Visible light-driven electron transfer and hydrogen generation catalyzed by bioinspired [2Fe2S] complexes

Article abstract of DOI:10.1021/ic702010w

Complexes [{(μ-SCH₂)₂NCH₂C ₆H₅}{Fe(CO)₂L¹}{Fe(CO) ₂L²}] (L¹ = CO, L² = P(Pyr) ₃, 2; L¹ = L² = P(Pyr)₃, 3) were prepared, which have the lowest reduction potentials for the mono- and double-CO-displaced diiron complexes reported so far. Hydrogen evolution, driven by visible light, was successfully observed for a three-component system, consisting of a ruthenium polypyridine complex, the biomimetic model complex 2 or 3, and ascorbic acid as both electron and proton donor in CH ₃CN/H₂O. The electron transfer from photogenerated Ru(bpy)₃⁺ to 2 or 3 was detected by laser flash photolysis. Under optimal conditions, the total turnover number for hydrogen evolution was 4.3 based on 2 and 86 based on Ru(bpy)₃²⁺ in a three-hour photolysis.

Full text of DOI:10.1021/ic702010w

Inorg. Chem. 2008, 47, 2805-2810
Visible Light-Driven Electron Transfer and Hydrogen Generation
Catalyzed by Bioinspired [2Fe2S] Complexes
Yong Na,† Mei Wang,*,† Jingxi Pan,† Pan Zhang,† Björn Åkermark,‡ and Licheng Sun*,†,§
State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Centre on
Molecular DeVices, Dalian UniVersity of Technology (DUT), 116012 Dalian, China, Department
of Organic Chemistry, Arrhenius Laboratory, Stockholm UniVersity, 10691 Stockholm, Sweden,
and Department of Chemistry, Royal Institute of Technology (KTH), Teknikringen 30,
10044, Stockholm, Sweden
Received October 11, 2007
Complexes [{(µ-SCH2)2NCH2C6H5}{Fe(CO)2L1}{Fe(CO)2L2}] (L1 ) CO, L2 ) P(Pyr)3, 2; L1 ) L2 ) P(Pyr)3, 3) were
prepared, which have the lowest reduction potentials for the mono- and double-CO-displaced diiron complexes
reported so far. Hydrogen evolution, driven by visible light, was successfully observed for a three-component system,
consisting of a ruthenium polypyridine complex, the biomimetic model complex 2 or 3, and ascorbic acid as both
electron and proton donor in CH3CN/H2O. The electron transfer from photogenerated Ru(bpy)3+ to 2 or 3 was
detected by laser flash photolysis. Under optimal conditions, the total turnover number for hydrogen evolution was
4.3 based on 2 and 86 based on Ru(bpy)32+ in a three-hour photolysis.
Introduction
homogeneous catalysts for the light-driven hydrogen produc-
tion.11–18 Almost in all cases, either with hetero or homo-
geneous catalyst systems, noble metal-based (Pt, Rh, Pd, etc.)
compounds were used as catalysts. Inspired by the remark-
Hydrogen is known as a clean fuel and a promising
alternative to fossil fuels, which in addition to being in
limited supply have well-known detrimental effects on the
environment. Conversion of solar energy into hydrogen is a
challenging task and has attracted extensive attention during
recent decades.1,2 Numerous studies on photochemical
hydrogen evolution by visible light-driven water splitting
have been reported since the late 1970s.3–6 While hetero-
geneous photocatalysis by multiple components has been
extensively studied,7–10 there are relatively few reports on
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* To whom correspondence should be addressed. E-mail: symbueno@
dlut.edu.cn.
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M.; Görls, H.; Henry, W.; Vos, J. G. Angew. Chem., Int. Ed. 2006,
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†
Dalian University of Technology.
Stockholm University.
Royal Institute of Technology.
‡
§
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10.1021/ic702010w CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 7, 2008 2805
Published on Web 03/12/2008

Na et al.
