Diamond-shaped [Ag4]4+ cluster encapsulated by silicotungstate ligands: Synthesis and catalysis of hydrolytic oxidation of silanes

Article abstract of DOI:10.1002/anie.201108372

An Ag₄ diamond is encapsulated by silicotungstate ligands in TBA₈[Ag₄(DMSO)₂(γ-H₂SiW ₁₀O₃₆)₂]·2 DMSO·2 H₂O (Ag4; DMSO=dimethyl sulfoxide, TBA=tetra-n-butylammonium), which was obtained by reaction of TBA₄H₄[γ-SiW₁₀O ₃₆] with AgOAc in an organic medium. Polyoxometalate Ag4 (see picture) selectively catalyzes hydrolytic oxidation of various silanes to the corresponding silanols in high yields (72-96 %). Copyright

Full text of DOI:10.1002/anie.201108372

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DOI: 10.1002/anie.201108372
Polyoxometalates
Diamond-Shaped [Ag4]4+ Cluster Encapsulated by Silicotungstate
Ligands: Synthesis and Catalysis of Hydrolytic Oxidation of Silanes**
Yuji Kikukawa, Yoshiyuki Kuroda, Kazuya Yamaguchi, and Noritaka Mizuno*
The design and synthesis of well-defined polynuclear metal or
metal–oxygen clusters have attracted much attention in many
fields of science such as catalysis, magnetism, electrochemis-
try, and photochemistry.[1] Generally, well-defined polynuc-
lear clusters have been synthesized by using structure-
directing agents such as oxygen- and/or nitrogen-donor
organic ligands.[1] Polyoxometalates (POMs) are a large
family of anionic metal–oxygen clusters consisting of the
group V and VI metals in their highest oxidation states and
are thermally and oxidatively stable in comparison with
commonly utilized organic ligands.[2] In addition to high
durability, their chemical properties, for example, redox
potentials, (multi)electron-transfer properties, acidities, and
solubilities, can be finely tuned by choice of constituent
elements and countercations, and diverse structures can be
synthesized.[2] These properties make POMs a class of
potential materials for catalysis.[2] In particular, lacunary
POMs are very attractive and can act as efficient homoge-
neous catalysts for H2O2-based oxidation.[3] Moreover, they
can be regarded as “inorganic multidentate oxygen-donor
ligands”. Diverse lacunary POMs ranging from mono- to
trilacunary have readily been synthesized. By employing
these lacunary POMs as inorganic ligands (structural motifs),
various kinds of metal–oxygen clusters (substituted into the
lacunary pockets[4] or encapsulated by two or more lacunary
POMs[5]) with unique structures and interesting catalytic
properties have been designed.
coproduct. To date, several efficient catalysts based on
precious metals such as gold,[7] ruthenium,[8] rhenium,[9]
platinum,[10] iridium,[11] and silver[12] have been reported for
hydrolytic oxidation of silanes. In particular, we paid atten-
tion to catalysis by silver, because silver has a high potential to
catalyze various functional-group transformations[12,13] and is
cheaper and more abundant than other precious metals for
hydrolytic oxidation of silanes.
