Crystal structure of hexaammonium octaaquadichromium-hexakis(μ3- oxo)-icosikaidikis(μ2-oxo)-tetradecaoxo-dodecatungsten hexahydrate, (NH4)6[Cr2(H2O)8W12O42] · 6H2O

Cr2H52N6O56W12, orthorhombic, Pbca (no. 61), a = 14.147(2) Å, b = 15.964(2) Å, c = 22.507(2) Å, V = 5083.0 Å, Z = 4, Rgt(F) = 0.032, wRref(F) = 0.076, T = 298 K. Source of material All commercially obtained reagent-grade chemicals were used without further purication. A mixture of CrCl3 (0.1 mmol, 0.016.g), (NH4)2WO4 (0.1 mmol, 0.029 g) and 2-methylpyridine (4 mL) was added into 20 mL water with 50 % (v/v) ethanol and heated for 12 h at 414 K. The solution was filtered after cooling to room temperature. Courless flake single crystals suitable for Xray diffraction measurements were obtained after a few weeks by evaporation of the solvent. 2-Methylpyridine does not take part in the reaction. Discussion Polyoxometalates (POMS) have definite sizes, shapes and resemble discrete fragments of metal oxide structures [1]. As one kind of metal oxide clusters with nanosize and abundant topology, POMs represent a vast class of inorganic materials with a virtually unmatched range of physical and chemical properties relevant for applications as diverse as medicine and biology [2], magnetism [3], materials science [4], and catalysis [5]. It is this unique range of properties which qualifies POM-based materials as prime candidates for the designed construction of molecularframework materials. The title crystal structure is composed of the well-known +metatungstate [W12O42] heteropolyanion, and two sixcoordinated Cr cations, which coordinated by four water molecules and two terminal oxygen atoms from the heteropolyanion. In addition, six lattice water molecule and six NH4 cations are present. The Keggin unit acts as tetra-dentate ligand coordinating two chromium cations through the terminal oxygen atoms of four (WO6) octahedra. The surface of the cluster is decorated by two secondary metal-ligand subunits Cr(H2O)4, the Cr site exhibits distorted octahedral environment through bonding to the oxygen atoms of the water molecules and the terminal oxo-group of the tungsten site of the +-metatungstate. Such linking of secondary metal-ligand subunits to polyoxoanion clusters appears to be a common characteristic for the hydrothermal chemistry of these oxides. The Cr—O distances vary from 2.354 to 2.410 Å. The bond angles of O–Cr–O range from 72.9° to 155.9°. Four kinds of oxygen atoms exist in the cluster according to the manner of oxygen coordination: Ot the terminal oxygen atom connecting only to one W atom; Ot’ the terminal oxygen atom linked to Cr and W atom; Ob the double-bridging oxygen atom; Oc oxygen atom connecting edge sharing WO6 octahedrons in the same W3O13 unit. The W—O distances very from 1.717 2.256 Å. It is also striking feature of the structure of title compound exhibiting extensive hydrogen-bonding interactions between water molecules, NH4 cations, and oxygen atoms in the Keggin anions. Z. Kristallogr. NCS 225 (2010) 231-232 / DOI 10.1524/ncrs.2010. 0100 231 © by Oldenbourg Wissenschaftsverlag, München Crystal: colorless flake, size 0.05 × 0.12 × 0.13 mm Wavelength: Mo K+ radiation (0.71073 Å) . : 275.69 cm−1 Diffractometer, scan mode: Bruker Smart Apex CCD, #/' 2,max: 52° N(hkl)measured, N(hkl)unique: 24858, 4996 Criterion for Iobs, N(hkl)gt: Iobs > 2 )(Iobs), 3792 N(param)refined: 343 Programs: SHELXS-97 [6], SHELXL-97 [6], SHELXTL [6] Table 1. Data collection and handling. H(1C) 8c 0.8129 0.6769 0.7231 0.050 H(1D) 8c 0.7535 0.7499 0.7350 0.050 H(1A) 8c 0.8486 0.7445 0.7619 0.050 H(1B) 8c 0.7744 0.6902 0.7835 0.050 H(2C) 8c 0.7672 0.7086 0.5426 0.044 H(2D) 8c 0.8315 0.6777 0.4967 0.044 Table 2. Atomic coordinates and displacement parameters (in Å). Atom Site x y z Uiso