Absorption of SO2(g) by TDAE[O2SSO2](s) to Give TDAE[O2SS(O)2SO2](s): Related Reactions of [NR4]2[O2SSO2](s) (R = CH3, C2H5)

One mole equivalent of gaseous SO2 is absorbed by purple TDAE­[O2SSO2](s), producing red, essentially spectroscopically pure TDAE­[O2SS­(O)2SO2](s); under prolonged evacuation, the product loses SO2(g), regenerating TDAE­[O2SSO2](s). Similarly, [NR4]2[O2SS­(O)2SO2](s) (R = Et, Me) can be prepared, albeit at lower purity, from the corresponding tetraalkylammonium dithionites (prepared by a modification of the known [NEt4]2[O2SSO2](s) preparation). While the [NEt4]+ salt is stable at rt; the [NMe4]+ salt has only limited stability at −78 °C. Vibrational spectra assignments for the anion in these salts were distinctly different from those for the anion in salts containing the long-known [O3SSSO3]2– dianion, the most thermodynamically stable form of [S3O6]2– (we prepared TDAE­[O3SSSO3]·H2O(s) and obtained its structure by X-ray diffraction and vibrational analyses). The best fit between the calculated ((B3PW91/6-311+G­(3df) and PBE0/6-311G­(d)) and experimental vibrational spectra were obtained with the dianion having the [O2SS­(O)2SO2]2– structure. Vibrational analyses of the three [O2SS­(O)2SO2]2– salts prepared in this work showed that the corresponding [O3SSO2]2– salts were present as a ubiquitous decomposition product. The formation of these new [O2SS­(O)2SO2]2– dianion salts was predicted to be favorable for [NMe4]+ and larger cations using a combination of theoretical calculations (B3PW91/6-311+G­(3df)) and volume based thermodynamics (VBT). Similar methods accounted for the greater stabilities of the TDAE2+ and [NEt4]+ salts of [O2SS­(O)2SO2]2– compared to [NMe4]2[O2SS­(O)2SO2](s) toward irreversible decomposition to the corresponding [O3SSO2]2– salts. These salts represent the first known examples of a new class of poly­(sulfur dioxide) dianion, [SO2]n2– in which n > 2.