Hydrolytic oxidation of dimethylphenylsilane (1a) to
dimethylphenylsilanol (2a) was initially carried out with
AgOAc under the conditions described in Table 1. In this
case, formation of glossy precipitates (i.e., silver metal), was
clearly observed just after addition of 1a to the reaction
solution containing AgOAc, and a low yield of 2a resulted
(Table 1, entry 1). Formation of silver metal could not be
avoided in the presence of commonly utilized nitrogen-donor
ligands such as pyridine and 2,2’-bipyridyl (Table 1, entries 6
and 7), as well as fully occupied silicotungstate TBA4[a-
SiW12O40] (SiW12, TBA = tetra-n-butylammonium; Table 1,
entry 2). Hydrolytic oxidation of 1a efficiently proceeded in
the presence of lacunary POMs such as TBA4H4[a-SiW11O39]
(SiW11) and TBA4H4[g-SiW10O36] (SiW10; Table 1, entries 3
and 4). In particular, the effect of SiW10 was very significant;
1a was selectively transformed into the desired silanol 2a in
88% yield without formation of the corresponding disiloxane,
and the stoichiometric amount of gaseous hydrogen with
respect to 2a was formed (92% H2 yield at complete
conversion of 1a; Supporting Information Figure S1a).[14]
The yellow color of the reaction solution did not change,
and no formation of silver metal was observed on allowing the
solution to stand at 508C for 24 h (Supporting Information
Figure S1b). Even when 1a (0.5 mmol) and water (5.6 equiv
with respect to 1a) were newly added to this solution,
Herein we focus on the development of efficient catalysts
(or precatalysts) for hydrolytic oxidation of silanes. Selective
oxidation of silanes to silanols is of paramount importance in
organic synthesis in academia and chemical industry because
of the versatile use of silanols as synthons for silicon-based
functional (polymeric) materials and cross-coupling reac-
tions.[6] In addition, hydrolytic oxidation using water and/or
dioxygen theoretically produces only hydrogen or water as
Table 1: Hydrolytic oxidation of 1a to 2a (effects of POM ligands).[a]
Entry
Catalyst (mol%)
t
Yield
[%]
[*] Dr. Y. Kikukawa, Dr. Y. Kuroda, Dr. K. Yamaguchi,
Prof. Dr. N. Mizuno
[min]
1
2
3
4
AgOAc (0.2)
4
4
4
4
12
4
4
60
60
4
7
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
AgOAc (0.2)+SiW12 (0.1)
AgOAc (0.2)+SiW11 (0.1)
AgOAc (0.2)+SiW10 (0.1)
AgOAc (0.2)+SiW10 (0.1)
AgOAc (0.2)+pyridine (0.2)
AgOAc (0.2)+2,2’-bipyridyl (0.2)
SiW10 (0.1)
34
88
79
17
9
5[b]
6
[**] This work was supported in part by the Japan Society for the
Promotion of Science (JSPS) through its FIRST Program, Global
COE Program (Chemistry Innovation through Cooperation of
Science and Engineering), and Grants-in-Aid for Scientific
Researches from Ministry of Education, Culture, Sports, Science
and Technology. Y. Kikukawa is grateful for JSPS Research Fellow-
ship for Young Scientists.
7
8
9
<1
<1
None
[a] Standard conditions: 1a (0.5 mmol), water (5.6 equiv with respect to
1a), acetonitrile (2 mL), 508C, under 1 atm of air. Yields were
determined by GC with naphthalene as internal standard. [b] Acetone
(2 mL) was used instead of acetonitrile.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108372.
2434
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Angew. Chem. Int. Ed. 2012, 51, 2434 –2437

Angewandte
Chemie
formation of 2a and gaseous hydrogen was observed with
almost the same rate as for the first run (86% yield of 2a over
4 min based on newly added 1a; cf. Table 1, entry 4). There-
fore, it is likely that SiW10 can act as an efficient oxygen-
donor ligand to stabilize (active) silver species,[15] resulting in
promotion of the hydrolytic oxidation and prevention of the
aggregation of silver species as is observed for the reaction
without the SiW10 ligand. These results stimulated us to
synthesize a silver-containing POM by employing SiW10 as an
inorganic ligand, which would be expected to be active for
hydrolytic oxidation of silanes and other functional-group
transformations.[13]
Usually, transition metal containing POMs have been
synthesized by reaction of alkali metal salts of lacunary POMs
and the corresponding transition metal salts in aqueous
media, but isomerization and decomposition of lacunary
POMs often proceed in aqueous (acidic) media. Synthesis in
organic media from appropriate lacunary precursors (e.g.,
TBA salts) can avoid isomerization and decomposition of the
original lacunary frameworks. For example, we synthesized
zinc-containing sandwich-type silicotungstate TBA8[{Zn-
(OH2)(m3-OH)}2{Zn(OH2)2}2(g-HSiW10O36)2]·9H2O, which is
not possible to synthesize in aqueous media, by the reaction of
SiW10 with Zn(acac)2 (acac = acetylacetonato) in acetone.[5a]
Likewise, we attempted to synthesize a silver-containing
POM in organic media, while taking the results in Table 1 into
account. Silver-containing POM Ag4 could be synthesized by
reaction of SiW10 and AgOAc in acetone (70% yield based
on SiW10), and single crystals suitable for X-ray crystallo-
graphic analysis were obtained by recrystallization from
acetone/dimethyl sulfoxide (DMSO; see Supporting Infor-
mation).[16] The molecular structure of the anion part of Ag4 is
shown in Figure 1.
