Electrochemical method in determination of antioxidative activity using ferrocene derivatives as examples

Article abstract of DOI:10.1007/s11172-011-0100-4

Electrochemical method for the evaluation of antioxidative activity of compounds based on their reaction with the stable radical, 2,2-diphenyl-1- picrylhydrazyl, was suggested with monitoring of the reaction by cyclic voltammetry (CVA). Antioxidative prop

Full text of DOI:10.1007/s11172-011-0100-4

Russian Chemical Bulletin, International Edition, Vol. 60, No. 4, pp. 647—655, April, 2011
647
Electrochemical method in determination
of antioxidative activity using ferrocene derivatives as examples
V. Yu. Tyurin,a N. N. Meleshonkova,a A. V. Dolganov,b A. P. Glukhova,a,с and E. R. Milaevaa
aM. V. Lomonosov Moscow State University, Department of Chemistry,
1 Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (495) 939 5546. Eꢀmail: tyurin@org.chem.msu.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation
сMoscow Municipal Pedagogical University,
4 2ꢀi Sel´skokhozyaistvennyi proezd, 129226 Moscow, Russian Federation
Electrochemical method for the evaluation of antioxidative activity of compounds based on
their reaction with the stable radical, 2,2ꢀdiphenylꢀ1ꢀpicrylhydrazyl, was suggested with moniꢀ
toring of the reaction by cyclic voltammetry (CVA). Antioxidative properties of new ferrocene
derivatives Fc(L)R (where Fc is the ferrocenyl, R is the fragment of 2,6ꢀdiꢀtertꢀbutylphenol or
its aromatic analog, L is the spacer) were studied. Anodic oxidation of the compounds Fc(L)R,
which contain azomethine and 2,6ꢀdiꢀtertꢀbutylphenol moities, proceeds in three steps, that
suggests a possibility of intramolecular protonꢀcoupled electron transfer process. Conjugates of
ferrocene and 2,6ꢀdiꢀtertꢀbutylphenol are efficient antioxidants.
Key words: cyclic voltammetry, ferrocenes, 2,6ꢀdiꢀtertꢀbutylphenol, antioxidative activity,
2,2ꢀdiphenylꢀ1ꢀpicrylhydrazyl.
Nowadays, there are several methods for the evaluaꢀ
tion of antioxidative activity of compounds, however,
a search for the new efficient methods is still of significant
interest.1—2 Lately, a reaction with the stable radical, viz.,
2,2ꢀdiphenylꢀ1ꢀpicrylhydrazyl (DPPH), is used as a fast
and convenient test, which is based on the hydrogen atom
transfer from the antioxidant molecule to the DPPH radiꢀ
cal.3—5 This reaction is usually monitored by spectrophotoꢀ
metry from the decrease in intensity of the absorpꢀ
tion band of DPPH at 512 nm. However, this method
has significant disadvantages. First, it cannot be used for
the chromophoric substances having an absorption
band close to the absorption band of DPPH itself. Second,
since DPPH possesses a high extinction coefficient4
(ε = 1.16•104 L mol–1 cm–1), the method is limited to the
range of low concentrations of DPPH and antioxidant.
We suggested to use CVA for monitoring of this reacꢀ
tion. The voltamgram of DPPH in the anodic region shows
two oneꢀelectron reversible waves, corresponding to the
oxidation and reduction of the radical6,7 (Fig. 1). From
the Randles—Shevchik equation8 it follows that for the
given surface of the electrode and speed of potential sweep,
the proportions of currents in the peak of oxidation or
reduction of DPPH is determined only by the ratio of
concentrations
where I0 is the current at the initial concentration of DPPH
(C0), I is the current at the concentration of DPPH in the
given moment of time (C). Consequently, if the I value is
known for some given concentration of DPPH and a deꢀ
crease of this value can be tracked with time, a kinetic
curve of concentrations of DPPH versus time can be plotꢀ
ted. The relative change of concentration of DPPH in the
I/mA
0.015
0.010
0.005
0
–0.005
–0.010
–0.015
200
400
600
800
1000
E/mV
Fig. 1. Cyclic voltamgram of DPPH in МeCN at the speed of
potential sweep 100 mV s–1 (Ptꢀelectrode, the concentration was
0.5•10–3 mol L–1, Bun4NBF4, Ag/AgCl).
I/I0 = C/C0,
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 633—641, April, 2011.
1066ꢀ5285/11/6004ꢀ647 © 2011 Springer Science+Business Media, Inc.

