Photocatalytic Oxidation
COMMUNICATION
[
22]
3
II
2+
III
2+
stable at higher pH.
Under the reaction conditions, we
ACHTUNGTRNENGNU{ [Ru AHCTUNGTNERGUN( bpy) ] }* to [Co Cl ACHTGUNRETNNUG( NH ) ] . Thus, higher concen-
3 3 5
IV
have obtained the highest TON values among the catalytic
substrate oxidation reactions reported to date by using pho-
tochemical catalytic cycles.
trations of the reactive Ru –oxo complexes can be attained
to achieve more efficient substrate oxidation reactions to
gain higher TON values.
[9,10]
Time profiles of the photoca-
talytic oxidation were obtained for five substrates (4-methyl-
benzyl alcohol, 4-chlorobenzyl alcohol, 1-propanol, 2-propa-
nol, and styrene; see Figure S4 in the Supporting Informa-
tion). The reaction efficiencies depend on the number of
electrons to be removed from the substrates by the oxida-
tion reactions: Two-electron oxidation reactions of sub-
strates (benzyl alcohols and 2-propanol) are much faster
than four-electron oxidation reactions (1-propanol and a
water-soluble styrene derivative).
To investigate the photoefficiency of the photocatalytic
systems, we determined the quantum yield of each pro-
[24]
IV
A
H
U
G
R
N
N
cess.
The formation of the Ru –oxo species 4–6 in an
aqueous B.–R. buffer solution (0.2 mm, pH 1.8) under photo-
irradiation at 440 nm were monitored by the absorbance
changes at 620 for 1, 630 for 2, and 360 nm for 3 in the pres-
III
II
ence of [Co Cl
(0.1 mm). The overall quantum yields (Fox)
mined to be 0.19 for 1, 0.16 for 2, and 0.15 for 3. On the
ACHTUNGTRENN(NUG NH ) ]Cl (0.4 mm) and [Ru ACHTUNGRTUNNEGN( bpy) ]Cl
3 5 2 3 2
[24]
were deter-
III
In addition, among the substrates that undergo two-elec-
tron oxidation, one with lower bond-dissociation enthalpy
basis of the quantum yield (F ) of the formation of [Ru -
(bpy) ] under the same conditions,
RuIII
3
+
[25]
A
H
U
G
R
N
N
the efficiencies of
3
[23]
À1
II
(BDE)
(i.e., benzyl alcohol, 87.5 kcalmol ) is oxidized
the electron-transfer reactions from Ru -aqua complexes to
III
3+
IV
much faster than others with higher BDE (i.e., 2-propanol,
[Ru ACHTUNGERTNNUNG( bpy) ] to afford the corresponding Ru –oxo com-
3
À1
[26]
9
3.0 kcalmol ).
plexes are calculated to be 0.56 for 1, 0.48 for 2, and 0.45
for 3, respectively. In addition, we also determined the over-
all quantum yields (FSub) of oxidation of 4-methylbenzyl al-
cohol. The quantum yields of the reactions are highly de-
pendent on the reaction conditions, especially on the con-
To obtain higher TONs, we conducted the photocatalytic
reactions of 4-methylbenzyl alcohol as a substrate with re-
duced amount of the catalysts (0.1 mol%; Table 1, entry 2).
Under the diluted conditions, the TONs for complexes 1–3
reached over 8000: 8700 for 1, 8240 for 2, and 8060 for 3,
respectively. These TON values obtained here are 50–400
times higher than those of other photocatalytic systems re-
[10a]
centration of light-absorbing components.
As for the re-
action in a solution of 4-methylbenzyl alcohol (10 mm) in
B.–R. buffered D O at pD 1.8 with the catalysts (0.5 mm),
2
[9,10]
II
III
ported so far.
To estimate the TOF numbers of this cata-
[Ru AHCTNUGTRENNNUG( bpy) ]Cl (0.1 mm) and [Co Cl ACHNUTGTREUNNGN( NH ) ]Cl (25 mm), the
3 2 3 5 2
quantum yields determined by monitoring the formation
of the product (4-methylbenzaldehyde) with use of H NMR
[24]
lytic system, we employed a solar simulator of AM1.5 to fix
the quantity of light, and obtained the time profiles of the
reactions, whose slope in the linear regions were used for
1
spectroscopy were much improved to be 0.35 for 1, 0.33 for
2, and 0.31 for 3, respectively, by monitoring the formation
À1
the calculations of TOFs (Figure 2). As results, TOFs (h
1
of the product (4-methylbenzaldehyde) with H NMR spec-
troscopy. The quantum yields of the oxidation steps of 4-
IV
methylbenzylalcohol by a Ru -oxo complex are higher than
IV
those of the formation of the Ru –oxo complexes, because
under the catalyst-diluted conditions, the reaction efficiency
[27]
is much improved as mentioned above. Furthermore, irre-
spective of the spin states of 4–6, which are responsive spe-
[15,17]
cies of substrate oxidations (see above),
the efficiencies
in oxidation reactions are nearly the same among the three
catalysts, as indicated by the TONs, TOFs, and the quantum
yields. The slight difference is probably derived from differ-
ence in the stability of the catalysts under the reaction con-
Figure 2. Time profiles of photocatalytic oxidation reactions of 4-methyl-
benzyl alcohol as substrates (10 mm) in B.–R. buffer of D
2
O (pD 1.8) at
[17]
ditions.
À2
2
98 K under AM1.5 illumination (100 mWcm ) in the presence of a cat-
II
III
A proposed mechanism of the photocatalytic oxidation of
3 2 3 5 2
alyst (0.5 mm), with [Ru ACHTUNGTRENN(UNG bpy) ]Cl (0.1 mm) and [Co Cl AHCTUNTGRNEUN(G NH ) ]Cl
(
25 mm): 1 (*, c), 2 (
&
, g), and 3 (~
, a).
organic substrates by 1–3 as catalysts is described in
II
Scheme 1. Photoinduced electron transfer from [Ru -
2
+
III
2+
AHCTUNGTRNENUG( bpy) ] to [Co Cl ACHTUNGRETNNUN(G NH ) ] through the formation of the
3 3 5
À1
3
II
2+
(
s )) were determined to be 13800 (3.8) for 1, 11500 (3.2)
MLCT triplet excited state ( ACHNUTGTRENNUN{G [Ru ACHTUNGTRENNUNG
oxidant, that is, [Ru ACHTUNGERTUNNNG( bpy) ] . In this process, the Co
3
complex is reduced to a labile Co complex, which should
undergo thermal and fast decomposition to make the elec-
tron transfer irreversible. The oxidant reacts with the Ru -
3
III
3+
III
for 2, and 10500 (2.9) for 3, respectively. These results indi-
cate that the performance of each catalyst is significantly en-
hanced under the diluted conditions. The enhancement
should be attained by less disturbance of the photoabsorp-
tion of the sensitizer by the catalysts, which exhibit absorp-
II
II
IV
aqua complexes to form the Ru –oxo complexes (4–6) as
[15,17,18]
tion in the visible region.
Efficient photoabsorption of
the reactive intermediates for the substrate oxidation reac-
IV
the sensitizer allows us to gain higher concentration of the
tions. The Ru –oxo complexes oxidize the substrates effi-
3
II
2+
excited state (
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
{[Ru
A
H
U
G
R
N
U
G
ciently and selectively to give the oxidized products as listed
in Table 1. In this catalytic system, water acts not only as a
3
III
3+
species ([Ru ACHTUNGTRENNUNG( bpy) ] ) through electron transfer from
3
Chem. Eur. J. 2013, 19, 1563 – 1567
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1565