Superior anion-binding properties of a cryptand-like oligopyrrolic macrocycle

Article abstract of DOI:10.1002/asia.200900518

Recognition by strapped system: Cryptand-type calix[4]pyrrole are shown to exhibit a highly enhanced affinity towards anionic guests as compared to other reported calix[4]pyrroles (see figure). The results obtained provide some insights as to how calix[4]pyrrole-based anion receptors may be designed to achieve higher affinity and selectivity.

Full text of DOI:10.1002/asia.200900518

COMMUNICATION
DOI: 10.1002/asia.200900518
Superior Anion-Binding Properties of a Cryptand-like Oligopyrrolic
Macrocycle
Seong-Jin Hong,[a] Jaeduk Yoo,[a] Dae-Wi Yoon,[a] Juyoung Yoon,[b]
Jong Seung Kim,[c] and Chang-Hee Lee*[a]
Over the last decade or so, calix[4]pyrroles and their
modified congeners have been studied extensively as neutral
receptors for anions and substantial studies have been made
to improve their binding characteristics.[1–10] As a part of our
ongoing efforts along these lines, we have recently reported
the synthesis and anion binding properties of various strap-
ped calix[4]pyrroles.[10–15] Many of these systems act as versa-
tile anion receptors.[16] For instance, strapped calix[4]pyr-
roles, functionalized with amide[13] or fluorophor moieties,
have been shown to have particularly useful binding proper-
ties. This provides support for the hypothesis that the strap-
ping strategy can be generalized for a variety of customized
anion–receptor systems.[17] So far, however, the versatility of
this potentially broad approach to anion recognition has yet
to be extensively explored. One particular appealing direc-
tion, and the subject of this report, involves the use of strap-
question as to whether these systems, which are not based
on calixpyrrole, can function in this regard. Sessler and co-
workers have also reported two closely related rigid three-
dimensional oligopyrrolic systems.[19] However, these sys-
tems contain cavities that are too small to accommodate any
guest molecules, other than solvent molecules. In the course
of our efforts to develop topologically non-planar calixpyr-
role receptors, we recently prepared a calix[4]pyrrole con-
taining a single pyrrolic moiety in the strap. This system was
found to bind chloride and fluoride anions with high affinity
in 25% aqueous DMSO.[16] It proved to be a more superior
anion receptor, under forcing polar conditions, than any
other calixpyrrole-type receptor that we are aware of. In an
effort to understand the determinants of anion recognition
in pyrrole-strapped calixpyrrole anion receptors, we have
now designed and synthesized the strapped-calix[4]pyrrole
system, 1. This system differs from all earlier ones in that it
incorporates two pyrroles and one 1,3-disubstituted phenyl
moiety as parts of the cryptand-producing strap. As expect-
ed, this new system displays a high anion affinity and dis-
plays binding properties that differ from those of other
strapped calix[4]pyrrole systems.[10–15]
À
ping elements that contain ancillary pyrrolic N H hydrogen
bonding donor motifs. Such systems are expected to show
enhanced affinity and selectivity for anionic guests.
The first oligopyrrole-based cryptand-like receptor was re-
ported by Beer et al.[18] The three-dimensional cavity gener-
ated by these researchers was found to trap diamines; how-
ever, to the best of our knowledge, no studies of anion rec-
ognition were carried out and thus there remains an open
The synthesis of the target receptor 1 is shown in
Scheme 1. It commences with the nucleophilic substitution
of a,a’-dibromo-m-xylene 2 with pyrrole in the presence of
potassium carbonate, to afford bis(dipyrrylmethyl)-m-xylene
3 in 35% yield. The yield of this direct nucleophilic substitu-
tion is rather low, but the reaction is considered useful be-
cause it provides a convenient entry into benzylpyrrole de-
rivatives.[20]
[a] S.-J. Hong, J. Yoo, Dr. D.-W. Yoon, Prof. C.-H. Lee
Department of Chemistry
Kangwon National University
Chun Cheon, 200-701 (Republic of Korea)
Fax : (+82)33-250-8490
E-mail: chhlee@kangwon.ac.kr
Compound 3 was then converted to precursor 4 by sub-
jecting it to an InIII-catalyzed Michael-type addition to
methylvinylketone.[21] The resulting bis-alkylated compound
4, obtained in 70% yield, was then reacted with pyrrole to
afford the bis-dipyrromethane derivative 5 in 39% yield. An
acid-catalyzed condensation of 5 in neat acetone resulted in
the desired receptor 1 in 9% yield. This imparts a larger
degree of flexibility than is inherent in previous strapped
systems of this type. It was expected that this would be re-
[b] Prof. J. Yoon
Department of Chemistry
Ehwa Womenꢀs University
Seoul 120-750 (Republic of Korea)
[c] Prof. J. S. Kim
Department of Chemistry
Korea University
Seoul Korea 140-70 (Republic of Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900518.
