Photoinduced hydrogen evolution in supramolecular devices with a rhenium photosensitizer linked to FeFe-hydrogenase model complexes

Article abstract of DOI:10.1039/c2dt30468f

Coordination of the pyridyl-attached diiron azadithiolate hexacarbonyl complexes (2 and 3) through the pyridyl nitrogen to the Re on 10-phenanthroline rhenium (5a) and 2,9-diphenyl-1,10-phenanthroline rhenium (5b) forms novel [Re-Fe] complexes 7a, 7b and 8 respectively. Under visible light illumination using triethylamine as a sacrificial electron donor and [Re-Fe] type complexes (7a, 7b or 8) as catalysts, remarkably increased efficiency was observed for photoinduced hydrogen production with a turnover number reaching 11.8 from complex 7a and 8.75 from 7b. To the best of our knowledge, these are the best values compared to other [Re-Fe] photocatalysts reported so far. In contrast to the parent molecules, the turnover number by the intermolecular combination of complexes 6a and 2 showed a value of 5.23, and that from 6b and 2 is 3.8, while no H₂ was detected from 8a and 3 under the same experimental conditions. Obviously, the intramolecular combination of rhenium(i) and [2Fe2S] as a catalyst is promising for efficient H₂ evolution, and it is better than the intermolecular multi-component system.

Full text of DOI:10.1039/c2dt30468f

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PAPER
Photoinduced hydrogen evolution in supramolecular devices with a rhenium
photosensitizer linked to FeFe-hydrogenase model complexes
Jianhui Liu* and Weina Jiang
Received 4th January 2012, Accepted 8th June 2012
DOI: 10.1039/c2dt30468f
Coordination of the pyridyl-attached diiron azadithiolate hexacarbonyl complexes (2 and 3) through the
pyridyl nitrogen to the Re on 10-phenanthroline rhenium (5a) and 2,9-diphenyl-1,10-phenanthroline
rhenium (5b) forms novel [Re–Fe] complexes 7a, 7b and 8 respectively. Under visible light illumination
using triethylamine as a sacrificial electron donor and [Re–Fe] type complexes (7a, 7b or 8) as catalysts,
remarkably increased efficiency was observed for photoinduced hydrogen production with a turnover
number reaching 11.8 from complex 7a and 8.75 from 7b. To the best of our knowledge, these are the
best values compared to other [Re–Fe] photocatalysts reported so far. In contrast to the parent molecules,
the turnover number by the intermolecular combination of complexes 6a and 2 showed a value of 5.23,
and that from 6b and 2 is 3.8, while no H2 was detected from 8a and 3 under the same experimental
conditions. Obviously, the intramolecular combination of rhenium(I) and [2Fe2S] as a catalyst is
promising for efficient H2 evolution, and it is better than the intermolecular multi-component system.
