Design and synthesis of new functionalized 8-(thiophen-2-yl)-1,2,3,4-tetrahydroquinolines as turn-off chemosensors for selective recognition of Pd2+ions

Article abstract of DOI:10.1039/d0nj02272a

We have developed two novel chemosensors 6-morpholino-8-(thiophen-2-yl)-1,2,3,4-tetrahydroquinoline-5-carbonitrile (L1) and 6-(methylthio)-8-(thiophen-2-yl)-1,2,3,4-tetrahydroquinoline-5-carbonitrile (L2) for selective identification of highly toxic Pd2+

Full text of DOI:10.1039/d0nj02272a

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Design and synthesis of new functionalized 8-(thiophen-2-yl)-
1,2,3,4-tetrahydroquinolines as turn-off chemosensors for
Received 00th January 20xx,
Accepted 00th January 20xx
2+
selective recognition of Pd ion
a
a
b
a
a
c
DOI: 10.1039/x0xx00000x
Shally, Vijay Kumar, Ismail Althagafi, Ashish Kumar, Divya Singhal, Abhinav Kumar, Rajeev
a,
a,
Gupta * and Ramendra Pratap *
We have developed two novel chemosensors 6-morpholino-8-(thiophen-2-yl)-1,2,3,4-tetrahydroquinoline-5-carbonitrile
(L1) and 6-(methylthio)-8-(thiophen-2-yl)-1,2,3,4-tetrahydroquinoline-5-carbonitrile (L2) for selective identification of
highly toxic Pd2+ ion. The selectivity of L1 and L2 were examined with a series of metal ions, among which Pd2+ ions
recognized selectively with fluorescence turn-off performances in methanol with very low limit of detections (nM).
Another compound L3 was synthesized and studied to understand the role of thienyl ring during the complex L1+Pd2+ and
L2+Pd2+ formation. The fluorescence changes after addition of Pd2+ was also observed through "naked-eye" (under 365 nm
UV lamp). Quantum chemical (DFT) calculation has been carried out to acquire information about the stability
of chemosensors and its complex with Pd2+ (L1+Pd2+ and L2+Pd ), which suggested that complex L2+Pd is more stable
2+
2+
2
+
than L1, L2 and complex L1:Pd . A number of studies comprising Job's plots, Stern–Volmer plots and detection limits
illustrate the notable sensing abilities of chemosensors with Pd2+ ion in which probe L2 was found more applicable than L1.
2
+
and toxicity of Pd ion, new and simple methods for selective
2
+
Introduction
Pd recognition is required with low limit of detection and
cost. Nowadays, various analytical methods and equipment’s
have been used for the detection of palladium ion, such as
inductively coupled plasma mass spectrometry (ICP-MS),
atomic absorption spectrometry (AAS), solid-phase
Palladium is a key component in several industrial processes
due to its unique chemical and physical properties. However,
palladium is part of several alloys, jewelry, dental material,
fuel cells automobiles and used as a catalyst in organic
synthesis. Palladium coordinates with thiol-containing amino
1
microextraction
coupled
high
performance
liquid
2,3
1
0
chromatography, X-ray fluorescence, etc. These methods
suffer with requirement of costly equipment, serious sample
preparation and highly skilled workers. Fluorescent-based
chemosensors have growing interest to researcher due to high
acid, DNA, RNA, proteins and other macromolecules (vitamin
6
B ) and interrupt a varieties of cellular function, when
4
accumulated in the human body. Palladium-catalyzed organic
synthesis have great demand due to their property and
applicable in various name reactions; such as Heck,
1
1
sensitivity, selectivity, and easy handling. Therefore, many
chemosensors have been reported for selective recognition of
5
Sonogashira and Suzuki-Miyaura coupling reacions. These
1
2
13
cations, anions and neutral molecules with good to
reactions are highly important as involved in the formation
1
4
excellent limit of detection.
6
and breaking of C-C, C-N,C-O and other bonds. The residual
Palladium ion is well known as fluorescence quencher because
it has heavy atomic effect and open-shell electronic
palladium ions produced from the industrial waste and
chemical laboratories mainly pollute the environment and
1
5
configuration.
