Catalytic oxidation of diorganosilanes to 1,1,3,3-tetraorganodisiloxanes with gold nanoparticle assembly at the water-chloroform interface

Article abstract of DOI:10.1039/C8NJ04223C

The formation of the spherical self-assembly of gold nanoparticles (AuNPs) of 200 ± 20 nm size at the water-chloroform interface is achieved by employing the cyclotetrasiloxane [RSCH₂CH₂SiMeO]₄ (R = CH₂CH₂OH) as the stabilizing ligand. The interfacially stabilized AuNPs act as a versatile catalyst for selective hydrolytic oxidation of only one of the Si-H bonds in secondary organosilanes, RR¹SiH₂ (R, R¹ = alkyl, aryl, and sila-alkyl), to afford the high yield synthesis of 1,1,3,3-tetraorganodisiloxanes, (HRR¹Si)₂O. The study unravels for the first time the role of the photothermal effect arising from the excitation of the surface plasmon resonance of the AuNPs under visible light irradiation in enhancing the catalytic activity at ambient temperature.

Full text of DOI:10.1039/C8NJ04223C

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Catalytic oxidation of diorganosilanes to 1,1,3,3-
tetraorganodisiloxanes with gold nanoparticle
assembly at the water–chloroform interface†
Cite this: New J. Chem., 2019,
43, 813
Ravi Shankar, *a Asmita Sharma,a Bhawana Jangir,a Manchal Chaudharya and
Gabriele Kociok-Kohnb
¨
The formation of the spherical self-assembly of gold nanoparticles (AuNPs) of 200 ꢀ 20 nm size at
the water–chloroform interface is achieved by employing the cyclotetrasiloxane [RSCH2CH2SiMeO]4
(R = CH2CH2OH) as the stabilizing ligand. The interfacially stabilized AuNPs act as a versatile catalyst
for selective hydrolytic oxidation of only one of the Si–H bonds in secondary organosilanes, RR1SiH2
(R, R1 = alkyl, aryl, and sila-alkyl), to afford the high yield synthesis of 1,1,3,3-tetraorganodisiloxanes,
(HRR1Si)2O. The study unravels for the first time the role of the photothermal effect arising from the
excitation of the surface plasmon resonance of the AuNPs under visible light irradiation in enhancing the
catalytic activity at ambient temperature.
Received 18th August 2018,
Accepted 27th November 2018
DOI: 10.1039/c8nj04223c
rsc.li/njc
ready availability of stable organosilane precursors. In addition,
the formation of dihydrogen as the only byproduct during
Introduction
Discrete siloxane frameworks bearing Si–H bonds, also referred the reaction eliminates the need for a cumbersome work-up
to as hydrosiloxanes, represent an important family of molecular procedure. While transition metal-based catalysts have been
synthons, and the practical applications of these compounds widely investigated,7 recent efforts have been devoted to the use
as substrates in hydrosilylation, dehydro/dealkylative coupling of supported AuNPs or unsupported nano-porous gold as
reactions and thiol–ene click chemistry have been widely reusable heterogeneous catalysts.8 However, the method often
recognized to accomplish new silicon-based materials.1–3 The requires elevated temperatures for the activation of the sub-
use of 1,1,3,3-tetramethyldisiloxane as a reducing agent is also strates and leads to complete oxidation to either a mono-, di- or
well established to furnish products with a high degree of trisilanol depending on the number of Si–H bonds in the
regioselectivity and/or stereoselectivity in organic synthesis.4 organosilane precursor. In a seminal report, Shimada et al.
