Calix[4]arene-based ditopic receptors for simultaneous recognition of fluoride and cobalt(II) ions

Article abstract of DOI:10.1016/j.tetlet.2012.02.083

A series of calix[4]arene based ditopic receptors possessing bipyridyl and hydrazone units have been synthesized and evaluated for ionic recognition. It has been observed that the synthesized derivatives function as allosteric receptors for simultaneous r

Full text of DOI:10.1016/j.tetlet.2012.02.083

Tetrahedron Letters 53 (2012) 2244–2247
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Calix[4]arene-based ditopic receptors for simultaneous recognition
of fluoride and cobalt(II) ions
⇑
Har Mohindra Chawla , Satya Narayan Sahu, Rahul Shrivastava, Satish Kumar
Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of calix[4]arene based ditopic receptors possessing bipyridyl and hydrazone units have been syn-
thesized and evaluated for ionic recognition. It has been observed that the synthesized derivatives func-
tion as allosteric receptors for simultaneous recognition of Co2+and FÀ ions through non-covalent
interactions. Significant bathochromic shifts in the UV–visible spectrum with a profound colour change
promise their use to engineer novel applications.
Received 30 October 2011
Revised 13 February 2012
Accepted 19 February 2012
Available online 23 February 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Calix[4]arene
Cobalt(II) ion
Ditopic receptor
Fluoride ion
Bipyridyl receptors
Introduction
are known to form complexes with anions.7 A combination of
bipyridyl and hydrazone units in a suitable macrocycle scaffold
The design and synthesis of ditopic molecular receptors for
simultaneous sensing of cationic and anionic species constitute
important challenges in supramolecular chemistry. Such receptors
find inherent applications in salt solubilization, ionic extraction,
transport phenomenon and the development of molecular de-
vices.1 A ditopic molecular host exhibits cooperative binding of
cationic and anionic species through allosteric effects wherein
binding of one ion markedly influences the binding of the other
ion.2 Amongst such receptors for cations, the ones having capabil-
ity for the recognition of transition metal ions have special fascina-
tion since they are toxic when present in excess but are essential in
small amounts (such as iron, cobalt, copper and zinc) for the living
systems.1a,3 Similarly, amongst anions, the fluoride ion evinces ma-
jor interest due to its ubiquitous detrimental and beneficial role in
human health and the environment.4
Amongst several approaches adopted for the development of
ditopic receptors, the most important one consists of combining
the crown ethers (to bind cations) and functions containing urea,
thiourea and amide linkages (for the recognition of anions through
N–H–X hydrogen bonds).5 Since the electronic charge on anions is
more polarized than cations, one would expect that the cation and
anion binding would be sequential in nature.
would therefore be expected to yield ditopic receptors for the rec-
ognition of both cations and anions simultaneously. In this context
we stipulated that the calix[n]arene scaffold8 would offer a
suitable molecular architecture for the development of ditopic
receptors1a,9 since they can be functionalized to incorporate appro-
priate and complementary binding sites with well defined confor-
mational characteristics to facilitate interaction with targeted
guest species.
In this Letter, we report the synthesis and evaluation of a series
of novel calix[4]arene based ditopic receptors (4a–d) for the simul-
taneous binding of FÀ and Co2+ ions with a significant bathochro-
mic shift and distinct ‘naked eye’ colour change. To the best of
our knowledge, this is the first report on the utilisation of bipyridyl
and hydrazone bearing calixarene scaffolds for target applications.
Calix[4]arene 1 was obtained by utilizing the synthetic proto-
cols available in the literature10 while 2a and 2b were synthesized
by following the synthetic route depicted in Scheme S1 (Supple-
mentary data). Compounds 2a and 2b were separately refluxed
with calix[4]arene (1) in the presence of K2CO3 in CH3CN for
48–96 h to give 3a–b in good yields (86%). Reaction of 3a and 3b
with phenylhydrazines11 gave 4a–d in 88–92% yields (Scheme 1).
The receptors 4a–d gave characteristic spectroscopic analysis
for their indicated structures. For instance 4a gave a >C@N– signal
at 1605 cmÀ1 and a sharp pair of doublets at d 3.52 and d 4.24 for
axial and equatorial protons respectively in the 1H NMR spectrum
and a distinct signal at d 30.6 for the methylene carbons in its 13C
NMR spectrum to indicate a symmetric cone conformation for the
calix[4]arene scaffold.11,12 The structure of 4a was indicated by the
Bipyridyl compounds are known to form strong complexes with
transition metal ions under specific pH conditions6 and hydrazones
⇑
Corresponding author. Tel.: +91 11 26591517; fax: +91 11 26591502.
E-mail addresses: hmchawla@chemistry.iitd.ac.in, hmchawla@chemistry.iitd.
ernet.in (H.M. Chawla).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.02.083

