Hydroxylamine-derived Reagents for the Synthesis of Molecular Metals

The reagent hydroxylamine hydrochloride , NH₃⁺–OH Cl^-, is sometimes used in the identification of organic compounds and in organic syntheses , where it readily forms crystalline oxime derivatives with aldehydes and ketones :

The related compound hydroxylamine-O-sulfonic acid , NH₃⁺–O–SO₃^- , will similarly form oxime sulfates , and has many uses in organic reactions . Wallace has outlined various applications of hydroxylamine-O-sulfonic acid - which he abbreviated "HOSA" - in a comprehensive review (the references are below , at the end of this web page) . HOSA owes much of its utility to the fact that it will readily react with both electrophiles , from its nucleophilic amino end , and with nucleophiles , which will displace the sulfate group intact as sulfate anion .

A neat example of this difunctional behaviour is provided by Kemp and Woodward's synthesis of 1,2-benzisoxazole :

The oxime sulfate was first formed , as with the hydroxylamine hydrochloride shown above . Then in the alkaline conditions of the reaction , the phenolate anion displaced the sulfate anion from the intermediate to cyclize the heterocyclic ring to benzisoxazole . The synthesis is noteworthy in that the entire process was carried out in mild conditions at room temperature without either heating or cooling , and that it was accomplished in an almost quantitative yield . This indicates that both the oxime formation and the phenolate anion displacement of sulfate anion are both very facile , which are quite encouraging for the proposals I'll make below .

This essay is partly a recapitulation of a report I wrote several years ago about the design and synthesis of new molecular metals , based on the structure of poly(sulfur nitride) , (SN)_x . At that time I suggested the preparation of a new reagent derived from hydroxylamine-O-sulfonic acid , containing a "chunk" of (SN)_x , namely the –N=S=N– grouping of atoms . Combining the new reagent with sulfide-based anions would then produce the –S–N=S=N–S– grouping .This latter cluster of sulfur and nitrogen atoms contains both divalent and tetravalent sulfur , as does (SN)_x :

In (SN)_x resonance of the p MO electrons reproportionates (blends) the sulfur II and IV valences into sulfur (III) , which has a trigonal planar hybridization , like the sulfur atom in the thiophene molecule , or the nitrogen in pyridine . The five nitrogen valence electrons are readily accomodated in such a hybridization . However , the six sulfur valence electrons (3s² 3p⁴) can't all fit into this trigonal planar sp² configuration . In thiophene , the electron pair in the sulfur's 3p_z AO form an aromatic sextet with the other four carbon p electrons , but this can't happen in poly(sulfur nitride) . Instead , what I believe occurs is the promotion of the "sixth" sulfur electrons into the 4s frontier AOs all along the chain :

The 4s orbitals are spatially-extended , and can overlap along the S-N chain and throughout the crystalline solid to form a sigma XO (crystal orbital) in it , which is the metallic bond , or conduction band , in the polymer . It is stabilized by the S-N sigma covalent bonds , the pi MO bonds , and the metallic bond , all of which are individually rather weak , but which collectively permit (SN)_x a tolerably stable existence (it decomposes violently when heated , detonating at about 240 ºC) . Poly(sulfur nitride) behaves in most ways like a conventional metallurgical metal , complete with a golden colour and metallic luster . Its crystals are malleable to a certain extent , and can be rolled into flattened sheets . It even tarnishes to a dull appearance when left out exposed to humid air . (SN)_x has a respectable ambient electrical conductivity (4000 ohm^-1cm^-1) and becomes superconducting at 0.26 K . Such metallic properties are indicative of the use of the sulfur 4s frontier orbitals to form the metallic bond in poly(sulfur nitride) . Potassium metal , whose single valence electron is also 4s¹ , would be isoelectronic with (SN)_x , although the two materials are very different chemically , of course .

This , briefly , is the basis for the design of several new molecular metals proposed in the previous report and in the following discussion . A second objective was to outline the synthesis of another candidate compound , not described in the earlier work , which also might be a molecular metal .

