Selective complexation of UO ²⁺₂ by the calix[6]arene⁶⁻ anion: Structure and hydration studied by molecular dynamics simulations

Abstract

Based on molecular dynamics simulations, we describe the structure and solvation pattern of thep-Me-calix[6]arene⁶⁻ anion in the free state, and complexed with UO ²⁺₂ in aqueous solution. UO ²⁺₂ is coordinated in its equatorial plane to five phenolate oxygens, and has no direct coordination with water molecules. The closest water molecules are strongly hydrogen bonded to the ligand. The complex of thep-Me-calix[5]arene⁵⁻ anion with UO ²⁺₂ displays similar binding and solvation features. For the purpose of comparison, a complex ofp-Me-calix[6]arene⁶⁻ with a spherical divalent cation, referred to as ‘Cu²⁺’ is simulated under the same conditions. In the most stable conformer, Cu²⁺ is coordinated to four phenolic oxygens in a square planar arrangement, and to one water molecule. An energy component analysis suggests that the high binding selectivity of the calixarene for uranyl relates to a large cation-guest interaction, and to a better hydration of the complex. In the UO ²⁺₂ complex the strong hydration of the negatively charged ligand is not significantly prevented by the complexed cation, unlike in the spherical cation complexes. It is stressed that entropic effects, which are not amenable to such calculations, play an important role in the binding selectivity of these calix[6] and calix[5]arenes for uranyl.