Protophilicity, electrochemical property, and desulfurization of diiron dithiolate complexes containing a functionalized C2 bridge with two vicinal basic sites

Article abstract of DOI:10.1016/j.poly.2009.01.006

Two diiron dithiolate complexes [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅L] (L = PPh₃, 2; P(Pyr)₃, 3) containing a functionalized C₂ bridge with two vicinal basic sites were prepared and characterized as models

Full text of DOI:10.1016/j.poly.2009.01.006

Polyhedron 28 (2009) 1138–1144
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Protophilicity, electrochemical property, and desulfurization of diiron dithiolate
complexes containing a functionalized C2 bridge with two vicinal basic sites
a
a
a
a,b,
Ting-Ting Zhang a, Mei Wang a, , Ning Wang , Ping Li , Zheng-Yi Liu , Li-Cheng Sun
*
*
a State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Centre on Molecular Devices, Dalian University of Technology,
Zhongshan Road 158-46, 116012 Dalian, Liaoning, China
b Department of Chemistry, Royal Institute of Technology (KTH), 10044 Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Two diiron dithiolate complexes [{
l
-SC(NBn)CH(NHBn)S-
l
}Fe2(CO)5L] (L = PPh3, 2; P(Pyr)3, 3) containing
Received 1 December 2008
Accepted 14 January 2009
Available online 14 February 2009
a functionalized C2 bridge with two vicinal basic sites were prepared and characterized as models of the
FeFe-hydrogenase active site. The molecular structures of 2 and its N-protonated form [(2HN)(OTf)] were
determined by X-ray analyses of single crystals. In the solid state of [(2HN)(OTf)]. Each asymmetric unit
contains a molecule of [(2HN)(OTf)] and a molecule of water. The molecule of water is close to the iron
atom of the [Fe(CO)3] unit (FeꢀꢀꢀO(H2O), 4.199 Å). The complexes 2 and 3 are relatively protophilic. 31P
NMR spectra and cyclic voltammograms show that they can be protonated by the mild acids CCl3COOH
and CF3COOH. Electrochemical studies show that the first reduction peak of 3 at ꢁ1.51 V versus Fc+/Fc is
110 mV more positive than that (ꢁ1.62 V) found for the analogous diiron azadithiolate complex
Keywords:
Carbonyl displacement
Desulfurization
Diiron complexes
Electrochemical property
[FeFe]-hydrogenase
[{(
650 mV for the FeIFeI/FeIFe0 reduction potentials. The shifts are apparently larger than that (450 mV)
for protonation of 7. The reaction of the all-carbonyl complex [{ -SC(NBn)CH(NHBn)S- }Fe2(CO)5L] with
two equivalents of bis(diphenylphosphino)methane (dppm) in refluxing toluene affords a desulfurized
complex [( -S)( -dppm)2Fe2(CO)4] (6). The reaction process was studied. dppm mono-dentate
intermediate [{ -SC(NBn)CH(NHBn)S- }Fe2(CO)5( dppm -bridging species
1-dppm)] (4) and
[{ -SC(NBn)CH(NHBn)S- }Fe2(CO)4(
characterized.
l-SCH2)2N(CH2C6H5)}Fe2(CO)5P(Pyr)3] (7). Protonation of 2 and 3 leads to the anodic shifts of 610–
l
l
l
l
A
l
l
j
a
l
l
l
l
-dppm)] (5) have been isolated and spectroscopically
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
ble protonated forms of diiron azadithiolate complexes, both at the
bridging-nitrogen atom and the iron center, have also been spec-
troscopically characterized in situ [14–16]. Recently, we have re-
ported the crystal structure of a proton-hydride diiron complex
The chemistry of diiron dithiolate complexes has attracted
intensive attention in recent years since their close resemblance
in structure to the FeFe-hydrogenase active site (H-cluster) [1,2].
The DFT investigations suggest that the central atom in the dithiol-
ate bridge of the H-cluster is a nitrogen (SCH2NHCH2S) or an oxy-
gen atom (SCH2OCH2S) [3–5]. Both the theoretical calculations and
the studies on the synthetic model complexes have shown that the
internal base may act as a proton transfer relay in the [FeFe]-H2ase
active site (H-cluster) [6–9]. The protophilicity and the electro-
chemical property of diiron dithiolate complexes containing differ-
ent internal bases have been extensively reported. Protonation of
diiron azadithiolate complexes at the bridging-nitrogen atom has
been verified spectroscopically and crystallographically [8–13],
which results in considerable anodic shifts (370–420 mV) of the
FeIFeI-to-FeIFe0 reduction potentials for the diiron complexes. Dou-
[(
PCH2N(n-Pr)CH2PPh2) [17]. In addition, there is one report on the
double protonation at the
-donor ligand (CNꢁ) and the iron cen-
l-H)(l
-pdt){Fe(CO)3}{Fe(CO)(j2p,p0-PNHP)}](OTf)2 (PNP = Ph2-
r
ter, resulting in an unstable species [18]. The pyridyl group has
been tethered to the central carbon of the propane-1,3-dithiolato
(pdt) bridge of the diiron complex as an internal base and a
hemi-labile ligand [19]. The protonation and deprotonation of
the pyridyl group can control the extent of carbonylation of the
diiron model complex.
