Selective Inclusion of Fluoride within the Cavity of a Two-Wall Bis-calix[4]pyrrole

Article abstract of DOI:10.1021/acs.orglett.0c01440

We report what to our knowledge is the smallest bis-calix[4]pyrrole (2). It proved capable of trapping fluoride anions exclusively relative to other anions (Cl^-, Br^-, SCN^-, NO₃^-, H₂PO₄^-, HSO₄^-, SO₄^2-, and HP₂O₇^3- tetrabutylammonium salts), as confirmed by ¹H NMR spectroscopy (CDCl₃), X-ray diffraction analysis, DFT calculations, and molecular dynamics simulations. The F^- selectivity is ascribed to the small size of the cavity in 2.

Full text of DOI:10.1021/acs.orglett.0c01440

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Letter
Selective Inclusion of Fluoride within the Cavity of a Two-Wall Bis-
calix[4]pyrrole
∥
∥
∥
Shenglun Xiong, Fangyuan Chen, Tian Zhao, Aimin Li, Guangyu Xu,* Jonathan L. Sessler,*
and Qing He*
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ABSTRACT: We report what to our knowledge is the smallest bis-calix[4]pyrrole (2). It proved capable of trapping fluoride anions
−
−
−
−
−
−
2−
3−
exclusively relative to other anions (Cl , Br , SCN , NO , H PO , HSO , SO , and HP O ; tetrabutylammonium salts), as
3
2
4
4
4
2
7
1
confirmed by H NMR spectroscopy (CDCl ), X-ray diffraction analysis, DFT calculations, and molecular dynamics simulations.
The F selectivity is ascribed to the small size of the cavity in 2.
3
−
olecular recognition remains a central focus of supra-
strap and showed that the systems in question displayed
exclusively selective toward the fluoride anion. This selectivity
is attributed to the conformational restriction imposed by the
small strap on the calix[4]pyrrole moiety, which prevents the
calix[4]pyrrole from accessing the cone conformation consid-
ered most favorable for anion binding (Figure 1a). Other
approaches to achieving high fluoride anion selectivity,
1
−7
M
molecular and host−guest chemistry.
However,
synthetic molecular recognition systems that perform with
8
near-idealized specificity are rare, notwithstanding the fact
that they are commonplace in nature. A specific challenge
involves increasing the inherent selectivity of a given class of
receptor. Here, we show that linking formally two calix[4]-
pyrrole subunits via two single atom oxygen bridges creates a
receptor with essentially exclusive selectivity for the fluoride
21
including the use of small pyrrolic cages, are also known.
However, the importance of fluoride as a target justifies
continued efforts to capture it with high fidelity.
−
−
anion relative to other standard test anions, including Cl , Br ,
−
−
−
−
2−
3−
SCN , NO , H PO , HSO , SO , and HP O , as
−
−
3
2
4
4
4
2
7
Fluoride (F ) is the smallest anion except for hydride (H ).
Our thinking, therefore, was that a three-dimensional cavity
slightly larger than the volume of fluoride anion capable of
supporting hydrogen-bonding interactions would reject anions
larger than fluoride, resulting the exclusive fluoride selectivity.
With such a predicative postulate in mind, we designed what
1
inferred from H NMR spectroscopic studies carried out in
CDCl , as well as supporting X-ray diffraction analyses and
DFT calculations.
3
Calix[4]pyrrole (octamethylporphyrinogen) and its deriva-
tives have been extensively studied as both anion receptors and
9
−17
ion-pair receptors.
In the context of this effort, it was found
we believe is the smallest two-walled bis-calix[4]pyrrole (2)
that strapped calix[4]pyrroles, wherein one face of the
macrocyclic core is bridged by a connecting linker, are
particularly effective as both anion and ion pair receptors
displaying, inter alia, relatively enhanced affinities and
23,24
reported to date.
It consists of two simple calix[4]pyrroles
(
1) formally linked via two oxygen bridges.
1
8,19
Received: April 27, 2020
selectivities.
In most cases, the observed selectivity reflects
a greater affinity for a given targeted species over other
comparative guests. Nevertheless, examples that show exclusive
2
0
selectivity toward targeted guests over other species are rare.
