CuI complexes with 2,2′-biquinolyl-containing polymeric ligands as electrocatalysts for the oxidative coupling of alkynes in the presence of dioxygen

Article abstract of DOI:10.1007/s11172-008-0283-5

Electrocatalytic oxidative coupling of terminal alkynes in the presence of atmospheric oxygen catalyzed by Cu^I complexes with polymeric biquinolyl ligands leads to the corresponding diynes in high yield. The reaction proceeds under mild conditi

Full text of DOI:10.1007/s11172-008-0283-5

2090
Russian Chemical Bulletin, International Edition, Vol. 57, No. 10, pp. 2090—2092, October, 2008
I
Cu complexes with 2,2´ꢀbiquinolylꢀcontaining polymeric ligands
as electrocatalysts for the oxidative coupling of alkynes
in the presence of dioxygen
T. V. Magdesieva,a A. V. Dolganov, A. V. Yakimansky, M. Ya. Goikhman, and I. V. Podeshvo
a
b
b
b
aDepartment of Chemistry, M. V. Lomonosov Moscow State University,
1
Leninskie Gory, 119991 Moscow, Russian Federation
Institute of Macromolecular Compounds, Russian Academy of Sciences,
1 Bolshoi prosp., 199004 St. Petersburg, Russian Federation
b
3
Electrocatalytic oxidative coupling of terminal alkynes in the presence of atmospheric
I
oxygen catalyzed by Cu complexes with polymeric biquinolyl ligands leads to the correspondꢀ
II
I
ing diynes in high yield. The reaction proceeds under mild conditions at a potential of Cu /Cu
electroreduction (–0.55 V vs. Ag/AgCl/KCl) and is accompanied by O reduction to H O.
2
2
Key words: copperꢀcontaining polymers, biquinolyl complexes, electrocatalysis, oxidative
coupling, alkynes, dioxygen.
I
The Cu complexes with Nꢀcontaining ligands are
plex. The energy profile has been calculated; the obtained
small values of the activation energy nicely correspond to
the fact that the reaction proceeds at room temperature.
The electrocatalytic coupling of alkynes in the presꢀ
ence of atmospheric oxygen has not been investigated so
far. Furthermore, electrochemical activation allows perꢀ
forming reactions under mild conditions, at room
temperature and atmospheric pressure, which often
known to catalyze various aerobic oxidative processes inꢀ
cluding biochemical reactions (see, e.g., Refs 1—3 and
papers cited therein). The structures of Cu/O complexes
formed during the reactions were investigated in detail.
It was demonstrated that they depend strongly on the Cu
ion coordination environment and determine largely the
type of the catalytic process. The range of oxidative proꢀ
2
4
4
13
cesses involving molecular oxygen is very wide. Oxidaꢀ
ensures high selectivity of a process.
1
4
tive aerobic coupling of terminal alkynes catalyzed by Cu
complexes is an example. The interest in such a process
is, in particular, due to the fact that the polyyne systems
are widely used in preparation of molecular wires, lumiꢀ
nescent materials, etc.
Oxidative coupling of alkynes is usually performed
using Cu complexes with amineꢀcontaining ligands. The
Recently, we have elaborated an electrochemical
approach using the sacrificial anode technique to the new
I
polynuclear Cu complexes with polymeric ligands based
on polyamido acids (PA) with biquinolyl (biQ) fragments
in the polymer backbone. The presence of the polymeric
5
I
ligand, as well as the presence of the excess of Cu ions
II
(which is easily accessible using instrumental control by
optimization of the current and potential values) allow
yield of the dimer and the reaction rate strongly depend
on the nature of the alkyne, as well as on the ligand
I
one to obtain coordinatively unsaturated Cu complexes
type.6
—10
A comparison of the catalytic efficiency of
I
with only one biQ fragment in the Cu coordination enviꢀ
dendrimeric amineꢀcontaining copper complexes with that
of their lowꢀmolecularꢀweight analogs revealed the adꢀ
vantages of the former, which seems to be related to the
better complexing ability and to the increase in the local
concentration of the reagents due to the adsorption of the
ronment.
These complexes appeared to be capable of activating
molecular oxygen. They showed high electrocatalytic acꢀ
tivity in the aerobic oxidation of aliphatic alcohols to the
corresponding carbonyl derivatives. As follows from the
experimental results, the polymeric chain stabilizes coorꢀ
dinatively unsaturated Cu complexes ([Cu(PA)L ]BF )
that are catalytically active. Especially high catalytic
activity can be achieved when using polymer Cu comꢀ
plexes immobilized on a graphite electrode.
1
5
alkyne molecule on the dendrimer surface.1 Recently,
1
12
I
a detailed quantum chemical investigation (DFT comꢀ
putation) of the mechanism of the oxidative coupling of
alkynes in the presence of atmospheric oxygen has been
published. It revealed that the key step of the process
might be the oxidation of copper acetylenide formed with
2
4
I
1
5
In continuation of this research, we studied the appliꢀ
cability of the aboveꢀmentioned polymeric biquinoline
2
+
dioxygen yielding a binuclear [Cu (μꢀO ] dioxoꢀcomꢀ
2
2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2052—2054, October, 2008.
066ꢀ5285/08/5710ꢀ02090 © 2008 Springer Science+Business Media, Inc.
1

