New approach to [FeFe]-hydrogenase models using aromatic thioketones

Article abstract of DOI:10.1002/ejic.201101032

The reactions of triiron dodecacarbonyl with thiobenzophenone (2a) and 9H-thioxanthene-9-thione (2b) were investigated under different conditions. In the case of a 1:1 molar ratio of triiron dodecacarbonyl and 2a or 2b, the ortho-metallated complexes [Fe₂(CO)₆{μ,κ,S, SCH(C₆H₅)C₆H₄-η²}] (3a) and [Fe₂(CO)₆{μ,κ,S,SCH(C₆H ₄)-S-C₆H₃-η²}] (4a) were obtained as the major products, respectively. In contrast, the treatment of triiron dodecacarbonyl with an excess of 2a or 2b afforded [Fe ₂(CO)₆{μ-SCH(C₆H₅)C ₆H₄S-μ}] (3b) and [Fe₂(CO) ₆{μ-SCH(C₆H₄)-S-C₆H ₃S-μ}] (4b), respectively, which are both bioinspired models for the active site of [FeFe]-hydrogenase. In addition to these complexes, the two reactions afforded [Fe₂(CO)₆{μ-SC(C₆H ₅)₂S-μ}] (3c) and [Fe₂(CO) ₆{μ-SC(C₆H₄-S-C₆H ₄)S-μ}] (4c). Furthermore, [{Fe₂(CO) ₆{μ-SCH(C₆H₅)₂}} ₂(μ⁴-S)] (3d) was isolated from the reaction of Fe ₃(CO)₁₂ with 2a. The molecular structures of all of the new complexes were determined from the spectroscopic and analytical data and the crystal structures for 3c, 3d, 4b, and 4c were obtained. A plausible mechanism for the formation of the isolated complexes that involves dithiirane derivatives as the key intermediates is proposed. Herein, thioketones 2a and 2b act as sulfur transfer reagents. The electrochemical experiments showed that complex 3b behaves as a catalyst for the electrochemical reduction of protons from acetic acid. Copyright

Full text of DOI:10.1002/ejic.201101032

FULL PAPER
DOI: 10.1002/ejic.201101032
New Approach to [FeFe]-Hydrogenase Models Using Aromatic Thioketones
Ahmad Q. Daraosheh,[a] Ulf-Peter Apfel,[a][‡] Helmar Görls,[a] Christian Friebe,[b]
Ulrich S. Schubert,[b] Mohammad El-khateeb,[c][‡‡] Grzegorz Mloston,*[d] and
Wolfgang Weigand*[a]
Dedicated to Professor Heinz Heimgartner on the occasion of his 70th birthday
Keywords: Bioinorganic chemistry / Enzyme mimics / Hydrogenase models / Electrochemistry / Sulfur heterocycles /
Thioketones / Iron
The reactions of triiron dodecacarbonyl with thiobenzo-
phenone (2a) and 9H-thioxanthene-9-thione (2b) were inves-
tigated under different conditions. In the case of a 1:1 molar
ratio of triiron dodecacarbonyl and 2a or 2b, the ortho-metall-
ated complexes [Fe2(CO)6{μ,κ,S,SCH(C6H5)C6H4-η2}] (3a)
and [Fe2(CO)6{μ,κ,S,SCH(C6H4)–S–C6H3-η2}] (4a) were ob-
tained as the major products, respectively. In contrast, the
treatment of triiron dodecacarbonyl with an excess of 2a or
2b afforded [Fe2(CO)6{μ-SCH(C6H5)C6H4S-μ}] (3b) and
[Fe2(CO)6{μ-SC(C6H4–S–C6H4)S-μ}] (4c). Furthermore, [{Fe2-
(CO)6{μ-SCH(C6H5)2}}2(μ4-S)] (3d) was isolated from the re-
action of Fe3(CO)12 with 2a. The molecular structures of all
of the new complexes were determined from the spectro-
scopic and analytical data and the crystal structures for 3c,
3d, 4b, and 4c were obtained. A plausible mechanism for the
formation of the isolated complexes that involves dithiirane
derivatives as the key intermediates is proposed. Herein,
thioketones 2a and 2b act as sulfur transfer reagents. The
electrochemical experiments showed that complex 3b be-
haves as a catalyst for the electrochemical reduction of pro-
tons from acetic acid.
