Bis-Organosilicon based receptor for detection of Hg2+ ions: Low detection limit and excellent selectivity

Article abstract of DOI:10.1016/j.jorganchem.2020.121458

Mercury Contamination represents a major environmental and health concern for which new eco-friendly solutions are desired. Azide–alkyne cycloaddition and Claisen Schmidt reaction were attempted to trigger a flavanone based bis-siloxyl hybrid materials. T

Full text of DOI:10.1016/j.jorganchem.2020.121458

Journal Pre-proof
Bis-Organosilicon based receptor for detection of Hg2+ ions: Low
detection limit and excellent selectivity
Gurjaspreet Singh , Pinky Satija , Akshpreet Singh ,
Kavita Chowdhary , Geetika Sharma , Mohit , Shilpy ,
Jandeep Singh , Jasbhinder Singh
PII:
S0022-328X(20)30360-0
DOI:
Reference:
https://doi.org/10.1016/j.jorganchem.2020.121458
JOM 121458
To appear in:
Journal of Organometallic Chemistry
Received date:
Revised date:
Accepted date:
1 July 2020
18 July 2020
23 July 2020
Please cite this article as: Gurjaspreet Singh , Pinky Satija , Akshpreet Singh , Kavita Chowdhary ,
Geetika Sharma , Mohit , Shilpy , Jandeep Singh , Jasbhinder Singh , Bis-Organosilicon based re-
ceptor for detection of Hg2+ ions: Low detection limit and excellent selectivity, Journal of Organometallic
Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2020.121458
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Highlights
 Organosilicon based receptors have been synthesized using Claisen Schmidt and
Click reaction.
 The synthesized sensor showed selectivity towards recognition of mercury ions.

The detection limit of the sensor was calculated to be 0.10 µM for Hg2+ ions.
1

Bis-Organosilicon based receptor for detection of Hg2+ ions: Low detection
limit and excellent selectivity
Gurjaspreet Singha*, Pinky Satijaa, Akshpreet Singhb, Kavita Chowdharya, Geetika Sharmab,
Mohita, Shilpya, Jandeep Singhc, Jasbhinder Singhc
aDepartment of Chemistry, Panjab University, Chandigarh-160014, India
bDepartment of Chemistry, GGDSD College, Sector-32, Chandigarh-160032, India
cDepartment of Chemistry, Lovely Professional University, Phagwara-144411, India
*Corresponding Author
Prof. Gurjaspreet Singh
Department of Chemistry,
P.U., Chandigarh, India
+91-0172-2534428,
Email: gjpsingh@pu.ac.in
Abstract
Mercury Contamination represents a major environmental and health concern for which new
eco-friendly solutions are desired. Azide–alkyne cycloaddition and Claisen Schmidt reaction
were attempted to trigger a flavanone based bis-siloxyl hybrid materials. The synthesized bis-
organosilicon compound was used as receptor for recognition of Hg (II) ions in semi aqueous
medium. The presence of silicon makes the receptor impetus and has benefits in the field of
nanochemistry. All the final compounds were recognized by spectroscopic techniques like
IR, NMR and further corroborate by mass spectrometry.
Keywords: Azide alkyne cycloaddition reaction, Claisen Schmidt reaction, Organosilicon
compound, Mercury ion
1. Introduction
The sensing of metal ions has arisen as area of escalating significance. In spite of great
success in the development of various chemosensor, the tracking of cumulative concentration
of toxic chemicals is still the current focus of research interest. Mercuric ion (Hg2+) is one of
the most hazardous and bioaccumulative heavy metal ion that causes grisly serious human
health issues like cognitive and motor disorders, immunotoxic, genotoxic, neurototoxic
2

