Synthesis of α-Dawson-type silicotungstate [α-Si 2W18O62]8- and protonation and deprotonation inside the aperture through intramolecular hydrogen bonds

Article abstract of DOI:10.1002/chem.201400390

The design of structurally well-defined anionic molecular metal-oxygen clusters, polyoxometalates (POMs), leads to inorganic receptors with unique and tunable properties. Herein, an α-Dawson-type silicotungstate, TBA ₈[α-Si₂W_18<

Full text of DOI:10.1002/chem.201400390

DOI: 10.1002/chem.201400390
Full Paper
&
Polyoxometalates
8
ꢀ
Synthesis of a-Dawson-Type Silicotungstate [a-Si W O ] and
2
18 62
Protonation and Deprotonation Inside the Aperture through
Intramolecular Hydrogen Bonds
[
a]
Takuo Minato, Kosuke Suzuki, Keigo Kamata, and Noritaka Mizuno*
Abstract: The design of structurally well-defined anionic mo-
lecular metal–oxygen clusters, polyoxometalates (POMs),
leads to inorganic receptors with unique and tunable prop-
drogen bonds with retention of the POM structure. In con-
trast, the aperture of phosphorus-centered POM TBA [a-
6
P W O ]·H O (III) was not protonated inside the aperture.
2
18 62
2
erties. Herein, an a-Dawson-type silicotungstate, TBA [a-
The density functional theory (DFT) calculations revealed
8
Si W O ]·3H O (II) that possesses a ꢀ8 charge was success-
that the basicities and charges of internal m -oxygen atoms
2
18 62
2
3
fully synthesized by dimerization of a trivacant lacunary
were increased by changing the central heteroatoms from
5
+
4+
a-Keggin-type silicotungstate TBA H [a-SiW O ]·2H O (I) in
P
to Si , thereby supporting the protonation of II. Addi-
4
6
9
34
2
an organic solvent. POM II could be reversibly protonated
in the presence of acid) and deprotonated (in the presence
tionally, II showed much higher catalytic performance for
the Knoevenagel condensation of ethyl cyanoacetate with
benzaldehyde than I and III.
(
of base) inside the aperture by means of intramolecular hy-
6ꢀ
Introduction
[
a-P W O ] in acetonitrile, and the proton was delocalized
2 18 62
on the anion surface (see the Experimental Section). DFT calcu-
lations showed that the highest-occupied molecular orbital
Polyoxometalates (POMs) are a large family of structurally well-
defined anionic molecular metal–oxygen clusters, and their
chemical and physical properties can be finely tuned by select-
ing constituent elements, charges, structures, or by introducing
metal cations, and therefore they are of increasing interest in
broad fields including catalytic chemistry, materials science,
6ꢀ
(HOMO) of [a-P W O ] consisted of orbitals on the terminal
2
18 62
(W=O) and bridging (W-O-W) oxygen atoms of the anion sur-
face, and no orbital was observed on the internal m -oxygen
3
atoms (Figure 1a). Therefore, to increase the affinity of the
[
1,2]
and medical science.
Based on the closely aligned oxygen
donor ligands of the constituent M-O-M units and their nega-
tive charges, porous materials or apertures constructed by
POMs can act as inorganic receptors for capturing various
[
3]
metal cations. In particular, the accurate design of the sizes
of apertures and the directions of the lone pair of oxygen
donors enables selective metal-cation recognition in POM-
[
4]
based inorganic molecular receptors. Herein, we explore the
idea that POMs with basic small apertures would act as inor-
ganic proton receptors through intramolecular hydrogen
bonds by controlling the anion charges and local electron den-
sities of the apertures.
We focused on rugby-ball-shaped a-Dawson-type POMs
nꢀ
6ꢀ
8ꢀ
18 62
[
a-X W O ]
that possess
a
small aperture in which
2 18 62 2
Figure 1. HOMOs of a) [a-P W O ] and b) [a-Si W O ] . The orbitals are
2
18 62
represented by dark red and blue lobes (isosurface value 0.02). The atoms
are represented by spheres: W, orange; P, green; Si, yellow; O, white.
m3-oxygen donors are closely aligned. Initially, perchloric acid
was added to solution of phosphorus-centered POM
a
[a] T. Minato, Dr. K. Suzuki, Dr. K. Kamata, 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
oxygen atoms of the aperture toward protons, we attempted
mꢀ
to increase the negative charges by changing XO
units
4
3ꢀ
4ꢀ
mꢀ
from PO4 to SiO4 because charges of the central XO4
units in POMs largely affected those of the oxygen atoms of
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
mꢀ
[5]
XO4
units. As expected, the natural bond orbital (NBO)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400390.
8ꢀ
charges of the six internal m -oxygen atoms in [a-Si W O ]
3
2
18 62
Chem. Eur. J. 2014, 20, 5946 – 5952
5946
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
6
ꢀ
and [a-P W O ] were ꢀ1.10 and ꢀ1.19, respective-
2
18 62
Table 1. Crystal data and structure refinement parameters for H
·II ·II
2
·II, H
3
·II, and H·III.
