Intervalence electron transfer through a thiolate bridge ligand: A FeIIIS-R-FeII mixed valence complex

Article abstract of DOI:10.1016/S0277-5387(00)00587-8

The reaction of Cp(dppe)FeI with the ligands 2,2′- and 4,4′-dithiobispyridine (S₂(PY)₂) give the mononuclear or binuclear complexes of the type [Cp(dppe)Fe-S₂(PY)₂]PF₆, [Cp(dppe)Fe-SPy]PF₆ or [{Cp(dppe)Fe}₂-μ-SPy](PF₆)₂ depending on the reaction condition. Reaction of Cp(dppe)FeI with dithiobispyridines in presence of TlPF₆ as halide abstractor and using CH₂Cl₂ as a solvent gives the complexes [Cp(dppe)Fe-4,4′-S₂(PY)₂)₂ ]PF₆ (1) and [CpFe(dppe)-2,2′-S₂(PY)₂]PF₆ (2) whereas the same reaction using CH₃OH as a solvent and NH₄PF₆ as the halide abstractor leads to the formation of the Fe^III-thiolate complex [Cp(dppe)Fe-2,2′-SPy]PF₆ (3) and the mixed-valence complex [Cp(dppe)Fe^III-μSPy-Fe^II(dppe)Cp] (PF₆)₂ (4). Magnetic and ESR measurements are in agreement with one unpaired electron delocalized between them. Mo?ssbauer data indicate clearly the presence of two different iron sites, each one of the N-bonded and S-bonded iron atoms, with intermediate oxidation state Fe^II-Fe^III. An electron transfer intervalence absorption was observed for this complex at 780 nm (in CH₂Cl₂). By applying the Hush theory the intervalence parameters were obtained; α=0.028, H_ab= 361 cm^-1 which indicate Class II Robin-Day. Estimation of the rate electron transfer affords a value k_th = 6.5 × 10⁶ s^-1. Solvent effect on the intervalence transition follow the Hush prediction for high dielectric constants solvents which permit the evaluation of the outer and inner-sphere reorganizational parameters, which were analyzed and discussed. The electronic interaction parameters compare well with those found for electron transfer in metalloproteins.

Full text of DOI:10.1016/S0277-5387(00)00587-8

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Polyhedron 19 (2000) 2679–2687
Intervalence electron transfer through a thiolate bridge ligand:
a FeIII–S–R–FeII mixed valence complex
Carlos D´ıaz a,*, Alejandra Arancibia b
a Departamento de Qu´ımica, Facultad de Ciencias, Uni6ersidad de Chile, Casilla 653, Santiago, Chile
b Facultad de Quimica, P. Uni6ersidad Catolica de Chile, Casilla 306, Santiago, Chile
Received 15 May 2000; accepted 5 September 2000
Abstract
The reaction of Cp(dppe)FeI with the ligands 2,2%- and 4,4%-dithiobispyridine (S2(Py)2) give the mononuclear or binuclear
complexes of the type [Cp(dppe)Fe-S2(Py)2]PF6, [Cp(dppe)FeꢀSPy]PF6 or [{Cp(dppe)Fe}2-m-SPy](PF6)2 depending on the reaction
condition. Reaction of Cp(dppe)FeI with dithiobispyridines in presence of TlPF6 as halide abstractor and using CH2Cl2 as a
solvent gives the complexes [Cp(dppe)Fe-4,4%-S2(Py)2)2]PF6 (1) and [CpFe(dppe)-2,2%-S2(Py)2]PF6 (2) whereas the same reaction
using CH3OH as a solvent and NH4PF6 as the halide abstractor leads to the formation of the FeIII–thiolate complex
[Cp(dppe)Fe-2,2%-SPy]PF6 (3) and the mixed-valence complex [Cp(dppe)FeIII-mSPy-FeII(dppe)Cp](PF6)2 (4). Magnetic and ESR
measurements are in agreement with one unpaired electron delocalized between them. Mo¨ssbauer data indicate clearly the
presence of two different iron sites, each one of the N-bonded and S-bonded iron atoms, with intermediate oxidation state
FeIIꢀFeIII. An electron transfer intervalence absorption was observed for this complex at 780 nm (in CH2Cl2). By applying the
Hush theory the intervalence parameters were obtained; h=0.028, Hab=361 cm−1 which indicate Class II Robin–Day.
