A silicon-heteroaromatic system as photosensitizer for light-driven hydrogen production by hydrogenase mimics

Article abstract of DOI:10.1002/ejic.201300537

The utilization of light and inexpensive catalysts to afford hydrogen represents a huge challenge. Following our interest in silicon-containing [FeFe]-hydrogenase ([FeFe]-H₂ase) mimics, we report a new model approach for a photocatalytic [FeFe]-H₂ase mimic 1, which contains a 1-silafluorene unit as a photosensitizer. Thereby, the photoactive ligand is linked to the [2Fe2S] cluster through S-CH₂-Si bridges. Photochemical H₂ evolution experiments were performed and revealed a turnover number (TON) of 29. This is the highest reported photocatalytic efficiency for an [FeFe]-H₂ase model complex in which the photosensitizer is covalently linked to the catalytic center. We report a viable synthetic pathway for the construction of a new photoactive model of the [FeFe]-hydrogenase ([FeFe]-H₂ase) active site with a dithiolate bridge and 1-silafluorene as a photosensitizer. The [FeFe]-H₂ase mimic represents a very compact, easily accessible, and inexpensive photocatalyst for hydrogen generation. Copyright

Full text of DOI:10.1002/ejic.201300537

FULL PAPER
DOI:10.1002/ejic.201300537
A Silicon-Heteroaromatic System as Photosensitizer for
Light-Driven Hydrogen Production by Hydrogenase
Mimics
Roman Goy,[a] Ulf-Peter Apfel,[b] Catherine Elleouet,[c]
Daniel Escudero,[d] Martin Elstner,[a] Helmar Görls,[a]
Jean Talarmin,*[c] Philippe Schollhammer,[c] Leticia González,*[e]
and Wolfgang Weigand*[a]
Dedicated to Professor Wolfgang Beck
Keywords: Photocatalysis / Enzyme models / Iron / Sulfur / Silicon / Hydrogenase
The utilization of light and inexpensive catalysts to afford hy-
drogen represents a huge challenge. Following our interest
in silicon-containing [FeFe]-hydrogenase ([FeFe]-H2ase)
mimics, we report a new model approach for a photocatalytic
[FeFe]-H2ase mimic 1, which contains a 1-silafluorene unit as
a photosensitizer. Thereby, the photoactive ligand is linked to
the [2Fe2S] cluster through S–CH2–Si bridges. Photochemi-
cal H2 evolution experiments were performed and revealed
a turnover number (TON) of 29. This is the highest reported
photocatalytic efficiency for an [FeFe]-H2ase model complex
in which the photosensitizer is covalently linked to the cata-
lytic center.
genase ([FeFe]-H2ase). Multicomponent systems containing
a surplus of ruthenium photosensitizers or organic fluoro-
phores and [2Fe2S] clusters were investigated and showed
moderate H2 development upon irradiation with light.[7–11]
Multiple covalently linked dyads were synthesized with por-
phyrin or ruthenium units as the photosensitizer.[12–18]
These complexes revealed low turnover numbers (TON Ͻ
0.15) for H2 generation.[12–18] To the best of our knowledge,
there is only one system containing a rhenium photosensi-
tizer covalently connected to the [2Fe2S] cluster by an aza-
dithiolato linker and it showed significant H2 development
with a turnover number of 11.8.[19] Furthermore, supra-
molecular assemblies comprising [2Fe2S] model com-
pounds with an InP nanophotocathode,[20] ZnS nanopar-
ticles,[21] multichromophoric hexad self-assemblies,[22] or
Mn2Ru complexes[23] as light-harvesting molecules were re-
ported. Also, micellar systems[24] and dendrimer-based
mimics[25] were utilized to allow for photocatalytic H2 de-
velopment in aqueous media.