648
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 4, April, 2011
Tyurin et al.
course of the reaction can be used for the evaluation of
antioxidative activity of compounds under study.
retical interest from the point of view of the search for new
model systems.9—16 In the present work, we studied ferꢀ
rocene conjugates Fc(L)R containing redoxꢀactive 2,6ꢀdiꢀ
tertꢀbutylphenol substituents R and spacers L. Such proꢀ
cesses as intramolecular electron transfer with subsequent
proton abstraction, or proton abstraction and electron
transfer, or protonꢀcoupled electron transfer in one eleꢀ
mentary step can take place in such polytopic systems.9,10
Note that these processes are responsible for the mechanism
of the antioxidant action,17—19 which include 2,6ꢀdiꢀtertꢀ
We set a task to develop a new method for the moniꢀ
toring of this reaction by CVA and its testing on the redoxꢀ
active compounds, for which spectrophotometric DPPHꢀ
test is of little use. Organometallic derivatives of 2,6ꢀdiꢀ
tertꢀbutylphenol, in which a protonꢀcoupled electron
transfer (PCET) process is possible, have been chosen as
such compounds. These processes play an important role
in biochemistry and are of significant practical and theoꢀ
Fc stands for ferrocenyl

Ferrocene phenol antioxidants
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 4, April, 2011
649
Scheme 2
butylphenols. There is certain correlation between the
redoxꢀproperties and antioxidative activity of these comꢀ
pounds.20 Thus, it seems reasonable to study antioxidative
properties of compounds together with their electrochemꢀ
ical behavior.
The present work is aimed at the study of electrochemꢀ
ical properties of ferrocenes Fc(L)R 1a—d, 2a—g, 3a,b,
4a,b, and 5a—c (R is the fragment of 2,6ꢀdiꢀtertꢀbutylpheꢀ
nol or its aromatic analog) in different solvents (MeCN,
CH2Cl2, DMF) and evaluation of their antioxidative acꢀ
tivity using the DPPHꢀtest, whose monitoring was perꢀ
formed by CVA.
Results and Discussion
Chalcones 2b—g were synthesized by the condensaꢀ
tion of ferrocenecarboxaldehyde with substituted acetoꢀ
phenones in alkaline medium according to the method
described earlier.21 New α,βꢀunsaturated ferrocene carboꢀ
nyl compound 2a containing the 2,6ꢀdiꢀtertꢀbutylphenol
fragment was obtained under similar conditions (Scheme 1).
R = Fc (3a), FcC(O)CH=CH (4a), 1ꢀacetylꢀ3ꢀferrocenylꢀ4,5ꢀdiꢀ
hydropyrazolꢀ5ꢀyl (5a), 1ꢀacetylꢀ5ꢀferrocenylꢀ4,5ꢀdihydropyrꢀ
azolꢀ3ꢀyl (5b)
CH2Cl2, the shift of potential peaks to the anodic region is
observed (Table 1). Such a displacement was observed for
the oxidation of other substituted ferrocenes, that is apꢀ
parently due to the change of solvation energy when anꢀ
other solvent is taken.24,25
Scheme 1
At the same time, two irreversible oxidation waves are
observed on the CVA curve of compound 1c in DMF in
anodic region at low speeds of potential sweep (lower
200 mV s–1) (Fig. 3). Apparently, at low speeds of potenꢀ
tial sweep phenoxy radicals formed in the second wave of
oxidation are unstable and readily involved into further
chemical transformations. This agrees with the ESA specꢀ
troscopic data, according to which stability of phenoxy
radical formed by oxidation of compound 1c is low.26 When
the speed of potential scanning is higher, the third peak
Amino derivatives 4a,b and 5c were obtained by the
reduction of the corresponding nitroꢀsubstituted phenylꢀ
ferrocene, ferrocenylchalcone, and ferrocenylꢀ4,5ꢀdihydroꢀ
pyrazole with SnCl2 in hydrochloric acid.22 The synthesis
of compounds 3a, 4a, and 5a,b was accomplished by acylꢀ
ation of ferrocene amino derivatives with 3,5ꢀdiꢀtertꢀbuꢀ
tylꢀ4ꢀhydroxybenzoyl chloride (Scheme 2).
