Small molecular chromogenic sensors for Hg2+: A strong "push-pull" system exists after binding

Article abstract of DOI:10.1002/ejoc.200601086

Two small molecular chromogenic sensors 1 and 2 for detection of Hg ²⁺ are described. After coordination with Hg²⁺, a red shift of about 100 nm was observed in the UV/Vis spectra and the color of the solution changed from pale yellow to red which could easily be detected by the naked eye. The results indicate a strong push-pull system was formed after coordination of Hg²⁺. The ¹H NMR spectra and control experiments showed the binding sites of 1 and 2 to be the aniline groups rather than the azine bridge. Moreover, mercury test papers were made by adsorbing 1 onto filter paper, allowing heterogeneous sensing of Hg²⁺ in aqueous solution. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200601086

FULL PAPER
DOI: 10.1002/ejoc.200601086
2
+
Small Molecular Chromogenic Sensors for Hg : A Strong “Push-Pull”
System Exists after Binding
Yanyan Fu,[a,b] Hongxiang Li,* and Wenping Hu*
[a]
[a]
Keywords: Chemosensors / Mercury / Molecular recognition / Push-pull effects
Two small molecular chromogenic sensors 1 and 2 for detec- showed the binding sites of 1 and 2 to be the aniline groups
2
+
2+
tion of Hg are described. After coordination with Hg , a
red shift of about 100 nm was observed in the UV/Vis spectra
and the color of the solution changed from pale yellow to red
which could easily be detected by the naked eye. The results
rather than the azine bridge. Moreover, mercury test papers
were made by adsorbing 1 onto filter paper, allowing hetero-
geneous sensing of Hg2 in aqueous solution.
+
indicate a strong push-pull system was formed after coordi- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
2+
1
nation of Hg . The H NMR spectra and control experiments
Germany, 2007)
Introduction
directly giving the ratio of the species, and 3) mercury test
devices can be made by adsorbing chromogenic sensors
onto a substrate (e.g., mercury test paper can be made by
adsorbing a chromogenic sensor onto filter paper), which is
convenient.
There continues to be a need for new and improved
methods for the detection of certain transition- and post-
[31]
transition-metal ions that have detrimental effects on hu-
mans or other animals.[
1,2]
Of these metal ions, mercury is
It is well known that the optical properties of a molecule
are strongly dependent on its electron distribution. Bearing
this in mind and considering the fact that amine is a good
ligand to mercury ions, we have designed structure-simple
receptors 1 and 2. Both receptors are electron-rich com-
pounds and contain two amino substituents. We reason that
when these receptors are exposed to a mercury ion, one of
the amine groups will coordinate with the ion and a strong
metal-induced intramolecular electron transfer (MICT) will
occur and a push-pull system will be formed (the other
amine group acts as a donor and because of MICT this
amine will have a very low affinity for the mercury ion)
causing a distinct change in the optical properties of the
considered a highly toxic and hazardous pollutant that
poses severe risk to human health and the environment.
[
3–5]
Despite its toxicity, mercury and mercuric salts are widely
used in industrial processes and products such as gold min-
[
6–8]
ing and fungicides.
Once elemental or ionic mercury has
been converted by aquatic organisms into methylmercury,
it can enter the food chain and be ingested by humans caus-
[
9–11]
ing many mercury-pollution-related diseases.
Great ef-
2
+
forts have been made to develop Hg sensors using chro-
[
12–18]
[19–27]
mogenic,
fluorogenic,
or electrochemical meth-
However, most of these chemical sensors suffer
[
28–30]
ods.
from limitations, such as a tedious synthesis, an inability to
be used in pure water thereby needing special or expensive
apparatus instrument, and interference with other metal
ions.
receptor.
In this paper, the syntheses, optical properties, and Hg2+
binding properties of receptors 1 and 2 (Scheme 1) are de-
We are interested in exploring further the use of chromo-
scribed. The binding site of these receptors was confirmed
2
+
genic sensors to selectively detect Hg because 1) chromo-
genic detection can be carried out without spectroscopic
instrumentation and therefore is simple, easy and low-cost,
1
by H NMR spectra and control experiments. Moreover,
the application of mercury test papers made by adsorbing
sensor 1 onto filter paper in aqueous solution is also pre-
sented.