values (1.12–1.25 for O20, O21, O56, and O57) found in Ag4
suggest that protons are likely located on these oxygen atoms.
The X-ray, elemental analysis, and thermogravimetric data
show that the formula of Ag4 is TBA8[Ag4(DMSO)2(g-
H2SiW10O36)2]·2DMSO·2H2O. Compound Ag4 can form
according to Equation (1).
2 TBA4H4½g-SiW10O36ꢀ þ 4 AgOAc !
ð1Þ
TBA8½Ag4ðg-H2SiW10O36Þ2ꢀ þ 4 AcOH
As shown in Figure 1, Ag4 has a diamond-shaped
tetrasilver [Ag4]4+ cluster cation encapsulated by two SiW10
subunits. Two DMSO molecules are coordinated to Ag2 and
Ag4. The Ag1 and Ag3 atoms are bridged by two SiW10
subunits in a slightly distorted square-planer environment,
ꢁ
and the average Ag OW distance is 2.38 ꢀ. The Ag2 and Ag4
atoms are coordinated to two oxygen atoms of the central
ꢁ
{SiO4} tetrahedrons with an average Ag OSi distance of
2.46 ꢀ. Thus, the [Ag4]4+ cluster is stabilized in the lacunary
ꢁ
ꢁ
ꢁ
pocket of SiW10. The Ag1 Ag2, Ag2 Ag3, Ag3 Ag4, and
ꢁ
Ag4 Ag1 distances are 2.962(2), 3.070(2), 3.016(2), and
3.074(2) ꢀ, respectively. Since they are shorter than twice
the van der Waals radius of the silver atom (3.44 ꢀ), they
suggest the existence of argentophilic interaction between
silver atoms in Ag4. It is well known that d10 metals tend to
show closed-shell interactions,[17] and such argentophilic
interactions were often observed in previously reported
organometallic complexes with similar [Ag4]4+ cores.[18] The
Ag1···Ag3 and Ag2···Ag4 separations are 4.868(2) and
3.683(2) ꢀ, respectively. The Ag1-Ag2-Ag3 and Ag2-Ag3-
Ag4 angles are 105.66(6)8 and 74.47(5)8, respectively. The
ꢁ
above-mentioned Ag Ag distances, Ag···Ag separations, and
angles in Ag4 are within the ranges of reported complexes
with [Ag4]4+ cores.[18]
Although various kinds of POMs substituting or encap-
sulating metal–oxygen clusters have been synthesized until
now,[2,4,5] we note that POMs containing multimetallic cores
with metal–metal interactions are very rare.[19–22] Polyoxo-
metalate assemblies with dimeric[19] or infinite[20] structures
[19a,b,20]
ꢁ
ꢁ
connected by metal–metal interactions (Ag Ag
or Rh
Rh[19c]) have been reported. The dirhodium-containing POMs
were synthesized by attaching dicarboxylatodirhodium com-
plexes to monolacunary POMs.[21] The g-Keggin silicotung-
states [g-SiW10M2S2O38]6ꢁ (M = W or Mo) with [M(O)(m-
S)2M(O)]2+ oxothio core show M M interactions.
All
[22]
Figure 1. Polyhedral and ball-and-stick representation of the anion part
of Ag4 (see Figure S2 of the Supporting Information for ORTEP). The
{WO6} units and a {SiO4} unit are shown as green octahedra and
a gray tetrahedron, respectively. Black, red, green, blue, and pink
spheres indicate silver atoms, oxygen atoms of DMSO (ODMSO), oxygen
atoms of the {WO6} units (OW), oxygen atoms of the {SiO4} units
(OSi), and monoprotonated oxygen atoms, respectively. DMSO mole-
cules, except for the oxygen atoms, are omitted for clarity.
