Formation of mono- and binuclear neodymium(iii)-gluconate complexes in aqueous solutions in the pH range of 2-8

The complex formation between Nd(iii) and d-gluconate (Gluc^-) is of relevance in modelling the chemical equilibria of radioactive waste repositories. In the present work, the formation of Nd_pGluc_qH_-r complexes at 25 °C and pH = 2-8 was studied via spectrophotometry, potentiometry, freezing point depression, conductometry and NMR spectroscopy. In addition to the four mononuclear complexes (pq-r = 110, 120, 130 and 11-2), the formation of two binuclear, so far unknown complexes (pq-r = 23-2 and 24-2) was revealed. Between pH = 5.5 and 7, with the increasing metal ion and ligand concentrations, the Nd₂Gluc₃H_-2⁺ species becomes progressively predominant. Under the conditions characteristic of waste repositories, however, the formation of these complexes can be neglected. Regarding the binding sites of Gluc^-, C2-OH and C3-OH groups, in addition to the carboxylate ion, were identified from ¹H and ¹³C spectroscopic measurements. Above pH = 6, the metal-ligand interactions became stronger implying the formation of deprotonated complexes involving the C2-OH group, while the displacement of the second proton at the C3-OH is also possible. The metal ion induced deprotonation of the ligand was confirmed by DFT calculations.