2) UV/Vis titration shows the appearance of a new band
and the disappearance of the initial band, the two bands
[
[
a] Beijing National Laboratory for Molecular Sciences (BNLMS),
Organic Solid Laboratory, Institute of Chemistry, Chinese
Academy of Sciences,
Results and Discussion
Synthesis
Beijing 100080, China
Fax: +86-10-62527295
E-mail: lhx@iccas.ac.cn
The syntheses of compounds 1 and 2 as well as model
compounds are outlined in Scheme 1. Compounds 1–4 were
obtained in moderate yields by reacting corresponding alde-
hydes with hydrazine hydrate. Compound 5 was synthesized
b] Graduate School of Chinese Academy of Sciences,
Beijing 100039, China
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2007, 2459–2463
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Y. Fu, H. Li, W. Hu
FULL PAPER
Figure 1. The UV/Vis response of 1 (CH
3
CN, 5ϫ10–6 ) upon ad-
2
+
dition of Hg
.
2+
ity of receptor 1 for these ions. With the addition of Cu ,
a new peak in the UV/Vis spectrum was observed at 336 nm
with the disappearance of the absorption at 393 nm. Con-
trol experiments showed Cu2 was not complexed by 1,
however, it did induce the decomposition of 1, as proved by
UV/Vis spectroscopy (see the Supporting Information). The
+
detection limit of Cu2 at 336 nm was 2ϫ10 .
+
–5
Scheme 1. Structures and synthetis of compounds 1–5.
by the Wittig–Horner reaction. All of the compounds were
1
totally characterized by H NMR, MS, and elementary
analysis.
Absorption Spectra
Figure 1 shows the UV/Vis response of compound 1
upon addition of a mercury ion. As expected, a new band
at 496 nm appeared with the disappearance of the initial
band at 393 nm. The new band was red-shifted by about
103 nm in the visible light region causing the color of the
solution to change from pale yellow to red which could be
detected easily by the naked eye. This new band was as-
cribed to a MICT absorption. The presence of a well-de-
fined isosbestic point indicated the existence of a two-state
equilibrium, which means there is only one stable complex
in solution. The UV/Vis titration profile showed the forma-
–
5
3
Figure 2. Top: UV/Vis response of 1 (CH CN, 1ϫ10 ) upon
addition of different metal ions (including Hg , Cu , Cd2+, Co
2
+
2+
2+
,
2
+
2+
2+
2+
2+
Ni , Pb , Zn , Ag+, Mg , and Zn ). Insert: Partial UV/Vis
2+
spectra in the range of 450–600 nm (the spectra of Hg has been
2
+
tion of a 1:1 adduct between receptor 1 and Hg , as ex- omitted). Bottom: color changes observed for 1 (MeCN/H O = 2:1,
2
–
4
1
ϫ10 ) upon addition of 2 equiv. of metal ions. From left to
right: free 1, Hg , Zn , Ni , Co , Cu , Ag , and Cd .
pected because of the existence of the MICT absorption.
2
+
2+
2+
2+
2+
+
2+
4
The binding constant was calculated to be 2.6ϫ10 . A
2
+
µ detection limit of receptor 1 for Hg was deduced from
the absorption spectrum.
The UV/Vis spectra (Figure 3) recorded on addition of
The sensing specificity of 1 for the mercury ion over in- mercury ions to receptor 2 are similar to those obtained
2
+
2+
2+
2+
terference metal ions, including Ni , Co , Zn , Cd , with 1. The absorption at 400 nm decreased gradually with
2
+
+
+
+
+
2+
Cu , Ag and main-group metal ions Li , Na , K , Mg , the observation of a new absorption at 500 nm and the
Ca , and Ba , was tested. Figure 2 shows the UV/Vis re- color of the solution changed from pale yellow to red (see
2
+
2+
sponse and color change of 1 upon addition of metal ions the Supporting Information). UV/Vis titration experiments
2
+
(
(
main-group metal ions have been omitted). From Figure 2 indicated a 1:1 adduct was formed between 2 and Hg with
4
top, inset) we could see that MICT absorptions were also a binding constant of about 6ϫ10 . No change in the
observed with Zn , Co , and Mg . However, their fluorescence spectrum was observed after the addition of
2
+
2+
2+
2+
MICT absorptions were very weak, indicating the low affin- Hg (see the Supporting Information).
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Eur. J. Org. Chem. 2007, 2459–2463

Small Molecular Chromogenic Sensors for Hg2+
FULL PAPER
Figure 3. UV/Vis response of 2 (MeCN, 1ϫ10–5 ) upon addition
1
of Hg2+
Figure 4. H NMR spectra of 1 before and after the addition of
2+ 2+
.
0
.2 equiv. of Hg . Top: free 1. Bottom: 1 + 0.2 equiv. of Hg .
