‘Quick CuAAC’ Chemistry for Hg(II) and Mn(II) ion sensing via 9H-carbazole derivatives

Article abstract of DOI:10.1016/j.ica.2021.120560

The synthesis of 1,2,3-triazole merged 9H–carbazole derivative is being reported with excellent yield but drastic reduction in the reaction time. The 1,2,3-triazole linker has been synthesized under thermal conditions within 20 min and yield of more than 90%, using Cu(I) as catalyst. Using UV–Vis spectral technique, the alkyne product displayed excellent detection capability towards Hg(II) and Mn(II) ions in methanol with the detection limit as low as 20 μM and 10 μM, respectively, whereas the triazolyl product can efficiently sense Cu(II) ions.

Full text of DOI:10.1016/j.ica.2021.120560

Inorganica Chimica Acta 527 (2021) 120560
Contents lists available at ScienceDirect
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Research paper
‘
9
Quick CuAAC’ Chemistry for Hg(II) and Mn(II) ion sensing via
H-carbazole derivatives
a
a
b
b
a
Alisha Rani , Parveen Saini , Gurjaspreet Singh , Sushma , Harminder Singh ,
c,
*
a,*
Gurpreet Kaur , Jandeep Singh
a
Department of Chemistry, Lovely Professional University, Phagwara 144411, Punjab, India
b
c
Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh 160014, India
Department of Chemistry, Gujranwala Guru Nanak Khalsa College, Civil Lines, Ludhiana 141001, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
The synthesis of 1,2,3-triazole merged 9H–carbazole derivative is being reported with excellent yield but drastic
Click Chemistry
Metal ion sensing
CuAAC
reduction in the reaction time. The 1,2,3-triazole linker has been synthesized under thermal conditions within
2
0 min and yield of more than 90%, using Cu(I) as catalyst. Using UV–Vis spectral technique, the alkyne product
displayed excellent detection capability towards Hg(II) and Mn(II) ions in methanol with the detection limit as
Carbazole
low as 20 µM and 10 µM, respectively, whereas the triazolyl product can efficiently sense Cu(II) ions.
Quick reaction
1
. Introduction
Click chemistry has been extensively used since its discovery by Sir
derivative has led to an innovative design that can proficiently sense the
presence of target metal ions.
There have been numerous reports on the conventional methodology
of CuAAC reaction which proceed either under thermal or photochem-
ical conditions. The optimized reaction time for thermal and photo-
chemical cycloaddition reaction is typically varies from 3 to 12 h and
1–24 h, respectively. Thus, since the advent of CuAAC, there have been
several modifications in the catalyst, reactants, temperature conditions,
mole percent of catalyst, etc. to enhance the yield and improve the re-
action [34–37]. But the reaction that yields 91% product within few
minutes of reaction has not been reported so far. Herein we report first
such Click reaction that occurs ‘quickly’ i.e. 15 times faster as compared
to conventional synthesis, which further acts as a chromophore due to
the existence of electron rich species in the product which have led to its
role as Hg(II) ion sensor.
Sharpless and co-workers in 2001, and has gained worldwide applauds
for simple reaction technique leading to the product formation with in-
offensive by-products [1–3]. The reaction involves fusion of a terminal
alkyne with an azide in the company of Cu(I) catalyst which undergoes
3
+ 2 cycloaddition to 1,4 disubstituted 1,2,3-triazole as product [4–7].
The reaction has been efficiently carried out under thermal and photo-
catalytic [8–12] conditions to give products containing various donor
sites from linkers and N-atom from triazole. The use of Copper(I)
Catalyzed Alkyne-Azide Cycloaddition (CuAAC) reaction have created a
library of molecules that can act as ion sensors [13–16], drug discovery
[
17–22], material and polymer science [23–24].
