Tetrasulfur tetranitride

Tetrasulfur tetranitride
Preferred IUPAC name Tetrasulfur tetranitride[citation needed]
Systematic name 1λ4,3,5λ4,7,2,4,6,8-Tetrathiatetrazocine[citation needed]
Other names Cyclic sulfur(III) nitride tetramer
Identifiers
CAS number	28950-34-7
PubChem	141455
ChemSpider	124788 Y
Jmol-3D images	Image 1 Image 2
SMILES n1snsnsns1 N=1SN=S=NSN=S=1
InChI InChI=1S/N4S4/c1-5-2-7-4-8-3-6-1 Y Key: LTPQFVPQTZSJGS-UHFFFAOYSA-N Y InChI=1/N4S4/c1-5-2-7-4-8-3-6-1 Key: LTPQFVPQTZSJGS-UHFFFAOYAC
Properties
Molecular formula	N4S4
Molar mass	184.287 g mol-1
Appearance	Vivid, orange, opaque crystals
Melting point	187 °C, 460 K, 369 °F
Y tetranitride (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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. 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.

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 separated by 2.586 Å, resulting in a cage-like structure as determined by single crystal X-Ray diffraction.^[3] 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 Waal's distances^[4] 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 enthalpy of formation of +460 kJ mol⁻¹. This endothermic enthalpy of formation anticipates its inherent instability, and originates in the difference in energy of S₄N₄ compared to its highly stable decomposition products:

2 S₄N₄ → 4 N₂ + S₈

Although many complex molecules are unstable in a thermodynamic sense yet stable kinetically, this is 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

S₄N₄ was first prepared in 1835 by W. Gregory, by the reaction of sulfur monochloride with ammonia.^[5]

Until recently, S₄N₄ was prepared by the reaction of ammonia with SCl₂ in carbon tetrachloride followed by extraction into dioxane, producing sulfur and ammonium chloride as side-products:^[6]

24 SCl₂ + 64 NH₃ → 4 S₄N₄ + S₈ + 48 NH₄Cl

A related synthesis employs sulfur monochloride and NH₄Cl instead:^[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, trimethylsilyl chloride, and sulfur dioxide:^[7]

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

Acid-base reactions

S₄N₄·BF₃

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 delocalization 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 to form a tetrafluoroborate salt:

S₄N₄ + HBF₄ → [S₄N₄H⁺][BF−
4]

The soft Lewis acid CuCl forms a polymer containing intact S₄N₄ rings as the bridging ligands:^[1]

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

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

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

Reactions with metal complexes

This area has been reviewed.^[2][8]

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.^[9] Reaction with piperidine generates [S₄N₅]⁻:

3 S₄N₄ + 4 C₅H₁₀NH → (C₅H₁₀NH₂)⁺[S₄N₅]⁻ + (C₅H₁₀N)₂S + ⅜ 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 + NMe₄N₃ → NMe₄[S₃N₃] + ⅛ 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 a salt containing the blue [NS₄]⁻ anion:^[10]

2 S₄N₄ + PPN(N₃) → PPN[NS₄] + ½ S₈ + 5 N₂

The anion NS₄⁻ has a chain structure described using the resonance [S=S=N-S-S]⁻ ↔ [S–S–N=S=S]⁻.

Reaction with acetylenes

S₄N₄ reacts with electron poor acetylenes.^[11]

Polythiazyl

Passing gaseous S₄N₄ over silver metal yields the low temperature superconductor polythiazyl or polysulfurnitride (transition temperature (0.26±0.03) K^[12]), 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 molecule.^[13]

Se₄N₄

The selenium compound Se₄N₄ is known and has been the subject of some research.^[14][15] In addition, adducts of aluminium chloride with Se₂N₂ have been isolated, this is formed from Se₄N₄.^[16]

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

1. ^ ^{a b c d e f g h i} Greenwood, N. N.; Earnshaw, A. Chemical Elements; 2nd edition; Butterworth-Heinemann: Boston, MA, 1997, pp 721-725.
2. ^ ^{a b} Chivers, T. “A Guide To Chalcogen-Nitrogen Chemistry” World Scientific Publishing Company: Singapore; 2004. ISBN 981-256-095-5
3. ^ Brahama D. Sharma and Jerry Donohue (1963). "The crystal and molecular structure of sulfur nitride, S₄N₄". Acta Crystallographica 16 (9): 891–897. doi:10.1107/S0365110X63002401. 
4. ^ H. S. Rzepa and J. D. Woollins (1990). "A PM3 SCF-MO Study of the Structure and Bonding in the Cage Systems S₄N₄ and S₄N₄X (X=N [+], N[-], S, N₂S, P[+], C, Si, B[-] and Al[-])". Polyhedron 9 (1): 107. doi:10.1016/S0277-5387(00)84253-9. 
5. ^ Jolly, W. L.; Steven A. Lipp (January 1971). "Reaction of tetrasulfur tetranitride with sulfuric acid". Inorganic Chemistry 10 (1): 33–38. doi:10.1021/ic50095a008.  edit
6. ^ Villena-Blanco, M.;Jolly, W.L.; Tyree (1967). "Tetrasulfur Tetranitride, S₄N₄". Inorg. Synth. 9: 98–102. doi:10.1002/9780470132401.ch26. 
7. ^ Maaninen, A.; Shvari, J.; Laitinen, R.S.; Chivers, T; (2002). Inorg. Synth. 33: 196–199. doi:10.1002/0471224502.ch4. 
8. ^ Paul. F. Kelly, Alexandra. M.Z. Slawin, David J. Williams and J. Derek Woollins (1992). "Caged explosives: metal-stabilized chalcogen nitrides". Chemical Society Reviews 21 (4): 245. doi:10.1039/CS9922100245. 
9. ^ Bojes, J.; Chivers, T; Oakley, R. D. (1989). "Binary Cyclic Nitrogen-Sulfur Anions". Inorg. Synth. 25: 30–40. doi:10.1002/9780470132562.ch7. 
10. ^ Bojes, J.; Chivers, T; Oakley, R. D. (1989). "Binary Catena-Nitrogen-Sulfur Anions". Inorg. Synth. 25: 35–38. doi:10.1002/9780470132562.ch8. 
11. ^ P. J. Dunn and H. S. Rzepa (1987). "The Reaction between Tetrasulphur Tetranitride (S4N4) and Electron-deficient Alkynes. A Molecular Orbital Study". Journal of the Chemical Society, Perkin Transactions 2 (11): 1669–1670. doi:10.1039/p29870001669. 
12. ^ R. L. Greene, G. B. Street and L. J. Suter, Superconductivity in Polysulfur Nitride (SN)_x, Phys. Rev. Lett. 34, 577–579 (1975) doi:10.1103/PhysRevLett.34.577
13. ^ Konarev, D.V. et al. (2000). "Donor-acceptor Complexes of Fullerene C₆₀ with Organic and Organometallic Donors". Journal of Materials Chemistry 10 (4): 803–818. doi:10.1039/a907106g. 
14. ^ Kelly, P.F. and Woollins, J.D., (1993). "The Reactivity of Se₄N₄ in Liquid Ammonia". Polyhedron 12 (10): 1129–1133. doi:10.1016/S0277-5387(00)88201-7. 
15. ^ Kelly, P.F., Slawin, A.M.Z. and Soriano-Rama, A. (1997). "Use of Se₄N₄ and Se(NSO)₂ in the preparation of palladium adducts of diselenium dinitride, Se₂N₂; crystal structure of [PPh₄]₂[Pd₂Br₆(Se₂N₂)". Dalton Transactions (4): 559–562. doi:10.1039/a606311j. 
16. ^ Kelly, P.F. and Slawin, A.M.Z. (1996). "Preparation and crystal structure of [(AlBr₃)₂(Se₂N₂)], the first example of a main-group element adduct of diselenium dinitride". Dalton Transactions (21): 4029–4030. doi:10.1039/DT9960004029.