Macrocyclic Ligands - Web del Profesor

16 MACROCYCLIC LIGANDS
O
S
O
N N
Cu
O O
O
N
Ba
Cu
(63)
occurs for the potassium ion, which has a radius of 1.38 ˚A, thus
correlating well with the cavity radius. However, 18-crown-6
forms extremely stable complexes with all of the alkali and
alkaline earth metal ions. Hence, Gokel argues that the data
indicate that the hole size concept is inapplicable, since the
binding constants for sodium, potassium, ammonium, and
calcium ions are the largest for the 18-crown-6 compared to
almost all of the other simple crown ethers. 119 Hancock has
proposed that chelate ring size is the critical factor, and that the
high stabilities observed for the crown ethers with large metal
ions is a result of the presence of five-membered chelate rings.
Thus the high affinity of these macrocycles for the potassium
ion is explained by the fact that potassium is the right size for
the five-membered chelate rings of the crown ethers. 78
O
O
O
(64)
O
O
O
O
O
(65)
O
O
O
N
O
O
S
O
O
O
O
O
(66)
A number of reviews of the structural aspects of crown
ethers can be found. 115–117 These structures vary considerably
in complexity. An example of the flexibility of the crown ethers
can be seen in the variation in the structures as a result of ring
size of three different benzo crowns. When the cavity of the
O
O
O
crown matches the radius of the metal ion, the metal ion can be
readily incorporated in the cavity, such as in the structure of the
rubidium thiocyanate complex with the dibenzo-18-crown-6
(67). In cases where the cavity of the crown is too large to
surround the metal ion snugly, a folded structure can result,
as with the dibenzo-30-crown-10 (68) and the potassium ion.
For very large metal ions incapable of fitting into smaller
macrocyclic cavities, sandwich-type structures can occur, as
in the benzo-15-crown-5 (69) with the potassium ion. 115
O
O
O
O
O
O
(67)
O
O
O O O
O O O
O
(68)
O
O O
(69)
Molecular mechanics studies indicate that the lowest energy
conformer of the uncomplexed ligand is not necessarily
that required for complexation, i.e. oxygen donors may be
exodentate as in the thia macrocycles. This means that in order
for complex formation to occur, the ligand must undergo both
reorganization as well as desolvation. A general rule of thumb
with respect to size, however, is that the larger macrocycles
are more flexible and subject to adaptability, while the smaller
macrocycles are more rigid and, in that sense, ‘preorganized’.
Cram has provided an excellent treatise on preorganization. 118
His principle of preorganization is that ‘the more highly
hosts and guests are organized for binding and low solvation
prior to their complexation, the more stable will be their
complexes.’ 118 �G values for a variety of macrocyclic oxygen
donors indicate that the ‘prearranged’ ligands in general bind
O
O
O