Features of the oxidation of certain hydroxy derivatives of ferrocene with molecular oxygen in organic solvents

Article abstract of DOI:10.1134/S107036320807013X

Features of the oxidation of hydroxymethylferrocene with molecular oxygen in organic solvent in the presence of weak and strong Broensted acids were studied. The resulting data were compared with published data on ferrocene and hydroxyferrocene oxidation, which made it possible to assess the trend in the oxidation mechanism in going from ferrocene π complexes to their hydroxyferrocene and hydroxymethylferrocene analogs. A conclusion was drawn that each of the ferrocenes reacts with oxygen in quite a specific way.

Full text of DOI:10.1134/S107036320807013X

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 7, pp. 1361–1370. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.M. Fomin, A.E. Shirokov, N.G. Polyakova, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 7, pp. 1125–1134.
Features of the Oxidation of Certain Hydroxy Derivatives
of Ferrocene with Molecular Oxygen in Organic Solvents
V. M. Fomin, A. E. Shirokov, and N. G. Polyakova
Lobachevskii Nizhny Novgorod State University,
pr. Gagarina 23, Nizny Novgorod, 603950 Russia
e-mail: niih325@bk.ru
Received December 12, 2007
Abstract—Features of the oxidation of hydroxymethylferrocene with molecular oxygen in organic solvent in
the presence of weak and strong Broensted acids were studied. The resulting data were compared with
published data on ferrocene and hydroxyferrocene oxidation, which made it possible to assess the trend in the
oxidation mechanism in going from ferrocene π complexes to their hydroxyferrocene and hydroxymethyl-
ferrocene analogs. A conclusion was drawn that each of the ferrocenes reacts with oxygen in quite a specific way.
DOI: 10.1134/S107036320807013X
The ability of π complexes of transition metals to
react with molecular oxygen can be considered as one
of the fundamental properties of these compound [1,
2]. However, even though the chemistry of ferrocene is
extensively developed, almost no information on
oxidation of ferrocene derivatives is available.
analysis of published data on ferrocene oxidation
(Scheme 1).
Scheme 1.
K1
Cp2Fe + H+
Cp2Fe–+H,
(1.1)
(1.2)
K2
Cp2Fe–+H + O2
Cp2Fe–+H···O2 ,
The aim of the present work was to study features
of the reaction of hydroxymethylferrocene (I) with
molecular oxygen in organic solvents and to compare
our results with published data on the oxidation of
hydroxyferrocene (II) and ferrocene (III). Thus we
expected to obtain evidence to show how the oxidation
mechanism changes in going from ferrocene π-
complexes to their hydroxyferrocene and hydroxyme-
thylferrocene analogs.
K3
Cp2Fe–+H···O2 + Cp2Fe–+H
2Cp2Fe+ + H2O2, (1.3)
Cp2Fe–+H + H2O2 → Cp2Fe+ + H2O + HO·, (1.4)
Cp2Fe–+H + HO· → Cp2Fe+ + H2O.
(1.5)
As known, ferrocene (III) is highly resistant to
oxygen and can be oxidized exclusively in the
presence of strong Brønsted acids (HX) according to
The mechanism was based on the following
evidence related, directly or indirectly, to properties of
ferrocene (III):
Eq. (1) [4, 5].
(1) ability to fast equilibrium protonation with
strong acids [3, 9, 10];
.
4Cp2Fe + O2 + 4H+ → 4Cp2Fe+ + 2H2O.
(1)
(2) ability of protonated ferrocene (Cp2Fe–+H) to
The reason for such an inertness of ferrocene (III)
lies in the fact that its standard redox potential
{E0(Cp2Fe+|Cp2Fe) 0.59 V [6]} is much higher than
reaction with oxygen to form the ferricinium ion
.
Cp2Fe+ and H2О [4];
–
that of oxygen {E0(O2 |O2) –0.56 V [7]}. In a strongly
(3) low energy of the Fe–H bond in Cp2Fe–+H,
equal to 209.2 kJ mol–1 [10], which allows the latter
molecule to be considered as an efficient donor of
atomic oxygen to oxidant;
acidic media, the redox potential of oxygen increases
to 1.23 V, which makes reaction (1) thermo-
dynamically favorable. The mechanism of reaction (1)
has been suggested in [8] as a result of a detailed
1361

1362
FOMIN et al.
