Synthesis of a new colorimetric receptor based on calix[4]pyrrole binding oxime for selective recognition of fluoride ions

Article abstract of DOI:10.1080/10610278.2013.830725

A colorimetric anion sensor -meso-heptaethylcalix[4]pyrrole-meso-3- aminophenyl-p-nitrobenzaldoxime was synthesised and characterised by various spectroscopic techniques. Anion-binding studies were carried out using UV-vis, and ¹H NMR titrations, revealing that the receptor exhibits selective recognition towards F^- over other anions. The selectivity for F ^- among the halides is attributed mainly to the hydrogen bond interaction of the receptor with F^-. Receptor showed colour change from colourless to yellow in the presence of tetrabutylammonium fluoride with 1:2 stoichiometry. Cyclic voltammetry studies, carried out in CH3CN, provided evidence of an anion-dependent electrochemical response with F^- ion. This response was particularly dramatic in the case of receptor after the addition of ～1 equiv. of F^O ion.

Full text of DOI:10.1080/10610278.2013.830725

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Synthesis of a new colorimetric receptor based on
calix[4]pyrrole binding oxime for selective recognition
of fluoride ions
Bilge Tanera, Onder Alicia & Pervin Devecia
a Department of Chemistry, Faculty of Science, Selcuk University, Konya, 42031, Turkey
Published online: 19 Sep 2013.
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To cite this article: Bilge Taner, Onder Alici & Pervin Deveci (2014) Synthesis of a new colorimetric receptor based on
calix[4]pyrrole binding oxime for selective recognition of fluoride ions, Supramolecular Chemistry, 26:2, 119-124, DOI:
10.1080/10610278.2013.830725
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Supramolecular Chemistry, 2014
Vol. 26, No. 2, 119–124, http://dx.doi.org/10.1080/10610278.2013.830725
Synthesis of a new colorimetric receptor based on calix[4]pyrrole binding oxime for selective
recognition of fluoride ions
Bilge Taner*, Onder Alici and Pervin Deveci
Department of Chemistry, Faculty of Science, Selcuk University, Konya 42031, Turkey
(Received 2 May 2013; accepted 18 July 2013)
A colorimetric anion sensor a-meso-heptaethylcalix[4]pyrrole-meso-3-aminophenyl-p-nitrobenzaldoxime was synthesised
and characterised by various spectroscopic techniques. Anion-binding studies were carried out using UV–vis, and 1H NMR
titrations, revealing that the receptor exhibits selective recognition towards F2 over other anions. The selectivity for F2
among the halides is attributed mainly to the hydrogen bond interaction of the receptor with F2. Receptor showed colour
change from colourless to yellow in the presence of tetrabutylammonium fluoride with 1:2 stoichiometry. Cyclic
voltammetry studies, carried out in CH3CN, provided evidence of an anion-dependent electrochemical response with F2 ion.
This response was particularly dramatic in the case of receptor after the addition of ,1 equiv. of F2 ion.
Keywords: calix[4]pyrrole-based receptor; oxime; anion recognition; cyclic voltammetry
1. Introduction
oximes are materials with interesting properties. They have
remained an important and popular area of research due to
their simple synthesis, versatility and potential applications
in many areas, such as medicine (22), bio-organic systems
(23), catalysis (24), electro-optical sensors (25) and quartz
crystal microbalance sensor (26). For that reason, we think
that synthesis of oxime compounds that functionalised with
calix[4]pyrrole derivative can generate new materials with
interesting properties due to their above-mentioned specific
complexation abilities with different anions.
