Zn2+ selective luminescent 'off-on' probes derived from diaryl oxadiazole and aza-15-crown-5

Article abstract of DOI:10.1016/j.tet.2007.08.031

New photo-induced electron transfer (PET) probes OMOX and OBOX, carrying an additional binding site in the form of 'oxadiazole nitrogen' have been designed to evaluate binding interactions with biologically significant Li⁺, Na⁺, K

Full text of DOI:10.1016/j.tet.2007.08.031

Tetrahedron 63 (2007) 11093–11100
Zn2D selective luminescent ‘off–on’ probes derived
from diaryl oxadiazole and aza-15-crown-5
a
a
b
Sabir H. Mashraqui,a, Subramanian Sundaram, Tabrez Khan and A. C. Bhasikuttan
*
aDepartment of Chemistry, University of Mumbai, Vidyanagari, Santacruz-E, Mumbai 400098, Maharashtra, India
bRadiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India
Received 28 April 2007; revised 25 July 2007; accepted 10 August 2007
Available online 15 August 2007
Abstract—New photo-induced electron transfer (PET) probes OMOX and OBOX, carrying an additional binding site in the form of
‘oxadiazole nitrogen’ have been designed to evaluate binding interactions with biologically significant Li+, Na+, K+, Ca2+, Mg2+, and
Zn2+ including environmentally toxic Ba2+ and Cd2+ using optical spectral techniques. While Li+, Na+, and K+ did not appreciably perturb
either the absorption or emission spectra, Ba2+, Ca2+, Mg2+, Zn2+, and Cd2+ induced slight red shifts (2–8 nm) in the UV–visible spectra as
well as pronounced chelation induced enhanced fluorescence (CHEF). Both OMOX and OBOX exhibited the highest CHEF in contact with
the zinc ion, whereas Ba2+, Ca2+, Mg2+, and Cd2+ induced relatively less emission enhancements. OBOX, which is a poorer emitter
(Ff¼0.0062) than OMOX (Ff¼0.015), showed highly promising 160-fold emission enhancement in the presence of Zn2+. Potential, therefore
is available in OBOX to function as a selective luminescent ‘off–on’ sensor for Zn2+ in the presence of coordinatively competing Ba2+, Ca2+
,
Mg2+, and Cd2+ ions.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
6-methoxy-(8-p-toluenesulfonamido)quinoline,7 Zinquin,8
Zinbo-5,9 and the Zinpyr family of chemosensors.10 Lumi-
nescent zinc sensors have also been designed using high
zinc affinity ligands, e.g., dipicolyl amine,10c,11 cyclam,11f,h
and iminodiacetic acid.12 However, with a few excep-
tions,12a,13 most Zn2+ probes provide fewer than 15-fold
CHEF upon zinc coordination.11d,14 Moreover, optical spec-
tral interference arising from the coordinatively competing
Ba2+, Ca2+, Mg2+, and Cd2+ ions are a mitigating factor,
which limit the application of these sensors.11f,h,15 There-
fore, selective discrimination of zinc ions is still deemed
an appealing area of research.12e,f,14b,c,e,16
Designing functional luminescent probes for the selective
detection of chemically and biologically significant metal
ions continues to engage worldwide attention. Given the di-
verse roles of Zn2+ in a multitude of cellular functions, its
trace detection is of special interest. Zinc ion is implicated
as a structural cofactor in metalloproteins, regulation of
gene expression,1 and cellular apoptosis.2 Zn2+ is also a con-
stituent of most DNA and RNA polymerases.3 A disorder of
zinc metabolism is believed to give rise to many neurologi-
cal conditions such as Alzheimer’s disease, amyotrophic lat-
eral sclerosis, Guam ALS-Parkinsonism dementia, and
hypoxia-ischemia and epilepsy.4 Additionally, clinical evi-
dence shows that zinc nutrition aids in wound healing
through a family of zinc dependent endopeptidases.5 Also,
zinc metal is a soil pollutant, significant concentrations of
which cause phytotoxic effects on soil microbes.6
Though, monoaza-15-crown-5 is known to bind zinc ion,
surprisingly Zn2+ sensors incorporating this receptor are
scarce in the literature.13a,16a,17 In the context of our interest
in fluoroionophores,18 we recently reported a new class of
PET based sensors, MOX and BOX (Scheme 1). These
probes were selective for Mg2+ among selected alkali and
divalent metal ions examined by us.19 However, in MOX
and BOX, only the aza-crown ring(s) participate in metal
ion complexation, with conceivably no binding interaction
stemming from the remotely placed oxadiazole. Since, par-
ticipation of the nitrogen of the oxadiazole ring in metal ion
coordination is precedented,20 we have presently designed
new analogs, designated as OMOX and OBOX with
a view to inducing lariat-type participation of oxadiazole
in the complexation process. Structurally, fluoroionophores
Most zinc sensors typically operate via PET, a process that
induces CHEF upon metal ion coordination with the aza-
receptor domains. Some well-known Zn2+ probes include
aryl sulfonamide derivatives of 8-aminoquinoline, such as
Keywords: Diaryl oxadiazoles; Synthesis; Photo-induced electron transfer;
UV–visible; Fluorescence; Selective Zn2+ sensor.
