SOLUTION-STATE, SOLID-STATE, AND CALCULATED STRUCTURES OF AN ALPHA-LITHIATED MONOPHOSPHAZENE

The structure of lithiated P-diphenyl(methyl) (N-phenyl)phosphazene (Li(+)1a(-)) has been determined. The crystal structure consists of monomeric units containing a four-membered ring with the lithium bonded to the nitrogen and methylene carbon atoms of the phosphazene. The coordination sphere of the metal is completed by two molecules of THF. The carbon bonded to lithium has a pyramidal configuration. Short intermolecular Li-phenyl-P distances (average 3.10 Angstrom) have been measured in the crystal packing. In THF at -111 degrees C, a 93:7 mixture of two compounds was found. The structure of the major component was determined by multinuclear H-1, Li-6, C-13, N-15 and P-31 NMR spectroscopy of isotopically labeled (6)Li(+)1a(-). From the (2)J(PLi)-coupling constant measured and the NOEs observed in 2D HOESY spectra, it was concluded that the major component retained a structure similar to that found in the solid state. N-15 Characterization was carried out through P-31,N-15 triple resonance experiments. MNDO calculations reproduced well the main structural features of Li(+)1a(-). Two local minima interconnected by two transition states were located through MNDO calculations. The preference for the formation of the C-P-N-Li four-membered ring was considered to be of electrostatic nature. No P-Li bonding interaction was predicted, and the calculated dissociation energy for the C-Li was 30.8 kcal . nol(-1).