Fluorescent chemosensors for mercury(II) cations

Article abstract of DOI:10.1134/S1070428016100110

The condensation of 4,5-dimethoxy-2-(morpholin-4-yl)aniline with anthracene-9-carbaldehyde gave N-(anthracen-9-ylmethyl)-4,5-dimethoxy-2-(morpholin-4-yl)aniline, a selective and efficient fluorescent chemosensor for mercury(II) cations.

Full text of DOI:10.1134/S1070428016100110

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2016, Vol. 52, No. 10, pp. 1436–1439. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © I.E. Tolpygin, K.S. Tikhomirova, Yu.V. Revinskii, A.D. Dubonosov, V.A. Bren’, V.I. Minkin, 2016, published in Zhurnal
Organicheskoi Khimii, 2016, Vol. 52, No. 10, pp. 1446–1449.
Fluorescent Chemosensors for Mercury(II) Cations
I. E. Tolpygina, K. S. Tikhomirovaa, Yu. V. Revinskiib, A. D. Dubonosovb,*
V. A. Bren’a, and V. I. Minkina, b
a Institute of Physical and Organic Chemistry, Southern Federal University,
pr. Stachki 194/2, Rostov-on-Don, 344090 Russia
b Southern Scientific Center, Russian Academy of Sciences,
pr. Chekhova 41, Rostov-on-Don, 344006 Russia
*e-mail: aled@ipoc.sfedu.ru
Received May 18, 2016
Abstract—The condensation of 4,5-dimethoxy-2-(morpholin-4-yl)aniline with anthracene-9-carbaldehyde
gave N-(anthracen-9-ylmethyl)-4,5-dimethoxy-2-(morpholin-4-yl)aniline, a selective and efficient fluorescent
chemosensor for mercury(II) cations.
DOI: 10.1134/S1070428016100110
1
Various chemosensors are widely used in organic
chemistry, biochemistry, and environmental studies to
monitor cations, anions, and neutral molecules [1–6].
An important problem is the design of organic chemo-
sensors for heavy metal ions [7–9]. Here, preference is
given to fluorescent chemosensors characterized by
high efficiency, sensitivity, and selectivity [10–12].
Our previous studies [13–16] showed that N-(2-amino-
phenyl)-N-(9-anthrylmethyl)amine derivatives can be
used as efficient fluorescent chemosensors for mer-
cury(II) ions. With the goal of obtaining new fluoro-
genic derivatives of this series we have modified the
receptor entity.
In the H NMR spectra of 2 and 4, protons of the
methoxy groups resonated as two singlets in the region
δ 3.70–4.09 ppm. The spectrum of 2 characteristically
showed doublet signals of the NH (OH) (δ 15.88 ppm)
and N=CH protons (δ 9.40 ppm), and amine 4 dis-
played signals from the CH2 and NH groups at δ 5.20
and 4.85 ppm, respectively. Schiff base 2 was found to
exist in solution as two tautomers A and B (Scheme 1)
at a ratio of 32 :68, as followed from the coupling
constant of the NH proton (J = 8.9 Hz) in the 1H NMR
spectrum [17].
Compound 4 in acetonitrile solution showed anthra-
cene type fluorescence, in particular three emission
maxima were observed in the region λ 390–460 nm
(λexcit 365 nm). Fluorophore-free compound 2 dis-
played a weak emission band with its maximum at
λ 405 nm (λexcit 340 nm). The chemosensing properties
of 2 and 4 for H+, Zn2+, Cd2+, Ni2+, Co2+, Cu2+, Pb2+,
and Hg2+ ions were assessed in acetonitrile by fluores-
cence spectroscopy. For this purpose, 5 equiv of tri-
fluoroacetic acid or the corresponding metal acetate
was added to a solution of 2 or 4 in acetonitrile; the
fluorescence spectra were recorded before and after
addition of the test species. The addition of mer-
cury(II) acetate to a solution of 4 induced increase
of the fluorescence intensity by a factor of 72
(λmax 422 nm). The fluorescence spectrum of 4
changed after addition of Cu2+: a new long-wave
emission band appeared at λmax 512 nm. Compound 2
By nitration of veratrole we synthesized 1,2-di-
methoxy-4,5-dinitrobenzene. One nitro group in the
latter was replaced by morpholine residue, and the
other nitro group was then reduced to amino with
hydrazine hydrate over Raney nickel. 4,5-Dimethoxy-
2-(morpholin-4-yl)aniline (1) thus obtained was
brought into condensation with 2-hydroxynaphtha-
lene-1-carbaldehyde, which afforded Schiff base 2
(Scheme 1). The condensation product of amine 1 with
anthracene-9-carbaldehyde, Schiff base 3, turned out to
be poorly soluble and was therefore unsuitable for use
as chemosensor. By reaction of 1 with anthracene-
9-carbaldehyde in the presence of sodium tetrahydrido-
borate we obtained amine 4. Compound 4 can also be
synthesized by direct alkylation of 1 with 9-(chloro-
methyl)anthracene in anhydrous pyridine.
1436

