Hydrogen release from dialkylamine-boranes promoted by Mg and Ca complexes: A DFT analysis of the reaction mechanism

Mg and Ca β-diketiminato silylamides [HC{(Me)CN(2,6-iPr 2C6H3)}2M(THF) n{N(SiMe3)2}] (M=Mg, n=0; M=Ca, n=1) were studied as precatalysts for the dehydrogenation/dehydrocoupling of secondary amine-boranes R2HNBH3. By reaction with equimolar quantities of amine-boranes, the corresponding amidoborane derivatives are formed, which further react to yield dehydrogenation products such as the cyclic dimer [BH2-NMe2]2. DFT was used here to explore the mechanistic alternatives proposed on the basis of the experimental findings for both Mg and Ca amidoboranes. The influence of the steric demand of amine-boranes on the course of the reaction was examined by performing calculations on the dehydrogenation of dimethylamine-borane (DMAB), pyrrolidine-borane (PB), and diisopropylamine-borane. In spite of the analogies in the catalytic activity of Mg- and Ca-based complexes in the dehydrocoupling of amine-boranes, our theoretical analysis confirmed the experimentally observed lower reactivity of Ca complexes. Differences in catalytic activity of Mg- and Ca-based complexes were examined and rationalized. As a consequence of the increase in ionic radius on going from Mg2+ to Ca2+, the dehydrogenation mechanism changes and formation of a key metal hydride intermediate becomes inaccessible. Dimerization is likely to occur off-metal in solution for DMAB and PB, whereas steric hindrance of iPr2NHBH 3 hampers formation of the cyclic dimer. The reported results are of particular interest because, although amine-borane dehydrogenation is now well established, mechanistic insight is still lacking for many systems. Dehydrocoupling of amine-boranes assisted by Mg- and Ca-based complexes was studied by DFT calculations to assess the viability of the pathways of the mechanistic scheme proposed on the basis of experimental evidence and to determine whether different ionic radii and charge densities are responsible for the observed differences in reactivity of Mg and Ca complexes (see scheme). The influence of the steric demand of amine-boranes on the course of the reaction was examined by performing calculations on the dehydrogenation of dimethylamine-borane, pyrrolidine-borane, and diisopropylamine-borane. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.