An artificial [FeFe]-hydrogenase mimic with organic chromophore-linked thiolate bridges for the photochemical production of hydrogen

Article abstract of DOI:10.1007/s11696-016-0049-8

An artificial [FeFe]-hydrogenase ([FeFe]-H₂ase) mimic 3II, consisting of dual organic chromophores covalently assembled to the [Fe₂S₂] active site, was constructed for light-driven hydrogen evolution. The structural confor

Full text of DOI:10.1007/s11696-016-0049-8

Chem. Pap.
DOI 10.1007/s11696-016-0049-8
ORIGINAL PAPER
An artificial [FeFe]-hydrogenase mimic with organic
chromophore-linked thiolate bridges for the photochemical
production of hydrogen
Shang Gao1 Wei-Yi Zhang1 Qian Duan1 Qing-Cheng Liang1 Da-Yong Jiang1
•
•
•
•
•
Jian-Xun Zhao1 Jian-Hua Hou1
•
Received: 19 April 2016 / Accepted: 30 May 2016
Ó Institute of Chemistry, Slovak Academy of Sciences 2016
Abstract An artificial [FeFe]-hydrogenase ([FeFe]-H2ase)
mimic 3II, consisting of dual organic chromophores
covalently assembled to the [Fe2S2] active site, was con-
structed for light-driven hydrogen evolution. The structural
conformation of synthetic photocatalyst was characterized
crystallographically and spectroscopically. The photo-in-
duced intramolecular electron transfer was evidently
demonstrated by the combination of electrochemical,
steady-state, and transient absorption spectroscopic studies.
Finally, a remarkable activity was obtained in the present
photocatalytic system, indicating the covalent incorpora-
tion of photosensitizer and catalytic center as a promising
strategy to construct inexpensive, easily accessible [FeFe]-
H2ase model photocatalysts.
and Pickett 2009; Lubitz et al. 2014). Among these distinct
H2ases, the active site of [FeFe]-H2ase features a butterfly
[Fe2S2] subunit coordinated by a cysteine-linked [Fe4S4]
cluster, carbon monoxide, and cyanide ligands (Scheme 1).
The two low-valent iron atoms are held together by a
dithiolate bridge, and serve as the catalytic center for
proton reduction (Adams and Stiefel 1998; Cammack
1999; Nicolet et al. 1999). On the other hand, the active site
of [NiFe]-H2ase contains nickel and iron atoms, which are
bridged by two cysteine residues (Scheme 1). The low-spin
iron center is octahedrally coordinated by one CO and two
CN- ligands, while the other two cysteine residues are
bound to the nickel in a terminal fashion (Lubitz et al.
2007).
Since the elucidation of the detailed picture of H2ases,
thousands of synthetic Fe/Ni and Fe/Fe thiolate complexes
have been extensively investigated as the structural or
functional models for both [NiFe]- and [FeFe]-H2ase active
sites (Ogo et al. 2013; Rauchfuss 2015; Denny and
Darensbourg 2015; Artero et al. 2015; Wombwell et al.
2015; Xu et al. 2015). On the aspect of converting solar
energy into chemical fuel, the artificially photocatalytic
systems consisting of photosensitizer (PS), [Fe2S2] catalyst,
sacrificial electron donor (D), and proton source have
already accomplished the light-driven H2 evolution with
relatively high turnover numbers (TONs) (Lomoth and Ott
2009; Wang et al. 2012; Wu et al. 2014; Wang et al. 2015).
However, the utilization of organometallic photosensitizers
containing precious metals, such as Pt, Pd, Ru, Ir, Rh et al.,
or toxic quantum-confined semiconductors (QDs) resulted
in undesirable resource and environmental impacts. The
construction of a noble-metal-free and easily accessible
photocatalyst still represents one of the greatest challenges.
Recently, we reported a photocatalytic [FeFe]-H2ase
mimic with organic photosensitizer covalently imbedded to
Keywords Hydrogenase Á Enzyme mimic Á
Photocatalysis Á Intramolecular electron transfer Á
Hydrogen evolution
Introduction
Hydrogen generation from solar energy represents a
promising solution to produce a renewable, nonpolluting,
sustainable fuel for looming global energy challenges. In
this context, hydrogenases (H2ases), a class of metalloen-
zymes catalyzing the reversible reduction of proton and
oxidation of H2, have attracted intensive attention (Tard
& Shang Gao
custgaoshang@126.com
1
School of Materials Science and Engineering, Changchun
University of Science and Technology, 7989 Weixing Road,
Changchun 130022, People’s Republic of China
123

Chem. Pap.
