Monomeric lithium bis(diphenylphosphino)amide,
LiN(PPh2)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. 31P and 6Li
NMR spectroscopy show that the monomeric structure
of 5 in THF solution is similar to the X-ray structure of
solid 5·3THF. Dynamic 31P
NMR
spectroscopy gave an 8.1 kcal/mol rotation barrier around the PN bonds.
The 6Li−CP/MAS
spectrum of 5·3THF has a single 6Li
line, whereas the 31P CP/MAS spectrum reflects
the
chemical nonequivalence of the phosphorus sites observed by X-ray
analysis. The appearance
of two 31P signals in the solid-state NMR spectrum at +25
°C suggests a minimum activation
barrier of the P,P-exchange process of ΔG
⧧
> 12.6 kcal/mol in the solid state. Ab
initio
calculations on the simplified, unsolvated models,
PH2NH2,
(PH2)2NH,
PH2NH-,
(PH2)2N-,
PH2NHLi, and (PH2)2NLi,
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.