Phosphazenes

Phosphazenes, formerly known as phosphonitrilic compounds, are characterized by the presence of the group P=N. Known compounds, particularly those containing the —p=n— group, are very numerous and they have important potential applications. 

In this bonding type, the P atom uses sp2 hybrid orbitals, one of which contains the lone-pair electrons. This type of phosphazene compounds exists in a bent configuration. For example, the structure of (SiMe3)2NPN(SiMe3) is shown below  

The compound (Me3Si)2N-P(NSiMe3)2 belongs to this type, as shown below  

The simplest compound of this type is iminophosphorane, H3P=NH, whose derivatives are very numerous, including R3P=NR, ChP=NR, (RO)3P=NR, and Ph3P=NR. In these compounds the P atom uses its sp3 hybrid orbitals to form four a bonds, and is also strengthened by djr-pjr overlap with N and other atoms. 

The main products of refluxing a mixture of PCI5 and NH4CI using tetra-chloroethane as solvent are the cyclic trimer (PNCl2)3 and tetramer (PNCl2)4, which are stable white crystalline compounds that can be isolated and purified by recrystallization from nonpolar solvents. 

Phosphazenes are a large family of ylide-type phosphorus-nitrogen compounds with the R3P+-N R unit, where R and R are monovalent groups. The following is a general synthesis  

The Cl groups on the (NPCl2) may be substituted by different nucleophiles such as fluoride, alkoxide, and amide anions, as shown below  

Suggest a mechanism for the general phosphazene synthesis mentioned above (5B.49). 

The highly basic phosphazenes thus provide an alternative to organolithium bases such as lithium dialkylamides and alkyllithiums. Milder reaction conditions and better solubility are two key advantages associated with phosphazenes. Another important consideration is the lack of a coordinating cation naked enolates and other anions obtained with phosphazenes often exhibit enhanced reactivity, relative to their lithium salts. 

Phosphazene synthesis is conceptually rather simple, as illustrated below for a PI base. The P-N skeleton is first put together via a series of nucleophilic displacements (presumably of the 8 2-81 type)  

The exceptional basicity of phosphazenes (iminophosphoranes) has been discovered by Schwesinger [83]. The phosphazene derivatives have been proved to be chemically very stable, kinetically active and highly versatile, and the large number of these bases has been synthesized. The gas phase and solution equilibrium basicity measurements for a large number of phosphazenes are conducted by Kaljurand et al. [84], and their PA and pAia values are published in various papers. These measurements show that phosphazenes surpass in their basicity the derivatives of acychc or bicyclic guanidines, amidines and vinamidines (Table 2.11). Extraordinary basicity of phosphazenes was theoretically rationalized by Maksic et al. [85], in terms of effective resonance stabilization of protonated molecules [86]. 

Branching of phosphazene bases leads to higher basicity, as evident in the P3 series where branched base 108 is more basic than linear base 101. This increase in basicity of branched compared to hnear stmctures has been ascribed mainly to effect of stability differences of the free bases, rather than of the cations, since linear conjugation in neutral phosphazenes is more effective than cross conjugation. Similarly, we could observe that in the P5 series linear base 117 is weaker than the branched 110, but in this case differences in resonance of the cations are more significant. 

The nature of the bonding in phosphazenes has been the subject of much discussion which does not seem to have led to any definite and widely accepted conclusions 

Formally unsaiuraicd PN compounds arc called phosphazenes and contain P in the 

More recently they have been made via a reaction associated with the name of A. V. Kirsanov (1962), c.g.  

As expected, the P -N distance is short and the angle at N is 120 , e.g. (a) and (b) above. Over 600 such compounds are now known, especially those with the C P N— group.  

Diphosphazenes can be made by reacting PCI5 with NH4CI in a chlorohydrocarbon solvent under mild conditions  

The inverse of these compounds are the phosphadiazenc cations, prepared by halide ion abstraction from diaminohalophosphoranes in CH2CI2 or SO2 solution, c.g.  

This review covers phosphazene literature over the period June 1999 to June 2000 Chemical Abstracts Vols. 131 and 132) and discusses linear phosphazenes including compounds derived thereof (Section 2), cyclophosphazenes (Section 3) and poly phosphazenes (Section 4). Structural data have been summarized in Section 5. 