* Corresponding author. Tel.: +91 022 26526091; fax: +91 022 26528547;
e-mail: sh_mashraqui@yahoo.com
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.08.031

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S. H. Mashraqui et al. / Tetrahedron 63 (2007) 11093–11100
O
O
N
N
N
N
N
N
O
N
O
O
O
O
O
O
O
O
O
O
O
MOX
BOX
O
O
N
N
N
N
N
N
O
O
N
O
O
O
O
O
O
O
O
O
O
OMOX
OBOX
Scheme 1.
OMOX and OBOX are characterized by the presence of one
and two aza-15-crown-5 receptors at the 2 and 2,20 positions
of the diaryl-oxadiazole motif, respectively. The photo-
physical behaviors of OMOX and OBOX, being fluoro-
phore-spacer-receptor models, are expected to be governed
by the PET mechanism. Furthermore, the availability of ad-
ditional ligating site in the form of ‘oxadiazole nitrogen’ in
OMOX and OBOX, (a feature lacking in MOX and BOX) is
expected to alter their metal binding characteristics to offer
different or improved photo-physical responses in the com-
pany of interacting metal ions.
of OMOX and OBOX, which are also PET probes, the
observed small red shifts, particularly with Zn2+ (8 nm),
Mg2+ (6 nm), and Cd2+ (5 nm) could be a consequence of
lariat-type binding of the oxadiazole nitrogen with the
aza-crown-bound cations. Such an interaction could induce
a marginal degree of enhancement in the charge transfer in-
teraction between the donor aryl ring(s) and the acceptor,
metal-coordinated oxadiazole, thereby giving rise to the
slight red shifts.
Excitation of OMOX and OBOX at their absorption max-
ima at 275 and 270 nm produced, respectively, emission
bands at 356 and 339–350 nm regions. These emission
bands presumably originate from the locally excited states.
The quantum yields Ff for OMOX and OBOX, determined
with reference to 2,20-biphenyldiol in acetonitrile
(Ff¼0.29)21 were found to be 0.0150 and 0.0062, respec-
tively. Relatively poor Ff for OBOX compared to OMOX
is understandable in view of a more efficient PET quenching
arising from two aza-crown ether rings in OBOX as against
just one aza-crown ether quencher in OMOX. This behavior
is in accord with many known fluoroionophores carrying
bis-(aza)-crown ether, which also display highly quenched
fluorescence.19,22
2. Results and discussion
Synthesis of OMOX and OBOX was carried out as shown in
Scheme 2. Condensation of o-toluolyl chloride 1 with 4-tert-
butylbenzoic hydrazide 2 in refluxing POCl3 provided diaryl
oxadiazole 3 in good yield. Bromination of 3 with NBS/CCl4
containing a catalytic amount of dibenzoyl peroxide af-
forded bromide 4. The reaction of 4 with a slightly more
than 1 equiv of monoaza-15-crown-5 under acetonitrile/
K2CO3 condition, followed by SiO2 column purification
afforded OMOX as a light yellow oil. Toward the synthesis
of OBOX, o-toluolyl chloride 1 was condensed with o-toluic
hydrazide 5 in POCl3 under reflux. The resulting oxadiazole
6 was subjected to two-fold bromination with NBS/CCl4
containing a catalytic amount of dibenzoyl peroxide to
afford dibromide 7. Finally, the coupling of 7 with 2.2 equiv
of monoaza-15-crown-5 under the conditions described
for OMOX gave after SiO2 purification OBOX as light
yellow oil.
