Isomerization of the hydride complexes [HFe2(SR) 2(PR3)x(CO)6-x]+ (x = 2, 3, 4) relevant to the active site models for the [FeFe]-hydrogenases

Article abstract of DOI:10.1039/b910147k

The stepwise formation of bridging (μ-) hydrides of diiron dithiolates is discussed with attention on the pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations occurring at a single Fe center, followed by isomerization to a series of μ-hydrides. Protonation of Fe₂(edt)(CO)₄(dppv) (1) gave a singleμ-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe₂(pdt)(CO)₄(dppv) (2) followed an isomerization pathway similar to that for [1H]⁺, except that a pair of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO) ₃ or Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine Fe₂(edt)(CO) ₃(PMe₃)(dppv) (3) was a single μ-hydride wherein PMe₃ is apical and the dppv ligand spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a mixture of three other isomeric hydrides. Protonation of Fe₂(pdt)(CO) ₃(PMe₃)(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 °C). Low-temperature protonation of the bis-chelates Fe₂(xdt)(CO)₂(dppv)₂ produced exclusively the terminal hydrides [HFe₂(xdt)(μ-CO)(CO)(dppv)₂] ⁺ (xdt = edt and pdt), which subsequently isomerized to a pair of μ-hydrides. At room temperature these (dppv)₂ derivatives convert to an equilibrium of two isomers, one C₂-symmetric and the other C_s-symmetric. The stability of the terminal hydrides correlates with the (C₂-isomer)/(C_s-isomer) equilibrium ratio, which reflects the size of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent polarity, and the presence of D₂O. This isomerization mechanism is proposed to be intramolecular, involving a 120° rotation of the HFeL₃ subunit to an unobserved terminal basal hydride as the rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which also highlight the instability of the basal terminal hydrides. Isomerization of the μ-hydride isomers occurs on alternating FeL₃via 120° rotations without generating D₂O-exchangeable intermediates.

Full text of DOI:10.1039/b910147k

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Isomerization of the hydride complexes [HFe2(SR)2(PR3)x(CO)6-x]+
(x = 2, 3, 4) relevant to the active site models for the [FeFe]-hydrogenases†
Bryan E. Barton,a Giuseppe Zampella,b Aaron K. Justice,a Luca De Gioia,*b Thomas B. Rauchfuss*a and
Scott R. Wilsona
Received 22nd May 2009, Accepted 13th August 2009
First published as an Advance Article on the web 16th September 2009
DOI: 10.1039/b910147k
The stepwise formation of bridging (m-) hydrides of diiron dithiolates is discussed with attention on the
pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations
occurring at a single Fe center, followed by isomerization to a series of m-hydrides. Protonation of
Fe2(edt)(CO)4(dppv) (1) gave a single m-hydride with dppv spanning apical and basal sites, which
isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of
Fe2(pdt)(CO)4(dppv) (2) followed an isomerization pathway similar to that for [1H]+, except that a pair
of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO)3 or
Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine
Fe2(edt)(CO)3(PMe3)(dppv) (3) was a single m-hydride wherein PMe3 is apical and the dppv ligand
spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a
mixture of three other isomeric hydrides. Protonation of Fe2(pdt)(CO)3(PMe3)(dppv) (4) proceeded
similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very
low temperatures (-90 ◦C). Low-temperature protonation of the bis-chelates Fe2(xdt)(CO)2(dppv)2
produced exclusively the terminal hydrides [HFe2(xdt)(m-CO)(CO)(dppv)2]+ (xdt = edt and pdt), which
subsequently isomerized to a pair of m-hydrides. At room temperature these (dppv)2 derivatives convert
to an equilibrium of two isomers, one C2-symmetric and the other Cs-symmetric. The stability of the
terminal hydrides correlates with the (C2-isomer)/(Cs-isomer) equilibrium ratio, which reflects the size
of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent
polarity, and the presence of D2O. This isomerization mechanism is proposed to be intramolecular,
involving a 120◦ rotation of the HFeL3 subunit to an unobserved terminal basal hydride as the
rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which
also highlight the instability of the basal terminal hydrides. Isomerization of the m-hydride isomers
occurs on alternating FeL3 via 120◦ rotations without generating D2O-exchangeable intermediates.
The interchange of bridging and terminal hydrides in clusters
is well established (e.g. for H2Os3(CO)10(PPh3)).3 Some hydrides
Introduction
are known to exist as isomers (Scheme 1).4 As demonstrated
in the current report, some diiron dithiolate hydrides protonate
at a single metal followed by intramolecular isomerization to a
bridging site. The present results suggest that protonation of more
complex metal clusters, which invariably affords m-hydrides, could
occur at a single metal followed by rapid transfer of the hydride to
a bridging position.