I/mA
Electrochemical redoxꢀpotentials for compounds 1a—d,
2a—g, 3a,b, 4a,b, and 5a—c were measured in different
solvents. Similarly to the picture earlier observed23 in
МeCN, the voltamgram of compound 1a in DMF in the
anodic region has three waves of oxidation (Fig. 2). The
irreversible character of the first oneꢀelectron wave indiꢀ
cates that the process of electron transfer is followed by
the fast chemical or electrochemical step. The second wave
is reversible and the third is irreversible. Such a shape of
CVA can probably be explained by the following sequence
of steps: oxidation of the ferrocene fragment to ferriciniꢀ
um cation with subsequent intramolecular reduction of
the Fe3+ ion due to the electron transfer from the phenol
fragment and elimination of proton (the first wave), reꢀ
peated oxidation of Fe2+ to Fe3+ at more positive anode
potential (the second wave), and oxidation of the phenoxy
radical formed.23 When МeCN is replaced by DMF and
0.012
0.008
0.004
0
0
200
400
600
800 1000
E/mV
Fig. 2. Cyclic voltamgram of compound 1a in DMF at the speed
of potential sweep 100 mV s–1 (Ptꢀelectrode, the concentration
was 1•10–3 mol L–1, Bun4NBF4, Ag/AgCl).

650
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 4, April, 2011
Tyurin et al.
Table 1. Electrochemical potentials of compounds 1a,c (Ptꢀ
electrode, 10–3 mol L–1, 0.5 М Bu4NBF4, Ag/AgCl/KCl,
I/mA
the scanning rate was 0.2 V s–1
)
0.015
red1
1
red2
red3
Eox /E
Eox /E
Eox /E
2
3
Comꢀ
pound
V
0.010
0.005
0
MeCN
1a
1c
0.48*
0.64*
0.75/0.67
0.81/0.75
1.01/0.92
1.05/0.97
DMF
1a
1c
0.60*
0.61*
0.85/0.75
0.78/0.61
1.03
–0.005
CH2Cl2
0
200 400 600 800 1000 1200 E/mV
1a
1c
0.64
0.77/0.68
—
1.13
1.13/1.03
0.96/0.88
Fig. 4. Cyclic voltamgram of compound 1a in CH2Cl2 at the
speed of potential sweep 100 mV s–1 (Ptꢀelectrode, the concenꢀ
tration was 1•10–3 mol L–1, Bun4NBF4, Ag/AgCl).
* Irreversible wave of oxidation.
appears at the potential corresponding to the oxidation of
phenoxy radicals. It is possible that when the speed of
sweep increases, amount of unreacted phenoxy radicals
increases, and potential of their oxidation is reached faster,
that leads to the registration of the corresponding peak on
the CVA curve.
When compound 1a is oxidized in CH2Cl2, three peaks
of oxidation on the direct scan of the cyclic voltamgram
are observed (the second peak is of low intensity), while
there are three waves on the reverse scan, apparently, corꢀ
responding to the conjugate reduction of the oxidation
products of this substance (Fig. 4). Three reversible waves
are observed on the CVA curve of derivative 1c in CH2Cl2
(Fig. 5), with the height of the second wave of oxidation
being considerably lower than that of the first and the
third. Obviously, in this case the rate of chemical step,
which follows the oxidation of ferrocenyl fragment of the
molecule (i.e., intramolecular electron transfer and proꢀ
ton abstraction or PCET), is low, which is indicated by
the low values of current in the peak of the wave correꢀ
sponding to the repeated oxidation Fe2+ → Fe3+. This
agrees with the known data on higher deprotonating abiliꢀ
ty of acetonitrile as compared to CH2Cl2. Oxidation of
thus formed phenoxy radical occurs at less positive potenꢀ
tials than in DMF, whereas the reversibility of the peak
indicates formation of stable phenoxonium cation.
To sum up, the study of electrochemical behavior of
ferrocene derivatives 1a and 1c in МeCN, DMF, and
CH2Cl2 indicates that processes of intramolecular elecꢀ
tron transfer and proton abstraction can proceed in these
I/mA
I/mA
2
0.010
0.008
0.006
0.004
0.002
0
0.016
0.012
1
0.008
0.004
0
–0.002
0
200
400
600
800
1000 E/mV
0
200 400 600 800 1000 1200 E/mV
Fig. 3. Cyclic voltamgram of compound 1c in DMF at the speeds
of potential sweep 100 (1) and 300 mV s–1 (2) (Ptꢀelectrode, the
concentration was 1•10–3 mol L–1, Bun4NBF4, Ag/AgCl).
