Oxidative cyclization of N-acylhydrazones. Development of highly selective turn-on fluorescent chemodosimeters for Cu2+

Article abstract of DOI:10.1039/b811612a

A series of N-acylhydrazones were synthesised and found to be "turn-on" fluorescent chemodosimeters for Cu²⁺. Among the tested transition metal ions such as Cu²⁺, Pb²⁺, Zn ²⁺, Cd²⁺, Hg²⁺, and Ni²⁺, a prominent fluorescence enhancement of up to 1000-fold was only observed for Cu²⁺ in acetonitrile (CH₃CN). This was indicated by an onset of unprecedented structured emission. Detailed experiments established that the highly Cu²⁺ selective fluorescence enhancement resulted from an oxidative cyclization by Cu²⁺of the originally nonfluorescent N-acylhydrazones into highly fluorescent rigid 1,3,4-oxadiazoles, n-dope type blocks in optoelectronic materials. The chemodosimeters can be applied to sense Cu²⁺ at nM levels in CH₃CN and sub-M levels in neutral aqueous environments, despite a slower response in the latter case. It is expected that these redox-based chemodosimeters might be of general applicability.

Full text of DOI:10.1039/b811612a

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Oxidative cyclization of N-acylhydrazones. Development of highly selective
turn-on fluorescent chemodosimeters for Cu2+†
Ai-Fang Li, Hui He, Yi-Bin Ruan, Zhen-Chang Wen, Jin-Song Zhao, Qiu-Ju Jiang and Yun-Bao Jiang*
Received 8th July 2008, Accepted 10th October 2008
First published as an Advance Article on the web 10th November 2008
DOI: 10.1039/b811612a
A series of N-acylhydrazones were synthesised and found to be “turn-on” fluorescent chemodosimeters
for Cu2+. Among the tested transition metal ions such as Cu2+, Pb2+, Zn2+, Cd2+, Hg2+, and Ni2+, a
prominent fluorescence enhancement of up to 1000-fold was only observed for Cu2+ in acetonitrile
(CH3CN). This was indicated by an onset of unprecedented structured emission. Detailed experiments
established that the highly Cu2+ selective fluorescence enhancement resulted from an oxidative
cyclization by Cu2+of the originally nonfluorescent N-acylhydrazones into highly fluorescent rigid
1,3,4-oxadiazoles, n-dope type blocks in optoelectronic materials. The chemodosimeters can be applied
to sense Cu2+ at nM levels in CH3CN and sub-mM levels in neutral aqueous environments, despite a
slower response in the latter case. It is expected that these redox-based chemodosimeters might be of
general applicability.
Introduction
N-Acylhydrazones have been widely employed in organic1 and
analytical2 chemistry, mainly in terms of metal ligands.2,3 It is
known that N-acylhydrazones coordinate strongly with a variety
of transition metal ions, forming complexes of varied biological
and pharmaceutical activities. The development of sensitive and
selective fluorescent chemosensors for biologically important
metal ions is of intense current interest because these metal
ions play important roles in living and environmental systems.4
Special attention has been focused on the design of fluorescent
chemosensors for Cu2+ due to its essential yet toxic nature.5 We
previously found that N-(p-dimethylaminobenzoyl)hydrazone (1,
Scheme 1), bearing an intramolecular charge transfer (ICT) fluo-
rophore, p-dimethylaminobenzamide, showed a highly selective
fluorescence response toward Cu2+ in CH3CN, despite similar
absorption spectral variations being observed with other metal
ions such as Pb2+, Zn2+, and Hg2+ too.6 This finding suggested
that the N-acylhydrazone in this case might act not only as a
ligand. Although several explanations were proposed, the exact
mechanism, however, could not be clarified with just one molecule
that also bears complicated excited-state ICT photophysics.6 In
Scheme 1 Molecular structures of N-acylhydrazones 1–4.
order to understand the responding mechanism, we decided
to remove the ICT channel in 1 to simplify the excited-state
photophysics and data rationalization. We therefore extended
our investigation to a variety of N-benzoylhydrazones 2 and 3
bearing substituents X less electron-donating than p-NMe2 in 1
(Scheme 1). A highly selective fluorescence response toward Cu2+
was again observed in both CH3CN and CH3CN–H2O solutions.
In particular, the observed enhanced emission is unprecedentedly
structured in these polar solvents. Detailed experiments allowed
us to establish that the enhanced fluorescence was due to an
oxidative cyclization by Cu2+ of the originally nonfluorescent
N-acylhydrazones into highly fluorescent rigid 1,3,4-oxadiazoles.
