Reactivity of ferrocenium cations with molecular oxygen in polar organic solvents: Decomposition, redox reactions and stabilization

Article abstract of DOI:10.1016/j.jorganchem.2005.02.009

The behaviour of chemically or electrochemically generated ferrocenium cations has been studied in some polar organic solvents (DMF, DMSO, acetonitrile, acetone, methylene chloride) under molecular oxygen. Adducts between oxygen and ferrocenium species can differently evolve according to the solvent (oxidizable or not) and the absence or the presence of another reagent. A rapid decomposition of ferrocenium cations is observed in the absence of another substrate. In the presence of some substrates and antioxidants, the stability of ferrocenium cations towards molecular oxygen notably increases and in some cases redox reactions take place with formation of ferrocene.

Full text of DOI:10.1016/j.jorganchem.2005.02.009

Journal of Organometallic Chemistry 690 (2005) 1829–1839
www.elsevier.com/locate/jorganchem
Reactivity of ferrocenium cations with molecular oxygen
in polar organic solvents: Decomposition, redox reactions
and stabilization
*
J.P. Hurvois, C. Moinet
´
´
Laboratoire dÕElectrochimie et Organometalliques, Institut de Chimie de Rennes, Universite de Rennes 1, Campus de Beaulieu,
35042 Rennes Cedex, France
Received 12 January 2005; accepted 8 February 2005
Available online 17 March 2005
Abstract
The behaviour of chemically or electrochemically generated ferrocenium cations has been studied in some polar organic solvents
(DMF, DMSO, acetonitrile, acetone, methylene chloride) under molecular oxygen. Adducts between oxygen and ferrocenium spe-
cies can differently evolve according to the solvent (oxidizable or not) and the absence or the presence of another reagent. A rapid
decomposition of ferrocenium cations is observed in the absence of another substrate. In the presence of some substrates and anti-
oxidants, the stability of ferrocenium cations towards molecular oxygen notably increases and in some cases redox reactions take
place with formation of ferrocene.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Ferrocenium cations; Oxygen-decomposition; Redox reactions; Antioxidants
1. Introduction
observed in DMF but not in acetonitrile, acetone or
methylene chloride [5]. However, at a higher time scale
than cyclic voltammetry, the decomposition of chemi-
cally or electrochemically generated ferrocenium cations
was observed in these latter solvents. The influence of
molecular oxygen and the specific role of the solvent were
highlighted during the anodic oxidation of Cr(CO)5CNꢀ
anions [6]. More recently, Zotti et al. [7] studied the
oxygen-decomposition of ferrocenium cations in aceto-
nitrile and suggested a decomposition mechanism
which involved an unstable dimeric complex 1e with an
oxygen bridge between two cyclopentadiene rings
(Scheme 1).
The decomposition of ferrocenium cations in organic
solvents was reported a long time ago [1]. The nucleo-
philic character of the solvent was previously proposed
to explain the reactivity of ferrocenium species in organic
solvents [2,3]. It was observed that ferrocenium cations
decomposed more rapidly in DMSO or DMF than in
acetone or acetonitrile. Later, by using electrochemistry,
Sato et al. [4] underscored the role of molecular oxygen
about this instability. The cyclic voltammogram of an
oxygen free solution of ferrocene in DMSO showed the
typical ferrocene/ferrocenium reversible system, while
in the presence of oxygen, a loss of reversibility was
observed. Quasi similar cyclic voltammograms were
The presence of this initially produced complex could
explain the trapping of cyclopentadienols by strong
dienophiles during the decomposition of various ferr-
ocenium cations in acetonitrile [8].
*
Corresponding author. Tel.: +33 2 23 23 67 88; fax: +33 2 23 23 59
This paper is devoted to a general study of the behav-
iour of ferrocenium species in polar organic solvents in
67.
E-mail address: claude.moinet@univ-rennes1.fr (C. Moinet).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.02.009

1830
J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
O O
O2
Fe+
2
Fe+
Fe+
1e
1b
O O
O O
O2
H2O
Fe3+ + 1/2 Fe2O3 + 2
+
Fe+
Fe+
1e
1f
Scheme 1.
the presence of oxygen and various reagents (oxidizable
species and anti-oxygen). Particularly, new intermedi-
ates are proposed to explain the reactivity or the stabil-
ization of ferrocenium cations in the presence of some
reagents and molecular oxygen.
