Electrochemical reduction of halopyridines catalyzed by Ni0(bipy)2

Article abstract of DOI:10.1023/A:1012354111715

The possibility of electrochemical reduction of halopyridines in the presence of Ni⁰(bipy)₂ (bipy = 2,2′-bipyridine) was shown, and the features of homo- and cross-coupling with participation of PyX (Py = 2-or 3-pyridyl, X = Cl, Br) were studied.

Full text of DOI:10.1023/A:1012354111715

Russian Journal of General Chemistry, Vol. 71, No. 1, 2001, pp. 128 131. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 1, 2001,
pp. 140 144.
Original Russian Text Copyright
2001 by Budnikova, Kargin.
Electrochemical Reduction of Halopyridines
Catalyzed by Ni0(bipy)2
Yu. G. Budnikova and Yu. M. Kargin
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
Kazan State University, Kazan, Tatarstan, Russia
Received July 13, 1999
Abstract The possibility of electrochemical reduction of halopyridines in the presence of Ni0(bipy)2
(bipy = 2,2 -bipyridine) was shown, and the features of homo- and cross-coupling with participation of PyX
(Py = 2- or 3-pyridyl, X = Cl, Br) were studied.
Formation of pyridines with functional substitu-
ents by replacement of halogen in the C Hal bond by
the pyridine fragment receives a particular attention.
At the same time, electrochemical studies in this di-
rection are scarce [1 3]. Thus, it is urgent to examine
the possibility of electrochemical reduction of halo-
Ni(bipy)3(BF4)2 in the presence of chloropyridines
we observed no catalytic increase in the current at
the potentials of NiII reduction wave ( 6%, which is
close to the experimental error), with significant de-
crease in the Ni0 oxidation current (Figs. 1 and 2,
curves 2). In both cases the rate constant k0 of
pyridines catalyzed with an Ni0 complex and the fea- the oxidative addition of PyCl to the reduced species
tures of homo- and cross-coupling with participation
of PyX (Py = 2- or 3-pyridyl, X = Cl, Br). The cata-
lytic cycle of reduction of organic halides in the pres-
ence of electrochemically generated complexes Ni0Ln
(L = -acceptor ligand) involves the stages of the ox-
idative addition to the catalyst and reductive elimina-
Ni0(bipy)2 [scheme (2)] can be calculated from the rel-
ative changes of the anodic current Ia /Ic at various
scanning rates of potential and large excess of sub-
strate (Table 1) [4]:
Ni2+(bipy)3 + 2e
Ni0(bipy)2 + PyCl
Ni0(bipy)2 + bipy,
(1)
(2)
tion of
organic complexes of nickel.
k 0
PyNiCl(bipy)2.
As catalyst precursors Ni(bipy)3(BF4)2 and
NiBr2(bipy) (bipy = 2,2 -bipyridine) can be used.
The voltammetric studies of the catalytic properties
of these complexes with various numbers of ligands
show that the catalyst activity during oxidative ad-
dition depends on the number of ligands. Different
coordination numbers typical for nickel in various
oxidation states determine the loss of one bipy lig-
and in the course of reduction [scheme (1)]. For
The rate constant k0 of oxidative addition for
3-ClPy is higher than for 2-ClPy; for NiBr2(bipy) it is
higher than for Ni(bipy)3(BF4)2. In all the tests, with
addition of bromopyridines to the solutions of nickel
complexes the catalytic increase in the current due to
the regeneration of Ni0 catalyst [e.g., reactions (3 7)]
is observed. As a result, direct calculation of k0 by
common methods is impossible.
1
Table 1. Rate constants of oxidative addition of PyX to Ni0 (k0, mol 1 l 1 s , procedure [4]) and rate constants of
1
catalyst regeneration (kap, mol 1 l 1 s , procedure [5]) [c(PyX) 0.126 M]
Complex
PyX
(Ip In)/In,a
%
k0
kap
Complex
PyX
(Ip In)/In,a
%
k0
kap
Ni2+(bipy)3(BF4)2 2-ClPy
6
6
100
43
10
22
Ni2+Br2(bipy) 2-BrPy
3-ClPy
110
3
35
3-ClPy
2-BrPy
3-BrPy
83
30
13
3-BrPy
33
a
I
and I are the reduction currents of complex in the presence and absence of the substrate.
p
n
1070-3632/01/7101-0128$25.00 2001 MAIK Nauka/Interperiodica

