Robust Cooperative Photo-oxidation of Sulfides without Sacrificial Reagent under Air Using a Dinuclear RuII–CuII Assembly

Article abstract of DOI:10.1002/cssc.201700930

A molecular chromophore–catalyst assembly containing a chromophore ruthenium(II) center (Ru^II_chro) and a catalytic copper(II) center (Cu^II_cat) has been prepared easily. The assembly was employed for photocatalyt

Full text of DOI:10.1002/cssc.201700930

DOI: 10.1002/cssc.201700930
Communications
Robust Cooperative Photo-oxidation of Sulfides without
Sacrificial Reagent under Air Using a Dinuclear RuII–CuII
Assembly
Duobin Chao* and Mengying Zhao[a]
A molecular chromophore–catalyst assembly containing a
sulfides can generate sulfoxides that are an important class of
compounds in organic synthesis and pharmaceuticals.[8] Many
homogeneous photocatalytic systems have been developed
for this transformation using metal complexes and organic
dyes.[9] However, loading of catalysts is still relatively high. For
instance, several chromophore–catalyst assemblies containing
ruthenium(II)-based chromophore were studied for oxidation
of sulfides using water as an oxygen source.[4b,c,6c,10] However,
these photocatalytic procedures displayed limited turnover
numbers (TONs) of about 1000. In addition, sacrificial reagents
are indispensable. Organic dyes such as BODIPY (boron-dipyr-
romethene) can also be used for photocatalytic oxidation of
sulfides, whereas these dyes suffered low stability upon irradia-
tion in the presence of O2 and afford low TONs of about
200.[11] Molecular O2 is known as a green and abundant oxi-
dant. Highly efficient activation of O2 for selective oxidation of
II
chromophore ruthenium(II) center (Ru ) and a catalytic cop-
chro
II
cat
per(II) center (Cu ) has been prepared easily. The assembly
was employed for photocatalytic oxidation of sulfides without
sacrificial reagent in the presence of dioxygen under blue light
irradiation. Unprecedented turnover number (TON) up to
32000 was achieved. It was elucidated that an electron trans-
II
II
*
ferred from excited state of chromophore Ruchro to Cucat along
with generation of CucI at that was further activated by O2.
These results demonstrate a promising strategy for efficient
cooperative photocatalytic reactions under air using the
chromophore–catalyst assembly.
Transition metal complexes play an important role in photo-
catalytic reactions.[1] In addition to mononuclear metal com-
plexes,[2] supramolecular complexes known as chromophore– organic substrates is promising and challenging. In nature, O2
catalyst assemblies are widely employed for various photocata-
lytic processes.[3] These assemblies commonly contain a light-
absorbing chromophore such as [Ru(bpy)3]2+ and a catalytic
site such as high-valent metal–oxo species RuIV(O) to achieve
efficient cooperative catalysis.[4] They are usually used in the
presence of sacrificial electron donors or electron acceptors.
For instance, triethylamine (TEA), triethanolamine (TEOA), and
ascorbic acid have been employed as electron donors in pho-
tocatalytic hydrogen evolution and CO2 reduction.[5] Electron
is capable of participating in redox processes by reacting with
the metal centers of various enzymes.[12] Inspired by these en-
zymes, great efforts have been devoted to developing efficient
aerobic catalysis. Copper compounds are usually utilized in
aerobic reactions to activate O2 because of the facile accessibil-
ity of versatile copper oxidation states.[13] Nevertheless, photo-
catalytic aerobic reactions using copper-based catalytic sites
are rarely investigated.[14] A ruthenium(II)–copper(II) dyad was
used for photocatalytic oxidation of organic sulfides in the
presence of a sacrificial electron donor (TEOA) under O2 atmos-
phere.[15] The highest TON was found to be 300. It has been re-
ported that peripheral ligands and bridging ligands in chromo-
phore–catalyst assemblies have significant impacts on the
overall catalytic efficiency and even the catalytic mecha-
nism.[6d,16] Therefore, we anticipate much more efficient photo-
catalytic process for organic sulfides using a new chromo-
phore–catalyst assembly without sacrificial reagents under air.
Herein, we report a new chromophore–catalyst assembly
2À
acceptors including S2O8 and [Co(NH3)5Cl]Cl2 can be utilized
in photocatalytic oxidation of H2O and organic substrates.[6]
These sacrificial reagents are essential to fulfill the whole cata-
lytic cycle. However, abundant sacrificial reagents are convert-
ed into useless byproducts, resulting in relatively low-atom
economy. Moreover, sacrificial reagents may also enhance the
complexity of photocatalytic processes. Consequently, it is
promising to develop highly efficient photocatalytic proce-
dures without sacrificial reagents.
