Highly efficient and selective photocatalytic oxidation of sulfide by a chromophore-catalyst dyad of ruthenium-based complexes

Article abstract of DOI:10.1021/ic5020972

Electronic coupling across a bridging ligand between a chromophore and a catalyst center has an important influence on biological and synthetic photocatalytic processes. Structural and associated electronic modifications of ligands may improve the efficiency of photocatalytic transformations of organic substrates. Two ruthenium-based supramolecular assemblies based on a chromophore-catalyst dyad containing a Ru-aqua complex and its chloro form as the catalytic components were synthesized and structurally characterized, and their spectroscopic and electrochemical properties were investigated. Under visible light irradiation and in the presence of [Co(NH₃)₅Cl]Cl₂ as a sacrificial electron acceptor, both complexes exhibited good photocatalytic activity toward oxidation of sulfide into the corresponding sulfoxide with high efficiency and >99% product selectivity in neutral aqueous solution. The Ru-aqua complex assembly was more efficient than the chloro complex. Isotopic labeling experiments using ¹⁸O-labeled water demonstrated the oxygen atom transfer from the water to the organic substrate, likely through the formation of an active intermediate, Ru(IV)=O.

Full text of DOI:10.1021/ic5020972

Article
pubs.acs.org/IC
Highly Efficient and Selective Photocatalytic Oxidation of Sulfide by
a Chromophore−Catalyst Dyad of Ruthenium-Based Complexes
†
‡
‡
‡
†
†
Ting-Ting Li, Fu-Min Li, Wei-Liang Zhao, Yong-Hua Tian, Yong Chen, Rong Cai,
,
†,‡
and Wen-Fu Fu*
†Key Laboratory of Photochemical Conversion and Optoelectronic Materials and HKU-CAS Joint Laboratory on New Materials,
Technical Institute of Physics and Chemistry and University of Chinese Academy of Sciences, CAS, Beijing 100190, People’s
Republic of China
‡
College of Chemistry and Engineering, Yunnan Normal University, Kunming 650092, People’s Republic of China
*
S Supporting Information
ABSTRACT: Electronic coupling across a bridging ligand between a
chromophore and a catalyst center has an important influence on
biological and synthetic photocatalytic processes. Structural and
associated electronic modifications of ligands may improve the
efficiency of photocatalytic transformations of organic substrates.
Two ruthenium-based supramolecular assemblies based on a
chromophore−catalyst dyad containing a Ru−aqua complex and its
chloro form as the catalytic components were synthesized and
structurally characterized, and their spectroscopic and electrochemical
properties were investigated. Under visible light irradiation and in the
presence of [Co(NH ) Cl]Cl as a sacrificial electron acceptor, both
3
5
2
complexes exhibited good photocatalytic activity toward oxidation of
sulfide into the corresponding sulfoxide with high efficiency and >99%
product selectivity in neutral aqueous solution. The Ru−aqua complex assembly was more efficient than the chloro complex.
1
8
Isotopic labeling experiments using O-labeled water demonstrated the oxygen atom transfer from the water to the organic
substrate, likely through the formation of an active intermediate, Ru(IV)O.
INTRODUCTION
Previous studies have shown that the oxygenation of organic
■
t
8
sulfides can be carried out using either H O or BuOOH.
2
2
Development of photocatalytic methods for organic trans-
formations is an important aspect of renewable energy
technologies. The enormous potential of solar energy as a
clean and economical energy source motivates the development
of new methods to convert solar to chemical energy. In
oxygenic photosynthesis, oxidation of water into molecular
However, these compounds have disadvantages in terms of
their thermal instability and poor selectivity, which limit their
practical application in industry. Photocatalytic sulfide oxygen-
ation by high-valent metal−oxo complexes in aqueous systems
remains relatively unexplored because the reactions involve a
1
9
oxygen is activated by a high-valent manganese−oxo cluster in
the dark at photosystem II. This reaction has inspired
two-electron and two-proton transfer process. In this context,
2
photocatalytic sulfide oxygenation systems have the following
requirements: (1) the chromophore should efficiently harvest
light in the visible spectrum, (2) the system should feature
rapid and effective excited-state electron transfer with minimal
recombination between the catalyst and chromophore, and (3)
water should be the source of the oxygen atom.
numerous efforts toward developing photocatalytic conversions
3
of organic substrates. The use of high-valent metal−oxo
complexes in organic transformations is essential because of
their irreplaceable roles as reactive intermediates in biological
4
and synthetic redox reactions. Since the pioneering work
reported by Gray and his colleagues, many Fe(IV)−,
5
Combining a chromophore with a catalytic fragment to form
a supramolecular photocatalyst could meet these requirements.
This would accelerate the successive electron transfer steps in
the correct direction, diminish electron recombination, and
enable more efficient performance of a dyad compared with a
bimolecular system. Additionally, this may simplify the study of
Mn(IV)−, and Ru(IV)−oxo complexes have been found to
take part in a stepwise photoinduced proton-coupled electron
transfer (PCET) through the interaction of their initial low-
valent metal−aqua complexes and oxidized photosensitizers
III
3+
III
3+
such as [Ru (bpy) ] or [Ru (tpy) ] (bpy = 2,2′-
3
3
6
bipyridine; tpy = 2,2′:6′2″-terpyridine). Recently, light-driven
sulfide oxygenation reactions using chromophore−catalyst dyad
photocatalysts have attracted considerable attention from many
research groups for their fundamental interest and potentia7l
applications in the pharmaceutical and petroleum industries.
