Synthesis, characterization and catalytic oxidation of organosilanes with a novel multilayer polyoxomolybdate containing mixed-valence antimony

Article abstract of DOI:10.1016/j.mcat.2018.04.013

Oxidation of organosilanes is one of the pivotal reactions in organic synthesis and the corresponding products of silanols are widely as raw materials in industrial processes. In this paper, a new type of polyoxomolybdate containing mixed-valence antimony, [Sb^VSb^III₄Mo₁₈O₆₆]^7? (1a), has been isolated as tetramethyl ammonium salt in aqueous solution. The compound was structurally characterized by FT-IR, XPRD, TG, XPS, ESI–MS etc. It is the first time that the containing mixed-valence antimony polyoxomolybdate was used as a heterogeneous catalyst to efficaciously catalyze the oxidation of organosilanes to silanols under mild reaction conditions. Furthermore, the catalyst was stable and maintained its catalytic activity after three reaction cycles.

Full text of DOI:10.1016/j.mcat.2018.04.013

MolecularCatalysis452(2018)167–174
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Synthesis, characterization and catalytic oxidation of organosilanes with a
novel multilayer polyoxomolybdate containing mixed-valence antimony
Yaping Wang1, Jingkun Lu1, Xinyi Ma, Yanjun Niu, Vikram Singh, Pengtao Ma, Chao Zhang,
Jingyang Niu⁎, Jingping Wang⁎
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University,
Kaifeng 475004, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Heterogeneous catalysis
Oxidation
Oxidation of organosilanes is one of the pivotal reactions in organic synthesis and the corresponding products of
silanols are widely as raw materials in industrial processes. In this paper, a new type of polyoxomolybdate
containing mixed-valence antimony, [SbVSb4IIIMo18O66 7− (1a), has been isolated as tetramethyl ammonium salt
]
Silanols
in aqueous solution. The compound was structurally characterized by FT-IR, XPRD, TG, XPS, ESI–MS etc. It is the
first time that the containing mixed-valence antimony polyoxomolybdate was used as a heterogeneous catalyst
to efficaciously catalyze the oxidation of organosilanes to silanols under mild reaction conditions. Furthermore,
the catalyst was stable and maintained its catalytic activity after three reaction cycles.
Mixed-valence antimony polyoxomolybdate
Reusable catalyst
1. Introduction
hydrogen peroxide have been devised. They are environmentally
friendly and offer the advantage of cleanly producing silanols along
Silicon materials have attracted great attention due to their various
applications in fields of organic synthesis, biochemistry as well as ma-
terial science [1–3]. In particular, silanols are compounds having
SieOH groups and are silicon analogues of alcohols, which have been
utilized in industry for the production of silicon-based polymeric ma-
terials and as nucleophilic partners in metal-catalyzed cross-coupling
reactions [4,5], organocatalysts for activating carbonyl compounds
[6,7], intramolecular guiding groups for CeH bond activation reactions
in organic synthesis [8,9]. Furthermore, several organosilanols have
shown to exhibit bioactivity and some of them were applied as struc-
tural components in the transition-state analogues of metalloprotease
inhibitors [10–12]. Especially noteworthy is that organosilanols have
been recently identified as challenging bioactive agents in experimental
pharmacology and employed in catalysis due to their anion recognition
ability [13]. Previously, silanols have often been synthesized by the
activation of silanes, especially the oxidation of the SieH bond. The
classical methods for the oxidation of silanes to silanols involve the use
of stoichiometric amounts of oxidants such as silver salts, peracids,
permanganate, dioxiranes, osmium tetroxide, oxaziridines etc [14–18].
But these methods are not environmentally benign because of the for-
mation of toxic by-products and generation of disiloxanes as side pro-
duct consequent from the condensation of silanes. To overcome these
drawbacks, catalytic systems involving a green oxidant like water or
with molecular hydrogen or water as the only side-product. In this
context, even though some homogeneous transition metal catalysts
have emerged to overcome some of the above mentioned problems,
their use is escorted with a difficulty of residual metal impurities in the
final products and they suffer from reusability standpoint [19–21].
Hence, heterogeneous catalytic systems are more attracting consider-
able attention because of the inherent operational advantages such as
ease of handling, separation and recovery of the catalyst from the re-
action mixture for reusability. Currently, the heterogeneous systems
rely heavily on different nanohybrid catalyst systems utilizing support
materials with anchored noble metal nanoparticles like Ag, Au, Pd, etc
[22–27]. And the homogeneous and heterogeneous catalyst systems
selective for the production of silanols in the oxidation of silanes are
briefly summarized in Table S1.
