Tetrasulfur tetranitride

{{Chembox | Verifiedfields = changed | Watchedfields = changed | verifiedrevid = 470605090 | Name = | ImageFile = Tetrasulfur-tetranitride.png | ImageFile_Ref = {{chemboximage|correct|??}} | ImageSize = 160 | ImageName = Stereo, skeletal formula of tetrasulfur tetranitride with some measurements |ImageFile2=Resonance_structures_of_tetrasulfur_tetranitride.svg |ImageCaption2=Resonance structures of tetrasulfur tetranitride | ImageFileL1 = Tetrasulfur-tetranitride-from-xtal-2000-3D-balls.png | ImageFileL1_Ref = {{chemboximage|correct|??}} | ImageNameL1 = Ball and stick model of tetrasulfur tetranitride | ImageFileR1 = Tetrasulfur-tetranitride-3D-vdW.png | ImageFileR1_Ref = {{chemboximage|correct|??}} | ImageNameR1 = Space-filling model of tetrasulfur tetranitride | OtherNames = {{Ubl|Cyclic sulfur(III) nitride tetramer{{Citation needed|date=January 2011}}|Tetranitrogen tetrasulfide|1λ⁴,3,5λ⁴,7,2,4,6,8-Tetrathiatetrazocine{{Citation needed|date=January 2011}} }} | IUPACName = Tetrasulfur tetranitride | SystematicName = 1λ⁴,3,5λ⁴,7-tetrathia-2,4,6,8-tetrazacycloocta-1,4,5,8-tetraene<ref>https://pubchem.ncbi.nlm.nih.gov/compound/141455#section=IUPAC-Name&fullscreen=true</ref> | Section1 = {{Chembox Identifiers | CASNo = 28950-34-7 | CASNo_Ref = {{cascite|correct|CAS}} | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 9BXS997HR6 | PubChem = 141455 | ChemSpiderID = 124788 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | SMILES1 = N1=[S]N=[S]N=[S]N=[S]1 | StdInChI = 1S/N4S4/c1-5-2-7-4-8-3-6-1 | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = LTPQFVPQTZSJGS-UHFFFAOYSA-N | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} }} | Section2 = {{Chembox Properties | Formula = {{chem2|S4N4}} | MolarMass = 184.287 g/mol | Appearance = Vivid orange, opaque crystals | MeltingPtC = 187 }} | Section3 = | Section4 = | Section5 = | Section6 = }} '''Tetrasulfur tetranitride''' is an inorganic compound with the formula {{chem2|S4N4|auto=1}}. 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.<ref name="Greenwood">{{ cite book |last1= Greenwood |first1= N. N. |last2= Earnshaw |first2= A. |title= Chemical Elements |edition= 2nd |publisher= Butterworth-Heinemann |location= Boston, MA |year= 1997 |pages= 721–725 }}</ref><ref name=Chivers>{{cite book |last= Chivers |first= T. |title= A Guide To Chalcogen-Nitrogen Chemistry |publisher= World Scientific Publishing |location= Singapore |year= 2004 |isbn= 981-256-095-5 }}</ref>

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 {{chem2|S4N4}} as their parent.

==Structure== The bonding in {{chem2|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.

{{chem2|S4N4}} adopts an unusual "extreme cradle" structure, with D_2d 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&nbsp;Å, resulting in a cage-like structure as determined by single crystal X-ray diffraction.<ref>{{cite journal |last1= Sharma |first1= B. D. |last2= Donohue |first2= J. |title= The Crystal and Molecular Structure of Sulfur Nitride, S₄N₄ |journal= Acta Crystallographica |year= 1963 |volume= 16 |issue= 9 |pages= 891–897 |doi= 10.1107/S0365110X63002401 |bibcode= 1963AcCry..16..891S }}</ref> The nature of the transannular interaction between those sulfur atoms remains a matter of investigation, especially in the context of molecular orbital theory.<ref name="Greenwood" /> The distance is significantly shorter than the sum of the van der Waals radii<ref>{{cite journal |title= 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⁻) |authorlink1= Henry Rzepa |last1= Rzepa |first1= H. S. |last2= Woollins |first2= J. D. |journal= Polyhedron |year= 1990 |volume= 9 |issue= 1 |pages= 107–111 |doi= 10.1016/S0277-5387(00)84253-9 }}</ref> and seems to be similar to the strength of a hydrogen bond.<ref>{{cite journal |last1= Chivers |first1= Tristram |last2= Oakley |first2= Richard T. |title= Dissociation Without Detonation: A DFT Analysis of the Thermally Induced Fragmentation of Binary Sulfur–Nitrogen Rings and Cages |journal= Inorganic Chemistry |date= 2024 |volume= 63 |issue= 44 |pages= 21238–21251 |doi= 10.1021/acs.inorgchem.4c03656 |pmid= 39437165 }}</ref>

