Stability constants of aza-oxa-crown ether complexes with silver(I) in nonaqueous polar solvents

Complex formation between the silver(I) ion and monoaza- and diaza-crown ethers has been studied in five polar, nonaqueous solvents using potentiometric techniques. The stability constants are spread over seven orders of magnitude, and the major contributor to this variation is the solvation of the silver(I) cation. The ring-size dependence of the stability constants is determined both by the correspondence of the diameters of the silver(I) ion and the ring-cavity size of the macrocycles, and by specific ligand effects including association complex formation between the ligands and the solvent molecules and conformational effects in the free ligands. In the 1:1 complexes of the silver(I) ion with monoaza- and diaza-crown ethers the dominant interaction is between the silver(I) ion and the ligandʹs nitrogen atoms, but the ether–oxygen atoms also interact with the silver(I) ion. In the AgL2 + complexes (L=monoaza-crown ether), however, the monoaza-crown ethers coordinate as monodentate ligands, as with simple mono-amine complexants. In these complexes the ethoxy-chains prevent direct interaction between the silver(I) cation and the solvent molecules, with the result that in dipolar aprotic solvents the free energies of transfer of the ligands and the complexes are equivalent, i.e. the so-called cryptate hypothesis holds for the AgL2 + complexes.