Thermodynamic and kinetic studies of silver ion complexes with cyclic and open chain diaza-polyethers including cryptands in acetonitrile + water mixtures

Abstract

.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, ΔG_tr, 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 ΔG_tr(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.