The role of neutral Rh(PONOP)H, free NMe2H, boronium and ammonium salts in the dehydrocoupling of dimethylamine-borane using the cationic pincer [Rh(PONOP)(η2-H2)]+ catalyst

E. Anastasia K. Spearing-Ewyn, Nicholas A. Beattie, Annie L. Colebatch, Antonio J. Martínez-Martínez, Andrew Docker, Timothy M. Boyd, Gregg Baillie, Rachel Reed, Stuart A. Macgregor, Andrew S. Weller

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Abstract

The σ-amine-borane pincer complex [Rh(PONOP)(η1-H3B·NMe3)][BArF4] [2, PONOP = κ3-NC5H3-2,6-(OPtBu2)2] is prepared by addition of H3B·NMe3 to the dihydrogen precursor [Rh(PONOP)(η2-H2)][BArF4], 1. In a similar way the related H3B·NMe2H complex [Rh(PONOP)(η1-H3B·NMe2H)][BArF4], 3, can be made in situ, but this undergoes dehydrocoupling to reform 1 and give the aminoborane dimer [H2BNMe2]2. NMR studies on this system reveal an intermediate neutral hydride forms, Rh(PONOP)H, 4, that has been prepared independently. 1 is a competent catalyst (2 mol%, ∼30 min) for the dehydrocoupling of H3B·Me2H. Kinetic, mechanistic and computational studies point to the role of NMe2H in both forming the neutral hydride, via deprotonation of a σ-amine-borane complex and formation of aminoborane, and closing the catalytic cycle by reprotonation of the hydride by the thus-formed dimethyl ammonium [NMe2H2]+. Competitive processes involving the generation of boronium [H2B(NMe2H)2]+ are also discussed, but shown to be higher in energy. Off-cycle adducts between [NMe2H2]+ or [H2B(NMe2H)2]+ and amine-boranes are also discussed that act to modify the kinetics of dehydrocoupling.

Original languageEnglish
Pages (from-to)14724-14736
Number of pages13
JournalDalton Transactions
Volume48
Issue number39
Early online date20 Sep 2019
DOIs
Publication statusPublished - 21 Oct 2019

ASJC Scopus subject areas

  • Inorganic Chemistry

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    Spearing-Ewyn, E. A. K., Beattie, N. A., Colebatch, A. L., Martínez-Martínez, A. J., Docker, A., Boyd, T. M., Baillie, G., Reed, R., Macgregor, S. A., & Weller, A. S. (2019). The role of neutral Rh(PONOP)H, free NMe2H, boronium and ammonium salts in the dehydrocoupling of dimethylamine-borane using the cationic pincer [Rh(PONOP)(η2-H2)]+ catalyst. Dalton Transactions, 48(39), 14724-14736. https://doi.org/10.1039/c9dt03358k