10
the different processes for mononuclear homologues.
Received: August 28, 2014
©
XXXX American Chemical Society
A
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Inorganic Chemistry
Article
Our group is interested in the design of a supramolecule that
combines a chromophore and a catalytic fragment within the
same entity to improve the photocatalytic reaction efficiency
λ = 355 nm laser from a frequency-tripled Nd:YAG (neodymium-
doped yttrium aluminum garnet) laser device. A single crystal of the
ligand L was obtained by slow diffusion of diethyl ether into a
dichloromethane solution containing L at room temperature.
Diffraction data were collected on a Rigaku R-AXIS RAPID IP X-ray
diffractometer using a graphite monochromator with Mo Kα radiation
II
2+
compared with that of bimolecular systems. One [Ru (bpy) ]
3
II
2+
or [Ru (tpy) ] moiety was chosen to act as the light-
3
harvesting antenna, and another unit behaved as a catalyst
connected by a bridging ligand. Our investigations of the
photophysical properties and catalytic activity as well as other
groups’ work revealed that the bridging ligands play a crucial
role in catalytic performance and impact the lifetime of the
excited states, the directionality of the electron transfer, and the
(
λ = 0.071073 nm) at 113 K. The molecular structure of the prepared
compound was resolved by direct methods and refined by full-matrix
least-squares methods on all F data (SHELXL-97). Non-hydrogen
2
atoms were refined anisotropically. The position of the hydrogen
1
5
atoms was calculated and refined isotropically. CCDC reference
number 987878 contains the supplementary crystallographic data for
ligand L. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
11
redox potentials of both units. We have also found that the
photocatalytic efficiency of the two moieties in a supramolecule
linked through a carbon−carbon single bond is superior to that
Electrochemical measurements were carried out on a CHI660C
electrochemical potentiostat. Cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) experiments were performed using a three-
electrode cell in sodium phosphate buffer. A glassy carbon disk
(diameter 3 mm), a platinum plate, and a Ag/AgCl electrode (3 M
KCl aqueous solution) were used as the working, counter, and
reference electrodes, respectively. The working electrode was
successively polished with 3 and 1 μm diamond pastes and sonicated
in ion-free water before use. The electrolyte was degassed with
of a double bond connection in either photocatalytic CO
2
11a−c
reduction or light-driven oxygenation of alcohols.
We have
now begun to focus on designing supramolecular catalysts to
improve the performance of photocatalytic sulfide oxygenation.
An assembly with the well-studied photosensitizer [Ru-
+
(
bpy)3]2 and the versatile catalyst [Ru(bpy)(tpy)(X)]n+ (X =
Cl, H O) has been used in a variety of chemical trans-
2
1
2
formations, including water oxidation, organic substrate
oxygenation,
the induction of [Ru(bpy)3] having a long excited-state
lifetime and a single bond linkage may improve the photo-
catalytic efficiency of sulfide oxygenation under visible light
irradiation.
9,10e,11a,b
13
nitrogen for 30 min. All potentials are reported vs the normal
and reduction of CO . We assumed
2
3+
hydrogen electrode (NHE), and the redox couple [Ru(bpy) ] /
2
+
3
2+
[
Ru(bpy) ] , E = 1.26 V vs NHE, was used as a standard; the scan
3 1/2
−1
rate was 100 mV s .
Photocatalytic oxidation of sulfide was investigated in a quartz tube
containing the dyad (0.01 mM), the sulfide derivative (10.00 mM),
and [Co(NH ) Cl]Cl (20.00 mM) in 0.1 M phosphate buffer (5 mL,
Herein we report the synthesis and spectroscopic and
electrochemical properties of two new chromophore−catalyst
dyads comprising a Ru−aqua complex and its chloro complex.
Under visible light irradiation and in the presence of a sacrificial
electron acceptor, both complexes exhibited high catalytic
activity and product selectivity for photocatalytic oxidation of
sulfide into its corresponding sulfoxide in neutral aqueous
solution. A PCET process involving formation of a Ru(IV)−
oxo intermediate was proposed on the basis of photophysical
and electrochemical studies, as well as an isotopic labeling
experiment.
3
5
2
pH 6.8). The reaction tube was equipped with a magnetic stirring bar
and carefully deaerated with argon for 20 min. The reaction tube was
then irradiated by a 3 W white light-emitting diode (LED) light
source. After the reaction, the resulting solution was extracted with
CH Cl (3 × 10 mL). The combined organic layers were dried over
2
2
anhydrous sodium sulfate overnight and filtered, and the solvent was
removed by rotary evaporation to afford the product. The conversion
1
and turnover number (TON) were determined through H NMR
spectroscopy by quantitative analyses of the ratio of integrated peak
intensities of the products and the corresponding substrates.
A standard energy meter (Newport model 842) was used for the
apparent quantum yield (Φ) determination of photocatalytic oxidation
of sulfide. The power of incident monochromatized light of λ = 450
EXPERIMENTAL SECTION
−2 −1
■
nm was determined as 60.0 mW cm s , and the irradiated area was
2
Materials and Characterization Methods. RuCl ·3H O, 4,4′-
confined to 1.0 cm . The 0.1 M phosphate buffer (5 mL, pH 6.8)
3
2
dimethyl-2,2′-bipyridine, 2-acetylpyridine, [Co(NH ) Cl]Cl , 2,2′-
containing complex 3 (0.01 mM), 4-methoxythioanisole (10.00 mM),
3
5
2
bipyridine, NH PF , and other chemicals were purchased from
and [Co(NH ) Cl]Cl (Co(III); 20.00 mM) was irradiated for 1 h,
3 5 2
4
6
1
Shanghai Energy Chemicals Corp. unless otherwise noted. Standard
phosphate buffer solutions at pH 6.8 (0.1 M) were prepared from the
sodium phosphate monobasic and dibasic salts (NaH PO /Na HPO ;
and the photocatalytic reaction was monitored with a H NMR
spectrometer.
The spectroelectrochemical oxidation experiment was conducted on
an electrochemical potentiostat and a spectrophotometer. A platinum
mesh and a platinum wire were employed as the working electrode
and counter electrode, respectively. The Ag/AgCl wire was used as the
reference electrode. The three electrodes were immersed in 0.01 mM
sample solutions in phosphate-buffered saline (PBS) in an electro-
chemical vessel with an optical cell of 1.0 cm optical path length. This
system was loaded with a 1.0 V vs NHE potentiostatic voltage for 2.0
min, and the spectral changes were monitored by UV−vis absorption
spectroscopy simultaneously.
Synthesis of Ligand L (tpy−bpy). 4-Methyl-2,2′-bipyridine-4′-
carbaldehyde was obtained by reacting 4,4′-dimethyl-2,2′-bipyridine
and selenium dioxide in refluxing 1,4-dioxane with stirring for 4 h
under a nitrogen atmosphere. 4-Methyl-2,2′-bipyridine-4′-carbalde-
hyde (400 mg, 2.01 mmol), 2-acetylpyridine (752 mg, 6.02 mmol),
2
4
2
4
Sigma). In pH-dependent experiments, the pH of the sample solutions
was finely adjusted by controlled microvolumetric additions of 3.0 M
sodium hydroxide or 3.0 M triflic acid solution, and the pH values of
the solutions were measured with a pH meter. Organic solvents used
in synthetic procedures were of analytical grade and were used without
further purification. Ru(bpy) Cl , Ru(bpy) Cl , [Ru(tpy)(bpy)Cl]-
2
2
3
2
[
Cl], and [Ru(tpy)(bpy)H O][Cl ] were synthesized according to
2 2
1
literature methods. The purity of each complex was confirmed by H
NMR spectroscopy on the basis of the reported data.
14
1
H NMR spectra were recorded on a Bruker Avance 400
spectrometer with chemical shifts (δ, ppm) reported relative to
tetramethylsilane. Mass spectra were measured on an APEX II model
FT-ICR mass spectrometer. Matrix-assisted laser desorption/ioniza-
tion time-of-flight (MALDI-TOF) mass spectra were obtained on a
Bruker BIFLEX III mass spectrometer. Elemental analyses were
performed on a Vario EL III instrument. UV−vis absorption spectra
were recorded on a Hitachi U-3010 spectrophotometer. Flash
photolysis was carried out on an Edinburgh Instruments LP900 flash
photolysis spectrometer. The mixture was excited with 8 ns pulses of a
and aqueous NH (10 mL, 12.04 mmol) were dissolved in ethanol (50
3
mL). NaOH (240 mg, 6.00 mmol) was added to the solution, which
was then heated at reflux for 48 h. After being cooled to room
temperature, the resulting brown solution was kept in a refrigerator.
After 12 h a residue was collected by filtration and washed with water
B
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Inorganic Chemistry
Article
Scheme 1. Synthesis of the Chromophore-Catalyst Supramolecules
(
50 mL) followed by cold EtOH (50 mL). The precipitate was
(ppm): 158.2, 157.3, 157.2, 157.1, 156.8, 156.5, 155.1, 152.3, 152.0,
151.9, 151.7, 150.9, 150.5, 150.0, 147.2, 146.6, 138.5, 138.3, 129.5,
128.5, 126.6, 125.5, 125.3, 125.2, 125.0, 122.9, 121.8, 119.5, 21.3.
purified by column chromatography using silica gel and dichloro-
methane/methanol (70:1, v/v) as the eluent. The solvent was
evaporated to give L as a a white solid (300 mg, 75% yield).
