Diferrous cyanides as models for the Fe-only hydrogenases

Article abstract of DOI:10.1021/ja051584d

The first systematic study of diferrous dicyano dithiolates is described. Oxidation of [Fe₂(S₂C₂H₄)(CN) ₂(CO)₄]^2- in the presence of cyanide and tertiary phosphines and of Fe₂(S₂C₂H ₄)-(CO)₄(PMe₃)₂ in the presence of cyanide affords a series of diferrous cyanide derivatives that bear a stoichiometric, structural, and electronic relationship to the H _ox^air state of the Fe-only hydrogenases. With PPh ₃ as the trapping ligand, we obtained an unsymmetrical isomer of Fe₂(S₂C₂H₄)(μ-CO)(CN) ₂(PPh₃)_2- (CO)₂, as confirmed crystallographically. This diferrous cyanide features the semibridging CO-ligand, with Fe-μC bond lengths of 2.15 and 1.85 A. Four isomers of Fe₂(S₂C₂H₄)(μ-CO)(CN) ₂(PMe₃)₂(CO)₂ were observed, the initial product again being unsymmetrical but more stable isomers being symmetrical. DFT calculations confirm that the most stable isomers of Fe ₂(S₂C₂H₄)(μ-CO)(CN) ₂(PMe₃)₂(CO)₂ have cyanide trans to μ-CO. Oxidative decarbonylation also afforded the new tetracyanide [Fe ₂(S₂C₂H₄)(μ-CO)(CN) ₄-(CO)₂]^2-. Insights into the oxidative decarbonylation mechanism of these syntheses come from the spectroscopic characterization of the tetracarbonyl [Fe₂(S₂C ₂H₄)(μ-CO)(CN)₃(CO)₃]. This species reacts with PEt₃ to produce the stable adduct [Fe ₂(S₂C₂H₄)(μ-CO)(CN) ₃(CO)₂(PEt₃)]^-.

Full text of DOI:10.1021/ja051584d

Published on Web 07/14/2005
Diferrous Cyanides as Models for the Fe-only Hydrogenases
Christine A. Boyke,† Jarl Ivar van der Vlugt,† Thomas B. Rauchfuss,*,†
Scott R. Wilson,† Giuseppe Zampella,‡ and Luca De Gioia*,‡
Contribution from the Department of Chemistry, UniVersity of Illinois at Urbana-Champaign,
Urbana, Illinois 61801, and Department of Biotechnology and Biosciences, UniVersity of
Milano-Bicocca, Piazza della Scienza 1 20126-Milan
Received March 11, 2005; E-mail: rauchfuz@uiuc.edu
Abstract: The first systematic study of diferrous dicyano dithiolates is described. Oxidation of
[Fe2(S2C2H4)(CN)2(CO)4]2- in the presence of cyanide and tertiary phosphines and of Fe2(S2C2H4)-
(CO)4(PMe3)2 in the presence of cyanide affords a series of diferrous cyanide derivatives that bear a
air
stoichiometric, structural, and electronic relationship to the Hox state of the Fe-only hydrogenases. With
PPh3 as the trapping ligand, we obtained an unsymmetrical isomer of Fe2(S2C2H4)(µ-CO)(CN)2(PPh3)2-
(CO)2, as confirmed crystallographically. This diferrous cyanide features the semibridging CO-ligand, with
Fe-µC bond lengths of 2.15 and 1.85 Å. Four isomers of Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2 were
observed, the initial product again being unsymmetrical but more stable isomers being symmetrical. DFT
calculations confirm that the most stable isomers of Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2 have cyanide
trans to µ-CO. Oxidative decarbonylation also afforded the new tetracyanide [Fe2(S2C2H4)(µ-CO)(CN)4-
(CO)2]2-. Insights into the oxidative decarbonylation mechanism of these syntheses come from the
spectroscopic characterization of the tetracarbonyl [Fe2(S2C2H4)(µ-CO)(CN)3(CO)3]-. This species reacts
with PEt3 to produce the stable adduct [Fe2(S2C2H4)(µ-CO)(CN)3(CO)2(PEt3)]-.
bimetallic subunit of the H cluster to the diamagnetic Hoxair state,
which is proposed to be [FeIIFeII].
Introduction
The Fe-only hydrogenases are highly efficient catalysts for
hydrogen evolution and oxidation. The unusual structure of the
active site of these enzymes, together with the technological
implications of their reactivity, has attracted intense interest from
both biological and chemical scientists.1,2 The H-cluster active
site contains CO and CN- ligands as well as a novel dithiolate
cofactor, as deduced from a combination of crystallographic
analysis3 and spectroscopic data.4 Four principal states of the
active site are known (Figure 1), with Hox and Hred as being
catalytically active. The collective evidence points to the
oxidation state assignments [FeIIFeI] (S ) 1/2) for Hox and
[FeIIFeII] for Hred, respectively, the latter being a protonated
[FeIFeI] species, hence the formal Fe(II) assignment.5
Initial modeling studies focused on substituted modifications
of Fe2(SR)2(CO)6 and their protonated derivatives.10,11 Our early
studies identified [Fe2(S2C3H6)(CN)2(CO)4]2- as a first genera-
tion structural model for the active site,12 while biomimetic
catalysis was discovered with HFe2(S2C3H6)(CN)(PMe3)(CO)4.13
Subsequent work has confirmed the general applicability of
diiron dithiolates for catalytic production of H2 by proton
reduction.14-17
A major challenge is the synthesis of active site models that
more faithfully replicate the structural features of the natural
system, as identified crystallographically.18 To this end, we
recently described a new class of diiron dithiolates of the
formula [Fe2(S2CnH2n)(µ-CO)(CNMe)6]2+ obtained from oxida-
Inhibition of Hox (and Hred) by CO yields Hox
also an S )
,
CO 6-8 which is
/ species. The exogenous CO occupies the
2
1
(9) De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, E. C.; Fernandez,
V. M. J. Am. Chem. Soc. 2000, 122, 11232-11233.
coordination site proposed to be directly involved in dihydrogen
activation and production.9 Air deactivation converts the
(10) Evans, D. J.; Pickett, C. J. Chem. Soc. ReV. 2003, 32, 268-275.
(11) Georgakaki, I. P., Darensbourg, M. Y. In ComprehensiVe Coordination
Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.: Amsterdam, 2004.
(12) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999,
121, 9736-9737.
† University of Illinois at Urbana-Champaign.
‡ University of Milano-Bicocca.
(13) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.; Be´nard, M.; Rohmer,
M.-M. Inorg. Chem. 2002, 41, 6573-6582.
(1) Frey, M. ChemBioChem 2002, 3, 153-160.
(2) Cammack, R.; Frey, M.; Robson, R. Hydrogen as a Fuel: Learning from
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J. Am. Chem. Soc. 2004, 126, 16988-16999.
(3) Nicolet, Y.; Lemon, B. J.; Fontecilla-Camps, J. C.; Peters, J. W. Trends
Biochem. Sci. 2000, 25, 138-143.
(15) Cheah, M. H.; Borg, S. J.; Bondin, M. I.; Best, S. P. Inorg. Chem. 2004,
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(4) Pierik, A. J.; Hulstein, M.; Hagen, W. R.; Albracht, S. P. J. Eur. J. Biochem.
1998, 258, 572-578.
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Darensbourg, M. Y. J. Am. Chem. Soc. 2004, 126, 12004-12014.
(17) Chong, D.; Georgakaki, I. P.; Mejia-Rodriguez, R.; Sanabria-Chinchilla,
J.; Soriaga, M. P.; Darensbourg, M. Y. Dalton Trans. 2003, 4158-4163.
(18) Tard, C.; Liu, X.; Ibrahim, S. K.; Bruschi, M.; De Gioia, L.; Davies, S. C.;
Yang, X.; Wang, L.-S.; Sawers, G.; Pickett, C. J. Nature 2005, 433, 610-
614.
(5) Liu, Z.-P.; Hu, P. J. Am. Chem. Soc. 2002, 124, 5175-5182.
(6) Lemon, B. J.; Peters, J. W. Biochemistry 1999, 38, 12969-12973.
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(8) Chen, Z.; Lemon, B. J.; Huang, S.; Swartz, D. J.; Peters, J. W.; Bagley, K.
A. Biochemistry 2002, 41, 2036-2043.
9
11010
J. AM. CHEM. SOC. 2005, 127, 11010-11018
10.1021/ja051584d CCC: $30.25 © 2005 American Chemical Society

Diferrous Cyanides as Models for Fe-only Hydrogenase
A R T I C L E S
Figure 1. Structures of the four states of the active sites of Fe-only hydrogenases.
tive decarbonylation of the classical19-21 substituted derivatives
of Fe2(SR)2(CO)4L2 species. The resulting complexes replicate
key features of Hoxair: a face-sharing bioctahedral structure, dia-
magnetism, a short Fe-Fe distance, and a µ-CO ligand.22 Sev-
eral structural aspects of the coordination environment remain
to be modeled, including the incorporation of naturally occurring
ligands on the diferrous framework, in particular CN- and CO
ligands, as observed in all forms of the binuclear active site.
Diferrous cyanides were first detected by Pickett et al. in
studies on the electrochemical oxidation of [Fe2[(SCH2)2-
CHCH2SMe](CN)2(CO)4]2-, a di-subferrous species containing
a pendant thioether.23 Low-temperature oxidation of this species
afforded an unstable intermediate with FT-IR and EPR signa-
A. Fe2(S2C2H4)(µ-CO)(CN)2(PPh3)2(CO)2 (2). Oxidation of
(Et4N)2(1) in the presence of excess PPh3 (Scheme 1) gave stable
homogeneous solutions. Despite the simplicity of the IR
spectrum, the complex 31P NMR spectrum indicated that the
reaction mixture contained several isomers. Using a combination
of column chromatography and fractional crystallization, we
were able to isolate one isomer of Fe2(S2C2H4)(µ-CO)(CN)2-
(PPh3)2(CO)2 (2) in 10% yield. 31P NMR analysis of the crude
reaction mixture indicates that 2 is the major product of the
reaction, formed in ca. 25% yield. 31P NMR spectroscopy
revealed that the two phosphine ligands in 2 are chemically
nonequivalent. The FT-IR spectrum of 2 exhibited weak bands
at 2119 and 2104 cm-1 assigned to νCN, strong bands at 2040
and 1993 cm-1 assigned to νCO, and a broader band of medium
CO
tures characteristic of a mixed valence derivative. This Hox
intensity at 1904 cm-1 attributed to νµ-CO
.
model undergoes further 1e- oxidation to give an unstable
diferrous derivative. Diferrous cyanides formally arise from the
protonation of [Fe2(S2C3H6)(CN)2(CO)4]2- and [Fe2(S2C3H6)-
(CN)(CO)4(PMe3)]-,13,24 although these species contain µ-H
ligands. Liaw and co-workers have described routes to both
monoferrous cyanides25 including [Fe(SR)2(CN)(CO)2L]-
(L ) CO, SR2), [Fe(SR)(CN)3(CO)2]2-, and diferrous tetra-
The crystallographically determined structure for 2 confirms
its face-sharing bioctahedral core (Figure 2, Table 1).22,27 The
bridging CO ligand is coordinated asymmetrically, reflecting
the electronically disparate trans ligands, PPh3 and CN-, with
bond lengths of 2.15 Å for Fe(1)-C(39) and 1.85 Å for
Fe(2)-C(39). The shorter Fe(2)-C(µ) distance is attributed to
the influence of the strongly σ-donating trans cyanide. The semi-
bridging character28 of the µ-CO-ligand is further indicated by
the Fe-C(µ)-O angles 129.8° and 151.4°. The Fe(1)-Fe(2)
distance is 2.55 Å, which is close to the values found in the
enzymes CpI and DdH,3 but this parameter is rather insensitive
for a wide range of bioctahedral diiron species. We have
previously discussed the fact that species of the type FeI2(SR)2L6
cyanides [Fe2(µ-SEt)2(CN)4(CO)4]2- 26
.