Cramer studied the reaction of aniline with sulfur tetrafluoride , which produced a novel compound with the –N=S=N– group :

The amino group in hydroxylamine-O-sulfonic acid might similarly be condensed with SF₄ :

Sulfur tetrafluoride is a gas at room temperature (b.p. –38 ºC) , and is a very toxic , highly reactive electrophile . It is widely used in chemical syntheses as a fluorinating agent , being quite effective at replacing oxygen in both organic and inorganic compounds with fluorine atoms . It should be able to condense with HOSA's amino group to provide the desired –N=S=N– function . The resulting product , sodium bis[(N-sulfate)imido]sulfurane , may prove to be a valuable new reagent in the synthesis of electronically active materials , including molecular and synthetic metals .

As the neutral sodium salt , this reagent could be combined with nucleophilic sulfide anions in various forms , which would displace the sulfate groups from it (as sulfate anions) , and so produce compounds with the desired electronically active –S–N=S=N–S– grouping . An example of this sort of reaction is sketched below :

The hypothetical compound , bis[(phenylthio)amido]sulfurane , was discussed in the previous study of molecular metals , as was the combination of the new reagent with sodium sulfide :

If it succeeded , this would be a remarkably simple and straightforward synthesis of poly(sulfur nitride) . However , it would probably produce the compound as a fine precipitate , and not as the larger single crystals resulting from the vacuum sublimation of S₄N₄ vapour through silver wool , which is the traditional method of preparing (SN)_x . Undoubtedly SN oligomers , such as S₄N₄ , would also form as by-products in the reaction .

Another interesting polymer that might be prepared using the new hydroxylamine-derived reagent , sodium bis[(N-sulfate)imido]sulfurane , is poly-bis[(phenylthio)imido]sulfurane . This is really an extension of the related small molecule of the same name , mentioned above . The corresponding difunctional thiol , 1,4-benzenedithiol , could be used as the sulfide source instead of the phenylthiolate anion. Several different synthesis routes to this compound were discussed in another web page . After treatment with two equivalents of aqueous sodium hydroxide , it would be reacted with an equimolar quantity of sodium bis[(N-sulfate)imido]sulfurane in water , which should cause the polymer to precipitate out of solution :

The neutral polymer would have to be activated by treatment with a one-electron oxidizer such as a nitrosonium salt , which would remove an electron from each –S–N=S=N–S– group . That would set up a resonance in them , in which the divalent and tetravalent sulfurs would be reproportionated to the "unnatural" sulfur(III) . In the process we would hope that the "extra" sixth valence electrons on the sulfurs would be promoted up into the 4s frontier orbitals over the sulfurs . The partially-filled 4s AOs could overlap throughout the polymer , creating a metallic bond in it . The polymer should then have electrical properties typical of the common metals : a fairly high electrical conductivity , and an inverse temperature-conductivity relationship . It might also be superconducting at a very low temperature . A related metallic polymer is discussed in greater detail in another web page .

The S₃N₃⁺ Cation

Two years ago [2005] I studied another hypothetical compound , 1,3,5-trithiabenzenium hexafluorophosphate , as a possible molecular metal . I thought it might even have a chance of being an ambient superconductor , if it could indeed be synthesized :

In the above formula , the three sulfur atoms have a trigonal planar hybridization , like the sulfur atom in thiophene . Then , aromatization can occur in the pi MO over the ring . However , there will be three "left-over" valence electrons from the sulfurs . One of them has been removed by a one-electron oxidizer , and replaced with an external anion . Based on the experience with (SN)_x , it might be possible to promote the remaining two extra electrons into the 4s frontier orbitals over the sulfurs . This admittedly is an unorthodox concept ; most chemists would assign those two electrons to non-bonding or antibonding orbitals , considering the 4s sulfur AOs to be at too high an energy level . This may be true , but to continue on , if they do go into the 4s AOs , they could form an intramolecular mixed-valence resonance system (with the empty AO as the "positive hole") , making them resonate over the three sulfur 4s AOs . They could constitute an aromatic pair (4n+2 , n = 0) in the upper sulfur AOs , which would overlap to form an MO . The trithiabenzenium molecule would be stabilized by three sets of bonds : the low energy C-S sigma covalent bonds ; the mid-level C-S pi MO aromaticity ; and the upper energy level S-S sigma MO aromatic pair .

