Chemistry Reference
In-Depth Information
As mentioned above, one of the most important problems in the molecular design of magnets based
on coordination compounds of transition metals is to provide self-assembly of high-dimensional (layered
polymer or framework) structures, which favors volume magnetization. The bridging organic or inorganic
ligands should additionally perform the function of effective exchange channels between the paramagnetic
centers. The most complex and occasionally insurmountable problem in the design of magnetoactive high-
dimensional (2-D and 3-D) systems based on mononuclear complexes is the lack of coordination sites at
the transition metal ion. Since the molecular design of magnets is mainly carried out based on metal ions
from the first row of transition elements, the maxium possible number of coordination sites is six. The
blocking of coordination sites by monodentate anion ligands, or ligands that are not capable of performing
the bridging function, or simply donor solvent molecules, precludes the construction of extended structures
in one, two, or three directions in space. This disadvantage can be avoided by using an approach based
on the assembly of a high-dimensional structure whose sites contain multinuclear structures, but not the
individual transition metal ions. These multinuclear structures may have the desired or preset number of
terminal metal ions; by modifying the bridging ligands in the multinuclear fragment, a definite spatial
orientation of these ligands can, in prinple, be set, thus directing the self-assembly of a high-dimensional
structure with preset topology as desired.
Before considering the particular examples of the chemical design of multinuclear complexes with
nitroxides, note that thousands of multinuclear compounds of transition metals have already been struc-
turally defined, among which complexes with various carboxylates are most numerous.
20,172 - 176
The
greatest progress in the development of the synthesis of molecular nanomagnets was achieved on metal
carboxylates.
177 - 189
Special attention was paid to the fact that the development of the chemistry and
technology of nanosized clusters would largely depend on progress in the understanding of the principles
governing the synthesis, structure, and stability of these compounds.
176
The results of studies of complex heterospin systems based on multinuclear pivalates of transition metals
with nitroxides are summarized below.
172,190 - 198
Attention is concentrated on the chemical behavior of
the multinuclear fragment.
13.4.1 Reactions whose products retain both the multinuclear fragment and nitroxide
The kinetically stable hexanuclear [Mn
6
(O)
2
Piv
10
(Thf)
4
] contains four terminal manganese(II) ions, which
coordinate the readily replaceable thf molecules, lying at the vertices of a flat tetrahedron.
199
The presence
of coordinated thf molecules is extremely important because NIT-R, which are the products of oxidation of
the corresponding dihydroxy precursors having no functional groups apart from
N-
•
O, are weak donors
and can only replace weaker ligands. Indeed, the interaction of [Mn
6
(O)
2
Piv
10
(Thf)
4
] with L
20
in nonpolar
heptane or carbon tetrachloride led to the replacement of all coordinated thf molecules and self-assembly
of [Mn
6
(O)
2
Piv
10
(L
20
>
)
2
] (Figure 13.3), having a framework diamond-like structure.
190 - 192
In the reaction
of [Mn
6
(O)
2
Piv
10
(Thf)
4
] with L
20
in dichloromethane, only some part of coordinated thf molecules was
replaced, and the [Mn
6
(O)
2
Piv
10
(Thf)
2
(L
20
(L
20
)
Mn
6
(O)
2
Piv
10
(Thf)(CH
2
Cl
2
)
)
] chain polymer was always
isolated as a solid irrespective of the initial [Mn
6
(O)
2
Piv
10
(Thf)
4
]/L
20
ratio (Figure 13.3).
The reaction of [Mn
6
(O)
2
Piv
10
(Thf)
4
] with L
20
in ethyl acetate led to crystallization of dumbbell-
shaped [(EtOAc)
3
Mn
6
(O)
2
Piv
10
(L
20
Mn
6
(O)
2
Piv
10
(EtOAc)
3
] molecules, in which two [Mn
6
(O)
2
Piv
10
]
hexanuclear fragments are linked by one bridging nitroxide molecule (Figure 13.3). Since all heterospin
complexes formed irrespective of the initial [Mn
6
(O)
2
Piv
10
(Thf)
4
]/L
20
ratio, the solvent used for the syn-
thesis obviously plays a critical role in the isolation of a particular product. This was evidenced by
the fact that treatment, for example, of [(EtOAc)
3
Mn
6
(O)
2
Piv
10
(L
20
)
)
Mn
6
(O)
2
Piv
10
(EtOAc)
3
] with addi-
tional L
20
in heptane always led to the framework compound [Mn
6
(O)
2
Piv
10
(L
20
)
2
]. Also note that both
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