ly,
and
the
difference
between
8ꢀ
6ꢀ
[
a-Si W O ] and [a-P W O ] (D) was 0.09 (Fig-
H
2
H
3
H·III
2
18 62
2
18 62
ure S1 in the Supporting Information). In contrast,
the D values of the terminal oxygen atoms (W=O)
and the bridging oxygen atoms (W-O-W) were less
than 0.03 and small. Additionally, in the HOMO of
formula
M [gmol
r
C
98Cl
4
N
6
O
65Si
2
W
18
C
88
N
9
O
62Si
2
W
18
88 9 62 2 18
C N O P W
5546.0
monoclinic
C2/c (no. 15)
26.0134(2)
15.45140(10)
36.1433(3)
90
105.6676(3)
90
13987.79(19)
4
123(2)
2.634
ꢀ
1
]
5808.2
orthorhombic
Pna2 (no. 33)
34.6054(2)
22.6385(1)
19.9623(1)
90
5540.3
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
monoclinic
C2/c (no. 15)
26.15030(10)
15.50760(10)
35.9046(3)
90
105.6917(3)
90
14017.68(16)
4
123(2)
2.625
1.099
0.0468
0.1296
1
8ꢀ
[
a-Si W O ] , orbitals were observed at the
2 18 62
m -oxygen atoms within the aperture, thus suggest-
3
8ꢀ
a [8]
b [8]
ing that [a-Si W O ] can be protonated inside the
2
18 62
90
90
aperture (Figure 1b; Figure S2 in the Supporting In-
formation). However, the synthesis of silicon-centered
a-Dawson-type POMs has been considered to be
quite difficult because of the following three fac-
g [8]
3
V [ꢁ ]
15638.75(14)
4
153(2)
2.467
0.904
0.0409
0.1364
Z
T [K]
ꢀ
3
[
6–8]
10ꢀ
1calcd [gcm
]
tors:
1) trivacant lacunary [a-SiW O ] , a precur-
9 34
GOF
1.100
0.0466
0.1302
8ꢀ
sor for [a-Si W O ] , is metastable in an aqueous
[a]
2
18 62
R1 (I>2s(I))
[
a]
media and easily isomerizes to monovacant lacunary
wR2
[
6]
or fully occupied silicotungstates; 2) dimerization is
inhibited by the electrostatic repulsion because of
the higher negative charges than those of
2
o
2
c
2 2 1/2
o
[
a] R
1
=ꢀj jF
o
jꢀjF
c
j j/ꢀjF
o
j. wR
2
={ꢀ[w(F ꢀF )]/ꢀ[w(F ) ]}
.
nꢀ
[7]
[
a-XW O ] (X=P, As, n=9; X=S, Se, n=8); and
9 34
10ꢀ
3
) dimerization of alkali-metal salts of [a-SiW O ] in aqueous
revealed that the product was an a-Dawson-type POM
(Table 1; Figure S4a in the Supporting Information). The bond
valence sum (BVS) values of tungsten (5.95–6.21) and silicon
(3.90 and 3.96) indicate that respective valences are +6 and
+4. Six TBA cations per anion could crystallographically be as-
signed in accord with the elemental analysis data, thus indicat-
ing the existence of six TBA cations and two protons per anion
(H ·II). The CSI-MS spectrum of H ·II in acetonitrile showed two
9
34
media is inhibited by strong coordination of alkali-metal cat-
1
0ꢀ [8,9]
ions to vacant sites of [a-SiW O ]
.
In this work, by utiliz-
trivacant lacunary
a-Keggin-type POM, TBA H [a-SiW O ]·2H O (I), we successful-
9
34
ing
an
organic-solvent-soluble
4
6
9
34
2
ly synthesized a ꢀ8-charged a-Dawson-type POM, TBA [a-
8
Si W O ]·3H O (II), in an organic solvent. POM II could be pro-
2
18 62
2
tonated (in the presence of acid) and deprotonated (in the
presence of a base) inside the aperture through intramolecular
hydrogen bonds with the retention of the POM structure, and
II showed much higher catalytic performance for the Knoeve-
nagel condensation of ethyl cyanoacetate with benzaldehyde
than I and III.
2
2
signal sets at m/z 3149.5 and 6056.5 assignable to
2
+
+
[TBA H Si W O ] and [TBA H Si W O ] , respectively (Fig-
8
2
2
18 62
7
2
2
18 62
1
ure S5a in the Supporting Information). The H NMR spectrum
of H ·II in CD CN showed a signal at d=10.85 ppm (2H per
anion) assignable to hydrogen-bonded protons (Figure S6a in
2
3
[
10]
29
the Supporting Information). The Si NMR spectrum showed
a signal at d=ꢀ79.4 ppm (Figure S7a in the Supporting Infor-
1
83
Results and Discussion
mation), and the W NMR spectrum showed five signals at
d=ꢀ118.5, ꢀ134.1, ꢀ143.1, ꢀ145.0, and ꢀ151.8 ppm with the
respective ratio of 1:2:2:2:2, thus suggesting that the overall
symmetry of the anion is C2v (Figure 2a). These NMR spectro-
Synthesis of a-Dawson-type silicotungstates
A tetra-n-butylammonium (TBA) salt of a trivacant lacunary
Keggin-type silicotungstate I was synthesized by mixing
scopic data indicate that two protons in H ·II are located
2
[
6]
Na [a-SiW O ]·3H O and TBABr in an acetate buffer solution
within the aperture without delocalization (H ·II=TBA [a-
10
9
34
2
2
6
(pH 3.70) at 38C. The cold-spray ionization mass (CSI-MS) spec-
H Si W O ]·3H O). The BVS values of bridging oxygen atoms
2 2 18 62 2
trum of I in dimethyl sulfoxide (DMSO) showed a signal set as-
(W-O-W) are 1.92–2.13. In contrast, the BVS values of internal
m3-oxygen atoms (O19, O47, O20, O48, O21, O49: 1.38–1.71;
average: 1.59) and terminal oxygen atoms (W=O; O13ꢀO18,
O32ꢀO34, O41ꢀO46, O60ꢀO62: 1.59–1.81; average: 1.70) are
+
29
signable to [TBA SiW O ] (m/z 3390.9). The Si NMR spec-
5
9
31
trum of I in [D ]DMSO showed a signal at d=ꢀ83.2 ppm, thus
6
1
83
showing the presence of the single species. The
W NMR
spectrum showed three signals at d=ꢀ153.1, ꢀ169.9, and
190.7 ppm with the respective ratio of 1:1:1, which suggests
lower than those of other oxygen atoms of H ·II (Table S1 in
2
ꢀ
the Supporting Information). The BVS values of internal m3-
that the overall symmetry of the anion is C .
oxygen atoms of H ·II are lower than those of other
3
2
The CSI-MS spectrum of I in dichloromethane showed two
new signal sets at m/z 3400.0 and 6575.5 assignable to
a-Dawson-type POMs (Table S2 in the Supporting Information;
1.75–2.12), whereas the values of terminal oxygen atoms of
2
+
+
[
TBA H Si W O ] and [TBA H Si W O ] , respectively, thus
H ·II are almost equal to those of other non-protonated
2
10
2
2
18 63
9
4
2
18 64
indicating that dimerization of I proceeds (Figure S3 in the
Supporting Information). The pale yellow single crystals were
obtained by dimerization of I in a mixed solvent of dichloro-
methane and ethyl acetate. The X-ray crystallographic analysis
a-Dawson-type POMs (Table S3 in the Supporting Information;
average 1.67–1.95). Additionally, the distances between
m3-oxygen atoms (O19···O47, O20···O48, O21···O49) are in the
range of 2.72–2.79 ꢁ and shorter than those of other
Chem. Eur. J. 2014, 20, 5946 – 5952
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Figure 3. Profiles for the potentiometric titration of H
2
·II in a mixed solvent
of acetonitrile and water (9:1 v/v) with 1m aqueous TBAOH as titrant. The
potentials are relative to a standard Ag/AgCl electrode.
