Photocatalytic Oxygenation of Sulfide and Alkenes by Trinuclear Ruthenium Clusters

Article abstract of DOI:10.1021/acs.inorgchem.5b02518

Trinuclear Ru cluster photocatalysts that contain two Ru(II) photosensitizers and a Ru(II) reaction center are prepared, and their activity in the photocatalytic oxygenation of a sufide and alkenes is investigated. Photoirradiation (visible light) and the use of a Co salt ([Co(NH₃)₅Cl]Cl₂, as an electron acceptor) are found to be essential for these catalytic reactions. The O atom of the water solvent (pH 6.8) is transferred to the substrate by a stepwise electron transfer and deprotonation of an aqua ligand at the reaction center. Through these processes, the aqua ligand coordinated at the reaction center is converted to a Ru^V=O species, which is the active intermediate in the sulfide and alkene oxygenation.

Full text of DOI:10.1021/acs.inorgchem.5b02518

Article
pubs.acs.org/IC
Photocatalytic Oxygenation of Sulfide and Alkenes by Trinuclear
Ruthenium Clusters
Siwas Phungsripheng,† Kazuyuki Kozawa,‡ Munetaka Akita,‡ and Akiko Inagaki*,†
†Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Minami-Osawa 1-1,
Hachioji, Tokyo 192-0397, Japan
‡Chemical Resources Laboratory, Tokyo Institute of Technology, R1-27, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
S
* Supporting Information
ABSTRACT: Trinuclear Ru cluster photocatalysts that contain two Ru(II) photo-
sensitizers and a Ru(II) reaction center are prepared, and their activity in the photo-
catalytic oxygenation of a sufide and alkenes is investigated. Photoirradiation (visible
light) and the use of a Co salt ([Co(NH3)5Cl]Cl2, as an electron acceptor) are found to
be essential for these catalytic reactions. The O atom of the water solvent (pH 6.8) is
transferred to the substrate by a stepwise electron transfer and deprotonation of
an aqua ligand at the reaction center. Through these processes, the aqua ligand
coordinated at the reaction center is converted to a RuVO species, which is the active
intermediate in the sulfide and alkene oxygenation.
interactions via the bridging ligands will largely affect to the
photoreactivities.10
1. INTRODUCTION
Utilization of clean and inexhaustible sunlight energy toward
molecular transformation is attracting a great deal of attention,
since it provides a new pathway for the synthesis of various
materials without consuming limited oil resources.1 Nowadays,
many researchers devote much of their effort into development
of highly active molecular catalysts, which can produce dihydro-
gen or dioxygen from abundantly present water,2 because
there is an urgent need to resolve future energy problems and
realize artificial photosynthesis.3 In this work, ruthenium(II)
trisdiimine complexes and their derivatives are used as photo-
sensitizers to collect visible-light energy, because of their
excellent photophysical properties;4 the collected energy is
used to drive electron transfer reactions. Although only little
attention has been paid to apply these systems for organic
syntheses, the continuous pursuit of organic chemists for
developing green reactions has led to rapid expansion in the use
of photoredox catalysts.5 Recently, Hamelin’s group reported
light-driven sulfide oxygenation using water as the oxygen
source; this reaction was catalyzed by a dyad of a ruthenium
photosensitizer and a catalytic center linked by a 1,10-
phenanthroline-based nonconjugated linker.6 In this reaction,
a high-valent RuIVO is generated as the active species for
oxygenation through a proton-coupled electron-transfer proc-
ess, which is a new way to use light energy for the catalytic
molecular transformation of organic compounds. Our group
has also been working toward the development of catalytic
organic transformation reactions driven by visible-light-active
photocatalysts.7 For these studies, we synthesized various
bridging ligands, which act as a linker between a photosensitizer
and a reaction center.8,9 We also considered that electronic
Previously, we developed a pyridine-substituted 2,2′-
bipyrimidine ligand (6-pyridyl-2,2′-bipyrimidine, pybpm) and
synthesized a dinuclear ruthenium catalyst, [Ru(bpy)2(pybpm)-
Ru(bpy) (OH2)]4+, linked by the pybpm ligand. Inspired by
the work of Hamelin and Men
́
age,11 the dinuclear ruthenium
catalyst was investigated for sulfide oxygenation, and was
found to show higher activity than the mixture of mononuclear
ruthenium components.9 Based on these results, we extended
this work to include trinuclear ruthenium cluster catalysts
having stronger electronic conjugation between the metals.
Normally, many multinuclear photocatalysts are designed by
linking the mononuclear components with nonconjugated
linkers; this is because it is a safe way to maintain the photo-
chemical performance of each component. Several multinuclear
catalysts have been designed for photocatalytic water oxidation,3a,12
alcohol oxidation,13 and sulfide and alkene oxygenation.6,11,14 In
contrast, in this work, we designed highly conjugated trinuclear
complexes that exhibit characteristic properties different from that
expected from a combination of the mononuclear components.
Here, we report synthesis of trinuclear catalysts containing two
ruthenium photosensitizers and one ruthenium reaction center,
which are linked by conjugated pybpm and/or bpm ligands, and
their reactivities, with a focus on the characteristic properties of this
electronically conjugated system.
Received: November 3, 2015
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Trinuclear Cluster Catalysts
Figure 1. Catalysts used for the photocatalytic oxygenation of sulfides.
