Structure of Lithium Bis(diphenylphosphino)amide: An X-ray, NMR, and Modeled Theoretical Study

Monomeric lithium bis(diphenylphosphino)amide, LiN(PPh 2 ) 2 ( 5 ), solvated with tetrahydrofuran, 5 ·3THF, was characterized by single-crystal X-ray analysis. The pentacoordinated lithium environment, a distorted trigonal bipyramid, includes two THF oxygens and the nitrogen of the bis(phosphino)amide in the equatorial positions; the third oxygen and a phosphorus are apical. 31 P and 6 Li NMR spectroscopy show that the monomeric structure of 5 in THF solution is similar to the X-ray structure of solid 5 ·3THF. Dynamic 31 P NMR spectroscopy gave an 8.1 kcal/mol rotation barrier around the PN bonds. The 6 Li−CP/MAS spectrum of 5 ·3THF has a single 6 Li line, whereas the 31 P CP/MAS spectrum reflects the chemical nonequivalence of the phosphorus sites observed by X-ray analysis. The appearance of two 31 P signals in the solid-state NMR spectrum at +25 °C suggests a minimum activation barrier of the P,P-exchange process of Δ G ⧧ > 12.6 kcal/mol in the solid state. Ab initio calculations on the simplified, unsolvated models, PH 2 NH 2, (PH 2 ) 2 NH, PH 2 NH -, (PH 2 ) 2 N -, PH 2 NHLi, and (PH 2 ) 2 NLi, show that the strong stabilization of the amino anions by the α-phosphino substituents is due to negative hyperconjugation and phosphorus polarization. The presence of the metal counteracts much of the α-substituent stabilizing effect in the free anions. The metal cations in lithiated model compounds show little tendency to bridge.