1
83
Figure 2. W NMR spectra of a) H
2 3
·II, b) H·II, and c) II in CD CN. Insets: Top
views of a) H ·II, b) H·II, and c) II, and the atoms are represented by spheres:
2
W, dark gray; Si, light gray; H, black.
a-Dawson-type POMs (W-based POMs, 2.83–3.10 ꢁ; Mo-based
POMs, 2.93–3.26 ꢁ; Table S4 in the Supporting Information).
These results show that two protons are sandwiched between
Figure 4. Schematic representation of condensation of I and reversible ex-
traction and insertion of protons. The atoms are represented by polyhedrals
and spheres: W, orange; Si, yellow; H, blue.
two pairs of oxygen atoms among the six internal m -oxygen
3
[
11]
atoms with intramolecular hydrogen bonds.
1
same signals as those of H ·II (Table 2). The H signals of H ·II
2
2
Reversible protonation and deprotonation inside the aper-
ture of a-Dawson-type silicotungstates
and H·II were observed at much lower fields than those of the
previously reported POMs that contain protons inside their
structures (d=2.1–8.1 ppm), thus indicating the stronger hy-
The potentiometric titration of H ·II with TBAOH showed in-
2
flection points at 1 and 2 equivalents of TBAOH with respect
drogen bonds of H ·II and H·II (Table S5 in the Supporting In-
2
[13]
to H ·II (Figure 3). Singly protonated H·II (TBA [a-
formation). Proton sponges are a class of compounds with
unusually high basicity and can capture protons through
2
7
HSi W O ]·H O) was formed by the addition of 1 equivalent of
2
18 62
2
[14]
TBAOH to the solution of H ·II in acetonitrile and was charac-
strong intramolecular hydrogen bonds. Upon addition of the
proton sponge (1,8-bis(dimethylamino)naphthalene (DMAN))
2
1
29
183
terized by CSI-MS and H, Si, and W NMR spectra (Figur-
es S5b, S6b, and S7b in the Supporting Information; Figures 2b
to the solution of H·II in CD CN (20 equivalents with respect to
3
and 4). Upon further addition of 1 equivalent of TBAOH to the
H·II), the signal intensity of H·II did not change and no signal
of DMANH (d=8.02, 7.88, 7.67, and 3.04 ppm) was observed
1
+
solution of H·II in CD CN, the H NMR spectrum showed no
3
signal assignable to a hydrogen-bonded proton (Figure S6c in
(Figure S8 in the Supporting Information). These results sug-
the Supporting Information). The product (II) was precipitated
gest that II is more basic than DMAN and the pK value of the
a
1
83
+
by the addition of diethyl ether to the solution. The W NMR
spectrum showed two signals at d=ꢀ134.6 and ꢀ164.6 ppm
with the respective ratio of 1:2 (Figure 2c), thereby suggesting
that the overall symmetry of the anion is D3h and that the
conjugated acid of II is higher than that of DMANH (18.18 in
[15]
acetonitrile). The Knoevenagel condensation of ethyl cyanoa-
cetate with benzaldehyde in the presence of II efficiently pro-
ceeded to give ethyl trans-a-cyanocinnamate in 96% yield.
The yield of ethyl trans-a-cyanocinnamate was about three
times higher than that with I [Eq. (1)]. The reaction rate of II
[12]
proton is released from H·II (II=TBA [a-Si W O ]·3H O).
8
2
18 62
2
A proton-free II can capture two protons within the aper-
4
ꢀ1
ture. Upon addition of 2 equivalents of nitric acid to the solu-
(1.1ꢂ10 mmmin ) was approximately 2000 times higher than
29
183
ꢀ1
tion of II in CD CN, the Si and W NMR spectra showed the
that of I (5.6 mmmin ). In addition, the reaction did not pro-
3
Chem. Eur. J. 2014, 20, 5946 – 5952
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Full Paper
with protons (Figure S10b in the Supporting Informa-
tion). These results are in sharp contrast with the se-
lective metal encapsulation within a closed dimer
[
a]
2 3 3
Table 2. Chemical shifts [ppm] of I, II, H·II, H ·II, H ·II, III, and H·III in CD CN.
1
29
183
H
Si
W
[4g]
[
b]
[c]
TBA [Si W O ]. The distances between three pairs
8 2 20 68
I
–
–
ꢀ83.17 ꢀ151.3(1), ꢀ169.9(1), ꢀ190.7(1)
ꢀ87.97 ꢀ134.6(1), ꢀ164.6(2)
[
d]
II
H·II
H·II
H
H
II+2HNO
3
H ·II
II+3HNO
of six internal m -oxygen atoms of the Si-O-W bond-
3
10.35 (1H) ꢀ83.49 ꢀ132.4(2), ꢀ147.6(1), ꢀ156.3(2), ꢀ157.8(2), ꢀ167.8(2)
ing of II (2.73–2.79 ꢁ) were significantly shorter than
[
b]
[c]
10.27 (1H) ꢀ83.61
–
those of TBA [Si W O ] (3.98 ꢁ). In addition, the en-
8
2
20 68
2
·II
·II
10.85 (2H) ꢀ79.44 ꢀ118.5(1), ꢀ134.1(2), ꢀ143.1(2), ꢀ145.0(2), ꢀ151.8(2)
[
b]
[c]
trance of the aperture of II (height: 3.07–3.08 ꢁ;
width: 4.27–4.30 ꢁ) was also smaller than that of
TBA [Si W O ] (height: 3.89–3.90 ꢁ; width: 3.98–
2
10.76 (2H) ꢀ78.99
–
3
10.85 (2H) ꢀ79.61 ꢀ120.2(1), ꢀ135.9(2), ꢀ144.8(2), ꢀ147.6(2), ꢀ153.3(2)
10.75 (2H) ꢀ79.13 ꢀ120.3(1), ꢀ136.1(2), ꢀ144.1(2), ꢀ145.7(2), ꢀ152.9(2)
[
b]
8
2
20 68
[c]
3
–
ꢀ79.48 ꢀ120.0(1), ꢀ135.7(2), ꢀ144.6(2), ꢀ147.6(2), ꢀ152.9(2)
5.14 ꢁ). These results indicate that the aperture of II
1
31
183
is too small to capture the metal cations, and only
protonation proceeds.
