Crystal structure of [Y6(μ6-O)(μ3-OH)8(H2O)24]I8·8H2O

In the crystal structure of a hexanuclear Y3+ compound, the six Y3+ cations are arranged octahedrally around an μ6-O atom at the centre of the cationic complex. Each of the eight faces of the Y6 octahedron is capped by an μ3-OH group in the form of a distorted cube. The proximity of the cationic complexes and lattice water molecules leads to the formation of a three-dimensional hydrogen-bonded network of medium strength.

The crystal structure of the title compound {systematic name: octa-3hydroxido-6 -oxido-hexakis[tetraaquayttrium(III)] octaiodide octahydrate}, is characterized by the presence of the centrosymmetric molecular entity [Y 6 ( 6 -O)( 3 -OH) 8 (H 2 O) 24 ] 8+ , in which the six Y 3+ cations are arranged octahedrally around a 6 -O atom at the centre of the cationic complex. Each of the eight faces of the Y 6 octahedron is capped by an 3 -OH group in the form of a distorted cube. In the hexanuclear entity, the Y 3+ cations are coordinated by the central 6 -O atom, the O atoms of four 3 -OH and of four water molecules. The resulting coordination sphere of the metal ions is a capped square-antiprism. The crystal packing is quite similar to that of the orthorhombic [Ln 6 ( 6 -O)( 3 -OH) 8 (H 2 O) 24 ]I 8 Á8H 2 O structures with Ln = La-Nd, Eu-Tb, Dy, except that the title compound exhibits a slight monoclinic distortion. The proximity of the cationic complexes and the lattice water molecules leads to the formation of a three-dimensional hydrogen-bonded network of medium strength.

Chemical context
Rare-earth-based oxido-hydroxido polynuclear complexes are of interest because of their unique luminescence (Chen et al., 2010;Le Natur et al., 2013;Petit et al., 2009), magnetic properties (Abbas et al., 2010;Xu et al., 2011) or structural characteristics (Zheng, 2001;Andrews et al., 2013). Actually, in this kind of complex, the spatial proximity between metal ions affords cooperative/synergetic effects or energy-transfer mechanisms workable in terms of optical properties. For more than a decade, our group has been involved in the synthesis and the characterization of such rare-earth-based hexanuclear complexes (Calvez et al., 2010). The hexanuclear complexes crystallize in different structures depending on the counteranion (e.g. nitrate, perchlorate, iodide: Zak et al., 1994;Wang et al., 2000;Mudring et al., 2006), the number of lattice water molecules and/or the radius of the involved lanthanide ion. Since the pioneering work of Zak et al. (1994), we have developed a systematic synthetic procedure for the nitrate counter-anion complex with most of the rare earth elements (Calvez et al., 2008(Calvez et al., , 2010. In this context, we have undertaken the study of a series of complexes based on the iodide counteranion which have never been obtained with heavier rare earth ions. We report here the synthesis and crystal structure of the yttrium derivative.
Since the 6 -O atom is located on an inversion centre and binds to six Y 3+ cations, a slightly distorted anion-centred [OY 6 ] octahedron results (Fig. 2). The average of the YÁ Á ÁY distances between adjacent cations in the octahedron is found to be 3.536 Å . The mean Y-( 6 -O) distance is 2.537 Å , while the averaged Y-( 3 -OH) is 2.34 Å . The hydroxide ions are situated above the eight faces of the OY 6 octahedron and form a distorted cube around the octahedron (Fig. 2).

Supramolecular features
The hexanuclear [Y 6 ( 6 -O)( 3 -OH) 8 (H 2 O) 24 ] 8+ units are arranged in a body-centred fashion in the crystal structure. Each of these units is surrounded by twelve iodide anions, connecting the units to each other through Coulombic interactions. Although the hydrogen atoms of the water molecules and hydroxide groups could not be located, the range of OÁ Á ÁO distances between the cationic complex and the lattice water molecules suggest the formation of medium-strength The OY 6 octahedron in the complex [Y 6 ( 6 -O)( 3 -OH) 8 (H 2 O) 24 ] 8+ cation. Y atoms are green and O atoms are red.

Figure 3
The crystal structure of [Y 6 ( 6 -O)( 3 -OH) 8  hydrogen bonds (Table 1). These interactions lead to the formation of a three-dimensional network in the structure (Fig. 3).

Synthesis and crystallization
Yttrium oxide Y 2 O 3 (2 g, Strem Chemicals 4M) was dissolved in fresh hydroiodic acid (9 ml, 57wt%, unstabilized from Acros Organics) under gentle heating (323 K). If the acid used is not fresh, it should be distilled twice. The clear solution was exposed to air under isothermal conditions (6 weeks). At this stage, the pH of the solution remains acidic. Large pale-yellow polyhedral crystals were separated manually from the solution and were mounted into a glass capillary.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms from the water molecules or hydroxide could not be assigned reliably and thus were not included in the refinement. However, they were taken into account for the chemical formula sum, moiety, weight, as well as for the absorption coefficient and the number of electrons in the unit cell.

Special details
Experimental. 6336 sampling points Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.