|Jmol-3D images||Image 1
|Molar mass||184.287 g mol−1|
|Appearance||Vivid orange, opaque crystals|
|Melting point||187 °C; 369 °F; 460 K|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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.
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.
- 1 Structure
- 2 Properties
- 3 Synthesis
- 4 Acid-base reactions
- 5 Reactions with metal complexes
- 6 S4N4 as a precursor to other S-N compounds
- 7 Reaction with alkynes
- 8 Polythiazyl
- 9 Miscellaneous facts
- 10 Se4N4
- 11 Safety
- 12 References
S4N4 adopts an unusual “extreme cradle” structure, with D2d 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. 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 but has been explained in the context of molecular orbital theory. 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.
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. Purer samples tend to be more explosive. Small samples can be detonated by striking with a hammer.
- 24 SCl2 + 64 NH3 → 4 S4N4 + S8 + 48 NH4Cl
A related synthesis employs sulfur monochloride and NH4Cl instead:
- 4 NH4Cl + 6 S2Cl2 → S4N4 + 16 HCl + S8
- 2 [(CH3)3Si]2NLi + SCl2 → [((CH3)3Si)2N]2S + 2 LiCl
- [((CH3)3Si)2N]2S + SCl2 + SO2Cl2 → S4N4 + 4 (CH3)3SiCl + SO2
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.
- 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−
- n S4N4 + n CuCl → (S4N4)n-μ-(-Cu-Cl-)n
- 2 S4N4 + 6 OH− + 9 H2O → S2O2−
3 + 2 S3O2−
6 + 8 NH3
- S4N4 + 6 OH− + 3 H2O → S2O2−
3 + 2 SO2−
3 + 4 NH3
Reactions with metal complexes
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
- 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]+.
- 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.
- 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 alkynes
Passing gaseous S4N4 over silver metal yields the low temperature superconductor polythiazyl or polysulfurnitride (transition temperature (0.26±0.03) K), 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.
- S4N4 + 8 Ag → 4 Ag2S + 2 N2
- S4N4 → (SN)x
S4N4 is shock-sensitive, thus grinding solid samples should be avoided. Purer samples are reportedly more sensitive than those contaminated with elemental sulfur.
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