Synthesis and anion recognition studies of a dipodal thiourea-based sensor for anions

Article abstract of DOI:10.1016/j.tetlet.2013.11.050

A new dipodal thiourea-based sensor (L) has been synthesized and its binding affinity toward a variety of anions has been investigated in solution and solid state. The colorimetric studies show a significant color change of L in the presence of fluoride, dihydrogen phosphate, and acetate in DMSO. The data obtained from UV-vis titrations is consistent with the results of the colorimetric observations. As investigated by ¹H NMR titrations, the sensor binds an anion in a 1:1 binding mode showing the binding trend in the order of F^- > Cl^- > Br^- > I ^- for halides and H₂PO4- > CH₃COO ^-, SO42- > HSO4- > NO3-, ClO4- for oxoanions. The data demonstrates that the binding is primarily influenced by the basicity of anions.

Full text of DOI:10.1016/j.tetlet.2013.11.050

Tetrahedron Letters 55 (2014) 438–440
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and anion recognition studies of a dipodal thiourea-based
sensor for anions
⇑
Maryam Emami Khansari, Kendrick D. Wallace, Md. Alamgir Hossain
Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A new dipodal thiourea-based sensor (L) has been synthesized and its binding affinity toward a variety of
anions has been investigated in solution and solid state. The colorimetric studies show a significant color
change of L in the presence of fluoride, dihydrogen phosphate, and acetate in DMSO. The data obtained
from UV–vis titrations is consistent with the results of the colorimetric observations. As investigated by
1H NMR titrations, the sensor binds an anion in a 1:1 binding mode showing the binding trend in the
order of Fꢀ > Clꢀ > Brꢀ > Iꢀ for halides and H2POꢀ > CH3COOꢀ, SO2ꢀ > HSOꢀ4 > NO3ꢀ, ClOꢀ4 for oxoanions.
Received 11 October 2013
Revised 11 November 2013
Accepted 13 November 2013
Available online 21 November 2013
4
4
Keywords:
Anion sensor
Thiourea
The data demonstrates that the binding is primarily influenced by the basicity of anions.
Ó 2013 Elsevier Ltd. All rights reserved.
Anion complex
Molecular recognition
Acyclic receptor
Because of the significant roles played by common anions in
both environmental and biological systems,1 anion recognition by
artificial receptors is a current area of research in supramolecular
chemistry.2,3 Although polyamine-based receptors have been
extensively explored for binding of anions, their binding ability is
primarily depended on solution pH.4 On the other hand amides,5–7
thioamides,8,9 ureas,10,11 and thioureas12 are neutral receptors
providing H-bond donors, which bind anions by hydrogen bonding
interactions regardless of solution pH. In particular, a molecule
with a thiourea functionality can provide two H-bond donors for
an anion. Additionally, electron withdrawing nature of the sulfur
atom on thiourea group can make the ligand more efficient to
interact with an anionic guest.13 In 1999, Wu and coworker re-
ported the synthesis and anion binding studies of three chromo-
genic naphthyl thiourea-based receptors including one, two, and
three thiourea moieties. The results showed that the tripodal
develop neutral receptors,11,17–20 we synthesized a simple dipodal
thiourea receptor (L), which shows strong selectivity for fluoride
and dihydrogen phosphate in DMSO, displaying a visible color
change for each.
The receptor L was synthesized by reacting 2,20-diamino-N-
methyldiethylamine and 4-nitrophenylisothiocyanate in CH2Cl2
(Scheme 1). The choice of selecting the nitrophenyl group as a sub-
stituent was due to its electron withdrawing character as well as
its optical properties,21 making the receptor potential as an effec-
tive sensor for anions.
Solution binding studies of L for anions were performed with 1H
NMR titrations using [n-Bu4N]+Aꢀ (Aꢀ = Fꢀ, Clꢀ, Brꢀ, Iꢀ, ClO4ꢀ, NOꢀ3 ,
HSOꢀ, H2POꢀ, CH3COOꢀ), and ZnSO4 in DMSO-d6. In the 1H NMR
4
4
spectra of free L, one NH (H1) resonance is observed at10.28 ppm
and the other one (H2) is merged with the aromatic CH peak at
8.18 ppm. The addition of n-Bu4NF to L resulted in a significant
receptor was able to complex H2POꢀ and HSO4ꢀ anions in DMF.12
downfield shift for both NH peaks (
Dd = 3.43 ppm for NH1 and
4
A series of fluorinated tripodal receptors were reported by Gale
and co-workers, which were able to transport chloride anions
through a lipid bilayer.14 Das and coworkers reported a tris
(4-nitrophenyl)thiourea receptor which was shown to form a 1:1
complex with fluoride.15 This receptor was also found to encapsu-
late the phosphate anion within a dimeric capsular assembly of the
ligand. A simple dipodal thiourea-based molecule containing a
mesitylene group and a p-nitrobenzene reported recently by
Hundal and coworkers has also been proven as an effective
receptor for a variety of anions.16 In our continuing interests to
Dd = 1.10 ppm for NH2 upon the addition of 1.5 equiv of n-Bu4NF
to L), indicating strong interactions between the ligand and the
anion. A similar change, but to a lesser extent, was observed for
NO2
S
NH2
NH2
N
H
N
H
(i), ((ii)
H3C
N
NO2
SCN
H3C
N
H
N
H
N
S
NO2
L
⇑
Corresponding author. Tel.: +1 601 979 3748; fax: +1 601 979 3674.
E-mail address: alamgir.hossain@jsums.edu (M.A. Hossain).
Scheme 1. Synthesis of L. Reagents and conditions: (i) CH2Cl2; (ii) reflux, 75 °C, 6 h.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.11.050

