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

Ten monoaza- and diaza-crown macrocycles containing aromatic carbocyclic an d N-heterocyclic side-arms linked to the aliphatic ring nitrogen atoms have been prepared and their complexing properties with silver(I) cations inves tigated. Pendant arms comprising aromatic carbocyclic structures lead to on ly small enhancements in the stabilities of the silver(I) complexes in pola r 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, aceton itrile and propylene carbonate show that (i) methanol strongly influences c omplex formation through hydrogen-bond formation with the free ligands, and (ii) the complexed silver ion is effectively completely shielded from inte ractions with the solvent molecules. Solvent dependence of the stability co nstants can be used to derive quantitative estimates of the single ion-free energies of transfer of Ag between polar aprotic solvents in a manner anal ogous 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 direc tly into the solvated Ligand, are independent of the solvent. (C) 1999 Else vier Science S.A. All rights reserved.