Organosilatranes with thioester-anchored heterocyclic ring assembly: Cu2+ ion binding and fabrication of hybrid silica nanoparticles

Article abstract of DOI:10.1039/c5ra09004k

This work presents the design, synthesis, UV-Vis absorption properties and Cu²⁺ ion binding of the organo-silicon complexes (3a-h) with different coordination abilities that are derived from mercaptopropylsilatrane (MPS) and respective heteroaromatic carboxylic acids (1a-h). The prepared thioester based organosilatranes (ThE-OS) have been meticulously characterized by a series of characterization techniques such as elemental analyses, FT-IR, NMR (¹H, ¹³C), LC-MS, and structure of 3e was unambiguously determined by X-ray single crystal analyses. All the compounds have shown judicious absorption enhancement in the intensity as well as λ_max values on binding with Cu²⁺ ions compared to other surveyed metal ions. In addition, it is for the first time that the hybrid silica nanoparticles (H-SiNPs) bearing a thioester linkage in the silica framework are reported. The synthesis was achieved conveniently by an in situ co-condensation reaction of tetraethyl orthosilicate (TEOS) with the corresponding ThE-OS. The derivatization of silica is confirmed by FT-IR, ¹³C and ²⁹Si solid state CP-MAS NMR, UV-Vis, TEM, XRD, TGA and EDX techniques. Furthermore, the H-SiNPs have exhibited greater affinity towards Cu²⁺ ions than the parent ThE-OS.

Full text of DOI:10.1039/c5ra09004k

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PAPER
Organosilatranes with thioester-anchored
2
+
heterocyclic ring assembly: Cu ion binding and
Cite this: RSC Adv., 2015, 5, 65963
fabrication of hybrid silica nanoparticles†
a
a
a
a
a
Gurjaspreet Singh,* Sunita Rani, Amandeep Saroa, Shally Girdhar, Jandeep Singh,
a
b
b
Aanchal Arora, Darpandeep Aulakh and Mario Wriedt*
2+
This work presents the design, synthesis, UV-Vis absorption properties and Cu ion binding of the organo-
silicon complexes (3a–h) with different coordination abilities that are derived from mercaptopropylsilatrane
(MPS) and respective heteroaromatic carboxylic acids (1a–h). The prepared thioester based
organosilatranes (ThE-OS) have been meticulously characterized by series of characterization
a
1
13
techniques such as elemental analyses, FT-IR, NMR ( H, C), LC-MS, and structure of 3e was
unambiguously determined by X-ray single crystal analyses. All the compounds have shown judicious
2+
absorption enhancement in the intensity as well as lmax values on binding with Cu ions compared to
other surveyed metal ions. In addition, it is for the first time that the hybrid silica nanoparticles (H-SiNPs)
bearing a thioester linkage in the silica framework are reported. The synthesis was achieved conveniently
by an in situ co-condensation reaction of tetraethyl orthosilicate (TEOS) with the corresponding ThE-OS.
Received 14th May 2015
Accepted 23rd July 2015
13
29
The derivatization of silica is confirmed by FT-IR, C and Si solid state CP-MAS NMR, UV-Vis, TEM,
2+
DOI: 10.1039/c5ra09004k
XRD, TGA and EDX techniques. Furthermore, the H-SiNPs have exhibited greater affinity towards Cu
www.rsc.org/advances
ions than the parent ThE-OS.
Heterocyclic aromatic systems containing nitrogen, oxygen
and sulfur atoms comprise a fundamental class of organic
Introduction
The development of molecular receptors with optical charac- compounds possessing myriad of applications in catalysis,
6
teristics that change upon binding to a specic guest analyte pharmaceuticals, and photoluminescence. Secondly, the
1
has emerged as an inherent topic of scientic research. The heterocyclic ring appended chemosensors for metal ions are
selective detection and quantication of vital cations such as appealing as they provide handy scaffolds with the appropriate
+
+
2+
2+
2+
2+
2+
Na , K , Mg , Cu , Fe , Ca and Zn etc., is particularly more structural adaptability and lone pair of electrons on hetero
7
important due to their plethora of activities in various biological atoms for efficient coordination. Consequently, their
and environmental processes. Copper is an essential so complexes with metal ions are well-known.
2
transition metal ion, third in abundance in the human body
Based on the ‘thio’ functionality, numerous chemosensing
and has a crucial role in many physiological systems such as systems have been explored for the effective and fast detection
haemoglobin biosynthesis, bone growth and nerve function of heavy transition metal ions. For instance, thioamide, thio-
3
2+
regulation. However, irregular levels of free Cu ions can cause urea, thiosemicarbazide, thiohydrazide, thioether and thio-b-
major syndromes such as Indian childhood cirrhosis, prion enaminone based ionophores have been used as a signalling
disease, Menkes disease, Parkinson's disease, Wilson disease, handle in a variety of probes having high selectivity and sensi-
4
8–13
lipid metabolism and inammatory disorders. For these tivity.
These results encouraged us to develop new effective
reasons, much effort has been devoted to the design of selective chemosensors with in-built thio functionality. We have selected
2
+
5
Cu ion chemosensors.
thioester motif since thioesters can make up strong metal ion
chelators owing to the presence of donor S atom and –C]O
14
group. In addition, it is a key synthon in various organic
transformations such as acyl transfer reactions, asymmetric
additions reactions, aldol and Michael reactions etc. Thio-
a
15
Department of Chemistry, Panjab University, India
b
Functional Materials Design & X-ray Diffraction Lab, Department of Chemistry & esters are also involved in fatty acid, polyketide biosynthesis
Biomolecular Science, Clarkson University, Box 5810, Potsdam, NY 13699, USA.
E-mail: gjpsingh@pu.ac; mwriedt@clarkson.edu
and possess stimulating roles in bactericidal, fungicidal, HIV
16,17
and tumour suppression activities.
†
Electronic supplementary information (ESI) available. CCDC 1400318. For ESI
and crystallographic data in CIF or other electronic format see DOI:
0.1039/c5ra09004k
On the other hand, siliceous materials such as silica nano-
spheres, silica shells and molecular sieves are attracting
1
This journal is © The Royal Society of Chemistry 2015
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massive interest as inorganic solid supports for organic–inor- 2400 CHNS elemental analyser. Mass spectral measurements
ganic hybrid materials because of their excellent thermal and (ESI† source with capillary voltage, 3000 V) were carried out with
mechanical stability, connection feasibility with other func- a WATERS, Q-TOF micro MASS spectrometer. The morphology
tional groups, high surface area, biocompatibility and surface of H-SiNPs was scrutinized using transmission electron
18
modication exibility. Organosilanes containing three alkoxy microscopy (TEM) at 80 kV (Hitachi H-7500). X-Ray Diffraction
groups on silicon end have been used as the convenient silica (XRD) technique was employed using PANalytical's X'pert PRO
19
13
precursors for the surface modication purpose. The method, spectrophotometer with Cu-Ka radiation. The solid state
C
2
9
however, generally involve agglomeration and polymerization and Si cross polarization magic-angle spinning (CP-MAS)
during deposition, resulting in irregular surface morphology. NMR spectra were recorded on a JEOL (400 MHz) solid state
Silatranes are triethanolamine-capped trialkoxysilanes, where ready NMR spectrometer. The Energy Dispersive X-ray (EDX)
silicon is locked into a penta coordinate position, and have an analysis was performed on JEOL JSM-6610 LV using voltage of
edge over alkoxysilanes in terms of hydrolytic and environ- 15 kV. Thermogravimetric (TG) analyses were carried out on
mental stability, purication ease, and fewer polymerization SDT Q 600 V20.9 Build 20 TGA instrument. The sample was
2
0
ꢀ
ꢁ1
chances. Subsequently, several advanced materials have also loaded in alumina pans and ramped at 10 C min from 25–
ꢀ
ꢁ1
been formulated with the use of silatranyl group as a superior 1000 C in dry air at 60 ml min
.
substrate to link organic molecules to the metal oxide surface.
