Organosilatranes with Acylthiourea Derivatives – Metal-Ion Binding, Substituent-Dependent Sensitivity, and Prospects for the Fabrication of Magnetic Hybrids

Article abstract of DOI:10.1002/ejic.201600204

A variety of topologically interesting acylthiourea-tethered organosilatranes (AcTu-OS) were prepared, and their function as metal-ion binding sites was investigated. The prepared compounds have been characterized by elemental analysis; FTIR, UV/Vis and NMR (¹H and¹³C) spectroscopy; and mass spectrometry. The organosilicon complexes 4a–4e possess diverse coordination abilities for the surveyed metal ions (Cu²⁺, Cd²⁺, Hg²⁺and Pb²⁺), as was appraised by the corresponding absorption shifts in the UV/Vis spectra. In addition, a facile preparatory route for the covalent grafting of the most efficient receptor 4e onto a silica-encrusted magnetic nanosupport was implemented. The resultant organic–inorganic hybrid nanoparticles (H-NPs) were characterized by physicochemical techniques such as FTIR spectroscopy, XRD, thermogravimetric analysis (TGA), TEM, field-emission SEM (FE-SEM) and energy-dispersive X-ray spectroscopy (EDX). The grafting of the sensory module afforded active sites for the adsorption of metal ions from the aqueous solution which is outlined using the Langmuir adsorption isotherm. The potential in sensing, sorbent properties and facile magnetic recovery of the hybrid evinces the separation process practical to undertake environmental issues.

Full text of DOI:10.1002/ejic.201600204

DOI: 10.1002/ejic.201600204
Full Paper
Organosilatranes
Organosilatranes with Acylthiourea Derivatives – Metal-Ion
Binding, Substituent-Dependent Sensitivity, and Prospects for
the Fabrication of Magnetic Hybrids
Gurjaspreet Singh*[a] and Sunita Rani[a]
Abstract: A variety of topologically interesting acylthiourea- implemented. The resultant organic–inorganic hybrid nanopar-
tethered organosilatranes (AcTu-OS) were prepared, and their ticles (H-NPs) were characterized by physicochemical tech-
function as metal-ion binding sites was investigated. The pre- niques such as FTIR spectroscopy, XRD, thermogravimetric anal-
pared compounds have been characterized by elemental analy- ysis (TGA), TEM, field-emission SEM (FE-SEM) and energy-dis-
sis; FTIR, UV/Vis and NMR (1H and 13C) spectroscopy; and mass persive X-ray spectroscopy (EDX). The grafting of the sensory
spectrometry. The organosilicon complexes 4a–4e possess di- module afforded active sites for the adsorption of metal ions
verse coordination abilities for the surveyed metal ions (Cu2+
,
from the aqueous solution which is outlined using the Lang-
Cd2+, Hg2+ and Pb2+), as was appraised by the corresponding muir adsorption isotherm. The potential in sensing, sorbent
absorption shifts in the UV/Vis spectra. In addition, a facile pre- properties and facile magnetic recovery of the hybrid evinces
paratory route for the covalent grafting of the most efficient the separation process practical to undertake environmental is-
receptor 4e onto a silica-encrusted magnetic nanosupport was sues.
Introduction
straints. Accordingly, the desire for simple molecular chemosen-
sors that respond through modifications to their optical signals
upon specific binding with a guest analyte is constantly mount-
ing.[9]
In today's technology-driven society, the recognition of cations
in aqueous or nonaqueous media is an imperative research di-
rection, as their ‘surplus value' induces lethal health afflic-
tions.[1] Considering the heavy- and transition-metal (HTM) ions,
Urea and its derivatives are considered as “privileged groups”
in both organic and inorganic chemistry owing to their multifar-
ious traits.[10] In particular, the N-acylated thiourea derivatives
(AcTu) are receiving attention in the fields of heterocyclic chem-
istry, polymer chemistry and optoelectronics and have dis-
played an array of pharmacological activities.[11,12] Furthermore,
on account of its structural framework comprising multiple liga-
ting sites, namely, C=O, C=S and N–H, the AcTu group is the
most prominent chelating ionophore for HTM ions.[13] Its inbuilt
coordinative fondness for both soft and hard metal ions sup-
ports the architectural versatility of these AcTu ligands. In the
present scenario, acylthiourea binding units have been ap-
pended onto heterocyclic chromophores, for instance, furan,
thiophene, pyridine and pyrazine. The rationale behind the se-
lection of these heterocycles is that they contain one or more
heteroatoms with an unshared pair of electrons to provide sen-
sible scaffolds for cogent coordination with metal ions.[14] Fur-
thermore, these rings may boast differential sensory aptitudes
towards particular metal ions.
On the other hand, silatranes [RSi(O1CH2CH2)3N1(O1–N1)] are
intramolecularly caged pentacoordinate silicon complexes that
can be fabricated readily by the reactions of precursor alkoxysi-
lanes with triethanolamine.[15] The variably functionalized orga-
nosilatranes have important applications in organic synthesis,
medicine and agriculture.[16] Recently, the modification of metal
oxide solid surfaces (SiO2, TiO2, Al2O3Fe3O4, etc.) with silatrane
through Si–O covalent tethering was explored.[17] If silatranes
mercury and its derivatives (Hg0, Hg2 and MeHg+) are highly
2+
deleterious, carcinogenic and notorious environmental pollu-
tants, which cause a sequence of disorders such as headaches,
nerve pain, nose bleeds, brain damage and splanchnic dam-
age.[2] On the other hand, Cu2+ ions play vital roles in numerous
biological systems including metalloenzymes, metalloproteins,
haemoglobin, superoxide dismutase and cytochrome c oxi-
dase.[3] Nevertheless, irregular levels of free Cu2+ ions may lead
to colon cancer, Indian childhood cirrhosis and gastrointestinal,
kidney and neurodegenerative diseases.[4] Similar consequen-
ces are encountered for Cd2+, Zn2+ and Pb2+ ions, and for all
these reasons, the recognition and proficient removal of HTM
ions even at low concentrations are strongly desirable.[5] In this
context, various approaches including electrochemical meth-
ods, atomic absorption, and chromatography have been devel-
oped.[6–8] However, highly sophisticated and pricey instrumen-
tation, time-consuming and tedious experimentation and un-
suitability for on-site monitoring are some of the major con-
[a] Department of Chemistry, Panjab University,
Chandigarh, 160014, India
E-mail: gjpsingh@pu.ac.in
http://chemistry.puchd.ac.in/show-biodata.php?qstrempid=
139&qstrempdesigcode=9
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201600204.