Ipc/Ipa = 1). However, when the scan is anodic enough
to oxidize triphenylphosphine (Epa = 1.08 VSCE) or
diphenylisobenzofuran (Epa = 0.75 VSCE), a loss of the
reversible ferrocene/ferrocenium system 1a/1b is ob-
served. Moreover compounds often used as antioxidants
in industry (phenols or amines) proved to be efficient for
stabilizing ferrocenium cations 1b in DMF under
oxygen. In this way, by adding 2,6-diterbutyl-4-methyl
phenol (ionol) in ionol/ferrocene molar ratio P1 or N-
cyclohexyl-N0-phenyl-1,4-benzene diamine (flexzone
6H) in flexzone/ferrocene molar ratio P0.5, a complete
reversibility of the system ferrocene/ferrocecium 1a/1b
and a total disappearance of the cathodic wave located
at ꢀ0.45 VSCE are observed in DMF saturated in oxy-
gen. It is worth noting that flexzone and ionol are irre-
2. Results
2.1. Cyclic voltammetry of ferrocenes in the presence of
oxygen
2.1.1. In DMF or DMSO
As previously reported [4,5], the cyclic voltammo-
gram at a platinum or a glassy carbon electrode of fer-
rocene 1a in an oxygen-saturated solution of DMSO
or DMF does not show the reversible system (Scheme
2) normally observed under nitrogen at a scan rate of
versibly oxidized before ferrocene (Epa = 0.34 VSCE
and after ferrocene (1.65 VSCE), respectively.
)
0.1 V sꢀ1
.
2.1.2. In acetonitrile, acetone or methylene chloride
In the presence of oxygen, the anodic wave increases
while the cathodic wave decreases and a new irreversible
cathodic wave appears at a lower potential (Epc #
ꢀ0.45 VSCE). The phenomenon is less pronounced with
1,10-dimethylferrocene 2a in an oxygen-saturated solu-
tion of DMF, the cathodic wave continuously decreases
with the appearance of an irreversible cathodic wave at
Epc # ꢀ0.40 VSCE after a repetitive cycling. Conversely,
a complete reversibility is again observed for decamethyl-
ferrocene in an oxygen-saturated solution of DMF. The
great stability of octamethyl- and decamethylferrocenium
cations in acetonitrile under oxygen was previously
observed [9].
In oxygen-saturated solutions of acetonitrile, acetone
or methylene chloride, the electrogenerated ferrocenium
cations 1b appear practically stable at the time scale of
cyclic voltammetry (0.1 V sꢀ1). However, as reported
by Daschbach [10] and later by Zotti et al. [7], the cur-
rent intensities of the anodic and cathodic peaks contin-
uously decrease during a repetitive scanning, with at the
same time, formation of a yellowish deposit on the
electrode.
2.2. Reactivity of ferrocenium cations in the presence of
oxygen
After addition of a stoechiometric amount of triphe-
nylphosphine or 1,3-diphenylisobenzofuran to the
oxygen saturated DMF solution, ferrocene 1a is
2.2.1. In DMF or DMSO
2.2.1.1. In the absence of reagents. In DMF or DMSO,
the blue color of ferrocenium cations 1b instantaneously
turns orange on addition of oxygen. At the same time,
the disappearance of the initial cathodic wave ((E1/2)c =
0.48 VSCE, in DMF) of ferrocenium cations and the
reversibly oxidized (Epa = 0.53 VSCE, Epc = 0.44 VSCE
,
appearance of an anodic wave ((E1/2)a = 0.50 VSCE
)
Fe +
- e
Fe
attributable to the ferrocene oxidation, are observed at
a platinum rotating disc electrode. The height of the lat-
ter wave is half of the former. After work up, ferrocene
Scheme 2.

J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
1831
1a was isolated in 47% yield from the DMF solution and
characterized. Previously, Prins et al. [2] observed a ra-
pid decomposition of ferrocenium cations in DMF,
DMSO and HMPTA with formation of ferrocene in
50% yield determined by spectrophotometric measure-
ments. The role of oxygen in the decomposition was
not considered.
2.2.1.2. In the presence of oxidizable reagents. As pre-
dictable by cyclic voltammetry, the stability of ferroce-
nium cations in DMF, under oxygen, increases by
adding stoechiometric amounts of triphenylphosphine
or 1,3-diphenylisobenzofuran to the solution. Ferrocene
1a is quantitatively produced as shown by voltammetry
at a rotating disc electrode, whereas triphenylphosphine
or diphenylisobenzofuran are oxidized (Figs. 1 and 2). It
is worth noting that under inert atmosphere of nitrogen,
the redox reaction is very slow (Fig. 2) and could result
from trace amounts of dissolved oxygen. The oxidation
of triphenylphosphine did not give triphenylphosphine
Fig. 2. Redox reaction between ferrocenium cations 1b and 1,3-
diphenylisobenzofuran in DMF + 0.1 M Bu4NBF4. Under nitrogen
until 2070 s, then oxygen. d, diphenylisobenzofuran disappearance; j,
ferrocenium disappearance; m, ferrocene formation.
oxide (absence of absorption frequency at 1190 cmꢀ1
)
not detected after decomposition. Moreover, we ob-
served that ferrocene 1a and ferrocenium cations 1b
simultaneously disappear from a mixture of these two
compounds in acetonitrile under oxygen (Fig. 3). The
rates of disappearance were dependent on the ratio fer-
rocene/ferrocenium.
but probably a phosphonium salt as proposed for elec-
trochemical oxidation of triphenylphosphine in non
aqueous media [11]. One mole of 2-benzoylbenzophenon
was obtained per mol of ferrocenium cations after indi-
rect oxidation of diphenylisobenzofuran (Scheme 3). A
quantitative formation of ferrocene shows that the pro-
posed intermediate 1e [7] cannot be retained as oxidizing
species.
