Efficient Separation of Europium Over Americium Using Cucurbit-[5]-uril Supramolecule: A Relativistic DFT Based Investigation

Abstract

Achieving an efficient separation of chemically similar Am(3+)/Eu(3+) pair in high level liquid waste treatment is crucial for managing the long-term nuclear waste disposal issues. The use of sophisticated supramolecules in a rigid framework could be the next step toward solving the long-standing problem. Here, we have investigated the possibility of separating Am(3+)/Eu(3+) pair with cucurbit-[5]-uril (CB[5]), a macrocycle from the cucurbit-[n]-uril family, using relativistic density functional theory (DFT) based calculations. We have explored the structures, binding, and energetics of metal-CB[5] complexation processes with and without the presence of counterions. Our study reveals an excellent selectivity of Eu(3+) over Am(3+) with CB[5] (ion exchange free energy, ΔΔGAm/Eu > 10 kcal mol(-1)). Both metals bind with the carbonyl portals via μ(5) coordination arrangement with the further involvement of three external water molecules. The presence of counterions, particularly nitrate, inside the hydrophobic cavity of CB[5], induces a cooperative cation-anion binding, resulting in enhancement of metal binding at the host. The overall binding process is found to be entropy driven resembling the recent experimental observations (Rawat et al. Dalton Trans. 2015, 44, 4246-4258). The optimized structural parameters for Eu(3+)-CB[5] complexes are found to be in excellent agreement with the available experimental information. To rationalize the computed selectivity trend, electronic structures are further scrutinized using energy decomposition analysis (EDA), quantum theory of atom in molecules (QTAIM), Mülliken population analysis (MPA), Nalewajski-Mrojek (NM) bond order, and molecular orbital analyses. Strong electrostatic ion-dipole interaction along with efficient charge transfer between CB[5] and Eu(3+) outweighs the better degree of covalency between CB[5] and Am(3+) leading to superior selectivity of Eu(3+) over Am(3+).

Role of Ligand Straining in Complexation of Eu3+–Am3+ Ions by TPEN and PPDEN, Scalar Relativistic DFT Exploration in Conjunction with COSMO-RS

“…30,31 The minimum-energy structures using both the COSMO and COSMO-RS approaches of the hydrated clusters of La 3+ , Sm 3+ , Eu 3+ , and Am 3+ are displayed in Figure S1, and the calculated structural parameters are presented in Table S1. The average M–O distances using both COSMO-RS and COSMO were found to be in fair agreement with the experimentally determined values using EXAFS spectroscopy 30,31 as well as the earlier computed results 32−34 (see Table S1). Overall, the calculated average bond distance was found to follow the increasing trend: Eu 3+ < Sm 3+ < Am 3+ < La 3+ as per the experimental findings and thus confirms the acceptance of the present computation method, which is subsequently used for the study of the complexes of TPEN and PPDEN in the next section.…”

An Insight into the Complexation of Trivalent Americium Vis‐à‐Vis Lanthanides with Bis(1,2,4‐triazinyl)bipyridine Derivatives

“…The degree of covalence in a metal–ligand bond can directly be correlated with the mixing coefficient between the metal and ligand orbitals, which is lower in the Eu–BTBP than in the Am–BTBP complex . Recently, a similar non‐participation of Eu f‐based orbitals in the valence MO of its cucurbit[5]uril complex was reported, whereas in the respective Am 3+ complex the f orbitals contribute significantly to the formation of the higher‐lying MOs. This is due to the lower energy of the 4f orbitals of Eu 3+ as compared to the 5f orbitals of Am 3+ as indicated in the literature .…”

Achieving highly efficient and selective cesium extraction using 1,3-di-octyloxycalix[4]arene-crown-6 in n-octanol based solvent system: experimental and DFT investigation

“…The above methodology was earlier reported to produce good correlation with the experimental results. [54][55][56] Finally, binding free energy of the metal binding reactions was calculated using following reaction scheme:…”