Conventionally, such siloxanes are synthesized by careful showed that the Au(I)PPh3/PPh3 system acts as an efficient
hydrolysis of the corresponding haloorganosilanes, HR2SiX or catalyst for dehydrogenative coupling between an organosilanol
H2RSiX (X = Cl, Br, and F; R = alkyl, aryl).5 The method, and an organosilane to afford di-/trisiloxanes bearing SiH2 or
however, has limited applications in view of the sensitivity of SiH groups.9
Si–H/Si–O bonds under acidic conditions.6 Another impediment
It is well established that the liquid–liquid interface offers
in this approach is the limited availability of appropriate organo- a useful platform to stabilize the self-assembly of metal nano-
silicon precursors. As a result, catalytic routes that enable a high particles with interesting structural attributes and optical
degree of selectivity are being explored to expand the library of properties.10 Although there exist intriguing possibilities of
hydrosiloxanes.
such NPs to act as a catalyst for the hydrolytic oxidation
The catalytic method for the oxidation of Si–H bonds in of organosilanes, there has been no precedence to validate this
organosilanes using water as an oxidant is a useful approach to concept until the publication of recent reports from our group.11
construct Si–O bonds. The reaction is of relevance in view of the AuNPs were synthesized by employing a triblock copolymer,
PiBA20-b-PDMS75-b-PiBA20 [PiBA = poly(isobornylacrylate), PDMS =
a Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas,
New Delhi-110016, India. E-mail: shankar@chemistry.iitd.ac.in
b Department of Chemistry, University of Bath, Bath BA2 7AY, UK
† Electronic supplementary information (ESI) available: XPS, UV-Vis spectro-
scopy, HRTEM, FESEM, and GC-MS. Crystallographic data. CCDC 1871681 for
7. For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8nj04223c
poly(dimethylsiloxane)], as the stabilizing matrix. Nevertheless,
NPs stabilized at the water–chloroform interface tend to agglo-
merate and are not robust enough to exhibit reusability in
subsequent cycles during the hydrolytic oxidation of primary
organosilanes to linear poly(hydrosiloxane)s, {R(H)SiO}n (R = alkyl
or aryl). There has been ample precedence in the literature
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highlighting the role of small molecules (ligands) such as plasmon resonances in the visible (640 nm) and NIR (1120 nm)
amines, phosphines, thioethers, etc. to achieve a fine balance regions. It is well known that plasmonic coupling in self-
between stability and surface activity of the NPs during a assembled metal nanostructures leads to a red-shift in the
catalytic event.12 Taking precedence from our earlier work13 visible and NIR regions, as observed herein.18
and that reported by others,14 herein, we present the self-
assembly of AuNPs at the water–chloroform interface (hereafter
referred as PIC-1) by employing [RSCH2CH2SiMeO]4
(R = CH2CH2OH) as the stabilizing ligand.
Synthesis of 1,1,3,3-tetraorganodisiloxanes
As a case study, methylphenylsilane was examined as a model
substrate for hydrolytic oxidation of the Si–H bond using PIC-1
as the catalyst. Notably, the liquid–liquid interface stabilized by
the AuNPs provides a platform for the catalytic reaction. The
reaction was performed with 0.01 mol% Au (with respect to the
silane) at 25 1C under ambient conditions and monitored by
1H NMR spectroscopy. As the reaction progresses, the substrate
concentration decreases steadily, as is evident from a decrease
in the intensity of the signal at d 4.34 due to the Si–H proton
(Fig. 1). Concomitantly, a new signal at d 5.1 grows with time,
suggesting the formation of product(s) bearing Si–H bonded
siloxy group(s). After complete consumption of the substrate,
the product in the organic layer was identified by GC-MS
analysis (Fig. S5, ESI†). The result reveals the formation of
the disiloxane (HMePhSi)2O, 1, (retention time (tR) = 15.08 min
and m/z = 258) along with minor amounts (3–4%) of MePhSi-
(OSiMePhH)2 (tR = 26.9 min, m/z = 394). The 29Si{1H} NMR
spectrum shows distinct signals at d ꢂ11.5, ꢂ13.3 (major)
and ꢂ30.2 (minor), which were assigned to HMePhSiO and
MePhSiO2 units, respectively. In the IR spectrum, characteristic
absorptions for Si–O and Si–H groups appear at 1060 and
2131 cmꢂ1, respectively. Based on these results, it is inferred
that the reaction proceeds selectively with the formation of
[HMePhSiOH] as the intermediate, which subsequently under-
goes condensation to yield (HMePhSi)2O, 1. Further oxidation
of the siloxy group containing the Si–H bond is extremely slow
to yield the trisiloxane as the minor product. No perceptible
change in the product composition was observed when the
Notably, the NPs thus formed act as a superior interfacial
catalyst for the hydrolytic oxidation of phenylsilane to linear
poly(hydrophenylsiloxane), H2PhSiO[PhHSiO]nSiPhH2, in com-
parison to the polymer-stabilized AuNPs reported earlier11a
(for details, see the Experimental section). As part of our
ongoing study, the efficacy of PIC-1 as a catalyst has been
investigated for the oxidation of diorganosilanes. The method
is a simple and viable approach for the high yield synthesis of
1,1,3,3-tetraorganodisiloxanes, (HRR1Si)2O [R = Me, R1 = Ph (1),
n-Hex (2), cyclo-Hex (3), n-Octyl (4), Me2PhSiCH2CH2 (5),
2-ThMe2SiCH2CH2 (6); R = R1 = Ph (7)]. A salient outcome
emanating from this study is the revelation of the plasmon-
induced photothermal effect arising from the excitation of the
surface plasmon resonance of the AuNPs under visible light
irradiation for the activation of Si–H bonds during the catalytic
event. The results are described herein.