Fig. 5. Cyclic voltamgram of compound 1c in CH2Cl2 at the
speed of potential sweep 100 mV s–1 (Ptꢀelectrode, the concenꢀ
tration was 1•10–3 mol L–1, Bun4NBF4, Ag/AgCl).

Ferrocene phenol antioxidants
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 4, April, 2011
651
compounds with their rates being significantly dependent
on the nature of the solvent. At the same time, compounds
1b and 1d, containing no phenol groups, undergo oneꢀ
electron and reversible oxidation in МeCN, DMF, and
CH2Cl2 at the potential close to the oxidation potential of
unsubstituted ferrocene.
with a lone electron pair on the heteroatom. It should be
noted that in the case of compound 2a in DMF and
CH2Cl2, no oxidation wave of phenol group is observed in
the operating region of potentials. Obviously, this is due to
the significant electronꢀwithdrawing effect of the oxidized
ferrocene fragment, which shifts the peak potential to the
region of discharge of the solvent molecules.
The CVA curve of compound 2a in МeCN exhibits the
oxidation peak of ferrocenyl fragment and twoꢀelectron
irreversible wave of oxidation of phenol substituent. This
indicates that the presence of the carbonyl group in the
spacer between the phenol and ferrocenyl fragments of the
molecule interferes with the intramolecular electron transfer.
The CVA curves of chalcones 2b—g in МeCN, DMF,
and CH2Cl2 exhibit a oneꢀelectron reversible wave in
the anodic region, which corresponds to the oxidation
Fe2+ → Fe3+ in ferrocene. Potentials of the oxidation peaks
of chalcones 2b—g are displaced toward the anodic region
as compared to those for unsubstituted ferrocene (Table 2),
that is due to the acceptor character of substituents in the
cyclopentadienyl ring. In the case of compounds 2a and
2c—e in МeCN, there is linear correlation between
the Brown constants27 and potentials of oxidation peaks
(Eox = 0.675 + 0.063σ+). This indicates the presence of
conjugation of the substituent and ferrocenyl fragment of
the molecule, which is stronger in the case of substituent
Compounds 4a,b and 5a—c containing an amide fragꢀ
ment also undergo oneꢀelectron and reversible oxidation
in МeCN, DMF, and CH2Cl2 at potentials more positive
than oxidation potential of ferrocene. The strongest shift
of potential of the redoxꢀtransition Fe2+ → Fe3+ is obꢀ
served in the case of compound 4a, in which the ferrocenyl
fragment is conjugated with the carbonyl group. The secꢀ
ond oneꢀelectron wave is observed in oxidation of comꢀ
pounds 4a,b and 5a—c, which apparently is due to the
oxidation process of the amino group.28 Three waves of
oxidation are observed on the CVA curve of compound 5c
in МeCN with the second wave having reversible characꢀ
ter (Fig. 6), that indicates stability of the Nꢀcentered radꢀ
ical cation formed and slow rate of its deprotonation. The
third peak, probably, corresponds to the oxidation of the
radical formed (Scheme 3).
The CVA curves of compounds 3a,b exhibit reversible
oneꢀelectron wave corresponding to the redoxꢀtransition
Fe2+ → Fe3. No peaks of oxidation corresponding to the
Nꢀcentered oxidation were observed.
Table 2. Electrochemical oxidation potentials of compounds
To sum up, research on electrochemical behavior of
ferrocenes 1a—d, 2a—g, 3a,b, 4a,b, and 5a—c by CVA
showed that potential of the first oxidation peak of these
compounds is more anodic than potential of the reduction
peak of DPPH (Table 1, 2). Consequently, the compounds
under study are convenient objects for the use in the sugꢀ
gested electrochemical method for evaluation of antioxiꢀ
dative activity. It should be noted that spectrophotometric
method is little suitable in the case of ferrocenes 2a, 3a,
1b,d, 2a—g, 3a,b, 4a,b, and 5a—c (Ptꢀelectrode, 10–3 mol L–1
,
)
0.5 М Bu4NBF4, Ag/AgCl/KCl, the scanning rate was 0.2 V s–1
red1
1
Comꢀ
pound
Eox /E /V
MeCN
DMF
CH2Cl2
1b
1d
2a
0.72/0.64
0.54/0.46
0.62/0.55,
1.62*
0.72/0.64
0.69/0.60
0.70/0.64
0.85/0.72
0.78/0.68
0.76/0.69
2b
2c
2d
2e
2f
2g
3a
3b
0.65/0.57
0.72/0.64
0.66/0.58
0.68/0.59
0.59/0.53
0.74/0.63
0.63/0.55
0.45/0.35,
1.10/0.90
0.74/0.65,
1.38*
0.75/0.67,
1.15/1.07
0.66/0.59,
1.30*
0.56/0.48,
1.07/0.99,
1.33*
0.73/0.62
0.70/0.63
0.74/0.66
0.67/0.68
0.60/0.53
0.76/0.65
0.59/0.49
0.46/0.37,
1.02*
0.82/0.71,
1.43*
0.78/0.68,
1.22*
0.80/0.70
0.61/0.54
0.84/0.68
0.85/0.72
0.77/0.68
0.79/0.70
0.79/0.70
0.68/0.60,
1.31*
0.85/0.76,
1.48*
0.82/0.75,
1.41*
I/mA
0.006
0.004
0.002
0
4a
–0.002
–0.004
–0.006
4b
5а,b
5с
0.64/0.55,
1.36*
0.62/0.53,
1.12/1.11
0.84/0.76,
1.44*
0.71/0.62,
1.31*
0
200 400 600 800 1000 1200
E/mV
Fig. 6. Cyclic voltamgram of compound 5c in МeCN at the
speed of potential sweep 100 mV s–1 (Ptꢀelectrode, the concenꢀ
tration was 1•10–3 mol L–1, Bun4NBF4, Ag/AgCl).
* Irreversible wave of oxidation.

652
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 4, April, 2011
Tyurin et al.
Scheme 3
of current in the oxidation peak of the DPPH radical anꢀ
ion is observed. The data obtained were used for the plotꢀ
ting of kinetic dependencies and evaluation of antioxidaꢀ
tive activity (A) of studied compounds. Figure 8 shows the
A values determined in the test with DPPH obtained by
CVA (the time from the beginning of the reaction is 10 min,
A = (1 – C/C0)•100(%)).
Antioxidative activity of ferrocenes containing the
2,6ꢀdiꢀtertꢀbutylphenol fragment changes in the followꢀ
ing order:
4a > 3a > 1a > 1c > 5a 5b > 2a.
It is possible that such a dependence is due to the
stability of phenoxy radicals formed in the reaction of
these compounds with DPPH. Compounds 1c, 1a, 3a,
and 4a, proceeding from the activity values A > 50%
(56, 61, 64, 74%, respectively), can be related to the class
of efficient antioxidants.
Ferrocene derivatives 1b,d and 2b—e containing no
phenol fragments do not possess antioxidative activity (the
A values are close to zero). At the same time, compounds
bearing an amino group can be also involved into the reacꢀ
tion of hydrogen atom abstraction by the radical DPPH.
However, the A values of compounds 3b, 4b, and 5c are
considerably lower (8, 6, and 6%, respectively) than those
of the corresponding phenol derivatives (see Fig. 8). Conꢀ
sequently, a conclusion can be drawn that the fragment of
spatially hindered phenol contributes the most to the antiꢀ
oxidative activity of these compounds.
4a, and 5a,b, since the absorption band of these comꢀ
pounds virtually coincides with the absorption band of
DPPH. Figure 7 shows the change of the reduction wave
of DPPH in time when compound 4a is added. It is seen
that while the reaction progresses, a decrease in the value
While the reaction progresses, the height of the second
oxidation wave of compound 1a decreases in parallel with
A (%)
80
I/mA
70
60
50
40
30
20
10
0.015
0.010
0.005
0
–0.005
–0.010
–0.015
0
500
1000
1500
E/mV
1a
1c
2a 3a
3b 4a
4b 5a 5c
Fig. 7. The change of the CVA curves in the course of the reacꢀ
tion of DPPH with compound 4a (Ptꢀelectrode, the speed of
Fig. 8. The values of antioxidative activity of compounds 1a,c,
2a, 3a,b, 4a,b, and 5a,c determined by CVA with the use of the
DPPHꢀtest (the time from the beginning of the reaction was 10 min,
the ratio of concentrations was 1 : 1, C0 = 0.5•10–3 mol L–1).
potential sweep was 200 mV s–1, concentration 0.5•10–3 mol L–1
,
the ratio of concentrations 1 : 1, Bun4NBF4, МeCN, Ag/AgCl,
the reaction time was 20 min).

