Acid-base catalyses by dimeric disilicoicosatungstates and divacant γ-Keggin-type silicodecatungstate parent: Reactivity of the polyoxometalate compounds controlled by step-by-step protonation of lacunary W{double bond, long}O sites

Article abstract of DOI:10.1016/j.jorganchem.2006.05.061

The catalytic properties of disilicoicosatungstates, [{γ-SiW₁₀O₃₂(H₂O)₂}₂(μ-O)₂]^4- (2) and [H(γ-SiW₁₀O₃₂)₂(μ-O)₄]^7- (3), and their parent divacant γ-Keggin type silicodecatungstate, [γ-SiW₁₀O₃₄(H₂O)₂]^4- (1), toward C-C bond formation reactions have been investigated. The disilicoicosatungstate 2 with aquo ligands exhibits the acidic nature and catalyzes the Mukaiyama-aldol condensation and carbonyl-ene reaction, while 1 and 3 are rather basic and catalyze the Knoevenagel condensation. Therefore, the acid-base properties of a series of lacunary γ-Keggin silicotungstate derivatives 1-3 are clearly different, and the catalyses of 1-3 depend on the molecular structures while 1-3 are composed of a common [SiW₁₀O₃₂] fragment.

Full text of DOI:10.1016/j.jorganchem.2006.05.061

Journal of Organometallic Chemistry 692 (2007) 455–459
www.elsevier.com/locate/jorganchem
Acid–base catalyses by dimeric disilicoicosatungstates and
divacant c-Keggin-type silicodecatungstate parent:
Reactivity of the polyoxometalate compounds controlled by
step-by-step protonation of lacunary W@O sites
a
a,b,1
a,b,
*
Akihiro Yoshida , Shiro Hikichi
, Noritaka Mizuno
a
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
b
Received 2 March 2006; received in revised form 2 May 2006; accepted 8 May 2006
Available online 30 August 2006
Abstract
The catalytic properties of disilicoicosatungstates, [{c-SiW O (H O) } (l-O) ] (2) and [H(c-SiW O ) (l-O) ] (3), and their
4
ꢀ
7ꢀ
4
1
0
32
2
2
2
2
10 32 2
4
ꢀ
parent divacant c-Keggin type silicodecatungstate, [c-SiW10
O
34(H
2
O)
2
]
(1), toward C–C bond formation reactions have been investi-
gated. The disilicoicosatungstate 2 with aquo ligands exhibits the acidic nature and catalyzes the Mukaiyama-aldol condensation and
carbonyl-ene reaction, while 1 and 3 are rather basic and catalyze the Knoevenagel condensation. Therefore, the acid–base properties
of a series of lacunary c-Keggin silicotungstate derivatives 1–3 are clearly different, and the catalyses of 1–3 depend on the molecular
structures while 1–3 are composed of a common [SiW10
2006 Elsevier B.V. All rights reserved.
O32] fragment.
ꢀ
Keywords: Polyoxometalate; C–C bond formation; Acid and base catalyst
1
. Introduction
thesis, structural characterization, and chemical properties
of novel POMs have been attractive subjects from the view-
points of the traditional coordination chemistry [2].
POMs are utilized as catalysts because of their acid–base
nature and redox properties [3]. For example, Keggin type
heteropoly acids, H [XM O ] (M = Mo(VI), W(VI);
(n, X) = (3, P), (4, Si); abbreviated as HPAs), are exten-
sively utilized as acid catalysts for various organic transfor-
mations because of their strong acidity and the lacks of side
reactions of sulfoxidation, chlorination, nitration, etc. in
comparison with conventional mineral acids [4]. Homoge-
neous and silica-supported HPA catalysts have been
reported to be efficient for the conventional reactions such
as hydration of unsaturated bonds, esterification,
condensation, etc., and the research interests in the applica-
tion of HPA catalysts toward fine organic transformations
are much growing. To date, a few C–C bond formation
reactions such as Diels–Alder reaction, Friedel–Crafts
Polyoxometalates (= POMs) are metal–oxygen cluster
molecules constructed by the self-assembly of M–O/M–
OH components [1]. The chemical properties of POMs will
be finely controlled by the modification of the molecular
structure as well as the composition. On the other hand,
however, the structural reorganization (i.e., ‘‘isomeriza-
tion’’) of POMs occurs in the solution state depending on
the conditions such as temperature, concentration, and
pH of the solution. Therefore, POMs show the structural
variation and unique properties, and the studies on the syn-
n
12 40
*
Corresponding author. Fax: +81 3 5841 7220.
E-mail address: tmizuno@mail.ecc.u-tokyo.ac.jp (N. Mizuno).
