Influence of a redox-active phosphane ligand on the oxidations of a diiron core related to the active site of Fe-only hydrogenase

Article abstract of DOI:10.1002/ejic.201000972

Modulation of the Fe redox levels within the diiron dithiolato carbonyls with the coordination of a redox-active phosphane is reported. Treatment of [Fe₂{μ-S(CH₂)₂NR(CH₂) ₂S}(CO)₆]₂

Full text of DOI:10.1002/ejic.201000972

FULL PAPER
DOI: 10.1002/ejic.201000972
Influence of a Redox-Active Phosphane Ligand on the Oxidations of a Diiron
Core Related to the Active Site of Fe-Only Hydrogenase
Yu-Chiao Liu,[a] Chia-Hsin Lee,[a] Gene-Hsiang Lee,[b] and Ming-Hsi Chiang*[a]
Keywords: Fe-only hydrogenases / Metalloenzymes / Electrochemistry / Iron / Phosphane ligands
Modulation of the Fe redox levels within the diiron dithiolato
carbonyls with the coordination of a redox-active phosphane
is reported. Treatment of [Fe2{μ-S(CH2)2NR(CH2)2S}(CO)6]2
and [Fe2(μ-pdt)(CO)6] with mppf affords the phosphane sub-
stituted species, [Fe2{μ-S(CH2)2NR(CH2)2S}(CO)5(mppf)]2 (1
and 2) and [Fe2(μ-pdt)(CO)5(mppf)] (3), respectively. These
mppf-substituted complexes have been structurally and
related species ligated by the redox-inactive phosphane with
approximately the same σ-donating ability. Stability of the
oxidized species is improved on the electrochemical time
scale. For the complex 3, three reversible oxidation events
are observed. The mppf unit is oxidized at 0.16 V, which ex-
erts an influence on the oxidation potential of the diiron core.
In contrast to the PPh3 analogue, oxidation of both Fe centers
spectroscopically characterized. Results of the electrochemi- is accessible at 0.47 and 0.77 V for 3. A large ΔE1/2 value
cal study on 1 and 2 indicate that oxidation of the mppf-sub-
stituted complexes occurs at a potential 100 mV less than the
suggests a substantial electron delocalization within the Fe2
core.
Introduction
An important function of Fe-only hydrogenase is energy
cycling in biological systems.[1,2] It can either store reducing
power in the form of molecular hydrogen or metabolize hy-
drogen molecules to generate reducing equivalents.[3] When
protons are transported through the proton channel to the
catalytic metal site, electrons are transferred to the Fed for
reduction.[4,5] In the reverse pathway, heterogeneous cleav- Scheme 1. The proton–electron transfer pathway within the active
site of Fe-only hydrogenase.
age of hydrogen molecules assisted by the aza nitrogen co-
factor facilitates generation of protons and hydrides. The
latter initially coordinates to the Lewis acidic Fe center and
then migrates to the neighboring Lewis basic sites, i.e. the
shifted by 120 mV to –0.86 V compared to the unattached
N- and/or S-containing residues, in concert with reduction
analogue. Intramolecular electron transfer from the reduced
of the metal center.[6,7] In both forward and reverse reac-
cubane to the Fe2 subset, suggesting interplay of redox
tions, the determining role that mediates electron flux
states between two units, is evidenced from a simulated
shuttling between the active site and the rest of the protein
mechanism and generation of the detached [4Fe4S] cluster
is the ferredoxin cluster that is covalently bonded to the
in the oxidized form.
[Fe2S2] subset through a cysteinyl thiolate on the peptide
It is of interest to examine a {Fe2S2}–mppf system [mppf
chain (Scheme 1).[8,9]
= (C5H5)Fe(C5H4PPh2)]. We designed this system for the
Electronic communication between two linked units, the
following reasons: an irreversible oxidation proceeds at
[4Fe4S] cluster and the [Fe2S2] subset, was first attested by
about 0.75 V (vs. Fc/Fc+) for both [Fe2{μ-S(CH2)2NR-
Pickett et al.[10] It was shown that the reduction potential
(CH2)2S}(CO)6]2 and [Fe2(μ-pdt)(CO)6] (pdt = 1,3-pro-
of the [4Fe4S] cluster in [Fe4S4(L)(Fe2{CH3C(CH2S)3}-
panedithiolate).[11–16] The potential decreases by about
{CO}5)][NBu4]2 {L = 1,3,5-tris(4,6-dimethyl-3-mercapto-
300 mV when the CO is replaced by one phosphane while
phenylthio)-2,4,6-tris(p-tolylthio)benzene} is positively
irreversibility of the redox process persists. In addition,
mppf is a redox-active phosphane ligand and its single-elec-
[a] Institute of Chemistry, Academia Sinica,
tron oxidation at 0.14 V is reversible.[17] Decrease of the po-
Nankang, Taipei 115, Taiwan
tential difference between these two closely separated oxi-
dation energy levels upon coordination of mppf to the Fe2
core could facilitate electron transfer in between and pro-
vide an avenue to direct observation of the influence of an
E-mail: mhchiang@chem.sinica.edu.tw
[b] Instrumentation Center, National Taiwan University,
Taipei 106, Taiwan
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000972.
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attached redox-active fragment on the Fe oxidation poten- solution. A similar synthetic procedure was employed to
tial. generate [Fe2(μ-pdt)(CO)5(mppf)] (3). Its IR peak pattern is
Previously, we have reported preparation of [Fe2{μ-S- identical to that of 1 and 2, suggesting that they all have
(CH2)2NR(CH2)2S}(CO)6]2 and [Fe2{μ-S(CH2)2NR(CH2)2- the same {Fe2S2} structural core. Energies of the IR peaks
S}(CO)5(PPh3)]2.[11] The electrochemical properties of these
complexes have been studied. The syntheses and electro-
chemistry of the pdt analogues are well documented.[12–16]
[Fe2(μ-xdt)(CO)5(mppf)] (x = N-p-C6H4CH3, S) have been
synthesized and the adt [adt = (p-C6H4CH3)N(CH2S–)2] de-
rivative has been structurally characterized.[18,19] However,
none of these compounds have been explored electrochemi-
cally. Herein, we report the synthesis and characterization
of [Fe2{μ-S(CH2)2NR(CH2)2S}(CO)5(mppf)]2 and [Fe2(μ-
pdt)(CO)5(mppf)]. Similar ν(CO) band energies are ob-
served for the PPh3- and mppf-substituted species. By
choosing phosphane ligands with approximately the same
σ-donating ability, the role of redox-active ligands on the
HOMO of the Fe2 core can be disentangled. It is concluded
that distinct electrochemical features in [Fe2{μ-S(CH2)2-
NR(CH2)2S}(CO)5(mppf)]2 and [Fe2(μ-pdt)(CO)5(mppf)]
can be ascribed to the involvement of redox orbitals of
mppf with those of the {Fe2S2} unit.
Results and Discussion
Synthesis and Characterization of 1–3
When a toluene solution of [Fe2{μ-S(CH2)2NR(CH2)2S}-
(CO)6]2 (R = iPr, nPr) was treated with mppf in the presence
of Me3NO at ambient temperature, cis-[Fe2{μ-S(CH2)2NR-
(CH2)2S}(CO)5(mppf)]2 (R = iPr, cis-1; nPr, cis-2) was iso-
lated as a major product in solution, accompanied by some
precipitation of the trans species as a minor product
(Ͻ 10%). Identification of the complexes was made by the
Figure 1. Molecular structures of (a) cis-1 with thermal ellipsoids
IR signatures: 2045 s, 1978 vs, 1956 sh, 1923 w and 2046 s,
drawn at the 30% probability level and (b) trans-1 with thermal
1978 vs, 1957 sh, 1923 w cm–1 in CH2Cl2 solution for cis-1
ellipsoids drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and angles [°] for cis-
1: Fe–Fe, 2.5005(8); Fe–S, 2.2689(11); Fe–P, 2.2226(12); Fe–CCO,ap
1.811(9); Fe–CCO,ba, 1.775(4); S–Fe–S, 79.74(4); S–Fe–Fe, 56.56(3);
Fe–S–Fe, 66.88(3); trans-1: Fe–Fe, 2.5093(9); Fe–S, 2.2710(13); Fe–
P, 2.2343(14); Fe–CCO,ap, 1.822(6); Fe–CCO,ba, 1.777(5); S–Fe–S,
and cis-2, respectively. The trans isomer was obtained as the
sole product when the reaction was performed with heating
(Scheme 2). The relatively low solubility of the trans species
in common organic solvents makes the isolation easy but
,
limits characterization and electrochemical investigation in 79.37(5); S–Fe–Fe, 56.46(4); Fe–S–Fe, 67.07(4).
Scheme 2. Reaction scheme for the synthesis of 1 and 2.
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Influence of a Redox-Active Phosphane on the Oxidations of a Diiron Core
of the complexes 1–3 reveal no significant difference from
those of their PPh3 derivatives. This suggests that PPh3 and
mppf have approximately the same donor strength. The mo-
lecular structures of the complexes cis-1 and trans-1 are
shown in Figure 1 and that of cis-2 in Figure S1. Figure 2
displays the molecular structure of 3. Selected metric pa-
rameters are listed in the figure captions. No significant
variation for the mppf species is observed compared to their
PPh3 analogues.
Figure 2. Molecular structure of 3, with thermal ellipsoids drawn at
the 50% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths [Å] and angles [°]: Fe–Fe, 2.5170(9); Fe–S,
Figure 3. 31P{1H} NMR spectra of 3 in the CD2Cl2 solution at
various temperatures.
2.2638(13); Fe–P, 2.2377(13); Fe–CCO,ap, 1.805(6); Fe–CCO,ba,
1.781(5); S–Fe–S, 84.66(5); S–Fe–Fe, 56.23(4); Fe–S–Fe, 67.55(4).
Two different behaviors in the 31P{1H} NMR spectra are
observed for the adt and pdt species in CD2Cl2. For the
complexes cis-1, and cis-2, a single resonance remains in the
temperature range of 300–183 K. The signal broadens at
T ≈ 230 K and becomes sharp again at lower temperature
suggesting the presence of only one species. The 31P NMR
spectra of 3 reveal significant temperature dependence (Fig-
ure 3). The singlet of 55.9 ppm broadens upon cooling. The
appearance of two new signals at δ = 51.6 and 63.1 ppm
with unequal integrations suggests that three isomers are
present, resulting from the apical/basal fluxional process
along with the flipping process of the methylene bridge-
head. From the ratio of 1 (δ = 63.1 ppm):21 (δ =
54.2 ppm):6 (δ = 51.6 ppm) it can be seen that the major
species in the solution preserves its structure in the solid
state.