The afore-mentioned internal bases of the FeFe-Hase mimics,
namely, the nitrogen atoms in the azapropanedithiolato bridge,
the CNꢁ ligand, and the pyridyl group, can be protonated in organic
solvents only in the presence of strong acids [8–19]. Studies on the
diiron complexes featuring an internal basic site, which can be
readily protonated by weak acids with a large influence on the
FeIFeI/FeIFe0 reduction potential, are of interest for design of iron-
based electrochemical and photochemical catalysts for proton
* Corresponding authors. Tel.: +86 411 88993886; fax: +86 411 83702185.
E-mail address: symbueno@dlut.edu.cn (M. Wang).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.01.006

T.-T. Zhang et al. / Polyhedron 28 (2009) 1138–1144
1139
reduction to dihydrogen. In this work, we prepared and character-
ized two diiron dithiolate complexes [{ -SC(NBn)CH(NHBn)S-
}Fe2(CO)5L] (L = PPh3, 2; P(Pyr)3, 3) containing a functionalized
C2 bridge with two vicinal basic sites. We found that the reaction
of the complex [{ -SC(NBn)CH(NHBn)S- }Fe2(CO)6] (1) with two
equivalents of bis(diphenylphosphino)methane (dppm) in reflux-
ing toluene afforded a desulfurized complex [( -S)( -dppm)2Fe2-
(CO)4] (6) via dppm mono-dentate intermediate [{
SC(NBn)CH(NHBn)S- }Fe2(CO)5(
1-dppm)] (4) and a dppm
bridging species [{ -SC(NBn)CH(NHBn)S- }Fe2(CO)4(
2.3. Synthesis of [{l-SC(NBn)CH(NHBn)S-l}Fe2(CO)5PPh3](OTf)
l
l
HOTf (1.5 equivalents) was added to a solution of 2 (0.81 g,
1.0 mmol) in diethyl ether (50 ml). The red solution was stirred
for 5 min and stood for 3 h. Protonated species [(2HN)(OTf)] was
precipitated as red powder from the solvent and washed three
times with cold diethyl ether. Yield: 0.83 g (87%). Anal. Calc. for
C40H32F3Fe2N2O8PS3: C, 49.81; H, 3.34; N, 2.90. Found: C, 49.43;
l
l
l
l
a
l
l
-
-
l
j
H, 3.42; N, 2.65%. IR (KBr):
m(CO) 2061 (s), 2009 (s), 1948 (s)
l
l
l
-dppm)]
cmꢁ1 1H NMR (CDCl3): d 7.09–7.43 (25H, 5Ph), 4.77, 4.58 (2d,
.
(5). The complexes 2 and 3 can be protonated by mild acids
CCl3COOH and CF3COOH in acetonitrile, resulting in a 610–
650 mV anodic shift of the FeIFeI/FeIFe0 reduction potentials. Here,
we report the preparation and characterization of 2–6, the
molecular structures of 2, its protonated species [(2HN)(OTf)],
and 6, the protophilicity of 2 and 3, as well as the electrochemical
properties of 2 and 3 compared with the analogous diiron azadithi-
JH–H = 13.2 Hz, 2H, C@NCH2), 4.40–4.80 (br, 2H, NH2), 4.10, 3.87
(2d, JH–H = 12.8 Hz, 2H, NHCH2), 3.59 (s, 1H, SCH) ppm. 31P NMR
(CDCl3): d 59.10 ppm.
2.4. Synthesis of [{l-SC(NBn)CH(NHBn)S-l}Fe2(CO)5(dppm)] (4)
Ligand dppm (0.39 g, 1.0 mmol) was added to a solution of 1
(0.58 g, 1.0 mmol) in toluene (20 ml). The mixture was refluxed
for 0.5 h. The resulting solution was evaporated to dryness under
reduced pressure, and the residue was chromatographed on an alu-
mina column with hexane/CH2Cl2 (2:1, v/v) as eluent. The complex
4 was obtained as red powder from the collected red band after re-
olate complex [{(l-SCH2)2N(CH2C6H5)}Fe2(CO)5P(Pyr)3] (7).
2. Experimental
moval of solvent. Yield: 0.66 g (69%). IR (KBr):
(vs), 1934 (m) cmꢁ1. 1H NMR (CDCl3): d 7.10–7.35 (30H, 6Ph), 4.52,
4.20 (2d, JH–H = 15.8 Hz, 2H, C@NCH2), 3.92, 3.61 (2d, JH–H
13.2 Hz, 2H, NHCH2), 3.40 (s, 1H, SCH), 3.18 (d, JH–H = 8.0 Hz, 2H,
PCH2P), 2.17 (s, 1H, NH) ppm. 31P NMR (CDCl3): d 53.18 (d, JP–P
m(CO) 2045 (s), 1986
2.1. General procedures and materials
=
All reactions and operations related to organometallic com-
plexes were carried out under dry, oxygen-free dinitrogen with
standard Schlenk techniques. Solvents were dried and distilled
prior to use according to the standard methods. Commercially
available chemicals, Me3NO ꢀ 2H2O, HOTf, CCl3COOH, CF3COOH,
PPh3, and dppm, were of reagent grade and used as received. Li-
=
91.5 Hz, coordinated dppm), ꢁ26.31 (d, JP–P = 91.5 Hz, non-
coordinated dppm) ppm. ESI-MS: m/z 937.1 [M+H]+.
2.5. Synthesis of [{l-SC(NBn)CH(NHBn)S-l}Fe2(CO)4(l-dppm)] (5)
gand P(Pyr)3, diiron carbonyl complexes [{
l-SC(NBn)CH(NHBn)S-
l
}Fe2(CO)6] (1) and [{( -SCH2)2N(CH2C6H5)}Fe2(CO)5P(Pyr)3] (7)
l
The toluene solution (20 ml) of 1 (0.58 g, 1.0 mmol) and dppm
(0.39 g, 1.0 mmol) was refluxed for 1.5 h. Complex 5 was obtained
by chromatography of the residue on an alumina column with hex-
were prepared following the literature methods [20–22].
