Tetrasulfur tetranitride

Tetrasulfur tetranitride is an inorganic compound with the formula S4N4. This gold-poppy coloured solid is the most important binary sulfur nitride, which are compounds that contain only the elements sulfur and nitrogen. It is a precursor to many S-N compounds and has attracted wide interest for its unusual structure and bonding.

Nitrogen and sulfur have similar electronegativities, which is the ability to attract electrons. When atoms are so evenly matched, they often form extensive families of covalently bonded structures. Indeed, a large number of S-N and S-NH compounds are known with S4N4 as their parent.

Structure
S4N4 adopts an unusual “extreme cradle” structure, with D2d point group symmetry. It can be viewed as a derivative of a hypothetical eight-membered ring of alternating sulfur and nitrogen atoms. The pairs of sulfur atoms across the ring are separated by 2.586 Å, resulting in a cage-like structure as determined by single crystal X-Ray diffraction. The nature of the "transannular" S---S interactions remains a matter of investigation because it is significantly shorter than the sum of the van der Waal's distances. but has been explained in the context of molecular orbital theory. The bonding in S4N4 is considered to be delocalized, which is indicated by the fact that the bond distances between neighboring sulfur and nitrogen atoms are almost the same.

Properties
S4N4 is stable to air. It is, however, unstable in the thermodynamic sense with a positive heat of formation of +460 kJ mol−1). This endothermic heat of formation anticipates its inherent instability, and originates in the difference in energy of S4N4 compared to its highly stable decomposition products:
 * S4N4 →  2 N2  +  ½ S8

It is not really very unusual for complex molecules to be unstable in a thermodynamic sense yet stable kinetically; this situation describes many compounds. This combination of kinetic stability and thermodynamic instability is, however, uncommon for very simple compositions, such as sulfur nitride. Because one of its decomposition products is a gas, S4N4 is an explosive. Purer samples tend to be more explosive. Small samples can be detonated by striking with a hammer.

S4N4 is thermochromic, changing from pale yellow below −30 °C to orange at room temperature to deep red above 100 °C.

Synthesis
Until recently, S4N4 was prepared by the reaction of ammonia with SCl2 in carbon tetrachloride followed by extraction into dioxane.
 * 6 SCl2 +  16 NH3    →    S4N4  +  S8  +  12 NH4Cl

A related synthesis employs NH4Cl in place of ammonia:
 * 4 NH4Cl +  6 S2Cl2  →  S4N4  +  16 HCl  +  S8

A more recent synthesis entails the use of {[Me3Si)2N]2S} as a precursor with pre-formed S-N bonds. {[Me3Si)2N]2S} is prepared by the reaction of lithium bis(trimethylsilyl)amide and SCl2.
 * 2 [(CH3)3Si]2NLi +  SCl2  →  [(CH3)3Si)2N]2S  +  2 LiCl

The {[(CH3)3Si)2N]2S} reacts with the combination of SCl2 and SO2Cl2 to form S4N4.
 * [(CH3)3Si)2N]2S +  SCl2  +  SO2Cl2  →   S4N4  +  4 (CH3)3SiCl  +  SO2

Acid-base reactions
S4N4 serves as a Lewis base by binding through nitrogen to strongly Lewis acidic compounds such as SbCl5 and SO3. The cage is distorted in these adducts, thus delocolization of electrons may be disrupted.
 * S4N4 +  SbCl5 →  S4N4.SbCl5
 * S4N4 +  SO3 →  S4N4.SO3

The reaction of [Pt2Cl4(PMe2Ph)2] with S4N4 is reported to form a complex where a sulfur forms a dative bond to the metal, this compound upon standing is isomerised to a complex in which a nitrogen atom forms the additional bond to the metal centre.

It is protonated by HBF4:
 * S4N4 +  HBF4   →  [S4N4H+] [ BF4−]

The soft Lewis acid CuCl forms a polymer containing intact S4N4 rings as the bridging ligands:
 * nS4N4 + nCuCl  →  (S4N4)n-μ-(-Cu-Cl-)n

S4N4 is sensitive to hydrolysis in the presence of base. Dilute NaOH hydrolyzes S4N4 as follows:
 * 2S4N4 +  6 OH−  +  9 H2O   →   S2O32−  +  2 S3O62−  +  8 NH3

Whereas more concentrated base yields sulfite:
 * S4N4 +  6 OH−  +  3 H2O  →  S2O32−  +  2 SO32−  +  4 NH3

Reactions with metal complexes
This area has been reviewed.

Reactions of S4N4 where the ring remains intact
S4N4 reacts with Vaska's complex ([Ir(Cl)(CO)(PPh3)2] in an oxidative addition reaction to form a six coordinate iridium complex where the S4N4 binds through two sulfur atoms and one nitrogen atom. This compound arises by the formal breaking of one S-N bond in the oxidative addition, followed by the coordination of the lone pair on another sulfur atom to form a dative bond. A related Pt(IV) compound arises from Zeise's salt.

Reactions of S4N4 where the ring does not remain intact
The reaction of S4N4 with the [Pd2Cl6]2− anion forms a series of three palladium complexes in which the S4N4 ring has been fragmented.

S4N4 as a precursor to other S-N compounds
Many important S-N compounds are prepared from S4N4. Reaction with piperidine generates [S4N5]−:


 * 3 S4N4 +  4 C5H10NH  →  (C5H10NH2)+[S4N5]−  +  (C5H10N)2S  +  ⅜ S8  +  N2

It is indicative of the richness of this area that a related cation is also known, i.e. [S4N5]+.

Treatment with tetramethylammonium azide produces the heterocycle [S3N3]−:


 * S4N4 +  4 NMe4N3  →  NMe4[S3N3] +  ⅛ S8  +  2 N2

In the language of electron counting, [S3N3]− has 10 pi-electrons: 2e−/S plus 1e−/N plus 1e− for the negative charge.

In an apparently related reaction, the use of PPN+N3 gives the blue perthionitrite salt:


 * 2 S4N4 +  PPN(N3)  →  PPN[NS3] +  ½ S8  +  5 N2

The anion NS3− is a chain described often as S=N-S-S−.

Reaction with acetylenes
S4N4 reacts with electron poor acetylenes.

"SNx"
Passing gaseous S4N4 over silver metal yields the low temperature superconductor polythiazyl or polysulfurnitride (transition temperature (0.26±0.03) K ), often simply called "(SN)x." In the conversion, the silver first becomes sulfided, and the resulting Ag2S catalyzes the conversion of the S4N4 into the four-membered ring S2N2, which readily polymerizes.
 * S4N4 +  8 Ag  →  4 Ag2S  +  2 N2
 * S4N4 →   (SN)x

Miscellaneous facts
S4N4 has been shown to co-crystallize with benzene and the C60 molecule.

Se4N4
The selenium compound Se4N4 is known and has been the subject of some research. In addition, adducts of aluminium chloride with Se2N2 have been isolated, this is formed from Se4N4.

Safety
S4N4 is shock-sensitive, thus grinding solid samples should be avoided. Purer samples are reportedly more sensitive than those contaminated with elemental sulfur.