2
0
In 2015, Panda and co-workers reported a series of
calix[4]pyrroles with what may well be the shortest possible
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c01440
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Figure 1. Two strategies leading to exclusive selectivity for the
fluoride based on calix[4]pyrroles: (a) strategy used by Panda and co-
workers; (b) strategy reported here. Color codes in (b): thick pink
lines, methylene units; gray/black lines, methyl groups; green lines,
bridging oxygen atoms; and blue cones, calix[4]pyrrole units.
Figure 2. Single-crystal structures of 2·2TBAF: (a) side on view; (b)
20
−
+
and (c) top and side views of the core 2·2F complex. For (a), TBA
−
−
and F ions are shown in space-filling form. For (b) and (c), the F
anions are shown in space-filling form while the counter cations are
omitted for clarity.
To prepare bis-calix[4]pyrrole 2, we started with commer-
cially available 2-propynyl ether. Hydrolysis catalyzed by HgO/
H SO (Kucherov’s reaction) yielded 2-propanone ether (3)
2
4
2
2
(Scheme S1). Condensation between 3 and pyrrole (4) gave
cup of the calix[4]pyrrole domain through presumed
the key O-linked bis-dipyrrolemethane intermediate (5) in a
cooperative C−H···π interactions and cation···π interactions
9
,25−29
yield of 39% (Scheme 1). A one-pot, BF -catalyzed, multi-ring-
(Figure 2a).
Each of the two fluoride anions
3
concurrently trapped within the inner cavity of 2 is bound to
one calix[4]pyrrole unit in its corresponding cone conforma-
tion. The top calix[4]pyrrole moiety was found tilted at ∼26
degrees relative to the lower one (Figure S5); presumably, this
reflects steric clashes between the peripheral methyl groups
attached to the meso positions. The net result is the formation
of two relatively independent small pockets defined by the
conelike calix[4]pyrrole subunits, the meso methyl groups, and
the two linking ether groups.
Scheme 1. Chemical Structure of Calix[4]pyrrole 1 and the
Synthetic Route Used to Prepare Receptor 2
The two bound fluoride anions are nestled into the pockets
of 2 in spite of the potential electrostatic repulsion such
−
−
cobinding is likely to engender (Figure 2b,c). The F ···F
distance was estimated to be 3.02 Å, which is consistent with a
degree of anion−anion repulsion interaction. However, this
repulsion is apparently stabilized by multiple cooperative
hydrogen bonding interactions, as has been seen in the case of
30
other dianion complexes. For instance, average pyrrole N···
−
−
F distances of 2.69 Å are seen. In addition, C···F distances of
−
−
−
3
.30 Å (C21···F ), 3.39 Å (C22···F ), and 3.20 Å (C27···F )
are seen for the methylene CHs and methyl CHs subunits,
respectively (Figure 2c). These are very short distances and
could reflect strong CH···F interactions. For instance, a
search of the Cambridge Crystallographic Data Centre
−
forming condensation of 5 with acetone then gave the desired
trimacrocyclic bis-calix[4]pyrrole 2 in 1% yield. Compound 2
was characterized by NMR spectroscopy and high-resolution
mass spectrometry (Figures S1−S3) as well as via single-crystal
X-ray diffraction analysis (Figure S4).
(CCDC) revealed roughly 40 calix[4]pyrrole−fluoride anion
−
structures. Most of the C···F distances (where C refers to
either a methylene or methyl carbon atom) were found to fall
2
6,31−33
−
Initial evidence that receptor 2 could act as an effective
receptor for the fluoride ion came from an X-ray diffraction
analysis of single crystals obtained by allowing a CH Cl /
in the 3.50−4.00 Å range.
Only few with C···F
34−36
distances in the 3.50−3.45 Å range were found.