Infrared spectra were recorded on JASCO FT/IR 430 spectro-
photometer. Proton and 31P NMR spectra were collected with a
Varian INOVA 400 NMR Instrument. Mass spectra were recorded
on an HP1100 MSD mass spectrometer. Elemental analyses were
performed with
analyzer.
ane/CH2Cl2 (1:1, v/v) as eluent. Yield: 0.60 g (66%). IR (KBr):
mCO
1995 (s), 1964 (vs), 1931 (s) cmꢁ1 1H NMR (CDCl3): d 7.11–7.55
.
(30H, 6Ph), 4.92, 4.73 (2br, 2H, C@NCH2), 4.35, 4.01 (2br, 2H,
NHCH2), 4.10 (s, 2H, PCH2P), 3.46 (s, 1H, SCH), 2.20 (br, 1H, NH)
ppm. 31P NMR (CDCl3): d 57.23 (d, JP–P = 59.9), 56.31 (br), 56.03
(br), 54.30 (d, JP–P = 53.4) ppm. ESI-MS: m/z 909.1 [M+H]+.
a
Thermoquest-Flash EA 1112 elemental
2.2. Synthesis of [{l-SC(NBn)CH(NHBn)S-l}Fe2(CO)5L] (L = PPh3, 2;
P(Pyr)3, 3)
2.6. Synthesis of [(l-S)(l-dppm)2Fe2(CO)4] (6)
CO-Removing reagent Me3NO ꢀ 2H2O (0.11 g, 1.0 mmol) was
added to a solution of 1 (0.58 g, 1.0 mmol) in MeCN (20 ml). The
red solution was stirred for 5 min and the color turned dark red.
Triphenylphosphine (0.27 g, 1.0 mmol) was added to the solution,
and it was stirred for 30 min. The solvent was removed under re-
duced pressure. The residue was purified by chromatography on
a silica gel column with hexane/CH2Cl2 (2:1, v/v) as eluent. The
complex 2 was obtained as red powder from the collected red band
after removal of solvent. Yield: 0.70 g (86%). Anal. Calc. for
C39H31Fe2N2O5PS2: C, 53.99; H, 3.71; N, 3.15. Found. C, 53.78; H,
Ligand dppm (0.78 g, 2.0 mmol) was added to a solution of 1
(0.58 g, 1.0 mmol) in toluene (20 ml). The solution was refluxed
for 6 h. The complex 6 was precipitated as red powder from the
solution and washed three times with acetone. Yield: 0.72 g
(70%). Anal. Calc. for C54H44Fe2O4P4S: C, 63.30; H, 4.33. Found: C,
63.27; N, 4.35%. IR (KBr):
mCO 1967 (m), 1925 (s), 1898 (m), 1875
(w) cmꢁ1 31P NMR (CH2Cl2): d 44.20 ppm.
.
2.7. X-ray crystal structure determination
3.78; N, 3.07%. IR (KBr): m .
(CO) 2048 (s), 1988 (s), 1938 (m) cmꢁ1
Single crystals of 2, [(2HN)(OTf)], and 6 were obtained by recrys-
tallization from dichloromethane/hexane. All of the single crystal
X-ray diffraction data were collected on a Bruker Smart Apex II
1H NMR (CDCl3): d 7.00–7.52 (25H, 5Ph), 4.54, 4.39 (2d, JH–
H = 15.8 Hz, 2H, C@NCH2), 3.87, 3.59 (2d, JH–H = 13.8 Hz, 2H,
NHCH2) 3.11 (s, 1 H, SCH), 2.10 (s, 1 H, NH) ppm. 31P NMR (CDCl3):
d 60.24 ppm.
CCD diffractometer with graphite monochromated Mo K
a radia-
tion (k = 0.71073 Å) at 298 K using the
x
ꢁ2h scan mode. Data pro-
The complex
described protocol. Yield: 0.50 g (80%). IR (KBr):
2007 (s), 1991 (s) cmꢁ1 1H NMR (CDCl3): d 7.13–7.27 (10H, 2Ph),
3
was prepared according to the above-
cessing was accomplished with the SAINT program. Intensity data
were corrected for absorption by the SADABS program. The structures
were solved by direct methods and refined on F2 against full-ma-
trix least-squares methods by using the SHELX-97 program package.
All non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were placed by geometrical calculation and refined in a
riding model, and the proton on the N atom of [(2HN)(OTf)] was
m(CO) 2059 (s),
.
6.63, 6.21 (2s, 12H, 3Pyr), 4.79, 4.73 (2d, JH–H = 14.4 Hz, JH–
H = 27.2 Hz, 2H, C@NCH2), 3.94, 3.70 (2d, JH–H = 8.8 Hz, 2H, NHCH2),
3.28 (s, 1H, SCH), 2.21 (s, 1H, NH) ppm. 31P NMR (CDCl3): d
140.01 ppm.

1140
T.-T. Zhang et al. / Polyhedron 28 (2009) 1138–1144
Table 1
Crystallographic data and processing parameters for 2, [(2HN)(OTf)], and 6.