Design, synthesis and development of new receptors for
molecular recognition are currently receiving importance in
the field of supramolecular chemistry (1–5). In particular,
the studies of novel artificial receptors towards anion species
are a subject of current interest due to their importance in
ubiquitous roles in chemical, biochemical and environmen-
tal fields (6, 7). Synthetic macrocycles such as calixpyrroles
should be of particular interest because they showed good
anion-binding capability both in the solution phase and in the
solid phase (8, 9). One of the more attractive developments
involves the construction of calix[4]pyrrole-based anion
sensors in both the optical (10) and electrochemical realms
(11, 12), which are of particular interest in the field of
recognition and sensing of anionic analytes (13, 14). These
macrocycles bind anions by means of hydrogen bonding
interactions between the polar NH units and the electron-rich
guests (15, 16). To date, various functionalised calix[4]
pyrrole derivatives bearing electron-donating or electron-
withdrawing substituents, chromophores in the pyrrolic or
meso-like positions (17–19), calix[n]pyrrole (n . 4) (20),
calix[n]bipyrrole (21) and so on have been designed for
tuning the anion-binding affinities and selectivities.
Although a great number of calix[4]pyrrole derivatives
have been synthesised and reviews have been published
giving an overview of the synthesis and properties of calix[4]
pyrrole derivatives, to the best of our knowledge, studies on
structural analyses and anion-binding behaviour of calix[4]
pyrrole-functionalised oxime ligands in modern supramole-
cular chemistry are very rare (18, 19). Like calix[4]pyrroles,
With this in view, we wish to first report the formation
of calix[4]pyrrole–oxime and show that the coloured
aggregation can be used as a powerful naked-eye sensor
for selective detection of F2 in solution. We also describe
the anion-binding behaviour of this sensor, as inferred
1
from the UV–vis titration study, H NMR spectroscopic
titration and electrochemical analyses.
2. Results and discussion
2.1 Synthesis and characterisation
In this study, treatment of a solution of a-chloro-p-
nitrobenzaldoxime (1) in tetrahydrofuran (THF) with
3-aminophenyl-calix[4]pyrrole gave a-meso-heptaethylca-
lix[4]pyrrole-meso-3-aminophenyl-p-nitrobenzaldoxime
(3), which was easily isolated (40%) by column
chromatography (Scheme 1). The synthesised receptor is
1
characterised by infrared (IR), H NMR, 13C NMR and
mass spectroscopy (MS) techniques, and the results are
given in Section 2. The receptor showed characteristic
*Corresponding author. Email: bilgetaner@gmail.com
q 2013 Taylor & Francis

120
B. Taner et al.
Scheme 1. Routes for the synthesis of the receptor.
vibrations of NH (3426cm21), OH (3384 cm21), CvN
(1626 cm21) and NO (926cm21) in FT-IR spectra. When
the 1H NMR spectra of the receptor in DMSO were
examined, peak corresponding to N–OH proton [10.56ppm
(s, 1H)] was observed downfield. This assignment was
further substantiated by disappearance of the corresponding
OH peak when receptor underwent deuterium exchange.
Two broad singlets were observed, one at 8.88ppm and the
other at 9.29ppm for pyrrole N–H groups. For ligand, the
chemical shift that belongs to the –NH protons was
observed at 7.98 ppm as a singlet. In addition, the
quaternary hydroxyimino carbon atoms (CvNZO)
appeared at 151.23ppm. In the mass spectrum of receptor,
the peaks observed at m/z 754.94 can be assigned to the
molecular (M þ H)þ ion peak for receptor. The peak at m/z
724.87 may be assigned to the loss of the –NOH group
from the receptor.
visual inspection of solution of receptor before and after
the addition of anions indicates that fluoride ion interacts
more strongly due to higher electronegativity and smaller
size compared with other anions (27, 28).
The titration profile of receptor upon the addition of
increasing quantities of F2anions depicted in Figure 3
shows the visible spectral changes of receptor in CH3CN
solution (1.0 £ 1024) upon the addition of increasing
quantities of F2 anion (from 1 to 10 equiv.), as well as the
nonlinear curve fitting of the absorbance band centred at
2.2 Anion-binding studies
The binding studies of new receptor with the tetrabuty-
lammonium (TBA) salts of the anions F2, Cl2, Br2, I2,
NO23 , AcO2, ClO42, HSO42 and H2PO24 were carried out by
UV–vis titrations in CH3CN solution. The response of
receptor in the presence of 10equiv. of the selected anions
can be seen in Figure 1(a). The addition of Cl2, Br2, I2,
NO23 , ClO24 , HSO42 and H2PO24 induced negligible
changes in the UV–vis profiles, whereas AcO2 promoted
a small increase in the absorbance of the visible bands.