The whole process occurs via nucleophilic attack by the surface Single crystal X-ray diffraction measurements
hydroxyl groups on the silicon site forming siloxyl bonds on the
Data collections were performed on single crystals coated with
Paratone-N oil and mounted on Kapton loops. Single crystal X-
ray data of all compounds were collected on a Bruker Kappa
21
surface. Such hybrid materials have found promising activities
in many elds including heterogeneous catalysis, solar photo-
voltaic cells, DNA imaging, and are recently involved in the
Apex II X-ray diffractometer outtted with a Mo X-ray source
22,23
active eld of chemosensing.
˚
(
sealed tube, l ¼ 0.71073 A) and an APEX II CCD detector
Keeping the above perspectives in mind and taking forward
our research interests that include the synthesis and application
of silatrane based molecular receptors for selective analyte
equipped with an Oxford Cryosystems Desktop Cooler low
temperature device. The APEX-II soware suite was used for
27
data collection, cell renement, and reduction. Absorption
24
sensing, we herein report systematic investigations of a family
of organosilatranes tailored with topologically interesting thio-
ester linkage conjugated to various heterocyclic rings and their
28
corrections were applied using SADABS. Space group assign-
ments were determined by examination of systematic absences,
E-statistics, and successive renement of the structures. Struc-
ture solutions were performed with direct methods using
SHELXT-2014, and structure renements were performed by
2
+
applicability as Cu ion sensors. Moreover, by embedding one of
the derivatized silatranyl modules onto the surface of silica
nanospheres, a Cu(II)-sensing system covalently graed onto the
silica surface is constructed and then fully characterized.
2
least-squares renements against |F| followed by difference
29–31
Fourier synthesis using SHELXL-2014.
All non-hydrogen
atoms were rened with anisotropic displacement parameters.
The C–H atoms were positioned with idealized geometry and
were rened with xed isotropic displacement parameters
Experimental
General materials and methods
[
(
U
eq(H) ¼ ꢁ1.2Ueq(C)] using a riding model with dC–H ¼ 0.95 A˚
The organic solvents were dried according to standard proce-
˚
aromatic) and 0.99 A (methylene) (ESI†).
25
dures. Furan-2-carboxylic acid (AVERA), thiophene-2-carboxylic
acid (Aldrich), pyrazine-2-carboxylic acid (Merck), picolinic acid
Synthesis of ThE-OS derivatives (3a–h)
(Aldrich), nicotinic acid (Aldrich), isonicotinic acid (Aldrich), 2,6-
In a round bottomed ask, 1 equiv. of heteroaromatic carboxylic
pyridinedicarboxylic acid (Spectrochem), 2,3-pyridinedicarboxylic
acid (Spectrochem), N,N-carbonyldiimidazole (Aldrich), 3-mer-
captopropyltrimethoxysilane (Aldrich), TEOS (Aldrich) and
triethanolamine (Merck) were used directly as received. Thionyl
chloride (SDFCL) and triethylamine (Merck) were distilled prior
to use. MPS was prepared by following a method already
reported in the literature.
Melting points were measured in a Mel Temp II device using
sealed capillaries. The H and C NMR spectra were recorded
on a JEOL (AL 300 MHz) spectrometer using CDCl as an
internal reference and chemical shis were reported relative to
tetramethylsilane. Electronic spectral measurements were
carried out using JASCO V-530 UV-Vis spectrophotometer. The
infrared spectra were recorded as neat spectra on a Thermo
Scientic NICOLET IS50 spectrophotometer. The Raman spec-
trum was recorded using Renishaw Raman spectrometer.
0
acid (1a–h) and 1.20 equiv. of N,N -carbonyldiimidazole (CDI)
were added in THF (30 ml) at room temperature. CO gas started
2
releasing with increase in temperature and the reaction mixture
was heated to reux for 1 h. Aer cooling to room temperature,
subsequent addition of MPS (1 equiv.) was carried out. The
reaction mixture was heated further for 8 h and then diluted
with water. The aqueous layer was extracted with ethyl acetate
and the combined organic layer was washed with brine (25%)
and dried over anhydrous Na SO . The mixture was concen-
26
1
13
2
4
3
trated in vacuo to give a crude material that was then puried by
silica gel column chromatography (ethyl acetate : hexane, 1 : 2)
to afford pure organosilatranyl derivatives.
Spectroscopic data for compounds (3a–h)
S-3-(2,8,9-Trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1-yl)propyl-
Elemental analyses were obtained on a Perkin-Elmer Model furan-2-carbothioate (3a). The reactants used were as follows: 1a
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(
0.50 g, 4.46 mmol), CDI (0.72 g, 5.36 mmol), MPS (1.11 g, 4.46
S-3-(2,8,9-Trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1-yl)propyl-
ꢀ
mmol). Yield: 79%; mp ¼ 169–171 C; empirical formula: C - pyridine-4-carbothioate (3e). The reactants used were as follows:
1
4
H NO SSi: anal. calcd: C, 48.9; H, 6.1; N, 4.1; S, 9.3; found: C, 1e (0.50 g, 4.06 mmol), CDI (0.79 g, 4.87 mmol), MPS (1.01 g, 4.06
2
1
5
ꢁ1
ꢀ
4
8.8; H, 6.0; N, 4.2; S, 9.2. IR (neat, cm ): 1642 (n C]O), 1098 (n mmol). Yield: 80%; mp ¼ 155–157 C; empirical formula: C -
1
5
1
ꢀ
~
3
22 2 4
Si–O), 693 (n C–S), 581 (n NSi). H NMR (300 MHz, CDCl , 25 C): d H N O SSi: anal. calcd: C, 50.8; H, 6.2; N, 7.9; S, 9.0; found: C,
ꢁ1
¼
2 2 2
0.35 (m, 2H, SiCH ), 1.58 (m, 2H, CCH C), 2.65 (t, 6H, NCH , J 50.7; H, 6.1; N, 7.8; S, 9.2. IR (neat, cm ): 1646 (n C]O), 1094 (n
1
ꢀ
¼
5.7 Hz), 2.90 (t, 2H, CH
2
S, J ¼ 7.3 Hz), 3.59 (t, 6H, OCH
2
, J ¼ 5.7 Si–O), 685 (n C–S), 582 (n N~ Si). H NMR (300 MHz, CDCl , 25 C): d
3
3
2
Hz), 6.37 (t, 1H, H , J ¼ 4.5 Hz), 6.98 (d, 1H, H , J ¼ 4.6 Hz), 7.41 (s, ¼ 0.38 (m, 2H, SiCH ), 1.65 (m, 2H, CCH C), 2.70 (t, 6H, NCH , J
2
2
2
4
13
ꢀ
1
H, H ). C NMR (75 MHz, CDCl
3
, 25 C): d ¼ 9.37 (SiCH ), 15.84 ¼ 5.4 Hz), 3.00 (t, 2H, CH S, J ¼ 7.4 Hz), 3.63 (t, 6H, OCH , J ¼ 5.4
2
2
2
3
2,5 3,4 13
(
1
CCH C), 31.57 (CH S), 50.98 (NCH ), 57.56 (OCH ), 111.92 (C ), Hz), 7.65 (d, 2H, H , J ¼ 4.6 Hz), 8.65 (d, 2H, H , J ¼ 4.6 Hz).
