Dehydrocoupling of phosphine-boranes using the [RhCp∗Me(PMe 3 )(CH 2 Cl 2 )][BAr F4 ] precatalyst: Stoichiometric and catalytic studies

Abstract

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 F4 ], 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.