Bis(4-aminopyridinium) μ6-oxido-dodeca-μ2-oxido-hexaoxido[rhenium(VII)tetratungsten(VI)vanadium(V)]ate heptahydrate

In the title organic–inorganic hybrid compound, (C5H7N2)2[ReVW4O19]·7H2O, the Lindqvist-type polyoxido anion has crystallographically imposed mm2 symmetry and is built up by six MO6 (M = W, V, Re) edge-sharing distorted octahedra. The Re and V atoms share the same crystallographic site in a 0.5:0.5 ratio. The 4-aminopyridinium cations lie on a mirror plane. Three of the four independent water O atoms in the asymmetric unit are located on a mirror plane whereas the remaining O atom has mm2 site symmetry. In the crystal, the cations, anions and water molecules are linked into a three-dimensional network through O—H⋯O and N—H⋯O hydrogen-bonding interactions.

In the title organic-inorganic hybrid compound, (C 5 H 7 N 2 ) 2 [ReVW 4 O 19 ]Á7H 2 O, the Lindqvist-type polyoxido anion has crystallographically imposed mm2 symmetry and is built up by six MO 6 (M = W, V, Re) edge-sharing distorted octahedra. The Re and V atoms share the same crystallographic site in a 0.5:0.5 ratio. The 4-aminopyridinium cations lie on a mirror plane. Three of the four independent water O atoms in the asymmetric unit are located on a mirror plane whereas the remaining O atom has mm2 site symmetry. In the crystal, the cations, anions and water molecules are linked into a three-dimensional network through O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen-bonding interactions.

Experimental
Crystal data (C 5 Table 1 Hydrogen-bond geometry (Å , ). The asymmetric unit of (I) contains 1.75 water molecules, one half of a 4-aminopyridinium cation and one fourth of a [ReVW 4 O 19 ] 2polyanion, in which Re and V metals share the same site in a 0.5:0.5 ratio. All constituents are completed by imposed crystallographic symmetries (Fig. 1). Three out of the four independent water O atoms in the asymmetric unit have mirror symmetry, while a fouth O atom has mm2 site symmetry. It is to be mentioned that the O3W and O4W oxygen atoms associated to water molecules have relatively high thermal desorder and could not be anisotropically refined. In the crystal packing, cations and solvent water molecules assemble the discrete [ReVW 4 O 19 ] 2anion complexes through OW-H···O and N-H···O hydrogen bonding interactions (Table 1) (Brown & Altermatt, 1985) gives effective bond valences of 4.9847 for the V cation, 7.1447 for the Re cation and 6.0697 and 6.1369 for the two independent W cations. These values are consistent with the oxidation states V(V), Re(VII) and W(VI). In an attempt to shed more light on the structure of the hexametalate anion, we contemplated 51 V and 183 W NMR studies on the title complex. In fact, regarding the [M 6 O 19 ] ncomplex structure, the vanadium and rhenium heteroatoms may occupy cis or trans positions in the octahedral structure. The 183 W NMR study shows two signals with relative intensity of ca 2:2, which can be assigned to the two equatorial (Weq) and the two axial (Wax) tungsten atoms respectively. On the other hand, the 51 V NMR spectrum presents one signal at 508.7 ppm. These results are consistent with the C 2v symmetry of the disubstitued hexametalate structure with preferential cis configuration as reported in literature (Chen et al., 2004;Domaille, 1984;Fedotov & Maksimovskaya, 2006;Leparulo-Loftus & Pope, 1987). All these observations corroborate supplementary materials sup-2 Acta Cryst. (2013). E69, m661-m662 the structural results in suggesting that both rhenium and vanadium atoms (with 0.5/0.5 occupation) occupy preferentially the same site thus leading to a cis-[X 2 W 4 O 19 ] 2-(X = Re 0.5 V 0.5 ) anion configuration.

Experimental
The title compound was prepared by the reaction of vanadium(V) oxide (0.18 g, 1 mmol), rhenium(VII) oxide (0.48 g,1 mmol), Na 2 WO 4 .2H 2 O (2 g, 6 mmol) and 4-aminopyridine (0.19 g, 2 mmol) dissolved in 50 ml of distilled water and then stirred for 1 h. Yellowish single crystals were obtained after two weeks by slow evaporation at room temperature (yield:

Refinement
An initial attempt to refine the crystal structure with the Re atom disordered over three independent sites resulted in rather high atomic displacement parameters for the heaviest atoms (Re1, W1 and W2) as a possible consequence of thelarge number of restraints required by this model. The refinement of a model implying the Re atom sharing only the site occupied by the vanadium atom with an occupancy factor of 0.5 rapidly converged to a plausible result with low residuals. These observations are also supported by NMR study as detailed in Comment section. In spite the crystal selected for the X-ray analysis appeared to be of good quality, its diffraction ability was very poor. This may account for the rather high residual peaks, high R values and unresolved disorder affecting part of the water molecules. In fact, anisotropical refinement of the O atoms associated to water molecules resulted in unreasonable U ij values for atoms O3W and O4W, which were therefore isotropically refined. The water H atoms could not be located and were placed

Bis(4-aminopyridinium) µ 6 -oxido-dodeca-µ 2 -oxido-hexaoxido[rhenium(VII)tetratungsten(VI)vanadium(V)]ate heptahydrate
Crystal data (C 5  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 > σ(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 )
x y z U iso */U eq Occ.