The protonation of di- and polymetallic compounds almost
invariably results in products containing m-hydride ligands.1 The
kinetic barriers for protonation are determined by reorganiza-
tional energies and are expected to be lower for protonation
at main group vs transition metal centers.2 Consistent with this
view, low temperature studies on the protonation of anionic metal
carbonyls indicate that the carbonyl oxygen is often the kinetic
site of protonation followed by a slower transfer of the proton to
the metal.3 Polymetallic complexes, in principle, contain multiple
possible sites of protonation, each with their own kinetic and
thermodynamic preferences.
aDepartment of Chemistry, University of Illinois at Urbana-Champaign,
Urbana, IL, 61801. E-mail: Rauchfuz@uiuc.edu
Scheme 1 Two isomers of HFe(CO)3(m-PR2)Pt(PR’3)2.
bDepartment of Biotechnology and Bioscience, University of Milano-
Bicocca, Piazza della Scienza 1 20126, Milan, (Italy). E-mail: luca.
degioia@unimib.it
The structure of dimetallic hydrides is relevant to the properties
of the two major hydrogenase enzymes, the [FeFe]- and the [NiFe]-
hydrogenases.5 The [NiFe]-hydrogenases are proposed to feature
m-hydrides in catalytically significant states, whereas in the [FeFe]-
hydrogenases, substrate binding and turnover are assumed to be
† Electronic supplementary information (ESI) available: Experimental
details. CCDC reference number 733498. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b910147k
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 3011–3019 | 3011
©

localized at one apical site of the [2Fe]H cluster (Fig. 1). We have
proposed that the regiochemistry of the hydride ligands in these
active sites determines the predominant physiological function of
the enzyme—the [NiFe] centers are optimal for H2 oxidation and
the [FeFe] centers are primarily reducers of protons.6 The (kinetic)
reactivity of hydride ligands is well known to be affected by
their geometry:7 bridging hydrides are more acidic than terminal
hydrides. A parallel pattern has long been noted for the boron
hydrides.8
affords three isomeric m-hydrides, which upon warming to room
temperature equilibrate fully to a single isomer.18
In this report, we describe and analyze protonations of a series
of substituted diiron(I) dithiolates. Reactions were monitored
at low temperature by both 1H and 31P NMR spectroscopies,
techniques that usually provide definitive structural assignments,
allowing us to deduce reaction sequences. Our results point to a
general mechanism for the protonation of electron-rich diiron(I)
dithiolates, one that begins with terminal protonation.
Results and discussion
Isomers of [HFe2(edt)(CO)4(dppv)]BF4 and
[HFe2(pdt)(CO)4(dppv)]BF4
To provide reference spectra for relatively simple systems, we
first examined the protonation of dichloromethane solutions
of Fe2(edt)(CO)4(dppv) (1) and Fe2(pdt)(CO)4(dppv) (2) with
HBF4·Et2O. The structures of the resulting hydrides can be
deduced from the coupling patterns since in diiron dithiolato
Fig. 1 Structures and reactivity of the [FeFe]- and [NiFe]-hydrogenases
with proposed oxidation states.
hydrides trans JP-hydride coupling constants are smaller (JPH
5 Hz) than the corresponding cis coupling constants (JPH
~
>
Almost all hydrides derived from protonation of diiron(I)
dithiolato complexes are bridging. The only crystallographically
characterized terminal hydride [HFe2(edt)(CO)2(PMe3)4]BF4 was
found to isomerize unimolecularly to the m-hydride isomer at room
temperature with a half-life of minutes.† The regiochemistry has
significant implications for catalysis since the terminal hydrides
are more susceptible to protonolysis9 and more readily reduced10
than the m-hydride isomers. In light of the differing reactivity for
the different binding modes, it may be useful to understand the
factors leading to the preferential formation of terminal hydride
species and the pathway for conversion from terminal- to m-H
species.
It has long been known that diiron(I) dithiolates proto-
nate to give m-hydride derivatives. Early studies by Poilblanc
led to the phosphine-substituted complexes such as [Fe2(m-
H)(SMe)2(CO)4(PPhMe2)2]+.11 After these compounds were found
to resemble the active sites of the [FeFe]-hydrogenases,12 analogous
protonations were described for the ethane- and propanedithiolato
diiron complexes.12–14 The catalytic properties of the derived m-
hydrido complexes have been examined.12,15
25 Hz).19 When protonations of 1 and 2 were monitored at
lower temperature, we observed intermediates. In the case of 1,
the low temperature 31P NMR spectrum featured two signals,
1
corresponding to the H NMR signal at d -16.1 (dd, JPH = 26,
6 Hz). This pattern is consistent with dppv spanning apical and
basal sites (Scheme 2). Upon warming to room temperature, this
hydride quantitatively converted to a new isomer with a single 31
P
=
1
NMR signal at d 85 and H NMR signal at d -14.7 (t, JPH
21 Hz).
Upon protonation of 2, the resulting hydride isomerized by the
same sequence as seen for 1. However, under the same conditions
as for the protonation of 1, terminal hydrides were observed upon
protonation of 2 at low temperatures. The formation of a pair
of isomeric hydrides was indicated by characteristically20–22 low
field signals at d -4.4 (s) and d -3.3 (t, JPH = 70 Hz). The
predominant (~92%) product, [2H-Term]+ is characterized by
31P NMR signals at d 97 and 71, consistent with a terminal
hydride on the Fe(CO)3 subunit and an adjacent Fe(CO)(dppv)
subunit with an apical–basal dppv. The minor (~8%) product,
[2H-Term*]+, is an isomeric terminal hydride isomer characterized
by a single 31P NMR signal at d 89, consistent with a terminal-
apical hydride on the Fe(CO)(dppv) subunit wherein the dppv
is dibasal. Isomerization of the major isomer commences at a
convenient rate around -30 ◦C, and upon warming the sample
to room temperature, a single m-hydride, [2H-A]+, is formed with
The diiron(I) dithiolates are fluxional species.16,17 For substi-
tuted derivatives, the various rotamers could exhibit differing
basicities. Recently, the low temperature protonation of several
diiron(I) dithiolates has revealed the intermediacy of terminal
hydrides, which subsequently form the more stable m-hydrides.
