Synthesis and anion recognition of molecular tweezers receptors based on acyl-thiourea

Article abstract of DOI:10.1080/10426500701613154

Three title compounds were designed and synthesized in high yields as novel and simple anion receptors. They had a higher selectivity for F- over AcO-. The binding properties for anions of the receptors were examined by UV-Vis and 1H NMR spectroscopy. The spectrographic data indicate that a 1:1 stoichiometry complex is formed between the receptors and the anions through hydrogen bonding interactions in DMSO solution. In addition, these molecular tweezers receptors have more binding points of anions, larger association constant, and better recognition ability. They can be used as better recognition receptors for F- and CH3COO-. Copyright Taylor & Francis Group, LLC.

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Phosphorus, Sulfur, and Silicon
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Synthesis and Anion
Recognition of Molecular
Tweezers Receptors Based on
Acyl-Thiourea
Tai-Bao Wei a , Wei Wei a , Cheng Cao a & You-Ming
Zhang a
a College of Chemistry and Chemical Engineering,
Gansu Key Laboratory of Polymer Materials ,
Northwest Normal University , Lanzhou, Gansu, P. R.
China
Published online: 09 May 2008.
To cite this article: Tai-Bao Wei , Wei Wei , Cheng Cao & You-Ming Zhang (2008)
Synthesis and Anion Recognition of Molecular Tweezers Receptors Based on Acyl-
Thiourea, Phosphorus, Sulfur, and Silicon and the Related Elements, 183:5, 1218-1228,
DOI: 10.1080/10426500701613154
To link to this article: http://dx.doi.org/10.1080/10426500701613154
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Phosphorus, Sulfur, and Silicon, 183:1218–1228, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701613154
Synthesis and Anion Recognition of Molecular Tweezers
Receptors Based on Acyl-Thiourea
Tai-Bao Wei, Wei Wei, Cheng Cao, and You-Ming Zhang
College of Chemistry and Chemical Engineering, Gansu Key Laboratory
of Polymer Materials, Northwest Normal University, Lanzhou, Gansu,
P. R. China
Three title compounds were designed and synthesized in high yields as novel
and simple anion receptors. They had a higher selectivity for F− over AcO−.
The binding properties for anions of the receptors were examined by UV-Vis and
1
H NMR spectroscopy. The spectrographic data indicate that a 1:1 stoichiometry
complex is formed between the receptors and the anions through hydrogen bond-
ing interactions in DMSO solution. In addition, these molecular tweezers recep-
tors have more binding points of anions, larger association constant, and better
recognition ability. They can be used as better recognition receptors for F− and
CH COO−.
3
Keywords Anion recognition; Acyl-thiourea; Hydrogen bonds; Molecular tweezers
INTRODUCTION
The sensing and recognition of simple-structure anion receptors have
attracted much attention1–2 because of the important roles of an-
ions not only in biological processes, but also in chemical processes,
the possibilities of applying anion receptors in sensors and molecu-
lar self-assembly, and even medical or environmental problems.3–11
Although various receptors containing different functional groups
for selective binding of F− and AcO− have been reported,2–16 the
anion recognition of the simple-structure molecular tweezers re-
ceptors are very few.1 In recent years, acyl thiourea-based molec-
ular tweezers compounds has received considerable attention in
Received 11 June 2007; accepted 17 July 2007.
We gratefully acknowledge the support of the National Natural Science Foundation of
China (No. 20671077), and the Natural Science Foundation of Gansu province (3YS051-
A25-01, 3ZS061-A25-027).
Address correspondence to You-Ming Zhang, College of Chemistry and Chemical En-
gineering, Northwest Normal University, Lanzhou, 730070, Gansu, P.R. China. E-mail:
zhangnwnu@126.com
1218

