsNO3) is used. Thus, selectivity
for CsF is seen (Figure S12, Supporting Information). As such,
our work serves to extend the findings of Beer and co-workers
whonotedthatappropriatelydesignedheteroditopiccalix[4]diquinone
receptors will form complexes with several ions and/or contact
ion pairs, albeit not with the selectivity demonstrated by 2.20
The strong preference for CsF displayed by 2 in 10% CD3OD
in CDCl3 (v/v) stands in marked contrast to what is true for
simple calix[4]pyrrole 3. This latter system has been observed
to bind a number of different anions under a wide variety of
conditions and thus high selectivity for CsF was not expected.17
Indeed, when tested under conditions identical to those used
above (i.e., exposure to a mixture of cesium salts), this
unfunctionalized “parent” system was found to interact with a
variety of anions in 10% CD3OD in CDCl3 (v/v), rather than
just fluoride, as seen for 2 (cf. Figure S13, Supporting
Information).
The high selectivity for CsF seen in the case of 2 relative to
3 is thought to reflect favorable ion pairing interactions.
However, it may also reflect a reduced accessibility to the anion
binding site compared to simple calix[4]pyrrole 3.10c Presum-
ably, this reduced accessibility is a consequence of the greater
rigidity of the calix[4]pyrrole core that results from the use of
relatively inflexible ethylene glycol spacers between the
calix[4]pyrrole and calix[4]arene subunits. The phenoxy groups
flanking the calix[4]pyrrole binding site may also serve to limit
accessibility to the cavity.
Single crystals of the CsF complex of 2 (2·CsF) were
obtained by subjecting a chloroform/methanol solution of
receptor 2 to slow evaporation in the presence of excess cesium
fluoride. The resulting structure revealed that 2 forms a 1:1
complex with cesium fluoride, and that the latter is bound in
the form of an ion pair wherein the constituent ions are separated
by a water molecule. As expected, the Cs+ cation is bound
strongly to the 1,3-alternate conformation of the calix[4]arene
through ion-dipole interactions with two phenoxy oxygen atoms
and π-cation interactions involving the two arene rings:21
Cs+ · · ·O distances of 2.98 and 3.09 Å; Cs+ · · ·arene centroid
distances of 3.28 and 3.40 Å (Figure 5). The Cs+ cation is also
coordinated by a molecule of methanol (Cs+ · · ·O distance )
3.04 Å) and a water molecule (Cs+ · · ·O distance ) 3.22 Å).
The distal oxygen atoms of the ethylene glycol spacers exhibit
long Cs+ · · ·O distances, 3.66 and 4.25 Å, leading us to suggest
that they do not play a significant role in cation binding (Vide
infra). The F- anion is bound to the NH protons of the
calix[4]pyrrole with distances of 2.79-2.81 Å (N· · ·F- interac-
tion), and is also bound to the water molecule with a distance
of 2.52 Å (O · · ·F- interaction). The distance between the Cs+
cation and the F- anion in 2·CsF is found to be 5.62 Å, which
is much shorter than the distance (10.92 Å) seen in the CsF
complex of 1. It is also much longer than the intra- (3.69 Å)
Figure 5. Two different views of the single-crystal structure of
2·CsF·CH3OH·H2O. Displacement ellipsoids are scaled to the 50%
probability level. Most hydrogen atoms have been removed for clarity.
and intercomplex (2.77 Å) distances seen in the solid-state
structure of the CsF complex of simple meso-
octamethylcalix[4]pyrrole 3 (Figure 6).10c In the previously
reported CsF complex of 3, the Cs+ cation is symmetrically
encapsulated with the cone-like cavity of the calix[4]pyrrole
Via apparent π-cation interactions with a distance of 3.39 Å
between the Cs+ ion and the centroids of the pyrrole rings.10c
Molecular mechanics calculations were performed to gain
further insight into the ion-pair binding interactions (see
Supporting Information for coordinates of all structures).
Definition of van der Waals parameters for the Cs+ cation,22
allowed calculations to be performed with the MMFF94 force
field model.23 Starting from the X-ray coordinates, geometry
optimization of the solvated (2·CsF) complex resulted in only
minor changes to the structure. Figure 7a shows the superposi-
tion of the optimized geometry on the X-ray geometry, yielding
a root mean squared displacement for heavy atom positions of
0.24 Å. After removal of the two solvent molecules, further
optimization results in a displacement of the Cs+ cation from
the calix[4]arene pocket toward the F- anion, going from a
water-separated ion-pair Cs · · ·F distance of 5.33 Å to a contact
ion-pair distance of 2.79 Å (see Figure 7b). Thus, the addition
of a water spacer is required in order for both the cation and
anion of this ion pair to contact their corresponding binding
sites within 2.
