Sulfur dioxide is an important solvent in superacid chemistry, e.g., as the medium for reactions with so-called “magic acid” (FSO₃H/SbF₅) or as a solvent for carborane acids. In this context, protonated species based on SO₂ have been detected and studied using quantum-chemical calculations. However, the properties and structure of protonated SO₂ had not been studied in-depth so far.

Andreas J. Kornath, Ludwig-Maximilian University of Munich, Germany, and colleagues have synthesized and isolated hemi- and monoprotonated SO₂-based cations (pictured) in the form of their Sb₂F₁₁⁻ salts. The team used [FS(OX)₂][SbF₆] (X = H,D) as a precursor, which can be obtained from SO₂ and SbF₅ in anhydrous hydrogen fluoride.

This compound was recrystallized in aprotic solvents at low temperatures. In SO₂, the researchers observed the formation of hemiprotonated SO₂ as [(SO₂)₂H][Sb₂F₁₁] and the corresponding deuterated species. In 1,1,1,2-tetrafluoroethane, monoprotonated sulfur dioxide was formed from the deuterated precursor as the salt [OSOD][Sb₂F₁₁].

The products were characterized using Raman spectroscopy and single-crystal X-ray structure analysis. For hemiprotonated SO₂, the work represents the first observation of the respective ν(SO) and δ(OSO) vibrational frequencies.

• Synthesis and Structure of Protonated Sulfur Dioxide,
  Christoph Jessen, Andreas J. Kornath,
  Angew. Chem. Int. Ed. 2024.
  https://doi.org/10.1002/anie.202401953