A Ruthenium(II)-Copper(II) Dyad for the Photocatalytic Oxygenation of Organic Substrates Mediated by Dioxygen Activation

Article abstract of DOI:10.1002/anie.201501180

Dioxygen activation by copper complexes is a valuable method to achieve oxidation reactions for sustainable chemistry. The development of a catalytic system requires regeneration of the Cu^I active redox state from Cu^II. This is usually achieved using extra reducers that can compete with the Cu^II(O₂) oxidizing species, causing a loss of efficiency. An alternative would consist of using a photosensitizer to control the reduction process. Association of a Ru^II photosensitizing subunit with a Cu^II pre-catalytic moiety assembled within a unique entity is shown to fulfill these requirements. In presence of a sacrificial electron donor and light, electron transfer occurs from the Ru^II center to Cu^II. In presence of dioxygen, this dyad proved to be efficient for sulfide, phosphine, and alkene catalytic oxygenation. Mechanistic investigations gave evidence about a predominant ³O₂ activation pathway by the Cu^I moiety.

Full text of DOI:10.1002/anie.201501180

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201501180
German Edition: DOI: 10.1002/ange.201501180
Photocatalysis
A Ruthenium(II)–Copper(II) Dyad for the Photocatalytic
Oxygenation of Organic Substrates Mediated by Dioxygen
Activation**
Wissam Iali, Pierre-Henri Lanoe, StØphane Torelli,* Damien Jouvenot, FrØdØrique Loiseau,
Colette Lebrun, Olivier Hamelin,* and StØphane MØnage
Abstract: Dioxygen activation by copper complexes is a val-
uable method to achieve oxidation reactions for sustainable
chemistry. The development of a catalytic system requires
the peroxo state and subsequent OÀO bond cleavage. In
dinuclear centers, the second metal ion acts as a reductant,
while in mononuclear active sites, an external electron donor
is required. This is the case for copper metalloenzymes such as
dopamine b-monooxygenase (DbM), peptidylglycine a-
hydroxylating monooxygenase (PHM), and the recently
I
II
regeneration of the Cu active redox state from Cu . This is
usually achieved using extra reducers that can compete with the
II
Cu (O ) oxidizing species, causing a loss of efficiency. An
2
alternative would consist of using a photosensitizer to control
identified insect tyramine b-monooxygenase (TbM) that are
II
I
the reduction process. Association of a Ru photosensitizing
involved in the transformation of various substrates by Cu /O
2
II
[3]
subunit with a Cu pre-catalytic moiety assembled within
chemistry. The catalytic activity is achieved owing to the
a unique entity is shown to fulfill these requirements. In
presence of a sacrificial electron donor and light, electron
transfer occurs from the Ru center to Cu . In presence of
dioxygen, this dyad proved to be efficient for sulfide, phos-
presence, in close proximity, of a reducing co-factor that
I
allows a regeneration of the active Cu species from the final
II
II
II
inactive Cu state (Scheme 1, top). Several bio-inspired mono
phine, and alkene catalytic oxygenation. Mechanistic inves-
3
tigations gave evidence about a predominant O activation
2
I
pathway by the Cu moiety.
F
rom an environmental point of view, there is a growing
interest in designing catalysts capable of using O as oxygen
2
atom source to perform oxidation reactions to avoid strong,
[
1]
toxic, and expensive oxidants. Indeed, O2 is nowadays
considered as an appealing oxidant for sustainable oxidation
chemistry. Nature has developed a large panel of metal-
containing enzymes to perform selective and efficient oxida-
[2]
tion reactions through dioxygen activation. O activation at
2
a metal center generally entails its two-electron reduction to
[
*] Dr. W. Iali, Dr. S. Torelli, Dr. O. Hamelin, Dr. S. MØnage
Laboratoire Chimie et Biologie des MØtaux
UniversitØ Grenoble-Alpes
CEA, DSV/iRTSV, LCBM—CNRS, UMR5249
Scheme 1. Copper-based oxygen activation/reaction (X: substrate, XO:
oxidized substrate).
