Hexachlorophosphazene
Hexachlorophosphazene conventional formula and bond lengths
Hexachlorophosphazene ball-and-stick model
Names
IUPAC name
2,2,4,4,6,6-Hexachloro-1,3,5,2λ5,4λ5,6λ5-triazatriphosphinine
Other names
  • Phosphonitrilic chloride trimer
  • Hexachlorotriphosphazene
  • Hexachlorocyclotriphosphazene
  • Triphosphonitrilic chloride
  • 2,2,4,4,6,6-hexachloro-2,2,4,4,6,6-hexahydro-1,3,5,2,4,6-triazatriphosphorine
Identifiers
3D model (JSmol)
ChEMBL
ChemSpider
ECHA InfoCard 100.012.160
EC Number
  • 213-376-8
UNII
  • InChI=1S/Cl6N3P3/c1-10(2)7-11(3,4)9-12(5,6)8-10 checkY
    Key: UBIJTWDKTYCPMQ-UHFFFAOYSA-N checkY
  • InChI=1/Cl6N3P3/c1-10(2)7-11(3,4)9-12(5,6)8-10
    Key: UBIJTWDKTYCPMQ-UHFFFAOYAJ
  • N1=P(N=P(N=P1(Cl)Cl)(Cl)Cl)(Cl)Cl
Properties
P3N3Cl6
Molar mass 347.64 g·mol−1
Appearance colourless solid
Density 1.98 g/mL at 25 °C
Melting point 112 to 114 °C (234 to 237 °F; 385 to 387 K)
Boiling point decomposes (above 167 °C)
60 °C at 0.05 Torr
decomposes
Solubility in CCl4 24.5 wt % (20 °C)
35.6 wt % (40 °C)
39.2 wt % (60 °C)
Solubility in cyclohexane 22.3 wt % (20 °C)
36.8 wt % (40 °C)
53.7 wt % (60 °C)
Solubility in xylene 27.7 wt % (20 °C)
38.9 wt % (40 °C)
50.7 wt % (60 °C)
−149×10−6 cm3/mol
1.62 (589 nm)
Structure
orthorhombic
62 (Pnma, D16
2h)
D3h
a = 13.87 Å, b = 12.83 Å, c = 6.09 Å
4
chair (slightly ruffled)
0 D
Thermochemistry
−812.4 kJ/mol
55.2 kJ/mol
76.2 kJ/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 mild irritant
GHS labelling:
GHS05: Corrosive
Danger
H314
P260, P264, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P405, P501
Flash point Non-flammable
Related compounds
Related compounds
 Hexafluorophosphazene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)
Infobox references

Hexachlorophosphazene is an inorganic compound with the formula (NPCl2)3. The molecule has a cyclic, unsaturated backbone consisting of alternating phosphorus and nitrogen centers, and can be viewed as a trimer of the hypothetical compound N≡PCl2. Its classification as a phosphazene highlights its relationship to benzene.[1] There is large academic interest in the compound relating to the phosphorus-nitrogen bonding and phosphorus reactivity.[2][3]

Occasionally, commercial or suggested practical applications have been reported, too, utilising hexachlorophosphazene as a precursor chemical.[2][4] Derivatives of noted interest include the hexalkoxyphosphazene lubricants obtained from nucleophilic substitution of hexachlorophosphazene with alkoxides,[4] or chemically resistant inorganic polymers with desirable thermal and mechanical properties known as polyphosphazenes produced from the polymerisation of hexachlorophosphazene.[2][4]

Structure and characterisation

Bond lengths and conformation

Hexachlorophosphazene has a P3N3 core with six equivalent P–N bonds, for which the adjacent P–N distances are 157 pm.[1][2][5] This is characteristically shorter than the ca. 177 pm P–N bonds in the valence saturated phosphazane analogues.[3]

The molecule possesses D3h symmetry, and each phosphorus center is tetrahedral with a Cl–P–Cl angle of 101°.[5]

The P3N3 ring in hexachlorophosphazene deviates from planarity and is slightly ruffled (see chair conformation).[2] By contrast, the P3N3 ring in the related hexafluorophosphazene species is completely planar.[2]

Characterisation methods

31P-NMR spectroscopy is the usual method for assaying hexachlorophosphazine and its reactions.[6][7][8] Hexachlorophosphazine exhibits a single resonance at 20.6 ppm as all P environments are chemically equivalent.[7][8]

In it IR spectrum, the 1370 and 1218 cm−1 vibrational bands are assigned to νP–N stretches.[7][8] Other bands are found at 860 and 500–600 cm−1, respectively assigned to ring and νP–Cl.[8]

