New tripodal and dipodal colorimetric sensors for anions based on tris/bis-urea/thiourea moieties

Article abstract of DOI:10.1080/10610278.2011.593629

Seven neutral tripodal and dipodal receptors having mesitylene/ triethylbenzene as core moiety, urea/thiourea as binding groups and p-nitrobenzene as signalling unit have been reported. The receptors act as selective colorimetric, naked eye sensors for sm

Full text of DOI:10.1080/10610278.2011.593629

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Supramolecular Chemistry
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New tripodal and dipodal colorimetric sensors for
anions based on tris/bis-urea/thiourea moieties
Vimal K. Bhardwaj a , Sanyog Sharma a , Narinder Singh b , Maninder Singh Hundal a & Geeta
Hundal a
a Department of Chemistry, Centre for Advanced Studies in Chemistry, Guru Nanak Dev
University, Amritsar, 143005, Punjab, India
b Department of Chemistry, Indian Institute of Technology Ropar, Ropar, Punjab, India
Version of record first published: 28 Nov 2011.
To cite this article: Vimal K. Bhardwaj, Sanyog Sharma, Narinder Singh, Maninder Singh Hundal & Geeta Hundal (2011): New
tripodal and dipodal colorimetric sensors for anions based on tris/bis-urea/thiourea moieties, Supramolecular Chemistry,
23:12, 790-800
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Supramolecular Chemistry
Vol. 23, No. 12, December 2011, 790–800
New tripodal and dipodal colorimetric sensors for anions based on tris/bis-urea/thiourea moieties
Vimal K. Bhardwaja, Sanyog Sharmaa, Narinder Singhb, Maninder Singh Hundala and Geeta Hundala*
aDepartment of Chemistry, Centre for Advanced Studies in Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India;
bDepartment of Chemistry, Indian Institute of Technology Ropar, Ropar, Punjab, India
(Received 6 September 2010; final version received 26 April 2011)
Seven neutral tripodal and dipodal receptors having mesitylene/triethylbenzene as core moiety, urea/thiourea as binding
groups and p-nitrobenzene as signalling unit have been reported. The receptors act as selective colorimetric, naked eye
sensors for small and spherical F2 ion with some interference from tetrahedral H2PO42 ions. Thiourea derivatives form
stable 1:1 H-bonded complexes with F2 anion and to some extent with H2PO24 anions, whereas for urea derivatives, the
recognition is simply based on acid–base reaction between ureidic protons and basic F2 ions and is a completely reversible
phenomenon. The pre-organisation of thiourea derivatives coupled with their high-intrinsic acidity is supposed to help them
in the formation of strong H-bonded complexes with F2 anion. The urea-based receptors do not respond at lower
concentrations of the anion, but at higher concentrations, they undergo completely reversible deprotonation concomitant
with a colorimetric change, with the production of stable [HF2]2 anion.
Keywords: 1,3,5-trimethyl/triethyl benzene core; urea/thiourea binding units; H-bonding; deprotonation; anion sensing
1. Introduction
and pyridinium-based tripodal receptors have also been
reported (12a, b, d) which have used weak C–H· · ·X
interaction for anion binding. Among various types of
neutral chemosensors mentioned above, those based on
urea and thiourea give strong, directional H-bond donors
and can easily be accommodated in other pre-organised
scaffolds (14). There are many examples in which the
monopodal (15) and dipodal (16) urea/thiourea-based
receptors have been used for the recognition of different
anions, but very few reports on their tripodal (11, 12d, 17)
analogues are available, specifically on the neutral ones.
Steed and coworkers have reported N core-based neutral
tris-urea receptors (18) and benzene core-based tris-urea
pyridinium receptors (12d) sensing Cl2 ion. In an
extensive study of the geometric effects, Davis and co-
workers have reported ‘cholapods’ which are steroid-
based tris-urea/thiourea receptors (19).
Simple synthetic receptors capable of recognising anions
serve as important tools for detecting chemically,
biologically and environmentally important analytes (1,
2). Many systems reported are based on amide (1c, 3),
pyrrole (4), urea/thiourea (5), azo dyes (6), naphthalene/-
naphthalimide (7), porphyrin containing neutral hosts (8)
or polyammonium (9), guanidinium, amidinium and
thiouronium (10) containing cationic hosts. While
designing and synthesising anion binding hosts, pre-
organisation, charge and lability of the receptors are some
of the features to be taken into consideration (11). As for
the pre-organisation factor, binding of guests with the pre-
organised macrocyclic systems is relatively simple to
understand, but the binding processes of ‘conformationally
flexible’ podands remain intangible.