3
8041 Grenoble (France)
E-mail: stephane.torelli@cea.fr
ohamelin@cea.fr
Homepage: http://www-dsv.cea.fr/irtsv/lcbm/bioce
and dinuclear copper complexes have been reported, but few
of them act catalytically in homogeneous conditions. This
Dr. P.-H. Lanoe, Dr. D. Jouvenot, Prof. F. Loiseau
DØpartement de Chimie MolØculaire
[4]
UniversitØ Grenoble-Alpes; CNRS UMR 5250, BP53
assessment might originate from the competitive reduction of
3
8041 Grenoble (France)
II
the active Cu (O ) oxidative species (that is, superoxo or
2
C. Lebrun
peroxo) by the additional sacrificial reductant (Scheme 1,
top) that would short-circuit the catalytic cycle. Exploiting
our expertise in the development of eco-aware photocatalysts
Laboratoire de Reconnaissance Ionique et
Chimie de Coordination—UniversitØ Grenoble-Alpes
CEA, DSV/iRTSV, LCBM—CNRS, UMR5249
[5]
for small molecules activation (such as water ), we report
3
8041 Grenoble (France)
II
II
herein a unique example of a Ru -Cu dyad capable of sulfide
and hydrocarbon catalytic oxidations, using O2 as unique
[
**] This work has been supported by the Labex ARCANE (ANR-11-
LABX-0003-01) and the ANR program (ANR wateract ANR-11-BS07-
0
004). Jacques PØcaut is acknowledged for X-ray diffraction analysis.
oxygen atom source. The covalent combination of a Ru-based
II
photosensitizer with a Cu catalyst for O activation and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501180.
2
associated with an appropriate sacrificial electron donor
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allows a controlled delivery of reducing equivalents that
complexes are luminescent, with broad emissions centered at
618 nm (Supporting Information, Figure S4). The presence of
the Cu moiety leads to a 70% drop of the quantum yield in
II
overcomes the undesired Cu (O ) reduction.
2
3+
II
The [(bpy) Ru(bpbpa)Cu(OTf)(CH CN)] (bpy = 2,2’-
2
3
bipyridine, bpbpa = N-(4-((5H-dipyrido[3,2-c:2’,3’-e]azepin-
(7H)-yl)methyl)benzyl)-1-(pyridin-2-yl)-N-(pyridin-2-ylme-
thyl)methanamine) catalyst reported here, abbreviated as
air-equilibrated solution (from 1.05 ns to 0.29 ns) and 60%
under Ar (Supporting Information, Table S1). Previous
studies on comparable heteropolymetallic complexes featur-
6
II
II
2+
II
Ru phot-Cu
(Scheme 1, bottom), covalently associates
ing [Ru(bpy)3] and polypyridyl-Cu moieties ascribed the
cat
2+
a light-absorbing photosensitizing [Ru(bpy) ] -like frag-
emission quenching to either energy or electron transfer or
3
[
6]
[14]
ment, known to be an efficient chromophore, and a bio-
inspired copper site as the proposed catalytic oxidation locus.
It was efficiently prepared in two steps from the reported
both processes.
This point is discussed along with the
transient absorption spectroscopy. The lifetime decay for
II
Ru
is mono-exponential and air-sensitive, which is
phot
[
7]
[8]
3
[
(bpy) Ru(bpBr )][PF ] and l-NH -bpa precursors. The
characteristic of the MLCT excited state radiative decay
pathway (Supporting Information, Figure S5).
2
2
6
2
2
II
[13]
catalytic moiety (Cu cat) was synthesized from the l-Pht-bpa
protected ligand (Supporting Information, Scheme S1). The
syntheses and the characterization for all these compounds
The dyad
displays a bi-exponential lifetime decay with a short (16 ns in
air equilibrated solution and 31 ns under Ar) and a long
component (148–328 ns; Supporting Information, Figure S6).
By comparison with the mono-exponential decay determined
(
NMR and UV/Vis spectroscopy, elemental analysis, ESI-
MS) are provided in the Supporting Information. The nature
of the copper subunit coordination sphere is proposed on the
II
for Ru phot, the latter is ascribed to the radiative deactivation
II
3
basis of the X-ray crystal structure resolved for Cu cat
of MLCT excited state. Considering i) that energy transfer
(
Supporting Information, Figure S1).
