Crystal structure of potassium sodium heptahydrogen hexamolybdocobaltate(III) octahydrate: an extra-protonated B-series Anderson-type heteropolyoxidometalate

A novel hexamolybdate(VI) polyanion protonated by an extra H atom is an unexpected polyanion species among the B-series Anderson-type polyoxometalates (POMs). The extra (seventh) H atom does not lie on a crystallographic centre of symmetry, but is located at the mid-point between two μ2-O atoms of adjacent polyanions, and forms a very short hydrogen bond.

Considering the geometry of the interpolyanion hydrogen bonds by an extra H atom (seventh H atom), observed via electron-density maps around the protonated 2 -OB atoms and bond valence sums (BVSs; Brown & Altermatt, 1985;Brese & O'Keeffe, 1991) of the protonated 2 -OB atoms in the polyanion, we can determine that the positions of the extra H atoms follow a pseudosymmetric model in the polyanion. Sometimes a short hydrogen bond (OÁ Á ÁO h 2.60 Å ), in which the H atom lies on a crystallographic centre of symmetry, occurs in this class of structure (Lee et al., 2010;Joo et al., 2015b). The focus of this report is to clarify the position of the extra H atom of the polyanion in the title compound. Mo 2 -OC-Co). The protonated O atoms in the polyanion were confirmed by the BVSs, the charge balance, the bondlength elongation and the interpolyanion hydrogen bonds ( Fig. 3 and Table 1). Consider the symmetry relation of O7B and O10B atoms, the electron density of the H atom between atoms O7B and O10B in the difference Fourier map (Fig. 2) and the very short O7BÁ Á ÁO10B distance of 2.430 (5) Å . Also consider the bond elongations by protonation of Mo1/2-O7B and Mo4/5-O10B, and the bond angles of Mo-OB-Mo. These data suggest that O7B or O10B in the polyanion should be protonated.
Confirmation of the protonated O atom was strongly supported by the BVS analysis.  (Brown, 2002). This showed that atom H7 in the polyanion has a distance of 1.21 Å with 0.41 v.u. As a result, the valence sums around O7B and O10B are 2.04 and 2.01 v.u., respectively. Therefore, these valence unit values satisfy the protonation conditions of O7B and O10B atoms in the polyanion. As a result, these data suggest that H7 is located on the midpoint between O7B and O10B ii atoms (the symmetry code corresponds to that in Fig. 3). However, the H7 atom contributes to the short hydrogen bonds, and does not lie on a crystallographic centre of symmetry; also, the electron density is not symmetric in the polyanion (Fig. 2), although we expect H7 atom to lie in the middle of the bond, which corresponds to a pseudosymmetric short hydrogen bond. This means that an extra H atom is co-shared by an adjacent polyanion; for example, 2 -O7BÁ Á ÁH7Á Á Á 2 -O10B ii (Fig. 3).
The BVSs for the K1, K2, and Na1 ions are 0.50, 0.55, and 1.26 v.u, respectively, in the title compound (NaÁ Á ÁO h 2.50 Å and KÁ Á ÁO h 3.00 Å ). BVS calculations for K1 and K2 reveal a considerable under-saturation in terms of valence units, which we ascribe to the disordered character of the K + position. All  (7) Symmetry codes:

Figure 1
The polyanion structure and the cations as well as the lattice water molecules in the title compound. Displacement ellipsoids are drawn at the 50% probability level for non-H atoms. H atoms are drawn as small spheres of arbitrary radius.

Figure 2
Difference Fourier map between atoms O7B and O10B, where H atoms were absent.
the BVSs agree well with the charge-balance requirements.

Synthesis and crystallization
Title compound was obtained from the ion-exchanged solution (ca pH 1.4) of K 3 [H 6 CoMo 6 O 24 ]Á7H 2 O (Lee et al., 2001) by Amberlite IR120. The resulting solution was concentrated in a hot water bath. After 1 d, stable blue crystals were obtained at room temperature. The Na + ion in the title compound is considered to have been a contaminant from the ion-exchange resin.

Refinement
The crystal data, the data collection and the structure refinement details are summarized in Table 2. All H atoms in the polyanion and all H atoms in the water molecules were located from difference Fourier maps. All H atoms of the polyanion were refined with a distance restraint of O-H = 0.85 (3) Å , except O7B-H7, and were included in the refinement with U iso (H) = 1.5U eq (O). The bond lengths of O7B-H7 and O10B-H7 i (the symmetry code corresponds to that in Fig. 3) were constrained by using the SADI ( = 0.03) command; they were set to be equal with an effective standard uncertainty to locate the shared H atom on the pseudocentre between atoms O7B and O10B.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (