Electron transfer and hydrogen generation from a molecular dyad: Platinum(ii) alkynyl complex anchored to [FeFe] hydrogenase subsite mimic

Article abstract of DOI:10.1039/c1dt11923k

A PS-Fe₂S₂ molecular dyad 1a directly anchoring a platinum(ii) alkynyl complex to a Fe₂S₂ active site of a [FeFe] H₂ase mimic, and an intermolecular system of its reference complexes 1b and 2, have be

Full text of DOI:10.1039/c1dt11923k

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PAPER
Electron transfer and hydrogen generation from a molecular dyad:
platinum(II) alkynyl complex anchored to [FeFe] hydrogenase subsite mimic
Wen-Guang Wang, Feng Wang, Hong-Yan Wang, Chen-Ho Tung and Li-Zhu Wu*
Received 12th October 2011, Accepted 11th November 2011
DOI: 10.1039/c1dt11923k
A PS–Fe2S2 molecular dyad 1a directly anchoring a platinum(II) alkynyl complex to a Fe2S2 active site
of a [FeFe] H2ase mimic, and an intermolecular system of its reference complexes 1b and 2, have been
successfully constructed. Time-dependence of H2 evolution shows that PS–Fe2S2 1a as well as complex
2 with 1b can produce H2 in the presence of a proton source and sacrificial donor under visible light
irradiation. Spectroscopic and electrochemical studies on the electron transfer event reveal that the
I
reduced Fe Fe0 species generated by the first electron transfer from the excited platinum(II) complex to
the Fe2S2 active site in PS–Fe2S2 1a and complex 2 with 1b is essential for photochemical H2 evolution,
I
while the second electron transfer from the excited platinum(II) complex to the protonated Fe Fe0
species is thermodynamically unfeasible, which might be an obstacle for the relatively small amount of
H2 obtained by PS–Fe2S2 molecular dyads reported so far.
or porphyrin PS to the Fe2S2 active site.19–21 On the other hand,
Introduction
Ott and Hammarstro¨m et al. made use of the second approach
Developing efficient photocatalysts to produce hydrogen (H2) is
to attach a phenylphosphine-derived ruthenium(II) ligand to one
one of key issues for solar energy conversion.1,2 Nature, long
ago, figured out how to use photosynthetic complexes to capture
sunlight, and to store its energy in a chemical form, H2, in which
hydrogenases (H2ases) catalyze the reversible reduction of protons
to H2 with remarkable activity.3 Since crystal analysis revealed
that the structures of the [FeFe] H2ases active site resembled the
well-known complexes [(m-SR)2{Fe-(CO)2L}2],4,5 extensive studies
have been focused on such a Fe2S2 cluster in order to understand
the proton binding and reduction of the [FeFe] H2ases active
site.6–10 From a photochemical point of view, multi-component
systems11–17 containing a photosensitizer (PS), a proton reduction
catalyst (Fe2S2), a sacrificial electron donor and proton source have
been developed, where electrons generated by the light-driven PS
are cascaded to the Fe2S2 cluster. To date, however, photoinduced
electron transfer from a PS to a Fe2S2 cluster mimic in a PS–Fe2S2
molecular dyad is rarely observed.
