Electrocatalytic reduction of organic halides with cobalt bipyridine complexes

Article abstract of DOI:10.1023/A:1021355606296

The electrocatalytic reduction of organic halides by the [Cobpy] ⁺ complexes coordinationally unsaturated with bpy (at potentials of the first wave) and by the coordinationally saturated [Cobpy₂] ^- complexes (at potentials of the second wave) was observed. The apparent rate constant of the process decreased with an increase in the difference of the reduction potentials of the substrate and catalyst in a large range of the driving force of the process.

Full text of DOI:10.1023/A:1021355606296

1702
Russian Chemical Bulletin, International Edition, Vol. 51, No. 9, pp. 1702—1708, September, 2002
Electrocatalytic reduction of organic halides with cobalt bipyridine complexes
Yu. H. Budnikova,ꢀ A. G. Kafiyatullina, Yu. M. Kargin, and O. G. Sinyashin
A. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center, Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 75 2253. Eꢀmail: yulia@iopc.knc.ru
The electrocatalytic reduction of organic halides by the [Соbpy]+ complexes coordiꢀ
nationally unsaturated with bpy (at potentials of the first wave) and by the coordinationally
saturated [Соbpy2]– complexes (at potentials of the second wave) was observed. The apparꢀ
ent rate constant of the process decreased with an increase in the difference of the reduction
potentials of the substrate and catalyst in a large range of the driving force of the process.
Key words: cobalt complexes, 2,2´ꢀbipyridine, organic halides, homogeneous electroꢀ
chemical catalysis.
The electroreduction of the cobalt complexes in the abꢀ
sence and presence of the starting substrates and organyl haꢀ
lides was studied by cyclic voltammetry (CV). Cyclic voltammoꢀ
grams were recorded on platinum (1.5 mm in diameter) or gold
(1.0 mm in diameter) electrodes. A silver electrode Ag/AgNO3
served as the reference electrode, and 0.01 М solutions of AgNO3
in МеCN were used. A platinum wire 1 mm in diameter and
10 mm in length was used as the auxiliary electrode. Measureꢀ
ments were carried out in a cell with the temperature mainꢀ
tained at 25 °C in an argon atmosphere. Voltammograms were
recorded on a PIꢀ50ꢀ1 potentiostat with a PRꢀ8 programmer
and an electrochemical cell, which was connected by the threeꢀ
electrode scheme. The curves were recorded at a linear potenꢀ
tial scan rate of 50 mV s–1 on a twoꢀcoordinate recorder. In
experiments on ER, a substrate was added to a solution of the
cobalt complex (1•10–2 mol L–1) using a 10ꢀµL syringe.
Preparative electrolysis was carried out using a B5ꢀ49 d.c.
power source in a 4ꢀmL threeꢀelectrode cell with a constant
temperature in DMF. The potential of the working electrode
was measured using a Shchꢀ1312 d.c. voltmeter relatively to
the reference electrode Ag/0.01 М AgNO3 in the correspondꢀ
ing solvent. In experiments on undivided electrolysis a magneꢀ
sium rod was used, whose working surface was thoroughly
cleaned with an emery paper before electrolysis.
In experiments with divided electrolysis a paper was used as
the diaphragm, a glassꢀcarbon plate with a working surface
area of 4 cm2 served as the anode, and the anolyte was a
saturated solution of the background, which was used in the
catholyte, in the corresponding solvent. In all cases, a nickel
network with a geometric surface area of 15 cm2 was the cathꢀ
ode. During electrolysis the electrolyte was stirred with a magꢀ
netic stirrer at a constant argon flow.
To prepare the СоbpyBr2 complex, СоBr2•6Н2О
(5•10–2 mol) was dissolved in EtOH (100 mL), the mixture
was stirred for 3 h until the salt dissolved completely, and
2,2´ꢀbipyridine (5•10–2 mol) was added with continuous stirꢀ
ring. After 10—12 h the precipitated residue of СоBr2bpy was
filtered off and dried for 1 day in a vacuum desiccator at 30 °C.
σꢀComplexes of transition metals with organic ligands
can often be key intermediates of electrocatalytic proꢀ
cesses. Data on their nature and electrochemical and
chemical processes are necessary for the simulating of
particular stages and elucidation of the reaction mechaꢀ
nism. Such a diagnostics using electrochemical methods
provides preliminary conclusions on the reaction mechaꢀ
nism without isolation of intermediates from the appearꢀ
ance of new reduction or oxidation waves.