2.2.2.2. In the presence of oxidizable reagents. The rate
of ferrocenium cation 1b disappearance is notably slo-
wed down by adding triphenylphosphine to an oxygen
saturated solution of acetonitrile containing the former
compound (Figs. 4 and 5). This rate decreases with
increasing the ratio triphenylphosphine/ferrocenium cat-
ions. As an example, for triphenylphosphine/ferroce-
nium ratios equal to 1.06 and 1.6, about 25% of
ferrocenium cations are reduced into ferrocene after
25 mn and 3 h, respectively. With a triphenylphos-
phine/ferrocenium cations ratio less than 1, a rapid
decomposition was observed after a quasi complete
transformation of triphenylphosphine (Fig. 5).
2.2.2. In acetonitrile, acetone or methylene chloride
2.2.2.1. In the absence of reagents. In oxygen saturated
solution of acetonitrile, acetone or methylene chloride,
ferrocenium cations 1b totally disappear in few minutes
with formation of a brown precipitate and a light brown
solution as previously observed [5,7]. Ferrocene 1a was
A mixture of ferrocenium cations 1b and 1,3-diph-
enylisobenzofuran (molar ratio 1/1) in acetonitrile ap-
pears totally stable after one hour under oxygen. The
cathodic wave of ferrocenium cations and the anodic
wave of diphenylisobenzofuran are not altered.
As previously shown by cyclic voltammetry (cf. Sec-
tion 2.1.1), the addition of antioxidants (ionol, flexzone
6H) to solutions of ferrocene 1a under oxygen, avoids
the rapid decomposition of electrogenerated ferroce-
nium cations 1b. Flexzone shows a higher efficiency than
ionol to stabilize ferrocenium cations 1b in acetonitrile
under oxygen. In this way, we do not observe any
decomposition of ferrocenium cations in the presence
of small amounts of flexzone (molar ratio flexzone/ferr-
ocenium 3.5%) after 3 h at room temperature, then heat-
ing at 60 ꢁC for 4 h (neither change in the blue color nor
Fig. 1. Redox reaction between ferrocenium cations 1b and triphe-
nylphosphine in DMF + 0.1 M Bu4NBF4 under oxygen. d, triphenyl-
phosphine disappearance; j, ferrocenium disappearance; m, ferrocene
formation.

1832
J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
O
C6H5
O
O2
C6H5
C6H5
Fe +
+
+
Fe
DMF, Bu4NBF4
C6H5
O
Scheme 3.
Fig. 3. Chemical decomposition of a mixture ferrocene 1a + ferroce-
nium cations 1b in acetonitrile + 0.1 M Bu4NBF4 under oxygen.
Ferrocene/ferrocenium molar ratio 0.72: d, ferrocenium disappear-
ance; j, ferrocene disappearance. Ferrocene/ferrocenium molar ratio
1.67: ., ferrocenium disappearance; m, ferrocene disappearance.
Fig. 5. Decomposition of ferrocenium cations 1b in the presence of
triphenylphosphine in acetonitrile + 0.1 M Bu4NBF4, under oxygen.
d, triphenylphosphine disappearance; j, ferrocenium cations disap-
pearance; m, ferrocene formation.
0.51 VSCE) and ionol (E1/2 = 1.3 VSCE) quantitatively
gives ferrocene 1a (Fig. 6). In order to avoid a possible
decomposition of ferrocenium cations, flexzone was
added (molar ratio flexzone/ ferrocenium 3.5%). We con-
trolled the lack of the redox reaction between 1b and io-
nol under nitrogen in the same medium. A similar redox
reaction takes place between ferrocenium cations 1b and
Fig. 4. Redox reaction between ferrocenium cations 1b and triphe-
nylphosphine in acetonitrile + 0.1 M Bu4NBF4, under oxygen. d,
triphenylphosphine disappearance; j, ferrocenium cations disappear-
ance; m, ferrocene formation.
modification of the cathodic wave). We only observe the
decomposition of ferrocenium cations after heating at
60 ꢁC for 4.5 h. With the same amount of ionol, ferroce-
nium cations are decomposed in few minutes at room
temperature.
Fig. 6. Redox reaction between ferrocenium cations 1b and 2,6-
diterbutyl-4-methyl phenol (ionol) in the presence of flexzone
(1.4 · 10ꢀ4 M); acetonitrile + 0.1 M Bu4NBF4, LiOH saturated. Under
nitrogen until 900 s, then oxygen. d, ionol disappearance; j,
ferrocenium disappearance; m, ferrocene formation.
In oxygen-saturated solution of acetonitrile contain-
ing lithium hydroxide to generate phenate anions, a re-
dox reaction between ferrocenium cations 1b (E1/2
=

J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
1833
OH
O
t-Bu
t-Bu
t-Bu
t-Bu
O2
Fe+
+
+
Fe
CH3CN, Bu4NBF4, LiOH
OCH3
O
Scheme 4.
3,5-diterbutyl-4-hydroxy anisole (E1/2 = 0.98 VSCE) in
the same oxygen-saturated medium (Scheme 4).
