Binuclear iron-sulfur complexes with bidentate phosphine ligands as active site models of Fe-hydrogenase and their catalytic proton reduction

Article abstract of DOI:10.1021/ic0610278

The displacement of CO in a few simple Fe(I)-Fe(I) hydrogenase model complexes by bisphosphine ligands Ph₂P-(CH₂) _n,-PPh₂ [with n = 1 (dppm) or n = 2 (dppe)] is described. The reaction of [{μ-(SCH₂)₂CH₂}Fe ₂(CO)₆] (1) and [{μ-(SCH₂) ₂N(CH₂CH₂CH₃)}Fe₂(CO) ₆] (2) with dppe gave double butterfly complexes [{μ-(SCH ₂)₂CH₂}Fe₂(CO)₅(Ph ₂-PCH₂)]₂ (3) and [{μ-(SCH₂) ₂N(CH₂CH₂CH₃)}Fe₂(CO) ₅(Ph₂PCH₂ (4), where two Fe₂S ₂ units are linked by the bisphosphine. In addition, an unexpected byproduct, [{μ-(SCH₂)₂N(CH₂CH ₂CH₃)}Fe₂(CO)₅{Ph ₂PCH₂CH₂(Ph₂PS)}] (5), was isolated when 2 was used as a substrate, where only one phosphorus atom of dppe is coordinated, while the other has been converted to P=S, presumably by nucleophilic attack on bridging sulfur. By contrast, the reaction of 1 and 2 with dppm under mild conditions gave only complexes [{μ-(SCH ₂)₂CH₂}Fe₂(CO)₅(Ph ₂PCH₂-PPh₂)] (6) and [{μ-(SCH ₂)₂N(CH₂CH₂CH₃)}Fe ₂(CO)₅(Ph₂PCH₂PPh₂)] (8), where one ligand coordinated in a monodentate fashion to one Fe ₂S₂ unit. Furthermore, under forcing conditions, the complexes [{μ-(SCH₂)₂CH₂}-Fe ₂(CO)₄{μ-(Ph₂P)₂CH₂}] (7) and [{μ-(SCH₂)₂N(CH₂CH ₂CH₃)}Fe₂(CO)₄{μ-(PH ₂P)₂CH₂}] (9) were formed, where the phosphine acts as a bidentate ligand, binding to both the iron atoms in the same molecular unit. Electrochemical studies show that the complexes 3, 4, and 9 catalyze the reduction of protons to molecular hydrogen, with 4 electrolyzed already at -1.40 V versus Ag/AgNO₃ (-1.0 V vs NHE).

Full text of DOI:10.1021/ic0610278

Inorg. Chem. 2007, 46, 1981−1991
Binuclear Iron−Sulfur Complexes with Bidentate Phosphine Ligands as
Active Site Models of Fe-Hydrogenase and Their Catalytic Proton
Reduction
Weiming Gao,† Jesper Ekstro
Bjo
1
m,‡ Jianhui Liu,*,† Changneng Chen,§ Lars Eriksson,# Linhong Weng,¶
|
1
rn Åkermark,*,‡ and Licheng Sun*,†,
State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular
DeVices, Dalian UniVersity of Technology, 116012 Dalian, China, Department of Organic Chemistry,
Arrhenius Laboratory, Stockholm UniVersity, 10691 Stockholm, Sweden, State Key Laboratory of
Structural Chemistry, Fujian Institute of Research on the Structure of Matter, 350002 Fuzhou,
China, DiVision of Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, 10691
Stockholm, Sweden, Chemistry Department, Fudan UniVersity, 200433 Shanghai, China, and KTH
Chemistry, Organic Chemistry, Royal Institute of Technology, 10044 Stockholm, Sweden
Received June 9, 2006
The displacement of CO in a few simple Fe(I)
(CH2)n PPh2 [with n 1 (dppm) or n 2 (dppe)] is described. The reaction of [{µ-(SCH2)2CH2
{µ-(SCH2)2N(CH2CH2CH3) Fe2(CO)6] (2) with dppe gave double butterfly complexes [{µ-(SCH2)2CH2
PCH2)]2 (3) and [{µ-(SCH2)2N(CH2CH2CH3) Fe2(CO)5(Ph2PCH2)]2 (4), where two Fe2S2 units are linked by the
bisphosphine. In addition, an unexpected byproduct, [{µ-(SCH2)2N(CH2CH2CH3) Fe2(CO)5 Ph2PCH2CH2(Ph2PS)
(5), was isolated when 2 was used as a substrate, where only one phosphorus atom of dppe is coordinated, while
the other has been converted to P S, presumably by nucleophilic attack on bridging sulfur. By contrast, the
reaction of 1 and 2 with dppm under mild conditions gave only complexes [{µ-(SCH2)2CH2 Fe2(CO)5(Ph2PCH2-
PPh2)] (6) and [{µ-(SCH2)2N(CH2CH2CH3) Fe2(CO)5(Ph2PCH2PPh2)] (8), where one ligand coordinated in a
monodentate fashion to one Fe2S2 unit. Furthermore, under forcing conditions, the complexes [{µ-(SCH2)2CH2
Fe2(CO)4{µ-(Ph2P)2CH2 ] (7) and [{µ-(SCH2)2N(CH2CH2CH3) Fe2(CO)4{µ-(Ph2P)2CH2 ] (9) were formed, where
−
Fe(I) hydrogenase model complexes by bisphosphine ligands Ph2P
Fe2(CO)6] (1) and
Fe2(CO)5(Ph2-
−
−
)
)
}
[
}
}
}
}
{
}]
d
}
}
}-
}
}
}
the phosphine acts as a bidentate ligand, binding to both the iron atoms in the same molecular unit. Electrochemical
studies show that the complexes 3, 4, and 9 catalyze the reduction of protons to molecular hydrogen, with 4
electrolyzed already at
−1.40 V versus Ag/AgNO3 (−1.0 V vs NHE).
Introduction
have been determined by X-ray crystallography.6-15 The
majority of these complexes have very negative redox
The realization that the active site of two bacterial
hydrogenases consists of a Fe2S2 cluster1,2 has initiated
extensive studies of such clusters.3-5 A number of model
complexes have recently been prepared, and their structures
(6) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc.
1999, 121, 9736-9737.
(7) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. Angew. Chem., Int. Ed. 1999, 38, 3178-3180.
(8) Lawrence, J. D.; Li, H.; Rauchfuss, T. B. Chem. Commun. 2001,
1482-1483.
* To whom correspondence should be addressed. E-mail: liujh@dlut.edu.cn
(J.L.), lichengs@kth.se (L.S.).
(9) Lawrence, J. D.; Li, H.; Rauchfuss, T. B.; Be´nard, M.; Rohmer, M.
Angew. Chem., Int. Ed. 2001, 40, 1768-1771.
† Dalian University of Technology.
‡ Department of Organic Chemistry, Stockholm University.
§ Fujian Institute of Research on the Structure of Matter.
# Division of Structural Chemistry, Stockholm University.
¶ Fudan University.
(10) Gao, W.; Liu, J.; Åkermark, B.; Sun, L. Inorg. Chem. 2006, 45, 9169-
9171.
(11) Liaw, W.-F; Tsai, W.-T; Gau, H.-B; Lee, C.-M; Chou, S.-Y; Chen,
W.-Y; Lee, G.-H. Inorg. Chem. 2003, 42, 2783-2788.
(12) Cloiree, A. L.; Best, S. P.; Borg, S.; Davies, S. C.; Evans, D. J.;
Hughes, D. L.; Pickett, C. J. Chem. Commun. 1999, 2285-2286.
(13) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 2001, 123, 3268-3278.
(14) Lawrence, J. D.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2002,
41, 6193-6195.