In 2009, Mizuno and co-workers reported highly selective oxidation of
organosilanes to silanols with hydrogen peroxide catalyzed by divacant
lacunary polyoxotungstate (TBA)4[γ-SiW10O34(H2O)2] [TBA = (n-
C4H9)4N+] [28]. Subsequently, they reported the hydrolytic oxidation of
hydrosilanes using a polyoxometalate (POM) containing the diamond-
shaped [Ag4]4+ core in 2012 [29]. As we all know, POMs are a large
family of anionic metal-oxides of V, Mo, W and Nb, etc. in their highest
oxidation states, and they are thermally and oxidatively stable compared
with commonly used organic ligands. Most importantly, they have several
⁎
Corresponding authors.
E-mail addresses: jyniu@henu.edu.cn (J. Niu), jpwang@henu.edu.cn (J. Wang).
These authors contributed equally.
1
https://doi.org/10.1016/j.mcat.2018.04.013
Received 11 February 2018; Received in revised form 29 March 2018; Accepted 11 April 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

Y. Wang et al.
MolecularCatalysis452(2018)167–174
advantages as catalysts which make them economically and en-
vironmentally attractive. The chemical properties of POMs such as redox
potential, electron-transfer properties, acidities, and solubility, etc. can be
finely tuned by adjusting the metal ions, the heteroatoms and the counter
ions. Especially, strong Brønsted acidic and efficient oxidants exhibit fast
reversible multielectron redox transformations under rather mild condi-
tions; which clearly indicates that they could be acidic and redox-bifunc-
tional catalysts in homogeneous and heterogeneous systems [30–33]. In
addition, scores of antimony-containing POMs have been widely studied
spectra were obtained on a Perkin-Elmer Optima 2000 ICP-OES spec-
trometer. The X-ray powder diffraction pattern (XPRD) patterns were
obtained using a Bruker AXS D8 Advance diffractometer instrument
with Cu Kα radiation (λ = 1.54056 Å) in the angular range 2θ = 5–50°
range at 293 K. Thermal gravimetric analysis (TGA) was measured by
on a Mettler-Toledo TGA/SDTA 851e thermobalance in N2 flow with a
heating rate of 10 °C min−1 from 25 °C to 900 °C. X-ray photoelectron
spectra (XPS) were recorded with an Axis Ultra X-ray photoelectron
spectrometer. Elemental analyses of C, H, and N were performed by
using a Perkin-Elmer 2400-II CHNS/O analyzer. ESI–MS measurements
were performed on an ABSCIEX Triple TOF 4600 spectrometer oper-
ating in negative ion mode and data was analyzed using the Peakview
2.0 software provided. GC chromatogram was measured on Bruker 450-
GC (FID) instrument, which was equipped with a 30 m column (GsBP-5,
0.25 um film thickness and 0.25 mm internal diameter) with nitrogen as
carrier gas.
till now, however among them are mostly Sb(III)-containing poly-
9−
oxotungstates such as [SbW9O33
]
, , [Cs2Na
[Sb2W22O74(OH)2]12−
(H2O)10Pd3(α-SbW9O33)2]9−, [Fe4(H2O)10(β-SbW9O33)2]6−, [NaSb9W21
-
18−
O86]
,
[(VO)3(α-SbW9O33)2]12−
,
[Sb2W22O74(OH)2]12−
,
[Sb2W20-
Fe2O70(H2O)6]8− and [Sb6W65O217(H2O)7]26− et al [34–40]. Contrast-
ingly, their molybdate analogues which offer some novel structures and
intriguing properties, are still less explored. Since the report of the first Sb-
containing polyoxomolybdate [H2SbVMo6O24 5– [41], addressed by Sasaki
]
in 1982, only several compounds were sporadically addressed [42–44].