(4) results of kinetic research [11], according to
which the ferrocene oxidation rate is described by Eq. (2).
electrons to the CP ligand by the conjugation me-
chanism [13] and thus reduce the redox potential of the
metal complex. Moreover, the OH group can act as a
Broensted acid (pKa 10.16 [12]) and take part in
binding of oxygen with the metal complex due to H-
bond formation with the oxygen molecule. This
increases the redox potential of the oxidant. Evidence
for these suggestions about reasons for the effect of the
OH substituent on the reactivity of compounds is
provided by the three obrservables: (1) methyl and
ethyl ether of hydroxyferrocene (II) fail to react with
oxygen in the absence of acids [12]; (2) the ferrocene
(III)–О2-weak carboxylic acid system (pKa ~4–5; c0HX/
cIII 1–10) undergoes no changes in organic solvents for
several weeks [14]; (3) ferrocenylacetic acid which is a
bifunctional reagent containing an electron-donor
(metal) and an electron acceptor (COOH group)
reaction centers that take part in the initial binding of
oxygen and further transformations of the resulting
oxygen complex is readily oxidized according to
Scheme 2 [14].
ν = kapp[Cp2Fe]2[O2][H+]2.
(2)
The kinetic analysis of the suggested analysis under
the assumption that the limiting stage is reaction (1.3),
gives rate Eq. (3).
k3K 12K2[Cp2Fe]02[O2]0[H+]02
.
ν =
(3)
(1 + K1[H+]0)2
At K1[H+]0 << 1 this equation transforms in Eq. (2),
where kapp = k3K12K2. The thermodynamic character-
istics of reactions (1.2), (1.3) and (1.4), (1.5) show that
the latter two reactions have higher negative standard
enthalpies and Gibbs energies (see table), which, in
terms of the thermodynamic criteria, confirms the
suggested
mechanism,
even
though
these
characteristics relate to the gas phase [8].
A different picture is observed in the oxidation of
hydroxy derivatives of ferrocene, which are highly
reactive toward oxygen. Monohydroxyferrocene
C5H5FeC5H4OH (II) and 1,1'-dihydroxyferrocene (IV)
are readily oxidized with air oxygen both in an ether
solution and in the crystalline state at room
temperature in the absence of any protic acids [12].
The example of compound IV was used to show that
the reaction destroys the sandwich structure of the
complexes and results in preferential formation of
cyclopentadienone dimer and an inorganic Fe3+
derivative.
Scheme 2.
CH2COO
CH2COOH
Fe
+ O2
Fe
O2
H
A
V
CH2COO_
CH2COO
а
Fe+
+ HO2
A
OOH _
b
Fe+
Such an essential effect of the ОH substituent on
the reactivity of metal complexes and their oxidation
products points to the fact that ferrocene and hydroxy-
ferrocene (II) react with oxygen by a radically different
mechanism. This difference is associated with direct
involvement of the substituent in the oxidation process.
Fe(O)OH + CO2 + organic compounds.
The CH2CООH substituent is not an electron
donor, like OH, but it has a much higher acidity than
hydroxyl (pKa ~5 [3], which probably explains the fact
that acid V is quite reactive toward oxygen.
Such involvement can be considered quite possible
in view of the known ability of the ОH group to donate
There is no information in the literature,
concerning oxidation kinetics of compound IV, but
Scheme 2 and data on the oxidation products of
dihydroxyferrocene IV can be used as the basis for the
probable mechanism of its oxidation (Scheme 3). This
scheme can also be extended to hydroxyferrocene (II).
Thermodynamic characteristics of ferrocene oxidation
[reactions (1.2), (1.3), (1.4), and (1.5)]
ΔrH 0
(298.15),
kJ mol–1
–168.1
–82.4
ΔrS 0
ΔrG 0
(298.15), (298.15),
Reaction
kJ mol–1
–29.5
kJ mol–1
–159.3
–115.7
–290.3
a
2Cp2Fe–+H + O2 → P1
Cp2Fe–+H + H2O2 → P2
Cp2Fe–+H + HO· → P3
Path a in Scheme 2 involves proton abstraction of
the oxygen molecule being reduced from the acidic
substituent, is most likely not realized in the case of
112.3
–297.2
–23.0
a (Pi ) Reaction products shown in Scheme 1.