Estimation of the rate electron transfer affords a value kth=6.5×106 s−1. Solvent effect on the intervalence transition follow the
Hush prediction for high dielectric constants solvents which permit the evaluation of the outer and inner-sphere reorganizational
parameters, which were analyzed and discussed. The electronic interaction parameters compare well with those found for electron
transfer in metalloproteins. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Thiolate-bridged complexes; Mixed-valence; Electron-transfer; Iron
and is probably a cysteinyl sulfur atom but this can not
be proven from the X-ray data. The assimilatory sulfide
1. Introduction
reductase from Desulfo6ibrio 6ulgaris (DvSiR) has been
proposed to contain a moiety Fe4S4ꢀSꢀSiroheme, how-
ever no X-ray structure data is available for such
enzymes [9]. Mixed-valence complexes in which two
metals enter are connected by a bridging group provide
a simple and useful model for comparison with most
complex biological systems [10,11]. In the case of iron–
A variety of metalloproteins are known to contain
sulfur as part of the coordination environments of
metal ions [1–9]. Iron, copper and the molybdenum ion
are especially common in metalloenzymes containing
sulfur ligands. In several of such metalloproteins the
metalꢀthiolate bond is involved in the active site [5–9],
specifically iron–thiolate clusters are involved in the
thiolate containing metalloproteins, no available iron–
electron transfer behavior of ferredoxin and cy-
tochrome P450, [5–8]. Thus, the enzyme sulfite reduc-
tase from Escherichia coli (EcSiR) which catalyzes the
reduction of sulfite to sulfide contains a covalently
bridged cluster-siroheme assembly [9]. The bridging lig-
thiolate–iron mixed valence models have been reported
[12–15].
In previous work [16,17] we have shown that the
products of the reaction of organic disulfide with
Cp(dppe)FeI depend on the reaction conditions i.e.
solvent and halide abstractor used (see Scheme 1).
Now we have investigated the reaction of Cp(dppe)-
FeI with 2,2%-dithiobispyridine and with 4,4%thiolate–
irondithiobispyridine ligands. Potentially, dithio-
* Corresponding author. Tel.: +56-2-6787-396; fax: +56-2-2713-
888.
E-mail address: cdiaz@abello.dic.uchile.cl (C. D´ıaz).
0277-5387/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0277-5387(00)00587-8

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C. D´ıaz, A. Arancibia / Polyhedron 19 (2000) 2679–2687
1
bispyridines can act as a N and S monodentate ligand
and as a N and S bridging ligand between two metal
centers. Examples of some of these coordination types
have appeared recently [18,19].
as those of the PySSPy ligand and PF6 anion [27]. H
NMR signals of the disulfide ligands are observed
normally [28]. Values are given in Section 4. The 13P
NMR spectra of the complexes 1 and 2 show the
typical singlet signal of dppe in [Cp(dppe)FeL]PF6 com-
plexes [29,30], and the septet corresponding to the PF6−
anion. UV–Vis spectra of the complexes exhibit one
d–d absorption band around 450 nm typical of
[Cp(dppe)FeL]PF6 complexes where L is neutral donor
ligand [29,30]. The formation of the FeII-dithioeter
complex instead of the FeIII-thiolate complex can be
explained by the low dielectric medium of the reaction
(CH2Cl2), as well as by the low acidic environment
provided by the iodide abstractor, TlPF6 salt [16,17].
In complex 3, the formation of a thiolate ligand
instead of the disulfide complex can be due to the high
dielectric (CH3OH) and acidic medium (NH4PF6). On
the other hand the possible formation of the binuclear
complex by coordination of the pendant nitrogen pyri-
dine side of the 2,2%-dithiopyridine to an Cp(dppe)Fe+
fragment (as occurs with the 4,4%-dithiopyridine reac-
tion, see below) is precluded probably due to steric
effect of the bulky Cp(dppe)Fe moiety [16,17].
2. Results and discussion
The reaction of Cp(dppe)FeI with 2,2%-dithio-
bispyridine and with 4,4%-dithiobispyridine afford differ-
ent products (see Scheme 2) depending on the solvent
and halide abstractor used. Thus, the reaction of
CpFe(dppe)I with dithiobispyridines in the presence of
TlPF6 and using CH2Cl2 as a solvent gives the com-
plexes [Cp(dppe)Fe-2,2%-S2(py)]PF6 (1) and [Cp(dppe)-
Fe-4,4%-S2(py)]PF6 (2). On the other hand the same
reaction with 2,2-dithiopyridine and using methanol as
the solvent and NH4PF6 as the halide abstractor gives
the blue thiolate iron(III) complex [CpFe(dppe)-
Spy]PF6 (3).
Thus the reaction of Cp(dppe)FeI with 4,4%-dithio-
bispyridine, in CH3OH as the solvent and in presence
of NH4PF6 affords the unexpected thiolate bridged
The violet mixed-valence complex [Cp(dppe)FeIII-4-
S(Py)-FeII(dppe)Cp](PF6)2 (4) was characterized by ele-
mental analysis, IR, ESR, Mo¨ssbauer and UV–Vis
spectroscopy as well as electrochemical and magnetic
methods. An NMR characterization of this complex
was precluded due to their paramagnetism. On the
other hand, despite many attempts, it was not possible
to grow single crystals of this complex, therefore struc-
tural determination could not be made. The IR spec-
trum showed the typical bands for the Cp(dppe)Fe
moiety [26] and the expected band at 838 cm−1 corre-
sponding to w(PF6). Bands of the thiobispyridine group
valence
mixed
complex
[Cp(dppe)FeIII-4-SPy-
FeII(dppe)Cp](PF6)2 (3).
