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Mn
Cu
Cu
Mn
Mn
O
Mn
L
35
Cu
N
Cu
Cu
N
+
L
35
Cu
Mn
Mn
N
Cu
Mn
O
L
35
Mn
C
u
Cu
Cu
Mn
Mn
Mn
Cu
Cu
Mn
Scheme 13.17
of the anisotropy of magnetic properties
90,91,123,140,141
and spin density distribution.
127,134,142,143
These
single crystals are transparent and very beautiful (Figure 13.2).
One of the most interesting findings of Kahn
et al
. was the synthesis of a multispin compound
(L
35
2H
2
O, where R was the 2-imidazoline nitroxide cation, and L was the
o
-phenylenebis(oxamato) anion.
144,145
The three-dimensional structure of the compound is formed by
a system of interpenetrating and intertwining nets, each of which represents a distorted graphite-like
motif. The manganese(II) ions lie at the vertices of each hexagon of the graphite-like motif, and the
copper(II) ions center the edges (Scheme 13.17). The role of a bridge between manganese(II) and
copper(II) is played by the octadentate
o
-phenylenebis(oxamato) anion. In addition, each oxygen atom
of the nitronyl nitroxide is coordinated to copper(II) ions from different subnets; this leads to the
formation of additional polymer chains in the crystals of the compound, which is ordered as a ferrimagnet
below
T
c
=
)
2
Mn
2
[CuL]
3
(DMSO)
2
·
5 K. Using this approach in the design of framework heterospin systems, the authors
obtained a series of molecular ferromagnets [L
+
]
2
M
2
[CuL]
3
, where L
+
22
.
is 2-(4-N-alkylpyridinium)-
4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl, and M
=
manganese(II), nickel(II), cobalt(II), for which
22-37K.
146
An original strategy in the development of the syntheses of polynitroxides, preferably with
T
c
=
N-
•
O
groups in the
meta
position of the benzene ring (so as to favor intraligand ferromagnetic coupling) such
as L
36
-L
38
(Scheme 13.18),
11,147 - 149
and their complexations with M(hfac)
2
141,150 - 161
allowed Iwamura,
Inoue
et al
. to obtain a series of molecular magnets capable of cooperative magnetic ordering at 3.4 - 46 K.
The 3-D heterospin complex [Mn(hfac)
2
]
3
(L
37
>
)
2
experienced a magnetic phase transition at the highest
temperature, 46 K.
141
In summary, the data presented in this section show that, due to the high kinetic stability of nitroxides,
as well as the unlimited possibilities for their chemical modification and the possibility of changing the
coordination mode of radicals due to selective combination of functional groups, the design of molecular
magnets based on complexes of paramagnetic metal ions with nitroxides is one of the most effective
methods for the preparation of various magnetoactive compounds. The lack of coordination sites at the
metal ion sometimes remains a challenge in constructing high-dimensional structures. Therefore, it would
be reasonable to discuss the synthetic approach to the design of high-dimensional heterospin systems
based on polynuclear compounds of metals with nitroxides, which has been developed over recent years
and removed the limitations on the number of coordination sites.
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