Interactions of Bis(2,4,4-trimethylpentyl)dithiophosphinate with Trivalent Lanthanides in a Homogeneous Medium: Thermodynamics and Coordination Modes.

Complexation of trivalent lanthanides with a sulfur-bearing ligand, bis(2,4,4-trimethylpentyl) dithiophosphinate, was studied in ethanol under identical conditions by optical spectroscopy, microcalorimetry, luminescence lifetime measurement, and extended X-ray absorption fine structure (EXAFS). Three successive complexes, LnL2+, LnL2+, and LnL3, where Ln and L denote the trivalent lanthanide and the dithiophosphinate ligand, respectively, formed in the solution. In contrast to the general findings that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, the complexation strength between Ln(III) and dithiophosphinate first increases from La(III) to Nd(III) and then decreases gradually toward heavier Ln(III) across the lanthanide series. This trend agrees well with the results of solvent extraction using the same ligand as an extractant. The complexation is driven by highly positive entropies and opposed by endothermic enthalpies. The enthalpies of complexation become less endothermic from lighter to heavier Ln(III), suggesting that less energy is required for desolvation for the complexation of heavier Ln(III). EXAFS study shows that, from lighter to heavier Ln(III), the number of sulfur atoms in the primary coordination sphere decreases while the number of oxygen atoms increases, which confirms that fewer solvent molecules are desolvated from heavier Ln(III) during the complexation process. A correlation between the thermodynamics trends and the coordination modes has thereby been well established.