ꢁ
reported POMs with metal–metal interaction have dimetallic
cores, and multimetallic cores consisting of more than two
metals, as observed in Ag4, have not been reported so far, to
the best of our knowledge.[23]
Finally, we investigated catalysis of hydrolytic oxidation of
silanes by Ag4. As we expected, various structurally diverse
silanes including aromatic, double bond containing, and
aliphatic ones, could selectively be converted to the corre-
sponding silanols in high yields in the presence of Ag4
(Table 2). Selectivities to the desired silanols were greater
than 99% (except for 1d). The hydrolytic oxidation of
commonly examined 1a efficiently proceeded with Ag4
(0.05 mol%), giving 2a in 96% yield under the conditions
described in Table 2.[14,15] In this case, the stoichiometric
Eight TBA cations per Ag4 anion could be crystallo-
graphically assigned, in accord with the result of the elemental
analysis. The bond valence sum (BVS) values of silicon (3.98
and 4.02), tungsten (5.83–6.26), and silver atoms (0.91–1.07) in
Ag4 indicate that the respective valences are + 4, + 6, and
+ 1. In addition, the four oxygen atoms with lower BVS
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Table 2: Hydrolytic oxidation of various silanes.[a]
the case of double bond containing silane 1e, the desired
unsaturated silanol 2e was selectively obtained in 96% yield
without formation of over-reduced products. It has been
reported that hydrolytic oxidation of aliphatic silanes hardly
proceeds in the presence of AgHAP catalyst.[12] In contrast,
Ag4 could promote hydrolytic oxidation of various aliphatic
silanes, such as 1i, 1j, 1k, and 1l (72–92% yields).
Entry Substrate
t
Product
Yield
[%]
1[b]
2
1a 4 min
1b 2 min
1c 1 h
2a 96 (88)
2b 83 (88)
2c 90 (94)
2d 92 (92)[d]
As mentioned in Table 1, a simple mixture of SiW10 and
AgOAc (“in-situ-prepared catalyst”) efficiently promoted
hydrolytic oxidation of 1a, whereby SiW10 could act as an
oxygen-donor ligand to stabilize active silver species.[15] It
would be advantageous for practical applications if the
hydrolytic oxidation could be carried out with such an in-
situ-prepared catalyst, because the reaction can readily be
performed even without isolation of truly active species.[24]
The catalyst prepared in situ by simply mixing the required
components of SiW10 and AgOAc showed very similar
performance to Ag4 (see values in parentheses in Table 2).
In summary, we have demonstrated that it is possible to
synthesize a novel POM containing the diamond-shaped
[Ag4]4+ core (Ag4) by reaction of SiW10 with AgOAc in an
organic medium. This POM is a rare compound which
contains a multimetallic core with metal–metal interaction.
In the presence of Ag4, hydrolytic oxidation of various
structurally diverse silanes selectively proceeded to give the
corresponding silanols in high yields (72–96%). Moreover,
the catalyst prepared in situ by simply mixing the required
components of precursors (SiW10 and AgOAc) showed
similar performance (71–98% yields) to Ag4.
3
4[c]
1d 3 h
5
6
1e 7 min
1 f 5 min
2e 96 (98)
2 f 91 (96)
7[c]
1g 3 h
2g 88 (77)
2h 94 (90)
78 (88)
78 (93)
2k 92 (90)
2l 72 (71)
8[c,e]
9
Bn3SiH
Et2MeSiH
Et3SiH
1h 3 h
1i 10 h
1j 8 h
1k 24 h
Bn3SiOH
Et2MeSiOH
Et3SiOH
nPr3SiOH
iPr3SiOH
2i
2j
10[c]
11[c]
nPr3SiH
12[c,f] iPr3SiH
1l
24 h
[a] Standard conditions: Ag4 (0.5 mol%), silane (0.25 mmol), water
(5.6 equiv with respect to silanes), acetonitrile (1 mL), 508C, under
1 atm of air. Yields were determined by GC with naphthalene as internal
standard. The values in parentheses are yields with the catalyst prepared
in situ by mixing SiW10 and AgOAc. [b] Ag4 (0.05 mol%), 1a
(0.5 mmol), water (5.6 equiv with respect to 1a), acetonitrile (2 mL),
508C, under 1 atm of air. [c] 708C. [d] The corresponding disiloxane was
formed as a byproduct (2% yield for Ag4; 3% yield for the catalyst
prepared in situ). [e] Acetonitrile (2 mL). [f] Ag4 (1 mol%).
