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

Alasdair P. M. Robertson, Erin M. Leitao, Titel Jurca, Mairi F. Haddow, Holger Helten*, Guy C. Lloyd-Jones, Ian Manners

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

38 Citations (Scopus)
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Abstract

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 center dot 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(mu-H)(mu-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 = kappa(3)-1,3-(OPtBu2)(2)C6H3)), 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.

Original languageEnglish
Pages (from-to)12670-12683
Number of pages14
JournalJournal of the American Chemical Society
Volume135
Issue number34
DOIs
Publication statusPublished - 13 Aug 2013

Keywords

  • AMINE-BORANE ADDUCTS
  • N-HETEROCYCLIC CARBENE
  • MOLECULAR-WEIGHT POLYAMINOBORANES
  • FREE HYDROGEN-TRANSFER
  • AMMONIA-BORANE
  • TRANSITION-METAL
  • METALLOCENE COMPLEXES
  • AMINOBORANE COMPLEXES
  • DIMETHYLAMINE-BORANE
  • METHYLAMINE-BORANE

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