New 1,8-naphthyridine-based probes for the selective fluorescence signalling of toxic cadmium: Synthesis, photophysical studies and molecular modelling

Article abstract of DOI:10.1080/10610278.2010.491153

Newly synthesised 1,8-naphthyridine-based molecular probes, NAP-1 and NAP-2, exhibit highly selective fluorescence responses towards the toxic cadmium over coordinatively competing Zn²⁺ and several other metal ions examined. On the one hand, NA

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Supramolecular Chemistry
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New 1,8-naphthyridine-based probes for the selective
fluorescence signalling of toxic cadmium: synthesis,
photophysical studies and molecular modelling
Sabir H. Mashraqui a , Rupesh Betkar a , Mukesh Chandiramani a , Kiran Poonia a , David
Quinonero b & Antonio Frontera b
a Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz-E, Mumbai,
400098, India
b Department of Quimica, Universitat de les Illes Balears, Crta. De Valldemossa km 7.5,
07122, Palma, Baleares, Spain
Published online: 23 Jun 2010.
To cite this article: Sabir H. Mashraqui , Rupesh Betkar , Mukesh Chandiramani , Kiran Poonia , David Quinonero & Antonio
Frontera (2010): New 1,8-naphthyridine-based probes for the selective fluorescence signalling of toxic cadmium: synthesis,
photophysical studies and molecular modelling, Supramolecular Chemistry, 22:9, 524-531
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Supramolecular Chemistry
Vol. 22, No. 9, September 2010, 524–531
New 1,8-naphthyridine-based probes for the selective fluorescence signalling of toxic cadmium:
synthesis, photophysical studies and molecular modelling
Sabir H. Mashraquia*, Rupesh Betkara, Mukesh Chandiramania, Kiran Pooniaa, David Quinonerob and Antonio Fronterab
b
aDepartment of Chemistry, University of Mumbai, Vidyanagari, Santacruz-E, Mumbai 400098, India; Department of Quimica,
Universitat de les Illes Balears, Crta. De Valldemossa km 7.5, 07122 Palma, Baleares, Spain
(Received 3 March 2010; final version received 27 April 2010)
Newly synthesised 1,8-naphthyridine-based molecular probes, NAP-1 and NAP-2, exhibit highly selective fluorescence
responses towards the toxic cadmium over coordinatively competing Zn2þ and several other metal ions examined. On the
one hand, NAP-1 (MeOH:H2O, v/v 80:20, pH 7.4) exhibits ca. 1.5 order of magnitude higher stability constant for Cd2þ over
Zn2þ; on the other hand, NAP-2 in MeOH offers unique selectivity only towards Cd2þ, exhibiting both absorbance and
emission red shifts as well as fluorescence enhancement. By 1H NMR analysis, the tetra-coordinated binding is indicated at
least for the NAP-1 þ Cd2þ complex and theoretical calculations reveal relatively stronger binding of Cd2þ over Zn2þ
.
Keywords: 1,8-naphthyridine; fluorophore photophysical studies; fluorescent cadmium sensors; NMR analysis; molecular
modelling
Introduction
in marine diatoms thriving in a zinc-depleted environment
(7). Consequently, monitoring Cd uptake in human and
environment is an increasingly important goal in biology
and health sciences. One major limitation associated with
many currently known fluorescent Cd2þ sensors is the
competing binding from the coordinatively similar Zn2þ
(8). Although receptors such as pyridyl(di)amines,
quinolylamines or their derivatives in conjunction
with suitable fluorophores have afforded many efficient
Zn2þ-selective fluorescent sensors (9, 10), despite exten-
sive reports (11), the Cd2þ-selective optical probes are
comparatively less common (12). Thus, it is a challenge to
seek new chemosensor motifs which could
allow the selective discrimination of Cd2þ, while
excluding or minimising interferences of Zn2þ and other
metal ions (13).
