Tertiary phosphine adducts of mixed halogens, R3PlBr; synthesis and structure in the solid state and solution

Abstract

The first extensive solid-state study of compounds of formula R3PIBr [R3=(p-CIC6H4)3, Ph3, Ph2Prn, Ph2Me, PhMe2 or Bun3] has been made, most having not previously been described. The crystal structure of Ph3PIBr shows it to be a surprisingly complex molecule containing dual occupancy of the halogen sites within the unit cell, existing predominantly as Ph3PIBr. Nevertheless, it is clear that the molecule is a further example of the four-co-ordinate molecular ‘spoke’ structure, Ph3P–X–X, previously established for Ph3PX2(X = Br or I) and Ph3ASI2. X-Ray powder diffraction studies on analytically pure samples of Ph3PIBr prepared the same way indicated that subtly different compounds are formed containing varying degrees of occupancy for the two sites described above; however, the molecule is always predominantly Ph3P–I–Br. Raman spectroscopic studies on all the R3PIBr compounds illustrate the dominance of the P–I bond in the spectrum; however, in some cases a minor peak assignable to v(P–Br) was noted, in agreement with the crystallographic studies. Solution 31P-{H} NMR studies in CDCI3 indicate complete ionisation of the compounds R3P–I–Br to [R3PI]Br; no evidence for the corresponding [R3PBr]I was observed. Results for the compound Bun3PIBr were inconclusive however. This suggests halogen exchange occurs in solution, in agreement with the findings of previous workers.