effects, damages to the central nervous system, endocrine system, kidney, acrodynia,
Hunter−Russell syndrome, Minamata disease, Alzheimer’s disease, DNA damage and
mitosis impairment which can be attributed to easy coordination with biological ligands such
as proteins, DNA and enzymes [1-7]. The major sources of this pollutant are power plants, oil
refining, fertilizer industries, gold mining, ore processing, industrial wastes, solid waste
incineration, combustion of fossil fuels, wood pulping, rubber processing, coal and fossil fuel
combustion and solid waste incineration [8-10]. World Health Organization (WHO) has
prescribed limit regarding maximum contamination level for mercury in drinking water,
which is ∼2µg L-1[11-12]. Although there are various analytical tools to detect Hg2+ ions such
as atomic absorption/emission spectroscopy, electrochemical, voltammetric, potentiometric
and inductively coupled plasma mass spectrometry (ICP-MS), but these require expensive
and sophisticated instrumentation, are time consuming and laboratory based, which curb their
application in Hg2+ monitoring [13-14]. Hence new detection methods are required which
have advantages such as cost effective, practical, real time monitoring, facile, sensitive,
selective and sustainable and uses eco friendly materials [15-16].
Therefore, we were motivated to take the next step for improvement of simple and elegant
chemosensor to track Hg2+ with high sensitivity and efficacy. Our quest is about the synthesis
of a silicon based complex molecular system for the tracking of Hg2+ions in the semi aqueous
medium. The insertion of two triazole rings via click reaction into the functional system
imparts multiple coordination sites to the receptor. The chemosensor reported here has high
potential for tracking Hg(II) ions and produces biodegradable and non-toxic byproducts.
Moreover, the presence of organosilicon unit in the prepared chemosensor can lead to the
construction of silicon capped nanoparticles which can equip the receptor to have advanced
applications.
2. Experimental Aspects
2.1.Materials and methods
Propargyl bromide (Aldrich), 4-hydroxyacetophenones (AVRA), 4-hydroxy -3-methoxy
acetophenone (Alfa Aesar), 2-hydroxy naphthaldehyde (Alfa Aesar), potassium carbonate
(Avra), sodium hydroxide (Avra), [CuBr(PPh3)3](Aldrich), 3-chloropropyl triethoxysilane
(Aldrich), Sodium azide (Avra) were used as received. 3-Azidopropyltriethoxysilane
(AzPTES) was prepared from (3-chloropropyl)triethoxysilane and Sodium azide according to
a procedure reported in the literature. Infrared spectrum was obtained on a Thermo Scientific
3

13
Fischer spectrometer. The NMR spectra (1H and C) were recorded on a BRUKER (400
MHz) spectrometer. JASCO V-530 double beam spectrophotometer is used for electronic
spectral measurements. The Schlenk technique was engaged to perform reactions under
nitrogen atmosphere.
2.2.Synthesis of acetylinic napthaldehyde and acetophenone
In 5 ml DMF, 2-hydroxy naphthaldehyde was dissolved followed by the addition of
anhydrous K2CO3. The resulting suspension was stirred for 30 min and the colour change
from brown to olive green was observed. To this, addition of propargyl bromide was done
and the resultant mixture was again allowed to stir for 16 h. The workup was done by ice cold
water leading to the formation of brown coloured precipitates of acetylinic napthaldehyde.
Similar procedure was followed for preparation of acetylinic acetophenone.
2.3.Synthesis of acetylinic chalcones (5a and 5b)
To ethanolic solution of substituted acetylenic acetophenone (1.0 eq), 10 ml of NaOH (5%
w/v) was added. After stirring for 20 min, ethanolic solution of the acetylinic naphthaldehyde
(1.0 equiv) was added dropwise and the solution was stirred for a period of 5h. Mustard
coloured precipitates were obtained, filtered, recrystallized using ethanol to afford the
crystalline product.
Synthesis of 5a: Yellow solid, M.P.: 118-121°C, Yield: 95 %. IR(cm-1): 3285 (C≡C-H), 2124
(C≡C), 1653 (C=O), 1169, 1268 (O-CH2).1H(CDCl3, 400 MHz,): δ = 8.45-8.48 (d, J = 15.7
Hz, 1H, Hβ), 7.41-8.30(m,10H,Ar-H), 7.08-7.11 (d, J = 15.7 Hz, 1H, Hα), 4.94(d, J=2.5Hz, -
OCH2(Naphthalene)), 4.81 (d, J = 2.4 Hz, 2H, -OCH2(phenyl)), 4.01(s, 3H, -OCH3),2.57-
13
2.61 (m, J = 2.5 Hz, 2H, -C≡CH). C (CDCl3, 101 MHz):δ= 189.4, 161.1, 154.8, 136.4,
131.2, 130.8, 128.4, 127.7, 124.2, 123.5, 118.7, 114.6, 114.3, 77.8, 76.0, 69.5, 57.1, 55.3.
Synthesis of 5b: Yellow solid, M.P.: 132oC-135°C, Yield: 93 %. IR(cm-1): 3294 (C≡C-H),
2124 (C≡C), 1659 (C=O), 1169, 1265 (O-CH2).1H (CDCl3, 400 MHz): δ = 8.45-8.48 (d, J =
15.7 Hz, 1H, Hβ), 7.42-8.32 (m, 9H, Ar-H), 7.11-7.14 (d, J = 15.4 Hz, 1H, Hα), 4.94(d,
J=2.3Hz, -OCH2(Naphthalene)), 4.89 (d, J = 2.2 Hz, 2H, -OCH2(phenyl)), 2.56-2.60 (m, J =
13
2.3 Hz, 2H, -C≡CH). C (CDCl3, 101 MHz,):δ= 189.3, 150.3, 149.4, 136.9, 134.5, 132.4,
131.2, 128.4, 127.5, 124.7, 123.5, 122.5, 118.5, 114.3, 112.4, 111.2, 78.4, 76.0, 63.0, 56.4.
4