Various research groups involved in
7
hampered eco-system and health of living being. The
development of turn-off fluorescence chemosensor for the
proposed dietary intake of palladium for a person is <1.5-15
mg/day (10 ppm of palladium active pharmaceutical
2
+
16
recognition of Pd ion. Liping Duan et al. has designed a
naphthalimide based complex with thio-heterocycle and
8-9
ingredients as a threshold). Due to the limitation of intake
2
+
17
alkyne for Pd recognition (Fig. 1). In this example, the
prominent design component was the presence of a thiophene
group attached to a methylene-amine fragment, both of which
acted together in the form of a chelating functional group. We
anticipated that the similar tetrahydroquinoline based
molecules containing appended thiophene groups may
function as the noteworthy chelate-based chemosensors (Fig.
a. Department of Chemistry, University of Delhi, North Campus, Delhi-110007
India.E-mail: rpratap@chemistry.du.ac.in;
ramendrapratap@gmail.com;rgupta.chemistry@gmail.com Tel: +91 1127666646
Department of Chemistry, Umm Al-Qura University, Makkah, Saudi Arabia
Department of Chemistry, University of Lucknow, Lucknow, Uttar Pradesh-226009
India
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
b.
c.
1
).
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Based on these design concepts, in this work, we present The designed ligands L1-L3 were synthesized by thVeiewreAartcictleioOnnlionef
two novel fluorescent chemosensors, L1 and L2, offering compound 3 or 4 with N-boc-3-piperidDoOnI:e10(.510)39in/D0DNMJ0S2O272inA
tetrahydroquinoline based molecules containing appended presence of sodamide at room temperature. The obtained
thiophene groups. In addition, another related molecule, L3, intermediate was treated with TFA in DCM to afford the
1
8
was also designed to understand the importance of the chemosensors L1-L3 (Scheme 2). All the synthesized
1
13
appended thiophene ring in the sensing application. Our compounds were characterized by H and C NMR, IR and
results illustrate that both chemosensors L1 and L2 acted as HRMS (see ESI, Fig. S1-S6). The absorption and emission
the bidentate ligands and coordinate with the Pd(II) ion spectra were recorded in methanol at room temperature. L1
whereas L3 did not show such kind of interaction towards the and L2 show absorption maximum (λmax) at 378 and 389 nm
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Pd(II) ion.
respectively.
Detection of Pd ion
2
+
R
O
N
O
To evaluate the effective interaction between chemosensors
and metal ions, we studied the interaction of various metal
O
CN
Pd
NH
+
2+
3+
2+
3+
2+
2+
2+
2+
2+
+
ions (Na , Ca , Cr , Fe , Fe , Mn ,Co , Ni , Cu , Zn , Ag ,
S
2+
2+
2+
2+
2+
3+
S
HN
Cd , Hg , Pb , Pt , Pd and Al ) through UV-vis and
fluorescence spectroscopy. The study shows that
Fig. 1: An earlier example by Duan and co-workers17 for the detection of Pd(II)
ion and the design of tetrahydroquinoline based sensors in the present work.
2+
chemosensors L1 and L2 selectively identify the Pd ion. The
UV-vis spectrum shows the absorption peak at 390 nm, 378
and 381 nm for chemosensors L1, L2 (Fig. 2) and L3 (see ESI,
Fig. S13) respectively due to π−π* transition. Interestingly, the
peak of L1 at 390 nm was disappeared and new peak appeared
at 325 nm, but in case of L2, absorption peak at 378 nm was
affected and one new peak was generated at 430 nm (Fig. 2).
The significant colour change from colourless to yellow was
observed in case of L2 only and peak was shifted in visible
region, which can be observed effortlessly through naked eye
Results and discussion
Synthesis and characterization of chemosensors
Chemosensors
tetrahydroquinoline-5-carbonitrile (L1), 6-(methylthio)-8-
thiophen-2-yl)-1,2,3,4-tetrahydroquinoline-5-carbonitrile (L2)
and 6-morpholino-8-phenyl-1,2,3,4-tetrahydroquinoline-5-
6-morpholino-8-(thiophen-2-yl)-1,2,3,4-
(
carbonitrile (L3) were synthesized by multi-steps reactions by
using functionalized 2H-pyran-2-ones as precursor. The
precursor 3 was synthesized by addition-elimination followed
by cyclization reaction of 2-cyano-3,3-bis(methylthio)acrylate
and acetophenone or 2-acetylthiophene under basic condition.
The products 4a and 4b were synthesized by amination of
compound 3 with morpholine in ethanol under reflux
conditions (Scheme 1).
(Fig. 7).
