Calix[4]tetrahydrothiophenopyrrole: A ditopic receptor displaying a split personality for ion recognition

Article abstract of DOI:10.1021/ol5026537

A calix[4]pyrrole fused with 2,5-dihydrothiophene, possessing both a deep, π-electron-rich pocket upon anion binding and chelating ligands on the periphery, was developed. The receptor selectively forms an ion-pair complex with CsF through H-bonding and a

Full text of DOI:10.1021/ol5026537

Letter
pubs.acs.org/OrgLett
Calix[4]tetrahydrothiophenopyrrole: A Ditopic Receptor Displaying a
Split Personality for Ion Recognition
Indrajit Saha,† Kyung Hwa Park,† Mina Han,‡ Sung Kuk Kim,§ Vincent M. Lynch,§ Jonathan L. Sessler,§
and Chang-Hee Lee*,†
†Department of Chemistry, Kangwon National University, Chun Cheon 200-701, Korea
‡Department of Molecular Design & Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
§Department Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
S
* Supporting Information
ABSTRACT: A calix[4]pyrrole fused with 2,5-dihydrothiophene,
possessing both a deep, π-electron-rich pocket upon anion binding
and chelating ligands on the periphery, was developed. The receptor
selectively forms an ion-pair complex with CsF through H-bonding
and a cation−π interaction. In the process, it adopt a conforma-
tionally fixed cone conformation. The receptor displays exceptionally
high affinity toward the Hg(II) ion and forms stable complexes while
maintaining a rigid 1,3-alternate conformation. This metal ion-
induced conformational locking is unprecedented in calix[4]pyrrole
chemistry.
he design and synthesis of smart receptors that are capable
Tof sensing various environmentally and chemically
important anions has become an area of increasing interest in
supramolecular and molecular recognition chemistry owing to
the key role played by anions in numerous chemical and
biological processes.1 Considerable efforts have been devoted to
improve selectivity and affinity toward the desired guest
molecules. Among the various anion receptors reported to
date,2 calix[4]pyrroles are known to be efficient for anionic
species, and many modified systems have been developed.3,4
These modifications typically focus on either the β-pyrrolic
positions or the meso-positions.4,5 The strapped systems
generally have superior affinity and guest selectivity. Introduction
of substituents on the β-pyrrolic positions usually decreases the
anion affinity because of destabilizing steric interactions incurred
upon anion binding followed by conformational changes.6
Currently, there are few calix[4]pyrrole-based anion receptors
that contain a bicyclic pyrrole.7 A good ion-pair receptor must
possess functionalities that recognize both cations and anions
with high affinity.8 There have been numerous reports detailing
efforts to improve the anion affinity of the calix[4]pyrrole
platform compared with relatively few attempts to improve its
cation binding affinity. We considered it likely that the
introduction of chelating functions on the periphery of
calix[4]pyrroles could serve to enhance their cation-binding
capability. Recently, it was established that octamethylcalix[4]-
pyrrole can form an ion-pair complex with cesium halide salts.9 In
these systems, the cation is believed to be held inside the cone-
shaped cavity through a cation−π interaction that is maximized
by the proper cone angle. However, the cation−π interaction is
weaker than the H-bonding interaction and results in a reduced
binding affinity for the ion-pair complex compared to what might
otherwise be possible. We report here a calix[4]pyrrole with
fused 2,5-dihydrothiophene subunits at the β-pyrrolic carbons.
This system (1) proved to be a Hg(II)-selective receptor, as well
as a selective ion-pair receptor for CsF.
The synthesis of receptor 1 was achieved according to Scheme
1. Diethyl pyrrole-3,4-dicarboxylate 2 was obtained from the
reaction of p-tosylmethylisocyanide and diethyl fumarate in the
presence of potassium tert-butoxide and 18-crown-6 in dry
THF.10,11 After N-protection using tosyl chloride, the ester
Scheme 1. Synthesis of Receptor 1
Received: September 7, 2014
© XXXX American Chemical Society
A
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Letter
functions of 3 were reduced to diol 4 with LAH. Conversion to
the dibromo compound 5 followed by cyclization with sodium
sulfide in boiling ethanol afforded the dihydrothiophenopyrrole
6 directly in moderate yield. Acid catalyzed condensation of 6 in
the presence of acetone gave the desired calix[4]dihydrothio-
phenopyrrole 1 in 14% yield. Receptor 1 was characterized by
spectroscopic means, as well as by single-crystal X-ray diffraction
analysis.