Synthesis of Supramolecule 2 (Ruphot−Ru −Cl). Complex 1
cat
+
MALDI-TOF-MS (m/z): calcd 402.2 [M + H] , found 402.2; calcd
(600 mg, 0.70 mmol) and RuCl ·3H O (200 mg, 0.70 mmol) were
3
2
+
1
4
8
24.2 [M + Na] , found 424.2. H NMR (400 MHz, CDCl ): δ (ppm)
dissolved in ethanol (20 mL). The resulting solution was refluxed for 5
h. After the solution was cooled to room temperature, the solvent was
removed by rotary evaporation, and the resulting brown solid was
3
.90 (d, 1H, J = 1.0 Hz), 8.86 (s, 2H), 8.82 (d, 1H, J = 5.1 Hz), 8.74
(
d, 2H, J = 4.0 Hz), 8.68 (d, 2H, J = 7.9 Hz), 8.60 (d, 1H, J = 4.9 Hz),
.31 (s, 1H), 7.89 (td, 2H, J = 7.8, 1.7 Hz), 7.80 (dd, 1H, J = 5.0, 1.0
8
dissolved in an ethanol/H O solution (30 mL; 3:1, v/v) followed by
2
the addition of 2,2′-bipyridine (bpy) (124.2 mg, 0.80 mmol), LiCl
(170 mg, 4.00 mmol), and triethylamine (0.20 mL). The solution was
degassed by argon bubbling and heated at reflux for 12 h. The hot
mixture was filtered, and the solvent volume was reduced by rotary
evaporation. The complex was precipitated by the addition of excess
NH PF and filtered. The crude product was purified by column
H). 13C NMR (101 MHz, CDCl ): δ (ppm) 158.1, 157.2, 156.7,
4
6
in an ethanol−water solution (20 mL; 1:1, v/v). The resulting solution
was degassed for 15 min by argon bubbling and then refluxed for 5 h.
A red precipitate was collected by filtration from the hot solution. This
material was dissolved in a minimum volume of water and
reprecipitated by the addition of excess NH PF . After filtration,
complex 1 was obtained as a red powder (80 mg, 66% yield).
Electrospray ionization mass spectrometry (ESI-MS; m/z): calcd 407.6
chromatography using silica gel and a mixture of acetonitrile/saturated
KNO aqueous solution (20:1, v/v) as the eluent to give a red solid;
3
this solid was dissolved in a minimum volume of water and
reprecipitated by the addition of excess NH PF to give 2 (210 mg,
4
6
+
60% yield). HR-ESI-MS (m/z): calcd 1398.0762 [M − PF ] , found
4
6
6
2
+
1398.0798; calcd 626.5560 [M − 2(PF )] , found 626.5554; calcd
6
3+
1
369.3826 [M − 3(PF )] , found 369.0458. H NMR (400 MHz,
6
2
+
1
[
(
2
M − 2(PF )] , found 407.6. H NMR (400 MHz, DMSO-d ): δ
CD CN): δ (ppm) 10.21 (d, 1H, J = 4.9 Hz), 9.11 (d, 1H, J = 1.7 Hz),
6
6
3
ppm) 9.30 (s, 1H), 9.13 (s, 1H), 8.86 (t, 6H, J = 10.7 Hz), 8.77 (d,
H, J = 4.2 Hz), 8.71 (d, 2H, J = 8.0 Hz), 8.19 (dd, 4H, J = 14.6, 7.7
Hz), 8.08 (td, 2H, J = 7.7, 1.6 Hz), 8.00 (dd, 1H, J = 6.0, 1.6 Hz), 7.88
d, 1H, J = 5.6 Hz), 7.85(d, 1H, J = 6.0 Hz), 7.79 (t, 2H, J = 6.1 Hz),
.76 (d, 1H, J = 5.4 Hz), 7.61 (d, 1H, J = 5.8 Hz), 7.56 (m, 6H), 7.42
d, 1H, J = 5.6 Hz), 2.56 (s, 3H). 13C NMR (101 MHz, DMSO-d ): δ
8.88 (s, 2H), 8.76 (s, 1H), 8.63 (d, 1H, J = 8.2 Hz), 8.56 (m, 6H), 8.31
(m, 2H), 8.09 (m, 5H), 7.99 (m, 2H), 7.92 (m, 3H), 7.81 (d, 2H, J =
5. Six Hz), 7.77 (d, 1H, J = 5.0 Hz), 7.70 (d, 3H, J = 6.2 Hz), 7.63 (d,
1H, J = 5.8 Hz), 7.46 (m, 4H), 7.32 (m, 4H), 6.94 (t, 1H, J = 7.2 Hz),
2.62 (s, 3H). Anal. Calcd for C H ClF N P Ru ·1.5H O: C, 42.85;
(
7
(
56
43
18 11
3
2
2
H, 2.95; N, 9.82. Found: C, 42.68; H, 2.82; N, 9.65.
6
C
dx.doi.org/10.1021/ic5020972 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Synthesis of Supramolecule 3 (Ruphot−Ru −H O). Complex 2
cat
2
(
50 mg, 0.041 mmol) and AgClO (90 mg, 0.41 mmol) were dispersed
4
in a water/acetone solution (30 mL; 4:1, v/v) and heated at reflux for
2 h in the dark under an argon atmosphere. After the solution was
1
cooled to room temperature, the white solid AgCl was filtrated, and
the filtrate was concentrated in vacuum. Addition of excess NH PF6
4
led to precipitation of a red solid. The crude product was recrystallized
from diethyl ether to give a deep red solid (33 mg, 66% yield). HR-
+
ESI-MS (m/z): calcd 1382.1112 [M − 2(PF ) − H] , found
6
2
+
1
382.1079; calcd 618.0768 [M − 3(PF ) − H] , found 618.0717.
