Contrasting protonation behavior of diphosphido vs dithiolato diiron(I) carbonyl complexes

Article abstract of DOI:10.1021/om300997s

This paper reports on the protonation of phosphine-substituted diiron diphosphido carbonyls, analogues of diiron dithiolato centers at the active sites of hydrogenase enzymes. Reaction of the diphosphines (CH₂) _n(PPhH)₂ (n = 2 (edpH₂) and n = 3 (pdpH ₂)) with Fe₃(CO)₁₂ gave excellent yields of Fe₂(edp)(CO)₆ (1) and Fe₂(pdp)(CO)₆ (2). Substitution of Fe₂(edp)(CO)₆ with PMe₃ afforded Fe₂(edp)(CO)₂(PMe₃)₄ (3; ν_CO 1855 and 1836 cm^-1). Crystallographic analysis showed that 3 adopts an idealized C₂ symmetry, with pairs of phosphine ligands occupying apical-basal sites on each Fe center. Relative to that in the dithiolato complex, the Fe-Fe bond (2.7786(8) A) is elongated by 0.15 A. Treatment of 3 with H(OEt₂)₂BAr ^F₄ (Ar^F = C₆H₃-3,5- (CF₃)₂) gave exclusively the C₂-symmetric μ-hydride complex [HFe₂(edp)(CO)₂(PMe₃) ₄]⁺. This result contrasts with the behavior of the analogous ethanedithiolate Fe₂(edt)(CO)₂(PMe ₃)₄ (edt = 1,2-C₂H₄S₂), protonation of which gives both the bridging and terminal hydride complexes. This difference points to the participation of the sulfur centers in the formation of terminal hydrides. The absence of terminal hydride intermediates was also revealed in the protonation of the diphosphine diphosphido complexes Fe₂(pdp)(CO)₄(dppv) (4; dppv = cis-1,2-C₂H ₂(PPh₂)₂) and Fe₂(edp)(CO) ₄(dppbz) (5; dppbz = 1,2-C₆H₄(PPh ₂)₂). Protonation of these diphosphine complexes afforded μ-hydrido cations with apical-basal diphosphine ligands, which convert to the isomer where the diphosphine is dibasal. In contrast, protonation of the dithiolato complex Fe₂(pdt)(CO)₄(dppv) gave terminal hydrides, which isomerize to μ-hydrides. In a competition experiment, 4 was shown to protonate faster than Fe₂(pdt)(CO)₄(dppv).

Full text of DOI:10.1021/om300997s

Article
pubs.acs.org/Organometallics
Contrasting Protonation Behavior of Diphosphido vs Dithiolato
Diiron(I) Carbonyl Complexes
Riccardo Zaffaroni, Thomas B. Rauchfuss,* and Amy Fuller
Department of Chemistry, University of Illinois, Urbana, Illinois 61801, United States
Luca De Gioia and Giuseppe Zampella
Department of Biotechnology and Biosciences, University of Milano-Bicocca, 20126-Milan, Italy
S
* Supporting Information
ABSTRACT: This paper reports on the protonation of phosphine-substituted diiron diphosphido carbonyls, analogues of diiron
dithiolato centers at the active sites of hydrogenase enzymes. Reaction of the diphosphines (CH2)n(PPhH)2 (n = 2 (edpH2) and
n = 3 (pdpH2)) with Fe3(CO)12 gave excellent yields of Fe2(edp)(CO)6 (1) and Fe2(pdp)(CO)6 (2). Substitution of
Fe2(edp)(CO)6 with PMe3 afforded Fe2(edp)(CO)2(PMe3)4 (3; νCO 1855 and 1836 cm−1). Crystallographic analysis showed
that 3 adopts an idealized C2 symmetry, with pairs of phosphine ligands occupying apical−basal sites on each Fe center. Relative
to that in the dithiolato complex, the Fe−Fe bond (2.7786(8) Å) is elongated by 0.15 Å. Treatment of 3 with H(OEt2)2BArF
4
(ArF = C6H3-3,5-(CF3)2) gave exclusively the C2-symmetric μ-hydride complex [HFe2(edp)(CO)2(PMe3)4]+. This result
contrasts with the behavior of the analogous ethanedithiolate Fe2(edt)(CO)2(PMe3)4 (edt = 1,2-C2H4S2), protonation of which
gives both the bridging and terminal hydride complexes. This difference points to the participation of the sulfur centers in the
formation of terminal hydrides. The absence of terminal hydride intermediates was also revealed in the protonation of the
diphosphine diphosphido complexes Fe2(pdp)(CO)4(dppv) (4; dppv = cis-1,2-C2H2(PPh2)2) and Fe2(edp)(CO)4(dppbz) (5;
dppbz = 1,2-C6H4(PPh2)2). Protonation of these diphosphine complexes afforded μ-hydrido cations with apical−basal
diphosphine ligands, which convert to the isomer where the diphosphine is dibasal. In contrast, protonation of the dithiolato
complex Fe2(pdt)(CO)4(dppv) gave terminal hydrides, which isomerize to μ-hydrides. In a competition experiment, 4 was
shown to protonate faster than Fe2(pdt)(CO)4(dppv).