N-Acylhydrazones were therefore shown to be a kind of redox-
based “turn-on” fluorescent chemodosimeter for Cu2+, a new
entrance to the active subject of “turn-on” fluorescent chemosen-
sors for Cu2+, a strongly quenching paramagnetic species.7–9 It
should be pointed out that there have been several nice Cu2+
chemodosimeters with enhanced fluorescence signal outputs,
which however follow hydrolysis8 or rearrangement reactions.9
Department of Chemistry, College of Chemistry and Chemical Engineering
and the MOE Key Laboratory of Analytical Sciences, Xiamen University,
Xiamen 361005, China. E-mail: ybjiang@xmu.edu.cn; Fax: (+86)592-218-
5662; Tel: (+86)592-218-5662
† Electronic supplementary information (ESI) available: Absorption and
fluorescence spectral titration traces and NMR spectra for compounds
2–13. See DOI: 10.1039/b811612a
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Results and discussion
N-Acylhydrazones 2 and 3 were facilely synthesized by a
simple one-step reaction in ethanol of the corresponding N-
benzoylhydrazine with 2-methoxybenzaldehyde and furan-2-
carbaldehyde, respectively. 2a in CH3CN exhibits three absorption
bands centred at 287, 298, and 323 nm with respective molar
absorption coefficients of 2.67 ¥ 104, 2.59 ¥ 104, and 2.93 ¥
104 M-1 cm-1, indicative of the (p, p*) transition character. In
the presence of Cu2+, the band at 323 nm is attenuated and a
shoulder at ca. 365 nm is developed (Fig. 1a). Hg2+ and Pb2+ exert
a similar effect, whereas other heavy transition metal ions such as
Zn2+, Cd2+, and Ni2+ exert a minor influence on the absorption
spectrum. 2b–d behave similarly in their absorption spectral
response toward these metal ions (Fig. S1-S4†). 3, analogues of
2, exhibit spectral variation profiles (Fig. S5-S8†) similar to those
of 2. 2 and 3 in CH3CN emit extremely weak fluorescence. In
the presence of Cu2+, however, an instant response was observed
by a dramatic enhancement of up to 1000-fold, despite the well-
known quenching character of Cu2+ (Fig. 1b, 2, S9, and S10†). The
fluorescence response profiles of 2 and 3 toward Cu2+ were found
to be simpler than that of 1,6 in that there was no emission band
shift and no fluorescence quenching at higher Cu2+ concentration
with 2 and 3 (Fig. 2). Removing the excited-state ICT channel in 1,
much higher fluorescence enhancement for 2a and 3a by Cu2+ was
observed (Fig. 2) than that for 1 which was ca.180-fold.6 Assays of
the fluorescence response of 2 and 3 in CH3CN toward a variety
of other transition metal ions such as Pb2+, Zn2+, Cd2+, Hg2+, and
Ni2+ indicated that a prominent fluorescence enhancement was
again only observed for Cu2+ whereas the other transition metal
ions tested exerted little influence (Fig. 2). This means that the
fluorescence responses of 2 and 3 are highly selective for Cu2+, as
is 1.6 The fluorescence enhancement factors (FEFs) of 2 and 3 by
Cu2+ were found to be higher with increasing electron-donating
ability of the substituent X and the FEFs of 3 were much higher
than those of 2 (Fig. 2 and Table 1). It therefore appears that higher
electron density at the hydrazone moiety and the relatively more
exposed furan oxygen atom in 3 are important for higher FEFs.
The fact that the fluorescence in CH3CN of 4 (Scheme 1), a control
molecule for 3a, does not show any response toward Cu2+, despite
Fig. 2 Plots of fluorescence enhancement factor (FEF, I/I0) in CH3CN
versus concentration ratio of metal ion to 2 and 3. R = 2a–d or 3a–d, [2] =
[3] = 10 mM.
a substantial absorption variation (Fig. 3), nicely indicates that the
=
vinyl proton CH in 3a plays an essential role in the fluorescence
enhancement of 3a by Cu2+.
Fig. 3 Absorption spectra of 4 (10 mM) in CH3CN in the presence of
increasing concentrations of Cu2+ (0–200 mM). The excitation wavelength
for acquiring fluorescence spectra given in the inset is an isosbestic
wavelength of 274 nm.
It was surprising that an unexpected structured emission was ob-
served for 2 and 3 in the presence of Cu2+ (Fig. 1b and 4). The trend
shown in Fig. 4, that the emission becomes less structured while the
FEF becomes lower with decreasing electron-donating ability of
the substituent X, seems to suggest that the FEF value is related
to the structured extent of the fluorescence spectrum. In order
to test this correlation, an extended variety of N-acylhydrazones
(5–12, Scheme 2) were prepared and their fluorescence response
toward Cu2+ was monitored in CH3CN. The fluorescence of 5–
12 was similarly found to be enhanced by Cu2+, confirming that
N-acylhydrazones could in general act as an excellent family of
“chemosensors” for Cu2+. The structured extents of the emission
spectra and the FEF values shown in Fig. 5, however, indicate
that the apparent correlation between the structured extents and
the FEF values reached in Fig. 4 does not hold in general, the
rigidity of the ligand itself obviously contributing to the structured
extent of the final emission spectrum. The unexpected structured
Fig. 1 Absorption (a) and fluorescence (b) spectra of 2a (20 mM) in
CH3CN in the presence of increasing concentrations of Cu2+ (0–200 mM).
The excitation wavelength was 267 nm, an isosbestic wavelength observed
in absorption titrations.
