Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 144

Fig. 22 Calculated structures (PM3) of the i + i + -[10.10.10] diammonium cage (a) and of its
inclusion complex with chloride; (b) lateral, and (c) longitudinal view

deuterated trifluoroacetic acid and D 2 O, exists as an equilibrium mixture of the out,
out ( o + o + ) isomer, in which the ammonium N-H fragments point outside the cavity
(23%) and of the in,in ( i + i + ) isomer, with N-H fragments pointing inside the cavity
(77%). The interconversion equilibrium is fast. Quite interestingly, addition of
NaCl makes the resonance of

-CH 2 protons shift upfield, which indicates the
occurrence of a fast and reversible anion encapsulation process. The equilibrium
constant K for the inclusion equilibrium was estimated to be

a

10. Similar
behaviour was observed on addition of Br and I , which accounts for the flexible
nature and consequent poor selectivity of the i + i + -[10.10.10] diammonium cage.
No crystal structures have been reported for salts of the i + i + -[10.10.10]
diammonium cage nor of its chloride inclusion complex. The structures in Fig. 22
have been calculated by a semiempirical method (PM3) and may give an idea of the
encapsulation process. Structures suggest that the i + i + -[10.10.10] isomer contracts
its cavity in order to accommodate the anion and to establish collinear hydrogen
bonding interactions N-H···Cl···H-N.
Park and Simmons gave triple-stranded diammonium derivatives of the type
described above the trivial name of “katapinands”, from the ancient Greek
katap

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nein, to swallow, thus imagining them like big fishes quietly swimming in
solution and looking for smaller fishes (anions) to ingest. It is worth noting that the
trivial name katapinand (reserved for anion inclusion [ 48 ]) precedes by 1 year that
of cryptand (metal ion inclusion [ 35 ]), which is much more widely used.
Anion coordination chemistry then grew sleepily for 7 years, until Lehn reported
the macrotricyle 13 [ 49 ], whose formula and calculated structure are shown in
Fig. 23 . According to the orientation of the four sp 3 nitrogen atoms, whether in or
out , the molecule may give rise to five topological forms. The structure in Fig. 23b
refers to the all- in form, in which the four amine nitrogens are located at the corners
of a tetrahedron and the six ethereal oxygens are at the corners of an octahedron.
The tetrahedron is inserted into the octahedron and the centres of the two polyhedra
coincide (Fig. 23c ). In the pertinent article [ 49 ], Lehn noted: “In addition to its
chemical interest, such a ligand is not without aesthetic appeal!” Oxygen and
nitrogen atoms lie on the same sphere, which account for the trivial name of
“spherical cryptand”.

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