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Tetrasulfur tetranitride

Names
IUPAC name Tetrasulfur tetranitride
Systematic IUPAC name 1,3,5,7-tetrathia-2,4,6,8-tetraazacyclooctan-2,4,6,8-tetrayl
Other names Cyclic sulfur(III) nitride tetramer Tetranitrogen tetrasulfide 1λ,3,5λ,7,2,4,6,8-Tetrathiatetrazocine
Identifiers
CAS Number	28950-34-7
3D model (JSmol)	Interactive image
ChemSpider	124788
PubChem CID	141455
UNII	9BXS997HR6
CompTox Dashboard (EPA)	DTXSID20183147
InChI InChI=1S/N4S4/c1-5-2-7-4-8-3-6-1Key: LTPQFVPQTZSJGS-UHFFFAOYSA-N
SMILES N1=N=N=N=1
Properties
Chemical formula	S4N4
Molar mass	184.287 g/mol
Appearance	Vivid orange, opaque crystals
Melting point	187 °C (369 °F; 460 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). N verify (what is  ?) Infobox references

Tetrasulfur tetranitride is an inorganic compound with the formula S₄N₄. This vivid orange, opaque, crystalline explosive 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. When the properties of atoms are so highly similar, they often form extensive families of covalently bonded structures and compounds. 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 D_2d point group symmetry. It can be viewed as a derivative of a (hypothetical) eight-membered ring (or more simply a 'deformed' 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 Waals radii but has been explained in the context of molecular orbital theory. One pair of the transannular S atoms have valence 4, and the other pair of the transannular S atoms have valence 2. 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 nearly identical. S₄N₄ has been shown to co-crystallize with benzene and the C₆₀ molecule.

Properties

S₄N₄ is stable to air. It is, however, unstable in the thermodynamic sense with a positive enthalpy of formation of +460 kJ/mol. This endothermic enthalpy of formation originates in the difference in energy of S₄N₄ compared to its highly stable decomposition products:

2 S₄N₄ → 4 N₂ + S₈

S₄N₄ is shock and friction sensitive and because one of its decomposition products is a gas, it is considered a primary explosive. Purer samples tend to be more sensitive. 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

S₄N₄ was first prepared in 1835 by M. Gregory by the reaction of disulfur dichloride with ammonia, a process that has been optimized:

6 S₂Cl₂ + 16 NH₃ → S₄N₄ + S₈ + 12 [NH₄]Cl

Coproducts of this reaction include heptasulfur imide (S₇NH) and elemental sulfur, and the latter equilibrates with more S₄N₄ and ammonium sulfide:

16 S + 16 NH₃ ↔ S₄N₄ + 12 (NH₄)S

A related synthesis employs [NH₄]Cl instead:

4 [NH₄]Cl + 6 S₂Cl₂ → S₄N₄ + 16 HCl + S₈

An alternative synthesis entails the use of (((CH₃)₃Si)₂N)₂S as a precursor with pre-formed S–N bonds. (((CH₃)₃Si)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, trimethylsilyl chloride, and sulfur dioxide:

2 (((CH₃)₃Si)₂N)₂S + 2 SCl₂ + 2 SO₂Cl₂ → S₄N₄ + 8 (CH₃)₃SiCl + 2 SO₂

Acid-base reactions

S₄N₄ is a Lewis base at nitrogen. It binds to strong Lewis acids, such as SbCl₅ and SO₃, or H[BF₄]:

S₄N₄ + SbCl₅ → S₄N₄·SbCl₅

S₄N₄ + SO₃ → S₄N₄·SO₃

S₄N₄ + H[BF₄] → [S₄N₄H][BF₄]

The cage is distorted in these adducts.

S₄N₄ reacts with metal complexes, but the bonding situation may be quite complex. The cage remains intact in some cases but in other cases, it is degraded. For example, the soft Lewis acid CuCl forms a coordination polymer:

n S₄N₄ + n CuCl → (S₄N₄)_n-μ-(−Cu−Cl−)_n

Reportedly, [Pt₂Cl₄(P(CH₃)₂Ph)₂] initially forms a complex with S₄N₄ at sulfur. This compound, upon standing, isomerizes to additionally bond through a nitrogen atom. S₄N₄ oxidatively adds to Vaska's complex ([Ir(Cl)(CO)(PPh₃)₂] to form a hexacoordinate iridium complex where the S₄N₄ binds through two sulfur atoms and one nitrogen atom.

Dilute NaOH hydrolyzes S₄N₄ as follows, yielding thiosulfate and trithionate:

2 S₄N₄ + 6 OH + 9 H₂O → S₂O2−3 + 2 S₃O2−6 + 8 NH₃

More concentrated base yields sulfite:

S₄N₄ + 6 OH + 3 H₂O → S₂O2−3 + 2 SO2−3 + 4 NH₃

As a precursor to other S-N compounds

Many S-N compounds are prepared from S₄N₄.

In electrophilic substitution or 1,3-dipolar cycloaddition reactions, S₄N₄ behaves as a combination of the dithionitronium synthon and the sulfide synthon. Thus it adds to arenes and electron-rich alkynes to give 1,2,5‑thiadiazoles. Electron-poor alkynes attack S₄N₄ to give a different cycloadduct of stoichiometry RC(NS)₂SCR′. With electron-rich alkenes, S₄N₄ behaves as a Diels-Alder diene.

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

x S₄N₄ → (SN)_4x

Oxidation of S₄N₄ with elemental chlorine gives thiazyl chloride, but milder reagents give S₄N
₃:

3 S₄N₄ + 2 S₂Cl₂ → 4 Cl

S₄N₄ + RC(=O)Cl → Cl + RNCO

That cation is relatively non-electrophilic and planar, with a delocalized π system. However, it adds triphenylphosphine to give ₃, a triimide analogue to sulfur trioxide. Conversely, S₄N
₃ salts react with aluminum azide to recover S₄N₄.

Treatment with tetramethylammonium azide produces the similar 10-π heterocycle [S₃N₃]:

8 S₄N₄ + 8 [(CH₃)₄N][N₃] → 8 [(CH₃)₄N][S₃N₃] + S₈ + 16 N₂

In a related reaction, the use of the bis(triphenylphosphine)iminium azide gives a salt containing the blue [NS₄] anion:

4 S₄N₄ + 2 [PPN][N₃] → 2 [PPN][NS₄] + S₈ + 10 N₂

[NS₄] has a chain structure approximated by the resonance [S=S=N−S−S] ↔ [S−S−N=S=S].

Reaction with piperidine generates [S₄N₅]:

24 S₄N₄ + 32 C₅H₁₀NH → 8 [C₅H₁₀NH₂][S₄N₅] + 8 (C₅H₁₀N)₂S + 3 S₈ + 8 N₂

A related cation is also known, i.e. [S₄N₅].

Triphenylphosphine abstracts a sulfur atom, replacing it with another triphenylphosphine moiety:

S₄N₄ + 2 PPh₃ → S₃(PPh₃)N₄ + SPPh₃

Safety

S₄N₄ is a categorized as a primary explosive that is shock and friction sensitive. While comparable to pentaerythritol tetranitrate (PETN) in terms of impact sensitivity, its friction sensitivity is equal to or even lower than lead azide. Purer samples are more shock-sensitive than those contaminated with elemental sulfur.

Related compounds

• The selenium analogue Se₄N₄, tetraselenium tetranitride.

References

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• Explosive chemicals
• Inorganic compounds
• Sulfur–nitrogen compounds
• Nitrides
• Eight-membered rings