This latter MO is shown as a yellow circle in the above sketch of the trithiabenzenium molecule . It should have been placed on top of the ring , but due to limitations in my chemistry software , this wasn't possible . As in (SN)_x , the 4s AOs are spatially extended , and can overlap continuously throughout the crystalline solid to form a crystal orbital (XO) , which is the metallic bond (conduction band) in the material . The really interesting thing in this system is : will the aromatic pairs of electrons in the XO stay intact at higher temperatures , as they move through the crystal under an applied p.d. (voltage) , as the electrical current in the solid ? Normally , valence electrons in a metallic bond XO will be distributed into vast numbers of energy levels by the Fermi-Dirac distribution , with only a few of them (typically about 1%) remaining as singlet electrons above the Fermi level . If the Fermi level in trithiabenzenium occurs between the pi MO energy level and that of the S-S sigma XO , then all of the aromatic pairs will be preserved intact . And , if they remain intact as they move through the XO , they will in effect be the Cooper pairs in an ambient superconductor .

If the CH groups in trithiabenzenium are replaced by nitrogen atoms with a trigonal planar hybridization similar to that of the nitrogen atom in pyridine , the resulting S₃N₃⁺ cation should be isoelectronic with it :

The trithiabenzenium cation appears to be unknown in the chemical literature , but the analogous S₃N₃⁺ cation is well-known , at least in the theoretical sense . The S₃N₃^- anion , with 10 p electrons in its pi MO and hence a certain amount of aromatic stabilization , is fairly stable and has been comprehensively studied since its discovery in in 1977 . However , the elusive S₃N₃⁺ cation has only 8 p electrons ; while it may have an underlying 6 p electron aromaticity in its pi MO , the remaining two extra electrons are thought to be free radicals , destabilizing the molecule . This is reminiscent of the triangulene molecule , discussed in another web page . In any case , all attempts to prepare the S₃N₃⁺ cation as a stable , isolable salt have apparently been futile :

"The preparation of salts of the S₃N₃⁺ cation has been described by several groups , but all these reports have later been retracted or refuted . If it existed as a planar ring , S₃N₃⁺ would possess an open-shell diradical configuration" (Chivers et al. , 1983 , footnote 8 , p. 2429) .

The usual synthesis approach to the S₃N₃⁺ cation has been via oxidation of the S₃N₃^- anion , which in turn is prepared by reduction of the readily available S₄N₄ . I wonder if hydride abstraction from the protonated anion might produce the cation :

At least three techniques have been studied for the extraction of hydride from organic compounds . The procedure is usually employed in cases where aromatic or otherwise electronically-stabilized products are formed in the reaction . Trityl salts (triphenylcarbenium cation compounds) , Ph₃C⁺ X^- (X is a non-nucleophilic spectator anion such as BF₄^- , PF₆^- , AsF₆^- , SbF₆^- , SbCl₆^- , NO₃^- , or ClO₄^-) , can replace a hydrogen atom on the substrate with a positive charge , as illustrated by two examples from the research of Bonthrone and Reid :

The by-product in these preparations , not shown in the sketches above , is triphenylmethane , Ph₃CH , m.p. 94 ºC .

A second method of hydride abstraction was developed by Olah and co-workers , using a powerful one-electron oxidizer , the nitrosonium cation , to remove the hydrogen atom . Note that two equivalents of NO⁺ will be consumed per equivalent of substrate , as one of their spectator anions is used to scavenge the extracted hydrogen atom :

NO⁺   +   e^-    ---------------->   NO (g)        E⁰_red = 1.45 V

Substrate –H  +  2  NO⁺ X^-   --------->  Substrate⁺ X^-  +  2  NO(g)  +  H –X

Two examples are shown below of their experimental results using this technique :

A third method of hydride abstraction from hydrocarbon substrates utilizes the mildly oxidizing reagent antimony pentachloride , SbCl₅ , b.p. 79 ºC (22 mm) . The stoichiometry involved is :

R–H   +   2  SbCl₅   --------------------->   R⁺ SbCl₆-   +   SbCl₃   +  HCl (g)

Two examples of this reaction are provided by the experimental results of Holmes and Pettit :

They reported excellent yields for all their carbocation products . However , the SbCl₆^- anion , while non-nucleophilic in nature , is expected to retain its oxidizing power , as its base of Sb(V) is mildly oxidizing :

Sb(V)   +   2 e-   --------------->   Sb(III)   ;   E⁰_red = 0.82 V  (chloride environment) .

