Fe-S complexes containing five-membered heterocycles: Novel models for the active site of hydrogenases with unusual low reduction potential

Article abstract of DOI:10.1039/b615037c

Three biomimetic 2Fe2S complexes [{(-SCH₂)₂NCH ₂(2-C₄H₃O)}](Fe₂(CO)₆) (6a), [{(-SCH₂)₂ NCH₂(2-C₄H ₃S)}](Fe₂(CO)₆) (6b) and [{(-SCH ₂)₂NCH₂(5-Br-2-C₄H ₂S)}Fe₂(CO)₆] (6c) were prepared as models for the active site of Fe-only hydrogenase by the convergent process from [(-S ₂)Fe₂(CO)₆] and N,N-bis(hydromethyl)-2-furan and thiophene. The structures of these complexes were identified spectroscopically and crystallographically. The electrochemical behavior of the complexes 6a and 6c was unique as they showed catalytic proton reduction with a low reduction potential at -1.13 and -1.09 V vs Fc/Fc⁺, respectively, in the presence of HClO₄. This journal is The Royal Society of Chemistry.

Full text of DOI:10.1039/b615037c

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Fe–S complexes containing five-membered heterocycles: novel models for the
active site of hydrogenases with unusual low reduction potential†‡
a
a
a
a
b
a,c
˚
Shi Jiang, Jianhui Liu,* Yu Shi, Zhen Wang, Bjo¨rn Akermark and Licheng Sun*
Received 17th October 2006, Accepted 9th January 2007
First published as an Advance Article on the web 24th January 2007
DOI: 10.1039/b615037c
Three biomimetic 2Fe2S complexes [{(l-SCH2)2NCH2(2-C4H3O)}](Fe2(CO)6) (6a), [{(l-SCH2)2
NCH2(2-C4H3S)}](Fe2(CO)6) (6b) and [{(l-SCH2)2NCH2(5-Br-2-C4H2S)}Fe2(CO)6] (6c) were prepared
as models for the active site of Fe-only hydrogenase by the convergent process from [(l-S2)Fe2(CO)6]
and N,N-bis(hydromethyl)-2-furan and thiophene. The structures of these complexes were identified
spectroscopically and crystallographically. The electrochemical behavior of the complexes 6a and 6c
was unique as they showed catalytic proton reduction with a low reduction potential at −1.13 and
−1.09 V vs Fc/Fc+, respectively, in the presence of HClO4.
Introduction
The Fe-only hydrogenases are important enzymes which catalyze
the reduction of protons to molecular hydrogen in numerous
microorganisms.1,2 Since the elucidation of the structures of the
Fe-only hydrogenases isolated from Desulfovibrio desulfuricans
and Clostridium pasteurianum,3,4 transition metal sulfides have
been the subject of much research aimed at structural and
functional mimics of the Fe-only hydrogenase active site. While the
natural enzymes catalyze proton reduction at potentials around
−0.4 V vs NHE (ca. −1 V vs Fc/Fc+),5 the early synthetic
models such as 1a, containing cyanide ligands,6 required potentials
below −2 V. Similarly, the mixed cyanide–phosphine 1b7 and the
bisphosphine complexes 1c8,9 required very negative potentials for
reduction. For the all carbonyl complex 1d, less negative potentials
had been measured around −1.7 V,7,9–11 as might be expected. It
was found that the reductive potential of a mixed phosphine–
cyanide complex 1b moved to ca. −1.6 V on protonation by a
strong acid.8,12 In addition, catalytic hydrogen evolution could
be detected at −1.4 V. A similar behavior was observed for
bisphosphine complexes.8 By contrast, addition of acid to the
carbonyl complexes had little effect on the reductive potentials.9,13
It has been suggested by recent crystallographic, spectroscopic
and theoretical studies that the structure of the bond between the
two iron atoms of H-cluster might be azadithiolate (ADT).14–16 The
possibility of nitrogen functionalization induces the potential for
a great structural versatility, and a few 2Fe2S model compounds
with an aryl or alkyl substituent on the N-bridged atom have
Chart 1
been synthesized.11,17–20 After protonation, the nitrogen-bridged
complex itself lowered the reduction potential from around −1.6 V
to between −1.16 (strong acid)11 and −1.36 V (acetic acid).18
Hydrogen evolution could be observed around −1.5 V with strong
acid and −1.6 V with acetic acid. With benzyl as a substituent on
the nitrogen, the phenyl group seemed to have an influence on the
reductive potential and with a furan group, a reduction potential
of −1.13 V was observed,21 which was getting very close to the
potentials of the hydrogenases.
