A comparative molecular dynamics study on the complexation of alkali metal cations by a poly-ethylene-glycol type podand in water and in dichloromethane

Molecular dynamics (MD) was used to study the complexation of the alkali metal cations (Li+, Na+, K+, Rb+ and Cs+) by a poly-ethylene-glycol type podand with eight potential donor oxygen atoms, in water and dichloromethane. MD simulations for the free podand in water provided evidence for a solvent driven stabilizing effect of certain conformations. The complexes in water proved to be unstable as they all dissociated well within the simulation time used. In contrast, the complexes proved to be stable in the organic solvent. Analysis of the MD trajectories for the complexes in dichloromethane showed: (i) an enlargement of the podand OC-CO dihedral angles as the cation size increases; (ii) an increase of the number of strong interactions between the oxygen atoms as the cation size increases; (iii) a considerable degree of solvent penetration into the inner coordination sphere from the potassium to cesium complexes. Quantum calculations showed a higher energetic destabilization for the lithium and sodium complex forms. The results support the experimental evidence that the extraction efficiency shown by this podand (from water to dichloromethane) is higher for potassium, rubidium and cesium than for lithium or sodium.