Chemistry Reference
In-Depth Information
13 provides a spheroidal cavity made by six oxygen atoms (like, for instance
2.2.2-crypt) and includes alkali metal ions in pure water. In particular, 13 forms
with K
+
a complex less stable than that of 2.2.2-crypt (log
K
¼
3.4 and 5.4,
respectively), a Rb
+
complex of comparable stability (log
K
¼
4.2 and 4.3), and
a more stable Cs
+
complex (log
K
¼
2).
On the other hand, the spherical cryptand can be turned in a powerful anion
receptor by acidifying the solution. In particular, in an aqueous solution adjusted to
pH
3.4 and
<
1.5 with HNO
3
, the macrotricycle exists in its tetraprotonated form LH
4
4+
,
which, as an
i
+
i
+
i
+
i
+
isomer, offers four highly polarised N-H fragments placed at
the corners of a tetrahedron and pointing inside the cavity, thus favouring the
encapsulation of spherical anions [
50
]. Indeed, in an acidic aqueous solution
13 H
4
4+
¼
forms an inclusion complex with Cl
, with an association constant
log
K
4, at least three orders of magnitude higher than that estimated for
[10.10.10] katapinand under the same conditions. The cascade process from the
spherical cryptand 13 to the chloride complex [13H
4
···Cl]
3+
is illustrated in Fig.
24
.
>
Fig. 23 (a) Lehn's spherical cryptand, 13, and (b) calculated structure (MM+) of 13, in which
each
sp
3
nitrogen atom points its lone pair inside the cavity. (c) The six oxygen atoms are placed at
the corners of an octahedron, and the four nitrogen atoms at the corners of a tetrahedron. The
tetrahedron is inserted into the octahedron. Hydrogen atoms have been omitted for clarity
Fig. 24 Consecutive equilibria for protonation of the spherical cryptand 13 and inclusion of Cl
in the tetraprotonated receptor 13H
4
4+
. In the [13H
4
···Cl]
3+
complex, the anion is tetrahedrally
coordinated by four highly polarised N-H fragments, as shown in the crystal structure. Unavail-
ability of coordinates at the Cambridge Crystallographic Database prevented any redrawing of
such a structure
