An efficient size-selective anion binding cleft-shaped receptor: A novel [F2(H2O)3]2- cluster with pseudo-encapsulated F- ion

Article abstract of DOI:10.1039/c2ce06286k

A cleft-shaped receptor 1 was synthesized and its anion binding properties have been investigated. Receptor 1 can selectively recognize fluoride ion by naked-eye color change and UV-vis spectral changes in aqueous-acetonitrile solvent. The single crystal

Full text of DOI:10.1039/c2ce06286k

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Volume 14 | Number 5 | 7 March 2012 | Pages 1493–1886
COVER ARTICLE
Akhtarul Alam, Guchhait et al.
An efficient size-selective anion binding cleft-shaped receptor:
A novel [F2(H2O)3]2– cluster with pseudo-encapsulated F– ion

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COMMUNICATION
An efficient size-selective anion binding cleft-shaped receptor: A novel
[F2(H2O)3]2À cluster with pseudo-encapsulated FÀ ion†
a
b
c
a
Sasanka Dalapati, Md. Akhtarul Alam, Rajat Saha,‡ Sankar Jana and Nikhil Guchhait
a
*
*
Received 28th September 2011, Accepted 22nd November 2011
DOI: 10.1039/c2ce06286k
anion receptor contains both non-hydrated9 and hydrated fluoride
ions.8
A cleft-shaped receptor 1 was synthesized and its anion binding
properties have been investigated. Receptor 1 can selectively
recognize fluoride ion by naked-eye color change and UV-vis spec-
tral changes in aqueous-acetonitrile solvent. The single crystal X-ray
analysis of 1 with fluoride ion shows that there are two types of
fluoride ions in the unit cell, one of which is pseudoencapsulated
within the cleft-shaped cavity and the other type forms a dimer
through [F2(H2O)3]2À fluoride-water cluster.
The cleft-shaped receptor 1 was synthesized by reacting iso-
phthaloyl chloride with two equivalents of 4-nitro-o-phenylenedi-
amine in the presence of pyridine (Scheme 1).10 The compound 1 was
characterized by 1H and 13C NMR and IR spectra (ESI, Fig. S1 and
S2†). The anion sensing capability and binding affinity of 1 was
studied by absorption spectral changes in the presence of various
anions such as FÀ, H2PO4À, AcOÀ, ClÀ, BrÀ, IÀ, NO2À, NO3À, ClO4
À
,
À
À
HSO3 and HSO4 in aqueous-acetonitrile medium (CH3CN, H2O
The recognition and sensing of anions have attracted considerable
attention in the past decades because of the important roles played by
anions in biological, industrial, and environmental processes.1–3 In
particular, the selective sensing of fluoride ion has gained tremendous
attention due to its significance in clinical treatment for osteoporosis
and fluorosis.4,5 In this regard, the development of chemosensors for
fluoride ion detection in aqueous medium is one of the most chal-
lenging targets for anion recognition due to its small size, high elec-
tronegativity, and high hydration enthalpy.6 Thus, the fluoride ion
detection limit is restricted in organic solvent.7 However, during
crystallization fluoride ion combines with a water molecule and forms
a fluorine water cluster.8
and DMSO v/v 95 : 4 : 1).
Visual color change of receptor l (1.0 Â 10À5 M) was investigated in
aqueous-acetonitrile medium. Upon addition of two equivalents of
FÀ ion to the solution of 1, a detectable naked-eye color change was
observed from colorless to intense yellow color (Fig. S3†). However,
À
under similar experimental conditions, addition of H2PO4 and
AcOÀ ions exhibit only a faint yellow color. Other ions, such as ClÀ,
BrÀ, IÀ, NO2À, NO3À, ClO4À, HSO3À and HSO4À did not exhibit any
detectable naked-eye color change.
To get a better insight into the cause of colorimetric detection of
anions by receptor 1, we have investigated the UV–vis spectral
changes upon addition of different anions. Receptor 1 in the absence
of any anionic guest shows an absorption band at ꢀ278 (sh) nm and
ꢀ364 nm (Fig. 1). The higher energy band at ꢀ278 nm corresponds
to the p–p* transition of the aromatic system and the lower energy
broad band at ꢀ364 nm arises due to charge transfer processes
occurring in the entire molecular unit. However, upon adding
increasing amounts of FÀ ion to the aqueous-acetonitrile solution of
receptor 1, the peak at 364 nm gradually decreases in its intensity with
a gradual appearance of a new absorption band at 395 nm (Fig. S4,
ESI†). Interestingly, two distinct isosbestic points at 377 and 325 nm
were observed during the titration process indicating the existence of
a host–guest H-bonding complexation equilibrium between the bare
In this regard, here, we report a cleft-shape molecular receptor 1
having amide and amine binding sites with an electron withdrawing
nitro group, namely; signaling unit, can selectively detect fluoride ion
through the naked eye color change as well as UV-vis spectral
changes in aqueous-acetonitrile medium. Furthermore, the single
crystal X-ray analysis of 1 with fluoride ions shows that there are two
types of fluoride ion in the unit cell, one of which is pseudoencapsu-
lated within the cleft-shape cavity and the other fluoride ion takes part
in dimer formation through a [F2(H2O)3]2À fluoride-water cluster. To
the best of our knowledge this is the first example where a single
aDepartment of Chemistry, University of Calcutta, India. E-mail:
nguchhait@yahoo.com; Fax: +9133 23519755; Tel: +913323408386
bDepartment of Chemistry, Aliah University, Kolkata, India. E-mail:
alam_iitg@yahoo.com; Fax: +91 3327062124; Tel: +91 3327062125
cDepartment of Physics, Jadavpur University, Kolkata, 700 032, India
† Electronic supplementary information (ESI) available: Experimental
details, Synthesis, 1H, 13C NMR, Naked-eye color change, UV-vis
spectra ORTEP diagram and crystallographic tables. CCDC reference
number 842781. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2ce06286k
‡ Responsible for X-ray crystal analysis.
Scheme 1 Synthetic route of receptor 1.
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CrystEngComm, 2012, 14, 1527–1530 | 1527

