Polynuclear Complexes of Nd and Dy with N2O3 Donor Ligands: Solution Speciation and Selective Precipitation Studies.

This study explores the formation and selective separation of polynuclear neodymium and dysprosium complexes with N2O3 donor ligands derived from N,N-bis(salicylidene)-1,3-diamino-2-propanol (H3LH). The research focuses on the structural characterization and solution speciation of Nd3+ and Dy3+ complexes by using ligands with different peripheral substituents: H3LH, H3Lp-OMe, and H3Lo-tBu. These substituents significantly influence the nuclearity of the resulting complexes. For Dy3+, single-crystal X-ray diffraction (SC-XRD) revealed a range of molecular architectures, from dinuclear to hexanuclear species, each with distinct solubility profiles, whereas for Nd3+ an heptanuclear molecular structure with H3Lo-tBu was obtained. Separation experiments with Nd:Dy ratios of 1:1 and 4:1 demonstrated the ability of the ligands to give a partial selective precipitation of Nd3+ and Dy3+ complexes, depending on both metal identity and ligand structure. In particular, H3LH achieved a separation factor (SNd/Dy) of 12.0 (±2.0), concentrating Nd3+ in the solid phase. In contrast, H3Lo-tBu favored Dy precipitation, yielding a separation factor of 20.0 (±4) after just 10 min. In various instances, the low separation factor values were ascribed to the formation of mixed-metal polynuclear species, which was confirmed through Electrospray Ionization Mass Spectrometry (ESI-MS) and by the structural characterization of a heteronuclear complex with H3Lo-tBu.