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Table 1 Absorption and fluorescence spectral parameters of 2 and 3, and 2 and 3 in the presence of 25 equivalents of metal ion in CH3CN
l
abs /nm
e/104 M-1 cm-1
l
flu/nm
FEF a
U b
2a
287/298/323
365
365
365
364
368
375
286/297/323
365
368
365
366
372
375
286/298/323
364
370
364
364
361
375
287/299/324
364
365
366
367
364
375
254/310
315/378
296/345
326
328
338
2.67/2.59/2.93
0.32
0.50
0.10
0.08
0.56
0.26
2.12/2.10/2.50
0.25
0.33
0.10
0.04
0.40
0.39
1.92/1.92/2.37
0.20
0.30
0.05
0.05
0.47
0.50
1.74/1.71/2.10
0.23
0.41
0.10
0.03
0.50
0.75
1.01/3.75
2.94/0.51
2.66/1.55
3.76
4.06
4.15
369
364
358
360
367
364
364
370
358
351
353
370
357
355
370
358
355
358
370
357
357
370
364
362
364
373
363
364
370
367
366
365
381
368
367
371
358
361
364
379
364
366
370
372
360
372
385
365
366
371
370
368
369
396
371
372
—
1.9
280
5.7
1.4
1.5
1.4
—
1.1
140
5.7
1.1
2.4
1.4
—
1.1
55
1.9
2.2
1.7
1.3
—
1.1
80
1.9
2.2
1.7
1.3
—
1.4
1000
5.2
1.5
29
1.1
—
1.1
260
1.3
1.1
8.1
1.1
—
1.1
145
1.2
1.1
6.3
1.1
—
1.1
136
1.0
1.0
2.9
1.1
0.0007
0.0008
0.34
0.0013
0.0009
0.0025
0.0012
0.0008
0.0010
0.30
0.0013
0.0010
0.0018
0.0011
0.0007
0.0011
0.20
0.0027
0.0012
0.0024
0.0015
0.0008
0.0010
0.26
0.0014
0.0011
0.0042
0.0015
0.0004
0.0016
0.49
0.0038
0.0024
0.0032
0.0034
0.0004
0.0032
0.36
0.0018
0.0025
0.0047
0.0025
0.0005
0.0029
0.33
0.0030
0.0007
0.0047
0.0038
0.0004
0.0024
0.29
2a + Ni2+
2a + Cu2+
2a + Zn2+
2a + Cd2+
2a + Hg2+
2a + Pb2+
2b
2b + Ni2+
2b + Cu2+
2b + Zn2+
2b + Cd2+
2b + Hg2+
2b + Pb2+
2c
2c + Ni2+
2c + Cu2+
2c + Zn2+
2c + Cd2+
2c + Hg2+
2c + Pb2+
2d
2d + Ni2+
2d + Cu2+
2d + Zn2+
2d + Cd2+
2d + Hg2+
2d + Pb2+
3a
3a + Ni2+
3a + Cu2+
3a + Zn2+
3a + Cd2+
3a + Hg2+
3a + Pb2+
3b
333
3.69
238/308
310/365
327
313
319
333
330
228/308
309/365
319
311
314
332
330
236/309
310/371
319/376
311/374
314
334/390
332
1.02/3.37
3.10/0.23
1.71
3.23
3.31
3.43
3.22
0.94/3.01
3.03/0.24
1.01
2.87
2.95
3.07
2.74
1.27/3.26
2.92/0.29
1.31/0.12
3.11/0.06
3.15
3.11/0.18
2.88
3b + Ni2+
3b + Cu2+
3b + Zn2+
3b + Cd2+
3b + Hg2+
3b + Pb2+
3c
3c + Ni2+
3c + Cu2+
3c + Zn2+
3c + Cd2+
3c + Hg2+
3c + Pb2+
3d
3d + Ni2+
3d + Cu2+
3d + Zn2+
3d + Cd2+
3d + Hg2+
3d + Pb2+
0.0017
0.0010
0.0039
0.0026
a Fluorescence enhancement factor, the ratio of the intensity of 2 and 3 in the presence of 25 equivalents of metal ion to that in the absence of metal
ion. b Fluorescence quantum yields of 2 and 3, and 2 and 3 in the presence of 25 equivalents of metal ion were measured using quinine sulfate as a
standard (0.546 in 0.5 M H2SO4: Demas, J. N.; Crobys, G. A., J. Phys. Chem. 1971, 75, 991–1024). The measurement errors were up to 50% and 15% for
N-acylhydrazones and their metal complexes, respectively.
emission and the high selectivity for Cu2+ therefore could not be
simply attributed to Cu2+ coordination to N-acylhydrazones.
The fluorescence emission of a ligand can in principle be affected
not only by metal ion coordination, but by a metal ion involved
reaction as well.10 The fact that the enhancement by Cu2+ of the
fluorescence of 2 and 3 becomes higher when the substituent
X is more electron-donating suggested that a redox reaction
might occur. Indeed, it was reported that N-acylhydrazones
underwent oxidative cyclization to 1,3,4-oxadiazoles by several
oxidants.11 Cu(ClO4)2 in CH3CN could be an effective oxidant,
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hypothesis on the basis of redox potentials data. Alternatively,
crucial evidence supporting this hypothesis was obtained from
the independent syntheses of 1,3,4-oxadiazoles by reactions of
N-acylhydrazones 3 with Cu(ClO4)2 in CH3CN. The oxidative
cyclization products 1,3,4-oxadiazoles 13 (Scheme 3, Table S1†)
1
were fully characterized by HRMS, H NMR, and 13C NMR.
Fluorescence excitation and emission spectra of the synthesized
oxidative cyclization product 13a were found identical to those
of 3a in the presence of 1.0 equivalent of Cu2+ (Fig. S11†).
Obviously the observed fluorescence enhancement of 3a by Cu2+
was not caused by Cu2+ coordination but instead by the oxidative
cyclization reaction. In order to confirm the role of Cu2+ in the
reaction of N-acylhydrazones in CH3CN, EPR experiments in
CH3CN at 100 K were carried out in which the Cu2+ concentration
was made constant while the 3a concentration varied. It was
found that the EPR signal of Cu2+ was indeed attenuated with
increasing 3a concentration (Fig. 6). This points to the conversion
of Cu2+ into diamagnetic Cu+ that can be stabilized in CH3CN.
A mechanism of the oxidative cyclization by Cu2+ was therefore
suggested (Scheme 3), which forms the basis of this new kind of
redox-based chemodosimeter for Cu2+.
Fig. 4 Normalized fluorescence spectra of 2 and 3 in the presence of 25
equivalents of Cu2+ in CH3CN at 25 ◦C.
Fig. 5 Normalized fluorescence spectra of 2c and 5–12 and FEF values
in the presence of 25 equivalents of Cu2+ in CH3CN at 25 ◦C.