{{chem2|S4N4}} has been shown to co-crystallize with benzene and the {{chem2|C60}} molecule.<ref>{{cite journal |last1= Konarev |first1= D. V. |last2= Lyubovskaya |first2= R. N. |last3= Drichko |first3= N. V. |last4= Yudanova |first4= E. I. |last5= Shulga |first5= Yu. M. |last6= Litvinov |first6= A. L. |last7= Semkin |first7= V. N. |last8= Tarasov |first8= B. P. |display-authors= 3 |title= Donor-Acceptor Complexes of Fullerene C₆₀ with Organic and Organometallic Donors |journal= Journal of Materials Chemistry |year= 2000 |volume= 10 |issue= 4 |pages= 803–818 |doi= 10.1039/a907106g }}</ref>

==Properties== {{chem2|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 {{chem2|S4N4}} compared to its highly stable decomposition products: :{{chem2|2 S4N4 → 4 N2 + S8}} {{chem2|S4N4}} is shock and friction sensitive and because one of its decomposition products is a gas, it is considered a primary explosive.<ref name="Greenwood" /><ref name=ExplosiveTest>{{cite journal|year=2021|title=Analysis of the explosive properties of tetrasulfur tetranitride, S₄N₄|last1=Koch|first1=EC|last2=Suceska|first2=M|journal=Zeitschrift für Anorganische und Allgemeine Chemie|volume=647|issue=4|pp=192&ndash;199|doi=10.1002/zaac.202000406}}</ref> Purer samples tend to be more sensitive.<ref name=":0">{{Citation |last1=Ebrahimian |first1=G. Reza |title=Tetrasulfur Tetranitride |date=2009-03-15 |encyclopedia=Encyclopedia of Reagents for Organic Synthesis |editor-last=John Wiley & Sons, Ltd |url=https://onlinelibrary.wiley.com/doi/10.1002/047084289X.rn00933 |access-date=2024-05-24 |place=Chichester, UK |publisher=John Wiley & Sons, Ltd |language=en |doi=10.1002/047084289x.rn00933 |isbn=978-0-471-93623-7 |last2=Fuchs |first2=Philip L.|url-access=subscription }}</ref> Small samples can be detonated by striking with a hammer.

{{chem2|S4N4}} is thermochromic, changing from pale yellow below −30&nbsp;°C to orange at room temperature to deep red above 100&nbsp;°C.<ref name="Greenwood" />

==Synthesis== {{chem2|S4N4}} was first prepared in 1835 by M. Gregory by the reaction of disulfur dichloride with ammonia,<ref>{{Cite journal| last1 = Jolly | first1 = W. L.| last2 = Lipp | first2 = S. A.| title = Reaction of Tetrasulfur Tetranitride with Sulfuric Acid| journal = Inorganic Chemistry | volume = 10 | issue = 1| pages = 33–38 | year = 1971| doi = 10.1021/ic50095a008| url = https://escholarship.org/uc/item/7xj1q0zf}}</ref> a process that has been optimized:<ref name="Greenwood" /> :{{chem2|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.<ref>{{Cite journal |last=Roesky |first=Herbert W. |date=Feb 1979 |title=Structure and Bonding in Cyclic Sulfur-Nitrogen Compounds |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.197900913 |journal=Angewandte Chemie International Edition in English |language=en |volume=18 |issue=2 |pages=91–97 |doi=10.1002/anie.197900913 |issn=0570-0833 |ref={{harvid|Roesky|1979a}}|url-access=subscription }}</ref> Nevertheless, one variant on this reaction uses anhydrous ammonia for simplicity:<ref name="Blanco">{{cite book |last1= Villena-Blanco |first1= M. |last2= Jolly |first2= W. L. |chapter= Tetrasulfur Tetranitride, S₄N₄ |display-authors= etal |series= Inorganic Syntheses |year= 1967 |volume= 9 |pages= 98–102 |doi= 10.1002/9780470132401.ch26 |isbn= 978-0-470-13168-8 |title= Inorganic Syntheses |editor= S. Y. Tyree Jr}}</ref> :{{chem2|6 S2Cl2 + 16 NH3 → S4N4 + S8 + 12 [NH4]Cl}} Coproducts of this reaction include heptasulfur imide ({{chem2|S7NH}}) and elemental sulfur; the latter equilibrates with more {{chem2|S4N4}} and ammonium sulfide:<ref>{{cite book|url=https://archive.org/details/cftri.2662nonaqueoussolven0000ludw/page/44/|page=44|title=Non-aqueous solvents|first1=Ludwig&nbsp;F.|last1=Audrieth|first2=Jacob|last2=Kleinberg|publisher=John Wiley & Sons|location=New York|year=1953|lccn=52-12057}}</ref> :{{chem2|16 S + 16 NH3 ↔ S4N4 + 12 (NH4)S}} An alternative synthesis pre-forms S–N bonds with lithium bis(trimethylsilyl)amide and {{chem2|SCl2}}: :{{chem2|2 ((CH3)3Si)2NLi + SCl2 → (((CH3)3Si)2N)2S + 2 LiCl}}

