Oxidatively induced reactivity of [Fe2(CO)4(κ 2-dppe)(μ-pdt)]: An electrochemical and theoretical study of the structure change and ligand binding processes

Article abstract of DOI:10.1021/ic201601q

The one-electron oxidation of the diiron complex [Fe₂(CO) ₄(κ²-dppe)(μ-pdt)] (1) (dppe = Ph ₂PCH₂CH₂PPh₂; pdt = S(CH ₂)₃S) has been investigated in the abse

Full text of DOI:10.1021/ic201601q

Article
pubs.acs.org/IC
Oxidatively Induced Reactivity of [Fe2(CO)4(κ2-dppe)(μ-pdt)]: an
Electrochemical and Theoretical Study of the Structure Change and
Ligand Binding Processes
Dounia Chouffai,† Giuseppe Zampella,*,‡ Jean-Francois Capon,† Luca De Gioia,*,‡ Frederic Gloaguen,†
̧
́ ́
Franco̧
is Y. Petillon,† Philippe Schollhammer,*,† and Jean Talarmin*
́
†UMR CNRS 6521, Chimie, Electrochimie Molec
Techniques, Cs 93837, 29238 Brest-Cedex 3, France
́ ́
ulaires et Chimie Analytique, Universite de Bretagne Occidentale, UFR Sciences et
‡Department of Biotechnology and Bioscience, University of Milano-Bicocca, Piazza della Scienza 2, 20126 Milan, Italy
S
* Supporting Information
ABSTRACT: The one-electron oxidation of the diiron
complex [Fe2(CO)4(κ2-dppe)(μ-pdt)] (1) (dppe =
Ph2PCH2CH2PPh2; pdt = S(CH2)3S) has been investigated in
the absence and in the presence of P(OMe)3, by both
electrochemical and theoretical methods, to shed light on the
mechanism and the location of the oxidatively induced structure
change. While cyclic voltammetric experiments did not allow to
discriminate between a two-step (EC) and a concerted, quasi-
reversible (QR) process, density functional theory (DFT)
calculations favor the first option. When P(OMe)3 is present,
the one-electron oxidation produces singly and doubly
substituted cations, [Fe2(CO)4−n{P(OMe)3}n(κ2-dppe)(μ-
pdt)]+ (n = 1: 2+; n = 2: 3+) following mechanisms that were
investigated in detail by DFT. Although the most stable isomer of 1+ and 2+ (and 3+) show a rotated Fe(dppe) center, binding of
P(OMe)3 occurs at the neighboring iron center of both 1+ and 2+. The neutral compound 3 was obtained by controlled-potential
reduction of the corresponding cation, while 2 was quantitatively produced by reaction of 3 with CO. The CO dependent
conversion of 3 into 2 as well as the 2+ ↔ 3+ interconversion were examined by DFT.
the models,4 which is less effective toward formation of H2 than
INTRODUCTION
One obvious difference between the active site of [FeFe]-
hydrogenases, the H-cluster, and the synthetic compounds of
general formula [Fe2(CO)6−x(L)x(μ-dithiolate)] is the geometry
■
the terminal hydride5 probably formed at the enzyme active
site.3,6−8
Recent research has been undertaken to identify the factors
that would favor the inverted geometry found at the enzymes’
diiron subsite, the so-called rotated state. Theoretical
calculations showed that one of these factors is a dissymmetric
of the diiron core, where the pyramids about the metal centers
are eclipsed (Fe(I)Fe(I) models) rather than inverted with a
bridging or semibridging carbonyl (enzymes) (Scheme 1).1−3
A
substitution at the iron centers,9 which stimulated the synthesis
of model compounds bearing chelating ligands. Although
crystallographic data proved that the [Fe2(CO)4(κ2-LL)(μ-
dithiolate)] models still adopted an eclipsed geometry in the
solid,10−15 some aspects of their chemistry,13 and particularly
the transient formation of terminal hydrides upon proto-
nation10d,14 suggested an easier access to the rotated state than
in the symmetrically disubstituted analogues.
Scheme 1. Schematic Representation of the Active Site of the
[FeFe]H2ases (H-cluster) and of Model Compounds in the
Eclipsed and Rotated Geometry
A few years ago, a Fe(I)Fe(II) complex with both a coordina-
tion sphere and spectroscopic properties similar to those of the
CO-inhibited [FeFe]-hydrogenases16 was obtained by oxida-
tion of a Fe(I)Fe(I) precursor.17 More recently, Fe(I)Fe(II)
compounds with a rotated geometry were crystallographically
consequence of this structural difference is to promote the
formation of a bridging hydride upon protonation of most of
Received: July 26, 2011
Published: November 22, 2011
© 2011 American Chemical Society
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characterized.18−21 Detailed studies of the role of the bridging
dithiolate and of the terminal ligands on the geometry about the
iron centers in the Fe(I)Fe(II) oxidation state, and of the reactivity
CH2Cl2-[NBu4][PF6] at room temperature yielded very low
amounts of the expected cation (E1/2red = −0.25 V, Epox = 0.74 V)
along with unidentified products. Electrolysis of 1 carried out
at −10 °C produced 1+ almost quantitatively40 after the transfer
of 1 F mol−1 1. 1+ was also obtained by chemical oxidation of
1 (1 equiv ferrocenium, −10 °C) in CH2Cl2. Both the redox
potential of 1 and the infrared spectrum of 1+ in CH2Cl2
(ν(CO) = 2079 (s), 2021 (s), 1920(w) cm−1) are similar
to those of the (κ2-dppv) analogue (dppv = Ph2PCH
CHPPh2).21
Previous experimental and theoretical studies have demon-
strated that one-electron oxidation of [Fe2(CO)4(L)(L′)(μ-
dithiolate)] or [Fe2(CO)4(κ2-LL)(μ-dithiolate)] complexes in
which the geometry of the pyramids about the metal centers is
eclipsed (E) resulted in a cation with a rotated structure
(R+)18−25,35 (Scheme 2). Structural changes induced by
of the oxidized complexes with CO, were also reported.19−25
The oxidatively induced structure change, which may lend
itself to electrochemical investigations,26−32 generates an open
coordination site in the apical position on the rotated iron
center, whose reactivity with substrates may also be investigated
by electrochemical means. The characterization of a complex
resulting from the reaction of a substrate at this vacant site
would also provide indirect evidence as to which of the iron
centers underwent isomerization. In relation with these issues,
we now report a combined electrochemical and density
functional theory (DFT) investigation of the oxidation of a
dissymmetrically disubstituted diiron complex, [Fe2(CO)4(κ2-
dppe)(μ-pdt)] (1),14a in the absence and in the presence of
P(OMe)3 (dppe = Ph2PCH2CH2PPh2; pdt = S(CH2)3S).
