Diiron dithiolate complexes containing intra-ligand NH...S hydrogen bonds: [FeFe] hydrogenase active site models for the electrochemical proton reduction of HOAc with low overpotential

Article abstract of DOI:10.1039/b715990k

Four diiron dithiolate complexes containing ortho-acylamino-functionalized arenethiolato ligands, [(μ-S-2-RCONHC₆H₄) ₂Fe₂(CO)₆] (R = CH₃, 1; CF ₃, 2; C₆H₅,

Full text of DOI:10.1039/b715990k

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Diiron dithiolate complexes containing intra-ligand NH · · · S hydrogen bonds:
[FeFe] hydrogenase active site models for the electrochemical proton reduction
of HOAc with low overpotential†
Ze Yu,a Mei Wang,*a Ping Li,a Weibing Dong,a Fujun Wanga and Licheng Sun*a,b
Received 17th October 2007, Accepted 8th February 2008
First published as an Advance Article on the web 7th March 2008
DOI: 10.1039/b715990k
Four diiron dithiolate complexes containing ortho-acylamino-functionalized arenethiolato ligands,
[(l-S-2-RCONHC6H4)2Fe2(CO)6] (R = CH3, 1; CF3, 2; C6H5, 3; 4-FC6H4, 4), were synthesized and well
characterized as biomimetic models of the Fe–Fe hydrogenase active site. The molecular structures of 3
and 4 were determined by X-ray crystallography. The intra-ligand NH · · · S hydrogen bonds were
studied by the X-ray analysis and by the 1H NMR spectroscopy. The contribution of the NH · · · S
hydrogen bonds to the reduction potentials of complexes 1–4 was investigated by electrochemistry. The
first reduction potentials of complexes 1–4 exhibit large positive shifts, that is, 220–320 mV in
comparison to that of the analogous complex [(l-SPh)2Fe2(CO)6] and 370–470 mV to that of
[(l-pdt)2Fe2(CO)6] (pdt = propane-1,3-dithiolato). Complex 4 is capable of electrocatalysing proton
reduction of acetic acid at relatively low overpotential (ca. 0.2 V) in acetonitrile.
electrocatalytic proton reduction with relatively large overpoten-
tial ca. (0.75 to 1.22 V) after protonation on the iron atoms
or on the bridged-N atom by moderate to strong acids, HOTs,
Introduction
Although the complexes of the general formula [(l-SR)2Fe2(CO)6]
have been known for over 70 years,1 the chemistry of di-
iron dithiolate complexes did not attract extensive attention
HBF4·Et2O, H2SO4, and HClO4 in CH3CN. The native Fe–Fe
hydrogenase can catalyse the reversible reduction of protons to
until the striking resemblance in structure between the Fe–
molecular hydrogen at neutral pH and at an incredibly low
potential (ca. −0.4 V vs. NHE).22,23 Biomimic of the property
Fe hydrogenase active site and the well-known complexes [(l-
SR)2Fe2(CO)6] was disclosed.2,3 In recent years, a great progress
and reactivity of the Fe–Fe hydrogenase at such a low potential in
has been achieved in structurally biomimicking the [Fe2S2] subunit
aqueous medium is undoubtedly a challenge and an arduous task.
of Fe–Fe hydrogenase active site. A large number of diiron
While much effort has focused on adjusting the redox potential,
dithiolate complexes, either with the alkylene bridges, e.g. pdt
the protophilicity, and the water solubility of diiron dithiolate
(propane-1,3-dithiolato)4 and edt (ethane-1,2-dithiolato),5–7 or an
model complexes by CO-displacement with various ligands,
adt (2-azopropane-1,3-dithiolato) bridge,8–11 were prepared and
characterized. Most reported structural models, such as [(l-
pdt)Fe2(CO)6],12 [(l-pdt){Fe(CO)3}{Fe(CO)2L}] (L = P(NC4H8)3,
N-heterocyclic carbene (NHC)),13,14 [(l-pdt){Fe(CO)2L}2] (L =
PMe3, 1,3,5-triaza-7-phosphaadamantane (PTA)),12 and [(l-pdt)-
such as cyanide,24–26 tertiary phosphines and phosphites,27–31
isocyanides,32,33 and N-heterocyclic carbenes,14,34 little work has
been reported on tuning the reactivity and the electrochemical
property of the diiron center by varying the dithiolato part. It
was reported that in the presence of HOAc diiron complexes con-
taining l-SEt bridges displayed similar activity for electrocatalytic
proton reduction as the analogous pdt-bridged complex at almost
identical reduction potentials.35 The alkyl in the l-SR bridge and
the alkylene in the l-SRS bridge have little effect on the redox
potential of the diiron center.36 Introduction of the functionalized
R group to the dithiolato bridge may give an additional approach
to adjust the redox property of the diiron center.
