Mechanochemical synthesis and micro-electron diffraction analysis of rare earth–aminopolycarboxylate coordination compounds

Abstract

Mechanochemical synthesis offers synthetic pathways to new materials that are inaccessible via traditional solvent-based approaches. In this work, we evaluated how mechanochemical synthetic variables (e.g. time, frequency, liquid assisted grinding (LAG), metal precursors) impacted the products obtained from reactions containing La(III) and ethylenediaminetetraacetic acid (EDTA). We found that tuning mechanochemical parameters (i.e., time and frequency) affected reactivity and use of different La(III) salt precursors (La₂O₃, LaCl₃·7H₂O, LaPO₄·xH₂O, La(NO₃)₃·6H₂O, and La(OOCCH₃)₃·1.5H₂O) led to variations in solid-state products. Reactivity trends were largely consistent with trends in the relative lattice energies of the lanthanum starting materials, with the outlier (La₂O₃) potentially undergoing additional hydroxylation on particle surfaces during LAG. Two products were successfully isolated and structurally characterized using electron diffraction, including a 1-D chain and a 2-D sheet prepared from La₂O₃ (LaEDTA1) and LaCl₃·7H₂O (LaEDTA2), respectively. Detailed structural analysis revealed protonation sites on EDTA ligands that contribute to overall charge neutrality of both compounds. Infrared spectroscopy further confirmed ligand protonation in LaEDTA1 and LaEDTA2, while thermogravimetric and elemental analysis measurements provided complementary characterization information. Finally, field emission scanning electron microscopy results confirmed the elemental compositions of both products, with trace levels of iron observed that likely originate from stainless-steel milling media.