So it isn't recommended for use in this particular hydride extraction application , as the residual SbCl₆^- anions in the product might unfavourably distort any metallic bond appearing in it . The distortion would appear as charge density waves , in which the oxidizing SbCl₆^- anions , in periodic arrays in the crystalline solid , attract the mobile free electrons in the metallic bond , forming zones of higher electron density around them , with zones of lower electron density elsewhere in the solid . This localization of electron density can cause an otherwise electronically-active solid to become semiconducting or insulating . Non-nucleophilic , redox-inactive spectator anions such as PF₆^- , AsF₆^- , BF₄^- , NO₃^- , and  ClO₄^- are therefore generally used in molecular metals .

In this "hydride extraction approach" to the S₃N₃⁺ cation , the protonated anion S₃N₃H will be the target molecule :

Its aromatic form on the right looks a little odd , but remember it has ten p electrons , not six . The "ordinary" form of the molecule (left side) , contains both sulfur(II) [sulfide] and sulfur(IV) [sulfurane] , and the key atomic grouping –N=S=N– which will come from the hydroxylamine-derived reagent , sodium bis[(N-sulfate)imido]sulfurane . Conversion of S₃N₃H into the S₃N₃⁺ cation via hydride abstraction is outlined below :

Hopefully that two p electrons in the cation will be promoted into the sulfur 4s AOs , where they can resonate over the three sulfur atoms (with the positive "hole") to form an aromatic pair . The cation would then be stabilized by the low-energy level sigma S-N bonds ; by the pi MO aromatization ; and by the upper level S-S sigma MO aromatization . This latter MO could in theory overlap continuously throughout the crystalline solid to form a crystal orbital (XO) , which is the metallic bond (or conduction band) in the material :

On the other hand , there is a strong possibility that too much energy will be required for the promotion , and the two electrons will enter lower energy level NBAOs to form the cation diradical , which unfortunately is very unstable and decomposes to various products .

Taking an optimistic , positive outlook , let's proceed with an experimental scheme to produce the target molecule and attempt the hydride extraction on it . The first approach would be to synthesize the stable S₃N₃^- anion by a known method , then treat it with a weak acid (such as acetic acid) to protonate the ring , as mentioned above . A second synthesis route to S₃N₃H , using the proposed new reagent sodium bis[(N-sulfate)imido]sulfurane , is sketched below :

The researcher will have to hunt about on the Internet to find a supplier of the starting material , dichloramine-T , which has been known for over a century as an antiseptic and disinfectant . The following sketch outlines the preparation of dichloramine-T , both on a small laboratory scale and on a more ambitious industrial scale :

A related compound , N,N-dichlorourethane (DCU) could also be tried as the starting material . It is commercially available (eg. Aldrich Chemicals) , and can be prepared in the laboratory by the method of Foglia and Swern :

An advantage to using DCU is that it would be easier to hydrolyse its product to S₃N₃H than the sulfonamide product . Indeed , Foglia and Swern avoided the use of NaOH with urethane , instead employing the mildly basic sodium acetate to absorb the by-product HCl from the chlorination . I would be concerned about the possible hydrolysis of the –N=S=N– group to sulfamide if the reaction conditions were too severe :

Finally , I would like to propose an "all-inorganic route" to S₃N₃H , first deriving another new reagent based on hydroxylamine-O-sulfonic acid : its N,N-dichloro derivative , prepared by the same chlorination technique used for the synthesis of dichloramine-T or DCU :

Note that sulfur-based anions are all strongly nucleophilic . For example , HS^- is considered to be comparable in nucleophilic strength to cyanide and iodide anions . The phenylthio anion - see above in the proposed synthesis of bis[(phenylthio)imido]sulfurane - is said to be an even stronger nucleophile than them , in comparison to the weakly nucleophilic phenoxide anion . The sufide anion should thus be able to effect the displacements shown above .