Introduction
The conversion of light into a storable energy source is
a highly desired endeavor. In particular, the photocatalytic
reduction of water into hydrogen affords an ideal fuel,
which is easy to store in large quantities.[1–3] In addition,
the combustion of hydrogen to water has a high specific
energy value (142 MJkg–1)[4] and affords no polluting emis-
sions.[5]
Hydrogen is part of the biological cycle and appears as
a biological energy source and transporter.[6] Numerous
structurally modified and photocatalytically functionalized
models have been inspired by the structure of [FeFe]-hydro-
[a] Institut für Anorganische und Analytische Chemie, Friedrich-
Schiller-Universität,
Humboldtstraße 8, 07743 Jena, Germany
Fax: +49-3641-948102
E-mail: wolfgang.weigand@uni-jena.de
Homepage: http://www.uni-jena.de/Prof_W_Weigand.html
[b] Institut für Anorganische Chemie, Ruhr-Universität Bochum,
Universitätsstraße 150, 44780 Bochum, Germany
[c] Chimie, Electrochimie Moléculaires et Chimie Analytique,
Université de Bretagne Occidentale,
However, water splitting by utilizing light and inexpen-
sive catalysts to afford hydrogen still represents a huge chal-
6 Avenue Victor le Gorgeu, CS93837, 29238 Brest cedex 3, lenge, as most complexes show a lack of reactivity and sta-
bility.[26–31]
France
[d] Max-Planck-Institut für Kohlenforschung,
In continuation of our research on silicon-containing
[FeFe]-H2ase mimics,[32–34] we aimed to synthesize a small,
compact, heavy-metal-free, and easily accessible photocata-
lytic [FeFe] model complex (Scheme 1) by utilizing a silicon-
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
[e] Institut für Theoretische Chemie, Universität Wien,
Währinger Str. 17, 1090 Wien, Austria
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300537.
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containing heteroaromatic system. Silicon-containing aro-
matics are well known for their good optical properties such
as light-emission and absorption in the longer wavelength
area or their electroluminescence properties[35–41] as well as
for their interesting physical properties.[42–46]
Scheme 2. Reaction pathway towards 1.
photoelectron transfer between the photosensitizer and the
diiron center. A comparable orbital interaction between the
σ(Si–C) orbital and a 3p(S) orbital in 1 is, therefore, very
likely. A similar interaction is not reported for [Fe2(CO)6-
(pdt)] (pdt = propanedithiolate), which shows an angle be-
tween the C–C–C and S–CC–S planes of 137.09°.[49] Cyclic
voltammetry (CV) experiments at slow-to-moderate scan
rates (0.05 Յ v Յ 1 Vs–1) show that the electrochemical re-
Scheme 1. Schematic representation of the light-driven production
of hydrogen by [FeFe]-H2ase mimic 1.
Herein, we present a new model approach for a photo-
catalytic [FeFe]-H2ase mimic 1, which contains 1-silafluor-
ene as a photosensitizer. Thereby, the photoactive ligand is
linked to the [2Fe2S] cluster through S–CH2–Si bridges, and
the photoactive 1-silafluorene is directly connected with the
redox-active iron center (Scheme 1).
Results and Discussion
The 1-silafluorene complex 1 was prepared according to
Scheme 2. The reaction of 2,2Ј-dibromobiphenyl (2), n-but-
yllithium, and bis(chloromethyl)dichlorosilane afforded
1,1Ј-bis(chloromethyl)-1-silafluorene (3) as a colorless oil in
70% yield. Subsequent reaction with [(μ-S)2Fe2(CO)6] ac-
cording to known procedures gave [FeFe]-H2ase mimic 1 as
a red-brown solid in 32% yield and it was characterized by
1H and 13C{1H} NMR spectroscopy, IR spectroscopy, and
mass spectrometry.[47]
The molecular structure of 1 (Figure 1, crystal data in
Table 1) shows the characteristic [2Fe2S] butterfly core. The
Si atom is surrounded in a distorted tetrahedral fashion.
Notably, all of the C–Si–C angles [92.2(2)–114.2(3)°] differ
significantly from those of an ideal tetrahedron. Addition-
ally, both Si–C–S angles [123.3(3) and 122.9(3)°] are best
explained by sp2 rather than sp3 hybridization. Similar ob-
servations were recently reported for other [2Fe2S(Si)] com-
plexes.[32] Additionally, Glass and co-workers reported a re-
lated [2Fe2S(Sn)] complex.[48] Consistent with our observa-
tions, enhanced S–C–Sn angles were observed and an inter-
action between a σ(Sn–C) orbital and a 3p(S) orbital was
verified by photoelectron spectroscopy.[48] We assume that
the large angle (170.54°) between the planes generated by
C1–Si1–C2 and S1–C1C2–S2 is an indicator for an effective Figure 1. ORTEP view of 1 (ellipsoids at the 50% probability level).