Fig. 6 EPR spectra of 1.0 mM Cu2+ in CH3CN at 100 K with increasing
concentration of 3a.
Scheme 2 Structures of the extended variety of N-acylhydrazones.
since the Cu2+/Cu+ couple in CH3CN reportedly has a high
reduction potential due to the stabilization of Cu+ by solvent
coordination.12 We therefore hypothesized that Cu2+ acted as
an oxidant in CH3CN to result in an oxidative cyclization of
N-acylhydrazones into 1,3,4-oxadiazoles. As no credible signals
were detected in the cyclic voltammograms of the reported N-
acylhydrazones in CH3CN, we were unable to directly confirm this
Scheme 3 Proposed oxidative cyclization of N-acylhydrazones by Cu2+
in CH3CN.
The absorption and fluorescence spectra of the oxidative cycliza-
tion product 13a in CH3CN, hardly changed upon the addition of
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up to 25 equivalents Cu2+ (Fig. S12†). This explained the observed
level-off of FEFs of 2 and 3 at higher Cu2+ concentration (Fig. 2).
The observation that the absorption spectrum of 4 in CH3CN
undergoes substantial variation whereas it remains nonfluorescent
in the presence of Cu2+ (Fig. 3) suggests that a simple coordination
of Cu2+ to N-acylhydrazones in the ground state does not lead to
an enhancement in the fluorescence of N-acylhydrazones. It was
found that the fluorescence quantum yield of 13a (U = 0.725) in
CH3CN is ca.1800-fold that of 3a (U = 4 ¥ 10-4). It is hence made
clear that the dramatic fluorescence enhancement results fully from
the oxidative cyclization by Cu2+ in CH3CN of the nonfluorescent
N-acylhydrazones which leads to the highly fluorescent rigid
1,3,4-oxadiazoles. The fluorescence response selectivity for Cu2+ is
therefore due to its oxidation capability in CH3CN, which makes it
differ from the other metal ions tested. Hence the herein reported
N-acylhydrazones do not act as metal coordinating chemosensors
but instead fluorescent chemodosimeters for Cu2+.
is only slightly influenced by the addition of either 5 equivalents
of each or all of the interference metal ions, Fig. 8. In aqueous
solutions, however, the response reaction was substantially slowed
down, likely due to the efficient hydration of Cu2+. Means
of dehydrating Cu2+ and/or the stabilizing of Cu+ in aqueous
solutions are expected to enhance the reaction to a reasonable level
that might allow efficient aqueous phase assays. This is currently
underway in this laboratory.
The high fluorescence enhancement factor of 3a upon the
addition of Cu2+ in CH3CN shows great potential for application
to the fluorescent sensing of Cu2+ at a low concentration level.
The fluorescence of 3a (0.5 mM) in the presence of 0.5 equivalents
of Cu2+ levels off rapidly within 2 min (Fig. S13†). Fluorescence
titration of 3a (0.5 mM) shows a linear response toward Cu2+ over
25.0 nM to 0.25 mM in CH3CN, with a detection limit of 3.5 nM
(Fig. S14†). The fluorescent sensing of Cu2+ by 3a in H2O–CH3CN
solutions was also tested. The optimal conditions obtained for the
assay (Fig. S15-S17†) were to carry it out in a 20:80 (v/v) mixture
of CH3CN and H2O at pH 7.2 (5 mM Tris-HCl buffer, 0.1 M KCl)
after heating at 50 ◦C for 3 h. In this case the detection limit was
0.30 mM. The FEF was proportional to the Cu2+ concentration
over 1.0–160 mM at a 3a concentration of 10 mM (Fig. 7). The
fluorescence enhancement of 3a by Cu2+ was also found to be
Fig. 8 (a) Fluorescence enhancement factors of 3a with individual ions
(2.5 mM) and (b) Relative fluorescence responses to Cu2+ (100 mM) plus
interference metal ion (500 mM) in a mixture of CH3CN and Tris-HCl
(5 mM, pH 7.2, 0.1 M KCl) aqueous buffer solution (20/80, v/v). “All”
means the relative fluorescence response of 3a to Cu2+ (100 mM) plus all the
interference metal ions tested, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Mg2+, Ca2+
,
and Ba2+ at 500 mM each.
-
independent of the counter anions of the Cu2+ such as NO3 , Cl-,
AcO-, ClO4 , and SO42- in aqueous CH3CN solutions (Fig. S18†).
-
Conclusions
Highly selective and dramatic enhancements by Cu2+ of the
fluorescence in CH3CN and CH3CN–H2O of a variety of
N-acylhydrazones were observed and were shown to result from
an oxidative cyclization by Cu2+ of the originally nonfluorescent
N-acylhydrazones to the highly fluorescent rigid 1,3,4-oxadiazoles.
In agreement with this conclusion was the structured emission
from the CH3CN solution of N-acylhydrazones and Cu2+. To the
best of our knowledge, this is the first-set of reported chemod-
osimeters following an oxidative cyclization reaction that exhibits
outstanding selectivity for Cu2+ with a dramatic fluorescence
enhancement output. The chemodosimeters can be applied to
sense Cu2+ at nanomolar levels in CH3CN. Both high selectivity
and sensitivity over other metal ions tested were obtained in a
20:80 (v/v) mixture of CH3CN and Tris-HCl aqueous buffer
solution, despite a much slower reaction rate. Although at
the moment we are unable to set a structural limit for an
N-acylhydrazone to be facilely oxidized by Cu2+, as all those
reported here undergo efficient oxidation cyclization, we expect
that this oxidative cyclization reaction could be taken as an
alternative route to 1,3,4-oxadiazoles, efficient electron acceptors
employed in optoelectronic materials.13 As redox reactions are
easily made selective for a reductant or an oxidant and even made
reversible, creating chemodosimeters following the redox reaction
strategy illustrated here shall be of general applicability.
