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)

Abstract

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