Receptor with an active methylene group as binding site for extraction of inorganic fluoride ions from seawater

Article abstract of DOI:10.1002/cplu.201402018

Two new receptors R1 and R2 based on triphenylphosphonium salts with an active methylene group as binding site have been designed and synthesised for the detection of F^- ions. The detection limit of these receptors in organic media was found to

Full text of DOI:10.1002/cplu.201402018

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DOI: 10.1002/cplu.201402018
Receptor with an Active Methylene Group as Binding Site
for Extraction of Inorganic Fluoride Ions from Seawater
Madhuprasad and Darshak R. Trivedi*[a]
1
Two new receptors R1 and R2 based on triphenylphosphoni-
um salts with an active methylene group as binding site have
been designed and synthesised for the detection of FÀ ions.
The detection limit of these receptors in organic media was
found to be 0.2 ppm. Upon adding FÀ ions, a Dlmax of 188 nm
and 256 nm was observed for receptors R1 and R2, respective-
ly. The detection process followed deprotonation of the meth-
ylene proton, which has been confirmed by H NMR titration.
The receptors were evaluated for real-life applicability by ex-
tracting FÀ ions from aqueous media and seawater into organ-
ic media. Receptor R2 was able to extract FÀ ions from seawa-
ter with 99% efficiency. The level of FÀ ions present in seawa-
ter has been quantified and found to be 1.4 ppm, which is
comparable to the reported literature value.
Introduction
The design and synthesis of organic receptors for the detec-
tion of anions through their optical, electrochemical and mag-
netic resonance response have received considerable attention
owing to the vital applications of anions in various industrial,
chemical, biological and environmental processes. In this
regard, organic receptors with neutral binding sites for various
anions have been synthesised for decades.[1] Recently, cationic
receptors with ammonium, quinolinium, imidazolium and gua-
nidinium salts were used for anion detection.[2] These cationic
receptors utilise electrostatic interaction between anion and
cation for anion recognition. Along with the electrostatic inter-
actions, other weaker noncovalent interactions, such as anion–
p interaction, were used for anion detection.[3]
drogen-bond formation has been used for detection.[7] Un-
fortunately, the majority of them are capable of working only
in absolute non-aqueous conditions for the detection of organ-
ic fluoride sources such as tetrabutylammonium fluoride
(TBAF). On the other hand, only a few organic receptors have
been reported for the detection of FÀ ions in aqueous media.[8]
To the best of our knowledge, only one report has been pub-
lished so far on the selective extraction of the FÀ ion from
aqueous solution with measurable visual detection.[9]
Anion-binding receptors with alkyl triphenylphosphonium
salts with an active/acidic methylene group as a binding site
were not studied to the extent of receptors with ÀNH or ÀOH
binding sites. Vicens et al. synthesised alkyl triphenylphospho-
nium salts attached to calix[4]arene and reported the forma-
tion of ion-pair-type complexes with a range of anions such as
Among the wide range of anions, the design and synthesis
of an effective receptor for FÀ ion detection have received con-
siderable attention because of its vital role in industrial usage,
environmental pollution and significance in clinical applica-
tions. In contrast, excess fluoride consumption is always
a health concern as it can cause dental fluorosis[4] and skeletal
fluorosis.[5] Acute fluoride exposure may cause collagen break-
down, depression in thyroid activity, bone cancer, immune
system disturbance and anaemia.[6] Thus, the real-time moni-
toring of the FÀ ion is most significant over other anion detec-
tion.
2À
À [10]
halides, AcOÀ, HPO4 and ClO4
.
Das et al. synthesised alkyl
triphenylphosphonium salts attached to an anthraquinone
skeleton, which displayed high selectivity towards FÀ ions.[9] In
addition, this receptor displayed a unique anion extraction
from aqueous solution to organic solvent.
With this background, herein we report new receptors
based on triphenylphosphonium salts that contain an active
methylene (ÀCH2À) group as a binding site for anion detection.
The receptors were designed on the binding site–spacer–sig-
nalling unit approach, in which the binding site and signalling
unit were separated by a “spacer” diazo group (Scheme 1). Re-
ceptor R1 contains an ethoxyphenyl unit as signalling unit and
receptor R2 contains a coumarin as signalling unit. These re-
ceptors are used for the detection of FÀ ions in organic media
and to extract inorganic FÀ ions from aqueous media into or-
ganic media. The extraction process has been visualised by an
instantaneous optical change in organic media. The recep-
tors R3 and R4 were synthesised to evaluate the role of the
active methylene group in the detection process.
