Sulfur–Nitrogen Compounds
Authors: Chivers Tristram 
Published: March 2006

The quintessential sulfur–nitrogen compound, tetrasulfur tetranitride, S
4
N
4
, was first detected in 1835, just 10 years after the discovery of benzene. Its unusual structure, like that of benzene, was not elucidated for over 100 years. The application of diffraction techniques revealed the unusual cage arrangement with two weak cross‐ring sulfur–sulfur interactions. The details of the electronic structure of this fascinating molecule are still a matter for debate today. The polymer, (SN)

x

, was first obtained in 1910 and its metallic character was noted. However, it was the discovery in 1973 that a polymer comprising only nonmetallic elements behaves as a superconductor at 0.26 K that sparked widespread interest in sulfur–nitrogen (SN) chemistry. A year later it was proposed that planar SN heterocycles belong to a class of ‘
electron‐rich aromatics
’ that conform to the well‐known Hückel (4n + 2)π‐electron rule of organic chemistry. This suggestion, which was based on simple electron‐counting concepts, provided an additional impetus for both experimental and theoretical investigations of SN systems. In the past thirty years, the combination of structural studies, primarily through X‐ray crystallography, spectroscopic information, and molecular orbital calculations has provided reasonable rationalizations of the structure–reactivity relationships of these fascinating compounds. Interfaces with other areas of chemistry, for example, materials chemistry, organic synthesis, coordination chemistry, and biochemistry have been established and are under active investigation. For example, in the area of solid‐state chemistry, materials with unique magnetic and conducting properties that depend on intermolecular sulfur–nitrogen interactions between radical species have been designed. These new materials have potential applications in the construction of organic data recording devices. At the other end of the chemical spectrum,
S
‐nitrosothiols (RSNO) have been shown to be important species in the storage and transport of nitric oxide. As NO donors, these SN compounds have potential medical applications in the treatment of blood circulation problems.

This contribution begins with a short discussion of structure and bonding in cyclic SN species. This is followed by an overview of the various physical methods that are used to characterize SN compounds. The subsequent sections deal with specific classes of SN compounds starting with binary species, which include cations and anions as well as neutral molecules. The next sections are concerned with two important classes of reagents, SN halides and SN oxides. Heterocyclothiazenes, in which a sulfur atom in an SN ring is replaced by another atom, most commonly carbon, phosphorus, or a transition metal, constitute a continually expanding area of investigation as reflected in the relatively large section on this topic. Sulfanuric ring systems, which involve sulfur in the +VI oxidation state, have been known for many years and recent interest has revolved around polymers involving the NS(O)R repeating unit, which are isoelectronic with the well‐known polyphosphazenes. The next section describes the chemistry of cyclic sulfur imides, that are structurally related to the cyclic sulfur allotropes by the replacement of one or more sulfur atoms by an imido (NR) group. In the final sections, the unusual properties of SN chains, including the unique polymer (SN)

x

are discussed.