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KLIMOV et al.
In this study we examined the reactions of metallic
copper with various o-quinones: tetrachloro-1,2-
benzoquinone, 3,5-di-tert-butyl-4-R-1,2-benzoquino-
nes (R = Н, thiocyanato, tricyanovinyl). The reactions
were performed in THF or DMF. To stabilize the
copper cation, we additionally introduced acetyl-
acetone and pyridine.
In the absence of stabilizing ligands, quinones Q
and Cl–Q at room temperature react with copper
slowly. Therefore, the reactions with them were
performed at 60°С. In the absence of oxygen, the
reactions yield Cu(II) bis-o-semiquinolate complexes.
In air, the complexes are readily oxidized. Acetyl-
acetone reacts similarly with the formation of copper
bisacetylacetonate:
τ, min
ESR signal as a function of time τ. Solvent THF, 60°С. (R)
Concentration of paramagnetic species. Systems: (1) tetra-
chloro-1,2-benzoquinone–pyridine and (2) 3,5-di-tert-butyl-
1,2-benzoquinone–acetylacetone.
2Q + Cu → (SQ)2Cu,
yields copper powder, pyridine, and a mixture of the
quinone and catechol.
2АcАc + Cu → (АcАc)2Cu,
where SQ is the anion of the corresponding o-
Thermal decomposition of copper acetylacetonate
under various conditions yields acetylacetone and
copper.
semiquinone.
When the reactions of o-quinones in the presence of
acetylacetone and pyridine are performed in DMF and
THF, the copper dissolution rate increases. The highest
copper dissolution rates and the most unequivocal
reaction pathway were observed in the systems 3,5-di-
tert-butyl-1,2-benzoquinone–acetylacetone–THF and
tetrachloro-1,2-benzoquinone–pyridine–THF. The results
of measuring the intensity of ESR signals of the
paramagnetic Cu(ІІ) ion are shown in the figure.
Metals having a positive redox potential can be
oxidized in nonaqueous media with a suitable oxidant
in the presence of complexing agents that stabilize the
metal cation and decrease the redox potential of the
process. Therefore, it seems promising to use sterically
hindered o-quinones that can act simultaneously as
oxidants and complexing agents.
The activating complexation concept [4] allows
prediction of such processes. It well combines with the
concept of thermal electronic excitation as basic
physicochemical process [5]. The essence of these
concepts is mutual modification of the donor and
acceptor due to electron density redistribution in the
donor–acceptor complex formed, which leads to
stabilization of the complex. The electron transfer in
such systems is favored by formation, along with a
donor–acceptor bond, of an additional bridging bond
between the donor and acceptor via atom or molecule Х.
Analysis of the dependences obtained shows that
the reactions of formation of the copper complexes
follow the second-order law. The experimentally
calculated ratio of the mean rate constants of copper
dissolutions in the systems Q–АcАc and (Cl–Q)–Рy
gives the value of 1 : 10.
The reactions of oxidative addition of o-quinones to
copper are primarily determined by the oxidation
potentials of o-quinones. The oxidation potential of
3,5-di-tert-butyl-1,2-benzoquinone is considerably
lower than that of tetrachloro-1,2-benzoquinone.
Acetylacetone (dicarbonyl compound) occupies an
intermediate position [7].
Direct oxidation of copper in organic media in the
presence of o-quinones has been reported in the
literature, but these processes are complex, yield a
mixture of products, and are possible only in the
absence of oxygen. In such reactions, the formation of
products is strongly influenced by the choice of the
stabilizing ligand. Various compounds can be used as
stabilizing ligands, e.g., alkenes, 2,2'-bipyridine,
phosphines, and о-phenanthroline [6].
Generally, the processes of formation of copper
complexonates can be presented as follows:
Q + Cu + 2AcAc →← [Q···Cu···2AcAc]*
→ QH2 + (AcAc)2Cu,
Cl–Q + Cu + 2Py →← [Cl–Q···Cu···2Py]*
→ (Cl–Q)Cu·2Py.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 9 2010
Klimov