able catalytic activity (6000–9000 molecules H2 s-1 per site)
of Fe-Fe hydrogenases ([FeFe]H2ase) for proton reduction
to H2,19 many chemists are engaged in creating electro- and
photochemical hydrogen production catalyst systems based
on the diiron model complexes of the [FeFe]H2ase active
site.20–25
sensitizer to the catalyst thermodynamically unfavorable. To
overcome this problem, a stronger reductant Ru(bpy)3 ,
+
which can be generated via reductive quenching of the
excited-state of Ru(bpy)32+, might possess enough driving
force for the desired electron transfer reaction.35
+
We have recently demonstrated that the Ru(bpy)3 , pho-
togenerated via a reductive quenching using diethyldithio-
carbamate anion (dtc–) as an electron donor, could trans-
fer one electron to the diiron dithiolate complexes [{(µ-
SCH2)2X}{Fe(CO)3}2] (X ) CH2, NCH2C6H5),36 resulting
in the formation of Fe0FeI species. Such species are proposed
to be crucial intermediates in electrochemical hydrogen
evolution catalyzed by [2Fe2S] complexes.21–24,26–28 The
three-component system of Ru(bpy)32+, all-CO diiron com-
plex and dtc- has two shortcomings for photochemical
hydrogen generation. First, the all-CO diiron complexes are
gradually decomposed under long time irradiation. To
improve the photostability of the [2Fe2S] model complex,
phosphine-coordinated complexes could be considered as
catalyst candidates, but CO-displacement by common phos-
phine ligands results in a great cathodic shift of reduction
potentials for [2Fe2S] complexes,24,26,37,38 which makes
The first example for electrocatalytic proton reduction
based on the bioinspired diiron dithiolate complex [{(µ-
SCH2)2CH2}{Fe(CO)2PMe3}{Fe(CO)2CN}]- was reported
with six turnovers by Rauchfuss and co-workers in 2001.20
In the following years, many biomimetic diiron complexes
were found electrocatalytically active for proton reduction
to H2 with ca. 0.5-1.0 V overpotential in the presence of
weak acid (HOAc) in CH3CN,23,24,26–28 and with relatively
large overpotential (ca. 0.75-1.22 V) in the presence of
moderate and strong acids (HOTs, HBF4 · Et2O, H2SO4, and
HClO4) in CH3CN with the maximum turnovers of
25.20–22,29,30 While a number of synthetic diiron dithiolate
complexes have been widely studied for electrochemical
hydrogen production, catalysts based on bioinspired diiron
complexes for photochemical hydrogen production have not
been reported so far.
Ruthenium polypyridine complexes have been successfully
used as a light harvesting unit in many light-driven hydrogen
production systems.3–14,31,32 To build intramolecular pho-
toinduced catalysts, several biomimetic models of the
[FeFe]H2ase active site were covalently linked to the
ruthenium photosensitizer or to the porphyrin derivative.25,33
However, the diiron catalytic center cannot be directly
reduced by the excited photosensitizer, which was quenched
either by energy transfer or by unwanted reverse electron
transfer.34 It is mainly because the reduction potential of the
dinuclear iron complex is more negative than the oxidation
potential of the excited-state of the ruthenium complex,
making the direct electron transfer from the excited photo-
electron transfer from Ru(bpy)3 to FeIFeI species thermo-
+
dynamically unfavorable. Therefore we prepared diiron
complexes containing weak electron donating phosphine
ligand(s), tris(N-pyrrolyl)phosphine (P(Pyr)3),39 and used
them as catalysts for photochemical hydrogen production.
Second, the ionic quencher dtc– is protonated and decom-
posed in the presence of proton acid. To overcome this
problem, an acid-tolerant quencher should be used to replace
the S-containing electron donor. Ascorbic acid (H2A) is a
good candidate since it can act as a proton source and its
ascorbate anion (HA-) can function as reductive quencher
for *Ru(bpy)32+ to Ru(bpy)3 .11 Here we present the results
+
on photoinduced electron transfer and light-driven hydrogen
generation using a three-component catalytic system of
Ru(bpy)32+, H2A, and [{(µ-SCH2)2NCH2C6H5}{Fe(CO)2-
L1}{Fe(CO)2L2}] (L1 ) L2 ) CO, 1; L1 ) CO, L2 ) P(Pyr)3,
2; L1 ) L2 ) P(Pyr)3, 3) (Scheme 1).
Experimental Section
Reagents and Instruments. All reactions were carried out under
N2 atmosphere with standard Schlenk techniques. Solvents were
dried and distilled prior to use according to the standard methods.
4,4′-Dimethyl-2,2′-bipyridyl (dmbpy) and 2,2′-bipyridyl (bpy) were
purchased from Aldrich and used as received. Photosensitizers
[Ru(bpy)3](PF6)2, [Ru(dmbpy)(bpy)2](PF6)2, and [Ru(dmbpy)3]-
(PF6)2 were synthesized according to the literature procedures.40,41
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2006, 691, 5045–5051.
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N.; Petro, B. J.; Zakai, U. I.; Evans, D. H.; Glass, R. S.; Lichtenberger,
D. L. J. Am. Chem. Soc. 2007, 129, 12521–12530.
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Int. Ed. 2004, 43, 1006–1009.