The oxidation rate for the latter compound (about
40% oxidized after 1 h) is higher than for ionol (about
15% oxidized after 1 h). After complete reactions, 70%
and 85% of ionol and 3,5-diterbutyl-4-hydroxyanisole
respectively were oxidized when ferrocenium cations
were totally reduced. 2,6-diterbutyl-1,4-benzoquinone
was obtained and characterized from the latter phenol.
Two adducts 1c and 1d resulting from reactions be-
tween the dienophile and cyclopentadien-5-ol and cyclo-
pentadiene respectively were isolated and characterized.
The quantity of adduct 1c notably increases as the
dienophile–ferrocenium cations ratio increases, when
the quantity of adduct 1d is nearly constant (Table 1).
Addition of methanol (2.5% in volume) to the mix-
ture of ferrocenium 1b and dienophile 3b in acetonitrile
before decomposition, gives again 1c and 1d after
decomposition without formation of a methoxylated
adduct.
2.2.2.3. In the presence of a dienophile. In order to trap
cyclopentadiene derivatives resulting from the decompo-
sition of ferrocenium cations, we used 4-phenyl-1,2,4-
triazoline-3,5-dione 3b known as a powerful dienophile
[12,13]. Ferrocenium cations 1b and the dienophile 3b
were prepared from a co-oxidation in a flow cell fitted
with a porous anode, of ferrocene 1a and 4-phenyl-
1,2,4-triazolidine-3,5-dione (4-phenylurazole) 3a in acid-
ified acetonitrile [8]. A decomposition of ferrocenium
cations 1b occurred after adding solid sodium carbonate
to the electrolyzed solution under oxygen (Scheme 5).
The decomposition is complete after 1 h. When aqueous
solutions of sodium carbonate are used, ferrocene is pro-
duced by a redox reaction. It is worth noting that in
acidic acetonitrile, ferrocenium cations 1b are stable un-
der oxygen; however after adding solid sodium carbon-
ate in order to neutralize the acidic medium,
decomposition is obtained in few minutes in the absence
of dienophile 3b.
In the same way [8], 1,10-dimethylferrocenium cations
2b give 2c in 41% yield. 2b decompose more slowly than
ferrocenium cations 1b.
2.3. Controlled potential electrolyses of ferrocene 1a in
the presence of oxygen
2.3.1. In DMF or DMSO
Macroscale electrolyses at a platinum or a glassy car-
bon anode (working potential 1 VSCE) of ferrocene 1a in
an oxygen-saturated solution of DMF, involve 2 F per
mol. A rapid decomposition of the produced ferroce-
nium 1b occurs with simultaneously formation of CO2
which can be trapped by bubbling in a solution of cal-
cium hydroxide or sodium hydroxide for dosage in the
latter case. About 0.45 mol of CO2 is produced per
mol of starting ferrocene. It is worth noting that, for
the chemical decomposition of ferrocenium cations in
HN
N
N
NH
R
R
anodic co-oxidation
CH3CN (2% H2SO4)
R
R
Fe+
+
+
O
O
Fe
O
O
N
N
Ph
Ph
1a (R = H)
2a (R = CH3)
3b
3a
1b, 2b
+ O2 + solid Na2CO3
R
OH
O
O
N
N
Ph
+
N
N
N
Ph
N
O
O
1c, 2c
1d
Scheme 5.

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J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
Table 1
2.3.2.2. In the presence of oxidizable reagents. The elec-
trolysis at 0.7 VSCE, under oxygen, of ferrocene 1a
(2 mmol) in acetonitrile + LiClO4 containing triphe-
nylphosphine (2 mmol) leads to a blue solution after
consumption of 1 F per mol of ferrocene with a signifi-
cant decreasing of the current intensity (more than 90%
relatively to the initial current). The characteristic blue
color of ferrocenium cations 1b was persistent until con-
sumption with residual current of 3–3.5 F per mol of fer-
rocene, then a change of color occurs. At this time, the
anodic wave of triphenylphosphine and the cathodic
wave of ferrocenium cations totally disappear.
A similar electrolysis of ferrocene 1a (2 mmol), per-
formed at 0.6 VSCE, in the presence of 1,3-diphenyliso-
benzofuran (2 mmol) instead of triphenylphosphine,
involved 1 F per mol of ferrocene with a residual current
corresponding to 5% of the initial current. Then the ano-
dic wave of diphenylisobenzofuran very slowly decreased
(less than 15% after one night) without decomposition of
electrogenerated ferrocenium cations 1b characterized by
their cathodic wave.
Adducts 1c (with cyclopentadienol) and 1d (with cyclopentadiene)
resulting from the decomposition of ferrocenium cations 1b in the
presence of 4-phenyl-1,2,4-triazoline-3,5-dione 3b in acetonitrile under
oxygen
3b/1b molar ratio
Adducts (% yielda)
1:1
2:1
3:1
a
1c (46)
1c (69)
1c (80)
1d (12)
1d (14)
1d (15)
Calculated from initial ferrocene 1a.