Obviously, the introduction of an electron-rich five-membered
heterocycle (furan and thiophene) on the Fe–S cluster may
influence the reduction potentials of the complexes. Direct
attachment to the N-bridged atom can increase its electron
density, make the protonation easier, and lead finally to the
less negative reduction potential. The incorporation of the 5-
membered heterocycle can also be realized through one carbon
atom between the N-bridged atom and the heterocycle, which
seems more synthetically easy. In addition, the electron negative
oxygen of the furan ring may capture a proton by forming a
hydrogen bond. However, very few studies have systematically
varied the genre of heterocycles. At the early stage of this
study, we observed the low reduction potential at −1.13 V
caused by the furan-containing 2Fe2S complex in the strong
acidic conditions. This observation encouraged us to explore the
thiophene-attached 2Fe2S system. Moreover, the introduction of
a halogen atom to the heterocycle can provide a possible way
to connect the photosensitizer to Fe-only hydrogenase active site
model compounds via a Pd-catalyzed coupling reaction. Bromine
was identified as an ideal candidate. Herein, we report on the
synthesis, structure and electrochemical properties of the furan-
containing complex 6a and thiophene-containing complexes 6b
and 6c.
aState Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and
Research Center on Molecular Devices, Dalian University of Technology
(DUT), Zhongshan Road 158-40, Dalian, 116012, P. R. China. E-mail:
liujh@dlut.edu.cn; Fax: +86 411 83702185; Tel: +86 411 88993886
bDepartment of Organic Chemistry, Stockholm University, 10691, Stock-
holm, Sweden. E-mail: lichengs@kth.se
cDepartment of Chemistry, Organic Chemistry, Royal Institute of Technology
(KTH), 10044, Stockholm, Sweden
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Details of
the electrochemical studies of 6a–6c and bulk electrolysis. See DOI:
10.1039/b615037c
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Table 1 A comparison of CO bands of analogous diiron heterocyclic-
azadithiolate complexes
Results and discussion
Synthesis and characterization
Complex
CO bands/cm−1 Ref.
The readily accessible complexes 6a and 6b were prepared
in a moderate yield according to literature methods by the
reaction of [(l-S2)Fe2(CO)6] with 2-aminomethylfuran (2a) and
2-aminomethylthiophene (2b), respectively (Scheme 1).21,22 After
lithiation of [(l-S2)Fe2(CO)6] and subsequent H–Li exchange
reaction, the resulting [(l-HS)2Fe2(CO)6] reacted with the
diol [(HOCH2)2NCH2(2-C4H3X) (X = O, S)] derived from
the hydroxymethylation of 2a and 2b to give the 5-membered
heterocyclic 2Fe2S derivatives 6a and 6b in good yields. It is worth
noting that the synthesis is different from the usual procedure,
where the reaction was performed between these two compounds
[(l-LiS)2Fe2(CO)6] and the dichloromethyl amine. Since the
chloromethylation reagent SOCl2 decomposed the diol products,
3a and 3b, we selected to use the direct reaction of complex 5
with 3a and 3b. Attempts to functionalize the thiophene group
of 6b by bromination with Br2 led to the decomposition of
the starting substance. The use of the less aggressive reagent
N-bromosuccinimide (NBS) resulted, to our surprise, in the
generation of the starting complex 4 and the mechanism will not
be discussed here. The final preparation of 6c was obtained from
2-aminomethyl-5-bromothiophene (2c), which was obtained from
the hydroxymethylation of 5-bromothiophene with (CH2O)n and
6a
6b
6c
2073, 2031, 1995 This work
2073, 2031, 1994 This work
2072, 2031, 1994 This work
[{(l-SCH2)2NCH2(2-BrC6H4)}Fe2(CO)6] 2074, 2035, 1997 20
ORTEP diagrams in Fig. 1. According to the single-crystal X-
ray diffraction analysis, their structures are similar to the active
site of Fe-only hydrogenases,3,4 with the usual distorted square-
pyramidal geometry around the iron centre.17,23–26 The sum of
the C–N–C angles around N of complexes 6a, 6b and 6c are
334.5, 332.2 and 340.2◦ respectively, consistent with an sp3-
hybridization of the N atom. The methylene group between the
heterocycles and the N-bridged atom allows the substituted group
flexibility compared to the rigid structure of phenyl-substituted
ADT-bridged diiron complexes, accordingly influencing the bond
angles in the corresponding position. The change of heteroatom
does not influence the structures of 6a and 6b to any great extent
except for the heteroatom position. However, when compared
to the aryl-substituted hydrogenase model [{(l-SCH2)2NCH2(2-
BrC6H4)}Fe2(CO)6],20 the changes seem obvious. In addition to
an sp3-hybridization of the N-bridged atom instead of an sp2-
hybridization for complexes 6a, 6b and 6c, the length of bonds also
changed significantly. For example, the range of the C–S bond in
◦
NH4Cl at 70 C by a procedure similar to that used for complex
6b (Scheme 1).
