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

The sigma-amine-borane pincer complex [Rh(PONOP)(eta(1)-H3B center dot NMe3)][BAr4F] [2, PONOP = kappa(3)-NC5H3-2,6-((OPBu2)-Bu-t)(2)] is prepared by addition of H3B center dot NMe3 to the dihydrogen precursor [Rh(PONOP)(eta(2)-H-2)][BAr4F], 1. In a similar way the related H3B center dot NMe2H complex [Rh(PONOP)(eta(1)-H3B center dot NMe2H)][BAr4F], 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%, similar to 30 min) for the dehydrocoupling of H3B center dot Me2H. Kinetic, mechanistic and computational studies point to the role of NMe2H in both forming the neutral hydride, via deprotonation of a sigma-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.