Crystal structure and Hirshfeld surface analysis of bis[hydrazinium(1+)] hexafluoridosilicate: (N2H5)2SiF6

The title compound is a salt formed by a hydrazinium (1+) cation and a hexafluoridosilicate anion interconnected by N—H⋯N and N—H⋯F hydrogen bonds.

The crystal structure of N 2 H 6 SiF 6 is well described by Frlec et al. (1980) and by Cameron et al. (1983), whereas that of (N 2 H 5 ) 2 SiF 6 has not previously been reported to our best knowledge. However, this compound was characterized by chemical analysis, vibrational spectroscopy and X-ray powder photography by Gantar & Rahten (1986), who determined the unit-cell parameters and the space group. We now describe the synthesis, single crystal structure and Hirshfeld surface analysis of the title compound, (I), at room temperature.

Structural commentary
Compound (I) is an inorganic molecular salt built up from N 2 H 5 + cations and SiF 6 2À anions, as shown in Fig. 1. In this ISSN 2056-9890 structure, all atoms are in general positions except for the silicon atom, which is located at the Wyckoff position 2d on the inversion centre 1 of the space group P2 1 /n. Thus, the silicon atom is connected to three unique fluorine atoms and their symmetry equivalents, forming a slightly elongated octahedron with Si-F distances in the range of 1.6777 (4) to 1.7101 (4) Å . The minimum and maximum cis F-S-F angles are 89.26 (2) and 90.74 (2) , respectively. The N-N separation in the cation is 1.4416 (8) Å .

Supramolecular features and Hirshfeld surface analysis
In the extended structure of (I), the hydrazinium cations are linked by strong N-HÁ Á ÁN hydrogen bonds (Table 1), building an infinite zigzag chain propagating along the [010] direction as shown in Fig. 2. The [SiF 6 ] 2À anion interacts with the (N 2 H 5 ) + cations through electrostatic attraction and accepts no fewer than ten simple, bifurcated or trifurcated N-HÁ Á ÁF hydrogen bonds (Fig. 3, Table 1). This results in a threedimensional network in which the hydrazinium cations build zigzag chains parallel to the b-axis direction and the [SiF 6 ] 2À anions are stacked along the [100] direction (Fig. 4). The packing of (I) was further investigated and quantified with a Hirshfeld surface analysis (McKinnon et al., 2004;Spackman & Jayatilaka, 2009) and two-dimensional fingerprint plots generated using the CrystalExplorer package (Turner et al., 2017).
The acceptor atoms in the interactions are shown with negative electrostatic potentials (red regions), and donor atoms are shown with positive electrostatic potentials (blue regions). The N-HÁ Á ÁF interactions in the structure are apparent from the relatively bright red-spots on the Hirshfeld surface of (I) mapped over d norm (Fig. S1 in the supporting information). In order to provide quantitative information on the contribution of the intermolecular interactions to the crystal packing, the three-dimensional d norm surface is resolved into two-dimensional fingerprint plots, generated based on d e and d i distance scales and illustrated in Fig. 5 Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
Molecular structure of (I) with displacement ellipsoids drawn at the 50% probability level.

Synthesis and Infrared measurement technique.
Hydrazinium (1+) hexafluoridosilicate (N 2 H 5 ) 2 SiF 6 crystals in the form of colourless blocks were obtained by slow evaporation, at room temperature, of an aqueous solution containing stoichiometric amounts of hydrazine NH 2 NH 2 and H 2 SiF 6 . The infrared spectrum was recorded in the range 450-4000 cm À1 with a Vertex 70 FTIR spectrometer.

Bis[hydrazinium(1+)] hexafluoridosilicate
Crystal data   (10) 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.