Figure 4. FTIR spectral changes for the conversion of cis-1 to
trans-1 in the hot toluene solution. The spectrum inset highlights
the IR region between 2100 and 1990 cm–1. The short dashed lines
indicate the signals of trans-1.
Conversion of the kinetic species, cis, to the thermo-
dynamic product, trans, was monitored by IR and 31P
NMR spectroscopy. Figure 4 displays the IR spectral
changes with time for the toluene solution of cis-1 at 80 °C.
The IR bands of cis-1 decreased in concert with the precipi-
Electrochemistry
Cyclic voltammograms of cis-1 and cis-2 in the accessible
tation of trans-1. When the conversion process was com- electrochemical window of the CH2Cl2 solution at ambient
plete, the solution color became pale orange and most of temperature under N2 exhibits one irreversible redox event
the trans-1 product was present in solid form. A longer re- at –2.19 and –2.18 V (all potentials are referenced to Fc/
action time was required for the reaction proceeded at room Fc+) for cis-1 and cis-2, respectively, which are assigned to
temperature. No intermediate was observed during the con- the reduction of these complexes (Figure 5). These re-
version.
duction potentials are comparable to that of –2.17 V for
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Table 1. Electrochemical properties of 1–3 (1 mm, 0.1 m nBu4NPF6, υ = 100 mV/s) and related compounds.
[a]
[a]
Complex
Solvent
Epa
Epc
[11]
[Fe2{μ-S(CH2)2NnPr(CH2)2S}(CO)6]2
CH2Cl2[11]
CH2Cl2
CH2Cl2
CH2Cl2
+0.74
+0.35
–1.90
–2.17
–2.19
–2.18
[Fe2{μ-S(CH2)2NnPr(CH2)2S}(CO)5(PPh3)]2
cis-[Fe2{μ-S(CH2)2NiPr(CH2)2S}(CO)5(mppf)]2, cis-1
cis-[Fe2{μ-S(CH2)2NnPr(CH2)2S}(CO)5(mppf)]2, cis-2
+0.46, +0.26(qr)
+0.44(qr),
+0.27(qr)
+0.74[b]
[Fe2(μ-pdt)(CO)6]
CH3CN[13]
CH3CN[16]
CH3CN[14]
CH3CN[15]
THF[16]
–1.74[b]
–1.16(rv)[c]
–1.57[d]
–1.6[e]
+1.2[c]
+0.84[d]
+0.67[e]
[c]
–
–1.25(rv)[c]
–1.73[f]
–1.93
THF[12]
–
[f]
CH2Cl2
+0.84
[Fe2(μ-pdt)(CO)5(PPh3)]
CH3CN[14]
CH2Cl2
+0.70, +0.34[d]
+0.29(qr)
–1.76[d]
–2.12
CH2Cl2
CH2Cl2
CH2Cl2
+0.32(rv)[g]
+0.53, +0.22(rv)
0.77(rv), 0.47(qr),
0.16(rv)[g]
–2.04[g]
–2.04
[Fe2(μ-pdt)(CO)5(mppf)], 3
–2.14[g]
[a] All redox pairs are irreversible and the potentials are referenced to Fc/Fc+ unless noted otherwise. rv = reversible. qr = quasireversible.
Epa (or Epc) = E1/2 when the redox process is (quasi)reversible. [b] Under N2 or CO, 0.1 m nBu4NPF6, υ = 200 mV/s. [c] nBu4NPF6, υ =
200 mV/s. The oxidation potential was not reported in THF. Referenced to Ag/AgCl. [d] 0.1 m nBu4NPF6, υ = 100 mV/s. Referenced to
Ag/AgNO3. [e] 0.3 m nBu4NPF6, υ = 100 mV/s. [f] Under N2 or CO, 0.3 m nBu4NClO4, υ = 100 mV/s. The oxidation potential was not
reported. [g] 0.1 m nBu4NBArF24, see text.
[Fe2{μ-S(CH2)2NnPr(CH2)2S}(CO)5(PPh3)]2.[11] For the anion in CH2Cl2 solution. Three reversible oxidation and
oxidation responses, two quasireversible processes are re- one irreversible reduction events are recorded when the vol-
corded. The first occurs at 0.26 and 0.27 V for cis-1 and cis- tammograms are measured in the presence of nBu4-
2, respectively, and is assigned to the oxidation of mppf and NBArF24 (BArF24 = [B{C6H3(CF3)2}4]–) as a supporting
one Fe center on the basis that mppf is oxidized at 0.1 V. electrolyte (Figure 6b). The reduction potential is slightly
The second Fe center is oxidized at 0.46 and 0.44 V for cis-
1 and cis-2, respectively. Table 1 tabulates the redox poten-
tials of 1–3 and their analogues.
Figure 5. Cyclic voltammograms of cis-2 (1 mm) in CH2Cl2 (υ =
100 mV/s, 0.1 m nBu4NPF6, vitreous carbon electrode) under N2.
Inset is shown the differential pulse voltammogram for the oxi-
dation processes.
Complex 3 is reduced at –2.04 V in CH2Cl2 solution.
