Pentacoordinate iron complexes as functional models of the distal iron in [FeFe] hydrogenases

Article abstract of DOI:10.1039/c1cc14449a

Mononuclear pentacoordinate iron complexes with a free coordination site were prepared as mimics of the distal Fe (Fe_d) in the active site of [FeFe] hydrogenases. The complexes catalyze the electrochemical reduction of protons at mild overpotential.

Full text of DOI:10.1039/c1cc14449a

View Article Online / Journal Homepage / Table of Contents for this issue
This article is part of the
Artificial photosynthesis
web themed issue
Guest editor: Andrew Benniston
All articles in this issue will be gathered together
online at
www.rsc.org/photosynthesis

Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 11662–11664
www.rsc.org/chemcomm
COMMUNICATION
Pentacoordinate iron complexes as functional models of the distal iron in
[FeFe] hydrogenaseswz
Maryline Beyler, Salah Ezzaher, Michael Karnahl, Marie-Pierre Santoni, Reiner Lomoth*
and Sascha Ott*
Received 21st July 2011, Accepted 12th September 2011
DOI: 10.1039/c1cc14449a
Mononuclear pentacoordinate iron complexes with a free
coordination site were prepared as mimics of the distal Fe (Fed)
in the active site of [FeFe] hydrogenases. The complexes catalyze
the electrochemical reduction of protons at mild overpotential.
The development of renewable energy is nowadays one of the
most important challenges as global energy consumption is
rising significantly.1 Molecular hydrogen is a carbon-free fuel
that could become the energy carrier of the future, thus
making the reversible inter-conversion of protons to molecular
Chart 1 (a) The active site of the [FeFe] H2ase; (b) hexacoordinate
model complex 1 and (c) pentacoordinate Fe(II) complexes 2–5.
hydrogen a key process for future energy schemes. Nature is
able to catalyze this process by a class of enzymes called
hydrogenases (H2ases), one of which being the [FeFe] H2ases.
The X-ray crystallographic structure determination of the
enzymes2 has revealed their active site (Chart 1a) and allowed
the mechanistic understanding of the catalytic process.3
Mononuclear complexes that mimic Fed naturally avoid the
less reactive m-H structures, and we have recently reported
catalytic H2 formation with the mononuclear Fe(II) complex 1.11
Although being also coordinatively saturated, Fe-hydride for-
mation is enabled after reductive deligation of one sulfide of the
bidendate benzene-1,2-dithiolate (bdt) ligand. This rearrange-
ment from a hexacoordinate to a pentacoordinate Fe species
compromises however the stability of the reduced catalyst and
thereby limits total turnover numbers.
Inspired by these insights, many Fe2 model complexes of the
[FeFe] H2ase active site have been synthesized.4 However, the
compounds often exhibit large overpotentials and low turnover
rates for electrocatalytic proton reduction,5 in part also because
of their lack to mimic functionally important features of the
active site. For example, the enzymatic process occurs when both
iron centres are in the Fe(I) oxidation state,6 in contrast to many
catalytic Fe2 mimics that involve a formal Fe(0) state. Second,
nature employs terminal hydride intermediates that are formed at
a free coordination site on the distal iron centre (Fed).7 Synthetic
Fe2 model complexes are coordinatively saturated and often lack
a free site for substrate binding. Also, the oxidative addition of
protons is often slow in the model compounds,8 and yields less
reactive bridging hydrides.9 Reductive ligand elimination (CO,
NHR2) can create open coordination sites on the dinuclear
model complexes but the limited stability of the intermediates
contributes to catalyst degradation.10
Herein, we report on stable pentacoordinate Fe(II) complexes
2–5 which provide an open site for substrate binding without the
need for ligand dissociation (Chart 1).12,13 The complexes
catalyze H2 formation from weak acids at low overpotentials
and feature greatly improved stability under turnover conditions.
Pentacoordinate complexes 2–5 are accessible through the
use of bidendate bis-phosphane ligands that reside trans to the bdt
ligand, in contrast to hexa-coordinated 1 with monodendate
phosphanes in a cis relationship to the bdt. Compounds 2–5
differ in their second coordination sphere of the bis-phosphane
and/or chloride substituents at the bdt. Complexes 3–5 contain
an amine functionality in the bis-phosphane as a potential
protonation site. Additional ether chains at the amine provide
increased solubility in polar aprotic solvents and render 5 most
suitable for electrocatalysis investigations.
Department of Photochemistry and Molecular Science,
˚
Angstrom Laboratories, Uppsala University, Box 523, 75120 Uppsala,
¨
Sweden. E-mail: sascha.ott@fotomol.uu.se,
The N-containing bis-phosphane ligands were obtained by
reaction of two equivalents of Ph2PCH2OH (readily preformed
from equimolar amounts of (CH2O)n and Ph2PH) with the
appropriate primary amine in refluxing MeOH.14 With the bis-
phosphanes in hand, complexes 2–5 can be obtained by two
different routes: starting from dinuclear [Fe2(CO)6(Cl2bdt)]15 (6)
reiner.lomoth@fotomol.uu.se; Fax: +46-18-471-6844
w Electronic supplementary information (ESI) available: Synthesis,
crystal data of 5, electrochemistry data and details of overpotential
calculation. CCDC 811891. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c1cc14449a
z This article is part of the ChemComm ‘Artificial photosynthesis’ web
themed issue
c
11662 Chem. Commun., 2011, 47, 11662–11664
This journal is The Royal Society of Chemistry 2011

