Novel hydrazone-based tripodal sensors: Single selective colorimetric chemosensor for acetate in aqueous solution

Article abstract of DOI:10.1002/cjoc.201180275

A series of novel tripodal colorimetric anion sensors based on hydrazone CH=N - NH groups have been synthesized and their recognition behavior with anionic guests has been studied. In DMSO solutions, sensors 1 and 2 show colorimetric responses for F^-

Full text of DOI:10.1002/cjoc.201180275

FULL PAPER
Novel Hydrazone-Based Tripodal Sensors: Single Selective
Colorimetric Chemosensor for Acetate in Aqueous Solution
Zhang, Youming(张有明)
Li, Qiao(李乔)
Zhang, Qinsheng(张勤生)
Lin, Qi(林奇)
Cao, Cheng(曹成)
Liu, Mingxia(刘明霞)
Wei, Taibao*(魏太保)
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China; Key Laboratory
of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal
University, Lanzhou, Gansu 730070, China
A series of novel tripodal colorimetric anion sensors based on hydrazone CH＝ N— NH groups have been syn-
thesized and their recognition behavior with anionic guests has been studied. In DMSO solutions, sensors 1 and 2
－
show colorimetric responses for F , H2PO4 and AcO－ , while in DMSO/H2O (9∶ 1, V/V) solutions, sensor 1
－
－
1
shows single selectivity for AcO . H NMR titration confirms that the tripodal sensors could bind anions through
the collaboration of three hydrazone groups and anions residing in the central cavity of the sensors.
Keywords colorimetric sensors, hydrazone, tripodal sensor, anion recognition, acetate ion
Introduction
The challenge is that strong hydration in the aqueous
phase stops the sensors from recognizing the anions.20
So far, only a few sensors that are able to recognize
anions in the aqueous phase have been synthesized.
In view of these and being interested in the studies
of anion recognition,21-23 we want to develop a colori-
metric anion sensors which could selectively sense ace-
tate anions in water solution. The strategies for design
of the sensors (sensors 1 and 2 in Scheme 1) are as fol-
lows. Firstly, because tripodal sensor has a sufficient
number of hydrogen bond site to serve as binding sites
and the sensor with a cavity has a strong affinity to
guest,24-28 we introduce tripodal frameworks into the
sensor’s design, which could remarkably enhance the
anion binding abilities of sensors.27,29,30,31 Secondly, the
cavity structure could reduce the impact of solvent
molecules. Thirdly, we use hydrazone CH＝N—NH
groups as binding sites, which have the merits of having
two different hydrogen bond donor sites (CH and NH),
which could provide versatile binding abilities.32,33
Lastly, aiming at achieving colorimetric recognition,
nitrophenyl groups were introduced into the sensor as
signal group and nitro groups increase the acidity of N
and H groups, which makes deprotonation easy. In order
to confirm our strategies, we also designed and synthe-
siz another two sensors 3 and 4 (Scheme 1). Sensor 3 is
an analogous of sensor 1, but has three nitro groups.
Sensor 4 resembles the single pod of sensor 1.
Because anions play an important role in a wide
range of chemical, biological and environmental proc-
esses, the rational design and synthesis of efficient sen-
sors to selectively recognize special anionic analytes
appear to be particularly attractive. Since the first syn-
thetic sensor for inorganic anions was reported in 1968,1
great progress has been made.2-7 Although previous
works have developed a wide variety of sensors based
on electrostatic interactions, hydrogen bond donor
groups, Lewis acid groups and hydrophobic interactions,
it is still a challenge to design the anion sensors with
high selectivity and sensitivity in a competitive media.8
Recently, there is an increasing interest in the develop-
ment of colorimetric anion sensors because they have
the advantage of showing easily observable color
changes in the presence of target anion guests.9
Acetate (AcO－ ) is a critical component of numerous
metabolic processes. The rate of AcO－ production and
oxidation has been frequently used as an indicator of
organic decomposition in marine sediments.10 Com-
pared with the F ,11-13 and H2PO4 sensors,14,15 AcO－
－
－
sensors are limited for most of the AcO－ sensors usu-
ally have a stronger affinity for F－ .16,17 On the other
hand, in biological and environmental systems, anion-
sensor interactions commonly occur in aqueous solution,
therefore, much attention has been paid to developing
anion sensors that work in the aqueous phase.5,12,18,19
*
E-mail: weitaibao@126.com; Tel.: 0086-0931-7973191; Fax: 0086-0931-7973191
Received September 23, 2010; revised March 10, 2011; accepted March 18, 2011.