Results and discussion
Characterization of the catalyst
Following a modification of our earlier approach,11 the interfacial
catalyst PIC-1 was synthesized by the reduction of HAuCl4ꢁ3H2O
with six equivalents of triethylsilane in chloroform using
[RSCH2CH2SiMeO]4 (R = CH2CH2OH) as the molecular scaffold
and subsequent addition of deionized water (chloroform : water =
4 : 1 v/v) into the reaction mixture (for details, see the Experi-
mental section). The adsorption of AuNPs at the interface occurs
readily and is accompanied by the formation of non-coalescent
water droplets dispersed in chloroform and the visual dis-
appearance of blue color in the organic layer (Fig. S1, ESI†).
The NP assembly thus formed showed no signs of clouding and/or
precipitation for several weeks under ambient conditions.
The assembly of AuNPs in PIC-1 was characterized by XPS,
HRTEM, FESEM and UV-Vis studies (Fig. S2–S4, ESI†). The
results obtained from X-ray photoelectron spectroscopy (XPS)
show a doublet for Au 4f7/2 (84.1 eV) and Au 4f5/2 (87.7 eV)
corresponding to the valence state of Au(0) while the band due
to Au(I) at 84.9 eV is absent.15 The FESEM and HRTEM micro-
graphs reveal the formation of large aggregates of individual
NPs featuring quasi-spherical ensembles of 200 ꢀ 20 nm size
with no signs of independent existence. We surmise that the
self-assembly phenomenon likely arises due to the presence of
intermolecular hydrogen bonding interactions between the
appended hydroxyl groups16 as well as a contribution from
multidentate thioether ligands17 on the cyclosiloxane scaffold
that act as mediators for NP assembly. The UV-Vis spectrum of
the AuNPs deposited on a glass surface shows distinct surface
Fig. 1 1H NMR monitoring of hydrolytic oxidation of methylphenylsilane.
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reaction was performed at 80 1C. Compound 1 was obtained in homogeneous/heterogeneous conditions is prone to changing
pure form from the reaction mixture by fractional distillation. as a result of Ostwald ripening or digestive ripening processes,
The reaction can be represented as follows:
affecting the efficacy of the catalysts.19
Substrate scope
With the optimal conditions in hand, the scope of the PIC-1
catalyst towards the hydrolytic oxidation of other diorgano-
silanes was examined (Table 1). In each case, the corresponding
1,1,3,3-tetraorganodisiloxanes, (HRR1Si)2O, (2–7) were obtained
in excellent yields irrespective of the nature of the alkyl/aryl
substituents on the silicon atom. However, a marked variation
in the TOF values for the substrates, Me(R1)SiH2, (R1 = n-Hex,
n-Oct) with very similar alkyl chain lengths deserved a detailed
inspection. Therefore, we performed parallel experiments
on the hydrolytic oxidation of Me(n-Hex)SiH2 under similar
conditions using three PIC-1 catalysts that were prepared in
separate batches. In each case, a complete conversion of the
organosilane to 2 was observed in nearly 30 minutes and was
comparable with the TOF value given in Table 1. The reaction
when performed with the aged (7 days) sample of PIC-1 was
completed in the same time scale. Based on these results, it is
evident that subtle changes in size and consequently in the
surface to volume ratio of the AuNPs obtained from different
batches of PIC-1 do not influence the activity of the catalyst.