(15) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710-9711.
| Royal Institute of Technology.
(1) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science
1998, 282, 1853-1858.
(2) Cammack, R. Nature 1999, 397, 214-215.
(3) Evans, D. J.; Pickett, C. J. Chem. Soc. ReV. 2003, 32, 268-275.
(4) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. ReV. 2000,
206-207, 533-561.
(5) Frey, M. ChemBioChem. 2002, 3, 153-160.
10.1021/ic0610278 CCC: $37.00
Published on Web 02/13/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 6, 2007 1981

Gao et al.
potentials. In order to get complexes which are more readily
reduced, at least after protonation, a number of phosphine
complexes (PMe3, PPhMe2, P(OMe)3, PPh3)16-19 have been
prepared. Also, a bidentate ligand (dppf, 1,1′-bis(diphenylphos-
phino)ferrocene) has been used to give a complex where the
phosphine ligand coordinates to two Fe2S2 units (Figure 1a).20
With the aim of developing more efficient catalysts, we
decided to study the influence of bisphosphine ligands in
more depth. In principle, bidentate ligands should be capable
of forming two more structural types of complexes with the
Fe2S2 unit, one where the ligand coordinates to both iron
atoms (Figure 1b) and one where it chelates to one of the
same iron atoms (Figure 1c). In addition, the bidentate ligand
could coordinate with only one phosphorus atom, leaving
the other free for coordination to a photosensitizer, providing
the possibility to create photocatalysis.21 We have, therefore,
started to investigate the coordination of bidentate phosphine
ligands, which might be expected to be both more flexible
and better nucleophiles than dppf. Described herein are
exploratory studies of the ligands dppe and dppm, which
lead to the formation of seven new complexes, [{µ-(SCH2)2-
CH2}Fe2(CO)5(Ph2PCH2)]2 (3), [{µ-(SCH2)2N(CH2CH2CH3)}
Fe2(CO)5(Ph2PCH2)]2 (4), [{µ-(SCH2)2N(CH2CH2CH3)}-
Fe2(CO)5{Ph2PCH2CH2(Ph2PS)}] (5), [{µ-(SCH2)2CH2}-
Fe2(CO)5(Ph2PCH2PPh2)] (6), [{µ-(SCH2)2CH2}Fe2(CO)4{µ-
(Ph2P)2CH2}] (7), [{µ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)5-
(Ph2PCH2PPh2)] (8), and [{µ-(SCH2)2N(CH2CH2CH3)}Fe2
(CO)4{µ-(Ph2P)2CH2}] (9), which were characterized by
spectroscopy and X-ray crystal structure determination.
Figure 1. The possible coordination manners for a bidentate phosphine
ligand.
ligand substitutes CO in the double monodentate manner to
give a cluster of Fe2S2 cores antibridged by dppe. The
presence of Me3NO proved to be crucial for the success of
this substitution, and no product was obtained in its absence.
In the presence of excess Me3NO and at a higher reaction
temperature, the same product 3 was obtained in a pure state.
The same reaction with the ADT complex 2, prepared
according to Rauchfuss’s protocol,24 gave the expected
complex 4 in moderate yield. However, the reaction also gave
substantial amounts of a byproduct, the complex 5 with one
phosphorus atom coordinated to the Fe2S2 complex, while
the other has been converted to a noncoordinating, terminal
PdS group. Nucleophilic substitution on a coordinated thiol
by phosphine or oxidative formation of elemental sulfur,
which reacts with phosphine, can, perhaps, explain this
remarkable transfer of sulfur from iron to phosphine.
By contrast, the reaction with the ligand dppm, where the
two phosphine units are linked by only one carbon, afforded
different products. The substrate and the reaction conditions
are crucial for the transformation. When the reaction was
carried out in acetonitrile at room temperature with 1 as a
substrate, complex 6, where the ligand is monodentate,
proved to be the only product. However, when the reaction
was performed in refluxing toluene, complex 7 was isolated,
where the two phosphorus atoms of the dppm are connected
to the two iron atoms in the same molecule (Scheme 2). Ap-
plication of similar reaction conditions to 2 gave complexes
8 and 9, which are the analogues to 6 and 7 in a variable
ratio. Short reaction times, ca. 0.5 h, gave mainly 8, while 9
was the main product when the reaction time was prolonged
to 1 h. It is interesting to note that 2 is much more reactive
toward the dppm than 1. Under the same conditions (reflux,
1 h), the complex 9 was formed in 75% yield, whereas com-
plex 1 gave only 6 in low yield. Increasing the reaction time
and adding Me3NO gave the complex 7 in ca. 30% yield.
The results of the ligand substitution in the parent clusters
1 and 2 deserve some further comments. A site-selective
substitution reaction was possible by controlling reaction
conditions and substrates. It is obvious that the nature of
the central bridge atom (N/C), the kind of bidentate ligand
(dppe/dppm), and the reaction conditions (solvent and
Results and Discussion
Chemistry. Two types of Fe2S2 complexes were studied,
a propane dithiolate (PDT) complex 122 and an azapropane
dithiolate (ADT) complex 2. In order to form a vacant site
and promote phosphine coordination, Me3NO23 and propy-
lamine19 were used, the amine oxide being, by far, the most
efficient promoter and, therefore, being used in most cases.
The known complex 1 was obtained by refluxing Fe(CO)5
and 1,3-propanedithiol in toluene according to the literature
method.22 Treatment of 1 with dppe in the presence of Me3-
NO in a 1:1:1 molar ratio gave a red solid in good yield. It
was characterized as [{µ-(SCH2)2CH2}Fe2(CO)5(Ph2PCH2)]2
(3) (Scheme 1), and we found that, like dppf,20 the dppe
(16) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.;
Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917-
3928.
(17) Gao, W.; Liu, J.; Åkermark, B.; Sun, L. Inorg. Chem. 2006, 45, 9169-
9171.
(18) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.; Benard, M.; Rohmer,
M.-M. Inorg. Chem. 2002, 41, 6573-6582.
(19) Ott, S.; Borgstro¨m, M.; Kritikos, M.; Lomoth, R.; Bergquist, J.;
Åkermark, B.; Hammarstro¨m, L.; Sun, L. Inorg. Chem. 2004, 43,
4683-4692.
(20) Song, L.; Yang, Z.; Bian, H.; Hu, Q. Organometallics 2004, 23, 3082-
3084.
(21) Ekstro¨m, J.; Abrahamsson, M.; Olson, C.; Bergquist, J.; Kaynak, F.
B.; Eriksson, L.; Sun, L.; Becker, H.-C.; Åkermark, B.; Hammarstro¨m,
L.; Ott, S. Dalton Trans. 2006, 4599-4606.
(22) Seyferth, D.; Womack, G. B.; Gallagher, M. K.; Cowie, M.; Hames,
B. W.; Fackler, J. P.; Mazany, A. M., Jr. Organometallics 1987, 6,
283-294.
(23) Gloaguen, F.; Lawrence, J. D.; Schmidt, M.; Wilson, S. R.; Rauchfuss,
T. B. J. Am. Chem. Soc. 2001, 123, 12518-12527.
(24) Li, H.; Rauchfuss, T. B. J. Am. Chem. Soc. 2002, 124, 726-727.