Additionally, it is worth to mention that incorporation of both Sb (V) and
Sb (III) into polyoxomolybdate frameworks could result in unprecedented
structural features and novel properties. Currently, these compounds have
been developed to significant stage since after the advent of first mixed-
valence antimony polyoxomolybdate [Sb4VSb2IIIMo18O73(H2O)2]12− re-
ported in 2008 [45]. Thus, concomitant with their potential catalytic
property, there is a strong impetus for us to explore the containing mixed-
valence antimony polyoxomolybdate chemistry. As a continuation of our
recent research on the synthesis and catalytic application of novel poly-
oxomolybdates containing mixed-valence antimony, the main aim of the
current work is the investigation of their catalytic function toward the
mild heterogeneous oxidation. Hence, the present study aims to report the
2.2. Catalyst preparation and characterization
Sb2O5 (0.12 g, 0.37 mmol) and Sb2O3 (0.12 g, 0.41 mmol) were
successively dissolved in dilute HCl (10 mL, 3 mol L−1) solution (de-
noted as solution A). In a separate beaker, (NH4)6Mo7O24·4H2O (3.2 g,
16.32 mmol) was added to 30 mL distilled water and then heated to
50 °C (denoted as solution B). When all the solid materials had been
completely dissolved and both solutions were clear, solution A was
added into B dropwise under stirring. After cooling to room tempera-
ture, the pH value of the mixed solution was adjusted to 3.10 using
tetramethylammonium hydroxide (4 mol L−1). Subsequently, the re-
sulting solution was heated to 60 °C for 2 h, then cooled to room tem-
perature and filtered. The filtrate was allowed stand for crystallization
and black crystals were obtained after three days. Yield: 0.13 g [25.56%
Based on Mo]. Elemental analysis (%) calculated for compound 1: C
7.22, H 2.07, N 2.11; found for C 7.59, H 2.03, N 2.23. Select IR (KBr
pellets, cm−1): v = 3424, 3160, 1633, 1490, 1414, 1188, 945, 891,
777, 646 and 461.
synthesis, characterization and catalytic activity of
a novel poly-
oxomolybdate containing mixed-valence antimony, [(CH3)4N]6H
[SbVSb4IIIMo18O66]·9H2O (1). In this paper, tert-butyl hydroperoxide
(TBHP) was used as the reaction oxidant, which accelerated the silanol
production. Although TBHP as an oxidant is less environmentally than
water and H2O2, polyoxomolybdates were able to activate TBHP [46]. To
the best of our knowledge, catalytic property of containing mixed-valence
antimony polyoxomolybdates has not been reported till date.
2.2.1. Bond valence sum calculation (BVS) and XPS analysis
The BVS calculation of 1 manifests that the oxidation states of all
Mo atoms are +6, and the oxidation states of Sb atoms are +3 or +5
(Table S2) [47]. The X-ray photoelectron spectroscopy (XPS) spectrum
2. Experimental
2.1. Materials and methods
of Mo in 1a gives two peaks at 235.2 and 232.1 eV, attributable to
6+
Mo63+d3/2 and Mo
, respectively (Fig. 1a). Moreover, the Sb 3d region
3d5/2
All chemical reagents were purchased and used without further
features a spin–orbit doublet with Sb 3d3/2 and Sb 3d5/2. Both the Sb
3d3/2 and Sb 3d5/2 spectra can be divided into two characteristic peaks.
And four deconvoluted peaks were observed in the Sb 3d region at
540.2, 530.6, 538.8 and 529.4 eV, confirming the Sb valence states as
Sb5+ and Sb3+ [48], respectively (Fig. 1b).
purification. The FT-IR spectra were obtained on a Bruker VERTEX 70
IR spectrometer using KBr pellets in the range of 450–4000 cm−1
.
UV–vis absorption spectrum was determined on a U-4100 spectro-
photometer at room temperature. Inductively coupled plasma (ICP)
Fig. 1. a) Mo 3d XPS spectrum; b) Sb 3d XPS spectrum.
168

Y. Wang et al.
MolecularCatalysis452(2018)167–174
2.2.2. IR, UV and XPRD spectral studies
thoroughly with acetonitrile and dried at room temperature to be used
in the next recycling experiment.
The FT-IR spectrum of 1 using a KBr pellet in the region of
4000–450 cm−1 was shown in Fig. S1. It gives a strong band at
3424 cm−1 which is assigned to the OeH stretching modes, illustrating
the presence of lattice and coordinated water molecules. While a signal
band appearing at 3160 cm−1 originates from υ(NeO). The spectrum
has five characteristic peaks below 1000 cm−1 appearing at ca. 945(m),
891(vs), 777(s), 646(s) and 461(m), respectively. A strong band at
945 cm−1 is assigned to the characteristic absorption of υ(Mo–Od). In
addition, the strong bands at 891 cm−1 and 777 cm−1 can be ascribed
to the vibrations of Ob–Mo–Ob and Mo–Oc, respectively [49]. And the
absorption of SbeO stretching and bending vibrations is identified as
2.4. Crystallography
Suitable single crystal of 1 was prudentially selected and en-
capsulated in a capillary tube due to their quick efflorescence. Intensity
data were collected at 296(2) K on a Bruker Apex-II CCD diffractometer
using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).