Electrochemical coupling of alkynes
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 10, October, 2008
2091
O
O
BF4–
m : n = 8 : 2; L are the molecules of the solvent (Nꢀmethylpyrrolidone, MeCN)
I
Cu complexes as catalysts for oxidative dimerization of
terminal alkynes in the presence of atmospheric oxygen
under conditions of electrochemical activation.
ceeds at a low cathodic potential (–0.55 V) at room temꢀ
perature and atmospheric pressure of air. This potential
1
4
value has been demonstrated to correspond to the
II
I
Cu /Cu redoxꢀtransfer providing formation of the active
form of the catalyst. Direct electrochemical dioxygen
reduction on a Pt electrode in acetonitrile proceeds at a
Experimental
Acetonitrile (pure grade) was stirred with CaH for 12 h and
distilled, then refluxed for 2 h over P O and distilled again
collecting the fraction with b.p. 81—82 °C (760 Torr).
Preparative electrolysis was performed with a Pꢀ5827M
potentiostat in an undivided 10 mL electrochemical cell.
The working electrode was graphite fabric with immobilized
Cu (PA)L ]BF complex (see Refs 14, 15), the counter
electrode was a platinum plate. All potentials are referred to
Ag/AgCl/KCl aq. reference electrode (the potential versus
Fc/Fc in acetonitrile is about –0.43 V). The measured potenꢀ
tial values were recalculated considering ohmic drops.
GCꢀMS analysis of the reaction mixtures was carried out
with the GCꢀMS spectrometer Finnigan MAT SSQ 7000 with
ionization energy of 70 eV, with capillary silicon column НРꢀ5
2
16
potential of –1.1 V. When the Cuꢀpolymer is immobiꢀ
2
5
lized on the graphite electrode, an intensive increase in
II
I
the Cu /Cu reduction current at a potential of –0.55 V is
observed in acetonitrile depending on the O concentraꢀ
2
I
tion. This suggests the electroactivity of Cu ꢀcomplex with
I
[
2
4
respect to O and electrocatalytic O reduction in the
2
2
presence of the Cuꢀpolymer.
The oxidative coupling of alkynes was performed in
the undivided electrochemical cell in MeCN using the
polymeric complex [Cu(PA)L ]BF immobilized on the
+
2
4
1
4
graphite electrode surface as a catalyst. The flow of air
was passed through the electrochemical cell. As follows
from Table 1, the coupling of all three investigated comꢀ
pounds occurs with the high yield and current efficiency.
As one can see in Table 1, the yields of the dimers
are somewhat lower in the case of pentꢀ1ꢀyne and triꢀ
methylsilylacetylene, as compared with phenylacetylene,
probably, due to their volatility. Special measures to miniꢀ
mize evaporation of the starting alkynes during the
experiment were undertaken (see Experimental), but it
was not possible to avoid losses completely. Chromatoꢀ
graphic (GCꢀMS and GC) analysis of the reaction mixꢀ
tures showed the presence of the dimers only, without
even traces of the starting alkynes. This allows one to
make a conclusion that the catalytic cycle provides
the complete conversion of the alkyne to diyne and the
decrease in the yield is attributed to the partial losses of
the starting alkynes due to their evaporation in the air flow.
The amount of the employed catalyst ([Cu(PA)L ]BF )
(
30 m). Detector and injector temperatures were 50 °C.
Electrocatalytic coupling of alkynes with O in the presence of
2
I
[
Cu (PA)L ]BF immobilized on the graphite electrode. The elecꢀ
2
4
trolysis was performed in a potentiostatic regime at a potential
of –0.55 V in the cell equipped with the efficient reflux conꢀ
denser filled with ice water. An alkyne (0.1 mmol) was dissolved
in 10 mL of MeCN (5% H O) containing 132 mg of Bu NBF
4
2
4
–
1
(
0.05 mol L ). A 1 : 1 mixture of argon and air was bubbled
–
1
through the solution (24 mL min ) during the electrolysis. The
process was stopped after 2 F mol–1 of charge was passed (19.6 C
per starting alkyne). The obtained solution was analyzed using
GCꢀMS. The peaks of molecular ions were observed in all cases,
+
+
m/z (I (%): 202 [M] (100) (PhC≡C–C≡CPh); 106 [M] (100),
rel
+
+
9
[
1 [M –CH ] (80), 76 [M –2 CH ] (38) (PrC≡C–C≡CPr); 194
3
3
+
+
M] (20), 179 [M –CH ] (100) (Me Si–C≡C–C≡C–SiMe ).
3
3
3
The yield of the reaction products was estimated by GCꢀMS
using mesitylene as the standard.
Results and Discussion
2
4
is 3.2•10–8 mol (0.032 mol.%) relative to the starting
alkyne. The application of the cathodic potential that
Three types of terminal alkynes were chosen for invesꢀ
tigation: with an aryl, alkyl, and elementꢀcontaining subꢀ
stituent at the C≡C triple bond, viz., phenylacetylene,
pentꢀ1ꢀyne, and trimethylsilylacetylene. All these comꢀ
pounds were found to undergo dimerization when the
proposed14 catalytic system was used. The reaction proꢀ
II
I
corresponds to Cu /Cu redoxꢀtransition is necessary for
regeneration of the catalyst and for the accomplishment
of the catalytic cycle. Control experiments demonstrated
that no reaction occurs when the electrochemical cell is
switched off.