C
2
2
2
2
2
4
1
ꢀ
21.01 (C ), 145.47 (C ), 151.48 (C ), 180.86 (C]O). MS m/z NMR (75 MHz, CDCl , 25 C): d ¼ 10.25 (SiCH ), 15.79 (CCH C),
3
2
2
+
2,4
(relative abundance (%), assignment): 366 (100, (M + Na) ).
32.60 (CH S), 50.86 (NCH ), 57.37 (OCH ), 120.12 (C ), 143.52
2
2
2
3
1,5
S-3-(2,8,9-Trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1-yl)propyl- (C ), 150.47 (C ), 190.85 (C]O).
thiophene-2-carbothioate (3b). The reactants used were as
S-3-(2,8,9-Trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1-yl)propyl-
follows: 1b (0.50 g, 3.90 mmol), CDI (0.76 g, 4.69 mmol), MPS pyrazine-2-carbothioate (3f). The reactants used were as follows:
ꢀ
(
0.97 g, 3.90 mmol). Yield: 73%; mp ¼ 169–171 C; empirical 1f (0.50 g, 4.03 mmol), CDI (0.78 g, 4.81 mmol), MPS (1.00 g, 4.01
ꢀ
formula: C14
H21NO S
4 2
Si: anal. calcd: C, 46.8; H, 5.9; N, 3.9; mmol). Yield: 74%; mp ¼ 169–171 C; empirical formula: C -
1
4
ꢁ1
S,17.8; found: C, 46.6; H, 5.7; N, 3.8; S, 17.5. IR (neat, cm ): 1641 H N O SSi: anal. calcd: C, 47.3; H, 5.9; N, 11.8; S, 9.0; found: C,
2
1 3 4
1
ꢁ1
~
(
n C]O), 1096 (n Si–O), 692 (n C–S), 585 (n NSi). H NMR (300 47.2; H, 5.8; N, 11.6; S, 8.9. IR (neat, cm ): 1645 (n C]O), 1095 (n
ꢀ
1
ꢀ
MHz, CDCl , 25 C): d ¼ 0.39 (m, 2H, SiCH ), 1.64 (m, 2H, Si–O), 686 (n C–S), 580 (n N~ Si). H NMR (300 MHz, CDCl , 25 C): d
3
2
3
CCH
2
C), 2.69 (t, 6H, NCH
2
, J ¼ 5.7 Hz), 2.97 (t, 2H, CH
2
S, J ¼ 7.3 ¼ 0.39 (m, 2H, SiCH ), 1.62 (m, 2H, CCH C), 2.73 (t, 6H, NCH , J
2
2
2
3
Hz), 3.63 (t, 6H, OCH
(
MHz, CDCl
(
2
, J ¼ 5.7 Hz), 7.00 (t, 1H, H , J ¼ 4.5 Hz), 7.49 ¼ 5.7 Hz), 2.95 (t, 2H, CH S, J ¼ 7.3 Hz), 3.64 (t, 6H, OCH , J ¼ 5.7
2
2
2
4 13
4 3 2
d, 1H, H , J ¼ 5.0 Hz), 7.66 (d, 1H, H , J ¼ 3.7 Hz). C NMR (75 Hz), 8.57 (m, 1H, H ), 8.70 (t, 1H, H , J ¼ 7.8 Hz), 9.02 (d, 1H, H , J
ꢀ
13
ꢀ
3
, 25 C): d ¼ 9.38 (SiCH
2
), 15.80 (CCH
2
C), 36.61 ¼ 4.7 Hz). C NMR (75 MHz, CDCl , 25 C): d ¼ 9.34 (SiCH ),
3
2
3
2
2 2 2
2 2 2 2
CH S), 50.84 (NCH ), 57.40 (OCH ), 127.57 (C ), 130.32 (C ), 14.82 (CCH C), 36.02 (CH S), 51.03 (NCH ), 57.66 (OCH ), 143.62
31.66 (C ), 142.83 (C ), 183.82 (C]O). MS m/z (relative abun- (C ), 145.73 (C ), 147.88 (C ), 193.59 (C]O). MS m/z (relative
4
1
2
4
1,3
1
+
+
dance (%), assignment): 398 (100, (M + K) ).
abundance (%), assignment): 378 (100, (M + Na) ).
