Redox and structural properties of mixed-valence models for the active site of the [FeFe]-hydrogenase: Progress and challenges

Article abstract of DOI:10.1021/ic8007552

The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these low-molecular weight models for the H_ox state of th

Full text of DOI:10.1021/ic8007552

Inorg. Chem. 2008, 47, 7405-7414
Redox and Structural Properties of Mixed-Valence Models for the Active
Site of the [FeFe]-Hydrogenase: Progress and Challenges
Aaron K. Justice,† Luca De Gioia,*,‡ Mark J. Nilges,† Thomas B. Rauchfuss,*,† Scott R. Wilson,†
and Giuseppe Zampella‡
Department of Chemistry, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801,
and Department of Biotechnology and Biosciences, UniVersity of MilanosBicocca, Piazza della
Scienza 1, 20126 Milan, Italy
Received April 26, 2008
The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that
affect the oxidation state assignments, structures, and reactivity of these low-molecular weight models for the Hox
state of the [FeFe]-hydrogenases. The propanedithiolates Fe2(S2C3H6)(CO)3(L)(dppv) (L ) CO, PMe3, Pi-Pr3) oxidize
at potentials ∼180 mV milder than the related ethanedithiolates (Angew. Chem., Int. Ed. 2007, 46, 6152). The
steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure,
which forms upon oxidation. Electron Paramagnetic Resonance (EPR) spectra for the mixed-valence cations indicate
that the unpaired electron is localized on the Fe(CO)(dppv) center in both [Fe2(S2C3H6)(CO)4(dppv)]BF4 and
[Fe2(S2C3H6)(CO)3(PMe3)(dppv)]BF4, as seen previously for the ethanedithiolate [Fe2(S2C2H4)(CO)3(PMe3)(dppv)]BF4.
For [Fe2(S2CnH2n)(CO)3(Pi-Pr3)(dppv)]BF4; however, the spin is localized on the Fe(CO)2(Pi-Pr3) center, although
the Fe(CO)(dppv) site is rotated in the crystalline state. IR and EPR spectra, as well as redox potentials and
density-functional theory (DFT) calculations, suggest that the Fe(CO)2(Pi-Pr3) site is rotated in solution, driven by
steric factors. Analysis of the DFT-computed partial atomic charges for the mixed-valence species shows that the
Fe atom featuring a vacant apical coordination position is an electrophilic Fe(I) center. One-electron oxidation of
[Fe2(S2C2H4)(CN)(CO)3(dppv)]-resultedin2eoxidationof0.5equivtogivetheµ-cyanoderivative[FeI2(S2C2H4)(CO)3(dppv)](µ-
CN)[FeII2(S2C2H4)(µ-CO)(CO)2(CN)(dppv)], which was characterized spectroscopically.
Introduction
rejuvenated. Ongoing work in this and other laboratories has
shown that these complexes are versatile with respect to
redox, substitution, and functionalizability in ways that are
pertinent to the enzyme’s active site. A significant break-
through in the modeling efforts entailed the preparation of
the mixed-valence derivatives of the formula [FeIFeII-
(SR)2(CO)4-x(PMe3)Lx]+ (L ) carbene or diphosphine).4,5
These species exhibit the “rotated” stereochemistry for one
of the two Fe centers (Scheme 1). This stereochemistry
exposes a vacant coordination site on one Fe center, as had
been anticipated by numerous biophysical studies.2 This
geometry was rare in the otherwise extensive chemistry of
diiron dithiolato carbonyls.6 Now that the rotated geometry
Modeling of the diiron active site of the [FeFe]-hydroge-
nases has provided a rich and tractable set of challenges to
organometallic chemistry.1 Motivated by discoveries in
biophysical chemistry,2,3 the study of the Fe2(SR)2-
(CO)6-xLx system, previously well examined, has been
* To whom correspondence should be addressed. E-mail: luca.degioia@
unimib.it (L.D.), rauchfuz@uiuc.edu (T.B.R.).
†
University of Illinois at Urbana-Champaign.
University of MilanosBicocca.
‡
(1) (a) Liu, X.; Ibrahim, S. K.; Tard, C.; Pickett, C. J. Coord. Chem. ReV.
2005, 249, 1641–1652. (b) 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.
(2) (a) De Lacey, A. L.; Ferna´ndez, V. M.; Rousset, M.; Cammack, R.
Chem. ReV. 2007, 107, 4304–4330. (b) Fontecilla-Camps, J. C.;
Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. ReV. 2007, 107, 4273–
4303.
(4) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008–
7009.
(5) Justice, A. K.; Rauchfuss, T. B.; Wilson, S. R. Angew. Chem., Int.
Ed. 2007, 46, 6152–6154.
(3) (a) Lubitz, W.; Reijerse, E.; van Gastel, M. Chem. ReV. 2007, 107,
4331–4365. (b) Vignais, P. M.; Billoud, B. Chem. ReV. 2007, 107,
4206–4272.
(6) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B. Chem.
Commun. 2007, 2019–2021.
10.1021/ic8007552 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 16, 2008 7405
Published on Web 07/12/2008

Justice et al.
Scheme 1. Structure of the Diiron Active Site of the Hox State of the
[FeFe]-Hydrogenasesa
a
X is CH2, NH, or O.
Figure 1. Redox properties of diiron(I) dithiolates (CH2Cl2 soln). Unless
indicated otherwise, couples are reversible as judged from the finding that
ipc/ipa ∼ 1.
has been demonstrated to exist in the absence of the protein,
efforts can be expected to focus on the factors that govern
this geometry and its reactivity.7,8 In this work, we probe
the effects of both the dithiolate and especially the terminal
ligands on the electronic and molecular structure of the mixed
valence state.
Modeling of the [FeFe]-hydrogenases started with the
preparation of [Fe2(SR)2(CN)2(CO)4]2-,9 this anion being
notable because it features five of six known ligands in the
active site. The redox and protonation of this dicyanide has
been developed only to a limited extent.10 Such studies are
complicated by the reactivity inherent in the cyanide ligand,
which is known to bridge to other metals and to protonate
readily.11 The modeling efforts have advanced significantly
by using tertiary phosphine ligands in the place of the
cyanides. In this paper, we return briefly to the redox
chemistry of the diiron centers supported by a cyanide ligand.
Figure 2. IR spectra (CH2Cl2 solns) of [Fe2(S2C3H4)(CO)4(dppv)]BF4
([2pdt]BF4, A) and [Fe2(S2C3H6)(CO)3(PMe3)(dppv)]BF4 ([3pdt]BF4, B).
Results
was confirmed on a preparative scale by treating a CH2Cl2
solution of 2pdt with FcBF4 at -45 °C. The IR spectrum
of the product (νCO ) 2076, 2019, 1943 cm-1) proved to
be similar to that for the recently reported5 [Fe2(S2C2H4)-
(CO)3(PMe3)(dppv)]BF4 but shifted to higher energy by
∼50 cm-1, reflecting the effect of replacing PMe3 by CO
(Figure 2). Solutions of [2pdt]+ were found to react with
NO to yield [Fe2(S2C3H6)(CO)3(dppv)(NO)]+, with the NO
bound on the Fe(dppv) side of the molecule, consistent
with an open coordination site residing on the Fe(dppv)
subunit (eq 1).14
[Fe2(S2C3H6)(CO)4(dppv)]+. The cyclic voltammetry of
the recently reported complexes12 Fe2(S2CnH2n)(CO)4(dppv)
established that the propanedithiolate (2pdt) undergoes
oxidation at about 190 mV less positive than the ethanedithi-
olato analogue (2edt) (Figure 1). The redox potentials for
these couples were unchanged when the oxidations were
conducted under argon, dihydrogen, and dinitrogen; traces
of water also had no effect.
From the cyclic voltammetry (CV) results, the complex
Fe2(S2C3H6)(CO)4(dppv), 2pdt, was shown to undergo a
one-electron oxidation at a potential lower than the Fc0/+
couple (0.5 V in CH2Cl2 vs Ag/AgCl).13 This conversion
(7) Thomas, C. M.; Darensbourg, M. Y.; Hall, M. B. J. Inorg. Biochem.
2007, 101, 1752–1757.
(8) Justice, A. K.; Nilges, M.; De Gioia, L.; Rauchfuss, T. B.; Wilson,
S. R.; Zampella, G. J. Am. Chem. Soc. 2008, 130, 5293–5301.
(9) (a) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem.
Soc. 1999, 121, 9736–9737. (b) Lyon, E. J.; Georgakaki, I. P.;
Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 1999,
38, 3178–3180. (c) Le Cloirec, A.; Davies, S. C.; Evans, D. J.; Hughes,
D. L.; Pickett, C. J.; Best, S. P.; Borg, S. Chem Commun. 1999, 2285–
2286.
Oxidation of Fe2(S2C3H6)(CO)3(PMe3)(dppv). In a pre-
liminary report, we had described the one-electron oxidation
of the ethanedithiolate Fe2(S2C2H4)(CO)3(PMe3)(dppv)
(3edt).5 Oxidation of the related propanedithiolate (3pdt)
using FcBF4 yielded the corresponding derivative [Fe2(S2-
C3H6)(CO)3(PMe3)(dppv)]BF4 ([3pdt]BF4) but at a potential
that is milder by ∼180 mV. We verified the electrochemical
results with a preparative scale reaction. When a 1:1 mixture
(10) (a) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710–9711. (b)
Razavet, M.; Borg, S. J.; George, S. J.; Best, S. P.; Fairhurst, S. A.;
Pickett, C. J. Chem. Commun. 2002, 700–701.
(11) Boyke, C. A.; van der Vlugt, J. I.; Rauchfuss, T. B.; Wilson, S. R.;
Zampella, G.; De Gioia, L. J. Am. Chem. Soc. 2005, 127, 11010–
11018.
(12) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B.; van der
Vlugt, J. I.; Wilson, S. R. Inorg. Chem. 2007, 46, 1655–1664.
(13) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877–922.
(14) Olsen, M. T., Justice, A. K., Rauchfuss, T. B., Wilson, S. R., in
preparation.
7406 Inorganic Chemistry, Vol. 47, No. 16, 2008

Models for the ActiWe Site of the [FeFe]-Hydrogenase
Scheme 2. Pathways Proposed for the Carbonylations of [3edt]+ and [3pdt]+
of 3pdt and 3edt was treated with 1 equiv of FcBF4 (0.5
equiv per diiron), we observed complete consumption of
3pdt as indicated by the loss of the high energy νCO band,
uniquely assigned to the S2C3H6 derivative. Only upon the
addition of a second equiv of FcBF4 did 3edt undergo
oxidation. The structure of [3pdt]BF4 was confirmed crys-
tallographically (see below). The IR spectrum (Figure 2) of
[3pdt]BF4 salt was very similar to that for [2edt]BF4, the
greatest difference being for the νµCO band, which occurred
at 7 cm-1 higher energy for the S2C3H6 derivative.
A solution of [3pdt]+ was found to react with CO to yield
the adduct [3pdtCO]BF4. Intermediates were not observed.
The rate of carbonylation was slower (t1/2 ∼ 600 s at 1 atm,
-45 °C) than the carbonylation of [2edt]+ (t1/2 ∼ 30 s at 1
atm, -45 °C). The slowness of the carbonylation is attributed
to the required, sluggish relocation of the PMe3 ligand to a
basal position, apparently driven by avoidance of a steric
clash with the propanedithiolate strap. For diiron dithiolates
bearing seven terminal ligands, it is known that the pro-
panedithiolates (vs ethanedithiolate) tend to be less stable,
which is attributable to steric congestion.8,15 In contrast,
mixed-valence complexes with a propanedithiolate are more
stable than the ethanedithiolate analogues.
4edt was unremarkable, and the position of the νCO bands
(νCO(avg) ∼ 1927 cm-1) indicated that the complex resembles
3edt. Unexpectedly, the CV data (+140 mV vs Ag/AgCl)
showed that 4edt was more difficult to oxidize than the
analogous PMe3 derivative, which provided the first clue that
the bulk of the trialkylphosphine ligands might strongly affect
the electronic structure of these diiron compounds.
Oxidation of 4edt was effected in the usual way using
FcBF4 at -45 °C in CH2Cl2. IR measurements showed that
the oxidation shifts νCO by an average of ∼50 cm-1, as seen
for the corresponding oxidations of 2pdt, 3edt, and 3pdt.
The average of the terminal νCO bands was, however, lower
than that observed for [3edt]+, and the pattern was distinctly
different, indicating that the structures of [3edt]+ and [4edt]+
differ. These cations also differ in terms of their stabilities:
solutions of [3edt]+ are stable for a few seconds at room
temperature, whereas the decomposition of [4edt]+ is already
apparent near -20 °C. For reasons of preparative conven-
ience, further studies on bulky phosphines were conducted
with Pi-Pr3 instead of PCy3 (next section).
[Fe2(S2CnH2n)(CO)3(Pi-Pr3)(dppv)]0/+. Using the methods
to obtain [4edt]+, the complexes Fe2(xdt)(CO)3(Pi-Pr3)(dppv)
were prepared for both xdt ) edt and pdt. The IR spectra
for the trisubstituted diiron(I) complexes (4edt, 3pdt, 5edt,
and 5pdt) were very similar. The 31P DNMR properties for
this series differed, however. The DNMR properties of 3edt
and 3pdt have been previously discussed.12 At 20 °C, the
31P NMR spectrum of 5edt featured both a broad and a sharp
singlet assigned, respectively, to the dppv ligand and Pi-
Pr3. At -40 °C, the dppv signal decoalesced into a pair of
doublets, whereas the singlet assigned to Pi-Pr3 remained
sharp. The low barrier for dynamics at the Fe(CO)(dppv)
site is consistent, indirectly, with the location of the Pi-Pr3
in a basal site, which would destabilize the apical-basal
spanning dppv because of a steric interaction across the two
basal sites. For 5pdt, the 20 °C 31P NMR spectrum consisted
of two singlets in a 2:1 ratio, again assigned to the dppv
and Pi-Pr3ligands, respectively. The structures of 5edt and
The stereochemistry of [3pdtCO]BF4 can be assigned on
the basis of its solution EPR spectrum, wherein we observed
a 1:4:4:1 quartet resulting from three 31P hyperfine couplings
associated with three basal phosphine ligands. This pattern
CO
is consistent with the previously published analysis of Hox
models (see below).8 In addition to its slow formation,
[3pdtCO]+ differs from [3edtCO]+ with respect to νµCO
,
which is 1791 cm-1 in the propanedithiolate and 1783 cm-1
in the ethanedithiolate. The terminal νCO bands are also
shifted for [3pdtCO]+ but in the opposite fashion, reflecting
a compensating relationship between νµCO and νtCO. The
binding of CO is a reversible process: an N2 purge slowly
regenerated [3pdt]+. The EPR spectrum for [3edtCO]+ (see
below) differed from that for [3pdtCO]+, being consistent
with apical PMe3 and dibasal dppv (Scheme 2).