this robustness is associated with the strength of the C−P and
M−P bonds. The similar stereodynamic properties of bis-
(thiolato)- vs bis(arsenido)- vs bis(phosphido)-bridged diiron
hexacarbonyl complexes have been established.8
INTRODUCTION
■
The active site of the [FeFe]-hydrogenases is a well-known
bioorganometallic reaction center. Many publications describe
the behavior of this structurally unusual center because of its
novelty and the importance of the reactions that it catalyzes.1
Most modeling efforts focus on substituted derivatives of
Fe2(pdt)(CO)6, especially those containing phosphine ligands.2
Some studies have examined the effect of replacing the
dithiolate ligand with diamides,3 diselenoates,4 ditelluroates,5
diphosphides,6,7 and diarsenides.8,9 Studies on such species
should guide the design of bioinspired catalysts for hydrogen
production. Of these classes of diiron complexes, the
diphosphido complexes are particularly intriguing to us because
of the thermal robustness of the Fe2(PR2)2 core. Qualitatively,
This paper focuses on the protonation of diiron diphosphido
complexes as analogues of the biologically more relevant diiron
dithiolato complexes. The chemistry of diiron diphosphido
carbonyls is extensive10−14 and includes unsaturated derivatives
10,13,14
Fe2(PR2)2(CO)5‑x(PR3)x
that are not observed in diiron
dithiolato complexes. Since the acidities of typical secondary
phosphines R2PH are lower than those of typical thiols,15 the
Received: October 23, 2012
Published: December 27, 2012
© 2012 American Chemical Society
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diiron phosphides are expected to display enhanced basicity
leading to highly stabilized hydrides. Indeed, complexes of the
type Fe2(PPh2)2(CO)4L2 were described as “markedly more
basic” than the dithiolato complex Fe2(SPh)2(CO)4L2.16 Since
the publication of the active site structure of the [FeFe]-
hydrogenases,17 two reports have appeared on the related
diiron diphosphido carbonyls. In the complex
Fe2[(PhPCH2)2NCH2CH2OMe](CO)6,7 the organic backbone
features a basic amine, as seen in the azadithiolate cofactor of
the enzyme.18 This amine protonates in the expected way,19,20
but protonation of the Fe center was not observed. It is known
from FT-IR measurements that N-protonation of related
complexes diminishes the basicity of the diiron(I) core;2,19 it
is consequently not surprising that this ammonium diphosphi-
do complex would not readily tautomerize to the hydride. In a
study of hydrogen evolution by diiron diphosphido complexes,
the protonation and subsequent catalytic hydrogen evolution
have been described for the Fe(0) complex
[Fe2(PPh2)2(CO)6]2−.6,21
In this work we prepared complexes of the type
Fe2[(PhP)2(CH2)n](CO)6, analogues of the archetypal
Fe2(edt)(CO)6 and Fe2(pdt)(CO)6 (edt = 1,2-C2H4S2, pdt =
1,3-C3H6S2). We converted these diphosphido hexacarbonyls
to a series of substituted derivatives, which were expected to be
analogous to the better known diiron dithiolato complexes.2 It
is known that protonation of complexes of the type
Fe2(SR)2(CO)4−x(PR3)2+x can afford either terminal or
bridging hydrido derivatives.22,23 In this work we investigated
if similar reactivity patterns apply to the corresponding
diphosphido complexes (Scheme 1, X = PPh).
(CH2)n(PPhH)2 + Fe3(CO)12
→ Fe2[(PhP)2(CH2)n](CO)6 + Fe(CO)5 + CO + H2
1 (n = 2)
2 (n = 3)
(1)
Relative to the dithiolato complexes, 1 and 2 are noticeably
more electron-rich. This difference is indicated by the positions
of the CO bands in the IR spectra. For 2, νCO spans the range
2052−1983 cm−1, whereas for Fe2(pdt)(CO)6 these bands are
found in the range 2077−1950 cm−1 (for medium and strong
bands).24 Cyclic voltammetry experiments show that 1 and 2
reduce at −2.25 V (ipa/ipc = 0.11) and −2.15 V (ipa/ipc = 0.95),
respectively. For comparison, Fe2(edt)(CO)6 and Fe2(pdt)-
(CO)6 are reduced near −1.7 V.25
Treatment of dichloromethane solutions of the diiron
diphosphido hexacarbonyl complexes with H(OEt2)2BArF
4
afforded the corresponding diferrous hydrides. According to
31P{1H} NMR analysis, treatment of Fe2(edp)(CO)6 with 1.2
equiv of the acid at room temperature resulted in ∼92%
protonation (Figure S1, Supporting Information). The
observation of coexisting diferrous hydride and its parent
FeIFeI complex is consistent with slow intermetallic proton
exchange, as is typical for related compounds.20 The H NMR
1
spectrum confirmed the presence of the hydride with a triplet at
δ −17.2 (JPH = 38 Hz). Under the same conditions,
Fe2(edt)(CO)6 does not react noticeably with H-
(OEt2)2BArF .26 Qualitative tests suggest that 2 is more basic
4
than 1 since, as a CD2Cl2 solution, it quantitatively converted to
Scheme 1. Known (X = S) and Predicted (X = PPh)
Regiochemistry of Protonation of Diiron(I) Complexes
the hydride on treatment with 1 equiv of H(OEt2)2BArF . The
4
1H and 31P{1H} NMR spectra of the hydrido complex are
consistent with a symmetrical species: δ −15.91 (JPH = 39 Hz)
and δ 125.2, respectively.
Fe2(edp)(CO)2(PMe3)4 and Its Protonation. The sub-
stitution of CO ligands in 1 and 2 with phosphines has been
examined. Treatment of 1 with excess PMe3 followed by UV
irradiation afforded the tetrasubstituted complex Fe2(edp)-
(CO)2(PMe3)4 (3). The reaction conditions are comparable to
those employed for the conversion of the ethanedithiolate
Fe2(edt)(CO)6 into Fe2(edt)(CO)2(PMe3)4.22 Complex 3 was
isolated as a fine red powder and exhibited good solubility in
toluene, dichloromethane, and pentane. The new complex was
characterized by IR and NMR spectroscopy. The IR spectrum
of 3 shows νCO bands at 1855 and 1836 cm−1, whereas for 1,
these bands are in the range 2052−1966 cm−1. At room
temperature, the 31P{1H} NMR spectrum (Figure S5,
Supporting Information) features multiplets at δ 136.2 and
17.0, as well as a triplet at δ 15.8 with 1:1:1 integration,
confirming that 3 exists in solution with a bis(apical−basal)
disposition of the phosphine ligands.
The structure of 3 was confirmed by single-crystal X-ray
diffraction (Figure 1). The molecule has an idealized C2h
symmetry with a bis(apical−basal) disposition of the
phosphines. The phenyl substituents project away from the
Fe2 core.