Receptors with well-known functionalities, such as urea/thio-
urea, amide, imide, pyrrole and imidazolium, as binding sites
for the FÀ ion have been reported in which conventional hy-
[a] Madhuprasad, Dr. D. R. Trivedi
Supramolecular Chemistry Laboratory
Department of Chemistry
National Institute of Technology Karnataka (NITK)
Surathkal, Karnataka 575025 (India)
E-mail: darshak_rtrivedi@yahoo.co.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402018.
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Scheme 1. Molecular structures of receptors R1–R4.
Results and Discussion
A single crystal of receptor R3 suitable for X-ray diffraction
analysis was grown by slow evaporation of an ethanol/di-
chloromethane (1:1) solution at room temperature. The ORTEP
diagram (50% probability) of receptor R3 is given in Figure 1.
Receptor R3 was crystallised in a monoclinic lattice. Detailed
crystallographic data of receptor R3 are given in Table 1.
Figure 2. UV/Vis spectral change on addition of anions (2 equiv) to recep-
tor R1 solution (1.0ꢁ10À5 m) in dry CH2Cl2: a) FÀ ions, b)AcOÀ ions and c) R1
and other anions.
with the addition of different anions (Figure S7 in the Support-
ing Information).
In addition, receptor R1 was further evaluated for the colori-
metric detection of anions. The receptor R1 solution (1.0ꢁ
10À5 m) in dry dichloromethane showed significant colour
change from pale yellow to pink instantaneously with the ad-
dition of FÀ ions. However, no colour change was noticed
upon addition of other anions (Figure 3). The addition of
AcOÀshowed slight change in the absorbance of the UV/Vis
spectra; however, it failed to induce any significant colour
change to receptor R1.
Figure 1. ORTEP diagram (50% probability) of receptor R3.
Table 1. Crystallographic data of receptor R3.
chemical formula C15H16N2O
a [8]
b [8]
g [8]
V [ꢂ3]
Z
90.00
98.890(3)
90.00
1337.98
4
formula weight
crystal system
space group
a [ꢂ]
240.30
monoclinic
P21/n
8.1773(4)
7.5454(4)
21.9486(10)
b [ꢂ]
c [ꢂ]
crystal size [mm3] 0.48ꢁ0.35ꢁ0.29
R factor [%] 7.26
Figure 3. Change in colour after addition of different anions (2 equiv) as TBA
salts to the receptor solution in dry CH2Cl2 (1.0ꢁ10À5 m). a) Receptor R1,
b) FÀ, c) ClÀ, d) BrÀ, e) IÀ, f) NO3À, g) HSO4À, h) H2PO4À and i) AcOÀ.
A selective anion detection study for receptors R1–R4 was
performed with the help of UV/Vis spectroscopy. Solutions
(1.0ꢁ10À5 m) of receptors R1 and R2 in dry dichloromethane
solutions were treated with two equivalents of different
anions, such as fluoride, chloride, bromide, iodide, nitrate, hy-
drogen sulfate, dihydrogen phosphate and acetate in the form
of tetrabutylammonium (TBA) salts. In the case of receptor R1,
a significant shift in the absorbance was observed with the ad-
dition of FÀ and AcOÀ ions (Figure 2). The intensity of the ab-
sorption band in UV/Vis spectra for AcOÀ ions was much less
than that for FÀ ions. This signifies that the interaction be-
tween receptor R1 and the FÀ ion is stronger and the interac-
tion between receptor R1 and the AcOÀ ion is much weaker.