The cyclic voltammogram (CV) of Ru phot-Cu
between the triplet excited state of the ruthenium and the
singlet ground state of the copper moieties is a spin-forbidden
process; ii) that the weak extinction coefficient of the Cu
subunit makes energy transfer a possible but likely minor
II
II
cat
in
CH CN shows, in the cathodic region, three successive one-
3
electron processes between À0.7 and À1.7 V vs NHE
[14b,15]
corresponding to the reduction of the bipyridine ligands
component of the excited state quenching;
and iii) the
II
3
(
Supporting Information, Figure S2). Surprisingly, three
estimated redox potential of Ru * MLCT excited state
[10b]
II
waves were observed in the anodic region. On the basis of
previous reports and CVs recorded for Cu
(
(À0.70 V vs NHE ) and the redox potential of Cu
II
and [(bpy) Ru-
(+ 0.42 V vs. NHE), the quenching is most likely due to an
cat
2
2+
II
II 3
bpbpa)] metalloligand (abbreviated Ru phot), the two one-
electron transfer from the Ru MLCT excited state to the
II
[14c,16]
electron quasi-reversible waves at + 1.57 V and + 0.42 V vs
Cu moiety (as early observed
). Consequently, the short
III
II[9]
II
I
NHE were attributed to the Ru /Ru and Cu /Cu couples,
respectively (Supporting Information, Figure S2). Thermody-
namically, by considering the redox potential of the excited
lifetime is ascribed to the decay of the excited state quenched
by photoinduced electron transfer (PET). The corresponding
7
À1
rate, estimated to be 2.9 ” 10 s in argon equilibrated
III
II
[10]
II
II
state of the Ru subunit(E8 Ru /Ru *) of À0.70 V vs NHE,
acetonitrile solution for Ru phot-Cu , is artificially increased
cat
II
7
À1
an electron transfer from the photoexcited state Ru * to the
in air-equilibrated medium (5.6 ” 10 s ), owing to the addi-
tional quenching by energy transfer to the triplet ground state
of molecular oxygen. The presence of triethanolamine
(TEOA, sacrificial electron donor) in the catalytic conditions
slightly affects the spectroscopic properties (Supporting
Information, Table S1, Figures S3, S4, and S6). All these
results enforce the viability of an electron transfer from the
II
Cu is consequently favorable. The third irreversible oxida-
tion process at + 1.44 V vs. NHE (anodic peak given) was
assigned to the oxidation of the tertiary amine of the bridging
ligand (N , Scheme 1).
comparison with the metal-free bdda molecule (Supporting
Information, Scheme S1, Figure S2) showing a similar irre-
versible pattern with an anodic peak at + 1.40 V vs. NHE.
The reliability of the photocatalytic system based on an
intramolecular electron transfer between the excited Ru
moiety and the vicinal Cu center was probed by EPR and
photophysical studies. The photophysical properties of
[11]
This was further confirmed by
A
[
12]
II
II
excited Ru *
to the Cu cat, rather than a reductive
phot
II
quenching of the Ru *
by the electron donor (Supporting
phot
Information, Scheme S2).
The photo-assisted Cu reduction in Ru
catalytic conditions (that is, CH CN, [Ru phot-Cu cat] = 10 m,
II
II
phot
II
-Cu
under
cat
II
II
À4
3
II
II
II
cat
Ru
and Ru phot-Cu
have been investigated in CH CN
TEOA 200 molar equiv) was probed under Ar-saturated
atmosphere by EPR spectroscopy. Spectra were recorded
before (t = 0 min), and after 1 and 5 min of irradiation with
a Xe lamp equipped with a by-pass filter at 450 nm (Figure 1).
At t = 0 min, the EPR spectrum is characteristic of
phot
3
at room temperature, both in air equilibrated and de-aerated
conditions (Supporting Information, Table S1). Both com-
plexes display an intense absorption around 290 nm (e =
À1
À1
À1
À1
II
6
0500 Lmol cm and e = 56 800 Lmol cm for Ru
phot
II
II
and Ru phot-Cu cat, respectively) assigned to p–p* transitions
Supporting Information, Figure S3). The broad absorption
bands around 445 and 455 nm are attributed to the metal-to-
a mononuclear copper center in axial symmetry (g > g )
k ?
(Support-
cat
II
(
with identical features compared to isolated Cu
[17]
ing Information, Figure S7). Upon light exposure of the
dyad in the ruthenium MLCT transition region at 298 K, the
EPR signal rapidly decreased and completely vanished within
1
[13]
ligand charge transfer transition ( MLCT, dp –p*bpy). It is
worth noting that the Cu-centered d–d transitions for the
dyad are not observable owing to their weak extinction
coefficient (lmax = 615 nm and e = 100 Lmol cm
Cu
Ru
II
45 min. A complete recovery of the Cu features was
À1
À1
for
observed upon exposure to air. The decomposition of the
dyad has to be precluded, as confirmed by the full conserva-
II
Cat
; Supporting Information, Figure S3). Upon excitation
II
II
II
cat
II
in the MLCTabsorption band, both Ru phot and Ru phot-Cu
tion of the UV features after 45 min of irradiation of Ru phot-
8
416
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Chemie
II
electron transfer to the Cu from the ruthenium photo-
sensitizer in its excited state.