Molecular Tweezers Receptors Based on Acyl-Thiourea
1219
metallamacrocyclic complexes17,18 because the multiple hydrogen bind-
ing interactions of host can induce high recognition selectivity for
guest. To the best of our knowledge, however, the anion recog-
nition and sensing of acyl thiourea-based molecular tweezers in-
volving multiple hydrogen-bonding sites has seldom been found in
literature.
As far as we know, for construction of anion receptors, it would be
crucial to design spacer molecules bearing anion-binding sites. It is well
known that binding strength and anion selectivity depend on structures
of the spacers, acidity of H-donors, solvents, shape and size of anions.19
Moreover, receptors bearing two thiourea groups at suitable sites can
bind anions through hydrogen bonding interactions and enhance the
stability of the resultant complex and recognition selectivity.20,21 In our
previous work, we have had great interest in the research of crown ether
and thiosemicarbazide derivatives for anions.22–28 In view of these
facts, and as a part of our work on anion recognition of supramolecular
compounds and thiourea-based molecular tweezers receptors, herein we
report the synthesis and anion recognition properties of three novel sim-
ple and effective molecular tweezers receptors based on acyl-thiourea.
The synthetic route of receptors is illustrated in Scheme 1. Isophtha-
lyl chloride (1) was prepared from the reaction of isophthalic acid with
thionyl chloride, and it was treated with ammonium thiocyanate un-
der the conditions of solid-liquid phase transfer catalysis using 3%
polyethylene glycol-400 (PEG-400) as the catalyst to give isophthalyl
isothiocyanate (2). This compound does not need to be isolated and it
was treated immediately with substituted aromatic amines to obtain
title compounds (3) in good to excellent yields. The recognition proper-
1
ties of receptors have been examined by UV-VisVis absorption and H
NMR spectroscopy.
SCHEME 1 3a: R = CH3CO; 3b: R = H; 3c: R = CH3CONH.

1220
T.-B. Wei et al.
RESULTS AND DISCUSSION
UV-Vis Absorption Spectra
UV-Vis spectrometry is associated with the variations of absorption
spectra in coordinate systems, and it has been used as an effective and
simple measuring method of the association constants of complexes in
supramolecular systems.
The UV-Vis absorption spectra of receptors 3a, 3b, and 3c (2 ×
10−5mol/L, DMSO) in the presence of various anions (50 times with
respect to receptors) showed that the three receptors have the recogni-
tion selectivity of F− and AcO−, but have not the obvious interactions
with other anions such as Cl−, Br−, I−, HSO4−, and NO−3 . In each case
the counter cation was tetrabutylammonium. As shown in Figure 1,
for example, receptor 3a has the high recognition selectivity of F− and
AcO−, but has not the obvious interactions of other anions.
Titration experiments for anion recognition of receptors were per-
formed by UV-Vis spectroscopy in DMSO solution. The UV-Vis absorp-
tion spectra were obtained from a solution of these receptors in the
absence and presence of F− and AcO−. In each case the counter cation
was tetrabutylammonium.
As shown in Figure 2a, the UV-Vis absorption spectra of solution of
receptor 3a (2 × 10−5mol/L) in the course of the titration with different
concentrations of anions in DMSO have been observed. During titra-
tion with fluoride ions, the intensity of absorption peak, which is from
FIGURE 1 UV-Vis absorption spectra of 3a (2 × 10−5mol/L, DMSO) in the
presence of various anions (50 times with respect to receptor 3a), which showed
that receptor 3a, had the high recognition selectivity of F− and AcO−.

Molecular Tweezers Receptors Based on Acyl-Thiourea
1221
FIGURE 2 (a) The UV-Vis absorption spectra of 3a (2 × 10−5mol/L, DMSO)
with gradual increasing of the concentration of fluoride ions; (b) The UV-Vis
absorption spectra of 3a (2 × 10−5mol/L, DMSO) with gradual increasing of the
concentration of acetate ions
the benzene moiety, decreased at 278 nm, and a new absorption peak
was observed at 337 nm. An obvious isobestic point appeared at 302
nm. Figure 2b shows the absorption spectra of receptor 3a with adding
acetate ions, the peak at 278 nm decreased while the new peak located
at 337 nm. Isobestic point appeared at 301 nm. The formation of the
complexes between receptor 3a and fluoride, acetate ions was the best
reason for inducing these variations.
Similar to receptor 3a, the absorption spectra of solution of recep-
tor 3b (2×10−5mol/L) with different concentrations of anions in DMSO

