Synthesis and silver(I) coordination of N-functionalized aza-crown ethers with pendant aromatic carbocyclic or heterocyclic side-arms

Abstract

Ten monoaza- and diaza-crown macrocycles containing aromatic carbocyclic and N-heterocyclic side-arms linked to the aliphatic ring nitrogen atoms have been prepared and their complexing properties with silver(I) cations investigated. Pendant arms comprising aromatic carbocyclic structures lead to only small enhancements in the stabilities of the silver(I) complexes in polar solvents. However, aromatic N-heterocyclic groups, such as pyridyl, 2,2′-bipyridyl and 1,10-phenanthroline, increase the stability constants of the silver(I) complexes by up to five orders of magnitude, relative to those of the parent aza-macrocycles. Comparisons of the results in methanol, acetonitrile and propylene carbonate show that (i) methanol strongly influences complex formation through hydrogen-bond formation with the free ligands, and (ii) the complexed silver ion is effectively completely shielded from interactions with the solvent molecules. Solvent dependence of the stability constants can be used to derive quantitative estimates of the single ion-free energies of transfer of Ag + between polar aprotic solvents in a manner analogous to an earlier hypothesis based on macrobicyclic cryptand ligands (the cryptate hypothesis). It is shown that this (and the cryptate) hypothesis is equivalent to the requirement that the stability constants for metal ion ligation, i.e., the complexation of the metal ion from the gas phase directly into the solvated ligand, are independent of the solvent.