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are applied as the organic precursor, much better coating re- acyl chlorides generated acyl isothiocyanates on substitution
sults with greater ease are produced, in comparison with those with KNCS.
with conventional alkoxysilanes. This can be attributed to the
reduced propensity of silatranes towards hydrolysis, which leads
to the smooth coating of the metal oxide surface.[18] As a result,
the silatrane-coated silica nanohybrids have found some amaz-
ing applications in materials science, catalysis, chemosensing,
drug design, atomic force microscopy and so forth.[19] However,
the tedious separation of the hybrid somewhat hinders its prac-
tical utility as an advanced nanocomposite. To alleviate this is-
sue and move towards the goal of an ideal nanomaterial, the
encapsulation of a magnetic Fe3O4 core into a silica matrix is
an emerging trend.[20] The silicified magnetic nanoparticles ob-
tained with such core–shell hierarchical architecture have syner-
gistic contributions from both materials that result in high ro-
bustness, stability and magnetic retrievability, whereas the
potential of the silica template for further conjugation with or-
ganic moieties is conserved.[21]
The acyl isothiocyanates can undergo condensation reac-
tions with functional groups that feature a labile H atom such
as amines, alcohols, thiols, phenols and amino acids.[22] The ad-
dition or condensation reactions of acyl isothiocyanates are a
little more complex than the corresponding reactions of iso-
thiocyanates. During the reactions of the acyl isothiocyanates
with the amine, in addition to the expected addition product,
a simple substitution at C=O can also lead to the formation of
the corresponding amide derivative (Scheme 2). The formation
of the substituted product can be vastly suppressed only in
nonprotonic solvents such as acetonitrile or acetone. In addi-
tion, proper care must be taken during the drying of the sol-
vent, because even a slight amount of moisture will favour the
substitution rather than the addition. In our case, screening
both of these solvents independently for the addition reaction
of 3a with APS revealed that a significant yield could be ob-
In this paper, the UV/Vis absorption spectral behaviour of a tained only with acetonitrile as the solvent. We noticed that
series of organosilatrane derivatives upon interaction with HTM APS is only sparingly soluble in acetonitrile, whereas it dissolved
ions is investigated. In addition, to the best of our knowledge, readily in acyl isothiocyanate.[23] Therefore, in a further attempt
the successful immobilization of a heterocyclic-ring-appended to improve the yield, the optimized molar ratio of acid chloride
acylthiourea moiety onto the surface of magnetic silica is re- and APS was determined to be 1:0.8, and the excess acyl iso-
ported here for the first time. It is envisioned that such grafting thiocyanate helps with solvation and prompts the generation
might provide an easy pathway for the effective recognition of a highly pure product in a shorter time.
and removal of transition-metal ions.
Results and Discussion
Synthetic Approach
In the present work, a simple methodology was selected to
tether carboxylic acids to silatrane through acylthiourea linkers
to generate acylthiourea-tethered organosilatranes (AcTu-OS,
Scheme 1). Firstly, (3-aminopropyl)silatrane (APS) was prepared
from (3-aminopropyl)trimethoxysilane (APTMS) and attached to
the carboxylic acid derivatives; this route was used instead of
Scheme 2. Possible reaction profile of primary amines with acyl isothio-
cyanates.
Spectroscopic Characterization
using APTMS for the condensation reaction each time and then IR spectroscopy is one of the most widely used techniques for
reacting each condensation product with triethanolamine, as it the characterization of acylthiourea derivatives. Quantum me-
would be more time-consuming and tedious. In the next step, chanical studies on such compounds established that the two
the carboxylic acids were derivatized into more reactive acyl N–H groups are in different environments and, hence, their IR
chlorides by treatment with excess thionyl chloride, and the bands should be observed at different frequencies.[24] This
Scheme 1. Synthesis of AcTu-OS derivatives 4a–4e.
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trend was observed for the present set of compounds. For in- the acyl thiourea unit is prevalent in all derivatives, one can
stance, the N–H group attached to the carbonyl group is ob- safely surmise that it is the incorporation of different aromatic
served at ν = 3223–3403 cm–1, which is akin to the position of rings that affects the absorption maxima so significantly. For
˜
the band for an amidic NH group. On the other hand, the N–H instance, amongst all of the derivatives, compound 4a with a
group attached to the C=S group behaves differently, and its IR furan unit absorbs at the lowest value of λmax = 248 nm. The
bands are observed at ν = 3170–3183 cm–1, which provides replacement of the O atom (4a) in the heterocyclic ring by a S
˜
compelling evidence for the intramolecular H bonding of this atom (4b) instigates a pronounced redshift of 7 nm in the ab-
N–H group with the O atom of the C=O group. Another affirma- sorption spectra, which highlights the fact that the thiophene
tion of this H bonding stems from the lowered absorption fre- ring has more-delocalized π electrons than the furan ring.[26]
quencies of the C=O group (ν = 1642–1654 cm–1). The forma-
˜
tion of an atranyl cage is established by the N→Si stretching
vibrations in the range ν = 581–592 cm–1 for all AcTu-OS deriva-
˜
tives.[25]
The H and 13C NMR spectra of the compounds are consist-
ent with the respective structures. In the H NMR spectra, the
1
1
signal for the CH2NH protons splits into a multiplet, and a shift
occurs in its position from δ = 2.62 (APS) to 3.37–3.68 ppm (for
4a–4e). This shift supports the derivatization of the amine
group. Secondly, the chemical shifts of the N–H protons of the
AcTu group are significantly different (δ = 9.03–10.81 and
10.29–11.10 ppm) owing to the aforementioned intramolecular
H bonding. The peaks corresponding to the OCH2 and NCH2
protons of the silatranyl fragment appear as triplets in the ran-
ges δ = 2.76–2.83 and 3.70–3.77 ppm, respectively. The corre-
Figure 1. Normalized UV/Vis spectra of 4a–4e.
sponding peaks of the ring carbon atoms (NCH2 and OCH2) are
On switching from five- to six-membered heterocycles,
present in the ranges δ = 50.14–52.76 and 56.78–57.71 ppm, higher λmax values are obtained (Table 1, Entries 3–5). We infer
respectively, in the 13C NMR spectra. Collectively, the 13C NMR that this phenomenon is caused by the overall larger conjuga-
resonances ascribed to the C=O (δ = 156.44–163.56 ppm) and tion imparted by the six-membered rings compared with the
C=S (δ = 178.62–180.83 ppm) carbon atoms substantiate the analogous five-membered rings. Further, 4d exhibits a lower
product formation. The mass spectra of all AcTu-OS derivatives absorption maxima than 4c, and this can be attributed to the
coincide with the projected structures and have successfully electron-withdrawing inductive effect of the extra N atom intro-
depicted the respective molecular-ion peaks with the addition duced into the ring. On the other hand, compound 4e, contain-
of H+, Na+ or K+ ions.
ing a pyridine ring conjugated with bis(acylthiourea) moieties
on both sides of the central plane, absorbs at the highest λmax
value; this highlights the effect of conjugation length in the UV/
Vis absorption study of the present set of compounds.