2
{Fe(CO)3}{Fe(CO)(g -L)}] (L = 1-methyl-3-(2-pyridyl)imidazo-
le-2-ylidene (NHCMePy)),15 can electrocatalyse the proton re-
duction with 0.5 to 1.0 V overpotential in the presence of
weak acid (HOAc) in CH3CN.16 Some diiron dithiolate com-
plexes, [(l-pdt){Fe(CO)2(CN)}{Fe(CO)2(PMe3)}],17,18 [(l-pdt)-
2
{Fe(CO)3}{Fe(CO)(g -IMe-CH2-IMe)}] (IMe = 1-methylimidazol-2-
ylidene),19 [{(l-SCH2)2N(CH2C6H4-4-Br)}Fe2(CO)6],20 and [{(l-
SCH2)2NCH2(2-C4H3S)}Fe2(CO)6],21 were reported active for
The crystallographic and spectroscopic studies have shown that
the NH · · · S hydrogen bonds are biologically important for the
activity of electron transfer metalloenzymes, e.g. rubredoxins,37
ferredoxins,38 and high-potential iron-sulfur proteins.39 It has been
reported that the number and extent of the NH · · · S hydrogen
bonds in rubredoxin and ferredoxin peptide model complexes,40,41
as well as in molybdenum oxotransferase models,42,43 can effec-
tively control the redox potentials of the metal centers. Enlightened
by these reports, we designed and prepared the diiron complexes
containing ortho-acylamino-functionalized arenethiolato ligands,
on the purpose to tune the redox potential of the diiron center
aState Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and
Research Center on Molecular Devices, Dalian University of Technology
(DUT), Dalian, 116012, China. E-mail: symbueno@dlut.edu.cn; Fax: +86-
411-83702185; Tel: +86-411-88993886
bDepartment of Chemistry, Royal Institute of Technology (KTH), 10044,
Stockholm, Sweden
† Electronic supplementary information (ESI) available: Fig. S1 and S2:
packing geometry and the NH · · · S data for complexes 3 and 4; Fig. S3:
cyclic voltammograms of 3, 4 and [(l-SPh)2Fe2(CO)6] (1.0 mM) in CH2Cl2.
CCDC reference numbers 655667 and 655668. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b715990k
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Table 1 The mCO bands of 1–4, and related diiron dithiolate complexesa
by forming the intra-ligand NH · · · S hydrogen bond. Herein
we report the preparation and spectroscopic characterization of
diiron dithiolate complexes, [(l-S-2-RCONHC6H4)2Fe2(CO)6] (1,
R = CH3; 2, CF3; 3, C6H5; 4, 4-FC6H4). Complexes 3 and 4
were structurally characterized, which shows that these complexes
contain the intra-ligand NH · · · S hydrogen bond. As expected,
complexes 1–4 display considerable positive shifts (370–470 mV)
for the reduction potentials of the diiron centers as compar-
ison to that displayed by the well-known diiron complex [(l-
pdt)Fe2(CO)6], and complex 4 exhibit relatively low overpotential
(0.2 V) for electrochemically catalysed proton reduction of acetic
acid in CH3CN. The result drops a hint that the possible NH · · · S
hydrogen bond between the amido group in protein and the
sulfur bridge in the H-cluster may have a contribution to the low
reduction potential and the possible proton pathway of the Fe–Fe
hydrogenase active site.
