Iron Precatalysts with Bulky Tri(tert-butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine-Borane

In an attempt to prepare new Fe catalysts for the dehydrocoupling of amine-boranes and to provide mechanistic insight, the paramagnetic Fe^II dimeric complex [Cp′FeI]₂ (1) (Cp′=η⁵-((1,2,4-tBu)₃C₅H₂)) was used as a precursor to a series of cyclopentadienyl Fe^II and Fe^III mononuclear species. The complexes prepared were [Cp′Fe(η⁶-Tol)][Cp′FeI₂] (2) (Tol=C₆H₅Me), [Cp′Fe(η⁶-Tol)][BAr^F ₄] (3) (BAr^F ₄=[B(C₆H₃(m-CF₃)₂)₄]⁻), [N(nBu)₄][Cp′FeI₂] (4), Cp′FeI₂ (5), and [Cp′Fe(MeCN)₃][BAr^F 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][BAr^F 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.