The {{chem2|(((CH3)3Si)2N)2S}} then reacts with the combination of {{chem2|SCl2}} and {{chem2|SO2Cl2}} to form {{chem2|S4N4}}, trimethylsilyl chloride, and sulfur dioxide:<ref>{{cite book |last1= Maaninen |first1= A. |last2= Shvari |first2= J. |last3= Laitinen |first3= R. S. |last4= Chivers |first4=T |chapter= Compounds of General Interest |series= Inorganic Syntheses |year= 2002 |volume= 33 |pages= 196–199 |doi= 10.1002/0471224502.ch4 |editor-last= Coucouvanis |editor-first= Dimitri |title= Inorganic Syntheses|isbn= 9780471208259 }}</ref>

:{{chem2|2 (((CH3)3Si)2N)2S + 2 SCl2 + 2 SO2Cl2 → S4N4 + 8 (CH3)3SiCl + 2 SO2}}

==Acid-base reactions== thumb|right|150px|{{chem2|S4N4*BF3}} {{chem2|S4N4}} is a Lewis base at nitrogen. It binds to strong Lewis acids, such as {{chem2|SbCl5}} and {{chem2|SO3}}, or [[Tetrafluoroboric acid|{{chem2|H[BF4]}}]]: :{{chem2|S4N4 + SbCl5 → S4N4*SbCl5}} :{{chem2|S4N4 + SO3 → S4N4*SO3}} :{{chem2|S4N4 + H[BF4] → [S4N4H]+[BF4]−}} The cage is distorted in these adducts.<ref name="Greenwood" />

{{chem2|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.<ref name=Chivers/><ref>{{cite journal |last1= Kelly |first1= P. F. |last2= Slawin |first2= A. M. Z. |last3= Williams |first3= D. J. |last4= Woollins |first4= J. D. |title= Caged explosives: Metal-Stabilized Chalcogen Nitrides |journal= Chemical Society Reviews |year= 1992 |volume= 21 |issue= 4 |pages= 245–252 |doi= 10.1039/CS9922100245 }}</ref> For example, the soft Lewis acid CuCl forms a coordination polymer:<ref name="Greenwood" /> :{{chem2|''n'' S4N4 + ''n'' CuCl → (S4N4)_{''n''}\-μ\-(\sCu\sCl\s)_{''n''}|}} Reportedly, {{chem2|[Pt2Cl4(P(CH3)2Ph)2]}} initially forms a complex with {{chem2|S4N4}} at sulfur. This compound, upon standing, isomerizes to additionally bond through a nitrogen atom. {{chem2|S4N4}} oxidatively adds to Vaska's complex ({{chem2|[Ir(Cl)(CO)(PPh3)2]}} to form a hexacoordinate iridium complex where the {{chem2|S4N4}} binds through two sulfur atoms and one nitrogen atom.<ref name=Chivers/>

Dilute NaOH hydrolyzes {{chem2|S4N4}} as follows, yielding thiosulfate and trithionate:<ref name="Greenwood" /> :{{chem2|2 S4N4 + 6 OH− + 9 H2O → S2O3(2−) + 2 S3O6(2−) + 8 NH3}} More concentrated base yields sulfite: :{{chem2|S4N4 + 6 OH− + 3 H2O → S2O3(2−) + 2 SO3(2−) + 4 NH3}}