While the plausible mechanism of the observed activity is not
yet clearly understood, we surmise that the catalytic event is
governed by other factors such as the number of exposed
surface atoms on the NP ensembles, which function as the
active sites, as well as the relative diffusion of the reactive
species on the NP surface.20
All the compounds obtained herein except 7 are distillable
liquids and the relevant spectroscopic data along with the
isolable yield of the products are summarized in the experi-
mental section. A representative GC-MS profile of 2 is shown in
Fig. S8 (ESI†). The molecular structure obtained at 298 K as well
as selected bond lengths and bond angles of 7 [Si1A–O1 =
1.615(4), Si1B–O1 = 1.604(5) Å, Si1A–O1–Si1B = 162.2(4)1]
(for details, see the ESI†) was found to be comparable with
those reported earlier.21 Interestingly, compound 7 is known
to exhibit static or dynamic disorder at the oxygen atom and
undergoes a phase transition from monoclinic (P21/n) at 298 K
to triclinic symmetry at 200 K.
A plausible mechanistic pathway was assumed to be similar to
that reported earlier for the hydrolytic oxidation of triorgano-
silanes to the corresponding silanols using heterogeneous Au
catalysts. It has been suggested that the reaction proceeds with
the activation of the Si–H bond on the NP surface to generate
Au–Si and Au–H bonds.8b The silyl metal hydride intermediate
reacts with nucleophilic water to produce organosilanol
and dihydrogen. Since the existence of [HMePhSiOH] in the
present case is not detected in the 1H NMR and GC-MS spectra
during the reaction, it is inferred that the formation of
the silanol and its condensation product (1) occurs concomi-
tantly on the surface of the nanoparticles. We surmise that the
resulting product desorbs from the NP surface into the
organic medium and further oxidation is not favoured, thus
allowing selectivity to be achieved during the catalytic event.
We attempted to isolate MePh2SiOH in solution by the hydro-
lytic oxidation reaction of MePh2SiH under similar conditions.
However, monitoring the reaction using GC-MS did not reveal
the identity of the silanol, and 1,3-dimethyl-1,1,3,3-tetra-
phenyldisiloxane (MePh2Si)2O (8) was isolated in quanti-
tative yield.
The kinetic plots of ln[Mo]/[M], where [M] represents the
concentration of the organosilane, at time (t) vs. time for the
reaction of MePhSiH2 at 25 and 80 1C are linear (Fig. S6, ESI†)
and consistent with the first order rate law with respect to the
concentration of the silane. The observed rate constants are
k25 = 1.8 (0.13) ꢃ 10ꢂ2 (R2 = 0.98) and k80 = 4.8(0.43) ꢃ 10ꢂ1 minꢂ1
(R2 = 0.96), where values in parentheses represent standard
deviation, suggesting a significant enhancement in the rate at
elevated temperature.
The stability of the AuNP assembly in PIC-1 up to the fourth
cycle of the reaction for methylphenylsilane at 80 1C was
examined. Interestingly, the TOF values after the first and
fourth cycle (43 680, 39 200 hꢂ1) of the reaction were comparable,
suggesting no appreciable degradation of the catalytic activity.
Moreover, the HRTEM micrograph of the catalyst after the
fourth cycle (Fig. S7, ESI†) reveals large ensembles of the NPs
similar to those observed in the primitive PIC-1, suggesting
the interfacial stability of the NPs during the catalytic event.
To validate the scope of PIC-1 as a ‘‘true’’ interfacial catalyst,
a controlled experiment was performed wherein the organic
layer was separated and used for the hydrolytic oxidation of
methylphenylsilane in the presence of water as the oxidant.
