Hexachlorophosphazene

Hexachlorophosphazene is an inorganic compound with the chemical formula (NPCl2)3. The molecule has a cyclic, unsaturated backbone consisting of alternating phosphorus and nitrogen atoms, and can be viewed as a trimer of the hypothetical compound N≡PCl2 (phosphazyl dichloride). Its classification as a phosphazene highlights its relationship to benzene. There is large academic interest in the compound relating to the phosphorus-nitrogen bonding and phosphorus reactivity. Occasionally, commercial or suggested practical applications have been reported, too, utilising hexachlorophosphazene as a precursor chemical. Derivatives of noted interest include the hexalkoxyphosphazene lubricants obtained from nucleophilic substitution of hexachlorophosphazene with alkoxides, or chemically resistant inorganic polymers with desirable thermal and mechanical properties known as polyphosphazenes produced from the polymerisation of hexachlorophosphazene.

Structure and characterisation

Bond lengths and conformation Hexachlorophosphazene is a cyclic molecule, containing a P3N3 core with alternating nitrogen and phosphorus atoms, and two additional chlorine atoms bonded to each phosphorus atom. Hexachlorophosphazene molecule contains six equivalent P–N bonds, for which the adjacent P–N distances are 157 pm. This is characteristically shorter than the ca. 177 pm P–N bonds in the valence saturated phosphazane analogues. The molecule possesses D3h symmetry, and each phosphorus center is tetrahedral with a Cl–P–Cl angle of 101°. The P3N3 ring in hexachlorophosphazene deviates from planarity and is slightly ruffled (see chair conformation). By contrast, the P3N3 ring in the related hexafluorophosphazene species is completely planar.

Characterisation methods 31P-NMR spectroscopy is the usual method for assaying hexachlorophosphazene and its reactions. Hexachlorophosphazene exhibits a single resonance at 20.6 ppm as all P environments are chemically equivalent. In it IR spectrum, the 1370 and 1218 cm−1 vibrational bands are assigned to νP–N stretches. Other bands are found at 860 and 500–600 cm−1, respectively assigned to ring and νP–Cl. Hexachlorophosphazene and many of its derivatives have been characterized by single crystal X-ray crystallography.

Bonding

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.

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. Instead, a charge separated model is generally accepted. 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. 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.

Synthesis The synthesis of hexachlorophosphazene was first reported by von Liebig in 1834. In that report he describes experiments conducted with Wöhler. They found that phosphorus pentachloride (PCl5) and ammonia (NH3) react exothermically to yield a new substance that could be washed with cold water to remove the ammonium chloride ([NH4]Cl) coproduct. The new compound contained P, N, and Cl, on the basis of elemental analysis. It was sensitive toward hydrolysis by hot water. 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 exothermic reaction associated with the NH3/PCl5. Typical chlorocarbon solvents are 1,1,2,2-tetrachloroethane or chlorobenzene, which tolerate the hydrogen chloride (HCl) side product. Since ammonium chloride is insoluble in chlorinated solvents, workup is facilitated. 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).

Purification by sublimation gives mainly the trimer and tetramer. Slow vacuum sublimation at approximately 60 °C affords the pure trimer free of the tetramer. 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%.

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]− (tetrachlorophosphonium hexachlorophosphate(V)) and the reaction proceeds via nucleophilic attack of [PCl4]+ (tetrachlorophosphonium) by NH3 (from [NH4]Cl dissociation). 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.

Reactions

Substitution at P Hexachlorophosphazene reacts readily with alkali metal alkoxides and amides.

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

(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).

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