Anion-Dependent Binding-Mode Changes in meso-(5,6-Dichlorobenzimidazole) Picket Calix[4]pyrrole

Article abstract of DOI:10.1002/ejoc.201700733

A deep-cavity calix[4]pyrrole anion receptor bearing 5,6-dichlorobenzimidazole subunits at diametrically opposed meso positions was synthesized and its anion-binding properties were investigated. The synthesized receptor showed higher affinity for fluoride anions than for chloride anions with a high discrimination factor. Significant differences in the solid-state geometries between the fluoride and chloride complexes were observed.

Full text of DOI:10.1002/ejoc.201700733

A Journal of
Accepted Article
Title: Anion Dependent Binding Mode Changes in meso-(5,6-
Dichlorobenzimidazole)-picket Calix[4]pyrrole
Authors: Chang-Hee Lee, Endale Mulugeta, Ranjan Dutta, Qing He,
Jonathan Sessler, and Vince Lynch
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700733
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10.1002/ejoc.201700733
European Journal of Organic Chemistry
COMMUNICATION
Anion Dependent Binding Mode Changes in meso-(5,6-
Dichlorobenzimidazole)-picket Calix[4]pyrrole
Endale Mulugeta,[a] Ranjan Dutta, [a] Qing He, [b] Vince Lynch, [b] Jonathan Sessler[b] and Chang-Hee
Lee*[a]
Dedication ((optional))
Abstract: A deep-cavity calix[4]pyrrole anion receptor bearing 5,6-
dichlorobenzimidazole subunits at the diametrically opposed meso-
positions has been synthesized and the anion binding properties are
described. The synthesized receptor shows a higher affinity for the
fluoride anion over the chloride anion with high discrimination factor.
Significant differences in the solid state geometries between fluoride
complex and chloride complex are observed.
Most recently, Anzenbacher et al. reported a calix[4]pyrrole
bearing two pyrrole moieties at the meso-positions and showed
that the system exhibited a high inherent affinity for the fluoride
anion.[15] In this system, the diametrically positioned pyrrole
moieties do not act as hydrogen bond donors. Instead, the pyrrole
moieties serve to stabilize the bound anion via anion-π
interactions. The cooperative six-point hydrogen bonding with
bound anion is not favorable due to limited flexibility of the
macrocycle. With these regards, we designed a novel deep cavity
model of calix[4]pyrrole 2 which can possibly display superior
binding affinity with anions. The designed receptor contains the
5,6-dichlorobenzimidazolyl group at the diametrically meso-
positions. The anion recognition affinity of the designed receptor
2 is expected to be superior due to the presence of the electron
deficient benzimidazolyl moieties resulting stronger anion-π
interaction. The anion selectivity also can be enhanced. In
addition, designed receptor 2 could display size-dependent
binding mode changes.
The design and synthesis of molecular receptors that can
selectively bind targeted guest molecules remains an ongoing
challenge within the molecular recognition community. Anions
play crucial roles in various biological systems and the
development of artificial anion receptors has attracted significant
attention in recent years.[1] Various synthetic constructs
containing Lewis acidic moieties,[2] metal coordination sites,[3]
hydrogen bond donors,[4] anion-π interactions,[5] electrostatic
interactions,[6] and halogen bonding sites[7] have been designed
and studied in an effort to enhance affinity and selectivity toward
targeted anions. The hydrogen-bonding donor-based neutral
receptors are the most widely studied systems and receptors
based on urea/thiourea,[8] imidazole,[9a] triazole,[9b] or amide[10]
motifs are well-known. We and others[11] have focused on the
anion binding properties of the tetrapyrrolic macrocycles, so
called, calix[4]pyrroles. These non-aromatic tetrapyrrolic
macrocycles provide up to four NH hydrogen bond donors in
recognizing anions. Early studies of octamethylcalix[4]pyrrole
1,[11a, 12] revealed that these systems could function as versatile
hosts for anions, halide anions in particular. In recent years, a
Figure 1 Calix[4]pyrroles 1 and 2
variety of topologically non-planar receptors based on
a
calix[4]pyrrole core have been reported, and most of them show
enhanced binding affinity and selectivity compared with the parent
system.[13] In these context, the calix[4]pyrrole having different
substituents either at the β-pyrrolic positions or at the meso-
positions have been reported.[14] In spite of the large number of
calix[4]pyrrole derivatives known to date, systems bearing
ancillary hydrogen bond donors in the meso-substituents are rare.
