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

Abstract

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.

From Neutral to Ionic Species: Syntheses and X‐ray Crystallographic and Multinuclear NMR Spectroscopic Studies of Li···P(SiMe3)–PtBu2 and Its Solvent Complexes

“…[7] Driess et al report on Li(THF)(Is 2 (F)Si)P-P(NiPr 2 ) 2 , finding a covalent interaction between Li and P in this compound. The coordination sphere of Li is completed by chelatization (fluorine, nitrogen), and one additional molecule of THF.…”

“…No splitting of the P1 signal by scalar 7 Li-31 P coupling occurs; however, its lines are considerably broadened (W 1/2 ≈ 125 Hz). The difference of the (10) and Li(THF) 3 P(SiMe 3 )-P(NiPr 2 ) 2 (11).…”

“…Eight-membered ring structures as in 3 or 21 [5] are realized when the formation of four-membered rings is prevented by steric demanding substituents. 7 Li NMR spectra of the THF complexes 1 and 2 were measured in the temperature range from 183 to 343 K in order to test for exchange or dissociation-association processes in solution. The lower limit of the temperature range is due to the increasing viscosity of the solutions and the onset of crystallization, both causing a strong line broadening especially in the 1 H spectra.…”

“…[4b] In the 7 Li spectrum only a broad singlet without fine structure is observed at 2.48 ppm (W 1/2 ≈ 30 Hz). In the 29 Si{ 1 H} spectrum, a doublet (δ = 0.67 ppm, 1 J SiP = 29.4 Hz) at 298 K is observed, which splits into a doublet of doublets ( 1 J SiP = 38.1 Hz; 2 J SiP = 31.4 Hz) at 203 K. Considering that a contact species is proven for 1 in the solid state, one could expect to detect scalar 7 Li-31 P couplings both in the 31 P{ 1 H} as well as in the 7 Li NMR spectra.…”

“…In the 29 Si{ 1 H} spectrum, a doublet (δ = 0.67 ppm, 1 J SiP = 29.4 Hz) at 298 K is observed, which splits into a doublet of doublets ( 1 J SiP = 38.1 Hz; 2 J SiP = 31.4 Hz) at 203 K. Considering that a contact species is proven for 1 in the solid state, one could expect to detect scalar 7 Li-31 P couplings both in the 31 P{ 1 H} as well as in the 7 Li NMR spectra. The lack of any coupling at 298 K may either suggest the existence of a solvent-separated ion pair in solution or an exchange process which proceeds fast on the NMR time scale.…”