Encapsulation Compounds

Increased sophistication in chemical synthesis has led to the development of a wide range of molecules which can ‘wrap up’ or ‘encapsulate’ metal ions (as well as, in some cases, even whole small molecules). A range of topologies (or three-dimensional shapes) has been de vised for accommodation of either metal ions with different preferred coordination numbers or small molecules. The former usually involve covalent bonds the latter weaker noncovalent interactions (such as hydrogen bonds). Developing functionality for encapsulated

Figure 4.38 Examples of three- four-five- and six-donor macrocycles. Metal ion selection and binding strength is based in part on ion– hole ‘fit’.

or structurally confined systems has been a growing interest, directed towards the develop ment of molecular machines.

Macrocycles are a diverse family of large ring organic molecules (defined as having nine membered or larger rings) characterized by a strong metal binding capacity when several heteroatoms are present in the ring. Several simple examples have been introduced earlier. This class includes the so-called ‘pigments of life’– nature’s aromatic macrocycles built for important biochemical use in plants (the chlorophylls) and animals (the hemes). Visible light is absorbed in these systems by the presence of a sequence of alternating double and single bonds to give characteristic colours. Macro monocyclic (single large ring) molecules including several heteroatoms represent the simplest members of the family of macro cycles. They can bind metal ions and even small molecules reasonably efficiently, depending on size. A range of types, both aliphatic and aromatic are known, as exemplified in Figure 4.38. In a simple sense, what is important in these cyclic systems is the matching of cavity hole size to metal ion size; a good ‘fit’ means a stronger complex. One well-known group of macrocyclic ligands are polyamines. Many may not be capable of ‘wrapping up’ the metal fully through not having sufficient donor groups to satisfy fully its coordination sphere. However, with sufficient donor groups this can be achieved, and in a very large ring even more than one metal ion may be accommodated. Coordination of a saturated polyamine macrocycle however, introduces subtle stereochemical consequences relating to the disposition of amine (R–NH–R) hydrogen atoms on complexation. This is illustrated for a tetraamine bound to a square-planar metal ion in Figure 4.39; different amine proton dispositions possible are highlighted. At a simple level we can see this as ‘four up’, ‘three up and one down’, and two types of ‘two up and two down’; other options (such as ‘four down’ compared with ‘four up’) are

Figure 4.39Possible dispositions of the secondary amine hydrogen atoms in a coordinated macrocyclic tetraamine.

equivalent to one of those shown, as they are equated simply by inverting the macrocycle. In effect, these isomers equate to a different chirality set for nitrogen donors, fixed upon coordination; each secondary amine RRHN group, when coordinated, forms RRHN M, and then, with four nonequivalent groups covalently bound around the tetrahedral nitrogen, the N centre is chiral. They exist in addition to any other sources of isomerism and chirality in the molecule. Al though the N-based isomers may have slightly different physical properties, interconversion between these isomers can be readily achieved by raising the solution pH, which promotes N-deprotonation and exchange leading to formation of the thermodynamically most stable N-based isomer. Usually, this subtle N-based isomerism tends to be ignored, as it is a level of complication too far for most. Macrocycles carrying pendant groups also capable of binding metal ions produce the opportunity to ‘wrap up’ metal ions better (Figure 4.40). These ‘molecular wrappers’ have pendant groups that can comeonor off so they behave as‘hingedlids’. The pendant groups may be of any type, and carry any form of potential binding group– amine, carboxylic acid, thiol, alcohol, pyridine and others. These groups may themselves be further elaborated

Figure Simple macrocycles (a) may be augmented with pendant group(s) attached to either a heteroatom (b) or a carbon atom (c) of the ring. Those with two pendant groups (d) offer better opportunities for ‘wrapping up’ octahedral metal ions as the two pendant arms can supply additional donors that, by being linked to the ring enhance entrapment by ‘capping’ the metal as shown in (e).

or extended using standard organic reactions, including forming dimers linking them to biomolecules, or binding them to surfaces.

Macropolycycles Macropolycyclic molecules (with several large organic rings fused together) that include several heteroatoms can bind metal ions efficiently, and a range of types are known. One simple family is the sarcophagines, bicyclic (two fused large ring) amine molecules with the capacity to ‘cage’ metal ions. They have a cavity which offers six donors, usually saturated nitrogen groups, to metal ions. This cavity is too small for anything but single atoms or ions; the metal ion ‘fit’ in the cavity is excellent for small metal ions, less efficient for large metal ions. As their name implies, they are molecular ‘graves’– once a metal ion is interred in the cavity it is trapped, and can only be removed with difficulty (usually requiring very strong acid or cyanide solution). Metal ions are effectively in a prison with the frame of the macrobicycle as the bars. A typical example is shown in Figure 4.41.

Cryptands are cyclic (mainly) polyether molecules with usually three chains linked at nitrogen ‘caps’ at each end of the molecule (Figure 4.42), much like the sarcophagines but with a different capping atom and different donors. They can, depending on host cavity size, bind metal ions (alkali or alkaline earth ions preferred) or small molecules. A wide range of molecules of this family have been prepared. They can be effective in metal ion selection from a group of ions, useful in both analysis and separation of mixtures. They also help solubilize metal ions in aprotic solvents.

Figure 4.41 A simple macrobicycle can be considered to arise by addition of another chain or ‘strap’ of atoms to a macro monocycle to provide three chains joined at two capping carbon atoms (top), effectively two fused macro monocycles. This example offers six donor groups to a metal ion, as exemplified with ball and stick and space-filling models; the latter shows the small central cavity able to accommodate only a single cation. These readily form very stable octahedral complexes with a range of metal ions; the best-known example has L = NH (called ‘sarcophagine’).

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.) 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.

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مواضيع ذات صلة

Bedded Down– The rmodynamic Stability

Host–Guest Molecular Assemblies

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)