The
octamethylcalix[4]pyrrole
1
adopts
1,3-alternate
conformation as free host. It undergoes conformational change
upon complexation with anion in organic media. The adaptation
of the conformation so as to maximize hydrogen bonding
interactions. We envisaged that the introduction of electron
deficient aromatic system containing additional hydrogen bonding
donors at the meso-positions would serve to enhance the binding
affinity and selectivity through anion-π interaction and ancillary
hydrogen bond. The deep cavity walled binding domain may play
as a size discriminating motif. To explore this possibility, we
incorporate two 5,6-dichlorobenzimidazolyl groups into the meso-
position of the calix[4]pyrrole framework. This so-called,
‘benzimidazole- picket’ calix[4]pyrrole 2 is expected to provide
additional hydrogen bond donors. In addition, the electron
deficient aromatic system can interact through anion-π interaction
in a spatial orientation suitable for anion recognition. The
synthesis of receptor 2 is outlined in scheme 1. Briefly, The meso-
[a]
[b]
E. Mulugeta, R. Dutta, Professor Chang-Hee Lee
Department Chemistry
Kangwon National University (http://www.kangwon.ac.kr)
Chun Cheon 24341, Korea
E-mail: chhlee@kangwon.ac.kr
Q. He, V. Lynch, Professor J. L. Sessler
Department Chemistry
University of Texas at Austin
Austin, TX 78712, USA
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
[2-(5,6-dichlorobenzimidazoly)-methyl]dipyrromethane
synthesized in 23% yield via acid catalysed condensation of 2-
4
was
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10.1002/ejoc.201700733
European Journal of Organic Chemistry
COMMUNICATION
acetyl-5,6-dichlorobenzimidazole with freshly distilled pyrrole.
Subsequently, compound 4 was condensed with acetone in the
presence of acid catalyst, to afford two isomeric products 2 and 3
in 4% and 2% yield, respectively. These two isomeric products
are readily separated by simple column chromatography on silica
gel and can be easily identified using their 1H NMR spectroscopic
data. The two distinctive signals corresponding to the pyrrole NHs
and benzimidazole NHs appear at δ = 9.69 and 12.1 ppm,
respectively, whereas the signals of the β-pyrrole-Hs and meso-
methyl protons appear at δ = 5.76-5.66 and δ = 2.01, 1.60, and
1.51 ppm, respectively in DMSO-d6. Appearance of two sets of β-
pyrrolic protons and three distinctive signals of meso-methyl
protons clearly suggests the cis-isomer. Only two meso-methyl
signals is observed in the case of the trans-isomer 3, on the other
hand (Figure S11, S13).
protons is observed which is a typical code of behaviour in the
anion-bound complex of calix[4]pyrrole. The fact that the signals
of both pyrrole NHs and imidazole NHs shifted significantly to the
down field, clearly indicate the participation of imidazole NHs in
the formation of host-guest complex resulting
a six-point
hydrogen bonding complex. This results also suggest a minimal
interaction between bound anion and the benzimidazole π-
system.
Figure 2. Partial 1H NMR spectral changes during the titration of receptor 2
(10.38 mM) with TBAF in acetone-d6. (i) Free host 2, (ii) 2 + 0.15 equiv. F–, (iii)
2 + 0.3 equiv. F–, (iv) 2 + 0.45 equiv. F–, (v) 2 + 0.6 equiv. F–, (vi) 2 + 0.75 equiv.
F–, (vii) 2 + 1 equiv. F–, (viii) 2 + 1.5 equiv. F–, (ix) 2 + 2.0 equiv. F–.
On the other hand, different spectral pattern is observed when
receptor 2 is titrated with the chloride anion (Figure S22).
