“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 (Ln 3+ )-actinide (An 3+ ) group separation factors. Literature has shown that the placement and number of the -CF 3 functionalities on the aryl rings proximate to the ligating sulfur atoms can significantly impact Ln 3+ -An 3+ extraction and separation factors, but the complexation thermodynamics of -CF 3 -derivatized aryldithiophosphinates have not been considered to date. This systematic study considers the complexation of three CF 3 -substituted aryldithiophosphinates-bis(phenyl)dithiophosphinate (L I ), [ o -(trifluoromethyl)phenyl](phenyl)dithiophosphinate (L II ), and bis[ o -(trifluoromethyl)phenyl]dithiophosphinate (L III ), with Nd 3+ in an ethanolic environment. The chelating ability of Nd III by these ligands follows the order of L III > L II > L I , which is in line with the reported extraction efficiency. The positive Δ S , as well as positive Δ H , suggests that Nd 3+ chelation is entropy-driven and effective desolvation is critical to enabling Nd 3+ interaction with otherwise weakly interacting sulfur-containing ligands. Extended X-ray absorption fine structure results confirm thermodynamic investigations and suggest that L I can only form up to 1:2 (M-L) complexes, while L II and L III form up to 1:3 complexes with Nd 3+ . All three L III anions have bidentate interactions with Nd III , but two L II anions have bidentate interactions with Nd 3+ , while the third L II anion is monodentate. The significant increase in Δ S with each o -CF 3 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.