No hydride abstraction would be required in this "all-inorganic" route . Treatment of the product with aqueous barium chloride should remove the N-sulfate group to produce the S₃N₃⁺ cation directly as its chloride salt . The barium cation would "punch out" the sulfate leaving group as the highly insoluble barite (whose solubility is 2.22 mg/L at room temperature) . Actually , if the S₃N₃⁺ cation was stable enough under these conditions , the sulfate group might be spontaneously expelled from the ring and remain as a spectator anion with the cation .

The more expensive silver cation could be used to sequester the sulfate anion as the moderately insoluble silver(I) sulfate (5.7 g/L) ; use of the water-soluble AgF (1820 g/L) and AgNO₃ (1220 g/L) would leave the non-nucleophilic fluoride and nitrate anions behind with the S₃N₃⁺ cation .

One anticipated problem that could defeat this scheme might be the instability of the trisodium disulfide-sulfate intermediate . This could decompose as follows :

Na₃ ^–O–SO₂–O–NS^-₂   ------------>   Na₂SO₄   +   NaNS₂

The product NaNS₂ is the sulfur analogue of sodium nitrite , NaNO₂ . Or , the nucleophilic sulfides could displace the sulfate anion from it , producing complex sulfur-nitrogen compounds (plus sodium sulfate by-product) . Interesting chemistry perhaps , but far away from our desired S₃N₃⁺ cation !

In any event , the S₃N₃⁺ cation is an attempt to re-create in a small molecule the environment of poly(sulfur nitride) , in order to coax the extra pi electrons up into the sulfur 4s AOs , as they seem to be in the metallic polymer . If that can be achieved , we might in addition receive the invaluable bonus of observing ambient superconductivity in a stable metallic S₃N₃⁺ cation salt . This intriguing possibility would make another investigation of the mysterious S₃N₃⁺ cation , with novel synthesis approaches , both worthwhile and educational .

S₃N₃⁺ Analogues

One of the nitrogen atoms in the S₃N₃⁺ cation might be replaced by another analogous atom or group , as we saw with the trithiabenzenium cation above . For example , a Phenyl–C group could be substituted for a nitrogen :

The starting material is benzal dichloride , an inexpensive and readily available (eg. Aldrich) organic chemical . The phenyl group could help stabilize the compound by p MO resonance with the heterocyclic ring . Hydride abstraction from the C–H with trityl cation or nitrosonium cation should be straightforward .

In a second S₃N₃⁺ analogue a nitrogen is replaced by the phenyl–B group , derived from the dichlorophenylborane starting material . This latter chemical is very water-sensitive , and should therefore be reacted with an anhydrous sulfide source (in this case , lithium sulfide) in a dry , polar solvent :

The product is noteworthy in that it has no ionic charge , so its molecules might be able to stack in tall columns as in the metallic compound TTF-TCNQ . If a sigma XO metallic bond can form in the material , it might be somewhat malleable and ductile , compared to the "crunchy" ionic compounds described above . Such malleability would confer useful technological advantages on it , if it could be fabricated into wires .

As synthesized , the bora-diazene would probably have the "non-aromatic" , non-metallic structure shown on the top right side of the sketch above . Irradiating it , possibly with UV light , either as the crystalline solid or in solution , might excite the sulfur valence electrons into p MOs , allowing them to form the heterocyclic aromatic ring . The UV energy might further promote the two "leftover" p electrons up into the 4s AOs over the sulfur atoms , creating the aromatic pair in the sulfur 4s sigma MO , which in turn would make the compound metallic and maybe even superconducting . This would indeed be a fascinating system to synthesize and study !

Update

In a recent email , Professor Tristam Chivers (Department of Chemistry , University of Calgary , Alberta , Canada) brought to my attention information about the S₃N₃⁺ cation which invalidates sections of the web page above :

C.G. Marcellus , R.T. Oakley , A.W. Cordes , and W.T. Pennington , "The Preparation and the Crystal and Molecular Structure of S₃N₂NH₂⁺ BF₄^- ; A Molecular Orbital Study of the Protonation of the Trisulfur Trinitride Anion" , Can. J. Chem. 62 (9) , pp. 1822-1827 (1984) .

