Aromatic Phosphenium Cations

The aromatic stabilization of cyclic phosphenium cations (general type C2N2P+) was studied by experimental methods (synthesis, multinuclear NMR, single crystal X-ray crystallography) and quantum chemical calculations (ab initio methods). The structures of the 1,3,2-diazaphosphole derivatives [(tBuN–CH=CH–NtBu)P+]Cl− (1), (tBuN–CH2–CH2–NtBu)P–Cl (2), [(tBuN–CH=CH–NtBu)P]+PF6− (3) and [(tBuN–CH2–CH2-NtBu)]P+PF6− (4) were examined by single crystal X-ray diffraction. The chloro phosphane [(tBuN–CH=CH–NtBu)P]+Cl− (1) has an ionic P–Cl bond and contains an aromatically stabilized phosphenium cation [shortest distance P···Cl = 275.9(2) pm], while the CC-saturated compound (tBuN–CH2–CH2–NtBu)P–Cl (2) is covalent. The two chloro-phosphanes 1 and 2 differ sharply in their volatility and solubility in organic solvents. Compound 2 is soluble in hydrocarbons and sublimes readily at 90 °C/0.1 Torr but 1 is insoluble in hexanes and not volatile below 180 °C/0.1 Torr. The degree of aromatic stabilization in the phosphenium cation 1 was investigated by ab initio methods. For the model cations [RN–CH2–CH2–NR]P+ and [(RN–CH=CH–NR)P]+, thermochemical stabilization energies of 25.8 kcal · mol−1 (R = H) and 28.1 kcal · mol−1 (R = Me) were obtained from isodesmic hydrogenation reactions at the RHF/MP2/6–31G*//RHF/6–31G* level.