Scheme 1 Schematic
illustration of the active sites of
biological [FeFe]-H2ase (left),
[NiFe]-H2ase (right), and the
artificial photocatalytic mimic
(middle)
the [Fe2S2] active site through an azadithiolate linker (Gao
et al. 2014). With the aim of improving catalytic efficiency,
a new compact, inexpensive [FeFe]-H2ase model photo-
catalyst (Scheme 1) with dual organic chromophores was
constructed through molecular engineering. Herein, the
synthesis and structural conformation of the artificial
photocatalytic [FeFe]-H2ase mimic (coded as 3II) is pre-
sented. The desirable intramolecular electron transfer was
evidently detected, and the photocatalytic activity of the
molecular catalyst was demonstrated with a TON of 31.6 in
the presence of proton source and sacrificial donor under
light irradiation.
conventional three-electrode cell under argon and at
ambient temperature. The working electrode was a glassy
carbon disc (diameter 3 mm) that was successively pol-
ished with 3 and 1 lm diamond pastes and sonicated for
15 min prior to use. The reference electrode was a non-
aqueous Ag/Ag? electrode (0.01 mol L-1 AgNO3 and
0.1 mol L-1 n-Bu4NPF6 in CH3CN), and the counter
electrode was a platinum wire. The potentials were repor-
ted versus ferrocene (Fc)/ferrocenium (Fc?) couple.
Nanosecond transient absorption measurements were
manipulated on an LP-920 Laser flash photolysis setup
(Edinburgh Instruments Ltd.). The excitation source was
the unfocused third harmonic (355 nm, 7 ns fwhm) output
of a Nd:YAG laser (Continuum surelite II); 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.
Experiment
General procedures
All organometallic reactions and operations were carried
out under a dry, oxygen-free argon atmosphere with stan-
dard Schlenk techniques. All solvents were dried and dis-
tilled prior to use according to standard methods. Fe(CO)5,
LiEt3BH, 4-methylbenzoic acid, and o-aminothiophenol
were commercially available and used without further
purification. The starting material [(l-S)2{Fe(CO)3}2] was
prepared according to the literature method (Bogan et al.
1983).
1H and 13C spectra were collected on a Bruker
AVANCE II/400 NMR spectrometer with tetramethylsi-
lane (TMS) as the internal standard. IR spectra were
recorded on JASCO FT/IR 430 spectrophotometer. HR-MS
spectra determinations were made on a GCT-MS instru-
ment (Micromass, England). UV–Vis spectra were mea-
sured on a PerkinElmer Lambda 35 spectrophotometer.
Steady-state emission spectra were determined on JASCO
FP-6500 spectrophotometer.
Synthesis and characterization
Compound 1: 4-methylbenzoic acid (10.24 g, 75 mmol)
was added portionwise to polyphosphoric acid (30 mL) at
100 °C until a smooth paste was obtained. o-aminothio-
phenol (9.36 g, 75 mmol) was added, and the reaction
mixture was heated to 185 °C. After stirring at 185 °C for a
further 30 min, the mixture was poured into vigorously
stirred H2O. The resulting solution was extracted with
CH2Cl2, and washed by Na2CO3 aqueous solution and then
deionized water. The concentrated residue was further
purified on silica gel (CH2Cl2 as elute), providing 1 as
white solid (7.91 g, 70%).
1H NMR (400 MHz, CDCl3): d = 8.07 (d, J = 8.0 Hz,
1H, NC6H4S), 7.99 (d, J = 8.0 Hz, 2H, C6H4), 7.90 (d,
J = 8.0 Hz, 1H, NC6H4S), 7.50 (t, J = 7.6 Hz, 1H, NC6-
H4S), 7.38 (t, J = 7.6 Hz, 1H, NC6H4S), 7.31 (d,
J = 8.0 Hz, 2H, C6H4), 2.43 (s, 3H, CH3) ppm; 13C NMR
(100 MHz, CDCl3): d = 168.2, 154.2, 141.4, 134.9, 131.0,
129.7, 127.5, 126.3, 125.0, 123.1, 121.6, 21.5 ppm; HR-
MS (EI) : m/z calc. for [M?]: 225.0612; found: 225.0612.