H. M. Chawla et al. / Tetrahedron Letters 53 (2012) 2244–2247
2245
O
O
O
C
X
N
N
N
N
OH
1
OH
OH
HO
(a)
HO
HO
H2C
O
CH2Br
X
N
N
C
O
O
O
2a x=H
2b x=CH3
3a-b
(b)
Anion
binding
site
C
X
O
Cation
binding
site
Y
N
O
N
H
O
O
C
X
O2N
O2N
Comd.No
X
Y
N
N
N
4a
4b
4c
4d
H
NO2
NO2
HO
HO
CH3
H
CH3
H
H
N
H
N
X
C
O
N
Y
4a-d
Scheme 1. Synthesis of compound 4a–d. Reagents and conditions: (a) K2CO3, CH3CN, reflux, 16 h; (b) phenylhydrazine derivatives, ethanol.
appearance of two singlets for the CH2-bpy and OCH2-bpy at d 5.13
and d 5.29, two triplets at d 6.62 and d 6.85 and two doublets at d
7.01 and d 7.19 for aromatic protons of calix[4]arene skeleton. It
was further confirmed by observing D2O exchangeable singlets at
d 8.52 and d 11.3 which could be attributed to of –NH and –OH pro-
(Fig. 1a). For example, when 2 Â 10À5 M solution of 4a was treated
with tetrabutylammonium fluoride, it exhibited a significant bath-
ochromic shift (Dkmax) of 95 nm in its UV–visible spectrum with
the appearance of a new peak at 483 nm while its absorption
maxima at 388 nm remained unchanged with other anionic spe-
cies (ClÀ, BrÀ, IÀ, HSO4À, ClO4À and PFÀ6 ) as shown in Figure 1a. It
was observed that on gradual addition of a standard solution of tet-
rabutylammonium fluoride, the intensity of absorption peak at
388 nm progressively decreased while the intensity of the absorp-
tion peak at 483 nm increased simultaneously (Fig. 1b). However
the interaction of 4a with fluoride ions did not affect the absorp-
tion peak at 287 nm at all indicating that the FÀ ions possibly bind
at the hydrazone NH binding sites through hydrogen bonds as
observed earlier.10d
tons, respectively.
A non deutratable singlet at d 7.59 for
azo-methine proton (–N@CH) confirmed the structure for 4a as de-
picted in Scheme 1.
The cation and anion binding characteristics of receptors 4a–d
were examined by UV–visible spectroscopy in DMSO:CH3CN
(0.5:9.5 v/v). It showed two absorption peaks at 287 and 388 nm
that could be attributed to the absorption maxima for the bipyri-
dyl8 and hydrazone10 units present.
The anion binding ability of 4a–d was examined for various
anions byÀusing tetrabutylammonium salts of FÀ, ClÀ, BrÀ, IÀ,
HSOÀ4 , ClO4 and PF6À in DMSO:CH3CN (0.5:9.5 v/v). It was deter-
mined that the receptor 4a very selectively recognises the fluoride
ion from a mixture of various commonly encountered anions
The binding of 4a–d with fluoride ions was found to be
associated with visual colour changes which could be easily mon-
itored through a ‘naked-eye’ as depicted in Figure 2. It was found
that 4a exhibited a prominent and instantaneous colour change
Figure 1. (a) Absorption spectra of 4a (2 Â 10À5 M) upon addition of FÀ, ClÀ, BrÀ, IÀ, HSOÀ4 , ClOÀ4 and PF6À ions (as tetrabutylammonium salts) (1 Â 10À4 M) in DMSO/CH3CN
(0.5:9.5 v/v) (b) UV–visible titrations of 6a with 0–8 equiv of FÀ.

