Synthesis of (TDAE)(O2SSO2)(s) and discovery of (TDAE)(O2SSSSO2)(s) containing the first polythionite, [O2SSSSO2]2–

Gaseous SO2 reacts with tetrakis(dimethylamino)ethylene (TDAE) in acetonitrile in a 2:1 stoichiometric ratio to give analytically pure insoluble purple (TDAE)(O2SSO2) (1) in about 80% yield. Crystals of (TDAE)(O2SSSSO2) (2) were obtained from orange solution over the purple solid. The Raman spectrum of [TDAE]2+ was established using (TDAE)(A) salts [A = 2Br–, 2Br–·2H2O (X-ray), 2[Br3]− (X-ray)]. Vibrational spectroscopy showed that [O2SSO2]2– in 1 has C2h geometry. The X-ray structure of 2 showed that it contained [O2SSSSO2]2–, the first example of a new class of sulfur oxyanions, the polythionites. The geometry of [O2SSSSO2]2– consists of S2 with an S–S bond length of 2.003(1) A connected to two terminal SO2 moieties by much longer S–S bonds of 2.337(1) A. Calculations (B3PW91/6-311+G(3df)) show that the structural units in [O2SSSSO2]2– are joined by the interaction of electrons in two mutually perpendicular π* SOMOs of the triplet-state diradical S2 with unpaired electrons in the π*-antibonding orbitals of the two terminal [SO2]•– and polarized to delocalize the negative charge equally onto the three fragments. Thermodynamic estimates show 2 to be stable with respect to loss of sulfur and formation of 1, in contrast to [O2SSSSO2]2– salts of small cations that are unstable toward the related dissociation. Reaction of TDAE with an excess of liquid SO2 led to (TDAE)(O3SOSO3)·SO2 (preliminary X-ray, Raman), (TDAE)(O3SSSSO3)·2SO2 (preliminary X-ray, Raman), and (TDAE)(O3SSO2) (Raman).