M. Emami Khansari et al. / Tetrahedron Letters 55 (2014) 438–440
439
Clꢀ and Brꢀ anions. While in case of Iꢀ, there was a little change in
Table 1
a
the chemical shifts. Figure 1 shows a typical stacking of 1H NMR
titration spectra for chloride in DMSO-d6. The change in the chem-
ical shift of NH resonances of NMR spectra, as recorded with the
increasing amount of anion solution at room temperature, gave
the best fit for a 1:1 binding model (Fig. 2).22 The experimental
data obtained from the titration methods for all anions is summa-
rized in Table 1. For halide series, the binding trend is found to be
in the order of Fꢀ > Clꢀ > Brꢀ > Iꢀ. In case of oxoanions, the highest
binding was achieved by H2POꢀ, followed by SO2ꢀ, CH3COOꢀ and
Binding constants (LogK) of L for anions in DMSO-d6
Anion
Log K
Anion
Log K
Fluoride
Chloride
Bromide
Iodide
3.69
2.66
2.26
<1
Bisulfate
Sulfate
Phosphate
Acetate
2.38
3.10
3.79
3.08
<1
Nitrate
<1
Perchlorate
a
The binding constants were determined form a 1:1 binding model (Error limit is
less than 20%).
4
4
HSOꢀ4 . However, no significant shift was observed after the addi-
tion of ClOꢀ4 and NOꢀ3 ions. These results indicate that the binding
is mainly influenced by the relative basicity of the anions.23
The interactions of L with anions were also investigated in
DMSO ꢀby UV–vis titrations using [n-Bu4N]+Aꢀ (Aꢀ = Fꢀ, Clꢀ, Brꢀ,
Iꢀ, ClO4 , NOꢀ3 , HSO4ꢀ, H2PO4ꢀ, CH3COOꢀ), and ZnSO4. As shown in
Figure 3A, an increasing amount of H2POꢀ to the ligand solution,
4
led to a gradual bathochromic shift of the peak at 358 nm. This
shift change suggests the formation of phosphate complex with
L. The binding constant (K) was calculated from the titration plot
using a non-linear regression analysis,17 which gave the best fit
for a 1:1 binding mode (Supplementary data). We also observed
the same trend in the UV spectra for Fꢀ and CH3COOꢀ, while the
spectral shifts for SO42ꢀ were much weaker than Fꢀ, H2POꢀ, or
4
CH3COOꢀ. In contrast, the addition of other anions did not show
any appreciable change in kmax or intensity. Interestingly, in the
Figure 3. Changes in absorption spectra of L (2 ꢁ 10ꢀ5 M) with an increasing
amount of (A) H2POꢀ (2 ꢁ 10ꢀ3 M); (B) Fꢀ (2 ꢁ 10ꢀ3 M) in DMSO.
4
4
Figure 1. Partial 1H NMR titration of L showing changes in the NH chemical shifts
of the receptor (2 mM) with an increasing amount of Clꢀ (20 mM) in DMSO-d6.
(H1 = CSNHAr and H2 = CH2NHCS).
presence of Fꢀ, a new band appeared at 475 nm, that gradually in-
creased with the incremental addition of the anion (Fig. 3), which
could be an effect of deprotonation of NH by the strong basic Fꢀ. A
similar phenomenon was previously reported by Fabbrizzi et al. for
a urea-based receptor with fluoride ion.23
In order to justify the results of UV–vis studies, the host mole-
cule was further investigated for naked eye colorimetric studies
with anions in DMSO. As shown in Figure 4, a distinct color change
(from bright yellow to red) was observed after the addition of fluo-
ride. In the presence of acetate, the color changed to deep orange,
whereas it was orange with dihydrogen phosphate.
In summary, we report a simple dipodal thiourea colorimetric
sensor, showing distinct color change for fluoride, dihydrogen
phosphate, and acetate anions. The results from UV–vis titrations
are consistent with the colorimetric results, showing a red shift
for the peak at 358 nm upon the addition of respective anions. As
evaluated from 1H NMR titrations, the new ligand shows signifi-
cant selectivity for fluoride and dihydrogen phosphate anions. Fur-
ther studies on this and related anion sensors are underway.
Figure 2. 1H NMR titration plot of L (2 mM) with an increasing amount of Clꢀ
(20 mM) in DMSO-d6. (H1 = CSNHAr and H2 = CH2NHCS).

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M. Emami Khansari et al. / Tetrahedron Letters 55 (2014) 438–440
References and notes
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Figure 4. Colorimetric studies of receptor (L) with halides and oxoanions.
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Acknowledgments
The National Science Foundation is acknowledged for a CAREER
award (CHE-1056927) to M.A.H. Kendrick Wallace a high school
student from Madison Central High School (Mississippi) who was
involved in this project as a summer participant in 2013 at Jackson
State University supported by NSF CAREER (CHE-1056927). The
500 MHz NMR instrument used for this work was funded by the
National Science Foundation (CHE-0821357).
17. Russ, T. H.; Pramanik, A.; Emami Khansari, M.; Wong, B. M.; Hossain, M. A. Nat.
Prod. Commun. 2012, 7, 301.
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Chem. Commun. 2008, 1762.
Supplementary data
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Supplementary data (experimental procedure for the synthesis
of L, experimental procedure for titrations, and titrations data)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2013.11.052.
23. Dey, K. R.; Wong, B. M.; Hossain, M. A. Tetrahedron Lett. 2010, 51, 1329.