Electroreduction of 3-bromopyrene in DMF. A new organic electrochemical oscillator

Article abstract of DOI:10.1135/cccc19961715

The reduction of 3-bromopyrene in DMF at a mercury electrode for scan rates lower than 0.1 V s^-1 is accompanied by a slow adsorption of an intermediate, resulting in a catalytic behaviour. The current oscillations take place in a region of the negative faradaic impedance.

Full text of DOI:10.1135/cccc19961715

Electroreduction of 3-Bromopyrene
1715
ELECTROREDUCTION OF 3-BROMOPYRENE IN DMF. A NEW ORGANIC
ELECTROCHEMICAL OSCILLATOR
1
2
Jan S. JAWORSKI , Piotr LESZCZYNSKI and Slawomir PAWLOWSKI
Department of Chemistry, University of Warsaw, 1 Pasteur St., 02-093 Warsaw, Poland;
1
2
e-mail: jaworski@chem.uw.edu.pl, pleszcz@chem.uw.edu.pl
Received February 26, 1996
Accepted August 23, 1996
–
1
The reduction of 3-bromopyrene in DMF at a mercury electrode for scan rates lower than 0.1 V s
is accompanied by a slow adsorption of an intermediate, resulting in a catalytic behaviour. The cur-
rent oscillations take place in a region of the negative faradaic impedance.
Key words: Electroreduction; 3-Bromopyrene; Current oscillations.
The irreversible reduction of halogen substituted aromatic compounds (AX) in aprotic
1
–8
media involves in general the reversible formation of an anion radical
AX
+
e
AX–•
,
(A)
followed by cleavage of the carbon–halogen bond
AX–• A•
+
X–
.
(B)
A further reduction of the resulting neutral aryl radical proceeds either at the electrode,
by the ECE mechanism
A•
+
e
A –
(C)
(D)
or in the solution
A•
+
AX–•
A–
+ AX
by the DISP1 mechanism, if the forward reaction (B) is the rate determining step. The
–
final step is the protonation of the anion A yielding the hydrocarbon. Moreover, it was
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1
716
Jaworski, Leszczynski, Pawlowski:
2
shown that the abstraction of H-atom from the solvent like DMSO by some neutral
aryl radicals is a concurrent reaction to steps (C) and (D).
For the electroreduction of 3-bromopyrene in DMF the ECE mechanism can be ex-
2
–5,7
pected on the basis of the behaviour found for other bromoarenes
. However, a more
complicated reduction process leading to oscillations of current has been observed by
us at lower scan rates.
Spontaneous current or potential oscillations in heterogeneous systems have been
9
widely investigated in recent years . Advantages of those studies are straightforward
control of the parameters (e.g. a potential or a series resistance) which shifts the system
1
0
to a state of instability, and a relatively short timescale of oscillations . Different kinds
of electrochemical oscillators have been reported, including catalytic oxidation of or-
1
1,12
ganic molecules at noble metals like platinum
and the electrocatalytic reduction of
1
3–15
inorganic ions at mercury, e.g. indium in the presence of thiocyanate
of an organic oscillator is reported here.
. A new type
EXPERIMENTAL
Bromination of pyrene in CCl and crystallization from ethanol was used to synthesize16 and purify
4
3
-bromopyrene; 0.1 M tetrabutylammonium perchlorate (Fluka) was applied as a supporting electro-
lyte, N,N-dimethylformamide (DMF, Lab Scan) was dried over sodium hydride and purified using
copper sulfate; each step was followed by distillation under reduced pressure.
A three-electrode cell consisting of a static mercury dropping electrode (SMDE, Laboratorni pri-
+
stroje, Prague), a Pt counter electrode and an Ag/Ag reference electrode in acetonitrile were used.
All potentials reported here were recalculated to SCE. Cyclic voltammetric curves and amperometric
measurements were performed with a PAR 273A potentiostat controlled by a computer. A polaro-
graph PA4 and a recorder XY, model 4106 (Laboratorni pristroje, Prague) were also used to study
the current oscillations and an oscilloscope HO79 (Hameg, Frankfurt) to observe the potential oscil-
lations. Argon gas was bubbled through the solution before each experiment. Measurements were
performed at 22 °C. The temperature effects were investigated in the range between 1.5 and 40 °C,
using a thermostat U2C (MLW, Germany).
RESULTS AND DISCUSSION
Reduction at Higher Scan Rates
–
1
At higher scan rates (v > 0.1 V s ) at room temperature the electroreduction of 3-bro-
mopyrene in DMF at a mercury electrode occurs via the ECE mechanism, i.e. its beha-
2
–5,7
viour is similar to that described for other bromoarenes
. The irreversible,
two-electron reduction of the C–Br bond followed by the reversible, one-electron redox
reactions of pyrene and its radical anion, are observed in cyclic voltammograms. The
1
/2
current of the first peak is proportional to v and the peak potential E depends on v.
p
The value of coefficient ∂E /∂ log v = –29 mV per decade of the scan rate after IR
p
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Electroreduction of 3-Bromopyrene
1717
correction, is in agreement with the theory of both ECE and DISP1 mechanisms. To
1
,6
distinguish between them, the analysis of the parameter p is necessary .
