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 (<b>1</b>), Me₃N–BH₂–NHMe–BH₃ (<b>2</b>), and MeNH₂–BH₂–NHMe–BH₃ (<b>3</b>). The mechanisms of the thermal and catalytic redistributions of <b>1</b> 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 (<b>8</b>) and [Me₂N–BH₂]₂ (<b>9a</b>)), 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] (<b>5</b>) and NMe₃ as intermediates, with the former operating as a catalyst and accelerating the redistribution of <b>1</b>. Intermediate <b>5</b> was then transformed to amine-borane <b>8</b> and the cyclic diborazane <b>9a</b> by two different mechanisms. In contrast, under catalytic conditions (0.3–2 mol % IrH₂POCOP (POCOP = κ³-1,3-(OP<i>t</i>Bu₂)₂C₆H₃)), <b>8</b> was found to inhibit the redistribution of <b>1</b> by coordination to the Ir-center. Furthermore, the catalytic pathway involved direct formation of <b>8</b> and Me₂NBH₂ (<b>9b</b>), which spontaneously dimerizes to give <b>9a</b>, with the absence of <b>5</b> and BH₃ as intermediates. The mechanisms elucidated for <b>1</b> are also likely to be applicable to other diborazanes, for example, <b>2</b> and <b>3</b>, for which detailed mechanistic studies are impaired by complex post-redistribution chemistry. This includes both metal-free and metal-mediated oligomerization of MeNHBH₂ (<b>10</b>) to form oligoaminoborane [MeNH–BH₂]_<i>x</i> (<b>11</b>) or polyaminoborane [MeNH–BH₂]_<i>n</i> (<b>16</b>) following the initial redistribution reaction