Photoredox catalysis for oxygenation/deoxygenation between sulfides and sulfoxides by visible-light-responsive polyoxometalates

Article abstract of DOI:10.1039/c5nj01045d

In this paper, we report the unique visible-light-responsive photoredox catalysis of a divacant lacunary silicotungstate TBA₄H₄[γ-SiW₁₀O₃₆] (SiW10) for functional group transformations of sulfur-containing compounds; namely, (i) aerobic oxygenation of sulfides to sulfoxides and (ii) deoxygenation of sulfoxides to sulfides. In the presence of suitable additives, such as Ce³⁺ (electron transfer mediator for oxygenation) and an alcohol (electron and proton donor for deoxygenation), SiW10 shows visible-light-induced charge transfers by using the newly formed highest occupied molecular orbitals derived from the coordinating Ce³⁺ or alcohol at the vacant site of SiW10. Consequently, oxygenation of sulfides and deoxygenation of sulfoxides can selectively proceed by irradiation with visible light (λ > 400 nm) to afford the corresponding desired products in high yields. The SiW10 photocatalysts can readily be recovered and reused for these transformations. Based on evidence from several experiments, the roles of the additives as well as the reaction mechanisms for these transformations are also discussed.

Full text of DOI:10.1039/c5nj01045d

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New Journal of Chemistry
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DOI: 10.1039/C5NJ01045D
Submitted to New Journal of Chemistry as a Paper
(Themed issue on Emergent Polyoxometalates and Soft-oxometalates)
Photoredox catalysis for oxygenation/deoxygenation between sulfides and
sulfoxides by visible-light-responsive polyoxometalates
Kosuke Suzuki, Jinu Jeong, Kazuya Yamaguchi and Noritaka Mizuno*
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
Email: tmizuno@mail.ecc.u-tokyo.ac.jp
Abstract
In this paper, we report the unique visible-light-responsive photoredox catalysis of a divacant lacunary
silicotungstate TBA4H4[γ-SiW10O36] (SiW10) for functional group transformations of sulfur-containing
compounds; namely, (i) aerobic oxygenation of sulfides to sulfoxides and (ii) deoxygenation of
sulfoxides to sulfides. In the presence of suitable additives, such as Ce3+ (electron transfer mediator for
oxygenation) and an alcohol (electron and proton donor for deoxygenation), SiW10 shows visible-
light-induced charge transfers by using the newly formed highest occupied molecular orbitals derived
from the coordinating Ce3+ or alcohol at the vacant site of SiW10, and consequently oxygenation of
sulfides and deoxygenation of sulfoxides can selectively be proceeded by irradiation of visible light (λ
> 400 nm) to afford the corresponding desired products in high yields. The SiW10 photocatalysts can
readily be recovered and reused for these transformations. Based on the several experimental evidences,
the roles of the additives as well as the reaction mechanisms for these transformations are also
discussed.
1

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DOI: 10.1039/C5NJ01045D
Introduction
Polyoxometalates (POMs) are a class of anionic molecular metal-oxide clusters with a wide range of
structural versatilities.1 Because their redox properties, photoactivity, and acid/base properties can be
controlled by their structures, constituent elements, and additional metal cations, they are emergent and
attractive materials in various fields of science, such as catalysis, medicine, and multifunctional
materials.1,2 In addition, they possess high durability against both oxidation and reduction reactions
compared with commonly utilized organic and organometallic catalysts. In particular, by utilizing the
photo-induced O-to-W charge transfers of the tungstate-based POM frameworks, they show
outstanding photocatalysis for various functional group transformations.3 However, ultra violet light
should be required for the O-to-W charge transfers because of their wide energy gaps,3 and therefore,
the design of efficient visible-light-responsive POM-based catalysts is an important subject.
