Iron-only hydrogenase active site models containing a cysteinyl group coordinated through its sulfur atom to one iron atom of the diiron subsite

Article abstract of DOI:10.1002/ejic.200700821

We have developed a simple and convenient method for the synthesis of the first H-cluster models in which a L-cysteinyl group is coordinated to one of the two iron atoms of the diiron subsite through its sulfur atom. This synthetic method includes (i) treatment of the Boc-protected L-cysteine ester Boc-NHCH(CH₂SH)CO₂Et (1, Boc = tert-butoxycarbonyl) with EtONa to give the L-cysteinyl mercaptide NaSCH₂CH(NH-Boc)CO ₂Et (2); (ii) further treatment of 2 with [Cp(CO)₂FeI] to produce the metallothioether ligand Cp(CO)₂FeSCH₂CH(NH- Boc)CO₂Et (3); and (iii) treatment of the parent diiron complex [Fe₂(μ-SCH₂)₂CH₂(CO) ₆] (4), [Fe₂(μ-SCH₂)₂N(tBu)(CO) ₆] (5), or [Fe₂(μ-SCH₂)₂N(C ₆H₄OMe-p)(CO)₆] (6) with Me ₃NO·2H₂O followed by ligand 3 to afford the target model compounds [Fe₂(μ-SCH₂)₂CH ₂(CO)₅(ligand 3)] (7), [Fe₂(μ-SCH ₂)₂N(tBu)(CO)₅(ligand 3)] (8), or [Fe ₂(μ-SCH₂)₂N(C₆H ₄OMe-p)(CO)₅(ligand 3)] (9), respectively. All the new compounds 2, 3, and 7-9 have been characterized by elemental analysis and various spectroscopic techniques. The X-ray diffraction analysis of 8 has confirmed that these models contain a cysteinyl sulfur atom not only coordinated to one Fe atom of the diiron subsite, but also to the Fe atom of the Cp(CO)₂Fe moiety to form the linkage [Fe_Cp-(μ- cysteinyl-S)-Fe_subsite], which is similar to [Fe_cubane- (μ-cysteinyl-S)-Fe_subsite] found in natural enzymes. In addition, spectroscopic and electrochemical measurements have further demonstrated that the linkage [Fe_Cp-(μ-cysteinyl-S)-Fe_subsite] can provide substantial electronic communication between the diiron subsite and the Cp(CO)₂Fe moiety. Under electrochemical conditions, 8 has been shown to be a catalyst for HOAc proton reduction to dihydrogen, and a new type of E*2E2C mechanism for this catalytic reaction is suggested. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200700821

FULL PAPER
DOI: 10.1002/ejic.200700821
Iron-Only Hydrogenase Active Site Models Containing a Cysteinyl Group
Coordinated through Its Sulfur Atom to One Iron Atom of the Diiron Subsite
Li-Cheng Song,*[a] Jian-Hua Ge,[a] Jing Yan,[a] Hu-Ting Wang,[a] Xiang Luo,[a] and
Qing-Mei Hu[a]
Keywords: Bioinorganic chemistry / Cysteinyl ligand / Electrocatalysis / Dihydrogen evolution / Iron-only hydrogenase
We have developed a simple and convenient method for the
synthesis of the first H-cluster models in which a -cysteinyl
group is coordinated to one of the two iron atoms of the diiron
subsite through its sulfur atom. This synthetic method in-
by elemental analysis and various spectroscopic techniques.
The X-ray diffraction analysis of 8 has confirmed that these
models contain a cysteinyl sulfur atom not only coordinated
to one Fe atom of the diiron subsite, but also to the Fe atom of
the Cp(CO)2Fe moiety to form the linkage [FeCp-(µ-cystein-
yl-S)-Fesubsite], which is similar to [Fecubane-(µ-cysteinyl-S)-
Fesubsite] found in natural enzymes. In addition, spectroscopic
and electrochemical measurements have further demon-
strated that the linkage [FeCp-(µ-cysteinyl-S)-Fesubsite] can
provide substantial electronic communication between the
diiron subsite and the Cp(CO)2Fe moiety. Under electro-
chemical conditions, 8 has been shown to be a catalyst for
HOAc proton reduction to dihydrogen, and a new type of
E*2E2C mechanism for this catalytic reaction is suggested.
