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and Kato in 1995 [36] as they described the 2 : 1 cation radical salts of EDT-
TTF-I with Br
-
and [Ag(CN)
2
]
-
(Scheme 4). In these two salts, each anion is
“coordinated” by two partially oxidized EDT-TTF-I molecules.
Scheme 4
Typical halogen bonded motifs involving a partially oxidized TTF molecule
such as EDT-TTF-I with halide (
top
)orcyanometallate(
bottom
)counterions
This original report prompted the preparation of numerous salts based
on halogenated TTFs. Reviews on this topic have recently appeared [15, 37],
showing the generality of this concept and even the activation of halogen
bonding in these salts. In the following, we will therefore recall the essen-
tial conclusions that were reached from these compilations and then highlight
the most recent developments. Indeed, following a first period where halo-
gen bonding interactions were shown to be able to coexist with the overlap
interactions of open-shell molecules to afford conducting organic salts, ef-
forts have been pursued to demonstrate that the halogen bonding interaction
itself could carry some “information” by eventually (1) contributing to the
band dispersion of the organic stacks, (2) transmitting magnetic interactions
if both halogen bond donor and acceptor are themselves paramagnetic, or
(3) allowing for the formation of porous materials.
3.1
An Introduction to Conducting TTF Salts
Since the discovery of the first organic conductors based on TTF, [TTF]Cl in
1972 [38] and TTF - TCNQ in 1973 [39], TTF has been the elementary build-
ing block of hundreds of conducting salts [40]: (1) charge-transfer salts if an
electron acceptor such as TCNQ is used, and (2) cation radical salts when an
“innocent” anion is introduced by electrocrystallization [41]. In both cases,
a mixed-valence state of the TTF is required to allow for a metallic conductiv-
ity (Scheme 5), as the fully oxidized salts of TTF
+
cation radicals most often
either behave as Mott insulators (weakly interacting spins) or associate into
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