Bio-inspired hydrogenase models: Mixed-valence triion complexes as proton reduction catalysts

Article abstract of DOI:10.1039/c1cc13249k

Mixed-valence triiron complexes Fe₃(CO)_7-x(PPh ₃)_x(μ-edt)₂ (x = 0-2) have been prepared and are shown to act as proton reduction catalysts. Catalysis takes place via an ECEC mechanism with a reduced overpotential of ca. 0.45 V for Fe ₃(CO)₇(μ-edt)₂ as compared to the corresponding diiron complex.

Full text of DOI:10.1039/c1cc13249k

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COMMUNICATION
Bio-inspired hydrogenase models: mixed-valence triion complexes as
proton reduction catalystsw
Shishir Ghosh,a Graeme Hogarth,*b Katherine B. Holt,*b Shariff E. Kabir,*a
Ahibur Rahamana and David G. Unwinb
Received 1st June 2011, Accepted 24th August 2011
DOI: 10.1039/c1cc13249k
Mixed-valence triiron complexes Fe3(CO)7ꢀx(PPh3)x(l-edt)2
(x = 0–2) have been prepared and are shown to act as proton
reduction catalysts. Catalysis takes place via an ECEC mechanism
with a reduced overpotential of ca. 0.45 V for Fe3(CO)7(l-edt)2 as
compared to the corresponding diiron complex.
light onto the high activity of A2ꢀ 20,21
. Thus, it is proposed
that upon addition of two electrons the central iron-iron bond
of A is cleaved which in turn leads to rotation of the iron-
tricarbonyl groups and formation of bridging carbonyls and
vacant coordination sites, the latter being able to bind protons
efficiently and thus leading to high electrocatalytic ability
(Scheme 1).20,21
Over the past decade there has been intense interest in the
chemistry of dithiolate-bridged diiron complexes1–5 since they
closely resemble the two-iron unit of the H-cluster active site
of iron-only hydrogenases.6–8 As a result of these studies
important advances have been made in our understanding of
how this enzyme site functions,9–12 however, there are still
many challenges remaining in the area. Most notably, while it
is relatively easy to prepare complexes which bear a close
structural resemblance to the H-cluster site, the preparation of
good functional models has not yet been achieved. Thus, while
virtually all diiron(I) dithiolate-bridged complexes are able to
act as catalysts for the reduction of protons to hydrogen under
reducing conditions they are generally characterized by high
overpotentials and poor turnover numbers and frequencies.
In 2005, Pickett reported the serendipitous isolation of the
tetrairon cluster, Fe4(CO)8{m3-(SCH2)3CMe}2 (A) (Scheme 1)
formed upon reaction of Bosnich’s thiol with Fe3(CO)12.13
This is a relatively rare example of a linear 66-electron cluster
being characterized by three metal-metal bonds14–19 and is
formally a mixed-valence complex comprising of a chain of
Fe(I)-Fe(II)-Fe(II)-Fe(I) centers.20 Importantly with respect to
functional modelling of the hydrogenase active site, while the
one-electron reduction product Aꢀ was shown to be only a
moderate catalyst for proton reduction, addition of a second
electron resulted in formation of A2ꢀ shown to be an excellent
electrocatalyst, dihydrogen elimination being some 500 times
greater than found in related Fe2(CO)6(m-dithiolate) complexes.13
Later detailed electrochemical and DFT studies shed some
These results prompted us to consider other mixed-valence
iron-thiolate clusters as potential electrocatalysts for proton
reduction. In an early paper on the synthesis of diiron
dithiolate complexes, Huttner reported that reactions of
HS(CH2)nSH (n = 2, 3) with Fe3(CO)12 afforded small
amounts of trinuclear materials, Fe3(CO)7{m-S(CH2)nS}2.22
No later reports detail these mixed-valence complexes and
consequently we were drawn to investigate them as possible
electrocatalysts for proton reduction. Herein we describe our
initial studies in this area which have focused on the ethane-
dithiolate (edt) complex Fe3(CO)7(m-edt)2 (1) and phosphine-
substituted derivatives (Scheme 2).
Reaction of Collman’s reagent, Na2[Fe(CO)4], with
1,2-ethanedithiol at room temperature in thf gave 1 as a red
solid (5%), the major product being Fe2(CO)6(m-edt) (27%).