Scheme 2
RESULTS
■
1. Oxidation of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (1). The
cyclic voltammetry (CV) of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (1)
shows that the complex undergoes two oxidations, the first of
ox1
which is quasi-reversible with E1/2 = −0.24 ± 0.01 V in
CH2Cl2-[NBu4][PF6].33 The second, irreversible, oxidation
ox2
(Ep ∼ 0.7 V, Table 1) which gives rise to products detected
Table 1. Redox Potentials of the Complexes in CH2Cl2-
[NBu4][PF6]
electrode reactions may occur according to different mecha-
nisms, depending on whether the isomerization is concerted
with the electron transfer (quasi-reversible process, along the
diagonal in Scheme 2) or occurs as a following reaction (two-
steps EC process, Scheme 2), which leads to a square scheme
mechanism in case of chemical reversibility.
It is frequent that the only species detected by CV are the
starting material E and the rearranged oxidized derivative R+ (in
case of an oxidatively induced structure change, Scheme 2)
which makes the distinction between the two mechanisms
difficult.27−29 Experimentally, the two possibilities may be
distinguished if a supplementary redox event (reduction of E+
or oxidation of R, Scheme 2), proving the existence of a square
scheme, is observable under specific conditions. However, that
a supplementary redox step cannot be detected does not
disprove the existence of a square scheme.26−30,36
ox1
ox2
complex
E1/2 V/Fc Ep V/Fc
[Fe2(CO)4(κ2-dppe)(μ-pdt)] (1)
[Fe2(CO)3{P(OMe)3}(κ2-dppe)(μ-pdt)] (2)
[Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-pdt)] (3)
−0.24
−0.54
−0.88
0.68
0.5
0.14
by their reduction on the return scan (Ep ∼ 0 and −0.5 V)
(Figure 1) was not investigated.
Our experimental attempts to discriminate between the two
possibilities failed. In the present case, CV at low temperature
(−40 °C) only resulted in an increase of the anodic to cathodic
peak separation (ΔEp) compared to the CVs run at room
temperature. CV at different scan rates (up to 40 V s−1), that
also lead to an increase of ΔEp with increasing v, showed that
the oxidation of 1 becomes broader when v ≥ 2 V s−1. A
distinct shoulder was present around −0.07 V, while it was not
observed at lower v. However, this is assigned to the separate
oxidation of the two eclipsed isomers of 1 with basal-apical and
dibasal binding of the dppe chelate that were identified by
NMR spectroscopy,14a rather than to the oxidation of 1 in the
eclipsed and rotated geometries.
Figure 1. CV of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (1) (1.13 mM) in
CH2Cl2-[NBu4][PF6] (vitreous carbon electrode; v = 0.2 V s−1;
potentials are in V vs Fc+/Fc).
The comparison of the current function [ip/v1/2] for the first
oxidation of 1 and for the one-electron oxidation of
[Fe2(CO)4(κ2-dppe)(μ-pdt)(μ-H)]+ (Supporting Information,
Figure S1) shows that the former is also a one-electron
process.34 Controlled-potential electrolysis of a solution of 1 in
Although an EC mechanism cannot be ruled out on the basis
of the experimental results, we assumed the occurrence of a
quasi-reversible process to work out the apparent heteroge-
neous electron transfer rate constant from the scan rate
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dependence of ΔEp.37 Digital simulations38 of the electro-
chemical oxidation of 1 were also carried out according to the
quasi-reversible scheme. A comparison of the experimental and
simulated CVs is presented as Supporting Information
(Supporting Information, Figure S2). The agreement is
reasonable at slow scan rates, the difference noted at faster
scan rates is assigned to the fact that a single isomer of 1 was
considered in the simulations.
To complement the experimental investigation on the
mechanism of the oxidation of [Fe2(CO)4(κ2-dppe)(μ-pdt)]
(1), DFT calculations were carried out to characterize the
stereoelectronic properties of 1 and 1+. It turned out that the
minimum energy conformation of 1 corresponds to an eclipsed
form featuring basal-apical dppe, in agreement with exper-
imental results. As for 1+, the lowest energy isomer corresponds
to a structure characterized by rotation of the Fe(dppe) moiety
(Figure 2). Also an eclipsed 1+ isomer closely resembling the
temperature shows that the oxidation peak current is little
affected by the presence of P(OMe)3 while the chemical
reversibility of the 10/+ couple is scan rate dependent, which
demonstrates the occurrence of an EC process.33,39 The pro-
duct of the follow-up reaction(s) is detected by a reversible
reduction at −0.54 V (Figure 3) and an irreversible oxidation
at 0.5 V (not shown in Figure 3).
Electrosyntheses were carried out to isolate (see below) the
product of the reaction of 1+ with P(OMe)3. Controlled-
potential oxidation of a CH2Cl2-[NBu4][PF6] solution of 1 in
the presence of 1 equiv of P(OMe)3 at room temperature (Eel =
−0.2 V, graphite anode) was completed after the passage of
about 0.9 F mol−1 1. CV of the electrolyzed solution showed
that the expected product, with E1/2red = −0.54 V and Epox = 0.5
red
V, was accompanied by a second one, with E1/2 = −0.88 V
ox
and Ep = 0.14 V. Thus, reactions that are too slow for
detection by CV generated the latter on the longer time scale of
electrolysis. Electrolyses performed in the presence of larger
amounts of P(OMe)3 (2−4 equiv) resulted in the almost
quantitative formation40 of the second product (3+), that was
barely detected by CV before electrolysis (Figure 4). A reverse
Figure 2. DFT optimized structures of relevant 1+ isomers. The
energy (in kcal/mol) of each isomer, relative to the lowest-energy one,
is shown below each structure. For the sake of clarity atoms are shown
as follow. Carbon, gray; hydrogen, white; sulfur, yellow; oxygen, red;
phosphorus, light purple; iron, dark purple.
structure of the parent compound 1 (i.e., basal-apical dppe)
corresponds to an energy minimum. However, the energy
of the eclipsed 1+ form is 4.5 kcal/mol higher than for the
rotated isomer. Another rotated 1+ isomer, featuring rotation
of the Fe(CO)3 moiety, is 6.0 kcal/mol higher in energy
(Figure 2). Other 1+ isomers are even higher in energy (data
not shown).
Figure 4. CV of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (1) (1.2 mM) in the
presence of 3 equiv of P(OMe)3 before (black lines) and after (red
lines) controlled-potential electrolysis at room temperature (Eel = −0.2
V; 0.9 F mol−1 1) (CH2Cl2-[NBu4][PF6]; v = 0.2 V s−1; vitreous
carbon electrode; potentials are in V vs Fc+/Fc).
2. Oxidation of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (1) in
the Presence of P(OMe)3 and Characterization of
[Fe2(CO)4−n{P(OMe)3}n(κ2-dppe)(μ-pdt)] (n = 1, (2); n =
2, (3)). The CV of 1 in CH2Cl2-[NBu4][PF6] at room
Figure 3. CV of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (1) (1.1 mM) in CH2Cl2-[NBu4][PF6] in the absence (black lines) and in the presence (red lines) of
1 equiv of P(OMe)3 (vitreous carbon electrode; potentials are in V vs Fc+/Fc).