L
cludes (i) treatment of the Boc-protected
Boc-NHCH(CH2SH)CO2Et (1, Boc = tert-butoxycarbonyl)
with EtONa to give the -cysteinyl mercaptide
L-cysteine ester
L
NaSCH2CH(NH-Boc)CO2Et (2); (ii) further treatment of 2
with [Cp(CO)2FeI] to produce the metallothioether ligand
Cp(CO)2FeSCH2CH(NH-Boc)CO2Et (3); and (iii) treatment of
the parent diiron complex [Fe2(µ-SCH2)2CH2(CO)6] (4), [Fe2-
(µ-SCH2)2N(tBu)(CO)6] (5), or [Fe2(µ-SCH2)2N(C6H4OMe-p)-
(CO)6] (6) with Me3NO·2H2O followed by ligand 3 to afford
the target model compounds [Fe2(µ-SCH2)2CH2(CO)5(ligand
3)] (7), [Fe2(µ-SCH2)2N(tBu)(CO)5(ligand 3)] (8), or [Fe2(µ-
SCH2)2N(C6H4OMe-p)(CO)5(ligand 3)] (9), respectively. All
the new compounds 2, 3, and 7–9 have been characterized
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
hydrogen, the cubic Fe4S4 cluster is responsible for electron
transfer from and to the active site through the bridging -
cysteinyl group. Inspired by the structural studies of the H-
cluster, chemists have synthesized a variety of active site
models,[19,20] and some of them have been found to be cata-
lysts for H/D exchange[21] and proton reduction to dihydro-
gen under electrochemical conditions.[22–24] However, up to
now, a H-cluster model in which a cysteinyl group is coordi-
nated through its sulfur atom to one iron atom of the diiron
subsite has not appeared in literature, although some cystein-
yl-group-containing di- and trinuclear complexes are
known.[25] It should be noted that the synthesis of such a
kind of model is particularly of interest, as this provides the
possibility of understanding both the biological and chemi-
cal functions of the bridging -cysteinyl group in the H-
cluster of FeHases. So, we initiated a study on the synthesis
of such model compounds with a basic skeleton structure
as shown in Scheme 1b. We introduce the [Cp(CO)2Fe] moi-
ety into such models in order to replace the complicated
cubic Fe4S4 cluster and construct the linkage [FeCp-(µ-
cysteinyl-S)-Fesubsite] for mimicking the [Fecubane-(µ-cystein-
Introduction
Hydrogenases are enzymes, found in many microorga-
nisms, that catalyze both the reduction of protons to dihy-
drogen and the oxidation of dihydrogen to protons.[1] These
enzymes can be divided into two main groups on the basis
of the metals in them: Fe-only hydrogenases[2,3] and Ni-Fe
hydrogenases.[4–6] In recent years Fe-only hydrogenases
(FeHases) have attracted considerably more attention than
Ni-Fe hydrogenases, largely because of their unusual struc-
tures and particularly because of their catalytic function in
the production of the “clean” and highly efficient fuel: dihy-
drogen.[7–10] X-ray crystallographic,[11–14] FTIR spectro-
scopic,[15–17] and theoretical[18] studies revealed that the
active site of FeHases, the so-called H-cluster, is composed
of a dithiolate-bridged diiron subsite, one of whose iron
atoms is linked to a cubane-like Fe4S4 cluster through the
sulfur atom of the -cysteinyl group (Scheme 1a). While the
diiron subsite catalyzes the formation and activation of di-
[a] Department of Chemistry, State Key Laboratory of Elemento-
Organic Chemistry, Nankai University,
Tianjin 300071, China
yl-S)-Fesubsite
]
linkage found in natural enzymes.[11,12]
Thus, in the designed models the cysteinyl sulfur atom is
not only coordinated to one iron atom of the diiron subsite,
Fax: +86-22-23504853
E-mail: lcsong@nankai.edu.cn
164
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Eur. J. Inorg. Chem. 2008, 164–171

Iron-Only Hydrogenase Active Site Models
1
but also to the iron atom of the [Cp(CO)2Fe] moiety. Herein carbonyl functions. The H and 13C NMR spectra of 2 and
we report the first three such biomimetic models with their 3 show all the corresponding proton and carbon signals for
synthesis, structural characterization, and some properties. the Boc-protected -cysteine ester moiety, whereas 3 dis-
In addition, two intermediate products utilized for synthesis plays an additional singlet at δ = 4.96 ppm for the five
1
of such target model compounds are also described.
equivalent protons of its Cp ring in its H NMR spectrum
and another singlet at δ = 85.32 ppm for the five equivalent
carbon atoms of its Cp ring in its 13C NMR spectrum.
Synthesis and Spectroscopic Characterization of Target
Model Compounds [Fe2(µ-SCH2)2CH2(CO)5(ligand 3)] (7),
[Fe2(µ-SCH2)2N(tBu)(CO)5(ligand 3)] (8), and [Fe2(µ-
SCH2)2N(C6H4OMe-p)(CO)5(ligand 3)] (9)
Our target model compounds 7–9 were actually synthe-
sized by the Me3NO-promoted CO displacement reactions
of the parent diiron complexes [Fe2(µ-SCH2)2CH2(CO)6]
(4), [Fe2(µ-SCH2)2N(tBu)(CO)6] (5), and [Fe2(µ-SCH2)2N-
(C6H4OMe-p)(CO)6] (6) with the above-prepared ligand 3
via the M1–3 intermediates,[26] as shown in Scheme 3. It is
noteworthy that ligand 3 should be added after complete
consumption of the equimolar mixture of Me3NO·2H2O
and complex 4, 5, or 6, since 3 also contains carbonyl
groups that can easily be removed by Me3NO. In addition,
the yields of 8 (45%) and 9 (40%) are much lower than the
yield of 7 (80%), presumably because 8 and 9 have much
stronger steric repulsion between the bulky ligand 3 and
the bulky tBu or p-MeOC6H4 group at their bridgehead N
atom.
Scheme 1.
Results and Discussion
Synthesis and Spectroscopic Characterization of L-Cysteinyl
Mercaptide NaSCH2CH(NH-Boc)CO2Et (2) and Metallo-
thioether Ligand Cp(CO)2FeSCH2CH(NH-Boc)CO2Et (3)
According to our designed synthetic method for the tar-
get models, we should first prepare new compounds 2 and
3. As shown in Scheme 2, both 2 and 3 were prepared by
starting from the Boc-protected -cysteine ester Boc-
NHCH(CH2SH)CO2Et (1, Boc = tert-butoxycarbonyl).
Treatment of the Boc-protected -cysteine ester 1 with the
freshly prepared EtONa in thf at room temperature gave
rise to 2 in a nearly quantitative yield. Then, further treat-
ment of the isolated 2 with [Cp(CO)2FeI] in thf at room
temperature or direct treatment of the in situ prepared thf
solution of 2 with [Cp(CO)2FeI] at room temperature re-
sulted in formation of 3 in 90% and 94% yields, respec-
tively. Our initial attempt to prepare ligand 3 by direct reac-
tion of 1 with [Cp(CO)2FeI] in the presence of Et3N was
unsuccessful, as indicated by recovery of almost all the
starting material [Cp(CO)2FeI]. While 2 is a highly hygro-
scopic white solid, 3 is a slightly air-sensitive red oil. Both
2 and 3 were characterized by elemental analysis and spec-
troscopy. The IR spectra of 2 and 3 display two absorption
bands in the range 1738–1696 cm–1 for their two different
organic carbonyl groups, whereas 3 exhibits two additional
bands at 2022 and 1968 cm–1 resulting from its two terminal
Scheme 3.