1982 Inorganic Chemistry, Vol. 46, No. 6, 2007

Binuclear Fe-S Complexes with Bidentate Phosphine Ligands
Scheme 1. Synthetic Routea
a Conditions: (a) CH3CN/CH2Cl2, Me3NO, dppe, rt, 1 h, 80%; (b) CH3CN/CH2Cl2, Me3NO, dppe, rt, 1 h, 60% for 4 and 23% for 5.
a
1
determined by H and 31P NMR as well as by MS spectra,
Scheme 2
although both complexes exhibited broadened signals in 1H
NMR. The high-resolution mass spectrum of 5 gave M+ )
831.9957, which corresponds well to the formula weight with
the terminal PdS double bond. The proposed structure of 5
is also clear from crystal structure data, with the expected
PdS double bond being 1.95 Å, in accordance with the
typical value of the PdS bond length of ca. 2.0 Å (while a
PdO double bond should be ca. 1.4 Å). The 1H NMR spec-
trum of 6 contains the expected peaks for the PDT in the
region between 1.5 and 2 ppm and for the dppm at 3.3 ppm.
In the 31P NMR spectrum, a doublet for the coordinated P is
found at 58 ppm, and a doublet for the free phosphorus is
found at -25 ppm. The corresponding 31P NMR spectrum
for complex 7 displays one singlet at 53.65 ppm. The same
trend is seen for 8 and 9, with a pair of doublets at 57.76
and -25.2 ppm for 8 and a singlet at 54 ppm for 9. The IR
spectra of 3, 4, 5, 6, and 8 in KBr show strong absorption
bands in the region of 1920-2050 cm-1, corresponding to
the carbonyl stretching pattern for terminally bonded C-O
groups. However, for 7 and 9, the carbonyl bands appear at
lower frequencies, as might be expected for these more
electron-rich complexes. For 7, four bands are visible at 1895,
1916, 1946, and 1981 cm-1, and for 9, three bands are
observed at 1916, 1952, and 1986 cm-1.
a Conditions: (a) CH3CN, Me3NO, dppm, rt, 4 h, 77%; (b) toluene, dppm,
Me3NO, reflux, 18 h, 27%; (c) toluene, dppm, reflux, 0.5 h, 50%; (d)
toluene, dppm, reflux, 1 h, 75%.
temperature) significantly influence the product type of the
substitution. A complex of the type a (Figure 1) was the
major product when the Fe2S2 complexes reacted with the
dppe ligand, as also reported earlier for the dppf ligand.20
Usually, the use of CH3CN/Me3NO gave good yields for
this transformation, but with the dppm ligand, the second
phosphorus atom (e.g., 6) did not coordinate to the second
Fe2S2 unit, probably due to steric factors. However, in
refluxing toluene, an Fe2S2 structure with a bridging diphos-
phine, such as 7, could be obtained selectively. Similarly,
the complex 9 was readily formed, preceded by the complex
8. This suggests that complexes 6 and 8 are precursors of
complexes 7 and 9, which represent a new type of structure
for complexes between bisphosphine ligands and Fe2S2 units.
Characterization of the Complexes. The 1H NMR
spectrum of complex 3 consists of the expected signals from
the PDT and dppe subunits. The 31P NMR spectrum shows
a singlet in the typical region around 60 ppm. In the API-
ES mass spectra, the most intense peak for 3 was observed
at m/z 1148.7, corresponding to [M + Cl]- (negative mode),
and at m/z 1114.7, corresponding to [M + H]+ (positive
mode). The structures of complexes 4 and 5 could also be
The crystallographic structures of 3-9 are depicted in
Figures 2-8, together with the atomic labeling system.
Selected bond lengths and angles are given in Tables 1-7.
The main framework of clusters 3 and 4 (Fe-P-C-C-P-
Fe) is composed of zigzag structures with one Fe2S2 butterfly
unit attached to each end of the dppe bridge. Each Fe2S2
subunit has a square pyramidal structure, and both complexes
have C2V symmetry. The Fe2S2 cores in the tetranuclear com-
plex 3 and the parent complex 1 are geometrically similar.
The Fe-Fe bond distance is equal to that in 1 (2.51 Å),
showing that the ligand exchange has no effect on the
Fe-Fe bond length. The P(1)-Fe(1)-Fe(2) plane is almost
parallel to that of P(1A)-Fe(1A)-Fe(2A) because of an
Inorganic Chemistry, Vol. 46, No. 6, 2007 1983

Gao et al.
Figure 2. ORTEP (ellipsoids at 30% probability) diagram of 3.
Figure 4. ORTEP (ellipsoids at 30% probability) diagram of 5.
Table 3. Selected Bond Lengths and Angles for Complex 5
bond lengths (Å)
bond angles (°)
Fe(1)-Fe(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-C(6)
Fe(1)-C(7)
Fe(1)-P(1)
P(2)-C(1)
P(2)-S(3)
2.5165(12)
2.2557(15)
2.2566(14)
2.2481(16)
2.2688(17)
1.756(5)
1.756(6)
2.2169(13)
1.819(4)
Fe(1)-S(1)-Fe(2)
Fe(1)-S(2)-Fe(2)
S(1)-Fe(1)-S(2)
S(1)-Fe(2)-S(2)
C(18)-N(1)-C(39)
C(1)-P(2)-S(3)
67.94(5)
67.57(4)
84.27(5)
84.16(5)
113.9(4)
113.39(18)
1.9445(18)
a staggered (antiperiplanar), rather than eclipsed, conforma-
tion. The C(10)-P(1)-C(16) angle is 102.3°, significantly
smaller than the typical value (ca. 109°), which is an
indication of the steric bulk of the two Fe2S2 subunits. The
structure of 4 is very similar to that of 3. The C(11)-P(1)-
C(17) (not all labeled for clarity) angle value is 100.1°, even
Figure 3. ORTEP (ellipsoids at 30% probability) diagram of 4.
Table 1. Selected Bond Lengths and Angles for Complex 3
bond lengths (Å)
bond angles (°)
Fe(1)-Fe(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-C(1)
Fe(1)-C(2)
Fe(1)-P(1)
P(1)-C(10)
P(1)-C(16)
S(1)-C(6)
S(2)-C(8)
2.5108(14)
2.262(2)
2.2587(19)
2.262(2)
2.273(2)
1.774(8)
1.764(8)
2.2212(19)
1.826(6)
1.841(7)
1.821(7)
1.143(8)
Fe(1)-S(1)-Fe(2)
Fe(1)-S(2)-Fe(2)
S(1)-Fe(1)-S(2)
S(1)-Fe(2)-S(2)
P(1)-Fe(1)-Fe(2)
C(10)-P(1)-Fe(1)
C(10)-P(1)-C(16)
C(9)-P(1)-C(16)
C(9A)-C(9)-P(1)
67.42(6)
67.30(6)
84.54(7)
84.22(7)
151.23(6)
114.5(2)
102.3(3)
99.7(3)
112.3(5)
Table 2. Selected Bond Lengths and Angles for Complex 4
bond lengths (Å)
bond angles (°)
Fe(1)-Fe(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-C(4)
Fe(1)-C(5)
Fe(1)-P(1)
2.5101(8)
2.2626(13)
2.2632(12)
2.2625(13)
2.2686(13)
1.783(5)
Fe(1)-S(1)-Fe(2)
Fe(1)-S(2)-Fe(2)
S(1)-Fe(1)-S(2)
S(1)-Fe(2)-S(2)
P(1)-Fe(1)-Fe(2)
C(23A)-C(23)-P(1)
C(11)-P(1)-C(17)
67.38(4)
67.27(4)
84.27(5)
84.15(5)
151.42(4)
111.5(3)
100.14(19)
Figure 5. ORTEP (ellipsoids at 30% probability) diagram of 6.
