Convenient synthesis and anion recognition property of acylhydrazone-based molecular tweezer receptors

Article abstract of DOI:10.3906/kim-0903-47

Three acylhydrazone-based compounds were designed as novel neutral sensors for anions, and synthesized by simple steps in good yields. Their anion recognition properties were studied by UV-vis and ¹H-NMR spectroscopy. The results showed that the receptors 1, 2, and 3 all had a better selectivity for F^- and CH ₃ COO ^-, but no evident binding with Cl^- , Br ^- , 1^- , NO ₃^- , H₂ PO₄^- , or HSO₄^- .The results indicated that anion recognition was achieved via convergent hydrogen bond interactions from acylhydrazone functionality on the side arms. The UV-vis data indicated that a 1:1 stoichiometry complex was formed between compounds 1 , 2, or3 and anions. The binding and selectivity were also tuned by the change of the place of the nitro group attached to the phenyl. Moreover, receptor 3 can act as the colorimetric sensor for such anions as F ^- and CH ₃ COO ^- , and the recognition mechanism and binding mode were discussed. TUeBITAK.

Full text of DOI:10.3906/kim-0903-47

Turk J Chem
34 (2010) , 731 – 737.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-0903-47
Convenient synthesis and anion recognition property of
acylhydrazone-based molecular tweezer receptors
Hai-Xian REN∗, Ming-Gen ZHAO, Ying-Jin WANG
Xinzhou Teachers University Department of Chemistry, Xinzhou
Shanxi, 034000, CHINA
e-mail: hxren326@163.com
Received 18.03.2009
Three acylhydrazone-based compounds were designed as novel neutral sensors for anions, and synthesized
by simple steps in good yields. Their anion recognition properties were studied by UV-vis and 1 H-NMR
spectroscopy. The results showed that the receptors 1, 2, and 3 all had a better selectivity for F−
and CH3 COO− , but no evident binding with Cl− , Br− , I− , NO3− , H2 PO4− , or HSO−4
. The results
indicated that anion recognition was achieved via convergent hydrogen bond interactions from acylhydrazone
functionality on the side arms. The UV-vis data indicated that a 1:1 stoichiometry complex was formed
between compounds 1, 2, or 3 and anions. The binding and selectivity were also tuned by the change of the
place of the nitro group attached to the phenyl. Moreover, receptor 3 can act as the colorimetric sensor for
such anions as F− and CH3 COO− , and the recognition mechanism and binding mode were discussed.
Key Words: Anion recognition; molecular tweezer; acylhydrazone
Introduction
The selective recognition of biologically and/or chemically important anions by means of artificial molecular
receptors represents as an active field of research with applications spanning from analytical chemistry to
environmental science, toxicology, and biology.1 A variety of artificial receptors for anions have been reported,
including polyammonium macrocycles,2 guanidinium,3 amides,4 urea/thiourea,5 and calixarenes.6 However,
acylhydrazone-based receptors have not been reported. Furthermore, to encapsulate the particular guest more
efficiently, the calixarenes (and their derivatives) have been used as large macrocyclic organic building blocks, 7
on the other hand, molecular ‘tweezers’ for anion recognition has also received considerable attention in recent
∗Corresponding author
731