1222
T.-B. Wei et al.
have also obtained. The absorption spectra of receptor 3b with adding
fluoride ions showed the maximal absorption peak decreased at 267nm,
the new peak appeared at 321 nm, and the discerned isobestic point was
observed at 283 nm. Similarly, the absorption spectra of receptor 3b
(2 × 10−5mol/L) with acetate ions showed the maximal absorption peak
decreased at 266 nm, the new peak appeared at 322 nm, and isobestic
point was observed at 284 nm. Therefore, receptor 3b also has the se-
lective binding ability for fluoride, acetate ions.
Similarly, the variations of the original and new absorption peaks of
solution of receptor 3c with gradual increasing of the concentration of
fluoride ions have also obtained. The maximal peak at 275 nm decreased
while the new peak located at 325 nm. Obvious isobestic point at 296
nm was observed. Meanwhile, the variations of absorption spectra of
receptor 3c with acetate ions are also observed. The maximal peak at
276 nm decreased while the new peak located at 324 nm. Isobestic point
appeared at 291 nm. These observations clearly showed that receptor
3c has the selective binding ability for fluoride, acetate ions.
The association constants and correlation coefficients of receptors
3a, 3b, and 3c with fluoride, acetate ions.
The degree of binding ability, as further illustrated by non-linear
fitting curve,29 can be seen from Table I. Receptors 3a, 3b, and 3c
bind fluoride ions preferentially, acetate ion was the other anion bound
stably. The observed selectivity for fluoride ions can be rationally ex-
plained based on the anion structures. Because the spherical fluo-
ride ion was bound with receptor more easily, however, acetate ion
has two oxygen atoms, it may be bound in a different complex struc-
ture, and fluoride ion is bound stronger than acetate ion, as shown in
Table I.
Table I also showed that the orders of the association constants Ks
of receptors with fluoride and acetate ions both are 3a>3b>3c. These
TABLE I The Association Constants Ks (mol/L)−1 and Correlation
Coefficients R of Receptors 3a, 3b, and 3c with Fluoride and Acetate
Ions in DMSO, as Further Illustrated by Non-Linear Fitting Curve
Anions
3a
3b
3c
F−
Ks
R
1.7 × 106
7.2 × 104
6.2 × 104
0.9971
0.9989
0.9911
AcO−
Ks
R
7.6 × 104
1.6 × 104
1.4 × 104
0.9972
0.9955
0.9994

Molecular Tweezers Receptors Based on Acyl-Thiourea
1223
FIGURE 3 The plot of the absorbance of receptor 3a vs. concentration of ac-
etate ions in DMSO. ([3a] = 2 × 10−5mol/L, λ =336 nm, the correlation coeffi-
cient R is 0.9972).
results are likely to be explained that the electron-deficient moieties
(p-CH3CO) can extend the conjugated system and enhance the acidity
of –NH protons. In contrast, the electron-rich moieties (p-CH3CONH2)
can decrease the acidity of NH protons of thiourea groups.
FIGURE 4 1H NMR spectra of receptor 3c and the presence of F− in DMSO-d6.
(a) Receptor 3c; (b) Receptor 3c in the presence of F−.

1224
T.-B. Wei et al.
Meanwhile, the plot of the absorbance of 3a vs. concentration of ac-
etate ions had been obtained, as shown in Figure 3. The plot of 3a vs.
concentration of fluoride ions had also been obtained from the same
step. Similarly, the plots of 3b and 3c vs. concentration of fluoride and
acetate ions had also been obtained in the same conditions.
1H NMR Study
1H NMR spectroscopy plays an important role in the understanding
of receptor-anion interaction and is capable of providing a revealing
picture of the details of the interactions between receptors and an-
ions. Therefore it can give convincing evidence for the existence of in-
tramolecular hydrogen bonds between the NH of thiourea group and
anions. As shown in Figure 4, for example, the binding ability of the re-
ceptor 3c for fluoride ions is evident in 1H NMR titration experiments
in DMSO-d6. Upon addition of 1 mol equiv. of tetrabutylammonium flu-
oride to receptor 3c in DMSO-d6 caused the complete disappearance of
the signals for thiourea NH protons. Figure 4a reported that a partial 1H
NMR spectrum of receptor 3c. Figure 4b showed that upon addition of
SCHEME 2 Possible complexes structures of receptor 3 with fluoride and ac-
etate ions.

