Synthesis and Photoelectron Spectroscopic Studies of N(CH2CH2NMe)3P=E (E = O, S, NH, CH2)
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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 provedto be a stronger proton acceptor than proazaphosphatrane N(CH2CH2NMe)3P, since it was shown todeprotonate N(CH2CH2NMe)3PH+. The extremely strong basicity of the ylide is in accordance with its lowionization 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 fora non-conjugated phosphorus ylide. Computations reveal the existence of two bond strech isomers, andthe stabilization of the phosphorus centered cation by electron donation from the equatorial and the axialnitrogens. Similar stabilizing effects operate in the case of protonation of E. A fine balance of these differentinteractions 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 flattensthe pyramidality of this atom, increasing its electron donor capability. As a consequence, the PNax distancein the short-transannular bonded protonated systems and the radical cations is longer by about 0.5 Å inthe Neq(Me) than in the Neq(H) systems. Accordingly, isodesmic reaction energies show that a stabilizationof about 25 and 10 kcal/mol is attributable to the formation of the transannular bond in case of Neq(H) andthe experimentally realizable Neq(Me) species, respectively.
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