Synthesis and characterization of a series of model complexes of the active site of [Fe]-hydrogenase (Hmd)

Article abstract of DOI:10.1021/ic200580z

A series of Fe complexes were synthesized and characterized as small molecule mimics for the active site of [Fe]-hydrogenase (Hmd). The collection includes both structurally new compounds and analogues of previously reported models. These complexes contain the essential ligands of the enzyme, namely, acyl, CO, pyridone, and sulfur ligands. They serve as IR and Moessbauer spectroscopic models for the Fe center in [Fe]-hydrogenase. The field-dependent Moessbauer study of representative model complexes shows that the sign and absolute value of the quadrupole splitting are sensitive to the change in the ligand environment of the Fe center.

Full text of DOI:10.1021/ic200580z

ARTICLE
pubs.acs.org/IC
Synthesis and Characterization of a Series of Model Complexes
of the Active Site of [Fe]-Hydrogenase (Hmd)
Dafa Chen,† Annegret Ahrens-Botzong,‡ Volker Sch€unemann,‡ Rosario Scopelliti,† and Xile Hu*,†
†Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fꢀedꢀerale de
Lausanne (EPFL), SB-ISIC-LSCI, BCH 3305, Lausanne CH 1015, Switzerland
‡Fachbereich Physik, Technische Universit€at Kaiserslautern, D-67663 Kaiserslautern, Germany
S Supporting Information
b
ABSTRACT: A series of Fe complexes were synthesized and characterized as small
molecule mimics for the active site of [Fe]-hydrogenase (Hmd). The collection
includes both structurally new compounds and analogues of previously reported
models. These complexes contain the essential ligands of the enzyme, namely, acyl,
CO, pyridone, and sulfur ligands. They serve as IR and M€ossbauer spectroscopic
models for the Fe center in [Fe]-hydrogenase. The field-dependent M€ossbauer
study of representative model complexes shows that the sign and absolute value of
the quadrupole splitting are sensitive to the change in the ligand environment of
the Fe center.
’ INTRODUCTION
Hydrogenases are enzymes that catalyze the production and
utilization of hydrogen. Three classes of phylogenetically un-
related hydrogenases, namely, the [FeFe]-, [FeNi]-, and [Fe]-
hydrogenases, are now known.1ꢀ3 The [Fe]-hydrogenase, also
termed as H2-forming methylene-tetrahydromethanopterin de-
hydrogenase (Hmd), catalyzes the reduction of methenyl-tetra-
hydromethanopterin (methenyl-H4MPTþ) with H2 to form
methylene-tetrahydromethanopterin (methylene-H4MPT) and
Hþ (Figure 1), an intermediary step in the reduction of CO2 to
methane by methanogens grown under nickel-limiting
conditions.4 Recent studies reveal several unique features of this
enzyme. Unlike [FeFe]- and [FeNi] hydrogenases,5ꢀ9 [Fe]-
hydrogenase does not contain any FeꢀS clusters and requires
only one metal (Fe) for function.2 The Fe center in [Fe]-
hydrogenase, however, has a similar electronic structure (low-
spin) and is ligated by similar ligands (sulfur and CO) as the
distal Fe centers in [FeFe] and [FeNi]-hydrogenases.2
Figure 1. Hydride transfer reaction catalyzed by [Fe]-hydrogenase.
The composition and structure of the Fe-containing active site
have been elucidated, albeit with some uncertainty, using a wide
range of spectroscopic and crystallographic methods.10ꢀ19 The
current model suggests that the Fe ion is coordinated to two cis-
CO ligands, a cysteine sulfur atom, a bidentate pyridone mole-
cule through its nitrogen and acyl carbon atoms, and a yet
unidentified ligand (Figure 2).15 Because it is a recent discovery,
only a few small molecule mimics of [Fe]-hydrogenase have been
reported.3,20ꢀ30 Such model compounds would be useful in
studying the properties and catalytic activity of this interesting
Figure 2. Proposed active site of [Fe]-hydrogenase.
enzyme. We recently communicated the synthesis of several
structural mimics for the active site of [Fe]-hydrogenase, includ-
ing those reproducing the first coordination sphere of [Fe]-
hydrogenase.27ꢀ29 Here we report the synthesis and reactivity of
Received: March 22, 2011
Published: May 03, 2011
r
2011 American Chemical Society
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Figure 3. Synthesis of iron bis(carbonyl) pyridonate complexes.
Figure 5. Synthesis of iron bis(carbonyl) pyridonate thiolate complexes.
Figure 4. Solid-state structure of 3. The thermal ellipsoids are displayed
in 50% probability. Selected bond distances (Å) and angles (deg):
Fe1ꢀN1, 2.024(2); Fe1ꢀO1, 1.996(2); Fe1ꢀP1, 2.2828(8); Fe1ꢀ
C30, 1.793(3); Fe1ꢀC31, 1.796(3); Fe1ꢀI1, 2.6691(5); C30ꢀO2,
1.137(3); C31ꢀO3, 1.133(4); C5ꢀO1, 1.299(3); C5ꢀN1, 1.357(4);
C1ꢀN1, 1.357(4); C30ꢀFe1ꢀC31, 91.93(13); O1ꢀFe1ꢀN1,
66.08(9). A phenyl group in the PPh3 is disordered and modeled over
two sites. For clarity, only one site is shown.
Figure 6. Solid-state molecular structure of complex 7. The thermal
ellipsoids are displayed at 50% probability. Selected bond distances (Å)
and angles (deg): Fe1ꢀN1, 1.998(3); Fe1ꢀO3, 2.047(3); Fe1ꢀP1,
2.2778(11); Fe1ꢀC1, 1.788(4); Fe1ꢀC2, 1.773(4); Fe1ꢀS1,
2.3444(11); C1ꢀO1, 1.143(5); C2ꢀO2, 1.143(5); C25ꢀO3, 1.312(5);
C25ꢀN1, 1.354(5); C1ꢀFe1ꢀC2, 89.35(18); O3ꢀFe1ꢀN1, 65.80(12).
some additional Fe model complexes, as well as M€ossbauer
studies on representative structural models.
’ RESULTS AND DISCUSSION
1. Synthesis and Reactivity of Iron Pyridonate Com-
plexes. 1.1. Synthesis and Structure of Iron Pyridonate Iodide
Complexes. We initially used simple pyridonate ligands to mimic
the pyridone cofactor found in [Fe]-hydrogenase. We showed
earlier that reaction of Fe(CO)3(PPh3)I2 with sodium 6-methyl-
2-pyridonate (PyO1) yielded Fe(CO)2(PPh3)I(PyO1) (1).27
Analogous reactions produced several other iron bis(carbonyl)
pyridonate derivatives 2ꢀ4 (Figure 3). Complexes 2ꢀ4 are
characterized by IR, NMR, elemental analysis, and X-ray
crystallography.31 The two CO ligands are mutually cis, and
the pyridonate ligand binds Fe in a η2-κ-N,O fashion in all
complexes. The Fe-PyO fragment is best described by a mixture
of two resonance forms, in which PyO exists in either deproto-
nated pyridinol or deprotonated pyridone form. All Fe-ligand
and CꢀO bond distances are comparable in complexes 1ꢀ4. A
structural drawing of 3 is shown in Figure 4; those of 2 and 4 can
be found in the Supporting Information.
Figure 7. Synthesis of iron bis(carbonyl) pyridyl-2-thiolate complexes.
sought to install sulfur ligands to mimic the cysteine ligand of
[Fe]-hydrogenase. Complex 1 reacted with sodium aryl thiolate
to give Fe(CO)2(PPh3)(PyO1)SAr (Ar = 2,6-Me2C6H3 (5),
2-iPrC6H4 (6), 2,4,6-Me3C6H2 (7), Figure 5). The synthesis of 5
was briefly communicated, but no single crystals could be
obtained, precluding its structural confirmation.27 Single crystals
suitable for X-ray diffraction study, however, can be obtained for
complexes 6 and 7. No stable species could be isolated from the
reactions of 2ꢀ4 with ArSNa (Ar = 2,6-Me2C6H3, 2-iPrC6H4,
2,4,6-Me3C6H2).
1.2. Synthesis and Structure of Iron Pyridonate Thiolate
Complexes. With Fe pyridonate iodide complexes in hand, we
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Figure 8. Synthesis of complexes 10a and its isomerization to 10b.
Despite many attempts, we were not able to replace the I anion
in 8 and 9 with another anionic ligand. Abstraction of Iꢀ by Ag or
Tl cation did not lead to a stable compound. Reaction of 9 with
TlPF6 in the presence of PPh3 gave the bis(phosphine) complex
11 (Figure 9). The structure of 11 was confirmed by X-ray
crystallography.31
Interestingly, a different reactivity was observed with the iso-
cyanide substituted precursor Fe(CO)3(2,6-dimethyl-PhNC)I2
(12). Reaction of 12 with sodium pyridyl-2-thiolate gave com-
plex 13 (Figure 10). The structure of 13 is shown in Figure 11.
The Fe ion is coordinated to two cis-carbonyls, one iodide anion,
the isocyanide ligand, and an unexpected acyl, thiolate chelate.
The latter ligand could be considered as the product of CO
migration into the pyridyl-2-thiolate ligand. Presumably, the
reaction first yielded the substitution product 13* and one
molecule of CO, which then inserted into the nucleophilic FeꢀN
bond to give the 13 (Figure 10). Consistent with this hypothesis,
the yield of 13 was higher when the reaction was carried out in
the presence of CO.
3. IR Spectra of Model Complexes. Complexes 1ꢀ11, 13 all
display two strong IR absorption bands between 2050 and
1950 cmꢀ1, arising from the two terminal CO ligands
(Table 1). The intensities of the two bands are similar, indicating
that the two CO ligands bind in a cis-fashion, with a COꢀFeꢀCO
angle of near 90 °C. This result is consistent with the crystal
structures of the complexes. The averaged CO vibration fre-
quencies of all FeII model complexes are within the range found
for monomeric FeII bis(carbonyl) complexes.21,23 Substitution of
PPh3 by PEt3 increases the electron density at the Fe center, and
lowers the averaged CO stretching frequency for about 10 cmꢀ1
(1 vs 4). The substituent at the 6-position of the pyridone ligand
has a small but noticeable effect in the CO stretching frequencies
(compare 1ꢀ3). Substitution of I by SAr also makes the Fe
center more electron rich and lowers CO stretching frequencies
for about 10 cmꢀ1 (1 vs 6 and 7). The pyridyl-2-thiolate ligand is
slightly more electron rich than the pyridonate ligand (1 vs 9, and
2 vs 8), lowering the IR frequency of about 10 cmꢀ1. As expected,
the νCO in the cationic complex 11 is much higher than in the
neutral complex 9. Compared to [Fe]-hydrogenase and our
previouslyreportedFeacylcomplexes(e.g., 15ꢀ17, Figure12),28
the Fe centers in the current model complexes are slightly less
electron rich.
Figure 9. Synthesis of iron complex 11.
Complexes 6 and 7 exhibit the same “Fe(CO)2(PR3)(PyO)”
core with similar bond lengths and angles as complexes 1ꢀ4.
There is insignificant change in the CO bond distances upon
substitution of I with SAr, even though IR data suggest a higher
degree of metal-to-ligand π-backbonding in 6 and 7 (see below).
The thiolate ligands coordinate to only one Fe center in spite of
their tendency to form bridging dimers with Fe carbonyl com-
plexes. A structural drawing of 7 is shown in Figure 6; that of 6
can be found in the Supporting Information.
We found that a phosphine ligand is essential for the isolation
and stability of model complexes 1ꢀ7. Attempts to substitute the
phosphine ligand by another N donor ligand were unsuccessful.