[Fe2(S2C2H4)(CO)3(PCy3)(dppv)]0/+. Unlike the analogous
PMe3 derivative, the tricyclohexylphosphine complex
Fe2(S2C2H4)(CO)3(PCy3)(dppv) (4edt) could not be prepared
by thermally induced substitution of 2edt. Photochemical
substitution proved efficient however. The spectroscopy for
(15) Boyke, C. A.; Rauchfuss, T. B.; Wilson, S. R.; Rohmer, M.-M.;
Be´nard, M. J. Am. Chem. Soc. 2004, 126, 15151–15160.
Inorganic Chemistry, Vol. 47, No. 16, 2008 7407

Justice et al.
Table 1. Selected EPR Parameters for [Fe2(S2CxH2x)(CO)3L(dppv)]BF4
(Spectra Recorded at 110 K (-163 °C), in 1:1 CH2Cl2:Toluene)
complex (donor ligands)
[2pdt]+ (dppv)
gz
Azz
gy
Ayy
gx
Axx
2.1490 78, 78 2.0199 71, 71 2.0086 72, 72
2.1384 80, 80 2.0280 74, 74 2.0102 72, 72
[3edt]+ (dppv, PMe3)
[3pdt]+a (dppv, PMe3) 2.1310 82, 82 2.0223 76, 76 2.0085 74, 74
2.1410 81, 81 2.0268 73, 73 2.0064 75, 75
[4edt]+ (dppv, PCy3)
[5edt]+ (dppv, Pi-Pr3)
[5pdt]+ (dppv, Pi-Pr3)
a For [3pdt]+ the two species are present in a 2:1 ratio.
2.0958
2.0968
2.1046
69 2.0423
70 2.0437
80 2.0268
67 2.0098 73
71 2.0105 75
73 2.0149 80
Figure 3. IR spectra (absorbance mode, CH2Cl2 at -45 °C) showing the
effects of edt vs pdt for the oxidation of complexes containing bulky
phosphine ligands: [Fe2(S2C2H4)(CO)3(Pi-Pr3)(dppv)]BF4 ([5edt]BF4, top)
and [Fe2(S2C3H6)(CO)3(Pi-Pr3)(dppv)]BF4 ([5pdt]BF4, bottom).
Scheme 3. Proposed Solution Structures of [5edt]+ and [5pdt]+
Figure 4. X-Band EPR spectra (110 K, 1:1 CH2Cl2:toluene frozen solution):
[Fe2(S2C3H6)(CO)4(dppv)]+ ([2pdt]+, (A), [Fe2(S2C3H6)(CO)3(PMe3)-
(dppv)]+ ([3pdt]+, (B), [Fe2(S2C3H6)(CO)3(Pi-Pr3)(dppv)]+ ([5pdt]+, (C).
5pdt, which were verified crystallographically (see Support-
of the two νtCO bands is 20 cm-1 lower for [5edt]+ vs [3edt]+.
This difference is consistent with the presence of a
FeII(CO)2(PMe3) versus a FeI(CO)2(Pi-Pr3) site: the larger
phosphine redirects the site of oxidation. A similar pattern
applies also to the [3pdt]+/[5pdt]+ pair. Solutions of [5pdt]+
were found to be stable at 0 °C for several minutes, whereas
[5edt]+ was found to rapidly decompose above -20 °C.
Solid [5pdt]BF4 was rapidly precipitated by transferring a
CH2Cl2 solution at -45 °C into a large excess of hexane
cooled to -78 °C. Although we were unable to isolate an
analytically pure sample, the IR spectrum of finely powdered
ing Information) are as follows:
The cyclic voltammetry of CH2Cl2 solutions of 5edt and
5pdt showed that both undergo one-electron oxidations. The
separation, Epdt - Eedt (160 mV), is comparable to the
difference observed for 3edt versus 3pdt, but the absolute
values of the redox couples are strikingly different. Complex
5edt was found to oxidize at +170 mV, and 5pdt at +10
mV (vs Ag/AgCl). These potentials are about 150 mV more
anodic than seen for related 3edt and 3pdt (Figure 1). One-
electron oxidation of 5edt with FcBF4 gave the mixed-
valence salt [Fe2(S2C2H4)(CO)3(Pi-Pr3)(dppv)]BF4, [5edt]
BF4. IR spectra in the νCO region are virtually identical for
[5edt]+ and the PCy3-containing cation [4edt]+. One-electron
oxidation of 5pdt at -45 °C using FcBF4 afforded [5pdt]
BF4. Values for νt-CO for [5pdt]+ and [5edt]+ match closely
(Figure 3). A significant difference is, however, observed
for νµ-CO, which is 1830 cm-1 for [5pdt]+ vs 1870 cm-1 for
[5edt]+, consistent with a steric interaction between the
propanedithiolate and the bulky Pi-Pr3 ligand, which dis-
places the bridging CO toward a more symmetrically
bridging position, as anticipated by the DFT calculations (see
below, Scheme 3, for explanation).
[5pdt]BF4 (KBr pellet) matched the solution data (∆νµCO
∼
3 cm-1).
Solution EPR Studies of Mixed-Valence Models. The
X-band EPR spectra of frozen solutions were similar,
showing axial patterns with well defined hyperfine coupling
to phosphorus (Table 1). Thus, a frozen solution of [2pdt]BF4
showed triplets with A(31P) ∼ 75 MHz (Figure 4). For the
mononuclear Fe(I) complex trans-[Fe(CO)3(PPh3)2]+, the
hyperfine values are about 55 MHz.16 The magnitude and
multiplicity of the hyperfine couplings indicate that the spin
resides on the Fe(CO)(dppv) center wherein the two phos-
phine sites are equivalent. The EPR spectrum of the
analogous salt [3edt]BF4 also showed triplets with compa-
rable g-values and hyperfine constants (Figure 5).
An interesting feature of the EPR spectrum of [3pdt]+ is
the presence of pairs of separate features in a ∼2:1 ratio,
(16) (a) MacNeil, J. H.; Chiverton, A. C.; Fortier, S.; Baird, M. C.; Hynes,
R. C.; Williams, A. J.; Preston, K. F.; Ziegler, T. J. Am. Chem. Soc.
1991, 113, 9834–9842. (b) Baker, P. K.; Connelly, N. G.; Jones,
B. M. R.; Maher, J. P.; Somers, K. R. J. Chem. Soc., Dalton Trans.
1980, 579–585, and references therein.
Complex [5edt]+ was found to carbonylate more slowly
than [3edt]+ (∼30 min vs ∼30 s at -45 °C). The average
7408 Inorganic Chemistry, Vol. 47, No. 16, 2008

Models for the ActiWe Site of the [FeFe]-Hydrogenase
Table 2. Selected EPR Parameters for [Fe2(S2CxH2x)(µ-CO)
a
(CO)3L(dppv)]BF4
complex
gz
A3
gy
A2
gx
A1
[3edtCO]+ 1.9943 323, 39, 42 1.9740 263, 28, 30 2.0073 258, 30, 33
[3pdtCO]+ 1.9976 30, 40, 36 1.9814 24, 30, 26 2.0131 24, 31, 26
[5edtCO]+ 1.9939 313, 36, 33 1.9750 254, 30, 28 2.0075 255, 27, 27
a Spectra recorded at 110 K (-163 °C), in 1:1 CH2Cl2:toluene.
stable than the propanedithiolate analogues. Frozen solution
EPR spectra of the mixed-valence Pi-Pr3 and PCy3 complexes
exhibit patterns with doublet features indicating that the
SOMO is localized on the Fe(CO)2(PR3) center, in contrast
to the behavior of the PMe3 complexes (Figure 4). The EPR
data for [4edt]+, [5edt]+, and [5pdt]+ are very similar with
respect to g and A values (Table 1). The A(31P) values for
these doublets are similar to the previously observed A values
for the triplets seen for [3edt]+ and [3pdt]+. Upon warming
a sample of [5pdt]+ to -40 °C, whereupon the solvent melts,
the signal merges to give an isotropic doublet, consistent
with the SOMO being localized on the Fe(CO)2(Pi-Pr3)
center.