Table 4. Selected Bond Lengths and Angles for Complex 6
bond lengths (Å)
bond angles (°)
1.780(5)
2.2330(11)
Fe(1)-Fe(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(1)-P(1)
2.5651(8)
Fe(1)-S(1)-Fe(2)
Fe(1)-S(2)-Fe(2)
S(1)-Fe(1)-S(2)
S(1)-Fe(2)-S(2)
P(1)-C(6)-P(2)
C(21)-P(1)-C(31)
C(6)-P(2)-C(41)
C(6)-P(2)-C(51)
C(41)-P(2)-C(51)
69.14(3)
69.25(3)
85.05(4)
2.2582(12)
2.2591(13)
2.2607(11)
2.2555(10)
2.2400(10)
inversion center at the midpoint of the CH2CH2 bond. The
substitution of CO is site selective, and the phosphine in 3
occupies only the two apical positions, which is consistent
with the behavior of the usual monophosphine ligands.16-19
Viewed along the C(9)-C(9A) bond, P(1) and P(1A) are in
85.05(4)
113.12(18)
100.58(19)
100.08(16)
102.14(19)
103.23(19)
1984 Inorganic Chemistry, Vol. 46, No. 6, 2007

Binuclear Fe-S Complexes with Bidentate Phosphine Ligands
Figure 6. ORTEP (ellipsoids at 30% probability) diagram of 7.
Table 5. Selected Bond Lengths and Angles for Complex 7
bond lengths (Å)
bond angles (°)
Fe(1)-Fe(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-P(1)
Fe(2)-P(2)
2.5154(18)
2.255(3)
2.252(3)
2.256(3)
2.255(3)
2.220(3)
2.219(3)
Fe(1)-S(1)-Fe(2)
Fe(1)-S(2)-Fe(2)
S(1)-Fe(1)-S(2)
S(1)-Fe(2)-S(2)
P(1)-C(5)-P(2)
67.78(8)
67.85(9)
84.53(11)
84.44(11)
115.8(5)
Figure 8. ORTEP (ellipsoids at 30% probability) diagram of 9.
Table 7. Selected Bond Lengths and Angles for Complex 9
bond lengths (Å)
bond angles (°)
smaller than the corresponding angle C(10)-P(1)-C(16) in
3, probably due to the interaction with the nearby n-propyl.
For complexes 4 and 5, the bond lengths and angles at the
central dinuclear units are very close to those reported earlier
for complexes related to 1.19 In complex 5, the monodentate
phosphine ligand also coordinates to an apical position, as
visualized in Figure 4. A comparison of complexes 3, 4, and
5 shows that the Fe(1)-P(1) bond distances are almost the
same, all falling in the range of 2.22-2.23 Å. The angle of
C(9A)-C(9)-P(1) in 3 is 112.3°, a little larger than the
Fe(1)-Fe(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(1)-P(1)
Fe(2)-P(2)
2.5044(6)
2.2579(9)
2.2652(8)
2.2384(9)
2.2923(9)
2.2005(9)
2.2393(9)
Fe(1)-S(1)-Fe(2)
Fe(1)-S(2)-Fe(2)
S(1)-Fe(1)-S(2)
S(1)-Fe(2)-S(2)
P(1)-C(10)-P(2)
67.69(3)
66.66(3)
84.15(3)
84.34(3)
113.68(15)
corresponding angle in 4, which is 111.5°. The P-CH2-
CH2-P chain is thus a little more compressed in 4. Notably,
the propyl groups on the bridgehead N of both 4 and 5 point
toward the bulky dppe moiety, which occupies a thermody-
namically unstable position.
Passing from dppe to dppm, there are clear structural
changes, as shown by the structure 6. The Fe-Fe bond
distance increases from 2.51 Å in complex 3 to 2.56 Å in
complex 6. This somewhat exceeds the bond length in the
complex with a ferrocenyl-bisphosphine ligand (2.543 Å).20
The second phosphorus atom is neither coordinated to Fe
nor combined with the sulfur atom, in contrast to the dppe
ligand. In complexes 3, 4, and 5, the phosphine occupies an
apical position, whereas in complex 6, it occupies a basal
position, perhaps for steric reasons. The Fe(1)-P(1) bond
distance in 6 is ca. 2.24 Å, slightly longer than that in 3 and
4. In contrast to the basal coordination in 6, the phosphine
is in an axial position in 8. Both the Fe-Fe bond (2.52 Å)
and the Fe-P bond (∼2.22 Å) in 8 are shorter than the
corresponding bond in 6, and they are of approximately the
same length as the bonds in 3, 4, and 5.
Figure 7. ORTEP (ellipsoids at 30% probability) diagram of 8.
Table 6. Selected Bond Lengths and Angles for Complex 8
For complexes 7 and 9, the dppm bridges the two iron
atoms with one phosphorus atom bonded to each iron atom.
The phosphorus atoms both occupy basal positions. The Fe-
(1)-P(1) distance (2.20 Å) is a little shorter than the Fe-
(2)-P(2) distance (2.24 Å) in 9, but in 7, they are essentially
equal. The P-C-P bond angles in complexes 6, 8, and 9
bond lengths (Å)
bond angles (°)
Fe(1)-Fe(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(2)-P(1)
2.5217(6)
2.2434(9)
2.2539(10)
2.2577(10)
2.2639(11)
2.2168(9)
Fe(1)-S(1)-Fe(2)
Fe(1)-S(2)-Fe(2)
S(1)-Fe(1)-S(2)
S(1)-Fe(2)-S(2)
P(1)-C(35)-P(2)
68.15(3)
67.86(3)
83.96(3)
84.52(4)
112.32(16)
Inorganic Chemistry, Vol. 46, No. 6, 2007 1985

Gao et al.
0.56 V and an irreversible reduction peak at -2.11 V.
Controlled potential coulometry of complexes 3 and 4 at -2
V in the absence of acid showed a consumption of 2.10 and
2.13 electrons per molecule. This suggests that the two Fe2S2
centers are essentially identical and mutually independent
and are both reduced one step at ca. -2 V.
The influence of strong acid CF3SO3H (0-40 equiv) on
complexes 3, 4, 7, and 9 was studied by CV (Figures 11
and 12). At moderate acid concentration (0 to 20 equiv), the
oxidation peak at 0.62 V for complex 4 moved to a more
positive potential by 0.24 V. A new reduction peak appeared
at about -1.25 V when 2 equiv of CF3SO3H was added.
This shift upon protonation (ca 0.6 V) is greater than the
corresponding shift (0.23 V) for a related hexacarbonyl
complex,30 perhaps because of the replacement of a carbonyl
group by a phosphine ligand. As the acid concentration was
increased, the peaks at -1.25 and -2 V both grew and
moved to more negative potentials (Figure 11b). These fea-
tures are indicative of catalytic proton reduction. By contrast,
no new reduction peaks were detected for complex 3, and
the original one did not move to a more negative potential
when CF3SO3H was added, although it also grew larger
(Figure 11a). At high acid concentration, curve crossing was
observed for both complexes 3 and 4 (vide infra).31,36
The CVs of complexes 7 and 9 in the presence of triflic
acid in CH3CN gave similar results as 3 and 4. For 7, except
that the current intensities of all reduction peaks grew further
with the increase of the acid concentration, no other new
changes were observed. By contrast, for 9, a new reduction
peak appeared at about -1.6 V when 1 equiv of CF3SO3H
was added. This shift value upon protonation (ca. 0.5 V) is
comparable to that for 4 (0.6 V). However, with complexes
7 and 9, curve crossing could not be observed even at a high
acid concentration. The oxidation peaks of 9 were clearly
shifted to more positive potentials when acid was added in
a slightly more complicated manner (Figure 12). The second
oxidation peak at 0.56 V (FeIFeII/FeIIFeII) moved to more
positive potential (0.83 V).37 After addition of 2 equiv of
acid, the first oxidation at 0.22 V (FeIFeI/FeIIFeI) disappeared
completely, and only one oxidation peak at ca. 0.83 V was
observed. This was true also after addition of 40 equiv of
acid. A possible explanation is that the two iron atoms
become essentially equivalent when the nitrogen is proto-
nated.