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duction of [Fe2(CO)6{μ-SCH2Si(R)CH2S}] (1) at E1/2
=
reaction of an ECrevE process (see Scheme 3). This result is
in accordance with the observation by Evans and co-
workers,[60] and hence an EE mechanism can be discarded.
–1.55 V is quasireversible in CH2Cl2/[NBu4][PF6].[50]
Table 1. Selected bond lengths [Å] and angles [°] for 1.
Fe(1A)–Fe(2A)
Fe(1A)–S(1A)
Fe(1A)–S(2A)
Fe(2A)–S(1A)
Fe(2A)–S(2A)
S(1A)–C(1A)
S(2A)–C(2A)
C(1A)–Si(1A)
C(2A)–Si(1A)
Si(1A)–C(3A)
Si(1A)–C(14A)
C(3A)–C(8A)
C(9A)–C(14A)
C(8A)–C(9A)
2.5268(10)
2.2490(15)
2.2563(14)
2.2504(15)
2.2428(15)
1.817(6)
1.821(6)
1.880(6)
1.865(6)
1.867(6)
1.864(6)
1.418(8)
1.418(8)
1.481(8)
Scheme 3. Proposed ECrevE mechanism.
The UV/Vis absorption spectra of 1 and 3 are shown in
Figure 2 for comparison. Time-dependent density func-
tional theory (TD-DFT) calculations have been performed
(see computational details in the Supporting Information)
to get an insight into the UV/Vis characteristics of 1 and 3.
The theoretical UV/Vis spectra of 1 and 3 are also shown
in Figure 2, and the main electronic excited states are high-
lighted (for a complete description of the main electronic
TD-DFT excitations see Table S1). Both compounds have
intense absorption bands at 210–242 and 277–290 nm,
which are theoretically attributed to π–π* excitations within
the 1-silafluorene moiety (for example, see S2 and S4 for 3
and S33 and S56 for 1 in Figure 2 and Table S1). The UV/
Vis spectra of 1 and 3 differ in the low-energy regime. Only
1 has a broad band peaking at ca. 330 nm. The states re-
sponsible for this band have σ–σ* and d–σ* character and
involve the Fe–Fe unit (see S4 and S9 in Figure 2 and the
orbitals involved in these excitations in Figures S5 and S6).
As can be seen in Figure 2, this band is slightly energetically
underestimated by the TD-DFT calculations. Obviously,
Fe(1A)–Fe(2A)–S(1A)
Fe(1A)–Fe(2A)–S(2A)
Fe(1A)–S(1A)–Fe(2A)
Fe(1A)–S(2A)–Fe(2A)
S(1A)-(C1A)–Si(1A)
S(2A)–C(2A)–Si(1A)
C(1A)–Si(1A)–C(2A)
C(3A)–Si(1A)–C(14A)
C(1A)-Si(1A)–C(3A)
C(2A)–Si(1A)–C(14A)
C(3A)–Si(1A)–C(2A)
C(14A)–Si(1A)–C(1A)
55.86(4)
55.58(4)
68.33(4)
68.34(5)
123.3(3)
122.9(3)
112.8(3)
92.2(2)
112.9(3)
112.7(3)
114.2(3)
110.5(3)
A comparison of the potentials of the reduction of 1 with the latter metal-based intense band determines the photo-
those of bis(mercaptomethyl)silane [(SCH2)2Si(Me2); E1/2 physical and photochemical properties of 1. Thus, upon ex-
=
–1.52 V],[32] pdt [S(CH2)3S; E1/2 = –1.74 V],[51] and benzene- citation of the brighter π–π* band, new deactivation path-
dithiolate (bdt: SC6H4S; E1/2 = –1.44 V)[52] analogues, ways involving the σ–σ*/d–σ* states (either of singlet and
which were measured under similar experimental condi- triplet character) arise. Hereby, the energy absorbed by the
tions, indicates that the electronic effect exerted by the Si- 1-silafluorene chromophore can be transferred in the course
containing bridge of 1 is intermediate between that of the of photo-deactivation to the [(μ-S)2Fe2(CO)6] catalytic unit.