The last decade has witnessed booming research on (Scheme 1) the
metal ion sensors with the exploitation of UV–Vis and/or Fluorescence
spectroscopy, electrochemistry and atomic/emission absorption spec-
troscopy have been developed for detection of metal ions [25–28]. There
have been immense efforts to synthesize a selective Hg(II) ion sensor
from a low-cost material via highly efficient methodology which can be
operational via simple instruments like UV–Vis spectrophotometer
2. Experimental section
2.1. Synthesis of benzyl azide (1)
To a stirred solution of Benzyl chloride (5.0 ml, 4.34 mmol) in DMF
(50 ml), Sodium azide (14.0 g, 21.7 mmol) was added slowly and the
[
29–33]. It has always been a great challenge to design a molecule that
◦
be produced within minutes in pure form and can be directly used for
metal ion detection. The presence of conjugated system on carbazole
reaction mixture was stirred for 5 h at 90 C. After completion of reac-
tion, the mixture was quenched by the addition of ice–cold water and the
*
Corresponding authors.
E-mail addresses: chemgurpreet5@gmail.com (G. Kaur), singhjandeep@gmail.com (J. Singh).
https://doi.org/10.1016/j.ica.2021.120560
Received 29 June 2021; Received in revised form 5 August 2021; Accepted 5 August 2021
Available online 8 August 2021
0
020-1693/© 2021 Elsevier B.V. All rights reserved.

A. Rani et al.
Inorganica Chimica Acta 527 (2021) 120560
Scheme 1. Synthesis of 9–((1–benzyl–1H –1,2,3–triazol–4–yl)methyl)–9H–carbazole (3).
crude product was extracted using diethyl ether. The combined organic
layers was dried over the anhydrous sodium sulphate; and in vacuum
evaporation of the solvent yielded the desired compound. The product
Table 1
Optimized reaction conditions to carry out Quick Click Chemistry under thermal
conditions.
ꢀ 1
was isolated as yellow viscous oil. Yield: 88%; IR (neat, cm ): 3032
◦
–
–
– –
–
Entry
Reaction Duration (min)
Temperature ( C)
Yield (%)
(
–C
Cꢀ H), 2931 (–Cꢀ H), 2090 (–N
–
N
–
N–), 1674 (–C
–
C).
1
2
3
4
5
6
7
8
9
40
80
160
320
5
25
25
25
25
60
60
60
60
80
80
80
0
0
2
.2. Synthesis of 9–(prop–2–ynyl)–9H–carbazole (2)
0
0
In a 100 ml of round bottom flask, Carbazole (1.0 g, 5.98 mmol) was
dissolved in 20 ml DMF by continuous stirring till homogenous solution.
It was followed by addition of dry K CO (5.0 g, 36.1 mmol) and
20
55
90
91
19
53
88
10
15
20
5
2
3
propargyl bromide dropwise (0.92 g, 0.837 ml, 7.74 mmol) within 5
min. The reaction mixture was stirred continuously for 26 h at room
temperature. The completion of reaction was monitored using TLC
1
0
10
15
11
(
hexane: ethyl acetate (4:1)). Upon completion of reaction, the mixture
was quenched by pouring into ice cold water. The creamy yellow solid
product formed was filtered and dried for further use. Yellow powder;
Yield: 99%; Anal. Calcd. for C15H11N (%): C, 87.77; H, 5.40; N, 6.82;
Found (%): C, 87.12; H, 5.23; N, 6.12; Melting point: 83–84℃; IR (neat,
ꢀ
1
–
–
–
cm ): 3261 (–C
–
Cꢀ H), 3045 (–C
Cꢀ H), 2924 (–Cꢀ H), 2115
–
–
–
3
C), 784 (ortho subs.); H NMR (400 MHz, CDCl ) δ
1
(
–C
–
C), 1591 (–C
=
8.05 (d, J = 7.8 Hz, 2H), 7.47–7.40 (m, 4H), 7.23 (ddd, J = 8.0, 6.9,
1
3
1
.3 Hz, 2H), 4.91 (d, J = 2.5 Hz, 2H), 2.18 (t, J = 2.5 Hz, 1H); C NMR
(
101 MHz, CDCl
3
) δ = 139.93 (s), 125.99 (s), 123.36 (s), 120.54 (s),
1
19.68 (s), 108.82 (s), 77.95 (s), 72.32 (s), 32.32 (s).
