Dehydrocoupling of phosphine-boranes using the [RhCp∗Me(PMe₃)(CH₂Cl₂)][BAr^F ₄] precatalyst: Stoichiometric and catalytic studies

We report a detailed, combined experimental and computational study on the fundamental B-H and P-H bond activation steps involved in the dehydrocoupling/dehydropolymerization of primary and secondary phosphine-boranes, H₃B·PPhR′H (R = Ph, H), using [RhCp∗(PMe₃)Me(ClCH₂Cl)][BAr^F ₄], to either form polyphosphino-boranes [H₂B·PPhH]_n (M_n ∼ 15 000 g mol^-1, PDI = 2.2) or the linear diboraphosphine H₃B·PPh₂BH₂·PPh₂H. A likely polymer-growth pathway of reversible chain transfer step-growth is suggested for H₃B·PPhH₂. Using secondary phosphine-boranes as model substrates a combined synthesis, structural (X-ray crystallography), labelling and computational approach reveals: initial bond activation pathways (B-H activation precedes P-H activation); key intermediates (phosphido-boranes, α-B-agostic base-stabilized boryls); and a catalytic route to the primary diboraphosphine (H₃B·PPhHBH₂·PPhH₂). It is also shown that by changing the substituent at phosphorus (Ph or Cy versus^tBu) different final products result (phosphido-borane or base stabilized phosphino-borane respectively). These studies provide detailed insight into the pathways that are operating during dehydropolymerization.