Consistent with the red shifts observed in the UV spectra, the
emission bands of OMOX and OBOX (lem at 356 and
350 nm, respectively) were also slightly red shifted by
1–5 nm upon adding perchlorates of Li+, Na+, K+, Ba2+,
Ca2+, Mg2+, Zn2+, and Cd2+. Fluorescence emission changes
of OBOX with respect to increasing concentration of Zn2+
are depicted in Figure 3. With alkali metal ions Li+, Na+,
K+ (concn ꢂ100-fold, i.e., ꢂ2.97ꢀ10ꢁ4 M) emission-band
intensities increased only ca. 3–50%, however Ba2+, Ca2+,
Mg2+, Zn2+, and Cd2+ induced significantly higher chelation
induced enhanced fluorescence (CHEF) at relatively lower
concentrations (ꢃ1.18ꢀ10ꢁ5 M) in the emission spectra of
both OMOX and OBOX. In the presence of Ba2+, Ca2+,
Mg2+, Zn2+, and Cd2+, the CHEF experienced by OMOX
and OBOX were found to be 4.6, 8.5, 14.6, 30.0, and
11.0-folds and 7.5, 3.4, 28.0, 160.0, and 43.0-fold, respec-
tively. The fluorescence enhancements observed with
OMOX in the presence of various metal ions follow the order
Absorbance spectra of OMOX (Fig. 1) and OBOX (Fig. 2)
measured in MeCN showed a broad band at 275 and 270 nm,
with molar extinction coefficients, 3m of ca. 2.31ꢀ104 and
2.52ꢀ104 Mꢁ1 cmꢁ1, respectively. Addition of perchlorates
of Li+, Na+, K+, Ba2+, Ca2+, Mg2+, Zn2+, and Cd2+ to the so-
lutions of OMOX and OBOX in MeCN produced small red
shifts of 1–8 nm. Since, the PET sensors do not exhibit
ground state interactions between the receptor and the fluo-
rophore, their excitation energies remain essentially unper-
turbed upon exposure to metal ions. However, in the cases

S. H. Mashraqui et al. / Tetrahedron 63 (2007) 11093–11100
11095
1,4-Dioxane / POCl3
O
+
COCl
H2N-NH-OC
water-bath
80%
N
N
1
2
3
O
O
O
O
N
O
NBS / CCl4
76%
O
H
N
N
CH3CN / K2CO3
64%
N
N
N
Br
O
4
O
N
O
O
OMOX
O
1,4-Dioxane / POCl3
COCl + H2N-NH-OC
water-bath
74%
N
1
5
6
O
O
O
O
N
O
O
NBS / CCl4
67%
H
N
N
N
CH3CN / K2CO3
55%
N
Br
Br
N
N
O
O
7
O
O
O
O
O
O
OBOX
Scheme 2.
Zn2+>Mg2+>Cd2+>Ca2+>Ba2+>Li+yNa+yK+ while for
OBOX, theorderisZn2+[Cd2+>Mg2+>Ba2+>Ca2+>Li+>
K+>Na+. With the aza-crown receptor being common to
both hosts, the marked differences observed in the binding
profiles with different metal ions presumably reflect differing
degree of participation of the oxadiazole nitrogen with the
aza-crown bound metal ions. The relative fluorescent
enhancements observed with OMOX and OBOX with vari-
ous metal ions at their complete complexations are high-
lighted in Figures 4 and 5, respectively. Of particular note
is the observation that among various cations examined,
zinc ion in both OMOX and OBOX produced the highest
CHEF by a factor of 30 and 160-fold, respectively. Further-
more, OBOX, which is a poorer emitter than OMOX (Ff
0.8
0.8
Zn2+
Zn2+
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
OMOX
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Mg2+>Cd2+>Ca2+>Ba2+>Na+~Li+~K+
OBOX
Cd2+>Mg2+>Ba2+>Li+>Ca2+>K+~Na+
240 250 260 270 280 290 300 310 320 330 340 350
Wavelength (nm)
240 250 260 270 280 290 300 310 320 330 340 350
Wavelength (nm)
Figure 1. Absorption spectra of OMOX (2.55ꢀ10ꢁ5 M) in the absence and
presence of Li+ (5.35ꢀ10ꢁ5 M), Na+ (4.84ꢀ10ꢁ5 M), K+ (5.86ꢀ10ꢁ5 M),
Ba2+ (4.33ꢀ10ꢁ5 M), Ca2+ (3.98ꢀ10ꢁ5 M), Mg2+ (3.95ꢀ10ꢁ5 M), Zn2+
(2.80ꢀ10ꢁ5 M), and Cd2+ (2.89ꢀ10ꢁ5 M) perchlorates at their saturated
concentration in MeCN.
Figure 2. Absorption spectra of OBOX (2.17ꢀ10ꢁ5 M) in the absence and
presence of Li+ (7.48ꢀ10ꢁ5 M), Na+ (7.38ꢀ10ꢁ5 M), K+ (7.59ꢀ10ꢁ5 M),
Ba2+ (6.48ꢀ10ꢁ5 M), Ca2+ (6.42ꢀ10ꢁ5 M), Mg2+ (6.18ꢀ10ꢁ5 M), Zn2+
(5.86ꢀ10ꢁ5 M), and Cd2+ (6.08ꢀ10ꢁ5 M) perchlorates at their saturated
concentration in MeCN.

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S. H. Mashraqui et al. / Tetrahedron 63 (2007) 11093–11100
160
140
120
1080
960
840
720
600
480
360
240
120
0
8.02 x 10-6
M
100
80
60
40
20
0
0
Li(I) Na(I) K(I) Ba(II) Ca(II) Mg(II) Zn(II) Cd(II)
Metal ions
Neat
300 320 340 360 380 400 420 440 460 480
Wavelength (nm)
Figure 5. The relative fluorescence intensity of OBOX (2.97ꢀ10ꢁ6 M) on
addition of Li+ (1.02ꢀ10ꢁ5 M), Na+ (1.01ꢀ10ꢁ5 M), K+ (1.04ꢀ10ꢁ5 M),
Ba2+ (8.88ꢀ10ꢁ6 M), Ca2+ (8.79ꢀ10ꢁ6 M), Mg2+ (8.46ꢀ10ꢁ6 M), Zn2+
(8.02ꢀ10ꢁ6 M), and Cd2+ (8.32ꢀ10ꢁ6 M) perchlorates at their saturated
concentration, (lex¼270 nm, lem¼350 nm).