O
SMe
O
N
MeOOC
MeS
CN
O
CN
O
EtOH
CN
O
KOH/DMSO
r.t
2+
+
Reflux
Fig. 2: UV-vis spectra (a) L1 (black line) and solution of L1+Pd (red line); (b) L2
(black line) and solution of L2+Pd2+ (Red line)
Ar
N
H
SMe
Ar
O
Ar
O
2
1
3
4
3
3
a) Ar = C6H5; 77%
b) Ar = 2-Thienyl; 81%
4a)
Ar = C6H5; 86%
Chemosensors L1 and L2 exhibited excellent fluorescence
emission band at λem 515 and 485 upon excitation at 390 and
4b)
Ar = 2-Thienyl; 73%
Scheme 1: Synthesis of 6-aryl-2-oxo-4-methylsulfanyl/(morpholin-1-yl)-2H- 376 nm respectively (Table 1). The addition of various metal
pyran-3-carbonitriles (3/4)
ions to the solution of L1 and L2, did not affect the emission
intensity of solution but Pd changed the emission intensity.
In presence Pd ions, fluorescence emission intensity of L1
and L2 were quenched effectively, while no changes was
observed with L3. Fluorescence emission intensity of L2 was
quenched strongly as compared to L1 (Fig. 3)
Fluorescence titration spectral studies of L1 and L2 were
carried out at λex 390 and 378 nm which emits strong
fluorescence with maximum intensity at λem 515 and 485
2
+
X
X
O
X
N
2+
CN
CN
O
CN
NaNH2 / DMSO
r.t
DCM / TFA
r.t
+
N
Ar
Ar
O
Boc
Ar
Boc
HN
3/4
Ar = C6H5, 2-thienyl
5
L1-L3
O
O
SMe
N
N
CN
CN
CN
2
respectively. The gradual addition of PdCl to the solution
S
HN
L2
S
HN
HN
causes a noteworthy decrease in the emission intensity of both
chemosensors. Chemosensor L1 shows greenish and L2 shows
sky-blue fluorescence, which was changed to nearly colourless
as observed under UV chamber (365 nm) (Fig. 7). The
L1
L3
Scheme 2: Synthesis of chemosensors L1, L2 and L3
2
| J. Name., 2012, 00, 1-3
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of Pd2+ ion. The calculated detection limit of L1 and L2 for
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palladium ion is 49×10 M and 44 ×10 M respectively (see
ESI, Fig. S19-20). This result clearly indicate that L2 showed
lower detection limit with a high quenching constant as
compared to L1 which recommended that L2 showed high
2
+
sensitivity towards Pd ion (Table 2).
We have compared our LOD results with earlier reported Pd
2
+
chemosensors (see ESI, Table 4) and found that our study
provides promising result.
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Stability and selectivity studies
It is necessary for sensors to show excellent selectivity,
sensitivity and fast response towards the specific metal ion in
the presence of competitive metal ion. To analyze the
Fig. 3: Comparative emission spectra of chemosensors with and without metal
ions (a) chemosensor L1; (b) chemosensor L2; (c) chemosensor L3; (d) Bar
diagram exhibiting a change in emission intensity at 515 nm for L1 (Green pillars)
and at 485 nm for L2 (Red pillars) in presence of assorted metal ions
Table 1. Photophysical studies of chemosensorL1 and L2 with Pd2+ ion
Sample
λabs(n
λ
em
Stokes
Shift
Δν (cm )
Quantum
Yield
(Φ)
Theoretical
Band Gap
(eV)
m)
(nm)
-1
L1
L1+Pd2+
390
325
378
430
515
6223
-
0.62
0.11
0.75
0.09
2.01
0.20
1.66
0.15
515 (turn-off)
485
L2
L2+Pd2+
4620
485 (turn-off)
-
quenching of emission intensity of L1 and L2 was saturated at
certain concentration of PdCl
addition of 5.0 equivalents. Fluorescence intensity of L2
quenched more effectively than L1 in presence of PdCl
2
+
Fig. 5: Selectivity of L1 (a) and L2 (b) towards Pd ion in presence of other metal
2
+
and which was verified up to ions: L1+metal ions (blue pillars); and L1+metal ions+Pd ion (red pillars),
2+
L2+metal ions (purple pillars); and L2+metal ions+Pd ion (dark red pillars).
2
2
.
2
+
interference of other metal ions in the presence of Pd ion,
2
+
same concentrations of Pd and other metal ions were taken
for this study and tested with L1 and L2. The addition of
Fig. 4: Emission spectra of L1 (a) and L2 (b) (20 μM) on exposure to increasing
conc. of PdCl ion (0–50 μM), Inset; decreasing emission intensity upon addition
of PdCl
2
2
.
