Mechanisms of the thermal and catalytic redistributions, oligomerizations, and polymerizations of linear diborazanes

Article abstract of DOI:10.1021/ja404247r

Linear diborazanes R₃N-BH₂-NR₂-BH ₃ (R = alkyl or H) are often implicated as key intermediates in the dehydrocoupling/dehydrogenation of amine-boranes to form oligo- and polyaminoboranes. Here we report detailed studies of the reactivity of three related examples: Me₃N-BH₂-NMe₂-BH₃ (1), Me₃N-BH₂-NHMe-BH₃ (2), and MeNH ₂-BH₂-NHMe-BH₃ (3). The mechanisms of the thermal and catalytic redistributions of 1 were investigated in depth using temporal-concentration studies, deuterium labeling, and DFT calculations. The results indicated that, although the products formed under both thermal and catalytic regimes are identical (Me₃N·BH₃ (8) and [Me₂N-BH₂]₂ (9a)), the mechanisms of their formation differ significantly. The thermal pathway was found to involve the dissociation of the terminal amine to form [H₂B(μ-H)(μ-NMe ₂)BH₂] (5) and NMe₃ as intermediates, with the former operating as a catalyst and accelerating the redistribution of 1. Intermediate 5 was then transformed to amine-borane 8 and the cyclic diborazane 9a by two different mechanisms. In contrast, under catalytic conditions (0.3-2 mol % IrH₂POCOP (POCOP = κ³-1,3-(OPtBu ₂)₂C₆H₃)), 8 was found to inhibit the redistribution of 1 by coordination to the Ir-center. Furthermore, the catalytic pathway involved direct formation of 8 and Me₂Ni - BH₂ (9b), which spontaneously dimerizes to give 9a, with the absence of 5 and BH₃ as intermediates. The mechanisms elucidated for 1 are also likely to be applicable to other diborazanes, for example, 2 and 3, for which detailed mechanistic studies are impaired by complex post-redistribution chemistry. This includes both metal-free and metal-mediated oligomerization of MeNHi - BH₂ (10) to form oligoaminoborane [MeNH-BH ₂]_x (11) or polyaminoborane [MeNH-BH₂] _n (16) following the initial redistribution reaction.