An obvious approach to diferrous dithiolato dicyanides is the
oxidation of [Fe2(SR)2(CN)2(CO)4]2-, but our early attempts
afforded only insoluble mixtures.12 In this paper we revisit this
synthetic challenge, capitalizing on our recently discovered route
to [Fe2(SR)2(µ-CO)(CNMe)6]2+ which entailed the use of milder
oxidation conditions in the presence of soft trapping ligands
(eq 1).22
contain an Fe-Fe bond,29 whereas the related FeII (SR)2(µ-CO)-
2
L6 species feature 3-center-2-electron interactions involving the
bridging CO ligand.22,30 The different steric properties of the
axial CN- and PPh3 are reflected in the geometry around the
respective iron centers; i.e., the S(1)-Fe(1)-P(1) angle is
99.62°, while the related S(1)-Fe(2)-C(43)N angle is 90°.
(19) de Beer, J. A.; Haines, R. J. J. Organomet. Chem. 1972, 36, 297-313.
(20) de Beer, J. A.; Haines, R. J. J. Organomet. Chem. 1972, 37, 173-188.
(21) Fauvel, K.; Mathieu, R.; Poilblanc, R. Inorg. Chem. 1976, 15, 976-978.
(22) Boyke, C. A.; Rauchfuss, T. B.; Wilson, S. R.; Rohmer, M.-M.; Be´nard,
M. J. Am. Chem. Soc. 2004, 126, 15151-15160.
Results and Discussion
Synthesis of Diferrous Dicyanides. The basic method entails
oxidation of MeCN solutions of (Et4N)2[Fe2(S2C2H4)(CN)2-
(CO)4], (Et4N)2(1), with 2 equiv of FcPF6 in the presence of
tertiary phosphines. The reactions proceed more efficiently when
conducted under an atmosphere of CO at -40 °C. The complex
31P NMR spectra of the crude red reaction mixtures indicate
the presence of several products. Purification by recrystallization
and column chromatography proved useful for obtaining three
species in pure form (Scheme 1).
(23) Razavet, M.; Davies, S. C.; Hughes, D. L.; Barclay, J. E.; Evans, D. J.;
Fairhurst, S. A.; Liu, X.; Pickett, C. J. Dalton Trans. 2003, 586-595.
(24) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.; Darensbourg,
M. Y. J. Am. Chem. Soc. 2001, 123, 9710-9711.
(25) Chen, C.-H.; Chang, Y.-S.; Yang, C.-Y.; Chen, T.-N.; Lee, C.-M.; Liaw,
W.-F. Dalton Trans. 2004, 137-143.
(26) Liaw, W.-F.; Tsai, W.-T.; Gau, H.-B.; Lee, C.-M.; Chou, S.-Y.; Chen, W.-
Y.; Lee, G.-H. Inorg. Chem. 2003, 42, 2783-2788.
(27) Lawrence, J. D.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2002, 41,
6193-6195.
(28) Cotton, F. A.; Kruczynski, L.; Frenz, B. A. J. Organomet. Chem. 1978,
160, 93-100.
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J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005 11011

A R T I C L E S
Scheme 1
Boyke et al.
31P NMR and IR analysis. Workup by column chromatography
afforded Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2 (unsym-3), which
is structurally analogous to 2. The FT-IR (Figure 3) and 31P
NMR spectra are unexceptional, although the two phosphine
signals were observed as doublets in the 31P NMR spectrum,
with a coupling constant JP-P of 6 Hz. The IR band for the
µ-CO-ligand is present at 1884 cm-1, 20 cm-1 lower in energy
vs 2.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Fe2(S2C2H4)(µ-CO)(CN)2(PPh3)2(CO)2, 2
Fe(1)-S(1)
2.239(2)
2.268(2)
2.148(7)
2.260(2)
2.5461(17) S(1)-Fe(1)-P(1)
88.9(2)
Fe(1)-S(2)
Fe(2)-S(2)
Fe(2)-C(39)
Fe(2)-C(43)
2.256(2)
2.278(2)
1.853(7)
1.938(7)
99.62(8)
78.7(3)
Fe(2)-S(1)
Fe(1)-C(39)
Fe(1)-P(1)
Fe-Fe
S(1)-Fe(2)-C(43)
P(1)-Fe(1)-C(39)
Fe(1)-C(39)-O(1) 129.8(6)
3
Fe(1)-C(39)-Fe(2)
162.33(19)
C(43)-Fe(2)-C(39) 168.3(3)
Fe(2)-C(39)-O(1)
C(39)-Fe(2)-Fe(1)
151.4(6)
55.8(2)
We examined a complementary route to 3 via oxidative
C(39)-Fe(1)-Fe(2)
45.52(18)
13
decarbonylation of the bisphosphine Fe2(S2C2H4)(CO)4(PMe3)2
using cyanide as the trapping ligand (Scheme 1). This reaction
yielded a red-brown homogeneous solution, for which the 31P
NMR spectrum, however, did not indicate the presence of
unsym-3. Instead, an extractive workup using toluene afforded
a symmetrical isomer of Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2,
sym-3. In contrast to unsym-3, the 31P NMR spectrum of sym-3
consisted of a singlet for the two equivalent PMe3 ligands. The
FT-IR spectrum revealed a single medium νCN band at 2115
cm-1, as well as bands at 2020 (shoulder), 2003, and 1839 cm-1
(νCO), indicative of a C2 isomer (Figure 3). We were unable to
grow crystals of sym-3, but its structure was analyzed by DFT
calculations (vide infra).
B. Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2 (3). As in the
PPh3 case, oxidation of (Et4N)2(1) in the presence of PMe3
(Scheme 1) also gave a homogeneous mixture, as indicated by
Using 31P NMR and FT-IR spectroscopy, we observed traces
of sym-3 in samples of unsym-3. In situ NMR spectroscopic
measurements confirmed that unsym-3 indeed converts to sym-3
(Figure 4), i.e., that unsym-3 is a kinetic isomer formed from
oxidative decarbonylation of (Et4N)2(1). Interestingly, our in
situ experiments revealed two intermediates in the conversion
of unsym-3 into sym-3. The first intermediate is symmetrical
(one 31P NMR resonance), which we suggest is the meso isomer,
meso-3 (Figure 4). Concomitant with the disappearance of
(29) Dahl, L. F.; Wei, C. H. Inorg. Chem. 1963, 2, 328-333.
(30) Lawrence, J. D.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2002, 41,
6193-6195.
Figure 2. Structure of Fe2(S2C2H4)(µ-CO)(CN)2(PPh3)2(CO)2, 2. Thermal
ellipsoids are drawn at the 50% probability level.
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Diferrous Cyanides as Models for Fe-only Hydrogenase
A R T I C L E S
Figure 3. FT-IR spectra (MeCN) of unsym-3 (left) and sym-3 (right).
reactions of Et4NCN with sym- and unsym-3 did not proceed
cleanly, presumably indicative of the greater difficulty of
displacing PMe3 vs the weakly basic PPh3.
The relative efficiency of the oxidation of (Et4N)2(1) in the
presence of excess Et4NCN prompted us to investigate the
corresponding reaction using the propanedithiolate analogue.
Indeed, (Et4N)2[Fe2(S2C3H6)(µ-CO)(CN)4(CO)2] is cleanly gen-
erated and exhibited the expected spectroscopic properties.
Oxidation of (Et4N)2(1) in the presence of only 1 equiv of
Et4NCN afforded an unstable red-colored species, assigned as
[Fe2(S2C2H4)(µ-CO)(CN)3(CO)3]- (5) (Scheme 1). In its FT-
IR spectrum, the νCO bands are relatively high in energy (2073,
2048, 2022, 1927 cm-1). The implied high electrophilicity is
probably related to the compound’s instability. For example,
removal of solvent from solutions of 5 resulted in decomposi-
tion. Compound 5 was shown to cleanly convert to 4 upon
treatment with 1 equiv of Et4NCN (Scheme 1). Similarly,
addition of PEt3 to an MeCN solution of 5 afforded a single
isomer of the adduct [Fe2(S2C2H4)(µ-CO)(CN)3(CO)3(PEt3)2]-
(6, Scheme 1). In contrast to its precursor, 6 was sufficiently
stable to be characterized by IR, ESI-MS, and 31P NMR
spectroscopy.
Attempted Reduction of HoxCO Models to Hred Models. The
electrochemical and chemical reduction of 2, sym-3, and 4 were
surveyed. Cyclic voltammetry of 2 in MeCN solution revealed
a reversible reduction at -555 mV vs Ag/AgCl, while the related
PMe3 derivative sym-3 requires a potential of -894 mV. The
dianionic tetracyanide 4 irreversibly reduced at -985 mV. In
all three cases, only one reduction process was observed.
Treatment of 2 with 1 equiv of Cp2Co gave ca. 50% yield of 1
together with unreacted starting material. Using 2 equiv of
Cp2Co, 2 cleanly gave 1, as verified by IR spectroscopy (Scheme
1), indicative of a 2e- process.
Density Functional Theory Calculations. Twelve diaster-
eomers are possible for [Fe(CN)(PMe3)(CO)]2(µ-CO)(S2CnH2n),
corresponding to six isomers derivable from a Cs symmetric
parent (3.2), while the other six isomers come from a C2
symmetric parent (3.1). The DFT structures of the relevant
isomers of 3 are shown in Figure 5, while corresponding relative
energies, IR frequencies, and Fe-Fe and Fe-µ-CO distances
are collected in Table 2. Assuming that an Fe-µ-CO bond
breaks to enable turnstile rotation of the Fe(CN)(CO)(PMe3)
vertex, the six isomers related to 3.1 are mutually interconvert-
able, as are the other isomers, but these two sets cannot
interconvert by ordinary turnstile mechanisms.
Figure 4. Time-dependent 500 MHz 31P NMR spectra of a CD3CN solution
of unsym-3 at 60 °C. * Indicates signals for unsym-3; 1 denotes intermediate
isomer meso-3; [ represents signal for sym-3. Thus, the gradual disap-
pearance of unsym-3 accompanies the appearance of an intermediate at 19.7
ppm. Note also the transient formation of an unsymmetrical intermediate
with chemical shifts close to those for unsym-3.
meso-3 is the appearance of an unsymmetrical species, the 31
P
NMR pattern of which is quite similar to that for unsym-3. The
isomerization from unsym-3 to sym-3 could also be monitored
by FT-IR spectroscopy in the νCO region, and through such
studies we confirmed that addition of 10% Et4NCN did not
affect the rate of isomerization of unsym-3 to sym-3.
C. [Fe2(S2C2H4)(µ-CO)(CN)4(CO)2]2-. Oxidative decarbo-
nylation of (Et4N)2(1) in the presence of cyanide was found to
be particularly efficient (Scheme 1). The synthesis was ac-
complished by the simultaneous addition of both FcPF6 and
Et4NCN to a cold MeCN solution of (Et4N)2(1) under an
atmosphere of CO. The red-colored product was separated from
the cogenerated Et4NPF6 by precipitation of the latter from cold
MeOH. The FT-IR spectrum of [Fe2(S2C2H4)(µ-CO)(CN)4-
(CO)2]2- (4) consisted of the expected bands attributable to νCN
t-CO, and νµ-CO. Dianion 4 was also cleanly formed by
displacement of PPh3 with CN- from 2. The corresponding
,
ν
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J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005 11013

A R T I C L E S
Boyke et al.
