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Iron precatalysts with bulky tri(tert‐butyl)cyclopentadienyl ligands for the dehydrocoupling of dimethylamine‐borane

Research output: Contribution to journalArticle

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Author(s)

J Turner, N Chilton, Amit Kumar, G Whittell, A Colebatch, H I Sparkes, A S Weller, I Manners

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Abstract

n an attempt to prepare new Fe catalysts for the dehydrocoupling of amine‐boranes and to provide mechanistic insight, the paramagnetic FeII dimeric complex [Cp′FeI]2 (1) (Cp′=η5‐((1,2,4‐tBu)3C5H2)) was used as a precursor to a series of cyclopentadienyl FeII and FeIII mononuclear species. The complexes prepared were [Cp′Fe(η6‐Tol)][Cp′FeI2] (2) (Tol=C6H5Me), [Cp′Fe(η6‐Tol)][BArF4] (3) (BArF4=[B(C6H3(m‐CF3)2)4]), [N(nBu)4][Cp′FeI2] (4), Cp′FeI2 (5), and [Cp′Fe(MeCN)3][BArF4] (6). The electronic structure of the [Cp′FeI2] anion in 2 and 4 was investigated by SQUID magnetometry, EPR spectroscopy and ab initio Complete Active Space Self Consistent Field‐Spin Orbit (CASSCF‐SO) calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 16 were investigated as catalysts for the dehydrocoupling of Me2NH⋅BH3 (I) in THF at 20 °C to yield the cyclodiborazane product [Me2N‐BH2]2 (IV). Complexes 14 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra‐violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 14 proceeded via formation of the aminoborane Me2N=BH2 (II) as the major intermediate, whereas for 6 the linear diborazane Me2NH‐BH2‐NMe2‐BH3 (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me3N⋅BH3 were investigated in an attempt to identify Fe‐based intermediates in the catalytic reactions. The σ‐complex [Cp′Fe(MeCN)(κ2‐H2BH⋅NMe2H][BArF4] was proposed to initially form in dehydrocoupling reactions involving 6 based on ESI‐MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR spectroscopic evidence. The latter also suggests that these complexes function as precursors to iron hydrides which may be the true catalytic species.
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Original languageEnglish
Pages (from-to)14127-14136
JournalChemistry - A European Journal
Volume24
Issue number53
Early online date24 Aug 2018
DOIs
Publication statusPublished - 21 Sep 2018

    Research areas

  • Amines, Boranes, Catalysis, Dehydrocoupling, Iron catalysts

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