However the latter, by virtue of this property, becomes
more interesting for they may show anion-dependent
conformational behaviour, thus acquiring a conformation
which may best accommodate the guest in cognizance
with size and other electronic factors. There are many
examples for dipodal and tripodal hosts in the literature
which are based on substituted benzene as core and various
binding/reporting units to give neutral or cationic
receptors for optical anion sensing (6b, 12). Anslyn and
co-workers (13) have reported many neutral tripodal
receptors that are based on the principle of indicator
displacement assay. Imidazolium-, benzoimidazolium-
We herein report five tripodal and two dipodal
urea/thiourea-based neutral receptors (Scheme 1) with
nitrophenyl groups to enhance both hydrogen bond donor
tendency and acidity and to act as a colorimetric signalling
subunit. The aim of the study is to compare the binding
ability of the thiourea versus urea group as a part of the
tripodal and dipodal receptors. We have also tried to study
the effectiveness of the tripodal pseudo-cavity being
generated in the ‘three up’ conformation of the
hexasubstituted central benzene core (20) in solution to
hold ions of different geometry.
*Corresponding author. Email: geetahundal@yahoo.com
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2011.593629
http://www.tandfonline.com

Supramolecular Chemistry
791
O2N
X
R
N
H
N
H
S
NO2
H2N
R
S
R
S
HN
HN
~ 3 eq. XCN
NO2
R
S
S
X
S
R
CH2Cl2, Stirring at r. t.
NH2
NH
NH
R
NH2
X
(4a) R = –CH3
X = S
(4b) R = –C2H5
X = S
(1a) R = –CH3
(1b) R = –C2H5
(2a) X = S
(2b) X = O
(4c) R = –CH3
X = O
NO
2
(4d) R = –C2H5
X = O
H
H
S
S
~ 2 eq. XCN
CH2Cl2, Stirring at r. t.
NO2
NH
HN
X
S
S
X
NH
HN
NH2
H2N
(5a) X = S
(5b) X = O
(2a) X = S
(2b) X = O
(3)
NO2
NO2
NH2
S
H2N
S
~ 2 eq. CON
CH2Cl2, Stirring at r. t.
NO2
S
S
HN
HN
S
NH
NH2
S
O
NH
O
NH2
NO2
NO2
(1a)
(2b)
(4e)
Scheme 1.
2. Results and discussion
,1490–1590 cm21 and a band around ,3050–
3290 cm21 characterising the presence of NH groups.
The 1H NMR spectra of 4 and 5 show signals lying in the
range ,d 9.44–10.44 and ,d 8.41–10.08 ppm corre-
sponding to thioureidic and ureidic protons, respectively.
The appearance of only one signal for one kind of protons
indicates that the two/three podes in the receptors are
chemically equivalent which suggests an ‘all up’
conformation (20) for them. Alternatively, it may be due
Thiourea- and urea-based dipodal and tripodal receptors 4
and 5 were synthesised by reacting tripodal (21) 1 and
dipodal 3 amines with 4-nitrophenyl isothiocyanate or 4-
nitrophenyl isocyanate in dichloromethane. These were
characterised by elemental analyses, IR, 1H, 13C NMR and
UV–vis absorption spectroscopy. Their IR spectra show
the presence of CvO or CvS stretching bands around

792
V.K. Bhardwaj et al.
to very rapid flipping of one conformation over the other
due to rotation about CarylZCmethyl/ethyl bonds, a rotation
that has to be faster on the NMR scale. The available NMR
evidence really cannot decide between the two. However,
Anslyn and coworkers (20b) have already shown that
ababab kind of conformation in the case of 1,3,5-R-2,4,6-
R0 substituted arene-based receptors is thermodynamically
more stable than its next conformation. Therefore, there is
a very high probability of having a cis,cis,cis conformation
for these ligands, at least in the solution state. CHN data
are also in accordance with the molecular formulae.
spectrophotometric titrations we carried out. A gradual
decrease in the absorption of the band at 355 nm with a
simultaneous increase in the absorption at 410 nm was seen
(Figure 1, Figures S2 and S3 of the Supplementary
Information, available online) on increasing the concen-
tration of F2 ion solution for all three receptors. There is no
emergence of any other band at much higher concen-
trations of TBAF (even up to 50 £ 103 equivalents, Figure
S4 of the Supplementary Information, available online).
