Calix[4]pyrroles with Shortest Possible Strap: Exclusively Selective toward Fluoride Ion

Article abstract of DOI:10.1021/acs.orglett.5b01866

Four new calix[4]pyrroles with the shortest possible strap so far through ortho-linking of the aromatic unit have been synthesized, including a naphthalene-derived fluorescent receptor. They show exclusive selectivity toward the fluoride ion as confirmed by ¹H NMR, isothermal titration calorimetry, and fluorescence spectroscopic study. Anion affinity could also be modulated further via functionalization at the strap. Computational analysis displays calix[4]pyrroles binding to fluoride ion in a very unusual 1,3-alternate conformation where the anion resides on the opposite side of the strap.

Full text of DOI:10.1021/acs.orglett.5b01866

Letter
pubs.acs.org/OrgLett
Calix[4]pyrroles with Shortest Possible Strap: Exclusively Selective
toward Fluoride Ion
Ritwik Samanta,†,‡ B. Sathish Kumar,† and Pradeepta K. Panda*,†,‡
‡
†School of Chemistry and Advanced Centre of Research in High Energy Materials, University of Hyderabad, Hyderabad 500046,
India
S
* Supporting Information
ABSTRACT: Four new calix[4]pyrroles with the shortest possible strap so
far through ortho-linking of the aromatic unit have been synthesized,
including a naphthalene-derived fluorescent receptor. They show exclusive
1
selectivity toward the fluoride ion as confirmed by H NMR, isothermal
titration calorimetry, and fluorescence spectroscopic study. Anion affinity
could also be modulated further via functionalization at the strap.
Computational analysis displays calix[4]pyrroles binding to fluoride ion in
a very unusual 1,3-alternate conformation where the anion resides on the
opposite side of the strap.
he recognition of a guest by a supramolecular host depends
on how closely the two partners fit together. One of the
schematically illustrated (Figure 1) for calix[4]pyrrole with a
large strap (L) and a short strap (S), both interacting with small
T
most fundamental and first thoughts in this area was the “lock-
and-key” concept proposed by Fisher,1a which is still a
stimulating research area in “host−guest” chemistry. With a
given host and guest, the thermodynamic stability of the “host−
guest” complex is dependent on host preorganization and
complementarity with respect to a specific guest. From the very
first report on the anion receptors, the above theory was found to
be viable, where it was also found that due to proper size match
only one receptor was selective toward the chloride ion.1b,c In
later studies, Lehn and co-workers followed by various groups
explored this phenomenon extensively through their work.2
Among various classes of anion receptors, pyrrole-based
macrocycles are quite popular among chemists, and this class
of anion receptors has undergone a vast amount of modification
over the decades in search of better selectivity toward a specific
anion.3 A century-old macrocycle, calix[4]pyrrole,4a after
discovery of its anion-recognition abilities,4b also endured an
extensive amount of alteration for the same cause. In particular,
functionalization has been carried out at its meso5 and β-
positions,6 including core modification7 and core expansion.8
Further, among the calix[4]pyrrole-based anion-receptor family,
single side-strapped calix[4]pyrroles are unique, owing to the
large enhancement in their affinity toward anions.9 However, to
the best of our knowledge, there are only two reports that deal
with anion affinity and selectivity with regard to variable strap
sizes.9b,c No insight was found about the minimum length of the
strap needed to design these types of receptors and the possible
selectivity toward anions it may impart, owing to the small
binding domain. In this regard, our effort was to try to synthesize
calix[4]pyrrole with the shortest possible strap and explore its
consequences on anion binding and selectivity. In these types of
systems some interesting things can happen. For example, a
shorter binding domain will try to discriminate between the
smaller and bigger anions on the basis of their size. This has been
Figure 1. Schematic representation of possible modes of interaction
between a long (L) and short (S) strapped calix[4]pyrrole with a small
and a large anion.
and large anions. For L, there may not be much selectivity in the
interaction between the large and small anions, as the binding
domains have enough space to accommodate both types of
anions. However, in the case of S, owing to steric hindrance
caused by the shorter strap, the larger anion may not fit into the
binding domain, whereas the smaller anion may not have any
such difficulty in binding. This size-imposed discrimination may
eventually lead to better selectivity for the smaller anions.
However, another event can happen with S where an anion can
go to the other side of calix[4]pyrrole, i.e., opposite side of the
strap, and hence may not display any size-based discrimination
between anions.