2. RESULTS AND DISCUSSION
Table 1. Photocatalytic Oxygenation of Thioanisole by
a
Ruthenium Catalysts
Syntheses. The trinuclear aqua complex with the bridg-
ing pybpm ligand 3-OH2 was synthesized as follows. First,
[Ru(bpy)2(pybpm)]2+ (1) was treated by RuCl2(dmso)4 to
obtain the dinuclear complex 2, which was subsequently treated
with [Ru(bpy)2(bpm)]2+ (bpm = 2,2′-bipyrimidine) to give
trinuclear the Ru complex 3-Cl. Chloride abstraction by a silver
salt afforded the target aqua complex 3-OH2 in a quantitative
manner. Unlike the synthesis of 3-OH2, the trinuclear complex
with the two bridging bpm ligands (5-Cl) was synthesized by
directly treating 4 with 2 equiv of cis-Ru(bpy)2Cl2·2H2O. Sub-
sequently, 5-Cl is quantitatively converted to the target aqua
complex 5-OH2 by reaction with a silver salt (see Scheme 1).
Both trinuclear Ru complexes 3-OH2 and 5-OH2 have been
unambiguously characterized by nuclear magnetic resonance
(NMR) and electrospray ionization-mass spectroscopy (ESI-MS)
and elemental analyses.
conversion36h
[%]
quantum
yield, Φ
b
c
entry
catalyst
light
TON
1
2
3
4
3-OH2 (Ru3)
5-OH2 (Ru3)
6-OH2 (Ru2)
ON
ON
ON
ON
73
68
77
54
366
340
384
265
0.10
f
0.086
f
6-OH2 (Ru2) +
[Ru(bpy)3]Cl2
g
g
5
7-OH2 (Ru) +
2[Ru(bpy)3]Cl2
ON
53
28
0.015
6
7
8
3-OH2 (Ru3)
OFF
ON
ON
0
6
0
0
31
0
f
f
f
d (no catalyst)
e
3-OH2 (Ru3) (with-
out Co salt)
a
Unless otherwise noted, the reaction was conducted with 0.2 mol %
(20 μM) of catalyst in phosphate buffered D2O (pH 6.8, 0.067 M)
with 2 equiv (vs substrate) of [Co(NH3)5Cl]Cl2 for 36 h at 35 °C.
b
c
Photocatalytic Oxygenation of Sulfides and Alkenes.
Photocatalytic oxygenation of thioanisole by the trinuclear
Turnover number for the sulfoxide formation. Quantum yields were
calculated by using a Si photodiode sensor and 435 nm
monochromatic light. Details for the measurements are shown in
d
Figures S17 and S18 in the Supporting Information. Without catalyst.
e
f
g
Without [Co(NH3)5Cl]Cl2. Not measured. See Figure S2 in the
Supporting Information for the obtained products.
catalysts was tested, and its performance was compared with
9
those of related species, such as the dinuclear catalyst 6-OH2
and the mononuclear catalyst 7-OH215 (see Figure 1, as well as
B
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Table 1 and eq 1). The reactions were conducted in the
presence of 0.2 mol % Ru catalyst (20 μM) and 2 equiv (vs
thioanisole) of the Co salt, [Co(NH3)5Cl]Cl2 (a sacrificial
oxidant), in phosphate-buffered water (pH 6.8, 0.067 M). The
reaction vessel was kept at 35 °C in a water bath with vigorous
stirring under degassed and visible-light-irradiated conditions
(150 W Xe lamp, λ > 420 nm).
Under the above-mentioned conditions, thioanisole was con-
verted to methyl phenyl sulfoxide (Figure S1 in the Supporting
Information). All the catalysts except 7-OH2 (described later)
gave only the methyl phenyl sulfoxide as a single product.16
The trinuclear catalyst 3-OH2 and dinuclear catalyst 6-OH2
showed the highest activity, compared to the other catalysts.
The other trinuclear catalyst (5-OH2) exhibited activity
comparable to that of 3-OH2 (entries 1 and 2 in Table 1).
The activity of the dinuclear catalyst 6-OH2 with additional
photosensitizer unit ([Ru(bpy)3]2+) was lower than that
without the photosensitizer (entries 3 and 4 in Table 1). The
mononuclear catalyst (7-OH2) mixed with two equivalent
of photosensitizer ([Ru(bpy)3]2+) showed much less activity
(entry 5 in Table 1). Although the conversion of thioanisole
is comparable to the reaction by other trinuclear catalysts,
yield of sulfoxide (turnover number, TON) was very low,
because other side products such as diphenyl disulfide and
bis(phenylthio)methane (see entry 5 in Table 1, and Figure S2
in the Supporting Information) is formed.17 Entries 6−8 in
Table 1 clearly show that light, catalyst, and a Co salt are
essential for the catalysis reaction. After the reaction catalyzed
by 3-OH2, the reaction mixture was checked by ESI-MS
spectra, and detected the mass number of the trinuclear com-
ponent, which supports the stability of the trinuclear catalyst
throughout the reaction. Although photodissociation of the
monodentate pyridine ligand often occurs,12b,18 it was con-
firmed that the pyridine ligand in 5-OH2 was kept through the
irradiation (λ > 420 nm) in D2O for over 20 h (see Figures S3
and S4 in the Supporting Information):
aldehydes or ketones via stepwise CC bond cleavage
(mentioned later). For example, cis-stilbene yielded 2 mol of
benzaldehyde, and similarly, styrene yielded benzaldehyde and
formaldehyde, with the latter being detected by 1H NMR spectra.