Hexachlorophosphazine and many of its derivatives have been characterized by single crystal X-ray crystallography.[2][5]

Bonding

Depictions of P–N bonding in a general cyclotriphosphazene: left, a representation of alternating single and double P–N bonds (does not account for equal bond lengths), used as a matter of convention;[1] middle, the earlier proposed delocalised ring system (discredited due to infeasibility of P 3d participation[3]); right, the most accurate description to current knowledge, where the majority of the bonding is ionic[1][3]

Early analyses

Cyclophosphazenes such as hexachlorophosphazene are distinguished by notable stability and equal P–N bond lengths which, in many such cyclic molecules, would imply delocalization or even aromaticity. To account for these features, early bonding models starting from the mid-1950s invoked a delocalised π system arising from the overlap of N 2p and P 3d orbitals.[2][3]

Modern bonding models

Starting from the late 1980s, more modern calculations and the lack of spectroscopic evidence reveal that the P 3d contribution is negligible, invalidating the earlier hypothesis.[3] Instead, a charge separated model is generally accepted.[1][3]

According to this description, the P–N bond is viewed as a very polarised one (between notional P+ and N−), with sufficient ionic character to account for most of the bond strength.[1][3]

The rest (~15%) of the bond strength may be attributed to a negative hyperconjugation interaction: the N lone pairs can donate some electron density into π-accepting σ* molecular orbitals on the P.[3]

Synthesis

The synthesis of hexachlorophosphazene was first reported by von Liebig in 1834. In that report he describes experiments conducted with Wöhler.[9] They found that phosphorus pentachloride and ammonia react exothermically to yield a new substance that could be washed with cold water to remove the ammonium chloride coproduct. The new compound contained P, N, and Cl, on the basis of elemental analysis. It was sensitive toward hydrolysis by hot water.[2]

Modern syntheses are based on the developments by Schenk and Römer who used ammonium chloride in place of ammonia and inert chlorinated solvents. By replacing ammonia with ammonium chloride allows the reaction to proceed without a strong exotherm associated with the NH3/PCl5 reaction. Typical chlorocarbon solvents are 1,1,2,2-tetrachloroethane or chlorobenzene, which tolerate the hydrogen chloride side product. Since ammonium chloride is insoluble in chlorinated solvents, workup is facilitated.[10][11] For the reaction under such conditions, the following stoichiometry applies:

n [NH4]Cl + n PCl5 → (NPCl2)n + n HCl

where n can usually take values of 2 (the dimer tetrachlorodiphosphazene), 3 (the trimer hexachlorotriphosphazene), and 4 (the tetramer octachlorotetraphosphazene).[12]

The three major cyclophosphazene products resulting from the reaction of phosphorus pentachloride and ammonium chloride
The three major cyclophosphazene products resulting from the reaction of phosphorus pentachloride and ammonium chloride

Purification by sublimation gives mainly the trimer and tetramer. Slow vacuum sublimation at approximately 60 °C affords the pure trimer free of the tetramer.[6] Reaction conditions such as temperature may also be tuned to maximise the yield of the trimer at the expense of the other possible products; nonetheless, commercial samples of hexachlorophosphazene usually contain appreciable amounts of octachlorotetraphosphazene, even up to 40%.[6]

Formation mechanism

The mechanism of the above reaction has not been resolved, but it has been suggested that PCl5 is found in its ionic form [PCl4]+[PCl6]− and the reaction proceeds via nucleophilic attack of [PCl4]+ by NH3 (from [NH4]Cl dissociation).[2] Elimination of HCl (the major side product) creates a reactive nucleophilic intermediate

NH3 + [PCl4]+ → HN=PCl3 + HCl + H+

which through further attack of [PCl4]+ and subsequent HCl elimination, creates a growing acyclic intermediate

HN=PCl3 + [PCl4]+ → [Cl3P−N=PCl3]+ + HCl
NH3 + [Cl3P−N=PCl3]+ → HN=PCl2−N=PCl3 + HCl + H+, etc.

until an eventual intramolecular attack leads to the formation of one of the cyclic oligomers.[2]

Reactions

Substitution at P

Hexachlorophosphazene reacts readily with alkali metal alkoxides and amides.[1][2]

A SN2 substitution at hexachlorotriphosphazene. A trigonal bipyramidal transition state is proposed.
A SN2 substitution at hexachlorotriphosphazene. A trigonal bipyramidal transition state is proposed.