H NMR (400 MHz, (CD ) CO): δ (ppm) 9.74 (d, 1H, J = 5.1 Hz),
6
1
3
2
9
.35 (d, 1H, J = 1.4 Hz), 9.15 (d, 2H, J = 3.4 Hz), 8.98 (s, 1H), 8.81
(
dd, 2H, J = 8.1, 3.2 Hz), 8.77 (d, 1H, J = 8.2 Hz), 8.68 (t, 4H, J = 8.0
Hz), 8.48 (m, 2H), 8.21 (m, 6H), 8.13 (m, 3H), 8.04 (d, 1H, J = 5.4
Hz), 7.94 (m, 3H), 7.87 (d, 1H, J = 5.1 Hz), 7.84 (d, 2H, J = 5.4 Hz),
7
.74 (d, 1H, J = 5.8 Hz), 7.55 (m, 6H), 7.45 (d, 2H, J = 5.8 Hz), 7.20
(
t, 1H, J = 6.1 Hz), 6.05 (s, 2H), 2.74 (s, 3H). Anal. Calcd for
C H F N OP Ru ·2H O: C, 40.27; H, 2.72; N, 9.23. Found: C,
5
6
45 24 11
4
2
2
3
9.89; H, 2.52; N, 8.89.
RESULTS AND DISCUSSION
Figure 1. Perspective view and labeling scheme for ligand L with
thermal ellipsoids at 30% probability. Hydrogen atoms are omitted for
clarity.
■
Synthesis, Characterization, and Structure. The
synthetic route for supramolecules is shown in Scheme 1.
First, the monometallic complex 1 was prepared by refluxing
Ru(bpy) Cl with the bridging tpy−bpy ligand L. This complex
2
2
energy. This suggests that the extinction coefficient of the
MLCT for the assembly is almost the sum of the MLCT molar
was reacted further with RuCl ·3H O to generate a dinuclear
3
2
intermediate, which was treated with 1 equiv of bpy to afford
absorptions for Ruphot and Ru −Cl species and indicates weak
cat
−
Ruphot−Ru −Cl with three Cl anions. Finally, the complex
cat
electronic coupling between the two metal sites in complex 2.
Therefore, high electron injection efficiencies from the catalytic
center to the chromophore fragment may be expected, as well
as diminished excited-state electron recombination. However, it
should be noted that the weak electronic coupling between the
Ruphot and Rucat shifts the position of the MLCT transition
absorption for two moieties compared with their corresponding
mononuclear complexes (Table 1), whereas the UV−vis
absorption spectra of Ru −Ru −H O displayed one
−
was purified on silica gel and then converted to the PF6 salt by
addition of excess NH PF in moderate yield. The complex
4
6
Ruphot−Ru −H O was obtained by directly reacting the chloro
cat
2
16
complex with AgClO in aqueous solution at reflux. Dinuclear
complexes 2 and 3 were characterized by H NMR spectros-
4
1
copy, high-resolution mass spectrometry, and elemental
analysis. Complex 2 showed one distinct doublet at low field
(
δ ≈ 10.21 ppm), which was assigned to the bpy proton closest
phot
cat
2
to the chloro ligand. In the case of complex 3, this doublet was
shifted upfield (δ ≈ 9.74 ppm) due to the larger diamagnetic
anisotropy of the aqua ligand. The presence of H O as a
MLCT absorption with λmax at 495 nm, which corresponds
with the overlapping MLCT band of the Ruphot and Ru −H O
cat
2
17
2
fragments. Additionally, the lowest energy band shifts to a
1
shorter wavelength when the Cl− ligand in complex 2 is
ligand in complex 3 was demonstrated by a H NMR
spectroscopy exchange experiment. The singlet signal at 6.05
displaced by H O to form a Ru−aqua complex. This is caused
2
−
ppm measured in acetone-d disappeared immediately after
6
by the absence of a π-donation effect from the Cl ligand and
1
9
addition of 0.1 mL of D O, suggesting the existence of H O/
2
2
increases the HOMO−LUMO energy gap. The two
complexes showed negligible emission, and their emission
lifetimes were shorter than 1 ns, in contrast to the mononuclear
complex 1, which exhibits intense emission with a λmax at 630
nm. This weak emission can be attributed to quenching of the
excited state of the Ruphot site by the Rucat unit.
D O exchange. The mass spectrum of the chloro complex 2
2
displayed a single-charge ion at m/z = 1398.0798, which was
+
ascribed to [2 − PF ] , the another two ESI-MS signals at m/z
6
=
626.5554 and 369.0458 that could be assigned to [2 −
2+ 3+
2
(PF )] and [2 − 3(PF )] , respectively. The mass spectrum
6
6
of complex 3 showed an ESI-MS signal at m/z = 1382.1079,
The time-resolved absorption difference spectra of complex 3
were recorded following flash photolysis at 355 nm. As depicted
in Figure 3, the spectra showed absorption peaks at about 385
and 590 nm (τ = 73 ns). However, in the presence of electron
acceptor Co(III), the signal intensities are below the resolution
of our detection system, and the excited-state lifetime was
reduced to 41 ns (Figure S12, Supporting Information),
indicating addition of excess Co(III) promoted an intra-
−
which was consistent with the tetracation plus two PF6 ions
minus one H ion. A single crystal of ligand L was obtained by
+
slow diffusion of diethyl ether into dichloromethane solution
containing L at room temperature. The corresponding
molecular structure is shown in Figure 1. Detailed crystal
data are listed in Table S1 (Supporting Information).