The spectral changes were used to calculate the
binding constants of the receptors. Fitting the changes in
UV–vis spectra of these receptors using SPECFIT
program (22) showed that for all the three receptors, best
fits were obtained considering species with 1:1 stoichi-
ometry. This suggests a stable, symmetrical 1:1 H-bonded,
endo-complex (5c) in which the fluoride anion is supposed
to reside inside the pseudo-cavity formed by the receptor.
The binding constants calculated for receptors 4 and 5 are
given in Table 2. The stoichiometry of these complexes
formed was also determined by Job’s plot (23) and was
found to be 1:1 (Figure S5 of the Supplementary
Information, available online). To verify the phenomenon
of recognition, a titration of 4b with tetrabutylammonium
hydroxide (TBAOH) was carried out in similar conditions.
As TBAOH is a strong base, it is bound to produce
deprotonation to the ligand. Indeed, the spectral changes
here took place in two steps: first, the appearance of a new
band at lmax 412 nm which has been attributed to the
formation of a strong H-bonded complex as with TBAF. It
is followed by the appearance of a second band at lmax
477 nm, at higher equivalents corresponding to the
deprotonation of the receptor (Figure 2). However, even
with TBAOH, the deprotonation emerges at a very high
concentration of the OH2 ion (.100 equivalents) and a
proper band starts forming only after the addition of 200
equivalents and ultimately attains saturation. Therefore, a
band at lmax 477 nm is considered to correspond to the
deprotonation of the receptor.
2.1 Anion binding studies of chromogenic receptors
Investigations of anion recognition and anion selective
behaviour of various receptors were carried out using UV–
vis spectroscopy. The spectral changes upon the addition
of various anions were determined in DMSO. The UV–vis
spectra of each of the receptors 4 and 5 show a strong band
at lmax ,355 nm, characterised as an intra-ligand charge
transfer band (ICT) which changes significantly upon
addition of a small amount of tetrabutylammonium
fluoride (TBAF) and in some cases, of tetrabutylammo-
nium dihydrogen phosphate.
2.1.1 Thiourea derivatives
There were no changes in the spectra of thiourea receptors
(4a, 4b, 5a) upon addition of tetrabutylammonium salts of
other anions such as Cl2, Br2, I2, NO32, CN2, ClO42,
AcO2 and HSO24 (Figure S1 of the Supplementary
Information, available online).
2.1.1.1 Binding with fluoride ions. On addition of F2 ion
into thiourea ligands, the absorption band at ,360 nm
disappears and a new band at , 417 nm appears with a red
shift of Dlmax ,57 nm (Table 1) owing to the formation of
a H-bonded complex. The complex formation on addition
of F2 ion solution can be visually perceived through a
colour change from pale to bright yellow. To learn more
about the binding properties of these receptors for F2,
The recognition of tripodal 4a, 4b and dipodal 5a with
1
F2 was evident in the H NMR titration experiment. The
results of NMR titrations of 4a with TBAF solution in
Table 1. Optical response of receptors 4 and 5 to fluoride and phosphate ions showing changes in absorption band.
Original band
Shifted or new band
Shifted or new band
l
max (ligand)
(log 1 M21cm21)(nm)
l
max (ligand þ F2)
lmax (ligand þ H2PO24 )
(log 1 M21cm21) (nm)
Dlmax for H2PO24
Ligands
(log 1 M21cm21)(nm)
Dlmax for F2
(log 1 M21cm21
)
4a
4b
4c
4d
4e
5a
5b
360
355
351
348
350
358
353
417
410
478
474
477
417
475
57
55
127
126
127
59
409
370
474
475
473
383
475
49
15a
123
127
123
25
122
122
a Very small intensity, seen only at high anion concentration.

Supramolecular Chemistry
793
Figure 1. Changes in absorbance spectra of 5a (10 mM) upon
addition of TBAF (0–100 mM) in DMSO.
Table 2. Values of stability constants.
4a
4b
5a
Log Ka1 for F2
Log Ka1 for H2PO24
4.14
3.55
4.65
5.13
3.416
a
a The change was too small to be significant.
