Chemistry Reference
In-Depth Information
Fig. 43 (a) Crystal structure the tris-benzimidazolium cage 25
3+
[
84
]; only hydrogen atoms
belonging to C-H fragments of the imidazolium subunits are shown. (b) Top view showing only
the three imidazolium subunits; the
circle
is the centroid of the nearly regular equilateral triangle
obtained by linking the hydrogen atoms of the imidazolium C-H fragments
trications possess three imidazolium subunits that are potentially suitable for tight
binding of an encapsulated fluoride anion. Indeed, crystal structures of derivatives
24
3+
-26
3+
revealed that in all cases the three imidazolium fragments points towards
the cavity [
81
-
84
]. As an example, the structure of derivative 25
3+
is shown in
Fig.
43
[
84
]. The cage is highly symmetric and, most interestingly, the three
imidazolium C-H fragments point inside the cavity, ready to interact with an
included anion, if any. In particular, C-H hydrogen atoms lie at the corners of a
nearly regular equilateral triangle and the distances between these hydrogen atoms
and the centroid of the triangle are: 1.64, 1.77 and 1.84
˚
. These values fall in the
range observed for strong F···H hydrogen bonds in receptor-anion complexes and
allow one to predict effective fluoride inclusion in cage 25
3+
. Indeed, titration
experiments in CH
3
CN (CD
3
CN) solution, using UV-vis (and
1
HNMR)
techniques, indicated the formation of a stable complex of 1:1 stoichiometry
(log
K
7) [
84
]. Addition of Cl
and of any other mono- or polyatomic anion
did not cause any modification of the spectral pattern of the receptor, which
suggests exclusive fluoride inclusion. Formation of a 1:1 F
complex in solution
was also observed with receptor 26b
3+
, an event also corroborated by calorimetric
titration experiments and by ab initio calculations [
85
]. However, crystalline salts
containing the fluoride inclusion complex have not been obtained (yet) for any
of the 24
3+
-26
3+
tris-imidazolium receptors, a disappointing circumstance that
may not diminish the importance of the investigation, but leaves the readership
(viewership) unsatisfied.
This raises a point of some relevance to the story developed in this chapter and,
in general, to the subject of this volume. The beauty of any object, including a
molecule, is related to its shape, to the harmony of its design or to the combination
of novel and unexpected visual features. In the macroscopic world, there exists
a single and neutral instrument, the eye, which provides objective perception and
information that is subsequently elaborated to generate pleasure, surprise, fun,
desire or, in the opposite direction, disgust or revulsion. On the other hand, the
shape of a molecule is perceived as a structural formula, which may be a simple
drawing made with a pencil by the chemist, deduced on the basis of several physical
