Chelating Effect of Alizarin-Oxalate on La3+ and Nd3+ in Acidic, Basic and Neutral Medium: A DFT study

Abstract This study focuses on conducting a comparative study of the extraction capacities of alizarin-oxalate (AR-Ox) ligands with La3+ and Nd3+ in acidic, neutral, and alkaline mediums. Density functional theory (DFT) calculations at ωB97X-D/6-311 + + G(d,p)/SDD level have been performed for structural, thermo-chemical, frontier-orbital (highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO)), natural bond orbital (NBO), reduced density gradient (RDG), and density of state (DOS) analysis for alizarin-oxalate-La(III) (AR-Ox-La) and alizarin-oxalate-Nd(III) (AR-Ox-Nd) complexes. The bonding characteristics of La3+ & Nd3+ ions with alizarin-oxalate ligand have been analysed using the quantum theory of atoms in molecules (QTAIM), revealing the presence of an intermediate type of bond between closed-shell (CS) and shared-shell (SS) electrons in (La/Nd)-O, (La/Nd)-C. The reduced density gradient (RDG) and iso-surface generated through the Multiwfn program shows mostly hydrogen-like and Van-der-Waal interaction between La3+/Nd3+ and oxygen atoms of alizarin-oxalate ligand except for some of the complexes showing the presence of non-bonded/repulsive (La/Nd)-O interaction. Thermo-chemical, DOS, and natural bond orbital analysis reveals alizarin-oxalate-( La3+/Nd3+) complexes in the alkaline medium is more stable than in neutral and acidic medium, and the stability of AR-Ox-Nd complexes is more than AR-Ox-La complexes. This work illustrates the subtle differences in chelating properties of alizarin-oxalate ligands with La3+ and Nd3+ for designing new ligands for efficient selective lanthanide separation.