Figure 9. Cyclic voltammograms of 3 (a) and 4 (b) (1.0 mM) in 0.05 M
n-Bu4NPF6/CH2Cl2 at a scan rate of 100 mV/s.
Figure 10. Cyclic voltammogram of 7 (a) and 9 (b) (1.0 mM) in 0.05 M
n-Bu4NPF6/CH3CN at a scan rate of 100 mV/s.
are about 113°, although their coordination manners are
different. In 7, this angle is slightly bigger, 115.8°.
Electrochemistry. In order to get an estimate of the
capability to catalyze hydrogen production, cyclic voltam-
metry (CV) of the complexes 3, 4, 7, and 9 (Figures 9 and
10) was performed. Complex 3 exhibited an irreversible
oxidation wave at 0.67 V (FeIFeI/FeIIFeI) and an irreversible
reduction peak at -1.94 V (FeIFeI/Fe0FeI) (vs Ag/AgNO3).
Complex 4 showed a broad irreversible oxidation peak at
0.62 V and an irreversible reduction peak at -1.93 V. This
is in agreement with the published CVs of other types of
related complexes.18,25-29 Complex 7 showed two irreversible
oxidation waves at 0.36 (FeIFeI/FeIIFeI) and 0.74 V(FeIFeII/
FeIIFeII),30,37as well as an irreversible reduction peak at
-2.14 V (FeIFeI/Fe0FeI) (vs Ag/AgNO3). Similarly, complex
9 showed two irreversible oxidation peaks at 0.22 and
(25) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 9476-9477.
(26) Chong, D.; Georgakaki, I. P.; Mejia-Rodriguez, R.; Sanabria-
Chinchilla, J.; Soriaga, M. P.; Darensbourg, M. Y. J. Chem. Soc.,
Dalton Trans. 2003, 4158-4163.
(27) Liaw, W.-F.; Lee, N.-H.; Chen, C.; Lee, C.-M.; Lee, G.-H.; Peng,
S.-M. J. Am. Chem. Soc. 2000, 122, 488-494.
The mechanisms for H2 formation are not clear. An ECCE
(electrochemical-chemical-chemical-electrochemical) mech-
anism, which has been suggested for a related complex,31
can tentatively be assigned for complex 3, although an ECEC
(electrochemical-chemical-electrochemical-chemical) mech-
anism is also possible (vide infra). For 4, a somewhat
different mechanism seems probable since the first step is
protonation of the two bridgehead nitrogens, leading to a
well-defined complex 4H22+. This results in a new reduction
peak at -1.25 V, and the carbonyl frequencies in the IR
spectrum are shifted by 10-18 cm-1 to a higher frequency
relative to those of the parent 4. This is of the same
magnitude as those observed for a related bis(trimethylphos-
phine) complex.35 The methylene protons of 4H22+, in
(28) Razavet, M.; Davies, S. C.; Hughes, D. L.; Barclay, J. E.; Evans, D.
J.; Fairhurst, S. A.; Liu, X.; Pickett, C. J. J. Chem. Soc., Dalton Trans.
2003, 586-595.
(29) Liu, T.; Wang, M.; Shi, Z.; Cui, H.; Dong, W.; Chen, J.; Åkermark,
B.; Sun, L. Chem.sEur. J. 2004, 10, 4474-4479.
(30) Ott, S.; Kritikos, M.; Åkermark, B.; Sun, L.; Lomoth, R. Angew. Chem.,
Int. Ed. 2004, 43, 1006-1009.
(31) Mejia-Rodriguez, R.; Chong, D.; Reibenspies, J. H.; Soriaga, M. P.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2004, 126, 12004-12014.
(32) Houmam, A.; Hamed, E. M.; Still, I. W. J. J. Am. Chem. Soc. 2003,
125, 7258-7265.
(33) Kuchynka, D. J.; Kochi, J. K. Inorg. Chem. 1988, 27, 2574-2581.
(34) Saveant, J. M. Acc. Chem. Res. 1980, 13, 323-329.
(35) Schwartz, L.; Eilers, G.; Eriksson, L.; Gogoll, A.; Lomoth, R.; Ott, S.
Chem. Commun. 2006, 520-522.
(36) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.;
Wiley: New York, 2001.
(37) Li, P.; Wang, M;. He, C.; Li, G.; Liu, X;. Chen, C.; Åkermark, B.;
Sun, L. Eur. J. Inorg. Chem. 2005, 2506-2513.
1986 Inorganic Chemistry, Vol. 46, No. 6, 2007

Binuclear Fe-S Complexes with Bidentate Phosphine Ligands
Figure 11. Cyclic voltammogram of 3 (a) and 4 (b) (1.0 mM) with addition of CF3SO3H (0, 2, 6, 10, 16, 20, 40 equiv) in 0.05 M n-Bu4NPF6/CH2Cl2 at
a scan rate of 100 mV/s. The curves for acid concentrations between 20-40 equiv were omitted for clarity (see Figure S2).
Figure 12. Cyclic voltammogram of 7 (a) (1.0 mM) with addition of CF3SO3H (0, 1, 2, 3, 4, 5, 10 equiv) and 9 (b) (1.0 mM) with addition of CF3SO3H
(0, 1, 2 equiv) in 0.05 M n-Bu4NPF6/CH3CN at a scan rate of 100 mV/s. The curves for acid concentrations between 5 and 10 equiv for 9 were omitted for
clarity (see Figure S3).
Scheme 3. Possible Mechanism of the Proton Reduction
proximity to the ADT nitrogen (-NCH2S-), moved from
2.66 (singlet) and 2.12 ppm (singlet) in 4 to 4.30 (multiplet)
and 3.05 ppm (doublet) (ratio of 1:1). The peak in the 31P
NMR, finally, moved from 60.45 to 62.50 ppm (see
Experimental Section). The protonation could be completely
reversed by the addition of 5 equiv of aniline to regenerate
4. However, when the same amount of 4-cyano-
aniline was added, no change occurred, placing the pKa of
2+
2+
4H2 between 10.6 and 7.6. If we represent half of 4H2
by (FeIFeINH)+, (Fe0FeINH) should be formed by the
reduction at ca. -1.25 V. Protonation will then give (HFeII-
FeINH)+, which could react with a second proton to give
molecular hydrogen and (FeIIFeINH)2+. After a second
reduction, (FeIFeINH)+, corresponding to 4H22+, would be
regenerated, in accordance with the mechanism suggested
above for 3 and earlier for related complexes.31 However,
since the hydride in (HFeIIFeINH)+ might be expected to
have more proton than hydride character, the reaction
sequence may, in fact, be reduction to (HFeIFeINH) first,
generating a complex with stronger hydride character,
followed by protonation to give hydrogen and 4H22+ directly.
The mechanism could then tentatively be presented as a
CECEC (chemical-electrochemical-chemical-electrochemi-
cal-chemical) mechanism (Scheme 3). Upon short bulk
electrolysis (5-10 min) of 4 in a strongly acidic acetonitrile
solution, two sets of carbonyl frequencies can be observed
by IR spectroscopy, one at 2062 and 2003 cm-1 and the other
at 2155 and 2121 cm-1. The first most probably corresponds
to 4H22+. The second species could be (HFeIIFeINH)+ since
the shifts in the carbonyl frequencies, 93 and 118 cm-1, are
of the same magnitude as those observed upon protonation
of related FeIFeI-bis(trimethylphosphine) complexes,31,35
which are around 80 cm-1.
