[Fefe]-hydrogenase models: Overpotential control for electrocatalytic H2 production by tuning of the ligand π-acceptor ability

Article abstract of DOI:10.1002/ejic.201000304

In the search for synthetic competitive catalysts that function with hydrogenase-like capability, a series of (Pyrrol-1-yl)-phosphane-substituted diiron complexes [(μ-pdt)Fe₂(CO)₅L] [pdt = propanedithiolate, L = Ph₂PPyr (2), PPyr₃ (4); Pyr = pyrrolyl] and [(μ-pdt)Fe₂(CO)₄L₂] [L = Ph₂PPyr (3), PPyr₃ (5)] were prepared as functional models for the active site of Feonly hydrogenase. The structures of these complexes were fully characterized by spectroscopy and X-ray crystallography. In the IR spectra the CO bands for complexes 2-5 are shifted to higher energy relative to those of complexes with traditional phosphane ligands, such as PPh₃, PMe₃, and PTA (1,3,5-triaza-7-phosphaadamantane), indicating that (pyrrol-1-yl)phosphanes are poor σ-donors and better π-acceptors. The electrochemical properties of complexes 2-5 were studied by cyclic voltammetry in CH₃CN in the absence and presence of the the weak acid HOAc. The reduction potentials of these complexes show an anodic shift relative to other phosphane-substituted derivatives. All of the complexes can catalyze proton reduction from HOAc to H₂ in CH₃CN at their respective Fe^IFe⁰level. Complex 4 is the most effective electrocatalyst, which catalytically generates H2 from HOAc at-1.66 V vs. Fc+/Fc with only ca. 0.2 V overpotential in CH₃CN.

Full text of DOI:10.1002/ejic.201000304

FULL PAPER
DOI: 10.1002/ejic.201000304
[FeFe]-Hydrogenase Models: Overpotential Control for Electrocatalytic H2
Production by Tuning of the Ligand π-Acceptor Ability
Fengwei Huo,[a] Jun Hou,*[a,b] Guicai Chen,[b] Dongming Guo,[a] and Xiaojun Peng*[b]
Keywords: Bioinorganic chemistry / Hydrogenase / Phosphanes / Carbonyldiiron compounds / Phosphane ligands
In the search for synthetic competitive catalysts that function
with hydrogenase-like capability, a series of (Pyrrol-1-yl)-
phosphane-substituted diiron complexes [(µ-pdt)Fe2(CO)5L]
[pdt = propanedithiolate, L = Ph2PPyr (2), PPyr3 (4); Pyr =
pyrrolyl] and [(µ-pdt)Fe2(CO)4L2] [L = Ph2PPyr (3), PPyr3 (5)]
were prepared as functional models for the active site of Fe-
only hydrogenase. The structures of these complexes were
fully characterized by spectroscopy and X-ray crystallogra-
phy. In the IR spectra the CO bands for complexes 2–5 are
shifted to higher energy relative to those of complexes with
“traditional” phosphane ligands, such as PPh3, PMe3, and
PTA (1,3,5-triaza-7-phosphaadamantane), indicating that
(pyrrol-1-yl)phosphanes are poor σ-donors and better π-ac-
ceptors. The electrochemical properties of complexes 2–5
were studied by cyclic voltammetry in CH3CN in the absence
and presence of the the weak acid HOAc. The reduction po-
tentials of these complexes show an anodic shift relative to
other phosphane-substituted derivatives. All of the com-
plexes can catalyze proton reduction from HOAc to H2 in
CH3CN at their respective FeIFe0 level. Complex 4 is the
most effective electrocatalyst, which catalytically generates
H2 from HOAc at –1.66 V vs. Fc+/Fc with only ca. 0.2 V over-
potential in CH3CN.
Introduction
Hydrogen has attracted remarkable interest as a clean
and highly efficient energy carrier of the future.[1] However,
presently expensive platinum-containing catalysts are used
to efficiently catalyze hydrogen evolution from the re-
duction of protons and electrons.[2] Therefore, the search
for less expensive and more efficient catalysts to replace
platinum-based materials is an important goal for hydrogen
energy applications.[3,4]
Figure 1. Active site of [Fe]H2ase (A) and the synthetic FeIFeI elec-
[FeFe]-hydrogenase ([FeFe]-H2ase) can efficiently cata- trocatalysts for H2 production (B).
lyze the reversible reduction of protons to hydrogen with
high rates up to 6000 molecules of H2 per second per
mmol.[5–7] The active site, which generates H2, consists of a
Since the elucidation of the structures of [FeFe]-H2ase,
2Fe2S unit bridged to a 4Fe4S cluster by a cystein-S bridge,
as revealed by X-ray structure determinations[8,9] and IR
spectroscopic studies[10,11] illustrated in Figure 1A. The two
Fe atoms in the 2Fe2S unit are coordinated by CO and
CN–, as well as a bridging 1,3-dithiolato ligand.[12,13] Please
note that the L ligand has not been identified with certainty,
and it is possibly H2O, H or vacant based on different redox
states.
there have been numerous attempts aimed at the synthesis
of diiron complexes that mimic the active site of [FeFe]-
H2ase in the past few years.[14–18] Mono- and disubstituted
diiron complexes for electrocatalytic H2 production were
developed (Figure 1B).[19–24] However, the FeIFeI catalysts
synthesized so far require the harsh conditions of either
strong acids (i.e., HOTs or HClO4)[21–25] or a relatively high
overpotential (0.5–1.0 V).[19,20] As is well known, [FeFe]-
H2ase can generate H2 at neutral pH and at low potential
(ca. –0.8 V vs. Fc+/Fc). Thus, the major challenge now is to
search for synthetic competitive catalysts with a low over-
potential that function with [FeFe]-H2ase-like capability un-
der mild conditions for both proton and electron sources.
