Halogen Bonding in Conducting or Magnetic MolecularMaterials - Halogen Bonding: Fundamentals and Applications - page 191

3.3
Cation Radical Salts of Halogenated TTFs

As mentioned above, (EDT-TTF-I) 2 Br was the first, beautiful example of halo-
gen bonding in TTF salts (Scheme 4) [36]. It possesses most of the character-
istics of the other salts described so far, that is, a linear C - Hal

Hal - inter- ···

Hal distance, found at 3.213 A and to be compared
with the corresponding type II D anis value(Table1)forC- I

action with short Hal ···

Br interaction
(3.60 A ). The other salt described by the same authors (Scheme 4) involved
the linear [Ag(CN) 2 ] - , affording upon electrocrystallization of EDT-TTF-I
a similar 2 : 1 salt where now each nitrile of the [Ag(CN) 2 ] - anion is halogen
bonded to the iodine atom of the partially oxidized EDT-TTF-I molecule [36].
AlinearC

···

Idistanceat2.88 A ,to
be compared with the corresponding type II D anis value (3.36 A ). The remark-
ably short halogen interactions found in these two examples demonstrate
that, upon oxidation of the TTF derivative to the radical cation state, the
positive charge density in the polar region of the iodine atom is enhanced,
for a stronger interaction with the negatively charged counter ion [15]. The
other examples which have been described since can thus be organized into
two such series, one involving halide, polyhalide or polyhalometallate anions
through C - Hal

≡ N ··· I interaction was observed with N ···

Hal interactions, the other involving polycyanometallate
or organic nitrile anions through C - Hal

···

··· N ≡ C interactions. They will be

presented successively in the following sections.

3.3.1
Hal ··· Hal anion Interactions

3.3.1.1
Halide Anions as Halogen Bond Acceptors

The (EDT-TTF-I) 2 Br salt described above [36] and the 1 : 1 (TTFI 4 )Isaltre-
ported by Gompper [51] were the only structurally characterized salts with
simple halide anions until Imakubo recently described an extensive series of
Cl - and Br - salts from several ortho -diiodo tetrathiafulvalene, tetraselena-
fulvalene and dithiadiselenafulvalene derivatives (Scheme 8) [62]. The X-ray
crystal structure analysis of the nine salts described there show a variety of
halogen bonded motifs, demonstrating the adaptability of the supramolecu-
lar interactions to other structural requirements imposed by the nature of the
heteroatoms (O, S, Se) in the TTF frame. Indeed, in (EDT-TTF-I 2 ) 2 X

(H 2 O) 2
(X = Cl, Br), a bimolecular motif (Fig. 6) associates two partially oxidized
EDT-TTF-I 2 molecules with one Br - anion and one water molecule.
Note that the acute (C -)I

·

I(- C) angle observed here (85.35(2) ◦ )
has already been observed in similar situations where one Br - or Cl - an-
ion is bonded to two activated iodine atoms, as reported, for example,

Br - ··· ···