Chemistry Reference
In-Depth Information
depended on both the hyperfine splitting parameters and the possibility of varying the temperature of the
complex solution within wide limits. Moreover, it appeared that in Ni(L
26
2
type heterospin complexes,
different paramagnetic centers could induce paramagnetic shifts with different signs. If these peculiarities
of the NMR spectra of metal-nitroxide systems are correctly taken into account, it is possible to determine
the temperature range in which a certain conformation of the heterospin complex is dominant and obtain
a solid molecular magnet of a definite structure in crystallization at a chosen temperature.
237
)
13.6.3 Stabilization of nitroxides with
-hydrogen atoms
As is well known, nitroxides containing an hydrogen atom in the
β
position relative to the nitroxyl group are
mostly kinetically unstable and can be transformed into various derivatives, for example, disproportionation
products (Scheme 13.25).
8,238 - 242
It has been shown
243,244
that nitroxides of this type can be stabilized by complexation with metals
(Scheme 13.26) because the radicals acquired conformations that precluded their destruction. Although
copper(II) compounds can catalyze various oxidative processes, for Cu(hfac)
2
L
56
crystals grown in solution
no changes in the composition and properties were detected after storage for three months under the normal
conditions.
β
13.6.4 Increased reactivity
Nitroxides often behave as oxidants in different chemical processes.
8,245,246
For example, in the presence of
copper(II) salts, di(
tert
-butyl)nitroxide accelerates the oxidation of methanol to formaldehyde with oxygen
and is reduced to hydroxylamine.
247
with spin labeled
thiosemicarbazone HL
58
, the latter was quickly reduced in methanol or acetone to the corresponding
hydroxylamine.
248
In the presence of copper(II) salts in alkaline media, sterically hindered hydroxylamines
are quickly oxidized to the corresponding nitroxides. If oxygen is removed from the reaction mixture, these
redox processes can give complexes of metals with different degrees of oxidation, for example, copper(II)
In the reaction of copper(II) chloride (CuCl
2
)
2
N
O
N
N
+
H
H
OH
O
Scheme 13.25
CF
3
F
3
C
O
F
3
C
H
O
N
O
F
3
C
O
Cu
O
N
M
H
O
O
O
N
F
3
C
F
3
C
O
O
CF
3
CF
3
Cu(hfac)
2
L
56
M(hfac)
2
L
57
M=Mn,Ni,Co
Scheme 13.26
Search WWH ::
Custom Search