Fig. 7 (a) Fluorescence spectra of 3a (10 mM) in a mixture of CH3CN
and Tris-HCl (5 mM, pH 7.2, 0.1 M KCl) aqueous buffer solution (20/80,
v/v) in the presence of increasing concentrations of Cu2+ and (b) linear
response curve. The excitation wavelength was 283 nm.
The fluorescence of 3a in 80% H2O–CH3CN (v/v) was found to
be hardly altered by the other metal ions tested: Co2+, Ni2+, Zn2+,
Cd2+, Hg2+, Mg2+, Ca2+, and Ba2+. The selectivity for Cu2+ over
these metal ions remains remarkably high and the FEF of Cu2+
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122.4, 125.4, 127.5, 128.8, 130.4, 131.3, 141.6, 142.9, 157.6, 162.7
ppm; HRMS (ESI): m/z: calcd for C16H17N2O2: 269.1290 [M +
H+]; found 269.1295 [M + H+].
Experimental
Chemicals used for syntheses were commercially available. Sol-
vents for spectral titrations were redistilled CH3CN and deionized
water.
Absorption and fluorescence spectra were recorded on Varian
Cary 300 spectrophotometer and Hitachi F-4500 fluorescence
spectrophotometer, respectively. Solutions were measured in a
1 cm quartz cell. Fluorescence quantum yields were measured
N¢-(2-Methoxybenzylidene)benzohydrazide (2c). 1H NMR
(500 MHz, DMSO-d6, TMS): d = 3.87 (s, 3H), 7.04 (t, 1H, J =
7.5 Hz), 7.12 (d, 1H, J = 8.5 Hz), 7.43 (t, 1H, J = 7.5 Hz), 7.52
(t, 2H, J = 7.5 Hz), 7.59 (t, 1H, J = 7.0 Hz), 7.88 (d, 1H, J =
7.5 Hz), 7.93 (d, 2H, J = 7.5 Hz), 8.82 (s, 1H), 11.85 ppm (s, 1H);
13C NMR (125 MHz, DMSO-d6, TMS): d = 55.5, 111.6, 120.6,
122.3, 125.4, 127.5, 128.3, 131.4, 131.5, 133.3, 143.2, 157.7, 162.9
ppm; HRMS (ESI): m/z: calcd for C15H15N2O2: 255.1134 [M +
H+]; found 255.1141 [M + H+].
1
using quinine sulfate as a standard (0.546 in 0.5 M H2SO4). H
NMR and 13C NMR were acquired on Bruker AV400 and Varian
Unity+ 500 MHz NMR spectrometers. HRMS were obtained on
a Micromass LCT spectrometer using methanol as the solvent.
EPR experiments were carried out on a Bruker EMX-10/12
spectrometer.
N¢-(2-Methoxybenzylidene)-4-chlorobenzohydrazide (2d). 1H
NMR (500 MHz, DMSO-d6, TMS): d = 3.87 (s, 3H), 7.03 (t,
1H, J = 7.5 Hz), 7.12 (d, 1H, J = 8.0 Hz), 7.43 (t, 1H, J = 7.5
Hz), 7.61 (d, 2H, J = 8.0 Hz), 7.88 (d, 1H, J = 8.5 Hz), 7.96
(d, 2H, J = 8.5 Hz), 8.81 (s, 1H), 11.90 ppm (s, 1H); 13C NMR
(125 MHz, DMSO-d6, TMS): d = 55.5, 111.7, 120.6, 122.2, 125.4,
128.4, 129.4, 131.5, 132.0, 136.5, 143.5, 157.7, 161.8 ppm; HRMS
(ESI): m/z: calcd for C15H14ClN2O2: 289.0744 [M + H+]; found
289.0754 [M + H+].
All spectral titrations were carried out by keeping the sensor
concentration constant while varying the metal ion concentration.
Metal ions were used as their perchlorates. In the pH titration
experiments, the solution pH was adjusted by dilute NaOH and
HCl solutions that contained the same concentration of metal
ion. Potassium chloride was employed to maintain solution ionic
strength.
4-Ethoxy-N¢-(furan-2-ylmethylene)benzohydrazide (3a). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 1.35 (t, 3H, J = 7.0 Hz),
4.11 (m, 2H), 6.64 (m, 1H), 6.91 (d, 1H, J = 3.2 Hz), 7.04 (d, 2H,
J = 8.8 Hz), 7.85 (s, 1H), 7.87 (d, 2H, J = 8.8 Hz), 8.33 (s, 1H),
11.66 ppm (s, 1H); 13C NMR (100 MHz, DMSO-d6, TMS): d =
14.4, 63.3, 112.1, 113.1, 114.0, 125.1, 129.4, 136.9, 145.0, 149.5,
161.2, 162.4 ppm; HRMS (ESI): m/z: calcd for C14H15N2O3:
259.1083 [M + H+]; found 259.1077 [M + H+].
Preparation and characterization of 2–12 and 13.
2–12 were facilely synthesized from equimolar amounts of N-
(substituted-benzoyl)hydrazine (2 mmol) with aldehyde or ketone
(2 mmol) by a one-step reaction in ethanol, respectively. The
mixture were refluxed for 3 h and then cooled down to room
temperature. The crude products were isolated by filtration, and
then recrystallized from absolute ethanol. New compounds were
fully characterized by 1H NMR, 13C NMR, and HRMS.