pdt and the bdt ligands. A comparison of the reduction The population of low-lying σ–σ*/d–σ* triplet excited
peak current (ipred) of 1 with the oxidation peak current states (owing to strong spin–orbit couplings for Fe) guaran-
(ipox) of an equimolar bis-N-heterocyclic carbene (bis- tees that the lifetimes of the excited states are increased,
NHC) complex [Fe2(CO)4(κ2-IMe–CH2–IMe)(μ-pdt)] (IMe
=
and, hence, the quenching of photoluminescence and/or the
1-methylimidazol-2-ylidene), which was previously shown photochemical hydrogen evolution is favored. Indeed, the
to undergo a one-electron oxidation, demonstrates that the lowest triplet excited state is adiabatically only 0.64 eV
former involves the transfer of two electrons (Figure above the singlet ground state. Furthermore, the lowest trip-
S1).[53–55] The single-step two-electron transfer arises from let excited-state geometry shows longer Fe–Fe distances (see
an inversion of the potentials of the individual one-electron Figure S4). Such active species are then responsible for the
reduction processes, E°2 – E°1 Ͼ 0, as already observed for catalytic activity of 1, as the longer Fe–Fe distance allows
a variety of diiron hexacarbonyl complexes bearing dif- for the coordination of hydrogen at the Fe–Fe core, and
ferent dithiolate bridges.[32,52,56–60] Typically, a potential in- ultimately the H2 evolution in the photocatalytic center by
version is observed when a chemical reaction (most often the coordination of an additional hydrogen atom is favored.
linked to a structural change) makes the second electron The emission spectra of 1 and 3 upon excitation of the π–
transfer thermodynamically more favorable than the π* band with an excitation wavelength of 255 nm are shown
first.[61–65] The structural change can either be concerned in Figure 3.
with one of the electron transfers[32,52,56–59] or appears as
To test whether a photoinduced electron transfer (PET)
the intervening step of an ECE process.[50,60] In the present occurs in this system, the spectral change in the presence of
case, CV at faster scan rates (1 Յ v Յ 20 Vs–1) leads to a triethylamine was studied. As shown in Figure 3, the emis-
significant decrease of the current function (ipred/v1/2) for sion intensity of 3 decreases upon addition of triethylamine,
the reduction of 1 (Figures S1 and S2). This strongly sug- and the maximum of the emission shifts from 387 to
gests that the abovementioned rearrangement, probably in- 395 nm (3 + 150 equiv. NEt3). The decrease of the emission
volving the cleavage of the Fe–S bond, is the intervening intensity under these conditions is reasonable as triethyl-
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highest reported photocatalytic efficiency for an [FeFe]-
H2ase model complex in which the photosensitizer is cova-
lently linked to the catalytic center. Headspace analysis of
a mixture of 1, Et3N, and TFA stored in the dark revealed
only traces of H2 and further supports the necessity to
photoexcite the complex to achieve catalytic activity. To fur-
ther substantiate the importance of 1 for the photocatalytic
hydrogen generation, an acetonitrile solution containing
TFA (1 mmol) and Et3N (1 mmol) was irradiated (254 nm)
for 15 hours in the absence and presence of 3. In both ex-
periments, no significant generation of H2 was observed. As
reported for Fe3(CO)12 and Fe2(CO)9, irradiation with UV
light in the presence of a photosensitizer can lead to signifi-
cant CO dissociation and finally to the decomposition of
the catalyst.[70,71]
Figure 2. Experimental UV/Vis spectra for 1 and 3 (0.027 mm) in
hexane superimposed on the TD-DFT vertical excitations. The
main electronic states are highlighted (see Table S1 for assign-
ments).
