Theoretical Study on Homogeneous Hydrogen Activation Catalyzed by Cationic Ag(I) Complex

Recently, the Li group reported the first Ag-catalyzed hydrogenation of aldehydes in water, demonstrating the utility of Ag complexes in homogeneous catalytic transformations through hydrogen activation. In the present study, density functional theory methods have been used to study the mechanism of Ag-catalyzed hydrogen activation. Three possible pathways, including base-assisted hydrogen activation, ligand-assisted hydrogen activation, and oxidative addition were investigated. The ligand-assisted hydrogen activation is disfavored because the neutral biaryl phosphine ligand XPhos is not a competent proton acceptor and results in the destruction of the aromaticity of an aryl group. Oxidative addition of H₂ on Ag^I complexes was also found to be unlikely. The resulting Ag^III hydride complexes are highly unstable and can undergo spontaneous reduction due to the weakly electron-donating ligand and the relatively low electronegativity of hydrogen. By contrast, the base-assisted hydrogen activation mechanism is more favored. This mechanism mainly includes three steps: base-assisted heterolytic H–H bond cleavage, hydride transfer, and protonation. Hydride transfer is the rate-determining step of the whole catalytic cycle. In addition, the ligand XPhos was found to coordinate with the Ag center by both the phosphine and the isopropyl-substituted phenyl groups, and this coordination mode is able to facilitate hydrogen activation