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 alpha,beta-unsaturated compounds by the change of the direct conjugation between the lone electron pair and the pi-bond in the free phosphines to hyperconjugation of the sigma(P-B) bond with the unsaturated moiety in the corresponding complexed derivatives. In the case of beta,gamma-unsaturated derivatives the observed hyperconjugation in phosphines disappears on complexation.

Can an Amine Be a Stronger Acid than a Carboxylic Acid? The Surprisingly High Acidity of Amine–Borane Complexes

“…Hence, one important common feature of these interactions between closed‐shell systems is that the deformation of the interacting subunits usually triggers significant, even dramatic, changes in their chemical properties. This change in chemical properties has been found in the case of many complexes involving BH 3 and some of its derivatives, for which these interactions are particularly strong 10–15. The important point we want to emphasize here is that the deformation plays a crucial role when these complexes are formed, so that the strength of the interaction actually can only be correctly rationalized by taking into account the effects that the deformation has on the donor and the acceptor properties of the interacting systems 16.…”

Spontaneous dehydrocoupling in peri-substituted phosphine–borane adducts

“…28 In addition, steric hindrance arising from the peri-geometry is likely to further destabilise the bis(borane) adduct 4c with respect to the monoborane adduct 6. 28 In addition, steric hindrance arising from the peri-geometry is likely to further destabilise the bis(borane) adduct 4c with respect to the monoborane adduct 6.…”

Synthesis of 3-stannyl and 3-silyl propargyl phosphanes and the formation of a phosphinoallene

“…The unsaturated carbon centres are also heavily deshielded (δ c 137.4 ( J PC 19.0 Hz) C 1 ; 209.9 ( J PC 11.3 Hz) C 2 ), the latter in particular being more characteristic of an allenic, 40 rather than alkynic centre; indeed, these data are in good agreement with those for the limited range of phosphinoallenes § ( Table 2) described previously. 14,41 We thus confidently formulate 15 as (Et 2 N) 2 PC-(Ph)vCvCH 2 (Scheme 2).…”