Synthetic Routes to Cyclophosphazenes

There are a number of methods for the direct synthesis of chloro-and bromocyclophosphazenes and their alkyl and aryl derivatives. Other derivatives have invariably been prepared by replacement reactions of the halogenocyclophosphazene precursors. In this section, ring-forming reactions will be discussed and the following section will deal with the substitution reactions. 

Cyclophosphazenes are conveniently prepared by the reaction of halophosphoranes with ammonium halides, 

The reaction of phosphorus pentachloride with ammonium chloride is tantalizingly complex and has been the subject of numerous investigations (21). Cyclic oligomers and linear products are formed, and 

The mechanism of ammonolysis of phosphorus pentachloride is not completely understood. It has been shown (164) that it proceeds in two steps. Initially, the intermediate [C13PNPC13] + [PC16] is formed, probably via PC14NH2 and PC13NH (48, 380). In the second step, this intermediate reacts with NH4C1 to produce cationic phosphazene chains that undergo cyclization by the elimination of [PC14]+ (164, 256). 

For the preparation of bromocyclophosphazenes the use of a mixture of PBr3 and bromine in place of PBr5 and addition of bromine to the reaction mixture at frequent intervals leads to improved yields of (NPBr2)3,4 (128, 130, 236, 451). 1,2-Dibromoethane is preferred as the reaction solvent because tiny quantities of chlorobromocyclophospha-zenes are formed in sy/n-tetrachloroethane (130). The preparation of bromocyclophosphazenes is difficult and tedious, and as yet no effective catalyst has been found to reduce the lengthy reaction periods (15-20 days). Fluorocyclophosphazenes cannot be prepared by this route but are obtained by metathetical reactions (Section III,D). Attempts to prepare iodophosphazenes have been unsuccessful (249). 

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