Determination and quantification of these aldehydes were done
1
on the basis of H NMR, gas chromatography (GC), and gas
chromatography−mass spectroscopy (GCMS) spectral data. The
reaction by trinuclear catalyst 3-OH2 was also applicable to
aliphatic alkenes, such as cis-2-decene, to yield the corresponding
aldehydes, although reactivity and selectivity is low (entry 4 in
Table 2). In addition to 1-octanal and other unidentified
biproducts, a small amount of 1-heptanal and 1-hexanal was
formed, probably due to the isomerization of the double bond of
the starting compound or intermediate during the reaction.19
The higher activity of the trinuclear catalysts compared to
the dinuclear and mononuclear catalysts is clearly observed
(entries 2−4 in Table 2), even under the same Ru photo-
sensitizer/Ru catalyst ratio (Ruphoto/Rucat = 2:1). In these
reactions, trinuclear catalyst 5-OH2 showed activity comparable
to that of 3-OH2. Although the mechanistic details of the CC
bond cleavage needed to give aldehydes has remained unclear, we
detected correponding epoxides during the oxygenation of
styrene, α-methylstyrene, and cis-stilbene (see eq 3, as well as
Next, photocatalytic oxygenation reactions of various alkenes
were tested under the same conditions (see Table 2 and eq 2, as
well as Figures S5−S8 in the Supporting Information). Alkenes
such as styrenes (entries 1 and 2 in Table 2) and cis-stilbene
(entry 3 in Table 2) were converted to 2 mol (vs substrate) of
Figures S5, S6, and S7 in the Supporting Information).20When
the reaction with the corresponding intermediates such as
styrene oxide and α-methylstyrene oxide was tested, the results
a
Table 2. Oxygenation of Alkenes
a
Unless otherwise specified, the reaction was conducted under O2-free conditions with 0.2 mol % (20 μM) of catalyst in phosphate buffered D2O
b
(pH 6.8, 0.067 M) with 2 equiv (vs substrate) of [Co(NH3)5Cl]Cl2 for 36 h at 35 °C. TON was calculated from the oxygenated products
determined by GCMS, which data are calibrated with standard chemicals. 4 equiv of [Co(NH3)5Cl]Cl2 vs substrate was added. Benzophenone,
stilbene oxide is also formed. The main product was trans-stilbene. Not performed.
c
d
e
g
C
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indicate that diol was formed as the major product, together
with the corresponding final product (benzaldehyde or
acetophenone) (see eq 4, as well as Figures S9 and S10 in
In order to confirm the oxygen source of the product, labeling
experiments were conducted for the photocatalytic oxygenation
of α-methylstyrene in a phosphate buffered H218O (97% atom
18O). After the reaction, the product was isolated and analyzed by
GCMS to determine the isotope distribution (eq 6, Figure 2).
the Supporting Information). Reaction of 2-phenyl-1,2-
propanediol gave acetophenone as a single product (see eq 5,
When comparing the mass spectra shown in Figures 2a
and 2b, the mass number of the product (acetophenone) in
Figure 2b shows that the product contains mainly 18O, having
the same fragment peaks as those in Figure 2a. The results prove
that the source of O atom in the product is the solvent water.
Photophysical and Electrochemical Studies. The
electronic absorption spectra for the trinuclear complexes
3-OH2 and 5-OH2 observed in DMF at room temperature are
compared with those of the mononuclear and dinuclear com-
ponents (6-OH2, [Ru(bpy)2(bpm)]2+ and 1) (see Figure 3, as
Scheme 2. Stepwise Oxygenation of Alkene to Aldehydes
Figure 3. Ultraviolet−visible (UV-vis) absorption spectra (room
temperature, DMF solution).
Table 3. UV-vis Absorption Spectral Data in DMF (at Room
Temperature)
complex
λmax [nm]
ε [× 105 M−1 cm−1
]
3-OH2
5-OH2
6-OH2
426.5
597
0.33
0.12
416.5
591.0
0.34
0.11
Figure 2. Mass spectra (m/z) of the products (acetophenone) after
photocatalytic oxygenation of α-methylstyrene (a) in phosphate
buffered H216O, and (b) in phosphate buffered H218O.
428.5
522.0
0.26
0.17
as well as Figure S11 in the Supporting Information).
Photocatalytic oxygenation of water-soluble styrene to give
the corresponding benzaldehyde has been reported by the
Kojima and Sun groups,21 but the precise mechanism has not
been discussed. Based on these results, the oxygenation of
alkenes proceeds via epoxide and diol (see Scheme 2). Several
reactions of alcohols to give the corresponding aldehydes by
RuO species have been reported by other groups,13 and it
seems that the C−C bond is cleaved during the diol oxidation
step, yielding the aldehydes.22
[Ru(bpy)2(pybpm)](BF4)2 (1)
430
429
0.21
0.21
[Ru(bpy)2(bpm)](BF4)2
well as Figure S12 in the Supporting Information and Table 3).
