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, L1 ) and tridentate 2-(butylphenylphosphine oxide)-1,10-phenanthroline (BuPh-MPPhen, L2 ), 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 L1 , whereas 1 : 1, 1 : 2 and 1 : 3 lanthanide complexes formed for L2 in methanol. The formation of these species was validated by fluorescence spectrometry, and the corresponding stability constants for the complexes of NdIII with L1 and L2 were determined by using UV/vis spectrophotometry. Structures of the 10-coordinated 1 : 1-type complexes of EuL1 (NO3 )3 and [EuL2 (NO3 )3 (H2 O)] Et2 O in the solid state were characterized by X-ray crystallography. In solvent-extraction experiments, L1 exhibited extremely strong extraction ability for both AmIII and EuIII , whereas L2 showed nearly no extraction toward AmIII or EuIII due to its high hydrophilicity. Finally, the structures and bonding natures of the complex species formed between AmIII /EuIII and L1 /L2 were analyzed in DFT calculations.