Bidentate phosphine ligand based Fe2S2-containing macromolecules: Synthesis, characterization, and catalytic electrochemical hydrogen production

Article abstract of DOI:10.1021/ic061258e

The reaction of [Fe₂(CO)₆(μ-SCH₂) ₂NCH₂CH₂N(μ-SCH₂) ₂Fe₂(CO)₆] (1) with 1,2-bis(diphenylphosphino) ethane in the presence of Me₃NO·2H₂O affords two structurally different metallomacromolecules: a dimer of the type [{Fe ₂(CO)₅(μ-SCH₂)₂NCH ₂CH₂N(μ-SCH₂)₂Fe ₂(CO)₅}(Ph₂PCH₂)₂] (2) and a tetramer species containing eight iron centers with an overall formula of [{Fe₂(CO)₆(μ-SCH₂)₂NCH ₂CH₂N(μ-SCH₂)₂Fe ₂(CO)₅}₂(Ph₂PCH₂) ₂] (3). Their structures have been determined by X-ray crystallography, showing one macrocyclic complex (2) and one linear complex (3). Electrochemical hydrogen evolution catalyzed by these two complexes with ca. 80-90 single-run turnovers is observed, indicating good potential as catalysts for future applications.

Full text of DOI:10.1021/ic061258e

Inorg. Chem. 2006, 45, 9169−9171
Bidentate Phosphine Ligand Based Fe2S2-Containing Macromolecules:
Synthesis, Characterization, and Catalytic Electrochemical Hydrogen
Production
Weiming Gao,† Jianhui Liu,*,† Bjorn Åkermark,‡ and Licheng Sun*,†,§
1
State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, 116012 Dalian,
People’s Republic of China, Department of Organic Chemistry, Arrhenius Laboratory,
Stockholm UniVersity, 10691 Stockholm, Sweden, and KTH Chemistry, Organic Chemistry,
Royal Institute of Technology, 10044 Stockholm, Sweden
Received July 6, 2006
The reaction of [Fe2(CO)6(
(1) with 1,2-bis(diphenylphosphino)ethane in the presence of Me3-
NO 2H2O affords two structurally different metallomacromol-
ecules: a dimer of the type [ Fe2(CO)5( -SCH2)2NCH2CH2N(
SCH2)2Fe2(CO)5 (Ph2PCH2)2] (2) and a tetramer species containing
eight iron centers with an overall formula of [ Fe2(CO)6( -SCH2)2-
NCH2CH2N( -SCH2)2Fe2(CO)5 2(Ph2PCH2)2] (3). Their structures
have been determined by X-ray crystallography, showing one
macrocyclic complex (2) and one linear complex (3). Electrochemi-
cal hydrogen evolution catalyzed by these two complexes with
µ
-SCH2)2NCH2CH2N(
µ
-SCH2)2Fe2(CO)6]
electrochemical hydrogen evolution has been reported re-
cently with 25 single-run turnovers during bulk electrolysis
with the model complex [{(µ-SCH2)2NR}Fe2(CO)6] (R )
p-bromobenzyl).4 This observation has been the starting point
for the extension of our work in the design and synthesis of
novel systems with improved catalytic properties. A synthetic
approach based on multidimensional coordination complexes
obtained by self-assembly is of considerable interest. In a
previous publication, we reported the preparation of a -CH2-
CH2--bridged tetranuclear double-Fe2S2 complex [Fe2(CO)6-
(µ-SCH2)2NCH2CH2N(µ-SCH2)2Fe2(CO)6] (1),5 which dis-
played an Fe2S2-capped zigzag geometry. To extend this
study, we decided to use complex 1 as a building block and
the bidentate phosphine ligand 1,2-bis(diphenylphosphino)-
ethane (dppe) as a bridging ligand. In this contribution, we
have successfully isolated two interesting complexes based
on the Fe2S2 unit by reaction of the Fe-S complex 1 with
the dppe ligand. We describe herein our results concerning
complexes [{Fe2(CO)5(µ-SCH2)2NCH2CH2N(µ-SCH2)2Fe2-
(CO)5}(Ph2PCH2)2] (2), [{Fe2(CO)6(µ-SCH2)2NCH2CH2N-
(µ-SCH2)2Fe2(CO)5}2(Ph2PCH2)2] (3), and [{Fe2(CO)6(µ-
SCH2)2NCH2CH2N(µ-SCH2)2Fe2(CO)5}(Ph2PCH2CH2PPh2-
S)] (4). The potential for 2 and 3 as catalytic hydrogen
production centers is investigated.
‚
{
µ
µ-
}
{
µ
µ
}
ca. 80−90 single-run turnovers is observed, indicating good
potential as catalysts for future applications.