However, the most remarkable changes were observed upon
the addition of F2 for which the absorbance of the visible
bands was highly enhanced.
These changes were reflected in colour modulations
from colourless to yellow. The receptor showed dramatic
colour changes from colourless to yellow in the presence of
tetrabutylammonium fluoride (TBAF), whereas it did not
give any noticeable colour change with different anions
(see Figure 2). These H-bonds or deprotonations affect the
electronic properties of the chromophore, resulting in a
colour change along with a new charge-transfer interaction
between the fluoride-bound OH and pyrrolic NH. Thus,
Figure 1. (Colour online) (a) UV–vis absorption spectra and
(b) the absorption ratios at 460 nm of R (1.0 £ 1024 M) in
CH3CN after the addition of 10 equiv. of each of the different
guest ions.

Supramolecular Chemistry
121
Figure 2. (Colour online) The visible colour changes observed for L in CH3CN solution (1.0 £ 1024 M) upon the addition of 10 equiv.
of anions as TBA salts: from left to right: free R, H2PO24 , HSO42, NO32, ClO42, I2, Br2, Cl2, AcO2 and F2.
460 nm. As it can be seen, a clear and gradual absorbance
enhancement of the visible band was observed.
and the fluoride anion (see Figure 5). The observed
downfield shifts of the pyrrole NH resonances are an
indication of hydrogen bonding to fluoride anion, as well
as the simplification of the pyrrole CH signals, a
characteristic for transition from 1,3 alternate to a
symmetrical cone-like conformation. The observed
resultants are in agreement with previous studies (27, 31,
32). Further additions of F2 induce slightly upfield shifts
of the b-proton on the pyrrole ring, thereby indicating the
increase in the electron density on the pyrrole ring owing
to a bond propagation effect (33).
The mole ratio method (29) was used to determine the
stoichiometry of complexed form of receptor with fluoride
ions. The stoichiometries between the receptor and fluoride
were determined from the molar ratio plots using the UV–
vis spectra, which showed to be 1:2 (Figure 4). Benesi–
Hildebrand plots (30) derived from the changes at 460 nm in
the UV–vis spectra of receptor gave the association
constants. The association constants of F2 with receptor
were calculated as 8.23 £ 104 from the UV–vis titrations
(Supplementary data, available online).
1
To investigate the nature of anion coordination, H
2.3 Electrochemical investigations
NMR titration experiments were conducted for receptor
with F2 in DMSO-d6. A partial 1H NMR titration
spectrum of fluoride ion with the receptor was shown in
Figure 5. The pyrrole NH and oxime OH signals in
receptor appeared at 8.88, 9.29 and 10.56 ppm, respect-
ively. The oxime OH signal in receptor disappeared and a
part of pyrrolic NH signal was shifted to downfield after
the addition of 0.5 equiv. of TBAF to the receptor
solutions. Furthermore, after the addition of 0.5 equiv. of
TBAF to the receptor solution, the remaining pyrrolic NH
signals at 8.88 and 9.29 ppm disappeared, indicating the
formation of a complex through the intermolecular
hydrogen bonding between the oxime OH, pyrolic NH
NMR spectroscopy and UV studies revealed that the new
receptor is able to complex fluoride anions in solution.
However, this receptor was further designed to allow the
anion-binding events to be detected through anion-induced
changes in the electrochemical properties of calixpyrrole
unit. The electrochemical properties of the receptor were
investigated by cyclic voltammetry (CV) versus Ag/AgCl
in CH3CN medium using 0.1 M tetrabutyl ammonium
tetrafluoroborat (TBATFB) as the supporting electrolyte at
100 mV s21 scan rate. The receptor was first scanned
between 0 and 21500 mV, and an irreversible (992 mV)
Figure 3. (Colour online) UV–vis titration spectra for receptor
(1.0 £ 1024) at room temperature with an increase in the
concentration of TBAF in CH3CN.
Figure 4. (Colour online) Absorbance versus mole ratio ([F2]/
[R]) at 460 nm.