The nucleophilic polysubstitution of chloride by alkoxide proceeds via displacement of chloride at separate phosphorus centers:[1]

(NPCl2)3 + 3 NaOR → (NPCl(OR))3 + 3 NaCl
(NPCl(OR))3 + 3 NaOR → (NP(OR)2)3 + 3 NaCl

The observed regioselectivity is due to the combined steric effects and oxygen lone pair π-backdonation (which deactivates already substituted P atoms).[1][2]

Ring-opening polymerisation

Heating hexachlorophosphazene to ca. 250 °C induces polymerisation.[1][2][4][6] The tetramer also polymerises in this manner, although more slowly.[4] The conversion is a type of ring-opening polymerisation (ROP).[6][7] The ROP mechanism is found to be catalysed by Lewis acids, but is overall not very well understood.[7] Prolonged heating of the polymer at higher temperatures (ca. 350 °C) will cause depolymerisation.[2]

The structure of the inorganic chloropolymer product (polydichlorophosphazene) comprises a linear –(N=P(−Cl)2−)n chain, where n ~ 15000.[2][4] It was first observed in the late 19th century and its form after chain cross-linking has been called "inorganic rubber" due to its elastomeric behaviour.[4]

Hexachlorotriphosphazene ROP and subsequent nucleophilic substitution for desired polyphosphazene synthesis
Hexachlorotriphosphazene ROP and subsequent nucleophilic substitution for desired polyphosphazene synthesis

This polydichlorophosphazene product is the starting material for a wide class of polymeric compounds, collectively known as polyphosphazenes. Substitution of the chloride groups by other nucleophilic groups, especially alkoxides as laid out above, yields numerous characterised derivatives.[2][4][6]

Lewis basicity

The nitrogen centres of hexachlorophosphazene are weakly basic, and this Lewis base behaviour has been suggested to play a role in the polymerisation mechanism.[7] Specifically, hexachlorophosphazene has been reported to form adducts of various stoichiometries with Lewis acids AlCl3, AlBr3, GaCl3, SO3, TaCl5, VOCl3, but no isolable product with BCl3.[7]

Among these, the best structurally characterised are the 1:1 adducts with aluminium trichloride or with gallium trichloride; they are found with the Al/Ga atom bound to a N and assume a more prominently distorted chair conformation compared to the free hexachlorophosphazene.[7] The adducts also exhibit fluxional behaviour in solution for temperatures down to −60 °C, which can be monitored with 15N and 31P-NMR.[7]

Coupling reagent

Hexachlorophosphazene has also found applications in research by enabling aromatic coupling reactions between pyridine and either N,N-dialkylanilines or indole, resulting in 4,4'-substituted phenylpyridine derivatives, postulated to go through a cyclophosphazene pyridinium salt intermediate.[6]

The compound may also be used as a peptide coupling reagent for the synthesis of oligopeptides in chloroform, though for this application the tetramer octachlorotetraphosphazene usually proves more effective.[6]

Photochemical degradation

Both the trimer and tetramer in hydrocarbon solutions photochemically react forming clear liquids identified as alkyl-substituted derivatives (NPCl2−xRx)n, where n = 3, 4.[6] Such reactions proceed under prolonged UVC (mercury arc) illumination without affecting the PnNn rings. Solid films of the trimer and tetramer will not undergo any chemical change under such irradiation conditions.[6]

Applications

Hexalkoxyphosphazene derivatives

The hexalkoxyphosphazenes (especially the aryloxy species), resulting from the nucleophilic hexasubstitution of the hexachlorophosphazene P atoms, are valued for their high thermal and chemical stability and their low glass transition temperature.[4] Certain hexalkoxyphosphazenes (such as the hexa-phenoxy derivative) have been put to commercial use as fireproof materials and high temperature lubricants.[4]

Polyphosphazene derivatives

Polyphosphazenes obtained from polymerised hexachlorophosphazene (polydichlorophosphazene) have gathered attention within the field of inorganic polymers and probed investigations on the properties of elastomeric and thermoplastic derivatives.[2][4] Some of them appear promising for future applications as fibre- or membrane-forming high performance materials, since they combine transparency, backbone flexibility, tunable hydrophilicity or hydrophobicity, and various other desirable properties.[4]

Current commercial applications for polyphosphazene rubber components are in O-rings, fuel lines and shock absorbers, where the polyphosphazenes confer fire resistance, imperviousness to oils and flexibility even at very low temperatures.[2]