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ReV. 2006, 250, 1254–1266.
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3815.
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Electroanal. Chem. 2006, 595, 47–52.
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Sun, L. Eur. J. Inorg. Chem. 2005, 2506–2513.
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2007, 3812–3819.
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Catal. 1979, 6, 261–267.
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Org. Chem. 2001, 3871–3877.
(34) Eksröm, J.; Abrahamsson, M.; Olson, C.; Bergquist, J.; Kaynak, F. B.;
Eriksson, L.; Sun, L.; Becker, H.-C.; Åkermark, B.; Hammarström,
L.; Ott, S. Dalton Trans. 2006, 4599–4606.
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70, 2073–2086.
2806 Inorganic Chemistry, Vol. 47, No. 7, 2008

Electron Transfer and Hydrogen Generation Catalyzed by 2Fe2S
Scheme 1. The Homogeneous Molecular Three-Component System for
Photochemical H2 Generation Catalysed by [FeFe]H2ase Active Site
Mimics
Laser Flash Photolysis. Nanosecond transient absorption mea-
surements were performed on a LP-920 pump–probe spectroscopic
setup (Edinburgh). The excitation source was the unfocused third
harmonic (532 nm, 7 ns fwhm) output of a Nd:YAG laser
(Continuum surelite II), and the probe light source was a pulse-
xenon lamp. The signals were detected by Edinburgh analytical
instruments (LP900) and recorded on a Tektronix TDS 3012B
oscilloscope and computer. All samples in flash photolysis experi-
ments were deaerated with argon for ca. 20 min.
Photocatalysis. In a typical experiment, [Ru(bpy)3](PF6)2 (0.86
mg, 1 µmol), H2A (176 mg, 1 mmol), complex 2 (6.8 mg, 10 µmol)
and CH3CN (5 mL) were added to a Schlenk tube. Water (5 mL)
was added after the mixture was stirred under N2 atmosphere for 5
min. The suspension was then stirred for another 10 min until H2A
was completely dissolved. The solution was freeze–pump–thaw
degassed for three times and then warmed to room temperature
prior to irradiation. The Schlenk tube was connected to a small
balloon of pure N2 with an atmospheric pressure. The gas phase
volume is 59 mL, and the total amount of hydrogen gas evolved in
the photocatalytic reaction is no more than 1 mL. The reaction
solution was irradiated at 25 °C using an Xe lamp (500 W) with a
Pyrex-glass filter (λ > ca. 400 nm). The gas phase of the reaction
system was analyzed on a GC 7890T instrument with a thermal
conductivity detector, a 5 Å molecular sieve column, and with N2
as carrying gas. The amount of hydrogen evolved was determined
by the external standard method. Hydrogen in the resulting solution
was not measured and the slight effect of the hydrogen gas evolved
on the pressure inside the Schlenk tube was neglected for calculation
of the volume of hydrogen gas.
Complex [{(µ-SCH2)2N(CH2C6H5)}Fe2(CO)6] (1) was synthesized
as previously reported.36 Tris(N-pyrrolyl)phosphine (P(Pyr)3) was
prepared by following the literature protocol.39
IR spectra were recorded in CHCl3 on a JASCO FT/IR 430
1
spectrophotometer. H and 31P NMR spectra were recorded on a
Varian INOVA 400NMR instrument. Mass spectra were recorded
on an HP1100 MSD instrument. Elemental analyses were performed
on a VarioEL III elemental analyzer.
[{(µ-SCH2)2NCH2C6H5}{Fe(CO)3}{Fe(CO)2P(Pyr)3}] (2). The
CO-removing reagent Me3NO ·2H2O (56 mg, 0.5 mmol) was added
to the solution of complex 1 (240 mg, 0.5 mmol) in CH3CN (20
mL), and the mixture was stirred at room temperature for 10 min.
Ligand P(Pyr)3 (115 mg, 0.5 mmol) was added to the mixture. The
reaction solution was refluxed for 6 h. After solvent was removed
on a rotary evaporator, the crude product was purified by column
chromatography on silica gel with hexane/CH2Cl2 (4:1, v/v) as
eluent. The product was obtained in a yield of 260 mg (76%). Anal.