In the Eq. (1)
p = k c /k (Fv/RT)1/2
0
3/2
(1)
d
2
k and k are rate constants of the cleavage step (B) and the solution electron transfer
2
d
0
(
D), respectively, and c is the bulk concentration of the reactant. The value of p ap-
proaching zero indicates the predomination of the ECE pathway , whereas the DISP1
mechanism prevails when p > 10 . The disproportionation step (D) is not far from the
diffusion-controlled limit giving k ≈ 10 mol
1
,6
3
1
,5
8
–1 –1
0
–1
s and c = 0.5 mmol l . The cleav-
d
–
1
–1
age rate constant k is much higher than 5 000 s because even at v = 250 V s no
2
anodic peak and no significant decrease of the cathodic peak current are observed in
voltammograms. Using the above values, the upper limit of the competition parameter
can be calculated from Eq. (1); the value p = 14 indicates the ECE mechanism.
Adsorption/Desorption Phenomena
An unusual behaviour of the first process was found at the room temperature for scan
–
1
rates lower than 0.1 V s , as shown in Fig. 1. The peak current of the main peak Ia is
much higher than expected for a diffusion-controlled process and it increases with de-
crease of the scan rate, reaching the value which corresponds to a three-electron reduc-
–
1
tion at v = 0.015 V s . The height of this peak is strongly temperature-dependent,
–
1
giving ∂I /∂T = 0.18 µA K , whereas for the diffusion current corresponding to the
p
–
1
reversible reduction of pyrene (peak II) ∂I /∂T = 0.008 µA K . These results indicate
p
–
I, µA
Ia
Ib
2
6
FIG. 1
–
I, µA
1
Cyclic voltammogram of 0.5 mM 3-
2
4
t, s
II
bromopyrene in DMF with 0.1
M
1
–
TBAP at 22 °C, scan rate 0.03 V s .
Cathodic peaks correspond to the
catalytic process of bromopyrene
3
0
(
(
Ia), reduction of adsorbed species
Ib) and the reversible reduction of
pyrene (II). Insert: typical oscillatory
time series for the current recorded
at the potential E = –1.94 V
1.0
1.5
2.0
2.5
–E, V
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1
718
Jaworski, Leszczynski, Pawlowski:
the catalytic nature of the reduction process Ia. However, the value ∂E /∂ log v = –31 mV
p
per decade of the scan rate corresponds to the ECE or DISP1 mechanism.
Moreover, an additional post-peak Ib observed at more negative potentials (Fig. 1)
can be divided into smaller peaks due to the current oscillations, as it will be discussed
later. The temperature dependence of its height is much less pronounced, ∂I /∂T = 0.024
p
–
1
µA K , in contrast to the catalytic peak Ia. The characteristics of the peak Ib suggest
a strong but relatively slow adsorption of the reactant or an intermediate. At more
negative potentials the cathodic current rapidly decreases due to the desorption process,
until the reduction of pyrene occurs (peak II). This current decrease occurs at a poten-
tial independent of scan rate indicating that the desorption is fast. On the reverse scan
a sharp spike is found at potentials when the adsorption starts (Fig. 1) followed by high
cathodic current, similar to that observed during the forward scan. It supports the auto-
catalytic nature of the process Ia, in which the adsorbed species participate. At lower
temperatures, when the adsorption is stronger, a position of the spike shifts to more
positive potentials with an increase of the scan rate. The height of the spike decreases
at the same time (Fig. 2). That behaviour supports the view of the slow adsorption
process in the time scale used. The dependence of both reduction peaks Ia and Ib on the
scan rate is more evident at lower temperatures due to stronger adsorption and the
absence of current oscillations. A typical example for the temperature of 1.5 °C is
–
1
shown in Fig. 3. The cathodic peak Ia appears at scan rates higher than 0.03 V s ,
when there is not enough time for the adsorption process.
If the adsorbed species is the parent 3-bromopyrene, the peak Ib should have a simi-
lar catalytic nature as the peak Ia, and should be well developed at the lowest scan rate,
–
•
but this is not the case (Fig. 3). Thus, the adsorption of an intermediate radical (AX
•
or A ) can be postulated. Moreover, it is highly probable that the above adsorption
facilitates the disproportionation reaction (D) at the interface. Thus, the regeneration of
the parent reactant in step (D) can explain the catalytic nature of the peak Ia. According
1
7
2
–
I, µA
3
4
5
4
1
FIG. 2
Current spikes in the reverse part of
cyclic voltammograms of 3-bromo-
pyrene recorded at 1.5 °C. Scan rates
(
1) 15, (2) 18, (3 ) 30, (4) 50, (5)
–
1
1.80
1.84
1.88
1.92
–E, V
100 mV s
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Electroreduction of 3-Bromopyrene
1719
to the theoretical kinetic-zone diagrams1,17,18 a shift from the ECE to the DISP1 mecha-
0
nism is observed for low values of the rate constant k and high values of k and c . The
2
d
last condition is fulfilled because for the reaction at the interface the adsorption of a
reactant has the same effect as an increase of its concentration for a solution process.
