Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 140

Fig. 16 Structures of 2.2.2-crypt, 9,(a)[ 36 ] and of its potassium complex (b)[ 37 ], as obtained
from X-ray diffraction studies on the crystalline compounds. Hydrogen atoms have been omitted
for clarity
outside with properly oriented oxygen atoms (to the detriment of the complex
stability).
In metal polyamine chemistry, the evolution from macrocycles to cages took
decades. In polyether coordination chemistry, such progress took only 2 years.
In 1969, Jean-Marie Lehn, a young professor at the Universit´ Louis Pasteur,
Strasbourg, with students Bernard Dietrich and Jean-Pierre Sauvage, reported the
synthesis of a family of tridimensional analogues of crown ethers, which were given
the trivial name of “cryptands” (making reference to the secret Chapel of a
Cathedral, where the most precious objects of the worship are kept, hopefully
including the remains of the patron saint) [ 35 ].
Structural features of a classical cryptand are shown in Fig. 16 . The cryptand
contains six ethereal oxygen atoms, plus two tertiary amine nitrogen atoms that act
as pivots. Indeed, the two nitrogen atoms do not play a mere structural role, but are
usually involved in the coordination of the included metal. The name of the
cryptand in Fig. 16 has been abbreviated to 2.2.2-crypt, where the numbers indicate
the number of ethereal oxygen atoms contained in each one of the three
polyoxoethylene chains. Figure 16 shows the structure of 2.2.2-crypt, as it is in
the crystalline state, showing an ellipsoidal cavity [ 36 ]. On inclusion of the K + ion
(Fig. 16b ), the cryptand rearranges to provide the spherical metal with a spheroidal
cavity [ 37 ]. In the cryptate complex, the metal establishes coordinative interactions
with the six oxygen atoms and with the two nitrogen atoms.
Cryptands display enhanced size selectivity with respect to alkali metal ions.
Figure 17 refers to 2.2.2-crypt, which shows peak selectivity for K + , in a 95/5
MeOH/water mixture (circles in Fig. 17 )[ 38 ]. This effect is much more pronounced
than that exerted by 18-crown-6 in pure MeOH (triangles in Fig. 17 ), a behaviour
which may reflect the higher degree of steric restraint granted by a cage compared
to a ring.
Knowledge of structural details may help the interpretation of the peak selectivity
exerted by 2.2.2-crypt. Figure 18 shows the structures of the alkali metal complexes
of 2.2.2-crypt, obtained from X-ray diffraction studies on crystalline salts [ 39 - 42 ].
First, it is clear that the low stability of the Li + complex depends upon the
circumstance that the smallest alkali metal ion is lost in the cavity and misses the
interaction with an oxygen atom and with one tertiary amine group. The other