Isophorone-boronate ester: A simple chemosensor for optical detection of fluoride anion

Article abstract of DOI:10.1002/aoc.4688

A highly selective isophorone-boronate ester based chemosensor, (1), having a dicyanovinyl moiety as a convenient colorimetric probe, has been designed. Different types of anionic analyte such as CH₃COO^?, ClO₄^?,

Full text of DOI:10.1002/aoc.4688

Received: 20 August 2018
DOI: 10.1002/aoc.4688
Revised: 10 October 2018
Accepted: 15 October 2018
F U L L P A P E R
Isophorone‐boronate ester: A simple chemosensor for
optical detection of fluoride anion
1,3,4
1
1,3
Jakku Ranjith Kumar
| Eda Rami Reddy
| Rajiv Trivedi
|
2
2
3,4
3,4
Anil Kumar Vardhaman | Lingamallu Giribabu | Nedaossadat Mirzadeh | Suresh K. Bhargava
1
Catalysis and Fine Chemicals Division,
A highly selective isophorone‐boronate ester based chemosensor, (1), having a
CSIR‐Indian Institute of Chemical
Technology, Uppal Road, Tarnaka,
Hyderabad 500007, India
dicyanovinyl moiety as a convenient colorimetric probe, has been designed.
−
−
−
−
−
Different types of anionic analyte such as CH COO , ClO , Cl , F , PF ,
3
4
6
2
Polymer and Functional Materials
−
−
−
Br and HSO4 were tested and among them only highly nucleophilic F
Division, CSIR‐Indian Institute of
Chemical Technology, Uppal Road,
Tarnaka, Hyderabad 500007, India
anion displayed significant response towards the sensor. Addition of the fluo-
ride anion across the boron atom disrupts the π‐conjugation thereby shifts
the absorption wavelength towards the redshift region due to the decrease in
the HOMO‐LUMO energy gap and a colour change from yellow to blue is
observed under visible light condition. The detection limit of this probe was
calculated to be 3.25 × 10—8 M for fluoride anion. The binding constants and
the detection limits of the sensor were calculated using absorption titration
studies. The silica gel TLC strips dip‐coated by the chemosensor (1) revealed
a colour change from yellow to brick red to naked eye.
3
IICT‐RMIT Centre, CSIR‐ Indian
Institute of Chemical Technology, Uppal
Road, Tarnaka, Hyderabad 500007, India
4
Centre for Advanced Materials and
Industrial Chemistry (CAMIC), School of
Science, RMIT University, GPO Box 2476,
Melbourne 3001, Australia
Correspondence
Rajiv Trivedi, Catalysis and Fine
Chemicals Division, CSIR‐Indian Institute
of Chemical Technology, Uppal Road,
Tarnaka, Hyderabad 500007, India.
Email: trivedi@iict.res.in;
KEYWORDS
boron derivatives, chemosensor, fluoride anion sensing, optical Spectroscopy
rtrajiv401@gmail.com
[
5–8]
1
| INTRODUCTION
developed.
Apart from the role of fluoride anion in
the dental health as well as treatment of osteoporosis,
the fluoride overexposure can adversely affect bones,
brain, thyroid gland and pineal gland etc. Moreover,
the use of excess fluoride can also cause acute gastric
One of the most potential and active research field in
supramolecular chemistry has been the advancement of
optical chemosensor for neutral and charged species
[1]
[9]
detection.
Owing to its tremendous importance in
and kidney problems.
environmental and biological purposes, the selective
detection of inorganic anions has catapulted as an
important area of research during the past few
Typical methods employed to analyse fluoride
anion, involving the use of sophisticated instrumental
methods, such as titrimetric, voltammetric, potentiomet-
ric, electrochemical as well as ion exchange chromatog-
[2]
decades. The variations in the physical properties of
anions such as size, shape, charge distribution and the
solvation in polar and protic solvents make the detection
[
10]
raphy often exhibit high detection limits.
While these
techniques are time‐consuming, optical sensors which
rely on either colour change or fluorescence intensity
variations have been studied over the past few
[3]
of anion as an intriguing subject. The chemosensors
relying on the colorimetric and/or optical response
can be transformed into a cheap, operationally simple,
[
4,10]
decades.
Design of anion‐selective chemosensors is
[4]
low detection limit technologies.
A
range of
often based on the high nucleophilicity of fluoride
anions.
host/guest strategies to bind fluoride anion have been
Appl Organometal Chem. 2018;e4688.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
1 of 11
https://doi.org/10.1002/aoc.4688

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KUMAR ET AL.
By virtue of its high Lewis acid character, the boron
can disrupt the π‐conjugation thereby decreasing the
HOMO‐LUMO gap along with a redshift in the absorp-
tion spectra.
[
11]
centre has played key a role in frustrated Lewis pairs,
[12,13]
[14]
anion sensing
and carbohydrate detection.
However, there exists several means and ways to
increase the Lewis acidic character of boron atom in an
The isophorone unit present in the organoboron
derivative has been extensively studied for its sensing
abilities. This unit acts as a colour reporting group having
unique optical and biocompatibility. Isophorone is a well‐
known push‐pull charge transfer dye chemosensor which
can undergo intramolecular charge transfer (ICT). Lee
et al. reported a group of isophorone‐based fluorescent
[15]
ensemble.
These include for instance, linking of
electron withdrawing groups such as perfluorinated aryl
[
13,16]
12a, 17
moieties,
coulombic interactions with cations,
[18]
boron incorporated anti‐aromatic systems
containing extended π‐conjugated arrays.
and boron
As illus-
[19]
[
23]
trated in (Figure 1), different types of boron containing
fluorescent probes have been studied in diverse
materials as red emitters for use in OLEDs.
A series
of isophorone based near‐infrared solid state emitters
[20]
fashion.
Moreover, the Lewis‐acid based approaches
have been reported, and all of the crystalline compounds
[
24]
are most effective due to their ability for quick detection,
easy visualization and sensitivity.
were fluorescent in the solid state.
Linking
dicyanovinyl isophorone unit onto a Boron moiety might
augment enhanced sensing capabilities.