Comparison of the spectra of the mononuclear and dinuclear
complexes indicates that a new MLCT (dπRu → πp*ybpm) absorp-
tion band appears in the low-energy region (6-OH2, λmax
522 nm) as a result of dinucleation. For the absorption spectra
=
of the trinuclear complex 5-OH2, which possesses only bpm as
D
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Figure 4. Cyclic voltammograms of (a) 1, [Ru(bpm)](BF4)2, 3-Cl, and 3-OH2, (b) [Ru(pybpm)Ru(bpy)(OH2)](BF4)4, [Ru(bpm)Ru(pybpy)-
(OH2)](BF4)4, 3-OH2, and 5-OH2 in CH3CN (Ru = [(bpy)2Ru], [complex] = 1 mM, [nBu4NBF4] = 0.1 M, 100 mV/s, E vs Fc/Fc+).
a
Table 4. Summary of Electrochemical Data in CH3CN
E1/2 [V]
bpm/bpm• −
pybpm/pybpm• −
Ru /Ru
Ru
/Ru
photo photo
II
III
cat
II
III
complex
cat
[Ru(bpy)2(pybpm)](BF4)2 (1)
−1.39
0.99
[Ru(bpy)2(bpm)](BF4)2
−1.39
1.03
1.30
1.21
1.27
1.25
1.28
b
[Ru(pybpm)Ru(OH2)(bpy)](BF4)4 (6-OH2)
[Ru(bpm)Ru(OH2)(tpy)](BF4)4
−0.86
1.04
b
−0.89
−0.98
−0.95
0.90
b
c
c
e
[Ru(pybpm)Ru(Cl)(bpm)Ru](BF4)5 (3-Cl)
[Ru(pybpm)Ru(OH2)(bpm)Ru](BF4)6 (3-OH2)
[Ru(bpm)Ru(OH2)(py)(bpm)Ru](BF4)6 (5-OH2)
−0.82
−0.80
1.01
b
1.18
d
b
−1.01, −0.82
1.00
a
[complex] = 1 mM, [nBu4NBF4] = 0.1 M, 100 mV/s, E vs Fc/Fc+. Ru = [(bpy)2Ru], Rucat = Ru reaction center, Ruphoto = Ru photoredox center.
Irreversible wave; anodic peak. Redox potential of the two-electron process. Irreversible wave; cathodic peak. Quasi-reversible peak.
b
c
d
e
III
a bridging ligand, a new MLCT band based on the dπRu → πb*pm
transition (λmax = 591 nm)11 is observed. On the other hand,
the trinuclear complex 3-OH2 possesses a broad and wide
MLCT band that stretches from 390 nm to 800 nm, because of
the overlap of the two dπRu → π*pybpm and dπRu → π*bpm trans-
itions. In this way, formation of the dinuclear and trinuclear
clusters results in low-energy absorption bands below 500 nm,
which suggest strong interactions between the metal fragments.
Both trinuclear clusters show a dark green color, which is due
to the above-mentioned low-energy absorption bands.
CV data measured in CH3CN at room temperature (see
Figure 4, as well as Figure S13 in the Supporting Information)
are summarized in Table 4. A comparison of the CV data of the
aqua complex (3-OH2) with the corresponding chloride
complex (3-Cl) reveals that the latter exhibits sharper peaks.
Thus, for reliable assignment of the peaks of the aqua com-
plexes, the CV spectrum of the 3-Cl is also shown (Figure 4a).
A comparison of the redox potentials of the bridging ligands
(BL/BL• −), pybpm and bpm, reveals that reduction occurs at a
slightly higher potential for pybpm than that for bpm.8,23,24
Thus, in the CV data of the trinuclear clusters with mixed
bridging ligands, the redox wave (BL/BL• −) with higher poten-
tial is assigned to pybpm (Figure 4a). The redox waves with the
highest potential (∼1.3 V) with high reversibility are not
affected by the number of nuclei or bridging ligands, and the
wave can be assigned to the redox process of RuIpIhoto/Ru
.
photo
In the spectra of the trinuclear complexes 3-Cl, 3-OH2, and
5-OH2, redox waves of Ru /Ru are observed at ∼1.0 V. The
II
cat
III
cat
potential for 3-OH2 is higher than that of 5-OH2 (Figure 4b).
Mechanistic Studies. The redox potential of the Rucat
center of 3-OH2 is plotted in the range of pH 4−10, since the
peaks become broader in the highly acidic (pH <2) region or
the basic pH (pH >10) region (see the Poubiax diagram in
Figure 5, as well as Figure S14 in the Supporting Information).
From the plotted electrochemical data, two oxidation processes
are observed: a one-electron one-proton process and a two-
electron one-proton process. Thus, within this pH range, the
Ru −OH2, Ru −OH, and RuVcatO species can be formed
II
cat
III
cat
through photoredox processes. It is interesting that, under the
actual photocatalytic reaction conditions (pH 6.8), the high
oxidation state RuVO species can be generated at relatively
low potential.
In order to confirm the active species of the catalytic
oxygenation, we attempted chemical oxidation of the catalyst.