The formation of the bridging pyridinethiolate ligand
arises from the oxidative addition of the organometallic
fragment Cp(dppe)Fe+ by the 4,4%-dithiobispyridine lig-
and followed by coordination of the pyridine pendant
nitrogen by other CpFe(dppe)+ fragment. In this reac-
tion the ligand has the dual role: affording the SꢀN
moiety as well as oxidizing the fragment from
[Cp(dppe)FeII]+ to [Cp(dppe)FeIII]+2
PyꢀSSꢀPy+2e− “2SꢀPy−
(1)
appear around 1570 cm−1
.
A limited number of complexes of pyridine-4-thiol
are known; however, coordination occurs usually at the
sulfur atom in the thione form [18–23]. The most usual
pyridine-2-thiol can act as an unidentate, bidentate or
bridging ligand, the latter being the most common
coordination mode.
At room temperature a value of veff=1.4 BM in
solid state (considering the two iron atoms) is in good
agreement with the expectation of one unpaired elec-
tron by two iron atoms veff=1.22 BM, spin-only value.
In complexes 1 and 2, dithiopyridine acts as a
disulfide ligand [24,25]. Their IR spectra exhibit the
expected bands of the CpFe(dppe) moiety [26] as well
Scheme 1.
Scheme 2.

C. D´ıaz, A. Arancibia / Polyhedron 19 (2000) 2679–2687
2681
Table 1
EPR, Mo¨ssbauer and electrochemical data for complex 4
Temperature (K)
EPR
Solid
Mo¨ssbauer b
VC c
DEb
0.99
Solution a
l1
DEa
l2
E1/2
E1/2
0.11
d
e
300
77
2.0738
2.0841
2.0685
0.31
0.92
0.49
0.26
2.1481(g1)
2.0691(g2)
1.9316(g3)
a In dichloromethane.
b Iron as reference.
c Dichloromethane solution, [NBu4]PF6 as supporting electrolyte.
d Half-wave potential assigned to the iron link to the nitrogen atom.
e Half-wave potential assigned to the iron link to the sulfur atom.
distinct doublets in the spectrum is diagnostic of a
localized valence whereas observation of a single aver-
age doublet is diagnostic of detrapped valence. The
spectrum of complex 4 showed a two-doublet signal
with quadrupole splitting and isomer shift values corre-
sponding to intermediate states FeIIꢀFeIII. The parame-
ters found for complex 4 are in agreement with some
intermediate situation, showing some average valence
II–III, which suggests a certain degree of delocalization
of the electron. The presence of two similar signals
indicate two slight different iron sites which can arise
from the asymmetry of the bridging ligand, which
induce different iron environments (the asymmetry of
The same reduced value with respect to that calculated
(vteo=1.7 BM) can be due to some antiferromagnetic
interaction in solid state. Detailed magnetic tempera-
ture studies are in progress.
The ESR spectrum of complex 4 in solid state
showed a single broad band with no observable cou-
pling. The g values extracted from spectra are collected
in Table 1. The broadening can be attributed to the
exchange interaction between two magnetically
nonequivalent sites (FeꢀS; FeꢀN) [31]. A similar single
signal was observed in the ESR of the mixed valence
complex Cp*Fe(dppe)ꢀCHꢁCHꢀCHꢁCHꢀFe(dppe)Cp*
[32]. On the other hand, the ESR spectrum in CH2Cl2
solution at room temperature also exhibits a single
sharp signal at g=2.0685 G (see Fig. 1(a)). At low
temperature (77 K) in solid state a single, but broader
signal than expected, is observed (g=2.0841 G). In a
dichloromethane solution (see Fig. 1(b)) at 77 K, the
spectrum shows three separated features corresponding
to the three components of the g tensor as expected for
d5 low spin iron(III) complexes in pseudooctahedral
geometry [33,34]. Hyperfine coupling with the 2 equiv.
31P nuclei with partial resolution was observed
AP(43G) [31]. As recently documented [33], the obser-
vation of this behavior may constitute definitive evi-
dence for delocalization of mixed valence complexes in
the EPR time scale. Thus this suggests a decrease in
electron-delocalization at low temperature.
The 57Fe Mo¨ssbauer spectrum of 4 is shown in Fig.
2. The observed parameters are listed in Table 1. It is
well known that Mo¨ssbauer spectroscopy is very useful
to identify the oxidation states of iron in mixed-valence
compounds [35–42]. Mononuclear iron(II) complexes
of the type (h5-C5R5)Fe(dppe)R exhibits Mo¨ssbauer
parameters in the range l=0.13–0.28; DEQ=1.95–
2.08 while that for similar iron(III) derivatives, parame-
ters are in the range l=0.25–0.35 mm s−1 and
DEQ=0.76–0.8 mm s−1. Then DEQ is characteristic, of
the oxidation state of the iron metal. For binuclear
mixed valence iron complexes, the presence of two
Fig. 1. EPR spectra of complex 4 in dichloromethane solution (a) at
room temperature and (b) at 77 K.