S,S-Bis(3-(2,8,9-trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1-yl)-
S-3-(2,8,9-Trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1-yl)propyl-
pyridine-2-carbothioate (3c). The reactants used were as follows: propyl)pyridine-2,3bis(carbothioate) (3g). The reactants used
1
c (0.50 g, 4.06 mmol), CDI (0.79 g, 4.87 mmol), MPS (1.01 g, 4.06 were as follows: 1g (0.50 g, 2.99 mmol), CDI (1.16 g, 7.16 mmol),
ꢀ
ꢀ
mmol). Yield: 80%; mp ¼ 160–162 C; empirical formula: C15
-
MPS (1.49 g, 5.98 mmol). Yield: 80%; mp ¼ 170–172 C; empir-
22 2 4
25 39 3 8 2 2
H N O SSi: anal. calcd: C, 50.8; H, 6.2; N, 7.9; S, 9.0; found: C, ical formula: C H N O S Si : anal. calcd: C, 47.6, H, 6.2, N, 6.6,
ꢁ1
ꢁ1
50.5; H, 6.3; N, 7.6; S, 8.8. IR (neat, cm ): 1650 (n C]O), 1090 (n S, 10.2; found: C, 47.9, H, 6.8, N, 6.4, S, 10.8. IR (neat, cm ):
1
ꢀ
1
~
~
Si–O), 665 (n C–S), 576 (n NSi). H NMR (300 MHz, CDCl , 25 C): d 1650 (n C]O), 1087 (n Si–O), 689 (n C–S), 585 (n NSi). H NMR
3
ꢀ
¼
0.46 (m, 2H, SiCH ), 1.68 (m, 2H, CCH C), 2.73 (t, 6H, NCH , J (300 MHz, CDCl , 25 C): d ¼ 0.40 (m, 4H, SiCH ), 1.60 (m, 4H,
2
2
2
3
2
¼
5.8 Hz), 3.02 (t, 6H, CH S, J ¼ 7.3 Hz), 3.67 (t, 6H, OCH , J ¼ 5.7 CCH C), 2.68 (t, 12H, NCH , J ¼ 5.7 Hz), 3.05 (t, 4H, CH S, J ¼ 7.2
2
2
2
2
2
4
3
2
4
Hz), 7.28 (m, 1H, H ), 8.10 (m, 1H, H ), 8.58 (d, 1H, H , J ¼ 4.7 Hz), 3.63 (t, 12H, OCH , J ¼ 5.7 Hz), 7.57 (t, 1H, H , J ¼ 8.1 Hz),
2
5
13
ꢀ
5
3
13
Hz), 8.86 (s, 1H, H ). C NMR (75 MHz, CDCl
3
, 25 C): d ¼ 9.87 7.77 (d, 1H, H , J ¼ 9.1 Hz), 8.78 (d, 1H, H , J ¼ 9.1 Hz). C NMR
ꢀ
(
(
(
SiCH
OCH
2
), 14.89 (CCH
2
C), 36.85 (CH
2
S), 51.06 (NCH
2
), 57.66 (75 MHz, CDCl , 25 C): d ¼ 9.38 (SiCH ), 15.42 (CCH C), 36.65
3
2
2
2
4
3
5
2
4
2
), 128.29 (C ), 131.43 (C ), 135.42 (C ), 148.62 (C ), 155.50 (CH S), 51.09 (NCH ), 57.57 (OCH ), 124.34 (C ), 128.24 (C ),
2 2 2
1
3
1,5
C ), 190.32 (C]O). MS: m/z (relative abundance (%), assign- 149.37 (C ), 151.95 (C ), 191.15 (C]O), 192.59 (C]O). MS m/z
+
+
ment): 377 (100, (M + Na) ).
(relative abundance (%), assignment): 630 (100, (M + H) ).
S-3-(2,8,9-Trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1-yl)propyl-
S,S-Bis(3-(2,8,9-trioxa-5-aza-1-sila-bicyclo[3.3.3]undecan-1-yl)-
pyridine-3-carbothioate (3d). The reactants used were as follows: propyl)pyridine-2,6bis(carbothioate) (3h). The reactants used
1
d (0.50 g, 4.06 mmol), CDI (0.79 g, 4.87 mmol), MPS (1.01 g, 4.06 were as follows: 1h (0.50 g, 2.99 mmol), CDI (1.16 g, 7.16 mmol),
ꢀ
ꢀ
mmol). Yield: 82%; mp ¼ 155–157 C; empirical formula: C15
-
MPS (1.49 g, 5.98 mmol). Yield: 82%; mp ¼ 165–167 C; empirical
22 2 4
25 39 3 8 2 2
H N O SSi: anal. calcd: C, 50.8; H, 6.2; N, 7.9; S, 9.0; found: C, formula: C H N O S Si : anal. calcd: C, 47.6, H, 6.2, N, 6.6, S,
ꢁ1
ꢁ1
50.4; H, 6.1; N, 7.7; S, 8.8. IR (neat, cm ): 1655 (n C]O), 1092 (n 10.2; found: C, 47.0, H, 6.9, N, 6.9, S, 10.8. IR (neat, cm ): 1642 (n
1
ꢀ
1
~
~
Si–O), 687 (n C–S), 581 (n NSi). H NMR (300 MHz, CDCl
3
, 25 C): d C]O), 1099 (n Si–O), 697 (n C–S), 589 (n NSi). H NMR (300 MHz,
ꢀ
¼
0.43 (m, 2H, SiCH ), 1.67 (m, 2H, CCH C), 2.71 (t, 6H, NCH , J CDCl , 25 C): d ¼ 0.47 (m, 4H, SiCH ), 1.70 (m, 4H, CCH C), 2.71
2
2
2
3
2
2
¼
5.8 Hz), 3.03 (t, 2H, CH S, J ¼ 7.3 Hz), 3.65 (t, 6H, OCH , J ¼ 5.8 (t, 12H, NCH , J ¼ 5.7 Hz), 2.97 (t, 4H, CH S, J ¼ 7.3 Hz), 3.66 (t,
2
2
2
2
4
3
5
3
Hz), 7.29 (m, 1H, H ), 8.11 (d, 1H, H , J ¼ 5.7 Hz), 8.65 (d, 1H, H , J 12H, OCH , J ¼ 5.7 Hz), 7.87 (t, 1H, H , J ¼ 9.1 Hz), 8.01 (d, 2H,
2
1
13
ꢀ
2,4
13
ꢀ
¼
4.6 Hz), 9.06 (s, 1H, H ). C NMR (75 MHz, CDCl
3
, 25 C): d ¼
H
, J ¼ 7.9 Hz). C NMR (75 MHz, CDCl , 25 C): d ¼ 10.78
3
9.51 (SiCH
2 2 2 2
2 2 2 2
), 15.72 (CCH C), 32.63 (CH S), 51.19 (NCH ), 57.69 (SiCH ), 23.30 (CCH C), 37.75 (CH S), 51.35 (NCH ), 57.87
4
2
3
5
3
2,4
1,5
(
(
OCH ), 123.23 (C ), 128.39 (C ), 134.39 (C ), 148.62 (C ), 153.25 (OCH ), 123.32 (C ), 138.62 (C ), 155.82 (C ), 193.23 (C]O).
C ), 190.49 (C]O).
2
2
1
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reections were calculated, 4117 were independent and of
which 2944 [F > 4s(F )] were considered observed and used in
the structure analysis and renement. All atoms are located in
Results and discussion
Synthetic approach
0
0
Overall, in our study, the synthesis of the silatranyl allied crystallographically independent general positions. Other
potential silica precursors (3a–h) was efficiently achieved via a crystal structure details are given in Table T1, ESI.† An ortep
three-step route. The rst step was the formation of the MPS view of compound 3e with atom labeling scheme is given in
from 3-mercaptopropyltrimethoxysilane (MPTMS) and trietha- Fig. 1(A). In the asymmetric unit, the Si atom is bonded to three
nolamine (Scheme 1). The reaction was carried out in toluene at oxygen atoms with Si–O bond distances in the range 1.660–
ꢀ
˚
ꢀ
1
10 C for 6 h and driving force here was the continuous 1.669 A. The propyl chain with C–C–C angle of 119.03(19)
azeotropic removal of the by-product methanol using a Dean– imparts exibility to the entire unit thereby exhibiting a lineal
˚
Stark apparatus. Simple recrystallization from chloroform distance of 11.574 A between the nitrogen atoms at the two
afforded pure MPS with good yield.
ends.
