The Lewis acidity of fluorophosphonium salts: access to mixed valent phosphorus(iii)/(v) species

Abstract

Oxidative fluorination of the electron-deficient phosphine Ph₂P(C₆F₅) using XeF₂, followed by fluoride ion abstraction from the resulting difluorophosphorane Ph₂P(F)₂(C₆F₅), produces electrophilic fluorophosphonium salts [Ph₂P(F)(C₆F₅)][X] (X = FB(C₆F₅)₃ or O₃SCF₃). Variable temperature NMR spectroscopic analysis of [Ph₂P(F)(C₆F₅)][FB(C₆F₅)₃] demonstrates a fluxional process attributed to fluoride ion exchange between B(C₆F₅)₃ and [Ph₂P(F)(C₆F₅)]⁺, suggesting that these species have comparable Lewis acidities. This exchange can also be illustrated by adding phosphine Ph₃P to [Ph₂P(F)(C₆F₅)][FB(C₆F₅)₃] at ambient temperature to produce Ph₂P(F)₂(C₆F₅) and Ph₃P–B(C₆F₅)₃, while heating this mixture results in thermally induced para-substitution of Ph₃P at the C₆F₅ group of the phosphonium ion to generate [Ph₃P(C₆F₄)P(F)₂Ph₂][FB(C₆F₅)₃]. Such frustrated Lewis pair reactivity also can be exploited by reacting [Ph₂P(F)(C₆F₅)][O₃SCF₃] with silylphosphine Ph₂PSiMe₃ to afford the unique mixed-valent salt [Ph₂P(C₆F₄)P(F)Ph₂][O₃SCF₃], which upon the addition of fluoride is converted to Ph₂P(C₆F₄)P(F)₂Ph₂. XeF₂ reacts with [Ph₂P(C₆F₄)P(F)Ph₂][O₃SCF₃] at ambient temperature, producing equal proportions of the dicationic salt [Ph₂P(F)(C₆F₄)P(F)Ph₂][O₃SCF₃]₂ and the bis(difluorophosphorane) Ph₂P(F)₂(C₆F₄)P(F)₂Ph₂, the latter of which can then be quantitatively converted to the former by adding one equiv of Me₃SiO₃SCF₃.