Figure 5. X-Band EPR spectra (110K, 1:1 CH2Cl2: toluene frozen solution)
of [Fe2(S2C2H4)(CO)3(PMe3)(dppv)]BF4 ([3edt]BF4). Top is the simulation
and bottom is experimental data.
especially discernible in the lower field signal. Each set of
axial signals has a pattern (both g and A values) similar to
that for [3edt]+. The two isomers are proposed to differ with
respect to the fold of the propanedithiolate. The sensitivity
of the EPR spectrum to the orientation of the dithiolate
backbone shows that the singly occupied molecular orbital
(SOMO) of [3pdt]+ is affected by the central atom of the
dithiolate (eq 2). Upon warming the solution to -50 °C, only
a single isotropic triplet signal is observed.
Crystallographic Characterization of [Fe2(S2C3H6)(CO)3-
(PR3)(dppv)]BF4 (R ) Me, i-Pr). Crystals of these salts
were grown at low temperatures. The [3pdt]+ salt is
unsolvated whereas [5pdt]BF4 crystallized with a CH2Cl2
in the lattice; for both complexes the BF4- is well separated
from the cationic diiron centers (Figure 6 and Table 3). The
cations adopt the expected structure wherein the terminal
ligands on the quasi-octahedral Fe centers are staggered. The
Fe(CO)(dppv) adopts the “rotated” geometry. The CHsFe(1)
distance is 2.49 Å in [3pdt]BF4, respectively beyond the limit
recognized for an agostic interaction.17 The greater steric
bulk of the S2C3H6 versus S2C2H4 is manifested in the
rotation of the pair of phenyl groups that surround the vacant
coordination site. That angle increases as follows: 4°
([3edt]BF4), 14° ([3pdt]BF4), and 33° ([5pdt]BF4), reflecting
a slightly increased tilting of the dithiolate toward the rotated
Fe (the angle of the axial R3P-Fe-S, [3pdt]+ ) 105.11
(15)° and [5pdt]+ ) 107.93°), although the CHsFe(1)
distance does not contract further.
Small but significant differences are observed between the
Fe-P distances for [3pdt]BF4 and [5pdt]BF4. Specifically,
the Fe-Pi-Pr3 distance is 2.2830(15) Å, which is 0.06 Å
longer than the Fe-PMe3 distance in [3pdt]BF4. The
Fe-P(dppv) distances are the same in both [3pdt]BF4 and
[5pdt]BF4. This elongated Fe-P distance is consistent with
steric congestion at this site.
Because the EPR and the crystallographic data for [5pdt]-
BF4 do not agree, we considered the possibility that the solution
structure differs from that observed in the single crystals. Indeed,
an IR spectrum of a solid sample (KBr) prepared from single
crystals differ distinctly from spectra for [5pdt]+, both as a
solution and as a rapidly precipitated solid (Figure 7). Signifi-
cantly, the IR band at 1958 cm-1 for the rapidly precipitated
solid is absent in the spectrum of the crystals. Furthermore, the
νµCO band at 1825 cm-1 in the rapidly precipitated material is
A similar equilibrium was not observed for other pro-
panedithiolate derivatives, which we attribute to either (i)
greater “steric asymmetry”, which strongly favors the tilting
of the central methylene toward the open site on the rotated
iron center ([5pdt]+) or (ii) insensitivity of the SOMO to
the asymmetry because of the presence of CO ligands in
both apical positions ([3pdtCO]+, see below).
The stereochemistry of the CO adduct, [3pdtCO]BF4, was
indicated by EPR spectroscopy. For such HoxCO models, we
recently showed that phosphine ligands in the apical positions
give rise to large 31P NMR hyperfine coupling (A > 200
MHz), whereas hyperfine coupling is much smaller (<50
MHz) for phosphine ligands attached to the basal sites.8 Thus,
the EPR spectrum of [3edtCO]+ (fluid solution) is character-
ized by one large A-value (>250 MHz) and two smaller ones
(42, 28 MHz). A very different spectrum was obtained for
[3pdtCO]+ in fluid solution: the A(31P) values were quite
small (27, 39, 28 MHz; see Supporting Information),
indicative that all three phosphine ligands occupy basal
positions. Upon binding of CO, the PMe3 undergoes a change
in geometry, from apical in [3pdt]+ to basal in [3pdtCO]+
(Scheme 2). In general, reflecting steric crowding, the CO
adducts of ethanedithiolate derivatives are significantly more
(17) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci.
U.S.A. 2007, 104, 6908–6914.
Inorganic Chemistry, Vol. 47, No. 16, 2008 7409

Justice et al.
Table 3. Selected Structural Parameters for [Fe2(S2C3H6)(CO)3L(dppv)]BF4 (L ) PMe3, Pi-Pr3)
distances (Å)
L ) Pi-Pr3
L ) PMe3
angles (deg)
L ) Pi-Pr3
L ) PMe3
Fe(1)-Fe(2)
Fe(1)-S(1)
Fe(2)-S(1)
Fe(1)-C(1)
Fe(2)-C(1)
Fe(1)-P(1)
Fe(2)-C(2)
Fe(2)-P(2)
C(1)-O(1)
C(2)-O(2)
2.5824(10)
2.2745(10)
2.2544(11)
1.785(5)
2.833
2.2387(11)
1.774(4)
2.2830(15)
1.147(6)
1.139(5)
2.5768(8)
2.2730(8)
2.2460(8)
1.786(4)
2.678
2.2369(8)
1.787(3)
2.2273(15)
1.148(5)
1.142(4)
Fe(1)-S(1)-Fe(2)
Fe(1)-Fe(2)-S(1)
Fe(2)-Fe(1)-S(1)
Fe(2)-Fe(1)-C(1)
Fe(2)-Fe(1)-P(1)
P(1)-Fe(1)-C(1)
Fe(1)-Fe(2)-C(2)
Fe(1)-Fe(2)-P(2)
P(2)-Fe(2)-C(2)
C(2)-Fe(2)-C(2A)
Fe(1)-C(1)-O(1)
Fe(2)-C(2)-O(2)
69.53(3)
55.60(3)
54.87(3)
78.55(16)
136.39(3)
92.15(12)
103.25(13)
154.09(5)
94.63(13)
91.9(3)
69.53(3)
55.73(2)
54.74(2)
73.20(13)
135.44(2)
93.86(10)
108.92(10)
147.79(5)
94.37(18)
93.0(2)
173.0(5)
175.8(4)
171.2(4)
177.8(3)
-
Figure 6. Structure of the cations in [3pdt]BF4 and [5pdt]BF4 with thermal ellipsoids set at 35% level. Phenyl ellipsoids, hydrogen atoms, and the BF4
were omitted for clarity.
(S2CnH2n)(CO)3(PMe3)(dppv)]+ (corresponding to [3xdt]+),
and [Fe2(S2CnH2n)(CO)3(Pi-Pr3)(dppv)]+ (corresponding to
[5xdt]+). In each family, both edt and pdt complexes have
been considered. Hereafter, the Fe atoms coordinated by one
and two phosphorus ligands will be referred to as Fecc and
Fec, respectively.
