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retains its structure in another type of hexanuclear complex, shaped like a bathtub. If, however, two edge-
sharing nickel(II) ions, one of which lies at a vertex of the “open book” and the other on the binding line
of the “book,” are eliminated from the starting hexanuclear fragment, a tetranuclear fragment fixed in the
structure of [Ni
4
(OH)
3
Piv
5
(HPiv)
4
(L
42
] is obtained. The doubling of this tetranuclear fragment leads to
the formation of a highly symmetric [Ni
8
(OH)
4
(H
2
O)
2
Piv
12
] molecule, which is a new chemical variant
of cubane. The understanding of the topological scheme of mutual transformation of a certain group of
polynuclear fragments makes it possible to control the synthesis of these complex heterospin complexes
with nitroxides, reveal the relationship between their structures, and justifiably divide the polynuclear
fragment into smaller units in describing the magnetic properties of the compound.
193
In constructing a topological scheme of this kind, the greatest difficulty is to isolate the “key” compound
of the topological series, that is, the hexanuclear [Ni
6
(OH)
4
Piv
8
(HPiv)
4
] complex in this case, because
it is most susceptible to various transformations which depend on the conditions of synthesis. These
transformations often hinder the isolation of the “key” compound as a single crystal, which can sometimes
appear (and subsequently vanish with time) as an impurity in addition to the kinetically more stable solid
crystals. The structure solution of the “key” polynuclear compound is vital; otherwise, it is difficult to
construct even a fragment of the topological structural scheme. When this compound is detected, the
preparation of compounds with the new composition and/or new polynuclear or polymer structure will
possibly complement and enrich the single topological scheme, thus reflecting the electronic nature of
the transition element and the types of structure formed with a particular set of ligands. An example of
topological construction for a set of polynuclear carboxylates is given in.
175
)
13.4.3 Transformation of both the polynuclear fragment and the starting nitroxide
Processes that involve the transformation of both the polynuclear matrix and the nitroxide molecule
are most complex from a synthetic viewpoint. The amount of experimental data is yet insufficient
to predict the formation of definite heterospin compounds. Studies of the products of the reaction
of [Co
III
2
Co
II
4
(O)
2
Piv
10
Thf
3
H
2
O], whose polymetallic fragment is almost identical to the manganese
compound [Mn
III
2
Mn
I
4
(O)
2
Piv
10
] discussed in Section 13.3.1, with L
20
and/or its imine analog showed that
redox reactions resulted in rearrangements in both the polynuclear
matrix and the starting
nitroxide.
196
In nitroxide molecules, a methyl substituent in the second position of the 2-imidazoline ring
can be oxidized, which leads to heterospin products containing L
43
-L
47
{
Co
6
(O)
2
Piv
10
}
(Scheme 13.21).
O
H
H
H
N
O
−
N
O
−
N
O
−
O
−
O
−
N
N
O
N
O
O
L
44
L
45
L
43
O
O
O
N
N
H
H
N
N
N
O
O
N
N
N
O
−
O
−
O
−
O
O
L
46
L
47
Scheme 13.21
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