Electrochemical measurements were made with a BAS
100B electrochemical workstation at a scan rate of
100 mV s-1. All voltammograms were obtained in a
123

Chem. Pap.
Copmplex 3II: A mixture of compound 1 (9 g,
the range 2.06°^H^26.00°, completeness Hmax
99.9%, 6606 independent reflections, Rint = 0.0628, 4253
=
40 mmol), NBS (7.12 g, 40 mmol), and azodiisobutyroni-
trile (0.65 g) was refluxed in anhydrous CCl4 for 12 h. The
mixture was cooled to room temperature, concentrated in
vacuo, and the residue crystallized from cyclohexane
yielding 8 g 1b as white solid. Super hydride LiEt3BH
(1 M solution in THF, 8 mL, 8 mmol) was added to a fresh
prepared solution of [(l-S)2{Fe(CO)3}2] (2) (1.38 g,
4 mmol) in THF (30 mL) at -78 °C by syringe over
30 min. 1b (2.42 g, 8 mmol) was added to the resulting
dark-green solution. The mixture was stirred for 2 h at
-78 °C, and then 1 h at room temperature. The solvent
was removed on a rotary evaporator. The crude product
was purified by column chromatography (silica, 10%
dichloromethane in hexane as eluent) to give complex 3II
(2.62 g, 85%) as a red solid.
reflections with Fo [ 4r (Fo), 433 parameters, 0 restraints,
R1obs = 0.0431,
wR2obs = 0.0950,
R1all = 0.0828,
wR2all = 0.1073, Goodness-of-fit on F2 = 1.014, largest
-3
˚
difference peak and hole: 0.356/-0.300 e A
.
Photocatalytic H2 production
Photochemical hydrogen production was performed by
irradiating an acetonitrile solution (10 mL) of 3II
(2.5 9 10-4 mol L-1) in the presence of acetic acid
(0.01 mol L-1) and ethanethiol (0.01 mol L-1) with a
500 W Xe lamp in a Schlenk tube at 298 K (controlled by
the cooling jacket). Prior to irradiation, the solution was
freeze–pump–thaw degassed for three times and then
warmed to room temperature under N2. The gas phase of the
photocatalytic system was analyzed on a GC 7890T instru-
1H NMR (400 MHz, CDCl3): d = 8.16 (d, J = 8.0 Hz,
2H), 8.11 (d, J = 8.0 Hz, 1H), 8.05 (d, J = 8.0 Hz, 1H),
8.00 (d, J = 8.0 Hz, 2H), 7.94 (d, J = 8.0 Hz, 1H), 7.88
(d, J = 8.0 Hz, 1H), 7.55 (d, J = 8.4 Hz, 2H), 7.48 (m,
2H), 7.40 (m, 2H), 7.25 (d, J = 8.4 Hz, 2H), 3.71(s, 2H),
3.30 (s, 2H) ppm; 13C NMR (100 MHz, CDCl3):
d = 208.3, 167.4, 167.3, 141.7, 141.5, 135.1, 135.0, 133.3,
132.9, 130.0, 129.2, 128.1, 127.9, 126.4, 126.3, 125.4,
125.2, 123.3, 123.2, 121.7, 121.6, 42.6, 28.3 ppm; IR
˚
ment with a thermal conductivity detector, a 5 A molecular
sieve column (2 mm 9 2 m), and with N2 as carrying gas.
Results and discussion
Compound 1 was prepared in a high yield from
4-methylbenzoic acid and o-aminothiophenol (Palmer et al.
1971) as a starting material and reference to investigate the
photo-induced electron transfer process. [(l-S)2-
{Fe(CO)3}2]2- dianion, freshly derived from [(l-S)2-
{Fe(CO)3}2] in tetrahydrofuran (THF), was treated with the
halogenated product of 1 at -78 °C to generate the target
complex 3II in a reasonable yield (as depicted in
Scheme 2). The solids of 1 and 3II were stable in air and
very soluble in various solvents. Their structural identities
were fully characterized by a combination of 1H, 13C
NMR, IR spectroscopies, and mass spectrometry.
