Steric hindrance of phenanthroline diamides permits to reach a hundredfold increase of Am(III) extraction efficiency

Abstract

Three novel 1,10-phenanthroline-2,9-dicarboxamides (DAPhen) were prepared and investigated in detail. The presence of alkyl groups in ortho-positions of aryl substituent in the ligand amide functions results in completely inhibited rotation around N-Ar bond and complicated conformational behavior. Very significant effect of ligand steric hindrance on a solvent extraction of lanthanides(III) and Am(III) from nitric acid solution was demonstrated. The extraction tests indicated that all three ligands L1-L3 extract Am better than all lanthanides. Very high separation factor of Am from the early lanthanides (La, Ce) SFAm/Lа,Се > 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 for spent nuclear fuel (SNF) reprocessing. The extraction efficiency (distribution ratios) 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 coordination chemistry of novel DAPhen sterically hindered 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 nearly hundredfold increase of Am(III) extraction efficiency when moving from ortho-methyl substituents toward more bulky ortho-isopropyl groups.