Photocatalytic hydrogen evolution by [FeFe] hydrogenase mimics in homogeneous solution

Article abstract of DOI:10.1002/asia.201000087

To mimic [FeFe] hydrogenases (H2ases) in nature, molecular photocatalysts 1a, 1b, and 1c anchoring rhenium(I) complex S to one of the iron cores of [FeFe]-H₂ases model complex C, have been constructed for H ₂ generation by visible li

Full text of DOI:10.1002/asia.201000087

FULL PAPERS
DOI: 10.1002/asia.201000087
Photocatalytic Hydrogen Evolution by [FeFe] Hydrogenase Mimics in
Homogeneous Solution
Wen-Guang Wang, Feng Wang, Hong-Yan Wang, Gang Si, Chen-Ho Tung, and
Li-Zhu Wu*[a]
Abstract: To mimic [FeFe] hydrogenas-
es (H2ases) in nature, molecular photo-
catalysts 1a, 1b, and 1c anchoring rhe-
nium(I) complex S to one of the iron
cores of [FeFe]-H2ases model complex
C, have been constructed for H2 gener-
ation by visible light in homogeneous
solution. The time-dependence of H2
evolution and a spectroscopic study
demonstrate that the orientation of S
and the specific bridge in 1a, 1b, and
1c are important both for the electron-
transfer step from the excited S* to the
catalytic C, and the formation of un-
precedented long-lived charge separa-
tion for 1a (780 ms), 1b, and 1c
(>2 ms) in [FeFe]-H2ases mimics. The
fast forward electron-transfer step from
the excited S* to the catalytic C but
the slow back electron-transfer step of
the charge-recombination in the de-
signed photocatalysts 1a, 1b, and 1c
are reminiscent of the behavior of
[FeFe]-H2ases in nature.
Keywords: electron
transfer
·
hydrogen · iron · photochemistry ·
sensitizers
Introduction
linked Fe4S4 cluster, carbon monoxide, and cyanide ligand,
and by dithiolate bridging the two iron centers
a
Catalysis of photochemical hydrogen evolution by molecular
catalysts has aroused much attention over years because mo-
lecular hydrogen (H2) is envisaged as a sustainable alterna-
tive of classical fuels for the future.[1] The use of sunlight has
enormous potential, as solar energy is clean, abundant, and
economical. Long ago, nature figured out how to use photo-
synthetic complex to capture sunlight, and then to store its
energy in a chemical form, H2, in which hydrogen metabo-
lism, named hydrogenases (H2ases), acts as catalyst to oper-
ate the reversible oxidation of H2 to protons and electrons
[Eq. (1)][2]:
(Scheme 1), chemists started to build molecular photocata-
lysts anchoring a sensitizer to the model of [FeFe]-H2ases
active site for H2 generation using visible light.[8] Two differ-
ent strategies have been put forward. In the first strategy,
the dithiolate-bridge is employed to connect the sensitizer
and [FeFe]-H2ases catalytic unit.[9–11] The alternative ap-
proach involves anchoring the sensitizer to one of the iron
centers of the [FeFe]-H2ases active site.[12,13] Following the
two strategies, several covalently linked molecular systems
of sensitizer-[FeFe]-H2ases active site have been constructed.
However, examples of photochemically driven proton-re-
duction systems using molecular H2ases mimics as catalysts
for H2 evolution in homogeneous solution are quite scarce,
especially for the second approach. In this regard, Ott and
co-workers made use of phenylphosphine-derived RuII li-
gands to attach one of iron centers of the [FeFe]-H2ases
active site.[13] Electrochemical and photophysical studies sug-
gested that the more negative potential of the active site of
[FeFe]-H2ases with respect to the excited state reduction po-
tential of RuII sensitizer renders the photo-reduction of the
[FeFe]-H2ases active site thermodynamically unfeasible,
leading to intramolecular photochemical H2 production
from such systems rather challenging.[14] Very recently, van
2Hþþ2e $ H2
ð1Þ
With the structural elucidation of [FeFe]-H2ases,[3–7] fea-
turing a butterfly Fe2S2 subunit coordinated by a cysteine-
[a] W.-G. Wang, F. Wang, H.-Y. Wang, Dr. G. Si, Prof. C.-H. Tung,
Prof. L.-Z. Wu
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials
Technical Institute of Physics and Chemistry
The Chinese Academy of Sciences
Beijing 100190 (P. R. China)
Fax : (+86)10-8254-3580
E-mail: lzwu@mail.ipc.ac.cn
Leeuwen and Reek reported
a self-assembled [FeFe]-
H2ases-based photocatalyst, in which a pyridyl-functional-
ized [FeFe]-H2ases active site was directly coordinated to
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000087.
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Chem. Asian J. 2010, 5, 1796 – 1803

It is expected that the linear,
rigid, and conjugated bridge
would satisfy the precise con-
trol over the spatial separation
between S and C to achieve ef-
ficient intramolecular electron
transfer from the excited state
S* to the catalytic site C, and at
the same time, to aviod the fast
back electron transfer of their
charge-separated state.