Ferrocene phenol antioxidants
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 4, April, 2011
653
I/mA
In conclusion, the present work showed that electroꢀ
chemical method can be used for the quantitative evaluaꢀ
tion of antioxidative activity of chromophoric substances
by the reaction with DPPH, whereas ferrocenes 1a,c, 3a,
4a, and 5a,b having the 2,6ꢀdiꢀtertꢀbutylphenol fragment
as a substituent are efficient antioxidants.
0.020
2
3
0.015
0.010
0.005
0
4
1
Experimental
Electrochemical redoxꢀpotentials were measured in a threeꢀ
electrode cell using an IPC ProꢀМ digital potentiometerꢀgalvaꢀ
nometer (Vol´ta, Russia) connected to a personal computer.
A stationary platinum electrode 2 mm in diameter was used as
a reference electrode. Oxygen was removed from the cell by
purging with dry argon. Potentials were measured relatively to
the saturated Ag/AgCl electrode. Concentrations of solutions of
compounds under study were 1•10–3 mol L–1. Platinum elecꢀ
trode was thoroughly purified after registration of each curve.
Antioxidative activity of compounds 1a—d, 2a—g, 3a,b, 4a,b,
and 5a—c was studied by the reaction with stable radical
2,2ꢀdiphenylꢀ1ꢀpicrylhydrazyl (DPPH) in МeCN (the full reacꢀ
tion time was 20 min). The rate of the process was monitored by
the change of the DPPH reduction current intensity (with the
ratio of concentrations of compounds and DPPH being 1 : 1
(C0 = 0.5•10–3 mol L–1)). The CVA curves were recorded after
certain periods of time (30 s and 1 min). The values of antioxidaꢀ
tive activity A = (1 – C/C0)•100(%) of compounds under study
were determined for the moment of time 10 min from the beginꢀ
ning of the reaction. The values of I0, obtained by the equation
of a calibrating curve at a given concentration of DPPH were
used for the plotting of kinetic curves.
1H NMR spectra were recorded on a Bruker Avance 400
spectrometer (400 МHz) in CDCl3 relatively to Me4Si. IR spectra
were recorded on a IR 200 Fourierꢀspectrophotometer (Thermo
Nicolet) in KВr pellets. The solvents used (МeCN, DMF,
CH2Cl2) were purified according to the standard procedures.27
Compounds 1a,d were obtained by the reaction of ferroceneꢀ
carboxaldehyde (Aldrich, 98%) with the corresponding amines
according to the known procedures.21,23 4ꢀAcetylꢀ2,6ꢀdiꢀtertꢀ
butylphenol was obtained according to the reported method.26
4ꢀAminophenylferrocene (3b), 3ꢀ(4ꢀaminophenyl)ꢀ1ꢀferroꢀ
cenylpropꢀ2ꢀenꢀ1ꢀone (4b), 1ꢀacetylꢀ5ꢀ(4ꢀaminophenyl)ꢀ3ꢀferroꢀ
cenylꢀ4,5ꢀdihydropyrazole (5c), 1ꢀacetylꢀ3ꢀ(4ꢀaminophenyl)ꢀ5ꢀ
ferrocenylꢀ4,5ꢀdihydropyrazole were obtained by the reduction
of the corresponding nitro derivatives according to the reported
method.22,30
–0.005
–0.010
0
200 400 600 800 1000 1200 E/mV
Fig. 9. The changes of the CVA curves in the course of the
reaction of compound 1a with DPPH: the CVA curve of DPPH
in the absence of compound 1a (1), the CVA curves in 0.5 (2),
1.5 (3), and 20 min (4) after the reaction begins (Ptꢀelectrode,
the speed of potential sweep was 200 mV s–1, the concentration
was 0.5•10–3 mol L–1, the ratio of concentrations was 1 : 1,
Bun4NBF4, МeCN, Ag/AgCl).
the decrease of the DPPH reduction peak height (Fig. 9).
