C O MMU N I C A T I O N S
Table 1. Data of kq and Φ in the Case of 2a as the Photocatalyst
substrate 3a 3a-d-1 3a-d-4 3b 3c 3d 3g 5a 5c
8.7 5.7 5.6 4.5 3.8 4.5 no quenching
Scheme 3
kq (×109 M-1) 8.7 6.1
Φ
0.38 0.31 0.32 0.23 0.29 0.16 0.20 0.28 no reaction
mercury lamp. A glass filter was employed to cut off light with
wavelength below 450 nm; thus only the photocatalyst was
irradiated. The generated hydrogen was collected and analyzed by
GC and a hydrogen detector (Oldham Detector Ltd., Arras, France),
and its volume was measured at the atmosphere pressure and
constant temperature. After irradiation, the solvent was removed
under reduced pressure, and the oxidized products were isolated
by extraction with ethyl acetate and identified by 1H NMR
spectroscopy and on the basis of known retention times of authentic
compounds on GC. Control experiments showed that both the
photocatalyst and light were necessary for hydrogen production.
For substrates 3, only the dehydrogenated products 4 (Scheme 1)
were produced, and no trace of the dealkylated product was
detected. The molar ratio of hydrogen to 4 was ca. 1:1, and the
yield of 4 (therefore the yield of hydrogen) was ca. 100% based
on the consumption of the starting material for any substrate of
3a-3g. The quantum yields (Φ) of the photocatalytic hydrogen
production in the case of 2a as the photocatalyst were determined
for the substrates with the initial concentration of 1 × 10-2 M and
are listed in Table 1.
is smaller than those for 3a and 3a-d-4. (2) The N-alkyl substituted
substrate 5c cannot be photocatalyzed to dealkylation by the
photocatalyst. (3) 5a and 5b undergo photocatalytic dealkylation
rather than dehydrogenation. The generated radical pair, [Pt
complex]‚H and 3• (or 5•), in a solvent cage undergoes either back
hydrogen atom transfer to yield the starting materials or dispro-
portionation (step 3, Scheme 3) to produce pyridines 4, and [Pt
complex]‚H2 (in the case of 3 as the substrate) or [Pt complex]‚
HR1 (in the case of 5 as the substrate). Finally, elimination of
hydrogen and alkane from the respective intermediates [Pt complex]‚
H2 and [Pt complex]‚HR1 regenerates the photocatalyst Pt(II)
complex and completes the photocatalytic cycle. The data of kq
and Φ for deuterated 3a in Table 1 support the above mechanism.
Steps 1 and 3 in Scheme 3 involve the transfer of the hydrogen
atom at the 1- and 4-positions of the 1,4-dihydropyridines,
respectively. Deuterium substitution of these hydrogen atoms would
retard the atom transfer, and thus decrease the quantum yield for
product formation.
To summarize, we have proved that platinum(II) terpyridyl
complexes can photocatalyze hydrogen production from Hantzsch
dihydropyridine derivatives in quantitative yields. Our further efforts
will be focused on expanding the scope of the photocatalyst and
the substrate, which can produce hydrogen in high quantum yield
with large catalytic turnover.
In contrast, irradiation of the solutions of 5a and 5b in acetonitrile
in the presence of a photocatalyst under similar conditions described
above resulted in dealkylated pyridine 4a and alkanes 6 (Scheme
2), and no trace of hydrogen was produced. The molar ratio of 4a
to 6 was 1:1, and the mass balance was greater than 95%.
To ascertain the photocatalytic character for the hydrogen
production and the dealkylation reaction, we measured the yield
of the oxidation products (4) as a function of irradiation time and
found that the product formation rate was constant during the
irradiation, indicating the absence of photocatalyst decomposition.
On the basis of the molar ratio of the oxidation product to the
catalyst, the total turnover at the end of irradiation was over 1000.
To provide information on the mechanism of the photocatalytic
oxidation, we performed the deuterium substitution of the hydrogen
atom at the 1-position (3a-d-1) and the two hydrogen atoms at the
4-position (3a-d-4) of 3a. As 3a, the deuterated compounds also
quench the photoluminescence of 2a, and the quenching constant
for 3a-d-1 is evidently smaller than those for 3a and 3a-d-4 (Table
1). However, the quantum yield of production formation for 3a-
d-1 is comparable to 3a-d-4 and smaller than 3a (Table 1).
The mechanism for this photocatalytic hydrogen production and
dealkylation is not yet well understood. We believe that the lowest
energy excited state (3MLCT) of the photocatalyst is responsible
for these reactions. This triplet state can be considered to have
biradical character. Furthermore, as in the ground-state chemistry,
vacant coordination sites in the metal center of the platinum(II)
terpyridyl complexes also would play a role in the photocatalysis.14
As shown in Scheme 3, the radical character and the availability
Acknowledgment. Financial support from the Ministry of
Science and Technology of China (Grant Nos. G2000078104 and
G2000077502) and the National Science Foundation of China
(Grant Nos. 20125207, 20202013, 20272066, 20332040, and
20333080) is gratefully acknowledged.
Supporting Information Available: The synthesis and identity of
2, 3, 5, and deuterated 3a, absorption spectra and emission quenching
of 2, measurements of photocatalytic quantum yields and turnover, data
of 1H NMR spectroscopy of 4 (PDF). This material is available free of
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3
of open coordination sites make the MLCT state able to abstract
a hydrogen atom from the 1,4-dihydropyridines 3 (or 5) to form
[Pt complex]‚H and radical 3• (or 5•). We proposed that the first
step for the photocatalytic reaction involves the hydrogen atom
abstraction from the 1-position rather than the 4-position of 3 (or
5) on the basis of the following observations: (1) The kq of 3a-d-1
JA037631O
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