(CH2Cl2): m/cm-1 2044, 2016, 1994, 1959, 1916 (CO); HR-
~
MS (ESI): m/z calc. for [M ? H?]: 792.8981; found:
792.8976.
Crystal structure determination
Diffraction measurement of complex 3II was made on a
SMART APEX II diffractometer. Data were collected at
298 K using graphite monochromatic Mo-Ka radiation
˚
(k = 0.71073 A) in the x-2h scan mode. Data processing
was accomplished with the SAINT processing program
(Siemens Energy & Automation Inc. 1996). Intensity data
were corrected for absorption by the SADABS program
(Sheldrick 1996). The structures were solved by direct
methods and refined by full-matrix least-squares techniques
on F2O using the SHELXTL 97 crystallographic software
package (Sheldrick 1997). All non-hydrogen atoms were
refined anisotropically. All hydrogen atoms were located
using the geometric method, and their positions and ther-
mal parameters were fixed during the structure refinement.
Crystallographic data of 3II: C34H20Fe2N2O6S4,
Mw = 792.46 g mol-1, red block, size 0.33 9 0.29 9
0.11 mm, orthorhombic, space group Pbca, a = 12.8288
The molecular structure of 3II (Fig. 1, crystal data in
Table 1), was determined by X-ray crystallography. In
agreement with previously reported diiron thiolate com-
plexes (Heinekey 2009; Tard and Pickett 2009), 3II
exhibited the [Fe2S2] core with butterfly conformation. The
˚
Fe–Fe bond length [2.5198 (7) A] of 3II was slightly
longer than those of its azadithiolate derivatives (Darens-
bourg et al. 2003; Georgakaki et al. 2003; Tard et al. 2005;
Tard and Pickett 2009; Gloaguen and Rauchfuss 2009),
still within the expected range (Nicolet et al. 2001). Unlike
diiron bidentate dithiolates [(l-S)2R{Fe(CO)3}2], the
[Fe2S2] core of complexes [(l-SR)2{Fe(CO)3}2] usually
showed three possible stereoisomers with regard to the
orientations of S–R bonds (Fig. 2). Only two isomers had
been observed, isolated, and identified as anti and syneq
(Seyferth and Henderson 1981; Seyferth et al. 1986). The
synax isomer fell into the absent type in [(l-
˚
(14), b = 16.8812 (17), c = 31.089 (3) A, a =
3
˚
b = c = 90.00°, V = 6732.8 (12) A , T = 298 (2) K,
Z = 8, qcalcd. = 1.564 g cm-3, k (Mo-Ka) = 0.71073 A, l
˚
(Mo-Ka) = 1.158 mm-1, F (000) = 3216, 36569 reflec-
tions in h (-15/15), k (-20/20), l (-38/38), measured in
123

Chem. Pap.
Scheme 2 Synthetic procedures of reference compound 1 and artificial mimic 3II
Fig. 2 Orientational stereoisomers of [Fe2S2] core in [(l-SR)2
{Fe(CO)3}2] complexes
adt = azadithiolate). In the solid state of 3II, the two S–R
bonds (S1–C7, S2–C21) resided in the apical and equato-
rial positions of thiolate bridges, respectively. The mole-
cule took an anti conformation. Accordingly, the Feavg–S–
C angles were 112.8° for the apical C7 atom and 114.9° for
the equatorial C21 atom.