[Fe2(CO)6{μ-SCH(C6H4)–S–C6H3S-μ}]
(4b),
respectively,
which are both bioinspired models for the active site of
[FeFe]-hydrogenase. In addition to these complexes, the two
reactions afforded [Fe2(CO)6{μ-SC(C6H5)2S-μ}] (3c) and
organisms have used H2 as a primary fuel source for billions
of years and consume an enormous amount of H2 in dif-
ferent forms as an energy source and as a transporter.[9]
Inspired by the rapid and reversible proton reduction
that is catalyzed by these hydrogenase enzymes, consider-
able research has been devoted to the design and synthesis
of model species that mimic the active sites of the hydro-
genases.[10]
Recently, we investigated the oxidative addition of the di-
or tetra-substituted 1,2,4-trithiolans to iron carbonyl com-
pounds in an attempt to produce [FeFe]-hydrogenase model
complexes.[11a]
Introduction
Nature has developed highly efficient enzymes that regu-
late the generation and depletion of H2.[1–4] These enzymes
are called hydrogenases and can be classified into three
major groups according to the metal content of their active
sites, namely, [FeFe]-, [NiFe]-, and [Fe]-hydrogenases.[5] The
[FeFe]-hydrogenases have a higher hydrogen production
ability compared to that of other hydrogenases.[6–8] Micro-
[a] Institut für Anorganische und Analytische Chemie,
Friedrich-Schiller-Universität,
Humboldtstraße 8, 07743 Jena, Germany
Fax: +49-3641-948102
In an earlier investigation of the reaction of 3,3,5,5-tetra-
phenyl-1,2,4-trithiolane (1) with Fe3(CO)12,[11b] we ob-
served a different reaction pathway to that of the corre-
sponding tetra-alkyl-substituted analogues. The latter react
with iron carbonyl complexes to yield the oxidative addition
products that result from the cleavage of the S–S bond. In
contrast, the former undergoes a [2+3]-cycloreversion reac-
tion[12,13a] and the fragments [e.g., Ph2C=S (2a)] react with
the iron carbonyl complexes to yield the ortho-metallated
complex 3a as the major component of the reaction mix-
ture.[11b–11e] In the same paper, the ortho-metallated com-
E-mail: wolfgang.weigand@uni-jena.de
[b] Laboratory of Organic and Macromolecular Chemistry
(IOMC) and Jena Center for Soft Matter (JCSM),
Friedrich-Schiller-Universität Jena,
Humboldtstr. 10, 07743 Jena, Germany
[c] Faculty of Science and Arts at Alkamil, King Abdul Aziz Uni-
versity,
P. O. Box 110, Alkamil 21931, Kingdom of Saudi Arabia
[d] University of Lodz, Department of Organic and Applied
Chemistry,
Tamka 12, 91-403 Lodz, Poland
[‡] Current address: Department of Chemistry, Massachusetts
Institute of Technology,
Cambridge, MA 02139, USA
[‡‡]Permanent address: Chemistry Department, Jordan University
of Science and Technology, P. O. Box 3030, Irbid 22110, Jordan plexes 3e, 3f, and 3g (Figure 1) were obtained after the aro-
318
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New Approach to [FeFe]-Hydrogenase Models
matic thioketones 4,4Ј-bis(dimethylamino)-thiobenzo-
phenone, dibenzosuberenethione, and xanthione, respec-
tively, were treated with Fe3(CO)12.
Figure 1. The ortho-metallated complexes 3a and 3e–g.
These observations prompted us to investigate the reac-
tion of 2a and 9H-thioxanthene-9-thione (2b) with Fe3-
(CO)12, and to examine the reactivity of the complexes of
type 3 that were initially obtained under the applied reac-
tion conditions. The structures of the isolated ortho-metall-
ated complexes 3a and 3e–g (Figure 1) suggested that these
compounds can play the role of key intermediates in the
synthesis of new iron complexes that may be unattainable
otherwise.
In the present work we demonstrate the role of the ortho-
metallated complexes as precursors for the synthesis of the
new [FeFe]-hydrogenase model complexes. In addition, the
synthesis and the structural characterization of the two syn-
thetic targets 3b and 4b, as well as the proposed mechanism
(Scheme 3) of their formation, are described. To the best of
our knowledge, this is the first study to illustrate the synthe-
sis of the 1,3-dithiolato-diiron complexes from the symmet-
rical aromatic thioketones.
Scheme 1. The reaction of Fe3(CO)12 with 2a where (a) n is 1, the
reaction time is 20 min, 3a (major), 3b and 3c (traces), and (b) n is
3, the reaction time is 180 min, and the main products are 3b–d
and 5.
Results and Discussion
The reaction of Fe3(CO)12 with one equivalent of thio-
benzophenone (2a) or 9H-thioxanthene-9-thione (2b) in thf
at reflux for 20 min resulted in the formation of the ortho-
metallated complexes, [Fe2(CO)6{μ,κ,S,SCH(C6H5)C6H4-
η2}] (3a) and [Fe2(CO)6{μ,κ,S,SCH(C6H4)–S–C6H3-η2}]
(4a), respectively, as the major products. In addition to 3a,
complexes [Fe2(CO)6{μ-SCH(C6H5)C6H4S-μ}] (3b) and
[Fe2(CO)6{μ-SC(C6H5)2S-μ}] (3c) were produced from the
reaction of Fe3(CO)12 with 2a. Similarly, complexes
[Fe2(CO)6{μ-SCH(C6H4)–S–C6H3S-μ}] (4b) and [Fe2(CO)6-
{μ-SC(C6H4–S–C6H4)S-μ}] (4c) were isolated along with 4a
from the reaction of Fe3(CO)12 with 2b (Schemes 1 and 2).
It must be noted, however, that products 3b, 3c, 4b, and
4c were obtained in trace amounts in these reactions. In
contrast, the treatment of Fe3(CO)12 with an excess of 2a
or 2b in thf at reflux for ca. 3 h gave the [2Fe2S]-model
complexes, 3b–c and 4b–c, respectively, in moderate yields.
Unexpectedly, the tetranuclear complex, [{Fe2(CO)6{μ-
SCH(C6H5)2}}2(μ4-S)] (3d), and known tetraphenylethylene 4c.