Complex
2
[(2HN)(OTf)]
6
Empirical formula
Mw
C40H33Cl2Fe2N2O5PS2
899.37
C41H34Cl2F3Fe2N2O9PS3
1065.45
C54H44Fe2O4P4S
1024.53
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P2(1)/n
13.412(3)
15.090(3)
20.288(5)
90.00
94.815(3)
90.00
4092(2)
triclinic
P1
11.5545(7)
12.2831(8)
19.086(1)
71.785
86.715
64.865
2320.4(3)
2
1.525
triclinic
P1
ꢀ
ꢀ
12.242(2)
12.453(2)
18.658(3)
108.06(1)
97.30(1)
108.486(8)
2483.3(6)
2
1.370
1056
0.07 ꢂ 0.10 ꢂ 0.17
1.81/25.00
18416/8400
586
a
(°)
b (°)
c
(°)
V (Å3)
Z
4
qcalc (g cmꢁ3
F(000)
)
1.460
1840
0.06 ꢂ 0.49 ꢂ 0.52
2.21/27.16
19849/7145
472
1088
Crystal size (mm3)
hMin./Max. (°)
0.09 ꢂ 0.13 ꢂ 0.22
2.17/18.65
17183/7943
584
Reflections collected/unique
Parameters refined
Goodness-of-fit on F2
1.057
1.011
0.932
Final R1 [I > 2
r
(I)]
0.0629
0.0437
0.0704
Final wR2
0.1833
1.236, ꢁ1.094
0.1164
0.651, ꢁ0.487
0.1341
0.665, ꢁ0.543
Residual electron density (e Åꢁ3
)
located by the difference Fourier map. Crystal data and parameters
for data collections and refinements of complexes 2, [(2HN)(OTf)],
and 6 are listed in Table 1. The quality of the single crystal of the
complex 6 containing solvent was not good, resulting in the large
Rint value (0.1613).
2 and 3 is clearly indicative of the weaker electron donating capa-
bility of the P(Pyr)3 ligand as compared to the PPh3 ligand. The 1H
H
NHCH2Ph
PhH2CN
S
NHCH2Ph
PhCH2N
S
2.8. Electrochemistry
+ HOTf
S
S
Acetonitrile (Aldrich, spectroscopy grade) used for electrochem-
ical measurements was dried with molecular sieves and then
OC
PPh3
CO
PPh3
+ aniline
OC
Fe
Fe
CO
Fe
Fe
OC
freshly distilled from CaH2 under N2.
A solution of 0.05 M
OC
OC
CO
CO
OC
n[NBu4][PF6] (Fluka, electrochemical grade) in CH3CN was used
as electrolyte, which was degassed by bubbling with dry argon
for 10 min before measurement. Electrochemical measurements
were recorded using a BAS-100W electrochemical potentiostat at
a scan rate of 100 mV/s. Cyclic voltammograms were obtained in
a three-electrode cell under argon. The working electrode was a
glassy carbon disc (diameter 3 mm) successively polished with 3
[(2HN)(OTf)]
2
Scheme 1.
NHCH2Ph
NHCH2Ph
PhH2CN
PhH2CN
and 1 lm diamond pastes and sonicated in ion-free water for
10 min. The reference electrode was a non-aqueous Ag/Ag+ elec-
trode (0.01 M AgNO3 in CH3CN) and the auxiliary electrode was a
platinum wire. All potentials are reported relative to the Fc+/Fc
potential.
S
S
S
S
dppm
toluene
reflux 0.5 h
OC
CO
P
OC
CO
Ph
Ph
Fe
Fe
Fe
Fe
OC
OC
CO
P
Ph
OC
CO
OC
CO
Ph
1
4
3. Results and discussion
reflux, 1 h
NHCH2Ph
3.1. Preparation and spectroscopic characterization of the complexes
2, 3, and [(2HN)(OTf)]
The parent complex [{l-SC(NBn)CH(NHBn)S-l}Fe2(CO)6] (1)
was prepared according to the literature method [20], which is
an oil and very sensitive to air and moisture. More stable diiron
PhH2CN
Ph
Ph
Ph
Ph
OC
OC
P
Fe
P
P
dppm
S
S
complexes [{
l
-SC(NBn)CH(NHBn)S-l}Fe2(CO)5PPh3] (2) and [{l-
S
CO
reflux, 4.5 h
CO
Fe
CO
OC
SC(NBn)CH(NHBn)S-l}Fe2(CO)5P(Pyr)3] (3) were obtained by treat-
Fe
Fe
CO
ing 1 with a phosphine ligand in the presence of the CO-removing
reagent Me3NO ꢀ 2H2O in CH3CN. The complexes 2 and 3 are stable
in the solid state and in the solution. They were characterized by
IR, 1H and 31P NMR spectroscopy, and the elementary analysis.
OC
P
P
P
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
PhH2CN
NHCH2Ph
6
5
C
C
The complex 2 displays three
1938 cmꢁ1 in the IR spectrum, and the
at 2059, 2007, and 1991 cmꢁ1. Comparison of the
m
(CO) bands at 2048, 1988, and
(CO) bands of 3 appear
(CO) bands of
S
H
m
m
Scheme 2.

T.-T. Zhang et al. / Polyhedron 28 (2009) 1138–1144
1141
of the SCH group directly attached to the protonated-N atom is
moved down-field by ca. 0.31 ppm in the 1H NMR spectrum of
[(2HN)(OTf)]. The spectroscopic evidence strongly supports the
protonation of the N atom in the complex 2. Upon addition of an
equivalents of aniline to the solution of [(2HN)(OTf)], the 31P signal
at 58.0 ppm for the N-protonated species completely disappeared,
and in the same time the signal at 60.2 ppm for the complex 2 ap-
peared as the only peak in the 31P NMR spectrum, indicating a
reversible protonation and deprotonation process between 2 and
[(2HN)(OTf)] (Scheme 1).