Scheme 1 (a) Formation of 3 from a dinuclear Fe precursor; (b)
synthesis of complexes 2–5 from FeCl2.
in the presence of decarbonylation agent Me3NO (Scheme 1a),
addition of the bis-phosphane led to a disproportionation and
the formation of mononuclear complexes in a reaction similar to
that described of 6 with CNꢀ or PMe3.12,16 Following this route,
complex 3 was isolated in 36% yield. Complexes 2–5 can however
also be obtained directly from FeCl2, Cl2bdt or bdt and the
appropriate bis-phosphane in MeOH under a CO atmosphere
(Scheme 1b). Using this more direct approach, the yield of 3
could for example be increased to 63%. It is noteworthy that the
reaction is unique for a chelating dithiolate as the same reaction
of bis-phosphanes with two equivalents of thiolate (RSꢀ) affords
hexacoordinate cis,cis,cis-[Fe(RS)2(PPh2(CH2)3PPh2)(CO)2].17
The molecular structure of 5 could be resolved by X-ray
crystallography (Fig. 1) which confirms that the Fe centre is
pentacoordinated.w It has a square pyramidal geometry with
the apical carbonyl ligand trans to the free coordination site.
The IR spectra of all complexes show a single nCO band in
the same spectral range (1917, 1919, 1921 and 1912 cmꢀ1 for
2–5 in CH2Cl2).w The difference between 2–4 and 5 can be
attributed to the effect of the electron-withdrawing chloride
substituents while the different phosphanes have a similar
electronic impact on the Fe centre.
Fig. 2 IR spectra in CH3CN solution of 5 (2 mM) before (—) and
after addition of 40 mM TsOH (---). Inset: Absorbance at 1912 cmꢀ1
(K) and 1932 cmꢀ1 (J) as a function of acid concentration.
band was observed and a pKa of about 7.3 could be estimated for
5H+ from titration experiments in CH3CN (Fig. 2).
The 1H NMR spectrum of 5H+ shows no signal in the
hydride region but a new signal at 9.44 ppm. The signal in the
31P NMR is only slightly downfield shifted by Dd = 5.1 ppm.w
These data are consistent with protonation on the nitrogen of
the bis-phosphane ligand, similar to Fed in the oxidized form of
the [FeFe] H2ase active site which also carries a proton in the
second coordination sphere and not metal-bound as a hydride.6
The redox behaviour of 5 and its catalytic activity were
studied by cyclic voltammetry and controlled potential electrolysis
in CH3CN solution. In the absence of acid, voltammograms of
5 show a reversible one-electron reduction wave at E1/2
=
ꢀ1.66 V vs. Fc+/0 that can be attributed to the formal FeII/I
couple. In contrast, hexacoordinate complex 1 with two strongly
donating PMe3 ligands undergoes irreversible reductions with
Protonation studies were performed on 2–5. When one
equivalent of triflic acid was added to a solution of 2 or 4 in
CH2Cl2, no changes in the IR frequencies of the CO band were
observed. The lack of reactivity is explained by the absence of
an amine in 2, while the amine in 4 seems to be insufficiently
basic for N-protonation due to the electron withdrawing
CH2CF3 substituent.18 In contrast, exposing 3 or 5 to equivalent
conditions led to significant shifts of the respective nCO bands.
Upon protonation of 5 with stoichiometric amounts of triflic acid
(pKa = 2.6 in acetonitrile) or excess of p-toluenesulfonic acid
(TsOH, pKa = 8.3 in acetonitrile) a shift of 20 cmꢀ1 of the CO
Fig. 3 Electrochemistry of 5 in CH3CN solution with [(C4H9)4N][PF6]
(0.l0 M) as a supporting electrolyte. (a) Cyclic voltammetry (v =
0.100 V sꢀ1) of 5 (0.25 mM) with 0, 0.05, 0.10, 0.20, 0.30, 0.50 M AcOH
and background current on the glassy carbon electrode for 0.50 M AcOH
without catalyst (---). Inset: Peak/plateau current as a function of [AcOH]
with 0.25 mM 5 (K), 0.50 mM 5 (m), and 1.0 mM 5 (’). (b) Cyclic
voltammetry (v = 0.100 V sꢀ1) of 5 (0.25 mM) with 0, 1, 2, 3, 5, 10 mM
TsOH and background current on the glassy carbon electrode for 10 mM
TsOH without catalyst (---). Inset: Peak/plateau current as a function of
[TsOH] with 0.25 mM 5 (K), 0.50 mM 5 (m), and 1.0 mM 5 (’) at ꢀ1.4 V
(ꢀ1.69 V for [TsOH] = 0) and at ꢀ1.53 to ꢀ1.63 V (open symbols).
Fig. 1 ORTEP view with thermal ellipsoids at 50% probability level
of complex 5. The hydrogen atoms are omitted for clarity.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11662–11664 11663

