Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 157

0.2 ˚ ), whereas the other N-H fragment points outside of
the cavity (average distance 3.0

(average distance 2.0

0.5 ˚ ). Looking at the triangles obtained by
linking the anion-bound hydrogen atoms of each tren subunit, it appears that the
fluoride ion profits from an almost regular trigonal prismatic coordination. 18H 6 6+
emerges as an excellent receptor for fluoride recognition in water. However, it is
selective, but not specific. In fact, 18H 6 6+ also forms a 1:1 complex with chloride
under the same conditions, with an association constant that is seven orders of
magnitude lower than fluoride [ 77 ]. The crystal structure (Fig. 39d ) shows that Cl
receives six H-bonds from the six secondary ammonium groups, like F [ 79 ].
However, the coordination geometry is not exactly the same, but it is midway
between the trigonal prism and the octahedron (Fig. 39f ). Thus, the cage frame-
work, due to the presence of the -CH 2 CH 2 - spacers, is flexible enough to accom-
modate two anions of distinctly different size ( r F ¼

1.81 ˚ )
and does not exert size exclusion selectivity in favour of F . The much higher
stability of the fluoride complex may result from the higher basicity of F and
from its tendency to establish stronger H-bond interactions than Cl . Bromide
( r Br ¼

1.33 ˚ , r Cl ¼

1.96 ˚ ) is too large for encapsulation by 18H 6 6+ . In the isolated crystalline
complex salt [18H 6 ]Br 6 ·H 2 O, the cavity is empty and all the six bromide ions lie
outside [ 78 ].
On reaction of 18 with triethylorthoformate at 120 C in dry xylene, a white solid
can be obtained, which corresponds to the tris-imidazolidinium cage 23 3+ , shown in
Fig. 40 [ 80 ]. The trication 23 3+ contains three imidazolidinium subunits. The C-H
fragment of imidazolidinium is highly polarised and can behave as an effective
H-bond donor for anions. However, the crystal structure of the [23](ClO 4 ) 3 ·H 2 O salt
(shown in Fig. 40 ) indicated that the three C-H fragments do not point towards the
cavity, but outside, a circumstance unfavourable to anion encapsulation. The
interaction of 23 3+ with fluoride was investigated through 1 H NMR titration of a
D 2 O solution of the tris-imidazolidinium cage, adjusted to pD

¼

1, with NaF. The
formation of a 1:1 complex was ascertained, and an extremely high association
constant of log K

¼ 12.5 was measured (such a high K value could be determined

Fig. 40 (a) Crystal structure of the tris-imidazolidinium cage 23 3+ [ 80 ]. Only C-H fragments of
the imidazolidinium subunits are shown. (b) Top view of the trication indicates that such C-H
fragments do not point towards the cavity