Complexes 6a–6c were purified directly by column chromatog-
1
raphy and characterized by H NMR, 13C NMR, IR and MS
˚
complex 6c is 1.808(4)–1.826(4) A, much less than that of [{(l-
˚
spectrometry. The API-ES positive mode mass spectra show the
parent ion peaks at m/z 468 [M + H]+ for 6a, 484 [M + H]+ for 6b,
and 562 [M + H]+ for 6c. The IR spectra of complexes 6a, 6b and 6c
all show three strong CO bands in the region of 1990–2080 cm−1.
Table 1 shows a comparison of CO bands for complexes 6a, 6b and
6c and a related diiron aryl-azadithiolate [{(l-SCH2)2NCH2(2-
BrC6H4)}Fe2(CO)6].20 The values of CO bands of complexes 6a,
6b and 6c are quite similar to those reported for the analogues.
SCH2)2NCH2(2-BrC6H4)}Fe2(CO)6] (1.863(5)–1.866(5) A).
Furthermore, it is interesting to note that the conformations of
the two complexes 6b and 6c are different in the solid state, which
changes from periplanar to gauche. In addition, the nonbonding
C · · · N distance between the N-bridged atom and the nearest
˚
˚
˚
carbonyl carbon atom is 3.03 A for 6b, 2.997 A for 6a and 3.101 A
for 6c. In comparison to other complexes reported, the Fe–Fe
˚
bond length is shorter by 0.1 A, but the Fe–S bond lengths are
longer. The change in solid state is mostly due to the electron
withdrawing property of the Br atom.
Molecular structure
The molecular structures of complexes 6a, 6b and 6c were
further confirmed by X-ray crystallography and are shown by
There is a simple packing and no hydrogen bond in the crystal
cell packing graphs of complexes 6b and 6c, with the exception of
Scheme 1 Synthetic route for complexes 6a–6c.
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◦
˚
Fig. 1 Molecular structure (ellipsoids at 30% probability level) of 6a, 6b and 6c. Selected distances (A) and angles ( ) for 6a: Fe–Fe, 2.5127(8),
Fe–S, 2.2391(8)–2.2535(9), Fe–C, 1.784(3)–1.808(3), S–C, 1.818(3)–1.825(3), N–C, 1.439(3)–1.472(3), C(8)–N–C(9), 112.4(2), C(7)–N–C(9), 109.45(19),
C(8)–N–C(7), 112.3(2); for 6b: Fe–Fe, 2.5126(6); Fe–S, 2.2476(8)–2.2505(9); Fe–C, 1.784(3)–1.813(4); S–C, 1.820(3)–1.825(3); N–C, 1.441(4)–1.477(4);
C(8)–N–C(7), 112.0(3); C(8)–N–C(9), 110.3(3); C(7)–N–C(9), 109.9(3); for 6c: Fe–Fe, 2.5042(13); Fe–S, 2.2448(15)–2.2571(15); Fe–C, 1.760(5)–1.811(5);
S–C, 1.808(4)–1.826(4); N–C, 1.441(5)–1.473(5); C(8)–N–C(7), 113.6(4); C(8)–N–C(9), 113.8(3); C(7)–N–C(9), 112.8(3).
complex 6a. In the crystal of complex 6a, there is one molecule
in the asymmetric unit and inversion-related molecules are linked
to form dimers by C–H· · ·O hydrogen bonds between the alpha-
position H13A atom of the furan and an adjacent carbonyl O6
atom at (−x, 1 − y, −z) with C13· · ·O6 3.485(4), H13A· · ·O6
For complexes 6a, 6b and 6c, the role of the protonated nitrogen
functionality is probably twofold. Above all, the reductive poten-
tial of the system should be reduced, simply by the introduced
positive charge. In addition, the closeness of the proton on the
nitrogen to the iron atoms allows for proton coupled electron
transfer,27 which is likely to reduce the potential.28 The role of the
aromatic substituents in the complexes such as 6a and 6b is open
to speculation. It could form a hydrogen bond to the proton on
the nitrogen with the heteroatom21, as shown in Fig. 3. This kind
of hydrogen bond should be fairly weak, with a bond energy of a
few kcal mol−1, but such a bond could still be sufficient to help in
fine-tuning the reductive potential of the system.