Routine Lorentz and polarization corrections were applied, and a multi-
scan absorption correction was performed using the SADABS program.
The structure was solved by direct methods, developed by successive
difference Fourier syntheses. Using Olex2 [51], the structure was solved
with the ShelXS-1997 structure solution program using Direct Methods
and refined with the ShelXL refinement package using Least Squares
minimisation [52,53]. In the final refinement, all the Sb and Mo atoms
were refined anisotropically, while the C, N and O atoms were refined
isotropically. The lattice water molecules were determined by CHN
element analysis and TGA results. All the hydrogen atoms on the water
molecules were directly included in the molecular formula. The CCDC
number is 1567093 for 1. Selected crystal data and structure refinement
parameters were listed in Table 1.
around 646 cm−1
.
The UV spectrum of compound 1 in aqueous solution reveals one
characteristic peak in the range of 200–400 nm (Fig. S2), and the ab-
sorption band at 208 nm can be attributed to the pπ–dπ charge-transfer
transitions of the Ot → Mo bonds [50]. The X-ray powder diffraction
pattern (XPRD) matches well with the simulated one from single-crystal
data in key positions (Fig. S3), indicating the purity of compound 1.
And this intensity discrepancy between the experimental and simulated
XPRD patterns may be caused by the variation in the preferred or-
ientation of the powder sample during the collection of the experi-
mental XPRD.
3. Results and discussion
2.2.3. Mass spectrometric study of the 1
To investigate the cluster species in solution, electrospray-ionization
mass spectrometry (ESI–MS) was conducted. Single crystals of 1 were
dissolved in 20 mL acetonitrile. The ESI–MS result shows that there
3.1. Synthesis
In this paper, compound 1 has been prepared by a one-pot reaction
of (NH4)6Mo7O24·4H2O, Sb2O3, Sb2O5 and (CH3)4NOH·5H2O in aqueous
solution. The simple synthetic process of 1 was shown in Scheme 1.
During our preparations of 1, the parallel experiments indicated that
there are three essential reaction conditions, such as pH value, counter
cations and molar ratios. The rarely isolated compound 1 could be
obtained from an acidified SbV, SbIII, molybdate solution (pH = 3.1,
molar ratio of 1:1.2:8.8) and (CH3)4N+ as counterions at 60 °C. (1) In
our case, the optimal pH is 3.1 and that is a vital factor in the formation
and crystallization of 1. For a lower pH (pH < 2.7), only an abundance
were three peaks located at m/z 848.64, 867.49 and 1185.75 which
4−
belong
to
H3[SbVSbI4IIMo18O66
]
(calcd
848.62),
4−
TMAH2[SbVSb4IIIMo18O66
]
[TMA = (CH3)4N+] (calcd 867.28) and
TMAH3[SbVSbI4IIMo18O66(H2O)5]3− (calcd 1185.76), respectively
(Table S7, Figs. S4–S7). Samples were introduced to the spectrometer
via direct injection at a flow rate of 15 μL min−1 using a syringe pump.
Moreover, the electrospray source was used with the dying nitrogen gas
and the ion polarity for all MS data collections recorded was negative.
And the following parameters were consistent for all ESI–MS data col-
lections as follows: ionspray voltage: −4500 V, curtain gas flow, 25 PSI;
ion source gas 1, 15 PSI; ion source gas 2, 15 PSI; ion energy 1, −1.1 V;
pulser frequency, 11.332 KHz; pulse 1 duration, 3.902 us; declustering
potential, −10 V; collision energy, −5.0 V.
of some amorphous precipitates was formed; for
(pH > 3.6), the crystal quality of 1 was very poor and the yield was
reduced. (2) During the course of preparing 1, no crystals suitable for
a higher pH
Table 1
2.2.4. Thermal analysis
The data collection and refinement parameters of compound 1.