With a distance of 2.7786(8) Å, the Fe−Fe bond is much
longer than 2.631(1) Å for Fe2[(PhPCH2)NCH2CH2OMe]-
(CO)6.7). Even derivatives with bulky phosphide groups are
shorter, as in Fe2(PPh2)(PBut2)(CO)6 with an Fe−Fe distance
of 2.707(1) Å.14 Apparently the Fe−Fe bond elongates to
We expected that the diiron(I) diphosphido complexes
would protonate to give terminal hydrides that would be more
kinetically stable than the hydrides of diiron dithiolato
complexes. As we show below, these predictions proved to be
incorrect, and the explanation for this surprising difference
provides insights into the behavior of the diiron dithiolato
carbonyl complexes.
RESULTS
■
Preparation of Fe2(edp)(CO)6 and Fe2(pdp)(CO)6 and
Their Protonation. Heating the diphosphines
(CH2)n(PPhH)2 (n = 2, 3) with Fe3(CO)12 in toluene at 100
°C was found to afford the diiron hexacarbonyl derivatives in
high yields (eq 1). Chromatography on silica gel afforded clean
products in good yields of both the ethylene and trimethylene
derivatives Fe2(edp)(CO)6 (1) and Fe2(pdp)(CO)6 (2),
respectively. Both compounds are known,9,11 but the new
route is more efficient.
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by chromatography on silica gel. For 4, the room-temperature
31P{1H} NMR spectrum shows multiplets near δ 106.2 and
93.6 (Figure S9, Supporting Information). For 5, the 31P{1H}
NMR spectrum features multiplets at δ 103.8 for the
diphosphido bridge and at δ 89.3 (Figure 3). The 31P NMR
Figure 1. Structure of 3 with thermal ellipsoids set at the 50%
probability level. Hydrogen atoms have been omitted for clarity. Key
bond distances (Å) and angles (deg): Fe1−Fe2, 2.7786(8); Fe1−P1,
2.204(1); Fe1−P2, 2.191(1); Fe1−C7, 1.757(4); P4−Fe1−Fe2−P3,
95.89; P1−Fe1−Fe2−P6, 100.07.
Figure 3. 31P{1H} NMR spectra of a CD2Cl2 solution of Fe2(pdp)-
(CO)4(dppbz) (5) at various temperatures (from the top: +20, −30,
−60, and −90 °C).
accommodate the steric bulk of the four phosphine ligands. The
Fe−P distances are in the normal range, however.27
Protonation of 3 at −80 °C afforded a bridging hydride with
the same bis(apical−basal) configuration of the phosphines as
the starting material. The 31P{1H} NMR spectrum of the
product features three multiplets consistent with a single isomer
(Figure S7, Supporting Information). The H NMR signal for
the hydride ligand appears as a triplet of triplets of triplets at δ
−21.72 (Figure 2). This spectrum can be simulated with J(H,P)
signals for 1 and 2 are δ 131.6 and 166.4. Thus, in these mixed
phosphide−phosphine complexes, the low-field signals are
assigned to the diphosphido ligands. As can be seen by the
multiplicity of the signals at low temperatures, the diphosphine
must span apical−basal sites in 4 and 5. Variable-temperature
NMR studies demonstrate that the chelating diphosphine
oscillates rapidly between equivalent positions, as is seen for
Fe2(pdt)(CO)4(dppv) and related complexes (eq 2, Figure
3).29,30
1
The complex Fe2(pdt)(CO)4(dppv) exists as a 90:10 mixture
of apical−basal and dibasal isomers, respectively. This ratio
does not perceptibly change from room temperature to −90
°C.31 Thus, it appears that the dibasal isomer is less stabilized in
the diiron diphosphido complexes. Variable-temperature NMR
results show that cessation of the dynamics of the Fe(CO)-
(diphos) center coincides with the temperature range where
the signals for the phosphido ligands split.
Figure 2. 1H NMR spectrum of a CD2Cl2 solution of [(μ-
H)Fe2(edp)(CO)2(PMe3)4]+ in the hydride region at −80 °C (the
spectrum remains constant to room temperature).
Protonations of 4 and 5 with 1 equiv of H(OEt2)2BArF4 were
conducted at low temperatures and monitored by NMR
spectroscopy. For the protonation of a CD2Cl2 solution of 4,
1
the H NMR spectrum of the initially observed hydride (T =
= 38.3, 26.3, and 4.1 Hz. Previous work, e.g. on [μH−
Fe2(pdt)(CO)4(PMe3)2]+,28 shows that J(H,P)trans is typically 5
Hz, whereas J(H,P)cis is around 28 Hz.
−90 °C) complex shows a triplet of doublets at δ −15.4 (JPH =
49.5, 25.0 Hz; Figure S10, Supporting Information). The
corresponding 31P NMR spectrum shows three sets of complex
multiplets, consistent with an apical−basal disposition of the
dppv ligand. Above −30 °C, this kinetic product converts to a
more stable isomer wherein the diphosphine is dibasal, this
species showing a triplet of triplets at δ −15.96 (JPH = 40.5, 22.0
Hz). The 31P{1H} NMR spectrum of this second isomer
features equally intense resonances at δ 129.05 and 89.03 for
the diphosphido bridge and the dppv, respectively. The
protonation of Fe2(pdt)(CO)4(dppv) followed a very similar
Fe2(pdp)(CO)4(dppv) (4) and Fe2(pdp)(CO)4(dppbz) (5)
and Their Protonation. The new compounds Fe2(pdp)-
(CO)4(dppv) (4; dppv = cis-1,2-C2H2(PPh2)2) and Fe2(pdp)-
(CO)4(dppbz) (5; dppbz = 1,2-C6H5(PPh2)2) were prepared
by photosubstitution of 2 by the diphosphine. These reactions
proceeded more slowly than the synthesis of the related ethane-
and propanedithiolates Fe2(xdt)(CO)4(chel).29 Both com-
pounds were obtained as red-orange powders after purification
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pattern (Scheme 2; see below). The protonation of 5
proceeded in a very similar way (Figure 4).