An examination of the reaction mixture by 1H NMR spectro-
scopy revealed that the concentration of the substrate remains
practically unchanged even after 24 h. The results are quite
Table 1 Synthesis of 1,1,3,3-tetraorganodisiloxanes, (HRR1Si)2O, (1–7)
(HRR1Si)2O
Compound
R
R1
Yield (%)
TOF (hꢂ1
)
1
2
3
4
5
6
7
Me
Me
Me
Me
Me
Me
Ph
Ph
n-Hex
cyclo-Hex
n-Octyl
Me2PhSiCH2CH2
2-ThMe2SiCH2CH2
Ph
96 (4)
87 (13)
88 (12)
96 (4)
499
499
499
43 680
20 410
10 690
9800
890
816
4900
Conditions: 0.01 mol% Au; 80 1C; ambient light source. Yield based
on GC-MS data. Values in parentheses represent the yield of
significant since the size of the catalytic metal NPs under RR1Si(OSiRR1H)2.
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biphasic medium. The catalyst exhibits high activity and selec-
tivity for the hydrolytic oxidation of diorganosilanes to afford
the formation of 1,1,3,3-tetraorganodisiloxanes, (HRR1Si)2O, in
excellent yields. The present study also provides a manifesta-
tion of the photothermal effect arising from plasmonic AuNPs
at the water–chloroform interface.
Experimental
Materials and methods
Solvents were freshly distilled over sodium wire/benzophenone
(diethyl ether) and phosphorous pentoxide (acetonitrile) under
an inert atmosphere. Chloroform (Fischer, HPLC grade) was
used as received. Glassware was dried in an oven at 110–120 1C
and further flame-dried under vacuum prior to use. 2,4,6,8-
Tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane, lithium alumi-
nium hydride, diphenylsilane, methyldiphenylsilane, phenyl-
silane and azobisisobutyronitrile (AIBN) were purchased from
Sigma Aldrich and used as procured. 1H and 29Si{1H} NMR
spectra were recorded in CDCl3 on a Bruker AVANCE II
400 MHz NMR spectrometer and the chemical shifts are quoted
relative to Me4Si. IR spectra were recorded on a Nicolet Protege
460 ESP spectrophotometer using KBr optics. Electrospray
ionization (ESI) mass spectra were recorded on a micrOTOF-Q
II 10262 mass spectrometer in positive ion mode using
an internal standard. X-ray photoelectron spectroscopy (XPS)
studies were carried out using a modified SPECS system
equipped with a Mg Ka source (1253.6 eV, 100 W) and the pass
energy of the analyzer was fixed to 40 eV. The samples were
prepared by spin coating of the solution over a mica surface.
The measured XPS spectra were fitted using Fityk. GC-MS was
performed using an Agilent Technologies 240 Ion Trap GC/MS:
VF-5MS Column in toluene. Temperature was programmed
from 80 to 280 1C (with a flow rate of 8 1C minꢂ1 for the
temperature range 80–220 1C and 15 1C minꢂ1 for the tempera-
ture range 220–280 1C). The UV-Vis spectra of the samples were
obtained using a Perkin Elmer UV/Vis/NIR Lambda 1050
spectrophotometer. High resolution transmission electron
microscopy (HRTEM) studies were carried out using a carbon
coated copper grid on a Philips CM 20 electron microscope
operating at 100 kV. Field emission scanning electron microscopy
(FESEM) was performed using a glass slide on an FEI Quanta 200
FESEM. The molecular weight (Mw) and polydispersity index (PDI)
of the polymer, H2PhSiO[PhHSiO]nSiPhH2, were estimated using
Waters gel permeation chromatography (GPC) equipped with an
L-2414 refractive index detector and Waters styragel HR3 and HR4
Fig. 2 Substrate conversion (%) vs. time for the hydrolytic oxidation of
MePhSiH2 under different conditions.