Complex
mCO/cm−1
1
2074 (m), 2037 (s), 1998 (s)
2077 (m), 2042 (s), 2003 (s)
2075 (m), 2038 (s), 1999 (s)
2076 (m), 2040 (s), 2001 (s)
2073 (m), 2032 (s), 1994 (s)
2072 (m), 2028 (s), 1987 (s)
2
3
4
[(l-SPh)2Fe2(CO)6]
[(l-pdt)Fe2(CO)6]
a In CH2Cl2.
group at d 7.6–9.0, which are relatively sharp and shifted to
downfield compared with the corresponding signal of acetanilide
in the same solvent (CDCl3), giving a hint to the formation of
hydrogen bonds for the amido hydrogen (NH).47 The chemical
shifts of the signals for the NH group in complexes 1–4 did not
change as the concentration of the complexes was increased up
to eightfold. The insensitivity of the NH chemical shift toward
variation of the complex concentration provides an indirect proof
for the intramolecular hydrogen bonds in solution between the
amido hydrogen and the adjacent sulfur atom in complexes 1–
4.47 The intramolecular hydrogen bonds in the obtained diiron
complexes were further proved by the X-ray studies on the crystal
structures of complexes 3 and 4.
Results and discussion
Preparation and spectroscopic characterization of complexes 1–4
The preparation of diiron dithiolate complexes with the general
formula [(l-SR)2Fe2(CO)6] by reactions of various disulfides
and iron(0) carbonyl compounds can be traced back to the
1960s.44,45 Complexes 1–3 were prepared using this traditional
protocol (Scheme 1). Treatment of Fe3(CO)12 with 1 equiv of
bis(2-acylaminophenyl) disulfide in toluene at 70 ◦C afforded
the predesigned diiron complexes 1–3 as red crystalline solid
in moderate yields. Owing to the poor solubility of bis(2-(4-
fluorobenzoylamino)phenyl) disulfide in toluene, complex 4 was
prepared in refluxing THF following the essentially identical
procedure in 46% yield. To make a comparison with 1–4, the
known complex [(l-SPh)2Fe2(CO)6] was also prepared according
to the literature procedure.46
Crystal structures of complexes 3 and 4
The molecular structures of 3 and 4 are shown in Fig. 1 and 2,
and selected bond lengths and angles are listed in the captions.
Complex 3 exists as two conformational isomers in the crystalline
state, while only one conformation was found for complex 4.
The central [Fe2S2] structures of 3 and 4 are in the familiar
butterfly framework, and each Fe atom is coordinated in the
square-based pyramidal geometry as those in previously reported
[Fe2S2] models.4,25–29 The Fe–Fe distances (2.4994(8)–2.5123(4) A)
˚
˚
fall in the normal range of the Fe–Fe bond length (2.49–2.57 A)
found in the analogous diiron complexes.25–29 Viewing along the
Fe–Fe bond, we can find that the basal CO ligands are eclipsed
with each other in 3 and 4. Complexes 3 and 4 both possess an
axial/equatorial (a/e) configuration for the two thiolate ligands. It
is reasonable that the preparation of 1–4 either at 70 ◦C in toluene
or in refluxing THF afforded thermodynamically more stable a/e
isomers. We did not observe the conversion of the a/e isomer
Scheme 1
1
to the e/e isomer in CDCl3 by H NMR spectroscopy, which is
Complexes 1–4 were characterized by IR, 1H NMR spec-
troscopy and elemental analysis. The IR data of mCO bands for 1–4
are listed in Table 1, together with the mCO data of [(l-pdt)Fe2(CO)6]
and [(l-SPh)2Fe2(CO)6] for a ready comparison. Complexes 1–4
each show one moderate and two strong mCO bands in the region
of 1990–2080 cm−1. The average mCO bands of 1 and 3 move by
4–8 cm−1 to higher wavenumbers as comparison to the average mCO
bands of [(l-pdt)Fe2(CO)6] and [(l-SPh)2Fe2(CO)6], implicating
a certain decrease of the electron density in the diiron center of
complexes 1 and 3. Introduction of an electron-withdrawing group
CF3 or 4-FC6H4 to the acylamino substituent caused the further
blue-shift of the mCO bands of complexes 2 and 4.
