Predicting Stability Constants for Uranyl Complexes Using Density Functional Theory

Abstract

The ability to predict the equilibrium constants for the formation of 1:1 uranyl/ligand complexes (log K1 values) provides the essential foundation for the rational design of ligands with enhanced uranyl affinity and selectivity. We use density functional theory (B3LYP) and the integral equation formalism polarizable continuum model (IEF-PCM) to compute aqueous stability constants for UO2(2+) complexes with 18 donor ligands. Theoretical calculations permit reasonably good estimates of relative binding strengths, while the absolute log K1 values are significantly overestimated. Accurate predictions of the absolute log K1 values (root-mean-square deviation from experiment <1.0 for log K1 values ranging from 0 to 16.8) can be obtained by fitting the experimental data for two groups of mono- and divalent negative oxygen donor ligands. The utility of correlations is demonstrated for amidoxime and imide dioxime ligands, providing a useful means of screening for new ligands with strong chelating capability to uranyl.

Amidoximes as ligand functionalities for braided polymeric materials for the recovery of uranium from seawater

“…As was determined in our previous work [35], the DFT-based method permitted reasonably good estimates of relative binding strengths for a series of 13 oxygen donor ligands, whereas the absolute complexation energies of uranyl complexes were significantly overestimated. As the formation constants for the uranyl cation with acetam and bzam are accurately measured, we can now apply our computational protocol [35] to calculate the complexation free energies for amidoxime ligands (eq 4) and rank-order their stability in comparison with monovalent oxygen donor ligands.…”

“…The quantitative prediction of absolute stability constants for multivalent metal ions poses a formidable challenge, because the solvation free energy of a multivalent ion is not fully accounted for by treating only the first hydration shell around a metal ion explicitly [64]. As was determined in our previous work [35], the DFT-based method permitted reasonably good estimates of relative binding strengths for a series of 13 oxygen donor ligands, whereas the absolute complexation energies of uranyl complexes were significantly overestimated. As the formation constants for the uranyl cation with acetam and bzam are accurately measured, we can now apply our computational protocol [35] to calculate the complexation free energies for amidoxime ligands (eq 4) and rank-order their stability in comparison with monovalent oxygen donor ligands.…”

“…The results given in Table 2 indicate that two recommended [35] theoretical methods correctly determine the order of ligand stability, but significantly overestimate the absolute values of ΔG aq (2). We note that the less negative ΔG aq (2) values obtained using the M06 and SMD protocol in comparison with the B3LYP and IEF-PCM protocol is due to generally higher ion solvation free energies predicted with the SMD solvation model [27].…”

“…and SMD [34] solvation models in Gaussian. We used a combination of the B3LYP functional with the IEF-PCM solvation model and the M06 functional with the SMD solvation model that were shown in our previous study [35] to provide the best overall performance in predicting the log K 1 values of uranyl complexes with negative oxygen donor ligands.…”

Cu(i) and Ag(i) complex formation with the hydrophilic phosphine 1,3,5-triaza-7-phosphadamantane in different ionic media. How to estimate the effect of a complexing medium

“…It also has to be taken into account that the gas-phase free energies (G°g as ) are calculated with the standard state of an ideal gas at 1 atm, but the solvation free energies are computed for a 1 M standard state. 50 Thus, the free energy in water (G°w ater ) has to be calculated including the term correcting for the change of the standard state: …”