2246
H. M. Chawla et al. / Tetrahedron Letters 53 (2012) 2244–2247
Table 1
Association constants from UV–vis titrations for complexes of receptors 4a–d with
cationic guests in DMSO:CH3CN (0.5:9.5 v/v)a
Receptor
Cations log K (MÀ1
)
Co2+
Cu2+
Ag+
4a
4b
4c
4d
6.89
6.73
6.79
6.68
6.23
6.02
6.19
6.00
5.19
4.89
5.07
4.79
A
B
C
D
F
G
H
E
Figure 2. Selectivity of 4a for fluoride ion over other anions. Colour changes of 6a
M) in DMSO/CH3CN (0.5:9.5 v/v) with the addition of TBA anions (5 Â 10À5).
The Co2+ and Cu2+ were added in the form of their perchlorate salts while Ag+
were added in form of its nitrate.
a
(20
l
A = free ligand, B = FÀ, C = ClÀ, D = BrÀ, E = IÀ, F = HSOÀ4 , G = PF6À, H = ClOÀ4
.
from light yellow to dark purple only with fluoride ion in DMSO/
CH3CN (0.5:9.5 v/v), while no such colour change was observed
with other anions (such as ClÀ, BrÀ, IÀ, HSOÀ4 , ClO4À and PFÀ6 ) when
used in the form of their tetrabutylammonium salts under similar
experimental conditions. It was interesting to note that the addi-
tion of protic solvents (such as water or methanol) triggered the
disappearance of the observed colour of 4a–FÀ complex with the
immediate regeneration of the original colour of the receptor. This
suggested that FÀ-calix[4]arene interaction did not involve any
plausible covalent bond formation and that the complexation of
the fluoride ion with 4a was reversible in nature.
When solutions of 4a–d in DMSO:CH3CN (0.5:9.5 v/v) were
individually treated with perchlorate and nitrate salts of transition
and alkali metal ions (Co2+, Cu2+, Ag+, Fe3+, Ni2+, Na+, K+), only the
cobalt ion(in the form of its perchlorate at 2 Â 10À5 M concentra-
tion) exhibited a significant bathochromic shift of 21 nm at kmax
287 nm with the appearance of a new absorption at 308 nm with
no change in the absorption peak at 388 nm as depicted in Figure 3;
this observation clearly indicated the involvement of bipyridyl
binding sites with the minimum perturbation of the absorption
peak at 388 nm.
Evaluation of interaction between metal ion and the synthe-
sized molecular receptors through Job’s plot revealed maximum
complexation at 0.5 mol fraction of Co2+ions indicating a 1:1 bind-
ing-stoichiometry. Similar behaviour was observed on the addition
of standard solutions of Cu2+ and Ag+ ions. The association con-
stants (logK)13 of Co2+, Cu2+ and Ag+ ions upon complexation with
receptors 4a–d were calculated from the UV–visible titration
experiments depicted in Table 1.
Since 4a has been synthesized in such a way that it has two
distinct binding sites that comprise of bipyridyl and the phenyl
hydrazone units, it seems apparent that bipyridyl units interact
with transition metal ions while phenyl hydrazone units strongly
bind the FÀ ions.
for cations. When the cation is bound with one of the sites, the
other sites become available for binding anions. When both the
sites have appropriate guest ions, the receptors become fully occu-
pied. In the above context, therefore it was interesting to examine
the behaviour of competing FÀ ions and Co2+ ions on the binding
ability of 4a. Accordingly, the binding ability of receptor 4a for
FÀ ions in the presence of Co2+ was examined by UV–visible spec-
troscopy in DMSO:CH3CN (0.5:9.5 v/v). When Co2+-4a (1:1) adduct
with absorption maxima at 308 nm and 388 nm was treated with
(Bu)4NF a significant bathochromic shift (Dkmax = 95 nm) of the
absorption at 388 nm was observed with the appearance of a
new absorption peak at 483 nm (Fig. 4). This fluoride binding
was observed to be accompanied by a prominent ‘naked eye’
colour change from light yellow to dark purple. Similarlry when
4a–FÀ (1:1) complex with absorption maxima at 287 and 483 nm
was treated with Co(ClO4)2 solution, kmax at 287 nm shifted to
308 nm. Appearance of similar UV–visible spectral change and col-
orimetric behaviour indicates that 4a simultaneously and quanti-
tatively recognises Co2+ and FÀ ions at appropriate binding sites.
When experiments were repeated with other synthesized
receptors (4b, 4c and 4d), their kmax values were found to exhibit
similar bathochromic shifts when same concentrations of Co2+and
FÀ ions were added to their solution (Table S1, Supplementary
data). However it was determined that the cation binding sites
on the ditopic receptors were prone to interference with silver
and copper ions. For example, under identical conditions, when
one equivalent of Ag+ was treated with 20
lM solution of 4a in
DMSO:CH3CN (0.5:9.5 v/v), the absorption band at 287 nm was
shifted to 307 nm, while no shift in the absorption peak at
388 nm (fluoride binding sites) was observed. Gradual addition
of fluoride ions to 1:1 molar ratio of 4a–Ag+ complex solution re-
sulted in the displacement of silver ions which caused the disap-
pearance of the absorption peak at 307 nm and the regeneration
of the original absorption peak at 287 nm but no shift in the
The molecular receptor 4a seems to behave like enzyme mimic
with allosteric sites for anions and cations. For example, when one
binding site is blocked by an anion the other site becomes available
Figure 3. UV–visible titration curve of 4a (2 Â 10À5 M) upon addition of cobalt
Figure 4. Titration curve of compound 6a. Co(II) (1:1) upon addition of tetrabu-
tylammonium fluoride (0–100 equiv) in DMSO/CH3CN (1:9 v/v).
perchlorate (0–100 equiv) in DMSO/CH3CN (0.5:9.5 v/v).