UV/Vis Absorption Study
All AcTu-OS derivatives possess major absorption bands in the
region λ = 248–292 nm, which confirm the π–π* electronic tran-
sitions in the conjugated system. The normalized UV/Vis spectra
Recognition Studies Involving Cations
are presented in Figure 1, and the salient absorption features All of the recognition studies were performed at 25 °C, and
are listed in Table 1. The spectral characteristics of any chromo- ample time was provided for the solutions to become consist-
phore depend on its electronic environment and the degree of ent before the spectra were recorded. The sensing abilities of
conjugation encircling it, which can together allow a systematic all of the compounds were scrutinized through the addition of
fine-tuning. In the context of UV/Vis absorption (Figure 1), as Na+, K+, Mg2+, Al3+, Cu2+, Ni2+, Co2+, Zn2+, Cd2+, Hg2+ and Pb2+
Table 1. Observed yields, UV/Vis absorption data and chemosensing parameters of 4a–4e.
[b]
[c]
Entry
AcTu-OS
derivative
Synthetic yield[a]
[%]
λmax
εmax × 104
Spectral behaviour
[nm]
[L mol–1 cm–1
]
Detected M2+ ion
Stoichiometry (AcTu-OS/M2+
)
Association constant (Ka)
–1/2
1
2
4a
4b
78
80
249
3.5714
Cu2+
Cd2+
Cu2+
Cd2+
Hg2+
Cu2+
Cu2+
Cu2+
Pb2+
2:1
2:1
2:1
2:1
1:1
2:1
2:1
1:1
1:1
1.41 × 102
1.57 × 104
1.62 × 102
1.64 × 104
3.05 × 105
1.76 × 102
1.45 × 102
5.97 × 105
7.29 × 105
M
M
M
M
M
M
M
M
M
–1/2
–1/2
–1/2
–1
256
3.125
–1/2
–1/2
–1
3
4
5
4c
4d
4e
82
84
85
270
266
281
2.560
1.776
4.450
–1
[a] Isolated yield. [b] Maximum absorption wavelength. [c] Molar absorption coefficient at the maximum absorption wavelength.
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cations (up to 20 equiv.) to CH3CN solutions of each receptor stituted analogues, the anticlinal conformation (archlike shape,
(1 μ
M
; Figures S1–S5).
U) is favoured. In addition to these fundamental rearrange-
ments, the orientation of the donor site is highly affected by
the appending sites to which the acylthiourea unit is attached
(R group in Scheme 3).
In the metal complexes of thiourea-containing ligands, coor-
dination can occur through the N or S atom separately or both
simultaneously.[27] The incorporation of the acyl fragment into
the thiourea core increases the complexity of the binding sys-
tem. Depending on the dihedral angles between the acyl N
atom and the adjacent N–C bond, several feasible confirmations
can be categorized into four main forms, as outlined in
Scheme 3, in which the capital letters indicate the positions of
the C=O and C=S bonds with respect to the N–H bond. Quan-
tum mechanical calculations on a library of acylthiourea deriva-
tives indicated the prevalence of the S and U forms only.[28]
Further, in the complexes of monosubstituted derivatives, the S
form with an opposite orientation between the C=O and C=S
bonds is dominant; in this geometry, the C=O bond and H atom
of the adjacent N–H group form a pseudo-six-membered cavity.
When this intramolecular H bonding is prevented, as for disub-
The UV/Vis titration experiments disclosed that all of the re-
ceptors 4a–4e possess an approximately identical response to-
ward copper ions (Figure 2). Specifically, the injection of Cu2+
ions into the receptor solution resulted in the enhancement of
the absorption intensity and a simultaneous bathochromic shift
in the wavelength maxima. This trend may arise because in-
creased conjugation on complexation with copper ions leads to
a decrease in the π–π* transition-energy difference. During the
titrations, the absorption changes occurred in the UV region
only (below 400 nm); therefore, colour changes could not be
observed. As all of the compounds showed identical perturba-
tions in the presence of copper ions, we envisaged that the
heterocyclic ring might not be involved in the bonding, other-
wise radically different absorption patterns would have been
observed. To ascertain the exact role of the heterocyclic ring
in Cu2+ complexation, a model compound (MC) devoid of any
heteroatoms in the aromatic ring was prepared from benzoic
acid as the precursor. To our delight, the UV/Vis titration plot of
MC with Cu2+ ions was akin to that of the heterocyclic-cored
receptors (Figure S6); this strongly suggests that the heterocy-
clic rings do not participate in Cu2+ recognition.
Scheme 3. Possible conformations around the –C(O)NHC(S)N– moiety.
Figure 2. Series of UV/Vis spectra recorded in the course of the titrations of (i) 4a, (ii) 4b, (iii) 4c, (iv) 4d and (v) 4e (1 μM
in CH3CN) with Cu2+ ions (0, 0.4, 0.8,
1.2, 1.6, 2.0, 2.4, 2.8, 3.2, 3.6, 4.0, 4.4, 4.8, 5.2, 5.6, 6.0, 6.4, 6.8, 7.2, 7.6, 8.0, 8.4, 8.8, 9.2, 9.6, and 10.0 equiv.). Insets: the B–H plots for 2:1 complexation between
the receptors and Cu2+ ions (1:1 for 4e).
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As the actual binding site, that is, the acylthiourea unit em- philic mercuric ions would coordinate strongly with either the
bodies S, N and O donor atoms, it can act as a monodentate S S atom alone or the O and S donor atoms, according to the
donor, a bidentate O,S donor, or a bidentate O,N donor.[13] To Pearson hard and soft acid and base (HSAB) principle.[29] There-
further deduce the most pertinent coordination mode, the IR fore, some preorganization of the ligating sites of the molecular
spectra of the pure receptor (4a) and the receptor in the pres- platform must occur in the presence of Hg2+ ions to produce
ence of copper ions were compared (Figure S7). A close inspec- different complexation behaviour.
The recognition of Hg2+ ions by 4b is in accord with the
HSAB principle, and the coordination with the S atom of the
thiophene ring and the O atom of the adjacent C=O group
is the expected outcome (Figure 3). Further, the recognition
mechanism is confirmed by the IR spectra of 4b, which reveals
a 36 cm–1 decrease in the frequency of the band of the C=O
group in the presence of Hg2+ ions, whereas the peaks ascribed
to rest of the acylthiourea moiety were only affected minimally
(Figure S8).
tion unveils the absence of the broad peak ascribed to the N–
H group upon complexation and, therefore, indicates that the
N–H group is deprotonated (Figure S7B). This increases the elec-
tron density over the adjacent double bonds, as shown in Fig-
ure 3, and chelation is favoured. As a result, the characteristic
peaks of the C=O and C=S bonds are redshifted by 25 and
10 cm–1, respectively, which indicates that the ligand adopts
the monobasic bidentate (O,S) chelation (in the U-shaped con-
firmation) with Cu2+ ions, as shown in Figure 3.