Results and Discussion
The molecular photocatalysts
1a, 1b, and 1c in this work
could be synthesized in reason-
able yields, as shown in
Scheme 2. For 1a, the active
site of [FeFe]-H2ases model
complex attached by 4-iodo-
phenyl-isocyanide is coupled
with rhenium(I) tricarbonyl di-
AHCTUNGERTGiNNUN mine alkyne by the Sonoga-
Scheme 1. ACHTUNGTRENNUNG[FeFe] Hydrogenases in nature, [FeFe] hydrogenase mimics, and references.
shira reaction. For 1b and 1c,
the iron core of the [FeFe]-
H2ases model with 4-(4-alkynyl-
zinc(II)-porphyrin sensitizers.[15] Despite of not being entire-
ly understood, only H2 was produced as two different types
of zinc(II)-porphyrin complexes were present in the [FeFe]-
H2ases solution upon irradiation at l>530 nm.
In this contribution, we report three molecular photocata-
lysts: 1a, 1b, and 1c, as photocatalytic mimics for H2 evolu-
tion in homogenous solution at room temperature
(Scheme 1). Herein, rhenium(I) complex was selected as a
photosensitizer S owing to its visible-light absorption, long
excited-state lifetime, more negative excited state reduction
potential, thermal and photochemical stability;[12,16] the typi-
pyridyl)phenyl-isocyanide is anchored to
center by direct coordination. The references 2a, 2b, 2c,
and 1d {[m-S2A(CH2)3]Fe2(CO)5CN} were also prepared by
a rhenium(I)
CTHUNGTRENNUNG
similar procedures. All of the complexes are quite soluble in
various solvents and have been well-characterized by
1H NMR, MS, and elemental analysis.
The photochemical H2 evolution was carried out in de-
gassed solution of CH3CN/CH3OH/H2O (v/v/v=3:2:1) in
the presence of acetic acid (HOAc) as proton source at
room temperature. CH3OH, a well-known electron donor,[18]
served as sacrificed electron donor (D) in this work. Typical-
ly, the solution of the photocatalyst (1a, 1b, or 1c) and
HOAc (0.01m) in a Pyrex tube were irradiated by a 500-W
high-pressure Hanovia mercury lamp. A glass filter was
used to cut off light below 400 nm, and thus guarantee the
irradiation by visible light. The generated H2 was collected
and analyzed by GC with methane as the internal standard.
The response factor for H2/CH4 determined by calibration
with known amounts of H2 and CH4 was 3.0 under the ex-
perimental conditions. The time-dependence shows that the
amount of H2 evolution increased in the first 60 min
(Figure 1). Further irradiation can lead to the decomposition
of the catalysts, and thus slows down the rate of H2 evolu-
tion, similar to the other reported [FeFe]-H2ases
mimics.[11,15] Irrespective of H2 dissolution in the solvent,
with one hour of irradiation, the amount of H2 reaches
0.17 mmol for 1a, 0.26 mmol for 1b, and 0.28 mmol for 1c, re-
spectively. Control experiment shows that the amount of H2
production from 1a, 1b, and 1c are more than that of their
cal active site model of [m-S2ACHTNUTRGNEN(UG CH2)3]Fe2(CO)5CN was used
as a catalytic reaction center C. A cyanide (CN) group,
which is one of the most distinctive features of H2ases in
nature,[17] is incorporated in the bridge (L) to easily anchor
the rhenium(I) S to the iron core of Fe2S2 catalytic center C.
Abstract in Chinese:
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L.-Z. Wu et al.
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Scheme 2. a) Me3NO·2H2O, phenylisocyanide, RT; b) Me3NO·2H2O, 4-iodophenylisocyanide, RT; c) 4-ethynylpyridine, PdACTHUNRGTNEUNG(PPh3)2Cl2/CuI, THF/Et3N, Ar,
608C; d) NaH, 1-bromo-4-bromomethylbenzene, dry DMF, RT; e) 2-methyl-3-butyn-2-ol, Ar, Pd
toluene; g) 3, Pd(PPh3)2Cl2/CuI, THF/Et3N, Ar, 608C; h) Re(CO)5Br, toluene, Ar, 608C; i) NaH, 4-bromomethylbenzene, dry DMF, RT; j) 4, AgSO3CF3,
THF, Ar, reflux; k) 1-(4-pyridyl)-2-phenylethyne, AgSO3CF3, THF, Ar, reflux.
ACHTUGNTRNE(UNNG PPh3)2Cl2/CuI, dry hexahydropyridine, reflux; f) KOH,
ACHTUNGTRENNUNG
Figure 1. Time-dependence of H2 evolution from a solution of CH3CN/CH3OH/H2O (v/v/v=3:2:1, 6 mL) in the presence of HOAc (10 mm) photocata-
lyzed by molecular photocatalysts or their reference complexes (0.33 mm).
ꢀ
corresponding intermolecular multi-component systems: for
reference 2a, 2b, or 2c, and 1d only about 0.11 mmol H2
have been detected under the identical experimental condi-
tion. Accordingly, the turnover number of the generated H2
was calculated as 0.08 for 1a, 0.13 for 1b, 0.14 for 1c, and
about 0.05 for the intermolecular reference systems with re-
spect to the amount of photocatalysts used in the reactions.