Incremental addition of chloride anion resulted in gradual
downfield shift of both the benzimidazole NHs and pyrrollic NHs.
The chemical shift changes in the 1H NMR spectrum are
consistent with fast complexation/decomplexation kinetics.
Saturation was observed after addition of one equiv. of chloride
anion. Compared with what was observed in the titration of
receptor 2 with fluoride anion, the chloride anion induced relatively
smaller down-field shifts in both sets of signals, a finding that was
taken as evidence of a weaker anion-receptor interaction. In
contrast to what was seen in the case of fluoride and chloride
anion titration, all the proton resonances of receptor 2 remained
largely unperturbed upon titration with TBAI, while addition of the
bromide anion induces small yet detectable changes (Figures
S24 and S25). Incremental addition of bromide anion to the
solution of receptor 2 resulted in a gradual downfield shift of both
the pyrrole NHs and benzimidazole NHs and showed saturation
upon addition of one equiv. of bromide anion. Since the
benzimidazole moieties are somewhat flexible in terms of both
rotation and breathing-like motions, geometrical adaptation
seems to be obvious depending on the size of the anion. It was
thus expected that receptor 2 would be able to accommodate
anionic guests larger than halide anions. Thus, 1H NMR spectral
titrations were performed with other anions such as BzO–, AcO–
Scheme 1 Synthesis of calix[4]pyrrole 2 and 3.
The binding property of the receptor 2 with various anions such
-
as F-, Cl-, Br-, I-, AcO-, BzO-, H2PO4 (as their tetrabutylammonium
1
(TBA) salts) was studied using standard H NMR spectroscopic
and isothermal titration calorimetry (ITC) techniques. In a typical
1H NMR titration, a solution of the anion in question dissolved in
acetone-d6 was titrated into a solution of host 2 in acetone-d6 at
25 ºC. The changes in the chemical shifts were recorded
accordingly. In the case of the fluoride anion, the incremental
addition of the anion to the solution of 2 led to the distinct
downfield shift of the benzimidazole NHs as well as pyrrole NHs.
Addition of calc. ~0.6 equiv. of fluoride anion resulted in signal
broadening of both NH proton resonances. However, addition of
up to ~1.0 equiv. of fluoride anion resulted in the appearance of
new sets of signal at 11.28 ppm and 14.32 ppm. The original
signals appeared at 8.94 ppm and 11.45 ppm were completely
disappeared (Figure 2). Low temperature 1H NMR titration carried
out at -30 ºC clearly indicate a slow complexation-decomplexation
kinetics where distinctive two sets of signals for both free host and
anion bound complex are observed upon addition of one equiv. of
fluoride anion (Figure S17). Furthermore, the aryl C-H protons in
the benzimidazole subunits (originally appearing as two distinct
signals at 7.62 and 7.77 ppm) merge together and resonate at
7.69 ppm in the gradual increment of addition of fluoride anion,
particularly after 0.75 equiv. A slight up-field shift of the β-pyrrolic
–
and H2PO4 (Figures S18, S20 and S23). Quantitative
determinations of the anion binding affinities in acetonitrile at 25oC
were determined using isothermal titration calorimetry (ITC)
(Figures S26-S31). Association constants and thermodynamic
parameters corresponding to the interaction between various
anions (as the TBA salts) and receptor 2 are summarized in Table
1.
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10.1002/ejoc.201700733
European Journal of Organic Chemistry
COMMUNICATION
Table 1. Binding affinities and thermodynamic parameters for the binding of
various anions (as their tetrabutylammonium salts) with host 2 measured by
isothermal titration calorimetry (ITC) at 298K in CH3CN.