◦
˚
2.56 A and C13–H13A· · ·O6 is 173 .
Protonation
The protonation of complexes 6a, 6b and 6c showed similar
behaviour. When the protonation occurred under the effect of
excess HClO4 in CH3CN, the color of the solution changed
from red to orange. The protonation products in CH3CN were
1
characterized by H NMR and IR correspondingly as shown in
Electrochemistry
Fig. 2. With regard to complexes 6a, 6b and 6c, there was no peak
at d < 0 ppm in the 1H NMR, indicating that the protonation was
occurring on the N-bridged atom rather than the formation of l-
H. The peaks of the thiophene proton and the methylene proton
between thiophene and the N-bridged atom were generally shifted
to a lower magnetic field than the parent complexes. With the
exception of the methylene (–NCH2S–) peaks in the proximity of
the ADT nitrogen (–NCH2S–), these were split from 3.34 into 4.20
and 3.27 ppm in a ratio of 1 : 1 for complex 6b, with an incomplete
split but a lower magnetic field shift from 3.32 to 3.72 ppm for
corresponding peaks in complex 6c, and also with an incomplete
split from 3.38 to 3.66 ppm for 6a. The signal at d 5.37 ppm
for complex 6b and 7.20 ppm for complex 6c were derived from
excess HClO4, which was difficult to eliminate even after repeatedly
washing the protonation product with solvent.
As expected, the m(CO) IR bands shifted when the protonation
on N-bridged atom occurred,11,17,20 and there were suitable shifts
of 10–17 cm−1 for complex 6a from 2073, 2031, 1995 cm−1 to
2087, 2048, 2011 cm−1, from 2074, 2030, 1995 cm−1 to 2084, 2040,
2011 cm−1 for complex 6b and from 2072, 2031, 1994 cm−1 to
2086, 2046, 2009 cm−1 for complex 6c. The spectral data further
indicated that protonation occurred on the bridging N atom.
The iron core plays a vital role in the formation of Fe–H
hydride and the process of H2 evolution. Thus, the electrochemical
character of 6a, 6b and 6c was investigated by cyclic voltam-
mograms (CVs). The irreversible oxidation peaks in CVs for
complexes 6a, 6b and 6c appeared at +0.71, +0.65 and +0.72 V
(vs Fc/Fc+) for the FeIFeI/FeIIFeI couple, consistent with the
reported description.9 In addition, coulometry was performed
to establish that one electron was transferred in the irreversible
process. Quasi-reversible reduction peaks were observed at −1.55,
−1.64 and −1.54V (vs Fc/Fc+), respectively, for the FeI FeI/FeIFe0
couple (Table 2). The oxidation potential of 6b was less positive
than that of 6a and 6c, indicating that it was easier to oxidize.
The reduction potential of 6b was more negative than 6a and 6c
by around 100 mV, identifying a decreased reduction capability.
These observations suggested that the methane group near the
heterocycle may increase the electron density in the p system
by donating electrons by hyperconjugation (r conjugation), and
slightly decrease the nuclear electron density of the Fe through the
three atoms (N, C, S). Such an effect can be increased by the oxygen
atom in furan and bromine atom on the thiophene, resulting in
less negative reduction potentials. Although the heterocycle is far
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Fig. 2 1H NMR and IR of the protonation of 6a, 6b and 6c.