These researchers found that protonation of the S₃N₃^- anion induces severe distortion in the neutral molecule (S₃N₃H) such that it rearranges into a five-membered ring , S₃N₂NH₂⁺ :

They commented ,

"....the polarizability of the p-systems of sulfur-nitrogen rings , coupled with the occupation of antibonding p-orbitals , leads to cyclic structures that will distort or even collapse upon coordination of nitrogen" (p. 1826) .

Therefore , the proposal made above regarding hydride extraction from S₃N₃H must be abandoned .

Another point : Marcellus et al. made no comment about their new compound , S₃N₂NH₂⁺ BF₄^- , being metallic . In the above sketch - and assuming all the sulfurs and nitrogens have an sp² trigonal planar hybridization - we see that there would be seven p electrons in the ring . That is , no promotion of the "extra" seventh p electron into the sulfurs' 4s AOs occurred . It's possible that the compound remained "ordinary" (that is , not aromatic) :

In a second research paper , Prof. Chivers described the synthesis of the norbornene derivative of the S₃N₃⁺ cation :

A. Apblett , T. Chivers , A.W. Cordes , and R. Vollmerhaus , "Synthesis and Structure of the Norbornene Adduct of 1,3,5,2,4,6-Trithiatriazinium Tetrachloroaluminate [C₇H₁₀ . S₃N₃][AlCl₄] " , Inorg. Chem. 30 (6) , pp. 1392-1396 (1991) .

In this latter example , the S₃N₃⁺ ring is non-planar , and there is no p electron resonance over the entire sulfur-nitrogen ring . In short , it would seem to be impossible to synthesize a planar S₃N₃⁺ ring with p electron resonance without it collapsing into a stabler structure , so we must reluctantly abandon this system as a possible source of new molecular metals . Similarly , since the –S–N=S=N–S– group will invariably produce eight p electrons in any S-N ring system into which it is incorporated , and such eight p electrons will cause severe distortion and instability in the ring , we must also generally abandon any new S-N molecular ring designs with it . However , it may still be suitable in linear structures such as the bis[(phenylthio)imido]sulfurane candidate molecule shown above . The two S₃N₃⁺ analogues discussed above , with phenyl-C and phenyl-B replacing a ring nitrogen , may be more stable and isolable , and I think are still worth considering .

I now strongly suspect that it is impossible to promote two electrons simultaneously into a higher energy level atomic orbital . The computer analogy would be that it is impossible to execute two commands simultaneously on a computer ; it will "crash" . Readers who are more familiar than me with quantum mechanics might try to prove this ad-hoc theorem mathematically . Thus , the aza-bora compound discussed above could not be photochemically excited (eg. with UV) into the metallic state , with two p electrons promoted into the sulfurs' 4s AOs .

Let's assume the aza-bora compound is reasonably stable , and can be synthesized in its "ordinary" form as sketched above . Then treatment of it with a one-electron oxidizer such as NO⁺ should cause p electron resonance in the ring . This time , there are seven p electrons in the cation , so its salt may be stable and isolable . But will the seventh electron be promoted into the sulfurs' 4s AOs ? If it is , a metallic or at least conductive compound may result . Then it might undergo a subsequent one-electron reduction , for example with sodium electride in anhydrous liquid ammonia , to produce the neutral molecule with the aromatic electron pair in the 4s AOs , that could not be synthesized photochemically from the original aza-bora compound .

The same reasoning - the inability to simultaneously promote two valence electrons to a higher energy level - would apply to the CH analogue of S₃N₃⁺ , the 1,3,5-trithiabenzenium cation discussed elsewhere . But I think the design concept for an ambient superconductor discussed here remains valid , if experimentally challenging , and still deserves consideration from materials scientists .

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Prof. Chivers also pointed out in his email that S-N bonds are hydrolytically sensitive , and that the aqueous reactions described above would probably be unsuccessful . I suspected this , as the –N=S=N– group is really the anhydride of sulfamide , –HN–S(=O)–NH– . The proposed new reagent , sodium bis[(N-sulfate)imido]sulfurane , could be modified for use in anhydrous , polar , aprotic solvents (such as acetonitrile , nitromethane , DMSO , propylene carbonate , etc.) if it was insoluble in them . For example , the triethylammonium groups from its synthesis could be retained , or perhaps tetraethylammonium cations could be substituted for the sodiums . Lithium salts are also often more soluble in organic solvents than are the corresponding sodium salts .