of the iron centers of the [FeFe] H2ases mimic.22 Nevertheless,
examples of photochemically driven H2 evolution from a PS–
Fe2S2 molecular dyad are quite scarce, especially for the second
approach. Although emission quenching was observed in these
dyads, the desired forward electron transfer from the PS to the
Fe2S2 active site was found to be thermodynamically unfeasible,
and thus leading to H2 generation from these PS–Fe2S2 dyads is
rather challenging. Very recently, PS–Fe2S2 molecular dyads with
a PS to a Fe2S2 active site of [FeFe] H2ases mimics appeared at
the forefront as a result of progress in synthetic [FeFe] H2ases
chemistry.23–27 Reek, Hartl and van Leeuwen in 2008 successfully
linked two zinc porphyrins to a Fe2S2 active site by coordinative
bonds and realized photochemical H2 production by PS–Fe2S2
dyad.24 In 2010, Wasielewski and co-workers employed imide as
a linkage to bridge zinc porphyrin to an Fe2S2 active site and
observed the forward electron transfer from the PS to the Fe2S2
active site.25 Meanwhile, we reported three PS–Fe2S2 molecular
dyads directly anchoring rhenium(I) complexes to one of the iron
cores of the Fe2S2 active site. Time-dependence of H2 evolution and
spectroscopic study demonstrate that the orientation of the PS and
the specific bridge in these dyads are important for the electron
transfer from the excited PS to the catalytic Fe2S2 active site to form
To make PS–Fe2S2 molecular dyads, two different strategies
have been put forward. In the first approach, the dithiolate-
bridge is employed to connect a PS and a Fe2S2 active site of
[FeFe] H2ases mimics. The alternative one involves anchoring a
˚
PS to one iron center of the Fe2S2 active site. Sun and Akermark
reported the first PS–Fe2S2 dyad using the first synthetic strategy
to covalently link a ruthenium(II) complex to a Fe2S2 active
site of [FeFe] H2ases mimics.18 Later on, Sun, Akermark and
˚
I
the unprecedented, long-lived Fe Fe0 species.26 From the above
results, it is evident that selection of a PS with an appropriate
redox potential is crucial for the realization of electron transfer
from the excited PS to the Fe2S2 catalytic center of the [FeFe]
H2ases mimics.
The platinum(II) polypyridyl alkynyl complex represents
an important class of compounds that possess long-lived
Song developed several dyads bearing ruthenium(II) bipyridine
Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, the Chinese Academy of
Sciences, Beijing, 100190, P. R. China. E-mail: lzwu@mail.ipc.ac.cn
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metal-to-ligand-charge-transfer (3MLCT) excited states and tun-
able redox potentials.28–41 Their 3MLCT emissive states are known
to be capable of undergoing electron transfer with both elec-
tronic donors and acceptors.30,36–41 In 2004, we reported, for
example, that platinum(II) polypyridyl alkynyl complexes could
be used as photocatalysts for H2 generation from Hantsch
dihydropyridines.36 The primary reaction has been subsequently
proved to be initiated by photoinduced electron transfer from
Hantsch dihydropyridines or 3,4-diaryl-2,5-dihydropyrroles to
3
the excited MLCT state of platinum(II) complexes.37–38 In 2008,
Eisenberg and co-workers constructed a photocatalytic H2 pro-
duction system using a platinum(II) alkynyl complex as a PS
and a cobalt(III) complex as a catalyst.39 Accordingly, oxidative
quenching the excited 3MLCT state of the platinum(II) PS by the
cobalt(III) catalyst was considered as the first step in the whole
photocatalytic process. Much to our surprise, despite of the fact
that platinum(II) alkynyl complexes have been studied in multi-
component photocatalytic systems,35–40 a molecular dyad that
incorporates a platinum(II) alkynyl complex and a catalytic center
in one molecule, to the best of our knowledge, has not yet been
explored.
Fig. 1 Molecular dyad 1a and its model complexes 1b and 2 as well.
Results and discussion
As shown in Fig. 2, PS–Fe2S2 molecular dyad 1a and its references
1b and 2 could be synthesized in reasonable yields. Introduction
of the acetylene moiety at iodophenyl-equipped Fe2S2 carbonyl
complex 3 was achieved by the Sonogashira cross-coupling
reaction giving rise to the alkynyl-functionalized Fe2S2 derivative
4.26,42,43 Further reaction of 4 and the platinum(II) 4,4-tBu2-
(CŸNŸN) complex 5 in CH2Cl2 for 12 h under an Ar atmosphere
resulted in the formation of PS–Fe2S2 1a in a yield of 40% at
room temperature.31–34 Similar synthetic procedures were used to
prepare complex 2 in a good chemical yield. All of the complexes
are soluble in organic solvents under ambient conditions and
In the present work, we wish to report an artificial PS–Fe2S2
molecular dyad 1a covalently anchoring a platinum(II) alkynyl
complex to a Fe2S2 active site of the [FeFe] H2ases mimic for
H2 evolution in homogeneous solution. The time dependence
of H2 evolution shows that PS–Fe2S2 1a as well as its model
complex 2 with 1b can produce H2 in the presence of a proton
source and sacrificial donor under visible light irradiation (Fig.