DMF under oxygen, without electrolysis (cf. 2.2.1), no
CO2 evolution occurred. Attempt to characterize dim-
ethylamine failed. During the electrolysis the initial or-
ange color of the solution turns red.
Electrolyses of ferrocene 1a performed in oxygen-sat-
urated solution of DMSO consume approximately 2.5 F
per mol of starting substrate. A rapid decomposition of
the electrogenerated ferrocenium leading to a red solu-
tion is observed without production of CO2.
No products were isolated from solutions after elec-
trolyses in DMF and DMSO.
2.3.2. In acetonitrile, acetone or methylene chloride
3. Discussion
2.3.2.1. In the absence of reagents. Anodic oxidations
of ferrocene 1a in an oxygen-saturated solution of aceto-
nitrile, acetone or methylene chloride involve 0.3–0.5 F
per mol according to the amount of starting substrate.
A clear to dark brown solid precipitates during the elec-
trolyses. Elemental analyses of these solids isolated after
electrolyses in acetonitrile or methylene chloride con-
taining Bu4NBF4 as electrolyte (Table 2) revealed a dif-
ference of Fe/F ratio between the two solvents. Fe
content is higher and F content is lower in acetonitrile
than in methylene chloride. Solids are slightly soluble
in water and soluble in nitric or hydrochloric acid. Char-
acteristic colors of ferric complexes with thiocyanate
ions or hexacyanoferrate (II) are only observed after
acidification. After adding sodium hydroxide to the
acidic solution and filtration of ferric hydroxide, the
resulting solution presents acido-basic properties char-
acterized by a reversible change of colours (orange-yel-
low in acidic medium and deep red in basic medium).
Solids decompose for temperature higher than 250 ꢁC,
giving a black powder. When electrolyses are performed
in the presence of Bu4NClO4 as electrolyte, the isolated
precipitate explodes for temperature higher than 250 ꢁC.
3.1. Oxidation of organic substrates by ferrocenium-
oxygen adducts
In order to account the experimental results, the ini-
tial oxygen attack on ferrocenium cations cannot in-
volve a direct coupling with the cyclopentadienyl ring
leading to 1e (Scheme 1) as previously proposed [7,9]
by analogy with products resulting from reactions be-
tween oxygen and 19-electron complexes of cobalt [14]
or iron [15,16]. In this last case, the proposed coupling
reaction between molecular oxygen and 19-electron
complexes takes place through a redox reaction followed
Åꢀ
by a nucleophilic attack of O2 at arene ligands of oxi-
dized 18-electron complexes [16]. In fact, ferrocene
quantitatively produced by a redox reaction between
oxidizable substrates and ferrocenium cations under
oxygen cannot result from the reduction of 1e. In that
case, the radical coupling reaction probably takes place
at the iron center affording a l-peroxy iron radical cat-
ion 1g (Eq. 1) and (or) a l-peroxo di-iron complex 1h
(Eq. 2) (Scheme 6). l-peroxo di-iron complexes have
been frequently proposed as unstable intermediates. A
first detection and characterization of a such intermedi-
ate was made in the autoxidation of ferrous porphyrins
[17]. In a preliminary communication [5], we suggested
1h as intermediate in the decomposition of ferrocenium
cations in polar organic solvents under oxygen.
Table 2
Elemental analyses of isolated solids after electrolyses of ferrocene 1a
in acetonitrile (a) or methylene chloride (b) + 0.1 M Bu4NBF4, under
oxygen
The first radical coupling (Eq. 1) is probably an equi-
librium in which the concentration of complex 1g is very
low. To argue this assertion, the stabilization of ferroce-
nium cations, particularly in acetonitrile, is obtained
Solvent
C%
H%
N%
Cl%
Fe%
F%
(a)
(b)
22.95
23.70
3.54
3.70
0.10
–
–
1.67
31.62
23.55
4.63
7.40

J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
1835
breaking antioxidants for many years and the supposed
mechanism involves a reaction with peroxy radicals [21].
By analogy, we can propose 1g instead of 1h as oxidizing
reagent as exemplified in Scheme 7 for phenate ions.
The new peroxy radical issued from phenate ion
decomposes affording the final product. Oxidations of
triphenylphosphine or diphenylisobenzofuran can take
place according to a similar mechanism involving 1g.
The oxidation rate is higher in DMF than in acetonitrile
(cf. Figs. 1 and 4 for triphenylphosphine). This obser-
vation can explain the apparent stability of di-
phenylisobenzofuran and ferrocenium cations in
acetonitrile under oxygen for one hour (cf. 2.2.2.2) and
a very slow disappearance of diphenylisobenzofuran
during the electrolysis of ferrocene in acetonitrile in
the presence of oxygen (cf. 2.3.2.2).
Fe+
Fe O O
+ O2
(1)
(2)
1b
1g
2+
Fe+
Fe O O
Fe O O
+
Fe
1b
1h
1g
Scheme 6.
after adding small amounts of an active antioxidant
(flexzone).