Clearly, the combination of rhenium(I) S and Fe2S2 catalytic
C is promising for efficient H2 evolution. The difference in
the H2 evolution efficiency for 1a, 1b, and 1c implies that
the orientation of the rhenium(I) complex, the specific
bridge, and conjugation of the auxiliary ligand play crucial
roles in mediating the light-driven H2 production.
exhibit intense intraligand p!p*
ACHTUNGERTN(NUNG N N) absorption bands at
wavelengths below 350 nm with extinction coefficients (e) in
the order of 104 dm3 molꢀ1 cmꢀ1, and a broad dp(Re)!p*-
ꢀ
(N N) MLCT band in the region of 350–500 nm with e in
the order of 103 dm3 molꢀ1 cmꢀ1. For 1a, 1b, and 1c, the cata-
lytic C of [FeFe]-H2ases unit also show weak absorption in
the region of 420–600 nm (Figure S1 in the Supporting Infor-
ꢀ
mation). Excitation of the dp(Re)!p*
ACHTUNGTREN(NUNG N N) MLCT char-
acteristic absorption at around 400 nm leads to maximal
emission at 613 nm and 620 nm with a quantum yield of
1.9ꢁ10ꢀ3 and 3.6ꢁ10ꢀ3 for 1a and 2a, respectively
(Table 1). The luminescent efficiency of 1a is approximately
20% less than that of 2a, while with the addition of 1d into
the solution under the same conditions, not much change
could be observed in the luminescent character of 2a (Fig-
ure S2 in the Supporting Information). Analysis of the
decay showed that the luminescence of 1a and 2a was mo-
To gain insight into the intramolecular [FeFe]-H2ases
mimics for H2 evolution in homogeneous solution, we exam-
ined the photophysical properties of the molecular photoca-
talysts, together with their references in deaerated CH3CN
solution. The UV/Vis absorption spectra of each complex
AHCTUNGTREGnNNNU oexponential with a lifetime of 28 ns and 36 ns, respective-
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Chem. Asian J. 2010, 5, 1796 – 1803

Photocatalytic Hydrogen Evolution
Table 1. Luminescence and electrochemical data for [FeFe] H2ases mimics and references complexes.
the bleach occurs on the same
time scale, and can be well-de-
scribed by a monoexponential
function with an unexpectedly
long lifetime of 780 ms for 1a
and >2 ms for 1b and 1c (no
decay was observed within in-
strument limitation of 2 ms).
The generated new absorption
is different from the profiles of
2a, 2b, and 2c, but quite similar
to the reported FeIFe0 species
generated by the reduction of
[FeFe]-H2ases models.[21] The
red-shifted absorption maxima
for 1a, 1b, and 1c in compari-
son to that of the FeIFe0 species
of all CO [FeFe]-H2ases models
may have resulted from the
stronger electron-donating phe-
nylisocyanide ligand at the iron
Complex lmax [nm] Emission Lifetime E00[a] [eV] Epa [V][b] Epa [V][b] Epc [V][b] Epc [V][b]
DGo[c] [eV]
F
t [ns]
E1/2
E1/2
E1/2
Fe2S2
E1/2
ReI/ReII Fe2S2
Pybm[d]
1a
1b
1c
1d
2a
2b
2c
613
na
1.9ꢁ10ꢀ3 28
2.50
0.83
0.63,
0.35
0.62,
0.33
0.62,
0.34
0.62,
0.36
na
ꢀ1.62
ꢀ1.60
ꢀ1.59
ꢀ1.58
na
ꢀ1.64,
ꢀ2.04
ꢀ1.56
ꢀ0.05
ꢀ0.12
ꢀ0.40
na
na
na
na
na
na
na
2.53[e]
2.83[e]
0.81[f]
0.84
na
ꢀ1.52,
ꢀ1.76
na
na
na
na
620
570
550
3.6ꢁ10ꢀ3 36
2.50
2.53
2.83
0.83
ꢀ1.65,
ꢀ2.01
ꢀ1.57,
ꢀ1.86
ꢀ1.52,
ꢀ1.75
na
0.027
0.300
484
1478
0.81[f]
0.84
na
na
na
na
na
na
[a] The excited state energy, which was calculated by using the wavenumbers at the intersection between emis-
sion and excitation spectra. [b] All potentials versus Fc/Fc+. [c] DGo =E
N
G
the work term arising from Coulombic interaction.[19] [d] It refers to reduction on ligand. [e] We assumed that
the excited state energy for 1b and 2b, 1c and 2c are the same. [f] The reported values for ReI analogues.[20]
ly. The quenching constant and quenching efficiency were
determined as 1.3ꢁ107 sꢀ1 and 20% for 1a, similar to the lu-
minescence-efficiency determination. More strikingly, the lu-
minescence of 1b and 1c was completely quenched in
CH3CN although their references 2b and 2c were highly
emissive. Clearly, the intramolecular quenching of the excit-
ed S* by the catalytic C in 1a, 1b, and 1c operates. The
larger quenching extent for 1b and 1c than 1a suggests the
more efficient intramolecular quenching of the excited S*
by the [FeFe]-H2ases active site C.