1). Spectroscopic and electrochemical studies on the electron
I
transfer event indicate that the reduced Fe Fe0 species generated
from the first electron transfer of the excited platinum(II) complex
to the Fe2S2 active site in PS–Fe2S2 1a as well as complex
2 with 1b is essential for photochemical H2 evolution, while
the second electron transfer step from the excited platinum(II)
1
have been well characterized by H NMR, MS, and elemental
analysis.
The activities of molecular dyad 1a and the intermolecular
system of 2 with 1b for H2 evolution were evaluated at room
temperature. In the presence of a proton source and sacrificial
donor, the degassed solution in a sealed Pyrex tube were irradiated
by a 500 W high-pressure Hanovia lamp. A glass filter was used
I
complex to the protonated Fe Fe0 species is thermodynamically
unfeasible, which might be an obstacle for the relatively small
amount of H2 obtained by PS–Fe2S2 molecular dyads reported so
far.
Fig. 2 Synthetic routes to molecular dyad 1a and its reference complexes 1b and 2 as well. Reagents and conditions: (a) THF/Et3N, 1,4-diethynylbenzene,
Pd(PPh3)2Cl2/CuI, Ar, 50 ◦C, yield 90%; (b) CH2Cl2, CuI, Ar, RT, 12 h, yield 76%; (c) CH2Cl2, CuI, Ar, RT, 12 h, yield 87%.
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to cut off light below 400 nm to guarantee the irradiation by
visible light. The generated photoproduct of H2 was characterized
excitation at the MLCT absorption region (~410 nm) led to a
maximal emission at 565 nm with a quantum yield of Uem = 0.064
and a lifetime of 564 ns in de-aerated CH3CN. For the PS–Fe2S2
molecular dyad 1a, however, a very weak emission was observed
upon excitation at the region of its MLCT absorption. Under
the same conditions the addition of one equiv. of 1b into the
solution of 2 caused not much change in the emission character,
but with further increasing the concentration of 1b, the emission
of 2 was quenched gradually (Fig. 4b). Analysis of the decay gave
a quenching rate constant of kq = 5.69 ¥ 109 s-1 M-1, suggesting
that the intermolecular quenching was diffusion-controlled.
Flash photolysis investigation reveals the formation of reduced
˚
by GC analysis using a molecular sieve column (5 A), thermal
conductivity detector, and nitrogen carrier gas with methane as
the internal standard. The response factor of 3.0 for H2/CH4
was determined by calibration with known amounts of H2 and
CH4 under the experimental conditions. Fig. 3 shows the time
dependence of H2 evolution in the reaction systems. When acetic
acid was used as the proton source and methanol served as
the sacrificial electron donor, irrespective of H2 dissolved in the
solvent, the amount of H2, with one hour of irradiation, reaches
0.12 mmol for 1a and 0.10 mmol for the intermolecular multi-
component system, 2 with 1b, respectively. Control experiments
suggested that the absence of any of the components yielded
an unobservable amount of H2. These results indicate that the
combination of the platinum(II) complexes and Fe2S2 active site
model to build up a molecular dyad is a potential strategy for
H2 evolution in spite of it not yet being a catalytic light-driven
reaction.
I
Fe Fe0 species with pulsed excitation of the PS–Fe2S2 dyad 1a,
as well as 2 with 1b. Upon laser pulse by 355 nm light, a
strong characteristic transient absorption of the 3MLCT state
for complex 2 appeared immediately (Fig. 5a).32,38,41 The decay
throughout the absorption region and the recovery of the bleach
occurs on the same time scale, and can be well described by a
mono-exponential function. The lifetime of 519 ns obtained from
the decays of the absorptions and from the bleach recoveries is
in quantitative agreement with that of the emission (Fig. 5c). In
contrast, when the [FeFe] H2ases mimic 1b was introduced into the
solution of complex 2, the absorptions were gradually replaced by
a series of new absorptions with characteristic absorptions at ca.
420, 570 and 700 nm. Such a spectral feature was well recorded in
3
Fig. 5, where the typical MLCT absorptions of complex 2 were
clearly observed in 0.6 ms after laser excitation (Fig. 5e); while
as the time course reached 3.4 ms, these absorptions disappeared
completely accompanied with the formation of new peaks (Fig.
