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

Abstract

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 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 °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 H₂L¹ are stronger than with H₂L³, 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 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 Cu^II by H₂L² is sterically much more efficient than by H₂L¹ and is evidenced by a higher heat of complexation and a stronger ligand field.