When a positive potential is applied, oxidation of the mppf
fragment at 0.22 V and a striping response at 0.53 V are
observed (Figure 6a). Decomposition does not occur as re-
versibility of the mppf redox pair is sustained during ad-
sorption and desorption of the oxidized species. Avoidance
of the striping wave and disentanglement of the Fe oxi-
and (b) in CH2Cl2 (υ = 100 mV/s, 0.1 m nBu4NBArF24, vitreous
dations are facilitated by the use of a weakly coordinating carbon electrode, 275 K) under N2.
Figure 6. Cyclic voltammograms of 3 (1 mm) (a) in CH2Cl2 (υ =
225 mV/s, 0.1 m nBu4NPF6, vitreous carbon electrode) under N2
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Influence of a Redox-Active Phosphane on the Oxidations of a Diiron Core
shifted to –2.14 V and its reversibility does not show any is calculated from the plots of the Iccat vs. the square root of
cat
improvement. Two quasireversible oxidation events of the the acid concentration per {Fe2S2} unit: –Ic = nFA[cat]-
1/2
cat
Fe centers at 0.47 and 0.77 V are resolved instead of ab- Dcat
k
1/2[H+]1/2 where Ic is the catalytic current, n the
cat
sorption of the oxidized species. Likewise, the first oxi- number of electrons involved, F the Faraday constant, A
dation at 0.16 V is a one-electron event ascribed to the the surface area of the working electrode, [cat] the bulk con-
FeII/FeIII pair of mppf. The positive potential gain of centration of the catalyst, Dcat the diffusion coefficient of a
150 mV on average with respective to mppf for 1–3 suggests catalyst, and kcat the rate constant of the catalytic reac-
1/2
that the HOMO of mppf is substantially stabilized upon tion.[20] The slope is proportional to kcat while the acid
coordination to the {Fe2S2} core. The ΔE1/2 shift recorded concentration is larger than the concentration of the cata-
here compared to the observed result of 30 mV in Re(CO)4- lyst. A good linear relationship is obtained with [HOAc]/
Cl(mppf) reveals that the {Fe2S2} core exerts a stronger [{Fe2S2}] Ͼ 3 for all complexes, shown in Figure 7b. The
electron-withdrawing effect.[17]
value of the A[cat]Dcat1/2 product is calculated from the ini-
Electrocatalytic production of H2 by 1–3 was monitored tial peak current prior to addition of acids. The rate con-
under N2 with stepwise addition of acetic acid (HOAc). A stant kcat for each {Fe2S2} unit within cis-1 and cis-2 is
new reduction wave was observed at the potential about 20– estimated as 382 and 425 m–1 s–1, respectively. Compared to
40 mV more positive when one equiv. of HOAc per {Fe2S2} the rate constant kcat of 95 m–1 s–1 for 3, both cis-1 and cis-
unit is added (Figure 7a and S2). The peak current of the 2 are superior catalysts to 3 with respect to reduction of
reduction process increases linearly with sequential in- HOAc to H2.
crements of acid added, indicating an electrocatalytic event
Influence of the mppf Ligand
cat
(Figure 7b). The slopes of the Ic vs. [H+]/[complex] plots
suggest that the efficiency of cis-1 and cis-2 for proton re-
When 3 is chemically oxidized by 1 equiv. of [AcFc][BF4]
duction catalysis is better than that of 3. The rate constant (AcFc = acetylferrocenium) or [AcFc][BArF24] in CH2Cl2,
the IR bands of the parent molecule are shifted to higher
energies, as displayed in Figure S3. The average blueshift of
6 cm–1 (5 and 7 cm–1 for [BF4]– and [BArF24]–, respectively)
corresponds to the oxidation of the mppf fragment. The
peak pattern for both the oxidized and parent species is
identical. This indicates that single-electron oxidation
causes no substantial change to the molecular structure of
3.
The intriguing electrochemical feature of compounds 1–
3 is that the two Fe centers within the {Fe2S2} unit with
ligation of mppf are oxidized at two separate potentials.
These redox processes are quasireversible for 1 and 2 in the
electrochemical timescale (υ = 100 mV/s), which is in
marked contrast to that of their PPh3 counterpart. [Fe2{μ-
S(CH2)2NnPr(CH2)2S}(CO)5(PPh3)]2 exhibits one irrevers-
ible oxidation wave at 0.35 V under identical conditions.
This value is closely matched to the average oxidation po-
tential of 0.36 V for 1 and 2.
Cyclic voltammograms of [Fe2(μ-pdt)(CO)5(PPh3)] were
measured in CH2Cl2 for comparison. One irreversible re-
duction process of –2.12 V is observed, displayed in Figure
S4. The oxidation is a quasireversible event occurring at
0.29 V. One redox process is recorded at more positive po-
tential, which is due to the electrochemical response of the
decomposed species that is suppressed at faster scan rates.
Reversibility of the FeIFeI/FeIFeII redox pair is also im-
proved under the same conditions. The FeIFeI/FeIFeII redox
response is electrochemically reversible with substitution of
the supporting electrolyte to nBu4NBArF24, shown in Fig-
ure S5. The current of the irreversible reduction is approxi-
mately twice that of the reversible oxidation, indicating the
oxidation is a single-electron event in CH2Cl2.