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Prof. Chivers advised that the –S–N=S=N–S– group has been incorporated into polymers , linking phenylene groups . Details of such polymers are available in his book , A Guide to Chalcogen-Nitrogen Chemistry , World Scientific , 2005 . I have been unable to find a copy of this book locally . See the DOI of it at : http://www.worldscibooks.com/chemistry/5701.html . It's also available from Amazon.com .

I thank Prof. Chivers for his review of my web page , and for kindly pointing out errors and omissions which I hope I've corrected to a certain extent . It's always a great pleasure to receive feedback on my web pages , and I welcome positive , constructive criticism of my essays . At the same time , I sincerely hope that all my readers enjoy the chemistry discussions and descriptions in these pages , find them mentally stimulating , and perhaps even learn something new in them .

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I'll take advantage of this update to propose a new mechanism of electrical conduction in poly(sulfur nitride) , one that doesn't require the promotion of sulfur valence electrons into the 4s AOs , as described above . The conduction would occur entirely in the p MO along the polymer spine :

Each –S–N–S–N– group has six p electrons : two from each sulfur , and one from each nitrogen , as sketched above near the top of this web page . The aromatic sextets are reasonably stable , and can all shift together simultaneously along the polymer spines downfield under the applied potential difference . Perlstein had proposed such a mechanism of electrical conduction in the molecular metal TTF-TCNQ thirty years ago . However , note that this mechanism would be inconsistent with the conventional physical theory of the metallic bond , involving the Fermi distribution of the valence electrons in a vast number of energy levels . In this regard , the 4s XO metallic bond/conduction band for (SN)_x proposed here and in my other writings is actually in better agreement with the modern theory of metals , despite the unorthodox idea of the promotion of sulfur 3p valence electrons into the higher energy 4s orbitals .

References and Notes

Wallace : R.G. Wallace , "Hydroxylamine-O-sulfonic Acid - A Versatile Synthetic Reagent" , Aldrichimica Acta 13 (1) , pp. 3-11 (1980) . This article can be downloaded without charge from the Web (PDF document , 2758 KB) , at http://www.sigmaaldrich.com/aldrich/acta/al_acta_13_01.pdf . Note : the entire issue #1 must be downloaded , not just Wallace's review , hence the large file size .

Kemp : D.S. Kemp and R.B. Woodward , “The N-ethylbenzisoxazolium Cation – I . Preparation and Reactions With Nucleophilic Species” , Tetrahedron 21 (11) , pp. 3019-3055 (1965) . See p. 3029 for an experimental description of the reaction cited .

poly(sulfur nitride) : A.G. MacDiarmid et al. , “Synthesis and Selected Properties of  Polymeric Sulfur Nitride (Polythiazyl) , (SN)_x” , Ch. 6 , pp. 63-72 in Inorganic Compounds with Unusual Properties , R.B. King (ed.) , Adv. Chem. Series 150 , American Chemical Society , Washington , D.C. , 1976 ; M.M. Labes , P. Love , and L.F. Nichols , “Polysulfur Nitride – A Metallic , Superconducting Polymer” , Chem. Rev. 79 (1) , pp. 1-15 (1979) .

Cramer : R. Cramer , “Bis(phenylimino)sulfur” , J. Org. Chem. 26 (9) , pp. 3476-3478 (1961) .

sulfur tetrafluoride : supplied by (for example) Air Products & Chemicals , at http://www.airproducts.com/Products/Chemicals/Fluorination/sulfur.htm  , and Matheson-Tri-Gas at https://www.mathesontrigas.com/pdfs/products/Sulfur-Tetrafluoride-Pure-Gas.pdf .

nucleophilic : J. March , Advanced Organic Chemistry , Reactions , Mechanisms , and Structure , 4th edition , John Wiley , New York , 1992 ; Table 10.9 , "Nucleophilicities of Some Common Reagents" , p. 351 . Sulfhydride anion , HS- , is tabulated with cyanide and iodide anions . On p. 350 the nucleophilic strengths of alkylthiolate and arylthiolate anions are said to be superior to those of cyanide and iodide , and much stronger than that of the weakly nucleophilic phenolate anion .