Model compounds 7–9 are air-stable, brown-black solids,
but they are slightly air-sensitive in solution. These com-
pounds were also characterized by elemental analysis and
spectroscopic techniques. The IR spectra of 7–9 display two
absorption bands in the range 1734–1713 cm–1 for their two
different organic carbonyl groups and three absorption
bands in the region 2043–1909 cm–1 for their terminal metal
carbonyl groups. The 13C NMR spectra of 7–9 each exhibit
two signals at ca. 155 and 171 ppm for their two different
organic carbonyls; in addition, they display two to four sig-
Scheme 2.
Eur. J. Inorg. Chem. 2008, 164–171
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165

L.-C. Song, J.-H. Ge, J. Yan, H.-T. Wang, X. Luo, Q.-M. Hu
FULL PAPER
nals in the range 207–214 ppm for their terminal carbonyl group, that is, its C20 has an R configuration. This -cystei-
groups. Interestingly, relative to the highest νCϵO fre- nyl group is bridged between one iron atom (Fe2) of the
quencies of the parent complexes 4–6, those corresponding diiron subsite and the iron atom (Fe3) of the (dicarbonyl)-
to products 7–9 are shifted by 30–35 cm–1 towards lower cyclopentadienyliron moiety through an S-configured sul-
values (Figure 1). This implies that 3 has a stronger elec- fur (S3) atom to construct a linkage, [FeCp-(µ-cysteinyl-S)-
tron-donating influence than CO.[27] It should be pointed Fesubsite], like [Fecubane-(µ-cysteinyl-S)-Fesubsite] found in
out that the 30–35 cm–1 shift is much larger than the ca. natural enzymes.[11,12] The Fe1–Fe2 bond length
15 cm–1 shift of the νCϵO frequencies of the diiron subsite [2.5255(13) Å] is very close to the corresponding bond
caused by the formal replacement of the Me in the thioether lengths in the simple diiron model compounds [Fe2(µ-
ligand of [Fe2MeC(CH2S)2(CH2SMe)(CO)5] by the cubane SCH2)2CH2(CO)6] [2.5103(11) Å],[28] [Fe2(µ-SCH2)2NMe-
dianion, to afford Pickett’s model compound [Fe4S4L- (CO)6] [2.4924(7) Å],[29] and [Fe2(µ-SCH2)2O(CO)6]
{Fe2MeC(CH2S)3(CO)5}](NBu4)2.[19e] Obviously, this is [2.5113(13) Å],[30] but they are all slightly shorter than the
mainly due to the direct attachment of ligand 3 with the corresponding bond lengths in the oxidized and reduced
diiron subsite, but the cubane dianion is attached to the DdH hydrogenases (2.60 and 2.55 Å).[12,13] In addition, the
diiron subsite by the sulfur atom of the cubane-dianion- Fe2–S3 bond length [2.3248(16) Å] is shorter than the cor-
containing metallothioether ligand. In addition, it is worth responding bond length (2.5 Å) in the oxidized DdH, but
pointing out that such electron-donating influences should the Fe3–S3 bond length [2.3170(15) Å] is very close to the
be reciprocated in the shifts of the reduction potentials of corresponding bond length (2.3 Å) in the oxidized DdH.[12]
the diiron subsite in our model compounds 7–9 to more
negative potentials, as well as that of the iron atom of li-
gand 3 in our model compounds 7–9 (vide infra) to a more
positive potential.[19e]
Figure 2. ORTEP view of 8 with 30% probability level ellipsoids.
Table 1. Selected bond lengths [Å] and angles [°] for 8.
Figure 1. Comparison of the IR spectra of 4–6 with the corre-
Fe1–S1
Fe1–S2
Fe1–Fe2
Fe2–S1
2.2614(17)
2.2677(17)
2.5255(13)
2.2776(17)
2.2799(16)
84.60(6)
56.50(4)
56.49(4)
83.95(6)
99.77(6)
Fe2–S3
Fe3–S3
S3–C19
C21–O8
2.3248(16)
2.3170(15)
1.830(5)
1.199(6)
1.431(6)
55.89(5)
56.04(4)
144.30(5)
67.61(5)
67.47(5)
116.26(6)
sponding spectra of 7–9.
Fe2–S2
C20–N2
Crystal Structure of Target Model Compound 8
S1–Fe1–S2
S1–Fe1–Fe2
S2–Fe1–Fe2
S1–Fe2–S2
S1–Fe2–S3
S2–Fe2–S3
S1–Fe2–Fe1
S2–Fe2–Fe1
S3–Fe2–Fe1
Fe1–S1–Fe2
Fe1–S2–Fe2
Fe3–S3–Fe2
The molecular structure of 8 was determined by single-
crystal X-ray diffraction techniques. Figure 2 shows its
ORTEP plot. Table 1 lists some of its bond lengths and
angles. As shown in Figure 2, model compound 8 contains
a tert-butyl-group-substituted azadithiolate ligand bridged
between the two iron atoms of the Fe(CO)3 and Fe(CO)2
99.70(6)
It is interesting to note that in the unit cell of the crystal
units to form two fused six-membered rings. The six-mem- structure of 8 (Figure 3) there exists a pair of enantiomers:
bered ring Fe1–S1–C13–N1–C14–S2 has a boat conforma- one is the same as that shown in Figure 2, which contains
tion, but another six-membered ring Fe2–S1–C13–N1– an R-configured C20 and an S-configured S3 center, and
C14–S2 adopts a chair conformation. The tert-butyl group the other contains an S-configured C20A and an R-config-
is equatorially attached to the common nitrogen atom (N1) ured S3A center. It is believed that the latter S/R isomer
of the two six-membered rings. Figure 2 also shows that 8 was produced from the minor -cysteinyl-group-containing
contains
a
tert-butoxycarbonyl-substituted -cysteinyl ligand 3, whereas -cysteinyl ligand 3 was generated from
166
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Iron-Only Hydrogenase Active Site Models
-cysteinyl mercaptide 2 and the -cysteinyl 2 derived from reduction of the FeII atom in its ligand 3 from FeII to FeI.