Instead of one single-electron oxidation for the PPh3 an-
alogue, two one-electron oxidation events occur at 0.47 and
0.77 V for 3. The presence of [mppf]+ exerts an effect of
150 mV on the redox level of the Fe center within the
Figure 7. Cyclic voltammograms of (a) cis-1 (1 mm) with in-
crements of HOAc (0.0, 2.0, 4.0, 6.0, 8.0, and 10.0 equiv.) in
CH2Cl2 (υ = 100 mV/s, 0.1 m nBu4NPF6, vitreous carbon electrode)
under N2. (b) Dependence of peak catalytic current (Iccat) vs. equiv.
of HOAc added for 1–3 (at room temp. 1 mm in CH2Cl2). Inset is
cat
displayed the linear relationship between Ic and the square root
of the acid concentration per {Fe2S2} unit.
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Ag+ electrode (0.01 m AgNO3/0.1 m nBu4NPF6). All potentials are
measured in 0.1 m nBu4NPF6 or nBu4NBArF24 solution in CH2Cl2
and are reported against Fc/Fc+.
{Fe2S2} unit with respect to the first Fe oxidation. In ad-
dition, ligation of mppf facilitates access to the FeIIFeII re-
dox pair. Comproportionation constants (Kcomp) of 2422
and 752 are calculated for cis-1 and cis-2, respectively.
These small values indicate that weak communication is
Molecular Structure Determinations: The X-ray single crystal crys-
tallographic data collections for 1–3 were carried out at 150 K with
present between two Fe centers. On the other hand, a large a Bruker SMART APEX CCD four-circle diffractometer with
Kcomp of 1.2ϫ105 is obtained for 3, suggesting substantial
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) out-
fitted with a low-temperature, nitrogen-stream aperture. The struc-
electron delocalization within the Fe2 core. From these re-
tures were solved by direct methods, in conjunction with standard
sults it can be seen that access to further redox levels,
difference Fourier techniques and refined by full-matrix least-
namely oxidation, of the {Fe2S2} unit benefits from the in-
squares procedures. A summary of the crystallographic data for
troduction of mppf. Experimental and theoretical investi-
the complexes 1–3 is shown in Table S1. An empirical absorption
correction (multi-scan) was applied to the diffraction data for all
structures. All non-hydrogen atoms were refined anisotropically
gations of the mixed valence and fully oxidized species are
currently ongoing.
and all hydrogen atoms were placed in geometrically calculated po-
sitions by the riding model. All software used for diffraction data
Conclusions
processing and crystal structure solution and refinement are con-
tained in the SHELXL-97 program suites.[26] One carbonyl group,
We have successfully used mppf as a simple model for
C(6)O(6), and the cosolvent molecule, CH2Cl2, exhibit disorder in
the ferredoxin cluster in the H-cluster to examine the influ-
ence of a redox-active ligand on the redox levels of the Fe2
subunit. Our results indicate that incorporation of mppf ef-
fectively alters the oxidation responses of the Fe centers
without significant structural distortion. In addition, a sub-
stantial electron delocalization between the proximal and
distal Fe sites is attested. These observations pave the way
for the design of better artificial biocatalysts for hydrogen
oxidation and production.
cis-1. In trans-1, the cosolvent CH2Cl2 exhibits disorder. The propyl
group, C(17)–C(19), of the tertiary amine site is disordered in cis-
2. There are two molecules in the unit cell of 3, and two carbon
atoms, C(37) and C(38), of the pdt bridge in one of the molecules
are disordered.
Synthesis of cis-1: [Fe2{μ-S(CH2)2NiPr(CH2)2S}(CO)6]2 (400 mg,
0.44 mmol), mppf (375 mg, 1.01 mmol) and anhydrous Me3NO
(99 mg, 1.32 mmol) were placed in a 50 mL Schlenk flask in a dry
box. Toluene (30 mL) was added to give an orange-red solution.
The solution was stirred at ambient temperature for 1 h before the
solvent was removed under reduced pressure. The solid was redis-
solved in CH2Cl2/acetone (v/v 10:1) and the solution was filtered
through a silica gel plug. The filtrate was dried under vacuum to
afford an orange-red solid which was washed with several portions
of hexane. cis-[Fe2{μ-S(CH2)2NiPr(CH2)2S}(CO)5(mppf)]2 was ob-
tained after purification by chromatography on silica gel with
CH2Cl2 as the eluent. From the red band, an orange-red solid was
obtained in 34% yield (237 mg). Crystals of cis-[Fe2{μ-S(CH2)2-
NiPr(CH2)2S}(CO)5(mppf)]2·CH2Cl2 suitable for X-ray crystallo-
graphic analysis were grown from a CH2Cl2/MeOH solution at
Experimental Section
General Methods: All reactions were carried out using standard
Schlenk and vacuum line techniques under an atmosphere of puri-
fied nitrogen. All commercially available chemicals were of ACS
grade and used without further purification. Solvents were of
HPLC grade and purified as follows: diethyl ether and THF were
distilled from sodium/benzophenone under N2. Hexane was dis-
tilled from sodium under N2. Dichloromethane was distilled from
CaH2 under N2. Acetonitrile was distilled first over CaH2 and then
from P2O5 under N2. Deuterated solvents obtained from Merck
were distilled from 4 Å molecular sieves under N2 prior to use. The
preparation of [Fe2{μ-S(CH2)2NR(CH2)2S}(CO)6]2 (R = iPr, nPr)
has been described previously.[11] [AcFc][BF4],[21] Na[BArF24],[22]
Ag[BArF24],[23] nBu4NBArF24,[21] [AcFc][BArF24],[21] [Fe2(μ-
pdt)(CO)6],[24] and mppf[25] were prepared according to the litera-
ture.