consistent with the fact reported for [(l-SPh)2Fe2(CO)6].48
˚
For conformational isomer 3a the N(2) · · · S(2) distance (3.17 A)
is relatively long as the N(2)–H bond is directed opposite to
the S(2) atom, indicating there is no N(2)H · · · S(2) hydrogen
˚
bond (Fig. S1), while the short N(1) · · · S(1) distance (2.94 A)
suggests the presence of the N(1)H · · · S(1) hydrogen bond. The
N(2)H group in the equatorial l-SR ligand of conformational
˚
=
isomer 3a forms the N(2)H · · · O(7) C(13) (N(2) · · · O(7) 2.87 A)
intermolecular hydrogen bond. For conformational isomer 3b,
=
there also exist one NH · · · S hydrogen bond and one NH · · · O C
bond (Fig. 3). The location of the hydrogen atom of the N(4)H
group is close to the adjacent arenethiolate S(4) atom (N(4) · · · S(4)
˚
The proton signal for the NH group was studied to detect the
intramolecular hydrogen bonds in the diiron complexes obtained.
Complexes 1–4 display the signals for the hydrogen of the amido
2.93 A) in the equatorial l-SR ligand of 3b, favoring the
formation of the N(4)H · · · S(4) hydrogen bond, while the N(3)H
=
group in the axial l-SR ligand forms the N(3)H · · · O(8) C(26)
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Fig. 1 Molecular structure of the two conformational isomers of 3, with thermal ellipsoids at 30% probability. Hydrogen atoms except for
˚
those on nitrogen atoms have been omitted for clarity. Selected bond lengths (A) and angles (deg) for 3a: Fe(1)–Fe(2), 2.4998(7); Fe(1)–S(1),
2.2742(10); Fe(1)–S(2), 2.2661(10); Fe(2)–S(1), 2.2629(10); Fe(2)–S(2), 2.2579(10); Fe(1)–C(1), 1.811(5); Fe(1)–C(2), 1.778(4); Fe(1)–C(3), 1.787(5);
N(1) · · · S(1), 2.9389, N(2) · · · S(2), 3.1721; Fe(1)–S(1)–Fe(2), 66.87(3); Fe(1)–S(2)–Fe(2), 67.09(3); S(1)–Fe(1)–S(2), 79.96(4); S(1)–Fe(2)–S(2), 80.37(4);
C(7)–S(1)–Fe(1), 111.81(12); C(20)–S(2)–Fe(1), 114.56(12); C(1)–Fe(1)–S(1), 104.57(12); C(1)–Fe(1)–S(2), 100.08(11). For 3b: Fe(3)–Fe(4), 2.4994(8);
Fe(3)–S(3), 2.2969(12); Fe(3)–S(4), 2.2511(11); Fe(4)–S(3), 2.2666(12); Fe(4)–S(4), 2.2733(11); Fe(3)–C(33), 1.811(5); Fe(3)–C(34), 1.789(5); Fe(3)–C(35),
1.766(5); N(3) · · · S(3), 2.9380, N(4) · · · S(4), 2.9263; Fe(3)–S(3)–Fe(4), 66.42(3); Fe(3)–S(4)–Fe(4), 67.07(3); S(3)–Fe(3)–S(4), 78.65(4); S(3)–Fe(4)–S(4),
78.82(4); C(39)–S(3)–Fe(3), 114.39(13); C(52)–S(4)–Fe(3), 118.08(13); C(33)–Fe(3)–S(3), 114.31(13); C(33)–Fe(3)–S(4), 95.07(13).
Fig. 3 The N(4)H · · · S(4) and N(3)H · · · O(8) hydrogen bonds in the solid
state of complex 3.
Fig. 2 Molecular structure of 4, with thermal ellipsoids at 30% prob-
ability. Hydrogen atoms except for those on nitrogen atoms have been
˚
omitted for clarity. Selected bond lengths (A) and angles (deg): Fe(1)–Fe(2),
2.5123(4); Fe(1)–S(1), 2.2818(6); Fe(1)–S(2), 2.2738(6); Fe(2)–S(1),
2.2679(6); Fe(2)–S(2), 2.2730(6); Fe(1)–C(1), 1.812(2); Fe(1)–C(2),
1.790(3); Fe(1)–C(3), 1.792(2); N(1) · · · S(1), 2.9989, N(2) · · · S(2),
2.9741; Fe(1)–S(1)–Fe(2), 67.034(19); Fe(1)–S(2)–Fe(2), 67.084(18);
S(1)–Fe(1)–S(2), 80.33(2); S(1)–Fe(2)–S(2), 80.65(2); C(7)–S(1)–Fe(1),
116.20(7); C(20)–S(2)–Fe(1), 119.15(7); C(1)–Fe(1)–S(1), 98.33(8);
C(1)–Fe(1)–S(2), 109.90(8).
