Chemistry Reference
In-Depth Information
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Figure 4.44
A cucurbituril macropolycycle composed of six units, and a side-on view of the cucurbituril acting
as a host to an organic guest cation.
coordinate covalent bonds. However, in certain cases, the shape and outcome is reminiscent
of coordination chemistry, and it may be of value to exemplify it here in passing.
Host-guest interactions
occur where a cavity in one molecule (the host) permits selected
entry of another molecule (the guest) into the cavity space; it is, like the macrocycles with
metal ions, a case of 'best-fit', although here it is a whole molecule that is fitted into the
cavity. The host-guest terminology is usually applied to molecule-molecule interactions,
not to metal ion binding, however. The assembly which forms in solution (with measurable
stability, as a result of spectroscopic changes that occur when the guest occupies the different
environment of the host), is held together by noncovalent bonding forces - hydrogen
bonding and other weaker interactions. The 'container' molecules (hosts) are now many
and varied, but all have the common character of acting as hosts for guest molecules or
molecular ions.
Cyclodextrins, calixarenes and cucurbiturils are well established examples of host
molecules.
Cyclodextrins
are large-ring molecules made of linked sugar units. They are
made with typically at least four units in the ring, and are rich in polar ether and alco-
hol groups. There are several related structural types that differ depending on the class
of monomer units and linkages adopted in macrocyclic ring formation.
Calixarenes
(from
the Latin calix
bowl), are large cyclic molecules made up of typically four to eight
phenol units linked via CH
2
groups and containing large bowl-like cavities of dimen-
sions appropriate for sequestering a variety of small molecules, which is of great interest
for purification, chromatography, storage, and slow release of drugs
in vivo
. Hydrogen
bonding involving alcohol groups plays a crucial role in the supramolecular chemistry of
such assemblies.
Cucurbiturils
are a family of cyclic host molecules, with their common
characteristic features being a hydrophobic cavity, and polar carbonyl groups surrounding
the two open ends (Figure 4.44). Better defined as cucurbit[
n
]urils (where
n
=
the number
of monomer units in the ring), they can be water-soluble, depending on substituent groups.
For these different classes of hosts, the capacity to control the size of the rings and the
introduction of various functional groups make it possible to 'tailor' them for a variety of
chemical applications. Their sequestering properties can be exceptional, while appropriate
substitution renders the cavities shape-selective and thus suitable for molecular recognition.
Changes in a range of spectroscopic properties with increasing formation of the host-guest
=