The development of molecular probes for the selective
recognition of metal ions of biological and environmental
significance is currently one of the important goals in
supramolecular chemistry. Of the many methods known for
ion detection, the fluorescence modulation is finding
increasing popularity due to the advantages of high
sensitivity and practical conveniences (1). The design of
optical probes typically involves connecting a selective
receptor to a chromophore/fluorophore motif such that the
binding event triggers photophysical perturbations in the
probe, delivering easily quantifiable colorimetric, fluori-
metric or ratiometric responses (2). With regard to the
sensing strategy, severalattractivephotophysical processes,
which include photoinduced electron transfer (PET),
intramolecular charge transfer, monomer vs. excimer
formations and fluorescence resonance energy transfer
mechanism, are currently available to tune the optical
signalling in the presence of an interacting analyte (2a).
Cadmium is notorious for its well-recognised acute
toxic effects on human and environments. It can
contaminate food chain and environment through mining
and various industrial processes (3, 4). The recommended
levels of cadmium in drinking water is no more than 3 mg/l
(1, 5). Overexposures to cadmium beyond this level can
cause bone, renal, and nervous system diseases, calcium
metabolism disorders as well as cancers of many vital body
organs (6). Despite its health hazards, a cadmium-
dependent carbonic anhydrase has recently been detected
1,8-Naphthyridine core has found applications in the
design of supramolecular systems, as a recognition unit for
DNA bases as well as components of organic solar dyes
(14). Surprisingly, this photoemittive heterocycle, posses-
sing a rigid 1,3-dinucleation geometry so far, has found
limited utility in the design of sensors of few selected metal
ions, viz. Zn2þ, Cu2þ and Hg2þ (15). While the present
work was in progress, Xiao et al. (16) independently
reported a naphthyridine-based Cd2þ-selective sensor,
offering fluorescence response under a bi-nuclear coordi-
nation. However, a small fluorescence amplification of
approximately only 2-folds means that the sensitivity of this
Cd2þ chemosensor, though not reported bythe authors, may
not be appreciable.
*Corresponding author. Email: sh_mashraqui@yahoo.com
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2010 Taylor & Francis
DOI: 10.1080/10610278.2010.491153
http://www.informaworld.com

Supramolecular Chemistry
525
In continuation of our interest in metal ion
chemosensors (17), presently, we presumed that optical
chemosensors featuring tetra-coordination around the rigid
naphthyridine core might offer improved binding charac-
teristics with selected metal ion(s) of biological or
environmental interest (18). With this anticipation, we
have synthesised and investigated the metal ion sensitivity
of two potentially tetra-dentate naphthyridine probes,
designated as NAP-1 and NAP-2. Based on our
photophysical studies, we find these probes to function
as Cd2þ-selective fluorescence turn-on chemosensors with
less significant or no detectable optical interferences being
observed from the coordinatively competing Zn2þ as well
as from several other biologically or environmentally
relevant metal ions investigated.
under the DMF/K2CO3 condition afforded the correspond-
ing O-alkylated products 3 and 5, respectively. The
condensation of 2-aminopyridine-3-carboxaldehyde (6)
with 3 and 5 in 10% NaOH in refluxing alcohol (19),
followed by work-up and chromatographic purification,
afforded the target products NAP-1 and NAP-2, respectively,
as pale yellow solids in a reasonable yield.
Optical spectral studies with metal ions
The optical sensitivities of NAP-1 and NAP-2 towards
selected metal ions of biological and/or environmental
relevance were evaluated via optical spectroscopic
techniques. The UV–vis spectra of NAP-1 and NAP-2,
carrying a common chromophore motif, displayed similar
absorption behaviour with maxima located around 244 and
326 nm, attributable to the anisyl and naphthyridine
chromophores, respectively. Addition of perchlorates of
Results and discussion
Liþ, Naþ, Kþ, Ba2þ, Ca2þ, Zn2þ, Mg2þ, Cd2þ, Cu2þ
,
Synthesis
Co2þ, Ni2þ, Agþ and Pb2þ up to 1000 equiv. into a
solution of NAP-1 (2.83 £ 1025 M) in MeOH:H2O (v/v
80:20), HEPES buffer, pH 7.4, did not significantly alter
the absorption profile of the probe, except for the marginal
reductions in the absorbance by 10–25% (see Supporting
Information, available online). The absence of detectable
changes in the ground state of NAP-1 means that no
appreciable charge shift takes place within the chromo-
phoric system in the presence of metal ions.