Figure 5. DFT-optimized structures of the twelve possible diastereomers for Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2 (3).
The comparison of structures and energies revealed that
isomers featuring CO and/or PMe3 groups trans to the µ-CO
ligand are less stable than isomers with CN- trans to µ-CO.
Furthermore, placing the PMe3 ligand in the basal arrangement
results in less distortion of the iron environment from octahedral
geometry than isomers featuring one or two axial PMe3 groups
(not shown). Analysis of the isomers featuring CO and/or PMe3
groups trans to the µ-CO suggested, however, that the origin
of the destabilization was mainly electronic. Supporting this
view, the Fe-µ-CO distance is most strongly affected by the
identity of the trans ligand. This effect can be partly attributed
to the competition for π electrons by mutually trans π-accep-
tors. Consistent with these considerations, the less stable
[Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2] isomers (3.7-3.12) are
characterized by the presence of CO and/or PMe3 ligands trans
to µ-CO ligand.
The crystallographically characterized isomer of Fe2(S2C2H4)-
(µ-CO)(CN)2(PPh3)2(CO)2 is analogous to the computed com-
plex 3.5, which is shown to be only 5.7 kcal/mol less stable
than 3.1 (Table 2). Notably, and in very good agreement with
X-ray data, the µ-CO ligand is bridged in an asymmetric fashion,
with the two Fe-µ-CO bond distances differing by more than
0.1 Å (Figure 2, 3.5). As discussed above, the peculiar geometry
of 3.5 (and 3.3, which is slightly more stable and differs only
by the relative position of the basal CO and CN- groups on
Fe2) is due to the ligands trans to the µ-CO group. Cyanide
trans to µ-CO leads to shortened Fe-µ-CO bonds.
The effects of the nature of the ligands trans to the µ-CO
ligand is reflected not only in a modulation of key geometry
parameters but also in the corresponding IR spectra. IR spectra
computed for 3.1 and 3.2 are similar (Table 2) and in reasonable
agreement with the corresponding experimental data. In par-
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Diferrous Cyanides as Models for Fe-only Hydrogenase
A R T I C L E S
Table 2. Summary of DFT Calculations for Isomers of Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2
1
∆
E (referred to
CN, CO frequencies (cm-
)
Fe1−Fe2; Fe−µ-CO distances (Å)
1
model
isomer 3.1; kcal mol-
)
[experimentally observed]
[experimentally observed]
3.1
0
1814; 1993; 2003; 2132; 2134
2.495; 1.969; 1.969
[1839; 2003; 2020; 2115 (sym-3)]
1830; 1996; 2012; 2128; 2129
1870; 1985; 2015; 2121; 2130
1856; 1987; 2007.56; 2126; 2133
1871; 1978; 2019; 2127; 2131
[1904; 1993; 2039; 2104; 2119 (2)]
[1884; 1992; 2032; 2107; 2117 (unsym-3)]
1868; 1984; 2019; 2130; 2133
1874; 1992; 2001; 2131; 2131
1896; 1991; 2002; 2121; 2137
1916; 1992; 2004; 2128; 2129
1897; 1979; 1998; 2131; 2138
1913; 1983; 2002; 2130; 2131
1901; 1984; 2002; 2133; 2140
3.2
3.3
3.4
3.5
2.0
3.9
4.8
5.7
2.509; 1.974; 1.976
2.517; 1.915; 2.049
2.519; 1.906; 2.085
2.515; 1.923; 2.038
[2.55; 1.85; 2.15]
3.6
3.7
3.8
3.9
3.10
3.11
3.12
9.5
11.1
12.7
14.8
15.4
16.1
20.3
2.535; 2.099; 1.911
2.516; 1.990; 2.004
2.525; 1.955; 2.027
2.489; 1.984; 1.986
2.527; 1.975; 2.008
2.530; 1.981; 1.977
2.550; 1.974; 2.061
ticular, strong back-donation from both Fe centers to the µ-CO
ligand results in a weakening of the CO bond, with consequent
shift of CO stretching frequencies to lower values (1814 and
1830 cm-1 for 3.1 and 3.2, respectively). The progressive
exchange of CN- ligands with PMe3 or CO ligands, as observed
going from 3.1 to 3.12, results in a dramatic shift to higher
values of the µ-CO IR frequency, confirming the extent to which
νCO can be tuned.
Isomerism. A challenge to modeling the active site of the
Fe-only hydrogenases is the stereochemistry of the terminal
ligands on the diiron core. In the case of [FeLL′L′′]2(µ-CO)-
(S2CnH2n), 12 diastereoisomers are possible. The dinuclear active
site of HoxCO has nine diastereomeric possibilities reflecting its
simplified ligand set [Fe(CO)(CN)L][Fe(CO)2(CN)](µ-CO)-
(S2CnH2n) (where L is the thiolate of the 4Fe-4S cluster). Despite
the potential for isomeric complexity, only one isomer is
Another stable isomer is 3.4, where both the PMe3 groups
are basal, whereas a CO and a CN- group have axial orientation.
Isomer 3.4 differs from 3.3 only by the exchange of the PMe3
and CO ligands coordinated to Fe2. As observed in 3.3 and
3.5, the µ-CO group is coordinated in an unsymmetrical fashion
in 3.4. The computed IR frequencies for 3.4 (Table 2) quite
closely resemble those computed for 3.3 and 3.5, although
observed spectroscopically in the protein for the oxidized states
air 8,9
Hox, HoxCO, and Hox
.
The situation for Hred is less clear, as
reflected by the complexity of its IR spectrum in the νCO region;4
i.e., it is possible that in Hred isomers coexist.