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electrolysis at −0.96 V generated a brown solution of the
corresponding neutral complex after the transfer of about 0.9 F
mol−1 of the initial 1 (Supporting Information, Figure S8). The
neutral species, obtained in >80% yield,40 was identified as
[Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-pdt)] (3) on the basis of
IR and NMR spectroscopies and indirectly by its reactivity
toward CO which gives [Fe2(CO)3{P(OMe)3}(κ2-dppe)(μ-
pdt)] (2). The redox potential of the 30/+ couple (Table 1) and
the infrared spectra of 3 (ν(CO) in CH2Cl2 = 1899 (s), 1880
(s) cm−1) and 3+ (ν(CO) in CH2Cl2 = 1962 (s), 1887 (w)
cm−1) are similar to those of [Fe2(CO)2(κ2-dppv)2(μ-xdt)]0/+
(xdt = pdt or edt; edt = SCH2CH2S).22,41 Satisfactory 1H NMR
data of 3 could not be obtained, but the 31P-{1H} NMR
spectrum of 3 in CDCl3, displays two singlets at 177.5 ppm and
85.4 ppm, which are assigned unambiguously to the
trimethylphosphite42 and to the chelated diphosphine43 ligands,
respectively. Variable-temperature 31P-{1H} NMR experiments
revealed that 3 is fluxional. Several dynamic processes related to
the dppe, P(OMe)3 or μ-pdt groups may be operative in
solution and this makes the determination of the nature and
the number of isomers difficult to state unambiguously. (see
Supporting Information, Figure S3).
The singly P(OMe)3-substituted analogue, [Fe2(CO)3{P-
(OMe)3}(κ2-dppe)(μ-pdt)] (2), was obtained by bubbling CO
through the solution of 3. 3 was not recovered upon flushing
the CO-saturated solution with N2. X-ray analysis of a single
crystal of 2, obtained from dichloromethane-hexane (1:1)
mixture at −18 °C, established without any ambiguity the
structure, in solid state, of 2 with a basal-apical binding mode of
the diphosphine group and an apical position of the P(OMe)3
ligand (Figure 5). 2 is structurally analogous to other
89.4 ppm (2 P) in the 31P-{1H} NMR spectrum in CD2Cl2 and
assigned to P(OMe)3 and dppe ligands, respectively. The H
1
NMR spectrum displays the set of resonances expected for
trimethylphosphite, dppe and pdt groups (see Experimental
Section). A VT 31P-{1H} NMR experiment, in CD2Cl2,
revealed that 2 is fluxional. At −90 °C in CD2Cl2, two signals
are observed in the phosphite region as well as three resonances
for the phosphorus atoms of the diphosphine (see Supporting
Information, Figure S4). These observations suggest that two
isomers differing at least by the binding mode of the
diphosphine are present in solution in a ratio close to 1:1.
One of them is characterized by a singlet, at 94.1 ppm, that
could be assigned to two equivalent phosphorus atoms of a
symmetrically bonded dibasal dppe ligand with the phosphite
lying in an apical orientation. The other two signals at 90.1 and
94.5 ppm could be attributed to two inequivalent phosphorus
atoms of a basal-apical dppe group of the second isomer. This
would be consistent with the results of the X-ray diffraction
study.
Attempts to synthesize 2 by replacing one carbonyl group by
P(OMe)3 in 1 in refluxing toluene gave a mixture of 2 (minor
product) and of a disubstituted compound 4, having a pendant
dppe ligand, [Fe2(CO)4{P(OMe)3}(κ1-dppe)(μ-pdt)] (4), in a
ratio about 1:10 (see Supporting Information, Figure S5). Such
a CO-migration in the ligand substitution process has already
been reported for an analogous [Fe2(CO)4(κ2-PNP)(μ-pdt)]
compound (PNP = (EtO)2PN(Me)P(EtO)2).46 The reaction
producing 4 was not studied further. The reaction of 2 with
1
HBF4·OEt2 was also monitored by VT H NMR spectroscopy.
The protonation process is similar to that reported for
[Fe2(CO)3{PMe)3}(κ2-dppv)(μ-pdt)].47 The initial formation
of a transient terminal hydride species at low temperature
and its isomerization upon warming into, successively, three
bridging hydride forms with different phosphite and
diphosphine positions, are observed (see Supporting Information,
Figure S6).
Complex 2 undergoes a quasi-reversible oxidation at E1/2
=
−0.54 V (Figure 6a, red trace, Table 1) indicating that 2+,
formed along the reactions in Scheme 3, is the product detected
by CV before electrolysis (Figure 2). Digital simulations38 of
the EC process in Scheme 3 are in fair agreement with the
experimental CVs (Supporting Information, Figure S7).
The presence of 3+ after controlled-potential electrolysis is in
accord with its formation from 2+ and P(OMe)3 (Supporting
Information, Figure S9). In a way similar to the CO-induced
conversion of 3 to 2 (Figure 6a), complex 3+ reacts with CO to
generate 2+ (Figure 6b). However, the reaction is at equilibrium
(2+/3+: 1.2/1) and appears more complicated than a mere
substitution of P(OMe)3 by CO. The occurrence of a minor
homogeneous redox process is indicated by the presence of a
small irreversible oxidation peak at the same potential as the
oxidation of 1 [that is irreversible in the presence of P(OMe)3]
(Figure 6b, red trace). This peak, as well as a small reduction at
the same potential as the 2+/0 couple, are still observed after
flushing the solution with N2 for a prolonged period (Figure 6b,
blue trace). The presence of 1 is proposed to arise from the
disproportionation of a species formed in low concentration
under the present experimental conditions, by reaction of 3+
with CO (see Discussion).
Figure 5. Structure of 2·CH2Cl2,C6H14 showing 30% thermal
ellipsoids (CH2Cl2 and C6H14 are omitted for clarity). Selected
distances (Å) and angles (deg): Fe1−Fe2, 2.5474(13), P1−Fe1,
2.217(2), P2−Fe1, 2.199(2), P3−Fe2, 2.155(2), S1−Fe2, 2.2699(19),
S1−Fe1, 2.2728(19), S2−Fe1, 2.2626(19), S2−Fe2, 2.269(2), P2−
Fe1−P1, 88.05(7), Fe2−S1−Fe1, 68.22(6), Fe1−S2−Fe2, 68.40(6).
trisubstituted diiron complexes of general formula [Fe2(CO)3L-
(κ2-chelate)(μ-dithiolate)].44,45
2 was also characterized by elemental analyses and IR and
NMR spectroscopies. The IR spectrum of 2 in dichloro-
methane displays three ν(CO) bands at 1968(s), 1913(s), and
1892(sh) cm−1. Two singlets are observed at 179.7 (1 P) and
3. DFT Investigations of the Reactivity of 1+ and 2+
with P(OMe)3. To shed further light on the reactivity of 1+,
we have used DFT to dissect the reaction mechanism for the
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Figure 6. (a) CV of [Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-pdt)] (3) in CH2Cl2-[NBu4][PF6] before (black lines) and after (red lines) bubbling CO
through the solution (which generates 2); (b) CV of [Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-pdt)]+ (3+) in CH2Cl2-[NBu4][PF6] before (dashed lines)
and after (red lines) bubbling CO through the solution, and after flushing the solution with N2 (blue lines); the start potential and scan directions are
the same for the red and blue traces (v = 0.2 V s−1; vitreous carbon electrode; potentials are in V vs Fc+/Fc).