II
II
Numerous photocatalytic reactions have been well estab-
lished and photocatalytic oxidation of organic substrates has
recently received great attention.[7] Particularly, selective oxida-
tion of organic sulfides is of significant interest.[8] Oxidation of
(abbreviated as Ru –Cu ) for robust photocatalytic oxidation
chro cat
of organic sulfides (Scheme 1). The assembly contains a
[Ru(deeb)2(dmbpy)]2+ (deeb=4,4’-diethylester-2,2’-bipyridine;
dmbpy=4,4’-dimethyl-2,2’-bipyridine) unit as the light-absorb-
II
ing chromophore (Ru ) and one [Cu(tpy)Cl2] (tpy=2,2’:6’,2’’-
chro
II
cat
terpyridine) fragment as the catalytic site (Cu ). Sulfides were
[a] Dr. D. Chao, M. Zhao
selectively oxidized into sulfoxides under mild conditions with-
out sacrificial reagents. Molecular O2 served as the terminal oxi-
dant and oxygen atom source. More importantly, the highest
TON was calculated to be 32000. Regardless of the condition,
it is the highest TON reported to date as far as we know for
photocatalytic oxidation of sulfides.
School of Petroleum and Chemical Engineering
Dalian University of Technology
Panjin, Liaoning (P. R. China)
E-mail: chaoduobin@dlut.edu.cn
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cssc.201700930.
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II
chro
II
cat
Scheme 1. Photocatalytic oxidation of sulfides with Ru –Cu
.
II
chro
Figure 1. Photoluminescence (PL) spectra of Ru tpy (16 mm) upon addition
of Cu2+ (0–18 mm) in mixed acetonitrile/water (v/v, 8:2) solution. Inset: corre-
sponding plot of PL intensity at 650 nm vs. [Cu2+].
II
chro
II
cat
The dinuclear complex Ru –Cu was obtained through
II
facile synthetic process (Scheme S1). In the last step, Ru tpy
reacted with CuCl2 to form RuIcIhro–Cu at room temperature
chro
II
(Figure S7). The final spectrum was identical to that of
cat
II
II
II
II
by taking advantage of the high affinity between tpy ligand
and CuII.[17] First, Ru –Cu was characterized by high-resolu-
Ru –Cu , in accordance with the synthesis of Ru –Cu by
chro cat chro cat
II
II
II
reacting Ru tpy with CuCl2. The emission quenching of
chro
chro
cat
II
II
II
tion electrospray ionization mass spectrometry (HR-ESI-MS)
and elemental analysis. As shown in Figure S1, the signal peak
at 619.0797 m/z (calcd 619.0768) is assigned to [M]2+ (M=
Ru –Cu may be a result from the Cu group owing to
chro cat cat
photoinduced electron transfer or energy transfer between
II
chro
II
cat
II
Ru
and Cu . In consideration of no absorption of Cu ob-
cat
II
II
II
chro
Ru –Cu ), and the isotopic signals are located with an inter-
served in the emission wavelength of Ru
from 550 to
chro
cat
val of 0.5 (Figure S2), which is in consistent with its +2 charg-
es. In addition, the signal at 401.0628 m/z (calcd 401.0618)
with an isotopic interval of 0.33 is assigned to [M-Cl]3+ (Fig-
ure S3). These results confirm the chemical structure of
750 nm, and the thermodynamically favorable electron transfer
II
II
*
from Ruchro to Cucat, we believe that electron transfer is the
II
II
major factor for negligible emission of Ru –Cu instead of
chro
cat
energy transfer.
II
II
II
II
*
Ru –Cu and indicate the chloride ion is liable to leave.
Encouraged by an electron transfer from Ruchro to Cucat, we
were interested in its photocatalytic performance. Initially, thio-
anisole was selected as the substrate owing to its suitable oxi-
dation potential (+1.34 V vs. SCE).[19] The reaction was per-
chro
cat
II
chro
II
cat
The assembly Ru –Cu was then characterized by cyclic
II
chro
II
cat
voltammetry experiments. Ru –Cu shows two reversible
waves at +0.54 and +1.49 V versus SCE (standard calomel
II
II
electrode), respectively (Figure S4). The first wave at +0.54 V is
formed in acetonitrile solution containing Ru –Cu upon ir-
chro cat
II
assigned to CuIcat/Cu and the second wave at +1.49 V can
radiation with blue LEDs (450Æ10 nm, 3 W) under air at room
temperature. Surprisingly, we found trace of product (Table 1,
entry 1). However, after addition of a small number of water
(CH3CN/H2O, v/v, 9:1), the TON clearly increased to 1600
(Table 1, entry 2). This is probably because the key intermedi-
ate persulfoxide is stable through hydrogen bonding in the
presence of water.[20] We then found the mixed solvent
(CH3CN/H2O, v/v, 8:2) was appropriate for photocatalytic oxida-
tion of thioanisole (Table 1, entry 3). More addition of water
into acetonitrile could reduce the solubility of thioanisole, re-
cat
II
III
be assigned to Ru /Ru , by comparing with separated
chro
chro
Cu(tpy)Cl2 (Figure S5) and [Ru(deeb)2(dmbpy)]2+ [18]
The oxida-
.