Molecular Tweezers Receptors Based on Acyl-Thiourea
1225
1 mol equiv. of tetrabutylammonium fluoride, the signals for the amide
NH protons at 11.30 ppm (S, 2H) and 12.41 ppm (S, 2H) of receptor 3c at
80◦C decreased greatly, and completely disappeared, however, signals
for the amide NH protons at 9.82 ppm (S, 1H) were still unchanged.
These results indicated that receptor 3c and fluoride ions form 1:1
stoichiometry complex by hydrogen-bonding interaction involving the
thiourea NH groups, but the amide NH protons of substitute phenyls
have seldom been bound with fluoride ions. The similar results of the
signals for receptor 3c upon addition of 1 mol equiv. of acetate ions can
also be observed. Similarly, the variations of the signals for the thiourea
NH protons of receptors 3a and 3b upon addition of fluoride and acetate
ions can be obtained in the same conditions. However, such similar re-
sults of signals for receptors 3a, 3b, and 3c were not observed upon ad-
dition of other anions such as Cl−, Br−, I−, HSO4− and NO−3 in the same
conditions. Due to the above facts, possible structures for the complexes
of receptor 3 with fluoride and acetate ions are illustrated in Scheme 2.
More detailed studies of the hydrogen bonds between thiourea groups
of receptors and other anions are under investigation.
CONCLUSION
The novel simple and effective receptors 3a, 3b, and 3c were synthe-
sized in good to excellent yields. The UV-Vis absorption spectra and 1H
NMR spectra show that receptors 3a, 3b, and 3c can form 1:1 stoichiom-
etry complexes with F− and AcO− by multiple hydrogen-bonding inter-
actions. Receptor 3a has an excellent ability to recognition with fluoride
and acetate ions in comparison with receptors 3b and 3c, and these
recognition properties of receptor 3a indicates that receptor 3a can be
considered as an excellent recognition receptor with F− and AcO− be-
cause of its larger conjugated system. These molecular tweezers recep-
tors have more binding points of anions, larger association constants,
and better recognition ability. They can be used as better recognition
receptors for F− and CH3COO−.
EXPERIMENTAL
Measurements and Reagents
UV-Vis spectra were obtained on an Agilent-8453 spectrometer. Ele-
mental analyses were performed by a PE-2400 CHN instrument. IR
spectra were recorded on a Digilab FTS-3000 FT-IR spectrophotometer
(using KBr pellets). 1H NMR and 13C NMR spectra were measured on
a Varian Mercury plus-400 MHz spectrometer; DMSO-d6 was used as

1226
T.-B. Wei et al.
solvent and TMS as internal standard. Melting points were determined
with an X-4 digital melting-point apparatus, which was uncorrected.
Tetrabutylammonium fluoride was commercial product of highest
reagent grade and used as received. Tetrabutylammonium acetate was
obtained by the neutral reaction of 25% tetrabutylammonium hydrox-
ide and acetic acid, and it dried in P2O5 drier. DMSO and acetone were
analytical pure.
General Procedure for the Synthesis of Receptors
1,3-isophthalyl chloride (1.1 g, 5 mmol), NH4SCN (1.2 g, 15 mmol), PEG-
400 (0.18 g, 3% with respect to ammonium thiocyanate) and 20 mL of
acetone were added in a dried round-bottomed flask containing a mag-
netic stirrer bar and stirred at room temperature for 2 h to give yellow
1,3-isophthalyl isothiocyanate, then, p-acetyl aniline (1.35 g, 10 mmol)
was added dropwise. The mixture was stirred at room temperature for 1
h; the precipitate was filtered, washed with acetone to remove residual
isothiocyanate, with water to remove inorganic salts, then dried and
crystallized from DMF-C2H5OH-H2O to give receptor 3a as a white
powder.
3a
Receptor 3a was obtained in 92.6% yields (2.36 g). m.p. 250–251◦C.
IR (KBr): 3417, 3186, 3034 (N-H); 1668 (C=O); 1595, 1527 (C=C);
1269, 1155 (C=S) cm−1. H NMR (DMSO-d6): δ 12.62 (s, 2H, NHCO),
1
11.58(s, 2H, NHAr), 7.26–8.61(m, 12H, ArH), 2.55(s, 6H, CH3). 13C NMR
(DMSO-d6): δ 179.09, 167.21, 159.11, 135.82, 132.85, 128.56, 123.58. El-
emental analysis: calcd. for C26H22N4O4S2: C, 60.23; H, 4.25; N, 10.81%.
Found: C, 60.25; H, 4.28; N, 10.78%.
According to the same synthesis method as before, aniline (0.93 g,
10 mmol) was added dropwise into 1,3-isophthalyl isothiocyanate, and
used the above method to give receptor 3b as a yellow powder.
3b
Receptor 3b was obtained in 89.3% yields (1.94 g). m.p. 197–198◦C.
IR (KBr): 3298, 3223, 3173, 3033 (N-H); 1700 (C=O); 1599, 1526(C=C);
1348, 1246, 1145 (C=S) cm−1
.
1H NMR (DMSO-d6): δ 12.56 (s, 2H,
NHCO), 11.60(s, 2H, NHAr), 7.28–8.58 (m, 14H, ArH). 13C NMR
(DMSO-d6): δ 178.98, 167.29, 137.92, 133.28, 131.96, 129.14, 128.77,
126.53, 124.51. Elemental analysis: calcd. for C22H18N4O2S2: C, 60.83;
H, 4.15; N, 12.90%. Found: C, 60.86; H, 4.18; N, 12.86%.

