STABILITIES OF TRIVALENT METAL-COMPLEXES OF PHENOLIC LIGANDS RELATED TO N,N'-BIS(2-HYDROXYBENZYL)ETHYLENEDIAMINE-N,N'-DIACETIC ACID (HBED)

The stability constants and other equilibrium parameters of the trivalent metal ion complexes of three multidentate ligands have been determined: N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethyienediamine-N,N'-diacetic acid (Me4HBED); N,N'-bis(2-hydroxy-3-methyl-5-tert-butylbenzyl)ethylenediamine-N,N'-diacetic acid (t-BuHBED); and N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)-N-(2-hydroxyethyl)ethylenediamine-N'-acetic acid (HBMA). Alkylation of the aromatic ring significantly increases the basicity of the ligand, as measured by protonation constants, but lowers metal ion affinity, because of steric effects. Replacement of an acetate of Me4HBED by a hydroxyethyl donor group lowers the protonation constants of the ligand only slightly but lowers affinity for trivalent metal ions by 4-6 orders of magnitude. Stability constants involving trivalent metal ions are compared with those of five other sexadentate ligands, all of which contain two phenolate donor groups. Effectiveness of all eight ligands in binding trivalent metal ions in biological media is assessed by comparing pM values at physiological pH 7.4. © 1990, American Chemical Society. All rights reserved.