A naked-eye detection of fluoride with urea/thiourea receptors which have both a benzophenone group and a nitrophenyl group as a signalling group

Article abstract of DOI:10.1080/10610270903449973

Two new colorimetric anion sensors have been synthesised where both a benzophenone group and a nitrophenyl group were used as signalling units and urea/thiourea moieties as binding sites. The receptors, effectively and selectively, recognised fluoride and carboxylate anions from other anions such as chloride, bromide, iodide, perchlorate, hydrogen sulphate and nitrate in DMSO.

Full text of DOI:10.1080/10610270903449973

Supramolecular Chemistry
Vol. 22, No. 4, April 2010, 267–273
A naked-eye detection of fluoride with urea/thiourea receptors which have both a benzophenone
group and a nitrophenyl group as a signalling group
Jongmin Kanga*, Yu Jin Leeb, Sung kyu Leea, Ju Hoon Leeb, Jin Joo Parka, Youngmee Kimc,
Sung-Jin Kimc and Cheal Kimb*
b
aDepartment of Chemistry, Sejong University, Seoul 143-747, South Korea; Department of Fine Chemistry and Eco-Product and
Materials Education Center, Seoul National University of Technology, Seoul 139-743, South Korea; Department of Chemistry and
Nano Science, Ewha Woman’s University, Seoul 120-750, South Korea
c
(Received 4 September 2009; final version received 29 October 2009)
Two new colorimetric anion sensors have been synthesised where both a benzophenone group and a nitrophenyl group were
used as signalling units and urea/thiourea moieties as binding sites. The receptors, effectively and selectively, recognised
fluoride and carboxylate anions from other anions such as chloride, bromide, iodide, perchlorate, hydrogen sulphate and
nitrate in DMSO.
Keywords: anion receptor; urea/thiourea; colorimetric receptor; anion recognition
Introduction
Experimental
Ureas and thioureas participate in bifurcated H-bond
interactions and have been used as binding fragments in the
design ofneutralreceptorsforanions (1). Especially, ureaor
thiourea derivativesconnected with a series of chromogenic
and fluorogenic substituents are proved to be very efficient
for the anion sensors (2–19). The interaction with an anion
typically stabilises the excited state of chromophore and
induces redshiftof the chargetransferabsorption band, thus
providing an efficient way for qualitative and quantitative
evaluation of anion activity in solution (20, 21). They can be
often easily synthesised from commercially available
reagents even by a single-step procedure (22–24). We
have also reported on novel colorimetric receptors
containing a nitrophenyl group as chromogenic signalling
subunit and urea/thiourea as binding sites, which were
selective for fluoride or acetate ion (25, 26). The anion
recognition via hydrogen-bonding interactions could be
easily monitored by anion-complexation-induced changes
in UV–vis absorption spectra and with the naked eye.
As a part of our efforts to develop more efficient anion
receptors, we planned to design new urea/thiourea
receptors with both a benzophenone group and a
nitrophenyl group as a chromogenic signalling subunit.
We report herein on novel urea/thiourea receptors 1 and 2.
These receptors were found to be an efficient detector for
F2 by the change in UV–vis and 1H NMR and the naked-
eye observation.
Synthesis and characterisation
Compounds 1 and 2
To a methylene chloride (60 ml) solution of 4-nitrophenyl
isocyanate (0.85 g, 5 mmol) or 4-nitrophenyl isothiocya-
nate (0.93 g, 5 mmol), 3,4-diaminobenzophenone (1.09 g,
5 mmol) in methylene chloride (40 ml) was added slowly
while being stirred vigorously. Orange solid (1) and light
yellow solid (2) were precipitated, respectively, after
stirring for 1 day at room temperature, filtered and dried
(yield 97% for 1 and 55% for 2; Scheme 1). For 1, Anal.
Calcd for C20H16N4O4 (886.84): C, 63.82; H, 4.28; N,
14.89; O, 17.00. Found: C, 63.75; H, 4.22; N, 15.01%. 1H
NMR (DMSO-d6) d 9.52 (s, 1H), 8.18 (d, 2H), 8.03 (s,
1H), 7.60 (m, 9H), 6.81 (d, 1H), 5.96 (s, 2H). IR (KBr):
3452 (NZH), 3336 (NZH), 3224 (NZH), 1706 (CvO),
1617 (CvO), 1331 (NO2) cm21. For 2, 1H NMR (DMSO-
d6) d 10.38 (s, 1H), 9.45 (s, 1H), 8.21 (d, 2H), 7.92 (d, 2H),
7.64 (d, 2H), 7.53 (m, 5H), 6.82 (d, 1H), 6.12 (s, 2H). IR
(KBr): 3457 (NZH), 3357 (NZH), 3218 (NZH), 1623
(CvO), 1337 (NO2), 1113 (CvS) cm21
.
Crystal data for 2
C20H16N4O4S, MW ¼ 408.43, T ¼ 293 K, l ¼ 0.71073
˚
˚
˚
A, triclinic, a ¼ 8.6646(15) A, b ¼ 12.366(2) A, c ¼
˚
19.082(3) A, a ¼ 89.530(4)8, b ¼ 85.461(4)8, g ¼
3
˚
78.375(4)8, V ¼ 1996.3(6) A , Z ¼ 4, Dcalcd ¼ 1.359
mg/m3, m(Mo Ka) ¼ 0.196 mm21
,
F(000) ¼ 848,
crystal size, 0.15 £ 0.15 £ 0.08 mm. No. of reflections
*Corresponding authors. Email: kangjm@sejong.ac.kr; chealkim@snut.ac.kr
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2010 Taylor & Francis
DOI: 10.1080/10610270903449973
http://www.informaworld.com