the excited state reduction potential of a Ru(II) photosensitizer
does not allow the electron transfer to the [2Fe2S] catalytic
Introduction
The search for substances to replace fossil fuels is a much
demanding issue due to the high cost and expected scarcity of
the conventional fuels in the future. Hydrogen is considered to
be a promising candidate for a clean and renewable energy
without emitting any greenhouse gases.1–3 Although hydrogen
production from conventional sources, including natural gas
steam reforming, partial oxidation of hydrocarbons as well as
coal gasification, is still a dominating, hydrogen generated from
renewable sources is likely to play an important role as an
energy carrier in future energy supply. Hydrogenase, a group of
enzymes with an H-cluster and an 4Fe–4S moiety being the prin-
cipal part which is responsible for the catalytic process and the
electron transfer, can produce hydrogen with highly efficiency.4,5
Therefore there is a strong and continuous interest to mimic their
active centers both structurally and functionally.6–8 Although a
variety of hydrogenase mimics have been described in the litera-
ture, the work that relies on the use of a photosensitizer for the
construction of three-component systems seems to be more
efficient. Thus, quite a lot of studies have been focused on the
light-driven water splitting into hydrogen with heterogeneous or
homogeneous systems, multicomponent systems or supramolecu-
lar systems.9–12 In 2003, Sun et al. disclosed supramolecular
photocatalysts with a ruthenium-moiety as the photosensitizer
linked to [2Fe2S] complexes,13–16 but thermodynamically the
more negative potential of the [2Fe2S] complex with respect to
center. In 2006, Song and co-workers reported a light-driven
model compound in which a tetraphenylporphyrin was co-
valently bonded to the [2Fe2S] complex,17 but no hydrogen evol-
ution was recorded. Later on, Sun et al. described the design and
synthesis of a supramolecular assembly,18 in which a zinc tetra-
phenylporphyrin and a pyridyl-functionalized [2Fe2S] complex
was self-assembled via noncovalent axial coordination, leading
to photo-induced hydrogen generation. Subsequently Reek and
co-workers reported a photocatalyst in which a [2Fe2S] complex
is directly coordinated to two zinc porphyrin sensitizers.19 Very
recently, Wasielewski and co-workers reported covalently linked
electron donor–acceptor triads containing zinc tetraphenylpor-
phyrin sensitizers, naphthalene monoimide [2Fe2S] complexes
and ferrocene secondary electron donors,20 but its turnover
number was only 0.56 after 14 hours of irradiation.
In the past several years, researchers have noticed that
rhenium(I) tricarbonyl complexes with bidentate heterocyclic
ligands, in particular, display intense luminescence in the visible
region of the spectrum and are stable to photodecomposition, so
they are ideally suited for the use in solar energy
conversion.21–23 In our own work, we focused on the supramole-
cular [Re–Fe] complexes and reported one example of photo-
catalysts,24 unfortunately, the one in which rhenium is linked to
the [2Fe2S] complex by a phosphorus-containing ligand is not
stable under the process of irradiation. Later on, Wu and
co-workers reported some other novel examples of [Re–Fe]
photocatalysts in which a cyanide (CN) group or triple bond-
containing bridges were applied to connect the photosensitizers
to the [2Fe2S] complexes. They provided significant insights for
this field. However, the results was not ideal with the TON for
State Key Laboratory of Fine Chemicals, Dalian University of
Technology (DUT), Dalian, 116012, P. R. China.
E-mail: liujh@dlut.edu.cn; Fax: +86 411 84986245;
Tel: +86 411 84986249
9700 | Dalton Trans., 2012, 41, 9700–9707
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H2 evolution being no more than 0.14 and 0.35 respectively.25,26
More recently, we have established a novel trimetallic [Re–Fe]
complex 8,27 consisting of a phenanthroline rhenium photo-
sensitizer and a [2Fe2S] complex (connected by the axial co-
ordination of a pyridyl group). Visible light-driven H2 generation
was achieved by using triethylamine (sacrificial electron donor)
in the presence of the complex 8 (catalyst) in CH3CN/H2O, with
a turnover number reaching 1.5. Although still very low, this is,
so far as we know, the best results for [Re–Fe] photocatalysts. In
light of the encouraging data for H2 evolution, we designed two
other novel [Re–Fe] complexes to examine their efficiency for
the photoinduced proton reduction based on the following
hypothesis. We reasoned that the electron-withdrawing ability of
the ester group has an unfavourable effect on electron transfer. In
addition, the dominant conformation, as shown in Scheme 1,28
which is determined by the stereoelectronic effect, makes the
two parts of the Re-moiety and [2Fe2S] system far away (linear
rather than bent). Instead of using an ester linking-chain as in 8,
we choose a simple methylene to connect the [2Fe2S] and pyri-
dine as in 7a and 7b. Herein we wish to further report these
novel trimetallic [Re–Fe] photocatalysts (complexes 7a, 7b and
8) in which the phenanthroline rhenium photosensitizers and the
[2Fe2S] complexes are connected by a pyridine bridge, as shown
in Chart 1, leading to an increased efficiency for H2 formation
with a turnover number of 11.8 from complex 7a, and 8.75 from
7b, so far as we know the highest among [Re–Fe] photocatalysts.