Figure 3. Fluorescence spectra (corrected) of OBOX (2.97ꢀ10ꢁ6 M) on
titration with Zn2+ (0–8.02ꢀ10ꢁ6 M) in MeCN.
of 0.0062 vs 0.0150) showed, except for Ca2+ relatively
higher emission enhancements with the other divalent metal
ions examined. This behavior is due to a more efficient sup-
pression of PET process in OBOX as a consequence of block-
ing of electron transfer from both the aza-crown ether
domains. Similar results have been previously encountered
with certain diaza-crown ethers, which also exhibit signifi-
cantly higher fluorescence switch-on ability on complexation
compared to their monoaza-crown counterparts.23 The selec-
tivity ratios for zinc ions, expressed as Zn2+/M2+ for OMOX
with respect to interfering Ba2+, Ca2+, Mg2+, and Cd2+ were
found to be 6.5, 3.5, 2.1, and 2.7, respectively. However, the
corresponding selectivity ratios for OBOX are significantly
higher at 21.5, 46.6, 5.7, and 3.7, respectively. Clearly,
OBOX, compared to OMOX exhibits better selectivity and
improved optical discrimination for the zinc ion relative to
coordinatively interfering Ba2+, Ca2+, Mg2+, and Cd2+ ions.
hand, for OBOX, which contains two aza-crown receptors,
an approximate plateau is reached at ca. 2.7 equiv of Zn2+
ions (see the plot shown in Fig. 7). Unlike OMOX, where
1 equiv of Zn2+ produced the highest (30-fold) emission
enhancement, however, in contrast for the case of OBOX,
addition of ca. 1 equiv of zinc resulted in only a six-fold
enhancement in emission intensity. This observation implies
that when only one aza-crown is metal bound, the PET pro-
cess still remains substantially operative due to the non-
occupancy of the second aza-crown ring. It is only when
the Zn2+ concentration is increased beyond 1 equiv that
steeper increases in emission intensity are observed, reach-
ing a maximum of 160-fold at ca. 2.7 mole fraction of
zinc. In this case, 1:2 stoichiometry corresponding to
OBOX and Zn2+ was indicated by the Job’s plot method.
Ideally, we expected to observe two plateaus for OBOX,
the first plateau appearing at 1:1 and the second at 1:2 stoi-
chiometry with respect to OBOX and Zn2+. However, in
practice only a single plateau corresponding to approxi-
mately 1:2 stoichiometry was discernible in this case. Fail-
ure to observe two separate plateau could in parts be due
to (a) the formation of small amount of 1:2 complex even
at 1:1 stoichiometry and (b) relatively steeper increases in
Addition of ca.1 equiv of Zn2+ to a solution of OMOX in
CH3CN led to an approximate plateau indicating 1:1 com-
plexation stoichiometry (Fig. 6), which was further con-
firmed by applying the Job’s plot method. On the other
32
28
24
20
16
12
8
1800
1600
1400
1200
1000
800
600
400
200
0
4
0
Neat Li(I) Na(I) K(I) Ba(II) Ca(II) Mg(II) Zn(II) Cd(II)
Metal ions
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
Eq. of Zn2+ ions
Figure 4. The relative fluorescence intensity of OMOX (6.96ꢀ10ꢁ6 M) on
addition of Li+ (1.46ꢀ10ꢁ5 M), Na+ (1.32ꢀ10ꢁ5 M), K+ (1.59ꢀ10ꢁ5 M),
Ba2+ (1.18ꢀ10ꢁ5 M), Ca2+ (1.08ꢀ10ꢁ5 M), Mg2+ (1.08ꢀ10ꢁ5 M), Zn2+
(7.64ꢀ10ꢁ6 M), and Cd2+ (7.89ꢀ10ꢁ6 M) perchlorates at their saturated
concentration (lex¼275 nm, lem¼356 nm) in MeCN.
Figure 6. Variation in fluorescence intensity of a solution of OMOX
(6.96ꢀ10ꢁ6 M) in MeCN at 356 nm as a function of added equivalents
of Zn2+
.