The interpretation of the curve crossing that was observed
in the CVs of complexes 3 and 4 is not clear. This type of
behavior was recently also observed for the related complex
[(µ-PDT)Fe2(CO)4L2] (L ) PTA).31 The crossing was barely
visible for 3, even upon addition of 40 equiv of acid (Figure
11a), while a distinct new peak could be observed at
-0.96 V upon the return scan for 4 (Figure 11b). The reduc-
tion peak currents (ip) at ca. -1.94 V for 3 and -1.25 V for
4 were plotted versus scan rates from 60 to 800 mV/s in the
absence of acid and at an acid concentration of 40 equiv.
For 4, a square-root relationship between ip and the scan
rate was obtained, which is an indication that the “curve-
Inorganic Chemistry, Vol. 46, No. 6, 2007 1987

Gao et al.
Figure 14. Coulometry for bulk electrolysis: CF3SO3H (100 mM) in the
presence of 9 (- - -) and the baselines electrolyzing at -1.85 V (ss).
Electrolysis potentials: -1.85 V for 9 (vs Ag/AgNO3). Solution volume:
5 mL. Glassy carbon electrode surface area: 1.1 cm2; all measurements
were made on 1 mM solutions of the relevant complexes in CH3CN with
n-Bu4NPF6 (0.1 M) as the supporting electrolyte.
Figure 13. Coulometry for bulk electrolysis: CF3SO3H (100 mM) in the
presence of 3 (- ‚ -), 4 (‚ ‚ ‚), and the baselines electrolyzing at -1.40 V
(ss) and -1.95 V (- - -). Electrolysis potentials: -1.95 V for 3 and -1.4
V for 4 (vs Ag/AgNO3). Solution volume: 5 mL. Glassy carbon electrode
surface area: 1.1 cm2; all measurements were made on 1 mM solutions of
the relevant complexes in CH2Cl2 with n-Bu4NPF6 (0.1 M) as the supporting
electrolyte.
Conclusion
The reaction of the Fe2S2 complexes 1 and 2 with the
bisphosphines dppe and dppm has been found to give three
types of complexes, depending on reaction conditions and
the structure of the phosphine. With dppe, under fairly mild
reaction conditions, both 1 and 2 gave structures that contain
two terminal butterfly Fe2S2 cluster cores coordinated to a
bisphosphine ligand (3, 4). In the case of 2, also ca. 20% of
complex 5 was formed, where one phosphine had been
converted to a phosphine sulfide. Under more forcing
conditions, this became the major product. By contrast, using
the ligand dppm, the bisphosphine was only monodentate,
giving complex 6, probably because the combined bulk of
two phenyl groups and the coordinated complex decreased
the reactivity of the second phosphorus for steric reasons.
However, under forcing conditions, the reactions with dppm
gave complexes 7 and 9, in which the diphosphine ligand
bridges the two Fe atoms of the same molecule. This shows
that, depending on complex, phosphine, and reaction condi-
tions, a great variety of complexes can be prepared selec-
tively. Finally, it was found that complex 4 is a good catalyst
for electrochemical production of hydrogen with a relatively
low reduction potential in the presence of an acid.
crossing” peak is not the result of a surface deposition
process.31 For 3, this relationship was not obtained, indicating
that the surface deposition may occur. For 4, not only does
surface crossing take place, but the formation of a new
intermediate is suggested by the observation of a distinct
peak at -0.96 V subsequent to the reduction of 4 at ca. -2
V.32-34 As far as we are aware, this type of behavior has not
been observed before. Further work is, therefore, necessary
in order to establish the nature of this intermediate.
Finally, catalytic generation of hydrogen was studied by
controlled potential electrolysis of complexes 3, 4, and 9 in
CH2Cl2- or CH3CN-Bu4NPF6 (Figure 13 and 14). During
the recording of the CVs of complexes 3, 4, 7, and 9, we
could observe hydrogen bubbles evolving from the surface
of the glassy carbon disc electrode for three complexes when
enough CF3SO3H was added. The electrochemistry of the
complexes was, therefore, studied in more detail. As might
be expected, some uncatalyzed proton reduction could be
observed upon addition of the strong triflic acid, even in the
absence of catalyst. At the potential required for the reduction
of 3 (1.95 V vs Ag/AgNO3), the rate of uncatalyzed hydrogen
evolution was fairly high, but a small catalytic activity of 3
could be observed (Figure 13). At -1.4 V, which was
sufficient for reduction in the presence of complex 4, the
rate of uncatalyzed reduction was considerably smaller, and
a distinct catalytic activity of 4 could be observed. However,
in both cases, the catalytic activity decreased with time and
ceased, at last, for 3, after 50 min and for 4, after 1.5 h. On
the basis of each Fe2S2 unit, this corresponds to ca. 4 turn-
overs for 3 and 12 for 4. Since 7 required a reduction
potential of -2.1 V, we did not study its hydrogen evolution.
Complex 9 showed hydrogen evolution with a turnover num-
ber of ca. 14 at -1.85 V (Figure 14). The less negative elec-
trolysis potentials and relatively higher efficiency for 4 and
9 show that they are better catalysts for reduction of protons.
Experimental Section
All reactions and operations related to organometallic complexes
were carried out under a dry, oxygen-free, nitrogen atmosphere with
standard Schlenk techniques. All solvents were dried and distilled
prior to use according to the standard methods. Commercially
available chemicals, including n-propylamine, paraformaldehyde,
dppe, and dppm were used without further purification. The reagents
LiEt3BH and triflic acid were purchased from Aldrich.
Infrared spectra were recorded on a JASCO FT/IR 430 spectro-
photometer. 1H and 13C NMR (mainly CDCl3 solution) were
collected on a Varian INOVA 400 NMR spectrometer. Mass spectra
were recorded on an HP1100 MSD instrument, and HRMS was
performed on a HPLC-Q-TOFMS (Micro) system. Gas chromato-
grams were recorded on a GC7890, and nitrogen was selected as
the carrier gas.
Methylene chloride (Aldrich, spectroscopy grade), used for
performance of electrochemistry, was dried with molecular sieves
1988 Inorganic Chemistry, Vol. 46, No. 6, 2007

Binuclear Fe-S Complexes with Bidentate Phosphine Ligands
(4 Å) and then freshly distilled from CaH2 under N2. A solution of
0.05 M n-Bu4NPF6 (Fluka, electrochemical grade) in CH2Cl2 or
CH3CN was used as the electrolyte. Electrochemical measurements
were recorded using a BAS-100W electrochemical potentiostat. The
electrolyte solution was degassed by bubbling with dry argon for
10 min before measurement. 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
and 1 µm diamond pastes and sonicated in ion-free water for 10
min. The reference electrode was a nonaqueous Ag/Ag+ electrode
(1.0 mM AgNO3 in CH3CN), and the auxiliary electrode was a
platinum wire. The gas in the airtight vessel was carefully taken
out by syringe, which was hydrogen identified by gas chromatog-
raphy. Gas chromatography was performed under isothermal
conditions with nitrogen as a carrier gas and a thermal conductivity
detector (TCD).
Single-crystal X-ray diffraction patterns were recorded with an
Oxford Diffraction Excalibur diffractometer equipped with a
sapphire-3 CCD on a Mo radiation source (λ ) 0.71073 Å) with
ω scans at different φ’s to fill an Ewald sphere. The sample-
detector distance was 50 mm. The maximum 2θ was ≈63°.
Indexing, cell refinements, and integration of reflection intensities
were performed with the Crysalis software.38 Numerical absorption
correction was performed with the program X-RED,39 verifying
the crystal shape with the program X-shape.40 The structure was
solved by direct methods using SHELXS97,41 giving electron
density maps where most of the non-hydrogen atoms could be
resolved. The rest of the non-hydrogen atoms were located from
difference electron density maps, and the structure model was
refined with full-matrix least-squares calculations on F2 using the
program SHELXL97-2.42 All non-hydrogen atoms were refined with
anisotropic displacement parameters, and the hydrogens, which were
placed at geometrically calculated positions and let to ride on the
atoms they were bonded to, were given isotropic displacement
parameters calculated as ê‚Ueq for the non-hydrogen atoms with
ê ) 1.2 for methylenic (-CH2-) and aromatic hydrogens.