==As a precursor to other S-N compounds== Many S-N compounds are prepared from {{chem2|S4N4}}.<ref name="ch7">{{cite book |last1= Bojes |first1=J. |last2= Chivers |first2=T. |last3= Oakley |first3=R. D. |chapter=Binary Cyclic Nitrogen-Sulfur Anions |display-authors= etal |title= Inorganic Syntheses |series= Inorganic Syntheses |year= 1989 |volume= 25 |pages= 30–35 |doi= 10.1002/9780470132562.ch7 |editor-last= Allcock |editor-first= H. R. |isbn=9780470132562 }}</ref> In the simplest case, (NS)₂ units insert into a weak, polarized bond:<ref name=Return>{{cite journal|doi=10.1016/S0065-2792(08)60083-7|journal=Advances in Inorganic Chemistry and Radiochemistry|volume=22|title=Cyclic sulfur-nitrogen compounds|first=H. W.|last=Roesky|year=1979b|publisher=Academic Press|pp=285&ndash;286,289}}</ref>{{rp|285}} :Me₃SiNR₂&nbsp;+ S₄N₄&nbsp;→ 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{{nbh}}thiadiazoles (a 1,3-dipolar cycloaddition). Very electron-poor alkynes also attack {{chem2|S4N4}} to give a different, 7-membered cycloadduct, corresponding to addition of -S-N-S-N-S- across the triple bond.<ref>{{ cite journal |title= The Reaction Between Tetrasulphur Tetranitride (S₄N₄) and Electron-deficient Alkynes. A Molecular Orbital Study |last1= Dunn |first1= P. J. |authorlink2= Henry Rzepa |last2= Rzepa |first2= H. S. |journal= Journal of the Chemical Society, Perkin Transactions 2 |year= 1987 |volume= 1987 |issue= 11 |pages= 1669–1670 |doi= 10.1039/p29870001669 }}</ref><ref>{{harvnb|Roesky|1971}}{{pn|date=April 2026}} also incorrectly suggests that S₄N₄ behaves as a Diels-Alder diene with electron-rich alkenes. In fact, the Diels-Alder "products" are all structural mis-assignments for 1,2,5{{nbh}}thiadiazoles; see {{harvnb|Roesky|1979b}}, p.&nbsp;285.</ref>

Passing gaseous {{chem2|S4N4}} over silver metal yields the low temperature superconductor polythiazyl or polysulfurnitride (transition temperature (0.26±0.03)&nbsp;K<ref>{{cite journal |last1= Greene |first1= R. L. |last2= Street |first2= G. B. |last3= Suter |first3= L. J. |title= Superconductivity in Polysulfur Nitride (SN)_x |journal= Physical Review Letters |year= 1975 |volume= 34 |issue= 10 |pages= 577–579 |doi= 10.1103/PhysRevLett.34.577 |bibcode= 1975PhRvL..34..577G}}</ref>), often simply called "(SN)_''x''". In the conversion, the silver first becomes sulfided, and the resulting {{chem2|Ag2S}} catalyzes the conversion of the {{chem2|S4N4}} into the four-membered ring {{chem2|S2N2}}, which readily polymerizes.<ref name="Greenwood" /> :{{chem2|S4N4 + 8 Ag → 4 Ag2S + 2 N2}} :{{chem2|''x'' S4N4 → (SN)_{4''x''}|}}

Alcoholic tin(II) chloride reduces {{chem2|S4N4}} to {{chem2|(NH)4S4}}, valence-isoelectronic with octasulfur. Autoxidation does not recover {{chem2|S4N4}}, but instead polymeric (&ndash;S(O)N(H)&ndash;)_∞.<ref name=Return/>{{rp|286,289}}