Enhancement of the catalytic activity of PIC-1 by the
photothermal effect
An underlying mechanism of plasmon-mediated catalysis
involves the conversion of solar energy into heat energy (photo-
thermal effect) through plasmon–phonon coupling and harnes-
sing this energy to activate the substrate(s) during the catalytic
event.22 The method offers a promising alternative to chemical
transformations, which are conventionally performed by
thermal heating. Herein, we performed preliminary studies to
validate for the first time the role of the photothermal effect
arising from plasmonic AuNPs in PIC-1 in the hydrolytic
oxidation reaction of methylphenylsilane. In addition to the
reaction conditions (25 and 80 1C, sunlight) described in the
preceding section, separate experiments were performed to
evaluate this phenomenon. Surprisingly, the reaction when
performed in the dark at 25 1C did not proceed and the
substrate was recovered unchanged after 24 h. The reaction,
however, proceeds smoothly upon irradiation with an LED
source (Thorlabs LED680L) with a maximum wavelength (lmax
)
of 680 nm corresponding to the position of the plasmonic band
of the AuNPs. A plot of percentage conversion of the substrate
as a function of time (Fig. 2) reveals that the reaction profile
closely resembles that observed for the analogous reaction
performed by irradiation with sunlight and the turnover fre-
quency values (TOFs) are on the order of 4266 and 3840 hꢂ1
,
respectively. We also performed a separate experiment at 80 1C
without exposing the reaction mixture to light irradiation.
As is evident from Fig. 2, complete consumption of the sub-
strate was observed in a time scale similar to that observed
under illumination at 25 1C. These results provide a basis to
infer that the plasmon-mediated photothermal effect likely
contributes to activating the Si–H bond during the catalytic
event. Further studies are in progress to understand this
phenomenon in detail.
columns in series using THF as an eluent (flow rate: 1 mL minꢂ1
;
polystyrene standards).
Crystallographic study
The intensity data of 7 were collected at 300 K using a Bruker
APEX III CCD diffractometer, using graphite monochromated
Mo-Ka radiation (l = 0.71069 Å). Cell parameter analysis and
Conclusions
In summary, we have developed a simple approach to construct data reduction were performed using Bruker SAINT. Lorentz
a AuNP-based interfacial catalyst, PIC-1, in a water–chloroform and polarization effects and empirical absorption corrections
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were applied using SADABS from Bruker. The structure was IR (KBr, cmꢂ1): 3070, 3052 (n CH, aromatic), 2173 (n SiH), 1429
solved by a direct method, using SIR-92 and refined by a (d CH), 1085 (n SiO). Mw = 2.2 ꢃ 103, PDI = 1.3.
full-matrix least square refinement method23 based on F2,
Synthesis of 1,1,3,3-tetraorganodisiloxanes 1–7
using SHELX-2014/7.
In a typical procedure, hydrolytic oxidation of methylphenyl-
Synthetic methods
silane (2.0 mL, 16.39 mmol) was performed in the presence of
the PIC-1 catalyst under aerobic conditions. After complete
consumption of the substrate, the organic layer was separated
and dried over sodium sulphate. The solvent was stripped off to
obtain the desired product, 1. A similar procedure was followed for
the synthesis of related 1,1,3,3-tetraorganodisiloxanes, (HRR1Si)2O,
2–7 by using an appropriate diorganosilane precursor.
By following the procedure reported earlier,24 the diorgano-
silanes MeR1SiH2 (R1
= Ph, n-Hex, cyclo-Hex n-Octyl,
PhMe2SiCH2CH2, 2-ThMe2SiCH2CH2) were synthesized by the
reduction of the corresponding dichlorodiorganosilanes with
lithium aluminium hydride in diethyl ether.
(HMePhSi)2O, 1. B.p. 148 1C/2 mmHg, yield = 95%. 1H NMR
(CDCl3): d 7.56, 7.35 (m, 5H, Ph), 5.13 (s, 1H, SiH), 0.39 (s, 3H,
Me). 29Si{1H} NMR (CDCl3): d ꢂ11.5, ꢂ13.3 (SiO). IR (KBr,
cmꢂ1): 3069 (n CH, aromatic), 2960, 2902 (n CH, aliphatic),
2131 (n SiH), 1255 (n SiMe), 1061 (n SiO). GC-MS: m/z = 258,
retention time (tR) = 15.0 min.
{HMe(n-Hex)Si}2O, 2. B.p. 121 1C/2 mmHg, yield = 85%.
