Biomimetic hydrogen evolution catalyzed by an iron carbonyl thiolate [17]

Full text of DOI:10.1021/ja016516f

9476
J. Am. Chem. Soc. 2001, 123, 9476-9477
Biomimetic Hydrogen Evolution Catalyzed by an
Iron Carbonyl Thiolate
Scheme 1
Fr e´ d e´ ric Gloaguen, Joshua D. Lawrence, and
Thomas B. Rauchfuss*
Department of Chemistry
UniVersity of Illinois at Urbana-Champaign
Urbana, Illinois 61801
ReceiVed June 28, 2001
Scheme 2
1
Homogeneous catalysts for proton reduction are of interest
because they are amenable to systematic manipulation, and they
represent viable precursors to tailored heterogeneous catalysts,
including those using more economically attractive base metals
such as Fe. Hydrogenase enzymes represent a structurally unusual
but highly efficient hydrogen-processing catalysts that rely on
2
base metals (Ni, Fe). The structures of both major families of
hydrogenase enzymes, the Fe-only and the NiFe hydrogenases,
3
are known at high resolution. The active site of the Fe-only
hydrogenases consists of an Fe
O/H and a thiolate-linked Fe
core shares key structural features with organometallic complexes
2
(µ-SR)
2
(CN)
2 3 n
(CO) L core (L )
4
H
2
2
4
S
4
(SR)
4
cluster, Scheme 1). This
5
Fe
2
(µ-SR)
2
(CO)
6
that have been known since the 1920s. So stable
(CO) derivatives that such compounds form
are the Fe
2
(µ-SR)
2
6
under harsh conditions (e.g., 50-200 MPa at 250 °C) from
9
H).6
2
kinetic inertness of the hydride. Complexes of the type HFe -
primitive reagents (FeS, RSH, HCO
2
+
(
µ-SR)
2
(CO)6-x
L
x
were first described by Poilblanc without
We have reported that model complex {Fe
2
[µ-S
2 2 3 2
(CH ) ](CN) -
10
2-
examination of their redox properties. It is significant that
protonation of 2 occurs at Fe, not the N of cyanide, as both are
basic sites.11 Further treatment of 3 with toluenesulfonic acid
HOTs) gave a new and relatively air-stable species that exhibits
CO bands ca. 10 cm higher in energy than 3 (Figure 1). In
(
CO)
metric amounts of dihydrogen. Unfortunately acid also converts
(and related dianions) into insoluble and catalytically inactive
polymeric derivatives of unknown structure. The unsuitability of
as a catalyst is attributable to its highly reducing character,
4
}
(1, Scheme 1) reacts with protons to give substoichio-
7
1
(
ν
-
1
1
contrast, the conversion of 2 to 3 results in a shift in νCO of ca.
60 cm . The new species generated by protonation of 3 is
supported by the aforementioned ability to reduce protons directly
as well as by electrochemical measurements. This logic led us
-
1
8
+
+
-
assigned as {HFe
2 2 2 3 4 3
[µ-S (CH ) ](CNH)(CO) (PMe )} (3H ).
to investigate the complex {Fe
2) which is less reducing than 1. As described below, 2 is an
2
2 2 3 4 3
[µ-S (CH ) ](CN)(CO) (PMe )}
Proton reduction catalysis was demonstrated by cyclic voltam-
metry (CV) (Figure 2A). The voltammogram of a solution of 2
with 1 equiv of HOTs shows, in addition to the one-electron
(
active catalyst for proton reduction, and as such provides the first
functional link between organometallic models and the Fe-only
hydrogenases.
reduction of 2 (E
p
) -2.14 V vs Ag|AgCl), two new reduction
peaks at a potential ca. 1 V less negative. Shifts of reduction
In evaluating the catalysis, we first examined the protonation
12
potential parallel the changes in the νCO
ascribed to the reduction of 3H and 3 at E
-1.13 V, respectively. The value of ∆E
peak potentials for reduction and reverse oxidation) indicates that
the reduction of 3 is a one-electron process (Figure 2B). With
2 4
increasing acid concentration (HOTs, H SO , and HCl give similar
results) the height of the first reduction peak increases, and its
potential is shifted toward more negative potentials, as expected
for catalytic proton reduction. At more negative potentials the
only significant electrochemical event observed is the reduction
of 2. In the presence of excess protons (g3 equiv), solutions
derived from 2 are completely stable under catalytic conditions
for hours at room temperature. In a preparative-scale reaction, a
2 4
solution of 10 M of 2 with 50 equiv H SO was electrolyzed at
1.2 V (ca. -1 V vs NHE). Over the course of 15 min, 12 F per
mol of 2 were passed. This corresponds to six turnovers for the
.
These two peaks are
) -1.03 and E1/2
p
(difference between the
of 2. Dark red HFe
in analytical purity from MeCN solutions of 2 upon addition of
2
[µ-S
2 2 3 4 4
(CH ) ](CN)(CO) (PMe ) (3) precipitates
+
p
)
1
excess aqueous H
of this species shows a P-coupled doublet signal at δ ) -17
H-P ) 23 Hz), consistent with protonation of the Fe-Fe bond.
Amine bases do not convert 3 into 2, probably reflecting the
2 4
SO (see Scheme 2). The H NMR spectrum
31
(
J
(
1) Koelle, U. New J. Chem. 1992, 16, 157-169.
(
2) (a) Cammack, R. Nature 1999, 397, 214-215. (b) Collman, J. P. Nat.
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(
3) (a) Volbeda, A.; Garcin, E.; Piras, C.; de Lacey, A. L.; Fernandez, V.
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3-23. (e) Nicolet, Y.; de Lacey, A. L.; Vern e` de, X.; Fernandez, V. M.;
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1
bulk solution, close to the theoretical maximum because of the
(
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(
1
(
(
1
(
Rauchfuss, T. B. Submitted for publication. (b) Le Cloirec, A.; Best, S. P.;
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1
0.1021/ja016516f CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/01/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 38, 2001 9477
monocyanide {Fe
(CH ](CN)(CO)
}- (4)8b,14 is inactive
2
[µ-S
2
2
)
3
5
catalytically. IR measurements show that this species only
protonates at the N of cyanide, not at the Fe-Fe bond, emphasiz-
ing that the PMe
bond. The diphosphine derivative Fe
3
ligand in 2 enhances the basicity of the Fe-Fe
2
[S
2
(CH
2
)
3
](CO)
4
(PMe
3
)
2
(5,
]-
E
1/2 ) -1.86 V), is protonated (HOTs) to give {HFe
2
[S (CH
2
2
)
3
+
10
4 3 2
(CO) (PMe ) } . Although this hydrido species undergoes a
reduction at E1/2 ) -0.98 V (vs -1.13 V for 3), the voltammetric
response does not exhibit catalytic features with increased [HOTs].
It is clear that catalysis is quite sensitive to subtle electronic effects
3
such that replacement of one PMe by CNH (the difference
+
+
between 3H and 4H ) significantly alters the catalytic properties
of this bimetallic unit. We conclude that the catalytic properties
of 3 could be due to its ability to both sustain protonation at the
Fe-Fe bond15 and carry a proton at the terminal cyanide. The
1
6
diiron complex Cp
bond, is catalytically inactive.
Much effort has focused on homogeneous proton reduction by
2
Fe
2
(SC
2
H
5
)
2
(CO)
2
,
which has no Fe-Fe
1,13,17
metalloporphyrin catalysts.
The complex [Fe(tetraphenylpor-
phyrinate)]2 is particularly active, although it is electrochemically
-
generated and operates at -1.6 V versus SCE in DMF, which is
∼
500 mV more negative than for 3. The extractable cofactor of
Figure 1. FT-IR spectra of (A) 2 (THF solution), (B) 3 (THF solution),
and (C) 3H (MeCN solution).
+
nitrogenase (FeMoco) requires only a modest overpotential of
-
0.5 V, but this catalyst is unstable in acid,18 in contrast to our
organometallic catalyst, and can generate only micromolar
amounts of dihydrogen.19 Previous catalysts proceed via an initial
reduction which generates a basic center that accepts a pro-
ton.1
3,17,19,20
In the present case protonation precedes reduction.
Protonation enhances the ability of the dimetal center to accept
electrons, which explains the rather modest overpotentials ob-
served in this work.
Further studies are underway to more thoroughly describe
mechanistic details, but this work shows that relatively simple
analogues of the Fe-only hydrogenases can be effective catalysts
for the reduction of protons to dihydrogen, beginning with
protonation at the Fe-Fe bond. These systems are amenable to
modification so that it should be possible to prepare immobilized
and water-soluble catalysts, as well as derivatives featuring the
recently proposed azadithiolate cofactor.4 Whereas the Fe-
hydrogenases are proposed to bind protons at a single Fe center,
our catalyst protonates at the metal-metal bond. Understanding
this difference between the synthetic and natural catalyst is of
continuing interest.
e
Acknowledgment. This research was supported by NIH and DOE.
F.G. thanks the CNRS for a leave of absence and Dr. Jean Talarmin
(CNRS - Universit e´ de Brest) for helpful discussion.
Supporting Information Available: Experimental procedures (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. (A) Effect of x equiv of toluenesulfonic acid (HOTs) on the
cyclic voltammetry of 2, (a) x )1, (b) x ) 2, (c) x ) 3, (d) x ) 4. (B)
Cyclic voltammetry of 2 and 3. Conditions: 2.3 mM 2 or 2.8 mM 3 in
JA016516F
(14) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 2001, 123, 3268-3278.
-
1
4 6
MeCN-Bu NPF ; potential scan rate ) 200 mV s ; glassy carbon
(
15) Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J. Inorg. Chim. Acta
2
working electrode of surface area 0.0701 cm .
1
999, 294, 240-254.
16) Vergamini, P. J.; Kubas, G. J. Prog. Inorg. Chem. 1976, 21, 261-
82.
(
2
time constant of the electrolysis cell. In such large-scale experi-
ments, bubbles of H are obvious. Gas chromatographic analysis
2
showed that the dihydrogen yield was 100 ((10)%. A simplified
catalytic cycle is presented in Scheme 2.
Having uncovered the catalytic behavior of 3, we examined
the catalytic properties of structurally related species. The
(
17) Collman, J. P.; Ha, Y.; Wagenknecht, P. S.; Lopez, M. A.; Guilard,
R. J. Am. Chem. Soc. 1993, 115, 9080-9088.
(18) Burgess, B. K.; Lowe, D. J. Chem. ReV. 1996, 96, 2983-3011.
(19) Le Gall, T.; Ibrahim, S. K.; Gormal, C. A.; Smith, B. E.; Pickett, C.
J. Chem. Commun. 1999, 773-774.
20) Meilleur, D.; Rivard, D.; Harvey, P. D.; Gauthron, I.; Lucas, D.;
Mugnier, Y. Inorg. Chem. 2000, 39, 2909-2914.
(