An oxidized active site model for the FeFe hydrogenase: Reduction with hydrogen gas

Article abstract of DOI:10.1021/ic2009573

Models for the oxidized form of the FeFe hydrogenase active site have been prepared. These cationic complexes contain two iron atoms, carbonyl ligands, a propanedithiolate bridge, and one other bridging group. Reduction of these complexes with hydrogen ga

Full text of DOI:10.1021/ic2009573

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pubs.acs.org/IC
An Oxidized Active Site Model for the FeFe Hydrogenase: Reduction
with Hydrogen Gas
Steven L. Matthews and D. Michael Heinekey*
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
S Supporting Information
b
Models for the oxidized form of the active site have been less
ABSTRACT: Models for the oxidized form of the FeFe
hydrogenase active site have been prepared. These cationic
complexes contain two iron atoms, carbonyl ligands, a
propanedithiolate bridge, and one other bridging group.
Reduction of these complexes with hydrogen gas is
demonstrated.
studied. In contrast to facile and reversible reduction, CV shows
that 1 is oxidized only with difficulty and irreversibly.12 Advances
in oxidation chemistry have been demonstrated in a phosphine-
containing system that reacts with hydrogen and silanes
photochemically.13 In di-iron models with bridging hydride
ligands, photochemical reactions with H2 have been assayed by
scrambling of D2 to form HD.14 The chemical oxidation with
halogens of complexes similar to 1 leads to FeÀFe bond cleavage,
but the products were not structurally characterized.15 Diferrous
complexes resembling 1 have been prepared with cyanide and
isonitrile ligands, but neither are reported to react with
hydrogen.16,17 Some Fe(I)ÀFe(II) mixed oxidation species have
also been reported, including one example which reacts with
hydrogen.18 Here, we report our studies of the chemical oxida-
tion of 1, which afford well-characterized cationic diferrous
species which can be reduced with hydrogen gas.
ydrogenase enzymes catalyze the oxidation of dihydrogen
H
and the reduction of protons in nature. X-ray crystallogra-
phy and IR spectroscopy of the FeFe hydrogenases have shown
the active site to be comprised of a [2Fe2S] subunit linked to a
[4Fe4S] cluster by a cystenyl-S bridge.1,2 The two iron atoms in
the [2Fe2S] subunit are linked by a bridging dithiolate ligand and
are ligated by the biologically uncommon ligands carbon mon-
oxide (CO) and cyanide (CNÀ).3 Other features of the enzyme
include channels for proton and gas transport and a chain of
[4Fe4S] clusters for electron distribution.
We find that the oxidation of 1 is cleanly achieved with
[N(Ar2,4-Br)3][SbCl6] in CH2Cl2. Anion metathesis with Li
[B(C6F5)4] in CH2Cl2 yields [Fe2(μ-Cl)(μ-S2C3H6)(CO)6]
[B(C6F5)4] (2) as orange crystals in 78% yield (Figure 1).19
The infrared spectrum (KBr) of 2 in the carbonyl region
indicates an oxidized species, with five bands at 2142, 2125, 2094,
The crystal structure of Fe2(μ-S2C3H6)(CO)6 (1) has been
reported; the structural resemblance to the active site of the FeFe
hydrogenase was noted by Darensbourg and co-workers.4 A
number of active site model complexes have been reported,
including a very interesting Fe2S3 model system which mimics
the active site with a pendant thioether moiety.5 In related work,
electrocatalytic H2 oxidation has been demonstrated at a graphite
electrode impregnated with a NiFe hydrogenase.6 The efficiency
of the electrocatalysis was comparable to Pt electrodes. A
photocathode functionalized with an H-cluster analogue has also
been demonstrated.7,8
A number of reduced synthetic models of the FeFe hydro-
genase are reported to react with acids to form dihydrogen.9
Several reports have described the reduction chemistry of 1. For
example, Darensbourg et al. reported that the reduction of 1 led
to hydrogen evolution in the presence of acetic acid.10 Pickett
and co-workers reported hydrogen evolution from protic media
catalyzed by complex 1 under reducing conditions.11
2059, and 2046 cmÀ1
.
1
Complex 2 is diamagnetic; the H NMR spectrum exhibits
resonances for the propane dithiolate bridge at δ 2.46 (4H, t) and
1.90 (2H, quint). An X-ray diffraction study of 2 revealed an
FeÀFe distance of 3.005 Å, consistent with oxidation to a
diferrous state and a loss of the FeÀFe bond (Figure 1). The
structure resembles diferrous dimers such as [Fe2(μ-SMe)3-
(CO)6]+ (FeÀFe distance 3.06 Å) and [Fe2(μ-SMe)3(CO)4-
(PPhMe2)2]+ (FeÀFe 3.07 Å)20 but is longer than that observed
in Fe2(μ-S2C2H4)(μ-CO)(PR3)2(CN)2(CO)2(FeÀFe 2.55 Å)