Fig. 1 Molecular structure of 3II (ellipsoids at 30% probability
level). The hydrogen atoms have been omitted for clarity
˚
Table 1 Selected bond lengths (A) and angles (deg) for complex 3II
Bond lengths
Bond angles
Likewise, the atoms of the phenylbenzo[d]thiazole
moieties in 3II molecule were almost coplanar, and the
dihedral angle between the two planes was 3.7°. The
Fe (1)–Fe (2)
Fe (1)–S (1)
Fe (1)–S (2)
Fe (2)–S(1)
Fe (2)–S (2)
Fe (1)–C (1)
Fe (1)–C (2)
Fe (1)–C (3)
Fe (2)–C (4)
Fe (2)–C (5)
Fe (2)–C (6)
S (1)–C (7)
S (2)–C (21)
S (1)ÁÁÁS (2)
2.5198 (7)
2.2638 (10)
2.2529 (10)
2.2704 (10)
2.2530 (9)
1.812 (4)
1.790 (4)
1.776 (4)
1.783 (4)
1.779 (4)
1.786 (4)
1.845 (3)
1.826 (3)
2.9069 (12)
Fe (1)–S (1)–Fe (2)
Fe (1)–S (2)–Fe (2)
C (1)–Fe (1)–Fe (2)
C (6)–Fe (2)–Fe (1)
C (1)–Fe (1)–S (1)
C (1)–Fe (1)–S (2)
C (6)–Fe (2)–S (1)
C (6)–Fe (2)–S (2)
Fe (1)–S (1)–C (7)
Fe (2)–S (1)–C (7)
Fe (1)–S (2)–C (21)
Fe (2)–S (2)–C (21)
S (1)–C (7)–C (8)
S (2)–C (21)–C (22)
Dihedral*
67.52 (3)
68.00 (3)
˚
150.49 (11)
150.35 (14)
104.71 (11)
101.52 (11)
107.23 (14)
99.59 (14)
113.71 (11)
111.97 (12)
115.54 (13)
114.28 (11)
113.2 (2)
average value (ca. 2.26 A) of Fe–S bond lengths in 3II
exhibited no significant difference related to the corre-
sponding those in reported diiron thiolates (Tard and
Pickett 2009). Meanwhile, the Fe–S bonds with sulfur atom
˚
bonded to the axial carbon atom were 0.01–0.02 A longer
than the other two Fe–S bonds within the anti-isomer. The
C–Fe–Fe angles composed by the apical CO ligands and
the Fe–Fe bonds were almost identical [\C1–Fe1–
Fe2 = 150.49(11)°, \C6–Fe2–Fe1 = 150.35 (14)°], sug-
gestive of less steric encumbrance from the upwards apical
and outwards equatorial substitutes. It was noteworthy that
˚
the distance of S1ÁÁÁS2 [2.9069 (12) A] and the dihedral
112.0 (2)
angle between the planes defined by Fe1–S1–Fe2 and Fe1–
S2–Fe2 (78.5°) were somewhat enlarged as compared with
previously reported models with bridged dithiolate ligands
(Tard and Pickett 2009).
78.5
* Defined by the intersection of the two [SFe2] planes
SR)2{Fe(CO)3}2] cases due to steric repulsion, which could
be overcome in the linked thiolates [(l-edt/pdt/adt){Fe
(CO)3}2] (edt = ethanedithiolate; pdt = propanedithiolate;
The proton NMR spectrum provided information as to
the nature of configuration of 3II in solution. The CDCl3
123

Chem. Pap.
solution of 3II displayed two distinct categories of the
methylene resonances at 3.23 and 3.71 ppm with essen-
tially equal intensities (Fig. 3), suggesting that the two
signals originated from one species in different positions
(King 1962). Likewise, two groups of resonances assigned
to the phenylbenzo[d]thiazole substitutes fell into the range
defined by aromatic protons with a similar pattern as that of
the above-mentioned methylene. Unlike previously repor-
ted results (Zhong et al. 2008; Crouthers et al. 2015), no
signals arising from the syneq isomer of 3II and other
impurities could be detected during the NMR measure-
ment. The observations strongly suggested that in solution,
the anti-isomer of 3II was thermodynamically favored and
always predominated.
in analogously visible region. The emission intensity of 3II
was strongly quenched with a quenching efficiency of 93%
relative to that of 1. Under the same conditions, the addition
of equimolar 2 to the solution of 1 caused an extremely low
quenching efficiency of 8%. The results strongly suggested
that the intermolecular collision process and quenching
rarely operated (Baskaran et al. 2005). The remarkably
decreased intensity of the fluorescence band of 3II mainly
arose from the intramolecular oxidative process with elec-
tron from the excited phenylbenzo[d]thiazole moiety to
[Fe2S2] unit in 3II.