S. H. Mashraqui et al. / Tetrahedron 63 (2007) 11093–11100
11097
1200
1080
960
840
720
600
480
360
240
120
0
1200
1100
1000
900
800
700
600
500
400
300
200
100
OBOX+matrix+Zn2+
OBOX+matrix
OBOX
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Eq. of Zn2+ ions
300 320 340 360 380 400 420 440 460 480
Wavelength (nm)
Figure 7. Variation in fluorescence intensity of a solution of OBOX
(2.97ꢀ10ꢁ6 M) in MeCN at 350 nm as a function of added equivalents
Figure 8. Fluorescence spectra of OBOX alone (2.97ꢀ10ꢁ6 M), OBOX+
matrix, consisting of Ba2+ (8.88ꢀ10ꢁ6 M), Ca2+ (8.79ꢀ10ꢁ6 M), Mg2+
(8.46ꢀ10ꢁ6 M), and Cd2+ (8.32ꢀ10ꢁ6 M), at their complete complexation
and OBOX+above matrix+Zn(ClO4)2 (8.02ꢀ10ꢁ6 M) in MeCN.
of Zn2+
.
To substantiate the high preference for Zn2+ in the presence
of interfering cations, fluorescence spectra of OBOX
(2.97ꢀ10ꢁ6 M) were recorded both without and with a
matrix consisting of Ba2+, Ca2+, Mg2+, and Cd2+ at their
complete complexation. The fluorescent spectrum with the
matrix revealed ca. 40-fold emission enhancement in the
fluorescence spectrum of OBOX (Fig. 8). This emission en-
hancement is reminiscent of that observed with Cd2+, which
is one of the strongest complexing cations present in the
matrix. Upon adding Zn(ClO4)2 (8.02ꢀ10ꢁ6 M) to the above
matrix, we observed a jump in the fluorescence enhancement
by ca. 120-fold, the overall enhancement being 160-fold, the
same as measured with Zn2+ alone (see Fig. 3). Evidently,
the superior complexing zinc ion displaces the relatively
poorly interacting Ca2+, Ba2+, Mg2+, and Cd2+ to form the
corresponding OBOX–Zn2+ complex.
the emission intensity beyond 1:1 complexation stoichio-
metry. These factors together could obscure the first plateau
expected at 1:1 complexation stoichiometry.
Apparent stability constants (log Ks), derived from emission
intensity changes for OMOX and OBOX with different cat-
ions are compiled in Table 1. For alkali metal ions, fluores-
cence changes were too small to allow a reliable measure of
their log Ks. Relatively poor CHEF with alkali metal ions
could be ascribed to their weak affinities toward the aza-
crown ring. Significantly higher log Ks of 6.95 for Zn2+
compared to Ca2+ and Mg2+ (log Ksꢃ5.89) obtained with
the better of the two fluoroionophore, OBOX is an important
factor in favor of zinc discrimination since, under many
physiological conditions, Ca2+ and Mg2+ exist at relatively
higher concentration than Zn2+. Remarkably high recogni-
tion of Zn2+ compared to the divalent ions investigated could
presumably arise from relatively stronger binding of Zn2+
with the aza-crown ether to effectively block the PET pro-
cess. In addition, electrostatic interaction of the aza-crown
bound Zn2+ with the ‘oxadiazole nitrogen’ could cause re-
stricted rotation around the aryl-oxadiazole bonds, thereby
reinforcing the radiative return of the excited states over
that of the non-radiative deactivation.
3. Conclusions
New PET based fluoroionophores, OMOX and OBOX have
been synthesized and their absorption and emission charac-
teristics investigated in the presence of biologically impor-
tant alkali and alkaline earth metal ions, including toxic
Table 1. Relative quantum yield (Ff), CHEF, and log Ks of OMOX and OBOX in the presence of metal ions
Mn+
OMOX
CHEF
OBOX
CHEF
Ffa
log Ksb
Ffa
log Ksb
Free
Li+
0.0150
0.0169
0.0166
0.0145
0.0675
0.1200
0.2085
0.4200
0.1575
1
—
0.0062
0.0233
0.0105
0.0124
0.0434
0.0201
0.1670
0.9553
0.2616
1
—
c
c
1.13
1.11
1.03
4.60
8.50
14.58
30
*
1.52
1.22
1.37
7.45
3.43
28
160
43
*
c
c
Na+
K+
*
*
c
c
*
*
Ba2+
Ca2+
Mg2+
Zn2+
Cd2+
5.12ꢄ0.12
5.35ꢄ0.18
5.73ꢄ0.21
6.05ꢄ0.17
5.60ꢄ0.14
4.92ꢄ0.12
4.60ꢄ0.19
5.27ꢄ0.17
6.95ꢄ0.14
5.89ꢄ0.12
11
a
The fluorescence quantum yields (Ff) were determined by comparing the integrated fluorescence spectra of the samples with that of the reference 2,20-bi-
phenyldiol in acetonitrile (Ff¼0.29).21
The stability constants were determined by non-linear fitting curve of fluorescence data.24
log Ks could not be determined due to insignificant fluorescence enhancement.
