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 Li-6, P-31 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 En-isomers 10 and 15. As is shown by 2D Li-6, P-31 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.