Synthesis of new tripodal receptors-a 'PET' based 'off-on' recognition of Ag+

Article abstract of DOI:10.1016/j.tet.2008.03.013

A new set of tripodal receptors based upon an aromatic platform have been synthesized in high yields. The compounds have been characterized by spectroscopic techniques and by single crystal X-ray crystallography. These receptors are found to have good extraction ability and high transport rate for Ag(I). The receptor with imine linkages exhibits weak fluorescence emission bands at λ_max=413 and 540 nm, upon excitation at λ_max=365 nm. The fluorescence spectrum of the receptor shows enhancement in the intensity of the signal at 413 nm on binding with the Ag⁺ cation. No such significant changes are observed with other metal ions. An absorption at ～365 nm is typical of an intraligand (π-π^*) transition involving the imine chromophore, which produces a weak emission band at 413 nm due to quenching caused by PET from a neighboring -OH group. Participation of OH group in coordination to the metal ion reduces PET and an enhancement of fluorescence intensity is observed, signaling recognition of the metal ion.

Full text of DOI:10.1016/j.tet.2008.03.013

Tetrahedron 64 (2008) 5384–5391
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of new tripodal receptorsda ‘PET’ based ‘off–on’ recognition of Agþ
*
Vimal K. Bhardwaj, Ajay Pal Singh Pannu, Narinder Singh, Maninder Singh Hundal, Geeta Hundal
Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new set of tripodal receptors based upon an aromatic platform have been synthesized in high yields.
The compounds have been characterized by spectroscopic techniques and by single crystal X-ray crystal-
lography. These receptors are found to have good extraction ability and high transport rate for Ag(I). The
receptor with imine linkages exhibits weak fluorescence emission bands at lmax¼413 and 540 nm, upon
Received 2 October 2007
Received in revised form 7 February 2008
Accepted 6 March 2008
Available online 18 March 2008
excitation at l ¼365 nm. The fluorescence spectrum of the receptor shows enhancement in the inten-
max
sity of the signal at 413 nm on binding with the Agþ cation. No such significant changes are observed
with other metal ions. An absorption at w365 nm is typical of an intraligand (p–p ) transition involving
*
the imine chromophore, which produces a weak emission band at 413 nm due to quenching caused by
PET from a neighboring –OH group. Participation of OH group in coordination to the metal ion reduces
PET and an enhancement of fluorescence intensity is observed, signaling recognition of the metal ion.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
ligands based on salicylidimine unit. Therefore in our ongoing work
on tripodal receptors containing Schiff bases10 we have exploited
Currently the synthesis of fluorescent sensors for heavy transi-
tion metal ions (HTM) is gaining impetus because of their biological
and environmental importance.1 There are numerous reported
examples of fluoroionophores, which show excellent selectivity
for heavy metals,2 especially Hg2þ and Cu2þ ions. However, the
quenching nature of HTM via spin–orbit coupling or via energy/
electron transfer poses3 a disadvantage in high signal output. The
known receptors for HTM are now being based upon chelation in-
duced quenching (CHEQ) or enhancement of fluorescence (CHEF)
phenomenon.4 Quenching of fluorescence by metal ion during
complexation is a common phenomenon whereas enhancement
is rare and of much interest as it opens up the opportunity for pho-
tochemical applications of these complexes.5 For sensing of a single
analyte, the use of the receptor–spacer–reporter paradigm is one of
the good approaches.6 In this approach a receptor is covalently
bonded to a reporter, which may be a chromophore/fluorophore.7
If the spacer is not long, the receptor and reporter may overlap in
identity8 but there must exist some mechanism by which the re-
porter can transduce the binding of the analyte to the receptor.
This approach has been used to obtain many chromogenic/fluoro-
genic receptors for different metal ions.2 A number of reports
have appeared lately on the prospective use of fluorescence charac-
teristics on transition metal complexation of Schiff base ligands
having no other fluorophore (reporter) in the chemosensor except
for the imine group.9 These examples mainly include small dipodal
the fluorescence capability of a newly synthesized tripodal Schiff
base where the imine group may behave as a receptor and/or a re-
porter. Both, the amine 2 and the Schiff base 3, have showed good
recognition for Agþ ion with the latter also showing selectivity for
the Agþ ion. To the best of our knowledge there are only a few flu-
orogenic sensors11 known for Ag(I) ion, therefore this piece of work
further gains importance.
2. Synthesis and spectroscopic characterization
Compound 1 was prepared by the reported method.12 Receptor
2 was synthesized by the reaction of tripodal bromide 1 with 2-
aminothiophenol under phase transfer catalytic and dry conditions.
A condensation reaction of tripodal amine 2 with salicylaldehyde,
in the presence of catalytic amount of zinc perchlorate10b gave re-
ceptor 3 in good yields (Scheme 1). The compounds were character-
ized by elemental analyses and spectroscopic techniques. Receptors
2 and 3 were also characterized by single crystal X-ray diffraction
methods. A peak at d 4.36 in the 1H NMR of 2 clearly indicates
the –NH2 protons. Since there is one peak each for –CH3, –CH2,
–CH2S protons and for each of the aromatic protons, this indicates
10c,13
that the tripodal 2 is present in a cis, cis, cis conformation
in
the solution phase. In the IR spectrum bands at around 3388,
3469 cmꢀ1 characterized the presence of the amine group in the
molecule. Similarly for receptor 3, a sharp peak at 1613 cmꢀ1 and
a broad peak at 3419 cmꢀ1 in the IR spectrum indicates the pres-
ence of an imine group and a H-bonded –OH group present in
the compound 3. The same were established by the presence of
* Corresponding author. Tel.: þ91 183 250 2113; fax: þ91 183 225 8820.
E-mail address: geetahundal@yahoo.com (G. Hundal).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.03.013

V.K. Bhardwaj et al. / Tetrahedron 64 (2008) 5384–5391
5385
Scheme 1.
signals at d 8.55 and 13.14 in the 1H NMR spectrum and at d 162.42
and 172.63 in 13C NMR spectra, respectively. The chemical shift of
the hydroxyl protons is typical for the resonance-assisted hydro-
gen-bonded (RAHB) proton of O–H/N]C.14 The CHN data are
also in accordance with the molecular formulae.
3. X-ray crystal structure studies
Table 1 shows the cell parameters and refinement parameters
for 2 and 3. The solid state structure of 2 is shown in Figure 1.
The molecule is in the cis, cis, cis conformation.10c,13 The four aro-
matic rings are planar. The ring B (C8–C13), ring C (C17–C22), and
the ring D (C26–C31) have dihedral angles of 16.3(1), 71.3(1),
Figure 1. Showing the ORTEP diagram of (2) and the labeling scheme used.
Table 1
Crystal data and structure refinement for 2 and 3
33.6ꢁ(1) with ring A (C1–C6), respectively. Torsion angles about
S1–C7, S2–C16, and S3–C25 are 176.5(3),177.9(2), and ꢀ63.4ꢁ(3), re-
spectively. Thus the aromatic rings B and C are in an extended con-
formation with respect to ring A but ring D has taken a twist toward
the central ring A (Fig. 2). This change in conformation may be due
to the C–H/p interaction between C27–H27/centroid of ring A at
3.88 Å. Any threefold symmetry found in the solution phase has
been destroyed because of the different orientations of the three
arms of the tripods.
Inter and intra-molecular H-bonding interactions were found in
the unit cell (Table 1 in Supplementary data). The amine nitrogens
are intra-molecularly H-bonded to sulfur atoms in their respective
arms. Inter-molecular N1/N2 H-bonding forms helical, polymeric
chains running parallel to the a axis. These parallel helical chains
are linked to each other via N1/N3 H-bonding extending the net-
work in the ac plane (Fig. 3). Thus the crystal structure comprises of
cross-linked helices down the b axis.
Identification code
2
3
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
C
33H39N3S3
C54H51N3O3S3
886.16
295(2)
0.71069
Triclinic
573.85
295(2)
0.71069
Orthorhombic
Pna21
Space group
Unit cell dimensions
P-1
a¼16.063(5) Å
b¼10.920(5) Å
c¼18.327(5) Å
a¼90.0ꢁ
a¼11.98(4) Å
b¼12.595(18) Å
c¼15.91(2) Å
a¼82.02(10)ꢁ
b¼87.26(18)ꢁ
g¼81.44(16)ꢁ
2350(8)
b¼90.0ꢁ
g¼90.0ꢁ
Volume (Å3)
Z
3215(2)
4
2
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
1.186
0.256
1224
1.252
0.205
936
)
Crystal size (mm3)
Theta range
0.22ꢂ0.20ꢂ0.20
0.21ꢂ0.18ꢂ0.19
2.17–26.00
1.29–25.50ꢁ
for data collection (ꢁ)
Index ranges
The X-ray crystal structure of 3 is shown in Figure 4. All seven
aromatic rings are almost planar with maximum deviation of
ꢀ1ꢃhꢃ19
ꢀ1ꢃkꢃ13
ꢀ22ꢃlꢃ25
7520
0ꢃhꢃ12
ꢀ14ꢃkꢃ15
ꢀ19ꢃlꢃ19
8947
Reflections collected
Independent reflections
6318 [R(int)¼0.0225] 8500 [R(int)¼0.0620]
Completeness to theta¼26.00ꢁ (%) 100.0
97.0
Absorption correction
Refinement method
None
None
Full-matrix least-squares on F2
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2s(I)]
6318/1/352
0.933
8500/0/568
0.807
R1¼0.0543
wR2¼0.1380
R1¼0.0816
wR2¼0.1501
0.00(8)
R1¼0.0861
wR2¼0.1945
R1¼0.2330
wR2¼0.2993
d
R indices (all data)
Absolute structure parameter
Largest diff.
