Single-crystal neutron structure analysis of (NH4)21[H3Mo57V6(NO)6O183(H2O)18]•53 H2O

The crystal structure of (NH4)(21)[H3Mo57V6(NO)(6)O-183 (H2O)(18)] . 53 H2O, a supramolecular heteropoly cluster compound (space group P6(3)/mmc, Z=2, final R1 = 0.1302 (I > 2sigma(I)) for 1745 unique reflections), was redetermined by single-crystal neutron diffraction studies at 20K. The X-ray diffraction results reported in 1994 by Mijller et al. (Z. Anorg. Allg. Chem. 620, 599) are confirmed. Additionally, we could localize many hydrogen positions not found so far and establish a phase transition near 240 K. Many of the ammonium ions, the ligand and hydrate H2O molecules, and the hydroxy group are orientationally disordered, even at 20 K. The central cavity of the structure is built up by two twelve-membered rings consisting of six O-(HO)-O-... hydrogen bonds each. These strong hydrogen bonds are obviously decisive for the stability of the cluster. The hydrate H2O molecules are stronger-hydrogen-bond acceptor groups than the oxoligands of the cluster. (C) 2002 Elsevier Science (USA).