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

Abstract

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(μ2,κ4-(iPrO)2P­(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.

Pyridine-di-phosphonates as chelators for trivalent f-elements: kinetics, thermodynamic and interfacial study of Am(iii)/Eu(iii) solvent extraction

“…The 1 : 1 and 2 : 1 complexes were found in both experiments. Previously for the phenanthroline ligands, 2 : 1 complexes were not detected in similar conditions (CH 3 CN solutions, nitrates of f-elements) 32 since these ligands are tridentate and tetradentate, respectively. However, phenanthroline 2 : 1 complexes were found with non-complexing counter-ions such as perchlorates or in the high ligand concentration range.…”

“…However, phenanthroline 2 : 1 complexes were found with non-complexing counter-ions such as perchlorates or in the high ligand concentration range. 32,64 The stability constant log β 2:1 for PO-Py-cHex was 0.5 orders of magnitude higher than for PO-Py-iPr. This result, however, did not contradict the similar Eu(III) extraction ability for these ligands.…”

“…For these ligands, SF = 8-14 in the case of extraction from 1 mol L −1 HNO 3 . [30][31][32] Phenanthroline diphosphine oxides (Fig. 1b) had a higher extraction ability than diphosphonates, but these extractants did not provide separation of the Am/Eu pair.…”

New tetradentate N,O-hybrid phenanthroline-derived organophosphorus extractants for the separation and complexation of trivalent actinides and lanthanides

“…And then the effect of different alkyls and counter anions on the extraction and complexation properties of phosphonate ligands has been investigated. [52][53][54] Another type of phenanthroline-derived organophosphorus ligand 55,56 was designed by combining the merits of both the phenanthroline core and the O atoms of phosphine oxide which were similar to those of CMPO. 57 For instance, (1,10-phenanthroline-2,9-diyl)bis(butyl ( phenyl)phosphine oxide) (BuPh-BPPhen) 56 was not found to selectively separate Am(III) from Eu(III) but this ligand had a great extraction ability at a high HNO 3 concentration (D Am(III) > 100, D Eu(III) > 100).…”

Theoretical unraveling of the separation of trivalent Am and Eu ions by phosphine oxide ligands with different central heterocyclic moieties

“…Up to now, many similar phosphine oxide ligands with N-heterocyclic rings have been developed. [17][18][19] The An/Ln separation ability of these new extractants is still not strong, and the extraction kinetics and radio stability properties also need to be optimized. Moreover, the complex mechanism for the An 3+ selectivity of these phosphine oxide ligands and the reason for the difference among the three different N-heterocyclic cores have still not been investigated well.…”