Examination of the protonation of Fe2(pdt)(CO)4(PMe3)2 by low
temperature NMR spectroscopy reveals no terminal hydrides but
1
31P NMR signals at d 95 and 92 and H NMR signal at d -14.4
Scheme 2 Course of protonation of 2, Fe2(pdt)(CO)4(dppv).
3012 | Dalton Trans., 2010, 39, 3011–3019 This journal is
The Royal Society of Chemistry 2010
©

Scheme 3 Isomerization pathway for [HFe2(pdt)(CO)3(dppv)(PMe3)]+, [4H]+, and [HFe2(edt)(CO)3(dppv)(PMe3)]+, [3H]+, and their equilibrium isomer
distribution.
(dd JPH = 25, 5 Hz). The spectrum is consistent with an apical–
basal dppv (see Scheme 2, Fig. S3†), identical to the initially
observed m-hydride for the edt example. As in the edt case, the
sample further isomerized at room temperature to give exclusively
[2H-B]+, a dibasal dppv isomer with a hydride signal at d -14.8
(t, JPH = 21 Hz). Ezzaher et al. had observed a similar sequence
of reactions for the protonation of Fe2(pdt)(CO)4(chel) (chel =
dmpe18 (1,2-C2H4(PMe2)2 and dppe21 (1,2-C2H4(PPh2)2)—these
terminal hydrides convert to the apical–basal then the dibasal
isomer of the m-hydride. The isomerization pathway for the minor
isomer of the terminal hydride could not be studied because of
inadequate signal/noise ratio.
(room temperature) solution was found to contain 75% [3H-B]+,
20% [3H-C]+, and 5% [3H-D]+ (Scheme 3).
In a preparative-scale study, 3 was protonated with HCl followed
by ion exchange with PF6- to give the thermally and air-stable salt.
From an equilibrated solution of [3H]PF6, we grew crystals of
exclusively [3H-B]PF6. Crystallographic analysis confirmed the
assignment of the dppv orientation as apical–basal and trans-
basal PMe3 (Fig. 2). Fresh solutions of these crystals contained
only [3H-B]+. Upon allowing its solutions to stand, this species
isomerized to the equilibrium mixture of [3H-B]+, [3H-C]+, and
[3H-D]+, as discussed above.
Protonation of Fe2(edt)(CO)3(PMe3)(dppv) and isomerization of
the resulting hydrides
Protonation of room temperature solutions of Fe2(edt)(CO)3-
(PMe3)(dppv) (3)23 by HBF4·Et2O was observed to yield a mixture
of three isomeric m-hydrides, as indicated by three multiplets in
1
the high field region of the H NMR spectrum.13,24 When this
protonation was conducted at -90 ◦C, only a single m-hydride was
obtained, [3H-A]BF4. In this species, the dppv ligand spans apical
and basal sites as indicated by 31P NMR signals at d 21, 91, 96.23
The 1H NMR spectrum displayed a doublet of triplets at d -16.9
(JPH1 = 23, JPH2 ~ JPH3 = 3 Hz) (Fig. S6, S7†). This pattern is
consistent with the dppv ligand spanning apical (JPH ~3 Hz) and
basal positions (JPH ~23 Hz). Upon warming the sample to 20 ◦C,
[3H-A]+ was found to convert to [3H-B]+. For this isomer, 1H and
31P NMR data indicated that the dppv remains apical–basal, but
the PMe3 has shifted to a basal site. Over the course of ~20 h at
room temperature, two further isomerizations occur giving rise to
[3H-C]+ and [3H-D]+. Patterns for 31P and 1H NMR (high-field)
spectra also allowed us to uniquely identify the stereochemistry of
these species. In [3H-C]+, the PMe3 was found to remain basal, but
the dppv shifted to a dibasal orientation. In [3H-D]+, the PMe3
returns to an apical position to give [3H-D]+. The equilibrated
Fig. 2 Structure of the cation in [Fe2(edt)(m-H)(CO)3(dppv)(PMe3)]PF6
([3H-B]+) with thermal ellipsoids drawn at the 50% probability level,
phenyl hydrogen atoms omitted for clarity. Key distances (A): Fe–Fe,
2.5744 (5); Fe(1)–S(1), 2.2581 (7); Fe(1)–S(2), 2.2794 (6); Fe(2)–S(1), 2.2668
(7); Fe(2)–S(2), 2.2760 (6); Fe(1)–P(1), 2.2137 (6); Fe(1)–P(2), 2.2282 (6);
Fe(1)–H(1), 1.69 (2); Fe(2)–H(1), 1.64 (2).
˚
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 3011–3019 | 3013
©

Protonation of Fe2(pdt)(CO)3(PMe3)(dppv) and isomerization of
the resulting hydrides
turnstile rotations of the HFe(CO)2(PMe3) center, placing the
PMe3 either trans-basal ([4H-B]+) or cis-basal ([4H-B*]+). Upon
standing at -30 ◦C, the [4H-B]+/[4H-B*]+ mixture was found to
convert entirely to [4H-B]+. We also observed low concentrations
of an analogous pair of isomers for the ethanedithiolate (i.e.,
[3H-B]+ and [3H-B*]+). At room temperature, [4H-B]+ was found
to isomerize to [4H-C]+ and [4H-D]+. The equilibrium ratio for
[4H]BF4 was 71% [4H-C]+, 18% [4H-B]+ (0% [4H-B*]+), 11%
[4H-D]+. The propanedithiolate favors the isomer wherein all
phosphines are basal ([4H-C]+), whereas for the edt compound,
the major isomer features basal PMe3 but apical–basal dppv
([3H-B]+).