3.2. CO-displacement of 1 by dppm
The reaction of the complex
bis(diphenylphosphino)methane (dppm) in refluxing toluene for
0.5 h afforded the complex [{ -SC(NBn)CH(NHBn)S- }Fe2-
(CO)5(
1-dppm)] (4), in which dppm acts as a simple mono-
1 with an equivalents of
l
l
j
dentate ligand. When the toluene solution of 4 was further re-
fluxed for 1 h, an intramolecular CO-displacement occurred to
form a
(CO)4(
was not detected during the reaction by 31P NMR monitoring,
which is different from the reaction of ( -pdt)[Fe2(CO)6] and dppm
[23,24]. If another equivalents of dppm was added to the toluene
solution of 5, an unexpected complex [( -S)( -dppm)2Fe2(CO)4]
l-dppm bridging complex [{
l-SC(NBn)CH(NHBn)S-l}Fe2-
l
-dppm)] (5). The complex with a chelating dppm ligand
l
l
l
(6) was obtained after 4.5 h reflux. The complex 6 was formed by
simultaneous CO-displacement and desulfurization of 5 (Scheme
2). Sulfur-containing organic compound PhCH2N@CHC(S)NHCH2Ph
was detected in the reaction solution by HPLC–MS analysis. The
first two steps for the reaction of 1 and dppm to form the com-
plexes 4 and 5 are just similar to the reaction of (
and dppm [23,24], while the third step, desulfurization reaction, is
special for 1. The complex ( -pdt)[Fe2(CO)4( -dppm)], in analogy
to 5, did not react with dppm even in refluxing toluene for 72 h.
The results show that the [ -SC@NBnCH(NHBn)( -S)] bridge in 1
is apparently less stable than the pdt bridge. The complexes 4–6
were characterized by IR, MS, 1H and 31P NMR spectroscopy, and
the molecular structure of 6 was determined by X-ray diffraction
study.
l-pdt)[Fe2(CO)6]
Fig. 1. The IR spectra of complexes 4 (bottom), 5 (middle), and 6 (top).
l
l
l
l
signals for the two CH2 groups, the CH, and the NH group in the
complex 3 are all move to down-field by 0.11–0.17 ppm compared
to the corresponding signals of 2. The complexes 2 and 3 display
31P signals at 60.2 and 140.0 ppm, respectively.
Protonation of 2 in diethyl ether with addition of 1.5 equiva-
lents HOTf afforded the N-protonated species [(2HN)(OTf)] as red
precipitate in a good yield (87%) after being washed with cold
diethyl ether and recrystallization in hexane/CH2Cl2. In comparison
Except for the additional 1H signals of dppm, the complex 4 dis-
plays a similar 1H NMR spectrum with that of 2. In addition to the
signals at 7.10–7.35 ppm for phenyl groups of dppm and the C2
bridge, a typical doublet at 3.18 ppm (J = 8.0 Hz) for the CH2 group
of dppm is found in the 1H NMR spectrum of 4. Two doublets, at
53.2 ppm for the coordinate phosphorus and ꢁ26.3 ppm for the
non-coordinate one, appear in the 31P{1H} NMR spectrum of 4.
As the free phosphorus of dppm in 4 is coordinated to the other
iron atom by replacement of a CO ligand, the 1H signal for the
to the m(CO) bands of 2, the m(CO) bands of [(2HN)(OTf)] is shifted to
higher frequencies by ca. 15 cmꢁ1 in average, indicating that pro-
tonation of the amino-N atom on the C2 bridge has apparent influ-
ence on the electron density of the iron atoms. This shift value of
the
m(CO) bands is comparable to those reported for N-protonation
of other analogous diiron complexes [8–15]. The proton resonance
Fig. 2. Molecular structures of 2 (left) and (2HN)+/H2O with the thermal ellipsoids at 30% probability.

1142
T.-T. Zhang et al. / Polyhedron 28 (2009) 1138–1144
Fig. 3. Diagram of the dimeric structure of [(2HN)(OTf)] encircling two water molecules by hydrogen bonds (ellipsoids at 30% probability).
CH2 group of dppm ligand is moved from 3.18 to 4.10 ppm. There
are two doublets at 57.23 ppm (JP–P = 59.9) and 54.30 ppm
(JP–P = 53.4) as well as two broad signals at 56.31 and 56.03 ppm
in the 31P spectrum of 5, indicative of existence of two conforma-
tion isomers for the complex 5 in solution in an approximate equal
The complex 4 displays three
2061–1948 cmꢁ1 (Fig. 1), which are shifted by ca. 50 cmꢁ1 in aver-
age to lower frequency as compared to the (CO) bands of the com-
plex 1. The subsequent intramolecular CO/phosphine replacement
results in further red-shift of the (CO) bands to 1995–
1931 cmꢁ1. The complex 6 with two
-bridging dppm ligands
exhibits four
m(CO) bands in the region of
m
a
m
ratio. The isomerization caused by swing of the
l-bridging dppm
l
ligand is similar as that found for ( -pdt)( -PNP)[Fe(CO)PMe3]-
l
l
m
(CO) bands in the region of 1967–1875 cm-1. The
[Fe(CO)2] in our previous studies [25]. Because of the non-symmet-
IR, MS, 1H and 31P NMR data give clear evidence for the formation
of phosphine mono-, di-, and tetra-substituted complexes 4, 5, and
6, in the different stages of the CO-displacement of 1.
ric bridge of 5, the chemical shifts should be different for the two
phosphorus centers of the
l
-dppm ligand. Therefore four 31P sig-
nals are observed, and the two broad signals at 56.31 and
56.03 ppm result from the overlap of two doublets. The complex
6 displays only one 31P signal at 44.2 ppm for four coordinate phos-
phorus atoms, indicating that 6 has a symmetric molecular
structure.
