Trivinylphosphineborane (CH₂═CH)₃PBH₃ and related compounds

In an investigation of the vinyl-activation in trivinylphosphine by acceptor molecules the 1 ∶ 1 addition compounds with BH₃, BCl₃, BBr₃, and BI₃ have been prepared. The reaction of Vi₃P with BF₃ led to polymerization of the phosphine and its adduct. While the complexes (Vi₃P)BX₃ with X = Cl, Br and I are air-stable colorless crystalline solids, (Vi₃P)BH₃ is a colorless, distillable liquid. The NMR spectra show little variation in the chemical shifts and coupling constants of the nuclei of the vinyl groups with the nature of the halogen, but there are large differences as compared to the borane complex (X = H). The crystal structures of the three boron trihalide complexes are also very similar. The components Vi₃P and BX₃ have a staggered conformation and the vinyl groups are in an all-trans propeller orientation. The variations in the bond distances and angles are very small and do not indicate major trends within the series. As shown by the successful preparation and structure elucidation of the reference compound (Et₃P)BF₃, the instability of (Vi₃P)BF₃ is not due to an intrinsically low affinity of BF₃ to tertiary phosphines or a steric effect of the phosphine. (Et₃P)BF₃ has an exceedingly long P–B donor–acceptor bond. It appears that BF₃ forms a weak complex, but is most effective in activating the vinyl groups for polymerization. In (Vi₃P)BH₃ the three vinyl groups and the three B–H function would match for an intermolecular triple-hydroboration to give a three-dimensional polymer with C–C and P–B structural units, but neither of the two functionalities appears to be sufficiently activated to accomplish this reaction.