[RuIII(bpy)3](ClO4)3 was selected as a one-electron oxidant,
because it possesses suitable redox potential to oxidize the
reaction center, but not the photoredox center. While up to
4.7 equiv of [RuIII(bpy)3](ClO4)3 was added to an CD3CN
solution of 3-OH2, the corresponding spectral changes
1
were followed by UV-vis absorption (Figure 6), H NMR
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in RuII can be attributed to the formation of [RuII(bpy)3]2+
from the reduced oxidant, and the increase in RuIII is due to the
unreacted oxidant ([RuIII(bpy)3]3+), as seen in the spectra.
These results indicate that up to 3 equiv of [RuIII(bpy)3]-
(ClO4)3 are consumed via the three-electron oxidation of
3-OH2 to generate a RuVO species (eq 7). After this, further
addition of the one-electron reductant, Cp2Fe, led to the
disappearance of the absorption at 314 nm and an increase in
the intensity of the absorption at 289 nm, supporting the
assignment of both peaks. In addition, the MLCT band between
500 and 700 nm remained unchanged, indicating the stability of
the trinuclear catalyst against decomposition during the redox
processes. A similar result was obtained by a use of NOBF4 as an
oxidant (see Figure S19 in the Supporting Information).
Figure 5. Pourbaix diagram plotted in the range of pH 4−10.
(Figure S15 in the Supporting Information), and ESI-MS
(Figure S16 in the Supporting Information) spectroscopies.
In the UV-vis absorption spectra (Figures 6 and 7), absorp-
tion changes at ∼289 nm (RuII) and ∼314 nm (RuIII: oxidant)
1
In H NMR spectra, a broad signal corresponding to the
residual [RuIII(bpy)3]3+ can be observed after the addition of
over 3 equiv of the oxidant (see Figure S15); this result corro-
borates well with the change in the UV-vis absorption spectra.
In the ESI-MS spectra, an intermediate RuVO peak was
detected (see Figure S16).
Based on these results, a plausible reaction mechanism is
depicted in Scheme 3. The reaction proceeds via the formation
Scheme 3. Plausible Reaction Mechanism
Figure 6. UV-vis absorption spectral changes via the addition of
oxidant ([Ru(bpy)3]3+ in acetonitrile, at room temperature).
Figure 7. Spectral change in the UV-vis absorption spectra at 289 nm
(RuII) and 314 nm (RuIII) via the addition of [Ru(bpy)3]3+(RuIII).
of RuIII−OH and RuVO species. Excitation of 3-OH2 induces
RuIpIhIoto formation by a redox process between Ru*photo and CoIII
(eq (i), Scheme 3) and subsequent deprotonation produces
3III−OH (paths (a) and (b)). Then, two cycles of the
photoredox processes (eq (ii)) and deprotonation give the
RuVO species (paths (c)−(e)). The reaction between the
can be seen. When the amount of the added oxidant is below
3 equiv, an increase in the amount of RuII species is observed.
In contrast, an increase in the amount of RuIII is observed when
the amount of the added oxidant exceeds 3 equiv. The increase
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substrate and RuVO gives an oxygenated product (subO),
and reoxidation of CoII to CoIII by 3III−OH2 regenerates the
starting complex (eq (ii), Scheme 3).25 In total, two equiv of
the CoIII salt is consumed in one catalytic cycle. The
characteristic feature of the mechanism is the formation of
RuVO, resulting in the high catalytic activities of oxygenation.
Alkenes are oxidized by the RuVO species to epoxides, diols,
and then aldehydes via C−C bond cleavage, in a stepwise
manner (Scheme 2). The facile formation of the RuVO
species is due to the presence of dicationic species in the same
molecule. Thus, the photosensitizer units assist the intra-
molecular one-electron transfer process (eq (i)), and at the
same time, helps to produce the RuVO species. These results
show the potential ability of the trinuclear cluster catalysts.
method and an organic substrate (30 μmol) was then added under N2
atmosphere. The system was sealed, and the resulting solution was
exposed to the light produced by 150 W Xe lamp through a cut filter
(λ > 420 nm) and stirred during the reaction. The reaction was followed
1
by H NMR and GC-MS, after appropriate time intervals.
Quantum Yield Measurements. A reaction solution containing
Ru catalyst (20 μM) in 5 mL of phosphate-buffered H2O (0.067 M,
pH 6.8) with thioanisole 10 mM and [Co(NH3)5Cl]Cl2 sacrificial
electron acceptor (20 mM) was prepared. Before light irradiation, the
solution was degassed by a freeze−pump−thaw method in 20 mL of
transparent Schlenk tube. Light-emitting diode (LED) monochromatic
light (λ = 435 nm) was used as a light source, which consisted of an
optical lens, to maintain incident light intensity. The number of
photons absorbed by the solution was calculated by using a light power
meter (StarLite, OPHIR Photonics Solutions, Ltd.) at λ = 435 nm. A
plot of photon number (mmol) versus methyl phenyl sulfoxide product
(mmol) exhibited a slope that indicated the quantum efficiency.
Electrochemistry. All cyclic voltammetry was performed in 1 mM
of each Ru complex in acetonitrile by an electrochemical analyzer
(model 620B, CH Instruments, Inc.) using a platinum working
electrode (1.6 mm in diameter), a Ag/Ag+ reference electrode (0.01 M
AgNO3, 0.1 M tetrabutylammonium perchlorate in electrolyte
solution), and a platinum wire counter electrode. Ferrocene was
added to the solution as an internal standard.
