Crown ethers. Host-guest relationship

The relationship between crown ether and the ion that is transport is called a host-guest relationship. 

Guests with polar N—H bonds include amides, ureas and related sulfur compounds, substituted hydrazines, and aromatic amines.21-264,265 Complexes with these substrates exhibit a wide variety of stoichiometric and nonstoichiometric associations. A particularly popular association is the 1 2 guest host relationship as typified by the structure of the benzenesulfonamide complex with [18]crown-6 (84).266-267 The weak complexes formed between crown ethers and cryptands with certain proteins allow for solubilizing these species in organic solvents.268... 

As we have seen, X-ray studies of the ionophorous antibiotics and their cation complexes were able to explain many of the steric factors that determine the selectivity patterns shown by these ligands. However, more systematic investigations on the relationship between host-cavity size and guest-ion radius could only be carried out using simpler synthetic ligands as models. In 1967, Pedersen reported the synthesis and complexing properties of a new class of compounds named crown ethers which are able to mimic effectively their natural counterpieces. 

Coordination chemistry complex stability. The sizes of the hydrophilic cavities of the macrocyclic poly ethers (13) can be compared with the radii of unsolvated ions of alkali metal ions (1,4) (Table 2.1). The predicted Optimal Spatial Fit Concept (circular recognition) between 12-crown-4 (3) and Li" ", 15-crown-5 (16) and Na", 18-crown-6 (15) and is confirmed by experimentally determined thermodynamic stability constants (15) (iCJ, i.e. the better the fit of the guest cation into the cavity of the host the larger Kg. Figure 2.3 clearly demonstrates the relationship between cation radius/cavity radius size and the stability constant of the respective complexes (15). 

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