Linear Phosphazenes

Compound (7.382) with n 20 is typical of the short-chain linear phosphazenes which have terminal or stopper end groups. Various atoms or groups may act as chain stoppers and many compounds have been isolated which can be represented by the general formula Cl(PNCl2) , where X = PCI3, H, MePOClj, PSCI2 or SOjCl. 

Several compounds of general formula Cl(PNCl2) PCI4 have been isolated from reactions between PCI5 and NH4CI or (PNCl2)3. They are more reactive than the cyclic halides and may exist in alternative forms  

Cl(PNCl2) PCI4 Cl(PNCl2) PCVCP-Cl(PNCl2) PCI3 PClg- (Table 7.8) 

The first manbers of the series (a) and (c) with n = 0, correspond to the covalent and ionic forms of phosphorus pentachloride (Table 7.7). The first member of the series (b) is represented by crystalline phosphorus pentabromide PBr4+Br- (Chapter 4.6). 

Alternate manbers of series (c) may be regarded as ionised dimers of series (a), but they are different compounds and can be distinguished from the latter by their melting points and other characteristics. The simplest member of series (b) with n = 1, is formed together with more highly polymeric material, in a reaction between phosphorus trichloride and sulphur nitride. 

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Examples of cyclophosphazenes with ring systems containing elements other than phosphorus or nitrogen continue to be reported. The linear phosphazene [Ph2(H2N)Pi N.i P(NH2)Ph2]+Cl is cyclized by antimony pentachloride to give the compound (35). This result contrasts with... 

Linear phosphazene polymers, obtained from the reaction of ammonium chloride with phosphorus pentachloride in chlorobenzene, may be rendered hydrolytically stable by reaction of one of the terminal chlorine atoms with, for example, sodium phenoxide ... 

An alternative method of synthesis of N3P3Cl6 has been developed recently, based on the reaction of tris (trimethylsilyl) amine and phosphorus pentachloride (40). This reaction either preferentially leads to the formation of N3P3C16 or to an N-silylated phosphoranimine intermediate C13P - NSiMe3, depending on the reaction conditions used. Thus the reaction between tris (trimethylsilylamine) and PC15 in methylene chloride at 40° C affords a mixture of cyclo and linear phosphazenes, which has been shown by an NMR analysis to contain up to 76% of N3P3C16 (Eq. 2). 

It is exceedingly difficult to determine the molecular structure of a synthetic macromolecule. X-ray diffraction—the ultimate structural tool for small-molecule studies—yields only limited information for most synthetic high polymers, and crucial data about bond lengths and bond angles are difficult to obtain.47 However, that same information can be obtained relatively easily from single crystal X-ray diffraction studies of cyclic trimers, tetramers, and short-chain linear phosphazene oligomers. The information obtained may then be used to help solve the structures of the high polymeric counterparts. 

The air-stable linear phosphazene (3) prepared from the reaction of Ph2PCl3 with ammonia in chloroform (57, 225) has proved a useful intermediate for cyclization reactions (Fig. 1) (379). A noteworthy feature of this approach is the synthesis of cyclophosphazenes in which hydrogen is attached to phosphorus (56, 378, 382). The only other route (383) reported for the preparation of a hydridocyclophosphazene derivative is... 

This review covers phosphazene literature over the period June 2001 to December 2002 Chemical Abstracts Vols. 135, 136 and 137), and discusses linear phosphazenes including compounds derived thereof (Section 2), cyclophos-phazenes (Section 3) and polyphosphazenes (Section 4). Structural data have been summarized in Section 5. References have been given in Section 6. 

The reaction pathway consists of two stages, viz. the formation of CI3PNPOCI2 at 165 °C, followed by thermal decomposition of the linear phosphazene at about 225 The preparation of (NPCl2) at low temperature by... 

Many papers have appeared on the chemistry of linear phosphazenes, varying from electron-rich ligands in various systems to starting materials in the preparation of organo-substituted polyphosphazenes. 

X-Ray structure determinations of some miscellaneous compounds containing linear phosphazene units are summarized in Section 5. 

A new method for the preparation (NPPh2)3 from KNPPh3 in the presence of M0O3 and 18-crown-6 has been described.S Novel l,2,5,6-tetrahydro-l,3-oxaza-4-phospha-2-phosphorine-2-oxides have been synthesized by metallation of linear phosphazenes R R C-P(Ph2)=N-P(0)(0Ph)2 with Bu"Li and subsequent quenching by aldehydes or ketones. ... 

In phosphazene chemistry, preparation and reactions of small molecule, linear phosphazenes continue to attract interest. This year numerous papers have explored the utility of the aza-Wittig reaction in the synthesis of complex molecules with particular reference to nitrogen heterocycles. In cyclophosphazene chemistry, the shift of emphasis to materials which are monomers themselves or models for phosphazene polymers is apparent. The synthesis and characterization of poly(phosphazenes) is an active area with extensive interest in phosphazene based polyelectrolytes being noted. Solid state NMR has been shown to be a valuable tool for the study of both structure and phase changes in poly(phosphazenes). 

The high-polymeric linear phosphazenes are potentially useful materials as far as physical and mechanical properties are concerned, but they have been generally useless because of chemical (especially hydrolytic) instability. Recently, use of perfluoroalkoxy and other side groups has given promise that useful polymers may yet be developed.47 b> 51... 

The synthesis of linear phosphazene oligomers has been studied in some detail from the reaction of PCI5 with ammonium chloride in tetrachloroethane or chlorobenzene The following chain growth mechanism is believed to operate ... 

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