Microwave-Assisted Neat Synthesis of a Ferrocene Appended Phenolphthalein Diyne: A Designed Synthetic Scaffold for Hg2+Ion

Article abstract of DOI:10.1021/acs.inorgchem.0c01236

A C2-symmetric internally conjugated 1,3-dialkyne system 5, containing phenolphthalein as a fluorophore and ferrocene as a redox moiety, has been synthesized via a microwave-assisted synthetic procedure. Compound 5 was synthesized by Cu-catalyzed Glaser-H

Full text of DOI:10.1021/acs.inorgchem.0c01236

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Article
Microwave-Assisted Neat Synthesis of a Ferrocene Appended
Phenolphthalein Diyne: A Designed Synthetic Scaffold for Hg2+ Ion
Adwitiya Pal, Krishna Mohan Das, Bappaditya Goswami, and Arunabha Thakur*
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ABSTRACT: A C2-symmetric internally conjugated 1,3-dialkyne
system 5, containing phenolphthalein as a fluorophore and
ferrocene as a redox moiety, has been synthesized via a
microwave-assisted synthetic procedure. Compound 5 was
synthesized by Cu-catalyzed Glaser−Hay coupling using a
microwave reactor in neat condition for the first time. Compound
5 was found to be highly selective toward Fe3+, Cu2+, and Hg2+
ions via multichannels. Interestingly, Fe3+ and Cu2+ ions simply
promote the oxidation of ferrocene unit to ferrocenium ion
without binding to the receptor, whereas Hg2+ binds with the
receptor 5 (ΔE1/2 = 71 mV). The oxidation and binding
phenomena were investigated by optical and electrochemical
analyses. Furthermore, the binding site of Hg2+ ion with our
1
designed probe was confirmed by H, 13C NMR and IR titrations,
which indicated that conjugated dialkyne unit interacts with Hg2+
ion by a favorable soft−soft interaction. Both receptor 5 and its
metal complex, [5·2Hg2+], are stable in the physiological pH range
(pH = 6−7) and thermally stable up to 78 °C. The experimental results of metal binding have been further supported by quantum
chemical calculations (DFT), which explore the favorable geometry of the free ligand as well as its Hg2+ complex.
In spite of knowing for the last few decades that mercury has a
strong propensity toward alkyne, very few examples are known
where alkyne systems have been explored as structural building
blocks for metal ions especially for mercury ion sensing.20,21 In
this regard, a side-on coordination to Hg2+ ions by sterically
accessible π-conjugation has been documented in the case of
mercury−ethynyl interaction.22−24 A soft binding center (base)
prefers to bind to a soft metal ion (acid) according to the HSAB
principle.25 Keeping this idea in mind, we have deliberately
designed a structural motif, consisting of a 1,3-diyne system,
which can interact and detect a soft Lewis acid, mercury.
Mercury is one of the most investigated metal ions in the field of
sensing owing to its extreme toxicity to living species.26−28
Mercury, in any form (e.g., elemental, inorganic, or organic
forms) shows different kinds of toxicity. Hence, the design and
development of any potential probe, capable of detecting the
INTRODUCTION
■
1,3-Diynes represent a unique class of intermediates in organic
synthesis and one of the most vital blocks in many natural
products and functional materials.1,2 Conjugated alkynes are
always advantageous because of their cylindrical electronic
symmetry. The morphological beauty and beneficial properties
of conjugated diynes have fascinated scientists to explore their
scope of synthesis and utilization. The diyne moiety has been
extensively utilized in many fields starting from nanoscience,3
liquid crystals,4−7 to antiviral activities,8 providing a strong
motivation for modern research. Furthermore, they are used in
optoelectronics,9 rotaxanes,10,11 catenanes,12 pharmaceuticals,13
and polymer science.14 Mostly the diyne molecular systems have
been developed so far by alkyne homocoupling and a few by
heterocoupling15 using transition metal (e.g., Cu, Pd, Ni, and
Au) catalysts. Such compounds can be prepared by the
conventional Glaser coupling, Eglinton coupling, Hay cou-
pling,16,17 as also by their modified versions.10−12 As an
advancement of the synthetic methodology, microwave-assisted
reactions in solvent phase18 and in solvent-free medium under
thermal condition19 have also been explored for alkyne−alkyne
coupling reactions. However, a microwave-assisted homoge-
neous synthesis of 1,3-diyne system in neat condition has not
been explored earlier.
Received: April 26, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01236
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Scheme 1. Synthesis of Compounds 3 and 4
Scheme 2. Synthesis of Compound 5
released mercury, would be a step forward in making the
environment safer for living species.
ion. To the best of our knowledge, this is the first report of a
conjugated dialkyne system for the detection of Hg2+ ion. A
series of experiments and DFT calculations unravel the binding
mode of the present probe with Hg2+ ion.
The design of metal ion sensors based on conjugated alkyne
molecules may be the subject of growing interest because of its
configurational and conformational rigidity.29 Therefore, we
have performed homocoupling of triazole-based ferrocene
appended phenolphthalein alkyne in a dry medium to generate
a dialkyne system that can be used as a potential metal ion sensor
having multichannels. Fluorescence spectroscopy is a widely
used and reliable technique for the detection of various metal
ions in a precise and nondestructive manner.30−32 Phenolph-
thalein is taken as the fluorophore moiety so that fluorescence
detection of the metal ion is possible, and ferrocene is appended
to make the molecule redox active. Phenolphthalein is a popular
pH indicator, but its scope of being used as the fluorophore in
chemosensor molecules is not well-explored despite having a
well-defined extended conjugation. In fact, it is not considered as
an efficient fluorophore since it lacks structural rigidity due to
the C−C single bond rotation.33,34 Hence, the easiest way to
turn it into an effective fluorophore is by restricting its feasible
C−C bond rotation. Interestingly, we have circumvented this
issue, which is the root cause for being nonfluorescent, by
functionalizing the hydroxyl groups by bulky substituents, and
this modification has enabled us to convert it to a fluorescent
derivative. On the other hand, the robustness in aerobic
conditions, easy availability, economical affordability, ease of
handling and storage, and most importantly its precise redox
property make the ferrocene unit a topic of intensive research
that has been well-explored.35
RESULTS AND DISCUSSION
■
Synthesis of Compound 5. In this work, we have
developed a conjugated diyne bridged chromophore system
by the homocoupling of two terminal alkynes with the pre-
established protocol of the Eglinton coupling36 with some
advanced modifications. Microwave-assisted synthesis of the
diyne system by the homocoupling of two terminal alkynes are
reported in solvent phase;18 however, use of solvents, especially
chlorinated solvents, make this strategy disadvantageous toward
green synthesis. In this report, we have tried the microwave-
assisted homocoupling of two terminal alkynes in neat condition
at slightly elevated temperature with low catalyst loading to
obtain the desired product. After successfully synthesizing the
conjugated diyne bridged chromophore system via a modified
method, we have investigated the metal-ion-sensing properties
of the synthesized compound.
