"Sweeping" Ortho Substituents Drive Desolvation and Overwhelm Electronic Effects in Nd3+ Chelation: A Case of Three Aryldithiophosphinates.

Bis[o-(trifluoromethyl)phenyl]dithiophosphinate is a sulfur-donating ligand capable of providing the largest reported trivalent lanthanide (Ln3+)-actinide (An3+) group separation factors. Literature has shown that the placement and number of the -CF3 functionalities on the aryl rings proximate to the ligating sulfur atoms can significantly impact Ln3+-An3+ extraction and separation factors, but the complexation thermodynamics of -CF3-derivatized aryldithiophosphinates have not been considered to date. This systematic study considers the complexation of three CF3-substituted aryldithiophosphinates-bis(phenyl)dithiophosphinate (LI), [o-(trifluoromethyl)phenyl](phenyl)dithiophosphinate (LII), and bis[o-(trifluoromethyl)phenyl]dithiophosphinate (LIII), with Nd3+ in an ethanolic environment. The chelating ability of NdIII by these ligands follows the order of LIII > LII > LI, which is in line with the reported extraction efficiency. The positive ΔS, as well as positive ΔH, suggests that Nd3+ chelation is entropy-driven and effective desolvation is critical to enabling Nd3+ interaction with otherwise weakly interacting sulfur-containing ligands. Extended X-ray absorption fine structure results confirm thermodynamic investigations and suggest that LI can only form up to 1:2 (M-L) complexes, while LII and LIII form up to 1:3 complexes with Nd3+. All three LIII anions have bidentate interactions with NdIII, but two LII anions have bidentate interactions with Nd3+, while the third LII anion is monodentate. The significant increase in ΔS with each o-CF3 addition suggests aiding desolvation could be central in enabling f-element interaction with weakly interacting donor groups, and this report provides an approach to controlling f-element desolvation as an innovative f-element chelating strategy.