II
II
The Cu -assisted reduction by the Ru photosensitizer
II
havin been established, the photocatalytic activity of Ru phot-
II
cat
Cu
in oxidation reactions has been evaluated using various
substrates under O -saturated atmosphere in acetonitrile with
2
TEOA. A blue LED system emitting at 468 nm matching the
photosensitizer subunit MLCT transition was used as light
source. Irradiation was held for 8 h, corresponding to the
maximum conversion for 4-bromothioanisole as test substrate
(
Supporting Information, Figure S11). Oxidation products
1
were quantified by H NMR and the main results are
summarized in Table 1. It was observed that most of the
II
II
Figure 1. Evolution of the X-band EPR spectra of [Ru phot-Cu cat]
À4
(
10 m+200 molar equiv TEOA) in CH CN, under argon after irradi-
3
Table 1: Catalytic oxidation of sulfides, phosphine, and alkenes by
Ru phot-Cu cat.
ation at 450 nm. A) t=0, B) t=1 min, and C) t=5 min at 100 K.
II
II
[a]
[b]
Entry Substrate
Product
t [h] TON /Conversion [%]
1
2
3
4
5
6
7
8
PhS
4-BrC H S
4-MeOC H S
sulfoxide
sulfoxide
sulfoxide
sulfoxide
8
8
8
8
1.5
8
97/>98
94/>98
92/>97
II
À4
Cu under the same conditions (10 m with 200 molar equiv
6
4
cat
6
4
of TEOA; Supporting Information, Figure S8). We thus
conclude that the disappearance of the Cu signal is due to
the formation of a diamagnetic Ru phot-Cu species gener-
ated by a photoinduced electron transfer (PET).
Two different pathways may be suggested to explain the
formation of the Ru phot-Cu species from the photogen-
erated Ru *phot-Cu
Scheme S2). In a reductive process, the Ru MLCT state is
[c]
[c]
II
4-O
2
NBrC
6
H
4
S
10, 28 /9, 26
100/100
36/100
Ph3P
indene
Ph P=O
II
I
3
cat
cis-diol
cyclohex-2-enone 16
cyclohexene
cis-cyclooctene cyclooct-2-enone 16
57/57
42/42
II
I
II
II
cat
[
a] Standard catalytic conditions: [Ru phot-Cu cat] (ca. 0.5 mm), [substrate]
II
II
dyad (Supporting Information,
cat
(ca. 50 mm], [TEOA] (ca. 100 mm), CH CN, RT, LED irradiation
(468 nm), O bubbling. All given values are averages of at least two
experiments. [b] TON=n(product)/n(complex). [c] after 24 h of irradi-
ation.
3
II 3
2
I
II
reduced by TEOA leading to a Ru -Cu
intermediate
cat
phot
I
II
À
+
(
Ru
is used to symbolize [Ru (bpyC )(bpy)(bpbpa)] ). An
phot
intramolecular electron transfer from the ruthenium center to
II
I
the copper ion generates the Ru phot-Cu dyad. In a second
cat
hypothesis, PET can occur immediately after photon absorp-
sulfides, except 4-nitrothioanisole bearing an electron-with-
drawing group, were nearly quantitatively and selectively
converted into their corresponding sulfoxides (Table 1,
entries 1–4). Whatever the nature of the sulfide, no sulfone
was detected even after 24 h of irradiation. Finally, the
photocatalyst proved to be also efficient for phosphine and
alkene oxidations, affording only phosphine oxide and the
major corresponding enone respectively. Yet, while oxidation
of cycloalkenes selectively gave the corresponding enone,
oxidation of the aromatic indene led to the formation cis-diol
(36 TON, Table 1, entry 6) and the corresponding dicarbox-
ylic acid (not quantified). In this case, the formation of
indenone as a result of the oxidation of the benzylic position
was not observed.