Scheme 3
(see Scheme 5) reveals that the free energy barrier (computed
using as reference the lowest energy 1+ isomer) for P(OMe)3
binding to the Fe(CO)3 moiety is 9.9 kcal/mol and, more
importantly, 8.7 kcal/mol lower than the corresponding energy
barrier for P(OMe)3 binding to the iron atom coordinated by
dppe. Therefore, in the initial step the incoming P(OMe)3
ligand reacts with the Fe(CO)3 moiety of 1+. In the
intermediate species which is formed (hereafter referred to as
2-CO+; Scheme 4) one of the terminal CO ligand of the
Fe(CO)3 moiety has moved to a bridging position, while the
CO group that occupied a bridging position in 1+ has moved to
a terminal position.
reactions 1+ + P(OMe)3 → 2+ + CO, and 2+ + P(OMe)3 → 3+
+ CO (Scheme 4).
As discussed in the previous section, the lowest energy
isomer of 1+ corresponds to a rotated structure featuring a
vacant coordination site on the Fe(dppe) moiety. The
structural features of such an isomer might suggest that
P(OMe)3 will bind to the five-coordinated Fe atom where dppe
is bound. However, comparative analysis of the transition states
for P(OMe)3 binding to 1+ along different reaction channels
In the lowest energy 2-CO+ isomer both iron atoms are six-
coordinated, with dibasal dppe and apical P(OMe)3 coordina-
tion. Other 2-CO+ isomers, differing by the relative orientation
of dppe and P(OMe)3, are computed to be at least 1.9 kcal/mol
higher in energy (see Supporting Information).
a
b
Scheme 4. Computed Free Energy Profile for the Reactions 1+ + P(OMe)3 → 2+ + CO, and 2+ + P(OMe)3 → 3+ + CO
a
See Reference 48.
b
Energies in kcal/mol. For the sake of clarity, bonds which are formed or cleaved in transition states are shown in red.
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a
P(OMe)3, the reaction path characterized by the lowest energy
barrier does not imply binding of P(OMe)3 to the vacant
coordination site of the Fe(dppe) moiety, but reaction of the
phosphine ligand at the iron center where the first P(OMe)3
ligand was bound (Scheme 6).
Scheme 5. Relative Energies and Structures of the
Transition States for the Reaction between 1+ and P(OMe)3
a
Scheme 6. Relative Energies of the Transition States for the
b
Reaction between 2+ and P(OMe)3
a
In kcal/mol.
b
Transition states for the attack of P(OMe)3 to the Fe(dppe) moiety
are at least 9 kcal/mol higher in energy, and therefore have not been
shown in the scheme.
a
In kcal/mol.
The energy barrier from the lowest energy 2+ isomer to the
transition state leading to 3-CO+ is 14.8 kcal/mol. In 3-CO+
the dppe ligand has dibasal coordination, whereas the two
P(OMe)3 ligands occupy apical and basal positions. However, it
should be noted that several isomers of 3-CO+, differing for the
orientation of the phosphine ligands, are within 1 kcal/mol
(data not shown) and therefore might coexist in solution.
CO loss from 3-CO+, leading to 3+, takes place going
through a transition state (ΔG⧧ = 13.3 kcal/mol; Scheme 4) in
which the CO group is lost from the iron atom coordinated by
dppe (Supporting Information, Scheme S2). The lowest energy
3+ isomer is characterized by rotated geometry and a vacant
coordination site on the Fe(dppe) moiety. However, an
eclipsed 3+ isomer is only 0.4 kcal/mol higher in energy (See
Supporting Information, Scheme S3). Considering that such an
energy difference is well below the expected accuracy of present
DFT methods, both isomers can be simultaneously present in
solution.
The intermediate species 2-CO+ can lose a CO ligand
forming the rotated species 2+, which features dibasal
coordination of the dppe ligand (Figure 7). In fact, sampling
Figure 7. DFT optimized structure of the lowest energy 2+ isomer. For
the sake of clarity atoms are shown as follows. Carbon, gray; hydrogen,
white; sulfur, yellow; oxygen, red; phosphorus, light purple; iron, dark
purple.
of different reaction paths revealed that the lowest free energy
barrier route implies CO loss from the iron atom coordinated
by dppe (ΔG⧧ = 14.1 kcal/mol). Other 2+ isomers lie at least
3.9 kcal/mol higher in energy (see Supporting Information,
Scheme S1).
4. DFT Investigations of the Reactivity of 3 with CO.
To fully complement the experimental results, DFT was also used
to analyze the mechanism of the reaction between the Fe(I)Fe(I)
neutral species 3 and CO. In the reaction 3 + CO → 2 (Scheme 7)
the rate determining step corresponds to CO binding to the
Fe(dppe) moiety, and the reaction is predicted to be globally
exergonic by −15.9 kcal/mol.
2+ can then react with a second P(OMe)3 molecule forming
3-CO+. As observed for the analogous reaction between 1+ and
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a
Scheme 7. DFT Dissection of the Reaction 3 + CO → 2
a
Energy values are in kcal/mol. For the sake of clarity, only the lowest energy isomers of reactants, products, and intermediate species are shown. In
addition, the transition state connecting 3-CO′ and 3-CO is not shown.
atoms in the two isomers. In fact, while partial atomic charges
of both iron atoms are very similar in the two rotated isomers
(data not shown), the bulky phenyl groups of dppe strongly
hinder the accessibility of the five coordinated iron atom in the
lowest energy isomer (see Figure 2).
DISCUSSION AND CONCLUDING REMARKS
■
The results reported above underline the enhanced reactivity of
complex 1 upon one-electron oxidation, which is entirely
consistent with the exposure of a vacant site in the cation.
Whether the oxidatively induced structure change occurs in a
two-step EC process or in a concerted way could not be firmly
established experimentally. However, DFT results favor the
former possibility. Electron extraction from 1 initially leads to
an eclipsed 1+ isomer, which then spontaneously rearranges to
the lowest energy rotated form (Scheme 8). However, it should
be noted that the computed energy barriers for the
interconversion among 1+ isomers are very low (Berry
pseudorotations; energy barriers lower than 6 kcal/mol),
indicating that isomerization of 1+ is a very fast process, thus
supporting the observation that it can be very difficult to
experimentally distinguish between an EC and a QR process in
the oxidation of this class of compounds.