5f). The generated new absorptions were very weak but the profiles
Fig. 3 Time dependence of H2 evolution in 6 mL CH3CN/CH3OH/H2O
(v/v/v = 3/2/1) solution in the presence of HOAc (10 mM) and 0.33 mM
I
are quite similar to the Fe Fe0 species formed by electrochemical
reduction of the [FeFe] H2ases models.26,44–47 Strikingly, upon laser
excitation of PS–Fe2S2 1a, the strong transient absorptions at
420 nm, 580 nm and 700 nm emerged immediately (Fig. 5b). As
compared to that of 2 with 1b, the shifted absorptions for 1a are
possibly due to the different electronic effects for covalent-linked
1a and [FeFe] H2ase mimic 1b. It is clear that the electron transfer
of the excited platinum(II) complex to the Fe2S2 active site in both
PS–Fe2S2 1a, and 2 with 1b operates resulting in the formation
of 1a ( ) and the references 1b + 2 (᭡).
᭹
To shed light on the photochemical H2 evolution, the electronic
absorption and emission spectra of 1a and its references 1b and
2 were studied. As shown in Fig. 4a, the absorption spectra
of PS–Fe2S2 1a and 2 display a moderately intense low-energy
MLCT (dp(Pt)→p*(CŸNŸN)) absorption band in the range of
390–500 nm with an extinction coefficient on the order of 103
M-1 cm-1. Complex 1b, on the other hand, shows the respective
absorption bands of the Fe2S2 active site at 300–400 nm and
450–600 nm, analogous to that observed in 1a. For complex 2,
I
I
of Fe Fe0 species. The faster generation of Fe Fe0 species for 1a
than 2 with 1b indicates that the intramolecular electron transfer
from the platinum(II) PS to the Fe2S2 catalytic center is more
efficient, consistent with that observed in the emission quenching
efficiency (Fig. 4). The kinetics probed at 420 nm suggest that the
I
decay of Fe Fe0 species for PS–Fe2S2 1a could be described by a
bi-exponential function with a lifetime of 110 ns (51%) and 11
ns (49%), respectively (Fig. 5d). The longer lifetime is tentatively
I
attributed to the Fe Fe0 species produced by the photoinduced
intramolecular electron transfer from the platinum(II) PS to the
Fe2S2 active site, while the shorter one may be a result from
some decomposed by-products of 1a during irradiation, providing
evidence for photochemically unstable [FeFe] H2ases mimics.9
The electrochemical properties of PS–Fe2S2 1a, 2 with 1b were
also investigated by cyclic voltammetry in CH3CN (Fig. 6) under
an argon atmosphere. 1a displays two irreversible oxidation waves
at 0.40 and 0.72 V, and two quasi-reversible reduction processes
at -1.61 and -1.83 V (E1/2 vs. Fc/Fc+). The former, in line with
that of 1b (0.34 and 0.71 V), is assigned to the redox process
Fig. 4 (a) UV-Vis absorption and emission spectra of 1a, 2, and emission
spectra of 2 with 1b (1.5 ¥ 10-5 M) in deoxygenated CH3CN; (b) emission
quenching of 2 (1.5 ¥ 10-5 M) in CH3CN with a varied concentration of
1b from 0 to 1.6 ¥ 10-4 M; Inset: the corresponding Stern–Volmer plot of
[I0/(I - 1)] versus concentration of 1b.
I
I
I
II
of the Fe2S2 active site in the [FeFe] H2ase mimic (Fe Fe /Fe Fe
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Fig. 5 Transient absorption spectra of 2 (a), 1a and 2 with 1b (b) upon laser pulse by 355 nm light in deoxygenated CH3CN solution (0.05 mM) under
an Ar atmosphere; the kinetic decay recorded at 490 nm for 2 (c) and at 420 nm for 1a (d); transient absorption spectra obtained at 0.6 ms (e) and 3.4 ms
(f) following laser flash photolysis of a deoxygenated CH3CN solution of 2 (8.0 ¥ 10-5 M) with 1b (8.0 ¥ 10-5 M) under an Ar atmosphere.
It is known that molecular H2 is produced in natural systems
from two protons and two electrons under mild conditions, where
electrons are cascaded to the Fe2S2 active site of [FeFe] H2ases.2
In the case of the PS–Fe2S2 dyad 1a, the anticipated first electron
transfer takes place successfully from the excited platinum(II) PS to
I
the Fe2S2 catalytic center to produce Fe Fe0 species that can further
react with a proton for H2 evolution.26,49,50 Here arises a question:
is the amount of H2 photochemically obtained in this work
determined by the first electron transfer or the second electron
transfer? To answer this question, the determination of the driving
force for the second electron transfer in 1a became imperative.