–
•
On the other hand, it can be postulated that for the adsorbed radical anion AX the
value of the cleavage rate constant k is much smaller than that for the ECE process at
2
–
1
v > 0.1 V s . Moreover, the rapid desorption observed at negative potentials is fully
understood for the negative radical ion. However, the suggested mechanism needs fur-
ther confirmation.
It can be added here that the unusual reduction of 9-bromoanthracene and 9,10-dibro-
1
9
moanthracene in DMF has been reported in the literature , but the experimental sup-
port of the proposed explanation has been unsuccessful. On the other hand, a similar
2
0
behaviour has been also observed in the reduction of olefin–mercury(II) compounds
and has been ascribed to the formation of rapidly adsorbed radical intermediates.
Current Oscillations
The current oscillations in the potential range of the adsorption–desorption processes
are the most interesting feature in Fig. 1. Spontaneous current oscillations lasting more
than 20 min were observed in amperometric experiments. In the potential range corre-
sponding to the desorption process oscillations with a constant amplitude and period
were observed, as shown in diagram inserted in Fig. 1. A typical period was 0.87 ± 0.01 s
at the temperature of 22 °C. The conditions at which oscillations occur are similar to
1
3–15,21
–
those established
for the In(III)/SCN oscillator. Due to the desorption of a cata-
lyst a negative faradaic impedance with absolute value smaller than the ohmic cell
resistance appeared. The latter one is of the order of 1 kΩ, for the DMF solution,
although part of the ohmic drop is compensated. No external series resistance is necess-
Ia
4.5
5
Ib
–I, µA
4
3
3
.0
.5
FIG. 3
2
Cathodic peaks for the reduction of 3-
bromopyrene in DMF at 1.5 °C. Scan
1
rates (1) 15, (2) 18, (3) 30, (4) 50,
–
1
(
5) 100 mV s . For Ia and Ib see dis-
1
cussion in text
1.65
1.75
1.85
1.95
–E, V
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1
720
Jaworski, Leszczynski, Pawlowski:
ary for the appearance of oscillations in the system under investigation. In the recent
2
1
theory for electrochemical instabilities of this kind current oscillations under poten-
tiostatic control are conditioned by out-of-phase potential oscillations in two parts of
2
2
the circuit (the working electrode and an internal cell resistance) . That requirement is
just fulfilled in the system under investigation with a relatively high internal cell resist-
ance. Simultaneous out-of-phase potential oscillations at the working and the counter
electrodes during the current oscillations, with the same period for all three phenomena,
2
1
were observed as shown in Fig. 4. That result fully supports proposed theory . How-
–
ever, in contrast to In(III)/SCN oscillator the catalyst is formed spontaneously during
the overall process.
In the potential range where monoperiodic oscillations exist, their amplitude in-
creases with negative potential shift. In logarithmic coordinates an amplitude of oscil-
lations was found to be linear function of potential (expressed with respect to the
estimated bifurcation potential) with the correlation coefficient of 0.972 and the slope
of 0.49. This result is in agreement with the theoretical prediction for an ordinary
2
3,24
(
supercritical) Hopf bifurcation
, which is a smooth transition of a system from a
stationary state to a stable time-periodic behaviour (periodic orbit). It can be noted that
1
3
–
similar behaviour was found for the In(III)/SCN oscillator.
At more positive potentials triperiodic oscillations were found with two shorter am-
plitudes (S) followed by a longer one (L) in the series SSLSSL... The more complex
–
13
periodic and chaotic oscillations (observed e.g. for the In(III)/SCN system ) were not
found, but there is a wide range of potentials where aperiodic oscillations exist. Their
features depend strongly on the internal cell resistance and their reproducibility is low.
In conclusion, taking into account that the ECE mechanism is very common in or-
ganic electrochemistry in aprotic solvents of a higher resistance, it is probable that
similar oscillations can appear in a number of systems with a slow adsorption of inter-
mediates acting as catalysts.
E
c
0.5 s
20 mV
FIG. 4
E
w
Simultaneous potential oscillations at the
working electrode Ew and the counter elec-
trode E during the potentiostatic experiment
at E = –1.94 V
c
t
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Electroreduction of 3-Bromopyrene
1721
Financial support from BST-502-8/95 is gratefully acknowledged. We thank Dr M. Orlik from our
Department for helpful discussions.
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