Complexes 1, 6, and 7 do not react with H2 nor with CO.
Complexes 6 and 7 decompose rapidly (in hours) in solution at
room temperature.
2. Synthesis and Reactivity of Iron Complexes Containing
Pyridyl-2-thiolate Ligands. Parallel to the work with pyridone
ligands, we also employed pyridyl-2-thiolate ligands to mimic the
N,S ligands found in the active site of [Fe]-hydrogenase. Reac-
tion of Fe(CO)3(PPh3)I2 with sodium 6-R0-pyridyl-2-thiolate
(R0 = Me, H) yielded Fe(CO)2(PPh3)I(R0-PyS) (8 and 9,
Figure 7). In both complexes, the Iꢀ and PPh3 ligands are
mutually trans. The synthesis and structure of compound 8 were
recently reported by Darensbourg et al.23
When monitoring the reaction of Fe(CO)3(PPh3)I2 with
sodium 6-Me-pyridyl-2-thiolate by 31P NMR, we found that
besides the signal at 73.2 ppm which belongs to 9, there was
another weak peak at 52.3 ppm. Attempts to isolate this minor
species were unsuccessful. However, in the analogous reaction of
6-OMe-pyridyl-2-thiolate with Fe(CO)3(PPh3)I2 (Figure 8),
after 0.5 h in ether, cis-(I, PPh3)-Fe(CO)2(PPh3)I(OMe-PyS)
(10a) was isolated as the product. The 31P NMR spectrum of 10a
shows one signal at 51.1 ppm. 10a isomerizes to trans-(I, PPh3)-
Fe(CO)2(PPh3)I(OMe-PyS) (10b) quantitatively in solution
overnight. The X-ray structures of 10a and 10b are shown in the
Supporting Information. The 31P NMR spectrum of 10b shows
only one signal at 75.3 ppm, comparable to the 31P signal of 8 and
9. On the basis of these observations, the minor species observed
in the synthesis of 9 should be its isomer, cis-(I, PPh3)-Fe(CO)2-
(PPh3)I(Me-PyS). It appears that in all these complexes, the
trans-isomers are thermodynamically more stable.
€
4. Mossbauer Studies of Representative Model Com-
plexes. One of the most useful tools to study the electronic
structureof Fe compounds is 57Fe M€ossbauer spectroscopy.32ꢀ34
Field-dependent M€ossbauer studies of [Fe]-hydrogenase found
an isomeric shift of δ = þ0.06 mm sꢀ1 and a positive quadrupole
splitting (ΔEQ= þ0.65 mm sꢀ1) for the diamagnetic Fe center.12
Very few M€ossbauer studies have been conducted on model
complexes,21,23,27 and we earlier reported so far the only
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Figure 10. Synthesis of iron complex 13.
Table 1. Selected Infrared Data
complex
νCO (cmꢀ1
)
complex
νCO (cmꢀ1
)
1a
2
2032, 1987
2039, 1988
2035, 2001
2026, 1974
2021, 1967
2025, 1985
2021, 1973
2028, 1986
2018, 1968
2031, 1980
10b
2024, 1977
2050, 1989
2041, 2007
2016, 1968
2003, 1931
1998, 1932
2011, 1944
2020, 1981
2020, 1956
2031, 1972
11
3
13
15b
16b
17b
4
5a
6
7
Hmdc
8
Hmd þ COc
Hmd þ KCNc
Hmd cofactorc
9
10a
a Data from ref 27. b Data from ref 28. c data from ref 11.
Figure 11. Solid-state molecular structure of complex 13. The thermal
ellipsoids are displayed at 50% probability. Selected bond distances (Å)
and angles (deg): Fe1ꢀC6, 1.941(5); Fe1ꢀC7, 1.891(8); Fe1ꢀC8,
1.827(7); Fe1ꢀC9, 1.879(6); Fe1ꢀS1, 2.2940(17); Fe1ꢀI2,
2.6461(9); C6ꢀO1, 1.144(7); C7ꢀO2, 0.989(8); C8ꢀO3, 1.059(8);
C9ꢀN2, 1.155(7).
Information, Figure S10). The major component corresponds
to complex 17 while the minor component is attributed to a high-
spin FeII impurity (S = 2). This minor component is not
observable under a magnetic field, probably because of detection
limits. The diamagnetic ground state is found for all complexes.
Table 2 lists the M€ossbauer parameters of these 8 complexes as
well as those of 1. Figures 13ꢀ16 show the M€ossbauer spectra of
complexes 7, 14, 15, and 16. The spectra of the other complexes
can be found in the Supporting Information.
field-dependent M€ossbauer study of a model complex, 1.27 Field-
dependent study is required to determine the sign of the
quadrupole splitting and to confirm the diamagnetic ground
state of the Fe center. Through the current and earlier synthetic
efforts, we have access to a large number of Fe model complexes.
This prompts us to carry out a comprehensive M€ossabuer study
on representative model complexes. These complexes are shown
in Figure 12.
Two measurements were taken for each of the 8 complexes (5,
7, 9, 13, 14ꢀ17). One is taken at 55 K, 77 or 100 K in the absence
of an external field, and one is taken at 5 K with an external field of
5 T perpendicular to the γ-ray. All samples show one component,
except the sample for 17 which shows two components of 94%
and 6% abundance at zero field and 77 K (Supporting
The isomeric shifts (δ) for all Fe model complexes shown in
Figure 12 fall into a small range between 0.10 and ꢀ0.02 mm sꢀ1
.
The δ-values of model complexes are comparable to those found
for [Fe]-hydrogenase12 and its CO and CN-inhibited forms, and
those of other FeII model complexes.21,23 Thus, the low-spin Fe
bis(carbonyl) core in these complexes has a major influence on
their isomeric shifts. Nevertheless, examination of the δ-values of
this series of related complexes reveals the smaller but observable
influence of other ligands. Substitution of I with thiolate ligands
resulted in a small decrease in the isomeric shifts (compare 1 with
5 and 7, Table 2). Substitution of I with acyl ligand resulted in an
even larger reduction in the isomeric shift (compare 9 and 16,
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Figure 12. Fe model complexes for the M€ossbauer study; PPN = bis(triphenylphosphine)iminium.
