Chemistry Reference
In-Depth Information
O
N
NO
2
N
O
Scheme 5.20
p-Nitrophenyl nitronyl nitroxide
Tamura
et al
. reported the existence of ferromagnetic intermolecular interaction in the crystal structure
(
phase) of
p
-nitrophenyl nitronyl nitroxide (Scheme 5.20), with a transition temperature to a ferromag-
netically ordered state of 0.65 K.
89
Since then, different strategies have been applied to design single molecules or organic free radicals
based metallic complexes to get ferromagnetic interactions at the highest temperature possible. However,
the best result ever achieved was to observe ferromagnetic properties at 35K with a sulfur - nitrogen
radical.
90
Much work has been devoted to the pursuit of organic magnetic materials, and relevant reviews
are available.
27d,49b,91 - 94
Herein, some NNOs which are important building blocks for the design of
magnetic materials are mentioned.
Nitronyl nitroxides (NNOs), and to a lesser extent imino nitroxides (INOs), represent the most promising
compounds to be used as bridging ligands for the construction of metal ion-based organic magnets. It is
the approach chosen by Kahn, Caneschi, Rey, and Sessoli who pioneered in this field using nitronyl
nitroxide radicals to bind transition metal ions (M(hfac)
2
) (Scheme 5.21). They reported the magnetic
behavior of one-dimensional chain metal - nitronyl nitroxide complexes showing the efficiency of such
coordinating system.
95 - 101
The possibility of modeling the architecture of the whole molecule by using
different substituents (R) can modify the arrangement of the molecules in the complex and influence the
ferromagnetic properties.
To observe magnetization at high temperature will require increasing the dimensionality of the
metal - nitroxide network. Two possibilities are currently developed: (1) increase the number of
coordination sites on the nitronyl nitroxide radical by choosing the right substituents and (2) increase the
number of organic free radicals around the metal. In order to add coordination sites on the NNO radicals,
different groups developed the synthesis of multidentate NNOs (Figure 5.4 shows examples) and the
metal - free radical complexes obtained were crystallized.
27,102 - 109
To increase the number of radicals surrounding the transition metal ion, several groups developed the
synthesis of bis-nitronyl nitroxides-based tetradentate ligands, such as 2,2'-bipyridine and phenanthroline
N
-oxide
N
-oxyl biradicals used with copper(II) and nickel(II) (Scheme 5.22, Figure 5.4).
110 - 112
Some
other examples of stable chelate multiradicals used as building blocks for the preparation of molecule
based magnets are reported in Figure 5.4.
113 - 115
Novoa
et al
. reported recently the analysis of 47 different crystal structures of
β
-nitronyl nitroxide
radicals and the influence of the relative disposition of the radical centers in the crystals on the magnetic
properties.
116
α
Such theoretical study coupled with experimental data should allow the design of more
F
3
C
CF
3
R
O
O
O
O
N
N
M
N
N
O
O
O
O
R
F
3
C
CF
3
Scheme 5.21
Structure of nitronyl nitroxide radicals-M(hfac)
2
complexes
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