Present address: Department of Material and Life Chemistry, Faculty
1
of Engineering, Kanagawa University, Yokohama, Kanagawa 221-8686,
Japan.
0
022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.05.061

456
A. Yoshida et al. / Journal of Organometallic Chemistry 692 (2007) 455–459
4
ꢀ
8ꢀ
0
acylation, Fries rearrangement of aryl esters, carbonyl-ene
reaction, and silylcyanation of ketones have been reported,
while some other useful reactions such as Mukaiyama-
aldol condensation and Knovenagel reaction have not been
examined so far [5]. In order to expand the scope of appli-
cability to fine reactions and to develop more efficient cat-
alysts, the fundamental studies on the reactivity of various
POMs are necessary.
O)2] (2) and [(c-SiW O ) (l-O) ] (3 ; mono-proton-
10 32 2 4
7
ꢀ
ated form of this compound, [H(c-SiW O ) (l-O) ] , is
1
0
32 2
4
abbreviated as 3) [9]. As shown in Scheme 1, the proton-
ation of 1 gives 2, which has ‘‘S-shaped’’ structure and
involves the aquo ligands on the terminal tungsten atoms.
On the contrary, the dehydration of 1 resulted in the for-
mation of the ‘‘closed-shape’’ cluster 3’, which contains nei-
ther vacant sites nor terminal aquo ligands. The ‘‘S-
shaped’’ cluster 2 can catalyze the Baeyer–Villiger oxida-
tion of cycloalkanones to lactones with H O , while 1
and 3 are almost inactive although the compounds 1–3
are composed of a common [c-SiW O ] unit [9]. Such dif-
ferent catalytic performances of 1–3 may arise from the
acid or base properties. Herein we apply 1–3 to the C–C
bond formation reactions catalyzed by acids or bases and
investigate the acid–base properties.
VI
ðnþ4Þꢀ
39
Lacunary polyoxometalates such as ½XM O ꢁ
and
11
VI
9
ðnþ6Þꢀ
½
XM O34
ꢁ
, which contain the vacant site(s) due to the
2
2
lack of one or more metal ions from the fully occupied
VI
nꢀ
POMs ½XM O40ꢁ , are recognized to be the useful precur-
12
10 32
sors for mixed-metal oxide clusters, giant metal clusters,
and inorganic–organic hybrid compounds. Especially, the
c-isomer of a divacant Keggin-type silicodecatungstate,
8
ꢀ
[
SiW O ] (SiW10) [6], has been utilized to construct
10 36
dimetal-substituted clusters such as reactive dinuclear
M (l-O) site-containing POMs [7]. The oxo ligands at
2. Results and discussion
2
2
the vacant sites (i.e., cis-W(@O) components) are basic
2
+
enough to react with not only metal cations but also H :
In order to clarify the acid–base properties of a series of
lacunary c-Keggin silicotungstate derivatives 1–3, the cata-
lytic performances for acid- or/and base-catalyzed C–C
bond formation reactions (Mukaiyama-aldol condensa-
tion, carbonyl-ene reaction, Diels–Alder reaction, Knove-
nagel reaction, and cyanosilylation of ketones) were
examined. The catalytic performance of the ‘‘S-shaped’’
dimeric cluster 2 was clearly different from those of the
monomeric parent 1 and the ‘‘closed-shape’’ dimer 3, i.e.,
2 worked as an acid catalyst whereas 1 and 3 were rather
basic catalysts.
we have successfully synthesized a protonated silicodeca-
4
ꢀ
tungstate, [c-SiW O (H O) ]
(1), which efficiently
1
0
34
2
2
catalyzes the olefin epoxidation with H O . The single
2
2
crystal X-ray structural analysis of 1 reveals that two of
four oxo ligands located at the vacant sites are protonated
selectively to give the aquo ligands [8].