Intermolecular hydrogen bonding interactions can be
Following this, the CDI mediated protocol of thioester
synthesis was employed through a one pot reaction of MPS and observed in the crystal packing between the carbonyl oxygen O1
˚
carboxylic acid (Scheme 1). The reaction was performed without and C4–H4 of the pyridine ring [C4–H4/O1, 2.404 A; angle C4–
ꢀ
utilizing inert atmosphere using optimized reaction conditions H4–O1, 141.53 ] (Fig. 1(B)). In addition similar hydrogen
(
Table 1, entry 8). During the course of this reaction, imidazole bonding interactions can be observed for C5–H5 of the pyridine
˚
and acyl carboxy substituted imidazole were formed, which ring with O21 attached to the Si atom [C5–H5/O21, 2.460 A;
ꢀ
combined readily to yield corresponding acylimidazole. The last angle C5–H5–O21, 138.36 ]. These intermolecular hydrogen
step of the reaction involved the in situ coupling of the acyli- bonding interactions give rise to the polymeric chains along the
midazoles with MPS to provide ThE-OS derivatives.
crystallographic b axis.
Crystallographic studies for characterization of 3e
UV-Vis absorption studies
The compound 3e crystallizes in the monoclinic space group
Choice of solvent system. Analysing the response of a
˚
P12 /n1 with unit cell dimensions of a ¼ 11.5580 A, b ¼ chemoreceptor in different solvent systems is primarily impor-
1
˚
˚
ꢀ
7
1
.3752(7) A, c ¼ 19.4783(2) A and b ¼ 93.613 , Z ¼ 4, V ¼ tant as changes in the environment may reorient corresponding
3
ꢁ3
˚
657.1(3) A , r (calculated) ¼ 1.421 g cm . A total of 27 042 energy levels, which can signicantly affect the position, shape
Scheme 1 Synthesis of ThE-OS derivatives (3a–h).
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Table 1 Optimization of reaction conditions for synthesis of compound 3a
a
b
Entry
Catalyst/coupling agent
None
Amount employed
Solvent
Reaction conditions
Reaction duration (h)
Yield (%)
ꢀ
1
2
3
4
5
6
7
8
9
—
Toluene
Toluene
110 C, st
8
8
5
10
8
8
8
8
8
8
0
ꢀ
H
2
SO
4
0.2 equiv.
10 equiv.
1 equiv.
1.2 equiv.
1.3 equiv.
1.5 equiv.
1.2 equiv.
1.2 equiv.
1.2 equiv.
110 C, st
30
67
50
76
76
76
79
70
72
ꢀ
SOCl
CDI
CDI
CDI
CDI
CDI
CDI
CDI
2
CHCl
3
80 C, st
ꢀ
Toluene
Toluene
Toluene
Toluene
THF
110 C, st
ꢀ
110 C, st
ꢀ
110 C, st
ꢀ
110 C, st
ꢀ
65 C, st
ꢀ
CH
2
Cl
2
40 C, st
ꢀ
1
0
CHCl
3
60 C, st
a
b
The amount used is w.r.t. carboxylic acid (3a). Determined by 1H NMR analysis of crude sample. The bold values specify the best optimized
conditions for the reaction.
electronic transitions in these conjugate systems and the
features of the absorption peaks are presented in Table 2.
It is observed that the different heterocyclic rings have
signicantly affected the absorption maxima values. Consid-
ering the ve-membered heterocycles used in this manuscript,
3a containing a furan ring conjugated with thioester group has
two absorption bands at lmax values of 215 nm and 284 nm. As
compared to 3a, the bathochromic shi in compound 3b
featuring a thiophene unit (Fig. S2†) emphasizes the effect of
substitution of O by S atom along with a better degree of
33
conjugation.
Switching from ve- to six-member heterocyclic ring linker
3c–e), similar bathochromic changes are observed (Table 2).
(
These shis are a direct consequence of the extension of p-
24a
conjugation length.
On the other hand, the positional
isomeric effect in compounds 3c–e is inoperative to alter the
absorption behaviour as shown in Fig. S3.† Similarly, the elec-
tron withdrawing inductive effect of extra N atom in the pyr-
azine ring (3f) fails to make any considerable change in the
absorption pattern as compared to compounds 3c–e (Fig. S3†).
The di-silatranyl substituted pyridine ring derivatives (3g–h)
Fig. 1 (A) ORTEP showing the crystal structure of 3e with displace- have acquired higher lmax values than the mono ones, owing to
ment ellipsoids drawn at the 50% probability level, selected atoms are the additional thioester group conjugation (Fig. S4†).
labeled. (B) 3D supramolecular coordination network with view along
the crystallographic b axis (hydrogen atoms have been removed for
clarity).
Recognition studies involving cations. All the recognition
studies were performed at 25 C and before recording any
ꢀ
spectrum, adequate time was given to ensure the uniformity of
the solution. The sensing abilities of all compounds were
+
+
examined by adding various cations (20 equiv.) such as Na , K ,
and intensity of the absorption bands. For instance, 3a has
shown doublet type absorption bands with two lmax values in
CH CN (Fig. S1, ESI†). However, a broad peak has been
3
2
+
3+
2+
2+
2+
2+
2+
2+
2+
Mg , Al , Ni , Cu , Co , Fe , Zn , Cd , Hg to CH
3
CN
solution of each receptor (1 mM). The addition of all cations
2
+
except Cu caused only nominal spectrum changes in any of
the compound (Fig. S5–S12†).
observed in the CH OH solvent system. Similar behaviour is
3
shown by other derivatives as well. This means two absorption
UV-Vis titration experiments with the receptors 3a–h and
copper ions showed, in most of the cases, a similar behaviour,
namely the addition perturbed the absorption bands by altering
both intensity as well as wavelength maxima. The general trend
bands of each receptor merge to make a broad band in CH OH
3
due to the H-bonded solute–solvent complex formation,
32
replacing the ne structure present in CH
CH OH as a donor solvent may interact with the surveyed metal
ions during titrations. To avoid these interactions, all the
spectra were recorded in CH CN.
