Photochemical hydrogen evolution catalyzed by trimetallic [Re-Fe] complexes

Article abstract of DOI:10.1016/j.inoche.2011.11.038

In order to conduct photoactive catalysts for hydrogen production, a novel trimetallic [Re-Fe] complex 1, consisting of the phenanthroline rhenium photosensitizer and the [2Fe2S] complex (connected by the axial coordination of a pyridyl group), was prepared and spectroscopically characterized. The apparent fluorescence quenching of the complex 1 was observed in comparison with the reference complex 1b, suggesting the possibility for an electron transfer from the excited state of the rhenium moiety to the [2Fe2S] moiety. Visible light-driven H₂ generation was achieved by using triethylamine (sacrificial electron donor) in the presence of the complex 1 (catalyst) in CH₃CN/H₂O, with a turnover number reaching to 1.5. This is by far as we know the highest for the [Re-Fe] photocatalysts. In contrast to the molecular device, the multicomponent catalyst of complexes 1b and 1c, no H₂ was detected by GC analysis in the same experimental condition. The plausible mechanism for the photochemical H₂-evolution with this molecular device is discussed.

Full text of DOI:10.1016/j.inoche.2011.11.038

Inorganic Chemistry Communications 16 (2012) 81–85
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Photochemical hydrogen evolution catalyzed by trimetallic [Re–Fe] complexes
⁎
Weina Jiang, Jianhui Liu , Cheng Li
State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116012, P. R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to conduct photoactive catalysts for hydrogen production, a novel trimetallic [Re-Fe] complex 1, con-
sisting of the phenanthroline rhenium photosensitizer and the [2Fe2S] complex (connected by the axial co-
ordination of a pyridyl group), was prepared and spectroscopically characterized. The apparent fluorescence
quenching of the complex 1 was observed in comparison with the reference complex 1b, suggesting the pos-
sibility for an electron transfer from the excited state of the rhenium moiety to the [2Fe2S] moiety. Visible
light-driven H2 generation was achieved by using triethylamine (sacrificial electron donor) in the presence
of the complex 1 (catalyst) in CH3CN/H2O, with a turnover number reaching to 1.5. This is by far as we
know the highest for the [Re-Fe] photocatalysts. In contrast to the molecular device, the multicomponent cat-
alyst of complexes 1b and 1c, no H2 was detected by GC analysis in the same experimental condition. The
plausible mechanism for the photochemical H2-evolution with this molecular device is discussed.
© 2011 Elsevier B.V. All rights reserved.
Received 4 November 2011
Accepted 25 November 2011
Available online 8 December 2011
Keywords:
Rhenium
Hydrogen evolution
Photocatalysis
[2Fe2S] complex
Searching for new sources of energy remains as one of the most
urgent problems of the 21st century. Hydrogen, a sustainable and
environment-friendly energy source, is considered to be a promising
alternative to fossil fuels whose reserves are rapidly diminishing
[1–3]. Photo-induced proton reduction is supposed to be the most de-
sirable way of transformation in obtaining hydrogen, given the sun-
light readily and economically available (The amount of solar energy
reaching the planet earth is many orders of magnitude greater than
that required for human development). Different kinds of heteroge-
neous or homogeneous systems, being either multicomponent or
supramolecular, have been developed for the light-driven decompo-
sition of water into hydrogen [4–17]. However, the improvement of
the efficiency for the hydrogen evolution is still a great challenge. Re-
cent research in the development of the catalytic hydrogen evolution
is focused on the search of novel complexes to mimic the active cen-
ters of hydrogenase, which are known for their ability to catalyze the
reversible reduction of protons to hydrogen in a wide variety of mi-
croorganisms [18,19]. The attachment of a photosensitizer to the
[FeFe]H2ases is a widely used protocol for photo-induced hydrogen
production. A wide range of transition metals have been used in the
design and synthesis of the proton reduction catalysts or photosensi-
tizers, such as Ru, Ir, Pt, Pd and Co, and/or their combinations like Ru-
Co, Ir-Co [4,14,17]. Of all these systems established, the maximal TON
reported in a photochemical system for H2 was 9000, using a
rhodamine-based photosensitizer and cobaloxime type catalyst [9].
Some other results deserve to be mentioned: a TON of 500 for the
rhenium and cobaloxime complexes [10]; and noticeably, 1450 for
the iridium and Fe3(CO)12 [11] were recorded, while the natural hy-
drogenases have a fascinating activity 6000–9000 molecules of H2
per active site [18,19]. We have earlier reported the supramolecular
photocatalysts with ruthenium as the photosensitizers linked to the
[2Fe2S] complexes [20–24]. We note, however, that the more nega-
tive potential of the [2Fe2S] complexes with respect to the excited
state reduction potential of Ru(II) photosensitizer renders the
photo-reduction of the [2Fe2S] complexes thermodynamically unfea-
sible. Therefore, rhenium (I) complex was selected as the photosensi-
tizer for its visible light absorption, being long excited-state lifetime,
more negative excited state reduction potential, along with its ther-
mal and photochemical stability [25]. Recently, we have reported
one supramolecular [Re–Fe] photocatalyst [26], in which rhenium is
linked to the [2Fe2S] complex by the phosphorus-containing ligand.
Unfortunately, it is not stable under the process of irradiation. Later
on, some novel interesting examples of [Re–Fe] photocatalysts were
reported by Wu and co-workers in which cyanide (CN) group is in-
corporated as the bridge to anchor the rhenium (I) to the iron core
of [2Fe2S] catalytic center [27]. Using these constructs, they reported
that the turnover number is 0.14. In a more recent communication
[28], the same research group disclosed three other [Re–Fe]
complexes featuring Re(I) subunit and Fe–S active site connected by
triple bond-containing bridges, and this time the TON is still no
more than 0.14. Following our previous publication dealing with
the catalytic behavior of established [Re–Fe] complexes, we report
here a new trimetallic [Re–Fe] photocatalyst (complex 1), in
which the phenanthroline rhenium photosensitizers and the
[2Fe2S] complexes are connected by a pyridine bridge (Chart 1).
Their corresponding references (complex 1b and 1c) are described
for comparison.
⁎
Corresponding author. Tel.: +86 411 84986249; fax: +86 411 84986245.
E-mail address: liujh@dlut.edu.cn (J. Liu).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.11.038