Recently, we have found that the dehydrative condensa-
tion of 1 results in the formation of two structurally
different disilicoicosatungstates, [{c-SiW O (H O) } (l-
1
0
32
2
2 2
8
-
4-
2.1. Acid catalysis of the S-shaped dimer 2
O
O
W
W
O
O
W
W
A crossed-aldol reaction between benzaldehyde and the
silyl enol ether of cyclohexenone (namely ‘‘Mukaiyama-
aldol condensation [10]’’) in homogeneous nitromethane
solution was efficiently catalyzed by 2. The main products
were trimethylsiloxy derivatives of aldols because the reac-
tion was performed under anhydrous conditions as shown
in Table 1 [11]. The TON (= turnover number per mole-
W
W
OH2
W
W
H O
2
+
4H+
- 2H O)
O
O
(
W
2
SiW10
4H+
H2O
W
H O
2
W
+
W
4
-
H O
O
2
ꢀ
1
2
cule; 48) and TOF (= turnover frequency; 4.4 h ) of 2
W
W
OH2
W
'S-shape' dimer
O
dimerization
in
organic solvent
were comparable to or higher than those (TON: 3.4–56,
Acidic
W
ꢀ2
ꢀ1
TOF: 7.7 · 10 –1.7 · 10 h ) of the previously-reported
catalysts based on d-block elements [12]. In contrast, the
amount of aldol products by 1 was lower than stoichiome-
tric one and 3 was almost inactive under the present condi-
tions. The syn to anti ratio for 2 was 43:57, and different
from that for 1 (75:25). These differences of the catalytic
activities and diastereoselectivities among 1–3 suggest the
difference of the acid or base properties.
8
-
1
Basic
W
W
W
W
without H+
O
O
O O
W
(
- 4H O)
2
W
W
W
The ‘‘S-shape’’ dimer 2 also catalyzed the intramolecular
cyclization of R-(+)-citronellal (i.e., carbonyl-ene reaction
W
W
W
0
W
= [SiW O ]
3'
closed' dimer
non-protonated form)
1
0 32
'
[
13]), while 1 and 3 were inactive (Table 2) [14]. The main
(
product obtained by 2 was (ꢀ)-isopulegol, which is the
more valuable intermediate for (ꢀ)-menthol. Recently,
the same reaction catalyzed by silica-supported
H [PW O ] has been reported [5d].
3
: mono protonated form of 3'
Basic
0
Scheme 1. Formation of disilicoicosatungstates 2 and 3 from 1.
3
12 40

A. Yoshida et al. / Journal of Organometallic Chemistry 692 (2007) 455–459
457
Table 1
Table 2
Catalytic activities for Mukaiyama-aldol reaction.
Catalytic activities for carbonyl-ene reaction.
OTMS
MeNO (50 μL)
2
PhCHO
+
305 K / 11 h
CH Cl (3 mL)
2
2
(
62.5 μmol)
(125 μmol)
O
OH O
O
305 K
O
Me SiO
3
(
1 mmol)
Ph
Ph
+
Ph
+
4
5
6
Catalyst (mol%)
Yield of product (%)
+
OH
+
OH
+
OH
+
OH
4
(syn:anti)
5
6
1
2
3
(4.0)
(2.0)
(2.0)
2 (75:25)
91 (43:57)
n.d.
Trace
1
Trace
n.d.
4
Trace
8
7 (diastereo isomers)
Catalyst
mol%)
Time
(min)
Conversion
(%)
Yield of product (%)
(diastereo selectivity)
(
7
8
The Diels–Alder reaction of 2,3-dimethylbutadiene and
,4-naphthoquinone was also catalyzed by 2, while 1 and
were much less active (comparable to without catalyst)
1
3
1 (1.0)
1440
160
1440
<1
99
<1
<1
n.d.
4
n.d.
2
3
(0.5)
(0.5)
84 (1:3:58:38)
<1
as shown in Eq. (1) (the yields of 9: 86% for 2, 16% for
3
2
, 14% for 1, 12% for the reaction without catalysts). While
worked as an acid catalyst under homogeneous condi-
reaction. In contrast with the Knoevenagel reaction, 1 and
were much less active for the cyanosilylation. This may be
related to the basicity and the bulky environment of the
tions (yield 86%, 5 h, 305 K, MeNO solvent), 2 was less
active for same reaction under heterogeneous conditions
2
3
(
yield 20%, 6 h, 305 K, CH Cl solvent). In contrast, the
2 2
basic sites of 1 and 3.
crystalline solid of dodecatungstophosphoric acid,
H [PW O ], and its derivative supported on silica have
been reported to be active for the same reaction under het-
erogeneous conditions [5a].