Theoretic[26,27] and experimental[19,28–30] studies indicate
that the ligands at the diiron core have an important influ-
ence on the electrocatalytic capabilities of the 2Fe2S com-
[a] Key Laboratory for Precision & Non-traditional Machining of
the Ministry of Education, Dalian University of Technology
Dalian 116024, P. R. China
Fax: +86-411-84706059
E-mail: junhou@dlut.edu.cn
[b] State Key Laboratory of Fine Chemicals, Dalian University of
Technology
Dalian 116012, P. R. China
E-mail: pengxj@dlut.edu.cn
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[FeFe]-Hydrogenase Models: Electrocatalytic H2 Production
plexes. In this context, numerous σ-donor-ligand-substi- PPyr3-substituted complexes is relatively more difficult than
tuted diiron complexes, [(µ-pdt)Fe2(CO)5L] and [(µ-pdt)- that of Ph2PPyr-substituted derivatives, presumably because
Fe2(CO)4L2] [L = PMe3, PMe2Ph, PPh3, P(OEt)3, CN–, of the electronic effect. The reactivity of PPyr3 is unusual
1,3,5-triaza-7-phosphaadamantane (PTA)] have been exten- as compared to that of alkylphosphanes. For instance, PPh3
sively investigated.[19,20,28–30] (Pyrrol-1-yl)phosphanes have and PMe3 can readily react with 1 to give mono- and disub-
been widely used to tune metal reactivity and selectivity in stituted derivatives.[30] These observations are consistent
homogeneous catalysis because of their exceptional π-ac- with a greatly reduced nucleophilicity for the phosphorus
ceptor character.[31] Some (Pyrrol-1-yl)phosphane-substi- atom in PPyr3, presumably due to aromatic delocalization
tuted rhodium complexes, [Rh(acac)(CO)L] and [RhH- of the nitrogen lone pair into the ring.[31]
(CO)L] (acac = acetylacetonato), were reported for hydro-
The products obtained in analytically pure form are solu-
formylation with higher yields and better selectivity.[32,33] ble in CH2Cl2, THF, and acetone. All complexes are air-
However, to the best of our knowledge, there is as yet no and thermally stable in the solid state but moderately sensi-
report on the use of (pyrrol-1-yl)phosphane-substituted di- tive in solution, and were characterized by IR and NMR
iron dithiolates as [FeFe]-H2ase model complexes. This led spectroscopy as well as HR mass spectrometry, as detailed
us to explore 2Fe2S synthetic catalysts with small overpot- in the Experimental Section. The HR-MS analyses are in
entials by tuning ligand π-acceptor ability using (pyrrol-1- good agreement with the supposed molecular weight. The
yl)phosphane ligands. Thus, we herein report on (pyrrol- resonances of the 1,3-propanedithiolato methylene hydro-
1-yl)phosphane mono- and disubstituted diiron complexes gen and carbon atoms in their characteristic regions show
formed by CO/L exchange reaction to study the influence a high-field shift as compared to the all-CO complex 1, be-
of (pyrrol-1-yl)phosphanes on structure and electrochemi- cause of the shield effects of the aromatic rings at the bulky
cal properties. The issue of overpotential in proton re- tertiary phosphane ligands. The IR spectra of complexes 2–
duction by these complexes is also discussed.
5 exhibit three major νCO bands in the region 2056–
1962 cm–1. All compounds were identified by X-ray struc-
ture determinations.
Results and Discussion
Preparation and Spectroscopic Characterization of
Complexes 2–5
Molecular Structures of Complexes 2–5
These new complexes were obtained in moderate yield by
The crystal structures of 2–5 were determined by X-ray
refluxing toluene solutions of 1 with (pyrrol-1-yl)phos- crystallography and are shown in Figure 2. Selected bond
phane. The preparation of these phosphane-substituted di- lengths and bond angles are listed in Table 1.
iron complexes is summarized in Scheme 1.
The 2Fe2S centers of all complexes are six-coordinate
The reaction of 1 with 1 mol-equiv. of Ph2PPyr in re- and exhibit square-pyramidal geometries. The Fe–Fe dis-
fluxing toluene for 48 h gave monosubstituted complex 2 in tances [2.5222(10) Å in 2, 2.5188(6) Å in 3, 2.523(2) Å in 4,
good yield. Disubstituted complex 3 as a major product and 2.5237(8) Å in 5] are in good agreement with those
together with a small amount of monosubstituted complex found in tertiary phosphane-substituted diiron ana-
2 was obtained by refluxing a solution of 1 with 2 equiv. of logues.[19,20,30,34]
Ph2PPyr for 48 h. In a similar way, complexes 4 and 5 were
In monosubstituted derivatives 2 and 4, the coordination
prepared in refluxing toluene by treatment of 1 with 1 and configurations are nearly identical with the tertiary phos-
2 equiv. of PPyr3, respectively. However, the preparation of phane monosubstituted derivatives [(µ-pdt)Fe2(CO)5L] [L =
Scheme 1. (a) for 2, Ph2PPyr (1 equiv.), toluene, reflux, 48 h; for 3, Ph2PPyr (2 equiv.), toluene, reflux, 72 h; (b) for 4, PPyr3 (1 equiv.),
toluene, reflux, 48 h; for 5, PPyr3 (2 equiv.), toluene, reflux, 72 h.
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F. Huo, J. Hou, G. Chen, D. Guo, X. Peng
FULL PAPER
Figure 2. ORTEP (ellipsoids at 30% probability level) view of 2 (a), 3 (b), 4 (c), and 5 (d).
Table 1. Selected bond lengths and angles for 2–5.