Under an argon atmosphere, the acetonitrile solution of
complex 3II with 0.05 mol L-1 n-Bu4NPF6 as supporting
electrolyte displayed quasi-reversible and electrochemi-
cally irreversible reductions at -1.54 and -2.11 V,
respectively (Fig. 5a). The former, in line with those of
various all-CO diiron models bearing different thiolate
bridges (Capon et al. 2009; Felton et al. 2009), could be
assigned to the redox process of [FeIFeI] to [FeIFe0], and
demonstrated to be one-electron reduction process by bulk
electrolysis. A comparison of the reduction potentials of 3II
with that of reference 1 indicated that the [Fe2S2] species
accepted an electron more readily than the organic chro-
mophore. Likewise, the irreversible oxidation event of
[FeIFeI] to [FeIFeII] for 3II occurred at 0.47 V vs. Fc/Fc?.
It was noticeable that the current intensity ratio of the
oxidation and primary reduction peaks was 1.3. Therefore,
the oxidation event of 3II could be assumed to be a com-
bined result of two overlapping processes of [FeIFeI] to
[FeIFeII] and the oxidation of organic chromophore. Based
on the determined oxidation potential Eox of chromophore,
reduction potential Ered of [Fe2S2] active site, and the
excited state energy E00 of chromophore, which was esti-
mated from the intersection between the absorption and
emission spectra of compound 1, the free-energy change
for the photo-induced electron transfer in 3II was calcu-
lated to be -1.72 eV by the Rehm–Weller equation (Rehm
and Weller 1970). The electron transfer process from the
excited chromophore to [Fe2S2] centre in the artificial
mimic 3II was demonstrated to be exothermic and ther-
modynamically feasible.
~
Complex 3II displayed five infrared bands in the m (CO)
stretching region (1916–2044 cm-1), which obviously
shifted to lower frequencies as compared with those of the
precursor compound 2 (2073, 2037, and 1998 cm-1). The
observations reflected a relatively enhanced electron
accumulation on iron atoms, which led to a stronger back-
~
bonding from the iron atoms to CO and red-shifts of the m
(CO) frequencies. On the other hand, the more electron rich
iron atoms simultaneously elongated the Fe–Fe bond
length in complex 3II (vide supra).
The photophysical properties of artificial mimic 3II,
together with the multi-component references, were inves-
tigated to give a primary insight into the intramolecular
electron transfer process. Complex 3II exhibited an intense
absorption band peaking at ca. 315 nm with a shoulder at ca.
330 nm and weak absorption band in the region of
400–500 nm (Fig. 4). The maximal absorption band was
attributed to p–p* excitation within the organic chro-
mophore. The low-energy shoulder at ca. 330 nm and the
broad absorption in visible region were similar to the char-
acteristic bands of previously reported [FeFe]-H2ase mimics
(Eilers et al. 2007). The states responsible for the near-UV
absorption featured r–r* and d–r* character (Goy et al.
2013), and dominated the photophysical and photochemical
properties of 3II. Excitation at the maximal absorptions of
3II, 1, and the multi-component reference system consisting
of equimolar 1 and 2 resulted in fluorescence emission bands
Fig. 3 1H NMR spectrum of
complex 3II in CDCl3
123

Chem. Pap.
Fig. 4 UV–vis absorption (left)
and emission spectra (right) of
complex 3II, compound 1, and
an equimolar mixture of 1 and
2, recorded in CH3CN
Fig. 5 a Cyclic
voltammograms of complex 3II
and reference compound 1 in
CH3CN solution (0.05 mol L-1
n-Bu4NPF6). The scan rate is
100 mV s-1. b Dependence of
current heights of
electrocatalytic waves for 3II at
-2.11 V on the concentration of
HOAc
The behavior of electrochemical proton reduction for 3II
was studied by cyclic voltammograms in the presence of
HOAc. Upon the addition of HOAc, the primary reduction
wave (-1.54 V) of 3II showed little variation, whereas the
second redox peak (-2.11 V) exhibited a significant
electrocatalytic response. The current height of the second
redox event for 3II gradually increased and the potential
was shifted to a relatively more negative value with
sequential increments of HOAc concentration. Meanwhile,
the current height change displayed a good linear depen-
dence on the concentrations of added acid (Fig. 5b). All
these electrochemical behaviors featured an electrocat-
alytic proton reduction process (Chong et al. 2003; Capon
et al. 2009; Felton et al. 2009). Gas chromatography (GC)
analysis demonstrated that molecular hydrogen was pro-
duced during above electrolysis, further confirming the
electrocatalytic process.
