Mendeleev Commun., 2009, 19, 69–71
At one time the increasing of the first wave current and the
(a)
(b)
(c)
decreasing of next waves currents were observed. The current
increasing is consistent with an electrocatalytic process at
the potential of the CoII/Co0 redox couple. Apparently, the
dependence of voltammetric response on the scan rate is deter-
mined by the ratio of the rates of two processes: (1) coordina-
tion of the reduced form of 1 (Co0 intermediate complex) with
N2O, and (2) further reduction of this Co0 intermediate complex
to metal Co0. Obviously, an intermediate complex formed at the
first stage of the reduction is not ‘catched’ by a coordination
with N2O when the scan rates are too high (> 20 mV s–1) and
undergoes further reduction [Figure 1(a)]. But if the scan rate is
relatively low (£ 20 mV s–1), the intermediate coordinates with
N2O yielding new complex LCo0Cl2·N2O, which further forms
CoII complex as a result of intramolecular redox process with a
deliberation of N2 thus finishing a catalytic cycle. Only a small
part of primarily formed Co0 intermediate undergoes further
reduction resulting in decrease of posterior peaks intensity
[Figure 1(b)–(e)].
(d)
(e)
5 μA
The presumable mechanisms of the N2O interactions with 1
in an electrochemical cell both in the presence and absence of
alkene are shown in Scheme 1. Two parallel processes take place
in the electrochemical cell: (a) the initial reduction of complex
1 followed by complexation with N2O to 12– and regeneration
of 1 accompanying by N2 deliberation and (b) complexation of
1 with N2O at first followed by reduction of N2O-containing
complex with N2 evolution. The source of H+ for the formation
of H2O probably is the solvent (water); so, the whole reaction
is electrochemically catalyzed reduction of N2O. In the case of
chemical reactions, norbornene (Scheme 1) or PPh3 acts as the
electron donor and the reaction products are oxygen-containing
compounds (2 or 3) and N2O.
0.5
0.0
–0.5
E/V
Figure 1 Cyclic voltammograms of complex 1 in MeCN solution (GC
electrode, Bu4NBF4): in the presence of N2O (solid line) or without N2O
(dotted line). Scan rate: (a) 20; (b) 50; (c) 100; (d) 200 and (e) 500 mV s–1
.
It is known that electrochemical reduction of N2O occurs
only at very negative potentials.15 In both experiments (in the
solution and in the layer of hdab), the voltammetric response of
1/hdab in the presence of N2O and without it was the same at
scan rates of 500, 200, 100 and 50 mV s–1. But at slow scan rate
(20 mV s–1) the potential of first oxidation peak in the presence
of N2O shifts by 0.07 mV to less negative potential, whereas the
potential of second oxidation peak shifts by 0.05 mV to more
negative potential [Figure 1(a)]. These facts confirm the interac-
tion of both initial and reduced forms of complex 1 with N2O.
In electrochemical cell:
N2O
LCoIICl2·N2O
LCoIICl2
1
+ 2H+
+ 2e
+ 2e
– N2
– H2O
N2O
LCo0Cl2·N2O
LCo0Cl2
20 μA
In the reaction (on norbornene example):
N2O
LCoIICl2
LCoIICl2·N2O
1
– N2
O
+ LCoIICl2
0.5
0.0
E/V
–0.5
Scheme 1
In conclusion, complex 1 is capable of catalyzing norbornene
and Ph3P oxidation by nitrous oxide under ambient conditions.
To our knowledge, it is the first example of such catalyzed
alkene oxidation by N2O.
Figure 2 Cyclic voltammogram of complex 1 in hdab film (GC electrode,
H2O, LiClO4): in the presence of N2O (solid line) or without N2O (dotted
line). Scan rate, 20 mV s–1
.
‡
Electrochemical studies were carried out on a PI-50-1.1 potentiostat
in MeCN or H2O. Glassy-carbon (2 mm in diameter in MeCN; 4 mm in
diameter in H2O) disks polished by Al2O3 (< 10 μm) were used as working
electrodes; a 0.05 M Bu4NClO4 solution in MeCN or 0.1 M LiClO4 solution
in H2O served as the supporting electrolyte; Ag/AgCl/KCl(satur.) was
used as the reference electrode. All measurements were carried out under
argon; the samples were dissolved in the pre-deaerated solvent. N2O was
bubbled through the aqueous solution, and the positive pressure of N2O
was maintained in an electrochemical cell during experiment.
This work was supported by the Russian Foundation for
Basic Research (project no. 07-03-00584).
References
1 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th edn.,
John Wiley, New York, 1980.
2 A. V. Leont’ev, O. A. Fomicheva, M. V. Proskurnina and N. S. Zefirov,
Usp. Khim., 2001, 70, 107 (Russ. Chem. Rev., 2001, 70, 91).
3 B. Ohtani, S. Takamiya, Y. Hirani, M. Sudoh, S. Nishimoto and T. Kagiya,
J. Chem. Soc., Perkin Trans. 2, 1992, 175.
1/hdab film preparation. The solutions of hdab (0.1 mol dm–3, 10 μl)
and 1 (10–4 mol dm–3, 10 μl) in MeCN were cast onto the surface of
basal-plane-oriented GC electrode. The 1/hdab-modified electrode was
allowed to stand overnight in a close vessel and then dried in open air for
at least 12 h, then in the Ar stream for 10 min.
4 J. I. Panov, G. A. Sheveleva, A. S. Kharitonov, V. N. Romannikov and
L. A. Vostrikova, Appl. Catal., A, 1992, 319, 31.
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