Their corresponding references (complex 6a, 6b, 8a, 2 and 3)
are also described for comparison.
phenanthroline (4b) to give fac-(Ph2phen)Re(CO)3Cl (5b). 5a
and 5b were then respectively reacted with complex 2 in
refluxing ethanol in the presence of CF3SO3Ag to afford com-
plexes 7a and 7b. Complex 8 was prepared by the reaction of 5a
and 3 in a similar manner. For comparison, complexes 6a and
6b were synthesized by the coordination of pyridine to 4a and
4b respectively.
Steady-state absorption spectra
The UV-vis absorption spectra of complexes 7a, 7b and 8 in
(0.01 mM) CH3CN at room temperature are shown in Fig. 1.
From the previous studies,29 complexes 6a and 8a have weak
absorption at about 281 nm attributed to the ligand-centered tran-
sition (πL → π*L), a broad band at ca. 360 nm assigned as dπ
(Re) → π* (phen) (MLCT). The absorption of complex 6b is
observed at 260 nm, assigned to the ligand-centered transition
(πL → π*L), and an 380 nm transition due to the MLCT.30 For
complex 7a, an intense absorption is observed at 273 nm and a
broad weak absorption at about 350 nm. Based on the above
analyses, the first one at 273 nm is attributed to the ligand-
centered transition (πL → π*L), and the one at 350 nm is due to
the MLCT. As to complex 7b, it is similar to the rhenium com-
plexes. Absorption at 256 nm is attributed to the ligand-centered
transition (πL → π*L), while the lower-energy features at ca.
280–400 nm are the mixtures of the MLCT and d(Fe) → d*(Fe)
transition. However, for the complex 8, intense absorptions are
observed at 273 nm and 364 nm, with a broad and weak absorp-
tion at 319 nm.
Results and discussion
Fluorescence spectra
Preparation and characterization
The fluorescence spectra of complexes 7a, 6a, 7b, 6b, 8 and 8a
in a deoxygenated acetonitrile solution are shown in Fig. 2.
Similar to the previous results, complexes 7a, 6a, 7b, 6b, 8 and
8a displayed broad lower-energy fluorescence emissions at about
540 nm to 560 nm, which is characteristic of MLCT emissions
of the rhenium complexes, and we assign them to the dπ (Re) →
Complexes 2 and 3 were prepared as depicted in Scheme 2, and
5a, 6a, 7a 5b, 6b and 7b were synthesized as depicted in
Scheme 3. The commercially available Re(CO)5Cl was treated
with 1,10-phenanthroline (4a) in refluxing toluene to give
fac-(phen)Re(CO)3Cl (5a) and with 2,9-diphenyl-1,10-
3
Scheme 1 The conformation of complex 8.
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Chart 1 The structures for the complexes and reference compounds in this study.
π* (phen) emissions. The rhenium complexes 7a and 7b dis-
played strong fluorescence emissions, while complex 8 displayed
relatively weak fluorescence emission, thus they can be
employed as sensitive probes for excited state quenching. If elec-
tron transfer and/or energy transfer occurs from the excited state
of the rhenium moiety to the [2Fe2S] unit, the fluorescence
quenching should be readily detected. The fluorescence of the
excited rhenium moiety was indeed quenched in complex 8 in
contrast to the corresponding rhenium complex 8a. We assume
that it is mainly caused by an intramolecular oxidative process
with the electron transfer from the excited photosensitizer to the
[2Fe2S] center in complex 8. However, we did not find that the
fluorescence of the excited rhenium moiety was quenched in
complexes 7a and 7b in contrast to the corresponding rhenium
complexes 6a and 6b. This is quite different from the previous
reports.25,26 From the results of the fluorescence emissions, we
assume the mechanism of the photoinduced hydrogen evolution
by complexes 7a and 7b is quite different.