The syntheses of NAP-1 and NAP-2 were carried out in two
easy steps as outlined in Scheme 1. Alkylations of 2-hydroxy
acetophenone (1) with N,N-dimethylaminoethane hydro-
chloride (2) and 2-chloromethylpyridine hydrochloride (4)
The absorption profile of NAP-2 (2.5 £ 1025 M,
MeOH) was also virtually invariant to Naþ, Liþ, Kþ,
Mg2þ, Ca2þ, Ba2þ, Co2þ, Ni2þ, Cu2þ, Agþ, Pb2þ
,
including Zn2þ up to 3.5 £ 1023 M, thereby implying the
absence of any ground-state perturbations in the probe by
these metal ions (see Supporting Information, available
online). However, unlike NAP-1, the ground state of NAP-
2 was detectably perturbed in the presence of Cd2þ. As
shown in Figure 1, the spectrophotometric titration of a
probe’s solution (2.5 £ 1025 M in MeOH) as a function of
Figure 1. Spectrophotometric titration of NAP-2
(2.5 £ 1025 M) with cadmium perchlorate (0–2.84 £ 1023 M)
in MeOH.
Scheme 1. Synthesis of NAP-1 and NAP-2.

526
S.H. Mashraqui et al.
increasing Cd2þ (0–2.84 £ 1023 M) induced a red shift of
the probe’s 327 nm maximum to 339 nm. A clear
isosbestic point at 332 nm indicates the formation of a
well-defined equilibrium complexation. Although not yet
well defined, the observed red shift of 12 nm implies the
inducement of a moderate degree of intramolecular charge
transfer from the donor phenoxyl ring to the acceptor
naphthyridine ring upon interaction of NAP-2 with Cd2þ
.
Excitations of NAP-1 in MeOH:H2O (v/v 80:20) at pH
7.4 at 326 nm generated a structured emission in the
range of 350–600 nm, with a maximum intensity centred
at 430 nm, while the excitation of NAP-2 in MeOH at the
isosbestic point at 332 nm produced a structured emission
at 433 nm. The quantum yields of NAP-1 and NAP-2 were
determined to be 0.005 and 0.016, respectively, with
respect to anthracene as the standard. The lower quantum
efficiency of NAP-1 compared to that of NAP-2 may
presumably be due to PET-induced fluorescence quench-
ing via electron transfer from the saturated nitrogen to the
excited naphthyridine chromophore. Support for the
involvement of the PET process in NAP-1 is obtained
from the study of the changes in fluorescence intensity of
NAP-1 with respect to variations in the pH. As shown in
Figure 2, the fluorescence was nearly unchanged between
the 11.5 and 7 pH range, thereafter gradual increases in the
emission intensity at 430 nm occurred in going from the 5
to 3 pH range, giving a maximum of approximately 2-fold
emission enhancements. From this result, we calculated
the pKa of 4.65 assignable to the more basic tertiary amino
group. The increase in the fluorescence intensity under
the acidic pH clearly supports the PET nature of the probe.
A further decrease in pH below 3 changed the fluorescence
only slightly. Although the protonation of the less basic
naphthyridine ring is expected to take place below pH 3,
the absence of significant changes in the fluorescence
intensity precluded the determination of the pKa of this
ring by this method.
Figure 3. NAP-1 (1 £ 1026 M) on addition of cadmium
perchlorate (0–1.3 £ 1024 M) in MeOH:H2O (v/v 80:20) at pH
7.4.
Of the different metal ions examined for their
fluorescence sensitivity towards NAP-1, the most remark-
able fluorescence response was induced by Cd2þ only.
