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Reactions of H2 and R3SiH with Electrophilic Cobalt(III) Alkyl Complexes:  Spectroscopic Characterization, Dynamics, and Chemistry of [Cp*Co(L)(H)(η2-H2)][B(ArF)4] and [Cp*Co(L)(H)(η2-HSiR3)][B(ArF)4]

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posted on 03.04.2020, 16:33 by Mark D. Doherty, Brian Grant, Peter S. White, Maurice Brookhart
Co(III) agostic alkyl complexes [Cp*Co(L)(CH2CH2-μ-H)][B(ArF)4] (Cp* = C5(CH3)5, L = P(OCH3)3, 1a, or P(CH3)3, 1b; ArF = 3,5-(CF3)2C6H3) react with H2 to yield ethane and trihydride complexes characterized as η2-dihydrogen hydride species [Cp*Co(L)(H)(η2-H2)][B(ArF)], 2a and 2b, in which there is rapid scrambling between the η2-H2 ligand and the terminal Co−H. Complexes 2a and 2b react with a variety of neutral donor ligands (L‘ = RCN, PMe3, P(OMe)3, H2O, CH3OH) to yield [Cp*Co(L)(L‘)(H)][B(ArF)4] complexes. Reaction of silanes with either 1a,b in the presence of traces of water or 2a,b yields η2-silane hydrides, [Cp*Co(L)(H)(η2-HSiR3)][B(ArF)4]. Analysis of the dynamics of these species by NMR spectroscopy provides evidence for an extremely rapid process involving silyl migration between hydrogens and a slower process in which a cobalt-silyl η2-H2 complex is formed as an intermediate and results in hydrogen scrambling between the two diastereomers of [Cp*Co(L)(H)(η2-H-SiHMePh)][B(ArF)4]. The structures and dynamics of 2a,b and the η2-silane complexes imply that cleavage of the cobalt−alkyl bonds of 1a,b in hydrogenation and hydrosilation catalytic cycles occurs by the σ-CAM (σ-complex-assisted metathesis) process.