Calcd for C26H23N4O5PS2Fe2: C, 46.04; H, 3.42; N, 8.26. Found:
Results and Discussion
Preparation of Complexes 2 and 3. In the presence of
CO-removing reagent Me3NO ·2H2O, the reaction of equiva-
lent of 1 and P(Pyr)3 gave monosubstituted diiron complex
2 in a good yield. Further CO-displacement of 2 with excess
P(Pyr)3 in refluxing toluene afforded disubstituted complex
3 in moderate yield. Complexes 2 and 3 were characterized
by MS, IR, 1H and 31P NMR spectra and elemental analysis.
The results of the mass spectra and the elemental analyses
for 2 and 3 are in good agreement with the supposed diiron
complexes. Each complex displays three VCO bands as those
reported for other analogous PR3-mono and -disubstituted
diiron complexes.24,26,37,38 The VCO bands of P(Pyr)3-
monosubsituted complex 2 display an average red shift of
17 cm-1 in comparison to that for the parent all-CO diiron
complex 1,36 and the VCO bands of P(Pyr)3-disubstituted
complex 3 show a further 21 cm-1 shift to lower frequencies.
The average red shifts for VCO bands of complexes 2 and 3
are much smaller than those for the analogous PR3- (PMe3,
P(OEt)3, PTA (1,3,5-triaza-7-phosphaadamantane)) mono-
and disubstituted diiron complexes reported previously (Table
1),24,37,42 indicating the relatively weak electron donating
properties of the P(Pyr)3 ligand in comparison with those of
other PR3-coordinated diiron complexes described above.
Electrochemistry. Diiron complexes 2 and 3 display the
FeIFeI/Fe0FeI reduction events at –1.52 and –1.55 V in
CH3CN, respectively, which are the lowest reduction poten-
tials for the mono- and double-CO-displaced diiron dithiolate
1
C, 46.21; H, 3.41; N, 8.37%. H NMR (CDCl3): δ 2.28 (s, 2H,
SCH2), 2.99 (s, 2H, SCH2), 3.18 (s, 2H, PhCH2N), 6.31 (s, 6H,
pyrrolyl), 6.83 (s, 6H, pyrrolyl), 6.90 (s, 1H, Ph), 7.18 (s, 4H, Ph).
31P{1H} NMR: δ 141.19. IR (CHCl3): VCO 2055 (s), 2002 (s), 1983
(s) cm-1. MS (API-ES): m/z: 678.9 [M + H]+.
[{(µ-SCH2)2NCH2C6H5}{Fe(CO)2P(Pyr)3}2] (3). Ligand P(Pyr)3
(68 mg, 0.3 mmol) was added to the solution of complex 2 (102
mg, 0.15 mmol) in toluene (50 mL), and the mixture was refluxed
for 20 h. As solvent was evaporated under reduced pressure, the
product was purified by column chromatography on silica gel first
with hexane/CH2Cl2 (4:1, v/v) and then with hexane/CH2Cl2 (3:1,
v/v) as eluents. The product was obtained in a yield of 86 mg (65%).
Anal. Calcd for C37H35N7O4P2S2Fe2: C, 50.53; H, 4.01; N, 11.15.
1
Found: C, 50.58; H, 3.95; N, 11.25%. H NMR (CDCl3): δ 2.16
(s, 4H, SCH2), 2.69 (s, 2H, PhCH2N), 6.26 (s, 12H, pyrrolyl), 6.49
(d, 2H, Ph), 6.81 (s, 12H, pyrrolyl), 7.05 (m, 3H, Ph). 31P{1H}
NMR: δ 138.27. IR (CHCl3): V(CO) 2025 (s), 1983 (m), 1968 (s)
cm-1. MS (API-ES): m/z: 880.0 [M + H]+.
Electrochemistry. Electrochemistry measurements were re-
corded using a BAS-100W electrochemical potentiostat at a scan
rate of 100 mV/s. Cyclic voltammogram experiments were per-
formed in a three-electrode cell under argon. The working electrode
was a glassy carbon disk (diameter, 3 mm) polished with 1-µm
diamond paste and sonicated for 15 min prior to use, and the
auxiliary electrode was a platinum wire. The reference electrode
was an Ag/Ag+ electrode (0.01 M AgNO3 in CH3CN). A solution
of 0.05 M n-Bu4NPF6 in CH3CN or a mixture of 0.05 M n-Bu4NPF6
in CH3CN and 0.05 M KCl in water were used as electrolyte. The
electrolyte solution was degassed by bubbling with dry argon for
10 min before measurement. All potentials are versus Ag/Ag+ (0.01
M AgNO3 in CH3CN).
(42) Eilers, G.; Schwartz, L.; Stein, M.; Zampella, G.; de Dioia, L.; Ott,
S.; Lomoth, R. Chem. Eur. J. 2007, 13, 7075–7084.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2807

Na et al.
