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

The stability constants of alkaline-earth-metal complexes of several macrocycles derived from 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (H(3)L(1)) were determined by the potentiometric pH-titration method. The derivatives are formed by variation of the substituent R at N-10, i.e. R = Pr-n (H(3)L(2)), CH2C6H4NO2-p (H(3)L(3)), CH2CH(OH)CH3(H(3)L(4)), CH2CH(OH)CH2OH (H(3)L(5)), CH2CH(OH)CH2OCH3 (H(3)L(6)) and CH2CO2H (H(4)L(7)). Tn 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 (H(4)cdta). For H(3)L(1)-H(3)L(3), the stability trend is Cat > MgL > SrL > BaL; for H(3)L(4)-H(3)L(6) and H(4)L(7), 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 H(4)cdta. The latter trend is the same as that for the larger, more flexible, and calcium-selective ligand ethylenedioxydiethylenedinitrilotetraacetic acid. The selectivity of H(3)L(4)-H(3)L(6) and H(4)L(7) 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.