The study of complex formation ability of chemosensors
illustrate that L2 is more effective over L1 with respect of
Stern–Volmer constants (KSV), limits of detection and binding
constants (K
b
). Stern–Volmer graph was used to demonstrate
4
-1
the value of quenching constant (KSV) i.e. 7.23×10 M and
5
-1
1
.876×10 M for chemosensor L1 and L2 respectively (see ESI,
Fig. S17-18). The KSV of both chemosensor illustrate that L2 has
strong quenching properties compared to L1 in the presence
Table 2. Stern-Volmer constants (KSV), detection limits and binding
constants (K ) for L1 and L2 with Pd ion
b
2
+
-
1
Species
KSV (M )
Detection Limit (nM)
Binding constants
Fig. 6: Effect on emission spectra of (a) L1 and (b) L2in the presence of different
palladium salts (c) L1 and L2 in the presence of tetrabutylammonium chloride
(20 equivalents)
b
(K )
7.23 ×104
1.876 × 105
4
-1
-1
L1
L2
49
44
1.66 × 10 M
4
7.96 × 10 M
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various metal ions has negligible effect of fluorescence
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+
DOI: 10.1039/D0NJ02272A
intensity in presence of Pd ion (Fig. 5). We therefore 5.7 ppm respectively. The peaks of aliphatic protons were
2
+
conclude that chemosensors distinguished selectively Pd ion slightly shifted towards downfield due to the interaction of
even presence of competitive ions. tetrahydroquinoline nitrogen with palladium ion. In case of L1,
After selectivity studies of chemosensors, we also tried the N-H proton peak was broadened enough after addition of
different salts of palladium, such as Pd(PPh Cl , Pd(PPh Palladium salt upto equivalent of but complete
Pd(OAc) to understand the counter ion effect. For this deprotonation does not occur, while in case of L2, peak of N-H
analysis, freshly prepared stock solution of chemosensors and proton was completely disappeared after addition of 4.0
)
3 2
2
3 4
) ,
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metal salts were used. Addition of different salts of palladium equivalents Pd , which indicate that Pd coordinated with
shows that emission intensity of L1 and L2 was quenched with loan pair of N-atom via deprotonation. The results also
these salts of palladium but in case of PdCl
quenched more efficiently. This result indicated that both the palladium ion as compared with L1 (Fig. 8 and also see ESI, Fig.
chemosensors L1 and L2 are more selective towards PdCl (Fig. S10-12). In addition, emission spectra of L3+Pd solution did not
a & 6b). The detection of palladium ion by a chemosensor
2
emission intensity indicate that L2 responded better toward coordination with
2
6
depends on its coordination and therefore ionization of a
metal salt plays an important role. Out of various Pd(II) salts
used [Pd(PPh
3 2 2 3 4 2 2 2
) Cl , Pd(PPh ) , Pd(OAc) and PdCl ]; PdCl is
known to ionize better in solution to generate the solvated
2
+
species, [Pd(solvent)x] , that allows ease in its coordination by
a chemosensor to generate the Pd(II) complex, responsible for
1
5d,e
the emission quenching.
To check any other role of the counter anion, we checked the
quenching with chloride salt (tetrabutylammonium chloride)
to acquire information. No significant changes were observed
in emission intensity upon excess addition of chloride ions (20
equivalents) (Fig. 6c), which suggest that only chloride ion is
not interacting with L1 and L2. This result also confirms that
chloride ion has no role in fluorescence quenching and better
solvation property of chloride ion facilitates the complex
formation.
Nature of binding and stoichiometry
The binding stoichiometry of chemosensors L1 and L2 with
2
+
Pd ion was examined by using Job's plot (see ESI, Fig. S18-
2
+
1
9). The results of stoichiometry binding of L1 and L2 with Pd
ion are 1:1 ratio, which was confirmed by HRMS spectra (see
ESI, Fig. S8-S9). The probable binding mode of chemosensor is
mentioned in figure 7.
1
6
Fig. 8: H NMR titration of L1 and L2 with palladium chloride in DMSO-d .
