Hexachlorophosphazene

Hexachlorophosphazene is the chemical compound with the formula N₃Cl₆P₃. It is a cyclic compound, whose chemical formula is more descriptively written as (PNCl₂)₃. It is a trimer of the hypothetical compound N≡PCl₂.

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Synthesis

The reaction of PCl₅ and NH₄Cl affords substances with the empirical formula PNCl₂:^[1] Purification by sublimation gives mainly the trimer (PNCl₂)₃ and tetramer (PNCl₂)₄. These rings were characterized by Liebig in his study of the reaction of PCl₅ and NH₃:

PCl₅ + NH₄Cl → 1/n(NPCl₂)_n + 4 HCl

Typical solvents are chlorobenzene.

Inorganic rings

Chemists have long known of rings containing carbon, e.g. benzene, pyridine, and cyclohexane. Related cyclic compounds lacking in carbon have been the subject of focused study. Hexachlorophosphazene is an archetype of this class of inorganic rings. Others include borazine, S₄N₄, and cyclic siloxanes.

"Inorganic rubber"

Hexachlorophosphazene has attracted considerable interest as a precursor to "inorganic rubber".^[2] Upon heating to ca. 250 °C, the trimer polymerizes to give the linear polymer (PNCl₂)_n via a ring-opening polymerization process. This polymer is soluble in organic solvents such as THF and benzene, wherein it can be derivatized by replacement of the P-Cl bonds with P-OR or P-NR₂ bonds (R = alkyl, aryl). The organically modified polymer is hydrolytically stable and exhibits some attractive properties such as low glass transition temperatures.

References

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Mark, J. E.; Allcock, H. R.; West, R. “Inorganic Polymers” Prentice Hall, Englewood, NJ: 1992. ISBN 0-13-465881-7.