Table 2. Mo€ssbauer Parameters for Model Complexes and [Fe]-Hydrogenase
complex
T/K
B/T
rel. contribution %
δ/mmsꢀ1
ΔEQ/mmsꢀ1
Γ/mmsꢀ1
η
1a
5
55
5
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
94
0.10 ((0.02)
0.10 ((0.02)
0.06 ((0.02)
0.04 ((0.02)
0.04 ((0.02)
0.06 ((0.02)
0.10 ((0.01)
0.11 ((0.02)
0.01 ((0.02)
0.01 ((0.02)
0.06 ((0.01)
0.06 ((0.02)
(þ) 0.48 ((0.02)
(þ) 0.48 ((0.02)
(ꢀ) 0.83 ((0.02)
ꢀ0.87 ((0.03)
(ꢀ) 0.84 ((0.05)
ꢀ0.89 ((0.02)
(ꢀ) 0.35 ((0.03)
ꢀ0.36 ((0.03)
(þ) 0.29 ((0.02)
þ 0.29 ((0.04)
(ꢀ) 0.74 ((0.02)
ꢀ0.67 ((0.02)
(þ) 0.73 ((0.02)
þ 0.72 ((0.02)
(ꢀ) 1.14 ((0.02)
ꢀ1.14 ((0.04)
(þ) 0.89 ((0.02)
3.16 ((0.02)
þ 0.91 ((0.03)
þ0.65
0.57 ((0.03)
0.40 ((0.03)
0.58 ((0.02)
0.44 ((0.02)
0.49 ((0.02)
0.42 ((0.02)
0.45 ((0.05)
0.40 ((0.02)
0.45 ((0.05)
0.47 ((0.03)
0.38 ((0.02)
0.44 ((0.02)
0.36 ((0.02)
0.43 ((0.02)
0.36 ((0.02)
0.40 ((0.02)
0.57 ((0.02)
0.35 ((0.02)
0.47 ((0.02)
0.32
5
5
5
5
5
5
5
5
0
100
5
0.6 ((0.2)
0.2 ((0.2)
7
100
5
9
77
5
0 ((0.5)
13
14
15
16
17
77
5
0.6 ((0.4)
0.4 ((0.1)
0.5 ((0.1)
0.4 ((0.1)
77
5
100
5
0 ((0.02)
0.01 ((0.01)
0 ((0.02)
0 ((0.02)
77
5
77
ꢀ0.02 ((0.02)
0.98 ((0.02)
ꢀ0.01 ((0.02)
0.06
6
5
80
4.2
80
4.2
80
4.2
80
5
4
4
4
100
100
100
99
0.1 ((0.1)
0.3 ((0.2)
0.5 ((0.1)
0.6 ((0.1)
Hmd (pH = 8.0)b
Hmd þ COb
0.06
þ0.65
0.32
ꢀ0.03
ꢀ0.03
ꢀ0.001
ꢀ0.001
0.03
ꢀ1.38
0.29
99
ꢀ1.38
0.29
Hmd þ KCNb
100
100
84
ꢀ1.75
0.38
ꢀ1.75
0.38
extracted Hmd cofactor (pH = 6.0)b
a Data from ref 27. b Data from ref 12.
0.43
0.31
Table 2). The presence of π-acceptor ligands (CNꢀ, and RNC)
reduces the isomeric shifts, so complexes 13, 15, and 17 have δ-
values at the low end. This is consistent with the results from the
M€ossbauer studies of the enzymes which showed that binding of
external CO or CNꢀ lowered the isomeric shifts of the Fe ion.12
Interestingly, PPh3 ligand appears to have a similar influence as
CNꢀ and RCN (compare 16 with 15 and 17, Table 2).
The small change in the isomeric shifts of the model com-
plexes can be rationalized considering the electronic structure of
these complexes. Isomeric shifts depend on the electron density
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Figure 13. (Top) M€ossbauer spectrum of 7 obtained at T = 100 K. The
solid line represents a fit assuming Lorentzian line shape with parameters
given in Table 2. (Bottom) M€ossbauer spectrum of 7 obtained at 5 K and
5 T. The solid line represents a simulation assuming a diamagnetic
ground state, with the parameters given in Table 2.
Figure 15. (Top) M€ossbauer spectrum of 15 obtained at T = 100 K.
The solid line represents a fit assuming Lorentzian line shape with
parameters given in Table 2. (Bottom) M€ossbauer spectrum of 15
obtained at 5 K and 5 T. The solid line represents a simulation assuming
a diamagnetic ground state, with the parameters given in Table 2.
Figure 14. (Top) M€ossbauer spectrum of 14 obtained at T = 77 K. The
solid line represents a fit assuming Lorentzian line shape with parameters
given in Table 2. (Bottom) M€ossbauer spectrum of 14 obtained at 5 K
and 5 T. The solid line represents a simulation assuming a diamagnetic
ground state, with the parameters given in Table 2.
Figure 16. (Top) M€ossbauer spectrum of 16 obtained at T = 77 K. The
solid line represents a fit assuming Lorentzian line shape with parameters
given in Table 2. (Bottom) M€ossbauer spectrum of 16 obtained at 5 K
and 5 T. The solid line represents a simulation assuming a diamagnetic
ground state, with the parameters given in Table 2.
at the nucleus, and almost exclusively the s electrons.32,34 For Fe
compounds, a higher s electron density at the nucleus results in a
lower (more negative) isomeric shift. The outer s electrons are
shielded by the valence d electrons. The 8 model complexes are
all low spin Fe(II), so they have the same electronic configura-
tions. Therefore, their isomeric shifts are similar. When the
π-donor ligand iodide in a complex is replaced by thiolate or
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acyl ligands, or when the complexes contain π-acceptor ligands,
there is a small decrease in the d electron density at the metal
center. Consequently, the shielding of the outer s electrons
decreases, and hence the isomeric shifts slightly decrease.