268
J. Kang et al.
Scheme 1. The synthetic procedure for the anion receptors 1 and 2.
measured: total, 11,309; unique, 7694 (Rint ¼ 0.0569).
Refinement method: full-matrix least squares on F 2,
goodness-of-fit indicator 0.804, final R1 [I . 2.00s(I)] ¼
0.0567. The X-ray structure of 2 is shown in Figure 1.
The crystallographic data, and the selected bond lengths
and angles are given in Tables S1 and S2, respectively
(see Supplementary Data). CCDC-738483 contains all
crystallographic details of this publication and is available
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.
html or can be ordered from the following address:
Cambridge Crystallographic Data Centre, 12 Union Road,
GB-Cambridge CB21EZ; Fax: (þ44)1223-336-033; or
deposit@ccdc.cam.ac.uk.
Results and discussion
Receptors 1 and 2 display strong absorption bands at 345
and 360 nm in DMSO, respectively. The selective
recognition of urea compound 1 and thiourea compound
2 with F2 over other halides such as Cl2 and Br2 in
DMSO is evident in UV–vis and 1H NMR titration
experiments. Figure 2 shows the family of spectra obtained
over the course of the titration of solution 1 with
tetrabutylammonium fluoride in DMSO. When 70
equivalents of fluoride are added to the 20 mM solution
of 1, lmax of 1 shifts from 345 to 356 nm and the spectra
show a clear isosbestic point at 370 nm. This result
suggests that a typical hydrogen-bonding complex forms
between the receptor and the anion because the basicity of
the anion is insufficient to induce deprotonation of the
receptor at this fluoride concentration. However, when an
excess of fluoride ions is added, a new intense absorption
band develops at 481 nm, which is attributed to the
deprotonated receptor (27). In addition, the spectra show a
new approximate isosbestic point at 420 nm (Figure 2(a)).
The approximate isosbestic point indicates that deprotona-
tion is incomplete and the hydrogen-bonded complex and
the deprotonated compound 1 exist as a mixture.
Therefore, fluoride ion initially forms the hydrogen-
bonded complex, but with high excess of added anions, the
deprotonation occurs with the formation of the hydrogen-
bonded anion dimer F2H2 (Figure 3) (28).
The deprotonation of receptor 1 can be seen clearly
when the solution of receptor 1 is titrated with
tetrabutylammonium hydroxide (Figure 2(b)). As more
hydroxide ions are added, absorption band at 481 nm is
developed again as in the case of fluoride. However, no
isosbestic point is observed.
Figure 1. Crystal structure of 2.

Supramolecular Chemistry
269
Figure 2. Family of spectra recorded over the course of titration of 20 mM DMSO solution of receptor 1 with a standard solution of
(a) tetrabutylammonium fluoride, (b) hydroxide and (c) acetate.
Figure 3. The interaction of receptor 1 and fluoride.

