ALKALINE-EARTH AND URANYL CATION COMPLEXES OF A CALIX[4]ARENE-TETRAAMIDE - MD AND FEP SIMULATIONS IN AQUEOUS AND ACETONITRILE SOLUTIONS ANDX-RAY STRUCTURE OF ITS SR(PICRATE)(2) COMPLEX

We report the X-ray structure of the L . Sr(Picrate)(2) (L = tert-buty l-calix[4] arene-tetrakis(diethylamide)dagger and MD simulations on th e L . M(2+) complexes in vacuo, in water and in acetonitrile solutions (M(2+) = Mg2+, Ca2+, Sr2+, Ba2+), with a comparison of 'converging' L (C) and 'diverging' L(D) conformers. Ln the simulated and solid-state structures of the L . Sr2+ complex, the cation is completely encapsula ted within the polar pseudo-cavity of L, without coordination to its c ounterion in the crystal, or to solvent molecules in solution. Computa tions show that the L . M(2+) complexes are of converging type in wate r and in acetonitrile, This contrasts with the L . M(+) alkali cation complexes, which display conformational flexibility in solution. Subtl e structural changes from Mg2+ to Ba2+ are compared in the gas phase a nd in solution. In the L . UO22+ hypothetical complex, simulated for c omparison, the UO22+ cation is calculated to be less bound by L than t he 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-b ottom mechanical coupling in the calixarene. Based on free energy pert urbation calculations, we calculate a binding sequence of alkaline ear th cations (Ca2+ > Sr2+ > Ba2+ > Mg2+) in agreement with experiment.