Figure 3. Changes in NMR titrations of 4a (10 mM) with TBAF
(0–3.0 molar equivalents) in DMSO-d6.
anion (Figure S8 of the Supplementary Information,
available online). From the extent of the shifts and the
shapes of the spectra it is clear that the H-bonding
interactions with the H2PO24 ions are strongest for 4a
followed by 5a and 4b, in this order. From the formation
constants, we may deduce that for F2 ions, the acidity of
thiourea derivatives varies as 5a . 4b . 4a. With the less
basic dihydrogen phosphate anions, this order may be
formed as 4a . 5a . 4b. A comparison clearly shows that
the pre-organisation of the dipodal receptor is best suited
for the spherical fluoride ion, whereas the tripodal receptor
4a is more adapted to the tetrahedral dihydrogen
phosphate anion.
Figure 2. Changes in absorbance spectra of 4b (10 mM) upon
addition of TBAOH (0.1–14 £ 102 equivalents).
DMSO-d6 are shown in Figure 3. On addition of TBAF,
signals due to both thiouredic protons H1 and H2 become
broad and then disappear which may suggest their
deprotonation. In many cases, however, the strong H-
bonding also leads to the disappearance of the signals (5f,
24) which seems to be the case presently since the UV–vis
data (vide supra) have suggested H-bonding and not
deprotonation in the case of thiourea derivatives. 1H NMR
spectrum of 4b and 5a in DMSO-d6 (Figures S6 and S7 of
the Supplementary Information, available online) also
shows similar changes with the addition of F2 anion.
2.1.2 Urea derivatives
2.1.2.1 Binding with fluoride ion. The UV–vis spectro-
photometric titrations of receptors (4c–4e) with tetra-
butylammonium salts of various anions show chromogenic
response towards F2 ion (Figure S9 of the Supplementary
Information, available online). On addition of TBAF
(,1 M equivalent), the absorption band at ,351 nm of
receptors 4c (Figure S10 of the Supplementary Infor-
mation, available online), 4d (Figure 4), 4e (Figure S11 of
the Supplementary Information, available online) and 5b
(Figure S12 of the Supplementary Information, available
online) shows slight bathochromic shifts with a decrease in
2.1.1.2 Binding with dihydrogen phosphate ions. With
4a, 4b and 5a, H2PO24 ions show lesser bathochromic
shifts (Table 1) on increasing the concentration of the

794
V.K. Bhardwaj et al.
equivalents of the anion and there they undergo
deprotonation too fast to register any H-bonding
step. Again, the acid–base titration of 4d with TBAOH
was used to confirm it. The titration (Figure 5) shows the
emergence of a band at ,lmax 474 nm (Dl ,125 nm,
corresponding to deprotonation) with a simultaneous
decrease in the absorption of the band at lmax 348 nm and
the latter disappearing completely after addition of 100
equivalents of the anion. The reversibility of the binding
process observed in all the urea-based receptors further
confirms the deprotonation step being involved. For this
reason, at any given time, the accurate absorbance values
for any given concentration of the anion could not be
recorded; therefore, the formation constants of urea
derivatives could not be calculated.
Figure 4. Changes in absorbance spectra of 4d (10 mM) upon
addition of TBAF (0–1100 mM) in DMSO.
The 1H NMR titrations of the urea-based ligands offer
some interesting results that might help in explaining the
difference in behaviour between these urea/thiourea
receptors. The most significant difference between the
NMR spectra of thiourea and urea derivatives is that the
spectra of the latter show the signal of Hf proton between
the Hh and Hg protons. As a representative of the changes
in the NMR spectra on gradual addition of TBAF, figure 6
shows them for ligand 5b, which illustrates the presence of
Hf protons at d 7.928 ppm in between complex multiplets
for Hh (d 8.194) and Hg (d 7.672). This significant
downfield shift of Hf protons in comparison with their
counterparts in thiourea derivatives indicates the presence
of intramolecular H-bonding interactions between the Hf
protons and the oxygen of ZCvO group. Such intramo-
lecular H-bonding interactions have earlier been reported
in the simple monopodal urea-based receptors for F2 ions
(15a). On addition of 0.25 equivalents of TBAF in 4d, the
signals H1 and H2 from the ZNH protons (originally seen
at d 10.084 and 8.408 ppm) show a high-frequency shift
with Dd of 0.41 and 0.12 ppm, respectively. Subsequent
addition of F2 anion shows the disappearance of the ZNH
protons. It is well known that for the ZNH- containing
receptors, one of the reasons of the penchant for F2 ions is
the remarkable stability of HF22 ion (5c, 15a, 25, 26). The
presence of which may be clearly seen in the NMR spectra
of the representative compound 4d in the form of a telltale
triplet at ,d 16 ppm (Figure 7) taken on addition of 12
equivalents of the anion. Fluoride ion though is not a
specially strong base (pKa ¼ 15 in DMSO) (27), however,
the extreme stability of [HF2]2 is well documented (14a)
and it is known to behave as a very strong base, second to
OH2 only. The NMR titrations of 4c (Figure S13 of the
Supplementary Information, available online), 4d (Figure
S14 of the Supplementary Information, available online)
and 4e (Figure S15 of the Supplementary Information,
available online) with TBAF also show deprotonation in
the presence of F2 anion. The NH2 protons in 4e do not
participate in the H-bonding signifying that the anion
prefers to get encapsulated by the receptor instead of
Figure 5. Changes in absorbance spectra of 4d (10 mM) upon
addition of TBAOH (1–3 £ 103 equivalents) in DMSO.
its absorbance. From 1 M equivalent onwards, a new band
appears at 474 nm which further grows on increasing the
concentration of F2 ion (Table 1). These spectral changes
are accompanied by visual colour changes from colourless
to reddish yellow promising a naked eye detection of the
F2 ion. There are three important differences in the
behaviour of urea and thiourea derivatives here. First, the
thiourea derivatives start responding at lower molar
equivalents of the added anion solution and are saturated
at ,2–3 equivalents, whereas the urea derivatives initiate
significant response (after 1 equivalent) and are saturated
at relatively much higher molar equivalents of the added
anion (7–8 M equivalents). Second, the original band gets
bathochromically shifted and gradually decreases in
absorbance, but it does not disappear at any time and
sustains even at saturation which is achieved at 10 times
higher concentration of the anion. Finally, the spectral
changes are transient and go back to the original situation
shortly and the visual colour of the solution also reverts
back. Urea derivatives, therefore, are not very sensitive to
F2 ion at low concentrations due to their obviously low
acidity. They show any significant response at ,8–10

Supramolecular Chemistry
795
Figure 6. Changes in the NMR spectra of 5b on stepwise increase in the concentration of TBAF in DMSO-d6.
Figure 7. Appearance of a peak at d 16 ppm corresponding to [HF2]2 ion in 1H NMR on addition of 12 equivalents of TBAF to DMSO-
d6 solution of 4d.

796
V.K. Bhardwaj et al.
simply being H-bonded to a terminally attached potential
H-bond donor (ZNH2 group).
from the receptors at lower concentrations of the anion. At
higher concentrations, the receptors are simply deproto-
nated and a considerable concurrent change in the dipole
moment of the systems produces larger bathochromic shift
in the spectra. Again, the deprotonation is dependent on
the basicity of the anion, hence bathochromic shift is much
less for H2PO24 ion.
2.1.2.2 With dihydrogen phosphate ions. For dihydrogen
phosphate ions, only 4d shows some significant recognition
and the bathochromic shift of band starts with rather higher
equivalents, i.e. from 20 equivalents onwards (Figure S16
of the Supplementary Information, available online). At
higher concentrations of the anion, the band appears at
3. Conclusions
1
In this work, we have reported seven neutral tripodal and
dipodal receptors based on urea/thiourea, which have the
potential to act as colorimetric anion sensors. Thiourea
derivatives are proved to be very selective and sensitive
towards small and spherical F2 ion with some interference
from tetrahedral H2PO24 ions. Their recognition act
involves stable H-bonded complexes. Urea derivatives,
on the other hand, are also selective for F2 ions but have
very low sensitivity and work only at relatively higher
concentrations of the anion. Their sensing process is
simply based upon Lewis acid–base reaction which is
completely reversible with time. The difference in the
activity of the thiourea/urea receptors is highly dependent
on their intrinsic acidity and conformational adaptability
for F2 ions. The more acidic thiourea receptors are more
flexible and adapt well to form H-bonded complexes. Urea
derivatives, on the contrary, have to undergo significant
conformational changes and they loose their intramole-
cular (ZCvO· · ·Hf) H-bonding if they have to give a tight
size fit to small F2 ions that are not suitable to
encapsulating the anion effectively. Hence, their 1:1 H-
bonded complexes are not very stable. At higher
concentrations, however, these receptors undergo depro-
tonation concomitant with a colorimetric change which is
completely reversible. In general, the tripodal pseudo-
cavity being generated in the ‘three up’ conformation of
the hexasubstituted central benzene core in solution seems
more suitable to hold either spherical F2 ion or tetrahedral
dihydrogen phosphate anion over more basic ‘Y’-shaped
acetate ions, signifying the host–guest complementarity.