–20 °C. IR (CH Cl ): νCO = 2045 (s), 1978 (vs), 1956 (sh), 1923 (w)
˜
2
2
cm–1. IR (KBr): ν = 2044 (s), 1978 (vs), 1965 (s), 1954 (s), 1923
˜
CO
(m), 1895 (w, sh) cm–1. H NMR (500 MHz, CD2Cl2, 274 K): δ =
1
3
0.01 (br., 2 H, CH2), 0.78 (d, JHH = 6.5 Hz, 6 H, 2 CH3), 1.04 (d,
3JHH = 6.5 Hz, 6 H, 2 CH3), 1.45 (m, 2 H, CH2), 1.74 (t, JHH
=
3
11.5 Hz, 2 H, CH2), 1.97 (m, 4 H, 2 CH2), 2.07 (m, 2 H, CH2),
3
2.19 (m, 2 H, CH2), 2.48 (hep, JHH = 6.5 Hz, 2 H, 2 NCH), 2.56
(m, 2 H, CH2), 3.91 (s, 10 H, 2 Cp), 4.04 (s, 2 H, 2 PC5H4), 4.49
(s, 2 H, 2 PC5H4), 4.59 (s, 2 H, 2 PC5H4), 4.76 (s, 2 H, 2 PC5H4),
6.78–8.07 (20 H, 4 Ph) ppm. 13C{1H} NMR (125.7 MHz, CD2Cl2,
273 K): δ = 14.11 (2 CH3), 21.05 (2 CH3), 27.76 (2 CH2), 37.91 (2
CH2), 49.82 (CH2 + 2 NCH), 49.89 (CH2), 51.12 (2 CH2), 70.17 (2
C5H5), 71.40, 71.47, 71.70, 72.92, 72.96, 76.70, 76.86, 79.04 (ipso),
79.44 (ipso) (2 PC5H4), 127.95, 128.02, 128.57, 128.62, 128.70,
131.03, 131.11, 135.08, 135.12, 135.22, 135.37, 141.48 (ipso), 141.81
(ipso) (4 Ph), 215.13, 215.19, 216.24, 216.29 (10 CO) ppm. 31P{1H}
NMR (202.48 MHz, CD2Cl2, 273 K): δ = 54.65 (s) ppm. ESI-MS:
m/z = 1486.76 {cis-1 – 4CO + H+}+, 1599.75 {cis-1 + H+}+.
C68H68Fe6N2O10P2S4 (1598.55): calcd. C 51.09, H 4.29, N 1.75;
found C 51.01, H 4.61, N 1.70.
Infrared spectra were recorded with a Perkin–Elmer Spectrum One
using a 0.05 mm CaF2 cell. 1H, 13C{1H} and 31P{1H} NMR spectra
were recorded with a Bruker AV-500 or DRX-500 spectrometer op-
erating at 500, 125.7, and 202.49 MHz. Spectra are referenced to
1
tetramethylsilane (TMS) for H and 13C{1H} and 85% H3PO4 for
31P{1H} NMR spectroscopy. Mass spectral analyses were per-
formed with a Waters LCT Premier XE at the Mass Spectrometry
Center in the Institute of Chemistry, Academia Sinica. Elemental
analyses were performed with an Elementar vario EL III elemental
analyzer.
Electrochemistry: Electrochemical measurements were recorded
with a CH Instruments 630C electrochemical potentiostat using a
gastight three-electrode cell under N2. A glassy carbon electrode
and a platinum wire were used as the working and auxiliary elec-
trodes, respectively. The reference electrode was a nonaqueous Ag/
Synthesis of cis-2: A similar synthetic procedure was used as de-
scribed above for cis-1 with [Fe2{μ-S(CH2)2NnPr(CH2)2S}(CO)6]2
as the starting material. The yield was 77% yield (539 mg). Crystals
1160
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 1155–1162

Influence of a Redox-Active Phosphane on the Oxidations of a Diiron Core
of cis-[Fe2{μ-S(CH2)2NnPr(CH2)2S}(CO)5(mppf)]2·CH2Cl2 suit-
turned to yellow-orange over 1.5 h. A red-orange-brown solid was
able for X-ray crystallographic analysis were grown from a CH2Cl2/
obtained upon removal of the solvent under vacuum. The product
was extracted from the solid by washing with THF/hexane. The