It can be explained by the fact that the corresponding
phenoxy radical is formed in the presence of DPPH, and
oxidation of 1a proceeds in two steps (Scheme 4): oxidaꢀ
tion of ferrocenyl fragment and phenoxy radical to pheꢀ
noxonium ion. Thus, there are no waves characteristic of
the process of intramolecular electron transfer and proton
abstraction on the CVA curves of 1a and 1c with DPPH,
that confirms the mechanism of oxidation of these ferꢀ
rocene derivatives suggested earlier.23
Scheme 4
DPPH
1ꢀ(3,5ꢀDiꢀtertꢀbutylꢀ4ꢀhydroxyphenyl)ꢀ3ꢀferrocenylpropꢀ2ꢀ
enꢀ2ꢀone (2a). Concentrated aqueous NaOH (1 mL) was added
to a mixture of ferrocenecarboxaldehyde (0.11 g, 0.5 mmol) and
4ꢀacetylꢀ2,6ꢀdiꢀtertꢀbutylphenol (0.12 g, 0.5 mmol) in ethanol
(7 mL). The mixture was refluxed for 8 h under argon. After
cooling, the reaction mixture was weakly acidified with dilute
aq. HCl, and the product was extracted with diethyl ether. After
the solvent was evaporated, the product was isolated on a colꢀ
umn with Al2O3 (Brockmann II activity, elution with diethyl
ether—light petroleum, 1 : 1). First was eluted unreacted FcCHO,
then product 2a as a dark red oil. The yield was 0.13 g (59%).
Found (%): C, 62.89; H, 9.86. C17H32O2Fe. Calculated (%):
C, 62.96; H, 9.95. 1H NMR (CDCl3), δ: 1.52 (s, 18 H, C(CH3)3);
4.20 (s, 5 H, C5H5); 4.48 (m, 2 H, C5H4); 4.61 (m, 2 H, C5H4);
Independent biological testing of compounds 1a and
1c on the degree of peroxide oxidation of lipids in the rat
liver and brain homogenates showed that these compounds
possess a significant antioxidative effect and inhibit a Ca2+
ꢀ
dependent mitochondria swelling.29 These data correlate
with the results obtained by electrochemical method.

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Tyurin et al.
5.73 (s, 1 H, OH); 7.14 (d, 1 H, =CH, J = 15.0 Hz); 7.73 (d, 1 H,
=CH, J = 15.4 Hz); 7.89 (s, 2 H, C6H2). IR, ν/cm–1: 3619
(OH); 2956, 2911 (CH, C(CH3)3); 1648 (CO); 1107 (C5H5).
4ꢀ(3,5ꢀDiꢀtertꢀbutylꢀ4ꢀhydroxybenzoylamino)phenylferrocene
(3a). Thionyl chloride (6 mL) was added to 3,5ꢀdiꢀtertꢀbutylꢀ4ꢀ
hydroxybenzoic acid (Aldrich, 98%) (0.25 g, 1 mmol). The mixꢀ
ture was refluxed for 1 h, excess SOCl2 was evaporated in vacuo.
A solution of 4ꢀaminophenylferrocene (0.14 g, 0.5 mmol) in
CH2Cl2 containing Et3N (2 mL) (to scavenge the liberated
HCl) was added to the thus obtained white crystalline residue,
the mixture was kept for 16 h. The solvent was evaporated
in vacuo, the residue was separated on a column with Al2O3
(elution with diethyl ether—light petroleum, 2 : 1) to obtain
product 3a (0.24 g, 88%), m.p. 195—197 °C. Found (%): C, 73.20;
H, 7.24; N, 2.45. C31H35O2NFe. Calculated (%): C, 73.08;
H, 6.97; N, 2.75. 1H NMR (CDCl3), δ: 1.51 (s, 18 H, C(CH3)3);
4.22 (s, 5 H, C5H5); 4.54 (m, 2 H, H(β), C5H4); 4.88 (m, 2 H,
H(α), C5H4); 5.64 (s, 1 H, OH); 7.36, 7.54 (both d, 4 H, nꢀC6H4);
7.65 (m, NH); 7.70 (s, 2 H, C6H2). IR, ν/cm–1: 3619 (OH);
3089 (NH); 2958, 2923 (CH, C(CH3)3); 1644 (CO);
1107 (C5H5).
(s, 18 H, C(CH3)3); 2.37 (s, 3 H, COCH3); 2.95 (m, 1 H, CH2,
heterocycle); 3.52 (m, 1 H, CH2, heterocycle); 4.15 (s, 5 H,
C5H5); 4.42 (m, 2 H, H(β), C5H4); 4.53 (m, 1 H, H(α), C5H4);
4.69 (m, 1 H, H(α), C5H4); 5.49 (m, 1 H, CH, heterocycle);
5.62 (s, 1 H, OH); 7.20, 7.60 (both m, 4 H, C6H4); 7.71
(s, 2 H, C6H2); 8.07 (br.m, 1 H, NH). IR, ν/cm–1: 3621 (OH),
3436 (OH and/or NH), 2956 (CH, C(CH3)3), 1646 (C=O),
1100—1110 (C5H5).