All other anions did not show any change in UV/Vis spectra,
which indicates that these anions either did not interact with
receptor R1 or the interaction was not enough to produce any
changes in the spectra. Receptor R2 showed similar changes
To understand the nature of the receptor–anion interactions,
a UV/Vis titration experiment was performed between recep-
tor R1 and TBAF (Figure 4). With the incremental addition of
TBAF to receptor R1 (1.0ꢁ10À5 m), the absorbance at 356 nm,
corresponding to the phenyldiazene group in receptor R1, de-
creased constantly and a new absorption band at 544 nm ap-
peared and gradually increased. This observation is owing to
the formation of charge-transfer (CT) transitions involving the
electron-rich methylene functionality as donor group and the
phenylenediazene group as the acceptor unit. The intensity of
the absorption band attained saturation after adding two equi-
valents of FÀ ions. The bathochromic shift of 188 nm with the
formation of an isosbestic point at 401 nm was attributed to
the formation of a CT complex between the receptor and FÀ
ion. The binding stoichiometry between receptor R1 and FÀ
ions was determined by the Benesi–Hildebrand method using
UV spectrometric titration data at 544 nm. The linearity of the
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Figure 4. UV/Vis titration spectra of R1 (1.0ꢁ10À5 m) with increasing concen-
tration of TBAF (0–3 equiv) in dry CH2Cl2. Inset: Benesi–Hildebrand plot for
R1 at 544 nm.
Figure 6. UV/Vis titration spectra of R2 (1.0ꢁ10À5 m) with increasing concen-
tration of TBAF (0–3 equiv) in dry CH2Cl2. Inset: Benesi–Hildebrand plot for
receptor R2 at 573 nm.
plot was obtained by using UV spectrometric titration data at
573 nm. The plot showed linearity only at the square of the
concentration of FÀ ions. This clearly indicates the formation of
a 1:2 stoichiometric complex between receptor R2 and FÀ ions
(Figure 6, inset).
Figure 5. Change in colour after addition of different anions (2 equiv) as TBA
salts to the receptor solution in CH2Cl2 (1.0ꢁ10À5 m). a) Receptor R2, b) FÀ,
c) ClÀ, d) BrÀ, e) IÀ, f) NO3À, g) HSO4À, h) H2PO4À and i) AcOÀ.
Correspondingly, the UV/Vis spectroscopic titration was per-
formed by adding tetrabutylammonium acetate to the recep-
tors R1 and R2 in dichloromethane. The spectral pattern dis-
played similar changes to that of TBAF (Figure S12 for recep-
tor R1 and Figure S13 for receptor R2). However, the intensity
of the new absorption band was much less than that of TBAF.
This is perhaps a result of weaker interactions between recep-
tors and AcOÀ ions.
graph confirms the formation of a stable 1:2 receptor/FÀ ion
stoichiometric complex (Figure 4, inset).
Further, the colorimetric investigation was extended to re-
ceptor R2. The receptor R2 (1.0ꢁ10À5 m) was treated with
two equivalents of different anions in dry dichloromethane
(Figure 5). The addition of FÀ ions resulted in a significant
colour change from pale yellow to dark blue, whereas AcOÀ
ions showed slight variation in colour from pale yellow to light
blue. This difference in the colour intensity indicated a stronger
binding interaction between receptor R2 and FÀ ions and a sig-
nificantly weaker interaction between receptor R2 and AcOÀ
ions. On the other hand, other anions either did not interact or
interacted feebly with receptor R2 and did not show any visual
colour change.
The coumarin moiety is a well-known fluorophore, so it was
interesting to study fluorescence changes upon FÀ ion binding
to receptor R2. Therefore, a fluorescence titration was per-
formed by adding TBAF to receptor R2 in dichloromethane
(Figure S16). As expected, upon increasing the concentration
of FÀ ions the quenching of fluorescence was observed owing
to the deprotonation of receptor R2.
The binding constant for both receptors (R1 and R2) was
calculated using the Benesi–Hildebrand equation and found to
be (4.58Æ0.02)ꢁ107 mÀ2 for receptor R1 and (7.5Æ0.03)ꢁ
107 mÀ2 for receptor R2. This result shows that the FÀ ion binds
more strongly to receptor R2 than receptor R1. This sensitivity
is perhaps because of the presence of the coumarin unit in re-
ceptor R2, which is a strong signalling unit when compared
with the ethoxyphenyl group in receptor R1.
Further, the binding mechanism of receptor R2 to FÀ ions
was proposed by evaluating the results obtained from UV/Vis
titration and the Benesi–Hildebrand method. On compiling the
results of UV/Vis experiments, it is evident that the colorimetric
detection of the FÀ ion with receptor R2 is a two-step process.
