Chemistry Reference
In-Depth Information
Fig. 3
Detail of the O
I interaction in the binary system associating the 4-amino-2,2,6,6-
tetramethyl(piperidin-1-yloxyl) radical
7
with
p
TFDIB
···
tance (2.827(9)A) is much shorter than the sum of the van der Waals radii
(3.48A), and comparable to the I
O distances found in halogen bonded
systems with pyridine
N
-oxide (2.75
A
) [34]. The C - I
···
···
Obondisapproxi-
mately linear (175
◦
) and the N - O
I characteristics indicate that the halo-
gen bond forms along the lone pair direction of the oxygen. These two recent
examples demonstrate that halogen bonding is a promising, largely unex-
plored tool for the construction of novel nitroxide assemblies, comparable in
that respect to hydrogen bonding and transition-metal coordination.
The attractive possibility of spin delocalization through the halogen bond
was not specifically addressed for these compounds, because the central car-
bon atom of the ONCNO moiety is known to be a node of the SOMO of
nitronyl nitroxides, hence little delocalization is expected from substitution
at this point. However, the influence of halogen bonding on the - N - O
·
group was investigated recently in solution by EPR spectroscopy [35]. In-
deed, addition of an iodoperfluorocarbon to the TEMPO radical molecule
2
causes a substantial increase of the nitrogen hyperfine splitting,
a
N
,ofthe
nitroxide, a behaviour related to an increase of the spin density on the ni-
trogen atom upon halogen bond formation. Furthermore, the equilibrium
constants (
K
1
) for the formation of the halogen bonded systems were deter-
mined, since, at the timescale of the EPR spectroscopy, the experimental spec-
trum represents the concentration-weighted average of the spectra due to the
free and halogen bonded radicals. The strongest interaction was found with
iodoperfluoroalkanes and the temperature dependence of
K
1
afforded the
corresponding thermodynamic parameters,
···
H
◦
=- 7.0
0.4 kcal mol
-1
and
∆
±
H
◦
value of - 7 kcal mol
-1
compares favourably with strong hydrogen bonded systems.
S
◦
=- 18.1
1.4 kcal mol
-1
K
-1
. Note that this
∆
±
∆
3
Conducting Materials Based on Tetrathiafulvalenes
The involvement of halogen bonding in conducting molecular materials is
essentially based on the use of halogenated TTFs in electrocrystallization ex-
periments with counter ions of Lewis base character prone to act as halogen
bond acceptors. This concept was first successfully introduced by Imakubo
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