Importance of the Pd and Surrounding Sites in Hydrosilylation of Internal Alkynes by Palladium–Gold Alloy Catalyst

Recently,
the Pd–Au alloy catalyst has been developed for
the hydrosilylation of internal alkynes as well as α,β-unsaturated
ketones under mild conditions. In this work, the mechanism of the
hydrosilylation reaction of internal alkynes on the Pd–Au bimetallic
cluster and the Pd–Au(111) alloy surface has been investigated
by density functional theory calculations. The calculated energy profiles
show that the reaction follows the Chalk–Harrod mechanism.
The rate-determining step is the hydrometalation in the Pd–Au
cluster while it is the Si–C reductive elimination in the Pd–Au(111)
alloy surface. The Pd site acts as the adsorption site and the reactive
center as observed in experiments. The surrounding Pd–Au bridge
and Au sites are also relevant for the bond activation and accepting
the substrates or intermediates during the reaction, which is characteristic
in the Pd–Au alloy catalysts and not available in the homogeneous
catalyst. The hydrosilylation reaction proceeds without too much high
energy barriers and too stable intermediates in the Pd–Au alloy
catalyst. The monometallic Au and Pd catalysts are not preferable
because of the high energy barriers (Au) or too stable intermediates
(Pd). The present picture of the relevance of the Pd atomic site and
its surrounding Pd–Au bridge or Au sites will be useful for
developing the alloy catalysts for the related catalytic reactions.