inversion of the R-configured Cα of the Boc-protected - In addition, the second and the fourth reductions at –1.84
cysteinyl ester 1. Although some cases are known in which and –2.22 V could be assigned to the reduction of the two
the -cysteinyl group in -cysteine ester hydrochlorides FeI atoms in its diiron subsite from FeIFeI to FeIFe0 and
keeps its configuration during functional transformation re- FeIFe0 to Fe0Fe0, respectively. The reduction at –1.84 V
actions,[31,32] it is possible to cause some configurational in- might be further assigned to the reduction of the FeI atom
version of the Cα atom of 2 from R to S under basic condi- remote from the electron-donating ligand 3, whereas the re-
tions. This has been proved by the decrease in the specific duction at –2.22 V could be ascribed to the reduction of
rotation from the originally prepared 1 ([α]2D0 = +20, c = the FeI atom directly attached to 3. It follows that the first
1.1 g100 mL–1, CHCl3) to
1
([α]2D0
=
+18,
c
=
reduction potential of 8 is positively shifted by 320 mV rela-
1.1 g100 mL–1, CHCl3) obtained by acidification of 2 with tive to that (–1.82 V, Table 2) of free ligand 3, while the
HCl. Figure 3 also shows that in the unit cell this pair of second reduction potential at –1.84 V is negatively shifted
enantiomers of 8 are held together by means of the two by 150 mV compared to that (–1.69 V, Table 2) of the par-
intermolecular hydrogen bonds N2–H2····O8A and N2A– ent complex 5. This is reasonable, since the electron-donat-
H2A····O8. The distance between N2 and O8A or N2A and ing influence of ligand 3 in lowering the νCϵO of the parent
O8 is 2.990 Å, while the bond angle N2–H2····O8A or complex 5 should be reciprocated in the shifts of the re-
N2A–H2A····O8 is 146.1°. These geometric parameters are duction potential of the FeII atom in ligand 3 to a more
typical of the hydrogen-bonding interactions.[33]
positive potential and those of the FeI atoms in its diiron
subsite to more negative potentials, as mentioned above. Fi-
nally, it is noteworthy that the 320-mV potential shift is
considerably larger than the 120-mV shift caused by the for-
mal replacement of the Me of the thioether ligand in
[Fe2MeC(CH2S)2(CH2SMe)(CO)5] by the cubane dianion
to give Pickett’s model compound.[19e] The 320- and 150-
mV potential shifts, and the 30–35-cm–1 νCϵO shifts de-
scribed above indicate substantial electronic communication
occurring in the linkage [FeCp-(µ-cysteinyl-S)-Feesubsite] be-
tween the diiron subsite and the [CpFe(CO)2] unit through
the bridging -cysteinyl group. This electronic communica-
tion is apparently more efficient than that occurring in the
linkage [Fecubane-(µ-SR)-Fesubsite] of Pickett’s model com-
pound.[19e]
Figure 3. The hydrogen-bonding interaction between two molecules
of 8.
Electrochemistry of Ligand 3, Model Compound 8, and
Related Complexes
The electrochemical properties of 3 and 8 along with
complexes 5 and [Cp(CO)2Fe]2 were determined by CV
techniques under identical conditions. Figure 4 shows their
cyclic voltammograms, and Table 2 lists their electrochemi-
cal data. All the reduction events shown in Figure 4 and
Table 2 are one-electron processes, which has been con-
firmed by bulk electrolysis of 5 (20.7 mmol) at –1.91 V in
MeCN. It should be pointed out that for 3 and 8 the
irreversible one-electron reduction event at –2.00 V should
not be attributed to the reduction of the FeII atom in 3 and
the FeI atom in 8, but instead, should be attributed to the
reduction of the FeI atom in [Cp(CO)2Fe]2. It is believed
that [Cp(CO)2Fe]2 might be generated from the decomposi-
tion of the free ligand 3 or of the coordinated ligand 3 in 8
during the electrochemical process, because it also displays
a reduction peak at –2.00 V, and in particular, it has been
isolated from the final electrolytic solution of 3 or 8. In fact,
production of [Cp(CO)2Fe]2 is common in CV determi-
nations of the [Cp(CO)2Fe]-containing complexes.[34] Now,
we might assign the first one-electron reduction of 3 at
–1.84 V to the reduction of its FeII atom from FeII to FeI,
Figure 4. Cyclic voltammograms of 3, 5, 8 and [Cp(CO)2Fe]2
(1 m) in 0.1  nBu4NPF6/MeCN. Scan rate = 100 mVs–1.
Table 2. Redox potentials of 3, 5, 8, and [Cp(CO)2Fe]2 in MeCN
at ambient temperature.[a]
.