We have engaged in design of POM-based molecular catalysts for a long time. Specifically, we
focused on the unique properties of lacunary POMs because their vacant sites can be utilized as the
inorganic multidentate ligands for construction of not only multimetal clusters but also the
coordination sites for substrates and/or oxidants for several functional group transformations.4
Moreover, based on the coordination properties at the vacant site of a divacant lacunary silicotungstate
TBA4H4[γ-SiW10O36] (SiW10, Fig. 1a),4h we have found the unique visible-light-induced charge
transfers and the photoredox catalysis, for example, oxidation of amines and one-pot synthesis of N-
alkylated imines by using the newly formed highest occupied molecular orbitals derived from the
coordinating metals or substrates at the vacant site of SiW10.5
Oxygenation of sulfur-containing compounds is a noticeable kind of transformations utilized in
various fields, such as organic synthesis and pharmaceuticals.6–8 Catalytic oxygenation of sulfides to
2

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sulfoxides has often been performed by using hydrogen peroxide and tert-butylhydroperoxide as the
DOI: 10.1039/C5NJ01045D
oxidants.7 Molecular oxygen (O2) is the most desirable oxidant, and several efficient photocatalytic
systems for oxygenation of sulfides using O2 as the oxidant have recently been reported.8
Deoxygenation of sulfoxides to sulfides is also one of the important transformations and applicable to
valuable transformations, such as chiral alcohol synthesis and carbon atom homologation.9,10 Up to the
present, several efficient deoxygenation systems using various reductants have been reported.10
Fig. 1 The structures of the anion parts of (a) SiW10 and (b) CePOM. Green octahedra and light blue
tetrahedra represent {WO6} and {SiO4} units, respectively. Blue and red spheres represent cerium and
oxygen atoms, respectively.
Toward the development of the selective and efficient photocatalytic systems, molecular
design for catalytically active sites, redox potentials, and charge transfer sites should be required. From
the above-mentioned desirable properties of the lacunary POMs, we envisioned that efficient
photoredox catalysts for oxygenation and deoxygenation of sulfur-containing compounds would be
developed. Herein, we report for the first time the photo-induced functional group transformations in
3

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the presence of visible-light-responsive SiW10-based POM catalysts; namely, (i) aerobic oxygenation
DOI: 10.1039/C5NJ01045D
of sulfides to sulfoxides and (ii) deoxygenation of sulfoxides to sulfides. In the presence of SiW10
together with suitable additives such as Ce3+ (electron transfer mediator for oxygenation) and an
alcohol (electron and proton donor for deoxygenation), oxygenation of sulfides [eqn (1)] and
deoxygenation of sulfoxides [eqn (2)] could selectively be proceeded by irradiation of visible light (λ >
400 nm) to afford the corresponding desired products in high yields. In this paper, the roles of the
additives as well as the reaction mechanisms for these transformations are also discussed based on the
several experimental evidences.
O
SiW10, Ce3+
S
S
+ 1/2 O2
(1)
R1
R2
R1
R2
hν (λ > 400 nm), 30°C
O
S
SiW10
S
+
+
+ H2O (2)
R1
R2
R
OH
R
R1
R2
O
hν (λ > 400 nm), 30°C
Results and discussion
Photocatalytic aerobic oxygenation of sulfides to sulfoxides
Firstly, the sulfoxidation of thioanisole (1a) was investigated in the presence of SiW10 and
various metal acetylacetonates (M(acac)n) as the additives in 1 atm of O2 with irradiation of visible
light (λ > 400 nm). The effect of metal additives is summarized in Table 1. Under the present
conditions, SiW10 gave methyl phenyl sulfoxide (2a) in only 9% yield (Table 1, entry 14). Notably,
Ce(acac)3 dramatically accelerated the sulfoxidation, and 2a was obtained in 94% yield (Table 1, entry
1). In contrast, other metal additives, such as Co(acac)2, Zn(acac)2, Mn(acac)3, Ni(acac)2, Fe(acac)3,
Cu(acac)2, Ag(acac), and Y(acac)3, were much less effective for the sulfoxidation (2–32% yields of 2a;
4

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Table 1, entries 6–13). The sulfoxidation with Ce(acac)3 in the absence of SiW10 gaveD2OaI: 1i0n.10o39n/lCy5N6J%01045D
yield (Table 1, entry 15). A mixture of a fully-occupied silicotungstate TBA4[SiW12O40] and Ce(acac)3
was also less effective (28% yield of 2a; Table 1, entry 16). Thus, both “lacunary” SiW10 and Ce3+ are
the indispensable components to attain the high yields of 2a. The sulfoxidation without photo-
irradiation or in 1 atm of Ar hardly proceeded (Table 1, entries 17 and 18).