In order to achieve S-type calix[4]pyrrole, a preliminary
structural analysis using Chem3D program (ChemBioOffice
12.0) shows strapped calix[4]pyrrole with one carbon between
benzene and calix[4]pyrrole is less likely to result in macro-
cyclization. Therefore, we intended to prepare strapped
Received: June 29, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01866
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Organic Letters
Letter
calix[4]pyrroles with two atoms as a bridging unit and for the
sake of convenience chosen the −OCH2− unit as a spacer
between the calix[4]pyrrole core and the strapping aromatic unit,
e.g., 3−6. In all of the receptors, aromatic units were derived from
catechol or its derivatives, like our previously reported receptors
1 and 2.10 All strapped calix[4]pyrrole receptors were
synthesized by slightly modifying the method we have used
previously to synthesize 1 (Scheme 1).10a Our attempts to
alkylate the dihydroxy aromatic compounds using 1-bromoace-
tone in dry acetone, in the presence of anhyd potassium
carbonate, did not yield the desired products, probably owing to
decomposition of 1-bromoacetone at a higher reaction temper-
ature. Although the reaction at rt in acetone resulted in formation
of the product, it was always accompanied by some impurities,
making the purification very tedious. On the other hand,
replacing the solvent with acetonitrile led to satisfactory yields of
the diketo compounds. The desired diketo compounds (7−10)
were purified from the crude product by column chromatog-
raphy with moderate to good yields (42−84%) (Scheme S1,
Supporting Information). These diketo compounds were
converted to their respective bisdipyrromethanes 11−14 using
excess pyrrole and TFA at 60 °C with satisfactory yields (45−
78%). Finally, condensation of the bisdipyrromethanes in
acetone, using catalytic BF3·etherate, led to the formation of
the desired macrocylces after 10 min, as per TLC analysis.
Further, the solid-state structure of strapped calix[4]pyrroles 3,
4 (slow evaporation of ethyl acetate−hexane mixture), and 5
(slow evaporation of dichloromethane−methanol mixture)
could be unequivocally ascertained by single-crystal X-ray
diffraction analysis (Figure 2). Although so far we did not
Figure 2. Molecular structures of (a) front and (b) side views of 3, (c)
front and (d) side views of 4, and (e) front and (f) side views of 5 scaled
at 50% probability level. Hydrogen atoms are excluded for clarity. Color
code: C, gray; N, blue; O, red; Br, brown.
succeed in obtaining the solid-state structure of 6, however, we
were able to obtain the crystal structure for its precursor 14 by
slow evaporation of its chloroform solution (Figure S40,
Supporting Information). In all of the crystal structures,
macrocycles display a 1,3-alternate orientation of the pyrrole
units, and interestingly, the strap resides almost orthogonal to the
calix[4]pyrrole cores, not noticed in analogues with a longer
strap viz. 1.10a Further, the methylene units are found to be
outwardly disposed in all of the crystal structures.
Preliminary solution-phase anion-binding behavior of 3−6
was studied by 1H NMR titration in CD3CN with various anions
as their tetrabutylammonium (TBA) salts (i.e., F−, Cl−, Br−, I−,
CN−, HSO4 , H2PO4 , CH3CO2 , N3 , NO2 , NO3 , and
−
−
−
−
−
−
−
ClO4 ), and the study revealed that the four tested receptors
showed interaction only with fluoride ion (Figures S41−47,
Supporting Information). To our knowledge, this type of
exclusive selectivity toward a particular anion is not reported in
calixpyrrole chemistry except for the highly organized cone
calix[4]arene diester strapped calix[4]pyrrole11 and colorimetric
sensing of fluoride ion by meso-expanded calix[2]-
bispyrrolylethene reported by us recently.8h For receptors 3−6,
it was found that 1 equiv of anion was sufficient for the complete
conversion of free receptors to corresponding receptor−anion
complexes. Interestingly, less shift of pyrrolic-NH signals was
observed unlike for other strapped calix[4]pyrroles, whereas
those in the case of β-pyrrolic-CH signals were almost negligible.
This indicates that the hosts possibly undergo very minimal
conformation change during complexation, which thus resulted
in diminished interaction toward fluoride ion (Figures S41−47,
Supporting Information).
In order to evaluate the binding affinities, isothermal titration
calorimetry (ITC) was performed in HPLC-grade acetonitrile
(with ∼0.05% water) for the receptors 3−6 with fluoride ion.
The binding isotherm was found to follow 1:1 binding
stoichiometry in all cases. Binding processes are found to be
exothermic in nature, with maximum heat change occurred for 5
upon complexation and followed by 6, 3, and 4 (Figures S48−51,
Supporting Information). All of the processes were entropically
Scheme 1. Schematic Representations of Synthesis of
Receptors
Purification of the reaction mixtures using silica gel column
chromatography and subsequent washing of the product with
methanol led to the desired strapped calix[4]pyrroles 3−6 in
18−31% yields. Our attempt to synthesize analogous strapped
calix[4]pyrrole from 1,3-dihydroxybezene met with failure in the
final macrocyclization step, possibly indicating the strap length is
not sufficient for convergent synthesis of calix[4]pyrrole
(Scheme S2, Supporting Information). All of the compounds
were characterized with standard spectroscopic techniques.
