Molecular Structure of Phosphazenes

Molecular Structures of Phosphazenes Determined by X-Ray Diffraction Methods... 

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 crystal structures of glycylaminomethylphosphonic acid, and of methane-, ethane-, and propane-diphosphonic acids, have been determined. The unit cell of the last acid contains two molecules, with different conformations. The molecular structures of the constrained phosphite (161), the phosphate (162), and the thio-phosphate (163) have been compared. The nitrogen in the last compound is very nearly trigonal planar, and the large P—N distance (313 pm) shows that there is little P - - N interaction. The phosphazene (164) adopts a novel conformation, ... 

FIGURE 13.15 Molecular structure of poly[f -lris(lrilluoroethoxy)-lV-trimethylsilyl] phosphazene. 

Phosphazene chemistry is covered separately elsewhere within this volume but three notable papers involving theoretical and computational chemistry methods have been reported. Non-empirical RHF/6-31G and MP2/6-31G quantum chemical methods have been used to calculate the molecular structure of two trichlorophosphazenes (8) and (9). The corresponding geometric parameters obtained from the calculations were compared with XRD analysis data previously reported in the literature. Conformational differences between the two molecules, previously found by XRD analysis of the crystals, were confirmed by non-empirical quantum chemical calculations on the molecules in the free state and the features of their geometry were discussed. A DFT study of the structural organization and infrared spectra of hexakis 4-iV -dichloro(thio)phosphonyl-A -methyl-dia-zobenzene cyclotriphosphazene, a phosphorus-containing dendrimer built... 

With this synthetic and molecular structural diversity, polyphosphazene chemistry has developed into a field that rivals many areas of organic polymer chemistry with respect to the tailored synthesis of polymers for specific experimental or technological uses. Indeed, hybrid systems are also available in which organic polymers bear phosphazene units as side groups. This is discussed in another Chapter. 

Fig. 16.23 Structure of tnmcnc phosphazene. PjNjCU (a) contributing resonance structures (b) molecular structure as determined by X-ray diffraction. [From Builen. C. J J. Chem. Svc. (A) 1971. 1450. Reproduced with permission.]...

Step-by-step growth of phosphazene chains can be achieved and the five-phosphorus chain can be converted into a dendrimeric structure. The fascinating chemistry of dendrimeric phosphors-nitrogen molecular architecture has been reviewed. ... 

Relaxation, CIDNP, and N.Q.R.—Relaxation. Phosphorus-31 and carbon-13 relaxation times are reported for dimethyl methylphosphonate. The mobility of phosphazene polymers has been studied using P spin-lattice relaxation parameters. The structure and mobility of polycrystalline nitrilotrimethylphos-phonic acid was estimated by line shape analysis. Molecular interactions of guanosine monophosphate and ATP have been studied through their relaxation properties. ... 

A chiral example of phosphazene bases was synthesized by treatment of (5)-2-(dialky-laminomethyl)pyrrolidine derived from 5-oxo-(5)-proline, with phosphorus pentachloride and subsequent addition of gaseous ammonia. The phosphazenes were isolated as HBF4 salts in high yields and fully characterized by H, and P NMR spectroscopy, various ID and 2D NMR experiments and mass spectrometry (El). The molecular structure and the absolute configuration of the HBF4 salts were determined by X-ray analysis [16]. 

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