2+
quenched and suggest that Pd also coordinated with S-atom
of thiophene moiety. The binding stoichiometry was
determined by using Job’s plot; employing emission spectral
2+
data versus mole fraction of Pd ion that provided a 1:1
stoichiometry. However, both UV-vis and fluorescence spectral
Fig. 7: Possible binding structure of L1 and L2 with Pd2+ ion and fluorescence
2+
titrations were performed by taking 2.5 equivalents of Pd
visible changes before and after addition of palladium chloride under visible and
UV-light.
ion. In case of proton NMR spectral titration, N-H protons were
found to completely disappear on addition of up to 4.0
equivalents of a Pd salt in case of L2 but not in case of L1.
To demonstrate the interaction between chemosensors and
2+
2
+
1
Pd ion, H-NMR spectral titrations were carried out in DMSO-
4
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Therefore, both for UV-Vis and fluorescence spectral as well as orbitals in the molecules. Highest occupied molecular orbital
NMR spectral studies suggested the moderate value of the of the chemosensor L1 and L2 is tetrahydroquinoline-
binding constants (≈10 M ) that required higher equivalents morpholine and tetrahydroquinoline-thiomethyl in plane
of Pd2+ ions to completely deprotonate a chemosensor owing to presence of electron donating amino and thio groups
followed by its coordination to the Pd ion. Furthermore at 6 position respectively. In case of LUMO the electron
confirmation regarding the binding stoichiometry of
complexes, mass spectra was recorded (see ESI, Fig. S8-S9)
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DOI: 10.1039/D0NJ02272A
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
90.9554 corresponding to [L1‒H+Pd]+ and [L2‒H+Pd] ,
+
3
respectively.
Lifetime measurement
Fluorescent lifetime technique has been found to be a very
important factor in order to explain the excited state behavior
of fluorescence compounds. The nature of excited state of
1
6
two newly synthesized compound L1 and L2 is determined in
2
+
the absence and presence of Pd ion (Fig. 9 and ESI, Table.
S1). The decay profile of chemosensors of L1 and L2 exhibit tri-
exponential decay with average lifetime (τav) 7.83 and 7.94 ns,
respectively. The average lifetime (τav) of both compounds is
2
+
decreased when added one equivalent of Pd in the solution.
1
6c
Palladium ion is familiar as fluorescence quencher, when
2
+
Pd ion added to the solution of L1 and L2, they formed
complex L1+Pd2+ and L2+Pd and average life time (τav) is Fig. 10: DFT optimized structures of L1, L2 and their complex structure L1+Pd
2+
2+
2+
and L2+Pd .
decreases from 7.83 to 7.24 ns and 7.94 to 4.74 ns
respectively. According to decay curve, fluorescence life time
of complex L1+Pd2+ is higher as compared to L2+Pd . The life
density is very low in the thiophene ring due to presence of
electron withdrawing CN-group at 5 position. After formation
of complex with Pd , the electron density was slightly shifted
2+
2
+
time spectra signified that L2+Pd
quenching behavior means it has highly affinity toward L2 (Fig.
).
complex has high
2+
toward Palladium. The energy HOMO of chemosensors (L1 and
9
2+
2+
L2) are more as compared to complex L1+Pd and L2+Pd ,
which indicated that complex is more stable than ligand. In
figure 10, the bond length of Pd-N is shorter than Pd-S because
electron density is more toward N-atom as per HOMO orbital
diagram. We also performed the DFT of complex L1 and L2
with Palladium salt with N-H containing ligand, the energy of
these complexes is higher than chemosensor L1 and L2, which
indicated that the complexes having NH group is not suitable
Fig. 9: Lifetime profile diagram of chemosensors L1 and L2 in the absence and
2+
presence of Pd ion in methanol.
Computational Studies
Optimization of the geometry of L1, L2 and its complex with
2
+
Pd ion in gas phase has been succeeded using Gaussian 09W
computational program using B3LYP function with basis set 6-
2
0
31G and LanL2DZ respectively. Optimized structure of L1 and
L2 shows that the thiophene and 1,2,3,4-tetrahydroquinoline
ring is not in the same plane. Palladium ion is coordinated to
the nitrogen atom of 1,2,3,4-tetrahydroquinoline via
deprotonation of N-H group and S-atom of thiophene. With
the help of optimization of complex L1+Pd and L2+Pd ,
palladium is coordinated with N-atom and S-atom as Pd-N and
Pd-S bond with bond length nearly 2.71 Ǻ and 2.04 Ǻ Fig. 11: DFT optimized structure and HOMO-LUMO band gap of L1, L1 + Pd , L2
respectively in both complexes. The result reveals that L1 and
2
+
2+
2
+
2
+
and L2 + Pd .
2
+
L2 behave as bidented ligand with Pd ion (Fig.10).