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absorption peak at 388 nm until the addition of more than one
equivalent of fluoride ion. Further addition of tetrabutylammo-
nium fluoride exhibited a bathochromic shift of maximum at
388 nm to 483 nm with a distinct ‘naked eye’ colour change from
light yellow to dark purple.
The above observations clearly indicate that the addition of the
first equivalent of fluoride ion may lead to the sequestering of the
metal ion to form a stable ion pair with fluoride ion in solution
which effectively inhibits the anion binding with the receptor
molecule as reported earlier.14 However, when the sequestering
of metal ion gets completed, further addition of fluoride leads to
the enhancement in intensity of the 483 nm peak plausibly due
to the complexation of fluoride ion as expected. Similar results
were obtained when titration of 4a was performed with fluoride
ion in the presence of one equivalent of Cu2+ ion.
In conclusion, we have synthesized and characterized novel
ditopic calix[4]arene based receptors 4a–d that possess both
hydrazone and bipyridyl functions at the lower rim. The synthe-
sized molecular receptors have been determined to recognize
Co2+ and fluoride ions simultaneously. They exhibit a substantial
bathochromic shift to elicit a distinct ‘naked eye’ colour change
for fluoride ions. However, it was determined that Ag+ and Cu2+
ions interfere in the binding of fluoride ion with 4a.
Acknowledgments
The authors gratefully acknowledge the assistance received
from Sophisticated Analytical Instrument Facility, CDRI, Lucknow
for FAB-MS spectra, DST, DBT, MOEF, MFPI and MRD, Govt. of India
for financial assistance and the Council of Scientific and Industrial
Research, India for a senior research fellowship to R. S and S. N. S.
Supplementary data
Supplementary data (synthetic procedures, characterisation
data) associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2012.02.083.
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