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

Abstract

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