Synthesis of [{µ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)6] (2). A
solution of n-propylamine (3.3 mL, 40 mmol) and paraformaldehyde
(2.4 g, 80 mmol) in THF (40 mL) was stirred for 4 h, cooled to 0
°C, and (HS)2Fe2(CO)6 (2 mmol) in THF (40 mL) was added. After
further reaction for 5 h, the solution was filtered, and the solvent
was removed on a rotary evaporator. The crude product was purified
by chromatography on silica gel with hexane as the eluent to give
2 (770 mg, 90%) as a red solid. 1H NMR (CDCl3): δ 0.79 (s, 3H,
CH3), 1.31 (s, 2H, CH3CH2CH2), 2.61 (s, 2H, CH2CH2N), 3.51 (s,
4H, 2 × NCH2S) ppm. IR (KBr): ν(CO) 2073, 2030, and 1992
cm-1. MS (API-ES): m/z 430.0 [M + H]+. Anal. Calcd (%) for
C11H11Fe2NO6S2: C, 30.79; H, 2.58; N, 3.26. Found: C, 30.37; H,
2.82; N, 3.27.
on silica gel with CH2Cl2/hexane (1/2 v/v) as the eluent to give 3
(220 mg, 80%).
1H NMR (CDCl3): δ 0.87-0.90 (m, 4H, 2 ×
CH2CH2CH2), 1.49-1.50 (m, 4H, 2 × SCH2CH2), 1.76-1.79 (m,
4H, 2 × SCH2CH2), 2.58 (s, 4H, PCH2CH2P), 7.40-7.33 (m, 12H,
Ph), 7.51-7.49 (m, 8H, Ph) ppm. 31P NMR (CDCl3): δ 60.66 (s)
ppm. IR (KBr): ν(CO) 2040, 1982, and 1922 cm-1. MS (API-
ES): m/z 1114.7 [M + H]+, 1148.7 [M + Cl]-. HRMS (ESI): m/z
[M + Cl]- calcd for C42H36Fe4O10P2S4Cl, 1148.7759; found,
1148.7786. Anal. Calcd (%) for C42H36Fe4O10P2S4: C, 45.27; H,
3.26. Found: C, 45.75; H, 3.72.
Synthesis of [{µ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)5(Ph2PCH2)]2
(4) and [{µ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)5{Ph2PCH2CH2-
(Ph2PS)}] (5). A solution of 2 (0.215 g, 0.5 mmol) and Me3NO‚
2H2O (0.111 g, 1 mmol) in CH3CN (40 mL) was stirred for 5 to
10 min at room temperature. Then, a solution of CH2Cl2 (2 mL)
with dppe (0.199 g, 0.5 mmol) was added. After 1 h, the solvent
was evaporated, and the residue was washed with CH2Cl2/hexane
(1/1 v/v) to give 4 (181 mg, 60%) and 5 (95 mg, 23%). 4. 1H NMR
(CDCl3): δ 0.66 (s, 6H, 2 × CH3), 0.88 (s, 4H, 2 × CH3CH2CH2),
1.25 (s, 4H, 2 × CH2CH2N), 2.08 (s, 4H, PCH2CH2P), 2.12 (s,
4H, 2 × NCH2S), 2.66 (s, 4H, 2 × NCH2S), 7.37 (s, 12H, Ph),
7.62 (s, 8H, Ph) ppm. 13C NMR (CDCl3): δ 11.71, 29.89, 51.90,
61.81, 88.09, 128.71, 130.10, 132.60, 209.65 ppm. 31P NMR
(CDCl3): δ 60.45 (s) ppm. IR (KBr): ν(CO) 2040, 1970 and 1936
cm-1. MS (API-ES): m/z 1200.8 [M + H]+. Anal. Calcd (%) for
C46H46Fe4N2O10P2S4: C, 46.02; H, 3.86; N, 2.33. Found: C, 45.40;
1
H, 4.14; N, 2.33. 5. H NMR (CDCl3): δ 0.66 (s, 3H, CH3), 0.99
(s, 2H, CH3CH2CH2), 1.26 (s, 2H, CH2CH2N), 2.57-2.69 (m, 8H,
2 × NCH2S, PCH2CH2P), 7.40 (s, 12H, Ph), 7.69 (d, J ) 40.0 Hz,
8H, Ph) ppm. 13C NMR (CDCl3): δ 12.90, 29.90, 50.04, 61.18,
128.56, 129.04, 131.29, 132.31, 132.68, 209.66 ppm. 31P NMR
(CDCl3): δ 45.33 (d, J ) 132.0 Hz), 59.56 (d, J ) 132.0 Hz)
ppm. IR (KBr): ν(CO) 2043, 1978, and 1927 cm-1; ν(PdS) 693
cm-1. MS (API-ES): m/z 831.8 [M + H]+. HRMS (ESI): m/z
calcd for C36H36Fe2NO5S3P2, 831.9929; found, 831.9957. Anal.
Calcd (%) for C36H35Fe2NO5P2S3: C, 52.00; H, 4.24; N, 1.68.
Found: C, 52.39; H, 4.32; N, 1.70.
Synthesis of [{µ-(SCH2)2CH2}Fe2(CO)5(Ph2PCH2PPh2)] (6).
A solution of 1 (0.12 g, 0.31 mmol) and Me3NO‚2H2O (0.07 g,
0.63 mmol) in CH3CN (20 mL) was stirred at room temperature
for 15 min. Then, dppm (0.12 g, 0.31 mmol) was added. After
stirring for 4 h, the solvent was removed under reduced pressure,
and the crude product was purified by chromatography on silica
gel with pentane/EtOAc (10:1 v/v) as the eluent. Complex 6 was
obtained as a red solid (0.179 g, 77%). 1H NMR (CDCl3): δ 1.61-
1.65 (m, 4H, 2 × SCH2CH2), 1.71-1.92 (m, 2H, CH2CH2CH2),
3.15-3.45 (m, 2H, PCH2P), 7.10-7.40 (m, 16H, Ph), 7.40-7.80
(m, 4H, Ph) ppm. 31P NMR (CDCl3): δ 58.35 (d, J ) 207.3 Hz),
-25.29 (d, J ) 207.3 Hz) ppm. IR (KBr): ν(CO) 2040, 1974, 1969,
1960, and 1916 cm-1. Anal. Calcd (%) for C33H28Fe2O3P2S2: C,
53.39; H, 3.80; Found: C, 53.39; H, 3.87.
Synthesis of [{µ-(SCH2)2CH2}Fe2(CO)5(Ph2PCH2)]2 (3). A
solution of 1 (0.193 g, 0.5 mmol) and Me3NO‚2H2O (0.111 g, 1
mmol) dissolved in MeCN (40 mL) was stirred for 5 to 10 min at
room temperature. Then, a solution of dppe (0.199 g, 0.5 mmol),
dissolved in CH2Cl2 (2 mL), was added. After 1 h, the solvent was
evaporated, and the crude product was purified by chromatography
Synthesis of [{µ-(SCH2)2CH2}Fe2(CO)4{µ-(Ph2P)2CH2}] (7).
A solution of 2 (0.1 g, 0.26 mmol), dppm (0.1 g, 0.26 mmol), and
Me3NO‚2H2O (0.087 g, 0.78 mmol) in toluene (25 mL) was
refluxed for 18 h. The solvent was then removed under reduced
pressure, and the crude product was purified by chromatography
on silica gel with CH2Cl2/pentane (1/3 v/v) as the eluent. Complex
(38) Oxford Diffraction. Xcalibur CCD System, CrysAlis Software System,
Version 1.170; Oxford Diffraction Ltd.: Oxfordshire, U.K., 2003.