cathodic peak potentials at Epc = ꢀ2.21 V and Epc
=
contrast to the first mononuclear models of Fed in the [FeFe]
H2ases active site, the complexes are characterized by an open
coordination site that allows for substrate binding without
potentially destabilizing ligand dissociation. Catalytic H2 for-
mation from weak acids like AcOH at low overpotential could
be demonstrated with promising results with regard to cata-
lytic rate constants and catalyst stability. The reaction with
AcOH occurs via initial metal-based reduction, and with
stronger acids via initial ligand protonation. Both mechanisms
thus mimic the enzymatic reaction in the sense that hydride
formation at Fed also only occurs after preceding reduction
to the formal Fed(I)–Fep(I) oxidation level.6 Spectroscopic
characterization of catalytic intermediates and computational
investigations are currently in progress.
ꢀ2.45 V.11 The structural dissimilarities between 1 and 5 hence
result in a greatly improved stability of the one-electron
reduction product of the latter complex.
Addition of increasing amounts of acetic acid to
5
(pKA = 22.3) renders the reduction wave
(AcOH, CH3CN)
irreversible and results in increasing anodic currents at the
potential of the 5/5ꢀ wave. This behaviour is indicative for
electrocatalytic reduction of protons from acetic acid as direct
proton reduction at the glassy carbon electrode is negligible
at this potential (Fig. 3a). The catalytic current reaches its
half-maximum value at a potential of ꢀ1.65 V, i.e. only
0.17–0.24 V more negative than the half-wave potential
obtained for the reduction of acetic acid (0.1–0.5 M in
acetonitrile) on a Pt electrode.w The catalytic current increases
linearly with catalyst concentration over the investigated range
of catalyst (0.25–1.0 mM) and acetic acid (0.05–0.50 M) con-
centrations (inset of Fig. 3a). At the highest acid concentrations,
plots of catalytic plateau currents are approximately linear
with [AcOH]1/2 and a bimolecular catalytic rate constant of
1 ꢁ 103 Mꢀ1 sꢀ1 can be estimated from the slope, giving rise to
a turnover frequency of 500 sꢀ1 at [AcOH] of 0.5 M.19 w
Formation of H2 during controlled potential electrolysis
was evidenced by gas chromatography,w and, after 30 turn-
overs, IR analysis of the reaction mixture did not indicate any
degradation of 5. The mechanism of proton reduction from
acetic acid starts with one-electron reduction to 5ꢀ which
presumably renders the metal sufficiently basic to add a proton
as a hydride ligand to the open coordination site. Attack of a
proton on a reduced hydride species could then result in
formation and release of H2.
Financial support was provided by the Swedish Research
Council, the Swedish Energy Agency, the Knut & Alice
Wallenberg Foundation, the Wenner-Gren Foundation, and
EU (FP7 Energy 212508 ‘‘SOLAR-H2’’). Dr Stefanie Tschierlei
and Dr Matthias Stein (MPI Magdeburg) are acknowledged for
valuable discussions.
Notes and references
1 N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A.,
2006, 103, 15729; N. Armaroli and V. Balzani, Angew. Chem., Int.
Ed., 2007, 119, 52.
2 J. W. Peters, W. N. Lanzilotta, B. J. Lemon and L. C. Seefeldt,
Science, 1998, 282, 1853; Y. Nicolet, C. Piras, P. Legrand,
E. C. Hatchikian and J. C. Fontecilla-Camps, Structure, 1999, 7, 13.
3 J. C. Fontecilla-Camps, P. Amara, C. Cavazza, Y. Nicolet and
A. Volbeda, Nature, 2009, 460, 814.
4 C. Tard and C. J. Pickett, Chem. Rev., 2009, 109, 2245.
5 G. A. N. Felton, C. A. Mebi, B. J. Petro, A. K. Vannucci,
D. H. Evans, R. S. Glass and D. L. Lichtenberger, J. Organomet.
Chem., 2009, 694, 2681.