2
4
3
5
Bond lengths [Å]
Fe(1)–Fe(2)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(2)–S(1)
Fe(2)–S(2)
Fe–Pap
2.5222(10)
2.2609(15)
2.2594(15)
2.2636(15)
2.2635(15)
2.2247(1)
1.802(6)
2.5188(6)
2.2486(8)
2.2584(8)
2.2617(7)
2.2563(8)
2.1661(7)
1.801(3)
Fe(1)–Fe(2)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(2)–S(1)
Fe(2)–S(2)
2.523(2)
2.274(3)
2.270(3)
2.268(3)
2.263(3)
2.215(8)
1.767(6)
1.746(2)[e]
2.5237(8)
2.2768(12)
2.2778(12)
2.2733(12)
2.2509(12)
2.1725(1)
1.769(2)
[c]
Fe–Pap
Fe–CCO,ba
[d]
Fe–CCO,ap
Fe–CCO,ba
[a]
1.774(4)
1.761(4)
1.783(1)
P–N
1.709(3)[f]
P–N
1.7096(2)[b]
Bond angles [°]
Fe(1)–S(1)–Fe(2)
Fe(1)–S(2)–Fe(2)
P(1)–Fe(2)–Fe(1)
P(1)–Fe(2)–S(1)
P(1)–Fe(2)–S(2)
S(1)–Fe(2)–Fe(1)
S(2)–Fe(2)–Fe(1)
67.76(4)
67.79(5)
156.96(5)
110.99(5)
106.94(6)
56.07(4)
56.03(4)
67.90(2)
67.82(2)
155.11(2)
108.11(3)
107.22(3)
55.80(2)
56.13(2)
Fe(1)–S(1)–Fe(2)
Fe(1)–S(2)–Fe(2)
P(1)–Fe(1)–Fe(2)
P(2)–Fe(2)–Fe(1)
P(1)–Fe(1)–S(1)
P(1)–Fe(1)–S(2)
S(1)–Fe(1)–Fe(2)
S(2)–Fe(1)–Fe(2)
67.50(9)
67.64(9)
158.29(9)
154.16(10)
109.10(12)
110.19(11)
56.14(9)
67.37(3)
67.73(4)
160.60(4)
154.01(4)
110.74(4)
111.86(5)
56.25(3)
55.63(3)
56.06(8)
[a] Average of five Fe–CCO,ba bonds. [b] Average of three P–N bonds. [c] Average of two Fe–Pap bonds. [d] Average of four Fe–CCO,ba
bonds. [e] Average of two P–N bonds. [f] Average of six P–N bonds.
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[FeFe]-Hydrogenase Models: Electrocatalytic H2 Production
PMe3, PMe2Ph, PPh3, P(OEt)3].[30] The phosphane moieties
In PPyr3-substituted diiron complexes 4 and 5, as ex-
Ph2PPyr and PPyr3 occupy apical positions around the Fe pected, the nitrogen geometries are planar, with a sum of
atoms, and are roughly trans to the Fe–Fe bond. The Fe–P angles at N ranging from 358.6(9)° to 360.01(6)°. The bond
distance of 2.1661(7) Å in 4 is slightly shorter by 0.06 Å lengths within the pyrrole rings are also similar to those
than that of 2.2247(14) Å in 2, consistent with the poor found in the free ligand.[38]
donor (or good acceptor) ability of the PPyr3 ligand. One
phenyl ring in 2 and one pyrrolyl ring in 4 are nearly facing
the dithiaferracyclohexane ring similar to the PPh3-coordi- π-Acceptor Ability of (Pyrrol-1-yl)phosphanes
nating complex.[30] The angles of C(4)–S(1)–Fe(2)
In the IR spectra the νCO bands in the CO region provide
[113.13(11)°] and C(6)–S(2)–Fe(2) [114.88(13)°] for 4 are
a powerful tool for evaluating the structural and electronic
slightly larger than the corresponding angles of C(4)–S(1)–
Fe(1) [111.37(16)°] and C(6)–S(2)–Fe(1) [110.70(11)°]. This
indicates that the bulk ligand PPyr3 around the diiron unit
in 4 causes the dithiaferracyclohexane ring to slant towards
changes of carbonyl transition-metal complexes.[39] The IR
data of the CO bands for complexes 2, 3, 4, and 5 are listed
in Table 2. The spectra of the all-CO parent complex 1 and
its PPh3-monosubstituted derivative [(µ-pdt)Fe2(CO)5PPh3]
the Fe(CO)3 unit. The larger differences between the C(6)–
(6) are also included for comparison purposes. Compared
S(2)–Fe(2) [115.1(2)°] vs. C(6)–S(2)–Fe(1) [109.8(2)°] angles
with the all-CO parent complex 1, the νCO bands of 2–5
for 2 and C(4)–S(1)–Fe(2) [116.0(2)°] vs. C(4)–S(1)–Fe(2)
[110.1(2)°] angles for 4 further confirm the steric interac-
shift to lower frequencies, indicating an increase of electron
density on the iron cores when a CO ligand is replaced by a
tions when PPyr3 is replaced by Ph2PPyr. The angles of
Ph2PPyr/PPyr3 ligand. The monosustituted series of diiron
P(1)–Fe(2)–Fe(1) in 2 and 4 are ca. 8.1° and 6.1° larger
than the C(3)–Fe(1)–Fe(2) angles, respectively. The average
Fe(2)–CCO,ba distance of 1.781(2) Å for 4 is 0.02 Å longer
complexes 2, 4, and 6 shows a steady increase of νCO with
an increasing degree of replacement of the phenyl groups
by pyrrol-1-yl groups. It can be seen that the IR νCO bands
than that of 2 [1.760(2) Å], and the corresponding length of
of 6 shift by an average of 50 cm–1 to lower frequencies,
C–O bonds [av. 1.139(13) Å] for 4 is shorter than that of 2
whereas the νCO bands of complexes 2 and 4 shift to lower
[av. 1.143(6) Å]. These features demonstrate that the intro-
duction of the π-acceptor phosphane ligand decreases the
wavenumbers by ca. 32 cm–1 and 18 cm–1, respectively. This
indicates that the pyrrol-1-yl group plays a key role in
electron donation of the iron centers and thereby leads to
decreasing the π-back-bonding of electrons from the metal
weaker π-back-bonding from the iron atoms to the carbonyl
cores to the CO ligands. The νCO bands in 4 are higher than
atoms. It is noteworthy that the average C–O bond length
those of complexes 2 and 6, showing that complex 4 has a
of 1.136 in 4 is in significant agreement with that reported
better π-acceptor ability and a poorer donor ability. In ad-
for [(µ-pdt)Fe2(CO)6] (av. 1.136 Å),[34] indicative of the sim-
dition, the values of νCO for 4 and 5 are significantly higher
ilar electronic effects of both the PPyr3 ligand and CO.
