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 dimeric complex [Cp'FeI] (1) (Cp'=η -((1,2,4-tBu) C H )) was used as a precursor to a series of cyclopentadienyl Fe and Fe mononuclear species. The complexes prepared were [Cp'Fe(η -Tol)][Cp'FeI ] (2) (Tol=C H Me), [Cp'Fe(η -Tol)][BAr ] (3) (BAr =[B(C H (m-CF ) ) ] ), [N(nBu) ][Cp'FeI ] (4), Cp'FeI (5), and [Cp'Fe(MeCN) ][BAr ] (6). The electronic structure of the [Cp'FeI ] 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 Me NH⋅BH (I) in THF at 20 °C to yield the cyclodiborazane product [Me N-BH ] (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 Me N=BH (II) as the major intermediate, whereas for 6 the linear diborazane Me NH-BH -NMe -BH (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me N⋅BH were investigated in an attempt to identify Fe-based intermediates in the catalytic reactions. The σ-complex [Cp'Fe(MeCN)(κ -H BH⋅NMe H][BAr ] 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.