Using the Ni0 and [Ni0L]•– nickel complexes, we
have previously shown1 that the Ni0 complex acts as the
innerꢀsphere electron mediator reducing organic halides
(RX) to form the RNiX σꢀcomplexes, and the radical
anion [Ni0L]•– complexes exhibit intermediate properꢀ
ties characteristic of Ni0 and L•– (outerꢀsphere mediaꢀ
tor). In this work we studied the electrochemical reducꢀ
tion (ER) of the СоII complexes with 2,2´ꢀbipyridine
(bpy) in МеCN and DMF solutions with different numꢀ
bers of ligands in the presence of RX (R = Alk, Ar, Het).
The study also included determination of the reactive
form of the complexes, estimation of the rate constant of
catalyst regeneration, and comparison of these results
with the data obtained previously for the nickel comꢀ
plexes.2—6
Experimental
The products of preparative electrolysis at the controlled
current or potential were analyzed by 1Н NMR spectroscopy,
GLC, and elemental analysis. 1Н NMR spectra were recorded
on a Varian Tꢀ60 instrument (60 MHz) relatively to the interꢀ
nal standard Me4Si. GLC analysis was carried out on a Chrom
chromatograph (flameꢀionization detector, helium as carrier
gas, glass column (120 cm × 3 mm, 5% Silicon OVꢀ17 on
Chromaton NꢀAWꢀDMCS, 0.16—0.20 mm)).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1562—1568, September, 2002.
1066ꢀ5285/02/5109ꢀ1702 $27.00 © 2002 Plenum Publishing Corporation

Electrocatalytic reduction of organic halides
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 9, September, 2002
1703
The [Сo(bpy)3](BF4)2 complex was synthesized simiꢀ
larly, namely, Сo(BF4)2•6Н2О (5•10–2 mol) was dissolved in
EtOH until the crystals dissolved completely, 2,2´ꢀbipyridine
(1.5•10–1 mol) was added, and the mixture was stirred for
8—10 h. The precipitate of Сo(bpy)3(BF4)2 that formed was
filtered off and dried in a vacuum desiccator analogously to
СоbpyBr2.
–I
15 µA
Tetraalkylammonium tetrafluoroborates were used as supꢀ
porting salts. The Et4NBF4 salt was prepared by the neutralizaꢀ
tion of HBF4 with tetraethylammonium hydroxide, triply reꢀ
crystallized from aqueous EtOH, and dried in vacuo for 72 h at
30 °С. The solvent (MeCN) was purified by triple distillation
above Р2O5 and KMnO4, and the middle fractions were used
for voltammetry. Commercial DMF was distilled in vacuo, dried
for 3 days above molecular sieves 4А, preꢀcalcined in a vacuum
furnace at 100 °С, and dried for 1 day above anhydrous К2О,
which was prepared by the thermal decomposition of К2СО3.
Then the solvent was decanted and twice fractionally distilled
in vacuo.
1.0
1.4
1.8
–E/V
Fig. 1. Cyclic voltammogram for СоIIbpy3 (1•10–2 mol L–1) at
the Pt electrode in a solution of Et4NBF4 in MeCN.
scan rate. It is seen from the data in Table 1 that the
СоIbpyn complex forms at potentials of the first, oneꢀ
electron peak (Ер1), while at potentials of the second
peak (Ер2) the number of transferred electrons is 1e for
n = 1 and 2е for n = 2—3. Thus, the electrochemical
behavior of the cobalt complexes with 2,2´ꢀbipyridine
depends on the number of ligands bound to the metal. It
can be assumed that the unstable 17ꢀelectron Со0bpyn
Commercial reagents (reagent grade) used as references in
GLC were purified by either distillation (liquid) or recrystalliꢀ
zation from hexane to their unvariable physical constants.