On the other hand, when the receptors were titrated with
The binding behaviour of the receptors towards Cd2+ ions
Hg2+ ions, the expected shifts in the absorption spectra were was also studied by UV/Vis spectroscopy. The data reveal that
observed for 4b only [Figure 4 (i)], whereas the spectra of the only the spectra of 4a and 4b showed prominent changes (Fig-
other compounds remained unchanged even with an excess of ure 5), whereas those of the other compounds 4c–4e remained
Hg2+ ions (Figures S1–S5). This observation firmly rules out the unperturbed. This is in accord with previous reports, in which
formation of a binding cavity similar to that for copper ions. If furan-based acylthiourea ligands formed stable complexes with
the chelation occurred through an O,S-donor system, then even Cd2+ ions through coordination with the S atom of the C=S
better binding results would be expected, as the highly thio- group by rearranging into the “S” conformer (Figure 3).[30] Strik-
Figure 3. Proposed binding modes of the compounds with different ions.
Figure 4. Series of UV/Vis spectra recorded over the course of the titrations of (i) 4b (1 μ
ions. The insets show the corresponding linear B–H plots between the receptors and ions.
M M
in CH3CN) with Hg2+ ions and (ii) 4e (1 μ in CH3CN) with Pb2+
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Figure 5. Series of UV/Vis spectra recorded over the course of the titrations of (i) 4a (1 μ
ions. The insets show the corresponding B–H plots for 2:1 complexation between the receptors and ions.
M M
in CH3CN) with Cd2+ ions and (ii) 4b (1 μ in CH3CN) with Cd2+
ingly, only silatranes containing five-membered rings showed 4e only, and the plots for the other compounds showed sig-
sensitivity with Cd2+ ions to form the pseudocavities, whereas moid shapes. Alternatively, graphs of (A – Ao)–1 versus [Cu2+ –0.5
the substitution of the furan/thiophene ring in the OS deriva- were plotted to attain the required linearity. Therefore, in all
]
tives by larger six-membered moieties caused poor association
with Cd2+ ions; therefore, the steric encumbrance by the aro-
matic ring over the whole molecular skeleton leads to the inter-
ruption of the chelation process.
Similar screening experiments were performed for lead ions,
and only the spectrum of 4e showed clear divergence. Many
cases except 4e, the stoichiometry of the complex with copper
ions is 2:1; this suggests that two acylthiourea moieties are re-
quired to form a stable complex with copper ions, as shown in
Figure 3. The observation wavelengths for the B–H plots of 4a–
4e with Cu2+ ions were 284, 292, 285, 307 and 305 nm, respec-
tively. As discussed earlier, the heterocyclic ring is not directly
research groups have established that pyridine-2,6-dicarboxylic involved in the binding with copper ions; however, it signifi-
acid and its derivatives form a burgeoning class of ligand scaf- cantly affects the strength of the resultant complex. For in-
folds for numerous metal ions. This is because of the generation stance, amongst all of the derivatives, 4e exhibits the most
of an optimum cleftlike cavity for the metal ions through the prominent binding, as indicated by the highest Ka value, which
rearrangement of the carbonyl groups with the coplanar pyrid- was calculated from the slope of the linear fit of the corre-
ine ring.[31] As a consequence, only receptor 4e could recognize
Pb2+ ions by entrapping them in the cavity, and the binding
results are shown in the UV/Vis spectra [Figure 4 (ii)]. Decisively,
even the spectrum of 4c, which is a close analogue of 4e, did
not show noticeable change even with an excess of Pb2+ ions
(Figure S3). This emphasizes the worth of a suitable cavity for
the complexation with lead ions, as a higher number of binding
atoms and their orientation flexibility increases the stability of
the complex formed with Pb2+ ions.
sponding B–H graph. This is on account of its geometrical fea-
tures, through which two copper ions can be entrapped by the
acylthiourea ligands present on both sides of the central pyrid-
ine ring to form a 1:1 complex. A comparison of the Ka values
of silatranes 4a and 4b with five-membered rings discloses that
4b emerges as a better chelator for Cu2+ ions (Table 1). This
can be explained by the fact that thiophene is superior electron
donor than furan and leads to higher electron density at the
binding unit, which facilitates the coordination with copper
ions. On a similar basis, among the monosubstituted six-mem-
bered derivatives, compound 4c containing a pyridine ring
showed better binding than that of the pyrazine-containing
compound 4d, as the electron-withdrawing nature of the addi-
tional N atom in 4d disfavours the chelation process (Table 1,
Entries 3 and 4).
Benesi–Hildebrand Plots, Association Constants and
Stoichiometry Calculations for Receptor–Cation
Complexation
As the cation-recognition properties of receptors 4a–4e had
been explored by examining UV/Vis spectral changes upon the
addition of different cations, the altered absorbance data could
be evaluated further through the modified Benesi–Hildebrand
(B–H) equation to gauge the comparative strength of different
receptors towards the tested ions.[32] In general, the B–H rela-
tionship is used to determine the stoichiometry of complexa-
tion as well as the association constant (Ka), a measure of the
strength of the interaction between the receptor and the metal
ions in a complex.
For the Hg2+ binding system (4b), the heterocyclic ring is
actually included in the chelation and is not just an extension
to the absorbing chromophore (on the basis of the FTIR spec-
troscopy results). On the stepwise addition of Hg2+ ions to a
solution of 4b, the absorbance intensity increased, and satura-
tion was observed when 10 equiv. Hg2+ ions had been added.
From the Benesi–Hildebrand plot (wavelength 291 nm), it was
deduced that a 1:1 stoichiometry fits better for the complex of
4b with Hg2+ ions, and the association constant was deter-
–1
mined to be 3.05 × 105
M
(Table 1, Entry 2).
As the UV/Vis absorption plots (Figure 2) revealed the bind-
ing of all of the chemoreceptors with copper ions, we first
measured the stoichiometry of the complexation in each case.