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FEATURES OF THE OXIDATION OF CERTAIN HYDROXY DERIVATIVES
1363
(3.1)
Scheme 3.
OH
O2
OH
fast
OH
Fe
Fe
+ O2
OH
IV
A
O
slow
Fe+
OOH _
OH
A
(3.2)
(3.3)
B
OH
SH
_S
Fe+
OOH _
OH
B
C
OH
OH
H
_
O
Fe+
O
C
FeO(OH) + 2
OH
(3.4)
OH
OH
+ O2
_HO2
(3.5)
(3.6)
O
O
O
OH
O
O
OH
SH
_S
O
(3.7)
(3.8)
_H2O2
OOH
dimer.
2
O
compound IV, since the О–H group in its hydroxy
group is much less polar. The Fe–O2 bond in complex
A should be considered as donor–acceptor with a small
contribution, like in charge-transfer complexes, charge
transfer. This bond is formed by the higher occupied
molecular orbitals of compound IV (dxy and dx2–y2),
The anomalously high reactivity of compounds II,
IV, and V toward oxygen compared to the system
ferrocene (III)–weak HX is a typical manifestation of
the effect of mutual approach and orientation,
associated with thermodynamically unfavorable
entropy losses characteristic of intermolecular interac-
tions of a high kinetic order [15, 16], including the
suggested model reaction (4) [14].
*
localized on the metal atom [3], and low-lying 1πg
orbital of O2.
_
X_
]
//
=
δ
[Cp2Fe+δO2
H+δ
δ
As О2 coordinates with metal complexes, its
interaction with an electron-donor reaction center
enhances interaction eith electron-acceptor (and vice
versa), which stabilizes adduct A and its structurally
related transition state, and results in a considerable
gain of energy required for overcoming the energy
barrier during motion along the reaction coordinate.
This picture is characteristic of bifunctional reacting
systems that are common under enzymatic catalysis
conditions [15, 16].
1
1
Cp2Fe + O2 + HX
a
Cp2Fe+OOH + X
Cp2Fe+X_ + HO2
(4)
b
The effect of mutual approach and orientation
intrinsic in enzymatic polyfunctional catalysis is mea-
sured thermodynamically (ΔG#appr, orient 10–12 kcal mol–1)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 7 2008

1364
FOMIN et al.
and kinetically as the ratio of the rate constants of the
enzymatic and homogeneous catalytic reactions (107 М,
where М is the molarity of excess reagent) [15, 16].
CH2ОH group is already electron acceptor rather than
electron donor, and, moreover, it is a stronger electron-
acceptor than the CH2CООH substituent in compound
V [20]. The second consequence is that the energy of
the О–H bond in the CH2ОH group to ~102 kcal mol–1
[21] against 84 kcal mol–1 in the ОH substituent in
hydroxyferrocene (II) [20, 21]. Third, the CH2ОH
group has a lower acidity (pKa ≈ 16–18), which is
characteristic of alcohols RCH2ОH [22]. As a result,
compound I is much more resistant to О2. Hydroxy-
methylferrocene (I) is oxidized for a long time neither
in the solid state nor in organic solvents. For this
reason, the oxidation of this complex in organic
solvents was studied in the presence of strong and
weak Brønsted acids.
The conjugation chain developed in complex A
and including the metal atom, O2 molecule, and acid
group provides, by analogy with enzymatic processes
referred to as “conjugate catalysis” [16, 17], syn-
chronous electron transfer from the metal atom and
hydrogen abstraction from the ОH group by the
reduced O2 molecule, which decreases the activation
energy of the process. The transformation of hydro-
peroxide complex C, leading to final reaction products,
is explained by the lability of the hydroperoxide anion
bound with the metal atom [18]. If for compounds II
and V to be oxidized several chemical bonds between
О2 and reaction centers in the metal complex should
form, then here we can speak about quite a stringent
complementarity of the reacting molecules both at the
stage of formation of adduct A and during its motion
along the reaction coordinate, which provides a
substantial rate gain [19].
Various-strength acids quite specifically affect the
reaction of compound I with oxygen. As the metal
complex (c0I 0.01 M) is mixed with perchloric acid
(c0HX 0.2 M, pKa –10) in dioxane, ethanol, toluene, and
DMF in a vacuum or in an oxygen atmosphere, the
light yellow solution immediately gets green and
remains unchanged for a long time in the presence of
oxygen. No oxygen is therewith absorbed (20–50°C). The
electronic absorption spectrum of the resulting
compound in dioxane is similar to that of the ferri-
cinium ion (λmax 626, 453, 346, and 258 nm) [23]. The
long-wave absorption band (λmax 626 nm) is asym-
metric and has a shoulder at ~580 nm, which may imply
overlapping bands of two different ferrocene deriva-
tives. The changes of compound I under the action of
HClO4 are due to the formation of the ferrocenyl-
methyl cation which is prone to further conversion to
the ferricinium ion according to Scheme 4 [3, 24].
The aforesaid points a great similarity of the
oxidation mechanisms of the mentioned bifunctional
ferrocene derivatives and the mechanisms of a series of
known processes involving polyfunctional enzymatic
systems [15, 16], which predetermines, in the final
count, the high reactivity of hydroxyferrocene (II) and
ferrocenylacetic acid (V) toward oxygen.
Hydroxymethylferrocene (I) differs from hydro-
xyferrocene (II) by that the former contains a
methylene group between ОH and CP. This, first of all,
reverses the properties of the substituent, since the
Scheme 4.
CH2OH2+
H+
CH2OH
Fe
Fe
(1)
+
CH2
CH2
(2)
(3)
Fe+
Fe
redox isomerization
H2O
(K-R)
(K-K)
CH2 CH2
1
2
Fe+
Fe+
recombination
In dilute solutions of strong acids, all the stages of
Scheme 4 are reversible, except for dimerization of the
radical cation [25]. Therefore, the presented spectrum
of the reaction product of compound I with HClO4 is
better treated as a combination of the spectra of the
ferrocenylmethyl and ferricinium ions.
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FEATURES OF THE OXIDATION OF CERTAIN HYDROXY DERIVATIVES
1365
It is known that α-ferrocenylcarbenium ions are
NO
2
resistant to molecular oxygen in strongly acidic
solutions [3]. Therefore, according to the reaction
mixture containing reaction products of compound I
with HClO4 may absorb oxygen due to oxidation of the
radical cation (RC) and its dimer 1,2-diferricinio-
ethane. Monomeric unsubstituted ferricinium ion is
readily oxidized with decomposition of its sandwich
structure [26]. Since solutions of compounds I in the
presence of HClO4 absorbed no oxygen, we can
conclude that the steady-state concentration of RC is
low, and 1,2-diferricinioethane is much less reactive
toward oxygen than monomeric ferricinium ion. This
can be explained both by steric reasons and by that the
highly rigid structure of the dimer prevents the
ferricinium ion, on its reaction with О2, from adopting
a configuration with tilted Cp ligands, thus hindering
their subsequent π–σ rearrangement in the course of
further transformations of the resulting oxygen adduct
[27].
2.0
1.5
1.0
0.5
0.0
5
3
2
1
4
0
20
40
60
Time, min
Fig. 1. Kinetic curves of oxidation of hydroxymethyl-
ferrocene (I) in the presence of (1–4) benzoic (VI) and (5,
6) trifluoroacetic (VII) acid in (3, 5) dioxane, (2) toluene,
(1, 6) dioxane–water, and (4) ethanol. c0I 0.02, c0VI 0.55, c0VII
0.2, c(H2O) 2 M, P(O2) 0.47×105 Pa, Т 50°C.
No similar transformations are observed with
compound I in the presence of benzoic acid (VI) (pKa
4.2) in the above solvents and in the presence of
trifluoroacetic acid (VII) (pKa 0.3) in dioxane, but the
metal complex can be oxidized with oxygen in this
case.