To examine the thermal behavior of 1, the TG was implemented
under a N2 atmosphere. The TG curve of 1 undergoes two steps of
weight loss on heating up to a final temperature of 900 °C, giving a total
loss of 38.71% (Fig. S8). And the first weight loss of 4.23% from 25 to
200 °C is attributed to the release of nine lattice water molecules (calcd:
4.06%). Subsequently, the other weight loss of 34.48% between 200 °C
and 900 °C is approximately attributed to the dehydration of one proton
(in the form of constitutional water molecules) and the removal of six
[(CH3)4N]+ cations, along with the oxidation of Sb3+ and the partial
sublimation of the MoO3.
Complex
1
Empirical formula
C24H82N6O71Mo18Sb5
3926.62
296.15
monoclinic
P21/n
12.6582(14)
24.514(3)
Mr (g mol−1
T/K
)
Cryst. syst.
Space group
a/Å
b/Å
c/Å
31.028(4)
β/°
100.841(2)
9456.3(18)
4
3.801
7388.0
0.4 × 0.25 × 0.08
3.148–50.2
−14 ≤ h ≤ 15
−29 ≤ k ≤ 27
−37 ≤ l ≤ 35
48601
16798
16798/38/618
1.019
V (Å3)
Z
2.3. Oxidation of silanes to silanols
μ/mm‐1
F(000)
Cryst. size (mm3)
The catalytic oxidation of various silanes was carried out in a 50 mL
round-bottom tube equipped with a reflux condenser. A typical proce-
dure for catalytic oxidation was as follows: catalyst 1 (15 mg), silanes
(1 mmol), tert-butyl hydroperoxide (TBHP) (6 mmol) and acetonitrile
(3 mL) were charged to the glass tube containing a magnetic stir bar at
the set temperature. At regular intervals, an aliquot was taken directly
from the reaction solution with a microsyringe and the liquid was
periodically analyzed by gas chromatography (GC) using naphthalene
as the internal standard. After the reaction was completed, catalyst 1
was recovered by using a centrifuge at the end of each cycle, washed
Range for data collection
Limiting indices
No. of reflns collected
No. of independent refins
Data/restraints/parameters
GOF on F2
R1, wR2 [I > 2σ (I)]
R1, wR2 [all data]
R1 = 0.0584, wR2 = 0.1424
R1 = 0.0831, wR2 = 0.1613
169

Y. Wang et al.
MolecularCatalysis452(2018)167–174
Scheme 1. The simple synthetic process of compound 1.
single crystal X-ray diffraction could be obtained when organic cations
were absent, indicating the necessity of organic (CH3)4N+ in the crys-
tallization of title compound. (3) Another interesting aspect in the
molecular growth of 1 is the molecular ratio of 1:1.1 of Sb2O5 and
Sb2O3, and the Sb2O5 dosage should be controlled around 0.37 mmol.
We try to adjust the molecular structure by increasing the amount of
Sb2O5 to 0.74 mmol or decreasing to 0.185 mmol, however, the at-
tempts failed.
(Fig. 2b) as well as each {Mo2O10} moiety is formed by two edge-shared
{MoO6} octahedrons. As shown in Fig. S9, the six-coordinate Sb1 ion
adopts a distorted octahedral geometry defined by six oxygen atoms
(O1, O2, O9, O58, O59, O72). Additionally, the BVS value is +5 for the
Sb1 atom in layer 1. Next, it is to be noted that the layer 2 of
{SbVSbI4IIMo18} comprises three Sb atoms and three MoO6 octahedrons.
And the three MoO6 octahedrons are linked together by three Sb atoms
alternatively to form a crown-shaped {Sb3Mo3} ring (Fig. 2a). We can
also find that the three MoO6 octahedrons are linked to the {Mo2O10
}
moieties of layer 1 in the vertex-sharing mode. Interestingly, oxygen
atoms coordinated to the three six-coordinated Sb atoms of the ring, all
cannot constitute octahedral geometries (Fig. S10). Furthermore, the
BVS calculations of Sb atoms are 3.147 (Sb3), 3.177 (Sb4) and 3.124
(Sb5) in layer 2, respectively. Finally, layer 3 represents the typical
trivacant B-α Keggin unit {SbIIIMo9O33}. The bond connectivity of layer
2 and layer 3 are quite similar as they are connected by six oxygen
atoms (O4, O10, O15, O16, O21 and O24) (Fig. 2c). In 1a, there are four
kinds of O atoms: namely, 24 terminal O atoms (Ot), 22 bridging μ2eO
atoms, 9 bridging μ3eO atoms, and 11 μ4eO atoms. The MoeOt,
Moeμ2eO, Mo/Sbeμ3eO and Mo/Sbeμ4eO bond lengths in
{SbVSbI4IIMo18} range from 1.6523 Å to 1.7222 Å, 1.7482 Å to 2.3734 Å,
1.8011 Å to 2.4967 Å and 1.8920 to 2.4967 Å, respectively.