Compound
Ered [V]
Eox [V]
[b]
[Cp(CO)2Fe]2
–2.00
0.13[b]
[b]
3
5
8
–1.82[b], –2.00[b]
0.26
–1.69 [c], –2.20[b]
0.67[b]
0.27[b]
–1.50[b], –1.84[b], –2.00[b], –2.22[b]
[a] MeCN solution (0.1  nBu4NPF6) with a glassy carbon working
electrode (A = 0.071 cm2) referenced to Fc/Fc+. Counterelectrode:
and the first one-electron reduction of 8 at –1.50 V to the Pt. Scan rate = 100 mVs–1. [b] Irreversible. [c] Quasi-reversible.
Eur. J. Inorg. Chem. 2008, 164–171
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167

L.-C. Song, J.-H. Ge, J. Yan, H.-T. Wang, X. Luo, Q.-M. Hu
FULL PAPER
Investigation on Proton Reduction Catalyzed by 8
[Fe2(µ-SCH2)2CH2(CO)4(Me3P)2].[23] The latter involve
only the two FeI atoms in their diiron subsites, whereas the
former E*2E2C mechanism involves not only the two FeI
atoms in its diiron subsite, but also the additional FeII atom
in ligand 3. It follows that electrocatalytic H2 production
by the E*2E2C mechanism much resembles biological H2
evolution in natural enzymes, since in enzymes the reductive
activation begins with the reduction of the Fe4S4 cluster
from the 2+ to the 1+ state followed by transfer of the reduc-
ing equivalent from the Fe4S4 cluster to the Fe2S2 cluster.[38]
However, it is worthy of note that in the E*2E2C mecha-
nism the reduced CpFe(I)(CO)2 moiety in 8– does not really
transfer the reducing equivalent to the Fe2S2 cluster, but
controls the reducing order of the two Fe atoms in the Fe2S2
cluster by virtue of its strong electron-donating ability.
We further investigated the electrochemical behavior of 8
in the presence of acetic acid to see whether 8 has the cata-
lytic activity for proton reduction to dihydrogen. As shown
in Figure 5, when HOAc was sequentially added from 2 m
to 10 m, the original peak at –1.50 V did not change, the
peak at –1.84 V slightly increased, the peak at –2.00 V dis-
appeared, and the peak at –2.22 V grew dramatically. Obvi-
ously, the disappearance of the peak at –2.00 V means that,
in the presence of HOAc, 8 became so stable that no
[Cp(CO)2Fe]2 was generated. In addition, the continuous
increase in the peak at –2.20 V is characteristic of an elec-
trocatalytic process for HOAc proton reduction to dihydro-
gen catalyzed by 8.[22–24,35] Actually, the reduction potential
of HOAc with 8 as a catalyst is shifted positively by 400 mV
relative to that without 8. The bulk electrolysis of a MeCN
solution of 8 (0.5 m) with HOAc (25 m) at –2.25 V indi-
cated that a total of 17.3 F per mol of 8 passed through the
electrolysis cell during half an hour. This corresponds to a
turnover of 8.7. Gas chromatographic analysis showed that
the dihydrogen yield was about 90%.
Scheme 4. Proposed E*2E2C electrocatalysis mechanism for H2
production from 8 and HOAc.
Conclusions
We have synthesized the first H-cluster model com-
pounds (7–9) in which the -cysteinyl sulfur atom is bridged
between one Fe atom of the diiron subsite and the Fe atom
of the [Cp(CO)2Fe] moiety. The synthetic route to 7–9 is
simple and convenient; it includes first the synthesis of the
Figure 5. Cyclic voltammograms of 8 (1 m) with HOAc (2–
10 m) and without HOAc in 0.1  nBu4NPF6/MeCN. Scan rate
of 100 mVs–1.
The above-mentioned electrochemical and IR spectral intermediate products 2 and 3, followed by the Me3NO-
observations, and the previously reported similar cases[22–24] induced CO substitution of the parent complex 4, 5, or 6
enable us to propose an E*2E2C (E* represents an electro- by ligand 3. The X-ray diffraction analysis of 8 has revealed
chemical step outside the catalytic cycle; E and C represent that these models contain a linkage, [FeCp-(µ-cysteinyl-S)-
the electrochemical and chemical steps in the catalytic cy- Fesubsite], which is similar to [Fecubane-(µ-cysteinyl-S)-
cle) mechanism to account for H2 formation from acetic Fesubsite] found in natural enzymes. Particularly interesting
acid catalyzed by 8 (Scheme 4). Firstly, at –1.50 V the FeII is that the linkage [FeCp-(µ-cysteinyl-S)-Fesubsite] in these
atom of the electron-donating ligand 3 in 8 is preferentially models has been demonstrated by spectroscopic and elec-
reduced to the FeI atom to give monoanion 8–. Then, at trochemical measurements to provide more efficient elec-
–1.84 V the FeI atom remote from 3 in its diiron subsite is tronic communication between the diiron subsite and the
reduced to Fe0 to afford dianion 82–. Thirdly, at –2.22 V the [Cp(CO)2Fe] moiety than that provided by the [Fecubane-(µ-
FeI atom close to 3 in its diiron subsite is reduced to Fe0 to SR)-Fesubsite] linkage in Pickett’s model compound.[19e] Ad-
produce trianion 83–. After one of the Fe0 atoms in this ditionally, model compound 8 has been shown to have the
trianion is protonated by oxidative addition to give species catalytic ability for proton reduction to dihydrogen in
8H2–, it reacts further with another proton to afford the HOAc under electrochemical conditions. To improve the
starting monoanion 8– with H2 evolution to complete the catalytic function for H2 production, we will further modify
catalytic cycle. It follows that this E*2E2C mechanism is the structures of 7–9 by using the other 2e ligands to replace
different from those mechanisms suggested for the simple their CO ligands and by using various substituted Cp li-
diiron model complexes, such as 4,[36] 6,[37] [Fe2(µ-SCH2)2O- gands to replace their Cp ligands. Some of the studies along
(CO)6],[30] [Fe2(µ-SCH2)2CH2(CO)4(CN)(Me3P)],[22] and this line are in progress in our laboratories.
