Crystal structure of the bis(picrato-O,O ')tetraaquacalcium complex with 18-crown-6, [Ca(Pic)(2)(H2O)(4)] center dot 18C6

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 diffr action analysis: space group C2/c, a = 20.446 Angstrom, b = 14.985 Angstrom , c = 16.163 Angstrom, beta = 135.41 degrees, Z = 4. The structure of I was solved by the direct method and refined by the full-matrix least-squares m ethod in the anisotropic approximation to R = 0.047 over all 3044 measured independent reflections (CAD4 automated diffractometer, lambda MoKalpha rad iation). In the crystal, I is not a guest-host complex but exists as indivi dual [Ca(Pic)(2)(H2O)(4)] and 18C6 molecules joined by intermolecular hydro gen bonds and van der Whals interactions. The Ca2+ cation (CN 8) is located on a twofold crystallographic axis, its coordination polyhedron being a di storted square antiprism. The centrosymmetrical 18C6 molecule has a crown c onformation with approximate D-3d symmetry and has six neighboring water mo lecules (three on each side of its mean plane), which form eight hydrogen b onds involving all six O atoms of 18C6.