0.762 and ꢀ0.234
0.511 and ꢀ0.568
peak and hole (e Åꢀ3
)
Figure 2. Showing one of the arms of the tripod being turned inside to come above the
ring A.
CCDC No.
662439
662440

5386
V.K. Bhardwaj et al. / Tetrahedron 64 (2008) 5384–5391
similar conformation has earlier been reported by us for a similar
mesitylene based tripodal ligand.10a The crystal structure of the
compound shows the formation of C–H/p based, centrosymmetric
dimeric molecules (C17–H17/centroid of ring Gii 3.45(3) Å), which
are further joined to each other due to p/p bonding (C42–H42/
centroid of ring Fv 3.80(4) Å) forming polymeric chains running
diagonally in the ac plane (Fig. 5).
Figure 3. Showing cross-linked helices in the ac plane.
Figure 5. Showing formation of polymeric chains due to C–H/p and p/p bonding.
4. Fluorescence studies
Receptor
3 shows a broad absorption band (Fig. 6) at
l
¼355 nm in the absorption spectrum, which is due to an
max
intraligand p/p transition. When excited at 365 nm (within the
*
envelope of the 355 nm band) at room temperature (10 mM concen-
tration of 3 in CH3CN/H2O (8:2, v/v) with HEPES as the buffer), it
exhibits two weak fluorescence emission bands at l ¼413 and
max
530 nm (Fig. 7). The band at longer wavelength is typical of an ex-
cited state intramolecular proton transfer (ESIPT) phenomenon.
The general pathway of the photochromism in Schiff bases is as
follows.15 The basal enol form is excited to a (p–p
) state from
*
where the hydroxylic proton is quickly transferred to the nitrogen
of the imino atom: the resulting geometry corresponds to the fluo-
rescent cis-keto tautomer (also a p–p state), which may undergo
*
relaxation toward a ground state trans-keto isomer form (photo-
chromic tautomer). The steady state emission in such compounds
is dominated by the cis-keto tautomer since the decay of the
excited enol state is very fast. As compared to absorption and fluo-
rescent studies15 of SA (salicylideneaniline) and SN (salicylidene-1-
naphthylamine) in acetonitrile, it may be stated that the tautomer
responsible for the ground state absorption is the hydrogen-
bonded enolic form giving a band at 355 nm; the same was found
at 337 and 350 nm in SA and SN, respectively. In both SA and SN
the prominent bands in the fluorescence spectra, showing large
Stokes shifts appearing at 537 and 547 nm, respectively, are due
to emission from the cis-keto tautomer formed after photoinduced
Figure 4. Showing ORTEP of compound 3 and its partial labeling scheme. Hydrogens
and labels of carbon atoms have been removed for clarity.
0.05 Å for the central ring (ring A). One arm of the tripod is highly
unsymmetrical with respect to the other two, thus removing any
threefold symmetry expected of the compound as found in the so-
lution phase. The rings B (C8–C13), D (C22–C27), and F (C36–C41)
have dihedral angles of 91, 144, and 94ꢁ with ring A, respectively.
Similarly ring C (C15–C20) and ring G (C43–C48) are perpendicular
(dihedral angles 101 and 93ꢁ, respectively) but ring E (C29–C34) is
parallel (dihedral angle 174ꢁ) with respect to ring A. The torsion
angles S1–C7 and S3–C35 are anti, being ꢀ169 and ꢀ174ꢁ, respec-
tively, but S2–C21 is gauche 78ꢁ. These values show that the tripod
is not in its fully extended form but one of its arms is folded to make
a loop with ring E parallel to ring A. This loop is further stabilized by
many intramolecular interactions such as C–H/p interactions be-
tween methylene C7 and ring E (3.709 Å) and imine C28 and ring A
(3.459 Å), edge to edge p/p interactions between ring A and D, H-
bonding between methylene C7 and O2 and C49 and S2, imine C28
and O3, phenylene C26 and O3, methylene C51 and C53, with S3
and methyl C54 and S1 (Table 2 in Supplementary data).
There are other intra and inter-molecular H-bonding interac-
tions found in the unit cell. The hydroxyl oxygens O1 and O3 are
acting as double H-bond donors to imine nitrogens N1 and N3
and thioether sulfurs S1 and S3, respectively, in the two extended
arms. However, in the folded arm of the tripod, O2 forms intramo-
lecular H-bonding with imine nitrogen N2, but not to S2. A very
Figure 6. Showing absorption spectrum of 3 (1ꢂ10ꢀ5 M) in CH3CN/H2O (8:2, v/v).

V.K. Bhardwaj et al. / Tetrahedron 64 (2008) 5384–5391
5387
To learn more about the properties of 3 as a receptor for silver,
fluorescence titration was carried out. Figure 9 illustrates the emis-
sion response of chemosensor 3 with increase in concentration of
Agþ ion. The fluorescence intensity of a 10 mM solution of 3 at
413 nm enhanced monotonically with a slight ‘blue shift’ on addi-
tion of Agþ ion into the solution, while shape of the emission
band is not changed. The receptor 3 exhibited a high sensitivity to-
ward silver, enhancing its fluorescence intensity fourfold with
2.0 equiv of silver. The association constant Ka of 3 for silver was
calculated on the basis of the Benesi–Hilderbrand plot18 (Fig. 10),
and it was found to be (2.9ꢄ0.1)ꢂ103 Mꢀ1. The stoichiometry of
the complex formed was determined by Job’s plot (Fig. 11), and it
Figure 7. Changes in fluorescence intensity of 3 (10 mM) at lmax¼413 nm upon addi-
tion of 2.0 equiv of a particular metal ion salt in CH3CN/H2O (8:2, v/v) (10 mM HEPES
buffer, pH¼6.2) with excitation at 365 nm.
proton transfer. The long wavelength band found in 3 at 530 nm
may thus be of the same nature. In both SA and SN, apart from
this main band, a weak, short wavelength emission has also been
noticed, which has been associated with the emitting species pre-
ceding the appearance of the fluorescent cis-keto tautomer. The
emission band observed at 413 nm in the present compound is
thus assigned to intraligand 1(p–p
) fluorescence from the excited
*
enolic state.9e,16 The band has low intensity because of quenching
due to photoinduced electron transfer (PET) mechanism, which
comes into play due to the availability of the lone pairs of N of
the –C]N group9e and O of the OH group, which cause quenching.
To evaluate the metal binding affinity of receptor 3, the changes
in fluorescence intensity of 3 upon addition of different metal salts
were recorded (Fig. 7). Figures 7 and 8 clearly show that there is
a marked enhancement in fluorescence intensity only upon addi-
tion of silver salt. On the other hand, no such significant changes
in fluorescence spectra were observed when receptor 3a was ex-
posed to copper, nickel, cobalt, zinc or mercury salts under the
same experimental conditions. Upon complexation of the Agþ
ion, the N of the –C]N group and O of the OH group are involved
in coordination with the metal ion hindering the PET phenomenon
and fluorescence is restored resulting in an off-to-on signal. Such
a phenomenon has earlier been reported in Zn(II)-sensitive fluores-
cent chemosensor.9e Both these systems work in a way similar to
that found in some aminofluoresceins.17 Thus in the present case
Ag(I) increases the oxidation potential of the receptor 3.
Figure 9. Fluorescence spectra changes of 3 (10 mM) upon addition of silver salt
(0–10 mM) in HEPES buffered (10 mM, pH¼6.2) CH3CN/H2O (8:2, v/v).
0.7
y = 1.1494x + 0.0335
0.6
R
2 = 0.9857
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
1/[G]
0.4
0.5
0.6
Figure 10. Benesi–Hilderbrand plot for the determination of stability constant of silver
with the receptor 3.
Ag (I)
Hg (II)
Zn (II)
Co (II)
Ni (II)
3
2.5
2
1.5
1
Cu (II)
0.5
0
-2.5
-2
-1.5
-1
-0.5
I0–I/I0
0
0.5
1
1.5
0
0.2
0.4
0.6
0.8
1
Figure 8. Fluorescence ratio [(I0ꢀI)/I0] of
3 (10 mM) at 413 nm upon addition of
[H] / [H] + [G]
2.0 equiv of a particular metal ion salt in HEPES buffered (10 mM, pH¼6.2) CH3CN/
H2O (8:2, v/v), I0 and I are the intensities in the absence and presence of the metal
ion, respectively.
