Metal-ion recognition. The interaction of copper(II), silver(I) and lead(II) with 22-membered macrocycles incorporating O4N2-,O2S2N2- and S4N2-donor sets

Abstract

A comparative potentiometric investigation in 95% methanol of the complexation of 22-membered macrocyclic rings incorporating mixed (O₄N₂, O₂S₂N₂ and N₂S₄) donor-atom sets towards copper(II), silver(I) and lead(II) has been made. As expected, the introduction of ‘soft’ sulfur donors into the macrocycle framework increases the discrimination for silver(I) over lead(II) with that incorporating an N₂S₄ donor set showing discrimination of at least 10⁸ in favour of silver(I). Details of the binding of silver(I) and lead(II) to the O₄N₂ and O₂S₂N₂ systems have been assessed in CDCl₃–(CD₃)₂SO (1:4) using ¹³C NMR spin–lattice relaxation (T₁) studies as well as observation of the induced chemical shifts of ligand resonances on complex formation. Overall, the NMR results correlate well with the observed thermodynamic stabilities of the individual complexes. Competitive mixed-metal transport experiments across a bulk chloroform membrane have been performed using each of the three macrocycles as the ionophore. For each experiment the source phase contained equimolar concentrations of cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), silver(I) and lead(II) as their nitrate salts. In each case the silver(I) ion was the only one to exhibit significant transport under the conditions employed, with both macrocycles incorporating sulfur donors being more efficient ionophores than the O₄N₂- donor system.