2682
C. D´ıaz, A. Arancibia / Polyhedron 19 (2000) 2679–2687
Fig. 2. Mo¨ssbauer spectrum of complex 4 at room temperature.
the ligand, see structure in Scheme 2, is also shown by
the results of the UV–Vis, which shows the presence of
FeIIIꢀS as well as FeIIIꢀN chromophores, see below)
Consistent with this, the presence of two FeIII centers
having different auxiliary ligands and linked with a
symmetrical bridge exhibit two slightly different signals
(l=0.192, −0.033 mm s−1) but almost the same
quadrupole values (1.978, 1.958 mm s−1). A localized
valence situation may induce the presence of a signal
with high quadrupole values (\1.9 mm s−1) corre-
sponding to FeII as well as a signal at low quadrupole
values (B0.9 mm s−1) corresponding to FeIII. For
instance the FeII–FeIII localized mixed valence complex
[Cp*Fe(CO)2CꢂCCꢂCCp*Fe(dppe)]PF6 exhibit two
doublets slightly separated with DEQ 1,990; 0,865, cor-
responding to the FeII and FeIII centers, respectively
[39]. If complex 4 holds a localized FeII–FeIII situation,
a signal with a DEQ value near 1.9, as is observed in
[CpFe(dppe)(NCCH3]PF6 (having a FeꢀN bond ex-
hibits a quadrupole value of 1.96 mm s−1[42]), might
be observed. Hence the highest quadrupole value for
complex 4 suggests clearly the presence of a detrapped
valence situation. Thus data for complex 4 compares
well with data for another delocalized FeIIꢀFeIII valence
mixed complexes: Cp*Fe(dppe)ꢀCꢂCꢀC6H4ꢀCꢂCꢀ
Fe(dppe)Cp*)PF6 DEQ=1.1 mm s−1 [37], Cp*-
Fe(dppe)ꢁCꢁCHꢀCHꢁCꢁFe(dppe)Cp*)PF6 DEQ=1.3
mm s−1 [35], and (Cp*Fe(dppe)ꢀCꢂCꢀCꢂCꢀFe-
(dppe)Cp*)PF6 DEQ=1.0 mm s−1 [32]. The DEQ value
for the complex 4 is also in agreement with the
FeIIꢀFeIII average-mixed-valence complexes, biferrice-
nium+TCA−·2TCAA, DEQ=0.903, and 1%,6%-diiodobi-
ferrocene, DEQ=1.20 mm s−1 [40]. Most recently [41]
the detrapped FeIIꢀFeIII mixed-valence complexes
[FeIIFeIII(bpmp)(L)][BF4]2 exhibit DEQ values in the
range 0.88–1.02 mm s−1, similar to the values found
for complex 4. Mo¨ssbauer spectroscopy can also be
used to monitor the intramolecular electron transfer
rate in mixed valence compounds in solid state. When
the intermetallic electron transfer rate is slower than 106
s
−1, localized valence is observed, whereas observation
of a single average-valence quadrupole doublet is diag-
nostic of an electron transfer faster than 109 s−1. Then
the electron transfer rate for the complex 4 in solid
state could be near 109 s−1
.
The UV–Vis spectrum of the complex 4 (see Fig. 3)
exhibits, in addition (without maxima) to the character-
istic absorptions of [Cp(dppe)FeIIL]+ L=neutral
donor ligand around 450 nm [29,30], the typical absorp-
tion band of the iron(III)–thiolate chromophores
around 560 nm [16,17]. This corroborates the presence
of FeIIIꢀS as well as the FeIIꢀN bond in the proposed
structure of the mixed-valence complex. For compari-
son purposes, umax is 439 nm for the complex
[Cp(dppe)FeꢀNC5H5]PF6 [29,30], while umax for the
complex [Cp(dppe)FeꢀS2(Py)2]PF6 is 450 nm. The
broad band around 790 nm is not present in the
mononuclear precursors [Cp(dppe)FeꢀNC5H5]PF6], [29]
nor [Cp(dppe)FeꢀSR]PF6 [16,17], then it was assigned
to an intervalence transition (IT) FeII“FeIII [43,44].
Consistently with this, upon reduction with Red Al
(sodium bis (2-methoxy ethoxy) aluminum) this interva-
lence band disappears as is shown in Fig. 3.

C. D´ıaz, A. Arancibia / Polyhedron 19 (2000) 2679–2687
2683
1/2
2.06×10−2(mmaxwmaxDw1/2
)
Hab (cm−1)=
(3)
d
The value 361 cm−1 suggests a moderate electronic
interaction between the metal centers similar to that
found in mixed-valence complexes with large bridge
ligands containing pyridine groups linked to the metals
[53]. Comparisons of Hab values are displayed in Table
3. This value is close to that observed for some metallo-
proteins [5–8]. The thermal first-order electron transfer
rate constant can be expressed as [43,44,50,51,50–53]:
Kth=[H2ab/h][y/kTu]1/2e[−Ea/RT]
(4)
where u is the reorganization energy of the electron
transfer, which can be calculated from the maximum of
the IT absorption band of the electron transfer by
E
op=u and the activation energy Ea can be estimated
from:
Fig. 3. UV–Vis spectra of (a) the complex [{Cp(dppe)Fe}2-m-
S(Py)](PF6)2 before the reduction; (b) after reduction with Red Al.