Table 1. A Comparison of the ∆νCO and ∆E Values of 2 and 3 with
Those of Other PR3-Coordinated Diiron Complexesa
complex
∆νCO cm-1
∆E mV
note
[{(µ-SCH2)2NCH2C6H5}
Fe2(CO)5P(Pyr)3] (2)
17
30
this work
[(µ-pdt)Fe2(CO)5PMe3]
[(µ-pdt)Fe2(CO)5PTA]
[(µ-pdt)Fe2(CO)5P(OEt)3]
[{(µ-SCH2)2NCH2C6H5}
{Fe(CO)2P(Pyr)3}2] (3)
[{(µ-SCH2)2NCH2C6H5}
{Fe(CO)2PMe3}2]
56
52
45
38
290
200
160
70
ref 37
ref 24
ref 37
this work
95
620
ref 42
[(µ-pdt){Fe(CO)2PMe3}2]
95
86
510
440
ref 24
ref 24
[(µ-pdt){Fe(CO)2PTA}2]
a ∆νCO ) νCOav(all-CO) - νCOav(PR3-coordinated) and ∆E ) Epc1(all-CO)
-
Figure 1. Transient absorption spectra obtained at 0.2 µs and 10 µs
following laser flash photolysis of a deoxygenated CH3CN solution
Epc1(PR3-coordinated)
.
2+
containing Ru(bpy)3 (0.03 mM), dtc- (1.0 mM) and 2 (0.2 mM). Inset:
kinetic traces at 520 nm (a) and 400 nm (b).
complexes reported so far. As expected, only small cathodic
shifts, ca. 30 mV for 2 and 60 mV for 3, were observed for
the FeIFeI/Fe0FeI reduction potentials in comparison to that
of the all-CO diiron complex 1,36 indicating the exceptionally
weak donating property of the P(Pyr)3 ligand in comparison
with that of other previously reported PR3-coordinated
analogues, which is in accord with the results of IR spectra.
It is already known that Ru(bpy)32+ displays three reversible
reduction peaks for three bipyridyl ligands in the cyclic
2+
voltammogram with the reduction peak for Ru(bpy)3
/
+
Ru(bpy)3 at –1.61 V (E1/2). Therefore, the one electron
reduction processes of 2 and 3 by the photogenerated
Ru(bpy)3 are thermodynamically feasible. The ∆E values
+
Figure 2. Kinetic traces at 520 nm obtained following flash photolysis of
a deoxygenated mixture of CH3CN/H2O (1:1, v/v) containing (a) 0.03 mM
Ru(bpy)32+, 0.1 M H2A; (b) the same as mixture a but with 0.2 mM complex
2. The two decay curves are normalized for comparison.
listed in Table 1 show that the reduction potentials of all
the previously reported diiron dithiolate complexes are much
more negative than that of the corresponding P(Pyr)3-
coordinated diiron dithiolate complexes, so that the electron
transfer from Ru(bpy)3+ to the other PR3-coordinated diiron
complexes is thermodynamically unfavorable.
the characteristic transient absorptions of the one-electron
+
reduced species of 3, but the slow decay of Ru(bpy)3
thwarted the observation of the absorption change in the
range of 500-700 nm. However, the newly generated
absorption for the one-electron reduced species of 3 could
be clearly observed at 400 nm (Supporting Information,
Figure S1). The lifetime of Ru(bpy)3+ was shortened to 7.2
µs upon addition of 3 (0.15 mM, Supporting Information,
Figure S1). By varying the concentration of the diiron
complexes, the rate constant of the electron transfer reaction
Flash Photolysis. The kinetic feasibility for photoinduced
electron transfer processes of the three-component systems
was studied by laser flash photolysis. We have previously
+
demonstrated that complex 1 could be reduced by Ru(bpy)3 ,
resulting in the formation of Fe0FeI species with three
characteristic absorptions at 400, 580, and 700 nm in the
transient absorption spectrum.36 To get high-quality transient
absorption spectra for the electron transfer processes of
from Ru(bpy)3 to the FeIFeI species is determined to be
+
Ru(bpy)3 to complexes 2 and 3, we first used dtc- as
+
3.5 × 109 M-1 s-1 for 1, 2.0 × 109 M-1 s-1 for 2, and 6.7
× 108 M-1 s-1 for 3. It is noteworthy that the electron transfer
rate constant greatly decreases as the number of P(Pyr)3
ligands in the diiron complex increases. The cathodic shift
of the reduction potential caused by CO-displacement makes
the desired electron transfer less preferred.