On the other hand, the B–H plots obtained for the addition
of Cd2+ ions to 4a (wavelength 297 nm) and 4b (wavelength
From the assessment of the titration data, a linear regression 284 nm) involved 2:1 stoichiometries, and the binding con-
for the plot of (A – Ao)–1 versus [Cu2+ –1
]
could be obtained for stants are comparable (Table 1, Entries 1 and 2). The enhanced
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selectivity shown by 4b can be attributed to the electron-re- O bonds at ν = 1061 cm–1. Importantly, the intensity of the
˜
leasing effect of the thiophene ring. Furthermore, 4e showed previously observed Fe–O peak is largely reduced, which au-
sensitivity towards Pb2+ ions, and the corresponding complex thenticates the modification of the Fe3O4 cores. The final attach-
is formed in 1:1 stoichiometry with Ka = 7.29 × 105 –1. A com-
M
ment of the organic moiety (4e) by immobilization on the sili-
parison of the Ka values of the complexes of 4e with Cu2+ and
Pb2+ ions reveals that a more stable complex is formed with
Pb2+ ions. This can be explained on the basis of the higher ionic
radius of Pb2+ (0.98 Å) compared with that of Cu2+ (0.57 Å) and
suggests that the cavity size of this cleft-shaped molecule is
more suitable for larger cations.[33]
ceous magnetic matrix is confirmed by the intense peak corre-
sponding to the aromatic functionality (ν = 1410 cm–1); there-
˜
fore, the coating procedure was successful (Figure 6, C). The
peak at ν = 1627 cm–1 corresponds to the bending vibration of
˜
adsorbed water molecules.
The composition and arrangement of the proposed core–
shell architecture were elucidated by powder XRD. The XRD pat-
tern of the hybrid nanomaterial (Figure 7, A) matches very well
with that of standard Fe3O4 crystals with a cubic spinel struc-
ture (Figure S9). This confirms the validity of the synthetic pro-
cedure for the magnetite nanoparticles as well as their preser-
vation under the modification conditions.[35] In addition, a
broad peak at 2θ = 23° (Figure 7, A) validates the existence of
amorphous silica in the final hybrid. The important point that
needs to be highlighted is that no changes to the inherent
Characterization of the Hybrid Nanomaterial
An imperative technique to provide information about the at-
tachment of organic groups to an inorganic structure is the FTIR
spectrum of the material in the solid state.[34] The FTIR spec-
trum of pristine Fe3O4 nanoparticles displays an intense band
at ν = 1396 cm–1, which corresponds to the stretching vibra-
˜
tions of the Fe–O bonds (Figure 6, A). The broad band centred
at ν = 3007 cm–1 is attributed to O–H stretching. In the IR spec-
˜
trum of silica-encapsulated Fe3O4 nanoparticles (Figure 6, B), properties of the silica and Fe3O4 occur even after their modifi-
the strongest peak corresponds to the characteristic peak of Si– cation to form the core–shell arrangement.
Figure 6. FTIR spectra of (A) Fe3O4, (B) Fe3O4@SiO2 and (C) Fe3O4@SiO2-silatrane.
Figure 7. XRD pattern (A) and (B) TGA and DTA profiles of the hybrid composite.
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Further, to probe the thermal stability of the hybrid material, form silica coating over tiny agglomerated magnetic cores can
thermogravimetric analysis (TGA) was employed under a flow be visualized (Figure 8, A). The closer view elucidates the fact
of N2. The corresponding TGA and differential thermal analysis that the dense Fe3O4 particles have been strongly encompassed
(DTA) curves of the Fe3O4@SiO2-silatrane are depicted in Fig- by the silica shell rather than simply adhering physically or
ure 7 (B).
During the thermal analysis of silatrane immobilized on
Fe3O4@SiO2 particles, the weight loss (13.6 %) corresponding to
the initial heating stage (25–200 °C range) was attributable to
the thermodesorption of physically adsorbed water or solvent
molecules and indicates that these nanoparticles have hydro-
blending into the silica.[37] The aggregation of the Fe3O4 nano-
particles (which occurred before the coating procedure) led to
the imprisonment of more than one magnetic nanoparticle into
the silica shell, as shown in Figure 8 (B), and this could be bene-
ficial during the magnetic separation of the H-NPs.
Insights into the surface morphology and topography of the
philic surfaces.[36a] Moreover, above 240 °C, the decomposition silatrane-grafted magnetic nanoparticles were pursued by field-
of the attached organic moiety to form gases occurs, as speci- emission SEM (FE-SEM) analysis, and the corresponding images
fied by the weight loss of ca. 33.4 % in the TGA graph. As the recorded at different magnifications are shown in Figure 8 (C–
loss is much greater than that observed for the bare E). From the surface analysis, one can conclude that the parti-
Fe3O4@SiO2 (Figure S10), the presence of organic material over cles possess well-defined large spherical shapes with smooth
the core–shell matrix is confirmed. The DTA curve shows that surfaces and are uniformly distributed. During the preparation
exothermic transitions commence during the decay of the hy- of the hybrid nanoreceptor, a series of chemical reactions were
brid.[36b] It is worth mentioning here that the inorganic tem- performed, but apparently no clogging occurred between the
plates did not decompose up to the studied temperature, particles, and the particles maintained a regular spherical
which indicates the stability of the hybrid nanoparticles (H-NPs, shape, which implies that their inherent mechanical strength is
Figure 7, B), and the analysis revealed that the hybrid is com- preserved.[38]
posed of 48 % core–shell unit, whereas the organic contents
comprise 33.4 % of the hybrid. It is fascinating that the organic
content in the present nanocomposite is 0.905 mmol/g, which
is greater than that predicted on the basis of previous reports
and can be credited to the appliance of a silatrane as the cou-
pling agent. This feature can be highly advantageous for the
adsorption of metal ions, as discussed in the next section.
For the quantitative interpretation of the composition of the
hybrid nanomaterial, energy-dispersive X-ray spectroscopy
(EDX) was employed. The elemental mapping of the uncovered
Fe3O4@SiO2 nanoparticles is shown in Figure 9a, and discernible
peaks can be indexed to Fe, Si and O atoms; therefore, the EDX
results provide evidence for the encapsulation of iron oxide by
the silica shell. On the other hand, the EDX spectra of the cova-
The morphology, particle size and composition of the lently grafted nanoreceptor (Figure 9, b) possess some addi-
grafted nanohybrid were appraised by TEM. The sample was tional peaks for the C, N and S atoms of the organic precursor,
dispersed in absolute ethanol and ultrasonicated for 2 h before and the original peaks corresponding to core–shell architecture
the analysis, and the resultant micrographs at different magnifi- remained intact. This confirms the successful surface tailoring
cations are presented in Figure 8 (A and B). A completely uni- of the organosilica unit over the Fe3O4 nanoparticles.
Figure 8. (A and B) TEM and (C–E) FE-SEM micrographs of Fe3O4@SiO2-silatrane at different magnifications.
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Figure 9. EDX spectra of (a) Fe3O4@SiO2 and (b) Fe3O4@SiO2@silatrane.
Metal-Ion Adsorption Study
noadsorbent was first reacted with the ions for an adequate
time, after which the nanoparticles were separated by the appli-
cation of an external magnet, and the concentration of ions in
the supernatant was measured by UV/Vis spectroscopy. For
both ions, the adsorption increased with reaction time from 1
to 120 min and remained invariable for a reaction time of more
than 2 h. Therefore, the nanoadsorbent was treated with the
ions for 2 h before further analysis. The amount of metal ions
adsorbed by the hybrid adsorbent at equilibrium can be deter-
mined with Equation (1).