The reversibility of the binding of receptors 1 and 2 to
2
+
Hg was then tested. On addition of thiourea, adducts
·Hg2+ and 2·Hg decomplexed quickly, as confirmed by
UV/Vis spectroscopy and color recovery of the solutions
see the Supporting Information). From the above results,
2+
1
(
we concluded that receptors 1 and 2 are good chemosensors
for mercury ions.
Binding Site Study
1
,4-Disubstituted azines have recently been used as re-
2
+
[32,33]
cognition sites for Hg detection.
As compounds 1
and 2 contain both an azine bridge and aniline groups,
these two compounds have multiple binding sites. On the
basis that no fluorescence change was observed on addition
2
+
of Hg ions to 2, we supposed the binding sites of com-
pounds 1 and 2 to be the aniline groups rather than the
azine bridge. In order to confirm this supposition, 1
H
NMR studies as well as control experiments were carried
out. Before the addition of mercury ions (Figure 4), com-
pound 1 showed two doublets arising from the protons on
Figure 5. UV/Vis responses of compounds 3 and 4 (MeCN,
–
5
2+
1
ϫ10 ) upon addition of Hg (0–10 equiv.). a: compound 3. b:
the phenyl rings and one quartet due to the NCH protons
2
compound 4.
owing to the symmetry of 1. On addition of mercury ions,
the signals arising from the protons on the phenyl rings
and NCH were shifted downfield. From the splittings and
2
chemical shifts of these protons, we could conclude that the
electron densities on the two phenyl rings were different af-
ter the addition of the mercury ions, indicating a lack of
symmetry in the adduct. If compound 1 coordinated to the
mercury ion at the azine bridge, the adduct would have a
1
similar symmetry to that of 1, which means the H NMR
spectrum of the adduct should be similar to that of free 1
1
but with different chemical shifts. The H NMR experiment
therefore further confirmed our supposition.
For the control experiments, model compounds 3, 4, and
5
were designed and synthesized. Compounds 3 and 4 only
contained an azine bridge, whereas compound 5 only con-
tained an aniline nitrogen atom. In the titration of com-
2
+
pounds 3 and 4 with Hg , the UV/Vis spectra showed no
absorption response (Figure 5). Also, the fluorescence spec-
tra of 4 before and after addition of mercury ions were ne-
arly same (see the Supporting Information). The UV/Vis
spectrum of compound 5 exhibited an absorption band at
about 353 nm with a shoulder at 328 nm. On addition of
–5
Figure 6. a: The UV/Vis response of 5 (MeCN, 1ϫ10 ) upon
2+
addition of Hg (0–6 eqiv.). b: Fluorescent titration profile of 5
2
+
2+
Hg , the absorption band at 353 nm gradually red-shifted with 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 equiv. of Hg
.
Eur. J. Org. Chem. 2007, 2459–2463
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Y. Fu, H. Li, W. Hu
FULL PAPER
to 360 nm. Moreover, the addition of Hg2+ induced signifi- Experimental Section
cant fluorescence enhancement (Figure 6). The fluorescence
titration profiles of 5 with Hg are consistent with a sup-
Materials and Instrumentation: Chemicals and reagents were pur-
chased from commercial suppliers and used without further purifi-
cation unless otherwise noted. UV/Vis absorption spectra were re-
corded with a Hitachi U3500 spectrometer. Fluorescence spectra
2
+
posed 1:1 adduct and the association constant was about
4
1
.9ϫ10 , indicating that 5 could be used as a fluorescent
2
+
1
sensor for Hg . From both the H NMR spectra and the
1
were obtained with a Hitachi F-4500 spectrometer. H NMR spec-
control experiments, we believe there is no doubt that the tra were obtained with a Bruker DMX-400 spectrometer.
binding sites in receptors 1 and 2 are the aniline groups
rather than the azine bridges.
Compound 1: Hydrazine hydrate (0.17 g, 4.2 mmol) was added
dropwise to a solution of 4-(diethylamino)benzaldehyde (1.5 g,
8
.4 mmol) in acetic acid (50 mL). The reaction mixture was stirred
at room temperature for 5 h. The solvent was evaporated and the
residue purified by column chromatography (CH Cl as eluent).
Compound 1 was obtained as a yellow solid. Yield 1.085 g (57.4%).
Mercury Test Papers
2
2
1
As receptors 1 and 2 are not soluble in aqueous solution, M.p. 193 °C. H NMR (300 Hz, CDCl ): δ = 8.54 (s, 2 H), 7.67 (d,
3
to confirm the potential application of these receptors, mer- 4 H), 6.67 (d, 4 H), 3.42 (m, 8 H), 1.19 (m, 12 H) ppm. C22
cury ion test papers based on compound 1 were prepared (350.25): calcd. C 75.43, H 8.57, N 16.00; found C 75.73, H 8.77,
N 15.54.