There is no clear correlation between the IR and M€ossbauer
parameters (Supporting Information, Figure S11). In some cases,
lower isomeric shifts are found for compounds having lower νCO
(compare 1 with 5 and 7, Tables 1 and 2). In other cases, different
isomeric shifts are found for compounds having very similar νCO
(compare 7 with 9). There are also cases where similar isomeric
shifts are found for compounds having different νCO (compare
15 with 16, and 13 with 15). Finally, there are cases when lower
isomeric shifts are found for compounds having higher νCO
(compare 9 with 13). The discrepancy between IR and
M€ossbauer parameters reflects the different origin of these
spectroscopic data. The νCO is an indication of the Fe-to-CO
π-backbonding, whereas the isomeric shift is an indication of the
s electron density at the 57Fe nucleus.
The quadrupole splitting ΔEQ varies significantly. Complexes
containing iodide ligands (1, 9, and 13) have lower absolute ΔEQ
values (ca. 0.3 to 0.5 mmsꢀ1); the other complexes have absolute
ΔEQ values between 0.7 and 1.1 mmsꢀ1. According to Table 2,
substitution of one ligand on a given complex results in a
sustainable change in the absolute values of ΔEQ and sometime
even the sign of the quadrupole splitting. For example, substitu-
tion of Iꢀ with a thiolate ligand (from 1 to 5 and 7) changes ΔEQ
from þ0.48 to about ꢀ0.85 mmsꢀ1. Likewise, substitution of the
isocyanide ligand in 15 with PPh3 (in 16) changes ΔEQ from
þ0.72 to ꢀ1.14 mmsꢀ1; further substitution with CNꢀ (in 17)
then changes ΔEQ to þ0.91 mmsꢀ1. Similar changes in ΔEQ
were observed for [Fe]-hydrogenase: treatment of Hmd with
CO or KCN resulted in a change of ΔEQ from þ0.65 to ꢀ1.38
and ꢀ1.75 mmsꢀ1, respectively.12
The field-dependent M€ossbauer study of the model complexes
shows that the sign and absolute value of the quadrupole splitting
is very sensitive to the change in the ligand environment of Fe
centers.
’ EXPERIMENTAL SECTION
A. Chemicals and Reagents. All manipulations were carried out
under an inert N2(g) atmosphere using glovebox techniques. Solvents
were purified using a two-column solid-state purification system
(Innovative Technology, NJ, U.S.A.) and transferred to the glovebox
without exposure to air. Deuterated solvents were purchased from
Cambridge Isotope Laboratories, Inc., and were degassed and
stored over activated 3 Å molecular sieves. All other reagents were
purchased from commercial sources. Liquid compounds were degassed
by standard freezeꢀpumpꢀthaw procedures prior to use. Complexes
Fe(CO)3(PPh3)I2, Fe(CO)3(PEt3)I2, Fe(CO)3I2(2,6-Me2C6H3NC),
[Fe(CO)2(PPh3)I(hmp)] (1), and [Fe(CO)2(PPh3)(hmp){S(2,6-
Me2C6H3)}] (5) (Hhmp = 2-Hydroxy-6-methylpyridine) were pre-
pared as described previously.27,35,36
B. Physical Methods. The 1H and 31P NMR spectra were recorded
at 293 K on a Bruker Avance 400 spectrometer. 1H NMR chemical shifts
were referenced to residual solvent as determined relative to Me4Si (δ =
0 ppm). The 31P{1H} chemical shifts were reported in ppm relative to
external 85% H3PO4. IR spectra were recorded on a Varian 800 FT-IR
spectrometer. Elemental analyses were performed on a Carlo Erba EA
1110 CHN instrument at EPFL. X-ray diffraction studies were carried
out in the EPFL Crystallographic Facility. Data collections were
performed at low temperature using four-circle kappa diffractometers
equipped with CCD detectors. Data were reduced and then corrected
for absorption.37 Solution, refinement, and geometrical calculations for
all crystal structures were performed by SHELXTL.38 M€ossbauer spectra
were recorded with a spectrometer from WissEL GmbH coupled to a
closed-cycle cryostat from CRYO Industries of America Inc. The
analysis of the spectra has been performed with the Software package
Vinda assuming a Lorentzian line shape (http://whome.phys.au.dk/
∼hpg/vinda.htm). The spectra obtained at high fields were simulated
using the spin-Hamiltonian formalism.39
The change in the quadrupole splitting is attributed to the
change of ligand environment. The quadrupole splitting arises
from the interaction between the electric field gradient (EFG)
and the nuclear quadrupole moment.32,34 It is a sensitive probe
for the chemical environment of the Fe center which determines
the EFG. The total EFG can be separated into two contributions.
In a noncubic symmetry, the charges on ligands surrounding the
Fe atom give rise to the lattice contribution. On the other hand,
the noncubic distribution of electrons in partially filled valence
orbitals of the Fe ion generates the valence contribution. The
model complexes measured here are all low-spin d,6 and all t2g
orbitals are filled. The valence contributions vanish. The lattice
contributions, however, are very different when the ligands are
different. Changing one ligand will perturb the magnitude and
sign of the lattice contribution. Therefore, very different quad-
rupole splitting parameters were detected among the model
complexes, as well as different states of the enzymes.
C. Synthetic Methods. Synthesis of [Fe(CO)2(PPh3)I(hp)] (Hhp=2-
Hydroxy-pyridine) (2). Na(hp) (137 mg, 1.17 mmol), prepared by
mixing Hhp (111 mg, 1.17 mmol) and NaH (28 mg, 1.17 mmol) in
tetrahydrofuran (THF), was added to a solution of Fe(CO)3(PPh3)I2
(768 mg, 1.17 mmol) in ether (5 mL) under stirring conditions. After
1.5 h, the solvent was evaporated, and the solid residue was dissolved in
a minimum quantity of CH2Cl2 and filtered. Pentane was added to the
filtrate and a precipitate was formed. The precipitate was collected,
washed with pentane, and dried under vacuum to afford 2 (560 mg,
0.94 mmol, 80%) as red crystals.