2.4.Synthesis of organotriethoxysilanes (6a and 6b)
To the solvent mixture of 1:1 THF/Et3N taken in a two-neck round bottomed flask, bis
acetylinic chalcone (1 equiv) was treated with 2 mol of AzPTES with a pinch of catalyst
[CuBr(PPh3)3] loading and the mixture was refluxed at 60°C for 5 h. After cooling, the
reaction mixture was filtered to separate catalyst. Bis-1,4 triazole linked chalcone
triethoxysilane was extracted by solvent evaporation, which was followed by addition of
ether to obtain mustard coloured solid product.
Synthesisof 6a: Mustard Yellow, Charring above 250⁰C.Yield: 94 %. IR(cm-1): 2876, 2921,
2962 (propyl chain), 1658 (C=O), 1598(C=C), 1013, 743(Si-O).1H NMR (CDCl3, 400 MHz):
δ= 8.35-8.39 (d, J = 15.7 Hz, 1H,Hβ), 7.20-8.18 (m, 11H, Ar-H), 6.98-7.00(d, J = 15.8 Hz,
1H, Hα), 5.36(s, 2H,-OCH2(Naphthalene)), 5.20 (s, 2H, -OCH2(phenyl)), 4.23-4.31(m, 4H,-
N3CH2-), 3.65-3.75 (m,12H, -OCH2CH3), 1.89-1.98 (m, 4H, -CCH2C-), 1.07-1.15 (m, 18H, -
OCH2CH3), 0.45-0.55 (m, 4H, -SiCH2-).13C NMR (CDCl3, 101 MHz): δ =189.1, 161.7,
155.3, 143.3, 136.7, 132.9, 130.9, 129.3, 128.5, 127.3, 124.1, 122.9, 118.0, 114.5, 63.2, 61.5,
58.9, 52.2, 23.9,18.1, 7.3. ESI-TOF-MS (m/z): 815(M-OCH2CH3).
Synthesisof 6b: Mustard Yellow, Charring above 250⁰C,Yield:91 %. IR(cm-1): 2974 (C=C-
H), 1652 (C=O), 1595(C=C), 1074, 751 (Si-O). 1HNMR(CDCl3, 400 MHz,): δ= 8.43-8.47 (d,
J = 15.3 Hz, 1H,Hβ), 7.42-8.26 (m, 10H, Ar-H), 7.14-7.16(d, J = 15.3 Hz, 1H, Hα),
5.47(s,2H,OCH2(Naphthalene)),
5.38(s,2H,-OCH2(phenyl)),
4.31-4.38(m,4H,-N3CH2-),
3.95(s,3H,-OCH3), 3.69-3.85 (m,12H, -OCH2CH3), 1.95-2.05 (m, 4H, -CCH2C-), 1.16-1.26
(m, 18H, -OCH2CH3), 0.53-0.63 (m, 4H, -SiCH2-). 13C NMR (CDCl3, 101 MHz): δ = 187.8,
138.0, 131.9, 130.7, 121.9, 121.6, 113.2, 94.9, 61.3, 56.9, 51.1, 28.4, 23.2, 17.0, 6.3. ESI-
TOF-MS (m/z): 845(M-OCH2CH3).
Synthesis of 6a+Hg complex: In a 50 ml RBF containing dry acetonitrile, equimolar amount
of 6a and anhydrous mercuric chloride were added and the contents were stirred for 2 hrs.
The obtained precipitates were filtration, washed with acetonitrile and dried to yield the
desired complex which was then characterized by IR spectrum.
3. Results and Discussion
3.1.Synthesis
We have used base catalyzed Claisen–Schmidt reaction for preparation of naphthaldehyde
based chalcones. Commercially available hydroxy-acetophenones were reacted with
5