(39) X-RED, Absorption Correction Program, Version 1.09; Stoe & Cie
GmbH: Darmstadt, Germany.
1
7 was obtained as a red solid (0.6 g, 27%). H NMR (CDCl3): δ
(40) X-SHAPE, Crystal Optimisation for Numerical Absorption Correction,
version 1.2; Stoe & Cie GmbH: Darmstadt, Germany.
(41) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(42) Sheldrick, G. M. Computer Program for the Refinement of Crystal
Structures; Go¨ttingen, Germany.
1.91-2.01 (m, 2H, CH2CH2CH2), 2.09-2.23 (m, 4H, 2 × SCH2-
CH2), 3.10-3.30 (m, 1H, P-CH2-P), 3.61-3.81 (m, 1H, P-CH2-
P), 7.28-7.50 (m, 16H, Ph), 7.50-7.70 (m, 4H, Ph) ppm. 31P NMR
(CDCl3): δ 53.65 (s) ppm. IR (KBr): ν(CO) 1981, 1946, 1916,
Inorganic Chemistry, Vol. 46, No. 6, 2007 1989

Gao et al.
and 1895 cm-1. Anal. Calcd (%) for C32H28Fe2O4P2S2‚C5H12‚
H2O: C, 55.2; H, 5.22. Found: C, 55.4; H, 5.24.
Synthesis
of
[{µ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)5-
(Ph2PCH2PPh2)] (8). A solution of 2 (0.215 g, 0.5 mmol) and dppm
(0.192 g, 0.5 mmol) in toluene (40 mL) was refluxed for 0.5 h.
Then, the solvent was removed under reduced pressure, and the
crude product purified by chromatography on silica gel with CH2-
Cl2/hexane (1/2 v/v) as the eluent. Complex 8 was obtained as a
1
red solid (0.196 g, 50%). H NMR (CDCl3): δ 0.70 (t, J ) 7.2
Hz, 3H, CH3), 1.12-1.14 (m, 2H, CH3CH2CH2), 2.2 (s, 2H,
CH2CH2N), 2.74 (d, J ) 9.2 Hz, 2H, NCH2S), 2.90 (d, J ) 9.2
Hz, 2H, NCH2S), 3.29 (d, J ) 9.2 Hz, 2H, PCH2P), 7.19-7.21
(m, 8H, Ph), 7.26-7.29 (m, 8H, Ph), 7.62 (t, J ) 8.8 Hz, 4H, Ph)
ppm. 31P NMR (CDCl3): δ 57.76 (d, J ) 219 Hz), -25.20 (d, J )
219 Hz) ppm. IR (KBr): ν(CO) 2040, 1977, and 1927 cm-1. Anal.
Calcd (%) for C35H33Fe2NO5P2S2: C, 53.52; H, 4.24; N, 1.78.
Found: C, 53.56; H, 4.35; N, 1.77.
Synthesis
of
[{µ-(SCH2)2N(CH2CH2CH3)}Fe2(CO)4{µ-
(Ph2P)2CH2}] (9). A solution of 2 (0.215 g, 0.5 mmol) and dppm
(0.192 g, 0.5 mmol) in toluene (40 mL) was refluxed for 1 h. The
solvent was then removed under reduced pressure, and the crude
product was purified by chromatography on silica gel with CH2-
Cl2/hexane (1/2 v/v) as the eluent. Complex 7 was obtained as a
red solid (0.379 g, 75%). 1H NMR (CDCl3): δ 0.84 (s, 3H, CH3),
1.26 (s, 2H, CH3CH2CH2), 1.33 (s, 2H, CH2CH2N), 3.01 (s, 4H,
PCH2P), 3.81 (s, 2H, NCH2S), 3.92 (s, 2H, NCH2S), 7.32 (s, 12H,
Ph), 7.59 (s, 8H, Ph) ppm. 31P NMR (CDCl3): δ 54.24 (s) ppm.
IR (KBr): ν(CO) 1986, 1952, and 1916 cm-1. Anal. Calcd (%)
for C34H33Fe2NO4P2S2: C, 53.92; H, 4.39; N, 1.85. Found: C,
53.40; H, 4.54; N, 1.78.
Protonation of 4. A small amount of 4 (3 mg) was dissolved in
CD3CN (0.5 mL) in an NMR tube, and then, 2.2 µL of triflic acid
1
(5 equiv) was added directly to the solution for in situ H NMR,
1
31P NMR, and IR analysis. 4H22+. H NMR (CD3CN): δ 0.79 (s,
6H, 2 × CH3), 0.93 (s, 4H, 2 × CH3CH2CH2), 1.38 (s, 4H, 2 ×
CH2CH2N), 2.77 (m, 4H, PCH2CH2P), 3.05 (d, 8H, J ) 10.4 Hz,
4 × NCH2S), 4.30 (m, 8H, 4 × NCH2S), 7.46 (s, 12H, Ph), 7.62
(s, 4H, Ph), 7.76 (s, 4H, Ph) ppm. 31P NMR (CD3CN): δ 62.50 (s)
ppm. IR (CD3CN): ν(CO) 2058, 1986, and 1954 cm-1
.
2+
The solution of 4H2 in CD3CN was fully deprotonated when
aniline (5 equiv) was added. It gave the deposit of 4, which was
identified after filtration by 1H NMR, 31P NMR, and IR in CDCl3.
However, when the same amount of 4-cyanoaniline was added, no
change happened, thus placing its pKa between 10.6 and 7.6.
A similar procedure was done to 3 in CD3CN in an attempt to
give 3H22+. However, we did not obtain a solution just like that of
4H22+. The H NMR, 31P NMR, and IR spectra in (CDCl3) were
1
the same as those of with 3, just as expected, even when triflic
acid was added. This provided evidence that 3 did not react with
triflic acid without the participation of the electron during CV.
Control Experiment. In order to confirm the two-electron
assignment for the first reduction wave of complexes 3 and 4, a
control experiment was performed. When the total charge (Q)
passed during bulk electrolysis at an applied potential of -2.0 V,
calculated values for the passage of 1.50, 1.75, and 2.10 electrons
were approached.43 The CVs obtained at the theoretical values of
1.50 and 1.75 electrons per molecule of 3 and 4 also showed a
reduction wave at about -1.90 V, although the height of this wave
was decreased. However, a CV obtained at the theoretical value of
(43) Tye, J. W.; Lee, J.; Wang, H.-W.; Mejia-Rodriguez, R.; Reibenspies,
J. H.; Hall, M. B.; Darensbourg, M. Y. Inorg. Chem. 2005, 44, 5550-
5552.
1990 Inorganic Chemistry, Vol. 46, No. 6, 2007

Binuclear Fe-S Complexes with Bidentate Phosphine Ligands
ca. 2.10 and 2.13 electrons per molecule for 3 and 4 did not show
any reduction wave at -1.90 V, indicating the complete reduction
of the bulk solution.
Ms. Rong Zhang for the HRMS measurements and Ms. Kun
Jin for the NMR measurements.
Supporting Information Available: Crystallographic informa-
tion files (CIF) for 3-9 and cyclic voltammograms for 3, 4, 7, and
9. This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We are grateful to the Ministry of Sci-
ence and Technology of China (Grant No. 2001CCA02500),
the Chinese National Natural Science Foundation (Grant No.
20633020), the Swedish Energy Agency, and the Swedish
Research Council for financial support. We also thank to
IC0610278
Inorganic Chemistry, Vol. 46, No. 6, 2007 1991