2
9
.3. Synthesis of 9–((1–benzyl–1H –1,2,3–triazol–4–yl)methyl)–
H–carbazole (3)
The compound (2) (1.0 g, 4.8 mmol) was taken in a two-necked
round bottom flask, THF:TEA (1:1) was added with continuous stir-
ring, followed by the addition of benzyl azide(1) (0.6 ml, 4.8 mmol) and
0
3 3
.01 g of [CuBr(PPh ) ] catalyst was added in it. The reaction mixture
was refluxed at 60℃ for 20 min; instantaneously white color pre-
cipitates were formed. Solution was filtered and from residue, the
desired product was obtained. It was dried at room temperature. The
product was isolated as off white solid. Yield: 91%; Light Brown; Anal.
Fig. 1. Graphical representation of yield of conversion of reactants into 1,2,3-
triazole product with time.
Calcd. for C22
H
18
N
4
(%): C, 78.08; H, 5.36; N, 16.56; Found (%): C,
3. Result and discussion
ꢀ 1
7
7.34; H, 5.04; N, 16.01; Melting point: 150–151℃; IR (neat, cm ):
–
–
–
–
–
1
3
047 (–C
Cꢀ H), 2939 (–Cꢀ H), 1591 (–C
δ = 8.04 (d, J = 7.8 Hz, 2H), 7.39 (d, J = 3.1 Hz, 4H),
.25–7.20 (m, 5H), 7.07–7.04 (m, 2H), 6.97 (s, 1H), 5.52 (s, 2H), 5.24 (s,
C), 1384 (–N
–
N); H NMR
3.1. Synthesis
(
400 MHz, CDCl
3
7
2
1
1
The synthesis of 1–(azidomethyl)benzene (1) was carried out from
the nucleophilic substitution reaction of Benzyl chloride with Sodium
Azide. 9H–carbazole was transformed into its terminal alkyne derivative
i.e. 9–(prop–2–ynyl)–9H–carbazole (2) by the reaction with propargyl
1
3
H); C NMR (101 MHz, CDCl
3
) δ = 140.09 (s), 134.47 (s), 129.05 (s),
28.69 (s), 127.87 (s), 126.00 (s), 123.10 (s), 121.51 (s), 120.44 (s),
19.47 (s), 108.86 (s), 54.07 (s), 38.93 (s).
2 3
bromide in DMF facilitated by base K CO . Azide (1) and Alkyne (2)
were coupled via copper–catalyzed alkyne–azide cycloaddition reaction
[
CuAAC]
yielding
more
than
90%
of
(3)
9–
as
(
(1–benzyl–1H–1,2,3–triazol–4–yl)methyl)–9H–carbazole
2

A. Rani et al.
Inorganica Chimica Acta 527 (2021) 120560
Fig. 2. Metal ion sensing response of compound (2) with different metal salts, in methanol solvent at λmax = 283 nm, by the successive addition of 29 equiv. 10 mM
solution of Mn(II) ions and 14 equiv. 10 mM solution of Hg(II) ion solution. Right side: probe (3) with variant ions solution at 303 nm, positive result with successive
addition of 30 equivalent 1 mM solution of Cu (II) ions.
ꢀ
product. This strategy is based on the selectively efficient [CuBr
PPh N chemical set–up enhancing the speed of cycload-
]–THF–Et
dition reaction from conventional 300 min to less than 20 min.
sharp signal at 2090 cm 1 corresponds to the azide group –N
3
in the
(
3
)
3
3
product. Upon cycloaddition between alkyne and azide, the absorption
ꢀ
1
–
ꢀ 1
–
band at 3261 cm (–C
–
Cꢀ H), 2115 cm (–C
–
C) along with a signal
ꢀ
1
at 2090 cm for –N
3
goes missing in the IR spectrum of Carbazole
linked 1,2,3–triazole (3). Along with, the appearance of new signals at
3
.2. Optimization of time and product yield
ꢀ
1
–
ꢀ 1
–
–
N) support the 5-membered
3
047 cm (–C
–
C–H) and 1384 cm (–N
heterocycle formation. From this cumulative analysis, the generation of
The optimization of the reaction conditions was performed to eval-
1
,2,3–triazole ring with less time duration of the click reaction is
uate the minimum time required for the maximum yield and many trial
reactions were carried out, as tabulated in the Table 1 and represented in
Fig. 1. The analysis of results indicates that the reaction did not initiate
at the room temperature even when in spite of stirring it for conven-
tional standard time of 5 h. As soon as the reaction was warmed and the
established.
