Exploration of isomeric bis(phosphanyl)-substituted carbenium ions: Identification and synthesis of the most stable isomer

Isomeric bis(phosphanyl) carbenium ions have been investigated by theoretical [MP2/6-31G(d,p) level of theory] and experimental means. On the potential energy surface (PES) of phosphorus cations of the formula [P ₂CH₅]⁺, five stable isomers are found. Their structures follow the stability order (ΔE_rel/kcal mol ^-1): P-protonated diphosphiranium, [(H₂P)(HP)CH ₂]⁺ 1a (0.0) > C-phosphonio phosphaalkene, [H ₃P-HC=PH]⁺ 2a (4.5) ≫ bis(phosphanyl) carbenium, [CH(PH₂)₂]⁺ 3a (18.5) > C-phosphanyl methylenephosphonium, [H₂P-CH= PH₂]⁺ 4a (22.6) ≫ diphosphirenium, 5a (47.1). The stability order is very different for the family of the four related stable [P₂CH(NH₂) ₄]⁺ ions 1b, 2b, 4b, 5b in which the phosphorus-bound hydrogens (and one carbon-bound hydrogen in the case of 1b) have been replaced by NH₂ groups: 2b (-39.6) ≫ 1b (0.0) > 5b (2.2) ≥ 4b (3.7), None of these ions has a structure comparable to the classical amidinium ion [HC(NH₂)₂]⁺. While all amino carbenium ions have a positively charged carbon atom, all phosphorus analogues carry a negative charge on carbon. In some, even the p(π)-population exceeds unity suggesting that these should be considered as cationic methanides. Experimentally, a derivative of the diphosphiranium ion 1a could be prepared by methylation of 1,2-bis-(2,4,6-tri-tert-butylphenyl)diphosphirane with MeO ₃SCF₃, and this new compound 8 was characterized by NMR methods. Although the most stable member in this family, it could only be observed under stable ion conditions (T < -40°C, five-fold excess of MeO₃SCF₃) and decomposes at higher temperatures.