III
I
tion, leading to the formation of Ru phot-Cu
species.
cat
Subsequent reduction of the ruthenium center by TEOA
II
I
yields to the Ru phot-Cu dyad.
cat
Nanosecond time resolved absorption spectroscopy under
catalytic conditions was used to discriminate between these
two processes. The initial spectrum (black trace, Supporting
3
Information, Figure S9) is characteristic of the MLCTexcited
state of tris(bipyridyl)–ruthenium complex, with an intense
positive absorption band at 365 nm corresponding to the
formation of the anionic bpyCÀ radical. An intense negative
signal at 450 nm corresponding to the bleaching of the
1
[18]
ground-state MLCT absorption is also observed.
Both
1
2
bands display a bi-exponential decay (t = 20–23 ns and t =
1
50–160 ns) of the same magnitude to those observed in
In control experiments performed with 4-bromothioani-
1
2
luminescence decay (t = 16 ns and t = 148 ns). The associate
rates (k370 = 3.7 ” 10 s , k = 4.4 ” 10 s ) are in agreement
with that estimated by time-resolved luminescence spectros-
copy with TEOA (k = 5.9 ” 10 s ). It is worth noting that the
sole in absence of either light, catalyst or O , no oxidation
2
7
À1
7
À1
products were detected. Without TEOA, a low amount of
sulfoxide was formed (8 TON), suggesting the intervention of
a competitive oxidation process (see below). It has to be
450
7
À1
EPR experiment showed that after 45 min of irradiation, no
noted that when O was replaced by air bubbling, the catalyst
2
À
signal corresponding to the bpyC radical anion was found at
is still active but yielded a lower conversion (from 98 to 53%,
Table 2, entry 3). More interestingly, the intramolecular
electron transfer proved to be more efficient than the
intermolecular electron transfer. Indeed, the catalytic activity
increased at least four-fold for the dyad compared to the
[
19]
g = 1.998 (Supporting Information, Figure S10).
Further-
I
more, a Ru intermediate can be ruled out as no characteristic
positive absorption band at 510 nm could be detected even in
presence of an excess of electron donor. Therefore, this
spectroscopic evidence is in agreement with an oxidative
[20]
II
cat
2+
bimolecular combination of Cu
and [Ru(bpy)3] (Table 2,
3
quenching of the MLCT state leading to a photoinduced
entry 4). Finally, when the photosensitizer-TEOA pair was
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Table 2: Catalytic 4-bromothioanisole oxygenation under various conditions.
oxidation by dioxygen activation
(Path 1; copper-dependent catalytic
oxidation). In that reduction step,
sulfide can be used as electron
[e]
Entry
Complex
Electron donor
O source
2
TON
[%] conv
[
[
a]
c]
II
II
II
[b]
[b]
1
2
Ru phot-Cu
TEOA
TEOA
O2
O2
94, 300
>98, 60
>98, 98
79
cat
cat
II
Ru phot-Cu
98 (8 h) 195 (22 h)
50 (28 h)
+
donor yielding to a transient RSC
2
53
25
[
[
a]
d]
II
II
thiyl radical that further reacts with
3
4
Ru phot-Cu
TEOA
TEOA
air
O2
53
25
cat
+
[22c,23]
[Ru(bpy) ]
water (Path 3).
The latter pro-
3
2
II
+
Cu
cess was partly ruled out using
cat
[
a]
a]
II
5
6
Cu
ascorbate
or DTT
TEOA
O2
0
0
18
cat
OH (1 molar equiv with respect
2
to the substrate), as no labeling was
[
II
phot
Ru
O2
25
25
II
incorporated using only Ru phot.
II
[
a] [complex] (ca. 0.5 mm), [4-bromothioanisole] (ca. 50 mm], [electron donor] (ca. 100 mm). [b] [Ru phot-
Independently, the formation of
II
II
II
Cu cat] (ca. 0.5 mm), [substrate] (ca. 250 mm], [TEOA] (ca. 500 mm). [c] [Ru phot-Cu cat] (ca. 0.5 mm) [4-
1
O during the catalytic cycle was
2
bromothioanisole] (3 portions of 100 molar equiv spaced by 14 h, then 6 h), [TEOA] (3 portions of 200
II
II
confirmed by using 9,10-dimethy-
molar equiv spaced by 14 h then 6 h). [d] [Ru phot]=[Cu cat] (ca. 0.5 mm), [4-bromothioanisole] (ca.
0 mm], [TEOA] (ca. 100 mm). [e] TON=n(product)/n(complex).