Figure 4. Light-driven hydrogen production by
1 (0.6 μmol,
0.15 mm) in the presence of trifluoroacetic acid (1 mmol, 0.25 m)
and triethylamine (1 mmol, 0.25 m) in degassed acetonitrile at
25 °C. H2 was detected by gas chromatography. Black line: with 1,
grey line: no catalyst.
Figure 3. Photoluminescence spectra of
1 (0.27 mm) and 3
To test the stability of our system, a solution of 1 was
irradiated under the conditions described above and the
UV/Vis spectra were recorded (Figures S8 and S9). In the
(0.092 mm) in acetonitrile in the presence of triethylamine (exci-
tation wavelength 255 nm, KSV = 80.0Ϯ2.2 Lmol–1, see Figure S7).
amine acts as sacrificial electron donor to fill the hole gen- absence of TFA and Et3N, a new band at 295 nm with a
erated in the π orbital upon photoexcitation. The progress- stronger absorbance was observed and overlapped with the
ive addition of NEt3 to the solution of 3 quenched the lumi- band at 330 nm. Further photoexcitation of the solution for
nescence with a rate constant KSV of 80.0Ϯ2.2 Lmol–1 a prolonged period of time resulted in stronger absorbance
(Figure S7). Excitation at the characteristic absorption of intensities for all bands and a redshift of 30 nm for the band
both compounds at 255 nm results in a maximal lumines- initially observed at ca. 230 nm. After 3 h, a third band was
cence at 387 nm with a quantum yield of Յ 0.0003 for 1 growing in at 340 nm. Additionally, irradiation of the mix-
and 0.183Ϯ0.003 for 3 based on a 0.01 mm pyrene solution ture for 15 h resulted in the loss of the characteristic CO
(in cyclohexane) as the reference.[66]
resonances in the 13C NMR spectrum of 1, which indicates
Photochemical H2 evolution experiments were per- dissociation of the CO ligand. This process was further con-
formed by irradiating 1 (0.6 μmol) in the presence of tri- firmed by IR spectroscopy, which showed no remaining CO
fluoroacetic acid (TFA, 1 mmol) and triethylamine bands. These observations indicate that in the absence of a
(1 mmol) in acetonitrile at 254 nm with a 15 W mercury- sacrificial electron donor and proton source, irradiation of
vapor lamp. Although our experimental set up did not al- the complex results in CO dissociation, which finally deacti-
low us to excite at the absorbance maximum (240 nm), we vates the complex for the photocatalytic H2 generation.
were able to obtain 17.4 μmol H2 in the headspace of our Even though catalytic activity was still observed after
reactor after 13 hours irradiation with our system. No fur- 13 hours, experiments in the absence of Et3N and TFA sug-
ther H2 generation was observed after 13 hours. This gested decomposition of 1 after 7.5 hours. Thus, we assume
amount of H2 reflects a turnover number (TON) of 29 and that different PET quenching mechanisms under the cataly-
a turnover frequency (TOF) of 2.2 h–1 (Figure 4) and is the sis conditions are likely.
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solvents were dried and distilled according to standard methods
prior to use. Et3BHLi (1.0 m in THF), 1,2-dibromobenzene, and
bis(chloromethyl)dichlorosilane are commercially available and
were used without further treatment.
To test this hypothesis, we repeated the irradiation ex-
periment in the presence of TFA and Et3N. The UV/Vis
spectrum showed different spectroscopic features than the
UV/Vis spectrum of the mixture without sacrificial electron
donors and acid. This observation suggests a different reac-
tion pattern in the presence of TFA and Et3N, which is
most likely because of a PET quenching processes (Figure
S9). A significant redshift was observed for the band at
250–270 nm. Furthermore, in contrast to the experiments
without TFA and Et3N, the intensity of this band increases
considerably faster and visibly results in higher extinction
coefficients. After 240 min, a new band at 283 nm with a
strong absorbance was observed. An additional band at
295 nm was observed and shifts to 315 nm upon excitation.
Contrary to our experiments in the absence of TFA and
NEt3, no band at 340 nm was observed, which further con-
firms a different reaction pathway.