electron donor. The transient absorption spectrum at 0.2 µs
after laser excitation (532 nm, pulse width 7 ns) of the
deoxygenated solution of Ru(bpy)32+, complex 2, and dtc–
clearly shows the absorption of Ru(bpy)3+ at 520 nm, which
decays completely at 10 µs. At the same time the absorptions
at ca. 390 and 520 nm, attributed to the one-electron reduced
species of 2, appear in the spectrum (Figure 1). The Fe0FeI
species of 2 does not display an apparent absorption at 700
nm, which is different from what we found for all-CO diiron
complexes. Compared to the lifetime (τ ) 17 µs) of
Ru(bpy)3+ in the absence of 2, the lifetime of Ru(bpy)3+ was
shortened to 2.6 µs upon addition of 2 (0.2 mM). The
When dtc- was replaced by H2A, the lifetime (16.9 µs,
curve a in Figure 2) of Ru(bpy)3+ was shortened to 1.9 (curve
b in Figure 2) and 13 µs (Supporting Information Figure
S2), respectively, upon addition of 2 and 3 (0.2 mM). The
results indicate that electron transfer reactions from Ru(b-
py)3+ to P(Pyr)3-substituted diiron complexes are still feasible
when H2A is employed as electron donor under the condition
for photochemical H2 evolution.
+
concomitant kinetics for the decay of Ru(bpy)3 at 520 nm
and the buildup of the Fe0FeI species at 400 nm further
supports the conclusion that one electron transfers from
Ru(bpy)3+ to 2 (Figure 1 inset). Efforts were made to obtain
Photochemical Hydrogen Production. After photoin-
duced electron transfer processes of the three-component
systems were verified by laser flash photolysis, light-driven
2808 Inorganic Chemistry, Vol. 47, No. 7, 2008

Electron Transfer and Hydrogen Generation Catalyzed by 2Fe2S
Table 2. Effect of the CH3CN/H2O Ratio on Photochemical H2
Evolutiona
CH3CN/
H2O (v/v)
H2 evolved
(µmol)
turnovers
turnovers
based-on cat.b
based-on PSc
3:7
1:1
7:3
a
13
43
19
1.3
4.3
1.9
26
86
38
2+
Reaction conditions: Ru(bpy)3 (0.1 mM), 2 (1.0 mM), and H2A (0.1
M) in the solution of different CH3CN/H2O ratios, 3 h. b cat. ) com-
plex 2. c PS ) Ru(bpy)3
.
2+
Table 3. Effect of H2A Concentration on Photochemical H2 Evolutiona
concentration
of H2A
H2 evolved
(µmol)
turnovers
turnovers
based on cat.b
based-on PSc
2+
Figure 3. Reaction conditions: Ru(bpy)3 (0.1 mM), H2A (0.1 M), and
0.05 M
0.1 M
0.2 M
0.3 M
29
43
33
31
2.9
4.3
3.3
3.1
58
86
66
62
different diiron complexes (1.0 mM) in CH3CN/H2O (1:1, v/v).
ca. 1 h under irradiation, while the CO bands of complex 2
disappeared after ca. 2.5 h under irradiation. P(Pyr)3-
monosubstituted complex 2 showed much higher catalytic
activity than 1 and 3. These results indicate that CO-
displacement by a relatively weak electron-donating phos-
phine ligand P(Pyr)3 improves the photostability and catalytic
activity of [2Fe2S] model complexes in photocatalysis
reactions in a certain extent.
Several control experiments were carried out to ascertain
that the photoinduced hydrogen evolution was promoted by
the [2Fe2S] model complex. As expected, hydrogen forma-
tion was not observed when the reaction of Ru(bpy)32+, the
diiron complex, and H2A was carried out in the dark.
Moreover, omission of any of the components of the three-
component system did not generate detectable amount of H2,
suggesting that all three components are required in the H2-
producing process. The results of the control experiments
indicate that the hydrogen generation in the present system
is indeed photoinduced via the excited-state of Ru(bpy)3
and catalyzed by the [2Fe2S] model complexes.
Two other ruthenium polypyridine derivatives, Ru
(dmbpy)(bpy)2 and Ru(dmbpy)32+, were also used as
photosensitizers to drive the electron transfer and hydrogen
production with complex 2 as catalyst. As methyl groups
were introduced to bipyridine rings, the redox potential for
the Ru(dmbpy)32+/Ru(dmbpy)3+ process is negatively shifted
a Reaction conditions: Ru(bpy)3 (0.1 mM), 2 (1.0 mM) and different
2+
concentrations of H2A in a 1:1 CH3CN/H2O solution, 3 h. b cat. ) complex
2. c PS ) Ru(bpy)3
.