82
W. Jiang et al. / Inorganic Chemistry Communications 16 (2012) 81–85
Scheme 1. The synthetic procedures of complexes 1, 1a and 1b.
Complexes 1, 1a and 1b were synthesized as depicted in
displayed a strong fluorescence emission, thus can be employed as a
sensitive probe for excited state quenching. If electron transfer and/
or energy transfer occurs from the excited state of the phen rhenium
moiety to the [2Fe2S] unit in complex 1, the fluorescence quenching
should be readily detected. This hypothesis was substantiated by
the fact that the fluorescence of the excited rhenium moiety was
Scheme 1. The commercially available Re(CO)5Cl was treated with
1,10-phenanthroline (phen) in refluxing toluene to give fac-(phen)
Re(CO)3Cl (1a), which was then coordinated to the complex 1c and
ethyl isonicotinate in refluxing ethanol in the presence of CF3SO3Ag to
form the complexes 1 and 1b, respectively[29–31]. No significant im-
provements were observed for the very low conversion toward the co-
ordination product 1. We attribute this to be the possible steric effect of
the phen ligand and the weak coordination ability of the pyridine group.
Complexes 1 and 1b were characterized by MS, 1HNMR spectra and ele-
mental analysis. The result of elemental analyses for 1 is in good agree-
ment with the supposed compositions.
3.0
2.5
1
1b
The UV–vis absorption spectra of complex 1 in 0.01 mM CH3CN at
room temperature is shown in Fig. 1 with complex 1b for comparison.
Complex 1b has weak absorptions at 281 nm attributed to the ligand-
2.0
1.5
1.0
0.5
0.0
⁎
centered transition (πL →π L), characterizing the phenanthrolinyl
moiety. The additional broad band at ca. 360 nm is assigned as
⁎
dπ(Re)→π (phen)(MLCT). These assignments were based on com-
parison with data from [fac-(bpy)Re(CO)3Cl] and [(bpy)Re(CO)3(py-
PTZ)]+ [32–35]. For the complex 1, however, intense absorptions
are observed at 273 nm and 364 nm, with a broad and weak absorp-
tion at 319 nm. The first absorption at 273 nm is attributed to the
⁎
ligand-centered transition (πL →π L), the other two at 364 nm due
250
300
350
400
450
⁎
to MLCT, and 319 nm is assigned to d(Fe)→d (Fe) transition.
Wavelength (nm)
The fluorescence spectra of complex 1 and 1b in the deoxygenated
acetonitrile solution were studied (Fig. 2). The rhenium complexes 1b
Fig. 1. The UV–vis spectra for 1 and 1b (0.01 mM) in CH3CN.