Scheme 2
We briefly examined the edp analogues of 4 and 5, and the
results were similar. As seen for Fe2(edt)(CO)4(dppv),29 only
the apical−basal isomer was observed. When they were
protonated at room temperature, both the edp−dppv and
edp−dppbz compounds afforded the corresponding μ-hydrido
complexes, again with apical−basal diphosphine ligands. These
kinetic isomers were found to convert to the more stable
isomer with a dibasal configuration of the diphosphine within 1
h at room temperature.
1
Figure 4. H NMR spectra for various stages in the protonation of a
CD2Cl2 solution of Fe2(pdp)(CO)4(dppbz) with 1 equiv of
H(OEt2)2BArF4 (from the bottom: T = −90, −60, −30, and +20 °C).
(CO)2(PMe3)4]+ is stable for several minutes at room
temperature,22 but we detected none of the analogous [term-
HFe2(edp)(CO)2(PMe3)4]+.
slow: Fe2(pdp)(CO)4(dppv) + H+
→ [HFe2(pdp)(CO)4(dppv)]+
Protonation of Fe2(pdt)(CO)4(dppv). Prior to comparing
the protonation of diiron diphosphido and diiron dithiolato
complexes, we re-examined the protonation of the Fdithiolate
Fe2(pdt)(CO)4(dppv) (6) to address a discrepancy in the
literature.31 When a mixture of Fe2(pdt)(CO)4(dppv) and
(3)
fast: Fe2(pdt)(CO)4(dppv) + H+
→ [HFe2(pdt)(CO)4(dppv)]+
H(OEt2)2BArF in CD2Cl2 is allowed to thaw and then
4
(4)
immediately checked by NMR spectroscopy, we observe rapid
formation of the terminal hydrido complex. In this kinetic
isomer, the dppv is dibasal on the unprotonated Fe center. We
had previously misassigned this kinetic hydride as the apical−
basal isomer.31 When this experiment was examined in the
presence of an integration standard, we found that substantial
amounts of the starting FeIFeI species had not dissolved. The
insolubility of such diiron complexes plagues related experi-
ments conducted at very low temperatures, even when the
diiron(I) complex is fully dissolved prior to cooling the sample.
As described previously,31 the terminal hydrides [HFe2(pdt)-
(CO)4(dppv)]+ isomerize near −40 °C to the unsymmetrical
and then symmetrical bridging hydrides, wherein the dppv is
apical−basal and then dibasal.
fast: Fe2(pdp)(CO)4(dppv) + Fe2(pdt)(CO)4(dppv)
+ H+ → [HFe2(pdp)(CO)4(dppv)]+
+ Fe2(pdt)(CO)4(dppv)
(5)
The comparison of energy differences between unrotated
and rotated forms of Fe2(edp)(CO)4(dppv) and Fe2(edt)-
(CO)4(dppv) (Scheme 3) showed that in both cases unrotated
forms have lower energies. More importantly, the energy gaps
between rotated and unrotated forms (which always corre-
spond to transition states of FeL3 rotation) are almost the same
in Fe2(edp)(CO)4(dppv) and Fe2(edt)(CO)4(dppv). There-
fore, the absence of terminal hydride derivatives of Fe2(edp)-
(CO)4(dppv) cannot be ascribed to hindered formation of
rotated forms. Notably, the comparison of rotated forms reveals
that structures characterized by dibasal dppv are more
destabilized relative to basal−apical species in the edp complex.
To qualitatively evaluate the relative basicity of iron atoms in
rotated and unrotated complexes containing edt or edp ligands,
we have computed atomic partial charges (according to the
NBOnatural bond orbitalpartition scheme of the electron
density) for Fe2(edt)(CO)6, Fe2(edp)(CO)6, Fe2(edt)-
(CO)4(dppv), and Fe2(edp)(CO)4(dppv). We found that
iron centers in the edp complexes are more electron-rich
than in edt species (Scheme S1, Supporting Information).
Moreover, the partial charge of the five-coordinated iron atoms
is always slightly less negative in the rotated forms. Analyses of
NBO charges have been complemented by the computation of
proton affinities for Fe2(pdt)(CO)4(dppv) and Fe2(pdp)-
Protonation of Fe2(pdt)(CO)4(dppv) vs Fe2(pdp)-
(CO)4(dppv). At −90 °C, the protonation of 4 is slow,
requiring several minutes. In contrast, protonation of the
related dithiolato complex 6 is rapid and quantitative under
these conditions.28 Surprisingly, when a mixture of Fe2(pdt)-
(CO)4(dppv) and 4 was treated with H(OEt2)2BArF , the only
4
hydride product obtained (immediately upon mixing at −90
°C) was the hydride derivative of the diiron diphosphido
complex. It thus appears that 6 catalyzes the protonation of 4.
These surprising results are summarized in eqs 3−5.
Computational Experiments. To better understand the
absence of terminal hydride derivatives of the phosphido
complexes, we carried out DFT calculations to compare
structural and electronic properties of diiron complexes with
bridging diphosphido vs dithiolato ligands. Recall that
protonation of Fe2(pdt)(CO)4(dppv) with strong acids gives
the terminal hydride [term-HFe2(pdt)(CO)4(dppv)]+, which
converts near −40 °C to two isomeric μ-hydride derivatives.
Even more striking, the terminal hydride [term-HFe2(edt)-
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(CO)4(dppv) complexes (the two different terminal−axial
coordination modes and the diiron bridging coordination have
been tested; see Table S1 in the Supporting Information). As
expected, the Fe−Fe bond in the pdp complex has a higher
affinity for protons. The corresponding proton affinity for the
pdt complex is lower by about 3 kcal/mol.
Scheme 3. Structures and Energy Differences (kcal/mol)
between Rotated and Unrotated Forms in
a
Fe2(edt)(CO)4(dppv) and Fe2(edp)(CO)4(dppv)
CONCLUSIONS
■
These studies were enabled by the preparation of phosphine-
substituted diiron diphosphido complexes, only a few of which
were known prior to this work.13,14,32 The new complexes are
highly basic, readily affording the corresponding μ-hydrido
derivatives. To our surprise and in contrast with extensive
results on related diiron dithiolato complexes,2 protonation
experiments never revealed detectable amounts of the terminal
hydrido complexes. This finding is consistent with our recent
hypothesis that μ-thiolate ligands function as proton relays
leading to terminal hydrides.28 In the absence of such sites,
attack at the Fe−Fe bond is favored.