3.3. Molecular structures of 2, [(2HN)(OTf)], and 6
To obtain unambiguous evidence for the N-protonation of 2, the
molecular structures of 2 and its N-protonated species [(2HN)(OTf)]
were determined by X-ray analyses of single crystals, which are de-
picted in Figs. 2 and 3. The selected bond lengths and angles are
summarized in Table 2. The central 2Fe2S skeletons of 2 and
(2HN)+ are both in the butterfly framework and each Fe atom is
coordinated in the pseudo-square-pyramidal geometry similar to
previously reported 2Fe2S models [9–14,23,26]. The protonation
of 2 occurs at the nitrogen atom [N(1)] of the secondary amine.
The NꢀꢀꢀN distance (2.704 Å) is proper for intramolecular N–HꢀꢀꢀN
interaction, which can considerably promote the stability of the
N-protonated species. After protonation, both the bond lengths of
N(1)–C(6) and N(1)–C(8) are elongated by 0.037 and 0.034 Å,
respectively, while the bond length of N(2)–C(7) is shortened by
0.01 Å.
Fig. 3 shows that two molecules of water are caged by two
[(2HN)(OTf)] molecules through hydrogen bonds. In the solid state
of [(2HN)(OTf)], each asymmetric unit contains a molecule of
[(2HN)(OTf)] and a molecule of water. The molecule of water is
close to the iron atom of the [Fe(CO)3] unit with the Fe(1)ꢀꢀꢀO(H2O)
distance of 4.199 Å. It is proposed that H2O is coordinated to the
distal iron atom in the Hox state of the natural H-cluster [2,24].
The bond lengths and angles related to hydrogen bonds are given
in Table S1 (see Supplementary data).
Table 2
Selected bond lengths (Å) and angles (°) for 2 and [(2HN)(OTf)] ꢀ H2O.
Complex
2
(2HN)+
Bond lengths
Fe(1)–Fe(2)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(2)–S(1)
Fe(2)–S(2)
Fe(2)–P(1)
S(1)–C(6)
S(2)–C(7)
N(1)–C(6)
N(1)–C(8)
N(2)–C(7)
N(2)–C(9)
N(1)ꢀꢀꢀN(2)
2.527(1)
2.256(1)
2.269(2)
2.249(1)
2.266(1)
2.253(1)
1.847(4)
1.803(5)
1.437(6)
1.468(6)
1.260(6)
1.456(6)
2.728
2.5109(7)
2.2651(9)
2.274(1)
2.250(1)
2.263(1)
2.260(1)
1.816(3)
1.788(3)
1.474(4)
1.502(4)
1.250(4)
1.455(5)
2.704
Bond angles
S(1)–Fe(1)–S(2)
S(1)–Fe(2)–S(2)
P(1)–Fe(2)–Fe(1)
C(6)–N(1)–C(8)
C(7)–N(2)–C(9)
N(1)–C(6)–C(7)
N(1)–C(6)–S(1)
C(7)–C(6)–S(1)
N(2)–C(7)–S(2)
N(2)–C(7)–C(6)
C(6)–C(7)–S(2)
80.64(5)
80.87(5)
151.23(4)
114.7(4)
121.3(4)
110.8(4)
111.1(3)
107.9(3)
125.2(4)
119.4(4)
115.3(3)
81.14(3)
81.72(3)
151.05(3)
114.6(3)
121.5(3)
109.8(3)
112.0(3)
110.2(2)
127.5(3)
118.3(3)
114.1(2)
The molecular structure of 6 was also determined by single
crystal X-ray analyses, which is shown in Fig. 4, and selected bond
lengths and angles are given in Table 3. The central iron atoms of 6
are coordinated in the trigon-bipyramidal geometry, consistent
with the structure of [(
l-CO)(l-dppm)2Fe2(CO)4] [27]. Each iron
atom is coordinated with two terminal carbonyls, which are

T.-T. Zhang et al. / Polyhedron 28 (2009) 1138–1144
1143
Fig. 5. Cyclic voltammograms of 3 (1.0 mM) in the presence of CF3COOH in CH3CN.
SCH2)2N(CH2C6H5)}Fe2(CO)5P(Pyr)3] (7) was prepared. The cyclic
voltammograms (CVs) of 2, 3, and 7 were studied in CH3CN using
0.05 M [NBu4][PF6] as electrolyte. The redox potentials versus
Fc+/Fc are summarized in Table 4. The first reduction peaks of 2
and 3 appear at ꢁ1.66 and ꢁ1.51 V, respectively, using ferrocene
as an internal standard. The approximately equal current height
in the CVs of complexes 2 and 3 and ferrocene with an identical
molar concentration suggests that the primary reduction of com-
plexes 2 and 3 is a one-electron process (FeIFeI/FeIFe0). The reduc-
tion potential of 3 is 110 mV more positive than that (ꢁ1.62 V)
found for the analogous diiron azadithiolate complex 7. The
oxidation peaks, at +0.60 and +0.83 V for 2 and 3, respectively,
and +0.59 V for 7, are attributed to the FeIFeI to FeIIFeI process
[29,30].
