.The stability constants for Ag+ complexes of several diazapolyether ligands, L = (2,2,1), (2,1),
(2,2) and (2) (Scheme 1), have been measured in acetonitrile and water mixtures. Rate constants
for formation and dissociation of Ag(2,2,1)+, together with the free energies of transfer, ΔGtr,
of (2,2,1) among the mixtures, are also reported, and are compared with corresponding values
for the Ag+-(2,1,1) and Ag+-(2,2,2) systems. The solvent dependences of the stability constants and
hence of ΔGtr(AgLCIO4) - AG,,(L) are almost identical for all systems, except for small differences
at low water content between the behaviour of the bicyclic cryptand ligands [(2,2,2), (2,2,1), (2,1,1)]
and the monocyclic and acylic ligands, attributable to effects of NH solvation in the latter group.
The preferential solvation of Ag+ by acetonitrile in the mixtures results in a sharp decrease in the
stability constants as acetonitrile is added to water, but the effect is partly compensated for by a
corresponding decrease in the free energies of the complexes. A strong differentiation in the kinetic
behaviour of the bicyclic cryptand ligands is observed. For (2,1,1) the formation rates are almost
independent of solvent composition and the free energies of transfer of the activated complex,
ΔG‡tr (Ag+. . .(2,1,1),ClO4-) closely parallels that of AgCIO4, whereas for the larger cryptands the
formation rates show a stronger solvent dependence, and ΔG‡tr (Ag+. . .L,ClO4-) values much more
closely resemble those of the stable complexes, ΔG‡tr (AgLClO4). It is suggested that for the smaller
(2,1,1) ligand preferential solvation of Ag+ by acetonitrile still persists in the transition state, but
that in the activated complex for (2,2,1) and (2,2,2) the strong interactions between Ag+ and the
ligand nitrogens have already replaced the Ag+. . .N≡CCH3 interactions occurring for the
uncomplexed Ag+ ion in the mixtures.
PHYSICAL AND CHEMICAL CHARACTERISTICS OF SUPRAMOLECULAR COMPLEXES OF GLYCIRRIZINIC ACID WITH PHYTOHORMONES