The preparative reduction of CobpyBr2 (1.7•10–2 mol L–1
in the presence of MesBr (Mes is mesityl) (3.4•10–2 mol L–1
)
)
was carried out at –1.3 V in a divided cell using a solution of
Et4NBr in DMF as supporting electrolyte. After 1.2 F electricꢀ
ity per CobpyBr2 were passed, the MesСоBrbpy complex (0.16 g,
84%) was isolated from the reaction mixture. Found (%):
C, 56.0; H, 6.02; N, 5.12; Co, 13.03; Br, 19.83. Calculated (%):
C, 55.1; H, 4.59; N, 6.76; Co, 14.25; Br, 19.32. 1Н NMR
(CDCl3), δ: 8.56—8.57 (m, 2 H); 8.33—8.37 (d, 2 H); 7.79—7.82
(m, 2 H); 7.28—7.31 (m, 2 H); 6.45 (m, 2 H, C6H2); 2.59 (s,
6 H, Me); 2.17 (s, 3 H, Me).
–
complex (n = 1, 2) for n = 1 or 18ꢀelectron Соbpyn
complex for n = 2, 3 is formed at Ер (Fig. 1). No
2
paramagnetic cobalt species were found in the products
of electrochemical reduction of the [Cobpy3](BF4)2 comꢀ
plexes in the accessible potential region studied by ESR
(although it should be admitted that cobalt species are
complicated objects for ESR studies), i.e., unlike similar
nickel complexes for which paramagnetic Ni(0)[L•–] is
Results and Discussion
–
formed at the second wave,1 Соbpyn is rather an anꢀ
ionic complex.
Cyclic voltammetry of 2,2´ꢀbipyridinecobalt(II) comꢀ
plexes. The reduction of the Со2+bpyn (n = 1—3) comꢀ
plex cations at the Pt electrode in МеCN (Table 1) is
characterized by two diffusionꢀcontrolled peaks in the
voltammetric curve. This is confirmed by linear depenꢀ
dence of the current on the square root of the potential
Thus, bpy forms complexes with cobalt in different
oxidation states, and the presence of pairs of redox peaks
in the CV curves (see Fig. 1) point to the relative stabilꢀ
ity of these complexes. These facts indicate that the
Co—bpy system can be used in homogeneous electroꢀ
catalysis, for example, for the dehalogenation of organic
substrates.
It can be expected that CoIbpyn (formed at potentials
of the first peak) or Cobpyn assumed by us (formed at
potentials of the second peak) are the catalytically active
forms of the cobalt compounds with bpy. The reactivity
of these complexes can depend on the number of ligands
bound to the metal.
Table 1. Parameters of the peaks of electrochemical reduction
of the СоII complexes with 2,2´ꢀbipyridine in MeCN at the Pt
electrode
–
1
1
2
2
Complex
–Ер
ip
(ia/ic)1 –Ер
ip
(ia/ic)2
/V
/µA
/V
/µA
Catalysis by the cobalt complex coordinationally satuꢀ
rated with bpy. Let us consider the electrochemical beꢀ
havior of the [Со2+bpy3](BF4)2 complex in the presence
of RX. When RX (chloroꢀ, bromoꢀ, and iodobenzene,
1ꢀbromoꢀ2ꢀmethylbenzene, 2ꢀchlorothiophene, 2ꢀbroꢀ
mopyridine, 1ꢀiodohexane, 1ꢀiodoꢀ1ꢀmethylethane, and
1ꢀiodoꢀ3ꢀmethylbutane) is added to a solution of the
cobalt complexes, an increase in the current of the caꢀ
Cobpy(BF4)2
Соbpy2(BF4)2
Соbpy3(BF4)2
1.33
1.30
1.24
25
30
30
0.3
0.8
0.9
1.89
1.89
1.89
28
65
65
0.35
0.48
0.70
Note. The following designations were used: Ер1 and Ер2 are the
potentials and ip1 and ip2 are the currents of the first and second
peaks of reduction of complexes, respectively; ia/iс is the ratio
of the anodic and cathodic peaks of currents for the forward
and reverse potential scans.

1704
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 9, September, 2002
Budnikova et al.
kIeff/s–1
thodic peaks is observed in the voltammetric curves. This
effect can serve as a quantitative characteristic of the
catalytic properties of the complex. It was found that
under experimental conditions the CoI complex did not
manifest catalytic properties (the reversibility of the
CoII/CoI wave remained unchanged and the catalytic
effect was absent) toward all substrates except allyl broꢀ
mide. However, the height of the second peak correꢀ
sponding, as we believe, to the reduction of CoI to
[Cobpy2]– increases when the substrate is added, and the
anodic peak disappears at the reverse scan (i.e., the elecꢀ
trochemical "reversibility" disappears). Thus, in this case,
the complex anion, whose regeneration during electrolysis
provides an increase in the current, is catalytically active
(Scheme 1).