propargyl bromide using K2CO3 as base in DMF. The intermediate acetylenes underwent
aldol condensation with acetylinic naphthaldehyde in the presence of 5% NaOH solution
resulting into Bis-acetylinic chalcones (5a, 5b) in high yields. These bis-acetylinic chalcones
underwent click chemistry with 3-azidopropyltriethoxysilane (3-AzPTES) using
[CuBr(PPh3)3] as catalyst to afford Bis-1,2,3-triazole blended triethoxysilanes (6a, 6b)
(Scheme 1).
O
O
Br
(i)
(i)
+
OH
O
X
1
2
X
O
O
Br
O
OH
+
3a, 3b
4a, 4b
X
(ii)
O
X
O
O
O
O
+
O
O
5a, 5b
2
4a, 4b
(iii)
(iv)
O
Si
O
O
N
N
N
N
N
N
O
O
X
Si
O
O
O
O
6a, 6b
Scheme 1: Synthesis of chalcone allied bis-triazole encapped organosilanes (6a and 6b).
Reagents and conditions: (i) K2CO3, DMF, 16h stir (ii) 5% NaOH Solution, Ethanol, 5h stir
(iii) 3-azidopropyltriethoxysilane, THF:TEA (1:1) (iv) [CuBr(PPh3)3], 5h reflux
3.2.Spectroscopic analysis
6

In IR spectroscopy, C=O stretching vibrations of carbonyl bond appeared in the range of
1660-1652 cm-1. The signal appeared around 2100 cm-1 and 3300 cm-1is due to bis alkynes,
5a and 5b respectively and disappearance of this region in silanes 6a and 6b endorse the
cyclization of alkynyl moiety into triazole unit. Moreover, stretching vibrations in the range
of 2975-2964 cm-1 are due to C=CH of the triazole ring which also adds on to the
1
confirmation of [2+3] cycloaddition. In H NMR spectra, Doublets at around 6.98-7.16 ppm
and 8.35-8.49 ppm corresponding to α and β hydrogen atoms respectively with a J value of
15-16Hz endorse the (E)-configuration of the chalcones. Alkynyl proton appears at around δ
≈ 2.4 ppm that upon cyclization with 3-AzPTES shows a shift to δ = 7.3-7.7 ppm which
corroborates the cyclization of alkynyl moiety into triazole unit. Due to presence of
unsymmetrical aromatic rings, OCH2 protons associated with naphthalene ring appeared at
4.81-4.89ppm and OCH2 protons linked with phenyl ring appeared at 4.9. The shifting of -
OCH2 protons of acetylene group from δ = 4.81-4.94 ppm (5a and 5b) to δ ≈ 5.20-5.47 ppm
(6a and 6b) validate the formation of cyclised product. Also, a major shift in the protons of
the carbon attached with the azide group in 3-AzPTES from 3.2 ppm to 4.2 ppm authenticates
the formation of the desired product. The ethoxy groups of the silanes manifest multiplets in
the range 1.10-1.20 ppm and 3.69-3.79 ppm respectively. Similar shifting is scrutinized in the
13
carbon spectrum of the respective compounds. In C NMR spectra of the compounds, the
carbonyl carbon appears as the least shielded carbon in the region δ = 184-189 ppm. The
vinyl carbons of the chalcone moiety appear at δ = 120-124 ppm and δ = 142.1-144.9 ppm for
α and β carbons respectively. Two carbons of alkynyl moiety (5a and 5b) appear in the range
of δ = 76.0-78.8 ppm that shift around δ = 119.4-122.9 ppm and δ = 136.6-143.0 ppm in
cyclised compounds (6a and 6b) justify the cyclization of alkyne functionality into triazole
unit. The HRMS spectrum of synthesized Bis-triazole chalcone blended organosilane (6a)
showed a signal at m/z 815 corresponding to [M-OCH2CH3].
3.3. Photophysical study
The UV-Vis absorption spectra of bis-triazole-triethoxysilanes (6a) were recorded in
Acetonitrile: H2O (9:1) mixture at room temperature with concentration of 10µM. The UV-
Visible response of 10 μM of chalcone blended organosilane (6a) in the presence of 10 mM
of various metals ions (Al3+, Cr3+, Ce3+, Fe2+, Hg2+, Zn2+, Cd2+, Cu2+, Co2+, Ni2+, Mn2+, Mg2+,
Ca2+, Na+, K+ etc.) was analyzed under identical conditions. Gratifyingly, only Hg2+ caused a
decrease in the absorbance of compound 6a which suggests the specificity of this sensing
system towards Hg2+ ions as shown in Fig 1. The prepared receptor worked selectively only
7