Scheme 2. The reaction of Fe3(CO)12 with 2b where (a) n is 1, the
reaction time is 20 min, 4a (major), 4b and 4c (traces) and (b) n is
3, the reaction time is 180 min, and the main products are 4b and
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(5) were obtained from the reaction of 2a with Fe3(CO)12. thioketone to dithiiranes by means of heating the corre-
Complexes 3b–d and 4a–c are air-stable in the solid state sponding thioketone with S8.[13b] In addition, Huisgen and
for months and for several hours in solution. It is worth Rapp have also suggested that “the thioketone itself can be
noting that these complexes are fairly soluble in common converted to a sulfur donor that is capable of generating
organic solvents including the hydrocarbons. It is interest- the thione S-sulfide in an unidentified pathway”.[13a]
ing to note that there is only one sulfur atom in the struc-
tures of starting thioketones 2a and 2b. The reaction of
these compounds with Fe3(CO)12, however, furnished the
[2Fe2S] complexes, 3b, 3c, 4b, and 4c, and the [4Fe3S] com-
plex, 3d. Thus, an important question arose about the
source of the additional sulfur atom in these complexes. A
possible explanation is based on the assumption that these
thioketones act as sulfur transfer reagents. If this assump-
tion is true, then the question arises as to whether or not
these thioketones can be used as efficient precursors for
[FeFe]-hydrogenase model synthesis. In order to find con-
vincing answers for these questions, we investigated the re-
action of 3a with 2a. This reaction led to the formation of
complex 3b in a moderate yield, which suggests that 2a is
acting as a sulfur transfer reagent, while 3a is an important
intermediate in the multistep synthesis of the [FeFe]-hydro-
genase model complexes of the type 3b. A plausible mecha-
nism for the formation of complex 3b from 3a is shown in
Scheme 3. The postulated reactive intermediate 8 plays a
key role in the formation of 3b. A similar reaction pathway
has already been described by Eisch et al.[13c]
Scheme 4. The reaction pathway for the formation of 3c via the
intermediate diphenyldithiirane (7).
Scheme 3. The proposed mechanism for the formation of 3b from
3a.
Scheme 5. The proposed mechanism for the conversion of the ini-
tially formed 3a into the dinuclear complex 3d by means of a sulfur
transfer mechanism.
Complex 3c is believed to be produced by the oxidative
addition of Fe3(CO)12 along the S–S bond of the in situ-
1
The H and 13C{1H} NMR spectra of 4a exhibit signals
generated diphenyldithiirane (7). The latter could be formed at δ = 4.86 and 60.7 ppm, respectively, which were attrib-
from 2a by means of a stepwise mechanism (Scheme 4) un- uted to the methine group. These resonances, as well as the
1
der the catalytic influence of the carbonyliron complex that other signals in the H and 13C{1H} NMR spectra of 4a,
is present in the reaction mixture. Thiobenzophenone S- are in the same range as those observed for the analogue
1
sulfide (thiosulfine) (6) is believed to be a reactive interme- complexes 3a and 3e–g. The H NMR spectra for 3c and
diate in the formation of 7. On the other hand, compound 4c show a broad signal at δ = 7.57 ppm (for 3c) and two
6 could play the role of a sulfur transfer reagent in the pro- broad resonances at δ = 7.42 and 7.74 ppm (for 4c), which
cess that leads to the formation of complex 3d (Scheme 5). were attributed to the aromatic protons. In addition, there
Saito et al. described the conversion of a special type of are no signals at δ Ͻ 6.2 ppm to indicate the presence of
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New Approach to [FeFe]-Hydrogenase Models
1
methine protons in these complexes. The H NMR spectra
of 3b and 4b show a singlet at δ = 5.90 and 5.28 ppm,
respectively, which corresponds to the methine protons.
These values are shifted downfield compared to those of
1
the analogues 3a and 4a, respectively. The H NMR spec-
trum of 3d reveals the presence of two methine groups and
the resonances for these protons are found at δ = 4.21 and
4.66 ppm, respectively. The 13C NMR spectra of 3b–d and
4a–c display the resonances of the C=O groups in the range
of 207 to 210 ppm. Finally, the IR spectra of complexes 3b–
d and 4a–c display three major absorption bands in the re-
gion of 2075 to 1985 cm–1, which are typical for carbonyl
groups that are bonded to iron atoms.
The molecular structures of complexes 3c, 3d, 4b, and 4c
were confirmed by X-ray diffraction analysis and are shown
in Figures 2, 3, 4, and 5, respectively. The central [2Fe2S]
moieties of these complexes are in the “butterfly” arrange-
ment and have a distorted octahedral geometry around the
iron center. The thiolato sulfur atoms S(1) and S(2) are μ2-
coordinated to Fe(1) and Fe(2) in the structures of 3c, 4b,
and 4c. However, the two sulfur atoms of the bridging di-
thiolato ligand of complex 4b are connected to different
carbon atoms. One of the sulfur atoms is bonded to an ali-
phatic carbon while the other one is bonded to an aromatic
carbon. In complexes 3c and 4c, on the other hand, the
sulfur atoms are both bonded to the same aliphatic carbon.