Organometallic diiron dithiolate complexes of the type
[Fe2(µ-S2R)(CO)6] (R ) -(CH2)3-, -CH2NR′CH2-)1 have
been reported as structural models for the active site of iron-
only hydrogenase. The protonation on the bridgehead
nitrogen heteroatom2,3 of the disulfide ligand offers a
thermodynamically and kinetically favorable pathway for
hydrogen evolution in the natural system. Although attempts
to achieve closer structural models of the diiron subsite have
led to the development of bimetallic iron species, structural
studies and electrochemical properties of multinuclear models
of the active site of iron-only hydrogenase have appeared
only sporadically. A very effective iron-sulfur complex for
Recent studies on CO/PR3-exchange reactions suggest that
the replacement of a carbonyl ligand by phosphine can be
easily realized.6,7 Thus, the reaction of 1 with dppe in the
presence of Me3NO‚2H2O (molar ratio 1:1:1) affords two
different kinds of molecules: the [Fe2S2]2-type complex 2
and complex 3, which is based on a [Fe2S2]4-type arrange-
ment. In both complexes, dppe acts as a bidentate bridging
ligand. Complexes 2 and 3 were purified by chromatography
* To whom correspondence should be addressed. E-mail: liujh@
dlut.edu.cn (J.L.), lichengs@kth.se (L.S.).
† Dalian University of Technology.
‡ Stockholm University.
§ Royal Institute of Technology.
(1) (a) Evans, D. J.; Picket, C. J. Chem. Soc. ReV. 2003, 32, 268-275.
(b) Georgakaki, I. P.; Thomson, L. M.; Lyon, E. J.; Hall, M. B.;
Darensbourg, M. Y. Coord. Chem. ReV. 2003, 238-239, 255-256.
(c) Rauchfuss, T. B. Inorg. Chem. 2004, 43, 14-26. (d) Sun, L.;
A° kermark, B.; Ott, S. Coord. Chem. ReV. 2005, 249, 1653-1663.
(2) Wang, F.; Wang, M.; Liu, X.; Jin, K.; Dong, W.; Li, G.; A° kermark,
B.; Sun, L. Chem. Commun. 2005, 3221-3223.
(4) Ott, S.; Kritikos, M.; A° kermark, B.; Sun, L.; Lomoth, R. Angew.
Chem., Int. Ed. 2004, 43, 1006-1009.
(5) Gao, W.; Liu, J.; Ma, C.; Weng, L.; Jin, K.; Chen, C.; A° kermark, B.;
Sun, L. Inorg. Chim. Acta 2006, 359, 1071-1080.
(6) Gloaguen, F.; Lawrence, J. D.; Schmidt, M.; Wilson, S. R.; Rauchfuss,
T. B. J. Am. Chem. Soc. 2001, 123, 12518-12527.
(3) Schwartz, L.; Eilers, G.; Eriksson, L.; Gogoll, A.; Lomoth, R.; Ott, S.
Chem. Commun. 2006, 520-522.
(7) Gao, W.; Liu, J.; A° kermark, B.; Sun, L., manuscript submitted.
10.1021/ic061258e CCC: $33.50
Published on Web 10/14/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 23, 2006 9169

COMMUNICATION
Figure 1. ORTEP view of 2 with 30% ellipsoids.
on a silica gel column, giving isolated yields of 36% and
22%, respectively. Interestingly, an unexpected side product,
compound 4, was also obtained in a yield of ca. 7%. In this
species, one phosphorus atom coordinated to the iron-sulfur
part and the other phosphorus atom has been converted to a
noncoordinating terminal PdS group. The mechanism for
the formation of the PdS bond is described elsewhere.8
All of the complexes have been fully characterized by mass
Figure 2. ORTEP view of 3 with 30% ellipsoids.
of dppe occupies an apical position and the other one is in
a basal position of the Fe2S2 unit. The surprising basal
configuration attributes to a small distortion along the chain
-NCH2CH2N- in comparison with the single-crystal struc-
ture of 1.5 The Fe(1)-Fe(2) distance (2.5197 Å) is a little
shorter than that of Fe(3)-Fe(4) (2.5387 Å), elongated by
ca. 0.01 and 0.03 Å, respectively, compared with the Fe-
Fe distance (2.5062 Å) of 1 after partial rotation. Complex
3 consists of an octanuclear structure of the type [{Fe2(CO)6-
(µ-SCH2)2NCH2CH2N(µ-SCH2)2Fe2(CO)5}2(Ph2PCH2)2], where
the two [Fe2(CO)6(µ-SCH2)2NCH2CH2N(µ-SCH2)2Fe2(CO)5]
units are linked by a single dppe unit. The two phosphorus
atoms coordinate to two iron atoms in an anti mode for the
[Fe2S2(CO)5N]dppe[Fe2S2(CO)5N] unit but in a syn manner
for the [Fe2S2(CO)5(CH2CH2)Fe2S2(CO)6] unit. The dppe
ligand occupies the two apical positions. The midpoint of
the -CH2CH2- bond in dppe acts as an inversion center.