Electrochemistry
The electrochemistry of photocatalysts 7a, 7b and 2 were studied
to evaluate their redox properties and shown in Fig. 3. For com-
parison, redox potentials for complex 2 in the same solvent
system are also included. The cyclic voltammograms of 7a, 7b
and 2 were recorded in CH3CN with 0.05 M [n-Bu4N][PF6] as
electrolyte. Reduction and oxidation potentials were given
vs. Ag/AgNO3 in Table 1. In the case of the trinuclear complex
7a, the reduction peaks at −1.47 V and −2.04 V are assigned to
9702 | Dalton Trans., 2012, 41, 9700–9707
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Scheme 2 The synthetic procedures of complexes 2 and 3.
Scheme 3 The synthetic procedures of complexes 5a, 5b, 6a, 6b, 7a and 7b.
the processes FeIFeI/FeIFe0 and FeIFe0/Fe0Fe0 respectively. The
peaks at −1.71 V and −2.21 V were attributed to the reduction
of the 1,10-phenanthroline ligands.25 The irreversible oxidation
peak at +0.80 V is assigned to FeIFeI/FeIIFeI and at +1.59 V is
the ReI/ReII process. Similarly, the electroreduction behaviors of
7b and 8 are almost identical to that of complex 7a.27 Complex
7b shows oxidation peaks at +1.58 V and +0.78 V and reduction
peaks at −1.48 V, −1.91 V and −2.21 V. As for the new [2Fe2S]
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Fig. 1 The UV-vis spectra for complexes 7a, 7b and 8 (0.01 mM) in CH3CN.
complex 2, +0.62 V is ascribed to the process FeIFeI/FeIIFeI,
+0.91 V is the oxidation peak of the pyridine,31 and −1.54 Vand
−2.08 V are ascribed to the processes FeIFeI/FeIFe0 and FeIFe0/
Fe0Fe0 respectively. It is interesting to note that the reduction
potential of the ligand which possesses one phenyl is −1.48 V,
0.23 V higher than the parent molecules. This points to substan-
tial differences in the electron-donating ability of phenyl-
attached ligand when compared to the parent phenanthroline.
The more negative reduction potential observed in 7a (−1.71 V
vs. Ag/AgNO3) than that of 7b (−1.48 V vs. Ag/AgNO3) is in
agreement with the reduced ease of reduction of phenyl attached
phenanthroline as a result of its higher π* orbital energy.
intermolecular multi-component system: complexes 6a and 2 in
the same concentration with TEA as the electron donor, while
complexes 6b and 2 under the same conditions can reach 3.8.
No significant H2 was detected for complexes 8a and 3 in the
same concentration. As expected, the TONs for 7a and 7b
increase with decreasing the length of the carbon chain through
the substitution of –CH2CH2OCO– by the less electronegative
methylene group, which can be evidenced from the IR spectra.
Complexes 7a and 7b show similar wavenumbers of the CO
group, complex 7a: νmax/cm−1 2029 and 1916; complex 7b:
νmax/cm−1 2033 and 1916, while for complex 8: νmax/cm−1 2071
and 1923.27 Obviously, the wavenumbers of complex 8 are
higher compared to complexes 7a and 7b because of the elec-
tron-withdrawing effect of the ester group, and therefore dis-
favours the electron transfer from the Re-containing
photosensitizer to the [2Fe2S] catalytic center. Moreover, the
better performance of 7a and 7b compared to 8 may also be
attributed to the different mechanism of photoinduced H2 evolu-
Photocatalysis
Complexes 7a, 7b and 8 were used as photocatalysts for light-
driven hydrogen production with Et3N as a sacrificial electron
donor, as shown in Fig. 4. The reaction solution was irradiated at
25 °C using an Xe lamp (500 W) with a Pyrex-glass filter (λ >
ca. 400 nm). The amount of hydrogen evolved was determined
by GC analysis of the gas phase of the reaction system. Irrespec-
tive of H2 dissolved in the solvent with several hours of
irradiation, the TON of hydrogen is determined to be 11.8 for
complex 7a, 8.75 for complex 7b and 1.5 for complex 8. Under
the same experimental conditions, for reference, a turnover
number of 5.23 is obtained for their corresponding
1
tion. From the H NMR of complexes 7a and 7b, we could see
that these two molecules are symmetrical, so their dominant con-
formation is linear. The results of the fluorescence emissions
also confirm this. As to the plausible mechanism, further investi-
gations are in progress. The combination of rhenium(I) and
[2Fe2S] as a catalyst is promising for efficient H2 evolution, and
it is better than the intermolecular multi-component system. This
result is in a good agreement with the previous report.32 Even
so, the intermolecular multicomponent system from either
9704 | Dalton Trans., 2012, 41, 9700–9707
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Fig. 2 Fluorescence spectra for complexes 7a, 6a, 7b, 6b, 8 and 8a (0.01 mM) in CH3CN.