All of the iron atoms in tetranuclear complex 3d are bonded
to the same sulfur atom (S3) and, in addition, the thiolato
sulfur atoms S(1) and S(2) are μ2-coordinated to Fe(1),
Fe(2) and Fe(3), Fe(4), respectively. The Fe–Fe bond length
of 4b [2.5218(5) Å] is comparable to those reported for the
[FeFe]-hydrogenase model complexes[10i,10l,14–20] and to that
of 3d [2.5246 Å (mean)], but it is longer than the corre-
sponding bond lengths in the analogous complexes 3e
Figure 3. The ORTEP drawing of [{Fe2(CO)6{μ-SCH(C6H5)2}}2-
(μ4-S)] (3d) with the thermal ellipsoids set at the 50% probability
level. The hydrogen atoms were omitted for clarity. The selected
distances [Å] and angles [°] are Fe(1)–Fe(2) 2.5195(3), Fe(3)–Fe(4)
2.5297(4), Fe(1)–S(1) 2.2555(5), Fe(2)–S(1) 2.2625(5), Fe(1)–S(3)
2.2321(5), Fe(2)–S(3) 2.2443(4), Fe(3)–S(2) 2.2701(5), Fe(4)–S(2)
2.2637(5), Fe(3)–S(3) 2.2344(5), Fe(4)–S(3) 2.2379(5), Fe(1)–S(1)–
Fe(2) 67.789(16), Fe(1)–S(3)–Fe(2) 68.505(15), Fe(3)–S(2)–Fe(4)
67.831(14), Fe(3)–S(3)–Fe(4) 68.849(15), Fe(1)–S(3)–Fe(3) 136(74),
S(2)–Fe(4)–S(3) 76.324(17), and S(1)–Fe(2)–Fe(1) 55.974(13).
Figure 4. The ORTEP drawing of [Fe2(CO)6{μ-SCH(C6H4)–S–
C6H4S-μ}] (4b) with the thermal ellipsoids set at the 50% prob-
ability level. The selected distances [Å] and angles [°] are Fe(1)–
Fe(2) 2.5218(5), Fe(1)–S(1) 2.2415(6), Fe(1)–S(2) 2.2340(6), Fe(2)–
S(1) 2.2417(7), Fe(2)–S(2) 2.2412(7), Fe(1)–S(1)–Fe(2) 68.46(2),
Fe(1)–S(2)–Fe(2) 66.60(2), S(1)–Fe(2)–S(2) 85.05(2), and S(1)–
Fe(2)–Fe1 55.767(18).
[2.4993(6) Å][11b] and 4c [2.4867(4) Å]. In addition, the Fe–
S bond lengths of 4b [2.2396 Å (mean)] are significantly
shorter than those reported for the [FeFe]-hydrogenase
model complexes[21–27] and are about 0.02 Å shorter than
those of 4c [2.2694 Å (mean)] and of 3c [2.2673 Å (mean)].
The Fe–Fe bond length of 3c [2.4850(5) Å] is similar to that
of the reported analogous complex 3a [2.4986(6) Å].[11b]
The angles of S(1)–Fe(1)–S(2) [85.22(2)°] and S(1)–Fe(2)–
Figure 2. The ORTEP drawing of [Fe2(CO)6{μ-SC(C6H5)2S-μ}]
(3c) with the thermal ellipsoids set at the 50% probability level.
The selected distances [Å] and angles [°] are Fe(1)–Fe(2) 2.4867(4),
Fe(1)–S(1) 2.2785(6), Fe(1)–S(2) 2.2625(6), Fe(2)–S(1) 2.2757(6),
Fe(2)–S(2) 2.2608(6), Fe(1)–S(1)–Fe(2) 66.190(19), Fe(1)–S(2)–
Fe(2) 66.699(19), S(1)–Fe(1)–S(2) 72.21(2), and S(1)–Fe(2)–Fe(1)
56.618(17).
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S(2) [85.02(2)°] in 4b are within the same ranges as those oxidation peak appears at –0.80 V. This signal was only ob-
observed for the [FeFe]-hydrogenase model complexes.[14–27] served upon the initial one-electron reduction of the initial
However, these angles are wider than the corresponding [FeIFeI] species at –1.58 V. According to the literature, this
angles of S(1)–Fe(1)–S(2) [72.21(2)°] and S(1)–Fe(2)–S(2) might be the reoxidation of a chemically transformed
[72.29(2)°] in 4c, and of S(1)–Fe(1)–S(2) [72.26(2)°] and [FeIFe0] species.[29] At ca. +1.28 V the irreversible oxidation
S(1)–Fe(2)–S(2) [72.17(2)°] in 3c, which is attributed to the of the [FeIFeI] cluster can be observed. A corresponding
bonding of the two sulfur atoms of the dithiolato ligand to reduction signal appeared at –0.67 mV, which suggests that
the same carbon in 3c or 4c.
there was structural reorganization after the oxidation and
that it was not solely a simple reduction of the obtained
[FeIIFeI] complex as has been already described for similar
reduction processes.
Figure 5. The ORTEP drawing of [Fe2(CO)6{μ-SC(C6H4–S–C6H4)-
S-μ}] (4c) with the thermal ellipsoids set at the 50% probability
level. The selected distances [Å] and angles [°] are Fe(1)–Fe(2)
2.4850(5), Fe(1)–S(1) 2.2693(6), Fe(1)–S(2) 2.2629(6), Fe(2)–S(1)
2.2643(6), Fe(2)–S(2) 2.2728(7), Fe(1)–S(1)–Fe(2) 66.478(19),
Fe(1)–S(2)–Fe(2) 66.44(2), S(1)–Fe(1)–S(2) 72.26(2), and S(1)–
Fe(1)–Fe(2) 56.666(18).
Figure 6. The cyclic voltammetric reduction of [Fe2(CO)6{μ-
SCH(C6H5)C6H4S-μ}] (3b) in acetonitrile (1.0 mm) on a glassy car-
bon electrode where Fc/Fc+ was used as the internal standard and
[nBu4N][PF6] (0.1 m) was used as the supporting electrolyte.