For the trinuclear complex 3-OH2, pH-dependent electrochemistry
was done at pH 2−10 in water. Square wave voltammetry was mea-
sured using 1 mM 3-OH2 in Britton−Robinson buffer 0.04 M (pH 2)
with glassy carbon (diameter 1 mm) as a working electrode, Ag/AgCl
(3 M NaCl aqueous solution) as a reference electrode, and platinum
wire as a counter electrode. The solution was titrated by 0.2 M NaOH
to a modified pH and an actual detected pH using a pH/ion meter
(Laqua F-72, Horiba, Ltd.). Square wave voltammogram data were
plotted versus pH, as shown in Figure S14 (Ag/AgCl vs NHE: +0.197 V).
[Ru(bpy)3]Cl2 was used as an internal standard in aqueous solution
E(RuII/RuIII) = 1.26 V.
3. SUMMARY
In summary, we have synthesized trinuclear ruthenium cluster
catalysts that contain two photoredox sites and one reaction
center, and we have achieved photocatalytic oxygenation of
sulfide and alkenes in neutral water media using these catalysts.
Since these catalysts possess two dicationic photoredox units,
RuVO with high oxidizability can be formed at relatively
low potential, resulting in high activity, when compared to the
corresponding intermolecular system. Alkenes were converted
to 2 mol of aldehydes via the stepwise oxygenation of a CC
double bond, followed by C−C bond cleavage. Investigation of
the reaction mechanism revealed that the formation of the
RuVO species proceeds via the RuIII−OH intermediate. The
dual role of the photosensitizers is one of the key advantages of
using trinuclear catalysts. The conjugated multinuclear system
is a new approach to the catalyst design for various oxygenation
reactions.
Preparation of 2.
4. EXPERIMENTAL SECTION
General Procedures. Standard Schlenk and vacuum line tech-
niques under N2 atmosphere were employed for all the manipulations.
Acetone (molecular sieves), acetonitrile (P2O5), MeOH (Mg(OMe)2),
and EtOH (Mg(OEt)2), were treated with appropriate drying agents,
distilled, and stored under N2. Dehydrated solvent was purchased
from Kanto Chemical for Et2O. Other solvents (dimethyl sulfoxide
(DMSO), dimethylformamide (DMF), and nitromethane) were
purchased and used as received. The metal reagents, RuCl2(dmso)4,26
[(bpy)2Ru(bpm)]2+,27 cis-Ru(bpy)2Cl2·2H2O,28 (1),9,29 6-OH2,9 and 7-
OH2,15 were prepared according to the published procedures. Other
organic chemicals were purchased and used as received. Standard
phosphate buffer solutions at pH 6.8 (0.1 or 0.067 M) were prepared
from the sodium phosphate monobasic and dibasic salts (KH2PO4/
Na2HPO4). 1H and 13C NMR spectra were recorded on Bruker
AVANCE-500, Bruker AVANCE-400, JEOL EX-270, JEOL JNM-400,
and JEOL ECS-400 spectrometers. ESI-MS and GC-MS spectra were
recorded on Bruker MicroTOF II, and Shimadzu GC-17A/QP5050 mass
spectrometers, respectively. Solvents for NMR measurements were dried
over molecular sieves, degassed, and stored under N2. UV-vis and steady-
state emission spectra were obtained on a JASCO V-670 and Shimadzu
RF-5300PC spectrometers, respectively. ESI-MS spectra were recorded
on a ThermoQuest Finnigan LCQ Duo and Bruker micrOTOFII-SDT1
mass spectrometers. GC and GCMS chromatography was recorded on a
Shimadzu GC-17A and QP5050 spectrometers. Electrochemical measure-
ments were made with CH Instruments Electrochemical Analyzer Model
620B and Hokuto Denko HZ-5000 analyzers. In the following sections,
A mixture of [Ru(bpy)2(pybpm)](BF4)2 (1) (100 mg, 0.122 mmol)
and cis(Cl), fac(S)-[RuCl2(dmso-S)3-(dmso-O)] (65.0 mg, 0.134 mmol)
in EtOH-dmso (6.6 mL, 10:1 v/v) was refluxed for 36 h. After cooling to
room temperature (RT), the solvent was evaporated and the residue was
purified by recrystallization from CH2Cl2 containing 5% CH3NO2/
diethyl ether more than once to afford a blackish-brown solid (112 mg,
0.105 mmol, 86.0% yield). 1H NMR (400 MHz, CD3NO2, RT, δ/ppm):
9.22−9.19 (m, 2 H, Hg and Hm), 8.63−8.57 (m, 5 H, HD, HD′, and Hi),
8.31−8.06 (m, 10 H, HA or HA′, Hd, He, Hh, and Hk), 7.96−7.85 (m,
4 H, HA or HA′, Hf, and Hl), 7.55−7.46 (m, 4 H, HB and HB′), 2.96 (s,
1.5 H, Ru-dmso), 2.77 (s, 1.5 H, Ru-dmso), 2.64 (s, 1.5 H, Ru-dmso),
2.53 (s, 1.5 H, Ru-dmso). ESI-MS (m/z): 985.6 [M−BF4]+ (calcd:
984.99). Anal. Calcd for C35H31B2Cl2F8N9ORu2S, CH2Cl2: C, 37.36; H,
2.87; N, 10.89. Found: C, 37.67; H, 2.71; N, 11.09.
