Quantum Chemistry Study on the Extraction of TrivalentLanthanide Series by Cyanex301: Insights from Formation of Inner-and Outer-Sphere Complexes

摘要

The extraction of lanthanide series by Cyanex301, i.e., bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HC301), has been modeled by density functional theory calculation, taking into account the formation of both inner- and outer-sphere complexes. The inner-sphere complex Ln(C301)3 and the outer-sphere complex Ln(H2O)9(C301)3 are optimized, followed by the analysis of interaction energy, bond length, Laplacian bond orders, and Mulliken populations. The covalency degree increases in Ln–S and Ln–O bonds in the inner- and outer-sphere complexes, respectively, as the lanthanide series is traversed. Mulliken population analysis indicates the important role of the 5d-orbital participation in bonding in the formation of inner- and outer-sphere complexes. Two thermodynamic cycles regarding the formation of inner- and outer-sphere complexes are established to calculate the extraction Gibbs free energies (ΔGextr), and relaxed potential energy surface scan is utilized to model the kinetic complexation of C301 anion with hydrated metal ions. Light lanthanides can form both inner- and outer-sphere complexes, whereas heavy lanthanides only form outer-sphere complexesin biphasic extraction. After adopting the data of forming inner-spherecomplex for light Ln(III) and that of forming outer-sphere complexesfor heavy Ln(III), the trend of the calculated −ΔGextr agrees very well with that of the experimentaldistribution ratios on crossing the Ln(III) series. Results from thiswork help to theoretically understand the extraction behavior of Cyanex301with respect to different Ln(III).