The role of neutral Rh(PONOP)H, free NMe 2 H, boronium and ammonium salts in the dehydrocoupling of dimethylamine-borane using the cationic pincer [Rh(PONOP)(η 2 -H 2 )] + catalyst

The σ-amine-borane pincer complex [Rh(PONOP)(η 1 -H 3 B·NMe 3 )][BAr F 4 ] [ 2 , PONOP = κ 3 -NC 5 H 3 -2,6-(OP t Bu 2 ) 2 ] is prepared by addition of H 3 B·NMe 3 to the dihydrogen precursor [Rh(PONOP)(η 2 -H 2 )][BAr F 4 ], 1 . In a similar way the related H 3 B·NMe 2 H complex [Rh(PONOP)(η 1 -H 3 B·NMe 2 H)][BAr F 4 ], 3 , can be made in situ , but this undergoes dehydrocoupling to reform 1 and give the aminoborane dimer [H 2 BNMe 2 ] 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 H 3 B·Me 2 H. Kinetic, mechanistic and computational studies point to the role of NMe 2 H 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 [NMe 2 H 2 ] + . Competitive processes involving the generation of boronium [H 2 B(NMe 2 H) 2 ] + are also discussed, but shown to be higher in energy. Off-cycle adducts between [NMe 2 H 2 ] + or [H 2 B(NMe 2 H) 2 ] + and amine-boranes are also discussed that act to modify the kinetics of dehydrocoupling.