No
−
examples with C···F distances as short as 3.20 Å (as in the
present instance) were found. This contrast was taken as an
indication that the cavity provided by 2 is restricted and was
thus likely to provide for size-dictated selectivity in favor of
fluoride. While not a proof of binding exclusivity, all efforts to
obtain diffraction grade crystals of 2 with larger anions (than
2
2
CH CN solution of 2 to undergo slow evaporation in the
3
presence of excess tetrabutylammonium fluoride (TBAF)
(
1
Figure 2). The resulting structure revealed an encapsulated
:2 complex in the solid state. As is common for solid-state
TBA anion complexes of calix[4]pyrroles, the TBA counter
cations were found to be partially bound to the electron-rich
−
−
−
−
−
−
−
2−
F ion), i.e., Cl , Br , SCN , NO , H PO , HSO , SO ,
3
2
4
4
4
B
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Letter
3
−
and HP O as their TBA salts failed. This was true in spite of
intuitively appealing inference that the sequential binding of
2
7
−
the fact that a variety of crystallization conditions were tested.
The ability of receptor 2 to bind fluoride anions in solution
was probed via H NMR spectroscopy using CDCl as the
solvent. Although receptor 2 exhibits only a 1,3-alternate
conformation in solid state, spectroscopic analysis of
compound 2 revealed only one set of resonances in CDCl3
at room temperature (Figure S1), as would be expected for a
relatively flexible system that is in conformational equilibrium.
When excess TBAF was added into a 1.0 mM solution of 2 in
CDCl , the broad singlet ascribed to the pyrrolic NH signals at
.49 ppm (designated as a in Scheme 1) in free 2 disappeared
completely presumably as the result of fast exchange induced
by anion binding (Figure S6). Concurrently, the two peaks
ascribed to the β-pyrrolic protons, located at 6.01 ppm (c) and
two F anions is subject to negative cooperativity (a = 4K /
1
2
37,38
K11 = 0.34 < 1).
In other words, the binding of a firs−t
1
fluoride anion serves to reduce the affinity for the second F
guest. This is in line with the repulsive anion−anion
interaction inferred from the solid-state structural studies
(vide supra). Moreover, under conditions of HRMS analysis,
3
−
evidence for only a 1:1 2·F complex is seen; this finding is
interpreted in terms of the binding of a second fluoride anion
in the gas phase being unfavorable (Figure S10), a conclusion
further supported by gas-phase DFT calculations (Table S1).
Experimental support for the notion that receptor 2 was
capable of binding fluoride in solution with exclusive selectivity
3
7
1
came from H NMR spectroscopic analyses performed in
−
CDCl (Figure 4). It was found that adding 50 equiv of Cl ,
3
5
.86 ppm (b), respectively, were found to shift to around 5.71
ppm. These changes are consistent with the conclusion that
one or more fluoride anions is being bound effectively by 2.
To obtain greater insights into the binding of fluoride by 2,
1
H NMR spectroscopic titrations were carried out in CD Cl
3
using TBAF as the fluoride anion source (Figure 3). Under the
1
Figure 4. Selected regions of the H NMR spectra (CDCl , 298 K) of
3
−
solutions of 2 recorded in the absence or presence of 50 equiv of F ,
Cl , Br , SCN , NO , H PO , HSO , SO , or HP O
−
−
−
−
−
−
2−
3−
,
3
2
4
4
4
2
7
respectively, as their TBA salts.
Br , SCN , NO , H PO , HSO , SO , or HP O73−,
−
−
−
−
−
2−
3
2
4
4
4
2
1
respectively, as their TBA salts to solutions of 2 in CDCl3
failed to produce any noticeable changes in the proton signals
of 2. This is consistent with the absence of any appreciable
interaction between 2 and any of these anionic species in
question. In contrast, dramatic changes in the β-pyrrole proton
signals (i.e., from 6.01 and 5.86 ppm to 5.71 ppm, respectively)
were seen in the case of TBAF under otherwise identical
conditions. Meanwhile, the presence of one or more
potentially competing anions in excess had no adverse effect
on the presumed fluoride anion binding (Figure S11). Taken
together, these findings lead us to conclude that receptor 2 is
Figure 3. Selected regions of H NMR spectra (CDCl , 298 K)
3
acquired during the titration of 2 with increasing quantities of TBAF:
, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.5, 1.8, 2.1, 2.5, 3.2, 4.2, and
.0 equiv. The resonances of the β-pyrrolic protons (peaks in blue and
red) undergo an upfield shift to 5.71 ppm (peaks in green) upon the
addition of TBAF to a solution of 2, while the signals of the
methylene protons (in pink) disappear in monotonic fashion.