(6a and 2) or (6b and 2) performs more efficiently for the
catalytic activity than the current (best) intramolecular Re-based
complexes, due to the reduced possibility for back electron
transfer.
product was purified by column chromatography with toluene/
acetonitrile (4/1) as an eluent to give complex 7a as a red solid
(10.8 mg, 10%). H NMR (CD3COCD3): δ (ppm) 3.54 (s, 4H,
NCH2S), 3.92 (s, 2H, NCH2Ar), 7.67 (d, 2H, Ar), 8.34 (d, 2H,
Ar), 8.37 (s, 2H, Ar), 8.54 (d, 2H, Ar), 9.10 (d, 2H, Ar), 9.90 (d,
2H, Ar). MS (TOF-ES): m/z: 928.9358 [M–CF3SO3−]+.
IR νmax/cm−1: 2033, 2021 and 1916 (CO).
1
Experimental
Reagents and instruments
Synthesis of complex 7b. Complex 7b was synthesized by the
procedure similar to that for complex 7a, expect that complex 5b
was used in place of complex 5a. 1H NMR (CD3COCD3):
δ (ppm) 3.52 (s, 4H, NCH2S), 3.88 (s, 2H, NCH2Ar), 7.24 (d,
2H, Ar), 7.60 (d, 2H, Ar), 7.74 (s, 6H, Ar), 7.99 (d, 4H, Ar),
8.37 (s, 2H, Ar), 8.43 (d, 2H, Ar), 9.13 (d, 2H, Ar). MS
(TOF-ES): m/z: 1081.0648 [M–CF3SO3−]+. IR νmax/cm−1: 2029
and 1916 (CO).
Commercially available chemicals, including 1,10-phenanthro-
line,
2,9-diphenyl-1,10-phenanthroline,
paraformaldehyde,
ethanol amine, [Fe(CO)5], AgCF3SO3 and Re(CO)5Cl were used
without further purification. The reagent LiEt3BH was purchased
from Aldrich. Complexes (HOCH2)2-N-(pyridyl-4-mehyl)
methanamine, [(μ-S)2Fe2(CO)6], [(μ-HS)2Fe2(CO)6],33 com-
plexes 3, 5a, 5b, 6a, 6b, 8 and 8a were prepared according to
the literatures.27,29,34,35 1H NMR spectra were collected on a
Varian INOVA 400 NMR spectrometer. Mass spectra were
recorded on a ESI-Q-TOF MS (Micro) instrument.