Convenient synthesis and anion recognition property of ..., H. X. REN, et al.,
times.8 Continuing our efforts to develop a highly selective anion receptor,9−11 here we report the synthesis,
characterization, and anion recognition nature of a series of acylhydrazone-based tweezer receptors for the
first time. The properties of the anion recognition have been investigated by UV-vis in DMSO and 1 H-NMR
spectroscopy in DMSO-d6 . The results show that receptor 3 can be used as anion sensors in the solution of
DMSO. Furthermore, the anion binding is accompanied by a visually striking color change, giving naked eye
anion sensing.
Experimental
Materials and methods
The substituent aryloxyacetyl hydrazones have been prepared using the published literature.12 DMSO was dried
and distilled before using according to standard practice. All other commercially available reagents were used
without further purification. The tetrabutylammonium salts were used as anionic substrates. Melting points
were measured on an X-4 digital melting-point apparatus (uncorrected). The infrared spectra were obtained
on a Digilab FTS-3000 FT-IR spectrophotometer. Elemental analyses were determined with a PE-2400 CHN
elemental autoanalyzer. 1 H-NMR spectra and 13 C-NMR spectra were recorded on a Varian Mercury plus-400
MHz spectrometer. UV-Vis spectra were obtained on a Lab Tech UV-2100 spectrometer. Mass spectra were
recorded on a HP-5988 spectrometer.
Synthesis of receptors
The synthesis of receptors was carried out by condensation of substituent aryloxyacetyl hydrazide with isoph-
thalic aldehyde using HCl as the catalyst. The reaction mixture was filtered. The solid obtained was washed
with ethanol 3 times to give a pure product.
Information of receptors
1: Yield: 70%; mp: 246-247 ◦ ˚C; 1 H-NMR( CD3 SOCD3 , ppm): 5.4 (4H, O-CH2), 4.868 (2H, N=C-H), 11.79
(2H, -NH), 7.1-8.8 (12H, Ar); IR (cm−1 , KBr): 3190.57 (N-H), 2977.49 (N=C-H), 1681.90 (C=O), 1608.17
(C=N); Anal. Calcd. for C24 H20 N6 O8 (%): C: 55.39, H: 3.87, N: 16.15, O: 24.59; Found: C: 55.4, H: 3.89, N:
16.13, O: 24.54; MS(FAB): m/z: 180, 131.
2: Yield: 72%; mp: 249-251 ◦ ˚C; 1 H-NMR (CD3 SOCD3 , ppm): 4.88-5.37 (4H, O-CH2), 3.358 (2H,
N=C-H), 11.75-11.78 (2H, -NH), 7.43-8.4 (12H, Ar); 13 C-NMR (CD3 SOCD3 , ppm): 168.05, 179.15, 163.73,
115.63-148.6, 65.3-66.7; IR (cm−1 , KBr): 3185.24 (N-H), 2974.24 (N=C-H), 1685.78 (C=O), 1527.55 (C=N);
Anal. Calcd. for C24 H20 N6 O8 (%): C: 55.39, H: 3.87, N: 16.15, O: 24.59; Found: C: 55.42, H: 3.87, N: 16.15,
O: 24.52; MS(FAB): m/z: 180, 131.
3: Yield: 77%; mp: 277-280 ◦ ˚C; 1 H-NMR (CD3 SOCD3 , ppm): 4.89-5.38 (4H, O-CH2), 3.372 (2H,
N=C-H), 11.76-11.8 (2H, -NH), 7.14-8.37 (12H, Ar); 13 C-NMR (CD3 SOCD3 , ppm): 168.2, 163.5, 115.14-
147.39, 65.4-66.6; IR (cm−1 , KBr): 3263.28 (N-H), 2926.73 (N=C-H), 1702.64 (C=O), 1593.12 (C=N); Anal.
732

Convenient synthesis and anion recognition property of ..., H. X. REN, et al.,
Calcd. for C24 H20 N6 O8 (%): C: 55.39, H: 3.87, N: 16.15, O: 24.59; Found: C: 55.38, H: 3.91, N: 16.15, O:
24.52; MS(FAB): m/z: 180, 131.
Results and discussion
The resulting 1 H-NMR data displayed peaks shifts of the amide NH toward downfield (1, 2, and 3 peaked at
11.79, 11.77, and 11.76 (ppm), respectively). This implies the formation of intramolecular hydrogen-bonding
interactions. The IR spectra of 1, 2, and 3 (1, 2, and 3 peaked at 3190.57, 3185.24, and 3263.28 (cm−1
)
respectively) also illustrated this note. The peaks downshift more than 100 wavenumbers. The UV-Vis spectra
show the characteristic absorption band of 1, 2, and 3 centered at 287.50 nm, 285.50 nm, and 292.00 nm,
respectively, which illustrated that a better conjugated system was formed in the receptor 3. The possible
structures of the receptors are shown in Figure 1.
CH
HC
N
N
N
N
O
H
C
O
C
H
O
CH2
H2C
O
R
R
2: R=m-NO2; 3: R=p-NO2
CH
N
HC
N
N
N
O
O C
H
C O
H
O
O
O
N
H2C
O
N
O
CH2
Receptor 1
Figure 1. The possible structures of the receptors.
733