270
J. Kang et al.
Figure 4. 1H NMR spectra of 1 mM of 1 with increased amounts of tetrabutylammonium fluoride in DMSO-d6. The shift of urea and
NH2 peaks are designated by dotted lines. F2H2 peaks are shown in the inset.
1
This phenomenon can be confirmed by a H NMR
titration (Figure 4). When 0.5 equivalent of tetrabutyl-
ammonium fluoride is added to the 1 mM solution of 1
in DMSO-d6, two urea NZH signals (9.48 and 8.01 ppm)
and the adjacent NH2 signal (5.94 ppm) in the benzo-
phenone moiety shifted downfield (9.75, 8.40 and
6.20 ppm, respectively) with broadening of signals. This
suggests that the typical hydrogen-bonding complex forms
between the receptor and the anion and that not only urea
but also the adjacent NH2 group participates in hydrogen
bonding with fluoride ion. When 2 equivalents of
tetrabutylammonium fluoride are added, the urea NZH
signals disappear. However, the adjacent NH2 peak shifts
downfield more. In addition, most of the CZH peaks
became broad. Broadening of the CZH peaks suggests that
the hydrogen-bonded complex and the deprotonated
compound exist as a mixture. When 10 equivalents of
fluoride are added to the solution, we can observe the
following phenomena. First, a clear and sharp spectrum of
deprotonated receptor 1 appeared. Returning to sharp
NMR signals suggests that only deprotonated compound 1
exists in the solution with sufficient fluoride ions. Second,
the NH2 signal from benzophenone moiety shifts upfield
(5.92 ppm). This indicates that the hydrogen bonding
between receptor 1 and the fluoride ion does not exist
anymore as deprotonation of receptor 1 occurs with
fluoride ion. Third, around 16.1 ppm, we could observe the
triplet of F2H2 (inset in Figure 4). All of these evidences
are in good agreement with our proposed mechanism.
With other anions such as acetate and benzoate,
receptor 1 also shows a typical spectrum pattern for the
formation of hydrogen-bonded complex (Figure 2(c)).
Assuming a 1:1 stoichiometry, a Benesi–Hildebrand plot
Table 1. The association constants or equilibrium constants of
receptors 1 and 2 with various anions in DMSO.
Anion
1
2
F2
CH3CO22
C6H5CO22
1.3 £ 104
1.2 £ 104
5.0 £ 103
3.3 £ 104
2.5 £ 104
1.3 £ 104

Supramolecular Chemistry
271
Figure 5. 1H NMR spectra of 1 mM of 1 with increased amounts of tetrabutylammonium acetate in DMSO-d6. The shift of urea and NH2
peaks are designated by dotted lines.
Figure 6. Family of spectra recorded over the course of titration of 50 mM DMSO solution of receptor 2 with a standard solution of
(a) tetrabutylammonium fluoride and (b) benzoate.

272
J. Kang et al.
(29) by use of change in the 346 nm absorption intensity
gives association constants. From the experiments,
receptor 1 shows association constants 1.3 £ 104 and
1.2 £ 104 for fluoride and acetate, respectively. The order
operate based on a hydrogen bonding and an acid–base
equilibrium. In addition, receptors 1 and 2 have proved to
be an efficient naked-eye detector for the fluoride ion.
of association constants was F2 . CH3CO22
.
C6H5CO22 . The results are summarised in Table 1.
Supplementary Data
1H NMR titration with acetate also shows evidence of
a discrete hydrogen-bonded complex (Figure 5). Two urea
peaks move to downfield (from 9.48 and 8.02 ppm to 12.81
and 11.19 ppm) along with adjacent NH2 peaks (from 5.93
to 6.20 ppm) as tetrabutylammonium acetate is added.
During titration, urea signals undergo broadening.
However, after 6 equivalents of acetate were added, all
peaks including NZH urea peaks became sharp. This
phenomenon is also an indication of the formation of
discrete hydrogen-bonded complex. Other anions such as
chloride, bromide, iodide, perchlorate, hydrogen sulphate,
nitrate did not bind to receptor 1 in DMSO at all.
Supplementary Data (X-ray crystallography, crystal data
˚
and structure refinement for 2, bond lengths (A) and angles
(8) for 2, and crystal structure of 2 containing two
asymmetric molecules) associated with this article can be
found in the online version.
Acknowledgements
Financial support from the Korea Ministry Environment ‘ET-
Human Resource Development Project’ and the Korean Science
and Engineering Foundation (R01-2008-000-20704-0 and 2009-
0074066) is gratefully acknowledged.
The more acidic thiourea fragment (pKa ¼ 21.1 in
DMSO) typically interacts more strongly with anions than
urea (pKa ¼ 26.9 in DMSO) (30). Therefore, the addition
of fluoride to more acidic thiourea derivative 2 induces
immediate deprotonation in DMSO. Spectroscopic
titration of receptor 2 by tetrabutylammonium fluoride in
DMSO shows the appearance of the new band at 360 nm
characteristic of the deprotonation of the receptor
(Figure 6(a)). Also, the presence of the sharp isosbestic
point at 376 nm indicates that only two species are present
at equilibrium over the course of the titration experiment.
The equilibrium constant for the deprotonation (rather
than binding constant) is calculated as 3.3 £ 104 for
fluoride. Acetate shows a similar behaviour and its
equilibrium constant for the deprotonation is calculated as
2.5 £ 104. However, benzoate shows a spectrum pattern
only for the hydrogen-bonded complex (Figure 6(b)).
Benzoate is not basic enough to deprotonate the thiourea
moiety of receptor 2 in DMSO. The binding constant is
calculated as 1.3 £ 104.
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