Lithium Phosphoranylidene Carbenoids Mes-P(E)C(X)Li(THF)3 (E = NMes, C(SiMe3)2; X = Br, Cl, F):  Synthesis and Structural Investigations in Solution and Solid State

Phosphoranylidene carbenoids Mes*-P(E)C(X){Li(THF)3} (E = C(SiMe3)2, NMes*; X = F, Cl, Br, 10−13, 15) were synthesized and their molecular structures characterized by low-temperature X-ray diffractometry and multinuclear NMR studies. The experiments confirm the presence of monomeric molecules with THF-solvated metal atoms in both solution and solid state. The solid carbenoids display elongated C−X bonds and distortions of carbon bond angles which represent typical features of carbenoids and suggest an interpretation of the bonding situation as a contact ion pair between a carbanion and a solvated metal cation. This is corroborated by NMR studies which show that the dynamic exchange between the two components in solution can be directly monitored in 2D 6Li,31P shift correlations. While structural and spectroscopic data give no evidence for a continuously increasing stability between F-, Cl-, or Br-substituted carbenoids, they reveal the presence of marked structural differences between the E/Z-isomers 10 and 15. As is shown by 2D 6Li,31P NMR spectra, this deviation is accompanied by a noticeable difference in kinetic stabilities, which is assigned to be the cause for the known different reaction behavior of both isomers.