As shown in Figure 3, sequential addition of Cd2þ to a
solution of NAP-1 (1 £ 1026 M) in MeOH:H2O (v/v
80:20) at pH 7.4 gave rise to progressive fluorescence
enhancement at the same wavelength until at saturating
Cd2þ (1.3 £ 1024 M), the emission intensity reached a
maximum of 15-fold enhancement. The inset shows the
linear increase in the emission intensity as a function of
increasing concentration of Cd2þ. The probe showed no
emission spectral shift, a feature which is consistent with
the PET character of the probe.
In contrast to Cd2þ, the coordinatively competing Zn2þ
required significantly higher concentrations in order to
induce detectable fluorescence modulation. The fluor-
escence response of NAP-1 towards Zn2þ, together with
the effects of subsequently added Cd2þ, is shown in
Figure 4. First, the addition of Zn2þ at its saturating
concentration (1.2 £ 1023 M) caused a blue shift of the
probe’s emission maximum from 430 to 386 nm with
approximately 4-fold emission enhancement at the newly
Figure 4. NAP-1 (1 £ 1026 M) on addition of Zn2þ
(1.2 £ 1023 M) þ Cd2þ (1.3 £ 1024 M) in MeOH:H2O (v/v
80:20) at pH 7.4.
Figure 2. pH titration of NAP-1 (pH 2 to 11) by monitoring
fluorescence intensity at 430 nm.

Supramolecular Chemistry
527
generated maximum. Subsequent addition of 1.3 £ 1024
M
under both ground-state and excited-state conditions with
virtually no detectable optical interferences arising from
other metal ions even at relatively higher concentrations.
For the case of NAP-1, tethered with a saturated
nitrogen donor, the observed fluorescence turn-on
response upon Cd2þ binding (and to some extent with
Zn2þ) can be attributed to a combination of the chelation-
enhanced fluorescence (CHEF) effect and the metal ion-
induced suppression of the PET process prevailing in the
metal-free probe (20). However, in NAP-2, in the absence
of any saturated nitrogen donor, the involvement of the
PET process may be excluded. Therefore, in this case, the
CHEF effect is presumably the main factor contributing to
the fluorescence enhancement upon Cd2þ chelation.
The complexation stoichiometry of Cd2þ with both
NAP-1 and NAP-2 was found to be 1:1 on the basis of the
Job plots (see Supporting Information, available online).
The apparent stability constants, log Ks, were calculated
by applying a non-linear square regression analysis of
fluorescence changes against metal ion concentrations
(21). For NAP-1, the log Ks for Cd2þ and coordinatively
competing Zn2þ were found to be 2.99 and 1.3,
respectively. Thus, in comparison to Zn2þ, over 1.5 log
unit, higher stability constant for Cd2þ signifies its
superior binding interaction over Zn2þ. For other metal
ions, the concentration-dependent fluorescence changes
were too insignificant to allow a reliable measure of their
log Ks. On the other hand, for the case of NAP-2, which
interacts only with Cd2þ, the log Ks was determined to be
3.48. The detection limits of NAP-1 and NAP-2 towards
Cd2þ, calculated from the fluorescence data (22), were
found to be 1.51 £ 1026 M and 1.4 £ 1026 M, respectively
(see Supporting Information, available online). These
micromolar sensitivities compare well with many reported
Cd fluorescence sensors and, as such, may be useful for
environmental monitoring (13).
Cd2þ (ca. 10 times less than the saturating Zn2þ) into the
above solution of NAP-1 þ Zn2þ not only caused the
386 nm emission band to shift back to 430 nm, but also
displayed about 15-fold emission enhancement at this
emission maximum. These emission features are reminis-
cent of those observed when NAP-1 interacts with Cd2þ
alone (see Figure 3). This experiment clearly demonstrates
that Cd2þ, even in relatively lower concentrations, can
completely displace Zn2þ from its NAP-1 chelate. Thus,
we can conclude that the affinity of NAP-1 towards Cd2þ
is relatively more pronounced than with Zn2þ. For other
metal ions, namely Naþ, Liþ, Kþ, Mg2þ, Ca2þ, Ba2þ
,
Co2þ, Ni2þ, Cu2þ, Agþ and Pb2þ investigated, no
detectable fluorescence responses could be discerned up
to 1.2 £ 1023 M, suggesting either very weak or no binding
interactions of NAP-1 with these metal ions.
The fluorescence sensing of NAP-2 towards Cd2þ was
next examined. As depicted in Figure 5, fluorimetric
titration of NAP-2 (1 £ 1026 M in MeOH) with increasing
addition of Cd2þ caused the probe’s emission maximum at
433 nm to progressively red shift to a new location at
465 nm, with a distinct isoemissive point being observed at
403 nm. The red shift in emission band is consistent with
the similar shift also observed in the absorbance of NAP-2
with Cd2þ. At a saturation concentration of 7.4 £ 1024
M
Cd2þ, the fluorescence intensity at the new maximum
registered approximately 5-fold increase with respect to
that of the unbound probe at 433 nm. Interestingly, in
contrast to NAP-1 that offers only enhanced fluorescence
with Cd2þ, the fluorescence response of NAP-2 comprises
both the change in the emission wavelength and
fluorescence amplification. Consistent with the absence
of any detectable absorbance changes, essentially no
fluorescence responses were exerted by Naþ, Liþ, Kþ,
Ca2þ, Ba2þ, Mg2þ,Co2þ, Ni2þ, Cu2þ, Agþ, Pb2þ as well
as coordinatively Zn2þ up to 5.3 £ 1023 M. These findings
imply that, among the metal ions examined, only Cd2þ
exhibits uniquely selective complexation with NAP-2
1
1
Figure 5. NAP-2 (1 £ 1026 M) on addition of cadmium
perchlorate (0–7.4 £ 1024 M) in MeOH.
Figure 6. (a) H NMR spectra of NAP-1 in MeOH and (b) H
NMR of NAP-1 in the presence of ca. 5 equiv. of Cd(ClO4)2.

528
S.H. Mashraqui et al.
Figure 7. (a) Two conformations of NAP-1 and the NCCC(O) dihedral values (degrees) are shown. (b) Two conformations of NAP-2
and the NCCC(O) and NCCO dihedral values (degrees) are shown.
Evaluation of cadmium binding by NMR
Molecular modelling studies
We have also evaluated the Cd2þ binding interactions by
1H NMR analysis. The 1H NMR spectra of NAP-1 without
and with Cd2þ are depicted in Figure 6. The assignment of
various protons is based on the chemical shift positions
and spin multiplicities and further supported by 2D NMR
analysis (see Supporting Information, available online). It
can be clearly seen that upon Cd2þ complexation, all the
protons associated with the naphthyridine and the anisyl
rings exhibit downfield shifts in the range of 0.1–0.4 d and
0.05–0.20 d, respectively. In addition, the side-chain
protons (CH3)2NZ, . NZCH2Z and ZCH2ZOZ also
suffer downfield shifts by 0.20, 0.31 and 0.12 d,
respectively. These cation-induced downfield shifts of the
rings and the side-chain protons clearly imply that Cd2þ
interacts with NAP-1 via a tetra-dentate coordination. An
attempt was also made to evaluate Cd2þ interaction with
NAP-2 by 1H NMR analysis. Although some of the
protons associated with the ring systems as well as the side
To further understand metal ion interactions, we optimised
the conformations of NAP-1 and NAP-2 based on the
theoretical calculations using RI-BP86/def2-TZVP both
under the gas phase and taking into account the solvent
effects. The two most stable conformations for both NAP-
1 and NAP-2 are shown in Figure 7(a) and (b),
respectively. For NAP-1, the conformation designated as
the conformer B can establish two weak intramolecular
non-covalent interactions. Thus, the conformer B is
calculated to be more stable by 3.14 kcal/mol in the gas
phase and 1.98 kcal/mol in the solvent (1 ¼ 41.76)
compared to the conformer A lacking in any non-covalent
interaction. For the case of NAP-2, the conformer B, which
exhibits three weak intramolecular non-covalent
interactions, is more stable by 0.89 kcal/mol in the gas
phase and 1.69 kcal/mol in the solvent (1 ¼ 32.61)
compared to the conformer A showing only one weak
intramolecular non-covalent interaction.
chain do appear downfield shifted in the presence of Cd2þ
,
The most stable conformations of the complexes of
Cd2þ and Zn2þ with NAP-1 and NAP-2 are depicted in
Figure 8(a) and (b), respectively, and the computed
binding energies are summarised in Table 1. Contrary to
our experimental results, the gas-phase calculations
indicate that the zinc cation should form the most stable
complexes. However, when the solvent effects are taken
individual assignments are complicated by the broad and
overlapping nature of the resonances (see Supporting
Information, available online). Although a clear con-
clusion about the binding mode in NAP-2 could not be
extracted from the NMR study, we believe that the tetra-
coordinated chelation is possible in this case as well.
Figure 8. Optimised complexes of (a) NAP-1 and (b) NAP-2 with Cd2þ and Zn2þ
.

Supramolecular Chemistry
529
˚
Table 1. Binding energies (DE, kcal/mol), equilibrium distances (Re, A) and dihedral angles (8).
Probeþcation
DE (gas phase)
DE (solvent)
Re (solvent)
NCCC(O)
NAP-1 þ Cd2þ
NAP-1 þ Zn2þ
NAP-2 þ Cd2þ
NAP-2 þ Zn2þ
2297.5
2349.4
2299.9
2354.3
2140.7
291.5
2139.4
295.7
2.31, 2.24, 2.56, 2.19
2.18, 1.99, 2.30, 1.99
2.30, 2.53, 2.51, 2.13
2.17, 2.00, 2.27, 1.94
22.9
14.4
18.2, 18.0
5.1, 3.9
into account, cadmium exhibits more favourable inter-
action than zinc, the results being in conformity with our
experimental findings. From the calculations, it can also be
observed that the values of the NCCC(O) and NCCO
dihedral angles correlate well with the binding energies of
these metal ions. It can be seen that the tetra-coordinated
metal ion interaction is accompanied by certain degrees of
reduction in the dihedral angles between the naphthyridine
and the sphenoxyl rings. While the larger Cd2þ
structures around the naphthyridine core to enhance the
sensitivity towards Cd2þ with the possibility of physio-
logical measurements.
Experimental and theoretical methods
General
Commercially available compounds were used without
further purification. Reagents and solvents used were
purchased form S.D. Fine Chemicals, Mumbai, India or
Sigma-Aldrich, Bangalore, India and used as received.
UV–vis spectra were recorded using Shimadzu recording
spectrophotometer, model no. UV-2401PC. Fluorescence
studies were carried out using Shimadzu spectrofluo-
rometer, model no. RF-5301PC. IR spectra were recorded
using Perkin-Elmer FT-IR spectrometer. 1H and 13C
NMR were recorded on Bruker Model Avance II
300 MHz.
˚
(r þ ¼ 0.96 A) imposes a lower level of reduction in the
dihedral angles, the geometrical requirements of the small
cation Zn2þ (r þ ¼ 0.74 A) for the shorter metalZN,O
˚
distance provoke sharp reductions in these dihedral angles
and consequently there are higher energetic costs attached
to the zinc complexation with the probes (2a).
Conclusion
In summary, the photoemittive 1,8-naphthyridine has been
elaborated to access two weakly emissive chemosensors,
NAP-1 and NAP-2, via simple synthetic procedures. Upon
exposure to Cd2þ, NAP-1, a PET-based probe, delivers
high fluorescence amplification, whereas the coordina-
tively competing Zn2þ only at significantly higher
concentrations induces relatively small fluorescence
enhancement. The superior affinity of Cd2þ is evident by
its ca. 1.5 log unit higher stability constant over that of
Zn2þ. Moreover, physiologically important metal ions,
such as Naþ, Kþ, Ca2þ, Mg2þ, Cu2þ and Co2þ, as well as
Liþ, Ba2þ, Ni2þ, Agþ and Pb2þ, of some biological/envir-
onmental import do not exert measurable optical
perturbations. On the other hand, NAP-2 exhibits
remarkable selectivity only towards Cd2þ, affording the
absorbance and emission red shifts as well as fluorescence
turn-on signalling responses. Other metal ions, including
Zn2þ, do not pose any interference even in relatively
higher concentrations. The 1H NMR analysis supports the
formation of a tetra-coordinated complexation between
NAP-1 and Cd2þ. In agreement with our experimental
findings, the theoretical calculations also predict superior
binding interactions of Cd2þ over Zn2þ. Although the
micromolar detection offered by NAP-1 and NAP-2
towards Cd2þ augurs well for environmental monitoring,
the present sensitivity needs to be improved to make the
probes compatible for nanomolar biological tracking of
Cd2þ. Work is currently under way to modify the ligand
Theoretical methods
The geometry of all the complexes included in this study
was fully optimised at the RI-PB86/def2-TZVP level of
theory within the program TURBOMOLE version 5.7
(23). The binding energies were calculated at the same
level. The optimisation of the molecular geometries has
been performed without imposing symmetry constraints.
We have also performed calculations in the presence of a
solvent (methanol, e ¼ 32.61 and MeOH:H2O (v/v 80:
20), e ¼ 41.76) using the Conductor-like Screening Model
(COSMO) (24) as implemented in TURBOMOLE.
Preparation of compound 3
2-Hydroxy acetophenone 1 (680 mg, 5 mmol) and N,N-
dimethylaminoethane hydrochloride 2 (1.08 g, 7.5 mmol)
were dissolved in dry DMF (20 ml) containing anhydrous
K2CO3 (1.38 g, 10 mmol). The reaction mixture was
stirred and heated at 1008C for 48 h. The reaction mixture
was diluted with water and extracted with chloroform
(3 £ 50 ml). The organic extract was washed once with
brine, dried over anhydrous Na2SO4 and concentrated. The
crude oily product was purified by SiO2 column
chromatography (eluant CHCl3:MeOH, 9:1) to give the
desired product 3 as a colourless oil (550 mg) in 53% yield.
IR (liquid film, cm21): 3054, 2963, 2872, 1640, 1615,

530
S.H. Mashraqui et al.
1
1577, 1539, 1307, 1215, 840, 755. H NMR (CDCl3): d
2.37 (s, 6H), 2.63 (s, 3H), 2.84 (t, 2H, J ¼ 13.5 Hz), 4.15
(t, 2H, J ¼ 12 Hz), 6.97 (q, 2H), 7.45 (t, 1H, J ¼ 7.2 Hz),
7.73 (d, 1H, J ¼ 9 Hz). Anal. Calcd: C12H11NO2. C, 69.56;
H, 8.21; N, 6.76; Found: C, 69.55; H, 8.23; N, 6.77.
in ethyl alcohol (10 ml) containing ca. 20 mg of KOH.
The reaction mixture was heated to reflux under nitrogen
atmosphere for 12 h. After completion of the reaction, the
reaction mixture was concentrated by half, diluted with
water and the precipitate solid was filtered. The product
was purified by repeated crystallisation from CHCl3:
petroleum ether (50:50) to give NAP-2 (120 mg) in 41%
yield. Mp ¼ 169–1718C. IR (KBr, cm21): 1600, 1577,
1537, 1489, 1456, 1422, 1309, 1268, 1203, 1025, 845. 1H
NMR (CD3OD): d 5.38 (s, 2H), 7.25 (t, 1H, J ¼ 12.4 Hz),
7.39 (d, 1H, J ¼ 6.7 Hz), 7.41 (t, 1H, J ¼ 12.3 Hz), 7.58
(m, 2H), 7.71 (t, 1H, J ¼ 12.5 Hz), 7.85 (t, 1H,
J ¼ 17.7 Hz), 7.9 (d, 1H, J ¼ 7 Hz), 8.30 (d, 1H,
J ¼ 6.7 Hz), 8.49 (d, 1H, J ¼ 6.7 Hz), 8.5 (d, 1H,
J ¼ 7.9 Hz), 8.63 (d, 1H, J ¼ 6.4 Hz), 9.15 (d, 1H,
J ¼ 6.2 Hz). 13C NMR (CDCl3): 71.17, 112.86, 121.17,
121.52, 121.76, 122.58, 124.67, 129.06, 131.01, 132.52,
135.93, 136.62, 136.83, 149.09, 153.35, 156.12, 156.23,
156.95. Mass m/e ¼ 314. Anal. Calcd: C, 76.71; H, 4.80;
N, 13.41. Found: C, 76.97; H, 4.69; N, 13.49.
Preparation of compound 5
2-Hydroxy acetophenone 1 (1.36 g, 10 mmol) and 2-
chloromethyl pyridine hydrochloride HCl 4 (820 mg,
5 mmol) were dissolved in dry DMF (20 ml) containing
anhydrous K2CO3 (2.76 g, 20 mmol). The reaction mixture
was stirred and heated at 1008C for 18 h. The reaction
mixture was worked up as described for the preparation of
3. The crude oily product was purified by SiO2 column
chromatography (eluant CH3Cl) to give the desired
product 5 as a colourless oil (670 mg) in 59% yield. IR
(liquid film, cm21): 3014, 2973, 1659, 1593, 1504, 1436,
1387, 1368, 1269, 1120, 745. 1H NMR (CDCl3): d 2.84 (s,
3H), 5.38 (s, 2H), 7.27 (t, 1H, J ¼ 12.5 Hz), 7.4 (d, 1H,
J ¼ 6.8 Hz), 7.45 (t, 1H, J ¼ 12.0 Hz), 7.6 (m, 2H), 7.86 (t,
1H, J ¼ 17.0 Hz), 7.9 (d, 1H, J ¼ 7 Hz), 8.62 (d, 1H,
J ¼ 6.4 Hz). Anal. Calcd: C14H13NO2, C, 74.0; H, 5.72; N,
6.16. Found: C, 73.92; H, 5.75; N, 6.20.
Acknowledgement
Thanks are due to the UGC, India for financial assistance to
Rupesh Betkar and Mukesh Chandiramani.
Supporting Information: 1H NMR spectra of compounds 3
and 5, 1H NMR and 13C NMR spectra of NAP-1, NAP-2, UV–vis
profiles, Job’s plots and detection limits of NAP-1 and NAP-2,
2D NMR of NAP-1 and 1H NMR of NAP-2 with Cd2þ, available
online.
Preparation of NAP-1
Compound 3 (414 mg, 2 mmol) and 2-aminopyridine
3-carboxyldehyde 6 (244 mg, 2 mmol) were dissolved
in ethyl alcohol (15 ml) containing ca. 50 mg of KOH.
The reaction was heated to reflux under nitrogen
atmosphere for 12 h. After completion of the reaction,
the reaction mixture was diluted with water, the solid
precipitate was filtered and air dried. The crude solid
was purified by SiO2 column chromatography (eluant
CH3Cl) to give the desired product NAP-1 as a light
yellow solid (260 mg) in 44% yield. Mp ¼ 99–1018C.
IR (KBr, cm21): 3002, 2950, 2823, 2773, 1602, 1580,
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1537, 1432, 1262, 871, 750. H NMR (CD3OD): d 2.29
(s, 6H), 2.81 (t, 2H, J ¼ 10.8 Hz), 4.28 (t, 2H,
J ¼ 11.1 Hz), 7.16 (t, 1H, J ¼ 12.6 Hz), 7.23 (d, 1H,
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