Dehydrocoupling of phosphine-boranes using the [RhCp*Me(PMe)(CHCl)][BAr] 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, HB·PPhR'H (R = Ph, H), using [RhCp*(PMe)Me(ClCHCl)][BAr], to either form polyphosphino-boranes [HB·PPhH] ( ∼ 15 000 g mol, PDI = 2.2) or the linear diboraphosphine HB·PPhBH·PPhH. A likely polymer-growth pathway of reversible chain transfer step-growth is suggested for HB·PPhH. 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 (HB·PPhHBH·PPhH). It is also shown that by changing the substituent at phosphorus (Ph or Cy Bu) 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.