Synthesis, photoelectron spectroscopy and quantum chemical study of kinetically unstabilized phosphines complexed by borane

Abstract

Ethynyl- and allenylphosphine-boranes have been prepared by addition at low temperature of borane on the free phosphine. Purification was performed by selective trapping in vacuo and the complexes were characterized by NMR and infrared spectroscopy and mass spectrometry. A kinetic stability lower than that of the corresponding free systems was observed. A series composed of these compounds and methyl-, vinyl-, allyl- and propargylphosphine-boranes was investigated by photoelectron spectroscopy and B3LYP/aug-cc-pVTZ quantum chemical study in order to define the variation in the electronic effects between the free systems and the corresponding complexes. Although the complexation only led to minor changes in the unsaturated moiety, the P–C bond shortens in all cases because of the charge transfer from phosphorus to boron. Similar rotamers can be found in the complexes and the free systems, and the order of the relative stability is reversed only in the case of the allenyl derivative. The calculated complexation energies are between 80–100 kJ mol⁻¹ in agreement with flash vacuum thermolysis experiments. The photoelectron spectra can be easily described in the case of α,β-unsaturated compounds by the change of the direct conjugation between the lone electron pair and the π-bond in the free phosphines to hyperconjugation of the σ_P–B bond with the unsaturated moiety in the corresponding complexed derivatives. In the case of β,γ-unsaturated derivatives the observed hyperconjugation in phosphines disappears on complexation.