Preparation and characterisation of thermally stable (SeNSeNSe)n(AsF6)2containing 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)A]. There are significant cation–anion interactions in both salts. The Se–Se [2.334(3)A] and Se–N bond lengths [average: 1.74(3)(side), 1.69(3)A(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)A] 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, ν½= 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, ν½= 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.