13 was synthesized from 3 (2 mmol) with 5 equivalents of
Cu(ClO4)2 (10 mmol) in CH3CN that were refluxed with stirring
for 24 h, and then evaporated in vacuo. Aqueous ethylenediamine
(1.0 mol L-1 ¥ 25 mL) solution was added and the solution was
then extracted by ethyl acetate (3 ¥ 20 mL). The combined organic
layer was thoroughly washed with water (3 ¥ 20 ml), dried over
anhydrous Na2SO4, and then concentrated under vacuum, then
purified by column chromatography on silica gel with ethyl acetate-
petroleum ether (1:5) as eluent. 13 was fully characterized by
1H NMR, 13C NMR, and HRMS to confirm the occurrence of
oxidative cyclization reaction.
N¢-(Furan-2-ylmethylene)-4-methylbenzohydrazide (3b). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 2.38 (s, 3H), 6.64 (s,
1H), 6.92 (s, 1H), 7.33 (d, 2H, J = 7.6 Hz), 7.81 (d, 2H, J =
7.6 Hz), 7.85 (s, 1H), 8.34 (s, 1H), 11.73 ppm (s, 1H); 13C NMR
(100 MHz, DMSO-d6, TMS): d = 20.9, 112.1, 113.3, 127.5,
128.9, 130.4, 137.2, 141.7, 145.0, 149.4, 162.8 ppm; HRMS (ESI):
m/z: calcd for C13H13N2O2: 229.0977 [M + H+]; found 229.0977
[M + H+].
N¢-(Furan-2-ylmethylene)benzohydrazide (3c). 1H NMR
(400 MHz, DMSO-d6, TMS): d = 6.65 (s, 1H), 6.94 (s, 1H), 7.53
(t, 2H, J = 6.4 Hz), 7.60 (t, 1H, J = 6.8 Hz), 7.86 (s, 1H), 7.90 (d,
2H, J = 7.2 Hz), 8.35 (s, 1H), 11.80 ppm (s, 1H); 13C NMR (100
MHz, DMSO-d6, TMS): d = 112.1, 113.4, 127.5, 128.4, 131.7,
133.3, 137.5, 145.1, 149.4, 163.0 ppm; HRMS (ESI): m/z: calcd
for C12H11N2O2: 215.0821 [M + H+]; found 215.0819 [M + H+].
N¢-(2-Methoxybenzylidene)-4-ethoxybenzohydrazide (2a). 1H
NMR (500 MHz, DMSO-d6, TMS): d = 1.35 (t, 3H, J = 7.0
Hz), 3.87 (s, 3H), 4.09–4.13 (m, 2H), 7.02 (t, 3H, J = 8.0 Hz), 7.11
(d, 1H, J = 8.0 Hz), 7.41 (t, 1H, J = 7.5 Hz), 7.87 (d, 1H, J = 7.0
Hz), 7.92 (d, 2H, J = 8.5 Hz), 8.81 (s, 1H), 11.72 ppm (s, 1H); 13
C
4-Chloro-N¢-(furan-2-ylmethylene)benzohydrazide
(3d). 1H
NMR (125 MHz, DMSO-d6, TMS): d = 14.4, 55.6, 63.3, 111.7,
113.9, 120.6, 122.4, 125.2, 125.3, 129.4, 131.2, 142.4, 157.6, 161.1,
162.2 ppm; HRMS (ESI): m/z: calcd for C17H19N2O3: 299.1396
[M + H+]; found: 299.1395 [M + H+].
NMR (400 MHz, DMSO-d6, TMS): d = 6.65 (m, 1H), 6.96 (d,
1H, J = 3.2 Hz), 7.62 (d, 2H, J = 8.4 Hz), 7.87 (s, 1H), 7.93 (d,
2H, J = 8.4 Hz), 8.34 (s, 1H), 11.86 ppm (s, 1H); 13C NMR (100
MHz, DMSO-d6, TMS): d = 112.1, 113.7, 128.5, 129.4, 132.0,
136.5, 137.7, 145.2, 149.3, 161.9 ppm; HRMS (ESI): m/z: calcd
for C12H10ClN2O2: 249.0431 [M + H+]; found 249.0427 [M + H+].
N¢-(2-Methoxybenzylidene)-4-methylbenzohydrazide (2b). 1H
NMR (500 MHz, DMSO-d6, TMS): d = 2.38 (s, 3H), 3.87 (s,
3H), 7.03 (t, 1H, J = 7.5 Hz), 7.11(d, 1H, J = 8.5 Hz), 7.33 (d,
2H, J = 8.0 Hz), 7.42 (t, 1H, J = 7.5 Hz), 7.85 (d, 2H, J = 8.0
4-Ethoxy-N¢-(1-(furan-2-yl)ethylidene)benzohydrazide (4). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 1.35 (t, 3H, J = 7.0 Hz),
2.28 (s, 3H), 4.11 (m, 2H), 6.61 (m, 1H), 6.95 (s, 1H), 7.02 (d, 2H,
J = 8.8 Hz), 7.80 (s, 1H), 7.86 (d, 2H, J = 8.8 Hz), 10.52 ppm
Hz), 7.88 (d, 1H, J = 8.5 Hz), 8.82 (s, 1H), 11.78 ppm (s, 1H); 13
C
NMR (125 MHz, DMSO-d6, TMS): d = 20.9, 55.5, 111.7, 120.6,
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(s, 1H); 13C NMR (100 MHz, DMSO-d6, TMS): d = 13.7, 14.5,
63.3, 111.2, 111.8, 113.8, 125.7, 129.9, 144.5, 145.1, 148.2, 151.8,
161.1 ppm; HRMS (ESI): m/z: calcd for C15H17N2O3: 273.1239
[M + H+]; found 273.1245 [M + H+].