Figure 11. Job’s plot between the receptor 3 and silver. The concentration of [HG] was
calculated by the equation [HG]¼I/I0ꢂ[H].

5388
V.K. Bhardwaj et al. / Tetrahedron 64 (2008) 5384–5391
was found to be 1:1. The smooth shape of the Job plot reflects well
the rather small value of Ks.
gradually broadened and shifted from d 13.13 to 12.79. Thus the sig-
nal of the –OH proton was shifted by Dd 0.34. Small high frequency
shifts of Dd 0.02, 0.12, and 0.04 were also seen for the imine protons
and the aromatic protons of the salicylaldehyde unit. However, no
shift was observed for protons of the –SCH2 or ethyl group suggest-
ing the binding to be mainly through –OH group only.
5. NMR studies of the binding in receptors
The 1H NMR experiment was performed to understand the na-
ture of receptor–Agþ interactions. A comparison of 1H NMR spec-
trum of receptor 2 and 1H NMR of receptor 2 mixed with
1.0 equiv of Agþ salt (CD3CN/CDCl3 (1:9, v/v) solvent system) shows
lot of changes. A singlet for the –NH2 protons in the free amine
shifts to a higher frequency with Dd 0.50 ppm. The –CH2S protons
show a low frequency shift of Dd 0.05 and at the same time all pro-
tons of 2-aminothiophenol and those of ethyl group show a high
frequency shift in their chemical shift values (Fig. 12). The maxi-
mum shift (d 0.28) is found for the proton attached to C6 (see
Scheme 1 for atom labeling). These changes in the NMR suggest
a binding of Ag(I) through amine N and S atoms. As no signal split-
ting was observed, it can be concluded that all the three arms of
receptor are engaged in complexation.
6. Extraction and transportation studies
To evaluate a particular metal binding ability of receptors 2 and
3, competitive solvent extraction experiments were performed. In
a typical experiment, an aqueous solution containing metal salt
(1.0 mM) was shaken with the host solution (2.0 mM). These exper-
iments were carried out at neutral pH. The concentrations of metal
salts left in the aqueous phase after extraction and also in blank
analysis were determined by atomic absorption spectroscopy, and
these values were taken as a measure to determine the percentage
of metal ion extracted by the receptor. Figure 14 represents the per-
centage of metal ions extracted (%E) by the receptors 2 and 3, which
clearly shows that both the receptors have higher complexation
ability for Agþ than for any other metal ion, with receptor 3 being
better than receptor 2. As receptor 3 has an imine linkage, which
tends to hydrolyze, so this receptor is not a good candidate for
studies to be carried out at low pH value. Thus, we selected only
the receptor 2 for pH dependent metal extraction and transport
studies.
Extraction experiments help to determine the efficiencies of
a receptor to complex metal ions at the interface of aqueous source
phase and organic layer. However, a higher extraction ability of any
receptor alone does not warrant its use for applications in
100
80
2
3
60
40
20
0
Figure 12. 1H NMR spectrum of (A) receptor 2 and (B) receptor 2þ1.0 equiv of silver
salt in CD3CN/CDCl3 (1:9, v/v) solvent system.
Cu (II)
3
The results of 1H NMR titration experiment, which was per-
formed to decide the binding sites of receptor 3 for Agþ are shown
in Figure 13. In the presence of Agþ a shift in –OH proton signal
shows that this donor group plays an important role in binding to
the metal ion. During the course of titration the –OH signal was
Ni (II)
Co (II)
Zn (II)
2
Hg (II)
Ag (I)
Figure 14. Competitive solvent extraction (%E) of various metal ions with the receptors
2 and 3.
Figure 13. Partial 1H NMR spectra of receptor 3 on addition of silver salt in CD3CN/CDCl3 (1:9, v/v) on a 400 MHz Bruker instrument.

V.K. Bhardwaj et al. / Tetrahedron 64 (2008) 5384–5391
5389
separation chemistry. If a receptor forms a very strong complex
with a particular metal ion and yet the decomplexation of metal
ions from the receptor is not possible, then the receptor will be
wasted after a single use. Thus, metal decomplexation from a recep-
tor is also important. For the determination of decomplexation of
metal from the receptor 2, a decomplexation experiment was
performed by extracting the organic layer from the extraction ex-
periment with hydrochloric acid (at pH 1). The data are expressed
as the percentage of Agþ ion decomplexed from the receptor 2
(Fig.15). The concentration of metal salt released to the acidic aque-
ous phase was determined by atomic absorption spectroscopy.
extraction of Agþ and its extraction efficiency only decreased a little
after being used five times.