Organophosphorus Chemistry, Volume 32 The Royal Society of Chemistry, 2002 

Investigations into the scope of dendrimers with PNP moieties, reactions of phosphines PR R R and azides (3) have been claimed to yield A-thiophos-phorylated and A -phosphorylated phosphoranimines R R R P=N-P(X)(0C6H4Y-4)2. Analogous reactions have been carried out with the ferrocenyl derivatives, yielding (4). 

The P=N-P(X) linkage can be easily alkylated on the X atom by means of methyl or isopropyl triflate to yield compounds (5) with a [P(Ph2)NP(XAlk)] group. Treatment of [P(Ph2)NP(SAlk)] with P(NMe2)3 leai to desulfurization and formation of a P =N-P linkage, which offers the possibility for 

The preparation of vinyl compounds (6) according to the Staudinger procedure has been described. The conversion of the formyl directive (6b) to (8b) occurs via a fine step procedure of which the formation of (7) is the first step. The third generation dendron (8) has 16 chorine atoms at the surface and a reactive vinyl group at the core. It has been shown that suitable derivatiza-tion of (8) both at the core and the surface provides reactive blocks that can lead to fascinating dendritic architectures by core-core, core-surface or corecore-surface-core reactions.  

The reaction of an azide RN3 and a phosphine PR 3 yields the reactive phos-phoranimine (iminophosphorane) RN=PR 3 under elimination of a nitrogen molecule. Phosphoranimines play an important role in synthetic organic chemistry and are useful precursors for a subsequent aza-Wittig approach, leading to various nitrogen-containing compounds. A theoretical study shows the polarity, and consequently, the reactivity of the N=P bond to be dependent on the nature of the phosphorus substituents.  

The importance of the Staudinger procedure as intermediate step in the synthesis of N-containing organic molecules can be illustrated by many 

Reactions of the bis(diphenylphosphine) amines Ph2PN(R)PPh2 (R = Et, Pr , Bu ) with a 10% excess of N3SiMc3 have been reported to give the corresponding 

Force field calculations have been carried out for compounds CI3PNPOCI2 and CI3PNPCI2NPOCI2. Conformations, bonding and flexibility have been discussed. In order to get some insight in chain flexibility of phosphazene polymers, ab initio MO calculations have been applied to study conformation, chain flexibility, and charge density of valence electrons in the linear trimer Me(NPCl2)3Me.2 

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. 

Donor-acceptor complexes, e.g. [Znl2(Me3SiNPEt3)]2 (2), can be obtained from ZnX2 (X = 1, Cl) and the corresponding silylated phosphoranimine at 

Organomanganese phosphoraneiminato complexes with a heterocubane struc- 

There have been few important advances in this area during the past year. The first crystal structure of a phosph(m)azene, Bu (Me3Si)NP=NBu, has been reported, and the claim made last year, of the formation of a compound containing a phos-phazene linkage which forms part of a four-membered ring, has been shown to be erroneous. The formation and properties of cyclophosphazenes (mainly amino-derivatives) and the phosphazene polymers have been reviewed. The patent literature is the most important growth area, particularly where flameproofing applications are concerned. 

From Amines and Phosphonis(v) Halides.— Relatively few examples of this route to monophosphazenes have been reported, apparently because of the greater versatility of the azide route (see below). Examples of the former route are those by which (1), (2), and (3) have been prepared. The P chemical shifts of compounds (1) were linearly related to Hammett o-constants for the substituents R, and the novel compounds (2) presumably result from partial hydrolysis at some stage in the reaction. The synthesis of A-halogenoalkyl-phosphazenes RCCl2N=PCl3 Phn (/j=0,1, or 2) has been reported in general terms only.  

From Azides and Phosphoras(in) Compounds.— Although phosphorus(m) compounds and azides generally undergo a ready reaction, two examples (4) and (5) of the class of azidophosphines have recently been prepared. The bulky di-isopropylamino-groups in (4) confer thermal stability such that the compound can be distilled at reduced pressure. Compound (5) was identified spectroscopically at low temperatures, 

Interestingly, a mixture of cyclic phosphazenes (6 n = 3,4, or 5) is obtained from the reaction of chloro-phosphites with trimethylsilyl azide. 