The structural changes in 1a, 1b, and 1c that result in dif-
ferent amounts of H2 generation and luminescence-quench-
ing efficiency promoted us to evaluate the driving force DG8
of the photochemical reaction. The electrochemical behavior
of 1a, 1b, and 1c was studied by cyclic voltammetry in
CH3CN (0.1m nBu4NPF6 as electrolyte) under argon atmos-
phere. According to the determined oxidation potential Eox
of S, reduction potential Ered of C, and the excited state
energy E00 of S (Table 1), the free-energy change (DG8) was
calculated to be ꢀ0.05 eV for 1a, ꢀ0.12 eV for 1b, and
ꢀ0.40 eV for 1c, respectively. The results indicate that the
electron-transfer processes from the excited S* to the cata-
lytic site C in the designed photoinduced systems are exo-
thermic. The greater driving force is responsible for the
more effective photoinduced intramolecular electron trans-
fer, thereby, resulting in the luminescence quenching and H2
generation with higher efficiency.
core of the [FeFe]-H2ases model. Prolonged irradiation of
1a, 1b, and 1c in CH3CN solution led to no permanent
change, indicating that the FeIFe0 species formed by irradia-
tion is stable. If we assume that the generated new absorp-
tion originates from the FeIFe0 species of the reduced
[FeFe]-H2ases model alone, the rate (kCR) for the back elec-
tron transfer is determined to be 1.28ꢁ103 sꢀ1 for 1a and
<0.50ꢁ102 sꢀ1 for 1b and 1c, respectively.
On the basis of the above results, it could be speculated
that the photocatalytic H2 evolution involves two photo-
physical processes: one is the forward electron transfer from
the excited sensitizer S* to the [FeFe]-H2ases catalytic
center C, and the other is the back electron-transfer step of
the charge-recombination (S+·-Cꢀ·), as simulated in
Scheme 3. When the sensitizer S in the molecular photocata-
Flash photolysis investigation reveals the formation of a
long-lived charge separated state with pulsed excitation of
1a, 1b, and 1c. Upon laser excitation using 355 nm light, a
Scheme 3. Proposed photophysical pathway for the molecular photocata-
lysts.
3
strong transient absorption of the MLCT state for referen-
ces 2a, 2b, and 2c emerged immediately (Figure 2). In con-
trast, 1a, 1b, and 1c display intense absorption throughout
the near-UV and visible region with the maximum at 410 nm
for 1a, 430 nm for 1b, and 420 nm for 1c, the decay of
which throughout the absorption region and the recovery of
lyst is excited, photoinduced intramolecular electron trans-
fer (forward reaction) takes place to generate the reduced
FeIFe0 species. The greater driving force leads to the more
effective photoinduced intramolecular electron-transfer pro-
cess (1c>1b>1a). On the other hand, transient absorption
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L.-Z. Wu et al.
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photochemically driven H2 production because the long-
lived FeIFe0 species can react with a proton more efficiently
to generate a [FeIFeII·H] intermediate for H2 production. As
1c is the greatest of the three photocatalysts in terms of
driving force, and at the same time possesses the longest-
lived charge separation, its efficiency for photocatalytic H2
evolution is therefore the highest.
Conclusions
We have designed and synthesized three molecular photoca-
talysts 1a, 1b, and 1c with sensitizer S directly linked to one
of iron centers of [FeFe]-H2ases active site C by two differ-
ent linkage orientations. Time-dependence of H2 evolution
and spectroscopic study demonstrate that the orientation of
S and the specific bridge in 1a, 1b, and 1c are important
both for the forward electron-transfer step from the excited
S* to the catalytic C and the back electron-transfer step of
the charge-recombination (S+·-Cꢀ·). In terms of steady-state
and time-resolved techniques, we have demonstrated that in
1a, 1b, and 1c, the forward electron-transfer step is much
faster, but the back electron-transfer step of the charge-re-
combination is quite slow, which is reminiscent of the behav-
ior of [FeFe]-H2ases in nature.
Experimental Section
Materials and Instrumentation
All reactions and operations were carried out under a dry argon atmos-
phere with standard Schlenk techniques. All solvents were dried and dis-
tilled prior to use according to the standard methods.[22] Fe(CO)5 was pur-
chased from Lanzhou Institute of Chemistry and Physics, CAS. 1,10-Phe-
nanthroline, 2-pyridinylbenzoimidazole, Re(CO)5Br, 4-ethynylpyridine,
AgCF3SO3, 2-methyl-3-butyn-2-ol, Me3NO·2H2O, and dichlorobis[(tri-
phenyl)phosphine]-palladium were purchased from Aldrich and used as
received. (m-CACTHNUTRGNEUNG
(SCH2)2)[Fe(CO)3]2,[23] 4-iodophenylisocyanide, and phe-
nylisocyanide,[24] 1-bromo-4-bromomethylbenzene, and 4-bromomethyl-
benzene[25] were prepared according to the literatures. Sonogashira reac-
tion, the palladium-catalyzed cross-coupling reaction of terminal alkynes
with aryl halides, was carried out according to the modified procedures.[26]
1-(4-pyridyl)-2-phenylethyne and the ReI complexes were prepared by
following the reported procedures.[27]
Figure 2. Transient absorption spectra of a) 2a and 1a, b) 2b and 1b,
c) 2c and 1c upon laser pulse by 355 nm light in deoxygenated solution
of CH3CN (0.05 mm) under argon atmosphere. And, the kinetic trace re-
corded at 390 nm for 2a and 410 nm for 1a; 390 nm for 2b and 430 nm
for 1b; 395 nm for 2c and 420 nm for 1c, respectively.