Transport experiments involve complexation at the interface of
the source aqueous phase and the organic layer, while decomplex-
ation occurs at the interface of the organic layer and the receiving
aqueous phase. Hence a transport experiment is a continuous pro-
cess of metal extraction and metal decomplexation. A slow de-
crease in the concentration of metal ions in the source aqueous
phase signifies the inability of a receptor to extract ions from the
aqueous phase. On the other hand, a slow increase in the concen-
tration of ions in the receiving aqueous phase reflects a difficulty
in the release of the metal ions. We examined the transport ability
of receptor 2 by taking a carrier in the chloroform layer and metal
ions in the aqueous buffer phase (source phase). In these transport
experiments, the concentrations of the carrier in the membrane
and of the metal ions in the buffer source phase (pH¼4.6) were 1
and 5 mM, respectively. Hydrochloric acid (pH 1) was used for
the receiving phase. The concentration of Agþ was monitored in
both aqueous compartments (i.e., both in the source phase and
in the receiving phase) with atomic absorption spectroscopy. To
find the efficiency of the receptor properly, it is necessary to find
out the time dependence of the transportation in both the phases.
Therefore, the percentage of metal ion transported was monitored
as a function of time. Each reading was taken after a time interval of
2 h for 12 h and then after 24 h. This makes it possible to estimate
the rate of transportation of Agþ ion and also the total time required
(Fig. 17). The data show that the transport rate is fast for the first 4 h
and then after next 4 h (i.e., after total 8 h) most of the Agþ has been
transported. The actual transport rate has been calculated to be
1140ꢂ10ꢀ7 M/l/day.
79
78.5
78
77.5
77
76.5
76
75.5
Ag + Complexed
Ag + Decomplexed
Figure 15. Comparison of Agþ extraction (%E) and decomplexation (%D) efficiencies of
the receptor 2.
120
Source Phase
100
80
60
40
20
0
Receiving Phase
Figure 15 shows that the decomplexation ability of the receptor
for Agþ is almost as good as its complexation. The majority of the
extracted metal ions was released when the organic layer contain-
ing the metal–receptor complex was shaken with hydrochloric acid
(at pH 1). Due to the high acidity, the protons compete for the
amine nitrogens and metal ions are released at a high rate. Receptor
2 was also tested for its reuse in extracting Agþ from the buffered
aqueous phase. Before reuse, the receptor was neutralized with
a base. Figure 16 shows that 2 can be reused effectively for the
0
5
10
15
Time (h)
20
25
30
80
Figure 17. Transport rate of Agþ as a function of time.
78
76
74
72
Studies on aza-thioethers have revealed that presence of S and N
atoms in the macrocycle facilitate the coordination Ag(I) and thus
help in extraction of metal ion.19 In functionalized thioethers the
functional group attached to the pendent helps in the extraction
by coordination to metal ion as well stabilizations of anion by
H-bonding with the group on the pendent. It appears that in the
present study receptor 2 mimics the aza-thioethers as far as coor-
dination of S and N atoms with Ag(I) is concerned. However the re-
ceptor 3 adopts different coordination modes with Ag(I) and major
coordination sites appear to be mainly through –OH groups.
70
68
66
64
7. Conclusions
62
60
Silver(I) ion selectivity and specificity of the Schiff base 3 has
been optically transduced by the receptor-cum-reporter imine
unit. This has been achieved by avoiding the appendage of any spe-
cial fluorophore unit to the molecule, just by using the weak
0
1
2
Number of times
3
4
5
Figure 16. Reuse of receptor 2 for the extraction of Agþ
.

5390
V.K. Bhardwaj et al. / Tetrahedron 64 (2008) 5384–5391
fluorescence property of the Schiff base itself. The fluorogenic che-
mosensor was selective and sensitive for Ag(I), capable of detecting
the metal ion in a concentration of 1ꢂ10ꢀ5 M with the help of spec-
trophotometer, in the presence of other interfering metal ions. The
tripodal amine 2 has been successfully used to extract and trans-
port Ag(I) and the receptor has been shown to be useful for re-
peated extraction and transportation experiments.
(Ar–OH). Anal. Calcd % for C54H51N3O3S3: C, 73.19; H, 5.80; N,
4.74. Found: C, 72.94; H, 6.01; N, 4.84.
8.4. Competitive solvent extraction (%E)
The percentage extraction (%E) of the metal salt extracted from
the aqueous solution containing metal salts (1.0ꢂ10ꢀ3 M, 2.0 ml) to
the chloroform layer containing a receptor (2.0ꢂ10ꢀ3 M, 2.0 ml)
was determined at 25 ꢁC. The aqueous and organic mixtures were
shaken vigorously for a few minutes and the mixtures were left
to stand until phase separation was completed. Similarly, the blank
analysis was performed by taking metal salts (1.0ꢂ10ꢀ3 M, 2.0 ml)
dissolved in the aqueous solution and 2.0 ml of neat chloroform.