W. Jiang et al. / Inorganic Chemistry Communications 16 (2012) 81–85
83
60
25
20
15
10
5
1
40
1b
20
0
-20
-40
-60
0
2
1
0
-1
-2
450
500
550
600
650
700
E / V
Wavelength (nm)
Fig. 3. Cyclic voltammogram of 1, 1.0 mM in 0.05 M [Bu4N][PF6]/ CH3CN at a scan rate
Fig. 2. Fluorescence spectra for complexes 1 and 1b (0.01 mM) in CH3CN.
of 100 mV/s with Ag/Ag+ as a reference electrode.
indeed quenched in complex 1 in contrast to the corresponding rhe-
nium complex 1b. We assume that the fluorescence quenching is
mainly caused by an intramolecular oxidative process with the elec-
tron transfer from the excited photosensitizer to the 2Fe2S center in
the complex 1. The catalytic activity of 1 for photoinduced hydrogen
evolution, described in the following section, gives a support for this
argument.
component system. This result is in a good agreement with the previ-
ous report [39].
We have earlier synthesized complexes where a photosensitizer
was covalently linked to [2Fe2S] catalytic center [20–24]. However,
the driving force for an electron transfer from the photo-generated
excited state [Ru]2 + ⁎ to the [2Fe2S] complex was thermodynamically
unfavorable. Therefore they all failed for the hydrogen production. On
the contrary, the potential of the Re(I) complexes has an opposite ef-
fect. We examined the driving force ΔG° of the photo-induced elec-
tron transfer of complex 1. Based on the determined oxidation
potential of the Re(I) complexes, the reduction potential of the
[2Fe2S] complexes, and the excited state energy E00 of the Re(I) com-
plexes, the free energy change (ΔG°) was calculated to be −0.375 eV
for complex 1 [40]. From the thermodynamically favorable free ener-
gy change (ΔG°), we believe that the electron transfer process from
the excited Re(I) complexes to the [2Fe2S] complexes occurs. Accord-
ing to the previous report, the process for the photocatalytic hydro-
gen generation catalyzed by the rhenium photosensitizer and
[2Fe2S] has been studied [27,41]. The initial step is oxidatively
quenching of the excited Re(I) complex by [2Fe2S] as an electron ac-
ceptor (Scheme 2). The formed Re(II)• is subsequently regenerated by
electron transfer from the sacrificial electron donor TEA, while the
long-lived [Fe(I)Fe(0)] species is reacted with a proton for H2
evolution.
Electrochemical properties of photocatalyst 1 were studied to
evaluate for its redox potentials. The cyclic voltammograms for the
complex 1 was recorded in CH3CN with 0.05 M [Bu4N][PF6] as an elec-
trolyte. Reduction and oxidation potentials were given vs internal
standard NHE and compared to those of the reference compounds
1b [35] and 1c [36] (Table 1). In the case of the trinuclear complex
1, the first reversible reduction peak at −0.99 V is assigned to a mix-
ture of the two overlapping processes FeIFeI/Fe0FeI and phen/phen−
.
The reduction couples at −1.37 V, −1.67 V is the phen−/phen2− and
FeIFe0/Fe0Fe0 respectively. The irreversible oxidation peak at 1.23 V is
II
assigned to FeIFeI/FeIIFeI and at ca. 1.63 V is assigned to ReI/Re
(Fig. 3). These data follow the general trend discussed earlier for
our reported [2Fe2S] complexes [37,38].
Complex 1 was used as photocatalyst for light-driven hydrogen
production with Et3N as a sacrificial electron donor (Fig. 4). The reac-
tion solution was irradiated at 25 °C using a Xe lamp (500 W) with a
Pyrex-glass filter (λ>ca. 400 nm). The amount of hydrogen produced
was determined by GC analysis for the gas phase of the reaction sys-
tem. Irrespective H2 was dissolved in the solvent followed by
5.5 hours of irradiation the TON of the H2 reached to 1.5 for complex
1. In the same experimental condition, for reference, its correspond-
ing intermolecular multicomponent system: complexes 1b and 1c in
the same concentration with TEA as the electron donor, no significant
1.6
1
1b+1c
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
H
2 was detected after 5 hours of irradiation. Obviously, the combina-
tion of rhenium(I) and [2Fe2S] catalyst appears promising for the ef-
ficient H2 evolution, which is better than the intermolecular multi-
Table 1
Electrochemical data of complexes 1, 1b and 1c (E vs NHE).
a
Epa
Epa
Epc
Epc
Epc
Complex ReI/ReII FeIFeI/FeIIFeI FeIFeI/Fe0FeI FeIFe0/Fe0Fe0 L/L-
1
1b
1c
+1.63
+2.05
–
+1.23
–
−0.99
–
−1.67
–
−0.99, −1.37
−0.93, −1.31
–
0
1
2
3
4
5
Time / h
+1.21
−0.92
−1.63
Cyclic voltammetry was carried out at a scan rate of 100 mV/s in 0.05 M [Bu4N][PF6] in
CH3CN. All potentials are vs. NHE
Fig. 4. Photochemical hydrogen evolution from an oxygen-free acetonitrile solution
(10 mL) of Et3N (500 equiv).

84
W. Jiang et al. / Inorganic Chemistry Communications 16 (2012) 81–85
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(2009CB220009), the Program for Changjiang Scholars and Innova-
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1
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washed with anhydrous diethyl ether (40 mL) before filtration. A solution of
complex 1c (54 mg, 0.1 mmol) in EtOH (40 mL) was added to the filtrate and
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