Structure and Conformation of Molecules R₃PCHPR₂ and R₃SINHPR₂

Triphenylphosphonium (diisopropylphosphino)methylide, Ph₃PCHP(i-Pr)₂ (1), was synthesized from Ph₃P̿CH₂ and (i-Pr)₂PCl. Its NMR data suggest a conformation with the lone pair of electrons at the P^III atom in a trans orientation relative to the ylide C-H bond. The i-Pr₂PCH pyramid shows one of the highest activation barriers of inversion recorded for tertiary phosphines (≫40 kcal/mol). The ¹³C NMR spectrum of 1 displays diastereotopic methyl groups for the neat liquid even at +200 °C. Severe electronic repulsion of the carbanionic and phosphine lone pairs in the transition state are a plausible explanation for this phenomenon. The X-ray crystal structure analysis of Ph₃SiNHPPh₂,2 (containing an isoelectronic skeleton), confirmed the conformational predictions for 1. 2 crystallizes in the triclinic space group PÍ with α= 9.914(5) Å, b = 14.418 (8)Å, c = 19.558 (9) Å, α = 110.48 (4)°, β = 103.54 (4)°, γ = 99.21 (4)°, V = 2454.6 ų, and d_calcd= 1.244 g/cm³ for Z = 4 at-40 °C. Full-matrix least-squares refinement produced R = 0.083, R_w= 0.084, w = k/σ²(F₀), and k = 2.6 for 295 refined parameters and 5382 observed reflections with F₀ ≥ 4.0σ(F₀). Both crystallographically independent molecules show a trans conformation of the P^III lone pair and the N-H bond as a consequence of the minimization of steric and electronic repulsive interactions. A comparison with the structures of related molecules, such as diphosphinoamines or phosphino ylides, shows the close parallels.