Oxidation with elemental fluorine or silver(II) fluoride gives {{chem2|N4(SF)4}},<ref name=Return/> but milder reagents give S₄N{{su|b=3|p=+}}: :3&nbsp;S₄N₄&nbsp;+ 2&nbsp;S₂Cl₂&nbsp;→ 4 [S₄N₃]⁺Cl^&minus; :S₄N₄&nbsp;+ RC(=O)Cl&nbsp;→ [S₄N₃]⁺Cl^&minus;&nbsp;+ 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^&minus;]₃, a triimide analogue to sulfur trioxide. Conversely, S₄N{{su|b=3|p=+}} salts react with aluminum azide to recover S₄N₄.<ref name=Sulf>{{cite book|title=Sulfur in Organic and Inorganic Chemistry|volume=1|editor-first=Alexander|editor-last=Senning|year=1971|publisher=Marcel Dekker|location=New York|lccn=70-154612|isbn=0-8247-1615-9|first=H.&nbsp;W.|last=Roesky|chapter=The Sulfur&ndash;Nitrogen Bond|pages=14–18}}</ref>

Treatment with tetramethylammonium azide or other nucleophiles produces the similar 10-π heterocycle {{chem2|[S3N3]−}}: :{{chem2|8 S4N4 + 8 [(CH3)4N]+[N3]- → 8 [(CH3)4N]+[S3N3]- + S8 + 16 N2}} Excess {{chem2|S4N4}} can react with the {{chem2|[S3N3]−}} to form {{chem2|[S4N5]-}}.<ref>{{cite thesis|pp=16&ndash;17|institution=University of Calgary|url=https://ucalgary.scholaris.ca/server/api/core/bitstreams/87cfc535-9c7d-45c2-a6d1-dd9aeb71c11b/content|title=Synthesis and Spectroscopic Characterization of Acyclic Sulphur-Nitrogen and Sulphur-Nitrogen-Oxygen Anions|first=Kenneth J.|last=Schmidt|type=PhD|date=March 1990}}</ref>

In a related reaction, the use of the bis(triphenylphosphine)iminium azide gives a salt containing the blue {{chem2|[NS4]−}} anion:<ref name="ch7" /> :{{chem2|4 S4N4 + 2 [PPN]+[N3]- → 2 [PPN]+[NS4]- + S8 + 10 N2}} {{chem2|[NS4]-}} has a chain structure approximated by the resonance {{chem2|[S\dS\dN\sS\sS−] ↔ [−S\sS\sN\dS\dS]}}.

Reaction with piperidine generates {{chem2|[S4N5]−}}: :{{chem2|24 S4N4 + 32 C5H10NH → 8 [C5H10NH2]+[S4N5]− + 8 (C5H10N)2S + 3 S8 + 8 N2}} A related cation is also known, i.e. {{chem2|[S4N5]+}}.

Triphenylphosphine abstracts a sulfur atom, replacing it with another triphenylphosphine moiety:<ref name=Sulf/> :S₄N₄&nbsp;+ 2&nbsp;PPh₃&nbsp;→ S₃(PPh₃)N₄&nbsp;+ SPPh₃

S₄N₄ also reacts with metal salts, forming a wide variety of complexes: :Pb(NO₃)₂&nbsp;+ S₄N₄&nbsp;+ {{NH3}}&nbsp;→ Pb(NH₃)(S₂N₂)&nbsp;+&nbsp;... :Co(CO)₂C₅H₅&nbsp;+ S₄N₄&nbsp;→ CoC₅H₅(S₂N₂)&nbsp;+&nbsp;... :VCl₄&nbsp;+ S₄N₄&nbsp;→ VCl₂(S₂N₃)&nbsp;+ VCl₄(S₂N₃)&nbsp;+&nbsp;... In all these cases, the product is a metallacycle, with the N-S atoms forming a semicircular bidentate ligand.<ref>{{cite journal|doi=10.1016/S0277-5387(00)84411-3|pp=624&ndash;627|journal=Polyhedron|volume=5|issue=3|year=1986|location=Great Britain|publisher=Pergamon|title=The preparation and structure of complexes containing simple sulphur-nitrogen ligands|first1=Paul F.|last1=Kelly|first2=J. Derek|last2=Woollins|id=Report #14}}</ref>

==Safety== {{chem2|S4N4}} 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.<ref name=ExplosiveTest/> Purer samples are more shock-sensitive than those contaminated with elemental sulfur.<ref name="Blanco"/><ref name=":0" />

==Related compounds== * The selenium analogue {{chem2|Se4N4}}, tetraselenium tetranitride.

==References== {{Reflist|30em}} {{sulfur compounds}} {{Nitrides}}

Category:Explosive chemicals Category:Inorganic compounds Category:Sulfur–nitrogen compounds Category:Nitrides Category:Eight-membered rings