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Chem. Asian J. 2010, 5, 768 – 772

addition of fluoride anion. This
observed up-field shift of pyr-
role NH protons on the strap is
rather unusual. This upfield
shift of the pyrrole NH reso-
nance is attributed to the im-
posed anion–p interaction be-
tween bound fluoride anion and
two pyrrole ring protons on the
strap.
The complete disappearance
of the original signals for the
pyrrole NH protons shown at
d=9.27 and 9.77 ppm was ob-
served
upon
addition
of
2.0 equivalents of tetra-n-buty-
lammonium fluoride (TBAF).
The signal for the ArH between
the two alkyl substituents, origi-
nally appearing at d=6.89 ppm,
was seen to shift to d=
Scheme 1. Synthesis of the desired strapped calix[4]pyrrole based receptor, 1.
flected in both affinity and selectivity differences when com-
pound 1 was employed as anion receptor system. Prior to
carrying out such tests, however, efforts were made to char-
acterize the physical-chemical properties of the molecule.
In analogy to previous strapped calix[4]pyrrole systems,
the conformation of the calix[4]pyrrole core in 1 was expect-
ed to reside predominantly in the 1,3-alternate conforma-
1
tion.[10] The H NMR spectrum of 1 is consistent with this
expectation and serves to underscore the symmetric nature
of the compound. For example, in [D6]DMSO two different
À
sets of pyrrole N H signals were observed. They register as
two sets of singlets; one at 9.27 ppm, ascribed to the cal-
ix[4]pyrrole moiety, and another at 9.77 ppm, the strap
moiety. Although receptor 1 is sufficiently soluble in aceto-
nitrile to permit the recording of a proton NMR spectrum,
its limited solubility led us to consider an alternative solvent
system for the analysis of its anion binding properties. Con-
sidering its highly competitive nature, it was decided to com-
1
plete the studies in [D6]DMSO by using both H NMR spec-
troscopy and isothermal titration calorimetry (ITC).
Figure 1. 1H NMR spectral titration of receptor 1 in [D6]DMSO with the
addition of different concentrations of TBAF.
Initial quantitative assessment of an anion binding study
1
performed by H NMR spectroscopy indicated that the com-
plexation/decomplexation kinetics are slow and the results
provided some insight for the anion binding. For instance, if
receptor 1 was subject to titration with FÀ (as its tetrabuty-
lammonium salt) in [D6]DMSO, dramatic changes in the
1H NMR signals were observed (Figure 1). In particular, the
pyrrole NH proton signals of the calix[4]pyrrole moiety,
originally observed at d=9.27 ppm in the absence of any
anion, diminished as the titration progressed. Meanwhile, a
new signal appeared at d=12.47 ppm with an integrated in-
tensity that was increasing as the titration progressed. At
the same time, the pyrrole NH on the strap part, originally
shown at d=9.77 ppm, shifted upfield to d=9.51 ppm upon
7.08 ppm upon addition of a fluoride anion. This downfield
shift is a good indication of the existence of an Ar-H-FÀ hy-
drogen-bonding interaction.[7]
Titration with a chloride anion showed somewhat similar
result as those for the fluoride anion in [D6]DMSO
(Figure 2). In this case, the pyrrole NH protons, again ob-
served at 9.27 ppm in the absence of the added anion, shift-
ed to 10.69 ppm upon addition of only one molar equivalent
of anion. At the same time, the pyrrole NH on the strap
shown at 9.77 ppm was also seen to disappear completely
and concurrently a new signal was seen to appear at
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C.-H. Lee et al.
