Hexaphenyl-1,2-Diphosphonium Dication [Ph3P–PPh3]2+: Superacid, Superoxidant, or Super Reagent?

Abstract

The oxidation of triphenylphosphine by perfluorinated phenazinium^F aluminate in difluorobenzene affords hexaaryl-1,2-diphosphonium dialuminate 1. Dication 1²⁺ is valence isoelectronic with elusive hexaphenylethane, where instead the formation of a mixture of the trityl radical and Gomberg’s dimer is favored. Quantum-chemical calculations in combination with Raman/IR spectroscopies rationalize the stability of the P–P bonded dimer in 1²⁺ and suggest, akin to the halogens, facile homolytic as well as heterolytic scission. Thus, 1²⁺ serves as a surrogate of both the triphenylphosphorandiylium dication (Ph₃P²⁺) and the triphenylphosphine radical monocation (Ph₃P^·+). Treating 1 with dimethylaminopyridine (DMAP) or ^tBu₃P replaces triphenylphosphine under heterolytic P–P bond scission. Qualifying as a superoxidant (E vs Fc/Fc⁺ = +1.44 V), 1 oxidizes trimethylphosphine. Based on halide abstraction experiments (^–BF₄, ^–PF₆, ^–SbCl₆, ^–SbF₆) as well as the deoxygenation of triethylphosphine oxide, triflate anions as well as toluic acid, 1 also features Lewis superacidity. The controlled hydrolysis affords Hendrickson’s reagent, which itself finds broad use as a dehydration agent. Formally, homolytic P–P bond scission occurs with diphenyldisulfide (PhSSPh) and the triple bonds in benzo- and acetonitrile. The irradiation by light cleaves the P–P bond homolytically and generates transient triphenylphosphine radical cations, which engage in H-atom abstraction as well as CH phosphoranylation.