Ea=[u2/4u]
(5)
The value calculated Kth=6.5×106 is in general of
the order for rate constants for other mixed-valence
complexes (containing pyridine bridging ligands) as can
be viewed from Table 3. Thus this value compares well
with electron transfer rate estimated recently for some
metalloproteins [5–8].
Information about the electron transfer interaction
between the metallic centers can be obtained by apply-
ing the Hush theory [45–47]. According to this model,
the degree of electronic delocalization (h) can be calcu-
lated from Eq. (2) (Table 2).
Solvent dependence of intervalence transition of a
mixed valence complex gives information about reorga-
nizational energy arising from rearrangements of the
inner and outer coordination spheres [10,45–47,50,51].
From a dielectric continuum model, Hush derived the
expression:
(4.2×10−4)mmaxw1/2
h2=
(2)
wmaxd2
where wmax and w1/2 are given in cm−1 and the inter-
,
metallic distance in A. Since the d value for complex 4
,
is unknown, we have used an average value (7.6 A),
obtained from FeꢀS in CpFe(CO)2SC2H5 [48] and from
mean values [49] for SꢀC, CꢁC and FeꢀN (pyridine)
bonds. This is an approximation usually used and
accepted in the absence of the most reliable crystallo-
graphic data [43,44]. The estimated value in CH2Cl2
h=0.028 indicate an interaction Class II Robin–Day
[50,51]. Values calculated in other solvents are similar
as is shown in Table 3. The degree of interaction
between the metal center can be estimated through the
electronic coupling element Hab [10,43,44,52].
E
op=u=ui+u0=u
+e2(1/2a1+1/2a2−1/a)
×(1/Dop−1/Ds)
(6)
where u and u0 are the inner and outer-sphere reorgani-
zational parameters respectively, e is the unit electron
charge, a1 and a2 are the molecular radius of the redox
sites, d is the internuclear separation and Dop and Ds
are the optical and static dielectric constants of the
Table 2
Intervalence transition parameter for [{Cp(dppe)Fe}2-m-S(Py)](PF6)2 in several solvents
a
b
c
d
e
Solvent
Eop(cm−1
)
h
Dw1/2 (cm−l
)
Hab (cm−l
)
Ea (cm−l
)
kth×106 (s−l
)
Acetonitrile
Ethylmethylcetone
Acetone
Dichloromethane
Methanol
12 682.3
12 674.3
12 674.3
12 698.4
12 690.4
0.028
0.030
0.030
0.028
0.026
1615
1689
1592
1616
2233
362
388
385
361
331
3170.6
3168.5
3168.6
3174.6
3172.6
6.3
9.1
7.2
6.5
2.4
a Calculated using Eq. (2).
b Calculated using Dw1/2=(2310Eop
c Calculated using Eq. (3).
d Calculated using Eq. (5).
e Calculated using Eq. (4).
1/2
)
.

2684
C. D´ıaz, A. Arancibia / Polyhedron 19 (2000) 2679–2687
Table 3
considerable contribution from the vibrational modes
of the bridging ligand to the reorganization energy
[55,56]. The total reorganization energy u=12 697
cm−1 is slightly higher than those reported for electron
transfer in metalloproteins, which is reasonable because
of the most bulky environment of the active site of
metal proteins.
The cyclic voltammogram for the [{CpFe(dppe)}2-m-
SPy](PF6)2 complex exhibits two close, one electron
electrochemical quasi-reversible waves. Values are
shown in Table 1. The first at 0.26 V is chemically
Electronic coupling element Hab a, rate constant b and compropor-
tionation constant c for mixed-valence complexes containing pyridine
bridged ligands
Complex
Hab (cm−1
)
kth (cm−1
6.5×106
7.0×106
4.7×107
)
Kc
3.5×102
[Cp(dppe)FeꢀSꢀC5H4Nꢀ 361
Fe(dppe)Cp]+
[(NH3)5RuꢀNCC5H4Nꢀ
535
240
4.4×104
Ru(NH3)5]5+
[(NH3)5RuꢀNC5H4
(CHꢁCH)C5H4Nꢀ
Ru(NH3)5]5+
[(NH3)5RuꢀNC5H4
(CHꢁCH)2C5H4Nꢀ
Ru(NH3)5]5+
[(NH3)5RuꢀNC5H4-
(CHꢁCH)3C5H4Nꢀ
Ru(NH3)5]5+
10
quasi-reversible (ip /ip =1.25) and can be assigned to
220
180
1.1×108
6.1×108
10
9
a
the oxidation of thce Fe–Py moiety by comparison with
data for [CpFe(dppe)L]PF6 complexes [29,30]. On the
other hand the lowest oxidation at 0.11 V (although not
well defined) is electrochemically quasi-irreversible