Further calculations were performed to evaluate the participa-
tion of the distal ethylene glycol oxygen atoms in Cs+ binding.
Conformational analysis of the methoxy-substituted analog of
(2·CsF) yielded a C2 symmetric conformer as the global
minimum, 1.9 kcal/mol lower in energy than the X-ray
conformation. On removal of F- followed by optimization, the
Cs+ cation moves into the internal calix[4]arene cavity (Figure
8a) with a binding energy of -39.8 kcal/mol. As with the X-ray
structure, the Cs+ · · ·O distances to the calix[4]arene oxygen
atoms (2.91 Å) are significantly shorter than the distances to
the distal oxygen atoms (3.31 Å). Figure 8b shows the optimized
structure obtained after moving the Cs+ to the external
(20) Lankshear, M. D.; Dudley, I. M.; Chan, K.-M.; Cowley, A. R.; Santos,
S. M.; Felix, V.; Beer, P. D. Chem.sEur. J. 2008, 14, 2248–2263.
(21) (a) Hay, B. P.; Nicholas, J. B.; Feller, D. J. Am. Chem. Soc. 2000,
122, 10083–10089. (b) Nicholas, J. B.; Hay, B. P. J. Phys. Chem. A
1999, 103, 9815–9820.
(22) MMFF94 van der Waals parameters for the Cs+ cation were adapted
from the default values used for the K+ cation by altering the atomic
polarizability from 1.0 to 2.0 Å3.
(23) Halgren, T. A. J. Comput. Chem. 1996, 17, 490–519.
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5832 J. AM. CHEM. SOC. VOL. 132, NO. 16, 2010

Calix[4]arene Strapped Calix[4]pyrrole: Ion-Pair Receptor
A R T I C L E S
Figure 6. Binding modes of CsF ion pairs stabilized by receptors 1-3 and distances between the Cs+ and F- ions seen in the complexes as determined by
X-ray diffraction analysis. The structures of the CsF complexes of 1 and 3 were reported in references 12 and 10c, respectively.
of the overall energetics. Considered in a different light, the
differences seen within this series serve to underscore the fact
that the formation of complexes, 1·CsF, which represents a host-
separated ion pair, and 2·CsF, which represents a solvent-
bridged ion pair, allows the unfavorable energetics associated
with Cs+ and F- charge separation to be overcome. The fact
that a solvent-bridged ion pair was observed in the case of
(2·CsF) led us to consider that the cavity in this receptor might
accommodate Cs+ salts of larger anions, and thus be useful as
a cesium cation extractant.
To test whether receptor 2 could be used to effect cesium
cation extraction, preliminary extraction studies were carried
out using the set up described previously.24 Specifically, cesium
salts were subject to extraction from an aqueous phase into a
nitrobenzene layer containing 2 at 10 mM. Measuring the cesium
distribution ratios as a function of the cesium salt concentration
revealed that CsCl, CsBr, and CsNO3 were extracted as ion pairs
in accord with the following thermodynamic equation
Figure 7. (a) Superposition of calculated versus observed (thin black tubes)
geometries for the solvated form of (2·CsF). (b) Calculated geometry for
the encapsulated contact CsF ion pair obtained after solvent is removed.
K
Cs+ + X- + 2 T CsX2
¯
where X- represents nitrate, chloride, or bromide, and a
superscripted line denotes species present in the organic phase.
Preliminary thermodynamic modeling of the results is consistent
with the presence of dissociated ion pairs at low salt concentra-
tion and ion pairing at higher salt concentration, a finding that
leads us to suggest that conditions appropriate for crystal growth
could stabilize the formation of ion-paired complexes. Further
extraction studies are in progress and more complete results,
including quantitative analyses, will be presented elsewhere.
The nature of the extracted ion-paired complexes was
evaluated with further modeling studies.22,23 Calculations on
CsCl (Figure 9) indicate that the cavity of 2 is too small to
contain a water-separated ion pair, but too large for the contact
ion pair. When a water molecule is present it binds to the side
of the CsCl ion pair with a Cs · · ·O distance of 3.09 Å and a
Cl· · ·H distance of 2.14 Å. This microsolvation results in an
ion separation of 3.54 Å (Figure 9a). When the water is
removed, the separation distance decreases to 3.30 Å as the Cs+
cation is dragged toward the anion (Figure 9b). Although the
effect is not as dramatic as with CsF, where the addition of
solvent separates the two ions by 2.54 Å (see Figure 7), the
coordination of water to the side of the CsCl contact ion pair
Figure 8. Optimized geometries for (2 ·Cs+) with the Cs+ cation located
in the internal cavity (a) and external cavity (b).
calix[4]arene cavity. This position, which lacks the extra two
oxygen atoms, yields a Cs+ binding energy of -38.4 kcal/mol.