In this paper we demonstrated that if isomerism existed in
the oxidized states of the protein, such isomers would be readily
distinguished using IR spectroscopy. Intriguing questions persist
as to the reason that the CN- ligands in the H-cluster are basal
vs the diaxial geometry that is energetically preferred by the
model systems examined in this paper. We can envision several
agreement is not excellent for νµ-CO
.
Conclusions
Oxidative Decarbonylation. The oxidative decarbonylation
of diiron(I) dithiolates is shown to be a powerful method for
generating previously unknown diferrous cyanides. This conver-
sion is initiated by the oxidation of suitable diiron dithiolates
that are more reducing than ferrocene (∼400 mV vs SCE).31
Upon oxidation, the diiron dithiolate requires additional donor
ligands to stabilize the corresponding FeII oxidation states, as
illustrated by the successful preparation of tricyanide 5 vs the
decomposition of (Et4N)21 upon oxidation without any ad-
ditional CN- present. Even this tricyano-tetracarbonyl species
is labile and highly reactive toward nucleophiles such as CN-
and PEt3, which readily afford isolable adducts. This set of
experiments may guide future applications of the oxidative
decarbonylation method whereby nucleophiles can be installed
subsequent to the oxidation step, thus minimizing potential
incompatibilities between electrophilic oxidants and nucleophilic
trapping agents.
factors that could stabilize the dibasal structure. In Hox and Hred
,
strong σ donors, OH- and H-, respectively, would also stabilize
the trans µ-CO.32 Crystallographic analyses reveal numerous
hydrogen-bonding contacts between the protein backbone and
the two cyanide ligands (Figure 6). These interactions could
dictate the stereochemistry of the cyanide ligands as well as
weaken their σ-donor properties.
Four isomers of Fe2(S2C2H4)(µ-CO)(CN)2(PR3)2(CO)2 were
observed in this work; two were isolated. In contrast to the
relatively slow isomerization for these diferrous species, isomer-
ism is a low energy process for the usual [FeIL(CO)2]2-
(µ-S2CnH2n) species reflecting the facility of turnstile rotation
at the formally five-coordinate Fe(µ-SR)2L3 centers.34 For the
confacial bioctahedral diferrous species, we suggest that isomer-
ization begins with rupture of an Fe-µ-CO bond followed by
turnstile rotation at the resulting Fe(CO)(CN)(PMe3) center. The
asymmetric Fe-µ-CO-Fe bonding mode is consistent with this
mechanism, the longer bond being to the Fe center that must
undergo the isomerization according to the pathway proposed
in eq 2. Also consistent with an intramolecular mechanism is
The new dithiolates resemble the Hox, HoxCO, and Hoxair states
of Fe-only hydrogenases by virtue of the presence of the
Fe2(SR)2(µ-CO)(CN)2(CO)2L2 coordination sphere. The influ-
ence of the chelating dithiolate is indicated by the con-
trasting structures of [Fe2(S2C2H4)(µ-CO)(CN)4(CO)2]2- (4) vs
[Fe2(SEt)2(CN)4(CO)4]2- 26
. The chelating dithiolate destabilizes
the edge-shared bioctahedral structures.
(32) Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2004, 43, 4733.
(33) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998,
282, 1853-1858.
(34) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M. Y. J.
Am. Chem. Soc. 2001, 123, 3268-3278.
(31) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-922.
9
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A R T I C L E S
Boyke et al.
(Figure 7). A striking demonstration of the sensitivity of the
νµ-CO band is that sym-3 features a νµ-CO peak at 1839 cm-1
,
which is 45 cm-1 lower than that for unsym-3 (1884 cm-1);
this difference indicates the dominant influence of the trans
ligands. Further demonstration of the sensitivity of νµ-CO can
be seen upon methylation of the CN- ligands in compound
4, which leads to a shift in νCO from 1876 to 1969 cm-1 for
[Fe2(S2C2H4)(µ-CO)(CN)4(CO)2]2- and [Fe2(S2C2H4)(µ-CO)-
(CNMe)4(CO)2]2+ 22
An overview of the broad range of the
.
µ-CO IR bands as found in our FeIIFeII model compounds is
shown in Figure 7.
One benchmark for comparing synthetic models to Hox is
air
the position of the νµ-CO band in the IR spectrum. This ab-
sorption occurs at 1847 cm-1 in the DdH enzyme.36 The fact
that the enzyme has such a low frequency suggests that the
ligands trans to µ-CO are powerful donors, which is consistent
with the presence of hydroxide.37 Diiron dithiolates containing
such hard anionic ligands are not known.
Figure 6. H-cluster active site from C. pasteuranium hydrogenase I
showing hydrogen bonding interactions involving the CN- ligands as well
as the met- - -OC(µ) and dithiolate- - -HScys interactions.33
Experimental Section
the observation that Et4NCN does not affect the rate of
interconversion.
General Procedures. Methods and materials were recently de-
scribed.22
Fe2(S2C2H4)(µ-CO)(CN)2(PPh3)2(CO)2 (2). An orange solution of
0.10 g (0.16 mmol) of (Et4N)2[Fe2(S2C2H4)(CN)2(CO)4] and 0.21 g (0.80
mmol) of PPh3 in 20 mL of MeCN and 5 mL of toluene was saturated
with CO and cooled to -40 °C. The reaction mixture was treated with
a solution of 0.105 g (0.32 mmol) of [Cp2Fe]PF6 in 5 mL of MeCN.
After stirring for 2 min, the solution was concentrated in vacuo to ∼5
mL. Addition of 50 mL of Et2O precipitated a brown solid leaving a
yellow solution of Cp2Fe. The brown solid was extracted into toluene,
and the extract was filtered through Celite to remove Et4NPF6. The
resulting red-brown solution was concentrated in vacuo to ∼5 mL.