˚
(N(3) · · · O(8) 2.88 A) intermolecular hydrogen bond. Fig. 4 shows
the intramolecular hydrogen bonds in the solid state of complex 4.
˚
The N(2) · · · S(2) distance (2.97 A) in the axial arm of complex 4
Fig. 4 The N(2)H · · · S(2) hydrogen bond in the solid state of complex 4.
is short enough for the formation of the N(2)H · · · S(2) hydrogen
bond (Fig. 4).42 The oxygen atom O(7) in the equatorial arm of
4 forms intermolecular hydrogen bonds with both the hydrogen
atom of the N(1)H group and one of the ortho-Hs of the 4-FC6H5
group, forming a 7-membered ring (Fig. S2).†
Electrochemistry of complexes 1–4 and electrocatalytic property
of 4
Cyclic voltammograms (CVs) of complexes 1−4, shown in Fig. 5,
were recorded in anhydrous CH3CN with 0.05 M nBu4NPF6 as
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1–4 and [(l-pdt)Fe2(CO)6] suggests that the primary reduction
of complexes 1–4 is a one-electron process (FeIFeI/FeIFe0). The
control experiment showed that the second reduction peak did not
disappear in CO-saturated CH3CN solution, indicating that it does
not result from the CO-displaced species of the diiron complexes.
Introduction of the electron-withdrawing group, e.g. CF3 (2) and
4-FC6H4 (4), to the acylamino substituent results in a small anodic
shift (50–70 mV), compared with the corresponding potentials of
1 and 3, respectively.
The reduction potentials of 1–4, having an amide NH group at
the ortho position of an arenethiolato ligand, are positively shifted
by 220–320 mV in comparison to that of their analogous complex
[(l-SPh)2Fe2(CO)6] (−1.51 V vs. Fc/Fc+), and by 370–470 mV to
that of the familiar diiron complex [(l-pdt)Fe2(CO)6] (−1.66 V vs.
Fc/Fc+).35,36 To exclude the possible influence of the intermolecu-
lar hydrogen bonds between the amido NH group and solvent on
the reduction potentials, the cyclic voltammograms of 3, 4 and [(l-
SPh)2Fe2(CO)6] were measured in CH2Cl2, and the first reduction
potentials move to −1.57, −1.43, and −1.88 V vs. Fc/Fc+ (Fig.
S3†), respectively. The large positive shifts (310–450 mV) of
reduction potentials of 3 and 4 relative to that of the analogous
complex [(l-SPh)2Fe2(CO)6] in CH2Cl2 suggest that the shifts of
the reduction potentials of diiron dithiolate complexes containing
the ortho-amido NH group could result from the formation of the
intra-ligand NH · · · S hydrogen bonds as reported for other iron-
sulfur and molybdenum-sulfur complexes.40–43 The intermolecular
NH · · · O hydrogen bonds, which is far away from the iron center,
cannot display important influence on the reduction potential of
the iron center. And considering the large steric hindrance for the
ortho-amido group in complexes 1–4, the intermolecular NH · · · S
hydrogen bonds between two diiron complexes in a dilute solution
(1.0 mM) should be much less favored as comparison to the
intra-ligand NH · · · S hydrogen bonds. A puzzling phenomenon
is found that complexes 1–4 display the relatively large shifts in
reduction potentials but the small shifts of the average mCO bands in
comparison to those of [(l-pdt)Fe2(CO)6] and [(l-SPh)2Fe2(CO)6].