Similar to that of the first electron transfer, the free energy change
for the second photoinduced electron transfer could be estimated
by the Rehm–Weller equation, where the oxidation potential Eox
and the excited state energy E00 of the platinum(II) complex are
identical to those for the first electron-transfer process, but the
potential Ered of the Fe2S2 catalytic site is varied to either the
Fig. 6 Cyclic voltammograms of (a) 2 and (b) 1a, sample concentration
is 1.0 mmol in 0.1 M n-Bu4NPF6/CH3CN, scan rate 100 mV s-1, working
electrode: glassy carbon, reference electrode: Ag/Ag+. All potentials versus
Fc/Fc+.
I
reduction of Fe Fe0 to Fe0Fe0 or the reduction of the protonated
I
II
II
II
and Fe Fe /Fe Fe ), while the latter, consistent with that of 1b
(-1.59 V) and 2 (0.83, -1.83 V), can be ascribed to the reduction
I
Fe Fe0 species. Spectroscopic and electrochemical studies have
I
I
I
of Fe Fe to Fe Fe0, and 6-phenyl-2,2¢-bipyridine ligand reduction
process of the platinum(II) complex, respectively. The oxidation
potential at 0.83 V is the typical behavior of platinum(II) complexes
established that the order of electron and proton uptake by the
Fe2S2 models depends on the acid strength and the extent of CO/L
substitution.44–47 To better understand the electrochemical activity
I
of the Fe Fe0 species obtained toward acid, we determined the
II/III
(Pt ). Accordingly, based on the determined oxidation potential
potential (Ered) of complex 1b by cyclic voltammograms in the
presence of HOAc in CH3CN/H2O (v/v = 3 : 1). As shown in Fig.
7a, with the addition of 2 mM of HOAc acid, the current intensity
of the first reduction peak of Fe Fe /Fe Fe0 of 1b increased slightly
and shifted to a more negative position at ca. -1.78 V. The height of
the reduction peak was linearly dependent on the concentration
of acid, which is the characteristic of electrochemical catalytic
proton reduction. While in the absence of 1b, the reduction of
HOAc acid was rather slow at ca. -1.96 V (Fig. 7b). The reduction
E
ox of the platinum(II) PS, the reduction potential Ered of the Fe2S2
active site, and the excited state energy E00 of the platinum(II)
complex, which was read from the cross-point (502 nm, 2.48 eV) of
the emission and excitation spectra of complex 2, the free energy
change for the photoinduced electron transfer in the PS–Fe2S2
molecular dyad 1a was estimated to be -0.04 eV by the Rehm–
Weller equation.48 The results indicate that the electron transfer
from the excited platinum(II) complex to the Fe2S2 catalytic center
in the designed system is exothermic.
I
I
I
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first step of the electron transfer, as shown in Fig. 8. When the
platinum(II) PS in the molecular dyad 1a is excited, intramolecualr
photoinduced electron transfer takes place with the generation
I
of the reduced Fe Fe0 species, which has been well evidenced
by steady-state emission quenching and flash photolysis analysis.
I
The Fe Fe0 species is further reacted with a proton and sacrificial
electron donor to get the platinum(II) PS recovery and to produce
I
the protonated Fe Fe0 intermediate.26,49,50 As the second electron
I
transfer from the excited platinum(II) PS to the protonated Fe Fe0
species is thermodynamically impossible for PS–Fe2S2 1a, we
proposed that H2 gas for the light-driven systems comes from the
I
protonated Fe Fe0 species generated by the first electron transfer
step in 1a. The comparable low H2 production efficiency by the
PS–Fe2S2 molecular dyad 1a as well as the intermolecular system
of 2 with 1b is probably due to a number of factors: the obstacle for
I
the second electron transfer process, the short lifetime of Fe Fe0
species produced by light excitation, and the instability of the Fe2S2
active site complex towards light.