122
B. Taner et al.
Figure 5. (Colour online) Partial 1H NMR spectra of L (2.5 £ 1022 M) in DMSO-d6 at 258 C and the corresponding changes after the
addition of various equivalents of TBAF (1.0 £ 1021 M).
wave was observed. Although cyclic voltammogram was
scanned between þ200 and þ1500 mV, two irreversible
waves at the oxidation peaks (755 and 1012 mV) appeared,
which could be attributed to the calixpyrrole centre. The
two positive peaks can be attributed to the oxidation of
redox active calix[4]pyrrole ring, probably first to the
cation radical and then to the dication products (12). This
behaviour was also observed for porphyrins, which are
structurally related to the calixpyrroles. CV was used to
probe the changes in the redox potentials of the receptor
upon complexation with anions. Titrations of 1 were
undertaken by the progressive addition of a solution
containing F2, Cl2, Br2, I2, NO23 , AcO2, ClO42, HSO24
and H2PO24 as their respective TBA salts. The addition of
anions generally varied from 0.2 to 2.0 equiv. The effect of
fluoride binding on the cyclic voltammograms of receptor
in CH3CN is illustrated in Figure 6. The addition of F2 to
receptor solution led to the decrease in redox response.
And the oxidation peak potential moved to the more
negative potential gradually, which may be related to the
formation of F2 complex. In an effort to test for selectivity,
CV titration experiments were also carried out with
solutions containing Cl2, Br2, I2, NO23 , AcO2, ClO24 ,
HSO24 and H2PO24 ions. However, in the presence of these
anions, the events did not result in the expected shift of the
oxidation potential, meaning that no reliable results (or
sensing effects) could be obtained for receptor.
3. Conclusions
In conclusion, we have designed and synthesised a highly
sensitive oxime calix[4]pyrrole-based colorimetric sensor
for the detection of F2 ion selectively. The colorimetric
receptor can be utilised for the detection of F2 in the
presence of competing anions such as Cl2, Br2, I2, NO32,
AcO2, ClO42, HSO42 and H2PO24 ions. A colour change of
the receptor from colourless to yellow is detectable – by
the naked eye – at a low concentration of the receptor.
This novel anion receptor could provide a simple,
inexpensive and more general method for the rapid
detection and identification of F2 anions.
Figure 6. (Colour online) Titration of receptor and F2 anions in
CH3CN with 0.1 M TBATFB and at scan rate of 100 mV s21
.

Supramolecular Chemistry
123
4. Experimental
13C NMR (100 MHz, DMSO-d6, ppm): 8.46, 8.64, 9.01,
9.35, 27.11, 28.61, 30.15, 30.64, 42.33, 42.99, 44.38,
102.96, 104.31, 104.64, 104.88, 117.50, 119.42, 123.89,
128.70, 129.03, 135.17, 135.90, 136.21, 138.35, 140.00,
141.06, 147.67, 147.95, 151.23; MS for C46H55N7O3 m/z:
754.94 [M þ H]þ, 740.92, 724.87, 707.73.
4.1 Equipments and instruments
Elemental analyses (C, H and N) were determined using a
LECO CHNS-932 (LECO corporation, Michigan, USA)
CHNSO model analyser. H NMR and 13C NMR spectra
1
were recorded on a Bruker 400 MHz spectrometer (Bruker,
Bremen, Germany) in DMSO-d6 as the solvent with Me4Si
as an internal reference. The IR spectra of solid samples
were recorded in the range from 200 to 4000 cm21 on a
Perkin-Elmer Model 1605 FT-IR spectrophotometer
(Perkin-Elmer, Massachusetts, USA). UV–vis spectra
were obtained using Shimadzu UV-1700 visible recording
spectrophotometers (Shimadzu Scientific Instruments,
Columbia, USA). Mass spectra were acquired in the linear
mode with an average of 50 shots on a Bruker Daltonics
Microflex mass spectrometer equipped with a nitrogen UV-
laser operating at 337 nm. Melting points were determined
using an electrothermal apparatus and are uncorrected. All
the electrochemical experiments were carried out using a
Gamry Reference 600 workstation (Gamry, Pennsylvania,
USA) electrochemical analyser (Model 600C series)
equipped with BAS C3 cell stand. Working electrode was
glassy carbon disc (BAS) with a geometric area of
0.027 cm2. The reference electrode was a Ag/AgCl/
KCl(sat.) and the counter electrode was a Pt wire.