3.2. Structural description of compound 1
Single-crystal X-ray diffraction analysis shows that 1 crystallizes in
the monoclinic system with space group P21/n (see Table 1). The crystal
structure of 1 consists of a rarely isolated 1a unit with six (CH3)4N+
cations, one proton and nine lattice water molecules in asymmetric
unit. In 1a, all the Mo atoms remain in the same conventional six-co-
ordinated octahedron configuration completed by O atoms. As a whole,
the {SbVSb4IIIMo18} fragment could be observed as three layers as shown
in Fig. 2a. At first, the layer 1 can be regarded as a central Sb1 atom
connecting to three {Mo2O10} moieties. And the three {Mo2O10} moi-
eties are further combined by three μ3eO atoms (O7, O13, O40)
3.3. Catalytic oxidation of dimethylphenylsilane
To study the catalytic performance of 1, we used dimethylphe-
nylsilane (2a) as a model substrate in the presence of TBHP as the
oxidant for the model reaction (Scheme 2). Moreover, the effects of
various reaction parameters on the oxidation reaction were also in-
vestigated in detail (Table 2). In a blank experiment (without catalyst),
the reaction proceeds sluggishly and only 24.9% yield of dimethyl-
phenylsilanol (3a) is obtained (see Table 2, entry 1). However, the
oxidation reaction of 2a in the presence of catalyst 1 (15 mg, 0.38 mol%
with respect to substrate) gave 3a in 94.8% yield under the same ex-
perimental conditions (entry 2). For the sake of comparison, we also
envisaged
antimony
trioxide,
antimony
pentaoxide
and
(NH4)6Mo7O24·4H2O independently instead of 1 as a catalyst for the
oxidation of dimethylphenylsilane and found them to be less active
than that of 1 (Table S8). In addition, the catalytic activities of catalyst
1 and (NH4)6Mo7O24·4H2O were checked by taking into account iden-
tical molybdate atoms. Catalyst 1 gives 99% conversion with 96% se-
lectivity, while (NH4)6Mo7O24·4H2O gives 63% conversion with 71%
selectivity. On the other hand, knowledge on the effect of catalyst do-
sage is very important to the process optimization because the con-
centration of the catalytically active species increased with higher
catalyst dosage (Fig. 3a). The reaction conversion increases as a func-
tion of the reaction time at different catalyst dosage. The conversion
percentage of 2a was found to be 79%, 99% and 100%, corresponding
to the catalyst loading of 10 mg, 15 mg and 20 mg, respectively.
Therefore, the optimized catalyst dosage was proved to be 15 mg, at
which moderate conversion and excellent selectivity were achieved.
The yield of 3a was also strongly affected by the reaction temperature.
Fig. 2. Polyhedral/ball-and-stick representation of 1a (a), Layer 1 and layer 2
(b), Layer 2 and layer 3 (c); Colour code: Sb (yellow); Mo (cyan); O (red); MoO6
of layer 1 (orange); MoO6 of layer 2 (green); MoO6 of layer 3 (sky blue). (For
interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
170

Y. Wang et al.
MolecularCatalysis452(2018)167–174
Scheme 2. Model reaction of selective oxida-
tion of silanes.
As the temperature increased from 30 to 50–60 °C, the yield of product
reached 26.6%, 94.8% and 94.1%, respectively (entries 2, 5–6). The
reaction yield was maximum at 50 °C, surprisingly, slightly higher
temperature (above 50 °C) lowered the yield of 3a, may be due to less
selectivity of product (3a) along with the formation of corresponding
disiloxanes (4a). Additionally, further raising the reaction temperature
to 85 °C accelerated the reaction and obtained an 89.9% yield in nearly
0.5 h (entry 7). Several solvents have been used to investigate their
effects on the catalytic activity. Among the various solvents, the highest
yield was obtained in acetonitrile (yield: 94.8%) (entry 2). However,
the results show a lower yield of 23.2%, 56.8%, 66.9% for CH3OCH3,
benzonitrile and CH2Cl2, respectively (entries 8–10). Consequently, the
optimized solvent for such a model oxidation reaction is acetonitrile.