The catalytic proton reduction of complexes 6a, 6b and 6c
was investigated through cyclic voltammetry on addition of
HClO4 in CH3CN under CO, with the concentration of 0–
8 mM, respectively (Fig. 4). The voltammograms of 6a–6c with
1 equiv. of HClO4 showed new reduction peaks more positive
than those of the FeIFeI/FeIFe0 couple, which indicated the
catalytic proton reduction.12 The second reduction peak still
indicated the presence of FeIFeI/FeIFe0 couple. With increasing
concentration of acid, the current intensity of the first reduction
peak increased notably, and the potential shifted toward more
negative potential, as expected for catalytic proton reduction.29
When the acid concentration reached over 4 mM, the new
peak showed a sharp negative shift, and the potential change
was great. The reason cannot be explained at present, although
Fig. 3 The possible conformation of complexes 6a, 6b and 6c after
protonation.
from the 2Fe2S center, it does have a considerable effect on the
redox potential.
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Table 2 The redox potentials of 6a–6c in MeCNa
Complex
FeIFeI/FeIIFeI (V)
+0.71
FeIFeI/FeIFe0 (V)
6a
6b
6c
−1.55
−1.33
−1.64
−1.36
−1.54
−1.33
+0.65
+0.72
a nBu4NPF6 (0.1 M in CH3CN; scan rate: 0.1 V s−1; working electrode:
glassy carbon electrode of diameter 3 mm; reference electrode: non-
aqueous Ag/Ag+ electrode (0.01 M AgNO3 in CH3CN); counter electrode:
platinum wire.
similar observations have been recorded in the literature.30,31
Moreover, the reduction potential of the FeIFeI/FeIFe0 couple
shows negative shifts (ca. 20, 60 and 30 mV) for 6a–6c. Bubbles
could be observed on the surface of the glassy carbon disk
electrode (surface area: 1.17 cm2) during the recording of the
electrolysis with controlled potentials at −1.2, −1.3 and −1.2 V,
respectively, for the three complexes, which were identified as H2
by GC. H2 evolution as a result of bulk electrolysis has been
reported recently.11–13,31 The isolated protonated product, identified
1
by H NMR and IR spectroscopy, showed that the protonation
of the complexes occurred on the bridging nitrogen atom first,
suggesting a chemical–electrochemical–chemical–electrochemical
(CECE) process.
In contrast to the redox behavior of the furan-containing
complex 6a, where the catalytic electrochemical reduction of the
proton was at −1.13 V (vs Fc/Fc+),21 the thiophene-containing
analogues 6b and 6c exhibited catalytic reduction potential at
−1.20 and −1.09 V, respectively, as shown in Fig. 4. It seemed
that the introduction of a bromine atom did have a favourable
effect on the acid catalyzing proton reduction. To the best of our
knowledge, −1.09 V was the most positive potential recorded
for 2Fe2S-hexacarbonyl model compounds, which was 40 mV
lower than that of complex 6a and 90 mV lower than that of
[{(l-SCH2)2NCH2(p-BrC6H5)}Fe2(CO)6].11 Since these potentials
are getting closer to those of the natural hydrogenases and also
coming into a range where reduction by an excited photosensitizer
of Ru(bpy)3-type is becoming possible, even small improvements
are important.
Experimental
Fig. 4 Cyclic voltammograms of 6a, 6b and 6c with HClO4 (0–8 mM) for
the catalytic proton reduction. Sample concentration is 1.0 mmol in 0.05 M
n-Bu4NPF6–CH3CN, scan rate 100 mV s−1, working electrode: glassy
carbon, reference electrode: Ag/Ag+. All potentials are versus Fc/Fc+.
Reagents and instruments
The reactions and operations involving organometallic complexes
were carried out under a dry, oxygen-free nitrogen atmosphere.
THF was dried and distilled by standard methods. Formaldehyde
solution was analytical pure, and NBS was recrystallized in dis-
tilled water at room temperature. Commercially available reagents,
2-thiophenemethanamine, 5-bromothiophene and [Fe(CO)5] were
used without further purification. The reagent LiEt3BH was pur-
chased from Aldrich. [(l-HS)2Fe2(CO)6] and [(l-LiS)2]Fe2(CO)6]
were prepared according to literature procedures.11,13 IR spectra
were recorded on a JASCO FT/IR 430 spectrophotometer. Proton
and 13C NMR spectra were collected on a Varian INOVA 400
NMR spectrometer. Elemental analyses were performed on a
Carlo Erba MOD-1106 elemental analyzer. Diffraction measure-
ments were made on a Siemens SMART CCD diffractometer using
˚
graphite monochromated Mo Ka radiation (k = 0.71073 A). The
crystal data and parameters for data collection and refinement
are listed in Table 3. CCDC reference numbers 245082, 266225
and 276211. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b615037c
Synthesis of [{(l-SCH2)2NCH2(2-C4H3O)}Fe2(CO)6] (6a).