A typical Lewis acid probe consists of a binding site
and a covalently linked signalling unit. The binding site
captures the anion and subsequently the signalling unit
exhibits an optical or fluorescent change indicating anion
detection. The binding affinity, sensitivity and detection
Recently our group has designed multichannel sen-
+
2
+2
+2
+2
sors for the detection of Hg , Co , Cu and Zn
[
25,26]
cations using absorption and emission spectroscopy.
Hence, herein, we describe the synthesis, characteriza-
tion, anion sensing abilities and DFT calculations of a
pinacol boronate linked dicyano isophorone sensor (1).
−
limit of the F anion can be tuned by modulating the
[21]
binding site as well as the signalling unit.
In order to
design a turn‐on receptor, it is necessary to tune the
electronic interaction around boronic esters. One of
the promising approaches can be the introduction of an
electron‐withdrawing unit π‐conjugated with a tri‐
coordinated boron ester. The polarised π‐system thus
produced can significantly enhance the intramolecular
2 | RESULTS AND DISCUSSION
The pinacol boronate linked dicyano isophorone sensor,
(1) was synthesized in three steps as shown in Scheme 1.
The dicyanovinylene substituted isophorone (L‐1) was
prepared in the first step followed by the pinacol protec-
tion of 4‐formyl phenyl boronic acid (L‐2) according to
the previous reports. The Knoevenagel Condensation of
L‐1 and L‐2 under standard conditions formed the desired
[22]
charge‐transfer interactions.
Keeping this in mind, a
boron containing compound having a conjugated organic
signalling unit and a sensitive pinacol boronate binding
site has been designed. The coordination of a strong
−
Lewis base, like F anion onto the vacant Boron p‐orbital
FIGURE 1 Different types of boron containing chemosensors suitable for anion detection

KUMAR ET AL.
3 of 11
2
.1 | UV–Vis. Spectroscopic Studies of
−
sensor (1) in the absence and presence of F
anion
Sensor (1) was highly stable at 25 °C and was soluble in
tetrahydrofuran (THF), dichloromethane (DCM) and
acetonitrile (ACN). The solution of (1) appeared
yellow in colour (FIGURE S13 in the Supporting
Information) in THF and exhibited a band at 425 nm
−
1
−1
(ε = 73333 M
cm ) in the absorption spectra.
{(Figure 2) and (FIGURE S14 in the Supporting Informa-
tion)} The sensing ability of the chemosensor (1) was
examined by using colorimetric and fluorescence titration
experiments. It was observed that among most of the
−
−
−
−
6
−
−
anions such as OAc , ClO , Cl , PF , F , HSO and
4
4
−
SCHEME 1 Synthesis of Boron based chemosensor (1). Reagents
and reaction conditions used:Step (i): Piperidine, Dry Ethanol,
Br ions, the chemosensor (1) responded spontaneously
to F anion (TBAF is the source of fluoride ion).
−
6
0 °C, 8 hours. Step (ii): L‐1, Pinacol, Diethyl ether, 25 °C, 24 hrs.
Step (iii): Knoevenagel Condensation of L‐1 and L‐2, Dry Ethanol,
0 °C, 8 hrs
4
sensor (1). The L‐1, L‐2 and sensor (1) were characterized
1
13
by various spectroscopic tools such as, H NMR,
C
11
NMR, B NMR, HRMS, and IR analyses (Figure S1‐S12
in the Supporting Information). In the H NMR spectra,
1
the proton chemical shifts of the phenyl ring appeared
as two doublets at δ 7.75 ppm and δ 7.45 ppm, while those
for the conjugated ethylene protons appeared as a singlet
at δ 6.99 ppm and the conjugated olefinic proton of the
isophorone moiety appeared at δ 6.79 ppm. The two
methylene units of isophorone appeared as two doublets
around δ 2.53–2.40 ppm and a singlet at δ 1.01 ppm
appeared for the two methyl groups of isophorone. In
13
the C NMR spectra, the aromatic phenyl carbon
appeared as singlets around δ 123.25 ppm and δ
1
1
15.31 ppm. The ethylene carbons appeared at δ
02.15 ppm and the pinacol carbons were observed in
11
the regions δ 4.25 ppm and δ 10.71 ppm. The B NMR
spectra of the boronate ester (1) exhibit a broad singlet
at δ 22.29 ppm which suggests the presence of boronate
esters in the chemosensor (FIGURE S8 in the Supporting
Information). In addition, the probe (1) was also con-
firmed by HRMS (Figure S10 in the Supporting Informa-
tion). Similar way (1.F)— was also confirmed by B NMR
and HRMS mass spectrometry (Figure 4) (FIGURE S9,
S11 in the supporting information).
11
The coordination of Lewis bases at the Boron centre
results in the modification of the electronic properties
of the Boron centre and thus changes the UV–Vis.
absorption behaviour of (1). The complexation studies
were carried out with the fluoride anion as a strong
Lewis base.
FIGURE 2 Fluorescence emission spectral changes observed
when variable concentrations (1 × 10 ‐9 × 10 M) of F—
−7
−6
−6
solution were added to the solution of (1) (3 × 10 M) (TOP). UV–
vis. Spectral changes observed when variable concentrations (1 ×
10 ‐9 × 10 M) of F— solution were added to the solution of (1) (3
−
7
−6
−
6
× 10 M) (BOTTOM)

4
of 11
KUMAR ET AL.
The colour of sensor (1) changes from yellow to blue
FIGURE S13 in the Supporting Information) and gener-
and simultaneous rise of a new intense band at 676 nm
with two clear isosbestic points at 360 nm, and 490 nm
was observed. {Figure 2 (TOP)} HOMO‐LUMO energy
(
−
ates a new F adduct on addition of tetrabutylammonium
−
−
—
fluoride (TBAF) (Scheme 2). The F anion‐adduct (1.F)
gap of the (1) and (1.F) was found to be 2.50 eV and
exhibited absorption at 670 nm, which can be assigned to
the strong nucleophilic strength of F anion. {Figure 2
1.73 eV respectively. These energy gaps were calculated
from the onset absorption wavelengths 495.4 nm,
715.5 nm of (1) and (1.F), respectively.
