Visible-light-induced photoredox catalysis with a tetracerium-containing silicotungstate

Article abstract of DOI:10.1002/anie.201403215

The development of visible-light-induced photocatalysts for chemoselective functional group transformations has received considerable attention. Polyoxometalates (POMs) are potential materials for efficient photocatalysts because their properties can be p

Full text of DOI:10.1002/anie.201403215

.
Angewandte
Communications
DOI: 10.1002/anie.201403215
Photoredox Catalysis
Visible-Light-Induced Photoredox Catalysis with a Tetracerium-
Containing Silicotungstate**
Kosuke Suzuki, Fei Tang, Yuji Kikukawa, Kazuya Yamaguchi, and Noritaka Mizuno*
Abstract: The development of visible-light-induced photo-
catalysts for chemoselective functional group transformations
has received considerable attention. Polyoxometalates (POMs)
are potential materials for efficient photocatalysts because their
properties can be precisely tuned by changing their constituent
elements and structures and by the introduction of additional
metal cations. Furthermore, they are thermally and oxidatively
more stable than the frequently utilized organometallic com-
plexes. The visible-light-responsive tetranuclear cerium(III)-
containing silicotungstate TBA [{Ce(H O)} {Ce(CH CN)} -
400 nm) because of the large energy gaps, and therefore, the
design of visible-light-responsive POM-based catalysts is
required.
Lacunary POMs can be utilized as multidentate inorganic
macroligands and accommodate various types of metal
cations in their vacant sites for the construction of well-
defined mono-, di-, tri-, and multinuclear metal–oxygen
[
4]
clusters. We have recently shown that the divacant lacunary
silicotungstate TBA H [g-SiW O ] can act as an efficient
4
4
10 36
ligand for the construction of multinuclear metal–oxygen
6
2
2
3
2
[
5]
(
m -O)(g-SiW O ) ] (CePOM; TBA = tetra-n-butylammo-
clusters with unique catalytic and magnetic properties.
Herein, we describe the synthesis of a tetranuclear
cerium(III)-containing sandwich-type silicotungstate,
TBA [{Ce(H O)} {Ce(CH CN)} (m -O)(g-SiW O ) ]
4
10 36 2
nium) has now been synthesized; when CePOM was irradiated
III
with visible light (l > 400 nm), a unique intramolecular Ce -
VI
to-POM(W ) charge transfer was observed. With CePOM, the
6
2
2
3
2
4
10 36 2
photocatalytic oxidative dehydrogenation of primary and
secondary amines as well as the a-cyanation of tertiary
(CePOM; TBA = tetra-n-butylammonium; Figure 1), by the
reaction of TBA H [g-SiW O ] with [Ce(acac) ] in acetone
4
4
10 36
3
amines smoothly proceeded in the presence of O (1 atm) as
2
the sole oxidant.
T
he development of photocatalytic systems that utilize
visible light for chemoselective transformations of organic
[
1]
molecules has received much attention. Although organo-
metallic complexes have frequently been utilized as effective
chromophores for visible light, there are several concerns, for
example, their durability and reusability. Polyoxometalates
(
POMs) are a large class of structurally well-defined anionic
molecular metal–oxygen clusters and thermally and oxida-
[
2]
tively more stable than organometallic complexes. Their
chemical and physical properties, such as redox potentials,
charge-transfer properties, and acidities, can be accurately
controlled by their constituent elements, structures, and
charges. By utilizing the intramolecular oxygen-to-metal
charge transfer of POMs (LMCT) in response to ultraviolet
light, they are suitable photocatalysts for various transforma-
Figure 1. a,b) Polyhedral and ball-and-stick representation of the
anionic component of CePOM (side view; a) and the tetranuclear Ce
core in CePOM (top view; b). Carbon black, cerium yellow, nitrogen
light blue, oxygen red, silicon blue, tungsten gray.
III
[3a,b]
tions, including functionalization reactions of CꢀH bonds,
[
3c]
[3d]
H evolution, and CO reduction. However, it is difficult
2
2
to utilize the LMCT by irradiation with visible light (l >
(
acac = acetylacetonate). CePOM displays unique properties:
[
*] Dr. K. Suzuki, F. Tang, Dr. Y. Kikukawa, Dr. K. Yamaguchi,
Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
III
VI
1) Intramolecular Ce -to-POM(W ) charge transfer occurs
in response to visible light (l > 400 nm), 2) CePOM possesses
coordination sites for substrates on the tetranuclear cerium
core, and 3) it displays a sufficient oxidation ability, which is
derived from the Ce /Ce redox potential. When irradiated
with visible light (l > 400 nm), CePOM showed remarkable
catalytic properties for oxidative transformations of primary,
secondary, and tertiary amines (see the Supporting Informa-
tion for comparisons). Although visible-light-induced metal-
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
IV
III
[
**] 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).
VI
to-POM(W ) charge transfers have very recently been
observed upon anchoring Ce to [PW O ] on the solid
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403215.