A diffraction grade crystal was obtained by slow evaporation of
chloroform in methanol. In the resulting single-crystal structure
of 1, the receptor is seen to adopt a 1,2-alternate conformation
wherein the two solvent molecules (MeOH) are bound to two
adjacent pyrrolic N−H protons (Figure 1).
Figure 2. Schematic representation of electrostatic interactions that may
stabilize the nonion paired complex obtained when receptor 1 is treated
with TBAF in CDCl3.
Figure 1. Single-crystal structure of 1·(CH3OH)2. Two molecules of
methanol are H-bonded to two N−H’s each. H-atoms were removed for
clarity. Displacement ellipsoids are scaled to the 50% probability level.
1
Figure 3. Partial H NMR spectra recorded during the titration of
receptor 1 (3.03 mM) with TEAF in CDCl3.
The preliminary anion binding ability of 1 toward various
TBAF (Δδ = 5.00 ppm). In the presence of TEAF, the methylene
protons (Ha and Hb) appeared at δ = 4.12 and 3.68 ppm as two
doublets (J = 11.2 Hz). This was attributed to tight ion-pair
binding resulting in conformational locking of the complex into a
cone conformation. Further support for the fixed conformation
of the TEAF complex came from the split meso-methyl protons at
δ = 1.87 and 1.71 ppm (originally appearing as a singlet at δ =
1.54 ppm; see SI). The changes in the spectrum are essentially
complete upon the addition of ∼1.0 equiv of TEAF. The
resonances corresponding to the tetraethylammonium (TEA+)
cation moved downfield as the concentration of TEAF increased
(SI). This observation is explained by the fact that the TEA+
cation becomes encapsulated within the deep-concave cavity,
formed as a result of convergent H-bonding of the pyrrole N−
H’s with the anion.7 Further support for the conclusion that the
fluoride anion from TEAF is bound more strongly than that from
TBAF came from the observation that the N−H resonance of 1 is
split into a doublet (JH−F = 38.4 Hz) at rt in the presence of
TEAF. The appearance of a doublet is presumably due to the
coupling of the bound fluoride anion and the N−H protons.
These differences in cation-dependent binding affinities are
attributed to the limited size of the extended concave cavity,
which can better accommodate the smaller tetraethylammonium
(TEA+) cation compared with the rather larger tetrabutylammo-
nium (TBA+) cation.
Receptor 1 also displayed moderate affinity toward the
chloride anion. When a CDCl3 solution of 1 was subjected to
titration with TBACl, complete saturation is reached upon the
addition of >10 equiv of the anion with significant downfield
shifts in the pyrrole N−H signals being observed (SI). These
observations support the suggestion that 1 has a lower affinity for
the chloride anion than for the fluoride anion. Nevertheless, in
analogy to what was seen for the fluoride anion, the chloride
anion binding affinity of 1 was also enhanced when TEACl,
rather than TBACl, was used as the chloride anion source. This
was corroborated by the downfield shift of the pyrrole N−H
signal (ΔδN−H = 3.9 ppm). In contrast to what was observed for
anions (F−, Cl−, Br−, I−, CH3COO−, H2PO4 , ClO4 , CN−,
−
−
−
−
−
NO3 , HSO4 , and ClO4 , studied as their tetraalkyl ammonium
salts) were investigated through 1H NMR spectroscopic
titrations carried out in CDCl3. The receptor displays a very
strong affinity for the fluoride anion. The pyrrole N−H’s of 1
resonate at δ = 7.00 ppm. A singlet for methylene protons (Ha
and Hb) at δ = 3.97 ppm is also seen (see Supporting Information
(SI)). The appearance of only one signal for Ha and Hb was
attributed to the fast conformational motion of 1 on the 1H NMR
time scale at rt. Upon addition of ca. 1.0 equiv of
tetrabutylammonium fluoride (TBAF), a large downfield shift
in the pyrrole N−H signals (Δδ = 5.0 ppm) is observed. This
supports the proposal that receptor 1 interacts strongly with the
fluoride anion. Although no splitting of the meso-methyl protons
or the Ha and Hb signals was observed, a slight upfield shift in the
Ha and Hb peaks was noted (Δδ = 0.04 ppm). Furthermore,
changes in the spectrum are evident upon addition of ca. 1.0
equiv of TBAF that are consistent with the fluoride anion
forming a 1:1 complex with receptor 1.
However, initial addition of TBAF resulted in significant peak
broadening. Disappearance of the N−H signal was also observed
upon the addition of ca. 1.0 equiv of TBAF. These findings are
interpreted in terms of the 1·TBAF complex undergoing fast
complexation−decomplexation kinetics in the solution phase.
The single resonance line seen for the methylene protons
supports the proposed conformational flexibility of the complex.
The fact that no chemical shift changes corresponding to the
tetrabutyl ammonium cation were seen is interpreted in terms of
the countercation (TBA+) forming a loose ion pair with the
complex as suggested schematically in Figure 2.
The interaction of receptor 1 with tetraethylammonium
fluoride (TEAF) was also investigated using 1H NMR
spectroscopic titrations carried out in CDCl3. These studies
revealed dramatic differences relative to TBAF. As shown in
Figure 3, the pyrrole N−H signal is shifted further downfield (Δδ
= 5.45 ppm) compared with what is seen in the presence of
B
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TEAF, upon exposure to TEACl, the singlet for the Ha and Hb
protons became broadened and shifted only slightly upfield (δ =
0.05 ppm). The signal for the meso-methyl protons also broadens
and appears at ca. δ = 1.89 ppm in the presence of excess TEACl.
As the concentration of TEACl increases, the signals
corresponding to the ethyl group of the TEA+ cation gradually
shift to lower field. Such observations are consistent with the
TEA+ cation interacting more strongly with the π-rich cavity than
TBA+. The result is a tight, 1:1 ion-pair complex (Cl−·1·TEA+).
A significant downfield shift of the pyrrole N−H signal was
also observed upon titration with TEABr (ΔδN−H = 2.96 ppm
Figure 4. Schematic representation of the proposed CsF complex of 1.
conformational locking, the methylene protons (Ha and Hb)
become diastereotopic. On this basis, we suggest that receptor 1
can selectively form a tight ion-pair complex with CsF. Due to the
limited solubility of CsF in chloroform even after a prolonged
sonication time, only 30% of 1 was associated with CsF. On the
other hand, neither CsCl nor CsClO4 was found to form a
receptor-shared ion-pair complex under the solid−liquid
extraction conditions.
upon addition of 5.0 equiv) as compared with TBABr (ΔδN−H
=
0.8 ppm upon addition of 10.0 equiv; see SI). On the other hand,
both CH3COO− and H2PO4− interact very weakly with receptor
1 with almost no changes in the chemical shifts being observed
upon exposure to the I−, CN−, NO3 , HSO4 , and ClO4− anions
(TBA+ salts).
−
−
To complement the solid−liquid extraction experiments, the
possible formation of ion-pair complexes was also studied in
organic solution. For these studies, a CDCl3 solution of 1 was
titrated against CsF dissolved in CD3OD. Upon the addition of
1.0 equiv of CsF, evidence for formation of a tight ion-pair
complex was seen in the 1H NMR spectra (SI). Two doublets (J
= 11.5 Hz) appeared at δ = 3.75 and 4.04 ppm, corresponding to
the methylene (Ha and Hb) resonances. Two singlets were also
identified at δ = 1.89 and 1.68 ppm corresponding to the meso-
methyl resonance. On this basis, we conclude that the CsF
strongly binds to receptor 1 resulting in a fixed conformation on
the NMR time scale. Likewise, when 1 was subjected to titration
with CsCl under identical conditions, binding occurred with
conformational changes but to a lesser extent. Upon addition of
an increasing amount of CsCl, two doublets (J = 11.7 Hz) at δ =
3.75 and 4.04 ppm, corresponding to the Ha and Hb resonances,
and two singlets at δ = 2.05 and δ = 1.65 ppm, corresponding to
the meso-methyl signals, were observed. The splitting of Ha and
Hb and the meso-methyl groups upon complexation with CsF and
CsCl, but not with TBAF and TBACl, is consistent with 1 being a
more effective ion-pair receptor rather than an anion receptor.
Cation binding studies carried out in the solution phase
revealed that 1 is an excellent metal ion receptor. When 1,
dissolved in CDCl3, was subjected to titration with the Hg2+ ion
The significant peak broadenings as well as the substrate-
dependent disappearance of the N−H signal during the course of
the NMR spectral titrations of 1 with fluoride and chloride salts
precluded the use of this method for quantitative determination
of the binding affinities. Therefore, to quantify the anion binding
affinity as well as determine the thermodynamic parameters for
the complexation with various anions, isothermal titration
calorimetry (ITC) experiments were performed in chloroform
at 25 °C (see SI). In contrast to NMR techniques, ITC methods
provide information on the energetics of the binding event
without the need for a structural probe.12 The thermodynamic
parameters obtained from the ITC analyses revealed that
chloride anion binding is almost entirely driven by enthalpy
(see SI). In contrast, bromide binding is entropically unfavorable,
resulting in an overall lower Gibb’s free energy change and lower
binding affinity. As expected, relatively high affinities were
observed for the tetraethylammonium chloride and bromide salts
compared with the corresponding tetrabutyl ammonium salts.
For example, the association constant for chloride binding is
increased by ∼15-fold in the case of the TEA+ salt vs the TBA+
salt. Almost no interaction between 1 and TBABr is observed by
ITC. However, a weak affinity for TEABr is observed (Ka = 233
M−1).7
Because receptor 1 possesses a deep electron-rich cavity and
sulfur ligands that can coordinate with metal cations, we
investigated the ion-pair binding properties of 1. Initially, the
ion-pair recognition ability of 1 was explored using solid−liquid
extraction experiments in chloroform with salts such as CsF,
CsCl, and CsClO4. A suspension of 1 with 5 equiv of each salt in
CDCl3 was sonicated for 1 h. Then, the 1H NMR spectra were
recorded using the soluble portion of the mixture. In the case of
CsF, the spectra clearly revealed two different sets of distinguish-
able signals corresponding to the free host 1 and the ion-pair
complex [1·CsF] (see SI). A large downfield shift of the N−H
proton (Δδ = 4.97 ppm) was observed with concurrent splitting
to a doublet (J = 34.9 Hz). The methylene proton (Ha and Hb)
signals split into two doublets (J = 11.5 Hz). Furthermore, the
meso-methyl groups were also split into two singlets appearing at
Δδ = 1.89 and 1.68 ppm, respectively. Such splitting is attributed
to the formation of a tight ion-pair complex where the
conformation of receptor 1 is fixed in the cone conformation.
The slight upfield shift of the β-pyrrolic protons is consistent
with the existence of a cation−π interaction.9 These observations
also support the suggestion that the cone angle of the deep cavity,
cup-like conformation formed by anion binding is well suited for
accommodating the Cs+ cation (Figure 4). Due to this
−
(dissolved in CD3OD and studied as the ClO4 salt), the
methylene protons were split into two doublets appearing at δ =
5.46 and 4.68 ppm (J = 15.0 Hz), respectively, upon addition of
∼2.0 equiv of Hg2+. A white precipitate was instantly formed
during the titration. In contrast to what was seen in the case of
CsF binding, a significant downfield shift of the signal for the Ha
and Hb was seen, a finding attributed to the chelation of Hg2+ to
the S-atoms. The fact that the signal for the meso-methyl groups
appears as a singlet at 1.68 ppm is consistent with a 2:1 binding
stoichiometry (Hg2+/receptor). The diastereotopic nature of the
Ha and Hb signals and the single meso-methyl resonance also
supports the suggestion that receptor 1 is complexed to two Hg2+
cations while in the 1,3-alternate conformation (see SI).
The observed homotopic nature of all the meso-methyl groups
is taken as evidence that the Hg2+−receptor complex derived
from 1 adopts a 1,3-alternate conformation. The two Hg2+
cations serve to lock the conformation into a 1,3-alternate
fashion, and the immediate solid precipitation indicates
intermolecular complexation to form a polymeric structure
(Figure 5). Treatment of the preformed [1·CsF] complex with
−
Hg2+ (as its ClO4 salt) resulted in complete decomplexation of
the bound CsF and formation of the insoluble Hg(II) complex.
This observation reinforces the notion that receptor 1 reacts with
C
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Letter
Resource Training Project for Regional Innovation (No.
2013H1B8A2032078), a 2013 Research Grant from Kangwon
National University (No. 120131804), the U.S. National Science
Foundation (CHE-1402004), and the Robert A. Welch
Foundation (F-1018) is gratefully acknowledged.
Figure 5. Schematic view of the oligomeric complex formed with
Hg(II).
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amorphous polymeric structures in the solid state.13
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ASSOCIATED CONTENT
* Supporting Information
■
S
Single crystal X-ray data and solution phase spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: chhlee@kangwon.ac.kr.
Notes
(11) Anzenbacher, P.; Try, A. C.; Miyaji, H.; Jursíkova, K.; Lynch, V.
M.; Marquez, M.; Sessler, J. L. J. Am. Chem. Soc. 2000, 122, 10268.
(12) 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.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(13) X-ray diffraction (XRD) data were collected with Cu Kα radiation
on a Rigaku R-AXIS-IV X-ray imaging plate detector.
Support from the Basic Science Research Program of the Korean
N a t i o n a l R e s e a r c h F o u n d a t i o n ( N R F ,
2013R1A2A2A01004894), the NRF through the Human
D
dx.doi.org/10.1021/ol5026537 | Org. Lett. XXXX, XXX, XXX−XXX