Project supported by the National Natural Science Foundation of China (No. 20671077) and Natural Science Foundation of Gansu (No.
1010RT2A018).
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Zhang et al.
FULL PAPER
Scheme 1 Synthetic procedures for sensors 1—
4
Experimental
(DMSO-d6, 400 MHz) δ: 11.21 (s, 3H, NH), 8.35 (s, 3H,
HC＝N), 7.86 (d, J＝7.6 Hz, 3H, ArH), 7.26 (t, J＝7.2
Hz, 4H, ArH), 7.07 (t, J＝8.4 Hz, 11H, ArH), 6.96 (t,
J＝7.2 Hz, 3H, ArH), 4.19 (t, J＝5.6 Hz, 6H, CH2),
3.17 (t, J＝5.2 Hz, 6H, CH2); IR (KBr) v: 3447 (mb),
3278 (m), 1601 (m), 1543 (w), 1497 (m), 1471 (m),
1305 (s), 1252 (m), 837 (w) cm－ 1, Anal. calcd for
C45H39N13O15: C 53.95, H 3.92, N 18.17; found C 53.90,
H 3.89, N 18.22.
Materials and physical methods
1H NMR spectra were recorded with a Mercury-
400BB spectrometer at 400 MHz. 1H chemical shifts are
reported downfield from tetramethylsilane (TMS, δ
scale, with the solvent resonances as internal standards).
UV-visible spectra is recorded on a Shimadzu UV-2550
spectrometer. Melting points were measured on an X-4
digital melting-point apparatus (uncorrected). The in-
frared spectra were performed on a Digilab FTS-3000
FT-IR spectrophotometer. All reagents used were of
analytical grade. All the tetrabutylammonium (TBA)
salts were purchased from Alfa Aesar Company and
dried by P2O5.
Sensor 2: Yield 75%, m.p. 284—285 ℃; 1H NMR
(DMSO-d6, 400 MHz) δ: 11.22 (s, 3H, NH), 8.37 (s, 3H,
HC＝N), 8.09 (d, J＝9.2 Hz, 6H, ArH), 7.87 (d, J＝7.6
Hz, 3H, ArH), 7.24—7.28 (m, 4H, ArH), 7.06 (d, J＝8
Hz, 8H, ArH), 6.96 (t, J＝7.6 Hz, 3H, ArH), 4.18 (t, J＝
4.4 Hz, 6H, CH2), 3.17 (t, J＝4.4 Hz, 6H, CH2); IR
(KBr) v: 3446 (mb), 3277 (m), 1602 (m), 1543 (w),
－
1
Synthesis of sensors
1470 (m), 1305 (s), 1252 (m), 837 (w) cm . Anal.
calcd for C45H42N10O9: C 62.35, H 4.88, N 16.16; found
C 62.31, H 4.85, N 16.21.
Tri[2-(2-formylphenoxy)ethyl]amine 2,2',2''-[Nitri-
lotris(2,1-ethanediyloxy)]trisbenzal-dehyde was synthe-
sized according to literature methods,34 yield 88%, m.p.
102—104 ℃.