Molecular Tweezers Receptors Based on Acyl-Thiourea
1227
Similarly, p-acetyl amido aniline (1.5 g, 10 mmol) was added dropwise
into 1,3-isophthalyl isothiocyanate, and used the above method to give
receptor 3c as a yellow powder.
3c
Receptor 3c was obtained in 94.3% yields (2.59 g). m.p. >300◦C. IR
(KBr): 3401, 3143 (N-H); 1663 (C=C); 1400, 1325, 1255, 1159 (C=S)
cm−1.1H NMR (DMSO-d6): δ 12.41 (s, 2H, NHCO), 11.30 (s, 2H, NHAr),
9.82 (s, 1H, NHAr), 7.55–8.56 (m, 18H, ArH). 13C NMR (DMSO-d6): δ
178.42, 167.90, 166.80, 137.26, 132.59, 131.87, 128.11, 124.24, 118.95.
Elemental analysis: calcd. for C26H24N6O4S2: C, 56.93; H, 4.38; N,
15.33%. Found: C, 56.97; H, 4.41; N, 15.29%.
General Procedure for UV-Vis Spectrometry of Receptors and
Anions
The DMSO solution of receptor 3a (2 × 10−4mol/L, 1mL) was added in
a dry 10 mL colorimetric tubes, the DMSO solution of tetrabutylammo-
nium fluoride (mol/L, 1mL) was added in it, and diluted to 10 mL in
DMSO. Similarly, the DMSO solutions of receptor 3a on respective ad-
dition of tetrabutylammonium chloride (mol/L, 1mL), bromide (mol/L,
1mL), iodide (mol/L, 1mL), acetate (mol/L, 1mL), hydrosulfate (mol/L,
1mL) and nitrate (mol/L, 1mL) were obtained, and then detected im-
mediately the UV-Vis spectra of this series of solutions at room temper-
ature. (In this case, the concentration of receptor 3a in DMSO, which is
50 times to the concentration of anions, is 2 × 10−5mol/L.) According to
the above method as before, the spectrometry experiment of receptor 3b
and 3c with anions can be carried out according to the above method.
General Procedure for UV-Vis Spectrometry Titration of
Receptors with Fluoride and Acetate Ions
The DMSO solution of receptor 3a (2 × 10−5mol/L, 2mL) was added
in a dry colorimetric tube, the DMSO solution of tetrabutylammonium
fluoride (1.0 mol/L) in different volumes were respectively added in it,
then detected immediately the UV-Vis spectra of this series of solutions
at room temperature. (In this case, the concentration of receptor 3a in
DMSO invariably is 2 × 10−5mol/L, the concentration of fluoride ions
is changeable.) Similarly, the UV-Vis spectrometry titration of receptor
3a and acetate ions was carried out. According to the above method as
before, the titration experiment of receptor 3b and 3c with fluoride ions
can be carried out according to the above method.

1228
T.-B. Wei et al.
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