b
c

11098
S. H. Mashraqui et al. / Tetrahedron 63 (2007) 11093–11100
and polluting Ba2+ and Cd2+. While, the absorption spectra
are only marginally modified when exposed to various metal
ions, significant emission enhancements were however de-
tected only in the presence of divalent cations. In particular,
the highest emission enhancements were observed for both
OMOX and OBOX upon contact with Zn2+. The CHEF of
160-fold experienced by OBOX with Zn2+ is the one of
the highest among the known PET based Zn2+ sensors. Al-
though not yet defined, the participatory role of ‘oxadiazole
nitrogen’ in the complexation process could be a factor in
contributing to high fluorescence enhancements by impos-
ing conformational rigidity on the host molecules. The dra-
matic increase in the emission-band intensity and high
optical discrimination render OBOX, the better of the two
hosts a potential interesting luminescent ‘off–on’ probe for
Zn2+ in the presence of interfering Ca2+, Mg2+, Ba2+, and
Cd2+ ions.
(s, 3H, Ar–CH3), 1.38 (s, 9H, Ar–C(CH3)3). m/z: 293
(M+1)+, 275, 252, 211, 161, 137. Anal. Calcd for
C19H20N2O: C, 78.08; H, 6.85; N, 9.59. Found: C, 77.98;
H, 7.05; N, 9.74%.
4.1.2. 2-[2-(Bromomethyl)phenyl]-5-(4-tert-butyl-
phenyl)-1,3,4-oxadiazole (4). Oxadiazole
3
(3 g,
10.28 mmol) and N-bromosuccinimide (NBS) (2.19 g,
1.2 equiv) were dissolved in carbon tetrachloride. A cata-
lytic amount of benzoyl peroxide (BPO) was added as an ini-
tiator and the reaction mixture refluxed for about 5 h. The
reaction mixture was brought to room temperature and the
insoluble succinimide filtered off. The filtrate was concen-
trated to give crude solid. Crystallization from methanol
gave 4 as a colorless solid in 76% yield (2.89 g), mp 117–
119 ꢅC. IR (KBr, n, cmꢁ1): 3055, 2957, 1613, 1541, 1494,
1440, 1267, 1221, 1105, 1016, 845, 756, 712, 601, 557. 1H
NMR (300 MHz, CDCl3): d 8.11 (d, J¼8.1 Hz, 2H,
Ar–H), 7.61–7.49 (m, 6H, Ar–H), 5.16 (s, 2H, Ar–CH2Br),
1.38 (s, 9H, Ar–C(CH3)3). m/z: 372 (M+1)+, 342, 329,
315, 289, 274, 258, 211, 181, 137. Anal. Calcd for
C19H19BrN2O: C, 61.45; H, 5.12; N, 7.55; Br, 21.56. Found:
C, 61.75; H, 5.37; N, 7.43; Br, 21.47%.
4. Experimental
4.1. General
Metal perchlorates were prepared as described in the litera-
ture25 and dried under vacuum prior to use. The chemicals
and spectral grade solvents were purchased from S. D. fine
Chemicals (India) and used as received. IR spectra were
recorded on a Shimadzu FTIR-420 spectrophotometer.
1H NMR spectra were recorded in CDCl3 solution on
a 300 MHz Bruker-AMX-500 spectrometer with TMS as
an internal standard. Coupling constants J are given in hertz.
13C NMR spectra (75.5 MHz) were recorded on a Bruker
AC-300 instrument. Mass spectra were obtained using
LC–MS in ESI mode. Elemental analyses were done on
Carlo Erba instrument EA-1108 Elemental analyzer. UV–
visible spectra were recorded on a Jasco V-530 UV–visible
spectrophotometer and fluorescence spectra were recorded
on Hitachi F-4500 Fluorescence spectrophotometer. The
fluorescence quantum yields (Ff) were determined by com-
paring the integrated fluorescence spectra of the sample with
2,20-biphenyldiol in MeCN (Ff¼0.29).21
4.1.3. N-[2-(4-Methylphenyl)-5-(4-tert-butylphenyl)-
1,3,4-oxadiazole]aza-15-crown-5 (OMOX). Bromide 4
(0.371 g, 1 mmol) and monoaza-15-crown-5 (0.263 g,
1.2 mmol) were dissolved in dry acetonitrile. After the addi-
tion of anhyd K2CO3 (250 mg), the reaction mixture was
stirred and refluxed on water-bath for about 3 h. The reaction
mixture was then filtered and the filtrate concentrated. The
brown oily mass obtained was extracted with chloroform,
washed with water, and dried over Na2SO4. Crude product
obtained on solvent removal was purified by SiO2 column
chromatography (Rf¼0.27) using CHCl3:CH3OH (99:1) as
an eluent. The compound OMOX was obtained as pale yel-
low oil in 64% yield (0.326 g). IR (Nujol, n, cmꢁ1): 2962,
2870, 1614, 1547, 1496, 1415, 1362, 1250, 1128, 935,
1
844, 751, 716. H NMR (300 MHz, CDCl3): d 8.06 (d,
J¼8.5 Hz, 2H, Ar–H), 7.91 (d, J¼7.5 Hz, 1H, Ar–H), 7.78
(d, J¼7.6 Hz, 1H, Ar–H), 7.55 (d, J¼8.5 Hz, 2H, Ar–H),
7.51 (t, J¼7.5 Hz, 1H, Ar–H), 7.39 (t, J¼7.6 Hz, 1H,
Ar–H), 4.15 (s, 2H, pNCH2–Ar), 3.63–3.51 (m, 16H,
–OCH2CH2–), 2.77 (t, 4H, J¼8.2 Hz, pNCH2CH2O–),
1.37 (s, 9H). 13C NMR (CDCl3): d 164.90, 164.36, 155.27,
131.22, 130.29, 129.60, 127.10, 126.73, 126.07, 123.14,
121.19, 70.96, 70.84, 70.48, 70.11, 69.88, 59.10, 54.69,
35.08, 31.12. m/z: 511 (M+2)+, 467, 423, 373, 335, 292,
241, 219, 162. Anal. Calcd for C29H39N3O5: C, 68.37; H,
7.66; N, 8.25. Found: C, 68.07; H, 7.89; N, 8.44%.