Owing to the lethal side-effects of HTM ions, various advanced
techniques have been formulated for their separation and re-
moval from water, including ion exchange, coagulation, chemi-
cal precipitation, solvent extraction, complexion, distillation and
reverse osmosis.[39] Currently, almost all of these techniques are
associated with impediments such as partial removal efficiency,
high cost, complicated sample preparation, non-recyclability
and the generation of toxic waste. The liquid–solid phase ad-
sorption stimulated by the immobilization of metal-ion-recep-
tive sites onto inorganic materials possessing magnetic separa-
bility has been a favoured pursuit in this regard.[40] The immobi-
lization process is necessary for the spontaneous adsorption,
because bare inorganic nanoparticles cannot be used directly
as a lack of potential binding sites would largely inhibit the
adsorption process. Therefore, in the present work, a hybrid in-
organic sorbent, which has been tailored to the analysis and
efficacious removal of heavy-metal ions, is utilized for the ad-
sorption purpose. This particular assembly of a chelating dipico-
linic acid derivatized ligand (4e) anchored onto the magnetic
core–shell matrix (H-NPs) is deemed to be a versatile nanore-
ceptor for HTM ions.
(Ci – Ce)V
Qe =
(1)
M
Qe is the amount of ions adsorbed per unit mass of adsorb-
ent at equilibrium, Ci and Ce symbolize the initial metal-ion con-
centration and the concentration at equilibrium, respectively, V
is the volume of the solution, and M is the weight of the ad-
sorbent (g). To determine the saturated adsorption amount Qm
(mg/g), Langmuir adsorption was employed [Equation (2)].[39]
Ce Ce
Qe Qm KαQm
1
=
+
(2)
As the precursor organosilatrane (4e) has shown binding
The straight line obtained for a plot of Ce/Qe against Ce con-
with copper and lead ions (Table 1), both of these ions were firms the viability of a Langmuir adsorption phenomenon. Ex-
tested for their adsorptive removal. For this, the H-NPs (0.025 g) plicitly, Qm was calculated through the reciprocal of the slope
were added into aqueous solutions (10 mL) of Cu2+ and Pb2+ of the straight line generated by the Langmuir linear fitting of
ions of concentrations 0.5–2.5 mmol/L. The adsorption kinetics the adsorption isotherm curve (Figure 10). The high model-fit
was explored by UV/Vis spectroscopy, as the absorbance is di- parameters (R2) in both cases (Table 2) support the fact that the
rectly proportional to the concentration of metal ions. The na- adsorption phenomenon follows the Langmuir criterion, which
Figure 10. Langmuir adsorption isotherm for the adsorption of (A) Cu2+ and (B) Pb2+ ions.
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mide (three drops) were placed in a round-bottomed flask main-
tained at 0 °C. To this mixture, thionyl chloride in excess (10 equiv.)
was added slowly (Table T1, Supporting Information). The mixture
was then heated at 80 °C for 6 h, after which the evolution of HCl
gas declined. Dry toluene was then added to the mixture for the
azeotropic removal of excess thionyl chloride with a Dean–Stark
extraction trap. Petroleum ether was added to wash the residue,
and the final product was obtained after vacuum-filtration. No fur-
ther attempts were made to purify the acid chlorides, and the prod-
ucts were used as obtained in the subsequent reactions.
proposes that adsorption occurs at specific homogeneous sites
within the adsorbent and is suited to monolayer adsorption
processes.[40]
Table 2. The relevant parameters of the Langmuir fit.
Adsorbed metal ion
R2
Qm [mmol/g]
Ka [L/mmol]
Cu2+
Pb2+
0.9875
0.9925
0.3817
0.4210
1.67
2.16
Conclusions
General Procedure for the Synthesis of Acylthiourea-Linked Or-
ganosilatranes 4a–4e: To
a suspension of acid chloride
Acylthiourea-tethered heterocyclic rings have been attached to
silatrane through a C3 linker and probed as receptors for heavy-
and transition-metal ions. The tuning of the electronic proper-
ties by variation of the substituents in these derivatives resulted
in differential sensitivity and selectivity towards the tested
metal ions. In addition, a protocol for the fabrication of mag-
netic silica nanospheres functionalized by the pyridine-based
bis(AcTu-OS) is presented. In addition to supplying a straightfor-
ward methodology for the synthesis of magnetic core–shell-
structured organosilica nanocarriers, this study underlines their
strong adsorption activity for copper and lead ions. This new
type of silatrane-based advanced material can be applied in the
detection and removal of heavy-metal ions.
(1.00 equiv.) in freshly dried acetonitrile (20 mL), KNCS (1.50 equiv.)
was added. The mixture was heated under reflux for 2 h. A gradual
change from colourless/brown to dark orange along with the for-
mation of a precipitate signified the successful formation of the
corresponding organic acyl isothiocyanate (3a–3e). The byproduct
KCl was removed by vacuum-filtration, and the filtrate containing
acyl isothiocyanate was not further worked upon for isolation. To
the filtrate, APS (0.80 equiv.) was added, and the resultant mixture
was heated under reflux for 24 h. After cooling to room tempera-
ture, the mixture was poured into water, and chloroform (25 mL)
was added. The organic layer was gradually washed with brine and
dried with anhydrous sodium sulfate. The solvent was removed un-
der reduced pressure to furnish the corresponding AcTu-OS deriva-
tives in high yields.
N-[3-(Silatranyl)propylcarbamothioyl]furan-2-carboxamide (4a):
The reactants used were 2a (0.38 mL, 3.83 mmol), KNCS (0.56 g,
5.77 mmol) and APS (0.72 g, 3.10 mmol), m.p. 143–145 °C.