and related dicationic species (FeÀFe distances 2.49À2.63 Å).14,15
The FeÀFe distance in 1 is 2.510 Å.4
Complex 2 does not react with Ag+ or Na+ salts at room
temperature. Surprisingly, complex 2 fails to react with the
Received: May 6, 2011
Published: July 27, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
COMMUNICATION
Figure 1. ORTEP diagram of 2. Thermal ellipsoids are drawn at 50%
probability. Hydrogen atoms and B(C6F5)4 counterion omitted for clarity.
powerful chloride abstracting reagent [Et3Si][B(C6F5)4]. Re-
duction with the hydride donor LiHBEt3 rapidly forms complex
1 with evolution of the hydrogen gas. Over the course of several
hours at room temperature, the reaction of HSiEt3 with complex
2 affords complex 1, accompanied by hydrogen evolution.
Formation of ClSiEt3 was confirmed by 29Si NMR spectroscopy.
The reduction of complex 2 can be achieved with hydrogen.
The heating of 2 in fluorobenzene to 70 °C under 1 atm of
hydrogen gas results in reduction to 1 as well as the formation
of C6F5H and small amounts (ca. 5%) of the previously repor-
ted cationic bridging hydride [Fe2(μ-H)(μ-S2C3H6)(CO)6]
[B(C6F5)4].21
Figure 2. ORTEP diagram of 3. Thermal ellipsoids are drawn at
50% probability. Hydrogen atoms and the B(C6F5)4 counterion
omitted for clarity. A schematic diagram is included to elucidate the
structure.
an asymmetric bridge between the iron atoms. The FeÀC distances
observed are 2.188(2) and 2.318(3) Å.
Reactions of the B(C6F5)4 anion are unusual; Reed et al. has
reported boronÀcarbon bond cleavage in this anion under super-
acidic conditions.24 Thisreactionisobservedmorecommonly with
tetrakis(bis-3,5-(trifluoromethyl)phenyl)borate anions.25
Heating a sample of 3 under hydrogen gas gave the same
product distribution as the hydrogen reduction of 2, suggesting
that 3 may be an intermediate in the reduction reaction. We
propose that complex 3 is formed after the initial CO loss,
consistent with inhibition by CO gas. The highly Lewis acidic
pentacarbonyl fragment can react with the B(C6F5)4 counterion
or with H2. Our data do not distinguish secondary hydrogenation
of 3 from direct coordination of H2 to the pentacarbonyl
fragment derived from 2. In both cases, a reaction with hydrogen
gas ensues, resulting in overall reduction of the iron centers to
Fe(I) (after deprotonation). Pentacarbonyl fragments derived
from complexes 2 and 3 could resemble the structure of the
oxidized H cluster.
2
Repeating the reaction with D2 gas and monitoring by H
NMR spectroscopy led to the detection of the presence of DC6F5
and the cationic bridging deuteride. The presence of deuterium
in the reaction products confirms that H2 (D2) is the reductant.
The fluorobenzene solvent also incorporates deuterium in the
course of this reaction, consistent with the fluorine substituent
activating proton exchange.22
Further studies are underway to detect dihydrogen intermedi-
ates in the reduction reaction and to employ less reactive
counteranions.
The observation of C6F5H in the reduction reaction suggests
that activation of the fluorinated tetraphenylborate counteranion
has occurred. When 2 is heated in fluorobenzene under argon, a
slow reaction takes place that can be inhibited by the addition of
an atmosphere of CO gas. Reaction is complete after heating at
70 °C for 72 h. The 1H NMR spectrum exhibits new propane-
dithiolate signals at δ 2.40 (4H, t) and 2.01 (2H, quintet).
Layering with pentane yields red crystals of complex 3 (43%
yield). Complex 3 exhibits IR bands (KBr) in the carbonyl region
at 2117, 2082, and 2062 cmÀ1, consistent with an oxidized
species.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and de-
b
tails of characterization of new compounds. X-ray crystallo-
graphic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: heinekey@chem.washington.edu.
The structure was determined by X-ray diffraction and confirms
theformulationas[Fe2(μ-C6F5)(μ-S2C3H6)(CO)6][B(C6F5)4] (3).
The FeÀFe distance in 3 is 2.803 Å (Figure 2).23 The bridging
chloride ligand has been replaced with the C6F5 group that forms
’ ACKNOWLEDGMENT
This work was supported by the U.S. National Science Foundation.
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Inorganic Chemistry
COMMUNICATION
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’ NOTE ADDED IN PROOF
Mild oxidation of a FeFe hydrogenase active site model has
been shown to enhance reactivity with hydrogen: Camara, J. M.;
Rauchfuss, T. B. J. Am. Chem. Soc. 2011, 133, 8098À8101.
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