8
19
17
15
13
11
9
9
6
7
5
7
Scheme 1
5
4
1
3
2
3
1
1
3
5
7
CRX•10–2/mol L–1
Fig. 2. Plot for the apparent rate constant (kIapp) of regeneraꢀ
tion of the [Co(bpy)2]– (1—8) and [Cobpyn]+ (9) catalysts
vs. RX concentration: RX is 2ꢀchlorothiophene (1), PhBr (2),
2ꢀbromopyridine (3), 2ꢀbromotoluene (4), PhCH2Cl (5),
PriI (6), C6H13I (7), PhI (8), and AllBr (9).
The scheme proposed explains, as a whole, the efꢀ
fects observed: the increase in the current of the second
cathodic peak and the total irreversibility of the process
due to the fast subsequent chemical reaction. The latter
results in the formation of a species, which is the donor
of the R• radical or contains a weakly bound radical
capable of further transferring from one reactant to anꢀ
other. The generated R• species can be dimerized, reꢀ
duced to R–, enter into other reactions of the radical or
anionic type. The dependence observed in several cases
reaction center (bromomesitylene), which decreases the
rate constants of the oxidative addition and reductive
elimination.
It is of interest that in the presence of allyl bromide
catalytic regeneration is observed already at potentials of
the first peak of Co2+bpy3 reduction, i.e., the reduction
of RX by Co+bpy2 occurs rather rapidly (see Fig. 2). This
process is also characterized by a linear dependence of
for kI
on the substrate concentration7 indicates the
app
first order of the reaction (Fig. 2). All plots like those
presented in Fig. 2 are linear and pass through the origin
kI on the substrate concentration.
app
of coordinates because kI
error in calculation of kI
= kIIappСRХ. However, the
in the region of low СRХ is
Thus, the CV data prove the catalytic increase in the
current of reduction of the mediator complexes when
the RX substrates are added and allow the estimation of
the potential of regeneration of the cobalt complex in
the catalytic cycle. Based on the current characteristics
of the process, we attempted to estimate the rate constant
of the catalytic reaction. In the case of the [Cobpy3]2+
complex coordinationally saturated with bpy, the cataꢀ
lytic effect and the gain in the corresponding current are
observed only at potentials of the second reduction wave
corresponding to the formation of the intermediate
[Сobpy2]– complex (except the case of allyl bromide),
and the apparent rate constant of catalyst regeneration
app
app
rather high and, therefore, the straight line can bend
near the coordinate origin. Using the data in Fig. 2, we
can find the kIIapp rate constant and determine the series
of reactivity toward [Cobpy2]– for various substrates:
2ꢀchlorothiophene < PhBr < 2ꢀbromopyridine < 2ꢀbroꢀ
motoluene < PhCH2Cl < PriI < C6H14I < PhI.
The catalytic effect was found at potentials of the
assumed reduction Co+bpy2 → [Cobpy2]– for the folꢀ
lowing substrates: PriI, 2ꢀbromotoluene, nꢀhexyl iodide,
2ꢀbromopyridine, 2ꢀchlorothiophene, PhCH2Cl, PhBr,
and PhI. In the case of bromomesitylene, PhCl, and
BuCl, catalysis is absent, probably, due to the slower
cleavage of the С—Hal bonds or steric hindrance of the
kII
for the RX studied is 30—160 mol–1 L–1 s–1
app
(Table 2).

Electrocatalytic reduction of organic halides
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 9, September, 2002
1705
Table 2. Potentials of the reduction peaks (Ер) of organic haꢀ
lides (RX) on the Pt electrode and rate constants of catalyst
regeneration kIIapp in the presence of the СoIIbpy3 and СoIIbpy
catalyst precursors
coordinationally unsaturated with bpy exhibit a higher
catalytic activity in the reduction of organic halides. For
example, the catalytic gain in the current is observed
already at potentials of the first peak of reduction
[Соbpy]2+ + e– → [Cobpy]+ in the presence of iodoꢀ
benzene, 1ꢀbromoꢀ2ꢀmethylbenzene, 2ꢀchlorothiophene,
1ꢀiodohexane, and 1ꢀiodoꢀ3ꢀmethylbutane. The current
gains at the potentials of the second peak of reduction
[Cobpy]+ + e– → Co0bpy are often so high that they are
difficult to estimate because at high substrate concentraꢀ
tions this wave is superimposed with the reduction wave
of the organic halide. Only in the case of 1ꢀbromoꢀ2ꢀ
methylbenzene and 2ꢀchlorothiophene, the second waves
RXa
–Ер/V
kII •10–2/mol–1 L–1 s–1
app
СoIIbpy3
СoIIbpyc
b
PhBr
PhI
2.82
—
—
2.30
2.23
2.11
2.09
2.07
1.97
1.97
0.2
0.5
2.56
2.35
0.6
1.0
0.7
1.0
0.9
0.9
—
—
0.2
—
0.1
—
0.3
—
2ꢀChlorothiophene
2ꢀBromopyridine
2ꢀMeC6H5Br
PhCH2Cl
Me2CH(CH2)2I
Me2CHI
increase not so much, and the calculation of kII
beꢀ
app
comes possible. The kIIapp/mol–1 L–1 s–1 values are inꢀ
dependent of the substrate concentration, which is charꢀ
acteristic of the first order reactions with respect to orꢀ
ganic halide.