FLUORESCENT CHEMOSENSORS FOR MERCURY(II) CATIONS
1437
Scheme 1.
O
O
N
OMe
NH2
CHO
N
N
OMe
OH
OH
O
H
+
N
OMe
OMe
N
O
OMe
OMe
1
2A
2B
O
N
O
CHO
N
N
NaBH4
1 +
OMe
HN
OMe
OMe
OMe
3
4
CH2Cl
Pyridine
1 +
4
showed response to mercury(II) ions and protons; the
fluorescence intensity increased by factors of 35 and 8,
respectively. The selectivity of sensor 4 was estimated
by adding a mixture of Hg2+ and Cu2+ salts to a solu-
tion of 4. As a result, 70-fold increase of the fluores-
cence intensity was observed at λ 422 nm, while no
other bands appeared in the fluorescence spectrum.
Other metal salts did not induce appreciable variations
in the fluorescence spectrum. Chemosensor 2 showed
no analogous efficiency or selectivity in the presence
of other metal ions (see figure).
cence spectra were recorded on a Varian Cary Eclipse
spectrofluorimeter. The IR spectra were measured on
a Varian Excalibur 3100 FT-IR instrument equipped
with an ATR accessory (ZnSe). The mass spectra
(electron impact, 70 eV) were obtained on a Shimadzu
GCMS-QP2010SE instrument with direct sample
admission into the ion source. The melting points were
measured in glass capillaries on a PTP (M) melting
point apparatus. The progress of reactions and the
purity of the isolated compounds were monitored by
TLC on Silufol UV 254 plates using chloroform as
eluent; spots were visualized by treatment with iodine
vapor in a moist chamber.
Thus, the new fluorescent chemosensors based on
4,5-dimethoxy-2-(morpholin-4-yl)aniline showed sen-
sitivity to mercury(II) ions, and N-(anthracen-9-yl-
methyl)-4,5-dimethoxy-2-(morpholin-4-yl)aniline
turned out to be an efficient and selective fluorescent
chemosensor for mercury(II) ions.
1,2-Dimethoxy-4,5-dinitrobenzene was synthesized
as described in [18]. Yield 82%, mp 123–124°C (from
i-PrOH); published data [18]: mp 126–127°C.
4-(4,5-Dimethoxy-2-nitrophenyl)morpholine.
A solution of 6.8 g (30 mmol) of 1,2-dimethoxy-4,5-
dinitrobenzene in 10 mL of anhydrous morpholine was
heated for 10 h. Excess morpholine was removed on
a rotary evaporator under reduced pressure, the residue
was dissolved in 50 mL of chloroform, and the solu-
tion was washed with water (5×50 mL) and dried over
anhydrous sodium sulfate. The solvent was distilled
off, and the residue was recrystallized from isopropyl
EXPERIMENTAL
1
The H NMR spectra were recorded on a Varian
Unity 300 spectrometer (300 MHz) from solutions in
DMSO-d6; the chemical shifts were measured relative
to residual proton signal of the solvent (δ 2.50 ppm).
The electronic absorption spectra were taken on
a Varian Cary 100 spectrophotometer, and the lumines-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 10 2016