1
lanthracene
(a
known
O2
5
quencher) with the detection of
the corresponding endoperoxide
However, when the reaction was carried out
[24]
replaced by an excess of dithiothreitol (DTT) or ascorbate,
product.
[4b, 21]
known to reduce transition metals such as cupric ions,
no
without sacrificial electron donor, thus by-passing the copper-
dependent catalytic pathway, only 7 TON were achieved
instead of 94 in presence of TEOA. These results confirm the
involvement of O in the sulfide oxidation process, but to
a low extent.
Most importantly, the O -based process involving the
reactivity of a copper oxygen-activated species (Path 1) was
clearly established, because when a 1:10 mixture of the
reduced Cu (10 mmol) and 4-bromothioanisol in anhydrous
acetonitrile was open to air, sulfoxide was formed in 70%
yield with respect to Cu
To conclude, the Ru phot-Cu
represents, to the best of our knowledge, a unique example of
copper-based system capable of performing efficient catalytic
oxidation product was detected (Table 2, entry 5). This highly
suggests that, as proposed, the gradual electron delivery to the
copper subunit is a prerequisite to the catalytic activity.
The stability of the catalyst was also investigated. Under
catalytic conditions, on the basis of electronic absorption,
about 50% of the catalyst remains intact after 8 h of light
exposure (Supporting Information, Figure S12). Despite this
degradation, a full catalytic activity (up to 195 TON) is still
detected upon addition of extra substrate (100 molar equiv)
and TEOA (200 molar equiv). The activity then significantly
decreased after the third addition, as only 65 additional TON
were achieved within 6 h of irradiation (Table 2, entry 2).
Furthermore, up to 300 TON (Table 2, entry 1, see foot-
note [b]) were obtained in the presence of 500 molar equiv of
1
2
2
I
cat
I
cat.
II
II
dyad reported herein
cat
oxidation of organic substrates using O as unique oxygen
2
4
-methoxythioanisole and 1000 molar equiv of TEOA (60%
yield).
Additional experiments were carried out to obtain
atom source under light irradiation and mild conditions. It
represents an interesting alternative to other photoactivable
Ru-based systems reported and using PhI(OAc) , H O , and
2
2
2
[5a,25]
mechanistic insights for sulfides oxidation. Based on previous
reports, a general mechanism involving three main oxidation
pathways can be proposed (Scheme 2). In the first common
H O as co-oxidants.
2
This study unambiguously validates
our proof-of-concept in which a judiciously chosen associa-
tion of appropriate photosensitizer and sacrificial electron
donor can provide a gradual and controlled electron delivery
II
to a Cu subunit for its reduction prior to O activation. We
2
also showed the efficient synergistic effect between both
partners in the dyad compared to the bimolecular system for
which a diffusional contact is required for electron transfer.
The deciphering of the reaction pathway for alkene oxidation
is under progress.
Keywords: copper · dioxygen · oxygenation · photocatalysts ·
ruthenium
How to cite: Angew. Chem. Int. Ed. 2015, 54, 8415–8419
Angew. Chem. 2015, 127, 8535–8539
Scheme 2. Mechanistic pathways for O -dependent sulfide oxidations.
RS=sulfide; RSO=sulfoxide.
2
[
1] C.-J. Li, P. T. Anastas, Chem. Soc. Rev. 2012, 41, 1413 – 1414.
II
II
[2] a) B. J. Wallar, J. D. Lipscomb, Chem. Rev. 1996, 96, 2625 – 2658;
b) A. Decker, E. I. Solomon, Curr. Opin. Chem. Biol. 2005, 9,
step, photon absorption by Ru phot-Cu
yields to photo-
cat
II
II
1
excited Ru *phot-Cu which might be able to generate O by
energy transfer (Path 2) and/or Ru phot-Cu by electron
transfer. Subsequent reduction by TEOA generates Ru phot-
Cu able to form a Cu (O ) adduct responsible of substrate
cat
2
1
52 – 163.
[
22]
III
I
cat
[3] a) E. I. Solomon, D. E. Heppner, E. M. Johnston, J. W. Gins-
bach, J. Cirera, M. Qayyum, M. T. Kieber-Emmons, C. H.
Kjaergaard, R. G. Hadt, L. Tian, Chem. Rev. 2014, 114, 3659 –
II
I
cat
II
2
8
418
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Received: February 6, 2015
Revised: March 20, 2015
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Published online: May 27, 2015
Angew. Chem. Int. Ed. 2015, 54, 8415 –8419
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