Sulfur dioxide is a useful reagent for inorganic and organic syntheses. However, it is a toxic gas and, thus, hard to handle. One approach to solve this problem is the preparation of solid adducts of SO₂ with bases. Such adducts have been prepared using nitrogen bases. In contrast, no adducts between phosphines and SO₂ had been synthesized so far.

Fabian Dielmann and colleagues, University of Münster, Germany, have synthesized the first phosphine–SO₂ adducts (pictured). The team used phosphines with imidazolin-2-imine substituents, strong π-donors that increase the electron-donating character of the phosphine. These electron-rich phosphines were dissolved in benzene and subjected to gaseous SO₂ at a pressure of 2 bar. The gaseous SO₂ could also be replaced with stoichiometric amounts of a nitrogen base–SO₂ adduct. The team characterized the resulting phosphine–SO₂ adduct using ³¹P NMR spectroscopy and single crystal X-ray diffraction.

The adduct features a pyramidal P→SO₂ unit and is stable at room temperature. However, when other, less electron-donating substituents are used at the phosphine, the corresponding SO₂ adducts decompose at room temperature. In these compounds, SO₂ is activated and sulfur monoxide (SO) is released, resulting in the formation of phosphine oxides. Due to the high reactivity of SO, it can further react with intact phosphine–SO₂ adducts, which leads to a mixture of decomposition products. According to the researchers, further studies on the possible use of phosphine–SO₂ adducts as SO-transfer reagents are in progress.

• Crystalline, Room-Temperature Stable Phosphine–SO₂ Adducts: Generation of Sulfur Monoxide from Sulfur Dioxide,
  Florenz Buß, Philipp Rotering, Christian Mück-Lichtenfeld, Fabian Dielmann,
  Dalton Trans. 2018.
  https://doi.org/10.1039/c8dt01484a