Crystal structure of bis(diisopropylammonium) molybdate

The crystal structure of the title salt, (C6H16N)2[MoO4], results from N—H⋯O hydrogen-bonded rings formed through interconnections between the (iPr2NH2)+ cations and [MoO4]2− anions.

The organic-inorganic title salt, (C 6 H 16 N) 2 [MoO 4 ] or ( i Pr 2 NH 2 ) 2 [MoO 4 ], was obtained by reacting MoO 3 with diisopropylamine in a 1:2 molar ratio in water. The molybdate anion is located on a twofold rotation axis and exhibits a slightly distorted tetrahedral configuration. In the crystal structure, the diisopropylammmonium ( i Pr 2 NH 2 ) + cations and [MoO 4 ] 2À anions are linked to each other through N-HÁ Á ÁO hydrogen bonds, generating rings with R 12 12 (36) motifs that give rise to the formation of a three-dimensional network. The structure was refined taking into account inversion twinning (ratio of ca 4:1 between the two domains).
In the crystal structure of (CyNH 2 ) 2 MoO 4 Á2H 2 O (Cy = cyclohexyl; Pouye et al., 2014) the four Mo-O bond lengths are equal with 1.7613 (12) Å . Although in this structure similar N-HÁ Á ÁO intermolecular interactions between the (CyNH 2 ) + cation and the molybdate anion are present in comparison with the ( i Pr 2 NH 2 ) + cation in the title compound, the small differences in the hydrogen-bonding pattern result in slightly different Mo-O bond lengths between the two structures. On one hand this may be related to the presence of additional water molecules in (CyNH 2 ) 2 MoO 4 Á2H 2 O, on the other hand to steric hindrance between the four diisopropylammonium cations that surround each molybdate anion in (I). At least the strengths of the N-HÁ Á ÁO hydrogen bonds do not seem to have a noticeable effect on the different Mo-O distances in (I). Both hydrogen bonds are very similar in terms of NÁ Á ÁO distances and N-HÁ Á ÁO angles (Table 2).

Supramolecular features
In the crystal structure of (I), each [MoO 4 ] 2anion is linked to two pairs of symmetry-related diisopropylammonium cations through N-HÁ Á ÁO hydrogen bonds (Table 2). Contrariwise, each ( i Pr 2 NH 2 ) + cation is linked to two molybdate [MoO 4 ] 2À anions. The interaction of six molybdate anions with six diisopropylammonium cations leads to {( i Pr 2 NH 2 )Á Á ÁMoO 4 } 6 ring systems with an R 12 12 (36) motif (Etter et al., 1990). Each ring is linked to six adjacent rings giving rise to infinite layers extending parallel to (010) (Fig. 2). The connection of the rings into a three-dimensional network structure perpendicular to this plane is shown in Fig. 3.

Synthesis and crystallization
Compound (I) was obtained from a mixture of molybdenum trioxide (3.2 g, 22.23 mmol) and diisopropylamine (4 g, 44.46 mmol) in a 1:2 molar ratio in water. A clear, colourless solution was obtained after stirring for approximately one h. After twenty days of evaporation in an oven at 333 K, some colourless single crystals were obtained.  Symmetry code: (ii) y þ 1 2 ; Àx þ 1 2 ; z þ 1 4 .

Figure 1
Asymmetric unit view of (I) with displacement ellipsoids drawn at the 50% probability level and hydrogen atoms as spheres of arbitrary radius.
In the IR spectrum of (I) (Fig. 4a), the bands at 899 and 786 cm À1 can be attributed to symmetric and asymmetric Mo-O stretching modes, respectively. The disopropylammonium cation is characterized by a series of vibrational bands in the 3000-2200 cm À1 region, which can be attributed to (N-H), (C-H) and combination modes. The (N-H) bending vibrations probably contribute to the signal observed at 1598 cm À1 .
In the Raman spectrum of (I) (Fig. 4b), the band at 797 cm À1 is attributed to the antisymmetric stretching mode of the [MoO 4 ] 2À molybdate anion. The symmetric vibration, s (Mo-O), in the form of a weak shoulder at 839 cm À1 in the infrared spectrum, is very intense in the Raman spectrum at 896 cm À1 . In the high wavenumber region of the Raman spectrum, the bands between 3000 and 2800 cm À1 can be assigned to the (N-H) and (C-H) stretching vibrations of the diisopropylammonium cation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The structure was refined taking into account twinning by inversion (ratio of ca 4:1 between the two domains). H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with N-H distances of 0.89 Å and C-H distances of 0.96 Å for methyl and of 0.98 Å for methylene groups, and with U iso (H) = 1.2U eq (C,N) or 1.5U eq (C methyl ). IR (a) and Raman (b) spectra of (I). Refined as an inversion twin Absolute structure parameter 0.19 (7) Computer programs: CrysAlis PRO (Rigaku OD, 2015), SHELXS (Sheldrick, 2008), SHELXL (Sheldrick, 2015) and OLEX2 (Dolomanov et al., 2009).

Bis(diisopropylazanium tetraoxomolybdate
Crystal data (C 6  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.21 e Å −3 Δρ min = −0.31 e Å −3 Absolute structure: Refined as an inversion twin Absolute structure parameter: 0.19 (7) sup-2 Acta Cryst. (2018). E74, 1682-1685 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. Refined as a 2-component inversion twin.