Anion binding modes in meso -substituted hexapyrrolic calix[4]pyrrole isomers

Article abstract of DOI:10.1021/ja411391c

We report on the synthesis of a new receptor for anions, meso-substituted hexapyrrolic calix[4]pyrrole 1. The calix[4]pyrrole's core features two additional pyrrole side-arms suspended above or below the calix[4]pyrrole core. This hexapyrrolic calix[4]pyrrole 1 is formed as cis- and trans-configurational isomers, the structures of which have been determined by single crystal X-ray diffraction. The anion binding experiments revealed interesting difference in the binding mode: The cis-1 isomer binds anions in a mixed binding mode featuring a combination of hydrogen bonding and anion-π interactions resulting in an unexpected strong binding. On the other hand, the trans-1 isomer displays only hydrogen bonding and lower affinity for anions. This is unexpected as one would assume both isomers to display the same binding modes. Overall, the titrations of 1 using UV spectrophotometry and NMR titrations by anions reveal that cis-isomer 1 displays higher affinity (10⁵-10 ⁶ M^-1) and cross-reactivity for anions, while the trans-isomer 1 shows a more selective response to anions. Such differences in binding mode in configurational isomers are so far unexplored and a feature deserving further study.

Full text of DOI:10.1021/ja411391c

Article
pubs.acs.org/JACS
Anion Binding Modes in meso-Substituted Hexapyrrolic
Calix[4]pyrrole Isomers
Kai-Chi Chang, Tsuyoshi Minami, Petr Koutnik, Pavel Y. Savechenkov, Yuanli Liu,
and Pavel Anzenbacher, Jr.*
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403,
United States
S
* Supporting Information
ABSTRACT: We report on the synthesis of a new receptor for anions, meso-
substituted hexapyrrolic calix[4]pyrrole 1. The calix[4]pyrrole’s core features two
additional pyrrole side-arms suspended above or below the calix[4]pyrrole core. This
hexapyrrolic calix[4]pyrrole 1 is formed as cis- and trans-configurational isomers, the
structures of which have been determined by single crystal X-ray diffraction. The
anion binding experiments revealed interesting difference in the binding mode: The
cis-1 isomer binds anions in a mixed binding mode featuring a combination of
hydrogen bonding and anion−π interactions resulting in an unexpected strong binding. On the other hand, the trans-1 isomer
displays only hydrogen bonding and lower affinity for anions. This is unexpected as one would assume both isomers to display
the same binding modes. Overall, the titrations of 1 using UV spectrophotometry and NMR titrations by anions reveal that cis-
isomer 1 displays higher affinity (105−106 M−1) and cross-reactivity for anions, while the trans-isomer 1 shows a more selective
response to anions. Such differences in binding mode in configurational isomers are so far unexplored and a feature deserving
further study.
hydrogen bond to a bound anion. Furthermore, the pyrrole
side-arms have a carboxyl ester in their 2-positions enabling an
attachment of various dyes for sensing applications.24 Likewise,
the compounds 1 retain free β-CH positions for a conversion to
sensors. Thus, these derivatives offer an example opportunity as
building blocks for molecular devices, particularly capsules
utilizing the pyrrole side-arms and sensors based on the β-CH
substitution.15c−g
INTRODUCTION
■
Anions play important roles in medicine, natural and industrial
processes, and biological systems, and the development of
artificial anion receptors garnered significant attention.1 To
date, various supramolecular receptors were realized, including
Lewis acid2 or metal coordination,3 hydrogen bonding,4
anion−π electron interaction,5 electrostatic interactions,6
halogen bonding,7 and others. Typical hydrogen-bonding
based receptors for anions employ thiourea,8 imidazole/
triazole,9 or amide10 moieties. Recently, octamethylcalix-
[4]pyrrole (OMCP), first synthesized by Baeyer in 1886,11
was rediscovered by Sessler and co-workers as a powerful
receptor for anions.12 Since then, anion receptors based on
OMCP have been widely researched and used as a structural
platform for preparation of anion sensors.13 Attachment of
functional groups to OMCP is usually carried out at the β-CH
position14,15 or meso-position.16 Particularly β-CH substitution