A
solution of 3a (1.57 g, 10 mmol), prepared from 2a (0.97 g,
900 | Dalton Trans., 2007, 896–902
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Table 3 Crystal data for complexes 6a–6c
6a
6b
6c
Formula
Mw
C13H9Fe2 NO7S2
467.03
Triclinic
P-1
7.5684(15)
10.878(2)
12.426(3)
111.59(3)
96.93(3)
108.30(3)
870.2(3)
2
C13H9Fe2 NO6S3
483.09
Monoclinic
C2/c
25.2878(17)
7.5720(5)
22.6997(15)
90.00
121.103(10)
90.00
C13H8BrFe2NO6S3
561.99
Crystal system
Space group
Monoclinic
P2(1)/c
9.160(4)
18.101(8)
12.148(6)
90.00
109.265(8)
90.00
1901.3(15)
4
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
3721.7(4)
8
1.724
Z
qcalcd/g m−3
1.782
1.963
T/K
273(2)
1.941
273(2)
1.923
273(2)
3.986
l/mm−1
F[000]
468
7286/3804
3361
226
1.027
0.0335
0.0935
1936
11269/4356
3514
226
1.041
0.0427
0.1284
1104
11402/4300
2402
235
0.790
0.0433
0.0747
Reflns collected/unique
Feflns observed [I > 2r(I)]
Parameters
GOF (on F2)
R1 [I > 2r(I)]
wR2 [I > 2r(I)]
10 mmol) and HCHO–H2O (2 ml, 20 mmol) in THF (20 ml)
was added to a THF solution (20 ml) of 5, freshly derived from
[(l-S)2Fe2(CO)6] (4) (0.344 g, 1.0 mmol), reacted with LiEt3BH
(1 M in THF, 1.0 ml, 1.0 mmol) and F3CCO2H (0.076 ml,
1.0 mmol) at −78 ◦C. The mixture was stirred for 1 h at −78 ◦C,
then at room temperature for 2 h. During the course of the
reaction, the color of the solution turned dark red. After evapo-
ration of the solvent, the crude product was purified by column
chromatography with silica gel using hexane as the eluent to give
complex 6a as a red solid (0.19 g, 39%). Recrystallization in hexane
afforded crystals of 6a suitable for X-ray crystallographic study.
ESI-MS: m/z: 467.9 [M + H]+; IR m(CO): 2073, 2031, 1995 cm−1;
1H NMR (CD3CN) d 7.39 (s, 1H), 6.32 (s, 1H), 6.19 (s, 1H), 3.74
(s, 2H), 3.38 (s, 4H) ppm; 13C NMR (CDCl3) d 207.9, 150.3, 142.9,
119.5, 109.0, 54.4, 52.5 ppm. Anal. Calcd for C13H9NO7S2Fe2:
C 33.43, H, 1.94, N 3.00. Found: C, 33.44; H, 1.97; N, 3.08%.
After evaporation of the solvent, the crude product was purified by
column chromatography with silica gel using hexane as the eluent
to give complex 6b as a red solid (0.18 g, 37%). Recrystallization in
hexane afforded crystals suitable for X-ray crystallographic study.
ESI-MS: m/z 483.9 [M + H]+; IR m(CO) 2073, 2031, 1994 cm−1;
1H NMR (CD3CN) d 7.26 (s, 1H), 6.94 (s, 1H), 6.86 (s, 1H), 3.91
(s, 2H), 3.34 (s, 4H) ppm; 13C NMR (CDCl3) d 207.9, 139.1, 127.0,
126.1, 77.2, 56.5, 52.3 ppm. Anal. Calcd for C13H9NO6S3Fe2: C
32.32, H, 1.88, N 2.90. Found: C, 32.38; H, 2.05; N, 2.88%.
Protonation of [{(l-SCH2)2NCH2(2-C4H3S)}Fe2(CO)6] (6b).
To a solution of complex 6b (100 mg) in CHCl3 (3 ml) was added
HClO4 (8 lL). After the solution was stirred for 10 min, the red
solution turned to a lighter red and a yellow-orange solid was
precipitated. The solvent was decanted and the solid was washed
three times with hexane and dried in vacuo. The N-protonated
product was obtained in 62% yield (62 mg). IR m(CO) 2084, 2040,
2011 cm−1. 1H NMR (CD3CN): d 7.65 (d, J = 5.2 Hz, 1H), 7.29
(d, J = 3.2 Hz, 1H), 7.14 (t, J = 4.0 Hz, 1H), 4.57 (s, 2H), 4.20 (s,
2H), 3.27 (s, 2H) ppm.