1
Sensor 3: Yield 74%, m.p. 169—171 ℃; H NMR
(DMSO-d6, 400 MHz) δ: 10.13 (s, 3H, NH), 7.81 (s, 3H,
HC＝N), 7.57 (d, J＝6.8 Hz, 6H, ArH), 7.19 (t, J＝7.2
Hz, 6H, ArH), 7.04 (d, J＝7.6 Hz, 6H, ArH), 6.96 (d,
J＝6.8 Hz, 6H, ArH), 6.70 (t, J＝7.2 Hz, 3H, ArH),
4.11 (t, J＝5.2 Hz, 6H, CH2), 3.08 (t, J＝4.4 Hz, 6H,
CH2); IR (KBr) v: 3445 (mb), 3314 (m), 1601 (m), 1504
(m), 1294 (m), 1251 (m), 1250 (m), 1130 (m), 831 (w)
Synthesis and characterization of sensors 1— 3
2,2',2''-[Nitrilotris(2,1-ethanediyloxy)]trisbenzaldehyde
(1.38 g, 3 mmol), (2,4-dinitrophenyl)hydrazine (1.98 g,
10 mmol) and catalytic amount of acetic acid were
mixed in ethanol (40 mL). The solution was stirred un-
der reflux conditions for 8 H and the precipitate was
filtrated, washed with hot absolute ethanol three times,
then recrystallized with DMSO/H2O to get crimson
compound 1 (2.11 g, 2.11 mmol). The other compounds
2— 4 were prepared through the similar procedures.
－
1
cm . Anal. calcd for C45H45N7O3: C 73.85, H 6.20, N
13.40; found C 73.82, H 6.17, N 13.44.
1
Sensor 4: Yield 84%, m.p. 259—260 ℃; H NMR
(DMSO-d6, 400 MHz) δ: 11.69 (s, 1H, NH), 8.73 (s, 1H,
HC＝N), 8.88 (d, J＝2.8 Hz, 1H, ArH), 8.4 (d, J＝7.2
1
Sensor 1: Yield 70%, m.p. 274—275 ℃; H NMR
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Chin. J. Chem. 2011, 29, 1529— 1534

Single Selective Colorimetric Chemosensor for Acetate in Aqueous Solution
Hz, 1H, ArH), 8.13 (d, J＝9.6 Hz, 1H, ArH), 7.83—
7.79 (m, 2H, ArH), 7.5 (t, J＝7.6 Hz, 3H, ArH). IR
(KBr) v: 3444 (mb), 3286 (m), 1616 (m), 1585 (m),
－
1
1513 (m), 1419 (m), 1334 (s), 1089 (m), 827 (w) cm .
Anal. calcd for C13H10N4O4: C 54.55, H 3.52, N 19.57;
found C 54.52, H 3.54, N 19.58.
General procedure for UV-vis experiments
All UV-vis experiments were carried out in DMSO
or DMSO/H2O binary solutions on a Shimadzu UV-
2550 spectrometer. Any changes in the UV-vis spectra
of the synthesized compound were recorded on addition
of tetrabutylammonium salt while keeping the ligand
concentration constant in all experiments. Tetrabutyl-
ammonium salts of anions (F－ , Cl－ , Br－ , I－ , AcO－ ,
－
－
－
H2PO4 , HSO4 and ClO4 ) were used for the UV-vis
experiments.
1
General procedure for H NMR spectroscopy titra-
tion experiment
For 1H NMR spectroscopic titration experiment, two
stock solutions were prepared in DMSO-d6, one of them
containing host only and the other containing an appro-
priate concentration of guest. Aliquots of the two solu-
tions were mixed directly in NMR tubes.