Conclusions
Fig. 7 (a) Cyclic voltammograms of 1b (1.0 mM) in the presence of HOAc
(0–50 mM), inset: ic vs. [HOAc]/[1b]; (b) CH3CN/H2O with added glacial
HOAc (10–50 mM). Electrochemical parameters: 0.1 M n-Bu4NPF6 in
CH3CN/H2O (v/v = 3 : 1), scanning rate 100 mV s-1, glassy carbon as
working electrode and Ag/Ag+ as reference electrode.
In conclusion, we have developed an artificial PS–Fe2S2 molecular
dyad 1a by covalently anchoring a platinum(II) alkynyl complex to
a Fe2S2 active site of a [FeFe] H2ases mimic, and an intermolecular
system of its model complexes 2 and 1b, for H2 evolution in homo-
geneous solution. Time dependence of H2 evolution demonstrates
that PS–Fe2S2 1a as well as complex 2 with 1b can produce H2
in the presence of a proton source and sacrificial donor under
visible light irradiation. Spectroscopic and electrochemical studies
events clearly suggest that the Fe2S2 active site could be reduced
I
I
I
from Fe Fe to Fe Fe0 species that can couple with protons and
release H2 by a second electron transfer from the electrode at
-1.78 V. Combining electrochemical and spectroscopic studies, we
estimated the free-energy change (driving force) for the second
electron-transfer in 1a to be 0.13 eV, where the catalytic reduction
potential of H2 generation was assumed as -1.78 V. Clearly, the
second electron transfer from the excited platinum(II) complex
on the electron transfer event reveal that the reduced Fe Fe0 species
I
generated from the first electron transfer of the excited platinum(II)
complex to the Fe2S2 active site in PS–Fe2S2 1a and complex 2 with
1b is essential for photochemical H2 evolution, while the second
electron-transfer step from the excited platinum(II) complex to the
I
I
to the protonated Fe Fe0 species would be thermodynamically
protonated Fe Fe0 species is thermodynamically unfeasible, which
impossible.
might be an obstacle for the relatively small amount of H2 obtained
by either PS–Fe2S2 molecular dyads or multi-component systems
of [FeFe] H2ases mimics reported so far. The present results remind
On the basis of the above results, it could be speculated that
in this case the photochemical H2 evolution is determined by the
Fig. 8 Proposed light-induced H2 production process.
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us that the feasibility of the second electron transfer event should
be an important objective to pursue the design of efficient H2
evolution systems by [FeFe] H2ases mimics.
2038; n (CN) 2125. C15H10Fe2NO5S2I: Calcd. C, 30.69; H, 1.72; N,
2.39; Found C, 31.17; H, 1.97; N, 2.47%.
Synthesis of compound 4
To a well-degassed solution of 4-iodophenyl-isocyanide model
complex 3 (0.12 g, 0.20 mmol) and 1,4-diethynylbenzene (0.04 g,
0.30 mmol) in 15 mL of THF/NEt3 (5 : 1) were added successively
Pd(PPh3)2Cl2 (15 mg) and then CuI (5 mg). The resulting solution
was stirred at 50 ◦C and the reaction was monitored by TLC.
The mixture was concentrated in vacuo and purified by column
chromatography on silica gel with CH2Cl2 to afford complex 4
as red solids (yield 90%). ESI-MS (m/z): 590. 1H NMR (400
MHz, CDCl3, d (ppm)): 7.47–7.52 (m, 6H), 7.26–7.29 (m, 2H),
3.19 (s, 1H), 2.18 (b, 2H, SCH2), 2.05 (b, 2H, SCH2), 1.83 (b,
2H, SCH2CH2CH2). IR (CH2Cl2, cm-1): n (CO) 1961, 1997, 2037;
n (CN) 2126; n (C C) 2215. C25H15Fe2NO5S2·C4H8O: Calcd. C,
52.99; H, 3.53; N, 2.13; Found C, 52.51; H, 3.57; N, 2.27%.
