Crystal structure of an organic–inorganic supramolecular salt based on a 4,4′-methylenebis(3,5-dimethyl-1H-pyrazol-2-ium) cation and a β-octamolybdate anion

The crystal structure of the title novel organic–inorganic supramolecular salt is based in the in situ formation of 4,4′-methylenebis(3,5-dimethyl-1H-pyrazol-2-ium) cations, which are engaged in N—H⋯O hydrogen bonds with β-octamolybdate anions.


Chemical context
4,4 0 -Methylenebis(3,5-dimethylpyrazole) (H 2 mbdpz) is a flexible organic molecule which has been extensively used in the last few years by various research groups to design coordination-based and organic solids. While, on the one hand, the central methylene moiety confers some conformational flexibility to the entire molecular unit, on the other the two peripheral pyrazole rings permit not only the coordination to various types of metal atoms but also the involvement of these moieties in complex networks based on hydrogen bonds. It is, thus, not surprising to encounter a rich chemistry and structural diversity associated with this molecule. A search in the literature and in the Cambridge Structural Database (CSD; Allen, 2002;Groom & Allen, 2014) reveals, for example, that H 2 mbdpz has been used as an effective bending spacer to construct a large number of metal-organic frameworks (MOFs) or coordination polymers with various remarkable topologies based on a rather diverse range of d-block metals (Goswami et al., 2013;Mondal et al., 2008;Timokhin et al., 2015). H 2 mbdpz and its derivatives have also been used to prepare a range of supramolecular networks based on either neutral organic molecules or in the formation of salts with a wide range of anions (since, typically, the two pyrazole moieties appear protonated) (Basu et al., 2009;Basu & Mondal, 2010;Hazra et al., 2010). Most of these structural reports available in the literature either use H 2 mbdpz purchased from commercial sources or the authors prepare the molecule using published procedures. For the latter case, the standard method dates back to that reported by Trofimenko (1970), but more recent and alternative approaches are also employed to prepare the intended molecule (Kruger et al., 2000). ISSN 2056-9890 In this communication, we report the unexpected isolation of a new supramolecular salt in which 4,4 0 -methylenebis(3,5dimethyl-1H-pyrazol-2-ium) (H 4 mbdpz 2+ ) is prepared in situ, inside the autoclave reaction vessel, starting from 3,5-dimethylpyrazole in a reaction catalysed by Mo VI ions in the presence of hydrogen peroxide. To balance the cationic charge of the protonated H 4 mbdpz 2+ moiety, the crystal contains the well-known -octamolybdate anion. It is remarkable to note that, despite the intensive research on supramolecular structures based on H 2 mbdpz, only a couple of very recent reports contain polyoxidometalate-type anions. Indeed, Tian et al. (2014Tian et al. ( , 2015 described various Ag + -based MOFs (or coordination polymers) in which Mo VI or W VI Keggin and/or Wells-Dawson polyoxidometalates balance the positive charge of the cationic architectures.
The H 4 mbdpz 2+ cation exhibits the typical structural features found in related compounds. The considerable steric hindrance imposed by the two peripheral 3,5-dimethyl-1Hpyrazol-2-ium moieties induces a tetrahedral angle of the bridging methylene group of 113.56 (17) , which is very close to the median value found in similar structures (from the CSD: median of 114.7 from 109 hits with range of 111.0-120.0 ). Conversely, the dihedral angle subtended by these two peripheral moieties is significantly more dependent on the crystal structure itself, with the literature values (from 109 hits in the CSD) ranging from as low as 55.1 (a chiral coordination polymer with Cu 2+ described by Lin et al., 2014) to 90.0 (an Ni 2+ layered network described by Goswami et al., 2013). Nevertheless, the interplanar angle registered for the title compound, 77.85 (15) , agrees well with the median value of all structures deposited in the CSD (81.1 ).
The molecular geometrical parameters for the -octa-  (8) interval. This wide dispersion for the internal octahedral angles is a notable and well-known consequence of the marked trans effect created by the terminal oxido groups, which displace the metal atoms from the center of the octahedra. The intermetallic Mo VI distances within the -octamolybdate anion range from 3.1875 (5) Å (for the Mo1Á Á ÁMo2 distance) to 3.5810 (5) Å [for the Mo1Á Á ÁMo1 i distance across the inversion center of the anion; symmetry operation: (i) Àx, 1 À y, 1 À z].

