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 AgI complexes was also found
to be unlikely. The resulting AgIII 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.