Alkaline earth and uranyl cation complexes of a calix[4]arene-tetraamide: MD and FEP simulations in aqueous and acetonitrile solutions and X-ray structure of its Sr(Picrate)2 complex

Abstract

We report the X-ray structure of the L·Sr(Picrate)₂(L =tert-butyl-calix [4] arene-tetrakis(diethylamide) and MD simulations on the L·M²⁺ complexes in vacuo, in water and in acetonitrile solutions (M²⁺= Mg ²⁺, Ca²⁺, Sr²⁺, Ba²⁺), with a comparison of ‘converging’ L_c and ‘diverging’ L_D conformers, In the simulated and solid-state structures of the L·Sr²⁺ complex, the cation is completely encapsulated within the polar pseudo-cavity of L, without coordination to its counterion in the crystal, or to solvent molecules in solution. Computations show that the L·M²⁺ complexes are of converging type in water and in acetonitrile. This contrasts with the L·M⁺ alkali cation complexes, which display conformational flexibility in solution. Subtle structural changes from Mg²⁺ to Ba²⁺ are compared in the gas phase and in solution. In the L·UO₂²⁺ hypothetical complex, simulated for comparison, the UO₂^{2 +} cation is calculated to be less bound by L than the alkaline earth cations. The solvent content of the cone is shown to depend on the size of the complexed cation and modulated by the top–bottom mechanical coupling in the calixarene. Based on free energy perturbation calculations, we calculate a binding sequence of alkaline earth cations (Ca²⁺ > Sr²⁺ > Ba²⁺ > Mg²⁺) in agreement with experiment.