Initially, the FÀ ion binds to the active methylene (ÀCH2À)
group of receptor R2 through hydrogen bonding, which re-
sults in a 1:1 adduct to form the R1···FÀ complex. A second FÀ
ion leads to deprotonation of the active methylene group to
Receptor R2 was further analysed quantitatively with UV/Vis
spectroscopic titration by adding TBAF to dry dichloromethane
solution (Figure 6). The addition of TBAF resulted in the gener-
ation of a new absorption band at 573 nm. With the incremen-
tal addition of FÀ ions to the receptor R2 solution, the absorb-
ance band at 317 nm, corresponding to the phenyldiazene
chromenone unit, decreased gradually and simultaneously the
absorption band at 573 nm increased progressively owing to
the development of CT transitions between the electron-rich
methylene functionality, which it acts as an electron donor,
and the phenylenediazene chromenone group, which acts as
an electron acceptor unit. The intensity of this new absorption
band attained saturation after the addition of two equivalents
of TBAF. The stoichiometric complexation ratio was determined
with the Benesi–Hildebrand method. The Benesi–Hildebrand
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one equivalent, the signal corre-
sponding to this methylene
group disappeared completely,
which indicated a fast proton ex-
change between the methylene
proton and the FÀ ion. Further,
the addition two equivalents of
FÀ ions resulted in the deproto-
nation of receptor R2 to form
R2À and simultaneously the elec-
tron density over the receptor
molecule increased. Therefore,
the signal corresponding to the
protons of the ÀCH2À group
Scheme 2. Proposed binding mechanism of FÀ ions with receptor R2.
shifted downfield from d=5.32
to d=5.76 ppm. Concurrently,
form R2À (Scheme 2).[11] As a result, the electron density of re-
ceptor R2 increases, which leads to intramolecular CT interac-
tion between the electron-rich phenyldiazene chromenone
functionality and the electron-deficient [PPh3]+ group. Thus,
receptor R2 shows intense colour change instantaneously
upon addition of FÀ ions.
The binding mechanism was further confirmed by 1H NMR ti-
tration (Figure 7) of receptor R2 performed in deuterated di-
methyl sulfoxide ([D6]DMSO) solution. The 1H NMR signal at
the splitting pattern disappeared because of the fast proton
exchange within the receptor. The multiplet signals for aromat-
ic protons experienced a downfield shift (from d=7.24–7.63 to
d=7.54–7.64 ppm) and merged together to form two major
signals with multiple splitting. This result further confirms the
increase in electron density over receptor R2. In addition, upon
adding two equivalents of FÀ ions to the receptor R2 solution,
the active methylene group undergoes deprotonation. This de-
protonation process was confirmed from the appearance of
1
d=5.32 ppm corresponds to two protons of the methylene (À the H NMR signal at d=16.1 ppm (Figure 7, inset), which cor-
À [13]
responds to HF2
.
As evidence for the deprotonation mechanism, UV/Vis spec-
troscopic titration was performed by adding tetrabutylammo-
nium hydroxide (TBAOH) to dry dichloromethane solutions of
receptors R1 and R2 (Figure S14 for receptor R1 and Fig-
ure S15 for receptor R2). The UV/Vis spectra of TBAOH for both
the receptors showed similar changes to that of TBAF, which
clearly indicates the detection process follows the deprotona-
tion mechanism.
Receptors R3 and R4, which do not contain an active meth-
ylene group, were studied for their anion detection ability in
dichloromethane. As expected, these receptors neither showed
any colour changes (Figures S8 and S9) nor displayed any UV/
Vis spectral changes (Figures S10 and S11) even with the addi-
tion of 20 equivalents of anions. This finding clearly suggests
that the active methylene group is responsible for the FÀ ion
detection.
Inorganic fluoride such as NaF is an essential nutrient for
living organisms; however, at higher concentrations it is haz-
ardous to health. Therefore, the World Health Organization has
restricted the FÀ ion concentration level to 1 ppm in drinking
water.[14] Keeping this fact in mind, the real-life applicability of
extraction of FÀ ions from aqueous media with receptors R1
and R2 has been evaluated. Standard solutions of NaF at differ-
ent concentrations were prepared in water and used as inor-
ganic FÀ ion source for the extraction study. These standard
solutions were treated with receptor R1 and R2 solutions in di-
chloromethane. Upon vigorous shaking of these organo-aque-
ous mixtures, the receptor solutions were able to extract FÀ
ions from aqueous media. As a result, the colour of recep-
tors R1 and R2 changed from yellow to pink and yellow to
1
Figure 7. Partial H NMR titration spectrum of receptor R2 in [D6]DMSO after
the addition of FÀ ions. Inset: Appearance of the H NMR signal at
d=16.1 ppm corresponding to HF2À upon adding 2 equivalents of FÀ ions.
