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) SF_Am/La,Ce > 40 were observed. For all three ligands, the separation factors from all lanthanides are also high (SF_Am/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.