The interconversion of the P(OMe)3-substituted cations 2+
and 3+ deserves several comments. In the mechanism proposed
in the light of the experimental and DFT results, the incoming
ligand (P(OMe)3 or CO) pushes the semibridging CO toward
the neighboring metal center, which eventually results in the
loss of the ligand trans to the swinging carbonyl group (Scheme
9). Such a mechanism, that has been reported for neutral or
anionic diiron dithiolate complexes,12,13,49−52 was also recently
invoked to account for the incorporation of 13CO at both metal
centers of Fe(I)Fe(II) complexes, [Fe2(CO)4(PMe3)(NHC)-
(μ-pdt)]+ (NHC = imidazol-2-ylidene ligands), submitted to a
13CO atmosphere.23
Although we could not crystallographically characterize 1+
nor the cationic derivatives resulting from its reactions with
P(OMe)3, the presence of a Fe(CO)2{P(OMe)3} entity in the
neutral complex 2 is an indirect evidence for the accessibility of
an isomer of 1+ with a rotated Fe(CO)3. This is fully supported
by the DFT results. In fact, it turned out that the lowest energy
1+ isomer, which is characterized by a vacant coordination site
on the Fe(dppe) moiety, is much less reactive than the
corresponding isomer featuring a vacant coordination site on
Fe(CO)3. The origin of the different reactivity of the two
rotated 1+ isomers is revealed by a comparative analysis of the
electronic and steric properties of the five coordinated iron
The reversibility of the processes in Scheme 9 requires that
the rotated center in 2+ is not the same as in 3+. Actually, the
most stable form of 2+ was shown by DFT to have a rotated
Fe(dppe)(CO) center (that is the same as in 3+, Scheme 4).
Nevertheless, the reaction of P(OMe)3 with 2+ takes place at
the iron center already carrying the first P(OMe)3 ligand
(Scheme 6), so that Scheme 9 shows the most reactive, not the
most stable, isomer of 2+. This is reminiscent of the results of
recent theoretical studies of [Fe2(CO)3(PR3)(κ2-dppv)(μ-
pdt)]+ analogues of 2+ (R = Me or i-Pr) that demonstrated
that the identity of the most stable rotated cation was
dependent on the R group, and that when R = i-Pr, the
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Inorganic Chemistry
Article
a
Scheme 8. Schematic Representation of the 1+ Isomers That Can Be Formed upon One-Electron Oxidation of 1
a
Computed energies (in kcal/mol) of the different 1+ isomers are relative to the lowest energy form. The reaction pathway between 1+ and the
ligand L = P(OMe)3 characterized by the lowest energy barrier is also shown (in blue).
a
Scheme 9. Proposed Reaction Sequence for the Interconversion 2+ ↔ 3+
a
PP = dppe.
rotation of the Fe(CO)2(Pi-Pr3) and of the Fe(CO)(dppv)
centers were nearly isoenergetic.21
for the ground state geometry of reactants (eclipsed and rotated
for Fe(I)Fe(I) and Fe(I)Fe(II) species, respectively), as well as
for the formal oxidation state of the iron atom which interacts
with the incoming ligand, proceeds via a very similar mechanism.
It is also worth noting that the only species detected in the
2+ ↔ 3+ interconversion are 2+ and 3+. The fact that the
intermediate in braces (Scheme 9) corresponding to the species
3-CO+ characterized by DFT calculations, was not detected
during the conversion of 2+ to 3+ would suggest that the
slowest step of the sequence is the binding of P(OMe)3. On the
other hand, our results indicate that the reaction of 3+ with CO
involves two types of processes, that is, the replacement of a
P(OMe)3 ligand by CO (which produces 2+) and a minor
homogeneous redox reaction. The latter may be a sign of the
transient existence of the CO-bridged intermediate. Assuming
that the slow step of the 3+ → 2+ conversion is the loss of the
P(OMe)3 ligand, the intermediate may be sufficiently long-
lived, as suggested by our DFT results, to undergo competing
reactions, that is, homogeneous electron transfer and ligand
loss, when it is formed by reaction of 3+ with CO (Scheme 9).
As for the corresponding 3 → 2 conversion, DFT results
highlight that the process is thermodynamically favored, in good
agreement with the experimentally observed quantitative
production of 2 starting from 3. In addition, it is worth noting
that both Fe(I)Fe(I) and Fe(I)Fe(II) species, even if differing
EXPERIMENTAL SECTION
■
Methods and Materials. All the experiments were carried out
under an inert atmosphere, using Schlenk techniques for the syntheses.
Dichloromethane was predried using conventional methods and
distilled prior to use. The diiron complex [Fe2(CO)4(κ2-dppe)(μ-
pdt)] 1 was prepared according to reported methods.14a
The preparation and the purification of the supporting electrolyte
[NBu4][PF6] were as described previously.53 The electrochemical
equipment consisted of a PGSTAT 12 or a μ-AUTOLAB (Type III)
driven by the GPES software. A GCU potentiostat and a IG5-N
integrator (Tacussel/Radiometer) were used for controlled-potential
electrolyses and coulometry. All the potentials (text, figures) are
referred to the ferrocene-ferrocenium couple; ferrocene was added as
an internal standard at the end of the experiments.
1H, 31P-{1H} NMR spectra were recorded on a Bruker AMX 400
and AC300 spectrometer and were referenced to SiMe4 (1H) and
H3PO4 (31P). VT NMR experiments were carried out on a Bruker
DRX 500 spectrometer. The infrared spectra were recorded on a
Nicolet Nexus Fourier transform spectrometer. Chemical analyses
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Article
were made by the Service de Microanalyses I.C.N.S., Gif sur Yvette,
France. Crystal data (Table 2) for compound 2 were collected on a
diethyl ether (3 × 20 mL). Evaporation of the solvent gave 2 as a
brown solid. Single crystals of 2 were obtained from a hexane-
dichloromethane (1:1) solution at −18 °C.
1
2. IR (CH2Cl2, cm−1): ν(CO) 1968(s), 1913(s) and 1892(sh). H
Table 2. Crystallographic Data for Complex 2·CH2Cl2,
C6H14
NMR (CDCl3, 25 °C), δ: 7.94−7.40 (m, 20H, C6H5), 3.74 (d, 9H, J =
11.2 Hz, P(OCH3)3), 2.64 (m, 4H, P-CH2−CH2−P), 1.59 (m, 4H,
S(CH2)3S), 1.38 (m, 1H, S(CH2)3S), 0.92 (m, 1H, S(CH2)3S). 31P-
{1H} NMR (CDCl3, 25 °C), δ: 179.7 (P(OMe)3), 89.4 (dppe). Anal.
Calcd (%) for C35H39Fe2O6P3S2: C, 50.99; H, 4.77. Found: C, 50.23;
H, 4.73.
DFT Calculations. DFT calculations have been performed by
using the TURBOMOLE suite of programs,57 at the B-P86/TZVP58,59
level of theory, which is well suited for studying [FeFe]-hydrogenase
models.60−62
First-order saddle and minimum points on the potential energy
surface (PES) have been determined by means of energy gradient
techniques, and a full vibrational analysis has been carried out to
further characterize each stationary point. The optimization of
transition state structures has been performed according to a
procedure based on a pseudo-Newton−Raphson method. First,
geometry optimization of a guess transition state structure is carried
out by constraining the distance corresponding to the reaction
coordinate (RC). After performing the vibrational analysis of the
constrained minimum energy structures, the negative eigenmode
associated to the RC is followed to locate the true transition state
structure, that is, the maximum energy point along the trajectory which
joins two adjacent minima.
