METAL-ION COMPLEXATION IN AQUEOUS-SOLUTIONS OF 1-THIA-4,7-DIAZACYCLONONANE-N,N'-DIACETIC ACID, 1-THIA-4,8-DIAZACYCLODECANE-N,N'-DIACETIC ACID AND 2,5-DIAZAHEXANE-N,N'-DIACETIC ACID

The stability constants of the macrocyclic 1 -thia-4,7-diazacyclononane- and 1-thia-4,8-diazacyclodecane-N,N'-diacetic acid (H-2L1 and H-2L2) and of the open-chain 2,5-diazahexane-N,N'-diacetic acid (H-2L3) with Mg(II), Ca(II), Sr(II), Ba(II), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Pb(II) and La(III) have been determined in aqueous solution (25-degrees-C, 0.1 mol dm-3 KNO3) by pH potentiometry, and in some cases in combination with visible absorption spectrophotometry. The complexation enthalpies with Cu(II) have been determined by adiabatic calorimetry. The electronic absorption spectra of the complexes of Cu(II) and Ni(II) were also recorded. All metal-ion complexes with H-2L1 are stronger than with H-2L3, even for the harder metal ions. The presence of the thioether donor particularly enhances covalent bonding with Cu(II) as indicated by a higher heat of complexation, a more favourable entropy change and a stronger ligand-field strength. The change in stability of the complexes upon replacing H-2L1 with a five-membered chelate ring between the metal ion and the two tertiary nitrogens, by H-2L2 having a six-membered one, is dependent on the metal-ion size. The larger metal ions are destabilised relative to the small metal ions: the metal ion size-based selectivity for this pair of ligands is controlled by the chelate-ring size. Binding of Cu(II) by H-2L2 is sterically much more efficient than by H-2L1 and is evidenced by a higher heat of complexation and a stronger ligand field.