The behavior of Fe2(pdp)(CO)4(dppv) illustrates the
complexity of the protonation chemistry. The protonation of
this species appears to be catalyzed by Fe2(pdt)(CO)4(dppv). A
proposed structure for the bimolecular intermediate is depicted
in eq 6.
a
All rotated isomers correspond to the transition states.
P bonds was observed when the reaction was maintained at 0 °C. A
round-bottomed flask equipped with a dropping funnel was loaded
with a glass-coated stir bar and 1.2 g (0.17 mmol) of lithium wire cut
into small (∼5 mm) pieces, followed by 30 mL of freshly distilled
THF. The lithium suspension was cooled to 0 °C. The addition funnel
was charged with a solution of 6.5 g (0.016 mmol) of 1,2-
bis(diphenylphosphino)ethane in 75 mL of THF. The dppe solution
was added to a cold suspension of lithium over the course of 3 h. The
reaction mixture was stirred for 20 h, being careful to maintain the
temperature at 0 °C. The course of the reaction was monitored by
31P{1H} NMR spectroscopy. The reaction was judged complete when
the signal for dppe at δ −15.2 had vanished. A solution of 15 mL of
water in 35 mL of THF was slowly added through the dropping funnel
over the course of 1 h, carefully maintaining the mixture at 0 °C. After
the mixture was stirred for an additional 10 min, the solvent was
removed under vacuum. The sticky residue was washed with 50 mL of
water to remove LiOH. The product was extracted into 4 × 50 mL of
Et2O. The ether was transferred by cannula into a flask containing 3.5
g of MgSO4. After the MgSO4 suspension was vigorously stirred, the
ether solution was transferred into a new flask, and solvent was
removed under vacuum. The product was distilled at 5 mTorr (135−
140 °C). 31P{1H} NMR (CDCl3): δ −46.08, −46.66 (∼1:1 mixture of
diastereoisomers).36
HPhP(CH2)3PPhH (pdpH2). This compound was prepared by
following the same procedure described above for edpH2. The
following amounts were used: 2 g (28 mmol) of lithium and 5 g (28
mmol) of 1,3-bis(diphenylphosphino)propane. The mixture was
stirred overnight at 0 °C, and the reaction was judged to be complete
when the signal of the starting material had vanished in the 31P{1H}
NMR spectrum. Workup was similar to that for edpH2, except that it
was not necessary to distill the product since 31P{1H} NMR analysis
showed that the product was >98% pure. 31P{1H} NMR (CDCl3): δ
−55.88, −56.01 (∼1:1 mixture of diastereoisomers).36
A similar pathway was invoked in the bimolecular pathway
for the protonation of Fe2(pdt)(CO)2(PMe4)4 and Fe2(edt)-
(CO)2(PMe4)4.28 S-protonation would be expected to diminish
the basicity of the associated Fe centers toward further
protonation. The susceptibility of terminal thiolato ligands to
protonation is long established.33 Typical μ-thiolato ligands are
much less basic, although the apparent S-protonation of diiron
dithiolato carbonyls by strong acids has been observed by IR34
and NMR spectroscopy.28
The presence of phosphorus at the bridging sites provides an
opportunity to probe the new details of the stereodynamics of
this family of diiron complexes. Thus, we show that cessation of
the turnstile dynamics of the Fe(CO)(dppv) center in
Fe2(pdp)(CO)4(dppv) correlates with the splitting of the 31P
NMR signals for the phosphide groups. Overall, the barriers for
the stereodynamic behavior for the FeIFeI diphosphido and
dithiolato complexes are similar, which suggests that their
dynamic behavior does not involve breaking of Fe−(μ-X)
bonds (X  SR, PR2).9 Likewise, the similar rates of
isomerization of the μ-hydrides [HFe2(pdx)(CO)4(dppv)]+
for the pdt and pdp derivatives suggests that the breaking of
Fe−S and Fe−P(phosphido) bonds is not involved in the
isomerization of these μ-hydrido complexes, consistent with the
proposed twist mechanism.35
Fe2(edp)(CO)6 (1). A 300 mL round-bottom flask was charged with
2.939 g (5.835 mmol) of Fe3(CO)12 and 75 mL of dry toluene. The
reaction solution was then treated with a solution of 1.437 g (5.835
mmol) of edpH2 in 10 mL of toluene. The reaction mixture was stirred
at an oil bath temperature of 100 °C for 3 h. Volatiles were removed
under vacuum, and the yellow residue was extracted into ∼100 mL of
hexane. Removal of the solvent afforded the product in analytical
purity. Yield: 2.46 g (81%). 31P{1H} NMR (CDCl3): δ 166.4 (s). H
1
NMR (CD2Cl2): δ 7.42−7.45 (m, C6H5, 10H), 216 (d, PCH2, 4H). IR
(CDCl3): νCO 2052 (m), 2013 (m), 1990 (m), 1966 (s) cm−1. Anal.
Calcd (found) for C20H14Fe2O6P2: C, 45.85 (45.94); H, 2.69 (2.82).