As in the protonation of 2, low temperature protonation of
Fe2(pdt)(CO)3(PMe3)(dppv)23 (4) with HBF4·Et2O allowed the
observation of kinetic terminal hydrides. At -90 ◦C, the 1H NMR
spectrum displayed signals at both d -3.2 and -4.0, a chemical shift
range that is again diagnostic of terminal hydrides (Fig. S4†).20,21
Both signals were doublets (JPH ~ 70 Hz), consistent with pro-
tonation at the Fe(CO)2(PMe3) center, and cis orientation of the
terminal-apical hydride and PMe3. The presence of two doublets
is attributed to the relative orientation of the propanedithiolate
strap, which is known to refold slowly at low temperatures
(Scheme 4).25 Alternatively, the data do not exclude the possibility
that the two doublets indicate the presence of two stereoisomers
arising from either cis or trans basal phosphine ligands (as in
[3H-B]+ and [3H-B*]+, see below). 1H◦NMR spectra indicate that
[4H-Term]+ isomerizes quickly at -90 C (it isomerizes faster than
the tetracarbonyl [2H-Term]+). The initial m-hydride, [4H-A]+, is
stereochemically analogous to the edt compound [3H-A]+, having
apical–basal dppv and apical PMe3.
The kinetics of the transformation of [4H-B]+ to [4H-C]+ and
1
[4H-D]+ were measured by H NMR spectroscopy. The linear
dependence of ln([4H-B]) vs time (Fig. 4) indicated that the
isomerization is first-order. The rate constant of isomerizati◦on
of [4H-B]+ to [4H-C]+ was quantified as 1.1 ¥ 10-5 s-1 at 23 C
(t1/2 ~ 17 h). The rate was unaffected by 1 atm. H2. When the
isomerization of [4H-B]+ was conducted under an atmosphere of
D2, the hydride ligand did not exchange.
Scheme
4
Possible terminal hydride isomers observed during the
low-temperature protonation of 4
1H NMR spectra indicated that [4H-A]+ isomerizes at -30 ◦C to
afford approximately equal amounts of two isomeric m-hydrides,
labeled [4H-B]+ and [4H-B*]+ (Fig. 3). 1H NMR spectra indicate
that both isomers feature two basal and one apical phosphine and
that both also feature apical–basal dppv and basal PMe3 ligands
(the isomer with dibasal-dppv and apical PMe3 can be eliminated
since this isomer corresponds to [4H-D]+, to be described, see
Scheme 3). Two species ([4H-B]+ and [4H-B*]+) arise from the
same terminal hydride but form via clockwise and anticlockwise
Fig. 4 Plot of the isomerization of [4H-B]+ to [4H-C]+, as monitored by
1H NMR spectroscopy.
The equilibration of [4H]+ was examined by high temperature
NMR measurements. Between 80 and 120 ◦C, the signals for two
isomers ([4H-C]+ and [4H-D]+) broaden, but coalescence was not
observed. Consistent with the proposed dynamics, the two isomers
that undergo exchange are related by a single turnstile rotation.
After three days in dimethylformamide, samples of [4H]+ were
observed to degrade to a mixture of [Fe2(pdt)(m-H)(CO)2(dppv)2]+
(see below) and unidentified poly-phosphine hydrido complexes.
Protonation of Fe2(xdt)(CO)2(dppv)2 and isomerization of the
resulting hydrides (where xdt = edt, pdt)
Low-temperature protonation of Fe2(edt)(CO)2(dppv)2 pro-
duced the terminal hydride [HFe2(edt)(m-CO)(CO)(dppv)2]+,
[5H-Term]+. For protonations of the (dppv)2 derivatives, we
selected [H(Et2O)2]BArF4 to ensure precise stoichiometry, however,
replicate experiments with HBF4·Et2O provided qualitatively
similar results. At -20 ◦C, this terminal hydride was found to
rapidly isomerize to a m-hydride, [5H-A]BArF , wherein both
4
dppv ligands chelate in the apical–basal orientation. The situation
was similar for [HFe2(pdt)(CO)2(dppv)2]BArF4 ([6H-Term]BArF )
1H-NMR spectra (CD2Cl2, 500 MHz) for protonation of
4
Fig.
3
although the conversion of [6H-Term]BArF4 to [6H-A]BArF4 was
much slower. Between -10 and 20 ◦C, this isomerization was
Fe2(pdt)(CO)3(dppv)(PMe3) (4) with HBF4·Et2O and various stages in
the isomerization of the resulting hydrides.
3014 | Dalton Trans., 2010, 39, 3011–3019
This journal is
The Royal Society of Chemistry 2010
©

found to be first-order with respect to [6H-Term]+ with a half-
life of ~12 min at 23 ◦C. Qualitatively the isomerization was
found to be unaffected by the presence of excess acid. The rate
of isomerization was indistinguishable for CD3CN and CD2Cl2.
Additionally, no H/D exchange was observed when a solution of
[5H-Term]+ was allowed to isomerize in the presence of D2O at
10 ◦C. Upon warming solutions of [5H-A]+ and [6H-A]+ to room
temperature we observed the appearance of the unsymmetrical
isomers, [5H-B]+ and [6H-B]+, respectively. 1H NMR spectra
of these isomers display hydride signals—doublet-of-doublet-of-
doublet-of-doublets and four signals in the 31P NMR spectrum—
consistent with the one dppv ligand in the apical–basal orientation
and the other in the dibasal orientation.
terminal hydride isomers (by ca. 9.0 kcal/mol). DFT optimization
for the hydride complex [3H]+ indicates that the four bridging
hydrides observed experimentally are close in energy, differing
only by a few kcal/mol. The calculated stabilities for the isomeric
m-hydrides agreed quite well with the observed equilibrium values,
with [3H-B]+ being lowest in energy and [3H-C]+, [3H-D]+, and
[3H-A]+ being higher in energy by 1.2, 0.5, and 2.8 kcal/mol,
respectively (Scheme 5).
Within the manifold of terminal hydrides, the apical hydrides are
predicted to be more stable than the isomeric basal hydrides, with
the most stable apical hydride featuring a basal PMe3 and apical–
basal dppv. This observation conforms to the trend wherein the
strongly s-donating hydride ligand stabilizes the trans p-acceptor
CO ligand. In basal isomers, the hydride is trans to the S center of
the chelating edt ligand. In agreement with experimental findings,
protonation at the Fe(CO)2(PMe3) subunit is slightly favored (1.6
kcal/mol) relative to the Fe(CO)(dppv) side.
DFT investigation of the hydrides [HFe2(edt)(CO)2(PMe3)(dppv)]+
[HFe2(edt)(CO)3(PH3)3]+ and [HFe2(pdt)(CO)2(dppv)2]+
Protonation of Fe2(edt)(CO)3(PMe3)(dppv) (3) can in principle
occur at apical sites, at basal terminal sites, or at the Fe–Fe
bond (Scheme 5). Protonation at sulfur has also been analyzed
computationally.26 As expected, and in line with DFT results
obtained for other organometallic [2Fe]H model complexes,27 the
m-H isomers are predicted to be significantly more favored than the
In order to elucidate how the steric bulk of the phosphines
affects the relative stability of isomers of [3H]+, we conducted
calculations on simplified analogues in which dppv and PMe3
ligands were replaced by the sterically less demanding PH3 ligands,
i.e., [HFe2(edt)(CO)3(PH3)3]+. In general, the energetic ranking of
the isomers remains similar—m-hydrides are more stable and basal
hydrides are least stable. In these uncrowded complexes, the most
stable m-H isomer features two trans CO ligands, whereas in the
organophosphine derivatives steric effects slightly favor only one
trans CO (Scheme 6). Among the terminal hydrides, isomers with
FeH(CO)2(PH3) and FeH(CO)(PH3)2 subunits are almost equi-
energetic.
The influence of ligand bulk on the protonation regiochem-
istry is even more evident for [(dppv)(CO)Fe(pdt)Fe(dppv)(CO)]
(Scheme 7). Again, we observe the usual energetic trend of m-
hydride < apical–hydride < basal-hydride, but the gap between the
basal and apical hydrides is twice that seen for the (PMe3)(dppv)
system.
Discussion
The first suggestion of isomeric hydrides in the diiron dithiolates
came from the characterization of [Fe2(edt)(m-H)(CO)4(CN)2]-.13
1
The H NMR spectrum shows two hydride signals that are as-
signed to isomers of [(CO)2(CN)Fe(m-edt)(m-H)Fe(CO)2(CN)]-.28
Related derivatives of the type [Fe2(edt)(m-H)(CO)3L(CN)2]-
also exist as isomers (L = PPh3, CN-).28 Additionally, Scholl-
hammer and Hogarth et al. have demonstrated isomerism in
[HFe2(pdt)(CO)4(chel)]+ (chel = dmpe,18 dppe,21 phen22).29
Terminal protonation
Diiron complexes with terminal hydrides are rarely observed.30
Poilblanc proposed that protonation of the diiron dithiolates
occurs directly at the Fe–Fe bond.31 However, when these protona-
tions are monitored at low temperatures by NMR spectroscopy,
terminal hydrides are often observed, especially for complexes
containing chelating ligands.10,18,21,22 The chelating phosphine lig-
ands elevate the barrier for the conversion of the terminal hydrides
to the bridging hydrides. Even though terminal hydrides are not
observed consistently, results suggest that terminal hydrides are
almost always the kinetic product of protonation. In cases where
Scheme
5 Computed relative thermodynamic stability of relevant
[(dppv)(CO)Fe(edt)Fe(CO)2(PMe3)(H)]+ isomers, energies in kcal/mol.
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Dalton Trans., 2010, 39, 3011–3019 | 3015
©

Scheme
7
Computed relative thermodynamic stability of relevant
[(dppv)(CO)Fe(edt)Fe(CO)(dppv)(H)]+ isomers, energies in kcal/mol.