Infrared spectra were measured with a Bruker IFS 66 spectrometer
(resolution Ϯ 4 cm–1) with the samples dispersed in compressed
KBr pellets. Preparative column chromatography was performed
with silica gel (Fluka, Kieselgel 60). UV/Vis spectra were recorded
with a Specord S600 spectrometer, and fluorescence spectra were
recorded with a Perkin–Elmer LS50B spectrometer. 1H, 13C, and
29Si NMR spectra were obtained with either a BRUKER Avance
200 or Avance 400 spectrometer. Elemental analyses were per-
formed with a Vario EL III CHNS analyzer from Elementar Ana-
lysensysteme GmbH. Mass spectra were measured with
FINNIGAN MAT SSQ710 instrument.
a
Structure Determination: Single crystals suitable for X-ray analysis
were mounted on a fiber loop and placed in a cold, gaseous nitro-
gen stream on a Nonius KappaCCD diffractometer performing φ
and ω scans at 120(2) K. Diffraction intensities were measured by
using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å).
Data were corrected for Lorentz and polarization effects, but not
for absorption.[72,73] The structure was solved by direct methods
(SHELXS) and refined by full-matrix least-squares techniques
Conclusions
With the synthesis of 1, we provide a viable synthetic
pathway towards the first photocatalytic model complex of
the [FeFe]-H2ase active site with the photosensitizer directly
imbedded into the bridging dithiolate unit. Thus, the pho-
tosensitizer is in close proximity to the catalytic [2Fe2S]
cluster and allows for an effective electron transfer. In com-
parison to the influence of phosphanes,[67] cyanides[68] or
NHCs,[69] the implementation of the photosensitizer into
the bridge revealed only moderate influence on the [2Fe2S]
cluster; thus, the fluorophore can be changed without alter-
ation of the mechanism of H2 formation. The Si–C–S
against F0 (SHELXL-97).[74] All hydrogen atoms were included at
2
calculated positions with fixed thermal parameters. All non-hydro-
gen atoms were refined anisotropically.[74]
CCDC-905950 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Crystallographic Data of 1: C20H12Fe2O6S2Si, Mr = 552.21 gmol–1,
red-brown prism, size 0.06ϫ0.06ϫ0.05 mm, monoclinic, space
group P21/c, a = 13.8679(3), b = 26.8618(7), c = 11.6998(3) Å, β
angles from the X-ray-structure are in accordance with sp2 = 92.863(1)°, V = 4352.93(18) Å3, T = –140 °C, Z = 8, ρcalcd. =
1.685 gcm–3, λ(Mo-Kα) = 0.71073 Å, μ(Mo-Kα) = 16.15 cm–1,
hybridization of the carbon atom; therefore, a “filled–filled”
interaction between the σ(Si–C) orbital and the 3p(S) or-
F(000) = 2224, 26452 reflections in h(–18/17), k(–34/33), l(–15/15),
measured in the range 2.11° Յ Θ Յ 27.50°, completeness Θmax
=
bital is favored and, hence, there is direct communication
between the photosensitizer and the [2Fe2S] cluster. This
behavior was also investigated by DFT calculations and
confirmed by the photocatalytic H2 evolution with the
highest reported TON for such a small [FeFe]-H2ase model
complex. Even though photocatalytic systems with higher
turnover numbers exist, the elimination of Ir, Pt, Rh, or Re
complexes as photosensitizers makes this design a powerful
platform for the further development of proton reduction
catalysts. However, a precise statement about the nature of
the different intermediates during the photocatalytic hydro-
gen generation cannot be given, and further investigations
to discover the mechanism for the H2 development with 1,
possible degradation pathways, and visible-light-driven ca-
talysis with the presented core structure are currently in
progress. This will allow the properties of this platform to
be tuned to achieve, for example, excitation with visible
light and higher turnover numbers.
98.9%, 9877 independent reflections, Rint = 0.0915, 8063 reflections
with Fo Ͼ 4σ(Fo), 559 parameters, 0 restraints, R1obs = 0.0756,
wR2 = 0.1849, R1all = 0.0939, wR2 = 0.1977, Goodness-of-fit
obs
all
on F2 = 1.149, largest difference peak and hole: 1.265/–1.016 eÅ–3.