Further reading

  • Discovery of cyclophosphazenes: Liebig-Wöhler, Briefwechsel vol. 1, 63; Ann. Chem. (Liebig), vol. 11 (1834), 146.
  • First reports on their polymerisation: H. N. Stokes (1895), On the chloronitrides of phosphorus. American Chemical Journal, vol. 17, p. 275.H. N. Stokes (1896), On Trimetaphosphimic acid and its decomposition products. American Chemical Journal, vol. 18 issue 8, p. 629.
  • Example of hexalkoxyphosphazene synthesis from hexachlorophosphazene and structure description: Allcock, Harry R.; Ngo, Dennis C.; Parvez, Masood; Whittle, Robert R.; Birdsall, William J. (1991-03-01). "Syntheses and structures of cyclic and short-chain linear phosphazenes bearing 4-phenylphenoxy side groups". Journal of the American Chemical Society. 113 (7): 2628–2634. doi:10.1021/ja00007a041. ISSN 0002-7863.
  • Novel hexalkoxyphosphazene synthesis not starting from hexachlorophosphazene: Ye, Chengfeng; Zhang, Zefu; Liu, Weimin (2002-01-01). "A Novel Synthesis of Hexasubstituted Cyclotriphosphazenes". Synthetic Communications. 32 (2): 203–209. doi:10.1081/SCC-120002003. ISSN 0039-7911. S2CID 97319633.

References

  1. 1 2 3 4 5 6 7 8 9 10 Allen, Christopher W. (1991-03-01). "Regio- and stereochemical control in substitution reactions of cyclophosphazenes". Chemical Reviews. 91 (2): 119–135. doi:10.1021/cr00002a002. ISSN 0009-2665.
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  3. 1 2 3 4 5 6 7 8 9 Chaplin, Adrian B.; Harrison, John A.; Dyson, Paul J. (2005-11-01). "Revisiting the Electronic Structure of Phosphazenes". Inorganic Chemistry. 44 (23): 8407–8417. doi:10.1021/ic0511266. ISSN 0020-1669. PMID 16270979.
  4. 1 2 3 4 5 6 7 8 9 10 11 12 Mark, J. E.; Allcock, H. R.; West, R. “Inorganic Polymers” Prentice Hall, Englewood, NJ: 1992. ISBN 0-13-465881-7.
  5. 1 2 3 Bartlett, Stewart W.; Coles, Simon J.; Davies, David B.; Hursthouse, Michael B.; i̇Bişogˇlu, Hanife; Kiliç, Adem; Shaw, Robert A.; Ün, İlker (2006). "Structural investigations of phosphorus–nitrogen compounds. 7. Relationships between physical properties, electron densities, reaction mechanisms and hydrogen-bonding motifs of N3P3Cl(6 − n)(NHBu t ) n derivatives". Acta Crystallographica Section B: Structural Science. 62 (2): 321–329. doi:10.1107/S0108768106000851. PMID 16552166.
  6. 1 2 3 4 5 6 7 8 9 10 Allcock, H. R. (1972). Phosphorus-nitrogen compounds ; cyclic, linear, and high polymeric systems. New York: Academic Press. ISBN 978-0-323-14751-4. OCLC 838102247.
  7. 1 2 3 4 5 6 7 8 9 Heston, Amy J.; Panzner, Matthew J.; Youngs, Wiley J.; Tessier, Claire A. (2005). "Lewis Acid Adducts of [PCl2N]3". Inorganic Chemistry. 44 (19): 6518–6520. doi:10.1021/ic050974y. PMID 16156607.
  8. 1 2 3 4 Dhiman, Nisha; Mohanty, Paritosh (2019-10-28). "A nitrogen and phosphorus enriched pyridine bridged inorganic–organic hybrid material for supercapacitor application". New Journal of Chemistry. 43 (42): 16670–16675. doi:10.1039/C9NJ03976G. ISSN 1369-9261. S2CID 208761169.
  9. ↑ J. Liebig (1834). "Nachtrag der Redaction". Ann. Pharm. 11: 139–150. doi:10.1002/jlac.18340110202.
  10. ↑ R. Klement (1963). "Phosphonitrilic Chlorides". In G. Brauer (ed.). Handbook of Preparative Inorganic Chemistry, 2nd Ed. Vol. 1. NY, NY: Academic Press. p. 575.
  11. ↑ Nielsen, Morris L.; Cranford, Garland (2007) [1960]. "Trimeric Phosphonitrile Chloride and Tetrameric Phosphonitrile Chloride". Inorganic Syntheses. Inorganic Syntheses. Vol. 6. pp. 94–97. doi:10.1002/9780470132371.ch28. ISBN 9780470132371.
  12. ↑ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
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