Unfolding the extraction and coordination behaviors of trivalent lanthanides by two novel cyclohexyl o-oxydiamides ligands

Abstract

In this work, two cyclohexyl o-oxydiamides ligands: 2,2′-(cyclohexane-1,2-diylbis(oxy))bis(N,N-dioctylacetamide)(L1: Octy-CDA) and 2,2′-(cyclohexane-1,2-diylbis(oxy))bis(N,N-bis(2-ethylhexyl)acetamide) (L2: 2-ethylhexyl-CDA) have been synthesized to study the extraction and coordination behaviors with trivalent lanthanides in HNO3 media. The extractability of ligands increases with the atomic number of lanthanides increasing. The straight alkyl chain demonstrated better extraction ability than the branched chain, which was also most likely caused by the impact of steric hindrance effects. Coordination chemistry studies were evaluated through slope analysis, ultraviolet–visible spectroscopy, nuclear magnetic resonance spectroscopy, electrospray ionization mass spectrometry, Fourier transform infrared spectroscopy and fluorescence, and all the results indicating that lanthanides with the above ligands can form both 1:1 and 1:2 metal/ligand complexes during extraction, and two ligands are tetradentate ligands. Density functional theory (DFT) calculations shed light that the interaction between the carbonyl oxygen atom and Eu(III) in the ligand is much stronger than that of the ether-bonded oxygen atom, and the analysis of the bond-forming properties shows that the stability of the 1:2 complex is stronger than that of the 1:1 complex. This work enriched the coordination study of diamide with lanthanides, which is expected to improve our understanding of the chemical properties of lanthanides during coordination and provide new insight into the design and synthesis of novel ligands.