In 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)][BArF 4] (3) (BArF 4=[B(C6H3(m-CF3)2)4]−), [N(nBu)4][Cp′FeI2] (4), Cp′FeI2 (5), and [Cp′Fe(MeCN)3][BArF 4] (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 1–6 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 1–4 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 1–4 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][BArF 4] 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.
- iron catalysts