2092
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 10, October, 2008
Magdesieva et al.
Table 1. The yield and current efficiency of electrocatalytic
Scheme 1
oxidative coupling of alkynes (0.1 mmol) in MeCN : H O =
2
–
8
2
0 : 1 in the presence of the catalyst (3.2•10 mol) at a potenꢀ
tial of –0.55 V (vs. Ag/AgCl/KCl)
Substrate
Product
Yield (%)
by current by substance
PhC≡CH
PrC≡CH
PhC≡C—C≡CPh
PrC≡C—C≡CPr
81
63
75
81
66
77
Me SiC≡CH Me SiC≡C—C≡CSiMe
3
3
3
Reduction of dioxygen in the catalytic cycle is known16
to proceed with formation of either H O (the consumpꢀ
2
2
tion of two electrons) or H O (the consumption of four
2
electrons). The cyclic voltammetry investigation of the
solutions obtained after electrolysis showed no even traces
of hydrogen peroxide, which could be detected by its
characteristic reduction peak at a potential of –1.29 V
The work was financially supported by the Russian
Foundation for Basic Research (Project Nos 08ꢀ03ꢀ00142
and 07ꢀ03ꢀ90824).
(
see Ref. 16). The application of the chemical tests (e.g.,
with KI) also gave a negative result. Consequently, fourꢀ
References
electron reduction of O to water does occur in the invesꢀ
2
tigated catalytic cycle, thus making the reaction ecologiꢀ
cally friendly.
The formal reaction stoichiometry can be presented as
follows:
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–
+
Cu + O + 2 RC≡СН + e + 2 H →
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2
2
+
→
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of dioxygen to water, are supplied by the molecules of the
alkyne that is oxidized (two electrons per two molecules
II
I
of alkyne) and the electrode (due to the Cu /Cu reducꢀ
tion and the regeneration of the catalyst). Water is the
source of additional protons. Special experiments showed
that the process does not occur (the current value is zero)
in thoroughly dried acetonitrile and when the air specially
dried by passing through the vessel with concentrated
H SO is bubbled through the cell.
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1
1
1
1
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1
2
2
4
The elucidation of the reaction mechanism is not a
2
subject of the present study. However, based on literature
data,1
2,17
an active Cu complex with molecular oxygen
I
can be assumed to serve as an oxidant. Unfortunately, we
were not able to determine the type of the oxygenꢀconꢀ
taining adduct. The reason for that was the immobilizaꢀ
tion of the catalyst on the graphite electrode, which makes
impossible the registration of the electronic absorption
spectra, which usually serve as a major diagnostic criteꢀ
15. T. Magdesieva, A. Dolganov, A. Yakimansky, M. Goikhman,
I. Podeshvo, V. Kudryavtsev, Electrochimica Acta, 2008, 53,
3
960.
1
1
6. D. T. Sawyer , J. S. Valentine, Acc. Chem. Res. 1981, 14, 393.
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3
29
4
rion of the structure of the intermediate. The realization
1
8. V. L. Kornienko, I. V. Chaevko, G. V. Kornienko, in
1
8
of the catalytic cycle involving the peroxide anion might
Electrochemistry of Organic Compounds in the Beginning of
be considered as an alternative mechanism but, as it has
2
1st Century, Sputnik, Moscow, 2008, p. 147.
been showed earlier,1
4,15
in the presence of copper comꢀ
plexes this route seems to be unlikely.
Received June 26, 2008;
in revised form October 2, 2008
The catalytic cycle in shown in Scheme 1.