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

Abstract

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 C_2v. 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.