,474 nm, again signifying deprotonation. The H NMR
spectrum (Figure S17 of the Supplementary Information,
available online) also shows absence of ureidic protons.
The H-bonding tendencies of a given donor group (e.g.
the NZH fragment of urea/thiourea) depend upon its
protonic acidity. Thiourea is more acidic than urea
(pKa ¼ 21.1 and 26.9, respectively in DMSO) (27); thus,
it is expected that thiourea- containing receptors bind more
strongly with anions than their urea-based counterparts
through H-bond interactions. Apart from that, the basicity
and geometry of the anion, stability of HX22 anion formed,
pre-organisation of the receptor and polarity of the solvent
are also known to be important factors for deciding the
specific anion selectivity and sensitivity. In the present
case, the thiourea derivatives form stable H-bonded
complexes with anions such on H2PO24 and F2, which is
not strictly according to the basicity scale because acetate is
a stronger base than dihydrogenphosphate (28). It shows
that the pre-organisation effect of the tripodal and dipodal
structures of the receptors is effective in the recognition of
the tetrahedral H2PO24 and preferably the smaller spherical
F2 ions over other anions. The pre-organisation effect of
the tripodal structure has been earlier found to be effective
in reversing the selectivity pattern of these two oxoanions in
contrast to their basicity pattern (29). For the F2 ion,
flexibility of these podands seems to be good enough to
stabilise a particular conformer that is a paradigm of the
best compromise between the basicity and geometry and
size of the guest anion, with dipodal receptor 5a being much
better than the tripodal ones. Such adaptability of these
podands gives a very stable, symmetrical 1:1 H-bonded,
endo-complex (5c) in which the fluoride anion is supposed
to reside inside the pseudo-cavity formed by the receptor.
For the urea-based receptors, on the other hand, such a
conformation that can bring the widely separated binding
units to converge together is also induced only by the F2
ions or to a much lesser extent by the H2PO24 anion.
However, they are probably not capable of encapsulaing
the smaller F2 ion as efficiently as their thiourea
counterparts. The latter besides having bigger S atoms in
them face no risk of loosing stability on the account of
intramolecular (ArZH· · ·OvC) H-bonding interactions
on effectively encapsulating the anion and are thus more or
less pre-organised for binding with F2 ion. Thus, the urea
derivatives do not allow any significant colorimetric action
4. Experimental
4.1 General
All the commercially available chemicals were purchased
from Aldrich and used without further purification. All
solvents were dried by standard methods. Unless otherwise
specified, chemicals were purchased from commercial
suppliers and used without further purification. TLC was
carried out on glass sheets pre-coated with silica gel. The 1H
and 13C NMR spectra were carried out in CDCl3 and
DMSO-d6 with TMS as an internal reference, on a 300
and 400 MHz NMR spectrometer. The infrared spectrum
(KBr pellet) was recorded using PYE Unicam IR
spectrophotometer in the range 400–4000 cm21. The

Supramolecular Chemistry
797
electronic absorption spectra were recorded on a Shimadzu
Phramaspec UV-1700 UV–vis spectrophotometer. Com-
pounds 2a and 2b were commercially available. The
tripodalamines 1aand1bwereprepared asalready reported
by us (21).
11.31; S, 17.26%; IR (KBr, cm21) 1498 (CvS), 3065
(NH); dH (300 MHz, DMSO þ CDCl3, Me4Si) 1.20 (br,
ZCH3, 9H), 2.90 (br, ZCH2, 6H), 4.08 (s, ZCH2, 6H),
7.31 (br, Ar, 6H), 7.47 (br, Ar, 3H), 7.55–7.57 (br, Ar, 3H),
7.98–7.15 (m, Ar, 12H), 9.46 (s, ZNH, 3H), 10.44 (s,
ZNH, 3H); dC (75 MHz, DMSO, Me4Si) 15.01, 21.65,
32.26, 119.85, 122.82, 125.19, 126.12, 127.77, 128.74,
132.51, 135.75, 141.40, 142.04, 144.69, 144.00, 178.97.
4.2 Preparation of receptors
4.2.1 Dipodal amine (3)
Dipodal amine 3 was prepared by taking K2CO3 in dry
acetonitrile along with 2-aminothiophenol (246 mg,
2.2 mmol). The reaction mixture was refluxed for 20 min
and then dibromide (30) (306 mg, 1 mmol) was carefully
added to it. The reaction mixture was refluxed for the next
8 h and the progress of the reaction was monitored by TLC.