Supplementary information
Supplementary data associated with this article can be
found in the online version at http://dx.doi.org/10.1080/
10610278.2013.830725
Acknowledgement
This work was supported by the Research Fund of Selcuk
University.
References
´
(1) Pastor, A.; Martınez-Viviente, E. Coord. Chem. Rev. 2008,
252, 2314–2345.
´
(2) Alexeeva, Y.E.; Kharisovb, B.I.; Garcıab, T.C.H.; Gar-
novskiia, A.D. Coord. Chem. Rev. 2010, 254, 794–831.
(3) Wanga, F.; Wua, Y.; Lua, K.; Yeb, B. Electrochim. Acta
2013, 87, 756–762.
(4) Samanta, R.; Mahanta, S.P.; Chaudhuri, S.; Panda, P.K.;
Narahari, A. Inorg. Chim. Acta 2011, 372, 281–285.
(5) Tabakci, B.; Alici, O.; Karatas, I. Talanta 2013, 106, 92–96.
(6) Gale, P.A. Coord. Chem. Rev. 2006, 250, 2939–2951.
(7) Gale, P.A.; Garcia-Garrido, S.E.; Garric, J. Chem. Soc. Rev.
2008, 37, 151–190.
(8) Sessler, J.L.; Gross, D.E.; Cho, W.S.; Lynch, V.M.;
Schmidtchen, F.P.; Bates, G.W.; Light, M.E.; Gale, P.A.
J. Am. Chem. Soc. 2006, 128, 12281–12288.
(9) Gale, P.A.; Sessler, J.L.; Kral, V. Chem. Commun. 1998, 1–8.
(10) Miyaji, H.; Sato, W.; Sessler, J.L. Angew. Chem. Int. Ed.
2000, 39, 1777–1780.
(11) Gale, P.A.; Hursthouse, M.B.; Light, M.E.; Sessler, J.L.;
Warriner, C.N.; Zimmerman, R.S. Tetrahedron Lett. 2001,
42, 6759–6762.
4.2 Preparation of receptor (R)
All chemicals were of reagent grade quality and used
without purification; a-chloro-p-nitrobenzaldoxime (34)
and meso-heptaethylcalix[4]pyrrole-meso-3-aminophenyl
(18, 19, 35) were prepared according to the procedures
described in supplementary data, available online.
A solution of meso-heptaethylcalix[4]pyrrole-meso-3-
aminophenyl (0.59 g, 1 mmol) in 10ml dry THF was added
to a stirred mixture of a-chloro-p-nitrobenzaldoxime (0.2g,
1 mmol) and triethylamine (0.14 ml, 1 mmol) in dry THF
(20 ml). The mixture was stirred at room temperature for
24h and monitored by the thin layer chromatography.
Dichloromethane was added to the reaction mixture and the
solution was washed with the saturated NaCl (3ml
£ 15ml). The organic phase was collected, dried with
Na2SO4 and then the solvent was removed under reduced
pressure. The product was purified by column chromatog-
raphy (ethyl acetate:n-hexane, 1:1.5).
¨
˘
(12) Taner, B.; Ozcan, E.; Ustu¨ndag, Z.; Keskin, S.; Solak, A.O.;
Eksi, H. Thin Solid Films 2010, 519, 289–295.
(13) Kato, R.; Nishizawa, S.; Hayashita, T.; Teramae, N.
Tetrahedron Lett. 2001, 42, 5053–5056.
(14) Lee, K.H.; Lee, H.-Y.; Lee, D.H.; Hong, J.-I. Tetrahedron
Lett. 2001, 42, 5447–5449.
(15) Gale, P.A. Coord. Chem. Rev. 2001, 231, 79–128.