168
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Iron-Only Hydrogenase Active Site Models
NHCH(CH2SH)CO2Et (1, 0.050 g, 0.20 mmol), thf (10 mL), and
the freshly prepared EtONa (0.014 g, 0.20 mmol). After the mix-
ture was stirred for 10 min at room temperature, [Cp(CO)2FeI]
(0.061 g, 0.20 mmol) was added. The new mixture was stirred at
room temperature for 1 h. The solvent was removed, and the resi-
due was subjected to TLC with acetone/petroleum ether (v/v =
1:2.5) as eluent. From the main red band, 2 (0.080 g, 94%) was
obtained as a red oil.
Experimental Section
General Comments: All reactions were carried out by using stan-
dard Schlenk and vacuum-line techniques under an atmosphere of
nitrogen. Acetonitrile was purified by distillation once from P2O5
and then from CaH2, while thf was purified by distillation from
sodium/benzophenone ketyl. The decarbonylating agent
Me3NO·2H2O and -HCl·H2NCH(CH2SH)CO2Et ([α]2D0 = –11, c
= 8.0 g/100 mL, 1  HCl) used for preparing the Boc-protected -
cysteine ester Boc-NHCH(CH2SH)CO2Et ([α]2D0 = +20, c = 1.0 g/
100 mL, CHCl3) were available commercially. Boc-NHCH-
(CH2SH)CO2Et,[31] EtONa,[39] [Cp(CO)2FeI],[40] [Fe2(µ-SCH2)2-
CH2(CO)6],[41] [Fe2(µ-SCH2)2N(tBu)(CO)6],[20a] and [Fe2(µ-SCH2)2-
N(C6H4OMe-p)(CO)6][37] were prepared according to the published
procedures. Preparative TLC was carried out on glass plates
(26ϫ20ϫ0.25 cm) coated with silica gel H (10–40 µm). Specific
rotation ([α]2D0) values were obtained by using a Perkin–Elmer
Model 341 polarimeter. IR spectra were recorded with a Bruker
Vector 22 infrared spectrophotometer. 1H (13C) NMR spectra were
recorded with a Bruker Avance 300 NMR or a Varian Mercury
Plus 400 NMR spectrometer. Elemental analyses were performed
with an Elementar Vario EL analyzer. Melting points were deter-
mined with a Yanaco MP-500 apparatus and were uncorrected.
Preparation of [Fe2(µ-SCH2)2CH2(CO)5][Cp(CO)2FeSCH2CH(NH-
Boc)CO2Et] (7): A 100-mL three-necked flask fitted with a mag-
netic stirring bar, a rubber septum, and a nitrogen inlet tube was
charged with [Fe2(µ-SCH2)2CH2(CO)6] (4, 0.077 g, 0.20 mmol),
Me3NO·2H2O (0.022 g, 0.20 mmol), and MeCN (10 mL). The mix-
ture was stirred at room temperature for 0.5 h. At this point, TLC
showed that no [Fe2(µ-SCH2)2CH2(CO)6] was left. To this mixture
was added Cp(CO)2FeSCH2CH(NH-Boc)CO2Et (3, 0.085 g,
0.20 mmol), and the new mixture was stirred at room temperature
for 3 h. The solvent was removed, and the residue was subjected to
TLC using acetone/petroleum ether (v/v = 1:4) as eluent. From the
main brown band, 7 was obtained as a brown-back solid. Yield:
0.125 g, 80%; m.p. 68–70 °C. 1H NMR (400 MHz, CDCl3): δ =
1.27 (t, J = 6.8 Hz, 3 H, CH2CH3), 1.42 [s, 9 H, C(CH3)3], 1.72
(br.s, 1 H, CH2CHHCH2), 2.15, 1.85 (2br.s, 4 H, CH2CH2CH2),
2.33 (br.s, 1 H, CH2CHHCH2), 2.63 (br.s, 2 H, CpFeSCH2), 4.19
(q, J = 6.8 Hz, 2 H, OCH2), 4.35–4.41 (m, 1 H, CH), 5.09 (d, J =
7.6 Hz, 1 H, NH), 5.23 (s, 5 H, C5H5) ppm. 13C NMR (100.6 MHz,
CDCl3): δ = 14.27 (CH2CH3), 22.85 (CH2CH2CH2), 28.47 [C-
(CH3)3], 30.91 (CH2CH2CH2), 34.22 (CpFeSCH2), 54.73 (CH),
61.90 (OCH2), 80.28 [C(CH3)3], 87.11 (C5H5), 155.46 (NHC=O),
Preparation of NaSCH2CH(NH-Boc)CO2Et (2): A 100-mL three-
necked flask fitted with a magnetic stirring bar, a rubber septum,
and a nitrogen inlet tube was charged with Boc-NHCH(CH2SH)-
CO2Et (1, 0.050 g, 0.20 mmol), thf (10 mL), and the freshly pre-
pared EtONa (0.014 g, 0.20 mmol). The mixture was stirred at
room temperature for 10 min, and then solvent was removed under
vacuum to leave a residue. After the residue was thoroughly washed
with hexane (10 mL), it was dissolved in thf (5 mL) and filtered to
remove the insoluble impurity. The solvent was removed to give 2
as a highly hygroscopic white solid. Yield: 0.053 g, 99%; m.p.
