Phenanthroline
carboxamide compounds are promising for lanthanide
intra-series separation. This paper presents a study on the effect
of structure modification of phenanthroline carboxamides on the extraction
of the whole lanthanide series. The study consists of theoretical
calculations, extraction experiments of the 14 stable lanthanides,
and extended X-ray absorption fine structure (EXAFS) analyses of Nd
and Dy complexes. Tridentate monocarboxamides and tetradentate dicarboxamides
show different trends in series extraction, although both preferentially
extract the light lanthanides. The amide substituents, although not
directly coordinating the metal ions, were also found to impact the
distribution ratio, most probably due to a modification in the internal
polarity of the molecules. This latter effect, if extrapolated to
other nitrogen-based ligands such as pyridines or triazines, can be
used to further fine-tune extractants for a process improvement.