ThE-OS derivatives exhibit major absorption bands in the
3
CN. Additionally,
3
3
observed is as follows: adding copper ions to the CH CN solu-
tions of receptors resulted in the margining of two bands to a
single band. A signicant red shi was simultaneously observed
in these spectra, which may be attributed to the increase in
3
ꢁ
1
region of 215–295 cm , which are ascribed to the p–p*
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Table 2 Observed UV-Vis absorption parameters for chemoreceptors 3a–h and their metal complexes
4
3
max ꢂ 10
l
max aer
Stoichiometry of
complexation (ThE-OS : Cu )
Association
constant
ꢁ
1
ꢁ1
2+
2+
ThE-OS derivative
lmax (nm)
(L mol cm
)
saturated with Cu
5
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1/2
ꢁ1
ꢁ1
ꢁ1
ꢁ1
3
3
3
3
3
3
3
3
a
b
c
d
e
f
216, 256
250, 292
216, 272
223, 271
225, 272
222, 273
251, 291
240, 286
230, 283
10.70, 11.40
10.52, 11.33
15.57, 10.26
10.80, 10.27
7.86, 7.94
10.05, 9.97
10.11, 11.61
10.52, 10.80
—
261
—
1 : 1
1 : 1
1 : 1
1 : 1
1 : 2
1 : 1
1 : 1
1 : 1
1 : 1
1.63 ꢂ 10 M
5
3.81 ꢂ 10 M
5
302
300
296
283
318
292
297
3.61 ꢂ 10 M
5
3.15 ꢂ 10 M
3
3.82 ꢂ 10 M
5
2.64 ꢂ 10 M
5
g
h
3.73 ꢂ 10 M
5
4.93 ꢂ 10 M
5
H-SiNPs
5.24 ꢂ 10 M
donor, –C]O group. Consequently, two linkage isomers are
possible for the corresponding metal complexes formed
(
Scheme 2).
To date, research on exploring the binding attribute of
1
4
thioester moiety has been scarce. Howbeit, there are a few
reports on the crystal structures of metal complexes with oxo-
esters, where the coordination to a transition metal center can
take place either through the carbonyl O atom or through
alkoxy O atom. The former binding mode is predominantly
observed as C]O group has signicant coordinative ability
through its p orbitals, while the coordination from alkoxy O
atom is accompanied by unfavourable steric effects, which
restrict the latter's participation in the complexation. Thio-
esters, sulfur analogues of oxo-esters, are expected to behave
similarly.
Scheme 2 Possible linkage isomers for thioester–metal complexes.
conjugation and consequently decrease in the p–p* transition
3
6a
34
energy difference. The intensity of this red-shied band
2
+
increased with increasing concentration of Cu ions and
eventually reached to limiting value when the addition
2
+
continued (nearly 2 equiv. of Cu ions for 3h, 4 equiv. for 3b, 3c,
d, 3g and 10 equiv. for 3a, 3e and 3f). The relative intensity and
3
2
+
the position of the red-shied band with addition of Cu ions
mainly depend on the receptor used. Since the absorption
changes occurred in the UV region (below 400 nm), any colour
To determine the exact mode of binding, the vibrational
spectroscopic tool was used. The infrared spectrum of
compound 3h reveals that the characteristic peak for C]O
35
ꢁ1
change was not observed throughout the titrations.
bond of the thioester group at 1642 cm gets shied to 1590
ꢁ
1
Mode of binding. The thioester group is a fascinating
ambidentate functional group as it possesses two different
types of binding sites: the so S donor site and the p electron
cm in the presence of copper ions (Fig. 2). This shi can be
interpreted as a result of the symmetrical coordination of the
carbonyl groups of 3h with the copper metal center. Moreover,
2+
Fig. 2 IR spectra of 3h (A) and 3h$Cu complex (B).
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2+
Scheme 3 Proposed binding modes of 3e with Cu ions.
2+
3
Fig. 3 (A) Family of UV-Vis spectra taken in the course of the titration of 3h (1 mM in CH CN) with Cu ions (0–2 equiv. i.e. 0, 0.1, 0.2, 0.3, 0.4, 0.5,
0
.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0). Inset shows the isosbestic point; (B) B–H plot for 1 : 1 complexation between 3h and
2+
Cu at 292 nm (R ¼ 0.9975).
the higher energy shi of the vibrations ascribed to the pyridine spectra, which were not observed. This reluctance of binding
2
+
ring authenticates the participation of pyridine N in the with Hg ions by the receptors corroborate to the proposed
bonding. The binding of heterocyclic unit with the copper binding model. However, the thiophene ring allied silatrane
centre was also checked by Raman spectroscopy. The Raman
2
+
spectrum of 3h$Cu complex contains the key features asso-
ciated with the pyridine ring (Fig. S13†). The most intense
ꢁ1
Raman band is observed at the frequency of 1036 cm which is
higher as compared to the free pyridine ring of dipicolinic
36b
acid. This shi further certies the binding of copper ions
with pyridine N.
The proposed binding mode is further supported by
2
+
studying the binding behaviour of compounds with Hg ions
2
+
as Hg ions display great affinity for so coordination center S
on account of the Hard and So Acid and Base (HSAB)
37
concept. In our case, the receptors (except 3b) have not shown
2
+
any notable UV-Vis absorption change in the presence of Hg
ions (Fig. S5–S12†). If alkthioxy S atom had been involved in the
binding, some changes would have to be there in the absorption
Scheme 4 Modification of silica surfaces with silatrane 3h.
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(
3b) comes forth as an excellent Hg(II) binder since it is an not be formed in the solution as explicated by the lower value of
electron-rich moiety that contains p-electrons on the C]O Ka (Fig. S20,† Table 2).
bond as well as lone-pair electrons on the S-atom of thiophene Meanwhile, the replacement of pyridine moiety to pyrazine
ring. The titration prole of receptor 3b with Hg ions is shown group as part of the metal-binding unit in the organosilatranyl
2
+
2
+
in Fig. S14.†
congener leads to decrease in the affinity for Cu , underlying
Stoichiometry calculations and comparison of binding the electron withdrawing inductive effect of the second N atom
abilities. As all receptors have shown binding with copper ions, of the ring and weaker binding ability (Fig. S21†).
herein we endeavour to make a comparative study to estimate
2,3 Disubstituted pyridine ring derivative, 3g behaves similar
the strength of the interaction of copper ions with different to the corresponding monosubstituted o-derivative, 3c having
ThE-OS derivatives. An assessment of the absorption intensity minimal different Ka value, which hints that the second
data on modied Benesi–Hildebrand (B–H) method has been substitution at the 3-position of pyridine ring have no contri-
38
exploited for the said purpose. The B–H relation is used to bution to the bonding with copper ions (Fig. S22†).
determine the association constant (K ), a measure of the However, disubstitution at 2,6 positions improves the K
a
a
strength of the interaction between the ligand and the metal value extensively. The presence of isosbestic point demon-
ions that come together to form the complex, and to determine strates that only one product is generated from 3h upon binding
2
+
the stoichiometry of complexation.
to Cu ions with the stoichiometry of 1 : 1 (Fig. 3). In fact,
From the UV-Vis titration proles, a linear regression for the among the prepared compounds, 3h comes out to be the best
ꢁ
1
2+ ꢁ1
plot of [A ꢁ A ] vs. [Cu ] is obtained for all ThE-OS deriv- chemosensor for copper ions because the two thioester groups
o
atives, except 3e. For 3e, as shown in Fig. S20,† the B–H plot of are expected to be coplanar with the central pyridine ring with
ꢁ
1
2+ ꢁ0.5
[
A ꢁ A ] versus [Cu ]
provides a straight line throughout their C]O bonds turned inside the cle toward each other to
o
2
+
2+
the entire range of Cu ion concentration, indicating 1 : 0.5 construct a nice binding pocket where Cu ions can be
40
complex formation. To conrm our statement, we constructed entrapped efficiently. A comparative study is also carried out
the B–H plot for 1 : 1 complexation (Fig. S15†). The nonlinear with some reported receptors bearing ‘thio’ functionality (Table
2
+
plot in this case again supports that the 3e$(Cu )1/2 complex- T2†) and our receptors, particularly 3h exhibited better sensing
ation occurs only through 1 : 0.5 stoichiometry. response as shown by the higher value of K
On the basis of the above discussion, a conclusion can be
b solution, the absorbance intensity increased in both drawn that the structures of the heterocyclic rings and their
compounds. In 3a, the increase was gradual and the maximum position with respect to thioester linkage affect not only the
a
.