with Hg2+ ions can be attributed to the same cage size of the molecule as is the size of Hg2+
ions. The size of other metal ions is different from the molecule resulting in the selective
sensing of Hg2+ ions.
1.0
0.8
Analyte+Hg2+
0.6
Analyte + other metal ions
0.4
0.2
0.0
250
300
350
400
450
Wavelength
Fig 1: Variations in absorption spectrum of receptor (6a) solution in the presence of different
metal ions
To figure out the variation of UV−Vis spectra, we studied separately the absorption spectra of
chalcone blended triazolesilane (6a) in the presence of Hg2+ ions. Upon sequential addition of
Hg2+ ions, the systematic changes in the absorption spectra of compound 6a in CH3CN:H2O
(9 : 1; v/v) was observed and the absorption peaks at 373nm, 332nm and 295nm were blue-
shifted and broadened as shown in Fig 2.
8

0.4
0.2
0.0
295nm
373nm
Addition of Hg2+
332nm
250
300
350
400
450
Wavelength
Fig 2: Variation in UV–Vis absorption spectrum of receptor(6a) upon gradual addition of
Hg2+
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Metal ions
Fig 3: Bar graph representation at 373nm of Silane (6a) in the presence of various metal ions
9

Such an excellent selectivity and specificity may be attributed to the fact that Hg2+ has much
stronger affinity towards nitrogen and oxygen atoms as hard-soft interaction is more favored
here and may form coordination complex. Moreover, other metal ions have showed a
negligible change in absorbance (Fig. 3) even at higher concentration. Further, Job’s plot
indicates 1:2 stoichiometry of complexation between the bis-triazolesilane (6a) and Hg2+ (Fig
4).
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
Mole fraction
Fig 4: Job’s plot for sensor 6a
The detection limit is calculated using the equation 3σ/K, where σ is the standard deviation
and K is the slope of the plot of Hg2+ and the value was found to be 0.10µM. The association
constant of the mercury complex formed in solution has been evaluated by using the standard
Benesi–Hildebrand equation (Fig. 5) and is found to be 3.06x107M-1.
10

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
y = 8.5954x + 0.0285
R² = 0.9766
0
0.02
0.04
0.06
0.08
0.1
0.12
1/[Hg2+]
Fig 5: B-H Plot for sensor 6a
Various chemosensor have been employed for the recognition of Hg2+ ions as shown in Table
1. The observations pave the way for the silicon based probe to assess the Hg2+ ions.
Table 1: Comparison of few reported sensors for Hg2+ ions with the present receptor
Receptor
Measured Signal
Stiochiometry
LOD
Reference
Pyrene based
receptor
Fluorescence
1:1
7.8nM
[18]
Carbon
Fluorescence
1:1
42nM
[19]
Nanoparticle
BODIPY
Fluorescence
1:1
2:1
108nM
20µM
[20]
[21]
Thiophene based UV-Vis/Fluorescence
Schiffs base
Silicon based
Receptor
UV-Vis
1:2
0.10 µM
Present
Study
11

3.4. Probable Binding mechanism
Scheme 2 displays the sensing strategy of the chalcone blended organosilane (6a) toward the
target metal ion Hg2+. The silane have a flexible structure and the presence of multiple donor
sites like nitrogens in the triazole rings and carbonyl oxygen, in a single entity enhance the
binding ability towards Hg2+ ions.
O
O
O
Si
O
Si
O
O
N
Hg2+
N
N
N
N
N
Hg2+
O
O
O
O
Hg2+
O
Si
O
O
Si
O
N
N
O
N
N
N
O
N
O
O
Scheme 2: Proposed binding mode for sensor 6a with Hg2+ ion
4. Conclusion
In this work, we have developed sensitive, simple, rapid, and cost-effective sensing system
that hold great practical relevance for detection of mercury ions in semi aqueous medium
with low detection limit of 0.10µM. The sensor is based on Organosilicon compounds which
have been synthesized by employing Claisen Schmidt and Click Chemistry approach. We
expect that the developed sensor may offer opportunity for the fundamental research and
applications in the field of living sciences.
Conflict of Interest
There is no conflict of interest.
12

Acknowledgements
The authors are thankful to the DST (Inspire Fellowship), DST-PURSE and CSIR
(01(2950)/18/EMR-11) for providing financial support.
There is no conflict of interest.
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