III
3ꢀ
1
2
40
5
356
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Chemie
[
6]
+
surface of mesoporous silica or by introducing [Re(CO)3]
absorption band in the visible region was not observed for
TBA H [g-SiW O ], indicating that the absorption band at
1
0ꢀ [7]
into [a -P W O ]
,
selective functional group transfor-
2
2
17 61
4
4
10 36
III
mations by using these systems have not been investigated. To
the best of our knowledge, CePOM is the first efficient POM-
based visible-light-responsive photocatalyst for liquid-phase
chemoselective functional group transformations in the
absence of any additional sensitizers.
460 nm is likely derived from an intramolecular Ce -to-
VI
[6,9]
POM(W ) charge transfer.
Upon irradiation of an aceto-
nitrile solution of CePOM with visible light (l > 400 nm)
under argon atmosphere in the presence of 4-methoxybenzyl-
amine as an electron donor, the color of the solution changed
from reddish-brown to purple (Figure 2a). The intensity of
CePOM was synthesized by the reaction of TBA H [g-
4
4
SiW O ] with two equivalents of [Ce(acac) ] in the presence
1
0
36
3
of one equivalent of p-toluenesulfonic acid (TsOH) in
acetone (92% yield based on TBA H [g-SiW O ]). Upon
4
4
10 36
addition of the pale-yellow [Ce(acac) ] powder to a colorless
3
solution of TBA H [g-SiW O ] in acetone, the solution color
4
4
10 36
dramatically changed to reddish-brown, indicating the for-
mation of a complex with a characteristic charge transfer in
response to visible light. The positive-ion cold-spray ioniza-
tion mass spectrum (CSI-MS) in acetonitrile showed two sets
+
of signals, which may be assigned to [TBA Ce O(SiW O ) ]
(
7
4
10 36 2
2
+
centered at m/z 7158) and [TBA Ce O(SiW O ) ] (m/z
8 4 10 36 2
3
700; Supporting Information, Figure S1), indicating that
III
8ꢀ
CePOM is composed of four Ce and two [SiW O ] units.
The Si NMR spectrum in CD CN showed a single signal at
1
0
36
2
9
3
ꢀ
120 ppm (Figure S2), showing that CePOM is a single
1
83
species and stable in solution. The
W NMR spectrum
showed three signals at 64.4, ꢀ90.3, and ꢀ96.8 ppm with an
intensity ratio of 2:1:2 (Figure S3). The prominently down-
field-shifted signal at 64.4 ppm is assignable to tungsten atoms
III
with neighboring paramagnetic Ce centers.
Single crystals of CePOM that were suitable for X-ray
crystallographic analysis were successfully obtained by recrys-
tallization from a mixture of acetonitrile and diethyl ether.
The bond valence sum (BVS) values of the silicon (3.90, 4.09),
tungsten (5.76–6.52), and cerium (2.99–3.16) atoms in
CePOM indicate corresponding valences of + 4, + 6, and
+
3, respectively (Table S1). Six TBA cations per anion of
CePOM could be assigned crystallographically, which is in
accord with the elemental analysis data. The anionic compo-
nent of CePOM consists of a tetranuclear cerium(III) core,
Figure 2. UV/Vis spectra of a) CePOM (0.5 mm) upon irradiation with
visible light (l>400 nm) for 0–4 hours in the presence of 4-methox-
ybenzylamine (2m) in acetonitrile under Ar (1 atm) and of b) photo-
reduced CePOM before (c) and after exposure to air for 20 s (b).
1
0+
[
{Ce(H O)} {Ce(CH CN)} (m -O)] , which is sandwiched
2
2
3
2
4
8
ꢀ
between two [g-SiW O ]
units (Figure 1; see also
1
0
36
Tables S2, S3); and each cerium atom was eight-coordinated.