Spectroscopic Properties. The absorption properties of
complexes 2 and 3 in phosphate buffer at pH 6.8 were
investigated, and the data are listed in Table 1. As shown in
molecular electron transfer from Ru to *Ruphot through the
cat
1
6,19
intermolecular interaction.
Figure 2, Ruphot−Ru −Cl displayed strong and broad
Electrochemical Properties. The redox potentials of
complexes 2 and 3 in aqueous phosphate buffer were
investigated by CV and DPV, and the results are listed in
cat
absorption bands in the visible light region with λmax at 470
and 525 nm, attributed to metal-to-ligand charge transfer
(
MLCT) transitions of the Ru
moiety and Ru −Cl unit,
Table 1. CV of the chloro dinuclear complex Ruphot−Ru −Cl
phot
cat
cat
1
8
respectively. The MLCT absorptions originating from Ru
(Figure S13, Supporting Information) in phosphate buffer (0.1
phot
and Ru −Cl moieties in Ru −Ru −Cl are separated in
M, pH 6.8) exhibited one reversible redox process at 1.00 V
cat
phot
cat
D
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Inorganic Chemistry
Article
Table 1. Spectroscopic Data and Electrochemical Results for Complexes 2 and 3 and Their Mononuclear Moieties in 0.1 M
Aqueous Phosphate Buffer at pH 6.8
λmax/nm (ε/M− cm−1
1
)
E1/2 /V vs NHE (ΔE)
Ru(III/II,IV/III)cat
a
compd
ππ*
dπRu(phot or cat)π*
Ru(III/II)phot
1.26 (60)
[
Ru(bpy) ]2+
290 (88000)
420 (13000)
3
4
65 (17000)
[
[
Ru(bpy)(tpy)Cl]+
290 (48000)
290 (41000)
500 (10000)
480 (11000)
1.00 (58)
2
+
Ru(bpy)(tpy)H O]
0.75 (53), 0.80 (52)
2
3
20 (41000)
Ruphot−Ru −Cl
290 (90000)
470 (18000)
1.00 (71)
1.32 (63)
1.32 (65)
cat
5
25 (18500)
Ruphot−Ru −H O
290 (108000)
495 (22000)
0.77 (110)
cat
2
a
E1/2 = (Eox + E )/2 in volts, and ΔE = E − Ered in millivolts.
red
ox
present work, the single bond of the tpy−bpy bridge reduces
charge trapping between the Ru and Ru centers. CV of the
phot
cat
aqua complex 3 showed two apparently reversible redox
processes at 0.77 and 1.32 V in aqueous solution. As shown in
Figure 4, the redox occurring at 1.32 V is ascribed to the
2
+
+
Figure 2. Absorption spectra for [Ru(bpy) ] , [Ru(tpy)(bpy)Cl] ,
3
2+
Ruphot−Ru −Cl, [Ru(tpy)(bpy)H O] , and Ru −Ru −H O in
.1 M aqueous phosphate buffer at pH 6.8 and room temperature.
cat
2
phot
cat
2
0
2+
Figure 4. Cyclic voltammograms of [Ru(bpy) ] (black), [Ru(tpy)-
3
2+
(
bpy)H O] (red), and complex 3 (blue) in 0.1 M aqueous
2
phosphate buffer (pH 6.8) at 25 °C. The data were recorded at a
−
1
scan rate of 100 mV s in air.
Ru(II)/Ru(III) redox course of the Ru
center, while the
phot
2+
mononuclear [Ru(tpy)(bpy)H O] species displayed two
2
successive redox courses at 0.75 and 0.80 V under the same
conditions which are assigned to two PCET processes of
Ru(II)−OH /Ru(III)−OH and Ru(III)OH/Ru(IV)O,
2
1
2,22
respectively.
The different electrochemical behavior of the
Rucat site in complex 3 may be caused by the overlap of two
one-electron redox processes with an average potential of 0.77
V. To gain further insight into the redox process at the Rucat
site in complex 3, we measured the pH dependence of the
Figure 3. Transient absorption spectra of complex 3 in phosphate
buffer (0.1 M, pH 6.8) under an argon atmosphere with an 8 ns laser
pulse at 355 nm.
potentials for complex 3 in aqueous solution for pH values
ranging from 1.0 to 13.0 (Figure S14, Supporting Information).
The Pourbaix diagram for complex 3 shows that the two
consecutive PCET processes followed well-defined lines with
the slope close to −59 mV per pH unit over the range of pH
and one quasi-reversible redox course at 1.32 V vs NHE. In
comparison, the redox potentials of complex 2 are similar to
those of its corresponding mononuclear fragments [Ru(tpy)-
−
+
2−11, ascribed to two 1e /1H PCET processes of Ru(II)−
+
2+
20
(
bpy)Cl] (1.00 V) and [Ru(bpy) ] (1.26 V). Previous
OH /Ru(III)−OH and Ru(III)OH/Ru(IV)O redox
3
2
investigations reported by Rocha and his colleagues have found
that substitution of the tpy−tpy bridging ligand by a more
conjugated bridging backbone 2,3,5,6-tetrakis(2-pyridyl)-
pyrazine results in charge delocalization across the bridging
couples. The narrow E1 separations indicate a very efficient
/2
redox potential leveling in this catalyst. Moreover, the Ru(II)/
14
Ru(III) oxidation of the Ruphot center is pH-independent.