Moreover, the impact of TBHP/substrate molar ratio and solvents effect
on the oxidation reaction were also investigated. As shown in Fig. 3b,
the reaction was carried out at different TBHP/substrate molar ratios
under other idential reaction conditions and obvious differences on
conversions could be observed. The results showed that conversions of
products ranged from 47 to 90% to 99–97% when the TBHP/substrate
molar ratio was varied from 1:1 to 4:1 to 6:1 to 7:1 (entries 2, 11–13).
On increasing the TBHP/substrate molar ratio, the conversion and yield
of dimethylphenylsilanol reached to its maximum at a ratio of 6:1.
Therefore, the oxidation reaction was subsequently performed by using
the molar ratio of TBHP/substrate of 6:1. Additionally, residual TBHP
after the reaction was determined through the iodometrical titration
technique by using 0.1 M of sodium thiosulfate solution. The efficiency
of the TBHP utilization for the oxidation of phenyldimethylsilane was
76% (TBHP efficiency (%) = products (mol)/consumed TBHP
(mol) × 100).
addition of the substrates and TBHP at 50 °C are shown in Fig. 4a. After
three consecutive reuses, there was a general degradation in yield to
86.9% using the same catalyst which was isolated by simple cen-
trifugation without further purification (Table 2, entry 15). To confirm
the high stability of the catalyst, the structure and crystalline integrity
of the catalyst were further investigated using FT-IR (Fig. 4b) and XPRD
(Fig. 5). After three catalytic cycles, there are little changes of the IR
spectrum in the region 700–900 cm−1, indicating that the framework
structure of the catalyst was partially decomposed. To clarify whether
catalyst 1 could be leached into the reaction mixture or not, we carried
out the following experiments. When the conversion reached 40.3%,
the catalyst was separated from the reaction mixture by filtration
(Fig. 4c). Further stirring of the filtrate under the above reaction con-
ditions did not yield any additional product. Moreover, we measured
the amounts of antimony and molybdenum leached into the filtrate by
inductively coupled plasma optical emission spectrometry (ICP-OES).
As for the catalyst in this research, the leaching amounts are almost
negligible (< 0.1%).
Having established the optimized reaction conditions for oxidation
of organosilanes of the model substrate, we extened the study to a range
of structurally diverse silanes such as chloro-, alkenyl-, and alkynyl-
containing silanes and alkoxysilanes, etc (Table 3). The electronic effect
of substrates plays a key role in the process of oxidation reaction.
Oxidation of 2b smoothly proceeded to afford the corresponding silanol
in 93.1% yield with 97% selectivity (entry 2). In contrast, electron-poor
dimethylphenylsilane derivatives, for example chloro (2c) group re-
quired a longer reaction time to obtain corresponding silanol in 80.6%
yield (entry 3). Reactions of alkenyl-, and alkynyl-containing silanes
also proceeded selectively to form the corrseponding silanols 2d–2e
with poor selectivities (entries 4–5). Furthermore, methylphenylvi-
nylsilane (2f) was selectively oxidized to the corresponding unsaturated
silanol with 92% selectivity while retaining the olefinic group (entry 6).
Another important issue associated with the performance of a het-
erogeneous catalyst is its recovery and reusability. Regarding this point,
the reused performances of catalyst 1 in the oxidation of 2a by re-
Table 2
Catalytic oxidation of dimethylphenylsilane (2a) with TBHP.a
Entry
Catalyst [mg]
Solvent
Temp [°C]
TBHPb
Conversion [%]
Yieldc [%]
TONd
1
2
3
4
–
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3COCH3
benzonitrile
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
50
50
50
50
30
60
85
50
50
50
50
50
50
50
50
6
6
6
6
6
6
6
6
6
6
1
4
7
6
6
26
99
79
100
35
99
98
31
62
82
47
90
98
98
92
24.9
94.8
71.4
93.7
26.6
94.1
89.9
23.2
56.8
66.9
42.2
84.5
94.1
93.4
86.9
65.5
15
10
20
15
15
15
15
15
15
15
15
15
15
15
249.5
285.6
183.7
70
247.6
236.6
61.1
149.5
176.1
111.1
222.4
247.6
245.8
228.7
5
6
7e
8
9
10
11
12
13
14f
15g
a
Reaction conditions unless stated otherwise: 1 mmol of dimethylphenylsilane (2a), 15 mg (3.8 μmol; 0.38% vs. substrate) of catalyst, 6 mmol of TBHP, 3 mL of
CH3CN, T = 50 °C, t = 3 h, under air (1 atm).
b
TBHP/substrate ratio.
