Sulfur trioxide (SO₃) is a strong oxidant and a Lewis acid. There is a limited number of known Lewis acid-base complexes of SO₃, some of which can be used for the synthesis of sulfuric acid derivatives. Simple sulfate derivatives such as the fluoro- and the chloro-sulfate anion ([SO₃F]^– and [SO₃Cl]^–), can be considered Lewis acid-base complexes of SO₃ with F^– and Cl^–, respectively. The pseudo-halide analogue of the fluorido- and chlorido-sulfates [SO₃CN]^– has been considered unstable so far.

Mathias Wickleder, University of Cologne, Germany, and colleagues have found a suitable route to stabilize and synthesize the [SO₃CN]^– ion for the first time. The approach is based on the reaction of a suitable cyanide with the pyridine-sulfur trioxide complex py∙SO₃. The team chose a TBA salt (TBA = tetra-n-butyl-ammonium, [N(C₄H₉)₄]⁺) as the cyanide source and found that the reaction of TBA(CN) and py∙SO₃ in 1,4-dioxane at room temperature gives single crystals of [N(C₄H₉)₄][SO₃CN].

The product was analyzed using X-ray diffraction (structure of the [SO₃CN]^– anion pictured) and vibrational spectroscopy. The developed approach could also be suitable for the preparation of other, hitherto unknown pseudohalido sulfates. In addition, [N(C₄H₉)₄][SO₃CN] is soluble in organic solvents and could be a suitable starting material for the synthesis of related salts.

• The cyanido‐sulfate anion [SO₃CN]^–,
  Thomas Kasperowicz, David van Gerven, Mathias Wickleder,
  Chem. Eur. J. 2023.
  https://doi.org/10.1002/chem.202301761