Upon completion of the reaction, K2CO3 was filtered off
and acetonitrile was evaporated. The crude product was
recrystallised from chloroform–methanol solvent mixture
to get a pure white product. Yield 58–60%. mp 1188C;
found: C, 69.91; H, 6.91; N, 6.81. Calcd for C23H26N2S2:
C, 70.01; H, 6.64; N, 7.10%; IR (KBr, cm21) 3427 (s),
1301 (s), 1020 (s); dH (300 MHz, CDCl3, Me4Si) 2.30 (s,
ZCH3, 6H), 2.33 (s, ZCH3, 3H), 3.99 (s, ZCH2, 4H), 4.36
(s, ZNH2, 4H), 6.62–6.67 (m, Ar, 4H), 6.83 (s, phenyl,
1H), 7.15 (t, Ar, 2H, J ¼ 9.3 Hz), 7.35 (d, Ar, 1H,
J ¼ 6.9Hz); dC (75 MHz, CDCl3, Me4Si) 15.17, 19.73,
34.99, 114.84, 118.32, 118.60, 129.93, 130.22, 132.08,
136.12, 136.24, 136.46, 148.54.
4.2.4 Tripodal ligand (4c)
Compound 4c was prepared by dissolving 1a (531 mg,
1.0 mmol) and 2b (525.18 mg, 3.2 mmol) in dry dichlor-
omethane separately. The two solutions were mixed and
the reaction mixture was stirred for 1 h at room
temperature. After completion of the reaction, solvent
was evaporated and product was recrystallised from
methanol. Pure product is solid and yellow. Yield 40%. mp
2678C; found: C, 59.49; H, 4.79; N, 12.86; S, 9.87. Calcd
for C51H45N9O9S3: C, 59.81; H, 4.43; N, 12.31; S, 9.39%;
IR (KBr, cm21) 1635 (CvO), 3500 (NH); dH (300 MHz,
DMSO þ CDCl3, Me4Si) 2.07 (s, ZCH3, 9H), 4.0 (s,
ZCH2, 6H), 7.05 (d, Ar, 3H, J ¼ 8.1 Hz), 7.36–7.39 (m,
Ar, 6H), 7.65 (d, Ar, 6H, J ¼ 9.3 Hz), 7.86 (d, Ar, 3H,
J ¼ 8.4 Hz), 7.18 (d, Ar, 6H, J ¼ 9 Hz), 8.35 (s, ZNH,
3H), 10.0 (s, ZNH, 3H); dC (75 MHz, DMSO, Me4Si)
15.65 (ZCH3), 34.85, 54.97, 117.47, 121.91, 124.12,
125.27, 128.15, 131.40, 132.37, 138.28, 141.15, 143.00,
146.40, 151.96, 187.17.
4.2.2 Tripodal ligand (4a)
4.2.5 Tripodal ligand (4d)
Compound 4a was prepared by dissolving 1a (21a)
(531 mg, 1.0 mmol) and 2a (577 mg, 3.2 mmol) in dry
dichloromethane separately. The two solutions were mixed
and the reaction mixture was stirred at room temperature.
After 30 min, yellow precipitates separated out. These
were filtered and dried under vacuum. Yield 74%. mp
134–1388C; found: C, 57.41; H, 4.45; N, 11.13; S, 17.99.
Calcd for C51H45N9O6S6: C, 57.12; H, 4.23; N, 11.76; S,
17.94%; IR (KBr, cm21) 1507 (CvS), 3216 (NH); dH
(300 MHz, DMSO þ CDCl3, Me4Si) 2.40 (s, ZCH3, 9H),
4.09 (s, ZCH2, 6H), 7.23 (br, Ar, 6H), 7.45–7.46 (br, Ar,
3H), 7.57 (br, Ar, 3H), 7.959 (d, Ar, 6H, J ¼ 9.0 Hz), 8.13
(d, Ar, 6H, J ¼ 9.3 Hz), 9.50 (s, ZNH, 3H), 10.42 (s,
ZNH, 3H); dC (75 MHz, DMSO, Me4Si) 15.76, 33.86,
54.97, 112.58, 121.78, 124.40, 126.54, 127.66, 128.88,
131.17, 134.70, 136.47, 137.22, 142.54, 146.20, 180.35.