Upon addition of one equivalents of HOTf to the CH3CN solution
of 3, a new peak is observed at ꢁ0.90 V in the CV, attributed to the
reduction of the protonated species [(3HN)(OTf)] (Fig. S1,
Supplementary data). Essentially identical CVs were obtained for
protonation of 2 with a new peak at ꢁ1.01 V. In comparison, the
reduction peak of [(7HN)(OTf)] appears at ꢁ1.11 V (Fig. S1). Proton-
ation of 2 and 3 leads to the 610–650 mV anodic shifts of the
reduction peaks, while after protonation, the reduction peak of 7
is shifted by 450 mV. It shows that protonation of the internal base
of 2 and 3 exhibits a larger influence on the reduction potential of
the iron atoms compared to that displayed by the protonation of
the azadithiolato bridge of 7. It is noteworthy that the complexes
2 and 3 can be protonated by mild acids CCl3COOH and CF3COOH
(Fig. 5). In contrast, no new peak is detected in the CV when
CF3COOH is added to the solution of 7 (Fig. S2). The protonation
of 2 and 3 is further verified by in situ 31P NMR spectroscopy with
stepwise addition of CCl3COOH to the CDCl3 solution of 2 or 3
(Fig. S3). The results indicate that the complexes 2 and 3 are more
protophilic than the diiron azadithiolate the complex 7. In fact, in
organic solvents the internal bases of the FeFe-Hase mimics
reported so far can be protonated only in the presence of
strong acids, such as HClO4 [11,13], HBF4 [12,16,19], and HOTf
[10,14,18].
Fig. 4. Molecular structure of 6 with the thermal ellipsoids at 30% probability.
Table 3
Selected bond lengths (Å) and angles (°) for 6.
Bond lengths
Fe(1)–Fe(2)
Fe(1)–S(1)
Fe(2)–S(1)
2.838(2)
2.267(2)
2.240(2)
Fe(1)–P(1)
Fe(1)–P(3)
Fe(2)–P(2)
Fe(2)–P(4)
2.229(2)
2.204(2)
2.230(2)
2.215(2)
Bond angles
Fe(1)–S(1)–Fe(2)
S(1)–Fe(1)–Fe(2)
S(1)–Fe(2)–Fe(1)
P(1)–Fe(1)–P(3)
P(2)–Fe(2)–P(4)
P(1)–Fe(1)–Fe(2)
P(2)–Fe(2)–Fe(1)
77.98(8)
50.60(7)
51.42(7)
174.3(1)
172.4(1)
92.40(9)
91.19(9)
P(3)–Fe(1)–Fe(2)
P(4)–Fe(2)–Fe(1)
P(1)–Fe(1)–S(1)
P(2)–Fe(2)–S(1)
P(3)–Fe(1)–S(1)
P(4)–Fe(2)–S(1)
93.16(9)
93.25(9)
97.22(9)
93.1(1)
85.23(9)
84.8(1)
Table 4
Redox potentials of complexes 3 and 7 vs. Fc+/Fc.a
Complex
Epa (V) FeIFeI/FeIIFeI
Epc (V) FeIFeI/Fe0FeI
2
+0.60
–
+0.83
–
+0.59
–
ꢁ1.66
ꢁ1.01
ꢁ1.51
ꢁ0.90
ꢁ1.62
ꢁ1.17
[(2HN)(OTf)]
3
[(3HN)(OTf)]
7
[(7HN)(OTf)]
a
[NBu4][PF6] (0.05 M) in CH3CN under Ar; scan rate 0.1 V sꢁ1
.
essentially co-planar with the bridging-sulfur atom. This plane is
perpendicular to the plane defined by the four phosphorus atoms
and the two iron atoms. The Fe(1)–Fe(2) bond length of
2.8383(16) Å in
[( -CO)( -dppm)2Fe2(CO)4], resulting from the replacement of
the -CO bridge by -S bridge [28].
6 is notably longer than that (2.742 Å) in
l
l
4. Conclusion
l
l
The 31P NMR spectra and CVs show that the complexes 2 and 3
with a C2 bridge containing two basic sites are more protophilic
than other internal bases in the FeFe-Hase mimics reported so
far. The omplexes 2 and 3 can be protonated by CCl3COOH and
CF3COOH in organic solvent. Protonation and deprotonation
3.4. Cyclic voltammograms of the complexes 2, 3, and 7
To compare the electrochemical property and the protophilicity
of
2 and 3 with the azadithiolate-bridging complex, [{(l-

1144
T.-T. Zhang et al. / Polyhedron 28 (2009) 1138–1144
processes between
2 and [(2HN)(OTf)], as well as 3 and
References
[(3HN)(OTf)], are instant and reversible. The protonated species
[(2HN)(OTf)] is relatively stable because of the proper distance
for intramolecular N–HꢀꢀꢀN interaction, which is structurally char-
acterized. It is noteworthy that each asymmetric unit of
[(2HN)(OTf)] containing a molecule of water close to the iron atom
of the [Fe(CO)3] subunit (Fe(1)ꢀꢀꢀO(H2O) 4.199 Å), which reminds us
of H2O coordinating to the distal iron atom in the Hox state of the
natural H-cluster. The reduction potential of the complex 3 is
110 mV more positive than that of the analogous azadithiolate
complex 7, and protonation of the internal base of 2 and 3 exhibits
a larger influence on the reduction potential of the iron atoms than
that displayed by the protonation of the azadithiolato bridge of 7.
In addition, a desulfurization reaction is found in the reaction of
the complex 1 and two equivalents of dppm ligand to afford the
complex 6 in refluxing toluene. The reaction undergoes by three
[1] J.W. Peters, W.N. Lanzilotta, B.J. Lemon, L.C. Seefeldt, Science 282 (1998) 1853.