(DEp=0.08 V), but chemically irreversible and is as-
signed to the FeIIIꢀS/FeIIꢀS couple [57,58]. Thus an
irreversible oxidation peak at 1.3 V corresponding to
the oxidation of RꢀSSꢀR, as well as a quasi-irreversible
oxidation peak at −0.55 V assigned to the oxidation of
RS− were observed, behavior typical of the redox
properties of [CpFe(dppe)SR]PF6 [57] and other metal–
thiolate complexes [58]. Although the second oxidation
wave described above is chemically irreversible, a rough
estimation can be made of the comproportionation
constant Kc=3.5×102 for the reaction:
[(NH3)5RuꢀNC5H4SSC5
H4NꢀRu(NH3)5]5+
[(NH3)5RuꢀNC5H4SC5
H4NꢀRu(NH3)5]5+
[(NH3)5RuꢀNC5H4ꢀ
C(O)OꢀRu(NH3)5]4+
[(NH3)5RuꢀNC5H4ꢀ
C(O)NHꢀRu(NH3)5]4+
[(NH3)5RuꢀNC5H4ꢀ
C5H4NꢀRu(NH3)5]5+
[(NH3)5RuꢀNC5H4ꢀ
CH2ꢀC5H4Nꢀ
310
150
300
510
390
100
8.7×109
3.7×106
2.9×103
8.8×107
6.5×108
7.3×105
8.0×104
158
2.6×107
1.3×109
20
73
Ru(NH3)5]5+
[(NH3)5RuꢀNC5H4ꢀNHꢀ 500
9.6×108
3.8×107
26
14
[FeII−S−py−FeII]+
C5H4NꢀRu(NH3)5]5+
K
c
+[FeIII−Spy−FeIII]3+ ?2[FeII−Spy−FeIII]2+
[(NH3)5RuꢀNC5H4ꢀCꢁCꢀ 285
C5H4NꢀRu(NH3)5]5+
(7)
a Estimated from Eq. (3), and using data Refs. [9,15,16,20–22].
b Estimated from Eq. (4).
The relatively high value of Kc agrees with the rela-
tively high electron coupling between the iron centers
and also provides a more efficient electron delocaliza-
tion than X=methylene, ethylene and acetylene groups
(considering the NC5H4ꢀXꢀC5H4N moiety) [53]. Data
shown in Table 3 also suggests that the thiolate bridg-
ing ligand is more efficient in electron delocalization
than organic sulfide ligands (R2S), but somewhat less
efficient than the disulfide ligands. These conclusions
are in agreement with the rate of redox chromium
exchange reactions,
c Estimated from Eq. (7).
medium. The intercept of the near linear plot Eop versus
(1/Dop−1/Ds) for the several solvents (see Fig. 4) af-
fords the approximate ui value, ui=12 470 cm−1 (the
solvent CH2Cl2 fall out from correlation is due proba-
bly to the ion pairing effects [43,44]). From Eq. (6), the
value for u0 can be calculated (u0=227 cm−1).
The low value of the outer-sphere reorganization
contribution respect to the inner-sphere reorganization
appears to be associated to the spherical symmetry of
the groups (h5-C5R5)Fe(dppe)+ [11,43,44,54]. The high
variations of the metal–ligand bond distances during
the electron-transfer indicated by the high values of ui,
appears to be characteristic of mixed-valence complexes
containing the moiety (h5-C5R5)Fe(dppe)+ (compare
for instance with [{Cp*Fe(dppe)}2-m-(CꢂCꢀ(C6H4)nꢀ
CꢂC)](PF6)2, n=0, 1 ui=7100 cm−1, u0=500cm−1
[35]) however an inverse tendency u0\ui has been
reported recently for [{CpFe(dppe)}2-m-CN]PF6 [43,44].
The high delocalized behaviour for 4 in spite of the
high inner reorganizational energies can be due to a
Fig. 4. Plot of Eop versus (1/Dop−1/Ds) for the mixed valence
complex [Cp(dppe)Fe-S(Py)Fe(dppe)Cp] (PF6)2.

C. D´ıaz, A. Arancibia / Polyhedron 19 (2000) 2679–2687
2685
*Cr2+ +[(H2O)5CrSR]2+UCr2+ +[(H2O)*5 CrSR]2+
(8)
where the rates of reduction were found to be fast
mediated by the thiolate sulfur ligand which functions
as an extremely efficient electron-transfer bridge be-
tween chromium(II) and chromium(III) [1]. It is inter-
esting to note that the only sulfur ligand bridging in
mixed-valence complexes are sulfides and disulfides,
then the complex [{Cp(dppe)Fe}2-mS(Py)](PF6)2 is to
our knowledge the first thiolate acting as a bridge of the
form MꢀSRꢀM between metal centers in mixed valence
compounds [LnMꢀXꢀMLn]z+ X=bridging ligand
[59,60].
Fig. 5. Molecular orbitals probably involved in the electron transfer
process. Interaction of the pyridine and thiolate side of the
pyridinethiolate ligand with the iron fragments was deduced from
HOMO of thiolate and LUMO of pyridine (see text).