MeOH solution at –20 °C. IR (CH Cl ): ν = 2046 (s), 1978 (vs),
˜
2
2
CO
1957 (sh), 1923 (w) cm–1. IR (KBr): νCO = 2042 (s), 1976 (vs), 1961 solvent was removed from the extractant to afford an orange-red
˜
(s), 1952 (s), 1924 (m), 1897 (w, sh) cm–1. 1H NMR (500 MHz,
CD2Cl2, 273 K): δ = 0.19 (br., 1 H, CH2), 0.21 (br., 1 H, CH2), CH2Cl2/hexane (v/v 1:1) as the eluent to remove unreacted starting
solid, which was purified by chromatography on silica gel with
3
0.80 (t, JHH
NCH2CH2CH3), 1.65 (m, 4 H, 2 CH2), 1.82 (m, 4 H, 2 CH2), 1.95 were removed under reduced pressure to afford the red-orange solid
(m, H, NCH2CH2CH3), 2.09–2.21 (m, H, CH2 of [Fe2(μ-pdt)(CO)5(mppf)] in 68% yield (99 mg). Crystals of
NCH2CH2CH3), 2.26 (m, 2 H, CH2), 2.54 (m, 2 H, CH2), 3.92 (s, [Fe2(μ-pdt)(CO)5(mppf)] suitable for X-ray crystallographic analy-
10 H, 2 Cp), 4.40 (s, 2 H, 2 PC5H4), 4.50 (s, 2 H, 2 PC5H4), 4.57 sis were grown from a CH2Cl2/MeOH solution at –20 °C. IR
(s, 2 H, 2 PC5H4), 4.78 (s, 2 H, 2 PC5H4), 6.75–8.08 (20 H, 4 Ph) (CH Cl ): ν = 2044 (s), 1981 (vs), 1960 (sh), 1930 (w) cm–1. IR
=
7.0 Hz,
6
H,
2
CH3), 1.39 (m,
4
H,
2
material and the second red-orange band was collected. Solvents
2
4
+
˜
CO
2
2
ppm. 13C{1H} NMR (125.7 MHz, CD2Cl2, 273 K): δ = 12.00 (2
CH3), 19.63 (2 NCH2CH2CH3), 25.66 (2 CH2), 37.33 (2 CH2), (s), 1930 (m), 1901 (w, sh) cm–1. 1H NMR (500 MHz, CDCl3,
54.44 (2 CH2, overlapped with CD2Cl2), 56.20 (2 CH2), 56.54 (2 296 K): δ = 1.24 (m, 1 H, CH2), 1.32 (m, 2 H, CH2), 1.51 (m, 1 H,
(KBr): ν = 2050 (s), 2037 (m, sh), 1990 (m, sh), 1971 (vs), 1952
˜
CO
NCH2CH2CH3), 70.16 (2 C5H5), 71.41, 71.48, 71.76, 72.94, 72.97, CH2), 1.71 (m, 2 H, CH2), 3.94 (s, 5 H, C5H5), 4.50 (m, 4 H,
76.66, 76.82, 79.04 (ipso), 79.44 (ipso) (2 PC5H4), 127.93, 127.99,
128.61, 128.65, 128.73, 131.04, 131.11, 134.99, 135.10, 135.45,
PC5H4), 7.40, 7.62 (m, 10 H, 2 Ph) ppm. 13C{1H} NMR
(125.7 MHz, CDCl3, 297 K): δ = 21.51 (2 CH2), 29.94 (1 CH2),
141.50 (ipso), 141.83 (ipso) (4 Ph), 211.10 (br), 214.99, 215.06, 69.74 (C5H5), 71.22 (d, JPC = 6.9 Hz, 2 C, PC5H4), 74.26 (d, JPC
=
216.37, 216.42 (10 CO) ppm. 31P{1H} NMR (202.48 MHz, 11.6 Hz, 2 C, PC5H4), 78.66 (d, JPC = 44.0 Hz, 1 C, ipso-PC5H4),
CD2Cl2, 273 K): δ = 54.43 (s) ppm. ESI-MS: m/z = 1485.89 {cis-
2 – 4CO + H+}+, 1570.88 {cis-2 – CO + H+}+, 1598.78 {cis-2 +
127.85 (d, JPC = 9.6 Hz, 4 C, PC6H5), 129.72 (2 C, PC6H5), 132.95
(d, JPC = 11.0 Hz, 4 C, PC6H5), 138.81 (d, JPC = 42.7 Hz, 2 C,
H+}+. C69H70Cl2Fe6N2O10P2S4 (1683.49): calcd. C 49.23, H 4.19, ipso-PC6H5), 209.53 (3 CO), 214.31 (d, JPC = 10.9 Hz, 2 CO) ppm.
N 1.66; found C 49.20, H 4.24, N 1.59.
31P{1H} NMR (202.48 MHz, CDCl3, 298 K): δ = 55.87 (s) ppm.
FAB-MS: m/z = 728.0 {3 + H+}+. C30H25Fe3O5PS2 (728.16): calcd.
C 49.48, H 3.46, S 8.81; found C 49.04, H 3.43, S 8.83.