2·CH2Cl2, C6H14
empirical formula
formula weight
temperature/K
crystal system
space group
a (Å)
C42H55Cl2Fe2O6P3S2
995.49
170(2)
monoclinic
P21/n
16.9360(9)
10.7232(5)
25.4174(13)
b (Å)
c (Å)
α (deg)
β (deg)
93.334(4)
γ (deg)
V (Å3)
4608.2(4)
4
Z
ρcalc (Mg mm−3
)
1.435
μ (mm−1
)
0.985
crystal size (mm)
range of θ (deg)
reflections measured
Rint
0.23 × 0.07 × 0.03
2.80−25.35
31636
Free energy (G) values have been obtained from the electronic SCF
energy considering three contributions to the total partition function
(Q), namely, qtranslational, qrotational, qvibrational, under the approximation
that Q may be written as the product of such terms. Evaluation of H
and S contributions has been made by setting T and P values tot
298.15 K and 1 bar, respectively.
0.1164
unique data/parameters
R1 (I > 2σ(I)]
8405/508
0.0759
R1 (all data)
0.1469
wR2 (all data)
0.1829
An implicit treatment of solvent effect (COSMO,63 ε = 9.1,
dichloromethane) has been employed.
goodness of-fit on F2
Δρmax, Δρmin/e Å−3
1.060
1.389, −0.395
In light of available experimental data and considering the chemical
nature of the ligands, only low-spin species have been investigated.
Oxford Diffraction X-calibur-2 CCD diffractometer, equipped with a
jet cooler device and graphite-monochromated Mo Kα radiation (λ =
0.71073 Å). The structures were solved and refined by standard
procedures.54−56
Digital Simulations. All the simulations were performed with
DigiElch Special Build Version 3 (Build SB3.600).38 Details of the
procedure are given as Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in CIF format. Further details are given in
Figures S1−S9, Schemes S1−S3, and Tables S1−S2. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Electrosynthesis of [Fe2(CO)2{P(OMe)3}2(κ2-dppe)(μ-pdt)]+/0
(3+/0). In a typical experiment, 23.4 mg (3.21 mmol) of 1 and 11.4
μL of P(OMe)3 (9.67 mmol, 3 equiv) were dissolved in 20 mL of
CH2Cl2-[NBu4][PF6] at room temperature in the electrochemical cell.
The controlled-potential oxidation, carried out at −0.18 V (graphite
anode), was terminated after the passage of 0.93 F mol−1 1. The CV of
the electrolyzed solution showed that 3+, characterized by a reversible
reduction at −0.88 V and an irreversible oxidation at 0.14 V, was
formed in 92% yield.40
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jean.talarmin@univ-brest.fr (J.T.), philippe.
schollhammer@univ-brest.fr (P.S.), luca.degioia@unimib.it
(L.D.G.).
The solution of 3+ was submitted to a controlled-potential
reduction at −0.96 V (graphite cathode). The current decayed to
the background level after the passage of 0.94 F mol−1 of the initial 1.
CV of the catholyte showed that 3+ had been converted essentially
quantitatively to the neutral complex 3 (typical yield: 90%).40 The
solution of 3 was canulated in a Schlenk flask under N2, and the
solvent evaporated under vacuum. The residue was extracted by
diethyl ether (3 × 20 mL). After evaporation of the solvent, the solid
residue was dried under vacuum.
ACKNOWLEDGMENTS
CNRS (Centre National de la Recherche Scientifique), ANR
(Programme “PhotoBioH2” and “CatH2”), Universite
■
́
de
Bretagne Occidentale and University of Milano-Bicocca are
acknowledged for financial support. We are grateful to Dr. F.
Michaud for the crystallographic measurements of 4[PF6]2 and
to N. Kervarec for recording the VT H and 31P{1H} NMR
1
3. IR (CH2Cl2, cm−1): ν(CO) 1899 (s), 1880 (s). 31P-{1H} NMR
(CDCl3, 25 °C), δ: 177.5 P(OMe)3), 85.4 (dppe). No satisfactory
elemental analysis could be obtained for 3, because of the limited
stability of the complex.
spectra. D.C. is grateful to the Minister
̀
e de l’Education
Nationale, de l’Enseignement Superieur et de la Recherche for
́
providing a studentship.
Synthesis of [Fe2(CO)3{P(OMe)3}(κ2-dppe)(μ-pdt)] (2). 2 was
obtained by bubbling CO for a few minutes in a CH2Cl2-[NBu4][PF6]
solution of 3 obtained as described above. The solution of 2 was
canulated from the electrochemical cell in a Schlenk flask under N2,
and the solvent was then removed under vacuum. 2 was extracted with
REFERENCES
■
(1) (a) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C.
Science 1998, 282, 1853−1858. (b) Lemon, B. J.; Peters, J. W.
Biochemistry 1999, 38, 12969−12973.
12583
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Article
(2) (a) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.;
Fontecilla-Camps, J. C. Structure 1999, 7, 13−23. (b) Nicolet, Y.;
de Lacey, A. L.; Vernede, X.; Fernandez, V. M.; Hatchikian, C. E.;
Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123, 1596−1602.
(3) For recent reviews on the hydrogenases enzymes, see:
(a) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y.
Chem. Rev. 2007, 107, 4273−4303. (b) de Lacey, A. L.; Fernandez,
V. M.; Rousset, M.; Cammack, R. Chem. Rev. 2007, 107, 4304−4330.
(c) Lubitz, W.; Reijerse, E.; van Gastel, M. Chem. Rev. 2007, 107,
4331−4365. (d) Vincent, K. A.; Parkin, A.; Armstrong, F. A. Chem.
Rev. 2007, 107, 4366−4413. (e) Armstrong, F. A. Curr. Opin. Chem.
Biol. 2004, 8, 133−140. Frey, M. ChemBioChem 2002, 3, 153−160.
(4) For recent reviews on models of the hydrogenases enzymes, see:
(a) Gloaguen, F.; Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38, 100−108.
(18) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008−
7009.
(19) Justice, A. K.; Rauchfuss, T. B.; Wilson, S. R. Angew. Chem., Int.
Ed. 2007, 46, 6152−6154.
(20) Singleton, M. L.; Bhuvanesh, N.; Reibenspies, J. H.;
Darensbourg, M. Y. Angew. Chem., Int. Ed. 2008, 47, 9492−9495.
(21) Justice, A. K.; De Gioia, L.; Nilges, M. J.; Rauchfuss, T. B.;
Wilson, S. R.; Zampella, G. Inorg. Chem. 2008, 47, 7405−7414.
