New molecular receptors with cyclophosphazene subunits: Synthesis, reactivity, and structure-property relationships

A series of hydrophopic (2 and 3) and new hydrophilic (4-7) molecular recep tors of the PNP-lariat ether with tetra-substituted cyclotriphosphazene sub units have been prepared by the complete nucleophilic substitution of chlor ine atoms in the reactive PNP-crown precursor 1 with the respective sodium cation-paired oxyanions (phenoxy --> 2, beta-naphthoxy --> 3, and methoxytr ioxyethylenoxy --> 4) and aliphatic amines (n-propylamine --> 5 aziridine - -> 6, and pyrrolidine --> 7). Their structures were established by MS and P -31 NMR spectroscopy and their metal ion complexing properties tested by a TLC method. Comparison of the complexation behaviour for ligands 1-7 shows that the affinity for particular cations is strongly substituent-dependent and, in general, is significantly enhanced by cooperation of the side arm d onor atoms (O or N) with the parent PNP-crown structure in the binding proc ess. The remarkable affinity of some ligands for selected cations, in parti cular lithium, cesium, and silver ions, is interpreted in terms of structur e-property relationships.