The effect of increase in chelate ring size beyond six-membered on the metal ion size selectivity patterns of tetra-aza macrocycles

Abstract

The complex-formation constants and protonation constants of the ligand 1,4,7,10-tetra-aza-cyclopentadecane, which forms three five- and one eight-membered chelate ring, are reported. The protonation constants measured by glass-electrode potentiometry in 0.1 mol dm^–3 NaNO₃ at 25 °C are pK₁ 9.83, pK₂ 8.95, pK₃ 5.40, and pK₄ 1.6, while the complex-formation constants, log K₁, were 19.25 (Cu²⁺), 11.74 (Ni²⁺), 10.70 (Zn²⁺), 10.18 (Cd²⁺), and 9.50 (Pb²⁺). These constants were compared to those of other tetra-aza macrocycles with three five-membered chelate rings plus a fourth chelate ring which varied regularly from five- to seven-membered, as well as of series of open-chain ligands in whose complexes a single chelate ring varied in size in the same manner. It was concluded that (a) the size of the macrocyclic ring plays very little part in determining complex stability, (b) the main determining factor in controlling complex stability is the size of the chelate ring formed, (c) an increase in chelate ring size from five- to six-membered produces a differentiation in complex stability dependent on the size of the metal ion, such that complexes of larger metal ions are destabilized relative to those of small metal ions, and (d) an increase in chelate ring size beyond six-membered produces a fairly uniform decrease in complex stability which does not show any strong dependence on the size of the metal ion.