The concentration of the metal salts in the aqueous solution was
determined after extraction and also in blank analysis by an atomic
absorption spectrometer. The percentage extraction (%E) of metal
salts was determined by the formula %E¼(C1ꢀC2)ꢂ100/C1, where
C1 is the concentration of metal salts in the aqueous buffer solution
in blank analysis, and C2 is the concentration of metal salts remain-
ing in aqueous buffer solution after extraction. An average value of
three independent determinations is reported.
8. Experimental
8.1. General
All solvents were dried by standard methods. Unless otherwise
specified, chemicals were purchased from Sigma Aldrich and used
without further purification. TLC was performed on glass sheets
pre-coated with silica gel (Kieselgel 60 PF254, Merck). The elemen-
tal analyses were performed on a Flash EA 1112 elemental analyzer.
The 1H NMR spectra were either recorded on a 300 MHz JEOL or
400 MHz Bruker spectrometer, in CDCl3 or CD3CN–CDCl3 (2:8)
mixture. 13C NMR were run on a 75 MHz JEOL machine. The chem-
ical shifts are reported as d values (ppm) relative to TMS. IR spectra
were recorded on a PYE Unicam IR spectrometer for the compounds
in the solid state as KBr discs or as neat samples. TMS was used as
an internal reference. A Perkin–Elmer A Analyst100 atomic absorp-
tion spectrometer was used for the measurement of concentrations
of the analytes in extraction and transportation experiments. The
absorption spectra were recorded on a Shimadzu UV-1700 UV–vis
spectrophotometer. The fluorescence measurements were per-
formed on a Perkin Elmer LS55 Luminescence Spectrometer.
8.5. Decomplexation of metal ion (%D)
The percentage decomplexation (%D) of the metal salt from the
chloroform layer (containing complex 2.0 ml) to hydrochloric acid
was determined at 25 ꢁC. The chloroform layer of a competitive sol-
vent extraction experiment separated from the aqueous layer was
selected. This organic layer was shaken with 2.0 ml of hydrochloric
acid (pH¼1.0), and the solution was kept undisturbed until com-
plete separation had occurred. The concentration of metal salt in
hydrochloric acid was determined on an atomic absorption spec-
trometer. The percentage decomplexation (%D) of metal salt was
determined by the formula %D¼(C3/C1)ꢂ100, where C1 is the con-
centration of metal salts in the aqueous buffer solution in blank
analysis of competitive solvent extraction experiment, and C2 is
the concentration of metal salts released to 0.1 M HCl. An average
value of three independent determinations is reported.
8.2. Compound 2
Tripodal amine 2 was prepared by taking K2CO3 (1 g, mmol)
in dry acetonitrile along with 2-aminothiophenol (375 mg,
3.0 mmol). The reaction mixture was refluxed for 20 min and then
tribromide 1 (440 mg, 1 mmol) was carefully added to it. The reac-
tion mixture was refluxed for next 8 h and the progress of the reac-
tion was monitored by TLC. Upon completion of reaction, K2CO3 was
filtered off and whole of the acetonitrile was evaporated. The crude
product was recrystallized from chloroform–methanol solvent mꢀix1-
8.6. pH Dependent transport experiment
ture to get a pure white material. Yield 47%; mp 87 ꢁC; IR (KBr, cm
)
1306 (s), 1604 (s), 2884 (s), 3388 (w), 3469 (w); 1H NMR (300 MHz,
CDCl3, ppm) d: 1.21 (9H, t, J¼7.5 Hz, –CH3,), 2.91 (6H, q, J¼7.5 Hz,
–CH2), 3.98 (6H, s, –CH2), 4.35 (6H, s, –NH2), 6.71–6.77 (6H, m,
Ar), 7.15 (3H, t, Ar, J¼8.0), 7.40 (3H, d, Ar, J¼7.5); 13C NMR
(75 MHz, CDCl3) d: 16.1 (–CH3), 22.8 (–CH2), 34.5 (–CH2), 115.0
(Ar), 118.5 (Ar), 118.7 (Ar), 129.8 (Ar), 131.2 (Ar), 135.6 (Ar), 142.9
(Ar), 148.5 (Ar). CHN Anal. Calcd % for C33H39N3S3: C, 69.07; H,
6.85; N, 7.32. Found C, 69.09; H, 7.12; N, 7.23.