1H NMR spectra were recorded on a Bruker 400 FT-NMR spectrometer,
and chemical shifts are given relative to tetramethylsilane. Infrared spec-
tra were recorded on a Nicolet NEXUS 670 FTIR spectrophotometer.
Mass spectra were recorded with Trio-2000 GC–MS spectrometers. Ele-
mental analyses were determined on a FLASH EA1112 elemental ana-
lyzer. UV/Vis spectra were measured on a Shimadzu UV-1601PC spec-
trophotometer. Steady-state emission spectra were recorded on a Perkin–
Elmer LS50B spectrofluorimeter. Luminescence lifetimes were measured
by using an Edingburge LP920 apparatus from Analytical Instruments.
Elemental analyses were performed by using a Carlo Erba 1106 elemen-
tal analyzer. Room-temperature luminescence quantum yields, F, were
spectra provide direct evidence on the formation of long-
lived charge separated species S+·-Cꢀ· following the effective
photoinduced electron-transfer process. The rate (kCR) for
the backward reaction is quite slow. Consequently, the
charge-separated species is reacted with proton and sacrifi-
cial electron donor to get sensitizer recovery and to produce
a [FeIFeII·H] intermediate.[11,14] The orientation of the sensi-
tizer S and the specific bridge L in molecular photocatalysts
1a, 1b, and 1c was found to be of importance for the forma-
tion of the long-lived charge separated state. The unprece-
dented long-lived charge separation for 1a (780 ms), 1b
measured by using [RuACTHNUTRGNEUNG
(bpy)3]2+ as a reference.[28] In the transient absorp-
tion spectroscopy experiments, the samples (5.0ꢁ10ꢀ5 m) were purged
with argon for 30 min. Excitation was provided by using an Nd:YAG
laser (third harmonic, 10 ns) at 355 nm. The detector was a xenon lamp
on the Edingburge LP920 apparatus from Analytical Instruments. Vol-
tammetric experiment was manipulated on a Princeton Applied Research
Potentionstat-gravanostat model 283. Acetonitrile (Fisher Chemicals,
ACHTUNGTRENNUNG
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Chem. Asian J. 2010, 5, 1796 – 1803

Photocatalytic Hydrogen Evolution
HPLC grade) used for the performance of the electrochemistry was dis-
tilled under argon atmosphere once from CaH2, once from P2O5, and
CaH2 again. nBu4NPF6 (0.1m) in CH3CN was used as electrolyte in all
cyclic voltammetric measurement. The electrolyte solution was degassed
by bubbling with argon for 10 min before measurement. A three-elec-
trode system is used for the measurement and bulk electrolysis, with a
3 mm glass carbon working electrode, a platinum wire counter electrode,
and a nonaqueous Ag/Ag+ reference electrode. The working electrode
was polished with a 0.05 mm alumina paste and sonicated for 15 min
before use.
um ether (v/v=1:4) to afford complex 7 as red solids (90%). IR
(CH2Cl2): n˜ =1967, 1997, 2038 (CO); 2126 (CN); 2217 cmꢀ1 (CꢁC);
1H NMR (400 MHz, CD3CN): d=8.69 (s, 1H), 8.45 (s, 1H), 7.92 (s, 1H),
7.78 (s, 1H), 7.29–7.53 (m, 10H), 7.19 (d, J=7.44 Hz, 2H), 6.23 (s, 2H),
2.19 (b, 2H, SCH2), 2.08 (b, 2H, SCH2), 1.77 ppm (b, 2H, SCH2CH2CH2);
elemental analysis: calcd (%) for C36H24Fe2N4O5S2: C 56.27, H 3.15,
N 7.29; found: C 56.38, H 3.43, N 6.98.
8: Complex 8 was synthesized by the procedure similar to that of 5,
except that 4-bromomethylbenzene (0.60 g, 3.50 mmol) was used in place
of 1-bromo-4-bromomethylbenzene. The reaction mixture was stirred for
another 8 h and then poured into cold water (300 mL); the organic com-
ponents were extracted with CH2Cl2 (3ꢁ50 mL). The organic phase was
washed with water and dried over anhydrous sodium sulfate. After evap-
oration of the solvent, the residue was purified by silica gel column chro-
matography with acetic acid ethyl ester/petroleum ether (v/v=1:15) to
yield white powders (80%). 1H NMR (400 MHz, CD3CN): d=8.63 (d,
J=4.00 Hz, 1H), 8.38 (d, J=8.00 Hz, 1H), 7.93 (m, 1H), 7.76 (m, 1H),
7.40–7.46 (m, 2H), 7.22–7.30 (m, 7H), 6.21 ppm (s, 2H); MS (ESI): m/z
(%): 285.
Synthesis and Characterization
3: 3 was prepared according to the procedures described by literature.[29]
A mixture of (m-CACHTUNGTRENNUNG(SCH2)2)[Fe(CO)3]2 (0.40 g, 1.03 mmol), 4-iodophenyli-
socyanide (0.24 g, 1.03 mmol), Me3NO·2H2O (0.12 g, 1.03 mmol) in
CH3CN (20 mL) was stirred at room temperature for 2 h. The color of
the initially red solution became brownish. After evaporation of the sol-
vent, the residue was eluted with CH2Cl2/petroleum ether (v/v=1:10) on
a silica gel column to afford 3 as brown air-stable solids (60%). IR
(CH2Cl2): n˜ =1961, 1998, 2038 (CO); 2125 cmꢀ1 (CN); 1H NMR
(400 MHz, CDCl3): d=7.79 (d, J=8.68 Hz, 2H), 7.14 (d, J=8.52 Hz,
2H), 2.20 (b, 2H, SCH2), 2.06 (b, 2H, SCH2), 1.90 ppm (b, 2H,
SCH2CH2CH2); elemental analysis: calcd (%) for C15H10Fe2NO5S2I:
C 30.69, H 1.72, N 2.39; found: C 31.17, H 1.97, N 2.47.
1a: 7 (0.154 g, 0.20 mmol) and Re(CO)5Br (0.08 g, 0.20 mmol) were
heated at 608C in toluene (15 mL) for 8 h. After the mixture was cooled
to room temperature, the mixture was concentrated in vacuo, and the
residue was subjected to column chromatography on silica gel (acetic
acid ethyl ester/dichloromethane) to give complex 1a as orange solids
(90%). IR (CH2Cl2): n˜ =1896, 1967, 1998, 2020, 2038 (CO); 2124 (CN);
2213 cmꢀ1 (CꢁC); 1H NMR (400 MHz, CDCl3): d=9.21 ( d, J=5.16 Hz,
1H), 8.18 (d, J=8.04 Hz, 1H), 7.91(d, J=7.28 Hz, 1H), 7.83 (d, J=
7.48 Hz, 1H), 7.47–7.64 (m, 9H), 7.28 (d, J=5.64 Hz, 1H), 7.13 (d, J=
7.76 Hz, 2H), 5.89–5.93 (q, 2H), 2.17 (b, 2H, SCH2), 2.05 (b, 2H, SCH2),
1.81 ppm (b, 2H, SCH2CH2CH2); MS (TOF-EI): m/z (%): 1118; elemen-
tal analysis: calcd (%) for C39H24BrFe2N4O8ReS2: C 41.88, H 2.16, N 5.01;
found: C 41.71, H 2.36, N 4.86.
4: To a well-degassed solution of 4-iodophenylisocyanide, model complex
3 (0.12 g, 0.20 mmol) and 4-ethynylpyridine (0.03 g, 0.29 mmol) in THF/
NEt3 (5:1, 15 mL) were added successively PdACHTNUGRTNE(UGN PPh3)2Cl2 (15 mg) and
then CuI (5 mg). The mixture was concentrated in vacuo and filtered
after stirring under an argon atmosphere at 608C for 3 h. The resulting
black residue was further purified by column chromatography on silica
gel with CH2Cl2 to afford complex 4 as red solids (90%). IR (CH2Cl2):
n˜ =1967, 1995, 2037 (CO); 2122 (CN); 2231 cmꢀ1
ACHTUNGTRENNUNG
(400 MHz, CD3CN): d=8.63 (s, 2H), 7.63 (m, 2H), 7.43 (m, 4H), 2.19 (b,
2H, SCH2), 2.07 (b, 2H, SCH2), 1.91 ppm (b, 2H, SCH2CH2CH2); MS
(ESI): m/z (%): 562; elemental analysis: calcd (%) for C22H14Fe2N2O5S2:
C 47.00, H 2.51, N 4.98; found: C 47.20, H 2.89, N 5.00.
2a: 8 (0.15 g, 0.20 mmol) and Re(CO)5Br (0.08 g, 0.20 mmol) were
heated at 608C in toluene (15 mL) for 8 h. After the mixture was cooled
to room temperature, the mixture was concentrated in vacuo, and the
residue was subjected to column chromatography on silica gel (acetic
acid ethyl ester/dichloromethane) to give complex 2a as orange solids
(90%). IR (CH2Cl2): n˜ =1884, 1912, 2020 cmꢀ1 (CO); 1H NMR
(400 MHz, [D6]DMSO): d=9.17 (d, J=5.20 Hz, 1H), 8.37 (d, J=8.20 Hz,
1H), 8.27 (m, 1H), 8.02 (d, J=7.92 Hz, 1H), 7.92 (d, J=7.60 Hz, 1H),
7.64–7.76 (m, 3H), 7.28–7.36 (m, 3H), 7.11 (d, J=7.12 Hz, 2H), 6.26 ppm
(s, 2H); MS (MALDI-TOF): m/z (%): 635; elemental analysis: calcd
(%) for C22H15BrN3O3Re: C 41.58, H 2.38, N 6.61; found: C 41.44, H 2.41,
N 6.60.
5: NaH (0.13 g, 3.50 mmol) and 2-pyridinylbenzoimidazole (0.69 g,
3.50 mmol) were added to anhydrous N,N-dimethylformamide (DMF,
30 mL) in a flask (100 mL); the solution became clear after 30 min, and
then 1-bromo-4-bromomethylbenzene (0.90 g, 3.50 mmol) was added to
the solution. The reaction mixture was stirred for another 8 h, and then
poured into cold water (300 mL); the organic components were extracted
with CH2Cl2 (3ꢁ50 mL). The organic phase was washed with water and
dried over anhydrous sodium sulfate. After evaporation of the solvent,
the residue was purified by silica gel column chromatography with acetic
acid ethyl ester/petroleum ether (v/v=1:10) to yield white powders
(85%). 1H NMR (400 MHz, CDCl3): d=8.61 (d, J=4.76 Hz, 1H), 8.46
(d, J=7.96 Hz, 1H), 7.85 (m, 2H), 7.26–7.38 (m, 6H), 7.05 (d, J=
8.36 Hz, 2H), 6.14 ppm (s, 2H); MS (ESI): m/z (%): 364.
1b:
A solution of 2a (0.10 g, 0.15 mmol) and AgSO3CF3 (0.06 g,
0.22 mmol) in THF (20 mL) was refluxed for 8 h under an argon atmos-
phere in the dark. The mixture was filtered into a solution of 4 (0.08 g,
0.15 mmol) in THF (5 mL). After further refluxing for 12 h, the solution
was concentrated in vacuo. The crude product was purified by column
chromatography on silica gel with dichloromethane/ethyl acetate (v/v=
3:1) as eluent to give 1b (60%) as red solids. IR (CH2Cl2): n˜ =1963,
1995, 2043 (CO); 2133 (CN); 2225 (CꢁC); 1270, 1174 cmꢀ1 (CF3SO3ꢀ);
1H NMR (400 MHz, CD3CN): d=9.33 (d, J=5.00 Hz, 1H), 8.22 (d, J=
8.00 Hz, 2H), 8.15 (m, 3H), 8.04 (d, J=8.20 Hz, 2H), 7.71–7.81 (m, 4H),
7.58 (d, J=8.59 Hz, 2H), 7.39 (d, J=8.51 Hz, 2H), 7.28 (m, 5H), 6.98 (m,
2H), 5.91–5.95 (q, 2H), 2.18 (b, 2H, SCH2), 2.08 (b, 2H, SCH2),
1.91 ppm (b, 2H, SCH2CH2CH2); MS (ESI): m/z (%): 1118; elemental
analysis: calcd (%) for C45H29F3Fe2N5O11ReS3·CH3COOC2H5: C 43.44,
H 2.75, N 5.17; found: C 43.24, H 2.81, N 5.36.
6: To a solution of 5 (1.26 g, 3.46 mmol) in freshly distilled and saturated
in argon hexahydropyridine (30 mL), and 2-methylbut-3-yn-2-ol (0.36 g,
4.29 mmol), was added in this order, PdACTHUNRGTNEG(UN PPh3)2Cl2 (15 mg) and CuI
(5 mg). The mixture was refluxed for 20 h. After evaporation of hexahy-
dropyridine, dry toluene (50 mL) and potassium hydroxide (0.19 g,
3.45 mmol) were added to the residue, and the reaction mixture was re-
fluxed for 1 h under argon atmosphere. The solution was filtered and the
solvent was eliminated at reduced pressure. The residual oil was purified
by silica gel column chromatography using acetic acid ethyl ester/petrole-
um ether (v/v=1:5) to yield white powders (78%). 1H NMR (400 MHz,
CD3CN): d=8.61 (d, J=4.20 Hz, 1H), 8.38 (d, J=8.04 Hz, 1H), 7.93 (m,
1H), 7.76 (m, 1H), 7.28–7.44 (m, 6H), 7.13 (d, J=8.28 Hz, 2H), 6.20 (s,
2H), 3.33 ppm (s, 1H); MS (ESI): m/z (%): 309.
2b: Complex 2b was synthesized by the procedure similar to that of 1b,
except that 1-(4-pyridyl)-2-phenylethyne (0.03 g, 0.15 mmol) was used in
place of 4. After evaporation of the solvent, the residue was eluted with
dichloromethane/ethyl acetate (v/v=3:1) on a silica gel column to afford
2b as yellow solids. IR (CH2Cl2): n˜ =1911, 2030 (CO); 2224 (CꢁC);
7: To a well-degassed solution of model complex 3 (0.06 g, 0.10 mmol)
and 6 (0.04 g, 0.11 mmol) in THF/NEt3 (v/v=5:1, 15 mL) were added
successively PdACHTUNGTRENNUNG(PPh3)2Cl2 (15 mg) and then CuI (5 mg). The mixture was
ACTHNUTRGNEUNG
1258, 1154 cmꢀ1(CF3SO3ꢀ); 1H NMR (400 MHz, CDCl3): d=9.34 (d, J=
concentrated in vacuo and filtered after stirring under an argon atmos-
phere at 608C for 3 h. The resulting black residue was further purified by
column chromatography on silica gel with acetic acid ethyl ester/petrole-
5.02 Hz, 1H), 8.23 (d, J=8.08 Hz, 1H), 8.12–8.15 (m, 3H), 8.06 (d, J=
8.02 Hz, 1H), 7.71–7.81 (m, 4H), 7.54–7.56 (dd, J1 =8.36 Hz, J2 =1.68 Hz,
1H), 7.40–7.48 (m, 3H), 7.34 (m, 1H), 7.26–7.29 (m, 5H), 6.97–6.99 (m,
Chem. Asian J. 2010, 5, 1796 – 1803
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1801

L.-Z. Wu et al.
