METAL-ION RECOGNITION - THE INTERACTION OF COPPER(II), SILVER(I) AND LEAD(II) WITH 22-MEMBERED MACROCYCLES INCORPORATING O4N2-DONOR, O2S2N2-DONOR AND S4N2-DONOR SETS

A comparative potentiometric investigation in 95% methanol of the comp lexation of 22-membered macrocyclic rings incorporating mixed (O4N2, O 2S2N2 and N2S4) donor-atom sets towards copper(II), silver(I) and lead (II) has been made. As expected, the introduction of 'soft' sulfur don ors into the macrocycle framework increases the discrimination for sil ver(I) over lead(II) with that incorporating an N2S4 donor set showing discrimination of at least 10(8) in favour of silver(I). Details of t he binding of silver(I) and lead(II) to the O4N2 and O2S2N2 systems ha ve been assessed in CDCl3-(CD3)(2)SO (1:4) using C-13 NMR spin-lattice relaxation (T-1) studies as well as observation of the induced chemic al 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 experiment s 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), c opper(II), zinc(II), cadmium(II), silver(I) and lead(II) as their nitr ate salts. In each case the silver(I) ion was the only one to exhibit significant transport under the conditions employed, with both macrocy cles incorporating sulfur donors being more efficient ionophores than the O4N2-donor system.