3
.3.2. NMR spectroscopy
1H NMR studies confirm the formation of expected azide (1), alkyne
◦
(2) and triazole (3) with good yield. All protons in the synthesized
compounds can be identified in spectra and assigned a value in NMR.
temperature was raised to 60 C, there was significant advancement in
the progress of reaction within few minutes of thermal equilibrium. The
formation of white solid with first few minutes of reaction confirmed the
progress of reaction. With the advancement of time, the reaction was
found to be complete within 20 min. Thereafter, insignificant increase in
the amount of product was recorded. Any increase in temperature and
time did not affect the yield of reaction. From this study, it was
concluded that the reaction was complete within first very few minutes
and thus the nomenclature was termed as “Quick Click” reaction. This
reaction is first of its kind to be carried out within such a short span and
with high yield and have not been reported so far to the best of our
knowledge.
1
The H NMR of compound (2) is in agreement with the expected result
due to absence of –NH signal of Carbazole which was substituted by
terminal alkyne propyne group. Moreover, the presence of signal at 2.18
–
ppm (t, J = 2.5 Hz, 1H) for 1H of –C
–
Cꢀ H and 4.91 ppm (d, J = 2.5 Hz,
2
H) for 2H of –CH
2
signifies the successful nucleophilic substitution
reaction. The transformation of alkyne (2) into bi-substituted 1,2,3-tria-
1
zole (3) can be further established from the disappearance of H NMR
–
signal for 1H of –C
–
Cꢀ H at 2.18 ppm and popping-up of a new singlet
peak at 6.97 ppm for 1H of –C=Cꢀ H of triazole ring, clearly indicating
the complete cyclisation of reactants. Moreover, the signal due to –CH
from propargyl group shows an upward shift from 4.91 ppm in alkyne
2) to 5.24 ppm in 5-membered ring product due to decrease in electron
density due to linkage to aromatic heterocyclic ring.
2
–
(
3
3
.3. Spectroscopic analysis
1
3
1
C NMR spectra completely support the data obtained from H NMR
.3.1. IR spectroscopy
for all the synthesized compounds. All the desired carbon signals were
The newly synthesized compounds were examined by Infra–Red
ꢀ 1
obtained in accordance to the expected results. Carbon NMR for termi-
Spectroscopy in the range of 4000–400 cm as a neat spectrum. The IR
spectrum of the synthesized compounds was as expected and each signal
can be assigned to a specific bond stretch/bend. The spectra of starting
material Carbazole and its alkyne (2) were analyzed and the two new
–
nal alkyne (2) exhibits two prominent signals at 77.95 ppm for –C
and at 72.32 ppm for -C
–
CH
–
–
CH group indicating that the desired com-
pound was formed successfully. The complete conversion of alkyne (2)
into product (3) was confirmed by the absence of signals at 77.95 ppm
ꢀ
1
–
ꢀ 1
signals at 3261 cm corresponding to –C
–
Cꢀ H and 2115 cm for
–
–
–
for –C
1
–
CH and 72.32 ppm for –C
–
–
CH and appearance of new signal at
C
–
– C– group originated in the spectrum of compound (2) confirmed the
–
–
–
–
CH.
40.09 ppm for –C
CH of triazole ring and at 123.10 ppm for -C
presence of terminal alkyne in product. In case of Benzyl azide (1), a
3

A. Rani et al.
Inorganica Chimica Acta 527 (2021) 120560
Fig. 4. (a) Metal ion detection was recorded with successive addition of 29
equiv. 10 mM solution of Mn(II) ions solution at λmax = 283 nm, using methanol
solvent. (b) Relative change in absorption maxima (A
n
/A
o
), (A
o
= Absorption
maxima of compound [2]; A
Mn(II) ions). (c) The correlation plot, (A
obtained lowest detection limit of Mn(II).