Addition of hexanes precipitated the product. This solid was taken
up in 5 mL of CH2Cl2, and this extract was passed through a 32 ×
3.5 cm column of silica gel, eluting with CH2Cl2, MeCN, and
MeOH. The third band (and largest band) was concentrated to 5 mL
and diluted with 50 mL of hexane to precipitate a brown powder. Single
crystals, suitable for X-ray analysis, were grown by slow evapora-
tion of a CH2Cl2 solution. Yield: 0.014 g (10%). Anal. Calcd for
C43H34Fe2N2O3P2S2: C, 59.74; H, 3.96; N, 3.24. Found: C, 60.26; H,
4.79; N, 3.03. 31P NMR (202 MHz, CD3CN): δ 46.3 (s), 71.2 (s).
FT-IR (MeCN, cm-1): ν 2119 (m), 2104 (m), 2039 (s), 1993 (s), 1904
(w) cm-1. ESI-MS (m/z): 864 {Fe2(S2C2H4)(µ-CO)(CN)2(PPh3)2(CO)2},
1729 {Fe2(S2C2H4)(µ-CO)(CN)2(PPh3)2(CO)2}2. FAB-MS (m/z): 864
{Fe2(S2C2H4)(µ-CO)(CN)2(PPh3)2(CO)2}.
Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2 (unsym-3). An orange solu-
tion of 0.10 g (0.16 mmol) of (Et4N)2[Fe2(S2C2H4)(CN)2(CO)4] in 20
mL of MeCN was saturated with CO and cooled to -40 °C. The
reaction mixture was treated with a solution of 0.105 g (0.32 mmol) of
[Cp2Fe]PF6 in 5 mL of MeCN and 0.08 mL (0.8 mmol) of PMe3 in 5
mL of MeCN simultaneously followed by stirring for 2 min. The vol-
ume of the resulting brown solution was reduced in vacuo to
approximately 5 mL. Addition of 50 mL of Et2O precipitated a brown
solid leaving a yellow solution of Cp2Fe. An extract of this solid
in 5 mL of CH2Cl2 was purified by chromatography on silica gel,
eluting with CH2Cl2. The third band (and largest band) was concentrated
in vacuo to 5 mL, and the concentrated solution was diluted with
50 mL of hexane to precipitate unsym-3 as a brown powder. Yield:
0.012 g (15%). Anal. Calcd for C13H22Fe2N2O3P2S2: C, 31.73; H,
4.51; N, 5.96. Found: C, 31.79; H, 4.58; N, 5.53. 1H NMR (500 MHz,
Considerations that guide this mechanism are that intermedi-
ates with two COs, (PMe3)(CO), or two PMe3 ligands in axial
positions are high-energy species, as indicated by the DFT
calculations. The subsequent conversion of proposed meso-
isomer to the d,l-diastereomer (3.2 to 3.1) requires interchange
of terminal ligands. This conversion is accompanied by the
formation of an apparent intermediate with 31P NMR chemical
shifts such as those for unsym-3. We suggest that this species
has structure 3.3. Further analysis of the associated mechanistic
aspects will be pursued with other substituted derivatives in a
future paper. The described diferrous compounds are stereo-
chemically related to the species obtained by protonation of
diiron(I) precursors, i.e., [Fe2(SR)2(µ-H)(CO)4L2]z. Isomerization
in such bioctahedral (µ-SR)2(µ-H) species is sufficiently slow
that individual isomers can be detected by NMR spectroscopy.35
Comments on νµ-CO. The new complexes demonstrate that
νµ-CO is highly dependent on coligands and stereochemistry
(36) Nicolet, Y.; de Lacey, A. L.; Verne`de, X.; Fernandez, V. M.; Hatchikian,
E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123, 1596-1601.
(37) Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2003, 42, 4773-
4781.
(35) Zhao, X.; Hsiao, Y.-M.; Lai, C.-H.; Reibenspies, J. H.; Darensbourg, M.
Y. Inorg. Chem. 2002, 699-708.
9
11016 J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005

Diferrous Cyanides as Models for Fe-only Hydrogenase
A R T I C L E S
Figure 7. Schematic overview of “tuning” νµ-CO in synthetic FeIIFeII complexes.
CD3CN): δ 1.60 (d, 9H, PMe3, JP-P 11 Hz), 1.79 (d, 9H, PMe3, JP-P
11 Hz), 2.60 (m, 1H, edt), 2.92 (m, 1H, edt), 3.04 (m, 1H, edt), 3.18
(m, 1H, edt). 31P NMR (202 MHz, CD3CN): δ 25.0 (d, JP-P 6 Hz),
35.1 (d, JP-P 6 Hz). FT-IR (MeCN, cm-1): ν 2117 (m), 2107 (m),
2032 (s), 1992 (s), 1884 (w). FAB-MS (m/z): 492.0 {Fe2(S2C2H4)-
(µ-CO)(CN)2(PMe3)2(CO)2}.
Fe2(S2C2H4)(µ-CO)(CN)2(PMe3)2(CO)2 (sym-3). A solution of 0.15
g of (0.32 mmol) Fe2(S2C2H4)(CO)4(PMe3)2 in 30 mL of MeCN was
cooled to -40 °C under a stream of CO and then treated with a solution
of 0.24 g (0.73 mmol) of [Cp2Fe]PF6 in 10 mL of MeCN. After 20
min, a solution of 0.13 g (0.83 mmol) of Et4NCN in 10 mL of MeCN
was added. The reaction mixture was stirred for an additional 20 min
and cooled to -40 °C. The reaction solution was treated with a solution
of 0.105 g (0.32 mmol) of [Cp2Fe]PF6 in 5 mL of MeCN and 0.025 g
(0.16 mmol) of Et4NCN in 5 mL of MeCN simultaneously, followed
by stirring for 2 min. Hexane was added to remove ferrocene in a
biphasic extraction, leaving a red MeCN solution containing the prod-
uct and Et4NPF6. FT-IR (MeCN, cm-1): 2121 (w), 2073 (s), 2048 (s),
2022 (m), 1928 (w). ESI-MS (m/z): 394, {[Fe2(S2C2H4)(µ-CO)(CN)3-
(CO)3]}-; 669, {(Et4NPF6)[Fe2(S2C2H4)(µ-CO)(CN)3(CO)3]}-.
