The new cyanido-sulfate anion, [SO₃CN]⁻, is a complex of the Lewis base CN⁻ and the Lewis acid SO₃. The ion the a pseudo-halide analogon of the well-known fluorido- and chlorido-sulfates. Its formation is astonishing because the oxidation of the cyanide ion by SO₃ seems to be favorable. For the successful preparation it seems to be benefitial that the synthesis can be carried out at room temperature.

Abstract

The reaction of the pyridine adduct of SO₃ and tetra-n-butyl-ammonium cyanide, [N(ⁿBu)₄][CN] at room temperature leads to the unprecedented cyanido-sulfate anion [SO₃CN]⁻ stabilized in the salt [N(ⁿBu)₄][SO₃CN]. The anion is a pseudo-halogen congener of the well-known fluoro- and chloro-sulfates. The new anion has been studied by vibrational spectroscopy and theoretical calculations.

Sulfur trioxide acts as a strong oxidizer towards halides as well as pseudohalides. Therefore, reactions of sulfur trioxide with cyanide anions do usually not lead to the formation of cyanido-sulfates anion but to oxidation of the cyanide ion. This problem can be avoided if a sulfur trioxide-pyridine complex is used instead of neat sulfur trioxide. In this way, the oxidation power is sufficiently reduced, and the pyridine molecule can be replaced by the cyanide anion under formation of the cyanido-sulfate anion, SO₃CN⁻. Thus sulfur trioxide gets rid of the badly smelling pyridine molecule, however, at the expense of gaining the toxic cyanide anion. More information can be found in the Research Article by M. S. Wickleder and co-workers (DOI: 10.1002/chem.202301761).