II
tion potential of the excited state of lone Ru
moiety
chro
(Ruchro/Ruchro) was À0.53 V versus SCE,[18] which is significantly
II
III
*
II
cat
lower than the reduction potential of CuIcat/Cu (+ 0.54 V vs.
II
*
SCE). Thus, an electron transfer from the excited state Ruchro to
II
Cu is thermodynamically favorable. That is to say, a new spe-
cat
cies Ru –CuIcat would be generated when Ru –Cu is stimu-
III
chro
II
chro
II
cat
lated under light irradiation. CuIcat would possibly reduce an ox-
idant such as O2, whereas RuIcIhI ro would oxidize a reductant
such as an organic substrate. Then, Ru –CuIcat can return to
III
chro
II
chro
II
cat
Table 1. Photocatalytic oxidation of thioanisole into sulfoxide with
the ground state Ru –Cu . At this stage, a complete photo-
II
chro
II [a]
cat
Ru –Cu
catalytic cycle in the presence of reducing organic substrates
can be expected under air.
Entry
Solvent (ratio)
Atmosphere
TON[b]/Yield [%]
II
II
We further investigated Ru –Cu through absorption and
chro
cat
1
2
3
4
CH3CN
air
air
air
air
air
air
N2
air
trace
1600/16
3200/32
2800/28
2000/20
0/0
II
II
emission spectroscopy. Ru –Cu has moderate absorption in
CH3CN/H2O (9:1)
CH3CN/H2O (8:2)
CH3CN/H2O (7:3)
CH3CN/H2O (6:4)
CH3CN/H2O (8:2)
CH3CN/H2O (8:2)
CH3CN/H218O (8:2)
chro
cat
the visible region from 400 to 500 nm (Figure S6), indicating it
is suitable for light-driven reactions under blue light.
5
II
II
Ru –Cu was almost nonemissive when it was excited at
6[c]
7
chro
cat
450 nm at room temperature. Under the same conditions,
0/0
0/0[d]
II
chro
8
Ru tpy exhibited much stronger emission at 650 nm
(Figure 1). The emission clearly decreased upon addition of CuII
and finally was quenched. The process was also monitored by
absorption spectroscopy. The UV/Vis absorption spectra dis-
played several isosbestic points at 288, 318, 378, and 430 nm
[a] Conditions: thioanisole (1 mmol), Ru –Cu
(1”10À4 mmol,
II
chro
II
cat
0.01 mol%), room temperature, 6 mL solution, 5 h and blue LEDs (450Æ
10 nm, 3W). [b] TON=n(isolated product)/n(catalyst). [c] No light. [d] TON
and yield of 18O labeled product.
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III
chro
sulting in lower TON (Table 1, entry 5). No conversion was ob-
served in the absence of light or under N2, indicating it was
indeed an aerobic light-driven reaction (Table 1, entries 6 and
7). Additionally, no sulfones were detected, indicating the oxi-
dation process was highly selective. Previous studies demon-
strated that both water and dioxygen could be used as the
oxygen source in the photocatalytic oxidation of thioaniso-
le.[4b,6c,10] To ascertain the oxygen source, we replaced H216O
with H218O in the photocatalytic reaction (Table 1, entry 8).
