Monomeric and Oligomeric Amine−Borane σ-Complexes of Rhodium. Intermediates in the Catalytic Dehydrogenation of Amine−Boranes

A combined experimental/quantum chemical investigation of the transition metal-mediated dehydrocoupling reaction of H3B·NMe2H to ultimately give the cyclic dimer [H2BNMe2]2 is reported. Intermediates and model complexes have been isolated, including examples of amine−borane σ-complexes of Rh(I) and Rh(III). These come from addition of a suitable amine−borane to the crystallographically characterized precursor [Rh(η6-1,2-F2C6H4)(PiBu3)2][BArF4] [ArF = 3,5-(CF3)2C6H3]. The complexes [Rh(η2-H3B·NMe3)(PiBu3)2][BArF4] and [Rh(H)2(η2-H3B·NHMe2)(PiBu3)2][BArF4] have also been crystallographically characterized. Other intermediates that stem from either H2 loss or gain have been characterized in solution by NMR spectroscopy and ESI-MS. These complexes are competent in the catalytic dehydrocoupling (5 mol %) of H3B·NMe2H. During catalysis the linear dimer amine−borane H3B·NMe2BH2·NHMe2 is observed which follows a characteristic intermediate time/concentration profile. The corresponding amine−borane σ-complex, [Rh(PiBu3)2(η2-H3B·NMe2BH2·NHMe2)][BArF4], has been isolated and crystallographically characterized. A Rh(I) complex of the final product, [Rh(PiBu3)2{η2-(H2BNMe2)2}][BArF4], is also reported, although this complex lies outside the proposed catalytic cycle. DFT calculations show that the first proposed dehydrogenation step, to give H2BNMe2, proceeds via two possible routes of essentially the same energy barrier: BH or NH activation followed by NH or BH activation, respectively. Subsequent to this, two possible low energy routes that invoke either H2/H2BNMe2 loss or H2BNMe2/H2 loss are suggested. For the second dehydrogenation step, which ultimately affords [H2BNMe2]2, a number of experimental observations suggest that a simple intramolecular route is not operating: (i) the isolated complex [Rh(PiBu3)2(η2-H3B·NMe2BH2·NHMe2)][BArF4] is stable in the absence of amine−boranes; (ii) addition of H3B·NMe2BH2·NHMe2 to [Rh(PiBu3)2(η2-H3B·NMe2BH2·NHMe2)][BArF4] initiates dehydrocoupling; and (iii) H2BNMe2 is also observed during this process.