REACTIVE LIGANDS IN RADICAL PROCESSES CATALYZED BY COPPER COMPLEXES
87
Initial rate, mol l−1 h−
0.040
The set of experimental data allowed us to propose
the following reaction scheme for mercaptan oxidaꢀ
tion with DMSO in the presence of Cu(II):
1
Cu2+ + RSH
Cu+ + 0.5RSSR + H+;
2Cu2+ + (CH3)2S + H2O.
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0
2Cu+ + (CH3)2SO + 2H+
Note that this scheme satisfactorily describes the
observed stoichiometry of the interaction of the copꢀ
per complex with DMSO (2 mol of mercaptan per
mole of DMSO).
Published data indicate that Cu2+ complexes oxiꢀ
dize dimethyl sulfide to DMSO in accordance with
the following reaction scheme [15, 16]:
+•
Me2S + Cu2+
[Me2S⋅⋅⋅Cu2+]
2+
[Me2S⋅⋅⋅Cu+],
+•
[Me2S⋅⋅⋅Cu+] + O2
+Me S
[Me2S–OO–⋅⋅⋅Cu+]
0
20
40
60
80
100
µ
Volume of added DMSO,
l
2Me2SO + Cu2+
.
2
Fig. 4. Dependence of the initial rate of mercaptan oxidaꢀ
tion in the presence of the complex of copper with DMSO
on the volume of added DMSO. The initial concentration
Summarizing the above equations, we can conꢀ
struct a general reaction scheme for mercaptan oxidaꢀ
tion with oxygen in the presence of the test copper
complex. In the presence of DMSO, the consecutive
reduction of copper ions with thiol occurred followed
by metal reoxidation to a bivalent state by sulfoxide
and the reduction of this latter to dimethyl sulfide. The
catalytic effect can be explained by the partial reoxidaꢀ
tion of dimethyl sulfide to DMSO by atmospheric
oxygen with the participation of copper ions. Thus, a
catalytic cycle is produced, the lifetime of which is
restricted by the low efficiency of dimethyl sulfide oxiꢀ
dation to DMSO. As a consequence, the complexes of
copper with DMSO exhibit low stability (the reaction
TON is no higher than 23).
–3
of the complex was 2
× 10 mol/l, and the thiol concenꢀ
–2
tration was 5
was 25°С
× 10 mol/l. The reaction temperature
.
a practically important problem of the removal of merꢀ
captans from hydrocarbon materials in a new way. The
results of the study of the combined effect of reactive
ligands and metal complexes based on these ligands
made it possible to develop new active catalytic sysꢀ
tems for the oxidation of organic sulfur compounds
under mild conditions [14, 17].
ACKNOWLEDGMENTS
Note that the addition of an excess of a ligand had
different effects on the catalytic properties of the test
systems. In this case, the activity and stability of sysꢀ
tems containing benzylamine increased, whereas an
opposite behavior was observed in systems containing
DMSO (Fig. 4). The inhibiting effect of an excess of
DMSO was due to the heterogenization of the system.
The solubility of DMSO in isooctane is low; therefore,
the sulfoxide is separated as an individual phase if it
occurs in an excess. The reaction rate considerably
decreases because the metal complex mainly occurs in
the phase of DMSO, as can be observed visually based
on the appearance of a characteristic color, and the
mercaptan substrate occurs in the phase of isooctane.
The activity of the system can be increased by homogꢀ
enization; for this purpose, aqueous alcohol additives
are used [14].
Thus, in the test radical reactions catalyzed by copꢀ
per complexes, reactive ligands undergo various transꢀ
formations, which considerably affect the reaction
rate and mechanism. In spite of generally accepted
concepts, these transformations are not always undeꢀ
sirable, resulting in catalyst decay. The above examples
indicate that, in a number of cases, they are necessary
components of a catalytic cycle. This allows us to solve
This study was supported by the Russian Foundaꢀ
tion for Basic Research (project no. 09ꢀ03ꢀ00128).
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KINETICS AND CATALYSIS Vol. 52
No. 1
2011