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.^[1][2]

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.

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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 of alternating sulfur and nitrogen atoms. The pairs of sulfur atoms across the ring are further bonded, resulting in a cage-like structure with pentagonal S₃N₂ rings. The nature of the "transannular" S-S interactions is a matter of debate and computational investigation^[3] but has been explained in the context of molecular orbital theory.^[1] 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). 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 + 0.5 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.^[1] 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.^[1]

Synthesis

Until recently, S₄N₄ was prepared by the reaction of ammonia with SCl₂ in carbon tetrachloride followed by extraction into dioxane.^[4]

6 SCl₂ + 16 NH₃ → S₄N₄ + S₈ + 12 NH₄Cl

A related synthesis employs NH₄Cl in place of ammonia:^[1]

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.^[5]

[(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.^[1]

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:^[1]

nS₄N₄ + nCuCl → (S₄N₄)_n-μ-(-Cu-Cl-)_n

S₄N₄ is sensitive to hydrolysis in the presence of base. Dilute NaOH hydrolyzes S₄N₄ as follows:^[1]

2S₄N₄ + 6 OH^- + 9 H₂O → S₂O₃^2- + 2 S₃O₆^2- + 8 NH₃

Whereas more concentrated base yields sulfite:

S₄N₄ + 6 OH^- + 3 H₂O → S₂O₃^2- + 2 SO₃^2- + 4 NH₃

Reactions with metal complexes

This area has been reviewed.^[6][2]

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.^[7] Reaction with piperidine generates [S₄N₅]⁻:

3 S₄N₄ + 4 C₅H₁₀NH → (C₅H₁₀NH₂)⁺[S₄N₅]^- + (C₅H₁₀N)₂S + 3/8 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] + 1/8 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] + 1/2 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.^[8]

"SN_x"

Passing gaseous S₄N₄ over silver metal yields the low temperature superconductor polythiazyl or polysulfurnitride (transition temperature (0.26±0.03) K^[9]), 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.^[1]

S₄N₄ + 8 Ag → 4 Ag₂S + 2 N₂

S₄N₄ → (SN)_x

Miscellaneous facts

S₄N₄ has been shown to co-crystallize with benzene and the C₆₀.^[10]

Se₄N₄

The selenium compound Se₄N₄ is known and has been the subject of some research.^[11][12] In addition, adducts of aluminium chloride with Se₂N₂ have been isolated, this is formed from Se₄N₄.^[13]

Safety

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

References