Results and discussion
The colorimetric sensing abilities were primely in-
vestigated by adding various anions such as F－ , Cl－ ,
－
－
－
－
－
－
Br , I , AcO , H2PO4 , HSO4 and ClO4 (tetrabu-
tylammonium was used as a cation) to DMSO solution
－
of the sensors (1.0×10－ 5 mol•L ) respectively. Taking
1
sensor 1 as an example, when adding 50 equiv. of AcO－ ,
F－ or H2PO4 to the DMSO solutions of sensor 1 re-
－
spectively, in the UV-vis spectrum, a new and strong
absorption peak appears at 502 nm (Figure 1a), and
meanwhile dramatic color changes of these solutions
take place from orange to purple (Figure 2a). While
－
－
－
－
when adding 50 equiv. of Cl , Br , I , HSO4 or
ClO4 to the DMSO solutions of sensor 1 respectively,
－
no obvious color or UV-vis spectrum changes are ob-
served (Figures 1a and 2a). These results indicate that
sensors 1 could colorimetrically sense AcO－ , F－ and
Figure 1 UV-Vis absorption of sensors 1— 3 (c＝ 1.0× 10－
5
－
1
mol•L ) in the presence of 50 equiv. of various anions in DMSO
－
H2PO4 in DMSO solutions.
at room temperature: (a) 1, (b) 2, (c) 3.
In H2O/DMSO binary solution, the anion sense
abilities of the sensors vary with the volume ratio of
water. When the ratio of DMSO/H2O was adjusted to
9∶1 (V/V), adding AcO－ to sensors 1 led to an obvious
color change from orange to magenta, while under the
same conditions, other anions such as F－ , Cl－ , Br－ , I－ ,
H2PO4 HSO4 and ClO4 could not cause any color
change (Figures 2b and 3). Therefore, in DMSO/H2O
(9∶1) (V/V) solutions, sensor 1 has the single colori-
metric selectivity to AcO－ . With the aim of excluding
the possibility of these results being due to pH changes,
we carried out the same experiments in 9∶1
DMSO/H2O HEPES buffered solution at pH 7.1 for
sensors 1. Once more, similar color changes are
observed as above (Figure4). It should also be noted that
the pH of the solution did not change by more than 0.2
units during these experiments, clearly excluding any
pH effects.
The further binding properties of sensor 1 and anions
were studied by UV-vis titration experiments (Figure 5).
By nonlinear least-squares fitting35 at λmax＝502 nm, the
－
－
－
association constants Ka of the sensor 1 toward AcO－ ,
－
F－ and H2PO4 are 3.0×105 L•mol , 3.0×104 and
1
－
－
2.9×104 mol•L respectively in DMSO solution. And
the association constant Ka of the sensor 1 toward
AcO－ is 6.4×104 in DMSO/H2O 9∶1 (V/V) solutions.
Besides, a detection limit of acetate by sensor 1 is esti-
1
Chin. J. Chem. 2011, 29, 1529— 1534
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Zhang et al.
FULL PAPER
Figure 2 Color changes observed upon the addition of various
anions (50 equiv.) to solution of sensor 1 (1.0× 10－ mol/L): (a)
5
in DMSO solution; (b) in DMSO/H2O (9∶ 1, V/V) solution. From
－
－
left to right: free 1, F－ , Cl－ , Br－ , I－ , AcO－ , H2PO4 , HSO4
and ClO4 .
－
Figure 5 UV-vis spectral titration of sensor 1 (c＝ 1.0× 10－
mol•L ) with AcO (0— 21.1 equiv.) in DMSO solution.
5
－
－
1
Figure 3 UV-Vis absorption of sensors 1 (c＝ 1.0× 10－ mol•
5
H2PO4 in DMSO. The association constants Ka35 of
－
－
1
the sensor 2 toward AcO － , F － and H2PO4 are
－
L ) in the presence of 50 equiv. of various anions in DMSO/H2O
－
1
8.0× 103, 2.5× 102 and 4.3× 102 L•mol respectively,
which are relatively smaller than sensor 1. This phe-
nomenon is due to nitro groups of sensor 2 being less
than that of sensor 1, which indicates that the colorimet-
ric responses abilities of sensor 2 are relatively weaker
than sensor 1, and it is most important that the electron
withdrawing groups (nitro groups) introduced into the
sensor are able to strengthen hydrogen bond between
host and guest. Sensor 3 does not possess signal groups,
and it could not colorimetricaly sense any anions. These
results confirm nitro groups play an important role in
the colorimetric sensors. Moreover, although sensor 4
does not possess tripodal structure, sensor 4 could col-
(9∶ 1, V/V) binary solution at room temperature.