1H NMR (400.13 MHz, CDCl3): 7.86 (d, J = 5.2 Hz, 1H), 7.52ꢀ7.29
(m, 15H), 7.06 (t, J = 7.6 Hz, 1H), 6.35 (t, J = 6.0 Hz, 1H), 5.55 ppm (d,
J = 8.4 Hz, 1H). 31P NMR (162 MHz, CDCl3): 73.3 ppm. IR
(νCO, cmꢀ1): 2039 (s), 1988 (s). Anal. Calcd for C25H19FeINO3P: C,
50.5; H, 3.2; N, 2.4. Found: C, 50.9; H, 3.2, N, 2.6.
’ CONCLUSION
Synthesis of [Fe(CO)2(PPh3)I(hpp)] (Hhpp =2-Hydroxy-6-phen-
ylpyridine) (3). Using a similar procedure as described above, Na(hpp)
(97 mg, 0.50 mmol), prepared by mixing Hhpp (86 mg, 0.50 mmol) and
NaH (12 mg, 0.50 mmol) in THF, was added to a solution of
Fe(CO)3(PPh3)I2 (328 mg, 0.50 mmol) in ether (5 mL) under stirring
conditions. After 0.5 h, the solvent was evaporated, and the solid residue
was dissolved in a minimum quantity of CH2Cl2 and filtered. Pentane
was added to the filtrate and a precipitate was formed. The precipitate
was collected, washed with pentane, and dried under vacuum to afford 3
(280 mg, 0.42 mmol, 84%) as red crystals.
In summary, a series of Fe model complexes for the active site
of [Fe]-hydrogenase are synthesized and structurally character-
ized. Some of them are analogues to previously reported models
(complexes 2ꢀ4, 8, 9); others are structurally new compounds
(complexes 6, 7, 10, 13). These complexes serve as IR and
M€ossbauer spectroscopic models for the Fe center in [Fe]-
hydrogenase. The averaged CO vibration frequencies and the
isomeric shifts of the model complexes fall into a narrow range,
and are comparable to the values determined for the enzymes.
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ARTICLE
1H NMR (400.13 MHz, CDCl3): 7.94 (d, J = 7.2 Hz, 2H), 7.67 (t, J =
8.8 Hz, 2H), 7.52ꢀ7.22 (m, 16H), 7.09 (t, J = 8.0 Hz, 1H), 6.37 (d, J =
8.0 Hz, 1H), 5.59 ppm (d, J = 8.0 Hz, 1H). 31P NMR (162 MHz,
CDCl3): 71.2 ppm. IR (νCO, cmꢀ1): 2035 (s), 2001 (s). Anal. Calcd for
C31H23FeINO3P: C, 55.5; H, 3.5; N, 2.1. Found: C, 54.8; H, 3.5, N, 2.6.
Synthesis of [Fe(CO)2(PEt3)I(hmp)] (4). Using a similar procedure as
described above, Na(hmp) (87 mg, 0.67 mmol), prepared by mixing
Hhmp (73 mg, 0.67 mmol) and NaH (16 mg, 0.67 mmol) in THF, was
added to a solution of Fe(CO)3(PEt3)I2 (341 mg, 0.67 mmol) in
ether (5 mL) under stirring conditions. After 3.0 h, the solvent was
evaporated, and the solid residue was dissolved in a minimum quantity of
CH2Cl2 and filtered. Pentane was added to the filtrate, and a precipitate
was formed. The precipitate was collected, washed with pentane, and
dried under vacuum to afford 4 (200 mg, 0.43 mmol, 64%) as red
crystals.
Synthesis of [Fe(CO)2(PPh3)I{S(6-Me-C5H3N)}] (9). NaS(6-Me-
C5H3N) (130 mg, 0.89 mmol), prepared by mixing HS(6-Me-
C5H3N) (111 mg, 0.89 mmol) and NaH (21.4 mg, 0.89 mmol) in
THF, was added to a solution of [Fe(CO)2(PPh3)I2] (585 mg,
0.89 mmol) in ether (10 mL) under stirring in the dark. After 0.5 h,
the solvent was evaporated, and the solid residue was dissolved with a
minimum quantity of CH2Cl2 and filtered. Ether was added to the
filtrate, and a precipitate was formed. The precipitate was collected and
recrystallized from dichlomethane/ether at ꢀ30 °C to afford 9 (350 mg,
0.56 mmol, 63%) as red crystals.
1H NMR (400.13 MHz, CDCl3): 7.50ꢀ7.22 (m, 15H), 6.98 (t, J =
8.0 Hz, 1H), 6.52 (d, J = 8.0 Hz, 1H), 5.97 (d, J = 8.0 Hz, 1H), 2.37
ppm (s, 3H). 31P NMR (162 MHz, CDCl3): 73.2 ppm. IR (νCO, cmꢀ1):
2018 (s), 1968 (s). Anal. Calcd for C26H21FeINO2PS: C, 50.0; H, 3.4;
N, 2.2. Found: C, 50.1; H, 3.2, N, 1.9.
1H NMR (400.13 MHz, CDCl3): 7.35 (t, J = 8.0 Hz, 1H), 6.36 (d, J =
8.0 Hz, 1H), 5.89 (d, J = 8.0 Hz, 1H), 2.47 (s, 3H), 1.75 (m, 6H), 1.15
ppm (m, 9H). 31P NMR (162 MHz, CDCl3): 70.0 ppm. IR
(νCO, cmꢀ1): 2026 (s), 1974 (s). Anal. Calcd for C14H21FeINO3P: C,
36.2; H, 4.6; N, 3.0. Found: C, 36.2; H, 4.6, N, 3.1.