Synthesis of complex 2. A solution of (HOCH2)2-N-(pyridyl-
4-methyl)methanamine (0.5 g, 2.9 mmol) in THF (5 mL) was
added to a THF solution (20 mL) of [(μ-LiS)2Fe2(CO)6] at
−78 °C, freshly derived from [(μ-S)2Fe2(CO)6] (1.00 g,
2.9 mmol) and LiEt3BH (1 M in THF, 6.0 mL, 6.0 mmol). The
mixture was stirred for 2 h at −78 °C and the solution turned
dark red from dark emerald green. The solvent was removed
from the filtrate on a rotary evaporator. The crude product was
purified by column chromatography with silica gel by using
CH2Cl2 as eluent to give complex 2 as a dark red solid (0.69 g,
Synthesis of complex 7a. Complex 5a (48.6 mg, 0.1 mmol)
and AgCF3SO3 (25.7 mg, 0.1 mmol) were refluxed in THF
(20 mL) for 3 h. Then the solvent was removed and the residue
was washed with anhydrous diethyl ether (40 mL) before fil-
tration. A solution of complex 2 (47.8 mg, 0.1 mmol) in EtOH
(40 mL) was added to the filtrate and refluxed for 2 h. Then the
solvent was removed on a rotary evaporator and the crude
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Fig. 3 Cyclic voltammograms of 7a, 7b and 2, 1.0 mM in 0.05 M n-Bu4NPF6/CH3CN at a scan rate of 100 mV s−1 with Ag/Ag+ as a reference
electrode.
Table 1 Electrochemical data of complexes 7a, 7b and 2 (V vs. Ag/Ag+)
Epa
Epa
Epc
Epc
Epc
Complex
ReI/ReII
FeIFeI/FeIFeII
FeIFeI/FeIFe0
FeIFe0/Fe0Fe0
L/L−
7a
7b
2
+1.59
+1.58
—
+0.80
+0.78
+0.62
−1.47
−1.48
−1.54
−2.04
−1.91
−2.08
−1.71, −2.21
−1.48, −2.21
—
1
50%). H NMR (CD3COCD3): δ (ppm) 3.75 (s, 4H, NCH2S),
4.07 (s, 2H, NCH2Ar), 7.37 (s, 2H, Ar), 8.67 (s, 2H, Ar). MS
(TOF-ES): m/z: 478.9392.
and no significant H2 was detected for complexes 8a and 3
under the same experimental conditions. The combination of
rhenium(I)–[2Fe2S] catalyst is promising for efficient H2 evol-
ution, and it is better than the intermolecular multi-component
system. Although still low, the TON value, as we believe, can be
improved by modification of the axial ligands and the proximity
of the metal centers. These investigations are in progress.
Conclusion
The introduction of a mono-rhenium complex with a 1,10-phe-
nanthroline ligand to the [2Fe2S] active center to form a trimetal-
lic system is obtained. When a mixed solvent system was treated
with these trimetallic [Re–Fe] complexes (7a, 7b and 8) in the
presence of TEA, light-driven H2 generation was detected. The
model complex 7a which shows better reducing abilities has the
advantage of simplicity and relatively higher stability. The turn-
over numbers can reach 11.8 for complex 7a and 8.75 for 7b
after 5.5 hours of irradiation, which are the best results compared
to other hydrogen evolution systems with Re photosensitizers
linked to [2Fe2S] complexes. In contrast, much lower TON are
obtained for the intermolecular multi-component systems, which
are 5.23 for complexes 6a and 2, 3.8 for complexes 6b and 2,
Acknowledgements
We are grateful to the Ministry of Science and Technology of
China and the Chinese National Natural Science Foundation
(grant no 20633020), the National Basic Research Program of
China (2009CB220009), the Program for Changjiang Scholars
and Innovative Research Team in University (IRT0711) and the
K. & A. Wallenberg Foundation for financial support of this
work. We also thank the Swedish Energy Agency and the
Swedish Research Council for support.
9706 | Dalton Trans., 2012, 41, 9700–9707
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Fig. 4 Photochemical hydrogen evolution from an oxygen-free acetonitrile solution (10 mL) of Et3N (500 equiv).
18 X. Q. Li, M. Wang, S. Q. Zhang, J. X. Pan, Y. Na, J. H. Liu,
B. Åkermark and L. C. Sun, J. Phys. Chem. B, 2008, 112, 8198–
8202.
19 A. M. Kluwera, R. Kaprea, F. Hartl, M. Lutz, A. L. Spek,
A. M. Brouwer, P. W. N. M. van Leeuwen and J. N. H. Reek, P. Natl.
Acad. Sci. USA, 2009, 106, 10460–10465.
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