−
−
(
TOP) (FIGURE S15 in the Supporting Information)}
−
Further, various concentrations of F anions were
titrated and the corresponding absorptions were
recorded. Significant spectral changes were observed in
From the absorption titrations and Benesi–Hildebrand
[
27]
(B‐H) plot (equation 1 in the experimental section), the
5
−1
binding constant (K) was found to be K = 1.73 × 10 M
−
−
the UV–Vis. spectra. On addition of the F anion, a
for the complex (1.F) formed in the reaction. The
detection limit was obtained from the UV–Vis. data. The
standard deviation σ = 0.002 was determined from the
deviation in absorption spectra recorded over the concen-
gradual decrease of the band at 425 nm was observed
−
7
−6
tration range (1 × 10 M ‐ 9 × 10 M) on titration with
the fluoride salt. The slope from the absorbance versus
5
concentration plot was calculated to be k = 2.76 × 10 for
the fluoride anion (FIGURE S16 in the Supporting Infor-
mation). The chemosensor (1) was found to possess certain
virtues such as sensitivity, stability, and ambient working
−
SCHEME 2 Sensing of fluoride anion
conditions. A ratio of 1:1 for (1) and F anion was
obtained from the Job's plot using absorption titration
experiment. (FIGURE S18 in the Supporting Information).
−
8
−
The detection limit was calculated as 3.25 x 10 M for F
anion (FIGURE S19 in the Supporting Information).
2
.2 | Fluorescence Emission Studies of
−
sensor (1) in the absence and presence of F
anion
Further, in the fluorescence spectra, a weak band at
80 nm was obtained upon excitation of (1) at 435 nm,
5
while a similar excitation at 570 nm failed to produce any
spectra. However, the fluorescence titrations by varying
F anion concentrations led to an incremental increase of
−
FIGURE 3 Electrochemical [differential pulse voltammogram
−
(
dpv)] titration experiment of (1) with F ion
emission band at 693 nm on exciting at 570 nm. {Figure 2
1
1
FIGURE 4
CDCl ) and in the presence of one
equivalent of fluoride anion (1.F)
B NMR spectra of (1)
(
3
−

KUMAR ET AL.
5 of 11
(
BOTTOM)} The binding affinity of the fluoride anions
were evaluated from the (B‐H) plot and equation (2) in
the experimental section, while the binding constant (K)
5
−1
were determined to be K = 2.59× 10 M for the complex
−
(
1.F) (FIGURE S17 in the Supporting Information). The
standard deviation σ = 0.003 was determined from the
deviation observed when fluorescence intensity and con-
centration spectra were recorded over the concentration
−
7
−9
−6
range (1 × 10
M
× 10 M) on titration with the
fluoride salts. The slope from the fluorescence intensity
versus concentration plot was calculated to be k = 1.025
11
×
10
was calculated to be 8.77× 10
(FIGURE S20 in the Supporting Information).
and for the fluoride anion detection limit
—14
M for F— anion.
FIGURE 5 Chemical structure and DFT Optimized Geometry of
−
Chemosensor (1) and complex (1.F)
−
FIGURE 6 DFT energy levels diagram of (1) and (1F)

6
of 11
KUMAR ET AL.
2
.3 | Electrochemical Titration Studies of
The ground state optimized geometry of (1) undergoes
drastic twisting, upon interaction with F anion, from
—
—
Probe 1 with F anion
trigonal planar to tetrahedron boron. This twist increases
the Lewis base character of boron which enhances the
ICT of overall system, resulting in a considerable redshift
in the UV–Vis spectrum.
The electrochemical (dpv) titration experiments were
performed between (1) and F anions. Chemosensor (1)
−
has an oxidation potential at 1.27 V vs SCE and upon
subsequent titration with multiples of 0.33 eq. up to a total
of 1 eq. of fluoride anion, new peaks appeared at 0.112 V
The different BLA is observed in the conjugated car-
−
bon chain in (1). The binding of F to the chemosensor
−
and 0.446 V for F anion vs. SCE. The corresponding peak
(1) decreases the BLA upon formation of the complex
−
of the sensor (1) diminished in dpv on addition of up to one
(1.F) . This perhaps originates from the collective elec-
—
equivalent of F anion. This indicated the formation of
tron donating effects of the tetrahedral boron moiety
−
−
1
:1 ratio of the corresponding complex (1.F) . (Figure 3)
(BpinF) and the electron‐rich π‐bridge. (FIGURE S23
in the Supporting Information)
The HOMO‐LUMO energy levels of (1) as shown in
Figure 6 were calculated using this method. The
HOMO‐LUMO energy gap of probe (1) was 3.05 eV and
drastically decreasing after fluoride anion binding (1.F)
1
1
2
.4 | B NMR Studies
In order to confirm the coordination of the Lewis acidic
Boron on sensor (1) with the incoming F anion,
NMR was run in CDCl . An upfield shift in the NMR
signal was observed.
Addition of TBAF generates the corresponding adduct
1.F) . The formation of this complex is confirmed by an
upfield shift in the BNMR resonance at δ 4.85 ppm
Figure 4) as typically found in four‐coordinate aryl
−
11
B
−
is 1.34 eV these energy gaps are agreed with Experi-
3
mental UV–Vis. Spectra (Table S1 in the Supporting
Information). Compound (1) has only one probable bind-
ing sitei.e., the boron end, hence the optimized geome-
tries for (1) suggested that the electron density on both
HOMO and LUMO levels was located on electron
withdrawing vinyl cyanide end thereby generating an
electron deficiency at the boronic ester end which initi-
−
(
11
(
[28,29]
boronate esters.
—
ates the binding of F ions. According to the electro-
2.5 | Theoretical Studies
static potential (ESP) data, it was found that the boron
atom was more prone to the binding of F anion than
—
The mechanism was explained for the interaction between
dicyanovinyl quaternary carbon. ESP surfaces of mole-
cules (1) and (1.F) with Mulliken atomic charges at
—
−
(
1) and F anion using density functional theory (DFT)
calculations. These calculations are performed with
B3LYP functional using 6–31 + G(d) basis set with
[
28]
Gaussian 09 program package.
The optimized geome-
−
tries of (1) and (1.F) are shown in Figure 5. Through
the DFT calculations, the energy gap, dipole moment
−
and Bond length alteration (BLA) of probe (1) and (1.F)
were observed. However, the HOMO‐LUMO energy gap
−
(
eV) and dipole moment of (1.F) was found to be smaller
than the free probe (1) (Table S1 in the Supporting
Information).