Figure 4 shows that intramolecular electron transfer from
2
1
ligand and thus lowers the photocatalytic efficiency. In the
Ru −H O to the photogenerated Ru(III) moiety in 3, with
cat
2
phot
E
dx.doi.org/10.1021/ic5020972 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a driving force of 550 mV, would be thermodynamically
favorable in water via PCET processes. In contrast, because
there is no PCET process involved, even though the Cl ligand
supramolecular assembly exhibits superior photocatalytic
activity compared with a multicomponent system. Additionally,
control experiments confirmed that light, complex, and
sacrificial electron acceptors are necessary for photooxidation
of sulfide. Notably, when an additional 20 mM Co(III) was
added to the Ruphot−Ru −H O system after reaction for 8 h,
−
is a strong π-electron donor, the oxidation potential of
Ru(III) /Ru(II) in Ruphot−Ru −Cl is higher than that of
cat
cat
cat
the Ru(II)OH /Ru(IV)O couple for its aqua complex
2
cat
2
2
3
(
Table 1).
the resulting solution was irradiated for another 4 h and the
TON increased to nearly 1000. This highlights the excellent
photostability of the catalysts. Moreover, we measured the
apparent quantum yield (Φ) of this reaction. The phosphate
buffer (0.1 M, 5 mL, pH 6.8) containing complex 3 (0.01 mM),
4-methoxythioanisole (10.00 mM), and Co(III) (20.00 mM)
was irradiated by monochromatized light of λ = 450 nm (the
Photooxidation of Sulfide. Light-driven oxidation of
24
sulfide was performed using Co(III) as a sacrificial electron
acceptor in deoxygenated phosphate buffer (0.1 M, pH 6.8)
with a complex (2 or 3):substrate:Co(III) mole ratio of
1:1000:2000 under white LED irradiation (λ >380 nm) at room
temperature. The corresponding products were extracted with
1
−2 −1
dichloromethane and studied by H NMR spectroscopy. In the
power of the light is 60.0 mW cm s ) for 1 h, a TON of 60
presence of a 4-methoxythioanisole substrate, the sulfide was
oxidized into the corresponding sulfoxide with 99% product
selectivity and a TON of up to 709 was achieved on the basis of
Ruphot−Ru −H O after 8 h of irradiation. Ru −Ru −Cl is
was achieved, and the apparent quantum yield was determined
to be 0.7% by monitoring the formation of the product with H
1
NMR spectroscopy. Different substrates were also used to
study the scope of photocatalytic activities of both complexes.
The results are summarized in Table 2. All the sulfides were
converted into their corresponding sulfoxides without for-
mation of sulfones. Substrates bearing electron-donating groups
gave better results than those bearing electron-withdrawing
groups, suggesting an electronic effect. The photooxidation of a
cat
2
phot
cat
the precatalyst, and it would convert to its aqua form by a fast
Cl¯/H O ligand exchange to achieve a lower efficiency with a
2
TON of 645 under the same conditions, as shown in Figures 5
sulfide bearing a −NH substituent could be achieved in higher
2
TON than that of a sulfide bearing a −NO group for both
2
catalysts Ru −Ru −H O and Ru −Ru −Cl.
phot
cat
2
phot
cat
Mechanistic Interpretation. Inspired by the high
efficiency of the present system, we were motivated to study
the catalytic mechanism. The changes in species following
electrochemical oxidation of Ru(II)phot−Ru(II) −H O at an
cat
2
applied potential of 1.0 V were monitored by UV−vis
absorption spectra. As shown in Figure 7, the MLCT band
with λmax at 495 nm originating from the complex Ruphot
−
Ru −H O blue-shifted to 490 nm, and the absorption band
cat
2
with λ at 522 nm ascribed to an MLCT transition of the
max
Rucat moiety in 3 disappeared during the electrochemical
reaction. A comparison of the spectral data for MLCT bands in
Figure 5. Turnover number for the photocatalytic oxidation of 4-
methoxythioanisole by 2 (red) and 3 (black) under argon as a function
of the irradiation time.
Table 1 and Figure 7 indicates that oxidation of the Ru −OH
cat
2
unit into Ru(III) OH/Ru(IV) O occurred through
cat
cat
21a
application of a 1.0 V potential. The remaining λ at 490
max
and 6. The efficiency of the catalytic photooxidation of sulfide
nm is assigned to the MLCT excitation of the Ruphot moiety.
2
+
in a solution containing [Ru(bpy) ] (0.01 mM), [Ru(tpy)-
3
Moreover, this behavior was clearly observed for Ruphot−Ru −
2+
cat
(
bpy)H O] (0.01 mM), 4-methoxythioanisole (10 mM), and
2
Cl, as shown in Figure S15 (Supporting Information). Upon
Co(III) (20 mM) was markedly lower (with a TON of only
01 after 8 h) compared with that of the supramolecular system
irradiation of an aqueous solution containing Ruphot−Ru −
cat
1
OH and excess Co(III), we found that the intensity of the
2
at identical reaction conditions. This indicates that the
MLCT absorption at 522 nm, ascribed to Ru(II) −OH ,
cat
2
declined and that of the Ru(II)
unit became more obvious,
phot
indicating that a photoinduced electron transfer from Ru-
II) −OH to Ru(III)phot occurred. The presence of the
(
cat
2
Ru(IV)O intermediate was detected by HR-ESI mass spectra
1
1b
in our previous work under the same conditions.