FULL PAPERS
2H), 5.91–5.95 ppm (q, 2H); MS (MALDI-TOF): m/z (%): 884; elemen-
tal analysis: calcd (%) for C36H24N4O6ReSF3·CH3COOC2H5: C 49.43,
H 3.32, N 5.76; found: C 49.95, H 3.22, N 5.84.
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1c: Complex 1c was synthesized by the procedure similar to that of 1b,
except that [(1,10-phenanthroline)Re(CO)3Br] was used in place of 2a.
The crude product was purified by column chromatography on silica gel
with dichloromethane/ethyl acetate (v/v 3/1) as eluent to give 1c (40%)
as red solids. IR (CH2Cl2): n˜ =1925, 1970, 1998, 2031 (CO); 2120 (CN);
2225 (CꢁC); 1266, 1157 cmꢀ1 (CF3SO3ꢀ); 1H NMR (400 MHz, CD3CN):
d=9.60 (dd, J1 =5.12 Hz, J2 =1.28 Hz, 2H), 8.85 (dd, J1 =8.32 Hz, J2 =
1.28 Hz, 2H), 8.26 (m, 2H), 8.17 (m, 2H), 8.10–8.13 (m, 2H), 7.52 (dd,
J1 =8.24 Hz, J2 =5.16 Hz, 2H), 7.53 (d, J=8.64 Hz, 2H), 7.36 (d, J=
8.52 Hz, 2H), 7.24 (dd, J1 =5.36 Hz, J2 =1.64 Hz, 2H), 2.15 (b, 2H,
SCH2), 2.05 (b, 2H, SCH2), 1.91 ppm (b, 2H, SCH2CH2CH2); MS
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2c: The procedure for the synthesis of 2c is similar to that of 2b. The
yellow solids were purified by column chromatography on silica gel with
dichloromethane/ethyl acetate (v/v=1:1) as the eluent (42%). IR
(CH2Cl2): n˜ =1919, 2031 (CO); 2122 (CN); 2223 (CꢁC); 1265, 1157 cmꢀ1
(CF3SO3ꢀ); 1H NMR (400 MHz, CD3CN): d=9.61 (dd, J1 =5.12 Hz, J2 =
1.16 Hz, 2H), 8.86 (dd, J1 =8.28 Hz, J2 =1.16 Hz, 2H), 8.25 (m, 2H), 8.17
(d, J=6.60 Hz, 2H), 8.10–8.13 (m, 2H), 7.50 (d, J=6.80 Hz, 2H), 7.37–
7.47 (m, 3H), 7.24 ppm (dd, J1 =5.36 Hz, J2 =1.36 Hz, 2H); MS (ESI):
m/z
(%):
630;
elemental
analysis:
calcd
(%)
for
C29H17F3N3O6ReS·CH3COOC2H5: C 45.72, H 2.91, N 4.85; found:
C 46.21, H 2.78, N 5.37.
1d: Complex 1d was synthesized by the procedure similar to that of 3,
except that phenylisocyanide (0.11 g, 1.03 mmol) was used in place of 4-
iodophenylisocyanide. After evaporation of the solvent, the residue was
eluted with CH2Cl2/petroleum ether (v/v=1:10) on a silica gel column to
afford 1d as brown air-stable solids (40%). IR (CH2Cl2): n˜ =1963, 1997,
2037 (CO); 2129 cmꢀ1 (CN); 1H NMR (400 MHz, CDCl3): d=7.33–7.37
(m, 3H), 7.30 (d, J=7.28 Hz, 2H), 2.16 (b, 2H, SCH2), 2.04 (b, 2H,
SCH2), 1.82 ppm (b, 2H, SCH2CH2CH2); elemental analysis: calcd (%)
for C15H11Fe2NO5S2: C 39.07, H 2.40, N 3.04; found: C 38.97, H 2.51,
N 3.01.
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In a typical experiment, a solution of CH3CN/CH3OH/H2O (v/v/v 3/2/1,
6 mL) with the model complex (3.3ꢁ10ꢀ4 m, 2 mmol) and HOAc (0.01m)
were added to a Schlenk tube. The frozen solution was evacuated and ni-
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amounts of H2 and CH4, and determined before and after a series of
measurements.
Supporting Information UV/Vis absorption spectra, steady-state and
time-resolved emission spectra, transient absorption spectra, cyclic vol-
tammograms for photocatalysts 1a, 1b, 1c, and references 1d, 2a, 2b, 2c.
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We are grateful for the financial support from Solar Energy Initiative of
the Knowledge Innovation Program of the Chinese Academy of Sciences
(KGCXZ-YW-389), the National Science Foundation of China (20732007
and 50973125), the Ministry of Science and Technology of China
(2006CB806105, 2007CB808004 and 2009CB22008), and the Bureau for
Basic Research of the Chinese Academy of Sciences.
1802
www.chemasianj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1796 – 1803

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Chem. Asian J. 2010, 5, 1796 – 1803
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1803