To the solution of 5 in MeCN was added 0.025 g (0.16 mmol) of
Et4NCN under a CO atmosphere at -40 °C. After warming the reaction
to room temperature, solvent was concentrated in vacuo to 5 mL.
Addition of 50 mL of Et2O precipitated a brown powder spectroscopi-
cally identified as 4.
(Et4N)[Fe2(S2C2H4)(µ-CO)(CN)3(CO)2(PEt3)]. To the solution of
5 in MeCN, prepared as described above, was added 0.2 mL (1.6 mmol)
of PEt3 under a CO atmosphere at -40 °C. Upon warming the solution
to room temperature, the solvent was concentrated in vacuo to 5 mL,
and the addition of 50 mL of Et2O precipitated a brown powder spec-
troscopically identified as (Et4N)[Fe2(S2C2H4)(µ-CO)(CN)3(PEt3)(CO)2].
31P NMR (202 MHz, CD3CN): δ 65.7 (s). FT-IR (MeCN, cm-1): 2120
(w), 2116 (w), 2103 (w), 2025 (s), 1978 (s), 1898 (w). ESI-MS (m/z):
484{[Fe2(S2C2H4)(µ-CO)(CN)3(CO)2(PEt3)]}-, 759 {(Et4N)(PF6)-
[Fe2(S2C2H4)(µ-CO)(CN)3(CO)2(PEt3)]}-.
at -40 °C before being allowed to warm to room temperature. Solvent
was removed in vacuo to leave a crude brown solid. The product was
extracted twice with 30 mL portions of toluene; the extract was
evaporated to dryness, and the brown residue was washed with 25
1
mL of hexane. Yield: 0.040 g (25%). H NMR (500 MHz, CD3CN):
δ 1.57 (d, 18H, PMe3, JP-H ) 10.5 Hz), 3.08 (m, 4H, SCH2CH2S).
31P NMR (202 MHz, CD3CN): δ 23.8 (s). FT-IR (toluene, cm-1): ν
) 2115 (w), 2020 (sh, m), 2003 (s), 1839 (w). Anal. Calcd for
C13H22Fe2N2O3P2S2: C, 31.73; H, 4.51; N, 5.96. Found: C, 32.01; H,
4.56; N, 5.83. FAB-MS (m/z) 491.9 (100% [M]).
(Et4N)2[Fe2(S2C2H4)(µ-CO)(CN)4(CO)2] (4). An orange solution of
0.10 g (0.16 mmol) of 1 in 20 mL of MeCN was saturated with CO
and cooled to -40 °C. The reaction solution was treated simultaneously
with a solution of 0.105 g (0.32 mmol) of [Cp2Fe]PF6 in 5 mL of MeCN
and 0.050 g (0.32 mmol) of Et4NCN in 5 mL of MeCN followed by
stirring for 2 min. The resulting red-brown solution was concentrated
in vacuo to ca. 5 mL and then diluted with 50 mL of Et2O to precipitate
a brown solid leaving a yellow solution of Cp2Fe. The solid was
extracted into MeOH. Cooling this extract to ca. -90 °C precipitated
Et4NPF6, which was removed by filtration. After rewarming to room
temperature, the solution was concentrated in volume in vacuo and
diluted with 50 mL of Et2O to precipitate the brown powder. Yield:
0.074 g (70%). Anal. Calcd for C25H44Fe2N4O3S2 H2O: C, 44.78; H,
6.92; N, 12.53. Found: C, 44.31; H, 6.90; N, 12.31. FT-IR (MeCN,
cm-1): 2109 (w), 2012 (s), 1990 (s), 1876 (w). ESI-MS (m/z): 522,
{(Et4N)[Fe2(S2C2H4)(µ-CO)(CN)4(CO)2]}-; 1174, {(Et4N)3[Fe2(S2C2H4)-
(µ-CO)(CN)4(CO)2]2}-.
Computational Methods. Calculations have been carried out using
the Turbomole suite of programs38 in connection with the resolution
of the identity technique.39,40 Structure optimizations have been carried
out within the density functional theory formalism using the BP86
functional41,42 and an all-electron valence triple-ú basis set with
polarization functions on all atoms (TZVP).43 The adopted level of
theory was already shown to be well suited to investigate models of
the Fe-hydrogenase active site.44 Stationary points on the energy
(38) Ahlrichs, R.; Baer, M.; Haeser, M.; Horn, H.; Koelmel, C. Chem. Phys.
Lett. 1989, 162, 165-169.
(39) Eichkorn, K.; Treutler, O.; Oehm, H.; Haeser, M.; Ahlrichs, R. Chem. Phys.
Lett. 1995, 240, 283-290.
(40) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc.
1997, 97, 119-124.
(41) Becke, A. D. Phys. ReV. 1988, A38, 3098-3100.
(42) Perdew, J. P. Phys. ReV. 1986, B33, 8822-8824.
(43) Schaefer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829-
5835.
(Et4N)[Fe2(S2C2H4)(µ-CO)(CN)3(CO)3] (5). An orange solution of
0.10 g (0.16 mmol) of 1 in 20 mL of MeCN was saturated with CO
(44) Zampella, G.; Bruschi, M.; Fantucci, P.; Razavet, M.; Pickett, C. J.; De
Gioia, L. Chem.sEur. J. 2005, 11, 509-520.
9
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A R T I C L E S
Boyke et al.
hypersurface have been located by means of energy gradient techniques.
Full vibrational analysis has been carried out to further characterize
each stationary point.
Supporting Information Available: X-ray crystallographic
file (in CIF format) for complex unsym-3. This material is
available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This research was supported by NIH. We
thank Maurizio Bruschi for helpful advice.
JA051584D
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11018 J. AM. CHEM. SOC. VOL. 127, NO. 31, 2005