Selectivity of macrocyclic aminocarboxylates for alkaline-earth metal ions and stability of their complexes

Abstract

The stability constants of alkaline-earth-metal complexes of several macrocycles derived from 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (H₃L¹) were determined by the potentiometric pH-titration method. The derivatives are formed by variation of the substituent R at N¹⁰, i.e. R = Prⁿ(H₃L²), CH₂C₆H₄NO₂-p(H₃L³), CH₂CH(OH)CH₃(H₃L⁴), CH₂CH(OH)CH₂OH (H₃L⁵), CH₂CH(OH)CH₂OCH₃(H₃L⁶) and CH₂CO₂H (H₄L⁷). In general, the stabilities of these complexes are greater than those with non-cyclic ligands except in a few cases, e.g. trans-1-cyclohexane-1,2-diyldinitrilotetraacetic acid (H₄cdta). For H₃L¹–H₃L³, the stability trend is CaL > MgL > SrL > BaL; for H₃L⁴–H₃L⁶ and H₄L⁷, CaL > SrL > BaL > MgL. The former trend is similar to those found for smaller, non-cyclic ligands with six or less donor atoms such as H₄cdta. The latter trend is the same as that for the larger, more flexible, and calcium-selective ligand ethylenedioxydiethylenedinitrilotetraacetic acid. The selectivity of H₃L⁴–H₃L⁶ and H₄L⁷ for Ca²⁺, Sr²⁺ and Ba²⁺ over Mg²⁺ ion is presumably due to their ability to saturate the octahedral co-ordination environment of Mg²⁺ while still allowing the larger Ca²⁺, Sr²⁺ and Ba²⁺ to be fully eight-co-ordinated.