(16) Gale, P.A.; Anzenbacher, P.; Sessler, J.L. Coord. Chem.
Rev. 2003, 240, 183–191.
´
(17) Sessler, J.L.; Anzenbacher, P.; Miyaji, H.; Jursıkova, K.;
Yield 40%; mp: 2028C; elemental analysis (found:
C, 73.01; H, 7.12; N, 12.91%. Calcd: C, 73.28; H, 7.35; N,
13.00%); IR nmax/cm21: 3426 (N–H), 3384 (O–H),
2875–2965 (C–Haliph), 1626 (CvN), 926 (N–O);
1H NMR (400 MHz, DMSO-d6, ppm): 0.75–0.91 (m,
18H, CH3), 1.26–1.70 (m, 12H CH2), 2.14 (s, 3H, CH3),
5.36–5.11 (m, 8H, pyr-CH), 6.90–7.31 (m, 4H, CH), 7.79
(d, 2H, CH), 7.74 (d, 2H, CH), 8.37 (s, 1H, NH), 8.88 (br s,
2H, NH), 9.29 (br s, 2H, NH), 10.56 (s, 1H, N–OH);
Bleasdale, E.R.; Gale, P.A. Ind. Eng. Chem. Rev. 2000, 39,
3471, and reference therein.
(18) Taner, B.; Deveci, P.; Bereket, S.; Solak, A.O.; Ozcan, E.
Inorg. Chim. Acta 2010, 363, 4017–4023.
(19) Taner, B.; Deveci, P.; Solak, A.O.; Ozcan, E. J. Incl.
Phenom. Macrocycl. Chem. 2012, 74, 391–396.
(20) Sessler, J.L.; An, D.; Cho, W.-S.; Lynch, V. Angew. Chem.
Int. Ed. 2003, 42, 2278–2281.
(21) Allen, W.E.; Gale, P.A.; Brown, C.T.; Lynch, V.M.;
Sessler, J.L. J. Am. Chem. Soc. 1996, 118, 12471–12472.

124
B. Taner et al.
(22) Dilworth, J.R.; Parrott, S.J. Chem. Soc. Rev. 1998, 27, 43–55.
(23) Hall, I.H.; Bastow, K.F.; Warren, A.E. Appl. Organomet.
Chem. 1999, 13, 819–828.
(29) Belay, A. Int. J. Biophys. 2012, 2, 12–17.
(30) Benesi, H.A.; Hildebrand, J.H. J. Am. Chem. Soc. 1949, 71,
2703–2707.
(24) Malmstrom, B.G. Acc. Chem. Res. 1993, 26, 332–338.
(25) Sevagapandian, S.; Rajagopal, G.; Nehru, K.; Athappan, P.
Transition Met. Chem. 2000, 25, 388–393.
(26) Harbeck, M.; Sen, Z.; Gurol, I.; Gumus, G.; Musluoglu, E.;
Ahsen, V.; Ozturk, Z.Z. Sensors Actuat. Chem. B. 2011,
156, 673–679.
(31) Nielsen, K.A. Tetrahedron Lett. 2012, 53, 5616–5618.
(32) Erdemir, S.; Kocyigit, O.; Alici, O.; Malkondu, S.
Tetrahedron Lett. 2013, 54, 613–617.
(33) Wu, Y.; Peng, X.; Fan, J.; Gao, S.; Tian, M.; Zhao, J.; Sun,
S. J. Org. Chem. 2007, 72, 62–70.
(34) Kao, Y.T.R.; Yang, D.; Yuen, K.-Y.;. United Sates Patent
US 2011/0212975 2011.
(27) Chauhan, S.M.S.; Bhaskar, G.; Tanuja, B. Supramol. Chem.
2008, 12, 1–8.
(35) Anzenbacher, P.; Jursikova, J.K.; Sessler, J.L. J. Am. Chem.
Soc. 2000, 122, 9350–9351.
(28) Black, C.B.; Andrioletti, B.; Try, A.C.; Ruiperez, C.;
Sessler, J.L. J. Am. Chem. Soc. 1999, 121, 10438–10439.