Thiophosphoryl Complexes of Trivalent Lanthanide Cations: Importance of Counterions and Stoichiometry for Binding Energies. A Theoretical Study

Abstract

The intrinsic interaction energies ΔE between trivalent lanthanide cations (M3+ = La3+, Eu3+, and Yb3+) and thiophosphoryl R3PS (R = H, Me, Et, and Ph) ligands L are investigated by quantum mechanical calculations on charged LM3+ or neutral LMCl3 and L2MCl3 complexes. Counterions are found to markedly modulate the binding properties. First, in 1:1 complexes, the order of the alkyl and phenyl substituent effects is inversed in R3PM3+ (where ΔE increases in the order R = Me ≪ Et ≪ Ph) compared to R3PMCl3 (where ΔE increases in the order R = Ph < Me < Et). Second, the coordination mode of the thiophosphoryl bond evolves from a linear coordination in LM3+ (related to polarization of L induced by the "hard" M3+ cation) to a bent coordination in LMCl3 (related to the “soft” metal sulfur interaction), as in the protonated forms R3PSH+. In addition, the stoichiometry of the complexes is found to determine the selectivity in the cation series. In all charged or neutral 1:1 complexes, ΔE increases with the cation hardness (La3+ < Eu3+ < Yb3+) for a given L. However, in the 1:2 (Me3PS)2MCl3 complexes, ΔE follows the order Yb3+ < Eu3+ < La3+, due to the interplay between the ligands, the anions, and their interactions with the M3+ cation. These results are important for our understanding of the factors which determine the arrangements of ligands around M3+ cations in condensed phases where the first coordination sphere is saturated and generally involves neutralizing counterions. They also have bearing on the discrimination between hard vs less hard cations (like trivalent lanthanides vs actinides) by extractant molecules involving soft binding sites.