The effect of chelate ring size on metal ion size-based selectivity in polyamine ligands containing pyridyl and saturated nitrogen donor groups

Ligands containing pyridyl groups and saturated nitrogens as donor groups, DPA-2 (N,N′-di-2-picolylethylenediamine), DPA-3 (N,N′-di-2-picolyl-1,3-propanediamine), and TPEN (N,N,N′,N′-tetra-2-picolylethylenediamine) were studied with large metal ions in 0.1 M NaNO3 at 25 °C. The pair of ligands DPA-2 and DPA-3 which form five and six-membered chelate rings, respectively, between two central nitrogen donor atoms was studied with CdII, PbII, and BiIII by glass electrode potentiometry, by differential pulse polarography (DPP), and by differential pulse voltammetry (DPV). The complexes of TPEN with CdII, PbII, and BiIII were studied by DPP, and with HgII by metal electrode potentiometry. Glass electrode potentiometry could not be used for metal-TPEN systems because precipitation of complexes occurs during titration and complexes are formed much below pH 2. Protonation constants and complex formation constants for these ligands and metal ions are reported. It has been shown that change in complex stability, Δ log K1, on passing from DPA-2 to DPA-3, plotted as a function of a metal ion radius, supports the rule that the change of chelate ring size from five to six-membered stabilizes the complexes of small relative to large metal ions. The effect of chelate ring size on complex stability obtained for DPA-2 and DPA-3 ligands is compared with corresponding relations for the pairs of ligands 2,2,2-tet and 2,3,2-tet, and py-12-aneN4 and py-14-aneN4. The possible steric effects of the bulky pyridyl groups of DPA-2 and DPA-3 were analyzed by Molecular Mechanics calculation in relation to steric aspects of complex formation. The results obtained here and elsewhere for TPEN are discussed in relation to the affinity of each metal ion for nitrogen donor ligands.