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 gro ups, DPA-2 (N,N'-di-2-picolylethylenediamine), DPA-3 (N,N'-di-2-picoly l-1,3-propanediamine), and TPEN (N,N,N',N'-tetra-2-picolylethylenediam ine) were studied with large metal ions in 0.1 M NaNO3 at 25 degrees 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 Cd-II, Pb-II, and Bi-III by glass electrode potentio metry, by differential pulse polarography (DPP), and by differential p ulse voltammetry (DPV). The complexes of TPEN with Cd-II, Pb-II, and B i-III were studied by DPP, and with Hg-II by metal electrode potentiom etry. Glass electrode potentiometry could not be used for metal-TPEN s ystems because precipitation of complexes occurs during titration and complexes are formed much below pH 2. Protonation constants and comple x formation constants for these ligands and metal ions are reported. I t has been shown that change in complex stability, Delta log K-1, on p assing from DPA-2 to DPA-3, plotted as a function of a metal ion radiu s, supports the rule that the change of chelate ring size from five to six-membered stabilizes the complexes of small relative to large meta l ions. The effect of chelate ring size on complex stability obtained for DPA-2 and DPA-3 ligands is compared with corresponding relations f or the pairs of ligands 2,2,2-tet and 2,3,2-tet, and py-12-aneN(4) and py-14-aneN(4). The possible steric effects of the bulky pyridyl group s of DPA-2 and DPA-3 were analyzed by Molecular Mechanics calculation in relation to steric aspects of complex formation. The results obtain ed here and elsewhere for TPEN are discussed in relation to the affini ty of each metal ion for nitrogen donor ligands.