H
P
W
[
d]
III
–
–
–
–
ꢀ12.50 ꢀ113.6(1), ꢀ157.7(2)
ꢀ12.35 ꢀ115.5(1), ꢀ158.6(2)
[
b]
[c]
[d]
DFT calculations supported the protonation behav-
ior. In contrast to the orbitals on the internal
m3-oxygen atoms of HOMOs of II and H·II, those of
H·II and III possessed orbitals only on the surface
oxygen atoms. Additionally, the energies of HOMOs
decreased with increase in the anion charges (II>
III
H·III
III+HNO
[
b]
ꢀ12.33 ꢀ113.9(1), ꢀ156.9(2)
[
d]
[c]
3
ꢀ12.40
–
[
[
a] The numbers in parentheses are integrated intensity ratios. [b] Chemical shifts in
]DMSO (H ·II and H·III are insoluble in acetonitrile). [c] The spectra were not mea-
D
6
3
sured. [d] No signal assignable to a hydrogen-bonded proton was observed.
H·II>H ·II, III) and basicities decrease in this order
2
ceed in the presence of III. These results support the stronger
basicity of II.
(Figure S2 in the Supporting Information). These results sup-
port that only the aperture of II and H·II can be protonated.
Conclusion
In conclusion, a-Dawson-type silicotungstates TBA [a-
6
H Si W O ]·3H O (H ·II), TBA [a-HSi W O ]·H O (H·II), and
2
2
18 62
2
2
7
2
18 62
2
TBA [a-Si W O ]·3H O (II) were successfully synthesized by di-
8
2
18 62
2
Upon further addition of 1 equivalent of nitric acid or tri-
merization of a trivacant lacunary a-Keggin-type silicotung-
state TBA H [a-SiW O ]·2H O (I) in an organic solvent under
fluoromethanesulfonic acid to the solution of H ·II in acetoni-
2
4
6
9
34
2
29
183
trile, the CSI-MS and Si and W NMR spectra did not change
and no crystal was obtained (Table 2). In contrast, upon addi-
mild conditions. Silicon-centered II that possessed a ꢀ8 charge
could reversibly be protonated (in the presence of acid) and
deprotonated (in the presence of base) inside the aperture
through intramolecular hydrogen bonds with the retention of
the POM structure.
tion of 1 equivalent of perchloric acid to the solution of H ·II in
2
acetonitrile, pale yellow single crystals were obtained. Five TBA
cations per anion could crystallographically be assigned in
accord with the elemental analysis data, thus indicating that
five TBA and three protons are contained in the anion (H ·II)
3
(
Table 1; Table S6 and Figure S4b in the Supporting Informa- Experimental Section
1 29
tion). The H and Si NMR spectra of H ·II in [D ]DMSO showed
almost the same signals as those of H ·II in [D ]DMSO (Table 2).
The W NMR spectrum of H ·II showed five signals with the
respective ratio of 1:2:2:2:2, thereby suggesting that the over-
3
6
General methods
2
6
1
83
IR spectra were measured using a JASCO FT/IR-460 instrument
using KBr disks. UV/Vis spectra were measured by using a JASCO
V-570DS instrument. CSI-MS spectra were recorded using a JEOL
JMS-T100CS instrument. NMR spectra were recorded using a JEOL
3
all symmetry of the anion is C2v and the same as that of H ·II
(
2
Figure S9 in the Supporting Information). These data indicate
1
JNM EX-270 or JMN ECA-500 spectrometer ( H, 269.60 or
that two of three protons are located within the aperture and
the other one is delocalized on the surface of the anion, which
shows that II can capture up to two protons (Figure 4).
29
31
183
4
95.13 MHz; Si, 53.45 or 98.37 MHz; P, 109.05 MHz; W, 11.20
1
29
or 20.84 MHz) by using 5 mm (for H and Si (98.37 MHz)) or
29
31
183
1
0 mm (for Si (53.45 MHz), P, and W) tubes. Chemical shifts (d)
Finally, we examined protonation of II in the presence of var-
ious metal cations. Upon the addition of a mixture of 1 equiva-
lent of Co(OAc) , Zn(acac) , LiOTf, NaOTf, AgOTf, Pb(OTf) , KOTf
were reported in parts per million (ppm) downfield from SiMe
4
1
29
(solvent, CDCl ) for H and upfield for Si NMR spectra, H PO (sol-
3
3
4
31
vent, D
O) for P NMR spectra, and 1m Na
WO
(solvent, D
O) for
2
2
2
2
2
4
2
1
83
W NMR spectra. GC analyses were performed by using a Shimad-
(
OTf=trifluoromethane sulfonate, acac=acetylacetonate), and
1
zu GC-2014 instrument with a flame ionization detector equipped
with an RTX-1 capillary column (internal diameter=0.25 mm;
length=30 m). Mass spectra were recorded using a Shimadzu
GCMS-QP2010 equipped with a TC-5HT capillary column at an ioni-
zation voltage of 70 eV. Thermogravimetric and differential thermal
analyses (TG-DTA) were performed by using a Rigaku Thermo Plus
TG 8120. Inductively coupled plasma (ICP) atomic emission spec-
nitric acid to the solution of II in [D ]DMSO, the H NMR spec-
6
trum showed a signal at d=10.22 ppm assignable to the hy-
drogen-bonded proton of H·II (Figure S10a in the Supporting
29
Information). The Si NMR spectrum of the solution showed
a signal at d=ꢀ83.4 ppm assignable to H·II, thus indicating
that II is protonated upon coexistence of various metal cations
Chem. Eur. J. 2014, 20, 5946 – 5952
www.chemeurj.org
5949
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
troscopy (AES) analyses were performed by using a Shimadzu
ICPS-8100 instrument. Elemental analyses for C, H, and N were per-
formed by using a Yanaco MT-6 at the Elemental Analysis Center of
School of Science of the University of Tokyo. Acetonitrile (Kanto
Chemical) was purified by The Ultimate Solvent System (Glass Con-
solution of III in acetonitrile. Five TBA cations per anion could crys-
tallographically be assigned in accord with the result of elemental
analysis, thus indicating the existence of five TBA cations and one
proton per anion (Table 1; Table S6 and Figure S4c in the Support-
31
ing Information). The P NMR spectrum of H·III in [D ]DMSO
6
[16]
tour Company) prior to use. Substrates and other solvents were
showed a signal at d=ꢀ12.33 ppm, and the chemical shift was the
[17]
purified according to the reported procedures.
Na WO ·2H O
2 4 2
same as that (d=ꢀ12.35 ppm) of III (Table 2; Figure S11a in the
183
(
Nippon Inorganic Colour and Chemical), Na SiO ·9H O (Wako
Supporting Information). The W NMR spectrum showed two sig-
nals at d=ꢀ113.9 and ꢀ156.9 ppm with the respective ratio of 1:2
(Figure S11b in the Supporting Information), thus suggesting that
2
3
2
Chemical), Na CO (Nacalai Tesque), NaCl (Kanto Chemical), tetra-n-
2
3
butylammonium bromide (TCI), tetra-n-butylammonium hydroxide
37% methanol solution) (TCI), nitric acid (Kanto Chemical), acetic
1
(
the overall symmetry of the anion is D . The H NMR spectrum of
3h
acid (Kanto Chemical), sodium acetate (Kanto Chemical), phospho-
ric acid (Kanto Chemical), trifluoromethanesulfonic acid (TCI),
perchloric acid (Wako Chemical), Fe(OTf)2 (Wako Chemical), Co-
H·III in [D ]DMSO showed no signal assignable to a hydrogen-
6
bonded proton in the range of d=10–11 ppm (Figure S11c in the
Supporting Information). These NMR spectroscopic data indicated
that III cannot be protonated inside the aperture and the proton
(
OAc) ·4H O (Wako Chemical), Ni(acac) ·2H O (Kanto Chemical),
2 2 2 2
Cu(OTf) (TCI), Zn(acac) (Kanto Chemical), LiOTf (TCI), NaOTf (TCI),
was delocalized on the surface of the anion (H·III=TBA H[a-
2
2
5
Agacac (Aldrich), KOTf (Kanto Chemical), Br2 (Kanto Chemical),
P W O ]·H O).
2
18 62
2
NH Cl (Kanto Chemical), KCl (Nacalai Tesque), TBACl (TCI), PbCl2
4
(
Kanto Chemical), AgOTf (Aldrich), CaCl (Kanto Chemical), naphtha-
2
Synthesis and characterization of [(n-C H ) N] H [a-
4
9 4
4
6
lene (Wako Chemical), and deuterated solvents (CD CN and
[
synthesized according to the reported procedures.
3
SiW O ]·2H O (I)
9
34
2
D ]DMSO) (ACROS) were used as received. Na [a-SiW O ] was
6
10
6]
9
34
[
Na [a-SiW O ]·3H O (20.0 g, 7.97 mmol) was dissolved in acetate
10 9 34 2
buffer solution kept at 38C (300 mL, 1m, pH 3.70), and TBABr
100 g, 0.310 mol) was added. After vigorous stirring for 2 min, the
white precipitate formed was isolated by filtration and washed
(
X-ray crystallography
29
Diffraction measurements were carried out by using a Rigaku Mi-
croMax-007 Saturn 724 charge-coupled device (CCD) detector with
graphic-monochromated MoKa radiation (l=0.71069 ꢁ) at 153 or
with water (108C, 1 L) (7.01 g, 28% yield). Si NMR (53.45 MHz,
[D ]DMSO, 298 K, SiMe4): d=ꢀ83.17 ppm (Dn1/2 =3.4 Hz);
6
183
W NMR (11.20 MHz, [D ]DMSO, 298 K, Na WO ): d=ꢀ153.1 (Dn
6
2
4
1/
1
23 K. The data were collected and processed using CrystalClear
=22.3 Hz), ꢀ169.9 (Dn =32.3 Hz), ꢀ190.7 ppm (Dn =21.5 Hz)
2
1/2
1/2
[18]
[19]
software for Windows and HKL2000 for Linux. Neutral scatter-
ing factors were obtained from the standard source. In the reduc-
tion of data, Lorentz and polarization corrections were made. The
with the respective integrated intensity ratio of 1:1:1; IR (KBr
pellet): n˜ =2961, 2873, 1634, 1485, 1470, 1381, 1347, 1153, 1106,
1065, 1004, 952, 901, 819, 738, 565, 515, 454, 376, 361, 352, 337,
[
20]
ꢀ1
+
structural analyses were performed by using CrystalStructure,
287, 257, 251 cm ; MS (CSI, DMSO): m/z: calcd: 3390.9; found:
[21]
[22]
+
+
WinGX,
and Yadokari-XG.
All structures were solved by
3390.9 [TBA SiW O ] ; MS (CSI, dichloromethane): m/z: calcd:
5 9 31
3400.0; found: 3400.0 [TBA H Si W O ] ; calcd: 6575.5; found:
10 2 2 18 63
6575.5 [TBA H Si W O ] ; elemental analysis calcd (%) for TBA H -
9 4 2 18 64 4 6
[23]
2+
SHELXS-97 (direct methods) and refined by SHELXH-97.
metal atoms (Si, P, and W) and oxygen atoms in the POM frame-
works were refined anisotropically.
The
+
[SiW O ]·2H O (C H N O SiW ): C 23.74, H 4.79, N 1.73, Si 0.87,
9
34
2
64 154
4
36
9
CCDC-974406 (H ·II), -974407 (H ·II), and -974408 (H·III) contain the
W 51.09; found: C 23.42, H 4.75, N 1.67, Si 0.89, W 51.05.
2
3
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis and characterization of [(n-C H ) N] [a-
4
9 4
6
H Si W O ]·3H O (H ·II)
2
2
18 62
2
2
Ethyl acetate (4 mL) was added to a solution of I (186 mg,
7.4 mmol) in dichloromethane (12 mL). The pale yellow single crys-
Quantum chemical calculations
5
[
24]
The calculations were performed with Gaussian 09 software.
Anionic parts of H ·II, H·II, II, and III were optimized at the B3LYP
tals of H ·II suitable for X-ray crystallographic analysis were ob-
2
1
2
tained (47.0 mg, 28% yield). H NMR (269.60 MHz, CD CN, 298 K,
3
[25]
level of theory with 6-31++G** (for H and O), 6-31G** (for Si
SiMe ): d=10.85 (2H), 3.15 (m, 48H), 1.64 (m, 48H), 1.41 (m, 48H),
4
[26]
1
and P), and LanL2DZ (for W) by using the conductor-like polariz-
0.98 ppm (t, 72H); H NMR (269.60 MHz, [D ]DMSO, 298 K, SiMe ):
6
4
[27]
able continuum model (CPCM)
with the parameters of the
d=10.76 (s, 2H), 3.18 (m, 48H), 1.58 (m, 48H), 1.33 (m, 48H),
29
United Atom Topological Model (UAHF) for acetonitrile.
0.94 ppm (t, 72H); Si NMR (53.45 MHz, CD CN, 298 K, SiMe ): d=
3 4
2
9
ꢀ
79.44 ppm (Dn1/2 =2.1 Hz);
Si NMR (98.37 MHz, [D ]DMSO,
6
1
83
2
98 K,
SiMe4):
d=ꢀ78.99 ppm
(Dn1/2 =8.7 Hz);
W NMR
Experimental details for addition of acids to TBA [a-
6
(
ꢀ
11.20 MHz, CD CN, 298 K, Na WO ): d=ꢀ118.5 (Dn =2.7 Hz),
3
2
4
1/2
P W O ]·H O (III)
2
18 62
2
134.1 (Dn1 =2.8 Hz), ꢀ143.1 (Dn =2.7 Hz), ꢀ145.0 (Dn
=
1/2
/2
1/2
Upon addition of 1 equivalent of nitric acid or trifluoromethanesul-
3.0 Hz), ꢀ151.8 ppm (Dn =3.0 Hz) with the respective integrated
1/2
fonic acid with respect to TBA [a-P W O ]·H O (III) in acetonitrile,
intensity ratio of 1:2:2:2:2; IR (KBr pellet): n˜ =2961, 2933, 2872,
1735, 1631, 1483, 1382, 1267, 1230, 1153, 1031, 1007, 957, 936,
915, 899, 799, 770, 533, 513, 484, 446, 420, 379, 328, 303, 294,
6
2
18 62
2
the CSI-MS spectrum showed only two signal sets assignable to
2
+
+
[
TBA P W O ] (m/z 3151.4) and [TBA P W O ] (m/z 6060.3).
8
3
2
18 62
7
2
18 62
1
ꢀ1
3
The P NMR spectrum of the resulting solution in CD CN showed
253 cm ; UV/Vis (acetonitrile): l (e)=369 (2.07ꢂ10 ), 285 (4.42ꢂ
3
4
4
ꢀ1
ꢀ1
+
a signal at d=ꢀ12.40 ppm, and the chemical shift was the same
10 ), 242 nm (6.10ꢂ10 m cm ); MS (CSI, acetonitrile): m/z:
1
2+
as that (d=ꢀ12.50 ppm) of III (Table 2). No H signal assignable to
calcd: 3149.5; found: 3149.5 [TBA H Si W O ] ; calcd: 6056.5;
8
2
2
18 62
+
a hydrogen-bonded proton was observed in the range of d=10–
found: 6056.5 [TBA H Si W O ] ; elemental analysis calcd (%) for
7 2 2 18 62
11 ppm. The pale yellow single crystals (H·III) were obtained by ad-
TBA [H Si W O ]·3H O (C H N O Si W ): C 19.65, H 3.85, N 1.43,
6 2 2 18 62 2 96 224 6 65 2 18
dition of 1 equivalent of perchloric acid with respect to III to the
Si 0.96, W 56.39; found: C 19.63, H 3.84, N 1.44, Si 0.95, W 56.35.
Chem. Eur. J. 2014, 20, 5946 – 5952
www.chemeurj.org
5950 ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ꢀ
1
+
Synthesis and characterization of [(n-C H ) N] [a-
444, 379, 329, 291, 281, 256 cm ; MS (CSI, acetonitrile): m/z:
4
9 4
7
2
+
calcd: 3028.8; found: 3028.8 [TBA H Si W O ] ; calcd: 3149.5;
HSi W O ]·H O (H·II)
7
3
2
18 62
2
18 62
2
2
+
found: 3149.5 [TBA H Si W O ] ; calcd: 5815.1; found: 5815.1
8
2
2
18 62
TBAOH (37% methanol solution, 0.6 mL, 2 mmol, 1 equivalent with
respect to H ·II) was added to a solution (1 mL) of H ·II (50.0 mg,
+
[
TBA H Si W O ] ;
elemental
analysis
calcd
(%)
for
6
3
2
18 62
2
2
TBA H[H Si W O ]·4H O (C H N O Si W ): C 17.02, H 3.41, N
5
2
2
18 62
2
80 191
5
66
2
18
8
.60 mmol) in acetonitrile. Diethyl ether (10 mL) was added to the
1
5
.24, Si 1.00, W 58.62; found: C 17.27, H 3.74, N 1.04, Si 1.01, W
8.98.
solution and a white precipitate formed and was isolated by filtra-
tion and washed with diethyl ether (2 mL) (32.2 mg, 62% yield).
1
H NMR (269.60 MHz, CD CN, 298 K, SiMe ): d=10.35 (1H), 3.18 (m,
3
4
1
5
6H), 1.65 (m, 56H), 1.41 (m, 56H), 0.98 ppm (t, 84H); H NMR
Synthesis and characterization of [(n-C H ) N] [a-
4 9 4 6
(
5
495.13 MHz, [D ]DMSO, 298 K, SiMe ): d=10.27 (1H), 3.15 (m,
6H), 1.55 (m, 56H), 1.30 (m, 56H), 0.90 ppm (t, 84H); Si NMR
P W O ]·H O (III)
6
4
2
18 62
2
29
TBA [a-P W O ] was synthesized according to the reported proce-
6
2
18 62
(
53.45 MHz, CD CN, 298 K, SiMe ): d=ꢀ83.49 ppm (Dn =2.8 Hz);
3
4
1/2
[
13a,28]
2
9
dures.
Ethyl acetate (3.5 mL) was added to a solution (30 mL)
Si NMR (98.37 MHz, [D ]DMSO, 298 K, SiMe ): d=ꢀ83.61 ppm
6
4
1
83
of TBA [a-P W O62] (500 mg, 85.9 mmol) in dichloromethane. The
6 2 18
pale yellow single crystals of III suitable for X-ray crystallographic
analysis were obtained (357.7 mg, 71% yield). H NMR (269.60 MHz,
(Dn =2.4 Hz); W NMR (11.20 MHz, CD CN, 298 K, Na WO ): d=
1
/
2
3
2
4
ꢀ
132.4 (Dn1/2 =3.2 Hz), ꢀ147.6 (Dn =3.3 Hz), ꢀ156.3 (Dn
=
1/2
1/2
1
2
.9 Hz), ꢀ157.8 (Dn1 =3.3 Hz), ꢀ167.8 ppm (Dn =3.2 Hz) with
/2
1/2
CD CN, 298 K, SiMe ): d=3.15 (m, 48H), 1.64 (m, 48H), 1.40 (m,
3
4
the respective integrated intensity ratio of 2:1:2:2:2; IR (KBr pellet):
n˜ =2961, 2933, 2871, 1636, 1483, 1384, 1254, 1152, 1120, 1034,
1
2
3
1
4
8H), 0.98 ppm (t, 72H); P NMR (109.05 MHz, CD CN, 298 K,
3
3
1
H PO ): d=ꢀ12.50 ppm (Dn1/2 =3.4 Hz);
P NMR (109.05 MHz,
3
4
013, 946, 915, 899, 843, 795, 768, 556, 529, 452, 421, 383, 331,
ꢀ1
+
[D
]DMSO, 298 K,
6
H
3
PO
4
): d=ꢀ12.35 ppm (Dn1/2 =2.5 Hz);
95, 288, 253 cm ; MS (CSI, acetonitrile): m/z: calcd: 3270.2;
1
83
2
+
W NMR (20.84 MHz, CD
6.4 Hz), ꢀ157.7 ppm (Dn1/2 =5.7 Hz) with the respective integrated
intensity ratio of 1:2; W NMR (20.84 MHz, [D ]DMSO, 298 K,
3
CN, 298 K, Na
2
WO
4
) d=ꢀ113.6 (Dn1/2
=
found: 3270.2 [TBA HSi W O ] ; calcd: 6298.0; found: 6298.0
9
2
18 62
+
[
TBA HSi W O ] ;
elemental
analysis
calcd
(%)
for
8
2
18 62
183
6
TBA [HSi W O ]·H O (C112H255N O Si W ): C 22.15, H 4.23, N 1.61,
Si 0.92, W 54.48; found: C 22.52, H 4.62, N 1.57, Si 0.91, W 54.42.
7
2
18 62
2
7
63
2
18
Na WO ): d=ꢀ115.5 (Dn =19.9 Hz), ꢀ158.6 ppm (Dn =13.3 Hz)
2
4
1/2
1/2
with the respective integrated intensity ratio of 1:2; IR (KBr pellet):
n˜ =2961, 2933, 2872, 1631, 1483, 1380, 1153, 1090, 1019, 991, 955,
Synthesis and characterization of [(n-C H ) N] [a-
ꢀ1
+
4
9 4
8
913, 790, 598, 565, 527 cm ; MS (CSI, acetonitrile): m/z: calcd:
2
+
Si W O ]·3H O (II)
3151.4; found: 3151.4 [TBA P W O ] ; calcd: 6060.3; found:
8 2 18 62
2
18 62
2
+
6
060.3 [TBA P W O ] ; elemental analysis calcd (%) for TBA -
7 2 18 62 6
2
TBAOH (37% methanol solution, 75 mL, 5.5ꢂ10 mmol, 2 equiva-
lents with respect to H ·II) was added to a solution (2 mL) of H ·II
[
P W O ]·H O (C H N O P W ): C 19.76, H 3.77, N 1.44; found:
2 18 62 2 96 218 6 63 2 18
2
2
C 19.61, H 3.69, N 1.28.
(
1.60 g, 275 mmol) in acetonitrile. Diethyl ether (400 mL) was added
to the solution and a white precipitate formed and was isolated by
filtration and washed with diethyl ether (10 mL) (1.49 g, 93%
Synthesis and characterization of [(n-C H ) N] H[a-
4
9 4
5
1
yield). H NMR (269.60 MHz, CD CN, 298 K, SiMe ): d=3.15 (m,
3
4
P W O ]·H O (H·III)
2
18 62
2
29
6
4H), 1.64 (m, 64H), 1.40 (m, 64H), 0.98 ppm (t, 96H); Si NMR
(
53.45 MHz, CD CN, 298 K, SiMe ): d=ꢀ87.97 ppm (Dn =1.6 Hz);
HClO (70% aqueous solution, 33 mL, 447 mmol, 1 equivalent with
4
respect to III) was added to a solution (1.5 mL) of III (1.30 g,
223 mmol) in acetonitrile. Pale yellow single crystals of H·III suitable
3
4
1/2
1
83
W NMR (11.20 MHz, CD CN, 298 K, Na WO ): d=ꢀ134.6 (Dn
=
3
2
4
1/2
2
2
2
.3 Hz, J(W-O-W)=18.6 Hz), ꢀ164.6 ppm (Dn =2.3 Hz, J(W-O-
1/2
W)=19.4 Hz) with the respective integrated intensity ratio of 1:2;
IR (KBr pellet): n˜ =2961, 2932, 2872, 1635, 1484, 1384, 1152, 1107,
1
2
for X-ray crystallographic analysis were obtained after 1 d (1.10 g,
1
88% yield). H NMR (269.60 MHz, [D
]DMSO, 298 K, SiMe
): d=3.18
6
4
3
1
008, 946, 900, 844, 770, 563, 514, 475, 454, 421, 386, 328, 303,
(m, 40H), 1.59 (m, 40H), 1.32 (m, 40H), 0.95 ppm (t, 60H); P NMR
(109.05 MHz, [D ]DMSO, 298 K, H PO
): d=ꢀ12.33 ppm (Dn1/2
W NMR (11.20 MHz, [D ]DMSO, 298 K, Na WO ): d=
ꢀ1
+
93, 282, 254 cm ; MS (CSI, acetonitrile): m/z: calcd: 3390.9;
6
3
4
=
2
+
183
found: 3390.9 [TBA Si W O ] ; calcd: 6539.4; found: 6539.4
3.3 Hz);
6
2
4
1
0
2
18 62
+
[
TBA Si W O ] ;
elemental
analysis
calcd
(%)
for
ꢀ113.9 (Dn1/2 =10.1 Hz), ꢀ156.9 ppm (Dn1/2 =8.0 Hz) with the re-
spective integrated intensity ratio of 1:2; IR (KBr pellet): n˜ =2962,
2933, 2874, 1631, 1483, 1382, 1152, 1091, 1025, 962, 910, 801, 597,
9
2
18 62
TBA [Si W O ]·3H O (C128H293N O Si W ): C 24.21, H 4.67, N 1.76,
Si 0.88, W 52.10; found: C 24.14, H 4.89, N 1.91, Si 0.84, W 52.01.
8
2
18 62
2
8
65
2
18
ꢀ
1
5
MS
[
63, 530, 472, 389, 372, 332, 321, 303, 292, 287, 281, 256 cm ;
+
(CSI, acetonitrile): m/z: calcd: 3030.7; found: 3030.7
Synthesis and characterization of [(n-C H ) N] H[a-
H Si W O ]·4H O (H ·II)
2+
2+
4
9 4
5
TBA HP W O ] ; calcd: 3151.4; found: 3151.4 [TBA P W O ] ;
7 2 18 62 8 2 18 62
+
2
2
18 62
2
3
calcd: 6060.3; found: 6060.3 [TBA P W O ] ; elemental analysis
7
2
18 62
calcd (%) for TBA H[P W O ]·H O (C H N O P W ): C 17.18, H
HClO (70% aqueous solution, 25 mL, 172 mmol, 1 equivalent with
5
2
18 62
2
80 183
5
63
2
18
4
3
.30, N 1.25; found: C 17.08, H 3.34, N 1.22.
respect to H ·II) was added to a solution (1.5 mL) of H ·II (1.00 g,
2
2
1
72 mmol) in acetonitrile. The pale yellow single crystals of H ·II
3
suitable for X-ray crystallographic analysis were obtained after 1 d
Procedure for catalytic Knoevenagel condensation
1
(
737 mg, 77% yield). H NMR (269.60 MHz, [D ]DMSO, 298 K, SiMe ):
6 4
d=10.75 (s, 2H), 3.18 (m, 40H), 1.58 (m, 40H), 1.33 (m, 40H),
Catalytic reactions were carried out with a glass tube (15 mL) con-
taining a magnetic stir bar. The procedure for the catalytic reaction
was as follows: ethyl cyanoacetate (1, 1.0 mmol), benzaldehyde (2,
1.5 mmol), acetonitrile (1 mL), and internal standard (naphthalene,
ꢁ0.2 mmol) were charged in the reaction vessel. The reaction was
initiated by the addition of II (5 mmol), I (10 mmol), or III (5 mmol)
and the reaction solution was periodically analyzed by GC and GC-
MS.
2
9
0
.94 ppm (t, 60H); Si NMR (98.37 MHz, [D ]DMSO, 298 K, SiMe ):
6 4
183
d=ꢀ79.13 ppm (Dn =7.7 Hz); W NMR (20.84 MHz, [D ]DMSO,
1
/2
6
2
1
98 K, Na WO ): d=ꢀ120.3 (Dn1/2 =11.7 Hz), ꢀ136.1 (Dn1/2
=
2
4
2.4 Hz), ꢀ144.1 (Dn =12.0 Hz), ꢀ145.7 (Dn =12.5 Hz),
1/2
1/2
ꢀ
152.9 ppm (Dn =9.8 Hz) with the respective integrated intensi-
1/2
ty ratio of 1:2:2:2:2; IR (KBr pellet): n˜ =2962, 2933, 2873, 1629,
1
483, 1380, 1240, 1152, 1103, 1029, 972, 920, 802, 778, 530, 486,
Chem. Eur. J. 2014, 20, 5946 – 5952
www.chemeurj.org 5951
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[
[
8] a) N. Laronze, J. Marrot, G. Hervꢅ, Inorg. Chem. 2003, 42, 5857; b) N. Lar-
onze, J. Marrot, G. Hervꢅ, Chem. Commun. 2003, 2360.
Acknowledgements
8
ꢀ
9] There has been only one report on the synthesis of [a-Si W O ]
2 18 62
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 (MEXT) and by the Funding
Program for World-Leading Innovative R&D on Science and
Technology (FIRST Program).
under severe hydrothermal conditions (1408C, 6 d). Y. Ding, H. Chen, W.
Chen, E. Wang, J. Meng, Trans. Annu. Meet. Orthop. Res. Soc. Trans. Met.
Chem. 2009, 34, 281.
[
10] a) V. Bertolasi, P. Gilli, V. Ferretti, G. Gilli, J. Chem. Soc. Perkin Trans. 2
997, 945; b) R. Ishimoto, K. Kamata, N. Mizuno, Angew. Chem. 2012,
24, 4740; Angew. Chem. Int. Ed. 2012, 51, 4662.
11] T. Steiner, Angew. Chem. 2002, 114, 50; Angew. Chem. Int. Ed. 2002, 41,
8.
12] Although crystallization of II was attempted in various solvents such as
acetone, acetonitrile, nitromethane, dichloromethane, 1,2-dichloro-
ethane, DMSO, N,N-dimethylformamide, and dimethylacetamide, single
crystals could not be obtained.
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1
1
[
[
4
Keywords: cage compounds · catalysts · hydrogen bonds ·
nanostructures · polyoxometalates
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Received: January 30, 2014
Published online on April 2, 2014
Chem. Eur. J. 2014, 20, 5946 – 5952
www.chemeurj.org
5952
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim