Trithiatetrazocine Cations, [RCN₄S₃]⁺; Planar Sulfur−Nitrogen 10π Aromatics

The reaction of bicyclic sulfur−nitrogen heterocyles RCN5S3 (R = F3C, Ph, Me2N, 2-FC6H4, 2,6-F2C6H3) with [Hg(SO2)2][AsF6]2 in liquid SO2 yielded the corresponding trithiatetrazocinium-hexafluoroarsenates [RCN4S3][AsF6] as yellow solids and a red-brown insoluble byproduct with the approximate composition Hg3N2. Single crystal structure determinations of the salts and theoretical calculations on the trithiatetrazocine cations [RCN4S3]+ revealed the cations to be planar eight-membered heterocycles. The [RCN4S3]+ cations are the missing link in the series of known valence isoelectronic eight-membered 10π azocines, that is, [S4N4]2+ and RC(NSN)2CR. In contrast to neutral trithiatetrazocines and dithiatetrazocines RC(NSN)2CR carrying donor substituents, which have a folded butterfly structure with a transannular S−S bond, the positive charge on the [RCN4S3]+ cations always stabilizes the planar structure independent of the nature of the substituent. Experimentally obtained structures and theoretical calculations (geometries, frequencies, charges, nucleus-independent chemical shifts (NICS)) are in agreement with the description of [RCN4S3]+ as planar eight-membered 10π Hückel aromatic. The single crystal structure determinations of some 3,5-R-1,2,4,6-thiatriazinyl hexafluoroarsenates (R = Ph, 2-FC6H4, 4-FC6H4), which were isolated from the above reaction as well, are also included.