Dehydrocoupling of phosphine–boranes using the [RhCp*Me(PMe(3))(CH(2)Cl(2))][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, H(3)B·PPhR′H (R = Ph, H), using [RhCp*(PMe(3))Me(ClCH(2)Cl)][BAr(F)(4)], to either form polyphosphino-boranes [H(2)B·PPhH](n) (M(n) ∼ 15 000 g mol(–1), PDI = 2.2) or the linear diboraphosphine H(3)B·PPh(2)BH(2)·PPh(2)H. A likely polymer-growth pathway of reversible chain transfer step-growth is suggested for H(3)B·PPhH(2). 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 (H(3)B·PPhHBH(2)·PPhH(2)). It is also shown that by changing the substituent at phosphorus (Ph or Cy versus(t)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.