COMMUNICATION
Since the system was found to undergo slow complexa-
tion/decomplexation kinetics relative to the NMR timescale,
ITC was used to assess the interactions between anions and
receptor 1.[22] The traces of titration for receptor 1 with
anions are shown in Figure 3. The calculated association
constant K1 value for a first fluoride anion binding process
was 1.28ꢂ108 mÀ1. A second binding event K2 ꢀ1.35ꢂ105 mÀ2
could be observed in the presence of more than one molar
equivalent of fluoride anion. The affinity constant for chlo-
ride anion binding was 3.27ꢂ107 mÀ1 and that of acetate
anion was 9.35ꢂ106 mÀ1. These observed values for fluoride
and chloride anions are exceptionally large, and greater
than those for other strapped calix[4]pyrrole systems espe-
cially the pyrrole-strapped calix[4]pyrrole system.[23] This is
especially noteworthy considering that the DMSO was not
subject to any special drying.
The results noted above provide important support for
the notion that receptor 1 binds the chloride and acetate
anions in a 1:1 (receptor to anion) fashion. The fluoride
anion binds in a 1:1 mode if the guest concentration is about
one molar equivalent, whereas it is bound in a 2:1 (receptor
to anion) fashion if the guest concentration is higher than
one molar equivalent. Based on these observations, we con-
sider it reasonable to suggest that the construction of a pyr-
role-bearing strap on one side of the calix[4]pyrrole moiety
provides the best way of enhancing the anion affinities of
this class of receptors. This strategy may also be useful in
tuning the selectivity. To underscore this point, the appropri-
ate comparison data has been collected in Figure 4. Inspec-
tion of Figure 4 reveals that, as expected, the anion affinity
depends on the nature of the binding site. Specifically, the
observed binding affinity (3.27ꢂ107 mÀ1) of receptor 1 is
ꢀ25000 times larger than that of octamethyl-calix[4]pyrrole
(1.3ꢂ103 mÀ1) measured in the same solvent (DMSO).[8] The
increased binding affinity is attributed to a combination of
the ꢃfour-pointꢀ hydrogen bonding interaction and an anion–
p interaction between the receptor and the bound anion.
Figure 2. 1H NMR spectral titration of receptor 1 in [D6]DMSO against
various concentrations of tetra-n-butylammonium chloride (TBACl).
9.36 ppm. The differences in the chemical shift behavior
seen for the fluoride anion and chloride anion provide im-
portant implications in terms of the binding stoichiometry
under solution phase conditions.
The signal for the ArH between the two alkyl substituent
shifts to d=7.50 ppm upon addition of the chloride anion.
This larger downfield shift indicates that the interaction be-
tween ArH and the chloride anion is stronger than in the
case of the fluoride anion. Quantitative analyses of the solu-
tion phase fluoride, chloride, and acetate anion binding
properties of 1 were performed by using isothermal titration
calorimetry (ITC) in DMSO. As detailed below, we have
found that the Ka values measured in DMSO for 1 are sub-
stantially larger by at least a factor of 10 than those for
other strapped calix[4]pyrrole systems measured in acetoni-
trile.[10–15]
Figure 3. ITC titration curve for receptor 1 with a) FÀ, b) ClÀ, and c) AcOÀ, as their tetrabutylammonium salts measured in DMSO at 258C. The host
concentrations were; a) 0.754 mm, b) 0.625 mm, and c) 0.955 mm.
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Chem. Asian J. 2010, 5, 768 – 772

Binding by a Cryptand-like Macrocycle
7.5 Hz, 2H; Ar-H), 5.80–5.78 (m, 2H;
pyrrolic-H), 5.76–5.75 (m, 2H; pyrrol-
ic-H), 3.87 (s, 4H; CH2), 2.75 (m, 8H;
CH2), 2.12 ppm (s, 6H; CH3);
13C NMR (100 MHz, CDCl3, 258C):
d=209.6, 139.7, 130.5, 129.8, 128.9,
128.8, 126.7, 105.8, 105.1, 44.1, 34.2,
30.1, 21.5 ppm.
