Cyclophosphazenes Chlorocyclophosphazenes

In addition to classical substituents such as NH2, NHR, NR2, OR, SR in various combinations, in recent years some more exotic groups have been attached to phosphazene rings. Among the novel cyclic phosphazenes the carbo-rane substituted derivatives are worth mentioning. Other unusual derivatives include chiral cyclophosphazenes, cyano-, ferrocenylphenoxy-, and ferrocenylhydrazone-cyclophosphazenes. Another interesting development is the synthesis of crown ethers with chlorocyclophosphazene subunits (Scheme 54). ... 

Most cyclophosphazenes are relatively stable and have good shelf lives. Some are sensitive to moisture. Even, the chlorocyclophosphazenes, N3P3CI6 and N4P4CI8 can be readily handled in open air (in a well-ventilated hood). Most persubstituted cyclophosphazenes have very low dipole moments and this is reflected in their excellent solubility properties even in nonpolar organic solvents. Table 3.1 summarizes the melt-ing/boiling point data for some of the prominent members of the cyclo-phosphazene family. 

Chlorocyclophosphazenes are the most important members of the cyclo-phosphazene family. These serve as precursors and starting materials for the preparation of a large variety of other cyclophosphazenes. As mentioned before, the four-membered ring N2P2CI4 is not known. 

The traditional and the still widely used synthesis of ehloro-cyclophosphazenes consists of the reaction of finely ground ammonium chloride (the source of nitrogen) with phosphorus pentachloride (the source of phosphorus) (see Eq. 3.1). This reaction is carried out in a high boiling chlorinated solvent such as symmetrical tetrachloroethane, CI2CHCHCI2 or chlorobenzene. This reaction is quite complex. Ammonium chloride is not soluble in these solvents, while phosphorus pentachloride is. Thus, the overall reaction is heterogeneous in nature. A complex mixture of cyclic and linear products is formed in this reaction. Individual chlorocyclophosphazenes can be separated fi-om each other by various means. 

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