Way to Enforce Selectivity via Steric Hindrance: Improvement of Am(III)/Eu(III) Solvent Extraction by Loaded Diphosphonic Acid Esters

Hybrid donor extractants are a promising class of compounds for the separation of trivalent actinides and lanthanides. Here, we investigated a series of sterically loaded diphosphonate ligands based on bipyridine (BiPy-PO-iPr and BiPy-PO-cHex) and phenanthroline (Phen-PO-iPr and Phen-PO-cHex). We studied their complex formation with nitrates of trivalent f-elements in solvent extraction systems (Am and Eu) and homogeneous acetonitrile solutions (Nd, Eu, and Lu). Phenanthroline extractants demonstrated the highest efficiency and selectivity [SF(Am/Eu) up to 14] toward Am(III) extraction from nitric acid solutions among all of the studied diphosphonates of N-heterocycles. The binding constants established by UV-vis titration also indicated stronger binding of sterically impaired diphosphonates compared to the primary substituted diphosphonates. NMR titration and slope analysis during solvent extraction showed the formation of 2:1 complexes at high concentrations (>10(-3) mol/L) for phenanthroline-based ligands. According to UV-vis titrations at low concentrations (10(-5)-10(-6) mol/L), the phenanthroline-based ligands formed 1:1 complexes. Bipyridine-based ligands formed 1:1 complexes regardless of the ligand concentration. Luminescence titrations revealed that the quantum yields of the complexes with Eu(III) were 81 +/- 8% (BiPy-PO-iPr) and 93 +/- 9% (Phen-PO-iPr). Single crystals of the structures [Lu(mu(2),kappa(4)-(iPrO)(2)P(O)Phen(O)(2)(OiPr))(NO3)(2)](2) and Eu(Phen-PO-iPr)(NO3) (3) were obtained by chemical synthesis with the Phen-PO-iPr ligand. X-ray diffraction studies revealed a closer contact of the f-element with the aromatic N atoms in the case of sterically loaded P = O ligands compared with sterically deficient ligands. Density functional theory calculations allowed us to rationalize the observed selectivity trends in terms of the bond length, Mayer bond order, and preorganization energy.

Automated summary

Hybrid donor extractants based on sterically loaded diphosphonate ligands showed promising efficiency and selectivity in separating trivalent actinides and lanthanides. Among the studied ligands, phenanthroline-based extractants exhibited the highest efficiency in extracting Am(III) from nitric acid solutions, forming complexes with high quantum yields. X-ray diffraction studies revealed closer contacts of f-elements with aromatic N atoms in sterically loaded P = O ligands compared to sterically deficient ligands, providing insights into the observed selectivity trends.