DMSO-d6 solvent (Fig. 2). Upon addition of 0.5–2.0 equiv. FÀ ion,
the peaks at 9.87 and 6.59 ppm, which were assigned to the amide
(–CONH) and amine (–NH2) protons, respectively shifted to down-
field with spectral broadening, indicating the formation of a H-
bonding complex between 1 and FÀ ion. On further addition of FÀ
ion, the amide and amine proton peaks completely disappeared and
this might be due to deprotonation of the respective protons. Inter-
estingly, the aryl –CHb proton also exhibits a downfield shift.9 These
observed phenomena indicates a structural alteration of 1 that could
influence both the amide, amine and aryl –CHb protons due to
pseudo-encapsulation of the fluoride ion. However, in the case of
other anions no appreciable changes in the chemical shift were
observed, suggesting the non-interacting nature or very weak inter-
actions of the respective anions with receptor 1.
Although the basicity of the anions play an important role for
selective binding through strong hydrogen bonding interaction with
the receptor, however, in the present studies the size and structure of
the anion play the pivotal role. Receptor 1 displayed the strongest
binding affinity towards FÀ ion which might be due to its highest
electronegativity and the optimal shape complementarities between
spherical FÀ and cleft shaped receptor 1.
Fig. 1 UV-vis spectral changes of 1 in the presence of various anions in
aqueous-acetonitrile solvent.
receptor 1 and fluoride ion. The Benesi–Hildebrand (B–H) plot11 of 1/
[A À A0] vs. 1/[FÀ]2 for the titration of 1 and FÀ ion provides a straight
line (Fig S5†), indicating 1 : 2 complex formation with association
constant (K) 4.7 Â 108 MÀ2. The cause of these visual color changes
and UV-vis spectral changes might be due to the hydrogen bonding
interaction between the amide/amine proton and FÀ ion. The H-
bonding interaction of the amide/amine moiety caused a significant
increase in the charge density on the amide/amine nitrogen and an
internal charge transfer (ICT) occurred between the amide/amine
group (donor) to the electron withdrawing nitro group (acceptor),
resulting in bathochromic shifting in the absorption spectra.
To rationalize our experimental results obtained from colorimetric
test and spectroscopic (UV-vis and NMR) investigation, we have
successfully performed the crystal structure analysis of the 1-fluoride
complex, obtained from slow diffusion of diethyl ether into acetoni-
trile solution of a 1 : 2 mixture of 1 and Bu4NF. The compound was
crystallized in the space group P21/c (Table S1†) with the molecular
formula of [1-F2.(n-Bu4N)2$1.5H2O] (Fig. 3). The crystal structure
shows that two fluoride ions are in two different positions (Fig. S9†).
The F1 atom is situated at the centre of the two amide (–CONH–)
groups and two amine (–NH2) groups and are strongly hydrogen
bonded with four N atoms (distances F1/N2, F1/N3, F1/N7,
On the other hand, under similar experimental conditions, upon
adding increasing amounts of AcOÀ, H2PO4À and ClÀ ions, the peak
at 364 nm of receptor 1 is shifted gradually to the higher wavelengths
ꢀ
F1/N8 are 2.764, 2.780, 2.768, 2.752 A, respectively, Fig. 4 and
at 380, 370 and 366 nm, respectively (Fig. 1, S6–S8†). However, on
À
addition of other competitive anions such as BrÀ, IÀ, NO2À, NO3
,
Table S2†). Importantly the amide N–H protons are pointing
upwards with respect to the amine N–H and the amide C]O groups
are pointing downwards with respect to the mean molecular plane.
To avoid the steric repulsion between F1 and C–H hydrogen of the
phenyl ring (1,3-biscarboxamide), the C–H hydrogen is pointing
downwards with a 15ꢁ angle of deviation from the molecular plane
ClO4À, HSO3À and HSO4À, there are hardly any notable spectral or
color changes observed, indicating no interaction or complexation of
