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

The oxidation of triphenylphosphine by perfluorinated phenazinium^F aluminate in difluorobenzene affords hexaaryl-1,2-diphosphonium dialuminate <b>1</b>. Dication <b>1</b>^<b>2+</b> 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 <b>1</b>^<b>2+</b> and suggest, akin to the halogens, facile homolytic as well as heterolytic scission. Thus, <b>1</b>^<b>2+</b> serves as a surrogate of both the triphenylphosphorandiylium dication (Ph₃P²⁺) and the triphenylphosphine radical monocation (Ph₃P^·+). Treating <b>1</b> with dimethylaminopyridine (DMAP) or <i>^t</i>Bu₃P replaces triphenylphosphine under heterolytic P-P bond scission. Qualifying as a superoxidant (<i>E</i> vs Fc/Fc⁺ = +1.44 V), <b>1</b> oxidizes trimethylphosphine. Based on halide abstraction experiments (<b>^-</b>BF₄, <b>^-</b>PF₆, <b>^-</b>SbCl₆, <b>^-</b>SbF₆) as well as the deoxygenation of triethylphosphine oxide, triflate anions as well as toluic acid, <b>1</b> 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.