n
= Absorption maxima with successive addition of
o
–A )/A vs Concentration of Mn(II), to
n
o
Fig. 3. (a) Metal ion detection was recorded with successive addition of 14
equiv. 10 mM solution of Hg(II) ions solution at λmax = 283, using methanol
Thus, the spectroscopic data confirms the conversion of linear chained
reactants to di-substituted cyclic product.
solvent. (b) Relative change in absorption maxima (A
n
/A
o
), (A
o
= Absorption
maxima of compound (2); A
Hg(II) ions). (c) The correlation plot, (A
obtained lowest detection limit of Hg(II).
n
= Absorption maxima with successive addition of
0
–A )/A vs Concentration of Hg(II), to
n
0
3
.3.3. UV–vis spectra and metal ion sensing
The absorbance spectral properties of final product (3) were exam-
ined solvatochromically using chloroform, dimethyl sulfoxide, dime-
thylformamide and methanol as solvents. The studies concluded that
4

A. Rani et al.
Inorganica Chimica Acta 527 (2021) 120560
Fig. 5. Graphical representation of comparative metal ion detection by the 9H–carbazole derivative for Hg(II) and Mn(II).
3.4. Comparative data
methanol was ideal solvent for use, owing to high solubility and better
detection studies. To record the metal–ion effect on absorption spec-
trum, the standard solution of 1,2,3-triazole (3) with the concentration
of 0.2 mM in methanol was prepared. Same way, metal salt stock solu-
tions of Thorium (IV), Cerium (IV), Zinc (II), Cadmium (II), Copper (II),
Mercury (II), and Manganese (II) were prepared with 1 mM concentra-
tion. To access absorbance spectra, the metal salt solution was added
dropwise in cuvette containing already 2 ml of standard solution of
synthesized compound. The UV–Visible spectra were recorded at addi-
tion of one equivalent of metal salt solution in standard solution of
compound (3). Among the spectrally analysed set of metal ions, the
compound (3) exhibited significant shift in absorption maxima intensity
but negligible change was observed in absorption wavelength maxima
with Mercury (II) and Manganese (II) ions, as shown in Fig. 2.
The comparative results of metal ion detection by 9H–carbazole
derivative for Hg(II) and Mn(II) ions prove that the detection of Hg(II)
ions is nearly 2.5 times more effective in comparison to Mn(II) ions, as
shown in Fig. 5. In spite of detection of both these ions with d [5] (Mn
(II)) and d [10] (Hg(II)) system at wavelength of 283 nm, this difference
can be attributed to the binding of metal ion to the ligand. In terms of
HSAB principle, Hg(II) acts as soft acid and Mn(II) acts as hard acid and
the ligand contains donor N-atoms which are in resonance within the 5-
membered ring making them lesser hard due to utility of unpaired
electrons. As a result Hg(II) ions bind better to ligand as compared to Mn
(II) ions in the similar fashion.
3
.5. Response of compound (3) towards Cu(II) ions
3
.3.4. Response of compound (2) towards Hg(II) ions
The results of UV–vis spectra of Hg(II) titration presented a unique
To examine the response of carbazole triazole towards metal ion
response, with a swift increase in the absorption maxima associated with
an increase in absorbance value keeping value of wavelength constant.
The maximum absorbance value was achieved, by the successive addi-
tion 14 equiv. 10 mM solution of Hg(II) ions as shown in Fig. 3. The
changes in value of wavelength can be decayed because of this, the
unique behavior of Hg(II) ions can be explained as there is increase in
number of electronic transitions keeping energy gap between HOMO
and LUMO constant, due to which absorbance value increases from
sensing, different type of metals ion such as Zn(II), Hg(II), Th(IV), Cd(II),
Mn(II), Cu(II) and Ce(III) at 1 mM concentration were prepared in
methanol and investigated with the solution of 0.2 mM Carbazole tri-
azole in methanol. The outcome of UV–Visible data showed that only Cu
(
II) ion was bearing quenching sensations with a unique stimulated
changes in absorbance values while rest of metal ions were demon-
strating negligible quenching with target molecule as shown in bar
graph Fig. 2. The studies performed with remaining ions were not
remarked with “turn-on” quenching, except Cu(II) ion which exhibit
1
.2543 to 1.81291 . The lowest detection limit of Hg(II) ions, was
calculated using the plot of (A )/A vs concentration of Hg(II) ions,
– A
the detection limit 20 M of Hg (II) was obtained as shown in Fig. 3 (c).