Convenient synthesis and anion recognition property of ..., H. X. REN, et al.,
The sensing ability of the receptors 1, 2, and 3 with anions in dimethyl sulfoxide was monitored by using
UV-Vis absorption. The receptors exhibited selective recognition for CH3 COO− and F− over other anions,
such as Cl− , Br− , I− , HSO4− , and NO−3 in DMSO. Sensor 2 (2 × 10−5 mol/L) showed a broad absorption
band centered at 286 nm. Upon addition of F− anion (as their tetrabutylammonium salts), the band at 334 nm
increased in intensity at the expense of the 286 nm transition. Meanwhile, a clear isosbestic point was observed
at 313 nm. Upon addition of CH3 COO− anion, the band at 320 nm increased in intensity at the expense of the
286 nm transition. Meanwhile, a clear isosbestic point was observed at 308 nm. The change of 2 from colorless
to light yellow is observed in the presence of F− or CH3 COO− . Figure 2 showed the spectra of 2 obtained
during the titration with F− (a) and CH3 COO− (b). Absorbance values at 286 nm were taken for nonlinear
least-squares treatment to obtain the binding constants (Ka) of the host 2 with guests (F− , CH3 COO−). The
insets (Figure 2) represent the change in absorbance of 2 at 286 nm with varying molar equivalents of anions.
1.2
0.6
0.035
1.2
[G]/[H]
1
2
4
0.030
[G]/[H ]
1
3
7
1.0
0.4
obs
calc
0.025
1.0
obs
calc
6
0.020
15
27
42
72
0.2
8
0.8
0.8
0.6
0.4
0.2
0.0
11
14
17
20
24
28
32
38
50
58
68
83
0.015
0.010
0.005
0.0
0
1
2
3
-1
4
5
122
0.6
0.4
0.2
0.0
[F-]/mmol.L
0.5
1.0
1.5
2.0
2.5
-
-1
CH COO /mmol.L
3
300
350
Wavelength/nm
400
300
350
400
Wavelength/nm
Figure 2. The UV-Vis titration spectra of receptor 2 with F− (a) and CH3 COO− (b) in DMSO (298 K). [2]=2 ×
10−5 mol/L.
In addition, a more sensitive response of 2 toward F− in the titration spectra was also observed than
CH3 COO− from Figure 2, which may be due to different complexation states of the host 2 and guests.
Sensor 1 exhibited similar recognition for F− or CH3 COO− as Sensor 2. Sensor 3 (2 × 10−5 mol/L)
showed a broad absorption band centered at 292 nm. Upon addition of F− anion, the band at 355.5 nm
increased in intensity at the expense of the 292 nm transition. Meanwhile, a clear isosbestic point was observed
at 315 nm. The change of 3 from colorless to dark red is clearly evident to the naked eye. Upon addition of
CH3 COO− anion, the band at 323 nm increased in intensity at the expense of the 292 nm transition. Meanwhile,
a clear isosbestic point was observed at 316 nm. The change of 3 from colorless to dark yellow is clearly evident
to the naked eye.
We propose that these changes are consistent with the anions binding to the hydrazone moiety through
hydrogen bonding. These combined binding modes give rise to an enhanced ICT character with concomitant
color changes. The more obvious color change in receptor 3 than 1 and 2 in the presence of F− or CH3 COO−
734

Convenient synthesis and anion recognition property of ..., H. X. REN, et al.,
will be explained by the fact that only when the nitro-group was in the para position of the phenyl group was
internal charge transfer (ICT) the easiest.
Continuous variation methods were used to determine the stoichiometric ratios of the complexes formed
between the receptors and the anion guests. The total concentration of the host and the guest was kept constant
(4 × 10−5 mol/L) in DMSO, while the molar fraction of the guest {[G] / ([G] + [H])} was continuously varied.
Figure 3 shows the Job plot for 2 with F− in DMSO. When the molar fraction of the guest is 0.5, the absorption
reaches a maximum, demonstrating that receptor 2 formed a 1:1 complex with F− .
0.025
0.020
0.015
0.010
0.005
0.000
0.0
0.2
0.4
0.6
0.8
1.0
[G]/[G]+[H]
Figure 3. Job plot of 2 and F− at a total concentration of 4×10−5 mol/L. [G] and [H] represent the concentration of
the guest anion and the host, respectively.
Table. Binding constants (Ka) for the 1:1 complexes between these hosts (1-3) with guests (F− , CH3 COO−) in
DMSO at 298 K.
Host
Guesta
CH3COO−
F−
Ka (dm3 mol−1 b
)
R
7450
1930
0.991
0.9938
0.9918
0.9976
0.994
0.9926
1
CH3COO−
13700
1127
2
3
F−
CH3COO−
F−
18100
3080
a, Tetrabutylammonium salts were used as anion sources.
b, Binding constants were represented as the average of 2-3 runs. Experiments errors were estimated to be <10%.
Experimental results showed that F− was sensed effectively by receptors 1-3 with binding constants of
1930, 1127, and 3080 dm3 mol−1 , respectively; For CH3 COO− , higher Ka values are observed from Table
1. That illustrated that the receptors had a stronger affinity towards CH3 COO− than F− , which supported
735

