A considerable amount of research has been directed toward the elucidation of phosphazene structures using X-ray diffraction and spectroscopic techniques. NMR spectroscopy has been used extensively in phosphazene chemistry as a structural identification technique and as a means for probing bonding patterns. The investigations have included 1H, 31P and 19F studies.
1.6.2.1 X-ray diffraction
Approximately 30 cyclo- and polyphosphazenes have been investigated by X-ray diffraction techniques. The results of single crystal studies confirm that those compounds which have the formula (NPR2)3-8 are cyclic, while fiber diagram work indicates that the rubbery materials of
the formula (NPRz)n are long-chain polymers. Cyclic phosphazenes are found with both
planar and puckered phosphorus-nitrogen rings. Unlike organic aromatic species, moderate puckering of the ring appears to have little or no influence on molecular stability [214]. The majority of tetramers and higher cyclic species examined to date are non-planar. The exceptions are (NPF2)4, which forms a regular planar ring, and the cyclic pentamer,
(NPC12)5, which achieves near-planarity by indentation and loss of a regular cyclic shape
(Fig. 1.20). Octachlorocyclotetraphosphazene, (NPC12)4, exists in two crystallographic
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contains molecules in a tub conformation. The bromophosphazene, (NPBr2)4 occupies a
similar conformation to that of the K form of (NPC12)4. An unusual conformation is found
for [NP(OMe)2]8, in which the cyclic octameric ring consists of two planar sections. (Fig.
1.21) [214-219].
Fig. 1.20 Molecular structure of (NPC12)5 [214].
Fig. 1.21. Idealized molecular shape of phosphazene ring in[NP-(OMe)2]8 [214].
The conformation of one high polymer, (NPF2)n has been solved by structure factor methods.
Attemperatures below -56oC the stretched polymer occupies a cis-trans planar conformation (eqn 1.4), although at higher temperatures a non-planar arrangement predominates. Optical transform analysis of X-ray fiber patterns from (NPC12)n indicated the presence of a slightly
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(1.4)
It seems clear that the high polymer conformations are more a response to intra- and intermolecular nonbonding forces than to the restricting effects of phosphorus-nitrogen bonding. For cyclic phosphazenes, the planarity or nonplanarity of the ring depends on the need for the molecule to avoid skeletal angular strain [214].
1.6.2.2 Phosphorus-Nitrogen bond lengths
A phosphorus-nitrogen single bond length is usually assumed to be in the region of 1.77-1.78 Å. The bond lengths in cyclo- and polyphosphazenes are in the range of 1.47-1.62Å, and this represents an appreciable contraction. The shortest skeletal bonds are associated with highly electronegative substituents, and this bond contraction is ascribed to skeletal π-bond influences. A marginal trend also exists toward shorter skeletal bond lengths in cyclophosphazenes as the ring size increases for the smallest rings, although this trend does not continue to the high polymers [214,222-224].
Of particular importance is the fact that, if the substituents are symmetrically disposed around the ring, all the phosphorus-nitrogen bond lengths are equal. No separation into alternating long and short, σ and σ-π bonds is observed in the neutral system. This provides a sharp contrast with the situation encountered in cyclooctatetraene and in boronnitrogen and thiazyl fluoride heterocycles, where a separation into alternating long and short bonds occurs in the cyclic tetramer. However, the protonated phosphazenium cation in [(NPMe2)4H]2+CoC142-
shows some evidence of bond length alternation. It should be noted that asymmetric ligand arrangements lead to the presence of unequal bond lengths around the ring because of the ligand electronegativity influence [214,225-228].
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1.6.2.3 Nuclear magnetic resonance
The chemical shift of a proton located in a substituent group will be influenced by: the other nuclei within that same group; by the other group attached to the same phosphorus atom; by the substituent groups attached to nearby phosphorus atoms, and often by the size of the phosphazene ring. However, in general, the proton chemical shifts are comparable to those found in related organic compounds, as illustrated by the following examples (chemical shift to TMS in parentheses): NMe2 (7.27-7.79), OCH3 (6.29-6.46), OPh (2.8-3.2) and Ph (2.2-
2.7). Spin-spin coupling effects are also usually present. Thus, proton NMR spectra can be used for “fingerprint” identification and for the structure determination of unknown compounds [214,229,230].
31
P NMR spectroscopy is a valuable tool for structural identification, it cannot be used reliably for direct chemical identification by “group shift” methods. Cyclic trimers generally yield 31P shifts in the range of 0 to -46 ppm. However, the large positive shift of the PBrz unit
(+37 to +45 ppm) is characteristic. Furthermore, apart from bromophosphazene trimers, positive shifts appear to be characteristic of cyclic tetramers, pentamers, hexamers, heptamers, octamers, and linear high polymers. Cyclic tetrameric and higher ring systems resemble the high polymers in providing more opportunities for conformational changes than do the rather rigid cyclic trimers [214].
Fluorine atoms bound directly to the phosphazene ring show 19F NMR spectra which indicate an influence by the supporting phosphorus atom and by nearby ring phosphorus atoms. Fluorine chemical shifts range from 19.6 ppm (relative to CFC13) for the PFBr unit in
N3P3F5Br to 71.9 ppm for the PF2 group in (NPF2)3 [214, 230].