Determined by GC using naphthalene as an internal standard.
TON = Turnover number = number of moles of product per mol of catalyst.
.5 h.
Yield of the 2nd run in the recycling studies.
Yield of the 3rd run in the recycling studies.
c
d
e
f
g
171

Y. Wang et al.
MolecularCatalysis452(2018)167–174
Fig. 3. a) Effect of catalyst amount on the catalytic oxidation of dimethylphenylsilane (data were obtained by carrying out parallel experiments under the same
experimental conditions). b) Effect of different TBHP/substrate molar ratios on the catalytic oxidation of dimethylphenylsilane.
Fig. 4. a) Recycling of the catalytic system for the oxidation of dimethylphenylsilane to the corresponding dimethylphenylsilanol; b) FT-IR spectra of the catalyst
before and after the reaction; c) Experiment to prove the nature of heterogeneous catalyst.
[54]. Interestingly, the conversion of diphenylsilane to the corre-
sponding silanol increased with increasing reaction time but, however,
the selectivity is relatively low. This may be due to the presence of two
Si-H bonds and it is expected to give more than one silanols. In contrast,
aliphatic silanes for example 2i, was oxidized to the corresponding
products in high yield (90.1%) under optimized reaction conditions
(entry 10). Importantly, with increasing carbon atoms of aliphatic si-
lanes (2g), a low yield (57.8%) was obtained in CH2Cl2 solvent (entry
11). One hour later, a lower yield of 51.2% (entry 9) was obtained for
2e.
4. Conclusions
In summary, a new polyoxomolybdate containing mixed-valence
antimony, [(CH3)4N]6H[SbVSb4IIIMo18O66]·9H2O, has been successfully
synthesized and structurally characterized. Interestingly, catalyst 1
contains a new mixed-valence five antimony and 18 molybdenum-ox-
ocluster. The framework of 1a represents a rare example of mixed va-
lent multilayer structural compound. Additionally, catalytic activity
Fig. 5. XPRD patterns spectra of catalyst 1 before and after the reaction.
study of 1 has been investigated the heterogeneous catalysis reaction
for the selective oxidation of silanes. Catalyst 1 results in 94.8% yield at
50 °C and it could be recycled for three consecutive runs. The hot fil-
tered reaction also demonstrated the heterogeneous nature of catalyst
1. More importantly, catalyst 1 is the first example that exhibits in-
teresting catalytic property in the containing mixed-valence antimony
Aromatic silanes such as 2 g (triphenyl) (entry 2) and 2h (diphe-
nylmethyl) are known to be less reactive than the majority of the in-
vestigated silanes, as a result of their greater steric bulk (entries 7–8)
172

Y. Wang et al.
MolecularCatalysis452(2018)167–174
Table 3
Oxidation of various silanes with TBHP catalyzed by 1.a
entry
Substrate
Time (h)
Yieldb (%)
94.8
Silanol:Disiloxane
TONc
249.5
245.0
212.1
155.8
1
2
3
4
2a
2b
2c
2d
3
96:4
97:3
92:8
63:37
2.5
5
93.1
80.6
2
59.2
5
6
2e
2f
3
3
65.4
84.7
69:31
92:8
172.1
222.9
7
8
2g
2h
12
12
98.3
89.7
99:1
258.7
236.1
90:10
9
Et3SiH
Et3SiH
nBu3SiH
2i
2i
2g
1
3
3
51.2
90.1
57.8
95:5
98:2
98:2
134.7
237.1
83.8
10
11d
a
Reaction conditions unless stated otherwise: 15 mg (3.8 μmol; 0.38% vs. substrate) of catalyst, 1 mmol of silanes, 6 mmol of TBHP, 3 mL of CH3CN, T = 50 °C.
b
c
Determined by GC using naphthalene as the internal standard.
TON = Turnover number = number of moles of product per mol of catalyst.
3 mL of CH2Cl2.
d
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We gratefully acknowledge support from the NSFC (grant) and
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.04.013.
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