immediate production of hydrogen, which was collected
and determined by gas chromatography (GC) analysis with
methane as the internal standard. Control experiments
suggested that the presence of all of the catalyst 3II, sac-
rificial electron donor EtSH, and HOAc was essential for
the photocatalytic H2 production. The amount of generated
H2 increased steadily in the first 2 h, with a turnover fre-
quency (TOF) of 19.7 h-1 for the initial 1 h. Prolonged
irradiation might result in the decomposition of photocat-
alyst, then the rate of H2 evolution slowed down gradually,
similar to the other reported artificial photosynthetic sys-
tems (Kluwer et al. 2009; Wang et al. 2010). With four
hours of irradiation, the total amount of evolved H2 had
reached 79 mmol, defining a turnover number (TON) of
31.6 based on photocatalyst 3II (Fig. 6a).
To further confirm the intramolecular electron transfer
process in artificial mimic 3II during H2 evolution, time-
resolved absorption investigations were performed using
the nanosecond flash photolysis technique (Fig. 6b). Laser
excitation by 355 nm light resulted in an immediate for-
mation of the excited state of compound 1, which was
characterized by an absorption band at ca. 460 nm and
The photochemical H2 evolution was performed by
irradiating the artificial mimic 3II in the presence of
ethanethiol as sacrificial electron donor and acetic acid as
proton source in degassed acetonitrile solution. Irradiation
of the catalytic system with a 500 W Xe lamp led to
123

Chem. Pap.
Fig. 6 a Time dependence of H2 evolution from the artificial mimic
3II (2.5 9 10-4 mol L-1 in CH3CN solution consisting of EtSH
(0.01 mol L-1) and HOAc (0.01 mol L-1), and b transient absorption
spectra of 1 (5 9 10-5 mol L-1, 0.15 ls: dotted line) and 3II
(5 9 10-5 mol L-1, 0.2 ls: solid line) upon laser pulse by 355 nm
light in an acetonitrile solution under argon atmosphere
significantly quenched by oxygen. On the basis of these
results, the transient was proposed to be due to the triplet
excited state of 1 (Iijima et al. 2010; Yang et al. 2012). In
contrast, 3II displayed a transient absorption spectrum with
totally different profile as compared with that of 1
throughout the near-UV and visible region. The new
spectral features, such as absorptions at ca. 410, 580 nm,
indicated a newly generated species. According to the
spectroelectrochemical studies of one-electron reduced
states of similar diiron thiolate complexes (Borg et al.
2004; Na et al. 2007), the transient peaks at 410 and
580 nm could be ascribed to the absorption of [FeIFe0]
species of 3II. In addition, the formation of [FeIFe0]
intermediate had been demonstrated as one of the most
crucial processes in either electro- or photochemical H2
evolution catalyzed by [Fe2S2] mimic complexes (Capon
et al. 2009). In our case, all evidences indicated that the
generation of [FeIFe0] species in artificial mimic 3II orig-
inated from the effectively intramolecular electron transfer
from the photoexcited organic chromophore to the [Fe2S2]
active site, because energy transfer would not result in any
new absorptions in the measuring spectral window (Wol-
pher et al. 2003). The photochemical H2 evolution was
closely related to the photo-induced electron transfer, and
the molecular photocatalyst 3II had achieved a promising
efficiency for light-driven H2 production.
thermodynamical feasibility of photo-induced intramolec-
ular electron transfer and the resulted luminescence
quenching in mimic 3II. The subsequently reduced [FeIFe0]
species generating from the efficiently intramolecular
electron transfer was evidently detected by laser flash
photolysis. In the presence of sacrificial electron donor and
proton source, the photocatalyst 3II exhibited catalytic
activities for light-driven H2 evolution in homogeneous
solution. A remarkable TON of 31.6 was obtained for such
a simple and compact hydrogenase mimic after four hours
of irradiation. In view of the facile modifications based on
the present molecule platform, improved performance of
such [FeFe]-H2ase model photocatalyst could be expected.
Further investigations are now in progress.
Acknowledgements We are grateful to the National Natural Science
Foundation of China (Nos. 21201022, 61106050, and 61473132), The
Specialized Research Fund for the Doctoral Program of Higher
Education (New Teachers, No. 20122216120001), and the Scientific
and Technological Development Project of Jilin Province (No.
20150311086YY) for financial support.
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