at oxygen consumptions of up to NO ≈ 1 (NO is the
2
2
number of oxygen moles absorbed by the reaction
mixture per 1 mol of metal complex) in the presence of
acids VI and VII are ~0.5 and ~0.3 mol per 1 mol of
the starting complex, respectively. The yield of acid IX
is lower by an order of magnitude. Compounds formed
by oxidative destruction of compound I were also
found in the reaction mixture: cyclopentadiene,
cyclopentadiene dimer, and a Fe3+ derivative. A more
Figure 1 shows the kinetic curves of oxidation of
compound I in various solvents in the presence of
acids VI and VII. As seen from the figure, the
oxidation rate increases with increasing strength of the
carboxylic acid, and the solvent effect varies in the
series dioxane > toluene > dioxane–H2О >> ethanol,
which shows that solvation palys a considerable role in
the reaction in hand (dioxane > toluene) and on the
other, solvents are capable of inhibiting oxidation by
binding one of the starting reagent and/or intermediate
reaction products into inactive hydrogen complexes
(toluene > dioxane–H2О >> ethanol). When the
concentration of water in dioxane is 4 M, compound I
is scarcely oxidized, like in ethanol.
profound oxidation of compound I (NO > 2) gave CО,
2
CО2, and unidentified organic compounds. As one of
the intermediate reaction product we detected the
corresponding ferricinium ion whose electronic
absorption spectrum contains in the long-wave region
a band with λmax 616 nm, which distinguished this
spectrum of the spectrum of a compound obtained by
treatment of I with perchloric acid. Noteworthy is the
fact that the oxygen consumption in the oxidation of I
is much greater than in the oxidation of ferrocene
(~0.3).
Addition of pyridine which can form strong
complexes with acids in a quantity equimolar to the
concentration of acids VI and VII retards oxidation of
compound I completely.
The kinetic results show that the oxidation of
hydroxymethylferrocene (I) in the presence of acids
VI and VII is a first-order reaction in metal complex,
oxygen, and benzoic acid (Fig. 2). With trifluoroacetic
acid (VII), the maximum reaction rate Wmax linearly
varies with acid concentration only at low
concentrations of VII. As higher acid concentrations,
Oxidation of hydroxymethylferrocene (I) in the
presence of acids VI and VII involves the CH2ОH
substituent and gives rise to ferrocenecarbaldehyde
(VIII) and ferrocenecarboxylic acid (IX). The highest
detected yields of aldehyde VIII in the oxidation of I
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1366
FOMIN et al.
25
20
15
10
5
40
35
30
25
20
15
10
5
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
16
12
8
3
1
3
1
4
4
2
2
0
0
0
0
0
0.01
0.2
0.02
0.4
0.03
0.6
0.04
0.05
0
0
0
0.2
0.4
0.6
0.8
1
cVI, M
cI0, M
0.8
1
0.02
0.04
0.06
0.08
c0VII, M
0.1
P(O2) × 10–5, Pa
Fig. 2. Dependences of the oxidation rates of compound I
on (1, 2) its concentration and (3, 4) oxygen pressure in the
presence of (1, 3) benzoic (VI) and (2, 4) trifluoroacetic
(VII) acid. c0VI 0.55, c0VII 0.06 M; P(O2) (1, 2) 0.47×105 Pa;
c0I 0.02 M, Т 50°C.
Fig. 3. Dependences of the oxidation rates of compound I
on the concentration of (1, 2) benzoic (VI) and (3)
trifluoroacetic (VII) acid in (1, 3) dioxane and (2) toluene.
с0I 0.02 M, Т 50°C, P(O2) 0.47×105 Pa.
the reaction order in the acid becomes lower than 1
(Fig. 3). The reaction time in the kinetic experiments
was taken to be no longer than 10–15 min to exclude
variation in the concentration of compound I due to its
possible reaction with the acid and intramolecular
dehydration with ether formation.
itself develops? Since compound I is oxidized in miles
temperature conditions (30–50°C) and only in the
presence of acids, we can suggest that radicals are
generated by molecular oxidation of the metal complex
(Scheme 5).
Scheme 5.
To gain insight into the reaction mechanism, we
studied the effect of additives known as free-radical
inhibitors (Ionol, о-phenylenediamine) on the oxida-
tion rate of compound I. The results of this study,
illustrated in Fig. 4, show that the above additives
strongly decelerate oxidation. This finding implies that
the overall oxidation process is mostly contributed by
the radical chain path, and the revealed formal kinetic
regularities relate just to this path. Evidence for this
conclusion comes from the fact that the oxidation of
compound I in the presence of acid VII in methyl
methacrylate or in the dioxane–methyl methacrylate
system is accompanied by polymerization of the
monomer. The radical chain mechanism explains the
formation of aldehyde VIII and acid IX in the
oxidation of alcohol I, since aldehydes and carboxylic
acids are known to be the major products of the radical
chain oxidation of alcohols with oxygen [28].