Similar phenomena were also reported for the diiron dithiolate
complexes with different l-SR and (l-S)2R bridges.35,36 It is
noteworthy that the diiron dithiolate complexes with different
ligands, such as cyanide, tertiary phosphines, isocyanides and
N-heterocyclic carbenes, often display quite sympathetic shifts
for the mCO frequencies and reduction potentials, while the all-
carbonyl diiron dithiolate complexes with different l-SR and (l-
S)2R bridges sometimes display the relatively small shift of the mCO
frequencies compared with the large shift in reduction potentials.
The plausible explanation for this phenomenon might be that
the R group and the NH · · · S hydrogen bond, which are far
away from the CO ligand, have indirectly influences on the CO
electron density. Another alternate explanation is also possible.
The greater interaction of the bridging-S atom and the amido
group involves the reduced complex, resulting in the apparent
shift of the reduction events to the positive potential, while the
neutral complexes show only small shifts of the mCO bands in the
IR spectra.
Fig. 5 Cyclic voltammograms of (top) complexes 1 and 2, and (bottom)
3, 4 and [(l-SPh)2Fe2(CO)6] (1.0 mM) in CH3CN with 0.05 M nBu4NPF6
at a scan rate of 100 mV s−1
.
electrolyte. The redox potentials of 1–4 and the related complexes,
e.g. [(l-pdt)2Fe2(CO)6] and [(l-SPh)2Fe2(CO)6], were given in
Table 2. Complexes 1–4 each display an irreversible reduction
peak at −1.19 to −1.29 V and a weak peak at −1.66 to −1.73 V
vs. Fc/Fc+. In light of the previously reported electrochemical
and spectroelectrochemical studies on the all-CO complex [(l-
pdt)Fe2(CO)6],12,35,49,50 the current height in the CVs of complexes
1–4 is compared with that in the CV of [(l-pdt)Fe2(CO)6] with
identical concentration and under the same measuring condition.
The approximately equal current height in the CVs of complexes
Table 2 The redox potentials of 1–4 and the related diiron complexes in
CH3CNa
Complex
E
pa/Vb
E
pc1/V
Epc2, V
1
+0.74
+0.88
+0.75
+0.77
+0.72
+0.82
−1.29
−1.22
−1.24
−1.19
−1.51
−1.66
−1.73
−1.68
−1.71
−1.66
−2.24
−2.27
2
3
4
The behavior of catalytic proton reduction by 4 was investigated
by cyclic voltammograms in the presence of weak acids (HOAc) in
CH3CN. The blank electrochemical scan for acetic acid (40 mM)
in the absence of electrocatalyst was made and the obtained CV
curve is presented in Fig. 6 for a convenient comparison. The
[(l-SPh)2Fe2(CO)6]
[(l-pdt)2Fe2(CO)6]c
a In 0.05 M nBu4NPF6/CH3CN at a scan rate of 100 mV s−1. b All potentials
are versus Fc/Fc+. c Ref. 35 and 36.
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arenethiolate are under way to further improve the electrocatalytic
property of [Fe2S2] model complexes.
Experimental
All procedures were carried out under dry nitrogen atmosphere
with standard Schlenk techniques. Solvents were dried and
distilled under nitrogen before use. Bis(2-aminophenyl) disulfide,51
bis(2-acetylaminophenyl) disulfide,52 bis(2-(trifluoroacetyla-
mino)phenyl) disulfide,42 bis(2-benzoylaminophenyl) disulfide,52
bis(2-(4-fluorobenzoylamino)phenyl) disulfide,40 and [(l-SPh)2-
Fe2(CO)6] were prepared according to the literature procedures.46
Infrared spectra were recorded on a JASCO FT/IR 430
spectrophotometer. Proton NMR spectra were collected on a
Varian INOVA 400 NMR spectrometer with tetramethylsilane
(TMS) as internal standard. Elemental analyses were performed
with a Thermoquest-Flash EA 1112 elemental analyzer.
Fig. 6 Cyclic voltammograms of 4 (1.0 mM) and HOAc (0–40 mM) in
CH3CN.
