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in neutral radical nitroxides, and a longer one involving conducting materi-
als derived from halogenated tetrathiafulvalenes (TTFs). We will particularly
show that halogen bonding is not only compatible with a metallic conduc-
tivity in molecular solids, but also that it is activated in these salts where
it might contribute to the conductivity itself through its involvement in the
conduction band dispersion.
2
Neutral Radical Nitroxides
In the search for molecular magnetic materials, nitroxide free radicals have
been widely used as materials in their own right [16] or in combination
with paramagnetic ions [17]. The first organic ferromagnetic material was
reported in 1991 in these series, namely the
p
-nitrophenyl nitronyl nitrox-
ide [18]. The overall magnetic behaviour of these radical molecules is con-
trolled not only by the distribution of the spin density in the molecule and
the interactions between spins if there is more than one electron, but also by
the intermolecular interactions, through space and/or through non-covalent
bonds. In that respect, hydrogen bonding interactions between nitroxide-
based radical molecules have been extensively investigated as they bring
a strong control over the supramolecular arrangement of the radical species.
Let us mention, for example, the TANOL (
1
) [19, 20] or the 4-hydroxyimino
derivative
3
of TEMPO (
2
) [21] (Scheme 1) where O - H
···
O - Ninter-
actions were unambiguously identified. Similarly, OH
···
O - N [22, 23] or
C - H
O - N [12, 24, 25] interactions were observed in various nitronyl ni-
troxide derivatives. It is therefore highly surprising that halogen bonding
interactions were not used purposely in these extensive series as the N - O
groups, which proved to be good hydrogen bond acceptors, can also be con-
sidered as potential good halogen bond acceptors.
···
Scheme 1
Examples of neutral nitroxide radicals
2.1
Halogenated Neutral Nitroxides
A survey of the CCDC database on halogenated nitroxides revealed only
a few examples of structures showing halogen bonded motifs, which will be
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