Crystal Structure of the Bis(Picrato-O,O")Tetraaquacalcium Complex with 18-Crown-6, [Ca(Pic)2(H2O)4] · 18C6

Abstract

The crystal structure of the bis(picrato-O,O")tetraaquacalcium complex with 18-crown-6, [Ca(Pic)2(H2O)4] · 18C6 (I), was studied using X-ray diffraction analysis: space group C2/c, a= 20.446 A, b= 14.985 A, c= 16.163 A, β = 135. 41°, Z= 4. The structure of Iwas solved by the direct method and refined by the full-matrix least-squares method in the anisotropic approximation to R= 0.047 over all 3044 measured independent reflections (CAD4 automated diffractometer, λMoKαradiation). In the crystal, Iis not a guest–host complex but exists as individual [Ca(Pic)2(H2O)4] and 18C6 molecules joined by intermolecular hydrogen bonds and van der Waals interactions. The Ca2+cation (CN 8) is located on a twofold crystallographic axis, its coordination polyhedron being a distorted square antiprism. The centrosymmetrical 18C6 molecule has a crown conformation with approximate D3dsymmetry and has six neighboring water molecules (three on each side of its mean plane), which form eight hydrogen bonds involving all six O atoms of 18C6.