The mechanism of formation of cyclic triphosphenium ions; detection of transient intermediates in solution

Abstract

The mechanism of formation of cyclic triphosphenium ions [–(CH₂)_nP(R₂)PP(R₂)–]⁺3 from diphosphanes R₂P(CH₂)_nPR₂ and phosphorus(III) halides PX₃ (X = Cl or Br) has been unequivocally established for the six-membered heterocycles with R = Et, ⁱPr or c-Hex, n = 3, and for five-membered rings with R = Et, n = 2. The initial stage is the formation of an acyclic species, [R₂P(CH₂)_nP(R₂)PX₂]⁺X⁻1. The cation of this species cyclises to a symmetrical dication [–R₂P(CH₂)_nP(R₂)P(X)−]²⁺2 by loss of halide, where the middle P atom has an X group attached and is still formally P(III). The rate-determining step is then a redox reaction to form the final cyclic monocationic product 3, with a ‘bare’ middle P atom. Several transient intermediate species, including the precursor cyclic dication 2 in each case, have been identified by means of ³¹P NMR solution-state spectroscopy.