Cyclische Triphosphenium-Ionen: Selektiver intramolekularer Ligandenaustausch in einer nucleophilen Substitution am zweifachkoordinierten Phosphor [1] / Cyclic Triphosphenium Ions: Selective Intramolecular Exchange of Ligand in a Nucleophilic Substitution at the Two-Coordinate Phosphorus [1]

Phenyl substituted di-, tri- and tetraphosphines with the donor-functions in 1,2- or 1,3-position effectively replace Ph3P from Ph3P-P-PPh3 +AlCl4 −yielding cyclic five- and six-membered triphosphenium cations. In the 1:1-products of PhP(-CH2-CH2-PPh2)2 (4) and MeC(-CH2-PPh2)3 (2) the unused third phosphine function may interact as a nucleophile with the central phosphorus leading to a threefold T-shaped or pyramidal coordination thereof, respec­tively. From this attack, in the former case only, a rapid intramolecular exchange (ΔH≠ = 9.6 kcal mol-1; ΔS≠= −9.9 cal mol-1 K-1) results, involving the two phosphine functions in the apical sites of the T-shaped transition geometry. This mechanism is suggested to describe the nucleophilic substitution at two-coordinate phosphorus in general. The cyclic triphosphenium cations can be protonated (and in part alkylated) to yield cyclic triphosphine-dionium cations.