170 °C (dec., determined in a sealed capillary tube). 1H NMR
(400 MHz, CDCl3): δ = 1.28 (t, J = 7.2 Hz, 3 H, CH2CH3), 1.44
[s, 9 H, C(CH3)3], 2.99 (br.s, 2 H, SCH2), 4.21 (q, J = 7.2 Hz, 2 H,
OCH2), 4.49 (br.s, 1 H, CH), 5.37 (br.s, 1 H, NH) ppm. 13C NMR
(75.5 MHz, CDCl3): δ = 14.28 (CH2CH3), 28.46 [C(CH3)3], 35.61
(SCH2), 53.54 (CH), 61.93 (OCH2), 80.31 [C(CH3)3], 155.29
171.08 (CHC=O), 211.42, 211.69 (CϵO) ppm. IR (KBr disk): ν =
˜
2041 (vs), 1969 (vs), 1911 (s, CϵO), 1734 (s, CHC=O), 1713 (s
(NHC=O) cm–1. C25H29Fe3NO11S3 (783.2): calcd. C 38.34, H 3.73,
N 1.79; found C 38.50, H 3.71, N 1.76.
Preparation of [Fe2(µ-SCH2)2N(tBu)(CO)5][Cp(CO)2FeSCH2CH-
(NH-Boc)CO2Et] (8): The same procedure as that for 7 was fol-
lowed, but [Fe2(µ-SCH2)2N(tBu)(CO)6] (5, 0.089 g, 0.20 mmol) was
utilized instead of [Fe2(µ-SCH2)2CH2(CO)6] (4). With acetone/pe-
troleum ether (v/v = 3:4) as eluent, 8 was obtained as a brown-
back solid. Yield: 0.076 g, 45%; m.p. 55–57 °C. 1H NMR
(300 MHz, CDCl3): δ = 0.93 [s, 9 H, NC(CH3)3], 1.22 (br.s, 3 H,
CH2CH3), 1.38 [s, 9 H, OC(CH3)3], 2.42 (br.s, 2 H, CpFeSCH2),
2.92–3.15 (2br.s, 4 H, CH2NCH2), 4.15 (br.s, 2 H, OCH2), 4.41–
4.42 (m, 1 H, CH), 4.91 (s, 5 H, C5H5), 5.42 (br.s, 1 H, NH) ppm.
13C NMR (100.6 MHz, CDCl3): δ = 14.30 (CH2CH3), 26.51
[NC(CH3)3], 28.46 [OC(CH3)3], 34.16 (CpFeSCH2), 49.19
(CH2NCH2), 55.56 (CH), 57.19 [NC(CH3)3], 61.40 (OCH2), 79.74
[OC(CH3)3], 85.56 (C5H5), 155.41 (NHC=O), 171.69 (CHC=O),
(NHC=O), 170.70 (CHC=O) ppm. IR (KBr disk): ν = 1717 (s,
˜
CHC=O), 1696 (s) (NHC=O) cm–1. C10H18NNaO4S (271.3): calcd.
C 44.27, H 6.69, N 5.16; found C 44.21, H 6.53, N 5.11. [α]2D0
+1.8 (c = 0.74 g/100 mL, thf).
=
Preparation of Cp(CO)2FeSCH2CH(NH-Boc)CO2Et (3): Method 1:
A 100-mL three-necked flask fitted with a magnetic stirring bar, a
rubber septum, and a nitrogen inlet tube was charged with
NaSCH2CH(NH-Boc)CO2Et (2, 0.053 g, 0.20 mmol), thf (10 mL),
and [Cp(CO)2FeI] (0.061 g, 0.20 mmol). The mixture was stirred
for 1 h at room temperature. The solvent was removed, and the
residue was subjected to TLC with acetone/petroleum ether (v/v =
1:2.5) as eluent. From the main red band, 3 was obtained as a red
208.14, 211.47, 213.19 (CϵO) ppm. IR (KBr disk): ν = 2032 (vs),
˜
1972 (vs), 1909 (s, CϵO), 1733 (s, CHC=O), 1715 (s (NHC=O)
cm–1. C28H36Fe3N2O11S3 (840.1): calcd. C 40.02, H 4.32, N 3.33;
found C 39.80, H 4.47, N 2.93.
1
oil. Yield: 0.077 g, 90%. H NMR (300 MHz, CDCl3): δ = 1.25 (t,
J = 6.9 Hz, 3 H, CH2CH3), 1.42 [s, 9 H, C(CH3)3], 2.47 (br.s, 2 H,
Preparation of [Fe2(µ-SCH2)2N(C6H4OMe-p)(CO)5][Cp(CO)2Fe-
SCH2), 4.18 (q, J = 6.9 Hz, 2 H, OCH2), 4.30–4.33 (m, 1 H, CH), SCH2CH(NH-Boc)CO2Et] (9): The same procedure as that for 7
4.96 (s, 5 H, C5H5), 5.46 (br.s, 1 H, NH) ppm. 13C NMR was followed, but [Fe2(µ-SCH2)2N(C6H4OMe-p)(CO)6] (6, 0.099 g,
(100.6 MHz, CDCl3): δ = 13.96 (CH2CH3), 28.11 [C(CH3)3], 33.79
(CpFeSCH2), 55.29 (CH), 60.93 (OCH2), 79.28 [C(CH3)3], 85.32
0.20 mmol) was employed in place of [Fe2(µ-SCH2)2CH2(CO)6] (4).
With acetone/petroleum ether (v/v = 1:2.5) as eluent, 9 was ob-
(C5H5), 155.18 (NHC=O), 171.26 (CHC=O), 213.30 (CϵO) ppm. tained as a brown-back solid. Yield: 0.071 g, 40%; m.p. 50–52 °C.
