Credit: © 2008 Wiley

One of the chief chemical characteristics of sulfur is its ability to form bonds to other sulfur atoms. These bonds and their chemistry are found in many proteins, as well as synthetic compounds. In addition, sulfur can also form four-coordinate 'hypervalent' bonds, where its formal number of valence electrons exceeds eight. Compounds containing these bonds are known as sulfuranes. Only a few unstable sulfuranes have been reported before, and no organosulfuranes have been investigated for reactivity or structure.

Now, Takayuki Kawashima and colleagues at the University of Tokyo have combined1 these two properties in one compound — a sulfur-substituted organosulfurane. Reaction of a mercaptoalkyl sulfide with N-bromosuccinimide gave the target compound, with the O–S–S motif. The central four-coordinate sulfur atom has a distorted pseudo-trigonal bipyramidal geometry and the S–S bond is longer (∼2.21 Å) than the usual disulfide bond length (∼2.08 Å). Density functional theory calculations revealed that the sulfur–oxygen and sulfur–sulfur bonds were three-centre four-electron bonds and that the S–S part was very polar because of the relative positive charge on the four-coordinate sulfur atom.

The compound is stable to oxygen and can be handled in air at room temperature, but decomposes rapidly in water. Crucially, the compound can be reversibly reduced, cleaving both the S–S and S–O bonds to produce the sulfide again. The ease of this transformation suggests that S(IV)–S(II) bond formation and dissociation redox processes could be found in biological systems.