C6H13
CH2=CH—CH2Br
I
0.4
1.0
a The concentration of RX was 1.5•10–1 mol L–1
.
2
b The concentration was 5•10–3 mol L–1, Ep2 = –1.89 V.
It can be assumed that the first stage of the reaction is
oxidative addition with the formation of the cobalt
σꢀcomplex
c The concentration was 5•10–3 mol L–1, Ep = –1.31 V.
The logkII values decrease with an increase in the
app
[Cobpy]+ + RX
[RCo Xbpy]+.
difference of the reduction potentials of the complex (А)
A—RX
A
RX
and substrate (RX) ∆Еp
= Ep – Ep (Fig. 3).
This complex can undergo further transformations,
e.g., decomposion, disproportionation, or reduction by
[Соbpy]+. In some cases, the peak of this intermediate
product is detected in voltammograms. For example,
The reaction is likely innerꢀsphere, and reductive
elimination is the probable limiting stage, as it was found
for the nickel complexes with bpy.8 For processes inꢀ
volving iodobenzene, 1ꢀiodohexane, 1ꢀbromoꢀ2ꢀmethylꢀ
a new peak at –1.55 V with ip
∼
is observed
benzene, and 2ꢀchlorothiophene we determined kII
app
for iodobenzene reduction. In the cases of 1ꢀiodoꢀ
hexane and 1ꢀiodoꢀ3ꢀmethylbutane, similar peaks at
–1.60—–1.66 V first increase and then decrease with
an increase in СRX. Probably, this is related to different
orders with respect to the substrate for reactions, which
occur at this potential. In most cases described earlier
for the nickel(II) complexes with bpy and in this work for
the cobalt(II) complex with bpy, the currents of the reꢀ
duction peaks of intermediate products are proporꢀ
tional to the square root of the substrate concentration
for the reduction of one substrate by various complexes.
The slopes of the lines connecting the points for one RX
and different complexes (∆logkIIapp/∆∆ЕpA—RX) are
1/1200 mV–1 on the average, which is by an order of
magnitude lower than the value characteristic of classiꢀ
cal outerꢀsphere activation processes of intermolecular
electron transfer (1/120 mV–1).9
Catalysis by the complex coordinationally unsaturated
with bpy. The reduced forms of the [Cobpy]+ complex
log(kIIeff/L mol–1 s–1
1.9
)
i = a1
+ b1. If the plot of the current of the oxidative
addition products vs. substrate concentration is paraꢀ
bolic, then i = a2С 2 + b2С + d (a, b, and d are unknown
coefficients) with the parabola vertex facing up.
3
1
2
1.7
1.5
1.3
1.1
1´
Somewhat different pattern is observed for the reꢀ
duction of allyl bromide by the electrochemically generꢀ
ated [Соbpy]+ complexes unsaturated with bpy. In the
presence of allyl bromide, three peaks are observed (the
potential of the second peak corresponds to the initial
peak) instead of a single peak corresponding to the transꢀ
formation of [Соbpy]2+ into [Соbpy]+. The total current
corresponding to the two first peaks is much greater than
the catalytic currents of the peaks observed in the case of
other substrates under similar conditions. It can be asꢀ
sumed that allyl bromide acts as the substrate undergoꢀ
ing reduction at the С—Br bond and also as the ligand
4
2´
4´
3´
∆ЕpA—RX/V
0.2
0.4
0.6
0.8
Fig. 3. Plot of logkII
for reduction of organic halides RX by
app
the [Соbpy]+ and [Соbpy2]– cobalt complexes vs. difference of
reduction potentials of the complexes (А) and substrates
(RX) ∆ЕpA—RX: A = CoIIbpy3 (Ep –1.89 V) (1—4), CoIIbpy
2
1
(Ep –1.31 V) (1´—4´); RX = C6H13I (1, 1´), Me2CH(CH2)2I
(2, 2´), 2ꢀMeC6H5Br (3, 3´), and 2ꢀchlorothiophene (4, 4´).