Electrochemical Procedures: The electrochemical experiments were
conducted under an inert atmosphere of nitrogen or argon. The
preparation and purification of the supporting electrolyte
([NBu4][PF6]) was performed as described previously.[75] Trifluoro-
methanesulfonic acid (Aldrich) was used as received. Cyclic vol-
tammetry was performed in a three-electrode cell by using a radi-
ometer potentiostat (PGSTAT 128N or μ-Autolab III) driven by
the GPES software. The working electrode consisted of a vitreous
carbon disk, which was polished on a felt tissue with alumina,
thoroughly rinsed with water, and dried before each CV scan. The
Ag/Ag+ reference electrode was separated from the analyte by a
CH2Cl2–[NBu4][PF6] bridge. All the potentials are reported against
the ferrocene–ferrocenium couple; ferrocene was added as an in-
ternal standard at the end of the experiments.
Procedure for Photocatalytic H2 Evolution: Photochemical hydro-
gen evolution experiments were performed by irradiating an aceto-
nitrile solution of 1 in the presence of trifluoroacetic acid (TFA)
and triethylamine at 254 nm with a 15 W mercury-vapor lamp in a
quartz glass precision cell at room temperature. Prior to irradiation,
the solution was sealed with a septum cap, degassed, and flushed
Experimental Section
General Procedures: All reactions were performed under a dry ni-
trogen or argon atmosphere with standard Schlenk techniques. All
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with dry nitrogen. Hydrogen was detected by gas chromatography
by using a calibrated Varian CP-3800 Gas Chromatograph with a
thermal conductivity detector and argon as the carrier gas.
UV/Vis (hexane): λmax (logε) = 213.4 (4.77), 235.3 (4.69), 242.8
(4.64), 273.9 (4.27), 288.2 (4.19), 328.4 (4.05).
Supporting Information (see footnote on the first page of this arti-
cle): Electrochemical investigations, computational details, TD-
DFT results, emission quenching of 3, and irradiation of 1.
Dibromobiphenyl (2):[76] A solution of 1,2-dibromobenzene (21.5 g,
91.1 mmol) dissolved in THF (120 mL) was cooled to –78 °C, and
n-butyllithium (31.4 mL, 50.24 mmol, 1.6 m in hexane) was added
dropwise over a period of 30 min. Within 24 h the reaction mixture
was warmed to room temperature and then hydrolyzed at 0 °C by
using hydrogen chloride (100 mL, 0.5 m in water). The reaction
mixture was extracted with diethyl ether (4ϫ 50 mL), and the com-
bined organic fractions were dried with sodium sulfate. Evapora-
tion to dryness and crystallization from ethanol afforded colorless
Acknowledgments
Financial support for this work was provided by the Deutsche Bun-
desstiftung Umwelt (to R. G.) and the Studienstiftung des Deut-
schen Volkes as well as the Alexander-von-Humboldt foundation
(to U.-P. A.). The authors thank B. Dietzek, L. Zedler, and J. Popp
for discussions and K. Dubnack for experimental support.
1
crystals (11.1 g, 78%). H NMR (200 MHz, CD2Cl2): δ = 7.70 (m,
2 H, CHaromatic), 7.45–7.24 (m, 6 H, CHaromatic) ppm. 13C NMR
(50 MHz, CDCl3): δ = 142.09 (Cq), 132.59, 130.96, 129.38, 127.1
(CHaromatic), 123.52 (CBr) ppm. MS (DEI): m/z = 312 [M]+, 232
[M – Br]+, 152 [M – 2Br]+, 76 [M – C6H4Br2]+.