FIGURE
coordination complex (1.F)
8
Plausible mechanism for the formation of
−
FIGURE 7 ESP surfaces of (1) and (1.F)— (isovalue = 0.0004)

KUMAR ET AL.
7 of 11
FIGURE 9 Demonstration for F— ions
sensing (a) Solutions colours of (1) in the
presence of various anions (b) Strips
colour under the white light (c) Strips
colour under the UV light (long
wavelength)
dicyano quaternary carbon and boron in the conjugate
2.6 | Silica Gel Dip‐Strips of Probe (1) to
−
(
1) are shown in Figure 7. The Boron atom on the ester
detect F anions
with the Mulliken charge of +0.877 has more positive
potential than that of dicyano quaternary carbon
−
Superior sensing capability of (1) towards F ion
detection in solution phase was observed with the bare
eye {Figure 9 (a)} and further used for the in‐field device
level applications, wherein, solution of (1) was introduced
on the silica gel sheets and the response was inspected
(
Mulliken charge = +0.6930).
Similarly, TD‐DFT for (1) and (1.F)— was calcu-
lated to know the electronic contribution from
HOMO‐LUMO in different level. This was performed
with B3LYP/6–31 + G(d) basis set and TD‐DFT calcu-
−
towards F anion sensing. The colour was clearly identi-
[
29]
lations included the solvent effect (CPCM).
The
fied with the naked eye when the silica gel sheet
colour changed from yellow to brick red {Figure 9 (b)}
(FIGURE S26 in the Supporting Information) in the
presence of white light and also the distinguished fluores-
cent behaviour was observed under the portable UV lamp
{Figure 9 (c)}. These results can be helpful to the develop-
ment of a selective fluoride chemosensors of practical
convenience.
−
results of transitions of (1) and (1.F) are summarized
TABLE S2 in the Supporting Information. The calcu-
lated wavelengths in the absorption spectra of (1) and
−
(
1.F) were 437 nm and 1424 nm (FIGURE S24, S25
in the Supporting Information) respectively with
oscillator strengths of 1.67 and 0.32. The predicted
absorption spectra were commensurate with the
experimentally observed ones.
Finally, following the experimental (stoichiometric
UV–vis. and electrochemical titrations) and DFT studies,
a plausible mechanism can be proposed in the Figure 8,
wherein, in the first step, on addition up to one equiva-
3 | CONCLUSIONS
In conclusion, we have successfully synthesized newly
designed highly sensitive Lewis acidic (Boron) based
chemo sensor by introducing cyclicborane unit into pre-
cursor compound's signalling unit. The chemo sensor
(1) was well characterized by various spectroscopic tech-
—
—
lent of F anion to the chemosensor (1), the F anion
gets coordinated with the Lewis acidic B atom, without
the cleavage of five member cyclic ester ring, to form a
stable four coordinate boron adduct as reported in the
[
30]
1
13
11
literature.
Upon addition of another equivalents of
niques such as IR, H NMR, C NMR, B NMR, and
—
−
F
anion to previous adduct, no changes were observed
in B‐NMR and absorption spectrum. Thus, it can
HRMS. Further (1) was used for F anion sensing studies
in solvent media. By using UV–Vis. fluorescence, and
11
be concluded that the combination of sensor (1) and
electrochemical titrations the binding constants, detec-
tion limits, and stoichiometry between (1) and F were
—
−
F
anion in 1:1 ratio forms the four coordinated B. F
adduct.
evaluated. Application of probe (1) was showed in the

8
of 11
KUMAR ET AL.
practical use such as in‐field device level utilization.
slowly poured into cold water (200 ml) and extracted with
CH Cl , organic layer was evaporated and recrystalliza-
Based on the experimental and theoretical results
2
2
−
probable mechanism for F anion binding with probe
tion from hexane affords a brown solid. Yield: 4.5 g
1
(
1) was proposed.
(90%). H NMR (400 MHz, CDCl , ppm): δ 6.62 (dd,
3
J = 2.8, 1.4 Hz, 1H, ‐CH=), δ 2.51 (s, 2H, ‐CH ‐), δ 2.17
2
(
(
s, 2H, ‐CH ‐), δ 2.03 (d, J = 1.0 Hz, 3H, ‐CH ), δ 1.01
2
3
4
| EXPERIMENTAL SECTION
13
s, 6H, ‐CH of ‐CMe ). C NMR (101 MHz, CDCl3,
3
2
ppm): δ 170.40, 159.78, 120.60, 113.20, 112.42, 45.70,
2.66, 32.39, 27.84, 25.33.
All chemicals such as isophorone, malononitrile, 4‐
formyl boronic acid, pinacol, piperidine, fluoride and
other TBA salts were purchased from Sigma Aldrich
and used as supplied. All the reactions were carried out
in the absence of air using standard Schlenk techniques
unless stated otherwise. Solvents were deoxygenated,
purified and dried prior to use. Sensing experiments were
carried out and monitored in freshly distilled tetrahydro-
4
4.2 | Synthesis of compound 4‐(4,4,5,5‐
tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)
benzaldehyde (L‐2)
The mixture of 4‐formyl boronic acid (1.5 g, 10 mmol),
anhydrous pinacol (1.2 g, 11.0 mmol) in anhydrous diethyl
ether (15 ml) was stirred under nitrogen atmosphere till the
starting materials disappeared (detected by TLC plate).
After completion of the reaction, reaction mixture was
washed with water; the ether phase was collected and dried
1
13
furan (THF). H and C NMR spectra were recorded at
room temperature on Bruker Avance 400 (or) Varian
Inova 500 spectrometer in CDCl as solvent; Chemical
3
shifts (δ) for protons were reported in ppm down field
from TMS as internal standard and the carbon shifts were
13
referenced to the C signal of CDCl3 at 77.0 ppm.
Coupling constants (J) were expressed in Hz. MALDI
were recorded on a Shimadzu Biotech Axima. FT‐IR
spectra were recorded on a Thermo Nicolet Nexus 670
using spectrophotometer using KBr discs. Melting points
were determined using a Toshniwal apparatus and are
uncorrected. The UV–vis. Spectra were recorded on a
UV 3600 (Shimadzu) spectrophotometer over the range
with MgSO . The organic fraction was dried to give the pale
4
1
yellow solid compound. Yield: 92%. H NMR (500 MHz,
CDCl , ppm): δ 10.05 (s, 1H, ‐CHO), δ 7.96 (d, J = 8.1 Hz,
3
2H, Ar‐CH), δ 7.88–7.83 (m, 2H, Ar‐CH), δ 1.36 (s, 12H, ‐
1
3
CH of pinacol (4Me)). C NMR (101 MHz, CDCl , ppm):
3
3
11
δ 192.69, 138.13, 135.24, 128.72, 84.36, 24.91. B NMR:
(128 MHz, CDCl , ppm) δ 30.69 ppm.