As shown in Schemes 1 and 2, phenolphthalein, 1, is
converted into its dialkynyl derivative, 2, using K2CO3 as a base
and propargyl bromide in DMSO at room temperature. Further,
copper-catalyzed azide−alkyne [3 + 2] cycloaddition (CuAAC)
reaction of 2 with mono(azidomethyl)ferrocene afforded
compounds 3 and 4 in 35% and 20% yield, respectively.
Conventional Eglinton coupling of compound 3 using Cu-
(OAc)2·H2O and pyridine as a base as well as a solvent afforded
compound 5 in low yield (30%). As an advancement of
conventional Glaser coupling, Eglinton coupling, Hay coupling,
microwave-assisted heterogeneous as well as homogeneous
homocoupling of terminal alkynes is reported either in alumina
In this work, we have developed a ferrocene-appended
phenolphthalein diyne system by microwave synthesis in a neat
condition. The present system, an easy-to-make optical and
electrochemical sensor, displayed a high selectivity toward Hg2+
B
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Table 1. Optimization of Microwave-Assisted Eglinton Coupling Reaction in Our Specified Substrate
a
Sl. no.
equiv of 3
equiv of catalyst
equiv of base/solvent
temperature (°C)
time (minutes)
% conversion
% yield
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
15
15
15
1
2
3
3
3
3.7
118 (5 mL)
118 (5 mL)
118 (5 mL)
118 (5 mL)
118 (5 mL)
118 (5 mL)
118 (5 mL)
118 (5 mL)
3 (19 μL)
60
60
60
60
60
60
25
40
60
10
20
30
30
30
30
30
30
30
20
60
100
20
65
100
10
6
50
90
8
49
90
5
40
80
25
40
a
Catalyst is Cu(OAc)2·H2O.
Figure 1. Absorption spectra of compound 5 (7.8 × 10−6 M) in CH3CN solvent upon addition of up to (a) 2 equiv of Hg2+ (the inset shows the Job’s
plot); (b) 2 equiv of Cu2+ and (c) 2 equiv of Fe3+ at room temperature (the insets show the peak appearing due to oxidation of the ferrocene unit).
surface37 or in solvent phase.18 Here, we wish to report a fast,
solvent free microwave-assisted homogeneous Eglinton cou-
pling of terminal alkynes.
been characterized by 1H and 13C NMR spectroscopy, HRMS,
and elemental analysis.
UV−vis Absorption Studies. The UV−vis spectra of
compound 5 (7.8 × 10−6 M) with stepwise addition of solutions
of Na+, K+, Fe2+, Fe3+, Cu2+, Cu+, Hg2+, Ag+, Ca2+, Mg2+, Mn2+,
Zn2+, Pb2+, Co2+, Cr3+, Al3+ ions (7.8 × 10−6 M) are recorded
separately in CH3CN solvent. From this experiment, we have
observed that the receptor 5 selectively responded toward Hg2+,
Cu2+, and Fe3+ ions (Figure S1) among all the aforementioned
cations. The UV−vis spectra of compound 5 shows two
absorption bands at λmax∼ 228 nm (ε = 81 907 M−1 cm−1) and
273 nm (ε = 13 832 M−1 cm−1), corresponding to the excitation
of the phenolphthalein moiety.38 After the addition of each
aliquot of the metal ions, the absorbance decreases, and it
As shown in Scheme 2 and Table 1, compound 5 can be
achieved in 90% yield in just 30 min by employing microwave
excitation to 50 W at 60 °C with low catalyst loading (3 equiv)
compared with the conventional reaction method (15 equiv)
mentioned earlier. For the further advancement of the reaction
protocol, the reaction was performed in neat condition with only
3 equiv of pyridine (19 μL) as a base and 3.7 equiv of Cu(OAc)2·
H2O, giving 80% conversion, under microwave irradiation. This
is the first report of homogeneous Eglinton coupling in a solvent-
free condition done under microwave irradiation and without
any external solid support. All the synthesized compounds have
C
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Figure 2. (a) Fluorescence emission titration spectra of compound 5 (6.62 × 10−6 M) upon addition of Hg2+ up to 2 equiv at room temperature; inset
picture showing the probable interaction responsible for the quenching. (b) Mechanism of fluorescence quenching by fluorescence lifetime study of 5
with Hg2+ at room temperature: static quenching.
continues to fall gradually up to 2 equiv of each of Hg2+, Cu2+,
and Fe3+ ions (Figure 1). On careful observation of the
interaction of 5 with Cu2+ and Fe3+ ions (Figure 1b,c) inset), it is
found that there occurs a small peak around 630 nm (ε = 7024
M−1 cm−1), corresponding to the oxidation of Fe(II) of
ferrocene unit to Fe(III) of ferrocenium ion.39 This peak
diminishes gradually on addition of L-sodium ascorbate (LAS)
as a reducing agent (Figure S2). This phenomenon clearly
indicates that the oxidation−reduction of the ferrocene unit in 5
is reversible. Interestingly, no such peak at high wavelength was
observed upon addition of Hg2+ ion. It may be concluded that
the kind of interaction of receptor 5 toward Hg2+ ion is different
from that toward Cu2+ and Fe3+ ions. Anticipating that the
interaction can be of binding type, we have calculated the
binding ratio of Hg2+ with probe 5 by binding assays using the
method of continuous variation (Job’s plot) (Figure 1a, inset),
and it was found to be 2:1. Although our several attempts to
obtain the mass spectra of the [5·2Hg2+] complex were
unsuccessful, the binding ratio and composition were confirmed
by elemental analysis of the isolated complex (vide supra).
Fluorescence Studies. To understand the properties of the
synthesized molecule more clearly, fluorescence studies are
indispensable as fluorescence spectroscopy acts as a supreme
and precise analytical tool. We have modified the structure of
phenolphthalein in order to make it a good fluorophore moiety
and to obtain a considerable fluorescence emission intensity.