cation-π interactions in both complexes. In the case of the fluoride
anion complex, the N--F– distances ranges between 2.775(7) and
2.805(5) Å, whereas the F•H-N angles varies from 151 to 170o. In
the case of the chloride complex, the N--Cl– distances ranges
from 3.28(3) to 3.32(3) Å. The benzimidazole units in the chloride
complex adopt a near perfect “face-to-face” orientation (C2V) in
solid state. On the other hand, the two benzimidazole moieties
adopt a tilted orientation (Ci) in the fluoride complex, which
presumably permits an angled hydrogen bonding interaction
between the bound F– and the benzimidazole N-H protons (Figure
5 and Figure S32). However, no evidences were found for the
participation of the benzimidazole NHs to the anion binding in the
case of chloride complex. This latter observation stands in
contract to what was seen in solution, where the signal of the
benzimidazole NHs undergoes downfield shifts upon chloride
anion binding. This results indicate that weak hydrogen bonding
interaction is operating in chloride binding in solution. The non-
linear hydrogen bonding interactions are well known when steric
or conformational restrictions limit the linear orientation of the
hydrogen bond donor and acceptor.[17] The careful analysis of the
3D-packing structures showed the difference in packing angle
with respect to the anions. As shown in the Figure 4, the zig-zag
type geometry was adopted in order to maximize the Coulombic
interaction. Due to the different size of anion and cone angle, the
packing angle between fluoride complex and chloride complex
was ranging from 138.63o to 144.76o respectively. The fact that
the distance between the centroids of two diametrical opposed
benzimidazole rings is 8.09 Å in the case of the F– complex while
8.40 Å in the Cl– complex indicates the tolerable nature of the
meso-aryl substituents depending on the size of anion. Based on
these differences, we conclude that receptor 2 is capable of
binding both the fluoride and chloride anions and adopts
geometrical adaptation depending on the size of the guest anion
resulting maximized binding interactions.
Anion
Ka
ΔG
ΔH
TΔS
(kcal/mol)
(kcal/mol)
(kcal/mol)
6
5
F-
Cl-
1.81x10±4.82x10
-8.55
-6.29
-4.10
-6.93
-7.67
-5.95
-3.39
-8.46
-2.19
-10.8
-10.8
-7.21
-5.16
-2.17
1.91
4
3
4.12x10±1.27x10
3
2
Br-
1.10x10±3.12x10
5
4
AcO-
BzO-
H2PO4
1.12x10±1.64x10
-3.87
-3.13
-1.26
5
4
4.29x10±6.51x10
-
4
3
2.29x10±1.90x10
The largest binding affinity was found with fluoride anion, which
benefits from a favourable entropic term. Relatively strong binding
was also seen in the case of the acetate (AcO−) and benzoate
(BzO−). Interestingly, similar binding constant (~106) is reported
for host 1 with fluoride anion (acetonitrile, ITC) wherein the ITC
measurement was carried out with tetraethylammonium fluoride
at 303 K.[16] In case of the meso-pyrrolyl calix[4]pyrrole, similar
order of binding constant is also reported for fluoride anion.[15]
However, the differences in binding affinities between fluoride and
other anions are much larger than those of reported receptors
indicating higher selectivity of current receptor 2.
Attempted crystal growth with TBAF or TBACl was not successful.
However, single crystals which were suitable for X-ray diffraction
analysis were obtained by slow evaporation of a solution of 2 in
mixed solvent system (acetonitrile/methanol/DCM) using
tetramethylammonium fluoride or tetramethylammonium chloride.
Figure 3. Crystal structures of the complex of 2 with F– (left) and Cl– (right). 2 is
presented in an ellipsoid model, while anions and TMA counter ions are
presented in space-filling representations. Other solvent molecules are omitted
for clarity.
Figure 4. Packing diagrams for the crystal structures of the fluoride (left) and
chloride (right) complexes. The cup-bound cation interacts with a neighbouring
cavity-bound anion forming zig-zag type ion-pair network. The tilt angles
between the two complexes are 138.63o for the fluoride and 144.76o for the
chloride complexes, respectively.
