Chemistry Reference
In-Depth Information
Fig. 14 Log
K
values
for the equilibrium:
M
þ
þ
L
⇄
½
ML
þ
in MeOH,
at 25
C(M
¼
alkali metal;
L
¼
18-crown-6, 8)[
32
]
Fig. 15 Position of alkali metal ions with respect to the six oxygen atoms of 18-crown-6, 8, which
are linked by segments, in the corresponding complexes. Values in
˚
give the distance between the
metal and the mean least-squares plane of the six oxygen atoms of the ligand. These values are
related to the stability of the complexes in solution: the lower the distance, the higher the
complexation constant [
31
,
33
,
34
]
radius, e.g. alkali metals, offered the opportunity to evaluate geometrical effects on
the thermodynamic stability of the complexes and to introduce the concept of size
selectivity. This is illustrated, for instance, by the diagram in Fig.
14
, in which
log
K
values of the complexation equilibria of 18-crown-6 in MeOH, at 25
C, have
been plotted against the ionic radius of the alkali metal [
32
]. A sharp peak
selectivity in favour of K
+
is observed, which apparently has the right radius to fit
the cavity of the crown ether relaxed to its more stable conformation. Figure
15
shows the position of each alkali metal ion with respect to the six oxygen atoms of
18-crown-6, which have been linked together by segments, as observed from X-ray
diffraction studies on crystalline complexes. The distance of the ion from the least-
squares mean plane of the six oxygen atoms is also reported in the figure. K
+
is
perfectly coplanar with the O
6
mean plane (distance 0.00
˚
)[
31
]. Rb
+
(Jaenschke,
Wilde and Olbrich, private communication; CCDC refcode: XESWIB) and Cs
+
[
33
]
are too big and must stay above the plane, at 0.98 and 1.44
˚
, respectively, thus
profiting from less intense metal-ligand interactions. On the other hand, the small
Na
+
ion [
34
] would be lost if fully inserted in the cavity and prefers to interact from