ELECTROCHEMICAL REDUCTION OF HALOPYRIDINES CATALYZED BY Ni0(bipy)2
129
2+
2+
Fig. 1. Voltammograms of Ni (bipy) (BF ) (c 0.017 M)
Fig. 2. Voltammograms of Ni (bipy) (BF ) (c 0.017 M)
3 4 2
3
4 2
in the (1) absence and (2) presence of 2-ClPy (c 0.085 M),
(3) after passing of 2e per nickel(II) complex at 1.2 V,
and (4) after passing of 5.2e per nickel(II) complex in
the absence of 2-ClPy (vs. saturated calomel electrode).
in the (1) absence and (2) presence of 3-ClPy (c 0.17 M),
(3) after passing of 8e per nickel(II) complex at the poten-
tial from 1.20 to 1.25 V (I
0), and (4) after passing
of 16e per nickel(II) complex at 1.4 V.
At the same time, at significant current increase
In some experiments the increase in the current of
the apparent rate constant of catalyst regeneration reduction of Ni0(bipy) to Ni0(bipy) at a potential of
can be determined using the Saveant procedure [5]. 1.9 V vs. saturated calomel electrode is observed. In
The rate constant of pseudo-first-order reduction of this case we can calculate the apparent rate constant of
2-BrPy is close for both catalysts.
regeneration of the catalyst, which acts as an electron-
transfer agent similar in properties to the outer-spher-
ical radical anion (Table 2). This procedure is correct
k0
Ni0 (bipy)2 + PyBr
PyNiBr(bipy)2
(3)
Ni0 (bipy)2 + PyNiBr(bipy)2
PyNi1+ (bipy)2 + Ni1+ Br(bipy)2
Table 2. Apparent rate constants of regeneration of
Ni0(bipy) catalyst in the presence of halopyridines in
(4)
(5)
2
DMF [c(Ni2+Br2(bipy)) 1.7 10 M]
PyNi1+ (bipy)2 + PyBr
Py2Ni3+ Br(bipy)
Reaction products + Ni1+ Br(bipy)
Ni1+ Br(bipy)2 + e Ni0 (bipy)2 + Br
Py2Ni3+ Br(bipy)2
c(PyX),
kap ,
PyX
Ip/In[Ni(0)/Ni(0) ]
1
M
mol 1 l 1 s
3-ClPy
3-BrPy
2-ClPy
7.0
8.0
5.5
0.34
0.085
0.085
116
604
308
(6)
(7)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

130
BUDNIKOVA, KARGIN
2+
2+
Fig. 3. Voltammograms of Ni (bipy) (BF ) (c 0.017 M)
Fig. 4. Voltammograms of Ni (bipy) (BF ) (c 0.017 M)
3 4 2
3
4 2
in the (1) absence and (2) presence of 2-BrPy (c 0.034
and 0.085 M), (3) after passing of 2.2e per nickel(II)
complex at 1.20 V, and (4) after passing of 3.9e per
in the (1) absence and (2) presence of 3-BrPy (c 0.085 M),
(3) after passing of 2e per nickel(II) complex at 1.27 V
(I
0), and (4) after passing of 4e per nickel(II) com-
nickel(II) complex at 1.2 V (I
0).
plex at 1.7 V.
only if the rate of reaction of PyX with Ni0 forming
at the first-wave potentials is relatively small and no
waves of the reaction products and no current increases
are registered in the voltammograms, i.e., Ni0 is stable
under the experimental conditions and the rate of its
reaction with PyX is negligible. For example, such
calculations are incorrect for the reaction with 2-BrPy.
1
and 10 mol 1 l 1 s , respectively) for Ni(bipy)3(BF4)2
and more rapid as compared with 2-BrTol. As shown
previously, the reaction with chloropyridine is unfa-
vorable for cross-coupling. In the case of bromopyr-
idines [reactions (9) and (12)] the current of NiII
reduction at 1.2 V increases owing to the catalyst re-
generation, and thus evaluation of k0 is impossible.
We found that ArNi+(bipy) reacts with all halopyr-
idines; however, in the case of preliminary mixing of
aryl halides (1 : 1 molar ratio) the cross-coupling
with 3-bromopyridine during electrolysis is insignif-
icant [6].
Now, let us analyze the reduction products of halo-
pyridines in the presence of Ni0(bipy)2 catalyst and
evolution of voltammograms in the course of electrol-
ysis. During electrolysis of 2-ClPy of 2-BrPy in
the presence of Ni(bipy)3(BF4)2 (Figs. 1 4) rapid
regeneration of the catalyst at 1.2 V proceeds, and
the main product of the process is 2,2 -bipy (100%
for 2-ClPy and 70% for 2-BrPy). During electrolysis
of 3-ClPy or 3-BrPy nickel is gradually bound in
We studied voltammograms of nickel complexes
with bipy in the presence and absence of various halo-
pyridines. In the presence of 2- and 3-ClPy the NiII/Ni0
wave remains almost constant, whereas the reaction
with 3-ClPy is more rapid than with 2-ClPy (k0 22
the
complex PyNi2+X(bipy), and the catalyst is
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001

ELECTROCHEMICAL REDUCTION OF HALOPYRIDINES CATALYZED BY Ni0(bipy)2
131
(8)
2-ClPy
2-BrPy
2-ClPy
2-BrPy
(2-Py)NiLCl. . .
2,2 -bipy
(100%)
unstable
(2-Py)NiLBr. . .
2,2 -bipy
(70%)
(30%)
(9)
unstable
PyH
(10)
2e
Ni2+L
Ni0L
3-ClPy
3-BrPy
e, 1.45 V
(3-Py)NiLCl
more stable
[H+]
3,3 -bipy + PyH
(35%)
(11)
1.2 V
3-PyH
e, 1.45 V
(3-Py)NiLBr
more stable
[H+]
PyH + 3,3 -bipy
(12)
3-PyH
L = 2,2 -bipy.
blocked. Small amounts of 3,3 -bipy (35%) are formed
in reaction with 3-ClPy and only at more negative
potentials ( 1.48 and 1.7 V), and in the case of
3-BrPy [reactions (11) and (12)] this reaction does not
proceed at all.
As a result, the reaction of Ni0 with halopyridines
is unfavorable for cross-coupling, because the catalyst
is consumed in the reaction and almost no product is
formed.
dine, and Ni2+(bipy)3(BF4)2, from Ni(BF4)2 6H2O and
3 equivalents of 2,2 -bipyridine in ethanol. The solu-
tions were stored for 12 h. The precipitate was filtered
off, washed with ethanol, and dried in a vacuum at
70 C for 24 h. The saturated calomel reference elec-
trode was placed into the compartment filled with
DMF and Bu4NBF4 separated from the solution with
a glass membrane. The working electrode was a gold
disc 0.25 mm in diameter. For the preparative elec-
trolyses a cylindrical nickel gauze cathode was used.
The solvents and supporting electrolyte (Bu4NBF4)
were purified by the standard procedures [7]. The chro-
matographic analysis of the reaction mixtures and
products was performed on a Chrom-4 gas liquid
chromatograph [glass columns packed with Chroma-
ton N-AW, 5% Silicone SE-30 (0.125 0.160 mm)]
equipped with a flame-ionization detector.
Our tests showed that electrochemically generated
complexes Ni0(bipy)n are the effective catalysts for
dehalogenation of halopyridines, and the reactions
products are the corresponding bipyridines (from
2-ClPy), pyridine (from 3-ClPy), or their mixture.
The reaction of Ni0(bipy)n with PyX yields -pyridyl
complexes of nickel, which do not react with the other
organic halides RX and thus do not yield the cross-
coupling products:
REFERENCES
k0
Ni0L + PyX
PyNi2+LX,
(13)
1. Nedelec, J.Y., Perichon, J., and Troupel, M., Top. Curr.
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for 2-X:
PyNi2+LX + Ni0
for 3-X:
PyNi2+LX + e
PyNi1+L + PyX
2. Schiavon, G., Zotti, G., Bontempelli, G., and
Lo Coco, F., Synth. Met., 1988, vol. 25, no. 4,
pp. 365 373.
PyNi1+L + X ( 1.2 B), (14)
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Zh. Obshch. Khim., 1993, vol. 63, no. 6, pp. 1308 1311.
PyNi1+L + X ( 1.45 B), (15)
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Py2Ni3+LX
Py2 + Ni1+LX. (16)
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5. Investigation of Rates and Mechanisms, Bernas-
con, C.F., Ed., New York: Wiley, 1986, vol. 6, part 2.
EXPERIMENTAL
6. Budnikova, Yu.G., Kargin, Yu.M., and Sinyashin, O.G.,
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The kinetic characteristics of the process were stud-
ied using voltammograms registered for each substrate
metal complex system. Ni2+Br2(bipy) was prepared
from Ni2+Br2 3H2O and 1 equivalent of 2,2 -bipyri-
7. Elektrokhimiya metallov v nevodnykh rastvorakh (Elec-
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Kolotyrkin, Ya.M., Ed., Moscow: Mir, 1974.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 1 2001