In order to explain the role of flexzone, an hypothetic
formation of singlet oxygen which is a powerful oxidiz-
ing reagent, can be considered. It is well known that
flexzone inhibits singlet oxygen; as an example, flexzone
totally suppressed singlet oxygen produced in the photo-
oxidation of non vulcanized rubbers [18]. However, our
experimental results are not in accordance with the pres-
ence of singlet oxygen. Diphenylisobenzofuran which
has a larger rate constant for quenching singlet oxygen
[19] affording benzoylbenzophenon, does not react (or
very slowly) in acetonitrile containing ferrocenium cat-
ions under oxygen. Nevertheless, it was previously
found [20] that singlet oxygen generated in acetonitrile
from a redox reaction between ferrocenium cation and
superoxide anion both prepared by pulse electrolysis,
was trapped by diphenylisobenzofuran. Moreover, an
oxidation of substrate by hypothetic singlet oxygen
resulting from the equilibrium (1) would not give
ferrocene.
In the presence of some oxidizable substrates, com-
plexes 1g or 1h act as oxidizing reagents. We observed
that under oxygen, one mole of ferrocenium cations
which is reduced into ferrocene, practically oxidizes
one mole of diphenylisobenzofuran in DMF (Fig. 2)
or one mole of phenols in acetonitrile (Fig. 6). It is
worth noting that a catalytic oxidation of residual water
in the medium, can explain the incomplete oxidation of
phenols (#90%). We controlled that ferrocenium cations
were reduced into ferrocene in aqueous basic media un-
der oxygen; the rate of this reaction increases with the
pH values. Oxidations of phenols generally involve 2F
per mol, consequently reductions of 1g or 1h would con-
sume 2F or 4F, respectively. Phenols were used as chain-
In the presence of an antioxygen (flexzone), the oxi-
dizing properties of the mixture ferrocenium-oxygen
are preserved (Fig. 6). The presence of flexzone does
not prevent the formation of the adduct 1g but avoids
its irreversible rearrangement (cf. 3.2.2).
3.2. Decomposition of ferrocenium cations
3.2.1. Decomposition in DMF or DMSO
The experimental results show that an oxidation of
DMF or DMSO occurs during the chemical transforma-
tion of ferrocenium cations under oxygen. Firstly, in the
absence of electrolysis, we observed a partial reduction
(50%) of ferrocenium cations into ferrocene in oxygen
saturated solutions of DMF or DMSO, secondly, CO2
was produced during macroscale electrolyses of ferro-
cene in DMF in the presence of oxygen. Except for fer-
rocene, the only oxidizable species is the solvent and
only DMF can be a source of CO2. It is worth noting
that CO2 is not obtained during decomposition of ferr-
ocenium cations in DMF without electrolysis. Conse-
quently, as observed by an increase of the anodic peak
in cyclic voltammetry (cf. 2.1.1), a second electrochemi-
cal process takes place next to the first one which corre-
sponds to the oxidation of starting ferrocene. The
second electrochemical process can result both from an
oxidation of ferrocene partially produced from chemical
transformation of ferrocenium cations as observed in
solution and probably from an oxidation of an interme-
diate giving the unstable N,N-dimethyl carbamic acid,
(CH3)2NCO2H, which decomposes with CO2 formation.
O
O
t-Bu
t-Bu
t-Bu
t-Bu
Fe
Fe O O
+
+
O O
R
R
1a
1g
Scheme 7.

1836
J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
In order to account all the experimental results, we
ferrocenium cation 1b affords ferrocene 1a and peroxy
radical cation 1g. The part of hydroperoxy cation 1i pro-
duced at the anode surface according to the reaction 3
may be also electrochemically oxidized (reaction 7).
The faradic result (2F per mol of ferrocene) is in accor-
dance with the Scheme 8. Furthermore, an hydrogen
atom transfer from N,N-dimethylamine 4d to peroxy
radical cation 1g is conceivable leading to a new aminyl
radical and hydroperoxy cation 1i. The cathodic irre-
versible peak observed at ꢀ0.45 VSCE on the cyclic vol-
tammogram (cf. 2.1.1) can result from reduction of
peroxidic species.
The formation of ferrocene in 50% yield during chem-
ical evolution of ferrocenium cations in oxygen-satu-
rated solution of DMSO probably involves also
hydroperoxy cation 1i as intermediate. DMSO is well
known as a H-atom donating solvent.
can propose the mechanism outlined in Scheme 8. The
reactions 1–7 occur during the electrochemical oxidation
of ferrocene in oxygen-saturated solution of DMF. The
reactions 2–5 correspond to the chemical transformation
of ferrocenium cations in DMF under oxygen.
