Communications
DOI: 10.1002/anie.201001468
Polyoxometalate Chemistry
Zinc(II) Containing g-Keggin Sandwich-Type Silicotungstate:
Synthesis in Organic Media and Oxidation Catalysis**
Yuji Kikukawa, Kazuya Yamaguchi, and Noritaka Mizuno*
The versatility and accessibility of polyoxometalates (POMs)
has led to various applications in the fields of analytical
chemistry, medicine, electrochemistry, photochemistry, and
catalysis.[1] POMs have especially received much attention in
the area of oxidation and acid catalysis.[1] Since the synthesis
of the divacant [g-SiW10O36]8ꢀ (SiW10) ion was reported by
Tꢀzꢀ and Hervꢀ,[2] a number of monomeric dimetal-substi-
tuted POMs has been synthesized with SiW10 as a “structural
motif”.[3] Also, SiW10 has been utilized as a “macroligand” to
encapsulate multinuclear metal–oxygen cluster cores and
several POMs built up of two (or more) SiW10 subunits
sandwiching multinuclear metal–oxygen cluster cores have
been reported.[4] In particular, the interests in the catalysis of
these metal-substituted and metal-encapsulating POMs has
been increasing because of their unique reactivities that
depend on the compositions and structures of their active
sites.[1,3,4]
as a monomeric species from solution then occurs.[6b,7] There-
fore, K+ ions inhibit the formation of multimetal centers.
We have reported that the tetra-n-butylammonium
(TBA) salt of SiW10 (TBA4H4[g-SiW10O36]; TBA-SiW10)
can act as an efficient homogeneous catalyst for various kinds
of H2O2-based oxidations, such as the epoxidation of
alkenes,[8a,b] oxygenation of sulfides,[8b] and hydroxylation of
organosilanes.[8c] During the course of our investigations, we
now found that the effect of metal additives was very
significant for the H2O2-based oxidations with TBA-SiW10.
The oxidation of 2-cyclohexen-1-ol (1a) with TBA-SiW10 in
acetone preferentially gave the corresponding epoxy
alcohol 1c (Table 1, entry 1).[9] No oxidation occurred with
[Zn(acac)2] (acac = acetylacetonato; Table 1, entry 2). Sur-
Table 1: Oxidation of 2-cyclohexen-1-ol.[a]
The encapsulation and substitution of multinuclear 3d-
metal cations into SiW10 with retention of the g-Keggin
framework(s) have generally been considered to be very
difficult.[5,6] It has been reported that SiW10 is easily
isomerized and/or decomposed to tri- and tetravacant
POMs by the presence of 3d-metal cations in aqueous acidic
media.[5] For example, Kortz and co-workers reported that the
reaction of the potassium salt of SiW10 (K-SiW10) with 3d-
metal cations in aqueous media produced b-Keggin dimer
[{b-SiNi2W10O36(OH)2(OH2)}2]12ꢀ [5a] and a-Keggin dimers
[{B-a-SiM2W9O34(OH2)}2]12ꢀ (M = Mn2+, Cu2+, and Zn2+)[5b]
with isomerization and/or loss of tungsten atoms. Although a
series of 3d-metal-substituted g-Keggin sandwich-type POMs
[{K(OH2)}2M(OH2)2(g-SiW10O35)2]8ꢀ (M = Mn2+, Co2+, and
Ni2+) has been synthesized from K-SiW10 in aqueous
media,[6] these POMs have only one 3d-metal cation with
respect to two SiW10 subunits. For the formation of
[{K(OH2)}2M(OH2)2(g-SiW10O35)2]8ꢀ, the role of K+ ions is
very important: Two SiW10 subunits are initially linked
through two K+ ions and the insertion of the 3d-metal cation
Entry
Catalyst ([mol%])
Yield [%][b]
1b/1c ratio
1
2
3
TBA-SiW10 (0.8)
[Zn(acac)2] (1.6)
TBA-SiW10 (0.8)
+ [Zn(acac)2] (1.6)
75
18/82[c]
–
only 1b
No reaction
91
[a] Reaction conditions: Catalyst (0.8–1.6 mol% with respect to 1a), 1a
(0.5 mmol), 30% aq. H2O2 (0.25 mmol), [D6]acetone (1.5 mL), 568C,
40 min. The corresponding epoxy ketone was not produced in all cases.
[b] Total yield based on H2O2 used. Determined by GC and 1H NMR
analyses. [c] The syn/anti ratio of 1c was 32/68.
prisingly, the epoxidation was completely suppressed and the
alcohol oxidation exclusively gave the corresponding enone
1b in an almost quantitative yield when a simple mixture of
TBA-SiW10 and [Zn(acac)2] was used as the catalyst (Table 1,
entry 3).[10] This chemoselectivity is much different from that
of reported tungsten-based catalysts including TBA-SiW10
(Table 1, entry 1).[9] The positive-ion cold-spray ionization
mass (CSI-MS) spectrum of a mixture of TBA-SiW10 and
[Zn(acac)2] (1:2 molar ratio) in acetone exhibited signals
centered at m/z 7364.7 for a + 1 charged species assignable to
the [(TBA)9H4Si2Zn4W20O74]+ ion, and no signals arising from
TBA-SiW10 were observed (Figure 1). These results show
that TBA-SiW10 reacted with [Zn(acac)2] in acetone to form
[*] Y. Kikukawa, Dr. K. Yamaguchi, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7220
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
[**] We thank Drs. K. Uehara and S. Uchida (The University of Tokyo) for
their help with experiments. This work was supported in part by a
Grant-in-Aid for Scientific Research from the Ministry of Education,
Culture, Science, Sports and Technology of Japan. Y.K. is grateful for
a JSPS Research Fellowship for Young Scientists.
a
new
zinc-containing
ꢀ8
charged
polyanion
[H4Si2Zn4W20O74]8ꢀ, which would catalyze the alcohol oxida-
tion (rather than the epoxidation).
From the above results, we got the idea to synthesize
g-Keggin POM(s) with multinuclear 3d-metal cations: Our
Supporting information for this article is available on the WWW
6096
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6096 –6100