For [Fe2(S2CnH2n)(CO)3(PH3)3]+, the lowest energy isomer
(see Supporting Information) is characterized by PH3 ligands
in basal sites, and the Fecc(unrotated)-µ-C(O) distance (2.438
Å) is long. Analogous results are obtained for the
[Fe2(S2CnH2n)(CO)3(PMe3)(dppv)]+ family (see Supporting
Information), indicating that the replacement of PH3 ligands
with dppv and PMe3 does not strongly affect the properties of
Figure 7. IR spectra (KBr) of [5pdt]BF4 that was rapidly precipitated (A)
and of single crystals grown at -20 °C (B).
the binuclear cluster. In both families ((PH3)3 and dppv-PMe3),
the energy difference between the most stable isomer and the
not present in the crystalline material. When the single crystals
other isomeric forms is greater for pdt than edt because of
were redissolved in CH2Cl2 (-45 °C), the solution IR spectrum
increased steric clashing between apical P ligands and pdt. This
matched that for a solution of [5pdt]+. As a control, we
situation is reversed when PMe3 is replaced by Pi-Pr3, where
demonstrated that the similarity of the IR spectra for solid (KBr)
the calculations suggest that rotation of Fecc and Fec are nearly
and solutions of [3pdt]BF4, ∆νCO ) -3 cm-1 (see Supporting
Information). It was not practical to compare the spectra of
solution versus solid [5edt]BF4, which is far less stable than
[5pdt]BF4.
isoenergetic (∆E ) 0.8 kcal/mol, Scheme 4).
In the lowest-energy [5xdt]+ isomer, the apical vacant
coordination site is located on Fecc. Moreover, relative to
the corresponding (PH3)3 and (PMe3)(dppv) cases, the µ-CO
group is more symmetrically bridging. This finding is
consistent with our IR data that show that νµCO is 1830 cm-1
for [5pdt]+ versus 1889 cm-1 for [3pdt]+. The relative
stability of the isomers is strongly influenced by the bulkiness
DFT Calculations. Structures, relative stabilities, and
electronic properties of different mixed valence models were
studied by BP86/TZVP DFT calculations. Three families of
model complexes, differing in the size of phosphine ligands,
have been analyzed: [Fe2(S2CnH2n)(CO)3(PH3)3]+, [Fe2-
7410 Inorganic Chemistry, Vol. 47, No. 16, 2008

Models for the ActiWe Site of the [FeFe]-Hydrogenase
Scheme 4. Computed Relative Stabilities (kcal/mol), Spin Densities, and NBO Partial Atomic Charges (in Italics) for Relevant Isomers of
[Fe2(S2CnH2n)(CO)3(Pi-Pr3)(dppv)]+
of the monophosphine ligand. Steric clashing is minimized
when Pi-Pr3 occupies a basal position trans from the basal
end of the dppv chelate ring (Scheme 4 and Figure 8).
Our analysis of the electronic properties of [Fe2(S2CnH2n)-
(CO)3(PH3)3]+, [Fe2(S2CnH2n)(CO)3(PMe3)(dppv)]+, and [Fe2-
(S2CnH2n)(CO)3(Pi-Pr3)(dppv)]+ reveals that the unpaired
electron is always localized on the iron atom featuring a
vacant apical coordination position, that is, Fec in
[Fe2(S2CnH2n)(CO)3(PH3)3]+ and [Fe2(S2CnH2n)(CO)3(PMe3)-
(dppv)]+, and Fecc in [Fe2(S2CnH2n)(CO)3(Pi-Pr3)(dppv)]+.
This picture is consistent with previous DFT studies carried
out on models,7 as well as on the H-cluster.18,19 Recent
pulsed EPR analysis of the Hox state of the enzyme indicates
that the unpaired electron is more localized on the nonrotated
(proximal) iron.20 This apparent discrepancy between theo-
retical/model and biophysical data may reflect subtle effects
due to the H-cluster environment.18 Our oxidation state
assignments agree with earlier calculations by Brunold on
the entire 6Fe H-cluster,19 as well as the results from Hall
et al. on a related model containing a carbene ligand on the
rotated iron site.7
[Fe2(S2C2H4)(CN)(CO)3(dppv)], Bu4N[6edt].12 This anionic
complex was expected to be more easily oxidized than the
related PMe3 derivative 3edt and thus a potential precursor
to a mixed-valence cyanide. Cyclic voltammetry indicated
that this species oxidizes at -130 mV, 150 mV more
cathodic than the [3edt]0/+ couple. Unlike the voltammogram
for the PMe3 derivative, however, the oxidation of the
cyanide was electrochemically irreversible, it rereduces at
-600 mV (∆Ep ∼ 500 mV). The i-V response is reminiscent
of an EC process, whereby oxidation induces an irreversible
chemical reaction. The CV remained unchanged even at scan
rates up to 500 mV/s.
In a preparative-scale oxidation, treatment of a CH2Cl2
solution of Bu4N[6edt] with 1 equiv of FcBF4 afforded a
thermally stable, toluene-soluble complex that proved to be
diamagnetic. This species is assigned as a four-iron compound
[6edt]2. Unlike the behavior of related phosphine complexes
discussed above and elsewhere,5,8 this oxidation was unaffected
by the presence of CO, indicating the absence of long-lived
unsaturated species. The corresponding two-electron oxidation
of Bu4N[6edt] in MeCN solution gives Cs-symmetric
[Fe2(S2C2H4)(µ-CO)(CO)2(CN)(NCMe)(dppv)]+.12
Notably, the analysis of computed partial atomic charges
for [Fe2(S2CnH2n)(CO)3(PH3)3]+, [Fe2(S2CnH2n)(CO)3(PMe3)-
(dppv)]+, and [Fe2(S2CnH2n)(CO)3(Pi-Pr3)(dppv)]+ shows that
the more electrophilic metal center always corresponds to
the Fe atom featuring a vacant apical coordination position
(Scheme 4). Therefore, in this class of compounds, the
assignment of the Fe(I) oxidation state to the iron center
carrying the unpaired electron should be taken as purely
formal.
The 31P NMR spectrum of [6edt]2 consisted of a broad
singlet at δ 95 and a sharp singlet at δ 74.5. The low field
signal is characteristic of dppv dynamically spanning apical
and basal sites on diiron(I) species. A similar structure is
Oxidation of [Fe2(S2C2H4)(CN)(CO)3(dppv)]-. We ex-
amined the oxidation of the cyanide complex Bu4N-
(18) To probe the possibility that the spin could reside on the non-rotated
site, we have carried out time-dependent DFT calculations on
[Fe2(S2CnH2n)(CO)3(PH3)3]+, evaluating the possible presence of low-
lying excited states in which the unpaired electron is not localized on
the Fe atom featuring vacant apical coordination site. We found that
the lowest-energy excited state is about 23 kcal/mol higher in energy
than the ground state. More importantly, in the excited state the spin
density distribution was largely unaffected.
Figure 8. DFT structure of the lowest energy isomer of [Fe2(S2C3H6)(CO)3
(Pi-Pr3)(dppv)]+ ([5pdt]+). Atoms are color coded as follows: carbon, green;
oxygen, red; sulfur, yellow; phosphorus, purple; iron, cyan.
(19) Fiedler, A. T.; Brunold, T. C. Inorg. Chem. 2005, 44, 9322–9334.
(20) Silakov, A.; Reijerse, E. J.; Alfracht, S. P. J.; Hatchikian, E. C.; Lubitz,
W. J. Am. Chem. Soc. 2007, 129, 11447–11458.
Inorganic Chemistry, Vol. 47, No. 16, 2008 7411

Justice et al.
Scheme 5. Redox Reactivity Proposed for [Fe2(S2C2H4)(CN)(CO)3(dppv)]-
assumed for Fe2(S2C2H4)(CO)4(dppv), which exhibits a single
31P NMR resonance at δ 94.7.12 The sharp higher field signal
is consistent with dppv on dibasal sites on diiron(II)
derivatives, for example, [Fe2(S2C2H4)(µ-CO)(CO)2(PMe3)-
(NCMe)(dppv)](BF4)2 (δ 74.6).12 Upon cooling the solution,
each of these two signals splits. The lower field signal splits
more, consistent with the presence of both apical and basal
phosphine ligands on a diiron(I) center, whereas the splitting
of the higher field signal is small (2 ppm) because the
electronic asymmetry is modest, consistent with this dppv
being dibasal. The IR spectrum of this charge-neutral
complex resembles the sum of the spectra of [Fe2(S2C2H4)(µ-
CO)(CO)2(PMe3)(MeCN)(dppv)]2+ (2058 and 2016 cm-1)
and the starting complex (1952 and 1901 cm-1). The 1952
and 1901 cm-1 bands assigned to the Fe(I)-Fe(I) center
occur at ∼10 cm-1 higher energy than in Bu4N[6edt],
consistent with the attachment of an electrophile to the FeCN.
These data indicate the formulation [FeI2(S2C2H4)(CO)3-
lography shows, that the Fe(CO)(dppv) remains rotated in
[5pdt]BF4, but we suggest that the FeI(CO)2(Pi-Pr3) site is
rotated in solution. DFT calculations indicate that the bulkier
phosphine slightly favors rotation at the monophosphine site.
Pi-Pr3 and PMe3 have comparable pKa’s of ∼9.7 (for PCy3,
which we assume is similar to Pi-Pr3) and 8.65 (PMe3,
compare 2.73 for PPh3).20 Pi-Pr3 is, however, much larger
than PMe3 with a cone angle of 160° (vs 118°).21The
influence of PMe3 versus Pi-Pr3 on the regiochemistry of
the oxidation is also supported by the IR data as well as
the electrochemical results. In solution, [5pdt]+ resembles
the active site to the extent that it features two donor
ligands and one CO on the “proximal” Fe center and one
donor ligand and two CO’s on the “distal” (rotated) Fe
center (eq 3).
(dppv)](µ-CN)[FeII (S2C2H4)(µ-CO)(CN)(CO)2(dppv)] (Sch-
2
eme 5). Further characterization of the charge-neutral product
via mass spectroscopy was unsuccessful.
Discussion
For the sake of completeness, we note that the EPR results
qualitatively support two distinct descriptions, L3FeII-
(SR)2FeIL3, which we favor,5,7,22 and L3FeIII(SR)2Fe0L3,
which has not been discussed. The 0,III description would
be consistent with oxidation occurring at the more electron-
rich iron center and implies that oxidation and rotation induce
a redox change at both iron centers, that is, L3FeI(SR)2FeIL3
f L3Fe0(SR)2FeIIIL3. “Dative metal-metal bonds” have been
invoked to describe compounds with such disparate oxidation
states.23 Precedents for ferric carbonyls include [Cp2Fe2-
(SMe)2(CO)2]2+ (νCO ∼ 2060 cm-1)24 and the dithiocarbam-
ate [(C5Me5)Fe(S2CNEt2)(CO)]+.25 The oxidation state as-
signments will be further probed through studies on the
Mo¨ssbauer spectra of these compounds.
Oxidation of diiron dithiolato carbonyls produces highly
unsymmetrical derivatives featuring a single “rotated” iron
center and a semibridging CO ligand. Relief of steric strain
influences the regiochemistry of the oxidation: the bulkier
Fe center undergoes rotation. The rotation desymmetrizes
the diiron center, localizing the mixed valency as indicated
by EPR spectra. Addition of CO to the mixed valence species
resymmetrizes the diiron center, leading to a more delocal-
ized mixed-valence system.8 The central feature of these
models is that the iron center that undergoes the redox-
induced rearrangement remains in the Fe(I) oxidation state,
whereas the iron center that undergoes oxidation remains in
the same geometry and becomes coordinatively saturated.
The square pyramidal iron(I) center is unsaturated and
electrophilic.
Effects of Coligands on Mixed Valency. The EPR spectra
of [2pdt]+, [3edt]+, and [3pdt]+ are very similar as are the
crystal structures for the latter two. The rotated nature of
the Fe(CO)(dppv) center was crystallographically verified
for both [3pdt]+ and [3edt]+, and we assume that a similar
structure applies to the corresponding tetracarbonyl [2pdt]+.
In face of this pattern, the EPR results are strikingly different
when PMe3 is replaced by Pi-Pr3 and PCy3. Specifically, the
EPR results indicate that the spin is located on the
Fe(CO)2PR3 center in these bulkier complexes. The crystal-
(21) Tolman, C. A. Chem. ReV. 1977, 77, 313–348.
(22) Silakov, A.; Reijerse, E. J.; Albracht, S. P. J.; Hatchikian, E. C.; Lubitz,
W. J. Am. Chem. Soc. 2007, 129, 11447–11458.
(23) (a) Jiang, F.; Jenkins, H. A.; Biradha, K.; Davis, H. B.; Pomeroy,
R. K.; Zaworotko, M. J. Organometallics 2000, 19, 5049–5062. (b)
Jiang, F.; Biradha, K.; Leong, W. K.; Pomeroy, R. K.; Zaworotko,
M. J. Can. J. Chem. 1999, 77, 1327–1335.
(24) (a) de Beer, J. A.; Haines, R. J.; Greatrex, R.; van Wyk, J. A. J. Chem.
Soc., Dalton Trans. 1973, 2341. (b) Vergamini, P. J.; Kubas, G. J.
Prog. Inorg. Chem. 1976, 21, 261–282. (c) Madec, P.; Muir, K. W.;
Pe´tillon, F. Y.; Rumin, R.; Scaon, Y.; Schollhammer, P.; Talarmin, J.
J. Chem. Soc., Dalton Trans. 1999, 2371–2384.
(25) Delville-Desbois, M.-H.; Mross, S.; Astruc, D.; Linares, J.; Varret,
F.; Rabaaˆ, H.; Le Beuze, A.; Saillard, J.-Y.; Culp, R. D.; Atwood,
D. A.; Cowley, A. H. J. Am. Chem. Soc. 1996, 118, 4133–4147.
7412 Inorganic Chemistry, Vol. 47, No. 16, 2008

Models for the ActiWe Site of the [FeFe]-Hydrogenase
Table 4. Details of Data Collection and Structure Refinement for
Site Isolation. For low molecular weight models, a
recurring question concerns the extent to which the synthetic
models can or need to emulate the steric shielding provided
by the protein. The issue is pertinent because the H-cluster
is deeply imbedded in both of the structurally characterized
proteins.26 The present results highlight continuing problems
that hamper the preparation of unsaturated models featuring
cyanide ligands. Specifically, we showed that oxidation of a
diiron cyanide leads to aggregation via formation of a µ-CN
bridge. This conversion, a disproportionation, is explicable
in terms of an inner sphere mechanism facilitated by the
ambidentate nature of cyanide. For the present generation
of models, however, tertiary phosphine ligands serve as
useful surrogates for cyanide, with the advantages that their
electronic and steric properties can be manipulated as we
showed in this paper.
Outlook. The chemistry of mixed-valence models for the
diiron active site of the [FeFe]-hydrogenases has evolved
such that many details can be evaluated. For example, the
nature of the dithiolate as well as the basicity and size of
the coligands significantly influence the mixed valency and
affect the reactivity of these species toward CO. These results
will guide further efforts to understand how substituents
affect the interaction of diiron dithiolato centers toward small
molecules, including H2.
Crystallography
complex
[3pdtBF4]
[5pdtBF4]
chemical formula
temperature (K)
crystal size (mm3)
crystal system
space group
a (Å)
C35H37BF4Fe2O3P3S2 C41H49BF4Fe2O3P3S2
193(2)
193(2)
0.28 × 0.12 × 0.06
orthorhombic
Pnma
0.31 × 0.08 × 0.08
monoclinic
P21/m
23.5362(7)
18.3985(5)
8.5950(2)
90
9.2644(7)
16.1182(12)
16.1141(12)
90
b (Å)
c (Å)
R (deg)
ꢁ (deg)
90
90
105.462(2)
90
γ (deg)
V (Å3)
3721.90(17)
4
2319.2(3)
2
Z
density calcd (Mg m-3
)
1.537
1.475
µ(Mo KR)(mm-1
max./min. transn
)
0.71073
0.9422/0.8116
47014/3695
0.71073
0.9323/0.7729
27790/4544
4544/503/471
1.069
reflections measd/indep
data/restraints/parameters 3695/261/339
GOF on F2
1.083
Rint
0.0623
0.0380 (0.0552)
0.0431
R1 [I > 2σ] (all data)a
0.0505 (0.0642)
0.1328 (0.1414)
1.036/-0.548
2
wR2 [I > 2σ] (all data)b 0.0904 (0.1003)
max. peak/hole (e-/Å3)
0.715/-0.322
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {[w(|Fo| - |Fc|)2]/∑[wFo ]}1/2, where
w ) 1/σ2(Fo).
mM CuSO4 in a 20% glycerol solution was recorded at the same
power, modulation amplitude, and temperature as that used for the
diiron samples. In this way, the spin concentration of three samples,
nominally 1 mM, of [3pdt]BF4 were found to be 0.60, 0.60, and
0.70 mM.
Experimental Section
Materials and methods (EPR and IR spectroscopy and cyclic
voltammetry) have recently been described.8 [CpFe(C5H4Ac)]BF4
was prepared by literature methods.13 The diiron complexes
Fe2(S2C2H4)(CO)4(dppv), Fe2(S2C3H6)(CO)4(dppv), Fe2(S2C3H6)-
(CO)3(PMe3)(dppv), Et4N[Fe2(S2C2H4)(CN)(CO)3(dppv)] have been
previously described.12 NMR spectra were recorded at room
temperature on a Varian Mercury 500 MHz spectrometer (202 MHz
for 31P).
X-ray Crystallography. Crystals were mounted to a thin glass
fiber using Paratone-N oil (Exxon). Data, collected at 198 K on a
Siemens CCD diffractometer, were filtered to remove statistical
outliers. The integration software (SAINT) was used to test for
crystal decay as a bilinear function of X-ray exposure time and
sin(Θ). The data were solved using SHELXTL by Direct Methods
(Table 4); atomic positions were deduced from an E map or by an
unweighted difference Fourier synthesis. H atom U’s were assigned
as 1.2 Ueq for adjacent C atoms. Non-H atoms were refined
anisotropically. Successful convergence of the full-matrix least-
squares refinement of F2 was indicated by the maximum shift/error
for the final cycle.
DFT Calculations. DFT calculations were carried out using the
BP86 functional27 and a valence triple-ꢀ basis set with polarization
on all atoms (TZVP).28
[Fe2(S2C3H6)(CO)4(dppv)]BF4 ([2pdt]BF4). A solution of 0.250
g (0.344 mmol) of Fe2(S2C3H6)(CO)4(dppv) in 15 mL of CH2Cl2
at -45 °C was treated with a solution of 0.094 g (0.344 mmol) of
FcBF4 in 10 mL of CH2Cl2. The purple solution was rapidly
transferred into 400 mL of hexane cooled to -78 °C. The
supernatant solvent was siphoned away from the finely divided
solid. Yield: 0.152 g (54%). IR (CH2Cl2): νCO ) 2076, 2020, 1943
cm-1. IR (solid KBr): νCO ) 2075, 2012, 1934, 1906 (sh) cm-1
.
[Fe2(S2C3H6)(CO)3(PMe3)(dppv)]BF4 ([3pdt]BF4). A solution
of 0.200 g (0.258 mmol) of Fe2(S2C3H6)(CO)3(PMe3)(dppv) in 10
mL of CH2Cl2 at -45 °C was treated with a solution of 0.070 g
(0.258 mmol) of FcBF4 in 10 mL of CH2Cl2. The purple solution
was transferred into 400 mL of hexane at -78 °C. The supernatant
was removed by cannula. Crystals were grown by diffusion of
hexane into a CH2Cl2 solution at -20 °C. Yield: 0.171 g (70%).
IR (CH2Cl2): νCO ) 2015, 1962, 1889 cm-1. IR (solid KBr): νCO
) 2007, 1950, 1886 cm-1
.
EPR Spectroscopy. The following procedure is illustrative: a
solution of 0.010 g (0.0116 mmol) of Fe2(S2C3H6)(CO)3(Pi-
Pr3)(dppv) in 6 mL of CH2Cl2 (-45 °C) was treated with a solution
of 0.003 g (0.0112 mmol) of FcBF4 in 6 mL of CH2Cl2. The
resulting solution (1 mM) was transferred via cannula to a chilled
(-78 °C) EPR tube that had been purged with N2. The EPR tube
was cooled with liquid nitrogen and flame-sealed under vacuum.
Spin concentrations were determined by double integrating
baseline-corrected spectra. For calibration, a sample containing 1
When CH2Cl2 solutions of [3pdt]BF4 were exposed to CO (1
atm at -45 °C), the in situ IR spectrum showed the disappearance
of [3pdt]BF4 and the growth of new peaks, attributed to [3pdt-
CO]BF4. Because of the instability of [3pdtCO]BF4, all data were
collected in situ. IR (CH2Cl2): νCO ) 2018, 1984, 1791 cm-1
.
Conversion of [3pdt]+ to [3pdtCO]+ using ∼1 atm CO required
∼10 min.
Fe2(S2C2H4)(CO)3(PCy3)(dppv) (4edt). A solution of 0.300 g
(0.421 mmol) of Fe2(S2C2H4)(CO)4(dppv) in 40 mL of toluene was
(26) (a) Pandey, A. S.; Harris, T. V.; Giles, L. J.; Peters, J. W.; Szilagyi,
R. K. J. Am. Chem. Soc. 2008, 130, 4533–4540. (b) Nicolet, Y.;
Lemon, B. J.; Fontecilla-Camps, J. C.; Peters, J. W. Trends Biochem.
Sci. 2000, 25, 138–143.
(27) (a) Becke, A. D. J. Chem. Phys. 1986, 84, 4524–4529. (b) Perdew,
J. P. Phys. ReV. 1986, B33, 8882.
(28) Schaefer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829–
5835.
Inorganic Chemistry, Vol. 47, No. 16, 2008 7413

Justice et al.
treated with 0.130 g (0.421 mmol) of PCy3 in 10 mL of toluene.
The reaction mixture was photolyzed with a 100 W UV immersion
lamp, λmax ) 356 nm (Spectroline), until the conversion was
complete (∼36 h) as indicated by IR spectroscopy. The solvent
was removed in vacuum, and the product extracted into 20 mL of
CH2Cl2. The solution was concentrated to ∼5 mL, and the red-
brown product precipitated upon addition of 40 mL of hexane.
FcBF4 in 3 mL of CH2Cl2. Because of the instability of [5edt]BF4,
all data were collected in situ. IR (CH2Cl2): νCO ) 1987 (m), 1968
(s), 1870 (br) cm-1
.
When CH2Cl2 solutions of [5edt]BF4 were exposed to CO (1
atm at -45 °C). The in situ IR spectrum showed the disappear-
ance of [5edt]BF4 and the growth of new peaks, attributed to
[Fe2(S2C3H6)(µ-CO)(CO)3(Pi-Pr3)(dppv)]BF4, [5edtCO]BF4. Be-
cause of the instability of [5edtCO]BF4, all data were collected
in situ. IR (CH2Cl2): νCO ) 2022, 1987, 1968, 1787 cm-1. The
reaction of CO with [5edt]+ to form [5edtCO]+ required ∼30
min.
[Fe2(S2C3H6)(CO)3(Pi-Pr3)(dppv)]BF4 ([5pdt]BF4). A solution
of 0.150 g (0.175 mmol) of Fe2(S2C3H6)(CO)3(Pi-Pr3)(dppv) in 20
mL of CH2Cl2 at -45 °C was treated with 0.047 mg (0.175 mmol)
of FcBF4 in 10 mL of CH2Cl2. The solution was transferred via
cannula to 400 mL of hexane cooled to -78 °C to precipitate the
red-brown solid. Single crystals were grown by diffusion of a layer
1
Yield: 0.352 g (86%). H NMR (CD2Cl2): δ 7.98-7.29 (m, 20H,
C6H5), 2.01-0.89 (m, 70H, P(C6H22)3, SCH2CH2S). 31P NMR
(CD2Cl2): δ 93.96 (bs), 62.53 (s) at 20 °C. IR (CH2Cl2): νCO
)
1953, 1901 cm-1. FD-MS: m/z 964.4 ([Fe2(S2C2H4)(CO)3-
(PCy3)(dppv)]+). Anal. Calcd for C49H59Fe2O3P3S2 (Found): C,
61.00 (61.38); H, 6.16 (6.50).
[Fe2(S2C2H4)(CO)3(PCy3)(dppv)]BF4 ([4pdt]BF4). A solution
of 0.050 g (0.052 mmol) of Fe2(S2C2H4)(CO)3(PCy3)(dppv) in 5
mL of CH2Cl2 at -45 °C was treated with 14 mg (0.052 mmol) of
FcBF4 in 3 mL of CH2Cl2. IR (CH2Cl2): νCO ) 1987 (m), 1961
of hexane into a CH2Cl2 solution at -20 °C. IR (CH2Cl2): νCO
)
(s), 1870 (br) cm-1
.
2011 (sm), 1988 (m), 1968 (s), 1830 (br) cm-1. IR of rapidly
precipitated solid (KBr): νCO ) 1998 (m), 1958 (s), 1904 (sm),
1824 (br) cm-1. IR of pulverized single crystals (KBr): νCO ) 1990
Fe2(S2C2H4)(CO)3(Pi-Pr3)(dppv) (5edt). A solution of 0.300 g
(0.421 mmol) of Fe2(S2C2H4)(CO)4(dppv) in 40 mL of toluene was
treated with 0.27 mL (2.105 mmol) of Pi-Pr3. The reaction mixture
was photolyzed until the conversion was complete (∼24 h) as
indicated by IR spectra. The solvent was removed in Vacuo, and
the product was extracted into ∼20 mL of CH2Cl2. The solution
was concentrated to ∼5 mL, and the red-brown product was
precipitated upon addition of 40 mL of hexanes. Yield: 0.268 g
(75%). 1H NMR (CD2Cl2): δ 7.98-7.27 (m, 20H, C6H5), 2.00 (m,
3H, P(CH)Me2), 1.56 (m, 2H, SCH2), 1.26 (m, 18H, PCH(CH3)2),
1.18 (s, 2H, SCH2). 31P NMR (CD2Cl2): δ 92.96 (bs), 71.74 (s) at
20 °C. 95.9 (d, JP-P ) 25 Hz), 91.6 (d, JP-P ) 25 Hz), 71.1 (s) at
-60 °C. IR (CH2Cl2): νCO ) 1954, 1903 cm-1. FD-MS: m/z 844.4
(s), 1931 (s), 1903 (m) cm-1
.
[Fe2(S2C2H4)(CN)(CO)3(dppv)]2 ([6edt]2). A solution of 0.050
g (0.0525 mmol) of Bu4N[Fe2(S2C2H4)(CN)(CO)3(dppv)] in 5 mL
of CH2Cl2 was treated with 0.014 g (0.0525 mmol) of FcBF4 in
5.0 mL of CH2Cl2. Solvent was removed in vacuo, and the product
was extracted into 20 mL of toluene. The orange solution was
concentrated to 5 mL, and the product precipitated upon addition
of 30 mL of hexanes. 31P NMR (CD2Cl2, 20 °C): δ 92.96 (bs),
71.74 (s). 31P NMR (CD2Cl2, -60 °C): 99.1 (d, JP-P ) 21 Hz),
92.7 (d, JP-P ) 21 Hz), 75.9 (s), 74.0 (s). IR (CH2Cl2): νCN
2136, 2116 cm-1. IR (CH2Cl2): νCO ) 2074 (sh), 2058 (s), 2016
(m), 1975 (m), 1952 (s), 1901 (s) cm-1
)
([Fe2(S2C2H4)(CO)3(Pi-Pr3)(dppv)]+).
C40H47Fe2O3P3S2 (Found): C, 56.89 (56.97); H, 5.61 (5.45).
Anal.
Calcd
for
.
Acknowledgment. This research was supported by the
NIH.
Fe2(S2C3H6)(CO)3(Pi-Pr3)(dppv) (5pdt). See the preceding
preparation. Yield: 71%. 1H NMR (CD2Cl2): δ 8.00-7.30 (m, 20H,
C6H5), 2.38 (m, 3H, PCHMe2), 1.67 (s, 2H, SCH2), 1.55 (s, 2H,
SCH2CH2CH2S), 1.37 (m, 18H, PCH(CH3)2), 1.25 (s, 2H, SCH2).
31P NMR (CD2Cl2, 20 °C): δ 90.69 (s), 72.94 (s). At -80 °C, the
Supporting Information Available: Crystallographic data for
[Fe2(S2C3H6)(CO)3(PMe3)(dppv)]BF4 and [Fe2(S2C3H6)(CO)3(Pi-
Pr3)(dppv)]BF4, voltammograms, as well as IR, EPR, and NMR
spectra. For DFT calculations: atomic coordinates, energy values,
spin populations, and partial atomic charges for the DFT models
[Fe2(S2CnH2n)(CO)3(PH3)3]+, [Fe2(S2CnH2n)(CO)3(PMe3)(dppv)]+,
and [Fe2(S2CnH2n)(CO)3(Pi-Pr3)(dppv)]+. This material is available
free of charge via the Internet at http://pubs.acs.org.
δ 90.69 signal is broad. IR (CH2Cl2): νCO ) 1954, 1901 cm-1
.
FD-MS: m/z 858.3 ([Fe2(S2C3H6)(CO)3(Pi-Pr3)(dppv)]+). Anal.
Calcd for C41H49Fe2O3P3S2 (Found): C, 57.36 (57.01); H, 5.75
(5.69).
[Fe2(S2C2H4)(CO)3(Pi-Pr3)(dppv)]BF4 ([5edt]BF4). A solution
of 0.050 g (0.052 mmol) of Fe2(S2C2H4)(CO)3(Pi-Pr3)(dppv) in 5
mL of CH2Cl2 at -45 °C was treated with 14 mg (0.052 mmol) of
IC8007552
7414 Inorganic Chemistry, Vol. 47, No. 16, 2008