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

The mechanism of formation of cyclic triphosphenium ions [-(CH2)(n)P(R-2)PP(R-2)-](+) 3 from diphosphanes R2P(CH2)(n)PR2 and phosphorus(III) halides PX3 (X = Cl or Br) has been unequivocally established for the six-membered heterocycles with R = Et, Pr-i 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)(n)P(R-2)PX2]X-+(-) 1. The cation of this species cyclises to a symmetrical dication [-R2P(CH2)(n)P(R-2)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 P-31 NMR solution-state spectroscopy.