The Fe(1)-Fe(2) distance (2.5183 Å) is longer than that of
Fe(3)-Fe(4) (2.4864 Å) and almost the same as that of
Fe(1)-Fe(2) in 2. All of the phenyls in 2 and 3 are positioned
to minimize steric hindrance.
The electrochemical properties of model complexes 1-3
were studied by cyclic voltammetry in dry CH2Cl2. For 1
and 2, irreversible reduction peaks were observed at -1.73
and -2.08 V (vs the ferrocene/ferrocenium, Fc/Fc+, couple),
respectively.9 Upon the addition of excess triflic acid to CH2-
Cl2 solutions of 1 or 2 (1 mM), an additional reduction peak
appeared at about -1.28 V for both compounds, 50 and 400
mV more positive than those reported for the complex [{(µ-
SCH2)2NR}Fe2(CO)6] (R ) p-bromobenzyl) under similar
acidic conditions. Other reduction peaks are observed at
1
spectrometry, IR, H NMR, and 31P NMR spectroscopies,
and elemental analyses. The IR spectrum of the complex 2
displays three strong bands in the carbonyl region (2045,
1976, and 1921 cm-1). Four ν(CO) bands are observed for
3 (2074, 2041, 1979, and 1927 cm-1), which are related to
two sets of different carbonyl stretches corresponding to
carbonyls from the [Fe2(µ-S2R)(CO)5PPh2-] (2041, 1979,
and 1927 cm-1) and the [Fe2(µ-S2R)(CO)6] (2074 cm-1)
moieties. The smaller IR difference (2045, 1976, and 1921
cm-1 vs 2041, 1979, and 1927 cm-1) for the [Fe2(µ-
S2R)(CO)5PPh2-] units in both 2 and 3 indicates that there
is a negligible electronic distinction between the Fe2S2 centers
in 2 and 3. IR data for 4 also show [Fe2(µ-S2R)(CO)5PPh2-]-
type carbonyl stretching vibrations. A strong absorption for
the PdS bond located at 693 cm-1 is observed as found for
other similar PdS complexes.8 Complex 1 shows a singlet,
typical for a NCH2S at 3.54 ppm, while complex 2 gives
two similar singlets at 2.37 and 2.61 ppm of equal intensity.
For complex 3, NCH2S signals are observed at 2.52, 2.64,
and 3.39 ppm (1:1:2 integration). The corresponding 31P
NMR spectra recorded in CDCl3 display one singlet relative
to the dppe ligand at 60.46 ppm for 2 and 59.95 ppm for 3,
at about the same value found for other related Fe2S2
complexes coordinated with the dppe ligand.8 The little
change of ∆δ ) 0.51 ppm also provides the fact that the
interaction is weak between [Fe2(µ-S2R)(CO)6] and [Fe2(µ-
S2R)(CO)5PPh2-] moieties.
Single-crystal X-ray data for the complexes 2 and 3 have
been obtained (Figures 1 and 2). For the macrocyclic
complex 2, the molecular structure is based upon a tetra-
nuclear semisquare with a ring size of 14 atoms. The dppe
ligand is coordinated to the iron atom via the two phosphorus
atoms to form a 14-membered ring. In 2, one phosphine atom
(9) CH2Cl2 was used for better solubility of 2 and 3 in it for cyclic
voltammetry measurement. The different solvent systems were
compared by using [{(µ-SCH2)2NR}Fe2(CO)6] (R ) p-bromobenzyl)
as a referent agent, which is reduced at -1.56 V vs Fc/Fc+ in MeCN.4
In our system and under otherwise identical conditions, this complex
is reduced at -1.74 V, which gives rise to a ∆E value of 0.18 V
brought by CH2Cl2.
(8) Gao, W.; Ekstro¨m, J.; Liu, J.; Chen, C.; Eriksson, L.; Weng, L.;
A° kermark, B.; Sun, L., manuscript submitted.
9170 Inorganic Chemistry, Vol. 45, No. 23, 2006

COMMUNICATION
around -1.70 and -2.00 V for 1 and 2, respectively, with
the current intensity increasing and shifting to more negative
potential as the acid concentration are increased. These
features are indicative of the presence of a catalytic proton
reduction.10,11 For complex 3, the irreversible reduction peaks
occur at -1.71 and -2.02 V, which are close to the values
obtained for 1 and 2. In the presence of excess triflic acid,
the reduction peak at around -1.28 V is observed again,
while the reduction peak at around -2.00 V is increased
and moves to a more negative potential with an increase of
the acid concentration. In addition, its current intensity is
considerably higher than that of the reduction peak at around
-1.70 V.12 This provides further evidence that the PPh2-
coordinated Fe2S2 parts contribute dominantly to catalyzing
the electrochemical proton reduction to molecular hydrogen.
