Tetrasulfur tetranitride is an inorganic compound with the formula S4N4. 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. Nitrogen and sulfur have similar electronegativities and atomic radii. 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 S4N4 as their parent.
Structure 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 nearly identical. S4N4 adopts an unusual "extreme cradle" structure, with D2d molecular 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 four nitrogen atoms form an square and the four sulfur atoms an approximae tetrahedron. 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 interaction between those sulfur atoms remains a matter of investigation, especially in the context of molecular orbital theory. The distance is significantly shorter than the sum of the van der Waals radii and seems to be similar to the strength of a hydrogen bond. S4N4 has been shown to co-crystallize with benzene and the C60 molecule.
Properties S4N4 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 S4N4 compared to its highly stable decomposition products:
2 S4N4 → 4 N2 + S8 S4N4 is shock and friction sensitive and because one of its decomposition products is a gas, it is considered a primary explosive. Purer samples tend to be more sensitive. 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.
Synthesis S4N4 was first prepared in 1835 by M. Gregory by the reaction of disulfur dichloride with ammonia, a process that has been optimized:
4 [NH4]Cl + 6 S2Cl2 → S4N4 + 16 HCl + S8 Several variants on the reaction are known, and yields are generally maximized when the sulfur-to-chloride ratio is 3:1. Nevertheless, one variant on this reaction uses anhydrous ammonia for simplicity:
6 S2Cl2 + 16 NH3 → S4N4 + S8 + 12 [NH4]Cl Coproducts of this reaction include heptasulfur imide (S7NH) and elemental sulfur; the latter equilibrates with more S4N4 and ammonium sulfide:
16 S + 16 NH3 ↔ S4N4 + 12 (NH4)S An alternative synthesis pre-forms S–N bonds with lithium bis(trimethylsilyl)amide and SCl2:
2 ((CH3)3Si)2NLi + SCl2 → (((CH3)3Si)2N)2S + 2 LiCl The (((CH3)3Si)2N)2S then reacts with the combination of SCl2 and SO2Cl2 to form S4N4, trimethylsilyl chloride, and sulfur dioxide:
2 (((CH3)3Si)2N)2S + 2 SCl2 + 2 SO2Cl2 → S4N4 + 8 (CH3)3SiCl + 2 SO2
Acid-base reactions
S4N4 is a Lewis base at nitrogen. It binds to strong Lewis acids, such as SbCl5 and SO3, or H[BF4]:
S4N4 + SbCl5 → S4N4·SbCl5 S4N4 + SO3 → S4N4·SO3 S4N4 + H[BF4] → [S4N4H]+[BF4]− The cage is distorted in these adducts. S4N4 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. For example, the soft Lewis acid CuCl forms a coordination polymer:
n S4N4 + n CuCl → (S4N4)n-μ-(−Cu−Cl−)n Reportedly, [Pt2Cl4(P(CH3)2Ph)2] initially forms a complex with S4N4 at sulfur. This compound, upon standing, isomerizes to additionally bond through a nitrogen atom. S4N4 oxidatively adds to Vaska's complex ([Ir(Cl)(CO)(PPh3)2] to form a hexacoordinate iridium complex where the S4N4 binds through two sulfur atoms and one nitrogen atom. Dilute NaOH hydrolyzes S4N4 as follows, yielding thiosulfate and trithionate:
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
As a precursor to other S-N compounds Many S-N compounds are prepared from S4N4. In the simplest case, (NS)2 units insert into a weak, polarized bond:
Me3SiNR2 + S4N4 → Me3Si-N-S-N-S-NR2 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 unsaturated bonds to give 1,2,5‑thiadiazoles (a 1,3-dipolar cycloaddition). Very electron-poor alkynes also attack S4N4 to give a different, 7-membered cycloadduct, corresponding to addition of -S-N-S-N-S- across the triple bond. 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 x S4N4 → (SN)4x Alcoholic tin(II) chloride reduces S4N4 to (NH)4S4, valence-isoelectronic with octasulfur. Autoxidation does not recover S4N4, but instead polymeric (–S(O)N(H)–)∞. Oxidation with elemental fluorine or silver(II) fluoride gives N4(SF)4, but milder reagents give S4N+3:
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, S4N+3 salts react with aluminum azide to recover S4N4. Treatment with tetramethylammonium azide or other nucleophiles produces the similar 10-π heterocycle [S3N3]−:
8 S4N4 + 8 [(CH3)4N]+[N3]− → 8 [(CH3)4N]+[S3N3]− + S8 + 16 N2 Excess S4N4 can react with the [S3N3]− to form [S4N5]−. In a related reaction, the use of the bis(triphenylphosphine)iminium azide gives a salt containing the blue [NS4]− anion:
4 S4N4 + 2 [PPN]+[N3]− → 2 [PPN]+[NS4]− + S8 + 10 N2 [NS4]− has a chain structure approximated by the resonance [S=S=N−S−S−] ↔ [−S−S−N=S=S]. Reaction with piperidine generates [S4N5]−:
… excerpt ends here. Continue reading the full article.