Fe2(pdt)(Cy2PCH2PCy2)(CO)4, where the diphosphine links the
two Fe centers, protonates rapidly to give the m-hydride.32 The two
isomers of Fe2(SCH2CH2PPh2)2(CO)4, protonate with retention
of geometry (eqn (1)).33 These protonations are slow at room
temperature, however, possibly reflecting the high barrier to
protonation of the Fe–Fe bond. A recent report suggests that
protonation at a Fe–Fe bond can be accelerated by ligands that
can serve as proton relays.34 In such cases however, it is difficult to
exclude relay to a single Fe center followed by rapid rearrangement
to the m-hydride isomer. In contrast, the protonation of the
complexes described in this paper occur within the time of mixing
at room temperature.‡
Scheme
6 Computed relative thermodynamic stability of relevant
[(PH3)2(CO)Fe(edt)Fe(CO)2(PH3)(H)]+ isomers, energies in kcal/mol.
the terminal hydrides have not been observed spectroscopically,—
3, and Fe2(xdt)(CO)4(PMe3)2 (xdt = adt, edt, pdt)—the initially
observed m-hydrides are explicable as arising via intramolecular
rearrangement from the analogous terminal hydrides.
Potential exceptions to the universality of terminal protonation
come from studies on the protonation of diiron dithiolates
that are constrained by special chelating ligands. The complex
‡ Although fast at room temperature, the protonation of Fe2(edt)(CO)4-
(dppv) is noticeably slower than the protonation of Fe2(pdt)(CO)4-
(dppv) at -80 ◦C.
3016 | Dalton Trans., 2010, 39, 3011–3019
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higher barrier for the isomerization of [2H-Term]+, in comparison
to [4H-Term]+, is attributed to the instability of [2H-Basal]+ where
CO is trans to m-CO. The second step in the isomerization involves
the interchange of the basal hydride ligand with the m-CO ligand.
Precedent for the conversion of the terminal to the m-hydride
comes from the pairwise exchange of terminal and m-hydrides
in species such as HOs3(m-H)(CO)10L.37
(1)
Mechanism of terminal to l-hydride isomerization
Interconversions within the l-hydride manifold
In those cases where they can be observed, terminal hydrides
do not readily deprotonate. For example, separate NMR signals
for hydrides and their conjugate bases can be resolved readily,
consistent with a relatively large barrier (> 12 kcal/mol) to
intermolecular proton transfer. Indicative of their significant
kinetic robustness is the high barrier for deprotonation. Thus, the
terminal hydride [HFe2(pdt)(CO)2(dppv)2]+ resists deprotonation
by a large excess of NEt3, which is far more basic than the
diiron center (DpKa ~ 6).35 Thus, the isomerization of the terminal
hydrides to the m-hydrides does not occur via a deprotonation–
reprotonation pathway. Consistent with this view, conversion of
[4H-Term]+ to [4H-A]+ occurs without H–D exchange with D2O.
The terminal hydrides are however unstable with respect to
conversion to the m-hydrides. The rate of this isomerization
is strongly affected by the dithiolate, the pdt derivatives be-
ing slower to isomerize than the analogous edt compounds.36
Basicity effects alone do not explain the stability trends
for the terminal hydrides as indicated by the finding that
[HFe2(pdt)(CO)3(PMe3)(dppv)]BF4is significantly less stable than
[HFe2(pdt)(CO)4(dppv)]BF4 ([2H-Term]BF4). The t◦erminal hy-
dride [4H-Term]+ is observed only fleetingly at -90 C, whereas
[2H-Term]BF4 is stable until ca. -40 ◦C.§ We suggest the first step
in the isomerization is a rate-determining 120◦ “turnstile” rotation
that shifts the terminal apical hydride to a terminal basal site. DFT
calculations predict that the basal hydrides are significantly less
stable than the apical hydrides, consistent with the fact that basal
hydrides species have so far never been observed (Scheme 8).29 The
Two or more isomers of the m-hydrides are observed in all
cases reported here. In general, these isomers exist in dynamic
equilibrium, and the overall isomer distribution is dictated by
the energetic advantage associated with CO ligands trans to the
hydride balanced against the relief of steric congestion.28 The m-
hydrides are proposed to interconvert via 120◦ turnstile rotations
of FeL3 subunits. The isomerization occurs at alternating Fe
sites. Particularly telling mechanistically is the observation of
pairs of m-H isomers in the case of the pdt-(PMe3)(dppv) system
([4H-B]+/[4H-B*]+). These rotamers contain basal PMe3 ligands
that are mutually cis and trans with respect to the apical–basal
dppv ligand. These apparent rotamers arise from the clockwise
and anticlockwise rotations of the Fe(CO)2(PMe3) center. A high
1
temperature H NMR spectrum of [4H]+ showed that a pair of
isomers related by a single 120◦ rotation of the Fe(CO)2(PMe3)
subunit ([4H-C]+ and [4H-D]+) approached coalescence. The
scission of individual Fe–m-H bonds has been proposed to occur
upon photolysis of [Fe2(pdt)(m-H)(CO)4(PMe3)2]+.14,38
Concluding remarks
The protonation of the substituted diiron dithiolato carbonyls
provides new insights into protonation of polynuclear complexes, a
broadly important theme.39 The main pathway entails protonation
at a single metal followed by isomerization to a series of m-
hydrides. As we have described recently,6 the terminal hydrides
are mechanistically related to the active site of the [FeFe]-
hydrogenases, whereas the m-hydrides are more closely related to
the active sites of the [NiFe]-hydrogenases.
Experimental
Manipulations were conducted using standard Schlenk
techniques. Solvents were filtered through activated alumina and
subsequently degassed. 1H and 31P NMR spectra were acquired on
a Unity Varian 500 or a Unity Varian 600 spectrometer. IR spectra
were collected on a Mattson Infinity Gold FTIR spectrometer.