o
n
o
“
turn-on” retort for absorption at 303 nm accompanied by hyperchromic
shift with the difference of 0.14 in absorbance value , in contrast at λmax
39 nm and 325 nm absorbance was gradually decreased with the
μ
3
3
.3.5. Response of compound (2) towards Mn(II) ions
addition of metal ion solution, which was assigned to “turn-off” response
The UV–vis spectra displayed an increase in absorption maxima upon
as shown in Fig. 6(a). Isosbestic point was rise at 320 nm due to this
successive addition 29 equiv. 10 mM solution of Mn(II) ions. It was
analyzed that, there is an increase in absorbance value that is 1.2616 to
“
turn-on” and “turn-off” phenomena. The reason for this distinctive
sensing attributes to the electronic configuration of Cu (II) that is [Ar]
1
.7523, whereas the concentration of compound (2) was decreased by
9
3
d and the presence of 3 electronegative atoms in probe ligand that is
the slowly addition of metal salt solution as shown in Fig. 4. The reason
behind this can be attributed to increase in the number of electrons in
transition from HOMO to LUMO. The energy gap between HOMO and
LUMO remains same because in UV–Visible spectra there is no desirable
change was detected in the wavelength. The lowest detection limit of Mn
10
nitrogen which made interaction with Cu and complete the octet as 3d
via ligand to metal charge transfer, because of this the lowest detection
limit of testing of Cu(II) ions is 23.5 µM Fig. 6(c).
3
.6. Mechanism of metal ion sensing
(
II) ions, was calculated using the plot of (Ao–An)/Ao vs concentration
of Mn(II) ions, the detection limit 10
shown in Fig. 4 (c).
μM of Mn (II) was obtained as
Among the various metal ions examined, the modulations in the
absorption behaviour of 9H-carbazole derivatives has been observed
only on complexation with Hg(II), Mn(II) and Cu(II) ions. These are
5

A. Rani et al.
Inorganica Chimica Acta 527 (2021) 120560
transition metal ions having large number of vacant orbitals in their
outermost shell as well as inner d-orbitals and consequently have strong
tendency to form complexes by accepting electron pairs through coor-
dinate bonding from ligands having N/O/S as donor atoms. The 9H-
carbazole derivatives 2 & 3 reported in the present research work are
enriched with nitrogen as donor atoms to bind with metal ions. Electron
transfer from ligand to metal ion changes the electronic and/or molec-
ular structure of ligand and results into fluctuation in absorption max-
ima that can be measured with UV–Visible spectroscopy. The extent and
nature of modulation either ‘Turn-on’ or ‘Turn-off’ depends upon the
nature of metal ion.
4
. Conclusions
The use of click chemistry for the synthesis of 1,2,3–triazole having
Carbazole moiety via copper–catalyzed click approach was successfully
demonstrated. The resulting synthesized compounds were characterized
1
13
by IR spectroscopy and NMR ( H and C) spectroscopy. All techniques
proved that all the synthesized structures were in alignment with the
expected results. Results obtained from UV–Vis spectra of compound (2)
gives a valuable data. The compound (2) have capability to detect Mn
(
II) and Hg (II) ions, it was confirmed by observing the increase in
absorbance value by addition of 29 equiv. 10 mM solution of Mn(II) ions
and 14 equiv. 10 mM solution of Hg(II) ions. The compound (2) gave
detection limit 10
μ
M of Mn (II) and 20 M of Hg (II). Moreover com-
μ
pound (3) was effectively detected Cu(II) ion among all the inspected
metal ions and it depicted uncommon alteration in absorbance value by
the consecutive addition of 1 mM solution of Cu (II) ions, which gave
2
3.5 µM lowest detection limit of Cu(II) ion.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120560.
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solvent. (b) Relative change in absorption maxima (A
n
/A
o
), (A
o
= Absorption
maxima of compound (3); A
n
= Absorption maxima with successive addition of
Cu(II) ions). (c) The correlation plot, (A
obtain lowest detection limit of Cu(II).
o
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