General procedure for preparation of diiron dithiolate complexes
1–4, [(l-S-2-RCONHC6H4)2Fe2(CO)6]
addition of 0–40 mM HOAc does not affect the current height
and the potential of the first reduction peak of 4, while the
current height of the second reduction peak grows apparently with
increase of the HOAc concentration (Fig. 6). Such observation in
the CVs indicates that complex 4 is electrocatalytic active at the
second reduction potential (−1.66 V) for reduction of protons
from HOAc. The electrocatalytic property of 4 in the presence
of weak acid is quite similar to those of [(l-pdt)2Fe2(CO)6] and
[(l-SEt)2Fe2(CO)6] reported by Darensbourg and co-worker,12,35
which also shows that the first reduction peak is inert and
the second one at ca. −2.27 V vs. Fc/Fc+ is electrocatalytic
active with addition of 0–100 mM HOAc. It is noteworthy that
the overpotential for electrochemical reduction of protons from
weak acid catalysed by the well-known diiron complexes [(l-
pdt)2Fe2(CO)6] and [(l-SEt)2Fe2(CO)6] is ca. 0.8 V,16 while the
overpotential for catalysed proton reduction of HOAc based on
complex 4 is dramatically reduced to ca. 0.2 V. To the best of
our knowledge, it is the lowest overpotential so far reported for
electrochemically catalysed proton reduction of a weak acid based
on bio-inspired diiron dithiolate model complexes.
Iron carbonyl compound Fe3(CO)12 (3.0 g, 5.95 mmol) was
suspended in toluene (80 mL) followed by the addition of 1 equiv
of bis(2-acetylaminophenyl) disulfide (1.98 g, 5.95 mmol). The
reaction mixture was stirred at 70 ◦C until the color of the solution
changed from deep green to dark red (ca. 10 h). The reaction
mixture was cooled to room temperature and filtered. The red
filtrate was evaporated to dryness under vacuum. The residue
was subjected to silica gel chromatography and eluted with n-
hexane/dichloromethene to give pure complex 1 as red powder.
Complexes 2–4 were prepared with the essentially identical
procedure.
[(l-S-2-CH3CONHC6H4)2Fe2(CO)6] (1). Yield: 1.87 g (51%).
1H NMR (CDCl3): d = 2.17 (s, 3H), 2.27 (s, 3H), 6.94 (t, 1H) 7.02
(t, 1H), 7.09 (d, 1H), 7.19 − 7.41 (m, 3H), 7.62 (s, 2H), 8.06 (d, 1H),
8.15 (d, 1H). IR (CH2Cl2): mmax/cm−1 2074 (m), 2037 (s), 1998 (s)
(CO). Found: C 43.41; H 2.79; N 4.63. Calc. for C22H16Fe2N2O8S2:
C 43.16; H 2.63; N 4.58%.
[(l-S-2-CF3CONHC6H4)2Fe2(CO)6] (2). Yield: 1.67 g (39%).
1H NMR (CDCl3): d = 7.05–7.22 (m, 3H), 7.31–7.48 (m, 3H),
8.10 (d, 1H), 8.20 (t, 1H), 8.53 (s, 2H). IR (CH2Cl2): mmax/cm−1
2077 (m), 2042 (s), 2003 (s) (CO); Found: C 36.52; H 1.46; N 3.74.
Calc. for C22H10F6Fe2N2O8S2: C, 36.69; H, 1.40; N, 3.89%.
Conclusions
Four diiron complexes containing ortho-acylamino functionalized
arenethiolato ligands were prepared as a new type of model
complexes for the Fe–Fe hydrogenase active site. The presence
of the NH · · · S hydrogen bond in the solid state was established
by X-ray crystallographic analyses of complexes 3 and 4, and
[(l-S-2-PhCONHC6H4)2Fe2(CO)6] (3). Yield: 2.32 g (53%). 1H
NMR (CDCl3): d = 6.94 (br, 1H), 7.08 (br, 2H), 7.30–7.80 (m, 9H),
7.94–8.12 (br, 3H), 8.27 (br, 1H), 8.57 (br, 2H), 8.95 (s, 2H). IR
(CH2Cl2): mmax/cm−1 2075 (m), 2038 (s), 1999 (s) (CO). Found: C
52.38; H 2.81; N 3.84. Calc. for C32H20Fe2N2O8S2: C, 52.20; H,
2.74; N, 3.80.