Experimental Section
C
15H23N3O5SSi (385.51): calcd. C 46.7, H 6.0, N 10.9, S 8.3; found C
General Materials and Methods: Furan-2-carboxylic acid (Avra),
thiophene-2-carboxylic acid (Aldrich), picolinic acid (Aldrich), pyra-
zine-2-carboxylic acid (Merck), 2,6-pyridinedicarboxylic acid (Spec-
trochem), 3-aminopropyltrimethoxysilane (APTMS, Aldrich), tetra-
ethyl orthosilicate (TEOS, Aldrich) and triethanolamine (Merck) were
used directly as received. Thionyl chloride (SDFCL) was distilled be-
fore use. 3-Aminopropylsilatrane (APS) was prepared by following a
previously reported method.[41] Potassium thiocyanate was vac-
uum-dried for 2 h before use. The organic solvents were dried ac-
cording to standard procedures.[42]
46.4, H 6.2, N 10.7, S 8.4. IR (neat): ν = 3403 [ν(N–H)], 3178 [ν(N–H)],
˜
1642 [ν(C=O)], 1260 [ν(C=S)], 1096 [ν(Si–O)], 581 [ν(N→Si)] cm–1. 1H
NMR (300 MHz, CDCl3, 25 °C): δ = 0.49 (m, 2 H, SiCH2), 1.80 (m, 2
H, CCH2C), 2.81 (t, J = 5.7 Hz, 6 H, NCH2), 3.68 (m, 2 H, CH2NH), 3.77
(t, J = 5.7 Hz, 6 H, OCH2), 6.59 (t, J = 4.5 Hz, 1 H, H3), 7.30 (d, J =
4.6 Hz, 1 H, H2), 7.56 (d, J = 4.6 Hz, 1 H, H4), 9.03 (s, 1 H, NH), 10.46
(s, 1 H, NH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 13.34 (SiCH2),
23.95 (CCH2C), 49.19 (CH2NH), 51.11 (NCH2), 57.69 (OCH2), 113.17
(C3), 118.14 (C2), 145.39 (C4), 146.03 (C1), 156.44 (C=O), 178.62 (C=
S) ppm. MS: m/z (%) = 385 (100) [M]+.
The melting points were measured with a Mel Temp II device with
the compounds in sealed capillaries. The 1H and 13C NMR spectra
N-[3-(Silatranyl)propylcarbamothioyl]thiophene-2-carbox-
were recorded with a JEOL (AL 400 MHz) spectrometer with CDCl3 amide (4b): The reactants used were 2b (0.37 mL, 3.41 mmol),
as an internal reference, and the chemical shifts are reported rela- KNCS (0.50 g, 5.15 mmol) and APS (0.63 g, 2.71 mmol), m.p. 139–
tive to tetramethylsilane. The electronic spectra were recorded with 141 °C. C15H23N3O4S2Si (401.57): calcd. C 44.9, H 5.8, N 10.4, S 15.9;
a JASCO V-530 UV/Vis spectrophotometer. The infrared spectra of found C 44.7, H 5.7, N 10.7, S 15.7. IR (neat): ν = 3223 [ν(N–H)], 3121
˜
neat samples were recorded with a Thermo Scientific NICOLET IS50
[ν(N–H)], 1644 [ν(C=O)], 1261 [ν(C=S)], 1077 [ν(Si–O)], 592 [ν(N→Si)]
spectrophotometer. Elemental analyses were obtained with a Per- cm–1
.
1H NMR (300 MHz, CDCl3, 25 °C): δ = 0.51 (m, 2 H, SiCH2),
kin–Elmer Model 2400 CHNS elemental analyzer. The mass spectral
1.74 (m, 2 H, CCH2C), 2.81 (t, J = 5.7 Hz, 6 H, NCH2), 3.46 (m, 2 H,
measurements (ESI source with capillary voltage, 3000 V) were per- CH2NH), 3.77 (t, J = 5.7 Hz, 6 H, OCH2), 7.48 (t, J = 4.5 Hz, 1 H, H3),
formed with a WATERS Q-TOF micro mass spectrometer. The mor- 7.59 (m, J = 5.0 Hz, 1 H, H2), 7.80 (d, J = 3.6 Hz, 1 H, H4), 9.38 (s, 1
phology of the H-NPs was scrutinized by TEM at 80 kV (Hitachi H-
7500). XRD was performed with a PANalytical X'pert PRO diffractom- 13.92 (SiCH2), 23.74 (CCH2C), 48.13 (CH2NH), 50.14 (NCH2), 56.78
H, NH), 10.40 (s, 1 H, NH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ =
eter with Cu-Kα radiation. For the FE-SEM analysis, a Hitachi SU8010
(OCH2), 128.12 (C3), 131.77 (C2), 133.75 (C4), 136.97 (C1), 161.91 (C=
ultrahigh-resolution scanning electron microscope was used. The O), 179.27 (C=S) ppm. MS: m/z (%) = 424 (100) [M + Na]+.
EDX was performed with an adjacent EDAX Inc. system. The TG
analyses were performed with an SDT q600 V20.9 Build 20 TGA
N-[3-(Silatranyl)propylcarbamothioyl]pyridine-2-carboxamide
(4c): The reactants used were 2c (0.50 g, 2.81 mmol), KNCS (0.41 g,
instrument. The samples were loaded in alumina pans, and the tem-
4.22 mmol) and APS (0.52 g, 2.24 mmol), m.p. 127–129 °C.
perature was ramped at 10 °C/min from 25–1000 °C in dry air at
C16H24N4O4SSi (396.54): calcd. C 48.5, H 6.1, N 14.1, S 8.1; found C
60 mL/min.
48.4, H 5.8, N 14.5, S 8.3. IR (Neat): ν = 3375 [ν(N–H)], 1649 [ν(C=O)],
˜
General Procedure for the Synthesis of Acyl Chlorides 2a–2e: 1273 [ν(C=S)], 1082 [ν(Si–O)], 582 [ν(N→Si)] cm–1. 1H NMR (300 MHz,
Acid chlorides were prepared by the literature method.[43] In a typi- CDCl3, 25 °C): δ = 0.43 (m, 2 H, SiCH2), 1.67 (m, 2 H, CCH2C), 2.76 (t,
cal experiment, the carboxylic acid (1.00 g) and N,N-dimethylforma- J = 5.8 Hz, 6 H, NCH2), 3.37 (m, 2 H, CH2NH), 3.70 (t, J = 5.7 Hz, 6
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H, OCH2), 8.20 (m, 1 H, H4), 8.42 (m, 1 H, H3), 8.49 (d, J = 4.7 Hz, 1 of Fe3O4@SiO2 nanoparticles was established by following the mod-
H, H2), 8.58 (d, J = 5.0 Hz, 1 H, H5), 10.17 (s, 1 H, NH), 10.29 (s, 1 H,
NH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 13.46 (SiCH2), 24.60
ified Stöber method.[45] Firstly, the Fe3O4 nanoparticles (0.5 g) were
dispersed in ethanol (30 mL) by vigorous stirring for 15 min, and
(CCH2C), 50.75 (CH2NH), 52.58 (NCH2), 57.45 (OCH2), 127.19 (C2), then aqueous NH3 solution (25 %, 10 mL) was added to the mixture.