1
CH2À) group of receptor R2. The splitting pattern of this signal
appeared as a doublet because of coupling with the adjacent
phosphorus.[12] Upon adding 0.5 equivalents of FÀ ions, a slight
downfield shift from d=5.32 to 5.33 ppm and d=5.35 to
5.36 ppm was observed. This downfield shift resulted from the
formation of a hydrogen bond between the methylene proton
and FÀ ion. Upon increasing the concentration of FÀ ions to
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deep blue, respectively. Further, these receptors were tested
for the extraction of FÀ ions from seawater collected from the
Arabian Sea (latitude 1380’33.99“, longitude 74847’17.23”) to
examine the practical applicability.
The extraction experiment was performed by treating stan-
dard solutions of NaF (1–4 ppm) and seawater (1.5 mL each)
with dichloromethane solutions of receptor R2, in glass vials.
The organo-aqueous solutions were shaken well to extract the
FÀ ions. The vials containing above 1 ppm NaF solution
showed significant colour change from yellow to blue, which
clearly indicates that the receptor is capable of extracting FÀ
ions from water (Figure 8). The instantaneous colour change
(blue) observed in the organic layer during extraction experi-
ments suggests that receptor R2 can extract FÀ ions efficiently
Figure 9. Calibration curve for quantitative determination of FÀ ions in sea-
water and samples.
Figure 8. Extraction process of FÀ ions from aqueous solutions and seawater
using receptor R2 solution in CH2Cl2.
Figure 10. Extraction process of FÀ ions from aqueous solutions and seawa-
ter using receptor R1 solution in CH2Cl2.
from seawater, and thus confirms that other competing
anions, such as chloride, bromide, iodide, phosphate and so
forth, present in seawater did not interfere in the detection/ex-
traction process. Further, the experimental studies revealed
that receptor R2 was able to extract FÀ ions from aqueous NaF
solution and seawater with 99% efficiency.
yellow to dark blue along with a significant bathochromic shift
of 188 nm and 256 nm, respectively. This colour change and
bathochromic shift resulted from the charge-transfer transi-
tions in the receptors on adding FÀ ions. These receptors were
able to detect FÀ ions even at concentrations as low as
0.2 ppm in organic media, which is much less than the World
Health Organization permissible level. In addition, these recep-
tors were able to extract FÀ ions from aqueous media into or-
ganic solutions with a substantial colour change. The real-life
applicability of the receptors was evaluated by extracting FÀ
ions from seawater. Receptor R2 was able to extract FÀ ions
from seawater with 99% efficiency. In addition, the level of FÀ
ions present in seawater was determined by using receptor R2
and was found to be 1.4 ppm, which is in good agreement
with literature reports. Currently, we are concentrating on the
design and synthesis of new and more efficient receptors for
the extraction of FÀ ions from industrial wastes.
The extraction study was further quantified by UV/Vis spec-
troscopy. Standard solutions of NaF with varying concentra-
tions from 1 to 4 ppm were prepared in water. Portions (1 mL)
of these standard solutions were subjected to extraction (three
times for each NaF solution) using receptor R2 dissolved in di-
chloromethane. The organic phases of each solution were sep-
arated, diluted four times and the UV/Vis spectrum was mea-
sured. The same procedure was repeated for seawater. A cali-
bration curve of absorbance versus concentration of FÀ ions (in
ppm) was plotted (Figure 9), from which the concentration of
FÀ ions in seawater was found to be 1.4 ppm. This result was
comparable with the previously reported literature value.[14b]
Similar extraction experiments were performed for recep-
tor R1, which showed a significant colour change in the organ-
ic medium only with organo-aqueous solutions that contained
a minimum 1.5 ppm of NaF. Therefore, receptor R1 was unable
to extract the FÀ ions from seawater (Figure 10). This is per-
haps owing to the less extended conjugation as the ethoxy-
phenyl group is not participating in the detection process.