1H NMR (CDCl3): d 4.44 (br, 1H, SiH), 1.17 (br, 8H, Hex), 0.73
(m, 3H, C-Me), 0.47 (br, 2H, SiCH2), 0.07 (br, 3H, SiMe). 29Si{1H}
NMR (CDCl3): d ꢂ3.0, ꢂ5.3 (SiO). IR (KBr, cmꢂ1): 2959, 2923
(n CH, aliphatic), 2119 (n SiH), 1257 (n SiMe), 1056 (n SiO).
GC-MS: m/z = 274, retention time (tR) = 12.1 min.
{HMe(cyclo-Hex)Si}2O, 3. B.p. 142 1C/2 mmHg, yield = 86%.
1H NMR (CDCl3): d 4.35 (br, 1H, SiH), 1.6–1.14 (br, 10H, cyclo-
Hex), 0.61 (br, 1H, C–CH), 0.06 (br, 3H, SiMe). 29Si{1H} NMR
(CDCl3): d ꢂ1.7, ꢂ3.1 (SiO). IR (KBr, cmꢂ1): 2917, 2846 (n CH,
aliphatic), 2109 (n SiH), 1250 (n SiMe), 1058 (n SiO). GC-MS:
m/z = 270, retention time (tR) = 15.3 min.
Synthesis of [RSCH2CH2SiMeO]4; R = CH2CH2OH
The reaction between 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclo-
tetrasiloxane [CHQCH2SiMeO]4 (0.99 g, 1.0 mL, 2.8 mmol) and
2-mercaptoethanol (0.97 g, 0.88 mL, 12.5 mmol) was performed
in acetonitrile using 2,20-azobisisobutyronitrile (AIBN, 10 wt%)
as a free radical catalyst. The mixture was heated at 70 1C for
nearly 18 h. The oily liquid thus obtained was immiscible in
acetonitrile. Repeated washing of the oil with the solvent yields
the desired cyclosiloxane in pure form (yield: 85%).
ESI-MS (+ve mode, m/z): 679.1227 (obs.)/679.1201 (calcd)
[M + Na]+. 1H NMR (CDCl3): d 3.63 (m, 2H, CH2OH), 2.65–
2.55 (t, 2H, SCH2CH2OH), 2.5–2.4 (m, 2H, SCH2), 0.85 (t, 2H
SiCH2), 0.05 (m, 3H, SiCH3). 13C {1H} NMR (CDCl3): d 60.9
(CH2OH), 35.3, 26.7 (SCH2), 18.3 (SiCH2), 0.00, ꢂ0.08 (SiCH3).
29Si {1H} NMR (CDCl3): d ꢂ21.8 (SiO). IR (KBr, cmꢂ1): 3338
(n OH), 1070 (n SiO), 1261 (n SiMe).
Synthesis of interfacial catalyst (PIC-1)
{HMe(n-Octyl)Si}2O, 4. B.p. 138 1C/2 mmHg, yield = 95%.
1H NMR (CDCl3): d 4.44 (br, 1H, SiH), 1.11 (br, 12H, n-Oct), 0.74
(br, 3H, C–CH3), 0.47 (br, 2H, SiCH2), 0.04 (br, 3H, SiMe).
29Si{1H} NMR (CDCl3): d ꢂ3.0, ꢂ5.3 (SiO). IR (KBr, cmꢂ1):
2957, 2921 (n CH, aliphatic), 2117 (n SiH), 1252 (n SiMe), 1056
(n SiO). GC-MS: m/z = 330, retention time (tR) = 20.39 min.
{HMe(Me2PhSiCH2CH2)Si}2O, 5. B.p. 210 1C/2 mmHg, yield =
96%. 1H NMR (CDCl3): d 7.52, 7.37 (s, 5H, Ph), 4.64, 4.55
(br, 1H, SiH), 0.67, 0.57 (br, 4H, CH2), 0.28 (s, 6H, Me2PhSi),
0.15 (s, 3H, SiMe). 29Si{1H} NMR (CDCl3): d ꢂ1.7 (Me2PhSi),
ꢂ3.9 (SiO). IR (KBr, cmꢂ1): 3061, 3012 (n CH, aromatic), 2956,
2905 (n CH, aliphatic), 2115 (n SiH), 1051 (n SiO). GC-MS:
m/z = 430, retention time (tR) = 30.3 min.