The anion without outer ligands that are coordinated to the
cerium atoms had approximately inherent D symmetry,
2
d
1
83
V
VI
which is in good agreement with the W NMR spectrum
Figure S3). To the best of our knowledge, the present
CePOM is the first example of a cerium-containing POM
the band around 830 nm, which is assignable to the W -to-W
intervalence charge transfer, increased upon irradiation with
(
ꢀ
1
ꢀ1
visible light (e = 4200m cm
after 4 hours; Figure 2a),
[8]
with the g-Keggin framework.
indicating the reduction of CePOM (and the formation of
V
[10]
All of the above-mentioned data confirm that the formula
a W species; steps 1 and 2 in Scheme 1).
The UV/Vis
of CePOM is TBA [{Ce(H O)} {Ce(CH CN)} (m -O)(g-
spectrum of photo-reduced CePOM was almost identical to
that of a CePOM species that had been subjected to
6
2
2
3
2
4
SiW O ) ]. The formation of CePOM can be expressed by
1
0
36 2
[11]
Eq. (1).
electrochemical one-electron reduction (Figure S5). These
results support the hypothesis that the absorption band
III
VI
around 460 nm is derived from Ce -to-POM(W ) charge
2
TBA H ½SiW O ꢁ þ 4 ½CeðacacÞ ꢁ þ 2 TsOH þ 3 H O þ 2 CH CN
4
4
10 36
3
2
3
III
transfer, and one-electron transfer from Ce to the POM
framework was possibly induced by visible light.
!
TBA ½fCeðH OÞg fCeðCH CNÞg ðm -OÞðg-SiW O Þ ꢁ
ð1Þ
6
2
2
3
2
4
10 36 2
[
12]
The
þ2 TBAOTs þ 12 acacH
photo-reduced CePOM was readily reoxidized by exposure
to air for 20 seconds, and the UV/Vis spectrum of reoxidized
CePOM was almost identical to that of the original CePOM
sample (Figure 2b; step 3 in Scheme 1).
The UV/Vis spectrum of CePOM showed a characteristic
absorption band around 460 nm (Figure S4). In contrast, the
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[
a]
Table 1: Oxidation of benzylamine and control experiments.
Entry
Catalyst (mol%)
Yield [%]
1
2
3
CePOM (1)
96
2
35
<1
4
Scheme 1. CePOM-mediated visible-light-induced electron transfer
from an amine to O2.
TBA H [g-SiW O ] (2)
4
4
10 36
[Ce(acac) ] (4)
3
[
b]
c]
4
5
6
7
CePOM (1)
CePOM (1)
[
[Ce(acac) ] (4) and TBA H [g-SiW O ] (2)
56
37
The CSI-MS spectrum of an acetonitrile solution of
a mixture of CePOM and 4-methoxybenzylamine showed
3
4
4
10 36
[Ce(acac) ] (4) and TBA [a-SiW O ] (2)
3
4
12 40
two main signal sets, which may be assigned to
[a] Reaction conditions: catalyst, benzylamine (0.2 mmol), acetonitrile
(2 mL), 308C, hn (l>400 nm), O (1 atm), 24 h. Yields were determined
by GC using naphthalene as an internal standard. [b] Without photo-
irradiation. [c] Under Ar atmosphere (1 atm).
+
[
TBA Ce (CH OC H CH NH ) O(SiW O ) ] (centered at
2
7
4
3
6
4
2
2
4
10 36 2
m/z 7707)
and
[TBA Ce (CH OC H CH NH ) O
8 4 3 6 4 2 2 4
2
+
(
SiW O ) ] (m/z 3905), indicating the interaction of 4-
10 36 2
III
[13]
methoxybenzylamine with the Ce sites (Figure S6). The
interaction is likely responsible for the efficient electron
transfer from 4-methoxybenzylamine to CePOM and pre-
vents charge recombination (reverse reaction of step 1 in
Scheme 1).
[
a]
Table 2: CePOM-catalyzed oxidation of primary and secondary amines.
Entry
Substrate
t [h]
Product
Yield [%]
1
24
26
24
24
24
96
85
93
90
93
[
[
[
[
b]
c]
c]
c]
Next, we intended to apply the photocatalytic properties
of CePOM for aerobic oxidative transformations of amines in
the presence of the visible-light-responsive POM. Imines and
a-amino nitriles are important classes of compounds that
have been utilized as versatile synthetic intermediates for
nitrogen-containing pharmaceutical and biologically active
2
3
4
5
6
7
24
20
95
95
[
14]
compounds. The oxidative dehydrogenation of primary and
secondary amines as well as the a-cyanation of tertiary amines
provide convenient methods for the preparation of imines and
a-amino nitriles, respectively, and the one-electron oxidation
of an amine is an essential step of both reactions. Considering
that CePOM can readily accept one electron from an amine
8
9
48
94
54
24
45
90
94
76
(
step 2 in Scheme 1) and provide the electron to O (step 3),
10
2
the development of visible-light-induced photocatalytic trans-
formations of amines with CePOM using O2 as the sole
oxidant should be possible.
