Phosphazenes

 PHOSPHAZENES OR PHOSPHONITRILIC HALIDES

Nitrogen and phosphorus catenate together, forming an interesting series of polymers containing P-N single bonds and P=N double bonds. In these compounds, the phosphorus atom is in a +5 oxidation state, and the nitrogen atom is in a +3 oxidation state. The compounds are formally unsaturated.

PREPARATION OF PHOSPHAZENES

When PCl₅ is refluxed with NH₄Cl in presence of C₂H₄Cl₂ or C₆H₅Cl, or heated with solid NH₄Cl at 145⁰C–160⁰C, ring compounds of the type [NPCl₂]_n where (n = 3-8) and chain compounds of the type Cl₄P(NPCl₂)_nNPCl₃ are produced. The newly synthesized compounds are called phosphazenes or phosphonitrilic halides.

Preparation of phosphazenes

The bromo derivative is obtained by heating PBr₅ with NH₄Br and the fluoro derivative is obtained from the chloride on treatment with NaF in acetonitrile.

[NPCl₂]_n + 2n NaF -----> [NPF₂] + 2n NaCl

Alkoxy groups can be introduced in the polymer by replacing Cl.

[NPCl₂]_n + 2n NaOR -----> [NP(OR)₂]_n + 2n NaCl

The iodide derivatives are not yet known. The rings compounds where n=3 and n=4 are stable and can be prepared by under controlled conditions.

STRUCTURE OF PHOSPHAZENES

The structure involves sp² hybridised trivalent nitrogen (N) and roughly sp³ hybridised pentavalent phosphorus (P). Alternate single and double bonds are present between N and P. All the P―N bond distances (167 pm) become same through resonance. This distance is shorter than P―N single bond distance (178 pm). The multiple bonding can be explained by pπ―dπ bonding between filled pπ orbital of N and empty dπ orbital of P. Electron delocalisation similar to benzene and graphite occurs here also, this gives a pseudo aromatic character to the ring.

Cyclophosphazene

INORGANIC RUBBER

When excess PCl₅ is refluxed with NH₄Cl in S-tetra chloro ethane linear polymers are obtained. They are rubbery in nature, elastic, thermally stable, and insoluble in petroleum ether. A long chain polymer [NPCl₂]_x of molecular weight of 20,000 or more is obtained by heating [NPCl₂]₃ to 250⁰C-350⁰C. The end group of such high molecular weight polymers are not known. These rubbery substances are called inorganic rubber. These are water proof and fire proof.

Inorganic rubber

PROPERTIES OF PHOSPHAZENES

The phosphazenes range from fluids and definite solids to polymeric elastomers. The lower polymers are insoluble in water but soluble in organic solvents and resemble vulcanized rubber in properties and are often called inorganic rubber.

REACTIONS OF PHOSPHAZENES

The chlorine atoms in chlorophosphazenes are reactive and may be substituted by a variety of reagents.

[NPCl₂]₃ + 6CH₃MgI ------> [NP(CH₃)₂]₃ + 3MgCl₂ + 3MgI₂

[NPCl₂]₃ + 6C₆H₅Li ------> [NP(C₆H₅)₂]₃ + 6LiCl

[NPCl₂]₃ + 6NaOR ------> [NP(OR)₂]₃ + 6NaCl

[NPCl₂]₃ + 6NaSCN ------> [NP(SCN)₂]₃ + 6NaCl

[NPCl₂]₃ + 6H₂O ------> [NP(OH)₂]₃ + 6HCl

[NPCl₂]₃ + 6NH₃ ------> [NP(NH₂)₂]₃ + 6HCl

The lone pair of electrons present on each N atom in [NPCl₂]₃ molecule makes it basic and hence it forms adduct with Lewis acids like HClO₄, SO₃, AlCl₃ etc.

[NPCl₂]₃ + 2AlCl₃ ------> [NPCl₂]₃•2AlCl₃

[NPCl₂]₃ + 3SO₃ -------> [NPCl₂]₃•3SO₃

[NPCl₂]₃ + 2HClO₄ ------> [NPCl₂]₃•2HClO₄

USES OF PHOSPHAZENES

Phosphazenes or phosphonitrilic halides find uses in flame-proofing of fabrics, as plasticisers and as catalysts in the manufacture of silicones.