Fermi-Dirac : A.R. Mackintosh , “The Fermi Surface of Metals” , Scientific American , 209 (1) , pp. 110-120 (July , 1963) .

discovery : J. Bojes and T. Chivers , "Synthesis of the Trisulphur Trinitride Anion , S₃N₃^- " , J.C.S. Chem. Comm. 1977 , pp. 453-454 .

Chivers : T. Chivers , W. Cordes , R.T. Oakley , and W.T. Pennington , “ ¹⁵N NMR Study of the Oxidation of Trisulfur Trinitride Anion by Molecular Oxygen : A Comparison of the Molecular and Electronic Structures of the S₃N₃^- , S₃N₃O^- , and S₃N₃O₂^- Ions” , Inorg. Chem. 22 (17) , pp. 2424-2435 (1983) . See also : J. Bojes et al. , "Crystal and Molecular Structure of [n-Bu₄N⁺] [S₃N₃^-] and the Vibrational Assignments and Electronic Structure of the Planar Six-Membered Ring of the Trisulfur Trinitride Anion" , J. Amer. Chem. Soc. 101 (16) , pp. 4517-4522 (1979) ; and , T. Chivers and M.N.S. Rao , "Reaction of the Trisulfur Trinitride Anion , S₃N₃^- , with Halogens and Other Electrophilic Substrates" , Can. J. Chem. 61 (9) , pp. 1957-1962 (1983) . "Of particular interest to us is the preparation of monosubstituted derivatives , S₃N₃X , and the cation S₃N₃⁺ " (p. 1957) .

reduction : The reduction of S₄N₄ by cobaltocene to produce a novel type of molecular stack compound was studied by Jagg et al. :

4 cobaltocene   +   3  S₄N₄   ------------->   4  [cobaltocenium⁺] [S₃N₃^-]

P.N. Jagg et al. , "The Preparation , X-Ray Crystal Structure and Theoretical Study of [Co(cp)₂][S₃N₃] , (Cp = Cyclopentadienyl) , a Novel Stacking Compound Incorporating Multiple C-H...N (pp) Interactions" , J. Chem. Soc. Chem. Commun. 1991 , pp. 942-944 .

Bonthrone : W. Bonthrone and D.H. Reid , “Hydride Ions in Organic Reactions . Part I . Dehydrogenation by Triphenylmethyl Perchlorate” , J. Chem. Soc. 1959 , pp. 2773-2779 .

Trityl salts are commercially available (eg. Aldrich , Alfa-Aesar) , but are somewhat expensive . They can be readily prepared by the reaction of the inexpensive triphenyl carbinol with the desired mineral acid , for example :

Ph₃C–OH   +   HBF₄   ------------->   Ph₃C⁺ BF₄-   +   H₂O

See : H.J. Dauben Jr. , L.R. Honnen , and K.M. Harmon , “Improved Preparation of Triphenylmethyl Perchlorate and Fluoroborate for Use in Hydride Ion Exchange Reactions” , J. Org. Chem. 25 (8) , pp. 1442-1445 (1960) .

Olah : G.A. Olah , G. Salem , J.S. Staral , and T.-L. Ho , “Preparative Carbocation Chemistry . 13 . Preparation of Carbocations from Hydrocarbons via Hydrogen Abstraction with Nitrosonium Hexafluorophosphate and Sodium Nitrite – Trifluoromethanesulfonic Acid ” , J. Org. Chem. 43 (1) , pp. 173-175 (1978) .

Holmes : J. Holmes and R. Pettit , “Hydride Ion Abstraction with Antimony Pentachloride” , J. Org. Chem. 28 (6) , pp. 1695-1696 (1963) .

charge density waves : S. Brown and G. Gruner , "Charge and Spin Density Waves" , Scientific American 270 (4) , pp. 50-56 (April , 1994) ; see the discussion , "Density-Wave Materials" , p. 54 .

TTF-TCNQ : E.M. Engler , “Organic Metals” , Chemtech 6 (4) , pp. 274-279 (April , 1976) .

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