However, no labeled sulfoxide was found through HR-ESI-MS
(Figure S8). Thus, it can be concluded that molecular dioxygen
served as both oxidant and oxygen source in the photocatalyt-
(Table S1). The driving force between Ru
and the chloride
III
derivative (DE=0.12 V) is smaller than that between Ru and
chro
the methoxy derivative (DE=0.36 V). Therefore, the methoxy
derivative was oxidized into the corresponding sulfoxide in a
shorter time. A higher TON of 32000 was obtained when we
increased the amount of substrate and irradiation time
(Table 2, entry 1). Moreover, decomposition of the Ru–Cu
assembly was observed. Regardless of the optimized condi-
II
II
tions, Ru –Cu showed the highest TON of 32000 reported
chro
cat
2+
to date (Table S2). It is worth noting that Ru(bpy)3 exhibited
a TON of 196 in methanol under air.[11] This highlighted the sig-
nificance of covalent bonding of Cu(tpy)Cl2 in RuIcIhro–Cu . The
II
cat
II
II
II
II
ic oxidation of thioanisole using Ru –Cu . Notably, we found
complex Ru –Cu was also effective for the oxidation of thi-
chro cat
chro
cat
II
chro
II
cat
that the UV/Vis absorption spectrum of Ru –Cu had little
oanisole on a larger scale (Scheme S2). The nitro derivative
only exhibited a TON of 7600 even after 30 h of irradiation
(Table 2, entry 5). This can be mainly ascribed to the higher oxi-
dation potential of nitro derivative compared to other deriva-
tives (Table S1), in accordance with the oxidation of sulfides
change after irradiation, suggesting the dinuclear complex was
relatively stable under the reaction conditions (Figure S9). This
II
chro
may be ascribed to efficient stabilization of delicate Ru
by
II
Cu through electron transfer.
cat
II
II
II
II
Inspired by the favorable stability of Ru –Cu , we next in-
through electron transfer using Ru –Cu . Oxidation of sul-
chro cat
chro
cat
vestigated its application in the oxidation of other thioanisole
derivatives for a longer time under optimized conditions
(Table 2). Reactions were monitored by thin-layer chromatogra-
fides through singlet oxygen is negligible owing to emission
II
II
quenching of Ru
by Cu . A dialkyl sulfide such as dibutyl
cat
chro
sulfide was also efficiently oxidized into the corresponding
sulfoxide (Table 2, entry 6).
To gain more insights into the photocatalytic process, we
II
II
II
chro
II [a]
cat
Table 2. Photocatalytic oxidation of sulfides with Ru –Cu
further investigated the reaction of Ru –Cu and sulfides by
chro cat
emission spectroscopy, electron paramagnetic resonance spec-
troscopy (EPR) and HR-ESI-MS. As shown in Figure 2a, the
Entry
1
Substrate
Product
t [h] TON[b]/Yield [%]
II
II
emission of solution containing Ru –Cu and methoxy thioa-
20
50
18000/90
32000/80[c]
chro
cat
2
3
25
15
17400/87
18600/93
4
27
17600/88
7600/38
5
6
30
14
16600/83
(1”10À4 mmol,
II
II
[a] Conditions:
substrate
(2 mmol),
Ru –Cu
chro cat
0.005 mol%), room temperature, 6 mL solution (CH3CN/H2O, v/v, 8:2),
under air and blue LEDs (450Æ10 nm, 3 W). [b] TON=n(isolated prod-
uct)/n(catalyst). [c] Substrate (4 mmol).
phy (TLC) until full conversion was observed. All derivatives
containing an electron-withdrawing or electron-donating
group were selectively oxidized into corresponding sulfoxides.
Full conversion was found with a TON of 18600 after 15 h of ir-
radiation for the methoxy derivative (Table 2, entry 3). The
chloride derivative required longer time to achieve full conver-
sion with a TON of 17400 (Table 2, entry 2). This is probably
because of its higher oxidation potential (+1.37 V vs. SCE)
compared to the methoxy derivative (+1.13 V vs. SCE)
Figure 2. a) PL spectra of the CH3CN/H2O (v/v, 8:2) solution containing
II
II
Ru –Cu (16 mm) and methoxy thioanisole (10 mm) upon light irradiation
chro
cat
(blue LEDs) under N2 atmosphere for 3 min, lex =450 nm. Inset: photograph
of the solution under a 365 nm portable UV lamp. b) Corresponding partial
HR-ESI-MS spectrum of the irradiated solution.
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III
chro
III
chro
nisole was very weak under N2 atmosphere before irradiation.
However, the emission significantly increased upon 30 s of
blue light irradiation and the change could be directly ob-
served by the naked eye under a portable UV lamp, indicating
of Ru –CuIcat. Ru
has moderate oxidation ability and can
III
II
oxidize organic sulfides. As a result, Ru
is reduced to Ru
,
chro
chro
resulting in Ru –CuIcat, which is emissive as observed in Fig-
II
chro
ure 2a under N2 atmosphere. However, CuIcat is oxidized to
II
cat
formation of emissive species. The emissive species was proba-
Cu and further activated by molecular O2 in air, leading
II
bly formed as RuIcIhro–CucI at owing to the reduction of Cu upon
to emission quenching and copper–oxygen adduct
Ru –CucIIat(O2).[23] As shown in Scheme 2, the adduct is capable
cat
light irradiation. It is reasonable that Ru –CuIcat was emissive
II
chro
II
chro
because of the d10 electronic state of the CuI ion. We then no-
ticed the emission was quenched again after exposure to air.
