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.