The electron density of HOMO and LUMO of L1 and L2
indicates that the presence of more electron and low electron
for complex formation (see ESI, Table 3) and complexation
occur followed by deprotonation. The overall outcome
specified that complex L2+Pd have very low energy means
2
+
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more stable as compared to L1, L2 and complex L1+Pd2+ (Fig.
Journal Name
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Experimental
View Article Online
DOI: 10.1039/D0NJ02272A
1
1).
Fabrication of filter paper strips
Methods and Instrumentation
To develop a cost-effective and straight-forward method for All chemicals and metal salts were used as received without
selective sensing of analytes is a new challenge for any further purification. High-quality HPLC grade solvents were
16a
used for the UV-Vis and fluorescence measurements. Stock
solutions of metal ions (1 mM) were prepared in methanol and
spectra were recorded with a 1.0 cm path length cuvette at
researcher. In this context, we prepared a silica coated TLC
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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0
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2
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5
6
7
8
9
0
1
13
room temperature. The H NMR (400 MHz) and C NMR (100
MHz) recorded in CDCl solvent and coupling constants were
reported in Hz. The FTIR spectra were recorded in KBr disc on a
3
-1
Perkin-Elmer AX-1 spectrometer in wavenumber (cm ). The
absorption spectra were recorded with a Perkin-Elmer Lambda
25 spectrophotometer. Internal signal patterns were reported
as m, multiplet; dd, double doublet; t, triplet; d, doublet; s,
singlet. Fluorescence studies were performed with a Cary
Eclipse Fluorescence Spectrometer. ESI-MS mass spectra were
Fig. 12: (a) Images of TLC Strips: (a, c) and (b, d) are the images of TLC strips in obtained with Q-TOF LC/MS mass spectrometer of Agilent.
ambient light and under ultraviolet light, respectively. In all cases, i exhibits
images for only TLC strips, ii exhibits images after treatment with probe L1 (a and
b) and L2 (c and d), whereas iii exhibit images of test strips covered with L1 (a
Synthesis of chemosensors
2
+
and b) and L2 (c and d) after treatment with Pd ion solution and study was
performed in methanol solution
6-Morpholino-8-(thiphen-2-yl)-1,2,3,4-tetrahydroquinoline-5-
carbonitrile (L1):
strips for identification of Pd2+ ion. The solution of L1 and L2
(
0.1 mm) probes were prepared in methanol and TLC strips A solution of 4-morpholino-2-oxo-6-(thiophen-2-yl)-2H-pyran-
dipped to it and dried. The emission colour of L1 and L2 test 3-carbonitrile 4b (0.5 mmol, 0.144 g) and N-boc-3-piperidone
strips are greenish and sky-blue under UV-lamp, respectively. (0.6 mmol, 0.119 g), sodamide (1.0 mmol, 0.039 g) in DMSO
When these ligand coated strips were dipped into an aqueous (5.0mL) at room temperature was stirred for 2 hours. After
solution of Pd2+ ion (0.1 mm), color of emission intensity was completion, the reaction mixture was poured onto crushed ice
completely disappeared (Fig. 12). Thus, we can conclude that with constant stirring. Then the reaction mixture was
2
+
both the ligands are quit efficient to detect Pd with TLC strip. neutralized with 10% HCl and obtained precipitate was filtered
X-ray crystallography of chemosensor L1
and dried. The resulting precipitate was collected by filtration,
washed with H O, and dried. Thus obtained crude product was
treated with TFA (10 mmol) in DCM (3.0 mL) at room
temperature for 30-35 minutes. The reaction mixture was
2
The compound L1 crystallizes in triclinic crystal system with
centrosymmetric P-1 space group having a single molecule in
the asymmetric unit. The molecule is consisted of a central
poured onto crushed ice and neutralized with NaHCO
extracted with DCM (25.0 mL×3). The combined organic layer
was dried over anhydrous Na SO and then evaporated in
3
and
1
,2,3,4-tetrahydroquinoline ring joined with an electron-
donating thiophene moiety and electron-withdrawing
cyanogroup at C-5 and C-8 positions of aromatic ring along
with alicyclic six membered puckered ring at C-7 position. The
thiophene ring and cyano group are so-oriented where the
charge transfer can take place in a unidirectional fashion. The
planar core of the molecule encompasses the thiophene ring,
aromatic core of 1,2,3,4-tetrahydroquinoline moiety and cyano
group in one plane and the chair shaped alicyclic oxa-azo
counterpart and twist-boat shaped tetrahydroquinoline in
another plane with mutual inter planar angle of 82.43° (Fig. 13
and see ESI, Table S3).