TOLPYGIN et al.
1438
80
70
60
50
40
30
20
10
0
Zn2+
Cd2+
Cu2+
Co2+
Ni2+
Pb2+
Hg2+
Cu2+ + Hg2+
H+
Relative changes of the fluorescence intensity of compounds 2 and 4 in acetonitrile (c = 5.0×10–6 M) on addition of various cations
(c = 2.5×10–5 M).
alcohol. Yield 6.0 g (74%), mp 113–114°C (from
i-PrOH); published data [19]: mp 115–117°C.
aldehyde, 0.62 g (3 mmol), was added to a solution of
0.71 g (3 mmol) of amine 1 in 50 mL of ethanol–DMF
(3:1), and the mixture was refluxed for 1 h. The mix-
ture was cooled to 60–65°C, and 0.38 g (10 mmol) of
finely powdered sodium tetrahydridoborate was added
in portions with stirring. The mixture was cooled,
50 mL of water was added, and excess NaBH4 was
quenched with 10% acetic acid. After 1 h, the precip-
itate was filtered off, recrystallized from butan-1-ol,
and dried in air. Yield 0.9 g (71%), mp 221–222°C. IR
spectrum, ν, cm–1: 1615 (C=N), 1470, 1380 (C=C).
1H NMR spectrum, δ, ppm: 2.60–2.74 m (4H, NCH2),
3.29–3.52 m (4H, OCH2), 3.86 s (3H, OMe), 4.00 s
(3H, OMe), 4.85 br.s (1H, NH), 5.20 d (2H, CH2, J =
4.5 Hz), 6.73 s (2H, Harom), 7.45–7.57 m (4H, Harom),
8.06 d (2H, Harom, J = 9.2 Hz), 8.30 d (2H, Harom, J =
9.2 Hz), 8.49 s (1H, Harom). Fluorescence spectrum
(acetonitrile, c = 5×10–5 M): λmax 412 nm. Found, %:
C 75.75; H 6.65; N 6.45. m/z 428 [M]+. C27H28N2O3.
Calculated, %: C 75.68; H 6.59; N 6.54. M 428.54.
4,5-Dimethoxy-2-(morpholin-4-yl)aniline (1) was
synthesized as described in [19]. Yield 85%, mp 106–
107°C (from EtOH); published data [20]: mp 109°C.
1-{[4,5-Dimethoxy-2-(morpholin-4-yl)phenyl-
imino]methyl}naphthalen-2-ol (2). 2-Hydroxynaph-
thalene-1-carbaldehyde, 0.52 g (3 mmol), was added to
a solution of 0.71 g (3 mmol) of amine 1 in 10 mL of
ethanol, and the mixture was refluxed for 2 h. The
mixture was cooled, and the precipitate was filtered
off, recrystallized from butan-1-ol–DMF (1:1), and
dried in air. Yield 1.1 g (92%), mp 245–246°C. IR
spectrum, ν, cm–1: 3300 (OH), 1645 (C=N), 1465
1
(C=C). H NMR spectrum, δ, ppm: 2.80–2.93 m (4H,
NCH2), 3.70–3.98 m (10H, OCH2, OMe), 6.65–6.86 m
(2H, Harom), 7.10–7.27 m (1H, Harom), 7.32–7.74 m
(4H, Harom), 8.32 d (1H, Harom, J = 7.5 Hz), 9.40 d (1H,
CH=N, J = 8.9 Hz), 15.88 d (1H, NH or OH, J =
8.9 Hz). Fluorescence spectrum (acetonitrile, c =
5×10–5 M): λmax 405 nm. Found, %: C 70.45; H 6.24;
N 7.06. m/z 392 [M]+. C23H24N2O4. Calculated, %:
C 70.39; H 6.16; N 7.14. M 392.46.
b. 9-(Chloromethyl)anthracene, 0.91 g (4 mmol),
was added to a solution of 0.71 g (3 mmol) of amine 1
in 10 mL of anhydrous pyridine, and the mixture was
refluxed for 10 h. The mixture was diluted with 50 mL
of water and cooled, and 2 h after the precipitate was
filtered off and recrystallized from butan-1-ol. Yield
N-(Anthracen-9-ylmethyl)-4,5-dimethoxy-2-
(morpholin-4-yl)aniline (4). a. Anthracene-9-carb-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 10 2016

FLUORESCENT CHEMOSENSORS FOR MERCURY(II) CATIONS
1439
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This study was performed under financial support
by the Ministry of Education and Science of the
Russian Federation in the framework of the project
part of state assignment in the sphere of research
activity (project no. 4.88.2014/K).
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