4-Oxo-7-fluoro-1,10-phenanthroline-2,9-diamides: Synthesis, Structural Features, Lanthanide Complexes, and Am(III)/Ln(III) Solvent Extraction

Steric hindrance of phenanthroline diamides enables a hundredfold increase in Am(III) extraction efficiency

Three novel 1,10-phenanthroline-2,9-dicarboxamides (DAPhen) were prepared and investigated in detail. The presence of alkyl groups in the ortho-positions of the aryl substituent in the ligand amide functions results in completely inhibited rotation around the N-Ar bond and complicated conformational behavior. A very significant effect of ligand steric hindrance on the solvent extraction of lanthanides(III) and Am(III) from nitric acid solution was demonstrated. Extraction tests indicated that all three ligands L1-L3 extract Am better than all lanthanides. Very high separation factors of Am from early lanthanides (La and Ce) SFAm/La,Ce > 40 were observed. For all three ligands, the separation factors from all lanthanides are also high (SFAm/Ln > 10), which makes them suitable for practical applications in spent nuclear fuel (SNF) reprocessing. The extraction efficiency (distribution ratio) increases by almost two orders of magnitude for all tested f-elements while maintaining an overall extraction trend. Since the solvent extraction of Ln(III) and/or Am(III) involves the formation of corresponding complex compounds, a detailed investigation of the coordination chemistry of novel sterically hindered DAPhen ligands toward lanthanides and americium nitrates was performed. A combination of single-crystal X-ray analysis, spectral techniques and quantum-chemical calculations allowed us to explain the nearly hundredfold increase in Am(III) extraction efficiency when moving from ortho-methyl substituents toward more bulky ortho-isopropyl groups.

Remarkable Effect of Stereoisomerism on the Am(III)/Ln(III) Solvent Extraction. New Ligands for Highly Efficient Separation of Americium

Two novel 1,10-phenanthroline-2,9-diamide ligands were constructed on the basis of 2-phenylpyrrolidine and obtained as pure diastereomers. These ligands demonstrated advanced properties in liquid-liquid extraction tests. They revealed high efficiency of americium(III) extraction alongside with the record values of selectivity in the separation of americium from light lanthanides from strongly acidic media. An abrupt increase of extraction efficiency when moving along the lanthanide series from lanthanum to lutetium was observed. The examination of the extraction behavior of pure diastereomeric forms revealed noticeable differences in their selectivity while maintaining the overall extraction trend. The explanation of the discovered patterns was elucidated by a comprehensive study of the ability of the ligands to bind lanthanide nitrates in solutions. All the data collected (UV-vis and NMR titration, X-ray analysis of resulting complexes, solvation numbers estimation) were supported by quantum chemical calculation. These data clearly indicated that in the case of light lanthanides the formation of 1 : 1 complexes is most preferable. At the same time, complexes with heavy lanthanides, such as ytterbium and lutetium, exist as ionic pairs which may consist of [L2M]z+ cations counterbalanced by metallate anions, which may result in the formation of the species of unusual composition like L2M2 or even L4M5 clusters.

4,7-Substituted 1,10-phenanthroline-2,9-dicarboxamides: photophysics of ligands and their complexes with the Eu-Gd-Tb triad

The impact of substituents at the 4- and 7-positions of 1,10-phenanthroline-2,9-dicarboxamides on the photophysical properties of the ligands and their coordination compounds with the lanthanide triad-europium, gadolinium, and terbium-was analyzed. This study demonstrates how modification of the electronic nature of ligands through the incorporation of diverse functional groups affects the luminescence properties of their complexes. The introduction of various substituents leads to the appearance of intra-ligand or ligand-to-ligand charge transfer (CT) states. The highest luminescence efficiency was observed for LH·Eu(NO3)3 (Qin = 54.1% and QL = 9.6%), suggesting strong luminescence quenching of the CT state. It was found that a relatively low ΔE (∼3000 cm-1) supports direct energy transfer from S1 to T1 bypassing the CT state, even though it is outside Reinhoudt's optimal range. The introduction of fluorines leads to the strongest luminescence quenching among all the substituents.

Kinetic features of solvent extraction by N,O-donor ligands of f-elements: a comparative study of diamides based on 1,10-phenanthroline and 2,2'-bipyridine

A variant of microfluidic setup design for the study of extraction kinetics has been proposed. Mass transfer constants for Am(III) and Eu(III) and observed rate constants were obtained for N-,O-donor ligands featuring phenanthroline and bipyridyl cores. The possibility of determining rate constants for cations independently of each other makes it possible to observe the kinetic effect of separation. The extraction rate was found to be lower for the bipyridyl ligand, compared to phenanthroline. The values of the rotation barriers for the ligands were calculated using the DFT method. The values correlate with the obtained low extraction rate for the bipyridyl ligand. Also, crystallographic data showing anti-conformation for the bipyridyl ligand align with the kinetic data. Surface tension was also determined for the systems with the studied ligands. It is shown that at equal ligand concentrations, the value of surface tension agrees with the extraction rate. Furthermore, it is shown that for the bipyridyl ligand, prior contact of the organic phase with nitric acid significantly affects the surface tension.