Synthesis of cis-(I, PPh3)-Fe(CO)2(PPh3)I(OMe-PyS) (10a) and
trans-(I, PPh3)-Fe(CO)2(PPh3)I(OMe-PyS) (10b). NaS(6-OMe-C5H3N)
(76.1 mg, 0.47 mmol), prepared by mixing HS(6-OMe-C5H3N) (65.5
mg, 0.47 mmol) and NaH (11.2 mg, 0.47 mmol) in THF, was added to a
solution of [Fe(CO)2(PPh3)I2] (305 mg, 0.47 mmol) in ether (10 mL)
under stirring in the dark. After 0.5 h, the solvent was evaporated, and the
solid residue was dissolved with a minimum quantity of CH2Cl2 and
filtered. Ether was added to the filtrate, and a precipitate formed. The
precipitate was collected to afford 10a (160 mg, 0.25 mmol, 53%) as an
orange powder. Leaving the solution of 10a in CH2Cl2 overnight in the
dark afforded 10b quantitatively, which was isolated as a red powder
after evaporation.
Synthesis of [Fe(CO)2(PPh3)(hmp){S(2-iPrꢀC6H4)}] (6). NaS(2-
iPrꢀC6H4) (101 mg, 0.58 mmol), prepared by mixing HS(2-
iPrꢀC6H4) (88 mg, 0.58 mmol) and NaH (14 mg, 0.58 mmol) in
ether, was added to a solution of [Fe(CO)2(PPh3)I(hmp)] (1) (353 mg,
0.58 mmol) in CH2Cl2 (5 mL) under stirring conditions. The resulting
solution was stirred for 1.5 h and then the precipitate was filtered off. The
filtrate was evaporated in vacuum, and the solid residue was recrystal-
10a: 1H NMR (400.13 MHz, CDCl3): 7.80ꢀ7.38 (m, 16H), 6.44 (d,
J = 8.0 Hz, 1H), 6.33 (d, J = 8.0 Hz, 1H), 3.91 ppm (s, 3H). 31P NMR
(162 MHz, CDCl3): 51.1 ppm. IR (νCO, cmꢀ1): 2031 (s), 1980 (s).
Anal. Calcd for C26H21FeINO3PS: C, 48.7; H, 3.3; N, 2.2. Found: C,
49.0; H, 3.8, N, 2.1.
lized from dichlomethane/pentane at ꢀ30 °C to afford 6 CH2Cl2 (100
3
mg, 0.14 mmol, 24%) as red crystals.
1H NMR (400.13 MHz, CDCl3): 7.66 (d, J = 8.0 Hz, 1H), 7.52ꢀ7.26
(m, 15H), 7.16ꢀ7.09 (m, 2H), 6.92ꢀ6.85 (m, 2H), 6.05 (d, J = 8.0 Hz,
1H), 5.34 (d, J = 8.0 Hz, 1H), 5.30 (s, 2H, CH2Cl2), 4.10 (m, 1H), 2.21
(s, 3H), 1.21 ppm (dd, J1 = 8.0 Hz, J2 = 2.4 Hz, 6H). 31P NMR (162
MHz, CDCl3): 54.0 ppm. IR (νCO, cmꢀ1): 2025 (s), 1985 (s). Anal.
Calcd for C36H34Cl2FeNO3PS: C, 60.2; H, 4.8; N, 2.0. Found: C, 60.0;
H, 4.8, N, 2.2.
10b: 1H NMR (400.13 MHz, CDCl3): 7.57ꢀ7.23 (m, 15H), 7.11 (t,
J = 8.0 Hz, 1H), 6.04 (d, J = 8.0 Hz, 1H), 5.86 (d, J = 8.0 Hz, 1H), 3.84
ppm (s, 3H). 31P NMR (162 MHz, CDCl3): 75.3 ppm. IR (νCO, cmꢀ1):
2024 (s), 1977 (s). Anal. Calcd for C26H21FeINO3PS: C, 48.7; H, 3.3;
N, 2.2. Found: C, 48.4; H, 3.4, N, 2.0.
Synthesis of [Fe(CO)2(PPh3)(hmp){S(2,4,6-Me3C6H2)}] (7). Using a
similar procedure as described above. NaS(2,4,6-Me3C6H2) (204 mg,
1.17 mmol), prepared by mixing HS(2,4,6-Me3C6H2) (178 mg, 1.17
mmol) and NaH (28 mg, 1.17 mmol) in ether, was added to a solution of
[Fe(CO)2(PPh3)I(hmp)] (1) (712 mg, 1.17 mmol) in CH2Cl2 (5 mL)
under stirring conditions. The resulting solution was stirred for 15 min
and then the precipitate was filtered off. The filtrate was evaporated in
vacuum, and the solid residue was recrystallized from dichlomethane/
pentane at ꢀ30 °C to afford 7 (350 mg, 0.55 mmol, 47%) as red crystals.
1H NMR (400.13 MHz, CDCl3): 7.40ꢀ7.25 (m, 15H), 6.87 (t, J =
8.0 Hz, 1H), 6.82 (s, 2H), 6.07 (d, J = 8.0 Hz, 1H), 5.25 (d, J = 8.0 Hz,
1H), 2.55 (s, 6H), 2.31 (s, 3H), 2.17 ppm (s, 3H). 31P NMR (162 MHz,
CDCl3): 53.3 ppm. IR (νCO, cmꢀ1): 2021 (s), 1973 (s). Anal. Calcd for
C35H32FeNO3PS: C, 66.4; H, 5.1; N, 2.2. Found: C, 65.9; H, 5.1, N, 2.3.