2
4
vacuum. Further, purification of the compounds was done by
silica gel column chromatography using ethyl acetate/hexane
(10: 90) as an eluent.
Yellow solid; yield: 150 mg (92%); mp = 148–150°C. IR
-1
1
(
KBr/Cm ):, 2924, 2216-2310, 1606.; H NMR (400 MHz,
CDCI ): δ 1.99 (quint, J = 7.6 Hz, 2H), 2.95–3.03 (m, 6H), 3.24-
.26 (m, 2 H), 3.85-3.88 (m, 4H), 6.82 (s, 1H, ArH), 7.10–7.13
m, 1H, ArH), 7.19-7.20 (m, 1H, ArH), 7.38 (d, J = 4 Hz, 1H, ArH);
C NMR (100 MHz, CDCl ): δ 21.2, 26.4, 41.4, 52.8, 67.1,
3
08.3, 116.8, 119.6, 123.3, 126.1, 126.3, 126.7, 127.7, 138.7,
3
3
(
1
3
1
1
3
+
39.3, 145.8; HRMS (ESI): m/z [M + H] calcd for C18
20 3
H N OS:
26.1322; found: 326.1324.
6-methylsulfanyl-8-(thiphen-2-yl)-1,2,3,4-
tetrahydroquinoline-5-carbonitrile (L2)
A solution of 4-(methylthio)-2-oxo-6-(thiophen-2-yl)-2H-pyran-
-carbonitrile 3b (0.5 mmol, 0.115 g), N-boc-3-piperidone (0.6
3
Fig. 13: (a) ORTEP representation of L1 at 30% probability of ellipsoid; (b) The mmol, 0.119 g), sodamide (1.0 mmol, 0.039 g) in DMSO
perspective view of L1.
(
5.0mL) at room temperature for 1 hour and completion of
reaction was monitored by TLC. After completion of the
6
| J. Name., 2012, 00, 1-3
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1
1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
reaction, the reaction mixture was poured onto crushed ice Chemosensors selectively identified palladium ionVioewveArrtivclaerOionluines
with vigorous stirring and neutralized with dilute 10% HCl and metal ions. The limit of detection of pallaDdOiuI:m10i.o10n39b/yD0LN1Ja0n2d27L22A
the resulting precipitate was filtered. The resulting precipitate was found up to 49 and 44 nM respectively. The coordination
was washed with H
2
+
2
O, and dried. Crude product was treated and bonding ratio with Pd ion were determined through
1
with TFA (10 mmol) in DCM (3.0 mL) at room temperature for fluorescence data, H-NMR titration spectra and HRMS
3
0-35 minutes and then poured the reaction mixture onto spectrum shows that both chemosensors binds with thiophene
crushed ice. Reaction mixture was neutralized with NaHCO sulfur and hydroquinoline N-atom as bidentate ligand.
and extracted with DCM (25.0 mL × 3). The combined organic Theoretical DFT studies were showed that the charge transfer
layer was dried over anhydrous Na SO and than evaporated in occurred from ligand to complex. Energy of chemosensors and
vacuo. Crude products L2 purified with silica gel column complexes NMR titration study were suggested that complex
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
4
2
+
2+
chromatography using ethyl acetate/hexane (8:92) as eluent to L2+Pd is more stable than L1, L2 and complex L1:Pd . Job’s
afford the product in good yield. plot, LOD calculation and mass spectra of complexes were
Yellow solid; yield: 115 mg (80%); mp = 78-80°C; IR (KBr/Cm ): specified that chemosensors bind in 1:1 stoichiometry with
-1
1
2
6
=
3
924, 2218, 1606; H NMR (400 MHz, CDCI
3
): δ 1.99 (quint, J = very low limit of detection. The comparison of LOD of our
.8 Hz, 2H), 2.45 (s, 3H, SMe), 2.98 (t, J = 6.4 Hz, 2H), 3.28 (t, J chemosensors with earlier reported chemosensors
5.6 Hz, 2H), 7.10-7.12 (m, 1H, ArH), 7.17 (dd, J = 1.6 Hz, J = demonstrated the better efficiency and lower limit of
.6 Hz, 1H, ArH), 7.20 (s, 1H, ArH), 7.39 (dd, J = 0.8 Hz, J = 4.8 detection.