It is well known [21] that hydrogen atom can be
transferred from a substrate to a peroxy radical. In this
way, the peroxy radical cation 1g abstracts an hydrogen
atom from DMF affording the new radical 4b and the
hydroperoxy cation 1i (reaction 3). This reaction can
take place partly at the electrode during electrolysis,
thus, 4b can be oxidized at the anode giving a cation
which adds residual water from the medium producing
the unstable hydroxamic acid 4c (reaction 6). The other
part of 4b produced in solution can give subsequent
chemical reactions (radical coupling reactions with 1g)
with decomposition without CO2 formation (reaction
5). Consequently, about 45% of 4b are directly oxidized
at the anode during macroscale electrolyses and the
other part chemically reacts. By analogy with the oxida-
tion of hydroxyperoxide by metal ions [21], a redox reac-
tion (reaction 4) between hydroperoxy cation 1i and
3.2.2. Decomposition in acetonitrile, acetone or methylene
chloride
The trapping with a strong dienophile of cyclopenta-
dien-5-ol in high yields during decomposition of ferroce-
nium cations in acetonitrile under oxygen necessary
- e
Fe
Fe+
(1)
(2)
1a
1b
Fe+
+ O2
Fe O O
1g
1b
+
(3)
(4)
+
+
(CH3)2NCHO
(CH3)2NCO
Fe O OH
Fe O O
4a
4b
1g
1i
+
+ H+
Fe+
Fe
1a
+
+
Fe O O
Fe O OH
1g
1i
1b
(solution)
radical coupling reactions
and decomposition without CO2 evolution
(CH3)2NCO
(5)
(6)
4b
O
- e
(H3C)2N C
(CH3)2NCO
(CH3)2NH + CO2
+ H2O - H+
OH
4b
4c
4d
+
- e - H+
Fe O OH
Fe O O
(7)
1g
1i
Scheme 8.

J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
1837
R
R
"
2+
"
+
O
O
O
R
O
"
"
R
R
+
or
Fe
O
+
+
FeO
Fe
R
1j
1g
1k
N
N
Subsequent reactions
with decomposition
+
O
O
N
Ph
3b
R
OH
O
O
N
N
N
Ph
+
N
N
Ph
N
O
O
1c (R = H)
2c (R = CH )
1d
3
Scheme 9.
implies an irreversible oxygen transfer from iron of 1g to
a cyclopentadienyl ligand as shown in Scheme 9. A
nucleophilic attack of residual water in basic media can-
not be retained. Firstly, the addition of methanol to the
solution before the decomposition did not give methoxy-
lated adduct, secondly, the regio (for dimethylferroce-
nium cations) and stereoselectivity of the reaction are
in accordance with an oxygen transfer from central iron
(endo addition). A direct reaction of molecular oxygen
at cyclopentadienyl ring would probably afford an exo
addition. A decomposition in the presence of 18O2
would give an unambiguous answer about the origin
of oxygen. The transfer can involve either one oxygen
atom with breaking of the O–O peroxy bond, directly
affording alcoolate of cyclopentadienol 1j or the peroxy
group giving an unstable peroxy compound 1k. The
presence of an active antioxidant (flexzone) does not
avoid the formation of the peroxy radical cation 1g
for which oxidizing properties remain, but inhibits the
oxygen transfer to the ligand.
Elemental analyses (cf. 2.3.2.1) of solids isolated from
the oxygen decomposition of ferrocenium cations in ace-
tonitrile or methylene chloride is in accordance with a
new organometallic iron cation having an organic ligand
(probably equivalent to one cyclopentadienyl) with BF4ꢀ
as counter-ion (or ClOꢀ4 when Bu4NClO4 is used as elec-
trolyte). The observed difference of Fe/F ratio higher in
acetonitrile than in methylene chloride can be explained
by the presence in the former one of a most important
part of ferrate ions (FeO24ꢀ) next to BF4ꢀ as counter-
ion. Ferrate ions can be produced from complete
decomposition of ferrocenium cations. The small
amount (12–15%) of adduct between dienophile and
cyclopentadiene must correspond to this complete
decomposition of ferrocenium cations. After dissolving
solids in aqueous acidic medium, ferric ions probably re-
sult both from ferrate anions and decomposition of the
new organometallic compound. These results are par-
tially in contradiction with the assertion of Zotti et al.
[7] who did not consider the presence of a counter-anion
in the solid isolated after decomposition.
The simultaneous disappearance of ferrocene and
ferrocenium cations in oxygen saturated solution of ace-
tonitrile must involve oxidizing intermediates of decom-
position acting with ferrocene.
4. Conclusion
In polar organic solvents, ferrocenium cations are
very sensitive to molecular oxygen with which they
reversibly form peroxy radical cations. Chemical trans-
formations of these peroxy compounds change accord-
ing to the nature of the solvent and the presence or
not of various substrates in solution. Transformation
rates are higher in an oxidizable solvent as DMF or
DMSO than in a non-oxidizable one as acetonitrile, ace-
tone or methylene chloride with a complete decomposi-
tion in the latter solvents and formation of ferrocene in
50% yield in the former one. In the presence of oxidiz-
able reagents, whatever the solvent, the rates are notably
decreased; redox reactions can take place with ferrocene
formation without decomposition. Ferrocenium cations
in oxygen-saturated solution of polar solvents can be
stabilized for a long time at room temperature, by add-
ing active antioxidants in small amounts (molar ratio
<5%), but with preserving the oxidizing properties of
peroxy radical cation. In conclusion, our study shows
that it is necessary to use deaerated solutions even for
anodic or chemical oxidations, particularly when the
generated species are paramagnetic. Moreover, ferro-
cene has been currently used as an internal standard
for electrochemical measurements [22] and then, in this
case, oxygen must be completely eliminated. Because
sufficient methyl substitution of the cyclopentadienyl
rings can render ferrocenium stable to oxygen, Baskin

1838
J.P. Hurvois, C. Moinet / Journal of Organometallic Chemistry 690 (2005) 1829–1839
and Kinlen [9] proposed the bis(pentamethylcyclopenta-
dienyl) iron as oxygen-stable reference electrode.
in oxygenated solution of solvents were monitored by
voltammetry at a rotating disc electrode. Analyses were
generally performed in 30 mL of organic solvent with
Bu4NBF4 as electrolyte.
5. Experimental
5.3. Anodic oxidation of ferrocene at a planar electrode
under oxygen
5.1. Chemical and reagents
Solvents (reagent grade) were used without further
purification. Tetrabutylammonium tetrafluoroborate
(Bu4NBF4) and tetrabutylammonium perchlorate
(Bu4NClO4) were purchased from Fluka. The former
salt was crystallized in a mixture methanol–water in or-
der to eliminate traces of iodide ions and dried. Ferro-
cene and 1,10-dimethylferrocene (Aldrich) and other
chemical were reagent grade and used as received.
Ferrocenium cations were prepared by stirring ferro-
cene for 2 h in concentrated sulphuric acid in the pres-
ence of air. The resulting blue solution was poured on
ice and after extraction with diethyl ether, HPF6 (40%
in water) was added to the aqueous solution, then the
solid ferrocenium hexafluorophosphate was filtered,
washed with diethyl ether and dried.
As a general procedure for non oxidizable solvents,
the oxidation at a platinum anode of 0.95 g (5.1 mmol)
of ferrocene in 160 mL of acetone containing Bu4NBF4
(0.1 M) as electrolyte was performed at 1 VSCE. An oxy-
gen bubbling was continuously maintained during all
the electrolysis. After total disappearance of the starting
ferrocene and consumption of 0.4 F per mol, 0.9 g of a
brown precipitate was filtered. This last one was slightly
soluble in water without appearance of Fe3+ cations in
the resulting solution and soluble in nitric acid with for-
mation of FE3+ characterized by adding SCNꢀ. Addi-
tion of aqueous sodium hydroxide to this acidic
solution led to a ferric hydroxide precipitate and a deep
red solution which turned orange yellow after
acidification.
5.2. Apparatus and procedure
5.4. Anodic co-oxidation of ferrocenes and 4-phenyl-
urazole at a porous electrode
Conventional electrochemical equipment was used
for cyclic voltammetry and voltammetry at a rotating
disc electrode (EG & G Princeton Applied Research
Model 362 scanning potentiostat with an XY recorder).
For both cyclic voltammetry and voltammetry at a
rotating disc electrode (x = 2000 rpm), the working elec-
trode was a disc of glassy carbon (3 mm diameter) or
platinum (1 mm diameter). All potentials referred to
the saturated calomel electrode (SCE) and were not cor-
rected for the ohmic drop.
Controlled potential electrolyses were performed in a
cell [23] equipped with a planar electrode (4 cm diame-
ter), made of glassy carbon or platinum. The cell is
adapted to collect gas which could evolve from the elec-
trolyzed solution. The coulometric measurements were
determined with a current integrator Tacussel IG5N.
Electrolyses under continuous bubbling of oxygen gen-
erally involve 2–10 mmol of ferrocene in 150–180 mL
of solvent (DMF, DMSO, acetonitrile, acetone or meth-
ylene chloride) with Bu4NBF4 or Bu4NClO4 as
electrolyte.
Co-oxidations at a porous graphite felt anode of fer-
rocene or 1,10-dimethylferrocene (4 mmol) and 4-phe-
nyl-1,2,4-triazolidine-3,5-dione (4-phenylurazole) (4, 8
or 12 mmol) were performed in 400 mL of acidified ace-
tonitrile (2% H2SO4). The total current intensity calcu-
lated from FaradayÕs law was corresponding to 1 F
and 2 F per mol of ferrocene and 4-phenylurazole,
respectively. Solid sodium carbonate and molecular oxy-
gen were added to the electrolyzed solution at the outlet
of the cell. A complete decomposition of ferrocenium
and 1,10-dimethylferrocenium were observed after
respectively one hour and one night. After decomposi-
tion and filtration, the solution was evaporated by half
under vacuum, then after adding an equivalent volume
of water, the resulting solution was concentrated under
vacuum. The residue was extracted with methylene chlo-
ride. After drying, the organic solvent and the crude
reaction products were purified by column chromatog-
raphy (ether) leading to pure Diels–Alder adducts. Phys-
ical characteristics of these products were previously
reported in a preliminary communication [8].
Electrolyses in a flow cell [24] fitted with a porous
graphite felt (RVG 4000 Le Carbone Lorraine) anode
(5.2 cm diameter, 1.2 cm thickness) located between
two counter electrodes were carried out at controlled
current intensities calculated from the FaradayÕs law.
The solution of substrates was pumped through the cell
from a reservoir.
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