2+
hydrogen generation was studied. The reaction conditions
for photochemical hydrogen production were optimized with
complex 2 as catalyst. When the ratio of CH3CN/H2O was
changed from 1:1 (v/v) to 3:7 and then to 7:3, the amount
of H2 generated in 3 h was reduced to ca. 30–44% of that
evolved from a 1:1 CH3CN/H2O solution (Table 2). Presum-
ably, it mainly results from the poor solubility of the diiron
complex in the 3:7 CH3CN/H2O solution and, in a reverse
case, from inefficient dissociation of H2A in the 7:3 CH3CN/
H2O solution. When the concentration of H2A was varied
from 0.05 to 0.1 M, the total amount of the hydrogen evolved
grew from 29 to 43 µmol in a 1:1 CH3CN/H2O solution
(Table 3). Further increase in the H2A concentration led to
decrease in the hydrogen evolution. The same tendency was
also observed with different solvents and H2A concentrations
using complex 1 as catalyst (Tables S1 and S2). Tables 2
and 3 show the photochemical hydrogen generation reactions
that were made with 0.1 M H2A in the 1:1 CH3CN/H2O
solution.
2+
2+
The three-component system of Ru(bpy)32+, diiron com-
plexes 1-3, and H2A were then explored for photochemical
hydrogen generation. The GC analysis of hydrogen formation
as a function of reaction time for complex 2 shows that the
amount of hydrogen evolved increases steadily and levels
off after ca. 3 h with a 0.1 M concentration of H2A in a 1:1
CH3CN/H2O solution (Figure 3). The initial slopes of
hydrogen generation for three diiron complexes 1-3 were
measured to be 1.7 × 10-7 mol/min for 1, 6.7 × 10-7 mol/
min for 2, and 3.0 × 10-7 mol/min for 3 under conditions
indicated in Figure 3. The total amount of hydrogen evolved
is 4.3 × 10-5 mol with 4.3 turnovers based on 2 and 86
2+
by ca. 100 mV in comparison to that for the Ru(bpy)3
/
Ru(bpy)3+ couple,43 resulting in observable improvement of
the electron-transfer rate constant. The rate constants for the
electron transfer between Ru(dmbpy)(bpy)2+ and 2 and that
+
between Ru(dmbpy)3 and 2 were determined to be 2.0 ×
109 and 2.5 × 109 M-1 s-1, respectively. Αlthough the rate
constant for electron transfer from the photogenerated
ruthenium complexes to the diiron complex is enhanced, the
amount of hydrogen evolved slightly decreased with Ru
(dmbpy)(bpy)22+ as photosensitizer, and it significantly went
2+
turnovers based on Ru(bpy)3 in a 3-hour photolysis. The
2+
down with Ru(dmbpy)3 as photosensitizer (Table 4).
amount of hydrogen evolved is 1.7 × 10-5 mol with 1.7
Considering that the potential for the *Ru(dmbpy)32+/Ru
2+
turnovers based on 3 and 34 turnovers based on Ru(bpy)3
.
(dmbpy)3 couple (0.23 V vs Ag/Ag+ in CH3CN/H2O) is
+
The total amount of hydrogen evolved is 7.8 × 10-6 mol
with 0.78 turnover based on complex 1. The IR spectra
showed that without irradiation diiron complexes did not
decompose in the presence of ascorbic acid in CH3CN/H2O
solution under nitrogen atmosphere. The IR monitoring
showed that complex 1 was completely decomposed after
more negative than that for the *Ru(bpy)32+/Ru(pby)3
+
couple (0.37 V) calculated according to the literature,43 it
could be predict that the electron transfer from the HA- anion
(43) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85–227.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2809

Na et al.
seems that there are two possibilities: (1) the Fe0FeI complex
is first protonated to give an HFeIIFeI species which is then
reduced to HFeIFeI. This new species should have sufficient
hydride character to be protonated again to give H2, and
regenerating the FeIFeI catalyst; (2) the HFeIIFeI species first
undergoes a second protonation to generate [(η2-H2)FeIIFeI]+
in accordance with the mechanism proposed earlier for
electrochemical hydrogen generation catalyzed by the diiron
dithiolate complexes.23,24 The second protonation should
render the reduction potential of the [(η2-H2)FeIIFeI]+ species
Table 4. Photochemical H2 Evolution with Different Ru-Based
Photosensitizersa
H2 evolved
(µmol)
turnovers
turnovers
photosensitizer
based-on cat.b
based-on PSc
2+
Ru(bpy)3
43
39
19
4.3
3.9
1.9
86
76
38
2+
Ru(dmbpy)(bpy)2
Ru(dmbpy)3
2+
a Reaction conditions: H2A (0.1 M) and 2 (1.0 mM) in a 1:1 CH3CN/
H2O solution with different ruthenium complexes (0.1 mM), 3 h. b cat. )
complex 2. c PS ) Ru(bpy)32+, Ru(dmbpy)(bpy)22+, Ru(dmbpy)3
.