0
6
conditions of the titration, new peaks (in green) around 5.71
ppm are seen to increase in intensity, while those of the
original β-pyrrolic proton signals (in blue and red) decrease.
Such changes are thought to reflect a fluoride anion binding
process that is slow on the NMR time scale. Additionally, the
signals for the methylene protons at 3.71 ppm (d, in pink) in
the ether bridge were seen to disappear upon the incremental
addition of TBAF. This disappearance could be attributed to
an increase in the rigidity of this otherwise flexible portion of
the molecule brought about as the result of fluoride anion
binding to 2 (Figure S7) such that local motions are
intermediate on the NMR time scale.
The H NMR data corresponding to the pyrrolic C−H
proton signals were fitted to a 1:2 binding model and the
resulting binding constants were estimated to be K = (5.2 ±
.4) × 10 and K = (4.4 ± 1.4) × 10 M , respectively
Figures S8 and S9). These values are in accord with the
−
able to bind F exclusively in the presence of not only larger
−
−
spherical Cl and Br but also the more structurally complex
−
−
−
−
2−
3−
SCN , NO , H PO , HSO , SO , and HP O7 anions.
3
2
4
4
4
2
To provide further support for the hypothesis that the
−
exclusive F selectivity was dictated by the very limited three-
dimensional (3D) space within receptor 2, density functional
theory (DFT) calculations were carried out in the gas phase at
the X3LYP/6-31g* level. Only in the case of the calculated 2·
−
−
2F and 2·F complexes did the optimization converge to give
1
rational structures (Figures S12 and S13). Conversely, in the
−
cases of various other putative complexes, including 2·2Cl , 2·
−
−
−
2Br , 2·2SCN , and 2·2NO , all attempts to force the
11
3
3
2
−1
0
(
convergence to structures with anions entrapped within the
1
2
expected cavities of 2 failed(Figures S14 and S15). Further
C
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Letter
support for this notion came from gas-phase molecular
dynamics simulation studies (Figures S16−S21).
Aimin Li − State Key Laboratory of Chemo/Biosensing and
Chemometrics, College of Chemistry and Chemical Engineering,
Hunan University, Changsha 410082, P.R. China
In summary, we describe here a small two-wall bis-
calix[4]pyrrole (2) formally linked by two single oxygen
atom bridges. This putative anion receptor was characterized
by standard spectroscopic protocols, as well as by single X-ray
crystal diffraction analysis. Bis-calix[4]pyrrole 2 was found
capable of binding the fluoride anion in a 1:2 manner in the
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01440
Author Contributions
∥S.X., F.C. and T.Z. contributed equally to this work.
solid state as evidenced by a single crystal structural analysis. In
−
CDCl solution, receptor 2 was found to bind the F anion
Notes
3
−
−
−
well but not interact appreciably with the Cl , Br , SCN ,
NO , H PO , HSO , SO , and HP O7 anions, as
inferred from H NMR spectroscopic analyses, a conclusion
supported by DFT calculations and gas-phase molecular
dynamics simulation studies. This study paves the way to the
design of functional receptors with high or exclusive selectivity.
The authors declare no competing financial interest.
−
−
−
2−
3−
3
2
4
4
4
2
1
ACKNOWLEDGMENTS
■
This research was supported by the National Natural Science
Foundation of China (21901069 to Q.H.), the Science and
Technology Plan Project of Hunan Province, China
(2019RS1018 to Q.H.), and the Fundamental Research
Funds for the Central Universities (Startup Funds to Q.H.).
G.X. thanks the Science and Technology Plan Project of
Hunan Province (2018TP1017) for financial support. The
work in Austin was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
Separation Science program under Award No. (DE-FG02-
ASSOCIATED CONTENT
sı Supporting Information
■
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01440.
Synthetic procedures, NMR, HRMS, titration studies, X-
ray crystallography, and DFT calculations of the
inclusion complexes (PDF)
0
0
1ER15186 to J.L.S.) and the Robert A. Welch Foundation (F-
018 to J.L.S.). We are grateful to Xiaopeng Li and Yunpeng
Accession Codes
Cui from the Molecular Scale Lab of the University of South
Florida for their help in carrying out confirmatory MS analyses.
CCDC 1949125−1949127 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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■
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■
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