1-Acetyloxymethyl-1,3,5,7-tetraazaadamantan-1-ium hexafluorophosphate

In the crystal structure of the title salt, C9H17N4O2 +·PF6 −, the cations and anions are linked by weak C—H⋯F interactions while C—H⋯O interactions also occur between the cations.

The author thanks an anonymous advisor from the Ordered Matter Science Research Centre, Southeast University, for great help in the revision of this paper.

Ming-Liang Liu Comment
Recently much attention has been devoted to finding ferroelectric complexes. Ferroelectric materials that exhibit reversible electric polarization in response to an external electric field have found many applications such as nonvolatile memory storage, electronics and optics. The freezing of a certain functional group at low temperature forces significant orientational motions of the guest molecules and thus induces the formation of the ferroelectric phase (Zhang et al. 2009;Ye et al. 2009;Zhang et al. 2010). The title compound has been synthesized to investigate these properties.
There is a similar structure reported by Reddy et al. (1994).
The asymmetric unit of C 9 H 17 N 4 O 2 .PF 6 consists of one 1-meyhyl acetate-1,3,5,7-tetra-aza-adamantan cation and one hexafluorophosphate anion linked by ionic bond (Fig 1). The hexafluorophosphate anion is a distorted octahedron. The P -F bonds are in the range 1.531 (4) to 1.559 (4) Å, the difference of the P-F bond distances are likely due to the different environment of F atoms. The bond angles around each phosphorus range from 84.3 (4)° to 179.1 (4)°. There is no classical hydrogen bond in the structure. The hexafluorophosphate anion is quite mobile, but examination of a difference map in the plane of the fluorine atoms does not show that fluorine atoms exist as three distinct atoms.

Experimental
Hexamine, ammonium acetate and acetic anhydride were dissolved in water to give a solution refluxing at 373K, then hexafluorophosphoric acid was added to the above solution and filtered it. Single crystals suitable for X-ray structure analysis were obtained by the slow evaporation of the above solution after 10 days in air.
The dielectric constant of the compound as a function of temperature indicates that the permittivity is basically temperature-independent (ε = C/(T-T 0 )), suggesting that this compound is not ferroelectric or there may be no distinct phase transition occurring within the measured temperature range (below the melting point).

Refinement
H atoms were placed in calculated positions with C-H = 0.96-0.97 Å, and refined in riding mode, U iso (H) = 1.2U eq (C) for methyl H atoms and 1.2U eq (C) for the others.  The molecular structure of the title compound, showing the atomic numbering scheme with 30% probability displacement ellipsoids.

Computing details
1-Acetyloxymethyl-1,3,5,7-tetraazaadamantan-1-ium hexafluorophosphate Crystal data 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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Symmetry codes: (i) −x, y+3/2, −z+3/2; (ii) −x, −y+2, −z.