A series of A-trimethylsilylphosphazenes (7) have been obtained by the azide route, and their n.m.r. properties examined in some detail. It was shown last year 

Two elastomers have been commercialized with unique property profiles. One has fluoroalkoxy substituents that provide resistance to many fluids, especially to hydrocarbons. This material also has a broad use temperature range and useful dynamic properties. Aryloxy substituents provide flame retardant materials without halogens. 

Phosphazene polymers are normally made in a two-step process. First, hexachlorocyclotriphosphazene [940-71 -6J, trimer (1), is polymerized in bulk to poly(dichlorophosphazene) [26085-02-9], chloropolymer (2). The chloropolymer is then dissolved and reprecipitated to remove unreacted trimer. After redissolving, nucleophilic substitution on (2) with alkyl or aryloxides provides the desired product (3). 

Substitution of chloropolymer is possible using a variety of nucleophiles. The most common are sodium salts of alcohols and phenols. Thermoplastics are obtained using a single substituent, whereas multiple substituents of sufficiently different size lead to elastomers (2). Liquid crystal behavior similar to polysHoxanes has been noted in most homopolymers. The homopolymer formed using trifluoroethanol as a substituent has received a fair amount of academic scmtiny (7). 

Elastomers formed using alkoxide salts of trifluoroethanol and higher fluoroalcohols have been found to have an interesting range of thermal and fluid resistance. These materials were developed under U.S. Army sponsorship, and initially commercialized by Firestone using the name PNF, later sold by Ethyl Corp. as EYPEL-F elastomer. ASTM has reserved the designation FZ for elastomers of this class which have the nominal stmcture given as (4). 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Two commercial phosphazene elastomers were developed and marketed in the mid-1980s, namely, poly(fluoroalkoxyphosphazene) elastomer (ASTM International designation FZ) and poly(aryloxyphosphazene) elastomer (ASTM International designation PZ) [109]. The structure of the fluorinated product is as follows [110]  

In general, the synthesis of polyphosphazene polymers is unique in that, in theory, an infinite number of polymers with a variety of properties can be derived from the common polymeric intermediate, poly(dichlorophosphazene) (PNCI2), by replacing the chlorines with different nucleophiles. If the polydichlorophosphazene precursor is reacted with the sodium salts of trifluoroethanol and a mixed fluorotelomer alcohol, a poly(fluoroaIkoxyphosphazene) elastomer (FZ elastomer) is obtained. It contains a small amount of an unsaturated substituent as a curing site. The polymer is a soft gum, which can be compounded with carbon blacks and fillers and cured with sulfur or peroxides or by radiation. 

FZ elastomer offers a broad service temperature range, namely, from -65°C to 175°C (-85°F to 347°F) [110], excellent flex fatigue resistance, damping properties, and resistance to chemicals and fluids. 

Phosphazene elastomers were very successful throughout the 1980s, being used mainly in military and aerospace industry. However, because of their high cost and relatively small volume market, they are not available commercially other than on special orders. 

Gangal, S. V. in Kirk-Othmer Encyclopedia of Chemical Technology, Vol.ll, Third Edition, John Wiley Sons, New York, p. 2 (1980). 

Inclusion of RSiCls will lead to chain branching 

The largest group of inorganic heteroatomic chain compoimds with considerable industrial potential is the phosphazenes, which are based on a phosphorus-nitrogen skeleton. The electronegativity difference here (xp = 2.19, xn = 3.04) is less than in the siloxane link, but some multiple bonding is involved between N and P, which strengthens the framework. The 

Polydichlorophosphazenes can be made by a variety of methods, the most familiar of which is the reaction of phosphorus pentachloride with ammonium chloride in an appropriate medium such as chlorobenzene  

The tendency of the P—N skeleton of simple phosphazene polymers to hydrolyze slowly can be put to good use. The products of the hydroly- 

1 Phosphorus and arsenic vapors consist of molecules P4 and AS4, respectively, in which the four atoms occupy the vertices of a regular tetrahedron. Suggest reasons for the difference between these structures and that of gaseous nitrogen (which is also in Group 15). 

