Cobalt-Catalyzed Ammonia Borane Dehydrogenation: Mechanistic Insight and Isolation of a Cobalt Hydride-Amidoborane Complex

Joseph W. Nugent, Max García-Melchor, Alison R. Fout

Research output: Contribution to journalArticlepeer-review

Abstract

A Co(I) complex featuring the electron-rich monoanionic bis(carbene) aryl pincer ligand (MesCCC) (MesCCC = bis(2,4,6-trimethylphenyl-benzimidazol-2-ylidene)phenyl), (MesCCC)Co-py (1), was found to catalytically dehydrogenate ammonia borane (NH3BH3, AB) in THF at 60 °C. This process releases 1.7 ± 0.1 equiv of H2 per AB with simultaneous formation of both soluble (borazine and polyborazylene) and insoluble (poly(aminoborane)) BN-containing species. To help elucidate the Co species present under the catalytic conditions, 1 was reacted with a stoichiometric amount of AB in THF at room temperature, yielding (MesCCC)CoH(NH2BH3) (2), the first characterized hydride-amidoborane complex of a late transition metal. This complex was characterized by multinuclear (1H, 11B, and 13C) NMR and IR spectroscopies as well as X-ray crystallography. Formation of 2 via N-H activation of AB across the Co(I) center of 1 was confirmed by the reaction of 1 with the deuterated isotopologues of AB and was supported computationally by means of density functional theory (DFT) calculations. Isolated 2 was shown to catalyze AB dehydrogenation, forming BN-containing products similar to 1, albeit at a slower rate. In both reactions starting with 1 or 2 as the catalyst, 2 is observed throughout the catalytic dehydrogenation of AB. DFT calculations revealed plausible pathways for the formation of aminoborane (NH2BH2) from 2, the generation of soluble BN-containing products, as well as the on-metal oligomerization of AB to produce the insoluble polymeric species. The complexes reported herein represent rare examples of homogeneous cobalt catalysts for the dehydrogenation of AB.

Original languageEnglish (US)
Pages (from-to)2917-2927
Number of pages11
JournalOrganometallics
Volume39
Issue number15
DOIs
StatePublished - Aug 10 2020

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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