(ESI): m/z: calcd for C13H19N2O2: 235.1447 [M + H+]; found
235.1446 [M + H+].
4-Phenyl-N¢-(2-methoxybenzylidene)benzohydrazide (11). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 3.88 (s, 3H), 7.05 (t,
1H, J = 7.6 Hz), 7.13 (d, 1H, J = 8.4 Hz), 7.43 (m, 2H), 7.52 (t,
2H, J = 7.6 Hz), 7.76 (d, 2H, J = 7.6 Hz), 7.84 (d, 2H, J = 8.4
Hz), 7.91 (d, 1H, J = 6.8 Hz), 8.05 (d, 2H, J = 8.4 Hz), 8.86 (s,
1H), 11.92 ppm (s, 1H); 13C NMR (100 MHz, DMSO-d6, TMS):
d = 55.6, 111.8, 120.7, 122.3, 125.4, 126.6, 126.8, 128.1, 128.2,
129.0, 131.5, 132.0, 139.0, 143.1, 157.7, 162.5 ppm; HRMS (ESI):
m/z: calcd for C21H19N2O2: 331.1447 [M + H+]; found 331.1451
[M + H+].
N¢-(2-Ethoxybenzylidene)-4-ethoxybenzohydrazide
(5). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 1.34–1.41 (m, 6H),
4.09–4.15 (m, 4H), 7.00 (d, 1H, J = 7.6 Hz), 7.03 (d, 2H, J = 8.8
Hz), 7.09 (d, 1H, J = 8.0 Hz), 7.39 (t, 1H, J = 7.6 Hz), 7.88 (d,
1H, J = 7.6 Hz), 7.91 (d, 2H, J = 8.8 Hz), 8.80 (s, 1H), 11.75
ppm (s, 1H); 13C NMR (100 MHz, DMSO-d6, TMS): d = 14.5,
14.6, 63.3, 63.7, 112.7, 114.0, 120.6, 122.6, 125.2, 125.4, 129.5,
131.3, 142.5, 157.0, 161.2, 162.4 ppm; HRMS (ESI): m/z: calcd
for C18H21N2O3: 313.1552 [M + H+]; found 313.1556 [M + H+].
N¢-(2-Methoxybenzylidene)-2-naphthohydrazide
(12). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 3.89 (s, 3H), 7.06 (t,
1H, J = 7.2 Hz), 7.13 (d, 1H, J = 8.4 Hz), 7.45 (t, 1H, J =
7.6 Hz), 7.64 (m, 2H), 7.92 (d, 1H, J = 7.6 Hz), 8.01–8.09 (m,
4H), 8.58 (s, 1H), 8.88 (s, 1H), 12.03 ppm (s, 1H); 13C NMR (100
MHz, DMSO-d6, TMS): d = 55.6, 111.8, 120.7, 122.3, 124.2,
125.4, 126.8, 127.6, 127.8, 127.9, 128.0, 128.3, 130.6, 131.5, 132.0,
134.2, 143.1, 157.7, 162.8 ppm; HRMS (ESI): m/z: calcd for
C19H17N2O2: 305.1290 [M + H+]; found 305.1281 [M + H+].
N¢-(2-Propoxybenzylidene)-4-ethoxybenzohydrazide (6). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 1.05 (t, 3H, J = 7.2 Hz),
1.36 (t, 3H, J = 7.2 Hz), 1.79 (m, 2H), 4.02 (t, 2H, J = 6.4 Hz),
4.11 (m, 2H), 6.99 (d, 1H, J = 7.2 Hz), 7.04 (d, 2H, J = 8.8 Hz),
7.08 (d, 1H, J = 8.4 Hz), 7.38 (m, 1H), 7.87 (s, 1H), 7.91 (d, 2H,
J = 8.8 Hz), 8.80 (s, 1H), 11.76 ppm (s, 1H); 13C NMR (100 MHz,
DMSO-d6, TMS): d = 10.5, 14.4, 22.0, 63.3, 69.4, 112.6, 113.9,
120.5, 122.6,125.3, 125.4, 129.5, 131.2, 142.4, 157.1, 161.1, 162.4
ppm; HRMS (ESI): m/z: calcd for C19H23N2O3: 327.1709 [M +
H+]; found 327.1703 [M + H+].
2-(4-Ethoxyphenyl)-5-furan-2-yl-1,3,4-oxadiazole (13a). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 1.37 (t, 3H, J = 7.0 Hz),
4.14 (m, 2H), 6.83 (m, 1H), 7.16 (d, 2H, J = 8.8 Hz), 7.43 (d,
1H, J = 3.6 Hz), 8.00 (d, 2H, J = 8.8 Hz), 8.08 ppm (d, 1H, J =
1.2 Hz); 13C NMR (100 MHz, DMSO-d6, TMS): d = 14.4, 63.5,
112.6, 114.4, 115.1, 115.2, 128.5, 138.6, 146.8, 156.4, 161.4, 163.1
ppm; HRMS (ESI): m/z: calcd for C14H13N2O3: 257.0926 [M +
H+]; found 257.0927 [M + H+].