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Bis(chloromethyl)-1-silafluorene (3): A solution of 2,2Ј-dibromobi-
phenyl (2.0 g, 6.4 mmol) in Et2O (25 mL) was cooled to –78 °C,
and nBuLi (1.6 m in hexane, 8.2 mL, 13 mmol) was added. The
reaction solution was warmed to room temperature by removing
the cooling bath and subsequent stirring overnight. The resulting
solution was cooled to –78 °C, and a solution of Cl2Si(CH2Cl)2
(1.5 g, 7.6 mmol) in Et2O (5 mL) was added dropwise. The reaction
mixture was stirred at –78 °C for 4 h and at room temperature for
an additional 12 h. The mixture was filtered, and the solvents were
evaporated by vacuum transfer under an argon atmosphere. The
residue was purified by bulb-to-bulb distillation (130 °C/0.11 mbar)
to afford 3 as colorless oil (1.24 g, 70%). 1H NMR (200 MHz,
CDCl3): δ = 7.85 (t, J = 7.6 Hz, 2 H, CHaromatic), 7.55 (t, J =
7.6 Hz, 2 H, CHaromatic), 7.34 (m, 4 H, CHaromatic), 3.29 (s, 4 H,
CH2) ppm. 13C NMR (50 MHz, CDCl3): δ = 148.52 (Cq), 134.34
(CH), 131.78 (CH), 131.72 (CSi), 128.06 (CH), 121.15 (CH), 25.29
(CH2) ppm. 29Si NMR (79.5 MHz, CDCl3): δ = –6.07 ppm. MS
(DEI): m/z = 278 [M]+, 229 [M – CH2Cl]+, 193 [M – CH2Cl2]+,
179 [M – (CH2Cl)2]+, 152 [M – Si(CH2Cl)2]+. C14H12Cl2Si (279.24):
calcd. C 60.22, H 4.33, Cl 25.39; found C 60.27, H 4.41, Cl 25.10.
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IR ν = 3062 (m), 2995 (m), 2933 (m), 2872 (m), 1963 (w), 1928 (w),
˜
1894 (w), 1854 (w), 1820 (w), 1593 (vs), 1483 (m), 1459 (s), 1431
(s), 1384 (s), 1260 (s), 1128 (vs), 1095 (s), 786 (vs), 749 (s) cm–1.
UV/Vis (hexane): λmax (logε) = 211.7 (4.72), 233.6 (4.62), 241.2
(4.54), 277.3 (4.21), 289 (4.16), 321.7 (3.56) nm. Emission (hexane):
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[(C14H12SiS2)Fe2(CO)6] (1): A solution of [(μ-S)2Fe2(CO)6] (62 mg,
0.18 mmol) in THF (10 mL) was cooled to –78 °C, and Et3BHLi
(0.36 mL, 0.36 mmol) was added dropwise. The solution was stirred
for 15 min, and 1,1Ј-bis(chloromethyl)-1-silafluorene (50 mg,
0.18 mmol) was added. The mixture was warmed to room tempera-
ture and stirred at this temperature for 16 h. The volatiles were
removed under vacuum, and the residue was purified by column
chromatography (silica gel) with hexane as eluent. From the major
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1
red band, 1 was obtained as a red-brown solid (0.031 g, 32%). H
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NMR (200 MHz, CDCl3): δ = 7.81–7.11 (m, 8 H, CHaromatic), 1.89
(s, 4 H, CH2) ppm. 13C NMR (50 MHz, CDCl3): δ = 205.47 (CO),
145.89 (Cq), 133.21 (CSi), 131.39 (CH), 129.85 (CH), 126.41 (CH),
119.16 (CH), 0.23 (SiCH2S) ppm. MS (DEI): m/z = 552 [M]+, 496
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Fe(CO)3]+, 384 [M
–
Fe(CO)4]+, 356 [M
–
Fe(CO)5]+.
C20H12Fe2O6S2Si (552.21): calcd. C 43.50, H 2.19, S 11.61; found
C 43.62, H 2.21, S 11.47. IR ν = 3069 (w), 2925 (m), 2854 (w),
˜
2073 (vs), 2032 (vs), 2001 (vs), 1984 (vs), 1719 (w), 1628 (w), 1594
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(w), 1459 (w), 1432 (w), 1260 (w), 1132 (w), 782 (m), 750 (m) cm–1.
Eur. J. Inorg. Chem. 2013, 4466–4472
4471
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www.eurjic.org
FULL PAPER
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Published Online: July 4, 2013
Eur. J. Inorg. Chem. 2013, 4466–4472
4472
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim