Group 8 and 9 Bis(phosphino)silyl Pincer Complexes: Applications in Bond Activation and Catalysis
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Advances in homogenous transition metal catalysis are vital for the development of improved, sustainable methodologies in chemical syntheses. Tridentate pincer ligands are highly tunable and are compatible with late 3d-, 4d-, and 5d-metals with applications in bond activation and catalysis. Metal-ligand cooperativity has proven integral in catalysis mediated by Earth-abundant 3d-metals, such as iron. Research in the Turculet group is focused on the reactivity of bis(phosphino)silyl PSiP pincer complexes. A key feature of PSiP ligation is the strongly σ-donating silyl donor, which promotes challenging bond activation by platinum group metals. Furthermore, the facile formation of η2-(Si-H) species with 3d-metals may play a key role in catalysis. The versatility exhibited by PSiP ligation with precious (Rh and Ir) and base (Fe) metals is highlighted in this thesis. While (Cy-PSiP)IrI (Cy-PSiP = κ3-(2-Cy2PC6H4)2SiMe) undergoes facile N-H oxidative addition of various amines to form complexes of the type (Cy-PSiP)IrH(NHR), subsequent insertion of unsaturated substrates (e.g., alkenes, alkynes) into the Ir-NHR linkage proved challenging. To further explore this chemistry for the development of new hydroamination reactions, the synthesis of indolyl(phosphino)silyl (iPr-PSiPInd) Rh and Ir complexes was targeted. While (iPr-PSiPInd)MH(NHR) (M = Rh, Ir) complexes could be synthesized, (iPr-PSiPInd)IrI undergoes competitive N-H and C-H oxidative addition of anilines, while the Rh analogue proved relatively unreactive. Subsequent insertion of alkenes and alkynes into the Ir-NHR linkage remained challenging, whereas the facile insertion of CO2 afforded isolable Ir carbamato hydride complexes. The utility of (PSiP)Fe complexes in hydrogenation catalysis was also investigated. The MgBr2-bridged diiron complex [(Cy-PSiP)FeBr]2(MgBr2) was found to mediate diarylalkyne hydrogenation under mild conditions. Unusual selectivity for the complete hydrogenation of alkynes was observed, with no reactivity for alkene hydrogenation. The mechanism of this reaction is being investigated using DFT methods in collaboration with Dr. Erin Johnson (Dalhousie). Results suggest that metal-silyl cooperativity plays a key role in this catalysis, involving geminal transfer of H2 to a single alkyne carbon via an Fe(II) carbene intermediate. Comparative studies of (iPr-PSiPInd)Fe coordination chemistry and reactivity are also described.