Compound 4d was prepared by the same method as that of
4c except that (573 mg, 1.0 mmol) 1b was taken instead of
1a. Pure product is solid and light yellow. Yield 51%. mp
2108C; found: C, 60.21; H, 4.67; N, 11.27; S, 8.87. Calcd
for C54H51N9O9S3: C, 60.83; H, 4.82; N, 11.82; S, 9.02%;
IR (KBr, cm21) 1501 (CvO), 3290 (NH); dH (300 MHz,
DMSO þ CDCl3, Me4Si) 1.20 (br, ZCH3, 9H), 2.94 (br,
ZCH2, 6H), 4.07 (s, ZCH2, 6H), 7.13 (d, Ar, 3H,
J ¼ 7.2 Hz), 7.37 (d, Ar, 3H, J ¼ 6.9 Hz), 7.42–7.45 (m,
Ar, 3H), 7.66 (d, Ar, 6H, J ¼ 8.7 Hz), 7.92–7.95 (m, Ar,
3H), 7.95–8.11 (m, Ar, 6H), 8.30 (s, ZNH, 3H), 9.87 (s,
ZNH, 3H); dC (75 MHz, DMSO, Me4Si) 14.50, 21.43,
32.24, 115.80, 121.13, 122.82, 123.40, 125.37, 126.16,
128.84, 129.42, 136.04, 139.80, 141.68, 144.78, 150.7.
4.2.6 Tripodal ligand (4e)
4.2.3 Tripodal ligand (4b)
Compound 4e was prepared by dissolving 1a (531 mg,
1.0 mmol) and 2b (361.06 mg, 2.2 mmol) in dry dichlor-
omethane separately. The two solutions were mixed and
the reaction mixture was stirred at room temperature. After
30 min, the yellow precipitate separated. This was filtered
and dried under vacuum. Yield 68%. mp 2158C; found: C,
Compound 4b was prepared by the same method as that of
4a except that (573 mg, 1.0 mmol) 1b (21b) was taken
instead of 1a. Pure product is solid and light yellow. Yield
52%. mp 1258C; found: C, 58.78; H, 4.78; N, 10.97; S,
17.47. Calcd for C54H51N9O6S6: C, 58.20; H, 4.61; N,

798
V.K. Bhardwaj et al.
61.89; H, 5.09; N, 11.86; S, 10.87. Calcd for
C54H51N9O9S3: C, 61.45; H, 4.81; N, 11.40; S, 11.19%;
IR (KBr, cm21) 1497 (CvO), 3285 (NH); dH (300 MHz,
DMSO þ CDCl3, Me4Si) 2.21 (s, ZCH3, 3H), 2.25 (s,
ZCH3, 3H), 2.29 (s, ZCH3, 3H), 3.87 (s, ZCH2, 2H), 4.02
(s, ZCH2, 2H), 4.07 (s, ZCH2, 2H), 5.28 (s, ZNH2, 2H),
6.50 (t, Ar, 1H, J ¼ 7.2 Hz), 6.72 (d, Ar, 1H, J ¼ 8.1 Hz),
7.0–7.15 (m, Ar, 4H), 7.25–7.32 (m, Ar, 2H), 7.36–7.43
(m, Ar, 2H), 7.64–7.68 (m, Ar, 4H), 7.89 (t, Ar, 2H,
J ¼ 8.4 Hz), 8.16–8.23 (m, Ar, 4H), 8.35 (s, ZNH, 1H),
8.37 (s, ZNH, 1H), 10.0 (s, ZNH, 1H) 10.03 (s, ZNH,
1H); dC (75 MHz, DMSO, Me4Si) 15.79, 34.84, 79.41,
85.40, 114.47, 116.94, 117.04, 117.32, 119.37, 119.62,
123.50, 126.12, 125.27, 128.34, 129.83, 130.67, 132.25,
132.45, 133.82, 134.80, 135.10, 135.80, 136.14, 143.0,
145.80, 149.30, 149.46, 152.96, 163.26.
4.3 Anion recognition studies
Anion binding ability of receptors was determined by
generally preparing solutions containing 10 mM of
receptor (unless specified otherwise) and 100 mM of
tetrabutylammonium salts of a particular anion in DMSO.
Changes in the electronic absorption spectra of the
receptors were observed. The anion binding ability of
receptors with TBAF was investigated using UV–vis
titration experiments.
Acknowledgements
GH thanks the CSIR (India) for research grant No. 01
(2104)/07/EMR-II. VKB and SS thank CSIR and UGC for
fellowships.
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