The unexpected formation of the FeIII–FeII thiolate
complex can be explained assuming the formation of
the iron(III)–thiolate mononuclear complex — the ex-
pected product using CH3OH as a solvent and NH4PF6
as a halide abstractor — and subsequently the coordi-
nation of the nitrogen coordination pendant site by an
Cp(dppe)Fe+ fragment. To our knowledge this is the
first FeIIꢀFeIII mixed valence complex, containing an
thiolate ligand bridge [12–15,59,60] where the FeIII is
bonded to the sulfur–thiolate and the FeII is linked to
some donor atom contained in R.
The scarcity of mixed-valence compounds containing
this type of bridging ligand can be due to the difficulty
in obtaining the mixed-valence complex by oxidation of
the corresponding FeII-(m-thiolate)–FeII complex owing
to the known oxidation of the thiolate ligand instead
the oxidation of the metallic center:
[64] and those for the iron–thiolate side of the bridging
ligand was made according to iron–thiolate structural
data [16,17,48]. HOMO-thiolate [62,63] and LUMO-
pyridine [61] was according literature data. SOMO of
Cp(dppe)Fe2+ and HOMO of Cp(dppe)Fe+ has been
reported previously [16,17].
3. Conclusions
From the study of the reaction of Cp(dppe)FeI with
dithiopyridines, a new mixed valence binuclear complex
containing a thiolate bridge was isolated and identified.
The nature of the FeIII–FeII interaction through the
thiolate bridge, determined from ESR, Mo¨ssbauer and
the Hush parameters, appears to be similar to the
electron transfer in some metalloproteins, although the
reorganizational energy is greater in the binuclear
model than the biological systems. Recent studies on
electron transfer between sulfur surface modified gold
electrodes and cytochrome
c reveal that the 4-
The efficient electron transfer between the iron cen-
ters through the pyridinethiolate ligand can be ex-
plained by the effective p-system formed by overlapping
of the SOMO(Cp(dppe)FeIII)–HOMO(S-C6H5) orbitals
as well as HOMO(Cp(dppe)FeII)–LUMO(NC5H5)
overlap. The electron transfer from FeII to the pyridine
ring can involve the HOMO of the fragment
Cp(dppe)Fe which is p in character [16,17] and the
LUMO of pyridine which also has p character [61] (see
Fig. 5).
On the other hand electron transfer from the pyri-
dine–thiolate ligand to Cp(dppe)FeIII fragment [16,17]
through the thiolate side can involve the HOMO of the
thiolate [62,63] which has a p character and the p-
SOMO orbital of the FeIII fragment. The possible p
system involved in the FeIIꢀFeIII electron transfer is
depicted in Fig. 5. In this orbital interaction diagram
the structure of the iron–pyridine moiety was estab-
lished according to data from iron–pyridine complexes
pyridinethiolate ligand can act as an efficient bridge
conductor between the cytochrome and the electrode
[65].
4. Experimental
All reactions were carried out under purified N2 or
Ar using standard Schlenk techniques and the solvents
used (methanol, dichloromethane, diethyl ether and
n-hexane) were appropriately distilled and dried before
use. The precursor complex Cp(dppe)FeI was prepared
according to literature methods [42,66]. 4,4%- and 2,2%-
dithiobispyridine, [NBu4]PF6, NH4PF6, (Aldrich) were
used as received. TlPF6 (caution! Thallous salts are very
poisonous and should be handled with caution) was
prepared from Tl2CO3 and HPF6. IR spectra were
recorded as KBr pellets on
spectrometer.
a FT-Bruker 66V

2686
C. D´ıaz, A. Arancibia / Polyhedron 19 (2000) 2679–2687
NMR spectra were run in CD2Cl2 or (CD2)2CO
4.2. Reaction of Cp(dppe)FeI with 4,4%-dithiobispyridine
in CH2Cl2
solution at room temperature (r.t.) on a Bruker AMX
300 spectrometer with TMS(H) l=0.0 ppm as internal
standard or 85% H3PO4 and downfield positive to the
reference as external standard for the 31P measure-
ments. UV–Vis spectra were run on a Varian DMS-9
spectrophotometer with 1 cm optical path cuvettes.
Cyclic voltammetry was carried out at r.t. in CH2Cl2
A solution of Cp(dppe)FeI 0.15 g (0.23 mmol) and
4,4%-dithiobispyridine 0.08 g (0.36 mol) in the presence
of TIPF6 0.16 g (0.46 mmol) in 30 ml dichloromethane
was stirred at r.t. for 24 h. The separation procedure,
similar to that above for 2,2%-dithiobispyridine, affords
red–brown solid. Yield 0.14 g; 68%. Anal. Found: C,
55.7; H, 4.2; N, 3.2. Calc. for C41H37F6N2S2P3Fe: C,
55.65; H, 4.18; N, 3.16%.
using
a Parc model 370 electrochemistry system
equipped with a three electrode device, employing a
glassy carbon working electrode, platinum wire, coun-
ter electrode and saturated calomel reference electrode.
