Cyclic phosphazene model

A number of the synthetic prospects for the field have already been discussed. Recent work with cyclic phosphazene model compounds has shown that the prospect exists for the modification of side groups already attached to a phosphazene backbone. 

Fig. 4. The phosphazene ring (a) island delocalization model predicting nodes in TT-density at the phosphorus atoms (b) dynamic deformation density (at 0.1 eA 3) in the plane of the ring (c) theoretical deformation density (at 0.05 eA-3) of cyclic phosphazene was used as a model (reproduced with permission from Cameron et al. [43]).

Faucher, J.-R, Labarre, J.-F., and Shaw, R. A., Bonding in cyclophosphazenes. Quantum and experimental support for Dewar s island model, Z. Naturforsch., 31b, 677 (1976). Mitchell, K. A. R., Noodleman, L., and Paddock, N. L., Xa scattered wave calculations of the ionization energies of N3P3F6 and N4P4F8, Chem. Phys. Lett., 47, 265 (1977). Noodleman, L., Westwood, N. P. C., and Mitchell, K. A. R., Ionization energies and electronic structure of the cyclic phosphazene as determined by UV photoelectron spectroscopy and Xa scattered wave methods. Chem. Phys. Lett., 58, 252 (1978). 

Allcock, H. R., and Austin, P. E., Schiff s-base coupling of cyclic and high polymeric phosphazenes to aldehydes and amines Chemotherapeutic models. Macromolecules. 14, 1616, 1981. 

Cyclophosphazenes are a fascinating group of inorganic heterocyclic compounds whose chemistry is multi-faceted, well developed and reasonably well understood. They are closely related to the linear poly-phosphazenes this relationship is unlike any other existing between ring-polymer systems. Although cyclic siloxanes and polysiloxanes have a close interrelationship, the number and types of cyclophospha-zene derivatives that are known, together with their exact counterparts in polyphosphazenes, underscore the utility of cyclophosphazenes as models for the more complex polyphosphazenes. The literature on cyclophosphazenes has appeared earlier in the form of books (1,2), chapters of books (3-5), authoritative compilations of data (6,7), and several reviews (8-21). The current literature on this subject is reported annually in the Specialist Periodic Reports published by the Royal Society of chemistry (22). This review deals mostly with chlorocyclo-... 

The emphasis in the following sections will be on exploratory model reactions carried out with phosphazene cyclic trimers or tetramers, although the analogous macromolecules systems have also been studied in several cases. First, I will summarize the various types of metal binding sites that are accessible at the present time. Synthetic procedures leading to the incorporation of several of these sites and their role in metal binding will then be discussed. 

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