Preparation and crystal structure of the paramagnetic solid F3CCSSSCCF3AsF6: implications for the identity of RCSSCR˙+

Abstract

4,5-Bis(trifluoromethyl)-1,2,3-trithiolylium hexafluoroarsenate, which contains the 7π F₃C[graphic omitted]CF₃˙⁺ radical cation, the first example of a cationic carbon–sulfur heterocycle, has been prepared and characterised. It was prepared as a purple solid in 90% yield by the reaction of F₃CCCCF₃ with a 1 : 1 mixture of S₄(AsF₆)₂ and S₈(AsF₆)₂ in SO₂ at 50 °C for 2 weeks. Small amounts of a homopolyatomic sulfur cation (probably S₈²⁺) were removed by work-up with fresh F₃CCCCF₃ and AsF₅ in SO₂ at 50 °C for 8 d. The F₃C[graphic omitted]CF₃AsF₆ was characterised in the solid state by chemical analyses, IR, mass spectrometry [M⁺ at m/z 258 (100%)] and X-ray crystallography. The crystal structure consists of layers containing both monomeric, planar F₃C[graphic omitted]CF₃˙⁺ radical cations and AsF₆^– anions [space group Pnma, a= 12.241(4), b= 8.232(5), c= 37.39(3)Å, Z= 12 and R= 0.086]. A powder photograph of the bulk product correlated with the single-crystal data, and a variable-temperature magnetic susceptibility study indicated that F₃C[graphic omitted]CF₃AsF₆ is a paramagnetic solid, exhibiting ordinary Curie–Weiss behaviour, µ= 1.68, θ=–0.6 K (µ= 2.1 in SO₂ solution at room temperature). The ESR spectrum of F₃C[graphic omitted]CF₃˙⁺ in SO₂ is identical to that previously reported for F₃C[graphic omitted]CF₃˙⁺[g= 2.014, a(¹⁹F)= 1.3 G, ‘a(³³S)’= 8.6 G], which suggested that spectra long assigned to 1,2-dithiete cations are in fact due to 1,2,3-trithiolylium cations. This is supported by the ESR spectrum of MeO₂C[graphic omitted]CO₂Me˙⁺, which consisted of a singlet at g= 2.017, with two sets of ³³S satellites in a 2:1 intensity ratio, with very similar ³³S coupling constants (8.0 and 8.9 G). The salt F₃C[graphic omitted]CF₃AsF₆ is reduced by KI to give a mixture of neutral cyclic polysulfides including F₃C[graphic omitted]CF₃(43%) and the previously unknown F₃C[graphic omitted]CF₃(23%). Attempts to oxidise F₃C[graphic omitted]CF₃˙⁺ with AsF₅ were unsuccessful.