Dehydrocoupling of phosphine-boranes using the [RhCp*Me(PMe3)(CH2Cl2)][BAr4F] precatalyst: stoichiometric and catalytic studies

摘要

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 center dot PPhR'H (R = Ph, H), using [RhCp*(PMe3)Me(ClCH2Cl)][BAr4F], to either form polyphosphino-boranes [H2B center dot PPhH](n) (M-n similar to 15 000 g mol(-1), PDI = 2.2) or the linear diboraphosphine H3B center dot PPh2BH2 center dot PPh2H. A likely polymer-growth pathway of reversible chain transfer step-growth is suggested for H3B center dot 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, alpha-B-agostic base-stabilized boryls); and a catalytic route to the primary diboraphosphine (H3B center dot PPhHBH2 center dot PPhH2). It is also shown that by changing the substituent at phosphorus (Ph or Cy versus Bu-t) 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.