Phosphine substitution has been extensively utilised in work on
diiron hydrogenase biomimetics, primarily in order to increase
the electron-density on the metal center and thus facilitate
proton binding,23–29 but also to give complexes in which the
two iron atoms are both electronically and sterically differ-
entiated.29–35 Heating a benzene solution of 1 and PPh3
resulted in formation of Fe3(CO)6(PPh3)(m-edt)2 (2) (23%)
and Fe3(CO)5(PPh3)2(m-edt)2 (3) (12%)z along with binuclear
Fe2(CO)5(PPh3)(m-edt) (22%),36 which results from cleavage
of one metal-metal bond, and Ph3P = S (30%).
Crystal structures of 1–3 have been carried out (Fig. 1).y All
contain an approximately linear triiron core, Fe(1)-Fe(2)-Fe(3)
bond angles [Fe–Fe–Fe 151.50(6)–151.83(7)1], and anti
a Department of Chemistry, Jahangirnager University, Savar,
Dhaka-1342, Bangladesh. E-mail: skabir_ju@yahoo.com
b Department of Chemistry, University College London,
20 Gordon Street, London, WC1H 0AJ, United Kingdom.
E-mail: g.hogarth@ucl.ac.uk, k.b.holt@ucl.ac.uk
w Electronic supplementary information (ESI) available: Experimental
details, characterising data, ORTEP figures, CVs and acid-dependent
CVs for 1–3. CCDC 827274–827276. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1cc13249k
Scheme 1 Proposed catalytically active dianion A2ꢀn formed upon
two electron reduction of Fe4(CO)8{m3-(SCH2)3CMe}2 (A).
c
11222 Chem. Commun., 2011, 47, 11222–11224
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studies and illustrated in Fig. 3 for the unsubstituted cluster 1.
In the absence of acid 1 undergoes a quasi-reversible one-
electron oxidation at 0.5 V and a reduction at ꢀ1.47 V,
followed by a further irreversible reduction at ꢀ2.04 V. The
first reduction is irreversible at scan rates below 5 V sꢀ1 and its
peak current is consistent with a one-electron process. For
comparison, oxidation of the corresponding di-iron species
[Fe2(CO)6(m-edt)] occurs at 0.85 V and reduction at ꢀ1.90 V
(see ESIw) under the same experimental conditions. Incorporation
of the third Fe centre has therefore not only shifted the
reduction potential ca. 0.45 V more positive but has also
significantly reduced the energy gap between the HOMO
and LUMO. Although substitution with electron-withdrawing
ligands could also achieve the former, a similar shift positive of
the oxidation potential would usually be anticipated. The
decreased HOMO–LUMO energy observed in this case seems
therefore to be a consequence of the additional metal centre
and the mixed valency of the cluster.
Scheme 2 Preparation of triiron complexes.
Fig. 1 Molecular structures of 1–3.
For complex 1 in the presence of HBF4ꢁEt2O (pKa = 3)
catalytic proton reduction takes place at the potential of the
first reduction with Ep being about ꢀ1.54 V and icat B40 mA at
10 equivalents of acid. Further electrocatalytic processes are
evident at more negative potentials, associated with the reduction
process at ꢀ2.1 V. Fe2(CO)6(m-edt) likewise catalyses proton
reduction at the potential of its first reduction (ꢀ1.92 V).
The tetrairon complex A undergoes a reversible one-electron
reduction at ꢀ1.22 V followed by a quasi-reversible one-
electron reduction at ꢀ1.58 V vs. Fc/Fc+, becoming an active
catalyst for proton reduction at this latter potential.20 Both 1
and A therefore operate, like the hydrogenase enzyme active
site, in formal Fe(I)/Fe(II) oxidation states, in contrast to most
diiron models like Fe2(CO)6(m-edt), which cycle between
Fe(0)/Fe(I) states.
arrangement of dithiolate ligands. Iron-iron distances [Fe-Fe
2.5385(8)–2.584(2) N A] are consistent with the Fe(I)–Fe(II)
bond length of 2.543(5) A found in A13 but shorter that the
Fe(II)–Fe(II) contact of 2.651(9) A in the same molecule. In
both 2 and 3 phosphine substitution occurs at the apical site(s)
of the external Fe(I) centres, that is approximately trans to the
metal-metal bond [Fe–Fe–P 150.27(9)–154.11(8)1]. An inter-
esting feature of all three is the bending of the carbonyl bound to
the Fe(II) center [Fe(2)–C–O 167.6(4)–168.2(8)1] being asso-
ciated with a semi-bridging carbonyl. The semi-bridging inter-
action in 1–3 can also be seen in their IR spectra, a relatively
weak low energy absorption being observed at 1904 cmꢀ1 in 1
and 1869 cmꢀ1 in 3, compared with that at 1861 cmꢀ1 seen in
[Fe2(CO)3(PMe3)(IMes)(m-CO)(m-pdt)][PF6].37
In light of the comparison of 1–3 with diiron mixed-valence
complexes it is useful to consider the structure of these clusters
as being comprised of binuclear species with a third iron(I)
‘‘ligand’’. This is illustrated for 1 in Fig. 2. Thus the
Fe(2)–Fe(3) sub-unit looks like a non-rotated binuclear complex
of the type, Fe2(CO)4(m-dithiolate)(k2-chelate), with eclipsed
FeL3 centres.30,31 In contrast the Fe(1)–Fe(2) sub-unit closely
resembles the rotated structure of mixed-valence Fe(I)–Fe(II)
complexes such as [Fe2(CO)3(PMe3)(IMes)(m-pdt)][PF6].37
Thus the two Fe(CO)3 fragments in 1 are staggered and the
adoption of a semi-bridging carbonyl leads to the generation
of a vacant coordination site. In mono-substituted 2 it is the
phosphine-bound unit that is rotated with respect to the
central Fe(II) center.
The catalytic mechanism for 1 was simulated using DigiSim
and an ECEC catalytic cycle established (see Scheme 3 and
ESIw). The catalytic peak currents for the experimental and
simulated CVs are plotted vs. equivalents of acid in the inset to
Fig. 3. The results for the simulation and experiment match well
for the parameters derived even for this simple mechanism.
Phosphine-substituted 2–3 show similar electrocatalytic
behaviour to 1 but the reduction potentials of both are shifted
All three clusters are electrocatalysts for the reduction of
protons to hydrogen as shown by cyclic voltammetry (CV)
Fig. 3 Cyclic voltammetry of 0.5 mM complex 1 in 0.1 M TBAPF6 in
DCM under Ar, GC electrode, scan rate 100 mV sꢀ1. Black = no
protons; brown = 1 equivalent HBF4; green = 2 equivalents HBF4;
light green = 10 equivalent HBF4. Inset compares catalytic peak
currents obtained experimentally with those from simulation.
Fig. 2 The two ‘‘binuclear’’ fragments in 1 highlighting the non-
rotated (left) and rotated (right) sub-units.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11222–11224 11223

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At convergence, R1 = 0.1027, wR2 = 0.2069 [I 4 2.0s(I)] and
R1 = 0. 2394, wR2 = 0.2551 (all data), for 581 parameters. CCDC
827275.
1 I. P. Georgakaki, L. M. Thomson, E. J. Lyon, M. B. Hall and
M. Y. Darensbourg, Coord. Chem. Rev., 2003, 238–239, 255.
2 D. J. Evans and C. J. Pickett, Chem. Soc. Rev., 2003, 32, 268.
3 T. B. Rauchfuss, Inorg. Chem., 2004, 43, 14.
4 L. Sun, B. Akermark and S. Ott, Coord. Chem. Rev., 2005,
249, 1653.
5 X. Liu, S. K. Ibrahim, C. Tard and C. J. Pickett, Coord. Chem.
Rev., 2005, 249, 1641.
6 Y. Nicolet, C. Piras, P. Legrand, C. E. Hatchikian and
J. C. Fontecilla-Camps, Structure, 1999, 7, 13.
Scheme 3 Mechanism for electrocatalytic proton reduction by 1 by
an ECEC mechanism. See ESIw for parameter selections.
7 J. W. Peters, W. N. Lanzilotta, B. Lemon and L. C. Seefeldt,
Science, 1998, 282, 1853.
to more negative potentials, ꢀ1.72 and ꢀ1.82 V respectively,
consistent with a higher degree of electron-density on the
triiron centre. Further, neither undergoes facile protonation
and thus similar ECEC mechanisms are proposed. The catalytic
currents for 2 are higher than for 1, which indicates that a
higher turnover frequency can be achieved by the addition of
PPh3 ligands; however this is at the expense of overpotential.
This suggests that the protonation steps may be rate limiting in
the catalytic cycle of 1.
8 B. J. Lemon and J. W. Peters, Biochemistry, 1999, 38, 12969.
9 M. Bruschi, C. Greco, M. Kaukonen, P. Fantucci, U. Ryde and
L. De Gioia, Angew. Chem., Int. Ed., 2009, 48, 3503.
10 C. Greco, P. Fantucci, L. De Gioia, R. Suarez-Bertoa, M. Bruschi,
J. Talarmin and P. Schollhammer, Dalton Trans., 2010, 39, 7320.
11 C. Greco, G. Zampella, L. Bertini, M. Bruschi, P. Fantucci and
L. De Gioia, Inorg. Chem., 2007, 46, 108.
12 P. Surawatanawong, J. W. Tye, M. Y. Darensbourg and
M. B. Hall, Dalton Trans., 2010, 39, 3093.
13 C. Tard, X. Liu, D. L. Hughes and C. J. Pickett, Chem. Commun.,
2005, 133.
14 N. D. Feasey, N. J. Forrow, G. Hogarth, S. A. R. Knox,
K. A. Macpherson, M. J. Morris and A. G. Orpen,
J. Organomet. Chem., 1984, 267, C41.
15 L. H. Polm, W. P. Mul, C. J. Elsevier, K. Vrieze, M. J.
N. Christophersen and C. H. Stam, Organometallics, 1988, 7, 423.
16 E. Delgado, Y. Chi, W. Wang, G. Hogarth, P. J. Low,
G. D. Enright, S.-M. Peng, G.-H. Lee and A. J. Carty, Organo-
metallics, 1998, 17, 2936.
17 M. Bergamo, T. Beringhelli, G. D’Alfonso, P. Mercandelli,
M. Moret and A. Sironi, Organometallics, 1997, 16, 4129.
18 Y.-F. Tzeng, C.-Y. Wu, W.-S. Hwang and C.-H. Hung,
J. Organomet. Chem., 2003, 687, 16.
19 W. P. Mul, C. J. Elsevier, M. Van Leijen, K. Vrieze, W. J. J. Smeets
and A. L. Spek, Organometallics, 1992, 11, 1877.
In conclusion we have reported for the first time the use of
bio-inspired mixed-valence triiron complexes as proton reduction
catalysts. Catalysis occurs at the first reduction potential
consistent with formation of an Fe(I)Fe(I)Fe(I) state and at a
relatively low over-potential when compared to related diiron
complexes. This is remarkably similar to the observation of
Pickett and co-workers who found that tetranuclear A with an
Fe(I)Fe(II)Fe(II)Fe(I) ground state is a facile catalyst in its
doubly reduced state.
This research has been part sponsored by the Ministry of
Science and Information & Communication Technology,
Government of the People’s Republic of Bangladesh. We
thank University College London for the provision of a
studentship (DU) and the EPSRC for an Advanced
Fellowship (KBH).
20 M. H. Cheah, C. Tard, S. J. Borg, X. Liu, S. K. Ibrahim,
C. J. Pickett and S. P. Best, J. Am. Chem. Soc., 2007, 129, 11085.
21 P. Surawatanawong and M. B. Hall, Inorg. Chem., 2010, 49, 5737.
22 A. Winter, L. Zsolnai and G. Huttner, Zeitschrift Naturforschung,
1982, 37B, 1430.
23 E. J. Lyon, I. P. Georgakaki, J. H. Reibenspies and
M. Y. Darensbourg, Angew. Chem., Int. Ed., 1999, 38, 3178.
24 E. J. Lyon, I. P. Georgakaki, J. H. Reibenspies and
M. Y. Darensbourg, J. Am. Chem. Soc., 2001, 123, 3268.
25 X. Zhao, I. P. Georgakaki, M. L. Miller, R. Mejia-Rodriguez,
C.-Y. Chiang and M. Y. Darensbourg, Inorg. Chem., 2002, 41, 3917.
Notes and references
z Selected spectroscopic data: 1 IR n(CO) (CH2Cl2): 2073s, 2040vs,
2008s, 1974s, 1904w cmꢀ1 1H NMR (CDCl3): d 2.53 (m, 4H),
;
2.26 (m, 4H); 2 IR n(CO) (CH2Cl2): 2035s, 2012vs, 1963vs, 1882w cmꢀ1
;
31P{1H} NMR (CDCl3): d 65.0 (s, 65%), 52.7 (s, 35%); 3 IR n(CO)
(CH2Cl2): 2008s, 1965vs, 1965vs, 1953sh, 1912m, 1869w cmꢀ1
;
26 F. Gloaguen, J. D. Lawrence, T. B. Rauchfuss, M. Be
M.-M. Rohmer, Inorg. Chem., 2002, 41, 6573.
´
nard and
31P{1H} NMR (CDCl3): d 63.8 (s, 25%), 61.5 (s, 75%), 53.1 (s).
y Crystal data: [Fe3(CO)7(m-edt)2] (1): red block, dimensions 0.14 ꢂ
%
0.0311 mm, triclinic, space group P1, a = 6.423(1),
27 J.-F. Capon, S. El Hassnaoui, F. Gloaguen, P. Schollhammer and
J. Talarmin, Organometallics, 2005, 24, 2020.
28 P. Li, M. Wang, C. He, G. Li, X. Liu, C. Chen, B. Akermark and
L. Sun, Eur. J. Inorg. Chem., 2005, 2506.
29 F. I. Adam, G. Hogarth and I. Richards, J. Organomet. Chem.,
2007, 692, 3957.
30 S. Ezzaher, J.-F. Capon, F. Gloaguen, F. Y. Petillon,
P. Schollhammer and J. Talarmin, Inorg. Chem., 2007, 46, 9863.
31 F. I. Adam, G. Hogarth, I. Richards and B. E. Sanchez, Dalton
Trans., 2007, 2485.
0.14
ꢂ
b = 8.291(2), c = 18.331(4) A, a = 77.541(3), b = 83.384(3), g =
68.854(3)1, V = 888.2(3) A3, Z = 2, F(000) 544, dc = 2.049 g cmꢀ3
,
m = 2.918 mmꢀ1. 7497 reflections were collected, 3970 unique
[R(int) = 0.0333] of which 3453 were observed [I 4 2.0s(I)]. At
convergence, R1 = 0.0417, wR2 = 0.1248 [I 4 2.0s(I)] and R1 = 0.
0480, wR2 = 0.1411 (all data), for 226 parameters. CCDC 827274.
[Fe3(CO)6(PPh3)(m-edt)2] (2): orange plate, dimensions 0.26 ꢂ 0.10 ꢂ
%
0.03 mm, triclinic, space group P1, a = 8.799(5), b = 11.268(6),
c = 16.543(9) A, a = 90.591(7), b = 90.499(1), g = 109.469(8)1, V =
32 S. Ezzaher, J.-F. Capon, F. Gloaguen, F. Y. Petillon,
P. Schollhammer, J. Talarmin, R. Pichon and N. Kervarec, Inorg.
Chem., 2007, 46, 3426.
33 S. Lounissi, J.-F. Capon, F. Gloaguen, F. Matoussi, F. Y. Petillon,
P. Schollhammer and J. Talarmin, Chem. Commun., 2011, 47, 878.
34 F. I. Adam, G. Hogarth, S. E. Kabir and I. Richards, C. R. Chim.,
2008, 11, 890.
35 G. Hogarth and I. Richards, Inorg. Chem. Commun., 2007, 10, 66.
36 P. C. Ellgen and J. N. Gerlach, Inorg. Chem., 1973, 12, 2526.
37 T. Liu and M. Y. Darensbourg, J. Am. Chem. Soc., 2007,
129, 7008.
1546.2(1) A3, Z = 2, F(000) 792, dc = 1.680 g cmꢀ3, m = 1.751 mmꢀ1
.
12859 reflections were collected, 6918 unique [R(int) = 0.0526] of
which 5013 were observed [I 4 2.0s(I)]. At convergence, R1 = 0.0943,
wR2 = 0.2600 [I 4 2.0s(I)] and R1 = 0.1181, wR2 = 0.2787 (all data),
for 379 parameters. CCDC 827276. [Fe3(CO)5(PPh3)2(m-edt)2].C5H12
ꢁ
MeOH (3): red plate, dimensions 0.22 ꢂ 0.10 ꢂ 0.05 mm, orthorhombic,
space group C2/c, a = 25.52(2), b = 13.484(9), c = 30.21(2) A, b =
83.813(3)1, V = 10360(1) A3, Z = 8, F(000) 4432, dc = 1.391 g cmꢀ3
,
m = 1.096 mmꢀ1. 41674 reflections were collected, 12131 unique
[R(int) = 0.2646] of which 5012 were observed [I 4 2.0s(I)].
c
11224 Chem. Commun., 2011, 47, 11222–11224
This journal is The Royal Society of Chemistry 2011