Experimental section
All reactions and operations were carried out under a dry
argon atmosphere with standard Schlenk techniques. All solvents
were dried and distilled prior to use according to the standard
methods.51 Fe3(CO)12, Me3NO·2H2O and dichlorobis[(triphenyl)-
phosphine]-palladium were purchased from Aldrich and used as
received. (m-S(CH2)3S)[Fe(CO)3]2,26 4-iodophenyliso-cyanide and
phenylisocyanide,26 1,4-diethynylbenzene, 1-(2-(4-ethynyl phenyl)-
ethynyl)benzene,36 4,4-tBu2-(CŸNŸN)PtCl (5), Sonogashira reac-
tion, the palladium-catalyzed cross-coupling reaction of terminal
alkynes with aryl halides, were carried out according to the
modified procedures.42,43
UV/Vis spectra were measured on a Shimadzu UV-1601PC
spectrophotometer. Steady-state emission spectra were recorded
on a Perkin Elmer LS50B spectrofluorimeter. Emission lifetimes
were measured by using Edingburge LP920 apparatus from
Analytical Instruments. Elemental analyses were performed by
using a Carlo Erba 1106 elemental analyzer. 1H NMR spectra were
recorded on a Bruker 400 FT-NMR spectrometer and chemical
shifts are given relative to tetramethylsilane. Infrared spectra were
recorded on a Nicolet NEXUS 670 FT-IR spectrophotometer. In
the transient absorption spectroscopy experiments, the samples
were purged with argon for 30 min. Excitation was provided by
using an Nd:YAG laser (third harmonic, 10 ns) at 355 nm. The
detector was a xenon lamp on the Edingburge LP920 apparatus
from Analytical Instruments. Voltammetric experiments were
manipulated on a Princeton Applied Research Potentionstat-
gravanostat model 283. Acetonitrile (Fisher Chemicals, HPLC
grade) used for the performance of the electrochemistry was
distilled under argon atmosphere once from CaH2, once from
P2O5, and CaH2 again. 0.1 M of n-Bu4NPF6 in CH3CN was
used as electrolyte in all cyclic voltammetric measurement. The
electrolyte solution was degassed by bubbling with argon for
10 min before measurement. A three electrode system is used for
the measurement and bulk electrolysis, with a 3 mm glassy carbon
working electrode, a platinum wire counter electrode, and a non-
aqueous Ag/Ag+ reference electrode. The working electrode was
polished with a 0.05 mm alumina paste and sonicated for 15 min
before use.
Synthesis of compound 1a
5 (0.16 mmol), 4 (0.50 mmol), Et3N (3 mL) and CuI (5 mg)
in degassed CH2Cl2 (30 mL) were stirred for 18 h under argon
atmosphere at room temperature in the absence of light. The
reaction mixture was then evaporated to dryness under reduced
pressure. The crude product was purified by flash chromatography
on silica gel (CH3COOC2H5/CH2Cl2) to give PS–Fe2S2 molecular
dyad 1a as brown solids (yield: 76%). 1H NMR (400 MHz, CDCl3,
d (ppm)): 9.12 (d, J = 5.7 Hz, 1H), 7.82–7.98 (m, 2H), 7.41–7.60
(m, 11H), 7.28 (m, 1H), 7.18 (m, 1H), 7.08 (m, 1H) 2.17 (b, 2H,
SCH2), 2.05 (b, 2H, SCH2), 1.83 (b, 2H, SCH2CH2CH2), 1.47
(s, 9H), 1.46 (s, 9H). MALDI-TOF: 1159 (MS+K+). IR (CH2Cl2,
cm-1): n (CO) 1964, 1997, 2036; n (CN) 2123; n (C C) 2095, 2211.
C49H41Fe2N3O5PtS2·CH3COOC2H5: Calcd. C, 52.70; H, 4.42; N,
3.23. Found: C, 53.25; H, 4.24; N, 3.47%.
Synthesis of compound 1b
Complex 1b was synthesized by the procedure similar to that for
3, except that phenylisocyanide (0.11 g, 1.03 mmol) was used in
place of 4-iodophenylisocyanide. After evaporation of the solvent,
the residue was eluted with CH2Cl2/petroleum ether (v/v = 1/10)
on a silica gel column to afford 1b as brown air-stable solids (yield
40%). 1H NMR (400 MHz, CDCl3, d (ppm)): 7.33–7.37 (m, 3H),
7.30 (d, J = 7.28 Hz, 2H), 2.16 (b, 2H, SCH2), 2.04 (b, 2H, SCH2),
1.82 (b, 2H, SCH2CH2CH2). IR (CH2Cl2, cm-1): n (CO) 1963,
1997, 2037; n (CN) 2129. C15H11Fe2NO5S2: Calcd. C, 39.07; H,
2.40; N, 3.04. Found: C, 38.97; H, 2.51; N, 3.01%.
