Dehydrocoupling of Dimethylamine Borane Catalyzed by Rh(PCy₃)₂H₂Cl

The Rh­(III) species Rh­(PCy3)2H2Cl is an effective catalyst (2 mol %, 298 K) for the dehydrogenation of H3B·NMe2H (0.072 M in 1,2-F2C6H4 solvent) to ultimately afford the dimeric aminoborane [H2BNMe2]2. Mechanistic studies on the early stages in the consumption of H3B·NMe2H, using initial rate and H/D exchange experiments, indicate possible dehydrogenation mechanisms that invoke turnover-limiting N–H activation, which either precedes or follows B–H activation, to form H2BNMe2, which then dimerizes to give [H2BNMe2]2. An additional detail is that the active catalyst Rh­(PCy3)2H2Cl is in rapid equilibrium with an inactive dimeric species, [Rh­(PCy3)­H2Cl]2. The reaction of Rh­(PCy3)2H2Cl with [Rh­(PCy3)­H2(H2)2]­[BArF4] forms the halide-bridged adduct [Rh­(PCy3)2H2(μ-Cl)­H2(PCy3)2Rh]­[BArF4] (ArF = 3,5-(CF3)2C6H3), which has been crystallographically characterized. This dinuclear cation dissociates on addition of H3B·NMe2H to re-form Rh­(PCy3)2H2Cl and generate [Rh­(PCy3)2H2(η2-H3B·NMe2H)]­[BArF4]. The fate of the catalyst at low catalyst loadings (0.5 mol %) is also addressed, with the formation of an inactive borohydride species, Rh­(PCy3)2H2(η2-H2BH2), observed. On addition of H3B·NMe2H to Ir­(PCy3)2H2Cl, the Ir congener Ir­(PCy3)2H2(η2-H2BH2) is formed, with concomitant generation of the salt [H2B­(NMe2H)2]­Cl.