Protonation of complex 6a. To a solution of complex 6a
(100 mg) in CHCl3 (3 ml) was added 8 lL HClO4. The solution was
stirred for 10 min. The red solution turned to a lighter red and a
yellow-orange solid was precipitated. The solvent was decanted
and the solid was washed three times with hexane and dried
in vacuo. The N-protonated product was obtained in 65% yield
(65 mg). MS (API-ES): m/z 467.9 (M+); IR m(CO) : 2087, 2048,
2011 cm−1. 1H NMR (CD3CN): d 7.62 (s, 1H), 6.70 (s, 1H), 6.50
(s, 1H), 4.36 (s, 2H), 3.66 (d, J = 3.6 Hz, 4H) ppm.
Synthesis and protonation of [{(l-SCH2)2NCH2(2-(5-Br–
C4H2S)Fe2(CO)6)} (6c). (5-Bromothiophene-2-yl)methanamine
(3.4 g, 18 mmol) was prepared from the commercial avail-
able product 5-bromothiophene (24.5 g, 151 mmol) reacting
with HCHO–H2O (12.4 g, 413 mmol) and ammonium chloride
(11.2 g, 211 mmol) in acetic acid (5 g, 5 ml) at 70 ◦C for
3 h. A solution of di-(hydroxymethyl)-(5-bromothiophene-2-yl)
anamine (2.52 g, 10 mmol), prepared from 2-aminomethyl-5-
bromothiophene (1.91 g, 10 mmol) and HCHO–H2O (2 ml,
20 mmol) in THF (20 ml), was added to a THF solution (20 ml)
of [H2S2Fe2(CO)6], freshly derived from [(l-S2) Fe2(CO)6] (0.344 g,
1 mmol), LiEt3BH (1.0 M in T◦HF 20 ml, 1.0 mmol) and F3CCO2H
(0.076 ml, 1.0 mmol) at −78 C. The mixture was stirred for 1 h
Synthesis of [{(l-SCH2)2NCH2(2-C4H3S)}Fe2(CO)6] (6b).
A
solution of di-(hydroxymethyl)-thiopheneanamine (1.73 g,
10 mmol), prepared from 2-aminomethyl thiophene (1.13 g,
10 mmol) and HCHO–H2O (2 ml, 20 mmol) in THF (20 ml), was
added to a THF solution (20 ml) of [(l-HS)2Fe2(CO)6], freshly
derived from [(l-S2) Fe2(CO)6] (0.344 g, 1 mmol), LiEt3BH (1.0 M
in THF 20 ml, 1.0 mmol) and F3CCO2H (0.076 ml, 1.0 mmol) at
◦
◦
◦
−78 C. The mixture was stirred for 1 h at −78 C, then 2 h at
at −78 C, then 2 h at room temperature, until the color of the
room temperature, until the color of the solution turned dark red.
solution turned dark red. After evaporation of the solvent, the
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crude product was purified by column chromatography with silica
gel using hexane as the eluent to give complex 6c as a red solid
(0.19 g, 35%). ESI-MS: m/z: 561.8 [M + H]+; IR m(CO): 2072, 2031,
1994 cm−1; 1H NMR (CDCl3) d 6.88 (s, 1H), 6.62 (s, 1H), 3.82 (s,
2H), 3.32(s, 4H) ppm; 13C NMR (CDCl3) d 207.8, 141.0, 129.8,
127.2, 77.2, 56.7, 52.1 ppm; Anal. Calcd for C13H8BrFe2NO6S3:
C 27.78, H, 1.43, N 2.49. Found: C, 28.00; H, 1.59; N, 2.60%.
The orange solid N-protonated product of 6c from 6c (100 mg,
0.18 mmol) was obtained in a similar manner to that of 6b (yield:
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We are grateful to the Ministry of Science and Technology of China
and the Chinese National Natural Science Foundation (Grant
No. 20633020) for financial support of this work. We would also
like to thank the Swedish Energy Agency, the Swedish Research
Council and the K & A Wallenberg Foundation for their financial
support. This research was also supported by the Programme of
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902 | Dalton Trans., 2007, 896–902
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The Royal Society of Chemistry 2007
©