An overall decrease in the number of publications on this topic is apparent, although the volume of patent literature is still increasing. Notable developments include the synthesis of a monophosphazene, (Me3Si)2NP=NSiMea, containing tervalent phosphorus and of a three-co-ordinated quinquevalent phosphorus compound (Me3Si)2NP(=NSiMe3)2, and the incorporation of a phosphazene unit in a four-membered ring.  

A comprehensive survey of the synthesis and properties of cyclophospha-zenes has appeared (already somewhat dated), and the importance of phosphazenes in the inorganic polymer field has been emphasized.  

From Amides and Phosphorus(v) I des.— The Kirsanov reaction has been used to advantage in the synthesis of certain P-aryl-phosphazenes  

Further phenylation can be accomplished by reaction with phenylmagnesium bromide to give products only previously obtained by the potentially hazardous azide route. Sulphamic acid also reacts with phenylchlorophosphoranes in a similar way  

Niecke and W. Flick, Angew. Chem. Intermt. Edit., 1973, 12, 585. 

The Cl atoms in (NPCl2)3, 14.68, and (NPCl2)4, 14.69, readily undergo nucleophilic substitutions, e.g. the following groups can be introduced  

Two substitution pathways are observed. If the group that hrst enters decreases the electron density on the P centre (e.g. F replaces Cl), the second substitution occurs at the same P atom. If the electron density increases (e.g. NMe2 substitutes for Cl), then the second substitution site is at a different P centre. 

Small amounts of linear pol)oners, 14.70, are also produced in reaction 14.136, and their yield can be increased 

The Cl atoms in the polymers are readily replaced, and this is a route to some commercially important materials. Treatment with sodium aUcoxides, NaOR, yields linear pol5mers [NP(0R)2] which have water-resistant properties, and when R = CH2CF3, the pol5mers are inert enough for use in the construction of artificial blood vessels and organs. Many phosphazene polymers are used in fire-resistant materials (see Box 16.1). 

Vj Fig. 14.19 (a) Structural parameters for the phosphazenes (NPX2)3 (X = C1 or F) colour code P, orange, N, blue X, green, (b) Schematic representations of the P4N4 ring conformations in (NPF2)4 (saddle conformation only) and (NPCl2)4 (saddle and chair conformations). 

This chapter covers the literature of phospha(V)azenes with discussion of lower valent species being restricted to molecules which can be transformed, or related to a phosphorus(V) derivative. Primary literature, reviews monographs and patents are covered in order to give a global view of progress in this area of chemistry. Full manuscripts and poster abstracts from the Thirteenth International Symposium on Phosphorus Chemistry (Jerusalem, 1995) have been published. Highly focused reviews will be cited in the appropriate sections below. 

Before consideration of specific acyclic phosphazene syntheses and reactions, the synthesis of certain phosphazene precursors or related products bears mention. A noteworthy example is the tetraaminophosphonium ion. The reaction of PCI5 with liquid anunonia gives P(NH2)%C1 ° while the corresponding 

3-triphenylphoranimino-P-lactams. The Staudinger reaction of N-(o-azidoben-zoyl)-a-amino acids is followed by an intramolecular aza-Wittig step leading to 

These highly interesting compounds have also become useful in a variety of organic transformations, as has been described in reviews [39], and several of 

Polymer-supported phosphazene bases (of which one example is commercially available) can be used to deprotonate a phenol group for allylation in the generation of an intermediate during an efficient synthesis of the natural product carpanone [48]. Such a system was also employed for a dehydration step in the synthesis of 1,3,4-oxadiazoles [49]. 

Reactions catalyzed by phosphazenes have also been described. The catalytic enantioselective alkylation of the amino acid intermediate Ph2C = NCH2C02-t-Bu by alkyl halides was reported to occur efficiently in the presence of BEMP or BTTP [52], and such bases catalyze Michael additions in non-aqueous [53] and aqueous media [54]. Methyl [55] and butyl [56] methacrylates are anionically polymerized using a phosphazene base as an initiator in the presence of an ester that is apparently deprotonated in the process. Functioning as promoters in the 

22 Proposed reaction scheme for the formation of the cyclic phosphazene (NPCl2)3, and the structures of 

F using NaF (see above) NH2 using liquid NH3 NMe2 using Me2NH  

The first step in reaction 15.147 is the formation of [Cl3P=N=PCl3] [PCl6], which can be converted to the chloride salt by reaction 15.148. This is a convenient route to [Cl3P=N=PCl3] CP which is a precursor to higher polymers (e.g. eq. 15.149). 

