Comparative Investigation into the Complexation and Extraction Properties of Tetradentate and Tridentate Phosphine Oxide Functionalized 1,10-Phenanthroline Ligands toward Lanthanides and Actinides.

Two new phosphine oxide functionized 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 the 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 both 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 solution. The formation of these species were also validated by fluorescence spectrometry and the corresponding stability constants for the complexes of Nd(III) with L1 and L2 were determined using UV-vis spectrophotometry. Structures of the 10-coordinated 1:1-type complexes of EuL1(NO3)3 and [EuL2(NO3)3(H2O)]‧Et2O in solid state were characterized by X-ray crystallography. In solvent extraction experiments, L1 exhibited extremely strong extraction ability for both Am(III) and Eu(III) while L2 showed nearly no extraction toward Am(III) and Eu(III) due to its high hydrophilicity. Finally, the structures and bonding natures of those complexes species formed between Am(III)/Eu(III) and L1/L2 were analyzed using DFT calculations.