Fe2(pdp)(CO)6 (2). Compound 2 was prepared by following the
procedure for 1 with pdpH2 in place of edpH2. The following amounts
were used: 2.042 g (4.05 mmol) of Fe3(CO)12 and 1.055 g (4.05
mmol) of pdpH2. After the reaction mixture was stirred for 3 h, solvent
was removed under vacuum. The residue was extracted into 10 mL of
CH2Cl2. This extract was subjected to chromatography on a 2.5 × 30
cm column of silica gel with 3/7 CH2Cl2/hexanes as eluent. The first
yellow band was collected. Solvent was removed under vacuum to
afford the product. Yield: 2.05 g (94%). 31P{1H} NMR (CDCl3): δ
EXPERIMENTAL SECTION
■
Methods have been recently reported.28
HPhP(CH2)2PPhH (edpH2). In this reaction, lithium is used to
cleave phenyl−phosphorus bonds, but without care, we also observed
scission of P−CH2 bonds. Good selectivity for cleavage of the phenyl−
236
dx.doi.org/10.1021/om300997s | Organometallics 2013, 32, 232−238

Organometallics
Article
131.6 (s). 1H NMR (CD2Cl2): δ 7.83−7.40 (m, C6H5, 10H), 2.00 (m,
PCH2, 4H), 2.09 (m, PCH2CH2CH2P, 2H). IR: νCO 2052 (m), 2013
(m), 1990 (m), 1966 (s) cm−1. Anal. Calcd (found) for
C21H16Fe2O6P2: C, 46.88 (47.01); H, 3.00 (3.22).
phenomena. However, it was verified that solvent-corrected energies
do not vary significantly compared to those computed under vacuum.
In light of available experimental data and considering the chemical
nature of the ligands, only low-spin forms of FeFe complexes were
considered for DFT calculations. The resolution of the identity
procedure42 was used for approximating expensive four-center
integrals (describing the classical electron−electron repulsive con-
tribution to the total energy) through a combination of two three-
center integrals. This procedure was made possible by expanding the
density ρ in terms of an atom-centered and very large basis, the
auxiliary basis set.
Fe2(edp)(CO)2(PMe3)4 (3). In a 100 mL Schlenk tube containing a
solution of 300 mg (0.57 mmol) of 1 in 50 mL of toluene was added
0.5 mL (4.8 mmol) of PMe3. After 10 min of stirring, being careful to
vent the evolved CO, the reaction flask was irradiated at 450 nm for 20
h. Volatiles were removed under vacuum, after which the red solid
residue was extracted into 60 mL of pentane. Evaporation of the
pentane extracts afforded a red fine powder. Yield: 326 mg (80%).
31P{1H} NMR (CD2Cl2): δ 136.4 (m, diphosphido bridge), 17.05 (m,
broad), 15.82 (m, broad). 1H NMR (CD2Cl2): δ 1.96 (broad s, PCH2,
4H), 1.96 (m, PCH2CH2CH2P, 2H), 1.33 (d, PMe3, 36H). IR
(toluene): νCO 1855 (m), 1836 (m) cm−1. Anal. Calcd (found) for
C28H50Fe2O2P6: C, 46.95 (47.10); H, 7.04 (7.18).
Fe2(pdp)(CO)4(dppv) (4). In a 100 mL Schlenk tube charged with
a solution of 0.300 g (0.56 mmol) of Fe2(pdp)(CO)6 in 50 mL of dry
toluene was added a solution of 0.221 g (0.56 mmol) of dppv in 25 mL
of toluene. The reaction mixture was then irradiated with an LED
lamp (λ 450 nm). After 24 h, the solvent was removed under vacuum.
The product was obtained in analytical purity after chromatography on
silica, with a 1/1 toluene/pentane mixture as eluent. The first yellow
band was found to be the starting diiron hexacarbonyl compound, and
the second yellow band was an unidentified product. The third (red-
orange) band contained the product. Yield: 0.217 g (44%). 31P{1H}
NMR (CD2Cl2): δ 106.2 (PhPCH2, s), 93.6 (dppv, s). 1H NMR
(CD2Cl2): δ 7.83−7.40 (m, C6H5, 10H), 2.00 (m, PCH2, 4H) 2.09
(m, PCH2CH2CH2P, 2H). IR: νCO 2001 (m), 1938 (m), 1922 (m)
cm−1. Anal. Calcd (found) for C45H38Fe2O4P4: C, 61.53 (61.86); H,
4.46 (4.63).
ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR, 31P NMR, and IR spectra and cyclic voltammograms
for new complexes and schematic structures, partial atomic
charges, and relative energies for Fe2(edt)(CO)6, Fe2(edp)-
(CO)6, Fe2(edt)(CO)4(dppv), and Fe2(edp)(CO)4(dppv), a
table giving computed proton affinities for Fe2(pdt)-
(CO)4(dppv) and Fe2(pdp)(CO)4(dppv), and a CIF file giving
X-ray crystallographic data for 3. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rauchfuz@illinois.edu.
■
Notes
The authors declare no competing financial interest.
Fe2(pdp)(CO)4(dppbz) (5). In a 100 mL Schlenk tube flask
charged with 0.300 g (0.56 mmol) of Fe2(pdp)(CO)6 was added 50
mL of toluene followed by a solution of 0.248 g (0.56 mmol) of dppbz
in 25 mL of toluene. The reaction mixture was then irradiated at 450
nm for 24 h. The reaction was judged complete when the 31P{1H}
NMR resonance of the free diphosphine at δ −12 vanished. Volatiles
were removed under vacuum to give a red-orange solid material.
Compound 5 was obtained in analytical purity after chromatography
on silica, with a 1/1 CH2Cl2/hexanes mixture as eluent. The first
yellow band was found to be the starting diiron hexacarbonyl
compound, and the second red-orange band turned out to be the
product. Solvent was removed under vacuum to afford an fine orange
powder. Yield: 0.281 g (54%). 31P{1H} NMR (CD2Cl2): δ 103.8
(diphosphido bridge, s), 89.3 (dppbz, s). 1H NMR (CD2Cl2): δ 7.83−
7.40 (m, C6H5, 10H), 2.00 (m, PCH2, 4H), 2.09 (m, PCH2CH2CH2P,
2H). IR: νCO 2000 (m), 1937 (m), 1921 (m) cm−1. Anal. Calcd
(found) for C49H40Fe2O4P4: C, 63.39 (63.56); H, 4.34 (4.72).
