.
Angewandte
Communications
(
1 mmol), 1 (2 mmol), Ru PS (4 mmol), and 2 (8 mmol) to
Table 1: Light-driven H production and catalytic oxidation of hydro-
2
carbons from water.
visible light for over 40 h, we observed very little H2
formation. In the article by Beller and co-workers, Ir PS
was used to reduce 2 for H production. Furthermore, it
was shown that Ir PS acts as a better photocatalyst for H
production than Ru PS. We replaced Ru PS with Ir PS and
[
a]
[b]
Substrate
Product
H [mmol] TON
2
[
14]
2
styrene
NaSS
4
benzaldehyde
154
77
55
28
2
4-formylbenzenesulfonate 111
-methylbenzyl alcohol 4-methyl benzaldehyde 56
[
15]
tested the H formation in the presence of 2. However, only a
2
[
(
a] Conditions: Substrate (1 mmol), 1 (2 mmol), Ru PS (8 mmol), Ir PS
residual amount of H was detected from analysis of the
2
2 mmol), 2 (8 mmol), LED light (450 nm), 10 mL THF/H O (9:1), 18 h.
2
headspace gas by the GC-TCD (GC = gas chromatography,
TCD = thermal conductivity detector).
[
b] TON=n(H )/n(1).
2
Photo-induced oxidation of hydrocarbons has been ach-
ieved using Ru PS, and we tested the applicability of Ir PS for
(Table 1). Control experiments in the absence of any of the
[6b]
hydrocarbon oxidation.
When styrene was used as a
components (substrate, H O, 1, Ru PS, Ir PS, 2, or in the dark)
2
substrate, a turnover number (TON) of 28 was determined
for the oxidation of styrene to benzaldehyde in a system
showed no or only residual amounts of H (see Figure S2 in
2
the Supporting Information).
2
+
1
containing 1 (1 mm), Ru PS (10 mm), and [Co(NH ) Cl]
H NMR spectroscopy and GC-MS analysis of the result-
3
5
(
500 mm) after exposure to LED light (450 nm; LED = light-
ing solution after photolysis confirmed the formation of
benzaldehyde (see Figure S3 in the Supporting Information).
As shown in Figure S4 in the Supporting Information, a
molecular ion peak at 106 was observed, consistent with the
formation of benzaldehyde. To confirm the source of oxygen,
we carried out photocatalytic oxidation of styrene using
emitting diode) for 20 h. However, when Ir PS was used
instead of Ru PS, we observed much less oxidation of styrene
to benzaldehyde, with a TON of 7. The above-mentioned
experiments suggested that Ru PS is more active than Ir PS
for styrene oxidation when [Co(NH ) Cl] is used as electron
acceptor.
2
+
3
5
1
8
16
H2 O instead of H2 O. A molecular ion peak at 108,
1
8
Previous studies have shown that Ru PS displays two
corresponding to the incorporation of O into benzaldehyde,
redox potentials at 1.26 V and À1.36 V (vs. a saturated
was observed with a + 2 shift from the peak at 106 when
H2 O was used. This result clearly showed that water is the
III/II
À
16
calomel electrode, SCE), assignable to Ru
and bpy/bpy ,
respectively. Similarly, the two redox potentials for Ir PS are
source of oxygen for styrene oxidation.
IV/III
at 1.25 V and À1.42 V (vs. SCE), corresponding to Ir
and
To determine the source of H formation, we conducted
2
À
[15,16]
bpy/bpy , respectively.
Considering the similar redox
the photolysis reaction using D O instead of H O. The
2
2
properties of Ru PS and Ir PS, we reasoned that electron
transfer between Ru PS and Ir PS should be feasible. When
both photosensitizers are used, it may be possible to couple
the oxidation of hydrocarbons to proton reduction. Therefore,
formation of H and D can be distinguished by GC-TCD
2 2
analysis using He as carrier gas. As shown in Figure S5 in the
Supporting Information, when H O was used, a negative peak
2
appeared at 0.8 min in the GC-TCD chromatogram, consis-
we carried out photolysis of styrene in a THF/H O (9:1)
tent with the formation of H . In contrast, a positive peak at
2
2
solution containing 1, Ru PS and Ir PS, and 2 as catalyst for H2
0.8 min was observed when D O was used, suggesting the
2
production. As shown in Figure 1, no H was detected before
formation of D . These results clearly showed that water is the
2
2
photolysis. After exposure to LED light (450 nm) for 3 h, a
peak at 0.9 min appeared, corresponding to the formation of
H . Residual amounts of O and N with retention times at
source of hydrogen in the photolytic reaction. Therefore, our
studies from both GC-MS and TCD analyses clearly demon-
strated that H O is not only the oxygen source for olefin
2
2
2
2
1
.9 min and 3.3 min, respectively, were also observed. The
oxidation, but also the proton source for H generation.
2
peak at 0.9 min continued to increase over time and reached
the maximum after 18 h of light irradiation. In the end, a TON
The effects of different light sources on the activity of
water splitting were investigated using a visible light source
of 77 was found for the H production based on complex 1
and natural sunlight. Although H formation was observed
2
2
when a 500 W halogen lamp was used, a much longer time is
needed for the reaction to be complete and a smaller turnover
number (see Figure S6 in the Supporting Information) is
obtained. Oxidation of styrene under ambient sunlight was
also carried out. Although we observed similar conversion
yields to the yields obtained with the halogen lamp, a higher
amount of CO was detected from GC-TCD analysis, probably
because of faster decomposition of iron carbonyl compounds
under UV irradiation of sunlight. The decomposition of iron
carbonyls under UV light irradiaton is well-known, and CO
has been observed during photocatalytic H formation by iron
2
[14]
carbonyls.
To optimize the conditions for H2 production, the
concentration of each component was varied to investigate
Figure 1. GC/TCD chromatogram of H production over time. Condi-
tions: Styrene (1 mmol), 1 (2 mmol), Ru PS (8 mmol), Ir PS (2 mmol), 2
2
their effects on H evolution. Among the conditions listed in
2
(
8 mmol), LED light (450 nm), 10 mL THF/H O (9:1), 18 h.
Table S1 in the Supporting Information, the best activity for
2
1
654
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1653 –1656