(22) Justice, A. K.; Nilges, M. J.; Rauchfuss, T. B.; Wilson, S. R.;
De Gioia, L.; Zampella, G. J. Am. Chem. Soc. 2008, 130, 5293−5301.
(23) Thomas, C. M.; Liu, T.; Hall, M. B.; Darensbourg, M. Y. Inorg.
Chem. 2008, 47, 7009−7024.
(24) Thomas, C. M.; Liu, T.; Hall, M. B.; Darensbourg, M. Y. Chem.
Commun. 2008, 1563−1565.
(25) Thomas, C. M.; Darensbourg, M. Y.; Hall, M. B. J. Inorg.
Biochem. 2007, 101, 1752−1757.
(26) Evans, D. H. Chem. Rev. 2008, 108, 2113−2144.
(27) Macias-Ruvalcaba, N. A.; Evans, D. H. Chem.Eur. J. 2007, 13,
4386−4395.
́
(b) Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollhammer, P.;
Talarmin, J. Coord. Chem. Rev. 2009, 253, 1476−1494. (c) Tard, C.;
Pickett, C. J. Chem. Rev. 2009, 109, 2245−2274. (d) 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, 2681−2699.
(e) Heinekey, D. M. J. Organomet. Chem. 2009, 694, 2671−2680.
(f) Best, S. P.; Cheah, M. H. Radiat. Phys. Chem. 2010, 79, 185−194.
(5) van der Vlugt, J. I.; Rauchfuss, T. B.; Whaley, C. M.; Wilson, S. R.
J. Am. Chem. Soc. 2005, 127, 16012−16013.
(6) (a) Cao, Z.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3734−3742.
(b) Fan, H.-F.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3828−3829.
(7) (a) Liu, Z. P.; Hu, P. J. Am. Chem. Soc. 2002, 124, 5175−5182.
(b) Liu, Z.-P.; Hu, P. J. Chem. Phys. 2002, 117, 8177−8180.
(8) (a) Greco, C.; Bruschi, M.; De Gioia, L.; Ryde, U. Inorg. Chem.
2007, 46, 5911−5921. (b) Bruschi, M.; Greco, C.; Fantucci, P.;
De Gioia, L. Inorg. Chem. 2008, 47, 6056−6071.
(9) Tye, J. W.; Darensbourg, M. Y.; Hall, M. B. Inorg. Chem. 2006,
45, 1552−1569. Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem.
2004, 43, 3733−3741.
(28) Evans, D. H.; O’Connell, K. M. In Electroanalytical Chemistry;
Bard, A. J., Ed., Marcel Dekker: New York, 1986; Vol. 14, pp 113−207.
(29) Geiger, W. E. Organometallics 2007, 26, 5738−5765.
(30) Geiger, W. E. In Progress in Inorganic Chemistry; Lippard, S. J.,
Ed.; John Wiley & Sons: New York, 1985; Vol. 33, pp 275−352.
(31) Bond, A. M.; Colton, R. Coord. Chem. Rev. 1997, 166, 161−180.
(32) Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Lemos, M. A. N.
D. A. Coord. Chem. Rev. 2001, 219−221, 53−80.
(33) The parameters ip and Ep are respectively the peak current and
c
a
the peak potential of a redox process; E1/2 = (Epa + Ep )/2; Ep , ipa and
Ep , ipc are respectively the potential and the current of the anodic and
c
of the cathodic peak of a reversible process; ΔEp = Epa − Ep . The scan
c
rate in CV experiments is v (V s−1). An ECE process consists of a
chemical reaction (C) comprised between two electron transfer steps
(E).
(10) (a) Adam, F. I.; Hogarth, G.; Richards, I.; Sanchez, B. E. Dalton
Trans. 2007, 2495−2498. (b) Adam, F. I.; Hogarth, G.; Richards, I.
J. Organomet. Chem. 2007, 692, 3957−3968. (c) Hogarth, G.; Richards,
I. Inorg. Chem. Commun. 2007, 10, 66−70. (d) Adam, F. I.; Hogarth,
G.; Kabir, S. E.; Richards, I. C. R. Chim. 2008, 11, 890−905.
(11) (a) Duan, L.; Wang, M.; Li, P.; Na, Y.; Wang, N.; Sun, L. Dalton
Trans. 2007, 1277−1283. (b) Wang, N.; Zhang, T.; Li, P.; Liu, J.; Sun,
L. Chem. Commun. 2008, 5800−5802. (c) Wang, N.; Wang, M.; Liu, J.;
Jin, K.; Chen, L.; Sun, L. Inorg. Chem. 2009, 48, 11551−11558.
(12) Wang, N.; Wang, M.; Liu, T.; Li, P.; Zhang, T.; Darensbourg, M.
Y.; Sun, L. Inorg. Chem. 2008, 47, 6948−6955.
(34) (a) The deduction of the number of electrons involved in the
oxidation of 1 is made by comparison of the current functions for
equimolar solutions of 1 and of (Fe2(CO)4(κ2-dppe)(μ-pdt)(μ-H))+
(one-electron oxidation)34b at different scan rates. This method is valid
if the complexes have identical diffusion coefficients, which is a
reasonable assumption owing to their similar size and structure.
(b) Ezzaher, S.; Capon, J.-F.; Dumontet, N.; Gloaguen, F.; Petillon,
́
F. Y.; Schollhammer, P.; Talarmin, J. J. Electroanal. Chem. 2009, 626,
161−170.
(35) (a) Petro, B. J.; Vannucci, A. K.; Lockett, L. T.; Mebi, C.;
Kottani, R.; Gruhn, N. E.; Nichol, G. S.; Goodyer, P. A.; Evans, D. H.;
Glass, R. S.; Lichtenberger, D. L. J. Mol. Struct. 2008, 890, 281−288.
(b) Felton, G. A. N.; Vannucci, A. K.; Chen, J.; Lockett, L. T.;
Okumura, N.; Petro, B. J.; Zakai, U. I.; Evans, D. H.; Glass, R. S.;
Lichtenberger, D. L. J. Am. Chem. Soc. 2007, 129, 12521−12530.
(36) Geiger, W. E.; Camire Ohrenberg, N. C.; Yeomans, B.;
Connelly, N. G.; Emslie, D. J. H. J. Am. Chem. Soc. 2003, 125, 8680−
8688.
(37) (a) The peak separation, ΔEp, was measured at a clean electrode
surface for 0.05 V s−1 ≤ v ≤ 10 V s−1. However, values of ΔEp at slow
scan rates (0.05 V s−1 ≤ v ≤ 0.4 V s−1) were selected to minimize the
effects of uncompensated solution resistance. The apparent hetero
geneous electron transfer rate constant calculated according to ref. 37b
was ksapp = 0.005 ± 0.001 cm s−1. (b) Nicholson, R. S. Anal. Chem.
1965, 37, 1351−1355.
(38) For detailed information concerning DigiElch, see: (a) Rudolph,
M. J. Electroanal. Chem. 2003, 543, 23−29. (b) Rudolph, M.
J. Electroanal. Chem. 2004, 571, 289−307. (c) Rudolph, M. J. Comput.
Chem. 2005, 26, 619−632. (d) Rudolph, M. J. Comput. Chem. 2005,
26, 633−641. (e) Rudolph, M. J. Comput. Chem. 2005, 26, 1193−1204.