Furthermore, the initial MLCT absorption band of Ru-
II)phot−Ru(II) −H O could be recovered on addition of 4-
(
cat
2
methoxythioanisole to the above solution under dark
conditions, caused by electron transfer from sulfide to
Ru(IV)O (Figures S16 and S17, Supporting Information).
An investigation into the role of H O in the photocatalytic
2
reaction can help us to further understand the mechanistic
nature of this PCET process. An isotopic labeling experiment
was conducted using a 2:1 mixture of H216O and H2 O as the
solvent and 4-methoxythioanisole as the substrate. The
products of photocatalytic oxidation after irradiation for 8 h
were extracted by dichloromethane, and the solvent was
18
1
Figure 6. H NMR (CDCl ) spectra of the substrate (4-
3
methoxythioanisole) and product using 3 as the photocatalyst in the
photooxidation reaction at different reaction times.
F
dx.doi.org/10.1021/ic5020972 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 2. Photocatalytic Oxidation of Sulfide Using Complex 3 in Phosphate Buffer
a
Conditions: [cat] = 0.01 mM, [substrate] = 10 mM, [Co(III)] = 20 mM; the reaction was processed in 0.1 M phosphate buffer (pH 6.8) for 8 h
b
under an argon atmosphere using an LED (λ > 380 nm) as the light source. Addition of another 20 mM Co(III) into the reaction solution and
illumination for a further 4 h.
Figure 8. Corresponding labeled and unlabeled sulfoxide obtained in
an unbuffered degassed H2 O/H2 O (2:1) mixture. The fragments at
Figure 7. Changes of visible absorption spectra following the
spectroelectrochemical oxidation of Ru(II)phot−Ru(II) −H O
16
18
cat
2
m/z 193.0292, 194.0307, and 195.1731 are assigned to unlabeled
(
black) into its Ru(II)photRu(IV) O intermediate by applying a
cat
+
sulfoxide plus one Na adduct. The fragments at m/z 195.0610+,
potential of 1.0 V over 120 s.
1
96.0724, and 197.0699 are assigned to labeled sulfoxide plus one Na
adduct.
removed by rotary evaporation. The obtained product was
characterized by ESI-MS. The mass spectra (Figure 8; Figure
the Ru(II) −H O moiety was observed in CV experiments.
cat
2
+
The generated Ru(IV)O species is considered to be the
active intermediate for the oxygenation of sulfide to the
1
1e,f
corresponding sulfoxide with high efficiency and selectivity.
sulfoxides. These results confirm that water is the oxygen
source for the photooxidation reaction of the organic substrate
and that a PCET process occurs. On the basis of previous
reports and these results, a proposed catalytic mechanism is
In conclusion, we have synthesized two new and structurally
simple chromophore−catalyst dyads, Ru −Ru −Cl and
phot
cat
Ru −Ru −H O. The single bond in the tpy−bpy bridging
phot
cat
2
ligand decreases charge trapping at the linker between the
Ruphot and Rucat sites. Both complexes showed remarkable
visible-light-driven catalytic activities with TONs of up to 1000
(Ruphot−Ru −H O) and high product selectivity (>99%)
3
a,12,25
presented in Scheme 2.
The excited-state *Ru(II)phot
moiety can be quenched by the sacrificial electron acceptor
Co(III), which provides a driving force for oxidizing the
neighboring Ru(II) −H O moiety to generate Ru(IV)O
cat
2
toward oxidation of sulfide to the corresponding sulfoxide in
neutral aqueous solution. Spectroscopic and electrochemical
investigations revealed an excited-state *Ru(II)phot moiety
initiates the PCET process, which results in the formation of
cat
2
species through subsequent PCET processes. However, a
mechanism involving successive two-electron two-proton
oxidation processes cannot be entirely ruled out because an
overlapping two-electron electrocatalytic oxidation process for
an active Ru(IV) O intermediate in the presence of one-
cat
G
dx.doi.org/10.1021/ic5020972 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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ASSOCIATED CONTENT
Supporting Information
■
(
*
S
X-ray crystallographic data for ligand L in CIF format, NMR
spectra of the compounds, electronic absorption and HR-ESI
mass spectra of the complexes, results of electrochemical
studies, and photocatalytic oxygenation results. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
E-mail: fuwf@mail.ipc.ac.cn.
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Notes
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The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
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Huang, F.; Gao, Y.; Sun, L. Angew. Chem., Int. Ed. 2012, 51, 2417−
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This work was supported by the National Key Basic Research
Program of China (973 Program 2013CB834804) and by the
Ministry of Science and Technology of China (Grant
2
012DFH40090). We thank the National Natural Science
Foundation of China (NSFC Grants 21267025, 21273257,
21367026, and U1137606) for financial support.
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dx.doi.org/10.1021/ic5020972 | Inorg. Chem. XXXX, XXX, XXX−XXX