Synthesis of compound 3
Compund 3 was prepared according to the literature.26 A mixture
of (m-S(CH2)3S)[Fe(CO)3]2 (0.40 g, 1.03 mmol), 4-iodophenyl-
isocyanide (0.24 g, 1.03 mmol), Me3NO·2H2O (0.12 g, 1.03 mmol)
in CH3CN (20 mL) was stirred at room temperature for 2
h. The color of the initially red solution became brownish.
After evaporation of the solvent, the residue was eluted with
CH2Cl2/petroleum ether (v/v: 1/10) on a silica gel column
to afford 3 as brown, air-stable solids (yield 35%). 1H NMR
(400 MHz, CDCl3, d (ppm)): 7.79 (d, J = 8.68 Hz, 2H), 7.14
(d, J = 8.52 Hz, 2H), 2.20 (b, 2H, SCH2), 2.06 (b, 2H, SCH2), 1.90
(b, 2H, SCH2CH2CH2). IR (CH2Cl2, cm-1): n (CO) 1961, 1998,
Synthesis of compound 2
Complex 2 was synthesized by the procedure similar to that for
1a, except that 1-ethynyl-4-(2-phenylethynyl)benzene (0.50 mmol)
was used in place of 4. After evaporation of the solvent, the residue
was eluted with CH3COOC2H5/CH2Cl2 (v/v = 1/3) on a silica gel
1
column to afford 2 as orange solids (yield 87%). H NMR (400
MHz, CD3COCD3, d (ppm)): 9.09 (m, 1H), 8.50 (d, J = 1.7 Hz,
1H), 8.25 (d, J = 1.3 Hz, 1H), 7.90–7.93 (m, 2H), 7.85 (dd, J = 5.7
Hz, J = 1.9 Hz 1H), 7.64 (d, J = 6.4 Hz, 1H), 7.53–7.55 (m, 2H),
7.37–7.43 (m, 7H), 7.10 (m, 1H), 7.05 (m, 1H), 1.48 (s, 9H), 1.46
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Dalton Trans., 2012, 41, 2420–2426 | 2425
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(s, 9H). ESI-Ms: 740. IR (CH2Cl2, cm-1): n (C C) 2091, 2209.
C40H36N2Pt: Calcd. C, 64.94; H, 4.90; N, 3.79. Found: C, 64.42;
H, 4.86; N, 3.72%.
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Photocatalysis
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20 S. Ott, M. Borgstro¨m, M. Kritikos, R. Lomoth, J. Bergquist, B.
In a typical experiment, 6 mL CH3CN/CH3OH/H2O (v/v/v =
3/2/1) solution with the complex (3.3 ¥ 10-4 M, 2 mmol) and
HOAc (0.01 M) were added to a Schlenk tube. The frozen solution
was evacuated and nitrogen gas injected. After the suspension was
equilibrated to room temperature, 60 mL CH4 was injected as
internal standard for quantitative GC analysis. The sample was
then irradiated under a 500 W high-pressure Hanovia mercury
lamp and a glass filter was employed to cut off light with a
wavelength below 400 nm. The photoproduct H2 generated from
the systems was measured using a GC-14B (Shimadzu) using
˚
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˚
nitrogen as the carrier gas with a molecular sieve 5 A column (30
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m ¥ 0.53 mm) and thermal conductivity detector. The response
factor for H2/CH4 was 3.0 under experimental conditions, which
were established by calibration with known amounts of H2 and
CH4, and determined before and after a series of measurements.
Acknowledgements
We are grateful for financial support from the Ministry of Science
and Technology of China (2009CB22008, and 2007CB808004), the
National Science Foundation of China (50973125, 21090343 and
20732007), Solar Energy Initiative of the Knowledge Innovation
Program of the Chinese Academy of Sciences (KGCXZ-YW-389),
and the Bureau for Basic Research of the Chinese Academy of
Sciences.
31 Q.-Z. Yang, L.-Z. Wu, Z.-X. Wu, L.-P. Zhang and C.-H. Tung, Inorg.
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2426 | Dalton Trans., 2012, 41, 2420–2426
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