Solution was 10−3 M in the complex and 0.1 M in
Bu4PF6 as supporting electrolyte. Under the same ex-
perimental conditions E1/2 for the ferrocene/ferroce-
nium couple was 0.45 V with separations of about 0.060
V. Magnetic measurements were carried out by a
Gouys method at r.t. (25°C) on a Johnson Matthey
balance using mercury terathiocyanate cobaltate(II) as
the calibrate. ESR spectra were carried out using an
ESR Bruker ECS 106 spectrometer using a rectangular
mode cavity with a 50 Hz field modulation. The mea-
surements were made with the microwave band X (9.79
G Hz). For the solid sample the crushed powder was
used. The Mo¨ssbauer spectrum, as a fine powder sam-
ple was obtained in a conventional spectrometer with
constant acceleration. The source was 10mCi57 Co in a
Pd matrix. The gamma ray detection was carried out
with a Reuter–Stoke detector connected to an Ortec
amplifying and selecting system. A 1024 channel Cam-
berra multichannel analyzer was used for the acquisi-
tion of information. The reported shift values are
relative to iron.
Ir (Kbr, cm−1) 1082 w(C5H5); 694 w(dppe); 1565 w(S2
(Py)2); 847 w(PF6).
1H NMR (acetone-d6 cm−1) 7.62m, 7.54m, 7.41m,
7.19m S2 (Py)2; 7.33m (dppe) 4.76s C5H5; 3.4m CH2.
31P NMR (acetone-d6 ppm) 92.3 dppe; −138.8 PF6.
4.3. Reaction of Cp(dppe)FeI with 4,4%-dithiobispyridine
in CH3OH
Cp(dppe)FeI 0.15 g (0.23 mmol) was stirred with
4,4%-dithiobispyridine 0.08 g (0.36 mmol) in the pres-
ence of NH4PF6 0.06 g (0.37 mmol) in CH3OH (30 ml)
for 24 h at r.t. The solvent was evaporated under vacuo
and the black solid residue was extracted with
dichloromethane (15 ml). Upon addition of a 1:1 mix-
ture of n-hexane–diethyl ether, a blue–black micro-
crystals precipitated which were washed several times
with diethyl ether and dried under reduced pressure.
Yield approximately 70%. Anal. Found: C, 55.98; H,
4.17; N, 1.1. Calc. for C67H62NF12SP6Fe2: C, 55.93; H,
4.31; N, 0.97%.
Ir (KBr, cm−1) 1094 w(C5H5); 695 w(dppe); 1565
w(S2(Py)2); 839 w(PF6).
4.1. Reaction of Cp(dppe)FeI with 2,2%-dithiobispyridine
in CH2Cl2
4.4. Reaction of Cp(dppe)FeI with 2,2%-dithiobispyridine
in methanol
A mixture of 0.15 g (0.23 mmol) of Cp(dppe)FeI [31]
and 0.08 g (0.36 mmol) of 2,2%-dithiobispyridine in 30
ml dichloromethane and in the presence of 0.16 g
TIPF6 (0.46 mmol) was stirred at r.t. for 24 h. The
solution was filtered through Celite and the filtrate was
concentrated under reduced pressure to a volume of
:10 ml. Then 5 ml of an n-hexane–diethyl ether 1:1
mixture was added. The brown–orange precipitate was
washed twice with diethyl ether and dried under vacuo.
Yield 0.16 g; 78%. Anal. Found: C, 56.68; H, 4.09; N,
2.7. Calc. for C41H37N2F6S2P3Fe: C, 55.69; H, 4.18; N
3.16%.
Cp(dppe)FeI 0.15 g (0.23 mmol) and 2,2%-dithio-
bispyridine 0.08 g (0.36 mmol) in the presence of
NH4PF6 0.06 g (0.37 mmol) in CH3OH (30 ml) were
stirred for 24 h at r.t. Separation and purification
procedures similar to the above reaction with 4,4%-
dithiobispyridine afford a blue solid. Yield 0.13 g; 65%.
Anal. Found: C, 55.9; H, 4.3; N, 1.7. Calc. for
C36H33F6NSP3Fe: C, 55.82; H, 4.26; N, 1.8%.
Ir (KBr, cm−1) 1094 w(C5H5); 695 w(dppe); 1565
w(S2Py); 838 w(PF6)
Ir (KBr, cm−1): 1088 w(C5H5); 698 w(dppe); 1576
w(S2(Py2)); 840 w(PF6).
1H
NMR
(acetone-d6
solution,
ppm)
Acknowledgements
8.46m,7.68m,7.65m,7.23m S2(Py)2; 7.33m dppe; 4.76s
Cp; 3.4m CH2. 31P NMR (acetone-d6 ppm) 92.4 dppe;
−138.8 PF6.
This work was partially supported by Fondecyt (Pro-
ject 1000672).

C. D´ıaz, A. Arancibia / Polyhedron 19 (2000) 2679–2687
[37] N. LeNarvor, C. Lapinte, Organometallics 14 (1995) 634.
2687
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