The Lewis acidity of fluorophosphonium salts: access to mixed valent phosphorus(III)/(V) species.

Oxidative fluorination of the electron-deficient phosphine Ph(2)P(C(6)F(5)) using XeF(2), followed by fluoride ion abstraction from the resulting difluorophosphorane Ph(2)P(F)(2)(C(6)F(5)), produces electrophilic fluorophosphonium salts [Ph(2)P(F)(C(6)F(5))][X] (X = FB(C(6)F(5))(3) or O(3)SCF(3)). Variable temperature NMR spectroscopic analysis of [Ph(2)P(F)(C(6)F(5))][FB(C(6)F(5))(3)] demonstrates a fluxional process attributed to fluoride ion exchange between B(C(6)F(5))(3) and [Ph(2)P(F)(C(6)F(5))](+), suggesting that these species have comparable Lewis acidities. This exchange can also be illustrated by adding phosphine Ph(3)P to [Ph(2)P(F)(C(6)F(5))][FB(C(6)F(5))(3)] at ambient temperature to produce Ph(2)P(F)(2)(C(6)F(5)) and Ph(3)P-B(C(6)F(5))(3), while heating this mixture results in thermally induced para-substitution of Ph(3)P at the C(6)F(5) group of the phosphonium ion to generate [Ph(3)P(C(6)F(4))P(F)(2)Ph(2)][FB(C(6)F(5))(3)]. Such frustrated Lewis pair reactivity also can be exploited by reacting [Ph(2)P(F)(C(6)F(5))][O(3)SCF(3)] with silylphosphine Ph(2)PSiMe(3) to afford the unique mixed-valent salt [Ph(2)P(C(6)F(4))P(F)Ph(2)][O(3)SCF(3)], which upon the addition of fluoride is converted to Ph(2)P(C(6)F(4))P(F)(2)Ph(2). XeF(2) reacts with [Ph(2)P(C(6)F(4))P(F)Ph(2)][O(3)SCF(3)] at ambient temperature, producing equal proportions of the dicationic salt [Ph(2)P(F)(C(6)F(4))P(F)Ph(2)][O(3)SCF(3)](2) and the bis(difluorophosphorane) Ph(2)P(F)(2)(C(6)F(4))P(F)(2)Ph(2), the latter of which can then be quantitatively converted to the former by adding one equiv of Me(3)SiO(3)SCF(3).