Experimental Section
General information
All chemicals were purchased from Sigma–Aldrich, Alfa Aesar or
Spectrochem and used without further purification. All the solvents
were procured from SD Fine, India, were of HPLC grade and used
without further distillation.
Conclusion
The 1H NMR spectra were recorded on
a Bruker Avance II
Two new receptors with an active methylene group as binding
site have been designed and synthesised for the selective de-
tection of FÀ ions. On adding FÀ ions, the receptors R1 and R2
displayed a colour change from pale yellow to pink and pale
(500 MHz) instrument using TMS as internal reference and
[D6]DMSO as solvent. Resonance multiplicities are described as
s (singlet), d (doublet), t (triplet) and m (multiplet). Melting points
were measured on a Stuart SMP3 melting-point apparatus in open
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capillaries. Infrared spectra were recorded on a Thermo Nicolet
Avatar-330 FTIR spectrometer; signal designations: s (strong),
m (medium) and w (weak). UV/Vis spectroscopy was performed
with an Analytikjena Specord S600 spectrometer in standard
3.5 mL quartz cells (two optical windows) with 10 mm path length.
Fluorescence spectroscopy was performed with a JASCO FP-6200
spectrofluorometer in standard 3.5 mL quartz cells (four optical
windows) with 10 mm path length. Elemental analyses were done
using a Flash EA1112 CHNS analyser (Thermo Electron Corpora-
tion).
Scheme 5. Synthesis of receptor R4.
Synthesis of (E)-1-[4-(bromomethyl)phenyl]-2-(4-ethoxyphe-
nyl)diazene (3) and (E)-3-acetyl-6-{[4-(bromomethyl)phenyl]-
diazenyl}-2H-chromen-2-one (4; Scheme 6)
Synthesis of (E)-4-(p-tolyldiazenyl)phenol (1) and (E)-2-hy-
droxy-5-[(p-tolylimino)methyl] benzaldehyde (2; Scheme 3)
N-Bromosuccinimide (NBS; 0.162 g, 0.91 mmol for R3 and 0.183 g,
1.03 mmol for R4) and a catalytic amount of dibenzoyl peroxide
(Bz2O2) were added to a solution of R3 or R4 (0.2 g, 0.83 mmol or
0.288 g, 0.94 mmol) in CCl4 (20 mL). The resulting mixture was
heated at reflux for 8 h. The decomposed product of NBS was then
separated by filtration and the filtrate was evaporated to dryness
to afford a yellow solid. The solid was triturated with diethyl ether
to yield pure product 3 (0.24 g) or 4 (0.343 g).
The diazonium salt of p-toluidine was prepared by adding sodium
nitrate (3.5 mmol) to a stirred solution of p-toluidine (2.33 mmol) in
concd HCl (2 mL) at 08C. A mixture of R (phenol, 2.33 mmol or 2-
hydroxybenzaldehyde, 2.33 mmol) and NaOH (4.66 mmol) was
added slowly to the diazonium salt at 08C and stirred for 10 min.
The reaction mixture was then stirred at room temperature for
30 min and the pH was adjusted to 6 to obtain the solid product.
The solid was isolated by filtration, washed with water and dried
to obtain the desired products 1 (0.446 g) or 2 (0.5 g).
Scheme 3. Synthesis of intermediates 1 and 2.
Synthesis of (E)-1-(4-ethoxyphenyl)-2-p-tolyldiazene (R3;
Scheme 4)
Scheme 6. Synthesis of intermediates 3 and 4.
Anhydrous K2CO3 (0.488 g, 3.53 mmol) and ethyl iodide (0.231 g,
1.48 mmol) were added to a solution of 1 (0.25 g, 1.18 mmol) in
dry acetonitrile (ACN). The reaction mixture was stirred at 508C for
8 h. The completion of reaction was checked by thin-layer chroma-
tography (TLC). Excess of K2CO3 was removed by filtration and the
filtrate was evaporated under reduced pressure to yield pure
brown product R3 (0.226 g).
