Nitrogen analogues crown ethers

There has been recent interest in lanthanide complexes of ligands of the type (21). These are in some sense nitrogen analogues of the crown ethers. As Schiff bases, their lanthanide complexes may be synthesized by a template condensation, for example from 2,6-diacetylpyridine, 1,2-diaminoethane and lanthanum nitrate. These components react in alcohols to give [La(N03)3L], where the aza-crown L = (21 R = Me). This compound has a structure (Figure 6), quite analogous to that of [La(N03)3(18-crown-6)], in which La—N... 

The synthesis of phosphorus analogues of cryptate ligands such as (5.392) seems not unlikely in the near future. Like the multidentate nitrogen ligands, they may be expected to stabilise certain ions by encapsulation. Diphosphate crown ethers such as (5.393) have already been reported. These compounds are fairly easy to make (5.394). 

Hetero analogues, in which oxygen is replaced either partially or completely by other heteroatoms, especially sulfur or nitrogen, are also known [179]. These hetero crown ethers are also called coronands or corands [180]. 

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