Metal-ion complexation in aqueous solutions of 1 - thia-4,7-diazacyclononane-, 1-thia-4,8-diazacyclodecane 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 (H2L1 and H2L2) and of the open-chain 2,5-diazahexane-N,N′-diacetic acid (H2L3) with MgII, CaII, SrII, BaII, MnII, CoII, NiII, CuII, ZnII, CdII, PbII and LaIII have been determined in aqueous solution (25 °C, 0.1 mol dm–3 KNO3) by pH potentiometry, and in some cases in combination with visible absorption spectrophotometry. The complexation enthalpies with CuII have been determined by adiabatic calorimetry. The electronic absorption spectra of the complexes of CuII and NiII were also recorded. All metal-ion complexes with H2L1 are stronger than with H2L3, even for the harder metal ions. The presence of the thioether donor particularly enhances covalent bonding with CuII 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 H2L1 with a five-membered chelate ring between the metal ion and the two tertiary nitrogens, by H2L2 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 CuII by H2L2 is sterically much more efficient than by H2L1 and is evidenced by a higher heat of complexation and a stronger ligand field.