Synthesis of trans-1: To a Schlenk flask containing [Fe2{μ-S(CH2)2-
NiPr(CH2)2S}(CO)6]2 (500 mg, 0.55 mmol) and mppf (446 mg,
1.20 mmol) was added toluene (15 mL). The solution was heated
to 90 °C in an oil bath for 22 h. The solution was evaporated to
dryness and several portions of hexane were used to wash the solid
obtained. The solid was dried under vacuum and the yield was 86%
(753 mg). Crystals of trans-[Fe2{μ-S(CH2)2NiPr(CH2)2S}(CO)5-
(mppf)]2·4CH2Cl2·H2O suitable for X-ray crystallographic analysis
were grown at –20 °C from a CH2Cl2/wet MeOH solution. IR
Conversion of the cis-Isomers to the trans-Isomers: To a flask con-
taining cis-1 or cis-2 (50 mg) was added toluene (10 mL). The solu-
tion was stirred at an elevated temperature and monitored con-
stantly by FTIR spectroscopy. The length of conversion time de-
pended on the reaction temperature. The reaction required 20 h for
completion at 80 °C. When the reaction was finished, the pale yel-
low solution was removed. Several portions (3 mLϫ3) of toluene
were used to wash the remaining solid. The trans-isomers were ob-
(CH Cl ): ν = 2041 (s), 1978 (vs), 1966 (sh), 1957 (sh), 1921 (w)
˜
2
2
CO
cm–1. IR (KBr): ν = 2040 (s), 1978 (s, sh), 1970 (vs), 1963 (s),
˜
CO
tained in yields greater than 80%. cis-1: IR (toluene): ν = 2047
˜
CO
1951 (s), 1917 (m), 1899 (w, sh), 1890 (w, sh) cm–1. 1H NMR
(s), 1979 (vs), 1969 (sh), 1958 (m), 1926 (w), 1915 (sh) cm–1. trans-
3
(500 MHz, CD2Cl2, 273 K): δ = 0.12 (m, 2 H, CH2), 0.5 (d, JHH
1: IR (toluene): νCO = 2040 (s), 1978 (vs), 1968 (sh), 1951 (m), 1925
˜
3
= 6.0 Hz, 6 H, 2 CH3), 0.66 (d, JHH = 5.5 Hz, 6 H, 2 CH3), 1.76
(w), 1914 (sh) cm–1. cis-2: IR (toluene): ν = 2047 (s), 1980 (vs),
˜
CO
(m, 4 H, 2 CH2), 1.95 (m, 6 H, 3 CH2), 2.35 (m, 4 H, 2 NCH +
CH2), 2.48 (m, 2 H, CH2), 3.93 (s, 10 H, 2 Cp), 4.04 (s, 2 H, 2
PC5H4), 4.47 (s, 2 H, 2 PC5H4), 4.60 (s, 2 H, 2 PC5H4), 4.74 (s, 2
H, 2 PC5H4), 7.09–8.06 (20 H, 4 Ph) ppm. 31P{1H} NMR
(202.48 MHz, CD2Cl2, 273 K): δ = 54.78 (s) ppm. ESI-MS: m/z =
799.98 {trans-1 + 2H+}2+, 1599.00 {trans-1 + H+}+. C68H68Fe6N2-
O10P2S4 (1598.55): calcd. C 51.09, H 4.29, N 1.75; found C 51.06,
H 4.30, N 1.80.
1970 (sh), 1959 (m), 1926 (w) cm–1. trans-2: IR (toluene): ν
=
˜
CO
2041 (s), 1978 (vs), 1967 (sh), 1955 (m), 1926 (w) cm–1.
Oxidation of 3 by [AcFc][BF4] and [AcFc][BArF24]: Chemical oxi-
dation of 3 was conducted in CH2Cl2 under N2 below –70 °C.
When one equiv. of [AcFc][BF4] or [AcFc][BArF24] was added to
the reaction solution, an instant color change to dark brown was
observed. The in situ IR spectra were recorded with a Mettler To-
ledo ReactIR iC10 FTIR system equipped with a MCT detector
and a 0.625 inch SiComp probe. The original IR peaks at 2044 (s),
1980 (vs), 1961 (sh), and 1931 (w) cm–1 were shifted to higher ener-
gies at 2050 (s), 1985 (vs), 1963 (sh), and 1931 (w) and 2053 (s),
1987 (vs), 1969 (sh), and 1931 (w) cm–1 for the [BF4] and [BArF24]
salts, respectively. The IR band of 1662 cm–1 was assigned to the
carbonyl group of acetylferrocene.
Synthesis of trans-2: A similar synthetic procedure was used as de-
scribed above for trans-1 with [Fe2{μ-S(CH2)2NnPr(CH2)2S}-
(CO)6]2 as the starting material. The yield was 71% (621 mg).
Attempts to grow crystals of trans-2 were not made due to its very
low solubility in organic solvents. IR (CH Cl ): ν = 2041 (s),
˜
CO
2
2
1978 (vs), 1955 (sh), 1919 (w) cm–1. IR (KBr): νCO = 2034 (s), 1976
˜
(s), 1957 (s), 1949 (s), 1926 (m), 1899 (w, sh) cm–1. 31P{1H} NMR
(202.48 MHz, CD2Cl2, 300 K): δ = 54.72 (s) ppm. ESI-MS: m/z =
1570.9 {trans-2
C68H68Fe6N2O10P2S4 (1598.55): calcd. C 51.09, H 4.29, N 1.75;
found C 50.79, H 4.26, N 1.79.
Supporting Information (see footnote on the first page of this arti-
cle): Molecular structure of cis-2, cyclic voltammograms, IR spec-
tra, and crystallographic data.
– CO + +
H+}+, 1598.9 {trans-2 H+}+.
Synthesis of 3: To a flask containing [Fe2(μ-pdt)(CO)6] (77 mg,
0.2 mmol) and Me3NO·2H2O (22 mg, 0.2 mmol) was added
CH3CN (20 mL). An immediate color change to dark orange-
brown was observed. The solution was stirred for half an hour
before addition of mppf (74 mg, 0.2 mmol). The solution slowly
CCDC-792562 (for cis-1), -792563 (for trans-1), -792561 (for cis-2),
and -792560 (for 3) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Eur. J. Inorg. Chem. 2011, 1155–1162
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1161

Y.-C. Liu, C.-H. Lee, G.-H. Lee, M.-H. Chiang
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Received: September 13, 2010
Published Online: January 24, 2011
1162
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 1155–1162