Table 1 Oxygenation of 1a by irradiation of visible light using various catalystsa
O
O
O
catalyst
S
S
S
+
hν (λ > 400 nm), O2
1a
2a
2a'
30°C
Yield (%)
Entry
Catalyst
2a
94
94
2a'
6
1
2b
3
SiW10 + Ce(acac)3
SiW10 + Ce(acac)3
SiW10 + Ce(OAc)3
SiW10 + Ce(NO3)3
SiW10 + CeCl3
5
90
16
8
7
<1
<1
<1
2
4
5
6
SiW10 + Co(acac)2
SiW10 + Zn(acac)2
SiW10 + Mn(acac)3
SiW10 + Ni(acac)2
SiW10 + Fe(acac)3
SiW10 + Cu(acac)2
SiW10 + Ag(acac)
SiW10 + Y(acac)3
SiW10
3
7
15
4
8
<1
<1
2
9
2
10
11
12
13
14
15
16
17c
18d
19
19
13
7
1
<1
3
32
9
1
Ce(acac)3
6
<1
<1
<1
<1
5
TBA4[SiW12O40] + Ce(acac)3
SiW10 + Ce(acac)3
SiW10 + Ce(acac)3
CePOM
<1
<1
4
93
a Reaction conditions: catalyst (SiW10 2 mol%, metal salt 4 mol%, CePOM
1 mol%) 1a (0.2 mmol), acetonitrile/methanol (1 mL, 2/1 v/v), 30°C, hv (λ >
400 nm), O2 (1 atm), 28 h. Yields were determined by GC using naphthalene as
c
an internal standard. b This experiment used retrieved catalyst. Without photo-
irradiation. d Under Ar (1 atm).
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We have recently reported the synthesis and unique visible-light-responsiDvOeI: 1p0.h10o3Vt9ioe/wCre5ANdrtiJco0lex1O0n4l5inDe
property of a tetra-cerium-containing silicotungstate TBA6[{Ce(H2O)}2{Ce(CH3CN)}2(µ4-O)(γ-
SiW10O36)2] (CePOM, Fig. 1b).5a The DFT calculation showed that the energy gap between the Ce3+-
based highest occupied molecular orbital (HOMO) and the POM(W6+)-based lowest unoccupied
molecular orbital (LUMO) was 2.4 eV, and thereby CePOM could possess the visible-light-responsive
property (Fig. S1a, ESI†). By irradiation of visible light (λ > 400 nm), the intramolecular Ce3+-to-
POM(W6+) charge transfer was efficiently induced in CePOM, which could be utilized for the
photocatalytic oxidative dehydrogenation of primary and secondary amines and oxidative α-cyanation
of tertiary amines.5a The cold-spray ionization (CSI) mass spectrum of a mixture of SiW10 and two
equivalents of Ce(acac)3 in a mixed solvent of acetonitrile and methanol (i.e., the similar solution to
that of the photocatalytic sulfoxidation demonstrated in Table 1) showed the signal sets assignable to
[TBA8Ce4O(SiW10O36)2]2+ (m/z 3700) and [TBA7Ce4O(SiW10O36)2]+ (m/z 7158) (Fig. S2a, ESI†),
indicating the in-situ formation of CePOM by the reaction of Ce3+ and SiW10 under the conditions
described in Table 1. Actually, the sulfoxidation of 1a by CePOM also efficiently proceeded to give 2a
in 93% yield (Table 1, entry 19). The simple mixture of Ce(acac)3 and SiW10 showed almost the same
catalytic performance as that of CePOM and therefore could be utilized as the “in-situ-prepared
catalyst” (Table 1, entry 1 vs entry 19). From a practical point of view, the use of such in-situ-prepared
catalysts for functional group transformations would be very advantageous because the special
synthesis and isolation of the catalysts are not required. For this reason, we have recently developed
the several in-situ-prepared metal-containing POM catalysts in organic media for functional group
transformations of various kinds of organic substrates.11 In addition, several aqueous in-situ-prepared
POM catalysts have been reported to date.12
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The effect of the counter anions of Ce3+ was very significant. Ce(OAc)3 (OAcD=OI:a1c0e.10ta39te/C)5NwJa01s045D
equally effective as Ce(acac)3, and the sulfoxidation of 1a with Ce(OAc)3 in the presence of SiW10
also gave 2a in 90% yield (Table 1, entry 3). The color of the solution containing Ce(OAc)3 and
SiW10 was pale orange and the same as that containing Ce(acac)3 and SiW10. Thus, the visible-light-
responsive CePOM was also possibly formed when using Ce(OAc)3. In contrast, Ce(NO3)3 and CeCl3
were much less effective for the sulfoxidation (Table 1, entries 4 and 5). When using Ce(NO3)3 or
CeCl3, white solid precipitated out from the reaction solutions at the beginning of the sulfoxidation.
Recently, we have reported that the transition metal salts of the divacant lacunary silicotungstate can
be synthesized by the reaction of SiW10 with various metal nitrates.4d Similarly, the cation exchange
of TBA with Ce3+ (rather than the formation of CePOM) would take place to form the white Ce3+ salt
of POM. Most importantly, the simple cations exchange is not effective, and the interaction of Ce3+
and SiW10 at the lacunary pocket is indispensable for the present visible-light-responsive catalysis.
Therefore, the use of Ce(NO3)3 and CeCl3 should be avoided in order to generate CePOM in-situ.
The scope of the present Ce3+–SiW10-catalyzed photocatalytic sulfoxidation system was next
investigated (Table 2). Thioanisole (1a) and its derivatives with electron-withdrawing as well as
electron-donating substituents on the phenyl rings (1b–1d) could efficiently be converted into the
corresponding sulfoxides (2a–2d) in high yields (Table 2, entries 1–4). Notably, the sulfoxidation of
nitro-substituted thioanisole (1d) efficiently proceeded to afford the corresponding sulfoxide (2d) in
86% yield although some of the reported photocatalysts showed low catalytic activities for the
sulfoxidation of 1d.8 Ethyl phenyl sulfide (1e) was also converted into ethyl phenyl sulfoxide (2e) in
91% yield (Table 2, entry 5). It should be noted that not only aryl sulfides but also less reactive alkyl
ones (1f and 1g) could efficiently be oxygenated (Table 2, entries 6 and 7).
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By addition of an excess amount of diethyl ether (ca. 20 mL) to the reaction solution, the
DOI: 10.1039/C5NJ01045D
catalyst (originally Ce(acac)3 + SiW10) spontaneously precipitated out and could easily be retrieved by
simple filtration (≥95% recovery based on CePOM). The IR spectrum of the retrieved catalyst
indicated that the [γ-SiW10O36]8– framework was preserved after the photocatalytic sulfoxidation (Fig.
S3, ESI†). The CSI-mass spectrum of the retrieved catalyst in acetonitrile showed the signal sets
assignable to [TBA8Ce4O(SiW10O36)2]2+ (m/z 3700) and [TBA7Ce4O(SiW10O36)2]+ (m/z 7158),
indicating that a part of Ce3+ was introduced to the vacant site of SiW10 to form CePOM (Fig. S2b,
ESI†). Moreover, the retrieved catalyst could be reused for the sulfoxidation of 1a without an
appreciable loss of its high catalytic performance (Table 1, entry 2).
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Table 2 Scope of the Ce3+–SiW10-catalyzed oxygenation of sulfides by irradiation of visible lighta
View Article Online
DOI: 10.1039/C5NJ01045D
O
O
O
SiW10, Ce(acac)3
S
S
+
S
R1
R2
R1
R2
R1
R2
hν (λ > 400 nm), O2
1
2
2'
30°C
Yield (%)
Entry
Substrate
Time (h)
28
Product
2
2'
O
S
S
S
1
2
94
6
1a
1b
2a
2b
O
S
24
92
92
8
7
MeO
Br
MeO
Br
O
S
S
S
S
3
24
1c
1d
1e
1f
2c
2d
2e
O
S
4
5
24
24
86
91
12
8
O2N
O2N
O
S
S
6
7
28
32
S
93
90
7
9
O
2f
O
S
S
1g
2g
a
Reaction conditions: 1 (0.2 mmol), SiW10 (2 mol%), Ce(acac)3 (4 mol%),
acetonitrile/methanol (1 mL, 2/1 v/v), 30°C, hν (λ > 400 nm), O2 (1 atm). Yields
were determined by GC using naphthalene as an internal standard. Under the
conditions, sulfides (1) were quantitatively converted into the corresponding
oxygenated products (2 and 2').
A possible reaction mechanism for the present photocatalytic oxygenation of sulfides by Ce3+–
SiW10 (equally CePOM) is shown in Fig. 2. As above-mentioned, we have reported that CePOM
shows the intramolecular Ce3+-to-POM(W6+) charge transfer in response to visible light.5a By
irradiation of visible light to CePOM, the intramolecular Ce3+-to-POM(W6+) charge transfer
efficiently occurs, followed by one electron transfer from a sulfide to the resulting Ce4+ to afford a
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sulfide cation radical species. Then, the reduced POM(W5+) is reoxidized to POM(W6+) by O2V,iewaAnrtdicleaOnline
DOI: 10.1039/C5NJ01045D
superoxide anion radical species is likely formed.5a By the reaction of the sulfide cation radical and the
superoxide anion radical species, the corresponding sulfoxides are successively obtained as the final
products.13
Fig. 2 Visible-light-responsive photocatalytic oxygenation of sulfides by CePOM.
