Ammonium 4,4-difluoro-1,3,2-dithiazetin-2-ide 1,1,3,3-tetraoxide

The asymmetric unit of the title compound, NH4 +·CF2NO4S2 −, consists of two crystallographically independent ammonium cations and two 4,4-difluoro-1,3,2-dithiazetin-2-ide 1,1,3,3-tetraoxide anions all located in general positions. The S—C—S—N rings of both crystallographically independent anions are almost planar, with the N atom bent out of the plane by 9.82 (5) and 12.82 (4)°. The structure was determined from a crystal twinned by inversion, with refined components in the ratio 0.73 (4):0.27 (4). Anions and cations are connected via hydrogen bonds (N—H⋯O and N—H⋯N) to form a three-dimensional framework. This framework is composed of two different layers parallel to the ab plane, which are built by the ammonium cations on the one hand and the complex cyclic anions on the other.

The asymmetric unit of the title compound, NH 4 + ÁCF 2 NO 4 S 2 À , consists of two crystallographically independent ammonium cations and two 4,4-difluoro-1,3,2-dithiazetin-2-ide 1,1,3,3tetraoxide anions all located in general positions. The S-C-S-N rings of both crystallographically independent anions are almost planar, with the N atom bent out of the plane by 9.82 (5) and 12.82 (4) . The structure was determined from a crystal twinned by inversion, with refined components in the ratio 0.73 (4):0.27 (4). Anions and cations are connected via hydrogen bonds (N-HÁ Á ÁO and N-HÁ Á ÁN) to form a three-dimensional framework. This framework is composed of two different layers parallel to the ab plane, which are built by the ammonium cations on the one hand and the complex cyclic anions on the other.
The title compound ammonium 4,4-difluoro-1,3,2-dithiazetin-2-ide 1,1,3,3-tetraoxide ( Fig. 1) crystallizes in the orthorhombic non-centrosymmetric space group Pna2 1 with two independent cations and anions. The ammonium containing title structure is isotypical to the corresponding potassium salt whereas the rubidium salt crystallizes in the monoclinic space group P2 1 /c (Jüschke et al., 1997). The S-C-S-N ring is almost planar and the nitrogen atom is bent out of the plane by 9.82 (5) and 12.82 (4)°, respectively in the two anions. The bond lengths and angles of the cyclic anion in its [NH 4 ] + and K + salt are very similar, whereas slightly larger deviations are found for the Rb + salt. Medium to weak hydrogen bonds connect the [NH 4 ] + cations and the cyclic anions to form a three-dimensional framework (Fig. 2).
The cyclic anion accepts hydrogen bonds by the oxygen atoms of the SO 2 group and by its amide nitrogen atom. The N-H···N hydrogen bonds are, according to the derived distances weaker than the N-H···O bonds. In general N-H···N hydrogen bonds are rare in structurally related salts. For example in the structure of the ammonium triflamide salt, [NH 4 ] [F 3 C-SO 2 -N-SO 2 -CF 3 ] no hydrogen bond between the amide nitrogen and the ammonium counter cation are present (Davidson et al., 2003). The title structure can be understood as a layered material. The cations and anions are found in layers perpendicular to the c axis (Fig. 3). The ammonium cations appear ordered as a consequence of their hydrogen bonds. The resulting non-centrosymmetric, layered ammonium salt fits well in the general structural chemistry of ammonium salts with layered structures (e.g. Reiss, 2002;Plizko & Meyer, 1998;Bucholz & Mattes, 1988).

Experimental
The title compound was synthesized according to a literature procedure (Jüschke et al., 1997).

Refinement
In the final stages of refinement the Flack-parameter indicated inversion twinning. The refinement of the twin components (Flack, 1983;Sheldrick, 2008) gave a ratio of 0.73 (4): 0.27 (4). All hydrogen atom positions were identified in difference syntheses. In the final stages of refinement the hydrogen atom positions of these were refined with their N-H distances softly restrained to one common refined value (0.86 Å) with one common U iso value for each group. ; program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2011); software used to prepare material for publication: publCIF (Westrip, 2010).

Figure 1
The asymmetric unit of the title compound, with displacement ellipsoids drawn at the 50% probability level; H atoms are drawn as spheres of arbitrary radius.

Special details
Experimental. Absorption correction: CrysAlisPro, Oxford Diffraction Ltd., Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. 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. 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 > σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq S1 0.48379 (