Fluorescence spectra of 5 in the presence of Na+, K+, Fe2+, Fe3+,
Cu2+, Hg2+, Ag+, Ca2+, Mg2+, Mn2+, Zn2+, Pb2+, Co2+, Cr3+, Al3+
ions (as their perchlorate salts) and Cu+ (as [(CH3CN)4Cu]-
PF6) are recorded separately in CH3CN solution (6.62 × 10−6
M) with the excitation wavelength at 273 nm (low energy).
Although excitation at 228 nm also produced similar results, we
have avoided this high energy absorption band as the excitation
wavelength. No significant change was observed in the emission
spectra of 5 after addition of the metal ions except for Hg2+,
Cu2+, and Fe3+ ions (Figure S3). As shown in Figure 2, the
addition of Hg2+ ion up to 2 equiv exhibited a gradual
diminishing fluorescence emission intensity with a small blue
shift (Δλ = 10 nm) on excitation at 273 nm, and no further
change was observed on addition of Hg2+ ion beyond 2 equiv. It
is known that alkyne units have two perpendicular π-electron
clouds,40 and if two alkyne units are joined by a C−C single
bond, that connecting single bond has slightly reduced bond
length.41,42 Coordination of metal ion in this “triple−single−
triple” bond unit can lead to a perturbation to the conjugation to
a slight extent and eventually may lead to a blue shift in
fluorescence spectra.43 It is evident from 1H, 13C NMR and IR
spectral data that the alternate triple bonds are stable and
prominent in our synthesized probe 5. However, the change of
the emission spectra (Δλ = 10 nm) on addition of Hg2+ ion led
us to conclude that receptor 5 may bind Hg2+ ion through π-
cloud via soft−soft interaction according to HSAB principle.
The reversibility of this interaction of the probe 5 toward Hg2+
ion has also been investigated through the fluorescence
spectroscopy by using aqueous solution of disodium EDTA
(6.62 × 10−6 M) as the chelating agent (Figure S4). However,
repeating the same experiment up to 2 equiv of Cu2+ and Fe3+
ions in a CH3CN solution of compound 5, fluorescence
quenching, with no net shift of the intensity maxima, was
observed (Figures S5, S6). Fe3+ and Cu2+ ions are known as
efficient fluorescence quenchers44,45 may be due to their
paramagnetic nature and Hg2+ ion led to the fluorescence
quenching may be via ligand-to-metal charge transfer (LMCT),
which was verified by TD-DFT calculations (vide supra). To get
further insight into the quenching mechanism for Hg2+,
fluorescence lifetime measurement was performed. As shown
in Figure 2b, lifetime of the undisturbed molecule (1.9 ns)
remains almost constant even after addition of 2 equiv of Hg2+
ion (2.1 ns). Thus, the unchanged lifetime predicts the
formation of a nonfluorescent complex in the ground-state,46
indicating a static quenching phenomenon.
In order to understand the sensitivity of the probe 5 (6.62 ×
10−6 M), we have performed the fluorescence titration using
Hg2+ ion (6.62 × 10−6 M) in CH3CN solution. A detectable
change in fluorescence spectra was observed only in the
presence of 0.015 equiv of Hg2+ ion; hence, the detection limit
(DL) of Hg2+ with probe 5 is 9.9 × 10−8 M. Further, DL can also
be precisely obtained via 3σ/S method,47 where “σ” is the
standard deviation of the blank and S is the slope of the
calibration curve. A good linear relationship between the
fluorescence intensity (data extracted from Figure 2a) and the
concentration of Hg2+ was obtained (Figure S7). Interestingly,
the limit of detection was obtained in the same order as that
obtained from the above method. Further, binding constant
value with Hg2+ ion have been calculated according to Stern−
Volmer equation,48 I/Io = 1 + K[Q] (data plot Figure S8), where
K is called the quenching constant, I is the intensity of
fluorescence at the quencher concentration [Q], and Io is its
value without quencher. The binding constant value was found
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Figure 3. Evolution of (a) CV and (b) DPV of compound 5 (2.5 × 10−4 M) in CH3CN solvent using [(n-Bu)4N]ClO4 as supporting electrolyte when
Hg(ClO4)2 is added up to 2 equiv at room temperature measured at a scan rate of 0.05 V s−1.
Figure 4. 1H NMR titration of compound 5 with Hg(ClO4)2 in d6-DMSO solvent at room temperature.
to be 6.8 × 103 M−1, which indicates a weak interaction of Hg2+
with probe 5.
complex species, [5·2Hg2+]. On the other hand, compound 5
did not exhibit any significant shift of ΔE1/2 upon addition of up
to 2 equiv of Cu2+ and Fe3+ ions (Figures S10, S11), only the
current got diminished with each aliquot of metal addition,
indicating oxidation of ferrocene to ferrocenium ion.51 The DPV
experiments also corroborated the same results as obtained from
CV experiments (Figure 3b). A significant shift in potential of
the complexed species was observed upon addition of Hg2+ ion
and no net shift in potential, but rather current, was observed for
the Fe3+ and Cu2+ ions (Figure S10). The responses of probe 5
toward the three metal ions remain unaffected by the presence of
other metal ions.
Electrochemical Studies. The sole purpose of including
ferrocene into the model of the probe is to carry out the various
electrochemical studies of metal binding phenomenon because
of its inherent reversible electrochemical nature.49,50 Among the
various electrochemical studies known, cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) experiments
are explored for the cation recognition mechanism study for the
receptor 5. A reversible one-electron oxidation process is
observed for the receptor 5 (2.5 × 10−4 M) with a redox wave at
E1/2 = 0.504 V due to oxidation of the Fe(II) of ferrocene to
Fe(III) of ferrocenium ion and vice versa. The electrochemical
behavior of compound 5 was investigated in the presence of
Fe3+, Cu2+, and Hg2+ ions along with other metal ions (as their
perchlorate salts) and Cu+ as [(CH3CN)4Cu]PF6 in CH3CN
(2.5 × 10−4 M) solvent containing 0.1 M [(n-Bu)4N]ClO4 as
supporting electrolyte (Figure S9). No perturbation of the cyclic
voltammogram (CV) was observed except in the case of Fe3+,
Cu2+, and Hg2+ ions. As shown in the Figure 3a, stepwise
addition of Hg2+ up to 2 equiv induced a significant shift of ΔE1/2
= 71 mV of compound 5, indicating the formation of a new
1H and 13C NMR Titrations. From all the above experi-
ments, it may be concluded that the present probe, 5, binds with
Hg2+ ion, but the binding site cannot be assigned unambiguously
as 5 has multiple probable binding sites. Therefore, in order to
find out the actual binding site, 1H NMR titration was conducted
in d6-DMSO at room temperature. Initially, 1H NMR of
compound 5 in d6-DMSO was recorded, and then 2 equiv of
Hg2+ ion in d6-DMSO is added in two instalments consecutively
(1 equiv + 1 equiv). Then the corresponding 1H NMR spectra
were recorded. Surprisingly, as shown in Figure 4, no net shift of
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Figure 5. (a) 13C NMR titration of compound 5 with 2 equiv of Hg2+ ion in d6-DMSO solvent at room temperature. (b) Selected part from 65 to 86
ppm.