These values can be compared with the value of -38.6 kcal/
mol, which is obtained when this model is used to compute the
Cs+ binding energy to the 1,3-alternate conformation of
tetramethoxycalix[4]arene. The results confirm that the elongated
Cs+ · · · O distances observed in the X-ray structure represent
weak contacts that are worth less than 1 kcal/mol apiece; in
other words, the calix[4]arene donor sites account for the
majority of Cs+ binding in the case of 2.
Taken together, the solid state results obtained for the CsF
complexes of compounds 1-3, leads to the conclusion that the
Cs+ cation complexation is “opportunistic”, with the binding
taking place in a way that leads, as expected, to a minimization
(24) Wintergerst, M. P.; Levitskaia, T. G.; Moyer, B. A.; Sessler, J. L.;
Delmau, L. H. J. Am. Chem. Soc. 2008, 130, 4129–4139.
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A R T I C L E S
Kim et al.
to complex Lewis basic anions, such as fluoride, in a relatively
nonpolar organic solvent system (chloroform), was found to bind
a variety of anions (specifically F-, Cl-, Br-, and NO3-) in a
more polar environment (10% CD3OD in CDCl3) provided a
presumably cobound cesium cation is present; this underscores
the fact that under these experimental conditions compound 2
is acting as a bona fide ion-pair receptor.
Even though quantitative studies of affinities could not be
carried out, it is important to appreciate that small differences
in the spectra corresponding to the ion-pair complexes were
observed in the case of the various test cesium salts. For
instance, treatment with CsCl and CsBr complexes give rise to
similar chemical shifts, a reduction of the intensity for the
ꢀ-pyrrolic protons was seen in the case of CsCl, but not CsBr,
as discussed further below. Moreover, in the case of the CsNO3
complex, the various peaks are split further than in the case of
the CsCl and CsBr complexes (cf. Figure S9, Supporting
Information).
Figure 9. (a) Calculated geometry for (2·CsCl) with a water molecule
coordinated to the ion pair. (b) Calculated geometry for (2·CsCl).
Explanations for these small differences came from solid-
state structural analyses. For instance, unlike the 1:1 solvent-
bridged CsF complex obtained when receptor 2 is exposed to
Cs+ and F-, a 2:2 complex is obtained with CsCl (22 ·(CsCl)2),
at least in the solid state. Here, a single-crystal X-ray diffraction
analysis revealed two different binding modes (Figure 11). One
cesium ion is bound within the receptor pocket in analogy to
what is seen in the CsF complex. In contrast, the other cesium
ionformsasandwichcomplexwithtwocone-shapedcalix[4]pyrroles
(from different receptors). That is, one CsCl in 2 exists as a
“solvent-loosened” contact ion pair within the receptor cavity
as predicted by modeling (Figure 9) and the other is spatially
separated by the receptor (i.e., host-separated as per Figure 1).
The crystal structure of the CsCl complex also shows that the
Cs+ cation within the pocket, labeled as Cs2, is further
coordinated by a water molecule with a Cs+ · · ·O distance of
3.17 Å (Figure 11). This cesium cation interacts with the
chloride anion labeled as Cl1a, forming the “solvent-loosened”
contact ion pair that, in turn, is bound to NH protons of the
calix[4]pyrrole Via hydrogen bonds with distances of 3.32-3.39
Å (N· · ·Cl- interaction). In contrast, the other cesium cation,
labeled as Cs1, is sandwiched between two cone-shaped
calix[4]pyrrole Via π-cation interactions characterized by
Cs+ · · ·C(pyrrole) distances of 3.28-3.66 Å. The chloride anion
(Cl1), spatially separated from Cs1 by the calix[4]pyrrole
skeleton, is also bound to the NH’s of the calix[4]pyrrole and
an OH of a molecule of methanol Via hydrogen bonds with
distances of 3.24-3.27 Å for the N · · · Cl- interaction. Such
distances are shorter than those for the interaction between Cl1a
and the nitrogen atoms of the calix[4]pyrrole subunit, a finding
that is attributable to a stronger interaction of the chloride anion
with Cs2. The distances between the Cs+ cation and the Cl- anion
in 22 ·(CsCl)2 are found to be 3.60 Å for the contact ion pair
(Cs2· · ·Cl1a interaction) and to be 4.93 Å for the host-separated
ion pair (Cs1 · · ·Cl interaction), respectively. By contrast, the
distance between Cs1 and Cl1a is found to be 6.25 Å, which is
interpreted in terms of Cs1 interacting with Cl1 more strongly than
with Cl1a.
