Table 1 Reduction of various substrates with coupling CdS and
complex 1 under irradiationa
Substrate
Product
Time/h Selectivity (%) TON
6
6
100
100
381
220
6
3
100
100
320
790
Fig. 4 Comparison of the activities of iridium complex 1 for the
reduction of cyclohexanone at different pH conditions in water.
a
The initial pH of solution of water/lactic acid is 6.5. See Fig. 2 for
other conditions.
Reaction conditions: (
) aqueous NaCOOH (1 mM); (
) aqueous
NaCOOH (1 mM) and lactic acid (1 mL); (
) CdS (10 mg) and
lactic acid (1 mL) under irradiation (l 4 400 nm). TOF based on the
conversion for 1 h.
pyruvate delivered a TON of 790 in 3 h, and initial TOF
exceeds 500 hꢀ1
The maximum TOF of 540 hꢀ1 was obtained at pH 3.5 and the
catalytic activity was diminished sharply at pH higher than 4.
While adding lactic acid into aqueous HCOONa, the reaction
showed a similar trend but lower acitivities at corresponding pH
values. Interestingly, the photocatalytic system, coupling CdS
with the iridium complex, responds contrarily to the pH change.
The initial TOF is 100 hꢀ1, and the catalytic activity was steadily
enhanced with the increase of solution pH. The peak appeared
.
In summary, we developed a highly selective photocatalytic
system for organic reduction reaction under visible light
irradiation, by coupling CdS nanoparticles with iridium-based
complexes. In the hybrid systems, the semiconductor harvests
light energy, and complexes are activated by the photo-driven
electron, and catalyze the subsequent reactions. The strategy
of activating catalysts by photoexcited electron-transfer
provides a promising method for various photocatalytic
organic reactions, particularly for reductions and oxidation
reactions. Furthermore, the strategy can be expanded to the
utilization of solar energy to produce H2 and chemicals from
H2O and CO2.
at ca. pH 6.5, delivering the maximum TOF of 400 hꢀ1
.
The different response to solution pH in the photocatalytic
system indicates that the electron transfer between CdS and
the iridium complex is affected greatly by the pH of solution. In
contrast, we didn’t observe similar pH-dependence using the
iridium complex with the –OMe group under irradiation.
Iridium complex 1 with two phenolic hydroxyl groups
existed as a protonated form at pH below 2.6, and a deproto-
nated oxyanion form at pH 7.3, as reported by Himeda et al.13
As shown in Fig. 5, the iridium complex 1 with oxyanion form
has stronger adsorption ability than that with a phenolic
hydroxyl at the surface of CdS. Furthermore, the oxyanion
form of complex 1 possesses a unique conjugated structure
between the oxyanion and the phenyl group, possibly favoring
the electron transfer between CdS and complex 1.
This work was supported by the National Natural Foundation
of China (No. 21003120), Program for Strategic Scientific
Alliances between China and Netherlands (No. 2008DFB50130).
We also thank Prof. Qihua Yang and Dr Sheng-mei Lu for the
revision of manuscript.
Notes and references
1 T. P. Yoon, M. A. Ischay and J. N. Du, Nat. Chem., 2010, 2, 527–532.
2 K. Zeitler, Angew. Chem., Int. Ed., 2009, 48, 9785–9789.
3 M. A. Ischay, M. E. Anzovino, J. Du and T. P. Yoon, J. Am.
Chem. Soc., 2008, 130, 12886–12887.
4 O. Reiser, T. Maji and A. Karmakar, J. Org. Chem., 2011, 76,
736–739.
A series of other substrates were tested with the coupled
1/CdS system under neutral conditions, and the results are
summarized in Table 1. For example cyclohexanecarboxaldehyde
was reduced efficiently, giving a TON of 381 in 6 h. Aceto-
phenone was reduced with a TON of 220 in the same time. The
CQC bond was selectively reduced for benzylideneacetone
in neutral conditions, giving a TON of 318. Particularly
a-ketoester shows much higher activity. For instance, ethyl
5 D. A. Nicewicz and D. W. C. MacMillan, Science, 2008, 322, 77–80.
6 H. W. Shih, M. N. Vander Wal, R. L. Grange and D. W. C.
MacMillan, J. Am. Chem. Soc., 2010, 132, 13600–13603.
7 M. A. Zhang, C. C. Chen, W. H. Ma and J. C. Zhao, Angew.
Chem., Int. Ed., 2008, 47, 9730–9733.
8 F. Z. Su, S. C. Mathew, G. Lipner, X. Z. Fu, M. Antonietti,
S. Blechert and X. C. Wang, J. Am. Chem. Soc., 2010, 132,
16299–16301.
9 I. B. Rufus, B. Viswanathan, V. Ramakrishnan and J. C. Kuriacose,
J. Photochem. Photobiol., A, 1995, 91, 63–66.
10 D. H. Nam, S. H. Lee and C. B. Park, Small, 2010, 6, 922–926.
11 C. B. Park, S. H. Lee, E. Subramanian, B. B. Kale, S. M. Lee and
J. O. Baeg, Chem. Commun., 2008, 5423–5425.
12 H. K. Song, S. H. Lee, K. Won, J. H. Park, J. K. Kim, H. Lee,
S. J. Moon, D. K. Kim and C. B. Park, Angew. Chem., Int. Ed.,
2008, 47, 1749–1752.
13 Y. Himeda, N. Onozawa-Komatsuzaki, S. Miyazawa, H. Sugihara,
T. Hirose and K. Kasuga, Chem. Eur. J., 2008, 14, 11076–11081.
Fig. 5 Acid–base behavior of complex 1 at different pH.
7082 Chem. Commun., 2011, 47, 7080–7082
c
This journal is The Royal Society of Chemistry 2011