HX
K1
CH2OH
CH2OH
(5.1)
Fe
+ HX
Fe
A
HX
CH2OH
K2
A
+ O2
Fe O2
(5.2)
(5.3)
B
CH2OO
k
+ H2O
B
Fe+ X_
B
The key stage in this scheme is the formation of
ternary complex HX·I·О2 in which the О2 molecule is
simultaneously bound with the metal atom and the
electrophilic carbon atom, which strengthens each of
these donor–acceptor bonds. The C–O2 bond formation
The above-presented data raise two principal
questions: (1) How radicals are generated at the initial
stages of the process and (2) how the chain process
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FEATURES OF THE OXIDATION OF CERTAIN HYDROXY DERIVATIVES
1367
stimulates electron transfer from О2 to iron and, as a
N
O2
result, polarizes the C–ОH bond. This makes possible
proton transfer to the hydroxyl group from the
coordinated HX molecule and transformation of the
ternary complex according to Eq. (5.3). The formation
of water in this reaction makes it thermodynamically
favorable. Complex A can be considered as a
bifunctional reagent, and, therefore, it is safe to state
that the effect of mutual approach and orientation
contributes something to make reaction of this
complex with molecular oxygen possible in principle.
Scheme 5 can explain in part the retarding effect of
water and ethanol on the reaction in hand, since they
both can prevent the OH group from coordination with
HX, thus slowing down radical generation. At a
different mode of coordination of О2 with HX and I
[Eq. (5)], the oxidation of the latter becomes even less
thermodynamically favorable compared to ferrocene
[8], since alcohol I contains an electron-acceptor
substituent.
3
2.0
1.5
1.0
0.5
0.0
4
1
2
0
20
40
60
80
Time, min
Fig. 4. Effect of о-phenylenediamine (In) on the oxidation
rate of compound I in the presence of (1, 2) benzoic (VI)
and (3, 4) trifluoroacetic (VII) acid. с0I 0.02; с0In 4×10–4, c0VI
0.55, c0VII 0.2 M; P(O2) 0.47×105 Pa; Т 50°C.
[30]. At the same time, the concentration of the
C5H5FeC5H4CH(OH)OO· radicals is much lower than
the concentration of compound I. For example, the
steady-state concentration of C6H5CH(CH3)OO· in the
oxidation of ethylbenzene is ~10–6 M at the reaction
initiation rate ~10–5 mol l–1 s–1. In our case, the rate of
inhibited oxidation of compound I, which can be taken
as the rate of initiation by Scheme 5 is ~ 10–9 mol l–1 s–1.
Consequently, the steady-state concentration of
peroxide radicals in the oxidation of alcohol I will be
much lower than 10–6 M, from which it follows that
CH2OH
I + O2 + HX
Fe O2
HX
_CH2OH
+
Fe X
+ HO2
(5)
As seen, Eq. (5) is identical to model Eq. (4).
The principal stages of the radical chain process
are shown in Scheme 6.
· 2
ν6 >> νt = 2kt[RO2 ] . Rearrangement (6.6) is charac-
teristic of ionic peroxide complexes like
LnM+OOCH2R– (LnM is a transition metal complex),
when the carbon atom attached to the peroxide group
bears hydrogen [18]. Thus, Scheme 6 is analogous a
classical scheme of hydrocarbon oxidation not
complicated by degenerate chain branching [28].
Reaction (6.10) generates chain-propagating
radicals C5H5FeC5H4C·HOH but does not affect their
generation rate.
Reactions (6.2)–(6.4) are analogous to reactions
involved in the radical chain oxidation of ethanol,
leading to acetaldehyde and acetic acid [28]. Reaction
(6.5) can be considered as a chain-terminating
reaction, since it is known that ferrocene and its
derivatives easily react with peroxide radicals
(k ~105 l mol–1 s–1) according to Eq. (6) [29].
It is known that radical chain hydrocarbon oxida-
tion reactions are solvent-sensitive. This explains why
ethanol, water, and even carboxylic acids in con-
siderable quantities adversely affect reaction rate, since
solvents and compounds capable of forming strong
hydrogen bonds with radicals RO2· deactivate the latter
and thus inhibit radical chain reactions [28]. By the
same reason, acids differently affect the rates of
molecular and radical chain oxidation of compound I.