IR (KBr disk): ν = 2022 (s), 1968 (vs, CϵO), 1738 (s, CHC=O),
1H NMR (400 MHz, CDCl3): δ = 1.28 (br.s, 3 H, CH2CH3), 1.45
1705 (s (NHC=O) cm–1. C17H23FeNO6S (425.1): calcd. C 48.01, H [s, 9 H, C(CH3)3], 2.51 (br.s, 2 H, CpFeSCH2), 3.77 (s, 3 H, OCH3),
˜
5.45, N 3.29; found C 47.86, H 5.61, N 3.14. Method 2: A 100-
mL three-necked flask fitted with a magnetic stirring bar, a rubber
4.20–4.40 (m, 7 H, CH2NCH2, OCH2, CH), 4.98 (s, 5 H, C5H5),
5.50 (br.s, 1 H, NH), 6.72–6.87 (m, 4 H, C6H4) ppm. 13C NMR
(100.6 MHz, CDCl3): δ = 14.39 (CH2CH3), 28.55 [C(CH3)3], 34.22
septum, and
a nitrogen inlet tube was charged with Boc-
Eur. J. Inorg. Chem. 2008, 164–171
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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169

L.-C. Song, J.-H. Ge, J. Yan, H.-T. Wang, X. Luo, Q.-M. Hu
FULL PAPER
(CpFeSCH2), 50.72 (CH2NCH2), 55.73 (CH), 55.93 (OCH3), 62.02
(OCH2), 80.52 [C(CH3)3], 87.12 (C5H5), 115.31, 117.37 118.01,
solution was condensed to ca. 5 mL in vacuo, and then diethyl
ether (20 mL) was added to yield a brown precipitate. The precipi-
120.18, 120.23 (C6H4), 155.42, (NHC=O), 171.77 (CHC=O), tate was washed thoroughly with diethyl ether. The washings were
207.27, 208.08, 211.29, 213.22 (CϵO) ppm. IR (KBr disk): ν = combined, and then diethyl ether was removed in vacuo to leave a
˜
2043 (vs), 1974 (vs), 1911 (s), CϵO), 1733 (s, CHC=O), 1715 (s,
NHC=O) cm–1. C31H34Fe3N2O12S3 (890.3): calcd. C 41.82, H 3.85,
N 3.15; found C 41.85, H 4.07, N 3.19.
residue. The residue was subjected to TLC with acetone/petroleum
ether (1:4 v/v) as eluent. A brown band was developed, from which
[Cp(CO)2Fe]2 (0.011 g) was obtained as a dark-red solid. 1H NMR
(400 MHz, CDCl3): δ = 4.78 (s, 10 H, 2C5H5) ppm. This value is
virtually the same as that (δ = 4.79 ppm) reported for an authentic
sample.[41] In addition, its cyclic votammogram and cyclic voltam-
metric data are completely the same as the corresponding values
shown in Figure 4 and Table 2, respectively. Similarly, [Cp(CO)2-
Fe]2 (0.009 g) was isolated from the electrolytic solution of 8
(0.084 g, 0.10 mmol) produced under the same electrolytic condi-
tions.
X-ray Structure Determination of 8: Single crystals of 8 suitable for
X-ray diffraction analysis was grown by slow evaporation of the
diethyl ether/hexane solution of 8 at 4 °C. A single crystal of 8 was
mounted on a Rigaku MM-007 (rotating anode) diffractometer
equipped with Saturn 70CCD. Data were collected at room tem-
perature, by using a confocal monochromator with Mo-Kα radia-
tion (λ = 0.71070 Å) in the ω-φ scanning mode. Data collection,
reduction and absorption correction were performed by the CRYS-
TALCLEAR program.[42] The structure was solved by direct meth-
ods with the SHELXS-97 program[43] and refined by full-matrix
least-squares techniques (SHELXL-97)[44] on F2. Hydrogen atoms
were located by using the geometric method. Details of crystal
data, data collections, and structure refinements are summarized
in Table 3.
Electrochemistry: A solution of nBu4NPF6 (0.1 ) in MeCN was
used as an electrolyte in all cyclic voltammetric and bulk electro-
lytic experiments. All measurements were made with a BAS Epsilon
potentiostat. Voltammograms were obtained in a three-electrode
cell with a glassy carbon working electrode of 3 mm diameter, a
platinum counterelectrode, and a Ag/Ag+ (0.01  AgNO3/0.1 
nBu4NPF6 in MeCN) reference electrode under a nitrogen atmo-
sphere. The working electrode was polished with 0.05 µm alumina
paste and sonicated in water for at least 10 min prior to use. All
potentials are quoted against the ferrocene/ferrocenium (Fc/Fc+)
potential. Bulk electrolytic experiments were run with a glassy car-
bon rod (A = 2.9 cm2) in a two-compartment, gastight, H-type elec-
trolysis cell containing ca. 20 mL of MeCN. The electrolyses of
solutions were carried out under hydrodynamic conditions, by vig-
orously stirring the solutions to mitigate mass transport complica-
tions. Gas chromatography was performed with a Shimadzu Gas
Chromatograph GC-9A (column: carbon molecular sieves TDX-1,
2.5 mϫ5 mm; column temperature: 40 °C) under isothermal con-
ditions with N2 as a carrier gas and a thermal conductivity detec-
tor.
Table 3. Crystal data and structure refinement details for 8.
Formula
C28H36Fe3N2O11S3·O(C2H5)2
Mr [gmol–1]
Crystal size [mm]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
914.44
0.10ϫ0.08ϫ0.06
triclinic
¯
P1
10.349(3)
12.028(4)
18.482(7)
103.092(8)
101.725(9)
104.146(9)
2089.8(12)
2
γ [°]
V [Å]
Z
ρcalcd. [gcm–3]
µ [mm–1]
F(000)
1.453
1.233
948.00
Acknowledgments
2θmax [°]
54.84
14731
8452
–12ՅhՅ12
–15ՅkՅ15
–23ՅlՅ14
1.089
0.0658
0.1603
We are grateful to the National Natural Science Foundation of
China and the Research Fund for the Doctoral Program of Higher
Education of China for financial support.
Reflections collected
Independent reflections
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