The [Ph3P=N=PPh3] ion (commonly abbreviated to [PPN] ) is related to [Cl3P=N=PCl3], and is often used to stabilize salts containing large anions (see Box 24.1). 

Quantum yields of chain scission are dependent on the presence or absence of oxygen, the nature of the solvent and the wavelength of the incident light. Concurrent with photo chain scission is a photochemical 1,4-Diels-Alder addition of singlet oxygen  

Disruption of the 7t-system will lead to a diminished state of conjugation and consequently, a reduction in electronic conductivity and nonlinear optical activity. 

The main chain of poly(organophosphazenes) (4.88) consists of —P=N— bonds, which are very stable towards UV irradiation. However, photodegradation of phosphazenes occurs by photolysis of side-groups  

Reactions which appear during photodegradation are discoloration, changes in physical properties, formation of volatile products and in some cases, chain scission and crosslinking. 

In the absence of oxygen, the phosphazene radicals (4.89) crosslink with each other  

Rather there occurs rupture of the phosphazene ring with concomitant formation of the appropriate SnaRe compoimd. The species (7) is suggested as an intermediate in the reaction sequence.  

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Properties. One of the characteristic properties of the polyphosphazene backbone is high chain dexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105° C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deUberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

Phosphazene polymers are inherently good electrical insulators unless side-group stmctures allow ionic conduction in the presence of salts. This insulating property forms the basis for appHcations as wire and cable jackets and coatings. Polyphosphazenes also exhibit excellent visible and uv radiation transparency when chromophoric substituents are absent. 

Another valuable characteristic of many phosphazene polymers is their flame-retardant behavior and low smoke generation on combustion (13). This property is utilized in commercial appHcations. 

A remarkable feature of phosphazene polymers of types (1) and (2) is that appropriate substituents (which are readily attached) can be used as toggle switches to turn several properties, such as hydrolytic stabiHty and electrical conductivity, on and off (1). 

Applications. Among the P—O- and P—N-substituted polymers, the fluoroalkoxy- and aryloxy-substituted polymers have so far shown the greatest commercial promise (14—16). Both poly[bis(2,2,2-trifluoroethoxy)phosphazene] [27290-40-0] and poly(diphenoxyphosphazene) [28212-48-8] are microcrystalline, thermoplastic polymers. However, when the substituent symmetry is dismpted with a randomly placed second substituent of different length, the polymers become amorphous and serve as good elastomers. Following initial development of the fluorophosphazene elastomers by the Firestone Tire and Rubber Co., both the fluoroalkoxy (EYPEL-F) and aryloxy (EYPEL-A) elastomers were manufactured by the Ethyl Corp. in the United States from the mid-1980s until 1993 (see ELASTOLffiRS,SYNTHETic-PHOSPHAZENEs). 

PHOSPHAZENES CONTAINING SKELETAL CARBON, SULFUR, AND METAL ATOMS... 

The first phosphazene polymers containing carbon (79), sulfur (80,81), and even metal atoms (82) in the backbone have been reported. These were all prepared by the ring-opening polymerization of partially or fully chloro-substituted (or fluoro-substituted) trimers containing one hetero atom substituting for a ring-phosphoms atom in a cyclotriphosphazene-type ring. 

M. Zeldin, K. J. Wynne, and H. R. AUcock, eds.. Inorganic and Organometallic Poljmers, ACS Symposium Series, Vol. 360, American Chemical Society, Washington, D.C., 1988 regarding poIy(phosphazenes), poly(silanes), and other polymers. 

See Elastopiers.synthetic-phosphazenes Inorganic high polymers. 

Not inclusive see also Acrylic elastol rs, Phosphazenes, Cm OROSULFONATED polyethylene, Eethylene-acrylic elastot rs, Polyethers under the tide ElASTOT RS, SYNTTiETIC. 

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