Preparation of 3-Cl. [Ru(bpy)2(pybpm)RuCl2(dmso)](BF4)2 (2)
(100 mg, 0.0932 mmol) and [Ru(bpy)2(bpm)](BF4)2 (76.4 mg,
0.103 mmol) were dissolved in EtOH (6 mL) and refluxed for 36 h.
NH4BF4 (100 mg, 0.954 mmol) was added after cooling to room
temperature and the solution was stirred for 5 h. After the solvent was
evaporated under reduced pressure, the reaction mixture was dissolved
in CH2Cl2 containing 5% CH3NO2 and filtered through Celite to
remove insoluble inorganic salts. The filtrate was concentrated and the
residue was purified by recrystallization from CH3NO2/diethyl
ether more than once to afford a blackish-green solid (141 mg,
0.0788 mmol, 84.5% yield). 1H NMR (400 MHz, CD3NO2, RT,
δ/ppm): 10.24−10.21 (m, 1 H), 10.16−10.14 (m, 1 H), 8.72−8.37
(m, 27 H), 8.35−7.90 (m, 38 H), 7.86−7.40 (m, 25 H), 7.22−7.07 (m,
2 H). ESI-MS (m/z): 1706.4 [M−BF4]+ (calcd: 1706.12). Anal. Calcd
1
3JHH and JCH are abbreviated as J and JCH, respectively.
General Photocatalytic Reactions. A total of 0.6 mL of a
0.1 mM solution of the catalyst in CH3NO2 was added in the reactor.
After evaporation of the solvent, [CoCl(NH3)5]Cl2 (15 mg, 60 μmol)
and 3 mL of 0.067 M sodium phosphate buffer (pH 6.8) H2O solution
were added. The resulting solution was degassed by freeze−pump−thaw
G
DOI: 10.1021/acs.inorgchem.5b02518
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
1
for C61H47B5ClF20N17Ru3, /2CH3NO2, /2diethyl ether: C, 40.36; H,
2.97; N, 13.50. Found: C, 40.09; H, 2.73; N, 13.13.
extracted by water and the water was removed before purified by
recrystallization in MeOH/Et2O (0.1088 g, 0.061 mmol, 78%). H
1
NMR (500 MHz, CD3NO2, RT, δ/ppm): 10.04−9.94 (m, 1 H), 9.08−
8.84 (m, 2 H), 8.72−8.35 (m, 14 H), 8.28−7.99 (m, 11 H), 7.95−7.85
(m, 6 H), 7.79−7.62 (m, 5 H), 7.50−7.10 (m, 10 H). ESI-MS (m/z):
1706.87 [M−BF4]+ (calcd: 1706.06). Anal. Calcd for
C61H49B5ClF20N17Ru3·1.5CH3NO2·H2O: C, 39.46; H, 2.94, N,
13.62. Found: C, 39.65; H, 2.84; N, 13.33.
Preparation of 5-OH2. Complex 5-Cl (0.0252 g, 0.01 mmol) and
AgBF4 (0.0160 g, 0.08 mmol) was dissolve in H2O, and the solution
was heated to 60 °C for 10 h. After reaction, water was removed and
the obtained solid was dissolved in CH3NO2 to remove AgBF4 salt
through Celite filtration. Dark green solid powder was precipitated
from CH3NO2/Et2O−CH2Cl2 solution (0.0175 g, 0.01 mmol, 67%).
1H NMR (500 MHz, CD3NO2, RT, δ/ppm): 9.98−9.84 (m, 1 H),
9.07−8.75 (m, 2 H), 8.66−8.37 (m, 15 H), 8.23−7.90 (m, 14 H),
7.84−7.42 (m, 16 H), 7.29−6.96 (m, 1 H). ESI-MS (m/z): 1688.6
[M−BF4]+ (calcd: 1690.9) (M = [(bpy)2RuII(bpm)RuII(OH) (py)
(bpm)RuII(bpy)2](BF4)5).
Preparation of 3-OH2. A mixture of [Ru(bpy)2(pybpm)RuCl-
(bpm)Ru(bpy)2](BF4)5 (3-Cl) (120 mg, 0.0670 mmol), and AgBF4
(23.9 mg, 0.118 mmol) in H2O (6 mL) was refluxed for 3 h. After
cooling to room temperature, the solvent was evaporated and the
residue dissolved in CH3NO2 was filtered through Celite. The filtrate
was concentrated and added excessive amounts of diethyl ether. After
the resulting solid was washed with toluene and CH2Cl2, a blackish-
green solid product was obtained from CH3NO2/diethyl ether
(117 mg, 0.0628 mmol, 93.8% yield). 1H NMR (400 MHz,
CD3NO2, RT, δ/ppm): 10.02−9.98 (m, 1 H), 9.95−9.92 (m, 1 H),
8.71−8.40 (m, 28 H), 8.35−8.04 (m, 29 H), 8.00−7.84 (m, 10 H),
7.81−7.40 (m, 23 H), 7.20−7.06 (m, 2 H). ESI-MS (m/z): 1687.9
[M−BF4]+ (calcd: 1688.16) (M = [(bpy)2RuII(pybpm)RuII(OH)
(bpm)RuII(bpy)2](BF4)5). Anal. Calcd for C61H49B6F24N17ORu3·
3H2O·2CH3NO2: C, 37.16; H, 3.02; N, 13.07. Found: C, 37.03; H,
2.69; N, 12.93.