Fe2(edt)(CO)4(dppv),
Fe2(pdt)(CO)4(dppv),
Fe2(edt)(CO)3-
Fe2(xdt)(CO)2-
(PMe3)(dppv),
Fe2(pdt)(CO)3(PMe3)(dppv),
(dppv)2,10,23 [H(Et2O)2]BArF
were prepared according
40
4
to literature procedures. HBF4·Et2O and dppv (cis-1,2-
bis(diphenylphosphino)ethylene) were purchased from Aldrich
and used as received.
Scheme 8 Schematic reaction coordinate for the isomerization of the api-
cal-terminal hydrides. Basal-terminal hydrides are not observed, indicating
their rapid conversion to the m-hydrides.
[Fe2(edt)(l-H)(CO)4(dppv)]PF6 ([1H]PF6)
A solution of 0.110 g (0.155 mmol) of Fe2(edt)(CO)4(dppv) in
10 mL of CH2Cl2 was treated with 0.20 mL (0.40 mmol) of a
2.0 M solution HCl in Et2O, the reaction flask was immediately
stoppered. After the reaction solution stirred for 3 h, solvent
§ For similar reactions and identical rates at 250 vs 200 K, the difference
in the activation energies is ca. 20%.
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was removed in vacuum. The residue, a red powder, was then re-
dissolved in 5 mL of MeOH. The red solution was then treated with
0.025 g of NH4PF6 (0.160 mmol) in 5 mL of MeOH, precipitating
the product. The red solid was collected via filter cannula and
washed with 2 ¥ 10 mL Et2O. Anal. Calcd for C32H27Fe2O4F6P3S2
(found): C, 44.78 (44.70); H, 3.17 (3.02). 1H NMR (CD2Cl2): d 7.2–
8.3 (m, 22H, P(C6H5)2; C2H2(PPh2)2); 0.7–2.9 (m, 4H, S2C2H4);
d -15 (t, 1H, Fe–H). 31P NMR (CD2Cl2): d 85. FT-IR (CH2Cl2):
2098 (s), 2052 (m), 2038 (m), 1980 (m) cm-1.
slowly, and characterized by low temperature NMR studies with
a pre-cooled NMR spectrometer probe.
Isomerization of [HFe2(pdt)(CO)3(PMe3)(dppv)]PF6, [4H]PF6
In a J-Young NMR tube dried CD2Cl2 was distilled onto a
solid mixture of ~5 mg (0.005 mmol) of [HFe2(pdt)(CO)3(PMe3)-
(dppv)]PF6 ([4H]PF6) and ~1 mg (0.006 mmol) hexamethylben-
zene (as internal standard). The frozen J-Young tube was thawed
carefully and immediately inserted into the NMR spectrometer.
The signals of interest (the PCH3 signals and the hydride signals)
were integrated relative to the internal standard over the course of
several weeks.
[Fe2(pdt)(l-H)(CO)4(dppv)]PF6 ([2H]PF6)
A solution of 0.066 g (0.091 mmol) of Fe2(pdt)(CO)4(dppv) in
10 mL of CH2Cl2 was treated with 0.2 mL (0.40 mmol) of a
2.0 M solution HCl in Et2O, the reaction flask was then stoppered.
After the reaction solution stirred for 3 h, solvent was removed
in vacuum. The residue, a red powder, was then re-dissolved in
5 mL of MeOH. The red solution was then treated with 0.015 g
of NH4PF6 (0.092 mmol) in 5 mL of MeOH, precipitating the
product. The red solid was collected via filter cannula and washed
H/D exchange from D2 + [HFe2(pdt)(CO)3(PMe3)(dppv)]PF6
In a J-Young tube and following the conditions for the isomer-
ization of [4H]PF6, D2 (~ 1.0 atmosphere) was added to the
freshly prepared and freshly frozen solution of [4H]PF6 in that
solvent. The sample was then thawed carefully, shaken (to ensure
complete and immediate saturation of solution), and immediately
inserted into the NMR spectrometer for kinetic characterization.
Experiments were conducted in both ambient light and dark, and
no measurable differences were observed.
1
with 2 ¥ 10 mL Et2O. H NMR (CD2Cl2): d 7.2–8.5 (m, 22H,
P(C6H5)2; C2H2(PPh2)2); 2.3– 3.6 (m, 6H, S2C3H6); d -14.7 (t, 1H,
Fe–H, JPH = 23 Hz). 31P NMR (CD2Cl2): d 85.6. FT-IR (CH2Cl2):
2098 (s), 2052 (m), 2038 (m), 1980 (m) cm-1.
H/D exchange from D2O + [HFe2(pdt)(CO)2(dppv)2]BArF
4
[Fe2(edt)(l-H)(CO)3(PMe3)(dppv)]PF6 ([3H]PF6)
In a J-Young NMR tube dried CD2Cl2 was distilled onto a solid
A solution of 0.125 g (0.164 mmol) of 3 in 25 mL of MeCN
was treated with 8.2 mL (0.820 mmol) of a 0.10 M solution of
12 M HCl in MeCN. After the reaction solution stirred for 3 h,
solvent was removed in vacuum. The residue, an orange powder,
was rinsed with 30 mL of Et2O and then re-dissolved in 5 mL
of MeOH. This extract was treated with a solution of 0.026 g of
NH4PF6 (0.164 mmol) in 5 mL of MeOH, and the product was
precipitated by the addition of 30 mL of H2O. The orange solid was
collected and rinsed with 15 mL each of H2O and Et2O. Crystals
were grown via slow diffusion of hexanes into a CH2Cl2 solution.
