STRUCTURE OF LITHIUM BIS(DIPHENYLPHOSPHINO)AMIDE - AN X-RAY, NMR, ANDMODELED THEORETICAL-STUDY

Monomeric lithium bis(diphenylphosphino)amide, LiN(PPh(2))(2) (5), sol vated with tetrahydrofuran, 5 . 3THF, was characterized by single-crys tal X-ray analysis. The pentacoordinated lithium environment, a distor ted trigonal bipyramid, includes two THF oxygens and the nitrogen of t he bis(phosphino)amide in the equatorial positions; the third oxygen a nd a phosphorus are apical. P-31 and Li-6 NMR spectroscopy show that t he monomeric structure of 5 in THF solution is similar to the X-ray st ructure of solid 5 . 3THF. Dynamic P-31 NMR spectroscopy gave an 8.1 k cal/mol rotation barrier around the PN bonds. The Li-6-CP/MAS spectrum of 5 . 3THF has a single Li-6 line, whereas the P-31 CP/MAS spectrum reflects the chemical nonequivalence of the phosphorus sites observed by X-ray analysis. The appearance of two P-31 signals in the solid-sta te NMR spectrum at +25 degrees C suggests a minimum activation barrier of the P,P-exchange process of Delta G(double dagger) > 12.6 kcal/mol in the solid state. Ab initio calculations on the simplified, unsolva ted models, PH2NH2, (PH2)(2)NH, PH2NH-, (PH2N-, PH2NHLi, and (PH2)(2)N Li, show that the strong stabilization of the amino anions by the alph a-phosphino substituents is due to negative hyperconjugation and phosp horus polarization. The presence of the metal counteracts much of the a-substituent stabilizing effect in the free anions. The metal cations in lithiated model compounds show little tendency to bridge.