Bis{tris[3-(2-pyridyl)-1H-pyrazole]zinc(II)} dodecamolybdosilicate hexahydrate

The asymmetric unit of the title compound, [Zn(C8H7N3)3]2[SiMo12O40]·6H2O, consists of a complex [Zn(C8H7N3)3]2+ cation, half of a Keggin-type [SiMo12O40]4− heteropolyanion and three uncoordinated water molecules. The Zn2+ cation is surrounded in a distorted octahedral coordination by six N atoms from three chelating 3-(2-pyridyl)pyrazole ligands. In the heteropolyanion, two O atoms of the central SiO4 group ( symmetry) are equally disordered about an inversion centre. N—H⋯O hydrogen bonding between the cations, anions and the uncoordinated water molecules leads to a consolidation of the structure.

Financial support from the Chinese Academy of Sciences (Hundred Talents Program)

Comment
The design and synthesis of polyoxometalates has attracted continuous research interest not only because of their appealing structural and topological novelties, but also due to their interesting optical, electronic, magnetic, and catalytic properties, as well as their potential medical applications (Pope & Müller, 1991). In our research group, organic amines, such as 3-(2-pyridyl)pyrazole and pyrazine, are used to effectively modify polyoxomolybdates under hydrothermal condictions (Zhang, Dou et al., 2009;Zhang, Wei et al., 2009). Here, we describe the synthesis and structural characterization of the title compound.
As shown in Figure 1, the title compound consists of three subunits, viz. of a complex [Zn(C 8 H 7 N 3 ) 3 ] 2+ cation, a heteropolyanion [SiMo 12 O 40 ] 4anion and of uncoordinated water molecules. The zinc(II) ion is in a distorted octahedral coordination by six N atoms from three chelating 3-(2-pyridyl)pyrazole ligands. The Zn-N bond lengths are in the range of 2.134 (7)-2.196 (7) Å. In the Keggin-type heteropolyanion, each Mo atom is surrounded by six O atoms and the Si atom  Table 2).

Refinement
All hydrogen atoms bound to aromatic carbon atoms were refined in calculated positions using a riding model with a C-H distance of 0.93 Å and U iso = 1.2U eq (C). Hydrogen atoms attached to aromatic N atoms were refined with a N-H distance of 0.86 Å and U iso = 1.2U eq (N). The hydrogen atoms of the three uncoordinated water molecules could not be located unambiguously from difference Fourier maps, probably due to disorder of the water molecules. Thus the structure was supplementary materials sup-2 refined without the H atoms of the water molecules (which includes the water O atoms O1W, O2W, O3W). In the SiO 4 unit, the two oxygen atoms (O5 and O20) are equally disordered about the inversion centre. One of the bridging O atoms (O16) is also disordered and was refined with split positions and an occupancy ratio of 1:1. In the final difference Fourier map the highest peak is 2.93 Å from atom O3w and the deepest hole is 0.20 A Å from atom O17. The highest peak is located in the voids of the crystal structure and may be associated with an additional water molecule. However, refinement of this position did not result in a reasonable model. Hence this position was also excluded from the final refinement. Fig. 1. The building blocks of the title compound with the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level; H atoms are given as spheres of arbitrary radius.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )