Tetrasulfur tetranitride
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| Template:Longitem | Template:Chem2 |
| Molar mass | 184.287 g/mol |
| Appearance | Vivid orange, opaque crystals |
| Melting point | Template:Chembox CalcTemperatures |
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Tetrasulfur tetranitride is an inorganic compound with the formula Template:Chem2. This vivid orange, opaque, crystalline explosive 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 the properties of atoms are so highly similar, they often form extensive families of covalently bonded structures and compounds. Indeed, a large number of S-N and S-NH compounds are known with Template:Chem2 as their parent.
Structure
Template:Chem2 adopts an unusual "extreme cradle" structure, with D2d point group symmetry. It can be viewed as a derivative of a (hypothetical) eight-membered ring (or more simply a 'deformed' 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 Waals radii[4] but has been explained in the context of molecular orbital theory.[1] One pair of the transannular S atoms have valence 4, and the other pair of the transannular S atoms have valence 2.Script error: No such module "Unsubst". The bonding in Template:Chem2 is considered to be delocalized, which is indicated by the fact that the bond distances between neighboring sulfur and nitrogen atoms are nearly identical. Template:Chem2 has been shown to co-crystallize with benzene and the [[fullerene|Template:Chem2]] molecule.[5]
Properties
Template:Chem2 is stable to air. It is, however, unstable in the thermodynamic sense with a positive enthalpy of formation of +460 kJ/mol. This endothermic enthalpy of formation originates in the difference in energy of Template:Chem2 compared to its highly stable decomposition products:
Template:Chem2 is shock and friction sensitive and because one of its decomposition products is a gas, it is considered a primary explosive.[1][6] Purer samples tend to be more sensitive.[7] Small samples can be detonated by striking with a hammer. Template:Chem2 is thermochromic, changing from pale yellow below −30 °C to orange at room temperature to deep red above 100 °C.[1]
Synthesis
Template:Chem2 was first prepared in 1835 by M. Gregory by the reaction of disulfur dichloride with ammonia,[8] a process that has been optimized:[9]
Coproducts of this reaction include heptasulfur imide (Template:Chem2) and elemental sulfur, and the latter equilibrates with more Template:Chem2 and ammonium sulfide:[10]
A related synthesis employs Template:Chem2 instead:[1]
An alternative synthesis entails the use of Template:Chem2 as a precursor with pre-formed S–N bonds. Template:Chem2 is prepared by the reaction of lithium bis(trimethylsilyl)amide and [[sulfur dichloride|Template:Chem2]].
The Template:Chem2 reacts with the combination of Template:Chem2 and [[sulfuryl chloride|Template:Chem2]] to form Template:Chem2, trimethylsilyl chloride, and sulfur dioxide:[11]
Acid-base reactions
Template:Chem2 is a Lewis base at nitrogen. It binds to strong Lewis acids, such as [[Antimony pentachloride|Template:Chem2]] and [[Sulfur trioxide|Template:Chem2]], or [[Tetrafluoroboric acid|Template:Chem2]]:
The cage is distorted in these adducts.[1]
Template:Chem2 reacts with metal complexes, but the bonding situation may be quite complex. The cage remains intact in some cases but in other cases, it is degraded.[2][12] For example, the soft Lewis acid CuCl forms a coordination polymer:[1]
Reportedly, Template:Chem2 initially forms a complex with Template:Chem2 at sulfur. This compound, upon standing, isomerizes to additionally bond through a nitrogen atom. Template:Chem2 oxidatively adds to Vaska's complex (Template:Chem2 to form a hexacoordinate iridium complex where the Template:Chem2 binds through two sulfur atoms and one nitrogen atom.[2]
Dilute NaOH hydrolyzes Template:Chem2 as follows, yielding thiosulfate and trithionate:[1]
More concentrated base yields sulfite:
As a precursor to other S-N compounds
Many S-N compounds are prepared from Template:Chem2.[13]
In electrophilic substitution or 1,3-dipolar cycloaddition reactions, S4N4 behaves as a combination of the dithionitronium synthon and the sulfide synthon. Thus it adds to arenes and electron-rich alkynes to give 1,2,5Template:Nbhthiadiazoles.[14] Electron-poor alkynes attack Template:Chem2 to give a different cycloadduct of stoichiometry RC(NS)2SCTemplate:Prime.[15][14] With electron-rich alkenes, S4N4 behaves as a Diels-Alder diene.[14]
Passing gaseous Template:Chem2 over silver metal yields the low temperature superconductor polythiazyl or polysulfurnitride (transition temperature (0.26±0.03) K[16]), often simply called "(SN)x". In the conversion, the silver first becomes sulfided, and the resulting [[silver sulfide|Template:Chem2]] catalyzes the conversion of the Template:Chem2 into the four-membered ring Template:Chem2, which readily polymerizes.[1]
Oxidation of Template:Chem2 with elemental chlorine gives thiazyl chloride,Script error: No such module "Unsubst". but milder reagents give S4NScript error: No such module "Su".:
- 3 S4N4 + 2 S2Cl2 → 4 [S4N3]+Cl−
- S4N4 + RC(=O)Cl → [S4N3]+Cl− + RNCO
That cation is relatively non-electrophilic and planar, with a delocalized π system. However, it adds triphenylphosphine to give [S(NPPh3)3]3+[Cl−]3, a triimide analogue to sulfur trioxide. Conversely, S4NScript error: No such module "Su". salts react with aluminum azide to recover S4N4.[14]
Treatment with tetramethylammonium azide produces the similar 10-π heterocycle Template:Chem2:
In a related reaction, the use of the bis(triphenylphosphine)iminium azide gives a salt containing the blue Template:Chem2 anion:[13]
Template:Chem2 has a chain structure approximated by the resonance Template:Chem2.
Reaction with piperidine generates Template:Chem2:
A related cation is also known, i.e. Template:Chem2.
Triphenylphosphine abstracts a sulfur atom, replacing it with another triphenylphosphine moiety:[14]
- S4N4 + 2 PPh3 → S3(PPh3)N4 + SPPh3
Safety
Template:Chem2 is a categorized as a primary explosive that is shock and friction sensitive. While comparable to pentaerythritol tetranitrate (PETN) in terms of impact sensitivity, its friction sensitivity is equal to or even lower than lead azide.[17] Purer samples are more shock-sensitive than those contaminated with elemental sulfur.[9][7]
Related compounds
- The selenium analogue Template:Chem2, tetraselenium tetranitride.
References
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