N¢-(2-Isopropoxybenzylidene)-4-ethoxybenzohydrazide (7). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 1.32 (d, 6H, J = 6.0 Hz),
1.36 (t, 3H, J = 6.8 Hz), 4.11 (m, 2H), 4.70 (m, 1H), 7.00 (t, 1H,
J = 7.2 Hz), 7.03 (d, 2H, J = 8.8 Hz), 7.12 (d, 1H, J = 8.0 Hz),
7.37 (t, 1H, J = 8.0 Hz), 7.87 (d, 1H, J = 8.0 Hz), 7.91 (d, 2H,
J = 8.8 Hz), 8.76 (s, 1H), 11.72 ppm (s, 1H); 13C NMR (100 MHz,
DMSO-d6, TMS): d = 14.4, 21.8, 63.3, 70.4, 113.9, 114.5, 120.6,
123.6, 125.3, 125.6, 129.5, 131.1, 142.8, 156.0, 161.1, 162.3 ppm;
HRMS (ESI): m/z: calcd for C19H23N2O3: 327.1709 [M + H+];
found 327.1705 [M + H+].
2-Furan-2-yl-5-p-tolyl-1,3,4-oxadiazole (13b). 1H NMR (400
MHz, CDCl3, TMS): d = 2.44 (s, 3H), 6.62 (m, 1H), 7.23 (d, 1H,
J = 3.2 Hz), 7.33 (d, 2H, J = 8.0 Hz), 7.67 (d, 1H, J = 1.2 Hz), 8.01
ppm (d, 2H, J = 8.0 Hz); 13C NMR (100 MHz, CDCl3, TMS): d =
21.7, 112.2, 113.9, 120.8, 127.0, 129.8, 139.6, 142.5, 145.6, 157.4,
164.2 ppm; HRMS (ESI): m/z: calcd for C13H11N2O2: 227.0821
[M + H+]; found 227.0822 [M + H+].
4-Ethoxy-N¢-(thiophen-2-ylmethylene)benzohydrazide (8). 1H
NMR (400 MHz, DMSO-d6, TMS): d = 1.35 (t, 3H, J = 6.8
Hz), 4.11 (m, 2H), 7.04 (d, 2H, J = 8.8 Hz), 7.14 (m, 1H), 7.45
(d, 1H, J = 3.2 Hz), 7.66 (d, 1H, J = 4.8 Hz), 7.88 (d, 2H, J =
8.8 Hz), 8.67 (s, 1H), 11.68 ppm (s, 1H); 13C NMR (100 MHz,
DMSO-d6, TMS): d = 14.4, 63.3, 114.0, 125.1, 127.7, 128.6, 129.4,
130.5, 139.2, 142.2, 161.2, 162.3 ppm; HRMS (ESI): m/z: calcd
for C14H15N2O2S: 275.0854 [M + H+]; found 275.0855 [M + H+].
2-Furan-2-yl-5-phenyl-1,3,4-oxadiazole (13c). 1H NMR (400
MHz, CDCl3, TMS): d = 6.63 (m, 1H), 7.24 (d, 1H, J = 3.6 Hz),
7.52 (s, 1H), 7.54 (m, 2H), 7.67 (d, 1H, J = 1.2 Hz), 8.13 ppm (m,
2H); 13C NMR (100 MHz, CDCl3, TMS): d = 112.2, 114.0, 123.5,
127.0, 129.0, 131.8, 139.4, 145.7, 157.4, 163.9 ppm; HRMS (ESI):
m/z: calcd for C12H9N2O2: 213.0664 [M + H+]; found 213.0669
[M + H+].
N¢-Benzylidene-4-ethoxybenzohydrazide
(9). 1H
NMR
(400 MHz, DMSO-d6, TMS): d = 1.36 (t, 3H, J = 7.0 Hz),
4.09–4.14 (m, 2H), 7.05 (d, 2H, J = 8.8 Hz), 7.43–7.48 (m, 3H),
7.73 (d, 2H, J = 6.4 Hz), 7.91 (d, 2H, J = 8.8 Hz), 8.45 (s, 1H),
11.73 ppm (s, 1H); 13C NMR (100 MHz, DMSO-d6, TMS): d =
14.4, 63.3, 114.0, 125.2, 126.9, 128.7, 129.5, 129.8, 134.4, 147.0,
161.2, 162.5 ppm; HRMS (ESI): m/z: calcd for C16H17N2O2:
269.1290 [M + H+]; found 269.1289 [M + H+].
2-(4-Chlorophenyl)-5-furan-2-yl-1,3,4-oxadiazole
(13d). 1H
NMR (400 MHz, CDCl3, TMS): d = 6.63 (m, 1H), 7.25 (d, 1H,
J = 3.6 Hz), 7.51(d, 2H, J = 8.8 Hz), 7.68 (t, 1H, J = 0.8 Hz),
8.06 ppm (d, 2H, J = 8.4 Hz); 13C NMR (100 MHz, CDCl3,
TMS): d = 112.2, 114.3, 122.0, 128.2, 129.5, 138.1, 139.3, 145.8,
157.5, 163.1 ppm; HRMS (ESI): m/z: calcd for C12H8ClN2O2:
247.0274 [M + H+]; found 247.0272 [M + H+].
4-Ethoxy-N¢-(2-methylpropylidene)benzohydrazide (10). 1H
NMR (400 MHz, CD3CN): d = 1.10 (d, 6H, J = 6.8 Hz), 1.38 (t,
3H, J = 7.0 Hz), 2.51–2.59 (m, 1H), 4.07–4.12 (m, 2H), 6.96 (d,
2H, J = 7.2 Hz), 7.51 (s, 1H), 7.77 (d, 2H, J = 8.4 Hz), 9.67 ppm
(s, 1H); 13C NMR (400 MHz, DMSO-d6, TMS): d = 14.4, 19.6,
31.0, 63.2, 113.9, 125.4, 129.3, 155.9, 161.0, 162.2 ppm; HRMS
Acknowledgements
This work was supported by the NSFC of China (20425518,
20675069), NFFTBS (J0630429), and the Ministry of Education
(MOE) of China.
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