4 mg/mL) and drying them in air. Figure 7 shows the Compound 2: Compound 2 was synthesized in a similar procedure
changes in the color of the test papers on immersion in to that used for 1 and obtained as a light yellow solid. Yield 52%.
30 4
H N
2
by immersing filter papers (4ϫ1 cm ) into 1 in CH Cl
2
2
(
2
+
1
aqueous solutions containing different amounts of Hg at M.p. 243 °C. H NMR (300 Hz, CDCl
4 H), 7.34–7.27 (m, 8 H), 7.16 (d, 8 H), 7.11–6.96 (m, 8 H) ppm.
(542.25): calcd. C 84.13, H 5.54, N 10.33; found C 83.78,
H 5.83, N 9.92.
3
): δ = 8.57 (s, 2 H), 7.65 (d,
2
+
pH = 7. Clearly, the test paper could detect Hg in aque-
ous solution at a concentration of about 2.5ϫ10 . Other
ions such as Ag , Ni , Co , Zn , Cd , Li , Na , K ,
–
5
38 30 4
C H N
+
2+
2+
2+
2+
+
+
+
2
+
2+
2+
Mg , Ca , and Ba did not cause any detectable changes, Compound 3: Compound 3 was synthesized in a similar procedure
2
+
to that used for 1 and obtained as a light yellow solid. Yield 72%.
indicating the high selectivity of the test papers for Hg .
M.p. 98 °C (ref.[ 98 °C). H NMR (300 Hz, CDCl
34]
1
): δ = 8.66 (s,
3
2
H), 7.83 (m, 4 H), 7.44 (m, 6 H) ppm.
Compound 4: Compound 4 was synthesized in a similar procedure
to that used for 1 and obtained as an orange solid. Yield 70%.
M.p. 280 °C (ref.[ 276 °C). H NMR (300 Hz, CDCl
(
4
35]
1
): δ = 10.15
s, 2 H), 8.88 (d, 4 H), 8.61 (d, 2 H), 8.09 (d, 4 H), 7.67–7.63 (m,
H), 7.58–7.54 (m, 4 H) ppm.
3
Compound 5: 4-(Diethylamino)benzaldehyde (2.818 g, 15.9 mmol)
was dissolved in anhydrous THF (50 mL) and then NaH (0.8 g,
16.6 mmol, 50% w/w) was added. A solution of diethyl benzylphos-
phonate (3.6 g, 15.8 mmol) in anhydrous THF (50 mL) was then
added to the reaction mixture over 5 min, which was then stirred
overnight. Excess NaH was destroyed and solvent removed. The
residue was purified by column chromatography and compound 5
was obtained as a yellow solid. Yield 1.3 g (32.6%). M.p. 96 °C
Figure 7. Changes in the colors of the test papers based on 1 for
detecting Hg2+ in neutral aqueous solutions with different Hg
concentrations.
2+
ref.[ 98 °C). H NMR (300 Hz, CDCl
d, 2 H), 7.32 (m, 2 H), 7.19 (m, 1 H), 7.04 (d, 1 H), 6.88 (d, 1 H),
.66 (d, 2 H), 3.37 (m, 4 H), 1.20 (m, 6 H) ppm. C18 21N (251.17):
36]
1
(
(
3
): δ = 7.47 (d, 2 H), 7.39
Conclusions
6
H
calcd. C 86.05, H 8.37, N 5.57; found C 85.73, H 8.65, N 5.15.
Structure-simple and electron-rich chromogenic sensors
Supporting Information (see also the footnote on the first page of
1
and 2 for mercury ions have been described. Upon bind-
2
+
2
+
this article): UV/Vis titration profiles of sensors 1 and 2 with Hg
and Cu , fluorescence spectra, color changes.
ing with Hg , they become strong push-pull systems and
color changes could easily be detected by the naked eye.
Control experiments showed the binding site in sensors 1
2
+
and 2 to be the aniline group rather than the azine bridge. Acknowledgments
Furthermore, mercury ion test papers were prepared by ad-
The authors thank the National Natural Science Foundation of
China (20402015, 90401026), the Center for Molecular Sciences,
and the Chinese Academy of Sciences (CMS-CX200514) for sup-
port.
sorbing 1 onto filter paper and showed high selectivity
towards the mercury ion in aqueous solution. As George S.
Hammond mentioned “The most fundamental and lasting
objective of synthesis is not production of new compounds,
but production of properties” (Norris Award Lecture,
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syntheses will be helpful in the design of other kinds of
sensors for specific ions.
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Small Molecular Chromogenic Sensors for Hg2+
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