1
3
Hz, 1H, ArH); C NMR (100 MHz, CDCl
3
): δ 14.0, 20.7, 26.4,
4
1
C
1.2, 115.7, 116.4, 122.7, 125.9, 126.2, 126.4, 126.8, 127.8,
+
Conflicts of interest
32.0, 138.5, 141.8; HRMS (ESI): m/z [M + H] calcd. for
: 287.0671; found: 287.0667.
H N S
15 15 2 2
There are no conflicts of interest to declare.
6
-Morpholino-8-phenyl-1,2,3,4-tetrahydroquinoline-5-
carbonitrile( L3)
Acknowledgements
To the dried RB flask 4-morpholino-2-oxo-6-phenyl-2H-pyran-
We are grateful to acknowledge Council Scientific of Industrial
Research (CSIR) and Indian Council of Medical Research
ICMR), New Delhi for financial support. RP is thankful to the
University of Delhi for providing instrument facilities to
accomplish this work.
3
-carbonitrile 4 (0.5 mmol, 0.141 g), N-boc-3-piperidone 5 (0.6
mmol, 0.119 g), sodamide (1.0 mmol, 0.039 g) was placed and
DMSO (5.0 mL) was added to the mixture. The reaction
mixture was stirred at r.t. for 3 hours and completion of the
reaction was monitored by TLC. The crude mixture was poured
onto ice–water with constant stirring and neutralized with 10%
aq. HCl. The precipitate obtained was filtered and dried and
treated with TFA (10.0 mmol, 0.76 mL) in DCM (3.0 mL) at
room temperature till completion of reaction. The reaction
mixture was poured onto crushed ice followed by
neutralization with NaHCO
mL×3). The organic layer was separated and dried over
anhydrous Na SO . The crude products thus obtained was
purified by silica gel column chromatography using 1:9 ethyl
acetate:hexane as an eluent to afford L3 as yellow solid; yield:
(
Notes and references
1
(a) T. Schwarze; H. Muller; C. Dosche; T. Klamroth; W.
Mickler; A. Kelling; H. G. Lohmannsroben; P. Saalfrank and H.
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3
and extracted with DCM (25.0
2
010,16, 1819−1825; (c) T. Schwarze; C. Dosche; R. Flehr; T.
2
4
Klamroth; H. G. Lohmannsroben; P. Saalfrank; E. Cleve; H. J.
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1
1
6
52 mg (95%); mp = 130–132 °C; IR (KBr): 3417, 2922, 2217,
420- 1576 cm ; H NMR (400 MHz, CDCl
.8 Hz, 2H), 2.97 (t, J = 6.6 Hz, 2H), 3.02 (t, J = 4.2 Hz, 4H), 3.19
2
3
(a) T. Iwasawa, M. Tokunaga, Y. Obora and Y. Tsuji, J. Am.
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–
1 1
3
): δ 1.98 (quint, J =
(t, J = 5.6 Hz, 2H), 3.86 (t, J = 4.6 Hz, 4H), 6.67 (s, 1H, ArH),
1
3
7
.35-7.39 (m, 3H, ArH), 7.43-7.46 (m,2H, ArH); C NMR (100
): δ = 20.4, 26.1, 41.0, 52.3, 66.4, 106.6, 116.7,
19.2, 125.3, 127.7, 128.7, 128.8, 130.6, 131.0, 138.0, 145.5;
MHz, DMSO-d
1
6
+
HRMS (ESI): m/z [M + H] calcd for C20
22 3
H N O: 320.1757; found:
4
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Conclusions
In summary, two novel tetrahydroquinone based chemosensor
L1 and L2 were successfully synthesized by ring transformation
strategy for identification of Pd metal ions exclusively.
Chemosensor L3 was also synthesized by same approach for
authentication of coordination sites of L1 and L2.
2
+
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New Journal of Chemistry
View Article Online
DOI: 10.1039/D0NJ02272A
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Graphical Abstract
Design and Synthesis of New functionalized 8-(thiophen-2-yl)-1,2,3,4-
tetrahydroquinolines as turn-off chemosensors for selective recognition of Pd
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Shally, Vijay Kumar, Ismail Althagafi, Ashish Kumar, Divya Singhal, Abhinav Kumar, Rajeev Gupta*
and Ramendra Pratap*
Functionalized 8-thienyl-1,2,3,4-tetrahydroquinoline was synthesized as a fluorescent turn-off
chemosensor for selective recognition of Pd ion with low detection limit.
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