Comparative investigation into the complexation and extraction properties of tridentate and tetradentate phosphine oxide-functionalized 1,10-phenanthroline ligands toward lanthanides and actinides

Two new phosphine oxide-functionalized 1,10-phenanthroline ligands, tetradentate 2,9-bis(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-BPPhen, L${}^1$) and tridentate 2-(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-MPPhen, L${}^2$), were synthesized and studied comparatively for their coordination with trivalent actinides and lanthanides. The complexation mechanisms of these two ligands toward trivalent f-block elements were thoroughly elucidated by NMR spectroscopy, UV/vis spectrophotometry, fluorescence spectrometry, single-crystal X-ray diffraction, solvent extraction, and theoretical calculation methods. NMR titration results demonstrated that 1 : 1 and 1 : 2 (metal to ligand) lanthanides complexes formed for L${}^1$, whereas 1 : 1, 1 : 2 and 1 : 3 lanthanide complexes formed for L${}^2$ in methanol. The formation of these species was validated by fluorescence spectrometry, and the corresponding stability constants for the complexes of Nd${}^{III}$ with L${}^1$ and L${}^2$ were determined by using UV/vis spectrophotometry. Structures of the 10-coordinated 1 : 1-type complexes of EuL${}^1$(NO${}_3$)${}_3$ and [EuL${}^2$(NO${}_3$)${}_3$(H${}_2$O)] Et${}_2$O in the solid state were characterized by X-ray crystallography. In solvent-extraction experiments, L${}^1$ exhibited extremely strong extraction ability for both Am${}^{III}$ and Eu${}^{III}$, whereas L${}^2$ showed nearly no extraction toward Am${}^{III}$ or Eu${}^{III}$ due to its high hydrophilicity. Finally, the structures and bonding natures of the complex species formed between Am${}^{III}$/Eu${}^{III}$ and L${}^1$/L${}^2$ were analyzed in DFT calculations. (© 2021 Wiley‐VCH GmbH)