Phosphazene bond

The phosphazene backbone has a particularly high resistance to thermal treatment and to homolytic scission of the -P=N- bonds, possibly due to the combination of the high strength of the phosphazene bond and its remarkable ionic character [456]. As a consequence, the onset of thermal decomposition phenomena (as detected, for instance, by TGA) are observed at considerably high temperatures for poly[bis(trifluoroethoxy)phosphazene], [NP(OCH2CF3)2]n [391, 399, 457], for phosphazene copolymers substituted with fluorinated alcohols of different length [391, 399, 457], for polyspirophosphazenes substituted with 2,2 -dihydroxybiphenyl groups [458], and for poly(alkyl/aryl)-phosphazenes [332]. 

The phosphazene bond in a 1,2-azaphosphole also forms an unstable carbon dioxide [2+2] cycloadduct . Also, aminoiminoboranes react with carbon dioxide to give [2+2] cycloadducts The [2+2] cycloaddition of carbon dioxide across a W-C bond in 57 is also observed to give 58. ... 

Phosphoranimines can react as electrophiles in electrocyclic reactions with arynes. Specifically, P-vinyl-X -phosphazenes with varying substituents have been found to undergo electrocyclic ring formation with benzyne precursors. Curiously, they do not exhibit straightforward [2-1-2] or [2-1-4] addition involving the C=C double bond, rather they insert the benzyne into the phosphazene bond, Scheme 5, yielding a heteroatomic bicyclic ring system. 

Bonding. All phosphazenes, whether cyclic or chain, contain the formally unsaturated... 

In short, the bonding in phosphazenes is not adequately represented by a sequence of alternating double and. single bonds -N=P-N—P- yet it... 

Poly(phosphazenes) are similar, partly inorganic polymers in that they consist of inorganic backbone, in this case of nitrogen and phosphorus atoms. They are separated formally by alternating single and double bonds and carry organic groups on the phosphorus atoms (10.3). 

In contrast to the results above, the reaction of a-ketocarboxylic esters follows a different course giving 26, the product of P-H addition of the tautomeric PH phosphazene form to the keto C=0 double bond [126] (Scheme 27). 

The JV-silyl phosphinous amides present some particularities in their reactivity that make these compounds worth commenting on separately. They are stable and can be easily prepared in the usual way by reaction of AT-silyl substituted primary amines or hexamethyldisilazane with halophosphanes [48,49,128,129] or byJV-silylation of the appropriate phosphinous amides [72, 107]. The reductive Ph-P bond cleavage in AT-silyl phosphazenes Ph3P=NSiMe3 by the action of sodium is a peculiar example of preparing Ph2PNHSiMe3 [130]. 

As described in several review articles [409,452-454] and books [10,13,15], this is basically due to the inherent features of the d -p bond in phosphazenes, which allows the permanent overlapping of the 2pj orbital of the skeletal nitrogens with any one of the 3p orbitals of the phosphorus atoms [455]. Such a high chain flexibihty generated very low glass transition temperatures in these polymers, which can reach values of about -100 °C when suitable flexible substituent groups (e.g. n-butanol) are present on the skeletal phosphorus [274]. 

Three independent P-N bond 137 lengths. OPO = 102.7°, exo-cyclic groups twisted at 48° to average plane of phosphazene ring... 

The nomenclature and numbering system adopted in this article is the one currently widely used in the literature and corresponds closely to the one suggested by Shaw (13,17). Only the chemistry of chlorocyclo-phosphazenes is dealt with in this review. The structural and bonding aspects are not considered here. 

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