Tetrasulfur tetranitride

Tetrasulfur tetranitride
File:S4N4.jpg The structure of 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]
Properties
Solubility in water	Insoluble
Melting point	187 °C (460 K)(368 °F)
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.

Introduction

Gold-poppy colored S₄N₄ is the most readily prepared sulfur nitride. It is used as a starting material to synthesize many S-N compounds and has attracted much interest for its unusual structure and bonding.^[1]

Because nitrogen and sulfur have similar electronegativities one would expect that they would form covalently bonded structures, and indeed a large number of S-N and S-NH compounds are known. S₄N₄ adopts an unusual “extreme cradle” structure, which was determined in 1944. The topologically related hydrocarbon is (CH₂)₄(CH)₄. 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 have been explained in the context of molecular orbital theory. ^[2]

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.

Properties

S₄N₄ is thermochromic, changing from pale yellow below -30 °C to orange at room temperature to deep red above 100 °C. ^[3]

S₄N₄ is air stable, though its heat of formation is positive (460 kJ/mol). This endothermic heat of formation anticipates its instability, and originates in the difference in energy compared to its stable decomposition products:

S₄N₄ → 2 N₂ + 0.5 S₈

Because one of its decomposition products is a gas, S₄N₄ is an explosive.^[4] Purer samples tend to be more explosive. Small samples can be detonated by striking with a hammer.

Synthesis

Until recently, S₄N₄ was prepared by the reaction of ammonia with SCl₂ in carbon tetrachloride followed by extraction into dioxane.^[5]

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

A related synthesis employs NH₄Cl in place of ammonia:. ^[6]

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. This species reacts with SCl₂ and SO₂Cl₂. {[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.^[7]

[(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 that delocolization of electrons may be disrupted.^[8]

S₄N₄ + SbCl₅ ${\rightarrow }$ S₄N₄*SbCl₅

S₄N₄ + SO₃ ${\rightarrow }$ 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^[9]

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

S4N4 is sensitive to hydrolysis in the presence of base. Dilute NaOH hydrolyzes S₄N₄ as follows^[10]:

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

While more concentrated base will yield sulfite:

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

Precursor to other S-N compounds

Most important S-N compounds are prepared from S₄N₄.^[11] By reaction with piperidine is generated 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 Ph = phenyl) gives the blue perthionitrite salt:

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

"SNx"

Passing gaseous S₄N₄ over silver metal yields the low temperature superconductor polysulfurnitride, 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.^[12]

S₄N₄ + 8 Ag → 4 Ag₂S + 2 N₂

S₄N₄ → (SN)_x

References

1. ^ Greenwood, N. N.; Earnshaw, A. Chemical Elements; 2nd edition; Butterworth-Heinemann: Boston, MA, 1997, pp 721-725.
2. ^ Villena-Blanco, M.; Jolly, W.L.; Tyree, S.Y. Ed.: Inorganic synthesis; Wiley: New York, NY, 1967; Vol. 9, pp. 98-102
3. ^ Maaninen, A.; Shvari, J.; Laitinen, R.S.; Chivers, T; Inorganic Synthesis; (2002) Vol. 33, pp. 196-199
4. ^ Bojes, J.; Chivers, T; Oakley, R. D. Inorganic Synthesis (1989) Vol 25, pp. 30-40.