1706
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Budnikova et al.
Table 3. Products of electrochemical reduction of aryl broꢀ
mides in the absence and presence of the cobalt complexes
with bpy
capable of coordinating with the cobalt(II) ion, shifting
the reduction wave of the latter to less negative potenꢀ
tials. Such a shift is characteristic of ligands, which form
a stronger complex with the reduced form of cobalt. In
this case, the unbound form of the Со+ complex and the
form bound to allyl bromide should be in equilibrium,
and each of them can catalytically reduce the substrate.
Therefore, the total current characterizing the reduction
of allyl bromide at these two first waves was used for the
ArBr
PhBr
Precursor
of catalyst
Product
Product
distribution (%)
—
PhH
Ph—Ph
PhH
100
81
19
CobpyBr2
calculation of the kII value. However, we have to adꢀ
oꢀBrTol
—
TolH
Tol—Tol
TolH
100
52
48
app
mit that the obtained kII
constant corresponds to a
CobpyBr2
app
more complicated process than the process for other RX
and their comparison is insufficiently correct. We can
say that the efficiency of the catalytic regeneration of the
cobalt complex in the presence of allyl bromide is much
higher. This can be caused by a less strength of the С—Br
bond in the latter and its capability of coordinating with
the cobalt(II) or cobalt(I) ion, which facilitates reduction.
Preparative reduction of organic halides with the
[Соbpy]+ complexes. Many key stages of the mechanism
of reactions catalyzed by the CoI complexes with bpy
remain unclear. Therefore, it is of interest to study in
detail the routes of RX transformations and the nature of
intermediates. We chose PhBr, oꢀBrC6H4Me (oꢀBrTol),
and Me3C6H2Br (MesBr) as model halides. The use of
this series of substrates makes it possible to model and
compare the routes of reactions involving various interꢀ
mediates different in stability due to the Me group in the
orthoꢀposition of the benzene ring.
MesBr
CobpyBr2
MesH
100*
* After acid hydrolysis.
the same as those in heterogeneous reduction (in the
absence of adsorption). Therefore, the mechanism of
interaction of [Сobpy]+ with RX through oxidative addiꢀ
tion is more preferential due to the formation of a considꢀ
erable amount of the dimeric R—R products (Scheme 2).
Scheme 2
[Cobpy3]2+ + e–
[Cobpy3]+
[Cobpy3]+,
[Cobpy2]+ + bpy,
[RCoXbpy2]+,
(1)
(2)
(3)
(4)
(5)
[Cobpy2]+ + RX
[RCoXbpy2]+
In the absence of the cobalt complexes, the reducꢀ
tion of RX occurs at high negative potentials (∼–2.6 V)
and the polarographic limiting currents correspond in all
cases to the transfer of two electrons per the RX molꢀ
ecule. The reduction products are the corresponding
hydrocarbons (RH), and no R—R dimers form. Thus,
heterogeneous reduction is a twoꢀelectron process
R• + X– + [Cobpy2]2+
,
2 R•
R—R.
The absence of dimesityl and МеsH in the products
of MesBr reduction can be explained by a significant
stability of the σꢀmesitylcobalt complexes formed in reꢀ
action (3) (see Scheme 2). They are completely decomꢀ
pose to MesH in an acidic medium by the treatment of
an electrolyte.
In most cases, the [Сobpy]+ complex interacts slowly
with RX, which is indicated by the absence of the peak
caused by catalyst regeneration and additional peaks corꢀ
responding to the reduction of the addition products
(σꢀorganyl complexes) in the CV curves of the current
increase.
In order to elucidate whether the target electroꢀ
synthesis of σꢀorganometallic cobalt derivatives is posꢀ
sible, we studied the reduction of the PhBr, oꢀTolBr,
and MesBr substrates by [Cobpy]+. As mentioned above,
under CV conditions the coordinationally saturated comꢀ
plexes are reactive toward these substrates only at potenꢀ
tials of the second peaks of cobalt reduction. Therefore,
RX + 2 e + H+
RH + X–.
In all cases except that with MesBr, the preparative
reduction of the chosen aryl halides in the presence of
the cobalt complexes affords the R—R dimers, which are
formally the products of oneꢀelectron reduction of RX
(Table 3).
2 RBr + 2 e
R—R + 2 Br–.
It can be assumed that the reaction of the СоI comꢀ
plex with RX proceeds via one of the possible mechaꢀ
nisms: (a) outerꢀsphere mechanism of electron transfer;
(b) innerꢀsphere mechanism including transformations
in the coordination sphere of the СоI complex.
The outerꢀsphere mediator does not bind with subꢀ
strates in the transition state and works as an electrode
(electron source). In this case, the products should be

Electrocatalytic reduction of organic halides
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 9, September, 2002
1707
–I
–I
C3
C2
10 µА
C3
10 µА
2
1
C1
C1
2
C3
C1
1.2
1.6
–E/V
1
Fig. 4. Cyclic voltammogram for CoBr2bpy (1.7•10–2 mol L–1
in the absence (1) and presence (2) of aromatic bromides
in DMF.
)
1.2
1.6
–E/V
they will not be considered in this report. The coordiꢀ
nationally unsaturated complexes, including СоbpyBr2,
are more reactive. They do not exhibit the current gain,
but the reversibility of the first peak disappears, and a
new peak appears. The polarization curves of electroꢀ
chemical reduction of the СоbpyBr2 complex in the abꢀ
sence and presence of PhBr are presented in Fig. 4.
Fig. 5. Cyclic voltammogram for a solution of CoBr2bpy
(1.7•10–2 mol L–1) in a DMF solution of Et4NBF4 (0.1 mol L–1
)
in the presence of PhBr (5.1•10–2 mol L–1) before electrolyꢀ
sis (1) and after passing 1 F electricity (2).
peaks at Е C increased compared to the values before
3
C
3
It can be assumed that the peak at the Ep potential
corresponds to the reduction of the oxidative addition
product, viz., σꢀphenylcobalt complex
electrolysis (Fig. 5).
Further passing electricity until the complete conꢀ
sumption of PhBr does not substantially change the situꢀ
ation. The GLC analysis of the solution showed benzene
and biphenyl as the products (19 : 81). Thus, product of
oxidative addition, PhCoBr2bpy, is insufficiently stable
[Cobpy]+ + PhBr
[PhCoBrbpy]+.
Its further transformations can be decomposition, disꢀ
proportionation, or reduction by [Cobpy]+.
under the experimental conditions, and the catalyst reꢀ
generates at E C
.
1
The oxidation state of cobalt in the oxidative addiꢀ
tion product remains unclear. The redox potentials of
the initial СоII complex and its reduced СоI form are
different. If the twoꢀelectron process occurs during the
oxidative addition of CoI, the СоIII complex with a higher
redox potential than the initial СоII complex should be
formed. However, this is not observed in the voltammoꢀ
grams. Therefore, oneꢀelectron oxidation seems to be
more preferential. In the case of cobalt, the hydrocarbon
R group (at the initial RX molecule) can bind with the
metal due to combining electrons (one electron from
each fragment), i.e., the R group in the σꢀcomplex is
uncharged, and cobalt has the oxidation state 2+, as in
the initial CoII complex. In this case, the σꢀcomplex is
characterized by free radical reactions rather than nuꢀ
cleophilic reactions of the potentially carbanionic R–
group. At the same time, it is well known that these are
precisely the radical reactions which are characteristic of
The preparative reduction of oꢀBrTol by [Cobpy]+
(3 : 1) gives similar results. After electrolysis at the E C
1
potential (6 F ) the solution becomes violet, which corꢀ
responds to the TolCoBr2bpy σꢀcomplex formed due to
oxidative addition. As in the case of PhBr, all peaks are
retained in the CV curves. However, the height of the
first peak decreases considerably, and the C3 peak correꢀ
sponding to the σꢀcomplex becomes predominant. Thus,
the tolyl σꢀcomplex is more stable than the phenyl comꢀ
plex. The complex slowly decomposes in the air, and
after 1—2 h a solution of the electrolyte becomes green.
It can be assumed that CobpyBr2 forms. This assumpꢀ
tion is confirmed by the voltammetric data. The chroꢀ
matographic analysis of the solution showed toluene and
ditolyl (48 : 52) as the products.
The stability of the aryl σꢀcomplexes containing the
Me group in the orthoꢀposition of the aromatic ring is
due to steric factors preventing rotation around the
σꢀAr—Co bond and the axial attack of the reactants at
the cobalt atom.10 Therefore, it is reasonable to choose
MesBr with two Me groups in the orthoꢀposition for the
synthesis of more stable σꢀcomplexes. For example, the
combined electrochemical reduction of CobpyBr2 and
MesBr was carried out in a divided cell. After the end of
compounds with the assumed involvement of CoIII
.
We carried out the preparative electrolysis of the
СоbpyBr2 complex and PhBr (1 : 3) at the potential
Е C = –1.3 V in a cell with a magnesium anode. After
1
1 F electricity was passed through the electrolyte, the C1
and C3 peaks were still observed in the voltammogram.
The current of the C1 peak somewhat decreased, and the

1708
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 9, September, 2002
Budnikova et al.
–I
a rule, during isolation. The metal—carbon bond is very
reactive, and only several organometallic cobalt comꢀ
plexes are known to date. In some cases, the σꢀcomꢀ
plexes can be isolated and studied independently as model
compounds in the investigation of mechanisms of orgaꢀ
nometallic synthesis.
C3
5 µА
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos. 01ꢀ03ꢀ
33210ꢀa and 01ꢀ15ꢀ99353), Complex Program of the
Russian Academy of Sciences "New Principles and Methꢀ
ods for Development and Target Synthesis of Substances
with Specified Properties," and INTAS (Grant 00ꢀ0018).
1.3
1.7
–E/V
Fig. 6. Cyclic voltammogram for a solution of [MesCoBr2bpy]
(0.01 mol L–1), viz., product of the combined electrolysis of
CoBr2bpy and MesBr, in DMF (after passing 1.2 F electricity).
electrolysis (complete disappearance of the reduction peak
of the initial complex at ∼–1.3 V, ∼1.2 F electricity per
Co atom were passed), the voltammogram of the soluꢀ
tion exhibited one reversible oneꢀelectron wave at more
References
1. Yu. H. Budnikova, O. E. Petrukhina, and Yu. M. Kargin,
Zh. Obshch. Khim., 1996, 66, 1876 [Russ. J. Gen. Chem.,
1996, 66 (Engl. Transl.)].
2. Yu. H. Budnikova, A. M. Yusupov, and Yu. M. Kargin,
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1994, 64 (Engl. Transl.)].
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Akad. Nauk, Ser. Khim., 1992, 1674 [Bull. Russ. Acad. Sci.,
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negative potentials (ЕC = –1.75 V) (Fig. 6), which can
3
be attributed to the reduction wave of the σꢀmesitylcobalt
complex formed according to Scheme 3.
Scheme 3
[Cobpy]+ + MesBr
[MesCobpyBr]+,
MesCobpyBr.
[MesCobpyBr]+ + e–
This complex was isolated from a solution and charꢀ
acterized by spectroscopy and elemental analysis.
Note that the occurrence of two Me groups in the
orthoꢀposition inhibits not only reductive elimination but
also other possible reactions, for example, oxidative adꢀ
dition involving the σꢀmesityl complex. The GLC analysis
of the ether extract of the reaction mixture showed only
traces of MesH as the products of MesBr transformation
(because the major portion of MesH is bound to the
stable σꢀmesityl complex), and after acid hydrolysis MesH
formed in almost 100% yield.
5. Z.ꢀZ. Zhang, H. Cheng, S.ꢀM. Kuang, Y.ꢀQ. Zhou, Z.ꢀX.
Liu, J.ꢀK. Zhang, and H.ꢀG. Wang, J. Organometal. Chem.,
1996, 516, 1.
6. Yu. H. Budnikova and Yu. M. Kargin, Zh. Obshch. Khim.,
2001, 71, 140 [Russ. J. Gen. Chem., 2001, 71 (Engl. Transl.)].
7. M. Hammouche, D. Lexa, M. Momenteau, J.ꢀM. Saveant,
J. Am. Chem. Soc., 1991, 113, 8455.
8. J. M. Saveant, Bull. Soc. Chem. Fr., 1988, 225.
9. J. Y. Nеdelec, J. Perichon, and M. Troupel, in Topics in
Current Chemistry, Ed. E. Steckhan, Springer, Berlin, 1997,
185, 141.
10. G. Meyer, Y. Rollin, and J. Perichon, J. Organometal. Chem.,
1987, 333, 263.
Thus, the formation of cobalt organic σꢀcomplexes,
which are poorly studied compounds, was shown. Comꢀ
pounds with this structure are key intermediates for the
whole series of reactions. However, they decompose, as
Received July 20, 2001;
in revised form April 11, 2002