Transport experiments were carried out by stirring the organic
phase of bulk liquid membrane cell at a constantly slow speed so
that the interfaces between the organic membrane and two aque-
ous phases remained flat and well defined. The two aqueous phases
were separated by the organic phase. An aqueous source phase
(4.0 ml) containing Agþ (5ꢂ10ꢀ3 M) in acetate buffer (pH¼4.6)
and a receiving phase (8.0 ml) containing HCl (pH¼1.0) were sepa-
rated by the organic layer of chloroform (25 ml) containing a carrier
(1ꢂ10ꢀ3 M). The concentration of the carrier in the transport cell
containing compound 2 was 2ꢂ10ꢀ3 M. The metal ion concentra-
tions in both aqueous phases were monitored with an atomic
absorption spectrophotometer. An average value of three indepen-
dent determinations is reported.
8.3. Compound 3
This compound was prepared by stirring tripodal amine 2
(573 mg, 1.0 mmol) along with salicylaldehyde (390 mg,
3.2 mmol) in the presence of traces of zinc perchlorate taken in ace-
tonitrile–chloroform (9:1) solvent mixture. The color of the solu-
tion changed immediately to yellow and precipitate separated out
in quantitative yield upon addition of methanol. These precipitates
were filtered and dried. Yield 58%; mp¼197 ꢁC; IR (KBr, cmꢀ1) 1613
(s), 3419 (br m); 1H NMR (300 MHz, CDCl3) d: 1.17 (t, 9H, –CH3,
J¼7.5 Hz), 2.63 (q, 6H, –CH2, J¼7.5 Hz), 3.92 (s, 6H, –CH2), 6.89–
6.99 (m, 6H, Ar), 7.13–7.40 (m, 18H, Ar), 8.54 (s, 3H, –CH]N),
13.14 (s, 3H, –OH,); 13C NMR (75 MHz, CDCl3): d 16.2 (–CH3), 22.9
(–CH2), 32.1 (–CH2), 117.3 (Ar), 118.1 (Ar), 118.9 (Ar), 119.3 (Ar),
127.3 (Ar), 129.0 (Ar), 130.2 (Ar), 131.2 (Ar), 132.4 (Ar), 133.0 (Ar),
137.0 (Ar), 143.7 (Ar), 154.4 (Ar), 161.2 (Ar), 162.4 (CH]N), 172.6
8.7. Stability constant determination
The stability constant of receptor 3 with silver was determined
by preparing solutions containing 10 mM of receptor 3 and varying
amounts (0–20 mM) of silver salt in CH3CN/H2O (8:2, v/v) (10 mM
HEPES buffer, pH¼6.2). The emission was measured at 413 nm.
8.8. Stoichiometry determination
Mixtures of receptor–silver salt were prepared as 1:9, 2:8, 3:7,
4:6, 5:5, 6:4, 7:3, 8:2, 9:1. These solutions were kept at 25þ1 ꢁC

V.K. Bhardwaj et al. / Tetrahedron 64 (2008) 5384–5391
5391
Montalti, M.; Zaccheroni, N. Coord. Chem. Rev. 2000, 205, 59–83; (f) Valeur, B.;
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before recording their spectra. The plot of [HG] versus [H]/[H]þ[G]
was used to determine the stoichiometry of the complex formed.
The concentration of [HG] was calculated by the equation
[HG]¼DI/I0ꢂ[H].
4. Kubo, K. PET Sensors. In Advanced Concepts in Fluorescence Sensing. Part A: Small
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8.9. X-ray crystallography and structure determination
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The data for both the crystals were collected on a Siemens P4
single crystal X-ray diffractometer, using Mo Ka radiation
(0.71609 Å). Table 1 gives the details of data collection and refine-
ment for both 2 and 3. Both structures were solved by direct
methods and subsequently by difference Fourier Syntheses and re-
fined by full-matrix least-squares on F2 with WINGX.20 Lorentz and
polarization corrections were applied but no absorption correction
was applied. All non-hydrogen atoms were refined anisotropically.
Hydrogens were fixed geometrically and were not refined. They
were made to ride on their respective atoms with thermal param-
eters 1.5 times for a methyl carbon and 1.2 times for methylene and
aromatic carbons. The H-bonding calculations, torsion and dihedral
angles and plane were calculated by using PARST.21
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Acknowledgements
G.H. and V.K.B. are thankful to the CSIR, India for research grant
No. 01(2104)/07/EMR-II and research fellowship, respectively.
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Supplementary data
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Supplementary data associated with this article can be found in
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