Chemistry Reference
In-Depth Information
cryptand-422
triethanolamine
cryptand-111
cryptand-211
cryptand-222
Figure 4.42
The simple noncyclic analogue triethanolamine, and macrobicyclic cryptands, which have three
chains joined at two capping nitrogen atoms. The cryptand trivial names reflect the number of O-
atoms in each linking chain. (Cryptand-222 is shown as a model of the K
+
complex, based on X-ray
structural data; all six O atoms bind to the cation.)
4.5.2.4
Catenanes
A particularly unusual class of macrocyclic molecules are the
catenanes
, where, instead of
two side-by-side fused rings as in the cryptands above, two separate and interlinked rings
each offer donors to a single metal ion (Figure 4.43). With appropriate components in the
ring, a change in oxidation state of the metal ion can lead to a ring rotating into a different
position, a process that can be reversed if the metal ion oxidation state is turned back to
its original. This process of switching position may provide an electrochemically-driven
molecular 'switch'. Synthesis of these interlinked systems is not simple, so their potential
application may be limited by this aspect.
4.5.3
Host-Guest Molecular Assemblies
A close relative of simple coordination chemistry is
supramolecular chemistry
. Whereas
coordinate covalent bonds link the ligand host and the metal guest in coordination com-
plexes, in supramolecular compounds the guest is a whole molecule, not a metal ion, and
as such can only involve itself in weaker nonbonding interactions with the guest, typically
involving the most potent of these forces, hydrogen bonding. Although hydrogen bonds are
relatively strong compared with other nonbonding effects, they are still much weaker that
+ e
-
- e
-
Figure 4.43
A schematic representation of a catenane complex with donors from each of two interlinked rings
binding to a central metal ion, and (at right) the process by which a system with differing ring
components may operate as an electrochemically-driven molecular switch, involving ring rotation.