1
the evidence of H NMR spectra suggests the existence of the
intramolecular hydrogen bonds in solution. The reduction peaks
of complexes 1–4 show large positive shifts in comparison to
complexes [(l-SPh)2Fe2(CO)6] and [(l-pdt)Fe2(CO)6], attributed
to the fact that the formation of the intra-ligand NH · · · S hydrogen
bond reduces the electron donating capability of the sulfur atom.
Complex 4 can electrocatalyse the proton reduction of acetic acid
with a considerably low overpotential (ca. 0.2 V) in CH3CN. The
results show that it is effective to tune the redox potential of the
[Fe2S2] complexes by formation of the intramolecular NH · · · S
hydrogen bonds. The efforts to introduce electron-withdrawing
groups to the benzene ring of the ortho-acylamino functionalized
[{l-S-2-(4-FC6H4CONHC6H4)}2Fe2(CO)6] (4). Yield: 2.11 g
(46%). 1H NMR (CDCl3): d = 6.94 (s, 1H), 7.07 (s, 2H), 7.34 (br,
5H), 7.72 (br, 2H), 7.99 (br, 3H), 8.21 (s, 1H), 8.46 (s, 2H), 8.82
(S, 2H). IR (CH2Cl2): mmax/cm−1 2076 (m), 2040 (s), 2001 (s) (CO).
Found: C 49.72; H 2.41; N 3.65. Calc. for C32H18F2Fe2N2O8S2: C,
49.77; H, 2.35; N, 3.63.
2404 | Dalton Trans., 2008, 2400–2406
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Table 3 Crystallographic data and processing parameters for complexes 3 and 4
Complex
3
4
Formula
Mw
C32H20Fe2N2O8S2
736.32
C32H18F2Fe2N2O8S2
772.30
Crystal system
Space group
Monoclinic
P2(1)/n
Monoclinic
P2(1)/c
˚
a/A
11.4255(13)
20.178(2)
27.909(3)
98.886(6)
6357.2(12)
8
15.3983(3)
9.8861(2)
24.0297(4)
120.090(1)
3165.06(10)
4
˚
b/A
˚
c/A
b/deg
3
˚
V/A
Z
D
calcd/g cm−3
1.539
1.098
1.621
1.115
l/mm−1
Crystal size/mm
h range/deg
0.22 × 0.15 × 0.08
1.79–25.80
31825/11738
0.0460
7448/845
1.003
0.0447/0.1000
0.519/−0.337
0.48 × 0.22 × 0.15
1.96–27.94
39178/7384
0.0382
5460/441
1.013
0.0347/0.0797
0.331/−0.264
Reflns collected/indep reflns
Rint
Reflns with I > 2r(I)/parameters
GOF on F2
R1a [I >2r(I)]/wR2b
−3
˚
Residue electron density/e A
ꢀ
ꢀ
ꢀ
a R1 = ꢀFo| − |Fcꢀ. b wR2 = [ [w(Fo − Fc2)2]/ w(Fo2)2]1/2
.
2
Crystal structure determination† of complexes 3 and 4
was a platinum wire. All potentials are reported relative to the
Fc/Fc+ potential.
The crystals of 3 and 4 suitable for X-ray analysis were obtained by
recrystallization in hexane/CH2Cl2 (2:1 v/v). The single-crystal
X-ray diffraction data were collected with SMART APEXII
diffractometer for 3 and 4 with graphite monochromated Mo-
Acknowledgements
We are grateful to the Chinese National Natural Science Foun-
dation (Grant nos. 20471013 and 20633020), the Swedish Energy
Agency, the Swedish Research Council and K & A Wallenberg
Foundation for financial support of this work.
˚
Ka radiation (k = 0.71073 A) at 273 K using the x-2h scan
mode. Data processing was accomplished with the SMART and
SAINT software package of Siemens Energy & Automation
Inc. (Madison, Wisconsin). Intensity data were corrected for
absorption by the SADABS program. The structures were solved
by direct methods and refined on F2 against full-matrix least-
squares methods using the SHELXTL97 program.53 All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were placed on the calculated positions except for the hydrogen
atoms of the amides, which were located by Fourier difference
maps and adjusted by taking into account of the N–H bond
distance. Crystal data and parameters for data collections and
refinements of complexes 3 and 4 are listed in Table 3.
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