Synthesis of (E)-1-(4-ethoxyphenyl)-2-{4-[(triphenylphosphi-
no)methyl]phenyl}diazene bromide (R1) and (E)-3-acetyl-6-
({4-[(triphenylphosphino)methyl]phenyl}diazenyl)-2H-chro-
men-2-one bromide (R2; Scheme 7)
A solution of 3 or 4 (0.2 g, 0.63 mmol or 0.3 g, 0.78 mmol) and tri-
phenylphosphine (0.182 g, 0.693 mmol for
3
and 0.225 g,
0.858 mmol for 4) in dry chloroform (20 mL) was heated at reflux
for 4 h and then stirred at room temperature for 8 h. The reaction
mixture was evaporated under reduced pressure to afford a hygro-
scopic residue, which was stirred with diethyl ether until it formed
a solid residue. The solid was isolated by filtration and dried to
give the pure product R1 (0.34 g) or R2 (0.47 g).
Scheme 4. Synthesis of receptor R3.
Characterisation data
(E)-4-(p-Tolyldiazenyl)phenol (1): Yield: 90%; m.p. 135–1368C; FTIR:
n˜ =3356.9 (br m), 3040.3 (m), 2936.4 (m), 1601.2 cmÀ1 (s); elemental
analysis calcd (%) for C13H12N2O: C 73.56, H 5.70, N 13.20; found: C
73.59, H 5.66, N 13.19.
Synthesis of (E)-3-acetyl-6-(p-tolyldiazenyl)-2H-chromen-2-
one (R4; Scheme 5)
Ethyl acetoacetate (0.135 g, 1.04 mmol) was added to a mixture of
2 (0.25 g, 1.04 mmol) and piperidine (0.018 g, 0.21 mmol) in etha-
nol, and stirred for 5 h at room temperature. The completion of re-
action was confirmed by TLC. The solid obtained was isolated by
filtration, washed with ethanol and dried to obtain a yellow pure
product R4 (0.316 g).
(E)-2-Hydroxy-5-[(p-tolylimino)methyl] benzaldehyde (2): Yield:
89%; m.p. 119–1218C; FTIR: n˜ =3364.9 (br m), 3072.3 (w), 2835.7
(w), 1698.9 (s), 1602.3 cmÀ1 (s); elemental analysis calcd (%) for
C14H12N2O2: C 69.99, H 5.03, N 11.66; found: C 70.02, H 5.06, N
11.69.
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Extraction efficiency measurement of receptor R2
A solution of TBAF (1.0ꢁ10À4 m, 1 mL) was treated with recep-
tor R2 solution (1.0ꢁ10À5 m, 19 mL) in absolute dry CH2Cl2 (20 mL
total volume). The absorbance was recorded by a UV/Vis spectrom-
eter. An aqueous NaF solution (1.0ꢁ10À4 m) was prepared and ex-
tracted three times with receptor R2 solution in CH2Cl2. The ex-
tracted non-aqueous solutions were combined, diluted to 20 mL
and the absorbance recorded. The absorbance of both solutions at
573 nm was compared to obtain the extraction efficiency and the
receptor was found to be 99% efficient.
Scheme 7. Synthesis of receptors R1 and R2.
Acknowledgements
(E)-1-(4-Ethoxyphenyl)-2-p-tolyldiazene (R3): Yield: 80%; m.p. 126–
1278C; 1H NMR (500 MHz, [D6]DMSO, 258C): d=7.88 (d, 3J(H,H)=
3
3
We acknowledge The Director and The HOD (Department of
Chemistry), NITK, for providing the research infrastructure. Mad-
huprasad is grateful to NITK for the research fellowship. We
thank the Department of Science and Technology, Government of
India for providing the SCXRD facility under the FIST program,
CSMCRI-Bhavnagar and Manipal Institute of Technology-Manipal
for the spectral analysis. We thank the reviewers for their insight-
ful suggestions and comments to improve the manuscript scien-
tifically.
7 Hz, 2H; Ar-H), 7.77 (d, J(H,H)=8 Hz, 2H; Ar-H), 7.39 (d, J(H,H)=
3
3
8 Hz, 2H; Ar-H), 7.13 (d, J(H,H)=7 Hz, 2H; Ar-H), 4.15 (q, J(H,H)=
7 Hz, 2H; -CH2-), 1.38 ppm (t, 3J(H,H)=7 Hz, 3H; -CH3); FTIR: n˜ =
3041.4 (m), 2940.3 (m), 1602.4 cmÀ1 (s); elemental analysis calcd
(%) for C15H16N2O: C 74.97, H 6.71, N 11.66; found: C 74.95, H 6.69,
N 11.69.