The influence of compound 3b towards the electrochemi-
cal reduction of protons to dihydrogen was investigated be-
tween 0.0 and –2.5 V by the addition of acetic acid (pKa =
22.3 in CH3CN) (Figure 7). In the presence of acid, the ini-
tial one-electron reduction signal remains unchanged. Nei-
ther a significant increase nor a shift of the signal was ob-
served. An acid-dependent increase in the peak current
Electrochemical Investigations
The electrocatalytic dihydrogen formation of the [FeFe]-
hydrogenase model compounds has been well estab-
lished.[28] In order to show the ability of the new complexes
to act as catalyst for dihydrogen formation, cyclic voltam-
metry was performed for compound 3b in the presence and
absence of acetic acid. The cathodic scan of complex 3b
(Figure 6) reveals an irreversible reduction peak at Ep,red
=
–1.58 V. In comparison to the internal standard ferrocene,
this signal is most likely a one-electron reduction and was
therefore attributed to the [FeIFeI]Ǟ[FeIFe0]– process. The
signal remained completely irreversible at the different scan
rates (1.5, 1.0, 0.8, 0.1, and 0.05 V/s). This behavior suggests
an EC mechanism where the [FeIFeI] state is transferred
into [FeIFe0]– by a one-electron reduction, followed by a
fast change in the bonding properties within the molecule,
which is in good agreement with the literature results.[29,30]
This change in the bonding properties can be best described
by the cleavage of the Fe–Fe bond and/or the appearance
of a bridging carbonyl molecule.[29] At –2.15 V a further
reduction of the chemically changed [FeIFe0] species was
observed, which was attributed to the [FeIFe0]– Ǟ
[Fe0Fe0]2– process in accordance with Fe2(CO)6(pdt) (pdt =
propanedithiolato).[31] Two sparsely separated reoxidation
Figure 7. The cyclic voltammograms of [Fe2(CO)6{μ-SCH(C6H5)-
C6H4S-μ}] (3b) in acetonitrile (1 mm) in the presence of HOAc (0–
signals were observed at –2.07 and –2.00 V. An additional 10 mm), (potentials vs. Fc/Fc+).
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New Approach to [FeFe]-Hydrogenase Models
around –2.0 V was observed when the cathodic scan in-
The most remarkable feature of this investigation, how-
cluded more negative potentials. According to the literature, ever, is the assembly of a [FeFe]-hydrogenase active-site-
this behavior could be explained by the catalytic reduction core analogue from simple aromatic thioketones. This is of
of acetic acid by a reduced 3b.[28] However, a comparison particular interest to prebiotic chemistry since one can en-
of the peak currents at around –2.0 V and in pure acetic vision that in a hydrothermal vent environment that has a
acid reveals only moderate catalytic activity for compound higher CO concentration, where reduced hydrothermal
3b.
Since compound 3b revealed the structural properties of nificant concentrations of iron carbonyls and thioketones
[Fe2(CO)6(pdt)] (pdt
propanedithiolato) (Fe–S-alkyl might be formed.[32a] In a slightly different prebiotic reac-
fluids pass through the iron-/sulfide-containing crust, sig-
=
bond) and [Fe2(CO)6(bdt)] (bdt = benzenedithiolato) (Fe– tion that was reported by Cody et al., iron sulfide is con-
S-phenyl bond), and since both of the complexes revealed sumed in the presence of CO and alkylthiol to produce
different electrochemical properties, a short comparison be- [Fe2(RS)2(CO)6], sulfur, and hydrogen.[32b] These possible
tween the three complexes will be given here. In contrast prebiotic reactions that are emerging for the [FeFe]-hydro-
to 3b and [Fe2(CO)6(pdt)],[31] [Fe2(CO)6(bdt)][10h] shows an genase model systems are of great importance in the context
initial two-electron reduction to a [Fe0Fe0] complex at of the iron-sulfur world hypothesis.[32c]
–1.25 V (Table 1). This reduction, however, appears at two
different potentials. The one-electron reduction of [Fe2-
(CO)6(pdt)] and 3b is observed at –1.34 and –1.58 V, respec-
tively. In contrast to [Fe2(CO)6(bdt)], the second one-elec-
tron reduction is found at a distinctly lower potential
Experimental Section
General Comments: All of the reactions were carried out under an
argon atmosphere by using the standard Schlenk techniques. The
around –2 V for both of the complexes. When acetic acid
1H and 13C{1H} NMR and 2D NMR spectra were recorded with
was added to the complexes, the reduction of the protons
a Bruker AVANCE 200 or 400 MHz spectrometer at room tem-
to dihydrogen was observed for all of the complexes at
perature and the solvent was used as the standard. The Mass spec-
tra were obtained with a FINNIGAN MAT SSQ 710 instrument.
The infrared spectra were measured with a Perkin–Elmer System
around –2 V. Based on these results, complex 3b should be
considered to be a comparable model to the [FeFe]-hydro-
genase model complexes with a propanedithiolato back-
bone.
2000 FTIR spectrometer. Thiobenzophenone (2a)[12d] and 9H-
thioxanthene-9-thione (2b)[13a] were prepared according to the lit-
erature procedures. The solvents and Fe3(CO)12 were purchased
from Sigma–Aldrich. All of the solvents were dried and distilled
prior to use according to the standard methods. Silica gel 60
(0.015–0.040 mm) was used for the column chromatography. TLC
Table 1. The electrochemical data of the iron complexes 3b,
[{Fe2(CO)6}(pdt)], and [{Fe2(CO)6}(bdt)].
Ered 1 [V]
Ered 2 [V]
Eox [V]
was done with Merck TLC aluminum sheets, silica gel 60 F254
The elemental analyses were performed with a Vario EL III CHNS
(Elementaranalysen GmbH Hanau) as single determinations.
