Preparation and characterisation of thermally stable (SeNSeNSe)n(AsF6)2 containing the ‘electron-rich aromatic’ 6π(SeNSeNSe)2+(n= 1) and 7π SeNSeNSe˙+(n= 2)

Abstract

Crystalline thermally stable ([graphic omitted]e)_n(AsF₆)₂ containing the ‘electron-rich aromatic’ 6π [graphic omitted]e²⁺(n= 1) and 7π radical cation [graphic omitted]e˙⁺(n= 2) were prepared in high yields from reactions of Se₄N₄ with stoichiometric quantities of Se₄(AsF₆)₂(n= 1) or AsF₅(n= 1 or 2) in liquid SO₂ and their X-ray crystal structures determined. The structure of [graphic omitted]e(AsF₆)₂ consists of discrete planar [graphic omitted]e²⁺ cations and AsF₆^– anions, and that of ([graphic omitted]e)₂(AsF₆)₂ consists of two identical, but crystallographically different, discrete ([graphic omitted]e˙⁺)₂ cations and AsF₆^– anions. The centrosymmetric ([graphic omitted]e˙⁺)₂dimer contains two planar [graphic omitted]e˙⁺ radical cations weakly joined by two long Se ⋯ Se bonds [2 × 3.123(3), 2 × 3.149(3)Å]. There are significant cation–anion interactions in both salts. The Se–Se [2.334(3)Å] and Se–N bond lengths [average: 1.74(3)(side), 1.69(3)Å(top)] in [graphic omitted]e²⁺ are shorter than their corresponding distances in the 7π [graphic omitted]e˙⁺[average: Se–Se 2.398(3); Se–N 1.76(2)(side), 1.69(2)(top)Å] consistent with removal of the unpaired electron from the π* singly occupied molecular orbital of the monocation. Surprisingly only one peak, rather than the expected two, was observed in the ⁷⁷Se NMR spectrum of [graphic omitted]e(AsF₆)₂ consistent with fluxional behaviour in solution. The ⁷⁷Se chemical shift [–70 °C, δ(Me₂Se)= 2434, ν_½= 10 Hz] is the highest so far recorded and is consistent with the dipositive charge and ‘electron-rich’ 6π aromatic character. The ⁷⁷Se and ¹⁴N NMR [room temperature (r.t.), δ(MeNO₂)=–67.6, ν_½= 200 Hz] and the Raman spectrum in liquid AsF₃ at 10 °C are all consistent with retention of the [graphic omitted]e²⁺ ring structure in solution. The ESR spectrum of [graphic omitted]e˙⁺ in SO₂ solution at r.t. (g= 2.043, broad) and the spectrum of powdered [graphic omitted]e˙⁺ in frozen SO₂ at –160 °C were similar to but not identical with those of [graphic omitted]e˙⁺, [graphic omitted]˙⁺ and [graphic omitted]˙⁺ indicative of a planar 7π ring system.