Chemistry Reference
In-Depth Information
available, continuously growing collection of data on metal-nitroxide systems allows the sorting out of
both elaborate and new scientific directions based on these compounds. This present review largely reflects
the scientific preferences of the author and presents an attempt to highlight those aspects of the problem
that generally receive less attention from researchers.
13.2 Two types of nitroxide for direct coordination of the metal
to the nitroxyl group
Since the discovery of stable nitroxides, it was quickly realized that, apart from having high kinetic
stability in both the solid state and solution, they were capable of reacting in such a way that their free
valence remained intact. This stimulated studies of coordination compounds where the metal was directly
coordinated to the nitroxyl
N-
•
O group. The direct coordination of the nitroxyl group, which favors
direct exchange, created great interest because the energy of interactions between odd electrons in the
{
>
M-O
•
-N
exchange cluster (M is the paramagnetic metal ion) was maximum in this case. With the
maximum exchange energy, the temperature of the potential magnetic anomaly for the heterospin system
was also expected to be high.
In principle, many variants of M-O
•
-N
<
}
coordination are possible. They can be conventionally divided
into two groups if the synthetic approaches used are correlated with the molecular structure of nitroxide.
Nitroxides can also be divided into two types according to complexation:
<
N-
•
O, but no other functional groups; and
(b) nitroxides containing other donor functional groups along with
(a) nitroxides containing only
>
N-
•
O.
>
13.2.1 Complexes containing only
>
N-
•
O as a coordinating group
The synthesis of complexes in which only the nitroxide can coordinate requires a certain methodological
approach because these compounds are extremely weak donors in accordance with thermodynamic data
for complexes in solution.
21-25
Therefore, the syntheses of complexes with these nitroxides are generally
performed with reagents that are rather strong Lewis acids, for example, anhydrous halides, perchlorates,
hexafluoroacetyl acetonates, or halogenated metal carboxylates.
Studies on the synthesis and magnetochemistry of complexes with metal-nitroxide coordination were
started in 1967, with complexes of formula CoX
2
(L
1
Cl, Br, I). These heterospin complexes were
synthesized using carefully dehydrated reagents and solvents. Dilution of the CoX
2
(L
1
)
2
,(X
=
)
2
solution led to
increased dissociation of the complexes, which was indicative of their low thermodynamic stability. There
was a strong antiferromagnetic interaction between the odd electrons of
N-
•
O groups and Co(II).
26
>
The [CuCl
2
(L
2
] and [PdX(L)
−
]
2
dimer complexes (X
L
1
,L
2
)
=
Cl, Br and L
=
)
were also synthesized in
the absence of moisture (Scheme 13.1). [CuCl
2
(L
2
)
] was diamagnetic because of strong antiferromagnetic
exchange
27
and [PdX(L)
−
]
2
because of the reduction of nitroxide in the course of the reaction and further
2
-coordination of hydroxylamine anions.
28-31
Quantum chemical calculations confirmed that the organic
ligand in palladium(II) complexes was in reduced form.
32
η
The CuBr
2
(L
2
diamagnet, which is rather stable
in the absence of moisture and in which the nitroxide is in the very unusual
)
2
-coordination mode, was
described in.
33
Quantum mechanical calculation for this complex showed that the binding electrons of the
coordination unit were considerably delocalized, and singlet - triplet splitting was at least 5000 cm
−
1
.
32
Considerable (in energy) antiferromagnetic exchange interaction, which generally led to complete cou-
pling between the spins of coordinated nitroxyl groups and metal ions, was also recorded for compounds
η
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