Polynuclear EDTA Complexes in Aqueous Solution: A Proof of Concept

Ethylenediaminetetraacetic acid (EDTA) is, so far, the most known and widely used chelating agent. It is well known to form very stable mononuclear complexes with several metal ions in aqueous solution. However, its multidentate nature opens the possibility to simultaneously bind more than one cation forming polynuclear species, rarely observed for EDTA, but common for its analogues like, e.g., DTPA and TTHA. If formed, these species could significantly affect the speciation of both EDTA and metal ions in solution. As such, in this work we report the result of a potentiometric investigation on the chemical speciation of three model systems, namely: Sn2+/Zn2+/EDTA, Sn2+/Fe2+/EDTA and Zn2+/Fe3+/EDTA, at T = 298.15 K and I = 0.15 mol⋅dm‒3 (in NaNO3(aq) or NaClO4(aq)), as a proof of concept that simple and mixed polynuclear species can be also formed by EDTA. In fact, experimental data analysis evidenced the formation of not only the simple Sn2(EDTA), Zn2(EDTA) and [Fe2(EDTA)]2+ species, but also the mixed SnZn(EDTA), SnFe(EDTA) and [ZnFe(EDTA)]+. The formation of latter species was then analyzed in terms of thermodynamic extra stability with respect to simple species, and their influence on the speciation of EDTA and the investigated cations was evaluated. Furthermore, quantum mechanical calculations were also performed for a better insight on the binding mode of EDTA when forming dinuclear species, and to support the evidence concerning the extra stability of mixed ones. We could conclude that, when forming dinuclear species, EDTA behaves as a tridentate ligand, binding cations through two carboxylates and one amino group (MIDA-like mode).