Ӑ
C5H5FeC5H4X + RO2· → (C5H5)Fe (C5H4X)OOR– , (6)
Х = H, alkyl, CHO, COOH, CN, COR.
Quadratic chain termination on peroxide radicals is
not included in Scheme 6 by the following reasons.
The rate constant of reaction (6) compares with the
recombination rate constant of the peroxide radicals
formed by oxidation of alkylaromatic hydrocarbons
The oxidative destruction of hydroxymethyl-
ferrocene (I) can involve oxidation of the ferricinium
ion formed [26, 27] and its disproportionation [31] and
decomposition, provided it contains a perioxide anion
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 7 2008

1368
FOMIN et al.
Scheme 6.
CH2OO
HOCH2
HOCH
CH2OOH
Fe+
Fe
Fe+
Fe
+
+
(6.1)
(6.2)
OO
CHOH
CHOH
Fe
+ O2
Fe
OO
CHOH
OOH
CHOH
CHOH
HOCH2
Fe
Fe
Fe
+
+
Fe
(6.3)
(6.4)
(6.5)
OOH
CHOH
CHO
Fe+
O
Fe
Fe
+ H2O2
OO
CH2
CHOH
CH2OH
OH
_
Fe
Fe
Fe
+
OOCH(OH)
Fe
CH2OH
CH2OH
Fe+
Fe+
_
Fe
H
OOCH(OH)
O_
C
O
OH
C
CH2OH
_
Fe+
Fe
OH
OH
+
(6.6)
[18]. As known, CО and CО2 result from oxidative
decarbonylation and decarboxylation of aldehydes and
carboxylic acids, respectively.
radicals by reaction (6), and, therewith, the steady-state
chain termination rate νt is equal to the rate of R·
generation ν0. According to Eq. (6.5), the rate of chain
termination will be given by Eq. (9).
Let us find out whether Scheme 5 corresponds to
the kinetics of radical chain oxidation of compound I.
Kinetic equation (7) relates to the radical chain
reaction path involving the substituent.
νt = kt[I][C5H5FeC5H4CH(OH)–O–O·]
kt[RH][RO2·].
(9)
Equating the reaction rates given by Eqs. (7) and
·
ν = kapp[I][O2][HX].
(7)
(8), we obtain Eq. (10) that relates the RO2
concentrations to the concentrations of the starting
compounds. Substituting this expression in the chain
termination rate equation we obtain Eq. (11) for the
rate of chain initiation.
According [28], the rate of radical chain oxidation
of hydrocarbons is limited by the chain propagation
stage involving peroxide radicals.
ν = kp[RO2·][RH].
(8)
Let RO2· = C5H5FeC5H4CH(OH)OO· and RH = I.
Let us assume that the chain is terminated by peroxide
kapp
[RO2·] =
[O2][HX],
(10)
kр
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 7 2008

FEATURES OF THE OXIDATION OF CERTAIN HYDROXY DERIVATIVES
1369
The oxidation of ferrocenylmethanol (I) was per-
formed in a vacuum static device under vigorous
stirring. The reaction progress was followed by oxygen
absorption measured by manometry.
ktkapp
kр
(11)
ν0 = νt =
[RH][O2][HX] = k0[RH][O2][HX]
.
Analyzing Scheme 5 for the suggested molecular
oxidation of compound I under the assumption that the
process is quasi-equilibrium, and the concentrations of
complexes A and C are much lower than the
concentrations of the starting reagents, we can show
[32] that the rate of the radical generation process has
Eq. (12).
Compound I was synthesized by the procedure in
[33] and purified by column chromatography on Al2O3
using the Trappe solvent series [22]. Ferrocene-
carbaldehyde (VIII) was synthesized by the procedure
in [34] and crystallized from ether. Ferrocene-
carboxylic acid (IX) was synthesized according [35]
and crystallized from ligroin and ethanol.
ν = k1K1K2[I][O2][HX].
(12)
This equation can be shown to be equivalent to Eq.
(11) for the rate of chain initiation, obtained by an
analysis of the radical chain oxidation of compound I,
under the assumption k0 = k1K1K2.
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