(E)-3-Acetyl-6-(p-tolyldiazenyl)-2H-chromen-2-one (R4): Yield: 95%;
m.p. 177–1788C; 1H NMR (500 MHz, [D6]DMSO, 258C): d=8.82 (s,
3
1H; Ar-H), 8.50 (s, 1H; Ar-H), 8.22 (d, J(H,H)=9 Hz, 1H; Ar-H), 7.85
(d, 3J(H,H)=8.5 Hz, 2H; Ar-H), 7.66 (d, 3J(H,H)=8.5 Hz, 1H; Ar-H),
3
7.44 (d, J(H,H)=8 Hz, 2H; Ar-H), 2.61 (s, 3H; -COCH3), 2.43 ppm (s,
Keywords: colorimetry · extraction · fluoride · receptors ·
water chemistry
3H; -CH3); FTIR: n˜ =3042.4 (m), 2885.9 (m), 1699.4 (s), 1684.4 (s),
1601.5 cmÀ1 (m); elemental analysis calcd (%) for C18H14N2O3: C
70.58, H 4.61, N 9.15; found: C 70.54, H 4.58, N 9.18.
(E)-1-[4-(Bromomethyl)phenyl]-2-(4-ethoxyphenyl)diazene (3): Yield:
90%; m.p. 156–1588C; FTIR: n˜ =3056.9 (m), 2986.4 (m), 1601.4 (s),
1247.3 cmÀ1 (m); elemental analysis calcd (%) for C15H15BrN2O: C
56.44, H 4.74, N 8.78; found: C 56.49, H 4.76, N 8.80.
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(E)-3-Acetyl-6-{[4-(bromomethyl)phenyl]diazenyl}-2H-chromen-2-
one (4): Yield: 91%; m.p. 188–1908C; FTIR: n˜ =3040.2 (w), 2889.3
(m), 1700.3 (s), 1687.4 (s), 1602.5 cmÀ1 (m); elemental analysis calcd
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3.46, N 7.29.
(E)-1-(4-Ethoxyphenyl)-2-{4-[(triphenylphosphino)methyl]phenyl}dia-
1
zene bromide (R1): Yield: 92%; m.p. 146–1478C; H NMR (500 MHz,
[D6]DMSO, 258C): d=7.62–7.94 (m, 23H; Ar-H); 5.30 (d, J=16 Hz,
2H; P-CH), 4.16 (q, J=7 Hz, 2H; -CH2-), 1.38 ppm (t, J=7 Hz, 3H;
-CH3); FTIR: n˜ =3462.3 (m), 3045.6 (m), 2985.7 (m), 1604.3 cmÀ1 (s);
ESI-MS: m/z: calcd: 581.5; found: 581.1; elemental analysis calcd
(%) for C33H30BrN2OP: C 68.16, H 5.20, N 4.82; found: C 67.53, H
5.58, N 4.86.
(E)-3-Acetyl-6-({4-[(triphenylphosphino)methyl]phenyl}diazenyl)-2H-
1
chromen-2-one bromide (R2): Yield: 92%; m.p. 159–1618C; H NMR
(500 MHz, [D6]DMSO, 258C): d=7.24–7.64 (m, 23H; Ar-H); 5.36 (d,
J=16 Hz, 2H; P-CH), 2.42 ppm (s, 3H; -CH3); FTIR: n˜ =3462.3 (m),
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found: 648.5; elemental analysis calcd (%) for C36H28BrN2O3P: C
66.78, H 4.36, N 4.33; found: C 66.23, H 4.68, N 4.73.
CCDC 985440 (R3) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Received: February 13, 2014
Revised: March 18, 2014
Published online on && &&, 0000
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Watercolors: A receptor based on a tri-
phenylphosphonium salt that contains
an active methylene group has been
designed and synthesized for the colori-
metric detection and extraction of FÀ
ions from seawater with 99% efficiency.
The process results in an instantaneous
color change (see figure).
Madhuprasad, D. R. Trivedi*
&& – &&
Receptor with an Active Methylene
Group as Binding Site for Extraction of
Inorganic Fluoride Ions from Seawater
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