Anal. Calcd for C34H36Fe2O3F6P4S2 (found): C, 45.06 (44.89); H,
mixture of 10 mg (0.009 mmol) Fe2(pdt)(CO)2(dppv)2 and ~10 mg
(0.010 mmol) [H(Et2O)2]BArF . The frozen J-Young tube was then
4
thawed carefully at -40 ◦C (CH3CN: dry ice) and then after
complete thawing (~1 min) was frozen in liquid N2 and 1 drop
D2O was added. The sample was then evacuated on the high-vac
line and carefully thawed at -40 ◦C and immediately inserted into
the NMR spectrometer for characterization.
X-ray crystallography
Structure was phased by direct methods. Systematic conditions
suggested the ambiguous space group. The space group choice
was confirmed by successful convergence of the full-matrix least-
squares refinement on F2. The highest peaks in the final difference
Fourier map were in the vicinity of atoms F1, C9, and F4; the final
map had no other significant features. A final analysis of variance
between observed and calculated structure factors showed no
dependence on amplitude or resolution.
1
4.00 (3.93). H NMR (CD3CN): d 7.2–8.5 (m, 22H, P(C6H5)2;
C2H2(PPh2)2); 0.4–2.8 (m, 4H, S2C2H4); 1.6 (d, 9H, PMe3); d -17
(td, 1H, Fe-H). 31P NMR (CD3CN): d 92.3, 90.7 (d, t, dppv); d
22.8 (d, PMe3). FT-IR (MeCN): 2031, 1978 cm-1.
[Fe2(pdt)(l-H)(CO)3(PMe3)(dppv)]PF6 ([4H]PF6)
The sample was prepared following the procedure for [3H]PF6.
Anal. Calcd for C35H38Fe2O3F6P4S2 (found): C, 45.67 (46.20); H,
4.16 (4.22). H NMR (CD3CN): d 7.2–8.5 (m, 22H, P(C6H5)2;
DFT calculations
1
C2H2(PPh2)2); 0.4- 2.8 (m, 6H, S2C3H6); 1.6 (d, 9H, PMe3); d -15
(td, 1H, Fe-H). 31P NMR (CD3CN): d 91.6, 90.6 (t, d, dppv); d
21.6 (d, PMe3). FT-IR (MeCN): 2028, 1973 cm-1.
DFT calculations were carried out using the B-P86 functional and
a valence triple-z basis set with polarization on all atoms (TZVP),41
a level of theory which has been found to give reliable results for
analogous organometallic compounds.42 Stationary points of the
energy hypersurface have been located by means of energy gradient
techniques and full vibrational analysis has been carried out to
further characterize each stationary point. Relevant IR bands,
computed for all intermediate species, which are involved in the
mechanism of interconversion, are available as ESI.†
Procedure for protonation of 1, 2, 3, 4 at low temperature
Into a J-Young NMR tube dried CD2Cl2 was distilled onto 5 mg
diiron dithiolate. To the frozen solution ~5 mL HBF4·Et2O (6.91 M)
(0.035 mmol, 1 to 10 equiv.) was added was freeze-pump-thawed
to ensure a vacuum atmosphere. The frozen J-Young tube was
then placed directly into a dry ice/Acetone bath (-78 ◦C), thawed
All energy differences have been computed by correcting gas
phase data with the inclusion of an implicit treatment of solvent
3018 | Dalton Trans., 2010, 39, 3011–3019
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©

effect (COSMO).43 The dielectric constant value has been set to
37.5 (corresponding to acetonitrile).
21 S. Ezzaher, J.-F. Capon, F. Gloaguen, F. Y. Pe´tillon, P. Schollhammer,
J. Talarmin, R. Pichon and N. Kervarec, Inorg. Chem., 2007, 46, 3426.
22 P.-Y. Orain, J.-F. Capon, N. Kervarec, F. Gloaguen, F. Pe´tillon, R.
Pichon, P. Schollhammer and J. Talarmin, Dalton Trans., 2007, 3754.
23 A. K. Justice, G. Zampella, L. De Gioia, T. B. Rauchfuss, J. I. Van Der
Vlugt and S. R. Wilson, Inorg. Chem., 2007, 46, 1655.
24 L. Schwartz, G. Eilers, L. Eriksson, A. Gogoll, R. Lomoth and S. Ott,
Chem. Commun., 2006, 520.
25 E. J. Lyon, I. P. Georgakaki, J. H. Reibenspies and M. Y. Darensbourg,
J. Am. Chem. Soc., 2001, 123, 3268.
26 J. W. Tye, M. Y. Darensbourg and M. B. Hall, Theochem, 2006, 771,
123.
Acknowledgements
This research was supported by the U.S. National Institutes of
Health and the Petroleum Research Fund. We thank Tere´sa
Prussak-Wieckowska for assistance with the crystallographic
analyses.
27 M. Bruschi, C. Greco, P. Fantucci and L. De Gioia, Inorg. Chem., 2008,
47, 6056; M. Bruschi, C. Greco, M. Kaukonen, P. Fantucci, U. Ryde
and L. De Gioia, Angew. Chem., Int. Ed., 2009, 48, 3503.
28 C. A. Boyke, Ph.D. Thesis, University of Illinois, Urbana-Champaign,
2005.
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©