129.90 (C4), 137.05 (C3), 147.66 (C5), 151.54 (C1), 162.61 (C=O), Afterwards, TEOS (1.0 mL) was added, and the mixture was heated
179.92 (C=S) ppm. MS: m/z (%) = 420 (100) [M + Na + H]+.
under reflux for 4 h. The precipitated Fe3O4@SiO2 nanoparticles
were separated magnetically, washed with ethanol and dried. The
merit of this protocol is that the silica nanoparticles are prepared
only in the presence of Fe3O4 nanoparticles so that the magnetic
nanoparticles can be incorporated inside the silica shells during
the synthesis process itself and the extra step of grafting can be
eliminated.[46]
N-[3-(Silatranyl)propylcarbamothioyl]pyrazine-2-carboxamide
(4d): The reactants used were 2d (0.50 g, 2.79 mmol), KNCS (0.41 g,
4.22 mmol) and APS (0.52 g, 2.23 mmol), m.p. 135–137 °C.
C15H23N5O4SSi (397.52): calcd. C 45.3, H 5.8, N 17.6, S 8.1; found C
45.5, H 5.5, N 17.2, S 8.2. IR (Neat): ν = 3223 [ν(N–H)], 3183 [ν(N–
˜
H)], 1654 [ν(C=O)], 1251 [ν(C=S)], 1087 [ν(Si–O)], 586 [ν(N→Si)]
cm–1 1H NMR (300 MHz, CDCl3, 25 °C): δ = 0.30 (m, 2 H, SiCH2),
.
Fe3O4@SiO2-Silatrane Hybrid Nanoparticles (H-NPs): For the
organic coverage of the Fe3O4@SiO2 surface, the prepared
Fe3O4@SiO2 nanoparticles (200 mg) and 4e (300 mg) were sus-
pended in anhydrous toluene (20 mL). The mixture was heated un-
der reflux for 24 h. The resultant nanoparticles were isolated mag-
netically and washed with dichloromethane to remove excess or
physically adsorbed silatrane. The final product was obtained after
4 h of drying. The justification behind the selection of 4e as the
grafting precursor is that it is composed of two silatrane units in
the periphery to hook the organic units onto two surfaces simulta-
neously (Figure 11) along with the best chelating system, as dis-
cussed previously. The steric hindrance between the binding source
and the silica surfaces is relieved by the flexible intervening propyl
chains present on both sides. This kind of functional tailoring results
in the effective fastening of the chelating site by making it an inte-
gral part of the material. Such a protocol for the synthesis of or-
ganic–inorganic hybrid materials by the hydrolysis and condensa-
tion reactions of bridged organosilanes (RO)3Si–R′–Si(OR)3 has al-
ready been performed, and promising results such as better hydro-
thermal stabilities, large surface areas and ordered pore systems
encouraged us to undertake the present work.[47]
1.67 (m, 2 H, CCH2C), 2.83 (t, J = 5.7 Hz, 6 H, NCH2), 3.54 (m, 2 H,
CH2NH), 3.70 (t, J = 5.7 Hz, 6 H, OCH2), 7.80 (d, J = 4.6 Hz, 1 H, H4),
8.15 (s, 1 H, H3), 8.30 (d, J = 4.7 Hz, 1 H, H2), 10.81 (s, 1 H, NH), 11.10
(s, 1 H, NH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 13.61 (SiCH2),
29.54 (CCH2C), 50.78 (CH2NH), 52.76 (NCH2), 57.49 (OCH2), 145.35
(C2), 146.25 (C4), 149.34 (C1), 151.60 (C3), 163.56 (C=O), 179.48 (C=
S) ppm. MS: m/z (%) = 398 (100) [M + H]+.
N2,N6-Bis[3-(silatranyl)propylcarbamothioyl]pyridine-2,6-di-
carboxamide (4e): The reactants used were 2e (0.50 g, 2.08 mmol),
KNCS (0.60 g, 6.19 mmol) and APS (0.77 g, 3.32 mmol), m.p. 140–
142 °C. C27H43N7O8S2Si2 (713.97): calcd. C 45.4, H 6.1, N 13.7, S 9.0;
found C 45.1, H 6.3, N 13.4, S 9.2. IR (Neat): ν = 3256 [ν(N–H)], 3170
˜
[ν(N–H)], 1646 [ν(C=O)], 1240 [ν(C=S)], 1096 [ν(Si–O)], 587 [ν(N→Si)]
cm–1 1H NMR (300 MHz, CDCl3, 25 °C): δ = 0.52 (m, 4 H, SiCH2),
.
1.76 (m, 4 H, CCH2C), 2.81 (t, J = 5.7 Hz, 12 H, NCH2), 3.49 (m, J =
7.3 Hz, 4 H, CH2NH), 3.76 (t, J = 5.7 Hz, 12 H, OCH2), 8.12 (t, J =
9.1 Hz, 1 H, H3), 8.30 (d, J = 7.9 Hz, 2 H, H2,4), 10.53 (s, 2 H, NH),
10.71 (s, 2 H, NH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 13.26
(SiCH2), 25.15 (CCH2C), 49.24 (CH2NH), 51.13 (NCH2), 57.71 (OCH2),
124.54 (C3), 138.64 (C2,4), 149.33 (C1,5), 163.50 (C=O), 180.83 (C=S)
ppm. MS: m/z (%) = 791 (100) [M + 2K]+.
Fe3O4 Nanoparticles: The Fe3O4 magnetic nanoparticles were syn-
thesized by following the coprecipitation method without the aid
of any external stabilizer.[44] In general, a mixture of FeCl3·6H2O
(1.0 g, 3.70 mmol) and FeCl2·4H2O (0.367 g, 1.85 mmol) was dis-
solved in deionized water (30 mL) in a 100 mL round-bottomed
flask under an inert atmosphere (Figure 11). The solution was stirred
vigorously (ca. 10 min), and aqueous NH3 solution (25 %, 15 mL)
was added. Immediately, a black precipitate formed; the mixture
was stirred for 1 h, and the precipitate was magnetically separated
from the reaction vessel. Repeated washing (three times) with dis-
tilled water and ethanol led to the desired product.
Acknowledgments
The authors are grateful to the Council of Scientific and Indus-
trial Research (CSIR), New Delhi, the University Grants Commis-
sion (UGC) and Department of Science and Technology (DST),
New Delhi for providing financial support.
Keywords: Organic–inorganic hybrid composites ·
Nanoparticles · Core–shell structures · Sensors ·
Organosilatranes
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Received: February 28, 2016
Published Online: ■
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Organosilatranes
G. Singh,* S. Rani ............................ 1–13
Organosilatranes with Acylthiourea
Derivatives
– Metal-Ion Binding,
Substituent-Dependent Sensitivity,
and Prospects for the Fabrication of
Magnetic Hybrids
Acylthiourea-tethered organosilatra- ranes is further immobilized onto the
nes are synthesized by the coupling of surface of magnetic silica nano-
carboxylic acids with (aminopropyl)- spheres, and the hybrid is character-
silatrane through acyl isothiocyanates. ized and tested for the adsorption of
These derivatives are probed for copper and lead ions.
metal-ion sensing. One of the silat-
DOI: 10.1002/ejic.201600204
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