Figure 6. (a) Visual color changes observed for receptor 5 (10−4 M) in CH3CN after addition of 1 equiv of several metal cations (10−4 M). (b)
Irradiation of 5 and [5+Hg2+] at 365 nm UV light under darkness in CH3CN solvent.
any of the peaks (not even the peak at 8.18 ppm, corresponding
to −CH proton of triazole unit)52 in the 1H NMR spectra was
observed after the addition of 2 equiv of Hg2+ ion. It imparts the
idea that the heteroatoms (O, N) present in the probe are
probably not contributing to the binding of metal ion; had it
been so, the corresponding adjacent H atoms in the 1H spectra
of the complexed moiety would have been shifted. Nevertheless,
our original quest for determining the actual binding site
unambiguously still remained unanswered.
carbons and triazole carbons show no obvious changes. This
result clearly demonstrates that the conjugated alkyne part plays
a vital role in interacting with the Hg2+ ion. The binding mode
and this proposed mechanism has been further supported by IR
titration and DFT studies (vide supra).
IR Titration. To further verify the proposed binding mode
the IR titration was performed taking all the compounds in the
solid state. Upon gradual addition of Hg(ClO4)2 up to 2 equiv,
the peak at υ = 2086 cm−1, corresponding to CC stretching
frequency, disappeared (Figure S13). This observation
suggested the presence of some interaction between the dialkyne
unit and metal ion, and this result agreed with our previous
observation in 13C NMR titration. Interestingly, no shifts were
observed for the peak at υ = 1752 cm−1, corresponding to the
carbonyl moiety of the lactone ring, indicating no interaction
with the Hg2+ ion. There is also a strong peak at υ = 1085 cm−1,
In order to understand the binding site in detail, 13C NMR
titration was performed to shed some light on the binding mode
for Hg2+ ion. As shown in Figure 5, addition of 2 equiv of Hg2+
into d6-DMSO solution of compound 5 led to the slight upfield
shifts of the signals at 70.35 and 76.19 ppm. The signal
corresponding to 76.19 ppm shifted upfield by 1.67 ppm, and
another signal corresponding to 70.35 ppm was merged with the
chemical shifts of ferrocene carbons. The signals at 70.35 and
76.19 ppm are attributed to the alkyne carbons, which were
confirmed from 1H−13C HSQC spectral data (Figure S12) of 5
in d6-DMSO. All other signals corresponding to aromatic
corresponding to the Cl−O stretching frequency53 of the ClO4
̅
unit, which results because of the inclusion of Hg(ClO4)2 unit in
5. Therefore, combining all the results from NMR and IR
F
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titration, it can be concluded that the plausible binding mode of
Hg2+ ions is the conjugated dialkyne unit in probe 5.
temperature, and it remains in the unperturbed form for
months. To explore the stability over a range of temperatures, we
have recorded fluorescence spectra of both receptor 5 and its
Hg2+ complex, [5·2Hg2+] in different temperatures (25−78 °C
range) in acetonitrile solvent (Figure 8). The investigated results
showed that the fluorescence intensity did not differ significantly
with increasing temperature for both the free 5 (6.62 × 10−6 M)
and [5·2Hg2+] (6.62 × 10−6 M) system in acetonitrile. The
unfluctuation in fluorescence intensities clearly indicated that
the probe as well as the complexed entity are stable to
temperature variation within the mentioned range. Since
acetonitrile is used as the analytical solvent, fluorescence
responses at temperatures above 78 °C could not be recorded.
A fast response time is a promoting factor for a sensor to be
used in real time. Therefore, the response time of compound 5
toward Hg2+ ion (2 equiv) (6.62 × 10−6 M) in CH3CN solution
was investigated by recording the changes of the fluorescence
intensity of the complexed species with time at room
temperature. As depicted in Figure 9, Hg2+ ion interacted with
compound 5 within 1 min of its addition to the ligand solution.
This can be observed from the sudden minimization of
fluorescence intensity of the complexed species within the first
1 min. However, the fluorescence intensity remains almost
unaltered upon rerecording the spectra after certain time
intervals up to 10 min. From this study, we can conclude that the
present probe has the response time of <1 min, and this
information will certainly allow this molecule to be used as a
potential sensor probe.
Competitive Studies. After careful assessment about the
nature of interaction of 5 with the three selected metal ions and
the characteristics of the complexed species, we were curious to
know about the extent of restoration of those individual
interactions in the presence of each other metal ions (Hg2+,
Cu2+, and Fe3+). Therefore, a competitive experiment was
carried out by electrochemical analysis. At first, 2 equiv of Cu2+/
Fe3+ ion was added into the CH3CN solution of 5, and it
promoted the oxidation of ferrocene to ferrocenium ion in probe
5, with no shift in potential in CV. Further, addition of 2 equiv of
Hg2+ to this solution led to a shift in voltammetric wave toward
anodic potential. This experiment indicated that Hg2+ ion is able
to bind with the probe even after the oxidation of Fe(II) to
Fe(III) of ferrocene unit of the probe (Figure 10a,b).
Interestingly, the same result was observed even in the presence
of excess amount of Cu2+/ Fe3+ ions. This experimental result
demonstrated that the binding interaction of Hg2+ ion remained
unchanged in the presence of other responding cations even in
excess. Moreover, we have also performed the experiment in the
reverse way, that is, addition of 2 equiv of Hg2+ followed by the
addition of 2 equiv of Cu2+/ Fe3+ (Figure 10c,d). The results
showed that even after binding to Hg2+, oxidation of the Fe(II)
of ferrocene unit occurs in the presence of Cu2+/ Fe3+ ion.
Therefore, from this competitive experiment, we can conclude
that the course of oxidation and binding are mutually exclusive,
that is, they are independent of each other and do not interfere in
each other’s mechanism (Figure 11).