As shown in Figure 3, the receptor-shared, tightly bound ion pair
complexes are formed in both cases. The fluoride and chloride
anion reside within the cleft generated by the two benzimidazole
moieties and cone shaped, four point hydrogen bonding
interaction is obvious. The tetramethylammonium cation occupied
the concave π-cavity generated by four pyrroles possibly through
The extended array of the ion-pair complex likely contributes to
the overall stability of the system. Interactions between the
receptor, bound anion and cation results this zig-zag type array
that contains disordered solvent molecules (Figure 4). Again, this
difference is considered to reflect adaptation of the binding
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10.1002/ejoc.201700733
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COMMUNICATION
Synthesis of calix[4]pyrrole 2 : To a solution of compound 4 (1.0 g, 3.62
mmol) in acetone (400 mL) was added methanesulfonic acid (1.0 mL, 15.4
mmol). The mixture was then stirred for 24 hour at room temperature. The
reaction mixture was quenched via the addition of triethylamine (2.53 mL).
Excess acetone was removed under reduced pressure and the resulting
mixture was combined with water and extracted with CH2Cl2 (100 mL × 2).
The organic layer was dried over anhydrous Na2SO4 and the solvent was
removed in vacuo. Column chromatography on silica gel (ethyl
acetate/hexanes = 1/3) afforded 2 and 3. Yield for 2: 0.05 g (4%). 1H NMR
(DMSO-d6, 300 MHz) δ (ppm): 12.1 (s, 2H, benzimidazole NH), 9.69 (s,
4H, pyr NH), 7.80 (s, 2H, Ar-H), 7.63 (s, 2H, Ar-H), 5.76(t, J = 3Hz, 4H, β-
pyrrole-H), 5.66 (t, J = 3Hz, 4H, β-pyrrole-H), 2.01 (s, 6H, CH3), 1.60 (s,
6H, CH3), 1.51 (s, 6H, CH3); 13C NMR (100 MHz, CD3CN) δ (ppm): 161.1,
139.5, 134.2, 125.7, 104.8, 103.2, 41.3, 34.8, 28.1, 27.6, 24.9; MALDI-
TOF calcd. for C40H36Cl4N8 770.58., Found 771.08 (M+H)+.
geometry providing tight binding of fluoride anion relative to what
is seen in the corresponding chloride anion binding.
Figure 5. Geometry of the anion-receptor complexes. Fluoride-complex (left)
and Chloride-complex (right) clearly show the geometrical difference between
the two complexes.
Keywords: calix[4]pyrrole • benzimidazole • anion binding •
binding mode change • angled hydrogen bond.
In conclusion we have designed and synthesized a new deep
cavity anion receptor bearing 5,6-dichlorobenzimidazole units that
are attached to the diametrical meso-positions of the
calix[4]pyrrole framework. The new receptor 2 acts as efficient
receptor for small halide anions and carboxylate anions with high
selectivity. The receptor exhibits highest affinity toward fluoride
anion in organic media. Receptor 2 also displays a higher inherent
anion affinity than the parent system 1. Six-point hydrogen
bonding interactions derived from the four pyrrole N-Hs and the
two benzimidazole N-Hs are considered to underlie this higher
binding affinity. The non-linear hydrogen bonding interactions
between imidazole N-Hs and anion is obvious due to steric and
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can manifest subtle, but important, differences in their recognition
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Acknowledgements
The authors acknowledge support from the Basic Science
Research Program through the National Research Foundation of
Korea funded by the Ministry of Science, ICT & Future Planning
(2015R1A2A1A10052586). This study is supported by 2015
Research Grant from Kangwon National University (520150436).
The Central Laboratory at KNU is acknowledged for support. The
work in Austin was supported by the National Institutes of Health
(grant GM103790 to J.L.S.) and the Robert A. Welch Foundation.
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Experimental Section
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10.1002/ejoc.201700733
European Journal of Organic Chemistry
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A meso-benzimidazole-picket
Supramolecular Chemistry
calix[4]pyrrole have shown anion
dependent conformational switching
with a selectivity toward fluoride anion
Endale Mulugeta, Ranjan Dutta, Qing
He, Vince Lynch, Jonathan Sessler and
Chang-Hee Lee*
Page No. – Page No.
Title
Anion Dependent Binding Mode
Changes in meso-(5,6-
Dichlorobenzimidazole)-picket
Calix[4]pyrrole
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