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 EuL1(NO3)(3) and [EuL2(NO3)(3)(H2O)] Et2O 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.

Automated summary

Two new phosphine oxide-functionalized 1,10-phenanthroline ligands, L-1 and L-2, were synthesized and studied for their coordination with trivalent actinides and lanthanides. Different complexation mechanisms and stability constants were elucidated using various analytical methods. L-1 exhibited strong extraction ability for Am-III and Eu-III, while L-2 showed minimal extraction due to its high hydrophilicity. The structures and bonding natures of the formed complex species were analyzed using DFT calculations.