(466e) Relative Stability Complexes of Lanthanide-Ligand Complexes from Computation

Relative stability constants of lanthanide (Ln)-ligand complexes quantify the selectivity of a ligand toward specific lanthanide ions, which is crucial in the separation of rare earth elements from one another. Ln-ligand stability constants can be measured experimentally or calculated with binding energies. Electronic structure calculations were done to obtain binding energies of La³⁺, Eu³⁺, Gd³⁺, and Lu³⁺ with ethylenediaminetetraacetic acid (EDTA) based on the structures of Ln-EDTA complexes in solution resolved with ab initio molecular dynamics simulations. The binding energies were used to calculate the relative stability constants and compared to experimentally measured ones. Ab initio molecular dynamic simulations were done at both 25 °C and 90 °C to determine how variations in temperature affect complex stability. In addition, the EDTA ligand was simulated at two protonation states corresponding to pH ~7 (HEDTA^3-) and pH ~11 (EDTA^4-) to determine the effect of solution pH has on stability of Ln-EDTA complexes. Results demonstrate that the trend of increasing complex stability along the lanthanide series was found to be correlated to decreasing structural deviations with an increase in temperature. Moreover, by protonating one amine group of the EDTA ligand, the solution structure of the Ln-ligand complex is significantly altered in a way that reduces the stability of the complex.