Supramolecular features
The crystal packing of the title compound is essentially mediated by the presence of various N-HÁ Á ÁO hydrogenbonding interactions between the H 4 mbdpz 2+ cation (which acts as the donor -D) and the -octamolybdate anion (the acceptor -A) (Fig. 2a). As depicted in Table 1 Schematic representation of the molecular entities composing the asymmetric unit of the title compound. The -octamolybdate 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 violet lines indicate N-HÁ Á ÁO hydrogen-bonding interactions (see Table 1 for geometrical details). Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx; Ày þ 1; Àz; (ii) Àx; Ày þ 1; Àz þ 1.

D-HÁ
distances are relatively short, ranging between 2.730 (2) and 2.977 (2) Å . It is noteworthy that the latter is associated with the N2-H2 group which is engaged in a bifurcated interaction with the neighbouring -octamolybdate anion (as depicted in Fig. 2a), hence leading to an average increase of the interatomic distances. Besides these interactions, the crystal structure is also rich in weak hydrogen bonds of the C-HÁ Á ÁO type (not shown) involving mainly the terminal methyl groups of the organic molecule. The various C-HÁ Á ÁO interactions present in the crystal structure are rather weak, with CÁ Á ÁO distances ranging from 3.203 (3) to 3.457 (3) Å , with <(CHO) interaction angles in the 123-168 interval.
The aforementioned hydrogen bonds between cations and anions lead to the formation of a two-dimensional supramolecular network parallel to the (010) plane (Fig. 2b). Individual supramolecular entities close-pack perpendicular to (010) to produce the crystal structure of the title compound (Fig. 3).
A mixture of MoO 3 (0.349 g, 2.42 mmol), 3,5-dimethylpyrazole (0.116 g, 1.21 mmol), water (23 mL) and H 2 O 2 (2 mL) was heated in a Teflon-lined stainless steel digestion bomb at 433 K for 26 h, at 373 K for 25 h, and finally slowly cooled down to ambient temperature over a period of 13 h. Single crystals of the title compound were obtained inside the Teflon vessel along with a yellow aqueous mother liquor (pH = 6) and a blueish solid, which was confirmed by powder X-ray diffraction studies to be residues of unreacted MoO 3 . FT

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2 Schematic representation of the type and role of N-HÁ Á ÁO hydrogen bond interactions present in the crystal structure of the title compound: (a) description of all interactions which connect the crystallographically independent 4,4 0 -methylenebis(3,5-dimethyl-1H-pyrazol-2-ium) cation to two neighbouring -octamolybdate anions; (b) portion of the twodimensional supramolecular layer placed in the ac plane of the unit cell formed by the connection between the molecular units present in the title compound. For geometrical details of the represented hydrogen bonds (as violet dashed lines) see Table 1. Symmetry operations used to generate equivalent atoms: (i) Àx, 1 À y, 1 À z; (ii) Àx, 1 À y, Àz.

Figure 3
Ball-and-stick schematic representation of the crystal packing of the title compound viewed in perspective along the [100] direction. The figure emphasizes, on the one hand, how the inorganic component of the crystal structure is fully embedded into an organic matrix based on the 4,4 0methylenebis(3,5-dimethyl-1H-pyrazol-2-ium) cation. On the other it shows how supramolecular hydrogen-bonded layers pack closely perpendicular to (010). approximation with isotropic displacement parameters fixed at 1.2 or 1.5U eq , respectively, of the carbon atom to which they are attached.
Hydrogen atoms associated with nitrogen atoms were directly located from difference Fourier maps and 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 isotropic thermal displacement parameters fixed at 1.5U eq (N).  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.