1,3-Bis[(5-
tyl)-1H-pyrrole-2-yl)methyl]benzene
(5): To the solution of (0.27 g,
ACHTUNGTRNE(NUNG 3,3-bis-(1H-pyrrol-2-yl)bu-
4
0.77 mmol) and pyrrole (10 mL), was
added trifluoroacetic acid (0.07 g,
0.77 mmol) and the mixture was
Figure 4. Comparison of chloride anion (as its tetrabutylammmonium salt) binding affinities of various cal-
ix[4]pyrroles measured by ITC in DMSO.
stirred at 708C for 1 h. The mixture
was cooled to room temperature and
The results clearly indicate that the receptors bearing a pre-
organized binding site with auxiliary hydrogen-bonding
donors have superior anion affinities and even higher selec-
tivities than those lacking these features.
the reaction was quenched upon addition of aqueous NaOH (0.1n,
30 mL). The mixture was then extracted with CH2Cl2 (30 mLꢂ4) and the
organic layer was dried (Na2SO4) and the solvent was removed in vacuo.
The remaining solid was purified by using column chromatography on
silica (CH2Cl2/EtOAc=19:1). Yield: 0.17 g (39%); 1H NMR (400 MHz,
CDCl3, 258C): d=7.58 (brs, 4H; NH), 7.18 (t, J=7.5 Hz, 1H; Ar-H),
7.12 (brs, 2H; NH), 7.00 (d, J=7.5 Hz, 2H; Ar-H), 6.96 (s, 1H; Ar-H),
6.50–6.49 (m, 4H; pyrrolic-H), 6.10–6.08 (m, 4H; pyrrolic-H), 6.06–6.02
(m, 4H; pyrrolic-H), 5.78–5.77 (m, 2H; pyrrolic-H), 5.72–5.71 (m, 2H;
pyrrolic-H), 3.78 (s, 4H; CH2), 2.31–2.26 (m, 4H; CH2), 2.18–2.14 (m,
4H; CH2), 1.54 ppm (s, 6H; CH3); 13C NMR (100 MHz, CDCl3, 258C):
d=140.6, 138.0, 132.3, 129.7, 129.3, 129.1, 127.0, 117.8, 108.1, 106.7, 105.3,
104.8, 40.8, 39.4, 34.5, 26.6, 23.1 ppm.
In conclusion, we have synthesized a cryptand-like cal-
ix[4]pyrrole, 1, that displays an exceptionally high affinity
toward fluoride and chloride anions in DMSO. This receptor
recognizes fluoride anions through both hydrogen-bonding
and anion–p interactions. The current results provide some
insights as to how calix[4]pyrrole-based anion receptors may
be designed to achieve high affinity and selectivity. On the
basis of the present studies, for instance, we predict that the
combination of anion-binding and cation-binding sites
within a strapped calix[4]pyrrole system of appropriate di-
mensions will provide a ditopic receptor that is ideal for
ion-pair recognition and transport. Studies along this latter
line of investigation are in progress.
Cryptand calix[4]pyrrole (1): Compound 5 (0.27 g, 0.34 mmol) was dis-
solved in acetone (100 mL) and then BF3·OEt2 (0.02 mL, 0.17 mmol) was
added. The mixture was stirred for 30 min at room temperature and the
reaction was stopped by addition of triethylamine (1 mL). Solvent was
removed under reduced pressure and the remaining dark solid was dis-
solved in methylene chloride and washed with water. The organic layer
was then dried (Na2SO4) and the solvent was removed in vacuo. The re-
maining dark solid was purified by using column chromatography on
silica (CH2Cl2). Yield: 0.02 g (9%); 1H NMR (400 MHz, CDCl3, 258C):
d=7.67 (brs, 2H; NH), 7.35 (br s, 4H; NH), 7.30 (t, J=7.6 Hz, 1H; Ar-
H), 7.13 (d, J=7.6 Hz, 2H; Ar-H), 7.09 (s, 1H; Ar-H), 5.94–5.92 (m, 4H;
pyrrolic-H), 5.89–5.87 (m, 4H; pyrrolic-H), 5.82–5.80 (m, 2H; pyrrolic-
H), 5.68–5.66 (m, 2H; pyrrolic-H), 3.89 (s, 4H; CH2), 2.31–2.27 (m, 4H;
CH2), 2.09–2.05 (m, 4H; CH2), 1.50 (s, 12H; CH3), 1.46 ppm (s, 6H;
CH3); 1H NMR (400 MHz, [D6]DMSO, 258C): d=9.77 (brs, 2H; NH),
9.27 (brs, 4H; NH), 7.29–7.18 (m, 3H; Ar-H), 6.89 (s, 1H; Ar-H), 5.75
(d, J=2.5 Hz, 2H; pyrrolic-H), 5.64 (d, J=2.5 Hz, 2H; pyrrolic-H), 5.59–
5.48 (m, 8H; pyrrole CH), 3.67 (s, 4H; CH2), 2.20–2.18 (m, 8H; CH2),
1.60 (s, 6H; CH3), 1.51 (s, 6H; CH3), 1.39 ppm (s, 6H; CH3); 13C NMR
(400 MHz, CDCl3, 258C): d=140.3, 138.1, 136.6, 131.8, 129.8, 129.2,
128.9, 127.0, 105.9, 105.1, 104.3, 103.6, 77.2, 41.5, 39.5, 35.5, 34.3, 30.4,
28.8, 28.6, 23.6 ppm; MS (MALDI-TOF): m/z: calcd for C46H52N6: 688.43
[M+H]+; found: 689.35.
Experimental Section
Experimental Procedures and Spectroscopic Data for Compounds 1 and
3–5
1,3-Bis[(1H-pyrrol-2-yl)methyl]benzene (3): a,a’-Dibromo-m-xylene
2
(2.02 g, 7.65 mmol) and K2CO3(1.21 g, 8.75 mmol) were dissolved in neat
pyrrole (2.50 mL, 36 mmol) and the mixture was refluxed for 6 h. The
mixture was cooled to room temperature and was combined with water
(100 mL), then extracted with CH2Cl2. The organic layer was dried
(Na2SO4) and the solvent/pyrrole mixture was removed in vacuo. The re-
sulting dark-brown solid was purified by using column chromatography
on silica (EtOAc/hexanes=1:4). Yield: 0.63 g (35%); 1H NMR
(400 MHz, CDCl3, 258C): d=7.75 (brs, 2H; NH), 7.22 (t, J=7.0 Hz, 1H;
Ar-H), 7.05 (d, J=7.0 Hz, 2H; Ar-H), 7.04 (s, 1H; Ar-H), 6.63–6.62 (m,
2H; pyrrolic-H), 6.14–6.12 (m, 2H; pyrrolic-H), 5.97 (brs, 2H; pyrrolic-
H), 3.91 ppm (s, 4H; CH2); 13C NMR (100 MHz, CDCl3, 258C): d=140.4,
131.0, 129.5, 129.3, 127.2, 117.4, 108.8, 106.9, 34.4 ppm; MS (EI): m/z:
calcd for C16H16N2: 236.13 [M]+; found: 236.21.
Acknowledgements
This work was supported by Basic Science Research Program through
the NRF (2009-008713). The Central Instrumentation Facility at KNU
and BK21 program are acknowledged for support.
1,3-Bis[(5-(3-oxobutyl)-1H-pyrrol-2-yl)methyl]benzene (4): The solution
of 3 (0.58 g, 2.45 mmol) in CH2Cl2 (20 mL) was added dropwise to the
mixture of methylvinyl ketone (1.60 mL, 19.2 mmol), InCl3 (0.591 g,
2.67 mmol), and CH2Cl2 (10 mL) at 08C. The whole mixture was stirred
at 08C for 4 h. The temperature was then raised to 258C and stirred for
another 1 h. The mixture was combined with CH2Cl2 (30 mL) and
washed with water. The organic layer was dried over anhydrous Na2SO4
and the solvent was removed in vacuo. The resulting solid was purified
by using column chromatography on silica (CH2Cl2/EtOAc=19:1). Yield:
0.65 g (70%); 1H NMR (400 MHz, CDCl3, 258C): d=8.20 (brs, 2H;
NH), 7.22 (t, J=7.5 Hz, 1H; Ar-H), 7.06 (s, 1H; Ar-H), 7.05 (d, J=
Keywords: anion-binding · anion-p interaction · cryptand
calix[4]pyrrole · enhanced anion affinity
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Chem. Asian J. 2010, 5, 768 – 772
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C.-H. Lee et al.
COMMUNICATION
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[22] Isothermal titration calorimetery (ITC) measurements were per-
formed as follows: Solutions of the chosen receptor in acetonitrile
were made up so as to provide a receptor concentration range of
0.1–1 mm. These solutions were then individually titrated with the
appropriate alkylammonium salts at 25Æ0.018C. The original heat
pulse was normalized using reference titrations carried out by using
the same salt solution, but pure solvent, as opposed to a solution
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Received: October 1, 2009
Published online: February 9, 2010
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