Figure 10. (a) Calculated geometry for (2·CsNO3) with a water molecule
coordinated to the nitrate anion. (b) Calculated geometry for (2·CsNO3).
increases the ion separation by 0.24 Å; this allows a more
favorable interaction between the calix[4]arene binding site and
the Cs+ cation. Similar behavior is observed with CsBr (not
shown), but given the larger size of this anion, the increase in
ion separation on water coordination is smaller, 0.17 Å (Figure
S14, Supporting Information).
The largest ion pair, CsNO3, exhibits the best match to the
cavity of 2 (Figure 10). Unlike the cesium halide salts where
the water interacts with both members of the ion pair, in this
case the added water interacts only through hydrogen bonding
with the NO3- anion (Figure 10a). When the water is removed,
the ion separation decreases by only 0.06 Å (Figure 10b), with
a drop in Cs · · ·O distance of 3.17-3.11 Å. These calculated
values are consistent with Cs · · ·O(nitrate) distances observed
in the Cambridge Structural Database (CSD),25 3.20 ( 0.09 Å,
leading to the prediction that 2 should host CsNO3 as a true
contact ion pair.
The solvent extraction and modeling studies provide support
for the conclusion that receptor 2 should be able to include
cesium salts other than CsF, either as “solvent-loosened” contact
ion pairs in the case of CsCl and CsBr or as a true contact ion
1
pair in the case of CsNO3. H NMR spectral analyses were
carried out in 10% CD3OD in CDCl3 to test whether 2 would
function in this manner. This solvent mixture, dictated by
solubility considerations, permitted qualitative studies but,
unfortunately, precluded quantitative measurements.
Further evidence for the proposed interaction between the
cesium cation and the calix[4]pyrrole moiety came from the
finding that the ꢀ-protons of the pyrrole subunits undergo
D-for-H exchange in CD3OD in the presence of CsCl. This
is a reaction that has not hitherto been observed in the context
of calix[4]pyrrole chemistry. As shown in Figures S9 and
S15, Supporting Information, the peaks of the ꢀ-pyrrolic
As in the case of CsF, salt-induced spectral shifts were seen
in the presence of CsCl, CsBr, and CsNO3, consistent with the
formation of ion-pair inclusion complexes with 2 (Figure S9,
Supporting Information). It is of particular interest that 2, unable
(25) Allen, F. H.; Kennard, O.; Taylor, R. Acc. Chem. Res. 1983, 16, 146–
153.
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5834 J. AM. CHEM. SOC. VOL. 132, NO. 16, 2010

Calix[4]arene Strapped Calix[4]pyrrole: Ion-Pair Receptor
A R T I C L E S
Figure 11. Single-crystal structure of 22 ·(CsCl)2 ·CH3OH·H2O. Displacement ellipsoids are scaled to the 30% probability level. Most hydrogen atoms have
been removed for clarity.
protons of calix[4]pyrrole subunits are almost absent from
1
the H NMR spectra of 2 recorded roughly 10 min after the
addition of 5.0 equiv of CsCl in 10% CD3OD in CDCl3 (v/
v). In contrast, when the corresponding experiment is run
using 10% CH3OH (a proton-containing solvent) in CDCl3
(v/v), the peaks for the ꢀ-pyrrolic protons are still observed
in the presence CsCl (Figure S15c, Supporting Information).
