Redetermination of the crystal structure of 3,5-dimethylpyrazolium β-octamolybdate tetrahydrate

The title compound, (C5H9N2)4[Mo8O26]·4H2O, was reported previously from a room-temperature data collection from which only the metal atoms could be refined anisotropically [FitzRoy et al. (1989 ▸). Inorg. Chim. Acta, 157, 187–194]. The current redetermination at 180 (2) K models all the non-H atoms with anisotropic displacement parameters and fully describes the supramolecular N—H⋯O and O—H⋯O hydrogen-bonded network connecting the 3,5-dimethylpyrazolium cations, the water molecules of crystallization and the β-octamolybdate anion. All H atoms involved in the three-dimensional hydrogen-bonding network could be located from difference Fourier maps, with the exception of those of one disordered water molecule, firstly seen in this structural report [refined over two distinct locations with site-occupancy factors of 0.65 (2) and 0.35 (2)]. The complete β-octamolybdate anion is generated by a crystallographic inversion centre.


Related literature
For the previous determination of the title compound at room temperature (Cambridge Structural Database refcode: JAMFEI), see: FitzRoy et al. (1989). For a description of the Cambridge Structural Database, see: Groom & Allen (2014). For previous studies investigating recovered molybdenum(VI) catalysts, see: Amarante et al. (2015); Lysenko et al. (2015).
Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL2014.  (Amarante et al., 2015;Lysenko et al., 2015). Often, the use of drastic experimental conditions leads to the formation of secondary products, which crystallize in the medium as trace amounts of impurity compounds. It is, thus, imperative that most of these possible products are fully described in the solid state in the most accurate fashion.  (1989). Besides the fact that the authors did not fully elucidate the hydrogen bonding network of this material (hydrogen atoms were placed geometrically) and that only the metallic centers could be refined anisotropically (from a room-temperature determination), the unit-cell parameters reported in the main text and in the abstract do not match (viz. the c axis length).
A search and match at the Cambridge Structural Database (Groom et al., 2014) also seems to ignore the presence of Mo(VI) metal centers. In this context, we decided to recollect the crystal structure of the title compound at low temperature to fully elucidate its finer structural details.

S2. Structure description
The asymmetric unit of the title compound is composed of two 3,5-dimethylpyrazolium cations, (C 5 H 9 N 2 ) + , one half of the β-octamolybdate anion, β-[Mo 8 O 26 ] 4− , and two water molecules of crystallization. Noteworthy, while one water molecule was fully located in its crystallographic position and even the associated hydrogen atoms found from difference Fourier maps, the other was found to be disordered over two distinct locations, O2W and O3W (Fig. 1). This feature was not disclosed in the previous structural determination by FitzRoy et al. (1989). The β-octamolybdate anion, located in the center of the unit cell, interacts with the remaining chemical species through a series of both electrostatic interactions and hydrogen bonds (Fig. 2). One crystallographically independent 3,5-dimethylpyrazolium cation donates both the hydrogen atoms bound to nitrogen to form two strong [D···A distances of 2.801 (4) and 2.869 (5) Å] and relatively directional [<(DHA) angles of 120 and 152°] charged N-H···O interactions with the polyoxoanion. The other cation has, however, a completely distinct behaviour: the same hydrogen atoms are instead donated in similar (strong and highly directional) interactions with neighbouring water molecules: while the D···A distances are 2.689 (7) and 2.776 (5) Å, the <(DHA) interaction angles are close to linearity, being 171 and 164°. The water molecules are instead interacting with the β-octamolybdate anion as depicted in both Figs. 1 and 2. Indeed, as depicted in Fig. 3 the water molecules play a decisive role in the overall crystal packing, acting as molecular fillers to effectively occupy the available space left from the arrangement of inorganic anions and organic cations.

S3. Synthesis and crystallization
All chemicals were purchased from commercial sources and used as received without additional purification steps.
A Teflon-lined stainless steel vessel was charged with a reaction mixture composed of MoO 3 (0.34 g, 2.43 mmol), 3,5dimethylpyrazole (0.11 g, 1.21 mmol) and water (ca 25 ml) and heated in an oven at 160 °C for 26 h. The resultant blueish solid was filtered from the aqueous mother liquor and washed with an excess of water and (4×10 ml) diethyl ether, dried at ambient temperature and characterized in the solid state. Colourless plates of the title compound were directly harvested from the walls of the Teflon vessel.

S4. Refinement details
Hydrogen atoms bound to carbon atoms were placed at idealized positions with C-H = 0.95 or 0.98 Å (for the aromatic and methyl groups, respectively), and included in the final structural model in riding-motion approximation with the isotropic thermal displacement parameters fixed at 1.2 or 1.5×U eq , respectively, of the carbon atom to which they are attached.
Hydrogen atoms associated with nitrogen atoms have been directly located from difference Fourier maps and were included in the model with the N-H distances restrained to 0.95 (1) Å in order to ensure a chemically reasonable environment for these moieties. These hydrogen atoms were modelled with the isotropic thermal displacement parameters fixed at 1.5×U eq (N).
A total of two water molecules of crystallization were directly located from difference Fourier maps. Though O1W was included in the final structural model by assuming full site occupancy and a typical anisotropic displacement behaviour, the second molecule was found to be disordered over two close crystallographic positions: O2W and O3W. These species were included in the structural model with linked site occupancy [which ultimately refined to 0.65 (2) and 0.35 (2), respectively] and by assuming an independent isotropic displacement behaviour. For O1W the two hydrogen atoms were markedly visible in difference Fourier maps and were included in the final model with the O-H and H···H distances restrained to 0.95 (1) and 1.55 (1) Å, respectively, in order to ensure a chemically reasonable geometry for this molecule.
These hydrogen atoms were modelled with the isotropic thermal displacement parameters fixed at 1.5×U eq (O1W).

Figure 1
Schematic representation of the molecular entities composing the asymmetric unit of the title compound. The βoctamolybdate anion has been completed by inversion symmetry for the sake of chemical accuracy. All non-hydrogen atoms are represented as displacement ellipsoids drawn at the 60% probability level and hydrogen atoms as small spheres with arbitrary radii. Non-hydrogen atoms belonging to the asymmetric unit have been labelled for clarity. Dashed green broken lines indicate N-H···O and O-H···O hydrogen-bonding interactions (see Table for geometrical details).