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 center dot NMe2H to ultimately give the cyclic dimer [H2BNMe2](2) is reported. Intermediates and model complexes have been isolated, including examples of amine-borane sigma-complexes of Rh(I) and Rh(III). These come from addition of a suitable amine-borane to the crystallographically characterized precursor [Rh(eta(6)-1,2-F2C6H4)((PBu3)-Bu-i)(2)][BAr4F] [Ar-F= 3,5-(CF3)(2)C6H3]. The complexes [Rh(eta(2)-H3B center dot NMe3)((PBu3)-Bu-i)(2)][BAr(F)4] and [Rh(H)(2)(eta(2)-H3B center dot NHMe2)((PBu3)-Bu-i)(2)][BAr4F] have also been crystallographically characterized. Other intermediates that stem from either H-2 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 center dot NMe2H. During catalysis the linear dimer amine-borane H3B center dot NMe2BH2 center dot NHMe2 is observed which follows a characteristic intermediate time/concentration profile. The corresponding amine-borane sigma-complex, [Rh((PBu3)-Bu-i)(2)(eta(2)-H3B center dot NMe2BH2 center dot NHMe2)][BAr4F], has been isolated and crystallographically characterized. A Rh(I) complex of the final product, [Rh((PBu3)-Bu-i)(2){eta(2)-(H2BNMe2)(2)}][BAr4F], 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 H-2/H2B=NMe2 loss or H2B=NMe2/H-2 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((PBu3)-Bu-i)(2)(eta(2)- H3B center dot NMe2BH2 center dot NHMe2)][BAr4F] is stable in the absence of amine-boranes; (ii) addition of H3B center dot NMe2BH2 center dot NHMe2 to [Rh((PBu3)-Bu-i)(2)(eta(2)-H3B center dot NMe2BH2 center dot NHMe2)][BAr4F] initiates dehydrocoupling; and (iii) H2B=NMe2 is also observed during this process.