We report a theoretical study on the liquid−liquid interfacial behavior of the species involved in the extraction of Cs+ by a calix[4]arene-crown6 ionophore (L):  the free Cs+ Pic- and Cs+ Cl- salts, the LCs+ and LCs+ Pic- complexes, and uncomplexed L. Based on molecular dynamics simulations, we calculated the free energies changes for migration from the interface into the aqueous and the organic phases, respectively. For free L and for the LCs+ complex, with or without Pic- counterion, an energy minimum is found close to the interface, on the chloroform side, showing that these species behave as surfactants. This contrasts with the uncomplexed Cs+, which diffuses spontaneously from the interface to water and displays no energy minimum. A remarkable counterion effect is found with Pic- which displays a high affinity for the interface, while Cl- prefers the bulk aqueous phase. The questions of ion extraction by ionophores, counterions, concentration and synergistic effects in assisted cation transfer through the liquid−liquid interface between immiscible liquids are discussed from the interfacial point of view.

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We report an ab initio quantum mechanical study on the interaction of M(n)()(+) cations (M(n)()(+) = La(3+), Eu(3+), Yb(3+), Sr(2+), and Na(+)) with model ligands L for lanthanide or actinide cations: several substituted amides, pyridines, and the phosphoryl-containing OPPh(3) ligand. The interaction energies DeltaE follow trends expected from the cation hardness and ligand basicity or softness in the amide series (primary < secondary-cis < secondary-trans < tertiary) as well as in the pyridine series (para-NO(2) < H < Me < NMe(2)). Among all ligands studied, OPPh(3) is clearly the best, while the (best) tertiary amide binds lanthanides slightly less than the (best) pyridine-NMe(2) ligand. In the lanthanide 1:1 complexes, the energy differences DeltaDeltaE as a function of M(3+) (about 40 kcal/mol for all ligands) are less than DeltaDeltaE in the pyridine series (up to about 90 kcal/mol) where marked polarization effects are found. The conclusions are validated by a number of methodological investigations. In addition to optimal binding features, we also investigate the directionality of ion coordination to the ligands and the effect of counterions and stoichiometry on the structural, electronic and energetic features of the complexes. The results are discussed in the context of modeling complexes of lanthanide and actinide cations and compared to those obtained with analogous Na(+) and Sr(2+) complexes.

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We report a series of ab initio QM calculations on uranyl and Sr2+ complexes of OPR3 ligands (R = H Me Ph) to assess the role of substituents R and of NO3 - counterions on the intrinsic cation−ligand interaction energy. When there are no counterions, the binding sequence of UO2 2+ and of Sr2+ complexes follows the order R = H < Me < Ph, due to polarization and charge-transfer effects. However, in the presence of NO3 - counterions, the OPMe3 and OPPh3 complexes become of similar stability, due to the ligand−anion repulsive interactions. Complexes of OPR3 with the spherical Sr2+ cation are found to be less stable than those with the linear UO2 2+ cation. In the second part of the paper we report molecular dynamics simulations in water on 1:1 and 2:1 complexes of OPR3 with UO2(NO3)2. The changes in free energies of solvation upon electronic reorganization of the ligand and UO2(NO3)2 induced by complexation are investigated using statistical perturbation FEP techniques and found to be nearly independent of R. The importance of these results in the context of designing efficient ionophores for uranyl cations is discussed.

Molecular dynamics simulations performed on the Pic-···Pic- like-anion pair (Pic- = picrate = 2,4,6-trinitrophenoxide) show that its behavior is strongly dependent on the solvent. In water, the intimately stacked pair is stable, while in acetonitrile it dissociates. The stability of the stacked pair in water is confirmed by several methodological tests (choice of atomic charges; role of starting configuration; Ewald/no-Ewald) and by PMF (potentials of mean force) calculations. Simulations on (K+Pic-)2 and (K+Pic-)4 in water starting with unstacked anions lead to the formation of stacked dimers. The simulation of the dissolution of a (K+Pic-)27 fragment of the crystal also reveals a contrast in the behavior in water (formation of diluted stacks) compared to that in acetonitrile (formation of a “molten salt”). In the solid state, stacking arrangements of Pic- anions are common but diverse in form, supporting this theoretical prediction that the stacking of two like Pic- anions is common and highly environment dependent. Dimerization can be viewed as the primary stage of crystal nucleation. The results also have implications concerning the behavior of Pic- at liquid−liquid interfaces in electrochemical or assisted ion-transfer processes.

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