Synthesis of [Fe(CO)2(PPh3)I{S(C5H4N)}] (8). NaS(C5H4N) (156
mg, 1.17 mmol), prepared by mixing HS(C5H4N) (130 mg, 1.17 mmol)
and NaH (28.1 mg, 1.17 mmol) in THF, was added to a solution of
[Fe(CO)2(PPh3)I2] (770 mg, 1.17 mmol) in ether (10 mL) under
stirring in dark. After 1 h, the solvent was evaporated, and the solid
residue was dissolved in a minimum quantity of CH2Cl2 and filtered.
Ether was added to the filtrate, and a precipitate was formed. The
precipitate was collected and recrystallized from dichlomethane/ether
at ꢀ30 °C to afford 8 (430 mg, 0.74 mmol, 60%) as red crystals.
1H NMR (400.13 MHz, CD3CN): 8.05 (d, J = 4.0 Hz, 1H),
7.60ꢀ7.20 (m, 15H), 7.10 (t, J = 8.0 Hz, 1H), 6.67 (d, J = 6.0 Hz,
1H), 6.14 ppm (d, J = 8.0 Hz, 1H). 31P NMR (162 MHz, CD3CN): 75.8
ppm. IR (νCO, cmꢀ1): 2028 (s), 1986 (s). Anal. Calcd for C25H19FeI-
NO2PS: C, 49.1; H, 3.1; N, 2.3. Found: C, 49.4; H, 3.1, N, 2.3.
Synthesis of [Fe(CO)2(PPh3)2{S(6-Me-C5H3N)}]þ(PF6)ꢀ (11). TlPF6
(91 mg, 0.26 mmol) and PPh3 (68 mg, 0.26 mmol) were added into a
solution of [Fe(CO)2(PPh3)I{S(6-Me-C5H3N)}] (9) (81 mg, 0.13
mmol) in CH3CN (5 mL) under stirring. After 4.5 h, the solvent was
evaporated, and the solid residue was dissolved in a minimum quantity of
CH2Cl2 and filtered. Pentane was added to the filtrate, and a precipitate
was formed. The precipitate was collected and recrystallized from
dichlomethane/pentane at ꢀ30 °C to afford 11 (78 mg, 0.086 mmol,
66%) as red crystals.
1H NMR (400.13 MHz, CDCl3): 7.81ꢀ7.32 (m, 30H), 6.65 (t, J =
7.2 Hz, 1H), 6.49 (d, J = 7.2 Hz, 1H), 5.29 (d, J = 7.2 Hz, 1H), 2.09
ppm (s, 3H). 31P NMR (162 MHz, CDCl3): 49.6, ꢀ143.2 ppm. IR
(νCO, cmꢀ1): 2025 (s), 1989 (s). Anal. Calcd for C44H36F6FeNO2P3S
3
0.2CH2Cl2: C, 57.5; H, 4.0; N, 1.5. Found: C, 57.2; H, 4.0, N, 1.7.
Synthesis of [Fe(CO)2I(2,6-Me2C6H3NC)(SC5H4N-CO)] (13). NaS-
(C5H4N) (93 mg, 0.70 mmol), prepared by mixing HSC5H4N (77
mg, 0.70 mmol) and NaH (16.6 mg, 0.70 mmol) in THF, was added to a
solution of Fe(CO)3(2,6-Me2C6H3NC)I2 (364 mg, 0.70 mmol) in ether
(5 mL) under stirring. After 0.5 h, the solvent was evaporated, and the
solid residue was dissolved in a minimum quantity of CH2Cl2 and
filtered. Pentane was added to the filtrate, and a precipitate was formed.
The precipitate was collected and recrystallized from dichlomethane/
pentane at ꢀ30 °C to afford 13 (90 mg, 0.18 mmol, 25%) as orange
crystals. When the reaction was carried out under CO (1 atm), the yield
increased to 42%.
1H NMR (400.13 MHz, CDCl3): 8.42 (br s, 1H), 7.79 (d, J = 8.0 Hz,
1H), 7.58 (t, J = 8.0 Hz, 1H), 7.18 (d, J = 8.0 Hz, 1H), 7.10ꢀ7.00
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Inorganic Chemistry
ARTICLE
(m, 3H), 2.38 ppm (s, 6H). IR (νCO/NC, cmꢀ1): 2177 (m, NC), 2041 (s,
terminal CO), 2007 (s, terminal CO) 1700 (s, acyl CO). Anal. Calcd for
C17H13FeIN2O3S: C, 40.2; H, 2.6; N, 5.5. Found: C, 40.0; H, 2.6, N, 5.4.
(20) Wright, J. A.; Turrell, P. J.; Pickett, C. J. Organometallics 2010,
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Pickett, C. J.; Liu, X. M. Chem. Commun. 2008, 3555–3557.
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(23) Li, B.; Liu, T.; Popescu, C. V.; Bilko, A.; Darensbourg, M. Y.
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’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic details, struc-
b
tural drawing of complexes 2, 4, 6, 10a, 10b, 11, M€ossbauer
spectra of 5, 9, 13, 17. This material is available free of charge via
the Internet at http://pubs.acs.org.
(26) Royer, A. M.; Salomone-Stagni, M.; Rauchfuss, T. B.;
Meyer-Klaucke, W. J. Am. Chem. Soc. 2010, 132, 16997–17003.
(27) Obrist, B. V.; Chen, D. F.; Ahrens, A.; Schunemann, V.;
Scopelliti, R.; Hu, X. L. Inorg. Chem. 2009, 48, 3514–3516.
(28) Chen, D. F.; Scopelliti, R.; Hu, X. L. J. Am. Chem. Soc. 2010,
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: xile.hu@epfl.ch.
(29) Chen, D. F.; Scopelliti, R.; Hu, X. L. Angew. Chem., Int. Ed. 2010,
49, 7512–7515.
(30) Tanino, S.; Ohki, Y.; Tatsumi, K. Chem. Asian. J. 2010,
5, 1962–1964.
’ ACKNOWLEDGMENT
The work at EPFL is supported by the Swiss National Science
Foundation (project no. 200021_119663). V.S. acknowledges
the support of NANOKAT.
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