offers marked opportunities for sensor development.15 On the
other hand the meso-substitution was used to develop cavity
systems,17 cryptands,18 capsule,19 strapped calix[4]pyrroles,20
and calix[4]pyrroles with pendant arms.21 In addition, the
chemistry of calixpyrroles has been extended for expanded
analogues.22 In spite of the number of studies of calix[4]-
pyrroles, the attachment of pyrrole rings to meso-positions of
OMCP and corresponding anion recognition properties were
not fully investigated.23 To achieve enhanced anion affinity of
the OMCP receptor we decided to synthesize a hexapyrrolic
calix[4]pyrrole possessing two pyrrole side-arms above or
above and below the parent OMCP-macrocycle hoping the
NH- moieties of the pyrrole side-arms are able to form a
RESULTS AND DISCUSSION
■
The synthesis of cis- and trans-1 was performed using the
following methods: 2-ethoxycarbonyl-3-ethyl-4-methyl pyrrole
2 is available through Barton−Zard synthesis.25 Ethyl 5-acetyl-
3-ethyl-4-methyl-1H-pyrrole-2-carboxylate 3 was synthesized
according to the literature.26 The compound 3 was condensed
with freshly distilled pyrrole in dichloromethane, which
afforded ethyl 5-[1,1-bis(1H-pyrrol-2-yl)ethyl]-3-ethyl-4-meth-
yl-1H-pyrrole-2-carboxylate 4 in 58% yield. Subsequently, the
condensation of 4 with acetone in acetonitrile using trifluoro-
acetic acid gave the desired hexapyrrolic calix[4]pyrrole (1) as a
mixture of two configurational isomers. These isomers (cis and
trans) differ in the orientation of the meso-pyrrole side group
with regard to the plane of the calix[4]pyrrole macrocycle. The
mixture of isomers was purified by column chromatography on
silica, affording pure cis- and trans-1 (Scheme 1) with an overall
yield of ∼10%.
Received: November 12, 2013
Published: January 6, 2014
© 2014 American Chemical Society
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Scheme 1. Molecular Structure of 1
The structures of tripyrrole 4 and both isomers 1 were
confirmed by NMR spectroscopy, mass spectrometry, and X-
ray diffraction analysis. Figure 1 shows the X-ray structures of 4,
while Figure 2 shows the structures of receptors 1.
Figure 2. X-ray structures of (A) cis-1 and (B) trans-1; cis-1 adopts
1,3-alternate conformation, while trans-1 adopts 1,2-alternate con-
formation. Thermal ellipsoids are scaled to the 50% probability level.
Hydrogen atoms bound to carbon atoms are omitted.
10−4 M) was mixed with anions (1.0 × 10−3 M) such as
fluoride, chloride, acetate, dihydrogen phosphate, hydrogen
pyrophosphate, and benzoate. The selection of anions was
made to gain an insight into the binding of biologically more
relevant anions, phosphates, and carboxylates. The negative
mode ESI MS revealed that both isomers of 1 bind anions with
1:1 stoichiometry. The MS data of 1·anion complexes are
summarized in the SI. An example of ESI MS suggesting a
strong complexation between cis-1 and benzoate is shown in
Figure 3.
Because the absorption of the parent OMCP macrocycle
generally occurs in the deep UV region, the investigation of
binding affinity for anions is generally carried out by NMR and
ITC. Here, however, the hexapyrrolic calix[4]pyrrole 1
possesses pyrrole side-arms that absorb light in the near-UV.
This allows performing anion titrations using a UV spectros-
Figure 1. (A) X-ray diffraction analysis of 4. (B) The X-ray analysis
reveals an intermolecular hydrogen bond between pyrrole NH and
acyl oxygen in the solid state. Thermal ellipsoids are scaled to the 50%
probability level. Hydrogen atoms bound to carbon atoms are omitted.
The carbon atoms are shown in gray, the nitrogen in light blue, the
oxygen in red, and the hydrogen atoms in aqua blue.
The crystal structures of 1 confirm the assignment of the cis-
and trans-orientation of the meso-pyrrole substituents with
respect to the calix[4]pyrrole macrocycle. Here, cis-1 displays a
1,3-alternate conformation27 (Figure 2A), while trans-1 shows a
1,2-alternate conformation27 (Figure 2B). Recently, Panda et al.
reported cis- and trans-meso-diacylated calix[4]pyrrole to form
intermolecular hydrogen-bonded networks.28 However, cis- and
trans-1 do not exhibit similar behavior either in the solid state
1
or in the solution as confirmed by H NMR experiments with
varying concentration of 1 in acetonitrile-d3 (see the SI).
To clarify the anion binding stoichiometry of 1, three types
of experiments were performed: gas-phase experiments using
electrospray ionization (ESI) mass spectrometry, in solution
using UV spectroscopy, and by molecular modeling using DFT
method for geometry optimization in acetonitrile. ESI-MS
experiments: The acetonitrile solution of cis-1 or trans-1 (2.0 ×
Figure 3. (A) ESI MS spectrum of cis-1·BzO− complex. The spectrum
showed 1:1 binding stoichiometry. (B) Calculated isotope pattern for
−
C53H63N6O6 .
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copy. Figure 4 shows UV spectra of both isomers 1 upon the
addition of fluoride in acetonitrile. Both isomers exhibited a
affinity of cis-1 for anions is stronger than that of trans-1,
particularly in the case of chloride (116 times difference),
dihydrogen phosphate (6 times), and benzoate (126 times).
The cis-1 binds anions more efficiently than trans-1; the
receptor cis-1 generally shows binding constants Kas exceeding
104 M−1 as well as a high cross-reactivity.30 On the other hand,
trans-1 exhibits lower binding constants and higher selectivity.
This increased selectivity is attributed to the high directionality
of the hydrogen bonds. The relatively large affinity of 1 for
carboxylate derivatives suggests that the compound 1 would be
suitable as a building block for sensors for carboxylates such as
nonsteroidal anti-inflammatory drugs (NSAIDs).15c−e
To investigate the role of pyrrole side-arms in anion binding,
1H NMR titrations of anions were conducted with both isomers
1. Figure 5A shows 1H NMR spectra of cis-1 upon the addition
of fluoride. The assignments of each signal were achieved using
2D NMR (see SI). In these titrations, coupling between
calix[4]pyrrole NHs of cis-1 and the bound fluoride was
observed. Both calix[4]pyrrole NHs (Ha) and side-arm pyrrole
NHs (Hk) show slow exchange kinetics.
To gain further understanding of the recognition process, we
have measured 2D NOESY NMR of cis-1 in the absence
(Figure 5C) or presence of fluoride (Figure 5D). Figure 5D
shows new correlation signals between side-arm pyrrole NHs
and CH3 signals (Hb and He). Because cis-1 is fixed in the cone
conformation by fluoride, the side-arm pyrrole NHs and CH3
become close to each other (<5 Å). The downfield shift of the
calix[4]pyrrole NHs of cis-1 upon the addition of anions
follows the order: fluoride (12.8 ppm) > benzoate (11.8 ppm)
≈ dihydrogen phosphate (11.8 ppm) ≥ acetate (11.7 ppm) ≈
hydrogen pyrophosphate (11.7 ppm) ≥ chloride (11.4 ppm)
from the original 8.0 ppm. Figure 5A shows that the
calix[4]pyrrole NH resonances undergo a dramatic downfield
shift from 8.0 to 12.8 ppm and an upfield shift of the
calix[4]pyrrole β-CH signals. The respective coupling constant
is 41 Hz.31 This suggests that calix[4]pyrrole NHs interact with
fluoride through a hydrogen bond. However, the pyrrole NH
protons on the side-arm (Hk) do not undergo an appreciable
downfield shift upon the addition of fluoride. Interestingly, this
behavior is consistent with an anion−π interaction.32 This
suggests an anion−π interaction between these pyrrole rings
and the bound fluoride rather than hydrogen bonding.33
1H NMR titration of trans-1 by fluoride also revealed a fixed
cone conformation. In the case of the trans-1 each side-arm
resonance is split into two sets of signals (Figure 5B). One of
the pyrrole side-arms NH (Hk) shows a significant downfield
chemical shift in comparison with cis-1 suggesting a hydrogen
bonding between the side-arm pyrrole NH (Hk) and the bound
fluoride. The other side-arm NH (Hk′) shows almost no change
of chemical shifts upon the addition of fluoride suggesting that
one side-arm pyrrole does not participate in fluoride binding
presumably because it is below the plane of the calix[4]pyrrole.
Thus, the split signals arise from the nonparticipating side-arm
and the resulting loss in symmetry of the host−guest
conformation.34 The downfield shifts of the calix[4]pyrrole
NHs of trans-1 upon the addition of anions have the following
order: fluoride (12.8 ppm) > benzoate (11.4 ppm) ≈ acetate
(11.4 ppm) ≥ chloride (11.1 ppm) > dihydrogen phosphate
(10.3 ppm) ≥ hydrogen pyrophosphate (9.9 ppm) from the
original 8.1 ppm.
Figure 4. Absorption spectra of 1 (A: cis-1, B: trans-1) upon the
addition of tetrabutylammonium fluoride in MeCN at rt, [fluoride] =
0−90 μM.
small bathochromic shift (1−5 nm) of spectra upon fluoride
addition, suggesting that the side-pyrrole of both isomers
interacts with fluoride. However, spectral changes are clearly
different between cis- and trans-1 (Figure 4A,B), suggesting that
the binding mode of cis-1 is different from that of trans-1.
The binding affinities of cis- and trans-1 for various anions are
summarized in the Table 1. Overall, the binding constants of
both isomers 1 are higher than that of OMCP,12,29 which lacks
side-arm pyrrole assisting in anion binding. As expected, the
Table 1. Binding Constants (Ka, M−1) of 1 for Anions in
a
MeCN
anion
cis-1
trans-1
F‑
>1 000 000
220 000
>1 000 000
540 000
45 000
>1 000 000
1900
Cl−
AcO−
BzO−
H2Pi−
HPPi3‑
200 000
4300
7700
We calculated the energy-minimized structure of the complex
of 1 and fluoride in acetonitrile using a DFT method. Figure 6A
shows the energy-minimized geometry of the cis-1·fluoride
23 000
ND
a
All errors are <17%. ND: biphasic nature of the isotherm.
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1
Figure 5. H NMR (500 MHz) titration of 1 (A: cis-1, B: and trans-1) upon the addition of fluoride. (C) 2D NOESY NMR spectrum of cis-1
without fluoride recorded in CD3CN. (D) 2D NOESY NMR spectrum of cis-1 with fluoride recorded in CD3CN.
anions compared to cis-1. This result is also in agreement with
the NMR data in Figure 5B.
Finally, the anion binding modes have been confirmed by a
DFT study aimed at interpretation of shifts in the NMR spectra
corresponding to the energy-minimized model for 1 in the
resting state and in the complex with fluoride. Here we show
the DFT-GIAO magnetic shielding calculated for 1 and 1·F− in
their energy minima (Figures 7). The calculation shows the
same trend (shifts of the 1H-resonances) as observed during the
titration experiments of 1 with fluoride observed for 1 with
fluoride in acetonitrile-d3 (see Figure 5).
The different anion-binding modes in cis-1 and trans-1 are
surprising. Naturally, one would expect that the trans-1 isomer
would, similarly to cis-1 bind anions in a mixed mode of
hydrogen bonding and anion−π interaction, albeit with only
one pyrrole side-arm. In fact this is not observed.
Figure 6. Energy minimized geometry (DFT, B3LYP/6-31G* level)
of the complex of (A) cis-1 and (B) trans-1 with fluoride in acetonitrile
(fluoride is shown in CPK mode).
CONCLUSIONS
■
complex with both of the calix[4]pyrrole NHs binding fluoride.
The calculated model predicts the side-arm pyrrole to interact
with fluoride through anion−π interaction. The distances
between the centroid of the side-arm pyrrole ring and the
fluoride are ∼4.3 Å, which is rather long35 but still in agreement
with previously reported anion−π interactions between anions
and meso-tetraaryl calix[4]pyrroles.32 Furthermore, the distance
of the side-arm pyrrole NHs and CH3 (Hb and He) is <5 Å,
which is in good agreement with the 2D NOESY NMR results
(vide supra). The complex of trans-1 with fluoride shows that
one of the side-arm pyrrole binds fluoride through hydrogen
bonding, while the other side-arm does not bind the guest
(Figure 6B), a presumed reason for overall lower affinity for
In summary, the configurational isomers of a new hexapyrrolic
calix[4]pyrrole 1 have been synthesized, and their structure
unambiguously characterized by X-ray crystallography. The
binding stoichiometry of 1 and anions was investigated by MS,
NMR, and UV spectroscopy. Despite the structural similarity,
both receptors display different physical interactions to form
anion-receptor complex. In the case of the cis-1 isomer the UV
titration, NMR analysis, and computational DFT model reveal
that the pyrrole side-arms contribute to anion binding by 2-fold
anion−π interactions. In contradistinction, the trans-1 isomer
forms the receptor-anion complex based solely on hydrogen
bonding, i.e., without any contribution from anion−π
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the NSF
(CHE-0750303 to P.A.). We would like to thank Dr. V. M.
Lynch of University of Texas for solving the X-ray structures.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The details of synthesis and characterisation of compounds 1
and 4, X-ray data, UV absorption, MS, and NMR spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
(16) Lee, C.-H.; Miyaji, H.; Yoon, D.-W.; Sessler, J. L. Chem.
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Corresponding Author
pavel@bgsu.edu
Notes
The authors declare no competing financial interest.
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