Mechanisms of the Thermal and Catalytic Redistributions, Oligomerizations, and Polymerizations of Linear Diborazanes

Linear diborazanes R3N–BH2–NR2–BH3 (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 MeNH2–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-NMe2)­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-(OPtBu2)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 Me2N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 MeNHBH2 (10) to form oligoaminoborane [MeNH–BH2]x (11) or polyaminoborane [MeNH–BH2]n (16) following the initial redistribution reaction.