Additional information was obtained from NMR experi-
ments. When ca. 30 equiv of HOTf are added to complex 3,
(Figure S5). For complex [{(µ-SCH2)2NR}Fe2(CO)6] (R )
p-bromobenzyl), the rate of electrolysis slows down with time
as the proton concentration decreases, and after approxi-
mately 60 min, the total charge passed through the cell
approaches its maximum of 50 F, which is the equivalent of
25 turnovers. It achieves its maximum quantum in ca. 1 h.
For complexes 1-3, the initial electrolysis rates are almost
identical in the first 20 min, the rates of electrolysis are
essentially unchanged for complexes 2 and 3, and they are
only slightly lower for complex 1 during our electrolysis in
60 min. The rates of 2 and 3 are almost the same, indicating
that the two terminal hexacarbonyl Fe2S2 units do not
contribute significantly to the catalytic process. After con-
tinuous electrolysis for ca. 30 min further for complexes 1-3,
the rate of electrolysis slows down rapidly and becomes in
agreement with the initial baseline at ca. -1.70 or -2.10 V.
The data indicate that after about 1.5 h complex 1 produces
ca. 60 turnovers of dihydrogen and complexes 2 and 3 can
both give out ca. 80-90 turnovers. Taking the dimer structure
of complexes 1 and 2 into consideration, each Fe2S2 unit
will give ca. 30 and 40-45 turnovers, respectively. Though
their efficiencies are still rather low compared with the nature
hydrogenase enzymes with rates up to 6000 turnovers per
second quoted (30 °C),14 the fact that the present catalysts
are not derived from expensive platinum metals stimulates
us to continue our investigations in this direction.
1
the H NMR peak at 3.39 ppm associated with the NCH2S
grouping coordinated to the hexacarbonyl fragment is still
observed; however, the peaks of the NCH2S moieties that
are coordinated to PPh2-coordinated Fe2S2 originally ob-
served at 2.52 and 2.64 ppm have completely shifted to ca.
4.26-4.67 ppm. The chemical-electrochemical-chemical-
electrochemical (CECE) mechanism is strongly proposed.
Because of the increased electron density brought by the
phosphine ligand, it is first protonated on the bridging
nitrogen atom and then on iron atoms of the PPh2-
coordinated Fe2S2 units during the CC (chemical and
chemical) steps in the CECE mechanism. Further proof can
also be obtained from controlled potential electrolysis.
Controlled potential electrolysis in CH2Cl2-Bu4NPF6 was
performed to test the catalytic activities for complexes 1-3.
The complex [{(µ-SCH2)2NR}Fe2(CO)6] (R ) p-bromoben-
zyl) was used as a reference for its hitherto best electro-
chemical hydrogen evolution activity with 25 turnovers
(Figure S5).4,13 In the absence of FeS catalysts, the initial
electrolysis rate at -2.10 V is only a little higher than that
at -1.70 V, both of which show a slow proton reduction
In conclusion, we have obtained two iron-sulfur com-
plexes. The complexes exhibited totally different topological
structures, with 2 having a quasi-square structure and 3 being
a three-dimensional macromolecule. The compounds show
promising catalytic properties for the formation of hydrogen,
and we are currently trying to connect them to photosensi-
tizers to realize light-induced hydrogen production.
Acknowledgment. We acknowledge the following foun-
dation for financial support of this work: the Chinese
National Natural Science Foundation (Grants 20471013 and
20128005), the Swedish Energy Agency, and the Swedish
Research Council. We thank Prof. Han Vos, Dublin City
University, Dublin, Ireland, for very helpful discussion.
(10) Bhugun, I.; Lexa, D.; Save´ant, J.-M. J. Am. Chem. Soc. 1996, 118,
3982-3983.
Supporting Information Available: Experimental details and
crystallographic information files (CIF) for 2 and 3. This material
is available free of charge via the Internet at http://pubs.acs.org.
(11) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.; Be´nard, M.; Rohmer,
M.-M. Inorg. Chem. 2002, 41, 6573-6582.
(12) Differential pulse voltammograms were performed to investigate the
change of the reduction peaks when an excess HOTf was added (see
the Supporting Information).
IC061258E
(13) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 9476-9477.
(14) Adams, M. W. W. Biochim. Biophys. Acta 1990, 1020, 115-145.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9171