.
3b[a]
–1.58
–1.34
–2.15
–1.95
–1.23
+1.28
+1.14
irreversible
[{Fe2(CO)6}(pdt)][31][b]
[{Fe2(CO)6}(bdt)][10h][c] –1.27
[Fe2(CO)6{μ-SCH(C6H5)C6H4S-μ}] (3b), [Fe2(CO)6{μ-SC(C6H5)2}-
(S-μ)] (3c), and [{Fe2(CO)6{μ-SCH(C6H5)2S-µ}] (3d). Method A:
Fe3(CO)12 (100 mg, 0.2 mmol) and thiobenzophenone (2a)
(118 mg, 0.4 mmol) in thf (30 mL) were stirred at 65 °C under ar-
gon for a period of 3 to 4 h. The reaction mixture was cooled to
room temperature and the solvent was removed under vacuum. The
crude product was purified by column chromatography. Elution
with hexane gave an orange solution of complex 3c (Rf = 0.7),
elution with hexane/diethyl ether (1:1, v/v) afforded a reddish solu-
tion of complex 3b (Rf = 0.5) and elution with diethyl ether gave a
red solution of 3d (Rf = 0.5). The solutions were evaporated under
vacuum. Suitable crystals of 3c and 3d for X-ray analysis were ob-
tained by the slow evaporation of a concentrated pentane solution
at –25 °C.
[a] Glassy carbon electrode (potentials given in V, Ϯ0.01) vs. Fc/
Fc+ (0.01 m) in [nBu4N][PF6]/CH3CN (0.1 m) as the supporting
electrolyte. [b] CH3CN solution (0.1 m [nBu4N][PF6]) with a glassy
carbon working electrode standard vs. Fc/Fc+. [c] First scan, v =
0.1 Vs–1; solution in [nBu4N][PF6]/CH3CN.
Conclusions
In summary, we have succeeded in synthesizing two new
complexes, 3b and 4b, that are bioinspired models for the
active site of the [FeFe]-hydrogenases by using the aromatic
thioketones, 2a and 2b, as the starting materials. The syn-
thesis of 3b was accomplished by a multistep reaction. A 3b: Yield 22 mg (22%). C19H10Fe2O6S2 (509.9): calcd. C 44.74, H
1
1.98, S 12.57; found C 45.18, H 1.83, S 12.1. H NMR (400 MHz,
CDCl3, 25 °C): δ = 5.90 [s, 1 H, H(1)], 7.03–7.23 (m, 5 H, Ar-H),
7.32 [dd, 3J = 7.7 Hz, 1 H, H(10)], 7.37 [dd, 3J = 8.1 Hz, 1 H,
possible mechanism for the formation of 3b has been pro-
posed. Firstly, thioketone 2a reacts with Fe3(CO)12 to give
the ortho-metallated complex, 3a. Secondly, a further equiv-
alent of 2a, which is activated by a side-on coordination to
an iron atom, serves as a sulfur transfer reagent. Thirdly,
complex 3b is formed by the insertion of sulfur into the Fe–
C σ-bond of 3a. It was found that complex 3b behaves as a
catalyst for the electrochemical production of hydrogen in
the presence of a weak acid, for example acetic acid, at a
moderate potential.
3
3
H(11)], 7.75 [d, J = 8.0 Hz, 1 H, H(9)], 8.51 [d, J = 8.2 Hz, 1 H,
H(12)] ppm. 13C{1H} NMR (400 MHz, CDCl3): δ = 70.7 (C1),
123.8, 125.9, 126.7, 127.1, 128.2, 130.8, 138.3, 141.8, 143.7, 144.6,
152.5, 157.8 (2 Ph), 207.3, 208.9, 210.5 (CO) ppm. FTIR (KBr):
ν
˜
= 2073 (vs), 2035 (vs), 2008 (w, sh), 1994 (s), 1979 (s) cm–1.
CϵO
MS (DEI = 70 eV): m/z = 510 [M+], 482 [M+ – CO], 454 [M+
–
2CO], 426 [M+ – 3CO], 398 [M+ – 4CO], 370 [M+ – 5CO], 342
[M+ – 6CO].
Eur. J. Inorg. Chem. 2012, 318–326
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
323

G. Mloston, W. Weigand et al.
FULL PAPER
3c: Yield 25 mg (25%). C19H10Fe2O6S2 (509.9): calcd. C 44.74, H
H(9)], 7.36 [m, 1 H, H(10)], 7.39 [m, 1 H, H(7)], 7.62 [m, 1 H,
1.98, S 12.57; found C 44.96, H 1.72, S 12.23. 1H NMR (400 MHz, H(3)], 7.92 [m, 1 H, H(4)], 8.04 [m, 1 H, H(2)] ppm. 13C{1H} NMR
CDCl3, 25 °C): δ = 7.57 (br. s, 10 H, 2 Ph) ppm. 13C{1H} NMR
(400 MHz, CDCl3): δ = 95.0 (SCS), 123.6, 127.7, 128.5 (2 Ph), 128.2, 128.5, 131.0, 135.9, 136.8, 141.8, 154.8 (2Ph), 208.6, 209.4
(400 MHz, CDCl3): δ = 60.7 (CS), 125.0, 125.8, 126.6, 127.2, 127.5,
207.0, 208.2 (CO) ppm. FTIR (KBr): ν
= 2076 (vs), 2035 (vs), (CO) ppm. FTIR (C H ): ν
= 2071 (vs), 2037 (vs), 2001 (vs),
˜
˜
CϵO
5
12
CϵO
1990 (vs) cm–1. MS (DEI = 70 eV): m/z = 510 [M+], 482 [M+
CO], 454 [M+ – 2CO], 426 [M+ – 3CO], 398 [M+ – 4CO], 370 [M+
5CO], 342 [M+ – 6CO].
