Formation and stability of the mixed-chelator complexes of Sr2+, Mg2+, Ca2+, Ba2+, and Y3+ in solution with bio-relevant chelators.

The formation and equilibria of Sr2+, Mg2+, Ca2+, Ba2+, and Y3+ (M) complexes with a mixed-chelator comprising two biodegradable chelators (GLDA, LG, 2-[bis(carboxymethyl)amino] pentanedioic acid; HIDS, LH, 2-(1,2-dicarboxyethylamino)-3-hydroxy-butanedioic acid) in an aqueous matrix was evaluated. The potentiometric measurement results (ionic strength, 0.10 M; temperature, 25 ± 0.1 °C) confirmed the formation of 1:1:1 (M:LG:LH) complexes and the experimental data sets were further used to derive the equilibrium constants for the ternary complexes. The [MHLGLH]5- complex was the dominant ternary complex with Sr2+, Mg2+, Ca2+, and Ba2+, while Y3+ formed [M(OH)2LGLH]7- as the principal ternary species. The trend in the overall formation constants of the MLmix (Lmix, LG:LH = 1:1) complexes was in the order: Y3+ > Ca2+ > Mg2+ > Sr2+ > Ba2+. The ternary complexation trend was interpreted using the corresponding atomic radii and solution-phase electronegativities of the elements. The modes of interaction between the chelators and cations in the MLmix systems were subsequently deduced, and evaluated by using Gaussian 16W program. The relative stabilities of the ternary complexes (ΔlogK) were interpreted by comparison with the stabilities of the corresponding binaries, with negative ΔlogK values observed for all the MLmix complexes.