STRUCTURAL AND MOLECULAR MECHANICS STUDIES OF BIS(DIBUTYL PHOSPHATO)AQUASTRONTIUM 18-CROWN-6 AND ANALOGOUS ALKALINE-EARTH-METAL COMPLEXES

The stability of bis(dibutyl phosphato)aquastrontium-18-crown-6 (1) has been compared with analogous real and hypothetical alkaline-earth-metal complexes by use of molecular mechanics calculations. Trial molecular structures from which energy minimization proceeded were obtained by crystal structure analysis of 1 and from its previously analyzed Ba analogue. Existence of these molecules in solution was also verified by IR and NMR spectra. Crystals of 1 are monoclinic, P21/c, with a = 9.23 (1) Å, b = 27.24 (3) Å, c =31.55 (3) Å, β = 94.95 (6)°, and Z = 8. In the two independent molecules of the crystal each Sr2+ion is coordinated by six O atoms of a crown ether, by two monodentate dibutyl phosphato ions, and by one H2O molecule. The differences in calculated total energies among these complexes are due primarily to the Coulombic contributions, the strain in the crown molecules being relatively unimportant within wide geometrical limits. An estimate of the selectivity of the crown/dibutyl phosphate combination among the hydrated alkaline-earth-metal ions suggests that they are selected in the order Ba > Sr > Ca > Ra > Mg. © 1987, American Chemical Society. All rights reserved.