1
1
[
a] Reaction conditions: CePOM (1 mol%), amine (0.2 mmol), acetoni-
As expected, various kinds of structurally diverse primary
and secondary amines could be oxidized by irradiation with
visible light (l > 400 nm) in the presence of only catalytic
amounts of CePOM. For example, benzylamine was oxidized
to N-benzylidenebenzylamine in 96% yield (Table 1, entry 1),
and the catalytic performance of CePOM was superior to
those of TBA H [g-SiW O ] and [Ce(acac) ] (entries 2 and
trile (2 mL), 308C, hn (l>400 nm), O (1 atm). Yields were determined
2
by GC using naphthalene as an internal standard. [b] CePOM
(0.4 mol%). [c] These experiments used recovered CePOM: first reuse
(entry 3), second reuse (entry 4), and third reuse (entry 5).
The present system could also be applied to a heterocyclic
compound, 2-thiophenemethylamine, which afforded the
corresponding imine in 94% yield (entry 10). Furthermore,
the secondary amine dibenzylamine was converted into the
corresponding imine (entry 11). After the reaction, the
CePOM catalyst could easily be recovered by the addition
of diethyl ether (by precipitation). The IR and CSI-MS
spectra of the recovered catalyst showed that the structure of
CePOM was preserved after the oxidative dehydrogenation
of benzylamine (Figure S7 and S8). The recovered CePOM
could be reused at least three times without an appreciable
loss of its high catalytic activity, and the total turnover number
4
4
10 36
3
3
). The reaction did not proceed at all in the dark (entry 4)
and hardly proceeded under argon atmosphere (entry 5). The
catalytic performance of a mixture of fully occupied TBA [a-
4
SiW O ] and [Ce(acac) ] was almost the same as that of
1
2
40
3
III
[
Ce(acac) ], probably because the Ce centers could not
3
interact with the [a-SiW O ] ions in solution, and there-
4
ꢀ
1
2
40
fore, no charge transfer occurred in this case (entry 7).
Benzylamine and its derivatives with electron-donating
groups on the aryl rings were suitable substrates for this
transformation and gave the corresponding imines in high
yields (Table 2, entries 1, 2, 6, and 7). The reactions of para-
trifluoromethylbenzylamine and ortho-chlorobenzylamine,
which are benzylamines with electron-withdrawing groups,
also afforded the corresponding imines, but longer reaction
times were required to obtain high yields (entries 8 and 9).
[15]
for the repeated-reuse runs reached up to 372 (entries 3–5).
Inspired by the highly efficient CePOM-catalyzed aerobic
oxidative dehydrogenation reactions of primary and secon-
dary amines, the oxidative a-cyanation of tertiary amines was
5
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also studied. As summarized in Table 3, various kinds of
structurally diverse trialkylamines could react with trimethyl-
Chem. Rev. 1995, 143, 407 – 455; c) N. Mizuno, M. Misono,
Chem. Rev. 1998, 98, 199 – 218; d) R. Neumann, Prog. Inorg.
Chem. 1998, 47, 317 – 370; e) I. V. Kozhevnikov, Chem. Rev.
1998, 98, 171 – 198; f) C. L. Hill in Comprehensive Coordination
Chemistry II, Vol. 4 (Eds.: J. A. McCleverty, T. J. Meyer),
Elsevier Pergamon, Amsterdam, 2004, p. 679; g) D.-L. Long,
R. Tsunashima, L. Cronin, Angew. Chem. 2010, 122, 1780 – 1803;
Angew. Chem. Int. Ed. 2010, 49, 1736 – 1758.
[16]
silyl cyanide (TMSCN)
in the presence of CePOM
[15]
(
1 mol%), to afford the corresponding a-amino nitriles.
[
a]
Table 3: Oxidative a-cyanation of tertiary amines catalyzed by CePOM.
Entry
Substrate
t [h]
Product
Yield [%]
[
3] a) I. Ryu, A. Tani, T. Fukuyama, D. Ravelli, M. Fagnoni, A.
Albini, Angew. Chem. 2011, 123, 1909 – 1912; Angew. Chem. Int.
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Albini, Angew. Chem. 2007, 119, 2583 – 2586; Angew. Chem. Int.
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Z. Shi, S. Ouyang, J. Ye, J. Am. Chem. Soc. 2012, 134, 19716 –
1
26
90
19721; d) J. Ettedgui, Y. Diskin-Posner, L. Weiner, R. Neumann,
J. Am. Chem. Soc. 2011, 133, 188 – 190.
4] a) P. Putaj, F. Lefebvre, Coord. Chem. Rev. 2011, 255, 1642 –
2
3
14
24
54
70
[
1685; b) B. S. Bassil, M. Ibrahim, R. Al-Oweini, M. Asano, Z.
Wang, J. van Tol, N. S. Dalal, K.-Y. Choi, R. N. Biboum, B. Keita,
L. Nadjo, U. Kortz, Angew. Chem. 2011, 123, 6083 – 6087; Angew.
Chem. Int. Ed. 2011, 50, 5961 – 5964; c) Y. Hou, L. Xu, M. J.
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4
5
14
40
82
92
49, 4125 – 4132; d) P. Mialane, A. Dolbecq, E. Riviꢀre, J. Marrot,
F. Sꢁcheresse, Angew. Chem. 2004, 116, 2324 – 2327; Angew.
Chem. Int. Ed. 2004, 43, 2274 – 2277.
[
5] a) K. Suzuki, R. Sato, N. Mizuno, Chem. Sci. 2013, 4, 596 – 600;
b) R. Sato, K. Suzuki, M. Sugawa, N. Mizuno, Chem. Eur. J. 2013,
19, 12982 – 12990; c) K. Suzuki, Y. Kikukawa, S. Uchida, H.
Tokoro, K. Imoto, S. Ohkoshi, N. Mizuno, Angew. Chem. 2012,
124, 1629 – 1633; Angew. Chem. Int. Ed. 2012, 51, 1597 – 1601;
d) Y. Kikukawa, K. Suzuki, K. Yamaguchi, N. Mizuno, Inorg.
Chem. 2013, 52, 8644 – 8652; e) K. Suzuki, M. Sugawa, Y.
Kikukawa, K. Kamata, K. Yamaguchi, N. Mizuno, Inorg.
Chem. 2012, 51, 6953 – 6961; f) Y. Kikukawa, K. Suzuki, M.
Sugawa, T. Hirano, K. Kamata, K. Yamaguchi, N. Mizuno,
Angew. Chem. 2012, 124, 3746 – 3750; Angew. Chem. Int. Ed.
2012, 51, 3686 – 3690; g) Y. Kikukawa, K. Yamaguchi, N. Mizuno,
Angew. Chem. 2010, 122, 6232 – 6236; Angew. Chem. Int. Ed.
2010, 49, 6096 – 6100.
[a] Reaction conditions: CePOM (1 mol%), amine (0.2 mmol), TMSCN
(
0.4 mmol), acetonitrile (2 mL), 308C, hn (l>400 nm), O (1 atm).
2
Yields were determined by GC using naphthalene as an internal
standard.
Monocyanated products were selectively obtained in all cases
without formation of the di- and tricyanated products.
Cyanation of various N-methylalkylamines proceeded regio-
selectively at the a-methyl positions to give the corresponding
a-amino nitriles in moderate to high yields (Table 3,
entries 1–4). Even a sterically hindered trialkylamine was
efficiently converted into the corresponding a-amino nitrile
in high yield (entry 5).
[
6] The photocatalytic degradation of gaseous isopropanol has been
investigated; see: a) T. Takashima, A. Yamaguchi, K. Hashi-
moto, R. Nakamura, Chem. Commun. 2012, 48, 2964 – 2966;
b) T. Takashima, R. Nakamura, K. Hashimoto, J. Phys. Chem. C
In summary, we have synthesized the first visible-light-
responsive tetranuclear cerium(III)-containing silicotung-
state, CePOM. CePOM showed unique intramolecular Ce -
2009, 113, 17247 – 17253.
16ꢀ
3 2
[
[
7] Photocatalysis with [P W O {Re(CO) } ]
has not been
4
35 124
III
investigated; see: C. Zhao, Z. Huang, W. Rodrꢂguez-Cꢃrdoba,
C. S. Kambara, K. P. OꢄHalloran, K. I. Hardcastle, D. G. Musaev,
T. Lian, C. L. Hill, J. Am. Chem. Soc. 2011, 133, 20134 – 20137.
8] a) B. S. Bassil, M. H. Dickman, I. Rçmer, B. von der Kammer, U.
Kortz, Angew. Chem. 2007, 119, 6305 – 6308; Angew. Chem. Int.
Ed. 2007, 46, 6192 – 6195; b) K. Wassermann, M. H. Dickman,
M. T. Pope, Angew. Chem. 1997, 109, 1513 – 1516; Angew. Chem.
Int. Ed. Engl. 1997, 36, 1445 – 1448; c) M. Sadakane, M. H.
Dickman, M. T. Pope, Inorg. Chem. 2001, 40, 2715 – 2719; d) T.
Li, F. Li, J. Lꢅ, Z. Guo, S. Gao, R. Cao, Inorg. Chem. 2008, 47,
VI
to-POM(W ) charge transfer, which was induced by irradi-
ation with visible light. In the presence of CePOM, aerobic
photocatalytic oxidative transformations of various structur-
ally diverse primary, secondary, and tertiary amines smoothly
proceeded under irradiation with visible light with O (1 atm)
as the sole oxidant.
2
Received: March 12, 2014
Published online: April 16, 2014
5612 – 5615; e) F. L. Sousa, F. A. A. Paz, A. M. V. Cavaleiro, J.
Klinowski, H. I. S. Nogueira, Chem. Commun. 2004, 2656 – 2657.
Keywords: amines · cyanation · dehydrogenation ·
photoredox catalysis · polyoxometalates
[9] DFT calculations showed that the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
.
III
VI
(
LUMO) of CePOM were Ce - and POM(W )-based orbitals,
III
respectively (Figure S9). Furthermore, the Ce -based HOMO
was observed to lie between the POM(W )-based LUMO and
VI
[
1] a) C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Chem. Rev.
2
013, 113, 5322 – 5363; b) J. M. R. Narayanam, C. R. J. Stephen-
oxygen-based orbitals (HOMO-1). These results support the
hypothesis that the absorption band around 460 nm is due to
Ce -to-POM(W ) charge transfer.
son, Chem. Soc. Rev. 2011, 40, 102 – 113.
III
VI
[
2] a) M. T. Pope, Heteropoly and Isopoly Oxometalates, Springer,
Berlin, 1983; b) C. L. Hill, C. M. Prosser-McCartha, Coord.
[10] E. Papaconstantinou, Chem. Soc. Rev. 1989, 18, 1 – 31.
Angew. Chem. Int. Ed. 2014, 53, 5356 –5360
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5359

.
Angewandte
Communications
+
[
11] The cyclic voltammogram of CePOM in acetonitrile showed
assignable to [TBA SiW O ] (m/z 4087), indicating that 4-
5 12 40
one-electron reduction peaks at ꢀ0.61, ꢀ0.86, ꢀ1.18, and
methoxybenzylamine is coordinated to the tetranuclear cerium
8
ꢀ
ꢀ
1.35 V (vs. the normal hydrogen electrode (NHE)), which
core in CePOM rather than to the [g-SiW O ] frameworks.
10 36
VI
V
are assignable to successive W /W redox waves (Figure S10).
When CePOM was subjected to electrochemical one-electron
reduction at ꢀ0.61 V, the UV/Vis spectrum showed the broad
band around 830 nm and was almost identical with that of photo-
reduced CePOM (Figure S5).
[14] a) S.-I. Murahashi, Angew. Chem. 1995, 107, 2670 – 2693; Angew.
Chem. Int. Ed. Engl. 1995, 34, 2443 – 2465; b) D. Enders, J. P.
Shilvock, Chem. Soc. Rev. 2000, 29, 359 – 373; c) S.-I. Murahashi,
D. Zhang, Chem. Soc. Rev. 2008, 37, 1490 – 1501.
[15] The catalytic performance of the present system was compared
with those of the previously reported systems (Tables S4 and S5).
[16] We have recently reported that POMs can effectively activate
[
[
12] In sharp contrast, upon irradiation of a mixture of 4-methoxy-
benzylamine and TBA [SiW O ] in acetonitrile with visible
4
12 40
ꢀ
[5e,f]
light, the color of the solution did not change.
TMSCN to generate a nucleophile (CN ).
13] The CSI-MS spectrum of a mixture of TBA [SiW O ] and 4-
4
12 40
methoxybenzylamine in acetonitrile showed only a signal that is
5
360
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