6 P. E. M. Siegbahn, J. W. Tye and M. B. Hall, Chem. Rev., 2007,
107, 4414; 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.
7 F. A. Armstrong, Curr. Opin. Chem. Biol., 2004, 8, 133.
8 G. Eilers, L. Schwartz, M. Stein, G. Zampella, L. de Gioia, S. Ott
and R. Lomoth, Chem.–Eur. J., 2007, 13, 7075.
With stronger acids like TsOH, 5 can be protonated on the
ligand and reduction of 5H+ is observed as an irreversible
wave at Epc = ꢀ1.32 V. With increasing excess of TsOH two
catalytic waves emerge at ꢀ1.34 to ꢀ1.37 V and at ꢀ1.53 to
ꢀ1.63 V, respectively (1–10 mM TsOH) (Fig. 3b).
However, due to the strong acid required for formation of
5H+, the half wave potential of the first wave is already
0.65–0.79 V more negative than the corresponding potentials
on a Pt electrode.w It can be anticipated that reduction of 5H+
is also metal-centred and triggers the formation of a hydride
intermediate that forms H2 by reaction with acid. The second
catalytic peak is tentatively attributed to a mechanism that is
initiated by one-electron reduction of 5 to 5ꢀ. For both peaks,
the catalytic currents are largely independent of catalyst
concentration (0.25–1.0 mM) and depend linearly on [TsOH],
at higher concentrations also for the first peak that relies
on the protonation of 5. These features are indicative of a
‘‘total catalysis’’ situation where the acid substrate is rapidly
consumed in the vicinity of the electrode and the current is
limited by substrate diffusion from the bulk rather than
catalytic turnover frequency.19 From the anodic shift of the
second catalytic peak relative to E1/2 of the 5/5ꢀ couple a
catalytic rate constant on the order of 106 Mꢀ1 sꢀ1 can be
estimated for the reduction of TsOH with pseudo first order
rate constants of 104 sꢀ1 at [TsOH] = 0.01 M.
9 F. Gloaguen and T. B. Rauchfuss, Chem. Soc. Rev., 2009, 38, 100;
B. E. Barton and T. B. Rauchfuss, Inorg. Chem., 2008, 47, 2261.
10 F. Xu, C. Tard, X. Wang, S. K. Ibrahim, D. L. Hughes, W. Zhong,
X. Zeng, Q. Luo, X. Liu and C. J. Pickett, Chem. Commun., 2008,
¨
606; L. Schwartz, J. Ekstrom, R. Lomoth and S. Ott, Chem.
Commun., 2006, 4206.
11 S. Kaur-Ghumaan, L. Schwartz, R. Lomoth, M. Stein and S. Ott,
Angew. Chem., Int. Ed., 2010, 49, 8033.
12 T. B. Rauchfuss, S. M. Contakes, S. C. N. Hsu, M. A. Reynolds
and S. R. Wilson, J. Am. Chem. Soc., 2001, 123, 6933.
13 Preliminary DFT calculations show that the HOMO of 5 is an anti-
bonding linear combination of Fe dxz and the two S-based pz
orbitals. The bdt ligand is thus non-innocent and the Fe oxidation
state presumably less than expressed by the +II formalism. See
also: T. Liu, B. Li, C. V. Popescu, A. Bilko, L. M. Perez,
´
M. B. Hall and M. Y. Darensbourg, Chem.–Eur. J., 2010, 16, 3083.
14 S. E. Durran, M. R. J. Elsegood, N. Hawkins, M. B. Smith and
S. Talib, Tetrahedron Lett., 2003, 44, 5255.
15 L. Schwartz, P. S. Singh, L. Eriksson, R. Lomoth and S. Ott, C. R.
Chim., 2008, 11, 875.
16 D. Streich, M. Karnahl, Y. Astuti, C. W. Cady, L. Hammarstrom,
¨
R. Lomoth and S. Ott, Eur. J. Inorg. Chem., 2011, 1106.
´ ´ ´ ´
17 J. Takacs, L. Marko and L. Parkanyi, J. Organomet. Chem., 1989,
361, 109.
18 S. Ezzaher, A. Gogoll, C. Bruhn and S. Ott, Chem. Commun., 2010,
46(31), 5775.
19 J.-M. Saveant, Elements of molecular and biomolecular electro-
´
In summary, we have shown that chelating bis-phosphane
ligands favour the formation of pentacoordinate ferrous com-
plexes with a bidentate bdt ligand and a single CO ligand. In
chemistry: an electrochemical approach to electron transfer chemistry,
John Wiley & Sons, Hoboken, NJ, 2006.
c
11664 Chem. Commun., 2011, 47, 11662–11664
This journal is The Royal Society of Chemistry 2011