2
+
In the rst place, on stepwise addition of Cu ions to 3a and
3
2
+
red shied band was observed at 261 nm, when 10 equiv. Cu
ions were added (Fig. S16†). While in 3b, a plateau in the the stoichiometry of complexation with Cu ions, giving an
absorption graph was reached only aer 4 equiv. of Cu ions insight into the different donor abilities of the heterocyclic
had been added (Fig. S17†). It is clear from Benesi–Hildeband rings.
maximum absorption wavelengths, but also the strength and
2
+
2
+
plots that both chemosensors 3a and 3b formed complexes with
Effect of counter anion. Experiments were performed to
2
+
2+
Cu ions in 1 : 1 stoichiometry. This observation indicates that explore the counter ion effect on the Cu -selective properties of
2
+
the thiophene S-atom in 3b favoured the binding with Cu
the receptor 3h. Acetate, sulfate, bromide, nitrate, and
ions, which can be explained in terms of HSAB concept. The perchlorate counter anions had similar inuence as compared
thiophene S-atom in 3b being a so base is inclined to form a with the chloride counter ions used in this work (Fig. S23†),
2
+
stable complex with Cu , a so acid than the furan O-atom in demonstrating that the counter ion doesn't participate in the
a, which is a well-known hard acid. Further, the data in Table 2 complexation.
show that the pyridine-based copper complexes are more stable Synthesis of hybrid silica nanoparticles (H-SiNPs). Once the
3
than the furan-based ones. Since both N and O atoms belong to best suitability of receptors 3h towards Cu(II) was proved, its
the category of hard bases, the differences in stability are practical applicability was examined by anchoring it onto the
accredited to the s-donor ability of the heterocyclic donor rings silica surface. The synthesis was achieved by a two-step
4
1
(
donor number, DN ¼ 33.1 and 6 for pyridine and furan, sequence using a modied St ¨o ber method (Scheme 4).
39
respectively).
The H-SiNPs were prepared by stirring 2.00 ml of TEOS and
It is worth pointing out that with the change in the position 6.00 ml of aqueous (25%) ammonia solution in 45 ml of ethanol
of the thioester substituent relevant to the pyridine ring (3c–e), a at room temperature. Aer a time period of 1 h, the solution
signicant decrease in K value is observed on moving from o, m turned milky indicating the formation of silica nanoparticles
a
to p isomeric position (Table 2). The superior binding of Cu(II) (SiNPs). Further, to carry out the desired coating on these
by o- and m-isomers (3c and 3d respectively) suggest that here particles, 3h (0.629 g, 1.00 mmol) was added to the milky
the two active donor units are so oriented that their accessibility solution and stirred overnight. The resultant particles were
towards free copper ions is feasible, leading to better centrifuged and washed with anhydrous ethanol (3 ꢂ 10 ml).
complexation (Fig. S18 and S19†). While with 3e, the stoichi- The rationale behind the design of H-SiNPs lies in the fact that
ometry of complexation is 1 : 0.5 which suggests that two donor the chemosensor having copper-binding recognition sites is
units of the receptor coordinate to the metal centre in an bridged between two silica units via the inclusion of the propyl
alternate manner (Scheme 3), due to which a stable chelate may chains as linker units which reduce the steric hindrance
between the silica surfaces and the binding functionality.
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13
29
Fig. 4
C CP-MAS NMR spectrum (A) and Si CP-MAS NMR spectrum (B) of H-SiNPs.
Characterization of H-SiNPs. The IR spectra of ungraed physically adsorbed water. In H-SiNPs, additional peaks at 1659,
ꢁ1
silica nanoparticles and functionalized silica nanoparticles are 1541, 1447 cm are observed corresponding to C]O (thio-
ꢁ
1
shown in Fig. S24.† Absorption peaks at 1042 cm
corre- ester) and pyridine ring vibrations, respectively. This observa-
sponding to the asymmetric stretching vibration of Si–O–Si tion conrms the graing of organic moiety onto the silica
are observed. The absorption bands observed at 1630 and surface.
ꢁ
1
3
282 cm are due to the bending and stretching vibrations of
Fig. 5 Transmission Electron Micrograph (TEM) of H-SiNPs at different magnification (A–C).
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than the original silica nanospheres, conrming the coating of
the organic moiety on the silica surface. The morphological
homogeneity of silica nanospheres remains preserved aer the
surface functionalization is carried out. The silatranyl group
acts as a modier here with the release of the triethanolamine
anions during the hydrolysis process which prevent the aggre-
gation on silica surfaces leading to smoothness of coating.
The TGA curves of disilatrane 3h, pure SiNPs and H-SiNPs
were recorded to get an insight into the thermal stability and
loading capacity of SiNPs. All these compounds were heated
ꢀ
from 25 to 1000 C under nitrogen atmosphere. The TGA curve
of 3h is a two-step prole; rst step involved the weight loss due
ꢀ
to release of ethanol from 60 to 180 C (for ethanol calcd%:
Fig. 6 Thermogravimetric curves for 3h, SiNPs and H-SiNPs.
1
4.60%, expt%: 13.40%) (Fig. 6). The rest of the molecule was
decomposed aer crossing 300 C with the evolution of
ꢀ
common gases leaving only silica (calcd%: 19.07%, expt%:
The thioester decorated organic functionalization of the
1
1
8.70%) at the end, which was identied aer annealing to
000 C.
1
3
silica gel was conrmed by solid state C CP-MAS NMR spec-
ꢀ
44
1
3
troscopy. The C CPMAS NMR spectrum of H-SiNPs exhibited
three peaks each corresponding to propyl chain carbon atoms
Non-modied SiNPs showed two well dened weight loss
ꢀ
regions; the rst one, taking place in the 25–200 C range, was
due to the thermo-desorption of physically adsorbed water,
indicating the presence of a hydrophilic surface. The second
(
Fig. 4(A)). The occurrence of peaks corresponding to pyridine
ring C atoms and a well resolved peak at d ¼ 191.57 ppm
guarantees the integrity of complete framework. The solid state
ꢀ
weight loss region from 600 C onwards was attributed to the
2
9
Si CP-MAS NMR spectrum contains the upeld resonance
peaks attributed to native silica absorptions of siloxane
45
condensation of the free silanol groups. For H-SiNPs, only
2% weight loss occurred, which proves that the remaining part
2
(
ꢁ
Si(OSi) ) and silanol groups (Si(OSi) OH) at d ¼ ꢁ110.23 and
4
3
of silica nanoparticles is composed of siloxane and the amount
le is far more than that in 3h. Moreover, the weight loss in H-
SiNPs is greater than that observed for the SiNPs, in accordance
with the loss of additional organic material affixed to the
surface of H-SiNPs.