Triethylsilane (10 mL, 0.06 mmol) was added to a sonicated
solution of HAuCl4ꢁ3H2O (4.0 mg, 0.01 mmol) and
[RSCH2CH2SiMeO]4; R = CH2CH2OH (6.8 mg, 0.01 mmol) in
chloroform (HPLC, 25 mL). A gradual colour change of the
solution from yellow to blue within 4–6 h ensures the formation
of AuNPs. The solution was left for equilibration for 24 h
at room temperature and ultracentrifuged. The precipitated
NPs were dissolved in chloroform and used for analysis. The
solution of NPs (4.0 mL) was taken separately and mixed with
1.0 mL of deionized water with constant stirring. The confine-
ment of the AuNPs at the water–chloroform interface was
observed by the formation of non-coalescent water droplets in
chloroform. The self-assembly was denoted as PIC-1 and used
for subsequent studies.
{HMe(2-ThMe2SiCH2CH2)Si}2O, 6. B.p. 198 1C/2 mmHg,
yield = 95%. 1H NMR (CDCl3): d 7.46 (d, 1H, Th H-3), 7.11
(d, 1H, Th H-5), 7.06 (dd, 1H, Th H-4), 4.41 (br, 1H, SiH), 0.44,
0.58 (br, 4H, CH2), 0.21 (s, 6H, 2-ThMe2Si), 0.08 (s, 3H, SiMe).
Synthesis of poly(phenylhydrosiloxane), H2PhSiO[PhHSiO]nSiPhH2
In a typical procedure, the hydrolytic oxidation of primary 29Si{1H} NMR (CDCl3): d ꢂ1.7 (2-ThMe2Si), ꢂ3.5 (SiO). IR
organosilane, PhSiH3 (2.0 mL, 0.02 mmol), was performed in (KBr, cmꢂ1): 3074, 3017 (n CH, aromatic), 2956, 2879 (n CH,
the presence of Au-stabilized PIC-1 under aerobic conditions at aliphatic), 2114 (n SiH), 1250 (n SiMe), 1049 (n SiO). GC-MS:
80 1C. After complete consumption of the monomer, the organic m/z = 442, retention time (tR) = 30.1 min.
layer was separated and dried over sodium sulphate. The solvent
(HPh2Si)2O, 7. M.p. 48–49 1C, yield = 98%. 1H NMR (CDCl3):
was evaporated under vacuum and poly(phenylhydrosiloxane) was d 7.63, 7.31, (m, 10H, Ph), 5.53 (s, 1H, SiH). 29Si{1H} NMR
isolated as a viscous oil.
(CDCl3): d ꢂ19.8 (SiO). IR (KBr, cmꢂ1): 3062, 3011 (n CH,
1H NMR (CDCl3): d 7.24 (br, 5H, SiPh), 5.14 (br, 1H, SiH). aromatic), 2135 (n SiH), 1067 (n SiO). GC-MS: m/z = 382,
29Si{1H} NMR (CDCl3): d 27.0 [H2PhSiO], d 44 to 47 (HPhSiO). retention time (tR) = 31.5 min.
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(MePh2Si)2O, 8. Compound 8 was isolated as a colorless
NJC
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liquid from the hydrolytic oxidation of MePh2SiH by following a
procedure described above.
1
B.p. 157 1C/2 mmHg, yield = 97%. H NMR (CDCl3): d 7.52,
7.27 (m, 10H, Ph), 0.56 (s, 3H, Me). 29Si{1H} NMR (CDCl3):
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
¨
and H. Mohwald, Angew. Chem., Int. Ed., 2004, 43, 5639;
This work was supported by CSIR Grant 01(2851)/16/EMR-II.
Asmita Sharma is grateful to UGC-CSIR 19-06/2011(i)EU-IV for
providing the Senior Research Fellowship. We thank Nidhi
Mahavar (SRF) and Nivedita Roy (JRF) for their assistance in
the experimental work. We are grateful to one of the reviewers
for providing valuable information on the structure of 7.
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