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

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