101.74 ppm, respectively (Fig. 4(B)). The peak corresponding
to germinal silanol groups appeared as a small shoulder at d ¼
42
ꢁ
92.46 ppm. Conclusively, the peak at d ¼ ꢁ65.89 ppm cor-
responding to C–Si(OSi) group proves the covalent linkage of
3
organic moiety with the siloxane framework in the synthesized
hybrid silica nanoparticles.
The TEM images of the hybrid silica nanoparticles reveal a
dense layer of coating with the mean diameter of 250 nm. Few
micrographs with different magnications are presented in
Fig. 5.
The structural features of the silica surface before and aer
functionalization were checked by XRD measurements. The
XRD pattern for H-SiNPs shows a weak broad band at 2q ¼ 21–
ꢀ
2
5 , which is due to the amorphous silica shell formed
46
(
Fig. S25†). The elemental composition of the H-SiNPs was
investigated by EDX analysis. The characteristic peaks assigned
for carbon, oxygen, nitrogen, sulfur and silicon elements are
found in the EDX spectrum as shown in Fig. 7(A), which clearly
give evidence for the successful coating on the surface of the
SiNPs with the framework of pyridine ring conjugated with the
As the size of unmodied silica nanoparticles lies in the
43
range of 160–170 nm, this means aer functionalization by the
receptor 3h, the hybrid silica nanospheres grow slightly bigger
47
bis (thioester) linkage.
Metal ion binding studies with H-SiNPs. The UV-Vis spec-
trum of H-SiNPs shows broad absorption band in the region of
270–300 nm due to p–p* electronic transitions which is
comparable to those of the precursor 3h (Fig. S26†). This
conrms that 3h had been covalently anchored onto silica
surface without any degradation. Further, the copper ion
binding was tested with H-SiNPs and to our delight; we found
that under the mastery of silica support, better results were
obtained. The presence of one isosbestic point here also
conrmed the ratio of binding of the hybrid particles with metal
as 1 : 1 (Fig. S27†). In this case, the change in absorption
2
+
intensity caused by Cu addition was more prominent as
compared to change in 3h and the value of binding constant K
a
was also higher (Table 2). As the silica unit and the binding
parts are covalently bonded with each other, the hybrid mate-
rials display the distinct properties together, where the stability
2+
Fig. 7 Energy Dispersive X-ray (EDX) graphs of H-SiNPs (A) and Cu
loaded H-SiNPs (B).
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would be provided by the inorganic support and the organic
part is required for the necessary binding sites.
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2
+
Isolation of Cu $H-SiNPs complex. We have reacted
compound 3h with CuCl $2H O in CH CN in 1 : 1 mole ratio for
2
2
3
4
h under continuous stirring at room temperature. Then the
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solvent was removed in vacuo to yield blue coloured solid.
Further immobilization of this complex onto silica surface was
carried out by the following the same procedure as used for its
precursor 3h. Aer similar work up, we have recorded the EDX
spectrum of this material that clearly demonstrated the pres-
ence of copper ions in the hybrid structure (Fig. 7(B)). This
observation supports the effective copper ion binding by H-
SiNPs. A pictorial representation of pure SiNPs, H-SiNPs and
2
+
Cu $H-SiNPs have been depicted in Fig. S28.† The brown
colour of the H-SiNPs shows the coating of organic moiety onto
the silica surface. Similarly, blue colour of Cu $H-SiNPs
signies the presence of copper ions.
9 M. Vonlanthen, C. M. Connelly, A. Deiters, A. Linden and
N. S. Finney, J. Org. Chem., 2014, 79, 6054–6060.
10 L. Tang, P. Zhou, Q. Zhang, Z. Huang, J. Zhao and M. Cai,
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2
+
1
1 H. Zheng, Z. H. Qian, L. Xu, F. F. Yuan, L. D. Lan and J. G. Xu,
Org. Lett., 2006, 8, 859–861.
Conclusions
In nut-shell, we have synthesized a series of new organo- 12 T. Chen, W. Zhu, Y. Xu, S. Zhang, X. Zhang and X. Qian,
silatranes which are tethered to a variety of heterocyclic moie- Dalton Trans., 2010, 39, 1316–1320.
ties via thioester linker. It was found that the nature of the 13 H. S. Kumbhar, U. N. Yadav, B. L. Gadilohar and
heterocyclic base attached to the thioester ring greatly affect the G. S. Shankarling, Sens. Actuators, B, 2014, 203, 174–180.
optical properties and metal ion binding properties. All these 14 (a) L. Feng and Z. Chen, Sens. Actuators, B, 2007, 120, 665–
organo-silicon derivatives have the potential to be transferred
into silica based organic–inorganic hybrid materials. It is shown
that the pyridine ring integrated with bis(thioester) function-
668; (b) C. Hou, Y. Xiong, N. Fub, C. C. Jacquot,
T. C. Squier and H. Cao, Tetrahedron Lett., 2011, 52, 2692–
2696.
ality was covalently attached to the silica units through exible 15 (a) Y. T. Huang, S. Y. Lu, C. L. Yi and C. F. Lee, J. Org. Chem.,
propyl chains. The resultant H-SiNPs have shown better binding
with copper ions than the ungraed precursor. These ndings
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B. C. Benicewicz, Org. Lett., 2000, 2, 3213–3216.
indicate that the design and synthesis of such innovative hybrid 16 (a) J. Staunton and K. J. Weissman, Nat. Prod. Rep., 2001, 18,
materials can be substantially important in the eld of metal
ion detection.
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1
7 J. L. Jios, S. I. Kirin, T. Weyherm u¨ ller, N. Metzler-Nolte and
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Acknowledgements
One of the authors, Sunita Rani, is grateful to the Council of 18 (a) R. Seno and S. Kobatake, Dyes Pigm., 2015, 114, 166–174;
Scientic and Industrial Research (CSIR), India, for providing
nancial support in the form of CSIR-SRF (NET) fellowship.
(b) C. Sanchez, B. Juli ´a n, P. Belleville and M. Popall, J. Mater.
Chem., 2005, 15, 3559–3592; (c) K. Nagase, J. Kobayashi,
A. Kikuchi, Y. Akiyama, H. Kanazawa and T. Okano, ACS
Appl. Mater. Interfaces, 2012, 4, 1998–2008; (d) Y. Li, B. Yan
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65974 | RSC Adv., 2015, 5, 65963–65974
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