Naked-Eye Detection. A sensor, having a prominent color-
changing property in the visual range upon interaction with
analytes, is a potential candidate for the practical application,
and this particular property of a sensor makes the preliminary
qualitative analysis easier. To render the synthesized dialkyne
moiety as a sensing probe, using its ground state complexation
property, its visual detectability was qualitatively tested by naked
eye. For that, each aforementioned metal ion (10−4 M) was
added to a 10−4 M ligand solution, and it showed color changes
from yellow to pale bluish green for Hg2+ and to light blue for
Cu2+ and Fe3+ ions, respectively (Figure 6a), supporting the
results obtained from the previous experiments. Thus, naked-
eye detection makes the chemosensor molecule more
susceptible to be applied practically. In fact, the quenching of
fluorescence is also perceptible in the naked eye when the
respective solutions of 5, [5+Hg2+] (Figure 6), [5+Cu2+] and
[5+Fe3+] (Figure S14) are irradiated under a 365 nm UV lamp
in the dark. The irradiated pictures of 5 and [5+Hg2+] are given
in Figure 6b to understand the quenching of fluorescence easily.
pH, Time, and Temperature Effects. Phenolphthalein is
well-known to be pH sensitive,54 and therefore, it has widely
been used as an indicator in acid−base titration. Thus, our
designed probe, containing phenolphthalein, is expected to
show pH sensitivity. Although structural modification in the
phenolphthalein unit was carried out by functionalizing the
phenolic −OH groups, but still there is the lactone moiety that
has pH sensitivity.55,56 Hence, the effects of pH on the
fluorescence response of compound 5 and [5·2Hg2+] complex
are very important to be investigated to assess the overall
applicability of the probe. A wide range of pH solutions from 2 to
12 in acetonitrile were prepared taking 5 (6.62 × 10−6 M) and
[5·2Hg2+] (6.62 × 10−6 M), and their corresponding
fluorescence spectra were recorded immediately. The spectra
show that fluorescence intensities and characteristics of the free
ligand as well as the complexed moiety remain intact in the pH
range of 6−7 but change erratically on moving left or right in the
pH scale. This experiment evidently demonstrated that both
compound 5 and [5·2Hg2+] complex are stable in the biological
pH range of 6−7 (Figure 7).
Thermal stability is another primary condition to be fulfilled
for a molecule to be employed as a potential sensing probe. Our
developed probe has a high degree of stability at room
Real Sample Analysis. In order to investigate the practical
utility, we have exposed our synthesized probe 5 toward the
quantitative detection of Hg2+ ion in real water samples,
collected from Jadavpur University campus. The water samples
were filtered on a Whatman 41 filter paper, and their pH values
were maintained around 6.5−7. Known concentrations of Hg2+
ion were added to the water samples separately, and cyclic
voltammograms were recorded after addition of each aliquot of
Figure 7. Fluorescence response of compound 5 (6.62 × 10−6 M) as a
function of pH (pH 2−12) in the absence and presence of Hg2+ (2
equiv) (6.62 × 10−6 M) metal ion in CH3CN at room temperature.
G
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Figure 8. Temperature dependence of fluorescence intensity of (a) 5 (6.62 × 10−6 M) and (b) [5+Hg2+] (2 equiv) (6.62 × 10−6 M) in CH3CN
solution.
alkyne units, and this observation is in agreement with literature
reports.41,42 Further, it is confirmed by natural bond orbital
(NBO) analysis at the same level of theory. Wiberg bond indices
of the C1−C2, C2−C3, C3−C4, C4−C5, and C5−C6 bonds
are 1.071, 2.610, 1.214, 2.610, and 1.070, respectively. The
detailed bond lengths and Wiberg bond indices are depicted in
Table S1.
Now in order to understand the exact binding site of Hg2+
with receptor 5, we have performed theoretical calculation with
different model systems. We have tried to optimize all the
possible structures of metal complexes with different binding
sites and calculated their corresponding binding energies.
Among all the possible geometries, the structure in which two
Hg2+ ions bind with conjugated 1,3-diyne unit is the energy-
minimized structure with most stable conformation (Figure 13).
This result supports our experimental findings where the ligand
to metal binding ratio was found to be 1:2. The binding energy
of this optimized geometry is significantly high (13.64 kcal/mol)
as compared to the other optimized structures. Another
geometry optimized structure involving O atom of − OCH2-
unit adjacent to alkyne (Figure S18) also exhibits the similar
kind of binding energy (14.43 kcal/mol). However, from the
Figure 9. Time-dependent fluorescence intensity changes of
compound 5 (6.62 × 10−6 M) upon addition of 2 equiv of Hg2+
(6.62 × 10−6 M) in CH3CN solvent at room temperature.
aqueous solution of Hg2+ ion to the ligand solution in CH3CN
(6.25 × 10−5 M). With each addition, shifts in potential were
obtained for both water samples. The corresponding recovered
Hg2+ ion concentrations were calculated from the equations y =
(3642x − 0.014) and y = (2908x − 0.004), with correlation
coefficients R2 = 0.988 and R2 = 0.992 for pond water and tap
water samples, respectively (Figure S15). The experimental
concentration and percentage recovery values are shown in
Table 2.
DFT Studies. In order to get the detailed information about
the electronic structure and to understand the bonding scenario
of the receptor toward Hg2+, we have performed theoretical
studies at the DFT level of theory. At first, few possible starting
geometries are taken to optimize the geometry to obtain the
global minima and geometry optimization of 5 and [5·2Hg2+]
was done by DFT calculations at B3LYP-D3/def2-SVP level of
theory.57 Their corresponding molecular orbital pictures are
shown in Supporting Information (Figures S16 and S17). The
optimized structure of the receptor 5 showed a linear structure
of middle 1,3-dialkyne unit, and the two phenolphthalein-
ferrocene moieties are oriented in trans fashion with respect to
each other (Figure 12). Bond lengths are given respectively as
C1−C2:1.496 Å; C2−C3:1.250 Å; C3−C4:1.407 Å; C4−
C5:1.25 Å; C5−C6:1.496 Å. Analysis of bond lengths indicated
a relatively shorter C3−C4 single bond which connects the two
1
experimental results obtained from H NMR and 13C NMR
titrations, the possibility of this optimized model can be
1
eliminated as no shift of −OCH2− is observed in H NMR as
well as 13C NMR titration of receptor 5 after addition of Hg2+
ion. Furthermore, natural bond analysis (NBO) at B3LYP-D3/
def2-SVP level is performed to get insight of the bonding pattern
of the [5·2Hg2+]. Bond lengths (and WBI) are given as C1−
C2:1.457 Å (1.071); C2−C3:1.220 Å (2.589); C3−C4:1.370 Å
(1.215); C4−C5:1.220 Å (2.589); C5−C6:1.457 Å (1.072).
From the energy minimized structure of the 5 and [5·2Hg2+], we
have observed that there is a change in alkyne bond lengths (1.25
Å to 1.22 Å) and middle C3−C4 bond length (1.40 Å to 1.37 Å)
upon coordination of mercury to the receptor, indicating a weak
interaction of the receptor with Hg2+ ion. To confirm it further
we have done NBO calculation, which depicts a small change in
Weiberg Bond Index from 2.610 to 2.589. This small change in
WBI indicates the weak interaction of π-orbital of alkyne with
the mercury ion.
In the optimized structure, the bond distance between Hg and
carbon atoms are in the range of 3.2−3.6 Å which corroborates
the value reported in the literature.58 Therefore, from H, 13C
1
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Figure 10. Competitive experiments with (a) and (d) Fe3+ and Hg2+; (b) and (c) Cu2+ and Hg2+ in CH3CN solvent at room temperature.
Figure 11. Schematic representation of binding mode with Hg2+ ion and competitive nature of interaction with all three metal ions.
NMR titrations and the combined DFT analysis, it is confirmed
that 2 Hg2+ ions bind with the 1,3-dialkyne unit of the receptor
and provided the more stable conformation.
Further, to rationalize the detailed information about the
UV−vis spectra, time-dependent DFT (TD-DFT) calculations
at the B3LYP-D3/def2-SVP/CPCM (acetonitrile) level were
performed. TD-DFT vertical excited state calculations of the
receptor 5 predicts a HOMO−28 (phenolphthalein π type) to
LUMO+1 (phenolphthalein π* type) transition with the band
centered at 224 nm (oscillator strength f = 0.18) (Table S2),
I
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diyne system, which has been exploited as a Hg2+ ion sensor. The
synthesized molecule, containing ferrocene as redox unit and
phenolphthalein as a fluorophore, was found to be highly
selective toward the detection of Hg2+ ion whereas Fe3+ and
Cu2+ merely promote the oxidation of ferrocene unit. In
addition, our designed probe was successfully explored toward
the detection of Hg2+ ion in real water samples. Two Hg2+ ions
bind with two-alkyne motifs by “soft−soft” interaction and this
mode of binding was supported by NMR titration and DFT
calculations along with optical and electrochemical experiments.
We believe that we have demonstrated a new strategy using
conjugated 1,3-diyne unit for direct detection of Hg2+ using the
concept of HSAB principle.
Table 2. Determination of Recovered Concentrations of Hg2+
Ion in Pond Water and Tap Water Samples
added metal
concn (M)
recovered metal
concn (M)
samples
% of recovery % error
pond
water
tap water
1.69 × 10−5
1.68 × 10−5
99.40
0.5
2.35 × 10−5
2.31 × 10−5
98.29
1.7
which is in close agreement with the experimental results.
Similarly, TD-DFT calculations at the B3LYP-D3/def2-SVP/
CPCM (acetonitrile) level on [5·2Hg2+] predict a strong
HOMO−29 (phenolphthalein π type) to LUMO+4 (phenolph-
thalein π* type) (Table S3) transition with the band centered at
225 nm (oscillator strength f = 0.14), and this is also in close
agreement with the experimental UV−vis results. The calculated
gaps between the orbitals involved in major transitions for both
5 and [5·2Hg2+] complex are in the same range (5.91 and 5.92
eV, respectively), which demonstrates no significant shifts in
UV−vis absorption bands, and this is clearly concurrent with the
experimental findings.
Furthermore, to shed more light on fluorescence quenching
mechanism, TD-DFT calculations were done at the same level of
theory. TD-DFT calculations revealed that the mercury-
centered empty orbital (LUMO) lies between the fluorophore
(phenolphthalein) transition orbitals (Figure 14). As a result,
the electron in the excited state of the phenolphthalein-centered
orbital (LUMO+4) was transferred to the energetically
favorable mercury-centered empty orbital (LUMO) instead of
highly buried HOMO−29 state. Therefore, this type of
orientation of orbitals might be providing an additional pathway
for fluorescence quenching of receptor in the presence of Hg2+
ion.
EXPERIMENTAL SECTION
■
Materials and Reagents. Among the metal ions used, Na+, K+,
Fe2+, Fe3+, Cu2+, and Hg2+ were purchased from Sigma-Aldrich, and
Ag+, Ca2+, Mg2+, Mn2+, Zn2+, Pb2+, Co2+, Cr3+, and Al3+ were purchased
from Alfa Aesar, as their respective perchlorate salts, and used directly
without further purification. Cu+ was purchased from Sigma-Aldrich as
[(CH3CN)4Cu]PF6. NaH was purchased from Sigma-Aldrich, and
Glacial AcOH was obtained from Merck. DMF and acetonitrile
(HPLC) were purchased from Thermo Fisher Scientific and freshly
distilled prior to use. The remaining chemicals were purchased from a
local brand. Mono(azidomethyl)ferrocene was prepared according to
literature reported procedure.59 Chromatography was carried out using
60−120 mesh silica gel in a column of 2.5 cm diameter. All the
necessary solvents were dried by conventional methods and distilled
under N2 atmosphere. The cyclic voltammetry (CV) was performed
with a conventional three-electrode configuration consisting of glassy
carbon as working electrode, platinum as an auxiliary electrode, and Ag/
Ag+ as a reference electrode. The experiment was carried out with a 10−4
M solution of sample in CH3CN containing 0.1 M (TBAP) [(n-
C4H9)4NClO4] as supporting electrolyte. The working electrode was
cleaned after each run. The cyclic voltammograms were recorded at a
scan rate 0.05 V s−1, and readings were taken considering ferrocene/
ferrocenium couple as the standard. The UV−vis spectra were carried
out in CH3CN solutions at c = 10−6 M and the fluorescence spectra
were carried out at c = 10−6 M, as stated in the corresponding figure
captions.
CONCLUSIONS
■
Herein, we have designed and synthesized a C2-symmetric
internally conjugated 1,3- dialkyne system via microwave-
assisted synthetic procedure in neat condition for the first
time. The microwave-assisted synthesis of the probe in solution
phase as well as in neat condition has paved the way for further
modulation of greener ways in synthetic methodology. To the
best of our knowledge, this is the first report of a conjugated 1,3-
Preparation of IR Samples. The compound 5 and 1 and 2 equiv of
Hg(ClO4)2 were weighed separately and dissolved in acetonitrile and
then mixed together correspondingly to form 5+1 equiv of Hg2+ and
5+2 equiv of Hg2+. The acetonitrile was then dried in a rotary
Figure 12. (a) Optimized structure of the receptor 5. Zoomed structure of middle unit of receptor 5 to understand the bonding (inset). (b) Chemdraw
structure of the conjugated binding unit before and after binding with Hg2+ along with the C−C bond lengths.