O
OTMS
CN
3
12 40
1
,2-DCE (2 mL)
+ TMSCN
(2 mmol)
Catalyst
(1 mmol)
3
7
05 K
2 h
1
1
O
O
MeNO (4 ml)
Yield: 30 % (for 1)
24 % (for 3)
2
+
Catalyst
3
05 K
O
O
9
ð2Þ
5
h
(
1.2 mmol) (1 mmol)
Yield: 86 % (for 2)
1
1
1
6 % (for 3)
2.3. Explanation for the acid or base properties of 1–3
4 % (for 1)
2 % (without catalyst)
The acid catalysis by 2 is likely promoted by (1) the
+
ð1Þ
aquo ligand, which works as H donor (i.e., Brønsted
acid), or (2) the coordinatively unsaturated +6-tungsten
ion (Lewis acid site) at the lacunary site formed by the dis-
sociation of the coordinated aqua ligand. The diastereose-
lectivities for both Mukaiyama-aldol and carbonyl-ene
reactions are not so high in comparison with those for
2
.2. Base catalyses of 1 and 3
The condensation of benzaldehyde with malononitrile
i.e. ‘‘Knoevenagel-type reaction’’), is generally considered
(
the epoxidation reactions with hydrogen peroxide by 1,
to be catalyzed by weak bases [15]. In contrast with the
order of catalytic activity for Mukaiyama-aldol and car-
bonyl-ene reaction (2 ꢂ 1 ꢃ 3 ꢃ 0), the monomeric cluster
4ꢀ
[
c- SiV W O (OH) ] , and [{c- SiTi W O (OH) } (l-
2
10 38
2
2
10 36
2 2
1
and ‘‘closed-shape’’ dimer 3 exhibited high catalytic per-
Table 3
formance for the present reaction while the ‘‘S-shape’’ clus-
ter 2 was almost inactive (Table 3). Thus, 1 and 3 can
abstract protons from the activated methylene moiety.
The catalyses of 1–3 for cyanosilylation of 2-adamanta-
none with trimethylsilylcyanide (Eq. (2)) were also exam-
ined. This reaction is known to be catalyzed both acid
and base catalysts [5e]. The base catalysts 1 and 3 exhibited
moderate activity (the yields of 11 after 3 days: 30% for 1,
Catalytic activities for the Knovenagel reaction
CN
CN
CN
10
MeCN (1 mL)
05 K / 2.5 h
Ph
PhCHO
+
3
CN
(
1.5 mmol) (1 mmol)
Catalyst (amount, mol%)
Yield of 10 (%)
1 (0.5)
90
Trace
96
2
3
(0.25)
(0.25)
2
4% for 3), while the acid catalyst 2 decomposed during the

458
A. Yoshida et al. / Journal of Organometallic Chemistry 692 (2007) 455–459
8
ꢀ
O)2] , where the sterically hindered active oxygen species
were formed [8,16]. The contrast suggests that the present
reactions proceed through less hindered intermediates
and that 2 works as a Brønsted acid catalyst.
purification. The silicotungstate catalysts 1 [8a], 2 and 3
[9], and the authentic compounds of crossed-aldol reac-
tion products [10a] were prepared according to the
reported methods.
While 1 and 2 retained the ‘‘lacunary’’ sites and the aquo
ligands, the catalytic properties were quite different from
each other: 1 exhibited rather basic catalysis. From the
viewpoint of coordination chemistry, 1 would have ampho-
teric nature: The oxo ligand(s) at the divacant site works as
a base for the dimerization from 1, and would work as a
proton acceptor. On the other hand, the five-coordinated
tungsten centers formed by the dissociation of the aquo
ligand would bind the oxo ligands of another molecule
3.3. Catalytic reactions
Due to ionic nature of 1–3, the catalysts were soluble
in only polar organic solvent. Especially, the solubility
of 2 was quite low, and therefore, nitromethane and ace-
tonitrile were used as solvent for the homogeneous reac-
tion systems. The reaction was carried out in a glass
vial with stirring. The Mukaiyama-aldol condensation
reactions were carried out as follows: The catalyst con-
taining 25 lmol of W (i.e., 1: 2.5 lmol, 2 and 3:
1.25 lmol), solvent (nitromethane: 50 lL), and substrates
(benzaldehyde: 6.4 lL (62.5 lmol), 1-trimethylsiloxycyclo-
hexene: 24.0 lL (125 lmol)) were charged in the reaction
vial under argon (operated in glove-box). The reaction
was carried out at 305 K for 1.5 h. The yield and product
(
i.e., works as an acid for the dimerization process to give
2
H
and 3) [9]. Both base catalysts 1 and 3 were efficient for
abstraction from the less hindered methylene moiety.
+
The basic site in 3 is under investigation.
In conclusion, a series of lacunary c-Keggin silicotung-
state derivatives 1–3, which were composed of the same
[
SiW O ] fragments, catalyzed various C–C bond forma-
10 32
1
tion reactions; 2 showed acid catalyses toward Mukaiy-
selectivity were determined by HPLC, GC, and H NMR
ama-aldol, carbonyl-ene and Diels–Alder reactions, while
analyses. The carbonyl-ene reactions: The catalyst
containing 100 lmol of W (i.e., 1: 10 lmol, 2 and 3:
5.0 lmol), solvent (dichloromethane: 3.0 mL), and sub-
strates (R-(+)-citronellal: 182 lL (1.00 mmol)) were
charged in the reaction vial. The reaction was carried
out at 305 K. The yield and product selectivity were deter-
1
and 3 showed base catalyses toward Knoevenagel and
cyanosililation reaction.
