Macrocycles and affinity of metal ions towards macrocycles---Assignment-Inorganic Chemistry--

 

MACROCYCLES AND AFFINITY OF METAL IONS

 TOWARDS MACROCYCLES

A macrocycle is defined as a cyclic compound with nine or more members including at least three donor atoms. Macrocycles are special types of polydentate ligands in which the ligating atoms are constrained in a large ring encircling the metal atom.

Macrocycles are important and powerful ligands, ubiquitous in transition metal coordination chemistry for the following reasons:

 1) They mimic important biological ligands developed long ago by nature, for example the porphyrin prosthetic group of many metalloproteins.

 2) They impart thermodynamic and kinetic stabilities to their metal complexes uncommon or non-existent with ligands of lessor types.

Macrocycles can be classified as,

a.      Polyaza Macrocycles

b.      Polythia, Polyphospha, and Polyarsa Macrocycles

c       Mixed Donor Macrocycles

d.      Polyoxa Macrocycles

Examples: Polyethers, in polyethers the ether oxygen atoms separated by two methylene groups each lie in a nearly planar arrangement about the central metal atom and the remainder of the molecule lies in a crown arrangement, so the name is crown ether. These macrocycles have the property of forming stable complexes with alkali metals.

In the crown ethers the interactions between the ligand and metal ion are considered to be more electrostatic in nature, than the covalent binding observed for the transition metal complexes of the aza, thia, and phospha macrocycles. From X-ray and CPK models, cavity radii are determined as 0.86–0.92 ˚A for 15-crown-5, 1.34–1.43 ˚A for 18-crown-6, and about 1.7 ˚A for 21-crown-7. For complex formation between the alkali metal ions and 18-crown-6, the maximum stability occurs for the potassium ion having radius of 1.38A˚, correlating well with the cavity radius.18-crown-6 forms extremely stable complexes with all of the alkali and alkaline earth metal ions . When the cavity of the crown matches the radius of the metal ion, the metal ion can be readily incorporated in the cavity,example: the structure of rubidium thiocyanate complex with the dibenzo-18-crown-6 (1). In cases where the cavity of the crown is too large to surround the metal ion, a folded structure can result, example: the dibenzo-30-crown-10 (2) and the K⁺ ion. For very large metal ions incapable of fitting into smaller macrocyclic cavities, sandwich-type structures can occur,Exmple: the benzo-15-crown-5 (3) with the potassium ion.

 

1

2

3

Li⁺ should preferentially fit crown-4,in solution crown-4,crown-5,crown-6 and crown-7 all prefer K⁺.This is because of the five membered ring formed when any one of the above crown ethers binds to an alkali metal cation best fits the size of K⁺ ion.

Gas-phase studies shows that crown-5  ethers prefers Li⁺ more than any other alkali metal cations:

Crown-5:Li+>>Na+>K⁺>Cs⁺

On the other hand crown-6 shows more affinity for Na⁺ and crown-7 prefers K⁺

Crown -6:Na⁺>=K⁺>Li⁺>Rb⁺>Cs⁺

Crown-7:k⁺>Na⁺>=Rb^{+ +}>Li⁺>Cs⁺

The difference in affinities shown in gas and solution phases suggests that solvent effects are quite important these ligands have the unusual ability to promote the solubility of alkali salts in organic solvents as a result of the large hydrophobic organic ring. Example: alkali metal do not normally dissolve in ethers as they do in ammonia ,but they will do so if crown ligands are present.

K+crown-6®[K(crown-6)]⁺ + [e(solvent)]^-