[2] Y. Nicolet, C. Piras, P. Legrand, C.E. Hatchikian, J.C. Fontecilla-Camps, Structure
7 (1999) 13.
[3] Y. Nicolet, B.J. Lemon, J.C. Fontecilla-Camps, J.W. Peters, Trends Biochem. Sci.
25 (2000) 138.
[4] Y. Nicolet, A.L. Lacey, X. Vernède, V.M. Fernandez, E.C. Hatchikian, J.C.
Fontecilla-Camps, J. Am. Chem. Soc. 123 (2001) 1596.
[5] A.S. Pandey, T.V. Harris, L.J. Giles, J.W. Peters, R.K. Szilagyi, J. Am. Chem. Soc.
130 (2008) 4533.
[6] H. Fan, M.B. Hall, J. Am. Chem. Soc. 123 (2001) 3828.
[7] T. Zhou, Y. Mo, A. Liu, Z. Zhou, K.R. Tsai, Inorg. Chem. 43 (2004) 923.
[8] B.E. Barton, M.T. Olsen, T.B. Rauchfuss, J. Am. Chem. Soc. 130 (2008) 16834.
[9] S. Ezzaher, J.-F. Capon, F. Gloaguen, F. Pétillon, P. Schollhammer, J. Talarmin,
Inorg. Chem. 48 (2009) 2.
[10] J.D. Lawrence, H. Li, T.B. Rauchfuss, M. Bénard, M. Rohmer, Angew. Chem., Int.
Ed. 40 (2001) 1768.
[11] S. Ott, M. Kritikos, B. Åkermark, L. Sun, R. Lomoth, Angew. Chem., Int. Ed. 43
(2004) 1006.
[12] P. Das, J. Capon, F. Gloaguen, F.Y. Pétillon, P. Schollhammer, J. Talarmin, Inorg.
Chem. 43 (2004) 8203.
[13] F. Wang, M. Wang, X. Liu, K. Jin, W. Dong, G. Li, B. Åkermark, L. Sun, Chem.
Commun. (2005) 3221.
steps via a dppm mono-dentate intermediate 4 and a dppm
bridging species 5.
l-
[14] L. Schwartz, G. Eilers, L. Eriksson, A. Gogoll, R. Lomoth, S. Ott, Chem. Commun.
(2006) 520.
[15] G. Eilers, L. Schwartz, M. Stein, G. Zampella, L. de Gioia, S. Ott, R. Lomoth, Chem.
Eur. J. 13 (2007) 7075.
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (Grant No. 20633020), the National Basic Research Program
of China (Grant No. 2009CB220009), the Program for Changjiang
Scholars and Innovative Research Team in University (IRT0711),
the Swedish Energy Agency, the Swedish Research Council, and
K&A Wallenberg Foundation for financial support of this work.
[16] B.E. Barton, T.B. Rauchfuss, Inorg. Chem. 47 (2008) 2261.
[17] N. Wang, M. Wang, T. Zhang, P. Li, J. Liu, L. Sun, Chem. Commun. (2008) 5800.
[18] F. Gloaguen, J.D. Lawrence, T.B. Rauchfuss, M. Bénard, M.-M. Rohmer, Inorg.
Chem. 41 (2002) 6573.
[19] F. Xu, C. Tard, X. Wang, S.K. Ibrahim, D.L. Hughes, W. Zhong, X. Zeng, Q. Luo, X.
Liu, C.J. Pickett, Chem. Commun. (2008) 606.
[20] R. Siebenlist, H. Frühaufa, H. Kooijman, N. Veldman, A.L. Spek, K. Goubitz, J.
Fraanje, Inorg. Chim. Acta 327 (2002) 66.
[21] K.G. Moloy, J.L. Petersen, J. Am. Chem. Soc. 117 (1995) 7696.
[22] Y. Na, M. Wang, J. Pan, P. Zhang, B. Åkermark, L. Sun, Inorg. Chem. 47 (2008)
2805.
[23] F. Adam, G. Hogarth, I. Richards, J. Organomet. Chem. 692 (2007) 3957.
[24] W. Gao, J. Ekstrom, J. Liu, C. Chen, L. Eriksson, L. Weng, B. Åkermark, L. Sun,
Inorg. Chem. 46 (2007) 1981.
Appendix A. Supplementary data
CCDC 693477, 693476 and 693475 contain the supplementary
crystallographic data for complexes 2, [(2HN)(OTf)], and 6, respec-
tively. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2009.01.006.
[25] N. Wang, M. Wang, T. Liu, P. Li, T. Zhang, M.Y. Darensbourg, L. Sun, Inorg.
Chem. 47 (2008) 6948.
[26] Z. Gao, M.B. Hall, J. Am. Chem. Soc. 123 (2001) 3734.
[27] K.G. Moodley, D.W. Engel, J.S. Field, R.J. Haines, Polyhedron 12 (1993) 533.
[28] P. Thometzek, K. Zenkert, H. Werner, Angew. Chem., Int. Ed. 24 (1985) 516.
[29] D. Chong, I.P. Georgakaki, R. Mejia-Rodriguez, J. Sanabria-Chinchilla, M.P.
Soriaga, M.Y. Darensbourg, Dalton Trans. (2003) 4150.
[30] P. Li, M. Wang, C. He, G. Li, X. Liu, C. Chen, B. Åkermark, L. Sun, Eur. J. Inorg.
Chem. (2005) 2506.