3
of 200–550 nm in CH CN and THF solution. The cyclic
3
4
.3 | Synthesis of compound (E)‐2‐(5,5‐
voltammetry (CV) was performed with a conventional
three‐electrode configuration consisting of glassy carbon
working electrode, platinum wire auxiliary electrode
and saturated calomel (SCE) reference electrode. The
cyclic voltammograms were recorded on CHI620 model
electrochemical analyser in the presence of 0.1 M
tetrabutylammonium perchlorate (TBAP) supporting
dimethyl‐3‐(4‐(4,4,5,5‐tetramethyl‐1,3,2‐
dioxaborolan‐2‐yl)styryl)cyclohex‐2‐en‐1‐
ylidene) malononitrile (1)
Under argon, 2‐(3,5,5‐trimethylcyclohex‐2‐enylidene)
malononitrile (0.186 g, 1 mmol) and the corresponding
4
‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl) benzalde-
hyde (0.234 g, 1 mmol) was dissolved in dry ethanol
50 ml). Piperidine (0.01 mmol) is added and the solution
is stirred at 45 °C till starting material disappeared
detected by TLC plate). The solution is concentrated
−
1
electrolyte at a scan rate of 0.1 Vs . Elemental analyses
were performed with an Elementar Vario MICRO ana-
lyzer. For HRMS, m/z values were expressed in atomic
mass units.
(
(
and the product is purified by flash column chromatogra-
1
4
.1 | Synthesis of compound 2‐(3,5,5‐
phy (hexane: ethyl acetate 10:1, v/v). H NMR (500 MHz,
trimethylcyclohex‐2‐en‐1‐ylidene)
malononitrile (L‐1)
CDCl
(t, J = 10.0 Hz, 2H, Ar‐CH‐), δ 6.99 (s, 2H, ‐CH=CH‐), δ
.79 (d, J = 6.8 Hz, 1H, =CH‐), δ 2.53 (d, J = 6.2 Hz,
2H, ‐CH ‐), δ 2.40 (d, J = 9.9 Hz, 2H, ‐CH ‐), δ 1.28
3
, ppm): δ 7.75 (d, J = 8.1 Hz, 2H, Ar‐CH‐), δ 7.45
6
A catalytic amount of piperidine (23 mg, 0.276 mmol)
was added to the solution of isophorone (3.8 g, 27.6 mmol)
and malononitrile (1.82 g, 27.6 mmol) in dry ethanol
2
2
(s, 12H, ‐CH of pinacol (4Me)), δ 1.01 (s, 6H, ‐CH of
3
3
13
CMe2). C NMR (101 MHz, CDCl , ppm): δ 155.34,
3
(
6
150 ml) and resulting clear solution was stirred at
0 °C under nitrogen atmosphere till starting material
140.53, 123.15, 115.31, 112.38, 108.34, 102.15, 44.35,
31.10, 28.91, 25.12, 17.91, 17.46, 13.91, 11.63, 10.71, 4.25.
B NMR (128 MHz, CDCl , ppm): δ 22.29. IR (KBr)
3
11
disappeared (detected by TLC plate). The reaction mix-
ture was cooled to room temperature, the solution was
3195.74 (=C‐H), 2940.21 (=C‐H), 2816.76 (30‐C‐),

KUMAR ET AL.
9 of 11
−
2
1
218.82 (‐O‐C‐), 1600.32 (‐CH=CH‐) 1553.76 (=CN),
016, 765 (‐CH3), 740 (‐CH3) cm . HRMS
titration with varying F anion concentrations, K is the
binding constant determined from the slope of the linear
plots from the constructed graphs.
−
1
[
M + H] + = 401.00 (calculated = 400.32).
4
.4 | UV–Vis. and Fluorescence
4.7 | Evaluation of Detection Limits for
Probe (1)
Spectroscopic Studies
−
3
−
Stock solutions of (1) (1 × 10 M) and F anion (1 ×
Detection limits were determined by using UV–Vis. titra-
tion data between probe (1) and F anion. The standard
−
3
−
1
0
M) were prepared in anhydrous THF. For every
new experiment the stock solution of (1) was diluted to
deviations (σ) were calculated from the absorption spec-
−
6
3
× 10 M and 3 mL was transferred into a quartz
trum, upon averaging ten titrations between the sensor
—
cuvette, subsequently, various concentrations of anionic
solution was added with micropipette and the absorption
and emission spectra was recorded after the thorough
mixing of the solution. Emission spectral data was
collected between 435/850 nm upon exciting at 422 nm
and 570 nm wavelengths respectively.
and F anion, and the slope k was obtained from the
graph which was plotted between absorbance and
−
concentration of the [F ]. Further obtained values were
−
substituted in equation (3) to determine exact F anion
detection limit of sensor (1).
Detection limit ¼ 3 σ=k
(3)
4
.5 | Detection of Binding Constant for
—
the Complex (1.F) Formation
4.8 | Electrochemical Studies of the
Chemosensor (1)
Probe (1) solution was prepared in the THF solvent to get
−
6
final concentration 3 × 10 M and used for the UV–Vis.
−
3
Electrochemical titration experiments of (1) in the absence
and fluorescence spectral titration studies. 1 × 10
M
−
and presence of F were carried out using differential pulse
concentrations of anionic stock solutions were prepared
voltammogram (dpv) method (FIGURE S22 in the
Supporting Information). The concentration of the
chemosensor (1) was used 1 × 10 M and anionic solutions
and titrated with (1) by varying final concentrations
−
7
−5
between 5.5 × 10 to 1.45 × 10 M.
−3
−3
−3
were maintained in the range of 1 × 10 M to 3 × 10 M.
4
.6 | Calculations to evaluate Binding
Constants by Using Spectroscopic Data
ACKNOWLEDGMENTS
Binding constants were calculated for the formation of (1.