B
DOI: 10.1021/acs.orglett.5b01866
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Letter
favored owing to the release of water molecules by TBAF upon
complexation (as it was used as its trihydrate) and only
marginally favored thermodynamically, except for 5, in which
case the binding is favored by enthalpy. Further, the translational
and conformational freedom available to the host could also
contribute to the change in entropy, and this component is
possibly less in the case of 5 owing to the much bulkier bromo
groups in the strap, and no binding of chloride ion was observed
when TBACl was used. Association constants and thermody-
namic parameters are tabulated in Table 1. The data clearly
strap, in which case the nitro derivative is expected to exhibit
greater hydration owing to hydrogen-bonding interactions.
In order to make our receptor fluorescence active, the
naphthalene moiety was introduced instead of benzene in
receptor 4. Its anion-binding affinities with various anions were
explored using fluorescence spectroscopy. It was found that
quenching of fluorescence was observed only in the case of
fluoride ion (Figure S52, Supporting Information). Further, the
selective affinity of the host toward F− ion could also be verified
by competitive-binding experiments (Figure S53, Supporting
Information). However, the amount of quenching was found to
be less than that observed in the case of 2. Job plot analysis for
this binding event found to follow 1:1 binding stoichiometry
(Figure 3, inset). The association constant (Ka) derived from this
Table 1. Association Constants (Ka) and Thermodynamic
Data Derived from ITC Experiments for the Interactions of
Receptors 1−6 with TBAF·3H2O in CH3CN at 303 K
b
b
b
Ka (M−1
)
ΔH
TΔS
ΔG
a
b
110
1.17 0.33 × 106
2.42 0.68 × 105
2.42 0.42 × 105
3.77 0.41 × 104
1.57 0.18 × 104
1.16 0.22 × 104
2.01 0.22 × 105
5.52 0.40 × 104
−6.27
−14.20
2.16
−8.42
−7.47
210
−6.50
a
a
3
4
−0.84
−0.68
5.45
5.15
−6.29
−5.83
5
6
−3.77
−1.99
3.58
4.58
−7.34
−6.57
a
b
Determined by fluorescence spectroscopy. In kcal/mol.
Figure 3. (Left) Fluorescence spectra of 4 in CH3CN (0.04 mM)
excited at 310 nm showing the induced changes upon addition of
increasing quantities of TBAF·3H2O. (Right) Quenching effect of
TBAF·3H2O upon gradual addition to 4. (Inset) Job plot analysis of 4 vs
TBAF·3H2O.
indicate association constants are lower compared to the
previously reported strapped calix[4]pyrroles with larger
straps,10 and this may be attributed to the repulsive interaction
between lone pairs of oxygen atoms present in the strap and the
anion. For example, receptor 3 displayed about two orders and
receptor 4 one order weaker association constant with fluoride
ion as compared to analogous calix[4]pyrroles with longer strap,
1 and 2, respectively. On the other hand, association constant of
5 was found to be about 1 order higher when compared with
those of 3 and 4. It showed that the affinity toward fluoride ion
could be enhanced further, without compromising on the
selectivity, by introducing electron-withdrawing groups onto the
benzene moiety of the strap (in 5 and 6), which probably helps in
reducing the repulsion between the lone pairs on oxygen atoms
and the anion. In order to prove this further, we tried to
synthesize analogous strapped calix[4]pyrrole by replacing the
“O”s with methylene moieties in 3. However, the corresponding
bisdipyrromethane did not yield the desired macrocycle,
probably owing to nonbonding repulsion between the bridging
alkyl chains (Scheme S2, Supporting Information). Further, we
could observe that two bromine atoms (in 5) impart a stronger
effect than one nitro group when introduced onto the benzene
moiety of the strap (in 6). Our attempt to synthesize analogous
strapped calix[4]pyrrole with two nitro groups on the benzene
moiety again could not succeed. In order to minimize the
possible error owing to the hygroscopic nature of acetonitrile, we
tried to introduce water onto it (0.5%); however, this led to no
proper isotherm, probably due to the weak binding nature of
these receptors. However, our similar study in analogous
receptor 1 already indicated that the presence of water in
acetonitrile only reduces the affinity constant marginally.10
Interestingly, in the case of 5 and 6 endothermic heat flows were
noticed upon successive addition of F− to the host solution,
particularly after addition of more than 1 equivt of the anion
(Figures S50−51, Supporting Information). This effect was more
severe in the case of the latter and may be attributed to possible
interaction of released water with the functional groups on the
quenching experiment is 1.16 × 104, which is again found to be
less by 1 order of magnitude than that observed for its higher
homologue 2.10b On the other hand, the association constants
derived for 4 from two methods (fluorescence quenching and
ITC) were found to be in good agreement with each other, as
noticed in the case of 2.