2+
Scheme 2. The Proposed Photochemical Proton Reduction Processes
Catalyzed by the Biomimetic [2Fe2S] Complexes
+
positive enough to be reduced by Ru(bpy)3 , followed by
release of H2.
Conclusion
+
The strong reductant Ru(bpy)3 , generated from Ru
2+
(bpy)3 in the action of light, can transfer electrons to
complexes 2 and 3 containing P(Pyr)3 ligand(s) when the
reductive quencher dtc- was replaced by HA-. Visible light-
driven hydrogen evolution catalyzed by bioinspired [2Fe2S]
complexes could be observed for the three-component system
of Ru(bpy)32+, [2Fe2S] complexes (1-3) and H2A in a 1:1
CH3CN/H2O solution without requirement of extra proton
acid. In comparison to the P(Pyr)3-disubstituted complex 3
and the all-CO diiron complex 1, the P(Pyr)3-monosubstituted
analogue 2 displays better photostability and improved
catalytic activity in photochemical hydrogen generation. The
total turnover numbers of 4.3 based on 2 and 86 based on
2+
to *Ru(dmbpy)3 is less favorable than to *Ru(bpy)32+. In
light of the potential analysis, the decline in hydrogen
evolution is attributed to the inefficient reductive quenching
of the excited ruthenium photosensitizer by the ascorbate
anion.
Discussion on the Mechanism. Although Ru(bpy)3 is
thermodynamically capable of reducing water to molecular
+
2+
Ru(bpy)3 in a 3-hour photolysis indicate that the photo-
chemical hydrogen generation is catalytic both in the [2Fe2S]
complex and in the photosensitizer Ru(bpy)32+. Although the
turnover number is low at this stage, the results illustrate
the working principle, that is, bioinspired diiron complexes
may act as economical catalysts for conversion of solar
energy to a sustainable fuel, hydrogen gas.
+
hydrogen, the solution containing Ru(bpy)3 , formed by the
reaction of H2A and photoexcited Ru(bpy)32+, does not
produce detectable H2 without catalyst.11 Therefore, the
proton reduction center can be attributed to the diiron
dithiolate complex, which has been shown to be an effective
catalyst for electrochemical reduction of protons to H2.20–30
A plausible process for photochemical hydrogen evolution
is illustrated in Scheme 2. Upon irradiation with visible light,
Acknowledgment. This work was supported by the
Chinese National Science Foundation (Grant Nos. 20471013
and 20633020), the Swedish Energy Agency, the Swedish
Research Council, and K. & A. Wallenberg Foundation.
2+
the photosensitizer Ru(bpy)3 is excited and then reduced
to Ru(bpy)3+ by electron transfer from HA-. The generated
Ru(bpy)3 is then capable of reducing the diiron complex
to an Fe0FeI species. Considering the reduction capability
+
Supporting Information Available: Kinetic traces of the three-
component system, Ru(bpy)32+, dtc–, and 3; kinetic traces of the
+
of Ru(bpy)3 and the second reduction potentials of com-
2+
dyad system Ru(bpy)3 and H2A, and of the three-component
plexes 1-3 (-2.13 V for 1, –2.16 V for 2, and –2.20 V for
system Ru(bpy)32+, H2A, and 3; tables for effect of the CH3CN/
H2O ratio and the H2A concentration on photochemical H2 evolution
using complex 1 as catalyst. This material is available free of charge
via the Internet at http://pubs.acs.org.
3), further reduction of Fe0FeI to Fe0Fe0 by Ru(bpy)3 is
+
thermodynamically unfavorable. In addition, protonation of
the bridged-nitrogen atom of the diiron complex 2 was not
detected by the IR spectrum in the presence of ascorbic acid
(50 equiv) in CH3CN/H2O (3:1, v/v) solution. It therefore
IC702010W
2810 Inorganic Chemistry, Vol. 47, No. 7, 2008