Molecular dynamics study of 21C7 crown ether derivatives and their alkali cation complexes. Comparison with 1,3-alt-calix[4]arene-crown-6 compounds

Abstract

A molecular dynamics study in vacuo and in an explicit water phase was undertaken on 21C7 benzo and cyclohexano crown ether derivatives, in order to better understand the differences between these compounds and calixarene-crown-6 compounds regarding the complexation of alkali cations. Whatever the number and the position of the benzo or cyclohexano groups on the crown ether, this crown ether remains highly flexible when it is simulated in the gas phase. The most notable differences appear in simulations in an explicit aqueous phase and are discussed in terms of hydrophobicity of the free ligands and instability of non hydrated complexes in water. Calixarene-crown-6 compounds appear to be much better preorganized, with a smaller and less mobile ether loop due to its grafting onto the calixarene cavity. In contrast to the 21C7s, the crown can adapt perfectly only around caesium and penalises sodium particularly. Moreover, the calixarenes isolate better the cations from the bulk aqueous phase. These differences of preorganisation and cation accessibility to the solvent can explain the superior extracting properties and the stronger selectivity of calixcrowns.