F) complexes by substituting UV–Vis. and Fluorescence
spectral titration data in the Benesi‐Hildebrand (B‐H)
plot equations 1 and (2).
This work was financially supported by the CSIR‐IICT (in‐
house project MLP‐0007) and DST‐(EMR/2016/006410).
AKV acknowledges SERB‐NPDF (PDF/2016/001158). J.
Ranjith Kumar is grateful to the IICT‐RMIT Centre for the
award of a research fellowship. The authors are thankful
to Dr. A. V. S. Sharma for recording the B NMR spectra.
The authors are thankful to Mr. Govind Reddy for assisting
in fluorescence experiment as well as the DFT calculations.
−
1
=ðA − A0Þ ¼ 1=fK ðAmax
̶
A0Þ Cg þ 1=ðAmax
̶
A0Þ (1)
(2)
11
1
=ðI − I0Þ ¼ 1=fK ðImax
̶
I0Þ Cg þ 1=ðImax
̶
I0Þ
Where A , A are absorption intensities of (1) of the
0
absorption maximum at λ = 678 nm observed in the
absence and presence of F anion at a certain concentra-
tion (C). Amax was the maximum absorption intensity
ORCID
−
Jakku Ranjith Kumar
655
http://orcid.org/0000-0003-3110-
7
value that was obtained at λ = 678 nm, during titration
Eda Rami Reddy http://orcid.org/0000-0002-5162-445X
Rajiv Trivedi http://orcid.org/0000-0002-2256-710X
−
with varying F concentrations, K was the binding con-
stant which was determined from the slope of the linear
plots in constructed graphs. Also, I and I are emission
0
intensities of sensor (1) at emission maximum at
REFERENCES
−
6
93 nm in the absence and presence of F anion at a
[
1] a) S. Lee, K. K. Y. Yuen, K. A. Jolliffe, J. Yoon, Chem. Soc. Rev.
2015, 44, 1749; b) J. F. Zhang, Y. Zhou, J. Yoon, J. S. Kim,
Chem. Soc. Rev. 2011, 40, 3416; c) Y. Zhou, S. Lee, Z. Xu, J.
certain concentration (C). Imax is the maximum emission
intensity value that was obtained at λ = 693 nm during

10 of 11
KUMAR ET AL.
Yoon, Chem. Rev. 2015, 115, 7944; d) N. Busschaert, C.
Caltagirone, W. Van Rossom, P. A. Gale, Chem. Rev. 2015,
[8] For bidentate boron‐centered Lewis acids, see, for example a)
V. C. Williams, C. Dai, Z. Li, S. Collins, W. E. Piers, W. Clegg,
M. R. J. Elsegood, T. B. Marder, Angew. Chem. Int. Ed. 1999, 38,
3695; b) M. V. Metz, D. J. Schwartz, C. L. Stern, P. N. Nickias,
T. J. Marks, Angew. Chem. Int. Ed. 2000, 39, 1312; c) V. C.
Williams, G. J. Irvine, W. E. Piers, Z. M. Li, S. Collins, W.
Clegg, M. R. J. Elsegood, T. B. Marder, Organometallics 2000,
1
4
15, 8038 e) P. A. Gale, C. Caltagirone, Chem. Soc. Rev. 2015,
4, 4212.
[
[
2] a) H. Li, J. Fan, X. Peng, Chem. Soc. Rev. 2013, 42, 7943; b) K. P.
Carter, A. M. Young, A. E. Palmer, Chem. Rev. 2014, 114, 4564;
c) X. Li, X. Gao, W. Shi, H. Ma, Chem. Rev. 2014, 114, 590; d) P.
Mahato, S. Saha, P. Das, H. Agarwalla, A. Das, RSC Adv. 2014,
19, 1619; d) W. E. Piers, G. Irvine, V. C. Williams, Eur. J. Inorg.
Chem. 2000, 2000, 2131; e) J. D. Hoefelmeyer, M. Schulte, M.
Tschinkl, F. P. Gabbaı, Coord. Chem. Rev. 2002, 235, 93; f)
L. D. Henderson, W. E. Piers, G. J. Irvine, R. McDonald,
Organometallics 2002, 21, 340.
4
, 36140; e) R. Saini, N. Kaur, S. Kumar, Tetrahedron 2014, 70,
4
285; f) R. M. Duke, E. B. Veale, F. M. Pfeffer, P. E. Kruger, T.
Gunnlaugsson, Chem. Soc. Rev. 2010, 39, 3936.
3] a) F. P. Schmidtchen, M. Berger, Chem. Rev. 1997, 97, 1609; b)
J. L. Sessler, P. A. Gale, W. S. Cho, Anion Receptor Chemistry,
Royal Society of Chemistry, Cambridge, U.K 2006; c) P. D.
Beer, P. A. Gale, Angew. Chem. Int. Ed. 2001, 40, 486; d) W. J.
Marshall, S. K. Bangert, Clinical Chemistry, 5th ed., Elsevier,
Edinburgh 2004; e) R. Martinez‐Manez, F. Sancentin, Coord.
Chem. Rev. 2006, 250, 3081.
[
9] a) L. K. Kirk, Biochemistry of the Halogens and Inorganic
Halides, Plenum Press, New York 1991; b) D. Briancon, Rev.
Rhum. 1997, 64, 78; c) Y. Michigami, Y. Kuroda, K. Ueda, Y.
Yamamoto, Anal. Chim. Acta 1993, 274, 299; d) J. Aaseth, M.
Shimshi, J. L. Gabrilove, G. S. Birketvedt, J. Trace Elem. Exp.
Med. 2004, 17, 83; e) R. Carton, J. Fluoride 2006, 39, 163.
[
10] a) T. T. Christison, J. S. Rohrer, J. Chromatogr. A 2007, 1155,
[
4] F. Wang, L. Wang, X. Chen, J. Yoon, Chem. Soc. Rev. 2014,
31; b) Z. Xu, X. Chen, H. N. Kim, J. Yoon, Chem. Soc. Rev.
4
3, 4312.
2010, 39, 127; c) Y. M. Chung, B. Raman, D.‐S. Kim, K. H.
[
5] For recent reviews of anion binding, see, for example a) A. P.
de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M.
Huxley, C. P. McCoy, J. T. Rademacher, T. E. Rice, Chem.