such anions with receptor 1.
To get further evidence for the H-bonding interaction between 1
1
and FÀ ion in solution, we have performed H NMR titrations in
ꢀ
and weakly H-bonded with the fluoride ion (distance 2.79 A and :
D–H–A 109ꢁ). It is noteworthy to mention that the fluoride anion
Fig. 3 Molecular structure of 1.FÀ complex. Tetrabutylammonium (n-
Bu4N+) cations were omitted for clarity.
Fig. 2 1H NMR spectral shift of 1 in the presence of FÀ ion in d6-DMSO.
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Fig. 6 Space-filling view of dimeric unit (left) and [F2(H2O)3]2À cluster
(right).
Fig. 4 H-bonding network within the dimeric unit of 1.FÀ complex, n-
Bu4N+ ions were omitted for clarity.
ion or it might be unable to form such stable crystals as with fluoride
ion.
In summary, a simple cleft-shaped anion receptor 1 containing
amide and amine groups as recognition units with a –NO2 signalling
group was synthesized using minimal synthetic procedures. Receptor
1 can selectively sense fluoride ion over other competitive anions by
naked-eye color change and UV-vis spectroscopic changes in
aqueous-acetonitrile medium. The crystal structure analysis shows
that receptor 1 binds with two fluoride ions and forms a stable 1 : 2
hydrogen bonded complex. Importantly, one of the fluoride ions is
pseudoencapsulated or isolated within the cleft-shaped cavity of
receptor 1 and the other fluoride ion forms a dimer through
[F2(H2O)3]2À fluoride-water cluster formation. The hydrated fluoride
selectivity of receptor 1 has potential application to the selective
removal of fluoride ion in its hydrated form in aqueous medium.
Presently we are exploring such applications.
(F1) is not fully encapsulated inside the cleft-shaped macrocyclic
ꢀ
cavity of 1, but it is placed slightly upwards about 1.01 A from the
mean plane passing through two amide ‘N’ atoms and two amine ‘N’
atoms consecutively, producing a distorted square pyramidal like
structure where the fluoride ion is at the apex of the pyramid (Fig. 5).
The F2 fluoride ion is hydrogen bonded with the N8 atom of the
ꢀ
amine group (distance F2/N8 is 2.895 A) and 1.5 units of the water
molecule (O1w and O2w distances F2/O1w and F2/O2w are 2.709
ꢀ
and 1.992 A, respectively, Fig. 4 and S10†). On the other hand, the
hydrogen atom of another amine group (N7 atom) is strongly
hydrogen bonded with the O1w atom of the water molecule (distance
ꢀ
O1w/N7 2.931 A, Fig. 4). Importantly, the water molecule in the
crystal might come from the hydrated tetrabutylammonium fluoride
salt or solvent or atmospheric moisture.
A lattice diagram shows that one molecular unit of 1 connected
with another molecular unit through [F2(H2O)3]2À fluoride-water
cluster and formed a dimer (Fig. 4 and 6). The overall crystal struc-
ture description summarized as, the four hydrogen atoms (two from
amide groups and two from amine groups) are capable of holding an
isolated fluoride ion and the remaining two extra hydrogen atoms
from the two amine groups are responsible for forming a dimer
through [F2(H2O)3]2À cluster formation. In other words the fluoride-
water cluster facilitates the formation of the dimer or vice versa. All
the hydrogen bonding interactions are summarized in Table S2.†
Finally we have tried to get crystals from other anions with
receptor 1 but after several attempts we failed to get single crystals
with other anions. This might be due to the very small cavity of
receptor 1 and hence, it can not accommodate other anions except FÀ
Acknowledgements
This work is supported by grants from DST, India (Project No. SR/
S1/PC/26/2008) to NG. SD and SJ would like to acknowledge UGC
for Fellowship. MAA thanks to the VC of AU for giving him
permission to work at CU.
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