A. Thaler et al., Synthesis and silver(I) coordination of N-functionalized aza-crown ethers with pendant aromatic carbocyclic or heterocyclic side-arms, INORG CHIM, 285(1), 1999, pp. 1-9
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.