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by competition with the Zr
IV
cation present in excess, forming Zr
IV
F
3+
), which
suggested anion encapsulation. X-ray diffraction studies on the salt isolated under
the same conditions, [23H
2
···F](ClO
4
)
2
(BF
4
)
3
·H
2
O, disclosed the formation of
a fluoride inclusion complex, whose structure is shown in Fig.
41
[
80
]. The fluoride
ion does not interact with the imidazolidinium N-H fragments, which still point
outside of the cavity, but it receives two H-bonds from the protonated tertiary amine
nitrogen atoms, exhibiting a
in
configuration (F···H distances: 1.68 and 1.74
˚
).
Fluoride binding does not seem to induce any serious rearrangement of the receptor,
as judged from the distance between the two pivot nitrogen atoms, 5.23
˚
, only
slightly smaller than that observed in the uncomplexed unprotonated system 23
3+
(5.43
˚
). Very interestingly, titration with Cl
under the same conditions (pD
1)
did not induce any modification in the
1
H NMR spectrum of the receptor. Thus,
23H
2
5+
exerts specific recognition of F
. Notice also that 23H
2
5+
rightfully belongs
to the ancient class of katapinands. The short and rigid spacers linking the tertiary
ammonium groups (in their
i
+
i
+
configuration) afford size exclusion selectivity in
favour of the smallest anion.
However, there exists another class of even smaller cages that can afford
exclusive fluoride encapsulation, whose formulae are shown in Fig.
42
. These
¼
Fig. 41 (a) Crystal structure of the fluoride inclusion complex of the diprotonated tris-
imidazolidinium cage, 23H
2
5+
[
80
]. Only C-H fragments of the imidazolidinium subunits and
N-H fragments of the tertiary ammonium groups are shown. F
receives two H-bonds from the
two tertiary ammonium nitrogen atoms. (b) Top view of the pentacation indicates that
imidazolidinium C-H fragments do not point towards the cavity and are not involved in anion
coordination
Fig. 42 Family of tris-imidazolium receptors suitable for exclusive encapsulation of F