orimetrically sense AcO－ , F－ and H2PO4 in DMSO
－
sloutions like sensor 1 (Figure 6). The association con-
stants Ka35 of the sensor 4 toward AcO－ , F－ and
－
1
H2PO4 are 2.5× 103, 1.0× 102 and 1.1× 103 L•mol
－
Figure 4 UV-Vis absorption of sensors 1 (c＝ 1.0× 10－ mol•
5
L－ 1) in the presence of 50 equiv. of various anions in 9∶
1
respectively, which are relatively smaller than sensor 1,
and in DMSO/H2O (9∶ 1, V/V) solutions, sensor 4
could not recognize any anions. These results show that
the tripodal structure could obviously enhance the sen-
sor’s anion binding abilities.
DMSO/H2O HEPES buffered solution at pH 7.1.
－
1
mated to be 5 µmol•L .
The anion recognition abilities of sensors 2— 4 have
been investigated by similar experiment and the follow-
ing results could prove our design strategies. Because
the structure of sensor 2 is very similar to sensor 1,
sensor 2 could colorimetrically sense AcO－ , F－ and
In order to study the anion-sensor reaction mecha-
1
nism, a H NMR titration experiment in DMSO-d6 is
1
carried out, and the partial H NMR titration spectrum
of sensor 2 with AcO－ is shown in Figure 7. Before the
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Single Selective Colorimetric Chemosensor for Acetate in Aqueous Solution
Figure 6 UV-Vis absorption of sensors 4 (c＝ 2.0× 10－ mol•
5
－
1
L ) in the presence of 50 equiv. of various anions in DMSO at
room temperature.
－
1
addition of AcO , the H NMR chemical shifts of the
N— H and HC＝ N protons on sensor 2 are at δ 11.22
and 8.38 respectively. After the addition of 1 equiv. of
AcO－ , the resonances are shifted to δ 13.33 for N— H
and δ 8.49 for HC＝ N. These downfield shifts suggest
the formation of a complex between sensor 1 and AcO－
through N— H… O and N＝ C— H… O－ hydrogen
bonds (Figure 6). With the continuous addition of AcO－
(after addition of 2 equiv. of AcO－ ), the resonances for
N— H protons disappear and the signal of the HC＝ N
protons unceasingly shift to low field, which suggest
that the N— H groups perhaps underwent a deprotona-
tion process and sensor 2 bind the AcO－ by the N＝ C
— H… O－ hydrogen bonds (Figure 8). By this means,
through the collaboration of three hydrazone groups, the
tripodal sensors 1 and 2 could form cavity complexes
with AcO－ , which remarkably enhanced the affinities of
sensor 1 or 2 toward AcO－ . The deprotonation of N— H
groups led to an increase in electronic density of the
phenyl group, so the resonances for the phenyl proton
showed mildly upfield shifts, which also enlarged the
Figure 8 The recognition mechanism of the sensor with AcO－ .
conjugate system of the sensor 2. Therefore, with the
deprotonation of the N— H groups, the color of sensor 2
changes from orange to magenta.
Conclusion
In summary, we have synthesized a series of tripodal
colorimetric anion sensors bearing hydrazone CH＝ N—
NH groups. Among them, sensors 1 and 2 show
－
colorimetric responses for F , H2PO4 and AcO－ in
－
DMSO solution, moreover sensor 1 shows colorimetric
single selective recognition for AcO－ in DMSO/H2O
(9∶ 1, V/V) solutions. By comparison of sensor 1 and
sensor 3, we comfirm that signal group plays an
important role in the colorimetric sensors. Moreover,
the comparison of sensor 1 and sensor 4 proves that the
tripodal structure could also obviously enhance the
sensor’s anion binding abilities.
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Figure 7 Partial H NMR spectra of sensor 2 (0.1 mol•L ) in
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