Iron Precatalysts with Bulky Tri(tert‐butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine‐Borane. Issue 53 (24th August 2018) - Turner, Joshua
Abstract: 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′=η 5 ‐((1, 2, 4‐ t Bu)3 C5 H2 )) was used as a precursor to a series of cyclopentadienyl Fe II and Fe III mononuclear species. The complexes prepared were [Cp′Fe(η 6 ‐Tol)][Cp′FeI2 ] (2 ) (Tol=C6 H5 Me), [Cp′Fe(η 6 ‐Tol)][BAr F 4 ] (3 ) (BAr F 4 =[B(C6 H3 ( m ‐CF3 )2 )4 ] − ), [N( n Bu)4 ][Cp′FeI2 ] (4 ), Cp′FeI2 (5 ), and [Cp′Fe(MeCN)3 ][BAr F 4 ] (6 ). The electronic structure of the [Cp′FeI2 ] − anion in2 and4 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. Complexes1 –6 were investigated as catalysts for the dehydrocoupling of Me2 NH⋅BH3 (I ) in THF at 20 °C to yield the cyclodiborazane product [Me2 N‐BH2 ]2 (IV ). Complexes1 –4 and6 were active dehydrocoupling catalysts towardsI (5 mol % loading), however5 was inactive, and ultra‐violet (UV) irradiation was required for the reaction mediated by3 . Complex6 was found to be the most active precatalyst, reaching 80 % conversion toIV after 19 h at 22 °C. Dehydrocoupling ofI by1 –4 proceeded via formation of the aminoborane Me2 N=BH2 (II ) as the major intermediate, whereas for6 the linear diborazane Me2 NH‐BH2 ‐NMe2 ‐BH3 (III ) could be detected, together with trace amounts ofII . Reactions of1 and6 with Me3 N⋅BH3 were investigated in an attempt to identify Fe‐based intermediates in the catalytic reactions. The σ‐complex [Cp′Fe(MeCN)(κ 2 ‐H2 BH⋅NMe2 H][BAr F 4 ] was proposed to initially form in dehydrocoupling reactions involving6 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. Abstract : A series of paramagnetic and diamagnetic Fe complexes is reported and their behaviour as precatalysts for the dehydrocoupling of amine‐boranes is described. Mechanistic studies suggest the operation of complex mechanisms and that σ‐amine‐borane complexes are formed as intermediates in some cases. However, these species function as precursors to iron hydrides that may operate as the true active catalysts.