The average Fe–P bond lengths of 2.215(8) Å in 3 and
2.1725(12) Å in 5 are in good agreement with that in phos-
phane-disubstituted analogues; the Fe–P bond length of 5
is ca. 0.04 Å shorter than that of 3, further indicative of the
better π-acceptor character of the PPyr3 ligand relative to
Ph2PPyr. In the case of 3 and 5, two aromatic rings face the
than those found for alkylphosphane-substituted deriva-
tives, [(µ-pdt)Fe2(CO)5L] and [(µ-pdt)Fe2(CO)4L2] (L =
PMe3, PMe2Ph, PTA).[20,30] For instance, the CO stretching
frequency of 5 is shifted to higher energy by nearly 45 cm–1
upon replacement of PMe3 by PPyr3. The shifts to higher
frequencies for 4 and 5 indicate a significantly reduced de-
gree of π-back-bonding donation from the iron atoms to
the carbonyl ligands in these diiron complexes and therefore
demonstrate either a strong π-acceptor ability or a poor σ-
donor character of the PPyr3 ligand. As expected, the CO
stretching frequencies of 4 are very close to those of 1, in-
dicative of a similar π-acceptor ability of the PPyr3 and CO
propanedithiolato bridge, consistent with the fact that the
1H NMR signals for the methylene protons of the 1,3-pro-
panedithiolato bridge significantly shift to high field due to
the shield effects of the aromatic rings.
An interesting structural feature of disubstituted com-
plexes 3 and 5 is that the phosphane ligands are in apical/
apical (ap/ap) position and trans to the Fe–Fe bond, con-
trary to that of [(µ-pdt)Fe2(CO)4(PTA)2] featuring a ba/ba
ligands. This effect has also been observed in other PPyr3-
substituted carbonylmetal complexes.[40] Thus, the π-ac-
ceptor capabilities of these phosphane ligands exhibit the
coordination mode.[20] This observation is identical to that
following trend: CO ≈ PPyr3 Ͼ PhPPyr Ͼ PPh3. As de-
found for PMe2Ph-[30] and tBuNC-disubstituted[35] deriva-
tives. We note that PMe3 and cyano-disubstituted com-
plexes [(µ-pdt)Fe2(CO)4(PMe3)2] and [(µ-pdt)Fe2(CO)4-
(CN)2]2– possess an ap/ba configuration.[36,37] It can be in-
ferred that the steric interactions of bulk phosphane with
the dithiaferracyclohexane ring are not the main factor in
the rearrangement and that electronic effects probably play
a key role. That is, the better electron-donating ligands
favor rearrangements into the ba/ba or ap/ba configuration,
whereas the ligands with better π-acceptor capability prefer
the ap/ap coordination mode.
scribed for PPyr3-substituted carbonylmetal complexes by
Table 2. Summary of IR νCO bands for diiron complexes.
Complex
νCO [cm–1]
1
2
3
4
5
6
2072(m), 2034(s), 1997(s)
2049(s), 1991(s), 1964(w)
2008(s), 1964(m), 1947(s)
2056(s), 2003(s), 1990(w)
2028(s), 1984(m), 1970(s)
2044(s), 1981(s), 1930(m)
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F. Huo, J. Hou, G. Chen, D. Guo, X. Peng
FULL PAPER
Moloy et al., the good π-acceptor ability of PPyr3 is attrib- Table 3. Redox potentials for phosphane derivatives of 1.
uted to the aromatic delocalization of the nitrogen lone pair
Complex Epc [V]
FeIFeI/Fe0FeI
–1.62
Epa [V]
FeIFeI/FeIIFeI
Epa [V]
FeIIFeI/FeIIFeII
into the five-membered rings.[31]
1
0.84
0.46
0.32
0.65
0.62
–0.40
–
2
–1.74
–1.92
–1.66
–1.70
–1.84
0.68
0.72
–
–
–
Electrochemistry of Complexes 2–5
3
4
The cyclic voltammograms of complexes 2–5 shown in
Figure 3 were recorded in CH3CN solution (with 0.1 
nBu4NPF6 as electrolyte), they were initiated from the open
circuit potential and scanned in the cathodic direction as
indicated in Figure 3. A summary of the redox potentials
for 2–5 and the parent all-CO complex 1 and its PPh3 deriv-
atives, [(µ-pdt)Fe2(CO)5PPh3] (6), is given in Table 3. It has
been demonstrated that complexes 2 and 3 display two
irreversible oxidation peaks, whereas 4 and 5 show one
irreversible oxidation peak. In publications, those reported
for ADT-, PDT-, and ODT-bridged (ODT = oxadithiolato)
analogs,[19–21,30,41,42] were assumed to be the oxidation
events of FeIFeI to FeIIFeI and FeIIFeI to FeIIFeII, respec-
tively. In all cases discussed in this paper, it is noticeable
that the 1st oxidation for complexes 2–5 exhibits a current
intensity ca. twice that of the corresponding 1st reduction
event, which was confirmed as a one-electron process by
bulk electrolysis (vide infra).
5[a]
6
[a] Complex 5 was somewhat soluble in CH3CN.
FeIIFeI and FeIIFeI to FeIIFeII. Further oxidation of these
complexes probably arises from the degradation of oxidized
species or the ligand redox process, which is beyond the
scope of this article.
Complexes 2, 3, 4, and 5 exhibit an electrochemically
irreversible reduction at –1.74 V, –1.92 V, –1.66 V, and
–1.70 V, respectively. Bulk electrolysis of complexes 2–5 at
each reduction potential shows a net consumption of ca.
0.95 electrons per molecule, demonstrating these reduction
events are one-electron reduction processes from FeIFeI to
Fe0FeI. However, for the electron counting of the 1st re-
duction process of the parent complex 1, whether it is a
one-electron process or a two-electron process, is disput-
able.[19–21,30,41–44] For most model complexes including
ADT, PDT, and ODT derivatives, the concerned reduction
event was considered to be a one-electron process.
In comparison to complex 1, the first reduction poten-
tials of complexes 2–5 are shifted to a relatively more nega-
tive value, consistent with the increase of electron density
at the diiron core upon replacement of CO by the better
donor ligands. Noticeably, the reduction potentials of 4 and
5 show a minor negative shift by only 40 mV and 80 mV,
respectively, as compared to complex 1. The minor shift
suggests that the PPyr3 ligand is a somewhat weaker π-ac-
ceptor in comparison to CO and therefore, slightly affects
the redox capability of these diiron complexes. On the other
hand, the reduction shifts are smaller than those observed
for complex 6 and other reported phosphane-substituted di-
iron complexes.[19,20,30,42] Within the series of 2, 4, and 6,
we can see that the reduction potentials gradually shift to
more positive potentials with displacement of the phenyl
ring by the pyrrol-1-yl group, consistent with the change
trends of the IR νCO bands.
In all cases, a linear dependence of the peak currents (ip)
on the square root of the scan rate (v1/2) in CH3CN solution
indicates the electron-transfer reactions are diffusion-con-
trolled.[45]
Electrocatalytic Proton Reduction to H2
Figure 3. Cyclic voltammograms of complexes 2, 3, 4, and 5 in
CH3CN solution (0.1  nBu4NPF6) at a potential scan rate of
100 mVs–1.
The behavior of electrocatalytic proton reduction to H2
by 2–5 has been investigated by cyclic voltammetry in the
Therefore, we assume the 1st oxidation of complexes 2– absence and presence of the weak acid HOAc. The cyclic
5 is either a two-electron process of FeIFeI to FeIIFeII or a voltammograms recorded in CH3CN solutions with dif-
combined result of two overlapping processes of FeIFeI to ferent acetic acid concentrations (0–20 m) are shown in
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[FeFe]-Hydrogenase Models: Electrocatalytic H2 Production
Figure 4. As can be seen, when 1 m of HOAc was added, CH3CN,[47] the overpotentials of complexes 2–5 are 0.28,
an obvious increase in the current intensity of the first re- 0.46, 0.2, and 0.24 V, respectively. Darensbourg and co-
duction peak at –1.74 V, –1.92 V, –1.66 V, and –1.70 V for workers reported H2 evolution by electrolysis catalyzed by
2–5, respectively, was observed. The height of the reduction complex [(µ-pdt)Fe2(CO)5PTA] at –1.94 V vs. Fc+/Fc with
peak in each CV shows a further increase with the sequen- ca. 0.48 V overpotential for H2/H+ (HOAc).[20,48] It is note-
tial increments of acid concentration. The current height worthy that complex 4 can electrocatalytically generate H2
change at their respective reduction peak displays a good in weak acid at –1.66 V with only ca. 0.2 V overpotential.
linear dependence on the concentrations of HOAc. In ad- This potential is quite similar to that of [(µ-S-2-
dition, the reduction potentials slightly move to more cath- RCONHC6H4)2Fe2(CO)6] (R = 4-FC6H4) reported by Sun
odic values with increasing acid concentration. All of these and co-workers.[49] Although hexacarbonyldiiron azadithi-
features are clearly indicative of a catalytic proton reduction olate complexes, [{(µ-SCH2)2N(2-C4H3O)}Fe2(CO)6] and
process.[19–21,30,41–43,46] The CVs of complexes 2–5 indicate [{(µ-SCH2)2N(4-BrC6H4)}Fe2(CO)6], can catalyze the H2
they are electrocatalytically active at the first reduction po- production at lower potentials (ca. –1.13 to –1.48 V vs. Fc+/
tentials for proton reduction from HOAc. The electrocata- Fc),[24,25] a strong acid (HClO4) is needed. A summary of
lytic properties of complexes 2–5 are very similar to that the overpotentials for 2–5 and diiron dithiolates is given in
of [(µ-pdt)Fe2(CO)5PTA] reported by Darensbourg and co- Table 4. To the best of our knowledge, in terms of reduction
workers,[20] which also shows the first reduction peak is cat- overpotential, complex 4 is the most energy-efficient diiron
alytically active in the presence of a weak acid (HOAc). electrocatalyst for the H2 production in the presence of a
To further confirm the evolution of hydrogen at the first weak acid (HOAc) based on ligand-substituted 2Fe2S bi-
reduction peak, bulk electrolyses in CH3CN solutions for omimics. Complex 4 with the best catalytic capability for
2–5 were performed in a gas-tight H-type cell as described proton reduction displays a catalytic potential very close to
in the Experimental Section. The electrolysis of complexes that of the noble metal Pt. In our previous work, we re-
2, 3, 4, and 5 was carried out at –1.80 V, –1.98 V, –1.72 V, ported (pyrrolidin-1-yl)phosphane monosubstituted com-
and –1.76 V, respectively, in the presence of HOAc (50 m). plex [(µ-pdt)Fe2(CO)5P(NC4H8)3] can also catalyze the H2
When 12 C of charge had passed through the cell, a sample production in the presence of HOAc at –1.98 V with a rela-
of gas was collected and analyzed by gas chromatography, tively high overpotential (ca. 0.52 V).[29] However, its peak
showing hydrogen is the sole gaseous product.
current is higher than that of 4. Thus, it seems that a low
By using the standard potential (–1.46 V vs. Fc+/Fc) re- overpotential is achieved at the cost of low rates of catalysis.
ported by Evans et al. for the HOAc reduction in Further experiments will be performed to confirm this in
Figure 4. Successive cyclic voltammograms of 1.0 m solution of 2 (a), 3 (b), 4 (c), 5 (d) with HOAc (0, 1, 5, 10, and 20 m) in CH3CN
(0.1  nBu4NPF6 as supporting electrolyte) at a potential scan rate of 100 mVs–1.
Eur. J. Inorg. Chem. 2010, 3942–3951
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3947

F. Huo, J. Hou, G. Chen, D. Guo, X. Peng
FULL PAPER
future work. This significant electrocatalytic feature of 4 as ation of FeIFeIIH and a second electroreduction event, hy-
well as of the analogues 2, 3, and 5 are interpreted as fol- drogen is evolved, and the starting material is reclaimed to
lows. On replacement of the CO ligand by the PPyr3 ligand fulfill the catalytic cycle. Our results also show that the one-
(or PPyr2Ph and PPyrPh2), the low potential at the first electron reductive level Fe0FeI is electrocatalytically active.
reduction is maintained due to the small donating ability of
the phosphane ligands relative to the CO ligand. The re-
duced species (FeIFe0 level) of 4 provides a strong base for
proton uptake, whereas that of 1 at its FeIFe0 level is not a
strong enough nucleophile to react with the proton, because
the six strongly π-accepting CO ligands decrease the nucleo-
philicity of the diiron core. In contrast, the PPyr3 ligand
(or PPyr2Ph and PPyrPh2) with a smaller π-acceptor ability
modulates the nucleophilicity of the diiron core of the re-
duced species (FeIFe0 level) of 4 at a functional level for H2
production.
Conclusions
In the search for synthetic competitive catalysts that
function with [Fe]-H2ase-like capability, a series of π-ac-
ceptor (pyrrol-1-yl)phosphane-substituted diiron complexes
2–5 were prepared as functional models for the Fe-only hy-
drogenase. This work explored the derivative chemistry of
the precursor [(µ-S2R)Fe2(CO)6] by replacement of CO with
the exceptional π-acceptor (pyrrol-1-yl)phosphane ligands
and developed applications of (pyrrol-1-yl)phosphanes in
bio-organometallic chemistry and catalysis.
In the presence of the weak acid HOAc complexes 2–5
can catalyze proton reduction to H2 at the first reduction
level FeIFe0. The most effective electrocatalyst is the PPyr3-
substituted complex 4. As compared to that of other σ-
donor phosphane-substituted diiron complexes (most of
them catalyze proton reduction at the Fe0Fe0 level with high
overpotentials), the introduction of (pyrrol-1-yl)phosphane
ligands maintains the reduction of 2–5 at relatively positive
potentials due to the ligand π-acceptor character and im-
proves the electrocatalytic ability. Since minor ligand π-
acidity modulation has a significant influence on the proton
reduction of these diiron models, the approach as exem-
plified by complexes 2–5 is worthy of further investigation
in future functional biomimetic designs of [FeFe]-H2ase. In
contrast to electron-rich diiron models supported by mul-
tiple donors, diiron models with an appropriate π-acidity
ligand have two main advantages: (a) enhanced stability
towards O2 and, therefore, easy operation; (b) in terms of
thermodynamics, small overpotentials for electrocatalytic
H2 production.
Table 4. Summary of overpotential for phosphane-substituted
2Fe2S complexes.
Complex
Epc [V]
Eoverpotential [V]
FeIFeI/Fe0FeI
1
2
3
4
5
6
–1.62
–1.74
–1.92
–1.66
–1.70
–1.84
–1.94
–1.66
0.16
0.28
0.46
0.20
0.24
0.38
0.48
0.20
[(µ-pdt)Fe2(CO)5PTA]
[(µ-S-2-RCONHC6H4)2Fe2(CO)6][a]
[a] R = 4-FC6H4.
On the basis of the electrochemical observations de-
scribed above and similar cases of phosphane-substituted
2Fe2S complexes previously reported,[19–21,30,41–43] an
ECCE (electrochemical/chemical/chemical/electrochemical)
mechanism for the electrocatalytic proton reduction process
by 2–5 could be proposed, as presented in Scheme 2. The
FeIFeI complex initially undergoes an electrochemical re-
duction to generate a one-electron reduced intermediate
FeIFe0, which is singly protonated to form a hydride species
FeIFeIIH in the presence of HOAc. After a further proton-
Electrocatalytic proton reduction to H2 at moderate
overpotentials in the presence of the weak acid HOAc was
achieved through replacement of CO by moderate π-ac-
ceptor (pyrrol-1-yl)phosphanes. Ligand PPyr3 may be uti-
lized as a surrogate for CO in the diiron systems. The pyr-
rol-1-yl groups are amenable to further modification so that
it should be possible to prepare water-soluble electrocata-
lysts. Further investigations are underway to improve the
proton affinity of the diiron complexes by a built-in proton
relay site and to develop more advanced and effective elec-
trocatalysts for proton reduction.
Experimental Section
Materials and Techniques: Unless noted otherwise, all reactions and
operations were carried out under nitrogen by using standard
Schlenk techniques. All solvents were dried and distilled prior to
use according to standard methods. Pyrrole and Et3N were distilled
from Na and stored under nitrogen. The following materials were
commercial chemicals and used without further purification: 1,3-
propanedithiol, PCl3, Ph2PCl, and [Fe(CO)5]. The starting materi-
als P(pyrrol-1-yl)3 (PPyr3), Ph2P(pyrrol-1-yl) (Ph2PPyr), and [(µ-
Scheme 2. Proposed ECCE mechanism for catalytic proton re-
duction to H2 by complexes 2–5.
3948
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 3942–3951

[FeFe]-Hydrogenase Models: Electrocatalytic H2 Production
pdt)Fe2(CO)6] were synthesized according to literature pro-
cedures.[31,50] [(µ-pdt)Fe2(CO)5PPh3] (6) was prepared according to
the literature[30] as an IR and electrochemistry reference complex.
Infrared spectra were recorded with a Nicolet FT-IR spectropho-
tometer as solutions with CaF2 plates. 1H, 13C and 31P NMR were
collected with a Varian INOVA 400 M NMR spectrometer. The 1H
and 13C spectra were normally referenced to TMS, and the 31P
spectra were referenced to 85% H3PO4. HR-MS data acquisition
was carried out with a GCT-MS instrument (Micromass, England).
Elemental analysis was performed with a PE 2400 II Elemental
Analyzer (Perkin–Elmer).
113.6, 29.5, 22.6 ppm. 31P NMR (400 MHz, CDCl3):
δ =
140.58 ppm. HR-MS (EI): calcd. for [M]+ 586.9124; found
586.9153.
Synthesis of [(µ-pdt)Fe2(CO)4L2] (5) (L = PPyr3): Complex 5 was
prepared according to a procedure similar to that described above
for 3. A mixture of 1 (2.4 g, 6.20 mmol) with P(pyrrol-1-yl)3 (3.0 g,
13.1 mmol) in toluene (120 mL) was refluxed for 72 h. TLC showed
the main product was the disubstituted complex 5 with a small
amount of a mixture with the starting material 1 and the monosub-
stituted complex 4. Following solvent evaporation, the resultant
dark red residue was purified by column chromatography on silica
gel. The starting material 1 was eluted with hexane, and the mono-
substituted complex 4 was obtained after eluting with CH2Cl2/hex-
ane (1:10, v/v). The disubstituted complex 5 was obtained with
CH2Cl2/hexane (1:4, v/v) as elute. Following further recrystalli-
zation from n-pentane/CH2Cl2 at –30 °C, an orange solid was ob-
tained. Yield: 2.5 g (51%). Crystals suitable for X-ray studies were
grown from a mixed CH2Cl2/hexane solution. C31H30Fe2N6O4P2S2
Synthesis of [(µ-pdt)Fe2(CO)5L] (2) (L = Ph2PPyr): PPh2(pyrrol-1-
yl) (0.83 g, 3.3 mmol) in toluene (30 mL) was added to a red solu-
tion of [(µ-pdt)Fe2(CO)6], 1 (1.27 g, 3.3 mmol) in toluene (50 mL)
through a syringe. The reaction mixture was refluxed until TLC
indicated there was no remaining carbonyl complex of the starting
material. The solvent was removed under vacuum, and the result-
ant dark red residue was purified by column chromatography on
silica gel eluting with CH2Cl2/hexane (1:5, v/v). A red solid was
obtained by recrystallization from n-pentane/CH2Cl2 at –30 °C.
Yield: 1.20 g (68%). Crystals suitable for X-ray studies were grown
from a mixed CH2Cl2/hexane solution. C24H20Fe2NO5PS2 (608.92):
(787.99): calcd. C 47.23, H 3.84, N 10.66; found C 47.41, H 3.71,
1
N 10.82. IR (in CH Cl ): ν = 2028, 1984, 1970 cm–1. H NMR
˜
2
2
CO
(400 MHz, CDCl3): δ = 0.90 (s, 2 H), 1.31 (s, 4 H), 6.40 (s, 12 H),
6.94 (s, 12 H) ppm. 13C NMR (400 MHz, CDCl3): δ = 211.0, 123.7,
113.6, 29.0, 21.2 ppm. 31P NMR (400 MHz, CDCl3):
δ =
calcd. C 47.32, H 3.31, N 2.30; found C 47.51, H 3.20, N 2.48. IR
138.60 ppm. HR-MS (EI): calcd. for [M]+ 787.9944; found
787.9967.
1
(in CH Cl ): ν = 2049, 1991, 1964 cm–1. H NMR (CDCl3): δ =
˜
2
2
CO
1.53 (m, 4 H), 1.84 (m, 2 H), 6.44 (s, 2 H), 7.19 (s, 2 H), 7.52 (m,
10 H) ppm. 13C NMR (CDCl3): δ = 212.6, 209.2, 137.4, 137.0,
131.6, 131.5, 131.1, 128.9, 128.8, 126.5, 112.6, 29.9, 22.1 ppm. 31P
NMR (CDCl3): δ = 116.25 ppm. HR-MS (EI): calcd. for [M]+
608.9247; found 608.9219.
X-ray Structure Determinations: The single-crystal X-ray data were
collected with a Siemens SMART CCD diffractometer. The data
were collected at 293 K by using graphite-monochromated Mo-Kα
radiation (λ = 0.71073 Å) with the ω-2θ scan mode. Data pro-
cessing was accomplished with the SAINT processing program.[51]
Intensity data were corrected for absorption with empirical meth-
Synthesis of [(µ-pdt)Fe2(CO)4L2] (3) (L = Ph2PPyr): PPh2(pyrrol-
1-yl) (1.40 g, 5.6 mmol) in toluene (50 mL) was added to a red solu-
tion of [(µ-pdt)Fe2(CO)6], 1 (1.07 g, 2.8 mmol) in toluene (50 mL)
through a syringe. The reaction mixture was refluxed until TLC
indicated there was no remaining carbonyl complex of the starting
material. The solvent was removed under vacuum, and the result-
ant dark red residue was purified by column chromatography on
silica gel eluting with CH2Cl2/hexane (1:2, v/v). A red solid was
obtained by recrystallization from n-pentane/CH2Cl2 at –30 °C.
Yield: 1.4 g (61%). Crystals suitable for X-ray studies were grown
2
ods. The structure was solved by direct methods and refined on Fo
against full-matrix least squares using the SHELXTL-97 program
package.[52] All of the non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were located by geometrical calculation, but
their positions and thermal parameters were fixed during the struc-
ture refinement. A summary of the crystallographic data and struc-
tural determinations is provided in Table 5. CCDC-617988 (for 2),
-617989 (3), -617990 (4), and -617991 (5) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Center via www.ccdc.cam.ac.uk/data_request/cif
from
a mixed CH2Cl2/hexane solution. C39H34Fe2N2O4P2S2
(832.01): calcd. C 56.27, H 4.12, N 3.37; found C 56.16, H 4.28,
N 3.56. IR (in CH Cl ): ν = 2008, 1964, 1947 cm–1. 1H NMR
˜
2
2
CO
Electrochemistry: Acetonitrile used for electrochemical measure-
ments was distilled from P2O5 and freshly distilled from CaH2 un-
der N2. Measurements were made with a BAS 100 B/W electro-
chemical workstation controlled by a PC running BAS 100W 2.0
software.[??] ((Ͻ=AUTHOR: Please add reference for software!))
The working electrode was glassy carbon (Bioanalytical Systems)
of diameter 3 mm, successively polished with 3 µm and 1 µm alu-
mina and sonicated in ion-free water for 15 min prior to use. The
(400 MHz, CDCl3): δ = 0.81 (br., 6 H), 6.35 (s, 4 H), 7.19 (s, 4 H),
7.42 (m, 20 H) ppm. 13C NMR (400 MHz, CDCl3): δ = 213.8,
138.0, 137.6, 131.5, 130.7, 128.7, 126.6, 112.2, 29.8, 19.6 ppm. 31P
NMR (400 MHz, CDCl3): δ = 113.17 ppm. HR-MS (EI): calcd. for
[M]+ 832.0134; found 832.0156.
Synthesis of [(µ-pdt)Fe2(CO)5L] (4) (L = PPyr3): Complex 4 was
prepared acording to a procedure similar to that described above
for 2. A mixture of 1 (1.0 g, 2.58 mmol) with P(pyrrol-1-yl)3 counter electrode was a platinum wire. The experimental reference
(0.591 g, 2.58 mmol) in toluene (80 mL) was refluxed for 72 h. Af-
ter solvent evaporation, the resultant dark red residue was purified
by column chromatography on silica gel. The monosubstituted
electrode was a non-aqueous Ag/Ag+ electrode (0.01  AgNO3/
0.1  nBu4NPF6 in CH3CN). The supporting electrolyte was 0.1 
nBu4NPF6 (Fluka, electrochemical grade). Ferrocene was used as
complex 4 was obtained after eluting with CH2Cl2/hexane (1:10, an internal reference. All potentials are reported relative to Fc+/Fc.
v/v). Following further recrystallization from n-pentane/CH2Cl2 at
–30 °C, a dark red solid was obtained. Yield: 0.95 g (63%). Crystals
suitable for X-ray studies were grown from a mixed CH2Cl2/hexane
solution. C20H18Fe2N3O5PS2 (586.91): calcd. C 40.91, H 3.09, N
During the electrocatalytic experiments under argon, increments of
acid were added by microsyringe. Bulk electrolysis experiments
were performed under argon with a BAS 100 B/W electrochemical
analyzer, and carried out on a glassy carbon rod (A = 3.14 cm2) in
a gas-tight H-type electrolysis cell containing ca. 18 mL of CH3CN
7.16; found C 41.02, H 3.15, N 7.05. IR (in CH Cl ): ν = 2056,
˜
CO
2
2
2003, 1990 cm–1. H NMR (400 MHz, CDCl3): δ = 1.64 (m, 4 H), solution. Gas chromatography was performed with a GC 920 in-
1
1.92 (m, 2 H), 6.40 (s, 6 H), 6.90 (s, 6 H) ppm. 13C NMR strument equipped with a thermal conductivity detector (TCD) un-
(400 MHz, CDCl3): δ = 210.5, 210.4, 208.4, 123.7, 123.6 113.7,
der isothermal conditions with argon as a carrier gas.
Eur. J. Inorg. Chem. 2010, 3942–3951
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3949

F. Huo, J. Hou, G. Chen, D. Guo, X. Peng
FULL PAPER
Table 5. X-ray crystallographic data for 2–5.
2
3
4
5
Empirical formula
Mr [gmol–1]
λ [Å]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C24H20Fe2NO5PS2
C39H34Fe2N2O4P2S2
832.44
0.71073
monoclinic
P2(1)
9.373(3)
16.816(5)
12.407(4)
90
98.333(5)
90
1934.9(10)
2
C20H18Fe2N3O5PS2
587.16
0.71073
C31H30Fe2N6O4P2S2
788.37
0.71073
tetragonal
P4(3)
19.042(3)
19.042(3)
9.6956(19)
90
609.20
0.71073
monoclinic
P21/c
9.2405(17)
17.439(3)
16.549(3)
90
102.025(3)
90
2608.3(8)
4
triclinic
¯
P1
9.2604(19)
9.646(2)
14.459(3)
91.520(3)
100.732(3)
106.683(3)
1211.2(4)
2
90
90
γ [°]
V [Å3]
3515.8(10)
4
Z
T [K]
293
1.551
1.369
1240
293
1.429
0.982
856
293
1.610
1.472
596
293
1.489
1.079
1616
ρcalcd. [gcm–3]
µ [mm–1]
F [000]
Total reflections
Reflections observed
Parameters
Goodness-of-fit on F2
6099
3296
316
0.947
0.0653
0.1240
0.860/–0.574
6542
3938
460
1.011
0.0800
0.1444
0.782/–0.326
5394
4545
298
1.092
0.0398
0.1127
0.569/–0.739
7226
5019
424
1.017
0.0426
0.0619
0.322/–0.251
[a]
R1 [I Ͼ 2σ (I)]
wR2[b] [I Ͼ 2σ (I)]
Max. peak/hole [eÅ–3]
[a] R1 = (Σ||Fo| – |Fc||)/(Σ|Fo|). [b] wR2 = [Σw(Fo – Fc ) /Σw(Fo2)2]1/2
.
2
2 2
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China (Project 20472012) and the China Postdoctoral Science
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Received: March 16, 2010
Published Online: July 9, 2010
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