1ꢀAcetylꢀ3ꢀ[4ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxybenzoylamino)ꢀ
phenyl]ꢀ5ꢀferrocenylꢀ4,5ꢀdihydropyrazole (5b). Thionyl chloride
(4 mL) was added to 3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxybenzoic acid
(0.15 g, 0.6 mmol). The mixture was refluxed for 1 h under argon
and excess thionyl chloride was evaporated in vacuo. A solution
of 1ꢀacetylꢀ3ꢀ(4ꢀaminophenyl)ꢀ5ꢀferrocenylꢀ4,5ꢀdihydropyrꢀ
azole (0.19 g, 0.5 mmol) in CH2Cl2 (20 mL, containing several
drops of Et3N) was added to the residue with cooling. The mixꢀ
ture was refluxed for 1 h and kept for 16 h. The solvent was
evaporated in vacuo. The residue was diluted with diethyl ether,
the undissolved precipitate was filtered off, the solvent was evapꢀ
orated. The residue was diluted with hexane, a precipitate formed
was filtered off and washed with hexane to obtain compound 5b
(0.27 g, 87%), m.p. 163—165 °C. Found (%): C, 69.56; H, 6.75;
N, 6.59. C36H42O3N3Fe. Calculated (%): C, 69.67; H, 6.82;
N, 6.77. 1H NMR (CDCl3), δ: 1.51 (s, 18 H, C(CH3)3); 2.33
(s, 3 H, CH3CO); 3.56 (ABX system, 1 H, Ha, heterocycle); 3.66
(ABX system, 1 H, Hв, heterocycle); 4.03 (m, 1 H, C5H4); 4.13
(m, 2 H, C5H4); 4.16 (s, 5 H, C5H5); 4.49 (m, 1 H, C5H4); 5.48
(ABX system, 1 H, Hх, heterocycle); 5.69 (s, 1 H, OH); 7.75
(s, 2 H, C6H2); 7.80 (m, C6H4). IR, ν/cm–1: 3623 (OH),
3426 (OH and/or NH), 2960 (CH, C(CH3)3), 1648 (C=O),
1108 (C5H5).
3ꢀ[4ꢀ(3,5ꢀDiꢀtertꢀbutylꢀ4ꢀhydroxybenzoylamino)phenyl]ꢀ1ꢀ
ferrocenylpropꢀ2ꢀenꢀ1ꢀone (4a). Thionyl chloride (7 mL) was
added to 3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxybenzoic acid (0.375 g,
1.5 mmol). The mixture was refluxed for 1 h, then excess SOCl2
was evaporated in vacuo. A solution of 3ꢀ(4ꢀaminophenyl)ꢀ1ꢀ
ferrocenylpropꢀ2ꢀenꢀ1ꢀone (0.33 g, 1 mmol) in CH2Cl2 (20 mL,
containing a few drops of Et3N to scavenge the liberated
HCl) was added dropwise to the residue, 3,5ꢀdiꢀtertꢀbutylꢀ4ꢀ
hydroxybenzoyl chloride. A white smoke was observed together
with a slight heating of the reaction mixture. The solution obꢀ
tained was stirred for 30 min at room temperature, then refluxed
for 1 h, and kept for 16 h. The reaction mixture was concentratꢀ
ed in vacuo, and the product was isolated by column chromatogꢀ
raphy on Al2O3 (Brockmann II activity, elution with diethyl
ether). The starting compound (traces) was eluted first, then
product 4a. The yield was 0.3 g (53%), m.p. 190 °C (decomp.).
Found (%): C, 72.43; H, 6.70; N, 2.36. C34H37O3NFe. Calcuꢀ
lated (%): C, 72.47; H, 6.61; N, 2.48. 1H NMR (CDCl3), δ: 1.51
(s, 18 H, C(CH3)3); 4.24 (s, 5 H, C5H5); 4.61 (m, 2 H, H(β),
C5H4); 4.94 (m, 2 H, H(α), C5H4); 5.68 (s, 1 H, OH); 7.2 (d, 1 H,
=CH); 7.67 (m, 2 H, C6H4); 7.73 (s, 2 H, C6H2); 7.77 (m, 2 H,
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 09ꢀ03ꢀ00743,
08ꢀ03ꢀ0084, and 09ꢀ03ꢀ12261ꢀofiꢀm).
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Received June 2, 2010;
in revised form February 21, 2011