The role of neutral Rh(PONOP)H, free NMe2H, boronium and ammonium salts in the dehydrocoupling of dimethylamine-borane using the cationic pincer [Rh(PONOP)(η2-H2)]+ catalyst

Abstract

The σ-amine-borane pincer complex [Rh(PONOP)(η¹-H₃B·NMe₃)][BAr^F₄] [2, PONOP = κ³-NC₅H₃-2,6-(OP^tBu₂)₂] is prepared by addition of H₃B·NMe₃ to the dihydrogen precursor [Rh(PONOP)(η²-H₂)][BAr^F₄], 1. In a similar way the related H₃B·NMe₂H complex [Rh(PONOP)(η¹-H₃B·NMe₂H)][BAr^F₄], 3, can be made in situ, but this undergoes dehydrocoupling to reform 1 and give the aminoborane dimer [H₂BNMe₂]₂. NMR studies on this system reveal an intermediate neutral hydride forms, Rh(PONOP)H, 4, that has been prepared independently. 1 is a competent catalyst (2 mol%, ∼30 min) for the dehydrocoupling of H₃B·Me₂H. Kinetic, mechanistic and computational studies point to the role of NMe₂H in both forming the neutral hydride, via deprotonation of a σ-amine-borane complex and formation of aminoborane, and closing the catalytic cycle by reprotonation of the hydride by the thus-formed dimethyl ammonium [NMe₂H₂]⁺. Competitive processes involving the generation of boronium [H₂B(NMe₂H)₂]⁺ are also discussed, but shown to be higher in energy. Off-cycle adducts between [NMe₂H₂]⁺ or [H₂B(NMe₂H)₂]⁺ and amine-boranes are also discussed that act to modify the kinetics of dehydrocoupling.