Tetrasulfur tetranitride

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Tetrasulfur tetranitride
General
Systematic name	Tetranitrogen tetrasulfide tetrasulfur nitride
Molecular formula	S4N4
Molar mass	184.29 g/mol
Appearance	Orange solid
CAS number	[28950-34-7] [1]
Properties
Solubility in water	Insoluble
Melting point	187°C (460 K)
IR	928 768 727 700 630 553 548 and 529 cm-1
NMR	14N peak at -246ppm
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Tetrasulfur tetranitride is an inorganic compound with the formula S₄N₄. Gold-poppy colored S₄N₄ is the most readily prepared and most important sulfur nitride. It is a key starting precursor to many S-N compounds and has attracted much interest for its unusual structure and bonding.^[1]

Nitrogen and sulfur have similar ability to attract electrons, a property known as electronegativity. 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.

Contents 1 Structure of S4N4 2 Properties 3 Synthesis 4 Acid-base reactions 5 Precursor to other S-N compounds 6 "SNx" 7 External links 8 References 9 Further reading

[edit] Structure of S₄N₄

S₄N₄ adopts an unusual “extreme cradle” structure, which was determined in 1944. The structure of S₄N₄ can be viewed as an 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 consisting of interlocking five-membered S₃N₂ rings. The nature of the transannular S-S interactions is a matter of debate but has been explained in the context of molecular orbital theory.^[1] The bonding in S₄N₄ is considered to be delocalized, which is relevant to the equivalency of the S-N distances than expected S-N bond distances.

S₄N₄ has been shown to co-crystallize with benzene and the C₆₀.^[2]

[edit] Properties

S₄N₄ is stable to air. It is, however, unstable in the thermodynamic sense with a positive heat of formation of 460 kJ/mole). 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 + 0.5S₈

It is not really very unusual for complex molecules to be unstable in a thermodynamic sense yet stable kinetically; this situation describes the great majority of organic 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]

[edit] Synthesis

Until recently, S₄N₄ was prepared by the reaction of ammonia with SCl₂ in carbon tetrachloride followed by extraction into dioxane.^[3]

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

One might expect this reaction, which begins with an S(II) source, to produce (SNH)_n or S(NH₂)₂, vs. the oxidation state of S(III) in S₄N₄.

A newer 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.^[4]

[(CH₃)₃Si)₂N]₂S + SCl₂ + SO₂Cl₂ → S₄N₄ + 4 (CH₃)₃SiCl + SO₂

[edit] 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

It is protonated by HBF₄:

S₄N₄ + HBF₄ → S₄N₄H⁺BF₄

The soft Lewis acid CuCl forms a polymer with 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 + 6OH^- + 9H₂O → S₂O₃^2- + 2S₃O₆^2- + 8NH₃

While more concentrated base will yield sulfite:

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

[edit] Precursor to other S-N compounds

Many important S-N compounds are prepared from S₄N₄.^[5] Reaction with piperidine generates [S₄N₅]⁻:

3 S₄N₄ + 4 C₅H₁₀NH → (C₅H₁₀NH₂)[S₄N₅] + (C₅H₁₀N)₂S + 3/8 S₈ + N₂

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

Treatment with tetramethylammonium azide produces the 10 pi-electron heterocycle:

S₄N₄ + 4 NMe₄N₃ → NMe₄[S₃N₃] + 1/8 S₈ + 2 N₂

In an apparently related reaction, the use of azide as its PPN⁺ salt (PPN = Ph₃PNPPh₃, where Ph₃P = triphenylphosphine) gives the blue perthionitrite salt:

2 S₄N₄ + NMe₄N₃ → (PPN)[NS₃] + 1/2 S₈ + 5 N₂

[edit] "SNx"

Passing gaseous S₄N₄ over silver metal yields the low temperature superconductor polysulfurnitride (transition temperature (0.26±0.03) K^[6]), known as "(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 + 2N₂

S₄N₄ → (SN)_x

[edit] External links

• Links to external chemical sources.

[edit] References

[edit] Further reading

• Chivers, T. “A Guide To Chalcogen-Nitrogen Chemistry” World Scientific Publishing Company: Singapore; 2004. ISBN 981-256-095-5