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 complexes 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

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.

Figure 4.44 A cucurbituril macro polycycle 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 alcohol 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 CH₂ groups and containing large bowl-like cavities of dimensions 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 adduct in water allow us to identify and quantify host–guest formation. The stability of host–guest assemblies in water can be high, approaching the strength of some metal complexes. Aquated metal ions and other small metal complexes can also act as guests, depending on the ring size. The array and shapes of molecules that can act as ligands for metal ions and even as hosts for small molecules is truly extensive, and the field continues to grow. It is inappropriate to dwell on this diversity here. Rather, the reader is directed to specialist textbooks for a more extensive coverage of this fascinating field. For us, it is sufficient to recognize that the simple ligands which we employ largely at the introductory level are no more than the tip of the iceberg, and a wealth of chemistry remains hidden. Perhaps this isn’t such a bad thing as the field is demanding enough for a student new to its wonders.

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مواضيع ذات صلة

Bedded Down– The rmodynamic Stability

Encapsulation Compounds

Compounds of Polydentate Ligands

Sophisticated Shapes

Isomer Preferences

Constitutional (Structural) Isomerism

Isomerism– Real 3D Effects

Chameleon Complexes

Moulding a Relationship - Ligand Influences

Metallic Genetics– Metal Ion Influences

Higher Coordination Numbers (ML7 to ML9)

Six Coordination (ML6)