Syntheses and crystal structures of hydrated complexes of strontium and barium bromides with 18-crown-6

Abstract

Two complexes, namely, triaqua(18-crown-6)strontium dibromide monohydrate (I) and diaquabromo(18-crown-6)barium bromide (II), are synthesized. Their crystal structures are determined by X-ray diffraction analyses. For complex I, space group C2/c, a = 17.547 A, b = 10.246 A, c = 14.786 A, β = 123.08°, Z = 4. For complex II, space group Pnma, a = 17.753 A, b = 17.465 A, c = 6.629 A, Z = 4. The structures are solved by a direct method and refined by the full-matrix least-squares method in the anisotropic approximation to R = 0.056 (I) and 0.042 (II) for 2696 (I) and 2440 (II) independent reflections (CAD-4 automated diffractometer, λMoKα radiation). Both complex cations—randomly disordered [Sr(18C6)(H2O)3]2+ in complex I and [BaBr(18C6)(H2O)2]+ in complex II—are of the host-guest type. The Sr2+ (Ba2+) cation resides in the cavity of the 18-crown-6 ligand and coordinated by all six O atoms. In the structures complexes I and II, the coordination polyhedra of the Sr2+ and Ba2+ cations (coordination number 9) can be described as distorted hexagonal bipyramids with one apex at the O atom of the water molecule in complex I or at the Br− ligand in complex II and the other split apex at the O atoms of two water molecules.