Received: November 28, 2011
Published online: January 19, 2012
amount of gaseous hydrogen with respect to 2a was also
formed (98% H2 yield at complete conversion of 1a).
Notably, the amount of Ag4 could be much reduced. When
the hydrolytic oxidation of 1a was carried out with
0.004 mol% of Ag4 at 708C, 2a was obtained in 72% yield
over 10 min (Supporting Information Figure S3). In this case,
the turnover frequency (TOF) based on silver was 27000 hꢁ1
and the highest among previously reported catalysts including
AgHAP catalyst (silver nanoclusters supported on hydrox-
yapatite, TOF = 178 hꢁ1 at 808C[12], Supporting Information
Table S1).[7–12] The second highest turnover number (TON) of
15625 (based on silver) was obtained when using
0.0008 mol% of Ag4 (Supporting Information Figure S3 and
Table S1).[7–12]
The electronic effect of substrates is very significant.
Hydrolytic oxidation of 1b smoothly proceeded to afford the
corresponding silanol 2b in 83% yield over only 2 min. In
contrast, dimethylphenylsilane derivatives with electron-
withdrawing substituents such as chloro (1c) and trifluoro-
methyl (1d) groups required longer reaction times and/or
a higher reaction temperature (708C) to attain high yields
(90–92%) of the corresponding silanols. These results suggest
that formation of a positively charged intermediate, such as
a silyl cation, is likely involved in the present transformation.
The present system could be applied to sterically bulky silanes
such as 1 f (diphenylmethyl), 1g (triphenyl), and 1h (triben-
zyl), and gave the corresponding silanols in high yields (88–
94% yields), though the steric effect was also significant. In
Keywords: cluster compounds · oxidation · polyoxometalates ·
.
silver · synthetic methods
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originates from water and are in agreement with the formation
of a stoichiometric amount of gaseous hydrogen.
[15] The positive-ion cold-spray ionization mass spectrum of a solu-
tion of Ag4 in acetonitrile just after addition of 1a exhibited
several sets of signals centered around m/z 3623 (A), 3682 (B),
3818 (C), 3925 (D), and 4034 (E) possibly due to [SiW10O36]8ꢁ
,
[SiW10O36]8ꢁ
,
[SiW10O36AgI2]6ꢁ
,
[SiW10O36AgI2Ag0]6ꢁ
,
and
[SiW10O36AgI2Ag0 ]6ꢁ, respectively (Supporting Information Fig-
2
ure S4a). After the reaction was completed (complete consump-
tion of 1a or water), signal sets A, B, D, and E almost
disappeared, while signal set C was still observed (Supporting
Information Figure S4b).
A similar phenomenon was also
observed for use of the catalyst prepared in situ from SiW10
and AgOAc (Supporting Information Figure S4c and d). There-
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are possible active species in the present hydrolytic oxidation of
silanes. Once the reaction is complete, Ag0 species are likely
reoxidized and stabilized in the lacunary pocket of SiW10, which
prevents their aggregation for long times (see Supporting
Information Figure S1).
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under 1 atm of Ar, the reaction rate was almost the same as
under 1 atm of air, and 2a was obtained in 88% yield after 4 min
(cf. Table 2, entry 1). In this case, formation of gaseous hydrogen
was also observed. In addition, hydrolytic oxidation of 1a with
18O-labeled water (18O content: ꢂ 98%) under the conditions
described in Table 2 led to formation of 18O-labeled 2a as major
product (77% yield, 18O content in 2a: 94% by GC-MS
Angew. Chem. Int. Ed. 2012, 51, 2434 –2437
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