Tetrasulfur tetranitride

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 adopts an unusual "extreme cradle" structure, with D_2d molecular 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 four nitrogen atoms form an square and the four sulfur atoms an approximae tetrahedron. 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.^[4] The nature of the transannular interaction between those sulfur atoms remains a matter of investigation, especially in the context of molecular orbital theory.^[2] The distance is significantly shorter than the sum of the van der Waals radii^[5] and seems to be similar to the strength of a hydrogen bond.^[6]

S₄N₄ has been shown to co-crystallize with benzene and the C₆₀ molecule.^[7]

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.^[2][8] Purer samples tend to be more sensitive.^[9] 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.^[2]

S₄N₄ was first prepared in 1835 by M. Gregory by the reaction of disulfur dichloride with ammonia,^[10] a process that has been optimized:^[2]

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

Several variants on the reaction are known, and yields are generally maximized when the sulfur-to-chloride ratio is 3:1.^[11] Nevertheless, one variant on this reaction uses anhydrous ammonia for simplicity:^[12]

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

Coproducts of this reaction include heptasulfur imide (S₇NH) and elemental sulfur; the latter equilibrates with more S₄N₄ and ammonium sulfide:^[13]

16 S + 16 NH₃ ↔ S₄N₄ + 12 (NH₄)S

An alternative synthesis pre-forms S–N bonds with lithium bis(trimethylsilyl)amide and SCl₂:

2 ((CH₃)₃Si)₂NLi + SCl₂ → (((CH₃)₃Si)₂N)₂S + 2 LiCl

The (((CH₃)₃Si)₂N)₂S then reacts with the combination of SCl₂ and SO₂Cl₂ to form S₄N₄, trimethylsilyl chloride, and sulfur dioxide:^[14]

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

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.^[2]

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.^[3][15] For example, the soft Lewis acid CuCl forms a coordination polymer:^[2]

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.^[3]

Dilute NaOH hydrolyzes S₄N₄ as follows, yielding thiosulfate and trithionate:^[2]

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

Many S-N compounds are prepared from S₄N₄.^[16] In the simplest case, (NS)₂ units insert into a weak, polarized bond:^[17]: 285

Me₃SiNR₂ + S₄N₄ → Me₃Si-N-S-N-S-NR₂

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 unsaturated bonds to give 1,2,5‑thiadiazoles (a 1,3-dipolar cycloaddition). Very electron-poor alkynes also attack S₄N₄ to give a different, 7-membered cycloadduct, corresponding to addition of -S-N-S-N-S- across the triple bond.^[18][19]

Passing gaseous S₄N₄ over silver metal yields the low temperature superconductor polythiazyl or polysulfurnitride (transition temperature (0.26±0.03) K^[20]), 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.^[2]

S₄N₄ + 8 Ag → 4 Ag₂S + 2 N₂

x S₄N₄ → (SN)_4x

Alcoholic tin(II) chloride reduces S₄N₄ to (NH)₄S₄, valence-isoelectronic with octasulfur. Autoxidation does not recover S₄N₄, but instead polymeric (–S(O)N(H)–)_∞.^{[17]: 286, 289}

Oxidation with elemental fluorine or silver(II) fluoride gives N₄(SF)₄,^[17] but milder reagents give S₄N⁺
₃:

3 S₄N₄ + 2 S₂Cl₂ → 4 [S₄N₃]⁺Cl⁻

S₄N₄ + RC(=O)Cl → [S₄N₃]⁺Cl⁻ + RNCO

That cation is relatively non-electrophilic and planar, with a delocalized π system. However, it adds triphenylphosphine to give [S(NPPh₃)₃]³⁺[Cl⁻]₃, a triimide analogue to sulfur trioxide. Conversely, S₄N⁺
₃ salts react with aluminum azide to recover S₄N₄.^[21]

Treatment with tetramethylammonium azide or other nucleophiles produces the similar 10-π heterocycle [S₃N₃]⁻:

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

Excess S₄N₄ can react with the [S₃N₃]⁻ to form [S₄N₅]⁻.^[22]

In a related reaction, the use of the bis(triphenylphosphine)iminium azide gives a salt containing the blue [NS₄]⁻ anion:^[16]

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:^[21]

S₄N₄ + 2 PPh₃ → S₃(PPh₃)N₄ + SPPh₃

S₄N₄ also reacts with metal salts, forming a wide variety of complexes:

Pb(NO₃)₂ + S₄N₄ + NH₃ → Pb(NH₃)(S₂N₂) + ...

Co(CO)₂C₅H₅ + S₄N₄ → CoC₅H₅(S₂N₂) + ...

VCl₄ + S₄N₄ → VCl₂(S₂N₃) + VCl₄(S₂N₃) + ...

In all these cases, the product is a metallacycle, with the N-S atoms forming a semicircular bidentate ligand.^[23]

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.^[8] Purer samples are more shock-sensitive than those contaminated with elemental sulfur.^[12][9]

• The selenium analogue Se₄N₄, tetraselenium tetranitride.