4,7,13,18-Tetraoxa-1,10-diazoniabicyclo[8.5.5]icosane hexafluoridosilicate

The asymmetric unit of the title molecular salt, C14H30N2O4 2+·SiF6 2−, contains half of both the anion and the cation, both ions being completed by a crystallographic twofold axis passing through the Si atom. The cation has a cage structure with the ammonium H atoms pointing into the cage. These H atoms are shielded from intermolecular interactions and form only intramolecular contacts. There are short intermolecular C—H⋯F interactions in the structure, but no conventional intermolecular hydrogen bonds.

The asymmetric unit of the title molecular salt, C 14 H 30 N 2 O 4 2+ ÁSiF 6 2À , contains half of both the anion and the cation, both ions being completed by a crystallographic twofold axis passing through the Si atom. The cation has a cage structure with the ammonium H atoms pointing into the cage. These H atoms are shielded from intermolecular interactions and form only intramolecular contacts. There are short intermolecular C-HÁ Á ÁF interactions in the structure, but no conventional intermolecular hydrogen bonds.
This structure was originally solved as a hexafluorophosphate salt, but the P-F bond lengths appeared unusually long and corresponded to typical Si-F rather than P-F bonds. The highest difference peak was close to H-N + , indicating that the occupancy of the H atom was higher than 0.5, which would be required to ensure charge neutrality in a PF 6 salt. The most negative difference density was observed near the central P atom, showing that in reality there are less electrons there.
Furthermore, refinement of the data with Si gave slightly lower R factors than with P. Though similar long P-F bonds are not unprecedented (Braband et al., 2003;Llusar et al., 2001), the above observations very strongly suggested that this was a diprotonated SiF 6 2salt rather than a monoprotonated PF 6 salt.
The presence of both anions in the reaction product was confirmed by 19 F NMR data collected in a CD 3 OD solution.
SiF 6 2is assumed to be generated by the formation of HF upon the hydrolysis of PF 6 and by the consequent reaction of HF with the silica of the glass. A smilar case was reported by Anderson et al. (2006).
In the crystal of compound (I), the two ammonium hydrogen atoms of the diprotonated cryptand cage are pointing inwards. Cryptands are known to form proton crypts, in which the protons are very efficiently concealed inside a tight molecular cavity. No exception is observed here: the ammonium hydrogen atoms are not involved in intermolecular hydrogen bonding. They only form intramolecular contacts with the oxygen atoms of the cryptand.

Experimental
Reagents were purchased from Sigma-Aldrich and were used without further purification. Reactions were carried out under inert conditions by Schlenk-line techniques. The metal chloride (WCl 6 , 100 mg, 0.25 mmol) was allowed to stir for a minute in 10 ml toluene and then was reacted with a small excess of of AgPF 6 (381 mg, 1.51 mmol) to give AgCl as a precipitate and toluene for 30 minutes. The crude reaction product was obtained as dirty yellow mass after drying the solvent. Portions of the product were recrystallized from acetone which produced crystal (I).

Refinement
Hydrogen U iso 's were set at 1.2 times the U eq of the heavy atom to which the hydrogen was attached and refined in riding mode. C-H distances were fixed at 0.97 Å and the N-H distance at 0.91 Å.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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 > 2σ(F 2 ) is used only for calculating Rfactors(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