3
. Experimental section
1
13
3
.1. Instruments
mined by GC, and H and C NMR analyses. The Diels–
Alder reactions: The catalyst containing 100 lmol of W
(i.e., 1: 10 lmol, 2 and 3: 5.0 lmol), solvent (nitromethane:
4.0 mL), and substrates (2,3-dimethylbutadiene: 135 lL
(1.20 mmol), 1,4-naphtoquinone: 158 mg (1.00 mmol))
were charged in the reaction vial. The reaction was car-
ried out at 305 K for 5 h. The products were assigned
NMR spectra were recorded at room temperature on a
1
13
JEOL JNM-EX-270 ( H: 270 MHz, C: 67.9 MHz) spec-
trometer. Chemical shifts (d) were reported in ppm down-
field from internal SiMe . Gas chromatographic (GC)
4
analyses were carried out on a Perkin Elmer Autosystem
gas chromatograph equipped with a TC-WAX column
1
13
by H and C NMR spectroscopy and the yields were
determined by GC analyses. The Knoevenagel-type reac-
tions: The catalyst containing 50 lmol of tungsten (i.e.,
1: 5.0 lmol, 2 and 3: 2.5 lmol), solvent (acetonitrile:
1.0 mL), and substrates (benzaldehyde: 152 lL (1.50
lmol), malononitrile: 62 lL (1.00 mmol)) were charged
in the reaction vial. The reaction was carried out at
305 K for 2.5 h. The yield and product selectivity were
(
30 m, 0.25 mmID, 0.25 lmdf; GL-Science), and a Shima-
dzu GC-17A gas chromatograph equipped with a TC-1
column (30 m, 0.25 mmID, 0.25 lmdf; GL-Science).
GC–MS analyses were carried out on a Shimadzu
GCMS-QP2010 spectrometer. Liquid chromatographic
(
(
LC) analysis was carried out on a Shimadzu HPLC system
pump: LC-10AD, UV-detector: SPD-10A) equipped with
1
Chirapak-AD and Capcellpak C18 columns.
determined by GC, and H NMR analyses. The cyanosi-
lylation reactions: the catalyst containing 100 lmol of W
(i.e., 1: 10 lmol, 2 and 3: 5.0 lmol), solvent (1,2-dichloro-
ethane: 2.0 mL), and substrates (2-adamantanone: 150 mg
(1.0 mmol), trimethylsilylcyanide: 279 lL (2.0 mmol))
were charged in the reaction vial. The reaction was car-
ried out at 305 K for 72 h. The yield and product selectiv-
3
.2. Materials and methods
The solvents used for the syntheses and catalytic reac-
tions were treated with appropriate drying reagents
nitromethane: MgSO4, acetonitrile, dichloromethane,
and 1,2-dichloroethane: P O , toluene: Na/benzophe-
(
1
13
ity were determined by GC, and H and C NMR
analyses.
2
5
none), distilled, and stored under argon. Benzaldehyde
was distilled under reduced pressure in order to remove
contaminated benzoic acid, and used immediately after
the distillation. 2,3-Dimethylbutadiene was distilled in
vacuo prior to use in order to remove hydroquinone
Acknowledgements
This work was supported in part by the Core Research
for Evolutional Science and Technology (CREST) pro-
gram of Japan Science and Technology Agency (JST)
(
anti-polymerization reagent). The commercially available
reagents of the highest grade were used without further

A. Yoshida et al. / Journal of Organometallic Chemistry 692 (2007) 455–459
459
[7] (a) J. Canny, R. Thouvenot, A. Te
´z e´ , G. Herv e´ , M. Leparulo-Loftus,
and a Grant in-Aid for Scientific Research from the Minis-
try of Education, Culture, Sports, Science, and Technology
of Japan. We also thank to Dr. Kazuya Yamaguchi for
providing the authentic compounds of the catalytic reac-
tion products.
M.T. Pope, Inorg. Chem. 30 (1991) 976;
b) K. Wassermann, H.-J. Lunk, R. Palm, J. Fuchs, N. Steinfeldt,
R. St o¨ sser, M.T. Pope, Inorg. Chem. 35 (1996) 3273;
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Chem. 35 (1996) 30;
d) C. Nozaki, I. Kiyoto, Y. Minai, M. Misono, N. Mizuno, Inorg.
(
(
(
Chem. 38 (1999) 5724.
8] (a) K. Kamata, K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi,
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