The reaction of S2NAsF6 with halogens: preparation and X-ray crystal structure of bis(difluorothio)nitronium hexafluoroarsenate(V), (SF2)2NAsF6; preparation of (SBr)2NAsF6, and vibrational spectrum and normal-co-ordinate analysis of the (SX)2N+(X = Cl or Br) cations
Solutions of S2NAsF6 in liquid SO2 react with elemental chlorine and bromine yielding (SX)2NAsF6(X = Cl or Br), essentially quantitatively. No reaction was detected with iodine. The vibrational spectrum of (SBr)2N+ was similar to that of (SCl)2N+ of known structure, implying a similar structure for the bromine derivative. This conclusion was supported by a normal-co-ordinate analysis of (SX)2N+. The analysis was consistent with some positive interaction between the halogen atoms in (SX)2N+, possibly accounting for the cis planar geometry of these cations. Attempts to prepare (SF)2NAsF6 were unsuccessful. However, (SF2)2NAsF6 was synthesised by the reaction of S2NAsF6 and XeF2 in liquid SO2F2, essentially quantitatively. The structure of (SF2)2NAsF6 was determined by X-ray diffraction. The crystals are orthorhombic with a= 14.909(1), b= 9.843(4), c= 12.113(1)Å, and Z= 8. The structure was refined in space group Pbca to a conventional R factor of 0.076 for 902 independent reflections with I 2σ(I). It consists of discrete (SF2)2N+ and AsF6– with some cation–anion interactions. The (SF2)2N+ cation has approximate C2v. symmetry with essentially eclipsed fluorine–sulphur bonds as viewed along the sulphur–sulphur axis. The average S–N and S–F distances are 1.551(10) and 1.523(8)Å, and the average FSF and FSN bond angles are 94.0(5) and 100.2(6)°. The SNS bond angle is 121.1(6)°. The vibrational spectrum of (SF2)2NAsF6 is reported.