Inorganic Chemistry, Vol.59, No.1, 161-171, 2020
"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 -CF3 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 -CF3-derivatized aryldithiophosphinates have not been considered to date. This systematic study considers the complexation of three CF3-substituted aryldithiophosphinates-bis(phenyl)-dithiophosphinate (L-I), [o-(trifluoromethyl)phenyl](phenyl)-dithiophosphinate (L-II), and bis[o-(trifluoromethyl)phenyl]clithiophosphinate (L-III), with Nd3+ in an ethanolic environment. The chelating ability of Nd3+ by these ligands follows the order of L-III > L-II > L-I, which is in line with the reported extraction efficiency. The positive Delta S, as well as positive Delta H, suggests that Nd3+ chelation is entropy-driven and effective desolvation is critical to enabling Nd(3+ )interaction with otherwise wealdy 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 in form up to 1:3 complexes with Nd3+. All three L-III anions have bidentate interactions with Nd-III , but two L-II anions have bidentate interactions with Nd3+, while the third L-II anion is monodentate. The significant increase in Delta S 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.