Rev. 1997, 97, 1515; b) P. A. Gale, J. L. Sessler, V. Kral,
Chem. Commun. 1998, 1; c) J. J. Lavigne, E. V. Anslyn, Angew.
Chem. Int. Ed. 2001, 40, 3118; d) R. Martínez‐Manez,
F. Sancernon, Chem. Rev. 2003, 103, 4419.
Ahn, Chem. Commun. 2006, 2, 186; d) A. Safavi, N. Maleki,
H. R. Shahbaazi, Anal. Chim. Acta 2004, 503, 213; e) D. Shan,
C. Mousty, S. Cosnier, Anal. Chem. 2004, 76, 178; f) T. Suzuki,
A. Hioki, M. Kurahashi, Anal. Chim. Acta 2003, 476, 159; g) V.
K. Rao, S. R. Suresh, N. B. S. N. Rao, P. Rajaram, Bull.
Electrochem. 1997, 13, 327.
[
[
11] a) D. W. Stephan, G. Erker, Angew. Chem. Int. Ed. 2010, 49, 46;
b) I. Peuser, R. C. Neu, X. Zhao, M. Ulrich, B. Schirmer, J. A.
Tannert, G. Kehr, R. Frçhlich, S. Grimme, G. Erker, D. W.
Stephan, Chem. Eur. J. 2011, 17, 9640.
[
6] For previous examples of fluoride binding by boronic
acid/esters, see, for example a) Y. Kubo, A. Kobayashi, T.
Ishida, Y. Misawa, T. D. James, Chem. Commun. 2005, 2846;
b) Y. Kubo, T. Ishida, A. Kobayashi, T. D. James, J. Mater.
Chem. 2005, 15, 2889; c) R. Badugu, J. Lakowicz, C. D. Geddes,
Curr. Anal. Chem. 2005, 1, 157; d) R. Badugu, J. R. Lakowicz, C.
D. Geddes, Sensors Actuators B Chem. 2005, 104, 103; e) J. K.
Day, C. Bresner, N. D. Coombs, I. A. Fallis, L.‐L. Ooi, S.
Aldridge, Inorg. Chem. 2008, 47, 793; f) E. Galbraith, T. M.
Fyles, F. Marken, M. G. Davidson, T. D. James Inorg, Chem
12] a) T. W. Hudnall, C.‐W. Chiu, F. P. Gabbaï, Acc. Chem. Res.
2
2
009, 42, 388; b) Z. M. Hudson, S. Wang, Acc. Chem. Res.
009, 42, 1584; c) C. R. Wade, A. E. J. Broomsgrove, S.
Aldridge, F. P. Gabbaï, Chem. Rev. 2010, 110, 3958; d) E.
Galbraith, T. D. James, Chem. Soc. Rev. 2010, 39, 3831; e) R.
Nishiyabu, Y. Kubo, T. D. James, J. S. Fossey, Chem. Commun.
2011, 47, 1106; f) S. Yamaguchi, S. Akiyama, K. Tamao, J. Am.
2
008, 47, 6236; g) R. Ting, C. W. Harwig, J. Lo, Y. Li, M. J.
Chem. Soc. 2001, 123, 11372.
Adam, T. J. Ruth, D. M. Perrin, J. Org. Chem. 2008, 73, 4662;
h) R. Ting, C. Harwig, U. auf dem Keller, S. McCormick, P.
Austin, C. M. Overall, M. J. Adam, T. J. Ruth, D. M. Perrin,
J. Am. Chem. Soc. 2008, 130, 12045.
[13] a) T. W. Hudnall, F. P. Gabbaï, J. Am. Chem. Soc. 2007, 129,
11978; b) Y. Kim, H. Zhao, F. P. Gabbaï, Angew. Chem. Int.
Ed. 2009, 48, 4957. Angew. Chem. 2009, 121, 5057; c) A. E. J.
Broomsgrove, D. A. Addy, A. DiPaolo, I. R. Morgan, C. Bresner,
V. Chislett, I. A. Fallis, A. L. Thompson, D. Vidovic, S.
Aldridge, Inorg. Chem. 2010, 49, 157; d) Y. Kim, T. W. Hudnall,
G. Bouhadir, D. Bourissou, F. P. Gabbaï, Chem. Commun.
[
7] For fluoride binding by other three‐coordinate boron‐
containing systems, see, for example a) G. E. Herberich, U.
Englert, A. Fischer, D. Wiebelhaus, Eur. J. Inorg. Chem. 2004,
4
011; b) Z. Q. Liu, M. Shi, F. Y. Li, Q. Fang, Z. H. Chen, T.
2009, 3729.
Yi, C. H. Chuang, Org. Lett. 2005, 7, 5321; c) I. H. A. Badr, M.
E. Meyerhoff, J. Am. Chem. Soc. 2005, 127, 5318; d) T. Agou,
J. Kobayashi, T. Kawashima, Org. Lett. 2005, 7, 4373; e) S. Xu,
K. Chen, H. Tian, J. Mater. Chem. 2005, 15, 2676; f) T. Agou,
J. Kobayashi, T. Kawashima, Inorg. Chem. 2006, 45, 9137; g)
E. Sakuda, A. Funahashi, N. Kitamura, Inorg. Chem. 2006, 45,
[14] a) T. D. James in Boronic Acids (Ed.: D.G. Hall), Wiley‐VCH,
Weinheim, 2005, pp. 441–480; b) W. Ni, H. Fang, G.
Springsteen, B. Wang, J. Org. Chem. 2004, 69, 1999. c) T. D.
James, K. Sandanayake, S. Shinkai, Angew. Chem. Int. Ed. Engl.
1996, 35, 1910. Angew. Chem. 1996, 108, 203– d) L. I. Bosch, T.
M. Fyles, T. D. James, Tetrahedron 2004, 60, 11175. e) T. D.
James, Top. Curr. Chem. 2007, 277, 107. f) S. D. Bull, M. G.
Davidson, J. M. H. van den Elsen, J. S. Fossey, A. T. A. Jenkins,
Y.‐B. Jiang, Y. Kubo, F. Marken, K. Sakurai, J. Zhao, T. D.
James, Acc. Chem. Res. 2013, 46, 312. g) K. Lacina, P. Skládal,
T. D. James, Chem. Cent. J. 2014, 8, 60.
1
0670; h) A. Li, A. Tani, J. Shimada, Y. Yamamoto, J. Am.