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Figure 13. Optimized structure of the complex [5·2Hg2+].
used for analysis. Anal. Calcd for C74H56Fe2N6O8Hg2: C, 53.10; H,
3.34; N, 5.02. Found: C, 54.04; H, 4.01; N, 5.54.
1
Instrumentation. H and 13C NMR spectra were obtained with
BRUKER 400 and 300 MHz FT-NMR spectrometers, respectively. For
1
compound 2, both H and 13C NMR spectra have been taken in
BRUKER 300 MHz FT-NMR spectrometer. The chemical shifts are
reported in ppm, using tetramethylsilane as an internal standard, and
were referenced to the residual solvent as follows: CDCl3 = 7.26 (1H),
76.16 (13C) ppm, DMSO = 2.51 (1H), 39.50 (13C) at room
temperature. For 1H NMR, coupling constants J are given in Hz, and
the resonance multiplicity is described as s (singlet), d (doublet), t
(triplet), m (multiplet). IR spectral studies were done with a
PerkinElmer LX-1 FTIR spectrophotometer. The absorption spectra
were recorded with a SHIMADZU-2450 UV−vis spectrophotometer at
room temperature. Fluorescence was recorded with SHIMADZU RF-
5301pc spectrophotometer. CH Instruments Electrochemical Analyzer
was used to perform the cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) studies. HRMS were taken using Quadruple-TOF
(Q-TOF) micro MS system using electrospray ionization (ESI)
technique. CHN analysis was performed on a Vario EL elementar
CHNS analyzer. Microwave optimization has been done in CEM
microwave.
Figure 14. Schematic representation of fluorescence quenching due to
Caution! Metal perchlorate salts are potentially explosive in certain
conditions. All due precautions should be taken while handling perchlorate
salts!
Hg2+ ion binding to the receptor.
Synthesis of Compounds 2, 3, and 5. Synthesis of Compound
2. Phenolphthalein (500 mg, 1.57 mmol) was taken in a clean round-
bottomed flask and dissolved in 10 mL of DMSO solvent. K2CO3 (867
mg, 6.28 mmol) was added to the solution and kept to stir for 30 min
after which propargyl bromide (0.5 mL, 6.28 mmol) was added and
stirring was continued overnight. The reaction mixture was extracted
with EtOAc and dried over Na2SO4, and the solvent was dried under
vacuum. Column chromatography in 20% EtOAc in hexane gave pure
compound 2 (3,3-bis(4-(prop-2-yn-1-yloxy)phenyl)isobenzofuran-
evaporator and stored under vacuum for some time. Thus, we obtained
two sets of complexed species in the solid form. The IR spectra of the
free ligand with these two sets of samples were recorded in solid form.
Preparation of Samples for Elemental Analysis of the
Complex Species. For the elemental analysis of the [5·2Hg2+]
complex, pure 5 was taken in a minimum amount of dichloromethane;
then 2 molar equiv of Hg(ClO4)2 was dissolved in CH3CN, and the two
solutions were mixed up gradually. The solvent mixture was evaporated
in a rotary evaporator and then stored under vacuum for some time.
The resulting green solid compound was washed several times with
water to remove free perchlorate anion, followed by washed with
dichloromethane and kept under high vacuum for 5 h. This sample was
1
1(3H)-one) (499 mg, 80% yield). H NMR (CDCl3, 300 MHz) δ =
7.91 (d, 1H, Haromatic, J = 6 Hz), 7.70−7.67 (m, 1H, Haromatic), 7.58−7.52
(m, 2H, Haromatic), 7.30−7.25 (m, 4H, Haromatic), 6.96−6.91 (m, 4H,
K
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H
H
aromatic), 4.68 (d, 4H, J = 2.4 Hz, OCH2), 2.53 (t, 2H, J = 2.4 Hz,
alkyne); 13C NMR (CDCl3,75 MHz) δ = 169.78, 157.63, 152.37,
was put to microwave radiation in the reactor under 50 W power
maintaining a constant 60 °C for 30 min. After reaction vessel was
cooled, it was taken out, and the color of the reaction mixture was
observed to change from green (before the reaction) to yellowish green
(after the reaction). The reaction mixture was poured into a vessel
containing cold water (7 mL) and was left for sometime, after which
brown precipitate can be observed. It was filtered and washed
successively with water (5 × 10 mL) and finally with ethanol (2 mL).
The precipitate was redissolved in DCM and further purification was
done by silica gel column chromatography after which pure 5 was
obtained (91 mg, 0.072 mmol, yield 90%).
134.25, 133.92, 129.34, 128.57, 125.96, 125.49, 124.07, 114.72, 91.42,
78.32, 75.95, 55.81; HRMS m/z calcd for C26H18O4 [M + Na]+,
417.1102; found, 417.0885.
For the neat reaction, 3 (50 mg, 0.078 mmol) was grinded well with
3.7 equiv of Cu(OAc)2·H2O (58 mg, 0.29 mmol) until a homogeneous
powder was formed. Then it was poured into the glass tube for a closed
vessel reaction system along with 3 equiv of pyridine as base (19 μL,
0.234 mmol). It was mixed properly and then subjected to microwave
irradiation at 50 W power and 60 °C for 30 min. Then it was cooled
down and washed successively with water to remove any traces of
pyridine. Finally, work up in EtOAc and removing the solvent under
vacuum after drying on sodium sulfate gave compound 5 with 80%
conversion of compound 3.
Synthesis of Compound 3. 3,3-Bis(4-(prop-2-yn-1-yloxy)phenyl)-
isobenzofuran-1(3H)-one (2) (1g, 2.53 mmol) and 0.9 equiv of
mono(azidomethyl)ferrocene (554.48 mg, 2.3 mmol) were taken in a
100 mL round-bottom flask and dissolved in tBuOH (26 mL) solvent.