To obtain insight into the possible binding modes, we
performed density functional theory (DFT) calculations on the
host−anion complexes (fluoride and chloride).12 The analysis
clearly revealed only fluoride ion complexes with the host.
Interestingly, the anion resides on the other side of the strap with
the calix[4]pyrrole in cone conformation transformed to its 1,3-
alternate conformation (Table S1, Supporting Information). In
addition, optimization of the complexes with host in 1,2-alternate
conformation led to partial cone conformation and was found to
be very slightly higher in energy than that with 1,3-alternate
conformation. They in turn were found to be more than 4 kcal/
mol lower in energy compared to complexes where the host is in
reverse cone conformation, i.e., cone away from the strap (Table
S1, Supporting Information). To our knowledge, calix[4]pyrrole
binding anions in a 1,3-alternate conformation are not known;
therefore, we believe the short strap with two electronegative
oxygens possibly forces the anion to bind on the opposite side of
the strap. This presumption is well supported by the minimal NH
1
shift in the H NMR study as well as the weaker binding
constants obtained in ITC experiments of the fluoride
complexation of these calixpyrroles.
In conclusion, we have synthesized four new strapped
calix[4]pyrroles with the shortest possible strap so far. Anion-
binding studies revealed that these receptors display exclusive
selectivity toward the fluoride ion, albeit weakly compared to the
analogous calix[4]pyrroles with a longer strap. This may be
attributed to the conformational restriction imposed by the small
C
DOI: 10.1021/acs.orglett.5b01866
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Letter
Commun. 2011, 47, 4496. (f) Mahanta, S. P.; Panda, P. K. Org. Biomol.
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strap on the calix[4]pyrrole moiety not being able to attain its
most favorable cone conformation toward the anion. Therefore,
a slightly longer strap may make the host more effective toward
the anion, and at present, efforts are underway in this direction.
Further, for the first time we demonstrate that incorporation of
substituents at the aromatic unit of the strap can also help
modulate the anion affinity without compromising the guest
selectivity.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b01866.
CCDC nos. for compounds 3, 4, 5 and 14 are 1409307, 1409308,
1409309, and 1409310.
Crystallographic data for 3 (CIF)
Crystallographic data for 4 (CIF)
Crystallographic data for 5 (CIF)
Crystallographic data for 14 (CIF)
Detailed synthetic scheme, procedures, NMR, and ITC
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: pkpsc@uohyd.ernet.in, pradeepta.panda@gmail.com.
Notes
́
(8) (a) Gale, P. A.; Genge, J. W.; Kral, V.; McKervey, M. A.; Sessler, J.
L.; Walker, A. Tetrahedron Lett. 1997, 38, 8443. (b) Turner, B.;
Botoshansky, M.; Eichen, Y. Angew. Chem., Int. Ed. 1998, 37, 2475.
(c) Cafeo, G.; Kohnke, H.; La Torre, G. L.; White, A. J. P.; Williams, D. J.
Angew. Chem., Int. Ed. 2000, 39, 1496. (d) Cafeo, G.; Kohnke, H.; Parisi,
M. F.; Nascone, R. P.; La Torre, G. L.; Williams, D. J. Org. Lett. 2002, 4,
2695. (e) Cafeo, G.; Kohnke, H.; La Torre, G. L.; White, A. J. P.;
Williams, D. J. Chem. Commun. 2000, 1207. (f) Sessler, J. L.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by CSIR, India (Project No. 01(2449)/
10/EMR-II). R.S. and B.S.K. thank CSIR for fellowships, and
R.S. also thanks DRDO, India, for a research assistantship. We
thank the Center for Modelling Simulation and Design (CMSD),
University of Hyderabad, for providing computational facility.
We thank the anonymous reviewers for their suggestions, which
helped us to greatly improve the quality of our manuscript.
́
Anzenbacher, P., Jr.; Shriver, J. A.; Jursíkoυa, K.; Lynch, V. M.;
Marquez, M. J. Am. Chem. Soc. 2000, 122, 12061. (g) Cafeo, G.; Kohnke,
H.; La Torre, G. L.; Parisi, M. F.; Nascone, R. P.; White, A. J. P.;
Williams, D. J. Chem. - Eur. J. 2002, 8, 3148. (h) Mahanta, S. P.; Kumar,
B. S.; Baskaran, S.; Sivasankar, C.; Panda, P. K. Org. Lett. 2012, 14, 548.
(i) Chandra, B.; Mahanta, S. P.; Pati, N. N.; Baskaran, S.; Kanaparthi, R.
K.; Sivasankar, C.; Panda, P. K. Org. Lett. 2013, 15, 306. (j) Saha, I.; Lee,
J. T.; Lee, C. H. Eur. J. Org. Chem. 2015, 3859.
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