(39) (a) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods.
Fundamentals and Applications; Wiley: New York, 1980; Chapter 11,
(13) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B.;
van der Vlugt, J. I.; Wilson, S. R. Inorg. Chem. 2007, 46, 1655−1664.
́
(14) (a) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.;
Schollhammer, P.; Talarmin, J.; Pichon, R.; Kervarec, N. Inorg. Chem.
2007, 46, 3426−3428. (b) Morvan, D.; Capon, J.-F.; Gloaguen, F.;
Le Goff, A.; Marchivie, M.; Michaud, F.; Schollhammer, P.; Talarmin,
J.; Yaouanc, J.-J.; Pichon, R.; Kervarec, N. Organometallics 2007, 26,
2042−2052. (c) Orain, P.-Y.; Capon, J.-F.; Kervarec, N.; Gloaguen, F.;
Pet
Trans. 2007, 3754−3756. (d) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.;
Kervarec, N.; Petillon, F. Y.; Pichon, R.; Schollhammer, P.; Talarmin,
J. C. R. Chim. 2008, 11, 906−914. (e) Morvan, D.; Capon, J.-F.;
Gloaguen, F.; Petillon, F. Y.; Schollhammer, P.; Talarmin, J.; Yaouanc,
́
illon, F. Y.; Pichon, R.; Schollhammer, P.; Talarmin, J. Dalton
́
́
J.-J.; Michaud, F.; Kervarec, N. J. Organomet. Chem. 2009, 694, 2801−
2807.
(15) Song, L.-C.; Yang, Z.-Y.; Bian, H.-Z.; Liu, Y.; Wang, H.-T.; Liu,
X.-F.; Hu, Q.-M. Organometallics 2005, 24, 6126−6135.
(16) (a) Lemon, B. J.; Peters, J. W. Biochemistry 1999, 38, 12969−
12973. (b) Bennett, B.; Lemon, B. J.; Peters, J. W. Biochemistry 2000,
39, 7455−7460. (c) Lemon, B. J.; Peters, J. W. J. Am. Chem. Soc. 2000,
122, 3793−3794. (d) Chen, Z.; Lemon, B. J.; Huang, S.; Swartz, D. J.;
Peters, J. W.; Bagley, K. A. Biochemistry 2002, 41, 2036−2043.
(17) Razavet, M.; Borg, S. J.; George, S. J.; Best, S. P.; Fairhurst, S. A.;
Pickett, C. J. Chem. Commun. 2002, 700−701.
pp 429−485. (b) Saveant, J.-M. In Elements of Molecular and
́
Biomolecular Electrochemistry - An Electrochemical Approach to Electron
Transfer Chemistry; Wiley: New York, 2006; Chapter 2, pp 78−181.
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(40) The product yields of the controlled-potential oxidations were
estimated from the comparison of the oxidation peak current of 1 in
the absence of substrate, to the reduction peak current of the
electrogenerated cation, assuming identical diffusion coefficients.
Similarly, the yields of the controlled-potential reductions were
estimated by comparison of the peak current of the cation to that of
the neutral product formed by electrolysis.
(41) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B. Chem.
Commun. 2007, 2019−2021.
(42) Morvan, D.; Capon, J.-F.; Gloaguen, F.; Schollhammer, P.;
Talarmin, J. Eur. J. Inorg. Chem. 2007, 5062−5068.
́
(43) Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollhammer, P.;
Talarmin, J. Eur. J. Inorg. Chem. 2008, 4671−4681.
(44) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B.;
van der Vlugt, J. I.; Wilson, S. R. Inorg. Chem. 2007, 46, 1655−1664.
́
(45) Orain, P. Y.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.;
Schollhammer, P.; Talarmin, J.; Zampella, G.; De Gioia, L.; Roisnel, T.
Inorg. Chem. 2010, 49, 5003−5008.
(46) Wang, N.; Wang, M.; Liu, T.; Li, P.; Zhang, T.; Darensbourg,
M. Y.; Sun, L. Inorg. Chem. 2008, 47, 6948−6955.
(47) Barton, B.; Zampella, G.; Justice, A. K.; De Gioia, L.; Rauchfuss,
T. B.; Wilson, S. R. Dalton Trans. 2010, 3011−3019.
(48) The scheme has been conceived to show relative energy data
concerning relevant stationary points of the PES associated to the
reaction pathway going from 1+ to 3+. In fact, steps 1 and 3 in Scheme
4 do not necessarily correspond to actual elementary reaction acts,
since possible unimolecular rearrangements have not been explicitly
shown in this scheme. For example 1+ might rearrange so as to expose
a vacant coordination site on the Fe(CO)3 moiety, before L attack to
such site.
́
(49) S. Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.;
Schollhammer, P.; Talarmin, J. Inorg. Chem. 2007, 46, 9863−9872.
(50) George, S. J.; Cui, Z.; Razavet, M.; Pickett, C. J. Chem.Eur. J.
2002, 8, 4037−4046.
(51) Zampella, G.; Bruschi, M.; Fantucci, P.; Razavet, M.; Pickett, C.
J.; De Gioia, L. Chem.Eur. J. 2005, 11, 509−520.
(52) Daraosheh, A. Q.; Harb, M. K.; Windhager, J.; Gorls, H.;
̈
El-khateeb, M.; Weigand, W. Organometallics 2009, 28, 6275−6280.
́
(53) Capon, J.-F.; Ezzaher, S.; Gloaguen, F.; Petillon, F. Y.;
Schollhammer, P.; Talarmin, J.; Davin, T. J.; McGrady, J. E.; Muir,
K. W. New J. Chem. 2007, 31, 2052−2064.
(54) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
(55) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.
J. Appl. Crystallogr. 1993, 26, 343−350.
(56) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838.
(57) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem.
Phys. Lett. 1989, 162, 165−169.
(58) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100.
(59) Perdew, J. P. Phys. Rev. B: Condens. Matter 1986, 33, 8822−
8824.
(60) Bertini, L.; Bruschi, M.; De Gioia, L.; Fantucci, P.; Greco, C.;
Zampella, G. In Atomistic Approaches in Modern Biology: From
Quantum Chemistry to Molecular Simulations; Reiher, M., Ed.; Springer:
Berlin, Germany, 2007; Vol. 268, pp 146.
(61) Bruschi, M.; Zampella, G.; Fantucci, P.; De Gioia, L. Coord.
Chem. Rev. 2005, 249, 1620−1640.
(62) Zampella, G.; Greco, C.; Fantucci, P.; De Gioia, L. Inorg. Chem.
2006, 45 (10), 4109−4118.
(63) Schafer, A.; Klamt, A.; Sattel, D.; Lohrenz, J. C. W.; Eckert, F.
Phys. Chem. Chem. Phys. 2000, 2, 2187−2193.
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