CuSO4·5H2O and sodium-L-ascorbate were dissolved in a total of 13
mL of H2O and added to the reaction mixture one by one at room
temperature. After 1 h, a greenish suspension was formed, which was
allowed to stir overnight at room temperature. Reaction mixture was
diluted with ethyl acetate and washed with distilled water. The organic
phase was separated and dried over sodium sulfate, and the solvent was
removed under reduced pressure. Then the crude product was purified
by silica gel column chromatography. Elution with 40% EtOAc/hexane
gave yellow colored solid compound 3 (550 mg, 0.86 mmol, yield 35%)
along with some amount of compound 4 (0.5 mmol, yield 20%) as side
1H NMR (CDCl3,400 MHz)δ = 7.91 (d, 1H, J = 8 Hz, Haromatic), 7.67
(t, 1H, J = 8 Hz, Haromatic), 7.53(t, 1H, J = 8 Hz, Haromatic), 7.49 (s, 1H,
H
triazole), 7.48 (s, 1H, Haromatic), 7.21 (t, 4H, J = 8 Hz, Haromatic), 6.91−
6.85 (m, 4H, Haromatic), 5.26 (s, 2H, OCH2), 5.13(s, 2H, OCH2), 1.63(s,
2H, NCH2), 4.26(s, 2H, HFc), 4.21(s, 2H, HFc), 4.16(s, 5H, HFc); 13
C
NMR (d6-DMSO,75 MHz) δ = 169.41, 158.26, 157.22, 152.19, 142.77,
135.38, 134.07, 133.22, 130.20, 128.45, 128.42, 125.87, 124.70, 124.55,
118.49, 115.08, 91.16, 82.63, 76.19, 70.35, 68.95, 68.91, 68.71, 61.26,
56.01, 49.31; HRMS (ESI) m/z calcd for C74H56Fe2N6O8 [M + H]+,
1269.2937; found, 1269.2935; Anal. Calcd for C74H56Fe2N6O8: C,
70.03; H, 4.45; N, 6.63. Found: C, 69.43; H, 4.38; N, 6.39.
1
product. 3: H NMR (CDCl3,400 MHz) δ = 7.93 (d, 1H, J = 8 Hz,
H
aromatic), 7.69 (t, 1H, J = 8 Hz, Haromatic), 7.57 (s, 1H, Haromatic), 7.55 (s,
1H, Htriazole), 7.51 (d, 1H, J = 8 Hz, Haromatic), 7.22−7.28 (m, 4H,
), 6.92 (d, 4H, J = 8 Hz, Haromatic), 5.30 (s, 2H, OCH2), 5.15 (s,
H
aromatic
2H, NCH2), 4.68 (s, 2H, OCH2), 4.28 (s, 2H, HFc), 4.22 (s, 2H, HFc),
4.17 (s, 5H, HFc), 2.53 (s, 1H, Halkyne); 13C NMR (CDCl3,75 MHz) δ =
169.77, 158.32, 157.61, 152.42, 143.67, 134.12, 133.97, 133.52, 129.25,
128.65, 128.49, 125.96, 125.55, 124.02, 122.15, 114.70, 114.64, 91.45,
80.72, 78.27, 75.80, 69.09, 68.92, 62.061, 55.83, 50.16, 30.91; HRMS
m/zcalcd for C37H29FeN3O4 [M]+ 635.1507; found 635.1900; Anal.
Calcd for C37H29FeN3O4: C, 69.93; H, 4.60; N, 6.61. Found: C, 69.63;
H, 4.48; N, 6.42.
Computational Studies. All computational calculations presented
in this paper were performed by density functional theory (DFT)
method using the Gaussian 1660 program package. Full geometry
optimizations were carried out using the B3LYP (Becke, three
parameter, Lee−Yang−Parr)-D3 level of theory.61 The def2-SVP57
basis set was employed for all the atoms with the conductor-like
polarizable continuum model62 (CPCM) using acetonitrile as a solvent.
The vibrational frequency calculations were performed to ensure that
the optimized geometries represent the local minima and that there are
only positive eigenvalues. Natural bond orbital (NBO) analysis63 and
Wiberg bond indices (WBI)64 were performed at the same level57,61
using the NBO Version 3.1 program implemented in the Gaussian
package. Orbital diagrams are rendered in the Chemcraft visualization
software with an isosurface value 0.04.65
Synthesis of Compound 5. The coupling reaction of compound 3
had been performed by two ways: one by slight modification of the
already established procedure36 and another by taking the assistance of
a single focus microwave in a closed reaction system.
By the conventional method: Compound 3 (206 mg, 0.324 mmol)
and 15 equiv of Cu(OAc)2·H2O (971.25 mg, 4.86 mmol) were added in
a 100 mL round-bottom flask and dissolved in pyridine solvent (15
mL). The reaction mixture was stirred at room temperature for 2 h and
then at 60 °C for 2 h. The blue reaction mixture was cooled to room
temperature, and the stirring was continued for further 48 h. Cold water
was added after which brown precipitate appeared, and the reaction
mixture was stirred for another 10 min. Filtration of the brown
precipitate under suction and successive washings with water and finally
with ethanol gave us dry pyridine free product, which on further
purification by silica gel column chromatography (60% EtOAC/Hex)
gave a pale yellow colored solid compound 5 (61 mg, yield 29.67%).
Microwave-assisted reaction: This was done in closed reaction
system whereby compound 3 (100 mg, 0.16 mmol) was taken into the
closed system vessel containing a rice magnet, dissolved in pyridine
solvent (5 mL), and then the catalyst Cu(OAc)2·H2O (3 equiv, 100 mg,
0.5 mmol) was added. When the catalyst got dissolved totally, the vessel
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01236.
■
sı
1H, 13C and HRMS data of compound 2, 3 and 5. UV−vis
spectra, fluorescence spectra and cyclic voltammetric plot
with Fe3+, Cu2+ as also with different metal ions,
quantitative binding data and LOD by 3σ/S for 5 with
Hg2+, DFT optimized alternate structure of binding of
Hg2+ with 5, bond distance table and excited state
transitions with oscillator strength tables for both 5 and
L
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AUTHOR INFORMATION
■
Corresponding Author
Arunabha Thakur − Department of Chemistry, Jadavpur
University, Kolkata 700032, India; orcid.org/0000-0003-
4577-3683; Phone: 0332-4572779;
Email: arunabha.thakur@jadavpuruniversity.in,
babuiitm07@gmail.com
Authors
Adwitiya Pal − Department of Chemistry, Jadavpur University,
Kolkata 700032, India
Krishna Mohan Das − Department of Chemistry, Jadavpur
University, Kolkata 700032, India
Bappaditya Goswami − Department of Chemical Sciences, Indian
Institute of Science Education and Research Kolkata, Mohanpur
741246, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01236
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
RUSA 2.0 is gratefully acknowledged for financial support to
carry out the research work. AP and KMD acknowledge CSIR
and UGC for providing JRF fellowships respectively. BG is
thankful to the IISER Kolkata for the fellowship.
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