Such observations are completely consistent with the sug-
gestion that the disappearance of the ꢀ-pyrrolic proton peak
in the presence of CD3OD is due to deuterium exchange and
that this exchange is abetted by π-metal complexation
involving the cesium cation and the four pyrrolic subunits
that make up the calix[4]pyrrole core. It important to note
that facile D-for-H exchange is not seen in the case of the
CsBr or CsNO3 complexes of 2 (Figure S9, Supporting
Information), lending credence to the suggestion that the ion-
pair binding mode differs for these two salts.26
Considering the extraction studies, modeling results, and the
absence of D-for-H exchange seen in 10% CD3OD-CDCl3 (v/
Figure 12. Two different views of the single-crystal structure of 2·CsNO3.
v) (Vide supra), we considered it likely that 2 would bind CsBr
Displacement ellipsoids are scaled to the 30% probability level. Most
and CsNO3 as contact pairs. Concrete support for this conclu-
sion, at least in the solid state, came from single-crystal
hydrogen atoms have been removed for clarity.
diffraction analyses of the latter salt. The structure of the CsNO3
complex, obtained from crystals grown by slow evaporation of
a chloroform/methanol solution of receptor 2 in the presence
of CsClO4 and TBANO3, is shown in Figure 12. It reveals that
the nitrate anion is coordinated to the Cs+ cation with two
oxygen atoms and the third oxygen atom is bound to the
calix[4]pyrrole moiety Via hydrogen-bonding contacts character-
ized by N · · ·O- distances of 2.95-3.04 Å. The distance between
the cesium cation and the two oxygen atoms of the nitrate anion
(3.12 and 3.17 Å) are in excellent agreement with those
predicted by modeling (3.11 Å, Figure 10) and observed in other
crystal structures bearing contact CsNO3 ion pairs (3.20 ( 0.09
Å); confirming that the cesium cation and the nitrate anion exist
as a true contact ion pair in this complex.
crystal X-ray diffraction analysis. Taken in concert, the X-ray
crystal structural data and the 1H NMR spectroscopic analyses
revealthatinboththesolidstateandinmixedmethanol-chloroform
solution, receptor 2 forms a stable 1:1 complex with a
solvent-bridged CsF species, but only if both the anion and
cation are both present. Such behavior, which follows the
terms associated with an AND logic gate, stands in marked
contrast to what was seen for the earlier system, 1. This
previously reported receptor binds the two constituent ions
sequentially, with the Cs+ cation being bound before the F-
anion. Receptor 2 was also found to bind other cesium salts
in mixed methanol-chloroform solution on the basis of an
AND logic gate even though it fails to bind the constituent
counteranions (as the tetrabutylammonium salts) in pure
chloroform. Receptor 2 forms an unprecedented 2:2 complex
with CsCl characterized by two different ion-pair binding
modes, including one where the anion and cation are
separated by the host framework. In contrast, single-crystal
X-ray diffraction analyses and 1H NMR spectroscopic studies
reveal that 2 forms only a 1:1 complex with CsNO3, wherein
the anion and cation are held together in close proximity as
contact ion pairs. These findings reveal receptor 2 to be a
versatile ion-pair receptor whose binding behavior can be
modulated Via an appropriate choice of the counteranion.
In conclusion, an ion-pair receptor, 2, derived by coupling
a strong anion-binding site with a weak cation recognition
site, has been synthesized and characterized by standard
spectroscopic means, as well as through the use of single-
(26) The basis for the CsCl-selective deuterium exchange may reflect the
favorable cone angle established in this particular anion complex. The
cone angle is expected to be larger in the case of the corresponding
fluoride complex. This and the existence of more favorable ion-pair
binding modes would preclude the presence of substantial quantities
of Cs+ in the calix[4]pyrrole “cup”. Conversely, in the case of the
other cesium salts examined, CsBr and CsNO3, anion complexation
will produce structures with smaller cone angles. Thus, even though
binding within the cup of calix[4]pyrrole might occur to a certain extent
(it is not the dominant mode as noted in the text proper), the resulting
complexes would lack an optimal geometry for D-for-H exchange.
Further tests of these hypotheses are in progress.
Acknowledgment. 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 NRF Grant (MEST 2009-
0087013 to C.H.L.), a CRI project of the Korean Research Foundation
9
J. AM. CHEM. SOC. VOL. 132, NO. 16, 2010 5835

A R T I C L E S
Kim et al.
(J.S.K), and the Korean WCU program (study leave for J.L.S.). B.P.H.
and L.H.D. acknowledge support from the Division of Chemical
Sciences, Geosciences, and Biosciences, Office of Basic Energy
Sciences, U.S. DOE at Oak Ridge National Laboratory.
all modeled complexes, and X-ray structural data for 2 · CsF,
22 · (CsCl)2 · CH3OH · H2O, and 2 · CsNO3. This material is
available free of charge via the Internet at http://pubs.acs.
org.
Supporting Information Available: Synthetic details, NMR
spectroscopic data, ITC analyses, Cartesian coordinates for
JA100715E
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5836 J. AM. CHEM. SOC. VOL. 132, NO. 16, 2010