Convenient synthesis and anion recognition property of ..., H. X. REN, et al.,
the notion that complementary structure played an important role in anion recognition. As an example, the
binding mode of 1 for the anions is illustrated in Scheme.
CH
N
HC
HC
N
N
N
CH
H
H N
N
N
anion
O
C
C
O
anion
H2C
O
CH2
O
N
O
O C
C O
H
H O
O
O
N
H2C O
N
O CH2
O2N
NO2
Scheme. The binding mode of the receptor 1 for the anions.
At the beginning of the titration with the tested anions, no obvious change was observed in the spectra
of the receptors, which was because at the beginning the anions were too scarce to compete with the solvent
molecular for the hydrogen-bonding sites.
It was immediately apparent from Table 1 that receptor 3 had a strong ability to complex with the tested
anions in comparison with 1 and 2, which was explained by the nature of hydrogen-bonding. The substituent of
NO2 attached to phenyl, as an electron-withdrawing and meta directing group, resulted in the order 2 < 1, 3 in
the case of the acidity of N-H of amide unit; in addition, considering that 1 has a greater steric hindrance caused
by intramolecular hydrogen bonding (Figure 1) particularly for the bigger CH3 COO− , it was not surprising
that 3 took on the strongest hydrogen-bond ability for the tested anions. Selectivity depends both on energy
terms (related to the intensity of the receptor–substrate interaction) and on geometrical factors (size and shape
matching between receptor and substrate). For F− , it has a small volume, and the former play an important
role; while for the bigger CH3 COO− anion, the latter is more important. The fact resulted in the order 2 <
1 < 3 in the case of binding ability of the receptors for F− , and the order 1 < 2 < 3 for CH3 COO−
.
Introduction of protic solvents such as methanol into the dimethyl sulfoxide solution of 2 plus F− led to
a gradual recovery of the absorption spectrum of 2 itself, which supported the hydrogen-bonding nature of the
anion-receptor interaction, likely at the amide NH sites.
To confirm this assumption, 1 H-NMR titration in DMSO-d6 was carried out. Addition of equivalent
tetrabutylammonium fluoride to the DMSO-d6 solution of 3 resulted in a disappearance of the N-H resonance.
A similar observation was reported earlier and was presumably due to strong hydrogen bonding with the fluoride
ion.
736

Convenient synthesis and anion recognition property of ..., H. X. REN, et al.,
14
12
11
10
9
8
7
6
a) receptor 3 in the presence of F− ; b) receptor 3.
Figure 4. 1 H-NMR spectra of receptor 3 in DMSO-d6.
Conclusions
In summary, we reported 3 receptors that were synthesized by a simple method. Their properties of anion
recognition were studied by UV-vis spectroscopy. The results showed that receptors 1, 2, and 3 can form 1:1
complex with anions such as F− , CH3 COO− by multiple hydrogen bonding interactions, and the 3 receptors
had a strong binding to F− , CH3 COO− . The results described in this paper lead us to suggest a new approach
to a receptor for selective recognition of acetate anion in polar solvent. Most notably, the selectivity of receptors
to anions could be efficiently tuned by changing the place of the chlorine substituent group at the N-phenyl
moiety. Of course, other cooperative or allosteric systems could be developed by further modifying these
receptors. Extensive efforts are being directed toward this end.
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4. Saravanakumar, D.; Sengottuvelan, N. Tetrahedron Lett. 2005, 46, 7255-7258.
5. Jose, D. A.; Kumar, D. K.; Ganguly, B. Org. Lett. 2004, 6, 3445-3448.
6. Pichierri, F. J. Mole. Struc. 2002, 581, 117-127.
7. Troisi, F.; Russo, A. Tetrahedron Lett. 2007, 48(45), 7986-7989.
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