Chem. Soc. 2008, 130, 4222; i) R. Boshra, A. Doshi, F. Jakle,
Organometallics 2008, 27, 1534; j) Q. Zhao, F. Li, S. Liu, M.
Yu, Z. Liu, T. Yi, C. Huang, Inorg. Chem. 2008, 47, 9256; k)
G. Zhou, M. Baumgarten, K. Mullen, J. Am. Chem. Soc. 2008,
1
30, 12477.

KUMAR ET AL.
11 of 11
[
15] a) A. Y. Timoshkin, G. Frenking, Organometallics 2008, 27,
71; b) K. Huynh, J. Vignolle, T. D. Tilley, Angew. Chem. Int.
[24] a) Y. Ha, J.‐H. Seo, Y. K. Kim, Synth. Met. 2008, 158, 548; b)
B.‐S. Li, J. Li, Y.‐Q. Fu, Z.‐S. Bo, J. Am. Chem. Soc. 2004, 126,
3430; c) Y.‐S. Yao, Q.‐X. Zhou, X.‐S. Wang, Y. Wang, B.‐W.
Zhang, Adv. Funct. Mater. 2007, 17, 93; d) J.‐Y. Li, D. Liu, Z.
Hong, S.‐W. Tong, P.‐F. Wang, C.‐W. Ma, O. Lengyel, C.‐S.
Lee, H.‐L. Kwong, S. Lee, Chem. Mater. 2003, 15, 1486.
3
Ed. 2009, 48, 2835. Angew. Chem. 2009, 121, 287–
[
[
[
[
16] J. O. Huh, H. Kim, K. M. Lee, Y. S. Lee, Y. Do, M. H. Lee,
Chem. Commun. 2010, 46, 1138.
17] a) C. R. Wade, F. P. Gabbaï, Dalton Trans. 2009, 9169;
b) Y. Kim, F. P. Gabbaï, J. Am. Chem. Soc. 2009, 131, 3363.
[
25] a) C. K. Kumar, R. Trivedi, K. R. Kumar, L. Giribabu, B.
Sridhar, J. Organomet. Chem. 2012, 718, 64; b) C. K. Kumar,
R. Trivedi, K. R. Kumar, L. Giribabu, B. Sridhar, Eur. J. Inorg.
Chem. 2013, 2013, 6019.
18] S. Yamaguchi, T. Shirasaka, S. Akiyama, K. Tamao, J. Am.
Chem. Soc. 2002, 124, 8816.
[26] C. K. Kumar, R. Trivedi, S. Niveditha, K. Bhanuprakash,
19] a) I. B. Sivaev, V. I. Bregadze, Coord. Chem. Rev. 2014, 270‐271,
B. Sridhar, J. Organomet. Chem. 2015, 780, 20.
7
5; b) S. Miyasaka, J. Kobayashi, T. Kawashima, Tetrahedron
Lett. 2009, 50, 3467.
[27] H. A. Benesi, J. H. Hildebrand, J. Am. Chem. Soc. 1949, 71,
703.
2
[
20] a) Z. Guo, I. Shin, J. Yoon, Chem. Commun. 2012, 48, 5956; b)
D. Li, Y. Chen, Z. Liu, Chem. Soc. Rev. 2015, 44, 8097; c) R.
Badugu, J. R. Lakowicz, C. D. Geddes, Talanta 2005, 70,
[28] a) C. G. Zhan, J. A. Nichols, D. A. Dixon, J. Phys. Chem. A 2003,
107, 4184; b) F. Zhang, S. R. Wang, X. G. Li, Y. Xiao, L. J. Pan,
Adv. Mater. Res. 2014, 43, 252.
1
421; d) T. Agou, M. Sekine, J. Kobayashi, T. Kawashima,
J. Organomet. Chem. 2009, 694, 3833; e) J. O. Huh, Y. Do, M.
H. Lee, Organometallics 2008, 27, 1022; f) L. Wang, L. Li, D.
Cao, Sensors Actuators B Chem. 2017, 239, 1307.
[
29] a) V. Barone, M. Cossi, J. Phys. Chem. A 1998, 102, 1995; b) M.
Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem. 2003,
24, 669.
[
21] a) X. J. Li, X. Y. Zhang, X. M. Ma, D. Wu, Y. Zhang, B. Du, Q.
Wei, Biosens. Bioelectron. 2014, 55, 330; b) L. Zhang, S. Li, M.
Hong, Y. Q. Xu, S. S. Wang, Y. Liu, Y. Qia, J. Zhao, Org. Biomol.
Chem. 2014, 12, 5115; c) S. P. Wang, W. J. Deng, D. Sun, M.
Yan, H. Zheng, J. G. Xu, Org. Biomol. Chem. 2009, 7, 4017; d)
F. Huo, Y. Zhang, P. Ning, X. Meng, C. Yin, J. Mater. Chem.
B 2017, 5, 2798; e) J. Y. Park, S. Y. Gwon, N. S. Yoon, Y. A.
Son, S. H. Kim, Fibers Polymer 2011, 12, 692.
[30] T. Neumann, Y. Dienes, T. Baumgartner, Org. Lett. 2006, 8, 495.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
[
22] a) A. Oehlke, A. A. Auer, I. Jahre, T. Rìffer, P. Zoufal, H. Lang,
S. Spange, J. Org. Chem. 2007, 72, 4328; b) A. Oehlke, A. A.
Auer, K. Schreiter, K. Hofmann, F. Riedel, S. Spange, J. Org.
Chem. 2009, 74, 3316; c) N. DiCesare, J. R. Lakowicz, J. Phys.
Chem. A 2001, 105, 6834; d) N. DiCesare, J. R. Lakowicz, Anal.
Biochem. 2002, 301, 111.
How to cite this article: Ranjith Kumar J, Rami
Reddy E, Trivedi R, et al. Isophorone‐boronate
ester: A simple chemosensor for optical detection of
fluoride anion. Appl Organometal Chem. 2018;
e4688. https://doi.org/10.1002/aoc.4688
[
23] T. Liu, J. Lin, Z. Li, L. Lin, Y. Shen, H. Zhu, Y. Qian, Analyst
2
015, 140, 7165.