Synthesis and Photoelectron Spectroscopic Studies of N(CH₂CH₂NMe)₃PE (E = O, S, NH, CH₂)

The synthesis and the crystal and molecular structure of N(CH2CH2NMe)3PCH2 is reported. The P−Nax distance is rather long in N(CH2CH2NMe)3PCH2. The ylide N(CH2CH2NMe)3PCH2 proved to be a stronger proton acceptor than proazaphosphatrane N(CH2CH2NMe)3P, since it was shown to deprotonate N(CH2CH2NMe)3PH+. The extremely strong basicity of the ylide is in accordance with its low ionization energy (6.3 eV), which is the lowest in the presently investigated series N(CH2CH2NMe)3PE (E: CH2, NH, lone pair, O and S), and to the best of our knowledge it is the smallest value observed for a non-conjugated phosphorus ylide. Computations reveal the existence of two bond strech isomers, and the stabilization of the phosphorus centered cation by electron donation from the equatorial and the axial nitrogens. Similar stabilizing effects operate in the case of protonation of E. A fine balance of these different interactions determines the P−Nax distance, which is thus very sensitive to the level of the theory applied. According to the quantum mechanical calculations, methyl substitution at the equatorial nitrogens flattens the pyramidality of this atom, increasing its electron donor capability. As a consequence, the PNax distance in the short-transannular bonded protonated systems and the radical cations is longer by about 0.5 Å in the Neq(Me) than in the Neq(H) systems. Accordingly, isodesmic reaction energies show that a stabilization of about 25 and 10 kcal/mol is attributable to the formation of the transannular bond in case of Neq(H) and the experimentally realizable Neq(Me) species, respectively.