–
–
1984 (w, sh) cm–1. MS (DEI = 70 eV): m/z = 508 [M+], 480 [M+
–
–
CO], 452 [M+ – 2CO], 424 [M+ – 3CO], 396 [M+ – 4CO], 368 [M+
5CO], 340 [M+ – 6CO].
[Fe2(CO)6{μ-SCH(C6H4)–S–C6H4S-μ}] (4b) and [Fe2(CO)6{μ-
SC(C6H4–S–C6H4)S-μ}] (4c): The ligand, 9H-thioxanthene-9-
thione (2b) (163 mg, 0.48 mmol), was added to a solution of
Fe3(CO)12 (120 mg, 0.24 mmol) in thf (40 mL) under argon. The
reaction mixture was stirred at 65 °C for a period of 3 to 4 h. After
evaporation of the solvent, the residue was purified by coloum
chromatography on a silica gel column. Elution with hexane gave
an orange-reddish solution of complex 4c (Rf = 0.6). Elution with
hexane/diethyl ether (2:1, v/v) afforded a reddish solution of com-
plex 4b (Rf = 0.3). The two solutions were evaporated under vac-
uum. Suitable crystals of 4b and 4c for X-ray analysis were ob-
tained by the slow evaporation of a concentrated pentane solution
at –25 °C. 4b. Yield 48 mg (37%). C19H8Fe2O6S3 (540.1): calcd. C
3d: Yield 21 mg (10%). C38H22Fe4O12S3 (989.8): calcd. C 46.09, H
2.24, S 9.72; found C 46.52, H 2.47, S 9.39. H NMR (200 MHz,
CDCl3, 25 °C): δ = 4.21, 4.66 (s, 2 H, 2 SCH), 7.06, 7.44, 7.68 (br.
s, 20 H, 2 Ph) ppm. 13C{1H} NMR (200 MHz, CDCl3): δ = 38.1,
38.9 (2 CS), 121.3, 125.2, 126.5, 127.3 (2 Ph), 206.6, 207.8, 208.1
1
(CO) ppm. FTIR (KBr): νCϵO = 2072 (vs), 2059 (w, sh), 2039 (vs),
˜
1998 (vs) cm–1. MS (DEI = 70 eV): m/z = 990 [M+], 906 [M+
–
3CO], 878 [M+ – 4CO], 822 [M+ – 6CO], 794 [M+ – 7CO], 766
[M+ – 8CO], 738 [M+ – 9CO], 711 [M+ – 10CO], 655 [M+ – 12CO].
[Fe2(CO)6{μ-SCH(C6H5)C6H4S-μ}] (3b). Method B: Thioketone 2a
(18 mg, 0.1 mmol) was added to a solution of 3a (46 mg, 0.1 mmol)
in thf (30 mL) under argon and the mixture was stirred at 65 °C
for 3 h. The solvent was removed under vacuum and the crude
product was purified by column chromatography. Elution with hex-
ane/diethyl ether (1:1, v/v) gave a reddish solution (Rf = 0.5), which
was identified as complex 3b. Yield 21 mg (41%).
1
42.25, H 1.49, S 17.80; found C 42.58, H 1.68, S 17.30. H NMR
(400 MHz, CDCl3, 25 °C, assignment analogous to 4a): δ =
3
5.28 ppm. [s, 1 H, H(12)], 7.05 [dd, 1 H, J = 8.0 Hz, H(8)], 7.31
3
3
[m, 1 H, H(9)], 7.43[d, 1 H, J = 8.0 Hz, H(10)], 7.56 [d, 1 H, J =
3
3
[Fe2(CO)6{μ,κ,S,SCH(C6H4)–S–C6H4-η2}]
(4a):
Fe3(CO)12 8.0 Hz, H(7)], 7.76 [dd, 1 H, J = 8.0 Hz, H(3)], 8.03 [d, 1 H, J =
(140 mg, 0.28 mmol) was dissolved in thf (40 mL) and 9H-thio-
xanthene-9-thione (2b) (64 mg, 0.28 mmol) was added to the solu-
tion. The mixture was stirred at 65 °C for 20 min under argon. The
solvent was removed in vacuo. The crude product was purified by
column chromatography by using hexane as the eluent. The major
dark red band (Rf = 0.5) was collected and the solvent was re-
moved. The product was identified as complex 4a. Yield 92 mg
(65%). C19H8Fe2O6S2 (507.8): calcd. C 44.91, H 1.59, S 12.62;
found C 44.70, H 1.92, S 12.58. 1H NMR (400 MHz, CDCl3,
25 °C): δ = 4.82 [s, 1 H, H(12)], 6.94 [m, 1 H, H(8)] 7.26 [m, 1 H,
8.0 Hz, H(4)], 8.62 [d, 1 H, 3J = 8.0 Hz, H(2)] ppm. 13C{1H} NMR
(400 MHz, CDCl3): δ = 69.9 (C12), 125.3, 126.4, 126.5, 126.8,
126.9, 127.1, 127.8, 128.05, 129.3, 132.8, 142.1 158.2 (2 Ph), 207.0,
208.5, 209.2, 212.2 (CO) ppm. FTIR (KBr): νCϵO = 2075 (vs), 2038
˜
(vs), 2017 (w, sh), 2003 (s), 1985 (s) cm–1. MS (DEI = 70 eV): m/z
= 540 [M+], 512 [M+ – CO], 484 [M+ – 2CO], 456 [M+ – 3CO], 428
[M+ – 4CO], 400 [M+ – 5CO], 372 [M+ – 6CO]. 4c: Yield 41 mg
(32%). C19H8Fe2O6S3 (540.1): calcd. C 42.25, H 1.49, S 17.80;
found C 42.73, H 1.58, S 17.62. 1H NMR (400 MHz, CDCl3,
25 °C, assignment analogous to 4a): δ = 7.42, 7.74 (br. s, 8 H, 2
Table 2. The crystal data and refinement details for the X-ray structure determinations of the compounds 3c, 3d, 4b, and 4c.