4.1.1. 2-(2-Methylphenyl)-5-(4-tert-butylphenyl)-1,3,4-
oxadiazole (3). o-Toulic acid (3.40 g, 25 mmol) was taken
in benzene, to it was added SOCl2 (2.14 mL, 30 mmol)
and refluxed for about 2 h. Benzene and excess SOCl2
were removed by vacuum distillation. The resultant acid
chloride 1 was dissolved in dry 1,4-dioxane and 4-tert-butyl-
benzoic hydrazide 2 (4.61 g, 24 mmol) was introduced. The
reaction mixture was heated on water-bath for about 2 h, till
evolution of HCl gas ceased. To the reaction mixture was
then added POCl3 and the reaction continued to be heated
on water-bath for 3 h, till evolution of HCl gas ceased. The
reaction was brought to room temperature and poured care-
fully into 100 mL of ice-cold water and neutralized with
Na2CO3 to get a precipitate. The solid was filtered, washed
with water, and air-dried. Crystallization from methyl alco-
hol gave 3 as a colorless crystalline solid in 80% yield
(5.60 g), mp 80–83 ꢅC. IR (KBr, n, cmꢁ1): 3074, 2960,
1614, 1582, 1540, 1497, 1268, 1115, 1052, 847, 777, 727,
669, 560. 1H NMR (300 MHz, CDCl3): d 8.07 (d,
J¼8.4 Hz, 2H, Ar–H), 8.03 (d, J¼6.6 Hz, 1H, Ar–H), 7.56
(d, J¼8.4 Hz, 2H, Ar–H), 7.46–7.34 (m, 3H, Ar–H), 2.77
4.1.4. 2,5-Bis(2-methylphenyl)-1,3,4-oxadiazole (6). o-
Toulic acid (3.40 g, 25 mmol) was converted into acid chlo-
ride 1 as described earlier. The crude acid chloride 1 and
o-toluic hydrazide 5 (3.60 g, 24 mmol) were dissolved in
dry 1,4-dioxane (50 mL) and the reaction mixture heated
on a water-bath for about 2 h, till evolution of HCl gas
ceased. POCl3 (10 mL) was then introduced and the reaction
continued to be heated on the water-bath for further 3 h. The
reaction was poured carefully into 100 mL of ice-cold water,
neutralized with Na2CO3, filtered, and dried. Crystallization
of the crude with methyl alcohol gave 6 as a colorless solid
in 74% yield (4.44 g), mp 120–122 ꢅC. IR (KBr, n, cmꢁ1):

S. H. Mashraqui et al. / Tetrahedron 63 (2007) 11093–11100
11099
3. Lippard, S. J.; Berg, J. M. Principles of Bioinorganic
Chemistry; University Science Books: Mill Valley, CA, 1994.
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5. Ravanti, L.; Kahari, V. M. Int. J. Mol. Med. 2000, 6, 391–407.
6. (a) Voegelin, A.; Pfister, S.; Scheinost, A. C.; Marcus, M. A.;
Kretzschmar, R. Environ. Sci. Technol. 2005, 39, 6616–6623;
(b) Callender, E. Environ. Sci. Technol. 2000, 34, 232–238;
(c) Nowicki, J. L.; Johnson, K. S.; Coale, K. H.; Elrod, V. A.;
Lieberman, S. H. Anal. Chem. 1994, 66, 2732–2738.
7. Frederickson, C. J.; Kasarskis, E. J.; Ringo, D.; Frederickson,
R. E. J. Neurosci. Methods 1987, 20, 91–103.
3040, 2959, 1604, 1535, 1449, 1386, 1247, 1051, 1035, 782,
729, 678, 561. H NMR (300 MHz, CDCl3): d 8.05 (d,
J¼7.8 Hz, 2H, Ar–H), 7.47–7.34 (m, 6H, Ar–H), 2.79 (s,
6H, Ar–C(CH3)3). m/z: 251 (M+1)+, 238, 223, 209. Anal.
Calcd for C16H14N2O: C, 76.80; H, 5.60; N, 11.20. Found:
C, 77.05; H, 5.74; N, 10.96%.
1
4.1.5. 2,5-Bis[2-(bromomethyl)phenyl]-1,3,4-oxadiazole
(7). A mixture of 6 (3.75 g, 15 mmol) and N-bromosuccin-
imide (NBS) (5.73 g, 2.2 equiv) was dissolved in carbon
tetrachloride. A catalytic amount of BPO was added as an
initiator. The reaction mixture was refluxed for about 5 h
whereby the reaction was judged to be complete by TLC.
The reaction mixture was filtered to remove succinimide.
The filtrate was concentrated to give a crude solid. Crystal-
lization from 1:1 CHCl3/petroleum-ether gave 7 as a color-
less solid in 67% yield (4.10 g), mp 180–182 ꢅC. IR (KBr,
n, cmꢁ1): 3035, 1601, 1539, 1492, 1437, 1293, 1223,
8. Zalewski, P. D.; Forbes, I. J.; Betts, W. H. Biochem. J. 1993,
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2004, 126, 712–713.
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Hayashi, Y.; Sheng, M.; Lippard, S. J. Inorg. Chem. 2004,
43, 6774–6779; (b) Chang, C. J.; Nolan, E. M.; Jaworski, J.;
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1
1056, 1044, 884, 757, 783, 711, 679, 605, 566. H NMR
(300 MHz, CDCl3): d 8.14 (d, J¼7.5 Hz, 2H, Ar–H), 7.97
(d, J¼8.1 Hz, 2H, Ar–H), 7.70 (t, J¼6.9 Hz, 2H, Ar–H),
7.62 (t, J¼6.9 Hz, 2H, Ar–H), 5.20 (s, 4H, Ar–CH2Br). m/z:
409 (M+1)+, 407, 391, 375, 369, 346, 332, 329, 327, 298,
258, 252, 242, 233, 224. Anal. Calcd for C16H12Br2N2O:
C, 47.06; H, 2.94; N, 6.86; Br, 39.21. Found: C, 47.32; H,
3.10; N, 6.66; Br, 39.42%.
11. (a) Parola, A. J.; Lima, J. C.; Pina, F.; Pina, J.; de Melo, J. S.;
ꢀ
Soriano, C.; Garcıa-Espan˜a, E.; Aucejo, R.; Alarco˜n, J.
4.1.6. N-[2,5-Bis(2-methylphenyl)-1,3,4-oxadiazole]aza-
15-crown-5 (OBOX). This reaction was carried out as
described for OMOX by using dibromide 7 (0.408 g,
1 mmol), monoaza-15-crown-5 (0.657 g, 3 mmol), and an-
hyd K2CO3 (0.552 g, 4 mmol) in 10 mL of dry MeCN.
The brown oily mass obtained was extracted with chloro-
form, washed with water, and the organic layer dried over
anhyd Na2SO4. Crude oily product obtained upon solvent
removal was purified by SiO2 column chromatography
(Rf¼0.21) using CHCl3/CH3OH (98:2) as an eluent. The
compound OBOX was obtained as pale yellow oil in 55%
yield (0.376 g). IR (Nujol, n, cmꢁ1): 3065, 2963, 1615,
1552, 1496, 1443, 1311, 1270, 1200, 1088, 974, 851,
751, 715, 624. 1H NMR (300 MHz, CDCl3): d 7.94
(d, J¼8.7 Hz, 2H, Ar–H), 7.90 (d, J¼8.4 Hz, 2H, Ar–H),
7.53 (t, J¼6.7 Hz, 2H, Ar–H), 7.39 (t, J¼6.9 Hz, 2H,
Ar–H), 4.22 (s, 4H, pNCH2–Ar), 3.51–3.71 (m, 32H,
–OCH2CH2–), 2.82 (t, J¼5.5 Hz, 8H, pNCH2CH2O–). 13C
NMR (CDCl3): d 164.39, 131.53 130.36, 129.61, 129.49,
127.22, 122.89, 71.12, 70.73, 70.37, 70.10, 59.09, 55.02.
m/z: 687 (M+3)+, 673, 578, 507, 467, 423, 374, 290, 265,
157. Anal. Calcd for C36H52N4O9: C, 63.16; H, 7.60; N,
8.18. Found: C, 62.95; H, 7.89; N, 8.42%.
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Acknowledgements
Thanks are due to BRNS, Government of India for financial
support in the form of research grant 2004/37/29/BRNS.
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