Photocatalytic deoxygenation of sulfoxides to sulfides
Besides the sulfoxidation, we found that deoxygenation of sulfoxides to sulfides, that is, an inverse
reaction to sulfoxidation, efficiently proceeded with SiW10 in 1 atm of Ar using a suitable electron and
proton donor (reductant), 4-mehthoxybenzyl alcohol (3). Unlike the above-mentioned sulfoxidation,
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the SiW10-catalyzed deoxygenation did not require any metal additives; the deoxygenation of V2ieaw AwrtiictlehOnline
DOI: 10.1039/C5NJ01045D
SiW10 gave 1a in 99% yield, while the yield of 2a was 38% when using SiW10 and Ce(acac)3 (Table
3, entry 1 vs entry 4).
Table 3 Scope of the SiW10-catalyzed deoxygenation of sulfoxides by irradiation of visible lighta
O
SiW10, 3
S
S
R1
R2
R1
R2
hν (λ > 400 nm), Ar
30°C
2
1
Entry
Substrate
Time (h)
Product
Yield (%)
O
S
1
8
8
8
8
8
8
8
8
99
95
99
38
<1
<1
<1
2
S
2b
3b
4c
5d
6e
7f
2a
1a
8g
O
S
S
9
8
99
2h
1h
1i
O
S
S
S
8
3
99
98
2i
10
11
O
S
2j
1j
Cl
Cl
Cl
Cl
O
S
S
S
6
98
94
12
13
2k
1k
O
S
10
2l
1l
a Reaction conditions: SiW10 (1 mol%), 2 (0.2 mmol), acetonitrile/toluene (2 mL,
1/9 v/v), 4-methoxybenzyl alcohol (3, 1 mmol), 30 °C, hν (λ > 400 nm), Ar (1 atm).
b These experiments used retrieved SiW10; the first reuse (entry 2), the second
reuse (entry 3). c Without SiW10 (1 mol%) and Ce(acac)3 (2 mol%). d Without
SiW10. e Without 3. f Without photo-irradiation. g With TBA4H4[SiW12O40].
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The cyclic voltammogram showed that the reduction current of 2a was observed belowVi–ew1A.8rticlVe Online
DOI: 10.1039/C5NJ01045D
vs NHE in acetonitrile (light blue line in Fig. 3). Thus, the reduction of 2a is essentially difficult due to
its high reduction potential.14 Upon addition of SiW10 (1 mol% with respect to 2a) to the acetonitrile
solution of 2a, the reduction current due to 2a was significantly shifted to the more positive potential at
ca. –1.5 V (red line in Fig. 3), indicating that 2a was efficiently reduced by the presence of SiW10.
Indeed, upon addition of 2a (100 equivalents with respect to SiW10) to an acetonitrile solution of the
electrochemically two-electron-reduced SiW10, the dark blue solution (due to the reduced SiW10)
changed to colorless within 10 min, indicating the electron transfer from the reduced SiW10 to 2a
possibly took place to form 1a. In contrast, even by addition of 2a to the two-electron reduced “fully
occupied” TBA4[SiW12O40] in acetonitrile (100 equivalents of 2a with respect to TBA4[SiW12O40]), the
dark blue color of the solution did not change (even after 4 h), and the formation of 1a was hardly
observed. Needless to say, the deoxygenation of 2a hardly proceeded with TBA4[SiW12O40] under the
conditions described in Table 3. The CSI-mass spectrum of the mixture of SiW10 and 2a in
acetonitrile showed the signal sets at m/z 1932, 2003, 2072, 3763, and 3903 assignable to
[TBA6(SiW10O34)]2+, [TBA6(SiW10O34)(2a)]2+, [TBA6(SiW10O34)(2a)2]2+, [TBA5(SiW10O34)(2a)]+, and
[TBA5(SiW10O34)(2a)2]+, respectively (Fig. 4a). In contrast, the CSI-mass spectrum of the mixture of
TBA4[SiW12O40] and 2a showed only the signal sets assignable to [TBA6(SiW12O40)]2+ (m/z 2165) and
[TBA5(SiW12O40)]+ (m/z 4086) (Fig. 4b). These results indicate that the vacant site of SiW10 can act as
the coordination site for 2a, which is likely responsible for the efficient reduction of 2a by the electron
transfer from the reduced SiW10.