Computational Details. Density functional theory (DFT)
calculations were carried with the TURBOMOLE suite of programs.37
A high-quality level of theory (BP86/TZVP38) was employed to treat
explicitly (no effective core potential was used; inner shell electrons
were explicitly treated) the full electronic structure of all atoms of the
diiron species investigated. Such a DFT scheme has been shown to be
suitable for investigating hydrogenase models.35,39
All stationary points on the potential energy surface were
determined by means of energy gradient techniques, and a full
vibrational analysis was carried out to further characterize the nature of
each point. Transition state structures were searched by means of a
procedure based on a quasi-Newton−Josephson algorithm.40 As a
preliminary step, the geometry optimization of a putative transition
state structure was carried out by freezing the molecular degrees of
freedom corresponding to the reaction coordinate (RC). After the
vibrational analysis of the constrained minimum energy structures was
performed, the negative eigenmode associated to the RC was followed
to locate the true transition state structure, which corresponds to the
maximum energy point along the trajectory that joins two adjacent
minima (i.e., reactants, products, and reaction intermediates).
An implicit treatment of solvent effects (COSMO;41 ε = 9.1,
dichloromethane) was used to evaluate possible polarization
ACKNOWLEDGMENTS
■
This work was supported as part of the ANSER Center, an
Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Award Number DE-SC0001059. We
also acknowledge the support of the International Institute for
Carbon Neutral Energy Research (WPI-I2CNER), sponsored
by the World Premier International Research Center Initiative
(WPI), MEXT, Japan.
REFERENCES
■
(1) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245.
(2) Tschierlei, S.; Ott, S.; Lomoth, R. Energy Environ. Sci. 2011, 4,
2340.
(3) Volkers, P. I.; Rauchfuss, T. B. J. Inorg. Biochem. 2007, 101, 1748.
(4) Harb, M. K.; Niksch, T.; Windhager, J.; Gorls, H.; Holze, R.;
Lockett, L. T.; Okumura, N.; Evans, D. H.; Glass, R. S.; Lichtenberger,
D. L.; El-khateeb, M.; Weigand, W. Organometallics 2009, 28, 1039.
Harb, M. K.; Windhager, J.; Daraosheh, A.; Gorls, H.; Lockett, L. T.;
Okumura, N.; Evans, D. H.; Glass, R. S.; Lichtenberger, D. L.; El-
khateeb, M.; Weigand, W. Eur. J. Inorg. Chem. 2009, 3414.
(5) Harb, M. K.; Apfel, U.-P.; Kubel, J.; Gorls, H.; Felton, G. A. N.;
Sakamoto, T.; Evans, D. H.; Glass, R. S.; Lichtenberger, D. L.; El-
khateeb, M.; Weigand, W. Organometallics 2009, 28, 6666.
(6) Cheah, M. H.; Borg, S. J.; Bondin, M. I.; Best, S. P. Inorg. Chem.
2004, 43, 5635.
̈
̈
̈
̈
́
(7) Das, P.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollhammer,
P.; Talarmin, J.; Muir, K. W. Inorg. Chem. 2004, 43, 8203.
(8) Adams, R. D.; Cotton, F. A.; Cullen, W. R.; Hunter, D. L.;
Mihichuk, L. Inorg. Chem. 1975, 14, 1395.
(9) Flood, T. C.; DiSanti, F. J.; Campbell, K. D. Inorg. Chem. 1978,
17, 1643.
(10) van der Linden, J. G. M.; Heck, J.; Walther, B.; Boettcher, H. C.
Inorg. Chim. Acta 1994, 217, 29. Walther, B.; Hartung, H.; Reinhold, J.;
Jones, P. G.; Mealli, C.; Boettcher, H. C.; Baumeister, U.; Krug, A.;
Moeckel, A. Organometallics 1992, 11, 1542.
237
dx.doi.org/10.1021/om300997s | Organometallics 2013, 32, 232−238

Organometallics
Article
(11) Seyferth, D.; Wood, T. G.; Henderson, R. S. J. Organomet. Chem.
1987, 336, 163.
(33) Kuehn, C. G.; Isied, S. S. Prog. Inorg. Chem. 1980, 27, 153.
Petrou, A. L.; Koutselos, A. D.; Wahab, H. S.; Clegg, W.; Harrington,
R. W.; Henderson, R. A. Inorg. Chem. 2011, 50, 847.
(12) Seyferth, D.; Wood, T. G. Organometallics 1988, 7, 714. De, R.
L.; Wolters, D.; Vahrenkamp, H. Z. Naturforsch. 1986, 41b, 283.
Seyferth, D.; Wood, T. G.; Fackler, J. P., Jr.; Mazany, A. M.
Organometallics 1984, 3, 1121. Collman, J. P.; Komoto, R. G.; Siegl, W.
O. J. Am. Chem. Soc. 1973, 95, 2389. Treichel, P. M.; Dean, W. K.;
Calabrese, J. C. Inorg. Chem. 1973, 12, 2908. Schaefer, H. Z. Anorg.
Allg. Chem. 1980, 467, 105. McKennis, J. S.; Kyba, E. P.
Organometallics 1983, 2, 1249. Shyu, S.-G.; Wojcicki, A. Organo-
metallics 1985, 4, 1457. Seyferth, D.; Brewer, K. S.; Womack, G. B.;
(34) Apfel, U.-P.; Troegel, D.; Halpin, Y.; Tschierlei, S.; Uhlemann,
U.; Gorls, H.; Schmitt, M.; Popp, J.; Dunne, P.; Venkatesan, M.; Coey,
̈
M.; Rudolph, M.; Vos, J. G.; Tacke, R.; Weigand, W. Inorg. Chem.
2010, 49, 10117. Dong, W.; Wang, M.; Liu, X.; Jin, K.; Li, G.; Wang,
F.; Sun, L. Chem. Commun. 2006, 305. Ezzaher, S.; Gogoll, A.; Bruhn,
C.; Ott, S. Chem. Commun. 2010, 46, 5775.
(35) Zampella, G.; Fantucci, P.; De Gioia, L. Chem. Commun. 2010,
8824.
(36) Brooks, P.; Gallagher, M.; Sarroff, A. Aust. J. Chem. 1987, 40,
1341.
(37) Ahlrichs, R.; Baer, M.; Haeser, M.; Horn, H.; Koelmel, C. Chem.
Phys. Lett. 1989, 162, 165.
́
Archer, C. M. J. Organomet. Chem. 1993, 446, 167. Ballinas-Lopez, M.
́
́
́
́
G.; Padilla-Martınez, I. I.; Martınez-Martınez, F. J.; Hopfl, H.; Garcıa-
̈
Bae
́
z, E. V. Acta Crystallogr., Sect. E 2005, E61, m1475. Gimbert-
̃
Surinach, C.; Bhadbhade, M.; Colbran, S. B. Organometallics 2012, 31,
(38) Becke, A. D. Phys. Rev. A: Gen. Phys. 1988, 38, 3098. Perdew, J.
P. Phys. Rev. B 1986, 33, 8822.
3480.
(13) Knoll, H.; Hennig, H.; Walther, B.; Bottcher, H. C.; Stufkens, D.
̈
(39) Bertini, L.; Greco, C.; Bruschi, M.; Fantucci, P.; De Gioia, L.
Organometallics 2010, 29, 2013. Siegbahn, P. E. M.; Tye, J. W.; Hall,
M. B. Chem. Rev. 2007, 107, 4414. Zampella, G.; Bruschi, M.; Fantucci,
P.; De Gioia, L. J. Am. Chem. Soc. 2005, 127, 13180. Zampella, G.;
Greco, C.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2006, 45, 4109.
Zampella, G.; Fantucci, P.; De Gioia, L. J. Am. Chem. Soc. 2009, 131,
10909.
J. Inorg. Chim. Acta 1993, 210, 33.
(14) Walther, B.; Hartung, H.; Bambirra, S.; Krug, A.; Boettcher, H.
C. Organometallics 1994, 13, 172.
(15) Henderson, W. A.; Streuli, C. A. J. Am. Chem. Soc. 1960, 82,
5791.
(16) Arabi, M. S.; Mathieu, R.; Poilblanc, R. J. Organomet. Chem.
(40) Jensen, F. Introduction to Computational Chemistry; Wiley:
Chichester, U.K., 2007.
(41) Schafer, A.; Klamt, A.; Sattel, D.; Lohrenz, J. C. W.; Eckert, F.
Phys. Chem. Chem. Phys. 2000, 2187.
(42) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor.
Chem. Acc. 1997, 97, 119.
1979, 177, 199.
(17) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C.
Science 1998, 282, 1853. Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian,
C. E.; Fontecilla-Camps, J. C. Structure 1999, 7, 13.
(18) Nicolet, Y.; de Lacey, A. L.; Vernede, X.; Fernandez, V. M.;
Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123,
1596. Silakov, A.; Wenk, B.; Reijerse, E.; Lubitz, W. Phys. Chem. Chem.
Phys. 2009, 11, 6592.
(19) Lawrence, J. D.; Li, H.; Rauchfuss, T. B.; Ben
́
ard, M.; Rohmer,
M.-M. Angew. Chem., Int. Ed. 2001, 40, 1768.
(20) Kramarz, K. W.; Norton, J. R. Prog. Inorg. Chem. 1994, 42, 1.
(21) Cheah, M. H.; Best, S. P. Eur. J. Inorg. Chem. 2011, 2011, 1128.
(22) Zaffaroni, R.; Rauchfuss, T. B.; Gray, D. L.; De Gioia, L.;
Zampella, G. J. Am. Chem. Soc. 2012, 134, 19260.
(23) Jablonskyte, A.; Wright, J. A.; Pickett, C. J. Eur. J. Inorg. Chem.
̇
2011, 1033. Jablonskyte, A.; Wright, J. A.; Pickett, C. J. Dalton Trans.
̇
2010, 39, 3026. Liu, C.; Peck, J. N. T.; Wright, J. A.; Pickett, C. J.; Hall,
M. B. Eur. J. Inorg. Chem. 2011, 2011, 1080.
(24) Winter, A.; Zsolnai, L.; Huttner, G. Z. Naturforsch. 1982, 37b,
1430.
(25) Felton, G. A. N.; Mebi, C. A.; Petro, B. J.; Vannucci, A. K.;
Evans, D. H.; Glass, R. S.; Lichtenberger, D. L. J. Organomet. Chem.
2009, 694, 2681.
(26) Matthews, S. L.; Heinekey, D. M. Inorg. Chem. 2010, 49, 9746.
(27) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.;
Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917. Li, P.;
Wang, M.; He, C.; Liu, X.; Jin, K.; Sun, L. Eur. J. Inorg. Chem. 2007,
2007, 3718.
(28) Zaffaroni, R.; Rauchfuss, T. B.; Gray, D. L.; De Gioia, L.;
Zampella, G. J. Am. Chem. Soc. 2012, 134, 19260.
(29) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B.; van
der Vlugt, J. I.; Wilson, S. R. Inorg. Chem. 2007, 46, 1655.
(30) Adam, F. I.; Hogarth, G.; Richards, I. J. Organomet. Chem. 2007,
692, 3957. Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.;
́
Schollhammer, P.; Talarmin, J.; Pichon, R.; Kervarec, N. Inorg. Chem.
2007, 46, 3426. Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Kervarec, N.;
́
Petillon, F. Y.; Pichon, R.; Schollhammer, P.; Talarmin, J. C. R. Chim.
2008, 11, 906.
(31) Barton, B. E.; Zampella, G.; Justice, A. K.; De Gioia, L.;
Rauchfuss, T. B.; Wilson, S. R. Dalton Trans. 2010, 39, 3011.
(32) Hogarth, G.; Lavender, M. H. J. Chem. Soc., Dalton Trans. 1993,
143. Santelli-Rouvier, C.; Coin, C.; Toupet, L.; Santelli, M. J.
Organomet. Chem. 1995, 495, 91. Boettcher, H.-C.; Krug, A.;
Hartung, H. Polyhedron 1995, 14, 901.
238
dx.doi.org/10.1021/om300997s | Organometallics 2013, 32, 232−238