Preparation of 4.
Anal. Calcd for C61H51B6F24N17ORu3·1.5CH3NO2·2H2O·CH2Cl2:
C, 36.76; H, 2.99; N, 12.49. Found: C, 36.42; H, 2.69; N, 12.19.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02518.
Synthesis of cis-Ru(bpm)2Cl2. 2,2′-Bipyrimidine (0.11 g, 0.67 mmol)
was dissolved in DMSO/EtOH (1/9, 3 mL) and RuCl2(dmso)4
(0.1527 g, 0.32 mmol) was slowly added and refluxed for 5 h.
After Celite filtration, the solvent of the filtrate was removed under
vacuum. The obtained cis-Ru(bpm)2Cl2 was purified by Al2O3 column
chromatography (1/9MeOH−CH3NO2). After recrystallization
by CH3NO2/Et2O twice, cis-Ru(bpm)2Cl2 was obtained as a brown
solid (0.0604 g, 0.12 mmol, 39%). 1H NMR (500 MHz, CD3NO2 RT,
δ/ppm). 10.21 (dd, J = 5.7 and 2.1 Hz, 2 H, Ha), 9.08 (dd, J = 4.7 and
2.0 Hz, 2 H, Hc), 8.74 (dd, J = 4.6 and 1.8 Hz, 2 H, Hd), 8.17 (dd, J =
5.9 and 1.9 Hz, 2 H, Hf), 7.83 (t, J = 5.1 Hz, 2 H, Hb or He), 7.19
(t, J = 5.1 Hz, 2 H, Hb or He).
GC-MS data of the oxygenated products, ESI-MS, NMR,
UV-vis absorption, differential pulse voltammograms,
and square wave voltammograms data of compounds,
related intermediates, and reagents used in the experi-
ments. Details for quantum yield measurements, ORTEP
diagram and IR data of the resultant Co salt. (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel.: 81-42-677-2546. E-mail: ainagaki@tmu.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Professor Kotohiro Nomura and Shinsuke Takagi
(Tokyo Metropolitan University) for fruitful discussions. We
gratefully acknowledge the financial support of the Naito
Foundation (A.I.), Cooperative Research Program of “Network
Joint Research Center for Materials and Devices”. A part of this
work was supported by a JSPS Grant-in-Aid for Scientific
Research on Innovative Areas (“3D Active-Site Science”: Grant
No. 26105003) from The Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan. S. P. acknowl-
edges the Tokyo Metropolitan government (Asian Human
Resources Fund) for a predoctoral fellowship.
Excess pyridine was added to a cis-Ru(bpm)2Cl2 (0.0336 g,
0.06 mmol) MeOH (2 mL) solution, and the solution was refluxed
for 12 h. After the solution was cooled to room temperature, NH4BF4
(0.0070 g, 0.07 mmol) was added and vigorously stirred for 3 h. The
obtained product (4) was purified by Al2O3 column chromatography
eluting with CH3NO2. Deep red crystal was deposited from recrys-
1
tallization in CH3NO2/Et2O (0.0336 g, 0.05 mmol, 85%). H NMR
(500 MHz, CD3NO2, room temperature (RT), δ/ppm): 10.17 (dd, J =
5.7 and 2.1 Hz, 1 H, Ha), 9.15 (dd, J = 4.7 and 1.9 Hz, 1 H, Hf),
9.11 (dd, J = 4.8 and 2.1 Hz, 1 H, Hc), 8.95 (dd, J = 5.7 and 2.0 Hz,
1 H, Hd), 8.91 (dd, J = 4.7 and 2.0 Hz, 1 H, Hc′), 8.88 (dd, J = 4.7 and
1.9 Hz, 1 H, Hf′), 8.65 (s, 2 H, Ho), 8.50 (dd, J = 5.8 and 1.9 Hz, 1 H,
Hd′), 8.14 (dd, J = 5.8 and 2.0 Hz, 1 H, Ha′), 7.92 (t, J = 5.2 Hz, 1 H,
Hb), 7.83 (tt, J = 7.7 and 1.3 Hz, 1 H, Hp), 7.77 (t, J = 5.3 Hz, 1 H,
He), 7.41 (t, J = 5.3 Hz, 1 H, He′), 7.36 (t, J = 5.2 Hz, 1 H, Hb′), 7.83
(tt, J = 7.1 and 1.3 Hz, 1 H, Hm). ESI-MS (m/z): 531.8 [M-BF4]+
(calcd: 531.9). Anal. Calcd for C21H17BClF4N9Ru·CH3NO2: C, 38.87;
H, 2.97, N, 20.60. Found: C, 39.02; H, 2.90; N, 20.41.
Preparation of 5-Cl. Complex 4 (0.0479 g, 0.08 mmol) and cis-
Ru(bpy)2Cl2·2H2O (0.1497 g, 0.23 mmol) were refluxed in EtOH
(3 mL) for 20 h. After cooled to RT, the solution was stirred with
NH4BF4 0.1043 g (1.00 mmol) for 3 h to obtain the product as a green
powder. The product was washed with EtOH. The product was
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I
DOI: 10.1021/acs.inorgchem.5b02518
Inorg. Chem. XXXX, XXX, XXX−XXX