This could be ascribed to the oxidation of CuIcat by molecular
of reacting with the sulfur-centered free-radical cation to gen-
erate persulfoxide that is stablized by a protonic solvent such
as water and react with another sulfide to form sulfoxide.[7b,8,24]
II
II
O2. To confirm the reduction of CuIcIat upon light irradiation, EPR
Simultaneously, the catalyst Ru –Cu is recycled. The whole
chro cat
II
experiments were performed. RuIcIhro–Cu showed typical EPR
catalytic process shows efficient cooperative interactions be-
cat
signal of terpyridine-based CuII complex before light irradiation
(Figure S10).[21] Then the signal gradually decreased during 2 h
tween Ru and Cu .
chro cat
II
II
In conclusion, we successfully prepared a new chromo-
II
II
of light irradiation. The decline of the EPR signal strongly re-
phore–catalyst assembly (Ru –Cu ) for photocatalytic oxida-
chro cat
II
vealed that Cu was reduced to CuIcat. Therefore, we further
tion of sulfides under air. Sulfides were selectively oxidized into
sulfoxides with turnover a number up to 32000. The whole
catalytic process proceeded efficiently with approximataly
100% atom economy in the absence of sacrificial reagents.
Molecular O2 was used as both oxidant and oxygen atom
source. It was demonstrated that an electron transferred from
cat
tried to achieve more direct evidence of the generation of
II
chro
II
chro
II
cat
Ru –CuIcat by HR-ESI-MS. A solution containing Ru –Cu
and methoxy thioanisole was irradiated with blue light for
180 s under N2 atmosphere and was then immediately ana-
II
II
lyzed by MS. No signal peaks of Ru –Cu were found in the
chro
II
cat
II
II
II
*
HR-ESI-MS spectrum, suggesting Ru –Cu was probably con-
the excited chromophore Ruchro to the catalytic unit Cucat along
chro
cat
verted into Ru –CuIcat (Figure S11). Indeed, we found several
with the generation of CuIcat, which was then activated by O2.
Formation of Ru –CuIcat was detected by high-resolution elec-
II
chro
II
II
chro
signal peaks corresponding to Ru –CuIcat. The signal peaks
chro
centered at 402.7404 (calcd 402.7494) were assigned to aceto-
trospray ionization mass spectrometry. These results not only
reveal an efficient strategy for photoredox cooperative cataly-
sis using the chromophore–catalyst assembly but also provides
a promising pathway for reductive activation of molecular O2
through O2-binding site under light irradiation.
II
nitrile-substituted Ru –CuIcat with +3 charges (Figure 2b). The
chro
isotopic peaks were located with an interval of 0.33, as expect-
ed. More importantly, signal peaks corresponding to adduct of
II
chloride Ru –CuIcat and methoxy thioanisole were also ob-
chro
served (Figure S11), indicating the probable interaction be-
tween catalytic site and sulfides. Additionally, no typical transi-
ent absorption of RuI species around 510 nm was observed, in-
dicating no generation of RuI species and oxidative quenching
Acknowledgements
II
II
of excited state Ruchro by Cucat (Figure S12).[22] Hence, an elec-
This work has been supported by National Natural Science Foun-
dation of China (No. 21605013). We thank Dr. Xuewang Gao for
his kindness in the EPR experiments.
*
II
II
*
tron transfer from Ruchro to Cucat can be confirmed. These re-
sults allowed us to establish the generation of Ru –CucI at
II
chro
during the photocatalytic oxidation of sulfides.
On the basis of the above results, we proposed a plausible
mechanism involving multiple single-electron transfer process
Conflict of interest
II
II
*
is excited to Ruchro under irradiation
(Scheme 2). First, Ru
chro
The authors declare no conflict of interest.
II
II
*
with blue LEDs. Then, an electron transfer from Ruchro to Cucat
occurs and Cu is reduced to CuIcat along with the generation
II
cat
Keywords: copper · oxidation · photocatalysis · ruthenium ·
sulfides
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II
II
Ru –Cu under air.
chro
cat
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Manuscript received: May 26, 2017
Revised manuscript received: July 14, 2017
Accepted manuscript online: July 26, 2017
Version of record online: August 16, 2017
ChemSusChem 2017, 10, 3358 –3362
www.chemsuschem.org
3362
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