3c
3d
4b
4c
Formula
C19H8Fe2O6S3
540.13
C38H22Fe4O12S3
990.14
C19H7Fe2O6S3
539.13
C19H10Fe2O6S2
510.09
fw [gmol–1]
T [°C]
Crystal system
Space group
–140(2)
monoclinic
P21/n
–140(2)
–140(2)
monoclinic
P21/c
–140(2)
monoclinic
C2/c
triclinic
¯
P1
a [Å]
b [Å]
c [Å]
α [°]
10.3982(3)
9.6800(2)
19.7021(5)
90
99.020(2)
90
1958.58(9)
4
1.832
9.1211(2)
13.7459(2)
16.5123(3)
89.164(1)
83.405(1)
76.626(1)
2000.65(6)
2
7.8592(2)
14.9765(3)
17.2733(3)
90
92.011(1)
90
2031.87(7)
4
1.762
20.4829(12)
6.4767(5)
30.4653(17)
90
107.121(3)
90
3862.5(4)
8
1.754
β [°]
γ [°]
V [Å3]
Z
ρ [gcm–3]
1.644
16.39
μ [cm–1]
18.37
17.71
17.53
Measured data
Data with IϾ2σ(I)
Unique data/Rint
wR2 (all data, on F2)[a]
R1 [IϾ2σ(I)][a]
s[b]
12241
3895
4480/0.0313
0.0727
0.0306
1.027
0.475/–0.409
none
21290
8546
10330/0.0197
0.0701
0.0294
1.042
0.485/–0.368
none
12476
4262
4595/0.0231
0.0865
0.0341
1.011
1.765/–0.534
none
6437
3780
4106/0.0218
0.0733
0.0311
1.065
0.466/–0.349
none
Residual electron density [eÅ–3]
Absorption correction
2 2
2
[a] R1 = (Σ||Fo| – |Fc||)/Σ|Fo|; wR2 = {Σ[w(Fo2 – Fc ) ]/Σ[w(Fo2)2]}1/2; w–1 = σ2(Fo2) + (aP)2 + bP; P = [2Fc2 + max(Fo2)]/3. [b] s = {Σ[w(Fo
–
2 2
Fc ) ]/(No – Np)}1/2
.
324
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 318–326

New Approach to [FeFe]-Hydrogenase Models
Ph) ppm. 13C{1H} NMR (400 MHz, CDCl3): δ = 93.9 (SCS),
123.8, 126.3, 127.0,129.3, 132.9, 134.3, 137.0 (2 Ph), 207.0, 208.1
[6]
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= 2075 (vs), 2037 (vs), 2001 (vs)
˜
CϵO
cm–1. MS (DEI = 70 eV): m/z = 540 [M+], 512 [M+ – CO], 484
[M+ – 2CO], 456 [M+ – 3CO], 428 [M+ – 4CO], 400 [M+ – 5CO],
372 [M+ – 6CO].
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Characterization of 1,1,2,2-Tetraphenylethene (5): Colorless crys-
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70 eV): m/z = 332 [M+].
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Electrochemistry: The cyclic voltammograms were measured in a
three electrode cell with a 1.0 mm diameter glassy carbon disc
working electrode, a platinum auxiliary electrode, and Ag/AgCl in
CH3CN as the reference electrode. The solvent contained
[nBu4N][PF6] (0.1 m) as the supporting electrolyte. The measure-
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VA Standard galvanostat. Deaeration of the sample solutions was
accomplished by passing a stream of nitrogen through the solutions
for 5 min prior to the measurements, and the solutions were kept
under nitrogen for the duration of the measurements. All of the
data obtained were corrected against the Fc/Fc+ couple as an in-
ternal standard (E1/2 = 503 mV vs. Ag/AgCl in CH3CN).
Crystal Structure Determination: The intensity data for the com-
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corrected for Lorentz and polarization effects but not for absorp-
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[11]
2
refined by full-matrix least-squares techniques against Fo
(SHELXL-97).[36] All of the hydrogen atom positions were included
at the calculated positions with fixed thermal parameters. All of
the non-hydrogen atoms were refined anisotropically.[36] XP (SIE-
MENS Analytical X-ray Instruments, Inc.) was used for the struc-
ture representations.
[12]
[13]
CCDC-803654 (for 3c), -803655 (for 3d), -803656 (for 4b) and
-803657 (for 4c) contain 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/
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Acknowledgments
This work has been funded by the European Union (EU)
(SYNTHCELLS project, Approaches to the Bioengineering of
Synthetic Minimal Cells), grant number #FP6043359 (to A. D.).
U.-P. A. is thankful for a fellowship from the Studienstiftung des
deutschen Volkes.
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