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DOI: 10.1039/C5NJ01045D
Fig. 3 Cyclic voltammogram of 2a (light blue line), SiW10 (black line), and the mixture of 2a and
SiW10 (red line) in acetonitrile (SiW10 1 mM, 2a 100 mM, TBAClO4 100 mM).
Fig. 4 CSI-mass spectra of (a) the mixture of SiW10 and 2a in acetonitrile (100 equivalents with
respect to SiW10) and (b) the mixture of TBA4[SiW12O40] and 2a in acetonitrile (100 equivalents with
respect to TBA4[SiW12O40]). The signal sets at m/z 1932, 2003, 2072, 3763, 3903, 2165, and 4086
were assignable to [TBA6(SiW10O34)]2+, [TBA6(SiW10O34)(2a)]2+, [TBA6(SiW10O34)(2a)2]2+,
[TBA5(SiW10O34)(2a)]+, [TBA5(SiW10O34)(2a)2]+, [TBA6(SiW12O40)]2+, and [TBA5(SiW12O40)]+,
respectively.
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We have recently reported the visible-light-responsive multielectron redox catalysis ofViSewiWArtic1le0Online
DOI: 10.1039/C5NJ01045D
by the in-situ coordination of 3 to the vacant site of SiW10.5b By the coordination of 3 to the vacant
site, the new HOMO was generated as the electron donor level due to the molecular orbital of 3, and
the energy gap between the newly generated HOMO and the POM(W6+)-based LUMO became much
narrower (2.82 eV) than that between the POM(O2–)-based HOMO and the POM(W6+)-based LUMO
in SiW10 (4.58 eV) (Fig. S1b, ESI†). This would enable the visible-light-responsive charge transfer
from 3 to SiW10. As the result, up to two electrons and two protons could efficiently be transferred
from 3 and stored in SiW10 by irradiation of visible light. Therefore, the vacant site of SiW10 could
play a dual role on the present deoxygenation, that is, not only the electron transfer site from 3 to
SiW10 but also the electron transfer site from the reduced SiW10 to 2a. Based on these results, we
envisioned that the present visible-light-responsive photoredox system with SiW10 is desirable for
photocatalytic deoxygenation of various kinds of sulfoxides, and we next investigated the
photocatalytic deoxygenation of sulfoxides by SiW10 in 1 atm of Ar in more detail.
As we expected, the SiW10-catalyzed deoxygenation of 2a quantitatively proceeded to afford
1a by irradiation of visible light (λ > 400 nm) in the presence of 3 as the electron and proton donor
(Table 3, entry 1). By irradiation of visible light, the color of the reaction solution promptly changed
from pale yellow to dark blue, indicating the efficient electron transfer from 3 to SiW10 to form the
photo-reduced SiW10. 4-Methoxybezaldehyde (0.2 mmol, an equimolar amount with respect to 1a
produced) was also produced as the two-electron oxidation product of 3, and no hydrogen (H2) was
detected in the gas-phase confirmed by the GC-TCD (thermal conductivity detector) analysis. These
results support that the electrons stored in SiW10 can mostly be used for the deoxygenation of 2a. In
contrast, the reactions did not proceed without SiW10 (Table 3, entry 5), 3 (Table 3, entry 6), or photo-
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irradiation (Table 3, entry 7). As above-mentioned, when a fully-occupied TBA4[SiW12O40] was used,
DOI: 10.1039/C5NJ01045D
the color of the reaction solution did not change, and the reaction hardly proceeded (Table 3, entry 8).
Based on the above-mentioned experimental evidences, a possible reaction mechanism for the
present SiW10-catalyzed photocatalytic deoxygenation of sulfoxides to sulfides is proposed (Fig. 5).
By irradiation of visible light, the efficient electron transfer from the coordinated alcohol (3) at the
vacant site to SiW10 occurs, and up to two electrons and two protons are stored on SiW10. Then, the
sulfoxide coordinated at the vacant site is possibly deoxygenated to the corresponding sulfide by using
the electrons and protons.
Fig. 5 Visible-light-responsive photocatalytic deoxygenation of sulfoxides by SiW10.
Finally, we turned our attention to the examination of the scope of the present SiW10-
catalyzed deoxygenation system. Various kinds of aryl sulfoxides (2h–2k) could quantitatively be
converted into the corresponding sulfides (Table 3, entries 9–12). Notably, this catalytic system was
15

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applicable to sulfoxides with other reducible substituents, such as C=C double bond (2i) and VCiewl A(r2ticjle),Online
DOI: 10.1039/C5NJ01045D
and the deoxygenation of sulfoxides selectively proceeded to give the corresponding sulfides (1i and
1j) without reduction of the C=C and Cl moieties (Table 3, entries 10 and 11). Alkyl sulfoxide (2l) was
also converted into the corresponding sulfide (1l) though a longer reaction time was required (Table 3,
entry 13).15 After the reaction, the catalyst could easily be retrieved as precipitates by addition of an
excess amount of diethyl ether (ca. 15 mL) to the reaction solution (>95% recovery based on SiW10).
The CSI-mass and IR spectra showed that the structure of the retrieved catalyst was preserved (Figs.
S4 and S5, ESI†). In addition, the retrieved catalyst could be reused at least twice without an
appreciable loss of its high catalytic performance (Table 3, entries 2 and 3).
Conclusions
We have successfully developed for the first time the visible-light-responsive photoredox catalysis of
the lacunary polyoxometalate (TBA4H4[γ-SiW10O36]) for oxygenation/deoxygenation reactions
between sulfides and sulfoxides. In the presence of the simple mixture of TBA4H4[γ-SiW10O36] and
Ce3+
(in-situ-prepared
catalyst)
or
the
cerium-containing
polyoxometalate
(TBA6[{Ce(H2O)}2{Ce(CH3CN)}2(µ4-O)(γ-SiW10O36)2]), selective aerobic oxygenation of various
kinds of structurally diverse sulfides to the corresponding sulfoxides efficiently proceeded by
irradiation of visible light. The visible-light-induced Ce3+-to-W6+ charge transfer was the key for the
oxygenation. In the presence of the suitable electron and proton donor, the photoredox system of
TBA4H4[γ-SiW10O36] could be further utilized for deoxygenation of sulfoxides, giving the
corresponding sulfides with high efficiencies and selectivity even in the presence of other reducible
substituents. In the deoxygenation, the vacant site of TBA4H4[γ-SiW10O36] acted as not only for the
16

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New Journal of Chemistry
electron transfer site form an alcohol to TBA4H4[γ-SiW10O36] but also the electron transfer siVtieewfArrtoicmle Online
DOI: 10.1039/C5NJ01045D
the reduced TBA4H4[γ-SiW10O36] to a sulfoxides.
Experimental section
Materials
Substrates and solvents were obtained from Aldrich, TCI, Kanto Chemical, and Wako Pure Chemical
Industries, and purified prior to use (if necessary).16 Ce(acac)3 (Aldrich), Co(acac)2 (Kanto Chemical),
Zn(acac)2 (Kanto Chemical), Mn(acac)3 (Aldrich), Ni(acac)2 (Kanto Chemical), Fe(acac)3 (Aldrich),
Cu(acac)2 (Aldrich), Ag(acac) (Aldrich) and Y(acac)3 (Aldrich) were used as received. POMs
SiW10,4h CePOM,5a and TBA4[SiW12O40]17 were synthesized according to the reported procedures.
Instruments
IR spectra were measured on a Jasco FT/IR-4100 spectrometer using KBr disks. Cold-spray ionization
(CSI) mass spectra were recorded on JEOL JMS-T100CS. ICP-AES analyses were performed on a
Shimadzu ICPS-8100. Elemental analysis for C, H, and N were performed on a Yanaco MT-6 at the
Elemental Analysis Center of the School of Science, the University of Tokyo. GC analyses were
performed on a Shimadzu GC-2014 instrument with a flame ionization detector (FID) equipped with
an Rtx-1 capillary column (internal diameter = 0.25 mm, length = 30 m) or a TC-1 capillary column
(internal diameter = 0.32 mm, length = 30 m). GC analyses for H2 were performed on a Shimadzu GC-
8A with a thermal conductivity detector (TCD) with a 5 Å molecular sieve column (internal diameter =
3.2 mm, length = 3.0 m). GC mass spectra were measured on a Shimadzu GCMS-QP2010 at an
ionization voltage 70 eV.
Electrochemistry
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View Article Online
Cyclic voltammetric measurements were carried out with a Solartron SI 1287 Electrochemical
DOI: 10.1039/C5NJ01045D
Interface. A standard three-electrode arrangement was employed with a BAS glassy carbon disk
electrode as the working electrode, a platinum wire as the counter electrode, and a silver wire electrode
as the pseudoreference electrode. The voltage scan rate was set at 100 mV s−1, and TBAClO4 was used
as an electrolyte. The potentials were measured using Ag/AgNO3 reference electrode (10 mM AgNO3,
100 mM TBAClO4 in acetonitrile, 0.55 V vs NHE) and were converted to NHE. Electrochemical
reduction of TBA4[SiW12O40] (20 µmol) was carried out in acetonitrile (40 mL, 50 mM TBAClO4) at
–1.05 V vs NHE with an Electrochemical Analyzer ALS 600A using a BAS glassy carbon electrode as
the working electrode, a platinum wire as the counter electrode, and a silver wire electrode as the
pseudoreference electrode.
Oxygenation of sulfides
A typical procedure for oxygenation of sulfides is as follows: Into a Schlenk flask (volume: ca. 20 mL)
were successively placed SiW10 (4 µmol, 2 mol% with respect to 1a), Ce(acac)3 (8 µmol, 4 mol%
with respect to 1a), 1a (0.2 mmol), acetonitrile/methanol (1 mL, 2/1 v/v), and a Teflon-coated
magnetic stir bar under O2 (1 atm). The reaction was initiated by irradiation of visible light (λ > 400
nm) with a Xe-lamp equipped with a 400 nm cut off filter at 30°C. The detailed reaction conditions are
shown in the footnotes of Tables 1 and 2. The products were confirmed by comparison of their GC
retention times and GC-MS spectra with those of authentic data. The yields of products were
periodically determined by GC analysis using an internal standard technique. The reactions were also
carried out by using CePOM (2 µmol, 1 mol% with respect to 1a) instead of the mixture of SiW10 and
Ce(acac)3. After the reaction, the catalyst was retrieved as precipitate by addition of diethyl ether (ca.
18

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New Journal of Chemistry
20 mL) to the solution (>95% recovery based on CePOM). The retrieved catalyst was washVeiedw AwrtiictlehOnline
DOI: 10.1039/C5NJ01045D
diethyl ether, and then dried prior to being used for the reuse experiment.
Deoxygenation of sulfoxides
A typical procedure for deoxygenation of sulfoxides is as follows: Into a Schlenk flask (volume: ca.
20 mL) were successively placed SiW10 (2 µmol, 1 mol% with respect to 2a), 2a (0.2 mmol), 3 (1
mmol), acetonitrile/toluene (2 mL, 1/9 v/v), and a Teflon-coated magnetic stir bar under Ar (1 atm).
The reaction was initiated by irradiation of visible light (λ > 400 nm) with a Xe-lamp equipped with a
400 nm cut off filter at 30°C. The products were confirmed by comparison of their GC retention times
and GC-MS spectra with those of authentic data. The yields of products were periodically determined
by GC analysis using an internal standard technique. After the reaction, the catalyst was retrieved as
precipitate by addition of diethyl ether (ca. 15 mL) to the solution (>95% recovery based on SiW10).
The retrieved catalyst was washed with diethyl ether, and then dried prior to being used for the reuse
experiment.
Quantum chemical calculations
The DFT calculations were performed with Gaussian 09 software.18 For the calculation of CePOM, the
anionic part of CePOM was optimized at the B3LYP level theory19 with 6-31+G* (for H, O) and 6-
31G* (for Si), LanL2DZ (for W),20 and Stuttgart RSC 1997 ECP basis set (SDD, for Ce)21 by using the
polarizable continuum model (PCM) using the integral equation formalism variant (IEFPCM)22 with
the universal force field (UFF) radii for acetonitrile. For the calculation of SiW10 and the complex of
SiW10 and 3, anionic parts were optimized at the B3LYP level theory with 6-31++G* (for C, H, and
O), 6-31G* (for Si), and LanL2DZ (for W) by using conductor-like polarizable continuum model
(CPCM)23 with the parameters of the United Atom Topological Model (UAHF) for acetonitrile.
19

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DOI: 10.1039/C5NJ01045D
Acknowledgements
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).
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24